碱处理对ZnZrOx/SAPO-34催化合成气一步法制低碳烯烃的影响

Effect of alkali treatment on ZnZrOx/SAPO-34 bifunctional catalyst for catalytic synthesis of light olefins from syngas

采用ZnZrOx金属氧化物和SAPO-34分子筛物理混合制备了双功能催化剂,用于合成气一步法制低碳烯烃(STO)反应。考察了三乙胺、四甲基氢氧化铵和四乙基氢氧化铵三种有机碱溶液及不同浓度的三乙胺溶液处理对SAPO-34分子筛织构、结构和酸性的影响,采用XRD、SEM、N2吸附-脱附、NH3-TPD对分子筛进行了表征,并考察了碱处理后催化剂的STO反应性能。结果表明,采用0.06 mol/L的三种有机碱后处理均可在SAPO-34分子筛表面刻蚀出部分多级孔道,从而在STO反应中加速金属氧化物表面形成的中间过渡物种从金属氧化物表面扩散进入SAPO-34分子筛孔道,提高了催化剂在STO反应中CO的转化率,同时,三种碱处理均可降低SAPO-34分子筛的酸量及酸强度,从而提高催化剂在STO反应中低碳烯烃选择性;采用0.02-0.10 mol/L的三乙胺处理SAPO-34分子筛,均在SAPO-34分子筛表面刻蚀出的多级孔,提高了STO反应中CO的转化率,且0.02和0.06 mol/L的三乙胺溶液处理后的SAPO-34分子筛,酸强度和酸量的降低,抑制了甲烷的形成和烯烃的加氢,因此,随着碱处理浓度从0、0.02到0.06 mol/L逐步提高,催化剂对低碳烯烃的选择性逐步提高。其中,在400℃,3.0 MPa和GHSV=3600 mL/(g·h)条件下,采用0.06 mol/L的三乙胺处理的SAPO-34物理混合ZnZrOx,与未经处理的SAPO-34分子筛相比,CO转化率从24.0%提升至26.4%,低碳烯烃选择性从78.2%提升至84.7%,且该催化剂具有较好的催化稳定性。

A bifunctional catalyst was prepared by physical mixing of ZnZrOx metal oxide and SAPO-34 zeolite for the one-step conversion of synthesis gas to light olefins(STO)reaction.The effects of triethylamine,tetramethylammonium hydroxide and tetraethylammonium hydroxide solutions and different concentrations of triethylamine solution on the texture,structure and acidity of SAPO-34 zeolite were investigated.XRD,SEM,N2 adsorption and desorption,NH3-TPD were used to characterize the SAPO-34 zeolite and the STO reaction performance of the catalyst after alkali treatment was investigated.The results show that all three kinds of organic base with 0.06 mol/L post-treatment can etch some hierarchical channels on the surface of SAPO-34 zeolite,thus accelerating the diffusion of intermediate transition species formed on the surface of metal oxides into the channels of SAPO-34 zeolite in STO reaction,improving the CO conversion rate in STO reaction.At the same time,all three kinds of alkali treatments can reduce the acid amount and acid strength of SAPO-34 zeolite,thereby improving the selectivity for light olefins in the STO reaction.The treatment of SAPO-34 zeolite with 0.02−0.10 mol/L triethylamine resulted in the formation of hierarchical pores etched on the surface of SAPO-34 zeolite,which improved the conversion rate of CO in the STO reaction.Moreover,the acid strength and acidity of SAPO-34 zeolite treated with 0.02 and 0.06 mol/L triethylamine solutions decreased,inhibiting the formation of methane and the hydrogenation of light olefins.Therefore,as the concentration of alkali treatment gradually increased from 0,0.02 to 0.06 mol/L,the selectivity for light olefins gradually increases.Under the reaction conditions of 400℃,3.0 MPa and GHSV=3600 mL/(g·h),the CO conversion rate increased from 24.0%to 26.4%,and the selectivity of light olefins increased from 78.2%to 84.7%on the bifunctional catalyst composed of 0.06 mol/L triethylamine-treated SAPO-34 compared to untreated SAPO-34 zeolite,and the modified bifunctional catalyst had good catalytic stability.