The most common approach to the synthesis of zeolites involves interfacing sol–gel chemistry with organic structure‐directing agents (SDAs) as soft templates. In a classical sol–gel process, precursors especially those of alkoxides such as tetraethyl orthosilicate (TEOS) are first hydrolyzed to form alkoxysilanols and/or orthosilicic acid. Subsequent cross‐linking reaction through the dehydration of the hydroxyl moieties results in the formation of nuclei, and further polymerization yields amorphous silica particles that appear either as sol (well‐dispersed particles in solution medium) or gel (continuous network formed by particles throughout the solution medium). The physical sizes of these amorphous particles are strongly influenced by concentration, pH, and temperature of the reaction medium. In the presence of SDAs, typically amines or quaternary ammonium surfactants but in some cases inorganic ions, the cationic head of SDAs will bind strongly to the silicate anions. Under such situations, there exist concerted interactions between (i) the silicate and surfactant (functioning as structural stabilization and blocking agents), (ii) surfactant and surfactant (functioning as structural template for the micropores), and (iii) silicate and silicate (assembly of silicate network) during the self‐assembly of the crystalline zeolites. The term “crystalline” refers to the repeated assembly of the basic unit cells of the microporous silicate network. Studies have shown that the slow crystallization process takes place during the hydrothermal aging after the formation of the amorphous silica particles. The surfactant SDAs can be removed by simple calcination, leaving behind well‐ordered micropore channels within which catalytic reaction can take place. These micropores range from 8‐membered ring (8‐MR) (ultrasmall pore ∼4 Å), 10‐MR (∼5 Å) to 12‐ (∼7 Å) and 14‐MR (ultralarge pore, ∼8 Å) or above. Channels of 6‐MR or less are too narrow to allow molecules to pass through and hence considered nonporous.
The signature strong acidity of silicate‐based zeolites originates from the partial substitution of the silicate (SiO44−) building block with that of the aluminate (AlO45−). The additional charge deficiency brought about by the latter can be readily neutralized by a labile proton, i.e., Brønsted acid. The Brønsted acid site can be conveniently used as an ion‐exchange site to immobilize other cations for single‐atom catalysis (discussed below). Interestingly, ion‐exchanged Ca2+, Y3+, and La3+ sites are efficient catalytic sites for the pyrolytic carbonization of ethylene and acetylene. This produces homogeneous graphene‐like layers within the micropores that upon the removal of the zeolite template produce faithful carbon replica of the microporous framework [44]. Such zeolite‐templated carbon (ZTC) is interesting not only because of the electrically conductive and well‐ordered microporous framework that can now be utilized for electrochemical and fuel cell‐related reactions but also because the carbon, which can be easily removed by calcination, can potentially serve as secondary templates to synthesize other nonzeolite microporous catalysts.
Care should be taken not to confuse zeolites with well‐ordered mesoporous catalysts (e.g., MCM‐41, SBA‐15, KIT‐6), which belong to a different class of porous materials and, by definition, consist of pores in the range of 2–50 nm. The MCM‐41 (tunable pore size of 2–9 nm) and SBA‐15 (tunable pore size of 5–10 nm), discovered by Charles T. Kresge et al. at the Mobil Oil Corporation in 1992 [45] and Galen D. Stucky and coworkers at the University of Santa Barbara in 1998 [46], respectively, are arguably the gold standards for this class of catalytic materials. These mesoporous catalysts are templated through the addition of bulky micelles such as those formed by cetyltrimethylammonium bromide (CTAB) surfactant and Pluronic P123 triblock copolymer, and sol–gel silica particles will precipitate in between these self‐assembled soft templates. Because the micelles serve as long‐range structural templates (and none at short range like those used for the synthesis of zeolites), well‐ordered mesopores can be obtained, but the silica walls are basically amorphous. These glassy walls are catalytically inactive, in stark contrast with the crystalline walls of zeolites. Nevertheless, the mesoporous materials are attractive as high‐surface‐area supports with mesoporous channels large enough for the deposition of a wide range of active metals without pore blocking and at the same time accessible to bulky reactant molecules that otherwise could not penetrate the zeolite micropores. Because there is no requirement for short‐range ordering, these surfactant templates can be flexibly used to fabricate a plethora of other mesoporous metal oxides including TiO2, WO3, and Al2O3. Furthermore, the mesoporous silica can be used as hard templates for the synthesis of mesoporous carbon and metal oxide nanorods [47]. An area that is actively being pursued is the synthesis of hierarchical zeolites, where mesoporous channels are introduced in zeolites, in such a way that the wall of the mesoporous catalyst is no longer amorphous silica but that of catalytically active, microporous crystalline silicate. This allows the accessibility of acid sites by large reactant molecules while overcoming the mass diffusion limitation associated with the narrow micropores of zeolites during catalytic reactions. More details on the design and synthesis of such hybrid micro‐/mesoporous catalysts are presented in Chapter 7.
Metal–organic framework (MOF) is a term first coined by Omar Yaghi in 1995 to describe a class of crystalline porous solids formed by a continuous network of multivalent metal cations/clusters and organic linkers of at least two coordination positions [48]. It is analogous to the zeolites, except with different set of building blocks. The elegance of MOFs arises from the simplicity of the template‐free synthesis, and the micropore size can be easily tunable by adjusting the length of the organic linker. A classic example is the fabrication of UiO‐66 that involves the simple hydrothermal reaction between zirconyl chloride and 1,4‐benzenedicarboxylic acid (BDC) linker. By replacing the BDC with a longer 1,4‐biphenyldicarboxylic acid (BPDC), one can obtain UiO‐67 and an extension of the pore size from 7.5 and 12 Å to 12 and 16 Å, respectively. In fact, the design of MOFs is so flexible that it can be extended to fabricate mesoporous catalysts by manipulation of the linkers or using SDAs [49]. The catalytic active sites of MOFs may originate from the active metal atoms or compounds covalently functionalized on the linkers or the framework metal cation centers if made coordinatively unsaturated (without affecting the rigidity of the MOF structure). An elegant account on the different strategies in designing MOF catalysts can be found in Chapter 8. To date, MOFs find wide applications in organic synthesis, biomass conversion, photocatalysis, and electrocatalysis, among others. Because of their organic frameworks, MOFs are normally used in mid‐ to low‐temperature applications below 500 °C. A more recent sister class of compound is the covalent organic frameworks (COFs), first discovered by Yaghi in 2005, that are built entirely based on nonmetal centers [50]. In their pristine forms, some COFs are effective in catalyzing photocatalytic and electrocatalytic reactions, while their tunable porous structures can also be functionalized with the desired metal catalysts similar that of the mesoporous silica structure to catalyze a wider range of reactions, e.g., the Suzuki–Miyaura coupling reaction when deposited with the Pd2+ single‐atom catalyst.
The synthesis of porous anisotropic catalysts received significant interests since 2005 or so, especially for photocatalytic reactions such as solar water splitting, abatement of environmental pollutants, and CO2 reduction. Photocatalysts are composed of semiconductor materials, that is, they can photoexcited with photons equal to or larger than their bandgaps to produce usable charges for surface redox reactions. Photocatalytic reactions can be carried out in two ways: particulate photocatalysis where the redox reactions as mediated by the electron–hole pairs take place on the same photocatalyst particle/aggregate (see Chapter 11 on the art of photocatalysts design) and photoelectrocatalysis where the photocatalyst is made into a photoelectrode and connected with a counter electrode in such a way that the electron–hole pairs are separated across the two electrodes (see Chapter 36 on the basics of photoelectrocatalysis) [51]. One‐dimensional (1D) photocatalysts