Elucidating CO2 Chemisorption in Diamine-Appended Metal-Organic Frameworks

Article abstract of DOI:10.1021/jacs.8b10203

The widespread deployment of carbon capture and sequestration as a climate change mitigation strategy could be facilitated by the development of more energy-efficient adsorbents. Diamine-appended metal-organic frameworks of the type diamine-M₂(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc^4- = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) have shown promise for carbon-capture applications, although questions remain regarding the molecular mechanisms of CO₂ uptake in these materials. Here we leverage the crystallinity and tunability of this class of frameworks to perform a comprehensive study of CO₂ chemisorption. Using multinuclear nuclear magnetic resonance (NMR) spectroscopy experiments and van-der-Waals-corrected density functional theory (DFT) calculations for 13 diamine-M₂(dobpdc) variants, we demonstrate that the canonical CO₂ chemisorption products, ammonium carbamate chains and carbamic acid pairs, can be readily distinguished and that ammonium carbamate chain formation dominates for diamine-Mg₂(dobpdc) materials. In addition, we elucidate a new chemisorption mechanism in the material dmpn-Mg₂(dobpdc) (dmpn = 2,2-dimethyl-1,3-diaminopropane), which involves the formation of a 1:1 mixture of ammonium carbamate and carbamic acid and accounts for the unusual adsorption properties of this material. Finally, we show that the presence of water plays an important role in directing the mechanisms for CO₂ uptake in diamine-M₂(dobpdc) materials. Overall, our combined NMR and DFT approach enables a thorough depiction and understanding of CO₂ adsorption within diamine-M₂(dobpdc) compounds, which may aid similar studies in other amine-functionalized adsorbents in the future.

Full text of DOI:10.1021/jacs.8b10203

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Article
Elucidating CO2 Chemisorption in Diamine-
Appended Metal–Organic Frameworks
Alexander C. Forse, Phillip J Milner, Jung-Hoon Lee, Halle N Redfearn, Julia
Oktawiec, Rebecca L. Siegelman, Jeffrey D. Martell, Bhavish Dinakar, Leo B Porter-
Zasada, Miguel I. Gonzalez, Jeffrey B Neaton, Jeffrey R. Long, and Jeffrey A. Reimer
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10203 • Publication Date (Web): 02 Dec 2018
Downloaded from http://pubs.acs.org on December 2, 2018
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(a)
(b)
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Journal of the American Chemical Society
c
M
M
N
M
N
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R
R
R
N
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N
H
R
H
H
H
H
N
R
R
N
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n
n = 1,2
H
H
H
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N
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O
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31R
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ammonium carbamate chains
carbamic acid pairs

Pressure gauge
Moveable
plunger
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O-ring
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H2N
e-2 =
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1º amine
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Chemisorbed CO2
e-2–Mg2(dobpdc)
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O
Direct
Physisorbed CO2
4.3
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Mg
Excitation
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Carbamate
O
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2º Ammonium
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150 100
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Polarization
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dmpn:
H2N
161.2
H2N
NH2
dmpn–Zn2(dobpdc)
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carbamic
acid N–H
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NH
1 bar 13CO2
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NH2
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1H: RMSD = 1.3 ppm
13C: RMSD = 2.2 ppm
15N: RMSD = 6.3 ppm
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ammonium carbamates
carbamic acids
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Ammonium carbamate chains
Ammonium carbamate (mixed)
Ammonium carbamate (other)
Carbamic acid pairs
PCH–APTES
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IRMOF-74-III-(CH2NH2)2
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dmpn:
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NH2
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desorption
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(a) e-2–Mg2(dobpdc)
(b) dmpn–Mg2(dobpdc)
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ammonium carbamate chains
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Elucidating CO2 Chemisorption in Diamine-Appended
Metal–Organic Frameworks
Alexander C. Forse,a,b,d Phillip J. Milner,a,e,† Jung-Hoon Lee,c,f Halle N. Redfearn,a Julia Oktawiec,a Rebecca L.
Siegelman,a,e Jeffrey D. Martell,a Bhavish Dinakar,b,e Leo B. Porter-Zasada,a Miguel I. Gonzalez,a Jeffrey B.
Neaton,*c,f,g Jeffrey R. Long,*a,b,e Jeffrey A. Reimer*b,e
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aDepartment of Chemistry, bDepartment of Chemical and Biomolecular Engineering, cDepartment of Physics, and dBerkeley Energy
and Climate Institute, University of California, Berkeley, California 94720, U.S.A.
eMaterials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, U.S.A.
fMolecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, U.S.A.
gKavli Energy Nanosciences Institute at Berkeley, Berkeley, California 94720, U.S.A.
ABSTRACT: The widespread deployment of carbon capture and sequestration as a climate change mitigation strategy could be facilitated by
the development of more energy-efficient adsorbents. Diamine-appended metal–organic frameworks of the type diamine–M2(dobpdc) (M =
Mg, Mn, Fe, Co, Ni, Zn; dobpdc4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) have shown promise for carbon capture applications, although
questions remain regarding the molecular mechanisms of CO2 uptake in these materials. Here, we leverage the crystallinity and tunability of
this class of frameworks to perform a comprehensive study of CO2 chemisorption. Using multinuclear nuclear magnetic resonance (NMR)
spectroscopy experiments and van der Waals-corrected density functional theory (DFT) calculations for thirteen diamine–M2(dobpdc) vari-
ants, we demonstrate that the canonical CO2 chemisorption products—ammonium carbamate chains and carbamic acid pairs—can be readi-
ly distinguished, and that ammonium carbamate chain formation dominates for diamine–Mg2(dobpdc) materials. In addition, we elucidate a
new chemisorption mechanism in the material dmpn–Mg2(dobpdc) (dmpn = 2,2-dimethyl-1,3-diaminopropane), which involves formation
of a 1:1 mixture of ammonium carbamate and carbamic acid and accounts for the unusual adsorption properties of this material. Finally, we
show that the presence of water plays an important role in directing the mechanisms for CO2 uptake in diamine–M2(dobpdc) materials.
Overall, our combined NMR and DFT approach enables a thorough depiction and understanding of CO2 adsorption within diamine–
M2(dobpdc) compounds, which may aid similar studies in other amine-functionalized adsorbents in the future.
old pressure. This behavior arises as a result of an unprecedented
cooperative adsorption mechanism whereby CO2 inserts into each
INTRODUCTION
Carbon dioxide capture and sequestration at power stations and
metal–amine bond to form ion-paired ammonium carbamate
chemical manufacturing plants is anticipated to play an important
chains that propagate along the framework channels (down the
role in efforts to mitigate greenhouse gas emissions.1 Aqueous
crystallographic c axis, see Figure 1c).5,14,20 Importantly, these mate-
amine solutions are already used to capture CO2 at some locations,
rials exhibit high working capacities for CO2 removal, and moreo-
including the recently retrofitted Petra Nova power station.2 More
ver relatively small changes in pressure and temperature can be
energy efficient capture materials could accelerate the adoption of
used to trigger adsorption or desorption. Thus, these materials
CO2 capture as a mitigation strategy, and amine-functionalized
may afford substantial energy and cost savings when used in pro-
adsorbents have emerged as promising next-generation materials
cesses involving temperature or temperature-pressure swings.21–23
that benefit from the highly selective chemistry of amines with
CO2, similar to traditional amine solvents.3 These materials may
offer improvements over amine solutions including improved sta-
bility, reduced volatility, and lower regeneration temperatures.
Recent single-crystal X-ray diffraction characterization of a series of
diamine–Zn2(dobpdc) compounds revealed that the formation of
ammonium carbamate chains occurs for a number of different dia-
mines with diverse structures,14 corroborating the results of other
studies using powder X-ray diffraction.5,19 The cooperative adsorp-
tion mechanism has been further validated at the atomic level using
van der Waals-corrected density functional theory (vdW-corrected
DFT) calculations,5,20,24,25 which are able to predict binding en-
thalpies within a few kJ/mol of experiment. These binding en-
thalpies, in combination with a statistical mechanical model, have
been used to explain the step-shaped isotherm in terms of an abrupt
increase in the mean chain length at a critical CO2 pressure.26 Im-
portantly, the adsorption thermodynamics—and therefore the step
pressure at a given temperature—can be tuned by varying the met-
Amine-appended metal–organic frameworks with the formula (di-
amine)2M2(dobpdc)—subsequently referred to here as diamine–
M2(dobpdc)—in particular have shown exceptional promise for
CO2 capture4–13 and are obtained by functionalizing the coordina-
tively-unsaturated metal sites in the M2(dobpdc) framework
(M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc4− = 4,4′-dioxidobiphenyl-
3,3′-dicarboxylate) with diamines (Figure 1a,b).4,5,14–19 Rather than
displaying traditional Langmuir-type adsorption, many diamine–
M2(dobpdc) materials exhibit step-shaped isotherms, correspond-
ing to sudden adsorption of CO2 upon exposure to a given thresh-
1
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al5,15,20,27 or the diamine,14–16,18 allowing for the targeted design of
um analogues, as well as the presence of structural disorder in some
1
2
3
4
materials for specific separations involving CO2. While single-
crystal X-ray diffraction studies have been applied with great suc-
cess to diamine-appended Zn2(dobpdc) variants,14,15,19 the lack of
large single crystals for the most technologically relevant magnesi-
materials, necessitates the development of additional characteriza-
tion methods to fully elucidate the factors dictating cooperative
adsorption in this class of frameworks.
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Figure 1. Canonical CO2 chemisorption products in diamine–M2(dobpdc) materials. (a) Structure of the parent metal–organic framework,
M2(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn).28 (b) Schematic structure of diamine–M2(dobpdc) materials, prepared by post-synthetic grafting of
diamines to the open M2+ sites of M2(dobpdc). (c) Ammonium carbamate chain formation upon CO2 adsorption. d) Carbamic acid pair formation
upon CO2 adsorption. Ammonium carbamate chains and carbamic acid pairs are the only chemisorption products that have been confirmed crystal-
lographically to date in these materials.
We recently discovered that the material (dmpn)2Mg2(dobpdc)
(dmpn = 2,2-dimethyl-1,3-diaminopropane) possesses an ad-
sorption step in the optimum range for capture of CO2 from coal
flue gas, enabling a working capacity of 2.42 mmol/g (9.1 wt %)
with a modest 60 °C temperature swing for adsorbent regenera-
tion using pure CO2.15 This material is also stable for at least 1000
adsorption–desorption cycles under humid conditions. Interest-
ingly, gas adsorption, NMR, and X-ray diffraction measurements
indicated that this material chemisorbs CO2 by a mechanism that
does not involve purely ammonium carbamate chain formation
(Figure 1c). Indeed, NMR measurements showed that
(dmpn)2Mg2(dobpdc) exhibits a mixed chemisorption mecha-
nism that involves the formation of both ammonium carbamate
and carbamic acid species.15 Crystallographic and NMR studies of
the isostructural (dmpn)2Zn2(dobpdc) analogue revealed that,
rather than ammonium carbamate chains, carbamic acid pairs
form at CO2 gas pressures close to 1 bar,15 a species that was first
predicted computationally in this class of materials (Figure
1d).20,25,29 At pressures below 1 bar, a mixed mechanism was again
observed, with an unspecified arrangement of ammonium carba-
mate and carbamic acid groups.15
comprehensive study of CO2 chemisorption using a combination
of multinuclear NMR spectroscopy experiments and vdW–
corrected DFT calculations. Many different diamine-appended
variants can be synthesized and analyzed using this highly or-
dered system, making these materials invaluable for fundamental
studies of CO2 chemisorption. Furthermore, the techniques and
spectroscopic signatures identified here may allow diamine–
M2(dobpdc) frameworks to serve as a model system for the
broader class of amine-functionalized solids, such as porous sili-
cas, in which the host structure and spatial distribution of amines
are often more challenging to control and characterize.30–33 Spe-
cifically, we have endeavored to: (i) develop a combined NMR
and DFT approach to distinguish between ammonium carbamate
and carbamic acid formation, (ii) determine ranges of NMR pa-
rameters for these mechanisms so that researchers may more
readily make structural assignments, (iii) understand the mixed
chemisorption mechanism and unique CO2 adsorption proper-
ties of (dmpn)2Mg2(dobpdc), and (iv) to investigate the effect of
water on CO2 chemisorption mechanisms.
Here, we make use of the well-defined and readily modified struc-
tures of the diamine–M2(dobpdc) materials class to perform a
2
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able plunger. This dosing system is similar to those previously
RESULTS AND DISCUSSION
reported for studies of zeolites34 and amine-functionalized
silicas.35
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Characterization of Ammonium Carbamate Chains. The ma-
terial e-2–Mg2(dobpdc) (e-2 = N-ethylethylenediamine) was
recently proposed to form ammonium carbamate chains exclu-
sively upon adsorption of CO2 (Figure 1c),14 based on its step-
shaped adsorption isotherms that saturate at a loading of one
CO2 molecule per diamine, as well as infrared spectra of the CO2-
dosed material. These results were also consistent with data col-
lected for the isostructural framework e-2–Zn2(dobpdc), for
which the formation of ammonium carbamate chains upon expo-
sure to CO2 was also confirmed via in situ single-crystal X-ray
diffraction experiments. We thus chose to first investigate e-2–
Mg2(dobpdc) in detail with NMR spectroscopy, as a benchmark
material to confirm and explore the formation of ammonium
carbamate chains. Studies of gas-dosed samples were enabled by a
home-built apparatus (Figure 2) that allows rotors (sample hold-
ers) for magic angle spinning (MAS) NMR experiments to be
sealed at different pressures of 13CO2 gas (99% 13C).15 Following
evacuation, activated samples were dosed with 13CO2 at a pres-
sure of ~1 bar that is above the adsorption step for this material at
25 °C (see Figure S1). The samples were then allowed to equili-
brate before they were capped inside the manifold using a move-
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Figure 2. Schematic of home-built manifold for preparing gas-dosed
NMR samples. The white section on the left is made from glass, and
gray sections are made from stainless steel tubing. Swagelok® VCR
metal gasket face seal fittings were used for steel-to-steel connections,
and Swagelok® Ultra Torr fittings were used for glass-to-steel connec-
tions. Capacitance Manometers (MKS Instruments model 722B)
were used for pressure measurements.
Figure 3. Solid-state MAS NMR (16.4 T) spectra of e-2–Mg2(dobpdc)–CO2. (a) 13C NMR spectra by direct excitation (no 1H decoupling) and
cross-polarization (with continuous wave decoupling of H). A 1:1 CO2:diamine reaction affords a capacity of 4.0 mmol/g CO2, which is reached
1
above the adsorption step of the material (see Figure S1). Cross-polarization experiments allow the much faster observation of chemisorbed CO2
compared to direct excitation experiments (partly because of the long longitudinal relaxation time, T1 = 60 s, for chemisorbed CO2). Cross-
polarization is used for the majority of 13C experiments that follow in this work. Note that framework and amine carbons are only weakly observed in
these experiments, as they have the natural abundance of 13C (~1%). (b) 1H → 13C HETCOR (contact time 100 µs) spectrum and correlation assign-
ments. (c) Nitrogen-15 NMR spectra (natural isotopic abundance) of e-2–Mg2(dobpdc) before and after reaction with 13CO2 (1 bar). Recent X-ray
crystallography of toluene–solvated e-2–Zn2(dobpdc) showed that this diamine binds to zinc(II) from the 1° amine,14 with dynamics in the anticipat-
ed 1° amine–Mg interaction a likely cause of 15N line broadening for the 1° amine 15N resonance here. The magic angle spinning (MAS) rate was 15
kHz in all cases. 13CO2 gas dosing pressures were approximately 1 bar, and gas dosing and NMR experiments were performed at room temperature.
The 13C NMR spectra of 13CO2-dosed e-2–Mg2(dobpdc) exhibit
resonances assignable to both chemisorbed and physisorbed CO2
at 162.1 and 124.7 ppm, respectively (Figure 3a). Experiments on
two independent samples (see Supporting Information, Figure
S2), as well as experiments by another research group,36 have
shown that the 13C chemical shift for chemisorbed CO2 is repro-
ducible within ~0.1 ppm. The chemical shift of physisorbed CO2
is within 0.4 ppm of the values observed for physisorbed CO2 in
various diamine-appended Mg2(dobpdc) materials (Figure S3),
although the resonance is shifted by −3 ppm relative to that of
free gas-phase CO2 (127.7 ppm at 1 bar).28 This chemical shift
difference may arise in part due to aromatic ring currents in the
dobpdc4− linkers.37,38 Quantitative 13C NMR indicates that at 1 bar
only a small percentage (10–15%) of the adsorbed CO2 is phy-
sisorbed, and thus adsorption primarily occurs via chemisorption
at this pressure. As expected, cross-polarization from 1H → 13
C
revealed only the resonance from chemisorbed 13CO2, as the
strong 1H–13C dipole–dipole couplings necessary for efficient
cross-polarization are absent for the physisorbed gas.
Additional NMR experiments enabled the assignment of the
chemisorbed species as ammonium carbamate, consistent with
the conclusions of earlier work.14 In particular, a two dimensional
1H → 13C HETCOR spectrum with a short contact time (100 µs)
allowed detection of only those hydrogen atoms near the 13C
nucleus of chemisorbed 13CO2 (Figure 3b). This spectrum shows
1
a dominant H correlation at 4.3 ppm, as well as a minor correla-
tion at 13.2 ppm. We have assigned the former to the N–H of the
carbamate group (i.e., NHRCOO−), for which a dominant corre-
lation is anticipated because this hydrogen is the closest to the
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carbamate 13C carbon (13C···1H = 2.05 Å, from the DFT struc-
that would result in a ratio of one CO2 per two diamines—such as
dangling ammonium carbamate pairs.39
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ture). The N–H correlation confirms reaction of CO2 with the
primary amine group, rather than the secondary amine. Reaction
of CO2 with the secondary amine would form a tertiary nitrogen
with no attached hydrogens, and so the N–H correlation would
be absent. The minor correlation at 13.2 ppm is attributed to an
ammonium hydrogen (13C···1H = 2.43 Å from DFT) and sup-
ports the formation of ammonium carbamate chains, in which
carbamate and ammonium groups are ion-paired and strongly
hydrogen bonded. We note that ammonium carbamate chain
formation is necessary here to achieve the observed CO2 adsorp-
tion capacity of one CO2 per diamine (see Figure S1), which
excludes the formation of other ammonium carbamate species
The 15N NMR spectrum of e-2–Mg2(dobpdc)–CO2 (Figure 3c)
exhibits resonances assignable to carbamate (76 ppm) and am-
monium (42 ppm) groups, further supporting the formation of
ammonium carbamate chains via reaction of the primary amine
with CO2. The secondary ammonium resonance is shifted by +8
ppm relative to the secondary amine resonance present in the
activated material e-2–Mg2(dobpdc)—similar to previous 15N
NMR studies of ammonium carbamates.40–42 Overall, our multi-
nuclear NMR experiments confirm that ammonium carbamate
chain formation dominates for e-2–Mg2(dobpdc) and e-2–
Zn2(dobpdc) (Figure S4).
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Figure 4. MAS NMR (16.4 T) measurements of dmpn–Zn2(dobpdc) dosed with CO2 at a pressure of 1015 mbar. (a) 13C NMR cross-polarization
spectrum (with continuous wave decoupling of 1H). (b) 1H → 13C HETCOR (contact time 100 µs) spectrum and correlation assignments (the minor
correlation at ~0 ppm arises from the methyl groups on the diamine backbone). (c) 15N NMR (at natural isotopic abundance) cross polarization
spectra (contact time 1 ms) of dmpn–Zn2(dobpdc) before and after reaction with 13CO2. For 15N NMR the acquisition times were 20 h and 8.5 h for
dmpn-Zn2(dobpdc) before and after CO2 dosing, respectively. The presence of a single 15N resonance in activated dmpn–Zn2(dobpdc) suggests that
the free end of the amine and the metal-bound end are likely undergoing rapid exchange under the experimental conditions.5 Cross polarization kinet-
ics and variable temperature NMR experiments should be carried out in future to further investigate diamine dynamics in diamine–M2(dobpdc)
materials. The magic angle spinning (MAS) rate was 15 kHz in all cases. Gas dosing and NMR experiments were performed at room temperature.
Characterization of Carbamic Acid Pairs. The material dmpn–
Zn2(dobpdc) was selected as a model system for the detailed
study of carbamic acid pairs in this class of MOFs. Unlike dmpn–
Mg2(dobpdc), which adsorbs CO2 by a mixed chemisorption
mechanism (see below), dmpn–Zn2(dobpdc) forms carbamic
acid pairs when dosed at CO2 pressures close to 1 bar, as con-
firmed by single-crystal X-ray diffraction and 13C solid-state NMR
spectroscopy.15 Although molecular carbamic acids have also
been characterized crystallographically,43 this result represents
the only crystallographic characterization to date of a carbamic
acid pair structure in an amine-functionalized adsorbent, and so
serves as a valuable model system for further study by NMR.
3b). The two strong correlations at short contact times are antici-
pated for carbamic acid pairs, given the relatively short C···H
distances predicted from DFT calculations (1.95 Å for C···HCOOH
,
1
and 2.01 Å for C···HNHRCOOH). Furthermore, the H chemical shift
of the COOH carbamic acid hydrogen is consistent with the an-
ticipated value based on previous empirical correlations between
1H chemical shifts and O···O distances for O–H···O hydrogen
bonds, whereby shorter hydrogen bonds give rise to larger chemi-
cal shifts (Figure S6).47 Overall, the short contact time 1H–13C
correlation experiments allow carbamic acid pairs to be clearly
distinguished from ammonium carbamates (Figures 3b, 4b).
The 15N NMR spectrum of CO2-dosed dmpn–Zn2(dobpdc) ex-
hibits resonances at 81.7 and 13.6 ppm, assigned to the
NRHCOOH nitrogen and the metal-bound amine RNH2–Zn,
respectively, as supported by the DFT calculations below. The
chemical shift of the NRHCOOH nitrogen cannot be used to
identify carbamic acids, because it falls within the range of values
used for NRHCOO− in ammonium carbamates (Tables 1 and 2).
On the other hand, the absence of a 15N chemical shift change for
an unreacted amine helps to distinguish carbamic acids from
ammonium carbamates. The 15N chemical shift of the metal-
bound amine is very similar to that in the activated material (14.4
ppm) (Figure 4c and Table 2). This contrasts to ammonium
carbamates where larger 15N chemical shifts are observed for the
ammonium cations than the amines in the corresponding activat-
ed materials (Figure 3c).
Figure 4a shows the 1H → 13C cross-polarization spectrum of
dmpn–Zn2(dobpdc) dosed with 1015 mbar of 13CO2. The spec-
trum exhibits a major resonance at 161.2 ppm, similar to that
observed in our previous work,15 which we assign to carbamic
acid pairs. This chemical shift is similar to those previously as-
signed to carbamic acids in the literature.35,40,41,44–46 The weak
resonance at ~164 ppm likely arises from a minor amount of am-
monium carbamate (see Figure S5). Importantly, a two dimen-
sional 1H → 13C HETCOR experiment (short contact time)
1
revealed two major correlations at H chemical shifts of 12.2 and
5.7 ppm (Figure 4b), assigned to hydrogens of the carbamic acid
(NHCOOH and NHCOOH, respectively). Importantly, this
result contrasts to that for the ammonium carbamate chains,
where one major and one minor correlation are observed (Figure
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Table 1. NMR results for ammonium carbamate chains. Carbon-13 chemical shifts arising from chemisorbed CO2 and key 1H chemical shifts
1
measured by short-contact time H → 13C HETCOR experiments. Values from DFT are indicated in parentheses. “–“ indicates not meas-
ured/calculated. *It is assumed that RNH3+ undergoes rotation on the NMR timescale, and we thus report average DFT-calculated 1H chemi-
cal shifts for the three ammonium hydrogens. **i-2–Mg2(dobpdc) and ee-2–Mg2(dobpdc) display two adsorption steps in their CO2 iso-
therms.14 Gas dosing pressures were approximately 1 bar in all cases at room temperature and at least 30 minutes were allowed for equilibra-
tion. Errors in the experimental chemical shifts are approximately 0.1 ppm, 0.4 ppm for 13C and 1H, respectively. For 15N the errors are approx-
imately 0.2 ppm and 1 ppm for carbamate and ammonium nitrogens, respectively.
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δ 13C (ppm)
Experiment (DFT)
162.1 (160.1)
δ 1H (ppm)
δ 15N (ppm)
Compound
Amine structure
Ref. for
Experiment
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Experiment (DFT)
Experiment (DFT)
NHRCO2–: 75.8 (86.8)
NH2R2+: 42.1 (43.3)
NHRCO2–: 4.3 strong (5.7)
This work
e-2–
Mg2(dobpdc)
NH2R2+: 13.2 weak (12.6)
162.0 (159.5)
162.6 (159.9)
162.5 (160.1)
NHRCO2–: 4.0 strong (5.3)
NH2R2+: 12.8 weak (14.2)
NHRCO2–: 75.3 (85.6)
NH2R2+: 32.8 (31.5)
This work
m-2–
Mg2(dobpdc)
NHRCO2–: 4.8 strong (5.6)
NHR3+: 14.0 weak (16.4)
NHRCO2–: – (80.4)
NHR3+: – (37.5)
This work
This work
mm-2–
Mg2(dobpdc)
NHRCO2–: 4.7 strong (5.7)
NHR3+: 13.7 weak (16.0)
NHRCO2–: 85.0 (86.5)
NHR3+: 52.7 (58.2)
ee-2–
Mg2(dobpdc)**
163.8 (162.4)
163.1 (164.9)
NH2R2+: 15.5 weak (16.0)
NR2CO2–: 73.3 (79.5)
NH2R2+: 33.4 (32.9)
This work
This work
m-2-m–
Mg2(dobpdc)
NHRCO2–: 4.8 strong (4.7)
NH3R+: 7.6 weak (8.2)
NHRCO2–: 86.5 (81.0)
NH3R+: 33.7 (35.0)
mpn–
Mg2(dobpdc)
[15]
162.7 (165.4)
163.7 (162.1)
NHRCO2–: – (6.2)
NH3R+: – (8.2)
NHRCO2–: – (92.5)
NH3R+: – (41.9)
pn–
Mg2(dobpdc)
NHRCO2–: – (4.7)
NH3R+: – (8.4)
NHRCO2–: 77 (77.3)
NH3R+: 51 (58.6)
[15,18]
dmen–
Mg2(dobpdc)
[19]
[19]
163.4 (159.6)
161.6 (159.4)
NHRCO2–: 4.8 strong (5.2)
NH3R+: 7.2 weak (7.3*)
NHRCO2–: – (91.4)
NH3R+: – (36.4)
(R,R)-dach–
S-Mg2(dobpdc)
NHRCO2–: 3.9 strong (4.1)
NH3R+: 7.8 weak (7.6*)
NHRCO2–: – (98.7)
NH3R+: – (37.1)
(R,R)-dach–
R-Mg2(dobpdc)
“
NHRCO2–: 4.4 strong (5.8)
NH2R2+: 12-13 weak (13.6)
NHRCO2–: – (75.8)
NH2R2+: – (57.5)
i-2–
163.3, 162.5, 161.9
(161.5)
Mg2(dobpdc)**
This work
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Table 2. NMR results for carbamic acid pairs. As for Table 1, except data reflect carbamic acid pairs. Values from vdW-corrected DFT calacu-
lations are indicated in parentheses. The gas dosing pressure was 1015 mbar, with gas dosing carried out at room temperature. Note: dmpn–
Zn2(dobpdc) undergoes a different (mixed) mechanism at lower pressures (see below).
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δ 13C (ppm)
Experiment (DFT)
161.2 (162.3)
δ 1H (ppm)
δ 15N (ppm)
Compound
Amine structure
Experiment (DFT)
Experiment (DFT)
NHRCO2H: 81.7 (90.8)
Zn–NH2R: 13.6 (9.9)
dmpn–
Zn2(dobpdc)
COOH: 12.2 strong, (14.5)
NHCO2H: 5.7 strong, (7.6)
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We performed similar NMR measurements on a series of dia-
mine-appended Mg2(dobpdc) materials in which the diamine
was varied (Table 1). All of the materials exhibit step-shaped CO2
adsorption isotherms,14,15,19 and were studied under CO2 pres-
sures much greater than the step pressure. Under these condi-
tions, the materials listed in Table 1 all display NMR spectra con-
sistent with the formation of ammonium carbamate chains (Fig-
ures S7-S9), supporting the hypothesis that this mechanism dom-
inates for a wide range of diamines.14,15,19 The key evidence for
ammonium carbamate chain formation is the observation of 13C
resonances that correlate strongly with a carbamate N–H group
and weakly with an ammonium group in two-dimensional exper-
iments. The observation of ammonium resonances in the 15N
NMR spectra provides additional support for ammonium carba-
mate chain formation. Additionally, the measured chemical shifts
generally show good agreement with DFT-calculated values for
ammonium carbamate chain structures (see below). For addi-
tional discussion on m-2–Zn2(dobpdc) and i-2–Mg2(dobpdc),
see Figures S7 and S10, respectively.
oms was required to obtain reasonable agreement between calcu-
lation and experiment, suggesting that these primary ammonium
groups do rotate on the NMR timescale. An average of calculated
+
1H chemical shifts for RNH3 groups was also shown to result in
agreement between experiment and theory in previous studies of
glycine in the solid state, where room temperature rotation of
RNH3+ groups is known.48,49 In the α and γ polymorphs of glycine,
1
+
the H chemical shifts for the RNH3 group are 8.4 and 8.8 ppm,
respectively,48 similar to the H chemical shifts measured here for
1
the primary ammonium cations.
The foregoing spectral assignments are well-supported by vdW-
corrected DFT calculations of 1H, 13C, and 15N chemical shifts
(Figure 5). Note that vdW-corrections are important for the ac-
curate computational treatment of adsorption (and related)
1
properties of MOFs.5,24,25 For the H NMR spectra, good agree-
ment is achieved between experimental and calculated chemical
−
shifts for both the N–H carbamate (NHRCO2 ) and the ammo-
+
nium (NHR3 ) protons (Figure 5a), with a total root mean
square deviation (RMSD) of 1.3 ppm between experiment and
theory. The calculated and experimental 13C chemical shift values
have a slightly higher RMSD of 2.2 ppm, with the largest devia-
tions approaching 3 ppm (Figure 5b). While this agreement is
good given the large 13C chemical shift range (~200 ppm), the
discrepancies between experiment and theory are of similar order
to the range of 13C chemical shifts for different forms of chemi-
sorbed CO2 (~4 ppm, Tables 1 and 2). The RMSD value of 6.3
ppm for 15N is small compared to the large chemical shift range of
this nucleus (~900 ppm).
The large 1H chemical shifts of the ammonium resonances (15.1–
7.2 ppm) are consistent with the formation of hydrogen-bonded
1
ion pairs.14 The secondary and tertiary ammonium H chemical
shifts (15.1–12.3 ppm) are well predicted by our 0 K DFT calcu-
lations, indicating that the alkyl ammonium groups do not rotate
significantly on a millisecond timescale, likely due to steric re-
strictions within the pores of the material. In contrast, for primary
+
ammonium groups (RNH3 ), smaller chemical shifts of 7–8 ppm
are observed, together with somewhat stronger resonances. Aver-
aging of the DFT-calculated values for the three different H at-
Figure 5. Key experimental and vdW-corrected DFT-calculated chemical shifts for diamine–M2(dobpdc) compounds following chemisorption of
CO2 at approximately 1 bar at room temperature for at least 30 minutes. The overall root mean square deviations are given for each nucleus to quanti-
fy the typical discrepancies between experimental and DFT values. *Note that all compounds in this figure form ammonium carbamate chains, except
for dmpn–Zn2(dobpdc), which forms carbamic acid pairs. M2(dobpdc) is chiral and racemates were studied in all cases except for where indicated
with S or R labels. For 1H (a), left and right bars correspond to chemical shifts of the NHCOO– and NR3H+ groups, respectively, with the exception of
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dmpn–Zn2(dobpdc), where the right bar is the carbamic acid NRCOOH hydrogen. For 13C (b) the chemical shift of the carbamate (NRCOO–)
carbon is given, except for dmpn–Zn2(dobpdc), where the chemical shift of the carbamic acid (NRCOOH) carbon is given. For 15N (c), left and right
bars for each compound correspond to ammonium (NR3H+) and carbamate (NHCOO–) nitrogen atoms, respectively, except for dmpn–
Zn2(dobpdc), where left and right bars correspond to metal-bound amine (M–NRH2) and carbamaic acid (NRCOOH) nitrogen atoms, respectively.
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Figure 6. Summary of reported 13C chemical shifts for chemisorbed CO2 species in amine-functionalized adsorbents. Data for diamine–M2(dobpdc)
samples are from this work and Refs 15 and 19; data for amine-functionalized SBA-15 materials are from Refs 35, 41, and 45; data for IRMOF-74-III-
(CH2NH2)2 are from Ref 40; and data for PCH–APTES (porous clay heterostructure functionalized with 3-aminopropyltriethoxysilane) are from Ref
44. For discussion of mixed chemisorption structures, see below.
Overall our results indicate that the ammonium carbamate chain
formation dominates for diamine–Mg2(dobpdc) materials with
diverse diamine structures, in agreement with previous
work.5,14,15,17,18 A range of 13C chemical shifts from 161.6–163.8
ppm is observed for ammonium carbamate chains in these mate-
rials (Table 1, Figure 6), and the observed 13C chemical shift of
161.2 ppm for carbamic acid pairs falls just outside of this range
(Table 2). Factors such as the number of alkyl groups on the
nitrogen atom and the presence of backbone alkyl groups can
affect the 13C chemical shifts for ammonium carbamate chains,
while stereochemistry can also play a role for diastereomeric
compounds—M2(dobpdc) is chiral and forms as a racemate un-
der typical synthetic conditions.19
The 13C chemical shifts for chemisorbed species in diamine-
appended M2(dobpdc) materials, as well as other amine-
functionalized adsorbents from the literature35,40,41,44,45 are shown
in Figure 6. The new data for diamine–M2(dobpdc) materials
extends the known ranges of 13C chemical shifts for ammonium
carbamates and carbamic acids. Because the shift ranges for am-
monium carbamate and carbamic acid species nearly overlap
(Figure 6), the use of 13C NMR spectroscopy alone should be
treated with significant caution when making judgments regard-
ing chemisorption mechanisms in amine-functionalized materi-
als, especially given the RMSD value of 2.2 ppm between experi-
mental and calculated 13C chemical shift values. As a further note
of caution, ammonium bicarbonate formation was recently ob-
served following CO2 adsorption in amine functionalized meso-
porous silicas under wet conditions, with corresponding 13C
chemical shifts (162–163 ppm) falling within range of those ob-
served for the ammonium carbamate species identified here.50,51
Similar to the 13C chemical shifts, the 15N chemical shifts meas-
ured here also show variation from compound to compound,
further illustrating the importance of additional NMR experi-
ments beyond 1D 13C and 15N spectra to obtain conclusive mech-
anistic assignments. Indeed, recent work on amine-functionalized
silicas has shown that the use of the anisotropy of the 13C chemi-
cal shift (rather than simply isotropic chemical shifts) can enable
identification of neutral versus charged chemisorption products.52
Overall, these studies demonstrate that 1H, 13C, and 15N NMR
experiments, combined with DFT calculations, provide far more
reliable structural assignments than 1D NMR experiments and
calculations on a single nucleus.
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Figure 7. (a) CO2 adsorption isotherm of dmpn–Mg2(dobpdc) at 25 °C; data taken from our previous work (Note: a 1:1 CO2:diamine reaction gives
3.8 mmol of adsorbed CO2 per gram of material, which is reached at pressure close to 1 bar CO2).15 (b) MAS 13C NMR (16.4 T) spectra acquired by
cross polarization (contact time 1 ms) for dmpn–Mg2(dobpdc)–CO2 samples dosed at different gas pressures, which are approximated by blue circles
1
on the gas adsorption isotherm in a). (c) 15N NMR spectra acquired by cross-polarization from H (contact time 1 ms). An enlarged inset is shown
for dmpn–Mg2(dobpdc), with a broad asymmetric feature that could be deconvoluted into two resonances at 14 and 12 ppm, suggesting that the
1
exchange of the metal-bound and free ends of the diamine is slower than in dmpn–Zn2(dobpdc) (Figure 4c). (d) H → 13C HETCOR (contact time
100 µs) spectrum for dmpn–Mg2(dobpdc)–CO2 dosed at 1015 mbar. (e) Portion of the proposed structure (see Figure 8) with key 1H – 13C
HETCOR correlations indicated by blue arrows and labels A–E. Gas dosing times were >13 hours in all cases, and care was taken to avoid air expo-
sure between sample activation and gas dosing, as water vapor has a large effect on the chemisorption mechanism (see below). Gas dosing and NMR
experiments were performed at room temperature.
A Mixed Chemisorption Mechanism. Having established NMR
parameters to distinguish between ammonium carbamate chains
and carbamic acid pairs, we next studied the apparent mixed
chemisorption mechanism in dmpn–Mg2(dobpdc).15 The 25 °C
CO2 adsorption isotherm for this material is shown in Figure 7a,
and 13C NMR spectra collected at various gas dosing pressures
are shown in Figure 7b. At all pressures, the 13C NMR spectrum is
dominated by two main resonances at 162.4 and 161.0 ppm,
while two minor features are also present at ~164.6 and ~163.5
ppm at lower pressures (Figure 7b). These results suggest a dif-
ferent chemisorption mechanism from that resulting in the for-
mation of only carbamic acid pairs, as observed for dmpn–
Zn2(dobpdc) at CO2 pressures close to 1 bar (Figure 4). The
major 13C resonances at 162.4 and 161.0 ppm are assigned to
ammonium carbamate and carbamic acid, respectively, on the
basis of data in Tables 1 and 2. Following dosing with 13CO2 at
1015 mbar, the 15N NMR spectrum of dmpn–Mg2(dobpdc) ex-
hibits an ammonium resonance at 29 ppm (similar to the ammo-
nium resonance in the related material mpn–Mg2(dobpdc), see
Table 1) and a broad resonance at ~17 ppm, which we assign to a
metal-bound amine (Figure 7c). The 15N NMR spectrum was
reproducible for two independent samples (Figure S11), and
provides additional evidence for the formation of a mixture of
ammonium carbamate and carbamic acid. Only a single reso-
nance could be observed for NHCOO−/NHCOOH species,
likely due to their similar chemical shift values.42,45
The assignment of the 13C resonance at 161.0 ppm to carbamic
1
acid is further supported by data from H → 13C HETCOR ex-
periments at a short contact time of 100 µs (Figure 7d), which
1
show a strong H correlation assignable to the COOH group (1H
11.3 ppm, correlation A) and a correlation from a NHRCOOH
group (1H 3.1 ppm, correlation B). The 13C resonance at 162.4
ppm shows a strong NHCOO– correlation (1H 3.3 ppm, correla-
tion C), and surprisingly the carbamate carbon also shows a cor-
relation with the carbamic acid COOH group (correlation D),
although this correlation is weaker than it is for the 161.0 ppm 13
C
resonance. The latter correlation indicates that the hydrogen of
the carbamic acid –OH group is near the carbon of the assigned
ammonium carbamate (Figure 7e). Interestingly, the ratio of the
integrated areas of the two main resonances (as determined by
quantitative 13C NMR experiments) was found to be 1:1.0 and
1:0.9 (left resonance:right resonance) at pressures of 1015 and 26
mbar, respectively, suggesting that the two species are present in
nearly equal amounts over this pressure range. Notably, a similar
mixed chemisorption behavior was also observed for dmpn–
Zn2(dobpdc) at intermediate gas-dosing pressures—i.e., above
the adsorption step but below the pressures where conversion to
carbamic acid pairs occurs (see Figure S10). At lower gas pres-
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sures of 15 and 6.4 mbar, two additional minor resonances are ladder-like structure (Figure 8b). Our calculated chemical shifts
for this structure are in good agreement with experimental 1H
NMR shifts (Table 3), with a total RMSD of 1.3 ppm for the four
assigned 1H resonances. This proposed mixed chemisorption
structure notably serves to explain the key 1H–13C correlations
observed in the HETCOR experiments (Figure 7c): the major
correlation of the carbamic acid carbon to an acid –OH (correla-
tion A, Figure 7c) is anticipated given the short CCOOH···HCOOH
distance of 1.94 Å in the DFT-derived structure, as is the weak
correlation of the COOH hydrogen resonance with the ammoni-
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clearly seen for dmpn–Mg2(dobpdc) at ~164.6 and 163.5 ppm
(Figure 7b), assigned to ammonium carbamate species that are
not interacting with carbamic acids (see Figure S13).
9
−
um carbamate carbon (correlation D, CNHCOO ···HCOOH distance
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of 2.59 Å). This mixed structure also accounts for the apparent
1:1 ratio of these chemisorption products, with synergistic am-
monium carbamate and carbamic acid formation owing to the
hydrogen bonding between these groups.
Finally, Rietveld refinement of powder X-ray diffraction data of
CO2-dosed dmpn–Mg2(dobpdc) was carried out, using the DFT
structure (Figure 8) as a starting point (Figures S14 and S15,
Tables S1 and S2). Good agreement was obtained, though slight
discrepancies were observed between the simulated and experi-
mental diffraction pattern, which appear to be due to some disor-
der of the alkyl groups of the amine that could not be modeled.
Rietveld refinement using a pure ammonium carbamate structur-
al model from DFT showed a worse agreement to the data than
the mixed chemisorption structure.
To further explore the observed chemisorption products, we used
DFT to calculate the CO2 adsorption energies (ΔEads) associated
with their formation (Table 4). Recent vdW-corrected DFT cal-
culations have accurately predicted ΔEads for CO2 adsorption in
diamine–M2(dobpdc) materials.20,24,26,27 The experimental ad-
sorption enthalpy for CO2 in dmpn–Mg2(dobpdc) was deter-
mined to be approximately −70 kJ/mol at gas loadings ranging
from 0–2.8 mmol/g, and this value decreased to about −53
kJ/mol at a CO2 loading of 3.2 mmol/g.15 The mixed chemisorp-
tion structure introduced above is calculated to have ΔEads = −54
kJ/mol (for full CO2 capacity, i.e., 1 CO2 molecule per diamine,
see Table 4), agreeing with the heat of adsorption determined at
the highest experimental loading. The calculated value is very
slightly larger in magnitude than that predicted for carbamic acid
pairs (ΔEads = −53 kJ/mol), which are not observed experimental-
ly in dmpn–Mg2(dobpdc). Additional DFT calculations of a pure
ammonium carbamate chain structure yielded ΔEads = −69
kJ/mol, which is 15 kJ/mol larger in magnitude than that for the
mixed structure. While DFT predicts the pure ammonium car-
bamate chain structure to be the most enthalpically favored at full
CO2 loading (i.e., at ~1 bar CO2), our experimental NMR data
clearly exclude a pure ammonium carbamate mechanism under
these conditions (Figure 7). Kinetic or entropic effects,53 which
are not accounted for in the DFT calculations, may partly explain
the discrepancy between the calculated adsorption energies and
the structures observed by NMR spectroscopy.
Figure 8. Proposed mixed chemisorption structure with ammonium
carbamate and carbamic acid in a 1:1 ratio for dmpn–Mg2(dobpdc).
a) Lewis structure. b), c) Structure from DFT calculations shown for
two adjacent chains along the c axis (b) and for a projection in the ab
plane (c).
Overall, the NMR data for dmpn–Mg2(dobpdc) at CO2 pressures
close to 1 bar point toward a dominant new chemisorption mech-
anism with a 1:1 ratio of interacting ammonium carbamate and
carbamic acid species. Guided by our NMR results, a vdW-
corrected DFT survey of structures with a 1:1 ratio of ammonium
carbamate and carbamic acid led to the discovery of the structure
shown in Figure 8. Here, ammonium carbamate chains and car-
bamic acid chains propagate along the c axis, down the one-
dimensional channels of the framework. Hydrogen bonds are
present in the ammonium carbamate chain and also across the ab
plane between the carbamic acid hydrogen and the neighboring,
unbound oxygen atom of the carbamate, forming an extended
At CO2 pressures below 1 bar, the NMR results (Figure 7b) re-
veal a combination of ammonium carbamate chains and the
mixed chemisorption structure (carbamic acid–ammonium car-
bamate adducts). Interestingly, at 50% CO2 capacity, DFT calcu-
lations predict a possible structure containing 2/3 ammonium
carbamates and 1/3 carbamic acids (Figure S16 with ΔEads = –73
kJ/mol, close to the heat of adsorption determined at the corre-
sponding experimental CO2 loading. Note that to obtain equili-
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brated samples, we dosed freshly activated dmpn–Mg2(dobpdc) thermodynamic product. Finally, the carbamic acid pairs appear
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for at least 13 h. Shorter dosing times (as well as wet conditions,
see below) resulted in larger amounts of ammonium carbamate
species not interacting with carbamic acid (based on 13C NMR
data; see Figure S17), suggesting that ammonium carbamate
chains are the kinetic product, while the mixed structure is the
to be the most enthalpically favored chemisorption structure at
the maximum CO2 loading (Table 4) in dmpn–Zn2(dobpdc),
consistent with the observation of these species at high dosing
pressures
by
NMR
and
X-ray
diffraction.15
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Table 3. Experimental and vdW-corrected DFT-calculated chemical shifts for the proposed mixed structure shown in Figure 8. See
Figure S12 and the subsequent discussion for dmpn–Zn2(dobpdc).
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δ 13C (ppm)
δ 1H (ppm)
δ 15N (ppm)
Compound
Amine structure
Experiment (DFT)
Experiment (DFT)
Experiment (DFT)
NHRCO2H: 82* (64.8)
Zn–NH2R: 17 (12.1)
COOH: 11.3 strong, (12.1)
NHCO2H: 3.1 strong, (4.7)
NHCO2–: 3.3 strong, (4.5)
NH3R: 7.2 weak (8.5)
dmpn–
Mg2(dobpdc)
COO–: 162.4 (158.5)
COOH: 161.0 (157.4)
NHRCO2–: 82* (98.9)
NH3R+: 29 (25.6)
NHRCO2H: – (83.6)
Zn–NH2R: – (12.9)
COOH: 14.3 strong, (12.2)
NHCO2H: 6.0 strong, (5.7)
NHCO2–: 7.3 strong, (3.8)
dmpn–
Zn2(dobpdc)
COO–: 164.4 (158.1)
COOH: 159.9 (158.1)
NHRCO2–: – (80.4)
NH3R+: – (25.8)
*Separate NHRCO2H and NHRCO2– resonances could not be identified, likely due to the overlap of these resonances.
have been observed previously for a range of diamine-appended
Table 4. vdW-corrected DFT-calculated ΔEads values for
CO2 adsorption in dmpn–M2(dobpdc) compounds. The
calculations assume a CO2 capacity of 1 CO2 per diamine. See
Figure S12 for a discussion on dmpn–Zn2(dobpdc).
M2(dotpdc) variants, including nPr-2–Mg2(dotpdc) (nPr-2 = N-
propylethylenediamine),16 indicating that such materials are ca-
pable of cooperative adsorption by ammonium carbamate chain
formation. Indeed, NMR measurements of nPr-2–Mg2(dotpdc)
were fully consistent with the exclusive formation of ammonium
carbamate chains (Figure S18). Overall, our additional experi-
ments on dmpn–Mg2(dotpdc) indicate that interactions between
chemisorbed species in the ab plane of dmpn–Mg2(dobpdc) are
an important feature of the mixed cooperative mechanism, lend-
ing additional support to the structure proposed in Figure 8.
Compound
Structure
ΔEads (kJ/mol)
–54
–69
–53
Mixed
dmpn–
Mg2(dobpdc)
Ammonium carbamate chains
Carbamic acid pairs
Mixed
–39
–43
–53
The bulky methyl groups on the diaminopropane backbone of
dmpn offer a means to alter the chemisorption mechanisms and
the resulting thermodynamics for diamine–M2(dobpdc) materi-
als. Notably, our results help to explain the previously measured15
larger entropic penalty associated with CO2 chemisorption in
dmpn–Mg2(dobpdc) compared to the penalty in materials that
form only ammonium carbamate chains. Indeed, the interactions
between ammonium carbamates and carbamic acids in the mixed
structure presumably result in additional entropy penalties. The
results obtained here also help to explain the isotherm (and iso-
bar) shape observed for dmpn–Mg2(dobpdc), which features a
less sharp adsorption step than typical diamine–Mg2(dobpdc)
materials.15 At pressures associated with the step region, a mixture
of ammonium carbamate and carbamic acid forms and hinders
the full cooperative adsorption process that would result upon
exclusive ammonium carbamate chain formation. The mixed
mechanism may also account for the hysteresis observed upon
desorption (Figure 9a), corresponding to retention of approxi-
mately 50% of the adsorbed CO2.15 We tentatively propose that
the CO2 molecules bound as carbamic acids in the mixed struc-
ture are more easily removed upon desorption than the CO2 mol-
ecules bound within ammonium carbamate chains, and that the
ammonium carbamates thus account for the desorption hystere-
sis. Additional adsorption and desorption experiments showed
that this hysteresis is most pronounced as equilibrium conditions
dmpn–
Zn2(dobpdc)
Ammonium carbamate chains
Carbamic acid pairs
The mixed chemisorption mechanism proposed for CO2 in
dmpn–Mg2(dobpdc) yields interacting ammonium carbamate
and carbamic acid chains in the ab plane (Figure 8). To explore
the role of these interactions in driving the mixed product for-
mation, we sought to increase the distance between adsorption
sites in the ab plane by studying CO2 adsorption in dmpn–
Mg2(dotpdc) (dotpdc4−
= 4,4′′-dioxido-[1,1′:4′,1′′-terphenyl]-
3,3′′-dicarboxylate). The key structural difference between
Mg2(dobpdc) and Mg2(dotpdc) is the larger minimum M···M
distance across the ab plane—approximately 15 Å for
Mg2(dotpdc) compared to ~10 Å in Mg2(dobpdc).54 Importantly,
the distance between metal centers along the c axis is anticipated
to be very similar in both frameworks.16
The CO2 adsorption profile of dmpn–Mg2(dotpdc) is broad
compared to that of dmpn–Mg2(dobpdc), which suggests a pri-
marily non-cooperative adsorption mechanism (Figure 9a). In-
deed, NMR characterization of dmpn–Mg2(dotpdc) (Figure
9b,c) revealed a complex mixture of ammonium carbamate and
carbamic acid species, quantified overall in a 1:0.5 ratio, respec-
tively, rather than the 1:1 mixed structure observed for dmpn–
Mg2(dobpdc). We note that sharp CO2 adsorption isotherm steps
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are approached, suggesting that the mixed chemisorption struc- ture forms with slow kinetics (Figures S19 and S17).
Page 22 of 28
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Figure 9. a) CO2 isobars for dmpn–Mg2(dobpdc) and the expanded analogue dmpn–Mg2(dotpdc) collected by thermogravimetric analysis at at-
mospheric pressure using a temperature ramp rate of 1 °C/min. Solid lines show adsorption, dotted lines show desorption. The dashed line shows the
theoretical capacity, and depict the two MOF structures featuring different pore sizes and shapes). b), c) Magic angle spinning NMR (16.4 T) charac-
terization of dmpn–Mg2(dotpdc)–CO2 (gas dosing pressure 1022 mbar). b) 13C cross-polarization experiment (1 ms contact time). c) Short contact
1
time (100 µs) H → 13C HETCOR experiment. As with dmpn–Mg2(dobpdc), a long gas dosing time was used (here 14 h) before sealing the rotor.
MAS rate 15 kHz. NMR experiments and gas dosing were performed at room temperature.
Figure 10. 13C MAS NMR (16.4 T) experiments to investigate the role of water in directing CO2 chemisorption for (a) e-2–Mg2(dobpdc) and (b)
dmpn–Mg2(dobpdc). For wet experiments the sample was first exposed to wet N2 for 30 min before dosing with dry 13CO2. For dry experiments,
freshly activated samples were dosed directly with 13CO2. Gas dosing pressures were approximately 1 bar, and the MAS rate was 15 kHz for all meas-
urements. Gas dosing and NMR experiments were performed at room temperature.
The Role of Water in Directing Chemisorption. Given that
water vapor is present in most practical gas mixtures of interest
and that all of the measurements presented so far were performed
under dry conditions, it is critical to evaluate the effect of water
on the chemisorption mechanisms in diamine–Mg2(dobpdc)
materials. We first chose to study e-2–Mg2(dobpdc), which ex-
clusively forms ammonium carbamate chains as discussed above
(Figure 10a). At room temperature, activated e-2–Mg2(dobpdc)
was first exposed to wet N2 for 30 min before dosing with dry
13CO2 for an additional 30 min. Proton NMR data collected be-
fore and after dosing with wet N2 confirmed the adsorption of
water in the material (Figure S20), and 13C NMR data revealed a
spectrum very similar to that obtained under dry conditions, with
a single resonance at 162.0 ppm assigned to ammonium carba-
mate chains. Separate experiments in which e-2–Mg2(dobpdc)
was first exposed to dry 13CO2 and then wet CO2 at natural iso-
topic abundance gave very similar results (Figure S21). These
results suggest that ammonium carbamate chains can form and
persist in the presence of water, which is essential for the practical
application of these materials. Our results also help to explain
previous studies of diamine–Mg2(dobpdc) materials, wherein
measurements under dry and humid conditions resulted in simi-
lar step-like adsorption profiles and CO2 adsorption
capacities.16,39 Recent DFT calculations have suggested that am-
monium carbamate chains are stabilized by the presence of water
via hydrogen bonding interactions, indicating that water may
have a beneficial effect on the CO2 capture performance of these
materials.24
In contrast to e-2–Mg2(dobpdc), dmpn–Mg2(dobpdc) under-
goes a dramatic change in its chemisorption mechanism in the
presence of water (Figure 10b). At room temperature, activated
dmpn–Mg2(dobpdc) was exposed to wet N2 for 30 min before
dosing with 13CO2 for 15 h. Under wet conditions, the major
chemisorption products are assigned to ammonium carbamate
chains on the basis of the observed 13C chemical shifts and a two
dimensional HETCOR spectrum, which indicates at least two
different ammonium carbamate conformations (Figure S22).
This result deviates from that observed under dry conditions,
wherein the mixed chemisorption structure dominates at the
same gas pressures (Figure 10b). Despite the change of the chem-
isorption mechanism under wet conditions, the CO2 adsorption
capacity is maintained in dmpn–Mg2(dobpdc), as shown previ-
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biphenyl]-3,3'-dicarboxylic acid (9.90 g, 36.0 mmol, 1.00 equiv), N,N-
ously via breakthrough experiments under dry and wet condi-
tions.15 Our findings are fully consistent with the known instabil-
ity of carbamic acid under wet conditions in other materials,35,40 as
well as the stabilization of ammonium carbamates by hydrogen-
bonding interactions with water.24
dimethylformamide (90 mL), and methanol (110 mL). The mixture was
sonicated until the solids dissolved and was then filtered through filter
paper into a 350 mL screw-cap high-pressure reaction vessel equipped
with a stir bar. The reaction mixture was sparged with N2 for 1 h, and
then the reaction vessel was sealed and the reaction mixture stirred slowly
at 120 °C for 14 h, resulting in precipitation of a white solid from solu-
tion. The mixture was filtered to collect the solid, and the solid was quick-
ly transferred to a Pyrex jar filled with N,N-dimethylformamide (500
mL). This was placed in an oven heated to 60 °C and allowed to stand for
at least 3 h. At this time, the mixture was filtered and the solid was re-
turned to the jar with fresh N,N-dimethylformamide (500 mL) and
placed in an oven heated to 60 °C for a further 3 h. This washing process
was repeated a total of three times. The N,N-dimethylformamide was
replaced with methanol (500 mL), and this washing process was repeated
a further three times with methanol. A small portion of the solid was then
removed and placed in a vial under flowing N2. The remaining solid was
activated under flowing N2 at 180 °C for 24 h, transferred to a glass ad-
sorption tube equipped with a Micromeritics TransSeal, and activated for
an additional 24 h under reduced pressure (<10 μbar) at 180 °C. Activat-
ed Mg2(dobpdc) was obtained as a white solid. Langmuir surface area
determined from the 77 K N2 adsorption isotherm: 3900 m2/g.
Synthesis of Zn2(dobpdc). An Erlenmeyer flask was charged with
Zn(NO3)2∙6H2O (3.72 g, 12.5 mmol, 2.50 equiv), 4,4'-dihydroxy-[1,1'-
biphenyl]-3,3'-dicarboxylic acid (1.37 g, 5.00 mmol, 1.00 equiv), N,N-
dimethylformamide (250 mL), and methanol (250 mL). The mixture
was sonicated until the solids dissolved and filtered through filter paper
into a 1 L Pyrex jar. The reaction mixture was vigorously sparged with N2
for 1 h, and the jar was placed in an oven at 120 °C and allowed to stand
for 14 h, resulting in precipitation of an off-white solid from solution. The
non-homogenous mixture was filtered to collect the solid, and the solid
was quickly transferred to a Pyrex jar filled with N,N-dimethylformamide
(500 mL). The jar was placed in an oven heated to 60 °C and allowed to
stand for at least 3 h. At this time, the mixture was filtered and the solid
was returned to the jar with fresh N,N-dimethylformamide (500 mL) and
placed again in an oven heated to 60 °C for 3 h. This washing process was
repeated a total of three times. The N,N-dimethylformamide was re-
placed with methanol (500 mL), and this washing process was repeated a
further three times with methanol. The mixture was filtered, and the
collected solid was transferred to a Schlenk flask and activated under
flowing N2 at 180 °C for 8 h followed by heating under high vacuum at
180 °C for 24 h. The Schlenk flask was transferred into a N2-filled glove-
box, and the solid was transferred to a glass adsorption tube equipped
with a Micromeritics TransSeal and activated for an additional 24 h un-
der high vacuum (<10 μbar) at 180 °C. Activated Zn2(dobpdc) was ob-
tained as 1.89 g (94%) of an off-white solid. Langmuir surface area de-
termined from the 77 K N2 adsorption isotherm: 3130 m2/g. Note:
methanol-solvated Zn2(dobpdc) slowly degrades in the presence of air
and thus should be stored activated in a N2-filled glovebox when not in
use.
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CONCLUSIONS
We have carried out a systematic study of CO2 chemisorption in
materials of the type diamine–M2(dobpdc). The canonical chem-
isorption products of ammonium carbamate chains and carbamic
acid pairs have been thoroughly characterized using multinuclear
NMR experiments and vdW-corrected DFT calculations. Our
results demonstrate the importance of using multiple experi-
mental and calculated NMR parameters to make accurate struc-
tural assignments. In particular, 13C NMR experiments and calcu-
lations alone may be insufficient to make accurate structural as-
signments, owing to the small range of 13C chemical shifts for the
possible chemisorption products and relatively large discrepan-
cies between experimental and calculated shift values. Nonethe-
less, the ranges determined here for chemical shift parameters of
different chemisorption products can guide structural assign-
ments following CO2 adsorption in the wider family of amine-
functionalized materials (Figure 6). Ultimately, experiments and
calculations that give information on the connectivity of the vari-
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1
ous groups (such as H–13C two dimensional HETCOR experi-
ments) can enable clearer assignments of carbamic acid and am-
monium carbamate groups.
Our results also indicate that ammonium carbamate chain for-
mation dominates for a wide range of diamine–Mg2(dobpdc)
materials, with the exclusive formation of carbamic acid pairs only
observed thus far for dmpn–Zn2(dobpdc). Furthermore, we have
demonstrated that a new mixed chemisorption mechanism can
occur, wherein ammonium carbamate and carbamic acid form in
a 1:1 ratio. Using a combination of NMR spectroscopy and DFT
calculations, we have proposed a structure for the mixed chemi-
sorption product in dmpn–Mg2(dobpdc), whereby both ammo-
nium carbamate and carbamic acid chains form along the c axis
while interacting across the framework pore. This new mecha-
nism accounts for the unusual adsorption properties of this mate-
rial and helps to explain its excellent performance for CO2 capture
from coal flue gas. The further development of amine-appended
adsorbents with bulky functional groups therefore offers an im-
portant route for tuning the chemisorption of CO2 and the ther-
modynamics for CO2 capture at different partial pressures. Final-
ly, our results highlight the importance of water in directing CO2
chemisorption in diamine-appended MOFs, supporting the con-
clusions of previous studies that ammonium carbamate formation
is favored and carbamic acid formation is disfavored in the pres-
ence of water.35,40 The rich chemistry of amine-functionalized
materials will undoubtedly continue to reveal more complexity as
increasingly diverse materials are explored. We anticipate that
understanding and controlling this complexity will enable the
design of improved materials for CO2 capture.
Synthesis of H4(dotpdc). The ligand was prepared according to the
literature procedure.54 A 500 mL roundbottom flask was charged with a
Teflon-coated magnetic stir bar, K2CO3 (31.1 g, 225 mmol, 3.00 equiv),
and 5-bromosalicylic acid (9.80 g, 45.0 mmol, 1.00 equiv). N,N-
dimethylformamide (250 mL) was added, and the flask was placed in an
ice water bath. Iodomethane (8.40 g, 135 mmol, 3.00 equiv) was added
slowly (~1 drop/s), and the reaction mixture was stirred vigorously at
room temperature for 20 h. The reaction mixture was then poured into
1.5 L of ice water and vigorously stirred for 10 min. The resulting mixture
was filtered to yield 5-bromo-2-methoxybenzoate (7.28 g, 65.7% yield).
1H NMR (in CDCl3 solvent) was consistent with that reported in the
literature.54 δ 7.84 (d, J = 3 Hz, 1H), 7.50 (dd, J = 9, 3 Hz, 1H), 6.80 (d, J
= 9 Hz, 1H), 3.83 (s, 3H), 3.81 (s, 3H) ppm.
EXPERIMENTAL
Materials. Diamines and solvents were purchased from commercial
sources and used as received. The ligand H4dobpdc was purchased from
Hangzhou Trylead Chemical Technology Co and used as received.
Synthesis of Mg2(dobpdc). The framework was prepared according
to the literature procedure.14 An Erlenmeyer flask was charged with
Mg(NO3)2·6H2O (11.5 g, 45.0 mmol, 1.24 equiv), 4,4'-dihydroxy-[1,1'-
A 50 mL 3-neck roundbottom flask equipped with a Teflon-coated
magnetic stir bar and a reflux condenser was charged with 5-bromo-2-
methoxybenzoate (3.60 g, 14.7 mmol, 2.20 equiv), XPhos Pd G2 (0.234
g, 0.30 mmol, 0.05 equiv), and benzene-1,4-diboronic acid (0.234 g, 0.30
mmol, 0.05 equiv). The flask was placed under vacuum for 5 min, and
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Page 24 of 28
then back-filled with nitrogen. This process was repeated a total of 3
flowing N2. Meanwhile, freshly-ground CaH2 (~30 mg) was added to a
times. Next, degassed tetrahydrofuran (8 mL) and K2CO3 (8.11 g, 58.7
mmol, 4 equiv, dissolved in water at 0.5 M) were added via syringe and
the reaction mixture was allowed to stir under reflux for 20 h, during
which time a gray solid precipitated from solution. The reaction mixture
was cooled to room temperature and then poured into 75 mL of cold
water. The mixture was then filtered, and the precipitate washed with 3 ×
10 mL cold water. The gray solid was dissolved in 100 mL hot CH2Cl2
and then filtered through celite and eluted with an additional 300 mL
CH2Cl2. The resulting brown solid was then concentrated in vacuo. The
solid was triturated with 20 mL cold methanol, then filtered and washed
with 2 × 5 mL cold methanol. This trituration and filtration step was
repeated two times to yield off-white dimethyl 4,4”-dimethoxy-
[1,1':4',1''-terphenyl]-3,3''-dicarboxylate (2.32 g, 96.4% yield). 1H NMR
solution of 1 mL of e-2 and 4 mL of toluene in a 30 mL scintillation vial
equipped with a stir bar. The mixture was stirred at 100 °C under flowing
N2 for 30 min and then allowed to cool to room temperature and settle
overnight. The dried diamine solution and activated Zn2(dobpdc) were
transferred to a N2-filled glovebag, and the diamine solution was carefully
decanted from the CaH2 and added via syringe to the Zn2(dobpdc) sam-
ple. The vial containing the framework and diamine was then swirled
several times and allowed to stand at room temperature for 0.5 h. At this
time, the mixture was filtered, and the resulting powder was thoroughly
washed with successive aliquots of toluene (3 × 20 mL) and allowed to
dry on the filter paper for several minutes before proceeding to activation.
All diamine-appended MOF samples were activated under flowing N2
using activation times and heating temperatures specified in Table S3 in
the Supporting Information. Diamine loadings were determined by quan-
titative 1H NMR experiments of digested frameworks and were 100 5%
for the majority of the studied materials (Table S3). For digestion, ~5 mg
of diamine appended framework was added to a mixture consisting of 1
mL of DMSO-d6 and two Pasteur pipette drops of DCl solution (35 wt.
% in D2O, ≥ 99 atom % D).
NMR Sample Preparation. Activated diamine-appended framework
samples were packed into 3.2 mm rotors inside a nitrogen-filled glove
bag. Each sample was then evacuated inside a home-built gas manifold
(Figure 2) for at least 10 min. The key feature of this gas dosing manifold
is that rotors may be dosed at different gas pressures, then closed while
still inside the manifold.15 Samples were dosed with 13CO2 gas (Sigma-
Aldrich, 99 atom % 13C, < 3 atom % 18O) and allowed to equilibrate for at
least 30 min (unless otherwise stated). Gas pressures (see Figure cap-
tions) were recorded immediately before closing the rotor inside the
home-built gas manifold. Gas dosing was carried out at room tempera-
ture (24(2) °C). 13C NMR experiments as a function of time indicated
minimal changes to the chemisorbed CO2 resonance during the time
needed for acquisition of NMR data (Figure S23). An exception to this is
experiments on dmpn–Zn2(dobpdc) dosed at 1 bar 13CO2, where small
losses of CO2 from the rotor led to a transition from the carbamic acid
pair structure to the mixed chemisorption structure on a timescale of ~10
hours. Here experiment times thus had to be limited to avoid structural
changes during the NMR experiment (this accounts for the low signal to
noise ratio in the bottom spectrum in Figure 4c).
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(in CDCl3 solvent) was consistent with that reported in the literature.54
δ
8.07 (d, J = 2 Hz, 2H), 7.72 (dd, J = 9, 2 Hz, 2H), 7.61 (s, 4H), 7.05 (d, J
= 9 Hz, 2H), 3.94 (s, 6H), 3.91 (s, 6H) ppm.
A 100 mL roundbottom flask was equipped with a stir bar and reflux
condenser and charged with dimethyl 4,4''-dimethoxy-[1,1':4',1''-
terphenyl]-3,3′′-dicarboxylate (2.21 g, 5.40 mmol, 1.00 equiv). HBr and
AcOH (90 mL each) were added to the flask, and the solution was al-
lowed to reflux for 24 h. During this time, the mixture turned orange, and
a white solid precipitated from solution. The reaction was allowed to cool
to room temperature, then poured into 200 mL cold water and filtered.
By NMR spectroscopy, a 7% impurity of the starting material was found,
so the sample was refluxed under the same conditions and acid amounts
for an additional 48 h. It was then washed with 2 × 50 mL cold water to
yield
H4(dotpdc)
(4,4''-dihydroxy-
[1,1':4',1''-terphenyl]-3,3''-
dicarboxylic acid) (96.0% yield). 1H NMR (in DMSO-d6) was consistent
with that reported in the literature.54 δ 14.1 (bs, 2H), 11.2 (bs, 2H), 8.07
(s, 2H), 7.86 (d, J = 9 Hz, 2H), 7.71 (s, 4H), 7.07 (d, J = 9 Hz, 2H) ppm.
Synthesis of Mg2(dotpdc): A synthetic route very similar to that pre-
viously reported54 was followed. A 20 mL scintillation vial was charged
with H4dotpdc (4,4''-dihydroxy-[1,1':4′,1''-terphenyl]-3,3''-dicarboxylic
acid) (35.0 mg, 0.103 mmol, 1.00 equiv) and Mg(NO3)2∙6H2O (64.0 mg,
0.250 mmol, 2.50 equiv). Methanol (5.5 mL) and fresh N,N-
dimethylformamide (4.5 mL) were added to the vial, and the solution
was sonicated until all of the solids dissolved. The threads of the vial were
wrapped in Teflon tape, sealed with a Teflon lined cap, and heated at 120
°C in a heating block for 14 h, during which time a white solid precipitat-
ed from solution. The vial was cooled to room temperature and the mix-
ture filtered to collect the solid, which was washed thoroughly with fresh
N,N-dimethylformamide (15 mL). The solid was then transferred to a
vial filled with fresh N,N-dimethylformamide (10 mL) and allowed to
soak at 60 °C for at least 3 h. The supernatant was decanted and replaced
with fresh N,N-dimethylformamide (10 mL). This process was repeated
a total of three times. The N,N-dimethylformamide was replaced with
methanol (10 mL), and the off-white solid was soaked in methanol at 60
°C for at least 3 h. The supernatant was decanted and replaced with fresh
methanol (10 mL). This process was repeated a total of three times to
obtain the final Mg2(dotpdc) product as a pale yellow solid. Langmuir
surface area determined from the 77 K N2 adsorption isotherm: 5770
m2/g.
For experiments under wet conditions, each activated diamine–
Mg2(dobpdc) sample was initially characterized with NMR spectroscopy
and the sample was then exposed to wet N2 (generated by flowing N2 gas
through a bubbler containing deionized water) for 30 min, and further
NMR spectra were recorded. The sample was then inserted into the
home-built gas-dosing manifold, evacuated for 10 min, and then dosed
with dry 13CO2 before recording additional NMR spectra. Dosing times
were 30 min for e-2–Mg2(dobpdc) and 15 h for dmpn–Mg2(dobpdc).
Additional data was collected on initially activated framework samples
that were then first dosed with dry 13CO2 before exposure to wet natural
abundance CO2 gas (99.998% Praxair, research grade), see Figure S21.
NMR Experiments. All NMR experiments were carried out at 16.4 T
using a Bruker 3.2 mm magic angle spinning (MAS) probe with a MAS
rate of 15 kHz in all cases. Carbon-13 NMR spectra were in general ac-
quired by cross-polarization from 1H, though direct polarization was used
for the spectrum in Figure 3a. For quantification of 13C resonances, direct
excitation experiments were used with continuous wave decoupling and
with sufficiently long recycle delays to yield quantitative data. Dmfit
software55 was used to fit the spectra and quantify the proportions of the
different 13C resonances. Nitrogen-15 NMR spectra were acquired by
cross-polarization from 1H. All cross-polarization experiments were ac-
Synthesis of Diamine-Appended Metal–Organic Frameworks. Di-
amine-appended metal–organic frameworks were synthesized by directly
grafting diamines to methanol-solvated M2(dobpdc) materials, as previ-
ously reported.14 First, methanol-solvated materials (Mg2(dobpdc),
Zn2(dobpdc), or Mg2(dotpdc)) were filtered and washed with toluene
(~50 mL). The filtered framework was then added to 5 mL of a 20%
(v/v) solution of diamine in toluene. After soaking for at least 12 h, the
solid was obtained by filtration and washed with toluene (~50 mL) to
remove excess diamine before proceeding to activation. Note: for the
preparation of e-2–Mg2(dobpdc), waiting for 0.5 h was sufficient for
complete diamine-appending, although this faster route has not yet been
explored for other materials.
1
quired with continuous wave H decoupling at ~80 kHz radio frequency
field strength and with contact times of either 100 µs or 1 ms as specified
in the figure captions. For two-dimensional experiments, a HETCOR
experiment employing cross-polarization from 1H to 13C was used ac-
cording to the sequence: 90°(1H) – t1 – cross-polarization – t2, again
using continuous wave 1H decoupling. Proton NMR spectra were ac-
quired using a 90° pulse-acquire sequence. Proton, 13C, and 15N chemical
shifts were referenced to 1.8 ppm (adamantane), 38.5 ppm (adamantane,
For e-2–Zn2(dobpdc) a different method was used. Under N2, a 30
mL scintillation vial was charged with freshly-filtered methanol-solvated
Zn2(dobpdc) (~20 mg), and the vial was heated at 180 °C for 24 h under
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Journal of the American Chemical Society
tertiary carbon – left hand resonance), and 33.4 ppm (glycine),56 respec-
The Supporting Information is available free of charge on the ACS
Publications website.
Gas adsorption data, NMR spectra, powder X-ray diffraction patterns
and discussion, sample activation conditions and diamine loadings.
(PDF)
tively. NMR experiments were performed at room temperature without
temperature control. The temperature increase due to frictional heating
at 15 kHz MAS in a 3.2 mm is expected to be approximately 12 K.57
Computational Details. In order to elucidate the local structures of
the various diamine-appended frameworks, we performed first-principles
density functional theory (DFT) calculations. We used a plane-wave
basis and projector augmented-wave (PAW)58,59 pseudopotentials with
the Vienna ab initio Simulation Package (VASP) code.60–63 To include the
effect of the van der Waals (vdW) dispersive interactions on binding
energies, we performed structural relaxations with vdW dispersion-
corrected functionals (vdW-DF2)64 as implemented in VASP. DFT bind-
1
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3
4
5
6
7
8
Structure files generated by DFT calculations with the PBE function-
al and the D3 vdW correction (CIF).
AUTHOR INFORMATION
Corresponding Authors
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ing energies were calculated using ΔEads = E(CO2)–diamine–M2(dobpdc) – (E(diamine–
M2(dobpdc) + ECO2), where the contributions of the zero-point energy and
thermal energy corrections are neglected, as these are generally less than
2 kJ/mol.24 For these CO2 binding energy calculations, we used (i) a Γ-
point sampling of the Brillouin zone and (ii) a 600 eV plane-wave cutoff
energy. We explicitly treated two valence electrons for Mg (3s2), twelve
for Zn (3d104s2), six for O (2s22p4), five for N (2s22p3), four for C
(2s22p2), and one for H(1s1). All structural relaxations were performed
with a Gaussian smearing of the electronic occupations of 0.05 eV.65 The
ions were relaxed until the Hellmann-Feynman forces were less than 0.02
eV Å−1. The convergence threshold for self-consistency in the total ener-
gy was 10−5 eV.
For NMR chemical shift calculations, we performed structural relaxa-
tions with (i) a 1000 eV plane-wave cutoff energy, (ii) a 0.01 eV Å−1 force
criterion, (iii) a 1 × 1 × 3 k-point grid, (iv) a 10−7 eV self-consistency
criterion, and (v) the PBE functional66 with the D3 vdW correction;67,68
see Table S4 for optimized lattice parameters. In these input criteria, the
*reimer@berkeley.edu
*jrlong@berkeley.edu
*jbneaton@berkeley.edu
Present Addresses
†P.J.M. Department of Chemistry and Chemical Biology, Cornell
University, Ithaca, NY, 14853, U.S.A.
ACKNOWLEDGEMENTS
The experiments investigating dmpn-Mg2(dobpdc) and all of the
computational studies were supported by the U.S. Department of
Energy (DOE) under NETL grant FWP-00006194. All other exper-
imental work was supported through the Center for Gas Separations
Relevant to Clean Energy Technologies, an Energy Frontier Re-
search Center funded by the U.S. Department of Energy (DOE),
Office of Science, Office of Basic Energy Sciences, under Award DE-
SC0001015. We note that the original syntheses and initial character-
ization of dmpn–Mg2(dobpdc), mpn–Mg2(dobpdc), pn–
Mg2(dobpdc), Mg2(dotpdc), and nPr-2–Mg2(dotpdc), as reported
previously in Refs. 15 and 16, was supported by ExxonMobil Re-
search and Engineering Company. Work at the Molecular Foundry
was further supported by the Office of Science, Office of Basic Ener-
gy Sciences, U.S. Department of Energy, under Contract DE-AC02-
05CH11231, and computational resources were provided by DOE
(LBNL Lawrencium and NERSC). This research also used the Savio
computational cluster resource provided by the Berkeley Research
Computing program at the University of California, Berkeley (sup-
ported by the UC Berkeley Chancellor, Vice Chancellor for Re-
search, and Chief Information Officer). Use of the Advanced Photon
Source at Argonne National Laboratory was supported by the U. S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, under Contract No. DE-AC02-06CH11357. We thank the
Philomathia Foundation and Berkeley Energy and Climate Institute
for support of A.C.F. through a postdoctoral fellowship, the National
Institutes of Health for support of P.J.M. through a postdoctoral
fellowship (GM120799), and the Miller Institute for Basic Research
in Science for support of J.D.M. through a postdoctoral fellowship.
The content is solely the responsibility of the authors and does not
necessarily represent the official views of the National Institutes of
Health. We further thank Dr. Richard Bounds and Thomas M. Os-
born Popp for assistance with the development of gas dosing equip-
ment for NMR experiments and useful discussions. Maria Paley is
thanked for assistance with powder X-ray diffraction measurements,
Dr. Jun Xu is thanked for useful discussions, and Dr. Katie R. Mei-
haus is thanked for editorial assistance.
isotropic chemical shielding (σiso) converged within 0.1 ppm. The iso-
tropic chemical shift (δiso) was obtained using δiso = –(σiso – σref), where
σref is a reference shielding value that is 30.9 ppm, 170.5 ppm, and 225.0
1
ppm for H, 13C and 15N, respectively. The σref values for 1H and 13C were
obtained by first computing σiso values for cocaine (CSD entry code:
COCAIN10 was used as the starting point and the structure was geome-
try optimized before NMR calculation). Cocaine was used for shift refer-
encing as it has several 1H and 13C resonances for comparison between
experiment and calculation. DFT-calculated values were compared to
published experimental values,69 with σref given as the y-intercept of a
linear fit (with a fixed gradient of –1) in a plot of δiso (experimental) vs.
σiso (calculated). The σref value for 15N was determined by comparison of
DFT-calculated σiso and the experimental δiso value for glycine.
Gas Adsorption. The CO2 adsorption isotherm for e-2–
Mg2(dobpdc) was obtained by a volumetric method using a Micromerit-
ics 3Flex gas adsorption analyzer and research-grade (> 99.998%) CO2
and He. Approximately 80 mg of the framework was loaded into a pre-
weighed sample tube, which was first transferred to a Micromeritics 2420
degas manifold and heated to 100 °C at a rate of 5 °C/min under reduced
pressure (< 10 μbar). The sample was held under vacuum at 100 °C for
~3 h, after which the evacuated tube was weighed to determine the mass
of the degassed sample. The sample was then transferred air-free to the
analysis port of the 3Flex adsorption analyzer and immersed in a temper-
ature-controlled oil bath at 26(1) °C. Equilibration was defined as
< 0.01% change in average pressure over an equilibration interval of 30 s.
Gas adsorption isotherm data for dmpn–Mg2(dobpdc) and dmpn–
Zn2(dobpdc) at 25 °C was taken directly from a previous study.15
Thermogravimetric analysis (TGA) experiments were conducted us-
ing a TA Instruments TGA Q5000. Isobars were measured using a tem-
perature ramp rate of 1 °C/min, except where otherwise indicated. Sam-
ples were first activated at 130 or 150 °C (for dmpn–Mg2(dobpdc) and
Mg2(dotpdc), respectively) under flowing N2 for 20 min prior to switch-
ing the gas stream to CO2 and cooling. Masses are uncorrected for buoy-
ancy effects.
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