Coke formation during the methanol-to-olefin conversion: in situ microspectroscopy on individual H-ZSM-5 crystals with different Brønsted acidity

Coke formation during the methanol‐to‐olefin (MTO) conversion has been studied at the single‐particle level with in situ UV/Vis and confocal fluorescence microscopy. For this purpose, large H‐ZSM‐5 crystals differing in their Si/Al molar ratio have been investigated. During MTO, performed at 623 and 773 K, three major UV/Vis bands assigned to different carbonaceous deposits and their precursors are observed. The absorption at 420 nm, assigned to methyl‐substituted aromatic compounds, initiates the buildup of the optically active coke species. With time‐on‐stream, these carbonaceous compounds expand in size, resulting in the gradual development of a second absorption band at around 500 nm. An additional broad absorption band in the 600 nm region indicates the enhanced formation of extended carbonaceous compounds that form as the reaction temperature is raised. Overall, the rate of coke formation decreases with decreasing aluminum content. Analysis of the reaction kinetics indicates that an increased Brønsted acid site density facilitates the formation of larger coke species and enhances their formation rate. The use of multiple excitation wavelengths in confocal fluorescence microscopy enables the localization of coke compounds with different molecular dimensions in an individual H‐ZSM‐5 crystal. It demonstrates that small coke species evenly spread throughout the entire H‐ZSM‐5 crystal, whereas extended coke deposits primarily form near the crystal edges and surfaces. Polarization‐dependent UV/Vis spectroscopy measurements illustrate that extended coke species are predominantly formed in the straight channels of H‐ZSM‐5. In addition, at higher temperatures, fast deactivation leads to the formation of large aromatic compounds within channel intersections and at the external zeolite surface, where the lack of spatial restrictions allows the formation of graphite‐like coke.     

Space‐ and Time‐Resolved In‐situ Spectroscopy on the Coke Formation in Molecular Sieves: Methanol‐to‐Olefin Conversion over H‐ZSM‐5 and H‐SAPO‐34

Formation of coke in large H‐ZSM‐5 and H‐SAPO‐34 crystals during the methanol‐to‐olefin (MTO) reaction has been studied in a space‐ and time‐resolved manner. This has been made possible by applying a high‐temperature in‐situ cell in combination with micro‐spectroscopic techniques. The buildup of optically active carbonaceous species allows detection with UV/Vis microscopy, while a confocal fluorescence microscope in an upright configuration visualises the formation of coke molecules and their precursors inside the catalyst grains. In H‐ZSM‐5, coke is initially formed at the triangular crystal edges, in which straight channel openings reach directly the external crystal surface. At reaction temperatures ranging from 530 to 745 K, two absorption bands at around 415 and 550 nm were detected due to coke or its precursors. Confocal fluorescence microscopy reveals fluorescent carbonaceous species that initially form in the near‐surface area and gradually diffuse inwards the crystal in which internal intergrowth boundaries hinder a facile penetration for the more bulky aromatic compounds. In the case of H‐SAPO‐34 crystals, an absorption band at around 400 nm arises during the reaction. This band grows in intensity with time and then decreases if the reaction is carried out between 530 and 575 K, whereas at higher temperatures its intensity remains steady with time on stream. Formation of the fluorescent species during the course of the reaction is limited to the near‐surface region of the H‐SAPO‐34 crystals, thereby creating diffusion limitations for the coke front moving towards the middle of the crystal during the MTO reaction. The two applied micro‐spectroscopic techniques introduced allow us to distinguish between graphite‐like coke deposited on the external crystal surface and aromatic species formed inside the zeolite channels. The use of the methods can be extended to a wide variety of catalytic reactions and materials in which carbonaceous deposits are formed.     

Single‐Particle Spectroscopy on Large SAPO‐34 Crystals at Work: Methanol‐to‐Olefin versus Ethanol‐to‐Olefin Processes

The formation of hydrocarbon pool (HCP) species during methanol‐to‐olefin (MTO) and ethanol‐to‐olefin (ETO) processes have been studied on individual micron‐sized SAPO‐34 crystals with a combination of in situ UV/Vis, confocal fluorescence, and synchrotron‐based IR microspectroscopic techniques. With in situ UV/Vis microspectroscopy, the intensity changes of the λ=400 nm absorption band, ascribed to polyalkylated benzene (PAB) carbocations, have been monitored and fitted with a first‐order kinetics at low reaction temperatures. The calculated activation energy (E_a) for MTO, approximately 98 kJ mol^?1, shows a strong correlation with the theoretical values for the methylation of aromatics. This provides evidence that methylation reactions are the rate‐determining steps for the formation of PAB. In contrast for ETO, the E_a value is approximately 60 kJ mol^?1, which is comparable to the E_a values for the condensation of light olefins into aromatics. Confocal fluorescence microscopy demonstrates that during MTO the formation of the initial HCP species are concentrated in the outer rim of the SAPO‐34 crystal when the reaction temperature is at 600 K or lower, whereas larger HCP species are gradually formed inwards the crystal at higher temperatures. In the case of ETO, the observed egg‐white distribution of HCP at 509 K suggests that the ETO process is kinetically controlled, whereas the square‐shaped HCP distribution at 650 K is indicative of a diffusion‐controlled process. Finally, synchrotron‐based IR microspectroscopy revealed a higher degree of alkylation for aromatics for MTO as compared to ETO, whereas high reaction temperatures favor dealkylation processes for both the MTO and ETO processes.     

Large Zeolite H‐ZSM‐5 Crystals as Models for the Methanol‐to‐Hydrocarbons Process: Bridging the Gap between Single‐Particle Examination and Bulk Catalyst Analysis

The catalytic, deactivation, and regeneration characteristics of large coffin‐shaped H‐ZSM‐5 crystals were investigated during the methanol‐to‐hydrocarbons (MTH) reaction at 350 and 500 °C. Online gas‐phase effluent analysis and examination of retained material thereof were used to explore the bulk properties of large coffin‐shaped zeolite H‐ZSM‐5 crystals in a fixed‐bed reactor to introduce them as model catalysts for the MTH reaction. These findings were related to observations made at the individual particle level by using polarization‐dependent UV‐visible microspectroscopy and mass spectrometric techniques after reaction in an in situ microspectroscopy reaction cell. Excellent agreement between the spectroscopic measurements and the analysis of hydrocarbon deposits by means of retained hydrocarbon analysis and time‐of‐flight secondary‐ion mass spectrometry of spent catalyst materials was observed. The obtained data reveal a shift towards more condensed coke deposits on the outer zeolite surface at higher reaction temperatures. Zeolites in the fixed‐bed reactor setup underwent more coke deposition than those reacted in the in situ microspectroscopy reaction cell. Regeneration studies of the large zeolite crystals were performed by oxidation in O₂/inert gas mixtures at 550 °C. UV‐visible microspectroscopic measurements using the oligomerization of styrene derivatives as probe reaction indicated that the fraction of strong acid sites decreased during regeneration. This change was accompanied by a slight decrease in the initial conversion obtained after regeneration. H‐ZSM‐5 deactivated more rapidly at higher reaction temperature.     

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.     

Unraveling the Homologation Reaction Sequence of the Zeolite‐Catalyzed Ethanol‐to‐Hydrocarbons Process

Although industrialized, the mechanism for catalytic upgrading of bioethanol over solid‐acid catalysts (that is, the ethanol‐to‐hydrocarbons (ETH) reaction) has not yet been fully resolved. Moreover, mechanistic understanding of the ETH reaction relies heavily on its well‐known “sister‐reaction” the methanol‐to‐hydrocarbons (MTH) process. However, the MTH process possesses a C₁‐entity reactant and cannot, therefore, shed any light on the homologation reaction sequence. The reaction and deactivation mechanism of the zeolite H‐ZSM‐5‐catalyzed ETH process was elucidated using a combination of complementary solid‐state NMR and operando UV/Vis diffuse reflectance spectroscopy, coupled with on‐line mass spectrometry. This approach establishes the existence of a homologation reaction sequence through analysis of the pattern of the identified reactive and deactivated species. Furthermore, and in contrast to the MTH process, the deficiency of any olefinic‐hydrocarbon pool species (that is, the olefin cycle) during the ETH process is also noted.     

Reversible Nature of Coke Formation on Mo/ZSM‐5 Methane Dehydroaromatization Catalysts

Non‐oxidative dehydroaromatization of methane over Mo/ZSM‐5 zeolite catalysts is a promising reaction for the direct conversion of abundant natural gas into liquid aromatics. Rapid coking deactivation hinders the practical implementation of this technology. Herein, we show that catalyst productivity can be improved by nearly an order of magnitude by raising the reaction pressure to 15 bar. The beneficial effect of pressure was found for different Mo/ZSM‐5 catalysts and a wide range of reaction temperatures and space velocities. High‐pressure operando X‐ray absorption spectroscopy demonstrated that the structure of the active Mo‐phase was not affected by operation at elevated pressure. Isotope labeling experiments, supported by mass‐spectrometry and ¹³C nuclear magnetic resonance spectroscopy, indicated the reversible nature of coke formation. The improved performance can be attributed to faster coke hydrogenation at increased pressure, overall resulting in a lower coke selectivity and better utilization of the zeolite micropore space.     

π‐Interactions between Cyclic Carbocations and Aromatics Cause Zeolite Deactivation in Methanol‐to‐Hydrocarbon Conversion

The understanding of catalyst deactivation represents one of the major challenges for the methanol‐to‐hydrocarbon (MTH) reaction over acidic zeolites. Here we report the critical role of intermolecular π‐interactions in catalyst deactivation in the MTH reaction on zeolites H‐SSZ‐13 and H‐ZSM‐5. π‐interaction‐induced spatial proximities between cyclopentenyl cations and aromatics in the confined channels and/or cages of zeolites are revealed by two‐dimensional solid‐state NMR spectroscopy. The formation of naphtalene as a precursor to coke species is favored due to the reaction of aromatics with the nearby cyclopentenyl cations and correlates with both acid density and zeolite topology.     

Bridging the Gap between the Direct and Hydrocarbon Pool Mechanisms of the Methanol‐to‐Hydrocarbons Process

After a prolonged effort over many years, the route for the formation of a direct carbon?carbon (C?C) bond during the methanol‐to‐hydrocarbon (MTH) process has very recently been unveiled. However, the relevance of the “direct mechanism”‐derived molecules (that is, methyl acetate) during MTH, and subsequent transformation routes to the conventional hydrocarbon pool (HCP) species, are yet to be established. This important piece of the MTH chemistry puzzle is not only essential from a fundamental perspective, but is also important to maximize catalytic performance. The MTH process was probed over a commercially relevant H‐SAPO‐34 catalyst, using a combination of advanced solid‐state NMR spectroscopy and operando UV/Vis diffuse reflectance spectroscopy coupled to an on‐line mass spectrometer. Spectroscopic evidence is provided for the formation of (olefinic and aromatic) HCP species, which are indeed derived exclusively from the direct C?C bond‐containing acetyl group of methyl acetate. New mechanistic insights have been obtained from the MTH process, including the identification of hydrocarbon‐based co‐catalytic organic reaction centers.     

The porosity, acidity, and reactivity of dealuminated zeolite ZSM-5 at the single particle level: the influence of the zeolite architecture

A combination of atomic force microscopy (AFM), high‐resolution scanning electron microscopy (HR‐SEM), focused‐ion‐beam scanning electron microscopy (FIB‐SEM), X‐ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron‐based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM‐5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol‐to‐olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM‐5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM‐5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5–50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity.     

New Insight into the Hydrocarbon‐Pool Chemistry of the Methanol‐to‐Olefins Conversion over Zeolite H‐ZSM‐5 from GC‐MS,Solid‐State NMR Spectroscopy,and DFT Calculations

Over zeolite H‐ZSM‐5, the aromatics‐based hydrocarbon‐pool mechanism of methanol‐to‐olefins (MTO) reaction was studied by GC‐MS, solid‐state NMR spectroscopy, and theoretical calculations. Isotopic‐labeling experimental results demonstrated that polymethylbenzenes (MBs) are intimately correlated with the formation of olefin products in the initial stage. More importantly, three types of cyclopentenyl cations (1,3‐dimethylcyclopentenyl, 1,2,3‐trimethylcyclopentenyl, and 1,3,4‐trimethylcyclopentenyl cations) and a pentamethylbenzenium ion were for the first time identified by solid‐state NMR spectroscopy and DFT calculations under both co‐feeding ([¹³C₆]benzene and methanol) conditions and typical MTO working (feeding [¹³C]methanol alone) conditions. The comparable reactivity of the MBs (from xylene to tetramethylbenzene) and the carbocations (trimethylcyclopentenyl and pentamethylbenzium ions) in the MTO reaction was revealed by ¹³C‐labeling experiments, evidencing that they work together through a paring mechanism to produce propene. The paring route in a full aromatics‐based catalytic cycle was also supported by theoretical DFT calculations.     

Hierarchical ZSM‐5 Zeolites in Shape‐Selective Xylene Isomerization: Role of Mesoporosity and Acid Site Speciation

The isomerization of o‐xylene, a prototypical example of shape‐selective catalysis by zeolites, was investigated on hierarchical porous ZSM‐5. Extensive intracrystalline mesoporosity in ZSM‐5 was introduced by controlled silicon leaching with NaOH. In addition to the development of secondary porosity, the treatment also induced substantial aluminum redistribution, increasing the density of Lewis acid sites located at the external surface of the crystals. However, the strength of the remaining Brønsted sites was not changed. The mesoporous zeolite displayed a higher o‐xylene conversion than its parent, owing to the reduced diffusion limitations. However, the selectivity to p‐xylene decreased, and fast deactivation due to coking occurred. This is mainly due to the deleterious effect of acidity at the substantially increased external surface and near the pore mouths. A consecutive mild HCl washing of the hierarchical zeolite proved effective to increase the p‐xylene selectivity and reduce the deactivation rate. The HCl‐washed hierarchical ZSM‐5 displayed an approximately twofold increase in p‐xylene yield compared to the purely microporous zeolite. The reaction was followed by operando infrared spectroscopy to simultaneously monitor the catalytic performance and the buildup of carbonaceous deposits on the surface. Our results show that the interplay between activity, selectivity, and stability in modified zeolites can be optimized by relatively simple post‐synthesis treatments, such as base leaching (introduction of mesoporosity) and acid washing (surface acidity modification).     

Initial Carbon–Carbon Bond Formation during the Early Stages of the Methanol‐to‐Olefin Process Proven by Zeolite‐Trapped Acetate and Methyl Acetate

Methanol‐to‐olefin (MTO) catalysis is a very active field of research because there is a wide variety of sometimes conflicting mechanistic proposals. An example is the ongoing discussion on the initial C?C bond formation from methanol during the induction period of the MTO process. By employing a combination of solid‐state NMR spectroscopy with UV/Vis diffuse reflectance spectroscopy and mass spectrometry on an active H‐SAPO‐34 catalyst, we provide spectroscopic evidence for the formation of surface acetate and methyl acetate, as well as dimethoxymethane during the MTO process. As a consequence, new insights in the formation of the first C?C bond are provided, suggesting a direct mechanism may be operative, at least in the early stages of the MTO reaction.     

Staining of fluid-catalytic-cracking catalysts: localising Brønsted acidity within a single catalyst particle

A time‐resolved in situ micro‐spectroscopic approach has been used to investigate the Brønsted acidic properties of fluid‐catalytic‐cracking (FCC) catalysts at the single particle level by applying the acid‐catalysed styrene oligomerisation probe reaction. The reactivity of individual FCC components (zeolite, clay, alumina and silica) was monitored by UV/Vis micro‐spectroscopy and showed that only clay and zeolites (Y and ZSM‐5) contain Brønsted acid sites that are strong enough to catalyse the conversion of 4‐fluorostyrene into carbocationic species. By applying the same approach to complete FCC catalyst particles, it has been found that the fingerprint of the zeolitic UV/Vis spectra is clearly recognisable. This almost exclusive zeolitic activity is confirmed by the fact that hardly any reactivity is observed for FCC particles that contain no zeolite. Confocal fluorescence microscopy images of FCC catalyst particles reveal inhomogeneously distributed micron‐sized zeolite domains with a highly fluorescent signal upon reaction. By examining laboratory deactivated FCC catalyst particles in a statistical approach, a clear trend of decreasing fluorescence intensity, and thus Brønsted acidity, of the zeolite domains is observed with increasing severity of the deactivation method. By comparing the average fluorescence intensities obtained with two styrenes that differ in reactivity, it has been found that the Brønsted acid site strength within FCC catalyst particles containing ZSM‐5 is more uniform than within those containing zeolite Y, as confirmed with temperature‐programmed desorption of ammonia.     

Microsecond X‐ray Absorption Spectroscopy Identification of CoI Intermediates in Cobaloxime‐Catalyzed Hydrogen Evolution

Rational development of efficient photocatalytic systems for hydrogen production requires understanding the catalytic mechanism and detailed information about the structure of intermediates in the catalytic cycle. We demonstrate how time‐resolved X‐ray absorption spectroscopy in the microsecond time range can be used to identify such intermediates and to determine their local geometric structure. This method was used to obtain the solution structure of the Co^I intermediate of cobaloxime, which is a non‐noble metal catalyst for solar hydrogen production from water. Distances between cobalt and the nearest ligands including two solvent molecules and displacement of the cobalt atom out of plane formed by the planar ligands have been determined. Combining in situ X‐ray absorption and UV/Vis data, we demonstrate how slight modification of the catalyst structure can lead to the formation of a catalytically inactive Co^I state under similar conditions. Possible deactivation mechanisms are discussed.     

Direct Detection of Supramolecular Reaction Centers in the Methanol‐to‐Olefins Conversion over Zeolite H‐ZSM‐5 by 13C–27Al Solid‐State NMR Spectroscopy

Hydrocarbon‐pool chemistry is important in methanol to olefins (MTO) conversion on acidic zeolite catalysts. The hydrocarbon‐pool (HP) species, such as methylbenzenes and cyclic carbocations, confined in zeolite channels during the reaction are essential in determining the reaction pathway. Herein, we experimentally demonstrate the formation of supramolecular reaction centers composed of organic hydrocarbon species and the inorganic zeolite framework in H‐ZSM‐5 zeolite by advanced ¹³C–²⁷Al double‐resonance solid‐state NMR spectroscopy. Methylbenzenes and cyclic carbocations located near Brønsted acid/base sites form the supramolecular reaction centers in the zeolite channel. The internuclear spatial interaction/proximity between the ¹³C nuclei (associated with HP species) and the ²⁷Al nuclei (associated with Brønsted acid/base sites) determines the reactivity of the HP species. The closer the HP species are to the zeolite framework Al, the higher their reactivity in the MTO reaction.     

Kinetics of MTO-catalyzed olefin epoxidation in ambient temperature ionic liquids: UV/Vis and 2H NMR study

The kinetics of oxygen-atom transfer from the peroxo complexes of methyltrioxorhenium (MTO) to alkenes in ionic liquids have been investigated. Noncatalytic conversions of alkenes to epoxide were monitored by UV/Vis at 360 nm, where the monoperoxorhenium (mpRe) and diperoxorhenium (dpRe) complexes absorb. Water- and peroxide-free dpRe was prepared in situ by the reaction of MTO and urea hydrogen peroxide (UHP) in dry THF. The observed biexponential time profiles in conjunction with kinetic modeling allow the assignment of the fast step to the reaction of olefin with dpRe (k4) and the slow step to the analogous reaction with mpRe (k3). In most of the tudied ionic liquids, k4 approximately 5 x k3. 2H NMR experiments conducted with [D3]dpRe under non-steady-state conditions confirm the speciation of the catalytic system in ionic liquids and assert the validity of the UV/Vis kinetics. Deuteriated alkenes were used to study the catalytic epoxidation and dihydroxylation of alkenes by 2H NMR spectroscopy. The values of k4 for alpha-methylstyrene in several ionic liquids exceed what is observed in acetonitrile by an order of magnitude. While the rate of olefin epoxidation is unaffected by the nature of the ionic liquid cation, a discernible kinetic effect is observed with coordinating anions such as nitrate.     

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.     

Hexane Cracking over Steamed Phosphated Zeolite H‐ZSM‐5: Promotional Effect on Catalyst Performance and Stability

The nature behind the promotional effect of phosphorus on the catalytic performance and hydrothermal stability of zeolite H‐ZSM‐5 has been studied using a combination of ²⁷Al and ³¹P MAS NMR spectroscopy, soft X‐ray absorption tomography and n‐hexane catalytic cracking, complemented with NH₃ temperature‐programmed desorption and N₂ physisorption. Phosphated H‐ZSM‐5 retains more acid sites and catalytic cracking activity after steam treatment than its non‐phosphated counterpart, while the selectivity towards propylene is improved. It was established that the stabilization effect is twofold. First, the local framework silico‐aluminophosphate (SAPO) interfaces, which form after phosphatation, are not affected by steam and hold aluminum atoms fixed in the zeolite lattice, preserving the pore structure of zeolite H‐ZSM‐5. Second, the four‐coordinate framework aluminum can be forced into a reversible sixfold coordination by phosphate. These species remain stationary in the framework under hydrothermal conditions as well. Removal of physically coordinated phosphate after steam‐treatment leads to an increase in the number of strong acid sites and increased catalytic activity. We propose that the improved selectivity towards propylene during catalytic cracking can be attributed to local SAPO interfaces located at channel intersections, where they act as impediments in the formation of bulky carbenium ions and therefore suppress the bimolecular cracking mechanism.