Progressive steps and catalytic cycles in methanol-to-hydrocarbons reaction over acidic zeolites

Selectivity Descriptors of Glycerol Valorization over 10-Member Ring Zeolites

The shift to renewable feedstocks is essential for developing "smart drop-in" technologies to produce synthetic fuels and chemicals, playing a crucial role in industrial decarbonization. Herein, zeolite-catalyzed glycerol valorization offers a mature way to create higher-value chemicals. This study aims to identify the key factors influencing process efficiency by examining the reaction mechanisms of glycerol valorization using 10-membered ring zeolites: ZSM-5 (MFI, 3D), MCM-22 (MWW, 2D), and ZSM-22 (TON, 1D), each with unique topologies and channel structures. Strategic integration of catalytic studies with advanced characterization techniques, such as in situ UV-vis and solid-state NMR spectroscopy, reveals that zeolite topology plays a key role in determining catalytic performance and product selectivity. Zeolites with cages or intersectional voids (e.g., ZSM-5, MCM-22) promote the production of aromatic compounds, whereas the cage/void-free ZSM-22 favors acrolein formation. Simultaneously, zeolite channels balance the dual cycle mechanism by mediating between diffusion-friendly straight channels (i.e., in ZSM-5 and ZSM-22) and diffusion-restrictive sinusoidal channels (i.e., in ZSM-5 and MCM-22), affecting efficiency and selectivity. This study underscores the critical role of zeolite architecture in determining catalytic outcomes, providing mechanistic insights that bridge the structure-performance gap.

Methanol to Olefins (MTO): Understanding and Regulating Dynamic Complex Catalysis

The research and development of methanol conversion into hydrocarbons have spanned more than 40 years. The past four decades have witnessed mutual promotion and successive breakthroughs in both fundamental research and industrial process development of methanol to olefins (MTO), demonstrating that MTO is an extremely dynamic, complex catalytic system. This Perspective summarizes the endeavors and achievements of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, in the continuous study of reaction mechanisms and process engineering of the dynamic, complex MTO reaction system. It elucidates fundamental chemical issues concerning the essence of the dynamic evolution of the MTO reaction and the cross-talk mechanisms among diffusion, reaction, and catalyst (coke modification), which are crucial for technology development and process optimization. By integrating the chemical principles, the reaction-diffusion model, and coke formation kinetics of MTO, a mechanism- and model-driven modulation of industrial processes has been achieved. The acquisition of a deepening understanding in chemistry and engineering has facilitated the continuous optimization and upgrading of MTO catalysts and processes.

Adsorbate-driven dynamic active sites in stannosilicate zeolites

Elucidating the nature of the active sites in heterogeneous catalysts is fundamental for understanding their reactivity and catalytic performances. Although stannosilicate zeolites have tremendous application potential for catalyzing biomass-related compounds in aqueous media, the detailed local structures and features of the real active sites and their possible structural variations under reaction conditions are poorly understood to date. In this study, a dynamic transformation of framework Sn-O-Si sites to Sn-OH/Si-OH pairs and subsequently a pseudo-Brønsted acid in stannosilicate zeolites upon molecular adsorption, which is analogous to various adsorbates/reactants under working conditions, was identified by solid-state nuclear magnetic resonance (NMR) spectroscopy for the first time, which challenges the widespread assumption that the active center structures remain rigid/stable during the catalytic process. These results provide new comprehensive insights for the fundamental understanding of the dynamic and flexible active centers and involved reaction mechanisms of novel zeolite catalysts with heterometal atoms, such as Sn, Ti, and Zr.

Advances in the Catalytic Conversion of Ethanol into Nonoxygenated Added-Value Chemicals

Given that ethanol can be obtained from abundant biomass resources (e.g., crops, sugarcane, cellulose, and algae), waste, and CO2, its conversion into value-added chemicals holds promise for the sustainable production of high-demand chemical commodities. Nonoxygenated chemicals, including light olefins, 1,3-butadiene, aromatics, and gasoline, are some of the most important of these commodities, substantially contributing to modern lifestyles. Despite the industrial implementation of some ethanol-to-hydrocarbons processes, several fundamental questions and technological challenges remain unaddressed. In addition, the utilization of ethanol as an intermediate provides new opportunities for the direct valorization of CO and CO2. Herein, the recent advances in the design of ethanol conversion catalysts are summarized, providing mechanistic insights into the corresponding reactions and catalyst deactivation, and discussing the related future research directions, including the exploitation of active site proximity to achieve better synergistic effects for reactions involving ethanol.

Tracking the Impact of Koch-Carbonylated Organics During the Zeolite ZSM-5 Catalyzed Methanol-to-Hydrocarbons Process

A methanol-based economy offers an efficient solution to current energy transition challenges, where the zeolite-catalyzed methanol-to-hydrocarbons (MTH) process would be a key enabler in yielding synthetic fuels/chemicals from renewable sources. Despite its original discovery over half a century ago over the zeolite ZSM-5, the practical application of this process in a CO2 -neutral scenario has faced several obstacles. One prominent challenge has been the intricate mechanistic complexities inherent in the MTH process over the zeolite ZSM-5, impeding its widespread adoption. This work takes a significant step forward by providing critical insights that bridge the gap in our understanding of the MTH process. It accomplishes this by connecting the (Koch-carbonylation-led) direct and dual cycle mechanisms, which operate during the early and steady-state phases of MTH catalysis, respectively. To unravel these mechanistic intricacies, we have performed catalytic and operando (i.e., UV/Vis coupled with an online mass spectrometer) and solid-state NMR spectroscopic-based investigations on the MTH process, involving co-feeding methanol and acetone (cf. a key Koch-carbonylated species), including selective isotope-labeling studies. Our iterative research approach revealed that (Koch-)carbonyl group selectively promotes the side-chain mechanism within the arene cycle of the dual cycle mechanism, impacting the preferential formation of BTX fraction (i.e., benzene-toluene-xylene) primarily.

A computational investigation of the decomposition of acetic acid in H-SSZ-13 and its role in the initiation of the MTO process

The zeolite-catalyzed reaction of acetic acid is important in the direct utilization of biomass and also plays a role in the reactivity of oxygenates in the methanol-to-olefins (MTO) process.

Ketenes in the Induction of the Methanol-to-Olefins Process

Ketene (CH2 =C=O) has been postulated as a key intermediate for the first olefin production in the zeolite-catalyzed chemistry of methanol-to-olefins (MTO) and syngas-to-olefins (STO) processes. The reaction mechanism remains elusive, because the short-lived ethenone ketene and its derivatives are difficult to detect, which is further complicated by the low expected ketene concentration. We report on the experimental detection of methylketene (CH3 -CH=C=O) formed by the methylation of ketene on HZSM-5 via operando synchrotron photoelectron photoion coincidence (PEPICO) spectroscopy. Ketene is produced in situ from methyl acetate. The observation of methylketene as the ethylene precursor evidences a computationally predicted ketene-to-ethylene route proceeding via a methylketene intermediate followed by decarbonylation.

Conversion of methanol to hydrocarbons over H-MCM-22 zeolite: deactivation behaviours related to acid density and distribution

The deactivation of H-MCM-22 zeolites with different Si/Al ratios can be roughly divided into three stages: first the rapid deactivation of the supercages, the second reaction with slow coking and the deactivation stage with rapid coking mainly on the external pockets.