S/N/O-Enriched Carbons from Polyacrylonitrile-Based Block Copolymers for Selective Separation of Gas Streams

Abstract

A series of polyacrylonitrile (PAN)-based block copolymers with poly(methyl methacrylate) (PMMA) as sacrificial bock were synthesized by atom transfer radical polymerization and used as precursors for the synthesis of porous carbons. The carbons enriched with O- and S-containing groups, introduced by controlled oxidation and sulfuration, respectively, were characterized by Raman spectroscopy, scanning electron microscopy, and X-ray photoelectron spectrometry, and their surface textural properties were measured by a volumetric analyzer. We observed that the presence of sulfur tends to modify the structure of the carbons, from microporous to mesoporous, while the use of copolymers with a range of molar composition PAN/PMMA between 10/90 and 47/53 allows the obtainment of carbons with different degrees of porosity. The amount of sacrificial block only affects the morphology of carbons stabilized in oxygen, inducing their nanostructuration, but has no effect on their chemical composition. We also demonstrated their suitability for separating a typical N₂/CO₂ post-combustion stream.

Keywords: atom transfer radical polymerization; block copolymer templates; hierarchical pores; oxidative-stabilization; sulfur-doping.

Figure 1

XPS spectra of BC26/74-O: survey (a), the high-resolution of N 1s (b), and C 1S (c); XPS spectra of BC26/74-S: survey (d), the high-resolution of N 1s (e), and S 2p (f).

Figure 2

SEM images of BC10/90-O (a), BC26/74-O (b), BC43/57-O (c), PAN-O (d), PAN-S (e), and BC10/90-S (f). Scale bar 200 nm in all the images, except d (2 μm).

Figure 3

Adsorption (full symbols) and desorption (empty symbols) isotherms of N₂ (77 K) for carbons from copolymers in the absence of sulfur. BC10/90-O (blue), BC26/74-O (grey), BC43/57-O (red).

Figure 4

Adsorption (full symbols) and desorption (empty symbols) isotherms of N₂ (77 K) for carbons from copolymers doped with sulfur. BC10/90-S (green), BC26/74-S (yellow).

Figure 5

Adsorption isotherms corresponding to CO₂ (273 K) for carbons from copolymers with and without sulfur. BC10/90-O (blue), BC26/74-O (grey), BC43/57-O (red), BC10/90-S (green), BC26/74-S (yellow).

Figure 6

Effect of copolymer composition upon the surface area determined by the adsorption of N₂ at 77 K and CO₂ at 273 K. Without sulfur: (black empty circle) N₂ at 77 K, (black full circle) CO₂ at 273 K. With sulfur: (purple empty square) N₂ at 77 K, (purple full square) CO₂ at 273 K.

Figure 7

Adsorption isotherms experimental data for N₂ (full symbols) and CO₂ (empty symbols) at 273 K. BC26/74-O(grey), BC26/74-S (yellow).

Figure 8

Influence of microporosity, determined with N₂ at 77 K (A), and ultramicroporosity, determined by comparing surface area determined with N₂ at 77 K and CO₂ at 273 K on CO₂ selectivity data (B). (full symbols) apparent selectivity, (empty symbols) IAST selectivity.

Figure 9

Relation between ultramicroporous surface area (determined with CO₂ at 273 K) and CO₂ uptake. T = 273 K. P = 15 kPa.

Figure 10

Effect of temperature upon the CO₂ adsorption isotherms. BC26/74-S (yellow), BC26/74-O (grey). 273 K (circle), 298 K (square), 313 K (triangle).

Figure 11

Influence of copolymer composition used in the carbon precursor and temperature upon the CO₂ uptake at P = 101.3 kPa = 760 mmHg. Full symbols correspond to BC-S series; empty symbols correspond to BC-O series.

Figure 12

Examples of the fitting behavior of models used in present work for CO₂ adsorption experimental data at 273 K. BC26/74-S (◯, yellow), BC26/74-O (◯, green). Langmuir (blue), Freundlich (red), Toth (green).

Figure 13

Effect of material and surface coverage on the value of heat of adsorption.