The synthesized SOMPs were denoted as OMP-SO3H-x, where x stands for curing temperature. The porous structure of the synthesized SOMPs was characterized by XRD, N2 sorption and electron microscopy, as given in Figure 1. Figure 1a shows small-angle XRD patterns of various SOMPs, which display three well-resolved diffraction peaks at around 0.88°, 1.52° and 1.76°. These peaks can be indexed to (100), (110) and (200) reflections, suggesting the formation of ordered mesostructure with two-dimensional (2-D) hexagonal symmetry (p6mm) in these samples. However, wide-angle XRD patterns of the SOMPs display only two weak peaks at 17.8° and 42.5° (see Figure S1), suggesting their non-crystalline networks. Figure 1b shows N2 adsorption-desorption isotherms at −196°C of the SOMPs, which display type IV profiles with H1 hysteresis loops. Such N2 sorption isotherms indicate the presence of ordered hexagonal mesopores with abundant micropores in the SOMPs. The ordered hexagonal mesopores are undoubtedly formed by the oriented assembly of polymer precursors with F127 micelles.34,35 The micropores are originated from the embedding of some F127 chains in the mesopore walls,36,37 and the removal of which leaves the voids with sub-nanometer size. Figure 1c shows pore size distributions of the SOMPs, which display trimodal profiles with the pore size of 0.63~0.73, 1.17~1.35 and 6.34~13.60 nm. The textural parameters of the SOMPs calculated from the N2 adsorption data are summarized in Table 1. It can be seen that the surface areas, micropore volumes and total pore volumes of the SOMPs firstly increase but then decrease with the increase of curing temperatures. Thus, the OMP-SO3H-180 exhibits the highest porosity among the SOMPs. However, the pore size of the SOMPs gradually increase with curing temperatures. Thus, the OMP-SO3H-220 exhibits the largest pore size among these SOMPs.
Table 1
Textural parameters and sulfur contents of the SOMPs.
SOMP | SBETa) (m2/g) | Vmb) (cm3/g) | Vt c) (cm3/g) | Dpd) (nm) | Sulfur content e) (wt.%) |
OMP-SO3H-140 | 545 | 0.12 | 0.44 | 0.73, 1.26, 6.34 | 14.36 |
OMP-SO3H-160 | 620 | 0.13 | 0.77 | 0.73, 1.34, 8.69 | 11.41 |
OMP-SO3H-180 | 697 | 0.15 | 0.85 | 0.68, 1.35, 8.63 | 12.97 |
OMP-SO3H-200 | 614 | 0.14 | 0.66 | 0.73, 1.28, 13.12 | 9.38 |
OMP-SO3H-220 | 428 | 0.09 | 0.63 | 0.63, 1.17, 13.60 | 8.96 |
a) Surface area calculated by BET equation; b) micropore volume calculated by DFT method; c) total pore volume calculated by DFT method; d) pore size extrapolated from pore size distribution; e)sulfur content determined by elemental analysis. |
Figure 1d and Figure S2 show SEM images of the OMP-SO3H-180, exhibiting ample cavities with well-ordered features, which are similar to those in other SOMPs (Figures S3~S4). Meanwhile, TEM images of the sample show highly ordered mesopores viewed from (110) direction associated with 2-D hexagonal symmetry (p6mm) and open cylinder shape can be clearly observed in the OMP-SO3H-180 (Figure 1e). In addition to the ordered mesopores, starry worm-like holes with sub-nanometer size can be clearly observed in the sample, suggesting they were also rich in microporosity. Figures 1f~i show STEM image with elemental mapping of the OMP-SO3H-180. It can be seen that C, O and S atoms are uniformly distributed in the SOMPs. The S atoms are originated from the grafted −SO3H group in the SOMPs. According to the elemental analysis, the sulfur contents of the SOMPs are 8.96~14.36 wt.% (Table 1 and S1), corresponding to −SO3H densities of 2.80~4.48 mmol/g. Such concentration of the −SO3H group supports the enhanced sulfonation degree of the SOMPs, much higher than those of the sulfonated materials reported yet. It is also noted that the sulfur contents of the SOMPs decrease with the increase of curing temperatures. This might be a result of the more rigid structure of the SOMPs as the curing temperatures increase, so that the −SO3H group became more difficult to be grafted onto the frameworks. Meanwhile, the H+ capacity of the SOMPs were up to 4.4 mmol/g, which is a record value among the sulfonated porous solid acids yet.
The chemical structures of the SOMPs were characterized by solid-state NMR, XPS spectra, as shown in Figure 2. In comparison with non-sulfonated OMP, the FT-IR spectra of SOMPs displayed additional four characteristic peaks at around 1016, 1040, 1125 and 1171 cm−1, which are ascribed to the signals of −SO3H groups38,39 (Figure S5 and Figure S6). Figure 2a shows 13C solid-state NMR spectra of the SOMPs, displaying a series of peaks associated with the carbon atoms in different environments, which include the −CH2− bridges between benzene rings (20.1 and 40.1 ppm), aromatic carbon atoms without being attached by heteroatoms (128 ppm), aromatic carbon atoms attached by −OH and −O−SO3H groups (150 and 141 ppm), and terminal –CH3 and −CHO groups attached to benzene rings.28 To study the intrinsic acidity of the SOMPs, reliable and classical 31P solid-state NMR combining TMPO probe molecule were carried out (see Figure 2b). It can be seen a pronounced and symmetric peak at ~79 ppm, which were ascribed to TMPO adsorbed on the strong acidic −SO3H site with uniform distribution.44 High acid density combined with strong and uniform acid strength play crucial roles for their applications in selective capture of alkaline gas such as NH3. Figures 2c~d, Figure S7 and Figure S8 show XPS spectra of the SOMPs. The C1s spectra can be deconvoluted into three peaks at 284.4, 285.3 and 286.1 eV, ascribing to C−C/C−H bonds, C−S bond and C−O bond respectively.40,41 The O1s spectra can be deconvoluted into two peaks at 531.4 and 532.9 eV, ascribing to O−S bond and O−C/O−H bonds.42 The S2p spectra can be deconvoluted into two peaks at 166.5 and 167.8 eV, ascribing to S−C bond and S−O bond.26,43 All these results confirm successful grafting of −SO3H group onto OMPs' framework.
As a result, given the high acid density with suitable acid strength, artistic microporosity tandem ordered mesoporosity, the SOMPs are expected to have great potential application in the separation of NH3. The performance of the SOMPs for NH3 selective adsorption was firstly examined, as depicted in Figure 3. Figures 3a~b show the NH3 sorption isotherms of the SOMPs at 25 and 50°C, which give obviously non-ideal profiles, indicating the strong interaction of the SOMPs with NH3. However, the NH3 adsorption isotherm of non-sulfonated sample OMP-180 exhibits an almost ideal profile, with much lower NH3 capacities than those of the other SOMPs (see Figure S9). Therefore, the strong interaction of SOMPs with NH3 is originated from the acid-base affinity between grafted −SO3H and NH3 molecules, while −OH group with weak acidity in the SOMPs play relatively low role for the adsorption of NH3. It is noted that NH3 capacities of the SOMPs firstly increase but then decrease with curing temperatures. Thus, the OMP-SO3H-180 exhibits the highest NH3 capacities among the SOMPs. This is well consistent with the variation of porosity for the SOMPs with curing temperatures. Additionally, the NH3 capacities of the SOMPs decrease with the increase of temperatures, suggesting the exothermic nature of NH3 adsorption process. The isosteric heats of NH3 adsorption by OMP-SO3H-180 and OMP-SO3H-200 were calculated by the Clausius-Clapeyron equation, distributing in the range of −65 ~ −45 kJ/mol (Figure S10).12,19 The magnitude of NH3 adsorption heats agree well with the non-ideal adsorption of NH3 by the SOMPs. Overall, the NH3 capacities of the SOMPs are very high, with the value of 15.09 mmol/g for the OMP-SO3H-180 at 25°C and 1 bar, which is quite competitive in relative to those of other solid adsorbents reported in the literature (Table S2). Owing to the non-ideal adsorption of NH3 by SOMPs, the NH3 capacities of them at low pressures are even more significant. For example, the OMP-SO3H-180 can adsorb 6.6 mmol/g of NH3 at 25°C and 0.05 bar, being superior to most other solid adsorbents reported in the literature (Table S2). This result is particularly important, because the contents of NH3 in industrial gas are normally low, and the performance of solid adsorbents for adsorption of NH3 at low pressures is more concerned.45,46
For the separation of NH3, the most important task is to selectively adsorb NH3 from N2 and H2 mixture. Previous works have illustrated that the adsorption of inert gases on mesopore walls is much weaker than that on micropore walls due to the reduced capillary condensation effect.28,47 However, the adsorption of acidic gases over mesoporous adsorbents can be well maintained by grafting basic sites onto the mesopore walls. Thus, highly efficient and selective adsorption of acidic gases from inert gases can be realized by constructing abundant mesopores and grafting suitable basic sties in solid adsorbents. It is deduced that similar principles can also be applied for selective separation of NH3 from N2 and H2. Since the SOMPs have abundant mesopores and micropores as well as high −SO3H density, it is anticipated that the adsorption of inert gases N2 and H2 by them is much lower than that of basic gas NH3. As expected, the N2 and H2 adsorption isotherms of the SOMPs at 25°C (Figure S11~S12) display almost ideal profiles with the N2 and H2 capacities being about two magnitudes lower than the NH3 capacities. Figures 3c~d shows the selectivities of the SOMPs for NH3/N2 and NH3/H2 at 25°C and 1 bar, obtaining from ideal adsorption solution theory.48,49 It can be seen that when the molar fraction of NH3 in gas phase is 0.05, the IAST selectivities of NH3/N2 and NH3/H2 for the OMP-SO3H-180 reach 453 and 1768. Figures 3e~f show the breakthrough curves for NH3/N2/H2 mixed gas over the OMP-SO3H-180 at 25 and 50°C. It can be seen that N2 and H2 instantly penetrate the fixed bed, indicating negligible adsorption of the two gases. Interestingly, the penetration of NH3 starts at ~138 min/g and completes at ~263 min/g at 25°C. Similarly, the penetration of NH3 still starts at ~124 min/g and completes at ~255 min/g at 50°C. The accelerated penetration of NH3 is ascribed to the decreased NH3 capacities as the temperatures increase. By integrating the breakthrough curves, it is estimated that the OMP-SO3H-180 can adsorb 6.16 and 5.53 mmol/g of NH3 from NH3/N2/H2 (5/23.75/71.25 vol.) mixture at 25 and 50°C, respectively. The values agree well with the NH3 capacities at 0.05 bar calculated from the NH3 isotherms (6.28 and 5.03 mmol/g). More importantly, the above penetrated parameters of OMP-SO3H-180 are very well maintained even after being treated saturated water vapor at 50°C for 2 h, suggesting the excellent water resistance of SOMPs for NH3 adsorption (Figure S13). These results confirm that the SOMPs can be used as highly efficient and selective adsorbents for NH3 from N2 and H2, much better than variously reported adsorbents in the field.26,50,51
The adsorption/desorption behavior of NH3 on the SOMPs was then examined, as shown in Figure 4. Figure 4a shows recycling of the OMP-SO3H-180 for NH3 adsorption at 25°C and 1.0 bar, while desorption was performed at 150°C and 0.001 bar. Noting that the SOMPs are stable enough at such desorption temperature (Figure S14). It can be seen that the NH3 capacity of the OMP-SO3H-180 remains almost unchanged after 30th cycling, giving the attenuation ratio only 1.6%. In contrast, the attenuation ratio on NH3 capacity over MOFs adsorbents (e.g. Ce-MOF) was as high as 39.1% under identical condition (Figure S15). Therefore, the adsorption of NH3 on the SOMPs is totally reversible and quite durable. Figure 4b shows in-situ DRIFTS spectra of the OMP-SO3H-180 with 0.05 vol.% of NH3 purge at 25°C. As exposure time prolongs, four characteristic peaks associated with chemically adsorbed NH3 gradually appear with increased intensities. For instance, the characteristic bands (1720, 1539, and 1265 cm−1) correspond to the asymmetric, and symmetric bending vibrations of NH4+, and the band at 1450 cm−1 is assigned to NH3 interacts with -SO3H group.52,53 Figure 4c shows NH3-TPD MS curve of the OMP-SO3H-180, exhibiting two NH3 desorption peaks centered at around 113°C and 188°C. Considering the uniform acid strength of the sample determined by 31P solid-state NMR spectra, two desorption peaks should be resulted from the –SO3H located in different microenvironment of micropores and ordered mesopores. Similar phenomenon can also be confirmed by controlling NH3 adsorption behavior, as given in Figure 4d. Notably, the fresh OMP-SO3H-180 has two adsorption steps distributed at 0~0.1 bar and 0.1~1.0 bar, which show quadratic function and linear function characteristics respectively. The former was resulted from NH3 interacts with –SO3H in micropores due to capillary condensation effect, the latter was resulted from NH3 interacts with –SO3H in mesopores. After saturated adsorption, controllable desorption of NH3 in mesopores was performed by treating the sample at suitable temperature (see experimental section for the mesoporous adsorption measurements). The NH3 isotherm of the degassed sample shows only linear function characteristic, which is consistent with the NH3 isotherm in mesoporous adsorption part of the fresh OMP-SO3H-180. The result confirms novel sequential pore-space diffusion rule of NH3 in interconnected micropores and ordered mesopores, the diagrammatic sketch was depicted in Figure S16. As a result, the SOMPs show a record integrated performance in selective adsorption and separation of NH3, far better than the industrial H-ZSM-5 zeolite with pure micropores and sulfonated mesopores silica of SBA-15-SO3H (Figure S17, and Table S3).
Based on the above statement, the sequential pore-space diffusion in interconnected micropores and ordered mesopores plays a key role for their excellent integrated performance in NH3 selective capture and separation. To better understand the inherent diffusion rule of NH3, molecular dynamics simulations were performed as a complement to the experimental approaches, which can reflect the adsorption and diffusion dynamical process of NH3 in the SOMPs on theoretical level. Figure S18 shows the simulated frameworks of the OMP-SO3H-160 and OMP-SO3H-180 possess both microporous and mesoporous channels. The two best SOMPs with excellent NH3 adsorption performance. Noting that OMP-SO3H-180 owns more micropores while OMP-SO3H-160 owns more mesopores because of their density difference. Then the adsorption and diffusion of NH3 molecules in the two SOMPs were studied. Figure 5 shows the snapshot of NH3 molecules adsorption and diffusion trajectory of one representative NH3 molecule in the OMP-SO3H-180 at low, medium and high loadings. At low loading, due to the strong acid-base interaction between −SO3H group and NH3 molecule as well as the confinement effect of micropores, most of the NH3 molecules are adsorbed strongly (Figure 5a) and migrated slowly in the microporous channels (Figure 5d). At medium loading, some NH3 molecules transport from microporous channels to mesoporous channels, and part of NH3 molecules located in mesopores due to the micropores have already been completely occupied (Figure 5b and 5e). While at high loading, more NH3 molecules permeate into the mesoporous channels (Figure 5c and 5f). Meanwhile, the diffusion behavior could also be quantitatively analyzed by diffusion radius,54 and the rapid movement is generally accompanied by large diffusion radius (Rd). At low loading, as most of NH3 molecules hop between multiple −SO3H groups in microporous channel, Rd is small as well as diffusion is slow (Figure 5g). Interestingly, with the increase of loading, Rd is dramatically increased, and more NH3 molecules could diffuse from microporous to mesoporous channels (Figure 5h and 5i). The same adsorption and diffusion behavior of NH3 molecules can also be observed in OMP-SO3H-160 (Figure S19). Overall, sequential pore-space adsorption and diffusion mechanism of NH3 molecules inside microporous and mesoporous channels of the SOMPs has been illustrated, in which micropores preferential adsorption together with ordered mesopore subsequent storage of NH3, thus maximally utilizing the volume advantage of micro- and mesoporosity for NH3 selective capture. Overall, the OMP-SO3H-x show much better performance than industrial zeolites applied in NH3 selective adsorption and separation, offering great opportunities for their wide applications in the field. Although the NH3 capacity of the OMP-SO3H-x was a little lower than few special MOFs adsorbents, but their low cost, easy preparation, and good corrosion resistance are very favorable for their scalable usage in the industry.