The design and synthesis of heterogeneous catalysts for environmental applications

Rupak Chatterjee,a Piyali Bhanja b and Asim Bhaumik *aWith the rapid advancements in synthetic strategies, the field of heterogeneous catalysis has expanded enormously over the last few decades, and today it is one of the foremost areas in energy and environ-mental research. Various templating and non-templating routes for designing porous nanomaterial-based catalysts starting from precursor building blocks are highlighted here. CO2 and biomass are two major abundant resources that can be utilized as feedstocks for various heterogeneous catalytic processes. These are described in brief, together with environmental clean-up applications and future perspectives for addressing environmental issues.

Introduction
Catalysts play the most significant role in reducing the acti- vation energy of chemical reactions and thereby facilitating the rate of the reactions. Heterogeneous catalysis, in which the catalyst is present in the distinct solid phase, is in very high demand for achieving the targets of green and sustainable chemistry.1 Due to their easy work-up, recyclability and environmental safety, heterogeneous catalysts are highly favoured over their homogeneous counterparts for large-scale industrial production processes. The most significant features of a catalyst are its activity and selectivity, which can be further enhanced by increasing the surface area of the catalyst. In order to enhance the specific surface area of the material, it is necessary to introduce voids or pores with nanoscale dimen- sions, which can make the large number of catalytic sites present in the bulk available in the form of internal surface for catalysing the reaction.2 Heterogeneous catalysis mainly pro- ceeds via three steps: adsorption of the substrate onto the cata- lytic surface, reaction of the substrate on the catalytic surface, and desorption of the products from the catalytic surface.3,4 Due to the nanoscale size of the catalyst particles and the pores, they are often referred to as nanocatalysts.5
The steady decreases in the reserves of fossil fuel and the increasing price of petroleum oil are the driving forces for researchers to search for renewable sources of fuel. To address this energy issue, CO2 and biomass are being explored as renewable sources of carbon, as they could be converted into valuable chemicals by the use of various heterogeneous cata- lysts.6 In particular, catalysts are utilised in (i) minimizing the formation of by-products and optimizing the selectivity toward valuable products, (ii) eliminating environmentally hazardous reagents and solvents, (iii) maintaining the stability of the catalyst under vigorous reaction conditions (high temperature and pressure), and (iv) allowing renewable feedstocks to be used as a source of reactants and reagents.6 The aim of this paper is to highlight the major synthetic routes for designing heterogeneous catalysts and their utilization in key chemical transformations involving renewable resources, which is in demand from an environmental perspective.

Synthesis of heterogeneous catalysts

Porous catalysts can be classified into three major categories: inorganic, organic and organic–inorganic hybrids. Microporous zeolites and silicoaluminophosphates (SAPOs), mesoporous silica and porous metal oxide are purely inorganic materials, covalent organic frameworks (COFs) and porous organic polymers (POPs) are purely organic materials, and functionalized mesoporous materials (FMMs), periodic meso- porous organosilicas (PMOs), metal–organic frameworks (MOFs) and MOF-derived porous carbon supported metal/ metal oxide nanoparticles belong to the organic–inorganic hybrid category of porous materials. In the following section, we describe generalized synthesis strategies for designing these heterogeneous catalysts.
ZeolitesaSchool of Materials Science, Indian Association for the Cultivation of Science, 2A & 2B Raja S C. Mullick Road, Jadavpur, Kolkata 700 032, India.
Zeolites are crystalline, microporous, hydrated tectoaluminosi- licates7 in which tetrahedral [SiO ]4− and [AlO ]5− units are E-mail: [email protected] 4 bMaterial Chemistry Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, India connected through a common O atom to form secondary building units (SBUs). The negative charge dispersed on the surface of the zeolite framework is neutralized by mono- or bi- valent cations (such as Na+, K+, Ca2+, etc.), which are present at the surfaces of the micropores. The structural formula of zeo- lites is Mx/n[(AlO2)x(SiO2)y]·zH2O, where ‘M’ is an alkali or alka- line earth cation, ‘n’ is the valence of the cation, ‘z’ is the number of water molecules per unit cell, and x and y are the total number of tetrahedra per unit cell. By using organic amines and quaternary ammonium cations as a single mole- cule template or structure directing agent (SDA), a large number of synthetic zeolites have been reported. In  1a, we have illustrated the synthesis of the very widely studied zeolite ZSM-5 with MFI topology.8

SAPOs
Crystalline microporous aluminophosphates (AlPO4-n) with a large diversity of framework topologies have been synthesized
using various amines as SDAs.9 The introduction of Si4+ into the neutral AlPO4-n framework results in silicoaluminopho- sphates having one acidic site for each Si, similarly to in zeo- lites. Furthermore, due to the robustness of the framework and the presence of strong Brønsted acid sites, they are often employed in various industrially important reactions such as methanol to gasoline (MTG) and fluid catalytic cracking (FCC).10
Ordered and disordered mesoporous materials
The development of a soft templating pathway using a supra- molecular surfactant assembly for the synthesis of ordered mesoporous silicas added a new dimension to the synthesis of porous materials.11 Subsequently, a wide range of surfactants were used as templates for designing ordered and disordered mesoporous silicas.12 Anionic and non-ionic surfactants (tri- block copolymers) have been intensively used as soft templates for designing mesoporous materials.13 The incorporation of a
large number of transition as well as non-transition elements
into the mesoporous silica frameworks, as well as mesoporous metal oxides, has been achieved following these synthetic strat- egies.14 By virtue of their exceptionally high surface area and the presence of abundant nanopores, these materials are often employed as hosts for immobilizing reactive metal ions/oxides or metallic nanoparticles for exploring their applications in various catalytic transformations. Often, the surfaces of these mesoporous silicas are functionalized with reactive organic groups (such as –CO2H, –SO3H, –NH2, etc.) to obtain functio- nalized mesoporous materials (FMMs), which are extensively used in eco-friendly catalytic transformations.15–17
PMOs
PMOs are organic–inorganic hybrid materials synthesized using organic bridging dipodal alkoxysilanes of the formula (R′O)3Si–R–Si(OR′)3, where R = organic bridging group and R′ =
methyl or ethyl, as the Si source via a hydrothermal soft-tem-
plating route.18,19 Since organic functionalities are grafted inside the pore walls of these PMOs, there is greater room for the organic reactants when they are used as catalysts. Reactive metals such as Ti, Co, Cu, Ni, Pd, Rh, etc., can be grafted on the surfaces of the PMOs to carry out several important cata- lytic reactions.20 Recently, we reported a PMO containing thia- diazole and thiol groups inside the pore wall via a CTAB- assisted hydrothermal synthesis pathway21  that dis- played excellent Hg(II) adsorption behavior. Due to the robust- ness of their frameworks, PMOs are often functionalized with reactive organic groups and the resulting materials can offer huge opportunities in heterogeneous catalysis.22
MOFs
MOFs are an exciting class of crystalline, porous organic–in- organic hybrid materials with ultrahigh surface area and a wide range of pore sizes and chemical compositions.23 MOFs
are constructed via coordination bonds between metal ion
1 A schematic depiction of generalized syntheses of zeolites (a), SAPOs (b), ordered mesoporous silica (c), PMOs (d), MOFs (e), COFs (f), POPs (g), porous metal oxides and (h) metal/metal oxides supported on porous carbon (i).nodes [also known as secondary building units (SBUs)] and multipodal organic linkers. Their high specific surface area and the exceptional variability of the building units (organicand inorganic components) make MOFs a potential candidate for various important applications such as gas storage, clean energy and catalysis.24 The feasibility of designing the MOFs with a large number of organic ligands has opened wide opportunities in catalysis.25–27COFs

COFs are a type of crystalline porous organic material having nanoscale porosity and highly ordered polygon skeletons, which are formed by the covalent linking of organic moi- eties.28 For the periodic enlargement of COFs to take place, a reversible chemical reaction must occur between organic moi- eties. Highly crystalline COFs are synthesized via reactions such as boronic acid and catechol linkage to form a five-mem- bered boronic ester and the self-condensation of boronic acid to form boroxine.29 However, these COFs are unstable in pres- ence of water, alcohol and acid. More stable imine-linked COFs have been synthesized through the Schiff-base conden- sation of ditopic to polytopic aldehydes and amines.30–33Porous organic polymers (POPs)

The design principle of POPs is based on the extended polymerization of one or more reactive organic building units through various reactions such as free radical polymeriz- ation,34 diazo-coupling,35 Suzuki–Miyaura cross-coupling,36 Friedel–Craftss reaction,37 Schiff-base condensation,38 nitrile cyclotrimerization,39 click reaction,40 and extended aromatic substitution,41 among others. Due to the abundance of hetero- elements such as N, O, and S in their networks, they often bind active metal centers, and the resulting materials show excellent catalytic activity for a wide range of organic trans- formations, viz. C–C/C–N/C–S cross-coupling reactions, multi- component coupling, CO2 fixation, and the synthesis of bio- fuels and chemical intermediates from biomass resources.Porous metal oxides he synthesis of porous metal oxides can be carried out via both exotemplating and endotemplating methods. In exotem- plating or hard templating methods, porous SiO2 is generally used as the hard template, and the metal precursor is impreg- nated into the pores. After the removal of the template by chemical etching methods (treatment with hot aqueous NaOH/HF), porous crystalline metal oxide is prepared.42 In endotemplating or soft templating methods, a porous metal oxide is synthesized via a hydrothermal process. In this process, self-assemblies of various surfactants (cationic, anionic and non-ionic surfactants) are used as structure directing agents, and interact with metal salts via electrostatic interactions to form a metal–surfactant composite. After calci- nation or solvent extraction of the nanocomposite material, the surfactant molecule is removed and the porous metal oxide nanostructure is obtained.43 Similarly, the use of mul- tiple metal sources yields mesoporous mixed metal oxides,44 which often display high catalytic efficiency due to synergistic effects. Single atoms, clusters or nanoparticles bearingdifferent active metals45 are responsible for the activity and selectivity of the catalytic processes.Metal/metal oxide NPs supported on porous carbonThe carbonization of MOFs is a convenient route for the stabi- lization of metal/metal oxide nanoparticles on supported porous carbons.46 During the pyrolysis process, stable nano- clusters are formed, together with hierarchical porosity ranging from small micropores to large mesopores. The syner- gistic effect of the different metals/metal oxides, elements doped onto the carbon, hierarchical nanostructure and poro- sity plays a crucial role in their high catalytic activity for the remediation of toxic pollutants.47Catalysts for environmental applicationsCO2 fixation. The ever-increasing concentration of CO2 in the atmosphere has become a major cause of concern from an environmental perspective. Thus, scientists are devoting a great deal of attention to solving this problem by reducing CO2 or fixing it on reactive organic molecules. Heterogeneous cata- lysts play a very crucial role in this area. The high surface area of the catalysts results in a larger number of catalytic sites for the reaction, and their surface basicity could facilitate the acti- vation of CO2 molecules under relatively moderate conditions, which is otherwise not achievable. Further advancement could be made by replacing simulated CO2 gas from a flue gas mixture or atmospheric CO2 to make the system highly sustain- able and energy efficient. CO2 fixation is a process in which CO2 acts as a C1 source to functionalize organic molecules via chemical reactions.48 Some of the challenging CO2 fixation reactions are summarized in  2. Several catalytic processes for the synthesis of cyclic carbonates, carbamates and urea using CO2 as a reagent have been established. Ethanol syn- thesis using CO2 as carbon source is very challenging. A Cu– Fe–Zn catalyst promoted with Cs showed a promising result in terms of ethanol selectivity and the overall yield of the process.49 Yang et al. reported a Cu-BDC MOF with macro- and mesopores that showed high catalytic activity for carbonylative coupling between various benzyl halide derivatives and CO2 to form the corresponding formate derivatives.50 The N-formylation of aromatic amines, N-alkylation of anilines, carboxylation of terminal alkynes, and oxidative cyclization of reactive aromatics bearing two functional groups for the syn- thesis of heterocyclic compounds such as 2-oxazolidinones, 2,4-quinazolinedione, etc.,  was achieved over a range of supported metal catalysts51 under moderate reaction conditions.

Photochemical CO2 reduction. This is a process in which CO2 is reduced in the presence of light by a photocatalyst. This process consists of the following five steps: light absorption, charge separation, CO2 adsorption, surface redox reaction, and product desorption.52 In the presence of light, the valence bond (VB) electrons of the photocatalyst are excited to the con- duction band (CB) and produce a hole in the valence band.

 2 Various CO2 fixation reactions to form fine chemicals using porous nanomaterials as catalysts or catalyst supports.surface of the MOF to enhance the catalytic activity of the com- posite material.

Electrochemical CO2 reduction. In electrochemical CO2 reduction, the driving force is the external potential applied, which controls the formation of different products. Usually, CO and HCO2H are the major reduction products over various electrocatalysts,54 and mesoporosity enhances the faradaic efficiency of the process.
Photoelectrochemical (PEC) CO2 reduction. The PEC reduction of CO2 involves multistep electron transfer pro- cesses.55 The reduction of CO2 produces various valuable small molecules such as carbon monoxide, methanol, ethanol, methane, formic acid, etc. CO2 is reduced on the semiconductor surface by irradiation with light. PEC methods for CO2 reduction are often preferred over EC and PC, as they are highly efficient. Often, heterojunctions greatly facilitate the faradaic efficiency and overall catalytic efficiency in this photo-assisted process.
Photodecomposition of volatile organic compounds (VOCs).VOCs make a significant contribution as atmospheric pollu-
tants. They are highly toxic, and should be catalytically reduced to harmless components. Among the various tech-The electrons in the conduction band are then used to reduce CO2 to CO, formic acid, methanol, etc., and the holes in valence band are used to oxidize water. Wei et al. reported a hybrid MOF incorporating Fe(bpy)Cl3 (UiO-68-Fe-bpy), which showed better CO2-to-CO photoreduction activity and selecti- vity than the parent Zr(IV)-MOF and Fe(bpy)Cl3.53 This result suggested the advantages of functionalizing the porousniques available, the photocatalytic decomposition process is quite convenient for VOC treatment.56 A higher formaldehyde degradation efficiency was achieved over TiO2–SiO2 film (with mesoporous SBA-15 as a support) than commercial TiO2 ana- logues 57
Electrochemical water splitting. One of the two half-cell reactions, the hydrogen evolution reaction (HER), is involved

 3 A schematic illustration of various environmental applications of heterogeneous catalysts.in the generation of hydrogen fuel from H2O.58 Noble metals such as Pt NPs supported on porous carbons are known to be benchmark electrocatalyst under acidic pH conditions. Suib et al. obtained mesoporous FeS2 via the post-synthesis sulfuri- zation of mesoporous Fe2O3, and the resulting material showed good catalytic activity for electrochemical HER with

very low overpotential of 96 mV and a Tafel slope of 78 mV dec−1 under alkaline pH conditions.59 Thus, the suitable design of the nanocatalysts can provide very high efficiency in
the HER.
Photoelectrochemical (PEC) water splitting. PEC water split- ting is a promising technique to convert solar energy into hydrogen fuel via a cheap and sustainable route. The most important requirement is the band gap of the catalyst, which can be optimized by tuning the particle size, morphology and surface-to-volume ratio of the catalysts.60 Band gap engineer- ing via doping with different heteroelements, as well as the introduction of heterostructures, can considerably improve the overall solar-to-hydrogen energy conversion efficiency.
Advanced oxidation processes (AOPs). AOPs are fundamental processes that are utilized extensively for potable water treat- ment. In AOPs, a hydroxyl radical (•OH) acts as the oxidizing agent, and several hierarchically porous catalysts are known to improve the efficiency for the complete oxidation of organic dyes in the presence of visible light.61 Ordered mesoporous MnCo2O4 used in a cathode displayed high efficiency for the electro-Fenton degradation of antibiotics in a contaminated water source.62 The existence of Mn/Co redox pairs, together with fast electron transfer, enhances the rate of this AOP. Peroxomonosulfate-assisted oxidation also promoted redox shuttling and electron transfer in the degradation of phenolic dyes, which is generally known as the sulfate radical-advanced oxidation process (SR-AOP).63
Selective catalytic reduction (SCR) of NOx. NOx, which is released in the combustion of fossil fuel, is a major source of air pollution, which causes acid rain, photochemical smog and stratospheric ozone depletion. The SCR of NOx with NH3 is often employed as post-treatment technology to control NOx emissions in which NO is converted into nitrogen and water. The reduction of NO to N2 in the presence of NH3 proceeds as:
4NH3 þ 4NO þ O2 ! 6H2O þ 4N2
Sun et al. reported an Ni-MOF catalyst showing 92% NO conversion over a wide temperature range of 275–440 °C.64
Fluid catalytic cracking (FCC). The catalytic conversion of linear long-chain alkanes into short-chain alkanes, isomerisa- tion of linear to branched alkanes, dehydrogenation of cyclic olefins to aromatics, etc., are collectively known as FCC.65 Microporous zeolites and SAPOs are often used as catalysts for FCC in large-scale applications of this environmentally demand- ing route for the production of fuels and fine chemicals.
Fischer–Tropsch (FT) synthesis. FT synthesis is the process in which a mixture of carbon monoxide and hydrogen (syngas) is converted into hydrocarbons with the help of a catalyst. Often, Fe, Co or Ru is employed as the active metal in develop- ing an efficient catalyst for the FT synthesis.66 Due to the utiliz-ation of harmful CO and CO2 in the FT process, it is very envir- onmentally challenging to develop novel catalysts for this reaction.
Biomass to biofuel. Generally, biomass feedstocks are divided into three categories: sugar or stretchy feedstocks (starch, glucose, fructose, etc.), triglyceride feedstocks, and woody or lignocellulosic feedstocks (corn stover, wheat straw, wood, grass, bagasse, etc.). Lignocellulosic biomass contains three main components: hemicelluloses (25–30%), cellulose (40–50%), and lignin (15–20%).67 Using heterogeneous cata- lysts,68 lignocellulosic biomass can be converted into various valuable fuel components via platform chemicals such as 5-hydroxymethylfurfural (HMF),69 furfural, etc. HMF can undergo selective catalytic hydrogenation over a range of cata- lysts to form 2,5-dimethylfuran (DMF, 3), which is often used as a biofuel for gasoline-operated vehicles.
Biomass valorization. Obtaining triglycerides via the cata- lytic transesterification of vegetable oils produces a huge amount of glycerol ( 3). Glycerol can be converted into value-added products, such as glycerol carbonate70 or mono-, di- and triacetins of glycerol, which have wide applications in cosmetics, pharmaceuticals, the food industry, lubricants, Li- ion batteries, etc. Additionally, carbohydrate biomass can be converted into HMF over a wide range of catalysts. HMF can undergo catalytic oxidation, reduction or polymerization to yield a wide spectrum of valuable chemicals, fuels and func- tional materials. Abu-Omar et al. explored a robust bifunc- tional catalyst, Pt-ReOx supported on carbon, to transform biomass-derived sugar acid and mucic acid into adipic acid via simultaneous deoxydehydration and transfer hydrogenation.71
Pollutant remediation. Harmful pollutants such as Cr(VI), humic acid, organic dyes etc. are widely present in water and soil resources, and catalysts play a key role in their remediation processes.72 Often, selective catalytic reduction can be achieved over porous heterogeneous catalysts ( 3).
Methanol to hydrocarbons. Although methanol can poten- tially be used in engine fuel, its direct use in motors is restricted due to technical issues.73 The conversion of metha- nol into high-octane gasoline74 or hydrocarbons could be achieved over various heterogeneous catalysts, viz. zeolites, SAPOs, etc.75 through the proper choice of catalyst and reaction conditions.

Conclusions and future perspectives
We have explored various synthetic strategies for designing heterogeneous catalysts and their environmental applications. To improve the catalytic efficiencies of these processes, the surface modification/functionalization of porous nano- materials is often carried out. The specific surface area, pore size tunability, number of active sites on the catalytic surface, and stability of catalysts have been improved via new design strategies involving low-cost synthetic routes. Heterogeneous organocatalysis is another very promising area of research, in which the use of COFs, POPs, and FMMs bearing the desired organic functionalities has huge scope for exploration in the coming years. The direct utilization of CO2 from flue gas mix- tures or air could add a new dimension to heterogeneous cata- lysis for the conversion of CO2 into valuable chemicals and fuel over these porous catalysts in the coming years. Due to the rapid growth in this area of research, we expect that in the future, several hazardous environmental issues will be more promptly addressed through the reticular synthesis of these materials MSAB

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
RC acknowledges CSIR, New Delhi for a junior research fellow- ship. AB would like to thank DST-SERB, New Delhi for a core research grant (Project No. CRG/2018/000230).

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