Silylation enhances the performance of Au/Ti–SiO2 catalysts in direct epoxidation of propene using H2 and O2

Nanoparticles as an antidote for poisoned gold single-atom catalysts in sustainable propylene epoxidation

The development of sustainable and anti-poisoning single-atom catalysts (SACs) is essential for advancing their research from laboratory to industry. Here, we present a proof-of-concept study on the poisoning of Au SACs, and the antidote of Au nanoparticles (NPs), with trace addition shown to reinforce and sustain propylene epoxidation. Multiple characterizations, kinetics investigations, and multiscale simulations reveal that Au SACs display remarkable epoxidation activity at a low propylene coverage, but become poisoned at higher coverages. Interestingly, Au NPs can synergistically cooperate with Au SACs by providing distinct active sites required for H2/O2 and C3H6 activations, as well as hydroperoxyl radical to restore poisoned SACs. The difference in reaction order between C3H6 and H2 (nC3H6-nH2) is identified as the descriptor for establishing the volcano curves, which can be fine-tuned by the intimacy and composition of SACs and NPs to achieve a rate-matching scenario for the formation, transfer, and consumption of hydroperoxyl. Consequently, only trace addition of Au NPs antidote (0.3% ratio of SACs) stimulates significant improvements in propylene oxide formation rate, selectivity, and H2 efficiency compared to SACs alone, offering a 56-fold, 3-fold, and 22-fold increase, respectively, whose performances can be maintained for 150 h.

Review and perspectives on TS-1 catalyzed propylene epoxidation

Titanium silicate zeolite (TS-1) is widely used in the research on selective oxidations of organic substrates by H2O2. Compared with the chlorohydrin process and the hydroperoxidation process, the TS-1 catalyzed hydroperoxide epoxidation of propylene oxide (HPPO) has advantages in terms of by-products and environmental friendliness. This article reviews the latest progress in propylene epoxidation catalyzed by TS-1, including the HPPO process and gas phase epoxidation. The preparation and modification of TS-1 for green and sustainable production are summarized, including the use of low-cost feedstocks, the development of synthetic routes, strategies to enhance mass transfer in TS-1 crystal and the enhancement of catalytic performance after modification. In particular, this article summarizes the catalytic mechanisms and advanced characterization techniques for propylene epoxidation in recent years. Finally, the present situation, development prospect and challenge of propylene epoxidation catalyzed by TS-1 were prospected.

In situ growth of hierarchical phase junction CdS on a H-mordenite zeolite for enhanced photocatalytic properties

A kind of cadmium sulfide (CdS) nanocomposite with different crystalline phases was grown on the surface of H-mordenite zeolite (HMOR) by a chemical liquid-phase co-precipitation method. In this work, 2 wt% CdS@HMOR photocatalytic material with the coexistence phase (hexagonal phase and cubic phase) of cadmium sulfide was grown on the surface of HMOR by controlling the reaction temperature and ammonia concentration. Photocatalytic degradation of methylene blue (MB) was used as an index to detect the photocatalytic performance of materials. The results indicated that the photocatalytic degradation efficiency of the system with HMOR was significantly improved in comparison to that without HMOR (CdS, 40.34%, 0.2578 h^-1). It was found that 2 wt% CdS@HMOR had the best photocatalytic activity. The degradation rate of MB was 84.15% in 2 h, and the degradation rate constant was 0.8884 h^-1. When 1.5 ml H₂O₂ was introduced into the system, the degradation rate of MB was increased to 98.98%, and the degradation rate constant was 1.9976 h^-1. SEM, HRTEM, PL, EIS and photocurrent showed that the cubic and hexagonal phases of CdS were in contact with each other on the HMOR surface, forming a good electron transport. By XRD, XPS and SEM tests, the results of materials after four cycles of reactions showed that the structure of the 2 wt% CdS@HMOR was still stable. Therefore, HMOR may provide a good support for CdS, and the synergistic effect between them is beneficial for the occurrence of photocatalytic reactions. HMOR can act as an electron receptor to inhibit the recombination of carriers. The homo-junction between different phases of CdS on the surface of HMOR is beneficial to the separation of photo-induced carriers. These results indicate that the construction of phase heterojunctions on zeolites and the synergism among them are a method for improving the photocatalytic activity.

Performance, Reaction Pathway, and Pretreatment of Au Catalyst Precursor in H2/O2 Atmosphere for the Epoxidation of Propylene

Gas-phase epoxidation of propylene in the copresence of H2 and O2 was performed over the catalyst of Au on as-synthesized TS-1 that contained a small amount of anatase TiO2. The catalytic performance was studied by washing or nonwashing the catalyst precursor to modulate the content of purity (K, Cl) and then calcining the samples in O2 or H2 prior to reaction. The results show that the catalytic performance of Au/TS-1 can be improved without washing (more K+ and Au maintained) and O2 pretreatment. It was found that the calcination in O2 was able to maintain more metallic Au and form more surface-active oxygen species and thus providing a better yield of propylene oxide with the assistance of potassium. Interestingly, more acrolein can be produced over the catalysts with respect to the in situ calcination in O2 than that in H2 when the feed only contained 10% O2 and 10% propylene in argon, while there was no formation of propylene oxide. On the other hand, the catalyst precursor calcined in H2 prefers the formation of successive oxygenates of PO.

Nitrogen Monoxide and Soot Oxidation in Diesel Emissions with Platinum–Tungsten/Titanium Dioxide Catalysts: Tungsten Loading Effect

Compared with Pt/TiO2, tungsten-loaded Pt–W/TiO2 catalysts exhibit improved activity for NO and soot oxidation. Using catalysts prepared by an incipient wetness method, the tungsten loading effect was investigated using Brunauer–Emmett–Teller surface areas, X-ray diffraction, transmission electron microscopy (TEM), CO pulse chemisorption, H2 temperature-programmed reduction, NH3 temperature-programmed desorption (NH3-TPD), and pyridine Fourier transform infrared (FT-IR) spectroscopy. Loading tungsten on the Pt/TiO2 catalyst reduced the platinum particle size, as revealed in TEM images. CO pulse chemisorption showed that platinum was covered with tungsten and the dispersion of platinum decreased when 5 wt.% or more of tungsten was loaded. The NH3-TPD and pyridine-FT-IR results demonstrated that the number of strong acid sites and Brønsted acid sites in the catalyst were increased by the presence of tungsten. Therefore, a catalyst containing an appropriate amount of tungsten increased the dispersion of platinum, thereby increasing the number of active sites for NO and soot oxidation, and increased the acidity of the catalyst, thereby increasing the activity of soot oxidation by NO2

Advances in Designing Au Nanoparticles for Catalytic Epoxidation of Propylene with H2 and O2

Au nanoparticles, which can be used in various industrial and environmental applications, have drawn substantial research interest. In this review, a comprehensive background and some insights are provided regarding recent studies concerning the use of Au nanoparticles for catalytic propylene epoxidation with H2 and O2. Over the last two decades, substantial progress has been made toward the efficient production of propylene oxide (PO); this includes the design of highly dispersed Au catalysts on Ti-modified mesoporous silica supports, the optimization of catalytic epoxidation, and the determination of the mechanisms and reaction pathways of epoxidation. Particularly, the critical roles of catalyst synthesis, the types of material support, Au nanoparticle sizes, and the dispersion amounts of Au nanoparticles are emphasized in this review. In future studies, novel, practical, robust, and highly PO-selective Au nanoparticle catalyst systems are expected to be continually designed for the enhanced catalytic epoxidation of propylene.

Recyclable Au/SiO2-Shell/Fe3O4-Core Catalyst for the Reduction of Nitro Aromatic Compounds in Aqueous Solution

Highly stable gold nanoparticles immobilized on the surface of amine-functionalized nanocomposite microspheres possessing a magnetite (Fe₃O₄) nanoparticle core and a silica (SiO₂) shell (Au/SiO₂-shell/Fe₃O₄-core) were prepared. These gold nanocomposite catalysts were tested for 4-nitrophenol (4-NP) and 2-nitroaniline (2-NA) reduction in aqueous solution in the temperature range 293-323 K and in the presence of aqueous NaBH₄ reducing agent. The magnetically recyclable gold catalyst showed high stability (∼3 months), efficient recyclability (up to 10 cycles), and high activity (∼100% conversion within 225 s, ∼700 ppm 4-NP or 2-NA). The pseudo-first-order apparent reaction rate constants (k) of 4-NP and 2-NA reduction were 7.5 × 10^-3 and 4.1 × 10^-3 s^-1, respectively, and with an apparent catalytic activity of 4.48 × 10^-8 kmol/(m³ s).

Direct epoxidation of propene on silylated Au–Ti catalysts: a study on silylation procedures and the effect on propane formation

Silylation was employed on an active Au/Ti–SiO₂ catalyst, in order to enhance catalyst performance for the direct epoxidation of propene to propene oxide (PO) using H₂ and O₂.

Epoxidation of propene using Au/TiO2: on the difference between H2 and CO as a co-reactant

The role of the reducing gas in the direct epoxidation of propene to propene oxide (PO) using O₂ over a Au/TiO₂ catalyst was studied through experiments and density functional theory calculations.