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Molybdenum-based epoxidation catalysts are among the most investigated since the homogeneous Mo-based catalytic process claimed by Halcon International Inc. for liquid phase olefin epoxidation (EPO). While homogeneous Mo-based catalytic technologies reached industrial implementation for olefin EPO, the same does not apply for heterogeneous Mo-based ones, which have not reached industrial implementation (e.g., to meet catalyst productivity and stability requirements). In this work, EPO nanocatalysts consisting of oxides possessing molybdenum and M = Ta, Nb or W were prepared viaa simple, versatile methodology. The influence of the material synthesis conditions on the material properties was investigated to meet superior catalytic performances. Promising Mo-based solid catalysts were obtained which promoted the EPO of relatively bulky olefins such as fatty acid methyl esters (methyl oleate, methyl linoleate), using tert-butyl hydroperoxide as oxidant under mild conditions; e.g., the materials Mo(75D)M-0.3 with M = Nb, W (75 at% Mo relative to M, and MoO2Cl2as precursor), obtained in a fast synthesis of 0.3 h, led to 92–96% epoxide selectivity at 84–95% methyl oleate conversion, at 70 °C. To the best of our knowledge, these are the first Mo,M oxides reported for these reactions.

The discovery of selective heterogeneous catalytic systems for industrial oxidation processes remains a challenge. Molybdenum oxide-based polymeric hybrid materials have been shown to be oxidation catalysts under mild reaction conditions, although difficulties remain with catalyst recovery/reuse since most perform as homogeneous catalysts or possess low activity. The present study shows that the hybrid material [MoO3(2,2′-biimidazole)]·H2O (1) is a superior catalyst regarding these issues. The structure of 1was confirmed (by single crystal and synchrotron X-ray powder diffraction) to comprise one-dimensional chains of corner-sharing {MoO4N2} octahedra. Strong MoOH–N hydrogen bonds separate adjacent chains to afford parallel channels that are occupied by disordered water molecules. Hybrid 1was additionally characterised by FT-IR spectroscopy, 1H and 13C MAS NMR, scanning electron microscopy and thermogravimetric analysis. The catalytic studies highlighted the versatility of 1for oxidation reactions with tert-butylhydroperoxide as oxidant. By complementing with characterisation studies, it was verified that the reaction occurs in the heterogeneous phase, the catalyst has good stability and is recoverable viasimple procedures. The chemical reaction scope covered epoxidation and sulfoxidation, and the substrate scope included biomass-derived dl-limonene and fatty acid methyl esters to give renewable bio-products, as well as thiophene and thioanisole substrates.

The isomerization of d-glucose to d-fructose is an important transformation in food and biomass chemistry, for which the use of robust heterogeneous catalysts instead of restrictive enzyme-based systems is desirable, although continues to be very challenging. Herein, solid base catalysts possessing a relatively robust hierarchical MFI structure and (non-toxic) alkali (M = Na, K) and alkaline-earth (Me = Ca, Mg) metals were prepared viatop-down strategies and characterized viacomplementary techniques. The catalyst preparation and regeneration conditions were optimized to achieve superior performances. The highest catalyst stability was reached for the Mg, K-containing hierarchical zeotype prepared viasolid-state impregnation (SSI) of Mg, which led to 65% d-fructose selectivity at 45% conversion, 100 °C, 2 h, or 99% selectivity at 26% conversion, 75 °C. The SSI is an up-scalable process which advantageously avoids the use of solvents, pre-preparation of alkaline liquid solutions and solvent evaporation stages for manufacturing eco-friendly catalysts.

3-(1,2,4-Triazol-4-yl)adamantane-1-carboxylic acid (tradcH), a heterobifunctional organic ligand in which carboxylic acid and 1,2,4-triazole groups are united through a rigid 1,3-adamantanediyl spacer, was employed for the synthesis of a MoVIoxide organic hybrid. The ligand crystallized from water as tradcH·H2O (1), possessing a two-dimensional hydrogen-bonding network, and from ethanol as a cyclic molecular solvate with the composition (tradcH)3·2EtOH (2). Treatment of tradcH with MoO3under hydrothermal conditions afforded a new Mo trioxide hybrid, [MoO3(tradcH)]·H2O (3), which was structurally characterized. In 3, the molybdenum atoms form a polymeric zigzag chain of {μ2-O-MoO2}nwhich is supported by double triazole bridges, while the carboxylic acid termini are left uncoordinated. The coordination environment of the Mo centers appears as distorted cis-{MoN2O4} octahedra. The hybrid exhibits high thermal stability (up to 270 °C) and was employed for a relatively broad scope of catalytic oxidation reactions in the liquid phase. Its catalytic behavior may be compared to a reversible mutation, featuring the best sides of homogeneous and heterogeneous catalysis. The original solid material converts into soluble active species, and the latter revert to the original material upon completion of the catalytic reaction, precipitating and allowing straightforward catalyst separation/reuse (like a heterogeneous catalyst). This catalyst was explored for a chemical reaction scope covering sulfoxidation, oxidative alcohol dehydrogenation, aldehyde oxidation, and olefin epoxidation, using hydrogen peroxide as an eco-friendly oxidant that gives water as a coproduct.

Three organic ligands bearing 1,2,4-triazolyl donor moieties, (S)-4-(1-phenylpropyl)-1,2,4-triazole (trethbz), 4-(1,2,4-triazol-4-yl)benzoic acid (trPhCO2H), and 3-(1H-imidazol-4-yl)-2-(1,2,4-triazol-4-yl)propionic acid (trhis), were prepared to evaluate their coordination behavior in the development of molybdenum(VI) oxide organic hybrids. Four compounds, [Mo2O6(trethbz)2]·H2O (1), [Mo4O12(trPhCO2H)2]·0.5H2O (2a), [Mo4O12(trPhCO2H)2]·H2O (2b), and [Mo8O25(trhis)2(trhisH)2]·2H2O (3), were synthesized and characterized. The monofunctional tr-ligand resulted in the formation of a zigzag chain [Mo2O6(trethbz)2] built up from cis-{MoO4N2} octahedra united through common μ2-O vertices. Employing the heterodonor ligand with tr/–CO2Hfunctions afforded either layer or ribbon structures of corner- or edge-sharing {MoO5N} polyhedra (2aor 2b) stapled by tr-links in axial positions, whereas −CO2H groups remained uncoordinated. The presence of the im-heterocycle as an extra function in trhisfacilitated formation of zwitterionic molecules with a protonated imidazolium group (imH+) and a negatively charged −CO2–group, whereas the tr-fragment was left neutral. Under the acidic hydrothermal conditions used, the organic ligand binds to molybdenum atoms either through [N–N]-tror through both [N–N]-trand μ2-CO2–units, which occur in protonated bidentate or zwitterionic tetradentate forms (trhisH+and trhis, respectively). This leads to a new zigzag subtopological motif (3) of negatively charged polyoxomolybdate {Mo8O25}n2n–consisting of corner- and edge-sharing cis-{MoO4N2} and {MoO6} octahedra, while the tetradentate zwitterrionic trhisspecies connect these chains into a 2D net. Electronic spectra of the compounds showed optical gaps consistent with semiconducting behavior. The compounds were investigated as epoxidation catalysts via the model reactions of achiral and prochiral olefins (cis-cyclooctene and trans-β-methylstyrene) with tert-butylhydroperoxide. The best-performing catalyst (1) was explored for the epoxidation of other olefins, including biomass-derived methyl oleate, methyl linoleate, and prochiral dl-limonene.

Phosphonate- and yttrium-based metal–organic frameworks (MOFs), formulated as [Y(H5btp)]·5.5H2O (1), [Y(H5btp)]·2.5H2O (2), (H3O)[Y2(H5btp)(H4btp)]·H2O (3), and [Y(H5btp)]·H2O·0.5(MeOH) (4), were prepared using a “green” microwave-assisted synthesis methodology which promoted the self-assembly of the tetraphosphonic organic linker [1,1′-biphenyl]-3,3′,5,5′-tetrayltetrakis(phosphonic acid) (H8btp) with Y3+cations. This new family of functional materials, isolated in bulk quantities, exhibits a remarkable breathing effect. Structural flexibility was thoroughly studied by means of X-ray crystallography, thermogravimetry, variable-temperature X-ray diffraction, and dehydration and rehydration processes, ultimately evidencing a remarkable reversible single-crystal to single-crystal (SC–SC) transformation solely through the loss and gain of crystallization solvent molecules. Topologically, frameworks remained unaltered throughout this interconversion mechanism, with all compounds being binodal 6,6-connected network with a Schäfli symbol of {413.62}{48.66.8}. Results show that this is one of the most stable and thermally robust families of tetraphosphonate-based MOFs synthesized reported to date. Porous materials 2and 3were further studied to ascertain their performance as heterogeneous catalysts and proton conductors, respectively, with outstanding results being registered for both materials. Compound 2showed a 94% conversion of benzaldehyde into (dimethoxymethyl)benzene after just 1 h of reaction, among the best results registered to date for MOF materials. On the other hand, the protonic conductivity of compound 3at 98% of relative humidity (2.58 × 10–2S cm–1) was among the highest registered among MOFs, with the great advantage of the material to be prepared using a simpler and sustainable synthesis methodology, as well as exhibiting a good stability at ambient conditions (temperature and humidity) over time when compared to others.

Metal–organic frameworks (MOFs) have high potential as nanoplatforms for the storage and delivery of therapeutic gasotransmitters or gas-releasing molecules. The aim of the present study was to open an investigation into the viability of tricarbonyl-pyrazine-molybdenum(0) MOFs as carbon monoxide-releasing materials (CORMAs). A previous investigation found that the reaction of Mo(CO)6with excess pyrazine (pyz) in a sealed ampoule gave a mixture comprising a major triclinic phase with pyz-occupied hexagonal channels, formulated as fac-Mo(CO)3(pyz)3/2·1/2pyz (Mo-hex), and a minor dense cubic phase, formulated as fac-Mo(CO)3(pyz)3/2(Mo-cub). In the present work, an open reflux method in toluene has been optimized for the large-scale synthesis of the pure Mo-cubphase. The crystalline solids Mo-hexand Mo-cubwere characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), FT-IR and FT-Raman spectroscopies, and 13C{1H} cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy. The release of CO from the MOFs was studied by the deoxy-myoglobin (deoxy-Mb)/carbonmonoxy-myoglobin (MbCO) UV–vis assay. Mo-hexand Mo-cubrelease CO upon contact with a physiological buffer in the dark, delivering 0.35 and 0.22 equiv (based on Mo), respectively, after 24 h, with half-lives of 3–4 h. Both materials display high photostability such that the CO-releasing kinetics is not affected by irradiation of the materials with UV light. These materials are attractive as potential CORMAs due to the slow release of a high CO payload. In the solid-state and under open air, Mo-cubunderwent almost complete decarbonylation over a period of 4 days, corresponding to a theoretical CO release of 10 mmol per gram of material.

The application field of dichloridodioxidomolybdenum(vi) chelate complexes, which have been intensively investigated as catalysts for liquid-phase olefin epoxidation, is broadened to encompass acid catalysis, in particular, alcoholysis and acetalisation reactions. Complex [MoO2Cl2(L)] (1) with L = 4,4′-di-tert-butyl-2,2′-bipyridine was chosen as (pre)catalyst. Depending on the reaction conditions, 1either remained structurally intact or was transformed into different metal species, the structures of which were determined on the basis of single-crystal X-ray diffraction, spectroscopic techniques (FT-IR and 1H NMR), and elemental analysis. The first example of a mixed-ligand complex of the type [MoO2X(OR)(L)] (X = halide) is disclosed. This complex is one of only a handful of complexes known to date that exhibit an all-cisconfiguration instead of the usual cis-oxido, trans-X, cis-L configuration (X = anionic ligand). Mechanistic considerations of the formation of the metal species are made.

The sustainable conversion of biomass and biomass-derived platform chemicals demands efficient catalytic processes for which modified versions of zeolites can be strategically important. The catalytic potential of bulk and composite catalysts which simultaneously feature zeolite crystallinity, mesoporosity and Zr and Al sites were explored for the valorisation of furfural (Fur; industrially produced from hemicelluloses) viaintegrated reduction and acid reactions in alcohol media, to give useful bio-products (bioP), namely, furanic ethers, levulinate esters and angelica lactones. Different synthetic strategies were used starting from zeolite microcrystals or nanocrystals. A composite consisting of nanocrystals of Zr,Al-Beta embedded in a mesoporous matrix is reported for the first time. In a different synthesis approach, a bulk mesoporous zeotype material was prepared by post-synthesis alkaline/acid/impregnation treatments, and explored for the first time as a catalyst for a one-pot reduction/acid reaction system. Characterisation studies of the morphology, structure, texture and nature of the Al and Zr sites (27Al MAS NMR spectroscopy, FT-IR spectroscopy of adsorbed pyridine or deuterated acetonitrile) helped understand the influence of material properties on catalytic performance. These types of materials are active and stable catalysts for the integrated conversion of Fur to bioP.

A new organosilica hybrid mesoporous material HM containing phosphane oxide surface groups was prepared via the “dual” organosilane templated cocondensation of 4bistriethoxysilylbenzene and diphenylphosphanylpropyltriethoxysilane, followed by postsynthesis oxidation of the phosphane surface groups. The HM material was used to immobilise the [MoO2Cl2] complex. The resultant material HMMo is an active and selective catalyst in the liquidphase epoxidation of olefins [ciscyclooctene, R+limonene, trans2octene, 1octene] with tBuOOH at 55 °C and gives comparable epoxide yields to those observed for the free complex [MoCl2O2OPCH2CH3Ph22] 1, under similar reaction conditions: the crystal structure of complex 1remains the same after reaction with tBuOOH. The oxidant is essentially consumed in the HMMo or 1catalysed epoxidation of the olefin. In the case of limonene possesses endo and exocyclic C=C bonds, the molar ratio of 1,2epoxypmenth8ene1,2–8,9diepoxypmenthane at ca. 90  conversion reached after 24 h reaction is ca. 5 and 10 for 1and HMMo, respectively, indicating a favourable regioselectivity towards the epoxidation of the endocyclic C1–C2 double bond, especially in the case of the heterogeneous catalyst.

A new mesoporous hybrid material denoted LPTEPPPOMo has been prepared by tethering [MoO2Cl2] onto mesoporous silica previously functionalised with phosphane oxide spacer ligands. The LPTEPPPOMo material was tested in the liquidphase epoxidation of olefins [ciscyclooctene, R+limonene, trans2octene and 1octene] with tBuOOH, at 55 °C, and without a cosolvent, giving at least 90  selectivity to the corresponding epoxide at 36–77  conversion: in the case of limonene, regioselectivity favours the epoxidation of the endocyclic double bond, giving mainly 1,2epoxypmenth8ene. The catalytic system based on a liquid–liquid biphasic system containing the homogeneous complex [MoCl2O2OPCH2CH3Ph22] dissolved in the ionic liquid 1butyl4methylpyridinium tetrafluoroborate leads to lower epoxide selectivity 91  at 64  conversion in the reaction of cyclooctene in comparison to that observed for LPTEPPPOMo 100  at 87  conversion. The reused solid–liquid and liquid–liquid biphasic catalytic systems show partial loss of catalytic activity.

The catalytic performance of the complexes [MoO2Cl2L2] [L = N,Ndimethylformamide dmf, N,Ndimethylacetamide dma, N,Ndimethylpropionamide dmpa, N,Ndiethylformamide def and N,Ndiphenylformamide dpf] was examined in the epoxidation of ciscyclooctene with tertbutyl hydroperoxide tbhp at 55 °C and in the absence of a cosolvent. The complexes showed high turnover frequencies in the range of 561–577 mol molMo–1 h–1, giving the epoxide as the only product in ≥98  yield after 6 h. The reaction rates decreased significantly in consecutive runs carried out by recharging the reactors with olefin and oxidant. On the basis of the IR spectroscopic characterisation of the solids recovered at the end of the catalytic reactions, the decrease in activity is attributed to the formation of dioxidoμoxidomolybdenumVI dimers. Accordingly, the treatment of [MoO2Cl2dmf2] with an excess amount of tbhp led to the isolation of [Mo2O4μ2OCl2dmf4], which was characterised by singlecrystal Xray diffraction and found to exhibit a catalytic performance very similar to that found in the second runs for the mononuclear complexes. The kinetics of the reaction of [MoO2Cl2dmf2] with tbhp was further examined by UVVis spectroscopy, allowing rate constants and activation parameters to be determined. For the dpf adduct, the effect of different solvents on cyclooctene epoxidation and the epoxidation of other olefins, namely, R+limonene, αpinene and norbornene, were investigated. © WileyVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009

An organic–inorganic hybrid ligand silica was prepared by reaction of the ordered mesoporous silica MCM41 with 3triethoxysilylpropyl 4pyridylacetamide. Elemental analysis indicated that a pyridyl group loading of 0.93 mmol g–1was achieved. The derivatised material was further characterised by powder Xray diffraction, N2adsorption, thermogravimetric analysis, FTIR and Raman spectroscopy, and 13C and 29Si CP MAS NMR spectroscopy. Pyridylfunctionalised MCM41 L″ was treated with chloroform solutions of lanthanide trisβdiketonate complexes Lnnta3H2O2[Ln = Eu, Gd; nta = 12naphthoyl3,3,3trifluoroacetonate] to give the surfacebound monosubstituted species Lnnta3H2OL″. The residual coordinated water molecules were subsequently replaced by pyridine py or methyl phenyl sulfoxide mpso to give immobilised Lnnta3pyL″ and Lnnta3mpsoL″ species. Photoluminescence studies were carried out at room temperature and 14 K. The emission spectra of the Eumodified materials were dominated by the typical Eu3+red lines ascribed to transitions between the 5D0excited state and the ground multiplet 7F0–4, and only very weak or negligible emission from the organic ligands are observed. Variation of the excitation wavelength confirmed that all of the Eu3+ions occupied the same average local environment within each sample. By comparison with the photoluminescence data for the nonsupported Eunta3py2and Eunta3mpso2complexes, it is shown that the interaction between the host and guest has a strong effect on the excited states of the organic ligands; Eu3+sensitisation occurs exclusively through the ligands rather than by direct intra4f6excitation.© WileyVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008

Molybdenum(II) complexes [MoX(CO)2(η3-allyl)(CH3CN)2] (X = Cl or Br) were encapsulated in an aluminium-pillared natural clay or a porous clay heterostructure and allowed to react with bidentate diimine ligands. All the materials obtained were characterised by several solid-state techniques. Powder XRD, and 27Al and 29Si MAS NMR were used to investigate the integrity of the pillared clay during the modification treatments. 13C CP MAS NMR, FTIR, elemental analyses and low-temperature nitrogen adsorption showed that the immobilisation of the precursor complexes was successful as well as the in situ ligand-substitution reaction. The new complex [MoBr(CO)2(η3-allyl)(2-aminodipyridyl)] was characterised by single-crystal X-ray diffraction and spectroscopic techniques, and NMR studies were used to investigate its fluxional behaviour in solution. The prepared materials are active for the oxidation of cis-cyclooctene using tert-butyl hydroperoxide as oxidant, though the activity of the isolated complexes is higher. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

We present here Monte Carlo simulation and experimental results on the adsorption of nitrogen and water in aluminum methylphosphonate polymorph α (AlMePO-α). We have assumed a detailed atomic model for the material, using experimental information to construct the simulation cell. Nitrogen was modeled with two different approaches:  as a simple Lennard-Jones (LJ) sphere with no charges, and as a diatomic molecule with charges explicitly included. Water was represented by the TIP4P model. Experimental adsorption isotherms were used to tune the proposed molecular model for the adsorbent. Simulated adsorption capacities were in agreement with the experimental results obtained for the studied systems. The influence of the surface model on the adsorption behavior was taken into account by considering different values of the surface methyl group size parameter. Our results corroborate the strong sensitivity of the simulation results to this parameter, as previously observed by Schumacher and co-workers. It is also observed that charged models are essential to accurately describe the low-pressure region of the adsorption isotherm, where the solid−fluid interaction rules the system behavior. However, a simple uncharged molecular model for nitrogen is able to describe the three loci arrangement at maximum loading. Experimental and simulation results presented here also confirm the low water affinity of AlMePO-α. These results enforce the application of this methodology to achieve quantitative predictions on similar systems, with the appropriate transferability of the molecular parameters.

MoVI complexes with the general formula [MoO2X2L] [X = Cl, Br; L = 4,4'-dimethyl-2,2'-bipyridine (dmbp) or 4-hexyl-4'-methyl-2,2'-bipyridine (hmbp)] have been prepared and characterized by IR and solution NMR (1H, 13C, 17O and 95Mo) spectroscopy. The complexes were applied as catalysts in the homogeneous phase for the epoxidation of cyclooctene, with tert-butyl hydroperoxide (TBHP) as the oxygen source. The desired epoxide was the only product and turnover frequencies of up to about 9000 h–1 could be reached. The catalytic activities increased in the order [MoO2Br2(dmbp)] < [MoO2Br2(hmbp)] < [MoO2Cl2(dmbp)] < [MoO2Cl2(hmbp)]. A kinetic model was built up for a homogeneous batch reactor based on a simplified mechanism involving three steps: (i) reversible coordination of TBHP to the starting MoVI complex to give a MoVI alkylperoxide; (ii) irreversible oxidation of cyclooctene to cyclooctene oxide by the species formed in step 1, with formation of the starting complex and tert-butyl alcohol; (iii) reversible coordination of tert-butyl alcohol to the starting complex. This model is consistent with the observed kinetics. The first step in this reaction mechanism was characterized in more detail by studying the kinetics of the reaction of the starting complexes with TBHP in the absence of any reductant by UV/Vis spectroscopy. Rate constants, equilibrium constants, and activation parameters were determined. All ΔS‡ values were negative and therefore support an associative mechanism in which a seven-coordinate intermediate is formed. The results also suggest that the first step is not always the rate-limiting step of cyclooctene epoxidation with these complexes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The tricarbonyl complex [(η5-C5H4-COOMe)Mo(CO)3Cl] was prepared from the reaction of sodium (methoxycarbonyl)cyclopentadienide, (C5H4-CO2Me)Na, with (Bu4N)[Mo(CO)5I]. Heating the ester with 3-(triethoxysilyl)propylamine gave the amide derivative {[η5-C5H4-CONH-C3H6Si(OEt)3]Mo(CO)3Cl}. The functionalised tricarbonyl complex was immobilised in the ordered mesoporous silica MCM-41 with a loading of 13 wt.-% Mo (1.4 mmol·g−1) by carrying out a grafting reaction in dichloromethane. Powder X-ray diffraction and nitrogen adsorption−desorption analysis indicated that the structural integrity of the support was preserved during the grafting and that the channels remained accessible, despite significant reductions in surface area, pore volume and pore size. The success of the coupling reaction was confirmed by 29Si and 13C (CP) MAS NMR spectroscopy. A supported dioxo complex of the type [(η5-C5H4R)MoO2Cl] was subsequently prepared by oxidative decarbonylation of the tethered tricarbonyl complex using tert-butyl hydroperoxide (TBHP). The oxidised material is an active catalyst for the liquid phase epoxidation of cyclooctene with TBHP as the oxygen source. Similar catalytic results were obtained using the tethered tricarbonyl complex directly as a pre-catalyst since fast oxidative decarbonylation occurs under the reaction conditions used. For both systems, the desired epoxide was the only product and the initial activities were about 13 mol·molMo−1·h−1. The solid catalysts were recycled several times. Some activity was lost between the first and second runs but thereafter tended to stabilise. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

The dioxomolybdenumvi complex MoO2Cl2p-tolylCH3DAB has been prepared in good yield by reaction of the solvent adduct MoO2Cl2THF2with one equivalent of the bidentate ligand N,N-p-tolyl-2,3-dimethyl-1,4-diazabutadiene. Treatment of the dichloro complex with the Grignard reagent CH3MgCl gives the dimethyl derivative MoO2CH32p-tolylCH3DAB. The complexes are highly active and selective catalysts for the homogeneous epoxidation of cyclooctene using tert-butyl hydroperoxide TBHP as the oxidant. In both cases, the initial activity is ca. 175 molmol−1Moh−1and cyclooctene oxide is obtained quantitatively within 4 h. It was possible to recover the dimethyl complex at the end of the reaction and reuse it in a second run with only a small decrease in activity. The complexes are also active and selective for the epoxidation of other olefins, such as 1-octene, 2-octene, cyclododecene and R--limonene, with TBHP. The catalytic production of cyclooctene oxide was investigated in detail, varying either the reaction temperature or the initial concentrations of substrate, oxidant and catalyst precursor. Kinetic studies show that the catalyst precursor–oxygen donor complex formation is first-order in TBHP and in the metal complex MoO2Cl2p-tolylCH3DAB. A specific rate of 3.2 mol−1dm3s−1was found for catalyst formation at 25 °C. Activation parameters for this reaction have also been measured ΔH≠  48 ± 3 kJmol−1, ΔS≠  −112 ± 10 Jmol−1K−1.

The dioxomolybdenum(VI) complex [MoO2Cl2(L-L)], containing the bidentate 1,4-diazabutadiene ligand RN=C(Ph)−C(Ph)=NR [R = (CH2)3Si(OEt)3], was immobilised in the ordered mesoporous silica MCM-41 by carrying out a grafting reaction in dichloromethane. The grafted material (3.3 wt.-% Mo) was tested as a catalyst for the epoxidation of cyclooctene with tert-butyl hydroperoxide (TBHP) at 55 °C. Selectivity to the epoxide was very high and the observed kinetic profile was similar to that of the complex [MoO2Cl2(L-L)] in the homogeneous phase. On recycling several times, some activity was lost from the first to second runs, but thereafter stabilised. Tethered complexes of this type were also prepared by a stepwise approach. The mesoporous silicas MCM-41 and MCM-48 were first treated with a toluene solution of the ligand L-L under reflux. Before isolating the grafted materials, the silylating agent Me3SiCl was added to remove the residual Si−OH groups on the surface of the mesoporous materials. Oxomolybdenum species were subsequently introduced into the ligand-silicas by pore volume impregnation of a solution of the complex [MoO2Cl2(THF)2] in CH2Cl2 at room temperature. The modified materials have been characterised by powder X-ray diffraction, N2 adsorption, and solid-state MAS NMR (¹³C, ²⁹Si). Despite having lower metal loadings (1 wt.-% Mo), the materials exhibited similar catalytic activities for the epoxidation of cyclooctene to that of the modified MCM-41 with 3.3 wt.-% of Mo. However, selectivities to the epoxide were lower. The MCM supported catalysts have also been tested for the epoxidation of cyclododecene and α-pinene with TBHP. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Mo K‐edge XAFS spectra have been measured for ordered mesoporous silica MCM‐41 grafted with the complexes [MoO2X2(thf)2] (X=Cl, Br). For grafting reactions in the absence of triethylamine, materials with 1 wt. % Mo are obtained; the Mo K‐edge EXAFS results indicate the co‐existence of isolated surface‐fixed monomeric species {MoO2[(O)3SiO]2(thf)n} and {MoO2[(O)3SiO]X(thf)n}. When Et3N is used in the grafting reactions, materials with 4 wt. % Mo are obtained. The EXAFS data for the material prepared using [MoO2Cl2(thf)2] and Et3N indicate the presence of dinuclear species with two MoVIcentres, each with two MoO groups and each linked by one or two oxo bridges (Mo⋅⋅⋅Mo 3.27 Å). The molybdenum centres in the material prepared using the dibromo complex comprise mainly isolated four‐coordinate dioxomolybdenum(VI) and trioxomolybdenum(VI) monomeric species, with a small contribution from dimeric species. All materials were further characterised in the solid state by powder X‐ray diffraction, N2adsorption analysis, MAS NMR (13C, 29Si) and FTIR spectroscopy. The derivatised MCMs perform differently as catalysts in the liquid‐phase oxidation of various olefins and alcohols with tert‐butyl hydroperoxide. The highest alkene epoxidation activity was recorded for the catalysts with low metal loading, whereas the material containing oxo‐bridged dimers had the highest activity for oxidation of alcohols. The recyclability of all the catalysts was tested: the catalytic activity of the derivatised materials tended to stabilize with ageing.

The organotin–oxomolybdates [(R3Sn)2MoO4]·n H2O (R=methyl, n-butyl, cyclohexyl, phenyl, benzyl) have been prepared and tested as catalysts for the oxidation of benzothiophene with aqueous hydrogen peroxide, at 35 °C and atmospheric pressure. In all cases, the 1,1-dioxide was the only observed product. The kinetic profiles depend on the nature of the tin-bound R group and also on the addition of a co-solvent. For the tribenzyltin derivative, the apparent activation energies for sulfoxidation as a function of the co-solvent are in the order 1,2-dichloroethane (5 kcal mol⁻¹)&lt;ethanol&lt;acetonitrile&lt;water (27 kcal mol⁻¹). The solid could be recovered by centrifugation from the three-phase (solid–liquid–liquid) system containing 1,2-dichloroethane. The catalyst was reused in a second reaction cycle with no significant loss of activity. Increasing the oxidant/substrate ratio from 2:1 to 6:1 allows the corresponding sulfone to be obtained in quantitative yield within 24 h. Changing the nature of the tin-bound R group in the catalyst precursors modifies their physical properties and hence their catalytic performance. The variation in hydrophobic/hydrophilic character is important, since the Me, nBu and Cy derivatives crystallize as anhydrous compounds, whereas the other two derivatives are hydrates. The polymers also have different structures, as evidenced by Xray powder diffraction. Mo K-edge and Sn K-edge EXAFS spectroscopy confirmed that the structures arise from the self-assembly of tetrahedral [MoO4]²⁻ subunits and [R3Sn]⁺ spacers. The Mo···Sn separation in the trimethyltin derivative is a uniform 3.84 Å. By contrast, the EXAFS results revealed the coexistence of short (3.67–3.79 Å) and long (3.93–4.07 Å) Mo···Sn separations in the other coordination polymers. The catalyst precursors were also characterized in the solid state by thermogravimetric analysis, FTIR, and Raman spectroscopy, and MAS NMR (¹³C, ¹¹⁹Sn) spectroscopy.

The organotin–oxomolybdates [(R3Sn)2MoO4]⋅nH2O (R=methyl, n‐butyl, cyclohexyl, phenyl, benzyl) have been prepared and tested as catalysts for the oxidation of benzothiophene with aqueous hydrogen peroxide, at 35 °C and atmospheric pressure. In all cases, the 1,1‐dioxide was the only observed product. The kinetic profiles depend on the nature of the tin‐bound R group and also on the addition of a co‐solvent. For the tribenzyltin derivative, the apparent activation energies for sulfoxidation as a function of the co‐solvent are in the order 1,2‐dichloroethane (5 kcal mol−1)

Two new large-pore sodium vanadium silicates, denoted AM-15 and AM-17 (Aveiro-Manchester, structures number 15 and 17) with framework composition Na6Si14V4O39 and Na4CaSi10V2O27, respectively, are reported. Electron paramagnetic resonance (EPR), Fourier transform infrared (FTIR) and diffuse reflectance ultraviolet-visible (DR UV/Vis) spectroscopy reveal that as-prepared AM-15 contains both V^IV and V^V in a square pyramidal and tetrahedral coordination, respectively, while AM-17 contains predominantly octahedral V^IV. The thermal stability of AM-15 in air is modest (ca. 380 °C), while AM-17 is stable up to 450 °C. The acid-base and redox properties of AM-17 have been assessed by the conversion of isopropanol and ethanol oxidation, respectively. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Mesoporous benzene–silica hybrid materials have been obtained via surfactant-mediated synthesis using alkyltrimethylammonium surfactants with alkyl chain lengths from 14 to 18. By combining N₂ adsorption–desorption experiments and powder X-ray diffraction (PXRD) analysis we showed that these samples exhibited mesopore sizes in the range 3.2 to 3.9 nm and that the molecular-scale periodicity in the walls of these materials is maintained. Transmission electron microscopy (TEM), solid state nuclear magnetic resonance (NMR) and thermogravimetric analyses (TGA) have also been used to characterise the structure and stability of the samples.

Mesoporous benzene–silica hybrid materials have been obtained viasurfactant-mediated synthesis using alkyltrimethylammonium surfactants with alkyl chain lengths from 14 to 18. By combining N2adsorption–desorption experiments and powder X-ray diffraction PXRD analysis we showed that these samples exhibited mesopore sizes in the range 3.2 to 3.9 nm and that the molecular-scale periodicity in the walls of these materials is maintained. Transmission electron microscopy TEM, solid state nuclear magnetic resonance NMR and thermogravimetric analyses TGA have also been used to characterise the structure and stability of the samples.

The synthesis and characterisation of AM-18 (Aveiro-Manchester structure number 18), a large-pore framework sodium titanium silicate catalyst, is reported. The as-synthesised material was characterised by powder X-ray diffraction (XRD), thermogravimetry (TGA) and differential thermal analysis (DTA), scanning electron microscopy (SEM) and chemical analysis by energy dispersive analysis of X-rays (EDAX), nitrogen and benzene adsorption measurements, 23Na triple-quantum (MQ) magic angle-spinning (MAS) and 29Si MAS NMR, Fourier transform infrared (FTIR) and Raman spectroscopies. The latter two techniques indicate that this novel titanium silicate may contain five-coordinated titanium. AM-18 shows high thermostability (in excess of 650 °C). Isopropanol conversion was used as a model reaction to study the acid–base properties of as-synthesised AM-18. This material exhibits high catalytic activity (71% conversion) and selectivity for propene (74%).

Rhodium acetonitrile complexes have been immobilised in the mesoporous silica MCM-41 by pore volume impregnation of a solution of the compound [Rh₂(CH₃CN)₁₀](BF₄)₄ in acetonitrile. Powder XRD and N₂ adsorption–desorption analysis of the derivatised material confirmed that the textural properties of the hexagonally ordered host were preserved during the grafting experiment and that the channels remained accessible. The BET specific surface area decreased from 1047 to 820 m2 g−1, while the specific total pore volume decreased from 0.85 to 0.64 cm3 g−1. The functionalised material was also characterised by MAS NMR (11B, 13C and 29Si), FTIR and Raman spectroscopy, Rh K-edge EXAFS spectroscopy, UV–Vis and luminescence spectroscopy. These studies revealed the existence of surface-attached dimeric species, with a rhodium–rhodium interatomic distance (2.58 Å) similar to that in the compound [Rh₂(CH₃CN)₁₀](BF₄)₄ (2.61 Å). Exposure of this material to light promotes irreversible photodissociation of the dimeric species to give immobilised monomeric species.

The ordered mesoporous silica MCM-41 was covalently grafted with (3-chloropropyl)trimethoxysilane. Chloro substitution by the anion [4-CH₂-4'-Me-2,2'-bipyridine]− gave a ligand-silica containing ca. 0.3 mmol bipyridyl groups per gram. Powder X-ray diffraction and nitrogen adsorption–desorption analysis demonstrated that the textural characteristics of the support were preserved during the grafting experiments and that the channels remained accessible, despite sequential reductions in surface area, pore volume and pore size. The coupling reactions were monitored by 29Si MAS NMR and 13C CP MAS NMR spectroscopy. Bipyridyl-functionalized MCM-41 exhibits a high encapsulating ability, as evidenced by its interaction with a dichloromethane solution of MoO₂Cl₂(THF)₂. A material with a metal loading of 8.3 mass% was obtained. Molybdenum K-edge EXAFS analysis could not substantiate the formation of a tethered complex of the type MoO₂Cl₂(N–N), but instead indicated the formation of unidentate-bridged entities of the type [O₂Mo–X–MoO₂] with a metal–metal separation of 3.28 Å. The molybdenum-containing MCM was active as a catalyst for the epoxidation of cyclooctene by tert-butyl hydroperoxide. However, this activity is due, at least in part, to leached molybdenum species in solution.

Methyltrioxorhenium(VII) (MTO) has been immobilized in the mesoporous silica MCM-41 functionalized with pendant bipyridyl (bpy) groups of the type 4-[(−O)3Si(CH2)4]-4'-CH3-2,2'-bipyridine. Powder X-ray diffraction and N2 adsorption-desorption studies confirm that the regular hexagonal symmetry of the host is retained during the grafting reaction and that the channels remain accessible. Solid-state magic angle spinning NMR spectroscopy (²⁹Si, ¹³C) was also used to monitor the coupling reactions. The formation of a tethered Lewis base adduct of the type CH3ReO3·bpy is confirmed by IR, Raman, and X-ray absorption fine structure (XAFS) spectroscopy. The XAFS results do, however, indicate that not all rhenium ions are present in this form, and this agrees with elemental analysis which gives an Re/N ratio of 1:1.1 (0.55 mmol Re g⁻¹). It is likely that the excess rhenium atoms are present as uncoordinated MTO molecules assembled in the MCM channels. The model complexes (4-alkyl4'-methyl-2,2'-bipyridine)(methyl)trioxorhenium (alkyl = methyl, n-butyl) were also prepared and characterized. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

The chiral dioxomolybdenum(vi) complexes [MoCl{(1R,2S,5S)-8-trimethylsilyloxy-1-(2-pyridyl)mentholato}(O)₂- (THF)] and [Mo{(1R,2S,5S)-8-trimethylsilyloxy-1-(2-pyridyl)mentholato}₂(O)₂] have been prepared in good yields by reaction of the solvent substituted complex [MoCl₂O₂(THF)₂] with one or two equivalents of chiral 2'-pyridyl alcohol. The optically active aminoalcohol was obtained by reaction of 2-pyridyllithium with (-)-(2S,5S)-8-trimethyl-silyloxymenthone. The complexes are active catalysts in the homogeneous epoxidation of cyclic and linear olefins, dienes and terpenes by t-BuOOH. They present remarkable activity and excellent product selectivity in cyclooctene epoxidation (cyclooctene oxide was obtained in quantitative yield). In the case of limonene, regioselectivity is high in favour of the epoxidation of the internal cyclic double bond. Ring opening activity was also observed for α-pinene oxide, producing campholenic aldehyde and epoxy campholenic aldehyde.

Ruthenium(ii) complexes containing the macrocycle 1,4,8,11-tetrathiacyclotetradecane ([14]aneS₄) and nitrogen-coordinated nitrile ligands were prepared using cis-[Ru(DMSO)₄Cl₂] as the starting material. The first synthetic step consisted of introduction of the thioether ligand to give [Ru([14]aneS₄)(DMSO)Cl]Cl. Substitution of the DMSO ligand by the nitrile ligands NCCH₃ or NC(CH₂)₃Si(OEt)₃ (L) gave the complexes [Ru([14]aneS₄)(L)Cl]X (X = Cl⁻, PF₆⁻). Introduction of the triethoxysilyl ligand allowed the [Ru([14]aneS₄)Cl]⁺ fragment to be tethered to the surface of the ordered mesoporous silica MCM-41 by pore volume impregnation of a solution of the complex in dichloromethane. A high metal loading was achieved (0.56 mmol g⁻¹). Powder XRD and N₂ adsorption studies of the derivatised material indicated that the textural properties of the support were preserved during the grafting experiment and that the channels remained accessible, despite a large reduction in both surface area and pore volume (ca. 50%). IR, UV/VIS, photoluminescence and MAS NMR spectroscopy (¹³C, ²⁹Si) confirmed that the structural integrity of the Ru^II complex was retained during immobilisation, except for loss of ethoxide groups due to the reaction with pendant silanols on the silica surface.

The gas-phase oxidative dehydrogenation of cyclohexanol with air using ETS-10 and related materials is studied. At reaction temperatures below 200°C, ETS-10 was 100% selective to cyclohexanone and 75% cyclohexanol conversion was achieved. Catalyst deactivation was attributed to pore blockage. The catalyst is completely regenerated by calcination at 400°C. Cyclohexanol conversion increases with temperature and oxygen/cyclohexanol feed ratios. The introduction of Cr, Fe, K, or Cs in ETS-10 affects stability and decreases conversion and selectivity towards cyclohexanone. For comparison, some results on TS-1, zeolites NaX, and mordenite are also presented.

A series of microporous chromium-containing ETS-10 (ETCrS-10) with different Cr/Ti molar ratios have been synthesised and characterised by energy dispersive absorption of X-rays (EDAX), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ adsorption measurements, diffuse-reflectance ultraviolet spectroscopy (DR-UV), electron paramagnetic resonance (EPR) and ²⁹Si magic-angle-spinning nuclear magnetic resonance (MAS NMR). The diffractograms of the as-synthesised ETCrS-10 materials indicate that the ETS-10 structure has remained intact upon Cr substitution. DR-UV and EPR reveal that the as-prepared ETCrS-10 samples contain Cr³⁺ in octahedral coordination. The resonances in the ²⁹Si MAS NMR spectra of ETCrS-10 are considerably broader than those of ETS-10 and such a broadening may relate to isomorphous substitution of Cr for Ti in the ETS-10 framework. The acid–base properties of ETCrS-10 materials are responsible for the high dehydrogenation activity in the conversion of isopropanol to acetone and dehydration activity in the conversion of tert-butanol to isobutene. Ethanol oxidation over ETCrS-10 yields acetaldehyde as a main product.

The dioxomolybdenum(VI) fragment MoO2X2 has been confined within the ordered mesopores of pure siliceous hexagonal MCM-41 and cubic MCM-48 molecular sieves either by direct grafting (solvent impregnation) with MoO2X2(THF)2 or by using a spacer ligand [L = NC(CH2)2Si(OEt)3]. The materials have been characterised by elemental analysis, powder X-ray diffraction, N2 adsorption, IR spectroscopy, and magic-angle spinning NMR spectroscopy (¹³C, ²⁹Si). All catalysts, homogeneous and heterogenised, are active in the epoxidation of cyclooctene with tert-butyl hydroperoxide.

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Review: 85 refs.

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