Ion Irradiation-Induced Coordinatively Unsaturated Zn Sites for Enhanced CO Hydrogenation

Defect engineering critically influences metal oxide catalysis, yet controlling coordinatively unsaturated metal sites remains challenging due to their inherent instability under reaction conditions. Here, we demonstrate that high-flux argon ion (Ar+) irradiation above recrystallization temperatures generated well-defined coordinatively unsaturated Zn (CUZ) sites on ZnO(101̅0) surfaces that exhibited enhanced stability and activity for CO hydrogenation. Combining low-temperature scanning probe microscopy, ambient pressure X-ray photoelectron spectroscopy, and surface-ligand infrared spectroscopy with density functional theory calculations, we identified <12̅10> step edges exposing CUZ sites as the dominant active sites. These sites facilitate hydrogen-assisted CO dissociation through a mechanism distinct from formate-mediated pathways on stoichiometric ZnO. The ion-irradiation approach effectively addressed instability of Zn species, a major problem in ZnO catalysis, enabling stable performance in syngas conversion when combined with zeolites. Our atomic scale investigation provided spectroscopic fingerprints for active sites on the ZnO catalyst and insights into the structure-activity relationships of ZnO for CO hydrogenation. Our approach for engineering thermally stable defect sites in oxide catalysts provided opportunities for rational catalyst design beyond traditional preparation methods.

Synthesis of Hollow Zn/ZSM-5 Nanosheets via Different Alkali Treatments with ZIF-8 as a Zn Source for Efficient Aromatization of Methanol

Diffusion limitations and monofunctional acidity of ZSM-5 molecular sieves affect the catalyst stability and aromatic yield in the reaction of methanol to aromatics (MTA). In this study, based on ZSM-5 nanosheets as parent molecular sieves, Zn-modified hollow ZSM-5 nanosheets were obtained after hydrothermal treatment by adding ZIF-8 or zinc nitrate as a source of Zn while treating with different types and concentrations of alkali solutions. The physical and chemical properties of the fabricated samples and their catalytic performance of methanol aromatization were systematically investigated by a combination of XRD, TEM, N2 adsorption-desorption, NH3-TPD, Py-IR, 27Al MAS NMR, 29Si MAS NMR, XPS, and TG characterization analyses and MTA experimental evaluation. The results indicated that a hollow structure emerged in the samples after alkaline treatment, with a significant increase in the proportion of mesopores, which further increased with the concentration of the alkaline solution. Both alkaline treatment and the introduction of Zn led to changes in the acidity of the catalyst. The increase in tetrahedrally coordinated aluminum during the alkaline treatment resulted in a higher content of B acid sites, while the introduction of Zn formed Zn-Lewis acid sites. Among the prepared samples, the catalyst obtained using ZIF-8 as the Zn source and treated with a mixed alkaline solution of 0.15 M sodium hydroxide and tetrapropylammonium hydroxide (ZnZ8(N+T)/Z5) exhibited higher relative crystallinity, more appropriate micromesopore ratio, a greater amount of Zn(OH)+, and a suitable B/L ratio (0.74). Under the same conditions (450 °C, atmospheric pressure, weight hourly space velocity (WHSV) = 5 h-1), the aromatic product yield of ZnZ8(N+T)/Z5 reached 26.2%, which is 14 percentage points higher than that of the parent ZSM-5 and 4 percentage points higher than that of ZnZN(N+T)/Z5 modified with zinc nitrate as the Zn source. After 8 h of reaction, the aromatic yield over ZnZ8(N+T)/Z5 could still be maintained above 18.5% with good catalyst stability.

Discrimination of the Synergistic Effect of Different Zinc Active Sites with a Brønsted Acid in Zeolite for Dehydrogenation Cracking of n-Octane and Ethane Dehydroaromatization

The synergetic effect of different zinc active sites with a Brønsted acid site (BAS) in Zn-MCM-22 for n-octane dehydrogenation cracking and ethane dehydroaromatization was investigated. Zn-MCM-22 catalysts containing ZnO were prepared via incipient wetness impregnation (IM) using liquid ion grafting, whereas those containing [ZnOx]2+ were prepared via atom-planting (AP) using the gas dechlorination reaction. The synergetic effects of BAS with micropore incorporated [ZnOx]2+ and external surface ZnO species on the dehydrogenation of different molecule size reactants n-octane and ethane were compared. The synergistic effect of ZnO and BAS can improve ethane dehydrogenation through aromatization, whereas [ZnOx]2+ as the introduced Lewis acid site (LAS) can override the bridge Si-OH-Al hydroxyl group BAS to inhibit the generation of benzene-toluene-xylene (BTX) and is more favorable for ethane dehydrogenation. The AP method can effectively regulate the n-octane dehydrogenation cracking product distribution by adjusting the volatilization time of ZnCl2 vapors to regulate the ratio of LAS/BAS in zeolites. The kinetic analysis was used to correlate the roles of the [ZnOx]2+ site and BAS in the dehydrogenation, hydrogen transfer, and cyclization reactions of n-octane and ethane, respectively. Moreover, the hydroxyl group grafted [ZnOx]2+ sites have better activity and stability for higher temperature dehydrogenation.

The Effect of Water Co-Feeding on the Catalytic Performance of Zn/HZSM-5 in Ethylene Aromatization Reactions

During the methanol-to-aromatics (MTA) process, a large amount of water is generated, while the influence and mechanism of water on the activity and selectivity of the light olefin aromatization reaction are still unclear. Therefore, a study was conducted to systematically investigate the effects of water on the reactivity and the product distribution in ethylene aromatization using infrared spectroscopy (IR), intelligent gravitation analyzer (IGA), and X-ray absorption fine structure (XAFS) characterizations. The results demonstrated that the presence of water reduced ethylene conversion and aromatic selectivity while increasing hydrogen selectivity at the same contact time. This indicated that water had an effect on the reaction pathway by promoting the dehydrogenation reaction and suppressing the hydrogen transfer reaction. A detailed analysis using linear combination fitting (LCF) of Zn K-edge X-ray absorption near-edge spectroscopy (XANES) on Zn/HZSM-5 catalysts showed significant variations in the state of existence and the distribution of Zn species on the deactivated catalysts, depending on different reaction atmospheres and water contents. The presence of water strongly hindered the conversion of ZnOH+ species, which served as the active centers for the dehydrogenation reaction, to ZnO on the catalyst. As a result, the dehydrogenation activity remained high in the presence of water. This study using IR and IGA techniques revealed that water on the Zn/HZSM-5 catalyst inhibited the adsorption of ethylene on the zeolite, resulting in a noticeable decrease in ethylene conversion and a decrease in aromatic selectivity. These findings contribute to a deeper understanding of the aromatization reaction process and provide data support for the design of efficient aromatization catalysts.

A Fine Analysis of Zn Species Structure and Distribution in Zn/ZSM-5 Catalysts by Linear Combination Fitting Analysis of XANES Spectra

Investigating the distribution of different Zn species on Zn-containing zeolite catalysts is crucial for identifying the active sites and establishing the relationship between the catalyst's structure and its activity in the process of ethylene aromatization. By utilizing X-ray absorption near edge spectra (XANES) of various reference samples, this study employed linear combination fitting (LCF) analysis on XANES spectra of real samples to accurately measure the changes in the distribution of Zn species in Zn-containing HZSM-5 zeolites under different Zn sources and loadings. The results showed that ZnOH+, ZnO clusters, and ZnO crystalline structures coexist in Zn/HZSM-5 catalysts prepared through physical mixing and incipient wet impregnation methods. A similar trend was observed for catalysts prepared using different methods, with an increase in Zn content resulting in a decrease in the proportion of ZnOH+ and a significant increase in the amount of larger ZnO crystals. Furthermore, ZnO clusters were confined within the zeolite pores. The findings of this study established a direct correlation between the amount of ZnOH+ determined through LCF analysis and both the rate of hydrogen production and the rate of aromatics formation, providing strong evidence for the catalytic role of ZnOH+ as an active center for dehydrogenation, which plays a key role in promoting the formation of aromatics. The method of LCF analysis on XANES spectra allows for the determination of the local structure of Zn species, facilitating a more precise analysis based on the distribution of these species. This method not only provides detailed information about the Zn species but also enhances the accuracy of the overall analysis.