MIL-100Cr with open Cr sites for a record N2O capture

Jiangfeng Yang ab, Bingjie Du a, Jiaqi Liu a, Rajamani Krishna c, Feifei Zhang a, Wei Zhou d, Yong Wang b, Jinping Li *ab and Banglin Chen *e
aResearch Institute of Special Chemicals, College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China. E-mail: jpli211@hotmail.com
bShanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Taiyuan 030024, Shanxi, P. R. China
cVan ‘t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
dNIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, USA
eDepartment of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, USA. E-mail: banglin.chen@utsa.edu

Received 24th September 2018 , Accepted 12th November 2018

First published on 12th November 2018


Nitrous oxide (N2O) is considered as the third most important greenhouse gas after carbon dioxide and methane and needs to be removed from air. Herein, we reported the metal–organic framework MIL-100Cr with open Cr sites for record N2O capture capacities of 5.78 mmol g−1 at 298 K and 8.25 mmol g−1 at 273 K, respectively. DFT calculations showed that the static binding energy of N2O on the open-Cr site is notably higher than that of N2, 72.5 kJ mol−1vs. 51.6 kJ mol−1, which enforces MIL-100Cr to exhibit extremely high N2O/N2 ideal adsorbed solution theory (IAST) gas separation selectivity up to 1000.


Nitrous oxide (N2O) is the third most potent greenhouse gas after CO2 and CH4, and is involved in the depletion of stratospheric ozone with a 300-fold greater warming potential than CO2.1–4 Over the past few decades, N2O has drawn the attention of researchers in various fields because of its involvement in some key biological processes such as denitrification, usage as the anesthetic medicine and the potent oxidizer for the formation of transition-metal oxocations.5–7 It has been reported that about 40% of total N2O emissions come from human activities, which arise from agriculture, transportation and industrial activities.8–11 Along with the understanding of N2O emissions, the degradation of N2O into N2 and O2 is one of the recent research hotspots.12–16 However, the catalytic decomposition of N2O typically occurs at high temperature and N2O cannot be recovered as a valuable intermediate for the production of other fine chemicals.17,18 Therefore, the development of an efficient and economic technology to capture or concentrate N2O is very important. However, capturing the trace amounts of N2O (150 ppm or 0.015%) from air is an enormous challenge.19,20 Unlike other active NOX (X ≥ 1) gases, N2O is quite inert and exhibits kinetic stability that is very difficult to activate under mild conditions.7 Porous adsorbents for the adsorptive capture of N2O are the economic and effective materials for such a purpose.21–26

Previously, various porous adsorbents such as zeolite silicalite-1, zeolite-5A and porous carbon materials have been examined for N2O capture with the maximal N2O adsorption capacity of 4.10 mmol g−1 on zeolite-5A.22–24 The emergence of new porous materials termed as metal–organic frameworks (MOFs) has enabled us to reach record materials for the capture and storage of CO2, NH3, CH4, O2 and H2, attributed to their diverse crystal structures, high surface areas, tunable pore sizes and functional sites.27–34 However, the record N2O adsorption volume on MOFs was only 2.81 mmol g−1 (Ni-MOF),26 which is even lower than the one reported on zeolite-5A.

With more and more MOFs developed, some potentially useful porous materials such as ZIF-8/MAF-4, UiO-66, and MIL-101/100 with both high thermal and water stability have been realized.35–46 Typically, MOFs were activated under gentle conditions of comparatively low temperature and vacuum to sustain the framework structures. In certain cases, for example, for the MIL-100/MIL-101 series, such gentle activation might not be able to remove all the solvent molecules, particularly those strongly bound terminal solvent molecules with metal sites, so their adsorption performance might still have not been optimized as well. Chang et al. found that MIL-100Cr can be regenerated after being activated under a higher vacuum of 1 × 10−8 bar and a higher temperature of 523 K, exhibiting excellent performance for N2 capture from CH4 and O2, and thus for N2/CH4 and N2/O2 adsorption separation.47 This motivated us to do more in-depth studies on the activation of MIL-100Cr. To our surprise, the suitably activated sample of MIL-100Cr at 523 K and 1 × 10−10 bar for 12 h can take up N2O of 5.78 mmol g−1 at 298 K and 8.25 mmol g−1 at 273 K, setting record N2O capture capacities among the developed porous materials. The gas separation selectivity of N2O/N2 (0.015%/99.985%) is accordingly very high up to 1000 under ambient conditions.

MIL-100Cr was prepared in our lab via a literature reported method and with some modifications (details shown in the ESI).48 Before the gas sorption studies, the MIL-100Cr was activated at 373 K for 12 h until there is no free water molecule in the structure, and then further activated in the in situ activation station for another 12 h under ultra-high vacuum (1 × 10−10 bar) and high temperature (423–573 K). The activated samples were named MIL-100Cr-X (X = 150/200/250/−275/300) to indicate the different activation temperatures of 150, 200, 250, 275 and 300 °C, respectively. The sample in the powder form was characterized by X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) analysis.

As shown in TGA,46 MIL-100Cr is very stable up to 523 K. PXRD patterns (Fig. S1, ESI) demonstrate that MIL-100Cr-250 indeed shows a high crystalline feature. The N2O adsorption isotherms recorded for the activated MIL-100Cr at 298 K are shown in Fig. 1. When the activated temperature increases, the activated samples increase their uptakes for N2O in which MIL-100Cr-250 (activated at 523 K) has the highest adsorption capacity (5.78 mmol g−1). Compared with those reported materials such as zeolite and porous carbon for N2O capture, MIL-100Cr-250 also has the highest N2O capture capacity (Table 1).21–24 The increase from 1.95 mmol g−1 in MIL-100Cr-150 to 5.78 mmol g−1 in MIL-100Cr-250 is remarkable, although there is only about 20% increase of their N2 uptakes at 77 K (Fig. S2, ESI), as well as the increase in their corresponding BET surface area from 1764 m2 g−1 to 2118 m2 g−1. Apparently, the higher temperature activation has enabled us to further remove those highly bound solvent molecules (with the metal sites). In fact, water adsorption studies indicated that MIL-100Cr-250 takes up 41 mmol g−1 water, while MIL-100Cr-150 adsorbs 32 mmol g−1 water (Fig. S3, ESI). Once the activation temperature further increases above 250 °C, the N2O capture amounts of the samples MIL-100Cr-275 and MIL-100Cr-300 decrease to about 5.46 mmol g−1, which might be attributed to the partial collapse of the framework.


image file: c8cc07679k-f1.tif
Fig. 1 Adsorption isotherms of N2O at 298 K on MIL-100Cr activated at 423, 473, 523, 548 and 573 K.
Table 1 N2O adsorption capacity on several porous materials
Sorbent N2O (mmol g−1) Temperature (K) Ref.
Porous carbon 2.50 298 21
Silicalite-1 1.75 298 22
Zeolite-4A 3.50 298 23
Zeolite-5A 4.10 298 24
MOF-5 0.90 298 24
ZIF-7 2.50 298 25
Ni-MOF 2.81 298 26
MIL-100Cr-150 1.95 298 This work
MIL-100Cr-250 5.78 298 This work
MIL-100Cr-250 8.25 273 This work


As established before, there exists 2/3 terminal bound H2O and 1/3OH/F sites which are distributed on the metal sites in MIL-100.38–42 These terminal bound water molecules can be further removed at a very high temperature of 523 K and under vacuum below 1 × 10−8 bar to generate open Cr3+ sites (Fig. S4, ESI) for the possible binding of N2O molecules.49 We believe that these open Cr3+ sites play a crucial role in the significant increase of N2O uptakes from 1.95 mmol g−1 in MIL-100Cr-150 to 5.78 mmol g−1 in MIL-100Cr-250.

We calculated the isosteric heat of adsorption (Qst) of N2O in MIL-100Cr-250 bases on the adsorption isotherms (Fig. S5, ESI) at 273 K and 298 K. As shown in Fig. S6 (ESI), the Qst of N2O at low coverage is up to 80 kJ mol−1, much higher than the Qst of N2 (39 kJ mol−1[thin space (1/6-em)]47). Consistent with this, our DFT calculations (see the ESI for details) also indicate that the static binding energy of N2O at the open-Cr site is notably higher than that of N2, 72.5 kJ mol−1vs. 51.6 kJ mol−1 (the N2 value being similar to the previously reported result, ∼48.7 kJ mol−1[thin space (1/6-em)]47). N2O binds to the open-Cr through N, with a Cr–N bond distance slightly smaller than that found in N2 binding (see Fig. 2).


image file: c8cc07679k-f2.tif
Fig. 2 The DFT-calculated adsorption configurations of N2O and N2 on the open-Cr sites in MIL-100Cr, using a cluster model. (Chromium, oxygen, nitrogen, carbon, and hydrogen atoms are in green, red, blue, gray and white, respectively.)

We further used the adsorption selectivity used in the ideal adsorbed solution theory (IAST) to examine MIL-100Cr-250 to assess trace N2O (150 ppm) separation from N2 performance. The N2 adsorption isotherms are shown in Fig. S7 (ESI). The N2O and N2 adsorption isotherm data were fitted with the dual-site Langmuir and single-site Langmuir models, respectively. Compared with MIL-100Cr-250 for its high N2 selectivity from O2 and CH4,46 the gas separation selectivity of N2O from N2 (N2O/N2 = 0.015/99.985) on MIL-100Cr-250 is significantly higher up to 1000 (Fig. 3). This is remarkable, indicating that trace amount of N2O can be readily removed through a column packed with MIL-100Cr-250. As expected, MIL-100Cr-250 has much higher gas separation selectivities than MIL-100Cr-150 (about 100) as well. As shown in Fig. 4, simulated breakthrough curves further confirmed the significantly improved performance of MIL-100Cr-250 for this separation. The sorption data of CO2, O2 and CH4 and possible separation of N2O/CO2, N2O/O2 and N2O/CH4 are provided in the ESI (Fig. S8–S13).


image file: c8cc07679k-f3.tif
Fig. 3 Comparison of the ideal adsorbed solution theory (IAST) gas selectivity of 0.015/98.985 N2O/N2 mixtures in MIL-100Cr-150 and MIL-100Cr-250 at 298 K.

image file: c8cc07679k-f4.tif
Fig. 4 Transient breakthrough simulation of 0.015/98.985 N2O/N2 mixtures in a fixed bed packed with MIL-100Cr-150 (left) and MIL-100Cr-250 (right) at 298 K and a total pressure of 1 bar.

In conclusion, through the optimized activation of the well-known, highly stable MIL-100Cr, we realized MIL-100Cr-250 as the best material for N2O capture under ambient conditions with the capture capacity of 5.78 mmol g−1 at 298 K. Based on DFT calculations, the open Cr3+ sites are speculated to play crucial roles for the binding of N2O with a binding energy of 80 kJ mol−1. The promise of MIL-100Cr-250 for the removal of trace amounts of N2O from N2 was further evaluated via IAST studies, in which the gas separation selectivities of N2O from N2 on MIL-100Cr-250 are found to be the highest ones ever reported, up to 1000 and 100 times higher than those on MIL-100Cr-150. Given the fact that this MOF is highly stable, it might facilitate its practical use for this very important application. This work will also motivate us to explore more extensively those well-established MOFs to optimize their activation conditions and thus to maximize their multifunctional performances, particularly for gas storage and separation.

This work was funded by the National Natural Science Foundation of China (No. 21676175 and 51672186) and partly by Welch Foundation (AX-1730).

Conflicts of interest

There are no conflicts to declare.

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Footnote

Electronic supplementary information (ESI) available: Standard XRD pattern, adsorption isotherms of N2 and H2O on MIL-100Cr etc. See DOI: 10.1039/c8cc07679k

This journal is © The Royal Society of Chemistry 2018