Abstract
For MOF materials to become efficiently used, an understanding of adsorption mechanisms on a microscopic level is essential. Many information may be withdrawn from the analysis of adsorption isotherm, as the isotherm shape holds information on a variety of structure-property relations, and evolution of these parameters with temperature. The majority of MOFs are microporous, with pores of the sizes of 1-3 nm. Their adsorption isotherms are of Type I according to the IUPAC classification. The characteristic steep increase in adsorption at a relatively low pressure indicates that the adsorbate-adsorbent (gas-wall) interaction is the dominating contribution to the system energy that drives the adsorption process. However, as the highly symmetric and ordered structure of MOFs’ pore walls causes heterogeneous distribution of adsorption energy [1], the adsorbate-adsorbate (gas-gas) interaction may induce a more complex adsorption mechanism even in microporous system, in particular at low temperature. The goal of the present analysis is to understand how the external temperature and the pore size/geometry of MOFs influence together the competition between the gas-framework and gas-gas interaction during adsorption and modifies the shape of adsorption isotherm. As a case example we have chosen CO2 adsorption in a series of MOFs with increasing pore size (IRMOF-1, IRMOF-8 and IRMOF-10). Monte Carlo simulations have been carried out for range of external temperatures (above CO2 boiling point). The first results showed that that: (i) at 220K, at a single pressure within the jump in the isotherm (orange point on a left graph), fluctuations between high- and low-density state of adsorbed CO2 occur (right graph), suggesting that there is an energy barrier between them. (ii) At 195K such fluctuations were not observed; it suggests that the energy barrier depends on the external temperature, (iii) We also observed that the frequency in switching between the low and high density states may be pore size-depend.