Catalyst recycling—A survey of recent progress and current status

Direct additive-free N-formylation and N-acylation of anilines and synthesis of urea derivatives using green, efficient, and reusable deep eutectic solvent ([ChCl][ZnCl2]2)

In the current report, we introduce a simple, mild efficient and green protocol for N-formylation and N-acetylation of anilines using formamide, formic acid, and acetic acid as inexpensive, nontoxic, and easily available starting materials just with heating along stirring in [ChCl][ZnCl2]2 as a durable, reusable deep eutectic solvent (DES), which acts as a dual catalyst and solvent system to produce a wide range of formanilides and acetanilides. Also, a variety of unsymmetrical urea derivatives were synthesized by the reaction of phenyl isocyanate with a range of amine compounds using this benign DES in high to excellent yields. [ChCl][ZnCl2]2 showed good recycling and reusability up to four runs without considerable loss of its catalytic activity.

Membrane-catalysis integrated system for contaminants degradation and membrane fouling mitigation: A review

The integration of catalytic degradation and membrane separation processes not only enables continuous degradation of contaminants but also effectively alleviates inevitable membrane fouling, demonstrating fascinating practical value for efficient water purification. Such membrane-catalysis integrated system (MCIS) has attracted tremendous research interest from scientists in chemical engineering and environmental science recently. In this review, the advantages of MCIS are discussed, including the membrane structure regulation, stable catalyst loading, nano-confinement effect, and efficient natural organic matter (NOM) exclusion, highlighting the synergistic effect between membrane separation and catalytic process. Subsequently, the design considerations for the fabrication of catalytic membranes, including substrate membrane, catalytic material, and fabrication method, are comprehensively summarized. Afterward, the mechanisms and performance of MCIS based on different catalytic types, including liquid-phase oxidants/reductants involved MCIS, gas involved MCIS, photocatalysis involved MCIS, and electrocatalysis involved MCIS are reviewed in detail. Finally, the research direction and future perspectives of catalytic membranes for water purification are proposed. The current review provides an in-depth understanding of the design of catalytic membranes and facilitates their further development for practical applications in efficient water purification.

Synthesis Strategies towards Tagged Homogeneous Catalysts To Improve Their Separation

The recycling of homogeneous catalysts while keeping them in the homogeneous matrix is an ongoing challenge many reactions face if they are to find industrial applications. While a plethora of different synthetic approaches towards better, recyclable homogeneous catalysts exist, the literature shows a gap when one searches for a concise overview of the different catalyst modifications. This Review is designed to close that gap by summarising the existing synthesis pathways towards polar, non-polar, fluorous, and molecular-weight-enlarged catalysts and by examining their respective synthesis routes with a focus on modular and late-stage approaches. Furthermore, we map out the potential for a generally applicable tag library that allows straightforward catalyst modifications to tune them for each desired recycling strategy.

Recovery of Precious Metals: A Promising Process Using Supercritical Carbon Dioxide and CO2-Soluble Complexing Polymers for Palladium Extraction from Supported Catalysts

Precious metals such as palladium (Pd) have many applications, ranging from automotive catalysts to fine chemistry. Platinum group metals are, thus, in massive demand for industrial applications, even though they are relatively rare and belong to the list of critical materials for many countries. The result is an explosion of their price. The recovery of Pd from spent catalysts and, more generally, the development of a circular economy process around Pd, becomes essential for both economic and environmental reasons. To this aim, we propose a sustainable process based on the use of supercritical CO2 (i.e., a green solvent) operated in mild conditions of pressure and temperature (p = 25 MPa, T = 313 K). Note that the range of CO2 pressures commonly used for extraction is going from 15 to 100 MPa, while temperatures typically vary from 308 to 423 K. A pressure of 25 MPa and a temperature of 313 K can, therefore, be viewed as mild conditions. CO2-soluble copolymers bearing complexing groups, such as pyridine, triphenylphosphine, or acetylacetate, were added to the supercritical fluid to extract the Pd from the catalyst. Two supported catalysts were tested: a pristine aluminosilicate-supported catalyst (Cat D) and a spent alumina supported-catalyst (Cat A). An extraction conversion of up to more than 70% was achieved in the presence of the pyridine-containing copolymer. The recovery of the Pd from the polymer was possible after extraction, and the technological and economical assessment of the process was considered.

The Need for Flexible Chemical Synthesis and How Dual-Function Materials Can Pave the Way

Since climate change keeps escalating, it is imperative that the increasing CO₂ emissions be combated. Over recent years, research efforts have been aiming for the design and optimization of materials for CO₂ capture and conversion to enable a circular economy. The uncertainties in the energy sector and the variations in supply and demand place an additional burden on the commercialization and implementation of these carbon capture and utilization technologies. Therefore, the scientific community needs to think out of the box if it is to find solutions to mitigate the effects of climate change. Flexible chemical synthesis can pave the way for tackling market uncertainties. The materials for flexible chemical synthesis function under a dynamic operation, and thus, they need to be studied as such. Dual-function materials are an emerging group of dynamic catalytic materials that integrate the CO₂ capture and conversion steps. Hence, they can be used to allow some flexibility in the production of chemicals as a response to the changing energy sector. This Perspective highlights the necessity of flexible chemical synthesis by focusing on understanding the catalytic characteristics under a dynamic operation and by discussing the requirements for the optimization of materials at the nanoscale.

Atomically Dispersed Pd Sites on ZrO2 Hybridized N-Doped Carbon for Efficient Suzuki–Miyaura Reaction

Researchers studying heterogeneous catalysis are intrigued by single-atom catalysts (SACs) due to their ultrahigh atomic utilization. However, only a few reports on SAC-catalyzed classical organic transformations are available. In this work, atomically dispersed Pd sites are confined to a ZrO2 hybridized N-doped carbon skeleton with a smart design. UiO-66-NH2 is used to anchor Pd atoms by the coordination of the donor atoms including lone pairs of electrons and metal atoms. Subsequently, the in situ introduction of ZrO2 doping is achieved using pyrolysis, which helps improve the catalytic performance by modulating the electronic state. The Pd@ZrO2/N–C catalyst obtained from the unique design exhibits a high yield (99%) in eco-friendly media with an extremely low noble metal dosage (0.03 mol% Pd) for the Suzuki reaction. Moreover, Pd@ZrO2/N–C remains highly active after being reused several times and possesses versatility in a variety of substrates. This strategy offers a feasible alternative to designing SACs with atomically dispersed noble metals for heterogeneous reactions.

Iron(III) Arylhydrazone Complexes Immobilized on Amine-Functionalized Mesoporous Silica: Catalysts for the Valorization of Biomass-Derived Furfuryl Alcohol

The aquasoluble Fe^III complexes [Fe(H₂ O)₃ (L¹ )] ⋅ 4H₂ O (Fe1) and [Fe(H₂ O)₃ (L² )] ⋅ 3H₂ O (Fe2), bearing the basic forms of 5-chloro-3-(2-(4,4-dimethyl-2,6-dioxocyclohexylidene)hydrazinyl)-2-hydroxy-benzenesulfonic acid (H₃ L¹ ) and 3-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)-2-hydroxy-5-nitrobenzenesulfonic acid (H₃ L² ), were incorporated for the first time into amine-functionalized SBA-15 support via an impregnation method. The successful preparation of the composites was confirmed by Fourier-Transform Infrared spectroscopy (FT-IR), powder X-ray diffraction (XRD), scanning electron microscope (SEM/EDS), transmission electron microscopy (TEM), elemental analysis, and nitrogen adsorption-desorption isotherms. The resulting Fe1@aptesSBA-15 and Fe2@aptesSBA-15 composites were tested as the first SBA-15-based heterogeneous catalysts for the conversion of furfuryl alcohol under mild reaction conditions (80 to 100 °C) and with an environmentally friendly oxidant (TBHP, 70 % aq. sol. with 1 : 1 oxidant/substrate molar ratio). The influence of various factors, such as reaction time, amounts of oxidant and catalyst, was investigated. The reaction time can be fairly reduced by adopting a microwave-assisted method allowing it to reach complete conversion after 0.25 h, in the absence of any added solvent or additive. Under these conditions, a vigorous furfuryl alcohol polymerization process occurred, with furfural as a by-product. Recycling studies were carried out for Fe2@aptesSBA-15 and after four consecutive runs, the overall conversion of furfuryl alcohol remained high (≥99 %), without an appreciable change in the obtained yield.

Cadmium-Doped and Pincer Ligand-Modified Gold Nanocluster for Catalytic KA2 Reaction

A gold nanocluster Au₁₇ Cd₂ (PNP)₂ (SR)₁₂ (PNP=2,6-bis(diphenylphosphinomethyl)pyridine, SR=4-MeOPhS) consisting of an icosahedral Au₁₃ kernel, two Au₂ CdS₆ staple motifs, and two PNP pincer ligands has been designed, synthesized and well characterized. This cadmium and PNP pincer ligand co-modified gold nanocluster showed high catalytic efficiency in the KA² reaction, featuring high TON, mild reaction conditions, broad substrate scope as well as catalyst recyclability. Comparison of the catalytic performance between Au₁₇ Cd₂ (PNP)₂ (SR)₁₂ and the structurally similar single cadmium (or PNP) modified gold nanoclusters demonstrates that the co-existence of the cadmium and PNP on the surface is crucial for the high catalytic activity of the gold nanocluster. This work would be enlightening for developing efficient catalysts for cascade reactions and discovering the catalytic potential of metal nanoclusters in organic transformations.

PolyE-IL Is an Efficient and Recyclable Homogeneous Catalyst for the Synthesis of 5-Hydroxymethyl Furfural in a Green Solvent

5-Hydroxymethyl furfural (5-HMF) is a potential platform molecule with multidimensional applications and can be produced from biomass-based hexose sugars. In the present article, polyethyleneimine (PEI)-functionalized polymeric Bronsted acid ionic liquid (PolyE-IL) catalyst has been explored for fructose dehydration in the presence of isopropyl alcohol (IPA) as a green and low-boiling-point (LBP) organic solvent. The use of homogeneous PolyE-IL catalyst provides several specific advantages in terms of high yield, conversion, selectivity, ease of catalyst separation, recycle and reuse, and so forth. PEI with various Bronsted acid counterions such as H₂SO₄, H₃PO₄, TsOH, TfOH, and TFA provides the corresponding variables of PolyE-IL such as [PEI]⁺[HSO₃]^-, [PEI]⁺[H₂PO₄]^-, [PEI]⁺[CF₃CO₂]^-, [PEI]⁺[TfO]^-, and [PEI]⁺[TsO]^-, which are tested for fructose dehydration in the presence of IPA. Of the tested catalysts, only PolyE-IL with [HSO₄]^-, [CF₃CO₂]^-, [TfO]^-, and [TsO]^- counterions showed the formation of 5-HMF. [PEI]⁺[HSO₄]^- showed the maximum yield of 5-HMF (61%) and selectivity (70%) with (87%) fructose conversion. Thus, further process optimization study was conducted to obtain the maximum yield, conversion, and selectivity. The intensified process provides a maximum yield of 5-HMF of 75% with 85% fructose conversion and 90% selectivity. The catalyst recyclability study showed the consistency in 5-HMF yield (75%), conversion (85%), and selectivity (90%) for five consecutive recycle runs. However, the study of reaction kinetics showed the first-order kinetics with an activation energy of 12.4 kJ/mole by using [PEI]⁺[HSO₄]^- catalyst. Thus, the use of an easily recyclable and robust catalyst provides an efficient route for production of 5-HMF in the presence of a green solvent.

Ag-modified TiO2/SiO2/Fe3O4 sphere with core-shell structure for photo-assisted reduction of 4-nitrophenol

Nitrogen-containing contaminants, such as 4-nitrophenol (4-NP), cause detrimental effects when discharged into the environment and thus should be reduced or removed from ecosystems. In this study, an Ag-loaded TiO₂-SiO₂-Fe₃O₄ (TSF) with a core-shell structure was employed for the photo-assisted reduction of 4-NP. Fe₃O₄, SiO₂, and TiO₂ in the core-shell structure served as a magnetic center, protective layer, and light absorber, respectively. To improve the reduction activity of 4-NP, Ag was loaded onto TSF under stirring, with a variation of the temperature (2-130 °C) and reaction time (1, 2, and 4 h). Under the optimized conditions, 5Ag-TSF (with 5 wt% of Ag) could promote the reduction of aqueous 4-NP solution (2 × 10^-4 M, 75 mL) in the presence of NaBH₄ (0.1 M, 5 mL) under irradiation by a metal halide lamp, affording over 98% reduction within 5 min and a rate constant of 0.185 min^-1, demonstrating its promising activity. Moreover, due to the advantages of the core-shell structure, the magnetic properties of Fe₃O₄ were sufficient to enable facile recycling of the sample for further reaction; SiO₂ could protect the Fe₃O₄ center from oxidation or reduction; TiO₂ enabled Ag accommodation and absorbed light to generate electron-hole pairs. In summary, an Ag-loaded TiO₂-SiO₂-Fe₃O₄ sphere with high activity and recyclability for 4-NP reduction was prepared via a facile and simple stirring method, where the sample can be used as a promising material in environmental remediation.

Branched Tertiary Amines from Aldehydes and α-Olefins by Combined Multiphase Tandem Reactions

This study presents the transformation of olefins to branched amines by combining a hydroformylation/aldol condensation tandem reaction with the reductive amination in a combined multiphase system that can be recycled 9 times. The products are branched amines that are precursors for surfactants. Since the multiphase hydrofomylation/aldol condensation system has already been studied, the first step was to develop the partial hydrogenation of unsaturated aldehydes together with a subsequent reductive amination. The rhodium/phosphine catalyst is immobilized in a polar polyethylene phase which separates from the product phase after the reaction. Reaction and catalyst recycling are demonstrated by the conversion of the C₁₄ -aldehyde 2-pentylnonenal with the dimethylamine surrogate dimethylammonium dimethylcarbamate to the corresponding tertiary amine with yields up to 88 % and an average rhodium leaching of less than 0.1 % per recycling run. Furthermore, the positive influence of a Bronsted acid and carbon monoxide on the selectivity are discussed. Finally, the two PEG based systems have been merged in one recycling approach, by using the product phase of the hydroformylation aldol condensation reaction for the reductive amination reaction. The yields are stable during a nine recycling runs and the leaching low with 0.09 % over the two recycling stages.

Copper-Catalyzed Reactions of Aryl Halides with N-nucleophiles and Their Possible Application for Degradation of Halogenated Aromatic Contaminants

This review summarizes recent applications of copper or copper-based compounds as a nonprecious metal catalyst in N-nucleophiles-based dehalogenation (DH) reactions of halogenated aromatic compounds (Ar-Xs). Cu-catalyzed DH enables the production of corresponding nonhalogenated aromatic products (Ar-Nu), which are much more biodegradable and can be mineralized during aerobic wastewater treatment or which are principally further applicable. Based on available knowledge, the developed Cu-based DH methods enable the utilization of amines for effective cleavage of aryl-halogen bonds in organic solvents or even in an aqueous solution.

Poisoning and Reuse of Supported Precious Metal Catalysts in the Hydrogenation of N-Heterocycles, Part II: Hydrogenation of 1-Methylpyrrole over Rhodium

Poisoning effect of nitrogen on heterogeneous, supported precious metal catalysts, along with their recycling, was further examined in the liquid-phase hydrogenation of 1-methylpyrrole (MP) to 1-methylpyrrolidine (MPD) over rhodium on carbon or γ-alumina, in methanol, under non-acidic conditions, at 25–50 °C and 10 bar. Reusing a spent, unregenerated 5% Rh/C or 5% Rh/γ-Al2O3 catalyst, it was found that the conversion of this model substrate and the activity of the catalyst were strongly dependent on the amount of catalyst, the type of support, the catalyst pre- or after-treatment, the temperature, and the number of recycling, respectively. An unexpected catalytic behaviour of rhodium was observed when it was used in a prehydrogenated form, because no complete conversion of MP was achieved over even the fresh Rh/C or Rh/γ-Al2O3, contrary to the untreated one. In addition, there was a significant difference in the reusability and activity of these rhodium catalysts, depending on their supports (activated carbon, γ-alumina). These diversions were elucidated by applying dispersion (O2- and H2-titration), temperature-programmed desorption of ammonia (NH3-TPD), and transmission electron microscopy (TEM) measurements.

Chirality-driven self-assembly: application toward renewable/exchangeable resin-immobilized catalysts

Resin-immobilized catalysts were prepared through chirality-driven self-assembly. The method allows the resin-immobilized catalyst to be regenerated under mild conditions and in situ catalyst exchange to be carried out quantitatively. The uniqueness of the methodology was demonstrated by the preparation of a catalyst for TEMPO oxidation as well as a two-step sequential TEMPO oxidation/aldol condensation sequence enabled by facile catalyst exchange.

Vapor Phase Alkylation of Isomeric Cresols with Tert-Butyl Alcohol over Perlite Supported Sulfated Zirconia Catalyst

In the present study, perlite was thermally activated and then modified desirably to generate super acidity by loading different weight percentages of sulfated zirconia (SZ) via the two-step sol-gel method. As-prepared sulfated zirconia perlite (SZP) catalysts showed suitable catalytic potential in the vapor phase alkylation of o, m, and p-cresols with tert-butyl alcohol. The presence of crystalline phases in SZP catalysts was confirmed by XRD and FT-IR studies. TEM images revealed the nano size of the catalysts in the range of 9–25 nm. The presence of SZ on the surface of perlite was further confirmed by N2 adsorption–desorption, SEM, SEM-EDX, TGA, and UV-Vis DRS techniques. The pyridine FT-IR results confirmed the existence of Brønsted, Lewis acidic sites and their combination as super acidic catalytic active centers on the surface of catalyst utilized in the vapor phase alkylation of o, m, and p-cresols with tert-butyl alcohol. The regeneration and reusability of the preferred catalyst until the 5th reaction cycle without any considerable loss in catalytic activity demonstrated the stability of the catalyst. Comparative studies show that SZP can be regenerated and is superior compared to other catalysts previously used for other alkylation reactions with the potential for use on a large scale.

Depolymerization of Waste Plastic Using Bubble Column for Nano Alumina Blended Coating

In this study, we aimed to figure out how the depolymerization of polyethylene terephthalate produces monomers, dimers, trimers, and other oligomers of bis (2-hydroxyethyl) terephthalate. Polymerization was achieved in a bubble column reactor with 0.05 wt% 40–50 nm magnesium oxide nanoparticle as a catalyst. A bubble column reactor was used to perform the recycling process at the boiling point of ethylene glycol and atmospheric pressure. Depolymerized polyethylene terephthalate (DPET) was mixed with poly(methyl methacrylate) and reinforced with 1% Nano Al2O3. The nanoparticles acted as a composite coating in low carbon steel protection. Adhesion strength and mechanical and structural properties were investigated for the composite, and the average coating thickness was 28.39 µm. The results showed that the hardness and adhesion forces between the substrate and composite coating increased with an increase in the amounts of DPET and nano-Al2O3 per polymer resin. On the other hand, the thermal conductivity of the composite coating decreased with the addition of DPET because of an increase in the end chain movement in the composite coating induced by the retardant and an increase in cross-linking force. Furthermore, the bubble column demonstrated outstanding heat and mass transfer phenomena that reduced the reaction time to just 40 min for complete depolymerzation and also reduced energy consumption.

Pd-Pd/PdO as active sites on intercalated graphene oxide modified by diaminobenzene: fabrication, catalysis properties, synergistic effects, and catalytic mechanism

Pd–Pd/PdO nanoclusters well dispersed on intercalated graphene oxide (GO) (denoted as GO@PPD–Pd) were prepared and characterized. GO@PPD–Pd exhibited high catalytic activity (a TOF value of 60 705 h⁻¹) during the Suzuki coupling reaction, and it could be reused at least 6 times. The real active centre was Pd(200)–Pd(200)/PdO(110, 102). A change in the Pd facets on the surface of PdO was a key factor leading to deactivation, and the aggregation and loss of active centres was also another important reason. The catalytic mechanism involved heterogeneous catalysis, showing that the catalytic processes occurred at the interface, including substrate adsorption, intermediate formation, and product desorption. The real active centres showed enhanced negative charge due to the transfer of electrons from the carrier and ligands, which could effectively promote the oxidative addition reaction, and Pd(200) and the heteroconjugated Pd/PdO interface generated in situ also participated in the coupling process, synergistically boosting activity. Developed GO@PPD–Pd was a viable heterogeneous catalyst that may have practical applications owing to its easy synthesis and stability, and this synergistic approach can be utilized to develop other transition-metal catalysts.

Pd(200) and the Pd(200)/PdO(102, 110) interface generated in situ participated in coupling reactions via a synergistic effect, boosting the catalytic activity to a high level.

A Critical Review on the Economically Feasible and Sustainable Poly(3-Hydroxybutyrate-co-3-hydroxyvalerate) Production from Alkyl Alcohols

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co-3HV)) is the most studied short-chain-length polyhydroxyalkanoates (PHA) with high application importance in various fields. The domination of high-cost propionate and valerate over other 3-hydroxyvalerate (3HV) precursors owing to their wide preference among PHA-producing bacteria has hindered the development of diverse production processes. As alkyl alcohols are mainly produced from inexpensive starting materials through oxo synthesis, they contribute a cost-effective advantage over propionate and valerate. Moreover, alkyl alcohols can be biosynthesized from natural substrates and organic wastes. Despite their great potential, their toxicity to most PHA-producing bacteria has been the major drawback for their wide implementation as 3HV precursors for decades. Although the standard PHA-producing bacteria Cupriavidus necator showed promising alcohol tolerance, the 3HV yield was discouraging. Continuous discovery of alkyl alcohols-utilizing PHA-producing bacteria has enabled broader choices in 3HV precursor selection for diverse P(3HB-co-3HV) production processes with higher economic feasibility. Besides continuous effort in searching for promising wild-type strains, genetic engineering to construct promising recombinant strains based on the understanding of the mechanisms involved in alkyl alcohols toxicity and tolerance is an alternative approach. However, more studies are required for techno-economic assessment to analyze the economic performance of alkyl alcohol-based production compared to that of organic acids.

Highly efficient hydrogenation of phenol to cyclohexanol over Ni-based catalysts derived from Ni-MOF-74

Highly efficient Ni@C-400 catalyst for selective hydrogenation of phenol to cyclohexanol was developed from Ni-MOF-74.

Magnetic Silica-Coated Picolylamine Copper Complex [Fe3O4@SiO2@GP/Picolylamine-Cu(II)]-Catalyzed Biginelli Annulation Reaction

An efficient and heterogeneous novel magnetic silica-coated picolylaminecopper complex [Fe₃O₄@SiO₂@GP/Picolylamine-Cu(II)] was synthesized, characterized, and employed as a magnetically recoverable nanocatalyst in Biginelli condensation for the preparation of biologically active 3,4-dihydropyrimidinones. Fe₃O₄@SiO₂@GP/Picolylamine-Cu(II) was synthesized easily using chemical attachment of the picolylaminecompound on Fe₃O₄@SiO₂@GP, followed by treatment with copper salt in ethanol under reflux conditions. Fe₃O₄@SiO₂@GP/Picolylamine-Cu(II) was affirmed by various analyses such as Fourier transform infrared, thermogravimetric analysis, X-ray diffraction, vibrating-sample magnetometry, field-emission scanning electron microscopy, transmission electron microscopy, DLS, inductively coupled plasma, energy-dispersive X-ray spectrometry, X-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller. The resulting catalyst system was successfully used in the Biginelli reaction through a variety of compounds such as aromatic aldehyde, urea, and ethyl acetoacetate under solvent-free conditions or ethylene glycol at 80 °C and yielded the desired products with high conversions with powerful reusability. The current approach was convenient and clean, and only 0.01 g of the catalyst could be used to perform the reaction. The easy work-up procedure, gram-scale synthesis, usage of nontoxic solvent, improved yield, short reaction times, and high durability of the catalyst are several remarkable advantages of the current approach. Also, the Fe₃O₄@SiO₂@GP/Picolylamine-Cu(II) nanocatalyst could be recycled by an external magnet for eight runs with only a significant loss in the product yields.

Supramolecular Strategies for the Recycling of Homogeneous Catalysts

Supramolecular approaches are increasingly used in the development of homogeneous catalysts and they also provide interesting new tools for the recycling of metal-based catalysts. Various non-covalent interactions have been utilized for the immobilization homogeneous catalysts on soluble and insoluble support. By non-covalent anchoring the supported catalysts obtained can be recovered via (nano-) filtration or such catalytic materials can be used in continuous flow reactors. Specific benefits from the reversibility of catalyst immobilization by non-covalent interactions include the possibility to re-functionalize the support material and the use as "boomerang" type catalyst systems in which the catalyst is captured after a homogeneous reaction. In addition, new reactor design with implemented recycling strategies becomes possible, such as a reverse-flow adsorption reactor (RFA) that combines a homogeneous reactor with selective catalyst adsorption/desorpion. Next to these non-covalent immobilization strategies, supramolecular chemistry can also be used to generate the support, for example by generation of self-assembled gels with catalytic function. Although the stability is a challenging issue, some self-assembled gel materials have been successfully utilized as reusable heterogeneous catalysts. In addition, catalytically active coordination cages, which are frequently used to achieve specific activity or selectivity, can be bound to support by ionic interactions or can be prepared in structured solid materials. These new heterogenized cage materials also have been used successfully as recyclable catalysts.

Chitosan-starch biopolymer modified kaolin supported Pd nanoparticles for the oxidative esterification of aryl aldehydes

A mild and efficient green protocol has been disclosed for selective oxidative esterification of various aldehydes over a novel Pd fabricated chitosan-starch polyplex encapsulated Kaolin (Kaolin@CS-starch-Pd) as a heterogeneous and reusable biocompatible nanocatalyst. Molecular oxygen was used as an oxidizing agent to generate water as the sole by-product. A wide variety of aldehydes was converted to their methyl esters in high yields. The process involved gentle reaction conditions to avoid any type of pre-activation. Structural features of the catalyst were determined through FT-IR, FE-SEM, TEM, EDX, elemental mapping, XRD and ICP-OES analyses. The material was found to be stable enough toward Pd leaching. Durability of Kaolin@CS-starch-Pd was further justified by retaining its catalytic activity through successful reusability for several times.

Continuous Hydrogenation: Triphasic System Optimization at Kilo Lab Scale Using a Slurry Solution

Despite their widespread use in the chemical industries, hydrogenation reactions remain challenging. Indeed, the nature of reagents and catalysts induce intrinsic safety challenges, in addition to demanding process development involving a 3-phase system. Here, to address common issues, we describe a successful process intensification study using a meso-scale flow reactor applied to a hydrogenation reaction of ethyl cinnamate at kilo lab scale with heterogeneous catalysis. This method relies on the continuous pumping of a catalyst slurry, delivering fresh catalyst through a structured flow reactor in a continuous fashion and a throughput up to 54.7 g/h, complete conversion and yields up to 99%. This article describes the screening of equipment, reactions conditions and uses statistical analysis methods (Monte Carlo/DoE) to improve the system further and to draw conclusions on the key influential parameters (temperature and residence time).

Defining the Qualities of High-Quality Palladium on Carbon Catalysts for Hydrogenolysis

Palladium-catalyzed hydrogenolysis is often the final step in challenging natural product total syntheses and a key step in industrial processes producing fine chemicals. Here, we demonstrate that there is wide variability in the efficiency of commercial sources of palladium on carbon (Pd/C) resulting in significant differences in selectivity, reaction times, and yields. We identified the physicochemical properties of efficient catalysts for hydrogenolysis: (1) small Pd/PdO particle size (2) homogeneous distribution of Pd/PdO on the carbon support, and (3) palladium oxidation state are good predictors of catalytic efficiency. Now chemists can identify and predict a catalyst's efficiency prior to the use of valuable synthetic material and time.

Immobilization of Rh(I)-N-Xantphos and Fe(II)-C-Scorpionate onto Magnetic Nanoparticles: Reusable Catalytic System for Sequential Hydroformylation/Acetalization

Two heterogeneous catalysts, MNP@SiO2-N-Xantphos/Rh(I) and MNP@SiO2-NH-C-scorpionate/Fe(II), were prepared by reaction of chloro-functionalized MNP@SiO2 with N-Xantphos and amino-functionalized MNP@SiO2 with iron(II)/C-allyl-scorpionate through nucleophilic substitution and hydroaminomethylation reactions, respectively. All catalysts were characterized using standard spectroscopic means, transmission electron microscopy (TEM), thermogravimetry (TG), and inductively coupled plasma optical emission spectrometry (ICP-OES). An active and highly selective one-pot hydroformylation/acetalization homogeneous system for the transformation of terminal and highly substituted olefins (including terpenes) onto ethyl acetals is described. A synergic effect of bimetallic Rh(I)/P and Fe(II)/C-scorpionate catalysts is disclosed for the first time. The further sequential use of the heterogeneous catalysts, MNP@SiO2-N-Xantphos/Rh(I) and MNP@SiO2-NH-C-scorpionate/Fe(II) in hydroformylation/acetalization reactions allows the direct transformation of olefin onto ethyl acetals, keeping the activity and selectivity. Both catalysts were easily recovered by magnetic separation and reused with negligible loss of activity/selectivity, after six reutilization cycles.

Efficient Heterogeneous Palladium Catalysts in Oxidative Cascade Reactions

Palladium-catalyzed oxidations involving cascade processes provide a versatile platform for streamlined conversion of simple feedstocks into functional molecules with high atom and step economy. However, the achievement of high palladium efficiency and selectivity in Pd-catalyzed oxidative cascade reactions is still challenging in many cases, as a result of the aggregation of active palladium species to Pd black and the possible side reactions during each bond-forming step. The two current solutions for addressing these issues are either to utilize oxidant-stable ligands or to use electron transfer mediators (ETMs). The former solution, which includes the use of amines, pyridines, sulfoxides, and carbene derivatives, inhibits aggregation of Pd⁰ during the catalytic cycle, while the latter solution facilitates reoxidation of Pd⁰ to Pd^II to improve the activity and selectivity. Following our long-standing interest in Pd-catalyzed oxidations, very recently we developed heterogeneous catalysts to resolve the issues mentioned above in oxidative cascade reactions. The heterogeneous palladium catalysts (Pd-AmP-MCF or Pd-AmP-CNC) comprise palladium nanoclusters (1–2 nm) immobilized on amino-functionalized siliceous mesocellular foam (MCF) or on crystalline nanocellulose (CNC), exhibiting high activity, selectivity as well as excellent recycling ability.

In this Account, we will discuss the synthesis and characterizations of the heterogeneous palladium catalysts, as well as their catalytic behaviors, and the mechanisms involved in their reactions. An important aspect of these catalysts in oxidation reactions is the generation of active Pd(II) species within the heterogeneous phase. Typical oxidative cascade reactions of our recent research on this topic include oxidative carbocyclization-carbonylation, oxidative carbocyclization-borylation, oxidative alkynylation-cyclization, oxidative carbonylation-cyclization, and oxidative carbocyclization-alkynylation. These reactions provide access to important compounds attractive in medicinal chemistry and functional materials, such as γ-lactone/γ-lactam-based poly rings, cyclobutenols, highly substituted furans, and oxaboroles. During these processes, the heterogeneous catalysts exhibited much higher turnover numbers (TONs) than their homogeneous counterparts (e.g., Pd(OAc)₂) as well as unique selectivity that cannot be achieved by homogeneous palladium catalysts. The origin of the high efficiency and unique selectivity of the heterogeneous catalysts was also investigated. Asymmetric syntheses for the construction of optically pure compounds were realized based on the excellent selectivity in these heterogeneous processes. Kinetic studies revealed that the rate and yield of the reactions were essentially maintained during recycling, which demonstrates that Pd-AmP-MCF and Pd-AmP-CNC are robust and highly active in these oxidative cascade reactions. In addition, inductively coupled plasma optical emisson spectroscopy (ICP-OES) analysis and hot filtration test suggest that these processes most likely proceed via a heterogeneous pathway.

Recent progress in our group has shown that the activity of Pd-AmP-MCF and Pd-AmP-CNC could be improved even further by the addition of Ag⁺ to generate cationic Pd(II). Furthermore, intriguing solvent effects were observed in a Pd-AmP-MCF-catalyzed oxidative cascade process, and solvent-controlled chemoselective transformations were developed based on this property of the catalyst. The heterogeneous strategy of this Account provides solutions to palladium deactivation and selectivity issues in Pd(II)-catalyzed oxidative cascade reactions and enables efficient catalyst recycling, which will open up new opportunities in oxidative cascade reactions.

Recovery/Reuse of Heterogeneous Supported Spent Catalysts

The rapid separation and efficient recycling of catalysts after a catalytic reaction are considered important requirements along with the high catalytic performances. In this view, although heterogeneous catalysis is generally less efficient if compared to the homogeneous type, it is generally preferred since it benefits from the easy recovery of the catalyst. Recycling of heterogeneous catalysts using traditional methods of separation such as extraction, filtration, vacuum distillation, or centrifugation is tedious and time-consuming. They are uneconomic processes and, hence, they cannot be carried out in the industrial scale. For these limitations, today, the research is devoted to the development of new methods that allow a good separation and recycling of catalysts. The separation process should follow a procedure economically and technically feasible with a minimal loss of the solid catalyst. The aim of this work is to provide an overview about the current trends in the methods of separation/recycling used in the heterogeneous catalysis.