Molecular sieve-Shape-selective catalysis

Molecular sieve is a material containing precise and single tiny holes, which can be used to adsorb gas or liquid. Small enough molecules can be adsorbed through the pores, but larger molecules cannot. Unlike an ordinary sieve, it operates at the molecular level. Molecular sieve is a new type of catalyst with specific spatial structure. Because different molecular sieves have different channel sizes, they are often used as support for shape-selective catalysts.

image
1. Shape-selective catalysis theory [1-4]

Shape-selective catalytic chemistry is a science that combines chemical reactions with the adsorption and diffusion properties of molecular sieves. It can change the known reaction pathways and product selectivity. The traditional shape-selective catalysis theory is mainly reflected in the molecular sieve effect, mass transfer selectivity and transition state selectivity.

(1) Molecular sieve effect. In shape-selective catalysis, this effect is reflected in reactant selectivity or product selectivity. In the mixed raw materials, only molecules that can enter the pores of the carrier and contact the active centers in the pores and participate in the reaction can be used as reactants, and molecules larger than the pore size of the molecular sieve will be rejected from the pores and will not participate in the reaction. It is the selectivity of the reactants. The larger molecules formed in the pores may be converted into smaller molecules to escape in equilibrium, or block the pores in situ, and finally lead to the deactivation of the catalyst. This shows the selectivity of the product.

(2) Mass transfer selectivity. In shape-selective catalysis, not only is the shape-selective effect caused by the restriction of molecular penetration through the pores of the molecular sieve, but also the mass transfer is restricted after the molecules enter the inner pore. Especially when the molecular diameter of the reactant or product is close to the pore diameter of the molecular sieve, the diffusion of molecules in the crystal will be restricted due to the effect of the inner pore wall field and various energy barriers. The diffusion that occurs under this condition is different from the Kundsen diffusion and general gas phase diffusion that are common in amorphous porous materials for gaseous molecules. Small changes in the pore size of the molecular sieve or the diameter of the diffusion molecules will cause significant changes in the diffusion coefficient. This change is attributed to the change in configuration when the molecule penetrates the pores of the molecular sieve. At this time, diffusion is not only related to the length of the molecule, but also related to the internal movement of the molecule. This kind of diffusion, which Weisz calls configuration diffusion, mostly occurs in 0.4～1.0 nm. A reaction limited by configuration diffusion, the reaction rate will be affected by the catalyst grain size and activity.

(3) The selectivity of the transition state. When the reactant and product molecules can diffuse in the pores, but the transition state (reaction intermediate) required to form the final product is large, the size or orientation of the reaction intermediate requires a larger space, and the effective space in the molecular sieve pores However, it is very small and can not provide the required space, then the transition state cannot be formed in the pores of the molecular sieve, and the reaction cannot proceed at this time, so the reaction shows the transition state selectivity. This selectivity is different from the mass transfer selectivity. It has nothing to do with the size or activity of the molecular sieve crystals, but only depends on the pore size and structure of the molecular sieve. For high molecular weight n-alkanes, due to its slow diffusion in the molecular sieve pores and the difficulty of forming cyclopropyl carbanion intermediates in the pores, the traditional shape-selective catalysis theory is difficult to explain well. . Martens et al. proposed the concept of pore catalysis based on the detailed analysis of the hydrogen reaction products of i-C17H36 on Pt/ZSM-22. They believe that in the single side chain reaction, the reactant molecules do not pass through the pores, but are partially inserted into the pores of the molecular sieve. The skeletal isomerization reaction occurs on the molecules adsorbed on the pores and the outer surface of the molecular sieve. When one end of a single-side chain molecule is adsorbed in the pores of a molecular sieve crystal, the other end of the reactant molecular chain can also penetrate into the pores of the adjacent molecular sieve crystal and undergo an isomerization reaction. This mechanism is called a key. Lock catalytic. Although this mechanism is only a speculation, the concepts of pore catalysis and key-lock catalysis better explain the hydrocracking/isomerization product distribution of long-chain alkanes.

image
2. Several common molecular sieve catalysts

2.1 SAPO-11[5-6] molecular sieve catalyst

SAPO-11 molecular sieve catalyst is a mesoporous molecular sieve, with two-dimensional non-intersecting ten-membered ring elliptical pores, pore size 0.39nm*0.64nm, physical and chemical properties are similar to silica alumina zeolite, but also has some phosphoaluminate molecular sieves Characteristics.

SAPO-11 molecular sieve exhibits different acid strength due to different synthesis conditions, so it exhibits unique catalytic performance. At present, it has been used in various petroleum refining and petrochemical processes such as cracking, alkylation, and isomerization reactions. The acidity, redox characteristics, cleanliness and pore structure of the molecular sieve can be changed by loading and doping to realize the modification of the molecular sieve.

2.2 ZSM-5[8] molecular sieve catalyst

ZSM-5 molecular sieve catalyst has a unique pore structure and pore size, a stable framework and a wide range of adjustable silicon to aluminum ratio. It has excellent catalytic performance. There are two-dimensional ten-membered ring pores with a pore diameter of about 0.55nm, and high thermal stability and catalytic activity. Because of its unique pore structure and surface acidity and alkalinity, its catalytic reaction is mainly carried out in the acid-base center, which can be used in the process of converting methanol to hydrocarbons and the process of dehydrogenating low-carbon alkanes. At the same time, the high-silicon ZSM-5 molecular sieve is hydrophobic and has good activity and thermal stability for the conversion of methanol to hydrocarbons. ZSM-5 can be modified by steam method, ion exchange method, chemical vapor deposition method, and the modified molecular sieve can improve its catalytic performance.

2.3 Aluminum phosphate [9] molecular sieve catalyst

The framework of this type of molecular sieve catalyst is strictly alternated with AlO4 and PO4 tetrahedra, and the framework is electrically neutral. The central Al3+ and P5+ in the tetrahedron of the aluminum phosphate molecular sieve catalyst can be replaced by many different valence metal or non-metal elements to form heteroatom MeAlPO-n molecular sieve catalysts with different structures and properties. Because the AlPO4-5 molecular sieve catalyst has a three-dimensional microporous crystal structure, it is composed of phosphorus oxygen tetrahedron and aluminum oxygen tetrahedron, and is electrically neutral. Therefore, as a carrier, it has unique advantages that other substances do not have. Adding iron ions to the catalyst can effectively limit the formation of inactive graphite carbon and improve the stability of the catalyst.

2.4 TS molecular sieve [10] catalyst

The TS molecular sieve has a Ti4+ center in the framework structure, and the titanium silicon molecular sieve is a mesoporous molecular sieve with a crystal structure. Because the mesoporous structure of TS molecular sieve catalyst has not only redox performance but also weak Lewis acid performance, combined with the mesoporous structure of molecular sieve, it can be used for selective oxidation reaction, photocatalytic reaction and acid catalyzed reaction of macromolecular compounds, and it is environmentally friendly. Solid catalyst. TS molecular sieve catalyst can be successfully applied to the catalytic oxidation of cycloalkenes, cycloalkanes and unsaturated alcohols. With the improvement of titanium silicate molecular sieve, the size limit of the microporous inorganic framework is broken, and the possibility of catalyzing organic macromolecular substrates is provided. It has good application prospects in the field of fine chemicals and the pharmaceutical industry.

2.5 MCM molecular sieve [7] catalyst

MCM series catalysts are mesoporous catalysts, and their mesopores are disordered and amorphous. Their pores are arranged in order, and the pore size distribution is very narrow. After optimized synthesis conditions or post-treatment, it has certain hydrothermal stability, good thermal stability, large specific surface area (>400m2/g), high porosity, regular particle shape, adjustable composition, etc. . And it can maintain a high degree of pore order in the micrometer scale. It can be modified by changing the template, adding pore enhancer, adjusting the carbon chain length of the surfactant, and adding auxiliary agents. The modified MCM can also be used as adsorbents, catalysts and catalyst carriers, and can also be used in industries such as environmental protection, organic macromolecule synthesis, redox reactions, and petroleum refining.

2.6 SBA molecular sieve catalyst

SBA molecular sieve catalyst has a mesoporous structure, uniform pore diameter distribution, adjustable pore size, wall thickness and high hydrothermal stability. It has a large specific surface area (up to 2500 m2/g) and pore volume (up to 2.25 cm3/g) through methods such as increasing wall thickness, characterizing pore walls, doping with metal atoms, and coating water-absorbing membranes on the ion surface Its modification can increase its stability, thereby improving its application in the field of catalysis.


3. Conclusion

The application of molecular sieve catalysts has spread to petrochemical, environmental protection, bioengineering, food industry, pharmaceutical and chemical industries. Some molecular sieve catalysts are not very active in catalytic oxidation, so some researchers have modified them. Due to its small pore size, it is difficult for macromolecules to enter the pores, and the diffusion resistance is relatively large. The products formed in the pores cannot escape quickly, which greatly limits the application of microporous zeolite in the catalytic reaction of macromolecules. . Mesoporous molecular sieves can make up for the shortcomings of microporous molecular sieves and provide favorable spatial configurations for macromolecular reactions. However, because the pore wall of the mesoporous molecular sieve is in an amorphous state, its hydrothermal stability is poor, and its acidity is weak, which also limits its application range. Therefore, people are studying the synthesis of mesoporous composite materials with strong acidity and zeolite-type pore wall structure and microporous-mesoporous molecular sieve composite materials.