The efficient citral hydrogenation was achieved in aqueous media using Pd/CMS and alkali additives like K2CO3. The alkali concentrations, reaction temperature and the Pd metal content were optimized to enhance the citral hydrogenation under aqueous media. In the absence of alkali, citral hydrogenation was low and addition of alkali promoted to ~92% hydrogenation without reduction in the selectivity to citronellal. The alkali addition appears to be altered the palladium sites. The pore size distribution reveals that the pore size of these catalysts is in the range of 0.96 to 0.7 nm. The palladium active sites are also quite uniform based on the TPR data. The catalytic parameters are correlated well with the activity data.
Selective hydrogenation of α-β-unsaturated aldehydes was widely used for the synthesis of fine chemicals and pharmaceutical industries [
Scheme 1. Consecutive reaction scheme of citral hydrogenation.
agents in aqueous as well as non-aqueous solvents [
On the other hand, carbon supported materials have been the most studied in the selective hydrogenation of citral. However, non-traditional carbon materials such as carbon nanotubes, carbon nano-fibers, carbon xerogel and aerogels were widely studied for hydrogenation application, and few reports were also available on use of composite materials like carbon-TiO2 in the literature for this application [
Organic solvents were commonly employed for this transformation; however, to develop a more environmentally friendly process, the use of water as solvent is highly desirable. One of the drawbacks relies on the insolubility of the substrate in water. This problem could be circumvented by performing the hydrogenation in the presence of alkali additive. Indeed, these additives are well known to play a positive role in a large number of reactions in aqueous media. In fact, alkali additive with varying concentrations can interact with reactant substrate and solubilise them in aqueous media [
Present study focused on the selective hydrogenation of citral using Pd/CMS catalysts in water with the objective of establishing the influence of alkali additive with varying concentration.
The CMS supported Pd catalysts were prepared by wet impregnation method using Pd(OAc)2, solution (5% Pd wt/Vol) as the precursor. The catalyst’s detailed preparation method was described in earlier studies for carbon supported palladium catalysts [
The catalysts were characterized using XRD, H2-TPR and BET surface area. XRD of reduced Pd/CMS catalysts were recorded on a Miniflex (M/s. Rigaku Corporation) X-ray difractometer using Ni filtered Cu Kα radiation. The BET surface area and pore size distribution were performed on Autosorb 2120 by nitrogen adsorption using multipoint method. The temperature programmed reduction (TPR) studies conducted on Newchrom TPD/Pulse chemisorptions unit using 5% H2 in Ar the mixture. The temperature was raised to 850˚C at a rate of 10˚C/min and kept at isothermal for 10 min.
All the catalytic experiments were conducted in an autoclave (M/s. Micron engineers). In all the experiments, the hydrogen to the citral mole ratio was maintained at 6. Initially, the reactor was charged with the required amount of catalyst, substrate and solvent. The reactor was heated to 50˚C after stabilization of the temperature, the system was pressurized with H2 to get required H2 to the citral mole ratio. The products were collected and analyzed by a Varian-450 Gas Chromatograph (FID), and the quantification of the products was done using a multi-point calibration for each product. The qualitative analysis was done by GC/MS (Agilent 6890N) using an HP-5 capillary column.
Adsorption/desorption isotherms and pore size distributions of Pd/CMS catalysts were displayed in (
The TPR patterns (
The XRD patterns of reduced CMS and CMS supported Pd catalysts depicted in
The effect of solvents was studied using the acetonitrile, ethanol, hexane, water and the results were summarized
in
The effect of Pd on the conversion of citral was examined by varying the Pd loading from 0.5% to 5% (
The hydrogen pressure was varied from 10 bar to 30 bar at 50˚C in aqueous media by using Pd/CMS-6 catalyst. The results reveal that (
The effect of alkali additive with varying concentrations on hydrogenation of citral has been investigated (
Solvent | % Conversion | % Selectivity | ||
---|---|---|---|---|
Citronellal | Dihydrocitronellal | 3,7-Dimethyl-1-octanol | ||
Acetonitrile | ~40 | 65 | 17 | ~18 |
Hexane | 30 | 100 | 0 | 0 |
Ethanol | 13 | 100 | 0 | 0 |
Water | ~9 | 100 | 0 | 0 |
Reaction conditions: H2 = 10 bar, water/citral mole ratio = 65, H2/citral = 6, reaction temperature = 50˚C, reaction time = 2 hours, Pd/citral mole ratio = 0.006, Pd/CMS-2 catalyst.
Entry | H2 in Bars | % Conversion | % Selectivity | ||
---|---|---|---|---|---|
Citronellal | Dihydrocitronellal | 3,7-Dimethyl-1-octanol | |||
1 | 10 | 22 | 100 | 0 | 0 |
2 | 20 | 36 | 100 | 0 | 0 |
3 | 30 | 43 | 100 | 0 | 0 |
Reaction conditions: H2 = 10, 20, 30 bar, water/citral mole ratio = 65, H2/citral = 6, 12, 18 reaction temperature = 50˚C, reaction time = 2 hours, Pd/citral mole ratio = 0.033, Pd/CMS-6 catalyst.
mixture. With an increase in the NaHCO3 concentration, the citral hydrogenation activity increased marginally up to 18.7 m∙mol of NaHCO3, and further raised the concentration. The hydrogenation activity was slightly declined. On the other hand, the addition of Na2CO3 (relatively weak base) the effect of alkali on citral conversion was marginal. With an increase in Na2CO3 concentration, the hydrogenation activity slightly increased and goes through a maximum at 11.1 m∙mol. It was interesting to observe that when K2CO3 (stronger base) used as alkali additive (0.5 to 14.2 m∙mol) to the reaction mixture, the conversion levels were increased notably at 0.5 m∙mol gave more than 50% hydrogenation activity and attained 82% at 2.8 m∙mol Further, increase in content of K2CO3, the citral hydrogenation goes through a maximum at 2.8 m∙mol. The influence of Na2CO3/NaHCO3 was on citral hydrogenation activity was insignificant compared to that of K2CO3. The addition of alkali carbonate/bicarbonate has not been influencing any product selectivity; only citronellal was observed as a hydrogenated product under employed conditions. On the other hand, the addition of K2CO3 has governs the hydrogenation activity to 100% with influencing other products selectivity with respects to the temperature from 50˚C to 100˚C; this tendency might be due to a substrate solubility was increased with respects to temperature. These results revealed that, the citronellal formed could be re-adsorbed in the micro pores of CMS support via the C=O bond, which was weakened after the C=C bond hydrogenation, thus, leading to the formation of the dihydrocitronellal and 3,7-dimethyl-1-octanol upon addition of K2CO3 (strong base), the hydrogenation activity was influencing in the following two key factors, i.e. 1) solubility of hydrogen, citral, 2) electron donating ability of potassium cation to Pd active sites.
1) The alkali additive can interact with citral substrate and solubilise them in an aqueous medium, in contrast; it may reduce the solubility of the hydrogen [
In the case of Pt catalysts, it has been suggested [
However, in the case of Pd metals, the interaction of -C=C- will be optimized and the rate of hydrogenation enhanced by the presence of potassium cations.
At lower alkali amounts, the effect of alkali on H2 solubility is insignificant, on the other hand; the citral solubility is increased; hence the hydrogenation activity was considerably increased up to 8.5 m∙mol. A further increase in the alkali content it appears that the alkali addition is significantly influencing the both solubility of H2 as well as blocking or poisoning of the catalytic sites, which reduce the hydrogenation activity. The results reveal that the addition of alkali additives like K2CO3, increases the activity up to an optimum concentration, further increase may reduce the hydrogenation due to deactivation of Pd active sites. It was opined that the palladium catalysts; the effect of alkali may cause mainly by poisoning the activity of the catalyst, which leads to the further hydrogenation of hydrocinnamaldehyde. However, a detailed study of these effects is required to shed light on these exploratory ideas.
In conclusion, the citral hydrogenation activity was significant in acetonitrile as a solvent over Pd/CMS like ~40% with 65% selectivity to citronellal. In the aqueous media, these catalysts exhibited the ~9% activity with 100% selectivity to citronellal. However, the activity in aqueous media is lower compared to that of organic solvents. The Pd/CMS prepared by simple impregnation method also provided good activity and compared to the literature reported carbon catalysts. The addition of optimum potassium carbonates to the aqueous media clearly demonstrates that cost-effective way of increasing the yields of citronellal. On the other hand, the Pd/CMS catalysts are found to be promising for the hydrogenation of α, β-unsaturated carbonyls and specific to the citral and stable in alkali carbonated aqueous media.
One of the authors R. Krishna is indebted to the UGC-CSIR for awarding the fellowship. The authors are thankful to the Director DRDE and A. P. Bansod for providing the necessary support for the research. Authors are sincerely thankful to Dr. K. S. Ramarao, IICT, Hyderabad for providing the X-ray diffraction analysis.
The alkali additives like K2CO3, effective in enhancing the selective hydrogenation of -C=C- in aqueous media for α-β unsaturated carbonyls.
Selective hydrogenation can be achieved in the aqueous media for α-β unsaturated carbonyls.
RacharlaKrishna,ChowdamRamakrishna,KeshavSoni,ThakkallapalliGopi,GujarathiSwetha,BijendraSaini,S. ChandraShekar, (2016) Effect of Alkali Carbonate/Bicarbonate on Citral Hydrogenation over Pd/Carbon Molecular Sieves Catalysts in Aqueous Media. Modern Research in Catalysis,05,1-10. doi: 10.4236/mrc.2016.51001