Insight into the Effects of Water on the Ethene to Aromatics Reaction with HZSM-5

Dual-engine-driven realizing high-yield synthesis of Para-Xylene directly from CO2-containing syngas

The direct synthesis of light aromatics, especially para-xylene (p-X), from syngas/CO2 is drawing strong interest, but improving the space-time yield (STY) of p-X is a significant challenge. Here, a dynamic "dual-engine-driven" (DED) catalytic system is designed by combining two partners of ZnCr and FeMn (named "dual-engine") with Z5@SiO2 capsule zeolite. The DED catalyst of 1.0%FeMn&[ZnCr&Z5@SiO2] shows an extremely higher p-X STY of 36.1 gp-x·kgcat-1·h-1, about eight times higher than that of [ZnCr&Z5]. DED manipulates ZnCr engine for methanol formation and drives FeMn engine for light olefins generation together, and then the formed methanol and light olefins are coordinately converted in situ into p-X-rich aromatics over Z5@SiO2. The DED model boosts the driving force for syngas/CO2 conversion, simultaneously concerting the cooperation of "dual-engine" for p-X generation, resulting in extremely high STY of p-X. This study achieves non-petroleum p-X production at industrial-relevant level and advances knowledge in designing innovative heterogeneous catalysts.

Water structures on acidic zeolites and their roles in catalysis

The local reaction environment of catalytic active sites can be manipulated to modify the kinetics and thermodynamic properties of heterogeneous catalysis. Because of the unique physical-chemical nature of water, heterogeneously catalyzed reactions involving specific interactions between water molecules and active sites on catalysts exhibit distinct outcomes that are different from those performed in the absence of water. Zeolitic materials are being applied with the presence of water for heterogeneous catalytic reactions in the chemical industry and our transition to sustainable energy. Mechanistic investigation and in-depth understanding about the behaviors and the roles of water are essentially required for zeolite chemistry and catalysis. In this review, we focus on the discussions of the nature and structures of water adsorbed/stabilized on Brønsted and Lewis acidic zeolites based on experimental observations as well as theoretical calculation results. The unveiled functions of water structures in determining the catalytic efficacy of zeolite-catalyzed reactions have been overviewed and the strategies frequently developed for enhancing the stabilization of zeolite catalysts are highlighted. Recent advancement will contribute to the development of innovative catalytic reactions and the rationalization of catalytic performances in terms of activity, selectivity and stability with the presence of water vapor or in condensed aqueous phase.

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.

Unveiling the mechanisms of carboxylic acid esterification on acid zeolites for biomass-to-energy: A review of the catalytic process through experimental and computational studies

In recent years, there has been significant interest from industrial and academic areas in the esterification of carboxylic acids catalyzed by acidic zeolites, as it represents a sustainable and economically viable approach to producing a wide range of high-value-added products. However, there is a lack of comprehensive reviews that address the intricate reaction mechanisms occurring at the catalyst interface at both the experimental and atomistic levels. Therefore, in this review, we provide an overview of the esterification reaction on acidic zeolites based on experimental and theoretical studies. The combination of infrared spectroscopy with atomistic calculations and experimental strategies using modulation excitation spectroscopy techniques combined with phase-sensitive detection is presented as an approach to detecting short-lived intermediates at the interface of zeolitic frameworks under realistic reaction conditions. To achieve this goal, this review has been divided into four sections: The first is a brief introduction highlighting the distinctive features of this review. The second addresses questions about the topology and activity of different zeolitic systems, since these properties are closely correlated in the esterification process. The third section deals with the mechanisms proposed in the literature. The fourth section presents advances in IR techniques and theoretical calculations that can be applied to gain new insights into reaction mechanisms. Finally, this review concludes with a subtle approach, highlighting the main aspects and perspectives of combining experimental and theoretical techniques to elucidate different reaction mechanisms in zeolitic systems.

Imaging of Single Molecular Behaviors Under Bifurcated Three-Centered Hydrogen Bonding

The mechanism for interaction and bonding of single guest molecules with active sites fundamentally determines the sorption and subsequent catalytic processes occurring in host zeolitic frameworks. However, no real-space studies on these significant issues have been reported thus far, since atomically visualizing guest molecules and recognizing single Al T-sites in zeolites remain challenging. Here, we atomically resolved single thiophene probes interacting with acid T-sites in the ZSM-5 framework to study the bonding behaviors between them. The synergy of bifurcated three-centered hydrogen bonds and van der Waals interactions can "freeze" the near-horizontal thiophene and make it stable enough to be imaged. By combining the imaging results with simulations, direct atomic observations enabled us to precisely locate the single Al T-sites in individual straight channels. Then, we statistically found that the thiophene bonding probability of the T11 site is 15 times higher than that of the T6 site. For different acid T-sites, the variation in the interaction synergy changes the inner angle of the host-guest O-H⋅⋅⋅S hydrogen bond, thereby affecting the stability of the near-horizontal thiophene and leading to considerable bonding inhomogeneities.

Improving the Hydrothermal Stability of ZSM-5 Zeolites in 1-Octene Aromatization by Sequential Alkali Treatment and Phosphorus Modification

For the aromatization of olefins (in Fischer–Tropsch synthetic oil), especially for the fluidized bed reaction with steam as the fluidized medium, improving the catalytic and hydrothermal stabilities of ZSM-5 catalysts is a research focus because of both fundamental research interests and potential commercial applications. In this work, sequential alkali treatment and phosphorus modification were carried out for ZSM-5 samples. The results obtained from characterization and reaction evaluation show that the introduction of mesopores facilitates the dispersion of phosphorus species into the pores and improves the reaction stability for 1-octene aromatization. After the hydrothermal treatment, the P-0.5A-Z5-ST sample treated with an appropriate concentration of alkali retains the most acid sites and shows the highest aromatic selectivity (22.5%) at TOS = 660 min. Therefore, a moderate distribution of mesopores in a zeolite plays an important role in the diffusions of reactants and products, as well as in the distribution of phosphorus species.

Modulating inherent lewis acidity at the intergrowth interface of mortise-tenon zeolite catalyst

The acid sites of zeolite are important local structures to control the products in the chemical conversion. However, it remains a great challenge to precisely design the structures of acid sites, since there are still lack the controllable methods to generate and identify them with a high resolution. Here, we use the lattice mismatch of the intergrown zeolite to enrich the inherent Lewis acid sites (LASs) at the interface of a mortise-tenon ZSM-5 catalyst (ZSM-5-MT) with a 90° intergrowth structure. ZSM-5-MT is formed by two perpendicular blocks that are atomically resolved by integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM). It can be revealed by various methods that novel framework-associated Al (Al_FR) LASs are generated in ZSM-5-MT. Combining the iDPC-STEM results with other characterizations, we demonstrate that the partial missing of O atoms at interfaces results in the formation of inherent Al_FR LASs in ZSM-5-MT. As a result, the ZSM-5-MT catalyst shows a higher selectivity of propylene and butene than the single-crystal ZSM-5 in the steady conversion of methanol. These results provide an efficient strategy to design the Lewis acidity in zeolite catalysts for tailored functions via interface engineering.

The acid sites are important local structures to determine catalytic performances of zeolites. Here, the authors expand the interface engineering to the field of porous zeolites through the lattice mismatch of the intergrown zeolite to enrich the inherent Lewis acid sites at the interface of a mortise-tenon ZSM-5 catalyst.

Towards the development of the emerging process of CO2 heterogenous hydrogenation into high-value unsaturated heavy hydrocarbons

The emerging process of CO₂ hydrogenation through heterogenous catalysis into important bulk chemicals provides an alternative strategy for sustainable and low-cost production of valuable chemicals, and brings an important chance for mitigating CO₂ emissions. Direct synthesis of the family of unsaturated heavy hydrocarbons such as α-olefins and aromatics via CO₂ hydrogenation is more attractive and challenging than the production of short-chain products to modern society, suffering from the difficult control between C-O activation and C-C coupling towards long-chain hydrocarbons. In the past several years, rapid progress has been achieved in the development of efficient catalysts for the process and understanding of their catalytic mechanisms. In this review, we provide a comprehensive, authoritative and critical overview of the substantial progress in the synthesis of α-olefins and aromatics from CO₂ hydrogenation via direct and indirect routes. The rational fabrication and design of catalysts, proximity effects of multi-active sites, stability and deactivation of catalysts, reaction mechanisms and reactor design are systematically discussed. Finally, current challenges and potential applications in the development of advanced catalysts, as well as opportunities of next-generation CO₂ hydrogenation techniques for carbon neutrality in future are proposed.

Stabilizing the framework of SAPO-34 zeolite toward long-term methanol-to-olefins conversion

As a commercial MTO catalyst, SAPO-34 zeolite exhibits excellent recyclability probably due to its intrinsic good hydrothermal stability. However, the structural dynamic changes of SAPO-34 catalyst induced by hydrocarbon pool (HP) species and the water formed during the MTO conversion as well as its long-term stability after continuous regenerations are rarely investigated and poorly understood. Herein, the dynamic changes of SAPO-34 framework during the MTO conversion were identified by 1D ²⁷Al, ³¹P MAS NMR, and 2D ³¹P-²⁷Al HETCOR NMR spectroscopy. The breakage of T-O-T bonds in SAPO-34 catalyst during long-term continuous regenerations in the MTO conversion could be efficiently suppressed by pre-coking. The combination of catalyst pre-coking and water co-feeding is established to be an efficient strategy to promote the catalytic efficiency and long-term stability of SAPO-34 catalysts in the commercial MTO processes, also sheds light on the development of other high stable zeolite catalyst in the commercial catalysis.

Stability of zeolite catalysts is a highly desirable property for commercial methanol to olefins conversion but extremely challenging to achieve. Here, the authors combine the catalyst pre-coking and water co-feeding to develop an efficient strategy to enhance the long-term stability of SAPO-34 catalyst.

Advanced Materials for Energy-Water Systems: The Central Role of Water/Solid Interfaces in Adsorption, Reactivity, and Transport

The structure, chemistry, and charge of interfaces between materials and aqueous fluids play a central role in determining properties and performance of numerous water systems. Sensors, membranes, sorbents, and heterogeneous catalysts almost uniformly rely on specific interactions between their surfaces and components dissolved or suspended in the water-and often the water molecules themselves-to detect and mitigate contaminants. Deleterious processes in these systems such as fouling, scaling (inorganic deposits), and corrosion are also governed by interfacial phenomena. Despite the importance of these interfaces, much remains to be learned about their multiscale interactions. Developing a deeper understanding of the molecular- and mesoscale phenomena at water/solid interfaces will be essential to driving innovation to address grand challenges in supplying sufficient fit-for-purpose water in the future. In this Review, we examine the current state of knowledge surrounding adsorption, reactivity, and transport in several key classes of water/solid interfaces, drawing on a synergistic combination of theory, simulation, and experiments, and provide an outlook for prioritizing strategic research directions.

Effect of Pore Connectivity of Pore-Opened Hierarchical MOR Zeolites on Catalytic Behaviors and Coke Formation in Ethanol Dehydration

The hierarchical zeolite is one of the most promising materials for catalytic applications. However, the effect of its pore connectivity on catalytic behaviors and coke formation has not clearly been revealed. In this contribution, we demonstrate the visualization of the mesopore architecture in three-dimensional perspectives together with the pore connectivity network of pore-opened hierarchical mordenite (MOR), fabricated by the seed-assisted template-free synthesis followed by the fluoride treatment via the electron tomography (ET) technique. Interestingly, the pore-opened zeolites clearly display higher catalytic performance (approximately 80% of ethylene yield) in ethanol dehydration with respect to the parent one due to their additional pore-opened structures connected to the external surfaces of zeolites. In addition, the effect of pore connectivity network on the coke location and type obtained from ethanol conversion has been observed. It was found that the porous structure of the etched sample is directly connected to the external surface, and then, the large area of crystals can contribute to the reaction. Conversely, only a small amount of closed mesopores is observed inside the crystals in the case of the untreated sample, and therefore, the molecules cannot easily penetrate inside crystals for the catalytic reaction. These results open up promising perspectives for the development of hierarchical catalysts including fabrication by the template-free synthesis approach, pore-architecture characterization, and catalytic applications.

Kinetic analysis of Ag particle redispersion into ZSM-5 in the presence of coke using in situ XAFS

In the present study, the redispersion behavior of Ag particles on ZSM-5 in the presence of coke was observed using in situ X-ray absorption fine structure (XAFS) spectroscopy.