The reduction of CO2 emissions is a serious challenge to mitigate the effects of climate change. As the atmospheric CO2 concentration has exceeded 400 ppm, not only CO2 capture from point sources e.g. steel plants but also from the atmosphere has become crucial. Direct air capture (DAC) has emerged as a promising technology to potentially reduce the atmospheric CO2 concentration. DAC has been performed using various porous materials such as porous silica, cellulose, and metal-organic frameworks (MOFs). Hybrid ultramicroporous materials (HUMs) have garnered attention as potential materials for DAC applications due to their ultramicroporous framework structure which are composed of metal-organic units and inorganic pillars. The ultramicroporous channels (< 10 Å) in these HUMs offers an appropriate environment for interaction with CO2 molecules. In this study, we investigated the effect of pore size tuning on the CO2 sorption properties of an isoreticular series of NbOFFIVE-1-Ni (aka KAUST-7). Through synthesis optimization, we have successfully synthesized and solved the structure of a number of novel variants of KAUST-7. The novel isoreticular KAUST-7 HUMs contained different early transition metal pillars (M’OFFIVE=M’OF52−=Nb3+, Ta3+, V3+) and organic ligands (1=pyrazine and 2=aminopyrazine). The idea is to adjust the metal-oxygen bond lengths and in turn tune the pore size of these HUMs for improved CO2 capture performance. Experimentally, the CO2 uptake capacities of TaOFFIVE-1-Ni, VOFFIVE-1-Ni, NbOFFIVE-2-Ni, and TaOFFIVE-2-Ni was found to be 1.18, 0.67, 0.02, and 0.01 mmol g−1, respectively, at 40 Pa CO2 (400 ppm equivalent at 298 K). The adsorption capacity of TaOFFIVE-1-Ni was comparable to NbOFFIVE-1-Ni at 40 Pa (1.45 mmol g−1). The effect of pore size on the CO2 uptake properties is clear from our experimental studies. IR spectroscopy also showed that the CO2 asymmetric stretching ν3 band of the adsorbed CO2 differed slightly on the different variants of KAUST-7. In the proposed study, we will use DFT calculations to investigate the mechanism of CO2 sorption on KAUST-7s. In particular, we are interested in the orientation of the CO2 molecules adsorbed in the pores of the different types of KAUST-7. The differences in the frequency of the ν3 band suggest that CO2 adsorbs slightly differently in the variants of KAUST-7. We are also interested in using DFT calculations to obtain the CO2 binding energy (heat of adsorption), which is extremely difficult to measure on these materials experimentally due to the very steep adsorption isotherms at very low CO2 partial pressure (this is also the reason why KAUST-7 is a good materials for DAC of CO2). The results from the proposed study will provide highly valuable information on the CO2 sorption properties of KAUST-7, in particular the new variants synthesized by us. The results will also be essential for further optimization of KAUST-7 or HUMs for DAC of CO2.