SSCHE 14 075 Kralik Aniline Catalysis and CHemEngng
SSCHE 14 075 Kralik Aniline Catalysis and CHemEngng
SSCHE 14 075 Kralik Aniline Catalysis and CHemEngng
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PROCEEDINGS
41st International Conference of Slovak Society of Chemical Engineering
Hotel Hutnk
Tatransk Matliare, Slovakia
May 2630, 2014
Krlik, M., Turkov, M., Mak, I., Lehock, P.: Aniline - catalysis and chemical engineering, Ed-
itor: Marko, J., In Proceedings of the 41st International Conference of Slovak Society of Chemical
Engineering, Tatransk Matliare, Slovakia, 723733, 2014.
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Abstract
Aniline (AN) is an important organic chemical commodity with an annual worldwide
production at levels of MT. Main applications are in polymer chemistry; however production
of rubber chemicals, dyes, pharmaceuticals, and other chemical specialties is also important.
Nitration of benzene (B) and hydrogenation of the generated nitrobenzene (NB) to aniline are
main chemical steps in the industrial production of AN. In spite of many attempts to replace a
classical nitric-sulphuric acid mixture for the nitration of benzene by more ecological agents,
e.g. a direct nitration with oxides of nitrogen, industrial production of NB is exclusively based
on that classical procedure. Of course, modern reactor solutions (jet mixing, intensive mass
transport, precise control of temperature, etc.) ensure high yields of NB and sophisticated
treatment of wastes (regeneration of nitration mixture, thermolysis and biological treatment of
wastewaters) minimize ecological impacts.
The catalytic reduction of NB can be carried out either in the gas/vapour phase (G-S) or
in the liquid phase (L-S). Copper catalysts are favoured for the G-S and noble metals (mainly
palladium and platinum) are preferred for the L-S system. Besides high yields of AN (more
than 99%) inevitable requirement is utilization of the reaction heat (more than 500kJ/mol).
Modern reaction routes for production of AN are based on a direct amination of
benzene these are subjects for investigations. However results obtained so far do not give
chance to compete with the nitrobenzene technology.
Together with literature and patent data, brief information about the development of
aniline technology in VUCHT is also reported. A special focus is on reaction routes in the
hydrogenation of NB, deactivation of catalysts and relationships with their physicochemical
features, as well as reactor arrangement and its influence on mass and heat transport.
Necessity to combine theoretical (calculated) results with those from a model hydrogenation
unit is a key factor for design of efficient technology.
Introduction
History of AN has started in 1826, when the German chemist Otto Unverdorben isolated
by destructive distillation of indigo a substance that he called in that time Krystallin [1].
Increasing demands for AN led to synthetic routes, which started using the so called Bechamp
reduction published in 1854 [2].
4 NO2 + 9 Fe + 4 H 2 O 4 NH2 + 9 Fe 3 O4
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Later the reduction with iron was substituted by a catalytic reduction of nitrobenzene,
firstly applied in 1871 [3,4], and it is still the most utilized procedure for the preparation of
AN [5,6].
This paper is devoted mainly to the production of AN by the reduction of NB, especially
using palladium catalysts but some alternative routes are mentioned, too.
Preparation of nitrobenzene
Nitrobenzene is industrially prepared by the electrophilic substitution of hydrogen in the
benzene ring with the nitronium ion (NO2+):
H
+ NO2+ + NO2
H
+ NO2
+ HSO4- NO2 + H 2SO4
Table 1. Standard molar enthalpy (heat) of formation at 298.15 K and reaction heat of the
nitration of B to NB and hydrogenation of NB to AN a.
Compound fH/kJ mol1(L) fH/kJ mol1(G)
B 49.1 82.9
HNO3 -174.1 -133.9
NB 12.5 67.5
H2 0.
AN 31.6 87.5
Water -285.8 -241.8
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There are only a few suppliers in the world which provide good nitration technologies.
Probably the first successful process of adiabatic nitration was presented by DuPont [8,9] who
offers competitive technologies also today:
http://dupont.t2h.yet2.com/t2h/page/techpak?id=27458&sid=330&abc=0&page=tpprint).
Meisner company (http://www.josefmeissner.com/en/nitroaromatics/mononitrobenzene-
mnb) applies adiabatic jet reactor [10] with subsequent tube rectors to ensure high contact of
organic phase (NB + B) with the nitration mixture. Formation of by-products (nitrophenols,
picric acid and dinitrobenzene) is minimized just by an optimal reactant intermixing and
negligible back mixing inside the tube reactor. Due to the adiabatic technology, the
reconcentration of the sulfuric acid can be directly in the plant. The crude NB is fed to the
washing section consisting of MEISSNER compact washers and residual B in the purified
NB is separated by means of steam? (as a steam) and returned into the process.
The next supplier of the NB technology is NORAM (http://www.noram-
eng.com/groups/nitration-group-technologies.html), who uses patented electrophilic reactor
[11]. A high dispersion of benzene in the acid phase and consequent a fast reaction rate is
achieved by jet impingement of organic and water-acid phases. It is possible to minimize
residence time and virtually stoichiometric conversion of nitric acid and benzene is attained.
A decanter is used to separate the organic and acid phases. Spent acid is reconcentrated in a
Sulfuric Acid Flash Evaporator (SAFE). Energy requirements for the process are met largely
by pre-heating B and nitric acid feeds with waste reaction heat. The crude product is taken to
downstream facilities for purification. These treatments involve both standard and proprietary
procedures for washing and B stripping.
Plinke also offers adiabatic nitration of B in the liquid phase:
http://www.plinke.de/fileadmin/user_upload/pdf/Flyer/PLINKE_NB_08-2012.pdf. The
concentration of nitration mixture is adjusted to keep safety limits (increase in temperature
due to exothermic reaction). Washing and stripping are the next steps for the production of
high quality NB.
Treatment of wastes is rather sophisticated. Meissner offers thermolysis of organic
wastes. Plinke uses treatment of waste waters with oxygen and FeSO4 as a reactant and a
catalyst and biological treatment in the next step. Outgases contain portion of organics and
some NOx and are treated using common DENOX procedures.
Despite a lot of endeavour, successful laboratory research, technological applications and
investigations, nitration of benzene still provides a space for further improvements [12].
The field of gas-phase nitration is of interest; however this process is not competitive
with liquid-phase processes using sulphuric acid. The challenge remains to find an efficient
catalyst for the gas-phase nitration with oxides of nitrogen [13,14].
Catalytic reduction of NB to AN
Catalytic reduction was firstly described by Kolbe and Zaztseff [3,4].
cat.
NO2 + 3 H 2 NH2 + 2 H 2 O
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NO2 HO NH2
1 6
H
2 O
NO N N
2 5 N NH2
H H H
2 2 8
NHOH
3
H N N H
2 2
H 7
NH2 2
N N
4 H H
NH 2 9
H 10 14
2
H
2
H 2O H
2
NH 2 NH O OH
11 13 NH 3
15 16
- NH 3 NH2 - NH3 NH2
H 2O
OH
- NH3
12
N H
N
17
H
2
H
2 18
N N
H H
20 19
NO2 NO NH2
NOH
H2
HO H
N NH2 NH2 NH O OH
O
H2 H2O H2
N N
- NH 3
AN - NH 3
AN - H2O H
H
NH N
N N N N
H2
H - H2 H2
NH2 N N H H
H N N
Scheme 3. Gelders [17,18] interpretation of reaction pathways in the hydrogenation of NB.
Experience from the recent years pointed out importance of quantum mechanical
modelling for reactivity elucidation. Increase in computers power together with available
modelling programs allow to calculate approximate values of charge distribution and energies
for HOMO (willingness to donate electrons) and LUMO (willingness to accept electrons)
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orbitals in a rather simple way. The very important is a difference between energies of
HUMO and LUMO; the lower is this value the higher is their reactivity. To evaluate steric
effects structures of molecules are presented in Figure 1.
Table 2. Energies of the Highest Occupied Molecular Orbital (EHOMO) and the Lowest
Unoccupied Molecular Orbital Calculated (ELUMO) using HYPERCHEM lite program with the
MP3 method considering atoms charges.
Figure 1. Structure of NB (upper left), NOB (upper right), PHA (down left), AN (down right).
According to the work of Kochetova et al. [19], the catalytic steps of hydrogenation are
depicted in Scheme 4, where positive effect of electron withdrawing moieties on the
formation of H+ species is evident. The electron withdrawing effect can be ensured by a
support, e.g. activated carbon, or positively charged moieties (V5+, Zn2+, and other cations of
transition metals) [20].
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+H - +H+
H 2O
* -
* * *
H +
H
N + N N - N
H -
- H
-
O O O O HO O - O
H+ H + H+ H+ H+ H+ H + H
+
Pd Pd Pd Pd
+H - +H + + H+ + H-
H2 O
* -
* *
NH NH N H H N H N
- -
O- OH O H O H+ H H
H + H + H
Pd Pd Pd Pd Pd
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Figure 4. Possibilities for intensification of mass transport in the G-L-S reaction systems.
Types of reactors from the left: mechanically stirred; Buss; column reactor with the recycling
of G-phase; gas lift.
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From the point of minimizing consumption of the catalysts and energy utilization (see
Table 2) the DuPont technology represents a very significant contribution to solution of this
problem
http://dupont.t2h.yet2.com/t2h/page/techpak?id=27458&sid=330&abc=0&page=details). This
technology uses plug-flow hydrogenation reactor that contains a DuPont proprietary
supported noble metal catalyst. NB is fed together with hydrogen into this reactor and reaction
takes place in the liquid phase. The catalyst has a high selectivity and the NB conversion is
virtually 100% per pass. Excess hydrogen from the reactor effluent is vented and the reactor
product is sent to a dehydration column to remove the reaction water followed by a
purification column to produce high quality aniline product.
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aqueous ammonia over platinum-loaded titanium oxide photocatalyst [29]. However, potential
industrial applications are limited due to higher costs in comparison with the classical NB
route.
Conclusions
Probably no organic compound attracts so much interest like aniline. From its
identification in 1826, a vast research and technological applications to produce aniline were
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described. Economic, ecological and safety reasons favour the production of aniline via
nitration of benzene and its hydrogenation. Besides catalytic chemistry, mass and heat
transport and a proper reactor design are governing the success of the processes.
Based on theoretical background, laboratory research and especially results from the
continuous model hydrogenation unit allowed VUCHT to classify proper hydrogenation
catalysts and design the reactor system. High selectivity to aniline minimizes complexity of
the separation system.
A direct amination of benzene still remains as a challenge for the development of future
aniline technologies.
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Acknowledgement:
This publication is one of results of the project implementation: Hydrogenation in the liquid
phase, ITMS: 26220220144, supported by the Research & Development Operational
Programme funded by the ERDF.
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