High-Efficiency Conversion of Methanol to BTX Aromatics Over a Zn-Modified Nanosheet-HZSM-5 Zeolite

The synergistic interplay of hierarchy, crystal size, and Ga-promotion in the methanol-to-aromatics process over ZSM-5 zeolites

In the context of advancing social modernization, the projected shortfall in the demand for renewable aromatic hydrocarbons is expected to widen, influenced by industries like high-end materials, pharmaceuticals, and consumer goods. Sustainable methods for aromatic production from alternative sources, particularly the methanol-to-aromatics (MTA) process using zeolite ZSM-5 and associated with the "methanol economy", have garnered widespread attention. To facilitate this transition, our project consolidates conventional strategies that impact aromatics selectivity-such as using hierarchical zeolites, metallic promoters, or altering zeolite physicochemical properties-into a unified study. Our findings demonstrate the beneficial impact of elongated crystal size and heightened zeolite hierarchy on preferential aromatics selectivity, albeit through distinct mechanisms involving the consumption of shorter olefins. While metallic promoters enhance MTA performance, crystal size, and hierarchy remain pivotal in achieving the maximized aromatics selectivity. This study contributes to a deeper understanding of achieving superior aromatics selectivity through physicochemical modifications in zeolite ZSM-5 during MTA catalysis, thereby advancing the field's comprehension of structure-reactivity relationships.

Modulation of Al Distribution in High-Silica ZSM-5 Zeolites for Enhancing Catalytic Performance

The spatial distribution of framework Al (AlF) has been one of the important factors that affect the catalytic properties of zeolites in diverse chemical reactions; however, the synthesis of high-silica zeolites with special AlF distribution remains a challenge. In this study, we successfully synthesized high-silica ZSM-5 zeolites with a unique AlF distribution by employing pentaerythritol (PET) as an additive in the presence of a few tetrapropylammonium hydroxide (TPAOH). The results demonstrated that the introduction of PET led to a higher proportion of Al atoms located at the sinusoidal and/or straight channels. It was observed that the addition of PET prevented the interaction between TPA+ and tetrahedral [AlO4]- during the crystallization process, resulting in enhanced availability of TPA species in the form of ion-paired TPA+. This effect leads to AlF atoms dominantly distributed away from the intersection and located in narrow channels, where acidic sites more effectively inhibit hydrogen transfer and coke formation. In the reaction of dimethyl ether (DME) to olefins, the catalyst with a unique Al distribution exhibited a significant prolonged catalytic lifetime, surpassing traditional TPA-ZSM-5 by more than 2-fold and maintaining DME conversion above 90% for a maximum of 148 h. The results of multiple pulse experiments also showed that these PET-assisted ZSM-5 zeolites significantly enhanced the selectivity of propene and butene. This approach provides an effective strategy to regulate AlF distribution in high-silica ZSM-5 catalysts with the assistance of neutral alcohol. It holds great potential for application in the synthesis of other high-silica zeolites, thereby enriching the diversity of zeolite catalysis.

In Situ Synthesis of Metal-Containing ZSM-5 and Its Catalytic Performance in Aromatization of Methanol

Metal-containing ZSM-5 zeolites, denoted as M/ZSM-5 (M = Ga, Zn, and La), were prepared by an in situ synthetic method. The structural, textural, and surface acid properties of M/ZSM-5 were characterized by XRD, N₂ adsorption-desorption, NH₃-TPD, and FT-IR. The catalyst pore size and the distribution of strong acid and weak acid were affected by the introduction of metal. Considering the methanol-to-aromatics model reaction, the catalytic performance of these samples was investigated in a fixed bed reactor under the conditions of 450 °C and a liquid weight hourly space velocity (WHSV) of 5 h^-1. The results indicated that the Zn/ZSM-5 molecular sieve was most beneficial to aromatization; the relationship between the change of B/L caused by Zn content and aromatics selectivity was further studied. When the B/L value decreased, the selectivity of aromatics showed an initial increase and then a decrease. The B/L value was optimal, and the aromatics yield was the highest when the Zn loading was 0.35%.

Efficient Utilization of Hydrocarbon Mixture to Produce Aromatics over Zn/ZSM-5 and Physically Mixed with ZSM-5

A mixture of saturated and unsaturated light hydrocarbon was used as feed gas for the production of aromatics. Natural gas liquids (NGL) from gas fields and hydrocarbon molecules obtained in the middle of conversion processes could be considered a kind of light hydrocarbon mixture. Therefore, for the conversion of the mixture into aromatics compounds, Zn-impregnated ZSM-5 catalysts were prepared and evaluated by employing different loading of Zn. In addition, the catalytic performance was tested and compared by charging physically mixed two different kinds of catalysts in the bed. The NH3-TPD result showed that the impregnation of Zn led to an increase in the number of medium-strength acid sites, whereas those of weak and strong acid sites were decreased. From the results of the catalytic activity tests, 0.5Zn/ZSM-5 showed the highest aromatics yield. As the amount of Zn loading was further increased to 1 wt.%, the yield of aromatics decreased. The test result in the case of the physically mixed catalysts showed a slightly lower yield in terms of total aromatics, but showed the highest BTX yield. To reveal the relative contribution of each hydrocarbon conversion to aromatics yield, each C2 compound was separately tested for aromatization over Zn/ZSM-5.

Zn-P Co-Modified Hierarchical ZSM-5 Zeolites Directly Synthesized via Dry Gel Conversion for Enhanced Methanol to Aromatics Reaction

A unique method to prepare Zn-P co-modified hierarchical ZSM-5 zeolites was developed. The ZSM-5 zeolite was directly synthesized by a dry gel conversion without adding any templates or seeds. Afterwards, the hierarchical structure was endowed to the ZSM-5 zeolite by the sequential desilication-dealumination. Zn and P species were then introduced into the hierarchical ZSM-5 zeolites by the impregnation method and their activity in methanol to aromatics process was investigated. It was found that the Zn-P co-modified hierarchical ZSM-5 zeolites possessed more Zn-related Lewis acid sites, and the ratio of Zn(OH)+/ZnO was increased. The catalytic evaluation results revealed that the benzene, toluene and xylene (BTX) and aromatics selectivity were significantly improved from 20.59% and 29.41% of pristine ZSM-5 zeolite to 28.12% and 41.88% of Zn-P co-modified hierarchical counterpart (1.5Zn0.3P/HZSM-5), respectively. Owing to the introduced highly stable Zn-P co-modified hierarchical structures, the lifetime (conversion not less than 99%) of ZSM-5 zeolite during methanol to aromatics reaction was increased from 6 h to 18 h.