New Ti centers in AlPO-5 materials catalytically active and selective in the oxidation of cyclohexene

Abstract

Trabajo presentado en el 5th Early Stage Researchers Workshop in Nanoscience, celebrado en Madrid (España) del 17 al 18 de junio de 2019.Zeolites and zeotypes are nanoporous crystalline materials , based on a three dimensional T02 (T = Si, Al, P, etc.) network and potentially used as heterogeneous catalysts with shape selectivity to reactants and products. These networks allow the incorporation of a great number of metallic elements by isomorphous substitution , what increased their applications as suitable catalysts far redox or acid-base reactions. In that sense , Ti-containing zeolites have become relevan! industrial heterogeneous catalysts in oxidation processes [1]. On the contrary , their homologous Si-free zeotypes TAPO materials have shown scarce catalytic activity in !hose reactions , likely due to their different titanium environments within the nanoporous networks. However, AIP04 frameworks possess higher versatility !han zeolitic ones in terms of heteroatom incorporation as either Al(lll) and/or P(V) ions could be substituted. The incorporation of Sn(ll) in Al(lll) siles of the AIP0-5 framework , instead of the more conventional and stable Sn(IV), replacing P(V), resulted in a material presenting better catalytic behavior in acid reactions [2]. In this work , we have changed the incorporation mechanism of titanium ions into AIP0-5 framework (Figure) in arder to generate titanium environments different from !hose found in the conventional TAP0-5 , by using Ti(lll) sources [3] instead of Ti(IV) ones in the synthesis gel. The final aim was the modification of the catalytic activity of TAP0-5 in the oxidation of olefins , particularly cyclohexene , using H,O, as the oxidan!. The Ti(lll)AP0-5 materials synthesized by this strategy enhanced the catalytic activity compared to the conventional Ti(IV)AP0-5 ones. Furthermore , they could reach the conversion level of the Ti-beta zeolite under suitable reaction conditions. Unlike the latter, AIP04 type materials are hydrophilyc and, consequently , the catalytic activity of both TAP0-5 tested was greatly affected by the presence of water in the reaction medium. Regarding the selectivity, Ti(lll)AP0-5 catalysts run the reaction preferably through a radical pathway , being highly selective to the allylic oxidation products , rather !han through the epoxidation route almos! exclusively followed by Ti-betaPeer Reviewe