Enhanced selective nitroarene hydrogenation over Au supported on β-Mo2C and β-Mo2C/Al2O3
Graphical abstract
Au/β-Mo2C promotes hydrogenation of dinitrobenzene with exclusive production of nitroaniline.
Highlights
► First reported synthesis and characterisation of Au on β-Mo2C and β-Mo2C/Al2O3. ► First reported application of Au/Mo2C and Au/Mo2C/Al2O3 in nitroarene hydrogenation. ► Combined action of Au with Mo2C delivers higher hydrogenation TOF than Au/Al2O3. ► Exclusive m-dinitrobenzene → m-nitroaniline over Au/Mo2C; Au/Al2O3 is non-selective.
Introduction
It is known that molybdenum carbide exhibits catalytic properties that are similar to noble metals (e.g. Pd and Ru) [1]. Indeed, carbides have been shown to be promising catalysts in ammonia synthesis [2], thiophene [3] and dibenzothiophene [4] hydrodesulfurisation, Fischer–Tropsch processing [5], the water gas shift reaction [6], alcohol steam reforming [7] and n-butane hydrogenolysis [8]. In comparison with noble metal catalysts, carbides are less expensive to produce and are tolerant to sulphur poisoning [7]. Moreover, in hydrogenation reactions [9], carbides exhibit selectivities that are quite distinct from Group VIII metal catalysts but this response has yet to be fully exploited. Molybdenum carbide can adopt a face-centred cubic (fcc, α-MoC1−x), orthorhombic (α-Mo2C) and hexagonal close packed (hcp, β-Mo2C) crystal structure [9], [10]. The particular crystalline phase, which is dependent on the method of synthesis, exhibits different catalytic behaviour [11]. Molybdenum carbides can be prepared by the reaction of graphite with vapourised Mo metal or MoO3 but this method generates a low surface area material (1–2 m2 g−1) and delivers a low product yield [12]. The principal means of Mo carbide synthesis involves temperature programmed nitridation and carburisation or direct carburisation [9]. A reduction and nitridation of MoO3 (with NH3) generates fcc Mo nitride that, in a subsequent carburisation step (in e.g. CH4/H2), can be converted to fcc α-MoC1−x [5]. Alternatively, a direct carburisation of MoO3 (to 900–1000 K) yields hcp β-Mo2C [13]. Both approaches can produce a high surface area carbide (40–200 m2 g−1) at high yield where the variables that impact on surface area include heating rate [14], gas space velocity [15], carbon source (e.g. CH4, C2H6 and C3H8) [16], [17] and % v/v H2 in the feed [18]. It is also possible to deposit the Mo oxide precursor on a support (e.g. Al2O3), which with carburisation generates Mo2C/Al2O3 [3], [19]. The resultant dispersion on the carrier can result in enhanced catalytic performance relative to the bulk carbide [20], [21]. Moreover, the incorporation of a transition metal (e.g. Ni [22] or Pt [7], [23]) with Mo carbide has also been shown to influence catalytic activity [23] and selectivity [7]. In terms of hydrogenation, the focus of this study, Mo2C has been used to promote the conversion of cyclohexene [24], naphthalene [21], benzene [20] and toluene [25]. However, a search through the literature did not unearth any reported study of the gas phase hydrogenation of aromatic nitro compounds over Mo carbide.
Gold catalysts deliver lower hydrogenation activity relative to conventional transition metal (Pd, Pt and Ni) catalysts as a result of a less effective activation/dissociation of H2 [26]. In the hydrogenation of nitro compounds [27], ketones [28], aldehydes [29] and dienes [30], gold catalysts can exhibit enhanced selectivity to the target product. Activity and selectivity for hydrogenation reactions are dependent on Au particle size where significant catalytic activity requires a well dispersed Au phase (particles < 5 nm) [31]. Moreover, it has been shown that electron transfer from the support to nano-sized Au particles influences the catalytic response [32]. Of direct relevance to this study is the finding of Florez and coworkers [33] that charge transfer in Au-carbide systems is greater than that observed for oxide supports. In previous publications [32], [34], [35], [36], we have demonstrated exclusivity (with respect to –NO2 reduction) in the gas phase continuous hydrogenation of a range of polyfunctional nitroarenes over Au supported on Al2O3, TiO2, Fe2O3 and CeO2. The goal of this study is to combine the catalytic properties of Mo carbide and Au in order to achieve a synergy that elevates reaction rate while maintaining hydrogenation selectivity. para-Chloroaniline (p-CAN) and meta-nitroaniline (m-NAN) are commercially important chemicals in the manufacture of polymers, dyes and agrochemicals [37]. Conventional synthesis routes exhibit serious drawbacks in terms of low selectivity to the target amine with the generation of appreciable toxic waste [38]. We report here the first synthesis and use of Au/Mo2C and Au/Mo2C/Al2O3 to promote the hydrogenation of para-chloronitrobenzene (p-CNB) and meta-dinitrobenzene (m-DNB). We compare the catalytic response with that obtained using Au/Al2O3 as a benchmark and correlate the catalytic data with critical catalyst structural characteristics.
Section snippets
Catalyst preparation and activation
Bulk β-Mo2C was synthesised via the temperature programmed carburisation of ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O, Merck (99%)) in 80 cm3 min−1 20% v/v CH4 (99.995%, Air Liquide) in H2 (99.995%, Air Liquide). The temperature was ramped at 1 K min−1 to 973 K, maintained for 1 h and cooled to room temperature under H2 with sample passivation in 1% v/v O2/Ar (30 cm3 min−1) for 1 h. The passivation step was necessary to circumvent autothermal oxidation upon contact with air [39]. In the
Catalyst characterisation
Elemental analysis (see Table 1) indicates that the deposition–precipitation procedure was efficient in preparing Au loaded on Al2O3, Mo2C and Mo2C/Al2O3 with a gold content of ca. 1% w/w. The pH associated with the point of zero charge (pHPZC) is a critical property of the support that determines the solution pH requirements to ensure precursor-support interactions during catalyst preparation by deposition–precipitation [40], [44], [45]. The associated titration curves for the supports (Fig. 1)
Conclusions
We report the first synthesis of gold catalysts supported on molybdenum carbide (Mo2C and Mo2C/Al2O3); gold was deposited by deposition–precipitation of HAuCl4 (as precursor) with urea. The (β-phase) Mo2C support was prepared by a temperature programmed carburisation and confirmed by XRD analysis. Mo2C was characterised by a lower pHPZC (2.9) when compared with Mo2C/Al2O3 (3.7) and Al2O3 (7.5). In the synthesis of Au/Mo2C and Au/Mo2C/Al2O3, solution pH exceeded support pHPZC resulting in a less
Acknowledgments
The authors would like to thank Dr. Christophe Méthivier for performing the XPS analyses and Patricia Beaunier for the HRTEM measurements.
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