Characterization of CuCl2/PdCl2/activated carbon catalysts for the synthesis of diethyl carbonate

X-ray diffraction (XRD), low-temperature electron spin resonance (ESR) spectroscopy, and magnetometry are employed to determine the electronic and structural properties of over a dozen samples of fresh and KOH-treated CuCl2/PdCl2/activated carbon (AC) catalysts used in the synthesis of diethyl carbonate (DEC) from ethanol and CO. The percentage yields of DEC from the preceding paper by Dunn et al. are compared with the results from XRD and ESR to determine the nature of the active species. Temperature variation (4 to 300 K) of the ESR spectra of CuCl2/AC for different loadings of Cu reveals two Cu2+ species: Cu2+-carbon and nanoclusters of CuCl2 precipitated during impregnation. An excellent correlation is observed between the ESR intensity of Cu2+-carbon species and weight percent of DEC produced, although the maximum yield for CuCl2/AC is only about 4% for Cu loading of 9% (wt). For the CuCl2/PdCl2/AC catalysts, the yield increases to 10%, signifying the important role of Pd2+. For the CuCl2/PdCl2/AC catalysts treated with KOH for different ratios of OH/Cu = x between 0 and 5, wt % of DEC produced increases to 18% for x = 1, but drops to negligible amounts for x = 3 and 5. In XRD studies, the intensity of the paratacamite peaks correlates well with the % yield of DEC for all values of x, showing paratacamite as the active species in the KOH-treated catalysts. For x = 3 and 5, paratacamite converts to calumetite, corresponding to the drastic drop in the yield for DEC conversion.