2916 Chen et al.
Asian J. Chem.
actual condition. For this reason, many Chinese researchers
have carried out the study of coal to ethanol technology.
Ethanol can be produced from both syngas directly and
other various sources such as acetic acid, ethyl acetate, methyl
acetate and dimethyl ether, which are conversed from syngas
[15]. Research on the production of alcohols from syngas directly
has been going on for decades, however the reaction exhibits
poor selectivity of ethanol and remains challenging [2]. The
processes of syngas to methanol [16,17] and carbonylation of
methanol to acetic acid [18] are mature technologies. Thus,
the hydrogenation process of acetic acid to ethanol is a promi-
sing choice.
acetaldehyde appear to be kinetically significant steps. The
investigations showed that Pt/Sn-based catalysts are selective
for conversion of acetic acid to form acetaldehyde and ethanol,
whereas platinum catalysts completely decompose acetic acid
to gas productions [25-27]. Additionally, Zhang [27] added
esterification of acetic acid and ethanol to ethyl acetate occurred
on the oxide surface to the elementary steps, therefore, selectivity
of ethanol was led into the kinetics model.A more recent study
of hydrogenation of acetic acid over alumina or silica supported
Cu/In and Ni/In catalysts was carried out, and the results showed
that the activity dependence on the reactant partial pressures
denotes the rate-determining surface reaction in terms of
Langmuir-Hinshelwood kinetics [28,29].
Many patents have reported the catalysts for hydrogena-
tion of acetic acid. In these studies, most of the catalysts were
one or more noble metals in Group VIII, dispersed on Group
III or IV metal oxides. Rachmady andVannice [19-23] carried
out a series of researches on platinum catalysts supported on
TiO2, SiO2, Al2O3 and Fe2O3, and the results were compared
with that obtained without support. Several observations were
made about the kinetic behaviour of acetic acid hydrogenation
to form organic compounds over supported platinum catalysts:
(a) the reaction requires both metal and an appropriate oxide
phase in the catalyst; (b) oxides that are active for ketoniza-
tion are the best supports, implying the reaction can occur on
the oxide surface; (c) platinum acts as a source of activated
hydrogen, presumably hydrogen atoms [19]. Based on these
findings, a Langmuir-Hinshelwood-type mechanism was
proposed, in which acetaldehyde is formed as the first initial
product, and it can react with additional hydrogen atoms to
form ethanol [19,22]. The reaction model similar to that used
to describe acetic acid reduction over Pt/TiO2 was applied to
this reaction over Fe/SiO2, with the only difference being that
the rate-determining steps involved the addition of the first H
atom to an acetate species [19] rather than an acyl species [22].
Non-local density functional theory (DFT) calculations were
used to examine alternative mechanisms for the hydrogenation
of acetic acid to ethanol over different catalysts [24]. Using
the overall reaction energies to deduce a plausible mechanism
for acetic acid hydrogenolysis, Pallassana and Neurock [24]
found that the acetyl formation and acetyl hydrogenation to
Within all reported catalysts, it was difficult to obtain a
high conversion of acetic acid while keep a high selectivity of
ethanol simultaneously. Our research group developed a multi-
metallic based catalyst, which was proven to be more efficiency
in both conversion and selectivity. At this point, investigating
the kinetics behaviour of hydrogenation reaction of acetic acid
to ethanol over this multi-metallic based catalyst becomes
critical to scale up in the future.
EXPERIMENTAL
Acetic acid (99.5 % purity) was supplied by Sinopharm
Chemical Reagent Co. Ltd., China. Hydrogen from Air Liquide
(China) Holding Co. Ltd. was 99.99 vol. %. The kinetic tests
were carried out on a multi-metallic catalyst.
Procedure: Hydrogenation of acetic acid system is shown
in Fig. 1. High pressure hydrogen from the cylinder wasdepre-
ssurized by a pressure reducing valve, and the hydrogen flow
rate was regulated by a mass flow controller. The acetic acid
was pumped into the reaction system and was well mixed with
hydrogen by an on-line mixer. A fixed bed reactor similar to
an isothermal integral reactor was used for the kinetics testing.
Acetic acid was heated to vapour phase in the front of reactor.
Hydrogenation reaction was down in the catalyst bed located
in the mid of reactor. The final products were cooled through
a condenser and entered a gas liquid separator tank. The liquid
products including ethanol, ethyl acetate, acetaldehyde, other
trace components and unreacted acetic acid were collected at
FC
Condenser
Reactor
Mass flow
controller
Hydrogen
Coolant
Coolant
Furnace
Acetic acid
tank
Pressure indicator
PI
Gas products
Pump
Gas liquid
separator
TE
Liquid products
Thermocouple
Fig. 1. Schematic diagram of acetic acid hydrogenation system