## 4 Applications of Zeolites in Catalysis, Bioprocesses, and Environmental Remediation
### Enzyme Immobilization  
The high applicability of enzymes in industrial processes and their favorable characteristics for processes, such as low toxicity, high conversion rate in ambient production conditions, high selectivity and high specificity, have been decisive in the improvement of their use in the pharmaceutical and textile industries, food and paper production, and water treatment [308]. Some aspects of the use of enzymes as biocatalysts for industrial purposes include the improvement of their stability under conditions of pH, use of organic solvents, temperature and pressure. Their high solubility in aqueous media leads to a difficult separation from reaction medium. The conventional use of enzymes can cause their loss from the process, even if it presents high activity, requiring the insertion of more enzymatic load in the process, burdening it [309]. The immobilization methods employed influence the specificity, selectivity and activity of the enzymatic molecules. As they are fixed to a solid material, their removal from the medium is facilitated, minimizing contamination and inhibition of the final product, enabling its recovery, minimizing aspecific adsorption and microbial contamination [310; 311]. The immobilization consists of the confinement of the enzymatic structure in a different phase from the one that the substrate and the products are presented. This phase is called matrix or support, which can be zeolitic structures [312], and must be inert, have high physical resistance, stability, regenerability, ability to improve activity, and enzymatic specificity [313]. There are factors that can influence the performance of immobilized enzymes, such as the hydrophobic partition, the microenvironment in which the support is found, the presence of several binding points, the possibility of immobilization of different types of enzymes, diffusion constraints and the presence of substrates or inhibitors [314]. Other factors include the porosity of the support, the modes of enzyme-support binding and the physical nature of the support [311]. Among the characteristics of zeolites that provide utility for this process are their mechanical strength, enabling their resistance to different pressures and experimental conditions [315], and the possibility of surface modification, allowing the insertion of compounds that facilitate the binding of the enzyme [316]. The possibility of synthesizing these supports with ideal characteristics such as morphology, porosity and presence of micropores allows better interaction between the enzyme and the substrate [317]. The mechanical stability and chemical inertia of the support, in addition to its ecologically favorable and nonpolluting nature, close the useful characteristics of these materials to immobilization [318]. Figure 18 shows the types of enzyme immobilization on carriers such as zeolites. Figure 17: Lewis acid zeolite Sn-Beta catalyzing the formation of methyl lactate from a triose and hexose. Reproduced from [299]. Copyright 2011 Royal Society of Chemistry. adsorption or physisorption is the simplest and least expensive method, occurring due to aspecific weak-force interactions such as van der Waals interactions, hydrogen bonding, electrostatic interactions and hydrophobic interactions [319]. Because these interactions are weak, the enzyme can easily undergo a desorption process as a result of environmental parameters such as pH, ionic strength, temperature and pressure [320]. Such weak interactions do not change the structure of the enzyme, preserving its active site and maintaining its activity [321]. Protein adsorption has occurred in mesoporous silicates with narrow pore distribution range. In these porous materials, the presence of silanol groups is essential to effect adsorption by hydrogen bonding [311] (p. 37). There is good stability of the lipase enzyme against factors such as pH and temperature when immobilized by physical adsorption with respect to its free form.