Zeolite Catalysis Research Articles

Effects of residual alkali metals on the hydrothermal/thermal stability of silicalite-1

Understanding of the main factors affecting the hydrothermal stability of zeolites is vital for the application of zeolites in catalysis and separation. Although incorporation of extra-framework metal cations is an important stagey to improve the hydrothermal stability of Al-rich zeolites, how these metal cations affect the hydrothermal stability of high-silica zeolites is not well understood. In this work, the effects of residual alkali cations on the hydrothermal/thermal stability of silicalite-1, a pure-silica MFI-type zeolite, is explored. We show that the alkali impurities from commercial tetrapropylammonium hydroxide (TPAOH) are not negligible; the concentration and type of residual alkali cations dictate the hydrothermal/thermal stability of silicalite-1. Above a certain concentration (∼0.3–0.5 wt%), the alkali cations can lead to less silanol defects but much poorer hydrothermal/thermal stability. Below this concentration, the degrading effect is negligible. Removal of the alkali cations by proton exchange improves the stability significantly. Steaming (hydrothermal treatment) has two opposite effects on the silicalite-1 crystal structure: destruction by hydrolysis and perfecting by healing the defects. For silicalite-1 with considerable amounts of alkali cations, the destruction effect predominates; while the perfecting effect predominates for silicalite-1 with low alkali contents.

Cavity-controlled methanol conversion over zeolite catalysts.

The successful development and application in industry of methanol-to-olefins (MTO) process brought about an innovative and efficient route for olefin production via non-petrochemical resources and also attracted attention of C1 chemistry and zeolite catalysis. Molecular sieve catalysts with diversified microenvironments embedding unique channel/cavity structure and acid properties, exhibit demonstrable features and advantages in the shape-selective catalysis of MTO. Especially, shape-selective catalysis over 8-MR and cavity-type zeolites with acidic supercage environment and narrow pore opening manifested special host-guest interaction between the zeolite catalyst and guest reactants, intermediates and products. This caused great differences in product distribution, catalyst deactivation and molecular diffusion, revealing the cavity-controlled methanol conversion over 8-MR and cavity-type zeolite catalyst. Furthermore, the dynamic and complicated cross-talk behaviors of catalyst material (coke)-reaction-diffusion over these types of zeolites determines the catalytic performance of the methanol conversion. In this review, we shed light on the cavity-controlled principle in the MTO reaction including cavity-controlled active intermediates formation, cavity-controlled reaction routes with the involvement of these intermediates in the complex reaction network, cavity-controlled catalyst deactivation and cavity-controlled diffusion. All these were exhibited by the MTO reaction performances and product selectivity over 8-MR and cavity-type zeolite catalysts. Advanced strategies inspired by the cavity-controlled principle were developed, providing great promise for the optimization and precise control of MTO process.

Molecular volume-controlled shape-selective catalysis for synthesis of cinnamate over microporous zeolites

On account of the safety and environmental problems arose from the inorganic acids use in the traditional cinnamate synthesis technology, developing eco-friendly solid-acid catalysts with low cost, high efficiency and recyclability is indispensable in cinnamate synthesis. Due to the unique shape-selective catalysis and diverse acidic properties of microporous zeolites, three different solid acid formulations including H-ZSM-5, H-Y and H-β are selected as catalysts and applied in catalytic synthesis of cinnamate. The effect of acidic property and pore structure on the performance of esterification of cinnamic acid with alcohols is emphatically investigated for three different zeolites after optimizing the catalytic reaction conditions. H-β zeolite exhibits the highest catalytic activity among three zeolites in the esterification of cinnamic acid with ethanol, owing to its strongest acidic property and largest pore channel. Taking account of the different structural properties of three microporous zeolites, it is considered that the pore size plays a dominant role in the synthesis of cinnamate. Furthermore, H-β zeolite always shows the highest catalytic activity coupled with the outstanding recyclability and regeneration efficiency, even in the condition of different alcohols used in cinnamate esterification reaction. It is found that the conversion of cinnamic acid is closely related to the molecular volume of intermediates and product molecules from experiment data and theoretical calculation. Specifically, the smaller molecular volume of intermediates and product molecules inclines to obtain the higher conversion of cinnamic acid and vice versa, which can be well explained from the perspective of mass transfer efficiency. It demonstrates that the activity of esterification reaction for cinnamic acid over H-β zeolite is dominantly associated with molecular volume of intermediates and product molecules. The research work provides a successful paradigm for microporous zeolite in shape-selective catalytic synthesis of cinnamate controlled by molecular volume.

Nuclear quantum effects on zeolite proton hopping kinetics explored with machine learning potentials and path integral molecular dynamics

Proton hopping is a key reactive process within zeolite catalysis. However, the accurate determination of its kinetics poses major challenges both for theoreticians and experimentalists. Nuclear quantum effects (NQEs) are known to influence the structure and dynamics of protons, but their rigorous inclusion through the path integral molecular dynamics (PIMD) formalism was so far beyond reach for zeolite catalyzed processes due to the excessive computational cost of evaluating all forces and energies at the Density Functional Theory (DFT) level. Herein, we overcome this limitation by training first a reactive machine learning potential (MLP) that can reproduce with high fidelity the DFT potential energy surface of proton hopping around the first Al coordination sphere in the H-CHA zeolite. The MLP offers an immense computational speedup, enabling us to derive accurate reaction kinetics beyond standard transition state theory for the proton hopping reaction. Overall, more than 0.6 μs of simulation time was needed, which is far beyond reach of any standard DFT approach. NQEs are found to significantly impact the proton hopping kinetics up to ~473 K. Moreover, PIMD simulations with deuterium can be performed without any additional training to compute kinetic isotope effects over a broad range of temperatures.

Zeolite-encaged mononuclear copper centers catalyze CO2 selective hydrogenation to methanol.

The selective hydrogenation of CO2 to methanol by renewable hydrogen source represents an attractive route for CO2 recycling and is carbon neutral. Stable catalysts with high activity and methanol selectivity are being vigorously pursued, and current debates on the active site and reaction pathway need to be clarified. Here, we report a design of faujasite-encaged mononuclear Cu centers, namely Cu@FAU, for this challenging reaction. Stable methanol space-time-yield (STY) of 12.8mmol gcat-1 h-1 and methanol selectivity of 89.5% are simultaneously achieved at a relatively low reaction temperature of 513K, making Cu@FAU a potential methanol synthesis catalyst from CO2 hydrogenation. With zeolite-encaged mononuclear Cu centers as the destined active sites, the unique reaction pathway of stepwise CO2 hydrogenation over Cu@FAU is illustrated. This work provides a clear example of catalytic reaction with explicit structure-activity relationship and highlights the power of zeolite catalysis in complex chemical transformations.

Catalytic performance of coal tar by step conversion of single-component, multi-components, distillate and all-distillates over bifunctional HZSM-5: Model optimization and experiment verification

Bifunctional catalysis in zeolites with L and B acid sites provides a special possibility to improve the catalytic performance. Here, we propose a new way by combining the point (single-component model compound, S-CMC), line (multi-component model compounds, M-CMC) and surface (distillate oil) to explore the structure-activity relationship between the physical properties (HF, HF’, Smicro, Sexter, Vmicro, Vmeso, Daver, DM), chemical properties (CTA, CMSA, CWA and DAx-y), χc of the catalyst and the yield of the desired products (benzene (B), toluene (T), ethyl benzene (E), xylene (X) and naphthalene (N)) produced by catalytic conversion of S-CMCs (docosane, naphthalene and 1-naphthol), M-CMCs (0.08 * docosane +0.25 * naphthalene +0.6 * 1-naphthol) and distillate oil, and the conditions optimized by machine learning modeling and experimental verification. Detailed characterization analysis show that the DM, Vmeso and HF’ of HZSM-5 increase in varying degrees after modified by Ni and W species, and also accompany by appropriate DAx-y. Moreover, DM*Vmicro and YTEX have an approximate nonlinear quadratic function relationship, while DM*Vmeso has a linear positive correlation with YN. The order of influence degree ratio of the physicochemical parameters on YBTEXN was: Smicro > Sexter > Vmicro > Vmeso > CWA > CMSA.

Nonclassical Approaches and Behaviors in Synthesis, Structure Characterization, and Catalysis of Zeolites

Zeolite-based materials play critical roles to develop new technologies toward renewable energy and environmental improvement to face the global sustainability from rapid industrial development and population increase in the new century. In the past decade, novel approaches for the preparation of zeolites have been designed from the concept of green chemistry, and new insights on zeolites frameworks have been revealed by the new techniques. Furthermore, the wettability of zeolite and consequent sorption/diffusion were demonstrated as important roles in the catalytic processes, which are quietly different from the classical behaviors of zeolites. We believe that these nonclassical advances in synthesis, structure, and catalytic performance could benefit for the understanding on crystallization of zeolites and framework characters, and consequently, the design of efficient zeolite-based materials for energy and environmental improvement.

Elevating carbonium ion chemistry in zeolite catalysis

Elevating carbonium ion chemistry in zeolite catalysis

Recent advances in shape selectivity of MFI zeolite and its effect on the catalytic performance

MFI zeolite characterized by uniform pore size, adjustable acidity, and high-temperature resistance has a broad application prospect in catalytic reactions. However, controlling the product distribution of zeolite as a catalyst is still confronting great challenges and applications. It is considered as an effective way to control the product distribution by developing and improving new zeolites to modulate their shape selective effect. In recent years, researchers have achieved remarkable successes in investigating the shape selective modulation of zeolites on catalytic reaction and molecular diffusion. The microporous channels of MFI zeolite are the main places for the entry and exit of reactants or product molecules. This review provides the research progress of the shape-selective modulation of MFI zeolite channels and its influence on a series of catalytic performances in recent years. The shape-selective modulation of microporous channels of zeolite, encapsulation of micropores to metals, catalysis of mesoporous zeolite, and the distribution of framework Al were all systematically discussed. The development of advanced catalysts still faces great challenges and potential applications. Finally, we discussed the problems to be addressed urgently in the field of zeolite catalysts in the future.

Evaluating the Role of Descriptor- and Spectator-Type Reaction Intermediates During the Early Phases of Zeolite Catalysis

Evaluating the Role of Descriptor- and Spectator-Type Reaction Intermediates During the Early Phases of Zeolite Catalysis

Preparation of aromatic hydrocarbons from fast pyrolysis of waste medical mask catalyzed by modified HZSM-5

The safe and efficient utilization of waste medical masks has attracted worldwide attention. Fast pyrolysis with the catalysis of zeolites can provide a promising route for the production of value-added aromatic hydrocarbons from waste medical masks. Herein, HZSM-5 was employed and modified by metal loading (Ga, Mo) and alkali treatment for the ex-situ catalytic pyrolysis of mask filters. The non-catalytic pyrolysis of mask filters only led to a low yield of aromatics. When HZSM-5 catalysts were employed, the liquid long-chain olefins, cycloalkanes, and alcohols, as well as gaseous light hydrocarbons were mainly converted into aromatics. According to the micro-scale pyrolysis experiments, the alkali treatment as well as Ga and Mo loading further promoted the generation of aromatics, with aromatic yields of 52.8 wt%, 54.1 wt%, and 46.3 wt%, respectively. For alkali treatment, the NaOH concentration of 0.3 mol/L was most effective for aromatic production, due to the moderate change in the pore size. For metal loading, Ga loading had better performance than Mo loading because the former resulted in a higher specific surface area while the two modified catalysts possessed similar acid activity. When the two modification methods were combined, the yields of aromatics increased to 56.7 wt% and 60.3 wt% under the catalysis of Mo/Na-HZSM-5 and Ga/Na-HZSM-5. Based on the lab-scale experiments, the aromatic yield could reach 63.0 wt% with the catalysis of Ga/Na-HZSM-5, accompanied by a high selectivity of 87.6% for monocyclic aromatic hydrocarbons (MAHs). In summary, both MAHs and polycyclic aromatic hydrocarbons (PAHs) were promoted with these catalyst modification methods by expanding the mesopore size and adjusting acid site distribution, with greater promotion on the formation of MAHs.

Boosting molecular diffusion following the generalized Murray's Law by constructing hierarchical zeolites for maximized catalytic activity.

Diffusion is an extremely critical step in zeolite catalysis that determines the catalytic performance, in particular for the conversion of bulky molecules. Introducing interconnected mesopores and macropores into a single microporous zeolite with the rationalized pore size at each level is an effective strategy to suppress the diffusion limitations, but remains highly challenging due to the lack of rational design principles. Herein, we demonstrate the first example of boosting molecular diffusion by constructing hierarchical Murray zeolites with a highly ordered and fully interconnected macro-meso-microporous structure on the basis of the generalized Murray's Law. Such a hierarchical Murray zeolite with a refined quantitative relationship between the pore size at each length scale exhibited 9 and 5 times higher effective diffusion rates, leading to 2.5 and 1.5 times higher catalytic performance in the bulky 1,3,5-triisopropylbenzene cracking reaction than those of microporous ZSM-5 and ZSM-5 nanocrystals, respectively. The concept of hierarchical Murray zeolites with optimized structural features and their design principles could be applied to other catalytic reactions for maximized performance.

Shape-selective catalysts of Mg2+ ion exchange modified SAPO-11 molecular sieves for alkylation of 2-methylnaphthalene

The conversion of 2-methylnaphthalene (2-MN) and methanol by molecular zeolites to 2,6-dimethylnaphthalene (2,6-DMN) is significant for the preparation of polyethylene naphthalate. However, there is still a challenge to develop catalysts with efficient selectivity and activity. In this work, the hydrothermally synthesized SAPO-11 samples were modified by ion-exchange of Mg2+ for the catalytic alkylation of 2-MN, and thus the selectivity of 2,6-DMN was greatly improved. The results show that the Mg2+ ion-exchanged SAPO-11 molecular sieves could maintain the original main structure and morphology. However, the introduction of Mg2+ regulated the pore structure, acid content, acid strength and the distribution ratio of Brönsted acid and Lewis acid sites in the molecular sieves, and thus the Mg/SAPO-11 catalyst presented better catalytic performance than the pure SAPO-11. After 6 h of reaction, the selectivity of 2,6-DMN could still maintain 41.3% and the ratio of 2,6-/2,7-DMN was 1.87 over 0.06M-Mg/SAPO-11 sample. The deep isomerization of methylnaphthalene was reduced due to the Mg2+ ion exchange resulting in a shape-selective effect from passivation of the outer surface of the catalyst and adjustment of the open pore size. Therefore, this work presents a shape-selective catalysis in zeolites using Mg2+ ion exchange strategy for the alkylation of 2-MN.

Chemistry of Ketene Transformation to Gasoline Catalyzed by H-SAPO-11.

Although ketene has been proposed to be an active intermediate in a number of reactions including OXZEO (metal oxide-zeolite)-catalyzed syngas conversion, dimethyl ether carbonylation, methanol to hydrocarbons, and CO2 hydrogenation, its chemistry and reaction pathway over zeolites are not well understood. Herein, we study the pathway of ketene transformation to gasoline range hydrocarbons over the molecular sieve H-SAPO-11 by kinetic analysis, in situ infrared spectroscopy, and solid-state nuclear magnetic resonance spectroscopy. It is demonstrated that butene is the reaction intermediate on the paths toward gasoline products. Ketene transforms to butene on the acid sites via either acetyl species following an acetic acid ketonization pathway or acetoacetyl species with keto-enol tautomerism following an acetoacetic acid decarboxylation pathway when in the presence of water. This study reveals experimentally for the first time insights into ketene chemistry in zeolite catalysis.

Coking and decoking chemistry for resource utilization of polycyclic aromatic hydrocarbons (PAHs) and low-carbon process

Low-carbon process for resource utilization of polycyclic aromatic hydrocarbons (PAHs) in zeolite-catalyzed processes, geared to carbon neutrality-a prominent trend throughout human activities, has been bottlenecked by the lack of a complete mechanistic understanding of coking and decoking chemistry, involving the speciation and molecular evolution of PAHs, the plethora of which causes catalyst deactivation and forces regeneration, rendering significant CO2 emission. Herein, by exploiting the high-resolution matrix-assisted laser desorption/ionization Fourier-transform ion cyclotron resonance mass spectrometry (MALDI FT-ICR MS), we unveil the missing fingerprints of the mechanistic pathways for both formation and decomposition of cross-linked cage-passing PAHs for SAPO-34-catalyzed, industrially relevant methanol-to-olefins (MTO) as a model reaction. Notable is the molecule-resolved symmetrical signature: their speciation originates exclusively from the direct coupling of in-cage hydrocarbon pool (HCP) species, whereas water-promoted decomposition of cage-passing PAHs initiates with selective cracking of inter-cage local structures at 8-rings followed by deep aromatic steam reforming. Molecular deciphering the reversibly dynamic evolution trajectory (fate) of full-spectrum aromatic hydrocarbons and fulfilling the real-time quantitative carbon resource footprints advance the fundamental knowledge of deactivation and regeneration phenomena (decay and recovery motifs of autocatalysis) and disclose the underlying mechanisms of especially the chemistry of coking and decoking in zeolite catalysis. The positive yet divergent roles of water in these two processes are disentangled. These unprecedented insights ultimately lead us to a steam regeneration strategy with valuable CO and H2 as main products, negligible CO2 emission in steam reforming and full catalyst activity recovery, which further proves feasible in other important chemical processes, promising to be a sustainable and potent approach that contributes to carbon-neutral chemical industry.

Synergistic catalysis of Ru single-atoms and zeolite boosts high-efficiency hydrogen storage

Liquid organic hydrogen carriers (LOHCs) are promising hydrogen carriers that play an important role in the hydrogen economy. However, designing an efficient catalyst for realizing hydrogen storage with cost-effective and low-temperature is still a great challenge. Herein, we report a Ru single-atoms supported on *BEA zeolite catalyst (Ru(Na)/Beta), with the assistance of hydrogen spillover, which can remarkably enhance the hydrogenation of N-ethylcarbazole(NEC), N-propylcarbazole(NPC) and 2-methylindole (2-MID) at lower temperatures with lower Ru content (0.5 wt%). Notably, the obtained Ru(Na)/Beta catalyst exhibits excellent activity in the hydrogenation of NEC with the hydrogen uptake of 5.69 wt% and a conversion rate of > 99% within 1.5 h for the 6 MPa H2 at 100 °C, whereas the hydrogen uptake on traditional Ru/Al2O3 is only 2.97 wt% with the conversion rate of 67 % under the same conditions. It is found that highly dispersed Ru single-atoms boost hydrogen activation and the strong acid sites (Brønsted and Lewis) of zeolites promote the hydrogen spillover on the hydrogenation with N-heterocycles. Moreover, the synergistic effect of Ru single atoms and *BEA zeolite is crucial for accelerating the hydrogenation rate and lowering the activation energy (45.7 vs. 88.3 kJ/mol) compared with traditional Ru-based catalysts.

Quantitative principle of shape‐selective catalysis for a rational screening of zeolites for methanol‐to‐hydrocarbons

AbstractThe production of hydrocarbons for the synthesis of readily available energy and multifunctional materials is of great importance in modern society. Zeolites have proven to be a boon for the targeted regulation of specific hydrocarbon as shape‐selective catalyst in converting carbon resources. Yet our mechanistic understanding and quantitative description of shape‐selectivity of zeolite catalysis remains rather limited, which restricts the upgrade of zeolite catalysts. Herein, we proposed quantitative principle of shape‐selectivity for zeolite catalysis using methanol‐to‐hydrocarbons (MTH) as model. Combining with molecular simulations and infrared imaging, we unveil the competition of thermodynamic stability, preferential diffusion and favored secondary reactions between different hydrocarbons within zeolite framework are the essence of zeolite shape‐selective catalysis. Notably, we provide methodology to in silico search for the optimal combination of framework topology and acidity properties of zeolites with operating conditions that potentially outperform commercial MTH catalysts to achieve high selectivity of desired hydrocarbon products.

Toward Computing Accurate Free Energies in Heterogeneous Catalysis: a Case Study for Adsorbed Isobutene in H-ZSM-5.

Herein, we propose a novel computational protocol that enables calculating free energies with improved accuracy by combining the best available techniques for enthalpy and entropy calculation. While the entropy is described by enhanced sampling molecular dynamics techniques, the energy is calculated using ab initio methods. We apply the method to assess the stability of isobutene adsorption intermediates in the zeolite H-SSZ-13, a prototypical problem that is computationally extremely challenging in terms of calculating enthalpy and entropy. We find that at typical operating conditions for zeolite catalysis (400 °C), the physisorbed π-complex, and not the tertiary carbenium ion as often reported, is the most stable intermediate. This method paves the way for sampling-based techniques to calculate the accurate free energies in a broad range of chemistry-related disciplines, thus presenting a big step forward toward predictive modeling.

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis.

Zeolite chemistry and catalysis are expected to play a decisive role in the next decade(s) to build a more decentralized renewable feedstock-dependent sustainable society owing to the increased scrutiny over carbon emissions. Therefore, the lack of fundamental and mechanistic understanding of these processes is a critical "technical bottleneck" that must be eliminated to maximize economic value and minimize waste. We have identified, considering this objective, that the chemistry related to the first-generation reaction intermediates (i.e., carbocations, radicals, carbenes, ketenes, and carbanions) in zeolite chemistry and catalysis is highly underdeveloped or undervalued compared to other catalysis streams (e.g., homogeneous catalysis). This limitation can often be attributed to the technological restrictions to detect such "short-lived and highly reactive" intermediates at the interface (gas-solid/solid-liquid); however, the recent rise of sophisticated spectroscopic/analytical techniques (including under in situ/operando conditions) and modern data analysis methods collectively compete to unravel the impact of these organic intermediates. This comprehensive review summarizes the state-of-the-art first-generation organic reaction intermediates in zeolite chemistry and catalysis and evaluates their existing challenges and future prospects, to contribute significantly to the "circular carbon economy" initiatives.

Multiscale dynamical cross-talk in zeolite-catalyzed methanol and dimethyl ether conversions.

Establishing a comprehensive understanding of the dynamical multiscale diffusion and reaction process is crucial for zeolite shape-selective catalysis and is urgently demanded in academia and industry. So far, diffusion and reaction for methanol and dimethyl ether (DME) conversions have usually been studied separately and focused on a single scale. Herein, we decipher the dynamical molecular diffusion and reaction process for methanol and DME conversions within the zeolite material evolving with time, at multiple scales, from the scale of molecules to single catalyst crystal and catalyst ensemble. Microscopic intracrystalline diffusivity is successfully decoupled from the macroscopic experiments and verified by molecular dynamics simulation. Spatiotemporal analyses of the confined carbonaceous species allow us to track the migratory reaction fronts in a single catalyst crystal and the catalyst ensemble. The constrained diffusion of DME relative to methanol alleviates the high local chemical potential of the reactant by attenuating its local enrichment, enhancing the utilization efficiency of the inner active sites of the catalyst crystal. In this context, the dynamical cross-talk behaviors of material, diffusion and reaction occurring at multiple scales is uncovered. Zeolite catalysis not only reflects the reaction characteristics of heterogeneous catalysis, but also provides enhanced, moderate or suppressed local reaction kinetics through the special catalytic micro-environment, which leads to the heterogeneity of diffusion and reaction at multiple scales, thereby realizing efficient and shape-selective catalysis.