Metal organic frameworks (MOFs) are crystalline porous materials consisting of polymeric inorganic networks with high surface areas, well-defined pore sizes and adjustable chemical functionality. MOFs have found applications in areas such as gas storage and separation, heterogeneous catalysis, sensors and drug delivery systems among others. MOF chemical properties make them ideal in the development of thin films and membranes for the adsorption and separation of gases, as their chemistry can be tailored to serve a particular application; recently, one of the main focuses of MOF membrane research is the separation and conversion of CO2
from other small gas molecules as in the case of power plant emissions. This work focuses on the formation of microcrystalline Zn-based MOF membranes on porous supports and the study of their gas permeation properties and gas transport mechanisms. A previously reported Zn(II)-based MOF (Zn4
with promising properties for CO2
capture and separation, was studied for the formation of microcrystalline porous membranes. Porous alumina disks were used as the support material for membrane formation. Porous supports were modified by the formation of a functionalized silica layer followed by dip-coating in a dilute polyethylenimine (PEI) solution to enhance hydrogen-bond attachment and provide a smooth surface for membrane formation2
. Seed crystals of (1)
were synthesized as previously reported1
and crushed into microcrystals, they were subsequently attached to the modified supports by a manual assembly procedure, XRD patterns of the seed crystals were studied. A MOF membrane was formed on the seeded supports using a secondary growth method from a synthesis solution of (1)3
, XRD patterns of the synthesized membrane confirmed the formation of (1)
, while the surface and thickness of the membrane was studied by SEM imaging. As an additional step, a thin and low permeance polysulfone (PSF) layer (<500 nm) was used in order to seal the surface of the membrane and impart mechanical strength to the superficial MOF crystals, this thin film also corrects superficial defects of the synthesized films. Membranes containing PSF superficial layers were studied by SEM imaging to ensure that gas permeation through MOF crystals was not blocked by the polymeric layer. A constant volume-variable pressure system was used to measure the flux of gases through the (1)
gas fluxes were measured showing a high gas permeance compared to other reported MOFs and a strong relation between molecular weight and gas permeance, this suggested a Knudsen type transport mechanism through the pores of (1)
. This was further confirmed by calculating the permselectivity of gases and comparing the obtained values to the Knudsen values. The effect of temperature on the permeance of gases was studied under conditions pertinent to the separation of CO2
from flue gases. The transport mechanism of gases inside the pores of the membrane was confirmed by the formation of a thin MOF layer composed by only a few microcrystals in thickness, the permselectivity results calculated from the permeance of these films corroborated a Knudsen transport mechanism happening inside the pores of the MOF and not between the spaces among adjacent crystals. This is a method that can be potentially applied to the study of the diffusion mechanism of gases through the pores of different MOF crystals.
1Ahrenholtz, S. R.; Landaverde-Alvarado, C.; Whiting, M.; Lin, S.; Slebodnick, C.; Marand, E.; Morris, A. J. Inog. Chem. 2015, 54, 4328-4336.
2 Ranjan, R.; Tsapatsis, M. Chem. Mater. 2009, 21, 4920-4924.
3 Lai, Z.; Tsapatsis, M.; Nicholich, J. P. Adv. Funct. Mater. 2004, 14, 716-729.