HYDROGENATION OF HIGHLY UNSATURATED HYDROCARBONS OVER HIGHLY DISPERSED PALLADIUM

Transcription

1 Applied Catalyclis, 6 (1983) Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 41 HYDROGENATION OF HIGHLY UNSATURATED HYDROCARBONS OVER HIGHLY DISPERSED PALLADIUM CATALYST. PART I: BEHAVIOUR OF SMALL METAL PARTICLES J.P. BOITIAUX, J. COSYNS and S. VASUDEVAN Institut Fransais du Petrole, Rueil-Malmaison, France. (Received 12 October 1982, accepted 20 January 1983) ABSTRACT Hydrogenation of 1-butyne over palladium catalysts with dispersions between 9 and 100% reveals that the reaction is sensitive to metallic dispersion. A similar sensitivity is also exhibited by 1,3-butadiene and isoprene, whereas I-butene is insensitive to dispersion. This behaviour of the highly unsaturated hydrocarbons is explained by the formation of a strong complex of these hydrocarbons with very small particles at high dispersion. INTRODUCTION The selective hydrogenation of acetylenics (I-butyne and vinylacetylene) in the 1,3-butadiene rich cut of the steam cracking reaction is a commercially attractive process for meeting the stringent specification of polymer grade butadiene. Although the literature abounds with work on the selective hydrogenation of acetylene in the ethylene cut [1,23, there are very few publications, excluding patents, on the selective hydrogenation of C4 cuts [3,4]. Kinetic studies of I-butyne hydrogenation have been reported by Mann et al. [5-73 over different metals of Group VIII. This study was however performed under conditions far removed from those of industrial operations. Furthermore, there has been no work reported on the influence of metallic dispersion on the hydrogenation of highly unsaturated hydrocarbons. In a recent publication, we have shown that the hydrogenation of 1,3-butadiene is strongly influenced by the particle size [8]. We observed a marked decrease in the turnover number (by a factor of 12), when the metallic dispersion was increased from 20 to 100%. In this article we report a study of the influence of dispersion on the kinetics of hydrogenation of other unsaturated hydrocarbons, notably I-butyne, in order to genaralize the special behaviour of the small particles. EXPERIMENTAL Materials The 1-butyne used in our study was procured from Air Liquide (>95% pure). This was further purified by passage through a bed of 4A + 13X molecular sieves. The 1-butyne was diluted in n-heptane (>99.8% pure) in order to obtain a concentration 0166~9834/83/$ Elsevier Science Publishers B.V.

2 b Particle diameter calculated by assuming that the crystallite has cubic form with on side in contact with the support TABLE 1 Sumnary of the different catalysts used. wt% Support area Reduction in Desorption Dispersion Pd /m2 g-' H2/=C b O,, /nm I-Butyne hydrog. Turnover no. Butene hydrog. under N2/'C I% activity/m01 s -1 for hydrog. of activity/m01 s-1-1 g Pd-' I-butyne/s-' o Pd A A A A A A A A A A A s s S asupport A = alumina, S = silica. 'Specific activity for the hydrogenation of I-butyne determined at 70% conversion.

3 48 of approximately 10% by weight of I-butyne. The hydrogen was of ultra pure grade (>99.999%) and was further purified in a deoxo unit followed by 4A molecular sieves. The two precursors used for the preparation of the catalysts were palladium acetylacetonate [Pd (CSH702)2] for alumina impregnation and palladium tetramine nitrate [Pd (NH3141 (NO312 for silica impregnation. These were obtained from Johnson Matthey, Paris. Two types of alumina, an a-alumina and a y+_-alumina with surface areas between 9 and 100 m* g -1 were used as supports in this study. The silica had a surface area of 210 m* g -1 and a pore volume of 0.76 ml g-l. Catalysts Palladium based catalysts have been used in this study. The method of preparation and the conditions of treatment employed to obtain a high dispersion have been described in detail elsewhere C81 and are merely summarized here as follows. The catalyst was prepared by wet impregnation of the alumina carrier with a known quantity of palladium acetylacetonate dissolved in benzene. It was oven dried at llo"c, air calcined at 300 C and reduced with hydrogen at temperatures between 300 and 8OO"C, according to the metal dispersion required. The palladium on silica catalyst was prepared using the palladium tetramine complex as precursor, as described in the literature C91. This catalyst was calcined at 400 C and then reduced at two different temperatures to give two different dispersions. Table 1 summarizes the different catalysts used in this study, their conditions of treatment, the metallic dispersions obtained by CO chemisorption and the mean crystallite sizes (calculated on the basis of a fee structure). Some of these catalysts were examined in an electron microscope (CTEM Jeol-120 X) and the average crystallite diameter obtained was in close agreement with the diameter calculated from CO chemisorption values. Kinetic tests Hydrogenation reactions were carried out in the liquid phase, in batch mode in a CSTR reactor at 2 MPa (20 bar) constant pressure. The hydrogenation rate was followed by recording the pressure drop in a constant volume hydrogen ballast, upstream of the pressure regulator, as a function of time. Cooling water was circulated in the reactor jacket in order to maintain the temperature at 20 C. The hydrocarbon samples were analysed in an Intersmat IGC 112F chromatograph provided with a flame ionization detector, using a 6 m 66' ODPN supported on chromosat PAW column at room temperature.

4 44 RESULTS Hydrogenation of l-butyne In Figure 1 is reported the hydrogen consumption as a function of time for the hydrogenation of 1-butyne over two catalysts with 9 and 67% metallic dispersion, respectively. In both cases the hydrogenation started inediately, without any induction period, and the rate increased as the reaction progressed. As the hydrogenation rate varies with conversion, the catalyst activities should be compared at the same conversion. In our study the 1-butyne hydrogenation rates were evaluated at 70% butyne conversion. Curves A and B of Figure 1 reveal a negative effect of the dispersion: 1-butyne hydrogenated more quickly on low dispersion than on high dispersion catalysts. I! Tii(mn1 Tie (mud FIGURE 1 Consumption of hydrogen vs. time during the hydrogenation of I-butyne and I-butene over two catalysts of 9 and 67% dispersion. FIGURE 2 I-butyne consumption and evolution of different products over a catalyst of 85% dispersion. Figure 2 shows the change of the reaction mixture composition with time over a catalyst of 85% dispersion. During the hydrogenation of 1-butyne, the only product of reaction was I-butene (up to a conversion of roughly 80%) if we except the very small amounts of butane formed right from the beginning. No parallel isomerization to E-butyne occurred. Once the 1-butyne had almost completely disappeared, the 1-butene was consumed with a higher rate than the I-butyne (cf. Figure 1 also). This I-butene hydrogenated into butane and isomerized into 2- butenes (cis and trans) in two parallel ways. Unlike 1 -butyne, the 1 -butene reaction did not change with time, the apparent order of reaction being nearly equal to zero. Lastly, the 2-butenes formed hydrogenated (cis and trans success-

5 45 ively), but with a very slow rate. Thus the overall reaction scheme could be represented as: I-butyne4 I-butene + 2-butenes.. * J / butane Influence of the dispersion on the activity I-butyne hydrogenation. As is seen in Figure 1, a highly dispersed catalyst (67%) had an activity which was considerably lower than a catalyst having 9% dispersion. In order to verify this strange behaviour of the small particles, we carried out a series of hydrogenation tests over the whole range of dispersion. We report the activity for the hydrogenation of 1-butyne, expressed as turnover number (moles of 1-butyne converted per atom of palladium exposed per second). The turnover number was determined at 70% conversion of 1-butyne and is plotted in Figure 3 as a function of dispersion. We observed from the curve that the activity remained constant up to 20% dispersion and decreased sharply thereafter. The residual activity for very high dispersions close to 100% was around 12 times lower. The nature of this curve is similar to the variation of activity for the 1,3-butadiene hydrogenation which we have reported earlier C83. Furthermore, we L: 16utene~6lltane 16UtElW ~16utem?exl r I I------, H 0.6 / I DkpanionM I aspniolorl FIGURE 3 Variation of the turnover number as a function of dispersion for l- butyne hydrogenation. FIGURE 4 Variation of the hydrogenation activity for 1-butene as a function of dispersion.

6 verified the variation of turnover number of I-butyne hydrogenation at different conversions and found that at iso conversion the nature of the curve remained identical. I-butene hydrogenation. Referring again to Figure 1, we observed that, contrary to l-butyne, the rate of hydrogenation of 1-butene (formed from I-butyne) was higher at higher dispersions. In Figure 4 we plot the rate of hydrogenation of 1-butene (in moles of I-butene hydrogenated per second per gram of palladium) as a function of dispersion and we observed that the activity increased linearly with dispersion (i.e. the turnover number remained constant with dispersion). In order to compare this activity of I-butene (ex 1-butyne) with pure 1-butene, we have also included the values of the activity of pure I-butene hydrogenation at different dispersions in Figure 4 and we find that the values are similar and lie on the same line. This confirms that the turnover number of I-butene was not influenced by particle size nor by the previous hydrogenation of the acetylenic compound. Influence of dispersion on the selectivity 1-butyne hydrogenation. As reported previously, some butane was formed from the beginning of the reaction. The extent of this formation increased with dispersion as is seen in Table 2, where the formation of butane at 90 and 99% conversion of I-butyne is indicated. This behaviour is in line with the observations that at higher dispersions the I-butene hydrogenation rate remained high, whereas the 1-butyne hydrogenation rate decreased sharply. TABLE 2 Dispersion/% % butane at 90% I-butyne conversion : butane at 99% 1-butyne conversion butene hydrogenation. Pure l-butene and l-butene formed during the hydrogenation of 1-butyne disappeared with similar rates (Figure 4). Nevertheless, the similarity of behaviour is no so apparent if we consider the selectivities. The ratios butane/z Z-butenes and tr-2-butene/cis-2-butene are plotted in Figure 5 for both feedstocks. Two observations can be made: firstly, the selectivities of the parallel reactions remain constant over the whole range of dispersion and, secondly, there was a significant difference in the hydrogenation/isomerization ratio between pure I-butene and I-butene ex 1-butyne (values being 0,95 and 2.0 respectively). This fact is perhaps due to some inhibition of the double bond shift induced by trace amounts of 1-butyne remaining on the catalytic surface as it is well known that double bond shift is strongly inhibited by the presence of diolefins or acetylenes.

7 * TrBut~ne/ZCi&utem<~ 2Trans.Butene/2Cia.Buten~ 1.2. v y ButendIZButenes :; :;-.. t., t I I. * * c 3 e 1 I. I. 0.. c Dispersion Dispersion ("/.I FIGURE 5 Selectivity as a function of dispersion during the hydrogenation of 1-butene (pure and ex 1-butyne). FIGURE 6 Selectivity as a function of dispersion during the hydrogenation of 1,3-butadiene. 1,3-butadiene hydrogenation. The influence of dispersion on the rate of butadiene hydrogenation and on the butyne hydrogenation rate has been shown to be similar [S]. In Table 3 the formation of butane at 90 and 99% conversion of butadiene is shown. The influence of the dispersion on the selectivity for consecutive reactions is similar for butadiene and for butyne. TABLE 3 ~~ ~ Dispersion/% % butane at 90% 1,3-butadiene conversion % butane at 99% 1,3-butadiene conversion Contrasting with these results, the selectivities for parallel reactions (tr-2-butene/cis-2-butene and I-butene/z P-butenes) do not change with the metallic dispersion, as is clearly seen from Figure 6. The difference between the trans/cis ratios of the P-butenes formed from 1-butene (%1.6) and butadiene (12.0) may be attributed to the predominance of the anti form (95%) in the butadiene. DISCUSSION A brief summary of our results is as follows: the turnover number for the hydrogenation of I-butene (pure or ex 1-butyne) is constant, irrespective of the dispersion. However, the turnover number for the hydrogenation of higher unsaturated hydrocarbons, such as 1,3-butadiene and l-butyne, is strongly influenced by

8 48 the average particle size. In addition to hydrogenation of isoprene and again found dependent on dispersion and the nature of butadiene or 1-butyne. these hydrocarbons, we also studied the that the turnover number was strongly the curve was similar to that of 1,3- SsgO 5!6 125 m 412 1: Ihm Dispdon 0 FIGURE 7 Relative variation of turnover number with respect to dispersion for the hydrogenation of 1-butene,m ;l-butyne,n ;1,3_butadiene,+ and isoprene, A. In Figure 7 we have plotted the variation of the relative turnover number as a function of dispersion. The relative turnover number is the value obtained by dividing the turnover number by its maximum value for each hydrocarbon. Thus, for each hydrocarbon the maximum turnover number is normalised to the value of unity. This figure demonstrates that at higher dispersion there is a spectacular decrease in the activity of the hydrogenation of highly unsaturated hydrocarbons by a factor of around 12 to 15. It is worthwhile emphazing that the hydrogenation studies were performed over three different types of support, a-a1203, yt-a1203 and Si02, their surface areas varying between 9 and 200 m2 g-l. The metal loading was varied between 0.17 and 1.06 wt% and three different metal precursors, Pd(N03)2, Pd(C5H702j2 and [Pt(NH3j41 (N03J2 were used. In spite of all these variations, the turnover number seems to be strongly influenced only by the metallic dispersion. As far as the selectivities are concerned, the consecutive formation of butane increases with dispersion, whereas the selectivity for all parallel formations of unsaturated intermediates does not vary with dispersion. Several hypotheses can be proposed to explain these effects. Some of these are as follows: Polymerization. The higher dispersions could promote the polymerization of the highly unsaturated hydrocarbons that would partially plug the catalytic surface.

9 49 Such an hypothesis can be easily eliminated as consecutive hydrogenation runs over the same catalyst have been made with good reproducibility. Moreover, such a trivial modification would certainly affect the hydrogenation of 1-butene coming from 1-butyne. Reducibility. The sharp decrease of turnover number obtained for dispersions higher than 20% can not be solely explained by the presence of unreduced palladium at high dispersions. With this interpretation, it is very difficult to explain such a large effect on diolefin and alkyne hydrogenation (turnover number divided by 10) while I-butene is not affected. Moreover, all catalysts with high dispersion were prereduced at a temperature equal to or higher than 25O'C and controlled by CO chemisorption measurement at room temperature. The stoichiometry observed is close to one CO per palladium atom [83. In addition. 02-H2 titrations [9] have shown that 30, is adsorbed on one Pd atom and 3/2 H2 is consumed by one palladium atom during titration of this oxygen. Such stoichiometries can be explained only if the major part of the palladium is in the reduced form. Geometric effects. Geometric effects due to multiple site adsorption of the hydrocarbons [lo] could be invoked as an hypothesis to explain the decrease in activity at high dispersion, as multiple site adsorption normally becomes more difficult as dispersion increases. This effect can not satisfactorily explain our results, wherein we find that the three hydrocarbons butadiene, isoprene and I-butyne, which probably have different geometries of adsorption, behave in the same manner with respect to dispersion. Furthermore, the selectivities of all parallel reactions remain constant with dispersion, which is again hardly explained by this hypothesis. Support effects. It has been demonstrated by certain authors [ that the small metal particles attain the bulk properties only when they contain atoms (corresponding to a diameter of 20 A). This would correspond to a dispersion of about 50%. Certain authors proposed Cl33 that the metal-support interaction becomes strong when the metal crystallite hasveryfew atoms (less than 200). Our results demonstrate that the most spectacular fall in activity occurs between 20 and 40% dispersion, which would correspond to a total number of atoms in the crystallite. With such a large number of atoms present, one can assume that the particles show bulk metallic properties. In such particles the proportion of atoms directly influenced by the support must be very low. Hence, even if some support effect exists, it alone cannot explain the spectacular decrease of turnover number observed at dispersions higher than 20%. Hence it is difficult to attribute the low activity to these type of effects. Intrinsic metallic properties of the metallic site If geometric and support effects can be, to a first approximation, neglected, it is logical to suppose that this variation in activity is due to changes in the electronic properties of the metal as dispersion increases. At high metallic

10 50 dispersions the surface atoms of small metal particles present a high coordinative unsaturation. Under these conditions they can coordinate more strongly with different species. Our earlier work on the infrared study of CO adsorption showed that there is a displacement of the bands towards lower frequencies as the dispersion increases from 27 to 86%, thus confirming a reinforcement of the M-CO bond at higher dispersion [S]. FIGURE 8 Infrared spectra of the hydrogenation of I-butyne over two different catalysts. 1, introduction of I-butyne; 2, introduction of hydrogen; 3, after 5 min; 4, after 15 min; 4', after 12 h. We also tried-to simulate the hydrogenation of I-butyne in the IR cell and the result of this study is given in Figure 8. On introduction of I-butyne along with a stream of nitrogen over a catalyst wafer, we observe a peak at 2970 cm -1, characteristic of adsorbed I-butyne. Over a low dispersion catalyst this peak disappears completely in about 15 min when hydrogen is introduced, whereas over a highly dispersed catalyst this peak does not disappear even after 12 h, thus indicating the formation of a strong complex over the highly dispersed catalyst. This type of formation of a strong complex with acetylenics and diolefins is known to occur with metal complexes in coordination chemistry [14]. Over such homogeneous catalysts the activity for the hydrogenation of butadiene and I-butyne is known to be very low [15,16]. Thus it is logical to consider the highly dispersed catalyst as a supported homogeneous catalyst. This type of comparison has already been made in the literature Hence we suppose that the low activity shown by the highly dispersed metal might be due to its electron deficient character. The surface atoms

11 51 of small particles coordinate more strongly with species which are electron donors. This has been shown previously over platinum [18]. In our case we can consider that highly unsaturated hydrocarbons are electron donors, as assumed by several authors [14], thus explaining the formation of their strong complexes. If too strong complexation with highly unsaturated hydrocarbons is the reason why turnover is low at high dispersion, it would be possible, as in homogeneous catalysis, to increase the activity and selectivity by adding a convenient electron donating additive (ligand). We will show in a later paper how a nitrogen compound, such as piperidine, can produce such a ligand effect. ACKNOWLEDGEMENT One of the authors (S.V) wishes to thank Engineers India Ltd. (India), for study leave and the French Government for the grant of a scholarship to carry out this work. REFERENCES 1 W.T. McGown, C. Kemball, D.A. Whan and M.S. Scurrell, JCS Faraday Trans. I, 73 (1977) Al. Ammar and G. Webb, JCS Faraday Trans. I, 74 (1978) B. Nowicki, J. Klimeck and A. Masiarz, Przemysl Chemiczny, 55 (1976) H. Blume, R. Batz, R. Schubert and D. Bernhart, Chem. Techn., 25 (1973) R.S. Mann and K.C. Khulbe, J. Catal., IO (1968) R.S. Mann and K.C. Khulbe. J. Catal.. 13 (1969) R.S. Mann and K.C. Khulbe; J. Catal.; 17 (1970) 46. J.P. Boitiaux, J. Cosyns and S. Vasudevan, Proc. 3rd Int. Symp. on the Scientific Bases for the Preparation of Heterogeneous Catalysts, Belgium, Elsevier, (1982). S. Fuentes and F. Figueras, J. Chem. Sot. Faraday Trans. I, 18 (1978) 74. G. Webbs, in Comprehensive Chemical Kinetics, Vol. 20, Chap. 1. N.R. Avery and J.V. Sanders, J. Catal., 18 (1970) 129. D.N. Ross, K. Kinositha and P. Stonehart, J. Catal., 32 (1974) 163. J.R. Anderson, in Structure of Metallic Catalysts, Academic Press, (1975) 275. R. Ugo, Catal. Rev. Sci. Eng., 11 (1975) 225. E.W. Stern and P.K. Maples, J. Catal., 27 (1972) 120. V.M. Frolov, O.P. Parenago, L.P. Shokina and G.M. Gherkasis, React. Kinet. Catal. Lett., 16 (1981) 115. C. Carturan and G. Strukul, J. Catal., 57 (1979) 516. P. Gallezot, J. Datka, M. Primet and B. Imelik, Proc. 4th Int Cong. Catal., London, The Chemical Society, (1976), p S. Vasudevan, Thesis E.N.S.P.M. Paris (1982), Technip Ed.

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