Lewis Acid Catalysis of TiO4 Tetrahedra on Mesoporous Silica in Water

Stabilisation of molecular TiO4 species on the pore surface of mesoporous silica for photocatalytic H2 evolution

Although molecular tetrahedral Ti-oxo species exhibit unique electronic and photochemical properties due to their discrete energy levels, which are different from those of anatase and rutile, such Ti-oxo species are generally unstable and readily transformed to amorphous/crystalline TiO2 (bulk phases, nanoparticles and clusters) via hydrolysis and condensation. Here, molecular Ti-oxo species were immobilised within mesoporous silica SBA-15 by grafting titanium(IV) oxyacetylacetonate using the surface silanol groups of SBA-15 as a scaffold, followed by chemical etching with dilute hydrochloric acid to form molecular TiO4 species. These Ti species mainly exist as isolated tetrahedrally coordinated structures, as was confirmed by diffuse reflectance UV-vis and Raman spectroscopy. The SBA-15-immobilised molecular TiO4 exhibited higher photocatalytic activity for H2 evolution from an aqueous methanol solution than conventional Ti-incorporated mesoporous silica (Ti-MCM-41) and reference TiO2 (P25).

Acidity of SiO2-Supported Metal Oxides in the Presence of H2O Using the AEIR Method: 2. Adsorption and Coadsorption of NH3 and H2O on TiO2/SiO2 Catalysts

Two different TiO₂/SiO₂ compounds containing TiO₂ nanodomains dispersed over SiO₂ were investigated applying the AEIR method at the adsorption equilibrium of NH₃ and H₂O from 300 to 723 K, particularly for the measurement of the individual heats of adsorption of the different species on Lewis acidic sites (LAS) and Brønsted acidic sites (BAS) as evaluation of the strength of the sites. It revealed two types of NH₃ adsorption sites: the first ones could correspond either to NH₃ species H-bonded to free OH groups or to coordinated weak LAS (named L1). The second ones (L2) were attributed to strongest LAS similar to those present at the surface of TiO₂ nanocrystallites. They also correspond to the stronger adsorption sites of H₂O. Two types of Brønsted acid sites (BAS) were additionally evidenced by the AEIR method and proposed to be specifically located on the Si-O-Ti bridging bonds at the TiO₂/SiO₂ interface. The heats of adsorption of the different adsorbed species provided by the AEIR method were consistent with literature data on average values of the heats of adsorption of NH₃ and H₂O from microcalorimetry measurements. The surface acidity of the two compounds in the presence of H₂O was determined using NH₃-H₂O coadsorption. At T ≥ 473 K, the NH₃ species on the L2 sites were not significantly displaced from the surface whatever the partial pressure of H₂O studied in agreement with the Temkin competitive model using the individual heats of adsorption of the NH₃ and H₂O species. This model also revealed the presence of a small amount of H₂O species adsorbed on L2 sites allowing H₂O dissociation or/and hydrolysis of SiOTi or TiOTi bridges, leading to the formation of a much higher amount of BAS. Therefore, this original work combining the AEIR method and the Temkin competitive model provided new insights for understanding water effects on acidic oxide catalysts.

One-pot sol–gel synthesis of a phosphated TiO2 catalyst for conversion of monosaccharide, disaccharides, and polysaccharides to 5-hydroxymethylfurfural

Catalytic conversion of biomass or biomass-derived carbohydrates into 5-hydroxymethylfurfural (HMF) is an important reaction for the synthesis of bio-based polymers, fuels, and other industrially useful products.

Fabrication of Monodisperse Porous Silica Microspheres with a Tunable Particle Size and Pore Size for Protein Separation

Monodisperse porous silica microspheres with a tunable particle size and pore size were fabricated by utilizing porous polymer microspheres as a novel hard template during the sol-gel process followed by calcination to remove the polymer. The particle size and pore size could be simply tuned by the feature of the polymer template and reaction conditions such as different functionalization of the parent polymer template, particle size of polymer template, and amount of TEOS during the sol-gel process. EDA (ethylenediamine), APTES (3-aminopropyl)triethoxysilane, and TMA (trimethylamine hydrochloride) functionalization of porous poly(GMA-co-EGDMA) microspheres were carried out to study their effect on the synthesized porous silica microspheres. The TMA-functionalized polymer microspheres led to a higher yield, smaller silica nanoparticles, and no self-nucleation of TEOS due to their positive surface charge. Furthermore, no addition of NaOH during TMA functionalization and the amount of TEOS during the sol-gel process played key roles in determining the pore size and particle size of porous silica microspheres. Then, through poly(aspartic acid) coating of the APTES-functionalized monodisperse porous silica microspheres, the modified monodisperse porous silica microspheres were explored as the stationary phase of HPLC for protein separation. The effects of particle size and pore size on the chromatographic behavior were discussed. When the protein mixture composed of transferrin, hemoglobin, ribonuclease A, cytochrome C, and lysozyme was used as the model analytes, the as-prepared silica microspheres exhibited an excellent separation performance with a high protein recovery and good reproducibility.

Hybrid Organic-Inorganic Anatase as a Bifunctional Catalyst for Enhanced Production of 5-Hydroxymethylfurfural from Glucose in Water

Hybrid organic-inorganic anatase (hybrid-TiO₂ ) is prepared by a facile hydrothermal synthesis method employing citric acid. The synthetic approach results in a high surface-area nanocrystalline anatase polymorph of TiO₂ . The uncalcined hybrid-TiO₂ is directly studied as a catalyst for the conversion of glucose into 5-hydroxymethylfurfural (HMF). In the presence of the hybrid-TiO₂ , HMF yields up to 45 % at glucose conversions up to 75 % were achieved in water at 130 °C in a monophasic batch reactor. As identified by Ti K-edge XANES, hybrid-TiO₂ contains a large fraction of fivefold coordinatively unsaturated Ti^IV sites, which act as the Lewis acid catalyst for the conversion of glucose into fructose. As citric acid is anchored in the structure of hybrid-TiO₂ , carboxylate groups seem to catalyze the sequential conversion of fructose into HMF. The fate of citric acid bound to anatase and the Ti^IV Lewis acid sites throughout recycling experiments is also investigated. In a broader context, this contribution outlines the importance of hydrothermal synthesis for the creation of water-resistant Lewis acid sites for the conversion of sugars. Importantly, the use of the hybrid-TiO₂ with no calcination step contributes to dramatically decreasing the energy consumption in the catalyst preparation.

Lewis acidity quantification and catalytic activity of Ti, Zr and Al-supported mesoporous silica

Water-tolerant metal supported Lewis acids were synthesized for the catalytic amidation of electron-poor and bulky amines.

Catalytic cracking of n-pentane over CLD modified HZSM-5 zeolites

The possible role of CLD modification on H-ZSM5 zeolite in n-pentane cracking was proposed.

Titanate nanotube-promoted chemical fixation of carbon dioxide to cyclic carbonate: a combined experimental and computational study

Titanate nanotubes are efficiently used as air- and water-tolerant, and recyclable Lewis acid catalysts for CO₂ fixation to cyclic carbonate.

Recent progress in the development of solid catalysts for biomass conversion into high value-added chemicals

In recent decades, the substitution of non-renewable fossil resources by renewable biomass as a sustainable feedstock has been extensively investigated for the manufacture of high value-added products such as biofuels, commodity chemicals, and new bio-based materials such as bioplastics. Numerous solid catalyst systems for the effective conversion of biomass feedstocks into value-added chemicals and fuels have been developed. Solid catalysts are classified into four main groups with respect to their structures and substrate activation properties: (a) micro- and mesoporous materials, (b) metal oxides, (c) supported metal catalysts, and (d) sulfonated polymers. This review article focuses on the activation of substrates and/or reagents on the basis of groups (a)-(d), and the corresponding reaction mechanisms. In addition, recent progress in chemocatalytic processes for the production of five industrially important products (5-hydroxymethylfurfural, lactic acid, glyceraldehyde, 1,3-dihydroxyacetone, and furan-2,5-dicarboxylic acid) as bio-based plastic monomers and their intermediates is comprehensively summarized.

Copper-loaded polymeric magnetic nanocatalysts as retrievable and robust heterogeneous catalysts for click reactions

A novel heterogeneous copper catalyst based on poly(ionic liquid)-coated magnetic nanoparticles was prepared by the polymerization of 3-carboxymethyl-1-vinylimidazolium in the presence of surface-modified magnetic nanoparticles, followed by the coordination of the carboxylate units in the polymer chains with copper sulfate.

Nano magnetite supported metal ions as robust, efficient and recyclable catalysts for green synthesis of propargylamines and 1,4-disubstituted 1,2,3-triazoles in water

Superparamagnetic supported metals complex catalysts have been synthesized and applied for the synthesis of propargylamines and 1,2,3-triazoles.

Efficient Mukaiyama aldol reaction in water with TiO4 tetrahedra on a hydrophobic mesoporous silica surface

A new heterogeneous catalyst, hydrophobic TiO₄-deposited mesoporous silica, exhibited high catalytic performance as a reusable catalyst for the Mukaiyama-aldol condensation in water without a surfactant.