Three-stage Crystallization Synthesis And MTO Performance Study Of Small Crystal SAPO-34

SAPO-34 molecular sieve is an effective catalyst for the methanol to light olefins(MTO)process.The large surface area and short pore length of small crystal SAPO-34 molecular sieves favors the diffusion of methanol and olefins,which benefits the full utility of internal cavity of crystal,and hence inhibits the deactivation of catalyst,prolongs the catalytic life of catalyst and improves the selectivity of olefins.Generally,small crystal SAPO-34 molecular sieves are prepared with the expensive TEAOH as a template.The low-price TEA and MOR,however,tend to produce SAPO-34 molecular sieves in much larger size.In this work,a three-stage hydrothermal crystallization procedure is proposed for the synthesis of small crystal SAPO-34 with triethylamine and morpholine as template.Samples properties were characterized by XRD,SEM,N2 adsorption-desorption and NH3-TPD.MTO performance were evaluated on a fixed bed reactor.The main results and conclusions are as follows:1.Contrast to samples synthezed by the traditional one-stage and two-stage hydrothermal crystallization methods,SAPO-34 samples derived from three-stage process have smaller particle size and higher yield,When TEA was used as a template,the particle size of onetwo-and three-stage SAPO-34 are about 10,5 and 1.5 ?m,respectively.The yields are about 51.56%,69.55% and 74.21%,respectively.When MOR was used as a template,the particle size of one-two-and three-stage SAPO-34 are about 15,5 and 3 ?m,respectively.The yields are about 39.68%,66.64% and 68.96%,respectively.When TEAOH was used as a template,the particle size of one-two-and three-stage SAPO-34 are about 750,600 and 400 nm,respectively.The yields are about 38.23%,41.33% and 49.70%,respectively.2.Compared with one-stage and two-stage process,three-stage process improves the catalytic life and olefins selectivity of SAPO-34.For the TEA as template,life time is prolonged from 205 min of the one-stage samples to 330 min of the three-stage one,and the olefins selectivity(C2=~C4=)are improved from 92.66% of the one-stage samples to 94.68% of the three-stage samples.For the MOR as template,life time is prolonged from 180 min of the one-stage samples to 275 min of the three-stage one,and the olefins selectivity(C2=~C4=)are improved from 90.53% of the one-stage sample to 92.46% of the three-stage sample.For the TEAOH as template,life time is prolonged from 245 min of the one-stage samples to 475 min of the three-stage one,and the ethylene and propylene selectivity(C2=~C3=)are improved from 77.64% of the one-stage samples to 80.43% of the three-stage samples,the butene selectivity decreased slightly.3.The conditions of three-stage hydrothermal crystallization were optimized.when the crystallization time of the first high-temperature stage was controlled at about 60 min and the crystallization temperature of low-temperature stage was controlled at about 100~130? in favor of SAPO-34 molecular sieves with excellent catalytic performance and light olefins selectivity(C2=~C4=).4.The mechanism of three-stage hydrothermal crystallization was discussed.The first high-temperature stage,during primary and secondary structural units are formed,should be limited in a relative appropriate time(60min)to product plenty amount of primary and secondary structural units for the following formation of crystal core.The second lowtemperature stage,during nucleus were formed in favor of suitable temperature interval(100~130?).The third stage is a high temperature stage,for the growth of nucleus.