The Carbonic Anhydrase Promoted Carbon Dioxide Capture

Abstract

To match simultaneously the climate change mitigation with the increasing global demand for energy is a tremendous paradox for this century. To satisfy both criteria, carbon capture seems to be a mandatory technology for the development of sustainable energy infrastructures. Post-combustion capture is a mature and proven technology, but not economically attractive unless novel solvents and optimized processes are implemented. The use of carbonic anhydrase, inspired by the CO₂ metabolic process in cells, a natural fast biocatalyst, is a promising technique which can dramatically improve the implementation and economics of carbon capture under stringent environment demands. In this tutorial review, the authors address the state of the art of the carbonic anhydrase-driven processes for carbon capture, recent developments, current and prospective research and engineering achievements.

1.1 Introduction

Global warming , resulting from the continuously increasing Earth’s temperature , has become a major focus of the environmental agenda worldwide. The tremendous rise in greenhouse gases is thought to be one of the most contributing factors to global warming. Six gases have been identified and reported as major contributors to the global warming, i.e., carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), hydrofluorocarbons (HFC), perfluorocarbons (PFC), and sulfur hexafluoride (SF₆), among which, CO₂ plays a key role in global warming. Carbon dioxide can be emitted from various sources including the burning of coal and fossil fuels (Figueroa et al. 2008).

According to the IPCC (2017), there are four main anthropogenic greenhouse gases, namely, methane (CH₄), fluorinated gases, nitrous oxides (NO_x), and carbon dioxide (CO₂). In 2004, 1.1% of the total anthropogenic greenhouse gas emissions were attributable to fluorinated gases, 7.9% to nitrous oxides, 14.3% to methane, and 76.7% to CO₂ (IPCC 2017). It is well noted that CO₂ is by far the greenhouse gas of anthropogenic origin whose emissions are the most abundant. Coal, oil, and natural gas-fired power plants which release over nine billion metric tons every year worldwide of carbon dioxide are the overwhelming anthropogenic sources of CO₂ emission. The forecasted consumption of coal and fossil fuels is estimated by the US Department of Energy (DOE) to increase by 27% over the next 20 years, and the overall CO₂ emissions from India and China in 2030 from coal use will be around three times that of the Unites States (1371 million tons of CO₂ for India, 3226 million tons for the United States, and 8286 million tons for China) (DOE 2016).

Henceforth, it is mandatory at present to address the effect of global environmental changes due to increasing emission of CO₂. Many approaches have been suggested such as switching from fossil fuel to alternate renewable energy sources such as nuclear, solar, or wind energy (Sims et al. 2003).

Most of the political and apolitical organizations worldwide are urging the use of alternate energy sources with less or no greenhouse gas emissions. However, considering the increasing energy demand, the complete substitution of fossil fuels by clean energies is extremely difficult. Furthermore, steadying the atmospheric concentration of CO₂ is equally important and requires developing new technologies or improving existing ones to alleviate this issue through various mechanisms and protocols.

The CO₂ capture and sequestration has several features. A potential solution to stabilize and ultimately reduce the release of CO₂ into the atmosphere is the implementation of carbon capture and sequestration (CCS) technology which is a combinatorial implementation of CO₂ separation from industrial and energy-related sources, transport to a storage location, and long-term isolation from the atmosphere. This technology, which is one of the upcoming fields of interests, requires a thoughtful strategy development to mitigate the impact of CO₂ as greenhouse gas on environment (Wang et al. 2011). There are three methods for capturing CO₂ from industrial emissions (natural gas combustion fumes, coal, and fuel oil), pre-combustion, oxy-fuel capture, and post-combustion (Abu-Khader 2006), as detailed in Fig. 1.1.

Fig. 1.1

Carbon dioxide capture systems

• In the pre-combustion processes, the primary fossil fuel is gasified in a first reactor by injection of steam and air to produce a mixture of carbon monoxide and hydrogen . Subsequently, the mixture is introduced into a second reactor where steam is added. A mixture of mainly CO₂ and hydrogen is obtained from the reaction between steam and carbon monoxide. This mixture can be separated into a stream of CO₂ and a stream of hydrogen. The hydrogen flux separated from the mixture can be used as a “green” carbon-free fuel, and the CO₂ can be stored or used for many industrial purposes (Blomen et al. 2009).

• The oxycombustion process is characterized by the use of pure oxygen instead of air as oxidizer in the combustion of the primary fossil fuel, producing a gas mixture solely composed of water vapor and CO₂. Following oxycombustion, the gas flow is cooled down and compressed to separate the water and making it possible to recover a gas flow with a very high CO₂ content (more than 80 vol. %) (Blomen et al. 2009).

• Whereas, the post-combustion processes involve separation of dilute CO₂ from flue gas after fuel combustion where air is used, which results in a CO₂ low concentration flue gas. The post-combustion is the most promising technology among the three strategies of CO₂ capture , since it can be integrated to new power plants and applied to existing power plants. Besides, it has a relatively lower cost and provides flexibility to the power plant (Leung et al. 2014).

Various techniques have been developed to achieve CO₂ capture , the majority of which are, however, too expensive and of limited efficiency. At present, one of the most viable technology options uses amine solvents to remove CO₂ from flue gases. Therefore, some biological methodologies, also called “bio-mimetic” CO₂ capture systems, are being implemented as more economic and more sustainable technologies. These methods are based on the use of enzymes involved in the CO₂ biological processes, occurring naturally in living organisms such as the respiratory system in mammalian cells or photosynthetic systems in plant cells. The carbonic anhydrases (specifically the EC 4.2.1.1) catalyze the reversible hydration of the CO₂ molecule and could be efficiently used in these processes (Di Fiore et al. 2015). On the basis of CO₂-catalyzing enzymes, the “bio-mimic” CO₂ capture systems can show high performance and efficiency in CO₂ capture and release comparable to biomechanisms (Di Fiore et al. 2015). The last decade has seen the emergence of one of the most innovative technologies in the field of CO₂ capture , namely, the use of carbonic anhydrase, an enzyme that catalyzes the CO₂ hydration reaction very efficiently (k _h ≈ 10⁻⁶ s⁻¹) (Whitford 2005).

Although the number of new research articles published has recently increased significantly, only few papers, to our knowledge, have specifically reviewed the use of enzymes in this field such as Pierre (2012), Yadav  and coworkers (Yadav et al. 2014), Shekh and coworkers (Shekh et al. 2012), and Long and their respective coworkers (Long et al. 2017).

This paper aims to provide a state-of-the-art evaluation of the research programs carried out so far in the carbonic anhydrase accelerated carbon dioxide capture. This paper will introduce the beginners to this technology with a summary of literatures. For experienced scientists, this paper will review the available achievements and predict the progress of future research directions. In addition to previous publications on carbon dioxide capture (Figueroa et al. 2008) and enzyme accelerated CO₂ capture (Pierre 2012; Shi et al. 2015), this paper will:

• (a) Elaborate historical and recent discoveries of carbonic anhydrase and its usage for carbon capture

• (b) Draw the reader’s attention on enzyme kinetic mechanisms for carbon dioxide capture

• (c) Discuss thoroughly the carbonic anhydrase uses in free and immobilized forms

• (d) Provide an update of major research activities worldwide and important pilot plant studies

In Sect. 1.2, the major carbon capture techniques are thoroughly reviewed. Then, Sect. 1.3 overviews the enzymatic carbon capture with an emphasis on the enzyme immobilization . Section 1.4 is a detailed survey of the kinetics and catalytic mechanisms of carbon dioxide capture promoted by carbonic anhydrase. In Sect. 1.5, most recent achievements on enhanced enzymatic carbon capture are overviewed. Then, major research programs worldwide and experimental studies based on pilot plants are reviewed in Sect. 1.6. Conclusions are drawn at the end of the article.

1.2 Carbon Dioxide Capture Processes

Various methods are available for carbon capture from product gas streams. Some of the more commonly used methods include absorption (physical and chemical), membrane contacting, adsorption, and cryogenic separation. Conventional processes with some emerging technologies involving a combination of products and/or processes are briefly reviewed in this section.

1.2.1 Physical Absorption

The physical absorption of CO₂ into a solvent involves Henry’s law where atoms or molecules transfer from a gas phase into a liquid phase. The solubility of the solute is sensitive to the partial pressure of the gas to be removed. The solvent regeneration is achieved mainly by desorption, i.e., by pressure reduction (flashing), some additional heating and sometimes both. However, physical solvents can usually be stripped of impurities by reducing the pressure without any heat addition. The main energy requirements originate from the flue gas pressurization because the physical absorption takes place at high CO₂ partial pressures (IEA 2004). In addition, physical solvents can usually be stripped of impurities by reducing the pressure without any heat addition. Furthermore, heat requirements are usually much less for physical solvents than for chemical ones such as amines since the heat of desorption of the acid gas for the physical solvent is only a fraction of that for the chemical ones (Dindore et al. 2004a).

In physical absorption, CO₂ is transferred from gas to liquid phase without chemical reaction with the absorbent. This process is suitable for bulk removal of CO₂ from gas streams having a high CO₂ partial pressure. Furthermore, this technique is easy to design, not very toxic with a low solvent loss but has limited CO₂ selectivity . It is not, therefore, suitable for the treatment of power plant flue gases with low CO₂ partial pressure (Olajire 2010).

The physical absorption is therefore not economical for flue gas streams with CO₂ partial pressures lower than 15 vol. % (Chakravati et al. 2001). Henceforth, physical solvents tend to be favored over chemical solvents when the concentration of acid gases or other impurities is very high. Typical physical absorption solvents used in industry are propylene carbonate (Fluor), n-methyl-2-pyrrolidone (Purisol), methanol (Rectisol and Ifpexol), and dimethyl ethers of polyethylene glycol (Selexol), some of which are becoming increasingly efficient (Green et al. 2004).

1.2.2 Chemical Absorption

Chemical absorption is dictated by the chemical reaction of CO₂ with a solvent to form a weakly bonded intermediate compound which may be regenerated with heat addition producing the original solvent and a CO₂ stream. The form of this separation displays a relatively high selectivity and can produce a relatively pure CO₂ stream. These features make chemical absorption well suited for CO₂ capture for industrial flue gas treatment (Dindore et al. 2004a). The acidic nature of dissolved CO₂ in water dictates the types of physical and chemical solvents that would potentially be successful for efficient CO₂ absorption. Applicable chemical solvents include amine solvents and solutions, which result in CO₂ absorption by zwitterion formation and easy deprotonation by a weak base (Boucif et al. 2012). There are many possible solvents and solvent mixtures under investigation for CO₂ absorption, including amines, sterically hindered amines, carbonate solvents, as well as ionic liquids (Vaidya and Kenig 2007).

However, the disadvantage of this technology is the high energy penalty associated with solvent regeneration in the stripping column. Around 80–90% (depending on the process conditions and the solvent) of the total process energy is needed for the solvent regeneration in the desorber, making this the most important chapter in the operational costs (Svendsen et al. 2011; Notz et al. 2011).

Energy is needed to heat up the solution in the desorber to generate stripping steam and reverse the CO₂ reactions (Notz et al. 2011). To optimize the process costs, an obvious approach would be to select a solvent with higher reaction kinetics and lower heat of desorption. Unfortunately, the heat of desorption and the kinetics are interrelated (Svendsen et al. 2011). Solvents with substantially less energy needs (tertiary amine or carbonate salt solutions) require absorber tower heights of several hundred meters for the same separation task. On the other hand, solvents with high reaction rates (primary or secondary amines) need more energy in the desorber reversing the reactions (Penders-van Elk et al. 2013).

1.2.3 Membrane Gas Permeation

In the membrane-based CO₂ capture , gases dissolve and diffuse into polymeric thin film materials (membranes) which provide a selectivity to separate mixtures with respect to relative rates at which constituent species permeate (Powell and Qiao 2006) (Fig. 1.2). The permeation rates would differ based on the relative sizes of the molecules or diffusion coefficients in the membrane material. The driving force for the permeation is the difference in partial pressure of the components at either side of the membrane, and the acid gas is recovered at low pressure (Baker 2004; Boucif et al. 1986). The terms permeability and selectivity are used to describe the performance of a gas separation membrane.

Fig. 1.2

CO₂ capture by membrane gas permeation

The gas permeation rate is controlled by the solubility coefficient and diffusion coefficient of the gas membrane system. Polysulfone, polyimide, or polydimethylsiloxane are the most common membrane materials used in carbon capture in various geometries such as plane, spiral-wound, or hollow fibers (Henis and Tripodi 1980). In the mid-1980, Monsanto (Prism), Cynara (Natco), Separex (UOP), and Grace Membrane Systems started selling membranes made from cellulose acetate to remove CO₂ from CH₄ in natural gas (Ho and Sirkar 1992). For post-combustion carbon capture, block copolymers (such as polyetherblockamides, PEBA) have shown excellent trade-off performances for the CO₂/N₂ gas pair, with a CO₂/N₂ selectivity around 50 and permeability up to 2000 GPU.

Membrane separations are particularly appealing for carbon capture due to their lower energy consumption, good selectivity , easily engineered modules, and consequently lower costs. The main disadvantage of membrane separation is that multiple steps are required to reach high purity. A maximum of three stages is usually reported for industrial applications, due to the increasing cost of compressors for multistage systems. For instance, multistage separation is employed to capture a higher proportion of CO₂ incurring extra capital and operating cost (Chakravati et al. 2001). However, if the gas is available at a high pressure, physical solvents might be a better choice than chemical solvents. Membrane gas separation technique is generally considered as suitable for high CO₂ concentration applications (well above 20 vol. %) such as flue gas streams from oxy-fuel (Favre 2007).

Furthermore, membranes have been extensively used in many industrial separation processes in recent years. The polymeric membranes usually dominate in most industrial applications. The inorganic membranes are progressing faster in the development of new application fields such as membrane reactors and fuel cells. These membrane separation processes are overwhelming the classical processes (Xu et al. 2001).

Based on their structure, the inorganic membranes can be classified into two categories: porous and dense. In the first category, a porous thin top layer is casted on a porous ceramic or metallic support which provides mechanical strength but reduces mass transfer resistance. Alumina, carbon, glass, zeolite, and zirconia membranes are mainly used as porous inorganic membranes supported on different substrates, such as α-alumina, γ-alumina, zirconia, zeolite, or porous stainless steel. This modification changes the mean pore size and promotes an eventual specific interaction between the membrane surface and the permeating molecules enhancing the separation and improving the performance. Gas separation by means of porous inorganic membranes is achieved by four main transport mechanisms, i.e., Knudsen diffusion, surface diffusion, capillary condensation, and molecular sieving (Luebke et al. 2006).

The second category consists of a metallic thin layer such as palladium and its alloys or solid electrolytes such as zirconia. These dense membranes are highly selective for hydrogen or oxygen permeation in which gas transport occurs though a solution–diffusion mechanism. However, the low permeability across the dense inorganic membranes limits their wide applications as compared to porous inorganic membranes (Powell and Qiao 2006).

1.2.4 Membrane Gas–Liquid Absorption

The membrane gas–liquid CO₂ absorption technology act as contacting devices between the gas stream and the liquid solvent where the membrane, as a barrier, may or may not provide additional selectivity . In the gas–liquid membrane contactor concept, flue gas is passed through the lumen of a bundle of membrane fibers, while an absorbent solution is flowed through the shell side of the contactor (Fig. 1.3). CO₂ diffuses through the membrane and is absorbed in the absorbent solution, while the impurities are blocked from contact to amine, thus decreasing the loss of amine as a result of stable salt formation. It is also possible to achieve a higher loading differential between rich amine and lean amine. After leaving the membrane bundle, the amine is regenerated before being recycled (Darde et al. 2010).

Fig. 1.3

Membrane gas–liquid CO₂ absorption contactor

Several absorbents such as pure water, aqueous alkaline solutions, amines, and amino acids have been theoretically and experimentally studied for CO₂ absorption in gas–liquid contacting processes. An ideal absorbent for CO₂ absorption should have the following properties (Dindore et al. 2004b):

• Higher surface tension to prevent membrane wetting leading to a high breakthrough pressure, thereby reducing the membrane susceptibility to membrane wetting

• Chemical compatibility with the membrane (to not damage the membrane)

• Low viscosity to avoid high mass transfer resistance and pressure drop

Unfortunately, the absorbent satisfying all those criteria has not been found yet.

This chemical scrubbing process uses as liquid solvent various aqueous solutions of different alkylamines to remove CO₂. The most popular alkanolamines used in industry are monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA) and some sterrically hindered amines such as 2-amino-2-méthylpropanol (AMP) or a blend of some of them. In addition, ammonia has been identified as a possible alternative to the MEA solvent as it is relatively cheap and commercially available (Davison 2007; Darde et al. 2010).

This process offers some advantages over the conventional contacting devices such as packed towers for their high compactness and their low susceptibility to flooding, entrainment, channeling, or foaming. This process requires, however, that the pressures on the liquid and gas chambers are equal to allow and promote CO₂ transport across the membrane, and consequently, their separation efficiency depends on the CO₂ partial pressure. However, although the amines react with CO₂ rapidly, selectively, and reversibly, and are relatively nonvolatile and inexpensive, they are corrosive and require more expensive construction materials.

1.2.5 Adsorption

The adsorption carbon capture technology involves the contacting of a CO₂-containing phase with a solid adsorbent to which CO₂ (and potentially other components of the gas phase) is adhered, either via physical adsorption (physisorption) or chemical adsorption (chemisorption). The physisorption technique involves sorption through weak molecular interactions, namely, van der Waals forces. On the other hand, the chemisorption technique involves chemical bond formation between the adsorbate (molecule being adsorbed) and adsorbent (solid to which the molecules adsorb), causing it to be energetically favorable to bond to the surface of the adsorbate (Seader and Henley 2006).

The adsorption processes involve the use of “swings” in which the system cycles between states of high amount adsorbed (adsorption) and low amount adsorbed (desorption) to selectively separate components in a fluid stream (typically gas), where certain components of the stream preferentially adsorb over others. The adsorption processes can be categorized as pressure swing adsorption (PSA), vacuum swing adsorption (VSA), temperature swing adsorption (TSA), and electrical swing (ESA) (Seader and Henley 2006).

The adsorbing materials generally used are different types of activated carbon, alumina, molecular sieves, metallic oxides, or zeolites, depending on the gas molecular characteristics and affinity of the adsorbing material (Zhao et al. 2007). These adsorbing materials can preferably adsorb CO₂ from flue gas. The higher the pressure, the more gas is adsorbed and the gas is freed and desorbed while reducing the pressure.

When the adsorbed bed is close to saturation, the regeneration reaction takes place by reducing pressure, thereby freeing the adsorbed gases. It is then ready to cycle again. The advantages of PSA are the direct gas delivery at high pressure (no need of compression), and their disadvantages are high investment and operation costs with extensive process control. The process of VSA is a special case of PSA where the pressure is reduced to near-vacuum condition.

In the case of TSA, adsorbent regeneration is achieved by an increase in temperature as increasing temperature at constant partial pressure decreases the amount adsorbed in the gas phase (or concentration in the liquid phase) (Mason et al. 2011). A very important characteristic of TSA is that it is used exclusively for treating low adsorbate concentration feeds. TSA disadvantages are low energy efficiency and thermal ageing of the adsorbent. In ESA swing, a voltage is applied to heat the adsorbent and release the adsorbed gas. This technique is not very common in industrial practice (Emamipour et al. 2007).

1.2.6 Cryogenic Carbon Capture

The cryogenic CO₂ capture (referred to as CCC) is a physical process that operates at sufficiently low temperatures and moderately high pressures to separate CO₂ and other components according to their different boiling temperatures. This technique produces direct liquefied CO₂ or CO₂ vapor at high pressure saving the additional cost of compression for storage. This method is suitable only for concentrated CO₂ stream. For dilute stream, this technique is not economically sound and energetically viable (Maqsood et al. 2014a, b).

The technique of this process is based on the principle that different gases liquefy under different temperature and pressure conditions. It is a distillation process operated under very low temperatures (close to −170 °C) and high pressure (around 80 bars). The process consists of cooling and compressing the flue gas in order to liquefy CO₂, which is then easily separated from the flue gas. It allows direct production of liquid CO₂ at a low pressure, so that the liquid CO₂ can be stored or sequestered via liquid pumping instead of compression of gaseous CO₂ to a very high pressure, thereby saving on compression energy (Pierce et al. 1995).

This physical process is suitable for treating flue gas streams with high CO₂ concentrations considering the costs of refrigeration. This is typically used for CO₂ capture for oxy-fuel process where CO₂ can potentially be recovered at 99% purity. However, this type of process requires the use of a large amount of equipments and instruments such as turbines, compressors, distillation columns, and heat exchangers (Fig. 1.4).

Fig. 1.4

Simple schematic diagram of the cryogenic carbon capture (CCC) process

For this, the investment capital and operating costs are extremely high (Wellinger and Lindberg 2000). Cryogenic fractionation has the advantage that the CO₂ can be obtained at relatively high pressure as opposed to the other methods of recovering CO₂. This advantage may, however, be offset by the large refrigeration requirement (Hart and Gnanendran 2009).

1.2.7 Metal–Organic Frameworks

The metal–organic frameworks (MOF) are hybrid organic/inorganic structures containing metal ions geometrically coordinated and bridged with organic ligands which hold great potential as adsorbents or membrane materials in gas separation . This arrangement increases surface area for adsorption, enabling them to be used as sorbents or as nanoporous membranes (Furukawa et al. 2015).

The metal–organic framework materials are nanoporous crystals that combine metal–organic complexes with organic linkers to create highly porous frameworks to offer various important advantages for membrane separations such as high surface area, better porosity, low density, and both thermal and mechanical stabilities (Furukawa et al. 2013).

A major breakthrough in the chemistry of CO₂ capture came with the development of reticular chemistry (Fig. 1.5) (Li et al. 2011). The motivation on developing MOFs for CO₂ capture has focused on reversible adsorption, a process that significantly lowers the need for energy input during regeneration and overcomes a key challenge of using traditional sorbents such as alkanolamine solutions. Consequently, the MOFs structures have since been systematically developed, fine-tuned, and studied in detail. The MOF features are (1) the presence of accessible unsaturated metal sites in the pores; (2) the integration of heteroatoms within, as well as covalently linked functionality to, the backbone; (3) the specific interactions of MOF building units; (4) the hydrophobicity of the pores; and (5) a hybrid of these structural features (Trickett et al. 2017).

Fig. 1.5

Some structural design features of effective metal–organic framework (MOF) adsorbents for selective CO₂ capture . (a) Coordinatively unsaturated metal sites ; (b) covalently linked polar functionalities; (c) heteroatomic amines; (d) alkylamines, either primary, secondary, or tertiary; (e) specific nonmetallic interactions within the backbone (or pores) of a MOF structure; (f) hydrophobicity and/or pore metrics for selectively capturing CO₂ in the presence of water (Trickett et al. 2017)

The MOFs possess enormous potential due to the numerous possible structures that can be developed using various combinations of metal ions and organic ligands which can be tailor-made to suit a particular application such as CO₂ capture . MOFs containing zinc and magnesium ions provide higher CO₂ adsorption and are hence being thoroughly investigated (Trickett et al. 2017). The other advantage is the lower energy regeneration required compared to conventional sorbents and solvents. The study of metal–organic frameworks is still in its infancy, with investigations being made primarily on a laboratory scale.

1.3 Enzymatic Carbon Capture Overview

This chapter gives a definition of an enzyme and provides a general overview of the principles of enzyme reactions and describes in detail the mechanism for carbonic anhydrase. It also gives an up-to-date literature review on comparable mass transfer experiments for enzyme-enhanced CO₂ capture in lab and in pilot scale.

1.3.1 Historical Background

The existence of enzymes has been known for well over two centuries. In early nineteenth century, Persoz with Payen isolated in a malt extract a substance that catalyzes the transformation of starch into glucose (Payen and Persoz 1833). The scientists called this substance diastase, from the Greek to separate, due to its ability to separate the constitutive blocks of starch into individual units of glucose. This enzyme is also called g-amylase. This is the first time an enzyme is isolated, a compound that has the properties of an organic catalyst. The suffix ase of diastase has been since then used to name enzymes. However, the first enzyme in pure form was obtained in 1926 by James Sumner who isolated and crystallized the enzyme urease from the jack bean. Thereafter, Northrop and Stanley discovered a complex procedure to isolate pepsin by a precipitation technique and crystallized several enzymes (Roberts et al. 1997).

These enzymes are bulky proteins made of amino acid polymers linked by peptide bonds. They catalyze biochemical reactions occurring in living organisms. Like any other catalyst, they do not modify the thermodynamic equilibria, but allow them to be reached more rapidly. The catalytic properties of enzymes are related to the existence in their structure of an active site, which can be schematically described as having the shape of a cavity adapted specifically to the substrates to be transformed, to which they are fixed by weak chemical bonds, but able to eliminate the random aspect of the contacts prevailing during collisions in homogeneous medium. This active site is in fact subdivided into two parts:

• The binding site (fixation or recognition) consisting of amino acids, characterized by a complementarity of shape of the cavity with a specific substrate to be transformed

• The catalytic site which realizes the transformation of the substrate into a product

1.3.2 Enzymes Classification

The International Enzyme Commission , created at the Third International Biochemistry Congress held in 1955 in Brussels, in agreement with the International Union of Pure and Applied Chemistry (IUPAC), decided to divide the enzymes into six classes according to the chemical reaction they catalyzed, a classification kept up to date by the Nomenclatures Committee of the International Union of Biochemistry and Molecular Biology (Webb 1992). Hence, there are six classes called “EC n,” for “Enzyme Commission number,” where n stands for a number from 1 to 6 designating:

• EC 1: Oxidoreductases that catalyze oxidation–reduction reactions in which oxygen or hydrogen is gained or lost.

• EC 2: Transferases that transfer a functional group of the amino, acetyl, or phosphate type from one molecule to another.

• EC 3: Hydrolases which catalyze the hydrolysis (decomposition by water) of various bonds.

• EC 4: Lyases that catalyze the formation of a C–C, C–O, C–S, or P–O bond by processes other than hydrolysis or oxidation.

• EC 5: Isomerases that catalyze isomerization in a single molecule or allow intramolecular rearrangements.

• EC 6: Ligases that catalyze C–C, C–S, C–O, and C–N bonds in condensation reactions coupled with the use of adenosine triphosphate (= ATP).

Enzymes can be denatured and precipitated with salts, solvents, and other reagents. They have molecular weights ranging from 10,000 to 2,000,000.

1.3.3 Enzyme Catalytic Properties

Many catalysts such as arsenite, formaldehyde, hypochlorite , and sulfide have been used to catalyze the CO₂ absorption into various aqueous solutions (Sharma and Danckwerts 1963; Pohorecki 1968; Augugliaro and Rizzuti 1987). These catalysts can accelerate the CO₂–H₂O hydration reaction by 2–4 orders of magnitude. However, the most effective CO₂ hydration catalyst known to date is the CA family of enzymes. It has been reported that the turnover number of the CA enzyme could reach more than one million per second (Davy 2009).

Enzymes are biological catalysts that reduce the activation energy of chemical and biochemical reactions. Their function is dependent on the amino acid sequence and their three-dimensional structure forming an active site with a catalytic activity into which a certain reactant (substrate S) can bind (Grunwald 2011).

There are interactions between the enzyme (E) and its substrate (S), and usually, van der Waals forces and hydrogen bonding take place to form an enzyme–substrate (ES) complex. The enzyme being a much larger molecule, the substrate fits into an active site of the enzyme molecule. Figure 1.6 shows the simplest lock-and-key interaction model where the enzyme represents the lock and the substrate the key (Whitford 2005).

Fig. 1.6

Schematic reversible enzyme reaction key–lock mechanism

In enzyme catalytic process, the enzyme and its substrate build reversible enzyme–substrate (ES) complex first, and then a chemical reaction occurs with a rate constant kcat called turnover number. The kcat expresses the maximum number of substrate molecules converted into product molecules per active site of enzyme per unit time (Whitford 2005). The reaction rate is expressed by the Michaelis–Menten expression as:

$$ \mathcal{R}=\frac{k_{\mathrm{cat}}\left[\mathrm{E}\right]\left[\mathrm{S}\right]}{K_{\mathrm{M}}+\left[\mathrm{S}\right]} $$

(1.1)

A typical enzymatic reaction curve is shown in Fig. 1.7. The reaction curve can be represented by a Michaelis–Menten equation, Eq. (2.10).

Fig. 1.7

Michaelis–Menten enzymatic reaction curve

The effect of higher product concentration on the enzyme reaction rate is often regarded as product inhibition, but it is basically a reversible reaction between substrate S and product P where both steps are considered reversible and following the Michaelis–Menten kinetics. The decrease in reaction rate with higher product concentration can be explained as the substrate and product are competing for binding onto the enzymes active site and the enzyme becomes more occupied by the product when its concentration increases and therefore less substrate can bind.

1.3.4 Enzyme Immobilization

The enzymes provide high potentialities in a wide range of applications due to their high selectivity , specificity, and activity under mild conditions. These industrial biocatalysts offer tremendous advantages with regard to their short processing time, low energy need, cost-effectiveness, and nontoxicity. Singh and coworkers (Singh et al. 2016) reviewed in detail the current industrial enzyme application, in food, organic synthesis, pharmaceutical and diagnostics, textile, as well as waste treatments.

Nevertheless, the use of enzymes in the industrial applications could be limited by their high cost, their isolation and purification, the instability of their structures once they are isolated from their natural environment, and their sensitivity both to process conditions, resulting in a short processing lifetime. The retention of enzymes by immobilization may be a valid method to overcome these shortcomings (Krajewska 2004; Rodrigues et al. 2013; Dos Santos et al. 2015).

Various immobilization techniques (Fig. 1.8) are available providing a wide flexibility for the solid biocatalyst preparation with regard to the enzyme applications and reactor configurations. These techniques are divided, in general, in three main categories based on the nature of the interaction between the enzyme and other reagents/phases involved in the process (Moehlenbrock and Minteers 2011; Sirisha et al. 2016).

Fig. 1.8

The most common enzyme immobilization techniques : (a) physical adsorption, (b) entrapment, and (c) covalent attachment and cross-linking. (Spahn and Minteer 2008)

1.3.5 Physical Adsorption

This technique is characterized by the physical interactions between proteins and the surface of solid carriers by means of van der Waals forces, hydrogen bridge bonds, and electrostatic interactions (Moehlenbrock and Minteers 2011). The physical adsorption enzyme immobilization is quite simple and may have a higher commercial potential, a lower cost, and a higher retaining enzyme activity as well as a relatively chemical-free enzyme binding (Huang and Cheng 2008).

However, in general, the physical bonding is too weak to keep the enzyme fixed to the carrier and subject to enzyme leaching (Kumakura and Kaetsu 2003), resulting in a considerable contamination of the substrate.

1.3.6 Enzyme Entrapment

The enzyme entrapment is an irreversible enzyme immobilization technique where enzymes are entrapped in a support or inside fibers, either in polymer membranes or in the lattice structures of a material that filtrate the substrate and products from the enzyme (Chiang et al. 2004). The entrapment consists of a physical restriction of the enzyme within a confined network space. Mechanical stability, enzyme leaching, and chemical interaction with polymer are typically improved by the enzyme entrapment immobilization technique (Won et al. 2005). This method modifies the encapsulating material providing therefore an optimal microenvironment for the enzyme, i.e., matching the enzyme physicochemical environment with the immobilizing material. The ideal microenvironment could be optimal pH, polarity, or amphilicity which may be achieved with a variety of materials including polymers, sol–gels, polymer/sol–gel composites, and other inorganic materials (Mohamad et al. 2015).

1.3.7 Covalent Bonding and Cross-Linking

The covalent bonding enzyme immobilization technique is one of the most prominent methods. The formation of covalent bonding is required, for more stable attachment, and these are generally formed through reaction with functional groups present in the protein surface (Guisan 2006). The functional groups’ contribution to the enzyme binding involves side chains of lysine (e-amino group), cysteine (thiol group), and aspartic and glutamic acids (carboxylic group) (Guisan 2006). The activity of the covalent bonded enzyme depends on the coupling method, the carrier material composition, as well as its size and shape and specific conditions during coupling (Mohamad et al. 2015).

The cross-linking enzyme immobilization technique, also called carrier-free immobilization, is another irreversible method which does not require a support to prevent enzyme loss into the substrate solution (Mohamad et al. 2015). In this method, the enzyme acts as its own carrier, and virtually pure enzyme is obtained eliminating, therefore, the drawbacks associated with carriers (Sheldon 2011). The use of carrier leads ineluctably to an activity depletion due to the introduction of a large portion of non-catalytic aggregates, the percentage of which may reach and even exceed 90%, resulting in low space–time efficiencies with a considerable cost (Sheldon 2011). The production of cross-linked enzyme aggregates (CLEA) consists of the formation of enzyme aggregates made of insoluble supramolecular structures and the cross-linking with a bifunctional agent to stabilize the aggregates in the aqueous medium (Barbosa et al. 2014).

1.3.8 Enzyme Immobilization Overview

The quality of the solid biocatalyst depends on the selection of the immobilization technique. In many cases, immobilizing enzymes may cause alter their activity. It provides, however, a great stability improvement under the various process conditions (Rodrigues et al. 2013). Criteria for selecting solid supports include the mechanical properties. The ideal supports for biocatalyst utilization in (a) internal mechanical stirring reactors are flexible polymers such as agarose, cellulose, etc., and (b) fixed bed reactors are rigid structures such as inorganic supports like porous glass, silicates, etc. Besides, the immobilized enzyme entrapment in polymeric matrices may offer a good mechanical resistance (Bentagor et al. 2005).

Several techniques and materials have been used for the CA immobilization, and only the relevant ones are summarized in this review with their relative success in terms of activity, stability, and reusability.

Oviya and Yadav and coworkers obtained good results using chitosan-based nanoparticles or hydrogels (Yadav et al. 2011; Oviya et al. 2012). Both chitosan and alginate are biocompatible and were used in many enzymatic applications (Machida-Sano et al. 2012; Zhai et al. 2013). Sharma and collaborators have purified and immobilized live P. fragi cells to chitosan and were able to observe CaCO₃ precipitation, a measure of catalyzed conversion of CO₂ to bicarbonate (Sharma et al. 2011).

Vinoba and coworkers (Vinoba et al. 2013) immobilized bovine carbonic anhydrase (BCA) covalently onto functionalized Fe₃O₄/SiO₂ nanoparticles by using glutaraldehyde as a spacer. They observed that, after 30 cycles, the Fe–CA displayed strong activity, and the CO₂ capture efficiency was 26-fold higher than that of the free enzyme. They have shown that the magnetic nanobiocatalyst is an excellent reusable catalyst for the sequestration of CO₂. Vinoba and his group synthesized a biocatalyst by immobilizing human carbonic anhydrase onto gold nanoparticles assembled over amine/thiol-functionalized mesoporous SBA-15. They demonstrate that these nanobiocatalysts are highly efficient potential for industrial-scale CO₂ sequestration (Vinoba et al. 2011).

A group of researchers studied other methods to attach the enzyme, including covalent attachment, enzyme adsorption, and cross-linked enzyme aggregation. In general, the enzyme activity was similar to that of the free enzyme , but displayed additional desirable features such as stability, reusability, and storage endurance (Vinoba et al. 2012). Wanjari et al. used mesoporous aluminosilicate as a support for CA immobilization due to its large surface area and pore size. Interestingly, the K _M for the immobilized enzyme was higher compared to the free form, indicating decreased affinity of the enzyme for the substrate due to suboptimal substrate/product exchange (Wanjari et al. 2012).

The enzymes trapping in porous materials are also possible. The original irreversible enzyme entrapment protocol in polyurethane foam was introduced in the 1980s by Wood and his group (Wood et al. 1982). Bovine carbonic anhydrase was immobilized by covalent attachment within a polyurethane (PU) foam matrix (Ozdemir 2009). This process is relatively fast, and a high percentage of active enzymes are covalently obtained in the final PU. In contrast to other materials, after seven cycles, there was no detectable enzyme leaching or a reduction in CA activity. And, after 45 days of storage of the CA-PU foam at room temperatures, it was still 100% active, while the free enzyme was completely inactive after the same period at 4 °C (Ozdemir 2009).

Many groups have attempted to immobilize CA between thin liquid membranes for CO₂ extraction from flue gas. The process comprises a thin liquid containing CA layer sandwiched between two membranes made of some polypropylene derivative strengthened to prevent curving of the pliable membrane. Kimmel and his group studied the covalent immobilization of carbonic anhydrase on the surface of polypropylene hollow fiber membranes using glutaraldehyde-activated chitosan tethering to amplify the density of reactive amine functional groups (Kimmel et al. 2013). Hou and his group developed a novel “Janus” (hydrophilic–superhydrophobic) biocatalytic gas–liquid membrane contactor for CO₂ capture. The carbonic anhydrase (CA) was immobilized on the hydrophilic carbon nanotube (CNT) side, while the superhydrophobic porous side was located in the gas phase, resulting in a permanent hydration of the immobilized CA and a reduction of the CO₂ diffusion in the solvent. The authors confirmed that catalytic efficiency with immobilized CA has significantly improved compared with the equal amount of free CA, and effective enzyme coating regeneration lasted over five cycles.

1.4 Kinetics and Catalytic Mechanisms of Enzymatic Carbon Dioxide Capture

In 1933, the carbonic anhydrase was independently discovered by Meldrum and Roughton (Meldrum and Roughton 1933). CA was first characterized while searching a catalytic factor necessary for fast transportation of ${\mathrm{HCO}}_3^{-}$ from the erythrocyte to pulmonary capillary. Meldrum and Roughton purified the erythrocyte carbonic anhydrase, and Keilin and Martin (Keilin and Mann 1939) presented the role for Zn in catalysis by finding the fact that activity was directly proportional to the Zn content; hence, carbonic anhydrase was the first Zn metalloenzyme identified. CA regulates important biological processes within humans and other living organisms such as the acid–base balance within the blood, the photosynthesis mechanism in plants, and the carbon concentration mechanism in microorganisms (Bhattacharya et al. 2004).

1.4.1 Classes of Carbonic Anhydrase Enzymes

The carbonic anhydrase, an ancient enzyme widespread among the entire prokaryotic and eukaryotic domain, has been known to catalyze the reversible hydration of carbon dioxide as follows:

$$ \left[\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{HC}{\mathrm{O}}_3^{-}+{\mathrm{H}}^{+}\right] $$

(1.2)

The CA can be produced via fermentation , and it may be disposed of with minimum detrimental impact on the environment. CAs are typically classified into five different classes defined by the Enzyme Commission as EC 4.2.1.1, namely, αCAs (predominant within animals), βCAs (predominant within plants), γCAs (predominant within Archaea) (Aggarwal et al. 2013; Rowlett 2010), as well as δCA and ζCA found in diatoms and in other marine phytoplankton (Boone et al. 2013). CA’s are expressed in numerous plant tissues and in different cellular locations, the most prevalent of which are those in the chloroplast, cytosol, and mitochondria. This diversity in location is paralleled in the many physiological and biochemical roles that CAs play in plants (DiMario et al. 2017).

The most commonly investigated class of CA is the α form which is generally found in mammals. Figure 1.9 illustrates the structure of α, β, and γCA. In αCA, the enzymatic activity is derived from a ${\mathrm{Zn}}_2^{+}$ ion that is coordinated to three histidine residues near the center of the molecule in a cone-shaped cavity.

Fig. 1.9

Representative structures of α, β, and γ carbonic anhydrase (CA) enzymes with their respective metal active sites. (Aspatwar et al. 2018)

The catalytically active alpha carbonic anhydrases are similar in structure with their conserved motifs of the active site cavity. To date, the crystallographic structure of human CA-I, CA-II, CA-III, CA-IV, CA-VI, CA-VII, CA-VIII, CA-IX, CA-XII, CA-XIII, and CA-XIV has been determined and is available in the protein data bank (www.PDB.org). All the αCA have similar tertiary structure and centrally bind a divalent metal ion, most often a zinc (${\mathrm{Zn}}_2^{+}$), held as a prosthetic group (Aspatwar et al. 2018).

1.4.2 Carbonic Anhydrase Mechanism

The metal ion Zn atom in all αCAs is essential to catalysis. The structure-based mechanism of human carbonic anhydrase hCAII has been exemplified and detailed (Berg et al. 2010).

The catalytic mechanism of hCA II consists of five distinct steps as reported by many authors (Supuran 2016; Gladis et al. 2017) and detailed in Fig. 1.10.

Fig. 1.10

The overall catalytic mechanism of carbonic anhydrases (Gladis et al. 2017). PC stands for proton channel

The first step in this mechanism is the binding of a CO₂ molecule to the enzyme. The water molecule linked to the amino acid is replaced by a CO₂ molecule which is linked to the enzyme by a hydrogen bond. The formation of a bicarbonate molecule forms in the second step occurring by a nucleophilic attack of the hydroxyl ion bound to the zinc ion on the CO₂ molecule. The bicarbonate molecule is linked through three bonds, two hydrogen bonds and one ionic bond. In the third step, the bicarbonate molecule is released with the partial regeneration of the active site leaving space for a water molecule. In the fourth step, depicted as isomerization or intramolecular proton transfer step, where the proton is first transferred to an amino acid side chain called a proton channel (PC), the enzyme is activated with binding a hydroxyl ion to the zinc ion.

But after the product is released, a water molecule is bound to the zinc ion with a proton expulsion as a result. The fifth and final stage of the mechanism is the intermolecular transfer where a molecule of unprotonated cationic buffer recovers the proton bound to the residue. The cycle is then completed, a bicarbonate molecule is produced, a buffer molecule is protonated, and the enzyme regenerated to its active state. It has been demonstrated that the enzyme recycling is the rate-reaction controlling step in this cycle.

The simplest process may be schematically represented by the following reactions:

$$ \mathrm{C}\mathrm{A}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\kern0.5em {\displaystyle \begin{array}{c}{k}_1\\ {}\rightleftharpoons \\ {}{k}_{-1}\end{array}}\kern0.50em \mathrm{CA}\bullet \mathrm{C}{\mathrm{O}}_2\kern0.5em {\displaystyle \begin{array}{c}{k}_{\mathrm{cat}}\\ {}\rightleftharpoons \\ {}{k}_{-2}\end{array}}\ \mathrm{CA}+\mathrm{HC}{\mathrm{O}}_3^{-}+\kern0.5em {\mathrm{H}}^{+} $$

(1.3)

As a potential model, the linear approximation of the Michaelis-Menten kinetics equation (Eq. 1.4) is a satisfactory tool:

$$ {\mathcal{R}}_{\mathrm{CA}}=\frac{k_{\mathrm{cat}}}{K_{\mathrm{M}}}\left[\mathrm{C}\mathrm{A}\right]\left\{\left[\mathrm{C}{\mathrm{O}}_2\right]-{\left[\mathrm{C}{\mathrm{O}}_2\right]}_{\mathrm{eq}}\right\} $$

(1.4)

where K _M refers to the Michaelis constant of the reaction and k _cat is defined as the turnover number and ranges between 10⁴ and 10⁶ molecules of CO₂ per molecule of CA per second depending on the strain of CA that is being used (Tripp et al. 2001; Shekh et al. 2012). The [CO₂] term represents the quantity of CO₂ that is being converted into $\mathrm{HC}{\mathrm{O}}_3^{-}$, and the [CO₂]_eq term represents the concentration of $\mathrm{HC}{\mathrm{O}}_3^{-}$ that is being converted back into CO₂.

Table 1.1 summarizes typical kinetic parameters of the many carbonic anhydrase isozymes with various substrates.

Table 1.1 CA isozymes kinetic constants with CO₂

1.4.3 Catalytic Models of the CO₂ Conversion Activity

Nevertheless, further studies demonstrated that the CO₂ hydration kinetics may be substantially modified by the nature of a buffer mixed in the enzymatic solution, whereas the Michaelis-Menten rate equation model implies that this proton exchange is not rate limiting. Therefore, several models were developed to specifically correct this omission.

Steiner and coworkers (Steiner et al. 1975) proposed a model in a classical Michaelis-Menten reversible kinetics with two reagents, i.e., the substrate [CO₂] and the product $\left[\mathrm{HC}{\mathrm{O}}_3^{-}\right]$. As for the enzyme , it comes in two forms: the active form [E] and the form of a transient complex [ES]. Several steps of the reaction mechanism are omitted in this model; only two steps are represented, including the bonding of CO₂ and the release of bicarbonate $\left[\mathrm{HC}{\mathrm{O}}_3^{-}\right]$.

Jonsson and collaborators (Jonsson et al. 1976) improved the model by adding an isomerization step (intramolecular proton transfer) to improve the Michaelis–Menten kinetics proposed. The model still includes only two reagents, but the enzyme comes in three different forms: the active form [E], a transient complex form, [ES] and a form in which a water molecule is bound to the zinc ion [E_w].

Rowlett and Silverman suggested a model (Rowlett and Silverman 1982) where the enzyme is found in three forms: the active form [E], the form of a transient complex, [ES] and the form where the residue is protonated [_HE]. This model is divided into three stages, namely, CO₂ binding, bicarbonate release, and intermolecular proton transfer. It is identified by the Ter Bi Ping Pong kinetics where the term Ter means that the model includes three substrates, namely, CO₂, water, and buffer in basic form. The term Bi means that the model includes two products, bicarbonate and buffer, in the form of conjugated acid.

Larachi (2010) presented four models to correct the discrepancies observed in the previous models. The model has three substrates, CO₂, water, and buffer, in basic form, as well as two products, including bicarbonate and buffer, in the form of conjugated acid. In this model, the enzyme is present in three forms: the active form [E], the form where a water molecule is bound to the zinc ion [E_w], and the form where the residue is protonated [_HE]. This model does not include a transient complex [#ES]. The steps included in this model are CO₂ binding and product release, intramolecular proton transfer (isomerization), and intermolecular proton transfer. The novelty of this model comes from the fact that it includes inter- and intramolecular transfer. Model (a) is represented by ordered Ter Bi Iso Ping Pong kinetics. Model (b) is represented as a random Quad Quad Iso Ping Pong kinetic. Model (c) is an ordered Ter Bi Iso Ping Pong kinetics. This model is similar to model (a) except that it includes a transient complex. Model (d) is represented by random Quad Quad Iso Ping Pong kinetics. This model is very similar to model (b) except that it includes a transient complex:

$$ \left[\mathrm{E}\ \right]+\left[\mathrm{S}\right]\ {\displaystyle \begin{array}{c}{k}_1\\ {}\rightleftharpoons \\ {}{k}_{-1}\end{array}}\ \left[\mathrm{E}\mathrm{S}\right] $$

(1.5)

$$ \left[\mathrm{ES}\ \right]+\left[\mathrm{W}\right]\ {\displaystyle \begin{array}{c}{k}_2\\ {}\rightleftharpoons \\ {}{k}_{-2}\end{array}}\ \left[{\mathrm{E}}_{\mathrm{w}}\right]+\left[\mathrm{P}\right] $$

(1.6)

$$ \left[{\mathrm{E}}_{\mathrm{w}}\ \right]\ {\displaystyle \begin{array}{c}{k}_2\\ {}\rightleftharpoons \\ {}{k}_{-2}\end{array}}\ \left[{}_{\mathrm{H}}\mathrm{E}\right] $$

(1.7)

$$ \left[\mathrm{B}\ \right]+\left[{}_{\mathrm{H}}\mathrm{E}\right]\ {\displaystyle \begin{array}{c}{k}_4\\ {}\rightleftharpoons \\ {}{k}_{-4}\end{array}}\ \left[\mathrm{B}{\mathrm{H}}^{+}\right]+\left[\mathrm{E}\right]\kern3.5em \mathrm{and}\kern2.25em \left[\mathrm{P}\ \right]+\left[{}_{\mathrm{H}}\mathrm{E}\right]\ {\displaystyle \begin{array}{c}{k}_5\\ {}\rightleftharpoons \kern0.5em \\ {}{k}_{-5}\end{array}}\left[\mathrm{W}\right]+\left[\mathrm{S}\right]+\left[\mathrm{W}\right] $$

(1.8)

The reaction rate defined by the production of bicarbonate [P] is expressed according to the following differential equation:

$$ \frac{\mathrm{d}\left[\mathrm{P}\right]}{\mathrm{d}t}={k}_1\left[\mathrm{W}\right]\ \left[\mathrm{E}\mathrm{S}\right]-{k}_{-2}\left[\mathrm{P}\right]\ \left[{\mathrm{E}}_{\mathrm{w}}\right]+{k}_{-5}\left[\mathrm{W}\right]\ \left[\mathrm{S}\right]\ \left[\mathrm{E}\right]-{k}_5\left[\mathrm{P}\right]\ \left[{}_{\mathrm{H}}\mathrm{E}\right] $$

(1.9)

Using the method of King and Altman (1956), the reaction rate is written in the following form:

$$ \frac{\mathrm{d}\left[\mathrm{P}\right]}{\left[{\mathrm{E}}_0\right]\mathrm{d}t}=\frac{\left(\left[\mathrm{S}\right]\left[\mathrm{B}\right]-\frac{{\mathrm{K}}_{a2}}{{\mathrm{K}}_{a1}}\left[\mathrm{P}\right]\left[{\mathrm{BH}}^{+}\right]\right)\left(\frac{{\mathrm{K}}_{a1}{k}_3}{{\mathrm{K}}_{\mathrm{E}}{k}_1}{k}_4\left(\frac{{\mathrm{K}}_{\mathrm{E}}}{{\mathrm{K}}_{a1}}{k}_1+{k}_5\left(1+\frac{k_{-1}}{k_2\left[\mathrm{W}\right]}\right)\right)+{k}_4{k}_5\left[\mathrm{P}\right]\right)}{\begin{array}{l}\frac{k_3}{k_1}\left(\frac{{\mathrm{K}}_{\mathrm{E}}}{{\mathrm{K}}_{a1}}{k}_1+{k}_5\left(1+\frac{k_{-1}}{k_2\left[\mathrm{W}\right]}\right)\right)\left(2\frac{{\mathrm{K}}_{a1}}{{\mathrm{K}}_{\mathrm{E}}}\left[\mathrm{S}\right]+\left[\mathrm{P}\right]\right)+\frac{k_3}{k_1}{k}_4\left(1+\frac{k_{-1}}{k_2\left[\mathrm{W}\right]}\right)\left(\left[\mathrm{B}\right]+2\frac{{\mathrm{K}}_{a2}}{{\mathrm{K}}_{\mathrm{E}}}\left[{\mathrm{BH}}^{+}\right]\right)\\ {}+{k}_4\left(1+\frac{k_3}{k_2\left[\mathrm{W}\right]}\right)\left[\mathrm{S}\right]\left[\mathrm{B}\right]+\left(2{k}_5+\frac{k_3{k}_1}{k_1{k}_{-1}}\left(\frac{{\mathrm{K}}_{\mathrm{E}}}{{\mathrm{K}}_{a1}}{k}_1+{k}_5\left(1+\frac{k_{-1}}{k_2\left[\mathrm{W}\right]}\right)\right)\right)\left[\mathrm{S}\right]\left[\mathrm{P}\right]\\ {}+{k}_4\left(\frac{K_{\mathrm{E}}}{K_{a1}}\left[\mathrm{B}\right]+\frac{{\mathrm{K}}_{a2}}{{\mathrm{K}}_{a1}}\left(\frac{k_3}{k_{-1}}+1\right)\left[{\mathrm{BH}}^{+}\right]\right)\left[\mathrm{P}\right]+\frac{{\mathrm{K}}_{\mathrm{E}}}{{\mathrm{K}}_{a1}}{k}_5{\left[\mathrm{P}\right]}^2+\frac{{\mathrm{K}}_{\mathrm{E}}{k}_1}{{\mathrm{K}}_{a1}{k}_{-1}}{k}_5\left[\mathrm{S}\right]{\left[\mathrm{P}\right]}^2+\frac{{\mathrm{K}}_{\mathrm{E}}{k}_1}{{\mathrm{K}}_{a1}{k}_{-1}}{k}_4\left[\mathrm{S}\right]\left[\mathrm{B}\right]\left[\mathrm{P}\right]\end{array}} $$

These models being very complex and lengthy to develop herein, the interested readers are advised to refer to Larachi’s paper (Larachi 2010). Figure 1.11 represents the five sets of data compared to model (d). This model represents fairly well the transition between regimes where the intramolecular transfer is the stage controlling the rate of reaction and a regime where the intermolecular transfer controls the reaction rate (Larachi 2010).

Fig. 1.11

Measured CO₂ hydration rate versus calculated using the Larachi model (d). (Larachi 2010)

1.4.4 The Carbonic Anhydrase Biomimetic CO₂ Capture

The process design of the biomimetic CO₂ capture depends closely on the selection of the enzyme forms able to handle the severe operational conditions, such as high temperature, high salt concentration, and elevated alkalinity which may affect the enzyme performance. In general, the absorption processes are run at temperatures ranging between 40 and 60 °C, while the desorption temperature is around 100 °C, although it can be lowered when running the unit under vacuum (about 0.3 bar) (Russo et al. 2013). Furthermore, the enzyme performance could be seriously damaged by some pollutant present in the flue gases, such as Cl, Hg, NO_x, SO₂, and fly ashes. The enzyme characterizations under typical process conditions are expressed in terms of kinetic assessment and long-term stability. The CO₂ loading capacity is increased by adding solvents, usually inorganic carbonate salts or amines.

1.4.4.1 Bicarbonate

Bicarbonates are regarded as solvents with the highest potential for use with CA: they do not degrade, are less corrosive, and require low regeneration energy. Besides, CA has high stability in bicarbonate with stable activity for long period of time (Ye and Lu 2014).

Lu and coworkers studied the CO₂ absorption in a stirred cell reactor using a characterized CA form of microbial origin. Tests were run using pure CO₂ as gas phase and 20% wt K₂CO₃ aqueous solutions as liquid phase, at 25, 40, and 50 °C and at CA concentration of 300 mg/l . Results showed that the CA enhanced the CO₂ absorption rate of about 10, 5, and 4 times with respect to tests run without promoter at 25, 40, and 50 °C, respectively.

Zhang and Lu characterized an engineered CA form provided by Novozymes. They run CO₂ batch absorption tests into K₂CO₃ 20% in lean solvent conditions (20% CTB conversion) and rich conditions (55% CTB conversion) at 50 °C. They evaluated k _cat/K _M as 9.0 10⁸ M⁻¹s⁻¹, without any CTB conversion influence. They further developed a theoretical model to simulate the CA performance in a packed-bed column at the scheduled conditions including the measured kinetic parameters.

Hu and coworkers (Hu et al. 2017) characterized a CA form of microbial origin using of a wetted wall column absorption via the stop flow technique. They used K₂CO₃ 30% aqueous solutions as liquid phase at 50 °C and different carbonate to bicarbonate (CTB) conversions (0–20%). The CA Michaelis–Menten catalysis parameter k _cat/K _M was determined to be around 5.3 10⁸ M⁻¹s⁻¹ and showed a slight decrease with the CTB conversion. The decrease may be due to the CA catalysis of the backward reaction of CO₂ hydration that occurs at high bicarbonate concentration and that influences the apparent reaction rate. The CA retained more than 70% of its initial activity after incubation into K₂CO₃ 30% at 50 °C for 8 h.

Gladis and coworkers (Gladis et al. 2017) characterized a recombinant CA form provided by Novozymes through absorption tests run in a wetted wall column. They compared the activity of four different solvents: the primary amine (MEA), the sterically hindered primary amine (AMP), the tertiary amine (MDEA), and the carbonate salt solution K₂CO₃ with and without enzyme in concentrations ranging from 5 to 50 wt% and temperatures from 298 to 328 K. The results revealed that the addition of carbonic anhydrase (CA) dramatically increases the liquid side mass transfer coefficient for MDEA and K₂CO₃, AMP has a moderate increase, whereas MEA was unchanged. The results confirmed that only the bicarbonate forming systems benefit from CA, showing that the enzyme activity was particularly influenced by the temperature, reaching in all the cases a k _cat/K _M of about 5 · 10³ m³/kg. s at low solvent concentrations (5–15 wt%). On the other hand, at 20% wt K₂CO₃, a considerable increase of the rate constant was noticed, passing from 1.2 10⁴ m³/kg. s at 25 °C to 2.1 10⁴ m³/kg. s at 55 °C.

Iliuta and Iliuta (2017) developed an enzyme–CO₂ dynamic 3D model removal performance of countercurrent packed-bed column reactors based on continuity, momentum, and species balance equations in the liquid and gas phases with simultaneous diffusion and chemical reaction at the enzyme washcoat/liquid film scale level. They observed that the packed-bed column reactor performance with immobilized human enzyme hCA II on random packings can be enhanced by reducing the washcoat thickness, increasing the inlet buffer concentration and pKa constant, and increasing the liquid velocity maintaining a low pressure drop level. Also, operating with extra hCA II loadings allows obtaining higher CO₂ conversion and avoids the degradation of the CO₂ hydration rate in long-term operation attributable to the decrease of hCA II enzyme activity.

1.4.4.2 Solvents

Many other solvents (ammonia, amino acids, primary, secondary, tertiary, and hindered amines) have all been used with CA. For amine solvents, the noncatalyzed rate increases linearly with increasing pKa (Penders-Van Elk et al. 2016a, b).

Penders-van Elk and coworkers studied extensively the carbonic anhydrase kinetics for CO₂ absorption with various solvents using different process conditions (Penders-Van Elk et al. 2012, 2013, 2016a, b). They investigated in their first study (Penders-Van Elk et al. 2012) the kinetics of two types of carbonic anhydrase with MDEA at 298 K in a stirred cell reactor and reported that the CO₂ physical solubility is not affected by enzyme addition. They observed a neat overall reaction rate increase of the solvent with the enzyme concentration increase at a fixed solvent concentration, with a linear relationship at lower enzyme concentration and a flattening out at higher enzyme concentrations. They also examined several new alkanolamines (Penders-van Elk et al. 2015): N,N-diethylethanolamine (DEMEA), N,N-dimethylethanolamine (DMEA), monoethanolamine (MEA), triethanolamine (TEA), and triisopropanolamine (TIPA) at 298 K. In both TEA and DMEA, they observed a decrease in enzymatic activity. A very low MEA concentration was chosen (0.1 mol/l) for measuring the enzymatic reaction. In a most recent study (Penders-Van Elk et al. 2016a, b), they looked at the enzyme kinetics with the temperature dependency in MDEA solutions and derived a simplified kinetic model based on their experimental results in a temperature range from 278 to 313 K. The model, however, underpredicted the results obtained at 298 K and overpredicted the ones at 343 K.

Vinoba and coworkers (Vinoba et al. 2013) used a vapor–liquid equilibrium device to investigate the CO₂ absorption using MEA, DEA, MDEA, and AMP solutions enhanced by bovine carbonic anhydrase. The results showed that the overall CO₂ absorption flux and reaction rate constant followed the order MEA > DEA > AMP > MDEA in the absence or presence of CA. The hydration of CO₂ by MDEA in the presence of CA directly produced bicarbonate, whereas AMP produced unstable carbamate intermediate and then underwent hydrolytic reaction and converted to bicarbonate. The MDEA > AMP > DEA > MEA reverse ordering of the enhanced CO₂ flux and reaction rate constant in the presence of CA was due to bicarbonate formation by the tertiary and sterically hindered amines. They reported that CA increased the CO₂ absorption rate by MDEA by a factor of 3 relatively to the absorption rate by MDEA alone. Furthermore, the thermal effects suggested that CA yielded a higher activity at 40 °C.

Zhang and Lu carried out the same simulation considering 5 M MEA as liquid phase, in condition of lean (40% MEA conversion) and rich (90% MEA conversion) solvent. Their results pointed out that the overall rate of CO₂ absorption into 5 M MEA solution and into K₂CO₃ 20% promoted by 3g/l CA was about the same.

Recently, Gladis and collaborators (Gladis et al. 2017) studied the effect of carbonic anhydrase addition on the absorption of CO₂ in a wetted wall column apparatus where they compared four solvents, the MEA, AMP, and MDEA with K₂CO₃, in concentrations ranging from 5 to 50 wt% in a temperature interval from 298 to 328 K with and without an enzyme . The results showed that the addition of carbonic anhydrase increased dramatically the liquid side mass transfer coefficient for MDEA and K₂CO₃, AMP had moderately increased, whereas MEA was unchanged. The results confirmed that only bicarbonate forming systems benefit from the enzyme catalyst.

Sivanesan and his group (Sivanesan et al. 2015) used model complexes based on the carbonic anhydrase in aqueous tertiary amine medium to improve CO₂ sequestration. They used a stopped-flow spectrophotometer to follow pH changes coupled to pH indicator in a continuous stirred-tank reactor (CSTR) to determine the effect of substituents on the CA model complexes on CO₂ absorption and desorption. The CO₂ hydration rate constants were determined under basic conditions, and a compound which contained a hydrophilic group showed the highest absorption or hydration levels of CO₂ (2.860 10³L/(mol. s)). Furthermore, the CSTR experimental results for simple model CA complexes may be suitable for post-combustion processing.

1.4.5 Temperature Effect on Carbonic Anhydrase Activity and Structure

At high temperature, enzymes lose their biological activity and become irreversibly denaturated. This inactivation by heat denaturation has a profound effect on the enzyme productivity (Sheldon 2007). The temperature limitation of enzymes is an important parameter for industrial applications affecting the cost of the process if the enzyme could not be reused. Lavecchia and Zugaro studied the thermal behavior of bovine carbonic anhydrase (Lavecchia and Zugaro 1991) who reported that carbonic anhydrase was active under 60 °C, but it lost its activity between 60 and 65 °C.

Many authors reported recently a decrease in enzyme activity when exposed to higher temperatures for a longer time (Russo et al. 2013; Gundersen et al. 2014; Ye and Lu 2014). The positive results from the large-scale experiments encourage the application of CA in carbon capture and show that it is possible to develop thermostable enzymes through protein engineering.

1.5 Enhanced Enzymatic Carbon Capture Overview

A wide spectrum of reactor configurations is reported in literature. However, the absorption unit designs are still an open and challenging issue. The reactor configurations, in general, are strongly associated with the enzyme form used, i.e., dissolved (homogeneous catalysis) or immobilized (heterogeneous catalysis). Nevertheless, the use of heterogeneous catalysts provides numerous advantages, in particular:

• 1. The use of immobilized carbonic anhydrase allows its easy recovery and reuse.

• 2. The use of the dedicated enzyme immobilization technique improves substantially its stability under the industrial processing conditions (Garcia-Galan et al. 2011).

• 3. Because the CO₂ absorption requires high salts and enzyme concentrations, the free CA may aggregate and then reduce the homogeneous enzyme efficiency (Ye and Lu 2014).

• 4. The suitable immobilization technique allows the use of high enzyme loadings, concentrations larger than 300 mg/l (Ye and Lu 2014).

The morphology of the solid biocatalyst and the reactor configuration should be carefully designed to maximize the CO₂ absorption rate. Several authors (Iliuta and Larachi 2012; Russo et al. 2013) reported that the enzyme catalysis on the CO₂ absorption rate is enhanced by the immobilized enzyme availability at the gas–liquid interface, by virtue of which various technical designs are available in the literature.

Iliuta and Larachi (2012) proposed a novel conceptual model of a multiscale monolith slurry reactor where hCA II was covalently immobilized on a monolith wall. The monolith is a bundle of parallel channels (honeycomb like) with a 3 mm cross-sectional diameter. The solvent was permanently regenerated by ion-exchange beads (Amberlite IRN-150) which remove ions, preventing CA product inhibition and enhancing CO₂ hydration rate. The reactor was run continuously with respect to both liquid and gas phases in a cocurrent flow pattern. They simulated the effects of enzyme loading, channel washcoat thickness, resin concentration, buffer acid–base constant and concentration, fluid fluxes, gas composition, and channel length on CO₂ scrubbing for monolith three-phase slurry enzymatic reactor enabled assessment.

Zhang and his group used hollow fiber membrane reactor filled with immobilized carbonic anhydrase by nanocomposite hydrogel to study the CO₂ facilitated transport. They reported that simulated results of CO₂ and CA concentrations, and flow rate of feed gas on CO₂ removal performance were in agreement with the experimental data with a maximum deviation of up to 18.7%. Besides, they also investigated the effect of CO₂ concentration on the required membrane areas for the same CO₂ removal target (1 kg/day).

Hou and colleagues developed a novel biocatalytic gas–liquid membrane contactor for CO₂ capture with virgin and superhydrophobic PP hollow fibers. To promote CO₂ hydration, biocatalytic TiO₂ nanoparticles with covalently immobilized CA were suspended in the solvent absorbent. The CA immobilization on titania nanoparticles was proved beneficial for higher immobilization yields and easier biocatalyst recovery with respect to CA adsorbed to the inner wall of the membrane. They also showed that the enzymatic promotion is more efficient at low liquid Reynolds number, which correspond to operating conditions of most conventional gas–liquid membrane contactors.

Leimbrink and his group (Leimbrink et al. 2017a) compared the use of some intensified contacting devices (ICD), especially membrane contactor (MC) and rotating packed beds (RPB) to classical packed columns (PC) to achieve enzyme accelerated carbon capture . They investigated a 30 wt. % aqueous MDEA solution with and without dissolved CA in a packed column and in the two ICDs to evaluate the potential improvement of a joint application of the ICD intensified contacting devices and the application of CA absorption. While all three equipments show similar absorption performance without adding CA, the authors claimed that the RPB can handle exceptionally high gas loads, while the MC can be operated over a much wider range of liquid loads. When CA is added to the solvent, the PC and the RPB show superior performance compared to the MC.

Kim and colleagues (Kim et al. 2017) studied the use of carbonic anhydrase for the acceleration of CO₂ reaction in MEA and MDEA solutions in a lab-scale membrane contactor module. They used specific microporous membranes which have both hydrophilic (surface) and hydrophobic (bulk) properties in order to avoid wetting of solution and reduce fouling by the enzymes simultaneously. They reported that enzyme addition improved substantially the CO₂ absorption rate in MDEA solution but had a negative effect in MEA solution. They coated, in the meantime, the porous hydrophobic membranes with a highly selective polyionic liquid layer to increase the affinity of CO₂ towards the interfacial area and consequently the driving force. They obtained promising results with the activated membrane material to accelerate CO₂ transport in MDEA solution. They concluded that polyionic liquid membrane coating combined with enzyme enhances considerably the CO₂ absorption in MDEA solution.

Gaspar and collaborators (Gaspar et al. 2017) developed a rate-based model for CO₂ absorption using carbonic anhydrase-enhanced MDEA solution and validated it against pilot-scale absorption experiments. The authors reported that the developed model is suitable for CO₂ capture simulation and optimization using MDEA and MDEA enhanced with CA. Besides, they studied the accuracy of the enhancement factor model for CO₂ absorption/desorption using wetted wall column for various CO₂ loadings and temperatures.

Leimbrink and his team (Leimbrink et al. 2017b) studied the combination of the effects of an aqueous MDEA solution with carbonic anhydrase in a packed column pilot plant to offset the loss of separation efficiency caused by the lower driving force in CO₂ capture from power plant flue gases.

They explored two different CA application strategies as a biocatalyst in reactive absorption processes to understand their influence on absorption efficiency: (i) dissolution of the enzyme in the solvent to allow the enzyme to react in the liquid boundary layer. However, due to the enzyme temperature sensitivity, the enzyme recovery requires an additional operation before desorption at high temperatures. (ii) Immobilization of the enzyme inside the absorption column is an alternative to this drawback but may create additional mass transfer resistance at the solid particles. Although this strategy allows locating the enzyme at convenient process conditions and avoiding high temperature in the desorber, the enzyme immobilization and the suitable packing selection increase the difficulty of this strategy.

Absorption performance with enzyme dissolution was three times higher than of the enzyme immobilization under equivalent operating conditions, but the immobilized enzyme concentration used was 50 times lower. On the other hand, the authors (Leimbrink et al. 2017b) reported that, with a liquid inlet temperature of 20 °C, a 30 wt. % MDEA concentration, and a liquid flow rate of 24 m³/(m². h), the best absorption performance with the enzyme dissolution and the measured absorption rate was 7.57 times higher than without enzyme added.

1.6 Major Research Programs and Pilot Plants Worldwide

Several companies are developing novel carbonic anhydrase-based CO₂ capture technologies. These attempts are focusing to improve the enzyme forms and functions to develop new methods for the enzyme use in engineered systems and to develop specialized mass transfer unit operations to implement the enzyme function.

1.6.1 Enzyme-Enhanced Amines by CO ₂ Solution Inc.

CO₂ Solutions Inc. (CSI) of Québec, Canada, has been developing CO₂ capture systems based on the biocatalyst carbonic anhydrase use in packed-bed absorption tower-type absorber–stripper systems [CO₂ Solutions, 2009]. This concept allows solutions with low regeneration temperatures having low absorption rates technically viable candidates for post-combustion capture.

Recently (2015), CO₂ Solutions Inc. has successfully demonstrated a 10 tpd CO₂ enzyme accelerated solvent carbon capture project from a natural gas fired boiler in Salaberry-de-Valleyfield near Montreal, Canada (Fig. 1.12). The plant was successfully run for 2500 h with biocatalyst stable performance, negligible solvent deterioration, no toxic waste generation, and production of 99.95% pure CO₂ suitable for many reuse applications. The plant reached 95% CO₂ capture which illustrates a wide range achievement of performance objectives. The inventors found that the enzyme remaining in the solvent kept excellent activity throughout the test period and demonstrated an easy enzyme addition during plant running. CO₂ Solution Inc. (CSI), which has proved the ability to erect up to 300 tpd plant, is presently erecting a 30 tpd plant in Canada and start-up is estimated in late 2018.

Fig. 1.12

Carbonic anhydrase-catalyzed amine absorber plant for carbon capture from fuel-fired power plant flue gas. (www.CO2solutions.com)

Lalande and Tremblay of CO₂ Solutions Inc. (Lalande and Tremblay 2005) invented a process and built a CO₂ recovery and recycling unit for gas emissions from a cement clinker production plant. In that process, a gas/liquid CO₂ packed column absorption catalyzed by carbonic anhydrase is used and subsequent with the production of limestone (CaCO₃). The sequence is accomplished when the CaCO₃ is used as first class raw material for the fabrication of Portland cement.

In addition, Codexis Inc., in a joint venture with CO₂ Solution Inc., has built a pilot-scale CO₂ capture process at the National Carbon Capture Center in Wilsonville, Alabama, USA, in which the observed CO₂ absorption rate was enhanced 25-fold compared to the noncatalyzed absorption process (Alvizo et al. 2014).

1.6.2 NASA Thin Liquid Membrane System

The National Aeronautics and Space Administration (NASA) has initially developed another process to clean the ambient air in the confined inhabited crew cabins where CO₂ is captured in thin aqueous films with some immobilized CA (Ge et al. 2002; Cowan et al. 2003).

The CO₂ concentration of such ambient air is relatively low (≤ 0.1%). Figure 1.13 illustrates the membrane rector constructed by sandwiching a thin (330 μm thick) enzymatic solution layer CA containing phosphate-buffered solution between two polypropylene membranes, themselves retained by thin metallic screens to insure the liquid membrane thickness and rigidity. The incoming CO₂ from the ambient atmosphere dissolves immediately in the liquid membrane on one face and then diffuses across the liquid membrane and evaporates out on the liquid membrane opposite face, either in vacuum or in a carrier gas. Capture and release gases analysis showed a selective CO₂ diffusion in a ratio of 1400 to 1 compared to N₂ and 866 to 1 compared to O₂. The collected data elected this enzyme-based contained liquid membrane as a viable and suitable technique for NASA applications to control CO₂ in the crew cabins.

Fig. 1.13

Thin liquid membrane for CO₂ capture developed by NASA. (Ge et al. 2002)

1.6.3 Hollow Fiber Membrane Program by Carbozyme Inc.

Carbozyme, Inc. has developed a biomimetic CO₂ capture apparatus able to accept a wide spectrum of gas streams and generate a stream acceptable to a pipeline operator. The Carbozyme permeator design consists of two fibrous microporous membranes portioned by a thin liquid membrane (CLM). To optimize the conversion efficiency, the enzymatic biocatalyst is immobilized in the hollow fiber wall to insure an intimate contact between CO₂ and the carbonic anhydrase at the gas–liquid interface (Fig. 1.14).

Fig. 1.14

Carbozyme permeator operation diagram . (Trachtenberg et al. 2009)

Contained liquid membranes (CLM) are a gas-to-gas application and operated in the same way as simple selective membranes. Absorption and desorption are carried out in the same unit where CO₂ dissolves into the liquid in the membrane and is desorbed on the other side producing an ultrapure CO₂ stream. They may be used as flat membranes or hollow fiber membranes to increase contact area but increasing operating difficulty. Sweep gas (argon or nitrogen) is usually used for desorption in experimental setups, whereas it would be done with vacuum in industrial scale. The advantage of this aims reducing energy which is beneficial for the enzyme stability (Figueroa et al. 2008). However, the process requires energy to pressurize the incoming gas and to create vacuum on the exit side. In addition, solvent loss through evaporation in the membrane pores may also be a serious problem, and higher capture ratios often require exponentially higher energy needs (Russo et al. 2013). The application of such technology may be suitable for cases where the inlet CO₂ concentration is fairly high and sufficiently low carbon capture rates are needed. This technology has led to satisfactory experimental results on laboratory scale. Bao and Trachtenberg have shown that CA in bicarbonate gave higher carbon capture rates than both uncatalyzed bicarbonate and the secondary amine diethanolamine (DEA) (Bao and Trachtenberg 2006).

The Carbozyme system achieved 85% CO₂ removal from a 15.4% CO₂ feed stream in a 0.5 m² permeator as predicted by the model calculations (Trachtenberg et al. 2009).

Most recently, Carbozyme has reported on the use of a proprietary absorber–stripper arrangement based on the same concept of using carbonic anhydrase immobilized at the gas–liquid interface (Smith et al. 2010).

However, in this process, some technical difficulties may appear due to the drying of aqueous film during continuous longtime process running. To overcome this drawback, Trachtenberg and his group suggested humidifiers such as polysulfone to humidify the capture and release gases (Cowan et al. 2003). Nevertheless, for a better solution to this problem, the investigators adapted the technique to hollow microporous fiber networks where the flue gas and the release gases could flow (Bao and Trachtenberg 2006; Trachtenberg et al. 2009). Following this progress, Carbozyme developed a new technology based on hollow microporous propylene microfibers, separated by control separators made of thin oxide powders, the whole system bathing in an excess aqueous enzyme solution. The enzyme was directly immobilized on the external faces of the microfibers, and water vapor under moderate vacuum (15 kPa) was used as sweep gas at low flow rates in the release microfibers. The CO₂ content in the sweep gas almost reached 95%, for a flue gas containing 15% of CO₂. No significant loss of enzyme activity was observed during a 5-day continuous run, and a conservative run time of 2500 h was selected before needing to change the enzyme (Trachtenberg et al. 2009).

Yong and his team (Yong et al. 2016) developed a similar strategy to promote the reaction rate by the electrostatic adsorption of carbonic anhydrase onto the surface of both porous polypropylene (PP) and nonporous polydimethoxysilane (PDMS) hollow fiber membranes via layer-by-layer (LbL) assembly. They reported that CO₂ absorption rate into K₂CO₃ is increased approximately threefold when CA is adsorbed onto the PP membrane surface, while the absorption rate of the modified PDMS membrane was slightly lower, within 70–90% of the PP values. The CO₂ hydration is enhanced in all cases, and the wetting of the porous PP membranes is significantly reduced by the pore blockage induced by the LbL adsorption of the polyelectrolytes. The company Carbozyme is developing a similar hollow fiber membrane system.

Furthermore, Novozymes has deposited some patents, the latter of which proposed the combination of various CO₂ capture and release units, such as those developed by the CO₂ Solutions or Carbozyme companies interconnected by fluid circulation pipes (Saunders et al. 2010).

1.6.4 Other Miscellaneous Programs

1.6.4.1 Akermin, Inc.

Akermin, Inc. has developed a carbonic anhydrase immobilization–stabilization technique for CO₂ capture from flue gas. The conceptual idea is to encapsulate the enzyme in custom polymer structures, thus protecting the enzyme and allowing a long life. Besides, the enzyme is spread out in the capture solution to be present at all the gas–liquid interface, where it can provide higher benefit.

The immobilized biocatalyst was shown to enhance kinetic rates compared to coated packing, and modeling showed a 30% lower energy needs (Reardon et al. 2014).

Akermin has been working on the technology for approximately 5 years and was recently awarded a 2-year project to optimize its enzyme-containing solvent formulation and demonstrate process efficacy by treating up to 2 standard cubic meters of simulated flue gas per hour (US Department of Energy National Energy Technology Laboratory, NETL, 2016h).

Akermin, Inc. has carried out field tests with their surface-immobilized packing absorption device at the National Carbon Capture Center (NCCC) in Wilsonville, Alabama. They achieved 80% capture in an absorption column with around 0.21 m diameter and a total packing height of around 8 m with 20 wt% K₂CO₃ with a liquid to gas ratio of 7.88 (kg/kg) over a timeframe of 5 months and 1 month, respectively. A six to sevenfold higher mass transfer rates was observed with the use of the surface-immobilized enzyme (Reardon et al. 2014).

1.6.4.2 Sulzer BX Gauze Packing

Kunze and coworkers (Kunze et al. 2015) carried out laboratory-scale experiments and showed chemical capability and evaluated various solvents. They measured CO₂ absorption rates of 30 wt.% MEA, 30 wt.% MDEA, 30 wt.% DEEA, and 10 wt.% K₂CO₃ with the addition of 0.2 wt.% carbonic anhydrase. They identified aqueous solutions of 30 wt.% MDEA as well as 30 wt.% K₂CO₃ as promising solvents whose CO₂ absorption rate was accelerated by the enzyme , as the addition of 0.2 wt.% carbonic anhydrase led to an increase of the absorbed mole flow by a factor larger than 4. Next, they tested the technical feasibility of the enzyme–solvent concept packed columns to check for scaling of laboratory size performance to pilot size (56 mm diameter, 2.3 m high, Sulzer BX gauze packing). Absorption runs at 317 K and 15 vol. % CO₂ in the gas phase resulted in comparable intensification of absorption compared to the results from the spray reactor, and CE_CA values of 4.0–5.9 for K₂CO₃ and 3.3–4.2 for MDEA were reported. They reported a good agreement in the increase of the absorbed mole flow in pilot scale in the presence of biocatalyst with the laboratory-scale experiments and did not observed any undesired effects such as foaming or aggregation.

1.6.4.3 Sandia National Laboratories Ultrathin Liquid Membrane

The Sandia National Laboratories group in collaboration with the University of New Mexico (Fu et al. 2018) has very recently developed a CA-catalyzed, ultrathin liquid membrane nano-immobilized via capillary forces for CO₂ separation (Fig. 1.15). Using atomic layer deposition and oxygen plasma processing, the silica mesopores are engineered to be hydrophobic except for an 18-nm-deep region at the pore surface which is hydrophilic. Carbonic anhydrase enzymes and water fill spontaneously the hydrophilic mesopores through capillary condensation to form an array of immobilized enzymes with an effective concentration ten times greater than that achievable in solution.

Fig. 1.15

The ultrathin liquid membrane by courtesy of Sandia National Laboratories . (Fu et al. 2018)

The metalloenzyme rapidly catalyzes CO₂ and H₂O conversion into $\mathrm{HC}{\mathrm{O}}_3^{-}$. Fu and his team (Fu et al. 2018) found that the enzymatic liquid membrane separates CO₂ at room temperature and atmospheric pressure at a rate of 2600 GPU with CO₂/N₂ and CO₂/H₂ selectivities as high as 788 and 1500, respectively, the highest combined flux and selectivity yet assessed for ambient condition operation by minimizing diffusional constraints, stabilizing and concentrating CA within the nanopore array to a concentration ten times greater than achievable in solution.

The authors have created in this device a mechanically stable liquid membrane just 18 nm thick, whereas in the Carbozyme configurations (Trachtenberg 2011), the characteristic membrane thicknesses were limited to 10–100 μm invalidating, therefore, the potential advantage of the liquid membrane compared to a polymer membrane. Furthermore, the advantage of this membrane compared to Carbozyme’s is that the confinement within the close-packed array of hydrophilic nanopores allows for higher enzyme concentration. In addition, the higher hydrophilic nanopore density in this new membrane, when filled with CA, would provide a considerably higher local CA concentration.

1.7 Conclusion

Although the use of carbonic anhydrase for biomimetic CO₂ capture is still in its infancy, it is an effective and rapidly advancing technology. However, the industrial applications of enzymes in carbon capture processes are restricted by their high cost, low catalytic activity, poor stability in time, high sensitivity to temperature, low resistance to pollutants such as sulfur compounds, and reusability. To overcome these adversities, further developments are still needed so that improved economic feasibility and significant progress on several features may also be expected.

Although the well-understood physicochemical laws governing the carbon capture in aqueous and solvent mediums are allowing development of various efficient reactor types, much effort should be made not only to improve the state-of-the-art technology but also to develop several innovative chemical reactor concepts for enzymatic gas–liquid contactors.

Besides, carbon dioxide may be turned into chemicals and fuel using chemical, photochemical, electrochemical, and enzymatic methods such as conversion to carbon monoxide, to methanol, to formic acid, to glucose, or to methane. Although facing harsh barriers, the enzymatic conversion of carbon dioxide into useful chemicals is making great strides and could be used to recycle considerable amounts of carbon. The implementation of these technologies with enzymatic conversion would very likely enhance selectivity and productivity and ought to be given further attention in the future.

In addition, only few conceptual processes have been tested on a lab scale, but just a very few of them have demonstrated potential interest on an industrial scale. Emerging processes that have successfully completed smaller pilot-scale tests and are in the process of scaling up to larger demonstrations are likely to be available commercially in the next 5–10 years. Nonetheless, several interesting routes have not yet been sufficiently explored.

In conclusion, all these studies confirm the remarkable potential of some CA forms as biocatalysts, providing a realistic demonstration of the feasibility of the biomimetic CO₂ capture processes.