Using ab initio electronic structure calculations we investigated the adsorption of C4 alkane and alkene isomers in pure-silica and cation-exchanged ZSM-12 zeolites. Alkali metal cations (Li+, Na+, and K+) in the ZSM-12 zeolite framework promoted the adsorption of alkenes relative to alkanes, due to electrostatic interactions between the cations and alkene ir-electrons (cation-pi interactions). Pure-silica zeolites exhibited the opposite trends, favoring adsorption of alkanes over alkenes. Linear alkenes were generally found to exhibit stronger binding on the cation sites versus branched species due to the bulkier molecular dimensions and steric constraints imposed by the zeolite pore. Calculations at different levels of (ONIOM-embedded method) theory were in perfect agreement with respect to the overall adsorption trends in the pores impact of the zeolite pore to the adsorption of the species, especially for pore sizes close to the critical diameter of the adsorbates. Importantly, we demonstrate that the steric effects imposed by the zeolite pore, the alkali metal cation (size and electropositivity) exchange, and the degree of saturation of the adsorbates can all play a significant role in the adsorption behavior in zeolites. Our calculated adsorption energy differences between alkanes and alkenes in cation-exchanged zeolites can serve as a guide for experiments focusing on the separation of C4 hydrocarbons.