Impregnation of metal ions into porphyrin-based imine gels to modulate guest uptake and to assemble a catalytic microfluidic reactor

Article abstract of DOI:10.1039/c6ta01035k

A series of (metallo)porphyrin imine gels have been synthesized based on imine chemistry. The resulting aerogels have sponge-like porous networked structures consisting of interconnected nanoparticles with hierarchical porosity. The aerogels have high specific surface areas (up to 719 m² g^-1) and large pore volumes (up to 2.60 cm³ g^-1). The effect of metal ions on the uptake of gases in these aerogels was investigated. The impregnation of various metal ions (Pd(ii), Ni(ii), Mn(iii), Fe(iii) and Sn(iv)) enhances the uptake capacity of various gases (e.g., CO₂) despite their higher densities. Among the metal ions, Pd(ii) is the best to increase the adsorption capacity and the isosteric heat of CO₂ adsorption. The Pd-tapp-A4 aerogel exhibits CO₂ volumetric uptake (1.62 mmol g^-1, 7.13 wt% at 298 K, 1 bar) with a high isosteric heat of adsorption (40.0 kJ mol^-1). The gels also show potential applications in catalysis because of their unique hierarchical porosity and the availability of metal centers. In combination with microfluidic technology, a catalytic gel capillary reactor has been assembled with the Pd-tapp-A4 gel supported on the inner surface of a functionalized capillary.

Full text of DOI:10.1039/c6ta01035k

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Impregnation of metal ions in porphyrin-based imine gels to
modulate guest uptake and to assemble a catalytic microfluidic
reactor†
Received 00th January 20xx,
Accepted 00th January 20xx
Lihua Zeng, Peisen Liao, Haoliang Liu, Liping Liu, Ziwei Liang, Jianyong Zhang,* Liuping Chen and
Cheng-Yong Su
DOI: 10.1039/x0xx00000x
www.rsc.org/
A series of (metallo)porphyrin imine gels have been synthesized based on imine chemistry. The resulting aerogels have
sponge-like porous networked structures consisting of interconnected nanoparticles with hierarchical porosity. The
aerogels have high specific surface areas (up to 719 m2 g-1) and large pore volumes (up to 2.60 cm3 g-1). The effect of metal
ions on the uptake of gases in these aerogels was investigated. The impregnation of various metal ions (Pd(II), Ni(II),
Mn(III), Fe(III) and Sn(IV)) enhances the uptake capacity of various gases (e.g., CO2) despite their higher densities. Among
the metal ions, Pd(II) is the best to increase the adsorption capacity and the isosteric heat of CO2 adsorption. Pd-tapp-A4
aerogel exhibits CO2 volumetric uptake (1.62 mmol g-1, 7.13 wt% at 298 K, 1 bar) with high isosteric heat of adsorption
(40.0 kJ mol-1). The gels also show potential applications in catalysis arising from their unique hierarchical porosity and the
availability of metal centers. In combination with microfluidic technology, a catalytic gel capillary reactor has been
assembled with the Pd-tapp-A4 gel supported on the inner surface of a functionalized capillary.
macrocyclic ligands with rich coordination chemistry. A
number of metal ions can be readily incorporated into the
porphyrin centre, and the resulting metalloporphyrins are
Introduction
Aerogels are a class of porous materials with intriguing
properties, such as low density, high specific surface area,
large pore size distribution, etc. They receive much attention
due to their tremendous potential applications in catalysis,
absorbents, sensors and others.1 Besides conventional
aerogels like metal oxides, carbons and organic polymers,
several new catalogues of aerogels have been developed
recently including graphene,2 chalcogenide,3 metal-organic4
and dynamic covalent aerogels.5-7 Recently we make our
efforts to develop imines gels/aerogels based on dynamic
covalent chemistry.5-7 Imine gels are constructed from
polycondensation of bridging amines and aldehydes,8 affording
an extraordinary feature to engender desired functional
behaviour by choosing appropriate small molecular precursors.
The availability of various building blocks makes it possible to
construct highly tuneable aerogels with unique properties for
diverse applications, for example, offering readily accessible
active sites for mass transport and allow large guest molecules
to access the active sites inside the gel/aerogel.
known in light harvesting, natural and artificial catalysis and
oxygen delivery. They are good building units with accessible
coordinatively unsaturated metal centre, and have been used
in the synthesis of microporous materials, including porous
organic materials9-12 and metal-organic frameworks (MOFs).13-
16
Porous materials with porphyrin building units are easily
functionalized through built-in metal sites with various
applications in gas storage/separation,10,11,14 catalysis,11,12,15
and light-harvesting.16
Herein we incorporate porphyrin functional units into
imine gels to get a series of functional porphyrin imine gels by
reacting 5,10,15,20-tetrakis(p-aminophenyl)porphyrin (tapp)
and its metalated derivatives (M-tapp, M = Pd, Ni, MnCl, FeCl,
SnCl2) with di/multifunctional aldehydes (tetrakis-(4-
formylphenyl)methane, A4, 1,3,5-triformylbenzene, A3 and
1,4-diformylbenzene, A2) (Scheme 1). These hierarchically
porous gels offer readily accessible (metallo)porphyrin active
sites for mass transport. In addition the synthetic availability of
these porphyrin-based aerogels affords an opportunity to fine-
tune their structure and properties. The adsorption properties
of the aerogels have been studied including CO2, H2 and C2H4
gases. To show their potential application in catalysis, the Pd-
tapp gel has been supported inside walls of a capillary to
assemble a catalytic gel microfluidic reactor.
On the other hand, porphyrins are a kind of important
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H2N
NH2
NH2
H2N
OHC
OHC
CHO
OHC
OHC
CHO
NH
NH
NH
NH
N
N
+
N
N
CHO
CHO
NH2
NH2
A4
A4
H2tapp
H2N
H2N
NH2
NH2
(-CHO : -NH2 = 1:1)
N
H2tapp
N
CHO
NH
N
N
N
OHC
CHO
CHO
N
M
N
N
Cat. HOAc
DMSO, 80 o
NH
A3
C
N
N
H2N
NH2
OHC
M-tapp
(M = Pd, Ni, MnCl, FeCl, SnCl2)
A2
Scheme 1 Molecular structures of building units for imine gels, H2tapp, Pd-
tapp, Ni-tapp, Mn-tapp, Fe-tapp, Sn-tapp, A4, A3 and A2.
Scheme 2 Synthesis of H2tapp-A4 gel from H2tapp and A4.
Results and discussion
Synthesis and gelation tests
In this study, a free-base porphyrin 5,10,15,20-tetrakis(p-
aminophenyl)porphyrin (H2tapp) was chosen. It contains four
amine groups and can act as a building unit with multiple
functional ends for imine gels. H2tapp was synthesized via
condensation
between
dipyrrolemethane
and
4-
nitrobenzaldehyde followed by reduction of nitro groups
according to the literature procedure.17 The free-base
porphyrin H2tapp was subsequently metallated with metal
salts (PdCl2, NiCl2·6H2O, Mn(OAc)2·4H2O, FeCl2·4H2O and
SnCl2·2H2O) to obtain the corresponding metalloporphyrins M-
tapp (M = Pd, Ni, MnCl, FeCl and SnCl2) (Scheme S1, Fig. S1-
S3).18 Among the metalloporphyrins, M-tapp (M = MnCl, FeCl
and SnCl2) were obtained by air oxidation of corresponding
precursors during the synthesis. For metalloporphyrins
5,10,15,20-tetrakis(p-aminophenyl)porphyrinatopalladium
a
b
(Pd-tapp)
and
5,10,15,20-tetrakis(p-
aminophenyl)porphyrinatonickel (Ni-tapp) with divalent metal
ions (M = Pd(II) and Ni(II)), the axial positions of metal ions are
left uncoordinated. For metalloporphyrins with trivalent and
tetravalent metal ions (M = Mn(III), Fe(III) and Sn(IV)), the axial
positions are occupied by one chloride for 5,10,15,20-
tetrakis(p-aminophenyl)porphyrinatomanganese chloride (Mn-
tapp) and 5,10,15,20-tetrakis(p-aminophenyl)porphyrinatoiron
chloride (Fe-tapp) or two chlorides for 5,10,15,20-tetrakis(p-
aminophenyl)porphyrinatotin chloride (Sn-tapp).
c
d
Fig. 1 a) Photographic images of H2tapp-A4 wet gel, b) SEM and c,d) TEM
images of H2tapp-A4 aerogel (bars represent 500, 100 and 20 nm,
respectively).
Rigid building blocks with at least two functional groups are
necessary to create a porous structure for uptake of guest
molecules.5 H2tapp or M-tapp are rigid planar molecules
containing four amino functional groups. Based on these
considerations, various imine gels were successfully prepared
by reacting H2tapp or M-tapp with rigid bridging aldehydes
tetrakis-(4-formylphenyl)methane (A4), 1,3,5-triformylbenzene
(
A3) or 1,4-diformylbenzene (A2) in a 1:1 molar ratio of –NH2
and –CHO reactive groups in DMSO in the presence of a
catalytic amount of acetic acid (Scheme 2). Heating the
precursor solution at 80 oC accelerated the gelation to result in
opaque deep coloured gels (Fig. 1,2,S4) within 0.5-8 h (Table
S1). Heating for a longer time caused the gels to turn more
2 | J. Name., 2012, 00, 1-3
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firmly, and the gels are thermally irreversible. As aerogels by energy dispersive X-ray spectroscopy (EDX) (Fig.
representatives the gels formed with the porphyrin precursor S17) and ICP-AES (Table S2) revealed the presence of
concentration 0.030 mol L-1 were selected for subsequent corresponding metal atoms, Ni, Pd, Mn, Fe, Sn, and the
investigation5 in order to focus on the effect of impregnated corresponding Cl- counterions. The fitting of X-ray
metal ions. The aerogels were produced by subcritical CO2(l) photoelectron spectroscopy (XPS) curves of metal elements
drying of the corresponding wet gels to minimize changing of confirms the 3d3/2 and 3d5/2 state of divalent Pd(II) (binding
the original gel structure. The shrinkage percentage of the energies 344.0 and 338.7 eV) and the 2p1/2 and 2p3/2 state of
aerogels was 80-90% and the density of the aerogels was divalent Ni(II) (binding energy 873.2 and 855.8 eV) for Pd-tapp-
about 0.12 g cm-3.
A4 and Ni-tapp-A4, respectively (Fig. S18). The XPS analyses
Characterization
also confirm the presence of trivalent Mn(III), Fe(III) and
SEM and TEM investigations show that the imine aerogels tetravalent Sn(IV) for Mn-tapp-A4, Fe-tapp-A4 and Sn-tapp-A4
,
have sponge-like porous networked structures consisting of respectively.
interconnected nanoparticles for the aerogels of H2tapp-A2,
The aerogels were stable in neutral and alkaline solutions
(e.g., 3 mol L-1 NaOH) evidenced by SEM (Fig. S19), however,
H2tapp-A3, H2tapp-A4, Mn-tapp-A4, Fe-tapp-A4, Ni-tapp-A4
,
Pd-tapp-A4 and Sn-tapp-A4 (Fig. 1,2,S5-S10). The particle sizes they were disrupted in acidic solution (e.g., 3 mol L-1 HCl)
range around 20~50 nm. TEM reveals that macro- and arising from the dynamic nature of imine bonding. The
mesopores are distributed in the gel network. All the aerogels thermostability of the aerogels was studied by
and wet gels are amorphous, as revealed by the broad powder thermogravimetric analysis (TGA) (Fig. S20). The aerogels
X-ray diffraction patterns (Fig. S11).
H2tapp-A2, H2tapp-A3, H2tapp-A4, Mn-tapp-A4, Fe-tapp-A4,
Ni-tapp-A4, Pd-tapp-A4 and Sn-tapp-A4 have a high thermal
stability with significant decomposition only observed at
temperatures > 300 oC.
Porosity analysis
Nitrogen physisorption was performed at 77 K to characterize
the porosity of the porphyrin-based imine aerogels. H2tapp-A4
,
H2tapp-A3 and H2tapp-A2 aerogels with different aldehyde
precursors are compared first. The nitrogen isotherm for
H2tapp-A4 aerogel display a combination of Type I and Type IV
characteristics according to IUPAC classification. The isotherms
of H2tapp-A4 aerogel exhibit a steep rise of uptake within the
region P < 0.001 MPa, indicative of the presence of substantial
micropores. The isotherms are accompanied by a remarkable
hysteresis in the desorption isotherm showing a range of
mesopores are present. Meanwhile, H2tapp-A2 and H2tapp-A3
aerogels showed type II adsorption branch with a steep rise at
P > 0.09 MPa (Fig. 3a). Brunauer-Emmett-Teller (BET) surface
area and total volume analysis derived from the adsorption
data reveal that the H2tapp-A4 aerogel with free-base
porphyrin moieties has the highest BET surface area of 719 m2
g-1 and pore volume of 2.60 cm3 g-1. All the aerogels have
analogical non-local density functional theory (NL-DFT) and
Horvath-Kawazoe (HK) pore size distributions with a wide
range of mesopores up to ca. 50 nm (Fig. 3b,c,S21). Therefore,
precursors with tetrahedral structure, A4, prefer to create a
more porous structure.20 It may efficiently prevent the
aggregation of porphyrin moieties through formation of a
three-dimensional network and is used as a precursor for
further investigation of metalloporphyrin-based imine gels.
The effect of impregnated metal ions on the porosity
properties of the aerogels was investigated (Table 1). All the
a
b
c
d
Fig. 2 a) Photographic images of Pd-tapp-A4 wet gel, b) SEM and b,c) TEM
images of Pd-tapp-A4 aerogel (bars represent 500, 100 and 20 nm,
respectively).
The chemical composition of the aerogels has been
characterized by FT-IR. All the aerogels display a characteristic
Ar–C=N– stretching band at around 1600-1650 cm-1 (Fig. S12),
confirming formation of imine bonds via condensation of
aldehyde and amine groups. A broad band at around 3400 cm-
1
is due to the N-H stretching of unreacted amine functional
groups for both the wet gels and aerogels. C=O stretching
bands of unreacted aldehyde functional groups were also
detected at around 1665-1700 cm-1 for the aerogels. It
suggests that the condensation of aldehyde and amine groups
is incomplete in the aerogels. 13C cross polarization magic
angle spinning (CP/MAS) NMR of the aerogels showed a peak
at around 160 ppm and confirmed the formation of imine
bonds (Fig. S13-S16).19 Elemental composition analysis of the
nitrogen isotherms of Mn-tapp-A4, Fe-tapp-A4, Ni-tapp-A4
,
Pd-tapp-A4 and Sn-tapp-A4 aerogels display a combination of
Type I and Type IV characteristics according to IUPAC
classification (Fig. 4a). The BET specific surface areas of Pd-
tapp-A4 and Ni-tapp-A4 are 618 and 695 m2 g-1, slightly lower
than that of H2tapp-A4. However, Mn-tapp-A4, Fe-tapp-A4 and
Sn-tapp-A4 have significantly lower BET specific surface areas
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(358~384 m2 g-1). Ni-tapp-A4 and Pd-tapp-A4 aerogels also (759 m2 mmol-1). This indicates that the surface area
have high pore volumes, 2.380 and 1.909 cm3 g-1, respectively. differences arise from variation in the density of the building
NL-DFT pore size distributions show that these aerogels have a unit for H2tapp-A4, Pd-tapp-A4 and Ni-tapp-A4. In contrast,
wide range of mesopores up to ca. 50 nm, while HK micropore the surface areas are significantly lower for Mn-tapp-A4 (402
sizes are centred at around 0.6 nm (Fig. 4b,c,S21). In addition, m2 mmol-1), Fe-tapp-A4 (405 m2 mmol-1) and Sn-tapp-A4 (470
all the tap aerogels display similar NL-DFT pore size profiles m2 mmol-1) with the axial chlorides. The axial chloride ligands
with maxima for the same pore sizes at around 6, 17 and 29 in porous network are not effective porogens and result in
nm, showing that they have similar textures.21
lower pore size and BET surface area.22 Pore blockage or
partial occupation of the metal sites by the axial chlorides,
solvent molecules or unreacted ligands may be responsible for
the lower surface areas. Simultaneously aggregation of
porphyrin moieties may also affect the porosity.23 These
results suggest that impregnation of divalent metal ions in the
porphyrin-based imine aerogels does not change the porosity
significantly, while impregnation of trivalent or tetravalent
metal ions lowers the surface area of the aerogels.
a)
1800
H2tapp-A4
1600
H2tapp-A3
1400
1200
1000
800
600
400
200
0
H2tapp-A2
a)
1800
1600
1400
1200
1000
800
600
400
200
0
H2tapp-A4
Mn-tapp-A4
Fe-tapp-A4
Ni-tapp-A4
Pd-tapp-A4
Sn-tapp-A4
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
b)
0.10
H2tapp-A4
H2tapp-A3
H2tapp-A2
0.08
0.06
0.04
0.02
0.00
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
b) 0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
H2tapp-A4
Mn-tapp-A4
Fe-tapp-A4
Ni-tapp-A4
Pd-tapp-A4
Sn-tapp-A4
0
10 20 30 40 50 60 70 80
Pore width/nm
0.6
c)
0.5
0.4
0.3
0.2
0.1
0.0
H2tapp-A4
0
10
20
30
40
50
60
70
80
Pore width/nm
c)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
H2tapp-A3
H2tapp-A2
Sn-tapp-A4
Mn-tapp-A4
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Pore diameter/nm
Fe-tapp-A4
Pd-tapp-A4
Fig. 3 a) N2 adsorption and desorption isotherms at 77 K, b) NL-DFT pore size
distributions (Model: N2 at 77 K on silica (cylindr. pore, NLDFT adsorption
branch model)) and c) HK pore size distributions of H2tapp-A2, H2tapp-A3
and H2tapp-A4 aerogels.
Ni-tapp-A4
H2tapp-A4
0.4
0.6 0.8
1.0 1.2
1.4 1.6
1.8 2.0
Pore diameter/nm
Because surface area is reported as an area per given mass,
Pd-tapp-A4, Ni-tapp-A4, Mn-tapp-A4, Fe-tapp-A4 and Sn-tapp-
A4 are each composed of heavier metalloporphyrins and
therefore are expected to provide lower surface areas. When
the surfaces of the aerogels are converted to an area per
mmol porphyrin units, the difference is minimal for H2tapp-A4
(744 m2 mmol-1), Pd-tapp-A4 (705 m2 mmol-1) and Ni-tapp-A4
Fig. 4 a) N2 adsorption and desorption isotherms at 77 K, b) NL-DFT pore size
distributions and c) HK pore size distributions of H2-tapp-A4, Pd-tapp-A4, Ni-
tapp-A4, Mn-tapp-A4, Fe-tapp-A4 and Sn-tapp-A4 aerogels.
Gas sorption
4 | J. Name., 2012, 00, 1-3
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High BET surface areas and pore volumes make the present
aerogels potential candidates in gas adsorption. The present
aerogels also provide a platform to observe the effect of
60
50
40
30
20
10
0
a)
Pd-tapp-A4, 273 K
H2tapp-A4, 273 K
25
exposed metal species on gas sorption (e.g., CO2,24 H2 and
hydrocarbons26) in aerogels. CO2 adsorption isotherms indicate
that H2tapp-A4 has an uptake capacity of 1.05 mmol g-1 (4.62
wt%) at 298 K, 1 bar and 1.79 mmol g-1 (7.88 wt%) at 273 K, 1
bar (Fig. 5). Mn-tapp-A4, Fe-tapp-A4, Sn-tapp-A4 and Ni-tapp-
A4 have similar capacities (0.99~1.09 mmol g-1 at 298 K;
1.87~2.08 mmol g-1 at 273 K) (Fig. S22-S25), while Pd-tapp-A4
exhibits significantly higher CO2 volumetric uptake at 1 bar
(1.62 mmol g-1, 7.13 wt% at 298 K; 2.39 mmol g-1, 10.5 wt% at
273 K) despite its higher density (Fig. 5, Table 1). The isosteric
enthalpies of CO2 adsorption (Qst) were calculated by the
Clausius-Clapeyron equation from CO2 isotherms collected at
273 and 298 K (Table 1). The Qst at zero coverage was 32.9 kJ
mol-1 for H2tapp-A4 aerogel. After incorporation of metal ions,
the initial affinity turns higher (36.8~37.7 kJ mol-1) for Ni-tapp-
Pd-tapp-A4, 298 K
H2tapp-A4, 298 K
0.00
0.02
0.04
0.06
0.08
0.10
Pressure/MPa
45
40
35
30
25
20
15
10
b)
H2tapp-A4
Pd-tapp-A4
A4, Mn-tapp-A4, Fe-tapp-A4 and Sn-tapp-A4. Pd-tapp-A4
aerogel has the highest isosteric heat of adsorption (40.0 kJ
mol-1). The Qst at zero coverage is higher than those of some
imine-based (porphyrin) covalent organic frameworks (18~21
kJ mol-1)27 and those of imine-based amorphous networks
(31~35 kJ mol-1).28 These high Qst values may be primarily
attributed to the electrostatic field provided by the
impregnated metal ions together with the strong interaction of
CO2 with imine and residue amine groups. Adsorption
selectivity of CO2 over N2 was estimated using the ideal
adsorbed solution theory (IAST).29 At 298 K IAST selectivities
with bulk phase equilibrium partial pressure of 85 kPa N2 and
0.0
0.2
0.4
0.6
0.8
CO2/mmol g-1
Fig. 5 a) CO2 adsorption-desorption isotherms of H2tapp-A4 and Pd-tapp-A4
aerogels at 273 and 298 K, b) isosteric heats of adsorption (Qst) as a function
of gas loading, estimated from low pressure isotherms at 273 and 298 K by
applying the virial equation.
15 kPa CO2 were 24.4 and 34.1 for H2tapp-A4 and Pd-tapp-A4
,
respectively (Fig. S26).
Table 1 Porosity properties and CO2 adsorption performance of various imine aerogels.
a
c
d
e
Aerogel
SBET
Vtb
Vmicro
Vmeso
CO2 uptake at 1.0 bar /wt% (mmol g-1)
Qst
/m2 g-1
/cm3 g-1
/cm3 g-1
0.048
0.067
0.140
0.114
0.152
0.083
0.056
0.116
/cm3 g-1
0.957
1.454
2.481
1.796
2.220
2.274
1.565
1.382
/kJ mol-1
@273 K
@298 K
H2tapp-A2
H2tapp-A3
H2tapp-A4
Pd-tapp-A4
Ni-tapp-A4
213
0.98
1.49
2.60
1.91
2.38
2.33
1.60
1.50
267
719(744)
618(705)
695(759)
7.88 (1.79)
10.52 (2.39)
8.27 (1.88)
8.36 (1.90)
8.22 (1.87)
9.15 (2.08)
4.62 (1.05)
7.13 (1.62)
4.44 (1.01)
4.80 (1.09)
4.36 (0.99)
4.40 (1.00)
32.9
40.0
37.5
36.8
37.6
37.7
Mn-tapp-A4 358(402)
Fe-tapp-A4
Sn-tapp-A4
360(405)
384(470)
a SBET is the BET specific surface area in the units of m2 g-1 and m2 mmol(porphyrin)-1; bVt is the total specific pore volume; c Vmicro is the specific micropore
e
volume calculated using the SF method; c Vmeso is the specific mesopore volume calculated using the BJH method; Qst is the isosteric enthalpy of CO2
adsorption at zero coverage.
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orbital of olefin, thus showing stronger interaction between
C2H4 guest molecules and the host network.
a) 160
140
120
100
80
Pd-tapp-A4, 77 K
H2tapp-A4, 77 K
a)
Pd-tapp-A4, 273 K
35
30
25
20
15
10
5
Pd-tapp-A4, 87 K
H2tapp-A4, 87 K
H2tapp-A4, 273 K
Pd-tapp-A4, 298 K
60
40
20
H2tapp-A4, 298 K
0
0.00
0.02
0.04
0.06
0.08
0.10
Pressure/MPa
0
0.00
0.02
0.04
0.06
0.08
0.10
Pressure/Mpa
b) 8
H2tapp-A4
Pd-tapp-A4
b) 34
H2TAPP-A4
Pd-tapp-A4
7
6
5
4
3
32
30
28
26
24
22
20
18
16
0.0
0.5
1.0
1.5
2.0
2.5
H2/mmol g-1
0.0 0.1 0.2 0.3 0.4 0.5 0.6
C2H4/mmol g-1
Fig. 6 a) H2 adsorption-desorption isotherms of Pd-tapp-A4 and H2tapp-A4
aerogels at 77 and 87 K, b) isosteric heats of adsorption (Qst) as a function of
gas loading.
Fig. 7 a) C2H4 adsorption-desorption isotherms of Pd-tapp-A4 and H2tapp-A4
aerogels at 273 and 298 K, b) isosteric heats of adsorption (Qst) as a function
of gas loading.
Encouraged by the improved CO2 adsorption, Pd-tapp-A4 was
subjected for further H2 and C2H4 adsorption study with
H2tapp-A4 as reference (Fig. 6). H2 adsorption experiments
reveal that Pd-tapp-A4 and H2tapp-A4 adsorbs H2 gas up to
1.26 and 0.94 wt% respectively at 77 K and 1 bar, and 0.84 and
0.60 wt% at 87 K and 1 bar (Fig. 6). Pd-tapp-A4 aerogel again
exhibits higher H2 volumetric uptake despite its higher density,
which may be attributed to the incorporation of accessible
Pd(II) sites. The Qst of H2 adsorption was estimated by fitting
the H2 isotherms at 77 and 87 K to the virial equation. The
initial Qst of Pd-tapp-A4 is 7.75 kJ mol-1, while that of H2tapp-
A4 is 7.32 kJ mol-1. This probably indicates that the
impregnated Pd2+ ions exert effects on the interaction energy
between H2 and the aerogel at low loadings.30 Pd-tapp-A4 and
H2tapp-A4 have similar and decreased Qst at high H2 loadings.
This is consistent with their imine polymer nature.
200
MeOH
benzene
150
100
cyclohexane
V
50
hexane
0.015
0
0.000
0.005
0.010
Pressure/MPa
Fig. 8 a) Vapor sorption isotherms of methanol, benzene, cyclohexane and
hexane for H2tapp-A4 aerogel at 298 K (Filled shapes represent adsorption
and open shapes represent desorption).
Pd-tapp-A4 and H2tapp-A4 were also subjected for C2H4
adsorption (Fig. 7). Pd-tapp-A4 uptakes more C2H4 (2.18%) at
298 K, 1 bar than H2tapp-A4 (1.78 wt%). The initial Qst of Pd-
tapp-A4 is 32.2 kJ mol-1, while that of H2tapp-A4 is 31.2 kJ mol-
1. The impregnation of Pd(II) metal ions probably enhances the
isosteric heat of C2H4 adsorption at low loading.30 The Pd(II)
metal centres are more capable of accepting π electron
density and/or donating electron density into the empty π*
Organic vapour sorption. To further study the adsorption
properties, MeOH, benzene, cyclohexane and hexane vapour
sorption was carried out for H2tapp-A4 at 298 K (Fig. 8). MeOH
sorption isotherm is type II and show maximum uptake of 181
cm3 g-1, corresponding to 26 wt%. Benzene sorption isotherm
is also type II and show maximum uptake of 174 cm3 g-1 (61
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wt%). The sorption capacity for benzene is remarkably higher (v:v = 1:1). DMSO solvent in the gel was exchanged to DMF
than those for saturated hydrocarbons cyclohexane (51 cm3 g- prior to catalysis. The reaction system was assembled as
1, 19 wt%) and hexane (30 cm3 g-1, 12 wt%). Both the shown in Fig. 9. Specifically a homogeneous DMF-H2O solution
desorption branches of MeOH and benzene do not trace the of 4-bromoanisole (0.05 mol L-1), benzeneboronic acid (0.075
adsorption ones, leaving large hysteresis loops. Such mol L-1) and CsF (0.12 mol L-1) was introduced using a syringe
behaviours may arise from favourable π-π (H-bonding) pump and entered the capillary reactor. A capillary of 1000
interactions between guest benzene (MeOH) molecules and mm long with 0.53 mm in diameter coated with 2 μm catalytic
the aromatic (imine) framework. It may also be attributed to gel layer was used for the reactor. The reaction mixture was
the unique features of the hierarchical imine aerogels.
allowed to pass the capillary during a period of 30 min at 100
oC. The cross-coupling biaryl product was afforded in 85% yield
Catalytic gel microfluidic reactor
The hierarchically porous nature and the availability of metal for an aliquot of 50 μL substrate solution. In addition, the
centres of the present metalloporphyrin imine gels prompted present gel microfluidic reactor shows advantage in catalyst
us to investigate their catalytic activity. As a representative, recovery and ready separation from the products. The reactor
Pd-tapp-A4 gel was chosen and immobilized within the could be readily reused for the next run after flushing with
channels of a microfluidic flow reactor because a combination DMF-H2O (1 mL × 3). No significant loss of reactivity was
of gel catalysts and microfluidic techniques may significantly observed over at least five runs (Table S3).
enhance the catalytic system.
The catalytic performance of the capillary reactor was
compared to control experiments under batch conditions.
Over a range of different Pd-tapp-A4 gel catalyst loading (0.5-
10 mol%), optimal catalyst performance was found to be 1.0
mol% (Fig. S27). 34% yield was obtained after 30 min at 100 oC.
After 2.0 h, 87% yield was achieved. However, further
increasing of the gel catalyst amount did not improve the
performance. The catalytic performance of the capillary
reactor was also compared with that of Pd-tapp homogeneous
precursor. Under homogeneous conditions (1.0% Pd-tapp), the
cross-coupling biaryl product was obtained in only 15% yield
after 30 min and 70% after 2.0 h (Fig. S27). In comparison,
similar yields were achieved in much shorter reaction time
under the gel-capillary flow reaction conditions (85% yield
after 30 min). The faster reaction in the capillary reactor than
those of the homogeneous and heterogeneous reactions
should be contributed to the combination of gel catalysts with
microfluidic technology.32 Much larger surface area is exposed
and the regents interact with a much larger proportion of the
catalytic gel in the capillary reactor. Furthermore the rigid gel
matrix with hierarchical porosity further improves mass
transfer.
Fig. 9 Schematic representation of setup of the catalytic gel microfluidic
reactor using a capillary, and SEM images of a cross-section of the capillary
(ID 0.53 mm) coated with Pd-tapp-A4 gel with about 2 μm thickness (bars
represent 10.0 and 2.0 μm from left to right).
Pd-tapp-A4 gel was introduced into a fused-silica capillary
reactor via in situ gelation. First the surface of the inner wall of
the capillaries was modified with amine groups in order to
anchor the Pd-tapp-A4 gel via imine bonding. Pd-tapp and A4
precursors in the presence of AcOH catalyst reacted inside the
capillary upon heating to form the Pd-tapp-A4 gel. Following
our previously reported strategy,6 a gel layer with ca. 2 μm
(evidenced by SEM) was successfully coated onto the capillary
(Fig. 9).
Conclusions
In summary, a series of (metallo)porphyrin imine aerogels
have been synthesized. The aerogels have sponge-like porous
networked
structures
consisting
of
interconnected
nanoparticles with hierarchical porosity. The aerogels have
high specific surface areas (up to 719 m2 g-1) and large pore
volumes (up to 2.60 cm3 g-1). The effect of metal ions on the
uptake of gases was investigated for these aerogels. The
incorporation of divalent metal ions (Pd(II), Ni(II)) in the
porphyrin-based imine aerogels does not change the porosity
significantly, while impregnation of trivalent or tetravalent
metal ions (Mn(III), Fe(III) and Sn(IV)) lowers the surface area
of the aerogels. After incorporation of metal ions (Ni(II),
Mn(III), Fe(III) and Sn(IV)), the initial affinity turns higher, while
their CO2 adsorption capacities does not change notably.
Among the investigated metal ions, Pd(II) is the best to
increase the CO2 adsorption.Pd-tapp-A4 aerogel exhibits CO2
Pd-tapp gel
MeO
Br
B(OH)2
+
MeO
DMF/H2O
CsF
Scheme 3 Suzuki-Miyaura cross-coupling of 4-bromoanisole and
phenylboronic acid catalyzed by Pd-tapp-A4 gel in a capillary reactor.
The catalytic activity of the Pd-tapp gel microfluidic system
was investigated in Suzuki-Miyaura cross-coupling31 of 4-
bromoanisole and phenylboronic acid as a model reaction
(Scheme 3). Pd-tapp-A4 gel was used as catalyst in DMF-H2O
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Journal Name
volumetric uptake (1.62 mmol g-1, 7.13 wt% at 298 K, 1 bar;
2.39 mmol g-1, 10.5 wt% at 273 K) with high isosteric heat of
adsorption (40.0 kJ mol-1). Pd-tapp-A4 aerogel also shows
improved adsorption capacities for H2 and C2H4. Vapour
sorption shows that H2tapp-A4 aerogel show good adsorption
ability for both aromatic benzene and polar methanol at
saturated vapour pressure and room temperature.
7
8
W. Luo, Y. Zhu, J. Zhang, J. He, Z. Chi, P. W. Miller, L. Chen
and C. Y. Su, Chem. Commun., 2014, 50, 11942-11945
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The present hierarchically porous gels are prepared under
relatively mild synthesis conditions. It is thus possible to obtain
some materials (e.g., free-base porphyrin porous materials)
that are difficult to synthesize under other synthesis conditions
(e.g., at higher temperature and/or in the presence of metal
ions).33 The unique hierarchical porosity of gels offers better
diffusion channels for guest molecules than the related
microporous materials (e.g., MOFs), which enhances mass
transfer. Pd-tapp-A4 gel catalyst and microfluidic technique
have been successfully combined in the catalytic gel
microfluidic reactor. In the microfluidic reactor, a layer of Pd-
tapp-A4 gel is coated onto a functionalized capillary. In the gel
capillary reactor similar yields were achieved in much shorter
reaction time in Suzuki-Miyaura cross-coupling of 4-
bromoanisole and phenylboronic acid compared to the batch
processes. These materials thus show potential applications
not only in gas storage, but also in supported catalysis. The
present strategy may help develop new metal-decorated
polymeric materials.34
9
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Acknowledgements
We acknowledge the 973 Program (2012CB821701), the NSFC
(51573216, 21273007, 21103233 and 91222201), the Program
for New Century Excellent Talents in University (NCET-13-
0615), the NSF of Guangdong Province (S2013030013474) and
the FRF for the Central Universities (14lgpy05) for support.
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Table of Contents
A series of (metallo)porphyrin imine gels show tuneable gas adsorption and potential to assemble
a catalytic gel capillary reactor.