Li Sze Lai

Zeolitic Imidazolate Frameworks (ZIF): A Potential Membrane for CO2/CH4 Separation

Removal of carbon dioxide (CO2) from natural gas is of importance because the existence of CO2 in natural gas increases the cost of the sweetening process. In recent years, membrane technology has emerged as an attractive alternative in separating CO2 from CH4 due to its economical, efficient, and environmentally-friendly process. Here, we review the different types of membranes used in CO2/CH4 gas separation. Zeolitic imidazolate frameworks (ZIFs) membranes are emphasized and ZIF-8 membranes are selected for further discussion due to their remarkable properties, including high chemical and thermal stability, facile and controllable pore apertures, and high CO2 permeance. Different types of methods used for the synthesis of ZIF membranes and the challenges encountered in the growth of the membrane are summarized. Potential use of microwave technology in fabricating a continuous and low-defects ZIF membrane within a short period of time are discussed and highlighted. In conclusion, future direction and perspectives are indicated.

Effect of Synthesis Parameters on the Formation of Zeolitic Imidazolate Framework 8 (ZIF-8) Nanoparticles for CO2 Adsorption

Zeolitic imidazole frameworks-8 (ZIF-8) is a subclass of metal-organic frameworks (MOFs) with the transition metal cations (Zn2+) linked by imidazolate anions forming tetrahedral frameworks in zeolite-like topologies. This article reports on the synthesis of ZIF-8 nanoparticles by varying the synthesis parameters at room temperature. The crystallization duration, molar ratios, and pH of the mixture solution were varied in order to study the effects of these parameters on the formation of ZIF-8 nanoparticles. The structural and morphology transformation of the resultant particles were characterized using x-ray diffraction, field emission scanning electron microscopy, and Brunauer–Emmett–Teller (BET) surface analysis. The CO2 adsorption characteristics of ZIF-8 nanoparticles were tested using CO2 physisorption analysis. Mature structural evolution was observed for ZIF-8 synthesized at 60 and 1440 min, but insufficient crystallization was found for ZIF-8 synthesized at 5 min. Meanwhile, ZIF-8 nanoparticles synthesized under lower amount of methanol resulted in larger particle size and higher crystallinity. Poorly intergrown ZIF-8 nanoparticles were observed for samples synthesized using a mixture solution with pH 8.2. Although different particle sizes and relative crystallinities were obtained for the ZIF-8 samples, synthesis using different molar ratios of the mixture solution, insignificant variations of BET surface areas, and CO2 adsorption capacities were found.

CO2 and CH4 permeation through zeolitic imidazolate framework (ZIF)-8 membrane synthesized via in situ layer-by-layer growth: an experimental and modeling study

In this work, a general model representing the permeation of CO2 and CH4 through Zeolitic Imidazole Framework-8 (ZIF-8) membrane synthesized via in situ layer-by-layer growth under microwave irradiation is developed. The model is formed based on the pressure drop concept in order to predict the intercrystalline properties of the ZIF-8 membrane according to the experimental permeation data of CO2 and CH4. The model combines Knudsen diffusion, viscous flow and generalized Maxwell–Stefan models, which considered the support resistance, gas diffusivity and intercrystalline pores of the membrane layer. The simulated data are fitted well with the experimental gas permeation results and consistent with the physical characterizations, including X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results showed that, layer-by-layer growth managed to reduce the intercrystalline pores present in the ZIF-8 membrane layer, with the approximate pore radius of 2.1 × 10−7 m and porosity of 1.15 × 10−4. However, the presence of the small pores can significantly affect the performance of the ZIF-8 membrane which resulted in CO2/CH4 ideal selectivity of ∼1.

Fast Synthesis of Highly Crystalline ZIF-8 using Microwave-assisted Solvothermal Method

This paper presents the formation of highly crystalline ZIF-8 using microwave-assisted solvothermal method. The crystallinity of the ZIF-8 particles was characterized using X-ray diffraction. The lattice vibrations of the structure in the ZIF-8 framework were determined through Fourier transforms infrared spectroscopy. The morphology of the ZIF-8 particles was observed through scanning electron microscopy. The results showed that, 0.5 hour was sufficient for the formation of highly crystalline ZIF-8 particles using microwave-assisted solvothermal method under temperature 120 oC.

Effect of the Solvent Molar Ratios on the Synthesis of Zeolitic Imidazolate Framework 8 (ZIF-8) and its Performance in CO2 Adsorption

In this research work, ZIF-8 was synthesized at room temperature using different synthesis solutions contained different molar ratios of methanol to zinc nitrate. The properties of the resultant particles were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermal gravimetric analysis (TGA). The basicity of the ZIF-8 samples was measured using temperature programmed desorption (TPD). Results showed that ZIF-8 samples synthesized in the present work were thermally stable up to 700 oC. ZIF-8 synthesized using synthesis solution contained lowest molar ratio of methanol to zinc nitrate of 86.7 possessed highest crystallinity and largest average particles sizes of ~250 nm. Besides, this sample also demonstrated highest CO2 uptake rate due to the presence of highest amount of basic site of 0.70 mmol/g among the ZIF-8 samples.