ZSM-5沸石骨架铝迁移规律的研究 Ⅲ.ZSM-5沸石骨架铝迁移的可逆性

STUDIES ON THE MIGRATION OF FRAMEWORK ALUMINIUM OF ZSM-5 ZEOLITE Ⅲ.THE REVERSIBILITY OF THE MIGRATION OF FRAMEWORK ALUMINIUM OF ZSM-5 ZEOLITE

考察了水热条件下HZSM-5沸石骨架铝的迁出,气-固或液-固反应条件下外界铝进入骨架的影响因素及表面性质的变化。发现在相同的条件下,随ZSM-5沸石硅铝比的增加,铝迁入骨架的比率增大;经水热处理后较相同数量级硅铝比的未经处理的沸石的铝迁入骨架速率快,结合骨架铝迁移的机理对此现象进行了讨论。在400℃气-固反应时,进入沸石骨架的铝量达400—600℃总量的75—80％,和在400℃水热处理时骨架铝的迁脱量相当(80％),这部分铝主要对应于强酸中心位,说明沸石骨架中对应于酸中心较强而稳定性较弱的铝易迁出也易迁入,表明酸性中心是可逆的。

High temperature gas-solid reaction and liquid-solid reaction at 373K with AlC13 and NaAlO2(aq.) as alumination reagent respectively were used to incorporate Al into the HZSM-5 zeolite framework. The amount of Al incorporated into the framework increased with the increase of SiO2/Al2O3 ratio of the aluminated samples. The alumination was also influenced by hydrothermal treatment of the sample used.The 27A1 MA-SNMR and NH3-TPD results indicated that both the peak area of tetra-hedral Al and NH3-TPD increased with the gas-solid reaction temperature used for alumination. Interestingly, the amount of Al incorporated into the framework of HZSM-5(3400) after gas-solid reaction at 673K reached 75% of the maximum value incorporated at 773K and about the same portion (80%) of framework Al migrated out of the framework upon steaming HZSM-5(50) at 673K. The portion of Al migrated out or incorporated into framework was parallel to the amount of dissipation or generation of the total acidic sites.It was thus evidenced that the framework Al relevant to the strong acidic sites was quite mobile as compared to those relevant to the weak acidic sites. Sample dealuminated by steaming was used for alumination. The observed results of the restoration of crystal parameters, the content of framework Al and the surface acidity suggested that the migration/incorporation of Al in ZSM-5 zeolite catalyst is partially reversible.