The influence of zeolite pore topology on the separation of carbon dioxide from methane

The influence of framework topology on the separation of carbon dioxide from methane has been studied in a set of small pore pure silica zeolites with CHA, IHW, ITW, LTA, MTF and RWR structures. To isolate this effect, the composition of the materials has been kept constant by using pure silica materials, which have neither aluminium in their framework nor extraframework cations that can alter the adsorption behaviour. Pure component adsorption isotherms and breakthrough experiments with synthetic mixtures resembling biogas and natural gas have been conducted. Materials Si-RWR and Si-ITW, which present channel-like topology, show higher IAST selectivities towards CO2 due to a kinetic exclusion of methane, which in the case of Si-RWR is pore-size based and in the case of Si-ITW, topology based. High heat of adsorption of CO2 on Si-ITW is explained through a maximisation of the dispersive interactions between the tight channels and the adsorbate. The extremely small pores of Si-RWR lead to a reduced diffusivity not only of CH4 but also of CO2 precluding its use as adsorbent for this separation. Si-ITW arises as a promising material in the separation of CO2 from CH4, having high selectivity towards CO2 while keeping high diffusion rates and a moderate maximum capacity. A comparison of the performance of these materials with that of aluminosilicate zeolites LTA with Si/Al ratio of 5 and 13X shows that Si-ITW can be more convenient by lowering the energetic requirements of the regeneration step. Moreover, the hydrophobicity of this material can be a further advantage over traditional zeolites.