In this article, an assessment of the impact of alkali-metal-ion impregnation on metal-organic frameworks (MOF) is presented employing CH4 and CO2 adsorption isotherm data. At first, the parent MOF, MIL-101(Cr), is prepared by a fluorine-free hydrothermal reaction procedure and impregnated with Li, Na, and K alkali cations. These synthesised MOFs are characterized by N2 adsorption/desorption isotherm analysis, X-ray diffraction (XRD) measurement and scanning electron microscopy (SEM). The amount of CH4 and CO2 adsorption uptakes onto parent and alkali ions impregnated MIL-101(Cr) are conducted for wide ranges of pressures and temperatures. For understanding the effects of MOF synthesis process and alkali cations impregnation, CH4 /CO2 uptakes on perfect crystalline MIL-101(Cr) MOF are also calculated by Grand Canonical Monte Carlo (GCMC) simulation and the results are compared with experimental isotherm data of synthesised parent and alkali ions impregnated MIL-101(Cr) MOFs. It is found that the limiting uptakes and the isosteric heats are mainly influenced by the modified adsorbent structures due to alkali ions impregnation and the polarity of adsorbate molecules. Employing Dubinin-Astakhov (DA) equation, the energy distribution of synthesised parent and alkali doped MIL-101 (Cr) MOFs are also presented to identify the alkali cation effects and the surface heterogeneity.