Zhang Nan, Liu Chunli, Ma Jinghong, Li Ruifeng, Jiao Haijun
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, China.
Phys Chem Chem Phys. 2019 Aug 28;21(34):18758-18768. doi: 10.1039/c9cp04050a.
Determining the locations and distributions of Al substitution in zeolite-based catalysts and catalysis is always very challenging. Despite advanced experimental characterization techniques and improved theoretical models, this issue is not reasonably solved and this is because the locations and distributions of Al substitution in zeolites are more kinetically than thermodynamically controlled. In this work, we computed one Al substitution in the orthorhombic form of MFI (HZSM-5) which contains 12 distinct tetrahedral (T) centers on the basis of a periodic slab model containing 96 T centers including van der Waals dispersion correction (GGA-PBE-D3). For all 12 T centers, there are 48 acidic sites and each site can be considered for the adsorption of probe molecules. Thermodynamically, the energy span of the twelve most stable acidic sites is less than 15 kJ mol-1, and such a small energy difference enables all adjustable possibilities for the locations and distributions of Al substitution under suitable conditions. Excellent agreement between experiment and theory in the adsorption enthalpies of pyridine, methylamine, dimethylamine and trimethylamine shows that the location of Al substitution is most likely at T1, T3, T5, T7 and T11, while much less likely at the often used T12 site. These results provide the basis for identifying Al substitution in new synthesized HZSM-5 catalysts and for studying the acidic site-dependent catalytic activity of HZSM-5 in cracking and hydrogenation reactions.
确定铝在沸石基催化剂中的取代位置和分布以及催化作用一直极具挑战性。尽管有先进的实验表征技术和改进的理论模型，但这个问题仍未得到合理解决，这是因为沸石中铝取代的位置和分布更多地受动力学控制而非热力学控制。在这项工作中，我们基于包含96个T中心（包括范德华色散校正，即GGA - PBE - D3）的周期性平板模型，计算了正交晶系MFI（HZSM - 5）中一个铝的取代情况，该模型包含12个不同的四面体（T）中心。对于所有12个T中心，有48个酸性位点，每个位点都可考虑用于探针分子的吸附。从热力学角度看，十二个最稳定酸性位点的能量跨度小于15 kJ/mol，如此小的能量差异使得在合适条件下铝取代的位置和分布具有所有可能的调整情况。吡啶、甲胺、二甲胺和三甲胺吸附焓的实验与理论结果高度吻合，表明铝取代的位置最可能在T1、T3、T5、T7和T11，而在常用的T12位点可能性要小得多。这些结果为鉴定新合成的HZSM - 5催化剂中的铝取代情况以及研究HZSM - 5在裂化和氢化反应中依赖酸性位点的催化活性提供了依据。
