Heteropolyacids supported on boron nitride and carbon nitride for catalytic and catalytic photo-assisted alcohol dehydration

Keggin H3PW12O40 (PW12) and Wells-Dawson H6P2W18O60 (P2W18) heteropolyacids (HPAs) are photo(catalysts) for various acid-promoted and redox catalytic reactions. Their activity increases if they are deposited on certain supports as metal oxides or carbon materials. Although the surface area and acid-base interactions of the HPAs with supports are considered as key factors for the performance of the binary material, the role of the local structure changes in the HPAs upon their immobilization on solids must be also of primary importance, directly affecting both acidity and photoredox properties. Here, the (photo)catalytic performance of Keggin and Wells-Dawson heteropolytungstates supported on boron nitride (BN) and two types of carbon nitride (C3N4) in the gas-solid 2-propanol dehydration to propene has been studied. Apart from characterizing the materials by a set of conventional laboratory-scale structural and physical-chemical techniques, an insight into the variations of the local structure of supported HPAs is provided by means of X-ray absorption spectroscopy (XAS), and their influence on the supported HPAs (photo)catalytic activity is discussed. The irradiation with UV light increased the activity of both heteropolytungstates with respect to the catalytic experiments. The apparent activation energy calculated for the photo-assisted process resulted always lower with respect to that obtained for the catalytic one alone. Moreover, the materials that showed the highest apparent turnover frequency (TOF) were those in which the HPAs were impregnated on graphitic C3N4 or on BN and, in particular, the highest TOF values were observed at the highest temperature for the PW12/BN and P2W18/BN samples. Interestingly, the highest activity of HPAs supported on BN seems to be due to the stabilization of the W octahedral coordination, according to the XAS data.
LFL is grateful to progetto di Ricerca ARS01_00637 Energie per l’Ambiente-TARANTO (PNR 2015-2020) for financial support. IK acknowledges support by Spanish MINECO (MAT2016-78155-C2-1-R), Gobierno del Principado de Asturias (GRUPIN-ID2018-170) and the Alexander von Humboldt Foundation through the Humboldt Research Fellowship. S.V.-R. and J.I.P. acknowledge financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades (MCIU), the Spanish Agencia Estatal de Investigación and European Regional Development Fund (ERDF) through project RTI2018-100832B-I00, and partial funding by Plan de Ciencia, Tecnología e Innovación (PCTI) 2013-2017 del Principado de Asturias and the ERDF (project IDI/2018/000233). A.S acknowledges financial support from the MCIU through the projects MAT2017-86540-C4-1-R and RTI2018-095303-A-C52 and from the Comunidad de Madrid for an “Atracción de Talento Investigador” contract (No. 2017t2/IND5395). The European Synchrotron (ESRF), MCIU and Consejo Superior de Investigaciones Científicas (CSIC) are also acknowledged for the provision of synchrotron radiation facilities. We also thank the BM25-SpLine staff for the technical support beyond their duties.