A series of bifunctional catalysts CuO/ZnO/ZrO2/HZSM-5 with different ZrO2 contents were prepared and characterized by surface area, XRD and XPS analysis. The catalytic system was evaluated in the development of STD process (synthesis gas-to-dimethyl ether). Based on these results, it was deduced that the incorporation of ZrO2 in CuO lattice caused the formation of Cu+ on the catalyst surface during reduction, which seems to be crucial for the high selectivity of dimethyl ether (DME) and CO conversion in the present study. Also, the promoting effect of ZrO2 on the surface structure, such as Cu/Zn ratio and Cu concentration, was closely related to the DME synthesis activity. In addition, two kinds of active components in the bifunctional catalysts are finely dispersed, maintain close contact, and consequently exhibit a high degree of 'synergistic effect' for the effective conversion of CO to DME. In summary, Cu+/Cu-0/Zn(2-delta)+/Zn2+/ZrO2/HZSM-5 is considered to be the surface active state for the direct synthesis of DME. (C) 2003 Elsevier B.V. All rights reserved.