Hydrate-based CO2 capture and/or H-2 purification from integrated gasification combined cycle (IGCC) syngas has been a more and more attractive technology in both environmental and clean energy fields. This work focused on both microcosmic and macroscopic studies for the CO2/H-2 hydrate formation process with synergic additives comprised of tetra-n-butylammonium bromide (TBAB) and nano-Al2O3. The experiments were carried out with 0.5 wt % nano-Al2O3 and 1, 5, 10, and 11 wt % TBAB. The microcosmic study shows that, with the synergic additives, the CO2/H-2 mixture hydrate formed mainly on the nano-Al2O3 surface and formed a semiclathrate structure. Additionally, the macroscopic study shows that the synergic additives could remarkably promote the gas uptake and separation efficiency. Noteworthy, in comparison to unitary TBAB, tetrahydrofuran, and cyclopentane, the synergic additives could increase the gas uptake for the CO2/H-2 hydrate formation process by approximately 43.62, 230.56, and 173.27%, respectively. The experimental results indicate that the synergic effect of TBAB and nano-Al2O3 is helpful for hydrate-based CO2 capture from IGCC syngas.