2062
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 10, October, 2008
Deliy and Simakova
Comparison of the catalytic activity of various group
VIII metals in the isomerization and hydrogenation of
αꢀ and βꢀpinenes. The difference in the catalytic activity
of the platinum metals in the isomerization and hydrogeꢀ
nation of the pinene double bond can be explained by the
difference in the electronic properties of the Group VIII
metals and mechanisms of adsorption of olefin and hyꢀ
drogen on the metal surface.
The studies performed showed that the isomerization
rate of αꢀ and βꢀpinenes correlates with the activation
energy of hydrogen adsorption on the metal surface. The
Pd/C catalyst is most active in the isomerization of
the double bond in a pinene molecule. For the Pd surface
the activation energy of hydrogen chemisorption on the
{111} face is lowest, being25 3.1 kJ mol–1. The other cataꢀ
lytic systems Rh/C, Pt/C, Ru/C, and Ir/C exhibit conꢀ
siderable activity in the hydrogenation of αꢀ and βꢀpinene
(see Tables 2 and 3).
of the double bond of nꢀbutenes on the Pd/C, Rh/C, and
Pt/C catalysts. The mechanism includes the semihydroꢀ
genated alkyl intermediates adsorbed on the surface. Howꢀ
ever, further studies are required to propose probable
mechanisms of isomerization and hydrogenation of a mixꢀ
ture of αꢀ and βꢀpinenes on the catalysts based on other
Group VIII metals (Pt, Rh, Ru, and Ir).
Scheme of the mechanism of hydrogenation and isomerꢀ
ization of αꢀ and βꢀpinene on the Pd/C catalyst. The forꢀ
mation of hydride species on the metal surface is necesꢀ
sary for steps leading to the migration and hydrogenation
of the double bond of olefins on the Platinum Group
metals.34 The source of chemisorbed hydrogen can be
external hydrogen and hydrogen obtained by the βꢀelimiꢀ
nation of the alkyl intermediate adsorbed on the metal
surface. We found that in the absence of hydrogen the rate
of isomerization and hydrogenation of a mixture of
αꢀ and βꢀpinenes on the Group VIII metals was low. This
indicates no formation of the η3ꢀallylic intermediate,
which is necessary for the reaction to occur, via the mechaꢀ
nism of intramolecular hydrogen transfer promoted by
the palladium adsorption sites.
The assumed scheme of the mechanism of isomerizaꢀ
tion and hydrogenation of αꢀ and βꢀpinene (migration of
the С=С double bond) on the Pd/C catalyst includes the
following steps (Scheme 3): step 1, chemisorption of H2
molecules on the palladium surface; steps 2 and 5, αꢀ and
βꢀpinene molecules are reversibly adsorbed on the metal
surface with the formation of intermediates πꢀbonded with
the surface metal atoms; step 3, insertion of the activated
C=C double bond between the metal—hydrogen bond
affords the σꢀbonded pinanyl intermediate; step 4,
βꢀelimination of the hydrogen atom from the chemisorbed
σꢀpinanyl intermediate results in the formation of the
πꢀcomplex with the surface palladium atom; step 6, inserꢀ
tion of the second hydrogen atom at the metal—carbon
bond in the σꢀpinanyl complex followed by the desorpꢀ
tion of cisꢀ or transꢀpinane results in the formation of a
vacancy on the catalyst surface.
Based on the obtained experimental data, the orders
of decreasing activity for the metal in βꢀpinene isomerꢀ
ization to αꢀpinene are
25 °C: Ir < Ru < Pt <Rh < Pd,
80 °C: Ir < Pt < Ru < Rh < Pd,
which agree with the known literature data29 for the
isomerization of the double bond of olefins on the Platiꢀ
num Group metals.
A probable reason for differences in the catalytic
activity of the platinum metals in double bond isomerizaꢀ
tion can be different mechanisms of olefin adsorption on
the metal surface. According to the data of IR spectroꢀ
scopy,30 alkenes adsorbed on the surface of Platinum
Group metals can exist in at least three forms: πꢀbonded (I),
diꢀσꢀbonded (II), and ethylidine (III) species (Scheme 2).
For example, the study of adsorption of butenes31 showed
that πꢀallylic particles predominate on the surface of the
{111} face. These particles were shown32 to be primary
intermediates in the hydrogenation and migration of the
double bond.
The multiple reiteration of the steps of addition and
elimination of hydrogen results in the establishment of
equilibrium between the pinene αꢀ and βꢀisomers.
Scheme 2
Thus, the liquidꢀphase hydrogenation and isomerizaꢀ
tion of αꢀ and βꢀpinenes was conduced on the Pd/C, Ru/C,
Rh/C, Pt/C, and Ir/C catalysts in the temperature interꢀ
val 25 to 100 °C and at hydrogen pressures of 0.5—11 bar.
It was shown that among the studied catalysts the Pd/C
catalyst has the highest catalytic activity and selectivity
in αꢀpinene isomerization to βꢀpinene. The maximum
yield of βꢀpinene upon the isomerization of αꢀpinene at
T = 100 °C and under the hydrogen pressure p(H2) = 11 bar
is 2.8 mol.%, which is close to the thermodynamically
equilibrium value. It was found that the rate of βꢀpinene
isomerization to αꢀpinene has the first order with respect
to βꢀpinene and the order equal to 0.5 with respect to
(Ι)
(ΙΙ)
(ΙΙΙ)
The kinetic data obtained by us for the isomerization
and hydrogenation of a mixture of αꢀ and βꢀpinenes on
the Pd/C catalyst are consistent with the known mechaꢀ
nism proposed33 for the isomerization and hydrogenation