Probing sub-5 Ångstrom micropores in carbon for precise light olefin/paraffin separation

Abstract

Olefin/paraffin separation is an important but challenging and energy-intensive process in petrochemical industry. The realization of carbons with size-exclusion capability is highly desirable but rarely reported. Herein, we report polydopamine-derived carbons (PDA-Cx, where x refers to the pyrolysis temperature) with tailorable sub-5 Å micropore orifices together with larger microvoids by one-step pyrolysis. The sub-5 Å micropore orifices centered at 4.1-4.3 Å in PDA-C800 and 3.7-4.0 Å in PDA-C900 allow the entry of olefins while entirely excluding their paraffin counterparts, performing a precise cut-off to discriminate olefin/paraffin with sub-angstrom discrepancy. The larger voids enable high C₂H₄ and C₃H₆ capacities of 2.25 and 1.98 mmol g^-1 under ambient conditions, respectively. Breakthrough experiments confirm that a one-step adsorption-desorption process can obtain high-purity olefins. Inelastic neutron scattering further reveals the host-guest interaction of adsorbed C₂H₄ and C₃H₆ molecules in PDA-Cx. This study opens an avenue to exploit the sub-5 Å micropores in carbon and their desirable size-exclusion effect.

Fig. 1. Formation process and gas sorption properties of PDA-Cx.

a Schematic illustration of the synthetic procedure. Dopamine undergoes self-assembly to prepare polydopamine firstly and subsequent pyrolysis to synthesize PDA-Cx with narrowly distributed sub-5 Å micropore orifices; b Schematic illustrations of a side view of the pore system. The small micropore orifices in PDA-C800 (4.1–4.3 Å) and PDA-C900 (3.7–4.0 Å) were probed by a series of angstrom-sized gas probes ranging from 3.3 to 5.0 Å; PDA-C800 admits C₃H₆ (4.0 Å) while rejects C₃H₈ (4.3 Å) completely and PDA-C900 admits C₂H₄ (3.7 Å) while rejects C₂H₆ (4.1 Å) completely; such refined small micropores enable the production of high-purity olefins (>97%) through a one-step adsorption-desorption process.

Fig. 2. Characterization of PDA and PDA-Cx.

a ¹³C CP MAS NMR experiments recorded on PDA and PDA-Cx; b PXRD patterns of PDA and PDA-Cx. The dotted lines mark the (002) and (100) peaks; c The average pore size of PDA-Cx derived from the PALS. The dashed line is a linear data fit as a visual guide. The error bars represent the standard deviations based on the nonlinear least squares fitting process carried out using the LTv9 program.

Fig. 3. Sorption behavior of gas probes and porosity information.

Sorption equilibrium isotherms of various gas probes with minimum molecular dimensions ranging from 3.3 Å (CO₂) to 5.0 Å (i-C₄H₁₀) at 273 K and N₂ at 77 K on a PDA-C800 and b PDA-C900 materials; c The pore volumes of PDA-Cx calculated from different probe gases based on the Dubinin-Astakhov (D-A) equation (the data points from left to right were calculated from probing gases of CO₂, Ar, C₂H₄, C₃H₆, C₂H₆, C₃H₈, CF₄, and i-C₄H₁₀ for PDA-Cx. While C₃H₆, C₃H₈, and i-C₄H₁₀ with high polarizability were excluded for heteroatom-rich PDA-C300 and PDA-C500 due to stronger host-guest interaction); d The pore size distributions of PDA-Cx (the differential pore volume (∆V) of y-axis was obtained from probes with successive sizes. V_Ar calculated by Ar was subtracted from V_CO2, V_C2H4 from V_Ar, and so on).

Fig. 4. Static sorption property of olefins and paraffins.

Single-component gas adsorption isotherms of a C₃H₆ and C₃H₈ on PDA-C800 and b C₂H₄ and C₂H₆ on PDA-C900 at varied temperatures. The separation factor (S’) was calculated by the uptake ratio of olefin/paraffin at 298 K and 1.0 bar; c Comparison of the number of olefins adsorbed (Q) and olefin/paraffin separation factor of PDA-Cx with state-of-the-art porous carbon adsorbents (PDA-Cx marked as pentagram). The details are given in Supplementary Tables 7-8. d Comparison of the experimental INS of solid C₃H₆ and C₃H₆ in PDA-C800 (difference between PDA-C800 and PDA-C800 with C₃H₆ adsorbed); e Comparison of the experimental INS of solid C₂H₄ and C₂H₄ in PDA-C900 (difference between PDA-C900 and PDA-C900 with C₂H₄ adsorbed). Discrete phonon modes are highlighted by orange color, and three prominent vibrational modes are highlighted by blue, green and yellow colors.

Fig. 5. Olefin/paraffin dynamic separation and regeneration property.

a Time-dependent gas uptake profiles of C₃H₆ on PDA-C800 and C₂H₄ on PDA-C900 at 308 K and 0.5 bar. The time required to reach 80% of the saturated olefin uptake and equilibrium is depicted as dashed lines. Experimental column breakthrough curves for the mixtures of b C₃H₆/C₃H₈ (50/50, v/v) on PDA-C800 and c C₂H₄/C₂H₆ (50/50, v/v) on PDA-C900 at 298 K with a constant flow rate of 1.5 mL/min; d Desorption curves following a column breakthrough experiment under 10 mL/min flow of He at 353 K with PDA-C800 and PDA-C900, and the corresponding cumulative purity of olefins. C and C₀ are the concentrations of each gas at the outlet and inlet, respectively; e Cycling uptake tests for pure-component C₃H₆ on PDA-C800 and C₂H₄ on PDA-C900; f Heat of adsorption (Q_st) of C₃H₆ in PDA-C800 and C₂H₄ in PDA-C900.