Propylene Epoxidation Research Articles

Titania-decorated hybrid nano-architectures and their preliminary assessment in catalytic applications

Gold nanomaterials have been recognized as considerably active and effective catalysts toward several industrial and environmentally relevant reactions, among which are hydrocarbon selective oxidation, CO oxidation and propylene epoxidation. Engineered nanomaterials may be able to suppress some of the more severe issues that block their industrial applicability, such as recyclability, catalytic activity reduction during time due to sintering effects or leakage of the active material. In this regard, the composition of hybrid nanomaterials comprising encapsulated ultrasmall gold nanoparticles, also called ultrasmall-in-nano, will broaden and enhance their potential catalytic applications by including synergic features. Here, we report a systematic investigation on the design and production of optimized titania-decorated gold nano-architectures together with their efficiency evaluation on both formic acid (thermo/photo-induced) decomposition and cyclic carbonates formation by carbon dioxide fixation for carbon capture and storage technology. This work paves the way for the next development of ultrasmall-in-nano catalysts bearing an early–late transition metal combination for sustainable processes.

Effect of nickel promoter in Au/TS-1 catalysts for gas-phase propylene epoxidation

Effect of nickel promoter in Au/TS-1 catalysts for gas-phase propylene epoxidation

Combining trace Pt with surface silylation to boost Au/uncalcined TS‐1 catalyzed propylene epoxidation with H2 and O2

AbstractModification of bifunctional Au‐Ti catalysts for direct epoxidation of propylene with H2 and O2 is of burgeoning interest for potential commercialization requirements with desirable overall catalytic performance. Herein, we report a strategy by combining the addition of trace Pt into Au particles to enhance hydroperoxide species formation with the surface silylation to promote propylene oxide (PO) desorption. The resultant catalyst, that is, silylated Au20Pt1/uncalcined TS‐1, gives rise to significantly enhanced propylene conversion of 11%, PO selectivity of 88%, and H2 efficiency of 41%. The underlying nature of the above enhanced performance is elucidated by multiple techniques, such as high‐angle annular dark‐field scanning transmission electron microscopy, X‐ray photoelectron spectroscopy, Fourier transform infrared spectra, and theoretical calculations. The insights revealed here, the synergy of Au‐Pt‐Ti triple active sites together with the surface silylation, could guide the rational design of Au‐Ti catalysts for propylene epoxidation to boost PO production.

Methane oxidation to methanol over copper-containing zeolite

Methane oxidation to methanol over copper-containing zeolite

Selective silylation boosts propylene epoxidation with H2 and O2 over Au/TS-1

In this issue of Chem Catalysis , Wang et al. describe a selective silylation strategy using HMDSO to boost propylene epoxidation with H 2 and O 2 over Au/TS-1 with blocked micropores. Removal of silanol groups from TS-1 hinders propylene-oxide (re)adsorption, thereby enhancing product selectivity and catalyst stability. In this issue of Chem Catalysis , Wang et al. describe a selective silylation strategy using HMDSO to boost propylene epoxidation with H 2 and O 2 over Au/TS-1 with blocked micropores. Removal of silanol groups from TS-1 hinders propylene-oxide (re)adsorption, thereby enhancing product selectivity and catalyst stability.

Liquid-Phase Epoxidation of Propylene with H2O2 over TS-1 Zeolite: Impurity Formation and Inhibition Study

Liquid-Phase Epoxidation of Propylene with H₂O₂ over TS-1 Zeolite: Impurity Formation and Inhibition Study

The essence of Na+ and Zn species in propylene epoxidation by oxygen on Na2CO3 modified Cu-Zn/SiO2 catalysts

The catalytic performances of Cu-Zn/SiO2 with different Zn content and promoter on C3H6 epoxidation were investigated. Zn species was in favour of the formation of propylene oxide (PO), and PO selectivity was further increased after Cu-Zn/SiO2 was modified by Na+. The promotive effect of Na+ on C3H6 epoxidation activity was much superior to that of Cl−. The PO formation rate on Cu92Zn8/SiO2-Na2CO3 was nearly as high as 50 times that on Cu/SiO2. Zn and Na+ could synergistically promote the reduction of Cu2+ to Cu+ species. The variation tendencies between C3H6 epoxidation activity and Cu+ content were in good agreement with each other. The role of Zn and Na+ was to enhance the Cu+ content via promoting the reduction of Cu2+ to Cu+ induced by Zn and Na+. Cu+ species played a vital role in PO formation and Cu+ may be the active site for C3H6 epoxidation reaction by O2.

Precisely Engineered Supported Gold Clusters as a Stable Catalyst for Propylene Epoxidation.

Designing a stable and selective catalyst with high H2 utilisation is of pivotal importance for the direct gas‐phase epoxidation of propylene. This work describes a facile one‐pot methodology to synthesise ligand‐stabilised sub‐nanometre gold clusters immobilised onto a zeolitic support (TS‐1) to engineer a stable Au/TS‐1 catalyst. A non‐thermal O2 plasma technique is used for the quick removal of ligands with limited increase in particle size. Compared to untreated Au/TS‐1 catalysts prepared using the deposition precipitation method, the synthesised catalyst exhibits improved catalytic performance, including 10 times longer lifetime (>20 days), increased PO selectivity and hydrogen efficiency in direct gas phase epoxidation. The structure‐stability relationship of the catalyst is illustrated using multiple characterisation techniques, such as XPS, 31P MAS NMR, DR‐UV/VIS, HRTEM and TGA. It is hypothesised that the ligands play a guardian role in stabilising the Au particle size, which is vital in this reaction. This strategy is a promising approach towards designing a more stable heterogeneous catalyst.

Precisely Engineered Supported Gold Clusters as a Stable Catalyst for Propylene Epoxidation

AbstractDesigning a stable and selective catalyst with high H2 utilisation is of pivotal importance for the direct gas‐phase epoxidation of propylene. This work describes a facile one‐pot methodology to synthesise ligand‐stabilised sub‐nanometre gold clusters immobilised onto a zeolitic support (TS‐1) to engineer a stable Au/TS‐1 catalyst. A non‐thermal O2 plasma technique is used for the quick removal of ligands with limited increase in particle size. Compared to untreated Au/TS‐1 catalysts prepared using the deposition precipitation method, the synthesised catalyst exhibits improved catalytic performance, including 10 times longer lifetime (>20 days), increased PO selectivity and hydrogen efficiency in direct gas phase epoxidation. The structure‐stability relationship of the catalyst is illustrated using multiple characterisation techniques, such as XPS, 31P MAS NMR, DR‐UV/VIS, HRTEM and TGA. It is hypothesised that the ligands play a guardian role in stabilising the Au particle size, which is vital in this reaction. This strategy is a promising approach towards designing a more stable heterogeneous catalyst.

Mechanism-guided elaboration of ternary Au–Ti–Si sites to boost propylene oxide formation

Mechanism-guided catalyst design and engineering at the scale of active sites has been developed as a powerful tool for boosting catalytic performance. Herein, we report an efficient selective silylation strategy for the elaborate fabrication of ternary Au–Ti–Si sites to boost propylene epoxidation with H2 and O2. Kinetics (isotopic) analysis, Fourier transform infrared measurements, and theoretical calculations indicate an urgent necessity for selective consuming silanol sites not only to suppress propylene oxide (PO) ring opening to byproducts promoted by H2 spillover but also to minimize PO inhibition effects. A continuous silylation treatment was developed to encourage the kinetically favorable formation of ternary Au–Ti(–OH)–Si(–O–SiR3) moiety. This delivers significantly improved H2 efficiency of 42.5% in addition to the promising PO formation rate of 193 g h−1 kgcat−1 and PO selectivity of 95.7% with the long-term stability in excess of 200 h. These insights could pave the way for rationally fabricating catalyst active sites toward optimized performance.

Continuous Liquid-Phase Epoxidation of Ethylene with Hydrogen Peroxide on a Titanium-Silicate Catalyst

Ethylene epoxidation with hydrogen peroxide was studied in a laboratory-scale trickle bed reactor under a broad range of experimental conditions (15–80 °C, 2.5–8.5 bar) utilizing a commercial titanium-silicate catalyst (TS-1). The catalyst was very stable and selective over 150 h time-on-stream. The main reaction product was ethylene oxide, while 2-methoxyethanol and ethylene glycol were observed as kinetic byproducts. In most of the experiments, ethylene glycol was not detected at all. An increase in temperature and pressure affected negatively the ethylene oxide selectivity, while an increase in the hydrogen peroxide concentration improved both the ethylene oxide selectivity and ethylene conversion. Ethylene epoxidation was comparable with propylene epoxidation, displaying, however, important differences in activity and selectivity, which were attributed to the partial pressures studied in the present work. It was demonstrated that TS-1 is a very selective and active catalyst for the selective epoxidation of ethylene with hydrogen peroxide.

Synthesis of Low Cost Titanium Silicalite-1 Zeolite for Highly Efficient Propylene Epoxidation.

Developing an effective and low-cost system to synthesize titanium silicalite-1 (TS-1) zeolite is desirable for a range of industrial applications. To date, the poor catalytic activity of the synthesized zeolite due to the low amount of framework titanium and large crystal size is the main obstacle limiting the widespread application of this material. Moreover, a large amount of wastewater is often produced by the existing synthesis process. Herein, a green and sustainable route for synthesizing small-crystal TS-1 with a high fraction of framework Ti was demonstrated via a seed-assisted method using a tetrapropylammonium bromide (TPABr)-ethanolamine hydrothermal system. The influence of the synthesis conditions on the physicochemical properties and catalytic activities of TS-1 was investigated. With the assistance of nanosized S-1 seeds, the incorporation of Ti into the framework of TS-1 was promoted, and the crystallization rate was effectively accelerated. After alkaline etching, the obtained hierarchical TS-1 had higher catalytic activity towards propylene epoxidation with an extremely high turnover frequency of 1,650 h−1. Furthermore, the mother liquid during the hydrothermal reaction could be reused for the next synthesis procedure. Consequently, utilization ratios of both ethanolamine and TPABr exceeding 95% were achieved by recycling the mother liquid. This low-cost approach for reducing wastewater could be easily scaled up to provide a promising synthesis method for the industrial production of TS-1 and other topological zeolites.

Mesoporogen-Free Strategy to Construct Hierarchical TS-1 in a Highly Concentrated System for Gas-Phase Propene Epoxidation with H2 and O2.

Hierarchical TS-1 has attracted enormous attention from both academia and industry due to its remarkable catalytic performance in epoxidation reactions. However, sustainable synthesis of hierarchical-nanosized TS-1 without mesoporogens is still challenging. In this work, we report a facile and mesoporogen-free strategy to simultaneously manipulate pore structure and particle size of TS-1 employing the concentrated system. Taking advantage of the suspended nuclei in the concentrated system as confirmed by the DLS-PSD and atomic force microscopy, the novel TS-1 is demonstrated to have higher Ti concentration on surface, higher surface area (539 m2/g), abundant mesopores, and reduced crystal size (ca. 150 nm). Moreover, this Au-Ti bifunctional catalyst shows a good PO formation rate with enhanced catalytic stability due to the hierarchical structure. This strategy opens a novel way for the green synthesis of hierarchical-nanosized TS-1 and facilitates industrial development of the Au/TS-1 catalyst for propene epoxidation.

The Clean Synthesis of Small-Particle TS-1 with High-Content Framework Ti by Using NH4HCO3 and Suspended Seeds as an Assistant.

The synthesis of a TS-1 zeolite with high-content framework Ti and small particles has been developed by adding NH4HCO3 and suspended seeds as an assistant. With the addition of NH4HCO3, the Hofmann decomposition of the tetrapropylammonium cation (TPA+) decreased, and the framework Ti content of the zeolite increased first and then decreased while the particle became larger. With the assistance of suspended seeds, the TS-1 synthesized under a low-alkalinity system possesses small particle size and high-content framework Ti, and it shows the best catalytic activity among the prepared catalysts. Because the decomposition of TPA+ decreased, the mother liquid could be reused in the next run of preparation. Even though the recycled mother liquid was reused five times, all obtained TS-1 samples exhibited similar catalytic performances in propylene epoxidation. This work provides an efficient process for preparing TS-1 with good catalytic performance and reduces the discharge of the waste liquid.

Structured binder-free MWW-type titanosilicate with Si-rich shell for selective and durable propylene epoxidation

Selective and durable fixed-bed catalysts are highly desirable for developing eco-efficient HPPO (hydrogen peroxide propylene oxide) process. The powder titanosilicate catalysts must be shaped before being applied in industrial processes. As the essential additives for preparing formed catalysts, binders are usually the catalytically inert components, but they would cover the surface and pore mouth of zeolite, thereby declining the accessibility of active sites. By recrystallizing the binder (silica)/Ti-MWW extrudates with the assistance of dual organic structure-directing agents, the silica binder was converted into MWW zeolite phase to form a structured binder-free Ti-MWW zeolite with Si-rich shell, which enhanced the diffusion efficiency and maintained the mechanical strength. Meanwhile, due to the partial dissolution of Si in the Ti-MWW matrix, abundant silanol nests formed and part of framework TiO4 species were transferred into open TiO6 ones, improving the accumulation and activation ability of H2O2 inside the monolith. Successive piperidine treatment and fluoridation of the binder-free Ti-MWW further enhanced the H2O2 activation and oxygen transfer ability of the active Ti sites, and stabilized the Ti-OOH intermediate through hydrogen bond formed between the end H in Ti-OOH and the adjacent Si-F species, thus achieving a more efficient epoxidation process. Additionally, the side reaction of PO hydrolysis was inhibited because the modification effectively quenched numerous Si-OH groups. The lifetime of the modified binder-free Ti-MWW catalyst was 2400 h with the H2O2 conversion and PO selectivity both above 99.5%.

Kinetics simulation of propylene epoxidation over different Ti species in TS‐1

AbstractA thorough experimental investigation on the kinetic behavior of liquid‐phase propylene epoxidation over TS‐1 and tetrapropylammonium hydroxide (TPAOH) treated TS‐1 catalysts was conducted in a fixed‐bed reactor. The amounts of different coordinated Ti species in the catalysts were quantified by spectroscopies, and their catalytic performances of the epoxidation and alcoholysis of propylene oxide were measured by kinetic modeling. The study shows that the TPAOH treatment converted some of the tetrahedrally coordinated Ti to octahedrally coordinated Ti, and both species were active for the epoxidation and alcoholysis. The superior catalytic performance observed over the TPAOH treated TS‐1 is due to two factors, the increased percentage of active sites, and reduced energy barrier for epoxidation on the octahedrally coordinated Ti. In addition, as the H2O2 conversion increases, the adsorption equilibrium constant of propylene oxide plays a more decisive role than the activation energy for the selectivity of propylene glycol monomethyl ethers.

Improvement of Propylene Epoxidation Caused by Silver Plasmon Excitation by UV-LED Irradiation on a Sodium-Modified Silver Catalyst Supported on Strontium Carbonate

The effect that UV-LED irradiation exerted on a sodium-modified silver catalyst supported on strontium carbonate (Ag-Na/SrCO3) was examined during an epoxidation of propylene to propylene oxide. Based on our previous study, we used Ag(56)-Na(1)/SrCO3 in this study. The numbers in parentheses refer to the weight percentage of silver and sodium. Although this catalyst system did not contain typical photocatalysts such as titanium oxide or tungsten oxide, UV-LED irradiation of Ag(56)-Na(1)/SrCO3 resulted in an evident improvement in the selectivity and yield of propylene oxide. Such an advantageous effect of UV-LED irradiation could not be discussed based on the bandgap used in photocatalysts and, therefore, we proposed a mechanism based on the plasmon excitation of silver, which could be accomplished using the irradiation wavelength of UV-LED to produce electrons. Since the lifespan of these electrons is expected to be short, it is difficult to place them into direct contact with the gas phase of oxygen. Once the generated electrons move to SrCO3, however, the lifespan is improved, which could allow suitable contact with oxygen in the gas phase to form active oxygen. If the oxygen is active for epoxidation as hydrogen peroxide, this could explain the improvement in activity from UV-LED irradiation.

Solvent‐free gas‐phase epoxidation of propylene in fluidized bed reactor

AbstractIndustrial HPPO process suffers from the use of CH3OH solvent, leading to energy‐intensive and lengthy purification operations. Herein, by using a microspheric TS‐1 catalyst and a fluidized bed reactor, we realized a solvent‐free gas‐phase epoxidation of propylene, in which, 18% C3H6 conversion and 97% PO selectivity have been achieved with 73% H2O2 utilization. Furthermore, the gas‐phase epoxidation proceeds with an excellent stability, attributed to use of the fluidized bed reactor.

Synthesis of titanium silicalite‐1 with high titanium content and its catalytic performance of propylene epoxidation

AbstractTitanium silicalite‐1 (TS‐1) has unique catalytic properties for oxidation reactions due to the titanium atom in the framework. To increase the Ti content in the framework, polyvinylpyrrolidone (PVP) was added as a surfactant to modify TS‐1 (PTS‐1). The titanium content in PTS‐1 as the active site in the catalytic reaction increased to 0.87 wt%, compared with the traditional TS‐1 which has a titanium content of 0.64 wt%. Simultaneously, the average particle size of PTS‐1 is reduced to 180 nm, compared with TS‐1 (230 nm). The increase in Ti content makes the Au loading increase; thus, Au/PTS‐1 shows higher catalytic performance for propylene epoxidation, and the propylene oxide (PO) formation rate increases from 95 gPOh−1kgCat−1 with Au/TS‐1 to 190 gPOh−1kgCat−1 with Au/PTS‐1. In addition, the mechanism for the modification of Ti content by PVP is proposed.

Competition of propylene hydrogenation and epoxidation over Au-Pd/TS-1 bimetallic catalysts for gas-phase epoxidation of propylene with O2 and H2

Au and Pd play a collaborative effect in the formation of H2O2 from H2 and O2, which is beneficial for the gas-phase epoxidation of propylene over Au-Pd/TS-1 (i.e., Au-Pd bimetallic nanoparticles supported on titanium silicalite-1). However, the introduction of Pd into Au/TS-1 makes this reaction more complicated. In this work, GC–MS results demonstrated the presence of propylene hydrogenation. A small amount of O2 was found to significantly promote the propylene hydrogenation over Au-Pd/TS-1, while O2 concentration had less effect. Four catalysts were designed to discuss the active sites on Au-Pd/TS-1. It was concluded that propylene hydrogenation preferentially occurred at Pd sites and propylene epoxidation occurred at Au-Ti sites. Based on the catalytic performance of Au-Pd/TS-1 at 100 °C, a four-stage model was proposed for the first time to reveal the competition between propylene hydrogenation and epoxidation. Moreover, the addition of CO could abate the propene hydrogenation over Au-Pd/TS-1.