Gas separation using amino-modified ZIF/polymer based mixed matrix membrane

Abstract

Natural gas is one of the most abundant fuel resources in the world, combustion od which fulfils much of our energy needs. However, the presence of certain, non-combustible and hazardous impurities, may reduce the energy content of the fuel gas. Carbon dioxide has the most adverse effect in this regard. It is mainly present in the order of 3-4% in natural gas, and increased amount reduces energy content of the gas. Therefore, it is imperative to separate carbon dioxide from the feed natural gas. For this purpose, it is proposed to use a mixed matrix membrane, which consists of amino-modified Zeolitic Imidazolate Framework (ZIF) incorporated in a cellulose acetate (CA) matrix, or a polymer blend membrane. Amine modification of the ZIF helps in the capture of acidic gases like CO2, as amine group is basic in nature. The use of ZIF is helpful because of its crystalline structure and fixed but tunable pores. Blend membranes work in much the same way, by including such groups in the matrix which have affinity for carbon dioxide. In the present work, synthesis of a series of polymer blend membranes, consisting of a blend of polyallylamine (PAAm) and CA/PEG in an acetone-water solvent system, and mixed matrix membranes consisting of amino-modified ZIF-8 in CA matrix, synthesized from a solution of the polymer and filler in acetone is carried out. These two membranes were then tested for gas permeation using single and mixed gas permeation testing (in Gas Chromatogram), analysed for their tensile strength using the Ultimate Tensile Testing Machine, their morphology and surface characteristics studied using SEM analysis and the presence of functional groups was confirmed using FT-IR spectroscopy. It was found that the amino-modified ZIF based CA membranes gave much better and superior single and mixed gas selectivities of 34.86 and 39.49 respectively for the membranes containing 20% amino-modified ZIF-8 in the matrix, with permeabilities up to 18.56 barrers. The mixed matrix membranes also exhibited uniformly dense structures, with no pores observed under SEM even at magnifications of up to 30000x. Moreover, they had a maximum tensile strength of about 43.7673 MPa. On the other hand, the polymer blend membranes became porous at much lower loading (<3%), had a maximum tensile strength of only 10.3056 MPa and a maximum selectivity of 1.7. After studying these two types of membranes, it was concluded that the amino-modified ZIF based membrane is far more suitable for use in gas separation, than any other blend membrane that we have synthesized in this work.