211
pressure and temperature constant. Earlier studies have demonstrated that these two
variables including pressure and temperature have shown similar results for all the
metal catalysts. These studies have shown that the deuterium distributions in the
butenes were approximately the same. This could happen only when each of the
butenes was an initial product, and not the result of the isomerization of any other
as well as the absence of both the 1,2- and 1,4-addition processes. Another major
conclusion drawn from this study was that there was limited but adjustable exchange
of the hydrogen atoms of the reactant. Appearance of small amounts of hydrogen in
the deuterated reactants and products shows that the concentration of atoms on the
surface was small under prevailing experimental conditions [18, 21].
2.2 Reaction Order of Reactants: 1.3-Butadiene, Hydrogen,
1-Butene
As the reactants and few products of this reaction have the tendency to competitively adsorb on the catalytic sites, the reaction order was found to be different on
different catalysts, therefore understanding the reaction order with respect to the
reactants and products can provide valuable information about the product selectivity. The main parameters which characterize the hydrogenation process are (a) the
sum of all three butenes (i.e. total butenes) selectivity and (b) individual butene
isomer selectivity, i.e. the fraction of butenes that each isomer constitutes (trans,
cis-2- butene and 1-butene). Over many of the metal catalysts from Groups 8 to 10,
the selectivity to the sum of all three butenes was close to unity and the composition
was unaffected until the complete consumption of reactant 1,3-butadiene; therefore,
it was more strongly adsorbed than the products. Kinetics studies on hydrogenation
of 1,3-butadiene in gas phase or liquid phase have shown that the intrinsic reaction
rate was zero order with respect to the butadiene and approximately first order with
respect to hydrogen [7–15, 17, 28–31]. Strong and preferential adsorption of
1,3-butadiene over 1-butene were indicated from the measurements of reaction
orders and activation energies. Moreover, each unsaturated compound has different
adsorption strength and always selectivity for the hydrogenation reaction depends
on these adsorption energies. Compared to n-butenes, 1,3-butadiene has high
propensity to be hydrogenated. Only, when the concentration of 1,3-butadiene was
lower in the mixture, n-butene was found to be hydrogenated preferentially to
n-butane. Oudar et al. [32, 33] observed two kinetic regimes for the 1,3-butadiene
hydrogenation and H 2 -D 2 exchange over the Pt(110) surface and pressure conditions
used for this reaction are 200–400 Torr. Reaction orders with respect to the hydrogen
and butadiene were found to be 1 and 0, respectively at >130 Torr of hydrogen
pressures. It was also observed that the selectivity was independent of the hydrogen
pressure applied, and every two Pt atoms of the catalyst surface was covered by one
butadiene molecule. Below critical hydrogen pressure (~125 Torr); the reaction
order was 2 and 0 with respect to hydrogen and 1,3-butadiene, respectively. Another
Selective Hydrogenation of 1,3-Butadiene to 1-Butene: Review on Catalysts, Selectivity…
pressure and temperature constant. Earlier studies have demonstrated that these two
variables including pressure and temperature have shown similar results for all the
metal catalysts. These studies have shown that the deuterium distributions in the
butenes were approximately the same. This could happen only when each of the
butenes was an initial product, and not the result of the isomerization of any other
as well as the absence of both the 1,2- and 1,4-addition processes. Another major
conclusion drawn from this study was that there was limited but adjustable exchange
of the hydrogen atoms of the reactant. Appearance of small amounts of hydrogen in
the deuterated reactants and products shows that the concentration of atoms on the
surface was small under prevailing experimental conditions [18, 21].
2.2 Reaction Order of Reactants: 1.3-Butadiene, Hydrogen,
1-Butene
As the reactants and few products of this reaction have the tendency to competitively adsorb on the catalytic sites, the reaction order was found to be different on
different catalysts, therefore understanding the reaction order with respect to the
reactants and products can provide valuable information about the product selectivity. The main parameters which characterize the hydrogenation process are (a) the
sum of all three butenes (i.e. total butenes) selectivity and (b) individual butene
isomer selectivity, i.e. the fraction of butenes that each isomer constitutes (trans,
cis-2- butene and 1-butene). Over many of the metal catalysts from Groups 8 to 10,
the selectivity to the sum of all three butenes was close to unity and the composition
was unaffected until the complete consumption of reactant 1,3-butadiene; therefore,
it was more strongly adsorbed than the products. Kinetics studies on hydrogenation
of 1,3-butadiene in gas phase or liquid phase have shown that the intrinsic reaction
rate was zero order with respect to the butadiene and approximately first order with
respect to hydrogen [7–15, 17, 28–31]. Strong and preferential adsorption of
1,3-butadiene over 1-butene were indicated from the measurements of reaction
orders and activation energies. Moreover, each unsaturated compound has different
adsorption strength and always selectivity for the hydrogenation reaction depends
on these adsorption energies. Compared to n-butenes, 1,3-butadiene has high
propensity to be hydrogenated. Only, when the concentration of 1,3-butadiene was
lower in the mixture, n-butene was found to be hydrogenated preferentially to
n-butane. Oudar et al. [32, 33] observed two kinetic regimes for the 1,3-butadiene
hydrogenation and H 2 -D 2 exchange over the Pt(110) surface and pressure conditions
used for this reaction are 200–400 Torr. Reaction orders with respect to the hydrogen
and butadiene were found to be 1 and 0, respectively at >130 Torr of hydrogen
pressures. It was also observed that the selectivity was independent of the hydrogen
pressure applied, and every two Pt atoms of the catalyst surface was covered by one
butadiene molecule. Below critical hydrogen pressure (~125 Torr); the reaction
order was 2 and 0 with respect to hydrogen and 1,3-butadiene, respectively. Another
Selective Hydrogenation of 1,3-Butadiene to 1-Butene: Review on Catalysts, Selectivity…