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Electrically conductive metal-organic frameworks (MOFs) have emerged in the past few years as promising materials towards applications in (opto)electronics, electrocatalysis and energy storage, among others. One of the most common strategies for the design of conductive MOFs is based on the use of electroactive organic ligands and their partial oxidation/reduction to increase the number of charge carriers. Although perylene salts were reported as the first molecular conductors, they have been scarcely explored as building blocks for the construction of conductive MOFs. Herein we report the electrical conductivity enhancement of a microporous perylene-based MOF upon partial ligand oxidation by using two-probe single-crystal devices. The origin of the conductivity enhancement is rationalised by means of spectroscopic studies and quantum-chemical calculations, supporting a through-space hopping transport along the herringbone perylene packing. This study opens the way for the design of conductive MOFs based on perylene building blocks.

Heptazine‐based polymeric carbon nitrides (PCN) are promising photocatalysts for light‐driven redox transformations. However, their activity is hampered by low surface area resulting in low concentration of accessible active sites. Herein, we report a bottom‐up preparation of PCN nanoparticles with a narrow size distribution (ca. 10±3 nm), which are fully soluble in water showing no gelation or precipitation over several months. They allow photocatalysis to be carried out under quasi‐homogeneous conditions. The superior performance of water‐soluble PCN, compared to conventional solid PCN, is shown in photocatalytic H2O2 production via reduction of oxygen accompanied by highly selective photooxidation of 4‐methoxybenzyl alcohol and benzyl alcohol or lignocellulose‐derived feedstock (ethanol, glycerol, glucose). The dissolved photocatalyst can be easily recovered and re‐dissolved by simple modulation of the ionic strength of the medium, without any loss of activity and selectivity.

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.

This work reports a study on the reactivity of polymeric carbon nitride (PCN) and PCN-H2O2 adduct samples for sunlight photocatalytic selective oxidation reactions. The characterization of these materials was reported in a previous paper where the reactivity towards the partial oxidation of 5-hydroxymethylfurfural (HMF) was studied by using two different laboratory scale photoreactors; one irradiated by UV lamps and the other one by natural sunlight. In the present study it has been confirmed the effectiveness of these C3N4 based materials for the selective partial oxidation of HMF to FDC (2,5-furandicarboxaldehyde) in aqueous medium in a pilot plant photoreactor irradiated by natural sunlight. The reactivity results and, in particular, the selectivity to FDC formation have been very encouraging, mostly by considering that the reaction was carried out in water. Moreover, they are comparable with those obtained in the laboratory scale photoreactors irradiated by both UV lamps and natural sunlight. Interestingly, the pristine PCN sample has shown a higher HMF conversion with respect to that of the PCN-H2O2 adduct, but the last one is more selective to the FDC formation. A kinetic study indicates that the pseudo-first-order rate constant of HMF oxidation is higher in the case of pristine photocatalyst and that the equilibrium adsorption constants of HMF is higher in the case of PCN-H2O2 adduct catalyst. Finally, the partial oxidation of two aromatic alcohols, i.e. benzyl alcohol (BA) and 4-methoxy benzyl alcohol (4-MBA) to benzaldehyde (BAL) and 4-methoxy benzaldehyde (4-MBAL), respectively, has also been studied.

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.

Composite microporous SiO2-TiO2 spheres and micro/mesoporous TiO2 spheres were prepared via the template-free two-step synthetic route using aqueous peroxotitanate solution and tetraethyl orthosilicate (TEOS) as precursors. Both the composite SiO2-TiO2 and pure TiO2 spheres prepared by the solvent-exchange method were initially non-porous, but the applied reflux treatment in water-ethanol suspension successfully transformed them into microporous materials with high apparent surface areas approaching 500 m2·g− 1 and the micropore volume of 0.17 cm3·g− 1, while maintaining the same morphology. The prepared composites retained high values of pore volume and specific surface area up to 400 °C of thermal treatment temperature. The crystallization of TiO2 into the anatase phase in the mixed oxide occurred only at 700 °C, that process was also accompanied by the significant reduction of pore volume, as well as apparent surface area values. The prepared materials were tested as adsorbents for the lead(II) removal; they demonstrated high adsorption capacities, reaching 340 mg(Pb2 +)·g− 1. Moreover, the mixed silica-titania oxide was found to be more efficient adsorbent at low pH values.

The photocatalytic TiO2-assisted decomposition of 2-(4-methylphenoxy)ethanol (MPET) in aqueous solution has been studied for the first time. The intermediate compounds of MPET photodegradation have been also determined. A toxic p-cresol is formed in significant quantities during the photocatalytic reaction. A solvent-exchange approach for a template-free preparation of spherical TiO2 particles has been described, which is based solely on precipitation of hydrous titania from aqueous titanium peroxo complex by using organic solvents. The proposed method favours the formation of spherical titania particles with a mean size varying from 50 to 260 nm depending on the choice of solvent. The procedure for converting nonporous titania spheres into mesoporous material maintaining the same spherical morphology has been developed. The synthesized TiO2 spheres demonstrate a degree of MPET photo-degradation close to that of the commercial titania Aeroxide P25, besides being successfully recovered and reused for four reaction cycles without loss of photocatalytic activity. The effectiveness of the commercial Aeroxide P25 in MPET photodegradation, on the other hand, suffers 10-time drop during the third reaction cycle, which is attributed to its poor recoverability because the photocatalyst is composed of small particles of 20 nm size.

Extended X-ray absorption fine structure (EXAFS) investigation of the oxygen-rich titania formed via the thermal treatment of N-doped TiO has revealed that the removal of N-dopants is responsible for the creation of defect sites in the titanium environment, thus triggering at high temperatures (500-800 °C) the capture of atmospheric oxygen followed by its diffusion toward the vacant sites and formation of interstitial oxygen species. The effect of the dopants on Ti coordination number and Ti-O and Ti-N bond distances has been estimated. The photocatalytic p-cresol degradation tests have demonstrated that the interband states formed by the N-dopants contribute to a greater extent to the visible-light activity than the oxygen interstitials do. However, under the UV irradiation the oxygen-rich titania shows higher efficiency in the pollutant degradation, while the N-dopants in N-TiO play the role of recombination sites. The presence of the surface nitrogen species in TiO is highly beneficial for the application in partial photooxidation reactions, where N-TiO demonstrates a superior selectivity of 5-hydroxymethyl furfural (HMF) oxidation to 2,5-furandicarboxaldehyde (FDC). Thus, this work underlines the importance of a rational design of nonmetal doped titania for photocatalytic degradation and partial oxidation applications, and it establishes the role of bulk defects and surface dopants on the TiO photooxidation performance.

The unprecedented ability of g-C3N4 to chemoselectively photo-oxidise the methyl group of 2-(4-methylphenoxy)ethanol instead of the easily oxidised oxyethanol fragment has been demonstrated. When g-C3N4 is treated with H2O2, its selectivity enhances due to the blocking of surface sites responsible for the adsorption and the subsequent oxidation of the oxyethanol substituent.

A new completely inorganic method of preparation of silica–titania photocatalyst has been described. It has been established that the addition of silica promotes crystallinity of TiO2 anatase phase. Relative crystallinity and TiO2 crystal size in the silica–titania particles increase with the silica content until SiO2/TiO2 molar ratio of 0.9, but at higher molar ratios they start to decrease. The single-source precursor containing peroxo titanic (PTA) and silicic acids has been proved to be responsible for high crystallinity of TiO2 encapsulated into amorphous silica. It has been proposed that peroxo groups enhance rapid formation of crystalline titania seeds, while silica controls their growth. It has been concluded from the TEM that the most morphologically uniform anatase crystallites covered with SiO2 particles are prepared at SiO2/TiO2 molar ratio of 0.4. This sample, according to 29Si NMR, also shows the high content of hydroxylated silica Q3 and Q2 groups, and it is the most photocatalytically active in UV-assisted decomposition of methylene blue among the tested materials. It has been determined that the increase in the amount of the condensed Q4 silica in the mixed oxides leads to the decrease in photocatalytic performance of the material, despite its better crystallinity. High crystallinity, low degree of incorporation of Ti atoms in SiO2 in the mixed oxide and adsorption of methylene blue in the vicinity of photoactive sites on the hydroxylated silica have been considered as the main factors determining the high degradation degree of methylene blue in the presence of silica–titania.

A new solvent-exchange technique to prepare anatase nanoparticles with exceptional thermal stability and photocatalytic activity is described here. The process of preparation is accomplished by using organic solvents to precipitate hydrous titania particles from a basic aqueous medium containing a titanium peroxo complex. Undoped titanium dioxide formed via a solvent exchange method has unprecedented thermal stability against transformation to the rutile phase, as opposed to TiO2 prepared by the common method of the gelation of an aqueous titanium peroxo complex. On the basis of X-ray thermodiffraction experiments, it has been established that the thermal treatment at 1000 °C of the titania prepared by ethanol precipitation contains 100% pure anatase phase. The stabilization of anatase is induced by the high defectiveness of the TiO2 nanostructure, which is evidenced from band-gap energy estimation, PXRD and HRTEM studies. The prepared TiO2 nanoparticles show an outstanding photocatalytic activity comparable to the commercial Aeroxide P25 photocatalyst in the UV-assisted decomposition of methylene blue.