Titania-silica monolithic multichannel microreactors. Proof of concept and fabrication/structure/catalytic properties in the oxidation of 2,3,6-trimethylphenol

• Titania-silica multichannel microfluidic reactors were fabricated.
• Highly dispersed Ti ions were successfully incorporated into silica monoliths.
• Impact of the fabrication on the structural/catalytic properties was shown.
• High catalytic activity in selective oxidation of TMP to TMBQ was confirmed.

Titania-silica monoliths with 3D hierarchical porosity in μm and nm scales were fabricated using either direct or post-synthesis method, to find that they enable very fast continuous-flow oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethyl-1,4-benzoquinone with hydrogen peroxide as the oxidant. Extensive characterization of the monoliths using N2 adsorption, mercury porosimetry, SEM, energy dispersive X-ray mapping, FT-IR and UV–Vis spectroscopy showed a strong impact of the fabrication method on the structural properties and also coordination/dispersion of the titanium ions incorporated into the silica. Systematic study of the monolithic microreactors and corresponding powders in the continuous-flow and batch systems, revealed a large complexity of performance/structure/catalytic properties relationships. A direct method resulted in titanium active centres highly dispersed in microporous skeleton, and therefore larger TOF, compared to the surface titanium entities in monoliths obtained from the post-synthesis approach. However, owing to the lower porosity and much smaller flow-through (macro)pores the pressure drops were almost two orders of magnitude larger. The highly porous monolithic microreactors fabricated by the post-synthesis incorporation of titanium appeared to be superior; the substrate conversions of 85% were obtained in about 12 min compared to those of ca. 75% obtained in batch systems after 1 h. This could be explained by very intensive mass transport in macro- and mesopore size scales.

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