An Explicit Dual-Site BET Isotherm Model for the Prediction of Nonideal Competitive and Cooperative Adsorption Equilibrium

A dual-site extension is proposed to the previously derived
ideal
adsorbed solution theory analytical solution for two coadsorbing single-site
BET isotherms. The proposed dual-site BET (DSBET) isotherm model is
described in detail, along with multiple possible simplifications
for real systems, including modeling humid carbon capture (CO2/H2O mixtures) using amine-functionalized chemisorbents
and activated alumina. These simplifications allow nonideal binary
equilibrium predictions of single-site Langmuir or dual-site Langmuir
isotherm models (type-1 isotherms) coadsorbing with type-2 or type-3
isotherms as either a single-site BET or a dual-site BET isotherm
model. The GAB (Guggenheim-Anderson-de Boer) isotherm could also be
utilized in the proposed model via a change of variables. Many of
the same advantages seen by the extended dual-site Langmuir isotherm
are found for the dual-site BET isotherm. These include at least two
distinct multicomponent equilibrium predictions per binary gas pairing
and the ability to predict nonideal coadsorption. The choice of nonlinearity
parameters (and their corresponding adsorption-site) is discussed
along with their implications in coadsorption equilibrium prediction.
A generalized single-site and dual-site BET isotherm model was also
derived to extend multicomponent loading predictions to any number
of components in a coadsorbing mixture. The DSBET isotherm model is
able to predict nonideal adsorption equilibrium for both competitive
and cooperative systems. This is demonstrated using hypothetical binary
mixtures of DSBET isotherms and their IAST solutions, as well as loading
deviation plots quantifying their difference from the unary DSBET
prediction. Multiple DSBET isotherm model examples are shown for both
hypothetical and experimental systems. Experimental examples include
CO2/H2O mixtures on an amine-functionalized
polymer (Lewatit VP OC 1065) and F-200 activated alumina.