## 4 Applications of Zeolites in Catalysis, Bioprocesses, and Environmental Remediation
### Production of BTX Aromatics  
The compounds known as BTX have three identities--\"B\" from benzene, \"T\" from toluene and \"X\" from xylene--and are all aromatic compounds. They are derived from oil thermal cracking and naphtha reforming [266]. The use of light cycle oil (LCO), a middle distillate from FCC process, is also useful to obtain these aromatics, which may contain up to 90% of mono-, di- and triaromatic compounds. In BTX production, hydrotreating (HDT) and hydroracking (HCK) procedures are achieved with catalysts with important roles in hydrogenation in naphthalene structure and selective cracking of naphthalene structures, resulting in one-ring aromatic hydrocarbons with alkyl chains. The MTA (methanol to aromatics) reaction has been another process of BTX production, mostly due to the development of coal-, shale gas-, and biomass-derived methanol using the syngas route [267; 268]. BTX synthesis from syngas can reach good selectivity using a dual bed of MnCrO\({}_{\text{x}}\)-ZSM-512-MR zeolites. Silica modification of external acid sites and the topology of zeolites determine BTX selectivity. The presence of 12-membered rings in zeolite structures such as USY enhance the product formation. Thus, it has been used to design a catalyst for a one-step synthesis reaction of BTX using syngas [269]. Bifunctional catalysts are important in improving the production of these compounds, ideally with acid nature acting as a support and with a metal for hydrogenation-dehydrogenation steps following the HDT and HCK reactions before BTX production [270]. In this process, the strong acidity and excess of hydrogenation activity may crack molecules generating gaseous light hydrocarbons and become sensitive to coke formation [271]. In this scenario, zeolites are important as catalyst support, mostly because of their stronger acidity, great thermal and hydrothermal stability, higher resistance in the presence of sul  
Figure 13: Scheme of light olefin formation using zeolites as heterogeneous catalysts. Reproduced from [249]. Copyright 2020 ACS. fur and nitrogen compounds, reduced tendency for coke formation, and great capacity of regeneration [266]. Other characteristics include shape selectivity allowing the occurrence of specific reactions influenced by distribution of microporous Brensted acidity [272]. A catalyst composed of Ni, CoMo, NiMo, and NiSn when supported over zeolite H-beta presented good BTX yields that may be improved by adding ZSM-5 zeolite in combination with H-Y. This effect was ascribed to large pores of FAU zeolite used becoming an easy medium for tri-ring aromatic molecules [266]. The zinc phosphate groups can be anchored in the channel system of HZSM-5 zeolite, enhancing the hydrogen transfer pathway, essential for high BTX selectivity and avoiding the methylation of these compounds generating heavy aromatics [267]. Nonetheless, the application of mesoporous zeolites is limited because of such structural factors as poor hydrothermal stability and weak acidity. Therefore, hierarchical structures can be used because they combine good structural aspects of microporous and mesoporous zeolites, such as large surface area and better metal or acid/base dispersion, in comparison to amorphous silica-alumina supports. Modification of these structures, such as using ZSM-5/SBA-15 with a heteroatom, decreases acid generation and increases aromatic hydrocarbon formation. As an example, modification with Ga atoms gives high BTX catalytic selectivity [268]. The intensity of zeolite acidity (Brensted sites) is important to drive the glycerol conversion in aromatics. The yield of aromatics is also very dependent on Si/Al ratio, the high Al content (low Si/Al ratio) leading to higher aromatic production. The alternative addition of bentonite in catalyst support can increase selectivity for benzene. The coke formation is a drawback in this process that can be removed by calcination. During the regeneration step, the bentonite structure collapses and reduces the acid strength.