Propylene Oxide Selectivity Research Articles

Size Effect of Unsupported CuOx on Propylene Epoxidation by Oxygen

Propylene epoxidation by oxygen over Cu-based catalysts is a most ideal and economical process. The unsupported CuOx nanoparticles were synthesized by liquid phase reduction method. The crystalline phase and size of CuOx nanoparticles were characterized by XRD and TEM, respectively. The catalytic performances of CuOx with different pretreatment temperatures and atmospheres were also investigated. The as prepared unsupported CuOx nanoparticles were composed of Cu and Cu2O. With elevating the pretreatment temperatures under N2 atmosphere, the size of CuOx nanoparticles increased gradually from 25 nm to 58 nm. However, the relative content between Cu and Cu2O remained almost unchanged, indicating that the valence state of Cu species in CuOx was kept nearly constant. The C3H6 epoxidation activities showed a first increases and then decreases trend with enhancing the particle size of CuOx. The highest activity of propylene epoxidation was achieved when the particle size of CuOx was 41 nm. The conversion of propylene was 0.14% and the selectivity of propylene oxide was 30% respectively under the reaction temperature of 150 °C. The results indicated that the appropriate CuOx particle size was a key factor for propylene oxide formation on Cu-based catalysts. The C3H6 epoxidation activities showed a first increases and then decreases trend with enhancing the particle size of CuOx. When the CuOx particle size was 41 nm, the formation rate of PO reached maximum. The appropriate particle size of Cu species with low valence state was crucial to the generation of PO for C3H6 epoxidation reaction by O2 on Cu-based catalysts.

Uncalcined TS‐2 immobilized Au nanoparticles as a bifunctional catalyst to boost direct propylene epoxidation with H2 and O2

AbstractDeveloping stable yet efficient Au–Ti bifunctional catalysts is important but challenging for direct propylene epoxidation with H2 and O2. This work describes a novel strategy of employing uncalcined titanium silicalite‐2 (TS‐2‐B) to immobilize Au nanoparticles as a bifunctional catalyst for the reaction. Under no promoter effects, the Au/TS‐2‐B catalyst compared to the referenced Au/TS‐1‐B catalyst delivers outstanding catalytic performance, that is, exceptionally high stability over 100 hr, propylene oxide (PO) formation rate of 118 gPO·hr−1·kgcat−1, PO selectivity of 90% and hydrogen efficiency of 35%. The plausible relationship of catalyst structure and performance is established by using multiple techniques, such as UV–vis, high‐angle annular dark‐field scanning transmission electron microscopy, thermogravimetric analysis, and X‐ray photoelectron spectroscopy. A unique synergy of Au–Ti4+–Ti3+ triple sites is proposed for our developed Au/TS‐2‐B catalyst with the higher stable PO formation rate and hydrogen efficiency. The insights reported here could shed new light on the rational design of highly stable and efficient Au–Ti bifunctional catalysts for the reaction.

Structure and reactivity of single site Ti catalysts for propylene epoxidation

Propylene epoxidation using molecular oxygen and hydrogen mixture on Au-based catalysts has attracted attention because of high propylene oxide selectivity and the use of an inexpensive and environmental friendly oxidant. Single-site titanium on metal oxide supports plays an important role in achieving high reactivity and selectivity in propylene epoxidation. Here we used TiO2 atomic layer deposition (ALD) to synthesize single-site titanium imbedded in the SiO2 framework for propylene epoxidation. High temperature calcination was used as post-treatment to control the titania structure and TiO coordination number. Using UV–vis spectroscopy and X-ray absorption spectroscopy, we successfully established that under similar propylene conversion the selectivity to propylene oxide (PO) is strongly correlated to the TiO coordination number and bond length. Using a cluster model, density functional theory (DFT) calculations indicate that the partial charges of single TiSiO2 sites scale linearly as a function of the coordination number. Also, the predicted TiO bond lengths follow the same trend as found in the experiments, providing additional support for the observed experimental activity relationships.

Seed-assisted synthesis of hierarchical nanosized TS-1 in a low-cost system for propylene epoxidation with H2O2

Designing nanosized and hierarchically porous zeolite with intracrystalline mesopores can enhance mass transport ability to improve the catalytic performance of HPPO, which is of prime scientific and industrial importance. Nanosized titanium silicalite-1 (TS-1, 360 × 190 × 640 nm) was first synthesized in a tetrapropylammonium bromide (TPABr)-n-butylamine dry gel system using the seed-assisted method, and post-modification with pure ammonium fluoride (NH4F) solution. This strategy introduced uniform intracrystalline mesoporosity and increased the hydrophobicity. Reducing the TS-1 grain size and creating a hierarchical porous architecture can improve the diffusion of molecules. This solves challenges in mass transport and promotes the catalytic performance. This catalyst was used for propylene epoxidation. The conversion of hydrogen peroxide and selectivity of propylene oxide (PO) attained 98.4% and 97.6%, respectively. Even after 5 runs, the catalyst still maintained a stable performance after simple filtering.

Propylene epoxidation by oxygen over tungsten oxide supported on ceria-zirconia

Ce0.05Zr0.95O2 (denoted as CZ) was prepared by using a precipitation method to support tungsten oxide. For comparison, ZrO2 (denoted as Z) and CeO2 (denoted as C) supports were also prepared with the same method. The WOx/Z, WOx/CZ, and WOx/C catalysts were characterized by XRF, nitrogen adsorption–desorption, FE-SEM, SEM-EDX, XRD, Raman, H2-TPR, NH3-TPD, and CO2-TPD analyses. Propylene epoxidation by oxygen was carried out over WOx/Z, WOx/CZ, and WOx/C catalysts in a continuous flow reactor. In the propylene epoxidation by oxygen, WOx/CZ catalyst showed the desirable conversion of propylene (3.8%) and selectivity for propylene oxide (46.8%). Such superior catalytic performance of WOx/CZ catalyst compared to the other catalysts (WOx/Z and WOx/C) was attributed to the moderate reduction ability and the optimum ratio of acidity/basicity.

Research on Direct Epoxidation of Propylene on Modified Au / TS-1 Catalysts

Au/TS-1 catalyst was prepared by deposition-precipitation method. The catalytic performance of the modified catalyst for propylene direct vapour-phase epoxidation was investigated by modifying the support TS-1 and adding promoters during the preparation of the catalyst. Combined with the characterization results of X-ray powder diffraction (XRD), ultraviolet-visible diffuse reflectance (UV-Vis) and infrared diffuse reflectance (FT-IR), it was found that the activity of propylene epoxidation could be significantly improved by modified catalyst. The results showed that the catalyst modified by Sn increased the conversion of propylene and the selectivity of propylene oxide on the, which might be due to the coordination effect caused by the addition of Sn. The pore volume and specific surface area of Au/TS-1 have little effect on the vapour-phase epoxidation reaction, and the decomposition ability of Au/TS-1 is an important factor affecting its reactivity.

Enhanced propylene oxide selectivity for gas phase direct propylene epoxidation by lattice expansion of silver atoms on nickel nanoparticles

A series of surfactant-free nickel-core and silver-shell (Ni@Ag) nanoparticles encapsulated within the mesopores of SBA-15 were synthesized and tested as catalysts for direct propylene oxidation by molecular oxygen. The influences of temperature, Gas Hour Space Velocity (GHSV) and Ni/Ag ratio on catalytic activity were systematically investigated. Among the prepared samples, Ni1Ag0.4/SBA-15 exhibited the best catalytic performance with selectivity of 70.7% and PO production rate of 4.4 nmol/g/s under 1 bar at 220 °C with GHSV of 192 h−1. High selectivity was attributed to longer Ag-Ag interatomic distance obtained by careful engineering the thickness of Ag shell over preformed Ni nanoparticles. In addition, all prepared new Ni@Ag core-shell catalysts presented excellent stability, which could maintain the conversion and selectivity for at least 10 h. These results suggest that new designs based on Ag surface atoms tailoring might pave the way to highly efficient and robust Ag catalysts for direct propylene oxidation using molecular oxygen as sole oxidant.

The construction of Mo6−δO3−x-supported catalyst for low-temperature propylene gas-phase epoxidation by Cu modification

Selective epoxidation of propylene by molecular oxygen under mild conditions remains a great challenge to date. Here, we successfully designed and constructed molybdenum oxide in an electron-rich state by introducing the transition metal copper into MoO3–Bi2SiO5/SiO2 through a simple co-impregnation method. The new Mo species has the structure Mo6−δO3−x, and greatly improved propylene epoxidation performance at low temperatures. Propylene oxide formation rate increased from 106 to 336 g PO kgcat−1 h−1. X-ray photoelectron spectrometry (XPS) and UV–visible diffuse reflection (UV–vis DRS) characterizations suggest that the bridging oxygen bond between molybdenum and copper produces an oxygen vacancy, which facilitates the formation of the electron-rich state of molybdenum with lower 3d electronic binding energy. In addition, the incorporation of Cu effectively inhibits the formation of Bi2Mo3O12 and improves the dispersion of Mo species. Furthermore, kinetic studies show that Cu-modified catalysts maintain lower apparent activation energy for propylene epoxidation, which obviously reduced the selectivity of acrolein and then conspicuously improved the selectivity of propylene oxide. Our results not only offer a new solution for low temperature propylene epoxidation, but also provide a new strategy for the design of other types of low-temperature epoxidation catalysts.

Mechanistic insights into the direct propylene epoxidation using Au nanoparticles dispersed on TiO2/SiO2

Direct propylene epoxidation is an important gas-phase reaction for the future industrial production of propylene oxide (PO). However, the mechanistic understanding of this reaction system is still elusive, including the role of side reactions. Herein, kinetic Monte Carlo (KMC) simulations are applied to explore the underlying mechanistic aspects of propylene epoxidation over Au/TiO2/SiO2, and consistent results are obtained with respect to experimental benchmarks. The present study systematically probes the reaction mechanism, which involves acrolein formation on Au nanoparticles as well as the formation of PO and several byproducts at the Au/Ti dual interface sites, and it highlights the synergistic effect of interface sites in heterogeneous catalysis. The origin of the negative effect of rising reaction temperature on PO selectivity has been clarified, as it is correlated with decreasing atomic oxygen coverage on the Au surface. The varying feed concentration of H2/O2/C3H6 leads to different coverages on the catalyst surface, emphasizing the importance of an optimum H2/C3H6 inlet concentration ratio for enhancing PO selectivity and hydrogen utilization efficiency. Based on our model results, we propose that supports with acidic groups may promote hydroperoxy species transferring to the adjacent Ti sites, thereby decreasing its decomposition to atomic oxygen on the Au sites, which leads to byproduct formation.

Catalytic epoxidation of propylene glycol and its acetates

The base-catalyzed, gas-phase epoxidation of propylene glycol acetates to propylene oxide (PO) is carried out in a laboratory-scale fixed-bed reactor. Potassium salts (0.5–2.5 mmol K/g) on silica are identified as selective catalysts for the reaction. A temperature of 400 °C at high space velocity gives a maximum PO selectivity of 81%, while the maximum yield of 30% is achieved at the same temperature but lower space velocity. Pre- and post-reaction catalysts were characterized by N2 adsorption, TGA, XPS, and FTIR. High temperature and high potassium loading lead to collapse of the silica pore structure, and in severe cases to obstruction of flow in the reactor. Potassium carbonate, formed via ketonization of potassium acetate generated on the support during reaction with KOH as the catalyst material, is the stable potassium species on the catalyst at high loadings (>1.5 mmol K/g silica). At low loadings, surface potassium silicate analogs (Si-O-K) predominate. The use of potassium on a low surface area, annealed silica support gives similar activity and PO selectivity without degradation of pore structure.

An efficient Titanium silicalite-1 catalyst for propylene epoxidation synthesized by a combination of aerosol-assisted hydrothermal synthesis and recrystallization

Reducing the crystal size and creating intra/inter-crystalline mesoporosity or macroporosity are two options to enhance the mass transportation efficiency in microporous zeolites. In this study, an improved aerosol-assisted hydrothermal method was used to synthesize small-crystal Titanium silicalite-1 (TS-1) with low template amount. By using a more concentrated template agent the particle size of the TS-1 crystal can be reduced from 350-800 nm to 230–270 nm. A well-dispersion of the Ti species can be achieved under optimized synthesis condition. A post-treatment with tetrapropyl ammonium hydroxide (TPAOH) was used to further improve the diffusion property. The mesopore volumes and external surface areas are significantly increased due to the formation of the intracrystalline voids. All the TS-1 samples were evaluated in propene epoxidation. The conversion of H2O2 and the selectivity of Propylene Oxide are 99.9% and 98%, which are improved as compared to those of the parent TS-1.

Ag-(Mo-W)/ZrO2 catalysts for the production of propylene oxide: Effect of pH in the preparation of ZrO2 support

Direct oxidation of propylene to propylene oxide was carried out over Ag-(Mo-W)/ZrO2 (pH X) catalysts prepared by changing the pH of Zr precursor solution from 3 to 14 while preparing the ZrO2 support. It was found that the fraction of monoclinic phase and the ratio of acidity to basicity in the catalysts showed a volcano-shaped trend with respect to pH of Zr precursor solution. Such characteristics were well correlated with the selectivity of reaction towards propylene oxide. Ag-(Mo-W)/ZrO2 (pH 10) with the highest fraction of monoclinic phase of ZrO2 and ratio of surface acidity to basicity showed the highest selectivity (68%) for propylene oxide.

Selective propylene epoxidation in liquid phase using highly dispersed Nb catalysts incorporated in mesoporous silicates

Selective propylene epoxidation to propylene oxide (PO) with hydrogen peroxide (H 2 O 2 ) was carried out in a catalytic semi-batch reactor. High propylene epoxidation activity (44 h − 1 ) was observed over Nb based mesoporous silicate materials Nb-TUD-1 under mild operating conditions. The physical and chemical properties of the Nb based silicates characterized using BET, FTIR, TPD, TEM and UV–Vis revealed that the site isolation and surface acidity are crucial for PO production. Catalyst synthesis methods were investigated for their effects on PO productivity, PO selectivity and H 2 O 2 utilization efficiency. It is found that Nb-TUD-1 material synthesized by the sol–gel method is more active and selective than impregnated materials for liquid phase propylene epoxidation. Surface characterization confirms that thus synthesized Nb-TUD-1 catalysts have more Lewis acidity and less Bronsted acidity compared to the catalysts by impregnation.

TS-1 supported highly dispersed sub-5 nm gold nanoparticles toward direct propylene epoxidation using H2 and O2

We report a simple and efficient method for the preparation of highly dispersed Au nanoparticles (< 5 nm) on TS-1 substrate. The synthesis relies on the use of NaBH4 as a reductant for rapid Au atom generation, as well as PVA as a capping agent confining the particle size and dispersion. The samples were characterized by N2 physisorption, inductively coupled plasma mass spectrometry, power X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, CO pulse chemisorption and thermogravimetric analysis. The size of Au particles can be controlled in the range of 3–5 nm. The supported catalyst shows both good activity and selectivity for propylene oxide (PO) generation from direct propylene epoxidation. An optimal performance with PO formation rate of 102.94 gPO h−1 kg−1cat and selectivity of 84.83% is achieved over 2.0 wt% Au/TS-1 catalyst, which is prepared by controlling PVA/Au3+ mass ratio of 1.5/1 and NaBH4/Au3+ mole ratio of 5/1. After 50 h test at 200 °C, no significant decrement of both catalytic activity and PO selectivity can be observed, indicating the excellent thermally stability of the catalyst. Furthermore, a possible reaction mechanism is described on basis of the previous researches and our experimental results.

Activity and stability of titanosilicate supported Au catalyst for propylene epoxidation with H2 and O2

Highly dispersed Au catalysts supported on amorphous titanosilicate supports with three-dimensional wormhole-like mesoporosity were prepared by deposition–precipitation method, which show excellent performance in propylene epoxidation with H2 and O2. The best catalytic performance, propylene conversion of 12.8%, propylene oxide selectivity of 76.2% and PO space-time yield of 160.9 gpo/Kgcat/h, was obtained at the reaction temperature of 300 °C. The Ti/Si molar ratio of the supports and some experimental parameters, such as the urea amount, loading temperature and Au loading on the as-produced catalyst are optimized. In addition, with the assistance of in-situ Fourier transform infrared spectroscopy (In-situ FT-IR), adsorption and desorption of PO, propylene and the reaction mixture on the catalyst are conducted to further explore the deactivation mechanism. It suggests that propylene and PO can be irreversibly and strongly adsorbed on the catalyst surface and finally transform to carbonate adsorbed on the active sites Ti-OH, resulting in catalyst deactivation.

Silylated Pd/Ti-MCM-41 catalyst for the selective production of propylene oxide from the oxidation of propylene with cumene hydroperoxide

Titanium-containing mesoporous molecular sieves with regular pore structure, high specific surface area, tunable pore size and suitable active sites have attracted great attention in the epoxidation of alkenes. Herein, we report the silylated Pd/Ti-MCM-41 (PdTM-ZS) catalysts prepared by treating the series Ti-MCM-41 samples (Si/Ti molar ratio = 10, 15, 20, 25 and 30) with palladium loading followed by surface silylation with trimethylchlorosilane. Reaction results show that the PdTM-ZS catalysts can be applied as efficient catalysts for the selective production of propylene oxide from the oxidation of propylene with cumene hydroperoxide as the oxidant. The PdTM-20S catalyst exhibits an excellent catalytic performance with the selectivity of propylene oxide approaching 100%, which can be attributed to the synergistic effect of abundant active sites, palladium loading and surface silylation.

Analysis of the propylene epoxidation mechanism on supported gold nanoparticles

The direct propylene epoxidation reaction has been investigated experimentally in the past by several different groups, and gold-based catalysts tend to provide high selectivity for propylene oxide, but the conversion is relatively low. Models that can connect the atomistic catalytic details to the observed experimental data are desired, in order to identify new catalyst structures and formulations. While electronic structure calculations have been used to quantify some of the key reaction steps in the direct propylene epoxidation reaction, atomistic models for translating this information into more experimentally-relevant data are needed. Here, kinetic Monte Carlo (KMC) simulations are used to bridge this gap in the modeling hierarchy. Relevant data from previous experiments and electronic structure calculations are used to parameterize a KMC model for predicting propylene oxide production from an Au/TiO2/SiO2 catalyst. The model connects the H2/O2-related reactions occurring on the Au sites with the epoxidation step on the isolated Ti surface sites. In addition, the composition in the bulk gas phase is synchronized with the dynamic reaction events occurring on the surface. The KMC model is able to adequately reproduce the experimental trends with respect to temperature and different reactant partial pressures. However, this is only achieved by considering the re-adsorption of trace amounts of the oxidant (H2O2) from the gas phase, versus merely assuming that desorbed species are immediately swept away in the gas stream.

Synergetic photo-epoxidation of propylene with molecular oxygen over bimetallic Au–Ag/TS-1 photocatalysts

Au–Ag bimetallic nanoparticle-supported microporous titanium silicalite-1 catalysts were prepared via a hydrothermal-immersion method, and their structures were examined. These materials serve as efficient catalysts for the photosynthesis of propylene oxide via the epoxidation of propene. The Au/Ag mass ratio and reaction temperature were demonstrated to have significant effects on the catalytic activity and selectivity of propylene oxide. The optimal formation rate (68.3 μmol/g·h) and selectivity (52.3%) toward propylene oxide were achieved with an Au:Ag mass ratio of 4:1. Notably, the strong synergistic effect between Au and Ag resulted in superior photocatalysis of the bimetallic systems compared with those of the individual systems. A probable reaction mechanism was proposed based on the theoretical and experimental results.

K- and Ca-promoted ferrosilicates for the gas-phase epoxidation of propylene with O2

In the propylene epoxidation reaction with Fe-SiO2 catalysts the presence of iron oxide particles has a detrimental effect due to the total combustion of propylene on these iron species. Thus, the complete elimination of the iron oxide particles is presented as a preliminary strategy in order to increase the selectivity towards propylene oxide in iron-based catalysts. In this sense, a simple post-treatment of the catalysts with alkali or alkaline-earth elements (such as K or Ca, respectively) has proven effective in the total elimination of these iron oxide particles. Furthermore, the addition of K and Ca has modified the physico-chemical properties of the catalysts, decreasing their superficial acidity and (for higher K or Ca loadings) masking/blocking the active sites responsible for the catalytic reaction. With all this, it is shown that K has a higher efficiency removing the iron oxide particles compared with Ca (for the same molar ratios) and that a higher amount of K (compared to Fe) is required for the complete elimination of the iron oxide particles. A considerable propylene oxide selectivity enhancement (up to 65%) has been obtained for the K-promoted Fe0.005SiO2 and Fe0.01SiO2 catalysts using O2 as sole oxidant.

Simultaneously Enhanced Stability and Selectivity for Propene Epoxidation with H2 and O2 on Au Catalysts Supported on Nano-Crystalline Mesoporous TS-1

It is a challenging task to design efficient Au/Ti-containing catalysts for propene epoxidation with H2 and O2 that simultaneously achieve high catalytic stability and selectivity to propylene oxide (PO). This contribution first describes the synthesis of a nanocrystalline mesoporous titanium silicalite-1 (MTS-1) by dry-gel conversion that employs cheap triblock copolymers as template. Compared with TS-1, MTS-1 has a shortened reactant/product diffusion length as a result of smaller crystal size (<100 nm) and the presence of mesopores (ca. 3 nm). Surprisingly, this designed catalyst shows simultaneously improved PO selectivity over 95% as well as stability over 40 h, much better than the traditional Au/TS-1 catalyst. Moreover, the intrinsic reason for the enhanced performance is elucidated. The better mass transfer ability together with higher hydrophobicity of Au/MTS-1 results in lower coke weight and absence of refractory aromatic coke. In this way, the side reactions and deactivation caused by blocking...