Methanol-to-hydrocarbons Process Research Articles

Recent Progress on the Involvement of Formaldehyde in the Methanol-to-Hydrocarbons Reaction.

The conversion of methanol-to-hydrocarbons (MTH) over zeolite catalysts has been the subject of intense research since its discovery. Great effort has been devoted to the investigation of four key topics: the initiation of C-C bonds, the establishment of hydrocarbon pool (HCP), the adjustment of product selectivity, and the deactivation process of catalysts. Despite 50 years of study, some mechanisms remain controversial. However, an intermediate species, formaldehyde (HCHO), has recently garnered considerable attention for its influence on the entire MTH process. The discovery of HCHO and its significant role in the MTH process has been facilitated by the application of in situ analytical techniques, such as synchrotron radiation photoionization mass spectrometry (SR-PIMS) and photoelectron photoion coincidence spectroscopy (PEPICO). It is now revealed that HCHO is involved in the initiation, propagation, and termination process of MTH reaction. Such mechanistic understanding of HCHO's involvement has provided valuable insights for optimizing the MTH process.

PEPICO analysis of catalytic reactor effluents towards quantitative isomer discrimination: DME conversion over a ZSM-5 zeolite.

The methanol-to-hydrocarbons (MTH) process involves the conversion of methanol, a C1 feedstock that can be produced from green sources, into hydrocarbons using shape-selective microporous acidic catalysts - zeolite and zeotypes. This reaction yields a complex mixture of species, some of which are highly reactive and/or present in several isomeric forms, posing significant challenges for effluent analysis. Conventional gas-phase chromatography (GC) is typically employed for the analysis of reaction products in laboratory flow reactors. However, GC is not suitable for the detection of highly reactive intermediates such as ketene or formaldehyde and is not suitable for kinetic studies under well defined low pressure conditions. Photoelectron-photoion coincidence (PEPICO) spectroscopy has emerged as a powerful analytical tool for unraveling complex compositions of catalytic effluents, but its availability is limited to a handful of facilities worldwide. Herein, PEPICO analysis of catalytic reactor effluents has been implemented at the FinEstBeAMS beamline of MAX IV Laboratory. The conversion of dimethyl ether (DME) on a zeolite catalyst (ZSM-5-MFI27) is used as a prototypical model reaction producing a wide distribution of hydrocarbon products. Since in zeolites methanol is quickly equilibrated with DME, this reaction can be used to probe vast sub-networks of the full MTH process, while eliminating or at least slowing down methanol-induced secondary reactions and catalyst deactivation. Quantitative discrimination of xylene isomers in the effluent stream is achieved by deconvoluting the coincidence photoelectron spectra.

Liquid metals for boosting stability of zeolite catalysts in the conversion of methanol to hydrocarbons

Methanol-to-hydrocarbons (MTH) process has been considered one of the most practical approaches for producing value-added products from methanol. However, the commonly used zeolite catalysts suffer from rapid deactivation due to coke deposition and require regular regeneration treatments. We demonstrate that low-melting-point metals, such as Ga, can effectively promote more stable methanol conversion in the MTH process by slowing coke deposition and facilitating the desorption of carbonaceous species from the zeolite. The ZSM-5 zeolite physically mixed with liquid gallium exhibited an enhanced lifetime in the MTH reaction, which increased by a factor of up to ~14 as compared to the parent ZSM-5. These results suggest an alternative route to the design and preparation of deactivation-resistant zeolite catalysts.

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

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

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

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

Revealing Main Reaction Paths to Olefins and Aromatics in Methanol-to-Hydrocarbons over H-ZSM-5 by Isotope Labeling

The nature of hydrocarbon pool (HCP) intermediates in the methanol-to-hydrocarbons (MTH) process has been thoroughly investigated, especially for BEA- and CHA-type zeolite catalysts like H-β and H-SAPO-34. Herein, we further reveal the dynamic mechanistic details of the MTH process over the H-ZSM-5 catalyst at 400 °C, based on the dual-cycle mechanism and HCP in this medium-pore zeolite. Application of switching sequences of 13C-labeled and unlabeled methanol pulses over a model H-ZSM-5 catalyst combined with on-line MS analysis and a recently reported technique called “fast scanning-pulse GC analysis” provides a direct and quantitative insight into the MTH reactions under quasi-steady-state conditions. The transient product responses showed the almost instant formation of hydrocarbons upon a small pulse of methanol, followed by secondary formation of light aromatics via HCP decomposition and olefin alkylation–dealkylation, especially in a long catalyst bed when methanol is quickly consumed in the initial reaction zone in the catalyst bed. The isotopic analysis of typical aliphatic C3+ product responses after switching 13C-methanol pulses to the unlabeled methanol pulses showed a fast isotope scrambling in the formation of C3+ species. MS analysis of the light aromatics indicates a complete consecutive but slower isotope incorporation process of 12C into 13C-aromatics. Results provide direct experimental confirmation of the kinetically preferred olefin-based cycle over the aromatic-based cycle. The sequential isotopic incorporation strongly suggests that the paring reaction pathway through aromatic ring contraction and re-expansion steps is operative. In the appearance of aromatics upon pulsing methanol over larger catalyst beds, four processes are directly discerned, involving the displacement of adsorbed species by formed water, isotope incorporation yielding directly labeled and unlabeled products through the paring mechanism and direct aromatization, and HCP conversion through secondary reactions.

Selective Production of Gasoline Fuel Blendstock from Methanol using a Metal-doped HZSM-5 Zeolite Catalyst

The increasing demand for substitutes for petroleum has become a global concern due to the looming petroleum crisis and the need to reduce carbon dioxide emissions. This study seeks a viable approach for converting methanol into petrochemicals and fuel-range hydrocarbons. A ZSM-5 zeolite catalyst was synthesised and modified with 0.5 wt% transition metals (Co and Ni) to improve selectivity towards the desired liquid product (gasoline) in the methanol-to-hydrocarbon (MTH) conversion. The synthesised catalyst was characterised using various techniques. The catalysts were further evaluated for methanol conversion into fuels and petrochemicals (BTX) under varying weight hourly space velocity (WHSV) (7 and 12 h-1) at 350 oC. Results showed that the synthesised catalyst exhibited characteristic features of a typical MFI framework of a zeolite catalyst. The catalyst evaluation results revealed that changes in operating conditions affected product distribution. A WHSV of 12 h-¹ favoured the production of gasoline range hydrocarbons (C5-C12) with a yield of over 80 % for all catalysts. In addition, C12+ hydrocarbons were produced with a selectivity of 12.6 %, 0.7 % and 7.5 % for Ni-doped, Co-doped and HZSM-5 catalysts, respectively. In particular, the Co-doped catalyst showed a 7.1 % higher BTX yield under these specific operating conditions (12 h-1 and 350 oC). Compared to 7 h-1, increasing the WHSV to 12 h-1 favoured the production of liquid hydrocarbons. Incorporating a small amount of transition metal into the parent catalyst improved the selectivity of the target products and overall liquid hydrocarbon yield. The results concluded that the synthesised catalyst is promising for the MTH process, and catalytic performance can be enhanced by metal modification and optimising reaction conditions to improve biofuel production.

Direct discerning reaction pathways in methanol-to-hydrocarbons by transient operation – FASPA

Monitoring complex catalytic pathways under industrially-relevant conditions is one of the key challenges in catalysis chemistry and technology. Herewith we describe a direct technique called ‘fast scanning-pulse analysis’ (FASPA) that allows the direct characterization and detailed kinetic analysis of intimately interweaved catalytic pathways. The power and potential of the FASPA approach are demonstrated with an industrially relevant methanol-to-hydrocarbons (MTH) process over H-ZSM-5 zeolite. This reaction proceeds via a hydrocarbon pool (HCP) mechanism producing olefins and aromatics. The HCP is built-up upon exposure to methanol during the induction period, followed by a transition regime to a quasi steady-state MTH operation. This FASPA technique allows (sub-)second resolution of the full temporal products response upon a methanol pulse providing direct and quantitative insights into the MTH reactions. Globally, two consecutive pathways can be discerned: a very fast primary product formation in the presence of methanol in a narrow active MTH reaction zone, followed by a slower formation of light aromatics, which is closely related to the decomposition and release of HCP species and secondary reactions in absence of methanol in the downstream part of the catalyst bed. The time delay between the appearance of inert tracer and primary products represents the time needed to build-up the HCP in the induction period, where methane is observed prior to other products. The primary products (alkanes, olefins, and light aromatics) are nearly instantaneously formed from the pulsed methanol. These results demonstrate the highly dynamic character of the HCP in the MTH process over H-ZSM-5.

Revealing the roles of hydrocarbon pool mechanism in ethanol-to-hydrocarbons reaction

Ethanol-to-hydrocarbons (ETH) process, is investigated in this work due to the requirement of sustainable production route for light olefins such as ethene and propene. Majority of the mechanism insights of ETH process are principally proposed based on the methanol-to-hydrocarbons (MTH) process, which involves a homologous reaction system. The reaction intermediate species including cyclopentenyl cations and aromatics species, which are denoted as “hydrocarbon pool (HCP)” species in MTH process, were discovered and identified in the ETH process by in situ solid-state 13C NMR (ssNMR) experiments, advanced 2D 13C–13C INADEQUATE (Incredible Natural Abundance DoublE QUAntum Transfer Experiment) ssNMR experiment, 12C/13C-C2H5OH isotope switching experiments complemented by gas chromatography-mass spectroscopy (GC–MS). Especially, in the ETH reaction, ethylcyclopentenyl cations and their deprotonated form were first captured and exhibited relatively higher activity in the formation of ethene and propene. Multi-routes with the participation of these active intermediates were proposed and evaluated by density functional theory calculation (DFT), demonstrating that they can play important roles in the formation of olefins. Ethene formation are mainly formed from ethanol dehydration, whereas propene can be produced via multi-routes with the participation of the captured HCP species. Moreover, the detailed reaction routes may be modulated by the temperature. This work provides direct evidences of the critical function of HCP species in the ETH process and reveals the mechanism of olefin formation.

A novel microwave-assisted methanol-to-hydrocarbons process with a structured ZSM-5/SiC foam catalyst: Proof-of-concept and environmental impacts

This study presents a novel combination of microwave irradiation and structured zeolite/SiC foam catalyst to improve the methanol-to-hydrocarbons (MTH) process as well as its environmental sustainability. Accordingly, ZSM-5/SiC foam was studied under microwave irradiation at various reaction temperatures (400–550 °C) and space velocities (8 and 12 h−1) and compared to conventional ZSM-5 pellets. ZSM-5/SiC foam promoted higher selectivity towards C1−C5 hydrocarbons (∼95%) and particularly propylene (∼40% at 400 °C and 8 h−1) compared to ZSM-5 pellets (∼40% and 6–9%, respectively) due to the enhanced mass and heat transfers. The microwave-assisted MTH process with both the structured catalyst and pellets was also assessed on the environmental sustainability via life cycle assessment in comparison with the conventionally-heat process with the structured catalyst. The results showed that the microwave-based process with the structured catalyst could reduce the environmental impacts of propylene production by 22–30% compared to the conventionally-heated process with the same catalyst.

Two-in-One Catalyst Turns Carbon Dioxide in Base Chemicals

Cascading chemical reactions can be realized when the local structure and composition of two catalytically active phases are fine-tuned and combined in one material. In this issue of Chem, Fan et al. report an innovative approach for the conversion of carbon dioxide into light olefins with a metal oxide-zeolite tandem catalyst.

Formation and Fate of Formaldehyde in Methanol‐to‐Hydrocarbon Reaction: In Situ Synchrotron Radiation Photoionization Mass Spectrometry Study

AbstractHCHO has been confirmed as an active intermediate in the methanol‐to‐hydrocarbon (MTH) reaction, and is critical for interpreting the mechanisms of coke formation. Here, HCHO was detected and quantified during the MTH process over HSAPO‐34 and HZSM‐5 by in situ synchrotron radiation photoionization mass spectrometry. Compared with conventional methods, excellent time‐resolved profiles were obtained to study the formation and fate of HCHO, and other products during the induction, steady‐state reaction, and deactivation periods. Similar formation trends of HCHO and methane, and their close correlation in yields suggest that they are derived from disproportionation of methanol at acidic sites. In the presence of Y2O3, the amount of HCHO changes, affecting the hydrogen‐transfer processes of olefins into aromatics and aromatics into cokes. The yield of HCHO affects the aromatic‐based cycle and the formation of ethylene, indicating that ethylene is mainly formed from the aromatic‐based cycle.

Formation and Fate of Formaldehyde in Methanol-to-Hydrocarbon Reaction: In Situ Synchrotron Radiation Photoionization Mass Spectrometry Study.

HCHO has been confirmed as an active intermediate in the methanol-to-hydrocarbon (MTH) reaction, and is critical for interpreting the mechanisms of coke formation. Here, HCHO was detected and quantified during the MTH process over HSAPO-34 and HZSM-5 by in situ synchrotron radiation photoionization mass spectrometry. Compared with conventional methods, excellent time-resolved profiles were obtained to study the formation and fate of HCHO, and other products during the induction, steady-state reaction, and deactivation periods. Similar formation trends of HCHO and methane, and their close correlation in yields suggest that they are derived from disproportionation of methanol at acidic sites. In the presence of Y2 O3 , the amount of HCHO changes, affecting the hydrogen-transfer processes of olefins into aromatics and aromatics into cokes. The yield of HCHO affects the aromatic-based cycle and the formation of ethylene, indicating that ethylene is mainly formed from the aromatic-based cycle.

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.

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

AbstractAlthough 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.

Computational QM/MM investigation of the adsorption of MTH active species in H-Y and H-ZSM-5.

The transformation of methanol-to-hydrocarbons (MTH) has significant potential as a route to synthesise low-cost fuels; however, the initial stages of the zeolite catalysed MTH process are not well understood. Here, we use hybrid quantum- and molecular-mechanical (QM/MM) embedded-cluster simulations to develop our understanding of the interaction between methanol and the zeolite catalysts H-ZSM-5, and for comparison, the larger pore H-Y. Energies and structures, calculated using hybrid-level density functional theory (hybrid-DFT) and higher-level correlated methods, are compared with previous experimental and computational results. We show that hydrogen-bonds between methanol adsorbates, formed through polarizable O-H bonds, substantially influence the adsorption energetics, structural parameters and vibrational frequencies. Our observations are extended by considering polar solvent molecules in the environment, with the presence of both water or methanol around the adsorption site leading to barrier-less transfer of the zeolite proton to an adsorbed methanol, which will significantly influence the reactivity of the adsorbed methanol.

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

AbstractAfter 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.

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.

Solvents Molecular Mobility in Coked Catalyst ZSM-5 Studied by NMR Relaxation and Pulsed Field Gradient Techniques

Through gasification and methanol synthesis, biomass can produce methanol and then produce light olefins through the methanol-to-hydrocarbon (MTH) process. To make this new biomass based production route profitable, the efficiency of the MTH process is important. Catalyst deactivation is the main reason for the decline of conversion and selectivity. NMR was applied to detect the relaxation and diffusion of liquid molecules in a series of coked ZSM-5, investigating the effect of coke on molecular transport properties. n-Heptane was chosen as a probe molecule in describing pore network connectivity, whereas methanol was used for relaxation measurements. Though there were only minor differences of pore connectivity among the samples, the longitudinal relaxation time showed an almost linear relationship with coke contents, suggesting that the interaction between reactants and catalyst surface influences the catalyst performance within low coke contents. PFG-NMR and NMR relaxation, as fast and straightforward ...

Conversion of Methyl Mercaptan to Hydrocarbons over H-ZSM-5 Zeolite: DFT/BOMD Study.

Methyl mercaptan—a harmful impurity in natural gas—may be selectively converted into H2S and hydrocarbons [methyl mercaptan to hydrocarbon (M2TH) process], using zeolite catalysts. When M2TH is compared with the well-known MTH (methanol to hydrocarbons) process, significant differences emerge, essentially regarding the formation and distribution of products. Density functional theory (DFT) and Born–Oppenheimer molecular dynamics (BOMD) were employed to reveal possible origins for the experimentally observed differences. We established a close similarity between DFT intrinsic (electronic) reaction profiles in the stepwise mechanism of methanol and mercaptan dehydration, although no variance in reactivity was revealed. BOMD simulations at the experimental temperature of 823 K reveal rapid hydrogen abstraction from the methyl group in mercaptan, adsorbed in the zeolite cavity in the presence of the methoxy intermediate. The formation of •CH2SH radical is 10 times faster than that of •CH2OH at the same temperature. The varied reactivity of methanol and mercaptan in MTH and M2TH processes, respectively, can therefore first be attributed to very rapid hydrogen abstraction in mercaptan, which occurs in the zeolite cavity, following the formation of surface methoxy.