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RESEARCH NOTES Direct Gas-Phase Epoxidation of Propene with Hydrogen Peroxide on TS-1 Zeolite in a Microstructured Reactor Elias Klemm,*,† Enrico Dietzsch,† Thomas Schwarz,† Thomas Kruppa,‡ Armin Lange de Oliveira,§ Frank Becker,§ Georg Markowz,§ Steffen Schirrmeister,| Ru1 diger Schu1 tte,§ Karl J. Caspary,| Ferdi Schu1 th,‡ and Dieter Ho1 nicke† Department of Chemical Technology, Chemnitz UniVersity of Technology, Strasse der Nationen 62, 09111 Chemnitz, Germany; Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mu¨lheim an der Ruhr, Germany; SerVice Unit Process Technology & Engineering, EVonik Degussa, Rodenbacher Chaussee 4, 63457 Hanau-Wolfgang, Germany; and Research and DeVelopment, Uhde GmbH, Friedrich Uhde Str. 15, 44141 Dortmund, Germany
Direct epoxidation of propene with hydrogen peroxide vapor was conducted in a microstructured reactor containing TS-1 catalyst coatings. At 140 °C, 1 bar, 5 vol % hydrogen peroxide, and 15 vol % propene, productivities of more than 1 kg of propene oxide per kg of catalyst and hour are obtained in lab scale, which is in an industrially highly relevant range. Excellent selectivities to propene oxide based on propene of >90% were reached. Potential and need for improvement lie in the propylene oxide selectivity based on hydrogen peroxide, which was observed to be about 25%. By increasing the molar excess of propene to about 6.6, propylene oxide selectivities related to hydrogen peroxide of up to 60% have been observed in the pilot plant. 1. Introduction At present, great efforts are made to replace the conventional propylene oxide production processes (chlorohydrin process and cooxidation process) by environmentally and economically more advantageous direct epoxidation processes. However, direct epoxidation of olefins, such as propene, which have allylic hydrogen atoms, poses a great challenge. Undesired allylic oxidation reduces the selectivity of the epoxide drastically. Thus, when using molecular oxygen as oxidant, even at the optimum reaction temperature of 250 °C, the maximum obtainable selectivity of propene oxide is only 64%. Still, this result has only been obtained at propene conversions less than 5% by using a highly modified silver catalyst with nitrogen monoxide and halogenides as cofeeds.1 Thus, direct epoxidation with molecular oxygen seems to be far away from an industrial implementation. Much more promising results were reported by Clerici and co-workers at the beginning of the 1990s by using aqueous hydrogen peroxide solution as an oxidant, methanol as a cosolvent, and TS-1 (Ti containing silicalite-1) as a catalyst.2 In a semibatch operated slurry reactor, 95% of the hydrogen peroxide loaded initially into the reactor reacted with propene after 90 min, reaching a final propene oxide selectivity related to propene of 90%. The experiment was performed at a reaction temperature of 60 °C and a propene pressure of 4 atm, which was held constant. The main byproducts are propylene glycol * To whom correspondence should be addressed. Phone: +49 371 531 31510. Fax: +49 371 531 21279. E-mail:
[email protected]. † Chemnitz University of Technology. ‡ Max-Planck-Institut fu ¨ r Kohlenforschung. § Degussa AG. | Uhde GmbH.
and monomethyl ethers, which originate from propene oxide because of consecutive acid-catalyzed reactions in the aqueous hydrogen peroxide, namely, the hydration of propene oxide to propylene glycol and its etherification with the cosolvent methanol to monomethyl ethers. Almost no hydrogen peroxide decomposition was observed. In recent years, Degussa and Uhde, as well as BASF and Dow, developed commercial processes based on these findings of Clerici and co-workers, and these companies have announced plans for production plants with start-ups scheduled for 2008.3 For the liquid-phase epoxidation of propene with hydrogen peroxide on TS-1, the following reaction scheme is widely agreed:4 • Step 1: Formation of an electrophilic oxygen species upon decomposition of hydrogen peroxide on the titanium lattice center:
H2O2 + Ti f Ti-〈O〉 + H2O
(1)
• Step 2: Transfer of the electrophilic oxygen species to the double bond (desired epoxidation reaction):
Ti-〈O〉 + C3H6 f C3H6O + Ti
(2)
• Step 3: Interaction of the electrophilic oxygen species with a second hydrogen peroxide molecule (undesired decomposition of hydrogen peroxide):
Ti-〈O〉 + H2O2 f O2 + H2O + Ti
(3)
For the active Ti-〈O〉 species, two structures are discussed in the literature (see Figure 1). First, a five-membered ring formed after hydrolysis of the Ti-O-Ti-bond by H2O2 and coordination
10.1021/ie071343+ CCC: $40.75 © 2008 American Chemical Society Published on Web 02/22/2008
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Figure 1. Active Ti-〈O〉 species discussed in literature (the electrophilic oxygen species that is transferred to propene, forming propene oxide, is marked in bold face).4
of one protic solvent molecule. Second, a Ti(η2-OOH) species formed after hydrolysis of the lattice Ti-O-Ti-bond by H2O2 without coordination of a solvent molecule. Parallel to the liquid-phase epoxidation of propene with hydrogen peroxide, Degussa and Uhde also investigated the possibility of performing this process in the gas phase. Both active Ti-〈O〉 species, the five-membered ring and the Ti(η2OOH) species, should also be formed upon interaction of hydrogen peroxide with the Ti lattice center in gas phase. In the case of the five-membered ring, water can act as the protic molecule. Thus, there is no reason why the described liquidphase mechanism should not also be valid in the gas-phase reaction. However, running a gas-phase process involving hydrogen peroxide vapor is extremely challenging, since both the evaporation of hydrogen peroxide and the processing of a propene/ hydrogen peroxide gas mixture needs special precautions to minimize the risk of explosions. Both of these risks can be significantly reduced by developing appropriate microstructured apparatuses. Thus, this reaction also demonstrates the need for the development of technical concepts for heterogeneously catalyzed gas-phase reactions in microstructured devices. The present contribution focuses on measurements in a singlechannel lab-scale microstructured reactor and the transfer of these results to a multichannel technical scale microstructured reactor (“scale-up/scale-out”). 2. Experimental Section 2.1. Reactor Concept. The so-called DEMIS-microstructured reactor concept has been developed by the companies Degussa AG and Uhde GmbH together with several partners from academia and was supported by German Federal Ministry of Education and Research (BMBF).5 It is a concept with a promising future in the production of bulk chemicals and intermediates through heterogeneously catalyzed gas-phase processes.6 Main features of the DEMIS concept are the modular design and the formation of an array of parallel slitlike reaction chambers between every two adjacent modules within a stack of modules (see Figure 2, right).7 In lab scale, one pair of platelets has been used that provides only one slitlike reaction chamber with the same dimensions except of the length of the slit, which was 0.1 m (see Figure 2, left) compared to 1 m in pilot scale (see Figure 2, right). Thus, from lab to pilot scale, one slitlike reaction chamber had to be scaled up concerning its length and further scaled out by parallelization. In order to achieve a successful scale-up/scale-out, inlet concentrations, temperatures, and the modified residence times τmod must be kept constant (see Section 2.4, Scale-up/Scale-out). 2.2. Catalyst. TS-1 was prepared according to Martens et al.8 The TiO2 content of the resultant TS-1 was 2.7% (w/w). A micropore volume of 0.17 cm3 g-1 was determined by nitrogen physisorption at 77 K. Both in lab scale and in pilot scale, grooves with a depth of up to 1 mm and a width of 2 cm have to be coated (see Figure 2). As coating technology, a spray
technology was used and a slurry of TS-1 and a colloidal silica binder was sprayed in multiple cycles on the bottom of the grooves.9 The thickness of the coating was 500 micron as well as the hydraulic diameter of the gas channel. 2.3. Catalytic Measurements. 2.3.1. Laboratory Setup. Hydrogen peroxide (50 wt %) was fed into a special glass evaporator with the help of a syringe pump and was completely evaporated at ∼100 °C using nitrogen as carrier gas. Degussa AG supplied a special H2O2 quality with an adapted stabilizer system to avoid decomposition and to prevent deposition of the stabilizers. The evaporator and the pipes had to be pickled and passivated by a special multistep procedure. The basics of this procedure are described in ref 10. The catalytic measurements were performed at a reaction temperature of 140 °C at atmospheric pressure and at modified residence times τmod between 0.02 and 0.2 gcat s mL STP-1. The standard feed specification was chosen as follows: 5 vol % H2O2, 10 vol % H2O, molar ratio C3H6/H2O2 ) 1-6, nitrogen as balance.11 It has to be emphasized that the catalytic laboratory measurements have been performed in almost identical setups at Chemnitz University of Technology (CUT) and at Max-PlanckInstitute of Mu¨lheim an der Ruhr (MPI). This strategy has been chosen to guarantee reliable and reproducible results by comparison of the results in two independent setups with two independent operators. 2.3.2. Pilot Plant. Hydrogen peroxide (50 wt %) was fed into a microstructured falling-film evaporator12 and was partially evaporated at ∼100 °C using nitrogen as carrier gas. Propene was added with the help of micromixers. In order to compare with the laboratory results, a campaign of catalytic measurements was conducted at the same reaction temperature, feed composition, and modified residence times as in the laboratory setup. 2.4. Scale-up/Scale-out. For the scale-up/scale-out (see also Section 2.1. Reactor Concept) of fixed-bed reactors, conversions and selectivities at the reactor outlet do not depend on the fluid dynamic residence time but rather on the so-called modified residence time, often also called space time. This can be deduced from the mass balance of an isothermal packed-bed plugflow reactor, assuming absence of film and pore-diffusion limitations:13
τmod )
dc
∫cc R (ci ) i
i0
i
(4)
i
The modified residence time (also called space time) is defined as follows:
τmod ) mcat/V˙
(5)
Equation 4 provides a rule for the scale-up, which says that the same concentrations ci of all species at the reactor outlet both in lab and industrial scales can be obtained if the modified residence time τmod is kept constant and if the inlet concentrations ci0, temperatures, and catalyst (i.e., kinetics Ri) are the same. Equations 4 and 5 can be analogeously derived for microreactors with catalyst coatings (see Figure 2). Thus, the same design rule can be used when channel dimensions are changed from lab to industrial scale (e.g., length of channel, see Figure 2). The production volume can be further increased at the same conversions and selectivities by parallelizing the channels (scale-
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Figure 2. Lab-scale microreactor platelet (left); pilot-scale microreactor plate (right).
Figure 4. Conversions of propene and hydrogen peroxide at various modified residence times τmod. Measurements were conducted at Chemnitz University of Technology (CUT) (T ) 140°, 5 vol % H2O2, 10 vol % H2O, molar ratio C3H6/H2O2 ) 3, nitrogen as balance). Figure 3. Measured productivities and selectivities of propene oxide at various modified residence times τmod in the laboratory microstructured reactors at MPI and CUT (T ) 140°, 5 vol % H2O2, 10 vol % H2O, molar ratio C3H6/H2O2 ) 3, nitrogen as balance).
out), provided that an equal distribution of the feed on the channels can be guaranteed. 3. Results Figure 3 shows the measured dependence of the propene oxide (PO) productivity of the catalyst and the selectivity of propene oxide related to converted propene versus the modified residence time at the standard feed composition (see Experimental Section) and a reaction temperature of 140 °C. As can be seen from Figure 3, the measured propene oxide selectivity related to propene was between 90% and 95%. Thus, in the gas-phase process, the same very high selectivities can also be obtained versus in the liquid-phase process. Contrary to the liquid-phase process, in the gas phase, propylene glycol and monomethyl ethers are not formed as byproducts, because more than likely, hydration of propene oxide to propylene glycol needs an aqueous acidic phase, which is, however, not present in the gas phase. In the gas phase, the main byproducts are acetaldehyde and carbon dioxide. It can be assumed that, at the higher temperatures of the gas-phase reaction compared to the liquidphase reaction (140 °C instead of 60 °C), total oxidation will be the main side reaction. Acetaldehyde could be an intermediate in the oxidative degredation of propene oxide. Catalyst productivities of at least 1 kg of propene oxide per hour and per kg of catalyst can be obtained, which is a prerequisite for the production of commodities like propene oxide with typical plant capacities of 10.000 up to some 100.000 metric tons per year. Figure 4 shows the corresponding conversions of propene and hydrogen peroxide. It can be seen that complete conversion
Figure 5. Long-term stability of the propene oxide productivity at a modified residence time τmod of 0.2 s gcat mL STP-1 (T ) 140 °C). Measurements were conducted at Chemnitz University of Technology (CUT) (T ) 140°, 5 vol % H2O2, 10 vol % H2O, molar ratio C3H6/H2O2 ) 3, nitrogen as balance).
of hydrogen peroxide and propene is obtained after a modified residence time of 0.12 gcat s mL STP-1. From the initial slopes of the conversion curves versus modified residence time, initial consumption rates of propene and hydrogen peroxide can be calculated: Ri,eff,o )
dci dXi mole i consumed |τmodf0 ) -ci0 | dτmod dτmod τmodf0 kg cat.‚s
[
]
(6)
For the initial consumption rate of propene, a value of 29.0 mol gcat-1 s-1 can be calculated; for the initial consumption rate of hydrogen peroxide, a value of 128.0 mol gcat-1 s-1 can be calculated. In the calculation of the latter value, the blind decomposition in the setup of ∼10% (offset on the ordinate at τmod ) 0 gcat s mL STP-1) has been considered. To get a better
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Figure 6. Selectivity-conversion plots related to converted propene (left) and to converted hydrogen peroxide (right) for different propene-to-hydrogen peroxide ratios measured both in laboratory setup and pilot plant (T ) 140°, 5 vol % H2O2, 20 vol % H2O, nitrogen as balance).
idea about the consumption rates, time constants of the reactions τRi can be calculated according to the following equation:
τR,i )
ci0 (s) Ri,eff,o‚mcat/Vgas
(7)
With a value of mcat/Vgas of ∼1 g/mL, a time constant for the hydrogen peroxide consumption of 27 ms can be determined. The time constant for the propene consumption is ∼4.3 times larger. Hydrogen peroxide decomposition according to eq 3 is the main side reaction of hydrogen peroxide (monitored by an oxygen sensor in the off-gas of the setup) and is responsible for the enhanced hydrogen peroxide consumption. From the final conversions in Figure 4 and considering the corresponding initial concentrations of propene (15 vol %) and hydrogen peroxide (5 vol %), a propene oxide selectivity related to hydrogen peroxide of 25% can be calculated for the chosen reaction conditions. From the initial consumption rates, the initial propene oxide selectivity related to hydrogen peroxide is only 16%. It seems that, with the progress of reaction and the corresponding increase of the propene-to-hydrogen peroxide ratio, selectivity is increasing (see also discussion of Figure 6). One should note that the experiments shown in Figure 3 have been carried out in two similar, but not identical, reactor setups at the Chemnitz University of Technology (CUT) and at MaxPlanck-Institute of Mu¨lheim an der Ruhr (MPI), and as one can see from Figure 3, the recorded data obtained in both laboratories agree well with each other. Catalyst deactivation and lifetime of the catalyst are key questions that need to be clarified for the development of a new propene oxide process based on the heterogeneously catalyzed epoxidation of propene with hydrogen peroxide in the gas phase over TS-1. Thus, long-term catalytic measurement over a time period of 300 h have been performed. The above specified standard feed composition and a modified residence time of 0.2 gcat s mL STP-1 were chosen for these experiments. The reaction temperature was set to 140 °C. From Figure 5, it can be seen that, after an initial deactivation of ∼40% within the first 100 h, the activity remains almost constant up to >300 h time-on-stream. As the main deactivation mechanism, a blocking of the surface of the TS-1 catalyst seems to be most likely, because micropore volume decreased from an initial value of 0.17 cm3 g-1 to a final value of about 0.03 cm3 g-1. Elemental analysis of the spent catalyst gave a total formula of C2H2nO0.3n, which suggests the formation of polyether glycols through polymerization of propene oxide. In lab experiments, it could
be shown that ∼80% of the micropore volume can be restored by purging 3 h with 5 vol % H2O2 in nitrogen at reaction temperature, i.e., by shuting down the propene supply. Figure 6 shows a comparison of the measured selectivityconversion dependencies that have been measured in laboratory and pilot-plant scale for almost the same reaction conditions. The propene oxide selectivities related to propene roughly coincide on both scales with a little bit higher selectivities in the pilot-plant scale. The propene oxide selectivities related to hydrogen peroxide are distinctly higher in the pilot plant compared to the lab setup. The propene oxide selectivity related to hydrogen peroxide can be further increased to ∼60% by increasing the molar excess of propene to 6.6 (see Figure 6, right). On the other hand, both selectivities strongly drop for a stoichometric ratio of 1 (see Figure 6, left and right). This increase of the hydrogen peroxide efficiency with increasing molar excess of propene suggests that a significant part of hydrogen peroxide decomposition takes place on the catalyst itself (eq 3). Unfortunately, it was not possible in the present work to separate exactly the decomposition of hydrogen peroxide on the catalyst from that in the apparatus. Ongoing experiments in a new lab reactor where the catalyst coating is located directly opposite of the falling film will deliver an answer to that question. However, from an economic point of view, the total hydrogen peroxide efficiency, which includes the decomposition in the apparatus, is pivotal. 5. Conclusions The direct epoxidation of propene can be successfully performed with hydrogen peroxide vapor using a microstructured reactor with TS-1 catalyst coating. Productivities of >1 kg of propene oxide per kg of catalyst and hour could be observed, which are in an industrially highly relevant range. Excellent selectivities to propylene oxide based on propene of >90% were found. Potential and need for improvement lie in the propylene oxide selectivity based on hydrogen peroxide, which was observed to be ∼25% at standard conditions in the laboratory setup. By increasing the molar excess of propene to ∼6.6, propylene oxide selectivities related to hydrogen peroxide of up to 60% have been observed in the pilot plant. However, for economic reasons, a hydrogen peroxide efficiency of at least 90% must be reached in order to compete with the liquid-phase epoxidation process described in the introduction.
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Acknowledgment We gratefully acknowledge the BMBF (Federal Ministry of Education and Research) for granting financial support to the project DEMIS. Literature Cited (1) Cooker, B.; Gaffney, A. M.; Jewson, J. D.; Onimus, W. H. (Arco Chemical Technology L.P.) U.S. Patent 5,780,657, 1997. (2) Clerici, M. G.; Bellussi, G.; Romano, U. Synthesis of Propylene Oxide from Propylene and Hydrogen Peroxide Catalyzed by Titanium Silicalite. J. Catal. 1991, 129, 159. (3) Ford, B. HPPO races to Commercialization. Chem. Market Rep. 2006, 269, 24. (4) Clerici, M. TS-1 and Propylene Oxide, 20 Years later. Oil Gas Eur. Mag. (Int. Ed. Erdo¨l Erdgas Kohle) 2006, 32, 77. (5) Markowz, G.; Schirrmeister, S.; Albrecht, J.; Becker, F.; Schu¨tte, R.; Caspary, K. J.; Klemm, E. Microstructured Reactors for Heterogeneously Catalyzed Gas-Phase Reactions on an Industrial Scale. Chem. Eng. Technol. 2005, 28, 459. (6) Klemm, E.; Do¨ring, H.; Geisselmann, A.; Schirrmeister, S. Microstructured Reactors in Heterogeneous Catalysis. Chem. Eng. Technol. 2007, 30, 1615. (7) Markowz, G.; Albrecht, J.; Ehrlich, J.; Jucys, M.; Klemm, E.; Lange de Oliveira, A.; Machnik, R.; Rapp, J.; Schu¨tte, R.; Schirrmeister, S.; von Morstein, O. (Uhde GmbH and Degussa AG) WO2004/091771, 2004.
(8) Martens, J. A.; Buskens, P.; Jacobs, P. A.; van der Pol, A.; van Hooff, J. H. C.; Ferrini, C.; Kouwenhoven, H. W.; Kooyman, P. J.; van Bekkum, H. Hydrogenation of Phenol with Hydrogen Peroxide on EUROTS-1 Catalyst. Appl. Catal., A 1993, 99, 71. (9) Schirrmeister, S.; Bu¨ker, K.; Schmitz-Niederau, M.; Langanke, B.; Geisselmann, A.; Becker, F.; Machnik, R.; Markowz, G.; Schwarz, T.; Klemm, E. (Degussa AG and Uhde GmbH) WO 2006/111340, 2006. (10) Schumb, W. C.; Satterfield, C. N.; Wentworth, R. L. ACS Monogr. Ser. 1955, 523. (11) Schuette, R.; Markowz, G.; Esser, P.; Balduf, T.; Thiele, G.; Hasenzahl, S. (Degussa AG) DE 100 02 514, 2000. (12) Klemm, E.; Albrecht, J.; Lange de Oliveira, A.; Markowz, G.; Gross, S.; Schu¨tte, R.; Ehrlich, J.; Schirrmeister, S. (Uhde GmbH and Degussa AG) WO2004/036137, 2004. (13) van Santen, R. A.; van Leeuwen, P. W. N. M.; Moulijn, J. A.; Averill, B. A. Catalysis: An Integrated Approach. Stud. Surf. Sci. Catal. 1999, 123, 390.
ReceiVed for reView October 5, 2007 ReVised manuscript receiVed January 9, 2008 Accepted January 31, 2008 IE071343+
