Competing non ideal behaviour of SAPO-34 and Poly(hexafluoropropylene) in mixed matrix membranes

Abstract

Amorphous and glassy polyhexafluoropropylene (PHFP) has been used for the first time as the matrix of mixed matrix membranes (MMMs). The preparation and the gas transport modeling of defect-free MMMs based on PHFP and 200 nm SAPO-34 crystals are described. By grafting SAPO-34 with perfluorinated tails the filler was well dispersed in the polymer and a good interfacial adhesion was guaranteed. The nano-particles determine a marked increase in gas permeability and a modest enhancement of the CO2/CH4 permselectivity with respect to the neat polymer. Unexpectedly, the increase of permeability of the MMMs is not monotonous with the amount of SAPO-34. Following a recent approach, unrestricted gas diffusivity and solubility in SAPO-34 and PHFP are the inputs of the four-phase macroscopic modelling of gas transport through the MMMs. A four-phase approach to MMM modelling is dictated by the evidences of two distinct interfacial changes with respect to the ideal, nominal two phase MMM made of bulk-like SAPO-34 particles embedded in the pristine PHFP polymer matrix: a modified polymer phase of larger free volume (increasing permeability), which surrounds SAPO-34 particles, and barriers to the transport of gas (making the filler less permeable than PHFP), in the interior or on the surface of SAPO-34. The four-phase modelling, which delivers total gas transport features in excellent agreement with the experiments, is carried out by means of numerical optimization procedures based on a) Maxwell's effective medium analytical formulas, and b) a finite element solution of the transport differential equations. The latter approach reveals additional details, with respect to the first one, by delivering the different paths of gases through or around the SAPO-34 crystals in the MMMs. This work demonstrates the need to identify the real phenomena at work for a fruitful modelling of mass transport in MMMs, by overcoming the simplistic assumptions of MMMs obtained by the mere addition of the two original components.