Читать книгу Heterogeneous Catalysts - Группа авторов - Страница 15

One of the earliest applications of heterogeneous catalysts in the modern petrochemical industries (crude oil refineries) can perhaps be traced to the catalytic cracking process. In the early 1920s, French engineer Eugene Jules Houdry, E. A. Prudhomme (the pharmacist who discovered the reaction) and their team developed the catalytic lignite‐to‐gasoline process, whereby lignite was first pyrolyzed to high‐boiling‐point liquid hydrocarbons, followed by vaporization and catalytic conversion to the gasoline fractions [36]. The latter step is similar to noncatalytic, high‐temperature, and high‐pressure cracking of the heavier fractions of the crude oil to produce (low octane rating) gasoline developed by Standard Oil Company in the United States a few years earlier. Efforts were made to boost the octane rating of the synthetic gasoline including trial using aluminum chloride as the cracking catalyst but was found to be economically unfeasible. Thomas Midgley and Charles Kettering of General Motors patented the addition of tetraethyl lead to gasoline to improve its octane rating substantially, which was rather successful commercially but was banned worldwide many years later due to the release of toxic exhaust fumes [37]. Houdry discovered a more environmentally benign solution, that is, use of Fuller's earth, a naturally occurring aluminosilicate layered clay, as a cracking catalyst to produce extremely high‐quality gasoline from heavy crude.

Despite not having found much success in France, where the process was deemed not commercially viable, Houdry brought his catalytic cracking process to the United States in the 1930s for further development with Sonoco Vacuum Oil Company (later Mobil Oil Corporation and now ExxonMobil) and adapting the technology to the petrochemical processing. Upon overcoming various reactor engineering challenges to cope with the rapid catalyst coking during the cracking reaction, the Houdry process became a phenomenal success that revolutionized the petrochemical industry. His inventions paved the way for the development of the modern fluidized catalytic cracking (FCC) process, where catalysts were fluidized for continuous looping between the catalytic cracking reactor and adjacent regenerator unit (to remove coke by air oxidation). The Houdry process was so successful that the production of synthetic silica–alumina and magnesia–silica catalysts was commenced in the 1940s to meet the needs for catalytic cracking reaction [38]. In fact, the silica–alumina catalyst is still used to this day in industrial FCC, but in the form of synthetic zeolites, which have a much higher surface area than the clay minerals.

Synthetic zeolites, which constitute crystalline microporous (0.3–2.0 nm pores) aluminosilicates, have been actively developed since the late 1950s by the Union Carbide and Mobil Oil Corporation, resulting in the discovery of zeolites A (Linde Type A) and X (Linde Type X) in 1959 [39], zeolite Y (Linde Type Y) in 1964 [40], and ZSM‐5 in 1972 [41, 42]. These landmark catalysts continue to find important applications not only in FCC but also in the isomerization of hydrocarbons, synthesis of specialty chemicals, methanol‐to‐hydrocarbon conversions, and catalytic deNO_x, with a great deal of advancement achieved in the last decade in the conversion of biomass, among many others. Excellent accounts on the fundamentals as well as the state‐of‐the‐art progress in some of these topics are highlighted in Chapter 33 (on the conversion of lignocellulose to biofuels), Chapter 34 (on the conversion of carbohydrates to high‐value products), and Chapter 38 (on the abatement of NO_x). In fact, the discovery of new zeolites has been thriving since the 1980s, with a unique set of material compositions, frameworks, and pore dimensions being discovered annually. A large database of zeolites is maintained by the International Zeolite Association since 1977 through the Atlas of Zeolite Structure Types [43]. While silicate and aluminosilicate zeolites dominate a large extent of the database, other zeolites based on aluminophosphates, metallosilicates, germanosilicates, aluminoborates, and so on also exist. Among them, some of the most widely used zeolites in industrial catalysis besides zeolite Y and ZSM‐5 include zeolite X, MCM‐22 (Mobil Composition of Matter No. 22), MCM‐49, SAPO‐34, Beta zeolite, and SSZ‐13.

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 such as nanorod and nanotube arrays are particularly attractive to capitalize on the high surface‐to‐bulk ratio as well as the much sought‐after vectorial charge transport for efficient photocharge separation during photoelectrocatalytic reactions. A variety of synthesis techniques to obtain such structures have been developed, ranging from chemical vapor deposition, spray pyrolysis, and hydro/solvothermal synthesis to electrochemical anodization, producing efficient anisotropic photocatalysts of TiO₂ nanotubes, WO₃ nanosheets, Nb₂O₅ nanorods, Ta₂O₅ nanotubes, α‐Fe₂O₃ nanotubes, etc. The electrochemical synthesis of these fascinating array photocatalysts can be found in Chapter 3. In recent years, the interest has expanded to two‐dimensional (2D) photocatalysts such as the graphitic carbon nitride, molybdenum disulfide, tungsten disulfide, and MXenes. Besides maximizing the surface‐to‐bulk ratio, these materials exhibit unique quantum electronic properties seen only when made into atomic‐thin layers [52].