alumina

Applied Catalysis, 37 (1988) 105-114 Elsevier Science Publishers B.V., Amsterdam -

105 Printed in The Netherlands

Effect of Partial Reduction on the Formation of Oxygenated Compounds in the Hydrogenation of Carbon Monoxide on Rhodium/Alumina D. DUPREZ*, J. BARRAULT and C. GERON Laboratoire de Catalyse en Chimie Organique UA 350,40 Avenue du Recteur Pineau, 86022 Poitiers Cedex (France) (Received 12 February 1987, accepted 13 August 1987)

ABSTRACT The catalytic behaviour of two series of rhodium/alumina catalysts in carbon monoxide-hydrogen reactions was investigated. Chemisorption, titration and IR spectroscopy of chemisorbed carbon monoxide were used to characterize the catalysts. The first series which consisted of well-reduced and variously dispersed samples (dispersion 7% 500 o C) . The second series comprised catalysts partially reducible at 500” C, which had been prepared by air treatment at high temperature (extent of reduction 22% < Rc 100%). Well-reduced samples showed maximum selectivity to oxygenates (mainly ethanol) for a particle size of 3-4 nm. This high oxo-selectivity is correlated with a lower gem/linear ratio of the carbonyl bands. Surprisingly, partial reduction severely reduced the selectivity to oxygenated compounds. These results appear to be in contradiction to previous reports which ascribed the formation of oxygenated to oxidized rhodium centres or to dual Rho-Rh”+ sites. Possible explanation of these discrepancies will be examined. Differences in Rh”+ site densities appear as being the most likely reason for the discrepancies.

INTRODUCTION

Rhodium catalysts have been recognized to be very active in carbon monoxide-hydrogen reactions, yielding both hydrocarbons and oxygenated comFor the last ten years, attention has been paid to the possible pounds [l-5]. influence of Rhn+ species upon the selectivity of rhodium catalysts [ 6-101. In those previous studies, Rhn+ species were generated by using rhodium oxide [ 61 or adequate promoters (manganese, vanadium) of the rhodium catalysts [ 7-121. It is now well-known that air treatment at high temperature can stabilize Rh3+ions in the support of Rh/A1203 catalysts so that these solids may be considered to be Rho--Rh3+/A1203 mixed compounds [ 13,141. Recently, it was shown that the presence of those unreduced phases of rhodium could markedly alter the properties of rhodium catalysts in steam reforming reac-

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106

tions [ 151. In the samples, Rh3+ centres were stabilized by the support itself and not by a promoter as in refs. 7-12. It was therefore of the greatest interest to study the same catalyst samples in carbon monoxide-hydrogen reactions in order to compare the influence of the stabilization method of Rh3+ centres in Rh/A1203 catalysts. EXPERIMENTAL

Materials The Rhone Poulenc GFS C y-alumina was used as a support. The pellets were crushed and sieved to 0.1-0.2 mm and subsequently treated in an air or hydrogen flow at high temperature so as to avoid any sintering of the support during further pretreatments. The catalysts were prepared by exchange of the support with an aqueous solution of rhodium chloride hydrate. The “parent” catalysts (2 and 4% rhodium) were filtered and dried at 120°C. Partially reduced samples were obtained from the 2% rhodium parent catalyst by air calcination at T> 650’ C. These are referred to as RhZAx, where x is the calcination temperature, Air treatment induces both the formation of a diffuse oxide phase (D.O.P.), non-reducible at 5OO”C, and a correlated sintering of the still reducible fraction: an increase of x results in a decrease of the degree of reduction R and of the dispersion D of the metallic phase. Therefore, for purposes of comparison, entirely reducible and variously dispersed samples were also prepared by treating the 4% rhodium catalyst in a hydrogen flow at high temperature. These are referred to as Rh4AyH, where y is the temperature of the treatment hydrogen. The high metal loading in this series was chosen so as to obtain relatively large particles at moderate temperatures ( < 1000 ‘C ) . The sulphur contamination by sulphate reduction was prevented by pretreatment of the support itself at 1000°C prior to rhodium impregnation. The efficiency of this treatment was supported by the fact that scanning transmission electron microscopy examination did not reveal the presence of sulphur in these catalysts. Dispersion Dispersions D were measured by hydrogen chemisorption ( H,) , oxygen (0,) and hydrogen ( HT) titrations, using a pulse chromatographic apparatus, which has been described elsewhere [ 161. The degrees of reduction R were deduced from oxygen uptake at 500” C, given that the metallic fraction would be reoxidised, at this temperature, into Rh203. The catalyst characteristics are recorded in Table 1. As Hc, Or, and HT gave coherent results, only dispersions deduced from Or (O,/Rh, =2, where Rh, stands for accessible rhodium atoms) are reported in the table.

107 TABLE 1 Catalyst characteristics All samples were re-reduced at 500°C prior to chemisorption and titrations. A =air; H = hydrogen Sample No.

1

2 3 4 5’ 6

Name

Rh2A120 Rh4A500H Rh4A900H Rh4AlOOOH Rh4A650 Rh2A900

Successive pretreatment (“Cl support

Catalyst

A 900 H 1000 H 1000 H 1000 H 1000 A 900

A A A A A A

120 H 450 H 450 H 450 H 650 H 900 H

500 500 900 1000 500 500

R (%I

D (%I

100 100 100 100 75 22

70 55 25 7 32 27

*Obtained from sample 2 by additional pretreatment.

IR spectra IR spectra of chemisorbed carbon monoxide were recorded at room temperature using Fourier transform spectrometer Nicolet lo-MX. The resolution was better than 2 cm-’ in the range 2400-1600 cm-‘. The cell equipped with ceasium chloride windows was connected to a gas handling and a vacuum system (ultimate pressure lo-* Pa). The samples were compressed under 5. lo5 kPa pressure to obtain wafers (18 mm in diameter, 30-40 mg) . Prior to IR measurements, the catalysts were re-reduced for 2 h at 723 K in flowing hydrogen (4. lo4 Pa) and subsequently evacuated at the same temperature up to 10e4 Pa. They were cooled to room temperature (R.T.) , contacted with lo4 Pa of carbon monoxide and finally evacuated at R.T. (lob4 Pa, 5 min) . The IR spectra were recorded in absorbance for about the same weight of rhodium in the wafer. This was obtained by diluting the high rhodium loaded samples with pure alumina. Carbon monoxide-hydrogen

reactions

These were performed in a dynamic reactor at atmospheric pressure (temperature range 200-260°C hydrogen to carbon monoxide molar ratio of 1, timeon-stream tn, 2-12 h) . The volume hourly space velocity was varied from 3000 to 7500 h-’ to maintain the conversion at less than 5%. The products were analyzed by gas chromatography using a Porapak R column. The activities A are given in mole per g of catalyst and the turnover frequencies Nare calculated per metal site.

108

absorbance

In :

R100 -3

R 75 R 22 D) 2150

I

2000

wavenumber 1850

5 6 I cm

-1

1700

Fig. 1. IR spectra of carbon monoxide chemisorbed at 25°C on rhodium catalysts with: (a) total reduction and various dispersions D ( % ) , (b) similar dispersions (25-32%) and various degrees of reduction R ( %) . RESULTS

The IR spectra of carbon monoxide chemisorbed on the catalyst at 25 aC are shown in Fig. 1. They are composed of three main bands in the range 2000-2100 cm-’ and of a large band centred around 1860 cm-l. Numerous studies [ 17-211

109

TABLE 2 Activities and selectivities of well-reduced Rh/A1203 catalyst in CO/H, 200-260°C)

reactions (1 atm,

Sample 1

2

3

5

55

25

7

D (%) 70 T”C! TV(h)

255 5

Activity* TOF (h-‘1

8.8 6.3

Selectivity** co, % vol. Hydrocarbons CH, GH, GHG GH, f&H, C,H, GH,, C, G Ci Alcohols G C, C:,

255 8 6.8 4.9

5-8

219 2

252 6

4.5 2.1

20.3 9.2

Traces

207 5

256 10

3.4 3.4

226 11

232 3

41.3 41

7.5 7.5

5.2 19

Traces

67.5 0.8 6.7 8.4 4.4 3.3 4.1 3.0 -

72 0.8 5.5 9.3 3.8 3.2 3.9 2.0 -

38 0.8 4.6 9.5 3 4.3 5.3 8.8 7.9 -

49.5 0.2 5.3 7.5 8.6 3.6 6.6 7.0 4.9 -

33 1 0.6 10.6 5.1 1.7 8.5 4.2 -

49.5 0.3 2.9 11.6 5.0 4.3 5.9 7.2 4.5 -

40 0.8 1.3 12.3 -

15 2 1.3 19.0 -

5.4 3.5 7.5 5.0 -

11.8 4.5 20 12.0 10.0

0.6 -

0.7 -

0.5 12.4 trace

0.8 4.9 0.5

2.3 30.0 0.5

1.4 6.4 trace

4.0 20.3 0.5

0.8 4.0 1.5

*Activity in 1O-4 mole CO converted per hour and per g of catalyst. **Selectivity S, =g

+

x

100 (CO, excluded from the distribution).

have ascribed these bands to certain rhodium carbonyl species, namely gemdicarbonyl Rh (CO) 2 at 2030 and 2100 cm-l, linear Rh-CO at 2063 cm-’ and bridged Rh,CO at 1860 cm-l. The effect of dispersion upon each of these bands is shown in Fig. la. The ratio gem/linear increases from about 0.2 to 10 as the dispersion increases from 7 to 70%. The results is in agreement with a previous report by Solymosi and Pas&or [ 221. It is interesting to note that the variation

110

TABLE 3 Activities and selectivities of partially reduced Rh/Al,O, catalysts (dispersion 25-32% ) in CO/H, reactions Sample 3

5

6

75

22’

232

255

R (%I 100

T”C

226 7.5 7.5

Activity TOF (h-l)* Selectivity co, % Vol. Hydrocarbons CH, C,H, C,H, GH, C,Hs C,H, C,H,, C, C, C, Alcohols C,

Cp C3

256 41.3 41

-

Traces

-

-

40 0.8 1.3 12.3 5.4 3.5 7.5 5.0

4.0 20.3 0.5

4.3 4.7

-

0.3 2.9 11.6 5.0 4.3 5.9 7.2 4.5

20 1 6 11 3.2 6.6 6.1 18.5 13 10

1.4 6.4 trace

0.1 1.0 1.0

49.5

-

1.5 13

-

50 6.3 10.5 20.5 6.2 4.5 4

-

*Turnover frequency per metal site (surface Rh3+ sites not considered)

of the gem to linear ratio is mainly due to the contribution of the gem-dicarbony1 bands which are strongly lowered on large particles, whereas the linear band is relatively less sensitive to the dispersion state of the catalysts. In the presence of an unreduced phase of rhodium (Fig. lb), the gem/linear ratio increased so that the partially reduced catalysts behaved as if they were both totally reduced and better dispersed. It is worth noting that the position of the bands is strictly independent of the dispersion and of the degree of reduction: only the density of carbonyl species is affected by these factors. The catalytic performances of the well-reduced samples are recorded in Ta-

111

ble 2. From these results it can be deduced that (i) the proportion of methane seems to be correlated with the particle size and is highest for the best dispersed catalyst; the same tendency is observed for C, hydrocarbons (ii) on the other hand, the selectivities towards higher hydrocarbons (C,) follow an opposite trend and are conspicuously favoured on the largest particles (iii) in a more complicated manner, the yields of oxygenated compounds (mainly ethanol) largely depend on the dispersion state of the catalyst: alcohols appear in the products for catalysts exhibiting dispersions lower than 60% (compare samples 1 and 2). Their yields pass through a maximum for the catalyst with 25% dispersion, and then decrease for a very low dispersion. The formation of alcohols would thus be particularly favoured on particles of 30-40 A. In all instances ethanol is the main oxygenated product. The results concerning the partially-reduced catalysts are reported in Table 3. In these samples, dispersion is quite similar (25-32% ) and can be considered constant. For purposes of comparison, the results obtained on the well-reduced catalyst and having similar dispersion ( sample 3) are recalled in the Table. Obviously the presence of an unreduced phase of rhodium prevents the formation of oxygenates. This is particularly evident as regards ethanol which cannot be formed on catalyst 6, with a reduction degree of 22%. Another interesting result reported in Table 3 is that the decrease of ethanol yield is accompanied with an increase in C, hydrocarbon which shows that ethane and ethylene are partly formed at the expense of ethanol. Also, partial reduction increases noticeably the proportion of C2 and C, (especially the corresponding olefins) in the products, whereas the selectivity towards higher hydrocarbons (C,) is severely diminished. DISCUSSION

The results presented in this report confirm that rhodium supported on alumina is able to produce oxygenated compounds from synthesis gas in fairly large proportions, even at atmospheric pressure. A maximal yield is obtained on catalysts with metallic particles of 30-40 A in diameter. Air treatment at high temperature ( > 600’ C ) induces the formation of an unreducible phase of rhodium, stabilized by the alumina support. For these partially reduced samples, it was found that the formation of oxygenated compounds was severely inhibited. The presence of an unreduced phase of rhodium also induces profound modifications in the hydrocarbon distributions, and it particularly prevents any chain lengthening. These results appear to be in contradiction to certain previous studies which supported, precisely, that the oxygenated compounds would be formed on Rhn+ sites, or on dual sites Rho-Rh3+ [ 4-101. It should be noted, however, that some important restrictions to the possible role of rhodium ions in the formation of oxygenated compounds were reported recently by Van der Lee et al. [ 111 and by Sachtler and Ichikawa [ 121. In the paper of Van der Lee et al. [ 111, an antipathetic correlation was obtained between the amount of rhodium ions, extractable by acetylacetone, and the

112

yield of C&-oxygenates: according to Van der Lee et al., only methanol (and not ethanol) would be formed on rhodium ions. However, on our catalysts, even the formation of methanol is inhibited by the presence of an unreduced phase of rhodium (see Table 3). Sachtler and Ichikawa [ 121 proposed that the carbon monoxide insertion reaction, which is thought to be responsible for the formation of CZ oxygenates, does not take place over rhodium ions but over isolated rhodium atoms. Moreover, the presence of unreduced rhodium species is not systematically evidenced in catalysts which are selective to oxygenates. For instance, Ichikawa et al. [ 231 did not mention the role of rhodium ions, and, in a recent study [ 241, Gilhooley et al. concluded that there was no obvious relationship between the oxidation state of the rhodium and the selectivity to oxygenated products. Nevertheless two reasons may be put forward to explain the apparent discrepancies between our results and those of refs. 4-10. The most simple reason would be that Rh3+ ions of the diffuse oxide phase are deeply embedded in the alumina matrix and are no longer available at the surface for catalytic reaction. Nevertheless it has been shown that Rh3+ levels are quite visible in the XPS spectra of the partially reduced sample [ 14 J . Moreover, even in the Rh2A 900 sample, the Rh3+ 3d/Rh” 3d intensity ratio is close to the bulk Rh3+/Rh” ratio measured by oxygen absorption. This hypothesis can thus be discarded. Kip et al. [ 251 reported that chlorine can severely inhibit the formation of oxygenated compounds on supported rhodium catalysts. As the particle size of our catalysts was varied by using high-temperature treatment in hydrogen, this finding can explain the increase in the selectivity to alcohols when dispersion decreases from 70% to 25%. However, such an explanation cannot account for the maximum selectivity to oxygenates which was found with metal particles of 30-40 A (Table 2 ) . Moreover, the decrease in the yield of alcohols, which was observed on partially reduced samples (Table 3), cannot originate from chlorine effects, given that the amount of chlorine in the catalyst cannot increase upon high temperature treatment in air. Another source of discrepancies between our results and the correlations of references [ 4-101 could result from differing distributions in Rh” and Rhn+ species. It is very likely that the active sites required to form oxygenates (especially Cz-oxygenates) should be dual sites containing both rhodium species, on which carbon monoxide would be dissociated (sites I) and rhodium species on which carbon monoxide would be non-dissociatively absorbed (sites II). It is clear that catalysts containing only sites I will give essentially hydrocarbons. Comparisons of catalytic and IR properties show that sites I could rise from rhodium sites adsorbing carbon monoxide as gem-dicarbonyl species at 25°C. On the other hand, site II could be rhodium sites adsorbing carbon monoxide as linear species. The amounts of carbonyl species present on the catalysts under reaction conditions are not known and the distribution gem/linear /bridged is presumably deeply modified, However, the dissociation of carbon

113

monoxide on rhodium sites, absorbing carbon monoxide as geminal species at 25 ‘C was proposed by Primet in 1978 [191 and confirmed by Zaki et al. [ 261. Moreover recent studies have shown that gem-dicarbonyl species are more reactive than linear ones [ 271, so that the formation of oxygenates will require a high proportion of sites II, on the catalysts. This is in agreement with the IR results (Fig. 1) which show that the catalysts highly selective to alcohols present a very low gem/linear ratio. Our hypothesis is that sites II would be rhodium centres in a special environment inducing an electron deficit of the metal atoms. The formation of these sites at the metal surface is likely to be very sensitive to the extent of reduction of rhodium. We think that an optimal proportion of Rh”+ is required to obtain the adequate site II/site I ratio. This optimum could correspond to a relatively low amount of unreduced rhodium atoms. This is precisely the case in promoted catalysts or in catalysts prepared on a special support. Quite different is the situation in the Rh3+/Rho catalysts stabilized by high-temperature treatment in air, as is the case in the present study: the degree of reduction is then relatively low and this could modify the site II/site I ratio in an unfavourable direction. If this hypothesis holds, the partial reduction would not be an unique criterion that determines the selectivity to oxygenates. Further work in required to bear out this hypothesis and, particularly, to determine the optimal degree of reduction. ACKNOWLEDGEMENTS

We are indebted to H. Hadrane for his assistance in performing IR. REFERENCES

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