Adsorption of Benzene to Copper in CuHY Zeolite

Adsorption of benzene to copper-exchanged Y zeolites has been studied employing Fourier transform infrared spectroscopy and QM/MM calculations. Experimentally, both Cu(I) and Cu(II)-Y zeolites were used as adsorbents. Benzene adsorption was observed on both copper and Bronsted acid sites, but the copper site is preferred. The FTIR spectra show the activation of the v,3 in-plane ring stretching mode at 1471 and 1479 cm(-1) for Cu and Bronsted sites, respectively. With respect to the gas-phase value, Cu sites cause a red shift of 13 cm(-1) whereas Bronsted sites show a shift of 5 cm(-1) only. Theoretical investigations were performed on complexes of benzene with Cu-exchanged Y zeolites at two different sites, SII and SIII, with different Si/Al ratios. An eta(2)-coordination of benzene to copper was observed for all adsorption complexes. After adsorption, the 3-fold coordination of Cu to framework oxygens is reduced to 2-fold at site SII, whereas 2-fold coordination prevails at site SDI. Benzene binds with Cu more strongly at SIII than at SII. The calculated frequency shifts are slightly overestimated compared to experiment. Population analysis reveals that 3d pi back-donation from Cu dominates over p pi donation from benzene. Comparison between experimental and theoretical results allows for the prediction of the preferred benzene-copper adsorption complexes, which may be of importance in the catalytic phenol formation from benzene and dioxygen using copper-containing zeolites.