Macrocellular Titanosilicate Monoliths as Highly Efficient Structured Olefin Epoxidation Catalysts

Abstract

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...

Titanosilicate Epoxidation Catalysts: A Review of Challenges and Opportunities

“…Utilizing high internal phase emulsions (HIPEs) [249,250] as sacrificial templates, Debecker et al . recently proposed the synthesis of three‐level micro‐/meso‐/macroporous self‐standing Ti−SiO 2 monoliths (Figure 11, right) [251] . Titanium isopropoxide was pre‐hydrolyzed in acidic conditions and polycondensed in the emulsion with pre‐hydrolyzed TEOS.…”

Enzyme immobilization inside the porous wood structure: a natural scaffold for continuous-flow biocatalysis

“…[18][19][20][21][22] With such systems, effective continuous-ow biocatalysis is possible, for example for the production of biodiesel or ne chemicals. 23,24 Generally, the materials used for these applications are either inorganic silica-based monoliths synthesized by sol-gel chemistry, 25,26 porous polymer networks, 27,28 or polymer-inorganic composites. 29 Hence, they require tedious synthetic processes as well as energy-and resource-intensive production methods, which calls for more sustainable alternatives.…”

Biocatalytic transamination in a monolithic flow reactor: improving enzyme grafting for enhanced performance

“…[17][18][19][20] Instead of the classical powdery materials that can be used as carriers and packed in a xed bed reactor, a seducing approach for ow chemistry is to use self-standing porous monoliths. [21][22][23] Inorganic or polymer monoliths containing intricate pore networks can be obtained in various desired shape and provide unique advantages such as fast kinetics and high throughput. 21 Porous monolithic materials can have a high void fraction, thereby provoking only low pressure drop during the passage of a uid.…”