## 4 Applications of Zeolites in Catalysis, Bioprocesses, and Environmental Remediation
### Processes with P-Xylene  
Para-xylene is a volatile organic compound (VOC), easily becoming a gas molecule at room temperature, undesirable in the environment, and coming mainly from automobiles. The adsorption of this molecule is one of the main approaches to remove it from the air [279]. The main source of xylenes is naphtha reforming and pyrolysis of gasoline, where the compound exists in an equilibrium proportional to the other two isomers (meta- and ortho-xylene). P-xylene is much in demand in the plastic industry for the production of terephthalatic acid and dimethyl terephthalate, intermediates in PET production (polyethylene terephthalate). However, the separation from isomers mixture is very important to obtain pure p-xylene. The reactions involved in this process are the isomerization of C\({}_{8}\) aromatics and the disproportionation and transalkylation of toluene. Alkylation of toluene with methanol is used in the same manner [42] (p. 8). The separation of p-xylene with usual methods, such as distillation, is difficult because its isomers have similar molecular structures and close boiling points. To make this process easier, zeolites are used to promote adsorption processes alongside the fractional crystallization method. These processes demand a great amount of energy and batch operations. In this context, polycrystalline MFI zeolites have been used to separate p-xylene by pervaporation and vapor permeation processes. The physical and structural characteristics of MFI-type zeolites allow the diffusion of p-xylene molecules, such as straight channels in b directions and sinusoidal channels in a direction. The shape selectivity is the main property that enables this separation. The decrease in xylene adsorption level improves this property for isomer separation [280]. The form of zeolites used for this operation is membranes using ferrite (FER) zeolites as MFI zeolites. Therefore, the presence of pinholes as large as 1 nm makes the separation of p/m xylenes difficult, so the presence of narrow pores is undesirable, and the temperature is crucial for separation of binary mixture such as p/o xylene [281]. The zeolite SSZ-35 is used as catalyst for reaction of p-xylene alkylation with isopropyl alcohol. This zeolite has a 10-ring channel system and shallow cavities with 18-ring cages. Chemical modification with acidic treatment results in decreased void volume, such as the concentration of acid sites located in external surface. Desilication of the structure can increase the selectivity of the desired product (2,5-dimethylcumene), leading to the formation of a hierarchical structure [282]. Besides reactions of transformation p-xylene, there are chemical processes for production of this molecule using biomass-derived dimethylfuran, involving H-beta zeolite [283] and beta zeolite with nanosponge-like morphology [284], ZSM-5 and Y zeolites [285]. Sustainable processes of p-xylene production avoid the traditional obtainment from the petrochemical industry. One of these routes is based on the use of a biomass-derived compound called 2,5-dimethylfuran (DMF), producing a biobased p-xylene by Diels-Alder cycloaddition of DMF and ethylene followed by dehydration process of an intermediate. For this process, a beta zeolite with a nanosponge-like morphology was obtained from multi-ammonium surfactants as a meso-micro dual structure-directing agent, composed of ultrathin zeolitic nanocrystals with intracrystalline mesopores and a large number of Bronsted sites, both externally and in micropores. Mesopores in addition to acid sites make access easier for reactant molecules and void the coke formation [284]. The application of zeolites beta, ZSM-5 and Y in continuous flow synthesis presents better catalytic performance, acrylic acid and DMF being used in the process. The Si/Al ratio determined the complete conversion of DMF, with yield of 83% p-xylene.