Guest–host interactions of arenes in H-ZSM-5 and their impact on methanol-to-hydrocarbons deactivation processes

High Reactivity of Dimethyl Ether Activated by Zeolite Ferrierite within a Fer Cage: A Prediction Study

The zeolite-catalyzed conversion of DME into chemicals is considered environmentally friendly in industry. The periodic density functional theory, statistical thermodynamics, and the transition state theory are used to study some possible parallel reactions about the hydrogen-bonded DME over zeolite ferrierite. The following are the key findings: (1) the charge separation probably leads to the conversion of a hydrogen-bonded DME into a dimethyl oxonium ion (i.e., DMO+ or (CH3)2OH+) with a positive charge of about 0.804 e; (2) the methylation of DME, CH3OH, H2O, and CO by DMO+ at the T2O6 site of zeolite ferrierite shows the different activated internal energy (∆E≠) ranging from 18.47 to 30.06 kcal/mol, implying the strong methylation ability of DMO+; (3) H-abstraction by DMO+ is about 3.94-15.53 or 6.57-18.16 kcal/mol higher than DMO+ methylation in the activation internal energy; (4) six DMO+-mediated reactions are more likely to occur due to the lower barriers, compared to the experimental barrier (i.e., 39.87 kcal/mol) for methyl acetate synthesis; (5) active intermediates, such as (CH3)3O+, (CH3)2OH+, CH3CO+, CH3OH2+, and CH2=OH+, are expected to appear; (6) DMO+ is slightly weaker than the well-known surface methoxy species (ZO-CH3) in methylation; and (7) the methylated activity declines in the order of DME, CH3OH, H2O, and CO, with corresponding rate constants at 463.15 K of about 3.4 × 104, 1.1 × 102, 0.18, and 8.2 × 10-2 s-1, respectively.

Water-Induced Micro-Hydrophobic Effect Regulates Benzene Methylation in Zeolite

Water is a ubiquitous component in heterogeneous catalysis over zeolites and can significantly influence the catalyst performance. However, the detailed mechanism insights into zeolite-catalyzed reactions under microscale aqueous environment remain elusive. Here, using multiple dimensional solid-state NMR experiments coupled with ultrahigh magic angle spinning technique and theoretical simulations, we establish a fundamental understanding of the role of water in benzene methylation over ZSM-5 zeolite under water vapor conditions. We show that water competes with benzene for the active sites of zeolite and facilitates the bimolecular reaction mechanism. The growth of water clusters induces a micro-hydrophobic effect in zeolite pores, which reorients benzene molecules and drives their interactions with surface methoxy species (SMS) on zeolite. We identify the formation and evolution of active SMS-Benzene complexes in a microscale aqueous environment and demonstrate that their accumulation in zeolite pores boosts benzene conversion and methylation.

Simulating Crystal Structure, Acidity, Proton Distribution, and IR Spectra of Acid Zeolite HSAPO-34: A High Accuracy Study

It is a challenge to characterize the acid properties of microporous materials in either experiments or theory. This study presents the crystal structure, acid site, acid strength, proton siting, and IR spectra of HSAPO-34 from the SCAN + rVV10 method. The results indicate: the crystal structures of various acid sites of HSAPO-34 deviate from the space group of R3¯; the acid strength inferred from the DPE value likely decreases with the proton binding sites at O(2), O(4), O(1),and O(3), contrary to the stability order in view of the internal energy; the calculated ensemble-averaged DPE is about 1525 kJ/mol at 673.15 K; and the proton siting and the proton distribution are distinctly influenced by the temperature: at low temperatures, the proton is predominantly located at O(3), while it prefers O(2) at high temperatures, and the proton at O(4) assumedly has the least distribution at 273.15-773.15 K. In line with the neutron diffraction experiment, a correction factor of 0.979 is needed to correct for the calculated hydroxyl stretching vibration (ν(O-H)) of HSAPO-34. It seems that the SCAN meta-GGA method, compensating for some drawbacks of the GGA method, could provide satisfying results regarding the acid properties of HSAPO-34.

Electron Diffraction Enables the Mapping of Coke in ZSM-5 Micropores Formed during Methanol-to-Hydrocarbons Conversion

Unveiling the coke formation in zeolites is an essential prerequisite for tackling the deactivation of these catalysts in the transformations of hydrocarbons. Herein, we present the direct mapping of coke in the micropores of ZSM‐5 catalysts used in methanol‐to‐hydrocarbons conversion by single‐crystal electron diffraction analysis. The latter technique revealed a polycyclic aromatic structure along the straight channel, wherein the high‐quality data permit refinement of its occupancy to about 40 %. These findings were exploited to analyze the evolution of micropore coke during the reaction. Herein, coke‐associated signals, which correlate with the activity loss, indicate that the nucleation of coke commences in the intersections of sinusoidal and straight channels, while the formation of coke in the straight pores occurs in the late stages of deactivation. The findings uncover an attractive method for analyzing coke deposition in the micropore domain.

Probing coke formation during the methanol-to-hydrocarbon reaction on zeolite ZSM-5 catalyst at the nanoscale using tip-enhanced fluorescence microscopy

The deactivation mechanism of the widely used zeolite ZSM-5 catalysts remains unclear to date due to the lack of analytical techniques with sufficient sensitivity and/or spatial resolution. Herein, a combination of hyperspectral confocal fluorescence microscopy (CFM) and tip-enhanced fluorescence (TEFL) microscopy is used to study the formation of different coke (precursor) species involved in the deactivation of zeolite ZSM-5 during the methanol-to-hydrocarbon (MTH) reaction. CFM submicron-scale imaging shows a preferential formation of graphite-like coke species at the edges of zeolite ZSM-5 crystals within 10 min of the MTH reaction (i.e., working catalyst), whilst the amount of graphite-like coke species uniformly increased over the entire zeolite ZSM-5 surface after 90 min (i.e., deactivated catalyst). Furthermore, TEFL nanoscale imaging with ∼35 nm spatial resolution revealed that formation of coke species on the zeolite ZSM-5 surface is non-uniform and a relatively larger amount of coke is formed at the crystal steps, indicating a higher initial catalytic activity.

Inhomogeneities in coke formation during methanol-to-hydrocarbon reaction on the zeolite ZSM-5 catalyst are imaged with ∼35 nm spatial resolution using tip-enhanced fluorescence microscopy.

A quantitative multiscale perspective on primary olefin formation from methanol

The formation of the first C-C bond and primary olefins from methanol over zeolite and zeotype catalysts has been studied for over 40 years. Over 20 mechanisms have been proposed for the formation of the first C-C bond. In this quantitative multiscale perspective, we decouple the adsorption, desorption, mobility, and surface reactions of early species through a combination of vacuum and sub-vacuum studies using temporal analysis of products (TAP) reactor systems, and through studies with atmospheric fixed bed reactors. These results are supplemented with density functional theory calculations and data-driven physical models, using partial differential equations, that describe the temporal and spatial evolution of species. We consider the effects of steam, early degradation species, and product masking due to the inherent autocatalytic nature of the process, which all complicate the observation of the primary olefin(s). Although quantitative spectroscopic determination of the lifetimes, surface mobility, and reactivity of adspecies is still lacking in the literature, we observe that reaction barriers are competitive with adsorption enthalpies and/or activation energies of desorption, while facile diffusion occurs in the porous structures of the zeolite/zeotype catalysts. Understanding the various processes allows for quantitative evaluation of their competing energetics, which leads to molecular insights as to what governs the catalytic activity during the conversion of methanol to primary olefins over zeolite/zeotype catalysts.

Theoretical investigation of the side-chain mechanism of the MTO process over H-SSZ-13 using DFT and ab initio calculations

The side-chain mechanism of the methanol-to-olefins process over the H-SSZ-13 acidic zeolite was investigated using periodic density functional theory with corrections from highly accurate ab intio calculations on large cluster models.

Disentangling Reaction Processes of Zeolites within Single-Oriented Channels

Establishing structure-reactivity relationships for specific channel orientations of zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, a well-defined model system was developed to build structure-reactivity relationships for specific zeolite-channel orientations during various catalytic reaction processes, for example, the methanol- and ethanol-to-hydrocarbons (MTH and ETH) process as well as oligomerization reactions. The entrapped and effluent hydrocarbons from single-oriented zeolite ZSM-5 channels during the MTH process were monitored by using operando UV/Vis diffuse reflectance spectroscopy (DRS) and on-line mass spectrometry (MS), respectively. The results reveal that the straight channels favor the formation of internal coke, promoting the aromatic cycle. Furthermore, the sinusoidal channels produce aromatics, (e.g., toluene) that further grow into larger polyaromatics (e.g., graphitic coke) leading to deactivation of the zeolites. This underscores the importance of careful engineering of materials to suppress coke formation and tune product distribution by rational control of the location of zeolite acid sites and crystallographic orientations.

Light Olefin Diffusion during the MTO Process on H-SAPO-34: A Complex Interplay of Molecular Factors

The methanol-to-olefins process over H-SAPO-34 is characterized by its high shape selectivity toward light olefins. The catalyst is a supramolecular system consisting of nanometer-sized inorganic cages, decorated by Brønsted acid sites, in which organic compounds, mostly methylated benzene species, are trapped. These hydrocarbon pool species are essential to catalyze the methanol conversion but may also clog the pores. As such, diffusion of ethene and propene plays an essential role in determining the ultimate product selectivity. Enhanced sampling molecular dynamics simulations based on either force fields or density functional theory are used to determine how molecular factors influence the diffusion of light olefins through the 8-ring windows of H-SAPO-34. Our simulations show that diffusion through the 8-ring in general is a hindered process, corresponding to a hopping event of the diffusing molecule between neighboring cages. The loading of different methanol, alkene, and aromatic species in the cages may substantially slow down or facilitate the diffusion process. The presence of Brønsted acid sites in the 8-ring enhances the diffusion process due to the formation of a favorable π-complex host-guest interaction. Aromatic hydrocarbon pool species severely hinder the diffusion and their spatial distribution in the zeolite crystal may have a significant effect on the product selectivity. Herein, we unveil how molecular factors influence the diffusion of light olefins in a complex environment with confined hydrocarbon pool species, high olefin loadings, and the presence of acid sites by means of enhanced molecular dynamics simulations under operating conditions.

Molecular elucidating of an unusual growth mechanism for polycyclic aromatic hydrocarbons in confined space

Extension and clustering of polycyclic aromatic hydrocarbons (PAHs) are key mechanistic steps for coking and deactivation in catalysis reactions. However, no unambiguous mechanistic picture exists on molecule-resolved PAHs speciation and evolution, due to the immense experimental challenges in deciphering the complex PAHs structures. Herein, we report an effective strategy through integrating a high resolution MALDI FT-ICR mass spectrometry with isotope labeling technique. With this strategy, a complete route for aromatic hydrocarbon evolution is unveiled for SAPO-34-catalyzed, industrially relevant methanol-to-olefins (MTO) as a model reaction. Notable is the elucidation of an unusual, previously unrecognized mechanistic step: cage-passing growth forming cross-linked multi-core PAHs with graphene-like structure. This mechanistic concept proves general on other cage-based molecule sieves. This preliminary work would provide a versatile means to decipher the key mechanistic step of molecular mass growth for PAHs involved in catalysis and combustion chemistry.

Coke-induced catalyst deactivation draws increasing concerns in industrially catalytic processes. Here the authors provide a strategy integrating advanced mass spectroscopy and isotope labeling to uncover a cage-passing molecular route of coking species in molecular sieve catalysts.

Desorption products during linear heating of copper zeolites with pre-adsorbed methanol

Study of desorption products from Cu-zeolites (MFI and CHA) during methanol-TPD using a chemical flow reactor with a gas phase FTIR spectrometer.

Mechanisms of 1,4-Dioxane Biodegradation and Adsorption by Bio-Zeolite in the Presence of Chlorinated Solvents: Experimental and Molecular Dynamics Simulation Studies

The use of bioaugmented zeolite (bio-zeolite) can be an effective technology for irreversibly removing recalcitrant organic pollutants in aqueous mixtures. Removal of 1,4-dioxane by a bio-zeolite (Pseudonocardia dioxanivorans CB1190-bioaugmented ZSM-5) in the presence of several chlorinated volatile organic compounds (CVOCs) was superior to removal by adsorption using abiotic zeolite. Mixtures containing 1,1-dichloroethene (1,1-DCE) were an exception, which completely inhibited the bio-zeolite system. Specific adsorption characteristics were studied using adsorption isotherms in single-solute and bisolute systems accompanied by Polanyi theory-based Dubinin-Astakhov (DA) modeling. Adsorption behavior was examined using characteristic energy (E_a/H) from modified DA models and molecular dynamics simulations. While the tight-fit of 1,4-dioxane in the hydrophobic channels of ZSM-5 appears to drive 1,4-dioxane adsorption, the greater hydrophobicity of trichloroethene and cis-1,2-dichloroethene cause them have a greater affinity over 1,4-dioxane for adsorption sites on the zeolite. 1,4-Dioxane was desorbed and displaced by CVOCs except 1,1-DCE because of its low E_a/H value, explaining why bio-zeolite only biodegraded 1,4-dioxane in 1,1-DCE-free CVOC mixtures. Understanding the adsorption mechanisms of solutes in complex mixtures is crucial for the implementation of sorption-based treatment technologies for the removal of complex contaminant mixtures from aquatic environments.

Fast detection and structural identification of carbocations on zeolites by dynamic nuclear polarization enhanced solid-state NMR

Acidic zeolites are porous aluminosilicates used in a wide range of industrial processes such as adsorption and catalysis. The formation of carbocation intermediates plays a key role in reactivity, selectivity and deactivation in heterogeneous catalytic processes. However, the observation and determination of carbocations remain a significant challenge in heterogeneous catalysis due to the lack of selective techniques of sufficient sensitivity to detect their low concentrations. Here, we combine 13C isotopic enrichment and efficient dynamic nuclear polarization magic angle spinning nuclear magnetic resonance spectroscopy to detect carbocations in zeolites. We use two dimensional 13C-13C through-bond correlations to establish their structures and 29Si-13C through-space experiments to quantitatively probe the interaction between multiple surface sites of the zeolites and the confined hydrocarbon pool species. We show that a range of various membered ring carbocations are intermediates in the methanol to hydrocarbons reaction catalysed by different microstructural β-zeolites and highlight that different reaction routes for the formation of both targeted hydrocarbon products and coke exist. These species have strong van der Waals interaction with the zeolite framework demonstrating that their accumulation in the channels of the zeolites leads to deactivation. These results enable understanding of deactivation pathways and open up opportunities for the design of catalysts with improved performances.

Methanol-to-olefin conversion in ABC-6 zeolite cavities: unravelling the role of cavity shape and size from density functional theory calculations

We report a density functional theory (DFT) study to investigate methanol-to-olefin (MTO) conversion in four types of ABC-6 zeolite cavities (cha, avl, aft and h1) by varying their shape and size. Based on a side-chain alkylation mechanism, the reaction energies and barriers of methylations, deprotonations and eliminations for hydrocarbon pool (HP) intermediates are calculated. In the cha cavity, methylations and eliminations are found to possess low barriers as attributed to the strong confinement effect of the elliptical and small cha cavity. The avl and aft cavities also exhibit low barriers of the first and second methylations, and similar barriers of eliminations for producing olefins as in cha. Due to the narrow shape and large size of the h1 cavity, most of the reaction barriers in h1 are the highest. The stabilities of HP species and transition states in the four cavities are quantified by the Gibbs energy profiles. It is found that aft with a wide dimension is favorable for the stability, especially for the charged HP species. The DFT calculation results reveal that the activity and selectivity of MTO conversion in zeolite cavities are strongly governed by the confinement effect, which depends on cavity shape and size. We also predict that zeolites with aft cavities might have good performance for MTO conversion.

A theoretical study of the confinement effects on the energetics and vibrational properties of 4,4'-bipyridine adsorption on H-ZSM-5 zeolite

The confinement effects of a zeolite framework on the adsorption of a bidentate 4,4'-bipyridine (44BPY) ligand on the straight channel of H-ZSM-5 have been investigated by density functional theory calculations using B3LYP, B3LYP-D3, M06-2X, M06-2X-D3 functionals, and the MP2 method with two basis sets 6-31+G* and 6-31++G(2d,2p). The straight channel is simulated by a realistic cluster model of 32 tetrahedra (T), having two aluminum atoms located in the straight and intersection regions, respectively. The potential energy surface of the double proton transfer reaction from the Brønsted acid sites of H-ZSM-5 to 44BPY is characterized by three minima corresponding to two monodentate ion pair complexes 44BPYH⁺/32T^- and one bidentate ion pair complex 44BPYH₂²⁺/32T^2- formed consecutively via two distinct pathways. No energy minimum is found for a neutral hydrogen bonding structure. The relative stabilities of these 44BPY adsorption complexes and the transition states connecting them do not exceed 4.9 and 4.2 kcal mol^-1, respectively. Consequently equilibrium between the mono and bidentate complexes could be established. Our results clearly show that the adsorption energy for all complexes is substantially governed by the confinement effects executed through steric constraints and dispersive van der Waals interactions. The calculated vibrational frequencies and frequency shifts of 44BPY adsorbed as mono or diprotonated species are in good agreement with our Raman spectra of 44BPY occluded in H-ZSM-5 of different Si/Al ratios.

Effect of Al-distribution on oxygen activation over Cu–CHA

The Al-distribution affects the formation of Cu(NH₃)₂⁺-pairs during NH₃-SCR over Cu–CHA and oxygen dissociates with low barriers over such pairs.

Direct structural identification of carbenium ions and investigation of host-guest interaction in the methanol to olefins reaction obtained by multinuclear NMR correlations

Probing and determining the intermediates formed during catalytic reactions in heterogeneous catalysis are strong challenges. Using ¹³C labelling and two dimensional ¹³C-¹³C through-bond NMR correlations, we directly reveal the structures of a range of carbenium ion species formed during the conversion of methanol to olefins on acidic H-ZSM-5 zeolite by mapping the carbon-carbon bond connectivities. Additionally, we use ¹³C-²⁷Al and ²⁹Si-¹³C through-space NMR experiments to probe the interactions between the confined carbon species (including carbenium ions) and the framework of the zeolite, which quantitatively provide an estimate for the carbon-aluminium and carbon-silicon distances, respectively.

Comparative investigation of the deactivation behaviors over HZSM-5 and HSAPO-34 catalysts during low-temperature methanol conversion

Deactivation over HZSM-5 originated from overloaded HCP species, while that over HSAPO-34 was caused by methyladamantanes generated from the external surface.

Catalytic conversion of methanol to aromatics over nano-sized HZSM-5 zeolite modified by ZnSiF6·6H2O

ZnSiF₆-modified nano-sized HZSM-5 zeolites (NZ2, NZ3 and NZ4 catalysts) were prepared and investigated as catalysts for the conversion of methanol to aromatics.

Coke Formation in a Zeolite Crystal During the Methanol-to-Hydrocarbons Reaction as Studied with Atom Probe Tomography

Understanding the formation of carbon deposits in zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, atom probe tomography (APT) has been used to spatially resolve the 3D compositional changes at the sub‐nm length scale in a single zeolite ZSM‐5 crystal, which has been partially deactivated by the methanol‐to‐hydrocarbons reaction using ¹³C‐labeled methanol. The results reveal the formation of coke in agglomerates that span length scales from tens of nanometers to atomic clusters with a median size of 30–60 ¹³C atoms. These clusters correlate with local increases in Brønsted acid site density, demonstrating that the formation of the first deactivating coke precursor molecules occurs in nanoscopic regions enriched in aluminum. This nanoscale correlation underscores the importance of carefully engineering materials to suppress detrimental coke formation.

Computational insights into the reaction mechanism of methanol-to-olefins conversion in H-ZSM-5: nature of hydrocarbon pool

Periodic DFT calculations in H-ZSM-5 revealed that 1,2,3,5-tetramethylbenzene is the primary component of methylbenzene, and olefins themselves are the active hydrocarbon pool species for the methanol-to-olefins conversion.

Effects of Coke Deposits on the Catalytic Performance of Large Zeolite H-ZSM-5 Crystals during Alcohol-to-Hydrocarbon Reactions as Investigated by a Combination of Optical Spectroscopy and Microscopy

The catalytic activity of large zeolite H-ZSM-5 crystals in methanol (MTO) and ethanol-to-olefins (ETO) conversions was investigated and, using operando UV/Vis measurements, the catalytic activity and deactivation was correlated with the formation of coke. These findings were related to in situ single crystal UV/Vis and confocal fluorescence micro-spectroscopy, allowing the observation of the spatiotemporal formation of intermediates and coke species during the MTO and ETO conversions. It was observed that rapid deactivation at elevated temperatures was due to the fast formation of aromatics at the periphery of the H-ZSM-5 crystals, which are transformed into more poly-aromatic coke species at the external surface, preventing the diffusion of reactants and products into and out of the H-ZSM-5 crystal. Furthermore, we were able to correlate the operando UV/Vis spectroscopy results observed during catalytic testing with the single crystal in situ results.

Similarities and differences between aromatic-based and olefin-based cycles in H-SAPO-34 and H-SSZ-13 for methanol-to-olefins conversion: insights from energetic span model

Both aromatic-based and olefin-based cycles involve similar sequences of elementary steps. Energetic span model analysis indicates olefins are active hydrocarbon pool species in H-SAPO-34 and H-SSZ-13 for MTO conversion.

Host-Guest Interactions in Dealuminated HY Zeolite Probed by (13)C-(27)Al Solid-State NMR Spectroscopy

Host-guest interactions in dealuminated HY zeolite have been investigated by advanced (13)C-(27)Al solid-state NMR experiments. This analysis allows us to report new insights into the adsorption geometry of acetone and its interaction with acid sites in the zeolite channels.

Reactivity Descriptor in Solid Acid Catalysis: Predicting Turnover Frequencies for Propene Methylation in Zeotypes

Recent work has reported the discovery of metal surface catalysts by employing a descriptor-based approach, establishing a correlation between a few well-defined properties of a material and its catalytic activity. This theoretical work aims for a similar approach in solid acid catalysis, focusing on the reaction between propene and methanol catalyzed by Brønsted acidic zeotype catalysts. Experimentally, the ammonia heat of adsorption is often used as a measure of the strength of acid sites. Using periodic DFT calculations, we show that this measure can be used to establish scaling relations for the energy of intermediates and transition states, effectively describing the reactivity of the acid site. This allows us to use microkinetic modeling to predict a quantitative relation between the ammonia heat of adsorption and the rate of propene methylation from first principles. We propose that this is the first step toward descriptor-based design of solid acid catalysts.