## 4 Applications of Zeolites in Catalysis, Bioprocesses, and Environmental Remediation
### Enzyme Immobilization
This immobilization protects its structure when changes in pH and temperature occur, delaying its denaturation [309] (p. 37). In zeolite structures, adsorption can generate heterogeneous catalysts with high activity due to the presence of Si-OH groups, allowing catalysis of a reaction in a considerable number of cycles [322]. The Si/Al ratio is one of the determining properties of enzymatic adsorption, as this parameter determines surface properties as well as hydrophilicity/hydrophobicity and ion exchange capacity. This effect could be evaluated in the immobilization of pancreatic porcine lipase. The presence of heteroatoms in structures such as Fe in ZSM-5 has promoted improvement in immobilization. Immodilization of enzymes on the support through covalent bonding occurs due to the presence of amino acid side chains such as lysine, arginine, aspartic acid, cysteine, glutamic acid and histidine via their functional groups such as carboxyl, sulfhydryl, hydroxyl, imidazole, amino, epoxy, thiol and phenol [323]. The enzyme-support interaction is strong and generally requires activation of the support through Schiff bases and carbodiimide, where glutaraldehyde and glyoxal are the main groups used, as well as spacers in the structure, offering simplicity and enzyme preparations with stability [309, 310, 311] (p. 37). Zeolitic supports have been used for this immobilization, such as nanoporous zeolite, used in the immobilization of \(\alpha\)-amylase, resulting in an increase in stability and a decrease in denaturation [324]. Figure 18: Methods of immobilization of enzymes onto carriers. Reproduced from [316]. Copyright 2020 Elsevier. Glutaraldehyde has been the most used ligand in the immobilization of enzymes in zeolitic structures of different topologies, namely, glucose oxidase@MFI, amylase@LTA, laccase@SOD, lipase@MFI, lipase@GIS/LTA/BEA/FAU. In addition to this, other binding agents used are glyme and ethylenediamine. Sometimes metallic ions and metallic nanoparticles can be used as ligands according to their affinities. The formation of enzyme-zeolite composites can occur by the fusion of terminal carbon and terminal nitrogen of enzymes producing fused bifunctional proteins. The enzyme immobilization process by entrapment and encapsulation are irreversible processes in which the macromolecule is in physical confinement in the support structure. The entrapment matrix, composed of several subunits of the immobilized enzyme, differs from the encapsulation that consists of a support unit with the catalyst inside [325]. In this process, there is no chemical interaction between the enzyme and the support and thus it does not undergo structural modification [326]. These forms of immobilization can make the interaction of the enzyme with the substrate more difficult due to mass transfer limitations. Thus, the material must have adequate morphology and porosity to allow the passage of substrate and products through the pores and retention of the enzyme. Both forms of immobilization guarantee an increase in the mechanical strength of the system [309] (p. 37). The use of zeolitic supports in the encapsulation and entrapment of enzymatic macromolecules has occurred in ZIF-8 structures. This is an MOF-like structure with isomorphism to a zeolitic structure, presenting imidazolate structures in the network [327]. In this structure, the lipase enzyme could be encapsulated, modulated by a surfactant, showing good thermal stability in the range of 50 to 70 \({}^{\circ}\)C, good storage stability (20 days) and catalyst recovery in six cycles, and stability to solvent denaturation [325]. The use of this structure can occur in the encapsulation of the HRP enzyme and can also form a hybrid HRP@ZIF-8/DNA system [328]. In the cross-linking method, an agent containing functional groups is used for the interaction between enzymes or in the formation of the enzyme-support complex. This method can be used in combination with others with lower immobilization efficiency, such as adsorption, improving the stability of the macromolecule [329].