Non-hydrolytic sol–gel as a versatile route for the preparation of hybrid heterogeneous catalysts

The tools of sol–gel chemistry allow synthesizing a plethora of functional materials in a controlled bottom-up fashion. In the field of heterogeneous catalysis, scientists utilize sol–gel routes to design solids with tailored composition, texture, surface chemistry, morphology, dispersion, etc. A field of investigation which shows great promises is that of hybrid heterogeneous catalysts. Examples are flourishing to show that catalysts featuring a combination of inorganic and organic components often display improved activity, selectivity, or even chemical stability as compared to the purely inorganic counterparts. Yet, classic sol–gel methods face some well-known limitations—related to the different reactivity of the precursors and to the high surface tension of water—which complicate the tasks of chemists, specifically for the synthesis of hybrid catalysts. Non-hydrolytic sol–gel (NHSG) chemistry appears as a pertinent alternative. Being realized in the absence of water, NHSG routes allow reaching an excellent control over the solid properties and on the distribution of the organic and inorganic components at the nano- and microscales. In this review, we briefly recapitulate the main types of non-hydrolytic sol–gel routes and we present the modified protocols to hybrid materials. Then, we present an overview of the non-hydrolytic sol–gel approaches that have been proposed to synthesize hybrid heterogeneous catalysts. For both Class I and Class II hybrids, we discuss how the NHSG technique has allowed tailoring the key properties that command catalytic performance. From this panorama, we argue that NHSG has a prominent role to play for the development of advanced hybrid heterogeneous catalysts.