Cellulose carbon molecular sieves for hydrogen separation

Hydrogen (H2) production from natural gas is considered to be one of the most potential technologies for low-carbon energy in the future and reducing greenhouse gas emissions. Compared with conventional H purification technologies, membrane-based separation technologies have received extensive attention due to their higher energy efficiency and environmental friendliness. However, currently commonly used H2 and CO2 separation membranes usually suffer from low separation performance, high cost, and low stability under high temperature and high pressure. Therefore, it is still challenging to prepare commercially viable H2 purification membranes. Carbon molecular sieve (CMS) membranes are made by controlled carbonization of polymer precursors at high temperatures and have rigid pore structures. When the CMS membrane is made into a hollow fiber suitable for a membrane module, it is expected to have the properties of high temperature and high pressure resistance.
Cellulose has strong interchain and intrachain hydrogen bonds, which makes it poorly soluble in most solvents, with only a few solvents such as N-methylmorpholine-N-oxide (NMMO), ionic liquids and Inorganic salts can effectively disrupt their hydrogen bond network. However, obtaining an accurate cellulose/solvent/non-solvent ternary phase diagram is still challenging due to the huge viscosity of this system.
Based on this, Xuezhong He et al. from the Norwegian University of Technology prepared carbon hollow fiber membranes (CHFMs) by adjusting the solidification temperature and final carbonization temperature of the cellulose/ionic liquid/water system and used them for H2 separation.
The researchers prepared asymmetric cellulose hollow fiber precursors through a dry and wet spinning process, and then exchanged with water to remove the original solvent EmimAc and DMSO, and finally obtained the corresponding microporous structure through high-temperature carbonization. From the SEM images, it can be found that the asymmetric structures of the outer selective layer and the porous inner support layer of about 3 μm are still maintained when different carbonization temperatures are used. CHFM-550 at the lowest carbonization temperature has the lowest hardness and Young's modulus. With the increase of carbonization temperature, the hardness and Young's modulus increase gradually. The increase in hardness and modulus can be attributed to the internal structural changes caused by the increase in carbonization temperature. At the same time, with the increase of carbonization temperature, the pore peaks >5 Å weakened, while the pore peaks <5 Å increased, which indicated that the average pore size of the carbonized CHFMs decreased at high temperature. Furthermore, both the surface area and pore volume of CHFM decreased with increasing carbonization temperature, suggesting that CMS films tended to have denser packing when carbonized at higher temperatures.
The membranes prepared at higher carbonization temperatures have higher H2/CO2 selectivity, but lower H2 permeability, which indicates that the gas permeability is mainly determined by the motion diameter of gas molecules, i.e. molecular sieve transport transport mechanism. When the sp3/sp2 ratio decreased from 0.73 to 0.36, the H2 permeability decreased from 466.8 GPU to 148.2 GPU, while the H2/CO2 selectivity increased from 11.1 to 83.9, which also suggested that the gas separation performance can be tuned by adjusting the carbon structure. Due to the simultaneous presence of molecular sieve and surface diffusion transport of CO2 molecules, the apparent activation energy of CO2 is relatively lower compared to H2, which indicates that temperature has a greater effect on H2 permeability, so lower CO2 adsorption at higher temperatures This leads to an increase in H2/CO2 selectivity. When the membrane was exposed to the laboratory atmosphere for 50 days, its H2 permeability and H2/CO2 selectivity decreased by about 40% and 10%, respectively, and the gas permeability and gas permeability were effectively restored after heat treatment and helium purging. Optional.
CHFM exhibits excellent H2/CO2 selectivity and high H2 permeability compared to other membranes, of which CHFM-850 shows the highest overall gas separation performance with an ideal H2/CO2 selectivity of 83.9 at 130 °C, exceeding non-polymer films. At the same time, the selectivity of CHFM-850 to H2/N2 is >800 and the selectivity of H2/CH4 is >5700, which provides the possibility for H2 purification in some processes.
In summary, this work produced an asymmetric cellulose hollow fiber material by spinning microcrystalline cellulose and EmimAc. The obtained cellulose hollow fibers are carbonized at high temperature to obtain asymmetric hollow fiber membranes whose microporous structure helps them to separate H2 from other gases.