Adsorptive separation of C2/C3/C4-hydrocarbons on a flexible Cu-MOF: the influence of temperature, chain length and bonding character

Abstract

In this work, manometrically and gravimetrically measured adsorption isotherms on the flexible copper MOF material Cu₄(μ₄-O)(μ₂-OH)₂(Me₂trzpba)₄ (1) are presented and discussed. This includes nitrogen (N₂) adsorption at 77.3 K, carbon dioxide (CO₂) adsorption at 298.15 K (Supplementary Material) and the adsorption of the five hydrocarbons ethane, ethene, propane, n-butane and 1-butene at 273.15 K, 298.15 K and 323.15 K. Up to the saturation pressure, all isotherms show distinct inflection characteristics, that are relatable to structural transitions of the flexible solid. The number of inflections and the relative pressure at which the inflections manifest depend on the characteristics of the relevant adsorptive and on temperature. For the hydrocarbon adsorption isotherms, we studied the influence of i) measurement temperature, ii) bonding character, and iii) the chain length of the alkanes and alkenes. In addition, the origin of decreasing equilibrium pressure with increasing adsorbed volume in the nitrogen adsorption isotherms on the flexible MOF material 1 is discussed. Calculations using the Ideal Adsorbed Solution Theory (IAST), based on the unary isotherm fits, along with transient breakthrough simulations, are used to demonstrate that Cu-MOF 1 has the potential to separate 5-component ethane/ethene/propane/n-butane/1-butene mixtures to yield three different fractions with increasing carbon numbers.