A method of forming the highly selective ultra-small pore amorphous adsorbent includes introducing an ion-exchange material to a sodium aluminosilicate zeolite such that an ion-exchanged zeolite forms, calcinating the ion-exchanged zeolite at a calcination temperature such that the ion-exchanged zeolite collapses and forms the decationized amorphous adsorbent, and introducing a back ion-exchange material to the decationized amorphous adsorbent such that the highly selective ultra-small pore amorphous adsorbent forms. The highly selective ultra-small pore amorphous adsorbent has a pore aperture size operable to permit carbon dioxide to adsorb into the amorphous adsorbent and operable to deny methane from adsorbing into the amorphous adsorbent.