Characterizations
The morphologies of the MOF particles and MMMs were observed by using a field-emission scanning electron microscope (SEM) (Ultra-55, Zeiss Co.). X-ray diffraction (XRD) (D2 Phaser, Bruker CO.) was applied to characterize the crystalline structure of the prepared samples. X-ray photoelectron spectroscopy (XPS, A RBD upgraded PHI-5000C, ESCA system, PerkinElmer) and Fourier transform infrared spectroscopy (FTIR, IRTracer-100, Shimadzu CO.) were employed to study the chemical structure of the prepared samples. N2adsorption-desorption isotherms was collected by using a physisorption analyzer (Autosorb iQ Station 1, Quantachrome Co.) at 77 K to obtain the Brunauer-Emmett-Teller (BET) surface area and pore size distribution. The gas sorption properties of the prepared samples were tested by using the physisorption analyzer at 25 °C.
The adsorption selectivity selectivities of the MOF NPs and pMOF MSs were calculated by Henry’s Law:25
The Henry’s laws constants KH was defined:
\(\text{\ K}_{H}=\frac{\text{dn}}{\text{dp}}\) (4)
The Henry’s law selectivity for pure gases was obtained as:
\(S_{i,j}=\frac{K_{H,i}}{K_{H,j}}\) (5)
where i and j were different gases,KH,i and KH,j were Henry’s law coefficient.
A dual sorption theory was used to describe sorption isotherm of the MMMs:45,46
\(C=C_{D}+C_{H}=k_{D}p+\frac{c_{H}^{{}^{\prime}}\text{bp}}{\ 1+\text{bp}}\)(6)
where C (cm3 (STP) cm-3(MMM)) was solubility. CD andCH represented the amount of gas adsorbed in Henry and Langmuir sites, respectively. kD(cm3 (STP) cm-3(MMM) kPa-1) andcH (cm3(STP) cm-3 (MMM)) referred to Henry’s solubility and Langmuir saturation constants, respectively. b(kPa-1) and p (kPa) were the Langmuir affinity constant and pressure, respectively.
The solubility coefficient (S ) of gas in membranes was calculated by:
\(S=\frac{C}{p}=k_{D}+\frac{c_{H}^{{}^{\prime}}b}{\ 1+bp}\) (7)
The diffusion coefficient (D ) was calculated from permeability (P ) and S :
\(D=\frac{P}{S}\) (8)