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Direct oxidation of propylene and other olefins on precious metal containing Ti-catalysts
Applied Catalysis A: General 213 (2001) 163–171
Direct oxidation of propylene and other olefins on precious metal containing Ti-catalysts W. Laufer, W.F. Hoelderich∗ Department of Chemical Technology and Heterogenous Catalysts, University of Technology RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany Received 17 May 2000; received in revised form 19 September 2000; accepted 20 September 2000
Abstract The properties and performance of an O2 –H2 system used for the direct oxidation of olefins have been studied on the example of direct oxidation of propylene. Ti-catalysts modified by impregnation with palladium tetraminenitrate and platinum tetraminechloride and by followed reduction under various conditions were used as bifunctional catalysts. New investigations showed that the pre-impregnation of the catalyst with salt and the using of alcohols or ketones as solvent for the impregnation of the catalyst with precious metal improved the selectivity of propylene oxide (PO). Also the influence of other precious metals such as Ir or Au as promoters on titanium silicalite-1 (TS-1) has been studied. In addition to propylene, other olefins like styrene or terpenes were also tested in direct oxidation with an O2 –H2 system by using Pd/Pt/TS-1 or Pd/Pt/Ti-MCM-41, respectively. The use of these olefins as substrate lead to low yields of the desired product and showed the limits of the O2 –H2 oxidation system, so far. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Direct oxidation; Propylene oxide; Precious metal containing TS-1 and Ti-MCM-41
1. Introduction The discovery of titanium silicalite-1 (TS-1) by Taramasso et al. [1,2] opened a new route for the epoxidation of small olefins with hydrogen peroxide. TS-1 catalyzes the epoxidation of propylene with H2 O2 at high conversion rate and selectivity under mild conditions in the liquid phase [3]. Styrene is oxidized and rearranged to give phenylacetaldehyde as the main product [4]. For bulky olefins like dodecene, norbornene or terpenes Ti-catalysts with larger pores (Ti-ß, Ti-MCM-41) have been used [5–8].
∗ Corresponding author. Tel.: +49-241-806560; fax: +49-241-8888291. E-mail address: [email protected] (W.F. Hoelderich).
Due to the relatively high and steady changing costs of hydrogen peroxide such an epoxidation process has not yet been commercialized. For economic reasons, attempts have been made to substitute H2 O2 by employing a mixture of O2 and H2 that generates H2 O2 in situ. In these cases, the Ti-catalyst is modified with precious metals such as palladium and platinum which catalyze the direct synthesis of hydrogen peroxide. For the epoxidation of propylene with an O2 –H2 mixture, Sato and coworkers [9–11] and Müller et al. [12] employed a palladium impregnated TS-1 catalyst, while Haruta and Hayashi [13] tested a highly dispersed Au/TiO2 catalyst in a gas phase reaction. Dow Chemicals used Au on titanium silicalites as catalysts for the direct oxidation of propylene [14–16]. Pd/(Pt)/TS-1 catalysts were also tested for the oxidation of alkanes to alcohols and ketones, for the hydroxylation of
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benzene to phenol and for the oxidation of phenol to hydroquinone [17,18]. In the case of the direct synthesis of propylene oxide (PO) by using of Pd/Pt/TS-1 as catalyst, we have already reported on the influence of some reduction methods and of the addition of salt to the reaction mixture on the catalytic performance [19,20]. In this paper, we report about the direct oxidation of various olefins with oxygen in the presence of hydrogen by using of Pd/Pt/Ti-catalysts. The influence of various parameters including reaction conditions (temperature, pressure, O2 :H2 ratio, solvent, addition of salt), reduction and impregnation methods of the catalysts, and the procedure (batch or semi-batch) has been studied on the example of direct oxidation of propylene. New results are compared with our previous experiments. In addition to propylene, other olefins like styrene and terpenes were also tested in the direct oxidation.
2. Experimental 2.1. Catalysts synthesis, impregnation and reduction TS-1 was prepared according to the procedure described in the original TS-1 patent by Taramasso et al. [1] with tetraethyl-ortho-titanate (TEOT)/tetraethyl-ortho-silicate (TEOS) as the Ti/Si sources and with tetrapropylammonium hydroxide as the template. Ti-MCM-41 were synthesized according to the procedure described elsewhere [21] using (NH4 )3 [Ti(O2 )F5 ] as the titanium source and tetradecylmethylammonium bromide (TDTMABr) as the template. Catalysts containing palladium and platinum were prepared by addition of an aqueous, alcoholic or ketonic [Pd(NH3 )4 ](NO3 )2 and [Pt(NH3 )4 ]Cl2 solution to the suspended Ti-catalyst. For the impregnation with salt, aqueous NaBr solutions were used. The impregnated catalysts were exposed to different methods of reduction. Reduction was carried out under pure H2 , under 5 vol.% H2 + 95 vol.% N2 or under pure N2 at a heating rate of 1 K/min from room temperature to 50 or 150◦ C, respectively. The autoreduction under pure nitrogen atmosphere bases on the thermal decomposition of the NH3 ligand at 150◦ C [19,22].
Platinum promoted Pd/Ti-catalysts, prepared by impregnation and reduction were characterized by TEM, EDAX and ICP-AES. 2.2. Direct synthesis of propylene oxide Since the concentration of the reactants were within the flammable area of the gas mixture, special safety precautions were required. The reaction vessel and peripheral equipment was placed in a barricaded area and a remote control system was used to take samples and to start, monitor and terminate the reaction. The reactor consisted of a 200 ml stainless steel autoclave equipped with two inlets for the reactants, an outlet, a temperature indicator and a pressure indicator. Through an outer water jacket, a constant temperature could be maintained. The reaction mixture was stirred by a magnetic stirrer. First, the solvent and the catalyst were charged into the autoclave. By using a batch modus, propylene fed into the reactor and the vessel was pressurized with hydrogen, nitrogen and oxygen in succession. Liquid propylene was pumped into the autoclave and O2 , H2 and N2 were supplied through mass flow controllers. For the semi-continuous procedure, gaseous propylene, O2 , H2 and N2 were fed at 7 bar pressure through mass flow controllers during the reaction. The slurry was heated to the desired reaction temperature (normally 43◦ C) under pressure and vigorous stirring. The reactor was kept at the reaction temperature for 2 h and then cooled down to 15◦ C. After filtration of the catalyst an external standard (MTBE) was added to the liquid reaction mixture. Ethylene was added to the gas phase as a reference substance to determine the propane quantity. The liquid and the gas phase analyses were performed on GC. Liquid phase analysis was performed on a Carlo Erba Z 2300 gas chromatograph, using a flame ionization detector (FID) and a 4 m column Carbowax 1500 as the stationary phase. For the gas phase analysis, a 3 m Porapack 1500 column was used in connection with a thermal conductivity meter. 2.3. Direct oxidation of other olefins For the direct oxidation of styrene or terpenes, the autoclave was charged with catalyst, solvent and olefin. The gaseous components O2 and H2 were pressured for the batch modus or were fed at
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constant pressure during the reaction for the semibatch procedure. The product analysis was performed on GC by adding an external standard. The GC analysis was performed on an autosampler HP 6890 (Hewlett-Packard) gas chromatograph using a FID and a 60 m CP-SiL19 column as the stationary phase. The yield was calculated by relating the molar amount of product to the molar amount of the reactant which was charged to the reactor with the lowest molar amount compared to the other reactants. The selectivity of the molar amount of the desired product is based on the molar amounts of all organic products. 2.4. Catalysts characterization The catalysts tested were characterized by various analysis methods. Form, sizes and distribution of precious metal clusters were analyzed by transmission electron microscopy (TEM) while for the qualitative analysis, EDAX was used. The micrographs were taken using a JEOL JEM-2000 FX at an acceleration voltage of 200 kV. The precious metal loading and the molar ratio of Si:Ti were determined by an ICP-AES Spectroflame D. The incorporation of Ti into the zeolite framework was confirmed by DRIFT spectra with a band at 960/cm. The diffuse reflectance IR (DRIFT) was carried out by means of a Praying-Mantis-Unit from Harric. Diffuse reflectance-UV–VIS (DR-UV–VIS) spectra were obtained with a Lambda 7 spectrometer from Perkin-Elmer. 3. Results and discussion The direct oxidation with O2 in the presence of H2 is carried out on bifunctional catalysts and takes two reaction steps. The first step is the direct synthesis of hydrogen peroxide on precious metals like palladium or platinum. The second step is the epoxidation of olefin with in situ formed H2 O2 on Ti-catalyst as carrier (Fig. 1). The oxidation process with an O2 –H2 gas mixture reveals three major drawbacks. 1. The low H2 O2 formation leads to low yields of oxidation products. 2. Hydrogenation of olefins under reaction conditions (precious metal catalysts, H2 ) to form alkanes.
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Fig. 1. Synthesis steps using an O2 –H2 system for direct oxidation.
3. Working in the flammable area results in safety risks requiring a lot of precautions. The variation of different reaction conditions and of catalyst preparation should help to optimized direct oxidation by minimizing these disadvantages. 3.1. Direct oxidation of propylene Former investigations gave general information on the reaction conditions, catalyst preparation and process design. Our previous experience has shown that direct oxidation is favored when carried out in a “one-pot” reaction procedure [23]. Investigations of the direct synthesis of hydrogen peroxide [24,25] and the epoxidation of propylene with H2 O2 [3,26] showed that the first step — the H2 O2 formation on Pd/Pt-catalysts — is favored at low temperatures of about 0–10◦ C with water as solvent while the second step — the epoxidation with H2 O2 on Ti-catalysts (TS-1) — is optimally carried out in methanol at temperatures of about 40–50◦ C. Recently, we found that the direct oxidation of propylene is preferably carried out in a H2 O–MeOH mixture (3:1) at 43◦ C [19]. Furthermore, the best yields of PO were obtained by using an O2 –H2 mixture with O2 :H2 molar ratio of 1:1 to 2:1. We also observed that a positive effect on the PO formation by using an O2 –H2 gas mixture as oxidant can be achieved by adding an alkali salt to the reaction mixture [20]. The formation of H2 O2 in the first reaction step is favored by addition of promoters like HCl, H2 SO4 and NaBr or NaCl [27,28]. In our experiments only salts, no acids, were added because the addition of acids to the reaction mixture catalyze the formation of by-products (methoxy-propanols,
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Fig. 2. Influence of changing the oxidation procedure from the batch to the semi-continuous modus and of adding NaBr to the reaction mixture.
diols) and leads to low PO selectivities. The amount of added salt to the reaction mixture is an important parameter and should not exceed 5.0 mmol/l. Higher salt concentrations lead to strong decrease in the PO yields because more salt inhibits the second reaction step. Best results were obtained with a salt concentration of 0.5–1.0 mmol/l [20]. Fig. 2 shows the influence of added NaBr to the reaction mixture on PO formation. The positive effect of NaBr addition can be observed using a batch as well as a semi-continuous modus. In both cases, the PO yields can be increased by maintaining the same PO selectivity. Furthermore, Fig. 2 also illustrates the influence of the oxidation procedure. Changing from the batch to the semi-continuous procedure, the PO selectivity increased significantly. When using the semi-batch procedure, the concentration of the reactants propylene and H2 can be adjusted to low concentrations during the reaction that leads to a decrease in propane formation. The reduction method for the catalyst tremendously influences the direct oxidation with significant effects on the product formation and selectivity. The reduction stages of the precious metal are decisive for H2 O2 direct synthesis and for the hydrogenation of the olefin. In the case of PO synthesis, the best results were obtained by catalyst autoreduction in a pure nitrogen atmosphere at 150◦ C [19]. The catalyst is reduced by the thermal decomposition of the amine complex which creates locally a reducing atmosphere [22] Pd2+ + NH3 → Pd0 + 2H+ + 21 N2 + 21 H2
Using higher reduction temperatures than 150◦ C or H2 as the reducing medium, the PO yields were lower and the hydrogenation of propylene to propane was favored [19]. The addition of platinum as promoter activates the catalyst which is superior in the direct synthesis of H2 O2 [24]. This promoter positively influences the size and the dispersion of the metal clusters on the carrier surface. Therefore, minor amounts of platinum improve also the PO formation [19]. Unfortunately, higher amounts of platinum favored the hydrogenation of propylene to propane and leads to lower PO selectivity. The best results were obtained by using a Pd/Pt/TS-1 catalyst with 1% Pd and 0.01–0.02% Pt. New investigations give more insights about influence of impregnation and reduction methods and of using other precious metals as promoters on the PO yields and selectivity. The propane formation can be strongly suppressed by using “mild” conditions for the reduction of the catalyst. In this case, the catalyst was activated at lower temperature of 50◦ C instead of 150◦ C. Thereby, most of the amine complex was not decomposed. The remaining amine salt on the carrier inhibits the activity of the Ti-sites. This was the reason for very low PO yields. The influence of low reduction temperatures by using various reduction media on the propane formation and PO yield in comparison with the standard autoreduced catalyst is illustrated in Table 1. The catalysts used were Pd/Pt/TS-1 loaded with 1.0 wt.% Pd and 0.01 wt.% Pt. The use of other precious metals as promoter in the direct synthesis of H2 O2 was also reported in [29]. Especially, Pd/Ir- and Pd/Au (bimetal)-catalysts were reported for very high Table 1 Influence of low reduction temperatures on PO formation and hydrogenation of propylene to propanea Reduction method
150◦ C/pure N2 50◦ C/pure N2 50◦ C/5 wt.% H2 + 95 wt.% N2 Liquid phase 50◦ C/pure H2
Catalytic performance PO yields (%)
Propane formed (mmol)
16.8 0.6 1.8 1.4
0.33 0.11 0.09 0.07
a Conditions: (semi-batch): 0.2 g catalyst, 15 g MeOH, 5 g H O, 2 1.3 l/h H2 , 1.3 l/h O2 , 1.4 l/h N2 , 0.1 l/h propylene, 7 bar 43◦ C, 120 min. Catalyst: 1 wt.% Pd + 0.01 wt.% Pt on TS-1.
W. Laufer, W.F. Hoelderich / Applied Catalysis A: General 213 (2001) 163–171 Table 2 Comparison of catalytic performance by adding various promoters to Pd (Pt, Ir or Au)a Promoterb
Pt Ir Au
Catalytic performance Conversion (%)
PO selectivity (%)
Propane formed (mmol)
21 55 68
71 25 16
0.33 5.68 7.04
a
Conditions: (semi-batch): 0.2 g catalyst, 15 g MeOH, 5 g H2 O, 1.3 l/h H2 , 1.3 l/h O2 , 1.4 l/h N2 , 0.1 l/h propylene, 7 bar, 43◦ C, 120 min. Catalyst: 1 wt.% Pd + 0.01–0.02 wt.% promoter on TS-1, autoreduction at 150◦ C under pure N2 . b As [Pt(NH ) ]Cl , [Ir(NH ) ]Cl , AuCl . 3 4 2 3 4 2 3
activity. We tested Ir- and Au-promoted Pd/TS-1 catalysts in the direct oxidation of propylene in the O2 –H2 system. The results demonstrate that the addition of these promoters increases the activity of the catalyst. Unfortunately, more propane is produced, which was the reason for the low PO selectivity (Table 2). The high catalyst activity probably results from the formation highly dispersed Pd clusters on the carrier. In addition to Pd clusters with sizes range about 10–20 nm also very small clusters of 1–2 nm were found by TEM analysis (Fig. 3). By addition of
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iridium as the promoter, this precious metal was found only on the extra framework titania (impurity). Gold show an inhomogeneous dispersion on TS-1. The gold or iridium addition leads to formation of needle-like as well as circular Pd clusters (Fig. 3B). The circular clusters were localized on the external carrier surface and the needle-like ones were formed in the carrier channels. Another parameter that influences the Pd cluster sizes and their dispersion on the carrier is the impregnation method. Carrier impregnation with precious metal complexes was normally carried out using water as the solvent [9,12,19]. The use of alcohols or ketones as solvents leads to the formation of big cluster agglomerations during the reduction process. The bulky solvation shell of the Pd salt formed with solvents leads to high metal concentrations on the outer carrier surface because its diffusion into the carrier channels is hindered. During the reduction process, the metal agglomerated because of the high concentration and mobility of Pd clusters. The size of the agglomerates ranges from 400 to 500 nm (Fig. 4). They are distributed inhomogeneously on the outer carrier surface. That is the reason for low propane formation normally caused by small surface of the Pd-clusters. Different PO yields are obtained due to
Fig. 3. TEM micrographs of Pd/Ir-TS-1 catalysts.
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Fig. 4. TEM micrographs of Pd/Pt/TS-1 impregnated in water and MeOH.
inhomogeneous dispersion generated by the application of various alcohols. Further investigations are needed to achieve better dispersion of the clusters. Thereby, the use of water–alcohol mixtures as solvent for the impregnation should be studied. Table 3 illustrates the catalytic results obtained when various solvents have been used for impregnation. The PO selectivity increased from 71 up to 90% by means of alcohols or ketones. The PO selectivity can also be improved by suppressing the ring opening reaction in the liquid phase to give methoxy-alcohols, diols, etc. In the case of epoxidation reaction with H2 O2 , the consecutive ring opening reaction is catalyzed by the very weak acidic sites of TS-1 and can be decreased by impregnation of the catalyst with alkali salts [30]. This effect could
be also observed in direct oxidation. The catalyst was pre-impregnated with NaBr and calcined before the impregnation with the precious metal. Fig. 5 shows the influence of such a pre-impregnation on the catalytic performance. The impregnation with 0.5 wt.% NaBr on TS-1 reveals the best results. Higher amounts of salt deactivate slightly the Ti-sites and lead to decreased PO yields. Higher PO selectivities result only from suppressing the ring opening reaction (PO selectivities in the liquid phase). The impregnation with NaBr had almost no effect on the distribution and size of the Pd clusters and did not influence decisive the hydrogenation reaction (propane formation). Leaching tests showed that the NaBr was washed out of the catalyst during the oxidation reaction.
Table 3 Influence of solvent used for impregnation with precious metals on catalytic performancea Solvent used for impregnation
Water Methanol iso-Propanol Acetone
Catalytic performance Conversion (%)
PO selectivity (%)
Propane formed (mmol)
21 6 18 13
71 84 87 89
0.33 0.10 0.20 0.18
Conditions: (semi-batch): 0.2 g catalyst, 15 g MeOH, 5 g H2 O, 1.3 l/h H2 , 1.3 l/h O2 , 1.4 l/h N2 , 0.1 l/h propylene, 7 bar, 43◦ C, 120 min. Catalyst: 1 wt.% Pd + 0.01 wt.% Pt on TS-1, autoreduction at 150◦ C under pure N2 . a
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Fig. 5. Influence of pre-impregnation of catalyst with salt on PO selectivity.
3.2. Direct oxidation of other olefins After investigating the influence of various parameters in the direct oxidation in an O2 –H2 system for PO synthesis, we tested other olefins as substrates. Styrene oxidation can be catalyzed by Pd/Pt/TS-1 catalysts. Since epoxidation of bulkier terpenes such as ␣-limonene and ␣-pinene cannot be catalyzed by TS-1, Ti-catalysts with larger pores like Ti-MCM-41 have been used as carrier. The direct oxidation of styrene or terpenes cannot successfully take place with an O2 –H2 gas mixture (Table 4).
With these substrates, the necessary conditions for successful oxidation with an O2 –H2 system are not fulfilled. • The substrate should react faster with in situ H2 O2 before H2 O2 decomposition takes place. • High reaction rates should also be guarantied in a highly diluted H2 O2 mixture. • The olefin should be less sensitive against hydrogenation. • The carrier should not catalyze the formation of by-products.
Table 4 Direct oxidation of various olefins with an O2 –H2 gas mixturea Substrate
Propylene Styrene ␣-Pinene ␣-Limonene
Solvent
MeOH–H2 O MeOH Acetonitrile Acetonitrile
Catalyst
Pd/Pt/TS-1 Pd/Pt/TS-1 Pd/Pt/Ti-MCM-41 Pd/Pt/Ti-MCM-41
Catalytic performance Olefin conversion (%)
Selectivityb (%)
34 53 7 28
PO/69 PAA/3 CA/<2 LO/<2
a Conditions: (semi-batch): 0.2 g catalyst, 20 g solvent, 1.3 l/h H , 1.3 l/h O , 1.4 l/h N , 7 bar, 43◦ C, 120 min, 0.1 l/h propylene or 1.2 g 2 2 2 other olefins, 0.5 mmol/l NaBr. Catalyst: 1 wt.% Pd + 0.01 wt.% Pt on Ti-catalyst, autoreduction at 150◦ C under N2 . b PO: propylene oxide; PAA: phenylacetaldehyde; CA: campholenic aldehyde; LO: limonene oxide.
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Fig. 6. Reaction pathways for direct oxidation of styrene with O2 and H2 .
Many disadvantages emerged in the case of direct oxidation of styrene (Fig. 6). First, the styrene can be easily hydrogenated under reaction conditions (precious metal catalyst, H2 ) to ethylbenzene as the major product. Furthermore, the oxidation with in situ H2 O2 leads to the formation of phenylacetaldehyde, benzaldehyde and styrene oxide [4]. The polymerization of styrene was of minor importance and could be suppressed by addition of inhibitors such as 4-tert-butylcatechol. The direct oxidation of ␣-limonene or ␣-pinene leads to very low conversions and selectivities of the desired products limonene oxide or campholenic aldehyde, respectively.
4. Conclusions The direct oxidation with an O2 –H2 gas mixture in the presence of precious metal containing Ti-catalysts shows an alternative route to oxidize propylene to PO. Our investigations illustrated the strong influence of the reaction parameters such as reaction conditions and catalyst preparation. In the example of direct synthesis to form PO, this system could be
optimized. Best results can be obtained by carrying out the reaction in a semi-batch process at 40–45◦ C by using of O2 :H2 molar ratios of 1:1 to 2:1. The addition of small amounts of salt to the reaction mixture has a positive influence on catalytic performance by improving the H2 O2 direct synthesis. Since most of the olefins are sensitive against hydrogenation under the reaction conditions, the size and dispersion of the precious metals on the carrier surface decisively influence the selectivity. For this reason, the reduction and impregnation methods were of most interest. The autoreduction under pure nitrogen showed the best results. “Milder” reduction conditions at 50◦ C decreased the side reaction of hydrogenation but only very low PO yields were obtained. The pre-impregnation of the catalyst with salt and the use of alcohols or ketones as solvent for the impregnation of the catalyst with precious metal improved the PO selectivity. The investigations of the O2 –H2 system performance show that only propylene, which reacts fast with in situ H2 O2 even in very dilute H2 O2 -mixtures, can be oxidized with good results. The oxidation of other olefins like styrene or terpenes leads to very low product yields and selectivities.
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Acknowledgements Financial support of the former Hoechst AG is gratefully acknowledged. The authors express their sincere thanks to Prof. Dr. K. Kühlein and Dr. U. Dingerdissen both from Hoechst AG for helpful discussions. References [1] M. Taramasso, G. Perego, B. Notari, US Patent No. 4,410,501 (1983), Enichem. [2] M. Taramasso, G. Manara, V. Fattore, B. Notari, US Patent No. 4,666,692 (1987), Enichem. [3] M.G. Clerici, G. Belussi, U. Romano, J. Catal. 129 (1991) 159. [4] S.B. Kumar, S.P. Mirajkar, G.C.G. Pais, P. Kumar, R. Kumar, J. Catal. 156 (1995) 163. [5] R. Hutter, T. Mallat, D. Dutoit, A. Baiker, Top. Catal. 3 (1996) 421. [6] A. Corma, N.T. Navarro, J. Perez-Pariente, F. Sanchez, Stud. Surf. Sci. Catal. 84 (1994) 69. [7] A. Corma, M.A. Camblor, P. Esteve, A. Martinez, J. Perez-Pariente, J. Catal. 145 (1994) 151. [8] A. Corma, N.T. Navarro, J. Perez-Pariente, J. Catal. 156 (1995) 65. [9] M. Sato, T. Miyake, JP 4-352771 (1992), Tosoh. [10] A. Sato, M. Oguri, M. Tokumaru, T. Miyake, JP HE18-269029 (1996), Tosoh. [11] A. Sato, M. Oguri, M. Tokumaru, T. Miyake, JP HE18-269030 (1996), Tosoh. [12] U. Müller, P. Lingelbach, P. Baßler, W. Harder, E. Karsten, V. Kohl, J. Dembrowsky, N. Rieber, M. Fischer, DE 44 25 672 A1 (1996), BASF AG.
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Direct oxidation of propylene and other olefins on precious metal containing Ti-catalysts W. Laufer, W.F. Hoelderich∗ Department of Chemical Technology and Heterogenous Catalysts, University of Technology RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany Received 17 May 2000; received in revised form 19 September 2000; accepted 20 September 2000
Abstract The properties and performance of an O2 –H2 system used for the direct oxidation of olefins have been studied on the example of direct oxidation of propylene. Ti-catalysts modified by impregnation with palladium tetraminenitrate and platinum tetraminechloride and by followed reduction under various conditions were used as bifunctional catalysts. New investigations showed that the pre-impregnation of the catalyst with salt and the using of alcohols or ketones as solvent for the impregnation of the catalyst with precious metal improved the selectivity of propylene oxide (PO). Also the influence of other precious metals such as Ir or Au as promoters on titanium silicalite-1 (TS-1) has been studied. In addition to propylene, other olefins like styrene or terpenes were also tested in direct oxidation with an O2 –H2 system by using Pd/Pt/TS-1 or Pd/Pt/Ti-MCM-41, respectively. The use of these olefins as substrate lead to low yields of the desired product and showed the limits of the O2 –H2 oxidation system, so far. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Direct oxidation; Propylene oxide; Precious metal containing TS-1 and Ti-MCM-41
1. Introduction The discovery of titanium silicalite-1 (TS-1) by Taramasso et al. [1,2] opened a new route for the epoxidation of small olefins with hydrogen peroxide. TS-1 catalyzes the epoxidation of propylene with H2 O2 at high conversion rate and selectivity under mild conditions in the liquid phase [3]. Styrene is oxidized and rearranged to give phenylacetaldehyde as the main product [4]. For bulky olefins like dodecene, norbornene or terpenes Ti-catalysts with larger pores (Ti-ß, Ti-MCM-41) have been used [5–8].
∗ Corresponding author. Tel.: +49-241-806560; fax: +49-241-8888291. E-mail address: [email protected] (W.F. Hoelderich).
Due to the relatively high and steady changing costs of hydrogen peroxide such an epoxidation process has not yet been commercialized. For economic reasons, attempts have been made to substitute H2 O2 by employing a mixture of O2 and H2 that generates H2 O2 in situ. In these cases, the Ti-catalyst is modified with precious metals such as palladium and platinum which catalyze the direct synthesis of hydrogen peroxide. For the epoxidation of propylene with an O2 –H2 mixture, Sato and coworkers [9–11] and Müller et al. [12] employed a palladium impregnated TS-1 catalyst, while Haruta and Hayashi [13] tested a highly dispersed Au/TiO2 catalyst in a gas phase reaction. Dow Chemicals used Au on titanium silicalites as catalysts for the direct oxidation of propylene [14–16]. Pd/(Pt)/TS-1 catalysts were also tested for the oxidation of alkanes to alcohols and ketones, for the hydroxylation of
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benzene to phenol and for the oxidation of phenol to hydroquinone [17,18]. In the case of the direct synthesis of propylene oxide (PO) by using of Pd/Pt/TS-1 as catalyst, we have already reported on the influence of some reduction methods and of the addition of salt to the reaction mixture on the catalytic performance [19,20]. In this paper, we report about the direct oxidation of various olefins with oxygen in the presence of hydrogen by using of Pd/Pt/Ti-catalysts. The influence of various parameters including reaction conditions (temperature, pressure, O2 :H2 ratio, solvent, addition of salt), reduction and impregnation methods of the catalysts, and the procedure (batch or semi-batch) has been studied on the example of direct oxidation of propylene. New results are compared with our previous experiments. In addition to propylene, other olefins like styrene and terpenes were also tested in the direct oxidation.
2. Experimental 2.1. Catalysts synthesis, impregnation and reduction TS-1 was prepared according to the procedure described in the original TS-1 patent by Taramasso et al. [1] with tetraethyl-ortho-titanate (TEOT)/tetraethyl-ortho-silicate (TEOS) as the Ti/Si sources and with tetrapropylammonium hydroxide as the template. Ti-MCM-41 were synthesized according to the procedure described elsewhere [21] using (NH4 )3 [Ti(O2 )F5 ] as the titanium source and tetradecylmethylammonium bromide (TDTMABr) as the template. Catalysts containing palladium and platinum were prepared by addition of an aqueous, alcoholic or ketonic [Pd(NH3 )4 ](NO3 )2 and [Pt(NH3 )4 ]Cl2 solution to the suspended Ti-catalyst. For the impregnation with salt, aqueous NaBr solutions were used. The impregnated catalysts were exposed to different methods of reduction. Reduction was carried out under pure H2 , under 5 vol.% H2 + 95 vol.% N2 or under pure N2 at a heating rate of 1 K/min from room temperature to 50 or 150◦ C, respectively. The autoreduction under pure nitrogen atmosphere bases on the thermal decomposition of the NH3 ligand at 150◦ C [19,22].
Platinum promoted Pd/Ti-catalysts, prepared by impregnation and reduction were characterized by TEM, EDAX and ICP-AES. 2.2. Direct synthesis of propylene oxide Since the concentration of the reactants were within the flammable area of the gas mixture, special safety precautions were required. The reaction vessel and peripheral equipment was placed in a barricaded area and a remote control system was used to take samples and to start, monitor and terminate the reaction. The reactor consisted of a 200 ml stainless steel autoclave equipped with two inlets for the reactants, an outlet, a temperature indicator and a pressure indicator. Through an outer water jacket, a constant temperature could be maintained. The reaction mixture was stirred by a magnetic stirrer. First, the solvent and the catalyst were charged into the autoclave. By using a batch modus, propylene fed into the reactor and the vessel was pressurized with hydrogen, nitrogen and oxygen in succession. Liquid propylene was pumped into the autoclave and O2 , H2 and N2 were supplied through mass flow controllers. For the semi-continuous procedure, gaseous propylene, O2 , H2 and N2 were fed at 7 bar pressure through mass flow controllers during the reaction. The slurry was heated to the desired reaction temperature (normally 43◦ C) under pressure and vigorous stirring. The reactor was kept at the reaction temperature for 2 h and then cooled down to 15◦ C. After filtration of the catalyst an external standard (MTBE) was added to the liquid reaction mixture. Ethylene was added to the gas phase as a reference substance to determine the propane quantity. The liquid and the gas phase analyses were performed on GC. Liquid phase analysis was performed on a Carlo Erba Z 2300 gas chromatograph, using a flame ionization detector (FID) and a 4 m column Carbowax 1500 as the stationary phase. For the gas phase analysis, a 3 m Porapack 1500 column was used in connection with a thermal conductivity meter. 2.3. Direct oxidation of other olefins For the direct oxidation of styrene or terpenes, the autoclave was charged with catalyst, solvent and olefin. The gaseous components O2 and H2 were pressured for the batch modus or were fed at
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constant pressure during the reaction for the semibatch procedure. The product analysis was performed on GC by adding an external standard. The GC analysis was performed on an autosampler HP 6890 (Hewlett-Packard) gas chromatograph using a FID and a 60 m CP-SiL19 column as the stationary phase. The yield was calculated by relating the molar amount of product to the molar amount of the reactant which was charged to the reactor with the lowest molar amount compared to the other reactants. The selectivity of the molar amount of the desired product is based on the molar amounts of all organic products. 2.4. Catalysts characterization The catalysts tested were characterized by various analysis methods. Form, sizes and distribution of precious metal clusters were analyzed by transmission electron microscopy (TEM) while for the qualitative analysis, EDAX was used. The micrographs were taken using a JEOL JEM-2000 FX at an acceleration voltage of 200 kV. The precious metal loading and the molar ratio of Si:Ti were determined by an ICP-AES Spectroflame D. The incorporation of Ti into the zeolite framework was confirmed by DRIFT spectra with a band at 960/cm. The diffuse reflectance IR (DRIFT) was carried out by means of a Praying-Mantis-Unit from Harric. Diffuse reflectance-UV–VIS (DR-UV–VIS) spectra were obtained with a Lambda 7 spectrometer from Perkin-Elmer. 3. Results and discussion The direct oxidation with O2 in the presence of H2 is carried out on bifunctional catalysts and takes two reaction steps. The first step is the direct synthesis of hydrogen peroxide on precious metals like palladium or platinum. The second step is the epoxidation of olefin with in situ formed H2 O2 on Ti-catalyst as carrier (Fig. 1). The oxidation process with an O2 –H2 gas mixture reveals three major drawbacks. 1. The low H2 O2 formation leads to low yields of oxidation products. 2. Hydrogenation of olefins under reaction conditions (precious metal catalysts, H2 ) to form alkanes.
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Fig. 1. Synthesis steps using an O2 –H2 system for direct oxidation.
3. Working in the flammable area results in safety risks requiring a lot of precautions. The variation of different reaction conditions and of catalyst preparation should help to optimized direct oxidation by minimizing these disadvantages. 3.1. Direct oxidation of propylene Former investigations gave general information on the reaction conditions, catalyst preparation and process design. Our previous experience has shown that direct oxidation is favored when carried out in a “one-pot” reaction procedure [23]. Investigations of the direct synthesis of hydrogen peroxide [24,25] and the epoxidation of propylene with H2 O2 [3,26] showed that the first step — the H2 O2 formation on Pd/Pt-catalysts — is favored at low temperatures of about 0–10◦ C with water as solvent while the second step — the epoxidation with H2 O2 on Ti-catalysts (TS-1) — is optimally carried out in methanol at temperatures of about 40–50◦ C. Recently, we found that the direct oxidation of propylene is preferably carried out in a H2 O–MeOH mixture (3:1) at 43◦ C [19]. Furthermore, the best yields of PO were obtained by using an O2 –H2 mixture with O2 :H2 molar ratio of 1:1 to 2:1. We also observed that a positive effect on the PO formation by using an O2 –H2 gas mixture as oxidant can be achieved by adding an alkali salt to the reaction mixture [20]. The formation of H2 O2 in the first reaction step is favored by addition of promoters like HCl, H2 SO4 and NaBr or NaCl [27,28]. In our experiments only salts, no acids, were added because the addition of acids to the reaction mixture catalyze the formation of by-products (methoxy-propanols,
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Fig. 2. Influence of changing the oxidation procedure from the batch to the semi-continuous modus and of adding NaBr to the reaction mixture.
diols) and leads to low PO selectivities. The amount of added salt to the reaction mixture is an important parameter and should not exceed 5.0 mmol/l. Higher salt concentrations lead to strong decrease in the PO yields because more salt inhibits the second reaction step. Best results were obtained with a salt concentration of 0.5–1.0 mmol/l [20]. Fig. 2 shows the influence of added NaBr to the reaction mixture on PO formation. The positive effect of NaBr addition can be observed using a batch as well as a semi-continuous modus. In both cases, the PO yields can be increased by maintaining the same PO selectivity. Furthermore, Fig. 2 also illustrates the influence of the oxidation procedure. Changing from the batch to the semi-continuous procedure, the PO selectivity increased significantly. When using the semi-batch procedure, the concentration of the reactants propylene and H2 can be adjusted to low concentrations during the reaction that leads to a decrease in propane formation. The reduction method for the catalyst tremendously influences the direct oxidation with significant effects on the product formation and selectivity. The reduction stages of the precious metal are decisive for H2 O2 direct synthesis and for the hydrogenation of the olefin. In the case of PO synthesis, the best results were obtained by catalyst autoreduction in a pure nitrogen atmosphere at 150◦ C [19]. The catalyst is reduced by the thermal decomposition of the amine complex which creates locally a reducing atmosphere [22] Pd2+ + NH3 → Pd0 + 2H+ + 21 N2 + 21 H2
Using higher reduction temperatures than 150◦ C or H2 as the reducing medium, the PO yields were lower and the hydrogenation of propylene to propane was favored [19]. The addition of platinum as promoter activates the catalyst which is superior in the direct synthesis of H2 O2 [24]. This promoter positively influences the size and the dispersion of the metal clusters on the carrier surface. Therefore, minor amounts of platinum improve also the PO formation [19]. Unfortunately, higher amounts of platinum favored the hydrogenation of propylene to propane and leads to lower PO selectivity. The best results were obtained by using a Pd/Pt/TS-1 catalyst with 1% Pd and 0.01–0.02% Pt. New investigations give more insights about influence of impregnation and reduction methods and of using other precious metals as promoters on the PO yields and selectivity. The propane formation can be strongly suppressed by using “mild” conditions for the reduction of the catalyst. In this case, the catalyst was activated at lower temperature of 50◦ C instead of 150◦ C. Thereby, most of the amine complex was not decomposed. The remaining amine salt on the carrier inhibits the activity of the Ti-sites. This was the reason for very low PO yields. The influence of low reduction temperatures by using various reduction media on the propane formation and PO yield in comparison with the standard autoreduced catalyst is illustrated in Table 1. The catalysts used were Pd/Pt/TS-1 loaded with 1.0 wt.% Pd and 0.01 wt.% Pt. The use of other precious metals as promoter in the direct synthesis of H2 O2 was also reported in [29]. Especially, Pd/Ir- and Pd/Au (bimetal)-catalysts were reported for very high Table 1 Influence of low reduction temperatures on PO formation and hydrogenation of propylene to propanea Reduction method
150◦ C/pure N2 50◦ C/pure N2 50◦ C/5 wt.% H2 + 95 wt.% N2 Liquid phase 50◦ C/pure H2
Catalytic performance PO yields (%)
Propane formed (mmol)
16.8 0.6 1.8 1.4
0.33 0.11 0.09 0.07
a Conditions: (semi-batch): 0.2 g catalyst, 15 g MeOH, 5 g H O, 2 1.3 l/h H2 , 1.3 l/h O2 , 1.4 l/h N2 , 0.1 l/h propylene, 7 bar 43◦ C, 120 min. Catalyst: 1 wt.% Pd + 0.01 wt.% Pt on TS-1.
W. Laufer, W.F. Hoelderich / Applied Catalysis A: General 213 (2001) 163–171 Table 2 Comparison of catalytic performance by adding various promoters to Pd (Pt, Ir or Au)a Promoterb
Pt Ir Au
Catalytic performance Conversion (%)
PO selectivity (%)
Propane formed (mmol)
21 55 68
71 25 16
0.33 5.68 7.04
a
Conditions: (semi-batch): 0.2 g catalyst, 15 g MeOH, 5 g H2 O, 1.3 l/h H2 , 1.3 l/h O2 , 1.4 l/h N2 , 0.1 l/h propylene, 7 bar, 43◦ C, 120 min. Catalyst: 1 wt.% Pd + 0.01–0.02 wt.% promoter on TS-1, autoreduction at 150◦ C under pure N2 . b As [Pt(NH ) ]Cl , [Ir(NH ) ]Cl , AuCl . 3 4 2 3 4 2 3
activity. We tested Ir- and Au-promoted Pd/TS-1 catalysts in the direct oxidation of propylene in the O2 –H2 system. The results demonstrate that the addition of these promoters increases the activity of the catalyst. Unfortunately, more propane is produced, which was the reason for the low PO selectivity (Table 2). The high catalyst activity probably results from the formation highly dispersed Pd clusters on the carrier. In addition to Pd clusters with sizes range about 10–20 nm also very small clusters of 1–2 nm were found by TEM analysis (Fig. 3). By addition of
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iridium as the promoter, this precious metal was found only on the extra framework titania (impurity). Gold show an inhomogeneous dispersion on TS-1. The gold or iridium addition leads to formation of needle-like as well as circular Pd clusters (Fig. 3B). The circular clusters were localized on the external carrier surface and the needle-like ones were formed in the carrier channels. Another parameter that influences the Pd cluster sizes and their dispersion on the carrier is the impregnation method. Carrier impregnation with precious metal complexes was normally carried out using water as the solvent [9,12,19]. The use of alcohols or ketones as solvents leads to the formation of big cluster agglomerations during the reduction process. The bulky solvation shell of the Pd salt formed with solvents leads to high metal concentrations on the outer carrier surface because its diffusion into the carrier channels is hindered. During the reduction process, the metal agglomerated because of the high concentration and mobility of Pd clusters. The size of the agglomerates ranges from 400 to 500 nm (Fig. 4). They are distributed inhomogeneously on the outer carrier surface. That is the reason for low propane formation normally caused by small surface of the Pd-clusters. Different PO yields are obtained due to
Fig. 3. TEM micrographs of Pd/Ir-TS-1 catalysts.
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Fig. 4. TEM micrographs of Pd/Pt/TS-1 impregnated in water and MeOH.
inhomogeneous dispersion generated by the application of various alcohols. Further investigations are needed to achieve better dispersion of the clusters. Thereby, the use of water–alcohol mixtures as solvent for the impregnation should be studied. Table 3 illustrates the catalytic results obtained when various solvents have been used for impregnation. The PO selectivity increased from 71 up to 90% by means of alcohols or ketones. The PO selectivity can also be improved by suppressing the ring opening reaction in the liquid phase to give methoxy-alcohols, diols, etc. In the case of epoxidation reaction with H2 O2 , the consecutive ring opening reaction is catalyzed by the very weak acidic sites of TS-1 and can be decreased by impregnation of the catalyst with alkali salts [30]. This effect could
be also observed in direct oxidation. The catalyst was pre-impregnated with NaBr and calcined before the impregnation with the precious metal. Fig. 5 shows the influence of such a pre-impregnation on the catalytic performance. The impregnation with 0.5 wt.% NaBr on TS-1 reveals the best results. Higher amounts of salt deactivate slightly the Ti-sites and lead to decreased PO yields. Higher PO selectivities result only from suppressing the ring opening reaction (PO selectivities in the liquid phase). The impregnation with NaBr had almost no effect on the distribution and size of the Pd clusters and did not influence decisive the hydrogenation reaction (propane formation). Leaching tests showed that the NaBr was washed out of the catalyst during the oxidation reaction.
Table 3 Influence of solvent used for impregnation with precious metals on catalytic performancea Solvent used for impregnation
Water Methanol iso-Propanol Acetone
Catalytic performance Conversion (%)
PO selectivity (%)
Propane formed (mmol)
21 6 18 13
71 84 87 89
0.33 0.10 0.20 0.18
Conditions: (semi-batch): 0.2 g catalyst, 15 g MeOH, 5 g H2 O, 1.3 l/h H2 , 1.3 l/h O2 , 1.4 l/h N2 , 0.1 l/h propylene, 7 bar, 43◦ C, 120 min. Catalyst: 1 wt.% Pd + 0.01 wt.% Pt on TS-1, autoreduction at 150◦ C under pure N2 . a
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Fig. 5. Influence of pre-impregnation of catalyst with salt on PO selectivity.
3.2. Direct oxidation of other olefins After investigating the influence of various parameters in the direct oxidation in an O2 –H2 system for PO synthesis, we tested other olefins as substrates. Styrene oxidation can be catalyzed by Pd/Pt/TS-1 catalysts. Since epoxidation of bulkier terpenes such as ␣-limonene and ␣-pinene cannot be catalyzed by TS-1, Ti-catalysts with larger pores like Ti-MCM-41 have been used as carrier. The direct oxidation of styrene or terpenes cannot successfully take place with an O2 –H2 gas mixture (Table 4).
With these substrates, the necessary conditions for successful oxidation with an O2 –H2 system are not fulfilled. • The substrate should react faster with in situ H2 O2 before H2 O2 decomposition takes place. • High reaction rates should also be guarantied in a highly diluted H2 O2 mixture. • The olefin should be less sensitive against hydrogenation. • The carrier should not catalyze the formation of by-products.
Table 4 Direct oxidation of various olefins with an O2 –H2 gas mixturea Substrate
Propylene Styrene ␣-Pinene ␣-Limonene
Solvent
MeOH–H2 O MeOH Acetonitrile Acetonitrile
Catalyst
Pd/Pt/TS-1 Pd/Pt/TS-1 Pd/Pt/Ti-MCM-41 Pd/Pt/Ti-MCM-41
Catalytic performance Olefin conversion (%)
Selectivityb (%)
34 53 7 28
PO/69 PAA/3 CA/<2 LO/<2
a Conditions: (semi-batch): 0.2 g catalyst, 20 g solvent, 1.3 l/h H , 1.3 l/h O , 1.4 l/h N , 7 bar, 43◦ C, 120 min, 0.1 l/h propylene or 1.2 g 2 2 2 other olefins, 0.5 mmol/l NaBr. Catalyst: 1 wt.% Pd + 0.01 wt.% Pt on Ti-catalyst, autoreduction at 150◦ C under N2 . b PO: propylene oxide; PAA: phenylacetaldehyde; CA: campholenic aldehyde; LO: limonene oxide.
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Fig. 6. Reaction pathways for direct oxidation of styrene with O2 and H2 .
Many disadvantages emerged in the case of direct oxidation of styrene (Fig. 6). First, the styrene can be easily hydrogenated under reaction conditions (precious metal catalyst, H2 ) to ethylbenzene as the major product. Furthermore, the oxidation with in situ H2 O2 leads to the formation of phenylacetaldehyde, benzaldehyde and styrene oxide [4]. The polymerization of styrene was of minor importance and could be suppressed by addition of inhibitors such as 4-tert-butylcatechol. The direct oxidation of ␣-limonene or ␣-pinene leads to very low conversions and selectivities of the desired products limonene oxide or campholenic aldehyde, respectively.
4. Conclusions The direct oxidation with an O2 –H2 gas mixture in the presence of precious metal containing Ti-catalysts shows an alternative route to oxidize propylene to PO. Our investigations illustrated the strong influence of the reaction parameters such as reaction conditions and catalyst preparation. In the example of direct synthesis to form PO, this system could be
optimized. Best results can be obtained by carrying out the reaction in a semi-batch process at 40–45◦ C by using of O2 :H2 molar ratios of 1:1 to 2:1. The addition of small amounts of salt to the reaction mixture has a positive influence on catalytic performance by improving the H2 O2 direct synthesis. Since most of the olefins are sensitive against hydrogenation under the reaction conditions, the size and dispersion of the precious metals on the carrier surface decisively influence the selectivity. For this reason, the reduction and impregnation methods were of most interest. The autoreduction under pure nitrogen showed the best results. “Milder” reduction conditions at 50◦ C decreased the side reaction of hydrogenation but only very low PO yields were obtained. The pre-impregnation of the catalyst with salt and the use of alcohols or ketones as solvent for the impregnation of the catalyst with precious metal improved the PO selectivity. The investigations of the O2 –H2 system performance show that only propylene, which reacts fast with in situ H2 O2 even in very dilute H2 O2 -mixtures, can be oxidized with good results. The oxidation of other olefins like styrene or terpenes leads to very low product yields and selectivities.
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