Enhanced catalytic activity through the tuning of micropore environment and supercritical CO2 processing: Al(Porphyrin)-based porous organic polymers for the degradation of a nerve agent simulant

Article abstract of DOI:10.1021/ja405495u

An Al(porphyrin) functionalized with a large axial ligand was incorporated into a porous organic polymer (POP) using a cobalt-catalyzed acetylene trimerization strategy. Removal of the axial ligand afforded a microporous POP that is catalytically active i

Full text of DOI:10.1021/ja405495u

Communication
pubs.acs.org/JACS
Enhanced Catalytic Activity through the Tuning of Micropore
Environment and Supercritical CO2 Processing: Al(Porphyrin)-Based
Porous Organic Polymers for the Degradation of a Nerve Agent
Simulant
Ryan K. Totten, Ye-Seong Kim, Mitchell H. Weston, Omar K. Farha,* Joseph T. Hupp,*
and SonBinh T. Nguyen*
Department of Chemistry and the International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208-3113, United States
S
* Supporting Information
catalyst design,18 we reasoned that an all-organic Al-
ABSTRACT: An Al(porphyrin) functionalized with a
large axial ligand was incorporated into a porous organic
polymer (POP) using a cobalt-catalyzed acetylene
trimerization strategy. Removal of the axial ligand afforded
a microporous POP that is catalytically active in the
methanolysis of a nerve agent simulant. Supercritical CO2
processing of the POP dramatically increased the pore size
and volume, allowing for significantly higher catalytic
activities.
(porphyrin)-based POP (Al-PPOP) with the right pore
environment could mimic the aforementioned characteristics
while offering the additional advantages of recyclability and
enhanced catalysis rates based on preconcentration. Herein, we
report the synthesis of an Al(porphyrin)-based POP (Scheme
1) capable of catalyzing the methanolytic degradation of
PNPDPP. Its catalytic activity can be enhanced by over 1 order
of magnitude through a combination of tuning the micropore
environment and supercritical CO2 processing.
In designing catalytically active Al-PPOPs, we wanted to
control, through the incorporation of rigid spacers, the
micropore environment and thus the ability to encapsulate
organophosphate substrates for catalysis. For our Al(porphyrin)
supramolecular dimers18 and tetramers,19 the selection of a
proper template to control the cavity shape and size of the
resulting assemblies is quite important. While the spacing of
Al(porphyrins) within a highly cross-linked polymer network,
such as a POP, is significantly influenced by the choice of the
“crosslinking” nodes,8,21,22 we hypothesized that temporarily
complexing a large axial ligand to the AlIII metal center could
further displace porphyrin monomers during the polymer-
ization process to afford materials with larger and more
accessible pores, capable of effecting the catalysis of large
substrates such as PNPDPP. To test this strategy, we compare
Al-PPOPs made from each of two Al(porphyrins): one
possessing a small methoxide axial ligand and the other
possessing a much larger 3,5 di-tbutyl benzoate ligand.
ver the past decade, much efforts have been made to
O
heterogenize homogeneous catalysts by integrating them
into microporous materials, such as metal-organic frameworks
(MOFs),1−5 covalent organic frameworks (COFs),6,7 polymers
of intrinsic microporosity (PIMs),8,9 conjugated microporous
polymers (CMPs),10,11 and porous organic polymers
(POPs).12,13 Beyond the obvious advantages of site iso-
lation14,15 and recyclability,4,13,16 this type of incorporation
allows for the “close packing” of multiple active metal sites into
a porous environment that can be used synergistically to bind
and preconcentrate substrates for catalysis.1 Additionally,
nondirective interactions, such as van der Waals forces and
solvophobic effects, can be used to further enhance the
encapsulation of substrates inside the micropores for catalytic
transformations.17−19 With these advantages, reactions cata-
lyzed by microporous materials can begin to mimic those
catalyzed by biological or supramolecular analogues.
For the POP synthesis, we chose a cobalt-catalyzed acetylene
trimerization strategy23,24 given its lack of reactants or side
products (i.e., protonated species) that may compete with the
axial ligand of the Al(porphyrin) during polymerization. The
copolymerization of a Td-directing tetrakis(4-ethynylphenyl)-
methane monomer with either one of two Al(porphyrin)
monomers (Al-1, R = OMe; Al-2, R = 3,5 di-tbutyl benzoate;
Scheme 1) proceeded smoothly in the presence of Co2(CO)8
to afford the desired POPs in good yields as deep-purple solids.
Subsequent thermal treatment (100 °C) in excess acetic acid to
remove the cobalt and the initial axial ligands (see SI, section
Recently, we demonstrated the catalytic activity of
homogeneous supramolecular Al(porphyrin) dimers18 and
tetramers19 in the methanolytic degradation of p-nitrophenyl
diphenyl phosphate (PNPDPP), a common simulant for toxic
nerve agents.20 The most critical factors for observing enhanced
catalysis rates in these systems over that of the corresponding
monomers are the optimal positioning of Lewis acidic
Al(porphyrin) metal sites, one of which binds and activates
an organophosphate substrate so that a nearby second site can
deliver a methoxide nucleophile to the phosphorus center in a
cooperative fashion. The hydrophobic environment of the
porphyrinic cavity further enhances catalysis by solvophobically
attracting the relatively hydrophobic PNPDPP substrate in
polar methanol. In an effort to heterogenize this biomimetic
Received: June 1, 2013
Published: July 22, 2013
© 2013 American Chemical Society
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reports,5,12,25−27 this observation demonstrates that specific
surface area is not always an accurate indicator of catalytic
activity in microporous materials. Rather, the ability to access
the catalytic site via larger pores is more influential on the
catalytic rate.
Scheme 1. Synthesis of Al-PPOPs via a Cobalt-Catalyzed
Acetylene Trimerization
a
Encouraged by the significant improvement of catalysis rate
in Al-PPOP-2, we suspected that further enhancement would
be possible if the accessibility of catalyst sites inside the pores
can be further improved. To this end, we reasoned that the
larger pores in POPs, particularly “interparticle” mesopores,28
could be collapsing upon solvent removal at 150 °C under
vacuum, and thus limiting access of substrates to catalytic sites.
That is, forced solvent removal via conventional, thermal
activation may engender aggregation of POP particles, thereby
inhibiting reactant access to some catalyst sites. This hypothesis
is prompted by the observation that the syntheses of our Al-
PPOPs afforded gel-like materials that initially filled the entire
reaction volume (see SI, Figure S24) but then shrunk
significantly upon work up. This is akin to the observed
collapse of aerogels under fast solvent evaporation where strong
capillary forces cause shrinkage and ultimate collapse of
mesopores.29 A similar line of reasoning has been proposed
to explain why supercritical CO2 processing has greatly
improved the surface areas of organic-containing microporous
materials, such as MOFs,28 to achieve some of the highest
reported surface areas to date.30−32
To test the aforementioned hypothesis, we resynthesized Al-
PPOP-2 and activated the as-synthesized gel-like materials
using supercritical CO2 processing. To our pleasant surprise,
the surface area of the supercritically processed sample, scpAl-
PPOP-2, increased from 660 to 950 m2/g compared to the
thermally activated sample. Remarkably, the isotherm for scpAl-
PPOP-2 changed to a Type II (Figure 1b), indicating a gain in
mesoporosity (pore sizes between 20 and 50 Å), along with a 3-
a
POPs shown above are idealized representations of a completely
formed network. However, different substitution patterns (1,3,4 vs
1,3,5) may be present, as well as terminal olefins/dienes and unreacted
acetylene groups due to incomplete polymerization. Solid-state
1H−13C CP-MAS NMR analyses of the Al-PPOP compositions
shown above suggested that these contain significant amounts of
olefin/diene groups (see SI, Figures S5 and S6).
S4), results in two acetate-substituted Al(porphyrin) POPs, Al-
PPOP-1 and Al-PPOP-2, with N2-derived BET surface areas of
640 and 660 m2/g, respectively, upon activation at 150 °C.
Although both Al-PPOP-1 and Al-PPOP-2 exhibit similar
surface areas, their pore size distributions reveal distinct
differences in dominant pore size: Al-PPOP-2 has two
dominant pores at 9 and 12.5 Å, while Al-PPOP-1 has only
one dominant pore at 9 Å. This difference is clearly reflected in
their catalytic activities in the methanolysis of PNPDPP (Table
1), with Al-PPOP-2 being 65% faster than Al-PPOP-1,
accentuating the important role that the larger pore plays in
the catalysis by Al-PPOP-2. Together with previous
Table 1. Observed Initial Rates in the Methanolysis of
PNPDPP by Al-PPOPs
BET surface area
(m2/g)
observed initial
b
relative rate vs uncat
reaction
a
catalyst
rate (M/s)
Al-PPOP-1
Al-PPOP-2
640
660
2.8 × 10−7
4.8 × 10−7
17
28
Figure 1. (a) Photographic images of Al-PPOP samples (50 mg each)
that were either activated thermally (left) or processed with
supercritical CO2. (b) N2 isotherms measured at 77 K of Al-PPOP-
2 (blue diamonds) and scpAl-PPOP-2 (red squares). Closed symbols,
adsorption; open symbols, desorption.
a
Reaction conditions: PNPDPP (25 mM), [AlIII] (1 mM, 4 mol %),
MeOH, 60 °C. Initial rates were measured up to 10% conversion and
corrected against background reactions.
b
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Communication
fold increase in total pore volume from 0.34 to 1.15 cm3/g.
Indeed, its BET-derived pore size distribution shows dominant
pore sizes of 12, 15, and 18 Å, in addition to a broad mesopore
population that centers around 27 Å (see SI, section S12). As a
result, the bulk density of scpAl-PPOP-2 (0.53 g/cm3) is much
less than that of Al-PPOP-2 (0.98 g/cm3, see SI, section S16).
These differences are accompanied by a drastic change in
physical appearance, as scpAl-PPOP-2 is significantly more
“powdery” and flocculent than Al-PPOP-2, which is more
“chunky,” dense, and brittle (Figure 1a). Together, these data
suggest that catalyst sites within supercritically processed Al-
PPOPs should be much more accessible to substrates in the
methanolytic degradation of organophosphate esters.
Consistent with our hypothesis, the methanolysis of
PNPDPP is significantly enhanced by the supercritical CO2
processed Al-PPOPs compared to the thermally activated Al-
PPOPs. With a half-life of 90 min, the rate of PNPDPP
methanolysis by scpAl-PPOP-2 is 7 times that of Al-PPOP-2
and ∼200-fold greater than that of the uncatalyzed reaction
(Figure 2, see also Table 2).33 That the rate of PNPDPP
processed Al-PPOPs are significantly more “powdery” and
have larger surface areas than the thermally activated Al-PPOPs
(Figure 1a), the most important factors for enhanced catalysis
appear to be larger pore volumes and the presence of
mesopores that make catalyst sites more accessible to the
PNPDPP substrate for solvolysis: a finely ground sample of
thermally activated Al-PPOP-2 elicits only a 1.4-fold increase in
initial rate over the unground sample (see SI, section S7).
Surprisingly, the initial PNPDPP methanolysis rate of scpAl-
PPOP-2 drops by 10-fold when it is reused (see SI, Figure
S10). Upon closer examination of the reaction profile for the
second cycle, we observed a 30 min induction period, which
suggests that substrates/products were retained within the
pores and blocked new substrates from entering (see SI, Figure
S11).27 To remedy this problem, we Soxhlet-extracted the
catalyst with methanol after the first cycle (see SI, section S9).
Although a significant amount of the entrapped material was
observed and the induction period was eliminated, the catalytic
activity of the Soxhlet-extracted material was not restored. This
led us to an alternate hypothesis that the loss of mesoporosity,
engendered by POP aggregation, could have occurred during
the catalysis, causing the reduction in activity. Thus, we
reprocessed the scpAl-PPOP-2 sample with supercritical CO2
after the first cycle and reused it for PNPDPP methanolysis.
Remarkably, the catalytic activity of scpAl-PPOP-2 was almost
fully restored to its initial level (see SI, section S10).
Furthermore, the BET surface area and pore size distributions
of this recycled, supercritical-CO2-processed material were
similar to the as-synthesized scpAl-PPOP-2 sample (see SI,
sections S13 and S14), further demonstrating the importance of
mesoporosity in enhancing the accessibility of catalyst sites.
In summary, we have demonstrated that catalytically active
Al(porphyrin) moieties can be integrated into an all-organic
porous network for use in the methanolytic degradation of a
nerve agent simulant. While the presence of a large axial ligand
on the Al(porphyrin) monomer can afford Al-PPOPs with
larger micropores that are more accessible to substrates in
catalysis, the best catalysts are obtained after supercritical CO2
processing. In contrast to the conventional activation method
of heating the samples under vacuum, supercritical CO2
processing affords POPs with much larger pores and total
pore volumes, thus significantly enhancing substrate accessi-
bility and catalytic rates. Supercritical CO2 processing is also
quite important in maintaining catalytic activity during the
recycling. Together, these results indicate that supercritical
processing can be an effective strategy to generate highly active
reusable POP-based catalysts and suggest that it can be usefully
deployed with other applications beyond catalysis where low-
density, porous materials with very high total pore volumes are
desired.
Figure 2. Reaction profiles for the methanolysis of PNPDPP in the
presence of 4 mol % of Al-PPOP-2 (blue diamonds) and scpAl-PPOP-
2 (red squares).
Table 2. Pore, Surface, and Catalytic Properties of
Al(porphyrin)-Based POPs
total
NLDFT-
BET
surface
area
derived
pore
dominant
pore
diameter
(Å)
relative
rate vs
uncat
observed
volume
initial rate
a
b
c
catalyst
(m2/g) (cm3/g)
(M/s)
reaction
Al-PPOP-1
Al-PPOP-2
scpAl-PPOP-1
640
660
830
0.35
0.34
1.02
9
2.8 × 10−7
4.8 × 10−7
17
28
88
9, 12.5
12, 15, 18, 1.5 × 10−6
27
ASSOCIATED CONTENT
■
scpAl-PPOP-2
950
1.15
12, 15, 18, 3.5 × 10−6
205
S
* Supporting Information
27
a
Reaction conditions: PNPDPP (25 mM), [AlIII] (1 mM, 4 mol %),
Synthesis procedures and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
b
MeOH, 60 °C. Total NLDFT-derived pore volume from the N2
adsorption profiles at p/p0 = 0.98. Initial rates were measured up to
c
10% conversion and corrected against background reactions.
AUTHOR INFORMATION
■
Corresponding Author
methanolysis by scpAl-PPOP-2 is 2-fold faster than that for
scpAl-PPOP-1 again reinforces the aforementioned advantage of
templating, where the use of a large axial spacer can lead to
better catalytic activity. Although the supercritical CO2
o-farha@northwestern.edu; j-hupp@northwestern.edu; stn@
northwestern.edu
Notes
The authors declare no competing financial interest.
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Communication
(31) Farha, O. K.; Eryazici, I.; Jeong, N. C.; Hauser, B. G.; Wilmer, C.
E.; Sarjeant, A. A.; Snurr, R. Q.; Nguyen, S. T.; Yazaydın, A. O.; Hupp,
J. T. J. Am. Chem. Soc. 2012, 134, 15016.
(32) Farha, O. K.; Wilmer, C. E.; Eryazici, I.; Hauser, B. G.; Parilla, P.
A.; O’Neill, K.; Sarjeant, A. A.; Nguyen, S. T.; Snurr, R. Q.; Hupp, J. T.
J. Am. Chem. Soc. 2012, 134, 9860.
(33) As a comparison, the initial per-Al(porphyrin) PNPDPP
methanolysis rate for scpAl-PPOP-2 is ∼3 times faster than that for
a homogeneous Al(porphyrin) dimer and ∼9 times faster than an
Al(porphyrin) tetramer; see refs 18 and 19.
ACKNOWLEDGMENTS
■
̈
Financial support for this work is provided by DTRA
(agreement HDTRA1-10-1-0023). Y.-S. K. thanks the Wein-
berg College of Arts and Sciences, Northwestern University for
a Weinberg College summer 2011 research grant. Instruments
in the Northwestern University Integrated Molecular Structure
Education and Research Center (IMSERC) were purchased
with grants from NSF-NSEC, NSF-MRSEC, Keck Foundation,
the state of Illinois, and Northwestern University.
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