乙醇制丙烯反应热力学及HZSM-5/SAPO-34复合分子筛的催化性能

采用平衡常数法计算了乙醇制丙烯的主、副反应在不同温度下的反应热、Gibbs自由能变和热力学平衡常数,以及不同反应温度和乙醇分压下产物的热力学平衡组成。热力学分析结果表明,降低反应温度和提高乙醇分压有利于提高丙烯的平衡组成。研究了复合分子筛HZSM-5/SAPO-34催化剂的乙醇制丙烯性能以及催化剂的结构和酸性。实验结果表明,反应温度和乙醇分压对HZSM-5/SAPO-34催化剂上丙烯收率的影响与热力学计算结果略有差别,这是因为热力学分析未考虑烯烃齐聚物和芳烃的二次反应及催化剂的酸催化和择形催化作用。HZSM-5/SAPO-34催化剂的酸量和酸强度分布与其活性密切相关。在773 K、乙醇分压0.020 MPa时,m(HZSM-5)∶m(SAPO-34)=4的HZSM-5/SAPO-34催化剂上丙烯收率最高(34.5%),主要归因于该催化剂具有适宜的酸量和酸强度分布。

Thermodynamics of Preparation of Propylene from Ethanol and the HZSM-5/SAPO-34 Composite Zeolite Catalysts

The reaction heats,changes of Gibbs free energy and equilibrium constants of the reactions in preparation of propylene from ethanol were calculated by equilibrium constant method.Thermodynamic equilibrium composition of the products under different reaction temperature(T) and ethanol partial pressure(p(ethanol)) was estimated.The thermodynamic calculation results indicated that low T and/or high p(ethanol) were beneficial to increasing propylene content in the equilibrium composition.The preparation of propylene from ethanol over HZSM-5/SAPO-34 composite catalysts with different HZSM-5/SAPO-34 mass ratio was carried under different T and p(ethanol).The result was different from that obtained by the thermodynamic calculation,possibly due to the ignored acid catalysis in the thermodynamic calculation.The concentration and strength distribution of the acid sites on the catalyst were close relative to its activity.The propylene yield could be 34.5% under the optimal conditions of HZSM-5/SAPO-34 mass ratio in the catalyst 4,773 K and p(ethanol) 0.020 MPa,owing to the catalyst’s moderate concentration and strength distribution of the acid sites.