Immobilization of Enzymes on Hierarchically-Structured Supports

Abstract

In the last decades, the energy consumption of the world has been dramatically enlarged, owing to increased globalization and the economic growth. Thus, new renewable energy sources have been developed to overcome this drawback. In recent years, biomass, which is one of the most widespread renewable energy sources, promises to satisfy this demand. Biomass is also utilized as a sustainable resource to be situated for some valuable chemicals as well as biofuels and biopolymer intermediates. Conversion of biomass can be accomplished by catalytic processes in which enzymes play a key role as biocatalysts. Contrary to the side effects of many physical and chemical processes in industry, biocatalysis has been preferred due to their inhibition of the environmental pollution. Contrary to chemical catalysts, enzymes are used under mild conditions due to their proverbial sensibility and limited operational stability. These features of enzymes cause some challenges in recovery and reusability for a given industrial application. In order to enhance stability of enzymes-based catalysts, immobilization techniques were improved. Besides, many scientists investigate with a great interest the most effective way to obtain maximized exposure of enzyme’s active site in the biocatalytic reaction mixture, with an obvious impact in the catalyst activity. A suitable strategy for the above described issue is usually to be benefit from a porous support, which should provide suitable binding sites for enzymes in active conformation, and/or allow encapsulation of enzymes, whilst minimizing enzyme leakage and deactivation. Additionally, the characteristics of a carrier utilized in immobilization of enzyme have to be well-considered including its permeability, surface area, hydrophilic character, insolubility, mechanical and thermal stability. While keeping in mind of the above-described properties, it is not a surprise that inorganic porous materials are commonly preferred due to their cage-like configurations, which allow the constructions of suitable microenvironments for enzymes. Inorganic porous materials present very interesting features, such as tunable pore size and shape, high specific surface area, high chemical and structural stability, and a variety of chemical functionalities, which can be used as anchoring sites for hosted enzymes. They also possess many advantages, such as increasing enzyme stability, protecting enzymes from harsh reaction conditions, having ability of tailored-cages construction for different enzyme sizes, etc. In this thesis, several different inorganic porous supports were used for enzyme immobilization. These are gradient macroporous stainless steel discs (GMSDs), gradient nanoporous ceramic discs (GNCDs), microporous zeolites, metal organic frameworks (MOFs), and silica-based nanoparticles (SiO2). In addition to that, several chemical catalysts were employed in biomass conversion. Hierarchical nanosheet zeolites, zeolitic imidazolate frameworks (ZIFs) as well as zinc oxide (ZnO) were tested in isomerization of monosaccharides. From the point of biomass type, starch became prominent in this thesis since it is a kind of primary biomass which appears abundantly in nature. This polysaccharide is constructed from glucose units linked with each other via glycosidic bonds and highly exploited in numerous applications in industry. There are several enzymes available for converting starch molecules. Here, we used α-amylase (and thermostable α-amylase) and ß-amylase to hydrolyze the starch to glucose. Additionally, α-glucosidase to hydrolyze maltose to glucose, invertase to hydrolyze sucrose to glucose and fructose and glucose oxidase to oxidase β-D-glucose to D-glucono-δ-lactone were employed. Alongside of the carbohydrate enzymes, lipases are also commonly used in many industrial applications. In this thesis, lipase from Thermomyces lanuginosus to perform transesterification of vinyl propionate (VP) with 1butanol as well as lipase from Aspergillus niger to hydrolase p-nitrophenyl palmitate to p-nitrophenol were tested. The specificity of an enzyme, the versatile activation of a chemical catalyst and the utility of inorganic porous materials in immobilization of enzymes allow several different designs for process engineering. Consequently, combining of biocatalysts and inorganic porous materials in order to achieve much more efficient reaction mechanisms is mainly focused on in this thesis