HZSM—5沸石内外表面酸,孔大小和择形催化性能的研究

用吡啶、2,4-二甲基喹啉及NH_3吸附的TPD法测定了HZSM-5沸石的内、外表面酸量和酸分布,用环己烷吸附速度和吸附量表征孔大小,以乙苯歧化和对二乙苯异构化作研究择形催化反应.结果表明,HZSM-5外表面酸约占总酸量的20％,内、外表面酸均为非均匀分布.强酸位是对二乙苯和乙苯裂化的活性中心,而对歧化和异构化反应生成对位异构体的选择性影响不大.间二乙苯主要在内表面酸中心上生成,而对二乙苯在外表面酸位上异构化不是主要途径.对位选择性的提高与环己烷吸附速度减小有很好的对应关系,而与酸量变化关系不大.虽然中强和弱酸位对对位选择有一定影响,但影响对位选择性的主要因素是沸石的孔径大小.

Studies on Internal and External Surface Acid Pore Size and Shape-Selective Properties of HZSM-5 Zeolite

Internal and external surface acid amount and their distribution on HZSM-5 (SiO2/ A12O3 = 38) zeolite were measured by the TPD method with pyridine, 2,4-dimethylquinoline and ammonia adsorption. The pore size of zeolite is characterized by the adsorption rate of cyclohex-ane. Ethylbenzene disproportionation and para-diethylbenzene isomerization were used as the test reactions for examining the shape-selectivity of catalysts. The result shows that the external surface acid is twenty percent total acid approximately. Both internal and external acids contain strong and weak acid sites. The strong acid is the active site for creaking para-diethylbenzene and ethylbenzene and it has little effect on p-isomer selectivity of disproportionation and isomerization. The m-diethylbenzene is produced mainly on the internal surface acid site, but not by isomerization of p-diethylbenzene on the external surface. The increase of para-selectivity is in a good relation to the decrease of adsorption rate of cyclohexane, but poor to the acid amount variance. Thus, the major factor which affects the selectivity of ethylbenzene disproportionation to p-isomer is the pore size of HZSM-5 zeolite, but is not the strong acid site or the isomerization of p/>-diethylbenzene on the external surface acid.