287293 Selectivity Control in 3-Phase Hydrogenation of Alkynes
Selectivity control in 3-phase hydrogenation of alkynes.
Craig T. Barrett, Andrew Monaghan and S. David Jackson
Centre for Catalysis Research, WestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, Scotland, UK
Introduction.
Selective hydrogenation of higher molecular weight alkynes is often performed over a Lindlar catalyst. This catalyst is Pd/CaCO3 with lead and quinoline modifiers and gives high selectivity to the cis-olefin. A recent theoretical study [1] examined the role of both the Pb and quinoline and assigned different complimentary properties to each: Pb affecting the thermodynamic factor and the ability of Pd to form hydrides, while the quinoline reduced the coverage of hydrogen and alkyne inhibiting oligomerization. In this study we have examined the effect of valeronitrile and its respective amine, amyl amine, and 3-phenyl propionitrile and its respective amine, 3-phenyl propylamine on activity and selectivity of 1- and 2-pentyne hydrogenation.
Experimental
The catalyst used throughout this study was a 1% w/w Pd/q-alumina (Johnson Matthey, characterised by a BET area of 97.6 m2g-1, a pore volume of 0.49 ml g-1, a metal loading of 1 wt.% and a metal dispersion of 32.5%). All reactants were used without further purification. The reaction was carried out in a 0.5l Buchi stirred autoclave to which 0.05 g of catalyst was added to 330 ml of degassed solvent, methanol. Reduction of the catalyst was performed in situ by sparging the system with H2 for 30 min at 313 K. For both 1-pentyne and 2-pentyne, 1 ml was injected into an unstirred solution along with a quantity of modifier. In all reactions the quantity of alkyne was constant while the concentration of modifier was varied. The vessel was then pressurised with H2 to 2 barg. Liquid samples were analysed by GC and standard checks were undertaken to confirm that the system was not under mass transport control.
Results/Discussion.
The reactions studied were the hydrogenation of 1-pentyne and 2-pentyne. Four modifiers were used, an aliphatic nitrile and amine, pentane nitrile (PN, valeronitrile) and its respective amine, pentyl amine (PA, amyl amine), and an aromatic nitrile and amine, 3-phenyl propionitrile (3-PPN) and its respective amine, 3-phenyl propylamine (3-PPA). These modifiers were not hydrogenated under reaction conditions. The effect of these modifiers at a 1:1 ratio with the respective alkyne has been reported previously [2]. However in this study we were interested to see the effect of reducing the amount of modifier in solution yet still changing activity/selectivity. Ideally the aim is to maintain high selectivity even at high conversions. In table 1 the selectivity is compared at high conversion when there is a ratio of 1-pentyne:modifier of 100:1. It is clear from the pentane selectivity that the aromatic amine and nitrile are the most effective at maintaining high olefin selectivity. Given that aliphatic amines are more basic than aromatic amines, it seems likely that ring is participating in the bonding and is blocking a larger amount of the surface and it is this that dominates over the basic strength at low concentrations.
Table 1. Effect of modifier at 1 % of reactant concentration at high conversion.
| 1-pentene | cis-2-pentene | trans-2-pentene | Pentane | Conversion (%) |
no modifier | 40 | 6 | 18 | 36 | 94 |
1% PN | 41 | 6 | 17 | 37 | 93 |
1% 3-PPN | 45 | 8 | 18 | 29 | 95 |
1% PA | 26 | 7 | 21 | 47 | 95 |
1% 3-PPA | 46 | 7 | 18 | 29 | 94 |
We examined the effect of 3-PPN on the hydrogenation of 2-pentyne over three orders of magnitude and the effect on selectivity is shown in table 2. It can be seen that the system slowly changes as the modifier ratio is changed and that by a 1000:1 reactant:modifier ratio the system is very similar to that without any modifier. Figure 1 shows the effect of 3-PPN concentration on the first-order rate constant for 2-pentyne
Table 2. Effect of 3-PPN, at different concentrations, on 2-pentyne hydrogenation selectivity at 20% conversion.
Ratio 2-pentyne:3-PPN | pentane | trans-2-pentene | 1-pentene | cis-2-pentene |
1:1 | 8 | 13 | 0 | 80 |
1:0.1 | 12 | 19 | 0 | 69 |
1:0.01 | 13 | 24 | 0 | 62 |
1:0.001 | 19 | 27 | 0 | 54 |
no modifier | 19 | 22 | 0 | 59 |
Figure 1. The relationship between 3-PPN modifier concentration and rate constant for 2-pentyne.
The behaviour of the modifiers can be linked to the strength of adsorption, to the mode of adsorption and surface specificity of the reactant, and to the influence on the presence of sub-surface hydrogen. Further details illuminating these aspects will be reported in the full paper.
References.
1. M. García-Mota, J. Gómez-Díaz, G. Novell-Leruth, C. Vargas-Fuentes, L. Bellarosa, B. Bridier, J. Pérez-Ramírez, N. López, Theoretica Chimica Acta, 128 (2011) 663-673.
2. P.E. Garcia, A.S. Lynch, A. Monaghan, S.D. Jackson, Catalysis Today, (2010)
See more of this Group/Topical: Catalysis and Reaction Engineering Division