Vapor-phase cyclodehydration of diethylene glycol (DEG) into 1,4-dioxane (DOX) were performed over several solid acid catalysts, such as Al2O3, Sio(2)-Al2O3, Beta and MFI zeolites. All the catalytic activity gradually decreased with time on stream, especially Beta and MFI zeolites with strong acidic property. The catalysts were modified with silver metal for stabilizing the catalytic activity: Al2O3 modified with 1 wt.% Ag2O showed the best catalytic performance. Characterizations such as XRD, TPD and FIR were performed for analyzing the carbonaceous compound deposited on the catalysts after reaction and investigating the effect of the addition of Ag2O onto Al2O3. Polyethylene glycol was proposed as the carbonaceous compound by FTIR analysis. The modification of Ag was found to decrease the averaged acid strength of Al2O3, which is supposed to be effective for inhibiting carbon deposition and stabilizing the catalytic activity. It was also found that H-2, as a carrier gas, was indispensable for stabilizing the conversion of DEG into DOX. Silver metal would work as a remover of the product on the catalyst surface together with H-2 to prevent carbon deposition. Over Al2O3 modified with 1 wt.% Ag2O, a stable DOX selectivity of 90% with a complete conversion was achieved at 250 degrees C and a contact time of 156.8 g h mol(-1). (C) 2015 Elsevier B.V. All rights reserved.