Introduction

With the increasing consumption and rising prices of fossil fuels and petrochemical products, significant research is being invested in alternative energy sources. The chemicals derived from petroleum sources have diverse applications in the industrial, transportation sectors and for various consumer-based products. In the context of renewable sources, a vast part of these applications depends on the different biomass generated platform chemicals such as bio-alcohols, levulinic acid, 5-hydroxymethylfurfural, furfural, ethyl levulinate, lactic acid, 5-ethoxymethylfurfural and other furan or non-furan-based chemicals (Okolie et al. 2021a) (Fig. 1). These higher demands for the various chemicals and fuels have increased adequately due to the modernization of society with growing numbers of industries (Srivastava et al. 2020; Tang et al. 2020). Therefore, to fulfill the requirement of these chemicals through renewable ways, there is an increment of the research in the conversion of lignocellulosic biomasses by implementing different environmentally friendly approaches like hydrothermal, solvothermal and thermocatalytic techniques (Nanda et al. 2018; Quereshi et al. 2019; Kaloudas et al. 2021).

Lignocellulosic biomasses are majorly comprised of cellulose, hemicellulose and lignin in the ranges of 35-55 wt%, 20–40 wt% and 10–25 wt%, respectively (Nanda et al. 2013; Pattanaik et al. 2019). In this review, we have attempted to provide an insight into the conversion of these components and their monomeric or oligomeric building blocks, such as glucose, xylose, cellobiose, mannose, galactose and arabinose into various platform chemicals with the catalytic effects of various homogeneous and heterogeneous catalysts.

The thermocatalytic conversion of biomacss using heterogeneous or homogeneous catalysts has become prevalent for converting pentose and hexose sugars into biochemicals. Various acid catalysts viz. zeolites, metal oxides, mesoporous silica, mineral acids, acidic ionic liquids and metal-organic frameworks have been reported to convert sugars into platform biochemicals such as levulinic acid, furfurals and 5-hydroxymethylfurfural (Acharjee and Lee 2018). Water-resistance is a primary characteristic of acid catalysts during the conversion of sugars to chemicals during aqueous reactions. Heterogeneous acid catalysts are highly popularized for their thermal stability, lower separation cost and other environmental and industrial advantages. On the other hand, homogeneous catalysts including mineral acids such as HCl, H2SO4 and H3PO4 are well-established and commercialized materials for the conversion of carbohydrates into various platform chemicals like furfural, 5-hydroxymethylfurfural and levulinic acid.

Besides different chemical catalysts, biocatalysts such as hydrolytic enzymes play a vital role in the depolymerization of the complex carbohydrates and polysaccharides into monomeric sugars and their conversion into various liquid chemicals (Nanda et al. 2017). However, compared with the other chemical-based catalysis, biocatalysis is comparatively expensive with lesser outcomes (Chua et al. 2020). Investigating various aspects of different catalytic processes, heterogeneous acid catalysts, including ionic liquids, are found to be economically and environmentally sustainable for the biomass conversion process.

Fig. 1
figure 1

List of platform biochemicals derived from lignocellulosic biomass. Some notable sugars and biochemicals produced from cellulose include cellobiose, glucose, fructose, 5-hydroxymethylfurfural, levulinic acid, lactic acid and sorbitol. On the other hand, hemicellulose can yield sugars and biochemicals such as xylose, xylulose, furfural, furfuryl alcohol and xylitol. The basic biochemicals produced from lignin are phenols, catechol and vanillin. Catalysts play selective and significant roles in converting cellulose, hemicellulose and lignin to biochemicals of commercial and industrial relevance

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Global market of commodity chemicals

Furfural

The renewability and abundance of lignocellulosic biomass provide a reliable source to produce furfural. The hydrolysis of pentosans in lignocellulose followed by dehydration of the released pentose sugars results in the formation of furfural and its derivatives. Commercially it is produced by acid hydrolysis of agricultural and woody biomass. (Kabbour and Luque 2020). The market price for furfural is U.S.$ 1500/ton and its demand has reached 600 kilotons annually with a global market size of U.S.$ 815 million in 2020 (Grand View Research 2020a). The U.S. and Germany are accounted to consume 30% of furfural and Japan with 11% consumption. China using corn cobs as the principal feedstock material, having 200 furfural production units is the leading global producer of furfural, which accounted for over 70% of world supplies (Lee and Wu 2021). Furfural and its derivatives being versatile chemicals can be used as transportation fuels, resins, decolorizing agents, intermediate in the synthesis of fine chemicals, biopolymers and pharmaceuticals. It also finds its application in the role of soil enhancers, organic fertilizers, jet fuel blendstock and flavor enhancers for drinks and food.

Levulinic acid

Levulinic acid is produced from lignocellulosic biomass typically by the following steps such as: (i) pretreatment of biomass, (ii) hydrolysis of cellulose into glucose monomers, (iii) isomerization of glucose monomers to fructose monomers, (iv) dehydration of hexose sugars to 5-hydroxymethylfurfural, and (v) rehydration of 5-hydroxymethylfurfural to levulinic acid (Kang et al. 2018). The global market demand for levulinic acid was 2,606 tons in 2013 and it is expected to be 3,820 tons by 2021 (Grand View Research 2015). The global market size for levulinic acid was U.S.$ 19.7 million in 2020 demonstrating a compound annual growth rate or CAGR of 4.8% (Grand View Research 2015). The agriculture and pharmaceutical sectors accounted for over 66% of the overall volume in 2013 and are anticipated to grow at 5.4% CAGR (Grand View Research 2015). North America is the leading producer of levulinic acid by contributing to more than 41% of world supplies in 2013 while Europe and the Asia-Pacific are the emerging markets of levulinic acid. The outlook of levulinic acid application is outrageous in pharmaceuticals, agriculture, food additives and cosmetics (Grand View Research 2015).

Bioethanol

Bioethanol is a promising future fuel, which is produced by the hydrolysis of lignocellulosic biomass followed by fermentation (Karimi et al. 2021). The market size for bioethanol was U.S.$ 33.7 billion in 2020 and is expected to reach around U.S.$ 65 billion by 2025 at a CAGR of 14% between 2020 and 2025 (Markets and Markets 2020a). North America is the largest market for bioethanol due to large production facilities, while U.S., Canada and Mexico are the growing consumers (Intrado 2020). Bioethanol and biobutanol can be used as a transportation fuel, feedstock in the chemical industry, fuel for fuel cells, in the cosmetics industry and manufacturing processes owing to its clean-burning and easy availability (Nanda et al. 2014; Kumar et al. 2018).

Ferulic acid

Ferulic acid, recognized as a phytochemical, is found in the cell walls of plants. Ferulic acid is produced by the usage of dilute acid pretreatment and enzymatic hydrolysis of lignocellulosic biomass. While the market size of ferulic acid was U.S.$ 60 million in 2020, its consumption could exceed 750 tons by 2025 (Osorio-González et al. 2019; TBinsights 2020). Owing to its usage in different fields like pharmaceutical, cosmeceuticals and food sectors, Europe, Asia Pacific and North America shared U.S.$ 12.7, 24.2 and 14.6 million, respectively in the global market in 2020 (Barbara et al. 2015; TBinsights 2020).

Vanillin

Vanillin is one of the lignin-derived aromatic compounds and the bacterial consortium is found to be a good biocatalyst for its production from lignocellulosic biomass (Harshvardhan et al. 2017). The global market size for vanillin was estimated to be U.S.$ 395 million in 2016, which was increased to U.S.$ 480 million in 2020 (Grand Research View 2017). In 2016, the Asia-Pacific was dominating the market which accounted for 39.7% of the total revenue followed by Europe, which accounted for 27% and it is expected to demonstrate a CAGR of 7.1% from 2019 to 2025 (Grand Research View 2017). It is used in perfumery and beverage as well as an intermediate in pharmaceutical industries. It is also used as a flavoring agent in the food processing industries.

Eugenol

Eugenol is a phenolic component that can be obtained from clove oil, nutmeg oil and cinnamon extract and many wide ranges of plant sources like basil, ginger, pepper, oregano, thyme and turmeric. It was the first natural compound used in the synthesis of vanillin in the late 19th century and it is an industrial source to produce iso-eugenol and methyl eugenol. It can be produced by drying of biomass followed by roasting, staged condensation and purification of the mixture (Khalil et al. 2017).

Various chemicals like 5-hydroxymethylfurfural, lactic acid, furfuryl alcohol, xylitol, sorbitol and mannitol had the global market value of U.S.$ 55, 1100, 473, 877, 1470 and 382 million in 2020 (Fig. 2) (Market and Research 2019; Markets and Markets 2020b; Grand View Research 2020b, 2021; EMR 2020a, 2020b). These biochemicals are potentially used in biofuels, pharmaceuticals, food industries, confectioneries and several other value-added sectors.

Fig. 2
figure 2

source: Market and Research 2019; Markets and Markets 2020b; Grand View Research 2020b; EMR 2020a; EMR 2020b; Grand View Research 2021). Note the highest market share of bioethanol followed by sorbitol, lactic acid, xylitol and furfural. Bioethanol has the highest market share globally because of its diverse applications as a laboratory chemical, industrial commodity, biofuel and precursor for other value-added products. Other biochemicals have found applications in specialty industries such as food processing, beverages, pharmaceuticals, laboratory chemical manufacturing, bioplastics and fuel processing, to name a few

Global market size of different commodity biochemicals in 2019–2020. (Data

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Components of lignocellulosic biomass

The lignocellulosic biomasses are primarily made up of cellulose, hemicellulose and lignin. The proportion of these chief biopolymers vary for different biomasses (Table 1). For instance, the crop residues are comprised of 30-50 wt% cellulose, 20-38 wt% hemicellulose and 7-21 wt% lignin, whereas the biomass obtained from agro-processing industries consist of 21-45 wt% cellulose, 15-33 wt% hemicellulose and 5-24 wt% lignin (Pattanaik et al. 2019). The composition of given lignocellulosic biomass also changes with the environment, harvesting practice, collection time and anatomical portion (Srivastava et al. 2019). The selection of required lignocellulosic biomass depends on the targeted end product. For the synthesis of most commodity biochemicals, biomass rich in polysaccharides and containing a low amount of lignin is preferred (Srivastava et al. 2017; Pattanaik et al. 2019). However, depolymerization of lignin also results in some aromatic monomers, which are of significance for biofuel generation and other applications (Gall et al. 2017).

Cellulose is the most abundantly available homopolysaccharide consisting of linear chains of D-glucose units attached by β-1,4-glycosidic linkage whereas hemicellulose is a complex polysaccharide comprised of xylose, mannose, galactose, arabinose, glucose and acetic acid (Okolie et al. 2019). Lignin is the second most abundant biopolymer made of coniferyl, sinapyl and coumaryl alcohol (Okolie et al. 2021b).

Table 1 Composition of various lignocellulosic biomasses
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Different routes for the conversion of lignocellulosic biomasses to bioproducts

Biological conversion route

Figure 3 represents different biological and thermochemical routes for the conversion of lignocellulosic biomass into various commodity chemicals, syngas, sugars and bio-oil. The biological route for converting lignocellulosic biomass involves enzymatic hydrolysis of polysaccharides such as cellulose and hemicellulose to monomeric sugars, which further undergo fermentation and distillation to produce biofuels such as bioethanol and biobutanol (Nanda et al. 2014; Nanda and Berruti 2021a). The biological process for converting biomass is well explored and preferred for obtaining valuable chemicals and fuel at a large scale due to its environmentally benign nature. The robust structure of lignocellulosic biomass impedes the effective enzymatic hydrolysis and fermentation, thereby necessitating the pretreatment of biomass (Zeng et al. 2014; Nanda and Berruti 2021b).

There are various possible approaches for the pretreatment of biomass to facilitate its bioconversion. The chemical pretreatment method involves the use of acids, bases or organic solvents. Various greener pretreatment methods such as grinding, milling, ultrasound, microwave and steam explosion are found to enhance biomass hydrolysis to a significant extent (Zhang et al. 2021). The pretreated biomass is subjected to enzymatic hydrolysis during the biochemical conversion process to generate sugar monomers. The sugar monomers are further converted into valuable commodities during the fermentation stage. The lignin content and its structure were the most crucial factor affecting the enzymatic hydrolysis process. Higher lignin content and a lower ratio of syringyl/guaiacyl led to reduced accessibility of enzymes to the polysaccharides, thereby decreasing its efficiency (Santos et al. 2012). Besides generating fermentable sugars to be used for further production of biofuels and some added value chemicals, the enzymatic hydrolysis of lignocellulosic biomasses itself leads to the production of many valuable commodity chemicals. For example, the enzymatic hydrolysis of biomasses such as de-starched wheat bran, corn stover and sugarcane bagasse can produce ferulic acid (Damásio et al. 2013; Cao et al. 2015; Zhang et al. 2015).

Fermentation of monomeric sugar molecules produced by enzymatic hydrolysis of polysaccharides is another important step during the biochemical conversion of biomass to biofuel. The metabolic activities of microorganisms under aerobic or anaerobic conditions convert the organic matter present in the biomass to biofuels (Patinvoh and Taherzadeh 2019; Sarangi and Nanda 2020). The fermentation process can be operated in different modes such as batch, continuous, fed-batch and consolidated bioprocessing by the digestion. To make the biochemical conversion process cost-effective, integration of various stages, including pretreatment, saccharification and fermentation using a single reactor and microorganisms have been suggested.

Fig. 3
figure 3

Different routes for the conversion of lignocellulosic biomass to biofuel and biochemicals. The biological conversion processes involve microorganisms and their enzymes to degrade lignocellulosic biomass to monomeric sugars and further to bioethanol, biobutanol, biohydrogen and biomethane. On the contrary, thermochemical conversion technologies involving liquefaction, pyrolysis and gasification transform waste biomass to specific biofuel products such as bio-oil, bio-crude oil, syngas and biochar. Through catalytic hydrotreating and upgrading processes, bio-crude oil and bio-oil can be transformed into synthetic transportation fuels such as biodiesel, aviation fuels and bio-gasoline as well as biochemicals and high-value clean hydrocarbons

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Thermochemical conversion routes

The thermochemical conversion routes mostly involve high temperatures, optimal reaction times and catalysts to break down the organic compounds to volatile gases and liquids, which are further processed for use as biofuels or precursors for biochemicals. Thermochemical conversion routes are broadly categorized into pyrolysis, gasification and liquefaction for fuel and chemical production (Aravind et al. 2020; Parakh et al. 2020; Patra et al. 2021; Suresh et al. 2021).

Pyrolysis is an endothermic process, which induces mild depolymerization of lignocellulosic biomass under an inert atmosphere producing mainly a liquid intermediate product called bio-oil at a temperature 300–700°C, a high heating rate of up to 1000°C/s and residence time up to 2 s (Aravind et al. 2020; Patra et al. 2021). The bio-oil obtained via pyrolysis can be subjected to catalytic hydrotreating processes to produce hydrocarbon liquid fuels with the removal of heteroatoms such as oxygen, nitrogen and sulfur. Besides, liquefaction technology can also convert lignocellulosic biomass into bio-oil upon heating at 250–374°C in an aqueous or organic solvent medium at pressures higher than 22 MPa (Tekin et al. 2014). It is considered an advantageous methodology to process wet biomass. Liquefaction can be used to produce bio-crude oil and tar from biomass (Aresta et al. 2005; Zhang et al. 2019). Biochar obtained from pyrolysis and gasification can be used as a solid fuel, adsorbent for wastewater treatment, catalyst support and precursor for carbon-based high-performance materials (Nanda et al. 2016; Balajii and Niju 2019; Gunarathne et al. 2019; Chen et al. 2019; Gopinath et al. 2021).

Gasification is a thermochemical technique, which involves complete depolymerization of biomass in the presence of gasifying agents such as oxygen and steam to syngas at temperatures typically above 800 °C. The produced gas mainly comprises a mixture of H2, CO2, CO and CH4 (Nanda et al. 2019). Furthermore, gasification of biomass at temperature and pressures greater than 374°C and 22.1 MPa, respectively under supercritical water medium is termed supercritical water gasification. Supercritical water gasification is considered economically feasible as it reduces the energy input. The biomass components under supercritical conditions are decomposed into glycerol, alcohols, organic acids, phenolics, aromatics and permanent gases (Akalın et al. 2017; Kumar et al. 2019; Nanda et al. 2015, 2019). Syngas obtained from gasification can be converted to methanol and dimethyl ether can also be produced by dehydrating methanol. Gasification can also be utilized for the generation of fuels and electricity (Okolie et al. 2021c).

Catalytic conversion of polysaccharides

Homogeneous catalysts

Homogeneous catalysis infers to the condition when the catalyst is in the same phase as the reaction medium. Homogeneous acid catalysis plays an essential role throughout the biomass conversion process, starting from hydrolytic cleavage of bonds in polysaccharides to form monomers to upgrading steps for delivering valuable products from platform chemicals. The mineral acids such as sulfuric acid or H2SO4, hydrochloric acid or HCl and nitric acid or HNO3 are among the oldest and most effective homogeneous catalysts to produce biofuel and other value-added commodities like furfural and 5-hydroxymethylfurfural with high selectivity.

Table 2 shows various examples of the catalytic conversion of polysaccharides into different platform chemicals. The promising application of homogeneous catalysts for furfural synthesis has led to an increased interest of researchers (Zhao et al. 2021). Tan-Soetedjo et al. (2017) have reported a 22% conversion of sucrose into 5-hydroxymethylfurfural using 0.05 M H2SO4 at 140℃. The conversion of sucrose into platform chemicals was found to be susceptible to temperature and concentration of acid.

When the monomeric sugar fructose was directly used as the substrate, 53 mol% of 5-hydroxymethylfurfural was obtained at 166℃ with 0.005 M H2SO4 and 0.1 M of initial fructose concentration (Fachri et al. 2015). Approximately 27% yield of furfural was obtained from Miscanthus at 185℃ and 0.5 M H2SO4 (Dussan et al. 2013). 5-hydroxymethylfurfural can further be converted to levulinic acid and 2,5-furan dicarboxylic acid, which are important value-added chemicals. Acid-catalyzed hydrolysis of lignocellulosic biomass is among the most widely used process to produce levulinic acid (Kang et al. 2018). The most frequently reported acids used are H2SO4 and HCl, whereas HNO3, not being too strong, cannot produce levulinic acid significantly. This emphasizes the role of acid strength in converting biomass feedstock into levulinic acid (Rackemann and Doherty 2011).

Galletti et al. (2012) used HCl to produce levulinic acid from waste biomasses such as poplar sawdust, tobacco chops, paper sludge, wheat straw and tree pruning olive. The optimization of reaction temperature and time of 200℃ and 1 h, respectively led to a 12-32 wt% yield of levulinic acid with the lowest yield from olive tree pruning and the highest yield from paper sludge. The utilization of other biomasses and optimization of reaction parameters can further enhance the yield of levulinic acid obtained.

Lignin is an integral component of lignocellulosic biomass that has several applications in fuel, chemical and biomedicine (Spiridon 2020). Lignin depolymerization with the homogeneous catalyst to produce valuable polymer precursors and aromatic monomers has been studied (Wanmolee et al. 2016). The ether bonds of lignin can be cleaved by acid or base catalysts such as triflic acid and NaOH. Lavoie et al. (2011) used NaOH for the synthesis of monomers such as phenol, catechol and vanillin from lignin obtained by steam processing of softwood and hemp. The monomeric composition obtained was different for lignin from hemp and softwood, which could be attributed to the difference in the structure of the original lignin. Additionally, the composition varied with the temperature of the depolymerization process.

In the catalytic conversion of lignocellulosic biomasses or biomass-derived pentose and hexose sugars such as xylose and glucose, the first step is the hydrolysis of polysaccharides into monomeric sugars. The second step is the isomerization of the sugars, and the most important part is the dehydration of the isomerized sugars into high-value compounds like furfural, 5-hydroxymethylfurfural and other furanics. In this catalytic process, hydrolysis and dehydration reactions are catalyzed by the Brønsted acid sites present in the acid catalysts, whereas the isomerization process is catalyzed by the Lewis acid sites (Pattnaik et al. 2021).

Although mineral acid-based homogeneous catalysts show a significant yield of various platform chemicals, due to the absence of a prominent level of Lewis acid sites, mineral acid does not significantly catalyze the isomerization reactions, which is mentioned as the rate-determining step. Therefore, these catalysts show comparatively lower yields than the various heterogeneous catalysts. Other major drawbacks of the homogeneous catalysts are the environmental toxicity caused by the different mineral acids in addition to the higher cost of the downstream processes for the separation of the chemicals from the reaction medium. Therefore, heterogeneous acid catalysts and ionic liquids are promising for their cost-effective and environmentally friendly approaches towards biomass conversion.

Table 2 Summary of works on the catalytic conversion of polysaccharides
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Mesoporous silica-based catalysts

For the catalytic conversion of the biomass, mesoporous SiO2 catalysts are used due to their larger pore size, which can produce convenient passage to the larger polysaccharide molecules and other derived chemicals. For the synthesis of the mesoporous SiO2 catalysts, two major synthetic routes are adopted, such as the soft templating method and the hard templating method (Shi et al. 2011). In the silica material, the Si atoms are replaced with Al atoms, which enhances the acidic property of the catalysts, and they are categorized as aluminosilicates. The discovery of the M41S family by Mobil’s corporation made a breakthrough in the mesoporous aluminosilicate catalyst materials. In the footprint of Mobil’s corporation, various other families of aluminosilicates were developed such as SBA, KIT, COK and MSU. Under this category of catalysts, MCM-41, MCM-50, MCM-48, SBA-15, and SBA-16 are well established for the conversion of lignocellulosic biomass into liquid chemicals.

Zhang et al. (2012) used the mesoporous MCM-41 for the conversion of biomass-derived xylose into furfural. They have performed the experiments using the catalyst at the temperature of 170 ºC for 3 h in a biphasic medium containing water and 1-butanol in the ratio of 1:1.5. In this study, there was a xylose conversion of 97% and furfural yield of 44%, showing a comparatively better selectivity for the synthesis of furfural.

The major bottleneck of the aluminosilicates catalysis is the hydrothermally unstable nature of the various materials, especially at higher temperatures or prolong catalytic processes (Xiong et al. 2014). However, some of the doped mesoporous silica has proved to be hydrothermally stable. Peng et al. (2017) performed catalytic conversion of glucose to 5-hydroxymethylfurfural by using Niobium or Nb doped SBA-15 at the Si/Nb ratio of 1:40. The authors reported a significant glucose conversion and 5-hydroxymethylfurfural yield of 94% and 62%, respectively. The above catalytic conversion was performed at the temperature of 165 ºC for 3 h and using tetrahydrofuran as a co-solvent in the reaction medium. In this study, the researchers have performed the catalytic conversion of cellulose into 5-hydroxymethylfurfural, where they have concluded with the cellulose conversion of 92% and 5-hydroxymethylfurfural yield of 51% at the reaction temperature of 170 ºC for 8 h. However, with the increase in the temperature to 190 ºC, the 5-hydroxymethylfurfural yield decreased to 21% due to the polymerization of glucose into humin.

5-ethoxymethylfurfural is another high-value platform chemical like 5-hydroxymethylfurfural, which can be synthesized from various carbohydrates by using selective catalysts. Rao et al. (2020) used the SBA-15 catalyst modified by Sn-doped heteropolysilicate for the selective conversion of fructose to 5-ethoxymethylfurfural. The higher selectivity of the catalyst was attributed towards the appropriate Brønsted to Lewis acid site. In this conversion process, fructose was dehydrated to produce 5-hydroxymethylfurfural by the Brønsted site. On the other hand, the catalytic effect of the Lewis sites resulted in the conversion of 5-hydroxymethylfurfural into 5-ethoxymethylfurfural (Rao et al. 2020).

In various studies, aluminosilicates are used as the support materials for various solid acid catalysts like sulfonic acids, tungsten oxide, heteropoly acids and other metal oxides, which catalyze a wide range of lignocellulosic biomasses, and biomass-derived carbohydrates to obtain various platform chemicals. However, aluminosilicates such as amorphous zeolites are widely used solid acid catalysts with a well-defined porous and crystalline structure. These materials are also hydrothermally stable to several high-temperature reactions.

Zeolite-based catalysts

Zeolites are also considered solid acid catalysts under the category of aluminosilicates. However, unlike the previously described aluminosilicates such as MCM or SBA, zeolites are microporous crystalline materials and possess significant stability towards the high-temperature water-based reaction medium. Zeolites consist of SiO4 tetrahedra where some of the Si-point is replaced by the Al+3 ion creating AlO3 linkages and leaving a negative charge to the zeolite framework. Moreover, to compensate for this negative charge, positive ions such as ammonium ions or NH4+, sodium ions or Na+ and proton or H+ reside over the framework. Among these ions, zeolites containing H+ are known for their acidic sites, where the Al2O3 provides the Lewis acid sites, and the H+ ion provides the Brønsted acid sites (Fig. 4). Therefore, the proper synchronization of these Lewis and Brønsted sites can catalyze various acid-catalyzed reactions like isomerization, dehydration and hydrogenation. There are several pieces of literature available for the catalytic conversion of lignocellulosic biomasses and sugar into different furan-based chemicals such as furfural, 5-hydroxymethylfurfural, organic acids like levulinic acid and sugar alcohols like xylitol, sorbitol and mannitol.

The biphasic systems have been proved to be the efficient reaction media compared to the water. When an organic solvent is present in the reaction medium, the degraded products such as furfural and 5-hydroxymethylfurfural obtained from the catalytic conversion of sugars are extracted to the organic medium due to their higher solubility in the organic solvent than the water. Therefore, the products further do not react to the various polymeric products like humin, which subsequently increases the yield of decided products from the sugars.

Sahu and Dhepe (2012) reported using various zeolite catalysts such as H-β, H-USY and H-mordenite to convert the hemicelluloses present in lignocellulosic biomass into furfural. In this study, the yield of furfural in the water medium and water/p-xylene medium was obtained to be 17% and 56%, respectively, in the case of H-USY with Si/Al ratio of 15 at 170 ºC for 6 h, where other zeolites were concluded with a comparatively lesser yield of furfural.

Among the various zeolitic catalysts, ZSM-5 or various modified ZSM-5 are highly implemented for their cost-effective aspects. However, in the catalytic conversion of the lignocellulosic biomass, there exist three significant reactions, such as hydrolysis of polysaccharides to mono-or oligomers, isomerization of monomers and finally dehydration of the isomeric sugars, in which the hydrolysis and isomerization can be performed by the Lewis sites present in ZSM-5 or other zeolites whereas the hydration step needs strong acid sites. Therefore, sometimes the modification of zeolites with a stronger acid catalyst can increase the acidity and catalytic activity.

Bisulfite or HSO3 modified ZSM-5 can be used to convert the corncob to obtain 5-hydroxymethylfurfural and furfural. Here, this modification using HSO3 enhances the acid properties of the parent catalyst by increasing the strong acid sites, for which the conversion of corn cob was efficiently performed, yielding 49% 5-hydroxymethylfurfural and 89% furfural at the temperature of 150 ºC for 5 h in a biphasic system containing tetrahydrofuran and water (Hoang and Cuong 2021). In some reports, as with the HSO3, heteropoly acids are used as the Brønsted acid modifier in the zeolites. Heteropolyacids are strong solid acids and can provide their Brønsted acidity in the acid catalytic conversion of carbohydrates. Zhou et al. (2019) used H4SiW12O40 modified Sn-β-zeolite for the conversion of alpha-cellulose and microcrystalline cellulose to produce methyl levulinate in the presence of methanol in the reaction medium. The authors reported methyl levulinate yields of 55% and 62% from α-cellulose and microcrystalline cellulose, respectively.

Zeolites are widely used for the conversion of lignocellulosic biomasses or biomass-derived sugars into various platform chemicals. However, a few disadvantages associated with zeolitic catalysts are encountered such as lower hydrothermal stability, deactivation of acid sites and blocking of micropores leading to the hampered synthesis of large molecules (Silva et al. 2020). In the conversion of sugars, the catalysts should be hydrothermally stable, as the reaction medium in this catalytic process is water. Therefore, for the catalytic conversion of polysaccharides, metal oxides are proven to be potential catalysts with a higher level of hydrothermal stability.

Fig. 4
figure 4

Development of Brønsted and Lewis acid sites in zeolites. In the zeolite framework, when tetravalent silicon ions are replaced by trivalent aluminum ions, extra negative charges are formed. These negative ions are compensated by the monovalent positive ions such as protons. These protons behave as the Brønsted acid sites in the zeolite catalytic framework. It should also be noted that by the dehydration of aluminum-doped silicon-based zeolites, Lewis acid sites are formed as vacant orbitals. The synergistic effects of the Brønsted and Lewis acid sites in zeolites can catalyze various biomass conversion processes

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Metal oxides

Since the sugars are soluble in water, the reaction is usually performed under aqueous conditions. Therefore, under such conditions, the tolerance of acid catalysts to convert sugars into chemicals is a critical feature. Furthermore, some water-resistant metal oxides such as ZrO2, TiO2, SnO2, Ta3O5, Nb2O5 and Mo2O3 as well as mixed metal oxides such as WO3–TiO2, WO3–ZrO2, ZrO2–TiO2 and WO3–Ta3O5 can be potentially implemented to transform lignocellulosic biomass into fine chemical precursors. The metal oxides mentioned above consist of unsaturated metal ions, which are responsible for the Lewis acidity of the metal oxides and can catalyze the isomerization process of the monomeric sugars. Moreover, these metal oxides with pre-existing Lewis acid sites can be modified by some solid Brønsted acids, which subsequently aid the dehydration of isomerized sugars including fructose or xylulose.

With the synchronizing effect of these two types of catalytic sites, polysaccharides present in the lignocellulosic biomass can be converted into liquid chemicals. Therefore, to enhance the Brønsted sites, the metal oxides can be mixed with tungsten trioxide or WO3, sulfuric acid or H2SO4, phosphoric acid or H3PO4 and heteropoly acids, which will provide strong acidity to the conversion reactions. Among the various Brønsted acids, WO3 is the most common dopant used by several researchers (Pattnaik et al. 2021).

Pattnaik et al. (2021) used several metal oxides and mixed metal oxides for the conversion of sugar mixture obtained from the subcritical hydrolysis of common reed or Phragmites karka. WO3-ZrO2 showed significant catalytic activity with 92% sugar conversion, 51% furfural yield and 34% levulinic acid at 170 ºC for 2 h when compared with other catalysts such as ZrO2, TiO2, ZrO2-TiO2, WO3-TiO2 and WO3-ZrO2-TiO2. The higher yield of levulinic acid and furfural than the other used catalysts is due to the synchronization of the basic sites present in ZrO2 and Brønsted sites of the WO3. Figure 5 shows the dehydration of xylose and glucose into specific products (Pattnaik et al. 2021).

Besides the mixed metal oxides, individual metal oxides like TiO2, ZrO2 and Nb2O5 are used as the catalyst for the conversion of sugars or carbohydrates into respective dehydrated chemicals. In a catalytic reaction, one of the significant factors is the lower activation energy, which establishes an energy-efficient catalytic conversion. In a study by Gupta et al. (2017), Nb2O5 showed comparatively lower activation energy (83 kJ/mol) than the conventional homogeneous catalysts and a mineral acid such as HCl (107-131 kJ/mol). With Nb2O5, xylose conversion and furfural yield were found to be 93% and 72%, respectively at 120 ºC in a biphasic system containing toluene and water.

To increase the acidity of the metal oxides, these are modified by several Brønsted acids. Moreover, sulfated and phosphate metal oxides show comparatively higher performance than the WO3 based catalysts. Anatase phase TiO2 impregnated with H3PO4 is known as phosphate TiO2, which showed tremendous activity in the conversion of cellulose, starch, sucrose, cellobiose, glucose and fructose, obtaining a yield of 33%, 80%, 98%, 94%, 90% and 98%, respectively at the temperature of 175 ºC for glucose and fructose whereas 180 ºC for remaining of the substrates. Among the above substrates, cellulose delivered a relatively lower yield of 5-hydroxymethylfurfural compared with the other substrates due to the existence of slow rate-determining step and hydrolysis of cellulose into its monomeric sugars (Atanda et al. 2015). Although most of the metal oxides are highly resistant to hydrothermal processes, some tungsten-based oxides such as WO3 are slightly unstable in the water-based single-phase system. However, the reactions conducted in bi-phase or multi-phase systems provide a higher level of stability to the unstable metal oxide catalysts (Choi et al. 2018).

Fig. 5
figure 5

a Dehydration of xylose into furfural, and b dehydration of glucose into 5-hydroxymethylfurfural and levulinic acid. Note that in the catalytic conversion of xylose to furfural, two consecutive steps occur such as isomerization of xylose to xylulose and dehydration of xylose to furfural. Likewise, in the catalytic conversion of glucose, the first step is the isomerization of glucose to fructose followed by the dehydration of fructose to 5-hydroxymethylfurfural and rehydration to form levulinic acid. The isomerization reactions are catalyzed by the Lewis acid sites, whereas the dehydration and rehydration are promoted by the Brønsted acid sites

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Ionic liquids

Unlike other conventional organic solvents, ionic liquids are composed of different inorganic or organic ions in cationic and anionic forms. They possess various potential characteristics like lower vapor pressure, higher thermal stability and excellent solubility in water. Ionic liquids are used for the dissolution of the various components of lignocellulosic biomasses such as cellulose, lignin and hemicellulose by taking multiple combinations of the anions and cations. According to the need of the processes, the combinations of anions and cations are tailored, which are raised with acidic, basic or other prominent properties.

Some notable ionic liquids such as 1-ethyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium acetate and their chlorides, especially chloride of 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium cations can solubilize lignocellulosic biomass components such as cellulose, hemicellulose and lignin (da Costa Lopes and Bogel‐Łukasik 2015). Furthermore, some phosphorous-based ionic liquids and sulfate-based ionic liquids are reported to selectively dissolve cellulose and lignin, respectively. Besides degrading lignin, ionic liquids are also used as the reaction medium for hydrolysis of cellulose and hemicellulose as well as the conversion of monomeric or oligomeric sugars into furan and non-furan-based biochemicals such as furfural, 5-hydroxymethylfurfural and levulinic acid.

In the biomass conversion process, the major bottleneck is the hydrolysis of complex polysaccharides like cellulose and hemicellulose into mono-or oligomeric sugars, which is a Brønsted acid-catalyzed reaction. Among various green processes, performing conversion reactions in ionic liquids has been tremendously established and well propagated by researchers worldwide. Amarasekara and Owereh (2009) have synthesized acidic ionic liquids by functionalizing the sulfonic acid group or –SO3H with imidazolium and pyridine-based ionic liquids. In the acid hydrolysis reaction, water plays a vital role in the reaction, which produces hydronium ion or H3O+ by reacting with H+ ions generated from the acidic ionic liquids. These hydronium ions react with the β-(1,4)-glycosidic linkages to disintegrate the bonds or interaction, resulting in pentose or hexose sugars, including some oligomeric sugars like cellobiose and raffinose. In the conclusion of this study, the researchers found methylimidazolium ionic liquids to be the efficient medium to produce the highest reducing sugars yield from cellulose than the pyridinium ionic liquids (Amarasekara and Owereh 2009; da Costa Lopes and Bogel‐Łukasik 2015).

Some alkaline ionic liquids such as 1-butyl-3-methylimidazolium carbonate, 1-butyl-3-methylimidazolium benzoate, 1-butyl-3-methylimidazolium acetate, ethanolamine tetrafluoroborate and 1-butyl-3-methylimidazolium hydroxide are widely implemented for the acid-catalyzed reactions for the conversion of monomeric sugars, namely glucose, xylose and fructose, oligomeric sugars like cellobiose and pure microcrystalline cellulose into furfural, 5-hydroxymethylfurfural, levulinic acid and other furanics.

Du et al. (2017) reported an alkaline ionic liquid named 1,1,3,3-tetramethylguanidine tetrafluoroborate showing an excellent yield of 74% for 5-hydroxymethylfurfural from fructose. Both the acidic and alkaline ionic liquids are considered greener solvents or catalysts for the pretreatment of lignocellulosic biomasses. However, the separation of synthesized products from the reaction medium can increase the cost of the downstream process.

Metal–organic frameworks

Metal-organic frameworks or MOFs are three-dimensional solid catalysts, which comprise metal clusters connected with the organic ligands with the coordinate bonds (Tran et al. 2019; Cong et al. 2021). MOFs are often known for their hierarchal porosity and have attracted the attention of many researchers for their heterogeneity, considerably higher surface area and adaptable pore dimension. Although a majority of MOFs have Lewis acid sites, there exist few MOFs containing both Lewis and Brønsted acid sites. MOFs can also be modified with certain Brønsted acid. These modified MOFs and MOFs with both Lewis and Brønsted acid sites are used for the catalytic conversion of carbohydrates to produce various liquid chemicals.

Qu et al. (2019) have used Lysin coordinated phosphotungstic acid or Lys-PM2 as the MOF for the conversion of glucose into levulinic acid. The most important advantages of the above heterogeneous catalyst are the convenient or cost-effective synthesis of the catalyst and higher stability in the aqueous medium with the least leaching of the catalyst elements into the reaction medium. In this study, with the synchronizing effect of Lewis and Brønsted sites, a levulinic acid yield of 57% was obtained at the temperature of 150 ºC for 9 h in the single-phase aqueous medium. Besides this significant yield of levulinic acid, the intactness of the Brønsted acid sites makes the catalysts be used significantly for four consecutive catalytic runs without losing the effectiveness of the catalytic process (Qu et al. 2019).

NU-1000 is another thermodynamically and chemically stable MOF-based catalyst similar to Lys-PM2. It is a MOF made up of ZrO2 clusters isolated by pyrene linkers. Due to the presence of Lewis acid sites, it is used as the catalyst for the isomerization of glucose to fructose. Yabushita et al. (2017) used phosphoric acid-modified NU-1000, where the phosphoric acid provided Brønsted acidity for the dehydration of fructose into 5-hydroxymethylfurfural. With the simultaneous effect of the Lewis and Brønsted sites, a glucose conversion of 94% and 5-hydroxymethylfurfural yield of 24% were reported at 140 ºC in 5 h using a biphasic system containing water and 2-propanol in a ratio of 1:9 (Yabushita et al. 2017). Although MOFs establish a green technique for the catalytic conversion of the lignocellulosic biomass or biomass-derived carbohydrates, the absence of Brønsted sites adds up the process cost for the modification of the MOFs to have Brønsted acidity.

Conclusion

The production of biofuel platform chemicals from diverse integral units of lignocellulosic biomasses is anticipated to be the chief facet of the biobased economy. The conversion of biomass into platform chemicals can be acquired by a thermochemical or biochemical pathway. The biochemical route is an entirely green approach but is somewhat more expensive and results in lower yields requiring thorough scientific investigations and innovations for enhancing the competency of the process. The thermochemical approach utilizing various homogeneous or heterogeneous catalysts is successfully applied to biofuel and platform chemicals production. The use of homogeneous mineral acids as the catalyst for this bioconversion is most widely commercialized, but the toxicity associated with these mineral acids leads to the utilization of environmentally benign heterogeneous catalysts. Mesoporous silica-based catalysts, zeolite-based catalysts, metal oxides, ionic liquids and metal-organic frameworks are profitably being employed for effective and efficient greener conversion of polysaccharides into biofuel platform chemicals. The catalytic processes postulate immense possibilities for driving the world towards the biobased economy.