(762d) 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

David.jackson@glasgow.ac.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.

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.

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)