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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.

In the present work we scrutinize the effect of substituents in the phenyl ring of the benzyl alcohol on its photo-oxidation to the corresponding benzaldehyde driven by polymeric carbon nitride photocatalyst in aqueous medium. It has been established that electron donating (ED) substituents in para- and ortho-position with respect to the CH2OH-group promote the reactivity of the substrate without compromising the selectivity towards benzaldehyde formation, maintaining it in the range of 84–98 %, if compared to the unsubstituted molecule. The same observation is true for meta-substituted benzyl alcohol with an electron withdrawing (EW) group. On the other hand, the presence of ED-group in meta-position or EW-group in para-position with respect to the CH2OH-group reduces the reactivity as well as the selectivity towards the aldehyde production, resulting in the values of selectivity ranging from 40 to 80 %. Quantum chemical calculations have allowed to establish that the reactivity correlates with the positive charge on the benzylic carbon in benzyl alcohol cation intermediate, while the selectivity, most probably, is conditioned by a negative charge on the carbon atoms in the phenyl ring being a target for an electrophile attack. The modification of the polymeric C3N4 photocatalyst with H2O2 reduces the unselective benzyl alcohol oxidation, thus favouring the higher selectivity towards benzaldehyde production.