Metal nanoclusters supported on cross-linked functional polymers: a class of emerging metal catalysts

Supported metal catalysts, M0/S, are typically two- components materials built up with a nanostructured metal component, in which the metal centre is in the zero oxidation state (M0), and with an inorganic support (S), quite various in its chemical and structural features [1]. M0 is the component typically deputed to the electronic activation of the reagents involved in the catalyzed reac- tions. S is typically a microstructured component mainly deputed to the physical support and to the dispersion of M0 nanoclusters. The catalytic chemistry of M0 depends on the elemen- tary properties of M and on the structure and size of the M0 nanoclusters (\u2018\u2018quantum dots\u2019\u2019) [2]. S may play a role as a reactivity enhancer of M0/S as a whole (co-catalytic role) and/or as a promoter of its catalytic chemoselectivity (promotional role) [3,4]. The proposal of cross-linked functional polymers (CFPs) as supports of metal nanoclusters to be employed in cata- lytic applications dates back to 1969 [5]. The inventors of the relevant Bergbau Chemie patent, which describes the one-pot synthesis of the industrially important solvent me- thylisobutylketone (MIBK) from acetone and dihydrogen, recognised in CFPs the ability of carrying metal nanoclus- ters and of providing the catalyst with another specific function at the same time. They produced in this way a smart bifunctional catalyst for the acid-catalysed condensa- tion of acetone to 4-hydroxy-4-methyl-2-pentanone, its de- hydration to mesytylene oxide and the hydrogenation of the latter to MIBK [6]. Catalysts based on CFPs of this type are currently employed in at least three other industrial proc- esses, i.e. the synthesis of MTBE with feedstocks rich in un- saturated substrates different from isobutene (Erdolchemie process) [7], the removal of ppm amounts of O2 from in- dustrial waters [8] and the production of alkanes and of branched ethers for the manufacturing of green petrol (BP etherol process) [7]. The concept of bifunctionality can be suitably ex- panded (Scheme 1) to produce multifunctional catalysts bearing both diverse nanostructured metals and diverse chemical functionalities for performing complementary catalytic tasks. The fine concept embodied in the proposal of CFPs as supports of nanostructured metal phases, experienced in fact an almost total inattention in the academic Catalysis community until the mid-nineties, when a few research groups started a systematic exploration of the field [9\u201312]