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

Abstract

The pore structure of a microporous material H–SAPO-34 is modified using organometallic reagents such as dialkyl zinc (ZnR2, 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 13C 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 Brønsted 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.