Silver-Catalyzed Oxidation of Ethylene to Ethylene Oxide in a

Jan 22, 2002 - Ethylene oxide synthesis has been chosen as a benchmark case to evaluate the performance of a microreaction system in comparison to an ...
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Ind. Eng. Chem. Res. 2002, 41, 710-719

Silver-Catalyzed Oxidation of Ethylene to Ethylene Oxide in a Microreaction System Harry Kestenbaum, Armin Lange de Oliveira, Wolfgang Schmidt, and Ferdi Schu 1 th* Max-Planck-Institut fu¨ r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mu¨ lheim an der Ruhr, Germany

Wolfgang Ehrfeld, Klaus Gebauer, Holger Lo1 we, and Thomas Richter Institut fu¨ r Mikrotechnik Mainz (IMM) GmbH, Carl-Zeiss-Strasse 18-20, D-55129 Mainz, Germany

Dirk Lebiedz, Ingo Untiedt, and Harald Zu 1 chner Westfa¨ lische Wilhelms-Universita¨ t Mu¨ nster, Schlossplatz 4/7, D-48149 Mu¨ nster, Germany

Ethylene oxide synthesis has been chosen as a benchmark case to evaluate the performance of a microreaction system in comparison to an existing industrial process. This reaction was selected because microreaction technology provides equipment with very good mass- and heat-transfer conditions, which avoids hot spots inside the reactor channels that are known problems for the partial oxidation of ethylene. Furthermore, because the microstructured reactors are inherently safe with respect to explosions, gas compositions within the explosion limits are attainable and can be handled safely. For example, 15% ethylene in pure oxygen, which is in the middle of the explosive regime and far away from typical compositions for industrial processes, could be used. Space time yields of 0.14-0.78 tons h-1 m-3 calculated on the basis of the channel volume, in comparison to the values of 0.13-0.26 tons h-1 m-3 for an industrial reactor calculated on the basis of the reactor volume, have been achieved by using the microreactor. 1. Introduction In 1931, Lefort discovered the direct oxidation of ethylene to ethylene oxide by silver catalysts.1 Since that time, the industrial production of ethylene oxide by direct oxidation has become a widely used process.2 In 1995, the world capacity for ethylene oxide was near 11.2 × 106 tons per year, and the worldwide production capacity is still growing.3 For this reason, much research has been focused on this reaction from a fundamental point of view, as well as with respect to prospective applications. Despite many investigations, silver is still the only known catalyst that epoxidizes ethylene with sufficient activity and selectivity. Commercially available R-Al2O3supported catalysts have been reported to operate with selectivities in the range of 80%.2,4 In this work, clean silver surfaces, such as polycrystalline silver, were used, for which an initial selectivity to ethylene oxide of ca. 40% is reported.5,6 Ethylene may either be selectively oxidized with oxygen over a silver catalyst to form ethylene oxide or totally oxidized with the products carbon dioxide and water..2 Furthermore, subsequent deep oxidation of ethylene oxide inititally formed is possible. The ethylene oxide formation is moderately exothermal (-105 kJ mol-1), while the total oxidations of both ethylene and ethylene oxide are strongly exothermal (-1327 and -1223 kJ mol-1), so that the reaction must be quenched to prevent full oxidation. In the conventional industrial process, a total of -350 to -550 kJ mol-1 is generated, * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +49 208 306-2995.

causing hot spots and heat removal problems, which lead to bad selectivities.7 The enhanced surface-tovolume ratio of microreactors enables an effective heat management by efficiently removing the heat that is produced. Temperature profiles have been calculated for this reaction and the reactor used in this study. They show a maximum temperature gradient of 1 °C, even at high ethylene conversions.8 In the past few years the field of microreaction technology has received considerable interest from reaction engineering and chemical industries.9-11 Dimensions that are less than the quench distance for explosions result in a higher degree of safety, and the small size of the microreactor leads to only a small reactor inventory of hazardous chemicals.12 It was recently shown that even the H2/O2 reaction can be handled safely.13 Kursawe et al. proved that microstructured reactors, even without a micromixer, are suitable to handle safely a reaction like ethylene oxidation.14 This is a very important fact because the explosion limits for ethylene in air range from 3.1 to 32%,15 whereas for the product ethylene oxide, they are 2.6100% at ambient conditions.2 Safety issues must be addressed, and expensive equipment is normally required to handle explosive reaction conditions such as this. At higher pressures, such as those used in ethylene oxide production, the explosive range is even wider. Thus, the critical oxygen concentration at room temperature ranges from about 15 vol % at 1 bar down to 8 vol % at 20 bar.16 To study the exceptional properties of microreactors, the synthesis of ethylene oxide was chosen to demonstrate the performance of catalytic microstructured reactors and to compare it with current industrial standards.

10.1021/ie010306u CCC: $22.00 © 2002 American Chemical Society Published on Web 01/22/2002

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Figure 1. Overview picture of the stainless steel housing containing a mixing unit on the top, a diffusion path of 1 mm length in between, and a silver catalyst at the bottom. The reaction gases are injected from opposite sides and directed by a specific channel geometry to the diffusion path. The top of a single silver foil realized by Laser-LIGA is shown in the catalytic area. It contains nine channels of 500 µm width, 50 µm height, and 9.5 mm length.

The main aim of the reactor development was the demonstration of the basic feasibility of an industrial gas-phase process in microreaction systems. Specific optimization of the microreactor performance compared to the industrial process by fine-tuning the catalyst was not intended. The results should be regarded as a description of the potential of microreactors, e.g., for distributed production on-site and on-demand as well as a basis for future developments. 2. Experimental Section 2.1. Microstructured Mixer/Reactor. For this investigation, a microreactor designed and constructed by Institut fu¨r Mikrotechnik Mainz GmbH (IMM) was used. The microreactor is specially designed for use under reaction conditions analogous to those applied on the industrial scale for the oxidation of ethylene, i.e., temperatures up to 300 °C and pressures up to 25 bar. The microreactor is divided into three sections, namely, a mixing unit, a diffusion path, and the catalytic area (Figure 1). In the following, the individual system components and their function are described in more detail. A short description of the reactor has been given before,17 but for a better understanding of this paper, the most salient points shall be repeated here. The mixing unit structure was created by excimer laser ablation of a resist material, which provides the negative image of the mold for mixing channels. In this case, polymethyl methacrylate (PMMA) was the resist material. The larger areas were machined by conventional milling techniques. The final structure was achieved by an electrochemical deposition of nickel.18,19 The whole process is called Laser-LIGA. To avoid any catalytic activity within the gas mixing unit, the nickel structure was coated with gold by electrochemical deposition. In the mixing unit, the reactant gases are injected from opposite sides, which is established by a specific channel geometry that introduces the gases ethylene from the one side and oxygen from the other side at a 90° angle to the final flow. The thickness of each layer is about 300 µm; thus, a stack of 14 foils is used. The positions of the channels are chosen for the separate gases to achieve a perfectly laminated gas stream entering the diffusion zone. The diffusion path is an

Figure 2. Schematic of a single silver foil realized by Laser-LIGA with parallel shallow channels.

opening of 1 mm length between the mixing unit and the catalyst zone. Flow modeling with FLUENT showed that with this particular laminated flow a diffusion length of 1 mm is sufficient to obtain a homogeneity of the gas mixture of about 99% for all flow rates investigated. Note that the inner to outer channel length in the mixing unit is increasing while the width of the channels increases as well from 148 µm up to 469 µm. The different channel widths provide an even distribution, i.e., constant volume flow through all channels regardless of their length. Thus, the flow velocity in the narrow and short channels is about a factor of 10 higher than that in the longer ones having a larger cross section. This arrangement of the channels establishes a constant pressure drop for each channel in the mixer.20,21 The catalytically active part of the reactor consists of stacked and microstructured silver foils in parallel-flow geometry. The use of silver with its high thermal conductivity leads to a uniform heat distribution and therefore an enhanced process performance. The silver foils were produced via two different techniques: First, sets of foils (“Laser-LIGA catalyst”) were made by the Laser-LIGA process, as described for the mixing unit. These were created by electrochemical deposition of silver using AgCN (Argophan) as an electrolyte onto the resist patterns generated by laser ablation and milling. One stack of 4.2 mm height for the reactor contains 14 foils, each 300 µm thick and 9.5 mm wide, with nine channels, each 9.5 mm long, 50 µm high, and 500 µm wide (Figure 2). The depth of only 50 µm ensures short diffusion paths for each molecule to reach the catalyst surface.22 A second set of foils with the same dimensions and the same amount of channels per foil was produced by etching of silver foils (“etched cata-

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Figure 3. SEM micrograph showing the surface before (a) and after (b) OAOR treatment.

lyst”). It was possible to reduce the thickness of a single foil to 200 µm and to increase the height of each channel to 80 µm. This allowed the use of 21 foils for the catalytic reaction. As described above, silver on R-Al2O3 shows a better selectivity than polycrystalline silver catalysts. Therefore, a third set of foils was prepared with an R-Al2O3 layer as the support. Only R-Al2O3 gives catalysts with sufficient activity and selectivity; foils made from stainless steel coated with amorphous or γ-Al2O3 by chemical vapor deposition, such as those used for the hydrogen combustion in a microreactor,12 could not be used. For phase transformation to R-Al2O3, temperatures exceeding 1000 °C are necessary. Unfortunately, standard steel grades corrode at such temperatures and the iron oxide formed covers the Al2O3. An aluminum-containing stainless steel normally used as metallic supported for car exhaust catalysts (Krupp VDM Fe-20Cr-5Al “Aluchrom-catalyst”) was used. When this steel is heated to 1100 °C over 5 h, the aluminum segregates to the surface, where it reacts with air oxygen to R-Al2O3.23 The microstructured channels for these foils have been realized by sawing before heat treatment. Thus, nine channels, each 9.5 mm long and 500 µm wide, had been realized in parallel-flow geometry. Because of the thickness of 250 µm for each foil, a stack contains 17 foils. Through this technique the depth of the channels cannot be adjusted as precisely as with the other techniques. However, the deviation of (10 µm at a channel depth of about 90 µm is sufficient for catalytic experiments. Such R-Al2O3 layers can be used as a support for the catalyst, which was deposited by sputtering the foils with silver in an evacuated chamber on both sides. The sputter time has been varied for each set of foils to achieve different thicknesses of the catalyst layer on the support. Thus, three sets of foils have been prepared with 1, 5, and 100 nm nominal layer thickness. The reactor housing for the mixing unit and the stacks of silver foils was constructed from stainless steel by conventional milling techniques in combination with electro discharge machining (µ-EDM) die-sinking techniques.24 In this way, an exact fitting of the metallic layer into the housing was ensured. Standard tubes are welded to the housing to achieve an easy and fast connection for the reaction and the outlet gases. It has to be kept in mind that the diffusion zone is the only non-microstructured area where a small volume of explosive gas mixture is present under the reaction temperature. Thus, the potential danger in this

zone is negligible because the volume of explosive gases is just 0.042 cm3. Although the outlet gas stream contains explosive gases, an explosion could be avoided by fast quenching from reaction conditions to 70 °C. 2.2. Catalyst Modification. Because the catalyst is made of polycrystalline silver, an attempt was made to increase the catalytically active surface. Czandern et al. reported that plain silver surfaces would roughen if the silver had been treated with oxygen at 250 °C and thereafter outgassed and reduced with hydrogen at 350 °C. They found that with this so-called oxidation and outgassing reduction (OAOR) process the surface area can be increased by a factor of up to 10.25 For this procedure the catalysts were oxidized over 12 h at 250 °C in an oxygen atmosphere at a volume flow of 0.5 L h-1 of oxygen, followed by reduction over 12 h at 350 °C in a hydrogen atmosphere at the same flow rate. For evaluating this process for use on a microstructured silver foil, we selected silver wool (Aldrich 99.9+%) as a testing system, because it was hoped that surface area analysis would be possible. In general, the wool threads each were 50 µm in diameter and the behavior of a thread could be considered to be like an inverse channel of the microreactor. However, precise measurement of the surface area could not be accomplished because the surface area of the silver wool was at the detection limit even after the OAOR process. Therefore, scanning electron microscopy (SEM) was used for analysis. Figure 3 shows how the surface of the silver wool changes after OAOR treatment, with the previously smooth surface roughened afterward. Unevenness before the experiment was caused by physical damage with the pincer during preparation. A modified OAOR technique without outgassing in between for preparing the silver foils in the reactor was used because evacuation of the microstructured reactor was not possible. Additionally, this preparation step was helpful in removing organic impurities on the catalyst surface possibly resulting from the microstructuring process. 2.3. Reactor Operation. Heating of the reactor was applied externally, by placing the microreactor into a forced convection flow oven, to generate the necessary temperatures ranging from 200 to 360 °C. To preheat the gases before entering the microreactor, the reactant gas tubes were guided through the oven. The reaction gas temperature was measured by a Ni-CrNi thermocouple, which had been placed in the outlet gas stream with ca. 1 mm distance to the catalyst stack. Gas concentrations were regulated by using mass flow

Ind. Eng. Chem. Res., Vol. 41, No. 4, 2002 713 Table 1. Overview of Experimental Conditions for Each Experiment Using Laser-LIGA-, Etched- and Aluchrom-catalysts catalyst

ethylene concn [vol %]

oxygen concn [vol %]

temp [°C]

pressure [bar]

residence time [s]

etched Etched Laser-LIGA etched etched etched Aluchrom

3.4-16.4 15 3 15 7.5 5 15

50 5-85 16.5 85 42.5 50 80

290 290 239-289 230-290 290 290 270

4 4 5 2-20 4-15 20 5

0.235 0.469 0.124 0.235-2.350 0.469 0.5-8 0.275

30

290

Laser-LIGA

6

controllers (Bronkhorst), adjusting the gas concentration of ethylene from 1 to 15 vol % and oxygen from 5 to 85 vol %, respectively. Nitrogen was used as the carrier gas. The total gas flow varied from 1 to 6 L h-1. Furthermore, the pressure was regulated with a pressure transducer in the outlet gas stream (Bronkhorst) between 1 and 20 bar. To ensure that the results are comparable to existing industrially used processes, all experiments were carried out after the catalyst had reached a steady state (time to reach steady state up to 10 days). As a guideline through the results section, Table 1 reports the conditions for all experiments described in this paper. To avoid condensation of synthesized ethylene oxide, the exhaust gas tube between the microreactor and the gas chromatograph (GC) was heated as well. Product gas mixture analysis was carried out via an online GC (Carlo Erba GC8000 Top) with a flame ionization detector/thermal conductivity detector combination with a micropacked column (Restek HayeSep S) for gas separation. In this manner permanent gases such as oxygen, nitrogen, and carbon dioxide, as well as ethylene and ethylene oxide, can be separated at the same time. Thus, concentrations of ethylene oxide and carbon dioxide can be obtained simultaneously, which is crucial for the determination of mass balance and selectivity. Carbon mass balances typically closed within 1.5%. 3. Results and Discussion Initially the fresh microstructured silver foils showed no catalytic activity. Even after treatment of the silver foils with the previously described OAOR process for cleaning the surface from impurities, only a slight increase in the catalytic activity could be achieved. During the first 100 h of operation at reaction conditions, however, the formation of a catalytically active silver was observed, as had been previously described.26 Under steady-state conditions, the observed catalytic activity corresponds to the range of values given in the literature. At a temperature of 239 °C, a pressure of 5 bar, and a feed composition of 3 vol % ethylene and 16.5 vol % oxygen, a reaction rate based on the geometric surface area of the channels of 4.8 × 10-5 mol s-1 m-2 to ethylene was found.17 Tsybulya et al. reported a value of 1.7 × 10-6 mol s-1 m-2 at 230 °C, atmospheric pressure, and 2 vol % ethylene and 7 vol % oxygen for silver powder.27 Considering the kinetic effect of higher pressure used and the slightly higher temperature (see below), an increase by a factor of about 26 in comparison to the conditions used by Tsybulya et al. could be expected, so that an adapted reaction rate of 1.86 × 10-6 mol s-1 m-2 can be calculated by using a formal kinetic description based on the result from sections 3.1 and

5

0.124

comments see section 3.1 and Figure 4 see section 3.2 and Figure 5 see section 3.3 and Figure 6 see section 3.3 and Figure 7 see section 3.3 and Figure 8 see section 3.4 and Figure 9 see section 3.4 and Figure 12; silver layer thickness 1, 5, and 100 nm see section 3.6 and Figure 13; DCE concentration 0, 35, and 100 ppm

Figure 4. Reaction rate and selectivity of the etched catalyst over ethylene partial pressure. Reaction conditions: 50% oxygen, 290 °C, 4 bar, residence time 0.235 s, total gas flow 2 L h-1.

3.2, which is fairly close to the value reported by Tsybulya et al. Kursawe and Ho¨nicke found a reaction rate of 6.04 × 10-6 mol s-1 m-2 at 250 °C, 3 bar, and 20 vol % ethylene and 20 vol % oxygen using polycrystalline silver foils in the microstructured reactor, which are comparable to foils used in this work. Again taking into account the different reaction conditions, the value can be recalculated to the conditions used by Tsybulya et al., which results in a value of 1.42 × 10-7 mol s-1 m-2. At this time we have no explanation for the difference in the reaction rate observed. On the other hand, these authors could achieve high conversions up to 70% at selectivities close to 60% by using anodically oxidized aluminum foils which were coated with silver by gasphase deposition.14 3.1. Influence of the Ethylene Partial Pressure. Figure 4 illustrates how the ethylene partial pressure influences the reaction rate and selectivity to ethylene oxide at constant oxygen partial pressure. Obviously, the reaction rate is increasing with higher ethylene partial pressure for an etched catalyst, but not in the way as would be expected for a reaction of first order. The order of partial oxidation of ethylene under these conditions is lower than first order with respect to ethylene. The reaction rate for ethylene oxide increases from 2.28 × 10-5 mol s-1 m-2 for 3.4 vol % ethylene to 5.28 × 10-5 mol s-1 m-2 for 16.4 vol % ethylene, whereas the selectivity stays almost constant. From this result an averaged formal order of 0.53 for ethylene oxide production was calculated. 3.2. Influence of the Oxygen Partial Pressure. Industrial reactors operate at an oxygen concentration of 4-9 vol % and ethylene concentrations of up to 40 vol % to ensure gas compositions with a surplus of ethylene in order to avoid dangerous gas mixtures in

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Figure 5. Reaction rate and selectivity of the etched catalyst over oxygen partial pressure. Reaction conditions: 15% ethylene, 290 °C, 4 bar, residence time 0.469 s, total gas flow 1 L h-1.

the explosive regime. One advantage of the microreaction system is the safe use of lean gas compositions, corresponding to a surplus of oxygen, and gas compositions within the explosive regime. In Figure 5, the results for a sequence of different oxygen concentrations at a constant concentration of ethylene for an etched catalyst are shown. This experiment was conducted at a constant ethylene concentration of 15 vol %, a total pressure of 4 bar, and a gas flow of 1 L h-1 at 290 °C. It was found that the selectivity in the microreactor depends on the partial pressure of oxygen. Thus, the reaction rate to ethylene oxide increases with increasing oxygen pressure. Based on these results, a formal order for ethylene oxide production with respect to oxygen of 0.78 was obtained. As shown, the selectivity for ethylene oxide unexpectedly increases with the concentration of oxygen added up to an oxygen concentration of approximately 45%. The positive influence of oxygen concentration on conversion and selectivity had been reported earlier by Haul and Neubauer.28 However, these results were observed under low-pressure conditions as necessary for XPS analysis. The results reported here highlight the importance of reaction systems enabling reaction gas concentrations within the explosion limits because this could lead to significant process improvement. The microreactor system described here, with its inherent safety, allows the use of reaction gas mixtures in explosive regimes with minimal risks. 3.3. Influence of the Temperature and Pressure. Figure 6 shows how the conversion and selectivity of the Laser-LIGA catalyst are influenced by the temperature of the reactor. The reaction rate to ethylene oxide increases from 4.5 × 10-5 up to 1.5 × 10-6 mol s-1 m-2, whereas the selectivity decreases from 64.1% down to 45.1%, as is expected with higher temperature. Variation of the reaction temperature between 239 and 308 °C for both catalysts, Laser-LIGA and etched, allowed the estimation of the activation energy. The activation energy was found to be about 47 kJ mol-1 for the LaserLIGA set of foils and about 49 kJ mol-1 for the set of foils made by etching. This corresponds to the data given in the literature.28,29 In comparison to the aluminasupported silver, the catalyst used for the industrial epoxidation of ethylene, the selectivity of about 50% at around 10% conversion is lower than the values established in industrial processes. When the temperature is decreased to moderate values, e.g., 239 °C, a selectivity of 65% and a conversion of 4.5% can be reached.

Figure 6. Reaction rate and selectivity of the Laser-LIGA catalyst depending on the reactor temperature. Reaction conditions: 3% ethylene, 16.5% oxygen, 5 bar, residence time 0.124 s, total gas flow 5 L h-1.

Figure 7. Space time yield for the etched catalyst (calculated on basis of the reaction channel volume) depending on the pressure and temperature. The residence time is increasing from 0.235 s at 2 bar and 290 °C to 2.350 s at 20 bar and 290 °C. Reaction conditions: 15% ethylene, 85% oxygen, total gas flow 1 L h-1. Hatched columns correspond to partially deactivated catalysts (see text).

These values appreciably exceed those observed for polycrystalline silver catalysts.5,30 The oxidation of ethylene is usually carried out at high pressures between 10 and 30 bar in order to obtain high space time yields. For comparison to the industrial process, the microreactor was designed to handle pressures up to 25 bar. The system was limited, however, by the electronic pressure transducer to a maximum pressure of 20 bar, and for this reason, the experiments were carried out between 230 and 290 °C and pressures between 2 and 20 bar. In Figure 7 the space time yields for the various experiments are plotted versus temperature and pressure. It can be seen that the maximum space time yield of 0.67 tons h-1 m-3 was found at 5 bar and 290 °C, whereas a further increase in pressure does not lead to an increase in yield. Rather, a deactivation of the catalyst was observed at this temperature and the experiments were stopped before reaching the steady state at 10 and 20 bar. Thus, these data points for 290 °C are absent because the rapid deactivation made it impossible to calculate space time yields with

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Figure 8. Reaction rate and selectivity of the etched catalyst at a constant residence time of 0.469 s for different pressures. Reaction conditions: 7.5% ethylene, 42.5% oxygen, 290 °C. The broken line shows the reaction rate calculated from the kinetic parameters extracted from Figures 5 and 6.

reasonable precision. The lower space time yield for the other experiments at 10 bar can also be explained by this observed deactivation. These experiments were carried out immediately after the experiments at 20 bar, with the catalyst already having lost some of its activity. An explanation for the deactivation of the silver catalyst could be the fact that the mass flow during all experiments was constant. This resulted in a different residence time for the reactants at the various pressures. Thus, the residence time was 0.235 s at 2 bar and 290 °C, whereas an increase of the pressure to 20 bar increased the residence time by a factor of 10 to 2.350 s. The maximum space time yield was obtained at a residence time of 0.588 s. It appears that under these conditions too long residence times lead to deactivation, possibly via degradation of the ethylene oxide formed. To avoid this problem, experiments with varying pressures at constant residence times of 0.469 s were carried out. Figure 8 shows that an increase of the pressure up to 15 bar leads to an increase of the reaction rate, if the residence time is kept sufficiently short. Accordingly, the space time yield improved from 0.10 to 0.35 tons h-1 m-3, and at the same time a slight increase in the selectivity could be observed. This corresponds to approximately the expected values calculated by using the formal orders as determined above for ethylene and oxygen. 3.4. Influence of the Residence Time. As described above, long residence times lead to deactivation of the catalyst. To examine the influence of the residence time on the performance of the catalyst, experiments were carried out, where the gas composition, temperature, and pressure have been kept constant. Only the mass flow was varied, so that the residence time increased from 0.5 to 8 s. In Figure 9 it can been seen that the reaction rate and selectivity is decreasing rapidly with higher residence times. It has to be mentioned that the catalyst showed no deactivation up to a residence time of 4 s at a conversion level of 24%. This is in contrast to the deactivation at a residence time of 0.588 s at 20% conversion described above. An explanation can be given in the different partial pressure for ethylene and oxygen in both cases. For the experiment in Figure 9, the partial pressure of ethylene has only been increased from 7.5 × 104 to 1 × 105 Pa, whereas for oxygen it has been increased from 4.25 × 105 to 1 × 106 Pa. Here again

Figure 9. Reaction rate and selectivity of the etched catalyst depending on the residence time. Reaction conditions: 5% ethylene, 50% oxygen, 290 °C, 20 bar.

Figure 10. C 1s XPS spectra of the etched catalyst in active and inactive states.

increasing the partial pressure of oxygen has a positive influence on the catalyst and on the use of longer residence times. XPS measurements were carried out to investigate the cause for the observed deactivation. Figure 10 shows the C 1s binding energies of the carbon on the silver surface. It can be seen that no significant signal for the active catalyst is found, whereas a signal at 285 eV can be clearly detected for the deactivated catalyst. The value of 285 eV is characteristic for elementary carbon on a silver surface31 and can be allocated to the formation of carbon on the surface after reaction at long residence times. Measurements could not be carried out in situ, so that the silver surface had been in contact with air prior the XPS analysis, which could lead to changes at the surface. To avoid this problem, the surface has also been sputtered with argon ions to obtain depth profiles, as shown in Figure 11. In this connection the etching time of 1000 s corresponds to a sputter depth of about 2 µm. For both the active and inactive catalysts, the carbon concentrations decrease. However, the concentration of C 1s for the inactive catalyst is higher at any depth. For oxygen the opposite behavior is observed: The O 1s concentration for the active catalyst is about 10% higher at any depth. 3.5. Using r-Aluminum Oxide as the Support. To investigate the effect of R-alumina as the support, the

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Figure 11. XPS depth profiles of relative concentration of Ag 3d, O 1s, and C 1s depending on the etch time.

Figure 12. Space time yield and selectivity for the Aluchrom catalyst depending on the silver layer thickness. For comparison, results for the etched catalyst are given as well. Reaction conditions: 15% ethylene, 80% oxygen, 270 °C, 5 bar, residence time 0.275 s.

performance of silver on segregated Aluchrom foils has been studied. In Figure 12 the results for three different catalysts which differ in the thickness of sputtered silver layer are shown, in comparison to the results for the etched catalyst under the same conditions. The catalyst with 1 nm of silver shows a selectivity of only 42%, whereas the use of catalysts with 5 and 100 nm thicknesses gives a high selectivity of 55% and 58%, exceeding that observed for the etched catalyst, at better space time yields. The difference of space time yield and selectivity for the layer 1 nm in comparison to 5 and 100 nm could be explained with a size effect, which has been described for silver in the ethylene oxidation by Goncharova et al.32 In contrast to the sputter process used in this work, Goncharova et al. had prepared the silver catalyst by the incipient wetness procedure and found an increase of conversion and selectivity for particles from 25 to 50 nm, whereas an increase of the particle size to 2 µm did not show any further increase of conversion or selectivity. For the 1 nm layer, it is very probable that no coherent layer is present after heat treatment of the catalysts but that small, isolated particles have been found. Cosputtered comparison samples on TEM carbon grids clearly showed particle formation for the 1 nm sample. The investigation of sputtered thin films of silver on ZnO by Arab showed a abrupt increase in resistivity with decreasing silver layer thickness at 5 nm. They assumed that the silver

Figure 13. Space time yield and selectivity for the Laser-LIGA catalyst without and with DCE addition. Reaction conditions: 6% ethylene, 30% oxygen, 290 °C, 5 bar, residence time 0.124 s, total gas flow 5 L h-1.

crystallites at a normal film thickness below 5 nm are no longer sufficiently large to form a close layer and are likely present as an isolated island.33 These results are not directly transferable to Al2O3, but they are consistent with the interpretation given above. 3.6. Influence of Adding 1,2-Dichloroethane (DCE). Kinetic models developed by Haul and Neubauer28 and Force and Bell34 propose that ethylene is converted to ethylene oxide by an Eley-Rideal mechanism, where gas-phase ethylene reacts with adsorbed oxygen on the silver surface. Depending on the state of oxygen adsorbed, selective oxidation or total oxidation of ethylene is observed. The beneficial influence of DCE in the industrial process on selectivity could be due to two facts. First, chlorine formed on the surface decreases the oxygen coverage, and second, chlorine on the surface leads to a reduction of the valence charge of bonded oxygen.35 However, DCE, being a inhibitor, not only improves selectivity but also reduces the reaction rate. Experiments show (Figure 13) that by adding DCE the selectivity increases from 52 to 69%, whereas the space time yield decreases from 0.78 to 0.42 tons h-1 m-3, at 290 °C, 5 bar, 5 L h-1, and a gas composition of 6 vol % ethylene and 30 vol % oxygen. The inhibitor DCE has been added by a micro flow pump (Eldex MicroPro) by injecting the liquid DCE over a stainless steel capillary directly into the ethylene gas stream. The amount of DCE added could not be determined exactly because the experiments were performed at the detection limit of the GC for DCE. Moreover, at 0.01 µL/min, for 35 ppm DCE, the micro flow pump was operated at the lowest working limit (0.02 µL/min is given as the lower threshold by the manufacturer). The lowest concentration at 35 ppm is thus only an estimate. A rapid deactivation of the catalyst is observed after adding 300 ppm DCE. This can be explained by the formation of AgCl on the catalyst surface, which could even be detected by XRD. Nevertheless, this series of experiments clearly illustrates that also for the microstructured reactor selectivity can be improved by the addition of chlorine compounds. 3.7. Change of Surface Morphology. Figure 14 shows four SEM images of reaction channel surfaces. As mentioned before, two different production processes had been used for manufacturing the catalyst foils, and this leads to very different surface characteristics, as is shown in Figure 14. The Laser-LIGA set of foils

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Figure 14. SEM micrograph showing the change of surface morphology for the silver catalyst after over 1000 h of operation. The surface of the Laser-LIGA catalyst is shown (a) before the OAOR process and usage and (b) after usage. The surface of the etched catalyst is shown (c) before and (d) after 1000 h.

showed a smooth surface before use, only pitted by a few holes (Figure 14a), whereas the set produced by etching is full of cavities between the fused silver particles (Figure 14c). Both catalysts were in operation for over 1000 h under reaction conditions without any deactivation. The catalysts deactivated at high pressures and residence times but could be reactivated by running the catalyst under standard reaction conditions over several days. It can clearly be seen that the catalyst surface in the channels of the Laser-LIGA catalyst has been roughened (Figure 14b) during the reaction process. Moreover, the surface is pitted substantially, and an agglomeration of surface silver particles can be seen. This morphology change of silver after use in partial oxidation reactions has already been described by Ertl et al.36 On the other hand, the etched catalyst showed a strong sintering (Figure 14d) and agglomeration of the small particles, whereby most of the cavities are lost. Because no extensive loss in the activity was observed for this catalyst during operation, we believe that this loss of surface area took place during the modified OAOR process for roughening the surface and cleaning it from impurities and not during the catalytic reaction itself. 3.8. Comparison of the Microreactor and Industrial Reactor. Currently, two processes exist for the production of ethylene oxide. Both technologies are very similar with only small differences, depending on whether air or pure oxygen is used for oxidation. The reactors

consist of large bundles of several thousand tubes that are 6-12 m long with an internal diameter of 20-50 mm. The catalyst is packed in the tubes in the form of rings or spheres with a diameter of 3-10 mm. For heat removal, the tubes are surrounded by a coolant like water or high-boiling hydrocarbons.2 In contrast, the microreactor is equipped with small channels that are 9.5 mm long with 50-80 µm height and 500 µm width. The channel wall material is the catalyst and the cooling medium at the same time, so we had to find a suitable point of reference for comparison of both processes. The space time yield seems to be an appropriate standard for comparison. This value gives a good measure of the potential yield from either of the processes irrespective of the catalyst. In Table 2 the main process data for both industrial processes and the results achieved with the microreactor process are shown. Instead of the reaction rate, the conversion is given in the table, as is usually found in the literature. The concentrations of ethylene were similar for all processes, whereas we were able to use oxygen concentrations in a wider range than could be handled safely in an industrial reactor. The temperature and pressure were also similar, but it has to be kept in mind that the highest space time yield had been achieved at 5 bar in the microstructured system. The microreactor reaches the same conversions as those in the oxygen process, with a somewhat lower selectivity. A possible explanation for this is the use of polycrys-

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Table 2. Main Process Parameter for an Oxygen-Based Industrial Process in Comparison to the Parameters Achieved with the Laser-LIGA, Etched, and Aluchrom Catalysts parameter

oxygen-based industrial process

C2H4 concn, vol % O2 concn, vol % CH4 concn, vol % temp, °C pressure, bar typical residence time, s C2H4 conversion, % selectivity, % space time yield, tons h-1 m-3

15-40 5-9 1-60 220-275 10-22 0.9-1.8 7-15 80 0.13-0.26 (reactor)

microreactor-based process (Laser-LIGA)

microreactor-based process (etched)

microreactor-based process (Aluchrom)

1.5-6 10-41

3-15 5-85

15 85

240-290 5 0.1-0.2 2-15 44-69 0.01-0.07 (foils), 0.14-0.78 (channels)

240-290 2-20 0.1-1.5 5-20 38-69 0.03-0.13 (foils), 0.18-0.67 (channels)

270 5 1.2 2-6 42-58 0.01-0.06 (foils), 0.08-0.36 (channels)

talline silver as the catalytically active compound, in contrast to the optimized supported catalyst in the industrial process, where the addition of inhibitors such as DCE and promoters such as Cs or Rh is used for increasing the selectivity up to 80%.2 The preliminary, nonoptimized experiments, during which DCE was added, show that such steps will also improve the performance of the microreactor. Thus, it seems likely to reach, or even exceed, selectivities as high as those achieved in the industrial process. Polycrystalline silver is normally known for its poor selectivity. Therefore, the selectivities between 38 and 65%, obtained without using inhibitors or promoters in this work, are exceptionally good.5 The space time yields were calculated on the basis of two different reactor volumes: Assuming that the utilization of the whole stack volume could be further optimized, with the first step already taken from the Laser-LIGA set, with a ratio of a stack to channels volume of 12, to the etched set, with a ratio of 6, we calculated the space time yield using the volume of the channels, on the one hand (upper limit), and on the basis of the whole stack volume, on the other hand (lower limit), which is comparable to the volume of the tube reactor. Even using the pessimistic stack volume for the calculation, the microreactor reaches about 0.13 tons h-1 m-3, which is comparable to the industrial performance. Starting from the even more unfavorable assumption that the mixing unit, diffusion path volume, and catalytic area are an inseparable unit for microreactor application, the ratio for an etched catalyst is increasing to 9, leading to a decrease in the space time yield to 0.07 tons h-1 m-3. When the space time yield is calculated on basis of the channel volume, the microreactor surpasses the industrial reactor with 0.78 tons h-1 m-3 for a Laser-LIGA catalyst, in comparison to the reported space time yield of 0.26 tons h-1 m-3 for the industrial process.37

taneously in the microreactor is underway. It should be mentioned that it is possible to utilize reaction conditions like those for ethylene in pure oxygen, which would be impossible in a conventional reactor design. Thus, we were able to show that the use of oxygen concentrations higher than those used in industrial processes gives an increase in the conversion and selectivities. This enables space time yields exceeding those of industrial processes. Whether such or similar reactors will ever be used in industry for production will certainly depend on economic factors, in particular on the possibility to increase the selectivity to above 80% cost efficiently. At present, 80% of the process costs are dependent on the price of ethylene. Thus, any increase of selectivity can decide whether a new process will be used. For that, a reduction of cost per reactor volume for the microstructured reactor by several orders of magnitude is necessary. This work, however, was not undertaken to replace the industrial ethylene oxide process but to demonstrate the basic feasibility and compare the reaction in a microstructured catalytic reactor to a well-known industrial process. Possibly, these first promising results will initiate more research toward the adaptation of microsystems to industrial-scale processes.

4. Conclusions

Literature Cited

In general, the results which are obtained using a microreactor for ethylene oxide synthesis are comparable to the already known results of the industrial process. For the microreactor system, selectivities of up to 70% were found. The industrial process is operating with a higher selectivity of up to 80% if pure oxygen is used as the oxidant at similar conversion levels. However, these results were obtained in the presence of chlorine and usually other promoters on the optimized silver/R-Al2O3 catalyst, in contrast to the pure silver, which was used for nearly all investigations in this work. Adding DCE or working with R-Al2O3 as the support showed an increase in the selectivity of about 15%. Work with combining Ag/R-Al2O3 and DCE simul-

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Acknowledgment The authors thank Th. Scholl and A. Wolf at IMM for valuable help in microreactor development and D. Wolf (ACA) for valuable discussions. For the SEM pictures and sputter experiments, we thank H.-J. Bongard. Furthermore, the authors thank the BMBF (German Ministry of Education, Science, Research and Technology) for funding this work (16SV674/0 and 16SV671/8) and all of the partners in this project for their cooperation and discussions.

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Received for review April 2, 2001 Revised manuscript received October 24, 2001 Accepted October 30, 2001 IE010306U