Differences in the methanol-to-olefins reaction catalyzed by SAPO-34 with dimethyl ether as reactant

Recent advances in bifunctional synthesis gas conversion to chemicals and fuels with a comparison to monofunctional processes

In order to meet the climate goals of the Paris Agreement and limit the potentially catastrophic consequences of climate change, we must move away from the use of fossil feedstocks for the production of chemicals and fuels. The conversion of synthesis gas (a mixture of hydrogen, carbon monoxide and/or carbon dioxide) can contribute to this. Several reactions allow to convert synthesis gas to oxygenates (such as methanol), olefins or waxes. In a consecutive step, these products can be further converted into chemicals, such as dimethyl ether, short olefins, or aromatics. Alternatively, fuels like gasoline, diesel, or kerosene can be produced. These two different steps can be combined using bifunctional catalysis for direct conversion of synthesis gas to chemicals and fuels. The synergistic effects of combining two different catalysts are discussed in terms of activity and selectivity and compared to processes based on consecutive reaction with single conversion steps. We found that bifunctional catalysis can be a strong tool for the highly selective production of dimethyl ether and gasoline with high octane numbers. In terms of selectivity bifunctional catalysis for short olefins or aromatics struggles to compete with processes consisting of single catalytic conversion steps.

Modulation of Al Distribution in High-Silica ZSM-5 Zeolites for Enhancing Catalytic Performance

The spatial distribution of framework Al (AlF) has been one of the important factors that affect the catalytic properties of zeolites in diverse chemical reactions; however, the synthesis of high-silica zeolites with special AlF distribution remains a challenge. In this study, we successfully synthesized high-silica ZSM-5 zeolites with a unique AlF distribution by employing pentaerythritol (PET) as an additive in the presence of a few tetrapropylammonium hydroxide (TPAOH). The results demonstrated that the introduction of PET led to a higher proportion of Al atoms located at the sinusoidal and/or straight channels. It was observed that the addition of PET prevented the interaction between TPA+ and tetrahedral [AlO4]- during the crystallization process, resulting in enhanced availability of TPA species in the form of ion-paired TPA+. This effect leads to AlF atoms dominantly distributed away from the intersection and located in narrow channels, where acidic sites more effectively inhibit hydrogen transfer and coke formation. In the reaction of dimethyl ether (DME) to olefins, the catalyst with a unique Al distribution exhibited a significant prolonged catalytic lifetime, surpassing traditional TPA-ZSM-5 by more than 2-fold and maintaining DME conversion above 90% for a maximum of 148 h. The results of multiple pulse experiments also showed that these PET-assisted ZSM-5 zeolites significantly enhanced the selectivity of propene and butene. This approach provides an effective strategy to regulate AlF distribution in high-silica ZSM-5 catalysts with the assistance of neutral alcohol. It holds great potential for application in the synthesis of other high-silica zeolites, thereby enriching the diversity of zeolite catalysis.

The Oxygenate-Mediated Conversion of COx to Hydrocarbons─On the Role of Zeolites in Tandem Catalysis

Decentralized chemical plants close to circular carbon sources will play an important role in shaping the postfossil society. This scenario calls for carbon technologies which valorize CO2 and CO with renewable H2 and utilize process intensification approaches. The single-reactor tandem reaction approach to convert COx to hydrocarbons via oxygenate intermediates offers clear benefits in terms of improved thermodynamics and energy efficiency. Simultaneously, challenges and complexity in terms of catalyst material and mechanism, reactor, and process gaps have to be addressed. While the separate processes, namely methanol synthesis and methanol to hydrocarbons, are commercialized and extensively discussed, this review focuses on the zeolite/zeotype function in the oxygenate-mediated conversion of COx to hydrocarbons. Use of shape-selective zeolite/zeotype catalysts enables the selective production of fuel components as well as key intermediates for the chemical industry, such as BTX, gasoline, light olefins, and C3+ alkanes. In contrast to the separate processes which use methanol as a platform, this review examines the potential of methanol, dimethyl ether, and ketene as possible oxygenate intermediates in separate chapters. We explore the connection between literature on the individual reactions for converting oxygenates and the tandem reaction, so as to identify transferable knowledge from the individual processes which could drive progress in the intensification of the tandem process. This encompasses a multiscale approach, from molecule (mechanism, oxygenate molecule), to catalyst, to reactor configuration, and finally to process level. Finally, we present our perspectives on related emerging technologies, outstanding challenges, and potential directions for future research.

Multi-scale 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.

Mechanistic differences between methanol and dimethyl ether in zeolite-catalyzed hydrocarbon synthesis

Methanol conversion to hydrocarbons has emerged as a key reaction for synthetic energy carriers and light alkenes. The autocatalytic nature and complex reaction network make a mechanistic understanding very challenging and widely debated. Water is not only part of the overall conversion, it is also frequently used as diluent, influencing, in turn, activity, selectivity, and stability of the catalysts. Water directly and indirectly influences the processes that initiate the C–C formation via adjusting the chemical potential of methanol and dimethyl ether, with the latter being more efficient to generate highly reactive C₁ species via hydride transfer. The insight shows paths to optimize the stability of catalysts and to tailor the product distribution for H-ZSM-5–based catalysts.

Water influences critically the kinetics of the autocatalytic conversion of methanol to hydrocarbons in acid zeolites. At very low conversions but otherwise typical reaction conditions, the initiation of the reaction is delayed in presence of H₂O. In absence of hydrocarbons, the main reactions are the methanol and dimethyl ether (DME) interconversion and the formation of a C₁ reactive mixture—which in turn initiates the formation of first hydrocarbons in the zeolite pores. We conclude that the dominant reactions for the formation of a reactive C₁ pool at this stage involve hydrogen transfer from both MeOH and DME to surface methoxy groups, leading to methane and formaldehyde in a 1:1 stoichiometry. While formaldehyde reacts further to other C₁ intermediates and initiates the formation of first C–C bonds, CH₄ is not reacting. The hydride transfer to methoxy groups is the rate-determining step in the initiation of the conversion of methanol and DME to hydrocarbons. Thus, CH₄ formation rates at very low conversions, i.e., in the initiation stage before autocatalysis starts, are used to gauge the formation rates of first hydrocarbons. Kinetics, in good agreement with theoretical calculations, show surprisingly that hydrogen transfer from DME to methoxy species is 10 times faster than hydrogen transfer from methanol. This difference in reactivity causes the observed faster formation of hydrocarbons in dry feeds, when the concentration of methanol is lower than in presence of water. Importantly, the kinetic analysis of CH₄ formation rates provides a unique quantitative parameter to characterize the activity of catalysts in the methanol-to-hydrocarbon process.

n-Butene Synthesis in the Dimethyl Ether-to-Olefin Reaction over Zeolites

Zeolite catalysts that could allow the efficient synthesis of n-butene, such as 1-butene, trans-2-butene, and cis-2-butene, in the dimethyl ether (DME)-to-olefin (DTO) reaction were investigated using a fixed-bed flow reactor. The zeolites were characterized by N2 adsorption and desorption, X-ray diffraction (XRD), thermogravimetry (TG), and NH3 temperature-programmed desorption (NH3-TPD). A screening of ten available zeolites indicated that the ferrierite zeolite with NH4+ as the cation showed the highest n-butene yield. The effect of the temperature of calcination as a pretreatment method on the catalytic performance was studied using three zeolites with suitable topologies. The calcination temperature significantly affected DME conversion and n-butene yield. The ferrierite zeolite showed the highest n-butene yield at a calcination temperature of 773 K. Multiple regression analysis was performed to determine the correlation between the six values obtained using N2 adsorption/desorption and NH3-TPD analyses, and the n-butene yield. The contribution rate of the strong acid site alone as an explanatory variable was 69.9%; however, the addition of micropore volume was statistically appropriate, leading to an increase in the contribution rate to 76.1%. Insights into the mechanism of n-butene synthesis in the DTO reaction were obtained using these parameters.

Lower olefins from methane: recent advances

Modern methods for methane conversion to lower olefins having from 2 to 4 carbon atoms per molecule are generalized. Multistage processing of methane into ethylene and propylene via syngas or methyl chloride and methods for direct conversion of CH4 to ethylene are described. Direct conversion of syngas to olefins as well as indirect routes of the process via methanol or dimethyl ether are considered. Particular attention is paid to innovative methods of olefin synthesis. Recent achievements in the design of catalysts and development of new techniques for efficient implementation of oxidative coupling of methane and methanol conversion to olefins are analyzed and systematized. Advances in commercializing these processes are pointed out. Novel catalysts for Fischer – Tropsch synthesis of lower olefins from syngas and for innovative technique using oxide – zeolite hybrid catalytic systems are described. The promise of a new route to lower olefins by methane conversion via dimethyl ether is shown. Prospects for the synthesis of lower olefins via methyl chloride and using non-oxidative coupling of methane are discussed. The most efficient processes used for processing of methane to lower olefins are compared on the basis of degree of conversion of carbonaceous feed, possibility to integrate with available full-scale production, number of reaction stages and thermal load distribution.

The bibliography includes 346 references.

Propene Adsorption-Chemisorption Behaviors on H-SAPO-34 Zeolite Catalysts at Different Temperatures

Propene is an important synthetic industrial product predominantly formed by a methanol-to-olefins (MTO) catalytic process. Propene is known to form oligomers on zeolite catalysts, and paramters to separate it from mixtures and its diffusion properties are difficult to measure. Herein, we explored the adsorption–chemisorption behavior of propene by choosing SAPO-34 zeolites with three different degrees of acidity at various adsorption temperatures in an ultra-high-vacuum adsorption system. H-SAPO-34 zeolites were prepared by a hydrothermal method, and their structural, morphological, and acidic properties were investigated by XRD, SEM, EDX, and temperature-programmed desorption of ammonia (NH3-TPD) analysis techniques. The XRD analysis revealed the highly crystalline structure which posses cubic morphology as confirmed by SEM images. The analysis of adsorption of propene on SAPO-34 revealed that a chemical reaction (chemisorption) was observed between zeolite and propene at room temperature (RT) when the concentration of acidic sites was high (0.158 mmol/g). The reaction was negligible when the concentration of the acidic sites was low (0.1 mmol/g) at RT. However, the propene showed no reactivity with the highly acidic SAPO-34 at low temperatures, i.e., −56 °C (using octane + dry ice), −20 °C (using NaCl + ice), and 0 °C (using ice + water). In general, low-temperature conditions were found to be helpful in inhibiting the chemisorption of propene on the highly acidic H-SAPO-34 catalysts, which can facilitate propene separation and allow for reliable monitoring of kinetic parameters.

Deactivation of Zeolites and Zeotypes in Methanol-to-Hydrocarbons Catalysis: Mechanisms and Circumvention

Solid catalysts deployed in industrial processes often undergo deactivation, requiring frequent replacement or regeneration to recover the loss in activity. Regeneration occurs under conditions distinct from, and typically more harsh than, the catalysis, placing strict requirements on physicochemical material properties that divert catalyst optimization toward addressing regenerability over high activity and selectivity. Deactivation arises from mechanical, structural, or chemical modifications to active sites, promoters, and their surrounding matrices, and the prevailing mechanism for deactivation varies with the reaction, the catalyst, and the reaction conditions. Methanol-to-hydrocarbons processes utilize zeolites and zeotypes-crystalline, microporous oxides widely deployed as catalysts in the refining and petrochemical industries-as solid acid catalysts. Deposition and growth of highly unsaturated carbonaceous residues within the micropores congest molecular transport and block active sites, resulting in deactivation. In this Account, we describe studies probing the underlying mechanisms of deactivation in methanol-to-hydrocarbons catalysis and discuss examples of leveraging the acquired mechanistic insights to mitigate deactivation and prolong catalyst lifetime. These fundamental principles governing carbon deposition within zeolites and zeotypes provide opportunity to broaden versatility of processes for C₁ valorization and to relax constraints imposed by hydrothermal catalyst stability considerations to achieve more active and more selective catalysis. Methanol-to-hydrocarbons catalysis occurs via a chain carrier mechanism. A zeolite/zeotype cavity hosts an unsaturated hydrocarbon guest to together constitute the supramolecular chain carrier that engages in a complex network of reactions for chain carrier propagation. Productive propagation reactions include olefin methylation, aromatic methylation, and aromatic dealkylation. Methanol undergoes unproductive dehydrogenation to formaldehyde via methanol disproportionation and olefin transfer hydrogenation. Subsequent alkylation reactions between formaldehyde and active olefinic/aromatic cocatalysts instigate cascades for dehydrocyclization, resulting in the formation of inactive polycyclic aromatic hydrocarbons and termination of the chain carrier. Addition of a distinct catalytic function that selectively decomposes formaldehyde mitigates chain carrier termination without disrupting the high selectivity to ethylene and propylene in methanol-to-hydrocarbons catalysis on small-pore zeolites and zeotypes. The efficacy of this bifunctional strategy to prolong catalyst lifetime increases with increasing proximity between the active sites for formaldehyde decomposition and the H⁺ sites of the zeolite/zeotype. Coprocessing sacrifical hydrogen donors mitigates chain carrier termination by intercepting, via saturation, intermediates along dehydrocyclization cascades. This strategy increases in efficacy with increasing concentration of the hydrogen donor and provides opportunity to realize steady-state methanol-to-hydrocarbons catalysis on small-pore zeolites and zeotypes.

Enhanced production of aromatics from propane with a temperature-shifting two-stage fluidized bed reactor

A temperature-shifting two-stage fluidized bed reactor technology was used to convert propane and its intermediate products into aromatics. The first stage served for the aromatization of propane with a Ga/ZSM-5 catalyst at 570 °C. The second stage served for the alkylation of the intermediates of olefins at 300 °C. The increased yield of aromatics was attributed to the effective transformation of C₂–C₃ olefins as well as due to the suppression of the hydrogen transfer effect of the olefins.

High-yield production of aromatics from propane with a temperature shifting, two-stage fluidized bed reactor technology.

A phase-transfer crystallization pathway to synthesize ultrasmall silicoaluminophosphate for enhanced catalytic conversion of dimethylether-to-olefin

This work reports a new phase-transfer crystallization pathway to generate ultrasmall silicoaluminophosphate SAPO-34 for enhanced catalytic dimethylether-to-olefin conversion.

Mechanistic role of water in HSSZ-13 catalyzed methanol-to-olefins conversion

Attenuation of chain initiation and termination rates and higher turnover capacities in methanol-to-olefins (MTO) conversion observed with water co-feeds are plausibly due to formaldehyde hydrolysis.

Mesoporogen-Free Synthesis of Hierarchical SAPO-34 with Low Template Consumption and Excellent Methanol-to-Olefin Conversion

Significant interest has emerged in the development of nanometer-sized and hierarchical silicoaluminophosphate zeolites (SAPO-34) because of their enhanced accessibility and improved catalytic activity for methanol-to-olefin (MTO) conversion. A series of nanometer-sized SAPO-34 catalysts with tunable hierarchical structures was synthesized in a Al₂ O₃ /H₃ PO₄ /SiO₂ /triethylamine(TEA)/H₂ O system by using a mesoporogen-free nanoseed-assisted method. The nanometer-sized hierarchical S_H -3.0 catalyst (TEA/Al₂ O₃ =3.0) possessed the highest crystallinity, highest abundance of intracrystalline meso-/macropores, and the most suitable acidity among all obtained catalysts, showing the highest ethylene and propylene selectivity of 85.4 %. This is the highest reported selectivity for MTO reactions under similar conditions. Detailed analysis of the coke produced during the reaction revealed that the small-sized methyl-substituted benzene and bulky methyl-substituted pyrene were mainly located inside the crystals instead of on the surface of the crystals, which provided further insight into understanding the deactivation of the SAPO-34 catalyst during MTO reaction. Significantly, the simple and cost-effective synthetic process and superb catalytic performance of the nanometer-sized hierarchical SAPO-34 is promising for their practical large-scale application for MTO conversion.

Effect of Nano-Sized Cavities in SAPO-34 Zeolite on Thermodynamics of Adsorbed Gas Mixtures

Adsorption of dimethyl ether and ethene in SAPO-34 zeolite with the calorimetric (adsorption heat versus coverage) curve measured together with the adsorption isotherm showed two phases of adsorption: first, Type 1 adsorption on acid sites, and second, Type 2 adsorption elsewhere in the cages by physisorption that continued with increasing pressure. Binary gas mixture experiments showed that only the ideal adsorbed solution theory (IAST) gave correct surface concentrations, while the multicomponent Langmuir isotherm for competitive adsorption was incorrect even though the acid site concentration was the same for the adsorbates. This is because the adsorption occurred in two adsorption phases while the Langmuir isotherm model is based on a single adsorption phase.

Intensified Biobutanol Recovery by using Zeolites with Complementary Selectivity

A vapor-phase adsorptive recovery process is proposed as an alternative way to isolate biobutanol from acetone-butanol-ethanol (ABE) fermentation media, offering several advantages compared to liquid phase separation. The effect of water, which is still present in large quantities in the vapor phase, on the adsorption of the organics could be minimized by using hydrophobic zeolites. Shape-selective all-silica zeolites CHA and LTA were prepared and evaluated with single-component isotherms and breakthrough experiments. These zeolites show opposite selectivities; adsorption of ethanol is favorable on all-silica CHA, whereas the LTA topology has a clear preference for butanol. The molecular sieving properties of both zeolites allow easy elimination of acetone from the mixture. The molecular interaction mechanisms are studied by density functional theory (DFT) simulations. The effects of mixture composition, humidity and total pressure of the vapor stream on the selectivity and separation behavior are investigated. Desorption profiles are studied to maximize butanol purity and recovery. The combination of LTA with CHA-type zeolites (Si-CHA or SAPO-34) in sequential adsorption columns with alternating adsorption and desorption steps allows butanol to be recovered in unpreceded purity and yield. A butanol purity of 99.7 mol % could be obtained at nearly complete butanol recovery, demonstrating the effectiveness of this technique for biobutanol separation processes.

New insights into catalyst deactivation and product distribution of zeolites in the methanol-to-hydrocarbons (MTH) reaction with methanol and dimethyl ether feeds

The ability of a zeolitic catalyst to dehydrate methanol to dimethyl ether affects catalyst deactivation and product distribution during the methanol-to-hydrocarbons (MTH) reaction.

High yield production of C2–C3 olefins and para-xylene from methanol using a SiO2-coated FeOx/ZSM-5 catalyst

SiO₂-coated FeO_x/ZSM-5 catalyst exhibited long life time to produce C₂–C₃ olefins and para-xylene in high yield.

Note: Molecular diffusivity in a small pore zeolite measured by a variable pressure (piezometric) uptake method

The use of numerical analysis to solve the diffusion equation in the uptake method allowed the measurement of molecular diffusivity in a zeolite with a variable pressure around it. The diffusivity was obtained from the data in the measurement of the adsorption isotherm, which means that the diffusivity measurement now needs neither a special instrument nor procedure. The diffusivities of all the gases are readily available from the measurement of their adsorption isotherms and these data include how the diffusivity changes versus adsorbed concentration. The modeling introduced can also be used for a zeolite with a surface barrier.

Conversion of dimethyl ether to toluene under an O2 stream over W/HZSM-5 catalysts

The direct conversion of dimethyl ether (DME) to toluene without other aromatics is realized over W/HZSM-5 catalysts with high W contents.

A microscopic model of the Tian-Calvet microcalorimeter, cell design for a faster response, and measurement by a continuous procedure

The transient heat conduction equation was used as the microscopic model of the Tian-Calvet microcalorimeter. It was verified by comparing simulated and experimental calorimetric curves and used to guide sample cell design for a faster response time, for which it gave the guidelines to minimize the heat flow distance and use a heat flux that is uniform and onto the whole face of the thermopile sensor. The resulting sample cell was disc-shaped with the sample powder placed in it as a thin 0.2 mm layer on a stainless steel base with a wall thickness of 0.5 mm that covered the whole face of the thermopile on which it was placed. The rise time of the heat response curve to a step change in sample temperature, which is the response time for measuring the differential heat released, was 45 s. The response curve from a gas dose returned to the baseline within 400 s, which is the time needed to measure the integrated heat in a pulsed dosage. The accuracy of the heats measured by the calorimeter was verified by comparison with data in the literature on the adsorption of ethanol and ammonia on HZSM-5 and adsorption of methanol and ammonia on SAPO-34. The differential heat of methanol adsorption on SAPO-34 at 333 K and ammonia adsorption on HZSM-5 at 423 K were measured by both the conventional discontinuous procedure and a new continuous procedure. In the continuous procedure, gas was continuously dosed at a very slow flow rate that was kept slow enough for the gas and adsorbate to reach quasi-equilibrium. The continuous procedure has the advantages of high resolution results and a simpler experimental procedure, and a calorimetric curve could be measured within 3 h.