Epoxidation of Styrene by Anhydrous H2O2 over TS-1 and γ-Al2O3 Catalysts: Effect of Reaction Water, Poisoning of Acid Sites and Presence of Base in the Reaction Mixture

The styrene conversion and product (viz. styrene oxide, phenyl acetaldehyde, benzaldehyde) selectivity in the liquid-phase epoxidation of styrene by H₂O₂ (H₂O₂/styrene = 2) over TS-1 (Si/Ti = 80) and -Al₂O₃ are strongly influenced by the presence of water and/or base (viz. urea and pyridine) in the reaction mixture. The TS-1 showed high styrene conversion activity but no epoxide selectivity in the absence of any base. When anhydrous H₂O₂ (24% H₂O₂ in ethyl acetate), with the continuous removal of the reaction water (using the DeanStark trap), was used instead of 50% aqueous H₂O₂, both the conversion and epoxide yield are increased drastically for the -Al₂O₃, whereas for the TS-1, the increase in the conversion was quite small and there was also no improvement in the epoxide selectivity and/or yield. However, when urea or pyridine was added in the reaction mixture, the epoxide selectivity for both the catalysts was increased depending on the concentration of the base added; the increase in the selectivity was very large for the TS-1 but small for the -Al₂O₃. Poisoning of the acid sites of the -Al₂O₃ by the chemisorbed ammonia or pyridine (at 100 °C) caused a small decrease in the conversion, but it also caused a large decrease in the epoxide selectivity. However, the pyridine poisoning of the TS-1 caused a little beneficial effect, a small increase in the epoxide selectivity. The ammonia poisoning of the TS-1, however, resulted in a small decrease in the conversion with no improvement in the epoxide selectivity. As compared to the TS-1, the -Al₂O₃ catalyst showed a much better performance in the epoxidation by anhydrous H₂O₂ with the continuous removal of the reaction water. However, the reaction water, if not removed continuously, is detrimental to the -Al₂O₃, causing a large decrease in the catalytic activity and selectivity for styrene oxide but an increase in the selectivity for benzaldehyde.     

Synthesis of TS-1 by microwave heating of template-impregnated SiO2–TiO2 xerogels

TS-1 was prepared by microwave heating of a SiO₂–TiO₂ xerogel dry-impregnated with the template, TPAOH. A highly crystalline product was obtained within 30 min after microwave irradiation with yields over 90%. These are significant advantages over the TS-1 obtained by conventional oven heating using alkoxide precursors in liquid phase, which requires 1–2 day crystallization time with low product yields. Characterization of the TS-1 obtained was carried out using XRD, SEM, FT-IR, and UV-vis spectroscopy, and catalytic activity was examined for 1-hexene epoxidation using H₂O₂ as oxidant. These studies revealed that the material obtained by microwave heating of the mixed oxide gel shows essentially identical physicochemical properties to those prepared by conventional means.     

M/BCS (M=Ni, Co; BCS=30 mol% BaCl2/Sm2O3): Effective Catalysts for the Partial Oxidation of Ethane to a Feedstock Gas Suitable for Ethene Hydroformylation

The M (M = Ni, Co)-loaded BaCl₂-promoted Sm₂O₃ (BCS) catalysts have been investigated for the partial oxidation of ethane to a feedstock gas suitable for ethene hydroformylation. It is found that at a temperature of 700 °C and a C₂H₆/O₂ molar ratio of 2/1, the product mixtures over 5 wt% Ni/BCS and 7 wt% Co/BCS were with C₂H₄/CO/H₂ and C₂H₄/CO_x/H₂ (CO_x=CO+CO₂) molar ratios close to 1/1/1 at a contact time of 1.19×10^-4 h g ml^-1 and close to 1/1/2 at a contact time of 2.78×10^-4 h g ml^-1. It is suggested that, besides reaction conditions, defect structures, M loading, and M dispersion of the catalysts are governing factors for the generation of the required feedstock gas.     

环氧丙烷的洁净生产技术TS-1催化丙烯环氧化过程研究进展

介绍了钛硅分子筛催化剂（ＴＳ－１）和它催化下的丙烯与Ｈ２Ｏ２的环氧化反应,以及以此为基础发展起来的环氧化过程与Ｈ２Ｏ２生产相结合的集成过程。     

Significant enhancement of the oxidation of CO by H2 and/or H2O on a FeO x /Pt/TiO2 catalyst

Oxidation of CO on the FeO _x /Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O at 60 °C, but no such enhancement is observed on the Pt/TiO₂ catalyst, but shift reaction (CO + H₂O → H₂ + CO₂) does not occur on the FeO _x /Pt/TiO₂ catalyst at 60 °C. DRIFT-IR spectroscopy reveals that the fraction of bridge bonded CO increases while that of linearly bonded CO decreases on the FeO _x loaded Pt/TiO₂ catalyst. The in-situ DRIFT IR spectra proved that the bridged CO is more reactive than the linearly bonded CO with respect to O₂, and the reaction of the bridge-bonded CO with O₂ as well as of the linearly bonded CO is markedly enhanced by adding H₂ to a flow of CO + O₂. From these results, we deduced that the promoting effect of H₂ and/or H₂O is responsible for the preferential oxidation (PROX) reaction of CO on the FeO _x /Pt/TiO₂ catalyst, and a following new mechanism via the hydroxyl carbonyl or bicarbonate intermediate is proposed for the oxidation of CO in the presence of H₂O.      

Nitrogen-incorporated TS-1 zeolite: Synthesis, characterization and application in the epoxidation of propylene

A novel nitrogen-incorporated TS-1 zeolite (N-TS-1) was successfully synthesized by direct calcinating the as-synthesized TS-1 powder in NH₃ flow at high temperature. The samples were characterized by XRD, FTIR, UV-Vis, NH₃-TPD, ICP-AES, CHN, XPS and ²⁹Si MAS NMR techniques. The results showed that nitrogen was incorporated into the framework of TS-1, and N-TS-1 preserved the MFI structure well. The fresh N-TS-1 samples showed low H₂O₂ conversion in the epoxidation of propylene with dilute H₂O₂, which was due to the coverage of the active titanium sites by unstable nitrogen species. By refluxing the fresh N-TS-1 samples with methanol, the unstable nitrogen species were washed out and the conversion of H₂O₂ increased markedly. The stable nitrogen species incorporated into the zeolite framework could effectively decrease the acidity of TS-1 zeolite and inhibit the side reactions, thereby improving the propylene oxide (PO) selectivity. After the 20th run, the N-TS-1-850-5 catalyst gave a H₂O₂ conversion of 91.5%, a H₂O₂ selectivity of 92.0%, and a PO selectivity of 90.9%. Finally, a model for nitridation of the as-synthesized TS-1 powder at high temperature was proposed.     

Activation of Supported Pd Metal Catalysts for Selective Oxidation of Hydrogen to Hydrogen Peroxide

Catalytic activity of supported Pd metal catalysts (Pd metal deposited on carbon, alumina, gallia, ceria or thoria) showing almost no activity in the liquid-phase direct oxidation of H₂ to H₂O₂ (at 295 K) in acidic medium (0.02 M H₂SO₄) can be increased drastically by oxidizing them using different oxidizing agents, such as perchloric acid, H₂O₂, N₂O and air. In the case of the Pd/carbon (or alumina) catalyst, perchloric acid was found to be the most effective oxidizing agent. The order of the H₂-to-H₂O₂ conversion activity for the perchloric-acid-oxidized Pd/carbon (or alumina) and air-oxidized other metal oxide supported Pd catalysts is as follows: Pd/alumina < Pd/carbon < Pd/CeO₂ < Pd/ThO₂ < Pd/Ga₂O₃. The H₂ oxidation involves lattice oxygen from the oxidized catalysts. The catalyst activation results mostly from the oxidation of Pd metal from the catalyst producing bulk or sub-surface PdO. It also caused a drastic reduction in the H₂O₂ decomposition activity of the catalysts. There exists a close relationship between the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity in the oxidation process and the H₂O₂ decomposition activity of the catalysts; the higher the H₂O₂ decomposition activity, the lower the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity.     

Direct Oxidation of H2 to H2O2 and Decomposition of H2O2 Over Oxidized and Reduced Pd-Containing Zeolite Catalysts in Acidic Medium

Both the conversion and H₂O₂ selectivity (or yield) in direct oxidation of H₂-to-H₂O₂ (using 1.7 mol% H₂ in O₂ as a feed) and also the H₂O₂ decomposition over zeolite (viz. H-ZSM-5, H-GaAlMFI and H- ) supported palladium catalysts (at 22 °C and atmospheric pressure) are strongly influenced by the zeolite support and its fluorination, the reaction medium (viz. pure water, 0.016 M or 1.0 M NaCl solution or 0.016 M H₂SO₄, HCl, HNO₃, H₃PO₄ and HClO₄), and also by the form of palladium (Pd⁰ or PdO). The oxidized (PdO-containing) catalysts are active for the H₂-to-H₂O₂ conversion and show very poor activity for the H₂O₂ decomposition. However, the reduced (Pd⁰-containing) catalysts show higher H₂ conversion activity but with no selectivity for H₂O₂, and also show much higher H₂O₂ decomposition activity. No direct correlation is observed between the H₂-to-H₂O₂ conversion activity (or H₂O₂ selectivity) and the Pd dispersion or surface acidity of the catalysts. Higher H₂O₂ yield and lower H₂O₂ decomposition activity are, however, obtained when the non-acidic reaction medium (water with or without NaCl) is replaced by the acidic one.     

Activation of a Au/TiO2 Catalyst by Loading a Large Amount of Fe–Oxide: Oxidation of CO Enhanced by H2 and H2O

Catalytic activity of a 1 wt% Au/TiO₂ catalyst is markedly improved by loading a large amount of FeO_x, on which the oxidation of CO in excess H₂ is selectively promoted at temperature lower than 60 °C. Oxidation of CO with O₂ on the FeO_x/Au/TiO₂ catalyst is markedly enhanced by H₂, and H₂O moisture also enhances the oxidation of CO but its effect is not so large as the promotion by H₂. We deduced that activation of Au/TiO₂ catalyst by loading FeO_x is not caused by the size effect of Au particles but a new reaction path via hydroxyl carbonyl intermediate is responsible for the superior activity of the FeO_x/Au/TiO₂ catalyst.     

Preferential CO oxidation over Pt–SnO2/Al2O3 in hydrogen rich streams containing CO2 and H2O (CO removal from H2 with PROX)

The effects of reaction gases including CO₂ and H₂O and temperature on the selective low-temperature oxidation of CO were studied in hydrogen rich streams using a flow micro-reactor packed with a Pt–SnO₂/Al₂O₃ sol–gel catalyst that was initially designed and optimized for operation in the absence of CO₂ and H₂O. 100% CO conversion was achieved over the 1 wt% Pt–3 wt% SnO₂/Al₂O₃ catalyst at 110 °C using a feed composition of 1.0% CO, 1.5% O₂, 25% CO₂, 10% H₂O, 58% H₂ and He as balance at a space velocity of 24,000 cm³/(g h). CO₂ in the feed was found to decrease CO conversion significantly while the presence of H₂O in the feed increased CO conversion, balancing the effect of CO₂.     

NO reduction with H2 in the presence of excess O2 over Pd/MFI catalyst

The NO SCR (selective catalytic reduction) activity with H₂ in the presence of excess O₂ was investigated over Pd/MFI catalyst prepared by sublimation method. With GHSV=90?000 h⁻¹, a very high steady-state conversion of NO to N₂ (70%) is achieved at 100 °C. Significant reorganizations take place inside the catalyst upon its first contact with all reactants and products at the reaction temperature. Pd⁰, which has a significant role in the NO-H₂-O₂ reaction, is possibly the active site for NO reduction. The formation of Pd-β hydride deactivates the catalyst for NO reduction. Throughout the entire NO-H₂-O₂ reaction, no N₂O or NO₂ is formed; N₂ is the only N-containing product. The presence of O₂ inhibits the formation of undesirable NH₃. The rate of the NO+H₂ reaction is fast or comparable to that of the H₂+O₂ reaction. The oxidation of Pd⁰ and subsequent agglomeration of PdO are responsible for the decreased NO reduction activity at high temperature.     

Epoxidation of Propylene with Dilute H2O2 over Titanium Silicalite Containing Trace Aluminum

Titanium silicalite (TS-1) was successfully synthesized by using TPABr as the template and silica sol as silicon source in a 100 l stainless steel autoclave. IR, XRD, UV--vis, elemental analysis, and ²⁷Al and ³¹P MAS NMR were used to characterize the synthesized products. The results show that the synthesized material has an MFI structure with high crystallinity and large crystal size and two kinds of titanium species. Trace aluminum in silica sol is also incorporated into the zeolite framework. The synthesized TS-1 exhibits high activity in the epoxidation of propylene with dilute H₂O₂ with high selectivity to methyl mono-ethers and low selectivity to propylene oxide (PO). The low selectivity toward PO is due to the residual acidity onto TS-1. The selectivity of PO can reach up to 90% through adjusting the pH of the reaction mixture. Extra amounts of base decrease the H₂O₂ utilization and the H₂O₂ conversion. However, in over acid-treated TS-1 in which part removal of extra-framework titanium takes place, the utilization of H₂O₂ is quite different: for the low Si/Ti ratio of TS-1, the H₂O₂ utilization increases. But the utilization of H₂O₂ does not change for the high Si/Ti ratio TS-1. Thermal analysis shows that the as-synthesized TS-1 exhibits high activity and thermal stability in the calcined range 540-900 °C.     

Partial oxidation of methane to synthesis gas over corundum supported mixed oxides: One channel studies

Catalytic partial oxidation of methane (POM) over the monolithic catalyst LaNiO_x/CeO₂–ZrO₂/α-Al₂O₃ has been studied. Experiments were conducted with one channel of a monolith at a varied channel length, contact time (1–6 ms) and temperature using the diluted gas mixture (1% CH₄ + 0.5% O₂ in He). At increasing temperature and contact time, CO selectivity rises within the whole temperature range whereas the contact time dependence of H₂/CO ratio varies with the temperature. These results support the POM reaction scheme including primary formation of CO and H₂ followed by their oxidation in the presence of gas-phase O₂. Steam and dry methane reforming reactions occur in the part of monolithic channel where oxygen is absent, thus increasing syngas yield.     

Synergism Between Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Au/TiO<Subscript>2</Subscript> in the Low Temperature Oxidation of Propene

CO impedes the low temperature (<170 °C) oxidation of C₃H₆ on supported Pt. Supported Au catalysts are very effective in the removal of CO by oxidation, although it has little propene oxidation activity under these conditions. Addition of Au/TiO₂ to Pt/Al₂O₃ either as a physical mixture or as a pre-catalyst removes the CO and lowers the light-off temperature (T ₅₀) for C₃H₆ oxidation compared with Pt catalyst alone by ~54 °C in a feed of 1% CO, 400 ppm C₃H₆, 14% O₂, 2% H₂O.     

Tunability of Propane Conversion over Alumina Supported Pt and Rh Catalysts

Propane conversion over alumina supported Pt and Rh (1 wt% metals loading) was examined under fuel rich conditions (C₃H₈:O₂:He = 1:2.25:9) over the temperature range 450–650 °C. Morphological characteristics of the catalyst materials were varied by calcining at selected temperatures between 500 and 1,200 °C. X-ray diffraction and BET analysis showed the treatment generated catalyts metals with particle sizes in the range of <10 to >500 nm, and support surface areas in the range of 20–240 m²/g. Remarkably, both Rh and Pt yielded product compositions close to equilibrium values (with high H₂ and CO selectivity, complete oxygen conversion and almost complete propane conversion) so long as the metal particle size was sufficiently low, ≲10–15 nm. In cases where the particle size was large, primarily complete oxidation rather than partial oxidation products were observed, along with unreacted C₃H₈, indicative of a direct oxidation pathway in which gas-phase CO and H₂ are not present as intermediate species. It is proposed that the high resistance of Rh to coarsening is largely responsible for the observation of a higher selectivity of this material for syngas products when prepared by procedures similar to those for Pt. Overall, the tunability of the product composition obtained over Rh and Pt via processing steps has direct significance for the incorporation of such catalyts into the anodes of solid oxide fuel cells.     

The Mechanism for the Selective Oxidation of CO Enhanced by H2O on a Novel PROC Catalyst

Preferential oxidation (PROX) reaction of CO in H₂ catalyzed by a new catalyst of FeO _x /Pt/TiO₂ (Fe: Pt: TiO₂ = 100: 1: 100) was studied by dynamic in-situ DRIFT-IR spectroscopy. The oxidation of CO is markedly enhanced by H₂ and H₂O, and the enhancement by H₂/D₂ and H₂O/D₂O takes a common hydrogen isotope. Dynamics of DRIFT-IR spectroscopy suggests that the oxidation of CO with O₂ in the absence of H₂ proceeds via bicarbonate intermediate. In contrast, rapid oxidation of CO in the presence of H₂ proceeds via HCOO intermediate and the subsequent oxidation of HCOO by the reaction with OH, that is, CO + OH→ HCOO and HCOO + OH → CO₂ + H₂O. The latter reaction is a rate determining step being responsible for a common hydrogen isotope effect by H₂/D₂ and H₂O/D₂O.     

Isotope effect of H2/D2 and H2O/D2O for the PROX reaction of CO on the FeOx/Pt/TiO2 catalyst

The oxidation reaction of CO with O₂ on the FeOx/Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O, but no such enhancement occurs on the Pt/TiO₂ catalyst. Isotope effects were studied by H₂/D₂ and H₂O/D₂O on the FeOx/Pt/TiO₂ catalyst, and almost the same magnitude of isotope effect of ca. 1.4 was observed for the enhancement of the CO conversion by H₂/D₂ as well as by H₂O/D₂O at 60 °C. This result suggests that the oxidation of CO with O₂ via such intermediates as formate or bicarbonate in the presence of H₂O, in which H₂O or D₂O acts as a molecular catalyst to promote the oxidation of CO as described below.      

Low-temperature mild partial oxidation of ethanol over supported platinum catalysts

Supported platinum catalysts containing 1.2% Pt loaded on Al₂O₃ (1.2% Pt/Al₂O₃) and 1.9% Pt loaded on ZrO₂ (1.9% Pt/ZrO₂) were prepared by incipient wetness impregnation and sol–gel method, respectively. The activity of these catalysts in the partial oxidation of ethanol (POE) was examined in a fixed-bed reactor in a temperature range between 373 and 473 K. The results indicated that significant ethanol conversion (C_EtOH > 50%) was found at the low reaction temperature with a feed ratio of O₂/EtOH ratio >0.75. Oxygen molecules introduced in reactant were completely consumed in POE reactions performed. H₂, H₂O, CO and CO₂ were the major products detected. The selectivity of hydrogen (S_H2) and CO (S_CO) varied significantly with reaction conditions. High selectivity of hydrogen (S_H2 > 95%) and low selectivity of CO (S_CO  0%) were found from a mild oxidation at T_R = 373 K over Pt/ZrO₂. However, these two selectivities were drastically deteriorated through oxidation at high T_R, high O₂/EtOH ratio or over Pt/Al₂O₃ catalyst.     

Partial oxidation of ethane by in situ generated H2O2

Selective partial oxidation of ethane to ethanol and acetaldehyde by in situ generated H₂O₂ has been achieved under cathodic current passing through a carbon supported Nafion-H catalytic membrane. A correlation between H₂O₂ generation rate and reaction rate has been found.     

Enhancement of C2H6 Oxidation by O2 in the Presence of N2O over Fe Ion-Exchanged BEA Zeolite Catalyst

Selective catalytic reduction (SCR) of N₂O with C₂H₆ took place effectively over Fe ion-exchanged BEA zeolite catalyst (Fe-BEA) even in the presence of excess oxygen. The mechanism in the SCR of N₂O with C₂H₆ over Fe-BEA catalyst was studied by a transient response experiment and an in situ DRIFT spectroscopy. No oxidation of C₂H₆ by O₂ took place below 350 °C (in C₂H₆/O₂). In the N₂O/C₂H₆/O₂ system, however, it was found that the reaction of C₂H₆ with O₂ was drastically enhanced by the presence of N₂O even at low temperatures (200-300 °C). Therefore, it was concluded that N₂O played an important role in the oxidation of C₂H₆ (i.e., activation of C₂H₆ at an initial step). On the basis of these findings, the mechanism in the SCR of N₂O with C₂H₆ is discussed.