ABSTRACT. Liquid-phase selective hydrogenation of ethyl 1 inolate to ethyl oleate has been carried out on nickel catalysts supported on sepiolite as well as.
M. Guisnet et al. (Editors), Heterogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.
SELECTIVE HYDROGENATION OF FATTY ACID ETHYL ESTERS ON SEPIOLITE-SUPPORTED Ni and Ni-Cu CATALYSTS.
F.M. BAUTISTA, J.M. CAMPELO, A. GARCIA, R. GUARDERO, D. LUNA, J.M. MARINAS and M.C. ORDOREZ. Department of Organic Chemistry, University of Cordoba, San Albert0 Magno Av., E-14004 Cordoba, Spain. ABSTRACT Liquid-phase selective hydrogenation of ethyl 1 inolate to ethyl oleate has been carried out on nickel catalysts supported on sepiolite as well as on several different supports. The influence of metal loading and Ni-Cu alloying has been studied as well. The results indicate that catalytic activity and selectivity correlate closely with some textural and/or acidbase properties of the support and selectivity increases with metal loading. Furthermore, as a general rule, Ni-Cu alloying improves in selectivity. INTRODUCTION The usual, widely used catalyst in edible oil and fat hydrogenation is still the standard combination of nickel with kieselguhr. However, various promotors such as A1203, Ti02, Zr02 or MgO are actually added in the catalyst formulation [1,21. Kieselguhr, a natural silica, is used as a support not only due to its relatively low cost but also because it acts as a filter aid, promoting selectivity, and, of course, providing a high surface area. In this respect, clay mineral sepiolite, a hydrous magnesium silicate: SiI2Mg8O32.nH20 [ 31, used currerltly as an industrial adsorbent especially in processes involving refining of mineral or fatty oils, has also proven to be an excellent support for nickel catalysts [ 4 , 5 1 in selective hydrogenation reactions. This report evaluates the catalytic behaviour of sepiolite-supported metal catalysts in the hydrogenation of fatty acid esters in order to verify the potential use of such clay supports in production-scale hydrogenation of oils and fats. EXPER MENTAL The syntheses of Ni and Ni-Cu supported catalysts were carried out by impregnation o f the supports to incipient wetness with aqueous solutions of nickel or nickel and copper as previously described 15-11 1. Bulk nickel was
obtained by reduction of nickel oxide (Merck, p.a.). An as received natural sepiolite (Sep) from Vallecas (Madrid) supplied by Tolsa S.A. was used as support with nominal chemical composition of Si02 62.0, MgO 23.0, A1203 1.7, Fez03 0.5, CaO 0.5, K20 0.6, Na20 0.3, weight loss from 293 to 1273 K 10.5%. In order to evaluate the effect of the support, we used several supported nickel catalysts (20 wt% Ni) previously studied in the liquidphase selective hydrogenation of 1,4-butynediol [ 51 as a reference. The support components of these catalysts are not only habituals such as silica (SO2, Merck); alumina (A1203, Merck); or active carbon (C, Panreac), but they also include three different AlP04 prepared according to Kearby [121 by precipitation from aluminum chloride and phosphoric acid using ammonium hydroxide solution (A1PO4-F), ethylene oxide (A1P04-E) and propylene oxide (AlP04-P); and three A1P04-A1203 (75-25 wt.%) systems similarly obtained (A1P04-A1203-F, E and P, respectively). The detailed syntheses procedures and textural properties have been published elsewhere [ 5 .1 They are sumnarized in Table 1, where the surface acidity and basicity of supports are also shown. These values were determined by a spectrophotometric method [I31 that allows titration o f the irreversibly adsorbed benzoic acid (BA, pKa= 4.19) or pyridine (PY, pKa= 5.251, employed as titrant agents o f basic and acid sites, respectively. Three nickel catalysts (Harshaw Chemie B.V. were also used as a reference: a Ni-5333 T (20 w t % Nil, a Ni-5132 P (64 w t % Ni) and a Ni-3210 T (35 w t X Nil. The metal loadings of the catalysts were determined by Atomic Absortion Spectrometry (AAS) The corresponding metal surface areas were calculated from the average crystal1 ite diameter, obtained by X-Ray Diffraction (XRD) measurements as reported in previous papers [5-111. The values obtained are summarized in Tables 2 and 3. Some catalysts were also studied by Transmission Electron Microscopy (TEM) on a Philips EM 300 type equipment. An IBAS I 1 KONTRON digital image analyser [I4 1 was used for the automatic counting of metal particles by directly using negative images. Fig. 1 shows a positive image of the Ni/Sep catalyst obtained by a Canon color laser copier 200 after negativefpositive image digital treatment. The values of crystallite diameters obtained by this method are exhibited in Fig. 2, where the values obtained from XRD and final metal loadings from AAS are also shown. Both procedures give the same results. Hydrogenation reactions [ 5-11 I were carried out in a conventional low pressure hydrogenator at controlled temperature conditions. Ethyl 1 inoleate
TABLE 1 Textural and acid-base properties of different supports. V d Acidity Basicity SUPPORT SBET (m2 9-11 (ml 9-11 (nm) ( mol 9-11 ( mol g-1) 31 174 Sepiolite 203 0.54 5.3 A1 O3 72 0.24 2.7 23 191 366 0.68 3.5 206 164 Sd2 743 0.55 1.5 124 132 C 228 0.94 2.5 227 166 A1 PO4-P 242 0.52 4.3 267 266 A1 PO4-E 156 0.68 3.6 190 200 A1 PO4-F 31 9 0.68 4.2 326 774 A1 P04-A1203-P 242 0.54 4.5 208 577 A1 P04-A1203-E 244 0.37 3.1 187 535 A1 P04-A1203-F Ni-bulk 16
TABLE 2 and selectivity, Support influence on catalytic activity ( p m o l s-l g'l) S ( X ) , in the hydrogenation o f ethyl linolate, rL, and ethyl oleate, rO, on 20 w t X supported nickel catalysts. Nickel surface, SNi, in m2 gNi-l. SUPPORT 'N i rL PO RL,O KL,O 5 Sepiolite
40a 46.0 45 11.5 27 2 8.9 t12$3 26 5.9 AlP04-P 32 47.8 A1 PO4-E 66 58.4 AlP04-F 56 13.9 A1 P04-A1203-P 42 2.5 A1 POq-A1203-E 32 3.6 AlP04-A1203-F 103 6.3 Ni-bul k 13 3.9 Ni-5333 T 1 30b 2.7 Ni-5132 P 1 93b 6.4 Ni-3210 T 1 25b 8.7 a metal loading 28.3 wtj. b BET Surface area in m g-1.
0.91 2.70 3.45 0.98 1.92 1.02 0.70 0.31 0.23 0.20 0.01 1.35 3.15 4.39
3.6 I .4 0.3 1.3 15.8 4.7 2.6 4.4 9.3 14.6 27.2 16.3 30.3 39.8
0.1 0.3 0.1 0.2 0.6 0.1 0.1 0.6 0.6 0.5 0.1 8.3 15.1 20.1
88.4 74.4 50.0 73.2 97.1 90.8 84.5 90.3 95.2 96.9 98.3 97.2 98.5 98.8
and ethyl oleate (Fluka p.a. 1, hydrogen (99.999 X , S.E.O.) and methanol, ethanol, I-propanol, 2-propanol, 1 -butanol , cyclohexanol and THF (99X, Panreac) and DMF (Merck)were used as solvents without further purification. Most hydrogenation reactions were carried out in 25 ml o f 0.5 M solutions of substrate in methanol, at 323 K, under initial hydrogen pressure of 0.41 MPa with 0.3 g of catalyst (particle size ro and, on increasing dielectric constant values of alcohol solvents, rL and ro increase, as previously obtained in several olefinic compounds [ 1 6 1, as well as S, RL,o and KL,-,, according to the results obtained in the liquid-phase selective hydrogenation of 1,4-butynedioI [ 5 .1 Thus, most experiments were carried out using methanol, the best solvent. With respect to the support influences (Table 21, they promote important changes in catalytic activity and selectivity. Thus, in order to determine the influence of textural and acid-basic properties of the supports on the catalytic properties of supported nickel catalysts, a correlation matrix using all the data in Table 1 and Table 2 was built. Results of the regression analysis of the well correlated parameter pairs are shown in Table 4. According to these, it is seen that the catalytic activity decreases on those catalysts whose supports exhibits a higher number of basic sites. Furthermore, selectivity increases with higher BET surface area and/or high pore volume and/or high surface acidity, as measured with PY. Accordingly, the best results for rL are obtained with Ni/Si02 and NiISep and for ro with Harshaw catalysts and Ni/A1203, so that rL / ro values differ strongly for different supports, ranging between 2, in Harshaw catalysts, and 50, in Ni/Sep. However, selectivity and R L t O follow a different sequence due to the double dependence, RL,o = (rL / ro) KL,o. Thus, the high selectivity in Harshaw
TABLE 4 General expression of the correlation y = ax + b obtained between some surface properties o f the supports in Table 1 and the corresponding catalytic properties of supported nickel catalysts in Table 2. Y X a b Significance(%) In rL BA -0.0036 3.745 97.9 In ro BA -0.0035 0.942 99.8 -185.231 1 -0.634 94.8 1n KL,O $;ET -0.6374 -2.839 90.4 In KL,0 1I P Y -53.0550 -0.934 99.6
In s$0 In s
1 1$$ET 1$$ET 1/PY
-41 .3829 -0.1642 -1 0.0634 206.3400 0.7949 46.6001
4.618 4.740 4.531 -0.434 -0.992 0.044
97.5 97.9 97.7 99.6 99.5 99.0
catalysts is explained by its higher KL,0 value while in Ni/Sep it can be associated with the highest value in rL / ro. All these facts, as well as the influence of metal loading on catalytic activity and selectivity, may be associated with metal-support interaction effects. An increase in metal loading in Ni/Sep catalysts (and in the metal/support ratio) leads in general (Fig. 2 ) to an increase of activity as well as to a higher selectivity, due to the increase in both rL / ro and KL,O values. Finally, the influence of Cu as a second metal in the selectivity is closely related to its influence in the K L , ~parameter. Thus, regardless of which support is used, the highest selectivity 0 9 8 % ) is closely related to the significant increase in K L , ~ , especially when Ni-Cu are in the proportion 20-0.3. Besides, catalytic activity is lower in bimetallic catalysts. Thus, taking into account that changes in the relative adsorption constant values of competitively hydrogenate substrate pairs have been used to probe changes in the electronic structure of platinum and other Group VIII metals [ 1 7 1 as well as in Ni-Cu alloys 112 I, the electronic influence of Cu promoting such an important change in R L , ~ and KL,0 ought to be considered as being the first responsible for the selectivity improvement promoted by the addition of this element to supported nickel catalysts. CONCLUSIONS We can conclude that Sepiolite could be an adequate support component to enable tailored Ni-Cu catalysts in oil and fat hydrogenation by taking into account how similar the results are when sepiolite is used as the support
instead o f A1P04 (which is the best o f all studied supports) and its comparatively lower cost as well a s the fact that, the Ni-Cu alloy (especially with a 20/0.3 proportion) exhibits substantially higher selectivity than the monometallic supported nickel catalyst, including comnercial catalysts we studied. ACNOWLEDGEMENTS The authors gratefully acknowledge the subsidy received from the Comisidn de Investigacidn Cientf fica y TBcnica (DGICYT, Project P889-0340), Ministerio de Educacidn y Ciencia as well as the financial aid from the Consejerfa de Educacidn y Ciencia d e la Junta de Andalucfa. The authors would also like to thank Harshaw Chemie B. V. for providing some catalyst samples and wish to acknowledge the grammatical revision of the manuscript carried out by Prof. M. Sullivan and the valuable help provided by Prof. F. Gracia of the Cell Biology Department in the use o f IBAS I 1 KONTRON. REFERENCES 1 R. J. Grau, A. E. Cassano and M. A. Baltanas, Catal. Rev. Sci. Eng., 30 (1988) 1. 2 J. W. E. Coenen, Ind. Eng. Chem. Fundam., 25 (1986) 43. 3 Y. Grillet, J.M. Cases, M. Francois, J. Rouquerol, and J.E. Poirier, Clay and Clay Minerals, 36 (1988) 233. 4 Ger. Offen.2., 108 (1971) 276. 5 F. M. Bautista, J. M. Campelo, A. Garcfa, R. Guardeiio, D. Luna and J. M. Marinas, in "Heterogeneous Catalysis and Fine Chemicals II", Studies in Surface Science and Catalysis Vol. 59, M. Guisnet et al., Eds., Elsevier, Amsterdam, 1991, p.269. 6 J. M. Campelo, A. Garcia, D. Luna and J. M. Marinas, Appl. Catal., 3 (1982) 315. 7 J. M. Campelo, A. Garcia, J. M. Gutierrez, D. Luna and J. M. Marinas, Appl. Catal., 7 (1983) 307. 8 J. M. Campelo, A. Garcia, D. Luna and J. M. Marinas, J. Chem. SOC., Faraday Trans. I, 80 (1982) 223. 9 J. M. Campelo, A. Garcia, D. Luna and J. M. Marinas, J.Catal., 97 (1986) 108. 10 F. M. Bautista, J. M. Campelo, A. Garcfa, R. Guardeiio, D. Luna and J. M. Marinas, J. Catal., 125 (1990) 171. 1 1 F. M. Bautista, J. M. Campelo, A. Garcfa, R. Guardeiio, D. Luna and J. M. Marinas, J. Mol. Catal., 67 (1991) 91. 12 K. Kearby, in: Proc. 2nd. Inter. Congr. Catal., (Technip. Ed.), Paris, 1961, p. 2567. 13 J. M. Campelo, A. Garcia, J. M. Gutierrez, D. Luna and J. M. Marinas, Can. J. Chem., 61 (1983) 2567. 14 W. Kllditz, Praktische Metallographie, 18 (1981) 105. 15 L. Cerveny, J. Vopatova and V. Ruzicka, React. Kinet. Catal. Lett., 19 (1982) 223. 16' F. h. Bautista, J. M. Campelo, A. Garcia, R. GuardeRo, D. Luna, and J. M. Marinas, J. Chem. SOC. Perkin Trans. 11, (1989)493. 17 T. T. Phuong, J. Massardier and P. Gallezot, J. Catal., 102 (1986)456.