## 4 Applications of Zeolites in Catalysis, Bioprocesses, and Environmental Remediation
### Enzyme Immobilization
This method can be used in the formation of enzymatic aggregates forming structures called CLEA (cross-linked enzyme aggregates), allowing the retention of high activity in the biocatalyst, in addition to high stability and recycling [330]. Its formation process involves mixing an aqueous solution of proteins with organic solvents, salt anions or polymeric structures [309; 310] (p. 37). The use of zeolites as a support for this type of immobilization is not much explored; however, the use of ZSM-5 structures to incorporate the pectinase stands out, verifying that the bifunctional catalyst obtained was ultraefficient in the catalysis of the hydrolysis of bonds of \(\beta\)-glycosides, with good thermal stability and good reusability [331]. FAU Zeolites as Support for Peroxidase Enzymes  
Most of the need for production of FAU (faujasite) zeolites is aimed at its use in adsorption of NH\({}_{3}\)[332], as catalyst for hydrogenation of 1,4-bis(phenylethynyl) benzene [333], as catalyst-adsorbent for desulfurization of fuel oil [334], as a model by Monte Carlo molecular simulation in removal of ethyl mercaptan traces from natural gas [335], as catalyst for the synthesis of coumarins [336], in separation of CO\({}_{2}\)/N\({}_{2}\)O gases [337], and as a precursor of hierarchical zeolite used for fatty acid isomerization [338]. One of the most important characteristics of these structures is biocompatibility, allowing its use in biological systems and giving affinity in contact with the biological molecules of peroxidase. Therefore, FAU zeolites offer an alternative to peroxidase immobilization and may be used in future applications optimizing its functions as biocatalyst support, such as high sensitivity to environmental conditions, low storage stability, and low reusability. This increase in enzyme-supported performance allows better applications in oxidation of phenols, amines and aromatic compounds, often present as environmental pollutants [339]. In this scenario, the FAU zeolites offer adequate properties for immobilization of these macromolecules. There are two main possibilities for the construction of this hybrid system or enzyme-support complex. One possibility is due to the high specific surface area of most types of FAU structures, reaching values ca. 600 m\({}^{2}\)/g compared to other structures commonly used in many research fields, including enzyme immobilization, and MFI structures that reach specific surface areas of ca. 300 m\({}^{2}\)/g, these studies about the surface area are useful for strategies of zeolite modification with chemical modifiers [340; 341]. The large specific area allows an adsorption of the molecule in an adequate microenvironment for accommodation of its high molecular volume. The high framework density of 13.3 atoms T per 1000 A and the wide ring forming the alpha cavity makes the enzyme entrapment easier than other zeolites, increasing the stability of the immobilized biocatalyst. The second possibility for protein immobilization is the functionalization of the structure with consolidated substances that can bind covalently with amino acid residues from enzyme. Moreover, the FAU structures can be modified with good acceptability with aminosilanes such as 3-aminopropyltriethoxysilane (APTES), aminopropylmethyldiethoxysilane (APMDES) and 3-aminopropyldimethylethoxysilane (APMDES) [342]. This modification of FAU zeolites allows great stability of immobilized protein because of the formation of chemical bonds between the support and the biocatalyst with the chemical modifier in the interface. The Figure 19 illustrates this process. These two possibilities to use this consecrated structure as support for peroxidase immobilization are validated by the adequate properties of FAU zeolites.