The synergic effect of Cyclopentane (CP) and Tetra-n-butyl Ammonium Bromide (TBAB) on the hydrate-based carbon dioxide (CO2) separation from IGCC (Integrated Gasification Combined Cycle) syngas is investigated by measuring the gas uptake and the power X-ray diffraction (PXRD) patterns in this work. The CF with CP/TBAB solution ratio of 5 vol% added into the 0.29 mol% TBAB solution can remarkably increase the gas uptake at 4.0 MPa and 274.65 K. The PXRD patterns of the semi-clathrate (Sc) hydrate and structure II (sII) hydrate are obtained for the CP/TBAB/gas/H2O system. The synergic effect of the CF and the TBAB includes two aspects: On one hand, the CF molecules housed in the hollow centers of the large cavities together with TBAB cations (TBA(+)) make the sc hydrate more stable. On the other hand, the TBA(+) displaced out of the large cavities by the CF molecules make the ionization reaction of TBAB in the solution going toward the reverse direction. Thus, the more TBAB molecules exist in the solution and form the more sc hydrate, resulting in the considerable increase of the gas uptake. Crown Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.