Effect of hydrogen and propylene on the hydrogen peroxide decomposition over Pt, PtO and Au catalysts

The decomposition of hydrogen peroxide (H2O2) on Pt, PtO and Au catalysts has been investigated in the presence of nitrogen, propylene and hydrogen. H2O2 formation on the catalyst is known to be a key intermediate step for the direct synthesis of propylene oxide (PO) from hydrogen, propylene and oxygen. Therefore, during this reaction, H2O2 is in contact with the catalyst on which it is produced, propylene and hydrogen. In this work we investigate the effect of the simultaneous presence of a metal catalyst (Pt, Au) and these gases on the H2O2 decomposition. The presence of hydrogen favors the decomposition of H2O2 over all the studied catalysts. This is attributed to the combination of direct decomposition and hydrogenation reactions. Furthermore, hydrogen changes the catalyst from an oxidized to a more metallic state accelerating the H2O2 decomposition. We also observed the positive effect of propylene on decreasing the decomposition activity of Pt and Au. The experimental results were used to estimate reaction rate constants. The obtained rate constants hinted to changes of the catalyst during the decomposition as the main reason for reaction rate changes. This was confirmed to happen for Pt but not for the Au catalyst. The decrease of H2O2 decomposition on Au in the presence of propylene is due to their interaction where propylene blocks the Au active sites responsible for the H2O2 decomposition

The direct epoxidation of propene in the explosive regime in a microreactor - A study into the reaction kinetics

A microreactor system was used for a kinetic study into the direct epoxidation of propene over a gold-titania based catalyst system using a mixture of hydrogen, oxygen, and propene. The small dimensions of a microreactor allow for the safe operation of the reaction in the explosive regime. The effect of the concentration of hydrogen, oxygen, and propene on the reaction rate was determined as well as the effect on catalyst deactivation and reactivation. A simple algebraic expression was developed, based on published kinetics, which provided the three reaction rate constants as a function of the feed gas composition. It is shown that propene does not influence the epoxidation rate; however, higher propene concentrations significantly reduce the catalyst deactivation rate. Hydrogen significantly increases the rate of the epoxidation, while it has a minor influence on the deactivation and reactivation of the catalyst. Oxygen has a threefold beneficial effect on the reaction: it increases the epoxidation rate; it slightly decreases the deactivation rate, and increases the rate of the catalyst reactivation. Performing the direct propene epoxidation in a microreactor system inside the explosive regime at optimized high feed concentrations can increase the propene oxide formation rate by a factor of three, while at the same time lowering the deactivation rate

Direct synthesis of propylene oxide in the liquid phase under mild conditions

Here, we study the direct synthesis of propylene oxide (PO) on Pd-based catalysts operating under mild conditions (40 °C, 5.5 bar) and in a continuous flow microreactor. We show that the PO yield can be improved by a factor of two, with respect to the values present in literature, by using a Pd–Pt/TS-1 catalyst in excess of oxygen. Moreover, we compare the PO reaction with the hydrogen peroxide (H2O2) synthesis, being H2O2 (or OOH species) the intermediate and rate limiting step of the direct PO formation. We found that the optimal conditions for the PO synthesis are not advantageous for the H2O2 productivity. In this respect, the presence of Pt, which improves the PO selectivity by lowering the hydrogenation of propylene, negatively affects the H2O2 productivity due to an acceleration of its side reactions. This effect is more pronounced for the Pd–Au/TS-1 catalyst, which shows a high performance for H2O2 production. However, the PO formation remains relatively poor due to a very fast hydrogenation of propylene to propane. We conclude that the optimization of the H2O2 synthesis is not sufficient to improve the direct PO formation. Indeed, the hydrogenation of propylene needs also to be considered

Direct synthesis of hydrogen peroxide in a wall-coated microchannel over Au-PD catalyst : a performance study

The direct synthesis of hydrogen peroxide out of hydrogen and oxygen has been studied in a wall-coated microchannel. A silica wash-coat layer embedded with Au–Pd colloidal nanoparticles served as an active catalyst for the direct synthesis reaction. The influence of different reaction variables has been evaluated in order to identify optimal process conditions and to maximize the concentration of peroxide produced in the liquid phase. The gas mixture used in this study contained hydrogen and oxygen concentrations that would be considered explosive in conventional reactors. The results presented here show that combining the active Au–Pd catalyst with microchannel technology can lead up to 5 wt% peroxide solutions at hydrogen conversions as high as 80%

Synthesis and characterization of Au/Ti-SBA-15 catalysts for the vapour phase epoxidation of propylene

Au-Ti-SBA-15 catalysts have been synthesized using different preparation routes. Ti-SBA-15 supports with Si/Ti ratio ranging from 10 to 80 have been prepared by grafting and direct synthesis. X-ray fluorescence (XRF), X-ray diffraction (XRD), diffuse reflectance UV-visible spectroscopy (DRS), transmission electron microscopy (TEM) and nitrogen sorption were applied for the characterization of the prepared catalysts. The catalytic activities of the Au/Ti-SBA-15 materials have been investigated in the direct epoxidation of propene by using a mixture of H2 and O2 and its performance was compared with an Au/TiO2 catalyst. The activity results were then interpreted with reference to the characterization

A transient kinetic analysis of the epoxidation of propene over gold-titania catalysts

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Direct synthesis of hydrogen peroxide in a wall coated microreactor using Au-Pd catalyst

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Alkylation of benzyl cyanide in a microreactor - Effect of hydrodynamics on reaction rate and selectivity

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The direct epoxidation of propene in the explosive regime in a microreactor:a study into the reaction kinetics

\u3cp\u3eA microreactor system was used for a kinetic study into the direct epoxidation of propene over a gold-titania based catalyst system using a mixture of hydrogen, oxygen, and propene. The small dimensions of a microreactor allow for the safe operation of the reaction in the explosive regime. The effect of the concentration of hydrogen, oxygen, and propene on the reaction rate was determined as well as the effect on catalyst deactivation and reactivation. A simple algebraic expression was developed, based on published kinetics, which provided the three reaction rate constants as a function of the feed gas composition. It is shown that propene does not influence the epoxidation rate; however, higher propene concentrations significantly reduce the catalyst deactivation rate. Hydrogen significantly increases the rate of the epoxidation, while it has a minor influence on the deactivation and reactivation of the catalyst. Oxygen has a threefold beneficial effect on the reaction: it increases the epoxidation rate; it slightly decreases the deactivation rate, and increases the rate of the catalyst reactivation. Performing the direct propene epoxidation in a microreactor system inside the explosive regime at optimized high feed concentrations can increase the propene oxide formation rate by a factor of three, while at the same time lowering the deactivation rate.\u3c/p\u3

Direct synthesis of propylene oxide in a packed bed membrane reactor

In the present work, the direct synthesis of propylene oxide (PO) in the liquid phase is successfully performed in a packed bed membrane reactor. We show that this engineering device can be implemented under mild reaction conditions (low temperature and pressure) and outside the explosive regime (low hydrogen concentration), making it appealing for industrial applications. The ceramic membrane allows for a separate feed of the reactants, therefore addressing the PO selectivity issue related to the propylene hydrogenation. The reaction is operated in a continuous methanol flow fed with propylene inside the tubular membrane; hydrogen and oxygen are fed through the porous material. We observe that the feeding strategy of the reactants has an important effect on key parameters such as PO selectivity and productivity. By separating propylene from hydrogen, the propane formation was reduced with respect to conventional packed bed reactors. Moreover, the addition of small amounts of NaBr to the reaction medium increases the catalytic activity to PO, but also to propane formation. Therefore, this study provides a good starting point in the design of a membrane reactor device for the direct synthesis of PO where the main limitations such as propylene hydrogenation and water formation can be addressed