## 4 Applications of Zeolites in Catalysis, Bioprocesses, and Environmental Remediation
### Production of Light Olefins
The reduction in Si content in a zeolite structure enables a olefin/paraffin ratio of <6 in STO reaction and the CO conversion reaches 11%. This is tailoring the structure. Moreover, the acidity of zeolites in bifunctional catalysts in STO reactions improves CO conversion and olefin selectivity [256]. The use of SAPO-34 zeolite to produce light olefins is due to the facile diffusion of olefins in its channel structure, presenting eight-membered ring channels [254]. In ZSM-5, used in catalytic cracking of gasoline to generate C\({}_{2}\)-C\({}_{4}\) olefins, this process occurs at the expense of gasoline yield due to preferential cracking of low-octane gasoline with linear olefins on its internal acid sites [257]. The bimolecular hydrogen transfer reaction decreases in a medium with ZSM-5, thereby contributing to the formation of light olefins [42] (p. 8). MFI zeolites are usually added in the FCC process, increasing either the production of light olefins or the gasoline octane number [258]. The stability of the H-MFI structure is guaranteed by phosphorous impregnation and high yields of products [259]. The dual additives MFI and BEA zeolites may improve the production of light olefins, mostly after treatment of acid activation with phosphoric acid. Moreover, the dual additives do not impact the quality of naphtha in comparison with just one structure-based additive (e.g., MFI) [260]. The catalytic process to obtain olefins from CO\({}_{2}\) to MeOH conversion and then MeOH to olefins demands high temperature control using several oxides, such as In\({}_{2}\)O\({}_{3}\), ZnGa\({}_{2}\)O\({}_{4}\) ZnZrO, and others as catalysts. To complete this MTO (methanol to olefins) reaction, many zeolites have been used to produce light olefins from CO\({}_{2}\) hydrogenation, such as SAPO-34, SSZ-13 and ZSM-5. The degree of proximity and distance of two catalysts involved, metal oxide/zeolite mass ratio, and the operating conditions are relevant to reach a good yield of C\({}_{2}\)-C\({}_{4}\) olefins [261]. The conventional zeolites that are made with microporous channels impose the restriction of bulk molecule diffusion and frequently yield coke, besides side reactions. Therefore, the hierarchical zeolites composed of mesopores and macropores in the same structure come to give a solution for this problem in light olefin synthesis under a zeolite structure used as catalyst [262]. These structures include MFI, MEL, CHA, FER, FAU, BEA and MTW, exhibiting high olefin selectivity, high conversion, and long lifetimes [263]. The reactions applied for these structures range from MTO and CO\({}_{2}\)/CH\({}_{4}\) conversion and factors such as increase in external surface areas, decrease in diffusion path length, accessibility of guest molecules to acid sites, and low coke formation, are relevant for this application and suppressed if a heteroatom is inserted in the framework, modifying the acidity of the material [264]. A simulation of an increase in light olefins from MTO reaction using H-ZSM-5 has been reported [265]. Large-pore zeolites may increase the yields of light olefins such as ethylene and mostly propylene. With a combination of ZSM-5 and a large-pore zeolite, optimal conditions were obtained in Daqing heavy oil cracking. Good product yields were reached when the combination of the same MFI structure was made with RE-USY and AT-H\(\beta\)-4 was employed, with increased yields when pore size increased [262]. Figure 13 shows the application of Y/ZSM-5 zeolites to obtain light olefins from Fischer-Tropsch synthesis.