Tailoring new adsorbents based on pi-complexation: Cation and substrate effects on selective acetylene adsorption

Sorbents were prepared by dispersing various metal cations, M2+ (Fe2+, Co2+, Ni2+), by thermal monolayer dispersion of MCl2 on various high surface area substrates (gamma-Al2O3, SiO2, MCM-41 mesoporous zeolite). All sorbents showed reversible and highly selective adsorption for acetylene (C2H2) over other hydrocarbons. This was due to the formation of pi-complexation bonds between the M2+ cations and C2H2. The following trend was observed for all substrates studied: Fe2+ > Co2+ > Ni2+. Substrates played an important role in acetylene adsorption, with SiO2-based sorbents outperforming those based on Al2O3. For example, the ratio of C2H2/M2+ for FeCl2/SiO2 was 1.35, while for FeCl2/Al2O3 the ratio was 0.9. This result is due to the fact that SiO2 has less surface oxygen vacancies, and consequently there are more four-coordinated M2+ ions (which can bond C2H2) and less five-and six-coordinated M2+ (which cannot bond C2H2 for stereochemical reasons) on SiO2. This work demonstrates the effects of cations and substrates on selective acetylene adsorption.