The liquid-phase hydrogenation kinetics of benzene and three
monosubstituted alkylbenzenes,
toluene, ethylbenzene, and cumene, was determined in a semibatch
reactor operating at hydrogen
pressures of 20−40 atm and at temperatures of 95−125 °C.
Commercial preactivated catalyst
particles of nickel−alumina were used in all experiments. The
hydrogenation activity of the
compounds decreased in the order benzene ≫ toluene > ethylbenzene >
cumene. The main
reaction product was always the completely hydrogenated cycloalkane,
whereas only trace
amounts of cycloalkenes were detected. The hydrogenation rates had
a slow initial period
followed by a period of a virtually constant rate, which decreased at
the end of the reaction.
The analysis of the data with a reaction−diffusion model
revealed that the kinetics was influenced
by pore diffusion. Rate equations based on a sequential addition
mechanism of adsorbed
hydrogen to the aromatic nucleus were derived, and the kinetic
parameters were estimated
from the reaction−diffusion model with nonlinear regression analysis.
The rate equations were
able to describe all the features of the experimental
data.
The liquid phase hydrogenation kinetics of five di- and
trisubstituted alkylbenzenes, xylenes,
mesitylene, and p-cymene were determined in a semibatch
reactor operating at hydrogen
pressures of 20−40 atm and at temperatures of 95−125 °C.
Commercial preactivated catalyst
particles of nickel−alumina were used in all experiments. The
hydrogenation activity of the
aromatic compound was affected by both the number of substituents and
their relative positions
in the benzene ring. The trisubstituted benzene (mesitylene) had a
lower reaction rate than
the disubstituted compounds (xylenes). The activity of the
different substituent positions
decreased in the order para > meta > ortho. The main reaction
product was always the
completely hydrogenated cycloalkane; no cycloalkenes were detected.
The hydrogenation rates
were virtually constant at low and intermediate conversions of the
aromatics, but at high
conversions the rates decreased. Rate equations based on the
formation of partially hydrogenated
surface complexes were derived, and the kinetic parameters were
estimated from a heterogeneous
reactor model with nonlinear regression analysis. The rate
equations were able to describe the
features of the experimental data.
The liquid-phase hydrogenation kinetics of one
multiaromatic mixture and five binary aromatic
mixtures of toluene, ethylbenzene, xylenes, and mesitylene were
determined in a semibatch
reactor operating at a pressure of 40 bar and a temperature of 125
°C. Commercial preactivated
catalyst particles of nickel−alumina were used in the experiments.
In mixtures, the aromatic
compounds reacted in queues so that the most reactive components
started to react immediately
while the least reactive components did not react until the most
reactive components had been
hydrogenated completely. This type of reactivity decreased with
the increasing number of
substituents, i.e. in the order monosubstituted > disubstituted >
trisubstituted. The relative
positions of the substituents affected the reaction rate so that the
reactivity decreased in the
order ortho > para > meta. The queue effect was described with a
kinetic model based on the
rapid adsorption of aromatic compounds and hydrogen and sequential
addition of hydrogen to
the aromatic nucleus, the first hydrogen addition step being rate
determining. The kinetic
parameters were estimated from a heterogeneous reactor model with
nonlinear regression
analysis. The kinetic model was able to describe hydrogenation
kinetics of the binary aromatic
mixtures.
The hydrogenation kinetics of 2,2-dimethylol-1-butanal (TMP-aldol) over a supported nickel catalyst was determined with experiments carried out in a batchwise operating autoclave at 50-90 °C and 40-80 bar hydrogen. The reaction mixture was analyzed with gas and liquid chromatography. It was found that TMP-aldol can be hydrogenated with a 100% selectivity to the corresponding triol, trimethylolpropane. The effects of the catalyst activation procedure and the formaldehyde concentration on the hydrogenation kinetics were studied. The hydrogenation experiments revealed that catalyst reduction at a high temperature (400 °C) under hydrogen flow was favorable for the catalyst performance. The reason was a more effective reduction of nickel oxides which was confirmed with thermogravimetry and X-ray photoelectron spectroscopy. The presence of formaldehyde had a considerable retarding effect on the aldol hydrogenation kinetics: the hydrogenation rate was low until all of the formaldehyde was hydrogenated to methanol. The retarding effect was more prominent at higher temperatures than at lower temperatures, which indicates that formaldehyde forms oligomers on the catalyst surface as the temperature increases. A kinetic model was proposed for the aldol hydrogenation. The model includes adsorption, desorption, and surface reaction steps as well as the inhibitory effect of formaldehyde on the aldol hydrogenation kinetics. The model was able to describe the experimentally recorded hydrogenation kinetics of TMP-aldol in the presence and in the absence of formaldehyde.
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