Effect of Steam Treatment on the Properties of Mordenite and Its Catalytic Performance in a DME Carbonylation Reaction

Abstract

A series of dealuminated mordenite (MOR) samples with different acidities and various porosities were prepared through steam treatment of H + /Na + mordenite. The obtained zeolites were elaborately characterized by X-ray diffraction, N 2 adsorption−desorption, scanning electron microscopy, 29 Si nuclear magnetic resonance (NMR), 27 Al NMR, NH 3 temperature-programmed desorption, and Fourier transform infrared spectroscopy, and the catalytic performance for dimethyl ether (DME) carbonylation was investigated. The characterization results revealed that the kind of cation balancing the zeolite charge had a great effect on the crystalline structure and acidity of mordenite zeolite during steam treatment and ion-exchange processes. Dealumination was observed both on Na-MOR and H-MOR, but it was much milder on the Na-MOR zeolite comparatively. After Na-MOR was treated with steam, its structure was wellpreserved, but side pockets of mordenite were opened, resulting in an increased microporous volume and strong acid sites. Interestingly, it was found that the removal of framework Al in the 12MR channel was favored over the 8MR channel during steaming of Na-MOR, leading to the mordenite to possess more acid sites in the 8MR channel compared with those in the 12MR channel. As for the steamed H-MOR, both the crystalline structure and porosity were obviously damaged, resulting in a decreased microporous volume and acidity. The influence was more significant in the 8MR channel of H-MOR due to the obvious removal of framework Al and blockage of side pockets. The performance tests revealed that Na-MOR treated with steam was an effective way to enhance the catalytic performance for DME carbonylation reaction. The N-773 catalyst (steamed with Na-MOR at 773 K) showed a methyl acetate product rate of 0.33 gg −1 h −1 , which was much higher than that of the parent H-MOR (0.23 gg −1 h −1 ). Moreover, the dealuminated catalyst exhibited a slow deactivation rate, which was possibly ascribed to the reduced strong acid sites in the 12MR channels. Although the steam treatment of H-MOR resulted in a lower activity in the DME carbonylation reaction, it exhibited a higher MeOAc selectivity and improved stability for its overall decreased acidity.

Regulation of the Si/Al ratios and Al distributions of zeolites and their impact on properties

“…166 The Si/Al ratios determined by solid state 29 Si MAS NMR is consistent with that measured by the elemental analysis. 24,72,141 By comparing the values with those obtained by chemical analyses, the (Si/Al) 𝑁𝑀𝑅 amount of extra-framework aluminium can be calculated. However, Equation 1 works well with materials that have framework Si/AI ratios less than about 10.…”

“…199,200 Yang and co-workers observed that the dealuminated MOR zeolites exhibited a low catalytic activity but high MeOAc selectivity, indicating that the low BASs density of mordenite is beneficial for the formation of MeOAc. 141 However, a trade-off effect was also observed in the DME carbonylation catalysed by the MOR zeolite, i.e., the conversion of DME increased before the Si/Al ratio reached 11.8 and then decreased with further increasing the Si/Al ratio. 201 More investigations revealed that the strength of the BASs in the 8MR side pockets of MOR zeolite decreased with the increase of Si/Al ratios 202 and the catalytic performance of MOR zeolites on the DME carbonylation is mainly determined by the BASs in the 8MR channels.…”

“…201 More investigations revealed that the strength of the BASs in the 8MR side pockets of MOR zeolite decreased with the increase of Si/Al ratios 202 and the catalytic performance of MOR zeolites on the DME carbonylation is mainly determined by the BASs in the 8MR channels. 141,[201][202][203] The conversion of methanol to hydrocarbons (MTH) catalysed by zeolites including methanol to olefins (MTO), methanol to propane (MTP), methanol to gasoline (MTG), and methanol to aromatics (MTA) provides an alternative route for producing basic chemicals in a large scale from non-petroleum sources. However, there is a debate on the aromatic-based hydrocarbon pool (HCP) mechanism and the olefins methylation/cracking mechanism.…”

“…140 Recently, Yang and coworkers prepared dealuminated H-MOR (Si/Al = 15.26, 25.13, 40.65) and dealuminated Na-MOR (Si/Al = 9.96, 11.20, 12.61) by steaming H-MOR (Si/Al = 8.74) and Na-MOR, respectively, which increased the availability of acid sites in the side pockets of MOR zeolite. 141 Dealumination of zeolites can increase their Si/Al ratios, while desilication can decrease their Si/Al ratios. It is well known that hydroxide ions (OH − ) can effectively break the Si-O-Si bonds.…”

Dealumination and Characterization of Natural Mordenite-Rich Tuffs

“…Dealumination rendered positive effects on catalyst activity and stability during the transformation of hydrocarbons. Xu et al [16] reported the preparation of dealuminated mordenites via hydrothermal treatment, and evaluated the catalytic properties of the samples for the carbonylation of dimethyl ether. Viswanadham et al [17] studied the dealumination of mordenite with nitric acid, and observed the increase in the surface area and volume of micropores when the concentration of acid was increased.…”