Liquid phase hydrogenation of cinnamaldehyde over supported ruthenium catalysts: Influence of particle size, bimetallics and nature of support

Journal of Molecular Catalysis, 85 (1993) 215-228 Elsevier Science Publishers B.V., Amsterdam

215

M274

Liquid phase hydrogenation of cinnamaldehyde over supported ruthenium catalysts: influence of particle size, bimetallics and nature of support B. Coq*, P.S. Kumbhar, C. Moreau, P. Moreau and M.G. Warawdekar Laboratoire de Chimie Organique Physique et Cinktique Chimique Appliqukes, URA 418 CNRS, ENSCM, 8 rue de l%cole Normale, 34053 Montpellier Cedex 01 (France); (+33)67144353 (Received April 16,1993; accepted July 27,1993)

Abstract Hydrogenation of cinnamaldehyde was studied over supported Ru based catalysts. The influence of particle size, bimetallic formulation and nature of support on activity and selectivity to the desired product, cinnamyl alcohol, was investigated. Ru particle size had a pronounced effect on both TOF and selectivity. Larger Ru particles showed higher selectivity to cinnamyl alcohol, as well as higher TOF. Addition of Sn, Fe, Ge, Sb, Zn and Ag as a second metal improved both activity and selectivity. Ru-Sn/Al,OS showed sustained improvement in selectivity, even at higher conversions, probably due to the presence of Ru-SnJ+ sites and/or due to the electron transfer from Sn to Ru. ZrOz was found to be the most effective support in terms of selectivity. Possible reasons for this behaviour of ZrCz are delineated. Key words: bimetallic catalysts; cinnamaldehyde; hydrogenation; selectivity

Introduction Selective hydrogenation of cw,j?-unsaturated aldehydes and ketones to the corresponding unsaturated alcohols is of considerable interest due to its commercial importance. It is well established that over supported Group VIII metals the C-C bond is hydrogenated preferentially to the C-O bond. The research in this field has been recently reviewed by Gallezot et al. [ 11 with special emphasis on the hydrogenation of cinnamaldehyde. From the literature review it is evident that the selectivity to cinnamyl alcohol, the desired product, is better: (i) over Ir than Pd, Pt, Rh or Ru; (ii) on large metal particles; (iii) when graphite is used as a support in place of carbon or silica; (iv) on metals supported over zeolites containing alkali cations; (v) on bimetallic catalysts like Pt-Fe/C. In several other studies dealing with the hydrogenation of cinnamaldehyde, these results have been discussed in the general frame of the fol*Corresponding author.

0304-5102/93/$06.00

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lowing two points [ 2-61: (i) the electronic or ligand effect of the support, or the doped metal, wherein there is a transfer of electrons from the support, or the doped metal, to the active metal which results in an increase in the electron density at the active metal site and decreases the probability of hydrogenation of the C=C bond by lowering the interaction between the K bonds of C-C and the metal; (ii) formation of new catalytic sites involving promoter cations, which are able to co-ordinate the oxygen atom in the carbonyl group and activate it. However, the generality of the above conclusions has been always questioned due to the lack of an exhaustive study on several cw,&unsaturated aldehydes over the same family of catalysts, and due to the contradictory results obtained by changing the metal precursor. In particular, the role of residual chloride ions on the selectivity is still not clear. Nitta et al. [7] have reported an increase in the selectivity to unsaturated alcohol over Co catalyst containing chloride ions, whereas the reverse effect has been observed over Pt and Ru catalysts [8,9]. Considering the facts outlined above, we planned a programme to study Ru based catalysts for hydrogenation of series of @-unsaturated aldehydes to arrive at general guidelines for designing selective catalysts. To avoid the effect arising from residual chloride ions, the catalysts were prepared by using organometallic precursors. In the first part of this programme, vapour phase hydrogenation of acrolein [lo], 2-methylpropenal, 2-butenal and 3-methyl-2butenal [ 111 were studied on a series of well characterised supported monometallic and bimetallic Ru catalysts. In the present paper we report the results on liquid phase hydrogenation of cinnamaldehyde over the same family of catalysts. A comparison is made with the results reported in earlier papers. Special attention was paid to studying the effect of Ru dispersion on alumina, the bimetallic Ru based catalysts and the influence of the nature of the support.

Experimental Chemicals

Cinnamaldehyde (Aldrich, 99% ) and i-propanol (SDS, 99.7%) were used without further purification. Ruthenium acetylacetonate, silver nitrate, iron acetylacetonate, tetrabutyltin, tetrabutylgermanium, tributylantimony and diethylzinc were used as precursors for preparation of the catalysts in order to avoid the effect of residual ions. The supports for the catalysts were y-A1203 (Rhane-Poulenc, 200 m2 g-’ ), Si02 (Rhene-Poulenc, 175 m2 g-l), Ti02 (Degussa, anatase, 120 m2 g-l ), graphite (Lonza HSAG, 300 m2 g- ’ ) and Zr02 (200 and 140 m2 g-l). Zr02 was prepared in the laboratory by precipitation of zirconium hydroxide from ZrOC12*8H20 (Aldrich, 98% ) and ammonia in a fixed alkaline medium, followed by drying and calcination at 200’ C and 400” C, respectively.

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217

Catalyst preparation

The supported monometallic Ru catalysts were prepared by adsorption of the ruthenium acetylacetonate precursor. The requisite amount of the precursor was dissolved in benzene, then contacted with the support for 15-60 h, followed by filtration. The solids were treated under a stream of nitrogen at 423 or 523 K followed by reduction in a stream of pure or diluted hydrogen. Details of the preparation method are given elsewhere [lo]. All the bimetallic catalysts were prepared starting from the pre-reduced Ru/A1203 (referred as RuMGAl) catalyst. The Ru-Sn, Ge, Sb, and Zn catalyst, supported on alumina were prepared by using the “controlled surface reaction” method as described previously [ 121. In brief, the basic principle of this method constitutes reacting the parent Ru/A1203 catalyst (RuMGAl, reduced in situ before use) with the desired amount of the ( C4H9)4Sn, for instance, in n-heptane solution under hydrogen. The reaction was carried out at 353 K. The RuFe/A1203 catalyst was prepared by impregnation of the pre-reduced RuMGAl with Fe(acac)3 in toluene. The Ru-Ag/AlzOs catalyst was prepared by impregnation of RuMGAl catalyst with an aqueous solution of AgN03. All the bimetallic catalysts were dried at room temperature under vacuum and then reduced at 623 K using diluted hydrogen. In all the cases the atomic ratio of Ru to the second metal was adjusted to 1: 0.25. The samples were stored in sealed bottles under air without any extra precaution. The metal contents were determined from the UV absorbence of the precursor solutions before and after contact. For a few samples this was counterchecked by atomic absorption spectroscopy. The values determined by both these methods were comparable. Chracterisation

The catalysts were characterised by hydrogen chemisorption and transmission electron microscopy. The hydrogen chemisorption was carried out in a static volumetric apparatus. The chemisorption was performed following the method of Yang and Goodwin [ 131. The hydrogen adsorption isotherms were determined in the range of 6.6-33 kPa at 373 K. After the first adsorption isotherm the cell was outgassed for 5 min at the same temperature and a second adsorption isotherm was obtained in same manner. The difference between the two isotherms, extrapolated to zero pressure, gave the quantity of irreversibly adsorbed hydrogen (the stoichiometry of Hi,/Ru was assumed to be unity, as proposed by Yang and Goodwin [ 13 ] ) . The dispersion was determined from this value. For some catalysts the results of the chemisorption were checked by TEM using a JEOL 1OOCXmicroscope. The main features of the catalysts prepared are given in Table 1. Catalytic experiments

Hydrogenation of cinnamaldehyde was studied in a batch mode in a 100 ml autoclave (Autoclave Engineers) equipped with a Rushton turbine hollow

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TABLE 1 Main properties of the catalysts Catalysts

Support Ru H/M (wt.%)

Reduction step temp. time (h) gas (vol.%/vol.%) (K)

Ru sponge RuAblSAl RuEECIII RuM4AL RuMGAl RuM5Al RuMSAl” RuSn/A120sb RuFe/AlsOsb RuGe/A1,Osb RuSb/AlsOs’ RuZn/AlrOs’ RuAg/A120s8 Ru/G” Ru/TiOz( LTR)” Ru/TiOs(HTR)” Ru/SiOza Ru/ZrO*-A’ Ru/Zr&-Te

100 ~-Al&s 0.86 Y-AI& 4.0 Y-AIJ~ 0.68 ~-Also, 1.1 ~-Al&s 1.05 Y-AI& 0.68 Y-AU& 1.1 Y-A&G3 1.1 Y-AI& 1.1 Y-AI& 1.1 Y-A& 1.1 ~-Al&s 1.1 graphite 0.9 TiOz 1.1 TiOz 1.1 SiOz 1.29 ZrOz 1.2 ZrOz 1.01

25x10-’ 0.07 0.25 0.40 0.60 0.69 0.72 0.20 0.34 0.29 0.28 0.28 0.27 0.02 0.87 0.04 0.29 0.25 0.59

773 823 573 623 623 623 623 623 623 623 623 623 623 523 823 623 623 573

24 34 4 4 4 4 3 3 3 3 3 3 4 4 4 4 4 4

Hr+HrO (97.5+2.5) Hs+HsO (97.5f2.5) H2+N2 (10/90) Hs+Ns (20/80) H,+Ns (20/80) H2+N2 (10/90) H2+N2 (10/90) H2+Nz (10/90) Hs+Nr (10/90) H2+N2 (10/90) H2+N2 (10/90) Hz+N2 (10/90) H2+N2 (10/90) H2+N, (20/80) Hz H2+Ns (10/90) H2+N2 (10/90) H2+Nz (10/90)

Mean size from TEM (nm)

>lO 2.7 1.9 1.1 1.1 2.1 1.05 1.25 1.4 1.7 2.1

“Dried in Nz stream at 523 K before reduction; the rest were dried at 423 K. bRu/M (second metal) atomic ratio= l/0.25. “A = Amorphous, T = Tetragonal.

stirrer, a sample port, gas inlet and vent. The sample tube was modified by placing a filter at its end to prevent carry over of catalyst particles while sampling. The pre-reduced catalyst (0.26 g, unless otherwise specified) was reactivated in i-propanol (40 cm3) at 383 K and at an hydrogen pressure of 4.5 MPa for 2 h in the autoclave while stirring. After cooling the autoclave to room temperature, cinnamaldehyde (5 cm3) in 10 cm3 of isopropanol was injected through the charging port. The autoclave was purged with hydrogen and the temperature was raised to 383 K rapidly without stirring. At this temperature hydrogen (4.5 MPa) was introduced and the stirring was started. This was taken as the initial time. The stirrer speed was 1550 rpm in all cases and was chosen after confirming that there were no mass transfer limitations. Micro samples were withdrawn periodically and analysed on a gas chromatograph (Varian 3300) using a DBWax capillary column (30 m x 0.53 mm i.d. ). The calibration was done by using synthetic mixtures of pure components.

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219

Results and discussion

Fig. I shows a typicalproduct distributionas a function of reaction time obtained during the hydrogenation of cinnamaldehyde (CAL) over the RuM6AI catalyst.Under the experimental conditions used, the main reaction products are hydrocinnamaldehyde (HCAL), cinnamyl alcohol (COL), hydrocinnamyl alcohol (HCOL), hydrocinnamaldehyde diisopropylacetal,flmethylstyrene and 1-phenylpropane. As reported earlierby Galvagno et al. [14] the diacetal is formed by reaction between H C A L and the solvent,isopropanol, and was always in equilibrium with HCAL. Therefore, for simplification H C A L and the acetal are reported together. Itiswell establishedthat the hydrogenation of ~,fi-unsaturatedaldehydes 100

80

6O

40

[]

20

0 0

10

20

30

Time (h)

Fig. I.Hydrogenation of cinnamaldehyde over Ru/AI20s (RuM6AI) catalystas a function of time. TR----383 K, C ~ d ~ h y d . = 7.9 X 10-4 toolcm-3, Pkvd~os.n= 4.5 MPa, catalystweight = 0.78 g, (•) cinnamaldehyde, ([]) hydrocinnamaldehyde + hydrocinnamaldehyde diisopropylacetal, (O) cinnamyl alcohol, (®) hydrocinnamyl alcohol), (/x) ]~-rnethylstyrene,(•) phenylpropane.

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through the classical pathway shown in Scheme 1. Our results are fully in line with this network of parallel and consecutive reactions. occurs

i-pmpml

,0-m *

CH2CH2CH '0.rpr

di-isopropylacetal

Hydrocinnamaldehyde (HCAL)

of hydrccinnamaldehyde

\

(=J CH=CHCH*OH Cinnamyl alcohol (COL)

Hydrocinnamyl alcohol (HCOL)

f3-methylstyrene (STY)

Phenylpropane

Scheme 1.

The results of activity and selectivity to COL for different catalysts (viz. Ru/A1203 of different dispersions, bimetallic RuM/A1203 (M = Sn, Ge, Zn, Sb, Fe, Ag) catalysts and ruthenium supported on various carriers) are summarised in Tables 2-4. The selectivity to COL increased until 50-70% of CAL was converted, whatever the catalyst used. Two explanations have been proposed for this behaviour: (1) selective poisoning of the sites responsible for C-C hydrogenation, or an electronic modification of the metal sites by the chemisorbed CO molecule formed in the course of the reaction by decarbonylation of CAL [ 141; (2) HCAL formed at the beginning of the reaction remains adsorbed in large amounts on the metal surface and modifies the properties of TABLE 2 Catalytic properties of alumina-supported ruthenium catalysts for the hydrogenation of cinnamaldehyde at 383 K. C,tiwi,ti=7.9~ 10V4mol cme3, P-,=4.5 MPa Catalyst

Ru sponge RuARlSAl RuEECIII RuM4Al RuMGAl RuMSAl RuMSAl

H/Ru

25x lo-’ 0.07 0.25 0.40 0.60 0.69 0.72

Reaction rate (mol s- ’ g$ ) x 104

0.02 2.47 2.86 4.27 2.44 4.44 3.59

TOF (h-l) x10-2

29.8 12.84 4.16 3.86 1.48 2.34 1.81

Selectivity to cinnamyl alcohol at different conversions of cinnamaldehyde (mol% ) 0

10

25

80.2 48.0 30.0 23.0 22.0 23.0 23.3

74.0 55.0 47.1 41.6 42.0 41.3 44.4

75.0 60.0 52.0 50.0 55.0 55.0 56.2

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TABLE 3 Catalytic properties of RuM/A&Os (M = Sn, Ge, Zn, Sb, Fe, Ag, Ru/M = 0.25 atomic ratio) catalysts for the hydrogenation of cinnamaldehyde at 383 K. C-=7.9~10-” mol cmm3, P ,,-,=4.5 MPa Catalyst

Ru/A1203 RuSn/Al,03 RuGe/Alz03 RuZn/Alz03 RuSb/Alz03 RuFe/A1203 RuAg/Al,03

H/Ru

0.60 0.20 0.29 0.28 0.28 0.34 0.27

Reaction rate (mols-‘g&) x10’

2.44 4.31 2.70 3.72 3.27 9.40 2.19

TGF (h-l) x10-2

1.48 7.13 3.08 4.40 3.86 9.15 2.95

Selectivity to cinnamyl alcohol at different conversions of cinnamaldehyde (mol% ) 0

25

50

22.0 46.8 46.3 37.3 47.0 42.0 49.2

55.0 60.0 57.5 54.8 53.0 52.1 53.0

52.0 63.6 59.0 57.2 55.2 57.0 -

TABLE 4 Catalytic properties of ruthenium supported on various carriers for the hydrogenation of cinnamaldehyde at 383 K. Ccinnunaldshyds= 7.9 x lo-’ mol cm-‘, Phydmwn= 4.5 MPa Catalyst

Ru/SiO* Ru/graphite Ru/TiOz(LTR) Ru/TiOz (HTR) Ru/TiOn (HTR)” Ru/Zr&-A Ru/Zrt&-T

H/ Ru

0.29 0.02 0.87 0.04 0.04 0.25 0.59

Reaction rate (mol s-l g;;,‘) x 104

5.85 3.99 8.52 32.4 2.53 2.17 1.84

TOF (h-l) x10-2

7.34 72.6 3.56 291 23.0 3.13 1.13

Selectivity to cinnamyl alcohol at different conversions of cinnamaldehyde (mol% ) initial

25

39.0 66.0 undetected undetected 55.0 60.0 51.7

61.0 65.0 75.0 76.0 by-products by-products 69.0 74.0 69.0 70.0 60.0 60.0

50

“Reaction temperature: 333 K.

the active phase either by a ligand effect which increases the charge density on the metal, resulting in decreased probability for C=C activation, or by a steric effect which forces an oncoming CAL molecule to adsorb via the C=O group at the extremity of the molecule [ 2,141. In the present case we prefer the second explanation, taking into account that the highest gain in COL selectivity was obtained when initially there is large formation of HCAL. This can be seen from the comparative behaviour of RuMGAl and Ru/G catalysts (Table 4). To

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TABLE 5 Effect of hydrocinnamaldehyde addition to the initial reaction mixture on reaction rate and selectivity for hydrogenation of cinnamaldehyde over RuMGAlcatalyst. Reaction temperature 383 K, Ccin_ibh+= 7.9 X 10m4mol cm-‘, P,,h,=4.5 MPa Hydrocinnamaldehyde concentration at initial time (mol/ cm31 0

Reaction rate (mol 9-l g&l x 10” Initial selectivity to COL Selectivity to COL at 10% CAL conversion

7.9x 10-s

2.44

0.95

22.0

79.6

42.0

36.2

check the effect of adsorbed HCAL on the selectivity, a separate experiment was performed by adding HCAL to the initial reaction mixture (HCAL/ CAL = l/10, mol/mol). The results of this experiment are summarised in Table 5. The addition of HCAL decreased the initial rate of reaction. The initial selectivity to COL was very large; however this high initial selectivity was not sustained as the reaction progressed. The reason for this behaviour is not clear. Effect of Ru particle size in Ru/A&O, catalysts

The large influence of the Ru particle size on both the specific activity (TOF) and the selectivity to COL is clear for the series of Ru/A1203 catalysts having different Ru particle size (1.2-20 nm, Table 2). The TOF increases by a factor of lo-15 from small to large particles. This behaviour was not reported by Galvagno et al. [ 141 who found a constant TOF for the hydrogenation of CAL over Ru/C of different dispersions. However, Galvagno et al. [ 141 performed the experiments at atmospheric pressure. They also reported that ruthenium was more active than platinum under these conditions, but the reverse was true under 5 x lo5 Pa. The difference in behaviour is probably due to the different kinetic dependency between the rate and the hydrogen pressure for these two metals. It is therefore easy to conceive that the same behaviour could exist between Ru particles of different sizes. Our earlier results on acrolein hydrogenation also showed similar behaviour for the TOF with the same family of catalysts [lo]. The COL selectivity increases from 22% for a Ru particle size of 1 nm to 80% on large crystallites of bulk Ru, in full agreement with the previous report from Galvagno et al. [ 141. This effect is particularly pronounced when the Ru particles are larger than 3 nm (Fig. 2). Interestingly, we did not observe this behaviour for acrolein hydrogenation and the selectivity to ally1 alcohol did not change much with Ru particle size [lo]. It is generally considered that

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I) i

I -

RdTi02(HTR) 0

I -

A

u.0

0.2

0.4

0.6

0.8

1.0

HiRU

Fig. 2. Selectivity to cinnam yl alcohol formation as a function of hydrogen uptake for ruthenium catalysta on various supports. Z’a= 383 K, C= 7.9 x 10m4mol cmT3, Pqydmssn=4.5 MPa. (0) Ru/AIZO1, (0) Ru/Si02, (0) Ru/graphite, (8) Ru/Z&, (0) Ru/TiO,(HTR). Ru/ TiOz (HTR) was plotted at the hydrogen uptake of Ru/Ti02 (LTR) because the HTR treatment does not modify the mean size of Ru particles.

particles larger than 3 nm suffer small change of morphology and are mainly populated with low index planes [ 151. Therefore, it seems unlikely that the enhanced selectivity to COL on the large particles could be ascribed to the appearance of some specific sites, namely low coordination sites. Contrary to this, the behaviour of large particles can be well explained on the basis of the geometric orientation of the cinnamaldehyde on the metal surface as proposed by Gallezot et al. [ 11. On the flat metal surface of large particles, cinnamaldehyde will be tilted far from the surface due to repulsive interaction between the aromatic ring and ruthenium [l-5], thus protecting the C-C bond. Against this, the unhindered acrolein molecule can adsorb as a flat species independent of the morphology of the Ru particles and does not show any changes in ally1 alcohol selectivity [lo].

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224

Effect of the addition of a second metal to ruthenium

As discussed in the previous section, the size of Ru particles has a dramatic influence on COL selectivity. It was therefore essential to compare the bimetallic catalysts having similar particle sizes to avoid the effects arising due to the change in the particle size. The “controlled surface reaction” method that we have used to prepare the bimetallics allow us to reach this goal, as reported in our earlier paper [ 121. This was further confirmed by plotting the histograms of particle sizes determined from TEM pictures for Ru/A1203 and

R~lAl2O3 (Ru6MAl)

RUSlllAl2O3

::.‘.:.‘.’ .: ...... :::.:..::: .... ::: .. :;::i::::: :.:i::: :: :::_:_:::: ... . . . . . . . ::: ...... . .....

. . . .

:::::.:: :;:::::.;i ::::::;i

. .

:::.::.::j .::::.:. :. ::i:.:.:.: ::::i::::: ..... :::;:.:.:.

.....

:;:.:;::::j::.:jii:. :::::.i;:::::::::::: :::;::..:::::;:.:.:: :.:.j;:::.:i:.:;:::: ;;::..:i:

:.:::.::::

,.:::i:.: ::.:.

;;:::::j:: ..... ...

..:::j::::

::;i:;::. .:. :;::::::. :.:.:.:.: ......................

0

::.::...:: ...... :::.:::::: :.:::.:.::

.:.:...:.:. .:

2

.....

4 Particle size (nm)

Fig. 3. Distribution RuSn/AlsOs.

of Ru particle

6

8 TEM micrographs size and electron

micrograph

of Ru/AlsOs

(RuMGAl), and

B. Coq et al. /J. Mol. Catal. 85 (1993) 215-228

225

RuSnAlpOs (Fig. 3 ) . Thus the discrepancy between the hydrogen uptake (Table 1) and the TEM observations is due to partial coverage of the Ru surface of the parent catalyst (RuMGAl) by the second metal and is not due to the sintering of the metallic phase. The TOF is increased by a factor of 2-6 upon addition of the second metal. This promotion in activity is in line with the previous reports on the hydrogenation of saturated or cu,&unsaturated aldehydes and ketones over RuSn [16], RuK [17], NiCu [18], NiFe [19], PtNi [20], PtGe,Sn [21], PtFe [ 1,22,23], PtSn [ 61. It is generally proposed that M”+ species polarize the C-O bond and promote its attack by hydrogen atoms. This also improves the selectivity to the unsaturated alcohol. However, for acrolein hydrogenation such a large increase in selectivity to ally1 alcohol was not observed [ 11,21,22], except for the Ru-Sn, which showed a clear increase in selectivity to ally1 alcohol [ 11,211. As far as the COL selectivity is concerned, over bimetallic catalysts we always obtained high values of initial selectivity as compared to the parent RuMGAl catalyst. However, these high values of initial selectivity are not sustained at higher CAL conversions. Based on this, we suggest that the addition of the second element to Ru produces the same effect on initial selectivity as the catalyst which has HCAL adsorbed on it. That is, the hindering of CAL adsorption as a flat species by the chemisorbed HCAL [ 21. Indeed, the main effect of the second element appears to stem from dilution of the Ru surface by the inactive second element making the C=C bond activation more difficult. However, some electronic modification of ruthenium by tin for Ru-Sn catalyst cannot be ignored, since a better COL selectivity is obtained even at higher conversions. Effect of the nature of the support The influence of the nature of the support for CAL hydrogenation is reported in Table 4. Unfortunately, we did not succeed in comparing these catalysts having similar Ru particle size. In order to discriminate between the support effect and the effect of particle size, we have plotted in Fig. 2 the COL selectivity as a function of hydrogen uptake (H/Ru) for the catalysts supported on A1203, SiOz, TiOz, ZrOz and graphite. Based on these results the catalysts can be categorised in two groups: (1) This group constitutes more conventional catalysts like Ru/A1203, Ru/SiOz and Ru/graphite for which the COL selectivity falls more or less on the same curve as a function of H/Ru. Moreover, as quoted previously, the data for the CAL hydrogenation on Ru/C of different Ru particle size conforms with this general trend [ 141. This shows that, for these catalysts, the selectivity to COL is only dependent on the size of the Ru particles and not on the nature of the support. The high TOF value obtained for the Ru/G sample has to be considered with caution because the graphite contained 1100 ppm of iron which is an activity promoter.

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(2)In contrast to the catalysts in the above group, the Ru/TiOz and Ru/

ZrOzcatalysts do not show a correlation between the COL selectivity and H/ Ru. These catalysts show much higher selectivity than might be expected from H/Ru values (Table 4 and Fig. 2). Indeed, the high temperature reduced Ru/ TiOz (HTR) shows features characteristic of strong metal-support interaction (SMSI) behaviour, namely, a sharp decrease of hydrogen uptake (from 0.87 to 0.04) without any sintering of the metallic phase (Fig. 4). This has been

0

1

2

Particle size (nm) Fig. 4. Distribution TiO,(HTR).

3 TEM micrographs

of Ru particle size and electron micrograph of Ru/TiO,(LTR)

and Ru/

B. Cog et al. /J. Mol. Catal. 85 (1993) 215-228

227

ascribed to the migration of TiO, suboxide species (x < 2) onto the metal particles [ 241. The TiOp supported Ru catalysts (both LTR and HTR) exhibited very abnormal behaviour. At 383 K the reaction mixture showed turbidity and the material balance was unsatisfactory. The probable formation of undesired side products, not detected by GC analysis, could be responsible for these observations. However, the Ru/TiOz (HTR) showed high rates of reaction based on moles of cinnamaldehyde reacted per gram of Ru. At lower reaction temperature (333 K) the problem of turbidity and incorrect material balance did not arise. Under these conditions good COL selectivity and high TOF for Ru/ TiOa (HTR) catalyst were obtained. Such improvement in activity and selectivity for the hydrogenation of the C=O bond was previously observed for acetone and crotonaldehyde hydrogenation and was attributed to the polarization of the C-O bond by the migrating TiO, species [ 8,10,25,26]. Interestingly, no extension of this work to cinnamaldehyde hydrogenation on metal/TiO, catalysts has been reported. The Ru/ZrOz catalyst shows interesting behaviour. In terms of selectivity to COL, the catalyst behaves similar to that of Ru/TiOa and this is true for both cinnamaldehyde and acrolein hydrogenation [lo]. Several explanations can be suggested to explain this behaviour. (1) We proposed previously that a bimetallic RuZr phase could be formed during the reduction of Ru/ZrOz, as shown by Szymanski et al. [27] in the case of Pt/ZrOz. Electronic, or geometric modifications of ruthenium would result from the appearance of such a phase. This will produce the same effect as in the bimetallic catalysts reported above. (2) The interaction between Ru and ZrOz can induce an epitaxial growth of the metal particles, modifying the morphology of the particle, from a round to a flat shape. The flat shape being characteristic of the low index planes of large particles, selective to COL formation. Such an epitaxial growth was evident for Pt films vapour deposited onto a ZrOz (100) crystal [ 281. (3) The occurrence of specific sites at the periphery of the Ru particles. From TPR studies on Rh/ZrOz, Joswiak et al, [29] have suggested that ZrOz will be partially reduced, probably at the boundary with Rh particles. These Ru-ZrO, mixed sites will reproduce, in part, the behaviour of Ru/TiOz. Even though no report is available on SMSI effects in the Ru/ZrO, system, the occurrence of this type of specific interaction cannot be ruled out.

Acknowledgements

This work was supported by a financial grant from Indo-French Centre for Promotion of Advanced Research (IFC/306-1). The help of Mr. R. Dutartre in TEM experiments is gratefully acknowledged.

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