Tailoring New Adsorbents Based on π-Complexation: Cation and Substrate Effects on Selective Acetylene Adsorption

Sorbents were prepared by dispersing various metal cations, M 2+ (Fe 2+, Co 2+, Ni 2+ ), by thermal monolayer dispersion of MCl 2 on various high surface area substrates (γ-Al 2 O 3, SiO 2, MCM-41 mesoporous zeolite). All sorbents showed reversible and highly selective adsorption for acetylene (C 2 H 2 ) over other hydrocarbons. This was due to the formation of π-complexation bonds between the M 2+ cations and C 2 H 2 . The following trend was observed for all substrates studied: Fe 2+ > Co 2+ > Ni 2+ . Substrates played an important role in acetylene adsorption, with SiO 2 -based sorbents outperforming those based on Al 2 O 3 . For example, the ratio of C 2 H 2 /M 2+ for FeCl 2 /SiO 2 was 1.35, while for FeCl 2 /Al 2 O 3 the ratio was 0.9. This result is due to the fact that SiO 2 has less surface oxygen vacancies, and consequently there are more four-coordinated M 2+ ions (which can bond C 2 H 2 ) and less five- and six-coordinated M 2+ (which cannot bond C 2 H 2 for stereochemical reasons) on SiO 2 . This work demonstrates the effects of cations and substrates on selective acetylene adsorption.