SAPO-34上甲醇及二甲醚吸附等温线及吸附热测量与分析(英文)

在25,60和100°C下分别测定了甲醇及二甲醚在SAPO-34分子筛上的吸附等温线,同时用微量热法测定了微分吸附热与覆盖率的关系曲线(量热线),提出了吸附数据需要利用双吸附位Langmuir方程拟合,并获取了相应的吸附参数.对比测得的吸附等温线与量热线发现,在一定压力下,当甲醇及二甲醚在SAPO-34上达到一定吸附量后,随着吸附质分压增加,量热线快速下降,而吸附等温线显示出吸附量仍然继续增加.由此推断,在SAPO-34分子筛上存在两种吸附位——常规吸附位及弱吸附位,其中弱吸附位在高分压下继续吸附.如缺乏量热数据提供的常规吸附位饱和吸附量数据,对吸附等温线进行单吸附位拟合获取吸附参数极易导致错误结果,尤其是当吸附质分压较高时.建议采用双吸附位Langmuir方程,参照量热线提供的常规吸附位的饱和吸附量,通过拟合可以获得两种吸附位的吸附参数.

Adsorption isotherms of methanol and dimethyl ether on SAPO-34 measured together with differential adsorption heat measurement

Adsorption isotherms of methanol and dimethyl ether on a SAPO-34 zeolite were measured at 25, 60, and 100 ℃. The curves of differential heats of adsorption versus coverage (calorimetric curves) were simultaneously measured with a microcalorimeter, which showed that a simple interpretation of the adsorption isotherms would be incorrect. The calorimetric curves showed sharp drops, and the coverages at which these occurred were taken as the saturation amounts of adsorbed methanol and dimethyl ether on regular sites. However, beyond the saturation coverage, the adsorption isotherms still showed increasing coverages, which were attributed to adsorption on weaker sites, and which should not be used during curve fitting to get the Langmuir parameters of the adsorption isotherm for the regular adsorption sites. In the absence of the calorimetric data, these regions would have been assumed to be part of the adsorption isotherm for the regular adsorption sites, which would be erroneous. To account for the existence of another weaker adsorption site which continued to be populated at high pressures, in addition to the regular adsorption site, we used the dual-site Langmuir equation with the saturation amount on the regular adsorption site obtained from the calorimetric curves. This gave good fits also in the high pressure region where adsorption mainly occurred on the weaker sites.