Ind. Eng. Chem. Res. 2002, 41, 4921-4927
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APPLIED CHEMISTRY Flame-Made Titania/Silica Epoxidation Catalysts: Toward Large-Scale Production Wendelin J. Stark,†,‡ Hendrik K. Kammler,† Reto Strobel,†,‡ Detlef Gu 1 nther,§ Alfons Baiker,‡ and Sotiris E. Pratsinis*,† Particle Technology Laboratory, Department of Mechanical and Process Engineering, ETH Zentrum ML F 26, Sonneggstrasse 3, CH-8092 Zurich, Switzerland, Laboratory of Technical Chemistry, ETH Ho¨ nggerberg, CH-8093 Zurich, Switzerland, and Laboratory of Analytical Chemistry, ETH Ho¨ nggerberg, CH-8093 Zurich, Switzerland
Titania/silica catalysts were made continuously in flame reactors and evaluated for the demanding epoxidation of 2-cyclohexenol with tert-butylhydroperoxide. Silica- and titaniaprecursors were evaporated separately and mixed prior to feeding into a hydrogen-air diffusion flame. Upon reaction, the corresponding oxides formed highly agglomerated nanoparticles with titania confined to the surface of silica particles. The epoxidation activity was strongly dependent on the titania content of the catalyst. Scaling-up the production rate of these catalysts (6-500 g/h) in the flame did not affect their exceptionally high catalytic performance. Sample composition and homogeneity were determined by local analysis using laser ablation inductively coupled plasma mass spectroscopy. Large-scale flame-made catalysts (100-500 g/h) were compared to similar catalysts made in a laboratory flame microreactor (6 g/h) and to conventional ones prepared by wet-phase chemistry. Compared to classical catalysts (65-80% selectivity), the flamemade titania/silica showed excellent selectivity (up to 90%) at 80% conversion. Introduction Alkene epoxidation is a widely used reaction in industrial organic synthesis. Epoxides are key raw materials for a variety of products such as polymers, surfactants, and many pharmaceuticals. Much effort is devoted to the development of new catalysts with high selectivity to avoid the formation of large amounts of undesirable byproducts such as glycols and ketones.1-3 While the simplest alkene, ethylene, can be oxidized using oxygen on a supported silver catalyst, this method cannot be applied to larger alkenes.4 Titania in a silica matrix is one of the most selective systems for epoxidation using organic or hydrogen peroxide.5 Typically, epoxidation catalysts are made batchwise by wet chemistry at low temperatures followed by filtration, drying, and calcination to eliminate undesired residuals and activate and homogenize the catalyst surface.6 Wet-phase-made catalysts often have limited thermal and mechanical stability. Titania/silica aerogels can be prepared with specific surface areas as high as 1000 m2/g; however, these materials lack considerable long-term stability.7,8 Catalyst synthesis at high temperature favors dispersion of the active material on a carrier oxide, leading to enhanced activity and better selectivity.6 Flame aerosol synthesis, in particular, is quite attractive for the preparation of catalysts with well * Corresponding author. E-mail:
[email protected]. Fax: +411 632 1595. http://www.ivuk.ethz.ch/staff/pratsinis. † ETH Zentrum ML F 26. ‡ Laboratory of Technical Chemistry, ETH Ho ¨ nggerberg. § Laboratory of Analytical Chemistry, ETH Ho ¨ nggerberg.
defined characteristics, as it is a reasonably well understood process for manufacture of commodities.9,10 As a result, flame technology has been used on a laboratory scale to make photocatalysts for polymerization11 or for the photodegeneration of aqueous phenol12 and salicylic acid,13 vanadia phosphorus oxide for maleic anhydride synthesis,14 vanadia coated titania for the selective catalytic reduction of NO by ammonia,15 and platinum on titania for SO2 oxidation.16 Very recently, epoxidation catalysts (0.5-4 wt % TiO2/silica) with excellent selectivity were made in a flame microreactor.17 Even though the above studies showed that flamemade mixed oxide catalysts are, at least, as good as, if not better than, their wet-phase-made counterparts, they are not used in industrial scale catalysis yet. One reason is that large quantities of these flame-made mixed oxides are not available in the market, as is, for example, Degussa’s flame-made P25 titania, that is widely used in photocatalysis. The excellent selectivity and the high stability of some of these flame-made catalysts could make them the market standard if their superior performance in the laboratory could be demonstrated on a large scale. The present study aims to fill this void by exploring continuous synthesis and evaluation of titania/silica epoxidation catalysts on a relatively large flame reactor.18 A diffusion flame reactor is preferred here over premixed or counterdiffusion flame reactors for its flexibility in safe synthesis of particles with a broad spectrum of characteristics that are of prime interest in catalysis.10 With respect to laboratory flame reactors, here, the production rate is increased by 2 orders of
10.1021/ie020200e CCC: $22.00 © 2002 American Chemical Society Published on Web 09/25/2002
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Table 1. Experimental Conditions of Large-Scale Productiona
exptb 0Ti150 0.1Ti150 0.4Ti150 0.8Ti150 1.1Ti150 1.4Ti150 1.9Ti150 3.8Ti150 1Ti100 1.2Ti200 1.2Ti250 1.2Ti300 1.2Ti350 1.2Ti400 1.2Ti500 1.2Ti250 1Ti350 0.7Ti500
Ti content/ wt %
prodtn rate/ g h-1
BET areac/ m2 g-1
H2 flow rate/ m3 h-1
air flow rate/ m3 h-1
0.004d 0.12 0.33 0.80 1.03 1.35 1.84 3.75 0.97 1.19 1.12
150 150 150 150 150 150 150 150 100 200 250 300 350 400 500 250 350 500
218 214 211 203 195 191 181 161 190 174 165 153 150 144 142 159 152 132
0.73 0.73 0.73 0.73 0.73 0.73 0.73 0.73 0.47 0.97 1.21 1.42 1.68 1.89 2.38 1.21 1.68 2.38
5.19 5.19 5.19 5.19 5.19 5.19 5.19 5.19 3.32 6.85 8.52 10.1 11.9 13.4 16.7 8.52 11.9 16.7
1.27 1.17 1.20 1.00 0.69
a The air number λ ) 1.45 and the C/H ratio ) 0.096 are kept constant in all experiments. b Sample names are given as xTiy, where x denotes the titania content in weight percent titania and y denotes the mixed oxide production rate. c Typical reproducibility errors are (3%. d Limit of detection.
magnitude (up to 500 g/h). Subsequently, the product powders were tested for the demanding epoxidation of 2-cyclohexenol, an excellent model system for probing the epoxidation of allyl-alcohols and alkenes, both major industrial processes for the production of basic chemicals.4 The effect of catalyst composition and production rate on product selectivity and rate of conversion is systematically examined and related to catalyst characteristics and process synthesis. Experimental Section Catalyst Synthesis and Characterization. The experimental setup consists of the precursor supply unit, the burner, and the filter for particle collection.18 Hexamethyldisiloxane (HMDSO, Fluka, 99%, dried over molecular sieves 4A, distilled prior to use) was evaporated using a bubbler setup. To introduce titania, titanium tetraisopropoxide (TTIP, Aldrich, >97%, distilled under vacuum) was fed through a liquid mass flow controller (Bronkhorst Liqui-Flow L1) and vaporized into a nitrogen stream using a commercial evaporator (Bronkhorst CEM 100W). The accurate delivery of this setup has been shown recently.15 After the HMDSO containing gas stream was heated, the two precursors were mixed and fed into the central tube of the burner. Gas flow rates were monitored using calibrated gas mass flow controllers (Hastings Inc., HFC 203) and rotameters (Vo¨gtlin Instruments). A modified hydrogen-air burner (Deutsche Forschungsanstalt fuer Luft- und Raumfahrt e. V.) with four concentric tubes of 0.5 mm wall thickness for gas delivery was used.18 The spacing between the three tubes was 1, 0.5, and 3.5 mm. The HMDSO vapor-laden nitrogen (13 L/min, Pan Gas, 99.995%) was fed through the innermost tube (inner diameter 6 mm) into the diffusion flame. Hydrogen (Pan Gas, 99.5%) was provided through the inner two annuli, while air (PanGas, 99.5%) entered through the outermost annulus. The flow rates of hydrogen and air are given in Table 1. All gases were preheated to 150 °C to prevent precursor condensation. The flame was ignited with a spark plug that
was withdrawn after ignition. The flame was encased in a 350-mm steel cylinder (chimney) to ensure stable burning. Particle laden gases from the burner were filtered through a commercial jet filter unit (Friedli AG, FRR 4/1.6, 2.5 m high, 0.6 m diameter), equipped with four PTFE coated Nomex baghouse filters covering a total surface of 2.2 m2. The particles were removed from the filter bags by periodic pressure shocks and collected into a removable container at the bottom of the filtration unit.18 The specific surface area of the product powders was analyzed by nitrogen adsorption at 77 K using the BET method (Micrometrics Gemini 2360). Transmission electron microscopy (Hitachi H 600) was used to investigate the particle morphology. The Ti dispersion was confirmed by electron sensitive mapping (CM30ST microscope, Philips; LaB6 cathode, operated at 300 kV, point resolution ∼4 Å). X-ray diffraction patterns were collected on a Bruker AXS D8 Advance to check for titania crystallites. Catalyst thermal stability was checked by heating samples of 1 g to a set temperature for 2 h and measuring specific surface area and catalytic activity. The sample composition, the distribution of the elements, and especially the Ti content within various prepared samples were analyzed by laser ablation inductively coupled mass spectrometry. An ArF 193 nm excimer laser (MicroLas, Go¨ttingen) coupled to an ELAN 6100 DRC ICP-MS was used for the local analysis. The major advantage of the system is the use of homogenizing optics, allowing the controlled removal and transport of material into the excitation source (ICP-MS). Further details about instrumentation applied to local analysis are given by Gu¨nther et al.19,20 The crater diameter caused by the laser was adjusted to 80 µm, and a fluence of 20 J/cm2 (at 10 Hz) was used for the ablation of the material. Therefore, the transient recorded signals in the ICP-MS indicate the sample composition of the sample. The removal rate of the sample is almost constant (for a depth-to-diameter ratio of 1:1), which allows the determination of the changes in concentration.21 Catalyst Evaluation. The epoxidation reactions were carried out batchwise in a mechanically stirred, 50-mL, thermostated glass reactor equipped with a thermometer, a reflux condenser, and a septum for withdrawing samples. All reactions were performed under nitrogen (99.999%) to avoid the presence of oxygen or moisture. Figure 1 gives an overview of the reactions involved in the epoxidation of 2-cyclohexenol by TBHP.2 In a standard procedure, 200 mg of nanoparticles was predried in situ in the reactor under flowing nitrogen for 15 min at 423 K. After cooling, 8.05 mL of the solvent toluene (Fluka, 99.8%, stored over molecular sieve 4A) and 0.5 mL of dodecane (Fluka, 99%) as internal standard were added. The mixture was heated to 363 K, and 1 mL of 2-cyclohexenol (Fluka, 99%) was injected. The reaction was started by introducing 0.45 mL of tert-butylhydroperoxide (TBHP, Fluka, 5.4 M in decane, stored over molecular sieve 4A). The total reaction volume was 10 mL. Tests for Ti leaching were carried out for reaction mixtures after 50% peroxide conversion.22 About 2 mL of the reaction mixture was filtered hot through a dry, preheated membrane filter (pore diameter 0.2 µm). The filtrate without catalyst was kept at 90 °C, and the concentrations of all species were monitored as for normal runs. The mixtures were analyzed using a Trace
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Figure 1. Epoxidation of 2-cyclohexenol (1) by TBHP, mainly forming 2,3-epoxycyclohexanol (2), with small amounts of the corresponding ketone (4) being formed as a major byproduct (7%). The side reactions leading to the dimers (3 and 5) as well as condensation to oligomers are of minor importance (here, less than 1% formed). Adapted from ref 2.
2000 gas chromatograph equipped with a cool on-column inlet and a HP-FFAP capillary column. Products (see Figure 1) were identified by GC-MS and by comparison with authentic samples. In all runs, the internal standard method was used to quantitatively analyze all components.17 The standard way to compare epoxidation catalysts is the notation of rate and selectivity.4 The epoxide (compound 2, Figure 1) selectivity, related to the olefin (compound 1, Figure 1) consumed, Schxol, and the peroxide selectivity, Sperox, related to the TBHP consumed, are calculated as follows:
Schxol(%) ) 100{[epoxide]/([chxol]0 - [chxol])} Sperox(%) ) 100{[epoxide]/([TBHP]0 - [TBHP])} where the subscript 0 stands for the initial value and all concentrations are expressed on a molar basis. Both values are determined at 50% conversion. The rate of epoxide formation is determined after 30 min and given as millimoles of epoxide formed per gram of catalyst in 1 h.17 Results and Discussion Table 1 gives a summary of the experiments for production of titania/silica conducted at a constant carbon-to-hydrogen ratio C/H ) 0.096 and a constant air number λ ) 1.45, defined as the ratio of moles of oxidant present in the flame over the moles of oxidant required to fully convert the fuel23 and prevent carbon contamination of the catalyst.18 A first series of experiments served for determination of the optimal Ti content for epoxidation. The titania content was varied from 0 to 3.8 wt % at a catalyst production rate of 150 g/h (0Ti150 to 3.8Ti150). In a second series, the as-found optimal titania content of about 1.1 wt % was kept constant and the influence of the mixed oxide production rate on the catalytic performance was investigated from 100 to 500 g/h (1Ti100 to 1.2Ti500). In a third series of experiments, the Ti surface loading was kept around 0.7 µmol of Ti/m2, to study the effect of decreasing specific surface area with increasing production rate (1.1Ti150 to 1.1Ti250 to 0.7Ti500).
Influence of the Ti Content. Table 1 gives also the Ti content of the catalysts, the specific surface area (SSA), and the employed gas flow rates during catalyst synthesis. Figure 2 gives the BET surface area of titania/silica powders produced at 150 g/h as a function of the Ti content. This figure also shows pictures of the 40 cm long, luminescent hydrogen-air flames with increasing Ti delivery into the flame. A strong orange emission is seen with increasing titanium concentration. A continuous decrease of the SSA from 220 m2/g for pure silica with increasing Ti content is observed, consistent with similar experiments in a flame microreactor.17 Adding titania to the silica can decrease its melting point and enhance sintering. Therefore, particles containing increasing amounts of dopant TiO2 still coalesce at temperatures below the solidification of pure silica, resulting in a longer growth process and consequently larger particles with lower specific surface area. This effect has already been observed in the reverse case of titania doped by silica: Akhtar et al.24 found an increase in specific surface area when doping titania with increasing amounts of silica in the vapor phase oxidation of the corresponding halides in a hot wall reactor. Vemury and Pratsinis25 further confirmed this during the synthesis of silica/titania in laminar diffusion flames. In Figure 3, transmission electron microscopy reveals highly agglomerated silica nanoparticles. High-resolution TEM with down to 4 Å resolution showed the absence of any titania clusters. No single titania clusters or enclosures are discernible, corroborating a good titania distribution. Furthermore, none of the Ti doped silica powders exhibited any reflections in XRD, indicating that the titania may be dispersed on the silica surface. For the microreactor-made titania/silica,17 it was shown that Ti formed small isolated clusters with silica, such as [Ti(OSi)4] and [Ti(OH)(OSi)x]. The preferred formation of tetrahedral Ti sites and the lack of titania crystallites in flame-made Ti/silica could be confirmed recently by in-situ X-ray absorption near edge spectroscopy (XANES).26 The high viscosity of silica and the high concentration in the flame favor the formation of aggregates,9 which are desirable for particle collection and catalyst operation, since open agglomerate structure exhibits less pressure drop than compact particles. Figure 4 gives the rate of 2-cyclohexenol (see Figure 1, compound 1) consumption as a function of the Ti content for samples produced at 150 g/h. A steep increase in rate with increasing Ti content is found. At high titania content (3.8%), rates of conversion reach up to 30 mmol/g‚h. Powders produced in the microreactor show a fast, similar increase in rate.17 The selectivity of the reaction for microreactor-made catalysts strongly depends on the Ti content. In the largescale reactor, a maximum is found for catalysts with a Ti content of about 1%. Here, both peroxide and olefin selectivities are above 90%. Because of the high preparation temperature, the flame-made catalysts are remarkably stable against sintering. The catalyst 1.1Ti150 could be heated to 600 °C for 2 h without a significant change in specific surface area and retained its full catalytic performance. Influence of the Production Rate. Since the catalytic activity of titania/silica depends on the surface of the nanoparticles, two series of experiments for the variation of the production rate were performed. Powders containing about 1.2 wt % titania were produced
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Figure 2. Specific surface area (BET) for silica nanoparticles containing increasing amounts of titania. The size of the particles grows because of the enhanced sintering properties of the mixed oxide nanoparticles, and consequently their specific surface area declines. The pure flame (left) is transparent, while flames producing up to 3.8 wt % TiO2/silica (right) show strong emission of orange light, typical for titania.
Figure 3. Specific surface area for silica with 1.2 wt % titania produced at different rates. A high production rate is achieved by providing a larger precursor HMDSO concentration into the flame. This results in longer particle residence time at higher temperature and higher particle concentration that favors particle growth, thus decreasing the specific surface area. Inset: Transmission electron micrograph of a typical flame-made catalyst. The silica support forms highly agglomerated nanoparticles of 10-30 nm diameter. No crystalline titania particles are discernible.
at production rates ranging from 100 to 500 g/h at constant air number λ ) 1.45 by increasing the HMDSO and TTIP concentrations and adjusting the gas flows (see Table 1). The specific surface areas of these materials are given in Table 1 (1Ti100 to 1.2Ti500) and compared in Figure 3. With increasing production rate, the specific surface area decreased from around 190 to 140 m2/g. The high-temperature residence time of the nanoparticles increases with increasing production rate, since the flames are longer and hotter.18 Higher precursor concentrations in the flame result in larger particles with smaller specific surface area. The same decrease
Figure 4. Catalytic properties of titania/silica particles with different Ti contents. The rate of reactant conversion (right axis) increases with Ti loading. The selectivity to peroxide consumption (left axis, triangles) and olefin usage (circles) strongly depends on the Ti content. A maximum is found at a titania content of about 1 wt %.
in specific surface area is observed in production of pure silica when increasing its production rate in the flame reactor.18 Figure 5A shows the selectivity of catalysts made at powder production rates from 100 to 500 g/h. The selectivity to epoxide, with regard to olefin consumption, Solefin, is generally around 90% and very stable over a broad range of conditions. The rate of olefin conversion remains constant when the production rate for 1.2 wt % Ti/silica is increased and stays around 10 mmol/g‚h (not shown in Figure 5A). Only catalyst production at 100 g/h leads to a material of lower selectivity (Figure 5A). This may be attributed to an uneven Ti surface distribution resulting from insufficient heat provided by the low precursor and fuel concentrations in this flame.
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Figure 5. Catalytic behavior of nanoparticles as a function of production rate with about 1 wt % titania (A) and about 0.7 µmol Ti/m2 (B). The selectivity is constant around 90% both in terms of peroxide and olefin usage for samples above 100 g/h and illustrates that flame aerosol synthesis can be successfully applied to catalyst production over a broad range of conditions.
A similar trend is observed for peroxide selectivity, Sperox, which is mostly around 90% for catalyst production rates larger than 100 g/h. The 100 g/h sample, however, only uses some 50% of the peroxide to form the product epoxide. The rate of olefin conversion remains constant when the production rate for 1.2 wt % Ti/silica is increased and stays around 10 mmol/g‚h (not shown in Figure 5A). To investigate the influence of the production rate on the catalytic activity independently of the decrease of specific surface area, samples with similar surfaces (i.e. similar Ti/Si ratio at the surface, assuming all Ti stays on the surface17) were prepared up to 500 g/h. The optimal Ti content was found for sample 1.1Ti150 and measured 1.03 wt % titania on a silica powder with 195 m2/g specific surface area. This corresponds to a Ti surface loading of about 0.7 µmol Ti/m2. Since microreactor-made powders17 showed excellent performance for this Ti surface loading (Table 2), a similar ratio was preserved in catalysts produced at higher powder production rates by adjusting the Ti precursor delivery (Table 1: 1.1Ti150, 1.2Ti250, 1Ti350, 0.7Ti500). The catalytic properties of these materials are shown in Figure 5B. The selectivity is generally high and shows a maximum at 350 g/h. Here, both selectivities with regard to peroxide and olefin usage are above 90%, showing an efficient reaction. Selectivity is lower for sample 0.7Ti500, still a good catalyst (Figure 5B), despite its similar Ti/Si surface ratio (selectivity above 70%). From Figure 4, the selectivity of a material containing 0.7 wt % Ti, produced at 150 g/h, may be estimated to roughly 50%, much lower than that of 0.7Ti500. This clearly shows the importance of keeping the Ti surface loading around 0.7 µmol/m2 to produce catalysts with high selectivity. The rate of olefin conversion of catalysts in this series of experiments drops with decreasing Ti content (1.1Ti150 down to 0.7Ti500), as expected from the behavior in Figure 4, since less Ti is present.
Unstable TTIP delivery or segregation of the two precursors may affect the homogeneity of the catalyst and lead to reduced selectivity. The laser ablation ICPMS analysis gave the Ti/Si elemental distribution in the sample as a depth profile. Catalyst 1.1Ti150 showed excellent parallel Ti/Si signal intensities over more than 400 laser shots, indicating a rather homogeneous distribution of both elements on the micrometer scale. The decreased selectivity of 1Ti100 could not be traced to Ti/Si inhomogeneity, since the depth profile has a coarse resolution if compared to the mean particle size. However, samples 1.1Ti150, 1Ti100, and 1Ti350 contain a similar amount of titania and may be compared to illustrate the importance of the synthesis conditions. While 1.1Ti150 and 1Ti350 are excellent catalysts, the sample 1Ti100 has low selectivity. This cannot be due to different specific surface area, since 1.1Ti150 with 195 m2/g and 1Ti100 with 190 m2/g have almost the same specific surface area. However, 1.1Ti150 and 1Ti350 have a much different specific surface area; still, both show high selectivity. This underlines the importance of sufficient heat delivery during the synthesis. Comparison of Microreactor and Large-Scale Flame Synthesis of Catalysts. Table 2 provides a comparison of powder production in the microreactor and the large-scale reactor. While the microreactor operates at low Re, the large-scale reactor results in highly turbulent flames with large Re.18,27 The ratio of carbon to hydrogen in the flame is kept at 0.096 in the large-scale reactor, since carbon formation has to be suppressed.18 In the microreactor, carbon formation is not a problem, since pure oxygen is used, leading to highly oxidizing conditions and high temperature. To sustain rapid precursor combustion during flame synthesis, oxygen has both to reach the fuel and to be sufficient for complete combustion. The λ is varied between 1.5 and 7.5 in the microreactor when increasing the oxygen flow rate. Catalysts with good selectivity were obtained with more than 6 L of O2/min and 0.5 L of CH4 /min, corresponding to λ ) 4.5.17 Interestingly, the low Re in the microreactor can be compensated by using a higher λ. The Re number accounts for the flow conditions (turbulence), giving an estimate of the gas mixing in the flame. Similarly, the air number serves as an estimate of the availability of oxygen for complete combustion. Table 3 compares the catalyst activity and selectivity for catalysts made in the microreactor (production rate 6 g/h) and in the large-scale setup (production rate 100500 g/h). Catalysts with excellent activity can be produced in flames at rates up to 500 g/h. The catalytic activities (selectivity and rate of conversion) of powders from both reactors were very similar. The catalytic measurement is an extremely sensitive tool and allows comparison of surface properties.2,3 A critical parameter in the production of titania/silica catalysts for epoxidation is the formation of Ti clusters on the surface, leading to lower selectivity. This clustering is enhanced by long residence time, that is, long flames, at low O2 availability and low temperature, leading to insufficient homogenization of the surface. The latter may be
Table 2. Comparison of Microreactor and Large-Scale Production
microreactor (6 g/h) large reactor (500 g/h)
burner size/mm
Reynolds number, Re
air number, λ
C/H ratio
BET area/ m2 g-1
optimal Ti loading/ µmol m-2
5.5 19
460-1500 4500-18000
1.51-7.47 1.46
0.28 0.096
70-300 90-220
0.7-1.7 0.7
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Table 3. Epoxidation of 2-Cyclohexenol by TBHP or H2O2 with Different Titania/Silica Catalysts catalysta
production rate/g h-1
T/K
conv/%
Sperox/%
Solefin/%
6
363
1.03 wt % TiO2
150
363
1.2 wt % TiO2
500
363
50 80 50 80 50 80 84 85 85
96 93 95 93 88 85 84 65
88 83 90 90 90 85
3.2 wt % TiO2
Shell-type (4 wt % TiO2) aerogel (20 wt % TiO2) TS-1/H2O2 (1.2 wt % TiO2)
383 363 330
67 80
rateb/mmol g-1 h-1
ref
14 17 10 10 after 20 hc 61 5.5
this work 5 28 29
a TBHP as an oxidant if not specified. b After 0.5 h. c Duration of experiment; no rate reported. The selectivity and conversion were reproducible within (3%.
responsible for the rather poor performance of the catalyst 1Ti100. Comparison to Conventional Epoxidation Catalysts. Table 3 gives also the compositions of some representative wet-phase-made epoxidation catalysts as well as their activities and selectivities for epoxidation of 2-cyclohexenol. The table gives the temperature of the reaction mixture, the conversion at which the selectivities Sperox and Solefin are measured, and the overall reaction rate. The first such catalyst (Shell-type) is prepared according to the patent literature and consists of Ti grafted onto a silica surface.5 In the second catalyst, an aerogel, considerably higher Ti content is used and it is prepared by controlled hydrolysis of titanium and silicon precursors.8,30,31 The third catalyst, TS-1, is a titania-substituted silicalite, a molecular sieve type material with excellent activity for epoxidation using aqueous hydrogen peroxide.32 No data on the peroxide selectivity are given. Table 3 gives the selectivities of two large-scale flamemade titania/silica catalysts (150 and 500 g/h). They are the only catalysts with peroxide selectivities around 90% (top entries in Table 3). Furthermore, the catalyst 1.1Ti150 shows an excellent olefin selectivity (90%) in addition to a peroxide selectivity of 93%. These catalysts make better use of both reactants, olefin and peroxide, and produce less byproduct, thus reducing the amount of waste generated in the process. The increased selectivity of flame-made catalysts is attributed to a low hydration of the surface during their high temperature preparation.17 It is known from materials prepared by wet chemical routes that careful drying and heat treatment prior to testing lead to greatly improved catalytic behavior.33 Flame-made materials do not require this time-consuming pretreatment procedure and can be directly used after production. Conclusions Highly selective titania/silica catalysts for the liquid phase epoxidation of allyl-alcohols can be produced up to 0.5 kg/h in flame aerosol reactors. The material properties are conserved when increasing the production rate from 6 g/h in a microreactor to 0.5 kg /h in a largescale setup. Compared to conventional wet-phase prepared catalysts, the powders show an increased selectivity, which is attributed to a dehydrated surface. A comparison of the two flame reactors shows that highly active catalysts can be made for a broad range of conditions. Local analysis of the distribution of the elements (Ti/Si) underlined the importance of obtaining a homogeneous Ti dispersion within the powder. The catalytic behaviors of products from both microreactor and large-scale reactor are very similar, indicating that
the surface structure of the catalyst can be preserved even at high production rates. The broad window of operation in the large-scale flame reactor shows that flame aerosol synthesis is well suited for the production of titania/silica catalysts. The flame aerosol synthesis is a continuous process and offers an attractive alternative to conventional batch wet-phase catalyst preparation. Acknowledgment We thank Dr. Frank Krumeich (ETH) for the TEM measurements. Financial support by the Swiss National Science Foundation (ETH Gesuch Nr. 19/01-1) and the Swiss Commission for Technology and Innovation (Top Nano 21, 5352.1) is kindly acknowledged. Literature Cited (1) Sheldon, R. A.; Wallau, M.; Arends, I.; Schuchardt, U. Heterogeneous Catalysts for Liquid-Phase Oxidations: Philosophers' Stones or Trojan Horses? Acc. Chem. Res. 1998, 31, 485. (2) Beck, C.; Mallat, T.; Bu¨rgi, T.; Baiker, A. Nature of Active Sites in Sol-Gel TiO2-SiO2 Epoxidation Catalysts. J. Catal. 2001, 204, 428. (3) Beck, C.; Mallat, T.; Baiker, A. On the Limited Selectivity of Silica-Based Epoxidation Catalysts. Catal. Lett. 2001, 75, 131. (4) Dusi, M.; Mallat, T.; Baiker, A. Epoxidation of Functionalized Olefins over Solid State Catalysts. Catal. Rev.sSci. Eng. 2000, 42 (1 and 2), 213. (5) Wattimena, F.; Wulff, H. P. British Patent 1,249,079 (to Shell), 1971. (6) Moser, W. R.; Jach, J. D. L.; Cnossen, E.; Fraska, K.; Schoonover, J. W.; Rozak, J. R. The Preparation of Advanced Catalytic Materials by Aerosol Processes. In Advanced Catalysts and Nanostructured Materials: Modern Synthetic Methods; Moser, W. R., Ed.; Academic Press: San Diego, CA, 1996. (7) Kuchta, L.; Fajnor, V. S. About the Synthesis and Thermal Stability of SiO2-Aerogel. J. Therm. Anal. 1996, 46, 515. (8) Hutter, R.; Dutoit, D. C. M.; Mallat, T.; Schneider, M.; Baiker, A. Novel Mesoporous Titania-Silica Aerogels: HighlyActive Catalysts for the Selective Epoxidation of Cyclic Olefins. J. Chem. Soc., Chem. Commun. 1995, 163. (9) Ulrich, G. D. Flame Synthesis of Fine Particles. Chem. Eng. News 1984, 62 (Aug 6), 22. (10) Pratsinis, S. E. Flame Aerosol Synthesis of Ceramic Powders. Prog. Energy Combust. Sci. 1998, 24, 197. (11) Formenti, M.; Juillet, F.; Meriaudeau, P.; Teichner, S. J.; Vergnon, P. Preparation in a Hydrogen-Oxygen flame of Ultrafine Metal Oxide Particles. J. Colloid Interface Sci. 1972, 39, 79. (12) Fotou, G. P.; Vemury, S.; Pratsinis, S. E. Synthesis and Evaluation of Titania Powders for Photodestruction of Phenol. Chem. Eng. Sci. 1994, 49, 4939. (13) Fotou, G. P.; Pratsinis, S. E. Photocatalytic destruction of phenol and salicylic acid with aerosol-made and commercial titania powders. Chem. Eng. Commun. 1996, 151, 251. (14) Miquel, P. F.; Katz, J. L. Formation and Characterization of Nanostructured V-P-O Particels in Flames: A New Route for the Formation of Catalysts. J. Mater. Res. 1994, 9, 746.
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Received for review March 18, 2002 Revised manuscript received July 10, 2002 Accepted July 18, 2002 IE020200E
