Ti-containing MCM-41 catalysts for liquid phase oxidation of cyclohexene with aqueous H2O2 and tert-butyl hydroperoxide

Framework Ti-substituted and Ti-grafted MCM-41 mesoporous material has been prepared by direct hydrothermal synthesis and a post-synthesis grafting method. The materials have been tested as catalysts for cyclohexene oxidation with aqueous H₂O₂ and tert-butyl hydroperoxide (TBHP). With aqueous H₂O₂ in methanol, the major products were cyclohexene diol and its methyl ethers. No cyclohexene oxide was produced. Titanium leaching was a serious problem, and the catalyst lost its activity irreversibly after only one cycle of reaction. With TBHP, the selectivity for cyclohexene oxide was near 100%, titanium leaching was negligible, and the catalyst could be repeatedly used after regeneration without suffering significant activity loss. However, the reaction rate was lower than when H₂O₂ was used. Framework substituted material and catalysts prepared by Ti-grafting onto a MCM-41 support behaved similar, but the Ti-grafted MCM-41 is somewhat more active. The turnover frequency (TOF) per mole of Ti decreases with an increase of Ti content in the catalyst. This is caused by a reduced Ti dispersion within the silica matrix. This revised version was published online in July 2006 with corrections to the Cover Date.     

Microporosity in Mesoporous SBA-15 Supports: A Factor Influencing the Catalytic Performance of Immobilized Metalloporphyrin

MnTMPyP cationic metalloporphyrin was immobilized by means of ion exchange on a series of aluminated SBA-15 mesoporous silica supports prepared by different methods. The solids were characterized with XRD, HRTEM, HRSEM, chemical analysis, and nitrogen sorption isotherms. The catalysts were tested in the reaction of cyclohexene oxidation with iodozobenzene. It was found that immobilization significantly enhances catalytic activity as compared to the homogeneous system. In contrast to previously investigated metalloporphyrin catalysts immobilized on aluminated HMS, MCM-41 or FSM-16 type supports, where too narrow pores limited the formation of epoxide and enhanced allylic oxidation, the use of aluminated large pore SBA-15 solids favoured the epoxidation pathway and resulted in yields significantly higher than in the case of homogeneous reaction. The catalysts showed important differences in the level of allylic oxidation. Analysis of various factors potentially influencing the product distribution demonstrates that the key role in determining the contribution of allylic oxidation is the microporosity nature of the support, in particular, the lack or presence of supermicropores capable of accommodating metalloporphyrin species. The MnTMPyP centres confined in supermicropores experience steric limitations, which do not allow for the formation of epoxide and favour allylic oxidation.     

Efficient catalytic epoxidation of olefins with hierarchical mesoporous TS-1 using TBHP as an oxidant

A series of hierarchical mesoporous TS-1 zeolites with MFI structure were successfully synthesized via hydrothermal route using amphiphilic organosilanes as pore directing templates. These materials were characterized by nitrogen sorption (for surface area, pore volume and pore size distribution), FTIR, SEM and UV–Visible spectroscopy. These materials are efficient catalysts for liquid phase epoxidation of olefins using TBHP as the oxidant. All these TS-1 samples oxidized cyclohexene selectively into epoxide. For the TS-1 sample with 4.4 % Ti complete conversion of cyclohexene into epoxide was observed.     

Optimisation of olefin epoxidation catalysts with the application of high-throughput and genetic algorithms assisted by artificial neural networks (softcomputing techniques)

An olefin epoxidation Ti catalyst has been optimised by means of high-throughput experimentation involving materials synthesis, postsynthesis treatments, and catalytic testing. Softcomputing techniques for advanced experimental design have been used. The variables explored in the hydrothermal synthesis of Ti-silicate-based catalysts were: concentration of OH⁻, titanium, and surfactant. The probe reaction employed for the optimisation was the solvent-free epoxidation of cyclohexene, with tert-butylhydroperoxide as oxidant. The different catalyst groups detected by clustering analysis were studied by XRD and UV diffuse reflectance. Ti-mesoporous MCM-41 and MCM-48 molecular sieves were among the most active catalysts. The best performing catalysts were tested for epoxidation of different linear olefins.     

Catalytic activity of MnTUD-1 for liquid phase oxidation of ethylbenzene with tert-butyl hydroperoxide

Manganese was incorporated into silica matrix of TUD-1 using tetraethylene glycol (TEG) as the template. Three samples with Si/Mn ratio of 115, 44 and 18 were prepared and characterized by various techniques. MnTUD-1 is shown to be mesoporous with tetrahedrally coordinated Mn when Si/Mn?=?115; and nano-particles of manganese oxides are visible at higher loading of manganese (Si/Mn?=?44 and 18). The catalytic activity of MnTUD-1 was explored in the liquid-phase oxidation of ethylbenzene with tert-butylhydroperoxide (TBHP) as oxidant. Influence of various reaction parameters such as time, Si/Mn ratio, oxidant and solvent were studied. Finally the catalytic activity also compared with well-known microporous and mesoporous catalysts like MnAlPO-5, Mn containing MCM-41, MCM-48 and SBA-15.     

酒石酸钛配合物在MCM-41上的组装和催化苯乙烯环氧化

采用表面模板组装途径合成了MCM-41介孔材料,研究了这类载体上键合钛酸异丙酯+酒石酸二乙酯(Ti(OiPr)4+DET)和TiCl4+DET两类钛配合物的组装方法。XRD和FT-IR分别被使用来表征载体的晶体结构和载体与钛配合物的键合作用,结果表明:MCM-41在焙烧过程中,其介孔长程有序结构会发生退化,这种结构退化将导致1219和580cm-1红外带的消失,并诱发载体产生强的Si-OH畸变振动。在催化苯乙烯环氧化反应中,用焙烧载体组装钛催化剂显示了最好的氧化活性;而用含模板剂载体组装钛催化剂则显示出最高的环氧化选择性。胺基模板剂和DET配体对钛活性中心的调变作用将造成其活性下降,而环氧选择性上升。     

Unprecedented oxidative properties of mesoporous silica materials: Towards microwave-assisted oxidation of lignin model compounds

The unusual oxidative ability of mesoporous silicas towards oxidation of an important lignin model molecule, 1,2-(4-hydroxy-3methoxy-phenoxy) ethanol, apocynol under microwave irradiation is presented in this work. Mesoporous MCM-41, HMS, SBA-15 and amorphous silica were employed as catalysts in the present study. Different reactivities were obtained for the various silica materials. It was assumed that the substrate conversion and product selectivity were highly influenced by the nature of mesoporous silica materials. Based on the nature of the catalysts and reaction product profile, a plausible mechanism has been proposed.     

The effect of silylation on titanium-containing silica catalysts for the epoxidation of functionalised molecules

The epoxidation of functionalised substrates of interest as fine chemicals using mesoporous titanium-containing silicas is here reported and the role of silylation in changing the surface hydrophilic character of these catalysts is investigated. The silylation procedure was carried out on two titanium-grafted silicas with different morphologies. An ordered MCM-41 and a non-ordered commercial mesoporous silica were used as supports. The reactivity of bulky substrates with different characteristics (limonene, -terpineol, carveol and methyl linoleate) is studied and compared. The effect of silylation is more pronounced on Ti–MCM-41 than with low-surface area Ti–SiO₂ and it is shown that the catalytic performances are strongly dependent on the nature of the reactant. Purely alkenic molecules show better reactivity over silylated catalysts than over non-silylated ones. On the other hand, a hydrophilic environment around the titanium active sites has often a beneficial effect in the epoxidation of richly functionalised substrates.     

Effect of pore size on the performance of mesoporous material supported chiral Mn(III) salen complex for the epoxidation of unfunctionalized olefins

A series of mesoporous MCM-41 and MCM-48 materials with different pore sizes were synthesized and used as supports to immobilize chiral Mn(III) salen complex. The heterogeneous catalysts were characterized by XRD, FT-IR, DR UV–vis, and N₂ sorption, and the results indicated the successful immobilization of chiral Mn(III) salen complex. The confinement effect of pore size on the catalytic performance of the heterogeneous catalysts was studied for the asymmetric epoxidation of unfunctionalized olefins with m-chloroperoxybenzoic acid as an oxidant. It was found that the conversions and enantiomeric excess (ee) values were closely correlated with the pore sizes of parent supports. The catalysts immobilized on the large-pore mesoporous supports exhibited higher conversions, and for the catalysts immobilized on MCM-41 materials, the ee values improved with increasing pore size. However, for the MCM-48 material-supported catalysts, the compatible pore size of the support with the substrate was beneficial for obtaining higher enantioselectivity in olefin epoxidation.     

Synthesis of Dimethyl Ether over Modified H-Mordenite Zeolites and Bifunctional Catalysts Composed of Cu/ZnO/ZrO2 and Modified H-Mordenite Zeolite in Slurry Phase

A simple, cost effective and reusable transition metal (M = Ti, V, Mn)-grafted mesoporous silica spheres were synthesized by sol–gel technique and characterized by SXRD, FTIR, UV–Vis DRS, SEM, EDS, BET-surface area, pore volume and average pore diameter. Epoxidation of cyclohexene to cyclohexene oxide over these modified silica spheres using TBHP as oxidant was performed to compare the catalytic activities. Cyclohexene oxide selectivity was recorded to be 87.5% with 72.5% conversion over 8 wt% Ti-grafted silica spheres in comparison to V and Mn-grafted catalysts.     

Control of the Chemoselectivity in the Oxidation of Geraniol Over Lanthanum, Titanium and Niobium Catalysts Supported on Mesoporous Silica MCM-41

The selective catalytic oxidation of geraniol with hydrogen peroxide over lanthanum, titanium and niobium catalysts supported on mesoporous silica MCM-41 has been investigated. Among the various catalysts studied, Nb-MCM-41 showed an excellent selectivity for allylic epoxide. In contrast, La-MCM-41 and Ti-MCM-41 catalysts exhibited high selectivity to citral. Catalyst??s characterization and activity results clearly demonstrated that the differences in the product distribution were due to the presence of different acid sites and the better coordination of oxidant with the catalyst used. All the catalysts were characterized by nitrogen adsorption?Cdesorption isotherms at 77?K, TPD-pyridine and XRD.     

Ti-HMS合成、表征及其催化氧化性能研究

以有机金属二氯二茂钛为钛源,六方介孔二氧化硅分子筛(HMS)为载体,采用嫁接法合成了含钛量(mol)分别为2.8%、3.8%和4.8%的Ti-HMS,HMS合成原料组成中H2O/EtOH(v/v)分别为0.5、1、2、5、9.合成材料用X射线粉末衍射(XRD)、N2吸附-脱附等温线、漫反射紫外可见光谱(UV-VIS)进行了表征,并考察了它们以叔丁基过氧化物(TBHP)为氧化剂在对叔丁基甲苯液相氧化中的催化性能.结果表明,表面钛嫁接后的HMS介孔结构有所损失,比表面积和孔体积减少.载体HMS合成原料中的H2O/EtOH(v/v)影响介孔织构和钛的配位环境.UV-VIS资料表明,H2O/EtOH(v/v)=0.5和9时不利于钛着床于分子筛骨架,H2O/EtOH(v/v)=1、2和5的HMS适合作为嫁接钛的载体.催化剂的活性主要受四配位Ti、介孔织构的影响.钛嫁接的HMS在氧化反应中表现出较好的催化活性,载钛量为4.8%Ti、H2O/EtOH(v/v)=1的Ti-HMS显示最好催化活性,对叔丁基甲苯转化率为21.8%.     

硅烷化含钛介孔分子筛催化环己烯环氧化动力学

研究了气相沉积法制备的硅烷化含钛介孔分子筛(Ti/HMS)催化环己烯与异丙苯过氧化氢(CHP)的环氧化反应的宏观动力学.结果表明:环己烯与CHP的环氧化反应对催化剂的量表现为1级;对于环己烯和氧化剂CHP,随着浓度的增加,反应随之由1级向0级过渡,根据实验结果和反应机理,运用Rideal-Eley方法,提出了硅烷化Ti/HMS催化环己烯与CHP环氧化反应的动力学方程.     

Propane oxidative dehydrogenation over vanadia catalysts supported on mesoporous silicas with varying pore structure and size

The structural characteristics and the performance of vanadia catalysts (0.7–8 wt.% V) supported on mesoporous (MCM-41, HMS, MCF, SBA-15), microporous (silicalite) and non-porous (SiO₂) silicas in oxidative dehydrogenation of propane were investigated. The structure of vanadia species, the redox and the acidic properties of the catalysts were studied using in situ Raman spectroscopy, TPD- NH₃ and H₂-TPR. The only vanadia species detected on the surface of HMS and MCM-41 for V loadings up to 8 wt.% were isolated monovanadates indicating high vanadia dispersion. Additional bands ascribed to V₂O₅ nanoparticles were evidenced in the case of SBA-15 and MCF supported catalysts while these bands were the only ones identified on the surface of the catalysts supported on silicalite and non-porous silica. The catalysts supported on mesoporous HMS and MCM-41 materials showed the best performance achieving high propane conversions (35–40%) with relatively high propene selectivities (35–47%). Lower activity due to the lower degree of vanadia dispersion, caused by the partial destruction of the pore structure was observed for the SBA-15 and MCF supported catalysts. The degree of dispersion of the V species on the catalyst surface and not the pore size and structure of the mesoporous support or the acidity/reducibility characteristics mainly determine the catalytic activity towards propene production. In addition, it was shown that the pore structure and size of the mesoporous supports did not have any significant effect in the turnover rates (TOF values) of propane conversion (and propene formation at low propane conversion, below ca. 10%). However, the highest propene yield (up to 19%) and stable catalytic behavior was attained for catalysts supported on HMS mesoporous silica, and especially for those combining framework mesoporosity and textural porosity (voids between primary nanoparticles).     

Synthesis and Characterization of Dendrimer-Templated Mesoporous Oxidation Catalysts

We report here the use of 4th and 5th generation dendrimers poly(propylene)imine (CU-D32 and CU-D64) as templating agents for the synthesis of mesoporous titanosilicate and vanadosilicate oxidation catalysts via solgel techniques. The physical properties of these mesoporous materials were characterized by TGA, BET, PXD and SEM/EDX analyses and these showed that the transition metals are evenly distributed throughout these silicates, which have interconnected spherical pores (approx. 12Å in diameter) and high surface areas of about 650m²g^–1. Kinetic studies showed that all transition metal-doped catalysts were highly selective at oxidizing cyclohexene to the corresponding epoxide. Additionally, CU-D64-templated catalysts were more catalytically active for cyclohexene epoxidation than CU-D32-templated catalysts as a result of differences in pore size. All CU-D64-templated catalysts exhibited epoxidation catalytic activity comparable to that of titanium doped MCM-41 materials.     

Titanium–Silica Catalysts for the Production of Fully Epoxidised Fatty Acid Methyl Esters

Grafted titanium-containing mesoporous silica catalysts were used in the selective epoxidation of C-18 unsaturated fatty acid methyl esters (FAMEs). High yields in mono- and diepoxide derivatives were obtained under acid-free reaction conditions with TBHP as oxidant. Ti-MCM-41 showed the best performance in terms of activity over the three FAMEs. Easy separation of the desired products and recycling of the catalyst were demonstrated.     

Structural and oxidative properties of Mn-MCM41 catalysts synthesised by MOCVD method

Molecular organic chemical vapour deposition (MOCVD) is employed to implant various gaseous manganese precursors onto mesoporous MCM-41 silica. After heat treatment in air these samples show significantly higher catalytic activity than Mn/MCM-41 samples prepared by a conventional impregnation method for trans-stilbene epoxidation to trans-stilbene oxide using tert-butylhydroperoxide (TBHP) as an oxidant. In contrast, the same MOCVD samples gave much poorer activity than the impregnation samples in diphenylmethane oxidation reaction using air as the oxidant. Catalyst characterisation (infrared, temperature programmed reduction, EXAFS, etc.) indicates that discrete entities of Mn species (Mn-oxo and/or bridging Mn-oxygen structures, etc.) are formed on the surface of the MCM-41 by the MOCVD technique. This is attributed to initial reactions of gaseous organo-manganese complexes with surface silanol groups, followed by their oxidation in air during the heat treatment. On the other hand, in the wet impregnation samples, bulk Mn oxide phases are deposited onto the MCM-41 structure. It is thus believed that the discrete supported manganese-oxygen species of high electrophilicity are the active sites for the oxygen transfer reaction in the trans-stilbene oxidation. However, the nucleophilic lattice oxygen of the bulk Mn oxide phases prepared by the wet impregnation is responsible for the effective hydrogen abstraction in the diphenylmethane oxidation. It is also evident that the catalytic activity of the MOCVD samples depends crucially on the type of precursors used. Thus, the result clearly suggests that active and ultraselective Mn-oxygen catalytic sites for a particular oxidation reaction may be tailored through the use of different chemical precursors using the MOCVD technique.     

Polymerization of α-Olefins Using Benzamidinate Zirconium Complexes Supported on Mesoporous Silica Materials

The polymerization of ethylene and propylene were studied using an anchored dimethyl benzamidinate zirconium complex. As supports, mesoporous silicas MCM-41 and HMS were utilized. These inorganic materials were reacted with methylalumoxane to serve as the cocatalyst and to avoid the deactivation of the active cationic complex. The reaction of the octahedral benzamidinate zirconium complex with the supported cocatalysts results in the formation of active catalytic systems for the polymerization of olefins. The polymer properties were found to depend on the nature of the supports. The activity of the heterogeneous catalysts and the polymer characteristics were compared with those obtained with the homogeneous catalyst.     

Metalloporphyrins encapsulated mesoporous molecular sieves as efficient heterogeneous catalysts for oxidation of cyclohexene with iodosylbenzene

The cationic Fe, Mn or VO porphyrins were encapsulated in Si, Al, Ti and V-MCM-41. The catalytic activity was investigated for the oxidation of cyclohexene with iodosylbenzene (PhIO) as oxidant. Manganese porphyrin supported catalysts produces epoxide as major products whereas iron and vanadyl porphyrins gave allylic oxidation products as major constituents. The results obtained clearly shows that the selectivity of various products depends not only on the nature of the support materials but also on the combined effects of metalloporphyrins and the supports. The recyclability of the heterogeneous catalysts was also investigated. The decrease in catalytic activity and leaching of active centers were observed when porphyrins were supported on Si-MCM-41.     

Ti-MCM-41分子筛催化剂的合成、表征及催化性能

以正硅酸四甲酯为硅源,钛酸四异丙酯为钛源,在异丙醇和水的混合溶液中室温下合成了Ti-MCM-41介孔分子筛,采用TEM、N 2吸附-脱附、XRD、UV-Vis和Raman等研究了分子筛的结构特性。并以过氧化氢异丙苯为氧化剂,考察了硅烷化处理后的Ti-MCM-41分子筛催化剂在丙烯环氧化反应中的催化性能。结果表明,与骨架外六配位钛物种相比,骨架内四配位钛物种活化有机过氧化物的能力更强,速率更快;四配位钛物种是环氧丙烷生成的活性物种,而六配位钛物种的存在则会导致反应副产物的生成。合成时添加适量的异丁醇可有效促进钛物种进入分子筛骨架中,并增大比表面积和孔体积,进而改善Ti-MCM-41催化剂的丙烯环氧化催化性能。性能最佳的催化剂上过氧化氢异丙苯转化率可达94.7%,环氧丙烷选择性可达95.8%。