Preparation Of Composite Molecular Sieves By Gas-Solid Phase Method

Molecular sieve is a porous material made of TO₄ tetrahedra connected by oxygen bridge bonds,with uniform pore structure,large specific surface area and high thermal stability.Among them,ZSM-5 molecular sieve with crossed decadal ring pore structure,and Y molecular sieve with super cage structure are widely used as important catalysts in petrochemical,fine chemical and gas adsorption.The gas-solid method is a more environmentally friendly way to synthesize molecular sieves than the traditional method,and the special acidity and pore structure of the composite molecular sieves can play an excellent synergistic effect in the catalytic reaction.In this paper,a four-layer"cage drawer"structure is used to enhance the interphase mass transfer and to investigate the optimal feedstock ratio and synthesis conditions of each molecular sieve.Three types of solid-phase precursors,namely direct mixing,mixed steam-drying and silica-alumina precipitation,and two types of synthesis methods,namely steam-assisted and steam-transfer,were investigated.The direct mixing method is suitable for the preparation of molecular sieves with high silica-alumina ratio,and the crystallization of ZSM-5 molecular sieve is in indirect contact with the solvent,so there is no silica-alumina dynamic equilibrium between the two phases,and it is easier to prepare micro-meso-multiple pore structure with rough surface and good diffusivity;the silica-alumina precipitation method is used for low-silica molecular sieves,and the dissolution and homogeneous mixing of silica-alumina solute in the preparation of Y molecular sieve is the key to its nucleation,so it is easier to prepare particles Uniform and smooth surface microporous structure.It was found that the weak alkaline system can effectively prevent the formation of impurity crystals,and the ratio of the two molecular sieves can be adjusted by changing the dosage ratio of each molecular sieve,the silica-alumina ratio during secondary crystallization and the synthesis conditions.The activity and selectivity of each molecular sieve were investigated in the 450℃atmospheric pressure cracking reaction of dodecane,and the gas-solid ZSM-5 molecular sieve with large external specific surface area could better expose the reactive sites,and the conversion rate of dodecane was as high as 99.81%,while the industrial product was only 45.73%,because the multi-stage pore structure enhanced its diffusion performance and reaction activity.The higher percentage of micropores and larger pore size of the ZSM-5/Y composite molecular sieve enabled it to maintain 83.02%conversion for 40 h and showed higher selectivity for olefins and C₁-C₄small molecule products.