Direct Utilization of Elemental Sulfur in the Synthesis of Microporous Polymers for Natural Gas Sweetening

•Sulfur in porous polymer synthesis•Solvent- and catalyst-free approach•Exceptional CO2 separation•High-value application for sulfur Elemental sulfur, mainly produced as a byproduct of natural gas purification, is one of the most abundant elements in the world but is utilized in a relatively limited number of large-scale applications, such as gunpowder and sulfuric acid production. Herein, we report on a high-value and scalable application for this low-value commodity, that is, using elemental sulfur and organic linkers to synthesize highly porous polymers with ultrafine pores. These polymers showed high affinity toward CO2, a known primary greenhouse gas, thus allowing us to capture and separate CO2 from large point sources such as flue gas and natural gas streams. Elemental sulfur can therefore be directly utilized in the synthesis of porous polymers and recycled back for an efficient, sustainable CO2 separation process. These polymeric materials offer new possibilities for directly utilizing elemental sulfur to provide sustainable solutions for challenging environmental issues. Elemental sulfur, which is produced by a process called hydrodesulfurization mainly as a byproduct of the purification of natural gas, is one of the most abundant elements in the world. Herein, we describe solvent- and catalyst-free synthesis of ultramicroporous benzothiazole polymers (BTAPs) in the presence of elemental sulfur in quantitative yields. BTAPs were found to be highly porous and showed exceptional physiochemical stability. Moreover, in situ chemical impregnation of sulfur within the micropores increased CO2 affinity of the sorbent while limiting diffusion of CH4. As low-cost, scalable solid sorbents, BTAPs showed promising CO2 separation ability and high regenerability under vacuum swing adsorption for the simulated flue gas, natural gas, and landfill gas conditions. The fact that elemental sulfur can be directly utilized in the synthesis of BTAPs means that it can be recycled back to the natural gas sweetening process for efficient CO2/CH4 separation, thus offering a high-value, scalable, large-scale application for elemental sulfur. Elemental sulfur, which is produced by a process called hydrodesulfurization mainly as a byproduct of the purification of natural gas, is one of the most abundant elements in the world. Herein, we describe solvent- and catalyst-free synthesis of ultramicroporous benzothiazole polymers (BTAPs) in the presence of elemental sulfur in quantitative yields. BTAPs were found to be highly porous and showed exceptional physiochemical stability. Moreover, in situ chemical impregnation of sulfur within the micropores increased CO2 affinity of the sorbent while limiting diffusion of CH4. As low-cost, scalable solid sorbents, BTAPs showed promising CO2 separation ability and high regenerability under vacuum swing adsorption for the simulated flue gas, natural gas, and landfill gas conditions. The fact that elemental sulfur can be directly utilized in the synthesis of BTAPs means that it can be recycled back to the natural gas sweetening process for efficient CO2/CH4 separation, thus offering a high-value, scalable, large-scale application for elemental sulfur. Sulfur is one of the world's most versatile and common elements.1Nehb W. Vydra K. Sulfur. Wiley, 2006Crossref Google Scholar However, unlike that of other chemical commodities, production of sulfur is mostly “involuntary” in that it mainly arises from petroleum refineries and natural gas processing, thus creating a global surplus of sulfur.2Rauchfuss T. Under sulfur's spell.Nat. 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Inter. 2016; 8: 14648-14655Crossref PubMed Scopus (68) Google Scholar Most of these polymers, however, require either expensive rare-earth metals or stoichiometric amounts of catalysts for their synthesis, which significantly limits their scalability. In addition, post-washing steps have to be applied in order to remove unreacted monomers, by-products, and high-boiling-point solvents. Thus, the development of polymerization strategies to address these issues is critical to ensuring the low cost and scalability of the resulting microporous polymers. Here, we introduce a catalyst- and solvent-free polymerization strategy that enables direct utilization of elemental sulfur in the synthesis of highly microporous benzothiazole polymers (BTAPs) with Brunauer-Emmett-Teller (BET) surface areas up to 750 m2 g−1. We synthesized BTAPs in quantitative yields by mixing two-dimensional (2D) or three-dimensional (3D) organic linkers incorporating p-tolyl and 4-aminophenyl moieties with elemental sulfur at 275°C under an Ar atmosphere. Subsequent heating at 400°C was applied to remove excess sulfur and to activate the pores, thus eliminating the post-washing step. BTAPs showed exceptional thermal stability up to 550°C under nitrogen and 500°C under air conditions, which are some of the highest values reported to date. Although BTAPs showed high affinity toward CO2, we observed very low affinity toward CH4, presumably because of its limited diffusion in the presence of a high concentration of polar functional groups within the micropores. These results prompted us to carry out breakthrough experiments to test the performance of real gas separation. BTAPs showed excellent CO2/CH4 separation performance and high regenerability values for the vacuum swing adsorption (VSA) process. Thus, BTAPs as low-cost, microporous, highly stable porous solid sorbents could be ideal candidates for large-scale utilization of elemental sulfur; they can then be fully recycled back to the natural gas sweeting process to separate CO2 from CH4 and to capture CO2 from a flue gas mixture. Benzothiazole is an aromatic heterocyclic compound incorporating S and N atoms in its structure. Although there are various synthetic routes for its synthesis, we followed a catalyst- and solvent-free synthetic route involving the reaction of methyl- and amine-substituted aromatic rings in the presence of elemental sulfur.33Zhang L.F. Ni Z.H. Li D.Y. Qin Z.H. Wei X.Y. Convenient synthesis of 2-arylbenzothiazoles and 2-arylnaphthothiazoles.Chin. Chem. Lett. 2012; 23: 281-284Crossref Scopus (13) Google Scholar, 34Nguyen T.B. Ermolenko L. Dean W.A. Al-Mourabit A. Benzazoles from aliphatic amines and o-amino/mercaptan/hydroxyanilines: elemental sulfur as a highly efficient and traceless oxidizing agent.Org. Lett. 2012; 14: 5948-5951Crossref PubMed Scopus (139) Google Scholar We first synthesized (Figure S1) 2-phenylbenzothiazole as a model compound under these reaction conditions. In order to test this approach for the preparation of BTAPs, we synthesized (Figure 1; Supplemental Experimental Procedures and Figure S2) 2D organic linkers, 1,3,5-tris(4-tolyl)benzene (M1) and 1,3,5-tris(4-aminophenyl)benzene (A1), and 3D organic linkers, tetrakis(4-methylphenyl)methane (M2) and tetrakis(4-aminophenyl)methane (A2), and reacted them in the presence of elemental sulfur. To determine the optimal amount of sulfur for the polymerization reaction, we reacted (Figure 1) M1 and A1 in the presence of varying amounts (20, 40, 100, 200 equiv with respect to amine) of elemental sulfur to synthesize BTAP-1 at 275°C for 5 hr under an Ar atmosphere and subsequently at 400°C for 5 hr to remove excess sulfur. We carried out BET surface-area analyses for BTAPs by Ar adsorption at 87 K. On the basis of these results, 100 equiv of elemental sulfur loading was identified as the optimal condition for the polymerization reaction for the preparation of BTAPs with high surface area and the control polymers (cBTAPs), which were obtained without the additional thermal treatment at 400°C. To verify the formation of BTAPs, we carried out elemental analysis (EA), energy-dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), solid-state UV-visible spectroscopy (UV-Vis), cross-polarization magnetic-angle spinning (CP/MAS) 13C nuclear magnetic resonance (NMR), powder X-ray diffraction (PXRD), and scanning electron microscopy (SEM) analyses. We carried out EA to verify the sulfur content in BTAPs and cBTAPs, (Supplemental Experimental Procedures). BTAP-1, BTAP-2, and BTAP-3 showed 8, 11, and 10 wt % more sulfur content, respectively, than the calculated values, and the sulfur loading amounts reached up to 20.93, 24.94, and 24.27 wt %, respectively. We attribute this additional sulfur loading to the C–H insertion reaction of sulfur radicals during the synthesis of BTAPs35Hwang T.H. Jung D.S. Kim J.S. Kim B.G. Choi J.W. 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Formation of polycyclic thiophenes from reaction of selected polycyclic aromatic hydrocarbons with elemental sulfur and/or pyrite under mild conditions.Energy Fuels. 1988; 2: 220-223Crossref Scopus (29) Google Scholar given that elemental sulfur can be transformed into its radical linear chain form at high temperatures (>160°C) and is simultaneously inserted on the aromatic rings. During the polymer synthesis process at 275°C and subsequently at 400°C, inserted sulfur chains can be broken by a process called inverse vulcanization,6Chung W.J. Griebel J.J. Kim E.T. Yoon H. Simmonds A.G. Ji H.J. Dirlam P.T. Glass R.S. Wie J.J. Nguyen N.A. et al.The use of elemental sulfur as an alternative feedstock for polymeric materials.Nat. Chem. 2013; 5: 518-524Crossref PubMed Scopus (831) Google Scholar which can lead to the formation of short sulfur chains (S2–S4) and thiol groups. We observed higher sulfur contents of 29.79, 39.64, and 40.90 wt % for cBTAP-1, cBTAP-2, and cBTAP-3, respectively, as a result of the presence of elemental sulfur within the pores. The sulfur content of the BTAP particles was also verified (Figure S3) with EDS analysis, the results of which show that sulfur atoms were mostly located within the BTAP particles. To further verify the form of sulfur within the frameworks and analyze thermal stability of BTAPs and cBTAPs, we carried out TGA analysis (Figure 2A ). BTAPs were found to be stable up to 550°C under N2 and 500°C under air conditions (Figure S4), indicating that residual sulfurs were chemically impregnated in the framework. These values are comparable with, if not better than, those of the all-carbon frameworks, such as PPN-622Ben T. Ren H. Ma S. Cao D. Lan J. Jing X. Wang W. Xu J. Deng F. Simmons J.M. et al.Targeted synthesis of a porous aromatic framework with high stability and exceptionally high surface area.Angew. Chem. Int. Ed. Engl. 2009; 48: 9457-9460Crossref PubMed Scopus (1195) Google Scholar (520°C in air) and PPN-540Yuan D. Lu W. Zhao D. Zhou H.C. Highly stable porous polymer networks with exceptionally high gas-uptake capacities.Adv. Mater. 2011; 23: 3723-3725Crossref PubMed Scopus (510) Google Scholar (450°C in N2). In contrast, cBTAPs showed (Figures 2B and S5) significant weight loss in the temperature range of 150°C–400°C, pointing to the fact that sulfur is mainly trapped in the pores in its elemental form. We carried out PXRD analysis (Figure 2C) to evaluate the crystallinity of BTAPs, which were found to be amorphous. We performed XPS analysis (Figures 2D–2F) to further elucidate the form of sulfur within the BTAP structures. The strong S 2p peak at 164.0 eV indicates the presence of sulfur in a thiophenyl form,41Kelemen S.R. George G.N. Gorbaty M.L. Direct determination and quantification of sulphur forms in heavy petroleum and coals.Fuel. 1990; 69: 939-944Crossref Scopus (272) Google Scholar which suggests successful formation of benzothiazole moieties in BTAPs. The peak at 165 eV was assigned to the formation of S–S bonds between polymer chains, which could originate from the dimerization thiol groups generated by the cleavage of sulfur chains at increased temperatures by a process called inverse vulcanization.6Chung W.J. Griebel J.J. Kim E.T. Yoon H. Simmonds A.G. Ji H.J. Dirlam P.T. Glass R.S. Wie J.J. Nguyen N.A. et al.The use of elemental sulfur as an alternative feedstock for polymeric materials.Nat. Chem. 2013; 5: 518-524Crossref PubMed Scopus (831) Google Scholar We believe that the 3D interpenetrated framework structure favors the formation of disulfide bonds. We also observed (Figures 2D and S6) two additional peaks in BTAP-1 around 168.0 and 169.2 eV, which we attributed to the short sulfur chains (S2–S4), which is in good agreement with the recently reported 2D sulfur-embedded covalent triazine frameworks.38Talapaneni S.N. Hwang T.H. Je S.H. Buyukcakir O. Choi J.W. Coskun A. Elemental-sulfur-mediated facile synthesis of a covalent triazine framework for high-performance lithium-sulfur batteries.Angew. Chem. Int. Ed. Engl. 2016; 55: 3106-3111Crossref PubMed Scopus (258) Google Scholar Formation of benzothiazole linkages in BTAPs and cBTAPs was also evaluated (Figures S7–S9) by FT-IR analysis. We observed characteristic benzothiazole –C=N– and –C–S– stretching bands and an –N=C–S vibration band at 1,480, 1,390, and 667 cm−1, respectively, in the FT-IR spectra of the model compound, cBTAPs, and BTAPs, indicating the successful formation of benzothiazole linkages.42Klots T.D. Collier W.B. Heteroatom derivatives of indene. Part 2. Vibrational spectra of benzothiophene and benzothiazole.Spectrochim. Acta Mol. Biomol. Spectrosc. 1995; 51: 1273-1290Crossref Scopus (29) Google Scholar Moreover, the complete disappearance of free N–H stretching bands at 3,400, 3,300, and 3,200 cm−1 for the A1 and A2 monomers also supports quantitative formation of cBTAPs and BTAPs. We also carried out solid-state UV-Vis analysis (Figures S10 and S11); all BTAPs showed a characteristic benzothiazole absorption peak at 290 nm.43Ellis B. Griffiths P.J.F. The ultra-violet spectra of thiazole and benzthiazole.Spectrochim. Acta. 1965; 21: 1881-1892Crossref Scopus (22) Google Scholar Moreover, BTAPs also showed broad absorption in the visible range, which could be attributed to the formation of polycyclic thiophenes during the formation of networks, as described previously by White et al.39White C.M. Douglas L.J. Schmidt C.E. Hackett M. Formation of polycyclic thiophenes from reaction of selected polycyclic aromatic hydrocarbons with elemental sulfur and/or pyrite under mild conditions.Energy Fuels. 1988; 2: 220-223Crossref Scopus (29) Google Scholar and verified (Figures 2D–2F) by the XPS analysis. These observations point to the fact that BTAPs can also be promising candidates for light-harvesting applications. The molecular connectivity of BTAPs was elucidated (Figure S12) by solid-state CP/MAS 13C NMR spectroscopy. The CP/MAS 13C NMR spectra of BTAPs were found to be in a perfect agreement with that of the model compound. In particular, the carbon atom of the benzothiazole ring, which resonated at 179.0 ppm, provides clear evidence of the formation of benzothiazole rings. Additional aromatic carbon peaks located at 167.3, 158.1, and 152.3 ppm in the model compound were also observed in BTAPs. Moreover, the carbon peak located at 90.2 ppm in the CP/MAS 13C NMR spectra of BTAP-2, -3 was ascribed to the quaternary carbon core of tetraphenylmethane. In order to evaluate bulk-scale morphology of BTAPs, we have carried out (see Figure S13) SEM analysis. BTAPs formed spherical particles with an average particle size of 500, 650, and 800 nm for BTAP-1, BTAP-2, and BTAP-3, respectively. We also observed the formation of film-like superstructures in the case of BTAP-2 and BTAP-3, which could explain their relatively lower surface areas (Figure 3) than that of BTAP-1, presumably as a result of the decreased accessibility of micropores.Figure 3Analysis of the Textural Properties of BTAPsShow full caption(A) The effect of the amount of elemental sulfur on the surface area of BTAP-1. For the synthesis of BTAPs, 100 equiv of sulfur was identified as the ideal condition because it showed the highest surface area.(B) Ar uptake isotherms of BTAPs at 87 K; filled and empty symbols represent adsorption and desorption, respectively.(C) Calculated pore-size distribution of BTAPs according to the non-local density functional theory.See also Figures S14–S20.View Large Image Figure ViewerDownload (PPT) (A) The effect of the amount of elemental sulfur on the surface area of BTAP-1. For the synthesis of BTAPs, 100 equiv of sulfur was identified as the ideal condition because it showed the highest surface area. (B) Ar uptake isotherms of BTAPs at 87 K; filled and empty symbols represent adsorption and desorption, respectively. (C) Calculated pore-size distribution of BTAPs according to the non-local density functional theory. See also Figures S14–S20. The porosity of BTAPs and cBTAPs (Figures S14 and S15) was investigated by Ar adsorption-desorption isotherms measured at 87 K (Figure 3). All Ar isotherms showed a typical type I adsorption profile. The rapid uptake observed at the low relative pressure range, below 0.03 (P/Po), and the lack of desorption hysteresis suggest the permanent microporous nature of BTAPs. The BET surface areas of BTAPs were calculated from Ar adsorption isotherms over a valid relative pressure range of 0.01–0.12, which was determined from the Rouquerol plots (Figures S16 and S17). It is important to note that the loading amount of sulfur plays a crucial role in the determination of network porosity. We observed (Figure 3A) increasing surface area with increasing amounts of sulfur up to 100 equiv and observed a slight decrease in the BET surface area when sulfur was further increased up to 200 equiv. The BET surface areas of the BTAP-1 series in the presence of 20, 40, 100, and 200 equiv of elemental sulfur were 0, 545.3, 750.9, and 702.7 m2 g−1, respectively. Because the singlet state of sulfur can undergo a C–H insertion reaction,44Yin L. Wang J. Lin F. Yang J. Nuli Y. Polyacrylonitrile/graphene composite as a precursor to a sulfur-based cathode material for high-rate rechargeable Li–S batteries.Energy. Environ. Sci. 2012; 5: 6966-6972Crossref Scopus (446) Google Scholar the accessibility of the micropores could be reduced at high sulfur loading amounts. The BET surface areas of BTAP-1, BTAP-2, and BTAP-3 synthesized in the presence of 100 equiv of sulfur were found to be 750.9, 445.6, and 419.9 m2 g−1, respectively. We attribute the lower surface areas of BTAP-2 and BTAP-3 to the interpenetration of the framework structure because they both involve 3D organic linkers in their synthesis. We also carried out (Figure S18) Soxhlet extraction on BTAP-1 and analyzed its gas sorption properties. Notably, we did not observe any significant change in its surface area or CO2 uptake capacity after Soxhlet extraction, thus proving that our synthetic strategy can eliminate the post-washing steps for sorbent activation. cBTAP-1, cBTAP-2, and cBTAP-3 showed much lower surface areas of 328.9, 147.6, and 20.7 m2 g−1, respectively, along with pore volumes of 0.07, 0.04, and 0.003 cm3 g−1, respectively. We attribute the lower surface areas and pore volumes of cBTAPs to the elemental sulfur trapped within the pores because it blocks the accessibility of micropores to the gas molecules. The pore-size distributions of BTAPs were calculated from Ar adsorption isotherms via non-local density functional theory. BTAPs exhibited a very narrow pore-size distribution in the microporous region (Figure 3C and see also Figures S19 and S20). Specifically, a pore width below 0.7 nm is credited as ultramicroporosity,45Swaidan R. Al-Saeedi M. Ghanem B. Litwiller E. Pinnau I. Rational design of i