Synthesis, characterization, and catalytic application of a cationic metal-organic framework: Ag2(4,4′-bipy)2(O 3SCH2CH2so3)

Article abstract of DOI:10.1021/cm9032308

We report a silver-based cationic metal-organic framework with two mixed organic linkers directing the structure. The structure consists of ID Ag(4,4′-bipy) cationic chains arranged into closepacked layers. Weakly bound alkanedisulfonate anions charge-balance the layers, where only one oxygen of each sulfonate makes a long contact with the Ag. The unsaturated linear Ag centers likely allow the Lewis acidity displayed by the material for ketone protection as well as esterification. The material showed no reduction in yield after three catalytic runs with average 95% conversion yield for ketal formation and 57% for esterification without further water removal. In addition to hydrothermal conditions, the structure can be synthesized by reflux or room temperature, with almost identical catalytic ability. Other properties of this compound including chemical and thermal stability are also described.

Full text of DOI:10.1021/cm9032308

Chem. Mater. 2010, 22, 2027–2032 2027
DOI:10.1021/cm9032308
Synthesis, Characterization, and Catalytic Application of a Cationic
0
Metal-Organic Framework: Ag (4,4 -bipy) (O SCH CH SO )
2
2
3
2
2
3
Honghan Fei, Latisha Paw U, David L. Rogow, Marc R. Bresler,
Yashar A. Abdollahian, and Scott R. J. Oliver*
University of California, Santa Cruz, Department of Chemistry and Biochemistry, 1156 High Street,
Santa Cruz, California 95064
Received October 20, 2009. Revised Manuscript Received January 22, 2010
We report a silver-based cationic metal-organic framework with two mixed organic linkers
directing the structure. The structure consists of 1D Ag(4,4 -bipy) cationic chains arranged into close-
0
packed layers. Weakly bound alkanedisulfonate anions charge-balance the layers, where only one
oxygen of each sulfonate makes a long contact with the Ag. The unsaturated linear Ag centers likely
allow the Lewis acidity displayed by the material for ketone protection as well as esterification. The
material showed no reduction in yield after three catalytic runs with average 95% conversion yield for
ketal formation and 57% for esterification without further water removal. In addition to hydro-
thermal conditions, the structure can be synthesized by reflux or room temperature, with almost
identical catalytic ability. Other properties of this compound including chemical and thermal stability
are also described.
1
2
seeking stable and easily recoverable catalysts. As yet,
Introduction
there are limited systematic reports concerning hetero-
6,13
geneous catalysis by porous coordination frameworks.
Microporous MOFs have been investigated recently and
show that the active site is the Lewis acidic coordinatively
5,6,14,15
unsaturated metal.
Cationic metal-organic frameworks are a small sub-
group of MOFs where the positive charge of the metals is
stoichiometrically greater than the negative (or neutral)
charge of organic linker. Charge-balancing extraframe-
Hybrid inorganic-organic solid-state compounds are
an emerging class of materials with exponential growth in
recent years, especially metal-organic frameworks
1
-3
(
MOFs).
Considering some retain a robust and geo-
metrically defined structure at elevated conditions, this
group of extended coordination frameworks have a wide
4
-6
7,8
range of application, such as catalysis,
gas storage,
9
10
11
gas absorption, ion exchange, and drug delivery. Due
to the great variety of metal units and organic linkers,
these multifunctional materials have almost infinite pos-
sible combinations, and more recent efforts have been
made to establish their structure-property relationships.
Heterogeneous catalysis using MOFs has attracted
much effort due to increased environmental concerns,
work anions function as structure directing agents
16-18
(SDAs) and reside in the framework.
Positively
charged MOFs may lead to a greater degree of coordina-
tively unsaturated metals owing to the weak electrostatic
interaction between the metal and extraframework an-
ions. Our group, among others, is taking the initial steps
of applying cationic hybrid inorganic-organic frame-
1
9
*
Corresponding author. E-mail: soliver@chemistry.ucsc.edu.
1) Cheetham, A. K.; Rao, C. N. R.; Feller, R. K. Chem. Commun.
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works to catalysis. Nitrate, perchlorate, and ethanedi-
sulfonate anions have been shown to direct the growth of
cationic oxides and fluorides based on lower p-block
elements. Hydrogen bonding and/or electrostatic interaction
(
2
(
(
˚
˚
bond length to be 2.44 A; 92.7% (over 7000 hits) are less than 2.70 A.
(
(
(
4) Wu, C. D.; Hu, A.; Zhang, L.; Lin, W. B. J. Am. Chem. Soc. 2005,
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(
9) Kondo, A.; Noguchi, H.; Ohnishi, S.; Kajiro, H.; Tohdoh, A.;
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Lett. 2006, 6, 2581–2584.
(
(
(
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2
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11737–11741.
4
r 2010 American Chemical Society
Published on Web 02/11/2010
pubs.acs.org/cm

2
028 Chem. Mater., Vol. 22, No. 6, 2010
Fei et al.
between the cationic host and SDA anion give rise to
coordinatively unsaturated metal sites, thus yielding effi-
cient Lewis acidity.
A recent goal of our group is to isolate cationic
extended frameworks based on transition metals which
are thus more predictable in terms of coordination and
properties. Aside from the well-established layered dou-
ble hydroxides (LDHs)/hydrotalcite minerals, there are
very few reports detailing cationic extended coordination
temperature for 10 min and then transferred to a 15 mL Teflon
lined autoclave to 2/3 full. The autoclaves were heated at 150 °C
for 5 days under autogenous pressure, during which the pH of
the reactant solution increased from 1.8 to 2.2. Colorless large
block crystals were isolated after filtration and rinsed by acetone
1
9-23
(
yield: 0.54 g, 98.7% based on silver nitrate). IR (KBr pellets):
467s, 3050 m (O-H stretch); 1605s, 1535s, 1490s, 1418s
aromatic C-H stretch); 1328s (CH stretch); 1200 m, 1070 m
3
(
(
2
-
-1
SO3 stretch); 863s, 817s (aromatic C-H bending) (cm ).
0
Crystals of Ag
2
(4,4 -bipy)
2
(O
3
SCH
2
CH
2
SO
3
) 4H
2
O can also
3
2
3,24
polymers constructed from transition metals.
(
Silver-
I), cadmium(II), and copper(I) are isoelectronic, with
be synthesized with no need for autoclaves by refluxing or
stirring at room temperature. Reactions were carried out with
the same ratio of reactants and crystals were filtrated after ca.
60 h (refluxing) and 5 days (stirring) reaction time. The yield is
1
0
25
filled d shells that favor rigid, linear structures. Sev-
eral Ag(I) organosulfonate MOFs have been reported but
are condensed neutral or open anionic frameworks.
Catalysis application of open cationic structures based on
2
6-29
7
4.1% (0.40 g) with refluxing and 83.3% (0.45 g) with stirring
both yields again based on silver nitrate).
(
1
d
0
Heterogeneous Catalysis. A 100 mg (0.13 mmol) portion of
the as-synthesized crystal catalyst, 70 mmol of 2-butanone, and
0 mmol of ethylene glycol were introduced into 80 mmol of
shell metal ions remain largely unexplored and may
form low-coordinate open metal sites for possible Lewis
acidity.
Herein, we report a silver-based 1D cationic coordina-
7
toluene, the latter was used as the solvent for a ketal formation
reaction. The reaction was refluxed at 110 °C under Dean-
Stark conditions for specified time intervals. The catalyst was
isolated by filtration and reused on subsequent reactions with-
out further treatment. A 100 mg portion of SLUG-21 was also
applied for an esterification between 70 mmol acetic acid and 70
mmol ethanol in 8 mL toluene. The reactants were refluxed for
0
0
tion polymer with 4,4 -bipyridine (4,4 -bipy) linker and
templated by 1,2-ethanedisulfonate (EDS). The structure
is a rare example employing mixed organic linkers to
direct the cationic extended frameworks. Two different
organic linkers support Ag(I) with enough openness to
allow access by incoming guests. The synthesis, thermal
properties, catalytic reactivity, and reusability are inves-
tigated in detail.
8 h without further methods for water removal. All product
1
yields were determined by H NMR (Supporting Information).
Instrumental Details. Samples for powder X-ray diffraction
(
PXRD) were measured on a Rigaku Americas Miniflex Plus
diffractometer and were scanned from 2 to 60° (2θ) at a rate of 2°
2θ) per minute and 0.04° step size, under Cu-KR radiation
Experimental Section
(
˚
Reagents. Silver nitrate (AgNO
3
, Fisher g99.7%), 1,2-etha-
SCH CH SO H, TCI Inc. 95%),
, Acros Organics, 98%] were used
(λ = 1.5418 A). Single-crystal X-ray diffraction data was
obtained a Bruker SMART APEX II CCD area detector X-ray
diffractometer under graphite monochromated Mo-KR radia-
nedisulfonic acid (EDSA: HO
0
3
2
2
3
and 4,4 -bipyridine [(C
5
H
4
N)
2
˚
as-received for the synthesis. 2-Butanone (CH COCH CH ,
3
tion (λ = 0.71073 A). An empirical absorption correction was
2
3
Acros Organics, 99%), 2-pentanone (TCI America, 97%),
ethylene glycol (HOCH CH OH, Acros Organics, 99%), acetic
acid (glacial, CH COOH, MP Biomedicals, 99%), formic acid
applied using SADABS, and the structure was solved by direct
3
0
2
2
method and refined with SHELXTL. All non-hydrogen atoms
were refined with anisotropic thermal displacement parameters.
Crystal structure views were obtained using Diamond v3.2 and
rendered by POV-Ray v3.6.
3
(
(
Acros Organic, 99%), acetone (Fisher, 99.9%), and toluene
CH , Fisher, 99.7%) were used as-purchased for the
C
H
6 5
3
catalytic studies.
Synthesis. Colorless crystals of Ag (4,4 -bipy) (O SCH -
Thermogravimetric analysis (TGA) was performed using a
TA Instruments 2050 TGA by heating from 25 to 600 °C under
0
2 2 3 2
CH
2
SO
3
) 4H
2
O (which we denote as SLUG-21: University of
California, Santa Cruz No. 21) was synthesized under hydro-
2
N purge with a gradient of 15 °C/min. In-situ mass spectra
3
coupled to the TGA were collected on a Pfeiffer Vacuum
ThermoStar GSD 301 T3 mass spectrometer with a 70 eV
ionization potential. Scanning electron micrographs (SEM)
thermal conditions. A reactant solution with a molar ratio of
0
1
:1:1:400 for AgNO
3
:EDSA:4,4 -bpy:H
2
O was stirred at room
1
were collected on a Hitachi S-2700 SEM; H NMR spectra were
collected with a Varian Oxford 600 MHz spectrometer by
dissolving the sample in 700 μL of deuterated choloroform with
tetramethylsilane as the internal standard.
(
20) Rogow, D. L.; Zapeda, G.; Swanson, C. H.; Fan, X.; Campana,
C. F.; Oliver, A. G.; Oliver, S. R. J. Chem. Mater. 2007, 19, 4658–
4
662.
21) Tran, D. T.; Zavalij, P. Y.; Oliver, S. R. J. J. Am. Chem. Soc. 2002,
24, 3966–3969.
(
(
1
22) Gandara, F.; Puebla, E. G.; Iglesias, M.; Proserpio, D. M.; Snejko,
N.; Monge, M. A. Chem. Mater. 2009, 21, 655–661.
23) Oliver, S. R. J. Chem. Soc. Rev. 2009, 38, 1868–1881.
24) Forster, P. M.; Tafoya, M. M.; Cheetham, A. K. J. Phys. Chem.
Solids 2004, 65, 11–16.
25) Wu, J. H.; Hao, S.; Lan, Z.; Lin, J. M.; Huang, M. L.; Huang, Y. F.;
Li, P. J.; Yin, S.; Satot, T. J. Am. Chem. Soc. 2008, 130, 11568.
26) Makinen, S. K.; Melcer, N. J.; Parvez, M.; Shimizu, G. K. H.
Chem.;Eur. J. 2001, 7, 5176–5182.
Results and Discussion
(
(
Crystals of SLUG-22 can be synthesized hydrother-
mally between 125 and 150 °C with reproducible crystal
size and morphology (see optical micrographs in the
Supporting Information, Figure S1). Reflux without
applied pressure as well as stirring at room temperature
may also be used. Micrographs (Figures 1 and S1) show
that reflux also achieved high quality crystals with similar
(
(
(
(
(
27) Hoffart, D. J.; Dalrymple, S. A.; Shimizu, G. K. H. Inorg. Chem.
2
005, 44, 8868–8875.
28) Ma, J. F.; Yang, J.; Li, S. L.; Song, S. Y. Cryst. Growth Des. 2005, 5,
07–812.
29) Cote, A. P.; Ferguson, M. J.; Khan, K. A.; Enright, G. D.;
Kulynych, A. D.; Dalrymple, S. A.; Shimizu, G. K. H. Inorg.
Chem. 2002, 41, 287–292.
8
(30) SHELXTL Crystal Structure Determination Package; Bruker Ana-
lytical X-ray Systems Inc.: Madison, WI, 1995-2010.

Article
Chem. Mater., Vol. 22, No. 6, 2010 2029
Figure 1. SEM images of SLUG-21 under hydrothermal conditions (left), refluxing (middle), and room temperature stirring (right). The scale bar is 40 μm
for the left image, and 20 μm for both the middle and right images.
Table 1. Crystal Data and Structure Refinement of SLUG-21
empirical formula
22 2 4 10 2
C H28Ag N O S
crystal system
space group
triclinic
P1
788.36
formula weight (g/mol)
˚
a (A)
8.1174(17)
9.2860(19)
10.297(2)
71.589(2)
75.392(2)
71.286(2)
687.6(2)
1
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
volume (A )
Z
crystal size (mm )
3
˚
3
0.06 ꢀ 0.05 ꢀ 0.03
color of crystal
goodness-of-fit on F
colorless
1.045
2
final R indices I > 2σ(I)
R indices (all data)
R
R
1
1
= 0.0262, wR
= 0.0288, wR
2
2
= 0.0559
= 0.0569
almost linear geometry (two similar N-Ag-N angles
of 171.5(3)° and 172.3(3)°], with Ag-N bond lengths in
the range of 2.156(6)-2.170(7) A. Two weak interactions
occur between each Ag atom and the oxygen of the EDS
Figure 2. PXRD of as-synthesized SLUG-21 prepared from (a) hydro-
thermal methods, (b) refluxing, and (c) stirring. Theoretical data from the
structuresolved bysingle-crystalX-ray diffractionis shownatthebottom.
˚
anion, and the atom distances are between 2.711(7) and
˚
31
crystal morphology compared to the hydrothermal
product. Room temperature stirring gave lower crystal-
linity but higher yield than reflux and larger specific
surface area. PXRD of SLUG-21 prepared from the three
methods confirms that the products give the identical
phase and match the theoretical pattern based on the
single crystal solution (Figure 2). SLUG-21 crystallizes in
a triclinic crystal system with the P1 space group (Table 1).
All of the products are chemically stable in deionized
water and organic solvents, including chloroform, to-
luene, and acetonitrile (Figure S6).
2
.759(9) A. These distances are significantly longer than
accepted Ag-O covalent bond lengths, especially con-
sidering literature Ag(I) to sulfonate distances are in the
32-34
˚
range of 2.45-2.55 A.
EDS anions only interact electrostatically with the Ag(I)-
These values confirm that the
bipy cationic extended framework.
Four crystallographically identified water molecules
verify that SLUG-21 cannot be obtained under nonaqu-
eous medium, as attempts to use dimethylformamide,
pyridine, or 2-butanol as solvent were unsuccessful. The
presence of hydrogen bonds between the EDS anion and
intercalated water molecules (Figure 4 and Table 2)
further stabilize the structure. Indeed, crystallinity was
not lost after immersing SLUG-21 into unbuffered water
or other common organic solvents for several weeks
Single-crystal X-ray diffraction studies reveal that the
structure of SLUG-21 is a cationic one-dimensional ex-
0
tended MOF of alternating Ag centers and 4,4 -bipy
molecules bonded through Ag-N bonds (Figures 3 and 4).
The chains are arranged into layers, with ethanedisulfo-
nate (EDS) anions residing in the interlamellar space. The
(Figure S6).
The thermal stability of SLUG-21 was also investigated
in detail by TGA-MS under nitrogen flow and ex-situ
0
Ag-bipy layer is stabilized by 4,4 -bpy ligand π-π stack-
ing between adjacent 1D chains. Each end of the EDS
molecule interacts electrostatically with adjacent cationic
layers through one sulfonate oxygen, with Ag-O dis-
(
31) Nakade, S.; Saito, Y.; Kubo, W.; Kitamura, T.; Wada, Y.;
Yanagida, S. J. Phys. Chem. B 2003, 107, 8607–8611.
(
32) Seward, C.; Chan, J. L.; Song, D. T.; Wang, S. N. Inorg. Chem.
2
˚
tances between 2.711(7) and 2.759(9) A (Figure 4). The
003, 42, 1112–1120.
other two oxygens are involved in a hydrogen bonding
network with interlamellar waters, which were resolved
(33) Budka, J.; Lhotak, P.; Stibor, I.; Sykora, J.; Cisarova, I. Supramol.
Chem. 2003, 15, 353–357.
34) Aguado, J. E.; Canales, S.; Gimeno, M. C.; Jones, P. G.; Laguna,
A.; Villacampa, M. D. Dalton Trans. 2005, 3005–3015.
(
(
Table 2). Silver(I) atoms with a full d shell form an

2
030 Chem. Mater., Vol. 22, No. 6, 2010
Fei et al.
Figure 3. Crystallographic a-projection (left) and b-projection (right) of the structure with hydrogen atoms omitted for clarity (silver purple; carbon gray;
sulfur yellow; oxygen red; nitrogen blue).
Figure 4. ORTEP diagram and atomic labeling of SLUG-21. Thermal
ellipsoids are shown at 50% probability.
Table 2. Selected Bond Lengths, Angles, and Hydrogen
Bonding in SLUG-21
a
Ag(1)-N(1)
2.156(7)
2.128(7)
171.5(3)
Ag(1)-N(2)
2.171(7)
2.156(7)
172.3(3)
Ag(2)-N(3)
Ag(2)-N(4)
N(1)-Ag(1)-N(2)
N(3)-Ag(2)-N(4)
Figure 5. Thermogravimetric trace (black) and coupled mass spectra
(blue and red for m/e = 18 and 64, respectively) of SLUG-21.
D-H
3 3 A
d(D-H) d(H
3 3 3 A)
d(D
3 3 3 A)
<(DHA)
3
1
O(11)-H(12) O(12)2 0.81(4)
1.94(4)
2.711(15)
2.733(13)
2.784(13) 150(11);
157(4);
119(4);
3
3 3
in this temperature range. The TGA trace is then stable
state up to 400 °C, when another weight loss occurs. This
event corresponds to loss of EDS anion (observed 27.6%,
expected 29.2%), leaving a pure silver phase as confirmed
by ex-situ PXRD after 450 °C (Figure S2). The coupled
mass spectra at m/e = 64 appears to verify this decom-
O(14)-H(41) O(13) 0.77(5)
2.27(5)
2.24(8)
1.89(7)
2.05(7)
3
3 3
O(12)-H(21) O(3)
0.61(7)
0.96(6)
3
3 3
O(13)-H(31) O(7)
2.795(10)
2.724(13)
158(5);
148(6);
3
3 3
3
O(13)-H(32) O(11) 0.76(6)
3
3 3
O(11)-H(11) O(6) 4 0.67(17) 2.31(16)
2.823(12) 135(17);
2.751(12) 152(8);
3
3 3
O(12)-H(22) O(14) 0.74(9)
3 3
2.07(9)
3
a
Symmetry transformations used to generate equivalent atoms: (1) x,
y, z - 1; (2) x, y, z þ 1; (3) x þ 1, y, z; (4) x - 1, y, z.
position step, with removal of an SO fragment.
2
Considering Ag(I) is typically three to six coordinate
2
6-29,35-37
among coordination polymers,
the two-coor-
PXRD (Figures 5 and S2, Supporting Information,
respectively). An 8.0% weight loss occurs after heating
to 120 °C owing to the removal of most of the interla-
mellar water molecules (calculated: 9.13%). The increase
in the m/e = 18 signal also supports the loss of water. Ex-
situ PXRD shows reduction in crystallinity and slight
phase transformation, but an open structure is retained.
The higher temperature phase is stable up to ca. 300 °C,
when the bipyridine decomposes by 350 °C (observed
dinate Ag(I) sites in SLUG-21 are coordinatively unsa-
turated owing to the weak bonding by the EDS anions. In
addition, the linear geometry metal sites are accessible
by one-dimensional pores running along the a-axis
(Figure 3). The lack of any metal hydroxyl groups and
the observation of constant pH of 6.5 when SLUG-21
is placed in unbuffered deionized water indicates
(
35) Gu, X. J.; Xue, D. F. Cryst. Growth Des. 2006, 6, 2551–2557.
4
0.2%, expected 39.6%). No major signal for either
(
36) Yaghi, O. M.; Li, H. L. J. Am. Chem. Soc. 1996, 118, 295–296.
m/e = 18 or m/e = 64 also implies organic decomposition
(37) Gu, X. J.; Xue, D. F. Inorg. Chem. 2006, 45, 9257–9261.

Article
Chem. Mater., Vol. 22, No. 6, 2010 2031
Figure 6. Yield for ketal formation by SLUG-21. (left) Yield vs time plot. (right) High conversion yield retained upon recycling the material.
Scheme 1. Acid Catalyzed Ketal Formation between 2-Butanone
and Ethylene Glycol
Table 3. Conversion Yields for Catalytic Reactions
catalyst time (h) conv (%)
a
reaction
ketalization
ketalization
ketalization
ketalization
esterification no catalyst
esterification hydrothermal SLUG-21
no catalyst
12
12
12
12
8
1
97
96
95
<10
57
hydrothermal SLUG-21
reflux SLUG-21
room-temp stirring SLUG-21
8
the material is Lewis rather than Bronsted catalytic.
Considering these factors, we investigated the Lewis acid
catalytic activity of the compound.
a
1
The conversion efficiency is determined by H NMR.
Control experiments for the catalysis have been con-
ducted by filtering off SLUG-21 after 6 h reaction time
and then continuing the reaction for an additional 12 h.
This study was conducted to determine whether the
reaction is being catalyzed by the material or some other
species present in the reaction solution. The additional
Ketalization of aldehydes and ketones is important for
the protection of carbonyl groups in preparative organic
3
8
synthesis, especially drug design. While toluenesulfo-
nate is a standard catalyst, it is homogeneous and must be
separated from the product by concentration under va-
3
9
cuum. Recent heterogeneous catalysts such as tetrabu-
tylammonium tribromide experience at least one of
the following problems: (i) toxicity; (ii) high cost; (iii)
difficulty in preparation; (iv) low yield for catalytic
1
2 h led to less than 2% increase in ketal yield, proving
that the SLUG-21 material is responsible for the catalytic
activity. Negligible mass loss of SLUG-21 during the
reaction further supports its inherent stability. In addi-
tion to ketalization of 2-butanone, different ketone pre-
cursors were studied, namely acetone and 2-pentanone.
For acetone, the yield was ∼90%, again with negligible
yield in the absence of the catalyst (Figure S7, Supporting
Information). The ketalization of 2-pentanone displayed
slightly lower yield (87%, Figure S8, Supporting Information),
possibly due to slower diffusion into the MOF pores by
the more bulky precursor.
4
0-42
reaction.
The mechanism of the reaction requires
the Lewis acid catalyst to activate the oxygen of the
carbonyl group, allowing glycol to substitute for the
ketone group. Our cationic silver-based compound was
tested for ketal formation between 2-butanone and ethy-
lene glycol (Scheme 1). The results reveal that SLUG-21 is
a strongly active and reusable catalyst.
We used 0.127 mmol of SLUG-21 to catalyze a reaction
between 70 mmol 2-butanone and 70 mmol ethylene
glycol, for a catalytic ratio of 1 to 552. With a Dean-
Stark trap to constantly remove the water, ketal product
increases linearly until almost complete conversion after 12 h
As previously mentioned, the structure is slightly trans-
formed upon partial removal of intercalated water
(Figure S2). There is lattice shrinkage along the b-axis,
as evidenced by the shift of both (010) and (020) PXRD
peaks to higher angle. This direction is to be expected
since the EDS linker and intercalated water molecules
reside in the interlamellar area, perpendicular to the
layers as defined by the b-axis. The activated form is
likely responsible for the catalysis. There is an ca. 10%
mass loss of catalyst after the first cycle and negligible
weight loss thereafter. The activated form of SLUG-21 is
thus stable for this catalytic reaction and highly reusable,
achieving similar yields through three cycles (Figures 6
and 7). We also tested the catalytic ability of SLUG-21
(
Figure 6 left and Table 3). Compared with a blank experi-
ment without catalyst under the same conditions and period
of time, only negligible product formed (Table 3), confir-
ming that SLUG-21 is an efficient Lewis acid catalyst.
(
(
38) Tsuchiya, T.; Ohmuro, S. Tetrahedron Lett. 2002, 43, 611–615.
39) Patel, M. V.; Bell, R.; Majest, S.; Henry, R.; Kolasa, T. J. Org.
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(
(
(
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032 Chem. Mater., Vol. 22, No. 6, 2010
Fei et al.
carboxylic acid, the conventional homogeneous catalyst
4
3
being condensed sulfuric acid. We carried out the
esterification between acetic acid and ethanol with
SLUG-21 as catalyst. 0.18 mol % of crystals (for a
catalytic ratio of 556 to 1) gave a yield of 57% after 8 h
reaction without taking further steps to remove the water
byproduct (Table 3 and Scheme 2). Longer reaction time
gave no improvement to the yield. An additional esteri-
fication experiment between formic acid and ethanol with
SLUG-21 as catalyst reached over 70% yield (Figure S9,
Supporting Information). The higher yield in formic acid
is possibly due to the smaller precursor size, for greater
diffusion into the extended framework. These satisfactory
yields illustrate that SLUG-21 functions as an esterifica-
tion catalyst.
Conclusions
The cationic Ag(I)-based MOF was isolated using a
mixture of organic linker and anionic template. This
strategy may lead to the discovery of other cationic
extended frameworks with open metal sites for catalytic
applications. The compound exhibits chemical stability in
various solvents, as well as excellent Lewis acid catalytic
ability by virtue of its coordinatively unsaturated metal
sites. The catalyst can be continuously reused in the
heterogeneous catalytic reaction without losing activity
or significant mass. The material is synthesized in one step
under ambient conditions, ideal for scale-up to industrial
scale.
Figure 7. PXRD of (a) as-synthesized SLUG-21; postcatalysis after one
(b) and three (c) catalytic cycles of ketal formation.
Scheme 2. Acid Catalyzed Esterification between Acetic
Acid and Ethanol
prepared under ambient and reflux conditions and both
showed similarly high yield (Table 3). All of the as-
synthesized crystals thus have almost equivalent Lewis
acid catalytic ability as well as reusability. Both ambient
pressure routes also give the same activated form by TGA
and ex-situ PXRD, achieving similar yields and reusa-
bility through at least three runs (Table S1, Supporting
Information).
To further test the Ag(I) Lewis acid catalytic site
activity, we also carried out an esterification reaction.
This reaction has attracted increasing interest for poten-
tial application in biofuels. A stronger acid catalyst,
however, is required to activate the carbonyl group of
Acknowledgment. This research was supported by an NSF
Career Award (DMR-0506279). The single crystal X-ray
diffraction data in this work were recorded on an instrument
supported by the National Science Foundation, Major
Research Instrumentation (MRI) Program under Grant
No. CHE-0521569.
Supporting Information Available: Optical micrographs,
1
ex-situ PXRD, H NMR spectra, and crystallographic informa-
tion file (CIF) including refinement parameters and full bond
distances. This material is available free of charge via the
Internet at http://pubs.acs.org.
(43) Otera, J. Esterification Methods, Reactions and applications; Wiley
VCH Verlag Gmbh and KgaA: Weinheim, Germany, 2003.