Preparation and characterization of CuO-Al2O3 catalyst for dimethyl ether production via methanol dehydration

In the present study, a CuO-Al2O3 catalyst with CuAl2O4 spinel structure was prepared by a co-precipitation method and used for dimethyl ether (DME) production via methanol dehydration at 50 bar and different reaction temperatures (150, 250, and 350 degrees C). Upon XPS analysis of the copper and aluminum species in the fresh and used CuO-Al2O3 catalyst, CuAl2O4 was found to be the dominant species with more than 50% of total composition. Three reductive reactions and temperatures for the formation of CuH (102.3 degrees C), the interaction between Cu2+ and Al atoms (356.6 degrees C), and the reduction of CuO (520.1 degrees C) were analyzed by H-2-TPR. Furthermore, the copper oxidation state in the fresh and used catalyst was Cu(II), as determined by the XANES spectra. The fine structural parameters revealed that the coordination number of Cu changed from 2.75 to 2.44 during the catalytic reaction, and that the Cu-O bond distance increased from 1.94 to 1.98 angstrom due to strengthened Cu2+-Al interactions. On-line FTIR spectra revealed that the optimum temperature for the formations of HCOOH (by-product) and DME (product) were 150 and 250 degrees C, respectively. The catalytic reactions in the duration of DME synthesis were found that included methanol decomposition, methanol/formic acid formations, and methanol dehydration occurring at CuO, Cu, and Al2O3/CuAl2O4 active sites, respectively. The highest methanol conversion (67.3%) and DME yield (40.6%) were obtained at 250 degrees C and 50 bar, as demonstrated by the catalyst performance. In addition, optimum DME formation (equilibrium constant 1.76 x 10(-2) L mol(-1) h(-1) and activation energy 5.14 kJ mol(-1)) occurred at 250 degrees C, as determined from the linear regression of the second order model with a high R-2 value (0.98). The exothermal and non-spontaneous nature of DME formation at high temperature was evaluated through thermodynamic calculations of the reaction enthalpy, entropy, and Gibbs energy. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.