Fluctuating Storage of the Active Phase in a Mn‐Na2WO4/SiO2 Catalyst for the Oxidative Coupling of Methane

Structural dynamics of a Mn‐Na₂WO₄/SiO₂ catalyst were detected directly under reaction conditions during the oxidative coupling of methane via in situ XRD and operando Raman spectroscopy. A new concept of fluctuating storage and release of an active phase in heterogeneous catalysis is proposed that involves the transient generation of active sodium oxide species via a reversible reaction of Na₂WO₄ with Mn₇SiO₁₂. The process is enabled by phase transitions and melting at the high reaction temperatures that are typically applied.     

Effect of pretreatment gases on the performance of WO3/SiO2 catalysts in the metathesis of 1-butene and ethene to propene

Different gases were employed to pretreat WO₃/SiO₂ for the metathesis of 1-butene and ethene to propene. Air-pretreated WO₃/SiO₂ was inactive, whereas N₂-, N₂/H₂-, and H₂-pretreated WO₃/SiO₂ exhibited high 1-butene conversion and propene selectivity. Tetragonal WO₃ and partially reduced WO_2.92 were found to be the active phases/species, whereas monoclinic WO₃ was inactive. N₂/H₂- and H₂-pretreated WO₃/SiO₂ contained both W⁶⁺ and W⁵⁺ species (i.e., nonstoichiometric WO_3−x). Air- and N₂-pretreated WO₃/SiO₂ only contained monoclinic W⁶⁺ species (WO₃) before the reaction. However, the used N₂-pretreated WO₃/SiO₂ contained both W⁶⁺ and W⁵⁺ species, suggesting that the N₂ pretreatment can facilitate the formation of some W⁵⁺ species during the reaction. This in situ partial reduction can activate the N₂-pretreated WO₃/SiO₂ for the metathesis.     

An investigation on the assembling of WO3 particles on the matrix of silica solution

A facile way to prepare modified WO₃ structure by silica support through sol–gel method is reported. The WO₃/SiO₂ complex film was synthesized from a two steps process and dip-coating method. The films were characterized with laser particle analyzer, IR, Raman. The studies of gelation time and particle size distribution of WO₃/SiO₂ sol indicate that the silica addition could largely reduce the polycondensation of WO₃ clusters. The reaction between WO₃ and SiO₂ were further systematically investigated using IR spectra, and an insight of this reaction was illuminated. Results reveal that Si–OH in SiO₂ sols tended to crosslink with WO₃ at the corner-sharing W–O sites, by which only edge-sharing WO_x clusters could be detected. This modified the WO₃ structure was also approved by the Raman spectra, TEM and AFM images. Moreover, gas sensing properties of the WO₃/SiO₂ films were tested. The assembled WO₃ films exhibited more stable gas sensing stability than pure WO₃ films.     

NaOH改性WO3/SiO2催化剂催化2-丁烯和乙烯复分解制丙烯SurasaMaksasithorn,DamienP.Debecker,PiyasanPraserthdam,JoongjaiPanpranot,KongkiatSuriye,SirachayaKunjaraNaAyudhya简

A WO₃/SiO₂ catalyst is used in industry to produce propylene from 2-butene and ethylene metathesis. Catalysts with various WO₃ loading (4% to 10%) were prepared by impregnation and tested for the metathesis of ethene and trans-2-butene. Ion exchange of NaOH onto the WO₃/SiO₂ catalyst was used to mitigate the acidity of the catalysts in a controlled way. At low WO₃ loading, the treatment with large amounts of NaOH resulted in a significant decrease in metathesis activity concomitant with significant W leaching and marked structural changes (XRD, Raman). At higher WO₃ loading (6% to 10%), the treatment with NaOH mainly resulted in a decrease in acidity. FT-IR experiments after adsorption of pyridine showed that the Lewis acidic sites were poisoned by sodium. Nevertheless, the metathesis activity remained constant after the NaOH treatment. This suggested that the remaining acidity on the catalyst was enough to ensure the efficient formation of the carbene active sites. Interestingly, Na poisoning resulted in some modification of the selectivity. The mitigation of acidity was shown to favor propene selectivity over the formation of isomerization products (cis-2-butene, 1-butene, etc.). Moreover, treatment with NaOH led to a shorter induction period and reduced coke formation on the WO₃/SiO₂ catalyst.     

WO3–SiO2 nanomaterials synthesized using a novel template-free method in supercritical CO2 as heterogeneous catalysts for epoxidation with H2O2

A series of tungsten oxide-silica (WO₃–SiO₂) composite nanomaterials were synthesized through a novel, template-free sol-gel method, in which supercritical-CO₂ (scCO₂) was utilized as synthesis medium. The efficacy of the synthesis method stems from a tailored reactor design that allows the contact of the reactants only in the presence of scCO₂. Selected synthetic parameters were screened with the purpose of enhancing the performance of the resulting materials as heterogeneous catalysts in epoxidation reactions with H₂O₂ as environmentally friendly oxidant. A cyclooctene conversion of 73% with epoxide selectivity of > 99% was achieved over the best WO₃–SiO₂ catalyst under mild reaction conditions (80 °C), equimolar H₂O₂ amount (1:1) and low WO₃ loading (~2.5 wt%). The turnover number achieved with this catalyst (TON = 328), is significantly higher than that of a WO₃–SiO₂ prepared via a similar sol-gel route but without supercritical CO₂, and that of commercial WO₃. A thorough characterization with a combination of techniques (ICP-OES, N₂-physisorption, XRD, TEM, STEM-EDX, SEM-EDX, FT-IR and Raman spectroscopy, XPS, TGA and FT-IR analysis of adsorbed pyridine) allowed correlating the physicochemical properties of the WO₃–SiO₂ nanomaterials with their catalytic performance. The high catalytic activity was attributed to: (i) the very high surface area (892 m²/g) and (ii) good dispersion of the W species acting as Lewis acid sites, which were both brought about by the synthesis in supercritical CO₂, and (iii) the relatively low hydrophilicity, which was tuned by optimizing the tetramethyl orthosilicate concentration and the amount of basic solution used in the synthesis of the materials. Our optimum catalyst was also tested in the reaction of cyclohexene with H₂O₂, resulting in cyclohexane diol as main product due to the presence of strong Brønsted acid sites in the catalyst, whereas the reaction with limonene yielded the internal epoxide as the major product and the corresponding diol as side product. Importantly, the catalyst did not show leaching and could be reused in five consecutive runs without any decrease in activity.     

Effect of tungsten oxide loading on metathesis activity of ethene and 2-butene over WO<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> catalysts

The metathesis of ethene and 2-butene to propene was studied over WO₃/SiO₂ catalysts with various WO₃ loadings (2, 4, 8, 12, 16, and 24 wt%). The 2-butene conversion and propene selectivity increased greatly with WO₃ loading increasing from 2 to 8 wt%, reached maximum at 8–12 wt% WO₃ loading, and then decreased when the WO₃ loading was higher than 12 wt%. From the above results and taking the economics into account, the optimal amount of WO₃ loading was ~8 wt%. The catalysts were characterized by physico-chemical and spectroscopic techniques to elucidate the effect of different tungsten oxide loadings on the metathesis reactivity of ethene and 2-butene. The characterization data indicated that three types of tungsten species (i.e., surface tetrahedral tungsten species, surface octahedral polytungstate species, and WO₃ crystallites) were present in the catalysts. It was found that WO₃ was not the active centers, and surface tetrahedral tungsten species might be more active than octahedral polytungstate species in metathesis reaction. The reduced form of tungsten species [W⁺⁴, W⁺⁵, and W^+(6−y) (0 < y < 1)] may be the suitable state of W species acting as metathesis active centers.     

WO_3/Bi_(12)SiO_(20)光催化降解气相苯

用气液反应法和化学溶液分解技术(CSD)分别制备了WO3和Bi12SiO20粉末,并将二者耦合,合成出WO3/Bi12SiO20复合光催化剂.以气相苯的降解为探针反应,考查了催化剂的光催化活性.结果表明:耦合后的WO3/Bi12SiO20催化剂的催化活性显著提高,其中30%(w)WO3/Bi12SiO20在紫外光下对苯的降解率明显优于P-25,而且催化剂具有一定的可见光响应能力.采用X射线衍射(XRD)、扫描电子显微镜(SEM)、X射线光电子能谱(XPS)、N2吸附-脱附(BET)和紫外-可见漫反射(UV-VisDRS)等手段对催化剂进行了表征.结果表明:WO3与Bi12SiO20之间存在良好的能带协同作用.WO3与Bi12SiO20耦合后,催化剂的光响应范围拓宽,光生电子和空穴能有效地分离,光生电子和空穴产生速率增大,所以催化剂活性提高.     

Electrochemistry of solid-phase reactions: Phase formation in the In2O3-WO3 system. Processes at the WO3|In2O3 and WO3|In6WO12 interfaces

Reactions of interactions at the WO₃|In₂O₃ and WO₃|In₆WO₁₂ heterophase reaction interfaces, whose main product is In₂(WO₄)₃, are studied by electrochemical methods for the first time ever. Due to a far greater n type conductance inherent in the initial substances, the reactions are a model object for the development of methodology of the electrochemical approach. Both reactions are discovered to proceed at the expense of the transport of components of WO₃ and no evidence is discovered for the contribution of In³⁺ into diffusion and migration. Consisted data are obtained between the polarity of a spontaneously generated reaction difference of potentials and the direction of the field that accelerates the reaction: the current that is passed through electrochemical cells accelerates the reactions exclusively at the (−)-potential of a brick of WO₃. A difference is discovered between the charge and mass transport paths—spontaneous and field-induced mass transport of WO₃ or its components occurs via heterophase interfaces and adjacent areas and does not touch upon the In₂(WO₄)₃ grains. Shown is the antibatic character of the behavior exhibited by dependences of identical properties of cells (potential drop across a cell) following a change in the dc polarity. A possible role of a reactionless electrosurface transport of WO₃ in the mechanism of reaction and evolution of electrochemical properties of model electrochemical cells is demonstrated. The obtained data may or may not testify in favor of a hypothesis that presumes a prevailing role of the {WO₄}²⁻ mobility in the In₂(WO₄)₃ structure. Original Russian Text ? A.Ya. Neiman, T.E. Kulikova, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 6, pp. 714–726. Based on the report delivered at the 8th International Meeting on Fundamental Problems of Solid-State Ionics, Chernogolovka (Russia), 2006.     

Temperature-dependent Raman spectroscopy studies of phase transformations in the K2WO4 and the MgMoO4 crystals

Temperature-dependent Raman spectroscopy studies of K₂WO₄ and MgMoO₄ polycrystals were performed in order to obtain information about vibrational and structural changes in these materials as a function of temperature. The stability of the monoclinic phase for both K₂WO₄ and MgMoO₄ samples was assessed and our results indicated that this phase is stable in the 295–723 K and 300–770 K ranges for K₂WO₄ and MgMoO₄, respectively. It was observed that both samples underwent two phase transformations above room temperature. The first phase transformations which occur at about 633 K and 640 K for K₂WO₄ and MgMoO₄, respectively, is most likely connected with weak tilting and/or rotations of WO₄/MoO₄ tetrahedral units that lead to a disorder in the oxygen sublattice. Raman spectroscopy data also indicated that K₂WO₄ and MgMoO₄ exhibited a first-order phase transition at around 723 K and 770 K, respectively, changing from monoclinic to hexagonal symmetry.     

Effect of oxygen activity in the gaseous phase on the mechanism of reactions in the solid-state synthesis of In<Subscript>2</Subscript>(WO<Subscript>4</Subscript>)<Subscript>3</Subscript> and In<Subscript>6</Subscript>WO<Subscript>12</Subscript>

The effect of oxygen’s activity on the rate of In₂(WO₄)₃ and In₆WO₁₂ formation reactions was studied to determine the reaction mass transfer mechanism. It was established that the formation of In₂(WO₄)₃ in a model reaction cell is due to the transfer of WO ₄ ^2? components and electrons moving in opposite directions through the reaction product. The relation between the diffusion coefficients of the carriers was found. The rate of electron diffusion and the reaction rate were shown to vary according to the law \(K_p \approx D_{\lim } = D_e \sim a_{O_2 }^{ - 1/4} \). We conclude that the formation of electronic conductor In₆WO₁₂ is a two-region process: at the In₂(WO₄)₃ | In₆WO₁₂ interface, the product is formed on the In₂(WO₄)₃ surface due to {WO₃} escaping toward In₂O₃, and at the In₆WO₁₂ | In₂O₃ interface, the product is formed on the In₂O₃ surface via the reaction of diffuse {WO₃} with In₂O₃. The probable relationship between the diffusion coefficients of the In₆WO₁₂ components was obtained. A relation was developed for the process rate. The diffusion coefficients for the limiting component were calculated using the data on the estimated thickness of the product layers.     

煅烧温度对花朵状钨酸铋光催化活性的影响

钨酸铋在水相体系中能光催化降解有机污染物,而对于钨酸铋光催化活性影响因素的研究非常少.本文研究了煅烧温度对花朵状钨酸铋光催化活性的影响.以钨酸钠和硝酸铋为原料,在160°C下水热反应20 h,合成催化剂,并经不同温度煅烧3 h.用X射线衍射(XRD)、拉曼(Raman)光谱、紫外-可见漫反射吸收光谱(UVVis DRS)和荧光(PL)光谱表征样品.结果表明,这些样品具有十分相似的相组成和电子结构.然而,对于水中苯酚的降解,钨酸铋的光催化活性显示出了很大的差异.钨酸铋的光催化活性随着煅烧温度的升高,先上升后下降,最优煅烧温度为350°C,并且不受样品比表面积大小的影响.这些样品的活性差异主要归结于钨酸铋的结晶度、吸光性和表面缺陷对其光生载流子分离效率的综合影响.     

Calorimetric study of high-pressure polymorphs of Li2WO4 and Li2MoO4

Heats of transition among the Li₂WO₄ polymorphs, Li₂WO₄I (phenacite-type structure), Li₂WO₄II, Li₂ WO₄III, and Li₂WO₄IV, and that between Li₂MoO₄ (phenacite) and Li₂MoO₄(spinel) were measured by transposed temperature drop calorimetry. The heats of fusion of Li₂WO₄I and Li₂MoO₄(ph) were also obtained. Using these data, the phase boundaries among the polymorphs of Li₂WO₄ and of Li₂MoO₄ were calculated. The calculated phase diagrams were compared with those reported previously. They agree well for Li₂WO₄ but show significant discrepancies, perhaps related to problems in attaining equilibrium at lower temperature, for Li₂MoO₄.     

Catalytic Performance of Amorphous Ti/SiO2 in the Gas-Phase Epoxidation of Propylene with H2O2

The epoxidation of propylene with dilute H₂O₂ aqueous solution over titanium silicalite-1 (TS-1) zeolite catalyst is a green chemical reaction for propylene oxide (PO) production. Carrying out the reaction in gas-phase can get rid of problems caused by using methanol solvent. This paper reports an attempt of using non-zeolite catalyst for the gas-phase epoxidation. Amorphous Ti/SiO₂, obtained by grafting amorphous SiO₂ with TCl₄ in ethanol solvent in a chemical liquid-phase deposition (CLD) process, has been used as the catalyst. Results show that the CLD Ti/SiO₂ with appropriate Si/Ti molar ratio is an active catalyst for gas-phase epoxidation, achieving 9.8 % propylene conversion and 66.9 % PO selectivity with 40.3 % H₂O₂ utilization, which indicates that this amorphous Ti/SiO₂ catalyst deserves extensive studies in the future.     

Vapor Phase Synthesis of 3-Methylindole over a Ag/SiO2 Catalyst

Ag/SiO₂ was used as an efficient catalyst for the vapor phase synthesis of 3-methylindole from aniline and 1,2-propanediol at atmospheric pressure. When Ag/SiO₂ was calcined at 500 °C for 4 h and the molar ratio of H₂O to H₂ was 0.750 in the reaction, the yield of 3-methylindole could be up to 35%. X-ray diffraction characterization showed that the Ag/SiO₂ catalyst with silver crystallites was active for the synthesis of 3-methylindole.     

Growth mechanism and photocatalytic properties of flower-like Bi2WO6 powders synthesized by a microwave hydrothermal method

Pure orthorhombic phase Bi₂WO₆ powders were synthesized by a microwave hydrothermal method in the absence of surfactants and templates, using Bi(NO₃)₃·5H₂O and Na₂WO₄·2H₂O as raw materials. Photocatalytic properties of the samples prepared at different reaction temperatures were also studied with Rhodamine B (RhB) solution as the target catabolite under visible light. The results indicate that flower-like Bi₂WO₆ powders can be obtained by controlling the microwave reaction temperatures in the absence of any additives. The growth of flower-like Bi₂WO₆ powders is a multistage layer assembly process, in which the flower-like Bi₂WO₆ self-assembling with the uniform size about 2 μm is synthesized at 180 °C. At the same time, the photocatalytic reaction rate constant (k) gets up to 0.04167/min and the degradation rate of RhB solution is more than 96 % after being irradiated under visible light for 70 min.     

Construction of SiO2-TiO2/g-C3N4 composite photocatalyst for hydrogen production and pollutant degradation: Insight into the effect of SiO2

Using low-cost precipitated silica (SiO₂) as the carrier, a ternary SiO₂-TiO₂/g-C₃N₄ composite photocatalyst was prepared via the sol-gel method associated with a wet-grinding process. The as-prepared composite exhibits photocatalytic hydrogen production and pollutant degradation performance under solar-like irradiation. The effect of SiO₂ carrier on the properties of the heterostructure between TiO₂ and g-C₃N₄ (CN) was systematically studied. It is found that SiO₂ has important effects on promoting the interaction between TiO₂ and CN. The particle size of TiO₂ and CN was obviously reduced during the calcination process due to the effects of SiO₂. Especially, the TiO₂ particles exhibit monodispersed state with particle size below 10 nm (quantum dots), resulting in the improvement of the contact area and the interaction between TiO₂ and CN, and leading to the formation of efficient TiO₂/CN Z-scheme heterostructure in SiO₂-TiO₂/CN. Besides, the introduction of SiO₂ can increase the specific surface area and light absorption of SiO₂-TiO₂/CN, further promoting the photocatalytic reaction. As expected, the optimum SiO₂-TiO₂/CN composite exhibits 12.3, 3.1 and 2.9 times higher photocatalytic hydrogen production rate than that of SiO₂-TiO₂, CN and TiO₂/CN under solar-like irradiation, while the photocatalytic active component in SiO₂-TiO₂/CN is only about 60 wt%. Moreover, the rhodamine B degradation rate of SiO₂-TiO₂/CN is also higher than that of SiO₂-TiO₂, CN and TiO₂/CN.     

Spectral study of catalysts for the oxidative condensation of methane

Methods for preparing catalysts for the oxidative condensation of methane based on a metal support (FeCrAl foil) containing Al₂O₃ and SiO₂ and active ingredients (Na₂WO₄, Mn₂O₃, and PbO) are developed. The prepared catalysts are studied by XRD, XPS, and EM.     

A combined computational and experimental study of methane activation during oxidative coupling of methane (OCM) by surface metal oxide catalysts

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na₂WO₄ catalytic active sites for CH₄ heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO₄ catalytic active sites resulting in surface WO₅ sites with retarded kinetics for C–H scission. On the other hand, dimeric Mn₂O₅ surface sites, identified and studied via ab initio molecular dynamics and thermodynamics, were found to be more efficient in activating CH₄ than the poisoned surface WO₅ sites or the original WO₄ sites. However, the surface reaction intermediates formed from CH₄ activation over the Mn₂O₅ surface sites are more stable than those formed over the Na₂WO₄ surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO_x, in agreement with the experimental findings in the literature on Mn-promoted catalysts. Consequently, the Mn-promoter does not appear to have an essential positive role in synergistically tuning the structure of the Na₂WO₄ surface sites towards CH₄ activation but can yield MnO_x surface sites that activate CH₄ faster than Na₂WO₄ surface sites, but unselectively.

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not necessarily enhance the activity of the surface Na₂WO₄ catalytic active sites for CH₄ heterolytic dissociation during OCM.     

(NH4)2SiF6 evaporation in the presence of SiO2

The kinetics and mechanism of the solid-phase reaction between (NH₄)₂SiF₆ and silica at molar ratios of (0.5–5): 1 were studied. SiO₂ is bound with (NH₄)₂SiF₆ to form volatile NH₄SiOF₃, which abruptly enhances the ammonium hexafluorosilicate evaporation. The activation energy and rate constants of evaporation were calculated for an (NH₄)₂SiF₆ + SiO₂ (2: 1) mixture in the range 220–300°C. The reaction with crystalline SiO₂ has a higher yield than with an amorphous “white soot.” The role of the SiO₂ surface in the formation of the volatile products is discussed. The phase and chemical composition of the sublimates was studied.     

Untersuchung der thermischen Zersetzung von Aluminium-dodekawolframatosilicat

Investigation on the Thermal Degradation of Aluminium-12-Tungstosilicate The dehydration of aluminium-12-tungstosilicate AlH[SiO₄W₁₂O₃₆] · 29 H₂O gives the anhydrous salt at 440°C. By means of X-ray heating patterns, thermal analysis, and i.r. spectroscopy the formation of the new phase 1/2 Al₂O₃ · SiO₂ · 12 WO₃ (I) at 500°C is observed, stable at room temperature. Above 800°C from I tetragonal W₃, Al₂(WO₄)₃, and amorphous SiO₂ are formed. Amorphous SiO₂ crystallizes to high-temperature cristobalite at 1000°C. High-resolution ²⁷Al NMR (MAS-technique) is used to determine the coordination number of aluminium in the different phases.