Iron and chromium MOFs as sustainable catalysts for transfer hydrogenation of carbonyl compounds and biomass conversions

Article abstract of DOI:10.1039/d0nj00552e

MIL-88B is a class of metal-organic framework (MOF) that has been widely explored in catalysis. Iron- and chromium-based MIL-88B MOFs were prepared and characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and N₂ adsorption/desorption measurements. The MIL-88B MOFs were investigated as catalysts for transfer hydrogenation (TH) of carbonyl compounds (ketones and aldehydes) and biomass derivatives (levulinic acid and furfural) with 2-propanol as a hydrogen donor. Their conversion and selectivity were excellent. The recovered catalyst was reused ten times, with no significant loss in catalytic activity. The recovered catalyst after the tenth run retained its original crystallinity and morphology, which were confirmed by SEM and powder XRD studies. Catalyst reusability for levulinic acid (LA) conversion was also studied. The activation energy for the transformation of LA to γ-valerolactone (GVL) was 25.44 kJ mol^-1. The catalytic performance of MIL-88B(Fe) was compared with that of MIL-53(Fe).

Full text of DOI:10.1039/d0nj00552e

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Iron and chromium MOFs as sustainable catalysts for transfer
hydrogenation of carbonyl compounds and biomass conversions
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Punitharaj Vasanthakumar,† Dharmalingam Sindhuja,† Duraisamy Senthil Raja,‡,§ Chia-Her Lin‡, #
and Ramasamy Karvembu*,†
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† Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, India
‡ Department of Chemistry, Chung Yuan Christian University, Chung Li, Taoyuan 32023, Taiwan
§ Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
# Department of Chemistry, National Taiwan Normal University, Taipei 24449, Taiwan
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Abstract: MIL-88B is a class of metal-organic framework (MOF) which is widely explored in
catalysis. Iron- and chromium-based MIL-88B MOFs were prepared and characterized by X-
ray diffraction (XRD), scanning electron microscopy (SEM) and N2 adsorption/desorption
measurements. The MOFs (MIL-88B) were investigated as catalysts for transfer
hydrogenation (TH) of carbonyl compounds (ketones and aldehydes) and biomass derivatives
(levulinic acid and furfural) with 2-propanol as a hydrogen donor. Their conversion and
selectivity were excellent. The recovered catalyst was reused for ten times, with no significant
loss in catalytic activity. Recovered catalyst after the tenth run retained its original crystallinity
and morphology, which were confirmed by SEM and powder XRD studies. Catalyst reusability
for levulinic acid (LA) conversion was also studied. The activation energy for the
transformation of LA to γ-valerolactone (GVL) was 25.44 KJmol-1. The catalytic performance
of MIL-88B(Fe) was compared with that of MIL-53(Fe).
Keywords
Metal-organic frameworks; MIL-88B; Transfer hydrogenation; Acetophenone; Levulinic acid;
γ-valerolactone
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1. Introduction
Reduction of C=O and N=O is one among the fundamental transformations in the
nature to form alcohols and amines.1, 2 Enzymes which catalyze such reductions are termed
as oxidoreductases. Oxidoreductases such as horse liver alcohol dehydrogenase catalyze
transfer hydrogenation of carbonyl compounds to alcohols,3 which contain zinc4 as the metal
centre to transfer hydride from an alcohol.5, 6 The reduction of carbonyl and nitrosyl groups
to their respective alcohols and amines is one of the most scientifically popular processes,
which is predominantly carried out in chemical industries. The catalytic hydrogenation of C=O
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group is usually performed using a precious metal (Pd, Au, Pt, Ag, Rh, Ru or Os) and/or a
non-precious metal (Cu or Ni) catalyst(s) along with H2 gas.9 The use of molecular hydrogen
has drawbacks such as the requirement of high pressure and temperature, safety issues
during transportation and storage, and it also increases the demand for hydrogen as a
feedstock for chemical processes.10 Therefore, researchers have made attempts to find
alternative sources for H2 gas for hydrogenation reactions. One of the alternatives is to use
hydrogen from an alcohol, a process referred to as transfer hydrogenation (TH). This concept
is appealing for the following reasons: (i) Catalytic transfer hydrogenation (CTH) does not
require hazardous pressurized H2 gas, (ii) the hydrogen donors are readily available,
inexpensive and easy to handle, (iii) the primary side product can be recycled and (iv) the
catalysts that are involved are usually readily accessible and not sensitive.11, 12
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CTH has recently been employed for the hydrogenation of bioderived compounds
such as furfural using metal oxides, including zeolite beta, Ru/RuO2/C, MgO, Al2O3, ZrO2, ZnO
and SiO2 as catalysts. Either, the performance of these catalysts was relatively lower or the
reactions employed high temperature (~150-600 C) and pressure (~2-10 bar). In this
scenario, the choice of earth abundant metals such as Fe and Cr will help to reduce the cost
of the final product. Iron is known to form stable iron oxides13 as well as iron hydrides.14, 15
There are considerably a number of reports on iron based CTH of ketones but most of the
good catalysts are either sulphur, oxygen or phosphine-based ligands-embedded iron
nanoparticles or their corresponding metal complexes.16 Chromium, on the other hand
remains concealed, wherein its catalytic potential is still to be discovered.17-19 However, giving
importance to its abundancy and low cost, recent trend has been shifted towards Cr-based
catalysts for hydrogenation reactions.20-22
Crystalline porous materials are extremely important for developing catalytic systems
with high scientific and industrial impact.23 Metal-organic frameworks (MOFs) show unique
features that are yet to be fully exploited. MOFs are generally formed by metal and an organic
di or tri topic linker, the coordination is extended in all three-directions, creating a uniform
porous structure. MOFs have the advantage of tunability in the organic linker to impart
specific functions such as chirality and electronic property.24-26 MIL-88B is one such MOF
which has the property of breathing in presence of different solvents i.e., it can increase its
pore volume up to 270 times the normal in solvents such as pyridine.27 Thus, MIL-88B has the
ability to accommodate a bigger molecule too. The mechanism of swelling was investigated
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and explained by Ferey et al.,28 in which they inferred that “polar solvents interact with the
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metal trimers through hydrogen bonding” which gave us the clue that MIL-88B can transport
hydrogen from one molecule to another.29 MOFs were widely experimented for catalytic
oxidation reactions.30 There are only limited reports on usage of MOFs for CTH reactions. CTH
of ethyl levulinate to γ-valerolactone (GVL) using zirconium-based MOFs was reported by
Valekar et al. whereas Rojas-Buzo et al. described the use Hf-based MOFs for the CTH of
biomass derived carbonyls.31, 32 Reduction of the carbonyl attached to a heterocycle remains
challenging, because heterocyclic molecule may either give poor selectivity or poison the
catalyst. We have extended the simple CTH to the conversion of biomass derivatives such as
levulinic acid (LA) and furfural to value-based products GVL and furfuryl alcohol respectively
with an absolute selectivity. Further, to understand the importance of MIL-88B in catalysis,
its catalytic efficiency was compared with that of MIL-53(Fe) which also has swelling property
but possesses a different morphology.
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2. Experimental section
2.1 Materials
1,4-Benzenedicarboxylic acid (H2BDC), 2-amino-1,4-benzenedicarboxylic acid (H2BDC-
NH2), CrCl3·6H2O, Fe(NO3)3·9H2O, hydrofluoric acid (HF), dimethylformamide (DMF), ethanol
and 2-propanol (HPLC grade) were obtained commercially and used without any further
purification. MOFs were synthesized according to the published procedures33, 34 and activated
under vacuum at 150 C for 12 h before the characterization and catalysis (Schemes S1-S3).
2.2 Physical methods
Fourier transform infrared (FT-IR) spectra of the samples were recorded on a Nicolet
iS5 FT-IR spectrometer using iD3 ATR mode in the range of 4000-550 cm-1. Powder X-ray
diffraction (XRD) patterns of the samples were recorded on a PAN analytical B.V.-Empyrean
diffractometer using CuKα radiation (40 kV, 40 mA). Scans were performed over the 2θ range
from 5 to 80 at the rate of 0.3/min. The specific surface areas were determined from N2
adsorption/desorption isotherms at liquid N2 temperature by using a Belsorp Max system
with the Brunauer-Emmett-Teller (BET) method.
2.3 Catalysis
All the MOFs were explored as catalysts for the TH of ketones and aldehydes, and
conversion of biomass derivative LA to GVL. MIL-88B/NH2-MIL-88B (3 mg), 2-propanol (3 mL),
substrate (1 mmol) and NaOH (1 mmol) were placed in a 10 mL round bottom flask fitted with
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a reflux condenser, and heated at the desired temperature for a specific tiDmOeI: 1.0.H10a39v/iDn0gNJ00552E
acetophenone as a model substrate and MIL-88B(Fe) as a test catalyst, the CTH reaction was
investigated under various temperatures, time, amounts of the base, catalyst loadings and
hydrogen donors to arrive at an optimized reaction condition. The catalyst was separated by
centrifugation for 20 min at 1800 rpm. The supernatant was decanted, extracted with ethyl
acetate and concentrated to 3 mL.35 300 µL of the concentrate was diluted with 1.5 mL of 2-
propanol and subjected to GC/GC-MS analysis. GC and GC-MS analyses were done with
Shimadzu GC-2010 and Shimadzu GC-MS QP 2010 Ultra instruments respectively, equipped
with ZB-5M plus Phenomenex column of 30 m length and 0.25 mm inner diameter, and the
relative conversion and selectivity were calculated from the area under the peak. Mass was
matched with NIIST and Wiley libraries. All the GC-MS results have similar retention index
irrespective of time. Mass was analyzed by scan mode (m/z from 45 to 750).
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2.4 Mercury drop test
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Catalyst MIL-88B(Fe) (3 mg, 0.0039 mmol), 2-propanol (3 mL, 39.43 mmol),
acetophenone (117 µL, 1 mmol), NaOH (40 mg, 1 mmol) and two drops of Hg were placed in
a 10 mL round bottom flask fitted with a reflux condenser at 90 C for 12 h. The product was
extracted and subjected to GC-MS analysis.
2.5 Hot filtration test and atomic absorption spectrometry analysis
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Hot filtration test was performed with MIL-88B(Cr). In a typical reaction, MIL-88B(Cr)
(3 mg, 0.0042 mmol), 2-propanol (3 mL, 39.43 mmol), acetophenone (117 µL, 1 mmol) and
NaOH (40 mg, 1 mmol) were taken in a round bottom flask, and the reaction was proceeded
for 5 h at 90 C. Then the reaction mixture was centrifuged, and the catalyst was separated
off by a simple filtration process.36 Filtrate was divided into two parts. One part was refluxed
under the identical conditions as the former for another 15 h, and the sample was extracted
with ethyl acetate and analyzed by GC. The other part was used for GC and atomic absorption
spectrometry (AAS) analyses. GC analysis gave conversion at 5 h. AAS analysis gave the
amount of leached metal ion. The standard solutions (1, 2, 5, 10 and 15 ppm) were prepared
by dissolving CrCl3·6H2O in 2-propanol.
2.6 Reuse of the catalysts
All the four catalysts were reused for 10 cycles. The recycling of the catalysts was
performed by following a specific process. Firstly, after the reaction, the catalyst was
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separated by centrifugation. The supernatant was processed and subjected to GC-MS
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analysis. The catalyst was washed twice with water and acetone, dried and reused.
3. Results and discussion
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3.1 Synthesis and characterization of the catalysts based on MIL-88
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We have synthesized four MIL-88B materials, namely MIL-88B(Cr), NH2-MIL-88B(Cr),
MIL-88B(Fe) and NH2-MIL-88B(Fe). All the four catalysts were isostructural but differed in the
transition metal (Cr3+ or Fe3+) and anions present in the cavity, F is present in MIL-88B(Cr)
and NH2-MIL-88B(Cr) whereas NO3 is present in MIL-88B(Fe) and NH2-MIL-88B(Fe).37 F and
NO3 in the pores acted as counter ions for the charge balance and also gave structural rigidity
to the material (Table S1). Powder XRD patterns of the prepared catalysts matched well with
the simulated patterns. All the catalysts showed signature peaks at 2θ values 9.53±0.5 and
10.50±0.5, which corresponded to 100 and 101 planes respectively (Figs. 1a and S1).38
Introduction of electron-donating group (NH2) in 1,4-benzenedicarboxylic acid (BDC)
significantly altered the physio-chemical properties by providing additional basicity to the
framework, resulting enhanced CTH activity (According to Noyori’s outer sphere mechanism,
presence of NH2 in the catalyst favours CTH).33 N2 adsorption measurement was carried, and
specific surface area was found to be 40 m2/g (Figs. 1b and S2). SEM and TEM analyses of the
catalysts showed uniform bipyramidal hexagonal prism-like morphology (Figs. 2, S3-S8).39, 40
A sharp peak around 1590 cm-1 was observed in their FT-IR spectra, which corresponded to
C=O stretching.41 The disappearance of OH stretching frequency confirmed the bonding of
COOH with Cr3+ or Fe3+. Moreover, the weak and sharp peak at 757 cm-1 indicated the
presence of metal-oxygen (Cr3+O/Fe3+O) bonds. In the spectra of catalysts NH2-MIL-88B(Cr)
and NH2-MIL-88B(Fe), NH2 stretching and NH bending frequencies were observed at 3470-
3327 and 1253 cm-1 respectively (Fig. S9).42, 43 With all these spectral and analytical studies,
the structure and morphology of all the four catalysts were confirmed. We also synthesized
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MIL-53(Fe),44
and characterized by powder XRD and SEM (Figs. S10 and S11). All the
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catalysts were activated at 150 C under vacuo for 6 h.
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Figure 1 Powder XRD patterns [a] and N2 adsorption and desorption isotherm [b] of catalyst
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NH2-MIL-88B(Fe)
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Figure 2 TEM images of catalyst NH2-MIL-88B(Fe)
3.2 Catalytic properties of MIL-88B
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3.2.1 CTH of acetophenone to 1-phenylethanol
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Various parameters such as time, temperature, quantity of catalyst, base and solvent
for CTH of acetophenone (substrate) to 1-phenylethanol (product) were optimized using 2-
propanol (hydrogen donor) and NaOH (base).
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3.2.2 Effect of reaction temperature and time
The temperature dependence of this reaction was studied in the range of 25-100 oC.
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The yield of 1-phenylethanol at room temperature (25 C) was meagre (8 %), reflecting the
need for thermal energy for the substrates to cross the activation barrier to form 1-
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increased proportionally to 66 %. Further increase in temperature to 100 C gave a better
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yield of 96 % (Fig. S12). However, further rise in temperature (above 100 C) decreased the
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selectivity of 1-phenylethanol due to the formation of the corresponding acetate.
We have also studied the influence of reaction time on the CTH of acetophenone to
1-phenylethanol at 100 oC. Aliquots were taken at a specific time interval and subjected to GC
analysis. At a shorter reaction time of 1 h, the yield was as low as 5 %, whereas the increase
in reaction time increased the yield of 1-phenylethanol, and the maximum yield of 96 % was
obtained after 10 h of the reaction (Fig. S13).
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3.2.3 Effect of bases
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Further, the effect of various bases on CTH of acetophenone was evaluated. Weak
bases such as K2CO3 and Et3N gave very less conversion, whereas strong bases such as NaOH,
KOH and KOtBu accelerated the process with the conversion up to 96, 92 and 83 %
respectively (Fig. S14). From the above results, it was evident that the strong bases because
of higher basicity favoured deprotonation of alcohol (hydrogen donor), which in turn led to
easy formation of alkoxide.
3.2.4 Effect of hydrogen sources
In the same way, various hydrogen donors were screened. HCOOH/Et3N/H2O failed to
donate hydrogen in the presence of all the catalysts (1 % conversion). When ethanol was
used, better conversion was obtained (18 %). However, the well-known hydrogen donor – 2-
propanol gave an excellent conversion (96 %). 2-propanol was used both as solvent and
hydrogen donor to eliminate the interference of solvent in the system. It is also evident from
the previously reported computational study that secondary alcohols interact more efficiently
with the MIL-88B class of MOFs.46
3.2.5 Effect of functional group in the benzene ring of acetophenone
All the catalysts exhibited comparable activity towards the conversion of
acetophenone up to 98 % (Table 1, entries 1-4). Effect of substitution was examined with
electron-withdrawing and -donating groups (Scheme 1). Electron withdrawing group (phenyl
or halogen) substituted acetophenones were converted into their respective alcohols quickly
(8-9 h) when compared to acetophenone (10 h). Among them, benzophenone (phenyl
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Table 1 TH of different substrates catalyzed by the MIL-88B catalysts
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Entry
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Substrate
Product
Catalyst
MIL-88B(Cr)
NH2-MIL-88B(Cr)
MIL-88B(Fe)
NH2-MIL-88B(Fe)
MIL-88B(Cr)
NH2-MIL-88B(Cr)
MIL-88B(Fe)
NH2-MIL-88B(Fe)
MIL-88B(Cr)
NH2-MIL-88B(Cr)
MIL-88B(Fe)
NH2-MIL-88B(Fe)
MIL-88B(Cr)
NH2-MIL-88B(Cr)
MIL-88B(Fe)
NH2-MIL-88B(Fe)
MIL-88B(Cr)
NH2-MIL-88B(Cr)
MIL-88B(Fe)
NH2-MIL-88B(Fe)
MIL-88B(Cr)
NH2-MIL-88B(Cr)
MIL-88B(Fe)
NH2-MIL-88B(Fe)
MIL-88B(Cr)
NH2-MIL-88B(Cr)
MIL-88B(Fe)
NH2-MIL-88B(Fe)
MIL-88B(Cr)
NH2-MIL-88B(Cr)
MIL-88B(Fe)
NH2-MIL-88B(Fe)
MIL-88B(Cr)
NH2-MIL-88B(Cr)
MIL-88B(Fe)
NH2-MIL-88B(Fe)
MIL-88B(Cr)
NH2-MIL-88B(Cr)
MIL-88B(Fe)
NH2-MIL-88B(Fe)
MIL-88B(Cr)
NH2-MIL-88B(Cr)
MIL-88B(Fe)
NH2-MIL-88B(Fe)
S/C ratio*
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% Yielda
94
TOF (h-1)
11.2
12.6
12.6
13.5
14.1
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15.0
12.2
12.4
13.9
15.2
12.9
13.9
13.5
14.4
4.6
OH
O
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98
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57
58
59
60
99
94#
OH
O
91#
92#
88#
92ǂ
9
OH
O
87ǂ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
97ǂ
Cl
Cl
100ǂ
97ǂ
OH
OH
OH
O
98ǂ
94ǂ
Br
Br
95ǂ
58
73
69
O
6.2
5.9
7.4
7.6
7.9
5.6
5.6
MeO
MeO
81
95§
O
93§
62§
Me
Me
62§
92ǂ
12.3
13.2
13.7
13.9
11.7
12.4
12.5
13.1
11.9
12.7
12.9
13.6
11.9
12.7
12.9
13.6
11.9
11.8
12.9
13.6
O
OH
93ǂ
H
H
96ǂ
92ǂ
98
97
97
96
O
OH
100 (99)b
100 (99)b
100 (99)b
100 (99)b
100 (99)b
100 (99)b
100 (99)b
100 (99)b
100
O
OH
H
O
O
H
O
O
O
OH
O
O
O
90
81
86
NO2
NH2
44
2
3
Reaction conditions: Substrate (1.0 mmol), catalyst (3 mg, ~0.004 mol %), NaOH (40 mg, 1.0 mmol), 2-propanol (3 mL), 100
oC and 10 h, # 8 h, ǂ 9 h, § 15 h, a GC-MS yield, b Isolated yield, * Calculation is given in ESI - Eq. S3.
4
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into diphenylmethanol in 8 h (Table 1, entries 5-8), whereas 4-chloro or 4-bromo
4
5
6
7
8
2
acetophenone (halogen substitution) was converted into 4-chloro or 4-bromo-1-
phenylethanol within 9 h (Table 1, entries 9-16). Only when the reaction time was extended
to 15 h, substrates with electron-donating substituent (methyl or methoxy) gave good
conversions (90 or 80 % respectively) (Table 1, entries 17-24). The scope was further
extended to benzaldehyde and substituted benzaldehyde molecules (Table 1, entries 25-28).
In general, CTH of aldehydes (benzaldehyde) was faster when compared to ketones
(acetophenone). It was evident from these observations that electron-withdrawing
substituents enhance the reaction rate by making the target carbonyl group electron
deficient. Further, to check the versatility, 2-adamantanone (an alicyclic bicyclic compound)
was used as a substrate. 2-adamantanone was converted into 2-adamantanol (Table 1,
entries 29-32). This showed the compatibility of the catalysts with bicyclic carbonyl
compounds. GC-MS chromatogram are given in Figs. S15-S25.
3
4
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5
6
7
8
9
10
11
12
13
O
OH
OH
O
H
R
R
MIL-88B, NaOH, 100 oC, 8-12 h
R'
R'
R = H or CH3; R' = CH3, OCH3, Br, Cl or NO2
14
15
Scheme 1 CTH of carbonyl compounds
3.2.6 CTH of furfural and levulinic acid
16
17
18
19
20
21
22
23
24
25
Further, CTH was extended for the conversion of value-added biomass derivatives
such as furfural and LA. Complete conversion (100 %) of furfural to furfuryl alcohol (Scheme
2) was achieved with an excellent selectivity (99 %) (Table 1, entries 33-36). The
hydrodeoxygenation of LA to GVL is of great importance in the valorisation of biomass. Also,
GVL is a platform chemical for fuel additives and drugs. To our delight, with MIL-88B based
catalysts, LA was converted into GVL (Scheme 3) with an excellent selectivity (99 %) (Table 1,
entries 37-40), and all the catalysts exhibited good substrate to catalyst ratio (S/C). The best
S/C values were 240, 255, 258 and 273 for MIL-88B(Cr), NH2-MIL-88B(Cr), MIL-88B(Fe) and
NH2-MIL-88B(Fe) respectively.
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O
OH
H
OH
O
O
O
MIL-88B, NaOH, 100 oC, 10 h
9
Furfuryl alcohol
Furfural
10
11
12
13
14
15
16
17
18
19
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21
22
23
24
25
26
27
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29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
2
Scheme 2 CTH of furfural to furfuryl alcohol
3
O
OH
O
O
H
OH
O
O
MIL-88B, NaOH, 100 oC, 10 h
Levulinic acid
Valerolactone
 -
4
5
Scheme 3 CTH of LA to GVL
6
3.2.7 Chemoselective reduction of nitro over carbonyl via CTH
7
8
3-Nitroacetophenone was chosen to study the chemoselectivity. It was observed that
nitro functional group was preferentially reduced over carbonyl functional group (Table 1,
entries 41-44) to form 3-aminoacetophenone selectively. Except NH2-MIL-88B(Cr), all the
other catalysts gave excellent selectivity (97-100 %) towards nitro group reduction (Scheme
4).
9
10
11
O
OH
O
OH
OH
O
H
R
R
R
R
MIL-88B, NaOH, 100 oC, 10 h
H2N
H2N
O2N
O2N
4-amino-1-phenylethanol
(P2)
4-nitroacetophenone
4-aminoacetophenone
(P1)
4-nitro-1-phenylethanol
(P3)
12
13
Selectivity (%)
P2
Catalyst
P1
P3
MIL-88B(Cr)
NH2-MIL-88B(Cr)
MIL-88B(Fe)
100
97
100
100
-
3
-
-
-
-
-
NH2-MIL-88B(Fe)
-
14
15
16
17
Scheme 4 Chemoselective CTH of 4-nitroacetophenone to 4-aminoacetophenone
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3.2.8 Gram scale synthesis of GVL and furfuryl alcohol
4
5
6
7
8
2
Further, to explore the catalyst activity at higher volume, we have performed
reactions with 20 mmol of LA (2.3222 g) or furfural (1.9216 g). Quantity of MIL-88B(Fe) and
other reagents were increased accordingly. After the reaction, the catalyst was removed by
centrifugation, and crude was passed through a short silica column and extracted with excess
of ethyl acetate. After the removal of ethyl acetate by self-evaporation, we have obtained
1.98 g of GVL or 1.96 g of furfuryl alcohol. Activity and selectivity of the catalyst remained the
same, even on the gram scale. GVL and furfuryl alcohol were isolated and confirmed by NMR
analyses (Figs. S26-S29).
3
4
5
9
10
11
12
13
14
15
16
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19
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22
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31
32
33
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37
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40
41
42
43
44
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46
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48
49
50
51
52
53
54
55
56
57
58
59
60
6
7
8
9
10
3.2.9 Mechanism of CTH in the presence of MIL-88B
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12
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14
15
16
17
For illustration, we have considered catalyst MIL-88B(Fe) (Fig. 3). Initially, 2-propanol
is deprotonated by the base to form isopropoxide which interacts via CO with the Fe centre
of the catalyst (Step 1). Addition of substrate leads to the interaction of carbonyl carbon with
the FeH formed through the elimination of acetone (Step 2). In step 3, hydride is transferred
to carbonyl carbon.47, 48 This intermediate might be stabilized by the π-π interaction between
aromatic ring in the substrate and terephthalic acid in the framework.35, 49, 50 Simultaneously
proton already removed from 2-propanol attacks the oxygen, and alcohol is given out.
OH
+
OH
H
O
H
Fe
Fe
Fe
Fe
MIL-88B
O
Fe
O
Fe
H
O
O
Fe
O
Fe
Step 1
O
O
O
O
O
O
O
O
O
R
OH
R
H
Step 3
O
O
Fe
Fe
R
O
Fe
H
O
Fe
O
O
O
O
18
19
Figure 3 Plausible reaction mechanism for the CTH of carbonyl compound
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3.2.10 Effect of metal centre and NH2-substitution
In accordance with the TOF values obtained with various substrates (Fig. 4), Fe3+ based
MOFs showed higher catalytic efficiency than their Cr3+ counterparts, presumably due to the
higher Bronsted acidity of Fe3+. Also, the NH2 incorporated MOFs displayed better activity,
because of the additional basicity provided by NH2 group, which might stabilize the generated
isopropoxide.
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41
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53
54
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56
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58
59
60
7
TON and TOF calculations are given in ESI (Equations S1 and S2)
8
9
10
11
Figure 4 Influence of the metal centre and NH2-substitution on the TOF values
3.2.11 Efficiency of the MIL-88B catalysts
12
13
14
15
16
Conversion of LA to GVL was tested under the optimized conditions with MIL-53(Fe),
and the results are given in Table 2. From the results, it was evident that MIL-53(Fe) exhibited
very less catalytic activity (TOF, 1.8 h-1) as compared to MIL-88B(Fe) (TOF, 12.9 h-1). The
51
superior activity of MIL-88B(Fe) was attributed to its higher magnitude of swelling and
surface area (BET surface area: MIL-88B(Fe), 40 m2/g; MIL-53(Fe), 12 m2/g).52
17
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Table 2 Conversion of LA to GVL catalyzed by the MOFs
Entry
Substrate
Product
Catalyst
MIL-53(Fe)
MIL-88B(Fe)
% Yielda
52
TOF (h-1)
1.8
O
1
2
O
O
OH
100
12.9
O
10
11
3
12
4
Reaction conditions: LA (1.0 mmol), catalyst (3 mg), NaOH (40 mg, 1.0 mmol), 2-propanol (3 mL), 100 °C and 10 h, a GC-MS
yield.
13
5
6
7
14
15
16
Catalytic performance of the MOFs was compared with other catalysts in the
literature. CTH is generally performed in two ways: one with molecular H2 and other with
hydrogen donors (2-propanol, HCOOH, etc.). Thus, MIL-88B was compared with both kinds of
catalysis. From Tables S2 and S3, it was seen that MIL-88B showed better selectivity. Noble
metal catalysts gave good selectivity at low temperature or at shorter reaction time. Still,
most of the non-noble metal catalysts (Ni, Fe or Cu) require either high temperature or
extended reaction time. From the viewpoint of the reaction conditions, cost of the raw
materials and the convenience of preparation, MIL-88B based catalysts have advantages
compared to the noble and other transition metal catalysts.
17
18
8
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
3.2.12 Stability of the MIL-88B catalysts
All the four catalysts were recycled 10 times with various substrates. The catalysts
were reused after solvent washing and activation. In the first five cycles, acetophenone was
tested. The reused catalysts showed similar activity as that of the fresh catalyst, and the
conversion of 95 % was obtained even after the fifth cycle. In the sixth and seventh cycles,
CTH of benzaldehyde and benzophenone was tested respectively. There was no decrease in
the catalytic activity, i.e., conversion and selectivity remained unaltered. The eighth cycle was
performed with 4-methoxy acetophenone which gave 70 % conversion. Ninth and tenth
recycling of the catalysts were tested for the conversion of furfural and 2-
bromobenzaldehyde respectively; conversion was 98 % for both the substrates (Fig. 5). From
the recycling experiments, we confirmed that the catalysts were not poisoned (i.e., the active
sites remain unaffected even after extensive use) by the reactants and base, which was also
evident from SEM (Figs. S30-S33), XRD (Figs. 6 and S34) and FT-IR (Fig. S35) analyses of the
reused catalysts. Moreover, the use of different substrates in different cycles showed the
versatile character of the MOFs. Most importantly, the activity did not drop much. This
showed a very long catalytic life of the MIL-88B catalysts. Average particle size of the reused
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catalyst was smaller than that of the fresh catalyst (Figs. S7 and S36, Table S4). Large particles
of MIL-88B was divided into smaller particles during the catalysis, which exposed fresh
surfaces. The newly exposed surfaces might be responsible for the constant activity of the
reused catalysts.
10
11
12
13
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31
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44
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46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5
6
o
Reaction conditions: Substrate, MIL-88B catalyst, NaOH, 2-propanol, 100 C (Mole ratio of substrate:catalyst:base:solvent
was maintained as 1:0.004:1:150).
7
8
9
Figure 5 Reusability of the MIL-88B catalysts
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Figure 6 Powder XRD patterns of 10 times reused and fresh NH2-MIL-88B(Fe)
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3.2.13 Heterogeneity tests
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7
8
2
Hot filtration test results showed that the conversion of acetophenone remained
almost the same after separating MIL-88B(Cr) from the reaction mixture. Thus, it was evident
that the MOF functioned as a heterogeneous material (Fig. 7). Part of the reaction mixture
after hot filtration test was analyzed by AAS, and Cr leaching was found to be 3.52 ppm. This
trace leaching of the metal was one of the causes for the meagre increase in the conversion
in the absence of the catalyst. Further, the conversion remained unaffected (96%) in the
presence of mercury drops [Hg(0)], which proved that there were no free metal particles
formation in the reaction mixture.
3
4
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48
49
50
51
52
53
54
55
56
57
58
59
60
10
11
12
Reaction conditions: Acetophenone (117 µL,1.0 mmol), MIL-88B(Cr) (3 mg, 0.004 mol %), NaOH (40 mg, 1.0 mmol),
2-propanol (3 mL) and 100 oC.
13
Figure 7 Conversion of acetophenone in the presence and absence of MIL-88B(Cr)
14
15
3.2.14 Catalyst stability in biomass conversion
16
17
18
19
20
21
Reusability was also performed in the conversion of LA to GVL, to evaluate the
reusability in the biomass conversion. As expected, catalyst MIL-88B(Fe) did not lose its
activity even with LA. Fresh, 1st, 2nd, 3rd, 4th and 5th cycles gave 100, 100, 98, 99, 100 and 99 %
conversions respectively (Fig. 8). Though the conversions varied by
, the selectivity
± 1 %
remained constant in all the cycles. From these results, it was evident that MIL-88B based
catalysts are robust for the conversion of biomass derivatives such as LA.
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60
1
2
3
4
5
6
7
8
Reaction conditions: LA, MIL-88B(Fe), NaOH, 2-propanol, 100 oC and 10 h (Mole ratio of substrate:catalyst:base:solvent was
maintained as 1:0.004:1:150).
Figure 8 Reusability of catalyst MIL-88B(Fe) for biomass conversion
3.2.15 Kinetics for the conversion of LA to GVL
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20
21
To understand CTH of LA over MIL-88B, kinetic studies were performed at three
different reaction temperatures (60, 80 and 100 °C). In the present system, 2-propanol was
used in excess; so that, the reaction kinetics was not affected by the concentration of 2-
propanol. CTH of LA was assumed to proceed as a pseudo first order reaction, as a function
of LA concentration. Fig. 9 depicts the plot of ln(Ct/C0) versus time for the CTH of LA over MIL-
88B(Fe), in which C0 is the initial concentration of LA, and Ct represents the concentration of
LA at a particular time. As shown in Fig. 9, the reaction followed pseudo first order kinetics at
all the studied temperatures, and the rate constant values were determined to be 0.1126,
0.1697 and 0.3030 min−1 at 60, 80 and 100 °C respectively. The variation of the rate constant
(k) with temperature is shown as an inset in Fig. 9. Based on the Arrhenius plot, the activation
energy (Ea) was calculated to be 25.44 kJmol−1. The reported activation energy for LA to GVL
conversion was 37.4 kJmol-1,53 which was about 1.4 times higher than the present report.
Hence, we conclude that CTH of LA to GVL over MIL-88B based catalyst showed higher activity.
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1
21
22
23
24
2
Figure 9 First order kinetic fit for CTH of LA to GVL at different temperatures
3
Conclusions
25
26
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31
32
33
34
35
36
37
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40
41
42
43
44
45
46
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48
49
50
51
52
53
54
55
56
57
58
59
60
4
5
Iron and chromium MIL-88B and NH2-MIL-88B (MOFs) were prepared and
characterized. All the four MOFs exhibited MIL-88B morphology (bipyramidal hexagonal
prism). They have been successfully applied as catalysts towards catalytic transfer
hydrogenation (CTH) of various carbonyl compounds, including biomass derivatives (furfural
and levulinic acid). Excellent conversion with low catalyst loading was observed. The MIL-88B
catalysts offer many advantages which include high porosity, high activity, recyclability
(compatible with various substrates and biomass) and mild conditions in an environmentally
benign solvent system with a low activation energy of 25.44 KJmol-1 and high S/C of 280.
Excellent chemoselectivite reduction of NO2 was also achieved. The MIL-88B catalysts
performed an efficient CTH under mild conditions compared to many of the reported catalytic
systems. After a comparative study with MIL-53(Fe), we believe that the interaction of
secondary alcohols with metal trimers of MIL-88B catalysts is very similar to the metal hydride
bond. Thus, the metal acquired the capacity to transfer hydrogen from secondary alcohol to
carbonyl compound. Therefore, along with the swelling property, morphology, crystal
structure and surface area are also responsible for the enhanced catalytic activity of MIL-88B.
Exploiting such solvent-induced swelling property of the MOFs may open new opportunities
in the development of highly active heterogeneous transfer hydrogenation catalysts.
6
7
8
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DOI: 10.1039/D0NJ00552E
Graphical abstract
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Fe and Cr bassed MOFs (MIL-88B) act as an efficient and reusable catalyst for transfer
hydrogenation of carbonyl compounds including bio-derived substrates.
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O
OH
O
O
O
O
CH3/H
R
O
O
O
OH
OH
CH3/H
R
MIL-88B (Fe/Cr)