Synthesis And Application Of SAPO-34@Silicalite-1 Core-shell Molecular Sieve

The MTO reaction is an important part of the preparation process of olefins from coal or natural gas.SAPO-34 molecular sieve crystals have regular microporous channels,a large number of acidic sites distribution in the channel and high ethylene and propylene selectivities in the MTO reaction.The outer surface of the nuclear molecular sieve is covered with a layer of low acidity molecular sieve membrane,which can modify the surface acidity and pore structure of SAPO-34 crystals.Silicalite-1 molecular sieve is the full-silicon form of ZSM-5 molecular sieve with low crystal size and weak acidity,which is suitable to be a shell phase zeolite membrane for the modification of nuclear phase SAPO-34 molecular sieve.In this paper,the best preparation conditions of SAPO-34 molecular sieve crystals were studied by the comparison of template,crystallization time and crystallization temperature.The results showed that the optimum conditions for the preparation of SAPO-34 zeolite were as follows:TEAOH as template,215?,99h.At the same time,the effects of TPABr solvent,TPABr and SAPO-34 dosage ratio and the pretreatment time of poly cationic agent on the exchange efficiency of SAPO-34 were investigated,and the optimum pretreatment conditions of polyacetic acid reagent were determined.After that SAPO-34 molecular sieve crystals were pretreated with TPABr isopropanol solution,the SAPO-34@Silicalite core-shell molecular sieves were prepared by seed secondary growth method.The crystallinity,specific surface area,pore volume,pore size,morphology and acidity of the molecular sieve catalyst were investigated by means of X ray diffraction,N2 low temperature adsorption,scanning electron microscopy and NH3 temperature programmed desorption.The catalytic performance of SAPO-34 and SAPO-34@Silicalite-1 zeolite in MTO reaction was conducted in a fixed-bed reactor.The results showed that using SAPO-34@Silicalite-1 molecular sieve as the catalyst of MTO process,catalytic life and the selectivity of ethylene were improved compared with that of SAPO-34 zeolite catalyst but the selectivity of propylene was decreased,which leads to a reault that the selectivity of propylene and ethylene increased.