Chapter 2

Functionalized Mesoporous Materials as Sustainable Catalyst in Liquid Phase Catalytic Transformations

Nabanita Pal1,2,  and Asim Bhaumik1     1Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, West Bengal, India     2Surface Physics and Materials Science Division, Saha Institute of Nuclear Physics, Kolkata, West Bengal, India

Abstract

Heterogeneous catalysts offer some unique advantages in product separation, easy recovery from the reaction media and high recycling efficiency. Organically functionalized mesoporous silicas, metal-grafted functionalized silica and porous organic polymers are very important classes of materials that contribute in this recyclable heterogeneous catalysis owing to some unprecedented features such as tuneable pore dimension, high specific surface area and wide flexibility for grafting various reactive functional groups at the surfaces. This chapter reviews the recent developments in functional mesoporous solids: their synthesis and properties that are responsible for their utility in numerous liquid phase catalytic reactions, specially which are industrially, biologically and analytically important.

Keywords

Green synthesis; Heterogeneous catalysis; Metal grafting; Organic-inorganic hybrids; Surface functionalization

1. Introduction

The panoramic journey towards exploration of mesoporous materials starts with the Mobil scientists in the United States who found out a high surface area ordered inorganic porous silica material having a pore dimension of >2 nm, named MCM-41 (Mobil Composition of Matter-41). This was synthesized based on a self-assembled supramolecular templating mechanism [1]. Before this invention, the largest pore dimensions reported for a templated porous material was mostly based on microporous zeolites or silicalites with a pore diameter well below the 2.0-nm regime and these materials are conventionally synthesized hydrothermally by using a single molecule template [2,3]. In a limited time, mesoporous materials have attracted widespread attention and emerged as the most popular category of nanoporous solids particularly to scientists belonging to materials chemistry. Having an exceptionally high surface area, well-defined tuneable pore diameter and a good possibility of surface functionalization, mesoporous materials open various new windows while experimenting on their potential applications. Liquid phase catalytic reactions are one of the most challenging applications of functionalized mesoporous solids. The perception of catalysis formulated by Wilhelm Ostwald was that 'there is probably no chemical reaction which cannot be influenced catalytically' [4]. The economic development as well as advancement of a country largely depends on the industrial catalytic processes and >20% of the world's gross national products relies on these catalytic technologies [5]. In the present world, about 60% of the industrially important chemical products are produced in different chemical processes among which 90% are based on catalytic reactions [6]. The term 'catalysis' was coined by the Swedish chemist Jöns Jacob Berzelius in 1835, and it refers to the change in the rate of a chemical reaction mediated by a substance named 'catalyst' [4]. Actually, a catalyst fulfils the criteria (1) of offering an alternative pathway of a lower activation energy than the respective un-catalysed reaction, resulting in a faster reaction rate but (2) does not hamper the overall thermodynamics of the reaction (Figure 1). By facilitating a chemical reaction, a catalyst can be said to be a chemical marriage broker. Later, a well-accepted mechanism and kinetics of the catalytic reaction was proposed by Cyril Norman Hinshelwood in 1927 based on this proposition. Catalysis is of two types: (1) homogeneous where the catalyst remains in the same phase as that of the reaction media (all are generally present in the liquid phase) and (2) heterogeneous, where the catalyst is generally solid and remains as a distinct phase from the liquid or gaseous reaction media. The major problem of homogeneous catalysis is the difficulty of separation and recovery, which makes it unsuitable for application in industrial purpose. The deficiency has been overcome by replacing homogeneous with heterogeneous solids [6]. The birth of industrial catalysis took place in Europe with the production of sulphuric acid in the 'contact' process discovered by Knietsch in 1898 [7]. Industrial processes which depend extremely on homogeneous catalysts are modified by 'heterogenization' of those catalysts, which possess all the properties of their homogeneous counterparts but have reusability like heterogeneous [5]. Organic functionalization or immobilization of soluble numerous metal complexes over solid inorganic mesoporous silica surface can build suitable organic-inorganic hybrid functionalized silicas, which can act as true heterogeneous catalysts in various important chemical reactions [8].
Figure 1 A simple pictorial scheme for catalytic reaction and the corresponding energy diagram. Catalytic reactions consist of steps like diffusion and adsorption of the reactants on the catalyst surface, successful reaction on the active site located at the surface and then desorption and finally diffusion of the products to the bulk reaction media. Since the catalyst surface plays a crucial role in the process, mesoporous materials having a high surface area compared to the nonporous material can thus contribute significantly in catalysis. Moreover, functionalization of mesoporous silicas with different organic moieties largely fulfils the requirement of generating a wide range of active sites for different organic catalytic transformations. These reactions result in a high turnover frequency, TOF (TOF = the number of moles of the substrate converted per mole of the active site of the catalyst in unit time). Using examples from recent literature, this review illustrates and refines the synthetic outlines and applications of those sustainable heterogeneous hybrid catalysts in many traditional as well as new organic transformations [9].

2. Synthesis and Types of Functionalized Mesoporous Materials

To introduce organic functionalities or metal complexes at the surface of an inorganic porous support, mesoporous silica is used extensively owing to its high surface area and versatility to condense with other active organic groups [8]. Recently, purely organic porous polymers containing various functional groups have been designed, which can accommodate different metal complexes and result in the formation of organic-inorganic hybrid materials [10]. There are also some reports on hybrid metallophosphate-based materials and metal-organic frameworks, which are synthesized generally through a non-templating pathway. They contain small pores within their network and are also very effective in catalytic transformations [11]. This review mainly highlights mesoporous hybrid materials, and thus, we will give special emphasis to silica- and polymer-based organic-inorganic hybrid catalysts, which have shown good potentials in sustainable organic transformations. In this context, it is pertinent to mention that the concept of the introduction of bridging organic groups into the pore walls of mesoporous silica was first invented by Inagaki et al. who reported successful immobilization of the ethane/benzene ring within the mesoporous silica pore wall [12]. General chemical pathways and the mechanism to design such a type of organic-inorganic hybrid materials have been illustrated and reviewed in a huge number of journal articles [13,14]. Syntheses of these hybrid solids to form a chemical bond between an organic molecule and the pore wall of the inorganic support are usually carried out through several methods (Figure 2): (1) 'post-grafting' technique where the inorganic silica pore wall is 'grafted' by subsequent modification with other organic groups, (2) 'co-condensation' route leading to simultaneous condensation of inorganic silica and organosilica precursors to form hybrid silica, (3) sol-gel route for the formation of 'periodic mesoporous organosilica' (PMO), that is, bridging organic units are directly incorporated in the three-dimensional (3D) network structure of the silica matrix through two covalent bonds [14]. In the case of 'post-grafting' or post-functionalization, the -Si-OH groups of mesoporous pure silica surface are condensed with another silica precursor containing various organic groups via covalent bond formation and organo-grafted silica is formed. The organic group attached to silica can be further functionalized depending upon the application purpose (Figure 3(a)) [15]. The 'Co-condensation' route is a one-pot method for the formation of organic-inorganic hybrid silica and is preferable to 'post-grafting' if uniform surface modification with organic groups is required. However, the latter provides a hydrolytically more stable and well-defined structure than in the one-pot condensation approach. Here, simultaneous covalent bond formation occurs between inorganic silica and organosilica precursors in the presence or absence of a template to form hybrid silica with mesopores (Figure 3(b))...