Nickel catalyst for selective hydrogenation of pꢀDNB Russ.Chem.Bull., Int.Ed., Vol. 66, No. 1, January, 2017
35
Determination of catalytic activity. pꢀDinitrobenzene
(pꢀDNB) (98%, Acros Organics) was used as the initial substrate.
The reaction was carried out in a 100ꢀcm3 autoclave at a hydroꢀ
gen pressure of 0.5—2 MPa and a reaction temperature of
60—170 °C. In all experiments, the amounts of the initial pꢀDNB
and the catalyst were 0.4 and 0.2 g, respectively. Tetrahydroꢀ
furan (30 mL) was used as the solvent, as it was shown in prelimꢀ
inary experiments that hydrogenation to diaminobenzene takes
place in THF in the presence of nickelꢀcontaining reference
catalysts and the 5%Ni/Pꢀ25 catalyst. When ethanol was used as
the solvent, no products of nitro group hydrogenation were
formed even upon complete conversion of DNB, while in DMF,
the products of DMF reactions with the substrate and nitroꢀ
aniline were formed. The commercial Ni/kieselguhr catalyst (TU
38.101396ꢀ89Е) and nickel chromium catalyst (OST 113ꢀ03ꢀ
4001ꢀ90) were tested as the reference catalysts.
During the reaction, samples of the reaction mixture were
taken at intervals through a special highꢀpressure sampling valve.
The concentrations of the reactants and products in the samples
were determined on a CrystaLuxꢀ4000ꢀM chromatograph with
a 30 m×0.25 mm Supelco capillary column with the OVꢀ1 staꢀ
tionary phase and a flame ionization detector. Nitrogen was
used as the carrier gas. The evaporator and detector temperaꢀ
tures were 240 °C. Analysis was carried out in the temperature
programmed mode: the column was initially heated to 150 °C,
held at this temperature for 6 min, heated from 150 °C to 240 °C
at 20 °C min–1, and held at 240 °C for 10 min.
Eicosane (C20H42) was used as the internal standard for quanꢀ
titative calculations of pꢀDNB conversion and selectivity; the
retention time of eicosane is close to the retention times of the
reaction products, but the peaks are well resolved. Prior to the
experiment, an exact portion of the internal standard (100 mg
with a 0.1 mg accuracy) was added to the reaction mixture and
the sample was analyzed before the reaction. The analyte to
standard peak area ratio was used for determination of the conꢀ
centrations of compounds detected in the reaction mixture.
The results of chemical analysis of the reaction mixture were
used to calculate the DNB and internal standard peak area ratio,
and the change in this ratio was used to calculate the selectivities
to nitroaniline and diaminobenzene.
In order to search for a more active catalyst operating
under high loads and to select the optimal support, we
prepared catalysts with 5% nickel content on various supꢀ
ports and tested them in the hydrogenation of DNB with
variation of temperature and hydrogen pressure. All cataꢀ
lysts were preꢀreduced at 350 °C in a hydrogen flow and
then passivated in argon containing an oxygen admixture.
Table 1 and Figures 1—3 summarize the results of experiꢀ
ments on pꢀDNB transformation catalyzed by Ni/kieselꢀ
guhr and by catalysts containing 5% nickel on various
supports. It can be seen from these data that in the presꢀ
ence of a catalyst with a high nickel content, Ni/kieselꢀ
guhr, complete conversion of pꢀDNB occurs over a short
period of time, the selectivity to PDA being 54%; the reacꢀ
tion involves the intermediate formation of nitroaniline,
which is then fully hydrogenated to PDA (see Fig. 1).
Similarly, hydrogenation via nitroaniline was also observed
on the NTꢀ27ꢀ390H catalyst (see Fig. 2). However, in the
case of catalysts synthesized on silica gel, no nitroaniline
was detected. The absence of nitroaniline in the reaction
products can be attributed to the fact that hydrogenation
of the second nitro group is much faster than hydrogenaꢀ
tion of the first one. A different hydrogenation mechanism
without the intermediate formation of nitroaniline cannot
be ruled out either.
The nature of the support has a substantial effect on the
catalytic properties of the nickel catalysts. Aluminaꢀ and
carbonꢀsupported catalysts were shown to provide a nearly
complete conversion of pꢀDNB only at 160—170 °C in
4—5 h; however, diaminobenzene was not formed in these
cases and the selectivity to nitroaniline was relatively low
(see Table 1). The catalyst supported on the TiO2 Aerolyst
was even less active than other catalysts. However, cataꢀ
lysts containing TiO2 Aeroxide Pꢀ25 as the support proꢀ
vided complete conversion of pꢀDNB to nitroaniline and
to PDA via successive hydrogenation; furthermore, this
Results and Discussion
C (%)
100
The pꢀDNB hydrogenation conditions were selected
in the preliminary experiments carried out at a temperaꢀ
ture of 60 °C and a hydrogen pressure of 1.3 MPa using the
commercial Ni/kieselguhr catalyst. These experiments
showed that the pꢀDNB conversion to pꢀphenylenediꢀ
amine (PDA) was relatively slow under these conditions.
Indeed, the pꢀDNB conversion within 60 min was <10%
with a selectivity of 50%, while complete hydrogenation
of the nitro groups took place in 150 min. When the reacꢀ
tion temperature was increased to 85—100 °C, the time
required for complete reduction of the nitro groups deꢀ
creased. It was found that under relatively mild conditions
(85 °C, 1.3 MPa), complete conversion can also be attained
in 60 min (Fig. 1). However, this requires a low DNB : Ni
ratio (1.3), which is inapplicable for industrial processes.
1
3
2
80
60
40
20
10
20
30
40
50
60 τ/min
Fig. 1. Effect of the time (τ) of pꢀdinitrobenzene hydrogenation
catalyzed by Ni/kieselguhr on the composition of the reaction
mixture: pꢀdinitrobenzene (1), pꢀnitroaniline (2), pꢀphenyleneꢀ
diamine (3); 85 °C, hydrogen pressure 1.3 MPa.