Heterogeneous Catalysts

While cobalt is known to produce a large fraction of diesel and paraffin wax, the iron catalyst results in higher content of short‐chain olefins when carried out at high reaction temperatures (∼340 °C) or paraffin wax at much lower temperatures. As the reaction proceeds, the iron metal is gradually converted into iron carbide, which is an even more active phase [24, 34]. Compared with crude oil–derived fuels, the FTS‐derived diesel and gasolines are characterized by their exceptionally high cetane and octane ratings due to the high yields of straight‐chain paraffins for cobalt‐derived diesel and olefins/isomers in the iron‐derived gasoline, respectively. Although nickel and ruthenium catalysts are also active in FTS, they are rarely used as stand‐alone catalysts. Nickel, which forms carbonyl and decomposes to the metallic phase (like iron), has a high tendency to form methane instead of liquid fuels. Ruthenium, which is the most active FTS catalyst, is far more expensive than cobalt and iron to justify its bulk usage except as a promoter to cobalt catalysts. Incipient wetness impregnation is by far the most common technique for the synthesis of FTS catalysts [35].

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 de NO 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 (SiO 4 4−) building block with that of the aluminate (AlO 4 5−). 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 Ca 2+, Y 3+, and La 3+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.