Hydrogenation of aromatic compounds
contain a mixture of saturated, monounsaturated, and polyunsaturated fatty acids. The mono and
polyunsaturated fatty acids have double bonds, all in the normal “cis” formation. These bonds can
easily be broken down by oxygen. This produces compounds that make the oil rancid. Rancidity
produces off flavours in foods. To control this edible oil is hydrogenated in the food industry to produce
fats and oils with desirable melting properties and an improved shelf life.
Edible oil + H
2
→ Margarine 1
Beside the desired hydrogenation reaction eqn.1, trans isomers of fatty acids are formed as
well [49]. The trans isomer has been reported to be undesirable for human diet due to adverse health
effects [50]. It has similar effects as saturated fats increasing serum cholesterol levels in the blood,
believed to be a major cause of heart disease [51]. The reduction andor elimination and content of
trans fatty acids in the food supply has attracted worldwide interest [52 56]. The options to reduce
the trans levels in the hydrogenation of an edible oil are changing process conditions and applying
selective low trans heterogeneous catalysts [57].
The copper chromite [44] has been extensively studied due to the high selectivity shown by this
catalyst in the partial hydrogenation of vegetable oils [58,59]. In particular, many experiments have
been done to correlate catalytic properties with operational parameters such as temperature [60],
hydrogen pressure [60,61 63], hydrogen flow [64], catalyst
concentration [61,63],
substrate composition [62] and activation procedures [65].
Copper chromite catalysts have long been known in edible oils hydrogenation as the most
selective for the reduction of linolenate C
18:3
to oleate C
18:1
leaving unaffected linoleate C
18:2
, valuable component from the nutritional point of
view [59]. The major factor responsible for the relative instability of soybean oil and other
vegetable oils for food uses is widely recognized as the linolenate present in the oil [66]. A particular
feature of the copper chromites is their high selectivity which has been used to advantage in the
hydrogenation of edible oils and fats, where stronger hydrogenation catalyst, such as nickel can
lead
to excessive
saturation and
inferior nutritional quality of the final product [67].
Commercially employed Ni catalysts have limited linolenate selectivity in comparision to copper
chromite catalyst. Consequently, soybean oil hydrogenated over Ni catalyst to an iodine value
IV of 110 contains 3 linolenate, while copper chromite catalyst reducing it to 0.1 [66]. In room
odour evaluations, copper chromite hydrogenated soybean oil gave higher scores and lower fishy
responses than nickel hydrogenated soybean oil after both had been exposed to fluorescent light
[68].
The catalysts are usually charged into the oil in the oxidized form and are partly reduced to CuI
andor Cu0 during use. Pre reduced copper chromites have found to be strongly deactivated in
soybean oil hydrogenation due to disappearance of CuII and CuI species and to the decrement of
CuCr ratio on the catalyst surface [69]. Capece and co workers [70] determined the oxidation
states and surface composition of copper chromite at various stages of catalytic use and after
reductive pre treatments, and they concluded that Cu
1
is the active species for double bond isomerization
while Cu
is required
for hydrogenation of conjugated dienes. According to
Rieke et al. [71], activity and selectivity correlate well with the crystallinity of the copper chromite
surface; they
increase with
decreasing crystallinity.
Szukalska and Drozdowski [72] hydrogenated rapeseed oils with different erucic acid contents
with Adkins type copper chromite catalyst. The tested rapeseed oils, after the elimination of
linolenic acid by selective hydrogenation showed several times higher oxidative stability than the
initial raw material and retained the liquid state at ambient temperatures.