Selective photooxidation of methane into methanol by nitric oxide over V-MCM-41 mesoporous molecular sieves

The partial photooxidation of methane by nitric oxide selectively led to methanol formation over the V-MCM-41 catalysts under UV irradiation ( > 270 nm) at room temperature, while only a complete oxidation of methane could occur in the presence of oxygen. The yield of methanol corresponds with the yield of the photoluminescence of the isolated tetrahedrally coordinated V-oxides species, indicating that the charge transfer excited triplet state of these species are active sites in this photooxidation reaction.     

Weak base favoring the synthesis of highly ordered V-MCM-41 with well-dispersed vanadium and the catalytic performances on selective oxidation of benzyl alcohol

The key to improve the performance of heteroatom catalysts is to ensure the orderliness of catalysts and the good dispersion of heteroatoms. The alkalinity plays the indispensable role in synthetic process of V-MCM-41 catalyst.The excessive alkalinity of synthetic system will make the MCM-41 difficult to crystallize, even to dissolve. It is easy to accumulate for heteroatomic species in the system of low alkalinity. Herein, the highly ordered VMCM-41 with high vanadic content in framework is synthesized in the condition of excessive NH_3·H_2 O in this paper. A series of characterization results prove the good dispersion of vanadium species, and most of vanadium gets into the framework of MCM-41 with the states of tetravalence and pentavalence. Furthermore, the modified MCM-41 by other transition metals is successful synthesized by the method of V-MCM-41 in this paper. The VMCM-41 shows well catalytic activity for the selective oxidation of benzyl alcohol, which up to 74.83% for the conversion of benzyl alcohol and 96.20% for selectivity of benzaldehyde when initial V/Si = 0.10. The paper provides the possibility for industrial application of V-MCM-41 in the oxidation of benzyl alcohol for benzaldehyde.Besides, the work provides a significant idea for the synthesis of modified MCM-41 by well-dispersed transition metals.     

Synthesis of V-MCM-41 by template-ion exchange method and its catalytic properties in propane oxidative dehydrogenation

Vanadium has been introduced to MCM-41 without collapse of the mesoporous structure by exchanging VO²+ ions in the aqueous solution with the template cations in the uncalcined MCM-41. This template-ion exchange (TIE) method provides tetrahedrally coordinated vanadyl species dispersed in the channel of MCM-41. Such synthesized V-MCM-41 shows higher catalytic activity in the oxidative dehydrogenation of propane than that prepared by direct hydrothermal method.     

The C-radioisotopic study of methanol conversion on V-MCM-41: The influence of methyl iodide on the transformation

The methanol conversion and a feasible methanol co-reaction with methyl iodide were studied on a vanadium incorporated MCM-41 type (V-MCM-41) mesoporous catalyst, which was prepared by a direct hydrothermal synthesis method. Adsorption/desorption as well as conversion derivates of radioactive methanol were easily followed by radioactivity detectors on V-MCM-41. The transformation and co-reaction products were analyzed by a gas chromatograph equipped with Radio/FID detectors. The radiodetector was applied to distinguish ¹¹C-derivates from the non-radioactive methyl iodide and its derivates. The radio-labeling method proved methanol transformation to methylal and, in the presence of methyl iodide, decided the roles of methyl group and iodide of methyl iodide in newly synthesized methyl iodide formation in the absence of oxygen gas.     

Heterogeneously Catalysed Aldol Reactions in Supercritical Carbon Dioxide as Innovative and Non-Flammable Reaction Medium

Aldol reactions of several aldehydes have been investigated over acidic and basic catalysts in supercritical carbon dioxide at 180 bar and 100 °C. Both acidic (Amberlyst-15, tungstosilicic acid (TSA) on SiO₂ and MCM-41) and basic (hydrotalcite) materials showed interesting performance in this preliminary study under the entitled reaction conditions. Small and linear aldehydes, such as propanal, butanal, pentanal and hexanal, react more efficiently than the branched 3-methylbutanal, which is converted much slower. Whereas Amberlyst-15 showed the highest conversion based on the catalyst mass, tungstosilicic acid-based catalysts were significantly better if the rates were related to the number of acidic sites (>1000 h^?1). The rate depends both on the dispersion and the kind of support. Strikingly, tungstosilicic acid (TSA) on MCM-41 was also an effective catalysts for the selective C=C double bond hydrogenation of 2-butenal and is therefore a potential catalyst for the ??one-pot?? synthesis of 2-ethyl-2-hexenal and 2-ethylhexanal via combined hydrogenation and aldol reaction from 2-butenal. A number of characterisation techniques, such as temperature-programmed desorption of ammonia (NH₃-TPD), transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), etc. were used to get an insight into the catalyst structure, which support a high dispersion and strong acidity of the tungsten based species on silica and MCM-41.     

V-MCM-41催化环己烯环氧化性能

采用水热法合成了V-MCM-41,并用ICP-AES、FTIR、DRS-UV-Vis、XRD、BET等技术对其进行了表征,发现只有部分V进入分子筛,所合成的V-MCM-41具有较高的结晶度,并且其结晶度随晶化液中V/Si摩尔比的增大而升高。V-MCM-41催化环己烯/H₂O₂氧化反应结果表明,当V-MCM-41中V含量大于1.10%（摩尔分数）时,骨架V-O-V物种在催化环己烯氧化反应的同时,也可加速H₂O₂的分解;乙腈是该反应体系的最佳溶剂,最佳反应温度为70℃。最佳反应结果表明,环己烯的单程转化率和H₂O₂有效利用率可分别达到23.91%和95.4%。     

Vanadium-containing ordered mesoporous silicas: Synthesis, characterization and catalytic activity in the hydroxylation of biphenyl

A series of vanadium-containing ordered mesoporous MCM-41 materials (V-OMS) have been synthesized by direct hydrothermal (V-MCM-41) and grafting (V/MCM-41) methods using hexadecyl trimethyl ammonium bromide (HDTMABr) as the structure-directing agent. The physico-chemical properties of the vanadium-containing materials were characterized in detail by ICP-OES, XRD, FT-IR, N₂ adsorption–desorption, DRUV-VIS, TPR, XPS and SEM techniques. The redox performances of the vanadium-modified mesoporous materials were tested in the hydroxylation of biphenyl using aqueous H₂O₂ (30 wt.%) as oxidant. For a better exploitation of the catalytic activity, the reaction parameters are optimized in terms of temperature, solvent, oxidant, etc. A comparison between the catalytic activity values of the vanadium-containing mesoporous materials prepared by the two routes shows that vanadium-substituted (V-MCM-41) materials had increased activity and improved selectivity for mono hydroxyl products in the hydroxylation reaction of biphenyl compared to the V/MCM-41 catalysts. The heterogenity of the catalysts was verified by a series of leaching studies. Both the catalysts enhance the leaching of active vanadia species during the reaction; among them, V/MCM-41 shows the least heterogenity.     

Catalytic behavior of vanadium substituted mesoporous molecular sieves

A series of V-MCM-41 samples with different pore sizes has been systematically investigated with both gas and liquid phase reactions. Methanol oxidation has been performed in the gas phase on these catalysts. Turnover frequencies are normalized by oxygen uptake measured under reaction conditions. A strong effect of pore size on the catalytic activity was observed in the form of a “volcano curve” and may be correlated with a variation in the local bond angle of Si–O–V. Considering this effect, the catalytic activity may be ‘tuned' to maximum by varying the pore size. As expected, the same effect was observed in the liquid phase oxidation of cyclohexene. In order to interpret the pore size effect, the reaction rates of both the gas phase and liquid phase oxidation were then correlated with the variation of edge energies of V K-edge XANES on these MCM-41 catalysts. Additional reactions were performed in the liquid phase to compare the activity of V-MCM-41 with that of crystalline vanadium and titanium silicalite zeolites. The possible causes of the difference in the catalytic behaviors of these materials is also discussed.     

Preparation of Cr–Ti Binary Oxide Anchored Mesoporous Silica by CVD Method and Their Photocatalytic Activities

Cr–Ti binary oxide anchored mesoporous silica (Cr–Ti/MCM-41) was prepared by stepwise CVD treatments of MCM-41 with TiCl₄ and CrO₂Cl₂. Cr–Ti/MCM-41 exhibited higher efficiency for the photocatalytic polymerization of ethylene as well as the oxidation of CO into CO₂ than those on single component Cr⁶⁺-oxide anchored MCM-41 under UV and visible light.     

Selective Photo-Assisted Oxidation of Methane into Formaldehyde on Mesoporous VO x /SBA-15 Catalysts

Mesoporous VO _x /SBA-15 samples have been prepared by different impregnation methods, characterized by N₂ adsorption and diffuse reflectance UV–visible spectroscopy, and their photocatalytic reactivity evaluated for the selective oxidation of methane with oxygen at 220 °C under UV irradiation. Vanadium in dehydrated VO _x /SBA-15 samples was found to be predominantly as isolated four-coordinated V⁵⁺ species, especially at V loading below 2.5 wt%. VO _x /SBA-15 catalysts prepared by impregnation with an aqueous solution of ammonium metavanadate (AMV) showed a maximum rate of formation of formaldehyde of 525 mol g^-1 h^-1 (selectivity of 93.8 mol%) at a vanadium content of 2.65 wt%. Both the rate of formation and selectivity of formaldehyde were improved when vanadium was impregnated on the SBA-15 support from a vanadyl sulfate methanolic solution (rate of formation of HCHO = 733 mol g^-1 h^-1, selectivity = 95.4 mol%). VO _x /SBA-15 catalysts were seen to be more effective for the selective photo-assisted oxidation of methane than VO _x /SiO₂ catalysts.     

Room temperature synthesized spherical V-MCM-41: a catalyst for vapour phase oxidation of diphenylmethane

Mesoporous spherical Si-MCM-41 and V-MCM-41 materials with a variable Si/V ratio of 25, 50, 75 and 100 were prepared by an economical route at room temperature and characterized by various techniques for qualitative and quantitative understanding of vanadium species. The morphology and mesoporosity of the spherical particles were confirmed by SEM and TEM micrographs. The catalytic activity of the synthesized catalysts was tested for vapour phase oxidation of diphenylmethane to benzophenone using CO₂ free air as oxidant at atmospheric pressure. The conversion of diphenylmethane and selectivity for benzophenone were measured in the temperature range of 593?C713?K over V-MCM-41 catalysts synthesized by adopting room temperature method as well as by direct hydrothermal method. The catalytic performance was related to vanadium content on the surface of the catalysts.     

Mono (V, Nb) or bimetallic (V–Ti, Nb–Ti) ions modified MCM-41 catalysts: synthesis, characterization and catalysis in oxidation of hydrocarbons (aromatics and alcohols)

Mesoporous MCM-41 molecular sieves modified by single (V and Nb) or bimetal (Nb–Ti and V–Ti) ions with highly ordered hexagonal arrangement of their cylindrical channels were prepared by direct synthesis with two different silica sources (sodium silicate and TEOS) and characterized (as-synthesized samples and those used after reaction) by XRD, N₂ adsorption–desorption, TEM, SEM and FTIR techniques. Niobium and titanium were stabilized in the autoclavized gel by complexation with a ligand (acetylacetone and oxalic acid, respectively). V modified MCM-41 catalysts gave a very high activity in hydroxylation reaction of benzene and toluene and a low conversion in oxidation of styrene while Nb-modified MCM-41 samples showed very high conversion in oxidation of styrene but low oxidation conversion of benzene and toluene. Further introduction of titanium in V- and Nb-modified MCM-41 materials conducted to the less well-ordered hexagonal arrangement and gave very different effect on activity in oxidation of aromatics. Introduction of Ti into V-MCM-41 solids led to an increase in activity in oxidation of styrene and benzene but a decrease in conversion in oxidation of toluene. While the addition of Ti in Nb-MCM-41 resulted in a slight increase in conversion of styrene oxidation but a significant decrease in activity of benzene and toluene oxidation. It reveals that the introduction mode of oxidant in the reactors can also influence the activity and selectivity of reaction. An increase in reaction rate of oxidation, in the first 20 h, was obtained by introduction of H₂O₂ step by step during the reaction. The present paper evidenced that the side chain oxidation is main reaction of styrene oxidation while the hydroxylation is main reaction of benzene and toluene. That is why these two reactions need different catalytic centers as revealed by this paper. The oxidation of alcohols using V-modified MCM-41 as catalysts has also been explored and these catalysts showed a very low catalytic activity which increases in the order: hexanol相似文献   

甲烷选择性氧化制甲醛/甲醇多相催化剂研究进展

甲烷是天然气、页岩气等化石能源的主要成分,储量十分丰富.由于甲烷分子结构高度稳定,其高效活化与选择性转化具有很大的挑战性,被认为是催化反应领域的"圣杯",因此,如何在温和条件下实现甲烷选择性氧化为高附加值的含氧化合物(如甲醇、甲醛)成为研究热点.近几十年来,研究者在甲烷选择性氧化催化剂的设计与制备方面开展了大量卓有成效...     

Effect of surface Na+ or K+ ion exchange on hydrodesulfurization performance of MCM-41-supported Ni–W catalysts

Ni–W hydrodesulfurization (HDS) catalysts supported on MCM-41 synthesized from two different silica sources (sodium silicate hydrate and tetraethylorthosilicate) as well as on Na⁺ or K⁺ ion exchanged MCM-41 were prepared. These catalysts were used to investigate the influence of the surface properties of MCM-41 on the performance of HDS catalysts with DBT as the model molecule. The XRD and N₂ adsorption results indicated that the MCM-41 prepared from tetraethylorthosilicate (MCM-41(T)) exhibited the best structural properties. The mesostructure of MCM-41 synthesized from sodium silicate (MCM-41(S)) remained after ion exchange with Na₂C₂O₂ and K₂C₂O₂. Both pyridine FT-IR and Hammett indicators showed that only MCM-41(S) possessed some Brönsted and Lewis acid sites. Ni–W/MCM-41(S) showed the highest HDS and hydrogenation activities. The introduction of Na⁺ and K⁺ strongly inhibited the hydrogenation activity of Ni–W/MCM-41(S) but enhanced its hydrogenolysis activity. UV–vis and TPR studies indicated that the introduction of Na⁺ and K⁺ into MCM-41(S) may lead to the segregation of surface Ni species and may hinder the reducibility of the supported Ni–W oxides. Spillover hydrogen, which is “trapped” by Na⁺ and K⁺, may play an important role in the HDS activity and selectivity of Ni–W catalysts.     

Hydrodesulfurization of Dibenzothiophene Over Ni-Mo Sulfides Supported by Proton-Exchanged Siliceous MCM-41

Strong acid sites on the surface of mesoporous MCM-41 were generated by ion-exchanging siliceous MCM-41 with dilute HNO₃ solution (0.5 M). The XRF determination indicates that most of the sodium cations contained in MCM-41 can be removed by the proton exchange, and dealuminization was observed during the proton exchange. The acidity of the mesoporous materials was characterized by means of NH₃-TPD and the Hammett indicators. It is revealed that new strong acid sites (-5.6 > H₀ > -8.2) were generated after the first 2 h of ion exchange and that the following ion exchanges had little effect on the acidic properties. XRD patterns of the mesoporous materials indicate that the structure of siliceous MCM-41 was improved by HNO₃ ion exchange. When Ni-Mo sulfides were supported on the prepared solid acid (H⁺-MCM-41), high performance in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) was observed. However, the HDS activity was decreased while the selectivity of biphenyl (BP) was increased, when H⁺-Si-MCM-41 was ion exchanged with Na₂CO₃ aqueous solution. TPR profiles of the supported Ni-Mo oxides reveal that the acidic properties of the supports greatly influence the hydrogenation activities of the bimetallic oxides. The high performance of H⁺-MCM-41-supported Ni-Mo catalysts may be attributed to the enhanced hydrogenation activity. The introduction of Na cation into the support led to the decrease of the HDS activity due to the poor hydrogenation ability of the supported bimetallic oxides. The HDS activity is well correlated with the low H₂ consumption temperature in the TPR profiles.     

Methanol synthesis over Pd/SiO2 with narrow Pd size distribution prepared by using MCM-41 as a support precursor

All silicious MCM-41 was investigated as a support or a support precursor for Pd/SiO₂ and prepared catalysts were tested for methanol synthesis from CO and H₂. The methods of Pd loading on the MCM-41 were impregnation, seed impregnation and chemical vapor deposition (CVD). For both impregnations, most Pd existed outside of the pore as large particles, and only a small part of Pd was inserted into the pore of MCM-41 retaining the initial structure. On the contrary, in the catalyst prepared by CVD method, the MCM-41 structure was completely destroyed to become amorphous SiO₂. Yet the average Pd particle size in this catalyst was smaller and its distribution was narrower than those of the catalysts prepared by impregnation methods. In the methanol synthesis from CO hydrogenation the catalyst prepared by CVD showed higher methanol selectivity than other MCM-41-derived catalysts. This result was considered to be due to the more uniform distribution of the Pd particle size.     

纳米金复合催化剂制备及其低温选择催化环己烷氧化性能

采用光催化直接还原法将Au(0)负载在TiO2修饰的中孔分子筛MCM-41的孔道内外,采用XRD, N2吸附-脱附,FT-IR, TEM和EDS等手段对所制催化剂进行了表征,并考察了其在温和条件下对环己烷选择性氧化反应的催化性能. 结果表明,所制纳米金催化剂的分子筛载体仍具有较高的结晶度,金颗粒粒径在10 nm左右. 在环己烷氧化反应中,纳米金与TiO2光催化共同作用,使复合催化剂具有低温高催化活性. 在温度100℃、压力1.0 MPa及250 W紫外灯光照8 h的条件下,环己烷转化率高达3.9%,目的产物的总选择性为90.2%.     

Effects of Novel Supports on the Physical and Catalytic Properties of Tungstophosphoric Acid for Alcohol Dehydration Reactions

The catalytic behavior of tungstophosphoric acid supported on modified mesoporous silica materials for the dehydration of 2-butanol and methanol was studied. Specifically, the supports evaluated here consisted of unmodified MCM-41 and SBA-15 mesoporous silicas, and these materials coated with sub-monolayer quantities of alumina, titania, and zirconia. UV-Vis DRS and ³¹P-NMR spectroscopy showed that the tungstophosphoric acid species retained their chemical identity in the synthesized supported form, although the spectra were influenced by the specific support material used. In addition, their acidic properties were evaluated using temperature-programmed oxidation of isopropyl amine. The differences in reaction rates between the samples reflect both the diversity in the amount of Brønsted acidic sites available for catalysis and dissimilarities in coking resistance. These two characteristics depend, in turn, on the type of support modifier used to prepare the catalyst.     

Niobic acid and niobium phosphate as highly acidic viable catalysts in aqueous medium: Fructose dehydration reaction

All silicious MCM-41 was investigated as a support or a support precursor for Pd/SiO₂ and prepared catalysts were tested for methanol synthesis from CO and H₂. The methods of Pd loading on the MCM-41 were impregnation, seed impregnation and chemical vapor deposition (CVD). For both impregnations, most Pd existed outside of the pore as large particles, and only a small part of Pd was inserted into the pore of MCM-41 retaining the initial structure. On the contrary, in the catalyst prepared by CVD method, the MCM-41 structure was completely destroyed to become amorphous SiO₂. Yet the average Pd particle size in this catalyst was smaller and its distribution was narrower than those of the catalysts prepared by impregnation methods. In the methanol synthesis from CO hydrogenation the catalyst prepared by CVD showed higher methanol selectivity than other MCM-41-derived catalysts. This result was considered to be due to the more uniform distribution of the Pd particle size.     

Sb-V-Ox catalysts—Role of chemical composition of MCM-41 supports in physicochemical properties

SbVO_x binary oxides were loaded on MCM-41 matrices of various chemical compositions: silicate (MCM-41), Nb-silicate (NbMCM-41), and Al-silicate (AlMCM-41). Vanadium and antimony were introduced by the post synthesis wetness impregnation carried out step by step (first V next Sb). The materials were characterised by N₂ adsorption/desorption, XRD, UV–vis, ESR, H₂-TPR, FT-IR combined with pyridine adsorption, and test reaction–hydrosulphurisation of methanol. SbVO_x dispersion was much higher when the support contained transition metal (NbMCM-41). Tetrahedrally coordinated vanadium(IV) species were deduced on all prepared samples from UV–vis spectra and were the only registered species on SbV/NbMCM-41 and SbV/AlMCM-41, whereas octahedrally ones were also present on SbV/MCM-41 and SbV/SiO₂. In bulk SbVO_x octahedral coordination dominated. The chemical composition of mesoporous support determined acidic–basic properties of SbVO_x catalysts and influenced the activity and selectivity in methanol hydrosulphurisation. The presence of Lewis acid–base pairs in the SbV/AlMCM-41 and SbV/NbMCM-41 catalysts strongly activated thiol formation in the reaction between methanol and hydrogen sulphide, whereas bulk binary oxides and SbVO_x loaded on silicate MCM-41 were less active and exhibited different selectivity.