Formaldehyde-Induced Deactivation of ZSM5 Catalysts during the Methanol-to-Hydrocarbons Conversion

Abstract

Formaldehyde productivity and reactivity over zeolite catalysts are suggested to influence their selectivity and stability in the methanol-to-hydrocarbons (MTH) conversion. However, details of these relationships have yet to be established. In this study, we applied photoelectron photoion coincidence spectroscopy to examine the productivity of this intermediate over unpromoted and zinc-loaded ZSM-5 catalysts. Formaldehyde is the primary MTH product. Its production over unpromoted catalysts proceeds mainly via hydrogen transfer between methanol molecules, the rate of which is proportional to the concentration of strong Brønsted acid sites. In zinc-promoted ZSM-5, formaldehyde is mainly derived via methanol dehydrogenation, which provides up to a 12× higher productivity of this intermediate. The cumulative turnover capacity of the catalyst displays a strong negative correlation with its formaldehyde evolution activity. Formaldehyde evolution is enhanced at higher methanol concentrations and reaction temperatures, which are conditions also leading to enhanced coking. Complementary formaldehyde cofeeding experiments demonstrate that this intermediate enhances the formation of ethene and methylbenzenes over C 3+ alkenes, which is in agreement with high selectivities to the products of the arene cycle over the catalysts with high formaldehyde productivity. The addition of formaldehyde promotes most substantially the deactivation of the most stable ZSM-5 catalysts displaying inherently low productivity of this intermediate, rendering their lifetimes comparable to those of their less stable counterparts. Operando diffuse-reflectance UV−vis spectra show that small amounts of formaldehyde greatly promote the conversion of cyclopentadienyl and methylated benzenyl intermediates into bicyclic and polycyclic aromatics. Consistent with this, formaldehyde cofeeding results in increased coke accumulation in more stable catalysts, especially in their micropores, as supported by X-ray diffraction and photoelectron spectroscopy analysis. These findings represent direct evidence that the stability and product distribution of ZSM-5 catalysts are strongly governed by their formaldehyde evolution activity.

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

“…Conversely, aged Z 40 catalysts exhibit an increased HTI in comparison to the fresh catalyst, whereas the value of this parameter in the Z 15 catalyst is only slightly affected after long-term operation (Figure b). This indicates that aging of Z 40 catalysts promotes substantially the HT reactions that are considered essential for the formation of arene chain carriers and coke species. ,,, …”

“…These catalysts were selected due to the industrial significance of ZSM-5 zeolites, which, among other favorable characteristics, also exhibit enhanced hydrothermal stability. Moreover, Z 15 and Z 40 zeolites, which exhibit low and moderate Si/Al ratios, respectively, are frequently employed as benchmark catalysts in various studies on MTH conversion. ,,,,, Importantly, these ZSM-5 materials allow for the investigation of performance changes in low- and medium-to-high-coking-resistant catalysts during their prolonged operation in the MTH process. In addition, the aluminum concentration in these materials enables more accurate assessment of acidity and aluminum speciation (vide infra), which becomes increasingly challenging to analyze spectroscopically in materials with higher Si/Al ratios.…”

“…Particle sizes and textural properties of these materials are consistent with those previously reported (Figure S1 and Table S1). The MTH reaction was initially carried out at 673 K, reflecting a typical temperature window of the methanol-to-gasoline processes (Figure ). , In contrast to the previous studies on the structural stability of the MTH catalysts, ,,,, the catalyst performance was analyzed at high WHSV (76 g CH 3 OH g cat –1 h –1 ), aimed at reducing concentration gradients along the catalyst bed and enhancing the detection of any structural changes.…”

“…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. ,…”

Revealing Formaldehyde-Mediated Methanol-to-Aromatics Reactions over Zn-Modified Zeolites by Observing the Oxygenated and Polyunsaturated Intermediates

“…Recently, HCHO was thought to participate in the synthesis of increased aromatics over Zn-modified zeolites, − and the increase of aromatic selectivity is usually accompanied by the severer accumulation of coke and a higher deactivation rate. Hence, the online capturing of active intermediates, the elucidation of the formation pathway of HCHO, and the depiction of the HCHO-mediated reaction network are not only crucial for regulating the formation of aromatics but are also important for figuring out the deactivation mechanism of zeolite catalysts.…”