Selective hydrogenation of nitrostyrene to aminostyrene over Pt/TiO₂ catalysts: Effects of pressurized carbon dioxide and catalyst preparation conditions

Abstract

The present work has studied the influence of CO₂ pressurization on the hydrogenation of nitrostyrene (NS) using Pt/TiO₂. With CO₂ pressurization up to 12 MPa (CO₂-dissolved expanded liquid (CXL) phase), the over-all reaction rate and the selectivity to aminostyrene (AS) increased. At higher CO₂ pressures where all NS dissolved in scCO₂ (scCO₂-homogeneous phase), both of them decreased with increasing the CO₂ pressure. The phase behavior was an important factor for the present reaction system. It was also found that the presence of pressurized CO₂ gave higher selectivity to AS than in its absence at any conversion level. This was ascribed to retardation effects of dense phase CO₂ on the hydrogenation of AS to ethylaniline. Competitive adsorption of nitro and vinyl groups was suggested to determine the product selectivity. FTIR measurements showed that the pressurized CO₂ lowers the reactivity of the nitro group, and this effect is stronger in scCO₂-homogeneous phase than in CXL phase, resulting in the lower AS selectivity in the former phase. Lower Pt loadings and higher catalyst reduction temperatures gave Pt/TiO₂ catalysts more selective for the AS formation. FTIR measurements of adsorbed CO over the catalysts suggested that the presence of less-coordinated Pt atoms on edge, corner and kink sites was significant for the selective hydrogenation of NS to AS.

Introduction

Supercritical carbon dioxide (scCO₂) is an attractive alternative to conventional organic solvents, due to its environmentally benign, non-toxic, and non-flammable nature, complete miscibility with gases, adjustable dissolving power, and easy separation from liquid/solid products after reactions [1], [2], [3], [4]. In addition, scCO₂ can have several advantages, which are the absence of gas–liquid mass transfer limitations, relatively high rates of molecular diffusion and heat transfer, and the possibility of molecular interactions with the dissolved reacting species (substrates or catalysts). These can result in interesting effects of enhancing reaction rates and modifying product selectivity.

When a large quantity of substrates or solvents is used for reactions under pressurized CO₂, dissolution of CO₂ into the liquid phase causes an increase in the volume of the liquid phase. Such phases are called CO₂-expanded liquids (CXLs). The extent of expansion depends on the nature of the liquid used as well as CO₂ pressure and temperature. In recent years, several reviews demonstrated that CXLs are also promising reaction media [5], [6], [7], [8]. Promotional features of CO₂ can also appear under CXL conditions. Dissolved CO₂ will facilitate the dissolution of other coexisting gases such as O₂, CO, or H₂, and hence may accelerate the reactions involving these gaseous reactants although CO₂ is not a reactant but rather a diluent. It can also interact with substrates and/or catalysts, resulting in the modification of the reaction selectivity. Co-exiting organic solvents in CXLs easily dissolve substrates and catalysts whose dissolution into scCO₂ is difficult. This would be another advantage of CXL compared to scCO₂.

The catalytic hydrogenation of nitrobenzenes is commonly used to manufacture anilines, which are important intermediates for polyurethanes, dyes, pharmaceuticals, explosives, and agricultural products. Industrially, the reaction is operated in gas phase with Cu or Ni catalysts using H₂ near or slightly above atmospheric pressure (0.1–0.5 MPa) at high temperatures around 523 K [9], [10], [11], [12], [13]. It can also be performed in the liquid phase by using supported metal catalysts (Pt, Pd, and Ni) and organic solvents with H₂ of higher pressures (1–4 MPa) at temperatures around 323 K [14], [15], [16], [17], [18], [19], [20]. However, these catalysts sometimes show low selectivity to anilines because of the formation of several poisonous reaction intermediates such as nitrosobenzenes, phenylhydroxyamines, azoxybenzenes, and azobenzenes whose formation and accumulation should be avoided for the green production of anilines. The present authors reported that, when the hydrogenation of nitrobenzene to aniline was carried out in/under pressurized CO₂, the selectivity to aniline was much higher than that obtained in conventional organic solvents [21], [22], [23], [24], [25]. The higher selectivity is ascribed to interactions of CO₂ with the substrate and the intermediates. When nitrobenzenes have another reducible group, their chemoselective hydrogenation to corresponding substituted anilines is another important issue. Corma et al. [26], [27] reported the selective hydrogenation of nitrostyrene to aminostyrene using TiO₂ supported metal catalysts in toluene. For these catalysts, high-temperature reduction was required to obtain the high selectivity to aminostyrene [26]. This would sometimes cause lower hydrogenation activity because of strong metal-support interaction (SMSI).

The present authors studied the effects of pressurized CO₂ on the selective hydrogenation of α,β-unsaturated aldehydes (cinnamaldehyde and citral) to unsaturated alcohols [28], [29], [30], and the interactions of pressurized CO₂ with aldehyde compounds were also studied by in situ FTIR spectroscopy and computational calculations [28], [29], [31], [32]. Pressurized CO₂ enhanced the selectivites to the unsaturated alcohols, and this was ascribed to the interactions of CO₂ with the carbonyl CO bond of the aldehydes; however, the interaction between CO₂ and the CC bond was absent. On taking account of these results, in the case of nitrostyrene hydrogenation, pressurized CO₂ may interact with the vinyl and nitro groups of nitrostyrene in different modes, and hence may improve the overall reaction rate and/or the selectivity to aminostyrene. To our knowledge, no one has so far studied the influence of pressurized CO₂ on the title reaction. In the present work, this reaction was carried out with Pt/TiO₂ in the presence of pressurized CO₂. It has been shown that pressurized CO₂ improves the overall reaction rate and the selectivity to aminostyrene under CXL conditions. The influence of a few catalyst preparation conditions (Pt loading and the catalyst reduction temperature) on the reaction has also been examined.