Oxidative Dehydrogenation Of Methanol Research Articles

Catalytic Consequences of Protons in Methanol Oxidative Dehydrogenation on Molybdenum-Based Polyoxometalate Clusters

Catalytic Consequences of Protons in Methanol Oxidative Dehydrogenation on Molybdenum-Based Polyoxometalate Clusters

Kinetic Coupling of Redox and Acid Chemistry in Methanol Partial Oxidation on Vanadium Oxide Catalysts

Kinetic, isotopic, and spectroscopic studies establish the active site requirements for three kinetically coupled catalytic cycles catalyzed by redox, Brønsted, and Lewis acid–base sites, which occur during methanol and oxygen reactions on titania-supported vanadium oxide catalysts. The initial activation of methanol during its oxidative dehydrogenation to formaldehyde restricts the overall turnovers─this reaction proceeds via CH3OH dissociative adsorption followed by a kinetically relevant C–H bond scission of the CH3O intermediate on V–O redox site pairs found at the interface of VOx and TiO2. The Gibbs free energy change of these two steps (ΔGads and ΔG⧧, respectively) both decrease as V–O–V coordination decreases and V–O–Ti coordination increases, leading to higher turnovers per surface vanadium as VOx dispersion increases, except the extreme case of isolated VO4. Coupled kinetically with the oxidative dehydrogenation cycle is the Brønsted acid-catalyzed cycle that forms dimethoxymethane─this cycle proceeds via methanol- and formaldehyde-derived CH3OCH2OH adsorption to Brønsted sites at the VOx–TiO2 interface, followed by its kinetically relevant C–O bond scission. Its turnovers also increase with VOx dispersion following the same trend as the oxidative dehydrogenation cycle but at a much faster rate, so the reaction can readily approach chemical equilibrium. Alongside the other two catalytic cycles is the Tishchenko reaction that forms methyl formate from two formaldehyde molecules, produced by the methanol oxidative dehydrogenation cycle, on exposed Ti4+–O2– Lewis acid–base pairs uncovered by VOx─this cycle proceeds via kinetically relevant intermolecular C–O bond formation followed by a rapid 1,3-hydride shift. The interplay of coexisting redox, Brønsted, and Lewis sites, each with its unique catalytic roles, leads to different rates and yields of the three products. The active site structure and mechanistic knowledge established here allow us to optimize the product ratios required for downstream synthesis of larger oxygenates.

Activity, Selectivity and Initial Degradation of Iron Molybdate in the Oxidative Dehydrogenation of Ethanol

AbstractIron molybdate catalysts are applied in the industrial FormOx process to produce formaldehyde by oxidative dehydrogenation (ODH) of methanol. Only few studies are available about the applicability of iron molybdate catalysts for the ODH of ethanol to produce acetaldehyde. Herein, an iron molybdate synthesized by co‐precipitation (p) and an iron molybdate prepared by a ball‐milling solid‐state synthesis (bm) are applied as ethanol ODH catalysts. Both materials show attractive acetaldehyde selectivites of >90 % (280 °C: p‐Fe2(MoO4)3: YAcH=90.3 %; bm‐Fe2(MoO4)3: YAcH=60.4 %) and a stable performance. The bulk composition and crystal structure could be confirmed by various characterization techniques and is maintained during ethanol ODH. XPS reveals an enrichment of Mo on the catalyst surface which is slightly decreasing after the catalytic tests. This observation could be a first sign of long‐term deactivation like known from methanol ODH. Comparing the performance of both materials, p‐Fe2(MoO4)3 shows higher activity and aldehyde selectivity. We propose the higher Mo/Fe surface ratio and the lower acidity to be the reasons for these differences.

Chemical Grafting of Highly Dispersed VOx/CeO2 for Increased Catalytic Activity in Methanol Oxidative Dehydrogenation

Chemical Grafting of Highly Dispersed VOx/CeO2 for Increased Catalytic Activity in Methanol Oxidative Dehydrogenation

Feature of catalysis on bimetallic alloys Zr with V, Mo, and Fe in the reaction of methanol oxidation

Catalytic behaviors of bimetallic catalysts-alloys of zirconium with vanadium, molybdenum, and iron was investigated in the oxidative dehydrogenation of methanol. The conditions for the formation of the catalyst’s active surface were revealed. The conversion of methanol into formaldehyde, dimethyl ether, and dimethoxymethane on bimetallic catalysts was studied. The characterization of catalysts was performed by XRD, XPS, and SEM. It was shown that the activity of samples increases after О2 + Н2 treatment and was associated with segregation of the active components of alloys (V, Mo) on the surface of catalysts and realization of their optimal oxidation state under catalysis conditions.

Methanol oxidative dehydrogenation and dehydration on carbon nanotubes: active sites and basic reaction kinetics

In situ active site titration, carbonyl group containing model catalysts, and kinetic analysis have been applied to reveal the nature of oxidized carbon nanotubes catalyzed methanol dehydration and oxidative dehydrogenation reactions.

One-step synthesis of methylal via methanol oxidation by Mo:Fe(x)/HZSM-5 bifunctional catalyst

The traditional one-step process for preparing methylal bifunctional catalyst has multiple limitations, including high cost, high acidity and low yield. In order to overcome these limitations, the author designed and studied a series of Mo:Fe(x)/HZSM-5 function catalyst with different ferromolybdenum ratios. The one-step process for preparing methyl acetal is realized in a fluidized bed reactor. The effect of different ferromolybdenum ratios on its catalytic performance was investigated and the optimum ferromolybdenum ratio catalyst was found. The results show that this new process and catalyst can overcome the disadvantages of traditional bifunctional catalysts and achieve a relatively high DMM yield. For instance, the methanol conversion ratio and DMM selectivity of the Mo:Fe(2)/HZSM-5 bifunctional catalyst reached 87.44% and 93%, respectively. Moreover, the recorded DMM yield was 81.32%, which was much higher than that of other research results. At the same time, the XPS catalyst activity and activation energy results show that increased molybdenum loading catalytic effect. However, when Mo/Fe > 2, the mutual promotion effect would be larger and part of the formaldehyde obtained by oxidative dehydrogenation of methanol would be further oxidized to obtain the by-product formic acid, further producing more methyl formate. In addition, the self-made Mo:Fe(2)/HZSM-5 dual-functional catalyst exhibits high stability and carbon deposition resistance under severe operating conditions, compared to other bifunctional catalysts of this process.

Obtaining formaldehyde on a new catalytic system

Formaldehyde is widely used in many fields of industry. The increase in the need for formaldehyde led to an increase in scientific research, the purpose of which is to obtain the greatest yield of the product (formaldehyde) with minimal costs for raw materials, catalyst and its regeneration, energy carriers, etc. At industrial plants for the production of formaldehyde by oxidative dehydrogenation of methanol on the silver on pumice catalyst, the process temperature is maintained at 600 ° C. The process of obtaining formaldehyde by oxidation of methanol with air oxygen at the combination of catalysts "silver" and "silver on pumice" in the temperature range of 250–450 °C is investigated. The results showed the possibility of practical application of the combined catalyst. Chemical and technological parameters of the process with the use of a new catalyst are slightly lower than production indicators, however, the temperature of the pilot process is 2 times lower - this will reduce not only the energy costs, but also increase the life of the catalyst and the cost of its regeneration.

Oxidative dehydrogenation of methanol at ceria-supported vanadia oligomers

We study the adsorption and partial oxidation of methanol at a trimeric VOX species supported on ceria, which was found to be the thermodynamically most stable species under slightly reducing conditions. Results for the oxidation of methanol obtained using dispersion-corrected PBE+U and HSE are compared with a metastable monomeric VO2 species. The intrinsic barriers for monomers (103kJ/mol), trimers (133kJ/mol), and the pristine surface (136kJ/mol) are compared with the corresponding values derived from temperature-programmed desorption experiments, i.e. 100, 125, and 150kJ/mol, respectively. It was found that the VO2 trimer is more active than the pristine ceria (111) surface due to entropic effects, showing up in a higher pre-exponential factor. However, the VO2 trimer is less active than the monomer due to structural relaxation in the surface O layer of the latter.

Surface enrichment of Pt in stable Pt-Ir nano-alloy particles on MgAl2O4 spinel in oxidizing atmosphere

With the capability of MgAl2O4 spinel {111} nano-facets in stabilizing small Rh, Ir and Pt particles, bimetallic Ir-Pt catalysts on the same support were investigated, aiming at further lowering the catalyst cost by substituting expensive Pt with cheaper Ir in the bulk. Small Pt-Ir nano-alloy particles (<2nm) were successfully stabilized on the spinel {111} nano-facets as expected. Interestingly, methanol oxidative dehydrogenation (ODH) rate on the surface Pt atoms increases with oxidizing aging but decreases upon reducing treatment, where Ir is almost inactive under the same reaction conditions. Up to three times enhancement in Pt exposure was achieved when the sample was oxidized at 800°C in air for 1week and subsequently reduced by H2 for 2h, demonstrating successful surface enrichment of Pt on Pt-Ir nano-alloy particles. A dynamic stabilization mechanism involving wetting/nucleation seems to be responsible for the evolution of surface compositions upon cyclic oxidizing and reducing thermal treatments.

Partial oxidation of methanol catalyzed with Au/TiO2, Au/ZrO2 and Au/ZrO2-TiO2 catalysts

Mesoporous TiO2, ZrO2 and ZrO2-TiO2 mixed oxides were synthesized by the sol-gel method and the Au/TiO2, Au/ZrO2 and Au/ZrO2-TiO2 catalysts were prepared by deposition-precipitation method using urea solution as a precipitating agent. These materials were characterized by UV–vis spectroscopy, X-ray diffraction (XRD), N2 adsorption-desorption isotherms, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and in situ FTIR-pyridine (FTIR-Py) adsorption. XRD patterns of the samples confirmed the formation of ZrTiO4 phase in the ZrO2-TiO2 mixed oxides. TEM micrographs showed that nanosized gold particles on the catalyst had an average diameter smaller than 5nm. Metallic gold (Au0) and oxidized Au species (Aunδ+) on the surface of the catalysts were evidenced by UV–vis and XPS characterization. In the partial oxidation of methanol (POM) reaction, among the six catalysts, the high metallic Au0/Au+ ratio and low surface acidity in the Au/ZrO2 catalysts are chiefly responsible for the highest hydrogen production rate in the whole temperature range between 210 and 300°C. Methanol decomposition as secondary reaction was favored on TiO2-based catalysts at higher temperature, producing a large amount of CO. Formation of ZrO2-TiO2 solid solution resulted in generation of both Brønsted and Lewis acid sites; as a result, dehydrogenation and oxidative dehydrogenation of methanol was allowed over Au/ZrO2-TiO2 catalysts.

ChemInform Abstract: Hybrid Density Functionals Applied to Complex Solid Catalysts: Successes, Limitations, and Prospects

AbstractReview: 338 refs.

Hybrid Density Functionals Applied to Complex Solid Catalysts: Successes, Limitations, and Prospects

Density functional theory (DFT), employing semilocal approximations to describe electron exchange and correlation effects, tremendously advanced the research in the realm of computational catalysis. It allows to calculate atomic and electronic structure details of extended systems like bulk solids, surfaces or nanoparticles with reasonable accuracy at moderate computational cost. However, semilocal approximations suffer from shortcomings such as self-interaction errors (SIEs). This work discusses results obtained using two established and related approaches, namely DFT + U and orbital-dependent hybrid density functionals. Both methods partially alleviate some of the problems incurred by SIEs and are widely used in the computational community. We discuss four case studies involving reducible oxide materials: (i) the oxidative dehydrogenation of methanol at small vanadium oxide clusters supported on the CeO2(111) surface, (ii) the adsorption of Au atoms on the reduced CeO2(111) surface, (iii) stabilities of various terminations of the V2O3(0001) surface, and (iv) the adsorption of water on the Fe3O4(111) surface. Compared with semilocal functionals including DFT + U, we report substantial improvements in band gaps, defect formation energies, as well as activation barriers and emphasize the important role of state-of-the-art experiments for assessing DFT. Limitations of hybrid functionals due to the imposed computational workload and inherent functional approximations are discussed. To overcome these limitations, alternatives in terms of generalized RPA and embedded wavefunction-based methods are suggested.

Water co-catalyzed selective dehydrogenation of methanol to formaldehyde and hydrogen

The non-oxidative dehydrogenation of methanol to formaldehyde is considered a promising method to produce formaldehyde and clean hydrogen gas. Although Cu-based catalysts have an excellent catalytic activity in the oxidative dehydrogenation of methanol, metallic Cu is commonly believed to be unreactive for the dehydrogenation of methanol in the absence of oxygen adatoms or oxidized copper. Herein we show that metallic Cu can catalyze the dehydrogenation of methanol in the absence of oxygen adatoms by using water as a co-catalyst both under realistic reaction conditions using silica-supported PtCu nanoparticles in a flow reactor system at temperatures below 250°C, and in ultra-high vacuum using model PtCu(111) catalysts. Adding small amounts of isolated Pt atoms into the Cu surface to form PtCu single atom alloys (SAAs) greatly enhances the dehydrogenation activity of Cu. Under the same reaction conditions, the yields of formaldehyde from PtCu SAA nanoparticles are more than one order of magnitude higher than on the Cu nanoparticles, indicating a significant promotional effect of individual, isolated Pt atoms. Moreover, this study also shows the unexpected role of water in the activation of methanol. Water, a catalyst for methanol dehydrogenation at low temperatures, becomes a reactant in the methanol steam reforming reactions only at higher temperatures over the same metal catalyst.

Investigation of the Structure and Active Sites of TiO2 Nanorod Supported VOx Catalysts by High-Field and Fast-Spinning 51V MAS NMR

Supported VOx/TiO2-rod catalysts were studied by 51V MAS NMR at high field using a sample spinning rate of 55 kHz. The superior spectral resolution allows for the observation of at least five vanadate species. The assignment of these vanadate species was carried out by quantum chemical calculations of 51V NMR chemical shifts of model V surface structures. Methanol oxidative dehydrogenation (ODH) was used to establish a correlation between catalytic activity and the various surface V sites. It is found that monomeric V species are predominant at low vanadium loadings with two 51V NMR peaks observed at about −502 and −529 ppm. V dimers with two bridged oxygens result in a peak at about −555 ppm. Vanadate dimers and polyvanadates connected by one bridged oxygen atom between two adjacent V atoms resonate at about −630 ppm. A positive correlation is found between the V dimers, giving rise to the −555 ppm peak, and the ODH rate, and an even better correlation is obtained by including V monomer contributions. This result suggests that surface V dimers related to the −555 ppm peak and monomers are the primary active sites for the methanol ODH reaction. Furthermore, a portion of the V species is found to be invisible to NMR and the level of such invisibility increases with decreasing V loading levels, suggesting the existence of paramagnetic V species at the surface. These paramagnetic V species are also found to be much less active in methanol ODH.

Catalytic Consequences of Chemisorbed Oxygen during Methanol Oxidative Dehydrogenation on Pd Clusters

Surface oxygen contents and thermodynamic activities of nanosized Pd clusters and their connection to the reactivities for methanol oxidative dehydrogenation are established from rate measurements in the kinetically controlled regime and oxygen uptake studies at chemical equilibrium. First-order rate coefficients for methanol oxidative dehydrogenation (turnover rates divided by CH3OH pressure) are single-valued functions of the oxygen-to-methanol ratio in the contacting gas phase because this ratio determines the oxygen coverages, the relative abundance of chemisorbed oxygen and unoccupied Pd sites, and in turn, the identities of the kinetically relevant steps at Pd cluster surfaces. As the oxygen-to-methanol ratio increases, the most abundant surface intermediates on Pd clusters vary from uncovered to saturated with chemisorbed oxygen; in response to this shift in coverages, the kinetically relevant step concomitantly varies from oxygen dissociation, to CH3OH activation on oxygen adatom and oxygen vacancy pairs (O*–*), and then to CH3OH activation on oxygen adatom pairs (O*–O*), during which the first-order rate coefficients initially increase and then reach a maximum value before decreasing to a constant value that does not vary with the oxygen-to-methanol ratio. These dependencies reflect the dual catalytic functionality of chemisorbed oxygen, first promoting the methanol conversion as the oxygen coverages increase and then inhibiting the reaction at near O* saturation, as oxygen displaces the unoccupied Pd sites, thus removing the O*–* centers while replacing them with less reactive O*–O* centers for CH3OH activation. Methanol acts as a surface oxygen scavenger that reduces the oxygen chemical potential at Pd cluster surfaces during its catalytic turnovers. The oxygen-scavenging step by methanol increases with temperature to a larger extent than the O2 activation step. As a result of more effective oxygen removal (by reactions) at higher temperatures, the critical oxygen coverage required for the transition of regimes from *–* to O*–* as the predominant surface sites occurs at higher oxygen-to-methanol ratios. Larger Pd clusters are more reactive for methanol oxidative dehydrogenation than smaller clusters in all regimes because cluster dimension influences the relative abundance of chemisorbed oxygen and unoccupied Pd sites, the oxygen binding strengths, and their reactivities. The direct connection of first-order rate coefficient and oxidant-to-reductant ratio appears to be general for methanol oxidative dehydrogenation on other transition metals and for oxidation catalysis, for which the reductant scavenges the chemisorbed oxygen effectively, thus dictating their coverages and thermodynamic activities.

Methanol Oxidative Dehydrogenation on Oxide Catalysts: Molecular and Dissociative Routes and Hydrogen Addition Energies as Descriptors of Reactivity

The oxidative dehydrogenation (ODH) of alkanols on oxide catalysts is generally described as involving H-abstraction from alkoxy species formed via O–H dissociation. Kinetic and isotopic data cannot discern between such routes and those involving kinetically-relevant H-abstraction from undissociated alkanols. Here, we combine such experiments with theoretical estimates of activation energies and entropies to show that the latter molecular routes prevail over dissociative routes for methanol reactions on polyoxometalate (POM) clusters at all practical reaction temperatures. The stability of the late transition states that mediate H-abstraction depend predominantly on the stability of the O–H bond formed, making H-addition energies (HAE) accurate and single-valued descriptors of reactivity. Density functional theory-derived activation energies depend linearly on HAE values at each O-atom location on clusters with a range of composition (H3PMo12, H4SiMo12, H3PW12, H4PV1Mo11, and H4PV1W11); both barriers and ...

Reactions of Methanol with Pristine and Defective Ceria (111) Surfaces: A Comparison of Density Functionals

We study the dissociative adsorption and oxidative dehydrogenation of methanol at the pristine and O-defective ceria (111) surfaces to understand the role of surface oxygen vacancies. The accuracy of two density functional theory based approaches (PBE+U and the HSE hybrid functional) is assessed on available experimental data. In addition, the impact of dispersion correction and zero-point vibrational energy contributions is discussed. Calculated vibrational frequencies are compared with experimental IR spectra. Using dispersion-corrected PBE+U, we obtain comparably large intrinsic barriers for the oxidation step at the pristine (104 kJ/mol) and defective (119 kJ/mol) ceria surfaces. Compared to HSE+D, these barriers are underestimated by 20 kJ/mol. Adsorption energies for the pristine surface agree well between PBE+U+D (−88 kJ/mol) and HSE+D (−86 kJ/mol). However, adsorption energies for the defective surface vary by 25 kJ/mol (PBE+U+D: −231 kJ/mol; HSE+D: −206 kJ/mol). Nonetheless, adsorption into surfa...

Effects of CeO2 support facets on VOx/CeO2 catalysts in oxidative dehydrogenation of methanol

CeO2 supports with dominating facets, i.e., low index (100), (110) and (111) facets, are prepared. The facet effects on the structure and catalytic performance of supported vanadium oxide catalysts are investigated using oxidative dehydrogenation of methanol as a model reaction. In the presence of mixed facets, Infrared and Raman characterizations demonstrate that surface vanadia species preferentially deposit on CeO2 (100) facets, presumably because of its higher surface energy. At the same surface vanadium densities, VOx species on (100) facets show better dispersion, followed by (110) and (111) facets. The VOx species on CeO2 nanorods with (110) and (100) facets display higher activity and lower apparent activation energies compared to that on CeO2 nanopolyhedras with dominating (111) facets and CeO2 nanocubes with dominating (100) facets. The higher activity for VOx/CeO2 (110) might be related to the more abundant oxygen vacancies present on the (110) facets, evidenced from Raman spectroscopic measurements.

Effect of the composition and structure of the surface layer on the functional properties of a core(Al2O3)-Shell(VO x /TiO y ) composite

Molecular layering method was used to synthesize vanadium, titanium, and vanadium-titanium oxide structures of various compositions and structures on the surface of γ-Al2O3. The systems obtained and the starting aluminum oxide were identified by chemical-analytical and adsorption methods, X-ray diffraction analysis, and X-ray photoelectron spectroscopy. The catalytic properties of the products obtained were examined for the example of oxidative dehydrogenation of methanol. It is shown that the reactivity of the samples is affected not only by the concentration of catalytically active vanadium oxide structures, but also by their remoteness from the surface of the starting matrix, controlled by the thickness of the titanium oxide sublayer.