In the thermodynamic equilibrium composition, the proportion of 2, 6-dmN is very low, only 12%, lower than 1, 6-dmn, 1, 7-dmn, 1, 3-dmN. Without the participation of zeolite molecular sieve, it is difficult to obtain a high selectivity of 2, 6-dmN. Therefore, the key technology of alkylation reaction between naphthalene and methyl naphthalene is the preparation of efficient catalyst. The molecular sieve catalyst suitable for the selective synthesis of 2,6-DMN must have two basic conditions: (1) suitable acidity. The weak acid center is beneficial to the synthesis of 2,6-DMN, while the strong acid center promotes the occurrence of side reactions, which is not conducive to improving the selectivity of the target product.
On the premise of high selectivity of 2,6-DMN, suitable pore size is conducive to the diffusion of reactants and products, and can effectively improve the conversion rate and catalyst life.
Molecular sieve modification is the direction
In order to improve the catalytic performance of molecular sieve in the reaction, it is usually necessary to modify the catalyst. The main modification methods include ion exchange, impregnation, chemical vapor or liquid phase deposition and hydrothermal dealuminization. The skeleton SiO2/Al2O3 ratio of zeolite molecular sieve is a very important parameter. The acid center concentration of zeolite molecular sieve is larger than that of low zeolite molecular sieve. When it is used as a catalyst, the catalytic activity is higher, but the selectivity is relatively low, and the catalyst is easy to be deactivated due to carbon accumulation. Dealuminization by post-treatment can improve the ratio of SiO2/Al2O3 and thus improve the catalytic performance of the catalyst. The commonly used dealuminization methods include pickling, deep roasting, hydrothermal heat treatment, dealuminization modified by silicon-containing halide, modified by fluoride and so on. By modifying the pore size or pore zigzag degree of zeolite, a larger diffusion rate difference between different products was obtained, and the acid catalytic center on the outer surface of zeolite without shape selection was removed, so as to improve the selectivity of the target products on the catalyst.
