(732d) Amination of 1-Hexanol over Au-Pd/TiO2 Catalysts Prepared By Controlled Surface Reactions

AuPd bimetallic catalysts have been studied and applied for use in a range of reactions, including direct synthesis of hydrogen peroxide^1–3, vinyl acetate synthesis^4–6, CO oxidation^7,8, and ethylene and cyclohexene hydrogenation⁹, among others. The promotional effect over monometallic catalysts can be observed in enhanced reactivity, selectivity, and/or stability⁶. This enhancement can be attributed to electronic and/or geometric effects, depending on the chemistry involved. This work examines the promotional effect of AuPd/TiO₂ catalysts for amination of 1-hexanol using ammonia.

Amination is used in the production of a range of products, including pharmaceuticals, solvents, dyes, and polymers¹⁰. Typically, industrial alcohol amination with ammonia is carried out over Co, Ni, Cu, and Zr based catalysts; however, achieving high selectivities is challenging, as mixtures of primary, secondary, and tertiary amines are generally formed^11,12. Most fundamental studies on alcohol amination have been carried out on homogeneous catalyst systems using Rh, Ru, and Ir^10,12,13, with a few studies on heterogeneous RhIr catalysts^14,15 and PtSn/γ-Al₂O₃ catalysts^16,17. AuPd catalysts have been used for several related reactions, including amination of phenols and synthesis of N-benzylideneaniline and N-benzylaniline¹⁸, and amination of phenols¹⁹. Thus, we have synthesized AuPd catalysts with Pd:Au ratios between 0.06 and 0.67 using controlled surface reactions to create well defined, uniform structures to develop structure-activity and structure-selectivity relationships for hexanol amination.

A Au/TiO₂ parent catalyst was first prepared by deposition precipitation, and Pd was then deposited by controlled surface reactions, as has been previously employed for other metal systems^20–23. UV vis spectroscopy was used to determine the extent of interaction between the Pd precursor (cyclopentadienyl Pd allyl) and the Au nanoparticles and the TiO₂ support. It was determined that Cp(Pd)allyl does not interact with the TiO₂ support at low concentrations, whereas Cp(Pd)allyl was fully taken up by the Au/TiO₂ parent catalyst during synthesis, indicating selective deposition of Pd onto the Au particles. Furthermore, qualitative energy dispersive spectroscopy confirmed that monometallic Pd particles were not formed.

The catalyst surface was probed using CO chemisorption and FTIR spectroscopy of adsorbed CO. When compared to a monometallic Pd catalyst, the bimetallic AuPd catalysts have a lower dispersion which is attributed to the presence of subsurface Pd. Additionally, the surface Pd species are well isolated from one another at Pd:Au ratios between 0.06 and 0.23, as indicated by the presence of CO adsorbed in a linear configuration on Pd. As the Pd:Au ratio is increased to 0.67, some Pd ensembles of 2 or more atoms begin to form; however, isolated species still dominate. Additionally, a red-shift in the linear CO peak of approximately 15 cm^-1 is observed on the bimetallic catalysts compared to Pd/TiO₂, suggesting an electronic interaction between Au and Pd^7,24,25. X-ray absorption spectroscopy results indicate that the Pd-Au coordination number is between 8 and 11, while the Pd-Pd coordination number is less than 1.6. These values corroborates the tendency for Pd to be well diluted in the Au nanoparticles, as determined by FTIR.

The bimetallic AuPd/TiO₂ catalysts were studied for amination of 1-hexanol at 503 K and 1 atm. The rate of hexanol conversion was observed to increase from 8.7 µmol ks^-1 µmol_Pd^-1 over the Pd/TiO₂ catalyst to between 21 and 42 µmol ks^-1 µmol_Pd^-1 over the bimetallic catalysts with Pd:Au ratios between 0.67 and 0.06. A rate enhancement is also observed when the rate is normalized by total metal content.The increased activity for bimetallic catalysts based on both Au and Pd suggests that the active site for hexanol amination is a bimetallic site where Pd and Au are in close interaction with one another. Turnover frequencies were calculated using Pd site density as determined by CO chemisorption. The AuPd_0.06/TiO₂ catalyst exhibits a turnover frequency two orders of magnitude higher than that of Au/TiO₂ (Au sites were estimated using STEM particle size) and one order of magnitude higher than that of Pd/TiO₂. As the Pd loading of the AuPd catalysts decreases, an increase in the selectivity to the higher substituted products (N-hexylidene hexylamine, dihexylamine, and trihexylamine) is observed.

The rate enhancement and shift in selectivity observed for hexanol amination using ammonia over AuPd/TiO2 bimetallic catalysts is attributed primarily to an electronic interaction between Au and Pd. By synthesizing well defined, AuPd bimetallic structures using controlled surface reactions, the relationships between catalyst structure and selectivity and activity have been studied.

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