(689b) Characterization of Surface Species during Benzene Hydroxylation over a NiO on Ceria-Zirconia Catalyst

As an important intermediate in the chemical industry, phenol is currently produced in quantities of 10 million tons per year and has a market value of $1700/ton [1]. Phenol is typically coproduced with acetone in a 1:1 molar ratio via the cumene process. However, unequal global demands for phenol and acetone led to an excess of acetone on the world market in recent years, and thus motivates the development of an independent conversion pathway of benzene to phenol [2]. Catalytic processes that can use O₂ as the oxidant will be critical, as the use of N₂O or H₂O₂ as oxidants are generally prohibitively expensive. Currently, benzene hydroxylation reactions with O₂ as the oxidant suffer from strong activity-selectivity tradeoffs, as only low to moderate phenol yields have been reported and complete oxidation to CO₂ occurs at high O₂ partial pressure and temperature [3]. Although water vapor in the feed has been reported to increase the selectivity towards phenol [3], a key challenge is to design a catalyst that can selectively perform partial oxidation of benzene to phenol without over-oxidizing surface species to form CO₂.

Our previous work on selective oxidation of methane to methanol with NiO on ceria-zirconia (CZ) catalysts suggests that these and similar materials can catalyze the selective oxidation of benzene to phenol with high selectivity. In this study, transmission in-situ infrared (IR) spectroscopy is used to examine surface species formed from feeding benzene, O₂, and water onto a NiO/CZ catalyst. We will focus on the formation of phenolate species as a function of the catalyst pretreatment, temperature, and the sequence and ratio in which reactants are fed to the catalyst. Without O₂ or water present, the spectrum of surface species from dosing benzene at 180 ^oC show C-H bond activation and C-O stretching vibrations and signifies the formation of phenolate species. Subsequent introduction of water enhances these IR signals and introduces several additional peaks, indicating that a variety of different surface species are formed. Oxygen contributions from the catalyst support, O₂, and water will be discussed. These results will provide important mechanistic insight into the hydroxylation of benzene on NiO/CZ and will guide the design of catalytic processes for direct conversion of benzene to phenol.

References

1. ICIS https://www.icis.com/chemicals/channel-info-chemicals-a-z/
2. E.R. Snyder, M.L. Bols, R.A. Schoonheydt, B.F. Sels, E.I. Solomon, Chem. Rev. 118 (2018) 2718-2768.
3. T. Fukushi, W. Ueda, Y. Morikawa, Y. Morooka, T. Ikawa, Ind. Eng. Chem. Res. 28 (1989) 1587-1589.