Direct Gas-phase Epoxidation Of Propylene Research Articles

Effect on the introduction of hollow structure on Au/TS-1 to improve the stability for propylene epoxidation

In the direct gas-phase epoxidation of propylene, the channels of TS-1 are easily blocked by the carbon deposition which leads to poor stability and even deactivation of the catalyst. In this paper, H-TS-1 molecular sieve with hollow structure was successfully synthesized by the post-treatment of TS-1 molecular sieve with tetrapropylammonium hydroxide (TPAOH) solution. The experimental results show that the Au/H-TS-1 catalyst exhibits good stability compared with Au/TS-1. TG results show that the weight of carbon deposition is only ∼3.56 wt% in Au/H-TS-1 at the reaction time of 50 h compared with Au/TS-1 catalyst (∼8.55 wt%). The introduction of hollow structure improves the mass transfer ability of the catalyst which could reduce the formation of carbon deposition and improve the stability of Au/H-TS-1 in propylene epoxidation. The PO formation rate of Au/H-TS-1 only decreased by ∼10.3 % after 100 h, while that of Au/TS-1 decreased by ∼28.3 %.

Impact of Titanium in Controlling Silver Particle Size on Enhancement of Catalytic Performance of AgMoO3/Ti-HMS for Direct Epoxidation of Propylene.

In this work, direct gas-phase epoxidation of propylene (DPO) to propylene oxide by molecular oxygen has been studied by using Ag–MoO3 supported on titanium-containing hexagonal mesoporous silica (Ti-HMSn) of different Si/Ti molar ratios. The promotion effect of NaCl on the synthesized catalysts has also been investigated. Among the studied supports, the hexagonal mesoporous silica (HMS) with a Si/Ti ratio of 10 was the most suitable one for production of propylene oxide (PO). The optimal performance of the AgMo/Ti-HMS10 catalyst in DPO exhibited a selectivity to PO of 43.2% with a propylene conversion of 14.1%, at 400 °C, 0.1 MPa, and a space velocity of 12,000 h–1. The catalyst verified good stability over at least 20 h on stream. Only 2.7% PO selectivity with a propylene conversion of 10.1% was achieved over the AgMo/HMS sample. The incorporation of Ti into the HMS frame could optimize the particle size distribution of Ag, producing Ag nanoparticles with an average size of 6.8 nm compared with that of Ag/HMS (24.3 nm). The in situ Raman spectrum of AgMo/Ti-HMS10 heated in a stream of C3H6/He at 400 °C showed new bands at 616, 390, and 210 cm–1, characteristic of the AgxMoyOz intermediate phase. The obtained results suggested that this formed AgMo/Ti-HMS10 phase could most likely be relevant for selective epoxidation of propylene. However, during the reaction of C3H6 with AgMo/HMS, the formation of this intermediate was hardly detected. On the other hand, the hydrogen temperature-programmed reduction measurements indicated improved reducibility of MoO3 in the AgMo/Ti-HMS10 catalyst, which acknowledged the role of Mo6+ in gaining electrons from silver to form positively charged Ag. This could reduce the effective charge of the adsorbed oxygen on silver sites and in turn favor the epoxidation path of propylene rather than the combustion route. Also, during the reaction of C3H6 with NaAgMo/Ti-HMS10 at 400 °C, two new Raman bands were detected at 277 and 350 cm–1, characteristic of Ag2MoO4.

Propylene epoxidation by molecular oxygen using supported silver catalysts: Effect of support type, preparation method and promotion with alkali chloride and/or steam

Abstract The direct gas-phase epoxidation of propylene using molecular oxygen remains a challenging method to produce propylene oxide (PO) with minimum environmental impact. The epoxidation of propylene using Ag catalysts supported on non-porous or low surface area supports, such as α-Al3O2, CaCO3, TiO2 rutile, SiO2 and ZrO2, as well as on meso/macroporous ZrO2 and TiO2 anatase, was thoroughly investigated in the present work. The catalysts (Ag loading 10–50 wt. %) were prepared by wet impregnation followed by in-situ reduction with amines or by ex situ treatment with H2 at 350 °C. The combination of moderate surface area (ca. 60 m2/g) of the meso/macroporous ZrO2 and TiO2 anatase with the in situ reduction with amines method offered a relatively high Ag dispersion and small crystallite size of 25–35 nm, even at the high loading of 50 wt.%. Propylene conversion was as high as ca. 40% in the temperature range of 200–320 °C, depending on the type of the support and the preparation method, with the mesoporous ZrO2 and the amines reduction method affording the most active catalysts. The selectivity of PO was maximized (ca. 6%) at ∼220–260 °C, i.e. 6.1% at 33.0% conversion for the TiO2 rutile amine reduced catalyst. Promotion of the catalysts with NaCl (2–5 wt.%), led to significantly lower reactivity (conversion ≤ 15%) even at high temperatures, but the selectivity to PO increased up to ca. 14%, with the low surface area ZrO2 and the TiO2 rutile based catalysts being the most selective. However, propylene tends to react also with the chloride ions, being on the promoted catalyst surface, leading to allyl chloride co-production, thus leading to gradual decrease of Cl− ions and loss of their beneficial effect on PO formation. Co-feeding of the reactant gas mixture with steam offered an alternative method of promoting PO production, even in the absence of NaCl, with the TiO2 rutile and CaCO3 based catalysts being the most effective via this route.

High Stability of Ruthenium–Copper-Based Catalysts for Epoxidation of Propylene

The catalytic performance of RuO2–CuO/SiO2-based materials for the direct gas-phase epoxidation of propylene using molecular oxygen at atmospheric pressure was investigated as a function of the Ru/Cu weight ratio and the addition of NaCl, TeO2, Cs2O, or TiO2. The best performing catalysts were then assessed for stability. Doping with NaCl delivered the highest PO yield, but its catalytic activity dropped quickly within a few hours due to the loss of chloride content. The same catalyst doped instead with TeO2, Cs2O, or TiO2 could maintain stable catalytic performance for 6 h. However, the addition of TeO2 presented the best improvement in terms of PO selectivity. Further optimization of PO selectivity for the RuO2–CuO–TeO2/SiO2 catalyst system was accomplished by varying reactor temperature and total feed gas flow rate. The highest PO selectivity could be achieved at 47% (255 gPO h−1 kgcat −1, 0.35% propylene conversion). The time-on-stream test revealed excellent catalytic stability within the 12 h of test period without any deactivation.

Copper–Manganese Mixed Metal Oxide Catalysts for the Direct Epoxidation of Propylene by Molecular Oxygen

Cu–Mn mixed metal oxide (MMO) catalysts doped with NaCl were synthesized using combustion synthesis technique with a variety of fuels for the direct gas-phase epoxidation of propylene with molecular oxygen. The catalyst structure consisted of Mn2O3 and Cu1+xMn2–xO4 crystals. The catalyst crystallinity, primarily influenced by the combustion synthesis fuel choice and calcination temperature, has been found to be of particular importance in affecting the selectivity of the epoxidation reaction. The optimum catalyst composition was 10–20 mol % Cu in Mn doped with 0.1–0.2 mol % NaCl. It exhibited 30–37% propylene oxide selectivity and 1.3–1.5% propylene conversion.

Gold supported on nitrogen-incorporated TS-1 for gas-phase epoxidation of propylene

Nitrogen-incorporated titanium silicalite-1 (NTS-1) was used to prepare a supported gold catalyst for direct gas-phase epoxidation of propylene using H 2 and O 2 . The NTS-1 was synthesized by direct calcination of the TS-1 powder in a flow of NH 3 at high temperature. Gold catalyst supported on NTS-1 was synthesized by the deposition-precipitation method. The samples were characterized by X-ray diffraction, N 2 adsorption-desorption, diffuse reflectance ultraviolet-visible spectroscopy, inductively coupled plasma optical emission spectrometry, elemental analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy. The results indicated that nitrogen was incorporated into the TS-1 framework, and that the NTS-1 preserved the multidimensional fatigue inventory (MFI) structure well. It was found that NH 3 -treatment resulted in decreased acidity of the TS-1 zeolite support and improved both the Au capture efficiency and dispersion, which remarkably improved the catalytic performance. The enhanced activity was ascribed to the decrease in acidic surface sites and the ligand effect of the terminal NH 2 groups on the support surface. 以高温氨气处理制得含氮微孔钛硅分子筛 (NTS-1) 为载体, 用沉积-沉淀法制得了一系列纳米金催化剂. 考察了纳米金催化剂在氢气和氧气共存下催化丙烯气相环氧化制环氧丙烷反应中的催化性能. 结果表明, 高温氨气处理钛硅分子筛 (TS-1) 载体降低了其酸性, 提高了纳米金催化剂制备中金的利用率和纳米金催化剂中金的分散度, 显著提高纳米金催化剂的催化活性. 载体酸性降低和载体表面–NH 2 配位效应增强了催化剂活性. A novel gold catalyst was prepared by immobilization of gold nanoparticles on nitrogen-incorporated TS-1. This catalyst exhibits an excellent catalytic capacity for gas-phase epoxidation of propylene using H 2 and O 2 . Nitrogen-incorporation into TS-1 improved both gold loading and dispersion, and decreased the acidic sites of the support surface.

Direct gas-phase epoxidation of propylene over nanostructured molybdenum oxide film catalysts

Abstract Mixed molybdenum oxides are widely used catalysts for propylene oxidation to acrolein or acrylic acid or for methanol oxidation in the gas phase. In the present study, however molybdenum oxides exhibited an epoxidation activity in the gas phase. To achieve this activity, the oxidation state, particle sizes and accessibility of molybdenum oxides are needed to be controlled. Such active molybdenum species can be prepared by PVD deposition of molybdenum nanorods and their subsequent oxidation in a controlled atmosphere. Accordingly, monoclinic MoO 2 particles with a diameter of

Direct gas-phase epoxidation of propylene to propylene oxide through radical reactions: A theoretical study

The gas-phase radical chain reactions which utilize O 2 as the oxidant to produce propylene oxide (PO) are investigated through theoretical calculations. The transition states and energy profiles were obtained for each path. The rate constants were also calculated. The energetics for the competing pathways indicate that PO can be formed selectively due to its relatively low activation barrier (9.3 kcal/mol) which is in a good agreement with the experimental value (11 kcal/mol) of gas-phase propylene epoxidation. The formation of the acrolein and combustion products have relatively high activation barriers and are not favored. These results also support the recent experimental findings.

Superior Performance of Gold Supported on Titanium-Containing Hexagonal Mesoporous Molecular Sieves for Gas-Phase Epoxidation of Propylene with Use of H2 and O2

Titanium-containing hexagonal mesoporous silicas (Ti-HMS) with wormhole structure and Si/Ti molar ratios ranging from 10 to 40 have been prepared by using long-chain alkyl primary amines as template agents. The Ti-HMS supported Au catalyst (Au/Ti-HMS) was obtained by a deposition−precipitation method for direct gas-phase epoxidation of propylene with use of O2 and H2. The structures of Ti-HMS and Au/Ti-HMS samples were characterized by X-ray diffraction, N2-physisorption, scanning electron microscopy, transmission electron microscopy, UV−vis diffuse reflectance spectroscopy, UV Raman spectroscopy, ammonia-temperature-programmed desorption, and atomic adsorption spectroscopy. The results showed that the Au/Ti-HMS catalyst exhibited superior performance in terms of propylene conversion, propylene oxide (PO) selectivity, and H2 efficiency in comparison with the Au catalysts supported on the conventional Ti-containing mesoporous materials. Besides the Si/Ti molar ratio, the chain length of alkylamine for the Ti-HMS preparation was crucial for the enhancement of catalytic performance. Specifically, 9.0% of propylene conversion, 97.3% of PO selectivity, and 30.4% of H2 efficiency can be obtained at 373 K in the initial 30 min of time-on-stream on the Au/Ti-HMS catalyst, where the Ti-HMS having a Si/Ti molar ratio at 20 was prepared by using tetradecylamine as the template agent. Regeneration of the spent catalyst by calcination in air gave almost no change in the PO selectivity but about 25% loss in the propylene conversion. The enhanced catalytic performance of Au/Ti-HMS catalyst may be essentially attributed to the homogeneous dispersion and uniformity of titanium species in combination with accessible pore structure.

Direct gas-phase epoxidation of propylene to propylene oxide using air as oxidant on supported gold catalyst

Gold catalysts supported on SiO 2, TiO 2, TiO 2-SiO 2, and ZrO 2-SiO 2 supports were prepared by impregnating each support with a basic solution of tetrachloroauric acid. X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) techniques were used to characterize their structure and surface composition. The results indicated that the size of gold particles could be controlled to below 10 nm by this method of preparation. Washing gold catalysts with water could markedly enhance the dispersion of metallic gold particles on the surface, but it could not completely remove the chloride ions left on the surface. The catalytic performance of direct vapor-phase epoxidation of propylene using air as an oxidant over these catalysts was evaluated at atmospheric pressure. The selectivity to propylene oxide (PO) was found to vary with reaction time on the stream. At the reaction conditions of atmosphere pressure, temperature 325° C, feed gas ratio V(C 3H 6)/ V (O 2)= ½, and GHSV = 6000 h −1, 17.9% PO selectivity with 0.9% propylene conversion were obtained at initial 10 min for Au/SiO 2 catalyst. After reacting 60 min only 8.9% PO selectivity were detected, but the propylene conversion rises to 1.4% and the main product is transferred to acrolein (72% selectivity). Washing Au/TiO 2-SiO 2 and Au/ZrO 2-SiO 2 samples with magnesium citrate solution could markedly enhance the activity and PO selectivity because smaller gold particles were obtained.

The study of the uncalcined Au catalyst and inorganic salts on direct gas-phase epoxidation of propylene

Abstract This paper presents the results of the propylene epoxidation in co-presence of hydrogen and oxygen on both dried and calcined Au catalysts, which were prepared by liquid grafting method using Me 2 Au(acac) as a precursor and trimethylsilylated Ti-MCM-48 as a support. The calcined catalyst shows PO activity at temperatures much higher and wider than the dried one. Addition of inorganic salts greatly modified the catalytic behaviors. CsNO 3 could be a promising promoter whereas KBr led to the production of propionaldehyde instead of propylene oxide.

Direct Vapor Phase Propylene Epoxidation over Deposition-Precipitation Gold-Titania Catalysts in the Presence of H2/O2:  Effects of Support, Neutralizing Agent, and Pretreatment

The effects of titanium connectivity, deposition solution neutralizing agent, and catalyst pretreatment were examined for a series of Au-on-titanium-containing supports for the direct gas-phase epoxidation of propylene using hydrogen and oxygen. The degree of titanium isolation was examined using pure titania, monolayer-titania on silica, submonolayer-titania on silica, and titanium silicalite-1 (TS-1) supports. Activity and selectivity were shown to increase as the degree of titanium isolation increased, with TS-1 and submonolayer-titania supports providing the best stability and yield. Isolation of the titanium was found to significantly reduce the cracking of propylene to ethanal and carbon dioxide. Sodium carbonate was found to be the best neutralizing agent for catalysts prepared using deposition-precipitation (DP). DP with ammonium hydroxide gave catalysts with reduced selectivity and activity. Titania-modified silica was found to produce better catalysts when the support was not calcined prior to gold deposition. Similarly, calcination was detrimental to catalysts prepared via deposition of a 2 nm gold colloid onto titania-modified supports even though the gold did not sinter. The beneficial effects of Ti site isolation and support acid/base control are best seen at higher temperatures, where only a few catalysts can maintain selectivity.

Direct epoxidation of propylene with molecular oxygen over Ag–MoO 3/ZrO 2 catalyst

Direct gas-phase epoxidation of propylene to propylene oxide (PO) by molecular oxygen was studied over the MoO 3-modified supported-silver catalyst. The effect of promoter and support on the performance of the Ag–MoO 3 catalyst and their role were investigated by XPS, XRD, SEM, BET surface area, NH 3-TPD, CO 2-TPD techniques and so on. As the promoter, MoO 3 plays a great role of electron- and structure-type bi-functional promoter. Of all the supports studied, ZrO 2 is the most suitable one for the Ag–MoO 3 catalyst. The presence of support can regulate the size of the Ag–MoO 3 particles and the pore radii of catalyst, and decrease the loading of MoO 3 from ∼50 wt.% in the unsupported Ag–MoO 3 catalyst to ∼4 wt.% in the Ag–MoO 3/ZrO 2 catalyst. The properly weak Lewis acidic sites, the larger size of the Ag–MoO 3 particles and pore channel on the Ag–MoO 3/ZrO 2 catalyst are beneficial to improve epoxidation performance of the catalyst. The effect of the reaction temperature and space velocity on the catalytic epoxidation of propylene was also investigated. The low temperature or high space velocity can decrease the deep oxidation of propylene oxide to improve the selectivity to PO. Over the 20%Ag–4%MoO 3/ZrO 2 catalyst at 400 °C, 0.1 MPa and space velocity of 7500 h −1, the selectivity to PO of 60.3% was achieved with the O 2 conversion of 4.8%; under the space velocity of 12 000 h −1, the selectivity to PO was 71.5% with 2.5% O 2 conversion.