, 939 (2008);
et al.Rahul Banerjee,
Capture2Frameworks and Application to CO
High-Throughput Synthesis of Zeolitic Imidazolate
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NH3/HCl and ND3/DCl mixtures condensed in
an argon matrix have been reported (28). Their
spectra yield (v = 0→1) vibrational transition
values of 1371 and 1113 cm−1, respectively, for
the N..H/D..Cl stretching modes of the proton-
transferred species, NH4+Cl−and ND4+Cl−. Be-
cause our spacings are extracted from highly
anharmonic parts of the progressions and the
(0→1) vibrational frequencies are strongly
affected by matrix effects (28, 29), the agreement
is satisfactory. The first three calculated spacings
between the energy levels associated with various
excitations of the central hydrogen atom are 1718
(0 to 1), 1171 (1 to 2), and 1214 (2 to 3) cm−1.
The latter value of 1214 cm−1matches very well
the spacing between the two most prominent
peaks [1218 cm−1(0.151 eV)], suggesting that
the two strongest peaks in the (NH4+Cl−)−spec-
trum can be assigned as transitions from v″ = 0
in the anionic complex to v′ = 2 and 3, respec-
tively, in the neutral manifold (v″ denotes vibra-
tional quantum numbers in the anion, v′ signifies
those in the neutral).
The secondary vibrational structure in the
photoelectron spectra is likely associated with
low-frequency vibrational modes, a prime can-
didate being the Cl-N intermolecular stretching
mode. This is supported by the calculated de-
crease of the Cl-N distance by 0.249 Å from the
neutral to the anion (Fig. 3). Both for the neutral
and the anion, the theoretical calculations showed
strong coupling between the central hydrogen
and intermolecular stretching modes. (These two
modes are schematically depicted in Fig. 2.) We
calculated anharmonic spacings in the 155 to
172 cm−1range (~0.02 eV) for the first five ener-
gy levels of the intermolecular stretching mode
in the neutral complex.
These intermolecular stretching progressions
are similar to those seen by Lineberger and co-
workers (30) in their seminal work measuring the
photoelectron spectra of the alkali halide anions.
There are clear similarities between the current
system, NH40Cl−, and the alkali halide anions,
(MX)−, which have also been described as M0X−.
Their spectra, however, are dominated by the only
available degree of freedom, the M-X stretch,
whereas in the ammonia–hydrogen chloride sys-
tem, transitions from that mode are far less prom-
inent, with the N…H/D…Cl stretch giving rise to
the dominant transitions.
References and Notes
1. R. S. Mulliken, W. B. Person, Molecular Complexes,
A Lecture and Reprint Volume (Wiley-Interscience,
New York, 1969).
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3. A. C. Legon, Chem. Soc. Rev. 22, 153 (1993).
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5. F.-M. Tao, J. Chem. Phys. 110, 11121 (1999).
6. R. Cazar, A. Jamka, F.-M. Tao, Chem. Phys. Lett. 287, 549
7. I. Alkorta, I. Rozas, O. Mo, M. Yanez, J. Elguero, J. Phys.
Chem. A 105, 7481 (2001).
8. A. Brciz, A. Karpfen, H. Lischka, P. Schuster, Chem. Phys.
89, 337 (1984).
9. A. Famulari, M. Sironi, M. Raimondi, in Quantum
Systems in Chemistry and Physics (Springer, Netherlands,
2000), vol. 1, pp. 361–379.
10. Z. Latajka, S. Sakai, K. Morokuma, H. Ratajczak,
Chem. Phys. Lett. 110, 464 (1984).
11. G. Corongiu et al., Int. J. Quant. Chem. 59, 119 (1996).
12. S. Y. Reece, J. M. Hodgkiss, J. Stubbe, D. G. Nocera,
Philos. Trans. R. Soc. B 361, 1351 (2006).
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14. J.-Y. Fang, S. Hammes-Schiffer, J. Chem. Phys. 106, 8442
15. J. V. Coe, J. T. Snodgrass, C. B. Friedhoff, K. M. McHugh,
K. H. Bowen, J. Chem. Phys. 87, 4302 (1987).
16. MOLPRO, version 2006.1, a package of ab initio programs;
H. J. Werner et al. (www.molpro.net).
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Inc., Wallingford CT, 2004.
18. P. R. Taylor, Lecture Notes in Quantum Chemistry II,
B. O. Roos, Ed. (Springer-Verlag, Berlin, 1994).
19. We used augmented, polarized, correlation consistent
basis sets of double- and triple-zeta quality (31)
supplemented with additional diffuse s and p functions
centered on the nitrogen atom with exponents chosen to
describe the excess electron-charge distribution in the
dipole-bound anion (ClH…NH3)−.
20. B. C. Garrett, D. G. Truhlar, J. Am. Chem. Soc. 101, 5207
21. D. T. Colbert, W. H. Miller, J. Chem. Phys. 96, 1982
22. C. S. Brauer et al., J. Phys. Chem. A 110, 10025
23. C. Desfrançois et al., Phys. Rev. Lett. 72, 48 (1994).
24. R. N. Compton et al., J. Chem. Phys. 105, 3472 (1996).
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26. G. Herzberg, Annu. Rev. Phys. Chem. 38, 27 (1987).
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M. Gutowski, J. Chem. Phys. 122, 204304 (2005).
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Trans. II 80, 455 (1984).
29. M. J. T. Jordan, J. E. Del Bene, J. Am. Chem. Soc. 122,
30. T. M. Miller, D. G. Leopold, K. K. Murray, W. C. Lineberger,
J. Chem. Phys. 85, 2368 (1986).
31. R. A. Kendall, T. H. Dunning Jr., R. J. Harrison, J. Chem.
Phys. 96, 6796 (1992).
32. ChemCraft Version 1.5 (build 276), www.chemcraftprog.org.
33. This material is based in part on work supported by
National Science Foundation grant CHE-0517337
(K.H.B.). We also thank the Polish State Committee for
Scientific Research (KBN) for support under grants
DS/8000-4-0026-8 (M.G.) and N204 127 31/2963 (M.H.)
and the U.S. Department of Energy (DOE) Office of
Basic Energy Sciences Chemical Sciences program (M.G.
and G.K.S.). The research of R.A.B. was supported by the
Deutsche Forschungsgemeinschaft (DFG) through the
Center for Functional Nanostructures (CFN, Project No.
C3.3) and by a grant from the Ministry of Science, Research
and the Arts of Baden-Württemberg (Az: 7713.14-300).
M.H. is a holder of an award from the Foundation for
Polish Science (FNP). This research was performed in part
at the Molecular Science Computing Facility in the
William R. Wiley Environmental Molecular Sciences
Laboratory at Pacific Northwest National Laboratory,
operated for the U.S. DOE by Battelle.
Supporting Online Material
11 October 2007; accepted 21 December 2007
High-Throughput Synthesis of Zeolitic
Imidazolate Frameworks and
Application to CO2Capture
Rahul Banerjee,1* Anh Phan,1Bo Wang,1Carolyn Knobler,1Hiroyasu Furukawa,1
Michael O’Keeffe,2Omar M. Yaghi1*
A high-throughput protocol was developed for the synthesis of zeolitic imidazolate frameworks
(ZIFs). Twenty-five different ZIF crystals were synthesized from only 9600 microreactions of
either zinc(II)/cobalt(II) and imidazolate/imidazolate-type linkers. All of the ZIF structures have
tetrahedral frameworks: 10 of which have two different links (heterolinks), 16 of which are
previously unobserved compositions and structures, and 5 of which have topologies as yet
unobserved in zeolites. Members of a selection of these ZIFs (termed ZIF-68, ZIF-69, and ZIF-70)
have high thermal stability (up to 390°C) and chemical stability in refluxing organic and aqueous
media. Their frameworks have high porosity (with surface areas up to 1970 square meters per
gram), and they exhibit unusual selectivity for CO2capture from CO2/CO mixtures and
extraordinary capacity for storing CO2: 1 liter of ZIF-69 can hold ~83 liters of CO2at 273 kelvin
under ambient pressure.
in the synthesis of new crystalline solid-state
compounds remains relatively undeveloped.
Often, the products are either known compounds
or ones with condensed extended structures
(1–7). For multicomponent chemical systems,
such as in the synthesis of porous metal-organic
structures, it would be reasonable to assume
that the most energetically favored structures
igh-throughput methods are routinely
used in screening for activity of drug
molecules and catalysts, but their use
would result and that these would be known
structures and topologies. Another challenge in
solid-state synthesis is overcoming the propen-
1Center for Reticular Chemistry at California NanoSystems
Institute, Department of Chemistry and Biochemistry,
University of California at Los Angeles, 607 East Charles
E. Young Drive, Los Angeles, CA 90095, USA.2Department
of Chemistry and Biochemistry, Arizona State University,
Tempe, AZ 85287, USA.
*To whom correspondence should be addressed. E-mail:
firstname.lastname@example.org (R.B.); email@example.com (O.M.Y.)
VOL 31915 FEBRUARY 2008
on February 15, 2008
sity for producing multiple phases when mixed
linkers are used in the synthesis (i.e., several
phases each containing one kind of linker,
rather than one phase containing mixed “hetero”
We show that high-throughput methods
can be successfully applied to developing a
robust synthesis protocol for ZIFs. Not only
does this approach consistently yield only tet-
rahedral porous ZIF structures, but it also
allows access to ZIF topologies previously un-
realized in ZIFs or in other zeolite materials.
We also show that ZIFs with heterolinks can
be produced that provide a greater level of
complexity into the pore composition and
structure, thus impacting the selectivity and
multifunctionality of the pores. Specifically,
the high-throughput syntheses and structures
of 25 ZIFs are described and studied. All are
open tetrahedral structures: 10 have heterolinks,
16 are previously unreported compositions
and structures, and 5 have topologies hereto-
fore unobserved in zeolite science. Among them
are ZIFs with large (>10 Å) aperture pores,
which opens up possibilities for application of
these materials. We show that their synthesis
is scalable to multigram quantities, using the
same conditions used in the high-throughput
method, and that the frameworks have ex-
traordinary thermal and chemical stability, as
well as high porosity. We find that several
ZIFs with heterolinks have pores that can
affect exceptional selective capture and stor-
age of CO2.
We (8, 9) and others (10–14) have recently
reported the synthesis of ZIFs in which (i)
transition metal atoms (M, specifically Zn and
Co) replace T atoms (tetrahedral linkers such as
Si, Al, and P) and (ii) imidazolates (IMs)
(Scheme 1) replace bridging oxides in zeolites.
Given that the M–IM–M angle is near 145° and
that it is coincident with the Si–O–Si angle pre-
ferred andcommonly found in manyzeolites,we
were hopeful that the class of ZIF materials
would at least be as extensive as that of zeolites
(15, 16). We set out to develop the ZIF portfolio
and quickly found that the traditional synthesis
used to discover the original ZIFs was tedious,
unpredictable, and time consuming, and that it
required wastefully large amounts of solvents
and reagents (8, 9). We targeted, among other
goals, structures with heterolinks (Scheme 1)
based on awide variety of availableIM units and
studied their application to the selective capture
The general ZIF reaction examined is com-
posed of one or two of these nine different
Fig. 1. The ZIF crystals presented and discussed in this paper. Each row has
the nets (blue line and black dot drawings) shown stacked on top of the tiles
the net labeled with the three-letter net symbol (18), followed by the single-
crystal XRD structures of ZIFs corresponding to each of the nets. The largest
cage in each ZIF is shown with ZnN4tetrahedra in blue and CoN4in pink. The
yellow ball is placed in the structure for clarity and to indicate space in the
stick representation (C, black; N, green; O, red; Cl, pink)].
15 FEBRUARY 2008VOL 319
on February 15, 2008
IM-type links, which were reacted with either
zinc(II) nitrate or cobalt(II) nitrate in N,N´-
dimethylformamide or N,N´-diethylformamide.
In total, we used 100 plates (9600 wells, 0.30 ml
reactant volume per well) [supporting online
material (SOM) text S1]. The metal-to-linker
mole ratio was varied from 1:1 to 1:12. These
amounts were dispensed with an automated
dispensing unit charged with a stock solution
whose concentration was also varied from
0.075 to 0.20 M for both reactants. After
loading the mixture of reactants into the wells,
the plates were covered with a polytetrafluoro-
ethylene sheet, sealed, and then heated to a
temperature range of 65° to 150°C for 48 to
100 hours. Crystalline products of ZIFs were
obtained in this temperature range. Photo-
graphic images of wells containing crystals (0.1
to 1.0 mm) are shown in SOM text S2. After a
preliminary analysis by automated powder x-ray
diffraction (PXRD) (17), crystals for single-crystal
x-ray diffraction (XRD) studies were then se-
lected from those wells containing new ma-
terials (SOM text S2). In general, we found that
a concentration level of 0.20 M, a reaction time
of 72 hours, and an isothermal temperature of
85° or 100°C were optimal for ZIF synthesis
We isolated 25 different crystals using this
protocol for single-crystal structural characteriza-
tion (Table 1, Fig. 1, and SOM text S2). Among
the 25 crystal structures, 9 are based on ZIFs al-
to 23) (8, 9), whereas 16 have a new composition
Table 1. The ZIFs discovered by high-throughput synthesis. Dashes indicate no zeolite symbol.
*Discovered by traditional synthesis methods.†N is the number of vertices of the largest cage.
Fig. 2. Gas adsorption isotherms and CO2capture properties of ZIFs. (A) The
N2adsorption isotherms for heterolinked ZIF-68, 69, and 70 at 77 K. P/P0,
relative pressure; STP, standard temperature and pressure. (B) The CO2and
CO adsorption isotherms for ZIF-69 at 273 K. For (A) and (B), the gas uptake
and release are indicated by solid and open symbols, respectively. (C)
Breakthrough curves of a stream of CO2/CO mixture passed through a sample
of ZIF-68 showing the retention of CO2in the pores and passage of CO.
VOL 31915 FEBRUARY 2008
on February 15, 2008
metal-organic compounds, and five have tetrahe-
dral topologies (dia, cag, frl, lcs, and zni) not
occurring in zeolites. The nets of the structures
are denoted by a bold lowercase three-letter sym-
bol (18) that is often the same as that of the
corresponding zeolitenet (Table 1). Furthermore,
10 structures (ZIF-60 to 62, 68 to 70, and 73 to
76) contain two chemically different imidazolate
links (i.e., heterolinks). In the Cambridge Struc-
24 ZIF structures have been reported in the past
12 years; however, using the methodology de-
scribed here,in 3 months,we optimized the reac-
tion and identification conditions and produced
crystals of ZIF-2 to 77 from the examination of
only 9600 wells.
We analyze the complexity of the nets in
terms of their natural tiling, which is a unique
partition of space into tiles such that the set of
edges and vertices of the tiles is the same as that
of the net (19). A useful measure of structural
complexity is the transitivity pqrs, which records
that the tiling has p kinds of vertices, q kinds
of edges, r kinds of faces, and s kinds of tiles.
Of the 14 topologies recorded in Table 1 and
shown in Fig. 1, all but one (frl) have just one
kind of vertex (uninodal). Of the 176 recognized
zeolite topologies, 21 are uninodal; we have found
9 of these in this study. Of the uninodal zeolite
nets, only three (GIS, SOD, and ABW) have just
one kind of tile (isohedral), and we have found
two of them.
Of the five nonzeolite nets found and men-
tioned above, four are uninodal and three are
ZIF chemistry, we are selecting the simpler nets
net has transitivity 24 27 41 19 (20), which
suggests that there is vast potential for the use of
this high-throughput method in accessing ZIFs
with structures based on more complex zeolites.
The only binodal net (frl) found is of interest
because it is simply related to the net (sra) of the
missing isohedral zeolite net ABW, as detailed
elsewhere (21). The topology (lcs) of ZIF-72 is
also of interest because it is that of an invariant
lattice complex (symbol S), but, despite its
simplicity, it has not been previously reported in
any tetrahedral structure.
We tabulate the density of the ZIFs using
the traditional zeolite measure of the number of
The density (T/V) of an IM analog (i.e., ZIF) is
typically one-eighth that of a silicate zeolite be-
cause, in an IM framework containing zinc(II),
the Zn...Zn distance is ~6.0 Å, whereas the cor-
responding Si...Si distance in a silicate is ~3.0 Å.
For the structures reported here, T/Vis in the range
2.0 to 4.6 nm–3; whereas the density for oxide
zeolites is 12 to 20 nm–3. This difference
indicates that ZIF frameworks are more open
and more amenable to functionalization of their
The existence of two different types of IMs
with a side chain (especially an NO2or a CH3
group) or an aromatic ring on the link makes
series (Fig. 1). Furthermore, the diameter of the
ranges from as low as 0.7 Å to as high as 13.1 Å,
whereas the diameter of the largest sphere that
will fit into the cavities (dp) varies from 0.7 to
for H in determining the appropriate sphere size.
For ZIF-69, 71, 72, and 77, where the atoms
nearest to the center of the cages are either Cl
(69, 71, and 72) or O (77), van der Waals radii
of 1.8 Å (Cl) and 1.5 Å (O) were used. The
values of daand dpprovide a lower limit to the
cage volume because, in some cases, the cages
are ellipsoidal. The number of vertices of the
largest cage in each structure ranges from 10
(dia) to 48 (lta). The cage face symbol (in
which [...nm...] signifies that the cage has m
faces that are n rings) and the transitivities of
the nets are given in Table 1.
In all of the ZIFs, a Zn or Co atom is con-
nected to four IM or substituted IM linkers to
create a corresponding tetrahedron (Fig. 1 and
SOM text S2). The tetrahedra are linked by
corner-sharing into different three-dimensional
zeolitic frameworks. However, these ZIFs differ
in the nature of the functional groups decorating
(Table 1). Across the series, the metrics are
systematically varied in increments of less than
1 Å; such tunability is unusual and potentially
useful in gas adsorption and separation. We first
needed to show for the ZIFs of interest that the
microscale synthesis and the reaction conditions
used for their discovery in the high-throughput
instrument could be translated into multigram-
scale bulk synthesis. For seven chosen ZIFs of
heterolinks (ZIF-60, 61, 68 to 70, 74, and 76),
we found that the microsynthesis conditions
are scalable to 10-g scale and pure ZIF mate-
rials can thus be obtained (SOM text S1).
We targeted ZIF-68, 69, and 70 for adsorp-
tion studies because they all have the same
topology (gme) and large pores (7.2, 10.2, and
15.9 Å in diameter for ZIF-69, 68, and 70,
respectively) connected through tunable aper-
tures (4.4, 7.5, and 13.1 Å). These ZIFs are
permanently porous metal-organic frameworks
in which the pore walls contain heterogeneous
link functionality. We first examined the struc-
tural, thermal, and chemical stability, as well
as the porosity, of these ZIFs. Thermal grav-
imetric analysis (TGA) was performed on the
as-synthesized, solvent-exchanged, and acti-
vated ZIF products of ZIF-68 to 70, which re-
vealed a thermal stability range of up to 390°C.
Specifically, the TGA trace for these ZIFs
showed a gradual weight-loss step between
25° and 168°C. A plateau between 150° and
390°C indicates that the evacuated framework
has high thermal stability (SOM text S3). The
evacuated frameworks of ZIF-68 to 70 thus
produced have PXRD patterns that are coin-
cident with the corresponding patterns simu-
text S3), which indicates that heterolinked ZIF
frameworks have high structural and thermal
stability. Examination of their chemical sta-
bility was performed by heating the samples
in boiling benzene, methanol, and water for 7
days: conditions that reflect potential extreme
industrial requirements. Notably, all of the
ZIFs retained their structures under these con-
ditions, as evidenced by the sharp, unshifted
diffraction lines in their PXRD patterns (SOM
The permanent porosity of these ZIFs was
also demonstrated by N2adsorption measure-
exhibit type I adsorption isotherm behavior typi-
cal of materials of permanent porosity (Fig. 2A).
The Langmuir surface areas were 1220, 1070,
and 1970 m2g−1for ZIF-68, 69, and 70, respec-
tively; these surface areas are more than double
those of the most porous zeolites (22) and sig-
nificantly higher than those of other reported
ZIFs (8, 9).
The exceptional stability and metric at-
tributes of these ZIFs led us to evaluate their
behavior for a particularly difficult gas sepa-
ration: CO2from CO. The adsorption isotherms
for all three ZIFs show a disproportionately
high affinity and capacity for CO2(SOM text
S5), with ZIF-69 outperforming ZIF-68 and
ZIF-70, as well as the state-of-the-art material
BPL carbon (23) (Table 2 and Fig. 2B). Ad-
sorption is completely reversible, and we cal-
culate that 1 liter of ZIF-69 can store 82.6
liters (162 g) of CO2at 273 K. The selectivity
is further supported by preliminary break-
through experiments, which show complete
retention of CO2and passage of CO through
the pores of ZIF-68, 69, and 70 when they are
exposed to streams containing a binary mix-
ture of CO2/CO (50:50 v/v) at room tempera-
ture (Fig. 2C and SOM text S5). In comparison
Table 2. Comparison of gas separation selectivity of ZIFs and BPL carbon (SOM text S5).
Material Gas pairs ZIFs selectivityBPL carbon
15 FEBRUARY 2008VOL 319
on February 15, 2008
with that of BPL carbon, ZIFs have higher Download full-text
selectivity (Table 2). In terms of storage capacity
and selectivity to CO2, ZIF-69 and 70 outper-
form BPL carbon and all the other ZIFs that we
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17. The isolation of the sample array is accomplished
in parallel by sonication and transfer in a custom-
designed shallow metal plate, which allows the presence
of a small amount of solvent during the PXRD data
collection. The samples were then analyzed by a Bruker
D8 DISCOVER high-throughput PXRD instrument with a
movable horizontal x-y stage for automated analysis
and an image plate detector system. The data collection
time was 3 to 6 min per sample. The PXRD patterns
thus collected were compared against an in-house
library of PXRD patterns of known ZIFs and other
18. Reticular Chemistry Structure Resource (http://rcsr.anu.
19. O. Delgado-Friedrichs, M. O’Keeffe, O. M. Yaghi, Acta
Crystallogr. A 59, 22 (2003).
20. V. A. Blatov, O. Delgado-Friedrichs, M. O’Keeffe,
D. M. Proserpio, Acta Crystallogr. A63, 418 (2007).
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Powders and Porous Solids (Academic Press, London,
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24. The work was supported by Badische Anilin und Soda
Fabrik (BASF) Ludwigshafen for synthesis, the U.S.
Department of Energy (DEFG0206ER15813) for
adsorption and separations studies, and the U.S.
Department of Defense (W911NF-061-0405) for
equipment used for breakthrough experiments.
Crystallographic data for the structures reported in this
paper have been deposited with the Cambridge
Crystallographic Data Centre under reference numbers
CCDC 671067 to 671089. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.
html (or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK).
Supporting Online Material
SOM Text S1 to S5
Figs. S1 to S86
Tables S1 to S23
2 November 2007; accepted 3 January 2008
Rogue Mantle Helium and Neon
The canonical model of helium isotope geochemistry describes the lower mantle as
undegassed, but this view conflicts with evidence of recycled material in the source of ocean
island basalts. Because mantle helium is efficiently extracted by magmatic activity, it cannot
remain in fertile mantle rocks for long periods of time. Here, I suggest that helium with high
3He/4He ratios, as well as neon rich in the solar component, diffused early in Earth’s history from
low-melting-point primordial material into residual refractory “reservoir” rocks, such as dunites.
The difference in3He/4He ratios of ocean-island and mid-ocean ridge basalts and the preservation
of solar neon are ascribed to the reservoir rocks being stretched and tapped to different
extents during melting.
rium. In contrast, most of the3He present is a
regular stable nuclide. The relative abundances
of the two isotopes in oceanic basalts there-
fore reflect the evolution of the parent/daughter
ratio (U+Th)/He. These three elements are
strongly incompatible (i.e., excluded from the
structure of the major silicate materials), but
one of them (He) is markedly affected by out-
gassing. Helium preferentially partitions not only
into any gas phase present, but also into liquid
during melting (1). Contribution from primitive
undegassed mantle gives basalts very low4He/
3He ratios. The canonical model holds that high
4He/3He ratios characterize high (U+Th)/He
mantle sources, such as the mantle underneath
mid-ocean ridges, which were degassed during
successive melting events. If He is less incom-
patible than both Th and U, however, these
elium-4 is a radiogenic nuclide produced
in Earth and other planetary bodies by
the alpha decay of uranium and tho-
low3He/4He regions could instead signal man-
tle that was depleted in incompatible elements
upon melting (1, 2). However, the abundances
of rare gases differ between these two models,
being high for undegassed mantle and very low
for residual mantle.
For historical reasons, He isotopic compo-
sitions are reported upside down as R/Ratm, where
R denotes the3He/4He ratio and the subscript
signals the normalization to the atmospheric
ratio. More than 30 years of observations have
shown that mid-ocean ridge basalts (MORBs)
are characterized by a narrow range of3He/4He
ratios clustering about 8 Ratm, whereas values in
excess of 20 Ratmare found nearly exclusively
in ocean-island basalts (OIBs) (3–6). These data
support the idea that MORBs are derived from
parts of Earth’s mantle that are significantly more
degassed than the source of OIBs (5, 7, 8). If
pushed to the extreme, the assertion that OIBs
are tapping a deep, largely undegassed part of
the mantle implies that the lower mantle is pris-
tine and that mantle convection takes place as
separate layers (9, 10).
This canonical view, however, conflicts
with several critical observations on OIBs.
Nearly every isotopic system involving lithophile
the mantle source of MORB and OIB is de-
UMR CNRS 5570, Ecole Normale Supérieure et Université de
Lyon 1, 69007 Lyon, France. E-mail: firstname.lastname@example.org
diffusion length scale (m)
Age of Earth
diffusion time scale a 2/D (years)
Fig. 1. Relationship between the time and length
scales of diffusion in olivine, using the data of
Shuster et al. (20) at two different temperatures.
The two isotopes of He have comparable diffusion
rates. At a temperature of 1460°C, He and Ne have
the same diffusivity in quartz, which suggests that
He and Ne diffusivity at mantle temperatures in
mantle minerals may not be very different. The set
of values labeled “Age of Earth” show that over the
geological ages, He and Ne may have moved by
diffusion over distances in excess of several kilome-
ters. Assuming that melting takes place in the up-
permost 100 km and an upwelling velocity of 10 m
year–1beneath OIBs and 10 cm year–1beneath
MORBs gives time scales for diffusion; the corre-
sponding distances of diffusion relevant to melt
extraction for MORB and OIB can be read from
the curve (see text).
VOL 31915 FEBRUARY 2008
on February 15, 2008