Formation of defect site on ZIF-7 and its effect on the methoxycarbonylation of aniline with dimethyl carbonate

Abstract

The Zeolitic Imidazole Framework ZIF-7, (Zn(benzimidazole)(2)), is known to exhibit a unique gate-opening property depending on the temperature and pressure in the presence of guest molecules, making it useful for H-2, CO2, and paraffine separation technology. Besides this distinctive gas adsorption property, ZIF-7 can be used as a catalyst because it contains a Lewis acid site on Zn, and a Lewis basic site on the benzimidazole. In this study, for the first time, we demonstrate that ZIF-7 is a very promising material as a catalyst for the methoxycarbonylation of aniline with dimethyl carbonate (DMC) to produce methyl phenyl carbamate (MPC), an isocyanate precursor. Fresh ZIF-7 showed a high aniline conversion of over 95% at 190 degrees C for 2 h, and the yield MPC was 60-70% due to the formation of methylated side products, such as N-methyl aniline and N,N'-dimethylaniline. However, interestingly, when the ZIF-7 was reused, the yield of MPC gradually increased and reached over 94.7% by the catalyst's 6th run. SEM and TEM images revealed the crystalline structure of ZIF-7 collapsed during the reaction due to the leaching of benzimidazole from the ZIF-7, which created defect sites on the Lewis acidic zinc center. Based on H-1 NMR and XPS studies, it can be assumed that the DMC binds to the Zn defect sites on the ZIF-7, activating the carbonyl groups on DMC, resulting in the increased selectivity to methoxycarbonylation compared to methylation. Pretreating ZIF-7 with DMC at 190 degrees C for 6 h could directly activated the ZIF-7 for this methoxycarbonylation reaction. The activated ZIF-7 showed 91.0% MPC yield and the catalyst was reusable. (C) 2019 Elsevier Inc. All rights reserved.