Improved catalytic performance in gas-phase dimethyl ether carbonylation over facile NH4F etched ferrierite

Abstract

Gas-phase dimethyl ether (DME) carbonylation to methyl acetate (MA) initiates a promising route for producing ethanol from syngas. Ferrierite (FER, ZSM-35) has received considerable attention as it displays excellent stability in the carbonylation reaction and its modification strategy is to improve its catalytic activity on the premise of maintaining its stability as much as possible. However, conventional post-treatment methods such as dealumination and desilication usually selectively remove framework Al or Si atoms, ultimately altering the intrinsic composition, crystallinity, and acidity of zeolites inevitably. In this study, we successfully prepared a series of hierarchical ZSM-35 materials through post-treatment with NH₄F etching, which dissolved framework Al and Si at similar rates and preferentially attacked the defective sites. Interestingly, the produced pore systems effectively penetrated the [100] plane, offering elevated access to both the 8-membered ring (8-MR) and 10-membered ring (10-MR) channels. The physicochemical and acid properties of the pristine and NH₄F etched ZSM-35 samples were comprehensively characterized using various techniques, including XRD, XRF, FESEM, HRTEM, Nitrogen adsorption-desorption, NH₃-TPD, Py-IR, ²⁷Al MAS NMR, and ²⁹Si MAS NMR. Under moderate treatment conditions, the intrinsic microporous structure, acid properties, and crystallinity of zeolite were retained, leading to superior catalytic activity and stability with respect to the pristine sample. Nonetheless, severe NH₄F etching disrupted the crystalline framework and created additional defective sites, bringing about faster deposition of coke precursors on the interior Brønsted acid sites (BAS) and decreased catalytic performance. This technique provides a novel and efficient method to slightly enhance the micropore and mesopore volume of industrially pertinent zeolites through a straightforward post-treatment, thus elevating the catalytic performance of these zeolites.

Fig. 1. The schematic diagram of reaction apparatus system.

Fig. 2. (a) XRD patterns. (b) Nitrogen adsorption–desorption isotherms. (c) Micropore size distribution profiles using HK model. (d) Mesopore size distribution profiles with BJH model of the pristine and NH₄F etched ZSM-35 samples.

Fig. 3. FESEM images of the pristine and NH₄F etched ZSM-35 samples.

Fig. 4. HRTEM images of the pristine and NH₄F etched ZSM-35 samples.

Fig. 5. ²⁷Al MAS NMR spectra of the pristine and NH₄F etched ZSM-35 samples.

Fig. 6. (a) NH₃-TPD profiles. (b) Py-IR spectra of the pristine and NH₄F etched ZSM-35 sample.

Fig. 7. ²⁹Si MAS NMR spectra of the pristine and NH₄F etched ZSM-35 samples.

Fig. 8. (a) DME conversion. (b) MA selectivity of the pristine and NH₄F etched ZSM-35 samples with 0.4 g of catalyst for 70 h on stream (reaction conditions: T = 220 °C, P = 1 MPa, weight hourly space velocity (WHSV) = 2000 ml g⁻¹ h⁻¹, N₂/DME/CO (mol%) = 5/5/90).