Methanol Formation Rate Research Articles

CO2 hydrogenation to methanol over Pt functionalized Hf-UiO-67 versus Zr-UiO-67.

Sustainable methanol formation from CO2/H2 is potentially a key process in the post-fossil chemical industry. In this study, Hf- and Zr-based metal-organic framework (MOF) materials with UiO-67 topology, functionalized with Pt nanoparticles, have been tested for CO2 hydrogenation at 30 bar and 170-240°C. The highest methanol formation rate, 14 molmethanol molPt-1 h-1, was obtained over a Hf-based catalyst, compared with the maximum of 6.2 molmethanol molPt-1 h-1 for the best Zr-based analogue. However, changing the node metal did not significantly affect product distribution or apparent activation energy for methanol formation (44-52 kJ mol-1), strongly indicating that the higher activity of the Hf-based analogues is associated with a higher number of active sites. Both catalysts showed stable catalytic performance during testing under kinetic conditions, but the addition of 2 vol% water to the feed induced catalyst deactivation, in particular the Hf-MOFs. Interestingly, mainly methanol and methane formation rates decreased, while CO formation rates were less affected by deactivation. No direct correlation was found between catalytic stability and framework stability (crystallinity, specific surface area). Experimental and computational studies suggest that water adsorption strength to the MOF node may affect the relative catalytic stability of Hf-UiO-67-Pt versus Zr-UiO-67-Pt methanol catalysts.This article is part of the discussion meeting issue 'Green carbon for the chemical industry of the future'.

Enhanced methanol formation in CO2 hydrogenation through synergistic copper and gallium interaction

Methanol production from CO2 hydrogenation has attracted great attention, due to the demand for carbon neutralization. Ga-promoted Cu catalysts show promising properties for methanol formation in CO2 hydrogenation, urging a fundamental investigation of the active state. In this study, we synthesized CuGa species on mesoporous SiO2, varying the amount of Ga. The structural characterizations revealed that Ga existed mostly in highly dispersed GaOx species with Cu in the form of metallic Cu clusters. The addition of Ga significantly increased the methanol formation with an optimum Ga value compared to Cu and Ga on SiO2. The surface-sensitive measurements showed the mixed form of Cu+ and Cu0 on the Cu surface exhibited, where the Cu+ state was proportional to the methanol formation rate. Therefore, we conclude that the Cu+-GaOx interface constitutes an active site for methanol production. Furthermore, we found that the Ga addition influences the oxidation state and dispersion of Cu.

Cr as a promoter for the In2O3-catalyzed hydrogenation of CO2 to methanol

The utility of Cr as a promoter for In2O3 catalysts in the hydrogenation of CO2 to methanol is investigated. Uniform precursors to binary CrOx-In2O3 and ternary NiO-CrOx-In2O3 catalysts are prepared by flame spray pyrolysis. For the CrOx-In2O3 samples, the highest methanol rate is obtained at a Cr content of 2 mol%, exceeding the methanol rate of In2O3 by 55 %. With increasing Cr content, the CO2 conversion rate does not increase, albeit the methanol selectivity decreases. Characterization of the samples supported by density functional theory calculations provides insight into the role of Cr. At low content, Cr is mainly doped into the lattice of In2O3, which leads to more oxygen vacancy (Ov) sites. The In2O3 surface sites close to Cr-oxide clusters present on the surface are also activated towards Ov formation, offsetting the decrease in Ov due to coverage of the In2O3 surface by Cr-oxide with increasing Cr content. Cr2O3 dispersed on the surface of the In2O3 particles suppresses sintering of In2O3 under reducing conditions, which is especially evident at a Cr content above 2 mol% and higher reaction temperatures. Introducing Ni to the CrOx-In2O3 catalysts results in a higher methanol formation rate compared to CrOx-In2O3. The methanol rate increases with the Ni content with the highest activity obtained at Ni and Cr contents of 22 mol% and 8 mol%, respectively. The optimum Ni(22)-Cr(8)-In2O3 catalyst displays twice the methanol rate of a Ni(22)-In2O3 reference. Ni and Cr play different promoting roles in achieving an increased and more stable rate of methanol formation compared to In2O3: Ni promotes the hydrogenation of formate and methoxy surface intermediates to methanol, while Cr results in more Ov sites and suppresses sintering of In2O3.

The copper size effect of CuZn/CeO2 catalyst in CO2 hydrogenation to methanol

The escalating emission of CO2 has caused a significant environmental impact. Hydrogenation of CO2 to methanol stands as the primary objective in the liquid sunlight vision, aiming to mitigate CO2 emissions. CuZn-based transition metal catalysts, known for their cost-effectiveness, demonstrate commendable catalytic performance at moderate temperatures and pressures, showcasing considerable potential in industrial applications. Consequently, a series of catalysts, composed of CuZn-based species supported on CeO2 with strategically introduced appropriate oxygen vacancies, were meticulously prepared. It worth to noting that the size of Cu was precisely tuned, along with the enhanced ability to the adsorption and activation of CO2 and H2. As a result, the optimal CuZn/CeO2 catalyst exhibits high methanol formation rate, recording at 433.4 gMeOH kgcat−1 h−1 with selectivity of 68.5% at 260 °C and 3 MPa. Simultaneously, the reaction path and the evolution of intermediates in CO2 hydrogenation was thoroughly delineated.

Reactivity Switch of Platinum with Gallium: From Reverse Water Gas Shift to Methanol Synthesis.

The development of efficient catalysts for the hydrogenation of CO2 to methanol using "green" H2 is foreseen to be a key step to close the carbon cycle. In this study, we show that small and narrowly distributed alloyed PtGa nanoparticles supported on silica, prepared via a surface organometallic chemistry (SOMC) approach, display notable activity for the hydrogenation of CO2 to methanol, reaching a 7.2 molCH3OH h-1 molPt-1 methanol formation rate with a 54% intrinsic CH3OH selectivity. This reactivity sharply contrasts with what is expected for Pt, which favors the reverse water gas shift reaction, albeit with poor activity (2.6 molCO2 h-1 molPt-1). In situ XAS studies indicate that ca. 50% of Ga is reduced to Ga0 yielding alloyed PtGa nanoparticles, while the remaining 50% persist as isolated GaIII sites. The PtGa catalyst slightly dealloys under CO2 hydrogenation conditions and displays redox dynamics with PtGa-GaOx interfaces responsible for promoting both the CO2 hydrogenation activity and methanol selectivity. Further tailoring the catalyst interface by using a carbon support in place of silica enables to improve the methanol formation rate by a factor of ∼5.

Direct Oxidation of Methane to Methanol over Transition-Metal-Free Ferrierite Zeolite Catalysts.

Direct oxidation of methane to methanol was reported to be highly dependent on the transition- or noble-metal-loading catalysts in the past decades. Here, we show that the transition-metal-free aluminosilicate ferrierite (FER) zeolite effectively catalyzed methane and N2O to methanol for the first time. The distorted tetracoordinated Al in the framework and pentacoordinated Al on the extra framework formed during calcination, activation, and reaction processes were confirmed as the potential active centers. The possible reaction pathway similar to the Fe-containing zeolites was advocated based on the reaction results using different oxidants, N2O adsorption FTIR spectra, and 27Al MAS NMR spectra. The stable and efficient methanol production capacity of FER zeolite was ascribed to the two-dimensional straight channels and its distinctive Al distribution of FER zeolite (CP914C) from Zeolyst. The transition-metal-free FER zeolite performed better than the record in the literature and our recent results using transition-metal-containing catalysts in terms of selectivity and formation rate of methanol and stability. This work has great significance and prospects for utilizing CH4 and N2O as resources and will open new avenues for methane oxidation.

Elucidating the promotional effect of ultra-low Zn content on Cu for CO2 hydrogenation to methanol

CO2 hydrogenation to methanol is currently of great importance for energy and environmental objectives. Herein, the effect of ultra-low amounts of Zn on Cu/SiO2 catalysts for CO2 hydrogenation was studied, maintaining the average Cu particle size in the 2–4 nm range to minimize structure sensitivity effects. The addition of Zn amounts as low as Zn/Cu = 0.01 increases the methanol formation rate up to twelve times compared to Cu/SiO2 catalyst, highlighting the importance of isolated Zn atoms in methanol synthesis catalysis. At these very low Zn contents, CuZnX alloy sites are identified, with strong affinity for CO adsorption. When Zn content is increased, CuZn alloy disappears to give rise to mostly metallic Cu likely forming Cu-ZnOx interfacial sites. Steady state and transient Operando-DRIFTS evidence that the reaction pathways depend on the nature of the active sites with reverse water–gas shift and CO hydrogenation dominant for low Zn content catalysts and formate pathway for high Zn content catalysts.

Supported Cu catalysts on UiO-66 toward enhanced methanol selectivity by CO2 hydrogenation: Effect of Cu loading

The xCu/MOF catalysts were synthesized by incipient-wetness impregnation of copper precursor on UiO-66 support, in which the Cu loadings were 5, 10, and 20% (wt.%). To gain insight into the properties of the materials, a range of techniques, including XRD, TEM, TGA, N2 physisorption, CO2-TPD, EDS, N2O chemisorption, XPS, IR, and NMR, were employed. The effectiveness of catalysts was evaluated for synthesizing methanol through CO2 hydrogenation under various Gas Hourly Space Velocities (GHSVs) and reaction conditions, including pressures of 10 and 30 bar, CO2/H2 ratios of 1:3, and CO2/H2/N2 ratios of 1:3:0.14 M, all conducted at a temperature of 250 °C. The active site in the Cu/UiO-66 catalyst is the interface between Cu+ nanoparticles (NPs) with Zr oxide SBU [Zr6O4(OH)4(−CO2) and the defects related to unsaturated Zr sites of the Zr-Oxo cluster due to the increase of Cu NPs occupying missing linker defects of the MOF structure. 20Cu/MOF catalyst, with the highest copper loading, showed the best methanol formation rate and CO2 conversion due to the strong metal-support interaction and the highest amount of the missing linker defects of the MOF structure.

Combining Atomic Layer Deposition with Surface Organometallic Chemistry to Enhance Atomic-Scale Interactions and Improve the Activity and Selectivity of Cu-Zn/SiO2 Catalysts for the Hydrogenation of CO2 to Methanol.

The direct synthesis of methanol via the hydrogenation of CO2, if performed efficiently and selectively, is potentially a powerful technology for CO2 mitigation. Here, we develop an active and selective Cu-Zn/SiO2 catalyst for the hydrogenation of CO2 by introducing copper and zinc onto dehydroxylated silica via surface organometallic chemistry and atomic layer deposition, respectively. At 230 °C and 25 bar, the optimized catalyst shows an intrinsic methanol formation rate of 4.3 g h-1 gCu-1 and selectivity to methanol of 83%, with a space-time yield of 0.073 g h-1 gcat-1 at a contact time of 0.06 s g mL-1. X-ray absorption spectroscopy at the Cu and Zn K-edges and X-ray photoelectron spectroscopy studies reveal that the CuZn alloy displays reactive metal support interactions; that is, it is stable under H2 atmosphere and unstable under conditions of CO2 hydrogenation, indicating that the dealloyed structure contains the sites promoting methanol synthesis. While solid-state nuclear magnetic resonance studies identify methoxy species as the main stable surface adsorbate, transient operando diffuse reflectance infrared Fourier transform spectroscopy indicates that μ-HCOO*(ZnOx) species that form on the Cu-Zn/SiO2 catalyst are hydrogenated to methanol faster than the μ-HCOO*(Cu) species that are found in the Zn-free Cu/SiO2 catalyst, supporting the role of Zn in providing a higher activity in the Cu-Zn system.

Reactivity of Group 5 and 6 Single-Site Photocatalysts for Partial Oxidation of Methane: Comparison of Chromium, Niobium, and Tungsten-Doped Mesoporous Amorphous Silica.

Single-site transition-metal-doped photocatalysts can potentially be used for partial oxidation of methane (POM) at remote sites where natural gas is extracted and methane is often flared or released to the atmosphere. While there have been several investigations into the performance of vanadium, there has been no general survey of the performance of other metals. This work aims and examines Cr, Nb, and W metal oxide materials embedded in amorphous SiO2 to determine the viability of each metal in catalyzing the POM. Photoexcited states are examined to determine the nature of the photoactivated species, and then the subsequent POM reaction mechanisms are elucidated. Using the calculated energies of reaction intermediates and transition states, the rate of methanol formation is evaluated through the use of a microkinetic model. The findings indicate that all three metals are potentially more suitable for catalyzing POM than vanadium but that niobium shows the most favorable energy profile.

Combined role of Ce promotion and TiO2 support improves CO2 hydrogenation to methanol on Cu catalysts: Interplay between structure and kinetics

Ce promotion increases the rate of methanol formation by two orders of magnitude compared to Cu/TiO2 and Cu/SiO2 catalysts. The promotion proceeds via the generation of a new active site in which Ce3+ intervenes at the interface with Cu, without significant influence of the support identity. For higher promoter concentrations (Ce/(Cu + Ce) greater than 0.3), additional active sites are not further generated due to the saturation of the space around copper clusters. TiO2-supported samples presented higher methanol selectivity (ca. 80% at 220 °C and 8 bar) compared to the ones supported on SiO2 (ca. 20%), due to an inhibition of the r-WGSR related to the strong metal support interaction between Cu and TiO2, which blocks surface sites responsible for CO production. Operando – DRIFTS further verified that the mechanism for methanol formation is the same between both families of catalysts and that it likely proceeds via the generation of formates at the Cu – Ce interface.

Effects of Al distribution in the Cu-exchanged AEI zeolites on the reaction performance of continuous direct conversion of methane to methanol

The effects of the organic structure-directing agent (OSDA) with or without Na cations for the synthesis of AEI zeolite on the location and content of the Al atoms in the framework as well as the Cu speciation and acidic features of the exchanged Cu/AEI zeolite catalysts and their catalytic properties in the continuous direct conversion of methane to methanol (cDMTM) were investigated. The AEI zeolite synthesized with Na cations led to more Al and a higher percentage of Al pairs than the one prepared without Na cations. Consequently, the Cu/AEI zeolite catalysts synthesized with Na cations exhibited lower apparent activation energy of methane conversion, higher methanol formation rate, and less stable ability in the cDMTM reaction due to the more active dicopper species and higher acid amount than those prepared without Na cations. Adjustment of the close Al content in the framework of the two AEI zeolites by calcination, the Cu/AEI zeolite synthesized with Na cations evidenced a higher methanol selectivity and more stable reaction performance because of the different relationship between Cu species and acid sites, such as the distance, caused by the Al distribution. The methanol formation rate of 27.3 µmol·g−1·min−1 with about 50% selectivity and stable performance in the cDMTM reaction at 350 °C were obtained. Moreover, the stability of both 5Cu/AEI zeolite catalysts could be upgraded by further calcination to diminish the acid amount, the high methanol formation rate of the 5Cu/AEI zeolite synthesized with Na cations was maintained and the reaction stability was newly attained. The 5Cu/AEI zeolite synthesized without Na cations, merely the stable performance was reached, while the methanol formation rate and selectivity seriously declined. This work contributed to the development of AEI zeolite with different Al distributions and its significant impacts on the Cu speciation and acidic properties, thus providing a valuable strategy to obtain a robust Cu/AEI zeolite catalytic system for efficient and stable methanol production from methane.

Carbon coated In2O3 hollow tubes embedded with ultra-low content ZnO quantum dots as catalysts for CO2 hydrogenation to methanol

CO2 hydrogenation coupled with renewable energy to produce methanol is of great interest. Carbon coated In2O3 hollow tube catalysts embedded with ultra-low content ZnO quantum dots (QDs) were synthesized for CO2 hydrogenation to methanol. ZnO-In2O3-II catalyst had the highest CO2 and H2 adsorption capacity, which demonstrated the highest methanol formation rate. When CO2 conversion was 8.9%, methanol selectivity still exceeded 86% at 3.0 MPa and 320 °C, and STY of methanol reached 0.98 gMeOHh−1gcat−1 at 350 °C. The ZnO/In2O3 QDs heterojunctions were formed at the interface between ZnO and In2O3(222). The ZnO/In2O3 heterojunctions, as a key structure to promote the CO2 hydrogenation to methanol, not only enhanced the interaction between ZnO and In2O3 as well as CO2 adsorption capacity, but also accelerated the electron transfer from In3+ to Zn2+. ZnO QDs boosted the dissociation and activation of H2. The carbon layer coated on In2O3 surface played a role of hydrogen spillover medium, and the dissociated H atoms were transferred to the CO2 adsorption sites on the In2O3 surface through the carbon layer, promoting the reaction of H atoms with CO2 more effectively. In addition, the conductivity of carbon enhanced the electron transfer from In3+ to Zn2+. The combination of the ZnO/In2O3 QDs heterojunctions and carbon layer greatly improved the methanol generation activity.

C-Axis-oriented sheet-like Cu/AEI zeolite contributes to continuous direct oxidation of methane to methanol

The cubic and sheet-like AEI zeolites were prepared in the absence and presence of CTAB. Sheet-like Cu/AEI zeolite exhibited more stable and higher methanol formation rate than the cubic-shaped one due to the shortened straight channel length.

Engineering Cu+/CeZrOx interfaces to promote CO2 hydrogenation to methanol

Cu-based catalysts are widely employed for CO2 hydrogenation to methanol, which is expected as a promising process to achieving carbon neutrality. However, most Cu-based catalysts still suffer from low methanol yield with a passable CO2 conversion and lack insight into its reaction mechanism for guiding the design of catalysts. In this work, Cu+/CeZrOx interfaces are engineered by employing a series of ceria-zirconia solid solution catalysts with various Ce/Zr ratios, forming a Cu+-Ov-Ce3+ structure where Cu+ atoms are bonded to the oxygen vacancies (Ov) of ceria. Compared to Cu/CeO2 and Cu/ZrO2, the optimized catalyst (i.e., Cu0.3Ce0.3Zr0.7) exhibits a much higher mass-specific methanol formation rate (192 gMeOH/kgcat/h) at 240 °C and 3 MPa. Through a series of in-situ and ex-situ characterization, it is revealed that oxygen vacancies in solid solutions can effectively assist the activation of CO2 and tune the electronic state of copper to promote the formation of Cu+/CeZrOx interfaces, which stabilizes the key *CO intermediate, inhibits its desorption and facilitates its further hydrogenation to methanol via the reverse water–gas-shift (RWGS) + CO-Hydro pathway. Therefore, the concentration of *CO or the apparent Cu+/(Cu++Cu0) ratio could be employed as a quantitative descriptor of the methanol formation rate. This work is expected to give a deep insight into the mechanism of metal/support interfaces in CO2 hydrogenation to methanol, offering an effective strategy to develop new catalysts with high performance.

ML based catalyst modelling for photo-catalytic reduction of CO2 to methanol

Industrial revolution & globalization lead to the rampant increase in the anthropogenic CO2 emissions which has become the global concern. Photo-reduction of CO2 to other value-added products such as methanol can be observed as a technology that has drawn immense attention of numerous researchers these days. This work majorly focuses on developing the best machine learning (ML) model that can predict the formation rate of methanol from CO2. Present study considers the development of several ML models such as Linear Models; Tree based models and Neural Network Models wherein the extensive data mining (505 data points, from last decade) lays a great foundation. In this process of mining the data, 21 different experimental parameters have demonstrated huge influence on the methanol formation rate from CO2. On comparing their performances, it was observed that Neural Network models (R2 ∼ 0.70) were the most promising models that have exhibited the best performance amongst other ML models. Apart from obtaining the best model this work also throws light on the quantifying influence of experimental parameters (known as input contribution) which aids in arriving at the crucial parameters that can be tuned to accelerate the performance of the model. It was also observed that the catalyst composition, reaction temperature, source of light and its wavelength have created a huge impact on the formation of methanol.

Cu/Mo2CTx interface drives CO2 hydrogenation to methanol

In a recent <i>Nature Catalysis</i> paper, Müller and coworkers engineer Cu/Mo₂C<i>T</i>_x (MXene) interface and achieve high intrinsic methanol formation rate and selectivity. Due to the good mobility of Cu species under H₂ at 500°C and increased electron affinity of partially reduced Mo₂C<i>T</i>_x surface, the interface of Cu/Mo₂C<i>T</i>_x becomes strong Lewis acid as reduction time increases, which correlates with enhanced rate of CO₂ hydrogenation to methanol.

Nitrogen doping of indium oxide for enhanced photocatalytic reduction of CO2 to methanol

Herein, the nitrogen-doped indium oxide (N-In2O3) photocatalyst was confirmed to be highly active and stable for photocatalytic reduction of CO2 to methanol in an aqueous solution at ambient conditions. The efficiency of N-In2O3 in producing methanol can be flexibly improved by tuning the nitrogen doping content. The highest formation rate of methanol reaches to 394 μmol gcat-1 h-1, with a methanol selectivity of 63%, at a nitrogen doping content of 3.74%. Nitrogen doping generates mid-gap energy states and reduces the bandgap of In2O3, thus boosting photon absorption and electron-hole separation. Nitrogen doping creates more oxygen vacancies on In2O3, thus forming more active sites for CO2 adsorption and conversion. Nitrogen doping also enhances the activity of the surface frustrated Lewis pairs (SFLPs), which further promotes CO2 adsorption and activation. The multiple-role of nitrogen doping results in the highly active and stable photocatalytic reduction of CO2 to methanol.

Enhanced Methanol Production over Non-promoted Cu–MgO–Al2O3 Materials with Ex-solved 2 nm Cu Particles: Insights from an Operando Spectroscopic Study

Enhanced methanol production is obtained over a non-promoted Cu–MgO–Al2O3 mixed oxide catalyst derived from a Cu–Mg–Al hydrotalcite precursor (HT) containing narrowly distributed small Cu NPs (2 nm). Conversions close to the equilibrium (∼20%) with a methanol selectivity of 67% are achieved at 230 °C, 20 bar, and a space velocity of 571 mL·gcat–1·h–1. Based on operando spectroscopic studies, the striking activity of this Cu-based catalyst is ascribed to the stabilization of Cu+ ions favored under reaction conditions due to lattice reorganization associated with the “HT-memory effect” promoted by water. Temperature-resolved infrared–mass spectrometry experiments have enabled the discernment of monodentate formate species, stabilized on Cu+ as the intermediate in methanol synthesis, in line with the results of density functional theory calculations. These monodentate formate species are much more reactive than bridge formate species, the latter ones behaving as intermediates in methane and CO formation. Moreover, poisoning of the Cu0 surface by strongly adsorbed species behaving as spectators is observed under reaction conditions. This work presents a detailed spectroscopic study highlighting the influence of the reaction pressure on the stabilization of active surface sites, and the possibility of enhancing methanol production on usually less active non-promoted nano-sized copper catalysts, provided that the proper support is selected, allowing the stabilization of doped Cu+. Thus, a methanol formation rate of 2.6 × 10–3 molMeOH·gcat–1·h–1 at 230 °C, 20 bar, and WHSV = 28 500 mL·gcat–1·h–1 is obtained on the Cu–MgO–Al2O3 HT-derived catalyst with 71% methanol selectivity, compared to 2.2 × 10–4 molMeOH·gcat–1·h–1 with 54% methanol selectivity obtained on a reference Cu/(Al2O3/MgO) catalyst not derived from a HT structure.

Photocatalytic reduction of CO2 to methanol using Zr(IV)-based MOF composite with g-C3N4 quantum dots under visible light irradiation

Considering the global warming due to the accumulation of excess CO2 in the atmosphere, it is vital to reduce the CO2emissions by capturing or converting it into value-added chemicals and fuels. Among the known materials, MOFs have great significance in capturing the CO2 and its conversion to energy efficient fuels due to its tuneability, high porosity, large surface area, unique structure, and semiconducting behavior. In the present study, graphitic carbon nitride quantum dots coupled Zr (IV) based MOF composite (g-CNQDs@MOF) has been reported for the photoreduction of CO2 to methanol selectively under visible light irradiation. Graphitic carbon nitride quantum dots (g-CNQDs) in the synthesized Zr-based MOF composite play an important role for photocatalytic CO2 reduction due to its rich photo properties thereby improving the electronic conductance properties of the composite. g-CNQDs in the Zr (IV)-based MOF composite acted as co-catalyst which enhanced the electron-hole separation by elongating the lifetime of photogenerated charge carriers on the surface of MOF composite. These excess electrons accelerated the generation of catalytically active sites on the composite’s surface to reduce CO2 selectively into methanol with better selectivity and efficient methanol formation rate.