Preface to the Special Issue on Hydrogen Production, Storage, Application, and Sensing.

This issue focuses on the multifaceted nature of hydrogen, exploring the latest research, advancements, and applications in this rapidly evolving field. Topics covered include hydrogen production methods, advancements in storage technologies, diverse applications, and the significance of hydrogen sensing for monitoring its environmental impact.

The reducibility and oxidation states of oxide-supported rhenium: experimental and theoretical investigations.

The activity and stability of supported metal catalysts, which exhibit high efficiency and activity, are significantly influenced by the interactions between the metal and the support, that is, metal-support interactions (MSIs). Here, we report an investigation of the MSIs between supported rhenium (Re) and oxide supports such as TiO2, SiO2, Al2O3, MgO, V2O5, and ZrO2 using experimental and computational approaches. The reducibility of the Re species was found to strongly depend on the oxide support. Experimental studies including temperature-programmed reduction by H2 as well as Re L3- and L1-edge X-ray absorption near edge structure (XANES) analysis revealed that the valency of the Re species started to decrease upon H2 reduction in the 200-400 °C range, except for Re on MgO, where the shift occurred at temperatures above 500 °C. The dependence of the Re L3- and L1-edge XANES spectra of the oxide-supported Re catalysts on the size of Re was also examined.

Transesterification of Ethyl-10-undecenoate Using a Cu-Deposited V2O5 Catalyst as a Model Reaction for Efficient Conversion of Plant Oils to Monomers and Fine Chemicals.

Transesterification of ethyl-10-undecenoate (derived from castor oil) with 1,4-cyclohexanedimethanol over a recyclable Cu-deposited V2O5 catalyst afforded 1,ω-diene, the corresponding cyclohexane-1,4-diylbis(methylene) bis(undec-10-enoate), a promising monomer for the synthesis of biobased polyesters, in an efficient manner. Deposition of Cu plays an important role in proceeding the reaction with high selectivity, and both the activity and the selectivity are preserved for five recycled runs by the addition of the substrates. The present catalyst was effective for transesterification with other alcohols, especially primary alcohols, demonstrating a possibility of using this catalyst for efficient conversion of plant oil to various fine chemicals.

Direct Phenolysis Reactions of Unactivated Amides into Phenolic Esters Promoted by a Heterogeneous CeO2 Catalyst.

The direct catalytic esterification of amides that leads to the construction of C-O bonds through the cleavage of amide C-N bonds is a highly attractive strategy in organic synthesis. While aliphatic and aromatic alcohols can be readily used for the alcoholysis of activated and unactivated amides, the introduction of phenols is more challenging due to their lower nucleophilicity in the phenolysis of unactivated amides. Herein, we demonstrate that phenols can be used for the phenolysis of unactivated amides into the corresponding phenolic esters using a simple heterogenous catalytic system based on CeO2 under additive-free reaction conditions. The method tolerates a broad variety of functional groups (>50 examples) in the substrates. Results of kinetic studies afforded mechanistic insights into the principles governing this reaction, suggesting that the cooperative effects of the acid-base functions of catalysts would be of paramount importance for the efficient progression of the C-N bond breaking process, and consequently, CeO2 showed the best catalytic performance among the catalysts explored.

Selective Transformations of Triglycerides into Fatty Amines, Amides, and Nitriles by using Heterogeneous Catalysis.

The use of triglycerides as an important class of biomass is an effective strategy to realize a more sustainable society. Herein, three heterogeneous catalytic methods are reported for the selective one-pot transformation of triglycerides into value-added chemicals: i) the reductive amination of triglycerides into fatty amines with aqueous NH3 under H2 promoted by ZrO2 -supported Pt clusters; ii) the amidation of triglycerides under gaseous NH3 catalyzed by high-silica H-beta (Hß) zeolite at 180 °C; iii) the Hß-promoted synthesis of nitriles from triglycerides and gaseous NH3 at 220 °C. These methods are widely applicable to the transformation of various triglycerides (C4 -C18 skeletons) into the corresponding amines, amides, and nitriles.

Origin of Nb2 O5 Lewis Acid Catalysis for Activation of Carboxylic Acids in the Presence of a Hard Base.

The Nb2 O5 surface catalyzes the amidation of carboxylic acids with amines through Nb5+ Lewis acid activation of the C=O group. In this work, DFT calculations were applied to theoretically investigate the C=O bond activation of a model carboxylic acid (acetic acid) on θ-Al2 O3 (110), anatase TiO2 (101), and T-Nb2 O5 (100) surfaces. The adsorption sites, adsorption energies, reaction energy barriers, electronic properties, and vibrational frequency of acetic acid were examined in detail. It was found that the bond activation of the carbonyl group is most efficient on Nb2 O5 , although the adsorption energy is larger on Al2 O3 and TiO2 . The most efficient C=O bond activation on Nb2 O5 results in the lowest energy barrier of C-N bond formation during amidation. The Nb2 O5 surface also shows larger tolerance to methylamine and water molecules than Al2 O3 and TiO2 surfaces. These crucial factors contribute to the highest amidation catalytic reactivity on Nb2 O5 . Furthermore, the position of the mean density of states of the d-conduction band of the active metal site relative to the Fermi energy level correlates well with the efficiency in the C=O bond activation and, consequently, the catalytic activity for amidation. These results suggest that, unlike a classical understanding of strong acid sites of metal oxide surfaces, interaction of a carbonyl HOMO with an unoccupied metal d-orbital, or, in other words, covalent-like interaction between a carbonyl group and metal adsorption site, is relevant to the present system.

TiO2 -Supported Re as a General and Chemoselective Heterogeneous Catalyst for Hydrogenation of Carboxylic Acids to Alcohols.

TiO2 -supported Re, Re/TiO2 , was found to promote selective hydrogenation of carboxylic acids having aromatic and aliphatic moieties to the corresponding alcohols. Re/TiO2 showed superior results compared to other transition-metal-loaded TiO2 and supported Re catalysts for selective hydrogenation of 3-phenylpropionic acid. 3-phenylpropanol was produced in 97 % yield under mild conditions (5 MPa H2 at 140 °C). Contrary to typical heterogeneous catalysts, Re/TiO2 does not lead to the formation of dearomatized byproducts. The catalyst is recyclable and shows a wide substrate scope in the synthesis of alcohols (22 examples; up to 97 % isolated yield).

Direct Olefination of Alcohols with Sulfones by Using Heterogeneous Platinum Catalysts.

Carbon-supported Pt nanoparticles (Pt/C) were found to be effective heterogeneous catalysts for the direct Julia olefination of alcohols in the presence of sulfones and KOtBu under oxidant-free conditions. Primary alcohols, including aryl, aliphatic, allyl, and heterocyclic alcohols, underwent olefination with dimethyl sulfone and aryl alkyl sulfones to give terminal and internal olefins, respectively. Secondary alcohols underwent methylenation with dimethyl sulfone. Under 2.5 bar H2, the same reaction system was effective for the transformation of alcohol OH groups to alkyl groups. Structural and mechanistic studies of the terminal olefination system suggested that Pt(0) sites on the Pt metal particles are responsible for the rate-limiting dehydrogenation of alcohols and that KOtBu may deprotonate the sulfone reagent. The Pt/C catalyst was reusable after the olefination, and this method showed a higher turnover number (TON) and a wider substrate scope than previously reported methods, which demonstrates the high catalytic efficiency of the present method.

Versatile and sustainable synthesis of cyclic imides from dicarboxylic acids and amines by Nb2O5 as a base-tolerant heterogeneous Lewis acid catalyst.

Catalytic condensation of dicarboxylics acid and amines without excess amount of activating reagents is the most atom-efficient but unprecedented synthetic method of cyclic imides. Here we present the first general catalytic method, proceeding selectively and efficiently in the presence of a commercial Nb2 O5 as a reusable and base-tolerant heterogeneous Lewis acid catalyst. The method is effective for the direct synthesis of pharmaceutically or industrially important cyclic imides, such as phensuximide, N-hydroxyphthalimide (NHPI), and unsubstituted cyclic imides from dicarboxylic acid or anhydrides with amines, hydroxylamine, or ammonia.

Sustainable heterogeneous platinum catalyst for direct methylation of secondary amines by carbon dioxide and hydrogen.

Pt and MoO(x) co-loaded TiO2 is found to be highly effective for direct methylation of aliphatic and aromatic secondary amines by CO2 and H2 under solvent-free conditions. This is the first additive-free and reusable heterogeneous catalytic system with acceptable turnover number.

General and selective C-3 alkylation of indoles with primary alcohols by a reusable Pt nanocluster catalyst.

The platinum rule: Heterogeneous, additive-free C-3 selective alkylation of indoles by aliphatic and aromatic alcohols proceeded under transfer hydrogenation conditions with the reusable Pt/θ-Al2 O3 catalyst (see scheme; TON=turnover number).