b-Axis-Oriented ZSM-5 Nanosheets for Efficient Alkylation of Benzene with Methanol: Synergy of Acid Sites and Diffusion

Abstract

ZSM-5 nanosheets are promising catalysts for the catalytic reactions controlled by diffusion limitations. This study reveals its significant application in the alkylation of benzene with methanol. The b-axis-oriented ZSM-5 nanosheets with similar acid property but varied thicknesses of about 30, 90, and 300 nm were prepared to investigate the effect of thickness on their catalytic properties for alkylation reactions. Comparative results demonstrate that the sample with a thickness of 30 nm exhibits higher benzene conversion, xylene selectivity, and methyl selectivity (up to 97%), accompanied by an ultralong lifetime (up to 1000 h, 10 times longer than that of the sample with a thickness of 300 nm) and lower byproduct ethylbenzene selectivity. This is ascribed to the shortened straight channel length, increased specific surface area, and enlarged mesopore volume that significantly facilitate the diffusion of reactants and products, increase the accessibility of acid sites, and decrease the coke formation. Moreover, compared with conventional ZSM-5 nanocrystals, ZSM-5 nanosheets deliver a substantially extended lifetime due to fewer framework defects. Most significantly, this study unravels the diffusion effect on ethylbenzene selectivity over ZSM-5 nanosheets with different thicknesses and illustrates the role of strong Brønsted acid sites in the dynamic changes of ethylbenzene selectivity. In light of the above analysis, we developed a precoking strategy and an introducing-heteroatom strategy to precisely tailor the catalyst acidity, further suppressing the ethylbenzene formation (<0.3%) while maintaining long-term stable operation (>300 h).

Tailor-Made Nanoseeds for the Synthesis of Zeolites with Nanosized Dimensions

“…In contrast, the ZSM-5-2 sample has a more centralized and thinner thickness along the b -axis. The synthesis of ZSM-5 nanosheet is a hot topic in recent research. − In the present work, the successful preparation of nanosheet ZSM-5 zeolite in fluoride medium in the presence of NH 4 F treated seeds shows clearly the versatility of the chemical cutting strategy for nanozeolite synthesis. Considering that the synthesis was carried out at 150 °C with a medium NH 4 F content, we believe that an optimization of the synthetic parameters can decrease further the length and thickness of the sheet-like crystals. , …”

Integration of Nano-Sized HZSM-5 with ZnZrOx as a Bifunctional Catalyst to Boost Benzene Alkylation with Carbon Dioxide and Hydrogen

“…, the most significant coke formation occurred in the first 8 h (1.78 wt %), and there was a slight rise to 3.03 and 3.48 wt %. Overall, the degree of coke deposition in the benzene alkylation reaction with CO 2 /H 2 is significantly less than that in the MTA , and benzene alkylation with methanol due to the in situ consumption of CH 3 O* which depressed the methanol self-reaction, and the atmosphere rich in H 2 and H 2 O was conducive to the elimination of coke. The smaller size and the more moderate acid property in nHZ240 led to a better diffusion efficiency of coke precursors and fewer deep alkylation side reactions.…”

“…The GC-MS (Figure 8c) revealed more detailed information about the soluble coke species, and there were several oxygenated aromatics such as dimethyl-benzaldehyde, tert-butylphenol, and di-tert-butylphenol, indicating that the oxygenated aromatics were main coke precursors under an atmosphere of CO 2 , CO, and H 2 , quite different from the coking process of MTA 46,56 and benzene alkylation with the methanol reaction. 4 The oxygenated aromatics may be derived from the carbonylation reaction of aromatic spices and CO through the Gattermann−Koch reaction. 26,57,58 Peaks of methylethylbenzene and trimethylbenzene appeared in the GC-MS chromatogram of ZZO-mHZ60, while is absent in the chromatogram of ZZO-nHZ240.…”

“…Therefore, it is of commercial and scientific interest to convert excessive benzene to valuable toluene or xylene through an alkylation reaction. Current approaches mainly focus on using methanol, − syngas, − methane, , or CO 2 − as the methylation reagents to react with benzene. Among these, CO 2 -coupled benzene alkylation is considered as a doubly beneficial route, which can not only increase the production of the high value-added toluene and xylene but also utilize CO 2 to alleviate the greenhouse effect …”

Three-dimensional inhomogeneity of zeolite structure and composition revealed by electron ptychography

“…S13). Probe-assisted nuclear magnetic resonance (33)(34)(35) and infrared spectroscopy of adsorbed pyridine (Py-IR) (36,37) are commonly used techniques for identifying and quantifying Lewis acid sites in zeolites that are closely related to O vacancies. The Py-IR method was used to determine the concentration of Lewis acid sites in the investigated ZSM-5 sample.…”