SELECTIVE HYDROGENATION OF CINNAMALDEHYDE WITH Pt AND Pt-Fe CATALYSTS: EFFECTS OF THE SUPPORT

SELECTIVE HYDROGENATION OF CINNAMALDEHYDE WITH Pt AND Pt-Fe CATALYSTS: EFFECTS OF THE SUPPORT

A.B. da Silva, E. Jordão^*, M.J. Mendes^* and P. Fouilloux

^*DESQ/FEQ/UNICAMP - Caixa Postal 6066 - 13083-970, Campinas, SP, Brazil

E-mail: [e-mail], [e-mail]

(Received: November 5, 1997; Accepted: February 26, 1998)

Abstract - Low-temperature reduced TiO₂-supported Pt and Pt-Fe catalysts are much more active and selective for the liquid–phase hydrogenation of cinnamaldehyde to unsaturated cinnamyl alcohol than the corresponding carbon-supported catalysts. High-temperature reduced catalysts, where the SMSI effect should be present, are almost inactive for this reaction. There is at present no definitive explanation for this effect but an electronic metal-support interaction is most probably involved.

Keywords: Selective hydrogenation, catalysts, support.

INTRODUCTION

Hydrogenation of a , b ethylenic aldehydes to the corresponding unsaturated alcohols is a class of reactions of great interest to the fine chemicals industry (Cordier et al., 1984). Development of catalysts for this class of reactions is a challenging problem, since the carbon-carbon double bond is normally more reactive than the carbon-oxygen one.

Platinum-based bimetallic catalysts have been used for the selective hydrogenation of a , b ethylenic aldehydes. Addition of Ni, Co, and Fe to Pt/SiO₂ increases activity and selectivity for unsaturated alcohols, while addition of Ga, Sn, and Ge improves selectivity but decreases the activity of the catalysts (Englisch et al., 1997a).

It is also known that a titania support influences the activity and selectivity of platinum-based catalysts in the hydrogenation of a , b ethylenic aldehydes.

A slightly condensed form of this paper was originally presented at the 9^th Brazilian Congress of Catalysis, 1997.

However, the role of titania is frequently analysed in the context of the SMSI effect (Englisch et al., 1997b).

In this work the activity and selectivity of titania-supported Pt and Pt-Fe catalysts in the selective hydrogenation of cinnamaldehyde is studied. Cinnamaldehyde is frequently taken as a model compound for the hydrogenation of a , b ethylenic aldehydes. According to the reaction schema cinamaldehyde (ALDC) can either be partially hydrogenated to cinnamyl alcohol (ALCC) and hydrocinnamaldehyde (ALDHC) or totally hydrogenated to hydrocinnamyl alcohol (ALCHC).

EXPERIMENTAL

The supports used were an activated carbon (Degussa, C.10) and titanium dioxide (> 99% anatase) from Fluka. The supports have similar particle sizes of around 40 m m. The metal precursor salts were hydrated hexacloroplatinic acid from Sigma and iron(III) nitrate nonhydrate from Aldrich.

Four catalysts were prepared: Pt/C; Pt-Fe/C; Pt/TiO₂; Pt-Fe/TiO₂. The catalysts were prepared by impregnation using solutions of the salts in an ethanol-benzene mixture. Before impregnation carbon was treated by refluxing with hydrocloric acid, oxidizing in concentrated sodium hypoclorite and drying at 723 K in N₂. The TiO₂ powder was calcined in N₂ at 773 K for 4 h. The amount and concentration of the impregnating solutions were determined so that the final catalysts would have a nominal total metal load of 5wt% (for the bimetallic Pt-Fe catalysts, the nominal final metal load was 4.7wt%Pt-0.3 wt%Fe). After elimination of the solution in a rotary evaporator, the powders were dried at 393 K for 12 h resulting in the so-called catalyst precursors. The carbon-supported catalyst precursors were pre-reduced at 430^oC. The pre-reduction temperatures of the TiO₂-supported catalyst precursors were fixed after TPR experiments carried out under a 2 vol% H₂-N₂ flow with a 5^oC/min linear raise in temperature.

The catalytic hydrogenation tests were carried out in a semibatch 300 ml reactor at 60^oC and 4 MPa with continuous addition of hydrogen. The catalyst (0.2 g) and the solvent (10 ml water + 37.5 ml isopropyl alcohol + 2.5 ml of 0.1 N natrium acetate aqueous solution) were put into the reactor. After purging with nitrogen and hydrogen, the catalyst was preconditioned for 2 h at the final temperature (60^oC) and hydrogen pressure (4 MPa). The reaction was then started by injecting 0.1 moles of cinnamaldehyde. Liquid samples were regularly collected after passing through a frit. These samples were analysed on a gas chromatograph equipped with a column with 1/8 inch diameter and a length of 3.6 m which was filled with 47V300 silicon oil on chromosorb WAW80/100 (190^oC) and a TCD (280^oC).

Further details of the experimental methods can be found in Silva (1995).

RESULTS AND DISCUSSION

Temperature Programmed Reduction Tests (TPR)

The results of the TPR tests with the catalyst precursors are shown in Figure 1.

For the precursor Pt/C a peak occurs at 200^oC corresponding to the reduction of the Pt species. The hydrogen consumption pattern also presents a broader peak at 500^oC, which is probably due to the decomposition of superficial chemical species of carbon.

For the Pt-Fe/C precursor there is also only one peak at 220^oC, implying that Pt and Fe are reduced together. This result has already been observed by other authors who have shown, by means of magnetic and X-ray measurements, that under similar conditions platinum and iron form an alloy (Goupil et al., 1987).

The reduction patterns of the titania-supported precursors do not differ significantly from those of the corresponding carbon-supported ones. For the Pt/TiO₂ precursor the Pt reduction peak reaches a maximum at 170^oC. The Pt-Fe/TiO₂ precursor also presents a reduction peak only at 210^oC, allowing the conclusion that here also Pt and Fe form an alloy.

Based on these TPR results, the pre-reduction temperatures of the titania-supported catalyst precursors were fixed as indicated in Table 1.

Table 1:
Pre-reduction temperatures of the catalyst precursors

Catalyst Precursor	Pre-reduction Temperature, oC
Pt/C	430
Pt-Fe/C	430
Pt/TiO2	150
Pt-Fe/TiO2	190; 430*

* Temperature chosen to evaluate possible consequences of the SMSI effect

Figure 1:
TRP of mono and bimetallic platinum catalyst precursors.

Figure 2:
: Hydrogenation on carbon-supported catalysts. (s cinnamaldehyde; 6 cinnamyl alcohol; u hydrocinnamaldehyde; n hydrocinnamyl alcohol ) Catalytic Tests

Figure 2 shows the results obtained with the carbon-supported catalysts. It should be stressed that these results were obtained for use as a reference by comparison with similar results from the literature (Goupil et al., 1987). As shown, the monometallic Pt/C catalyst is initially very active in the hydrogenation of the carbon-carbon double bond but this activity decreases rapidly. Selectivity for unsaturated alcohol is, however, very poor. On the contrary, the Pt-Fe/C catalyst not only presents much higher overall activity but is also much more selective for unsaturated alcohol.

Delbecq and Sautet (1996) compared, by means of semiempirical extended Hückel calculations, the electronic properties of a Pt-Fe alloy with those of pure Pt. The authors found that in the alloy an electron transfer occurs from Fe to Pt resulting in increased electron density on the Pt surface atoms. As a consequence, not only is the Fermi level shifted up in the alloy but also the local density of surface states is modified. An important result is that the adsorption sites and the geometries of the adsorbates are not the same on the alloy and on pure Pt. More specifically, a , b ethylenic aldehydes change their dominant adsorption mode when Pt is alloyed with Fe: the di-s _CO adsorption mode becomes predominant over the di-s _CC mode. According to the authors, these changes in the adsorption modes explain the differences in catalytic behaviour of pure Pt and the PtFe alloy in regard to the selectivity for the hydrogenation reactions.

Figure 3:
Hydrogenation on low-temperature reduced titania-supported catalysts. (s cinnamaldehyde; 6 cinnamyl alcohol; u hydrocinnamaldehyde; n hydrocinnamyl alcohol)

Figure 3 shows the results obtained with the low-temperature reduced titania-supported catalysts. The comparison of the results in Figure 3 with those in Figure 2 evidences the dramatic effect of the titania support. The low-temperature reduced titania-supported catalysts are not only much more active but are also much more selective for unsaturated alcohol than the corresponding carbon-supported catalysts. It is well known that titania-supported platinum catalysts can exhibit a strong metal-support interaction (Haller and Resasco, 1989). This effect cannot, however, be used to explain the present results due to the low temperatures at which the catalysts were reduced (Table 1). At the present state of knowledge only some possible explanations for the observed promoting effect of the titania support can be presented.

a. Selective catalytic hydrogenation proceeds on the titania surface, whose properties are changed by the presence of activated hydrogen supplied by the platinum via a spillover mechanism. This possibility was suggested by Pestman et al. (1997) to explain their results on the selective hydrogenation of acetic acid to acetaldehyde on platinum/titania catalysts. According to the authors platinum activates the hydrogen, which migrates to the titania, where the actual reaction takes place. The spillover hydrogen can be used as a reactant in the hydrogenation reaction or to maintain the concentration of oxygen vacancies at an adequate level. There are, however, some arguments against this possibility: i) The authors^, conclusions are based only upon acetaldehyde yield and do not consider other possible reaction products such as ethanol or ethane; ii) When applied to the catalytic hydrogenation of cinnamaldehyde, this possibility could not explain the profound differences observed between the activity and/or selectivity of the Pt and PtFe catalysts shown in Figure 3. The possibility of hydrogen spillover in platinum/titania catalysts should not, however, be neglected. Conesa and Soria (1982), for example, have already detected the reversible formation of Ti⁺³ ions on Pt/TiO₂ (anatase) at low temperatures due to hydrogen adsorption and were able to interpret these results in terms of a spillover of H atoms from the metal to the oxide surface.

b. Selective catalytic hydrogenation proceeds on the metal surface, whose properties are changed by an electronic promotion effect of titania. This possibility, explained in terms of a charge transfer between the metal and the oxide, has a more historical value. In effect, such a charge transfer, even when it does occur, extends no farther than to the neighborhood of the immediately adjacent metal atoms and thus it cannot affect surface chemistry (Feibelman and Hamann, 1985).

c. Selective catalytic hydrogenation proceeds at the interface between platinum and titania, where special catalytic properties could arise. This possibility is probably the most commonly accepted one at the present. Special catalytic properties at the interface could be due to the presence of TiO_x species (x< 2) created by hydrogen spillover, as mentioned above. Such properties could also be due to changes in the Fermi level local density of states of the metal surface induced by the support (Feibelman and Hamann, 1985). These two effects probably work together to generate such special catalytic sites at the interface. It must, however, be admitted that such sites are particularly favorable for the adsorption of the carbonyl group, as is the case of the PtFe alloy.

Figure 4 shows the results obtained with the titania-supported PtFe catalyst reduced at 430^oC. As can be seen, the catalyst is almost inactive although still very selective. This fact can most probably be explained by the SMSI effect, that is, a geometric blocking of the metal sites by TiO_x species (Haller and Resasco, 1989).

Figure 4: Hydrogenation on Pt-Fe/TiO₂ reduced at 430 ^oC. (s cinnamaldehyde; 6 cinnamyl alcohol; u hydrocinnamaldehyde; n hydrocinnamyl alcohol )

CONCLUSIONS

- In the liquid phase hydrogenation of cinnamaldehyde low-temperature reduced TiO₂-supported Pt and Pt-Fe catalysts are not only more active but also more selective for unsaturated alcohol than the corresponding carbon-supported catalysts.

- The effect of Fe on the activity and selectivity of Pt catalysts for the reaction can be explained by the formation of a Pt-Fe alloy which alters the local density of surface states.

- The effect of titania as a support on the activity and selectivity of the catalysts for the reaction is most probably due to the combined effect of hydrogen spillover and electronic metal-support interaction, with the reversible creation of Ti⁺³ species and a change in the local surface density of states at the metal-support interface.

- High temperature reduction of titania-supported Pt catalysts creates the SMSI effect that makes the catalysts practically inactive for the reaction.

ACKNOWLEDGEMENTS

This work was undertaken thanks to the financial assistance of PADCT II/ FINEP and FAPESP and to a PICD grant .

REFERENCES

Conesa, J.C. and Soria, J., Reversible Ti³⁺ Formation by H₂ Adsorption on M/TiO₂ Catalysts, J. Phys. Chem., 86, 1392 (1982).

Cordier, G., Colleuille, Y. and Fouilloux, P., Hydrogénation Catalytique Sélective sur Métaux et Chinie Fine, in Imelik, B. et al. (ed.), Catalyse par les Métaux, CNRS, Paris (1984).

Delbecq, F. and Sautet, P., Electronic and Chemical Properties of the Pt₈₀Fe₂₀ Alloy Surface: A Theoretical Study of the Adsorption of Atomic H, CO and Unsaturated Molecules, J. Catal., 164, 152 (1996).

Englisch, M., Ranade, V.S. and Lercher, J.A., Hydrogenation of crotonaldehyde over Pt based bimetallic catalysts, J. Mol. Catal.A, 121, 69 (1997a) , Structure Sensitivity of the Hydrogenation of Crotonaldehyde over Pt/SiO₂ and Pt/TiO₂, J. Catal., 166, 25 (1997b).

Feibelman, P.J. and Hamann, D.R., Modification of Transition Metal Electronic Structure by P, S, Cl and Li Adatoms, Surf. Sci., 149, 48 (1985).

Goupil, D., Fouilloux, P. and Maurel, R., Activity and Selectivity of Pt-Fe/C Alloys for the Liquid Phase Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol , React. Kinet. Catal. Lett., 5, 185 (1987).

Haller, G.L. and Resasco, D.E., Metal-Support Interaction: Group VIII Metals and Reducible Oxides, Adv. Catal., 36, 173 (1989).

Pestman, R., Koster, R.M., Pieterse, J.A.Z. and Ponec, V., Reactions of Carboxylic Acids on Oxides 1. Selective Hydrogenation of Acetic Acid to Acetaldehyde, J. Catal., 168, 255 (1997).

Silva, A.B., Hidrogenação Seletiva do Aldeído Cinâmico a Álcool Cinâmico sobre Catalisadores de Pt Suportados em Carvão e Óxido de Titânio, Ph. D. diss., UNICAMP (1995).

Publication Dates

• Publication in this collection
  09 Oct 1998
• Date of issue
  June 1998

History

• Accepted
  26 Feb 1998
• Received
  05 Nov 1997