Continuous sol-gel synthesis of phosphated TiO₂ catalysts in a microreactor

Continuous microreactors, featuring µm-sized tubes and thus a fast mass and heat transfer, enable a precise control of chemical processes, which leads to improved efficiency and safety with respect to conventional large-scale reactors.^[1] In addition, their modular design adds unrivaled flexibility. While numerous catalyzed reactions in microreactors have been reported,^[1] the preparation of heterogeneous catalysts in these systems has been greatly overlooked even though a controlled design of catalysts will be key for the development of more sustainable chemical processes.^[2]

Therefore, we established a novel continuous sol-gel microreactor process for the production of phosphated TiO₂, which can be applied as cheap and eco-friendly catalyst for the selective synthesis of the platform chemical 5‑hydroxymethylfurfural (5-HMF) from bio-derived glucose.^[3] This approach allows a highly efficient catalyst preparation since all steps, i.e., (i) sol formation, (ii) functionalization by H₃PO₄, and (iii) gel formation, are carried out in one continuous process (Figure 1, left). The catalytic performance was correlated with the catalyst properties (e.g., surface area) and its synthesis parameters (e.g., temperature) through design of experiments (DoE, Figure 1) to obtain the optimal catalyst preparation process and the material with the best catalytic results. Due to these correlations, we could additionally gather a deeper mechanistic understanding of both catalyst synthesis and catalyzed reaction. This was essential to rationalize phenomena occurring such as fouling, i.e., blockage of the microchannels by large agglomerates, which can pose a critical challenge upon the catalyst production.

[1] V. Hessel, S. Hardt, H. Löwe, Chem. Micro Process Engineering, Wiley-VCH, Weinheim, 2004, 674 pp.
[2] H. Jacobsen, in Handbook of Green Chemistry, Vol. 2 (Eds.: T. Anastas, R. H. Crabtree), Wiley-VCH, Weinheim, 2013, pp. 93–116.
[3] L. Atanda, S. Mukundan, A. Shrotri, Q. Ma, J. Beltramini, ChemCatChem, 2015, 7, 781–790.