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Four different C 3 N 4 specimens have been prepared, a bulk one (MCN), a thermally etched (MCN-TE), a solid prepared by hydrothermally treating MCN with H 2 O 2 (MCN-H 2 O 2 ) and a polymeric carbon nitride-hydrogen peroxide adduct (MCN-TE-H 2 O 2 ). The principal aim of this work was to correlate the capability of the prepared material to generate reactive oxygen species (ROS), under irradiation, with their photocatalytic activities in terms of conversion and selectivity for partial oxidation reactions. Photoelectrochemical studies revealed that MCN-TE represented the best material in terms of photoconductivity, whereas MCN-H 2 O 2 was defective and evidenced a poor mobility of carriers. EPR studies showed a maximum generation of reactive oxygen species irradiating the MCN-TE sample. The photocatalytic activity of these materials in the selective oxidation of three different alcohols to the corresponding aldehydes, both under UV and natural solar light, showed that the highest conversion was obtained in the presence of the MCN-TE sample, whereas the most selective one was MCN-TE-H 2 O 2 . Under solar light irradiation the performances of the powders were generally better than those under UV light. The characterization of the C 3 N 4 -based materials well justified their photocatalytic activity. The pristine C 3 N 4 materials were more active but less selective than those prepared in the presence of H 2 O 2 .

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.

Polymeric carbon nitride-hydrogen peroxide adduct (PCN-H2O2) has been prepared, thoroughly characterised and its application for selective photocatalytic conversion of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxaldehyde (FDC) in aqueous suspension has been studied. The PCN-H2O2 adduct is stable in aqueous suspension under UV and solar irradiation up to 100 °C. It is also stable up to 200 °C if heated in air, while at temperatures close to 300 °C its decomposition takes place. Based on the obtained characterisation data it has been proposed that H2O2 attaches to the non-polymerised carbon nitride species and to the heptazine nitrogen atoms, thus producing strong hydrogen bonding within the PCN-H2O2 adduct. The blockage of the surface amino-groups in PCN-H2O2 by H2O2 hinders the interaction of HMF with these sites, which are responsible for unselective substrate conversion. PCN-H2O2, although being less active, possesses a superior selectivity in natural solar light assisted oxidation of HMF to FDC reaching 80% with respect to its thermally etched PCN counterpart, which gives rise to a 40–50% selectivity. We believe that the exceptional performance of the applied photocatalyst in the selective photocatalytic conversion of HMF to a high added value FDC in a green solvent under natural illumination makes a significant contribution to the development of environmentally friendly technologies for biomass valorisation.

[EN] Keggin [PW12O40]3– and Wells–Dawson [P2W18O62]6– heteropolyanions are nanosized transition-metal-oxygen clusters belonging to the heteropolyacids (HPAs) family. They are widely used as catalysts due to their high Brønsted acidity, and their dispersion on solid supports favors the accessibility to their acid sites generally increasing the catalytic activity. A series of binary materials composed of Keggin or Wells–Dawson HPAs and SiO2, TiO2, and ZrO2 have been prepared by impregnation or solvothermal methods. Remarkable differences have been found in the catalytic activities among the unsupported and supported HPAs. These differences have been correlated in the past to the structural changes of the HPAs due to the cluster–support interaction, which is different depending on preparation methodologies of the binary material. In the present work, the modes of interaction between the two types of HPA, Keggin and Wells–Dawson, and various supports have been studied by X-ray absorption spectroscopy. The obtained data have been compared to the characterization of the same materials reported before by using different bulk and surface physicochemical techniques. The characterization results were then used to correlate the interaction modes between the HPAs and the supports with the catalytic performances reported for 2-propanol dehydration to propene and for propene hydration to 2-propanol. The results reveal that the deposition of HPA by impregnation or solvothermal treatment may cause distortions in the H3PW12O40 cluster structure depending on the presence of stronger (TiO2 and ZrO2) or weaker (SiO2) basic sites in the support, respectively. Moreover, the type of preparation method affects the structure and acidic properties of the supported HPAs. In particular, during the preparation of TiO2 and ZrO2 with HPA by in situ solvothermal method, the reaction of the HPA with the products of metal alkoxides hydrolysis occurs with consequent destruction of the Keggin structure. Therefore, the catalytic activity of such materials is poor. These modifications, in addition to the bulk and surface features of the supported HPAs, affected the catalytic 2-propanol dehydration to a significant extent. On the contrary, the propene hydration was less influenced, probably, due to the propene nonpolar nature., I.K. acknowledges the financial support from Spanish MINECO (MAT2016-78155-C2-1-R), Gobierno del Principado de Asturias (GRUPIN-ID2018-170), and Ministry of Education and Science of the Russian Federation (grant No. 4.9722.2017/8.9). L.F.L. is grateful to progetto di Ricerca ARS01_00637 Energie per l′Ambiente-TARANTO (PNR 2015-2020) for financial support. The authors acknowledge The European Synchrotron (The ESRF), MINECO, and CSIC for the provision of measurements using the BM25-SpLine beamline. They also thank the BM25-SpLine staff for the technical support beyond their duties.