(29a) Selective Liquid Phase Hydrogenation of Chloronitrobenzene over Aupd Nanoclusters on TiO2 Catalysts

Application of heterogeneous catalytic reactions in the production of fine chemicals is of increasing interest. Such reactions are often carried out in batch and semi-batch reactors. In the case of three-phase heterogeneous catalytic reactions, the rate of process and its selectivity can be determined either by intrinsic reaction kinetics or by external diffusion as well as by internal diffusion through the catalyst pores. Careful analysis of mass transfer is important for the elucidation of intrinsic catalytic properties, for the design of catalysts, and for the scale-up of processes. Nevertheless, in scale-up the operating conditions are selected in a way that the process is shifted from the kinetic region to a diffusion-controlled one. That is why it is important to know how the shift will affect not only the conversion but also the selectivity.

Palladium catalyst has been well-known to have high activity for liquid phase hydrogenation reaction. However, it is too active to have high selective for intermediate compounds. How to increase selectivity of intermediate without losing overall activity is the key issue in catalyst design. Using bimetallic catalyst is an option to achieve this goal.

In this study, bimetallic AuPd was loaded on TiO₂ support. TiO₂ support was chosen because its isoelectric point is ~7, therefore it is easy to deposit Au and Pd on it by using deposition-precipitation technique with HAuCl₄ as the starting material. After preparation, the bimetallic catalyst was reduced by NaBH₄ at room temperature. Various Au/Pd atomic ratios were used. The catalysts were characterized by various techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR). Selective hydrogenation of chloronitrobenzene to chloroaniline was carried out in a Parr batch reactor. The conditions for hydrogenation reaction were 0.55 MPa as H₂ pressure, room temperature and 300 rpm. Methanol was used as the solvent, the concentration of p-CNB was 0.2 M (2.54 g p-CNB in 80 ml methanol) and the amount of metal-based catalyst was 0.5 g.

All of PdAu catalysts could reach 100% conversion of p-CNB on the same reaction conditions in short times. The reaction could take place at room temperature and low hydrogen pressure. Only very small amount of palladium could improve the reaction activity and selectivity. Adding palladium could reduce gold-valence state, increase reaction active sites. Therefore, gold and palladium catalysts could promote conversion and selectivity for hydrogenation reaction. While Pd and Au form nanoally, Au became surface-enriched. The presence of small amount of Au could suppress dechlorination significantly. Au and Pd form semispherical shape and have very strong interaction with TiO₂ support. H₂ is dissociatively adsorbed on Pd-TiO₂ interface. Since the reaction can be operated at room temperature, no extra heating energy is needed. The operation cost is decreased and it is very easy to operate. H₂ partial pressure is also low (4 atm). For hydrogenation of p-CNB, TiO₂ is a good support. TiO₂ could donate electrons to Au, made Au become electron-rich and more active. AuPd/TiO₂ showed high activity and selectivity. Adding small amount of Pd could increase activity of Au catalyst significantly. AuPd/TiO₂ catalyst had a very high activity and selectivity at room temperature. AuPd form nanoalloy particles. Particle size < 4 nm. Pd was enriched on the surface of nanoalloy. The optimum ratio Pd/Au= 1/6.