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 iii 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 iv 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.