Cooperative gas adsorption without a phase transition in metal-organic frameworks

摘要

Cooperative adsorption of gases by porous frameworks permits more efficientuptake and removal than does the more usual non-cooperative (Langmuir-type)adsorption. Cooperativity, signaled by a step-like isotherm, is usuallyattributed to a phase transition of the framework. However, the class ofmetal-organic frameworks mmen-M$_2$(dobpdc) exhibit cooperative adsorption ofCO2 but show no evidence of a phase transition. Here we show how cooperativityemerges in these frameworks in the absence of a phase transition. We use acombination of quantum and statistical mechanics to show that cooperativityresults from a sharp but finite increase, with pressure, of the mean length ofchains of CO2 molecules that polymerize within the framework. Our studyprovides microscopic understanding of the emergent features of cooperativebinding, including the position, slope and height of the isotherm step, andindicates how to optimize gas storage and separation in these materials.