Incorporation of Alkylamine into Metal–OrganicFrameworks through a Brønsted Acid–Base Reaction forCO2 CaptureHao Li+,[a] Kecheng Wang+,[a] Dawei Feng,[a] Ying-Pin Chen,[a, b] Wolfgang Verdegaal,[c] andHong-Cai Zhou*[a, b]

Introduction

Carbon capture has become increasingly important for the

pressing need to curb the climbing concentration of atmos-

pheric CO2, which could cause severe global warming.[1, 2] Thecapture of CO2 directly from air is also necessary for some ap-

plications, such as the removal of CO2 from the controlled at-mospheres of submarines or aircraft.[3–4] Tremendous effort has

been devoted to studying various materials for CO2 cap-ture.[3, 5–13] Among them, metal–organic frameworks (MOFs) areconsidered to be promising candidates for their high porosity,

tunable structure, and low heat capacity.[14–27] In most cases,CO2 is physically adsorbed in MOFs.[28–30] This leads to a verylow selectivity of CO2 over coexisting species with higher po-larity in flue gas or air, such as H2O,[5, 31, 32] which severely re-

stricts the CO2 adsorption capacity of MOFs under moist condi-

tions. The problem can be solved by introducing alkylamines

into the frameworks,[33–38] because the amine groups can spe-

cifically chemisorb CO2 from gas mixtures through reversiblechemical reactions.[5] So far, alkylamines have mainly been in-

corporated through coordination bonds between aminegroups and open metal sites (OMSs).[34–39] Although some alkyl-

amine-integrated MOFs with high CO2 uptakes have been ob-tained using this approach, intrinsic disadvantages of thismethod still exist. Firstly, a high density of OMSs in MOFs is

greatly desired to achieve a large alkylamine loading amount,which is a prerequisite for high CO2 uptake in MOFs. However,the OMS density in a MOF is restricted by the inherent struc-tures of the inorganic nodes and the organic linkers. They in-

evitably set an upper limit on the alkylamine density and cor-responding CO2 uptake in MOFs. Secondly, the method is only

applicable to MOFs with OMSs. Thus, it excludes manyMOFs,[40–42] covalent organic frameworks (COFs),[43] and porouspolymer networks (PPNs),[7, 44] which have no OMSs but retain

high chemical stability and survive in alkylamine tethering con-ditions. Therefore, introducing other amine grafting sites can

not only overcome the upper limit of alkylamine density inMOFs but also expand the range of material candidates for

amine incorporation, which is of great importance to advance

the development of CO2 adsorbents.Herein, by employing Brønsted acid sites in a highly stable

MOF, Cr-MIL-101-SO3H,[45, 46] we successfully integrated alkyla-mine into this mesoporous adsorbent. By tuning the alkyla-

mine structure, the solvent species, and other factors in the al-kylamine loading process, we determined the optimal experi-

The escalating atmospheric CO2 concentration is one of themost urgent environmental concerns of our age. To effectivelycapture CO2, various materials have been studied. Amongthem, alkylamine-modified metal–organic frameworks (MOFs)are considered to be promising candidates. In most cases, al-kylamine molecules are integrated into MOFs through the co-ordination bonds formed between open metal sites (OMSs)

and amine groups. Thus, the alkylamine density, as well as thecorresponding CO2 uptake in MOFs, are severely restricted by

the density of OMSs. To overcome this limit, other approaches

to incorporating alkylamine into MOFs are highly desired. Wehave developed a new method based on Brønsted acid–base

reaction to tether alkylamines into Cr-MIL-101-SO3H for CO2

capture. A systematic optimization of the amine tethering pro-

cess was also conducted to maximize the CO2 uptake of themodified MOF. Under the optimal amine tethering condition,the obtained tris(2-aminoethyl)amine-functionalized Cr-MIL-101-SO3H (Cr-MIL-101-SO3H-TAEA) has a cyclic CO2 uptake of2.28 mmol g@1 at 150 mbar and 40 8C, and 1.12 mmol g@1 at 0.4

mbar and 20 8C. The low-cost starting materials and simplesynthetic procedure for the preparation of Cr-MIL-101-SO3H-

TAEA suggest that it has the potential for large-scale produc-

tion and practical applications.

[a] H. Li,+ K. Wang,+ Dr. D. Feng, Y.-P. Chen, Prof. Dr. H.-C. ZhouDepartment of ChemistryTexas A&M UniversityCollege Station, Texas 77842-3012 (United States)E-mail : zhou@chem.tamu.edu

[b] Y.-P. Chen, Prof. Dr. H.-C. ZhouDepartment of Materials Science and EngineeringTexas A&M UniversityCollege Station, Texas 77843 (United States)

[c] W. VerdegaalProfusa, Inc.345 Allerton Ave. South San Francisco, California 94080 (United States)

[++] These authors contributed equally to this work.

Supporting Information and the ORCID identification number(s) for theauthor(s) of this article can be found under http://dx.doi.org/10.1002/cssc.201600768.

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Full PapersDOI: 10.1002/cssc.201600768

mental conditions to obtain a modified MOF with the maxi-mum CO2 uptake. The experimental results in the systematic

optimization are explained from a thermodynamic perspective.With cyclohexane (CH) as the solvent, the optimal adsorbent

was generated by modifying Cr-MIL-101-SO3H with 8 equiva-lents of tris(2-aminoethyl)amine (TAEA) at room temperature

(RT) in 5 min. The TAEA-functionalized MOF (Cr-MIL-101-SO3H-TAEA) has cyclic CO2 uptakes as high as 2.28 mmol g@1 at150 mbar and 40 8C, and 1.12 mmol g@1 at 0.4 mbar and 20 8C.

Results and Discussion

Owing to the strong electron donating ability of N atoms inamine groups, an alkylamine is not only a good ligand to coor-

dinate onto OMSs, but also a strong base that is able to formionic bonds with acids. Therefore, an alternative approach to

introducing alkylamine into MOFs is to utilize Brønsted acidsites. Because Brønsted acid groups can be conveniently inte-grated into many types of organic linkers,[47, 48] this approachcan enormously increase the number of MOF candidates forCO2 capture. To test our hypothesis, we chose Cr-MIL-101-SO3H

(Figure 1) as the pristine MOF to fix alkylamines for the follow-ing reasons: Firstly, Cr-MIL-101 is well-known for its ultrastabili-

ty under harsh environments,[49] which allows the framework

to remain intact in alkylamine solution during the post-syn-thetic treatment. Secondly, as a strong Brønsted acid, the sul-

fonic acid group can easily form ionic bonds with amine

groups, by which alkylamines can also be immobilized inMOFs. In Cr-MIL-101-SO3H, the density of the sulfonic acid

groups is 3.25 mmol g@1, which is even higher than that ofOMSs (2.17 mmol g@1).[45] This provides Cr-MIL-101-SO3H with

the potential to tether more alkylamine molecules than Cr-MIL-101 and therefore have a higher CO2 uptake. Thirdly, Cr-MIL-

101-SO3H is mesoporous.[49] The favorable pore volume permitsthe incorporation of large alkylamines with more amine

groups in a single molecule and thus promises a higher aminegroup density than that of MOFs only modified with small al-

kylamines. Finally, the organic ligand to synthesize Cr-MIL-101-SO3H, namely monosodium 2-sulfoterephthalic acid (BDC-

SO3Na(H)), is commercially available. This is extremely impor-tant because large-scale production of the absorbent is neces-sary for industrial application.

To discover the most suitable alkylamine tethering condi-tions for Cr-MIL-101-SO3H to generate an adsorbent with thehighest CO2 uptake for this material, we systematically studiedthe experimental parameters such as the alkylamine structure,

alkylamine quantity, reaction time and temperature, and sol-vent, and rationalized the results from a thermodynamic per-

spective.

Alkylamine structure

As the alkylamine structure significantly affects the CO2 capture

ability of the amine-functionalized MOFs according to the re-ported examples,[35, 37–39] the effect of the alkylamine structure

was first studied by conducting control experiments using a va-riety of alkylamines (Figure 2). Fully activated Cr-MIL-101-SO3H

samples were added to five different CH solutions containing2 equivalents of ethylenediamine (en), diethylenetriamine(DETA), triethylenetetramine (TETA), tetraethylenepentamine(TEPA), and pentaethylenehexamine (PEHA) (Supporting Infor-mation, Section 5). These suspensions were stirred at RT for

5 min before the modified MOFs (Cr-MIL-101-SO3H-X, X = en,DETA, TETA, TEPA, or PEHA) were separated from the solutions.

The MOFs were washed with anhydrous CH, and activatedbefore the measurements of their gas uptake.

As the molecular weight of the alkylamine increases from ento DETA, the CO2 uptake of alkylamine-modified MOFs increas-

es from 0.79 mmol g@1 to 2.23 mmol g@1 (the data are provided

at 150 mbar and 20 8C; unless otherwise mentioned, all theother CO2 adsorption data in the text are given at the same

pressure and temperature). As the amine weight further in-creases from TETA to PEHA, the CO2 uptake decreases from

1.94 mmol g@1 to 1.33 mmol g@1. The maximum uptake value isobtained using DETA- or TETA-modified MOFs. (Figure 3) We

Figure 1. (a) Cr-MIL-101-SO3H topology. Each lime ball represents a superte-trahedron unit, and green and blue polyhedra represent two types of cages.(b) Supertetrahedron unit in Cr-MIL-101-SO3H. Atom color legend: black(carbon), gray (hydrogen), yellow (sulfur), red (oxygen). (c) Illustration ofelectrostatic interaction between sulfonate groups and ammonium groupsafter proton transfer from sulfonic acid groups to amine groups.

Figure 2. Chemical structures of the alkylamine molecules and schematic il-lustration of their electrostatic interaction with Cr-MIL-101-SO3H.

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propose that the result is related to the amine group densityin the modified MOFs, which is determined by the number of

amine groups in a single alkylamine molecule and the amountof alkylamine actually incorporated into the MOF. Small mole-

cules such as en are able to reach most of the amine grafting

sites in Cr-MIL-101-SO3H, including those located in very smallpores. This leads to a large alkylamine loading amount in Cr-MIL-101-SO3H-en. However, the advantage is dramaticallyoffset by the number of amine groups in each en molecule.

Therefore, the overall amine group density in Cr-MIL-101-SO3H-en is rather limited. For large alkylamines such as TEPA, PEHA,

although each alkylamine molecule can bring more aminegroups into the framework, the bulky structures severelyhamper their access to the amine grafting sites. This also givesrise to a low overall amine group density in Cr-MIL-101-SO3H-TEPA or Cr-MIL-101-SO3H-PEHA. DETA and TETA are two alkyla-

mines that strike a balance between the number of aminegroup in a single molecule and the amount of alkylamine in-

corporated into the MOF. Thus, they excel the other linear al-

kylamines investigated and produce modified MOFs with thehighest amine group densities and highest CO2 uptakes. Our

explanation is well supported by the N/S atomic ratio obtainedby elemental analysis (Table S6).

We also tethered TAEA, the isomer of TETA, into Cr-MIL-101-SO3H, because these two alkylamines have the same number

of amine groups in a molecule. Therefore, a high CO2 uptakemight also occur in the case of Cr-MIL-101-SO3H-TAEA. It turns

out that the CO2 uptake of TAEA-modified Cr-MIL-101-SO3H(2.39 mmol g@1) was even higher than those of the TETA

(1.94 mmol g@1) and DETA (2.23 mmol g@1) counterparts. Wepropose that the branched geometry of TAEA is more compact

than the linear geometry of TETA, allowing TAEA to migrateinto Cr-MIL-101-SO3H and have access to the amine graftingsites more easily than TETA. Thus the larger alkylamine loading

amount in Cr-MIL-101-SO3H-TAEA leads to its higher aminegroup density (Table S6) and higher CO2 uptake. Based on theaforementioned, TAEA was chosen to be the optimal alkyla-mine to incorporate into Cr-MIL-101-SO3H.

Alkylamine quantity

The pore size distribution indicates that the population of mes-

opores in Cr-MIL-101-SO3H-TAEA significantly decreases (Fig-

ure S8), whereas its powder X-ray diffraction (PXRD) patternsuggests there is no collapse or phase change of the frame-

work (Figure S13). This indicated the successful incorporationof TAEA into the cages of the framework. To probe the chemi-

cal nature of the captured TAEA in Cr-MIL-101-SO3H-TAEA,Fourier transform infrared (FTIR) spectra of Cr-MIL-101-SO3H,

Cr-MIL-101-SO3H-TAEA, and anhydrous TAEA were collected

(Figure 4). Cr-MIL-101-SO3H-TAEA exhibited two new peaks at3234 and 3147 cm@1, which were attributed to the stretching

vibrations of @NH3+ .[50] This suggested that there was a proton

transfer from@SO3H to@NH2 and the formation of ammonium,which is consistent with our hypothesis.

The pore volume of the Cr-MIL-101-SO3H modified using

2 equivalents of TAEA is 0.32 cm3g@1 (Figure 3 a) and the meso-pores still exist (Figure S8). This indicated that there was stillextra room in the framework to accommodate more alkyla-

mine for a higher CO2 uptake. Therefore, a further attempt wasmade by increasing the quantity of TAEA in solution to 4, 8,

12, and 16 equivalents, and maintaining the other experimen-tal conditions. The order of CO2 uptakes in terms of TAEA

Figure 3. (a) N2 uptakes at 77 K and (b) CO2 uptakes at 20 8C of Cr-MIL-101-SO3H modified with 2 equivalents of different alkylamines. Control experi-ments were designed with the alkylamine structure as the only variable. Theother experimental factors were as follows: alkylamine quantity (2 equiva-lents), reaction time (5 min), reaction temperature (RT), and solvent (CH).

Figure 4. FTIR spectra of Cr-MIL-101-SO3H-TAEA (blue), pristine Cr-MIL-101-SO3H (green), and anhydrous TAEA (orange). (left) FTIR spectrum in therange of the stretching frequency with the region of interest highlighted ingray. (right) Expanded region of interest with characteristic peaks of Cr-MIL-101-SO3H-TAEA marked at 3234 and 3147 cm@1.

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quantity was 2 equivalents (2.40 mmol g@1)<4 equivalents(2.56 mmol g@1)<8 equivalents (2.75 mmol g@1)>12 equiva-

lents (2.56 mmol g@1)>16 equivalents (2.00 mmol g@1) (Fig-ure 5 b). If the TAEA quantity is small, the amount of TAEA

loaded into the MOF is the main limiting factor for CO2 uptake.

More TAEA molecules can be incorporated into Cr-MIL-101-SO3H as the TAEA quantity is increased, which leads toa higher CO2 uptake. As the TAEA quantity continues to in-crease, TAEA molecules occupy more space inside the frame-

work (Table S7), eventually making some of the amine graftingsites unapproachable for CO2. In this situation, the accessiblepore volume becomes the determining factor for the CO2

uptake of the adsorbent. Thus, a modest quantity of TAEA iscrucial to guarantee sufficient amine groups in MOF as well as

easy access to amine groups for CO2 molecules. With limitedexperiments, we chose 8 equivalents of TAEA to be the optimal

amount for alkylamine tethering in Cr-MIL-101-SO3H.

Time and temperature

Although many examples of alkylamine functionalization in

MOFs have been reported,[35–39] a comprehensive optimizationof the amine tethering process is still missing, which inspires

us to investigate if the maximum CO2 uptake for certain MOFshas been achieved. More importantly, it is unclear why certain

amine tethering conditions are adopted for a MOF and wheth-er they are applicable to other MOFs. To determine the optimal

experimental conditions of our system for the maximum CO2

uptake, as well as to reveal the key factors that influence the

integration of alkylamine into MOFs, we further studied otherexperimental parameters, namely the reaction time and tem-perature, for amine tethering.

Two series of control experiments were designed withamine tethering time and temperature as the only variable. In

the first set of experiments, the reaction time was 10 min,30 min, 1 h, and 12 h, whereas the reaction temperature wasmaintained at RT. Compared with Cr-MIL-101-SO3H-TAEA pre-pared in 5 min at RT (2.75 mmol g@1), the adsorbent prepared

in 10 min at RT exhibited almost the same high CO2 uptake

(2.65 mmol g@1) (Figure 6 b). However, Cr-MIL-101-SO3H-TAEAsamples prepared using a longer reaction time show a decline

in CO2 uptake, which might be attributed to the gradual de-composition of the frameworks, as supported by the attenuat-

ed peaks in the PXRD pattern of the sample prepared for 12 h.In the second set of experiments, the reaction time was kept

to 5 min, but the reaction temperatures were increased

to 40 and 60 8C. The results show that the samples preparedat RT (2.75 mmol g@1), 40 8C (2.78 mmol g@1), and 60 8C

Figure 5. (a) N2 uptakes at 77 K and (b) CO2 uptakes at 20 8C of Cr-MIL-101-SO3H modified by different amounts of TAEA. Control experiments were de-signed with the alkylamine quantity as the only variable. The other experi-mental factors were as follows: alkylamine (TAEA), reaction time (5 min), re-action temperature (RT), and solvent (CH).

Figure 6. (a) N2 uptakes at 77 K and (b) CO2 uptakes at 20 8C of Cr-MIL-101-SO3H-TAEA prepared for different periods of time. Control experiments weredesigned with the reaction time as the only variable. The other experimentalfactors were as follows: alkylamine (TAEA), alkylamine quantity (8 equiva-lents), reaction temperature (RT), and solvent (CH).

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(2.69 mmol g@1) in 5 min have almost the same amount of CO2

uptake (Figure 7 b).Both sets of experiments indicate that the distribution equi-

librium of TAEA between the solution phase and the MOFphase can be reached within 5 min at RT. This suggests that

this amine tethering method is very time and energy efficient

compared with the previous works in which the suspensionsare usually refluxed or stirred at RT for more than 12 h.[35–39]

We propose that there are two main reasons that account forthe easy achievement of the distribution equilibrium. Firstly,

the ionic bond between @SO3@ and @NH3

+ makes the alkyla-mine not as strongly localized to the amine grafting site as in

the case of a coordination bond, which facilitates the migra-tion of the alkylamine molecules deep into the framework. Sec-ondly, Cr-MIL-101-SO3H is a mesoporous structure with large

cage windows,[49] which makes it easy for large alkylamine mol-ecules to diffuse and reach the amine tethering sites located in

the interior part of the framework. Therefore, based on thecontrol experiments, the optimal amine tethering time and

temperature for this system were 5 min and RT.

Solvent

As mentioned, the incorporation of TAEA into Cr-MIL-101-SO3H

can be regarded as the distribution of TAEA between the solu-tion phase and the MOF phase. Therefore, the nature of the

solvent might also affect the amine loading process, which hasbeen significantly overlooked in previous studies. To determinethe optimal solvent for our system, we selected three other an-hydrous solvents in addition to CH, namely tetrahydrofuran(THF), dichloromethane (DCM), and methanol (MeOH), toperform control experiments. The CO2 uptakes of productsprepared in the solvents are CH (2.75 mmol g@1)>THF(2.43 mmol g@1)>DCM (2.29 mmol g@1)>MeOH (1.38 mmol g@1)

(Figure 8 b); the amine loading amount and porosity of theproducts are exactly in the reverse order (Table S8, and Fig-ure 8 a). This shows that the CO2 uptake of Cr-MIL-101-SO3H-

TAEA is higher if a larger amount of TAEA is loaded into theMOF provided the accessible pore volume is sufficient. In view

of the previous discussion, the distribution equilibrium of TAEAbetween the solution phase and the MOF phase is reached

within 5 min at RT in CH. Therefore, to explain the above se-

quence, efforts should first be devoted to clarifying whetheran equilibrium is also achieved in the case of the reactions in

THF, DCM, and MeOH.The case of MeOH was selectively examined because it gen-

erated a sample with the lowest CO2 uptake, which is as repre-sentative as the sample with the highest CO2 uptake prepared

in CH. Thus we conducted control experiments in MeOH with

an amine tethering time extended to 2 and 12 h. Comparedwith Cr-MIL-101-SO3H-TAEA prepared in 5 min in MeOH, nei-

Figure 7. (a) N2 uptakes at 77 K and (b) CO2 uptakes at 20 8C of Cr-MIL-101-SO3H-TAEA prepared under different temperatures. Control experimentswere designed with the reaction temperature as the only variable. The otherexperimental factors were as follows: alkylamine (TAEA), alkylamine quantity(8 equivalents), reaction time (5 min), and solvent (CH).

Figure 8. (a) N2 uptakes at 77 K and (b) CO2 uptakes at 20 8C of Cr-MIL-101-SO3H-TAEA prepared in different solvents. Control experiments were de-signed with the solvent as the only variable. The other experimental factorswere as follows: alkylamine (TAEA), alkylamine quantity (8 equivalents), reac-tion time (5 min), and reaction temperature (RT).

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ther the CO2 uptakes nor the porosities of these two samplesshowed any apparent differences (Figure 9), which indicated

that the distribution equilibrium of TAEA between the solutionphase and the MOF phase is reached within 5 min in MeOH at

RT. Therefore, it is reasonable to assume that equilibrium wasalso achieved in the case of THF and DCM, and the aforemen-

tioned sequence should be explained from a thermodynamicperspective.

Because TAEA contains many polar amine groups, the inter-action between TAEA and the solvent would be stronger in

a more polar solvent than that in a less polar solvent. As

a result, TAEA dispersed in less polar or nonpolar solvents suchas CH would have a relatively higher chemical potential

(Figure 10). This serves as a stronger driving force for TAEAmolecules to migrate from the solution phase to the MOF

phase and makes the modified MOF have a higher aminegroup density and a higher CO2 uptake. Therefore, there

should be a reverse relationship between solvent polarity and

TAEA loading amount. Our explanation is well supported bythe order of the solvent polarity (CH (0.006)<THF (0.207)<

DCM (0.309)<MeOH (0.762)).[51] Based on the above thermody-namic rationalization, a nonpolar solvent, such as CH, is the

optimal solvent for the incorporation of alkylamine into Cr-MIL-101-SO3H.

Optimal amine tethering condition for Cr-MIL-101-SO3H

According to the results of the above systematic study, the op-

timal condition to tether TAEA to Cr-MIL-101-SO3H is to im-

merse the MOF in CH solution with 8 equivalents of TAEA andto stir the suspension for 5 min at RT. The same procedure was

also applied to other alkylamines, and the gas uptakes of theresulting samples were measured. Cr-MIL-101-SO3H-TAEA still

excels the other alkylamine-modified Cr-MIL-101-SO3H in CO2

uptake (Figure 11 b), which is the best CO2 adsorbent we ob-

tained in this systematic investigation.

Figure 9. (a) N2 uptakes at 77 K and (b) CO2 uptakes at 20 8C of Cr-MIL-101-SO3H-TAEA prepared for different periods of time in MeOH. Control experi-ments were designed with the reaction time as the only variable. The otherexperimental factors were as follows: alkylamine (TAEA), alkylamine quantity(8 equivalents), reaction temperature (RT), and solvent (MeOH).

Figure 10. Graphical illustration of the relative chemical potentials of thesame alkylamine dispersed in four different solvents.

Figure 11. (a) N2 uptakes at 77 K and (b) CO2 uptakes at 20 8C of Cr-MIL-101-SO3H modified with 8 equivalents of different alkylamines. Control experi-ments were designed with the alkylamine structure as the only variable. Theother experimental factors were as follows: alkylamine quantity (8 equiva-lents), reaction time (5 min), reaction temperature (RT), and solvent (CH).

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Isosteric heat of adsorption and recyclability test

Isosteric heat of CO2 adsorption (denoted as @Qst) of Cr-MIL-101-SO3H-TAEA was calculated based on the CO2 uptake data

collected at 0, 20, and 40 8C with a triple-site Langmuir model(Section 8; Figure 12 a). The isosteric heat of CO2 adsorption is87 kJ mol@1 at zero coverage (Figure 12 b). This indicatesa strong interaction between CO2 and Cr-MIL-101-SO3H-TAEA,

which is likely to be as a result of a CO2 molecule interactingwith multiple amine groups in the framework.[52] The high @Qst

value is also consistent with its high CO2 uptake at very lowpressure. The N2 uptakes of Cr-MIL-101-SO3H-TAEA at 0, 20,and 40 8C are all too low to be accurately measured, suggest-

ing the CO2/N2 selectivity would be extremely high.

To assess Cr-MIL-101-SO3H-TAEA as a regeneratable adsorb-ent, we simulated temperature and vacuum swings by saturat-ing the sample with CO2 gas up to 150 mbar at 40 8C or0.4 mbar at 20 8C, followed by a high vacuum for 60 min at

80 8C. After 15 cycles, there was no apparent loss in CO2

uptake, which was 2.28 mmol g@1 at 150 mbar and 40 8C (Fig-

ure 12 c; the conditions relevant to CO2 capture from flue gas),and 1.12 mmol g@1 at 0.4 mbar and 20 8C (Figure 12 d; the con-ditions relevant to CO2 capture directly from air). Therefore, it

is evident that Cr-MIL-101-SO3H-TAEA is an efficient and regen-eratable adsorbent with a very strong affinity towards CO2,

suggesting the good potential for its application in CO2 cap-ture.

Conclusions

Based on the acid–base reaction between sulfonic acid groupsand amine groups, we successfully incorporated alkylamines

into Cr-MIL-101-SO3H using a fast and mild approach. We alsosystematically optimized the experimental conditions in the

amine tethering process to maximize the CO2 uptake of the re-sulting adsorbent. The experimental outcomes of the optimiza-

tion were rationalized from a thermodynamic perspective. Atthe optimal amine tethering conditions, the obtained Cr-MIL-

101-SO3H-TAEA had a high cyclic CO2 uptake of 2.28 mmol g@1

at 150 mbar and 40 8C, and 1.12 mmol g@1 at 0.4 mbar and

20 8C. Furthermore, the inexpensive starting materials andsimple synthetic procedure for the preparation of Cr-MIL-101-SO3H-TAEA hold great promise for its large-scale production

and application. Efforts to extend this amine tethering methodto other porous materials for CO2 capture are currently underway.

Experimental Section

Chemicals

Chromium (III) nitrate nonahydrate (Cr(NO3)3·9 H2O), monosodium2-sulfoterephthalic acid (H2BDC-SO3Na), hydrofluoric acid (HF), N,N-dimethylformamide (DMF), hydrochloric acid (HCl), methanol(MeOH), acetone, cyclohexane (CH), tetrahydrofuran (THF), di-chloromethane (DCM), ethylenediamine (en), diethylenetriamine(DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA),pentaethylenehexamine (PEHA), tris(2-aminoethyl)amine (TAEA)were purchased from VWR. All commercial chemicals were usedwithout further purification unless otherwise mentioned.

Instruments

Powder X-ray diffraction (PXRD) was performed with a BRUKER D8-Focus Bragg–Brentano X-ray powder diffractometer equipped witha Cu sealed tube (l= 1.54178 a) at 40 kV and 40 mA. Fourier trans-form infrared (FTIR) spectra were recorded on a SHIMADZU IR Af-finity-1 instrument. Surface images and elemental analyses of scan-ning electron microscopy (SEM)/energy dispersive X-ray spectros-copy (EDS) were taken by FEI Quanta 600 FE-SEM. The Quanta 600FEG is a field emission scanning electron microscope capable ofgenerating and collecting high-resolution images at a vacuumbelow 5 mTorr. It is equipped with a motorized x-y-z-tilt-rotatestage, providing the following movements: x = y = 150 mm (motor-ized); z = 65 mm (motorized); tilt + 70 degrees to @5 degrees (mo-torized); source: field emission gun assembly with Schottky emittersource. Voltage: 200 V to 30 kV. Beam current: >100 nA. Equip-ment associated with the Quanta 600 includes: conventional Ever-hart–Thornley detector, back-scattered electron detector, infraredcharge-coupled device (IR-CCD) chamber camera, Oxford EDSsystem equipped with X-ray mapping and digital imaging, HKL/Oxford electron backscatter diffraction (EBSD) system includinggeological phase database for phase ID, Gatan panchromatic cath-odoluminescence detector with RGB filters and a Zyvex S100 nano-manipulator. N2 and CO2 adsorption–desorption isotherms weremeasured using a Micrometritics ASAP 2020 system at differenttemperatures.

Preparation of Cr-MIL-101-SO3H

A mixture of Cr(NO3)3·9H2O (2.00 g, 5 mmol), H2BDC-SO3Na (2.70 g,10 mmol), deionized water (30 mL), and HF (48–51 wt %, 0.3 mL)was heated at 463 K for 24 h.[45] The as-synthesized solid waswashed with deionized water, DMF, and acetone in sequence andwas dried in air. The obtained green crystalline powder of Cr-MIL-

Figure 12. (a) CO2 adsorption data collected under 273, 293, and 313 K andrespective triple-site Langmuir fit curves. (b) Isosteric heat of CO2 adsorptionof Cr-MIL-101-SO3-TAEA at 293 K. (c) Fifteen cycles of CO2 adsorption at150 mbar and 40 8C. (d) Fifteen cycles of CO2 uptake at 0.4 mbar and 20 8C.

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101-SO3Na(H) was post-synthetically acidified in 60 mL of diluteHCl (0.08 m, the solution was prepared with 2 mL of concentratedHCl, 150 mL deionized water, and 150 mL MeOH) at 80 8C for 12 hto give Cr-MIL-101-SO3H.[46] The resultant green solid was washedsequentially with ionized water and acetone and finally dried over-night at 130 8C under vacuum prior to further use.

Procedure for tethering alkylamine onto Cr-MIL-101-SO3H

A sample of fully activated Cr-MIL-101-SO3H (100 mg, 0.108 mmol)was added to a 40 mL vial full of an anhydrous solution containing2 equivalents of alkylamine (en, DETA, TETA, TEPA, PEHA, or TAEA)that had already been dispersed. The aforementioned solvent wascollected fresh from a solvent still (CH, THF, DCM, or MeOH). Thesuspension was stirred at RT for 5 min before alkylamine-modifiedCr-MIL-101-SO3H was separated from the solution. The solid waswashed with anhydrous cyclohexane three times, desiccated undera Schlenk line and activated at 80 8C under vacuum before gasuptake measurements were taken. 1 equivalent refers to theamount of a certain alkylamine if we assume all the sulfonic acidgroups in Cr-MIL-101-SO3H are to be consumed in a 1:1 ratio reac-tion with an alkylamine. The volume of 1 equivalent of the respec-tive alkylamine was calculated and listed in Table S5.

Acknowledgements

This work was supported as part of the Center for Gas Separa-tions Relevant to Clean Energy Technologies, an Energy Frontier

Research Center funded by the U.S. Department of Energy, Officeof Science, Basic Energy Sciences under Award Number DE-SC0001015 (synthesis of MOFs and gas uptake measurements),by U.S. Department of Energy, Office of Fossil Energy, NationalEnergy Technology Laboratory under Award Number DE-

FE0026472 (FTIR and PXRD characterization) and by Welch En-dowed Chair to HJZ under A-0030 (EDS characterization). The FE-

SEM acquisition was supported by the NSF grant DBI-0116835,the VP for Research Office, and the TX Eng. Exp. Station.

Keywords: amine tethering · Brønsted acid–base reaction ·carbon dioxide capture · metal–organic frameworks ·systematic optimization

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Received: June 9, 2016

Published online on September 1, 2016

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