Chemistry

Fluorinated MOF platform for selective removing and sensing of SO 2 from flue gasoline and air

SO2 removing from flue gasoline

Just lately, our exploration of fluorinated MOF platforms, particularly KAUST-7 ([Ni(NbOF5)(pyrazine)2]·2H2O, obvious floor space 280 m2 g−1, estimated pore quantity zero.zero95 cm3 g−1) and KAUST-Eight ([Ni(AlF5(OH2)) (pyrazine)2]·2H2O, obvious floor space 258 m2 g−1, estimated pore quantity zero.102 cm3 g−1), resulted in lots of fascinating properties that embrace direct air seize44, propane-propylene separation45, gasoline/vapor dehydration46, and acid gasoline (H2S, CO2) removing47. Though each of the MOFs are isostructural, the delicate variations of their chemical compositions, pillared by the inorganic moiety (NbOF5)2− as a substitute of (AlF5(OH2))2−, allowed the modulation of their properties by various the content material and intermolecular spacing of pending fluoride teams realized through totally different tilts of pyrazine molecules (Fig. 1). Impressed by the superb stability and the modular nature of those MOF supplies44,45,46, we investigated their SO2 removing and sensing in artificial flue gasoline and air, respectively.

Fig. 1Fig. 1

Synthesis and crystal construction of KAUST-7 and KAUST-Eight. a [Ni(pyrazine)2]2+ sq. grid ensuing from the connection of Ni2+cations and pyrazine molecules. b Crystal construction of KAUST-Eight ensuing from the pillaring of [Ni(pyrazine)2]2+ sq. grid with (AlF5(OH2))2− pillar. c Crystal construction of KAUST-7 ensuing from the pillaring of [Ni(pyrazine)2]2+ sq. grid with (NbOF5)2− pillar

KAUST-7 was first investigated for SO2 sorption, which can also be among the finest physisorbent supplies for capturing CO2 at atmospheric focus44 with good water stability45. The steep, pure SO2 adsorption isotherm collected at 25 °C (Supplementary Determine 1) instructed a excessive affinity of the KAUST-7 framework for SO2. To achieve higher perception on the SO2 place throughout the framework and the governing interactions accountable for resultant sturdy affinity, we carried out in situ single crystal X-ray diffraction (SCXRD) examine on an appropriate single crystal of KAUST-7 at 296 Ok below four–5 bar of SO2-containing ambiance (7% SO2, 93% N2). The construction evaluation revealed that the resultant compound crystallized within the tetragonal house group P4/nbm with the unit cell parameters a = b = 9.9249(2) Å and c = 7.8387(2) Å (Supplementary Desk 1). An axial distortion of the octahedral (NbOF5)2− anion (Nb–O bond is shorter than Nb–Ftrans, 1.82(three) and a pair of.03(three) Å, respectively) was noticed and manifested in a dysfunction within the construction: the Nb atom was break up over two positions, whereas the oxygen and the trans-fluoride had been refined in the identical place with equal thermal parameters (Supplementary Determine 2a). The fluorinated pillars are turned by φ = ±Eight.5(2)° from the route towards the middle of the channel (Supplementary Determine 2b) relying on the central atom place. The (NbOF5)2− anion twist is stabilized by 4 pairs of C–H∙∙∙F hydrogen bonds with the adjoining pyrazine molecules: the H∙∙∙F distances and C–H∙∙∙F angles are 2.45 Å and 158° within the case of the Nb shifted towards that Ni(pyrazine)2 layer, and a pair of.52 Å and 170° when in the other way (Supplementary Determine 2c and second). The SO2 molecule was localized within the heart of the one-dimensional (1D) channel at a particular place, much like CO2 molecule within the KAUST-7 (CO2)construction44. As the positioning symmetry (−42m) is increased than the symmetry of the visitor molecule, the SO2 is disordered over 4 positions round axis −four with the overall occupancy refined to zero.424(four). Opposite to the linear CO2 molecule surrounded by 4 F-pillars, the electropositive sulfur atom of the triangular SO2 molecule interacts with solely two electronegative fluorine atoms of the adjoining (NbOF5)2− anions (S∙∙∙F distances of two.80(1) and a pair of.86(1) Å) proven in Fig. 2a–c. As well as, the SO2 molecule within the MOF is stabilized by 4 C–H∙∙∙O contacts with the hydrogen atoms of 4 totally different pyrazines (H∙∙∙O distance = 2.87 Å, angle = 120°).

Fig. 2Fig. 2

In situ visualization of SO2 in host frameworks. Intermolecular host–visitor interactions in SO2 loaded crystal buildings of a, c KAUST-7 and b, d KAUST-Eight

This statement is corroborated by density purposeful concept (DFT) calculations, which revealed excessive SO2/KAUST-7 interplay power of − 64.Eight kJ mol−1. This is because of a comparatively stronger interplay between the sulfur atom of SO2 and the F-pillars with attribute interatomic distances of two.9 Å (Supplementary Determine 3a) together with a cost switch between the visitor and this area of the MOF. Curiously, the SO2/KAUST-7 interplay power is as excessive as the worth calculated for CO2 (−54.5 kJ mol−1). This latter molecule occupies barely totally different websites than SO2, implying an interplay of the visitor molecule with each the F-pillars and the pyrazine teams (Supplementary Determine 3b). The so-predicted energetics and spatial distributions for each molecules as single elements counsel simultaneous seize of SO2 and CO2.

Cyclic adsorption column breakthrough exams with SO2/N2: 7/93 point out stability and good uptake (≈2.2 mmol g−1) of SO2 (Fig. 3a). Moreover, adsorption column breakthrough experiments with SO2/CO2/N2: four/four/92 gasoline combination confirmed simultaneous and equal retention time within the column for SO2 and CO2, demonstrating similar uptake of ≈1.1 mmol g−1 (Supplementary Determine four), which is in line with the simulated energetics tendencies. Upon lowering the SO2 focus with nitrogen within the vary generally noticed in flue gasoline (500 p.p.m.) (SO2/N2: zero.05/99.95 combination) and the instantly harmful to life or well being focus (100 p.p.m.), KAUST-7 nonetheless maintains a excessive SO2 uptake of about 1.four mmol g−1 at 500 p.p.m. SO2 focus (Fig. 3b). Curiously, adsorption column breakthrough experiments below mimicked flue gasoline circumstances with 500 p.p.m. of SO2 and 10% CO2 in N2 (SO2/CO2/N2: zero.05/10/89.95) resulted in equal and simultaneous retention time for each SO2 and CO2, resulting in uptakes of ≈zero.01 mmol g−1 and ≈2.2 mmol g−1, respectively (Fig. 3c). The direct co-adsorption experiments with totally different SO2 and CO2 compositions (four% SO2, four% CO2, steadiness N2 and 500 p.p.m. SO2, 10% CO2, steadiness N2) reveal that KAUST-7 reveals equal selectivity towards SO2 and CO2 (SO2/CO2 selectivity ≈1), which is fascinating for simultaneous CO2 and SO2 seize in flue gasoline (containing low SO2 concentrations). Nonetheless, temperature-programmed desorption (TPD) confirmed the presence of CO2 solely with an undetectable quantity of SO2 (Fig. 3d) within the adsorbed part as the quantity of SO2 adsorbed is negligible owing to its low focus. The efficiency of KAUST-7 utilizing humid (≈40% Relative Humidity (RH)) 250 p.p.m. SO2/steadiness N2 gasoline combination was additionally investigated (Supplementary Determine 5); it’s evident that SO2 has identical breakthrough time as that of water and ends in a decrease SO2 uptake below humid situation in contrast with the dry situation, which isn’t shocking contemplating the focus of water is round 50 instances increased than that of SO2. Along with beforehand reported wonderful water stability44,45, SO2 stability of the KAUST-7 was confirmed by powder X-ray diffraction (PXRD) comparability of the supplies earlier than and after dry and humid SO2 publicity (Supplementary Determine 6), CO2 breakthrough examine earlier than and after 7% SO2 breakthrough experiments (Supplementary Determine 7), and CO2 adsorption isotherm after the humid SO2 breakthrough experiment (Supplementary Determine Eight).

Fig. threeFig. 3

CO2/SO2 seize efficiency of KAUST-7. Adsorption column breakthrough experiments for KAUST-7 with a SO2/N2: 7/93 combination (10 cm3 min−1, circulation fee), b SO2/N2: zero.05/99.95 combination (40 cm3 min−1, circulation fee), and c SO2/CO2/N2: zero.05/10/89.95 combination (25 cm3 min−1, circulation fee). d Temperature-programmed desorption after preliminary adsorption within the column utilizing a mix akin to flue gasoline (SO2/CO2/N2: zero.05/10/89.95), suggesting an adsorbed part composition dominated by CO2

In our quest for a cloth with a extra favorable selectivity for SO2 removing from flue gasoline than CO2 (at 500 p.p.m. of SO2), we opted to research an analog of KAUST-7 with decrease CO2 interactions and doubtlessly increased SO2 interactions. Impressed by our outcomes with KAUST-Eight for dehydration of gases46 and simultaneous removing of H2S/CO247, we discovered it compelling to discover the structural SO2/CO2 co-adsorption property. KAUST-Eight reveals three pendant fluoride teams with barely increased F…F distance (three.613 Å) and one potential open metallic web site, whereas KAUST-7 accommodates 4 pendants fluoride with smaller F…F distance (three.210(Eight) Å) and no open metallic web site. Such minute variations in structural options led us to appreciate equal selectivity for CO2 and H2S over a variety of concentrations and temperatures47. Inspired by this construction–property tuning of H2S and CO2 adsorption affinity utilizing this MOF, we anticipated KAUST-Eight to be extra selective towards SO2 than CO2. Much like acquire higher perception of the KAUST-Eight system, SCXRD knowledge had been collected on the SO2-loaded KAUST-Eight crystal. The construction evaluation revealed the resultant compound crystallized within the tetragonal house group P4/mmm with the unit cell parameters a = b = 6.9996(2) Å and c = 7.7033(2) Å (Fig. 2b–d, Supplementary Desk 2). Opposite to KAUST-7, a rotational dysfunction of the (AlF5)2− is attributable to trigonal bipyramidal form of the anions positioned on the axis four amongst 4 1D channels within the construction. When one fluorine atom of a pillar is directed completely to the channel heart (φ = zero°), within the two different adjoining channels Al–F bonds of the identical anion are directed ± 24.1(four)° out the diagonal route, and within the fourth channel there aren’t any F-atoms from this pillar (Supplementary Determine 9a). Due to this fact, the only common cage aperture is fashioned by fluorine atoms of three adjoining (AlF5)2− anions solely. The pillar twist is stabilized by three pairs of C–H∙∙∙F hydrogen bonds between the pyrazine molecules and the fluorine atoms with H∙∙∙F distances and C–H∙∙∙F angles of two.38 Å and 178°, 2.48 Å and 134°, and a pair of.17 Å and 161°, respectively (Supplementary Determine 9b and 9c). Much like SO2-loaded KAUST-7 construction, the visitor SO2 molecule is disordered within the heart of the 1D channel (the particular place symmetry is four/mmm) of the KAUST-Eight MOF. The distances between electropositive sulfur atom of the SO2 molecule and fluorine atoms of adjoining F-pillars equals 2.86(2), three.11(2), or three.25(1) Å relying on the (AlF5)2− orientation (φ = zero° or ± 24.1(four)°, Supplementary Determine 9d) and every of them is lower than the sum of the S and F van der Waals radii (three.27 Å). Opposite to the KAUST-7 construction, the SO2 molecule participates in 4 C–H∙∙∙O contacts with solely two neighboring pyrazines (H∙∙∙O distance = 2.80 Å, angle = 105°). These observations had been supported by DFT calculations and it revealed a decreasing of the host/visitor interplay power of CO2 for KAUST-Eight in contrast with KAUST-7 (− 47.zero kJ mol−1 vs. − 54.5 kJ mol−1). Within the case of KAUST-Eight, the trigonal bipyramidal-like Al3+ setting doesn’t enable for additional optimum interactions between a carbon atom in CO2 and 4 F-pillars (Supplementary determine 3d), as seen in KAUST-7. DFT calculations had been additional carried out beginning with the SO2-loaded crystal construction mannequin elucidated from SCD knowledge. Curiously, the simulated preferential location of SO2 is barely pushed towards the pore wall, as in contrast with the situation in KAUST-7, with the formation of a twin interplay between its sulfur atoms and the 2 close by F-pillars, in addition to its oxygen atoms interacting with the pyrazine linker with shorter interacting distances (Supplementary Determine 3c). The ensuing geometry led to a slight enhancement of the SO2/host interplay power (− 73.9 kJ mol−1) and diminished affinity towards CO2, making KAUST-Eight a promising candidate to selectively adsorb SO2 over CO2.

Investigation of single element SO2 adsorption confirmed that KAUST-Eight additionally reveals steep adsorption isotherm at 25 °C (Supplementary Determine 10). The corresponding adsorption column breakthrough experiment with SO2/N2: 7/93 combination confirmed a better uptake of two.2 mmol g−1 (Fig. 4a). KAUST-Eight will be utterly regenerated by heating at 105 °C in a vacuum or inert gasoline setting (Supplementary Determine 11), confirming SO2 stability and recyclability. In the course of the adsorption column breakthrough experiments carried out with low SO2 (SO2/N2: zero.05/99.95) combination, KAUST-Eight nonetheless maintained a excessive uptake of SO2 (1.6 mmol g−1) (Fig. 4b, Supplementary Desk three). Subsequent TPD evaluation of the adsorbed part confirmed the adsorption of SO2 (Supplementary Determine 12) at p.p.m. degree. Supplementary Desk three compares SO2 uptake of various benchmark MOFs at low focus. Supplies with comparatively extra open construction akin to MFM-300(In, Al) have very excessive SO2 uptake at increased SO2 focus; nevertheless, they’ve decrease SO2 uptake at decrease concentrations near 500 p.p.m. (of curiosity for utility akin to SO2 seize from flue gasoline and SO2 sensing) in contrast with KAUST-7, KAUST-Eight, and a few SIF62− based mostly MOFs. Supplies akin to MFM-300(In, Al) with very excessive SO2 uptake at increased SO2 focus might have many essential purposes. MFM-300(In, Al) (very excessive uptake at increased focus), and KAUST-7 and KAUST-Eight (excessive uptake at very low focus) are complementary to one another and their makes use of depend on nature of the appliance. Adsorption column breakthrough experiments with artificial flue gasoline utilizing a SO2/CO2/N2: zero.05/10/89.95 combination confirmed that SO2 continues to be adsorbed for lengthy durations previous the CO2 breakthrough time (Fig. 4c). This means that the adsorbed CO2 is changed by SO2 from the gasoline combination, which is in line with a a lot increased estimated interplay power of SO2 over CO2. Subsequent TPD evaluation suggests an adsorbed part composition of 1.5 mmol g−1 for CO2 and zero.5 mmol g−1 for SO2, which is outstanding contemplating the big distinction in concentrations of CO2 and SO2 within the artificial flue gasoline (Fig. 4d). A selectivity of SO2/CO2 ≈ 66 obtained from the mix of breakthrough and TPD experiment exhibits that KAUST-Eight is a extremely environment friendly materials for SO2 removing at a p.p.m. degree and is promising for selectively eradicating SO2 from flue gasoline. The efficiency of KAUST-Eight utilizing humid (≈40% RH) 250 p.p.m. SO2/ steadiness N2 gasoline combination was additionally investigated (Supplementary Determine 13). For KAUST-Eight, additionally SO2 has nearly the identical breakthrough time as that of water; nevertheless, owing to its increased water adsorbing capability than KAUST-7, it will possibly adsorb extra SO2 in contrast with KAUST-7 below the identical circumstances. Uptake of SO2 by KAUST-Eight in humid situation is notable contemplating within the above experiment gasoline stream contained nearly 50 instances increased focus of water than SO2. Along with beforehand reported wonderful water stability46, SO2 stability of the KAUST-Eight was confirmed by PXRD comparability of the supplies earlier than and after dry and humid SO2 publicity (Supplementary Determine 14), CO2 breakthrough examine earlier than and after 7% SO2 breakthrough experiments (Supplementary Determine 15) and CO2 adsorption isotherm after the humid SO2 breakthrough experiment (Supplementary Determine 16).

Fig. fourFig. 4

CO2/SO2 seize efficiency of KAUST-Eight. Adsorption column breakthrough experiments for KAUST-Eight with a SO2/N2: 7/93 (10 cm3 min−1, circulation fee), b SO2/N2: zero.05/99.95 combination (40 cm3 min−1, circulation fee), and c SO2/CO2/N2: zero.05/10/89.95 (40 cm3 min−1, circulation fee). d TPD experiment suggests a substantial quantity of SO2 together with CO2 as adsorbed part after a breakthrough experiment with 500 p.p.m. SO2 within the presence of 10% CO2 and steadiness N2

Selective SO2 detection from air

From the adsorptive separation examine above, KAUST-Eight and KAUST-7 had been proven to exhibit tunable CO2/H2S selectivity, molecules which can be current in environments contaminated with SO2. To profit from the excellent properties of this platform, we discovered it compelling to discover the feasibility of depositing KAUST-Eight and KAUST-7 on a QCM electrode and unveiling their SO2 sensing properties within the presence and absence of humidity to imitate atmospheric circumstances. Weight-detectable sensor akin to QCM has been used on this examine to detect SO2 precisely when the disturbing presence of moisture is concerned. These suggest that QCM system will be an alternative choice to IDE48 when testing for a gasoline within the presence of humidity. A part of the rationale for this distinction is that IDE sensors are based mostly on the change in dielectric fixed (εH2O = 80 and εSO2 = 16) and in an effort to detect SO2 precisely within the presence of moisture the deposited supplies on the IDE-type electrode has to beat the excessive dielectric fixed for H2O.

The floor morphology of KAUST-Eight and KAUST-7 coated on QCM (see inset) was studied utilizing scanning electron microscopy. The skinny movies of each MOFs had been discovered to be compact and uniform. The densely packed MOFs crystals had been uniformly deposited on the QCM substrate with low intergranular voids and random orientation. As illustrated in Fig. 5, the coating of KAUST-7 led to cubic crystallites of ~150 nm, whereas for the KAUST-Eight movies the dimensions of the crystallites is considerably bigger at 30 μm. PXRD experiments had been carried out to substantiate the purity and crystallinity of the deposited MOFs (Supplementary Determine 17).

Fig. 5Fig. 5

Scanning electron microscopy of skinny movies. SEM micrographs at a excessive and low magnification of a KAUST-7 and b KAUST-Eight skinny movies coated on the gold electrode of QCM

The sensitivity (∆f/f)) of KAUST-Eight- and KAUST-7-coated QCM units had been measured for various concentrations of SO2, starting from zero to 500 p.p.m. in steadiness with nitrogen18. Uncoated QCM confirmed a negligible response to SO2. With the rise within the focus of SO2, each MOF-coated sensors responded with a nonlinear lower in sensitivity (Fig. 6) and (Supplementary Determine 18). The bottom detection restrict, outlined theoretically utilizing a largely accepted methodology by the sensing neighborhood42, was estimated to be about 100 p.p.b. (with noise drift within the resonance frequency of ± 1.5 Hz)49. By optimizing the system parameters, drift within the sensors resonance frequency, this detection restrict will be diminished to beneath zero.2 Hz50,51 and thereby enhancing the detection restrict to < 15 p.p.b. This discrepancy/hole between the theoretical and experimental detection limits (15 p.p.b. and 5 p.p.m.) was by no means mentioned beforehand within the open literature.

Fig. 6Fig. 6

Frequency shift comparisons. Frequency shift as a operate of the SO2 focus for the uncoated and KAUST-7- or KAUST-Eight-coated QCM sensors

After every publicity cycle, the system was in situ heated at 105 °C in ambient nitrogen for four h, permitting the reactivation of the evaluated MOF skinny movie for an additional sensing cycle.

Humidity is current in most environments, and so it is very important perceive a sensors response in its presence. Due to this fact, combined gasoline experiments had been carried out, exposing KAUST-7 and KAUST-Eight to SO2 in humid circumstances mimicking real-world circumstances for SO2 detection. Determine 7 exhibits the sensor sensitivity as a operate of SO2 focus in humid circumstances (60% RH) at RT for uncoated and coated KAUST-7 and KAUST-Eight QCMs. Uncoated QCM has a near-zero response to humidity and SO2. This corroborates that the sensing response to SO2 below humid circumstances is because of its affinity to KAUST-7 and KAUST-Eight movies.

Fig. 7Fig. 7

Sensor responses. Plots of sensors responses as a operate of SO2 focus in artificial air

The responses of the 2 sorts of sensors had been totally different. As seen in Fig. 7 and (Supplementary Determine 19), the resonance frequency of the QCMs initially decreased when the atmosphere was modified from dry to humid SO2 circumstances. Essentially the most outstanding distinction is the inversion within the sensor output as a result of introduction of SO2 at 60% RH however not in the identical method as in contrast with the dry SO2 case. Curiously, when uncovered to 25 p.p.m. of SO2 within the above-mentioned humid circumstances, the sensor resonance frequency for SO2 was diminished. Below humid circumstances, the sensitivity of the 2 MOFs barely diminished compared with dry circumstances. Nevertheless, KAUST-7 movies demonstrated a four-time increased sensitivity towards SO2 within the presence of humidity in contrast with KAUST-Eight.

To additional analyze the outcomes obtained, it’s needed to think about the particular options of the adsorption of SO2 and water on the floor of KAUST-7 and KAUST-Eight. As seen in Fig. 7, the presence of humidity (60% RH) didn’t considerably have an effect on the KAUST-7-based sensors response to the SO2 analyte. This can be as a result of affinity of SO2 molecules to switch a number of the adsorbed water molecules or/and coexist within the extremely confined pore system. Within the case of KAUST-Eight-based sensor, which is isomorphic to the KAUST-7, decrease sensitivity to SO2 within the presence of humidity was noticed. Though SO2 has the affinity to switch water molecules, the diminished sensitivity is attributed to the absence of accessible ultra-microporous morphology. The variety of SO2-adsorbing lively websites is diminished by the pre-adsorbed water, thereby limiting the accessible house within the pore system for adsorption. This statement is supported by the truth that the water molecules strongly work together with Al3+46 with increased host/visitor interplay power as in contrast with SO2. The TPD experiment outcomes (Supplementary Determine 20) present that within the case of KAUST-7, the SO2 can exchange the already adsorbed water molecules comparatively straightforward as in contrast with the KAUST-Eight, which has stronger water affinity. These outcomes are in settlement with the conclusion reported within the literature for the simultaneous seize of CO2 and H2O utilizing KAUST-846.

A very powerful parameters for a sensing system are its stability and reproducibility. These parameters had been investigated by cyclic publicity of the sensor to totally different SO2 concentrations after each 48 h at RT over a interval of 12 days (Fig. Eight). The three outcomes demonstrated the steadiness of the sensors uncovered to 50, 100, and 157 p.p.m. SO2 gasoline with no vital change within the resonant frequency over time.

Fig. EightFig. 8

Stability and reproducibility. Lengthy-term stability property of the a KAUST-7 and b KAUST-Eight sensors uncovered to 50, 100, and 157 p.p.m. SO2 gasoline


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