A spray drying technique is used to prepare hollow zeolite microparticles into which an enzyme can be entrapped. Via this “Lego-like” strategy, we create hybrid heterogeneous catalysts that can run multistep chemo-enzymatic cascade reactions.
Herein, we present the aerosol-assisted sol − gel preparation of hierarchically porous TiO 2 − SiO 2 catalysts having a spherelike shell morphology and a high Ti dispersion. In order to control the porosity at the micro-, meso-, and macrolevels, we use the evaporation-induced self-assembly (EISA) of a surfactant, possibly combined with polymer beads as hard templates. These catalysts are tested for the epoxidation of cyclohexene with cumene hydroperoxide as the oxidant, and their performance is compared to the reference TS-1 zeolite. The high catalytic performance observed with the catalysts prepared by aerosol stems from their high speci fi c surface area, but also from the short di ff usion path length generated by the meso-/macropore architecture which provides entryways for bulky reactants and products. Besides, these materials can incorporate a higher Ti loading than TS-1 zeolite, while ensuring a good control over the Ti speciation. Thus, the unique features of the aerosol process which is also known to be scalable allow us to prepare catalytic materials with high epoxidation activity, also for bulky olefins.
Epoxidation reactions are tremendously important for modern chemistry, as they lead to series of highly useful bulk and fine chemicals, monomers, and intermediates for organic synthesis. Progress in epoxidation processes goes hand in hand with the advancement made in catalysis science. In this context, heterogeneous catalysts, and particularly Ti‐based formulations, are playing a central role and have seen tremendous developments over the past two decades, leveraging on advanced materials science. The aim of this review is to illustrate the various strategies of titanosilicate catalysts preparation that can lead to more versatile, more performant, and greener epoxidation processes. We successively cover (i) supported catalysts, obtained by the grafting of Ti species onto preformed silica supports, (ii) microporous crystalline titanosilicates (zeolites), and (iii) amorphous titanosilicates obtained by sol‐gel chemistry. For each category, with an emphasis on catalyst preparation, the challenges that have to be tackled to boost catalyst performance are highlighted. From that point, we present a critical review of the different approaches that have been proposed in the primary literature to tailor the properties that govern catalysts performance (activity, selectivity, stability, ease of handling). This is done by better controlling the nature of the active surface species, adapting particles size and shape, optimizing texture, modifying surface chemistry, etc. These lines of attack encompass molecular approaches for the grafting of well‐defined species, top‐down and bottom‐up synthesis of hierarchically porous zeolites, advanced sol‐gel routes potentially performed in non‐conventional media or coupled with original processing, preparation of self‐standing monoliths, etc. Future research directions are discussed with emphasis on the application scope of new catalytic materials and possible approaches to increase catalyst performance.
Mononuclear complexes of formula [Fe(iptz) 6 ]X 2 {X = BF 4 -(1), ClO 4 -(2); iptz = 1-isopropyl-1H-tetrazole} have been synthesized and characterized spectroscopically and structurally. These materials display a gradual and incomplete spin conversion around 95 K on cooling, as well as a reversible thermochromic effect. This magnetic behavior was satisfactorily simulated using the Sorai and Seki domain model (ΔH = 5.9 kJ mol
Titanosilicates (Ti-SiO2) are well-known catalysts for the epoxidation of olefins. Isolated Ti inserted in the silica framework in tetrahedral coordination are the active species. Recently, adjusting the hydrophobic/hydrophilic balance of such catalysts’ surfaces has appeared as a promising tool to further boost their performance. However, adjusting the hydrophobic/hydrophilic balance via a one-pot classical sol-gel generally leads to a decrease in the Ti dispersion and/or collapse of the pore network. To overcome this limitation, hydrophobic mesoporous Ti-SiO2 were here synthesized by aerosol-assisted one-pot sol–gel, which allowed the simultaneous control of their Ti loading, degree of methyl-functionalization, and textural properties. Methyl-functionalization was achieved by a partial substitution of tetraethoxy silane (TEOS) by methyltriethoxy silane (MTES) in different ratios. Solid-state 29Si-NMR, FTIR, TGA, and vapor-phase water adsorption showed that methyl moieties were effectively incorporated, conferring a hydrophobic property to the Ti-SiO2 catalysts. ICP-AES, DRUV, XPS, and N2 physisorption demonstrated that Ti dispersion and textural properties were both successfully preserved upon the incorporation of the methyl moieties. In the epoxidation of cyclooctene with tert-butyl hydroperoxide as oxidant, the hydrophobic Ti-SiO2 showed higher catalytic performance than pristine Ti-SiO2 prepared without MTES. In addition to disentangling the positive effect of adjusting the hydrophobic/hydrophilic balance of epoxidation catalysts on their performance, this contribution highlights the advantages of the aerosol procedure to synthesize mesoporous functionalized catalysts with very high dispersion of active sites.
Self-standing macrocellular titanosilicate monolith foams are obtained using a one-pot sol-gel route and show excellent performance in the epoxidation of cyclohexene. Thanks to the High Internal Phase Emulsion (HIPE) templating method, the materials feature a high void fraction, a hierarchically porous texture and good mechanical strength. Highly dispersed Ti species can be incorporated in tetrahedral coordination in the silica matrix. These characteristics allow the obtained 'SiTi(HIPE)' materials to reach high catalytic turnover in the epoxidation of cyclohexene. The monoliths can advantageously be used to run the reaction in continuous flow mode.Titanosilicates are an important class of materials that catalyze selective oxidation reactions. [1] For example, the combination of TS-1 zeolite -in which Ti atoms substitute Si atoms in the crystalline framework -with H 2 O 2 is omnipresent in industrial applications, notably for hydroxylation and epoxidation reactions. [2,3] The crystallinity and the intrinsic hydrophobicity of TS-1 make this material particularly efficient to catalyze the oxidation of small substrates with high selectivities. [4] However, the microporosity of TS-1 makes it less efficient for the conversion of bulky olefins, as the restricted diffusion of the reactants and products in and out the porosity can severely lower the catalytic activity which is consequently mainly restrained to the external surface. [5,6] An additional limitation is linked to the fact that the Ti loading incorporated into the zeolitic framework can hardly be tuned -with a typical maximal Ti loading of 2.5 % (here and after the loading is expressed as mol Ti/(mol Ti + mol Si) × 100 %). [7] These issues triggered intensive research on the incorporation of titanium in silicabased materials with larger pores in order to broaden the range of applications for titanosilicates. [8][9][10] New strategies in this field are flourishing, taking benefits of bottom-up sol-gel routes [11] involving templating methods -possibly combined to supercritical [12,13] or aerosol [14,15] drying strategies -or realized under non-aqueous conditions. [16][17][18] Despite those successes, the resulting materials are often obtained in the form of fine powders which require further steps to shape the catalysts as extrudates, pellets or monoliths, before they could be used in industrial flow processes. [19] This shaping step represents an additional challenge as it necessitates a fine control of the mechanical properties and the diffusion regimes without sacrificing the intrinsic performance of the catalyst. Alternatively, the catalyst can be dispersed onto a pre-shaped solid support [20][21][22] that should meet several specifications (texture, surface composition, mechanical stability, etc.). With such strategy, a challenge is to maximize the loading and the dispersion of the catalyst while avoiding leaching and/or deactivation. [23] Thus, the one-step preparation of Ti-containing materials that do not require further shaping steps is highly sought f...
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