ZSM-5構造のベリロケイ酸塩およびアルミノベリロケイ酸塩の合成と触媒特性

抄録

Beryllosilicate (BeZSM-5) and aluminoberyllosilicate (AlBeZSM-5) with ZSM-5 structure were synthesized at 100°C from clear solutions in the system Na₂O-TPA₂O-BeO-SiO₂-H₂O. Crystals of these compounds grew up to 8μm in period of 21 to 28 days. The AlBeZSM-5 contained a small amounts of alumina which came from alumina impurity present in a silica source. The BeZSM-5 contains Be atoms in the range of 1.50 to 12.74 and Al atoms in the range of 0.03 to 0.06 per unit cell, whereas the AlBeZSM-5 contains Be atoms in the range of 0.58 to 4.59 and Al atoms in the range of 0.65 to 1.20 per unit cell. The isomorphous replacement of Si by Be did not lead the change in lattice constants, and hence it is considered that interatomic distance Be-O in these compounds is approximately equal to Si-O distance.
The relation between the catalytic activity for methanol to hydrocarbons conversion reaction over HBeZSM-5 and HAlBeZSM-5 catalysts and the Be contents in these catalysts were examined. The catalytic activities of HBeZSM-5 increased with increasing Be content, although they were lower than those of HZSM-5. The major products over the HBeZSM-5 were aliphatic hydrocarbons including C₂ and C₄ ones. This may be due to the fact that Be atom brings about an increase in the number of acid sites in this catalytic system. The HAlBeZSM-5 showed high activity ranging from 95 to 100% in the conversion of methanol to hydrocarbons. These results indicate that Al atoms in the framework of this catalyst enhance its catalytic activity and the acid strength of H-AlO₄ tetrahedron is stronger than that of H-BeO₄ one.
The ratios of aliphatic hydrocarbons to aromatic ones formed over HAlBeZSM-5 catalysts are high compared to those over HZSM-5 catalysts. The main aliphatics formed are n-and iso-C₄, but their product distributions shift to the side of lower molecular weight hydrocarbons with reaction time as the Be content in catalysts increases, and the formations of isoparafins and isoolefins are inhibited at the same time. The main aromatics formed are xylenes and trimethylbenzenes (or toluene), and their product distributions vary regardless of reaction time and the Be content in catalysts. However, the alkylation selectivity (trimethylbenzenes/xylenes) reduces with the Be content, and the product selectivity in xylene isomers is approximately constant.