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•Adding metals to Mo/ZSM-5 confers multifunctionality to the catalysts enhancing their performance in MDA.•Transition metal additives, Fe, Co, Ni, can change the nature of the C ...deposits formed in reaction.•Noble metal additives, Pt, Rh, Pd, Ir, Ru, Cu, hydrogenate carbon deposits formed in reaction in situ.•Metals additives like Cr, Ag, can enhance the acid-assisted heterolytic splitting of methane.•Multifunctional Mo-X/ZSM-5 catalysts must be coupled with process intensified reactor configurations to optimize MDA.
The drastic rise in shale gas production has encouraged the quest for alternative uses of methane as a chemical feedstock in the manufacturing industry. While two-step syngas routes for methane valorization are deployed commercially, direct one-step routes for methane conversion are attracting much attention. As steam cracking installations have shifted from using oil-based naphtha to shale-based natural gas liquids, production of aromatics has dropped. Methane dehydroaromatization (MDA) is a one-step reaction capable of valorizing methane to hydrogen and benzene. Challenges with the MDA reaction are two-fold: the reaction is thermodynamically limited with low one-pass methane conversion and even the best catalytic systems, Mo/zeolites, suffer rapid deactivation from coking. A catalyst design strategy to improve stability is the use of multifunctional Mo-X/zeolite systems where X is a dopant capable of modulating the stability. In this paper we provide a complete overview of the main Mo-X/zeolite systems used in MDA and critically draw connections among the different types of dopants (X) employed, as a function of the role they play in the reaction/deactivation pathway. We have also dedicated a section to emerging trends with non-Mo based catalysts. The goal of this review article is to establish a basis that will facilitate the identification of useful multifunctional catalytic systems, and recognize gaps in the knowledge of these systems that deserve more attention. Improving MDA systems to the point to which they can be commercially deployed requires a multifaceted approach that combines optimization of the designs of both the catalyst and the reactor configuration. We therefore also provide a brief overview of the most recent advances in process intensification strategies employed with different reactor configurations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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•A new approach to generating zeolite-supported Mo carbide active species with improved performance for methane aromatization is presented.•Mo carbides formed ex situ constitute more ...stable catalysts than Mo oxides where Mo carbides are formed in the reaction induction period.•10 wt% Mo carbide catalyst exhibits a benzene yield of 8% and little deactivation; in contrast the best Mo oxide catalyst deactivates completely.•The amount of carbon deposits formed after reaction does not play a role in the catalyst deactivation.•Careful preparation of supported Mo carbide species overcomes the rapid deactivation characteristic of Mo oxide supported systems.
The catalytic activity of HZSM-5 supported Mo-oxide (MoOx) and Mo-carbide (MoCy) for methane aromatization was studied using a packed-bed microreactor. MoOx/HZSM-5 catalysts with 3, 6, 10, and 12 wt.% Mo loading were prepared by incipient wetness impregnation method followed by calcination at 500 °C. The MoCy/HZSM-5 catalysts were prepared ex situ by treating the oxide catalysts by temperature-programmed reduction and carburization. The MoCy/HZSM-5 catalysts show significantly higher activities and stability compared to those of the MoOx/HZSM-5 catalysts. Unlike the oxide catalysts, not only methane conversion and benzene yield improve with higher Mo loading but also the deactivation rate becomes much slower for the carbide catalysts. The optimum carbide catalyst has a Mo loading of 10 wt% The catalysts were characterized by XRD, N2 adsorption, TPR, NH3-TPD, TPO,TGA and 27Al-NMR. The results show that the oxide catalysts at a high loading face pore blockage after few hours of reaction, which makes the active sites inaccessible to CH4. Carbide catalysts, on the other hand face no pore blockage even after a long period of reaction, making them much better catalysts than the oxides for nonoxidative methane aromatization reaction.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The influence of an Fe additive and different types of pretreatment were studied on HZSM‐5‐supported molybdenum oxide (MoOx) catalysts for methane aromatization. The catalytic behavior of catalysts ...that consist of 6 wt % Mo/ZSM‐5 with 0, 0.2, and 1 wt % Fe was tested with two types of pretreatment: 1) heating under He flow and 2) reduction in H2/CH4 and carburization in CH4. Under He pretreatment, the addition of 0.2 wt % Fe improved the benzene yield, but the addition of 1 % Fe decreased it slightly. Precarburization of the catalysts resulted in enhanced catalytic properties for all Fe loadings and improved the catalyst stability. The precarburized 6 wt % Mo–0.2 wt %Fe catalyst presented the highest benzene yield (6.9 %), which was almost stable in the subsequent 10 h test. The fresh and spent catalysts were characterized by using XRD, N2 adsorption, temperature‐programmed reduction, SEM, temperature‐programmed oxidation, and thermogravimetric analysis. The results show that the precarburized catalysts are more stable because of the formation of smaller amounts of carbon deposits and, consequently, less pore blockage. The addition of Fe causes the carbon deposits to be more reactive and easier to burn off. Higher Fe loadings are linked to the formation of carbon nanotubes.
Foot on the gas: The influence of an Fe additive and different types of pretreatment are studied on HZSM‐5‐supported MoOx catalysts for methane aromatization. The catalytic behavior was tested with two types of pretreatment: 1) heating under He flow and 2) reduction in H2/CH4 and carburization in CH4.
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FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
The preparation of silica membranes by chemical vapor deposition (CVD) is critically reviewed, with focus placed on the two most common supports used, Vycor glass and porous alumina. The major ...application of the membranes is hydrogen separation, but other gases can also be separated by functionalization of the silica. A description is given of the different gas transport mechanisms, and a comparison is provided to palladium membranes.
•Silica-based membranes for hydrogen separation are critically reviewed.•Chemical vapor deposition (CVD) on Vycor glass and alumina supports is described.•The main CVD geometries, “one-sided” and “opposing reactants” feed are discussed.•Different operative gas transport mechanisms are presented.•The use of silicon and non-silicon CVD precursor compounds is covered.
Hydrogen separation membranes are important in the gas separation field and among these, silica-based membranes have emerged as promising materials at high temperatures due to their high permeation rates, high selectivity, hydrothermal stability, resistance to poisons, and mechanical strength. A critical review of the preparation of silica membranes by chemical vapor deposition (CVD) is given, with special attention placed on the two major supports used, Vycor glass and porous alumina. The coverage includes the different gas transport mechanisms that occur through silica membranes, which are convective flow, Knudsen diffusion, molecular sieving, activated diffusion, and solid-state diffusion. A description is made of the two main CVD geometries, which are the “one-sided” feed and “opposing reactants” feed configurations. The results of numerous studies in which CVD precursors such as silicon alkoxides and organosilicon species, as well as non-silicon element compounds are used to control pore size and surface properties are presented. Also the effect of varying reaction conditions such as temperature, pressure, and reaction time are compared. Finally, the permeation properties of the silica membranes are compared to those of palladium membranes, both types of membranes are employed for hydrogen separation in membrane reactors, and the advantages of silica membranes in these reactor systems are discussed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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•ZSM5-supported Mo carbide catalysts modified with Co and Ni additives were more stable in methane dehydroaromatization.•A synergy is established between Mo and each additive, with ...increased benzene yield compared to an unmodified Mo catalyst.•Each additive presents an optimum loading for 6wt% Mo, resulting in the most stable catalyst (0.6 wt% Co and 0.2 wt% Ni).•The effect of the additives is only beneficial when the Mo carbide species are formed ex situ via previous carburization.•The additives interact with the Mo, decreasing its mobility and changing its reducibility.
Methane dehydroaromatization is studied using different loadings (0.2 wt%, 0.6 and 1 wt%) of Co and Ni additives on a 6 wt% Mo/ZSM-5 catalyst to evaluate the promoting effect of Ni and Co on reactivity and stability of the catalysts. A synergy between Mo and the additives is observed when combining a reduction/carburization pretreatment with an optimum additive loading: benzene yield and catalytic stability are both improved. The fresh and spent samples were characterized by X-Ray Diffraction, 27Al Magic-Angle Spinning Solid State Nuclear Magnetic Resonance Spectroscopy, Temperature-Programmed Reduction and Thermogravimetric Analysis. The results suggest that catalytic enhancement in precarburized Mo-Co and Mo-Ni samples is due to a combination of effects: extraction of Al from the ZSM-5 framework to form inactive Al2(MoO4)3 is reduced, interaction of Mo with the additives enhances the retention of Mo species within the zeolite channels, prevents Mo aggregation in reaction and modifies the reducibility of the Mo sites.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The direct oxidation of propylene to propylene oxide (PO) using molecular oxygen has many advantages over existing chlorohydrin and hydroperoxide process, which produce side products and require ...complex purification schemes. Recent advances in liquid-phase and gas-phase catalytic oxidation of propylene in the presence of only molecular oxygen as oxidant and in absence of reducing agents are summarized. Liquid-phase PO processes involving soluble or insoluble Mo, W, or V catalysts have been reported which provide moderate conversions and selectivities, but these likely involve autoxidation by homogeneous chain reactions. Gas-phase PO catalysts have been mostly Ag-, Cu-, or TiO
2
-based substances, although other compositions such as Au-, MoO
3
-, Bi-based catalysts and photocatalysts have also been suggested as possibilities. The Ag catalysts differ from those used for ethylene oxide production in having high Ag contents and numerous additives. The additives are solid-phase alkali metals, alkaline earth metals, and halogens, with the most common substances being NaCl and CaCO
3
. Nitrogen oxides in the form of gas-phase species or nitrates have also been found to be effective in enhancing PO production. Direct epoxidation by surface nitrates is a possibility. Titania catalysts supported on silicates have also been reported. These have higher PO selectivities at high conversion than silver catalysts.
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BFBNIB, GIS, IJS, KISLJ, NUK, PNG, UL, UM, UPUK
The effect of the Si/Al ratio of the ZSM-5 support on the structure and activity of Mo/HZSM-5 catalysts in methane dehydroaromatization (MDA) was studied. ZSM-5 (with Si/Al = 15, 25, and 40) ...supported Mo oxide catalysts (MoOx/ZSM-5) with 3 and 10 wt% Mo loading were prepared by incipient wetness impregnation followed by calcination in air at 500 °C. The as-prepared catalysts were activated by temperature programmed reduction in hydrogen in order to maximize Mo dispersion. To understand the structure-activity relationship, the fresh and spent samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N2 adsorption isotherms, Scanning Transmission Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (STEM-EDS), X-ray absorption spectroscopy (EXAFS and XANES), thermogravimetric analysis (TGA), and Raman spectroscopy. The results show that the Si/Al ratio does not influence the the local structure around the Mo centers. It does however affect the number of available Brønsted acid sites in the support and the amount of Mo species entering the zeolite channels. A lower Si/Al ratio and higher Mo loading resulted in a higher Mo occupation within the zeolite channels. The higher channel occupation directly correlated with higher benzene selectivity and yield, as well as a lower catalyst deactivation rate.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•Ex situ formation of supported Mo carbide species for CH4 aromatization was explored.•Mo carbide species formed at lower temperature by slow reduction/carburization were more stable ...in reaction.•Heating Mo oxides in He before exposing to CH4 caused a drop in surface area.•Mo carbide species formed by slow reduction/carburization we more dispersed.•This higher Mo dispersion was maintained even after reaction and Mo exposure remained high despite the formation of C deposits.
The synthesis of zeolite-supported Mo carbide species was studied by testing different reduction/carburization conditions applied to a zeolite-supported Mo oxide catalyst, with the aim to find the optimized treatment conditions necessary to form stable supported Mo carbide catalysts ex situ for application in methane dehydroaromatization reaction. Four types of treatment were performed and studied using temperature-programmed reduction and carburization profiles: (1) heating the catalyst in a reducing gas, H2, up to reaction temperature and switching to CH4; (2) heating the catalyst in a reducing gas, H2, mixed with dilute CH4; (3) heating the catalyst in CH4 up to reaction temperature; and (4) heating the catalyst in an inert gas (commonly He) up to reaction temperature and then switching to CH4 or to H2 followed by CH4 or to H2/CH4 mixture. Each of these processes were stopped at intermediate points to analyze the phases that were present in order to identify the structural evolution of the supported Mo carbides that originate from the supported Mo oxides. Once the supported carbides were formed, they were quenched under the same gas mixture, and then they were each tested in methane dehydroaromatization via previous heating to reaction temperature in He flow. Despite all of them showing only presence of Mo2C species on HZSM-5, the catalytic properties were dramatically different. Catalysts treated in H2 or CH4/H2 showed remarkably higher stability. These catalysts exhibited a higher Mo dispersion and thus exposure on the active surface.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Metal oxide-impregnated zeolites are employed in a wide variety of catalytic reactions, including methane dehydroaromatization (MDA). The most studied catalysts for MDA are Mo carbides supported on ...H-ZSM-5, formed through the carburization of Mo-oxide-loaded H-ZSM-5. A complete structural understanding of these materials has not yet been achieved, limiting the potential for rational catalyst design for improved performance. We hereby pursue experimental and theoretical investigations of these catalyst precursors to uncover rational design principles. We employ temperature-programmed oxidation and extended X-ray absorption fine-structure experiments, density functional theory calculations, and QuantEXAFS analysis to unveil Mo-oxide speciation in H-ZSM-5. We demonstrate that Mo-oxides exist within these systems as a combination of various motifs, and the relative abundance of these species is controlled through tailored preparation methods. The synergies exploited in this work may be leveraged in other related catalysts. The conclusions drawn are applicable to other relevant applications of zeolite-supported metal oxides.
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IJS, KILJ, NUK, PNG, UL, UM
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•Cu/ZSM-5 catalysts active for the water-gas shift reaction were characterized using in situ XRD, AP-XPS and DRIFTS.•WGS Activity increases with Cu loading (10% > 5% > 1%).•Cu exists ...in channel intersections, O 5-member rings, and as a segregated phase.•Metallic Cu aggregates exist when catalysts are most active.
Cu/ZSM-5 catalysts were found to exhibit activity for the water-gas shift (WGS) reaction at temperatures over 300 °C. Nominal Cu loadings of 1, 5, and 10% on ZSM-5 were synthesized using the wetness impregnation technique and evaluated for WGS activity; with a clear trend favoring increased Cu loading (10% > 5% > 1%). X-ray diffraction (XRD) confirmed high dispersion of small Cu at low loadings while CuO crystallized at higher loadings. In-situ XRD showed an evolving transformation of CuO → Cu metal, with the appearance of a metallic copper phase at the optimum reaction conditions, particularly at the highest copper loading. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed the ability of the surface to activate H2O and adsorb CO, by the presence of HO-Al/Si and transient CO-Cu+ surface species in two distinct locations (5 membered rings (2157 cm−1) vs channel intersections (2138 cm−1) within the ZSM-5 framework, that likely formed during the CuO → Cu transition. The lowest loaded Cu-ZSM-5 (1%) showed only the 5 membered ring occupied Cu+ while the higher loadings showed both, with comparable temperature dependent behavior for the channel intersection Cu+. The amounts of Cu+ species in each location were observed to be dependent on temperature while the 5-membered ring showed better thermal stability. In-situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) was used to further confirm the gradual changes to the chemical state of copper (CuO → Cu2O → Cu) under reaction conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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