Direct Synthesis of Propene Oxide from Propene, Hydrogen and

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Direct Synthesis of Propene Oxide from Propene, Hydrogen and Oxygen in a Catalytic Membrane Reactor Emila Kertalli, Dulce M. Perez Ferrandez, Jaap C. Schouten, and T. Alexander Nijhuis* Department of Chemical Engineering and Chemistry, Laboratory of Chemical Reactor Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands ABSTRACT: The direct gas-phase epoxidation of propene in a catalytic membrane reactor has been investigated. Powdered Au/Ti-SiO2 catalyst pressed into a disc was used as a membrane contactor, allowing different feeding of the reactants and providing resistance to their direct contact. The optimization of this reactor concept was performed by screening different propene, oxygen and hydrogen feeding strategies on the catalyst in order to maximize the hydrogen efficiency (propene oxide/ water formation ratio). Several membrane configurations were tested and it was found that the optimal configuration was obtained by feeding propene and oxygen separately from hydrogen. The system was operated as a membrane contactor where propene and oxygen flows were forced through the catalytic membrane and the hydrogen had to reach the catalyst by counter diffusion, thereby being the limiting reactant. This creates a favorable concentration profile of the reactants that enhances the performance of the membrane reactor. The membrane thickness and resistance (porosity-tortuosity) were experimentally found to be influential on the hydrogen efficiency. The experimental results were evaluated using a comprehensive reactor model. A membrane thickness of 0.2 mm was determined to be the minimum value needed to ensure a positive effect of the membrane reactor on the hydrogen efficiency. This value can be influenced by changing the resistance of the reactants through the catalytic membrane. The combination of experimental and theoretical information leads to the optimized membrane reactor design where an excess of propene is coupled with a controlled hydrogen supply by diffusion along the catalytic membrane. als.11−13 However, the industrial implementation of this process is limited by the explosive nature of the reactants and the low hydrogen efficiency. As it is well-known from the literature, hydrogen and oxygen form explosive mixtures in a large range of concentrations (4−96% H2 concentrations). In order to implement the direct gas phase epoxidation at a large scale, specific requirements of C3H6 conversion (10%), PO selectivity (90%), and hydrogen efficiency (50%) need to be achieved,14 which is not possible yet for the current catalysts. Previous work done in our group on the PO production10 shows that the hydrogen efficiency is strongly influenced by the reactant concentrations. Kinetic results proved that high concentrations of propene have a positive effect on the H2 efficiency.10 Higher concentrations of H2 favor not only the PO formation but also the undesired direct water formation rate. The first approach to address the low hydrogen efficiency in this reaction has been focused on the catalyst development. Gold titanium species, treated with promoters such as CsCl, showed lower H2 consumption keeping the conversion of propene constant.15 Also making the catalyst more hydrophobic shows promising results on hydrogen efficiency.14,16 However, instead of only focusing on the catalyst improvement, an alternative solution can be provided by an engineering approach. A catalytic membrane reactor/contactor enables H2 efficiency improvements by tuning reactant concentrations and avoiding the formation of explosive mixtures. The idea of feeding the

1. INTRODUCTION Propene oxide (PO) is an important compound used in the chemical industry as an intermediate for the manufacturing of different products such as polyether polyols (polyurethane), propene glycol (polyesters), and propene glycol ethers (solvents).1,2 The traditional processes utilized for the PO production are less attractive due to environmental and economical concerns. The chlorohydrin process leads to chlorinated side products and large amounts of waste salt (CaCl2). The hydroperoxide processes have the disadvantage of large quantities of coproducts being produced making the economy dependent on that of the coproducts.2 Alternative methods have been considered in order to address these problems. A processes overcoming these disadvantages has been developed by Dow in collaboration with BASF and SCG and two plants are currently running in Antwerp and Map Ta Phut, respectively.3,4 In this process, PO is synthesized starting from hydrogen peroxide and propene over titanium silicalite-1 (TS-1). This option is considered green, since the main byproduct obtained is water. However, the high H2 O2 production costs5 requires the integration of its production via the anthraquinone process with the process for the epoxidation of propene, which makes the overall process complex and only suitable for production on a large scale. A possible alternative to produce PO in a simple and costeffective manner is the direct synthesis starting from H2 and O2 and C3H6. Hydrogen and oxygen first react forming an intermediate hydroperoxy species that is used in the epoxidation of propene.5,6 Since Haruta first discovered gold as a very active catalyst to selectively produce PO,7 this topic has attracted significant interest in the literature mostly combining Au with titanium silicate8−10 or silicalite materi© XXXX American Chemical Society

Received: June 26, 2014 Revised: September 29, 2014 Accepted: September 30, 2014

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dx.doi.org/10.1021/ie502576n | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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gases separately and increasing their concentrations fulfills the requirements for a higher hydrogen efficiency and PO selectivity enhancement. Oyama et al.17 showed how a higher catalyst productivity, selectivity, and C3H6 conversion can be reached by using a packed bed membrane reactor configuration. The concentrations of hydrogen, fed through an inorganic Nanosil membrane, and oxygen were increased up to 40% and still allowed to operate in safe conditions. Further work in that direction has been done by the group of Sasidharan,18 where a zeolitic material was synthesized as a thin film over a membrane support (α-alumina tube) and impregnated with Au active metal. From an engineering point of view, a wall coated reactor minimizes the pressure drop and increases mass transfer of the reactants to the catalyst. However, the membrane reactor feeding strategy still needs to be determined. The reactor design can optimize the process by providing important information on reactant concentration profiles and membrane properties. The combination of this information should lead to a maximized hydrogen efficiency. In the present work, a membrane reactor concept has been developed. The benefits of using a membrane contactor system on the direct synthesis of propene oxide over Au/Ti-SiO2 catalyst have been investigated in order to maximize the H2 efficiency (PO/water formation). The reactants were fed through the catalytic membrane in different ways in order to observe the effect of the gas feeding configuration on the hydrogen efficiency. The experimental data are combined with a mathematical model of the reactor in order to extrapolate the optimal conditions for the best membrane reactor design.

Figure 1. Wicke-Kallenbach membrane reactor housing equipment for the direct synthesis of propene oxide. 1 = Wicke-Kallenbach cell. 2 = stainless steel membrane holder. 3 = catalytic membrane.

membrane is 20 mm and the inlet diameter of the chambers facing the membrane is 19 mm. Each chamber is provided with an inlet and an outlet. A macro porous sintered stainless steel plate is placed in between the membrane and the inlet chambers. This allows the concentration gradient to be the driving force for the gas diffusion. It is also possible to induce viscous flow in the system by forcing one of the feed streams through the membrane with the closure of one of the exits. The presence of valves after the mass flow controllers gives the flexibility of feeding the reactants according to different configurations. The reactants are diluted with nitrogen and helium. An online Micro GC (Varian CP-4900) equipped with two columns and thermal conductivity detectors (TCD) is used to analyze the composition of the exit gases. 2.3. Catalytic Testing. The Au/Ti-SiO2 catalyst was used in the direct propene oxide production from H2, O2, and C3H6. A total of 300 mg of the synthesized catalyst were pressed under different conditions (loading weight) in order to obtain membrane discs of different thickness and porosity. The catalytic membrane discs were 20 mm in diameter and were pressed into a super fine steel mesh (aperture = 0.039 mm) to provide additional mechanical strength. The diffusion mechanism of the gases through the membrane is not changing with the packing procedure. Since in all the cases the pores will remain in a macro scale level, the most important transport mechanism will be a combination of molecular diffusion and viscous flow (viscous flow only in the case of one blocked exit with forced flow through the membrane). Since the catalytic membranes were prepared under different conditions, the porosity and consequently the tortuosity of the bed are different leading to different effective diffusion coefficients.19 The synthesized catalytic membrane was placed in the WickeKallenbach system (Figure 1) and exposed to the reactant mixtures. The feed from one side of the membrane was forced through the catalytic membrane by closing one of the exits present in the Wicke-Kallenbach cell. Therefore, the cell had only a single exit resulting from the combination of the two inlets with the different reactants. No sweep gas was used. According to the way that the gases were distributed over the membrane, four different membrane contactor configurations were analyzed. These configurations were then compared with the conventional reactor system where all the reactants were fed together from the same side and forced through the membrane reactor. The hydrogen efficiency was evaluated as the ratio between the concentrations of PO and water produced. The different configurations are represented in Figure 2. The reaction was run at atmospheric pressure and 403 K.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. The catalyst preparation starts with the grafting of titanium on the silica support according to the method presented elsewhere.8 A total of 10 g of Davisil 643 (pore size 150 Å, 200−425 mesh, Sigma-Aldrich) silica support were first dried at 393 K for 2 h in order to remove the water present. Then, the powder was mixed with 120 mL of anhydrous 2-propanol (0.005% H2O, Merck) and 0.45 mL of TEOT (Sigma-Aldrich) under a nitrogen atmosphere. The obtained slurry was transferred to a rotating evaporator for removing the alcohol, dried in air overnight at 353 K, and calcined (873 K for 4 h). The Au deposition on the synthesized support was performed with a deposition precipitation method. One g of Ti/SiO2 was dispersed in 100 mL of demineralized water. Then 20 mL of solution containing 575 mg of HAuCl4 (30 wt % in HCl solution, Sigma-Aldrich) were added to the support. The pH of the mixture was controlled to 9.4 with the addition of ammonia (2.5 wt %). The solution was stirred for 1 h, filtered, washed, and calcined (673 K for 4 h). The Au particle size and loading were analyzed by TEM and ICP. 2.2. Experimental Setup. A schematic representation of the experimental setup is shown in Figure 1. The catalytic tests of Au/Ti-SiO2 were conducted in a Wicke-Kallenbach system. The Wicke-Kallenbach cell is standard equipment utilized for the study of fluid diffusion in a porous media. The particularity of the system consists in the equalized pressure of the two cells facing the porous material. The gradient concentration of the fluid becomes the driving force for the compound to diffuse. The stainless steel equipment consists of two separated chambers and a middle holder for hosting the catalytic membrane. The three different parts are coupled and sealed by the utilization of copper rings. The diameter of the B



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Figure 2. Membrane reactor configurations with different reactant combinations. 1 = configuration 1 (hydrogen is fed through the catalyst). 2 = configuration 2 (oxygen and propene are fed through the catalyst). 3 = configuration 3 (hydrogen and propene are fed through the catalyst). 4 = configuration 4 (oxygen is fed through the catalyst). Figure 5. Dependence of PO/H2O concentration ratios on the different catalytic membrane reactor configurations. Membrane pressed with 1 ton loading. T = 403 K. P = atmospheric pressure. Black square, Configuration 1. Red circle, Configuration 2. Blue triangle, Configuration 3. Pink triangle, Configuration 4. Green diamond, Conventional. 50 mL/min total gas flow rate (NTP). 10/ 10/10/70 vol % C3H6/H2/O2/N2.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Figure 3 shows a TEM picture of the Au−Ti/SiO2 catalyst before and after the

urations for the concentration ratios of PO/water. The highest hydrogen efficiency is reached with configuration 2. In this case, propene and oxygen are forced to flow through the catalytic membrane and the hydrogen is fed to the exit side of the membrane. The worst performance is obtained in configuration 1 where the hydrogen crosses the membrane and the diffusion of propene and oxygen is limited. The other configurations achieve similar hydrogen efficiency in between these two extremes. To explain the experimental results, a schematic reaction mechanism based on the common understanding in literature for PO and water formation over Au−Ti/SiO2 is presented in Scheme 1.

Figure 3. TEM image of Au−Ti/SiO2 catalyst: (a) fresh catalyst and (b) spent catalyst.

reaction. No sintering of gold nanoparticles was observed during the experiments. Figure 4 shows a gold particle distribution between 2 and 6 nm. Titanium and gold loadings were quantified by ICP elemental analysis. Loadings of 0.64% and 0.55% were obtained for titanium and gold, respectively. 3.2. Influence of Membrane Configuration. Figure 5 shows the comparison between different membrane config-

Scheme 1. Reactions Steps to Water and PO Formation20,21

Both water and PO are formed starting from the reaction of hydrogen and oxygen to form a hydroperoxy species over gold nano particles (reaction 1). The propene adsorbed on Ti sites further reacts with hydroperoxy species to PO (reaction 2). The hydrogenation of the hydroperoxy species leads to water formation (reaction 3) with a direct effect on the H2 efficiency (molPO/molwater).22 The kinetic study on the direct epoxidation by Chen et al.10 shows a strong dependence of the H2 efficiency on the reactant concentrations. The positive reaction order of hydrogen for water and propene oxide formation emphasizes the beneficial effect of higher hydrogen concentrations for both reactions. It was also found that propene has a positive effect on maximizing the propene oxide formation and suppressing the water formation.22 In our case, the improved performance of configuration 2 can be explained with the diffusivity of hydrogen into the catalytic membrane. This system has a limited hydrogen presence combined with higher propene concentrations. The hydroperoxy species formed over Au

Figure 4. Gold particle size distribution. C



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and oxygen) is fed at the exit of the reactor. The selectivites obtained for the different configurations are between 94 and 92% with CO2 being the main byproduct and propane. 3.3. Influence of Gas Flow Rates. Figure 7 shows the dependence of the PO/water concentrations ratio and propene conversion on the gas flow rates.

nanoparticles are preferentially consumed by propene to propene oxide rather than hydrogenated to water. This is translated into higher hydrogen efficiency. Configuration 1 can be considered the opposite of configuration 2 where hydrogen is in excess compared to propene. Hydrogen, flowing through the membrane hydrogenates a larger number of hydroperoxy species. The hydroperoxide hydrogenation reaction becomes more important over the epoxidation at higher hydrogen concentrations. This explains the worse performance provided by configuration 1. The other configurations (3, 4 and conventional) show almost no difference in the hydrogen efficiency. In these configurations, the oxygen concentration profile over the catalytic membrane is varied by feeding it in different ways. For these configurations propene and hydrogen are fed together from the same side of the reactor. This underlines the limited influence of oxygen on improving the hydrogen efficiency of the catalyst.10 Since the hydrogen and propene are fed from the same side of the catalytic membrane, there are no concentration gradients along the catalytic membrane. This means that the ratio H2/C3H6 along the membrane remains constant with no influence on the hydrogen efficiency which is determined by the kinetics of the reaction. In order to exclude the dependence of the different H2 efficiency results for the membrane configurations on the conversions, Figure 6 shows the propene conversions for the

Figure 7. Dependence of PO/H2O concentration ratios and the propene conversion on the gas flow rates. Membrane pressed with 5 ton loading. T = 403 K. P = atmospheric pressure. Figure 2 membrane configuration 2. 10/10/10/70 vol % C3H6/H2/O2/N2.

According to the obtained results, increasing the reactant flow rates (lower residence time) has a beneficial effect on the H2 efficiency. An explanation can be given by the concentration of hydrogen (diffusing gas) along the catalytic bed. Increasing the through-flow of propene and oxygen through the membrane limits the counter-diffusion of hydrogen along the catalytic membrane. The lower hydrogen flux leads to a relatively higher PO formation with increasing the H2 efficiency. However, in this system, hydrogen penetrates only in a relatively thin layer of the catalytic membrane, making the catalyst not efficiently utilized and lowering the conversions as can also be seen in Figure 7. In the present system, the hydrogen efficiency has been chosen as the main parameter for optimizing the catalytic membrane reactor. However, since during the different membrane configurations one of the gases is always fed at the exit of the reactor, the positive effect on the hydrogen efficiency is combined with lower conversions. A compromise between hydrogen efficiency and conversion needs to be made in the reactor design. A better utilization of the catalyst can be achieved by minimizing the reactant bypass and creating concentration profiles of the gases along the catalytic membrane. This can be obtained by tuning membrane properties such as thickness and the effective diffusion through the catalytic membrane contactor. 3.4. Influence of Membrane Thickness. The membrane thickness is a key parameter for optimizing the hydrogen efficiency. Different membrane thicknesses were considered. The experimental results obtained for thinner membranes did not show any influence of different membrane configurations on the PO/water formation. In order to produce the thicker membrane (operated at the same weight hourly space velocity), the same amount of catalyst (300 mg) was diluted with 200 mg of Davisil silica and pressed under a lower weight loading. Figure 8 shows the PO/water formation for different

Figure 6. Propene conversion for the different catalytic membrane reactor configurations. Membrane pressed with 1 ton loading. T = 403 K. P = atmospheric pressure. 50 mL/min total gas flow rate (NTP). 10/10/10/70 vol % C3H6/H2/O2/N2. Configuration 1 = selectivity 92%. Configuration 2 = selectivity 94%. Configuration 3 = selectivity 94%. Configuration 4 = selectivity 93%. Conventional = selectivity 93%.

different reactor concepts. It can be seen that the propene conversion remains almost constant for the different types of reactor configurations. Also the conversion of a conventional reactor system where the gases are fed all together along the catalyst is comparable with the rest of the conversions underlying the benefits of the second configuration feeding strategy among the others. Configuration 1 presents a slightly lower conversion compared to the rest. This can be attributed to the reactor feeding strategy which indicates a non optimal utilization of the catalyst while one of the reactants (propene D



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equations (diffusion, convection, and reaction terms) for each component. The two main products considered are water and PO. The kinetics utilized in the present work were obtained previously in our group from experimental data for this reaction and a similar catalyst.23 These (empirical) kinetics were chosen since the reported PO rate expression is consistent with the values presented in the literature.17,20 The rate expression used is a power law consistent with our experiments, and explicit expressions are available for both PO and water formation. The equations coupled with boundary conditions are summarized below. Generalized governing equations: Deff d2Ci 2

rm dx

Figure 8. Dependence of PO/H2O concentration ratios on the thickness of the membrane catalytic bed. One ton = Membrane pressed with 1 ton loading. Two ton = Membrane pressed with 2 ton loading. Five ton = Membrane pressed with 5 ton loading. T = 403 K. P = atmospheric pressure. Figure 2 - membrane configuration 2. Flow rate =50 mL/min (NTP). 10/10/10/70 vol % C3H6/H2/O2/N2.

2

Vthr dCi − (r1 + r 2) = 0 Arm dx

−

(4)

r1 = k1CO2 0.279CC3H6 0.198C H2 0.45

(5)

r2 = k 2CO2 0.124CC3H6−0.264C H2 0.85

(6)

Deff = DAB

ε τ

τ = ε−0.5

(7) (8)

Boundary conditions: at X = 0:

membrane thicknesses with the same amount of catalyst. The reaction was run at the same conditions for all the different cases. By increasing the thicknesses of the catalytic membrane the hydrogen efficiency is improved. A larger membrane thickness increases the resistance of the catalytic membrane and creates a more evident concentration gradient along the catalyst. This also reduces the mixing of propene and hydrogen that would lead to a conventional operating system with no improvement on the hydrogen efficiency for the catalytic membrane reactor. 3.5. Reactor Modeling. A theoretical interpretation of the experimental results was obtained with the help of a mathematical model which was numerically solved using the Athena Visual Studio program. The model describes the optimal configuration observed experimentally (configuration 2). The objective of the model is to explain the experimental results by analyzing the concentration profiles of the different components along the catalytic membrane. A schematic representation of the gas concentration profiles studied in the mathematical model is shown in Figure 9. The model provides an understanding of what is happening along the catalytic membrane. This can not be observed experimentally since we can only measure the outlet composition of the gases. The model consists of mass balance

CO2 = C0

(9)

CC3H6 = C0

(10)

Deff dC H2 A − VthrC H2 = 0 dx rm

(11)

at X = 1: C H2 = C0

(12)

Deff dCO2 A − VthrCO2 = 0 dx rm

(13)

Deff dCC3H6 A − VthrCC3H6 = 0 rm dx

(14)

Equation 4 is the mass balance for each component. In this balance, transport is by diffusion as well as by convection through the membrane and the reaction terms are also included. Equations 5 and 6 are the rate expressions for the PO and water generation, respectively. The effective diffusion of the gases (eq 7) is estimated from bulk diffusion and porosity-tortuosity correlations. An approximation of the tortuosity of porous materials was used (eq 824). For pelletized powders, the tortuosity and porosity values are lower than the one obtained from this correlation,25 which will result in minor inaccuracies in the model prediction. However, this will not affect the trends that will be observed. The boundary conditions were obtained from the feed concentrations (eqs 9, 10, and 12) and from mass balance of the compound diffusing through the catalytic membrane (eqs 11, 13, and 14). The molar flow rate of the compound at the boundary layer is equal to the diffusion of the compound itself. 3.6. Model Results. The absolute values of the PO/water concentration ratios obtained from the model are higher than the experimental values (Figure 5). This discrepancy is

Figure 9. Schematic illustration of concentration profiles studied in the mathematical model. E



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attributed to the difference in reaction conditions between the present work and experiments done to obtain the kinetics (Au loading).23 However, since in both cases the reaction mechanism is the same (same type of active sites present in the Au/Ti-SiO2 catalyst), this allows these kinetics to be used in our model to describe the experimental trends. The difference in operating conditions is translated into a lower reaction rate (as already remarked in the experimental section). In order to extrapolate information from the model and implement them for the optimization of the catalytic membrane reactor design, consistency between the experimental data and the model needs to be provided. In order to do so, Figure 10 shows the influence of the gas feeding strategy on the PO/water concentration ratio for configurations 1 and 2.

Figure 10. Model results for the dependence of PO/H2 O concentration ratios on the feeding strategies of the reactants.

Figure 10 shows the results from the model for the system operating under configurations 1 and 2. As already observed experimentally (Figure 5), the model confirms the improved performance of configuration 2 compared to configuration 1. The model describes the PO/water concentration ratio along the catalytic bed. The concentrations measured experimentally correspond to the exit of the membrane reactor (model position x = 1). Both experimental and theoretical values show an improvement of configuration 2 of 1.5 times higher compared to configuration 1. Figure 10 also shows the hydrogen efficiency to decrease along the catalytic membrane for configuration 2 and to increase for configuration 1. In order to explain these results, the reactant concentration profiles (hydrogen, oxygen, and propene) along the catalytic bed obtained from the model are shown in Figure 11. As experimentally observed and confirmed from the theoretical results (Figure 10), the improved performance of configuration 2 is due to the hydrogen feeding strategy. In configuration 1 (Figure 11a), hydrogen is fed in position x = 0 and flows through the catalytic membrane. This is translated into a higher hydrogen concentration in position x = 0 and lower one in x = 1. This indicates lower hydrogen efficiency for higher hydrogen concentrations. The model also confirms the opposite trend of configuration 2 compared to configuration 1. In configuration 2 (Figure 11b) the hydrogen is fed in x = 1 and its diffusion through the catalytic membrane is limited. This indicates that the efficiency close to x = 1 is lower compared to x = 0 due to lower hydrogen concentrations. The absolute values obtained from the model are higher compared to the

Figure 11. Concentration profiles of O2, C3H6, and H2 along the catalytic membrane. (a) Configuration 1. (b) Configuration 2. T = 403 K. P = atmospheric pressure. Membrane thickness = 0.2 mm.

experimental ones where the maximum hydrogen efficiency obtained is 0.14. However, the trends obtained from the present model are qualitatively consistent with experimental results. This allows us to implement the results from the model for the optimization of the catalytic membrane reactor design. It was experimentally demonstrated that the membrane thickness and effective diffusion have an important role on the membrane resistance influencing the PO/water concentration ratio. In the experimental part, the diffusion coefficient can not be controlled directly. However, by tuning the press loading of the catalyst it was possible to change the porosity of the catalytic membrane, thereby changing the effective diffusion coefficient. As shown in Figure 8, an increasing of the effective diffusion implies a decreasing of PO/water concentration ratio. 3.6.1. Membrane Thickness. Figure 12 shows the simulation results for the dependence of the PO/water concentration ratios on the thickness of the membrane for an effective diffusion coefficient of 10−7 m2/s. The hydrogen efficiency is calculated based on the PO and water concentration at the exit of the reactor (X = 1), since this was the value experimentally measured. The PO/water concentration ratio remains almost constant until a membrane thickness value of 0.2. However, after the critical value of 0.2 mm is reached, the PO/water formation is significantly improved by increasing the membrane thickness. F



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Figure 12. Dependence of PO/H2O concentration ratios on the thickness of the membrane catalytic bed for lower diffusion coefficients. Deff = 10−7 m2/s. T = 403 K. P = atmospheric pressure. Figure 2 - membrane configuration 2.

This suggests that thinner membranes do not provide enough resistance in order to avoid the complete mixing of the gases. In the specific case, the hydrogen diffuses through the membrane so fast that it is completely mixed with oxygen and propene. This leads to the absence of a H2 concentration profile along the catalytic membrane during its diffusion. In these conditions, the operation of the reactor is merely reduced to a conventional packed bed where the gases are fed together. The calculations allow to further investigate the gas concentration profiles along the catalytic membrane gaining information on their relation to hydrogen efficiency. Figure 13 shows the concentration profiles of PO and water and Figure 14 shows the concentration profiles of propene, oxygen, and hydrogen. Thinner membranes (0.1 mm) show that the propene oxide and water concentration profiles are almost flat over the catalytic bed. As the thickness of the membrane is increased, the concentration profiles become more prominent. The absolute concentrations of PO and water reach higher values at the hydrogen feed side and decrease at the opposite one. A more complete understanding of the system can be obtained by combining the information provided by the product concentration profiles (Figure 13) with the reactant ones (Figure 14). The constant concentration profiles of the reactants along the catalytic membrane show the mixing of the gases over the catalyst with no effect on the PO/water concentrations ratio. The increasing of PO and water formation on the right side of the membrane reactor (x = 1) confirms the positive effect of higher H2 concentrations on the catalyst productivity. Oxygen and propene are fed from the left side of the membrane (x = 0). Here, lower concentrations of hydrogen (the diffusing gas) are combined with excess of propene and oxygen. The ratio H2/ propene is lower increasing the hydrogen efficiency. Figure 15 shows the PO/water concentration profile along the catalytic bed for a membrane thickness of 0.8 mm. The concentrations ratio of PO/water is higher at the left side of the catalytic membrane (x = 0) and decreases to the right one (x = 1). In the left side of the reactor, the amount of water decreases more than PO due to propene excess that preferentially consumes hydroperoxy species to PO. As observed experimentally, the model confirms the need to

Figure 13. Concentration profiles of (a) propylene oxide and (b) water along the catalytic bed for different membrane thickness. Deff = 10−7 m2/s. T = 403 K. P = atmospheric pressure. Figure 2 - membrane configuration 2.

increase the membrane thickness in order to guarantee a concentration profile of the gases and observe an influence of the catalytic membrane on the PO/water formation. However, the thickness of the membrane, as already mentioned, is connected to the gas flow-diffusion along the catalytic bed. A more pronounced hydrogen concentration profile requires a thicker catalytic membrane in order to provide the concentration profiles. 3.6.2. Porosity and Tortuosity. The diffusion of the reactants through the catalytic membrane is influenced by the porosity (ε) values of the bed which are obtained under different compressions. The effect of the porosity was investigated. The calculations were performed for a membrane thickness of 1 mm, for which experimentally a clear positive effect on the H2 efficiency was observed. Figure 16 shows the dependence of the PO and water concentration profiles on the resistance provided by the catalytic membrane. Figure 16a,b represents the results for a densely packed catalytic membrane system (ε = 0.25). In this case the resistance of the catalytic membrane to the gas flow is higher. It can be observed that, close to the position x = 0 (propene and oxygen feeding side), the H2 efficiency is high due to the fact that PO produced is comparable to water formation. G



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Figure 15. PO/H2O concentrations ratio profile along the catalytic bed. Deff = 10−7 m2/s. T = 403 K. P = atmospheric pressure. Figure 2 membrane configuration 2. Membrane thickness = 0.8 mm.

Figure 16. Concentration profiles of propylene oxide and water along the catalytic bed for different membrane resistances. Black line, PO concentration profile for dense packed systems. Red line, water concentration profile for dense packed systems. Blue line, PO concentration profile for loosely packed system. Pink line, water concentration profile for loosely packed systems. Membrane thickness = 1 mm. Deff = 10−7 m2/s. T = 403 K. P = atmospheric pressure. Figure 2 - membrane configuration 2.

mainly attributed to the formation of concentration profiles along the catalytic bed that avoid the mixing of the reactants along the membrane. As already observed, the resistance is tuned by combining the porosity and the thickness of the membrane which decreases with having a highly packed system. Figure 16 also clearly shows the positive effect of high hydrogen concentrations on the catalyst productivity. As can be observed, for a loosely packed catalytic membrane, the production of PO is increased compared to the densely packed system (as already noticed in the experimental section). However, it is evident that the increased water formation is the result of the increasing the hydrogen concentration. The water productivity increases more compared to the PO one, as expected from the kinetics of the two reactions. The final membrane reactor design will have to deal with the compromise between catalyst productivity and H2 efficiency. This leads to some considerations on the optimal catalytic

Figure 14. Concentration profiles of (a) propene, (b) oxygen, and (c) hydrogen along the catalytic bed for different membrane thickness. Deff = 10−7 m2/s. T = 403 K. P = atmospheric pressure. Figure 2 membrane configuration 2.

Figure 16c,d shows similar trends for loosely packed membranes (lower resistance, ε = 0.5). It is noticeable that the concentration of water is always higher that of PO meaning that the hydrogen efficiency is always lower compared to the densely packed system. In the loosely packed catalytic membrane, the H2 concentration is higher due to low restriction. This allows hydrogen to easily diffuse along the catalytic membrane. This indicates that a higher membrane resistance has a positive effect on hydrogen efficiency. This is H



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combined with a more densely packed system to maximize the gas concentration profiles and to avoid their direct mixing.

membrane reactor concept. It was experimentally and theoretically shown that the hydrogen and propene should be fed separately. The propene flow should be forced through the catalytic membrane. The hydrogen diffusion must be controlled in order to create concentration gradients. In the present work, the experiments were conducted at constant reactant concentrations, paying attention to work outside the explosive regime. A further optimization of the catalytic membrane reactor system can be achieved by tuning the gas concentrations. Since the oxygen feeding on either side of the membrane was found not to affect the PO/water formation, an optimized system can be presented by feeding the oxygen and hydrogen together. The concentration of oxygen needs to be in excess in order to avoid explosive mixtures. The separation of propene and oxygen also allows the possibility to increase the initial concentration of propene without being limited from safety and making advantageous utilization of the catalytic membrane concept. The propene flow can then be forced along the membrane and H2 and O2 diffusivity can be limited creating concentration profiles along the catalytic membrane. This will be the optimal configuration for the catalytic membrane reactor design considering the combination of experimental results and theoretical calculations. 3.7. Productivity and Hydrogen Efficiency. The membrane reactor concept analyzed in the present work is focused on the optimization of the hydrogen efficiency as the main parameter for the direct PO synthesis. Moreover, hydrogen efficiency is an important parameter to be optimized also for other oxidation processes where the generation of in situ hydrogen peroxide is the rate limiting step. The optimal membrane reactor strategy, valid for the direct PO synthesis, can be then extended to other processes like hydroxylation of benzene, oxidation of methane, and oxidative desulphurization where the hydrogen and oxygen are fed together in a single reactor unit and the further hydrogenation of the intermediate H2O2 may become an issue.26 However, as already mentioned, a drawback of the present membrane reactor concept is related to the productivity of the catalyst. It was noticed, first experimentally, with a decrease of the conversions, and then theoretically, that increasing the hydrogen concentration along the bed increases the productivity of the catalyst. Higher hydrogen concentrations increase the PO production but also the water formation thus lowering the H2 efficiency. The benefit of lower hydrogen concentrations along the catalytic bed is also translated in a non optimal utilization of the catalyst where the most active part is the layer close to the exit of the reactants. The final reactor design needs to keep into consideration the compromise between these two parameters.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +31 40247 3671. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The research leading to these results has received funding from the European Community’s Seventh Framework Programme through the Collaborative Project INCAS, under agreement No. 245988. The authors would like to thank M. Fernanda Neira D’Angelo for helpful discussions.

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NOMENCLATURE DAB = bulk gas diffusion (m2/s) Deff = effective diffusion (m2/s) ε = porosity of the catalytic bed τ = tortuosity of the catalytic bed C0 = initial concentration (mol/m3) CA = compound concentration (mol/m3) rm = membrane thickness (mm) X = spatial coordinate A = membrane area (m2) r1 = reaction rate PO formation (mol/gcat s) r2 = reaction rate H2O formation (mol/gcat s) k1 = PO reaction constant (mol1‑(A+B+C) m3(A+B+C) /gcat s) k2 = H2O reaction constant (mol1‑(α+β+γ) m3(α+β+γ) /gcat s) Vthr = volumetric flow through the membrane (m3/s) i = H2, O2, C3H6, PO, H2O concentration ratio = (molA/m3)/(molB/m3) REFERENCES

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4. CONCLUSIONS The best membrane reactor performance for the direct epoxidation of propene over an Au−Ti/SiO2 catalyst is achieved in configuration 2 where hydrogen is fed separately from oxygen and propene. Propene and oxygen flows are forced through the catalytic membrane and hydrogen diffusion is limited. The concentration profiles of the gases along the catalytic membrane reactor have been appointed as key parameters in order to maximize the PO/water formation. Therefore, the membrane resistance plays an important role in obtaining an improvement on the catalytic membrane reactor performance. This can be controlled by the thickness and the effective diffusion. A large membrane thickness needs to be I



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Direct Synthesis of Propene Oxide from Propene, Hydrogen and

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