Green Chemistry of Recoverable Catalysts

Abstract

This chapter highlights the various aspects of recoverable catalysts, a green concept. Catalysts play a vital role in the chemical and pharmaceutical industries. Environmental considerations aim toward its recycling and reuse. Recovery of the catalyst after the reaction and its reuse focuses on the sustainable greener approaches in chemical industries. The question is, how effectively the catalyst recovery can be achieved. To address the issue of catalyst regeneration, scientists have developed a wide range of strategies. One approach to classify the catalysts is based on their method of separation, and the majority comes under heterogenized solid-supported catalysts. In this context, the development of new logical solutions for the regeneration and reuse of the catalysts without losing the activity is of prime importance.

1 Introduction

The recovery of catalysts after the completion of a reaction is of great importance in green chemistry [1]. Green chemistry addresses the impact of any process on the environment, and it is primarily concerned about preventing pollution [2]. Catalysis plays a vital role in the chemical industry [3]. The waste generated in chemical synthesis is mainly due to the usage of stoichiometric reagents, especially the metal-based reagents [4]. For example, stoichiometric reducing agents, both metals (Na, Mg, Zn, Fe) and metal hydrides (LiAlH₄, NaBH₄, etc.), oxidizing agents such as permanganate, manganese dioxide, and chromium(VI) reagents are the commonly used ones. Reactions such as sulfonation, nitration, halogenation, diazotization, and Friedel–Crafts acylation employ stoichiometric amounts of mineral acids (H₂SO₄, HF, H₃PO₄) and Lewis acids (AlCl₃, ZnCl₂, BF₃) [5].

The broad classification of catalysis is heterogeneous and homogeneous catalysis. In heterogeneous reactions, the phase of the catalyst is different from that of the reactants, and hence, the separation of the catalyst from the reaction medium is relatively easy. On the other hand, in homogeneous reactions, the catalyst and other reactants are in the same phase and are miscible with one another [6]. Solvent extraction, [7] filtration, nanofiltration, [8] catalyst leaching, chemical precipitation, [9] magnetic recovery, [10], and adsorption are catalyst recovery techniques typically employed in industrial and pharmaceutical processes. Nanofiltration is a relatively new membrane filtration technique used only in the homogenous process, and the catalysts are immediately reused. Materials such as activated carbon and ion exchange resins are used in the adsorptive recovery techniques and can include the recovery of either homogeneous catalysts or homogeneous ligands/catalysts. Magnetic regeneration of the catalysts allows the easy retrieval of catalysts with minimal loss.

2 Classification

Catalyst recovery can be broadly classified based on the recovery techniques used, and the choice of method for the catalyst separation is dependent on the nature of the catalysts. The separation techniques are time-consuming and have the limitations of cost, efficiency, generation of secondary wastes, etc. [11].

2.1 Metallodendrimers-Dendrimers Supported Catalysts

A widely used method for the easy recovery of the catalyst is the attachment of the metal catalysts to a solid support [12]. Generally used supports include the organic, inorganic, and hybrid, which work through heterogeneous catalysis. The solid support silica is the universal inorganic support used because of its chemically inert nature and physical strength. But the catalyst efficiency in this type of heterogeneous catalysis is inferior due to the very slow diffusion of the catalyst into the reaction medium, which decreases the rate of mass transfer and also the generally less activity of heterogeneous catalyst compared to the homogeneous ones. These limitations focus on the development of soluble catalyst support, which makes the reaction medium seamless. Dendrimers are such types of soluble catalyst supports used. The globular macrostructure of such supports enables the catalyst recovery effortless, and it is suited for membrane filtration [13]. The first example of a recoverable metallodendritic catalyst was reported in 1997 by Reetz for the Heck reaction using Pd dendrimers derived from the dendritic phosphines [14]. Potentially speaking, a dendrimer can be considered as a homogenous catalytic system as it is a combination of a large number of monomeric units with active sites. The total catalytic features such as selectivity, stability, and recyclability depend on the dendritic architecture of the metallodendrimers [15] (Fig. 1).

Fig. 1

Dendrimer architectures

A copper-free recoverable dendritic palladium catalyst for the Sonogoshira reaction was reported in 2003 by Heuze and co-workers [16]. The catalyst prepared bears two cyclohexyl substitutions on the phosphorous atom, and the catalyst is quickly recovered and reused by a simple precipitation method. They have developed three generations of the dendritic palladium complexes by treating Pd(OAc)₂ with aminophosphines. The catalytic activity of the synthesized compounds was tested on the coupling between phenylacetylene and iodobenzene or bromobenzene in a copper-free Sonogashira procedure (Scheme 1). The first two generations of the complex showed excellent catalytic activity. Still, for the third generation complex, the activity was less because of the increased steric effect of the dendrimer branch.

Scheme 1

Dendritic Pd catalyzed Sonogoshira reaction

In the year of 2004, Nlate and co-workers synthesized a stable polyoxometallate (POM) centered dendrimer and studied its catalytic activity in different oxidation reactions [17]. The core present in the dendritic catalyst was peroxophosphotungstate and was developed by the ionic bonding approach (Scheme 2). The application of this catalyst lies in the oxidation reactions using peroxides, and they were found to be highly air-stable and recoverable. The catalyst was efficient for performing the epoxidation of the alkene, selective oxidation of sulfide to sulfone, and also on the oxidation of cyclohexanol to cyclohexanone. The bulkier branches surrounding the POM center increase the stability of the catalyst, which prevents it from the degradation.

Scheme 2

POM-centered dendrimer synthesis

An efficient catalyst supported on phosphorous dendrimers and nanobeads of Co/C for the Michael addition of aldehydes to nitro olefins was developed by Keller and co-workers in 2013 [18]. The dendritic catalyst was a combination of Jorgensen-Hayeshi organocatalyst grafted via triazole linkers on phosphorus dendrimers (Scheme 3). A comparative study on the catalytic performances of dendrimer supported and magnetic nanoparticle supported organocatalysts on Michael addition was carried out. The dendrimer supported catalyst was efficient with maximum conversion at a reasonable time. It showed excellent recycling ability via precipitation method and were reused up to seven consecutive catalytic cycles.

Scheme 3

Preparation of Azido terminated phosphorous dendrimers

2.2 Fluorous Biphasic Catalyst-PTC

Phase transfer catalyst (PTC) allows the transfer of reagents from one phase and creates suitable conditions for the reaction to happen. Generally, organic salts with fluorinated anions act as phase transfer catalysts in organic transformations [19].

In the year of 2004, several cinchona derived alkaloids were anchored to different polystyrene supports by Chinchilla et al., resulting in the formation of chiral polymeric ammonium salts (Scheme 4) [20]. The catalyst prepared was used for the asymmetric alkylation of N-(diphenyl methylene) glycine esters. The best S-enantiomer selectivity was found for the Merrifield resin supported cinchonidium salt, and the R-selectivity was observed for the cinchonine derived salts. Simple filtration techniques were used to separate the catalysts after the reaction and were reused.

Scheme 4

Polystyrene anchored cinchona ammonium salts

Tetraalkyl ammonium salts containing PF₆- and BF₄- anions were developed by Kryshtal and co-workers and have used as efficient PTC for the trichloromethylation and dichlorocyclopropanation reactions of alkenes and aldehydes (Scheme 5) [21]. The fluorinated PTCs were readily recovered from the reaction medium and washed with H₂O and Et₂O. The catalyst retained its activity over several cycles of reactions.

Scheme 5

Cyclopropane synthesis using fluorinated PTCs

A silica nanoparticle supported phosphonium-palladium phase transfer catalyst matrix was reported by Hajipour et al. The application of this catalyst in the Heck reaction in neat aqueous media was studied [22]. The palladium complexes along with the quarternary phosphonium cations were covalently immobilized on a silica support. The catalyst was synthesized by a complex formation of quaternary phosphonium iodide with palladium chloride, which resulted in the formation of [PdCl₂I₂]²⁻, and it was reacted with SiO₂ nanoparticles (Scheme 6). The activity of Pd/PTC was tested in the coupling reaction of iodobenzene and methyl acrylate, using pure water as the solvent (Scheme 7). The catalytic recyclability was also checked in the same reaction conditions. Catalyst recovery from the medium was done expertly by centrifugation method and reused after washing it with ethanol and acetone. The synthesized catalyst exhibited better reactivity in terms of turnover numbers.

Scheme 6

Immobilization of Pd/PTC on silica

Scheme 7

Heck reaction catalyzed by immobilized Pd/PTC on silica

Commercially available cinchona alkaloids were used for the synthesis of different dendrimeric phase transfer catalysts by Rull et al. The quaternization of the quinuclidine nitrogen resulted in the formation of PTC, and they catalyzed asymmetric organic transformations [23] (Scheme 8). The catalytic activity was checked on the asymmetric alkylation of the glycinate Schiff base with benzyl bromide. The recovery and reusability of the catalyst were valid for five consecutive cycles, maintaining the activity. The enantioselectivity of the catalyst was observed to reduce slightly during the runs.

Scheme 8

Synthesis of benzyl chloride/iodide functional phosphorous dendrimers

2.3 Magnetically Recoverable Metal Nanoparticles

Magnetically recoverable catalysts are green chemistry aspects as they enable the environmentally friendly catalytic approach. Various strategies, like grafting, surface modification, and self-assembly, etc., provide a wide range of scope for the preparation of metal nanoparticles (MNPs). The separation of the catalyst from the reaction medium is also cheap, feasible, and straightforward. Most of the Fe-, Ni-, Co-based MNPs enhance the reaction in both homogenous and heterogeneous media [24].

Considering the photocatalytic application of magnetically recoverable catalysts, Boruah and co-workers synthesized Fe₃O₄/reduced graphene oxide catalysts [25]. The synthesis was carried out following eco-friendly solution chemistry. The performance of the prepared nanoparticle was checked on the dye degradation. Carcinogenic and mutagenic dyes such as methyl green, methyl blue, and rhodamine B were effectively degraded by Fe₃O₄/rGO (Scheme 9). The catalytic application was extended to the photoreduction of mutagenic aqueous Cr (VI) to nontoxic Cr (III) solution under sunlight. The activity was found to be valid up to ten reaction cycles and was stable enough.

Scheme 9

Photo dye degradation using Fe₃O₄/reduced graphene oxide

Vavasori et al. synthesized a new magnetically recoverable Pd catalyst by depositing 1% palladium on Poly-(1-oxo-trimethylene) with a 15% magnetite [26]. Alkoxycarbonylation of iodobenzene to benzoic esters was catalyzed using synthesized metal nanoparticles under phosphine free conditions (Scheme 10). The conversion was found to reach up to 100% within 3 h. The catalyst was recovered and recycled for about five times.

Scheme 10

Alkoxy carbonylation catalyzed by Pd/Fe₃O₄ with the mechanism

Later, in 2017, a Pd(0)-based magnetically recoverable nanocatalyst supported on nano Fe₃O₄–SiO₂ was developed by Khazaei et al. (Scheme 11) [27]. The prepared catalyst was used for the Suzuki coupling reaction (Scheme 12). Eggshell was utilized as a low-cost solid base for the reaction. The catalyst was characterized using SEM, EDS, XRD, TEM, etc. and confirmed the formation of Fe₃O₄@SiO₂–Pd. An external magnet recovered the nanoparticles and was further used certain times without losing the catalytic activity.

Scheme 11

Preparation of Pd-magnetic nanoparticles

Scheme 12

Suzuki coupling in the presence of Fe₃O₄@SiO₂-Pd

A catalytic reduction of nitroaromatic compounds using magnetically separable nanoparticles was reported by Hong et al. in 2018 [28]. The catalyst synthesized was a combination of 13 nm Au nanoparticles enclosed in a hollow mesoporous carbon. The nanocatalyst Au@hmc-FeCo/GC was prepared from the thermal decomposition of Fe and Co on silica support containing Au nanoparticles (Scheme 13). Au@hmc-FeCo/GC showed superparamagnetism at room temperature. Catalytic reduction of 4-nitrophenol, 4-nitrotoluene, etc. was carried out, and the results were confirmed to be better (Scheme 14). The same reaction cycle can be repeated at least five times without losing the initial catalytic properties.

Scheme 13

Synthesis of Au@hmc-FeCo/GC

Scheme 14

Au@hmc-FeCo/GC catalyzed nitroarene reduction

2.4 Organic Resin and Polymer-Supported Recoverable Catalysts

Insoluble organic resins and polymers such as polystyrene-based cross-linkers have been used as supports in heterogeneous catalysis, which allows the active recovery and reuse of many metal catalysts by simple methods. Organic polymer supports provide improved catalytic stability, surface activity, and high selectivity for the reaction.

Angelini et al. reported the conjugate addition of an N-nucleophile for the synthesis of β-amino carbonyl compounds catalyzed by polystyryl-DABCOF₂ (Scheme 15) [29]. The reaction was carried out in an aqueous medium. The porous polymeric materials were found to be more compatible with the conditions. DABCO, a diamine moiety carries high loading fluoride as the counter ions and efficient than the commercially available fluoride catalysts. The catalyst was used in the β-azidation of enones (Scheme 16). The reaction was carried out in a continuous flow method, and it helps for the recovery of the catalyst used.

Scheme 15

Preparation of PS-DABCOF₂

Scheme 16

β-Azidation in the presence of PS-DABCOF₂ based catalysts

Merrifield resin supported peroxomolybdenum (VI) compounds for the selective oxidation of organic sulfides was developed by Boruah et al. in 2013 [30]. The catalyst was prepared by the immobilization of dioxomonoperoxomolybdenum (VI) on amino acid-functionalized Merrifield resins (Scheme 17). The catalyst showed excellent activity and selectivity for the oxidation of thioethers and dibenzothiophene to sulfoxides/sulfones by peroxides (Scheme 18). H₂MoO₄ reacted with 30% H₂O₂, and amino acid-functionalized resin at neutral pH resulted in the catalyst [MoO₂(O₂)L₂]²⁻MR, L = Valine/Alanine; MR = Merrifield resin. The straightforward synthetic route, clean and safe reaction conditions, commercially cheap starting materials, easy recovery and reusability of the catalyst for several cycles, etc. are the compelling features of the catalyst.

Scheme 17

Synthesis of Merrifield resin supported PMO complexes

Scheme 18

Selective oxidation of methyl phenyl sulfide

Liang and co-workers studied soluble polymer supports for homogenous continuous flow reactions [31]. The transition metal entrapped polyisobutylene, and poly(4-dodecylstyrene) catalyzes the cyclopropanation, ring-closing metathesis, and nucleophilic catalysis. Reactions were carried out in the heptane phase, saturated with acetonitrile (Scheme 19a). The catalyst was finally separated via gravity-based liquid/liquid separation. Leaching helps the recovery of the catalyst after the reaction. The desired products were obtained in moderate to good yields, and the reaction outputs were compatible with the prior results obtained with the same phase insoluble catalyst. The utility of heptane soluble polymer-supported catalyst was first studied in a batch reaction of rhodium-catalyzed cyclopropanation. Heptane soluble PIB (Polyisobutylene)-bound Rh (II) carboxylate catalyzes cyclopropanation (Scheme 20) was reported. A commercially available PIB-bound iodide was used as the starting material. In the same way, a ruthenium-based PIB-bound Hoveyda-Grubbs catalyst affected in the ring-closing metathesis of diene substrates (Scheme 19b). The constant recovery and reuse of the catalyst were achieved because of the phase selectivity of PIB.

Scheme 19

a) Photo E/Z isomerization using PIB-Ir complex, b) reaction catalyzed by PIB-bound Hoveyda-Grubb’s catalyst

Scheme 20

Synthesis of PIB-Rh (II) Carboxylate

An efficient, recyclable catalyst based on ruthenium complexes immobilized on hyper cross-linked polymers for organic transformations was reported by Jia et al. (Scheme 21) [32]. The synthesized catalyst was characterized by employing physicochemical methods. It was observed to possess high thermal stability, chemical stability, and low synthetic cost. Organic synthesis of 2,4-diaryl substituted pyridine was carried out from acetophenones, NH₄OH, and DMF using ruthenium catalysts, and it also catalyzed the formation of dihydrofurans via cycloaddition reaction of diazodicarbonyl compounds with olefins. After the reaction, the HCP-PPh₃-Ru catalyst was recovered and reused without significant loss in reactivity. The study indicates that the PPh₃-Ru catalyst immobilized on external cross-linked HCP materials are quite robust for the organic transformations.

Scheme 21

Preparation of HCP-PPh₃-Ru catalyst

A highly recoverable polymer-supported ionic salen-Pd complex (PS-Pd-salen) was developed by Balinge et al. for the Suzuki- Miyaura cross-coupling reactions (Scheme 22) [33]. The catalytic activity was checked on the coupling of phenylboronic acid with different aryl halides using water as a solvent, under 20% catalyst loading. The PS-Pd-salen complex was found to be thermally stable, and it showed excellent reusability for five consecutive runs.

Scheme 22

Synthesis of polymer-supported Pd-Salen complexes

Very recently, Yang et al. developed a heterogeneous system in which chloride quinones supported on an organic resin was used for the synthesis of trisubstituted pyridines (Scheme 23) [34]. Recoverable Merrifield resins were used as organic support. The synthesized catalyst was characterized using FTIR, XPS, EDX techniques. The chemoselective synthesis of 2, 4, 6-trisubstituted pyridines offered good yields under mild conditions. The proposed mechanism was well established by the successful capture of the intermediate during the reaction pathway. The catalyst was recovered by simple centrifugation techniques, washed with water, dried, and used further for the synthesis.

Scheme 23

Supported NQ-MR catalyzed pyridine synthesis

Bartacek reported a heterogeneous catalyst for enantioselective Sharpless epoxidation in 2019 (Scheme 24) [35]. The metal present in the catalyst was Ti (IV). The titanium complexes were supported on cross-linked polystyrene beads with ethyl-(4-vinyl benzyl)-L-tartrate/ethyl-(2R,3R)-2,3-dihydroxy-4,5-oxo-5-(4-vinyl phenyl) pentanoate using suspension polymerization. The catalytic activity was checked on the cinnamyl alcohol epoxidation, and high conversion and enantioselectivity were observed for the reaction.

Scheme 24

Synthesis and application of L-(+)-tartaric acid esters immobilized on styrene on Sharpless epoxidation

Pisk and co-workers synthesized adipic acid via oxidation reaction using Molybdenum/Tungsten-based Keggin polyoxometallates (Scheme 25) [36]. The POM was surrounded by organic cations or functionalized Merrifield resins. Aqueous H₂O₂ (30%) was used as the oxidizing agent, and the starting materials used were cyclohexene, epoxy cyclohexane/cyclohexane diol. The catalyst loading was very low, about 0.001%- 0.007% for the POM, and adipic acid was obtained in excellent yields. The resin was finally isolated at the end of the catalytic cycle, washed with diethyl ether, dried, and used further.

Scheme 25

Merrifield supported heterogeneous catalysts- synthetic route

Very recently, Chen et al. demonstrated the fabrication of thermoresponsive polymer-based TEMPO nanoreactors [37]. The application of the catalyst lies in the selective oxidation of alcohols in aqueous media. Catalyst synthesis was carried out initially using an amphiphilic co-polymer NHS-P(MMA₂₅-b-OEGMA₇₅) consisting of PMMA and poly(oligo ethylene glycol) methyl ether methacrylate with terminal NHS (Scheme 26). Then, TEMPO was introduced to the terminal of the hydrophobic part. The self-assembly of the amphiphilic co-polymer in water is resulted in the core-shell type of polymeric nanoreactors, which catalyzed the selective oxidation of the alcohol and yielded excellent amounts of aldehydes and ketones. The thermoresponsive character of the catalyst helps for its easy recovery by precipitation.

Scheme 26

Synthesis of TEMPO-(PMMA₂₅-b-OEGMA₇₅)

2.5 Silica-Supported Organocatalysts

An efficient synthesis of silica-supported gold (I) complexes were developed by Shu et al. [38]. The catalyst was used for the regio- and enantio-selective lactonization reactions. The preparation of the catalyst was carried out by stirring silica materials in a CH₂Cl₂ solution of Ph₃PAuBF₄ at room temperature for 4 h (Scheme 27). Catalytic activity was observed on the nucleophilic addition reaction of allenes. High enantioselectivities for the lactonization were obtained using heterogeneous catalysts. The catalysts were quickly recovered and reused up to 11 consecutive cycles without loss in the enantioselectivity.

Scheme 27

One-step synthesis of heterogenous gold@SiO₂ catalysts

A novel and efficient boronic acid substituted silica-supported Pt was synthesized by Li et al. in 2019 [39]. The boronic acid-functionalized silica gel complex (SiO₂- BA) was prepared by grafting silica gel and 3-aminophenyl boronic acid using 3-chloropropyltrimethosilane, γ-glycidoxypropyltrimethoxysilane, and 3-aminopropyl tri methoxysilane as complexing agents. Further, Pt nanoparticles were dispersed on the complex using a solution of H₂PtCl₆.6H₂O in ethanol. The catalytic activity of the synthesized compounds was investigated by the hydrosilylation of olefins with methyl dichlorosilane (Scheme 28). The substrate scope was explored for a wide range of terminal aliphatic olefins and methyl dichlorosilane. The recycled catalyst was found to be useful up to 10 catalytic cycles without any noticeable loss in catalytic activity.

Scheme 28

Boronic acid substituted Pt@SiO₂ catalyzed reaction

Mesoporous recoverable silica-based nanomaterials for the synthesis of substituted pyridodipyrimidines were reported by Kohzadian and co-workers [40]. The catalyst preparation was straightforward and cost-effective (Scheme 29). The nanocatalyst [SiO₂-R-NMe₂SO₃H]Cl was detached from the reaction medium via centrifugation and decantation. Further, it was washed with methanol and reused. Synthetic protocol carried out for the preparation of pyrido[2,3-d: 2,3-d’]dipyrimidine was a one-pot multicomponent reaction of aryl aldehydes, 2-thiobarbituric acid, and NH₄OAc in the presence of the synthesized catalyst. The reaction time, yield, as well as the condition were better compared to the reported methods.

Scheme 29

Preparation of nano-[SiO₂-R-NMe₂SO₃H][Cl]

Post-transition metal triflates as solid catalysts are being used in heterogeneous green catalysis. An indium (III) triflate [SISAln(OTf)₂] supported on sulfoacetate modified silica for the synthesis of tetrasubstituted imidazoles was established by Vaid et al. (Scheme 30) [41]. 1,2,4,5-Tetrasubstituted imidazoles were obtained by the reaction of aromatic aldehydes, benzyl, aromatic amines, and ammonium acetate. To investigate the stability and recyclability, the catalyst was separated from the reaction mixture by simple filtration. The catalytic activity was found to be stable up to five consecutive runs.

Scheme 30

Multi-step synthesis of Sulfoacetate modified silica-supported indium (III) triflate [SiSAIn(OTf)₂]

2.6 Thermomorphic Catalysts

Thermomorphic catalysts are separated from the reaction mixture by varying the temperature of the system. This helps in the phase separation based on their temperature-dependent solubility behavior. The separation strategies include (i) cooling (for the catalysts which are soluble under elevated temperatures)—catalysts separate by lowering the temperature, (ii) temperature-dependent miscibility of solvent mixtures.

A novel thermomorphic system for the electrolytic Diels-Alder reaction was reported by Imada et al. [42]. They replaced the polar LiClO₄/CH₂NO₂ solution with a less polar LiTFSI/1-nitropropane solution (Scheme 31). During the electrochemical response, methylcyclohexane forms a monophasic condition with LiTFSI/1-nitropropane at rt. After the completion of the reaction, at −50 °C, it turns into a biphasic system. Cycloadducts move into the upper methylcyclohexane phase, and in the lower phase, LiTFSI/PrNO₂ gets separated, which can be used for further cycles.

Scheme 31

Electrocatalytic Diels-Alder reaction under thermomorphic conditions

Heijazifar et al. proposed the use of thermomorphic ionic liquid microemulsion applied for the Ru-catalyzed asymmetric transfer hydrogenation of ketones (Scheme 32) [43]. [C₁₂mim][(ⁱC₈)₂PO₂] and [C₁₂dmim][(ⁱC₈)₂PO₂] were found to be the best thermomorphic ionic liquids for the reaction (Fig. 2). Use of water-soluble ligands improve the immobilization of the catalyst, and it minimizes the chances of leaching into the organic layer during extraction. The catalytic system was found to be more effective for aromatic ketones than the aliphatic ones in terms of conversion and enantioselectivity. Moderately good yields were obtained during the recyclability study, and the reuse of the catalyst was effective.

Scheme 32

Asymmetric trans hydrogenation of ionic liquid microemulsion

Fig. 2

Surface active ionic liquids

Homogenously catalyzed reductive amination in a thermomorphic system was first reported very recently by Künnemann and co-workers (Scheme 33) [44]. Thermomorphic multiphase system was established for the recovery and reuse of the catalyst (Scheme 34). Commercially available PPh₃ ligand and Rh(acac)(cod) catalyst precursor were used for the reaction. The researchers have scaled up the reaction into a continuous system that recycles the catalyst inflow over 70 h. The overall reaction possessed above 90% yield and meager percentage of catalyst leaching, 1%/h.

Scheme 33

Thermomorphic Rh complex catalyzed reductive amination

Scheme 34

Representation of thermo multiphase system

3 Conclusion

The development of a recoverable catalyst is a rapidly growing area in catalysis. Effective recycling of the catalyst from the reaction medium can be performed using various methodologies. Depending on the nature of the catalyst used, different recovery methods are applied. Significant progress has been observed in the preparation of solid-supported catalysts—both dendritic and silica-supported, magnetically supported, and also organic resin supported catalysts. In most of the methods, the effective recovery of the catalysts is done without altering its efficiency. Moderate to high reaction rates can be sustained over multiple consecutive cycles.

Magnetically recoverable metal nanoparticles enhance the catalytic activity because of its increased surface area. The advantages of metallodendritic catalysts extend to both homogenous and heterogeneous catalysis, and these can be separated employing simple techniques such as precipitation, filtration, and centrifugation. These modified recoverable catalysts could benefit from the additional active supports (dendrimers, organic resins, silica, polymers, etc.), which helps in the catalyst formation and reaction. Ease of synthesis, the serenity of the reaction, recyclability, and cost-effectiveness make these methods more sustainable toward green chemistry approach.