Study On Modified Synthesis Of SAPO-34 And Its Application In Syngas Conversion To Light Olefins

Light olefins(C⁼_2-4,i.e.,ethylene,propylene and butene)are important basic organic chemical raw materials and the cornerstone of modern chemical industry,whose production strongly depends on petroleum resources.The scarcity of domestic petroleum resources,coupled with the growing demand for light olefins,has created an urgent need to develop a non-petroleum route for olefin production process.In recent years,methanol to olefin（MTO）technology has been extensively researched,which realizes the transformation from coal or natural gas to methanol and then to light olefins.To improve the conversion efficiency and reduce the equipment investment,direct conversion from syngas to light olefins can be achieved by using a bifunctional catalyst composed of oxide-zeolite（OX-ZEO）to catalyze the direct production of light olefins from syngas（STO）,thus overcoming the limitation of light olefins selectivity in the Fischer-Tropsch synthesis reaction.In the OX-ZEO catalyst,the activation and conversion of CO and H₂ occur on the surface of the metal oxides,and intermediates such as methanol and alkenone are generated,which are later converted to the target products by the zeolite.The type and distribution of target products are directly related to the particle size and acid properties of the zeolite,etc.SAPO-34 has a CHA structure and its special shape-selective effect is widely used in MTO.However,the particle size and physic-chemical properties of SAPO-34 in catalytic STO still need to be investigated.In this work,a series of SAPO-34 zeolites with different templates were synthesized by hydrothermal method,and the particle size of zeolites was regulated by selecting templates（diethylamine（DEA）,morpholine（MOR）and tetraethylammonium hydroxide（TEAOH））.The acid properties of SAPO-34 were regulated by the molar ratio of silicon to aluminum（Si O₂/Al₂O₃）,while Me SP-34 zeolite was prepared by in situ synthetic modification of SAPO-34 using different metals Me（Zn,Ce,Zr）and different Zr doping amounts.The effects of the modified synthesis conditions on the particle size and physicochemical properties of SAPO-34 were investigated with the assist of XRD,TEM,SEM-EDS,N₂adsorption-desorption,FTIR,NH₃-TPD,XPS,TG and GC-MS characterization.Finally,the synthesized SAPO-34 zeolites were physically mixed with Ga Zr O_x metal oxides reported by our group to prepare Ga Zr O_x/SAPO-34 bifunctional catalysts,and their performance in catalyzing the STO was investigated.In the first part of this thesis,the SAR0.4-M and SAR0.4-D zeolites synthesized with MOR and DEA templates,respectively,both had larger particle sizes,while the SAR0.4-T synthesized with TEAOH or SARx-TD synthesized with mixed templating agents both had smaller particle sizes and larger specific surface areas.The catalytic evaluation results showed that the SAPO-34 with small particle size was beneficial to improve CO conversion and C⁼_2-4selectivity,when the SAPO-34 particle size was small and the difference was not large,reducing the amount of Br（?）nsted acid in the zeolite could improve the selectivity of C⁼_2-4.Among them,SAR0.4-TD with Si O₂/Al₂O₃ molar ratio of 0.4 can achieve CO conversion of28.0%and C⁼_2-4 space-time yield of 261.0 m L·g^-1_cat·h^-1 when used for the preparation of Ga Zr O_x/SAR0.4-TD bifunctional catalysts.by the STO,the causes of catalyst deactivation were analyzed by the evolution of the carbon species accumulated on the catalyst after the reaction.In the second part,SAPO-34 was synthesized hydrothermally using different metal Me（Ce,Zn,Zr）and Zr content doping.The results showed that the doping of Zr decreased the zeolite particle size and increased the relative crystallinity.The particle size of 1.0%Zr SP-34synthesized with 1.0%Zr doping was the smallest with an average particle size of 0.53μm and moderate amount of Br（?）nsted acid(1.34 mmol·g^-1);the doping of excessive Zr led to the presence of excess Zr in the form of Zr O₂ on the surface of the zeolite,covering the strong acid center.Compared with the unmodified SAPO-34,the Ga Zr O_x/1.0%Zr SP-34 bifunctional catalyst prepared by 1.0%Zr-doped synthesis of 1.0%Zr SP-34 increased the CO conversion from 14.2%to 21.2%and C⁼_2-4 selectivity from 71.0%to 82.4%,and the catalyst did not show significant deactivation after 60 h of reaction.