The development of supported gold catalysts for selective hydrogenation applications
Abstract
An alternative cleaner route for the production of aromatic amino-compounds
under mild reaction conditions (P = 1 atm; 393 K ≤ T ≤ 573 K) via the continuous gas
phase reduction of aromatic nitro derivates has been investigated over (oxide and/or
carbon) supported Au, Ag, Pd, Ni, Ni-Pd, Au-Pd and Au-Ni catalysts. Taking the
hydrogenation of p-chloronitrobenzene as a model reaction, Pd/Al2O3 promoted the
exclusive production of nitrobenzene and aniline, i.e. hydrodechlorination with
subsequent -NO2 group reduction prevailed. In contrast, p-chloroaniline was the only
product detected over a series of supported Ni catalysts. This is the first time that such
product exclusivity has been achieved in gas phase operation. The synthesis of
bimetallic Pd-Ni/Al2O3 (prepared via co-impregnation) proved effective to enhance
catalytic activity while maintaining 100% selective -NO2 reduction, a result ascribed to
bimetallic particle formation as established by TPR, H2 chemisorption and XRD
analyses. Nevertheless, the three systems (supported Pd, Ni and Pd-Ni) suffered a loss
of activity with time-on-stream. Monometallic Au catalysts promoted the exclusive and
time invariant formation of p-chloroaniline. The incorporation of Au (as a modifier)
with Pd via reductive deposition to form Au-Pd/Al2O3 (Pd/Au=10 mol/mol) did not
influence catalytic performance, which was equivalent to that delivered by Pd/Al2O3,
i.e. aniline was the predominant product. On the other hand, the inclusion of Pd (as a
promoter) with Au (at Au/Pd≥20) via co-impregnation and/or co-deposition
precipitation resulted in increased hydrogenation rate while retaining exclusivity to pchloroaniline,
an effect resulting from a surface Pd-Au synergism demonstrated by
DRIFTS analysis. With the goal of elevating the catalytic activity of Au, the possible
role of the oxide (Al2O3 vs. TiO2) support to modify catalytic response was considered.
Au/TiO2 delivered a higher specific rate that was attributed to a combination of smaller
Au particle size (with higher number of defects) and possible p-chloronitrobenzene
activation via interaction(s) with TiO2 surface oxygen vacancies. This work was
extended to decouple the individual contribution of each factor by (i) considering a
series of oxide supports that exhibited a range of acid-base and redox surface properties,
i.e. Al2O3, TiO2, Fe2O3 and CeO2 and (ii) controlling the Au particle size using two
synthesis methods (deposition-precipitation and impregnation). The results
demonstrated that specific activity increased with decreasing particle size (from 9 to 3
nm), regardless of the nature of the support. Furthermore, in the case of Au/Fe2O3, XRD
and TPR analyses have established that Au can promote the partial reduction of the
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support (from α-Fe2O3 to Fe3O4), an effect more pronounced for smaller Au particles (<
5 nm) where H2-TPD suggests the participation of spillover hydrogen in this reduction
step. A similar effect was also found for the TiO2 allotropic phase transition (from
anatase to rutile), which can occur at lower temperatures due to the presence of Au, as
demonstrated by DRS UV-vis, XRD and BET measurements.
Having established that supported Au is effective in promoting the exclusive
reduction of p-chloronitrobenzene to p-chloroaniline, hydrogenation selectivity was
proved further by considering the reduction of m-dinitrobenzene. The reaction products,
i.e. m-nitroanline (partial -NO2 reduction) and m-phenylenediamine (complete -NO2
reduction) are both high value intermediates in the fine chemical industry but existing
routes can not achieve high selectivity to either product. It is shown that the nature of
the oxide support (for TiO2-rutile, TiO2-anatase, Al2O3, CeO2, Fe2O3) does not have a
direct effect on the rate of nitro-group reduction, which is controlled by Au particle size
where a mean size of 5 nm was found to be critical in that with larger particles, nitrogroup
reduction rate was structure insensitive. In contrast, the nature of the support has
a direct effect on the selectivity response. Au/TiO2 and Au/Fe2O3 promoted the
exclusive hydrogenation of m-dinitrobenzene to m-nitroaniline, Au/CeO2 delivered mphenylenediamine
as the sole product and Au/Al2O3 generated a mixture of both
products. This response can be accounted for on the basis of a modification to the
electronic character of the Au nanoclusters induced by the acid-base Lewis properties of
the support that impacts on the adsorption/activation of m-dinitrobenzene. A similar
alteration of the electronic nature of Au can also be induced by alteration of the Au
particle size or the introduction of a second metal (Ni). Taking the same reaction over
Au/Al2O3, Ni/Al2O3 and Au-Ni/Al2O3, it is established that it is possible to control
product composition in terms of partial (over Au) or complete nitro-group reduction
(over Ni) or a combination of both (over Au-Ni), which can be attributed to a surface
Au-Ni synergism as suggested by XPS and EDX mapping measurements. In order to
assess the impact of annealing treatment on catalytic response, the analysis was
extended to the preparation and application of an Au-Ni/Al2O3 alloy, formation of
which is demonstrated by XRD, DRS UV-Vis and HRTEM. While alumina supported
bimetallic and alloy both promoted the formation of both m-nitroaniline and mphenylenediamine
via a predominantly stepwise reduction mechanism, the annealed
(alloy) system delivered rate constants up to two orders of magnitude lower. In the final
section of this thesis, the catalytic behaviour of supported (TiO2) Au and Ag is
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compared in the selective hydrogenation of a series of para-substituted nitroarenes.
While both catalysts promoted the exclusive nitro-group reduction, Ag/TiO2 delivered
lower reaction rates. The reaction mechanism has been probed by adopting the Hammett
approach, where the linear correlation and positive slopes (higher for Au than Ag)
associated with the Hammett plots are consistent with an electron withdrawing
substituent activation effect, i.e. nucleophilic attack and a more effective reactant
activation over Au/TiO2.
The results presented in this thesis demonstrate, for the first time, that catalytic
hydrogenation over gold-based catalysts in continuous flow gas operation is a viable,
clean high throughput route to aromatic amines. The findings of this thesis show that Au
on reducible supports with dAu < 5 nm is the optimum monometallic formulation while
-NO2 group reduction rate and/or selectivity can be controlled (or tuned) by (i) changing
the acid-base Lewis character of the support, (ii) modifying Au dispersion or (iii)
incorporation of Pd or Ni as promotors. This work represents a critical advancement in
the sustainable production of high value fine chemicals.