Revealing Molecular Mechanisms in Hierarchical Nanoporous Carbon via Nuclear Magnetic Resonance

Abstract

•Fabricated a hierarchical nanoporous carbon following Murray's law by microwave heating•Probed a molecular-scale study of adsorbate adsorption via nuclear magnetic resonance•Provided an efficient way to quantitate adsorption of adsorbate in hierarchical carbons•Demonstrated presence of Schroeder's paradox by differences in liquid and vapor phase Hierarchical nanoporous carbons combining pore sizes of different length scales are highly important for separation processes. However, critical questions remain regarding the hierarchical structure regulation and the molecular mechanisms of gaseous adsorbate uptake and interactions within the hierarchical nanoporous carbons. These materials present characterization challenges in that there are no experimental techniques that can elucidate the molecular mechanisms of the organic compounds and CO2 within the materials. We deployed multidimensional solid-state nuclear magnetic resonance (NMR) to generate maps of guest-framework interactions as a function of adsorbate concentrations and adsorption times. The NMR spectra provide insight toward the design of effective hierarchical pore structures. Our materials show a high volatile organic compounds/CO2 physisorption capacity, which reveals promising application to carbon-capture strategies to mitigate global warming. Hierarchical nanoporous carbons (HNC) have been proven to be an effective adsorbent for the adsorption of volatile organic compounds (VOCs) and CO2. However, questions remain regarding the hierarchical structure regulation, the adsorption mechanisms of adsorbate uptake, and interactions within the HNC. We synthesize HNC from wood, using a microwave-induced heating method incorporating K2CO3 activation. Our HNC exhibit Murray's law multiscale structures, prompting a molecular-scale study of adsorbate adsorption via nuclear magnetic resonance (NMR). NMR chemical shifts are consistent with ring-current effects from the adsorbent. Our NMR technique provides a convenient way to quantitate adsorption of adsorbate in HNC. VOC vapor adsorption results show NMR chemical-shift changes with time, suggesting initial adsorption into mesopores, followed by diffusion into micropores. Schroeder's paradox is demonstrated by differences in observed shifts for adsorbed liquid vis-à-vis vapor phase in these HNC. These HNC show high CO2 adsorption capacity, portending applications to carbon capture. Hierarchical nanoporous carbons (HNC) have been proven to be an effective adsorbent for the adsorption of volatile organic compounds (VOCs) and CO2. However, questions remain regarding the hierarchical structure regulation, the adsorption mechanisms of adsorbate uptake, and interactions within the HNC. We synthesize HNC from wood, using a microwave-induced heating method incorporating K2CO3 activation. Our HNC exhibit Murray's law multiscale structures, prompting a molecular-scale study of adsorbate adsorption via nuclear magnetic resonance (NMR). NMR chemical shifts are consistent with ring-current effects from the adsorbent. Our NMR technique provides a convenient way to quantitate adsorption of adsorbate in HNC. VOC vapor adsorption results show NMR chemical-shift changes with time, suggesting initial adsorption into mesopores, followed by diffusion into micropores. Schroeder's paradox is demonstrated by differences in observed shifts for adsorbed liquid vis-à-vis vapor phase in these HNC. These HNC show high CO2 adsorption capacity, portending applications to carbon capture. Volatile organic compounds (VOCs) are common air pollutants, contributing to the formation of ground-level ozone and carcinogens, known to be harmful to human health.1Brüggemann M. Hayeck N. George C. 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Here, we synthesize HNC derived from pinewood that follows Murray's law of interconnected micro- and mesopores, using a microwave-induced method incorporating K2CO3 activation (Figure 1A).21Adinata D. Wan Daud W.M.A. Aroua M.K. Preparation and characterization of activated carbon from palm shell by chemical activation with K2CO3.Bioresour. Technol. 2007; 98: 145-149Crossref PubMed Scopus (333) Google Scholar,22Dahal N. García S. Zhou J. Humphrey S.M. Beneficial effects of microwave-assisted heating versus conventional heating in noble metal nanoparticle synthesis.ACS Nano. 2012; 6: 9433-9446Crossref PubMed Scopus (114) Google Scholar Several characterization methods have been used to investigate HNC's adsorption performance, such as adsorption capacity measurements via breakthrough experiments and gas adsorption isotherms.23Wang H. Jahandar Lashaki M. Fayaz M. Hashisho Z. Philips J.H. Anderson J.E. Nichols M. 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We compare solid-state NMR studies of three typical liquid and gaseous VOCs as adsorbates, namely acetone, toluene, and n-hexane, as well as CO2. These molecules were chosen because they are representative of environmental pollutants, with each molecule possessing different dimensions and polarity. We observe that the VOCs' adsorption from the liquid phase reflects uptake into the mesopores of HNC. The resulting chemical shifts show the effects of polyaromatic ring currents from the carbon adsorbent in all adsorbates. Integrating the NMR signals from liquid-adsorbed VOCs yields VOC mass uptakes that compare favorably with those determined by adsorption experiments. In an apparent manifestation of Schroeder's paradox, we found a difference in the observed NMR chemical shifts of VOCs obtained by gas-phase exposure and those obtained by liquid-phase exposure.32Schroeder P. Uber Erstarrungs- und Quellungserscheinungen von Gela-tine.Z. Phys. Chem. 1903; 45: 57Google Scholar Finally, CO2 physisorbed into HNC with surprising capacity. Together, these findings offer detailed insights into the interactions between liquid/gaseous adsorbates and HNC via NMR. The argon adsorption-desorption isotherm at 77 K of HNC is demonstrated in Figure 1B and Table S1. After microwave heating and K2CO3 activation, we observe that at relative pressure (P/P0) below 0.05, the argon uptake increases sharply with the increase in relative pressure, proving the existence of microporous pore-dominated structure. These adsorption isotherms are close to type-I to -IV hybrid shape defined by the BDDT (Brunauer-Deming-Deming-Teller) classification.33Román S. Ledesma B. Sabio E. González J.F. González C.M. Production of cost-effective mesoporous materials from prawn shell hydrocarbonization.Nanoscale Res. Lett. 2016; 11: 435Crossref PubMed Scopus (5) Google Scholar After the K2CO3/microwave treatment, the specific surface area by BET (Brunauer-Emmett-Teller) measurement and total pore volume of our HNC were remarkably improved (Table S1). The micropore surface area significantly rising from 30 to 1,857 m2/g, with the micropore volume of biochar increasing from 0.016 to 0.741 cc/g, indicating that the micropores developed in the mesopore walls while some mesoporous channels collapsed. The development of porosity is associated with the reaction of K2CO3 and C, which leads to the formation of K2O, K, CO, and CO2, whereby the high microwave temperature is assumed to accelerate the activation reaction.34Sun Y. Wei J. Wang Y.S. Yang G. Zhang J.P. Production of activated carbon by K2CO3 activation treatment of cornstalk lignin and its performance in removing phenol and subsequent bioregeneration.Environ. Technol. 2010; 31: 53-61Crossref PubMed Scopus (34) Google Scholar The potassium species formed during the activation step diffuse into the internal structure of the biochar matrix, which widens existing pores and creates new ones. Consequently, the presence of K2CO3 promotes the formation of dominant micropores, a small fraction of mesopores, a much larger surface area, and a larger pore volume. The measured surface areas are 3.4 times higher than those of carbons activated by K2CO3 via thermal heating of samples derived from tobacco stem.35Chen R. Li L. Liu Z. Liu M. Wang C. Li H. Ma W. Wang S. Preparation and characterization of activated carbons from tobacco stem by chemical activation.J. Air Waste Manag. Assoc. 2017; 67: 713-724Crossref PubMed Scopus (67) Google Scholar The pore-size distribution curves plotted in Figure 1C are derived from argon adsorption measurements using the Horváth-Kawazoe method,36Horvath G. Kawazoe K. Method for the calculation of effective pore size distribution in molecular sieve carbon.J. Chem. Eng. Jpn. 1983; 16: 470Crossref Scopus (1656) Google Scholar indicating that HNC manifest a wide pore-size distribution covering micropores (0.65–2 nm) and mesopores (2–50 nm). The overwhelming majority of pore sizes include micropores (<2 nm), supermicropores (0.7–2 nm), and even ultramicropores (<0.7 nm). Overall, these isotherms (Figure 1B) reveal a micropore distribution with a mean size of 0.8 nm (Dmicro). The K2CO3/microwave-activated HNC appear to follow Murray's law with three layers of the structure at the micro-, meso-, and macropore level, as well as abundant interconnected pores. Such a hierarchical pore structure aids in the diffusion of adsorbates and enhances the adsorption and desorption performance of the HNC. The transmission electron microscopy (TEM) image (Figure 1D) also shows a disordered hierarchical nanoporous structure containing mesopores. The large quantities of white spots between the disordered carbon layers suggest that abundant mesopores exist in the HNC from pinewood. Using an auto-threshold function (Figure S1), we transformed the real-space images to binary images to clearly observe wormhole-like pores, as well as the interconnectivity of micropores and mesopores. A representative scanning electron microscopy (SEM) image of HNC is depicted in Figure 1E. The perfect honeycomb structure and prismatic rectangular cells from the natural pinewood are maintained after chemical activation. The dimensions of the cells were ca. 20 μm, and the wall thickness was ca. 2 μm. It is important to note that there was no evidence of rupture of the pinewood pore walls, indicating that the wall material had a sufficient tensile strength to impregnate K2CO3 and remove dissolved K2CO3. Upon K2CO3/microwave activation, the pores were etched and developed during the reaction of K with carbon. The result is similar to previously published SEM images of carbonized and activated virgin cork.37Carrott P.J.M. Carrott M.M.L.R. Mourão P.A.M. Lima R.P. Preparation of activated carbons from cork by physical activation in carbon dioxide.Adsorpt. Sci. Technol. 2004; 21: 669-681Crossref Scopus (36) Google Scholar,38Foo K.Y. Hameed B.H. Mesoporous activated carbon from wood sawdust by K2CO3 activation using microwave heating.Bioresour. Technol. 2012; 111: 425-432Crossref PubMed Scopus (157) Google Scholar The present K2CO3/microwave activation of pinewood yields an interconnected HNC following Murray's law via a facile, inexpensive, and environmentally friendly process. As shown in Figure 2A, proton (1H) spin-echo magic-angle spinning (MAS) NMR was performed to obtain spectra of HNC loaded with acetone for the range 23 wt % to 141 wt %. The initial adsorption gives rise to a broad signal at −2.5 ppm. This signal was shifted from the signal for liquid acetone (2.2 ppm) to −2.5 ppm by 4.7 ppm and was assigned to “in-pore” acetone (Figure 2B). This shift to lower frequency results from ring currents emanating from the aromatic rings of the graphene planes in the pore walls.39Forse A.C. Griffin J.M. Merlet C. Carretero-Gonzalez J. Raji A.R.O. Trease N.M. Grey C.P. Direct observation of ion dynamics in supercapacitor electrodes using in situ diffusion NMR spectroscopy.Nat. Energy. 2017; 2: 16216Crossref Scopus (182) Google Scholar This has previously been confirmed experimentally on micro- and mesoporous carbon.39Forse A.C. Griffin J.M. Merlet C. Carretero-Gonzalez J. Raji A.R.O. Trease N.M. Grey C.P. Direct observation of ion dynamics in supercapacitor electrodes using in situ diffusion NMR spectroscopy.Nat. Energy. 2017; 2: 16216Crossref Scopus (182) Google Scholar The ring-current effect is strongly dependent on the distance between the NMR-observed nucleus and the center of the aromatic ring.40Harris R.K. Thompson T.V. Norman P.R. Pottage C. Adsorption competition onto activated carbon, studied by magic-angle spinning NMR.J. Chem. Soc. Faraday Trans. 1996; 92: 2615-2618Crossref Scopus (34) Google Scholar,41Harris R.K. Thompson T.V. Norman P.R. Pottage C. Phosphorus-31 NMR studies of adsorption onto activated carbon.Carbon N. Y. 1999; 37: 1425-1430Crossref Scopus (38) Google Scholar Upon increasing the acetone loading, the broad line grows in intensity until it reaches a plateau at higher loadings (63 wt %), suggesting pore-filling and saturation. As the loading level increases to 82 wt %, a narrower peak appears at 1.8 ppm, a shift close to that of the methyl protons in neat acetone. This peak is associated with liquid acetone external to the HNC pores assigned to “ex-pore” acetone (Figure 2B). The chemical-shift deviation between the in-pore resonance and neat acetone is quantified by Δδ=δin-pore−δneat, and has a value of −4.5 ppm (corresponding to the two peaks in Figure 2A at 141%). To quantitate the exchange between the in-pore and ex-pore environments,41Harris R.K. Thompson T.V. Norman P.R. Pottage C. Phosphorus-31 NMR studies of adsorption onto activated carbon.Carbon N. Y. 1999; 37: 1425-1430Crossref Scopus (38) Google Scholar we conducted two-dimensional 1H homonuclear exchange experiments42Devautour-vinot S. Maurin G. Serre C. Horcajada P. Cunha D.P. Guillerm V. Costa E.S. Taulelle F. Martineau C. Structure and dynamics of the functionalized MOF type UiO-66(Zr): NMR and dielectric relaxation spectroscopies coupled with DFT calculations.Chem. Mater. 2012; 24: 2168-2177Crossref Scopus (137) Google Scholar at various mixing times (0.001, 0.1, and 0.25 s). The results are shown in Figures 2C–2E. As expected, the cross peaks appearing at mixing times in excess of 0.1 s confirm that acetone exhibits slow exchange between “in-pore” and “ex-pore” environments. As shown in Figure S2, we obtained the 1H spin-echo MAS spectra of toluene and n-hexane adsorbed onto HNC as a function of loading. The qualitative features of these spectra are analogous to those observed from acetone; the methyl and aromatic proton resonances from toluene reveal in-pore and ex-pore environments, as do the CH3 and CH2 resonances from n-hexane. At a mass ratio of 62 wt %, the pores of HNC become “full” and the toluene/n-hexane loading reaches saturation. As the loading increases, the narrow lines emanating from ex-pore features become prominent. Therefore, the four peaks at low and high frequencies are assigned to in-pore and ex-pore toluene, respectively (Figure S2A). For these adsorbates, we calculate Δδ to be −4.2 ppm (acetone), −4.2 ppm (toluene), and −4.4 ppm (both CH3 and CH2 resonances from n-hexane) (Figure S2B). The chemical-shift deviations Δδ are very similar for the three adsorbates, indicating that the underlying mechanism is mainly due to ring-current shifts associated with the aromatic rings in the HNC. Quantitative NMR spectroscopy can be used to provide an alternative method to provide adsorption uptakes.43Anderson R.J. McNicholas T.P. Kleinhammes A. Wang A. Liu J. Wu Y. NMR Methods for characterizing the pore structures and hydrogen storage properties of microporous carbons.J. Am. Chem. Soc. 2010; 132: 8618-8626Crossref PubMed Scopus (44) Google Scholar The adsorption isotherms of acetone/toluene/n-hexane for our HNC were acquired using a sorption analyzer at 298 K with N2 as the carrier gas, shown in Figure 3C. The proton (1H) single-pulse NMR spectra of liquid acetone, toluene, and n-hexane spectra adsorbed at various loadings were deconvoluted and integrated using dmfit software (Figure S3),44Forse A.C. Milner P.J. Lee J.H. Redfearn H.N. Oktawiec J. Siegelman R.L. Martell J.D. Dinakar L.B. Porter-Zasada Gonzalez M.I. et al.Elucidating CO2 chemisorption in diamine-appended metal-organic frameworks.J. Am. Chem. Soc. 2018; 140: 18016-18031Crossref PubMed Scopus (49) Google Scholar thus providing the amount (in mmol) of adsorbed in-pore VOC per gram of HNC (Figure 3C). For comparison of NMR uptake with gas-sorption studies, the abscissa is given as mmol of VOC adsorbed. For the NMR data, this is the amount of liquid VOC placed into the sample; for the isotherm data, this is determined by converting the partial pressure (P/P0) to mmol via the Peng-Robinson equation of state.45Valiollahi S. Kavianpour B. Raeissi S. Moshfeghian M. A new Peng-Robinson modification to enhance dew point estimations of natural gases.J. Nat. Gas Sci. Eng. 2016; 34: 1137-1147Crossref Scopus (16) Google Scholar The result shows that the ultimate uptakes of adsorbates, as measured from adsorption isotherms, are in good agreement with those determined from the NMR spectra. The reason for the lag at low uptakes is unclear and requires further study. The adsorption capacity of the three compounds within the HNC was found to be in the order of toluene > acetone > n-hexane. This order of adsorption capacity clearly demonstrates the effects of the molecular dimension and polarity of these three VOCs.41Harris R.K. Thompson T.V. Norman P.R. Pottage C. Phosphorus-31 NMR studies of adsorption onto activated carbon.Carbon N. Y. 1999; 37: 1425-1430Crossref Scopus (38) Google Scholar,46Ania C.O. Cabal B. Parra J.B. Arenillas A. Arias B. Pis J.J. Naphthalene adsorption on activated carbons using solvents of different polarity.Adsorption. 2008; 14: 343-355Crossref Scopus (25) Google Scholar By way of comparison with the HNC synthesized in this work, VOC isotherms and adsorbate capacity on commercial activated carbon were also performed. Our HNC exhibit higher adsorption capacities for all VOCs compared with commercial activated carbon (Figure S4). The saturated toluene, acetone, and n-hexane adsorption capacities reached 11.9, 8.8, and 7.2 mmol/g, respectively, which are 1.5-, 1.6-, and 1.9-fold higher than those of commercial activated carbon, respectively. Consequently, our HNC demonstrate high adsorptive performance of VOCs, providing a cost-effective alternative to commercial activated carbon in many air-quality remediation and treatment applications. In many practical applications, VOC adsorption occurs from the vapor phase rather than the liquid phase, whereby the time required for adsorbents to equilibrate with dosed gas has significance for process swing designs. Therefore, we further examined vapor VOCs loaded onto HNC as a function of adsorption time. Figure 4B depicts the 1H NMR spectra of acetone vapor adsorbed onto HNC as a function of adsorption time at room temperature. After exposure to acetone vapor (Figure 4B) for 1 min, we observed a broad peak at −0.4 ppm (labeled “A”). With increasing adsorption time to 91 min, the intensity of peak A gradually decreases, ultimately disappearing after 150 min. A second upfield peak (∼2.5 ppm, peak B) increases to a maximum intensity, suggesting a saturation of the micropores. Figure 4D confirms that the total proton NMR signal, which is integrated from the “within-pore” environments (both peak A and peak B), changes as a function of adsorption time, where, as expected, the adsorption uptake increases with time and reaches saturation at ∼91 min. Compared with the liquid-phase loaded HNC spectrum (Figure 4A), the ex-p