论文部分内容阅读
研究了甲苯、甲醇在一系列ＺＳＭ５沸石催化剂上生成二甲苯的烷基化反应。以ＮＨ３为探针分子，用微分吸附量热法表征了这些催化剂的酸性，并与催化活性关联。结果发现，ＨＺＳＭ５沸石上既有强酸位，又有弱酸位，反应产物为热力学平衡组成的二甲苯异构体。浸渍了镁之后，强酸位被中和（对ＭｇＺＳＭ５（２），吸附热位于１３４～８０ｋＪ／ｍｏｌ之间），弱酸位吸附热和样品单位面积上的吸附覆盖度几乎没变。ＭｇＺＳＭ５（２）沸石上的对二甲苯产率为１４．１％，其选择性达到１００％。ＰＺＳＭ５沸石上的弱酸位和大部分强酸位都被中和，对二甲苯的产率较低（６．０％），但选择性也达到１００％。可见，催化剂的强酸性并不有利于甲苯甲醇烷基化反应的高对位选择性。更重要的是中和沸石外表面的强酸位以抑制初级产物在其上进一步发生次级异构化反应。
The alkylation of toluene and methanol to xylene over a series of ZSM5 zeolites was studied. Using NH3 as a probe molecule, the acidity of these catalysts was characterized by differential adsorption calorimetry and correlated with the catalytic activity. The results showed that both HZSM5 zeolites had strong acid sites and weak acid sites. The reaction products were xylene isomers with thermodynamic equilibrium. After impregnation of magnesium, the strong acid sites were neutralized (for MgZSM5 (2), the heat of adsorption was between 134 and 80 kJ / mol), the adsorption heats at weak acid sites and the adsorption coverage per unit area of the sample remained almost unchanged. The p-xylene yield on the MgZSM5 (2) zeolite was 14.1% with a selectivity of 100%. The weak acid sites and most of the strong acid sites on PZSM5 zeolite were neutralized. The yield of p-xylene was lower (6.0%) but the selectivity was also 100%. It can be seen that the strong acidity of the catalyst does not favor the high para-selectivity of the toluene-methanol alkylation reaction. More importantly, the strong acid sites on the outer surface of the zeolite are neutralized to inhibit further primary isomerization reactions on the primary product.