Efficient Acid-Catalyzed Conversion of Phenylglyoxal to Mandelates on Flame-Derived Silica/Alumina

Amorphous, nonporous silica/alumina (SA) made by flame-spray pyrolysis (FSP) efficiently catalyzes the direct conversion of phenylglyoxal (PG) to alkyl mandelates. The SAs exhibited a turnover frequency more than an order of magnitude higher than dealuminated zeolite Y, which hitherto has been considered as the most active solid acid for this reaction. The free diffusion of PG to surface acid sites and rapid removal of mandelate products are proposed to be at the origin of the superior performance of SAs. The recyclability of the catalyst was tested in five repetitive runs and showed no significant loss of catalyst performance

Tuning Phase Composition of TiO₂ by Sn⁴⁺ Doping for Efficient Photocatalytic Hydrogen Generation

The anatase–rutile mixed-phase photocatalysts have attracted extensive research interest because of the superior activity compared to their single phase counterparts. In this study, doping of Sn⁴⁺ ions into the lattice of TiO₂ facilitates the phase transformation from anatase to rutile at a lower temperature while maintaining the same crystal sizes compared to the conventional annealling approach. The mass ratios between anatase and rutile phases can be easily manipulated by varying the Sn-dopant content. Characterization results reveal that the Sn⁴⁺ ions entered into the lattice of TiO₂ by substituting some of the Ti⁴⁺ ions and distributed evenly in the matrix of TiO₂. The substitution induced the distortion of the lattice structure, which realized the phase transformation from anatase to rutile at a lower temperature and the close-contact phase junctions were consequently formed between anatase and rutile, accounting for the efficient charge separations. The mixed-phase catalysts prepared by doping Sn⁴⁺ ions into the TiO₂ exhibit superior activity for photocatalytic hydrogen generation in the presence of Au nanoparticles, relatively to their counterparts prepared by the conventional annealling at higher temperatures. The band allignment between anatase and rutile phases is established based on the valence band X-ray photoelectron spectra and diffuse reflectance spectra to understand the spatial charge separation process at the heterojunction between the two phases. The study provides a new route for the synthesis of mixed-phase TiO₂ catalysts for photocatalytic applications and advances the understanding on the enhanced photocatalytic properties of anatase–rutile mixtures

Cooperativity of Brønsted and Lewis Acid Sites on Zeolite for Glycerol Dehydration

Selective dehydration of glycerol, a byproduct from the biodiesel industry, on solid acids is an important reaction in the production of the value-added chemical acrolein for economic-sustainable biorefinery. Most efforts have been made on the development of strong Brønsted acid sites (BAS) to improve the production of acrolein, because the Lewis acid sites (LAS) generally promote the generation of the byproduct acetol. However, exclusively tuning the properties of BAS or LAS did not well-promote the acrolein production from glycerol as indicated in this work. We provide a new route for efficient and selective glycerol transformation to acrolein via the cooperative dehydration between the BAS and LAS. The role of LAS (extra-framework aluminum species on zeolites) was altered from competition with BAS to generate the byproduct acetol to cooperation with the neighboring BAS. It is very beneficial for the sequential two-step dehydration of the internal and terminal hydroxyl groups of glycerol to value-added acrolein. This cooperativity of BAS and LAS significantly improved the yield of acrolein from the selective glycerol dehydration

Bimetallic Ag–Cu Supported on Graphitic Carbon Nitride Nanotubes for Improved Visible-Light Photocatalytic Hydrogen Production

This work, for the first time, reports visible-light active bare graphitic carbon nitride nanotubes (C₃N₄ NTs) for photocatalytic hydrogen generation, even in the absence of any cocatalyst. Upon uniform dispersion of the cocatalysts, Ag–Cu nanoparticles, on the well-ordered bare C₃N₄ NTs, they exhibit twice the H₂ evolution rate of the bare C₃N₄ NTs. The improved activity is attributed to their unique tubular nanostructure, strong metal–support interaction, and efficient photoinduced electron–hole separation compared to their bare and monometallic counterparts, evidenced by complementary characterization techniques. This work reveals that the H₂ production rates correlate well with the oxidation potentials of the sacrificial reagents used. Triethylamine (TEA) outperforms other sacrificial reagents, including triethanolamine (TEOA) and methanol. Mechanistic studies on the role of various sacrificial reagents in photocatalytic H₂ generation demonstrate that irreversible photodegradation of TEA into diethylamine and acetaldehyde via monoelectronic oxidation contributes to the improved H₂ yield. Similarly, TEOA is oxidized to diethanolamine and glycolaldehyde, whereas methanol is unable to quickly capture the photoinduced holes and remains intact due to the low oxidation potential

Exploring the Origin of Enhanced Activity and Reaction Pathway for Photocatalytic H₂ Production on Au/B-TiO₂ Catalysts

Gold-embedded boron-doped TiO₂ (Au/B-TiO₂) photocatalysts were synthesized by a sol–gel hydrothermal method. The TEM images display that the gold nanoparticles were embedded into the B-TiO₂ framework. Hydrogen evolution under light irradiation showed that doping of boron into TiO₂ enhanced the photocatalytic activity. A further remarkable improvement of the activity was observed over the Au/B-TiO₂. Evidenced by B 1s XPS and ¹¹B MAS NMR spectra, the embedment of Au nanoparticles contributes to the formation of more interstitial boron species in B-TiO₂. In turn, it gives rise to surface or near-surface states facilitating the embedment of Au nanoparticles, as demonstrated by the Au 4f XPS spectra, which indicates the strong interaction between gold and the B-TiO₂ framework. This specific synergy significantly contributes to the enhancement of photocatalytic activity. For the first time, the isotopic tracer studies using a gas chromatograph isotope ratio mass spectrometer along with a series of control experiments reveal that the produced hydrogen originated mainly from water rather than methanol, whereas the direct oxidation of methanol did not lead to hydrogen generation. Acting as a sacrificial reagent, methanol could be oxidized to formaldehyde by protons/water under oxygen-free conditions

Sensitization of Pt/TiO₂ Using Plasmonic Au Nanoparticles for Hydrogen Evolution under Visible-Light Irradiation

Au nanoparticles with different sizes (10, 20, 30, and 50 nm) were synthesized using a seed-assisted approach and anchored onto Pt/TiO₂ employing 3-mercaptopropionic acid as the organic linker. The sizes of the Au nanoparticles were controlled within a narrow range so that the size-dependent surface plasmonic resonance effect on sensitizing Pt/TiO₂ can be thoroughly studied. We found that 20 nm Au nanoparticles (Au₂₀) gave the best performance in sensitizing Pt/TiO₂ to generate H₂ under visible-light illumination. Photoelectrochemical measurements indicated that Au₂₀-Pt/TiO₂ exhibited the most efficient “hot” electrons separation among the studied catalysts, correlating well with the photocatalytic activity. The superior performance of Au-supported Pt/TiO₂ (Au₂₀-Pt/TiO₂) compared with Au anchored to TiO₂ (Au₂₀/TiO₂) revealed the important role of Pt as a cocatalyst for proton reduction. To elucidate how the visible-light excited hot electrons in Au nanoparticles involved in the proton-reduction reaction process, Au₂₀/TiO₂ was irradiated by visible light (λ > 420 nm) with the presence of Pt precursor (H₂PtCl₆) in a methanol aqueous solution under deaerated condition. Energy-dispersive X-ray spectroscopy mapping analysis on the recovered sample showed that Pt ions could be reduced on the surfaces of both Au nanoparticles and TiO₂ support. This observation indicated that the generated hot electrons on Au nanoparticles were injected into the TiO₂ conduction band, which were then subsequently transferred to Pt nanoparticles where proton reduction proceeded. Besides, the excited hot electrons could also participate in the proton reduction on Au nanoparticles surface

Promoter Effects on Nickel-Supported Magnesium Oxide Catalysts for the Carbon Dioxide Reforming of Methane

The nickel catalysts supported on bare MgO and its binary Mg–Al, Mg–La, and Mg–Fe metal oxides were prepared and used for carbon dioxide reforming of methane to syngas. The effects of Al, La, and Fe metal oxides on the structural properties, reducibility, and metal–support interaction of the Ni catalysts supported on MgO-based binary metal oxide were investigated. The X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and hydrogen temperature-programmed reduction (H₂-TPR) analyses show that the nickel nanoparticles were highly dispersed on the supports. It is found that the Al ions can be well-incorporated into the MgO lattice to form uniform Mg–Al oxides, while isolated lanthanum oxides and iron oxides were observed in the Mg–La and Mg–Fe binary systems by TEM, respectively. Ni/Mg–Al metal oxide exhibits greatly improved catalytic activity, owing to the formation of a homogeneous Mg–Al oxide matrix with small particle sizes of Ni nanoparticles compared to bare Ni/MgO. Very low conversions for both CH₄ and CO₂ were obtained on Ni/Mg–La and Ni/Mg–Fe metal oxides, even at a high temperature of 800 °C, as a result of the incomplete reduction of the nickel nanoparticles

Effect of Dehydration on the Local Structure of Framework Aluminum Atoms in Mixed Linker MIL-53(Al) Materials Studied by Solid-State NMR Spectroscopy

The present study features ¹H and ²⁷Al MAS NMR spectroscopic investigations on mixed ligand metal−organic frameworks (MOFs) of MIL-53(Al) type with benzene-1,4-dicarboxylate (BDC) and 2-aminobenzene-1,4-dicarboxylate (ABDC) linkers. The excellent resolution of the ¹H spectra allowed an elegant and facile quantitative analysis of the organic linkers using solid-state NMR. The actual molar fraction of ABDC in the dehydrated mixed linker MOFs was determined by evaluating the intensity of the −NH₂ signal at 5.6 ppm. The incorporation of amine groups led to higher field shifts of the corner-sharing AlOH signals and a more homogeneous charge distribution in the local structure of framework aluminum atoms corresponding to a decrease of the quadrupole coupling constant by ∼1 MHz compared to that of aluminum coordinated to BDC. Upon rehydration, the local structures of the framework aluminum atoms exhibited a much lower symmetry, as indicated by an increase of the ²⁷Al quadrupole coupling constant by up to 3 MHz

Analysis of the Promoted Activity and Molecular Mechanism of Hydrogen Production over Fine Au–Pt Alloyed TiO₂ Photocatalysts

Fine metal nanoparticles (2–3 nm; Au, Pt, and alloyed Au–Pt) with a narrow size distribution were deposited on active TiO₂ through a facile chemical reduction method. Compared to the bare TiO₂, a remarkable enhancement of up to 10-fold for photocatalytic hydrogen evolution was achieved on the alloyed nanocomposites. By using core level and valence band XPS analysis, two electronic properties are shown to contribute to the promoted photocatalytic activity: stronger metal–support interaction between the alloyed structures and TiO₂ and higher electron population on the Au–Pt/TiO₂ photocatalysts in comparison with the bare TiO₂. Moreover, an improved charge separation over TiO₂ using Au–Pt nanoparticles was clearly evidenced by the significant increase of photocurrent responses obtained from the photoelectrochemical measurements. For the first time, in situ ¹³C and ¹H NMR spectroscopy was applied to monitor the gas–liquid–solid photocatalytic reactions under real working conditions. Via a two-electron oxidation pathway, the surface-adsorbed methanol was first oxidized to formaldehyde, followed by spontaneous hydrolysis and methanolysis to methanediol and methoxymethanol, rather than methyl formate and formic acid that have been previously reported in gaseous CH₃OH photocatalysis. The in situ monitoring also revealed that deposition of metal NPs would not alter the reaction pathways while making the reaction faster compared to the bare TiO₂

Tailoring and Identifying Brønsted Acid Sites on Metal Oxo-Clusters of Metal–Organic Frameworks for Catalytic Transformation

Metal–organic frameworks (MOFs) with Brønsted acidity are an alternative solid acid catalyst for many important chemical and fuel processes. However, the nature of the Brønsted acidity on the MOF’s metal cluster or center is underexplored. To design and optimize the acid strength and density in these MOFs, it is important to understand the origin of their acidity at the molecular level. In the present work, isoreticular MOFs, ZrNDI and HfNDI (NDI = N,N′-bis(5-isophthalate)naphthalenediimide), were prepared as a prototypical system to unravel and compare their Brønsted and Lewis acid sites through an array of spectroscopic, computational, and catalytic characterization techniques. With the aid of solid-state nuclear magnetic resonance and density functional calculations, Hf6 oxo-clusters on HfNDI are quantitatively proved to possess a higher density Brønsted acid site, while ZrNDI-based MOFs display stronger and higher-population Lewis acidity. HfNDI-based MOFs exhibit a superior catalytic performance in activating dihydroxyacetone (DHA) and converting DHA to ethyl lactate, with 71.1% selectivity at 54.7% conversion after 6 h. The turnover frequency of BAS-dominated Hf-MOF in DHA conversion is over 50 times higher than that of ZSM-5, a strong BAS-based zeolite. It is worth noting that HfNDI is reported for the first time in the literature, which is an alternative platform catalyst for biorefining and green chemistry. The present study furthermore highlights the uniqueness of Hf-based MOFs in this important biomass-to-chemical transformation