Journal of Molecular Catalysis, 39 (1987) 93 - 103 93

HOMOGENEOUS CATALYSIS OF CO-H, REACTIONS: HOMOLOGATION OF Cs ALCOHOLS

PHILIPPE ANDRIANARY, GERARD JENNER*, SUZANNE LIBS and GERARD TELLER

Laboratoire de Pidzochimie Organique, Chimie Organique Applique’e, (UA 469) ENSCS, et Laboratoire de Spectrochimie de Masse, Dbpartement de Chimie (G. T.) Universitk Louis Pasteur, 1, rue Blaise Pascal, 67008 Strasbourg (France)

(Received March 6, 1986;accepted July 11, 1986)

Summary

n-Propanol and isopropanol can be homologated into their next homologs by hydrocarbonylation mediated by cobalt-ruthenium catalytic mixtures. For given catalyst compositions, there is a depression in conversion due essentially to a dramatic inhibition of the hydrocarbon formation, whereas the yield and the selectivity to C4 products are highest. The effect of two important parameters (iodine promotion and total pressure) are investi- gated. Both alcohols lead to n- and isobutanol, suggesting a possible olefinic intermediate.

Introduction

Our previous investigations on the homogeneous catalysis of CO-H2 reactions were concerned with the hydrocarbonylation of methanol to acetaldehyde [l] and to ethanol [2] and with the homologation of ethanol to higher alcohols [3]. In the latter case, the combination of a cobalt cat- alyst with a ruthenium cocatalyst in appropriate Co:Ru ratios promotes substantially the formation of n-propanol and a small amount of n-butanol, which suggests a further homologation reaction. The yield of C4 alcohol how- ever is very low (propanol:butanol N 30 molar ratio) and furthermore pen- tanols are present in abysmally low yield. These results are in apparent accordance with a Schulz-Flory distribution [ 41.

The synthesis of higher alcohols from lower ones could be a valuable industrial reaction: for example, C4 products might be added to gasoline, since n-butanol and i-butanol have been recognized as adequate products to ensure solubilization of methanol in gasoline, and have been found to increase the miscibility of water in hydrocarbon-methanol mixtures [ 51.

*Author to whom correspondence should be addressed.

0304-5102/87/$3.50 @ Elsevier Sequoia/Printed in The Netherlands

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The synthesis of butanols from syngas can be achieved either by homologa- tion of alcohols or hydroformylation of propylene [ 61.

The homologation of n-propanol was considered by Wender et al. as early as 1949 [7] and Ziesecke in 1952 [8]. The catalyst was a cobalt or a cobalt-iron compound, and the other experimental conditions were quite unusual (temperature: 225 “C, pressure: 100 MPa, reaction time: 22 to 66 h). Under such conditions, propanol conversion reached 50% and reaction products included ethers (20%), butanols (30%)) pentanols (5%).

More recent patents describe the synthesis of C4 compounds by reductive carbonylation of propanol using cobalt compounds, iodine and water [ 91. Other patents detail the hydrocarbonylation of methanol employing an unusual catalytic system consisting of a cobalt compound, iodine and additional complex ligands which may be either tri(alkyl or aryl)- at-sines or stilbines or an alkyl(aryl)biphosphine disulfide [lo]: the best selectivity results regarding total C4 products achieved 27 - 31%. Finally, a dissertation reports the homologation of isopropanol in the presence of cobalt acetate [ 111.

We wish to report our homologation results, starting from n- or isopropanol in order to produce C4 and possibly C5 products. An earlier attempt performed in our laboratory [ 121 did not detect homologation products during the hydrogenation of carbon monoxide in propanol (acting as a solvent) in the presence of ruthenium catalysts. The only products observed were propyl formate and propyl acetate.

Experimental

In a typical run, 187 mg (0.75 mmol) cobalt acetate tetrahydrate are introduced into a titanium stainless steel autoclave together with 60 mg (0.15 mmol) ruthenium(II1) acetylacetonate and 380 mg (1.50 mmol) iodine and finally 5 ml of the considered C3 alcohol. The autoclave is closed, filled up with the synthesis gas in the appropriate CO:H, ratio and heated. After reaction, the autoclave is vented and the gas and liquid phases collected. In isopropanol runs, demixtion occurs, requiring careful separation of both layers. Diglyme (serving as external standard) is added to each phase. In propanol runs there is no demixtion. The liquids are analyzed by GC in conditions already described for the ethanol homologation runs [ 31.

A b brevia tions PI-OH: n-propanol iPrOH: isopropanol BuOH: n-butanol iBuOH: isobutanol PrCHO: n-butyraldehyde Przo: di-n-propyl ether

iPrOiBu: isopropyl isobutyl ether PrAc : n-propyl acetate iPrAc : isopropyl acetate BuAc: n-butyl acetate iBuAc : isobutyl acetate PrBu: n-propyl butyrate

95

z&o: di-isopropyl ether BuBu: n-butyl butyrate iPrOBu: isopropyl n-butyl ether iPriBu : isopropyl isobutyrate Coat?: cobalt acetate tetrahydrate Ru(acac)s: ruthenium tris(acetylacetonate) y= [Co]:[Co] + [Ru] p = [iBuOH] : [iBuOH] + [BuOH] + [PrCHO] .

The selectivity to a product E is the ratio (percent Cs alcohol converted into this product to percent propanol converted into products).

Results

The analysis of reaction products in the gas phase reveals the presence of carbon dioxide, propane, n-butane and isobutane. For the optimal y-value (see Abbreviations), concentration of gases are kept to a low level, thus denoting homogeneous catalysis. Using GC and MS, the liquid phase was shown to contain the products listed in Table 1. As revealed by Table 1, the mixture consists of alcohols, ethers and esters just as in the corresponding ethanol reaction. Some differences, however, should be noted.

In the case of propanol, in the liquid phase, the major undesirable product is dipropyl ether, thus confirming the tendency of the alcohol to

TABLE 1

Products in the hydrocarbonylation of propanol@

Productsb Starting alcohol: n-Propanol i-Propanol

product (“/,)b product (%)b

alcohols BuOH 23.5 BuOH 20 iBuOH 11.5 iBuOH 21.5 pentanoF 0.5 pentanoF 1.5

aldehydes PrCHO 14.5 ethers PQO 32 iPr,O 22

iPrOBu 6 iPrOiBu 7

esters Pr AC 12.5 iPrAc 3 PrBu 3 iBuAc 1 BuAc 1 iPriBu 2 BuBu 0.5

others - 7.5 alkanes propane 1.0 propane 8.0

butane traces butane 0.5 isobutane traces isobutane 0.2

aP 45 MPa, T 200 “C, t 2 h, Coacz 0.75 mm, Ru(acac)s 0.15 mmol, 12 1.5 mmol, CO/H2 = l/2. bWeight composition (based on PrOH). Water is neglected as a product. CPentanol-l + 3-methyl-1-butanol.

96

dehydrate easily under the acidic conditions imposed by the formation of HCo(CO), in situ. In comparison, the higher ethers (di-n-butyl ether and n- propyl butyl ether) are present only in trace amounts. The production of significant quantities of n-butyraldehyde is also noteworthy. C4 and C5 alcohols amount to 35% (50% if the C4 aldehyde is included) of the total products and demonstrates that the homologation takes place. Esters are formed in low yields, except for n-propylacetate which is a relatively impor- tant (and also unwanted) byproduct. It should be remembered that in the case of ethanol, ethyl acetate is also formed in large amounts.

In the case of isopropanol, for y = 0.83, the major products are the C4 and C, alcohols (43%), but no C4 aldehyde is formed. It should be noted that 35% of the products are ethers and only 6% are esters.

In order to find out how operational parameters may affect the course of the homologation process, our study was focused on some determining factors, such as catalyst, promoting agent and pressure effects on the basis of results derived from the former study performed on ethanol, in order to maximize the yield of C4 and possibly C, species.

Effect of catalyst composition Modification of the Ru:Co ratio affects conversion and product distri-

bution (Figs. 1, 2). At first sight, the progress of the homologation process is

CONVERSION

SELECTIVITY

Fig. 1. Homologation of n-propanol. Influence of catalyst composition (7 = [Co]/[Co] + WI).

97

CONVERSION SELECTIVITY

%

l hydrocarbons I

25

I I

0.5 1.0 II

Fig. 2. Homologation of isopropanol. Influence of catalyst composition (7 = [Co]/[Co] + WI).

considerably modified by the addition of ruthenium to the typical cobalt catalyst. As regards conversion, isopropanol shows higher reactions rates than n-propanol over the entire composition range. The behaviour of both alcohols is, nevertheless, similar: mixed Co-Ru catalysts cause an apparent depression in conversion in comparison with the conversion rates reached in the presence either of cobalt or ruthenium catalyst used alone. This is probably due to a slower dehydration of the alcohol to ether (at least for propanol, since the yield of iPr,O is only slightly decreased for Co-enriched catalyst composition) (Fig. 1). It should be observed that, for n-propanol and unlike the homologation of ethanol, the ether concentration remains high over the entire range of catalyst composition. Also the inhibition of gaseous hydrocarbon formation for optimal y-values (0.80 - 0.86) may be responsible for the lower conversion. This is an important feature of the homologation reaction (the phenomenon has already been observed in the case of ethanol [3]) and may give some indication of the function of the ruthenium catalyst.

A still more interesting point is that the yield of C4 products shows a net increase when the catalytic composition is gradually enriched with cobalt. For both alcohols, there is sharp increase in the forementioned cat- alytic range with an optimum for y-ratios between 0.80 and 0.86, which corresponds precisely to a minimum in conversion. This result can be paral- leled with the data reported in our previous paper [3] : yields and selec-

98

tivities to C4 products are highest when the ruthenium catalyst is combined with the starting cobalt catalyst. Surprisingly, while butyraldehyde is pro- duced in the case of propanol, no aIdehyde can be detected in the homologa- tion of isopropanol. The reason may be a certain reluctance of n-butyral- dehyde to be hydrogenated under given conditions, whereas i-butyraldehyde behaves differently, if produced at all. As a matter of fact, butyraldehyde is formed at any catalyst composition (Table 2). The result is indicative of the complex role played by the ruthenium cocatalyst.

Both Cs alcohols are homologated into the two butanols (n- and iso- butanol). Although an increased n-butanol concentration accompanies an increased isobutanol concentration, there is no proportionality. For both alcohols, the ruthenium catalyst used alone leads predominantly to the expected next higher alcohol (in the n-propanol case, n:iso = 25:l and with isopropanol, iso:n = 2:l). By addition of cobalt to the catalytic mixture, the other isomer is formed in increased quantities. Finally, pentanols are also formed, deriving from a subsequent homologation of the produced butanols.

As a provisional conclusion, mixed Co-Ru catalytic systems display a synergistic behavior for the production of C4 products, though the overall reaction rates are depressed (antagonistic effect).

Effect of the promoting agent The role of iodine promoters in propanol homologation is fundamental,

because no reaction is observed in the absence of a promoter, In order to investigate how molecular iodine and iodides can activate the hydrocarbon- ylation of propanols, the reaction was conducted in the presence of covalent or ionic iodides, or their mixtures (Table 3).

In the case of propanol, iodine and the covalent iodide CH31 give almost similar results. With the ionic iodide KI, the overall yield and, consequently, the yield of butanols are considerably depressed. Furthermore, the forma- tion of Pr,O is dramatically inhibited, so that the selectivity to C4 products improves significantly. We hoped to take advantage of this result by com- bining the ionic iodide (to maintain the selectivity at a high level) with either iodine or CHsI (to maintain a reasonable rate). The result was higher yields of C4 products (in comparison with run 1209), while the concentration of the ether remained low. The total selectivity to C4 products was considerably increased (72% for run 1210 and even 80% for run 1216, in comparison with 37% for run 1208). The butanol ratio p is not sensitive to the nature of the promoting agent (n-:isobutanol = 2:l). Propyl acetate is formed in an appre- ciable amount, independent of the iodide promoter.

As regards the homologation of isopropanol, the combination of KI and I, dramatically reduces the conversion, in contrast with the previous case of propanol.

Effect of pressure It is a well-known fact that, in the homologation of methanol, increased

pressures result in higher yields of ethanol [13] and that the product distri-

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TABLE 3

Effect of iodide promoter@ in the homologation of propanols

Run Promoters Conversion Products (mmol) (%)

PQO PrCHO BuOH iBuOH PrAc

Propanol 1208 I2 41 5.03 4.64 7.23 3.30 2.15 1212 CHsI 47 5.70 3.65 5.18 1.96 2.75 1209 KI 11 0.21 1.82 1.86 0.81 2.15 1211 CHsI + KIb 21 0.31 2.40 3.50 1.70 4.59 1210 KI + Izb 17 0.24 2.56 3.61 1.51 2.15 1216 KI + I$ 26 0.36 2.85 5.30 2.72 1.79 1264 KI + Izc 35 0.30 2.21 3.93 1.97 1.50

Isopropanol iPrzO iPrOBu iPrOiBu BuOH iBuOH

1224 I2 58 4.13 1.26 1.80 7.11 7.81 1259 KI + Iz 6 1.09 0 0 0.45 1.00

Wonditions as in run 1208 (footnote as in Table 2, [I]: 1.5 mmol). bConditions in run 1211: CHsI 0.16 mmol, KI 0.9 mmol; run 1210: KI 0.9 mmol, I2 0.16 mmol. CTemperature 200 “C (run 1216) and 210 “C (run 1264).

bution shifts increasingly to higher alcohols [3, 141. The role of pressure, nevertheless, is complex. In the hydrocarbonylation of ethanol, there is an optimum pressure between 40 and 45 MPa (at 180 “C). At higher pressures, the yield of propanol (homologation product) is lowered while, interestingly, the formation of butanols is favoured. Higher-boiling products are also formed, e.g. glycol ethers, as reported by Riley [ 151.

The pressure effect in the homologation of C3 alcohols was investigated with two promoting systems (I2 f KI), at least in the case of propanol (Figs. 3 and 4, Table 4). Common trends are observed, whatever the alcohol and the promoting system :

(iii)

(iv)

Conversion increases with increasing pressure. Increased pressures have detrimental effects on the etherification reac- tion. In the presence of the (KI + I,) system, the yield of ether (not shown in the figure) is kept to a very low level (less than 2%). The yield of pentanols is noticeably increased at higher pressures, partic- ularly in the case of isopropanol (cf. the former study performed on ethanol [3]). p decreases with increasing pressure, which means that pressure favours the unbranched alcohol. The same trend is observed for the further homologation products (pentanols) consisting of exclusively 1-pentanol from n-propanol as the starting alcohol, as well as l-pentanol and 3-methyl-1-butanol from the homologation of isopropanol; the unbranched/branched C5 alcohol ratio increases with increasing pressure.

101

25- . v

\

C4 mductr

v

Pr20

I I 50 100 P(MPa)

Fig. 3. Pressure effect in the homologation of n-propanol. conversion, Z: molar selectivity, p: see text).

P or

75

25

e

0.25

Promoter : 12, T: 200 “C; c:

q psntanolr I

50 100 P(MPa)

Fig. 4. Pressure effect in the homologation of n-propanol. Promoting system: KI + 12, KI:Iz = 5.5, T: 180 ‘C; C: conversion, z: molar selectivity, p: see text.

102

TABLE 4

Pressure effecta in the homologation of isopropanol

Run P Conversion Products (mmol) P (MPa) (%) iPrz0 iPrOBu iPrOiBu BuOH iBuOH Pentanols

1224 45 58 4.13 1.26 1.80 7.11 7.81 0.65 0.52 1270 70 72 3.48 1.16 1.00 7.27 7.92 1.33 0.52 1271 100 76 2.66 1.00 0.20 9.03 7.38 2.20 0.45

aConditions as in run 1224.

In the propanol reaction, the formation of C4 products (butanols + butyraldehyde) depends on the promoting system. When iodine is used as a promoter, there is an optimal pressure range, as in the corresponding ethanol reaction [3] (40 - 45 MPa at 180 “C). We are of the opinion that this is not due to a detrimental pressure effect on the catalytic activity, but to the formation of higher-boiling products (pentanols and unidentified products). With the promoting system (KI + 12), there is a continuous increase in con- centration of C4 products, at least within the investigated pressure range. At 140 MPa, more than one third of the starting propanol is transformed into C4 products. It should be observed that the selectivity to butanols is dramat- ically high: at the highest pressure, practically only alcohols are produced (96% butanols and 2.5% pentanols).

In the isopropanol reaction, only the promoting effect of iodine was investigated since the (KI + I,) system is unfavourable (cf. Table 3). The formation of butanols is only slightly enhanced by pressure. As already mentioned, the concentration of ethers decreases with increasing pressure.

Conclusions

The results from this study that are of particular interest include: (i) Association of cobalt and ruthenium in the appropriate concentration

produces a synergistic effect with respect to the selectivities of the homologation of either n-propanol or isopropanol. This result is of great value for production of butanols as possible additives to gasoline.

(ii) The presence of an iodide promoter is essential to the reaction. While elemental iodine is preferred, when maximum yield of butanols is the aim, a combination of iodine with an ionic iodide leads to higher selectivity for C4 products.

(iii) Operating pressures in the homologation reaction play an important role in the rate of product formation and in product distribution. No mechanistic scheme is suggested at this stage, since other alcohols

are still under investigation. The fact that n-propanol, as well as isopropanol, can be homologated to both n- and isobutanol is indicative of a possible

103

olefinic intermediate. Also the distribution of the products is similar to that found in the hydroformylation of propene [16], with the exception of isobutyraldehyde and the esters of isobutyric acid (probably because iso- butyraldehyde is easily hydrogenated into the corresponding alcohol, as suggested above, limiting the aldehyde available to react further according to Tischenko- or Cannizzaro-type reactions [ 171). Mechanistic details will be given in a forthcoming publication.

Acknowledgements

The authors gratefully acknowledge financial support from the GRECO CO and express their appreciation to Dr. Majowski for his help in preparing the English manuscript.

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