Catalytic Transfer Hydrogenation of Biomass-Derived Carbonyls over Hafnium-Based Metal–Organic Frameworks

Article abstract of DOI:10.1002/cssc.201701708

A series of highly crystalline, porous, hafnium-based metal–organic frameworks (Hf-MOFs) have been shown to catalyze the transfer hydrogenation reaction of levulinic ester to produce γ-valerolactone by using isopropanol as a hydrogen donor. The results are compared with their zirconium-based counterparts. The role of the metal center in Hf-MOFs has been identified and reaction parameters optimized. NMR studies using isotopically labeled isopropanol provide evidence that the transfer hydrogenation occurs through a direct intermolecular hydrogen transfer route. The catalyst, Hf-MOF-808, can be recycled several times with only a minor decrease in catalytic activity. The generality of the procedure has been demonstrated by accomplishing the transformation with aldehydes, ketones, and α,β-unsaturated carbonyl compounds. The combination of Hf-MOF-808 with the Br?nsted-acidic Al-Beta zeolite gives the four-step one-pot transformation of furfural to γ-valerolactone in good yield of 75 %.

Full text of DOI:10.1002/cssc.201701708

Accepted Article
Title: Catalytic Transfer Hydrogenation of Biomass-Derived Carbonyls
over Hafnium-Based Metal-Organic Frameworks
Authors: Avelino Corma, Pilar García-García, and Sergio Rojas-Buzo
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To be cited as: ChemSusChem 10.1002/cssc.201701708
Link to VoR: http://dx.doi.org/10.1002/cssc.201701708
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ChemSusChem
10.1002/cssc.201701708
FULL PAPER
Catalytic Transfer Hydrogenation of Biomass-Derived Carbonyls
over Hafnium-Based Metal-Organic Frameworks
[a]
[a]
[a]
Sergio Rojas-Buzo, Pilar García-García,* Avelino Corma*
Dedication ((optional))
Abstract: A series of highly crystalline, porous, hafnium-based
metal-organic frameworks (MOFs) have shown to catalyze the
transfer hydrogenation reaction of levulinic ester to produce γ-
valerolactone using isopropanol as hydrogen donor and the results
are compared with the zirconium-based counterparts. The role of
the metal center in Hf-MOFs has been identified and reaction
parameters optimized. NMR studies with isotopically labeled
isopropanol evidences that the transfer hydrogenation occurs via a
direct intermolecular hydrogen transfer route. The catalyst, Hf-MOF-
In the past two decades, highly porous metal-organic
frameworks (MOFs) with three-dimensional crystalline structures
composed of organic bridging ligands, that are coordinatively
bonded to metal ions or metal ion clusters, have emerged as
interesting materials for applications in heterogeneous
[
4]
catalysis. MOFs can have potential as catalyst supports and as
catalysts themselves because they can display many of the
properties of an ideal heterogeneous catalyst with well-defined
structures, active-site uniformity and high surface area and
[
5]
[6]
808, can be recycled several times with only a minor decrease in
porosity.
Furthermore, mesoporous MOFs,
can facilitate
catalytic activity. Generality of the procedure was shown by
accomplishing the transformation with aldehydes, ketones and α,β-
unsaturated carbonyl compounds. The combination of Hf-MOF-808
with the Brønsted acidic Al-Beta zeolite gives the four-step one-pot
transformation of furfural to γ-valerolactone in good yield of 75%.
transport of molecular reactants and products, making these
materials attractive as heterogeneous catalysts and catalyst
supports for liquid-phase reactions.
Metal-organic frameworks have been explored for a broad range
of catalytic transformations. Examples have been reported such
[
7]
[8]
[9]
as condensation,
ring opening,
cycloaddition,
N-
[
10]
[11]
[12]
[13]
alkylation, isomerization, hydrogenation, oxidation and
carbene reactions. MOF-based materials have been found to
be suitable catalysts for the production of certain fine
Recent studies are focusing on thermally and
chemically stable MOFs toward practical applications and Zr-
[
14]
Introduction
[
15]
chemicals.
The gradual depletion of fossil resources and the growing
demand for energy have conducted researchers to look for
alternative renewable resources. Among them, biomass appears
as a sustainable source of liquid fuels and chemicals, so far
mostly derived from fossil resources. Research in valorization of
waste biomass is therefore attracting a great deal of interest and
enormous efforts are being made to develop suitable catalytic
[16]
[17]
MOFs stand out by their exceptional stability.
Their unique
properties make them privileged materials and interesting
candidates in heterogeneous catalysis, finding use either as
[18]
catalysts or catalyst supports.
In the same family, although
much less explored, are hafnium-based MOFs, that share
properties with zirconium-based MOFs, such as high thermal
and chemical stability. We will show here that such solid
materials outperform the related zirconium-based MOFs in
catalytic reactions such as the hydrogen transfer reaction of
carbonyl compounds using isopropyl alcohol as hydrogen donor.
More specifically, we will show that they can catalyze the
hydrogenation of levulinic ester to γ-valerolactone and when in
combination with a zeolite Beta, it is possible to catalyze the
four-step process in one-pot transformation of furfural to γ-
valerolactone.
[1]
processes
for the production of fuels and value-added
chemicals. Nowadays, the most successful heterogeneous
[
2]
catalysts are oxide materials such as zeolites, aluminas or
[3]
aluminum silicates, and various forms of supported metals
such as metal nanoparticles, metal oxides or organometallic
complexes. In the field of heterogeneous catalysis, there is a
need for well-defined catalysts that are also stable and easily
attainable. In fact, many heterogeneous catalysts lack structural
uniformity and contain multiple kinds of sites, potentially differing
in both reactivity and selectivity. As a consequence, significant
effort must be devoted to their characterization and, in particular,
to the identification of the most catalytically relevant species.
[19]
Hf-MOFs have only been recently developed. Since the initial
[19]
report of UiO-66(Hf), several attempts have been devoted to
the synthesis of novel Hf-MOFs. However, traditional
solvothermal synthesis of Hf-MOFs usually ends up with either
amorphous solids or viscous gels. This fact has probably slowed
down development of practical applications of Hf-MOFs. The
use of “modulators” in MOFs synthetic procedures allow for the
construction of crystalline solid materials along with the
intentional positioning of defects in the framework that, in
[a]
S. Rojas-Buzo, Dra. P. García-García, Prof. Dr. A. Corma.
Instituto de Tecnología Química, UPV-CSIC, Universitat Politècnica
de València-Consejo Superior de Investigaciones Científicas.
Avenida de los Naranjos s/n 46022 Valencia, Spain.
E-mail: pgargar@itq.upv.es; acorma@itq.upv.es.
[
20]
several cases, appear to correlate with their catalytic activity.
[b]
Title(s), Initial(s), Surname(s) of Author(s)
This modulator strategy is particularly successful for building
crystalline Hf-MOFs. A modulator is a non-bridging monotopic
acid (e.g., acetic, formic or benzoic acid) that is added to the
reaction mixture (as many as 250 molar equivalents relative to
Department
Institution
Address 2
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
[
19b]
[19a]
[21]
the ligand). Acetic acid,
water
and formic acid
have
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ChemSusChem
10.1002/cssc.201701708
FULL PAPER
IV
been used as modulator in the synthesis of MOF UiO-66(Hf).
catalyst. Lewis acidic zeolite Beta with Hf sites in the
[
20]
Benzoic acid is reported to allow the synthesis of MOF UiO-
framework catalyzes the Meerwein-Ponndorf-Verley reduction of
and the reduction of
furfural with isopropanol. Hf-Beta zeolite has also shown good
[
20]
[35]
67(Hf). Hf-MOF-808
with the trimesate linker, makes use of
5-(hydroxymethyl)furfural with ethanol
[
36]
formic acid for its preparation.
Regarding to the catalytic applications, Hf-MOFs have been
used as catalyst supports for Rh(I) complexes for the catalytic
[
37]
activity in the reduction of methyl levulinate to γ-valerolactone.
In all of these procedures, zeolitic material Hf-Beta exhibits
higher activity as compared to other metal-substituted zeolite
materials such as Zr-Beta, Sn-Beta, Ti-Beta or mesoporous
siliceous solids such as Sn-MCM-41 and Zr-MCM-41 materials.
Recently, a new hafnium phosphonate organic-inorganic hybrid
catalyst has been used in the conversion of levulinic acid and
[
22]
hydrogenation of ethylene,
cobalt species able to catalyze a
[
23]
broad scope of hydrogenation reactions and organozirconium
single-site component as promising catalyst for ethylene and
stereoregular 1-hexene polymerization. Catalysis at the linker
in Hf-MOFs has also been reported for the selective fructose
dehydration to 5-hydroxymethylfurfural by sulfonic acid
[
24]
esters into γ-valerolactone using isopropanol as the hydrogen
[
25]
[38]
functionalized MOF: NUS-6 (Hf),
the hydrosilylation of
source.
While the latter affords good yield of the desired γ-
terminal olefins by a 2,2’,2’’-terpyridine-Fe complex included as
valerolactone product, the method uses relatively harsh reaction
conditions (150ºC) and a high amount of solid catalyst (52 mol%
Hf loaded is required).
[
26]
a bringing ligand in a Hf-metal organic layered material,
for the conversion of CO and epoxides into cyclic carbonates at
ambient conditions by Cu-porphyrin included in the Hf metal-
and
2
[
27]
organic framework named FJI-H7.
Hf-based MOFs in the transfer hydrogenation reduction of
biomass-based compounds
When considering the use of MOFs as catalysts, it is common to
make use of the metal oxide node component of the framework
as the active site. Bare Hf-MOFs have been employed almost
exclusively for Brønsted acid catalyzed regioselective ring
As part of our ongoing project devoted towards the development
of synthetic procedures for biomass-based molecules and
compounds of interest in fine chemistry under a clean and green
reaction profile, we have explored the possibility of using Hf-
based metal-organic framework in the Meerwein-Poondorf-
Verley reduction for the production of target compounds. We
focused our attention in Hf-based MOFs materials due to their
exceptional properties such as high thermal, chemical and
mechanical stability that would anticipate solid material integrity
during the catalytic procedure, along with the possibility of long
life and chances of reutilization. We were further encouraged
with the observation that previous reports on the Meerwein-
Poondorf-Verley reaction with heterogeneous catalysts show
superior performance of the Hf-based solids compared to the Zr-
relatives as discussed above. With this in mind, we have
[
20, 28]
opening reaction of epoxides
and in the thioanisole
oxidation with hydrogen peroxide. As far as we know, these are
all of the catalytic applications of Hf-MOFs as heterogeneous
catalyst. We here expand applicability of these exceptionally
stable MOF materials to the Meerwein-Pondorf-Verley reduction
of carbonyl compounds using isopropyl alcohol as hydrogen
donor. The reactivity presented here positions Hf-MOFs as good
candidates for heterogeneous catalysis in biomass-based
molecules transformations.
Results and Discussion
prepared several Hf-based MOFs of the UiO-family, such a UiO-
Transfer hydrogenation reaction such as the Meerwein-
Ponndorf-Verley reaction offers an attractive alternative to
molecular H for the reduction of targeted functional groups, by
2
the usage of alcohols as hydrogen donor. This strategy became
even more interesting if a heterogeneous catalyst is utilized
featuring non-precious metal and providing effective
performance resulting in a cost-effective alternative method to
[21]
66 (Hf)
2
and UiO-66-NH (Hf) both using formic acid as
modulator in the synthetic procedure (see Experimental Section
for details). UiO-66-NH (Hf) is reported here for the first time, as
2
far as we know. UiO-67 (Hf) with a biphenyl-4,4-dicarboxylate as
organic linker was prepared following the reported procedure
[20]
using benzoic acid as modulator.
As for the related UiO-66
(
Zr), the synthetic procedure used here is the same as the one
2
the use of molecular H or formic acid. Therefore, a variety of
employed in the synthesis of UiO-66 (Hf) to assure similar
degree of defects in both frameworks and a fair comparison of
metal activity. Therefore, formic acid was also utilized in the
synthetic procedure (see Experimental Section). While UiO-66
catalytic non-precious metal-contained solids have been
reported for this reaction, ranging from metal oxides such as
[
29]
ZrO
2
,
to crystalline porous materials including zeolites such as
[
30]
[30-31]
Zr-Beta, Sn-Beta,
UiO-66-Zr,
and metal organic framework such as
frameworks present ideally a 12-coordination in the metal center
[
32]
[32]
[33]
Zr-MOF-808
and Zr-MOF-808-P.
As for the
[20, 39]
(
Figure 1), MOF-808 materials
have been described
hafnium heterogeneous catalysts, organometallic hafnium
complex is reported to be immobilized on MCM-type materials –
mesoporous siliceous materials with mono- or tridimensional
pore structure- and used for the selective hydrogen transfer
recently and feature a 6-coordination environment appearing as
coordinatively unsaturated centers with promising catalytic
activity. Both Hf and Zr MOF-808 materials have been prepared
[20, 39]
in the present work following the reported procedures.
(See
[
34]
reduction
of
α,β-unsaturated
ketones.
Good
Supporting Information for characterization details of the MOFs
solids prepared within this work).
chemoselectivities are achieved for the allylic alcohol products
and the authors demonstrate higher efficiency for the Hf-MCM
solids as compared with similar Zr-MCM material. However, no
catalyst recycle or reutilization is reported for this Hf-MCM
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ChemSusChem
10.1002/cssc.201701708
FULL PAPER
Cat
O
OH
O
O
(8 mol% in metal)
H
isopropanol
100ºC
1
2
100
80
60
Hf-MOF-808
Zr-MOF-808
UiO-66(Hf)
40
UiO-66-NH (Hf)
2
UiO-66(Zr)
UiO-67(Hf)
HfCl4
20
HfO2
0
0
1
2
3
4
5
6
7
8
Time (h)
Figure 2. Hafnium and zirconium catalysts for the Meerwein-Poondorf-Verley
reduction of furfural with isopropyl alcohol as hydrogen donor. Reaction
conditions: 0.1 mmol furfural, catalyst (8 mol% in metal), dodecane (10 µL) as
internal standard, 2-propanol (0.4 mL) Tª = 100 ºC.
Figure 1. Node connectivity in zirconium and hafnium-based MOFs and
schematic representation of zirconium and hafnium-MOF topology.
In view of the good results attained with Hf-MOF-808 in the
transfer hydrogenation reaction of furfural 1 to furfuryl alcohol 2,
we were interested in expanding the scope of the procedure to
other biomass-based compounds. As shown in Figure 3, 5-
This series of highly crystalline, porous, hafnium- and zirconium-
based MOFs have been tested in the hydrogen transfer
reduction of the carbonyl group of furfural using isopropyl
alcohol at 100ºC. The reaction was followed over time using GC
chromatography and dodecane as internal standard. The
attained results are shown in Figure 2 (see Experimental Section
for details). As expected, the highest activity was observed for
both MOF-808 solids with the 6-connected metal nodes that
additionally feature the catalytically active sites easily accessible.
While high activity is observed for both solids under the reaction
conditions tested, superior performance is achieved with the Hf-
MOF-808, being its reaction rate two times higher than for the
(
hydroxymethyl)-furfural 3 was reduced to the corresponding
alcohol with analogous efficiency, giving the 2,5-
bis(hydroxymethyl)furan product 4 in a good yield of 98% after
only 1.5 hours.
γ-Valerolactone 6 has been recognized as versatile platform
chemical, which can be used as a fuel additive, as a green
solvent for biomass processing, as a food additive and a
precursor for more valuable chemicals such as liquid alkenes
and polymers. Reduction of ethyl levulinate 5 –being a ketone
substrate- probed more challenging compared to the aldehydes,
and higher temperature was required to achieve efficient
transformation. We were delighted to see that Hf-MOF-808
promotes the Meerwein-Poondorf-Verley reduction of ethyl
levulinate to γ-valerolactone and a good yield of 94% is
achieved after 8 hours reacting at 120ºC with 10 mol% of
catalyst loading (Figure 3 and Table 1). The intermediate
hydroxyester, after ketone reduction, was not detected at any
time since intramolecular transesterification to γ-valerolactone is
a fast step under the reaction conditions. The related Zr-MOF-
2
Zr-MOF-808. UiO-66 (Hf) and UiO-66-NH (Hf) also show fair
activity and a good yield of 80% is achieved after 8 hours
reaction time. Similar trend is observed here with the related
UiO-66 (Zr) displaying lower activity for the transformation. In the
case of UiO-67 (Hf) while having higher pore size around 12 and
16 Å and opening window of 8 Å (compared to the UiO-66 solid
with pore sizes of 8 and 11 Å with a 6 Å window) it displays in
this case lower activity. We attribute this reduced activity to the
synthetic procedure here employed that makes use of benzoic
acid as modulator instead of formic acid as in the UiO-66
materials. Therefore, the amount and nature of defects in the
framework should differ. For the comparative purposes in hand,
[32]
8
08 has already been described for this transformation, and it
was also tested here under otherwise similar reaction conditions,
showing three times lower rate in formation of the desired
product 6 (Figure 3).
4 2
we also tested HfCl and HfO within the same loadings, and
they displayed no activity in the transfer hydrogenation reaction
of furfural under the conditions tested (Figure 2).
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ChemSusChem
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FULL PAPER
O
Hf-MOF-808
or
8
OH
O
Hf-MOF-808
mol%)
OH
O
O
O
O
R
(
10 mol%)
R
(8
H
H
isopropanol
100ºC
isopropanol
100ºC
1
2
R
=
H 1
R = H
2
R = CH OH
3
R = CH OH
4
2
2
MOF-808
(
a)
(b)
O
mol%)
O
(10
O
100
100
O
isopropanol
20ºC
80
80
O
1
5
6
MOF-808-Hf
Filtrate
60
60
40
20
0
40
20
100
0
0
1
2
3
4
5
1
2
3
4
5
Time (h)
Runs
80
60
Figure 4. (a) Time-yield plots for the Meerwein-Poondorf-Verley reduction of
furfural with Hf-MOF-808 (black line) and removing the catalyst after 10
minutes (red line) and (b) reusability of the catalyst. Reaction conditions: 0.1
mmol furfural, catalyst (8 mol%), dodecane (10 µL) as internal standard, 2-
propanol (0.4 mL) Tª = 100, 2 hours reaction time.
4
0
MOF-808-Hf (furfural, 1)
20
MOF-808-Hf (5-hydroxymethylfurfural, 3)
MOF-808-Hf (ethyl levulinate, 5)
MOF-808-Zr (ethyl levulinate, 5)
0
0
1
2
3
4
5
6
7
8
Time (h)
the recovered material show a substantial decrease in the BET
surface area that was attributed to deposition of non-soluble
organic components that account for the observed diminished
activity after repeated use.
Figure 3. Hf-MOF-808 for the Meerwein-Poondorf-Verley reduction of furfural
, 5-hydroxymethyl furfural 3 and ethyl levulinate 5 with isopropyl alcohol as
hydrogen donor. Reaction conditions: 0.1 mmol starting material, catalyst (8 or
0 mol% in metal), dodecane (10 µL) as internal standard, 2-propanol (0.4 mL)
1
1
Mechanistic studies using NMR spectroscopy and
isotopically labeled isopropanol
Tª = 100 for 1 and 3, 120ºC for 5.
NMR studies were next conducted to gain insight into the
mechanism of the catalytic transfer hydrogenation reaction. Two
possible alternative pathways could be envisaged as the
transformation can take place via (1) direct hydrogen transfer
from isopropanol to the carbonyl starting material at the metal
active site center, or (2) a sequential metal hydride formation
and subsequent carbonyl reduction (see Supporting Information
for the schematic representation). In order to distinguish these
two pathways, isotopically labeled isopropanol-d8 was used
together with tert-butanol in a molar ratio of 1:3. tert-Butanol is
not able to act as H-donor since it lacks the β-H, but it can
replace deuterium on the metal surface with H, and it can also
exchange –OD group to –OH in the isotopically labeled
Catalyst heterogeneity and Reusability
The heterogeneity of Hf-MOF-808 in the catalytic procedure was
evaluated by the hot filtration test. The catalyst was removed
from the reaction mixture in the reduction of furfural 1 after 10
min reacting at 100ºC, and then the reaction was continued with
the filtrated for 5 hours. The results shown in Figure 4
demonstrate that no further reaction occurs after filtration,
indicating that no active species were leached to the reaction
medium and that Hf-MOF-808 behaves as a true heterogeneous
catalyst. Furthermore, the postreaction solution after full
conversion to product 2 (2 hours reaction time) and solid catalyst
filtration, was analyzed by Inductively coupled plasma (ICP)
spectroscopy, and the Hf-content was determined to be 137
ppm, demonstrating integrity of the material during the catalytic
procedure and little hafnium leaching to the liquid-phase. More
importantly, the Hf-MOF-808 could be reused at least five times.
Yield remained constant in the first and second run, decreasing
by 15% for the rest of the recycles (Figure 4b). In all cases,
selectivity for furfuryl alcohol remained above 98%. The
recovered Hf-MOF-808 after five cycles was characterized by
isopropanol. This experiment was performed with furfural 1, and
1
the product was analyzed by GC-MS and by
H NMR
spectroscopy (see Supporting Information). It was found that the
molecular ion peak of furfuryl alcohol 2 was m/z = 99 and
therefore with an additional 1 amu, meaning incorporation of one
1
deuterium. H NMR of the product further confirmed the result
2
since methylene protons in the “CH OH” group integrate as only
one proton meaning incorporation of one deuterium and
therefore a “CHDOH” structure. Since deuterium will not be
available in the metal-hydride route (pathway 2 above) we could
conclude that a direct hydrogen transfer is occurring with Hf-
MOF-808 under the tested reaction conditions. It can be
FT-IR, XRD, TGA, ICP analysis and N
see supplementary information), and the data indicate that Hf-
MOF-808 was stable under the reaction conditions. However,
2
physisorption isotherm
(
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ChemSusChem
10.1002/cssc.201701708
FULL PAPER
O
OH
O
postulated thereof, that contiguous acid-base sites (Hf-O) could
assist the formation of isopropoxide moieties on the Lewis acid
sites from 2-propanol. At the same time, the carbonyl group is
also activated by the Lewis acid site to form a six-membered
intermediate between alcohol, carbonyl compound and Lewis
acid site wherein β-H transfer occurs as in line with the
Meerwein-Poondorf Verley mechanism.
O
2
-butanol
H
Lewis Acid
1
2
OR
O
Brønsted
Acid
R = sec-butyl
7
Four-step one-pot transformation of furfural to γ-
O
O
O
2
-butanol
OR
valerolactone
Lewis Acid
In view of the good results attained in the catalytic transfer
hydrogenation of furfural 1 to furfuryl alcohol 2 and ethyl
levulinate 5 to γ-valerolactone 6, we were intrigued in the
utilization of this catalyst Hf-MOF-808 in combination with a
Brønsted acidic solid for the direct conversion of furfural 1 to γ-
valerolatone 6 as in the reaction pathway shown in Scheme 1.
Tranfer hydrogenation of furfural affords furfuryl alcohol which in
the presence of a Brønsted acid and the 2-butanol solvent forms
the corresponding ether 7. Brønsted acid also catalyzes the
hydrolysis of 2 and 7 to levulinic acid and ester, respectively.
Levulinic acid could be esterified in the reaction media in the
excess alcohol employed. Subsequent transfer hydrogenation of
the levulinic acid/ester would produce γ-valerolactone 6 after
intramolecular cyclization. Román-Leshkov and coworkers
reported on this integrated catalytic process by means of
6
8
O
OH
OR
O
Scheme 1. One-pot reaction for the production of γ-valerolactone from furfural
by the use of a combination of Lewis acid, Hf-MOF-808 and Brønsted acid Al-
Beta zeolite.
Table 1. Meerwein-Poondorf-Verley reduction of several carbonyl
[
a]
[40]
compounds with Hf-MOF-808.
zeolites with Brønsted and Lewis acidic sites.
Under their
optimized conditions, they achieved a 78% yield of the γ-
valerolactone product after 48 hours reacting at 120ºC. In our
case, the combination of Hf-MOF-808 (10 mol%) with Al-Beta
Entry Starting Material
Product
Tª
Time
(h)
Yield
(%)
o
[b]
(
C)
O
OH
(
Si:Al 12 mole ratio, 5 mol%, see Supporting Information for
O
O
1
100
100
120
2
1.5
8
97
H
Brønsted acid catalysts screening) was used to produce γ-
valerolactone 6 from furfural in 51% yield in 6 hours (with
significant amount of levulinate ester also formed). At this point,
we observed that the transformation didn’t proceed further when
allowed to react for longer time, which was attributed to a certain
deactivation of the catalyst. It was found that sequential addition
of catalysts to the batch reactor (see experimental section)
affords the desired product γ-valerolactone 6 in 75% isolated
yield after 10 hours at full conversion of the intermediate
compounds.
HO
O
HO
OH
O
O
2
3
95 (92)
94
O
O
O
O
O
O
OH
4
5
6
120
120
120
3
16
3
97
91
96
O
OH
Scope of the Transfer Hydrogenation Reaction
O
OH
Since Hf-MOF-808 exhibited remarkable activity toward
intermolecular
hydrogen
transfer
of
furfural,
5-
O
O
OH
OH
hydroxymethylfurfural and ethyl levulinate, the study was
expanded to a variety of carbonyl compounds (Table 1). Other
aldehydes were tested in the procedure. As an example,
benzaldehyde (Table 1, entry 4) can be efficiently converted to
produce benzylic alcohol in 97% yield. The method could also
be applied to the reduction of ketones as demonstrated above
for the ethyl levulinate and in Table 1, entries 5 and 6, since
aromatic and aliphatic ketones can be efficiently converted to
their corresponding alcohols in high yields by means of Hf-MOF-
7
8
120
120
3
5
96
92 (90)
[
a] Reaction conditions: 0.1 mmol carbonyl compound, catalyst (10 mol%),
dodecane (10 µL) as internal standard, 2-propanol (0.4 mL). [b] Determined
by GC analysis using an internal standard; Isolated yield in brackets on a
0.5 mmol scale.
808.
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The substrate scope was further expanded to the
chemoselective hydrogen transfer reaction of α,β-unsaturated
carbonyl compounds to allylic alcohols (Table 1, entries 7 and 8).
Allylic alcohols are important and versatile intermediates in fine
chemistry. The chemoselective reduction of α,β-unsaturated
carbonyl compounds by means of a heterogeneous catalyst is
therefore an attractive synthetic entry to these products
particularly since the starting materials are available from many
sources such as aldol condensation reaction. Hf-MOF-808
competently catalyzes the reduction of cinnamaldehyde to
cinnamyl alcohol (Table 1, entry 7). α,β-Unsaturated ketones
require longer time (Table 1, entry 8) and the corresponding
allylic alcohol is obtained in 93% yield after 5 hours reaction time
by means of Hf-MOF-808 and isopropanol as hydrogen donor. It
should be noted that the procedure is highly chemoselective
yielding exclusively the allylic alcohol product. Reduction of the
double bond was not observed, which is in agreement with the
concerted H-transfer reaction mechanism between the hydrogen
donor and the carbonyl compound as discussed above. The
Meerwein-Ponndorf Verley reduction is intrinsically selective
since only the carbonyl group coordinates with the Lewis acid
center, while the double bond remains unactivated. Saturated
carbonyl products could be formed under metal-hydride
mechanistic pathway which is not the case.
and operated under the same conditions. Organic solutions were
concentrated under reduced pressure on a Büchi rotary evaporator.
Flash column chromatography was performed using E. Merck silica gel
(60, particle size 0.040-0.063 mm). Chemical yields refer to pure isolated
substances unless stated otherwise. All the products obtained here are
1
13
known compounds and were characterised by IR, GC-MS, H- and C-
1
13
NMR. H and C NMR were recorded on a Bruker 300 spectrometer. C,
N, and H contents were determined with a Carlo Erba 1106 elemental
analyzer. Thermogravimetric and differential thermal analysis (TGA-DTA)
were conducted in an air stream with a Metler Toledo TGA/SDTA 851E
analyzer. FTIR spectra were recorded with a Nicolet 710 spectrometer (4
-1
cm resolution) using conventional greaseless cell. IR spectra were
recorded on KBr disks at room temperature or by impregnating the
windows with a dichloromethane solution of the compound and leaving to
evaporate before analysis.
Synthesis of materials
[21]
[20]
[20]
[20]
UiO-66(Hf),
UiO-67(Hf),
UiO-67(Zr),
Hf-MOF-808
were
synthetized according to methods reported in the literature. The
corresponding UiO-66(Zr) and Zr-MOF-808 were obtained by the same
procedures except ZrCl was used instead of HfCl .
4 4
4
Synthesis of Hf-MOF-808: A mixture of HfCl (160 mg, 0.5 mmol), 1,3,5-
benzenetricarboxylic acid (110 mg, 0.5 mmol), dimethylformamide/formic
acid (20 mL/20 mL) were sonicated for 30 min and then added to an
autoclave vessel and heated at 100 ºC for 72 h. The resulting solid was
filtered and washed with an excess of DMF and acetone. The as-
synthesized solid was activated at 120 ºC in vacuo during 12 h. Powder
X-ray diffraction pattern (see Supporting Information) confirmed that the
solid material has the crystalline morphology of MOF-808 type MOF. ICP
analysis shows a 46 % of Hf in the sample. Elemental analysis: 13.9 % C,
Conclusions
We have demonstrated that Hf-based MOFs exhibited
remarkable catalytic activity in the Meerwein-Ponndorf-Verley
reaction toward formation of valuable alcohols in high yields
from biomass-based carbonyl compounds via direct hydrogen
transfer. Catalytic performance of Hf-based MOFs was
compared to the related Zr-counterparts finding higher efficiency
for the Hf-based catalysts. The substrate scope was extended to
α,β-unsaturated carbonyl compounds in the catalytic highly
chemoselective transformation to allylic alcohols. This catalyst
shows chemical stability and could be recovered and reused for
several reaction cycles with only a minor decrease in catalytic
efficiency. The combination of Hf-MOF-808 with the Brønsted
acidic Al-Beta zeolite could be harnessed in the one-pot
transformation of furfural to γ-valerolactone in good yield of 75%
comprising four chemical transformations in a one-pot operation
procedure. This catalyst combination could be extrapolated to
other similar lignocellulose-derived substrates for biomass
upgrading.
1
.3 % H and 0.3 % N.
Synthesis of UiO-66-NH
2
4
(Hf): A mixture of HfCl (147 mg, 0.46 mmol), 2-
aminoterephthalic acid (84 mg, 0.46 mmol), formic acid (1.5 mL) in
dimethylformamide, was added to an autoclave vessel and heated at 120
ºC for 48 h. The resulting solid was filtered and washed with an excess of
DMF and acetone. The resulting solid was activated at 120 ºC in vacuo
during 12 h. Powder X-ray diffraction pattern (see Supporting
Information) confirmed that the solid material has the crystalline
morphology of UiO type MOF.
Catalytic reactions
Transfer hydrogenation of furfural. Catalysts screening: Furfural (0.1
mmol, 9.6 mg), catalyst (8 mol % in metal), dodecane (10 µL) as internal
standard and 2-propanol (0.4 mL) were added to a 2 mL glass crimp cap
vial. The reaction mixture was left to stir at 100 ºC. Yield was determined
by analysis by gas chromatography of aliquots taken from the reaction
mixture at different times.
Filtration test in the transfer hydrogenation of furfural: Furfural (0.1 mmol,
9.6 mg), catalyst (8 mol % in metal), dodecane (10 µL) as internal
standard and 2-propanol (0.4 mL) were added to a 2 mL glass crimp cap
vial. The reaction mixture was left to stir at 100 ºC. The solid was filtrated
after 10 minutes reaction time and the filtrated was left to stir further at
Experimental Section
General Information
1
00ºC. Yield was determined by analysis by gas chromatography of
Unless otherwise stated, all reagents were purchased from commercial
suppliers and used without further purification. Reactions were monitored
by gas chromatographic analyses performed in an instrument equipped
with a 25 m capillary column of 5% phenylmethylsilicone using dodecane
as an external standard otherwise indicated. GC/MS analyses were
performed on a spectrometer equipped with the same column as the GC
aliquots taken from the reaction mixture at different times.
Reuses study: Furfural (0.3 mmol, 29 mg), catalyst (8 mol % in metal),
dodecane (25 µL) as internal standard and 2-propanol (1.2 mL) were
added to a 2 mL glass crimp cap vial. The reaction mixture was left to stir
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ChemSusChem
10.1002/cssc.201701708
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at 100 ºC for 2 hours wherein completion is accomplished as determined
by gas chromatography. Solid catalyst was separated by centrifugation
and then washed with EtOAc and acetone. The catalyst was activated in
vacuo for 2 hours at room temperature and then used for the next run.
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General procedure for the transfer hydrogenation reaction of carbonyl
compounds: Carbonyl compound (0.1 mmol), Hf-MOF-808 (10 mol%),
dodecane (10 µL) as internal standard and 2-propanol (0.4 mL) were
added to a 2 mL glass crimp cap vial. The reaction mixture was left to stir
at 100 ºC or 120 ºC. Yield was determined by analysis by gas
chromatography of aliquots taken from the reaction mixture at different
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Title
Hf-based metal-organic framework Hf-MOF-808 catalyses the transfer
hydrogenation reaction of biomass-based carbonyl compounds such as furfural and
levulinic ester. The combination of Hf-MOF-808 with the Brønsted acidic Al-Beta
zeolite allows the four-step one-pot transformation of furfural to γ-valerolactone.
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