Ultra-selective carbon molecular sieve membranes for natural gas separations based on a carbon-rich intrinsically microporous polyimide precursor

Abstract

A highly contorted, carbon-rich intrinsically microporous polyimide (PIM-PI) made from spirobifluorene dianhydride and 3,3-dimethylnaphthidine (SBFDA-DMN) was employed as a precursor for the formation of carbon molecular sieve (CMS) membranes at pyrolysis temperatures from 550 to 1000 °C. The high carbon content of SBFDA-DMN (~84%) resulted in only 28% total weight loss during pyrolysis under a nitrogen atmosphere at 1000 °C. The development of the various microstructural textures was characterized by gas sorption analysis, Brunauer-Emmett-Teller (BET) surface area, X-ray diffraction, Raman spectroscopy, electrical conductivity, and gas transport properties. Heat treatment of a pristine SBFDA-DMN membrane at 550 °C resulted in reduced permeability for all gases (e.g.: P CO2 dropped from 4700 to 1500 barrer) as well as lower BET surface area from 621 to 545 m 2 g -1 . At 600 °C, new pores induced by pyrolysis increased the BET surface area to nearly that of the precursor and significantly improved gas separation performance. Above 600 °C, a progressive collapse of the micropores became evident with CMS membranes showing higher gas-pair selectivity but lower permeability. At 1000 °C, ultra-micropores comparable in size with the kinetic diameter of CH 4 emerged and induced a prominent molecular sieving effect resulting in very high CH 4 rejection. This strong size exclusion effect, further supported by gravimetric gas sorption measurements, resulted in unusually high N 2 /CH 4 and CO 2 /CH 4 selectivities of 35 and 1475, respectively.

Nano-Confinement Effects on Structural Development and Organic Solvent-Induced Swelling of Ultrathin Carbon Molecular Sieve Films

“… 10 The presence of micropores results in both PIMs and CMS outperforming the state-of-the-art polymeric membrane materials, sometimes by a very large margin. 11 In addition, CMS membranes are highly chemically and thermally stable even in long-term field operation. 12 , 13 However, similarly to other high free volume amorphous membrane materials, CMS membranes suffer from a progressive structural densification (physical aging) leading to significant reductions of permeance.…”

Ultraselective carbon molecular sieve membrane for hydrogen purification

“…4 depicts the WAXD patterns of the prepared CMS membranes. There are two broad peaks around the 2 θ diffraction angle of 23 °and 45 °, respectively, indicating that the CMS membranes have an amorphous carbonaceous structure with graphiticlike crystallites stacking [24] . These two peaks correspond to the (002) and (100) plane of the disordered graphite lattice, respec-…”

“…For the CMS-900 membrane, however, its d 002 (3.49 Å ) was much smaller than the molecular kinetic diameter (3.8 Å ) of CH 4 gas, implying the membrane would effectively prevent the diffusion of CH 4 , which could be found in the later section. In addition, the full width at half maximum (FWHM) of the (002) peak decreased with the carbonization temperature, and the estimated crystallite stacking thickness ( L c) and the layer-plane size ( L a) increased, indicating that more ordered graphitic structures were developed [40] . This phenomenon can also be observed in the TEM images of CMS membranes in Fig.…”

“…The nanoscale pore structures of CMS membranes are commonly described as slit-like with an idealized bimodal pore size distribution, which consists of the large micropores (7-20 Å ) acting as the sorption sites and determining diffusion jump lengths and https://engine.scichina.com/doi/10.1016/j.jechem.2020.03.008 the small ultramicropores ( < 7 Å ) providing molecular sieving function of CMS membranes [41] . The smaller CO 2 gas is often used as a molecular probe to provide an assessment of the ultramicropore size and distribution [40] . The CO 2 adsorption isotherms of the obtained CMS membranes and the corresponding ultramicropore size and distribution were shown in Fig.…”

“…The gas permeation performance of CMS membranes is mainly affected by the two critical parameters of polymeric precursors and final treatment temperature [13 , 17-21] . Many polymeric precursors were used to prepare CMS membranes, including aromatic polyimide and derivatives [22][23][24] , polyetherimide [25] , phenolic resin [26] , poly (furfuryl alcohol) [27] , polyphenylene oxide [28] , poly (phthalazinone ether sulfone ketone) [29 , 30] . For instance, Ma et al used an intrinsically microporous polyimide to prepare CMS membranes, and the membrane treated at 630 °C possessed H 2 permeability of 4690 Barrer with the H 2 /CH 4 selectivity of 81 [31] .…”