Introduction

Selective hydrogenation of cinnamaldehyde to cinnamyl alcohol is a very important reaction with respect to industrial and academic point of view, it has applications in pharmaceutical industries , fine chemicals mainly in field of flavor and fragrance chemistry, in organic synthesis, agrochemicals. In order to produce pure cinnamyl alcohol through a chemical process first we need to understand the chemistry of Cinnamaldehyde hydrogenation. Cinnamaldehyde has two main functional groups 1) olefinic group (C=C) 2) carbonyl group (C=O) , Hydrogenation of cinnamaldehyde(CAL) gives multiple products a) Hydrocinnamaldehyde(HCAL) b) Cinnamyl alcohol(COL) c) Hydrocinnamyl alcohol(HCOL) as shown in FIG 1. Out of the listed three products the most valuable product is Cinnamyl alcohol but unfortunately by natures will the formation of Cinnamyl alcohol is thermodynamically disfavored as a result we get low yield of cinnamyl alcohol which is also impure with other products. In order to attain high selectivity of Cinnamyl alcohol we have to manipulate kinetics of reaction to do so we have two probable solutions which are 1) Reducing agents 2) Heterogeneous catalysts. Reducing agents show reasonable selectivity and activity but they produce large amounts of hazardous waste, need costly chemicals, environmentally non eco friendly, can be used only once, since they are stoichiometric reactions amount requirement will be large . Whereas Heterogeneous catalysts have fairly reasonable selectivity, easier to separate, re-usable, economically cheap compared to reducing agents, Environmentally eco friendly, amount of catalyst required is small, so we can observe heterogeneous catalysts are advantageous compared to reducing agent except in selectivity and activity, so the way to improve the heterogeneous catalyst systems is to manipulate electronic and steric effects of metal and supports which will affect the kinetics of reaction. so, research work is going on in the direction to design catalysts which gives high selectivity and activity for the production of cinnamyl alcohol.

In order to design a heterogeneous catalyst for selective hydrogenation of α, β-unsaturated aldehydes (cinnamaldehyde) we need to understand the mechanism of reaction, it is assumed by researchers that hydrogenation of α, β-unsaturated aldehydes on metal surface takes place by Horiuti-Polyani mechanism which is based on Langmuir-Hinshelwood model. This mechanism was substantiated by studies of reduction reactions with deuterium. The mechanism involves di-σ C=C η2 , or di-σ C=O η2

or di-∏ η2 (η4 ) adsorbed states schmatically shwon in FIG 2 . Adsorption through di-σ C=O η2 gives the desired product COL whereas di-σ C=C η2 adsorption states give undesired produt CAL, but we never get pure compunds of either ones because there is always competitive adsorption of the C=C and C=O bonds on the metal surface so we get mixture of products.

FIG 1 Hydrogenation of cinnamaldehyde.

a b

FIG 2 Schematic descriptions of absorption states involved in mechanism of hydrogenation of α, β-unsaturated aldehydes. a) di-σ C=C η2 , b) di-σ C=O η2 .

FIG.3 shows a general view of different pathways which can takes place during hydrogenation of α, β-unsaturated aldehydes, 1,2 addition product gives unsaturated alcohol, 3,4 addition product gives saturated aldehydes, 1, 4 addition product gives and enol which may further get hydrogenated to get saturated alcohol. Sometimes there is a possibility of getting Hydrogenolysis product to get hydrocarbons.

FIG 3 Schematic description of different pathways possible in hydrogenation of α, β-unsaturated aldehydes. A part from mechanistic dependence of selectivity, other factors such as effect of support, electronic effects, steric effects, effect of Prereduction, effect of solvent, effect of reaction conditions like

1,2 addition

3,4 addition1,4 addition

Isomerisation

Carbonyl groupOlefinic group

Desired product

undesired rex

Desired rex

undesired rex

temperature, pressure, concentration also play a important role in selectivity and activity. Desired reaction, although thermodynamically less favored than the hydrogenation to saturated aldehydes, good selectivity and activity can be achieved efficiently provided that the proper catalysts and appropriate reaction conditions are chosen and their cumulative effects may contribute to enhancement in selectivity and activity.

Catalyst systems and other reaction factors

I. Metal catalysts(monometallic system) II. Bimetallic system

III. Tri metallic systemIV. Metal Complexes.V. Influence of reaction conditions

I. Metal catalysts(monometallic system)

A. Intrinsic selectivity.B. Effect of support( C AND G)C. Zeolite as support D. Steric effects of metal surfaceE. Electronic Effect of Supports and Ligands. F. Effect of preparation method

I A. Intrinsic selectivity

Unpromoted metals have specific selectivities to unsaturated alcohols known as intrinsic selectivity, Cordier noticed a trend for intrinsic selectivity for cinnamaldehyde hydrogenation shown below , theoretical calculations of Delbecq and Sautet , showed that intrinsic metal selectivities can be rationalized in terms of the different sizes of their d bands, the larger the band, the stronger the four-electron repulsive interactions with the C=C bond and the lower the probability of C=C adsorption hence considerable selectivity for cinnamyl alcohol can be expected.

0s > Ir > Pt > Ru > Rh > Pd Increasing order of intrinsic selectivity

I B. Effect of support

Table 1 Selectivity at 0% and 25% for cinnamaldehyde hydrogenation on activated C and G

As we know in heterogeneous catalysts Support plays an important role in selectivity and activity, as we can witness form Table 1 that the selectivity of Pt and Ru metals on C and G are vastly different, Pt and Ru which show low selectivity on C support but show increased selectivity on G support, the reason behind this effect is electric effect, G support increases electron density on metal sites which decreases the adsorption of C=C on metal surface due to electronic repulsion between the electron density on metal and electron density of C=C bond.

I C. Zeolite as support

As we have discussed above there is always competition for adsorption either through olefinic group or carbonyl group as a result it is bound to get mixed selectivity or mixture of products, now in order to reduce this completion for adsorption we need to create an environment around metal cluster where in we increase adsorption through only one functional group (carbonyl group) here comes the use of Zeolites which have the quality to orient the adsorption of reactant molecule in a desired direction to get maximum selectivity and it is observed practically that metal Zeolites have increased selectivity than normal metal support catalysts, the main reason for such a increased selectivity in Zeolite support is due to molecular constraints in the 3D pore systems of Zeolite which forces the reactant molecules to adsorb on encaged metal clusters in a end-on mode via C=O group and retarding the absorption via C=C group since bulky phenyl group is sterically hindered to enter the 3D pores , Table 2 compares the selectivity in hydrogenation of cinnamaldehyde on C and Zeolite Y . The ability of Zeolite to selectively direct adsorption of C=O group on metal cluster depends on the size shape of pores and metal clusters, the phenomenon of this kind of selectivity is known as shape selectivity in Zeolite. Zeolites have the ability to increase the selectivity but 3D pore system limits the diffusion of reactants and products and severely effects the activity or rate of reaction but anyways more research is required in Zeolite supports to overcome this disadvantage.

Table 2

I D. Steric effects of metal surface

It has been observed that different planes of Pt metal show varied selectivity towards cinnamaldehyde hydrogenation, the variation depends on the steric hindrance offered by surface. The close packed structure of Pt(111) shows good selectivity towards unsaturated alcohol when compared to Pt(110) plane, the reason for higher selectivity by Pt(111) is because of steric hindrance induced by the plane for the accommodation of bulky group like phenyl and thus inhibiting adsorption through C=C group and facilitating adsorption via C=O group leading to high selectivity for unsaturated alcohol. In the case of Pt(110) no such steric hindrance is induced towards bulky phenyl group as a result competitive adsorption of functional group persists leading to less selectivity.

Show steric hindrance for no steric hindrance forBulky groups good selectivity bulky groups poor selectivity

Effect of metal cluster size on selectivity

Pt (110) facept (111) face

It is observed that as metal cluster size is increased there is a considerable increase in selectivity towards cinnamyl alcohol, some observations for Pt and Rh are shown in Table 3. Initial selceti-

Table 3 Selectivity for cinnamyl alcohol as a function of metal cluster size

-vity of Pt supported on carbon has increased from 0% to 32% for the increase of metal cluster size from 1.3nm to 8.0nm. A 98% selectivity at 50% conversion was observed in Pt supported on Graphite when Pt was sintered leading to increase in cluster size from 1.3nm to 5.0nm. The increase in selectivity is attributed to the steric hindrance between the bulky phenyl group and surface of metal. Cinnamaldehyde is a planar molecule and it cannot adsorb parallel to flat metal surface because theoretical calculations show that the phenyl group must lie 0.3nm away from surface as there is energy barrier which prevent closer approach of phenyl group on to the metal surface, this steric effect is schematically shown if FIG 4

FIG 5 Scheme of adsorption of cinnamaldehyde on small metal cluster and on flat surface.

I E. Electronic Effect of Supports and Ligands

Increase in electron density on metal cluster have positive effect towards increased in selectivity towards cinnamyl alcohol, many reasons have been suggested regarding this effect, higher electron density on metal clusters on one hand increase electronic repulsion towards the electron rich C=C system as a result there will be an decrease in binding energy which further decrease possibility of C=C adsorption on metal surface, on the other hand high electron density on surface favors back bonding

interaction with the - 1∏*CO –orbital which strengthens adsorption of C=O on to the surface and weakens the C=O double bond as electrons are added into 1 ∏* of C=O as shown in FIG 6.

FIG 6 molecular orbital diagram of CO, showing LUMO where back bonding e- are added.Effect of alkali hydroxides

It was observed that when alkali hydroxide(Na, K hydroxides) were added to reaction medium of cinnamaldehyde hydrogenation taking place on Pt/C, a enhanced selectivity was observed towards cinnamyl alcohol. The reason behind this improvement was attributed to electron-donating effect of alkali hydroxide, there was a increase in rate it infers that alkali hydroxides enhance rate of C=O bond activation which happens because of interaction between alkali cation and lone pair of electrons on oxygen which causes polarization of C=O bond hence enhancing end-on adsorption of C=O on metal cluster leading to increase in selectivity and activity. Some results regarding effect of alkali hydroxides are shown in Table 4.

Table 4

I E. Effect of preparation method

The preparation methods employed for the preparation of catalyst has influence on selectivity and activity of catalyst because preparation method also have role to play in textural and surface properties of catalyst such as oxidation states of metal, dispersion of metal, distribution of metal on surface. As we have seen earlier in the section IB that Pt/C and Pt/Gex (prepared by ion exchange) showed vast difference in selectivity towards hydrogenation of cinnamaldehyde to cinnamyl alcohol, now Pt/G col a new catalyst prepared by decomposition of zero-valent platinum complex Pt(DBA)2 (DBA- Dibenzylidene acetone) and graphite powder suspended in CH2Cl2 solution , the catalyst Pt/Gcol though contain G as su

Table 5 Effect of preparation methods on selectivity in cinnamaldehyde hydrogenation.

-pport but it shows varied selectivity when compared to catalyst Pt/Gex , result of selectivity are shown in Table 5. We can observe in Table 5 that the selectivity of Pt/Gcol is intermediate to Pt/C and Pt/Gex , the reason behind such a change is said to be because when catalyst is prepared by decomposition of colloids on graphite than the metal particles get deposited on basal planes rather at the extremities of basal plane as a result the electron donation from G to metal cluster is hindered or because electron conduction is lower in the direction perpendicular to graphite plane or because the oxygenated functional groups are present only on the edges, so this infers the effect of preparation methods on selectivity.

II. Bimetallic system

The selectivity and activity of metal supported catalyst show drastic changes in association of a second metal specially more electropositive than the principal metal. For the present discussion we shell consider Pt system in association of few electropositive metals then Pt such as Fe, Co, Ge. The modifications brought by the electropositive metal on the catalyst system can be explained in following ways:

The electropositive metal or promoter acts as a electron donor that increases electron density on principal metal leading to increase in electronic repulsion between principal metal and C=C, the high electron density on metal clusters(Pt) also reduce binding energy of C=C and favors hydrogenation of C=O group leading to increase in selectivity and activity, the electronic effect caused by electropositive metal is similar to a electron donating

molecule or support. FIG 7 shows an example of Pt-Fe/C where in the selectivity was 85% with a good rate at Pt/Fe ratio = o.2.

Fig 7 Effect of Fe promoter Fe in Pt-Fe/C on selectivity and rate.

The electropositive metal species on the surface of principal metal (Pt) acts as electrophilic or Lewis sites which interact with the lone pair of electrons of oxygen leading to polarization of C=O bond which lead to activation and adsorption of C=O on the metal cluster, this mechanism is schematically shown in FIG 8.

FIG 8 Scheme of C=O bond activation by electropositive Fe on Pt surface.

In another case Pt catalyst promoted by Co supported on PVP (Polyvinylpyrrolidone) Pt-Co/PVP has shown greater selectivity and activity compared to Pt-Fe/C, the result of which are shown in Table 6.

Table 6

III. Tri metallic system

Earlier we have seen bimetallic system and their effect of the promoter on the selectivity and activity of catalyst, in the present discussion we shell discuss about a tri metallic system and try to see what extra advantages or disadvantages we might encounter. We have considered an Pt based catalyst supported on mesopores carbon and which is being promoted by Fe and Zn, the results are encouraging and there has been an substantial increase in activity and selectivity. Catalysts with different metal combinations were prepared to compare the performance with each other and establish the best one amongst, the mesopores carbon support was activated under 5% O2 , oxygen groups on support are expected to anchor the metal clusters , activated support is designated as C-SA, and unactivated support is designated as C-UA, The following sets of catalysts were prepared – Pt/C-SA, Pt-Fe/C-SA, Pt-Zn/C-S, Pt-Fe-Zn/C-SA and Pt-Fe-Zn/C-UA.

Table 7 gives the XPS data of the following catalyst system which shows varied properties on catalysts, in the monometallic Pt catalyst Pt/C-SA , Pt surface concentration(Table 7) was found to be close to its loaded amount (5 wt.%) with a good dispersions. On the other hand, P s of the bi- and tri-metallic catalysts were drastically higher than the Pt bulk concentration, indicating the presence of promoters Fe and Zn which lead to the segregation of Pt on to the surface. Pt and Zn surface concentrations in Pt-Fe-Zn/C-UA were found to be higher than those in Pt-Fe-Zn/C-SA, indicating the role of the carbon surface, Pt-Fe-Zn/C-UA catalyst shows higher amount of Pt0 (84.3%) in comparison with the other catalysts prepared over the standard oxidised support (around 80%).

Table 7

We can observe in Table 7(XPS data) that the binding energy of promoted catalyst are lower than the Unpromoted catalyst this happens because there is a transfer of electron density from promoter to Pt, this electronic effect is very significant because higher electron density on Pt will hinder the C=C bond adsorption and favor adsorption via C=O bond. When we observe the data of (HCMD+PPT) given in Table 8 it seems that Fe is particularly efficient to improve activity of C=O hydrogenation and Zn is better in terms of selectivity this inference is backed by the observation that the selectivity given by Zn promoted catalyst remained constant with lower amounts of imputes (HCMD + PPL). Table 8 compares the performance of all the catalysts in terms of selectivity and conversion, it is evident that tri metallic catalysts are superior in termsof conversion, selectivity, the reasons attributing to following results can be explained in

Table 8 performances of catalyst in terms of conversion and product selectivity.

following two mechanisms: (1) the electron transfer from Fe and Zn promoters to the Pt clusters, evidenced in the XPS data(Table 7) (2) origin of electrophilic or Lewis sites, which favors adsorption through C=O bond. The higher electron density on Pt decreases the probability of the C=C bond activation due to increase in the repulsive four-electron interaction, and favors the back- bonding interaction with the 1∏* CO orbital(FIG 6) and thus enhancing the hydrogenation of the C=O bond. We can see in Table 8 that the tri metallic catalyst has the highest selectivity amongst all the catalyst but it has less activity when compared to Pt-Fe/C-SA, the reason behind such a decrease in rate may be due to dilution of Pt-Fe surface with Zn. Among the two tri metallic catalysts Pt-Fe-Zn/C-UA ha higher selectivity, the reason for such a

result was attributed to the higher Zn surface concentration (Table 7) which led to higher probability of Pt-Zn species, low oxygen group concentration which facilitate easy transfer of electron density when compared to C-SA support, large cluster size of Pt in Pt-Fe-Zn/C-UA as we have seen in section I D the effect of cluster size. Out of the two tri metallic catalysts Pt-Fe-Zn/C-SA is showing higher activity than other one the reason attributed to this result was that the catalyst with lower oxygen groups on surface has poor dispersion of Pt leading to lower activity as we know that Pt-Fe-Zn/C-UA is unactivated catalyst which has less oxygen on surface.

The greater selectivity and activity in tri metallic system is due to combined contribution of Fe, Zn promoters, support properties and oxygen groups on surface which led to the increase in electrophilic sites hence increasing activation of C=O bond, and high electron density on Pt which led to decrease in activation of C=C bond lowering the possibility of formation of saturated aldehyde.

IV. Metal Complexes

The metal complexes generally come under homogenous systems of catalysts, these are generally complexes of metals belonging to second and third row transition metals which include Ru, Rh, Os, and Ir, these complexes mostly contain phoshines as ligands. These complexes give very good selectivity and activity as we can see from Table 9, it has been observed that high selectivity is observed when the cone angle of phophine is greater than 1400. The most important problem with homogenous catalysts is separation of the catalyst, it is very difficult to separate the catalyst, most of the metals used in the homogenous catalyst systems are very costly so use of homogenous catalysts lead to loss of money, so research work is going to bring better techniques to separate catalyst after its use.

Table 9 selective hydrogenation of α, β-unsaturated aldehydes by Transition metal complexes

V. Influence of reaction conditions

The reaction rate of a reaction which are generally dependent on thermodynamics which get effected by reaction conditions such as temperature, pressure, type of solvent , concentration of reactant, Prereduction of catalyst. Reaction conditions predominantly shows its effects in

liquid phase reactions, but unfortunately detailed systematic study of effect of reaction condition on catalyst performance in hydrogenation of α, β-unsaturated aldehydes have not been done yet. so, in present discussion we shell consider only effect of Prereduction on catalyst performance.

Effect of catalyst Prereduction

In the Hydrogenation reaction the catalyst is reduced in the reactor rather than prereducing the catalyst but Cordier et al. had reported that Prereduction of catalyst has substantially improved the selectivity in the case of Os, Ir, Pt results of these experiments are shown in Table 10 in fact longer the duration of Prereduction the greater the improvement in selectivity. In this way we can assume that bimetallic catalyst will have increased effect on selectivity because Prereduction may affect the oxidation state of the electropositive metal acting as a promoter which is involved in activation of C=O functional group in α, β-unsaturated aldehydes.

Table 10

Probable research aspects

Possible effects of various diffusional limitations: as we have seen earlier in section IC Zeolites when used as supports showed very good selectivity comparable with bi/tri metallic system but Zeolite show diffusional limitation which substantially affect the rate of reaction, so research work can be done to design Zeolite based catalyst focusing to improve the activity aspect.

The effects of reaction conditions on the activities and selectivities of catalytic reaction: much of research work has not been put to systematically analyze the effect of reaction condition on the performance of catalyst, as we know thermodynamics play important role in reaction rate so it becomes important to work on this area of research.

As there are very few data available on catalyst aging in the course of liquid-phase reactions, catalysts should be checked for any changes in their structure and morphology.

Finally, experiments involving the comparison of activity and selectivity data obtained for the same reaction conducted in the liquid phase and in the gas phase would be highly required.

Conclusion

As we have seen in the following discussion that though the selective hydrogenation of α, β-unsaturated aldehydes to unsaturated alcohol is thermodynamically unflavored in comparison with the hydrogenation to saturated aldehydes but still the reaction to unsaturated alcohol can be activated to a very good selectivity and activity bypassing the thermodynamics provided if we use proper catalyst system under suitable reaction conditions. It is the steric and electronic effect which helps us bypass the thermodynamics so we have to design our catalyst in a way that we can use the steric and electronic effects in a direction to enhance selectivity and activity towards unsaturated alcohol. Intrinsically the metals may have little selectivity and activity but they can be modified according to our desire within limited boundary of laws of nature as we have seen in the sections above and bring enhanced selectivity and activity.