Center for Molecular Modeling, Ghent University, Technologiepark 903, 9052 Zwijnaarde, Belgium.
Chemistry. 2011 Aug 8;17(33):9083-93. doi: 10.1002/chem.201100920. Epub 2011 Jul 19.
The role of naphthalenic species during the methanol-to-olefins (MTO) process in a silicoaluminophosphate zeolitic material exhibiting the chabazite topology (H-SAPO-34) has been studied from first principles. These species could either act as active olefin-eliminating compounds or as precursors for deactivating species. Results incorporating van der Waals contributions for finite large clusters point out that successive methylation steps of naphthalenic compounds are feasible. The calculated intrinsic activation barrier is relatively independent of the number of methyl groups already attached on the aromatic compound and is approximately 140 kJ mol(-1). The influence of the composition of the catalyst and hence the acidic strength on the intrinsic chemical kinetics was investigated in detail through comparison with the isostructural high-silicon material. Apparent chemical kinetics, starting from adsorbed methanol on the acid site, were also computed. The initiation steps of the side-chain route starting from a trimethylated naphthalenium ion were also examined. The actual side-chain methylation exhibits a high barrier and hence this mechanism involving methylated naphthalenes is not expected to be an active ethene-eliminating route in H-SAPO-34.
采用第一性原理研究了具有菱沸石拓扑结构（H-SAPO-34）的硅铝磷酸盐沸石材料中萘酚物种在甲醇制烯烃（MTO）过程中的作用。这些物种既可以作为活性烯烃消除化合物，也可以作为失活物种的前体。包含范德华贡献的有限大团簇的结果表明，萘酚化合物的连续甲基化步骤是可行的。计算出的内在活化能垒与已附着在芳香族化合物上的甲基数目相对独立，约为 140 kJ/mol。通过与同构高硅材料进行比较，详细研究了催化剂组成（即酸度）对内在化学动力学的影响。还计算了从酸性位上吸附的甲醇开始的表观化学动力学。还检查了从三甲基化萘翁离子开始的侧链途径的引发步骤。实际的侧链甲基化表现出很高的势垒，因此，这种涉及甲基化萘的机理预计不会是 H-SAPO-34 中活性乙烯消除途径。
