Journal of Molecular Catalysis, 78 (1993) 295-307 Elsevier Science Publishers B.V., Amsterdam

M3067

295

Cinnamaldehyde hydrogenation: dual catalytic chemistry of iron-rhodium/Grafoil catalysts

Jonathan Phillips* The Pennsylvania State University, Department of Chemical Engineering, University Park, PA 16802-4400 (USA)

Pierre Gallezot and Gerard Bergeret Institut de Recherches SW .!a Catalyse, 2, Avenue Albert Einstein, 69626 Villeurbanne, Cedex (i?ance)

(Received April 6, 1992; revised June 12, 1992)

Abstract

Work in our laboratory has shown that many bimetallic particles, including Fe/Rh, Fe/ Ir and Pd/Ag, have multiple phase structures that can be accessed through the proper sequence of treatments. Corresponding to the multiple bulk phase structures are a variety of surface structures and compositions. In this study it is shown for Fe/Rh that the different surfaces have completely different selectivities for cinnamaldehyde hydrogenation. Moreover, it is clear that the catalytic behavior of the alloy is not a simple linear combination of the behaviors observed for the parent metals, indicating that alloys may prove to have unique properties for low-temperature selective hydrogenations.

Introduction

This is a report on a study of the use of Fe/Rh supported on graphite as a catalyst for cinnamaldehyde hydrogenation. It is intended to complement earlier studies of the activity and selectivity of base metal/noble metal alloys as catalysts in low-temperature fine-chemical processes. A great deal of previous work has gone into the study of support and particle size effects on the selectivity of individual metals used in this type of reaction [l-4 1. There has also been a considerable amount of work done on the influence of promoters [5-71. More apropos of the present study is prior work with base metal/noble metal alloy systems. Earlier work suggested that base metals can significantly influence the selectivity of noble metals [S-lo], and that further exploration is warranted.

Iron/rhodium was chosen as the alloy for study because it is a well- understood system. Recent studies indicate this system has two interesting features which might yield particularly interesting catalytic surfaces for low- temperature fine-chemical catalysis [ 1 l-131. First, the surface has two com-

*Author to whom correspondence should be addressed.

0304-5102/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

296

pletely distinct reduced states. Each of these surfaces can be produced by a distinct set of pretreatments. Following oxidation and a low-temperature reduction the surface consists of segregated patches of each of the two reduced metals (segregated metal state). The metals are not present in alloy form. In contrast, following high-temperature reduction the surface is a true alloy of iron and rhodium (alloy state). Second, it was shown that in the alloy state the catalyst had a chemistry completely different from that of either parent metal. For example, used for 1 -butene hydrogenation the catalyst in its alloy state had a combination of chemistries: the selectivity toward isomerization of one of the metals (iron) and the high activity of the other metal (rhodium).

Graphite was chosen as the support material because it is clear that neither iron, rhodium, nor iron-rhodium interacts strongly with this support [13-l 5 1. Thus the results of this study permit an examination of the chemistry of the metals themselves, with minimal impact of support effects. Future studies in which the metals are placed on other supports can be contrasted with the information reported here.

Experimental

The support material used in this study was GTA grade Grafoil (Union Carbide), which is a high-purity carbon with a high surface area (22 m2 g - ‘), consisting primarily of basal plane [16-l 81. Before use the Grafoil was finely ground, then treated in flowing hydrogen at 1200 K for 12 h. This treatment is designed to anneal the surface of the material, thus reducing the number of nucleation sites for particle growth, and to partially remove sulfur impurities [ 19 1.

Metal was added to the Grafoil by the incipient wetness technique, using heat treatments designed to produce large particles in which the metals were thoroughly mixed. The alloy catalysts were fabricated using the following procedure. First, separate aqueous solutions of iron nitrate nonahydrate and rhodium chloride hydrate were prepared. Enough of each solution was added to produce a catalyst in which the atomic ratio of the two metals was l:l, and such that the total metal loading of the catalyst was 5 wt.%. Second, after solutions were added simultaneously to a batch of prepared Grafoil, and mixed for 10 min, the catalyst was dried by heating at 330 K for 18 h in the ambient atmosphere. Third, the dried material was calcined in flowing oxygen at 525 K for 3 h, and reduced in flowing hydrogen at 725 K for 12 h. Finally, the calcination and reduction procedures (step three) were repeated.

The standard pretreatment described above left the catalyst in the alloy state. Thus, after initial fabrication the catalyst was added directly to the reactor. In contrast, to prepare the segregated metal state it was necessary to treat the catalyst in flowing oxygen at 573 K for 3 h and then in flowing hydrogen at 400 K for 3 h.

Single-metal iron, rhodium and platinum catalysts were made on the same support material. Iron- and rhodium-only catalysts were made using

297

the corresponding salt listed above, and each was made to approximately 5 wt.% loading using the incipient wetness technique. Platinum catalyst was produced using [Pt(NH&](NO& also to 5 wt.% loading. These samples were dried at 330 K for 12 h in air and then treated in flowing hydrogen at 725 K for 4 h.

In some cases catalysts were recovered from the reactor and treated a second time to test for hysteresis effects on the catalytic behavior. Recovery was done using a simple filter and a pump. After recovery on the filter the catalyst was dried at 330 K in air and then treated to produce either the alloy or the segregated metal state, as described in the text.

The same procedure was followed for all kinetic studies. Following appropriate pretreatments catalysts were mixed with a standard solution consisting of 37.5 cm3 of isopropanol, 10 cm3 of water and 2.5 cm3 of sodium acetate and pretreated in a hydrogen atmosphere in a stirred (1500 rpm) high-pressure batch reactor at 4 MPa and 333 K for 150 min (except where noted otherwise). Next, immediately following pretreatment cinna- maldehyde (0.1 mol) was introduced into the catalyst slurry under hydrogen pressure and the liquid-phase reaction started immediately. A chromatographic system was used to analyze the composition of samples periodically removed from the reactor.

To determine the phase composition and size of the particles X-ray diffraction was employed.

Results and discussion

Kinetics Distinct kinetic results were obtained as a function of the state of the

particles. On the basis of a comparison of these results with the results obtained with rhodium-only particles produced on the same support material, certain clear conclusions can be drawn. First, the catalyst in the segregated metal state has a catalytic chemistry almost identical to that of pure rhodium in terms of selectivity. Second, the chemistry of the catalyst in the alloy state is completely different from that of either parent metal.

In Fig. l(a) the results of a kinetic study of rhodium supported on Grafoil are shown. The catalyst used for this test was reduced at 725 K for 4 h. Note that the selectivity does not change significantly after approximately 20% conversion and that at all times cinnamyl alcohol (COL) is the major product (Fig. 2)

A second test, not shown, was conducted on a Rh-only catalyst treated at 725 K for 4 h in flowing hydrogen, then oxygen at 573 K for 3 h and then flowing hydrogen at 400 K for 3 h. This was meant to duplicate the treatment given to produce alloy catalysts in the segregated metal state. There was no difference in selectivity found between the two samples and only a small difference in activity, clearly showing that the pretreatments designed to produce the segregated metal and alloy states of the catalysts

SELECTIVITY OF RHODIUM

l”>

0 HCAL 30 c - _

. HCOL aP

. COL

20 -

O- &::,:.,., , . , mw- , , , ,

0 10 20 30 40 50 60 70

% CAL Conversion

SELECTIVITY IN SEGREGATED METAL STATE

30 -

20 -

10 -

% CAL CONVERSION

(ccmhnued)

299

SELECTIVITY IN ALLOY STATE

l * l *

l *aw-- . . I I I I

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Fig. 1. Product concentration as a function of the extent of CAL conversion: (a) rhodium/ Grafoil catalyst; (b) Fe-Rh/Grafoil catalyst in the segregated metal state; (c) Fe-Rh/Grafoil catalyst in the alloy state.

had no influence on the final surface chemistry of rhodium-only samples. Earlier reports suggest that small rhodium particles are very selective for hydrocinnamaldehyde (HCAL) [ 11; however, the earlier work was for very small particles ( <cu. 3 run) and it is known that metal particle size can affect the selectivity in this reaction.

Studies of platinum-only and rhodium-only catalysts used for the same reaction have clearly shown that particle size can strongly influence selectivity [2-41. Indeed, previous interpretations have suggested that large particles of all metals should yield more cinnamyl alcohol (COL). The large (see below) rhodium particles studied in this paper are more COL-selective than the small particles studied earlier. Thus the rhodium-only selectivity results appear to be consistent with earlier work.

Tests were also run on iron-only samples. The activity of these catalysts was found to be too low for reliable measurement.

In Fig. l(b) the activity as a function of the extent of conversion for a fresh Fe/Rh catalyst in the segregated metal state is shown. The selectivity behavior of this catalyst (Fig. 2) is nearly identical to that of the rhodium- only catalyst, suggesting that the rhodium ‘patches’ in the surface of the particles in the segregated metal state dominate the catalytic chemistry.

The catalytic chemistry of the particles in the alloy state is totally different from that of either parent metal. This can be seen in Fig. l(c), which shows

300

SELECTIVITY

0.75

0.65

0.55

0.35

q

0 A

ImB AM

U4AA A AA

A DII

q A

q q A

m Cl q

q q fi

l AMA 0 AA

0 Alloy

q Segregated Metal

A Rhodium

m cum

ra A

l

I t I I

20 40 60 80

% CAL Conversion

Fig. 2. Percent selectivity toward COL as a function of extent of CAL conversion, plotted for the same three catalysts and the same three runs as in Fig. 1. Selectivity is defined here to be the fraction of the three product molecules that are COL.

the selectivity as a function of conversion for a fresh Fe/Rh catalyst in the alloy state. Note that the selectivity changes as a function of conversion. At low conversions the selectivity is similar to that of rhodium and the segregated metal material. (This could be seen even more clearly in other alloy studies, not shown. This particular data set was chosen for the figure because it is the only one for which data is available at conversions greater than 80%.) However, above about 20% conversion the selectivity changes and HCAL, becomes the dominant product. This basic selectivity remains fixed up to very high levels of conversion (Fig. 2). This unusual dependence on the extent of conversion was found to exist four times out of four trials, as described later.

301

It turned out to be difficult to explain the shift in selectivity with increasing conversion. One possible explanation is that the catalyst becomes ‘activated’ under the reaction conditions, because reduction is not complete prior to introduction of the reactant. In an effort to test this hypothesis in one case the reduction time prior to charging the reactor with reactant was increased from the normal 150 min at 333 K to 1500 min, with the temperature of pretreatment raised to 366 K. This had no impact on the subsequent results. Another possible explanation is that some sites are ‘preferentially blocked’ by product or reactant molecules, and that this blocking increases with increasing levels of conversion. A similar suggestion was advanced to explain the fact that for very small particles COL selectivity increases with increasing conversion [ 11. It has also been suggested that HCAL is an inhibitor for CAL hydrogenation on FWC [20]. This suggestion appears to be somewhat consistent with the finding, discussed below, that in some cases the rate of conversion does change early in the process. A third possible explanation is that the alloy surface promotes the conversion of COL to HCAL, but the experimental evidence appears to rule this out. For example, in one case the reaction was carried out for more than 4 days (twice the period used for other data). This data is shown in Fig. l(c). It was clear at the end that both COL and HCAL were being fully hydrogenated to hydrocinnamyl alcohol (HCOL), but there was no evidence of any interconversion between the two half-hydrogenated species. In fact, an effort was made to fit the data to kinetic schemes in which there is conversion of COL to HCAL. These efforts failed, suggesting that no interconversion takes place.

The reproducibility of the results is always an issue. In an effort to address this concern several tests were performed. First, the catalyst was recovered and reactivated several times. After each reactivation the catalyst was pretreated to produce the state that was not present previously. For example, if the catalyst was in the segregated metal state for the first catalytic test, it was treated to produce the alloy state (i.e., dried and then reduced in flowing hydrogen at 725 K for 12 h) prior to being placed in the reactor for the next catalytic test. It was anticipated that each treatment would lead to additional particle growth, as the interaction between Grafoil and iron, rhodium and iron/rhodium has been shown to be weak [ 12-151.

The actual sequence of experiments performed is described below. First, catalytic tests were conducted for the sample in the alloy state (test l), and then, following recovery by filtration, in the segregated metal state (test 2). The same procedure was then performed on a second catalyst sample taken from the same parent batch (tests 3, 4). In terms of both activity and selectivity, both ‘alloy’ samples were identical. The same was true for both ‘segregated metal’ samples. These two ‘twice used’ catalysts were combined, mixed and treated together to produce an alloy catalyst (test 5). This was done because there is some material loss (less than 10%) with each filtering, and to compare results it was judged best to have virtually the same mass of catalyst (approximately 0.4 g) for each study. Finally, a third catalyst sample, also taken from the common parent batch, was used to make

302

first a segregated metal catalyst (test 6) and then an alloy catalyst (test 7).

In all cases, the behavior of the catalyst in terms of selectivity was a function of the final treatment only. That is, there was no detectable hysteresis in the selectivity. In contrast, there was a decline in activity following each recovery. This decline is documented in Fig. 3(a). It should be noted that the data are plotted in such a manner that if the kinetics are first order, then the data will fall on a straight line. It is suggested that the primary cause of activity loss is loss of surface area resulting from sintering; however, there is not enough particle-size data to quantitatively support this model.

Note that for tests 1 and 3 (and, in fact, for tests 2, 5, 6 and 7 - not shown) there appear to be two straight-line regions, one at high rates, the second at lower rates. It is possible that this indicates two rate-limiting processes.

At low concentrations (times greater than cu. 300 min) the mechanism can be written as first order in cinnamaldehyde with constant concentrations of catalyst sites and hydrogen. Langmuir-Hinshelwood kinetics predict a first-order dependence if either the cinnamaldehyde adsorption is weak, or hydrogen, isopropanol or any other species is much more strongly adsorbed such that the fractional coverage of the surfaces by cinnamaldehyde is low. The rate constants derived on the basis of this assumption for the low- concentration regions in Fig. 3(c) are given in Table 1.

It is difficult to interpret the data at short times (times less than cu. 300 min). Many factors, including hydrogen diffusion to the surface, depletion of hydrogen reservoirs in the liquid or on the metal surface, could play a role. One explanation is attractive because of its simplicity: Initially virtually all the metal sites are active, yielding very high rates. As product molecules (COL, HCOL or HCAL) are produced, they block the majority of sites, severely lowering the activity and leading to the first-order dependence seen at longer times. Selective blocking of sites by product molecules could even lead to shifts in selectivity, as observed for the catalyst in the alloy state. The similarity in the rates among all the catalysts at very short times could represent a mass-transfer (hydrogen) limit.

Other interesting features of the results include the surprising finding that Fe-Rh in the segregated metal state has virtually the same selectivity as a Rh-only catalyst (Fig. 2), yet has higher activity (Table 1). Yet, earlier studies suggested that only on the order of 10% of the sites on the segregated metal surface are rhodium. That is, the X-ray data for rhodium zones in the segregated metal particles cannot be used to compute the rhodium surface area. Earlier work clearly showed that iron metal dominates the surface of particles in this state. Most of the rhodium is buried [ 131. In sum, the exposed rhodium surface area is expected to be far smaller than that suggested by the size of the rhodium zones. Given the similarity in particle size of the two catalysts (next section), this suggests that rhodium sites in the Fe-Rh catalyst are more active than those on rhodium only. This could be due to modest changes in heats of adsorption, resulting in higher cinnamaldehyde coverages.

303

E FelRh Alloy (Test #l)

. FelRh Segregated Metal (Test #3)

q FeiRh Alloy (Test #.I)

0 Platinum

n Rhodium

0 1000 2000 3000

TIME (minutes)

0.3 -

S

2

3 : 0.2 -

e

0.1 -

@>

Fig. 3.

. q :

q . q . :

q .

.

. q . .

1. : 0

100 2 0 0 300

TIME (minutes)

(continued)

Time (minutes)

Fig. 3. First-order rate plots. (a) For all highly active catalysts there are two linear regions, suggesting two different mechanisms as a function of conversion. (b) The initial rate is virtually the same for six out of the seven catalysts shown. (c) Only data taken after more than 290 min have elapsed are plotted. The data from each catalyst are fit to a single straight line. The rate constants derived are given in Table 1.

TABLE 1

Reaction rate constants

Catalyst Test no. Rate constant x 1 O4 ((g cat)-’ min-‘)

Fe-Rh alloy 3 13.9 Fe-Rh seg. metal 4 12.0 Fe-Rh alloy 5 6.3 Rh only 7.5

Note: All values determined from a curve fit of Fig. 3(c), and the weight of the sample.

One final subject that requires comment is the relationship between particle size and selectivity. All of the catalysts tested have a relatively large particle size and all have fairly high COL selectivity. There is strong variation of selectivity with structure, but this is consistent with earlier work, which indicated that a variety of factors, in particular steric factors [22], particle size, impurities and promoters [2 1 ] and identity of the metal, can influence selectivity.

305

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306

In order to demonstrate that the bimetallic particles were in the claimed state (i.e., segregated metal or alloy), X-ray diffraction studies were performed. In Fig. 4(a) an X-ray spectrum of a sample following the standard treatment used to produce an alloy catalyst is shown, and Fig. 403) shows the X-ray spectrum following treatment designed to produce the segregated metal catalysts. It is clear that following the low-temperature reduction treatment (Fig. 4(a)) iron and rhodium exist separately as reduced metals. It is only after the high-temperature reduction (Fig. 4(b)) that an alloy exists. As the positions of diffraction lines for the alloy are somewhat complex and generally depend strongly on composition, some explanatory comments are needed. The five lines superposed over the pattern (4(b)) show the positions of the lines of two Fe-Rh alloys (JPDS-ICDD file card no. 25-1408 [23]). Comparison of the pattern with this standard data shows the presence of FeRh alloyed particles and indicates that the Fe/Rh ratio is near one, as in the reference alloys (4:4 or 5:4). Thus it is probable that virtually all the metal is alloyed and very little is present as pure iron or pure rhodium.

Approximate average particle size can be obtained from the Sherrer equation. The rhodium zones in the segregated metal sample (Fig. 4(a)) are about 6 nm in size. The iron lines are too noisy for a confident measure to be produced, but it appears that the iron zones are considerably smaller than the rhodium zones. The alloy particles in Fig. 4(b) are about 5 nm in size, and the pure rhodium particles on the Rh/Grafoil sample were about 8 nm in size. These results suggest that the particles are similar in size in all these cases.

The X-ray data alone are not sufficient to establish the morphology of the particles. This data is presented only as a means to confirm that the various treatments produce the phases predicted on the basis of earlier work. In the earlier work the same support material, the same treatments and the same metals were used, but the metals were introduced via the thermal decomposition of a bimetallic cluster, and not by salts as in the present case. However, it is felt that the X-ray work serves as an adequate indication that the phase behavior of the catalysts used in the present study is very similar to that found in earlier work.

Conclusion

The results presented here clearly show that the two reduced surfaces of the Fe-Rh/Grafoil catalysts have very different activities and selectivities. In the segregated metal state the selectivity is very similar to that of pure rhodium, but the activity is higher. This result is not surprising, as in the segregated metal state the surface is thought to be composed of patches of reduced metallic iron (90%) and patches of reduced metallic rhodium (10%). The surface might be expected to show behavior similar to that of a physical mixture of the two metals. Since iron itself was shown to be inactive for

307

this reaction, it is not surprising that such a surface will be dominated by the behavior of the rhodium patches.

In contrast, the true alloy shows behavior unlike either metal. For example, the selectivity changes dramatically as a function of the extent of conversion and is quite different from that of rhodium. It is not clear what causes the selectivity to change; no explanation is fully satisfactory. In any event, this result suggests that additional study of bimetallic systems for fine-chemical catalysis is warranted. There is simply not enough data available for us to be able to predict the catalytic chemistry of such systems, as they do not necessarily have a chemistry that is a linear combination of the chemistries of the parent metals. It might prove possible to produce bimetallic catalysts with activities and selectivities better than either parent metal and better than any single metal.

References

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Int. Congr. Cutal., Vol. 3, The Chemical Institute of Canada, Ottawa, Ont. 1988, p. 1074. 5 S. Galvagno, 2. Poltarzewski, A. Donato, G. Neriaud and R. Pietropaolo, J. Chem. Sot.,

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