Pore Modification of H-SAPO-34 Using Dialkyl Zinc: Structural Characterization and Reaction Pathway

摘要

The pore structure of a microporous material H-SAPO-34 is modified using organometallic reagents such as dialkyl zinc (ZnR~2, R = Me, Et) via both solution and vapor deposition techniques. After quenching with methanol and air calcination, the modified material is obtained (designated as H-SAPO-34/ZnMe2). H-SAPO-34/ZnMe2 is different compared with framework incorporated ZnAPSO-34 and zinc cation exchanged Zn/SAPO-34. Diffuse Reflectance UV/vis (DRUV) reveals a blue shift in the charge transfer band (202 nm→ 195 nm) for H-SAPO-34/ZnMe2, indicating perturbations to the SAPO-34 framework as a result of ZnMe2 modification. Two new resonances (3717 and 924 cm~(-1)) associated with dimethylzinc treatment are observed in FTIR and are attributed to a new Zn-OH moiety. A combination of solid-state ~1H and ~(13)C NMR analyses confirms that zinc is in the form of Zn-OH attaching to the framework oxygen in H-SAPO-34/ZnMe2. The reaction of ZnMe2 with H-SAPO-34 has been monitored by NMR following an in situ Chemical Vapor Deposition (CVD) treatment. On the basis of this study, a detailed reaction pathway is proposed: the Bronsted acid sites in H-SAPO-34 react with ZnMe2 upon contact, forming methane and anchoring the Zn-Me species to the framework oxygen; subsequent quenching with methanol converts Zn-Me into Zn-OMe, which is converted to Zn-OH upon air calcination. Presence of the Zn-OH species in the pores reduces the pore volume, as measured by the uptake capacity for methanol.