Heterogeneous Catalysis

Introduction

Heterogeneous catalysis refers to the catalytic reactions occurring at the interface of two phase (solid-liquid, solid-gas, liquid-gas), which is one of the most important transformation technologies in the chemical industry, occupying the important position, because the industrial used in catalytic reactions mostly belongs to the heterogeneous catalysis. Any catalytic processes including heterogeneous catalysis are energy demanding, then how to a catalyst that is stable, recyclable, and efficient is a key fundamental issue in heterogeneous catalysis. Traditionally, in heterogeneous catalysis, inorganic and polymeric matrixes have been widely used for loading catalysts to immobilize on the surface. However, these heterogeneous catalysts encounter problems of low efficiency as a result of site hiding, random and amorphous distribution, and leakage of active sites. Therefore, more and more attention has been paid to the development of new heterogeneous catalysts. Recently, a representative molecular platform, i.e., crystalline porous metal organic frameworks (MOFs) and covalent organic frameworks (COFs), emerges as predesignable heterogeneous catalysts. In contrast to traditional heterogeneous catalysts, the MOFs and COFs show improved stability, high-activity and recycling ability.

Application of MOFs in Heterogeneous Catalysis

MOFs materials conform to the characteristics of heterogeneous catalysis such as reusability, stability, and easy isolation. And also own the advantages such as well defined active sites, high activity, and improved selectivity, thereby serving as novel and outstanding solid catalysts. The catalytic actions about MOFs in heterogeneous catalysis are described below.

• The way of catalytic active sources:(1) MOFs with intrinsic catalytic activity including active sites at metal nodes and active sites at organic linkers; (2) by functionalized modification, including the functionalized modification for metal nodes and functionalized modification for organic linkers; (3) by encapsulation of guest species in MOFs, MOFs are able to accommodate guest species (e.g., metal complexes, enzyme, and metal NPs (MNPs)) into their pore space, extending the application of MOFs in catalysis; (4) MOF derivatives, the much enhanced stability of MOF derivatives makes them suitable catalysts for reactions under harsh reaction conditions^[1].

Fig.1 Functional the catalytically active sites of MOFs by modification for metal nodes

• The types of catalytic reactions: Some typical heterogeneous catalytic reactions by MOFs are shown. (1) Oxidation reactions, such as oxidation of alcohols, hydrocarbons, CO, olefins, sulfur derivatives and others; (2) reduction reactions, such as unsaturated bonds hydrogenation, hydrogenation of diverse nitro compounds and so on; (3) dehydrogenation, hydrogen production from hydrogen-rich solid compounds, such as hydrazine, hydrazine borane, and ammonia borane; (4) C-C bond-forming reaction, such as Suzuki-Miyaura, cyanosilylation, and Knoevenagel condensation reactions, Friedel-Crafts reaction; (5) Cycloaddition, such as cycloaddition of CO₂; (6) polymerization reactions.

Fig.2 Schematic diagram showing the oxidization of CO to CO₂ by MOFs

Application of COFs in Heterogeneous Catalysis

COFs are fascinating as they can merge a series of structural features including stability, porosity, designability, and tunability in one material. The catalytic actions about COFs in heterogeneous catalysis are described below.

• The sources of catalytic active: Based on the structural features, the catalytic active of COFs based catalysts into four different structural origins: (1) catalysis based on backbones; (2) catalysts based on π skeletons; (3) catalysts based on side walls; (4) catalysts based on pore surface engineering which enables the integration of catalytic sites into the channels without any coordinative monomers. These different catalytic active can be applied for different types of transformations^[2].
• The types of catalytic reactions: Some typical heterogeneous catalytic reactions by COFs are listed. (1) Coupling reactions, such as Suzuki-Miyaura reaction, Heck reaction, Chan-Evans-Lam reaction, and Sonogashira reaction; (2) oxidation and reduction reactions; (3) condensation reactions, such as Knoevenagel condensation reaction, aldol condensation reaction, Mannich reaction, Henry reaction and so on; (4) Addition reactions, such as Michael addition, Diels−Alder reactions and others; (5) Isomerization reactions.

Advantages of using MOFs and COFs

• Uniform pore size/shape allows the accessibility of reaction substrates or products with specific shape/size, endowing the selective catalysis of MOFs and COFs;
• High surface area favors the adsorption and enrichment of substrate molecules around the active sites, benefiting the subsequent activation and catalytic conversion;
• Unique channel structure and tunable porosity provide efficient access to active sites and fast mass transport for catalytic reaction;
• Well-defined structures, which are of vital importance to understand the underlying mechanism and relationship between structure and catalytic performance.
• With more improved stability, high-activity and recycling ability.

What Can Alfa Chemistry Do

Alfa Chemistry provides various MOFs and COFs with high catalytic activity which can be used in different types of heterogenetic organic reactions. And our professional technology team also provides customers with high-quality MOFs and COFs design and customization services, no matter what design ideas you have, we will implement them together with you. In addition, Alfa Chemistry is committed to supporting customers a series of solutions in heterogeneous catalysis fields by using MOFs and COFs. Please contact us immediately to order or cooperate in research and development with high quality and reasonable price.