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Although ketene has been proposed to be an active intermediate in a number of reactions including OXZEO (metal oxide-zeolite)-catalyzed syngas conversion, dimethyl ether carbonylation, methanol to hydrocarbons, and CO

NMR experiments reveal a mechanism of syngas conversion in which CO reacts with OCH3 on the oxide surface, generating ketene intermediates, which can either form acetate or diffuse into zeolite.

Syngas chemistry has been under study since Fischer-Tropsch synthesis (FTS) was invented in the 1920s. Despite the successful applications of FTS as the core technology of coal-to-liquid and gas-to-liquid processes in industry, the product selectivity control of syngas conversion still remains a great challenge, particularly for value-added chemicals such as light olefins. Recent studies show that the catalyst design concept of OXZEO (oxide-zeolite-based composite) enables direct syngas conversion to mixed light olefins with a selectivity reaching 80% and to ethylene with a selectivity of 83% among hydrocarbons. They both well-surpass the limits predicated by the Anderson-Schultz-Flory model via the conventional FTS route (58% and 30%, respectively). Furthermore, this catalyst concept allows one-step synthesis of gasoline-range isoparaffins and aromatic compounds, which is otherwise not possible in conventional FTS. A rapidly growing number of studies demonstrate the versatility of this concept and may form a technology platform for utilization of carbon resources including coal, natural gas, and biomass via syngas to a variety of basic chemicals and fuels. However, the selectivity control mechanism is far from being understood. Therefore, we focus mainly on the catalytic roles of the bifunctionalities of OXZEO while reviewing the development of bifunctional catalysts for selective syngas conversion by taking syngas-to-light olefins as an example. With this, we intend to provide insights into the selectivity control mechanism of the OXZEO concept in order to understand the challenges and prospects for future development of much more active and more selective catalysts.

The OXZEO (oxide−zeolite) bifunctional catalyst concept has enabled selective syngas conversion to a series of value-added chemicals and fuels such as light olefins, aromatics and gasoline. Herein we report for the first time a dynamic confinement of SAPO-17 cages on the selectivity control of syngas conversion observed during an induction period. Structured illumination microscopy, intelligent gravimetric analysis, UV-Raman, X-ray diffraction, thermogravimetry and gas chromatography-mass spectrometer analysis indicate that this is attributed to the evolution of carbonaceous species as the reaction proceeds, which gradually reduces the effective space inside the cage. Consequently, the diffusion of molecules is hindered and the hindering is much more prominent for larger molecules such as C4+. As a result, the selectivity of ethylene is enhanced whereas that of C4+ is suppressed. Beyond the induction period, the product selectivity levels off. For instance, ethylene selectivity levels off at 44% and propylene selectivity at 31%, as well as CO conversion at 27%. The findings here bring a new fundamental understanding that will guide further development of selective catalysts for olefin synthesis based on the OXZEO concept.

Non-oxidative conversion of methane to olefins, aromatics and hydrogen (MTOAH) has been reported recently over metal single sites such as iron and platinum. The reaction was proposed to involve catalytic activation of methane followed by gas phase C−C coupling of methyl radicals. This study using H atom Rydberg Tagging time-of-flight technique provides direct experimental evidence for the formation of hydrogen radicals during MTOAH reaction over a catalytic quartz wall reactor containing embedded iron species (denoted as Fe-reactor). Fe-reactor gives 7.3% methane conversion at 1273 K with 41.2% selectivity toward C2 (ethane, ethylene and acetylene) and 31.8% toward BTX (benzene, toluene and xylene), respectively. The enhancing effects of hydrogen radicals on overall MTOAH performance are validated by cofeeding hydrogen donor benzene, which provides an additional route of methane activation apart from catalytic activation.

ACS catalysis 12(7), 3897 - 3908 (2022). doi:10.1021/acscatal.2c00092, The methane to olefins, aromatics, and hydrogen (MTOAH) process via Pt/CeO$_2$ catalysts poses an attractive route to improve yield and stability for the direct catalytic conversion of methane. In this study, two sets of samples, one composed of PtO$_x$ single sites on ceria and the other with additional Pt agglomerates, were prepared. Both sets of samples showed enhanced catalytic activity for the direct conversion of methane exceeding the performance of pure ceria. Pulsed reaction studies unraveled three reaction stages: reduction of the ceria support during activation, an induction phase with increasing product formation, and finally, stable running of the catalytic reactions. The reduction of ceria was confirmed by X-ray absorption spectroscopy (XAS) after conducting the MTOAH reaction. Operando X-ray absorption spectroscopy at challenging reaction temperatures of up to 975 °C in combination with theoretical simulations further evidenced an increased Pt–Ce interaction upon reaction with CH$_4$. Analysis of the extended X-ray absorption fine structure (EXAFS) spectra proved decoration and encapsulation of the Pt particles by the CeO$_2$/Ce$_2$O$_3$ support or a partial Ce–Pt alloy formation due to the strong metal–support interaction that developed under reaction conditions. Moreover, methyl radicals were detected as reaction intermediates indicating a reaction pathway through the gas-phase coupling of methyl radicals. The results indicate that apart from single-atom Pt sites reported in the literature, the observed Pt–Ce interface may have eased the activation of CH$_4$ by forming methyl radicals and suppressed coke formation, significantly improving the catalytic performance of the ceria-based catalysts in general., Published by ACS, Washington, DC

The metal oxide-zeolite (OX-ZEO) bifunctional catalyst concept has aroused wide interest recently for direct syngas conversion. However, the mechanism of product selectivity control is far from bei...

Boron-modified ZrO2 (B-ZrO2) was synthesized under various pH values (9, 10, and 11) and used as the supports of PtSn catalysts (PtSn/B-ZrO2-x) for non-oxidative dehydrogenation of propane. The NH3-TPD and pyridine IR show that only Lewis acid is present and the acid strength increases with the synthesis pH. PtSn/B-ZrO2-10 exhibits the best catalytic performance with an initial propane conversion of 36% and a deactivation rate constant (kd) of 0.0127 h−1. The XPS results indicate that the electronic properties of Pt and SnOx are affected not only by their interaction but also by the interaction with support. After a careful analysis of the oxygen storage capacity and activity in CO oxidation, it is hypothesized that the interaction between Pt and Sn becomes stronger following the order: PtSn/B-ZrO2-9

Despite significant progress achieved in Fischer-Tropsch synthesis (FTS) technology, control of product selectivity remains a challenge in syngas conversion. Herein, we demonstrate that Zn2+ -ion exchanged ZSM-5 zeolite steers syngas conversion selectively to ethane with its selectivity reaching as high as 86 % among hydrocarbons (excluding CO2 ) at 20 % CO conversion. NMR spectroscopy, X-ray absorption spectroscopy, and X-ray fluorescence indicate that this is likely attributed to the highly dispersed Zn sites grafted on ZSM-5. Quasi-in-situ solid-state NMR, obtained by quenching the reaction in liquid N2 , detects C2 species such as acetyl (-COCH3 ) bonding with an oxygen, ethyl (-CH2 CH3 ) bonding with a Zn site, and epoxyethane molecules adsorbing on a Zn site and a Bronsted acid site of the catalyst, respectively. These species could provide insight into C-C bond formation during ethane formation. Interestingly, this selective reaction pathway toward ethane appears to be general because a series of other Zn2+ -ion exchanged aluminosilicate zeolites with different topologies (for example, SSZ-13, MCM-22, and ZSM-12) all give ethane predominantly. By contrast, a physical mixture of ZnO-ZSM-5 favors formation of hydrocarbons beyond C3+ . These results provide an important guide for tuning the product selectivity in syngas conversion.

The effect of the Al2O3 structure on the performance of Pt/Ga/Al2O3 catalysts is investigated for the direct dehydrogenation of propane. The study unveils that the structure of Al3+ determines the bulk structure of catalysts, particularly a high content of coordinatively unsaturated Al3+ sites (penta-coordinated Al3+, denoted as Al3+penta) could lead to a remarkably improved dehydrogenation activity of the catalyst. The bulk characterization reveals that the sufficient amount of Al3+penta in Al2O3 benefit the dispersion of Pt and Ga2O3 on the Al2O3 support. At the same time, TPR results reveal that the presence of Pt facilitates the reduction of Ga2O3, likely due to the hydrogen spillover between the well dispersed Pt and Ga2O3, which consequently enhances the synergistic function between Pt and Ga2O3 in the dehydrogenation of propane. Recyclability tests demonstrate that the dehydrogenation activity stabilizes after three cycles over the Pt/Ga/Al2O3 catalyst.

Significant progress has been demonstrated in the development of bifunctional oxide-zeolite catalyst concept to tackle the selectivity challenge in syngas chemistry. Despite general recognition on the importance of defect sites of metal oxides for CO/H2 activation, the actual structure and catalytic roles are far from being well understood. We demonstrate here that syngas conversion can be steered along a highly active and selective pathway towards light olefins via ketene-acetate (acetyl) intermediates by the surface with coordination unsaturated metal species, oxygen vacancies and zinc vacancies over ZnGaOx spinel−SAPO-34 composites. It gives 75.6% light-olefins selectivity and 49.5% CO conversion. By contrast, spinel−SAPO-34 containing only a small amount of oxygen vacancies and zinc vacancies gives only 14.9% light olefins selectivity at 6.6% CO conversion under the same condition. These findings reveal the importance to tailor the structure of metal oxides with coordination unsaturated metal sites/oxygen vacancies in selectivity control within the oxide-zeolite framework for syngas conversion and being anticipated also for CO2 hydrogenation.

The measurement of the energy spectrum of cosmic ray helium nuclei from 70 GeV to 80 TeV using 4.5 years of data recorded by the DArk Matter Particle Explorer (DAMPE) is reported in this work. A hardening of the spectrum is observed at an energy of about 1.3 TeV, similar to previous observations. In addition, a spectral softening at about 34 TeV is revealed for the first time with large statistics and well controlled systematic uncertainties, with an overall significance of $4.3\sigma$. The DAMPE spectral measurements of both cosmic protons and helium nuclei suggest a particle charge dependent softening energy, although with current uncertainties a dependence on the number of nucleons cannot be ruled out., Comment: 11 pages, 13 figures, published in Phys. Rev. Lett. Add one more digit for first three columns in Table S2

Oxide-Zeolite (OX-ZEO) bifunctional catalyst design concept has been exemplified in several processes to direct conversion syngas to value-added chemicals and fuels such as mixed light olefins, ethylene, aromatics and gasoline. Herein we demonstrate that the product can be steered toward liquefied petroleum gas (LPG) with a selectivity up to 89% in hydrocarbons especially propane selectivity reaching 80% at CO conversion of 63% using ZnCrOx-H-SSZ-39 catalyst. Interestingly, the quantity of the acid sites of SSZ-39 does not influence obviously the hydrocarbon distribution but the strength is crucial for selective formation of propane. This finding provides an alternative route of LPG synthesis from a variety of carbon resources via syngas.

We report that methane conversion to olefins and aromatics over an iron-coated catalytic quartz reactor can be enhanced by hydrogen radicals, which are provided by the thermal decomposition of hydr...

Aromatics are important basic chemicals. However, direct conversion of syngas via the conventional Fischer-Tropsch synthesis produces little aromatics. We presented herein that a bifunctional composite of ZSM-5 in combination with ZnCrOx catalyzes syngas conversion to aromatics. Particularly, ZSM-5 crystals with a sheet-like morphology can enhance significantly the aromatization activity. The lower length ratio of the b/a axes of the crystals, the more aromatics form but without influencing the selectivity of small molecules such as CH4 and C2–C4. Since the acid properties and the Al chemical environment were not altered while the morphology changed, the enhanced aromatic selectivity is likely attributed to the favored diffusion of aromatics in these sheet-like crystals.

Mordenite (MOR) zeolite is known to possess a 12-membered ring (12 MR) and 8 MR channels, which are interconnected by 8 MR side pockets. Hyperpolarization (HP) 129Xe NMR reveals a new Xe adsorption...

Strong metal–support interaction (SMSI) has been widely recognized for platinum-group metals on reducible oxide supports. Herein we report that the catalytic activity of Ni catalyst in CO2 methanation is significantly suppressed over conventional anatase (a-TiO2) support due to the SMSI-induced formation of a titania overlayer around the Ni nanoparticles. Furthermore, CO is the only product . In contrast, the NH3-treatment and H2-treatment of the a-TiO2 support enhance remarkably the activity of Ni, i.e., CO2 conversion increases by 1 order of magnitude and CO2 is hydrogenated almost exclusively to CH4. X-ray photoelectron spectroscopy (XPS), H2 and CO chemisorption, and low temperature electron paramagnetic resonance (EPR) reveal that the enhanced CO2 methanation activity may be related with the Ti3+ species in the bulk that are generated by reduction treatment, which likely have altered the SMSI between Ni and a-TiO2 support. This simple reduction treatment approach may be applicable to modulate the SM...

Boron had been introduced into the structure of carbon material (BC), which was used as the support of Pd catalyst for hydrogenation of 4-carboxybenzaldehyde (4-CBA). The physical properties and chemical composition of the support and corresponding catalyst were characterized by N2 adsorption–desorption, Raman spectroscopy, inductively coupled plasma optical emission spectroscopy (ICP-OES), element analysis (EA), high-resolution transmission electron microscopy (HRTEM), CO-pulse chemisorption and X-ray photoelectron spectroscopy (XPS). The results demonstrate that Pd/BC catalyst exhibits a superior activity and good stability due to the more uniform dispersion of Pd nanoparticles, the presence of mesoporous structure and the enhanced interaction between Pd nanoparticles and the support, compared to carbon and N-doped carbon supported Pd catalysts (Pd/C and Pd/NC, respectively).

Carbon nanotubes (CNTs) have shown as unique nanoreactors to tune the catalytic activity of confined nano-catalysts. Here we report that the catalytic performance of molybdenum carbide nanoparticles (MoCx NPs) for the hydrogen evolution reaction (HER) process can be enhanced by encapsulation within single-walled carbon nanotubes (SWNTs) with a diameter of 1-2 nm. The catalyst with MoCx NPs located on the interior surface of SWNTs (MoCx@SWNTs) exhibits a lower onset over-potential and a smaller Tafel slope than the one with MoCx NPs attached on the exterior surface (MoCx/SWNTs). This is likely attributed to the much smaller particle size and the more reduced states of the confined MoCx NPs, as well as the larger specific surface area of MoCx@SWNTs compared with MoCx/SWNTs. In addition, the electronic structure of the confined MoCx NPs might be modified by the confinement effects of SWNTs, and hence the adsorption free energy of H atoms on the confined MoCx NPs, which could also contribute to their higher performance. These results suggest that the SWNTs can be further explored for constructing novel catalysts with beneficial catalytic performance.

Metal-exchanged zeolites with small pore sizes have attracted much attention in recent years due to their application in the selective catalytic reduction (SCR) of NOx in diesel engines. Typically, copper-chabazite (e.g. Cu-SSZ-13) has been gradually used as an SCR catalyst in heavy-duty diesel vehicles over the last decade due to its relatively excellent catalytic performance and stability. However, most SSZ-13 zeolites are still prepared via the traditional hydrothermal process in the presence of organic templates, requiring consecutive solid separation and thermal treatment steps to achieve the final zeolite products. In recent years, several strategies for the environmentally friendly preparation of zeolites have been reported, which are also applicable for the synthesis of zeolites for emission control applications. These concepts include copper–amine templating, organotemplate-free synthesis, and solvent-free synthesis. In this review, we briefly summarize the potential advantages of the environmentally friendly synthesis of zeolites for SCR.

The oxide–zeolite (OX–ZEO) bifunctional catalyst design concept has shown promising potential for selectivity control in syngas conversion by separating CO activation and C–C coupling onto oxides a...

Nonoxidative conversion of ethane to ethylene and/or BTX (benzene, toluene , and xylene) suffers rapid deactivation due to coke deposition. We report here the effects of phosphorus modification on the stability and activity of Mo/ZSM5 for nonoxidative conversion of ethane. The results show that the ethylene and BTX yield and stability are significantly enhanced upon modification with 2.5 wt.% P. NH 3 TPD, pyridine FTIR, 1H MAS NMR, 27Al MAS NMR, 31P MAS NMR, 129 Xe NMR, XPS, UV–visible diffuse reflectance spectra (UV–vis DRS), and nitrogen physisorption were carried out to understand the effects of P on the structure of Mo/ZSM5 and its correlation with catalytic performance. The presence of P reduces the acid strength and density, changes the channel system of ZSM5 by forming thermally stable SAPO-like interfaces with the framework Al, and improves the dispersion of molybdenum . Rapid deactivation still occurs on Mo/ZSM5 with 1 wt.% P due to the existence of denser silanol groups, more isolated Mo species, and reduced aperture size with little change in effective micropore volume. A higher P loading (2.5 wt.%) leads to less dense silanol groups and less reduced but stable molybdenum species, and simultaneously reduces channel diameter and internal volume. Consequently, the ethylene selectivity is enhanced and the formation of coke precursors is restricted, resulting in improved stability.

Nitrogen doping has been proved to be an effective way to modify the properties of graphene and other carbon materials. Herein, we explore a composite with nitrogen doped carbon overlayers wrapping SiC substrate as a support for Ni (Ni/CN-SiC) and evaluate its effects on the methanation activity. The results show that both the activity and stability of Ni are enhanced. Characterization with STEM, XRD, XPS, Raman and H2-TPR indicates that nitrogen doping generates more defects in the carbon overlayers, which benefit the dispersion of Ni. Furthermore, the reduction of Ni is facilitated.

NC@SiC prepared by a facile pyrolysis method was used as the support of Pd catalyst for the hydrogenation of 4-carboxybenzaldehyde. The structure of Pd/NC@SiC was characterized by N2 adsorption-desorption, ICP-OES, SEM, HRTEM and XPS. The Pd/NC@SiC catalyst showed superior activity compared with Pd/SiC due to its hierarchically mesoporous and microporous structure and the synergistic effect between Pd and N-doped carbon layer. In addition, Pd/NC@SiC also exhibited a good durability under high temperature and high pressure H2, which was attributed to its highly dispersed Pd nanoparticles stabilized by the N-doped carbon structure.

Carbon supported PdCl2 is highly active in catalyzing acetylene hydrochlorination reaction, but deactivates rather quickly. Upon nitrogen doping in the carbon structure, the stability of the PdCl2 catalysts is significantly improved. Furthermore, the results show that 900 °C is a preferred doping temperature. The acetylene conversion keeps above 90% even after 1200 min time on stream whereas the one without nitrogen doping drops to below 10% after 450 min. The stabilizing mechanism of nitrogen doping on catalyst was studied.

CO2 capture via solid adsorption technology relies on the development of low-cost and high-performance new solid adsorbents. The robust and large-scale produced hexagonal boron nitride (h-BN) porous materials are an ideal candidate but present low CO2 adsorption properties. Here, we report that in situ carbon doping of BN materials (BCN) can be facilely controlled by thermal annealing of boric acid and melamine mixtures in various atmospheres. CO2 uptake of the BCN materials is in the range of 3.74–3.91 mmol CO2 g−1 (165–172 mg CO2 g−1) at 298 K and ambient pressure, whereas it is only 1.16–1.66 mmol CO2 g−1 (53.0–73.1 mg CO2 g−1) over the pure BN materials. The separation coefficient between CO2 and N2 is up to 74 and no decrease in the adsorption performance was observed after nine adsorption/desorption cycles. The BCN material presents the best performance among known BN-based sorbents, which is comparable to many excellent carbon materials. Detailed characterization reveals that the incorporation of carbon into the BN matrix increases the density of ultramicropores (

This study presents the discovery that porous boron nitride (p-BN) is active in acetylene hydrochlorination, although boron nitride (BN) is generally considered chemically inert. An acetylene conversion of 99% is achieved with a vinyl chloride selectivity over 99% at 280 °C at a gas hourly space velocity (GHSV) of 1.32 mL min–1 g–1. By contrast, the commercially available crystallized hexagonal BN (h-BN) exhibits no catalytic activity. Furthermore, this p-BN is rather durable as demonstrated by a 1000 h lifetime test. Catalytic tests, spectroscopic characterization, and theoretical calculations indicate that the activity likely originates from the defects and edge sites. Particularly, the armchair edges of BN can polarize and activate acetylene, which then reacts with gaseous HCl giving vinyl chloride as the product.

The catalytic activity of metal catalysts can be modulated by confinement within the channels of carbon nanotubes (CNTs). Here, we show that the product distribution of cinnamaldehyde hydrogenation can be modified by confinement of Ru nanoparticles in CNTs. A catalyst composed of Ru nanoparticles dispersed on the exterior walls of CNTs gave hydrocinnamaldehyde as product. In contrast, confinement of the Ru nanoparticles within CNT channels facilitated hydrogenation of C=O bonds and complete hydrogenation, and both cinnamyl alcohol and hydrocinnamyl alcohol formed in addition to hydrocinnamaldehyde. High-resolution transmission electron microscopy, Raman spectroscopy, hydrogen temperature-programmed reduction, and hydrogen temperature-programmed desorption were used to investigate the characteristics of the catalysts. The results indicate that the different interactions between the confined Ru nanoparticles and the exterior and interior walls of the CNTs, as well as spatial restriction and enrichment within the narrow channels likely play important roles in modulation of the product distribution.

Acetylene hydrochlorination is an important coal-based technology for production of vinyl chloride, the monomer of one of the world mostly used plastics. Despite of the great potentials demonstrated for carbon-based catalysts to replace the toxic mercury chloride, the stability and the deactivation mechanism are rarely discussed, which is essential for real applications. Herein, we present a detailed study on the deactivation mechanism of nitrogen doped carbon based catalyst in acetylene hydrochlorination. The results show that the deactivation was likely caused by the carbon-like deposition over the catalyst, which can be regenerated with high temperature NH 3 treatment.

Understanding the properties and behavior of water molecules in restricted geometries, such as the nanopores of rocks, is of interest for shale gas exploitation. We present herein ex situ and in situ nuclear magnetic resonance (NMR) studies on the effects of water on the adsorption and diffusion of methane in nanopores. Silica materials with one-dimensional pores of ZSM-22, MCM-41, and SBA-15, with pore sizes ranging from 0.5 to 6 nm, were chosen as models. Hyperpolarized (HP) 129Xe NMR results show that water adsorption does not affect the pore sizes of ZSM-22 and MCM-41 but reduces that of SBA-15. The presence of water suppresses methane adsorption; this suppression effect is stronger in smaller pores. The self-diffusion coefficients of methane within ZSM-22 and MCM-41 are not significantly influenced by the presence of water, as measured by 1H pulsed field gradient (PFG) NMR. However, within SBA-15, which has a pore size of 6 nm, the diffusion coefficient of methane increases as the amount of water adsorption increases, peaks, and then decreases to a constant value with further water adsorption. These experiments reveal the effects of the pore size and the presence of water on methane adsorption and diffusion in constrained spaces, which could have important implications for flow simulations of methane in shales.

The key of syngas (a mixture of CO and H2) chemistry lies in controlled dissociative activation of CO and C–C coupling. We demonstrate here that a bifunctional catalyst of partially reducible manganese oxide in combination with SAPO-34 catalyzes the selective formation of light olefins, which validates the generality of the OX-ZEO (oxide-zeolite) concept for syngas conversion. Results from in situ ambient-pressure X-ray photoelectron spectroscopy, infrared spectroscopy, and temperature-programmed surface reactions reveal the critical role of oxygen vacancies on the oxide surface, where CO dissociates and is converted into surface carbonate and carbon species. They are converted to CO2 and CHx in the presence of H2. The limited C–C coupling and hydrogenation activities of MnO enable the reaction selectivity to be controlled by the confined pores of SAPO-34. Thus, a selectivity of light olefins up to 80% is achieved, far beyond the limitation of Anderson–Shultz–Flory distribution. These findings open up pos...

With increasing concern about the environmental impact of shale gas exploitation, nonaqueous fracturing with carbon dioxide has emerged as a promising alternative to increase gas production and, at the same time, to store large amounts of CO2. The key process of CH4 displacement by CO2 is worth a systematic investigation from aspects of both experiment and simulation. In this work, the CH4 and CO2 displacement was studied with in situ 13C NMR in the pores of silica (SBA-15), which were functionalized with organic groups such as phenyl and cyclohexyl, in order to model the organic matter in shale with different aromaticity. Due to the stronger adsorption strength and higher capacity of CO2 in SBA-15, CH4 can be easily stripped out of the pores by CO2, while the reverse process to displace CO2 with CH4 is not effective. Even though the displacement effect in the pores of SBA-15 with a higher aromaticity is relatively better at room temperature, the superiority is eliminated by high temperature. Furthermore,...

A composite catalyst combining the partially reducible ZnCrOx with zeolite ZSM-5 enables direct conversion of syngas to aromatics, with a selectivity to aromatics reaching 73.9% at a single pass CO conversion of 16.0%.