Tandem Synthesis of ?-Caprolactam from Cyclohexanone by an Acidified Metal-organic Framework

Article abstract of DOI:10.1002/cctc.202100425

Tandem synthesis of ?-caprolactam, one of the largest scaled commercial chemicals, is highly desired from the viewpoint of cost, energy, and environment. However, relevant studies have remained largely underexplored. By using a one-pot strategy, we encapsulated phosphotungstic acid (PTA) into a chromium terephthalate metal-organic framework (MOF), MIL-101, for the efficient tandem conversion of cyclohexanone to ?-caprolactam. The highly dispersed PTA in the MOF matrix showed a high yield of ?-caprolactam through a tandem oximation-Beckmann rearrangement reaction at 100 °C for 12 h. Moreover, MIL-101-PTA was recycled three times, with only a slight loss in their catalytic performance. To the best of our knowledge, this represents the first report using acidified MOF for a tandem oximation-Beckmann rearrangement reaction.

Full text of DOI:10.1002/cctc.202100425

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Tandem synthesis of ε-caprolactam from cyclohexanone by an
acidified metal-organic framework
Authors: Biying Zhang, Xinle Li, Jingwen Chen, Tianqing Liu, Andrew
Cruz, Yuchen Pei, Minda Chen, Xun Wu, and Wenyu Huang
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.202100425
Link to VoR: https://doi.org/10.1002/cctc.202100425
01/2020

10.1002/cctc.202100425
ChemCatChem
FULL PAPER
e
Tandem synthesis of -caprolactam from cyclohexanone by an
acidified metal-organic framework
Biying Zhang,[a] Xinle Li,[b] Jingwen Chen,[a] Tianqing Liu,[a] Andrew Cruz,[c] Yuchen Pei,[a] Minda
Chen,[a] Xun Wu[a,c], Wenyu Huang[a,c]
*
[a]
B. Zhang, Dr. J. Chen, T. Liu, Dr. Y. Pei, M. Chen, X. Wu, Prof. W. Huang
Department of Chemistry
Iowa State University
Ames, IA 50011, United States
E-mail: whuang@iastate.edu
Prof. X. Li
[b]
Department of Chemistry
Clark Atlanta University
Atlanta, Georgia 30314, United States
A. Cruz, X. Wu, Prof. W. Huang
Ames Laboratory
[c].
U.S. Department of Energy
Ames, IA 50011, United States
Supporting information for this article is given via a link at the end of the document.
Abstract: Tandem synthesis of e-caprolactam, one of the largest
scaled commercial chemicals, is highly desired from the viewpoint of
cost, energy, and environment. However, relevant studies have
remained largely underexplored. Herein, by using a one-pot strategy,
we encapsulated phosphotungstic acid (PTA) into a chromium
terephthalate metal-organic framework (MOF), MIL-101, for the
efficient tandem conversion of cyclohexanone to e-caprolactam. The
highly dispersed PTA in the MOF matrix showed a high yield of e-
caprolactam through a tandem oximation-Beckmann rearrangement
reaction at 100 °C for 12 h. Moreover, MIL-101-PTA was recycled
three times, with only a slight loss in their catalytic performance. This
represents the first report using acidified MOF for a tandem oximation-
Beckmann rearrangement reaction to the best of our knowledge.
realize the liquid-phase Beckmann rearrangement at low
temperatures, such as zeolites,[6] ionic liquid anchored on metal
oxide,[7] acidified carbon,[8] etc. Among them, Polyoxometalates
(POMs) have been proved as effective catalysts for the
transformation of oxime to yield CPL.[9] However, the low surface
area of POMs (1-10 m2/g) severely restricted the activity, thus
restraining their scale-up applications.
Cr3+
+
BDC
PTA
One-pot
+
Introduction
Cyclohexanone
oxime
Cyclohexanone
Caprolactam
e-Caprolactam (CPL), the monomer of nylon-6, is one of the
largest scaled commercial products. Based on recent research,
the annual production of CPL increases at a rate of 3% each year
since 2017 and is predicted to reach around 6.5 million tons by
the end of 2023.[1] Among the developed methods for the
synthesis of CPL, one traditional synthetic route is to start from
phenol substrates through phenol hydrogenation to form
cyclohexanone,[2] cyclohexanone-hydroxylamine condensation to
produce the cyclohexanone oxime,[3] and Beckmann
rearrangement to transform the oxime intermediate into the
CPL.[4] Conventionally, the step of Beckmann rearrangement is
carried out in fuming sulfuric acid (a.k.a., oleum) that serves as
both catalyst and solvent.[5] However, oleum causes severe
environmental and safety concerns. Moreover, a massive waste
of ammonium sulfate is produced from the neutralization of oleum
in the production of CPL. To circumvent these drawbacks, solid
acids have emerged as alternative candidates to oleum, and a
variety of heterogeneous catalysts have been developed to
Figure 1. Synthetic scheme of MIL-101-PTA and one-pot tandem synthesis of
CPL through a tandem oximation-Beckmann rearrangement reaction.
Metal-organic frameworks (MOFs), a class of crystalline porous
materials constructed by assembling metal ions/clusters and
organic ligands, have emerged as auspicious platforms for
heterogeneous catalysis.[10] MOFs have high porosity and
structural tunability that allows precise integration of various
active sites in their pores, including solid acid,[11] metal
complexes,[12] nanoparticles,[13] and others,[14] allowing for the
flexible design of multifunctional catalysts for tandem catalysis.
Inspired by this, we encapsulate phosphotungstic acid (PTA) in a
robust and high-surface-area MOF matrix to afford an acidified
MOF catalyst. In essence, the chromium terephthalate MOF, MIL-
101 (MIL for Matérial Institut Lavoisier), is rationally selected to
1
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10.1002/cctc.202100425
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support PTA (termed MIL-101-PTA) for the following
considerations (Figure 1): i) MIL-101 features high chemical and
thermal stability, which is critical for heterogeneous catalysis; ii)
the high surface areas and uniform mesopores of MIL-101 allows
for the well-dispersive and homogenous distribution of PTA; iii)
the nanoconfinement effects endowed by MIL-101 could lead to
the synergistic interplay between PTA and surrounded
frameworks, thus resulting in enhanced properties through
cooperative catalysis.[15] With these in mind, we firstly apply MIL-
101-PTA as acidic catalysts in a tandem oximation-Beckmann
rearrangement reaction that directly transforms cyclohexanone to
CPL in one pot (Figure 1). Remarkably, the acidified MOF
displays a high yield of e-caprolactam (89%) and could be
recycled three times with only a slight loss in their catalytic
performance. We further encapsulate Pd nanoparticles in the
MIL-101-PTA. The resultant multifunctional catalyst shows a high
yield of e-caprolactam in sequential tandem catalysis that
combines phenol hydrogenation, oximation, and Beckmann
rearrangement, despite significant Pd leaching after the first run.
The failure of recyclability of Pd loaded catalysts has been the
biggest challenge in reported efforts to develop multifunctional
catalysts for this transformation,[16] which will be the target for our
future studies.
arrows in Figure 2b, presumably due to the interaction between
MOF and PTA.[21]
(a)
PTA
MIL-101-PTA
MIL-101
Simulated-MIL-101
4
6
8
10
12
14
16
18
2 Theta (degree)
(b)
Red shift Blue shift
MIL-101-PTA
MIL-101
Results and Discussion
PTA
-1
886 cm
-1
978 cm
PTA was immobilized into MIL-101 following
a one-pot
-1
1078 cm
encapsulation method to yield the hybrid acidified MOFs, MIL-
101-PTA.[17] The Powder X-ray diffraction (PXRD) patterns of
MIL-101 and MIL-101-PTA match the simulated pattern,[18]
indicating that the presence of PTA did not interfere with the
formation of MIL-101 framework (Figure 2a). It is known that there
are two types of cages in MIL-101, with the diameter of 29 and 34
Å. The window size of the smaller cage is 12 Å that is slightly
smaller than the diameter of PTA molecule. Therefore, PTA
leaching problem can be largely suppressed by the size restriction
of the cage window. Another common problem encountered by
the loading of PTA is that some carrier materials, such as SiO2
and Al2O3, have strong interactions with PTA, which could lead to
the loss of the acidity of PTA or even partially decomposition
forming lacunary species.[19] To test the acidity of the PTA
encapsulated in MIL-101, the amount of acidic proton was
quantified by acid-base titration.[20] As shown in Table S1, the
proton amount increases with the PTA loading and agrees with
the corresponding theoretical value.
1600
1400
Wavenumber (cm
1200
1000
800
-1
)
Figure 2. (a) PXRD patterns of PTA, MIL-101, MIL-101-PTA, and simulated
MIL-101. (b) DRIFTS spectra of PTA, MIL-101, and MIL-101-PTA. The grey-
shaded regions represent the characteristic vibrational bands of PTA. The red
(blue) arrow indicates the red (blue) shift of the vibration band of PTA after the
encapsulation in MIL-101.
A comparison of the spectra obtained by diffuse reflectance
infrared Fourier-transform spectroscopy (DRIFTS) of MIL-101
and MIL-101-PTA confirms the presence of PTA in MIL-101-PTA
(Figure 2b). The three major characteristic absorption peaks of
PTA at 1078, 978, and 886 cm-1 are assignable to the vibrations
of P–O, terminal W=O, and edge-sharing W–O–W, respectively
(grey-shaded areas in Figure 2b).[11a] Moreover, these bands are
absent in the spectra of the pristine MIL-101. It is evidenced that
the band at 886 cm-1 will disappear when lacunary structures are
formed. Therefore, the presence of this band for MIL-101-PTA
indicates that the kegging structure is preserved during the MOF
synthesis and encapsulation process. It is noteworthy that there
is a 10 cm-1 blueshift for the characteristic band of edge-sharing
W–O–W (886 cm-1) and around 5 cm-1 redshift for the band of
terminal W=O vibration in PTA, represented by the blue and red
Figure 3. (a) HAADF-STEM image of MIL-101-PTA. EDS elemental mapping
analysis of (b) Cr, (c) W, (d) P, and (e) the composite. (f) TEM image of MIL-
101-PTA.
High-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) and energy-dispersive X-ray
spectroscopy (EDS) elemental mapping of MIL-101-PTA (Figure
2
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10.1002/cctc.202100425
ChemCatChem
FULL PAPER
3) show the well-mixed Chromium (Cr), Phosphorus (P), and
Tungsten (W) signals in MIL-101-PTA, which unambiguously
indicates the homogeneous and uniform distribution of PTA
throughout the MIL-101 framework. The smooth and uniform
surface of MIL-101-PTA (Figure 3a) also implies the well-
dispersed PTA in the MOF since dark, and small dots are
observed if PTA molecules aggregate on the surface of the MOF,
especially when using wetness impregnation method to load PTA
on MIL-101.[22] The loading of W from PTA is determined by
inductively coupled plasma-mass spectroscopy (ICP–MS) at
around 17% (Table S1). Nitrogen physisorption isotherms of MIL-
101 and MIL-101-PTA also confirm the presence of PTA inside
the pores (Figure 4). The Brunauer–Emmett–Teller (BET) surface
area drops from 2840 to 1630 m3/g, and the pore volume of MIL-
101 also decreases from 1.38 to 0.90 cm3/g after the
encapsulation of PTA (Figure 4a, Figure 4b, and Table S2).
(entries 2-4) shows no improvement in CPL selectivity, probably
due to the weak basicity of the solvent competing with the
adsorption of reactant that poisons the active acid sites.[6c] Other
solvents such as dioxane and nitrobenzene (entries 5, 6) do not
improve the selectivity. To our delight, when acetonitrile (MeCN)
is added to DCE in a 1:1 ratio, a high CPL yield (89%) and
cyclohexanone conversion (94%) are observed (entry 7). By
varying the ratio between MeCN and DCE, we find that a larger
ratio of MeCN/DCE leads to a higher selectivity towards CPL
(entries 7-9). Given that the proton transfer from nitrogen to
oxygen during the process of forming O-protonated oxime is the
rate-determining step, we speculate the MeCN is prone to
stabilize the charged intermediate during the Beckmann
arrangement to lower its’ activation energy.[23] We also find that it
is crucial to use an anhydrous solvent in the reaction. Adding 1%
of water to the reaction solvent decreases the conversion to 84%
and selectivity to 34% by facilitating the hydrolysis of
cyclohexanone oxime to cyclohexanone. The conversion and
selectivity decrease more when 10% of water is added to the
reaction (entries 10, 11). The reaction temperature (Figure 5) is
also investigated to show the highest cyclohexanone conversion
of 94% with a CPL yield of 89% at 100 °C. In a nutshell, the
optimal reaction condition (entry 7, Table 1) for the tandem
synthesis of CPL catalyzed by MIL-101-PTA is 100 °C in a binary
mixture of DCE/MeCN (1:1) as solvent.
1000
(a)
MIL-101
900
800
700
600
500
400
300
200
MIL-101-PTA
Table 1. Optimal reaction conditions screening in the oximation-Beckmann
rearrangement. [a]
0.0
0.2
0.4
0.6
0.8
1.0
Entry
Additive
Add./DCE
Conv. (%)
Yield. (%)
Oxime CPL
P/P
0
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(b)
1
2
-
DMSO
-
99
77
87
96
99
69
94
96
95
84
88
65
82
77
77
55
3
2
0
MIL-101
MIL-101-PTA
1:1
1:1
1:1
1:1
1:1
1:1
3:7
1:9
1:1
3
DMAC
0
4
DMF
0
5
Dioxane
Nitrobenzene
MeCN
2
6
0
7
89
79
41
29
1
2
3
4
5
8
MeCN
10
42
45
Pore Diameter (nm)
9
MeCN
10
MeCN, 1%water
Figure 4. (a) Nitrogen sorption isotherms of MIL-101 and MIL-101-PTA. (b) Pore
size distributions of MIL-101, MIL-101-PTA.
MeCN,
10%water
11
1:1
44
26
4
12[b]
13[c]
MeCN
MeCN
1:1
1:1
63
75
11
8
42
64
With the thorough characterizations of MIL-101-PTA in hand,
we set to optimize conditions for the tandem synthesis of e-
caprolactam through an oximation-Beckmann rearrangement
reaction (Table 1). We find that the dielectric constant or polarity
of the solvents plays a critical role in catalysis activity. When
dichloroethane (DCE) is solely used as the solvent (entry 1), no
selectivity to the CPL is observed, and the products are mainly
oxime (88%) and 2-chlorocyclohexanone. To increase the
selectivity to CPL, various polar aprotic organic additives are
added along with DCE. The addition of dimethylformamide (DMF),
dimethylacetamide (DMAC), or dimethyl sulfoxide (DMSO)
[a] Reaction condition: MIL-101-PTA (20 mg, 7 nmol PTA), cyclohexanone
(0.25 mmol), hydroxylamine hydrochloride (0.375 mmol), internal standard
(n-decane, 20 µL), solvent (1 mL, DCE and additive), 100 °C, 12 h stirring at
600 rpm. [b] No catalyst was added. [c] MIL-101 was used as the catalyst.
Considering the oximation-Beckmann rearrangement is
catalyzed solely by the acid sites, PTA plays a significant role in
ensuring the tandem transformation of cyclohexanone to CPL. As
expected, MIL-101-PTA shows a higher yield (89%, entry 7) of
3
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CPL compared to that catalyzed by MIL-101 (65%, entry 13). The
Brønsted acid sites of PTA boost the catalyst activity in Beckmann
rearrangement since the protonation of the nitrogen in the oxime
group is proved as the initial step. However, the hydroxylamine
hydrochloric acid (NH2OH·HCl) also possesses acid sites.
Therefore, we perform a blank experiment in which the MIL-101
or MIL-101-PTA is not added (entry 12). Interestingly, a 42% yield
(entry 12) of CPL is achieved without any catalyst, suggesting
that the hydrochloric acid in NH2OH•HCl also serves as a catalyst
(Figure S1).[24] Compared with the blank experiment, reaction
catalyzed by MIL-101 exhibited an increased yield of CPL.
Previous study demonstrates that Lewis acid can also catalyze
Beckmann rearrangement.[25] The increased production of CPL by
MIL-101 was likely attributed to the Lewis acidic Cr sites. By
changing the loading of PTA in the MOF, we synthesized MIL-
101-0.5PTA and MIL-101-2PTA and tested their catalytic
properties (Figure S2). Overall, we observed an increased yield
of CPL as we increased the loading of PTA in the catalyst from
MIL-101 to MIL-101-PTA, and the yield of CPL stopped growing
for MIL-101-2PTA, probably due to the high yield of CPL. To
differentiate the performance of these two catalysts, we ran the
reaction at 80 °C. MIL-101-2PTA exhibits a higher yield of CPL
(20%) than MIL-101-PTA (12%). We also test the oximation solely
with our catalyst and compare the result with reaction catalyzed
with MIL-101 and no catalyst (Figure S3). Since oximation is a
relatively mild reaction and can proceed even without the aid of
the catalyst. We conducted the reaction at 25 °C to maximize the
catalytic difference between the reactions with and without
catalyst. These results illustrate that MIL-101-PTA also facilitates
the oximation with 70% yield in 1 min compared with MIL-101
(56%) and no catalyst (6%). The above results indicate that even
though both Lewis and Bronsted acids can facilitate oximation-
Beckmann rearrangement reaction, Bronsted acid performs
better than Lewis acid in catalyzing the reaction. We recycle the
MIL-101-PTA catalyst in oximation-Beckmann reaction three
times. Compared with fresh catalyst, MIL-101-PTA exhibits ca.
25% decrease in the yield in the third run (Figure S4). By
comparing the DRIFTS spectra of recycled MIL-101-PTA with the
fresh catalyst, it is evident that the characteristic bands of the PTA
are at the same positions, which proves that the PTA molecules
are still in the Keggin structure (Figure S5). However, the PXRD
analysis reveals that the crystallinity of the recycled catalyst is
gradually lost during each cycle (Figure S6). The surface area and
pore size/volume of the recycled MOF catalyst also decrease after
three reaction cycles (Figure S7-S8). Therefore, the reduced
activity of the catalyst could stem from the partial collapse of the
MOF structure.
the reaction. However, hydroxylamine aqueous solution is not
preferred for the reaction because the presence of H2O leads to
the backward reaction of oxime to cyclohexanone.
Conversion
Yield to oxime
Yield to CPL
100
80
60
40
20
0
60
80
Temperature (°C)
100
Figure 5. Temperature effect on oximation-Beckmann rearrangement.
(Reaction condition: MIL-101-PTA (20 mg, 7nmol PTA), cyclohexanone (0.25
mmol), hydroxylamine hydrochloride (0.375 mmol), n-decane (20 µL), DCE (0.5
mL), MeCN (0.5 mL). The reaction was performed at different temperatures for
12 h under stirring at 600 rpm).
Encouraged by the successful one-pot synthesis of CPL from
cyclohexanone, we sought to add one more step to the tandem
reaction with selective phenol hydrogenation to cyclohexanone.
The whole tandem catalysis then combines phenol hydrogenation,
oximation, and Beckmann rearrangement in one pot. To achieve
phenol hydrogenation in the initial step, we loaded Pd precursor
into the MOF and reduced it to Pd nanoparticles in MIL-101-PTA
(termed Pd-MIL-101-PTA, Figure S11-13). The loading of Pd in
Pd-MIL-101-PTA is 4.9 wt.% measured by ICP-MS (Table S3). Pd
nanoparticles have been proved as highly selective catalysts in
the hydrogenation of phenol to cyclohexanone.[2,26] We first run
the phenol hydrogenation over Pd-MIL-101-PTA in DCE under 20
bar hydrogen at 60 °C, followed by the subsequent addition of
MeCN and hydroxylamine hydrochloride to conduct oximation-
Beckmann arrangement. As shown in Table S4, a remarkable
yield of CPL (92%) is achieved after 6 h of phenol hydrogenation
and 12 h of oximation-Beckmann rearrangement. However, Pd-
MIL-101-PTA cannot be recycled in such sequential tandem
catalysis since the recovered catalyst becomes inactive towards
phenol hydrogenation in the second run. Such a problem was also
encountered by Ju Yeon Shin et al., where Pd/Sc(OTf)3/ionic
liquid system was employed as the catalyst for the tandem
synthesis of CPL from phenol.[16] To account for such “quenched”
activity, we perform ICP-MS (Table S3) analysis of the used
catalyst and find that 40% of Pd leaches from the MOF support
after the first round of tandem catalysis, which could be induced
by the strong acidic condition and chloride ions which facilitate the
dissolving of Pd by coordination.[27] Currently, more endeavors
are ongoing to resolve the Pd leaching issue.
To account for the crystallinity loss of recycled MOFs, we did a
series of control experiments. First, we evaluated the stability of
MIL-101 after recycling for different times. The decreasing peak
intensity of the recycled MOF samples indicates that the
crystallinity of MIL-101 gradually lost during each cycle (Figure
S9), which is similar to what we observed in MIL-101-PTA (Figure
S6). The similar stability performances of MIL-101 and MIL-101-
PTA indicate that the collapse of the MOF structure during cycling
is not due to the encapsulation of PTA in the MOF matrix. To study
the role of HCl in the collapse of MOF structures, we conducted a
recycle test by using hydroxylamine aqueous solution instead of
hydroxylamine hydrochloride salt in the reaction. After recycling
three times, the PXRD pattern of the recycled MOF catalyst barely
changes compared with the pristine MOF (Figure S10), implying
HCl plays a major role in destroying MOF primary structure during
The core problem of the tandem reaction starting from phenol
is that the required reaction condition for oximation-Beckmann
rearrangement is quite harsh, which is under strong acidic and
28]
high-temperature.[16,
Pd nanoparticles cannot survive under
these conditions, most likely due to etching by the acid and
poisoning by the solvent such as CH3CN, resulting in the lost
activity of Pd in recycled catalysts for phenol hydrogenation.
4
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10.1002/cctc.202100425
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regia and diluted by 2 wt.% HNO3 to appropriate concentrations for the
elemental measurement. The acidity of the catalysts was measured by
acid-base titration.[20] 250 mg catalyst were suspended in 10 g saturated
NaCl solution and stirred at room temperature for 24 h. The solid was
collected by centrifuge and washed with 20 g deionized water. All of the
filtrates were titrated with 0.01 M NaOH solution.
Conclusion
In conclusion, we have developed acidified MOFs, MIL-101-PTA,
by encapsulating phosphotungstic acid (PTA) into MIL-101 for the
one-pot transformation of cyclohexanone to e-caprolactam. The
resultant MIL-101-PTA shows uniformly dispersed PTA
throughout the MOF matrix and high yield of e-caprolactam in the
tandem oximation-Beckmann rearrangement. MIL-101-PTA can
be recycled three times with a slight loss in their catalytic
performance. Moreover, a new multifunctional MOF catalyst is
developed by encapsulating Pd nanoparticles in MIL-101-PTA
and shows a high yield of e-caprolactam in sequential tandem
catalysis that combines phenol hydrogenation, oximation, and
Beckmann rearrangement in one pot, despite significant Pd
leaching after the first run. This work opens up new avenues to
the synthesis of e-caprolactam via tandem catalysis by
multifunctional acidified MOFs and uncovers the great potential of
using MOFs in the efficient synthesis of fine chemicals.
Reaction conditions for oximation-Beckmann rearrangement
reaction. Cyclohexanone (0.25 mmol, 26 µL), hydroxylamine
hydrochloride (0.375 mmol, 26.1 mg), n-decane (20 µL) and catalyst (20
mg) were added in a binary mixture of acetonitrile and DCE (1 mL, v:v
=1:1). The reaction was performed at 100 °C for 12 h with stirring in the
autoclave under 40 bar nitrogen in a 300 mL high-pressure vessel.
Reaction conditions for oximation reaction. Cyclohexanone (0.75
mmol, 78 µL), hydroxylamine (1.125 mmol, 74.4 µL, 50 wt. % in water), n-
decane (20 µL) and catalyst (20 mg) were added in a binary mixture of
acetonitrile and DCE (1 mL, v:v =1:1). The reaction was performed at 25 °C
for 1 min with stirring in 6-dram vial.
Reaction conditions for the one-pot sequential synthesis of CPL from
phenol. Phenol (0.250 mmol, 23.5 mg), n-decane (20 µL) and catalyst (20
mg) were added in 0.5 mL 1,2-dichloroethane (DCE) solution in a 6-dram
vial. The reaction was stirred under 20 bar H2 at 60 °C for 6 h. After release
the hydrogen pressure, acetonitrile (0.5 mL) and hydroxylamine
hydrochloride (0.375 mmol, 26.1 mg) were added to the reaction vial. The
reaction mixture was stirred at 100 °C for 12 h under 40 bar nitrogen. The
reaction was carried out in a 300 mL high-pressure vessel.
Experimental Section
Synthesis of MIL-101. MIL-101 was synthesized following the
hydrothermal method reported in previous literature.[17] Typically, a mixture
of Cr(NO3)3•9H2O (2.00 g, 5.00 mmol) and terephthalic acid (0.832 g, 5.00
mmol) in 20 mL deionized water was stirred at room temperature for 20
min. The mixture was transferred to a 100 mL autoclave and was heated
to 220 °C and maintained for 18 h. After cooling to room temperature, the
solid was collected by centrifugation and thoroughly washed with water
(three times) and methanol (three times). Then the materials were
activated by heating at 150 °C for 12 h under vacuum.
For all the reactions, the reactants/products were quantified by gas
chromatography (Hewlett Packard 5890 II) equipped with an FID detector
and an HP-5 column (30
m × 0.32 mm × 0.25 μm) and gas
chromatography-mass spectrometer (Agilent 6890N/5975 GC-MS)
equipped with an HP-5ms capillary column (30 m × 0.32 mm × 0.25 µm).
Synthesis of MIL-101-PTA, MIL-101-0.5PTA, and MIL-101-2PTA. MIL-
101-PTA was synthesized following a one-pot encapsulation method.[29]
A
mixture of Cr(NO3)3·9H2O (2.00 g, 5.00 mmol), terephthalic acid (0.832 g,
5.00 mmol), and phosphotungstic acid hydrate (1.00 g, 0.345 mmol) in 20
mL deionized water was stirred at room temperature for 20 min. Then the
mixture was transferred to a 100 mL autoclave and was heated to 220 °C
and maintained for 18 h. The washing and activation procedures are the
same as the synthesis of MIL-101. 0.5 g phosphotungstic acid hydrate was
used for MIL-101-0.5PTA, and 2 g phosphotungstic acid hydrate was used
for MIL-101-2PTA.
Acknowledgements
This work is supported by National Science Foundation (CHE-
1566445) and Iowa State University. A.C. thanks the support from
Science Undergraduate Laboratory Internship (SULI) program
through Ames Laboratory, sponsored by U.S. Department of
Energy Office of Science, and Office of Workforce Development
for Teachers and Scientists (WDTS).
Synthesis of Pd-MIL-101-PTA. 200 mg of MIL-101-PTA was dispersed in
2 mL chloroform and sonicated for 20 min to achieve a homogeneous
suspension. Afterward, 2 mL chloroform solution containing 29.5 mg
Pd(acac)2 was added dropwise in the MOFs suspension. After stirring for
24 h at ambient temperature, the mixture was vacuum dried and reduced
at 200 °C for 2 h under 10% H2/Ar at a flow rate of 50 mL/min to afford Pd-
MIL-101-PTA.
Keywords: Metal-organic frameworks (MOFs) • Tandem
reaction • e-caprolactam • phosphotungstic acid
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Entry for the Table of Contents
Cyclohexanone
Cr3+
+
Cyclohexanone
oxime
BDC
PTA
+
e
-Caprolactam
Using a one-pot strategy, we encapsulate phosphotungstic acid (PTA) inside a Cr-based metal-organic framework (MOF) matrix to
afford MIL-101-PTA catalysts. The acidified MOFs catalyze the tandem oximation-Beckmann rearrangement reaction that directly
transforms cyclohexanone to e-caprolactam. The acidified MOF displays a high yield of e-caprolactam (89%) and can be recycled three
times with a slight loss in their catalytic performance.
7
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