Cinnamaldehydes hydrogenation

Analogous observations can be made for the cinnamaldehyde hydrogenation over large Pt ensembles in the inner pore volume of Beta zeolites (53). 

Fig. 3. Cinnamaldehyde hydrogenation activity on the Pd/CNTs and Pd/AC at 80 °C and imder atmospheric pressure of hydrogen.

Fig. 4 shows the reaction pathway involved in the cinnamaldehyde hydrogenation, leading to compoimds containing hydrogenated C=C and/or C=0 bonds. 

As it is visible from Fig. 1, the selectivity towards COL increased with conversion. This was most clearly observed in non-reduced catalysts. Previously [23], it was demonstrated that in the liquid-phase hydrogenation of a similar molecule (e.g. crotonaldehyde), there was a clear increase of the selectivity as a function of conversion. This kinetic pattern did not obey the general behavior characteristic for parallel-consecutive reactions, thus calling for the introduction [23] of the reaction-induced formation of active sites. One can thus speculate that, also in cinnamaldehyde hydrogenation, during the reaction there is in situ formation of sites responsible for the hydrogenation of the carbonyl group (formation of unsaturated alcohol). 

Although common examples in the literature mention acrolein and crotonalde-hyde hydrogenation, it is probably best to consider research on cinnamaldehyde hydrogenation for determining the best conditions to apply to the hydrogenation of a,/ -unsaturated aldehydes. 

Promoters might be added to a PGM catalyst by the manufacturer, or directly to the reaction mixture. Each route has its benefits. In cinnamaldehyde hydrogenation the promoter is added to inhibit olefin hydrogenation during reduction of the carbonyl function. This has been achieved by addition of bases such as KOH or NaOH at 5-10% concentration [17], or more often by addition of other electropositive metals that are reduced on the catalyst surface in situ upon admission of hydrogen into the reactor. 

Pt/Al-A and Pt/Al-B The results of cinnamaldehyde hydrogenation on various monometallic catalysts, based on Al-A and Al- B, are given in Tables land 2 respectively. 

In Table 3 the results of cinnamaldehyde hydrogenation on two Pt-Ru bimetallic catalysts are given. A widely var3dng activity among these bimetallic catalysts is observed however, the selectivity pattern shows much less sensitivity towards catalyst composition than the activity trend. The significance of these results will be discussed in more detail in the following section. 

The activity and selectivity for various products of cinnamaldehyde hydrogenation on Pt/Al catalysts are dependent on the physical properties and the make of the support alumina. 

For parallel -consecutive reactions, as examplified (Figure 4.18) by cinnamaldehyde hydrogenation over a Ru-Sn sol-gel catalyst,... 

Figure 4.18. Parallel -consecutive reactions in cinnamaldehyde hydrogenation.

Figure 4.19. Selectivity vs. conversion dependence in cinnamaldehyde hydrogenation. (COL - cinnamylalcohol, PPAL- phenylpropanal)...

Figure 4.20. Reactants concentration as a function of time in cinnamaldehyde hydrogenation- mechanism (4.129) ( J. Hajek, J. Wama, D.Yu. Murzin, Liquid-phase hydrogenation of cinnamaldehyde over Ru-Sn sol-gel catalyst. Part II...

Figure 4.23. Selectivity v.v conversion at different hydrogen partial pressures in cinnamaldehyde hydrogenation-mechanism (4.136). (J. Hajek, J. Warna, D.Yu. Murzin, Industrial Engineering Chemistry Research, 43 (2004) 2039).

Figure 4.24. Selectivity vs conversion in cinnamaldehyde hydrogenation- mechanism over Ru/Y zeolite. (J. Hajek, N.Kumar, P. Maki-Arvela, T.Salmi, D.Yu.Murzin, Selective hydrogenation of cinnamaldehyde over Ru/Y zeolite, J. Molecular Catal. A. 217 (2004) 145).

Plot of substrate concentration vs normalised catalyst mass in cinnamaldehyde hydrogenation (Figure 9.26) indicates that gas-to-liquid mass transfer resistance does not play a significant role at the tested experimental conditions. 

Figure 9.26. Substrate concentration vs normalised catalyst mass in cinnamaldehyde hydrogenation.

An experimental approach to verify the impact of internal diffusion is to perform experiments with catalyst of different particle sizes. The experimental results on the impact of particle size in cinnamaldehyde hydrogenation are presented in Table 9.11, demonstrating that a decrease in mean particle size increases both the catalyst activity and selectivity and that the smallest catalyst fraction (< 45 pm) can be considered to be sufficient to safely eliminate the influence of internal diffusion, as confirmed by the calculations. 

Under the same experimental conditions great differences in the activity and selectivity could be observed. Indeed the catalyst chosen was about 3 times more active in cinnamaldehyde hydrogenation than in that of 2-pentyl-2-nonenal. Furthermore the selectivity for unsaturated alcohol (COL) was also very hi even at almost total conversion, while the selectivity for 2-pentyl-2-nonenol was about 20% under the reactional conditions that were chosen (the solvent was propylene carbonate instead of dodecane). Owing to the presence of an aromatic nucleus the ole c bond was harder to hydrogenate and this molecule can not be taken as representative of the selective hydrogenation of linear unsaturated carbonyl derivatives. 

Table 5 Effect of Metal Chlorides on Cinnamaldehyde Hydrogenation...

Gallezot, P., Giroir-Fendler, A., and Richard, D. (1990) Chemioselectivity in cinnamaldehyde hydrogenation iduced by shape selectivity effects in Pt-Y zeolite catalysts. Catal. Lett., 5, 169—174. 

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