Dynamic evolution of HZSM-5 zeolite framework under steam treatment

Abstract

Understanding the dynamic evolution of zeolite framework structures and the interactions between various hydroxyl groups or aluminum species under different steam conditions at the atomic scale is extremely crucial. Herein, using a series of characterization methods, the framework structures of HZSM-5 zeolites (Si/Al = 36) following exposure to steam in the temperature range of 100 to 500 °C are investigated. Under mild steam conditions (T ≤ 200 °C), dealumination is nearly absent, and the silanol nests directly condense to create new framework Si-O-Si bonds. Conversely, under severe steam conditions (T ≥ 300 °C), the framework tetrahedral aluminum atoms Al(IV)-1 can be sequentially converted to partially coordinated framework aluminum Al(IV)-2 and extra-framework aluminum (EFAL) through partial and complete hydrolysis, which cause an increase in the framework Si/Al ratio and a decrease in crystallinity. Al(IV)-2 is recognized as a significant intermediate species for framework complete dealumination. The Brønsted acid sites on Al(IV)-2 can be perturbed by the framework Al-OH groups due to hydrogen bonding interactions, leading to a shift in 1H chemical shifts to lower fields, appearing at 6.0-9.0 ppm and 12.0-15.0 ppm. The newly generated EFAL and silanol nests further evolve through condensation as well. Meanwhile, during dealumination, the spatial correlations (or interactions) of various hydroxyl groups on structurally distinct aluminum species [Al(IV)-1, Al(IV)-2, and EFAL] and aluminum species become extremely intricate. Based on these findings, the dynamic evolution path of HZSM-5 zeolite framework structures under mild and severe steam conditions is proposed.

Structural Changes of ZSM-5 Catalysts during Methanol-to-Hydrocarbons Conversion Processes

“…−106.5 ppm) decreases with respect to the signal arising from silicon atoms surrounded only with other silicon atoms in their first shell (Q 4 [0Al], deconvoluted into four components in the range of ca. −111 to −120 ppm, Figure b). ,, In addition, the aged catalysts also exhibit a decrease of a signal assigned to defect Si sites (Q 3 [0Al], ca . −102 ppm), which points to their healing upon prolonged catalyst use.…”

“…Several factors can lead to these structural alterations. In particular, the MTH conversion is associated with the formation of equimolar amounts of water, i.e ., steam, which can induce dissociation of Al F sites. ,,,− Besides, steam is also generated during the oxidation of coke. Although the regeneration treatment generates low amounts of steam, its high local concentration at very high combustion temperatures can lead to a profound dealumination effect.…”

“…A critical challenge in the industrial exploitation of the MTH processes is associated with the deactivation of the zeolite and zeotype catalysts. ,,− The most commonly studied deactivation mechanism is catalyst coking. ,,− It comprises transformation of active DCHP species into higher-molecular-weight ones, mostly polycyclic aromatic hydrocarbons (PAHs). These species deposit inside the micropores and on the outer crystal surfaces, thus occluding the active sites. ,,,, Coke formation is thermodynamically favored and proceeds via C–C coupling sequences coupled with hydrogen-transfer (HT) reactions. ,,− Coking is commonly considered as a transient deactivation process, since coke can be eliminated by oxidation at high temperatures (>823 K). ,, Nonetheless, typical MTH catalysts are also susceptible to structural alterations that permanently modify its performance. − The restructuring of the catalyst is typically ascribed to the generation of byproduct water, i.e., steam during both MTH conversion and subsequent oxidative regeneration steps. ,,,, More specifically, when performed at temperatures and steam concentrations equivalent to those existing during MTH reaction and regeneration, the steaming treatments of the zeolite catalysts such as ZSM-5 lead to dissociation of the framework aluminum (Al F ) sites, causing a decrease of the BAS concentration. ,− Depending on the steaming conditions, dislodged aluminum can take diverse forms, ranging from partially dissociated framework-associated (Al FA ) sites to completely displaced extraframework (Al EF ) species that may still interact with remaining Al F sites. ,− Although the exact structure of the Al FA and Al EF sites and their interaction with the framework remains elusive, their generation via steaming may modify the catalyst performance. ,…”

“…These LAS are proposed to strengthen the BAS acidity by coordinating to the bridging oxygen atom, ,, promoting the polarization of molecules, increasing the heat of adsorption and thus increase the concentration of reactants, or inducing steric constraints that lead to higher activation entropies . Moreover, steam-induced dealumination may induce changes in aluminum distribution across the T sites and crystal volume ,, and modulate the concentration of silanol defects, which can affect the MTH reaction performance. , …”

Highly selective production of renewable methyl acrylate via aldol condensation over Cu modified nitrogen-containing Beta zeolites

“…It can be seen that all samples have the typical Beta zeolite structure [35] . After nitridation and loading Cu, the positions of the main diffraction peaks of the zeolites remain unchanged, and there is no diffraction peak of CuO due to its high dispersion on the surface of zeolites [36] . Nevertheless, the intensities of the diffraction peaks slightly decrease.…”