Mesoporous materials: versatile supports in heterogeneous catalysis for liquid phase catalytic transformations

Abstract

Due to their unprecedented intrinsic structural features, like tunable pore diameter of nanoscale dimensions, huge BET surface areas and good flexibility to recognize/accommodate various functional groups and metals onto the surface, an inevitable linkage of nanoporous materials with catalysis has been built-up over the past few decades. As a result of which, a huge number of communications and articles dealing with these materials with nanoscale porosity have come to light. In this review, our objective is to provide a comprehensive overview of mesoporous solids, the most remarkable member of the nanoporous family of materials, the general strategy for their syntheses and application of functionalized porous materials in liquid phase catalytic reactions. In the latter part, the role of catalytic centres in various organic transformations over these functionalized mesoporous materials and their economical, environmental and industrial aspects are described in detail.

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Nabanita Pal

Nabanita Pal is presently working as a Postdoctoral Research Fellow at Saha Institute of Nuclear Physics in Kolkata, India. She received her Bachelor (BSc) and Master (MSc) degrees in Chemistry from the University of Calcutta in 2004 and 2006, respectively. Then she joined the Indian Association for the Cultivation of Science in Kolkata in 2007 for her doctoral research. She obtained a PhD degree in 2012 with several publications on catalysis over nanoporous materials. During 2012–2014, she was a Dr D. S. Kothari postdoctoral fellow in Jadavpur University, India and later a postdoctoral fellow in Sungkyunkwan University, South Korea. Her research interest mainly focused on nanoporous materials and their applications in catalysis, biosensing and adsorption, etc.

Asim Bhaumik

Asim Bhaumik is currently working as a senior professor at the Department of Materials Science, Indian Association for the Cultivation of Science (IACS), Kolkata, India. He received his PhD from NCL, Pune in 1997. After his postdoctoral research as a JSPS fellow at the University of Tokyo, Japan during 1997–1999 and as an associate researcher at Toyota Central R & D Labs. Inc., Japan during 1999–2001, in 2001 he joined IACS, India, as a faculty member. His research focuses on several aspects of energy, the environment and biomedical science, including designing porous nanomaterials for adsorption, gas storage, catalysis, sensing, photocatalysis, DSSCs and drug delivery applications. He is the coauthor of 270 research publications and the inventor of 13 patents. He is a board member of several journals and a Fellow of the Royal Society of Chemistry, UK.

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1. Introduction

Today, worldwide over 60% of industrially important chemical products are produced by different chemical process and the majority of these processes are accelerated by using suitable catalysts.¹ Wilhelm Ostwald rightly said that “there is probably no chemical reaction which cannot be influenced catalytically”.² Catalysts play a crucial role in the manufacturing process for the synthesis of fine chemicals, pharmaceuticals, commodity chemicals, energy resources, fuels, biofuels and so on. The extraordinary advantage of heterogeneous catalysis over homogeneous catalysis is the separation, recoverability and recycling capability, which leads to compliance to green synthetic pathways involving minimum environmental pollution.

Porous nanostructured materials with a huge BET surface area are suitable for catalytic reactions and in most cases the nature of the catalytically active components (e.g. metals, organic functional groups etc.) is most crucial in heterogeneous catalysts. Hence, researchers are very keen to design hierarchical porous nanomaterials bearing catalytically active centers at their surface. A number of reviews by Pérez-Ramírez et al.,³ Pal et al.,⁴ Lopez-Orozco et al.⁵ and others on the synthesis and applications of porous solids have been reported in recent times. The introduction of microporous solids, particularly zeolites in industrial manufacturing processes, has brought an enormous economical and environmental revolution.⁶ These microporous materials play a key role in shape-selective catalysis on the industrial scale.⁷ However, their small pore opening has motivated researchers to focus their attention on designing materials with pores of still larger dimensions in the nanoscale region. This ultimately leads to the discovery of ordered mesoporous silicas through supramolecular templating pathways.⁸ Later, other mesoporous supports like mesoporous oxides, phosphates, carbons, etc. were invented and they play a significant role in industrial manufacturing processes for fine chemicals. Liquid phase catalytic transformations in heterogeneous media over mesoporous solids are more favorable, and they lead to minimum diffusion as compared to other materials belonging to the porous categories.

A number of comprehensive reviews in the literature related to the catalytic application of mesoporous solids by Corma,⁹ Taguchi et al.,¹⁰ Perego et al.¹¹ and Martín-Aranda et al.¹² have arisen, which mainly discussed catalysis over ordered mesoporous silica based materials. The other mesoporous materials like oxides, phosphates, carbons, polymers and organic–inorganic hybrid periodic mesoporous organosilicas,¹³ which are of parallel importance as far as industrial catalysis is concerned, have had less emphasis in these reviews. The present review illustrates an overall picture of heterogeneous catalytic reactions mediated by a wide range of mesoporous materials invented so far and their environmental aspects for clean chemical synthesis.

2. What are mesoporous materials?

Traditional porous materials, the more commonly used term being ‘nanoporous’ materials, are defined as a continuous and solid network material filled through voids of ‘nanoscale pores’ of the order of say 100 nm or smaller. The International Union of Pure and Applied Chemistry (IUPAC) have classified nanoporous solids into three categories according to the pore size they possess: microporous with a pore diameter of less than 2 nm; mesoporous having a pore diameter in the range of 2–50 nm; and macroporous with a greater than 50 nm pore size.¹⁴ There are some recent reports on catalytic activity over macroporous materials,^15,16 but owing to the low surface area of nanoporous materials of this category they are not in such high demand for liquid phase heterogeneous catalysis. Also many microporous zeolites, aluminophosphates, organic–inorganic hybrid phosphonates, and metal organic frameworks (MOFs) are quite useful for environmentally benign green catalytic reactions,^17,18 but those are out of the scope of this review, where our discussion will be focused on mesoporous solids and their potential in heterogeneous catalysis.

The history of mesoporous molecular sieves (where, ‘meso’ the Greek prefix, meaning “in between”, adopted by IUPAC for dimensions of pores typically between 2 and 50 nm) begins in the last decades of the twentieth century when materials like MCM (Mobil Composition of Matter)-type materials⁸ were successfully synthesized by Mobil scientists. The discovery of the M41S family of ordered mesoporous materials with a pore dimension of 2–10 nm, high surface area (∼1000 m² g⁻¹) and large pore volume (∼1.0 cm³ g⁻¹), by using quaternary alkylammonium surfactants (e.g. cetyltrimethylammonium bromide, CH₃(CH₂)₁₅N(CH₃)₃⁺Br⁻) as a template, can be considered one of the most important milestones in the history of the porous materials world, resulting in huge expectations for their application as heterogeneous catalysts.^19,20

Among the ordered mesoporous materials discovered initially, mesoporous silicas, e.g. MCM-41, MCM-48, MCM-50, FSM-16,²¹ SBA-15 (ref. 22) etc., are well-known. Though there is some difference in the synthesis conditions and structural properties of these silicas, the basic strategy of synthesis is similar in all cases, and is based on the supramolecular self-assembly of the surfactants (or templates).²³ Rather than a small organic molecule as the single molecule template (SDA), long chain cationic surfactant molecules, like CTAB (cetyltrimethylammonium bromide), CPC (cetylpyridinium chloride) etc. or anionic surfactants like SDS (sodium dodecyl sulphate) are used during the synthesis of highly ordered silica based materials.⁸ In solution, co-operative self-assembly of surfactant molecules takes place at concentrations higher than the CMC when a spherical or rod shaped geometry is obtained around which hydrolyzed inorganic silica precursors (say, tetraethyl orthosilicate or TEOS) arrange themselves in order to form organic–inorganic metal-template composites in a highly basic medium. Through ageing for a particular time further condensation occurs and the composites take on an ordered arrangement such as cubic or hexagonal, etc. After removal of the surfactants from the composite materials through calcination or solvent-extraction we can obtain the final mesoporous solids free from organic templates. A detailed synthesis strategy for mesoporous silica and other mesoporous solids using a surfactant-templated route based on the selection of different types of surfactants has been extended in the review published recently.⁴ A typical synthesis scheme of highly ordered 2D hexagonal pure mesoporous silica MCM-41 mediated by a surfactant is shown in Fig. 1. Depending upon the type of surfactant used, the surfactant–silica ratio and the pH of the solution, the structure and mesopore size of the silica can be varied from hexagonal (e.g. MCM-41), to cubic (e.g. MCM-48), to lamellar (e.g. MCM-50). Mesoporous SBA-15 type materials are another family of silica with large mesopores, synthesized using non-ionic block copolymer surfactants under highly acidic conditions.²² In addition to the co-operative pathways, nanocasting using already formed ordered mesoporous materials as hard templates has been developed to synthesize mesoporous materials.²⁴ This method of nanocasting is highly effective for the synthesis of other non-siliceous porous oxide and carbon materials with ordered pore arrangements which are difficult to prepare directly by the surfactant-assisted route.²⁵

Fig. 1 Surfactant assisted synthesis of mesoporous silica with a 2D ordered hexagonal pore arrangement.

Non-siliceous mesoporous materials like oxides, mixed oxides,²⁶ metal sulphides,²⁷ metal phosphates,²⁸ polymers,²⁹ carbons³⁰ and carbon nitrides³¹ have also been synthesized successfully using surfactant templating routes. The long range applications of these materials extend from gas adsorption, ion-exchange, magnetism, sensing, catalysis and electrochemistry to many versatile fields of research. Some of these mesoporous materials are purely inorganic; some are organic–inorganic hybrid materials while the others are completely organic based. Types of mesoporous solids commonly known in these three categories are shown in Table 1.

Table 1 Possible types of mesoporous materials with examples

Framework composition	Type of materials	Examples
Purely inorganic	Mesoporous silicas	MCM-41/MCM-48,^8 SBA-15 (ref. 22) etc.
	Metal containing mesoporous silicas	Ti-MCM-41, Zn–silica etc.
	Mesoporous metal oxides and mixed metal oxides	TiO2, AlO2, ZrO2, ZnTiO3 etc.
	Mesoporous metallophosphates	Silicotitanium phosphate, silicoalumino phosphate etc.
Organic–inorganic hybrid	Periodic mesoporous organosilicas (PMOs)	Various metal containing PMOs etc.
	Metal oxophenylphosphates	Iron phosphonate, chromium phosphonate etc.
Purely organic	Mesoporous carbons	CMK-3
	Mesoporous polymers	Triazine based polymer, porphyrine based polymer

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3. Catalysis: homogeneous vs. heterogeneous

The phenomenon called ‘catalysis’ has been known since very ancient centuries, even though people knew nothing about the underlying mechanism and chemical process involved in it. The making of soap, the fermentation of wine to vinegar, and the leavening of bread are all processes involving catalysis. The name ‘catalysis’, first proposed in 1835 by Swedish chemist Jöns Jakob Berzelius (1779–1848), was taken from the Greek words kata, which means ‘down’ and lyein, meaning ‘loosen’. In a short paper summarising his concepts on ‘catalysis’ as a new force, Berzelius wrote: “It is, then, proved that several simple or compound bodies, soluble and insoluble, have the property of exercising on other bodies an action very different from chemical affinity. By means of this action they produce, in these bodies, decompositions of their elements and different recombination of these same elements to which they remain indifferent.” He called this new force a ‘catalytic force’.³² The actual basic role of a catalyst is to offer an alternative pathway of lower activation energy than the respective uncatalyzed reaction, resulting in a faster reaction rate without hampering the overall thermodynamics of the reaction. Enzymes are the most common and efficient catalysts found in nature. Not a single reaction in our body is possible without the aid of these biocatalysts. They have also been used as catalysts in food industries for many years. In the case of laboratory and industrial use, catalytic oxidation using oxygen, hydrogenation using metal catalysts, and many well known reactions are worthy of mention. Day by day the imperative role of catalysis in the manufacturing of the majority of chemicals used by our society is increasing rapidly. According to the survey report, in 2005, catalytic processes generated about $900 billion worth of products worldwide.

Catalytic reactions are usually categorized as either homogeneous or heterogeneous. Biocatalysis (enzymatic reactions) is often considered as a separate group. A homogeneous catalysis reaction is one where both the catalyst, the reactants on which it works and the products, are all in the same phase (solid, liquid, or gas), generally in the liquid phase. The first formal example of this type is the reaction involving the decomposition of starch into glucose in boiling water in the presence of a sulphuric acid catalyst studied by the Russian chemist Gottlieb Sigismund Constantin Kirchhoff (1764–1833) in 1812. Both the sulfuric acid and the starch were in aqueous solution during the reaction.³³ Organometallic catalysts are another example of homogeneous catalysts.³⁴ On the other hand, heterogeneous catalysis is that where the catalyst is generally solid and remains as a distinct phase from the liquid or gaseous reaction media on which it acts. Most of the reactions that occur in industry are driven by heterogeneous catalysts. The role of finely divided iron in the Haber process for synthesis of ammonia or the role of vanadium oxide for the production of sulphuric acid in the contact process, etc. are well-known examples of this type. In fact, with the production of sulphuric acid in the “contact” process solved by Knietsch in the year 1898 in Europe, the idea of industrial catalysis was developed.³⁵ Today porous materials are recognized as one of the most important materials in heterogeneous catalysis and they play the major role in industrial fine chemical synthesis.⁹

Homogeneous catalysis has some important properties like high selectivity of the products and good accessibility to catalytically active sites, etc. but with increasing public awareness regarding environmental issues, heterogeneous catalysis has become much more important over homogeneous catalysis.³⁶ This is because some major problems like corrosion, toxicity, the difficulty of catalyst separation, recovery, regeneration and reuse, high cost and the creation of huge amounts of solid waste, etc. in the case of homogeneous catalysis, make it unsuitable for application in industrial purposes in this environmentally conscious and economically pressured world. Although homogeneous catalysts are used in some industries like the food, fine chemical, pharmaceutical and agrochemical industry etc., in this century heterogeneous catalysis has been proven to be the ultimate goal of industrial catalysis and in engineering fields. The industrial processes which are highly dependent on homogeneous catalysts are recommended to be used only by ‘heterogenization’, which means immobilization of the catalysts on a solid insoluble support which possesses all the properties of the homogeneous counterpart, and also the advantage of recyclability like in heterogeneous catalysis.³⁷

Catalytic reaction on a heterogeneous support involves fundamental steps like diffusion and adsorption of the reactants onto a solid surface, successful reaction on it, then desorption of the products in bulk with regeneration of the catalyst for the next cycle. A schematic representation of a heterogeneous catalytic cycle taking place through different steps is shown in Fig. 2. Since the surface of catalyst plays a pivotal role in the related process, materials having high surface area are more desirable than nonporous materials to accomplish a significant part in industrial catalysis.^1,9 Although most of the chemical industries generally use zeolites or activated carbon based microporous materials as a stable support for catalysis, the number of fascinating organic transformations successfully carried out over different mesoporous supports is growing very rapidly. In light of this a systematic brief discussion on various mesopore mediated important heterogeneous catalytic transformations under liquid phase reaction conditions has been made and illustrated in the following part of this review.

Fig. 2 Steps of a heterogeneous catalytic cycle over a mesoporous solid.

4. Heterogeneous catalysis over mesoporous materials

The most important feature of mesoporous materials for which they show great potential for catalytic applications, is the possibility of controlling the morphology and local environment at the catalytic site depending upon the requirements of a particular reaction. Basically the reasons for which mesoporous materials act as promising candidates for heterogeneous catalysis are, (i) they have a high surface area and narrow pore size distribution essential for acting as a catalyst bed, (ii) the dimension of the pores can be tuned by varying the functional groups and ligands to obtain good selectivity for the product of interest, (iii) they have good adsorption properties, which allow the facile diffusion of reactant and product molecules during reactions, (iv) the ability to vary the type and concentration of surface functionality to change the textural properties and (v) the ability to tailor the polarity/hydrophobicity/hydrophilicity of the surface in order to influence the yield of a catalytic reaction. Moreover, they are relatively non-toxic, non-corrosive, non-air sensitive, highly reusable, completely pollution-free, environmentally benign supports for catalytic transformation in the liquid phase.¹⁰ The role and function of mesoporous silica and other non-siliceous materials in mediating sustainable liquid phase reactions e.g. acid–base, redox, polymerization, condensation and other organic coupling reactions, etc. are discussed herein.

4.1 Redox reactions over mesoporous materials

Although a pure mesoporous silica surface has high BET surface area (ca. 800–1400 m² g⁻¹) and a tunable pore diameter (2–50 nm), the material is not so effective for doing any catalytic reactions. By introducing one or more heteroatom and metal complex into the silica framework (metal doped silica) or often by suitable immobilization of organic functionalities (organosilica or organic–inorganic hybrid silicas) the desired catalytic activity of the material can be achieved. The advantage of choosing mesoporous silica as a support for incorporating other foreign atoms or functionalities is the inertness of the inorganic silica walls, where a tetravalent Si atom can be replaced by other reactive metal centers whilst retaining considerably good surface area and porosity.¹⁰ In spite of these benefits, silica structures like SBA-15, KIT-6, etc. have the limitation of losing porosity at elevated temperatures, which restricts their application in large scale heterogeneous catalysis. Metal incorporated mesoporous silica can be prepared using both a direct (or one step) or a post synthesis method.⁴ A number of related reports of mesoporous metal doped silica as heterogeneous catalysts can be mentioned in this respect, e.g. Zn–silica,²⁰ Cr–MCM-41,³⁷ CeO₂–silica,³⁸ Ti–MCM-41,³⁹ Ce–Fe-SBA-15,⁴⁰ Mn doped CeO₂–silica,⁴¹ Pt and Rh nanoparticles supported silica,⁴² and so on. Periodic mesoporous silica (PMO), prepared by immobilizing organic moieties onto the inorganic silica surface by a direct synthesis method, can also act as a good redox catalyst when grafted with transition metals like V, Ti, Au, Mo, etc.⁴³

Further, additional advantages of mesoporous transition metal oxides and mixed oxide catalysts are their high crystallinity and also their capability of existing in various oxidation states due to having vacant d orbitals, which can take part in electron transfer from reactants during a given catalytic process.⁴⁴ Thus, these metal oxides or metal doped silica based materials are attracting wide-spread attention in recent times as non-toxic and environmentally friendly mild catalysts replacing conventional homogeneous hazardous oxidants like chromate, permanganate or periodide. In the following section we summarize some redox reactions mediated by the mesoporous solids.

a. Oxidation of hydrocarbons. Oxidation, partial oxidation and epoxidation of hydrocarbons, especially olefins (Fig. 3), using peroxides or molecular oxygen under mild conditions are very important reactions in organic synthesis from both academic and industrial points of view, since a variety of chemicals in high demand like polyurethanes, nylons, unsaturated resins, glycols, etc. are manufactured by this pathway.⁹ Due to several over oxidation products, satisfactory conversion and isolation of epoxides from alkenes is quite difficult in many cases. Recently, a number of scientists have dealt with this difficulty and reported the epoxidation of various alkenes like styrene, cyclohexene, cyclooctene, norbornene etc. over Ti-PMO materials⁴⁵ under mild conditions. Ti-PMO, Ti-containing periodic mesoporous organosilica, was obtained using a co-synthesis method using an organosilica, an inorganic silica precursor (TEOS) and a Ti-precursor in the presence of CTAB surfactant under hydrothermal conditions. This mesoporous Ti–silica was tested for epoxide preparation under liquid phase conditions in the presence of tert-butylhydroperoxide (TBHP) oxidant. On the contrary, Cu-complex embedded MCM-41 was prepared using a post-synthesis method by surface modification of MCM-41 and then applied for the epoxidation of various substrates in the presence of TBHP.⁴⁶ Along with epoxide formation, partial oxidation of olefins sometimes produces other industrially used fine chemicals over various metal doped mesoporous silica supports. Another Cu(II)-complex grafted SBA-16 catalyst, synthesized in a similar way as reported by Jana et al.,⁴⁶ has been used for the oxidation of cyclohexene in the presence of a mild oxidant, H₂O₂, where other oxidation products like cyclohexanol and cyclohexanone etc. are produced in a significant amount along with epoxide.⁴⁷ Liquid phase allylic oxidation of highly strained α-pinene and β-pinene is also feasible over UO₂²⁺–MCM-41 producing a number of products along with the respective epoxides.⁴⁸ In this respect, styrene oxidation to produce benzaldehyde as a major product over Fe and Ti-SBA-1 mesoporous silica using H₂O₂, reported by Tanglumlert et al., can also be mentioned.⁴⁹ Instead of using any oxidant, oxidation of hydrocarbons using air or O₂ under mild conditions is more environmentally-friendly and is in high demand. Mesoporous silica acts as a good support for Au nanoparticles which efficiently catalyze the selective oxidation of cyclohexane in the presence of molecular O₂, as reported by Wu et al.⁵⁰

Fig. 3 Different oxidation products of cyclohexene, cyclohexanol and cyclohexanone with adipic acid as the final product.

Adipic acid is an industrially important dicarboxylic acid, which can be produced exclusively from the catalytic oxidation of cyclohexane, cyclohexene or cyclohexanone over mesoporous materials like WO₃/SiO₂,⁵¹ and the core–shell Fenton catalyst (Fe₂O₃/Al₂O₃)⁵² in the presence of H₂O₂ oxidant. In Fig. 3 the step by step formation of adipic acid from these hydrocarbons is shown. Over the core–shell Fenton catalyst, the process occurs through formation of a hydroxyl radical by the reaction of Fe³⁺ with H₂O₂, and that OH˙ helps to oxidize cyclohexanone to adipic acid through the intermediates caprolactone and 6-hydroxyhexanoic acid. Cr(VI) grafted mesoporous polyaniline, synthesized by radical polymerization of aniline with a Cr precursor, exhibits good conversion in the liquid phase oxidation of alkane and alkene in a non-toxic green solvent, water, with a reuse capability of more than five times.⁵³ Highly selective oxidation of cyclohexane to produce cyclohexanone is a very challenging task since cyclohexanol is always obtained as a major side product. Very recently, another porous co-ordination polymer was reported by Zhang et al. who have employed atmospheric O₂ as an oxidant.⁵⁴ This catalytic pathway provides a useful route for the selective conversion of β-isophorone to ketoisophorone, a significant reaction for the synthesis of vitamin E.

Porous carbon based materials are also utilized as effective supports for the oxidation of hydrocarbons under liquid phase reaction conditions. N doped nanoporous carbon with supported Pd nanoparticles has shown high catalytic efficiency for the selective aerobic oxidation of indane, diphenyl methane and ethyl benzene etc. thus providing a potential route for large scale fine chemical synthesis.⁵⁵ A new kind of boron- and fluorine-enriched polymeric mesoporous carbon nitride with high surface area has been prepared by Wang et al. who illustrate the potential of the material for selective cyclohexane to cyclohexanone conversion under mild conditions.⁵⁶ This organic semiconductor with controlled mesopore structures and narrow pore size distribution greatly facilitates this selective oxidation reaction with reusability for several times without losing any activity. Heterogeneous organocatalysts are particularly high in demand as they are devoid of toxic metal and thus free from the possibility of metal-leaching during the catalytic reactions. In this context porous graphitic carbon nitride (g-C₃N₄) has shown high catalytic activity in the selective oxidation of many branched hydrocarbons under mild environmentally-friendly conditions.^57,58

b. Oxidation of alcohols and sulfides. The aldehydes and ketones obtained from the oxidation of primary and secondary alcohols, respectively, are very essential intermediates in organic synthesis, and the pharmaceutical and agricultural industries, etc. Aerobic oxidation of long chain primary alcohols, like 1-octanol, has been achieved with excellent yields obtained using small Au nanoparticles deposited on ceria or iron oxide doped hexagonal mesoporous silica (HMS) materials.⁵⁹ Synthesis of high grade benzaldehyde is particularly attractive to modern researchers for application in perfumery and food industries. A highly selective chlorine-free method of benzaldehyde production without the formation of over oxidized products like acid or ester at room temperature is observed over a highly ordered mesoporous ceria–silica composite which was able to convert above 50% of benzyl alcohol to benzaldehyde exclusively under solvent-free conditions using TBHP. The silica supported catalyst was truly heterogeneous and stable to exhibit high recyclability up to three times towards this alcohol oxidation reaction with minimum Ce leaching.³⁸ A detailed study has also been undertaken by the authors to compare the product conversions at RT with those at an elevated temperature. A schematic representation is given in Fig. 4 to highlight the synthesis and catalysis conditions for this ceria–silica catalyst. Mesoporous metal oxides are also efficient in the oxidation of primary and secondary alcohols. This has been demonstrated by the catalytic potential of an interesting mesoporous nanocomposite of Cr₂O₃ and 12-phosphomolybdic acid (PMA), prepared via a “nanocasting” method using mesoporous SBA-15 as a hard template.⁶⁰ The catalyst Cr₂O₃–PMA solid-solution shows an extraordinary green oxidative pathway for oxidation of 1-phenylethanol to produce acetophenone with up to 85–87% yield using non-toxic H₂O₂ at a very low temperature (323 K). The synergetic interaction between the PMA clusters and Cr₂O₃ matrix via Cr–O–Mo bonds as well as the dispersion of PMA over large effective surface area of Cr₂O₃ play the key role for this selective oxidation of secondary alcohol. Using molecular oxygen from air as a clean oxidant, oxidation of benzyl alcohol to benzaldehyde with 72% conversion and 100% selectivity is reported over highly dispersed mesoporous carbon supported iron oxide (FeO_x/H-CMK-3) under mild reaction conditions.⁶¹ A Pd@core–shell nanosphere consisting of Pd metal clusters as the core and microporous silica as the outer shell also exhibits good catalytic efficiency and selectivity for the aerobic oxidation of long chain primary alcohols under solvent-free conditions.⁶²

Fig. 4 A schematic representation of the synthesis of a mesoporous ceria–silica composite and its catalytic application for oxidation of a primary alcohol (benzyl alcohol).

The oxidation of sulfides to obtain sulfoxide and sulfones; products playing an imperative role in organic and biological reactions as well as in anti-hypertension and as cardiotonic agents etc., using industrially important heterogeneous and mesoporous catalysts replacing the homogeneous ones like halogen compounds, nitrates and transition metal oxides etc., has won great scientific interest.⁶³ A Cr-grafted organic–inorganic hybrid mesoporous polymer, synthesized using an anionic surfactant, efficiently catalyzes the partial oxidation of different aromatic and aliphatic sulfides to their corresponding oxides with impressive yields, having excellent recyclability and no significant loss in activity.⁶³ The almost selective oxidation of sulfide to sulfoxide using H₂O₂ oxidant in methanol solvent was reported over WO₃ nanoparticles supported MCM-48 heterogeneous catalyst in the year 2005.⁶⁴ The activity and stability of Mn and Cu oxo complexes have been increased to a great extent by ‘heterogenization’ of these homogeneous catalysts, i.e. by post-grafting onto mesoporous MCM-41. The resulting supported catalyst was able to convert methyl phenyl sulphide to sulfoxide with a high yield and exhibited excellent reusability up to five times in the presence of TBHP.⁶⁵ Here, Mn^V=O species present in the Mn–oxo salen complex stabilized in the mesoporous silica framework play the active role for this sulphide oxidation. Mesoporous graphitic carbon nitride (g-C₃N₄) polymer has been established as a green oxidation catalyst for the selective oxidation of various sulphides using visible light at room temperature.⁶⁶

c. Oxidation of amines. Oxidation of primary amines, e.g., aniline and substituted anilines, to the corresponding azoxybenzene derivatives is a very common and fundamental reaction in organic chemistry, because azoxybenzene and its derivatives have great potential for application in the synthesis of industrially important fine chemicals.⁶⁷ In 2009 a very interesting cobalt-bound polymer was prepared by Chang et al. and the polymer was used as a template to prepare mesoporous silica containing cobalt-oxide. The material exhibited good activity and reusability in the catalytic oxidation of aniline and its derivates to the respective azoxybenzenes with above 60% conversion achieved at 298 K as well as 100% conversion at an elevated temperature in the presence of the non-toxic H₂O₂ oxidant.⁶⁷ Apart from Co, Ti- and Zr-containing mesoporous silicas have also been employed as catalysts for the oxidation of amines under variable reaction conditions.⁶⁸ An advanced approach was also made at the end of the 20th century when hexagonal mesoporous silica H_ISiO₂ was prepared using a non-ionic surfactant, Brij 76, at room temperature and then modified with 3-(aminopropyltriethoxysilane) (APTES), a well-known organosilica precursor to form a NH₂-functionalized organic–inorganic hybrid silica, which was immobilized with various metal aqua complexes (e.g. Mn^II, Cu^II, Co^II and Zn^II). These metal complex loaded mesoporous silicas were tested for the oxidation of amines.⁶⁹ Amine oxidation follows first order kinetics and the rate of the reaction is highly dependent on the oxidizing potential of the metal complex used in the catalyst.⁶⁹ In Fig. 5, the synthesis of Mn^II-aqua-NH₂ functionalized H_ISiO₂ catalyst and its oxidizing role for the oxidation of o-aminophenol is shown by a simple schematic diagram.

Fig. 5 The synthesis of Mn^II-aqua-NH₂ functionalized mesoporous H_ISiO₂ catalyst and its oxidizing role for amine (o-aminophenol) oxidation.

d. Ammoximation. Ammoximation of ketones with ammonia and dilute aqueous H₂O₂ oxidant to produce oximes is a very crucial reaction in modern synthetic chemistry because oximes play an intermediate role for the fine chemical synthesis of lactams and amides etc. via secondary chemical reactions. Many metallosilicates like TS-1 have been employed as efficient catalysts for the synthesis of oximes in the presence of NH₃ and aqueous H₂O₂.⁷⁰ The ammoximation of bulky ketones has been efficiently carried out over a Ti-grafted ethane bridged hybrid mesoporous silica support synthesized by a one pot surfactant assisted method.⁷¹ Cyclic ketones like cyclohexanone and cyclodecanone were catalytically converted with 100% selectivity for oxime using tert-butanol as a solvent at a reaction temperature of 353 K.

e. Hydroxylation of aromatics. Hydroxylation or introduction of a –OH group into various aromatic substrates like benzene, toluene, phenol etc. is a commercially important chemical reaction, because the resulting phenol and substituted phenol products are used in the fine chemical, agrochemical, photographic chemical, pharmaceutical and food industries.⁷² Fe-incorporated mesoporous silica was prepared by a simple co-precipitation method and employed for the catalytic hydroxylation of phenol to selectively produce dihydroxybenzene with proper optimization of the reaction conditions.⁷² Phenol hydroxylation was also successful over a redox active group ferrocene loaded nanocomposite made with mesoporous polymer and silica via a controlled co-polymerization route.⁷³ Hydroxylation of benzene to phenol is sometimes inhibited by the formation of by-products like catechol and hydroquinone etc. The highly selective conversion of benzene to phenol is reported over V-complex incorporated PMO prepared by direct co-condensation of the metal grafted organosilica with the inorganic silica precursor in the presence of CTAB. In this fashion, VO(acac)₂, a well-known homogeneous catalyst ‘heterogenized’ by immobilization onto a silica surface,⁷⁴ catalyzes this benzene to phenol transformation in the presence of H₂O₂ oxidant and the reaction is proposed to be mediated by the peroxovanadyl radical (V⁵⁺–O–O˙) as the main active species.⁷⁴ Another important example of heterogenization over a solid support is the loading of FeCl₃ over mesoporous carbon nitride (g-C₃N₄). This hybrid catalyst shows up to 38% conversion of benzene to phenol with high selectivity (97%) under visible light irradiation and mild conditions.⁷⁵ In Fig. 6, a schematic representation of the synthesis and catalytic role of mesoporous Zn doped aluminophosphate (ZnAlPO₄) is shown. Highly selective formation of phenol from benzene is observed over this Zn-doped mesoporous aluminophosphate with a minor amount of hydroquinone produced as a by-product. The material has been synthesized by a method of physical mixing using tetrapropylammonium bromide as a template (Fig. 6).⁷⁶

Fig. 6 The synthesis scheme and catalytic performance of mesoporous Zn-doped aluminophosphate (ZnAlPO₄) material.

f. Hydrogenation and other reductions. Hydrogenation of unsaturated compounds via treatment with hydrogen or reduction in the presence of Pt, Ir, Ru, Ni, Pd nanoparticles etc. over a heterogeneous support is an important chemical reaction industrially. An easy way to prepare a heterogeneous catalyst bearing the above mentioned reducing metals is via successful confinement of these metals into a mesoporous support. Hydrogenation of alkene, ketones, phenol and nitro substrates etc. are some potential examples of this category and worthy of mention in this . Instead of reduction in the gas phase under harsh conditions as reported in the case of Zr/Ni-doped mesoporous silica for the hydrogenation and ring opening of tetralin at a high temperature using H₂ gas,⁷⁷ liquid phase catalytic reactions under mild conditions are more desirable from an environmental view point. The asymmetric hydrogenation of various aryl substituted ketones and quinolones with high conversion was successful under very mild conditions over chiral Ir and Ru complexes supported mesoporous silica, synthesized through co-condensation of an organosilica, an organometallic complex, [Cp*IrCl₂]₂ or [RuCl₂(-C₆Me₆)]₂, and an inorganic silica precursor in the presence of non-ionic Pluronic (P123) and CTAB surfactants, respectively.^78,79

Mesoporous oxides or mixed oxides can also act as a support similarly for thioether nanoparticles such as Au, Ni, Pt and Pd and these confined nanoparticles can catalyze the hydrogenation of aromatic nitro compounds. Mesoporous oxides like TiO₂/Al₂O₃/SiO₂/ZrO₂ with confined PtPd/AuPd/AuPt thioether nanoparticles have been synthesized via a surfactant-assisted route and show highly selective synthesis of aniline from nitrobenzene under mild conditions using hydrazine hydrate as a hydrogenating agent.⁸⁰ On the contrary, mesostructured nickel–aluminium mixed oxide hydrothermally synthesized using lauric acid as a structure directing agent, was itself efficient for the satisfactory reduction of nitrobenzene and its derivatives in 2-propanol solvent which acts as a hydride donor to generate substituted anilines.⁸¹ The synthetic pathway for the mesoporous self-assembled Ni–Al mixed oxide nanoparticles and their application for hydrogenation of nitrobenzene to aniline is shown in Fig. 7. Hydrogenation of D-glucose over Ru nanoparticles impregnated mesoporous hypercrosslinked polystyrene was reported by Sapunov et al.⁸² They also provided an elaborate mechanistic pathway and kinetics for the formation of D-sorbitol. In a non-toxic solvent like water, selective hydrogenation of phenol to cyclohexanone over ultra-small Pd nanoparticles grafted mesoporous carbon is a green synthesis in high demand. Pd supported N-functionalized ordered mesoporous carbon was prepared by nitration of a typical mesoporous carbon FDU-15 using NH₃ followed by treatment with an aqueous solution of H₂PdCl₄.⁸³ The catalyst was highly heterogeneous in nature and showed reusability up to 6 times with a good product yield each time.

Fig. 7 A schematic representation showing the synthesis of mesoporous Ni–Al mixed oxide and its application for hydrogenation of nitrobenzene to aniline.

The above mentioned redox reactions are a few examples of a huge number of liquid phase catalytic reactions that have been carried out over the years using the mesoporous family of materials. In most of these cases the mechanisms of the reactions follow a free radical pathway mediated by one or two radicals generated in situ.^53,74 In Table 2 we have summarized all the relevant reactions in tabular form for the convenience of the readers.

Table 2 Different redox reactions in liquid phase over mesoporous materials

Type of reactions	Catalysts	Reaction involved	Product yield (%)	Ref.
Oxidation of hydrocarbons	Ti doped PMO	Epoxidation of styrene, trans-stilbene norbonene, etc.	65–80	45
	Cu-complex doped MCM-41	Epoxidation of cyclohexene, cyclooctene, styrene, etc.	92–99	46
	Cu(II)-complex grafted SBA-16	Partial oxidation of cyclohexene	21–77	47
	UO2^2+–MCM-41	Allylic oxidation of α-pinene, β-pinene etc.	80–88	48
	Fe- & Ti-SBA-1	Partial oxidation of styrene	59–70	49
	Au loaded mesoporous silica	Oxidation of cyclohexane	10–34	50
	Core–shell Fenton catalyst	Oxidation of cyclohexanone	69	52
	Cr–mesoporous polyaniline	Oxidation of alkane, alkene	70–98	53
	Porous co-ordination polymer	Conversion of β-isophorone to ketoisophorone	85	54
	Pd@N doped nanoporous carbon	Selective aerobic oxidation of indane, ethyl benzene	14–30	55
	Boron- and fluorine loaded mesoporous carbon nitride	Selective oxidation of cyclohexane to cyclohexanone	2–8	56
	g-C3N4 polymer	Oxidation of strained hydrocarbons	26–100	57 and 58
Oxidation of alcohols	Au–ceria or iron oxide–HMS	Oxidation of 1-octanol	10–79	59
	Mesoporous CeO2–silica	Oxidation of benzyl alcohol	51	37
	Mesoporous Cr2O3–PMA composite	Oxidation of 1-phenylethanol	85–87	60
	Iron oxide on mesoporous carbon (FeOx/H-CMK-3)	Oxidation of benzyl alcohol to benzaldehyde	72	61
	Pd@silica core–shell nanosphere	Solvent-free aerobic oxidation of long chain primary alcohols	10–61	62
Oxidation of sulfides	Cr–mesoporous polymer	Partial oxidation of different aromatic and aliphatic sulfides	7–97	63
	WO3 NP–MCM-48	Oxidation of different sulfides	100	64
	Mn and Cu oxo complex–MCM-41	Oxidation of methyl phenyl sulphide	65–100	65
	Mesoporous g-C3N4 polymer	Selective oxidation of sulphides	39–98	66
Ammoximation	Ti-grafted ethane bridged porous inorganic–organic hybrid silica	Oxidation of cyclohexanone, cyclodecanone	42–90	71
Oxidation of amines	Mesoporous cobalt oxide–silica composite	Oxidation of aniline and its derivatives	26–100	67
	Ti, Zr-doped mesoporous silica	Oxidation of aniline	100	68
	Mn, Cu, Co & Zn-NH2-grafted organic–inorganic hybrid silica	Oxidation of amine	90–100	69
Hydroxylation of aromatics	Fe incorporated mesoporous silica	Hydroxylation of phenol	20–50	72
	Ferrocene loaded nanocomposite of mesoporous polymer and silica	Conversion of phenol to form dihydroxybenzene	25–30	73
	VO(acac)2 incorporated PMO	Selective conversion of benzene to phenol	12–38	74
	FeCl3/g-C3N4	Conversion of benzene to phenol	38	75
	Mesoporous Zn doped aluminophosphate	Selective hydroxylation of benzene to phenol	99	76
Hydrogenation & other reductions	Chiral Ir and Ru complexes-loaded mesoporous silica	Asymmetric hydrogenation of aryl substituted ketones and quinolones	80–99	78 and 79
	PtPd/AuPd/AuPt-embedded TiO2/Al2O3/SiO2/ZrO2	Selective conversion of nitrobenzene to aniline	65–87	80
	Mesostructure NiO–Al2O3	Reduction of nitrobenzene and its derivatives	36–57	81
	Ru–mesoporous polystyrene	Hydrogenation of D-glucose	40–43	82
	Pd nanoparticles-loaded mesoporous carbon	Selective hydrogenation of phenol to cyclohexanone	23–80	83

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4.2 Acid–base reactions mediated by mesoporous materials

Lewis and Brønsted acid or basic sites present in a solid catalyst are the main reason for the generation of the acid–base properties in a given solid. The Lewis acidity originates due to the presence of metal ions coordinated to the mesoporous support whereas –COOH, –SO₃H, –NR₃⁺ groups etc. are generally responsible for the Brønsted acidity. In the same way, –NH₂, –NMe₂ etc. and –COO⁻ groups account for Lewis and Brønsted basicity, respectively. The number and strength of these sites determine the catalytic activity of the concerned materials and also their selectivity for the organic transformations. Moreover, surface properties i.e. huge surface area and pore width of the mesoporous material give an additional advantage compared to the other nonporous materials. Though inherent weak acid sites restrict the application of mesoporous materials in many petrochemical reactions, they still demonstrate good potential for the reactions which require a lower level of acidity, e.g. Friedel–Crafts alkylation of organics.¹⁰ A lot of scientific reports such as those on mesoporous perovskite ZnTiO₃,¹⁹ Zn–silica,²⁰ –COOH grafted functionalized mesoporous silica,⁸⁴ Mn-doped ceria–silica,⁴² sulphonated zinc phosphonate,⁸⁵ zirconium oxophosphate,⁸⁶ and so on have been published in the literature regarding the acid–base catalytic reactions over mesoporous supports. In this section we shall illustrate some of those heterogeneous reactions carried out in the liquid phase.

a. Friedel–Crafts reaction. Friedel–Crafts (FC) alkylation and acylation reactions are the set of organic reactions developed by Charles Friedel and James Crafts in 1877 to attach substituents to an aromatic ring and this is one of the most important chemical processes for laboratory synthesis and industrial production of value added organic fine chemicals.⁸⁷ These liquid phase reactions proceed via electrophilic aromatic substitution and are usually catalyzed by Lewis acidic groups. Instead of using corrosive H₂SO₄ or solid acids like AlCl₃, FeCl₃ or BF₃ etc. in the homogeneous phase, which suffers as a method from a lot of environmental hazards and inconvenience due to metal leaching, non-recyclability etc., mesoporous based catalysts can be a good replacement as non-toxic, heterogeneous, low-cost catalysts. Perovskite ZnTiO₃, a mesoporous oxide, was reported to be highly efficient for FC benzylation of p-xylene, toluene and benzene using benzyl chloride as the alkylating agent at a temperature of 343–348 K to produce 1-(2,5-dimethylbenzyl)benzene, 2,4-methylbenzyl benzene and diphenyl methane, respectively with above 90% conversions.¹⁹ The presence of strong Lewis acidic metal sites (Zn and Ti) in the perovskite material along with a good surface area helps such excellent conversion to substituted aromatics. Zhang et al. have synthesized a new mesoporous iron phosphate Fe–P–O catalyst using four different methods and applied it for benzylation of benzene and other substrates.⁸⁸ The alkylation carried out using benzyl chloride gives 100% selective formation of diphenyl methane after 3 h. On the contrary, an acylation reaction using acetic anhydride, benzoyl chloride or p-toluoyl chloride has been observed very frequently over functionalized mesoporous materials, e.g. p-toluoyl chloride is used for liquid phase FC benzoylation of anisole at 393 K over a silicotungstic acid loaded mesoporous alumina molecular sieve.⁸⁹ Instead of these hazardous acylating and alkylating agents, green non-toxic reagents like aliphatic and aromatic alcohols are also very popular for use in FC reactions. Mesoporous Nb oxide when sulphated catalyzes the benzylation of toluene and anisole in the presence of benzyl alcohol to produce substituted aromatics.⁹⁰ Alkylation of toluene using benzyl alcohol has been carried out efficiently over Mo-embedded mesoporous carbon microspheres which results in a 100% conversion of benzyl alcohol to produce p-, m-, and o-benzyl toluene after just 1 h at a reaction temperature of 383 K, and exhibits high reutilization for up to three cycles without any decay in activity. The high oxidation state of Mo(VI) along with the porous structure of carbon allowing free movement of reactants and products between embedded α-MoO₃ and the liquid medium mainly accounts for this excellent catalytic performance.⁹¹ Different FC products obtained from different substrates using different alkylating and acylating reagents are shown in Fig. 8.

Fig. 8 Friedel–Crafts products obtained from various substrates during reactions over different mesoporous catalysts.

FC reactions are also very common over mesoporous silica based materials as revealed by many recent reports. A 3D network of mesoporous gallosilicate (GaSBA-1) was prepared in an acidic medium using the cationic surfactant CTAB and an appropriate amount of Ga salt. This material has been tested for the benzylation of various aromatics using benzyl chloride. The catalytic role of the Ga species along with a schematic representation of a suitable mechanism has been mentioned and explained by the authors.⁹² A Cu(II) complex containing a chiral bis(oxazoline) ligand has been immobilized on mesoporous silica and the catalyst was employed for asymmetric benzoylation of 1,2-diphenyl-1,2-ethanediol which produces a highly enantioselective product using benzoyl chloride under mild conditions.⁹³ There are many other reports on the alkylation of aromatics using alcohols over metal doped mesoporous silica. Sulphated Zr–MCM–41 and Zr–MCM–48 solid acid catalysts have successfully catalyzed attached substituent of phenol aromatic ring in presence of tert-butyl alcohol to produce 4-tert-butyl phenol as major product with above 90% yield at 413 K temperature.⁹⁴

b. Esterification and transesterification reactions. Esterification and transesterification reactions are widely employed today for biofuel production, which is an alternative and renewable source of fuel in most of the developed countries. The condensation of carboxylic acid with alcohol in the presence of any acid catalyst was named as esterification by the German chemist Emil Fischer whereas the conversion of one ester to another carboxylic acid ester using alcohol and acid or base as a catalyst is called transesterification (Fig. 9). Generally, the reactions of triglycerides i.e. triesters of glycerol or any other fatty acids with short chain alcohols like methanol or ethanol are categorized as biodiesel reactions.⁹⁵

Fig. 9 (A) Esterification and (B) transesterification reactions for biodiesel production by acid or base catalysts.

Both Lewis and Brønsted acids are used as catalysts for the esterification of long chain fatty acids and the equilibrium of the reversible reaction can be right shifted towards the ester conversion by removing water from the reaction medium through azeotropic distillation or using molecular sieves or using an excess of alcohol.¹⁹ Replacing previously used strong Brønsted acids like H₂SO₄, p-TsOH (p-toluenesulphonic acid) various environmentally non-hazardous, recyclable functionalized mesoporous materials have been designed, which are employed successfully as heterogeneous catalyst for the biodiesel production. The mechanism of an esterification reaction over solid acid catalysts can proceed via activation of the carboxylic acid by the catalyst, then nucleophilic attack by the alcohol on the activated carbonyl carbon to form a tetrahedral intermediate and finally formation of an ester with elimination of one molecule of water.¹⁹ Esterification of a long chain fatty acid with alcohol and vice versa is observed over mesoporous zirconium oxophosphate material which not only produces the corresponding ester in high yield but also shows high reusability up to the 5th cycle, the catalytic activity being attributed to the large surface area and large number of acidic sites located at the surface of the phosphate material.⁸⁶ In Fig. 10, a simple scheme for the synthesis of this Zr-oxophosphate material is shown along with its application in the proposed mechanistic pathway for an acid catalyzed reaction. The presence of the acidic sites necessary for catalytic activity in the Zr-catalyst has been investigated and quantitatively estimated by using the temperature-programmed desorption of ammonia (NH₃-TPD) measurement.⁸⁶ The TPD result suggested that with the increase in surface Brønsted acidity a higher yield of the biodiesel products was obtained. Esterification over periodic mesoporous silica materials has been described in the review published by Yang et al. in 2009.⁹⁶ MCM-48 supported tungstophosphoric acid acts as a heterogeneous catalyst for esterification of a number of fatty acids like palmitic, lauric, stearic, isostearic, myristic etc. with alcohols, e.g. cetyl, butanol, hexanol, octanol etc. in 11 MPa super critical CO₂ medium at a temperature of 373 K to produce esters in an appreciable amount which increases with the increasing chain length of the fatty acids.⁹⁷

Fig. 10 A simple schematic pathway of the synthesis of mesoporous zirconium oxophosphate and a proposed catalytic cycle for the esterification reaction over this catalyst.

Transesterification reactions involve an acid or base mediated reaction where increasing the electrophilicity of a carbonyl carbon of fatty acids or the nucleophilicity of an attacking alcohol can improve the reaction rate considerably.⁹⁵ Sulphonated porous zinc phosphonate is a example of a green catalyst, which provides an eco-friendly route for biodiesel production via transesterification of long chain fatty acids in the presence of methanol at room temperature.⁸⁵ Thus, ordered Zn-doped mesoporous silica²⁰ or amine-functionalized mesoporous organosilica⁹⁸ or mesoporous polyoxometalate–tantalum pentoxide composite, H₃PW₁₂O₄₀/Ta₂O₅ (ref. 99), are very active acid catalysts to give a satisfactory yield of the respective esters under suitable conditions. In the mesoporous H₃PW₁₂O₄₀/Ta₂O₅ composite, (≡TaOH₂)_n⁺[H_3−nPW₁₂O₄₀]ⁿ⁻ species formed at the catalyst surface of the composite via a Ta–O–W bond facilitates the electron transfer from the terminal oxygen atoms of W=O groups to Ta₂O₅; encouraging a good amount of proton release to ensure high Brønsted acidity of the catalyst responsible for the biofuel conversion.⁹⁹ On the contrary, transesterification using base catalysts has been efficiently carried out over Mn-doped ceria–silica and MgO-functionalized mesoporous silica. etc. Mn doped ceria–silica catalyzes the transesterification of methyl benzoate, ethyl cyanoacetate and ethyl chloroacetate with methanol, butanol and octanol under solvent-less mild reaction conditions. The presence of basic sites in the MgO–silica was determined by CO₂ sorption measurements and the sites help to produce biofuel from vegetable oil in the presence of ethanol, achieving a conversion level of above 90% within 5 h reaction time.¹⁰⁰

c. Acetalization reactions. Acetalization is widely used in the synthesis of natural products or industrially important chemicals which are employed for the protection of active functional groups like aldehydes or ketones in a molecule and thus useful in designing a multi-functional molecule. Acetal formation occurs through the nucleophilic addition of an alcohol to carbonyl compounds in the presence of an acid.^101,102 Mesoporous solids are used extensively as acid catalysts for acetalization, e.g. methanol was used to acetalize cyclohexanone at room temperature over a mesoporous silica catalyst, which shows remarkable shape selectivity and a good correlation of catalytic activity with pore diameter of the corresponding mesopores.¹⁰³ Acetalization of cyclohexanone using methanol to produce dimethyl ketal is also successfully carried out over mesoporous zirconia functionalized with –SO₄ gr. and a conversion of over 90% with 100% selectivity is observed within a very short reaction period (45 min).¹⁰¹ Glycerol is also used for acetalization of acetone over a stable Ni–Zr supported mesoporous carbon catalyst, when selectively two products, 5-membered solketal and six-membered acetal, were obtained under solvent-less conditions (Fig. 11).¹⁰⁴ Another interesting acetalization process is the reaction of heptanal with 1-butanol over various sulphonic acid functionalized mesoporous organosilicas.¹⁰⁵ Three types of organosilica, viz. arene sulfonic acid ethane-silica (AS-MES), mercaptopropyl ethane-silica (Pr-SH-MES) and arene sulfonic acid functionalized SBA-15 were prepared using Pluronic non-ionic surfactant, P123 and then post-functionalized with an –SO₃H group via treatment with a H₂O₂–H₂SO₄ solution. These acid sites in the organosilica help to acetalize heptanal with butanol to form the corresponding acetal.¹⁰⁵

Fig. 11 Acetalization reaction between glycerol and acetone over a Ni–Zr supported mesoporous activated carbon catalyst.

d. Aldol condensation. Carbon–carbon bond formation reactions in various large biomolecule and industrial fine chemical syntheses are largely dependent on aldol condensation reactions, a well-known organic reaction of an enol or enolate ion with a carbonyl compound in order to form a β-hydroxyaldehyde or β-hydroxyketone or ‘aldol’ (aldehyde + alcohol), followed by dehydration of the intermediate to produce an α,β-unsaturated carbonyl compound. The reaction is catalyzed by both acid and base to form enol and enolate ion, respectively. Conventionally used Brønsted bases like NaOH, Ca(OH)₂, Ba(OH)₂, etc. and acids such as acetic acid, and Lewis acids like M(Otf)_n (M = Zn, Cu, Pb, Sc etc.) produce hazardous, toxic by-products causing a lot of environmental pollution. Hence, aldol reactions over mesoporous catalysts in heterogeneous media are in high demand.¹⁰⁶

Mesoporous carbon can act as a good support for metal nanoparticles or metal oxides to improve catalytic performances, e.g. Mg–Zr mixed oxide supported on mesoporous carbon catalyzes the cross-aldol reaction of furfural and acetone followed by hydrogenation–dehydration to form C₈ and C₁₃ alkanes at a temperature of 323 K, the conversion and selectivity being higher than when bulk Mg–Zr mixed oxide is used.¹⁰⁷ The reaction mechanism and the kinetic model of aldol condensation over the distributed basic sites of this supported catalyst are also explained by the authors. Bai et al. have synthesized mesoporous γ-Al₂O₃ based on a cation–anion double hydrolysis method and the material was proven to be truly efficient for the aldol reaction of cyclohexanone carried out in a water-separating device in the form of water–cyclohexanone azeotrope at a temperature of 438 K for 2 h when the conversion was as high as ∼70%. The remarkably high catalytic activity of crystalline γ-Al₂O₃ could be attributed to the huge surface area, precise mesopore diameter, absence of anionic species as well as the surface basicity generated due to aluminium and oxygen vacancies.¹⁰⁸ Based on the experimental results they also proposed a suitable mechanism for the formation of α,β-unsaturated ketone involving the absorption of α-H of cyclohexanone by the aluminium vacancies. Aldol condensation of benzaldehyde and heptanal to form jasminaldehyde under solvent free conditions is reported over novel Mg–Al mixed oxide supported on hexagonal mesoporous silica (Fig. 12).¹⁰⁹ The bi-functional behaviour of this catalyst, where the weak acid sites activate the aldehyde by protonation and the basic sites favour the formation of the enolate heptanal intermediate, helps in carrying out the reaction successfully.¹⁰⁹ In this context a completely metal-free organocatalyst has been developed via heterogenizing (S)-proline on MCM-41, which catalyzes the asymmetric aldol condensation of p-nitrobenzaldehyde and 2,2-dimethyl-1,3-dioxan-5-one under the influence of different solvents ranging from hydrophobic to hydrophilic, proving hydrophilic polar media as more suitable for catalysis.¹¹⁰

Fig. 12 The synthesis of hexagonal mesoporous silica supported Mg–Al mixed oxide catalyst and its application for the aldol condensation of benzaldehyde with heptanal.

e. Knoevenagel condensation. Unlike aldol reactions involving the condensation of two carbonyl compounds, Knoevenagel condensation involves the reaction of a carbonyl compound with a compound containing an active methylene group followed by dehydration to form a C–C bond resulting in an α,β-unsaturated compound. The reaction is generally mediated by a basic catalyst and is claimed to be one of the most important environmentally benign C–C bond formation reactions, because water is the only by-product produced in this condensation.¹ Instead of using soluble amine bases in homogeneous media, any amine based heterogeneous support or metal support with basic sites will be more suitable as a non-toxic pollution free catalyst for such reactions.

Mesoporous Ce_xZr_1−xO₂ solid consisting of nanometer sized particles was synthesized via a sol–gel method and it showed excellent chemoselectivity for the classical Knoevenagel condensation of benzaldehyde and malononitrile owing to the presence of pores of a large size (∼10 nm) and both surface acidic and basic sites.¹¹¹ The Knoevenagel reaction under mild conditions is also reported over ordered mesoporous Ni–Al mixed oxide obtained from Ni–Al-layered double hydroxides (LDHs) by ultrasonic irradiation using Pluronic-F127 surfactant. The catalyst exhibits high selectivity for the product with good reusability without significant loss of activity.¹¹² Mesoporous oxides without basic sites can also be used for Knoevenagel reaction after proper functionalization of the oxide surface with amine groups. By co-condensation of 3-aminopropyltrimethoxy silane and zirconium precursors in CTAB surfactant solution, mesoporous amine grafted zirconia can be prepared and used for condensation of benzaldehyde and a malonic ester, diethyl malonate, in methanol solvent at room temperature to produce cinammic acid exclusively.¹¹³ Besides this, as metal-free organocatalysts, guanidine base immobilized on mesoporous silica SBA-15 and urea embedded mesoporous polymer are highly recommended for Knoevenagel reaction.^114,115 Two types of Knoevenagel condensation, one between benzaldehyde and ethyl cyanoacetate, and another between cyclohexanone and benzylcyanide have been reported over basic guanidine functionalized silica at RT as well as at an elevated temperature.¹¹⁴ On the contrary, mesoporous urea-functionalized polymers synthesized through the non-ionic F127 surfactant-directed urea–phenol–formaldehyde oligomer self-assembly approach have been utilized for Knoevenagel reaction of aromatic aldehyde and ethyl cyanoacetate at mild temperatures in a non-hazardous aqueous medium. In Fig. 13, a simple pictorial representation for the synthesis of this mesoporous urea-polymer along with a proposed mechanism for the catalytic reaction over this polymer is shown. The synergic effect between the urea active sites embedded on the mesopore wall and their neighbouring surface phenolic –OH groups in the mesoporous support accounts for the success of this reaction along with the good stability of the catalyst inhibiting the leaching of active species as documented through successful reuse up to six reaction cycles.¹¹⁵

Fig. 13 The synthesis of urea functionalized mesoporous polymer and Knoevenagel condensation of substituted benzaldehyde with ethyl cyanoacetate over this polymer.

f. Baeyer–Villiger oxidation. Baeyer–Villiger (BV) oxidation involves the oxidative cleavage of a carbon–carbon bond adjacent to a carbonyl group, which forms esters and lactones from acyclic ketones and cyclic ketones, respectively. This reaction is important for the industrial production of bioactive chemicals and generally proceeds in the presence of peracids like C₆H₅CO₃H and CF₃CO₃H etc. or non-toxic, less hazardous hydrogen peroxide along with Lewis acidic or sometimes basic catalysts.¹¹⁶ The reaction has been successfully employed for the conversion of cyclohexanone, cyclooctanone, ethyl methyl ketone and methyl isopropyl ketone to their respective lactones and esters in the presence of a non-siliceous mesoporous Mg–Al mixed oxide catalyst using H₂O₂.¹¹⁷ The high surface area and nanoscale porosity alone with a considerably good amount of basic sites present in the porous MgAl₂O₄ catalyst accelerate the reaction process. However, the above reaction was carried out in benzonitrile as a co-solvent at an elevated temperature (343 K) for which formation of benzamide is observed as a by-product and this minimises the selectivity of the desired lactone and ester. The conversion and selectivity of cyclic and acyclic ketones with or without using benzonitrile is shown in Fig. 14. More convenient and mild conditions have been employed by Jeong et al. who carried out the BV reaction over metal doped PMO at quite a lower temperature (313 K) in the presence of molecular oxygen (O₂)/benzaldehyde system. For highly selective conversion of cyclohexanone to ε-caprolactone they used different metalloporphyrin (Fe, Cu, Sn) bridged periodic mesoporous organosilica materials among which an Fe-catalyst has been proven to be the most promising and efficient catalyst for this reaction.¹¹⁸ Using O₂/benzaldehyde as an oxidizing agent, an environmentally benign green catalytic route has been reported over a mesoporous zirconium phosphate material which exhibits a strong ability to catalyze the conversion of cyclic ketones, even bulkier ones such as adamantanone, to the corresponding lactones with a high yield and 100% selectivity at RT under solvent-free conditions.¹¹⁹ A free-radical reaction pathway facilitated by the Lewis acidic sites present in this mesoporous Zr-phosphate catalyst has been suggested. The catalyst is chemically stable and showed good recyclability, suggesting that the procedure is economically advantageous for industrial purposes.

Fig. 14 Baeyer–Villiger oxidation of cyclic and acyclic ketones over a mesoporous Mg–Al mixed oxide along with the conversion–selectivity representation in a bar diagram.

g. Beckmann rearrangement. Beckmann rearrangement, named after the German chemist Ernst Otto Beckmann, is another important acid-induced reaction in organic chemistry to form lactams and amides from cyclic and acyclic oximes, respectively. The reaction is generally catalyzed by strong acids like polyphosphoric acid and sulphuric acid etc. Instead of using these corrosive acids there are some reports of Beckmann rearrangements over mesoporous acid catalysts, but most of those reactions proceed under harsh conditions like high temperatures in the vapor or gas phase.¹²⁰ Liquid phase rearrangement of cyclohexanone oxime to form ε-caprolactam (with about 50% yield) was observed in a series of solvents over acidic sites of Al doped mesoporous MCM-41 molecular sieves by Ngamcharussrivichai et al. although the conversion and selectivity level are low.¹²¹ A few years later, sulphonic acid functionalized mesoporous organosilica with an SBA-15 type nanostructure was employed for this reaction, but the yield and selectivity of lactam did not improve by much. But the material is quite interesting as reports on heterogeneous metal free organocatalyst mediated catalytic transformations are comparatively scarce.¹²² The above acid–base reactions are summarized in Table 3. These acid–base reactions mentioned here are a few examples of a wide range of mesoporous catalyst-mediated liquid phase organic transformations reported in the literature.

Table 3 Different liquid phase acid–base reactions mediated by mesoporous materials

Type of reactions	Catalysts	Reaction involved	Product yield (%)	Ref.
Friedel–Crafts reaction	Mesoporous perovskite ZnTiO3	Reaction of p-xylene, toluene, benzene with benzyl chloride	90–93	19
	Mesoporous iron phosphate	Benzylation of benzene, toluene, p-xylene	100	88
	Silicotungstic acid-loaded mesoporous alumina	Benzoylation of anisole with p-toluoyl chloride	80–100	89
	Mesoporous sulphated Nb oxide	Reaction of toluene and anisole with benzyl alcohol	100	90
	Mesoporous Mo-doped carbon sphere	Alkylation of toluene using benzyl alcohol	100	91
	Mesoporous GaSBA-1	Benzylation of various aromatics using benzyl chloride	95–100	92
	Cu(II) complex–bis(oxazoline) loaded mesoporous silica	Asymmetric benzoylation of 1,2-diphenyl-1,2-ethanediol	29–40	93
	Sulphated Zr–MCM-41 and Zr–MCM-48	Reaction of phenol with tert-butyl alcohol	18–91	94
Esterification	Mesoporous ZnTiO3	Reaction of palmitic, lauric, oleic acid with methanol	73–92	19
	Mesoporous zirconium oxophosphate	Esterification of long chain fatty acid with alcohol and vice versa	46–91	85
	Tungstophosphoric acid-loaded MCM-48	Esterification of palmitic, lauric, stearic, isostearic, myristic acids with different alcohols	48–97	97
Trans-Esterification	Sulphonated porous zinc phosphonate	Transesterification of long chain fatty acids with methanol	35–81	84
	Zn-doped mesoporous silica	Transesterification of ECA, EClA, EAA, EA etc.	25–94	20
	Amine-grafted OMS	Transesterification of glyceryl tributyrate with methanol	75–95	98
	H3PW12O40/Ta2O5 porous composite	Reaction of palmitic, oleic, linoleic acid with methanol	20–68	99
	MgO-grafted mesoporous silica	Vegetable oil with ethanol	72–96	100
Acetalization reactions	Mesoporous silica	Acetalization of cyclohexanone with methanol	87–89	103
	Mesoporous sulphated zirconia	Reaction of cyclohexanone with methanol, ethylene glycol	72–98	101
	Ni–Zr supported mesoporous carbon	Reaction of acetone and glycerol	68–100	104
	Mesoporous SO3H-loaded organosilica	Reaction of heptanal with 1-butanol	39–97	105
Aldol condensation	Mg–Zr supported mesoporous carbon	Cross-aldol reaction of furfural and acetone	98	107
	Mesoporous γ-Al2O3	Homo aldol reaction of cyclohexanone	60–80	108
	Mg–Al mixed oxide loaded HMS	Condensation of benzaldehyde and heptanal	100	109
	(S)-Proline loaded MCM-41	Reaction of p-nitrobenzaldehyde & 2,2-dimethyl-1,3-dioxan-5-one	20–96	110
Knoevenagel condensation	Mesoporous CexZr1−xO2	Condensation of benzaldehyde and malononitrile	72–90	111
	Mesoporous Ni–Al mixed oxide	Reaction of benzaldehyde and malononitrile	99	112
	Mesoporous amine grafted zirconia	Condensation of benzaldehyde with diethyl malonate	80–98	113
	Guanidine base loaded SBA-15	Condensation of cyclohexanone with benzyl cyanide	30–65	114
	Urea embedded porous polymer	Condensation of benzaldehyde with ECA, EAA, etc.	21–99	115
Baeyer–Villiger oxidation	Mesoporous Mg–Al mixed oxide	BV oxidation of cyclohexanone, cyclooctanone, methyl isopropyl ketone, ethyl methyl ketone	62–89	117
	Metalloporphyrin-bridged PMO	Selective conversion of cyclohexanone to ε-caprolactone	10–100	118
	Mesoporous Zr-phosphate	Conversion of cyclic ketones like adamantanone to lactones	96	119
Beckmann rearrangement	Al doped MCM-41	Cyclohexanone oxime	46–50	121
	SO3H-grafted SBA-15	Cyclohexanone oxime to ε-caprolactam	14–52	122

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4.3 Different coupling reactions over mesoporous catalysts

The involvement of mesoporous solids for hydrosilylation, cyanosilylation, Diels–Alder, Suzuki reactions etc. in mediating C–C, C–N, C–H, C–S and C–O coupling and cross coupling reactions in heterogeneous media to synthesize various value-added organic fine chemicals is a widely recognized area of research nowadays. Transition metals like Cu, Ni, Fe etc. or noble metals like Au, Pt, Pd etc. supported mesoporous siliceous and non-siliceous materials, polymers or carbons, play an imperative role in these types of organic transformations.¹²³ A concise report on all these reactions is mentioned in the following sections and these are also summarized in Table 4.

Table 4 Various organic coupling reactions carried out under liquid phase conditions over mesoporous catalysts

Type of reactions	Reaction name	Reaction involved	Catalysts used	Product yield (%)	Ref.
C–C coupling reaction	Sonogashira coupling	Reaction of iodobenzene & phenylacetylene	PdNP loaded mesoporous starch	90–99	126
		Substituted aryl halides with phenylacetylene	Pd-grafted triazine functionalized mesoporous polymer	60–90	125
		Reaction of substituted aryl halides & phenylacetylene without Cu co-catalyst	Pd-loaded periodic mesoporous silica	72–90	124
		Reaction of substituted benzoyl chloride and phenyl acetylene	Mesoporous tin silicates	76–97	127
	Suzuki or coupling	Reaction of substituted aryl halide with sodium salt of phenyltrihydroxyborate	Pd-incorporated triazine functionalized mesoporous polymer	60–95	125
		Reaction of substituted aryl halides with substituted phenylboronic acids	Pd(II) complex anchored 2D-HMS	63–99	128
		Reaction of different aryl halides with phenyl boronic acids	Pd@imidazolium-functionalized SBA-15	87–99	129
		Substituted aryl halides with substituted phenyl boronic acids	PdNP@mesoporous carbon	>99	130
		Coupling of substituted aryl halides with different substituted phenyl boronic acids	Pd grafted mesoporous ionic-liquid framework organosilica	55–99	131
		Coupling of substituted aryl halides with different substituted phenyl boronic acids	Au Schiff-base complex anchored mesoporous support	84–95	132
	Heck reaction	Coupling of substituted aryl halides with acrylic acid or styrene	Pd loaded mesoporous polymer	50–95	125
		Coupling of substituted aryl iodides with styrene	Pd@mesoporous triallylamine polymer	88–95	133
		Reaction of substituted aryl iodides with substituted olefins	Pd supported TMG ionic liquid modified SBA-15	60–94	134
		Reaction of substituted aryl halides (Cl, Br, I) with Styrene	Pd supported NiFe2O4	72–97	135
	Hiyama coupling	Cross couplings of various substituted aryl halides with alkyl and aryl trimethoxysilane	Pd loaded phloroglucinoldiimine modified PMO	65–95	124
		Substituted aryl halides with different trimethoxysilane	Pd NP loaded SBA-15	70–95	136
	Ullmann reaction	Reaction of iodobenzene to form biaryl	Pd doped Ph-functionalized MCM-41	70–88	137
		Homo coupling of chlorobenzene in water	Pd@mesoporous silica–carbon nanocomposites	100	138
		Reaction of various aryl iodides	AuNP@phenylene containing PMO	80–95	139
	Stille reaction	Organotin compound with a sp^2-hybridized organic halide	Pd loaded triazole & benzene grafted PMO	60–80	140
	Negishi coupling	Cross coupling of an organozinc compound with an organic halide	—	—	141
C–S coupling reaction	—	Reaction of 4-chlorothiophenol & substituted aryl iodides	Mesoporous NiO–ZrO2 nanocrystals	39–89	145
		Reaction of thiols with different aryl halides	Mesoporous Cu–Fe-hydrotalcite	50–90	143
		S-arylation of aryl iodides with thiols	CuO well-dispersed on mesoporous silica	66–97	147
		Reaction of substituted aryl iodide with thiophenol	Cu(OAc)2 loaded mesoporous furfural functionalized silica	75–88	144
		Coupling of aryl halides, benzyl bromides with thiourea	Cu-grafted imine functionalized SBA-15	80–88	146
Three component coupling	Biginelli condensation	One pot reaction of aryl aldehyde, EAA and urea	Al–MCM-41	68–89	152
		Reaction of substituted aryl aldehyde, EAA and urea	Fe3O4 nanoparticles@cysteine loaded SBA-15	78–85	153
		One pot reaction of aryl aldehyde, EAA and urea	Phosphonic acid functionalized 2D HMS	83–92	154
	A^3 coupling	Coupling of aldehyde, alkyne and amine in glycol	AgNP doped mesoporous SBA-15	40–95	150
		Three component reaction of aldehyde, alkyne and amine	Oxidized Cu NP supported on titania	50–88	149
		Coupling of substituted aryl aldehyde, substituted phenylacetylene with different amine bases	Au@ionic liquid framework PMO heterogeneous support	75–88	151

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a. Reactions involving C–C coupling. Replacing metal based homogeneous catalysts which suffer from the drawbacks of low catalyst recovery, lack of regeneration of the original activity and corrosiveness, metal based heterogeneous mesoporous supports with exceptionally high surface area and tunable pore opening in nanoscale dimensions are highly desirable for different C–C homo or cross coupling reactions¹²⁴ as mentioned below.

The Sonogashira coupling reaction is a highly demanding reaction in organic synthesis, which is usually catalyzed by Pd metal to form a C–C bond between a terminal alkyne and an aryl or vinyl halide in the presence of an amine base and with or without a Cu co-catalyst.¹²⁴ Instead of using Pd metal directly, Pd-grafted mesoporous silica, polymer or carbons are highly appreciated as non-corrosive, stable, environmentally-friendly, highly reusable catalysts for the production of aryl or vinyl substituted alkynes using the Sonogashira reaction. Using Cu catalyst the mechanism is initiated with the simultaneous formation of copper acetylide complex when the copper ion catalyst attacks on the terminal alkyne in presence of base and oxidative addition of organic halides to Pd(0) occurs to form a Pd(II) intermediate complex. The Pd(II) intermediate on reaction with copper acetylide followed by reductive elimination gives the final C–C coupled product with regeneration of the Pd(0) catalyst.¹²⁵ The reaction is reported to be highly successful over a Pd-grafted triazine functionalized ordered mesoporous polymer material¹²⁵ and a Pd nanoparticles loaded mesoporous polysaccharide-derived material,¹²⁶ etc. The catalysis in the former case (Pd–polysaccharide) was carried out without a Cu co-catalyst under microwave conditions at a temperature of 403 K, while the latter (Pd–triazine polymer) employs a much lower temperature (363 K) and a green solvent, water, for the coupling of substituted aryl halides with phenylacetylene in the presence of triethanol amine and a Cu catalyst. A similar experiment was carried out very efficiently by Modak et al. with a Pd-loaded periodic mesoporous silica catalyst in aqueous media at a temperature of 393 K using hexamine base and without any Cu catalyst.¹²⁴ The yield was very satisfactory in this Cu-free Sonogashira reaction and a suitable mechanism has also been proposed showing the coupling between a Pd(II) intermediate and an alkyne to form an alkyne co-ordinated Pd complex which was followed by the attack of a base producing the final product with regeneration of the catalyst.¹²⁴ A comparative mechanism of Sonogashira coupling without and with a Cu co-catalyst over a Pd@PMO material and a mesoporous Pd@triazine polymer, respectively, has been proposed in Fig. 15. An extraordinary approach was made by Reddy et al. who tested an acyl Sonogashira coupling reaction using acyl halide (instead of aryl halide) over a mesoporous tin silicate material (an alternative to a Pd catalyst) synthesized by a simple surfactant assisted soft-templating route.¹²⁷ The reaction was performed at room temperature under solvent-free conditions using triethyl amine as a base as well as a solvent and the catalyst was highly stable for reuse for several cycles to produce a high yield of ynones.

Fig. 15 A comparative mechanism proposed for Sonogashira coupling without and with a Cu co-catalyst over Pd@PMO and mesoporous Pd@triazine polymer, respectively.

The Suzuki or Suzuki–Miyuara cross coupling reaction first published by Akira Suzuki in 1979 is the organic reaction of an organoboronic acid with an aryl or alkyl halide in the presence of Pd(0) to form a C–C coupled product with or without using a base.¹²⁴ Compared to the homogeneous counterparts, Pd-grafted mesoporous catalysts provide ease of product separation, catalyst recovery by simple filtration and excellent recycling efficiency. Recently, a number of reports have been revealed regarding this Suzuki coupling reaction studied in the presence of Pd immobilized heterogeneous mesoporous silica or polymer using mild reaction conditions. The reaction proceeds through oxidative addition of a halide to Pd(0) to form an organopalladium(II) halide. This species on reaction with an organic base followed by a nucleophilic attack by a boronate species via transmetalation forms an organopalladium(II) species. Finally, following reductive elimination of the desired biaryl or divinyl the original palladium(0) catalyst is restored.^128,129 Ordered mesoporous carbon with loaded Pd nanoparticles has also been utilized for a Suzuki coupling reaction in a DMF–water solvent at a temperature of 423 K (Fig. 16).¹³⁰ A base free route is suggested using a sodium salt of a boronic species instead of an organoboron compound (Fig. 16). Karimi et al. have carried out the reaction with Pd-grafted mesoporous ionic-liquid framework organosilica in aqueous media in the presence of K₂CO₃ under very mild conditions.¹³¹ The reaction is very successful at room temperature giving a satisfactory yield. In addition to Pd, Au metal anchored mesoporous support also has high efficiency and true heterogeneity for Suzuki homo coupling reactions under mild conditions, reports of which are not quite rare in the literature.¹³²

Fig. 16 Suzuki–Miyaura cross coupling reactions (A) with K₂CO₃ base over Pd doped ordered mesoporous carbon and (B) without base over Pd grafted mesoporous triazine polymer.

Another example of C–C cross coupling is the Heck or Mizoroki–Heck reaction involving an interaction between an unsaturated halide (or triflate) with an alkene in the presence of a base and a palladium catalyst or a palladium thioetherific-tethered catalyst to form a substituted alkene via C–C bond formation. Allowing substitution on a planar system, the reaction is very much important in organic synthesis with applications ranging from natural product and bioactive compound synthesis to the development of organic electronics.¹³³ Bases used in this reaction are generally triethylamine, potassium carbonate and sodium acetate, etc. The reaction occurs through the oxidative addition of a halide with a Pd(0) species to form a Pd(II) halide intermediate, then formation of a π-complex with an alkene, next some rearrangement and β-elimination to form a new palladium-alkene π-complex and finally reductive elimination to give the final product with regeneration of the catalyst.¹³³ The reaction in a non-hazardous water–ethanol mixture using a Pd loaded mesoporous polymer shows a high percentage of conversion in a very short time,¹²⁵ however there is always a positive effort to use more green solvents like water for such a reaction and a bit lower reaction temperature. Mondal et al. have successfully employed a Pd-loaded triallylamine based mesoporous polymer catalyst for the Heck coupling reaction.¹³³ The scheme in Fig. 17 illustrates a simple route for the synthesis of this Pd-based mesoporous polymer and a Heck coupling reaction over this catalyst. Another convenient and environmentally friendly method is the reaction in solvent-free conditions as reported by Ma et al. They employed a highly reusable catalyst, which consisted of Pd supported on 1,1,3,3-tetramethylguanidinium (TMG) ionic liquid modified mesoporous SBA-15, for Heck coupling of different aryl halides with substituted olefins at a temperature of 413 K to produce C–C coupled arylated olefins. The yield was high enough with respect to the reaction time and the amount of catalyst showing excellent activity for several reaction cycles without significant metal leaching.¹³⁴ On the contrary, Pd supported on NiFe₂O₄ catalyst, highly efficient for Mizoroki–Heck reactions of chloro, bromo and iodo substituted aryl halides with styrene in DMF solvent, has the advantage of magnetic separation from the reaction mixture.¹³⁵

Fig. 17 Mizoroki–Heck coupling reaction over mesoporous Pd doped triallylamine based polymer.

Hiyama coupling, another palladium-catalyzed cross-coupling reaction used in organic natural product synthesis involves reaction of organosilanes with organic halides to form a carbon–carbon bond (C–C bond) in the presence of an activating agent such as a fluoride ion or a base. The use of an expensive fluoride source and hazardous organic solvents like DMF or DMSO etc. has recently been modified by Modak et al. who suggested a suitable mechanistic pathway for the Hiyama coupling reaction without the need for any fluoride source at a moderate temperature over a newly developed Pd-grafted PMO.¹²⁴ They used NaOH to maintain the alkaline conditions and water as a solvent to make the process more environmentally-friendly. The heterogeneity of the Pd-catalyst was proven by different tests like a hot filtration test, three phase test and a solid phase poisoning test which showed minimum metal leaching with retention of catalytic activity. Recently, another interesting catalyst is a Pd nanoparticles loaded SBA-15 material fabricated through deposition of Pd nanoparticles in the micropores of SBA-15, consisting of hydrophobic trimethylsilyl (TMS) or triphenylsilyl (TPS) group functionalized mesopores.¹³⁶ In Fig. 18, we have shown a scheme for the synthesis of this novel catalyst. This nanocatalyst shows excellent activity for the Hiyama cross-coupling between various aryl triethoxysilanes and aryl halides under relatively mild reaction conditions (373 K) to form different biphenyl derivatives.

Fig. 18 A simple synthesis scheme for Pd nanoparticles doped trimethylsilyl group functionalized SBA-15 and the Hiyama coupling reaction over this material.

The Ullmann reaction, named after scientist Frietz Ullmann, is a homo coupling reaction of aryl halides to form a symmetric biaryl derivative in the presence of Cu, Pd or Au catalysts (Fig. 19). A number of reports have been published about the Ullmann reaction in aqueous media to provide a green route for the synthesis of biphenyls over heterogeneous catalysts. Ullmann reaction of iodobenzene to form a biaryl with a maximum ∼80% yield and high selectivity in water as a solvent was carried out over a novel Pd doped Ph and Al-functionalized MCM-41 catalyst.¹³⁷ Coupling of substituted chlorobenzene over heterogeneous palladium catalysts supported on ordered mesoporous silica–carbon nanocomposites was carried out both at normal and elevated temperatures showing product formation increasing with temperature.¹³⁸ The reaction is also catalyzed by a novel recyclable catalyst, comprising Au nanoparticles embedded bifunctional phenyl containing periodic mesoporous organosilica (Au@PMO),¹³⁹ with an unexpected high yield (above 90%) and negligible leaching of metal (Fig. 19).

Fig. 19 Ullmann homo coupling reaction over Au incorporated PMO and Stille reaction over Pd@PMO.

The Stille reaction, or Migita–Kosugi–Stille coupling, used extensively in organic synthesis is the reaction of an organotin or organo stannanes compound with an sp²-hybridized organic halide catalyzed by palladium usually performed under an inert atmosphere (Fig. 19). The mechanism involves three basic steps like in the coupling reactions: oxidative addition, transmetalation and reductive elimination. A highly efficient, reusable Pd mesoporous catalyst reported by Singh et al. was synthesized by grafting a Pd anchored triazole ligand onto the surface of a functionalized benzene containing periodic mesoporous organosilica with a uniform hexagonal arrangement prepared under basic conditions using CTAB surfactant.¹⁴⁰

Beside these reactions, there are also Negishi coupling involving C–C cross coupling of an organozinc compound with an organic halide over a nickel or palladium based homogeneous or heterogeneous catalyst,¹⁴¹ and pinacol coupling, which is an acid catalyzed homo coupling of a carbonyl compound to form a 1,2-diol.¹⁴² Though the reactions over metal doped mesoporous supports have not yet been reported, extensive research is ongoing in an effort to design suitable mesoporous catalysts for carrying out these coupling reactions under mild conditions.

b. Reaction involving C–S coupling. In order to design new target molecules in synthetic organic, biological and pharmaceutical research via carbon–sulfur bond formation, the C–S coupling reaction of aryl or substituted aryl halides with different sulfur containing reagents, e.g. thiol,¹⁴³ thiophenol,¹⁴⁴ thiourea,¹⁴⁵ etc., has drawn great attention from modern scientists. Replacing hazardous homogeneous catalysts containing transition metals like Fe, Ni, Pd, Cu and Co; metal based heterogeneous supports are much more effective for C–S coupling reactions to form diaryl sulfides in various polar organic solvents and an inert atmosphere (Fig. 20). Various metals, mixed metal oxide nanoparticles and metal complex supported mesoporous catalysts have been reported up to now suggesting that they provide an easy, reusable and simple mechanistic pathway for this aryl-sulfur coupling reaction.¹⁴⁴

Fig. 20 A general scheme for a C–S coupling reaction over a mesoporous solid catalyst.

The mesoporous Cu–Fe-hydrotalcite solid catalyst is a ligand free, novel, environmentally green catalyst for C–S coupling reactions using thiols with different aryl halides in DMF solvent and with K₂CO₃ base. The homogeneous mixed phase of two oxides (CuO and Fe₂O₃), good BET surface area (123 m² g⁻¹) and large pore diameter (∼15 nm) mainly contribute to the observed high conversion of products achieved within a reaction time of 4–10 h.¹⁴³ CuO well-dispersed on mesoporous silica prepared by a sol–gel route is a highly reusable heterogeneous support for S-arylation of aryl iodides with thiols in the presence of Cs₂CO₃ base and DMSO solvent. Cu(Oac)₂ loaded on organic inorganic hybrid mesoporous silica functionalized with furfural groups is also applied for aryl-sulfur coupling of thiophenol with aryl iodide in the presence of K₂CO₃ base and DMF solvent.¹⁴⁴ The yield observed for different substituted diaryl sulfides was very satisfactory with proper reusable efficiency shown up to the 5th cycle. A synthesis scheme of mesoporous Cu grafted furfural functionalized organic–inorganic hybrid silica is shown in Fig. 21. A probable mechanism has also been suggested showing generation of Cu(0) active species from Cu(II) grafted organosilica in the presence of a base, followed by the consecutive oxidation addition of aryl iodide and thiophenol to form another intermediate. As observed in the catalytic cycle the intermediate formed releases an aryl-sulfur coupled product with regeneration of the active species via a reductive elimination step (Fig. 21).¹⁴⁴

Fig. 21 Synthesis scheme of mesoporous Cu grafted furfural functionalized organosilica and its role in the mechanistic catalytic C–S cross coupling reaction cycle.

To avoid the toxicity and hazards of the polar solvents used in different C–S coupling reactions, we explored the reaction in a completely green solvent, water, over NiO–ZrO₂ self-assembled mesoporous nanocrystals at a relatively lower temperature (353 K).¹⁴⁵ 4-Chlorothiophenol is used as a sulfur source and different substituted aryl iodides are used to produce value added aryl-sulfur compounds with an appreciable yield. NiO species present in the material mainly take the active part in the catalysis which is illustrated with a proposed mechanism in Fig. 20. As an alternative to volatile and toxic thiol or foul-smelling thiophenol sulfur sources, which lead to enormous environmental and safety problems, thiourea is widely used nowadays for large scale operations. Three component one-pot coupling of aryl halides, benzyl bromides and thiourea in aqueous media is another significant approach towards green organic synthesis.¹⁴⁶ A Cu-grafted mesoporous organo functionalized SBA-15 catalyst synthesized via a simple post-grafting technique was employed for this one-pot thioetherification carried out at a temperature of 373 K. A significant yield of aryl sulfides is obtained using this heterogeneous, non-toxic Cu-mesoporous catalyst.¹⁴⁶

There are also other coupling reactions like C–N and C–O, etc. which to the best of our knowledge have not yet been observed to proceed over heterogeneous mesoporous materials.

c. Three component coupling reaction. There are some three component one-pot coupling reactions which are observed to be catalyzed by mesoporous catalysts in liquid phase heterogeneous media. These reactions of a variety of organic reagents to synthesize new heterocyclic compounds are highly important in different fields for synthesis of natural products, fine chemicals and pharmaceutical components, etc. One such reaction is the one-pot thioetherification involving C–S coupling which has already been mentioned in the previous section.¹⁴⁷

Another highly significant three component coupling reaction is A³ coupling, i.e. the coupling of aldehyde, alkyne and amine, used extensively in the manufacturing of various nitrogen-containing heterocycles, natural products and biologically active compounds, etc. to form propargylamine in a convenient approach. Though many metals viz. Au, Ag, Cu, In, etc. based homogeneous catalysts showed efficiency for A³ coupling reactions,¹⁴⁸ but due to some disadvantages for e.g. difficulty of separation of those catalysts from reaction mixture, lack of recycling efficiency and the possibility of metal leaching etc. metal immobilized heterogeneous supports are much more desirable for such reactions.¹⁴⁹ Ag nanoparticle supported mesoporous SBA-15 has been proven to be highly efficient for A³ coupling reactions in glycol as a ‘green’ solvent at a temperature of 373 K. The product conversion observed was believed to be highly dependent on Ag particle size and a high yield of propargylamines was obtained with ca. 8 nm optimized size Ag-loaded SBA-15.¹⁵⁰ A 2D-hexagonal periodic mesoporous organosilica containing imidazolium ionic liquid framework synthesized in acidic conditions with P123 surfactant has been functionalized with 0.2 mol% of Au and a thorough investigation was performed to study the catalytic ability of that Au loaded PMO heterogeneous support for the A³ coupling reaction under mild conditions.¹⁵¹ In Fig. 22 we have illustrated the synthesis of the Au@ionic liquid PMO catalyst and its application in an A³ coupling reaction. Chloroform has been proven to be the most appropriate solvent for the reaction at a relatively lower temperature (333 K) with a high average yield of around 70–80% for the corresponding propargylamine derivatives.

Fig. 22 A synthesis scheme for mesoporous Au nanoparticles doped imidazole functionalized PMO and an A³ coupling reaction over this catalyst.

Biginelli condensation is another three component one-pot reaction which creates 3,4-dihydropyrimidin-2(1H)-ones from aryl aldehyde, ethyl acetoacetate (EAA) and urea in the presence of a Brønsted or Lewis acid as a catalyst. In 2010, an Al–MCM-41 mesoporous catalyst was employed for this reaction in octane solvent to obtain various essential substituted dihydropyrimidinones with high yields and a high catalytic yield was obtained with a small loss of activity after repeated use.¹⁵² Later in 2012, superparamagnetic Fe₃O₄ nanoparticles impregnated on cysteine functionalized mesoporous SBA-15 was designed and it has been proven to be a highly reusable and magnetically recoverable catalyst for the one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones via the Biginelli reaction in pollution free ethanol solvent.¹⁵³ A method of synthesis of the Fe₃O₄@cysteine functionalized SBA-15 material is shown in Fig. 23. An idea of a one-pot reaction and separation of the catalyst with a large bar magnet is also illustrated in this figure. The catalyst allowed for a high yield of products using different substituted aldehydes within a very short reaction time and a suitable catalytic cycle was suggested to be taking place over the Lewis acidic Fe₃O₄ active sites of the catalyst. A more environment friendly approach was made by Pramanik et al. who synthesized a novel phosphonic acid functionalized 2D hexagonal mesoporous organosilica and applied this metal free catalyst for a Biginelli condensation reaction under solvent-free conditions at 333 K.¹⁵⁴ The reaction pathway is very clean with a very high yield of the products obtained and good recycling efficiency. The high activity of this heterogeneous organocatalyst can be attributed to the high Brønsted acidity of the phosphonic group along with the huge surface area and large pore diameter of the silica support.

Fig. 23 Magnetically recoverable Fe₃O₄@cysteine grafted mesoporous SBA-15 catalyst and its role in a one-pot Biginelli condensation reaction.

Besides the above mentioned organic reactions taking place over mesoporous supports, other reactions like Diels–Alder reactions, hydroformylation of olefins, aldehydes, polymerization reactions etc. are also found to be efficiently catalyzed by some metal supported or metal-free mesoporous materials under mild reaction conditions.^155–157 Further, hierarchical porosity can be introduced in the ordered porous silica composites by using anionic polystyrene spheres and triblock copolymers as templates¹⁵⁸ and the resulting materials can potentially be utilized as supports in heterogeneous catalysis for bulkier molecules where diffusion of the reactant molecules could be facilitated.

5. Future perspectives and conclusions

To give an overview of the potential of mesoporous siliceous and non-siliceous materials in liquid phase heterogeneous catalysis, the present review offers a concise idea showing numerous examples of organic transformations with proper illustrations. Additionally, an outline of the synthesis strategies of the functionalized mesoporous materials presented here helps with suggestive information about how to design and prepare the mesoporous catalysts, tune their pore structure and specific surface area as well as how to impregnate them with various active transition metal nanoparticles, complexes or organic functional groups at the pore surface. Based on this one can prepare a number of new metal doped and organically functionalized mesoporous materials and explore their potential in versatile catalytic transformations under liquid phase reaction conditions. Replacing conventional hazardous, toxic homogeneous catalysts, which create serious environmental problems and ecological imbalance, many reactions mediated by those metallocatalysts or metal-free heterogeneous organocatalysts are carried out in aqueous solvents or without solvent, at lower temperatures or without the use of any toxic reagents. These green catalytic pathways are in high demand for the development of industrial manufacturing processes. Thus, it is expected that in future, more and more applications of these green catalysts in each and every industrial fine chemical synthesis will create a pollution free world for the benefit of all human beings.

List of abbreviations

nm	Nanometer
3D	3 dimensional
CMC	Critical micelle concentration
$	Dollar sign
VO(acac)2	Vanadyl(IV) acetylacetonate
Cp*	Cyclopentadienyl
MPa	Megapascal
RT	Room temperature
HMS	Hexagonal mesoporous silica
EA	Ethyl acrylate
ECA	Ethyl cyanoacetate
EclA	Ethyl chloroacetate
EAA	Ethyl acetoacetate
TMG	1,1,3,3-Tetramethylguanidinium
NP	Nanoparticle
BET	Brunauer–Emmett–Teller
DMF	Dimethyl formamide
DMSO	Dimethyl sulfoxide
AIBN	2,2′-Azobis(isobutyronitrile)

Acknowledgements

AB wishes to thank DST, New Delhi for financial support through DST-SERB and DST-UKIERI research grants. NP is grateful to Department of Atomic Energy, Government of India for financial support.

Notes and references

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