Catalytic transfer hydrogenation of ethyl levulinate to γ-valerolactone over zirconium-based metal-organic frameworks

Article abstract of DOI:10.1039/c6gc00524a

A series of highly crystalline, porous, zirconium-based metal-organic frameworks (Zr-MOFs) with different ligand functionalities and porosities were applied for catalytic transfer hydrogenation of ethyl levulinate (EL) to form γ-valerolactone (GVL), using isopropanol as a hydrogen donor. The roles of the ligand functionality and the metal center of the Zr-MOFs were identified and reaction parameters optimized, for selective production of GVL. The maximum yield of GVL (up to 92.7%) was achieved in 2 h at 200 °C with UiO-66(Zr). Interestingly, zirconium trimesate (MOF-808) emerged as the most suitable candidate, with the highest GVL formation rate (94.4 μmol g^-1 min^-1) among the catalysts tested at 130 °C. It was also found to be effective in conversion of EL to GVL in an open system using the solvent refluxing method. Both the catalysts (UiO-66(Zr) and MOF-808) were recycled at least five times under their specified reaction conditions without a notable change in catalytic activity and product selectivity. Fresh and recycled catalysts were characterized in detail using X-ray powder diffraction (XRD), N₂ adsorption-desorption, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) in order to understand the stability and structural changes that occurred in the catalysts. Finally, a plausible reaction mechanism was presented on the basis of active sites present in catalysts confirmed by characterization results.

Full text of DOI:10.1039/c6gc00524a

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ARTICLE
Catalytic Transfer Hydrogenation of Ethyl Levulinate to γ-
Valerolactone over Zirconium-based Metal-Organic Frameworks
a,c
a
a,c
a,c
a,c
a,c
Anil H. Valekar, Kyung-Ho Cho, Sachin. K. Chitale, Do-Young Hong, Ga-Young Cha, U-Hwang Lee, Dong Won
Received 00th January 20xx,
Accepted 00th January 20xx
a,c
d
a,b
a,c
Hwang, Christian Serre, Jong-San Chang* and Young Kyu Hwang*
A series of highly crystalline, porous, zirconium-based metal-organic frameworks (Zr-MOFs) with different ligand
functionality and porosity were applied for catalytic transfer hydrogenation of ethyl levulinate (EL) to form γ–valerolactone
(GVL), using isopropanol as a hydrogen donor. The role of ligand functionality and the metal center of the Zr-MOFs were
identified and reaction parameters were optimized, for selective production of GVL. Maximum yield of GVL (up to 92.7%)
DOI: 10.1039/x0xx00000x
www.rsc.org/
o
was achieved in 2 h at 200 C with UiO-66(Zr). Interestingly, zirconium trimesate (MOF-808) emerged as the most suitable
o
candidate, with the highest GVL formation rate (94.4 μmol/g/min) among the catalysts tested at 130 C. It was also found
effective in conversion of EL to GVL in an open system using the solvent refluxing method. Both the catalysts (UiO-66(Zr)
and MOF-808) were recycled at least five times under their specified reaction conditions without notable change in
catalytic activity and product selectivity. Fresh and recycled catalysts were characterized in detail using X-ray diffraction
2
(XRD), N adsorption-desorption, thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and
scanning electron microscopy (SEM) in order to understand the stability and structural changes that occurred in the
catalysts. Finally, a plausible reaction mechanism was presented on the basis of active sites present in catalysts confirmed
by characterization results.
melting and high boiling points; remarkably low vapor pressure,
even at elevated temperature and its ready miscibility with water
1
. Introduction
1
8-20
without forming an azeotropic mixture.
The growing demand for energy to improve the quality of life and
the gradual depletion of fossil resources along with environmental
deterioration, have left researchers in a quest for alternative
renewable resources. Among several renewable resources, only
biomass some of which is inexpensive and abundant in nature is a
sustainable source of liquid fuels and chemicals otherwise
Generally, three major strategies have been explored for GVL
production from levulinic acid (LA) and its ester, based on diversity
2
1
of the hydrogen source. Molecular H2 is the most common
hydrogen source used for this reaction, which occur in the presence
of various metal catalysts (e.g., Ru, Pt, Pd, Ni, Co, Cu, Rh, Re and
22-28
1
, 2
Mo).
Formic acid (FA), which forms in equimolar amount with
conventionally derived from fossil resources.
Enormous efforts
LA during acid hydrolysis of carbohydrates, is also utilized as a
hydrogen source to produce GVL from LA to embodies the principle
of atom economy. Several catalytic systems including RuRe/C,
are being made to develop suitable catalytic processes for the
production of various platforms and value-added chemicals from
3
-11
carbohydrate, or directly from lignocellulosic biomass.
Among
Au/ZrO -VS, ruthenium immobilized functionalized silica, nickel
2
these chemicals, γ-Valerolactone (GVL) is recognized as a versatile
building block that could be utilized as an additive in liquid fuel for
promoted copper-silica and Ag-Ni-ZrO nanocomposites have been
2
1
2, 13
successfully utilized for quantitative conversion of LA into GVL,
transportation,
as precursor for the production of polymeric
and for creation of various value-added chemicals
2
9-33
1
4, 15
while consuming equimolar FA as a hydrogen donor.
However,
monomers,
1
6, 17
both the hydrogenation strategies mentioned above have some
limitations (e.g.., harsh reaction conditions, use of corrosive acids,
use of precious metals and non-environmental friendly solvents)
including organic solvents and bio-oxygenates.
GVL itself has
proved an excellent green solvent for biomass processing due to its
extraordinary physicochemical properties, which include low
3
4
that restrict their large scale application to a certain extent.
Recently, Dumesic et al. reported for the first time, a catalytic
transfer hydrogenation (CTH) method based on the principle of
Meerwein-Ponndorf-Verley (MPV) reduction for the hydrogenation
of LA and alkyl levulinates to GVL over heterogeneous catalysts
using secondary alcohols as hydrogen donor. They demonstrated
that ZrO2 displayed better activity than the other metal oxides
3
5
tested due to its amphoteric nature. The high chemical selectivity
of MPV reduction toward the carbonyl groups of aldehyde and
ketones, replacement of molecular H2 by alcohols, and effective
performance of catalysts comprising non-precious metals provide a
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Journal Name
cost-effective alternative for the production of GVL. Therefore,
within a short period of time, a variety of catalytic systems mainly
Ethyl levulinate (99%), γ-valerolactone (99%), 2-propanol (99.5%),
naphthalene (98%), ZrOCl .8H (98%), ZrCl (99.5%), 1,4-
benzenedicarboxylic acid (H BDC, 98%), 1,2,4-benzenetricarboxylic
acid (H BTC, 99%), 2-aminoterphthalic acid (99%), 1,3,5-
benzenetricarboxylic acid (H BTC, 95%), benzoic acid (99.5%),
2
2
O
4
3
6
37
38
based on zirconium such as ZrO
2
, ZrO(OH)
2
, Al-Zr mixed oxides
2
and amorphous Zr-complexes [zirconium 4-hydroxybenzoate (Zr-
3
3
4
39
HBA), zirconium phosphonate (Zr-PhyA) ] have been reported for
this reaction. Very recently, GVL was produced via vapor phase and
3
acetic acid (99.7%), hydrochloric acid (HCl, 37%) and N,N-
dimethylformamide (99.8%) were obtained from Sigma-Aldrich.
Formic acid (99%) and fumaric acid (99%) were purchased from
Samchun Pure Chemicals (South Korea) and iosopropyl 4-
oxovalerate (95%) was supplied by Beta Pharma Co. Ltd. (Shanghai,
China). All the chemicals were used without further purification.
4
0
41
2
microwave assisted systems over ZrO /SBA-15 and Ru/C ,
respectively. In the category of crystalline porous material, several
zeolites such as Zr-Beta, Ti-Beta, Sn-Beta, Al-Beta and Al-MFI-ns
42 ,43
were tested for GVL production from LA and its ester.
Moreover, Linh Bui et al. used Brönsted and Lewis acid zeolite
4
3
mixture for selective production of GVL directly from furfural.
2
.2 Catalyst preparation
Metal-organic frameworks consist of metal nodes or clusters,
bridged through organic ligands to form a well ordered, and highly
crystalline porous network. Its exceptional properties such as large
surface area, tunable pore size, coordinatively unsaturated sites
Synthesis of UiO-66(Zr) and its functionalized analogs:
UiO-66(Zr) synthesis was carried out by reflux method. Typically,
5
6
4
3
.62 g (27.8 mmol) of H
2
BDC was initially dissolved in DMF (23.6 g,
(CUSs), and functionality of metal ions and organic ligands provide
22 mmol) at room temperature in a 100 ml round flask. Then, 8.96
additional advantages in catalysis over non-porous and zeolitic
4
4-46
2 2
g (27.8 mmol) of ZrOCl ·8H O and 4.63 mL of 37% HCl (5.47 g, 150
materials.
From the point of view of catalysis, the high surface
mmol) were added to the mixture. The molar ratio of the final
ZrOCl ·8H O/H BDC/DMF/HCl mixture was 1:1:11.6:5.4. The
area and permanent porosity of MOFs, accessible through
continuous and permeable channels, can provide a greater number
of active sites for the reactant. The presence of larger channels or
cages further facilitates diffusion, which enhances the interaction
between reactant and the active sites present in the porous
network. Subsequently, the rate of reaction and turn over
frequency (TOF) increases, which is highly desirable from the
perspective of green chemistry. Nevertheless, the relatively lower
hydrolytic and thermal stability of various MOFs, compared to
2
2
2
reaction mixture was vigorously stirred to obtain a homogeneous
o
gel. The mixture was then heated to 150 C and kept at this
temperature for 6 h, leading to a crystalline UiO-66(Zr) solid. The
resulting product was recovered from the slurry by filtration, re-
o
dispersed in DMF at 60 C for 6 h while stirring, and then recovered
by filtration. The same procedure was repeated twice, using MeOH
instead of DMF. The solid product was finally dried at overnight
o
4
7, 48 100 C.
zeolite restricted its practical applications in various fields.
UiO-66(Zr)-NH
.94g of 2-amino- terphthalic acid (10.7 mmol) was dissolved in
38.4 ml H O and 9.2 ml acetic acid (160.1 mmol) solution in a 100
2
was synthesized by the reflux method. Initially,
Fortunately, Zr-MOFs exhibit unprecedented chemical, thermal, and
mechanical stability due to higher co-ordination number of Zr in the
1
4
9, 50
2
framework.
a variety of applications such as gas and water adsorption,
Recently, several authors explored this material for
ml round flask. Then, 3.44 g of ZrOCl
to the solution with continuous stirring. The molar ratio of the final
ZrOCl ·8H O/H BDC-NH /H O/CH COOH mixture was 1:1:200:15.
2 2
·8H O (10.7 mmol) was added
5
1-55
separation, sensing and catalysis.
2
2
2
2
2
3
In this work, a series of zirconium MOFs were selected to
study the CTH of EL to GVL using isopropanol as H-donor.
Isopropanol has already been proven as the preeminent H-donor
o
The reaction solution was ramped to 100 C and kept for 24 h. After
cooling to room temperature, the precipitate in the reaction
solution was filtered. To remove the residual precursor and ligands,
the precipitates were sufficiently washed with water and ethanol at
3
7
alcohol due to its low reduction potential. We preferred EL as
feedstock over LA considering as a model compound which possibly
o
o
3
5
80 C and 60 C, respectively. Finally, the product was dried
would not block the basic sites of catalyst as LA does. In addition,
there are no safety and economical concern over corrosion of
reactors because of its acid free form. To the best of our knowledge,
no report has been focused on GVL formation from LA or its ester
using porous crystalline metal-organic frameworks (MOFs) prior to
this article. In our study, we have explored the role of the metal
center, ligand functionality, and porous properties of Zr-MOFs for
the conversion of EL and selective formation of GVL. Effect of
different process parameters on the selectivity of products was
investigated and a plausible reaction mechanism was proposed
based on findings of active centers present in the MOFs. Detailed
characterizations of fresh and used catalysts were conducted to
check their stability and structural changes the under applied
reaction conditions.
o
overnight at 100 C.
UiO-66(Zr)-COOH was also synthesized using the reflux method.
Briefly, 14g of 1,2,4-benzenetricarboxylic acid (66.7 mmol) and
1
3
0.74 g of ZrOCl
2
·8H
O and 20.4g of benzoic acid (166.6 mmol) in a 100ml round
The molar ratio of the final
O/H BTC/H O/C COOH mixture was 1:2:50:5. Then, the
2
O (33.3 mmol) were dissolved in a solution of
0 ml H
2
flask.
ZrOCl
2
·8H
2
3
2
6 5
H
o
reaction solution was ramp to 100 C and kept for 24 h. Work-up
was done exactly the same way mentioned in the procedure used
above for UiO-66(Zr)-NH
Synthesis of MOF-808: H
3.3 g, 0.5 mmol) were added to a solvent mixture of DMF/formic
2
.
3
2 2
BTC (4.8 g, 0.5 mmol) and ZrOCl ·8H O
(
acid (270 mL/360 mL). Reaction mixture was transferred to a 1L
Teflon-lined pressure autoclave and heated at 135°C for two days.
White precipitate was collected by centrifugation and washed with
DMF (24 h), followed by ethanol (24 h). The respective solvents
2
. Experimental
2.1 Chemicals and materials
2
| J. Name., 2012, 00, 1-3
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were replaced twice during that period. Finally, the product was In a typical run, 4 mmol of EL, 400 mmol of 2-propanol, 0.24g of
o
dried at 100 C for 12 h.
naphthalene as an internal standard and respective amounts of
Synthesis of MOF- 801: MOF-801, commonly known as Zr- catalyst were filled to a 100 mL stainless steel reactor containing an
Fumarate MOF, has been synthesized according to the procedure inner lining of Pyrex glass and equipped with magnetic stirrer. The
5
7
published earlier. Briefly, ZrCl
4
(2.585 mmol, 1 eq) was dissolved in reaction was conducted at a certain known temperature for the
5
0 ml water. Then, formic acid (258.5 mmol, 100 eq) as a modulator desired time. Catalyst was separated by filtration and washed
and fumaric acid (7.75 mmol, 3 eq) as a linker molecule was added thoroughly with ethanol-water system (95:5). The filtrate were
to the solution of the metal precursor. The reaction mixture subjected to quantitative analysis using gas chromatography (GC,
o
transferred into a Teflon-lined pressure vessel and heated at 120 C FID detector and HP-5 column) and identification of the products
for 24 h. White precipitate was collected by centrifugation and was done by GC-MS (Agilent 6890N GC and 5973 N MSD). For the
washed with ample water and ethanol. Finally, the product was open-system solvent-reflux method, reactions were carried out in
o
dried at overnight 100 C.
two-neck, 50 mL round-bottom flasks equipped with septum ports
Synthesis of ZrO(OH)
2
and ZrO
2
: Zirconium oxyhydroxide were and reflux condensers. EL conversion and yield of products were
3
7
synthesized as reported elsewhere. Concentrated NH
4
OH was defined using the following equations shown in eqn. (1) and (2).
added into aqueous solution (100 g/L) of ZrOCl ·8H O with vigorous
2
2
ꢏꢄꢐꢇꢂꢄꢑꢂꢀꢁ
stirring to adjust the solution pH in between 9 to 10. Resulting
emulsion was aged for 24 h, filtered and then thoroughly washed
ꢀꢁꢂꢃꢄꢅꢆꢇꢈꢉꢊꢄꢅꢂꢋ%ꢌ ꢍ ꢋ1 ꢎ
ꢌꢂꢕꢂ100ꢌꢂ… … … ꢋ1ꢌ
ꢒꢅꢊꢓꢊꢔꢐꢂꢏꢄꢐꢇꢂꢄꢑꢂꢀꢁ
O
with water. Solid precipitate dried at 100 C overnight to get
O
ZrO(OH)
2
. ZrO
2
was prepared by calcination of ZrO(OH)
2
at 500 C
ꢏꢄꢐꢇꢂꢄꢑꢂꢚꢈꢄꢗꢘꢃꢓ
ꢒꢅꢊꢓꢊꢔꢐꢂꢏꢄꢐꢇꢂꢄꢑꢂꢀꢁ
for 5 h in air.
ꢂꢂꢂꢖꢈꢄꢗꢘꢃꢓꢂꢙꢊꢇꢐꢗꢋ%ꢌ ꢍ ꢋ
ꢌꢂꢕꢂ100ꢂ … … … … … … . ꢋ2ꢌ
2
.3 Catalyst characterization
3
. Results and discussions
Powder diffraction patterns were obtained by Rigaku
diffractometer using Ni-filtered Cu Kα- radiation (40 kV, 30 mA,
The MPV reduction reaction for different substrates can be
λ=1.5406 Å). The
N
2
adsorption-desorption isotherms were
effectively catalyzed by several Zr-containing catalysts, including
measured at 77 K using a Micromeritics Tristar 3000. The samples
were dehydrated under vacuum at 423 K for 12 h prior to analysis.
The specific surface areas were evaluated using the Brunauer-
Emmett-Teller (BET) method and the pore volume was determined
35, 36
porous and non-porous materials such as metal oxides,
metal
42, 43
37
34, 39
hydroxides, amorphous metal complexes
and zeolites.
Owing to the potential of porous materials for MPV reduction
reaction, various zirconium-based metal-organic frameworks (Zr-
MOFs) were tested for CTH of EL to GVL as shown in Scheme 1
0
by the single point method at p/p = 0.99. The micropore size
distribution was determined from Ar sorption techniques using the
Horvath-Kawazoe method. Thermal gravimetric analysis (TGA) of
the catalysts was performed on a Sinco TGA-N 1000 thermal
3
.1 Role of metal center/cluster and ligand functionality of Zr-
MOFs in CTH of EL
o
analyzer. The samples were run at a heating rate of 5 C/min in the
o
range of 25 to 700 C under constant flow of nitrogen at 20 ml/min.
FTIR spectra were recorded on a Nicolet FTIR spectrometer
(
MAGNA-IR 560) using KBr pellets. Morphological properties of the
catalysts were studied by scanning electron microscopy (SEM)
Tescan Mira 3 LMU FEG with accelerating voltage of 10 kV). Acidic
and basic properties of the catalysts were measured using NH -TPD
and CO -TPD, respectively. TPD profiles of catalysts were measured
(
3
2
Scheme 1 Catalytic transfer hydrogenation of EL to GVL using Zr-
on a Micromeritics AutoChem II 2920 V3.05 apparatus equipped
MOFs
with a thermal conductivity detector. Samples were activated at
o
1
50 C for 12h in a helium flow, prior to the adsorption step.
Subsequently, activated samples were exposed to NH
3
or CO
at 50 C for 30 minute with a flow rate of 50 mL/min. Initially,
physically adsorbed NH or CO gases were removed by purging
with helium gas for 1 h at the same temperature and flow rate. TPD
2
gases
o
3
2
o
o
data were recorded from 50 C to 300 C with heating rate of
o
5
C/min. Inductively coupled plasma (ICP) analysis was used to
determine the metal leaching from the Zr-MOFs. Elemental analysis
EA) was performed to determine the carbon and hydrogen content
in the catalyst.
(
Fig. 1 Representative structure of UiO-66(Zr) and its functionalized
2
.4 Catalytic test and product analysis
analogs with different ligands.
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Table 1 Physico-chemical properties and catalytic activity of UiO-
Journal Name
100
6
6(Zr) and its functionalized analogs in CTH of EL to GVL
80
b
c
d
e
f
S
BET
PV
Conv.
Y
GVL
Y
IPL
60
Entry
Catalysts
Conversion
YieldGVL
2
3
(
m /g)
(cm /g)
(%)
(%)
(%)
4
0
None
----
----
8.9
2.7
1.0
YieldIPL
1
2
20
UiO-66(Zr)
1046
1.65
100
53.5 44.2
13.9 13.5
UiO-6(Zr)-
COOH
0
3
575
0.80
1.8
28.5
97.6
120
140
160
180
200
UiO-6(Zr)-
ο
4
1006
27.3 64.6
Temperature ( C)
NH
2
a
Reaction conditions: EL 4 mmol, isopropanol 400 mmol; catalyst Fig.2 Effect of reaction temperature in catalytic transfer
O
b
0
.8 g; naphthalene 0.24g; temperature 150 C; time 4 h. S BET = BET hydrogenation of EL to GVL over UiO-66(Zr): Reaction conditions: EL
c
d
surface area as a nominal surface area, PV = Pore Volume, Conv. 4 mmol, isopropanol 400 mmol, catalyst 0.8 g, naphthalene 0.24g,
e
f
=
Conversion,
YGVL = GVL yield and
Y
IPL= Yield of isopropyl and reaction time 4 h.
levulinate (IPL).
MOFs possess multifunctional properties derived both from a metal slow diffusion of EL into the narrow pores of UiO-66(Zr)-COOH or
cluster and ligand functionality. Therefore, it is highly desirable to the reaction could be limited only to the external surface area of
understand the origin of active centers (either of metal cluster or UiO-66(Zr)-COOH. On the other hand, nearly complete conversion
ligand), to design a material superior for the respective applications. of EL was observed using UiO-66(Zr)-NH . However, in product
2
Considering this point, we synthesized UiO-66(Zr) and its distribution, IPL was the major product with 64.6% yield. This
functionalized analogs distinguished on the basis of ligand clearly indicates that the transesterification reaction was
functionality, as shown in Fig. 1. Their structures were confirmed predominant in presence of amine functionalized UiO-66(Zr). It has
using XRD, FTIR, N
the supplementary information (Fig.S1). The obtained results were silicas
2
adsorption and TG analysis; and are reported in been reported that basic catalysts such as amine functionalized
6
2, 63
64
and carbon nanotubes are highly efficient in
transesterification reactions. Furthermore, the presence of electron
The physicochemical properties and catalytic activity of UiO- donating or withdrawing groups on ligands can change the charges
5
8, 59, 60
well matched with earlier reports.
6
5
6
6(Zr) and its functionalized analogs with acid (COOH) and base on metal cluster which significantly affects the catalytic activity.
NH ) group in the framework of the ligand are summarized in Table
This result indicated that large surface area, along with larger
. UiO-66(Zr) catalyst has a large BET surface area (1046 m /g) and pore size, balanced acid-base property or charge on the metal
pore volume (1.65 cm /g). Replacement of the BDC ligand by cluster and easy access to the active center are the key factors in
electron donating and withdrawing groups functionalized BDC, selective conversion of EL into GVL. Ligand functionality hampers
significantly altered its chemical and physical properties. The the catalytic activity by changing electronic and the porous
presence of carboxylic acid functionality in the ligand delivers extra properties of the original UiO-66(Zr) framework.
(
2
2
1
3
acidity to the UiO-66(Zr) framework. However, considerable loss in
surface area and pore volume of UiO-66(Zr)-COOH was observed, 3.2 Effect of reaction temperature and time.
probably due to partial consumption of available space in the pores
The temperature dependence of this reaction was exhibited in the
by free carboxylic groups. In contrast, the presence of amino groups
o
range 120 - 200 C for 4 h reaction time (Fig.2). In most of the
in the ligand provides additional basicity to the UiO-66(Zr) structure
previous studies, transfer hydrogenation of LA or EL was carried out
with minor loss in surface area. This was ascribed to smaller size of
34 35, 36,
o
o
6
1
at moderate to high reaction temperature (120 C to 250 C). ,
2
NH groups compared to COOH groups. Previous literature and
3
7, 39,
Furthermore, extraordinary thermal and chemical stability of
pore size distribution curves from Ar-physisorption (Fig. S2) shows
4
9 66
UiO-66(Zr) was already been proved,
which allowed us to
that, the pore size of UiO-66(Zr) and its functionalized analogs
o
extend the reaction temperature to 200 C. The reaction conversion
increased in the order UiO-66(Zr)-COOH < UiO-66(Zr)-NH
6(Zr).
Entry 1 in Table 1 shows that without catalyst, the reaction
2
< UiO-
o
was 83% at 120 C and reached to completion after increasing the
6
o
temperature by 30 C. Yield of GVL increased with reaction
o
temperature and reached maximum to 92.7% at 200 C. At the
proceeded to a minor extent. Isopropyl levulinate (IPL), the
transesterified product of EL, in the presence of 2-propanol, was
the major side product observed in all cases. Among the tested
catalysts, UiO-66(Zr), which has no ligand functionality, showed
maximum EL conversion and GVL yield. Only 28.5% EL conversion
and 13.9% GVL yield was achieved using UiO-66(Zr)-COOH catalyst.
It should be noted that the low conversion was possibly due to the
same time, the IPL yield decreased progressively with rise in
o
o
reaction temperature from 67.9% at 120 C to 0% at 200 C. IPL is
the major side product at lower reaction temperatures, most
probably due to the presence of excess amount of isopropanol in
reaction mixture. Metal center of UiO-66(Zr) in presence of excess
isopropanol is likely to undergo transesterification reaction more
rapidly to form IPL than CTH does to produce GVL. Simultaneous
4
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Conversion
Yield
100
100
80
80
6
0
Conversion
YieldGVL
60
40
40
YieldIPL
20
20
0
0
0
30
60
90
120
150
180
1
2
3
4
5
Time (min.)
Cycle
Fig.3 Effect of reaction time in catalytic transfer hydrogenation of EL Fig. 4 Recycle test of UiO-66(Zr) catalyst in catalytic transfer
to GVL over UiO-66(Zr) : Reaction conditions: EL 4 mmol, hydrogenation of EL to GVL: Reaction conditions: EL 4 mmol,
isopropanol 400 mmol, catalyst 0.8 g, naphthalene 0.24 g, and isopropanol; 400 mmol, catalyst 0.8 g, naphthalene 0.24 g,
O
O
reaction temperature 200 C.
reaction temperature 200 C and reaction time 2 h.
3
3
etherification and transfer hydrogenation reaction of oxygenates in volume (1.65 cm /g and 1.63 cm /g) of the catalyst was observed
presence of alcohol has been reported previously for Lewis acid after five tests (Fig.5 b).
67
zeolites. Raising reaction temperature improved, GVL yield
Nevertheless, differences in the TG curve of fresh and used
improved significantly because IPL undergoes CTH as EL more catalysts were observed (Fig.5 c). Fresh catalyst absorbed more
effectively does at higher temperature. This observation concluded water than did the spent, which was determined by slightly greater
o
that GVL could be formed at lower temperature; however the rate weight loss before 75 C and this was well supported by a small loss
of GVL formation increases with increasing reaction temperature.
in BET surface area and pore volume of spent, compared to fresh,
We also examined the influence of reaction time on CTH of EL UiO-66(Zr). The number of BDC ligands present in the fresh and
o
to GVL at 200 C. Results show that, even at short duration (15 min) used UiO-66(Zr) was calculated from the weight loss that occurred
o
o
EL conversion reached completion (Fig. 3). By increasing the in the temperature range of 350 C to 550 C. In spite of the good
reaction time from 15 to 120 min, GVL yield improved from 70 to crystalline structure, fresh UiO-66(Zr) has one linker deficiency; this
9
2.7%. Allowing the reaction continues for 3 h, did not change GVL fact is in line with the results obtained in other studies where HCl
6
8,
yield. To the contrary, the IPL yield progressively declined with was used as a deprotonating agent in the synthesis of UiO-66(Zr).
6
9
increasing reaction time. This observation supports that IPL can
undergo MPV reaction as EL does, most likely with a lower reaction groups. In this case the proposed formula for fresh UiO-66(Zr)
rate.
was Zr (OH) (OH) (BDC) . After five cycles, the catalyst lost one
Missing linker defects were assumed to be capped by the -OH
6
9
6
O
4
4
2
5
more BDC linker from its framework (combining BDC anions with
two protons from OH of a hexanuclear Zr cluster, and a departure
3
.3 Catalyst recyclability and characterizations
of neutral acid) leaving
Zr (OH) (BDC) . The charge imbalance generated due to linker
deficiency was compensated by partial replacement of μ
a
material with the formula
A recycling test of UiO-66(Zr) for CTH of EL to GVL was studied
under optimized reaction conditions, and the results are shown in
Fig. 4. After every cycle, the catalyst was recovered by filtration,
washed with an ethanol-water system (95:5) and dried prior to the
next run. There was no marginal difference observed in the EL
conversion and GVL yield, even after five cycles, indicating no loss
of the active sites present in the catalyst.
6
O
6
4
4
–
3
–OH ions
3
–O ions. The stability of UiO-66(Zr) with two linker
2
–
68
by μ
6
8
deficiency has also been reported previously. FTIR analysis
-
showed that the characteristic sharp peak at wavenumber 3673 cm
1
70, 71
assigned to bridged OH stretches
in fresh catalyst, reduced
considerably after recycle tests, which supported our hypothesis on
charge balance. That the morphology of the catalyst remains
unchanged was confirmed by SEM analysis (Fig. S3).
In general, it is very difficult to predict the stability of MOFs
under specific reaction conditions despite the high thermal stability
provided by their chemical durability. Consequently, it is always
necessary to check for changes in the crystalline structure, after
reaction. Therefore, recycled UiO-66(Zr) catalyst was carefully
studied using various characterization techniques, to confirm
changes in its structural and morphological properties after five
recycle tests (Fig. 5). In the XRD patterns of used UIO-66(Zr), all
peaks were preserved with little decrease in peak intensity,
compared to fresh samples (Fig.5 a). Similarly, a negligible
ICP analysis did not detect any zirconium in the reaction filtrate.
Additionally, higher Zr content was observed in the used catalyst
than in the fresh catalyst. Contrarily, elemental analysis results
show that carbon and hydrogen content decreased in the used
catalyst (Table S1). This confirmed leaching of organic moiety, not
of zirconium, from the MOF structure.
All the above observation reinforced the fact that, UiO-66(Zr)
has a unique property in the ability to undergo structural
rearrangement in ways that stabilize its structure. This unique
property developed due to the high connectivity of zirconium in the
2
2
difference in BET surface area (1046 m /g and 1000 m /g) and pore
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b)
a)
1200
Adsorption
Desorption
1000
8
00
00
00
00
0
6
4
2
Fresh
Used
0.0
0.2
0.4
0.6
0.8
1.0
5
10
15
20
25
30
35
40
45
50
2θ
(deg)
Relative pressure (P/Po)
c)
d)
Fresh
Used
100
Used
80
Fresh
4.0 BDC
60
5.0 BDC
40
3673
20
0
100
200
300
400
500
600
700
4000
3500
3000
2500
2000
1500
1000
ο
-1
Temperature ( C)
Wavenumber (cm )
2
Fig.5 Characterization of the fresh and used UiO-66(Zr) after 5 cycles: a) XRD pattern, b) N adsorption-desorption isotherm at 77K , c)
TGA curves, and d) FT-IR patterns.
UiO-66(Zr) framework. In spite of the structural rearrangement, the However, MOF-808 with metal centers-to-linker co-ordination
catalytic activity of UiO-66(Zr) was unaffected. This further number “6”possesses porous properties different from other two
Indicated that the metal nodes are the active sites, rather than the Zr-MOFs (Table2 and Fig.S5). Anticipating pronounced potential of
organic ligands in the selective transformation of EL to GVL.
MOF-808 due to its large surface area and bigger pore size, it was
decided to check the performance of the catalyst at moderate
3
.4 Evaluation of Zr-MOFs possessing different physico-chemical reaction temperature to represent the principle of green chemistry.
properties for CTH of EL to GVL
Table 3 demonstrates the catalytic activity of various Zr-based
catalysts at moderate reaction temperature. Three different MOFs
After confirming the active sites in UiO-66(Zr) for transfer were compared in terms of conversion, yield, GVL formation rate
hydrogenation of EL, we planned to check the possibility that of and turn-over frequency (TOF) (entry number 1− 3 in Table 3).
other Zr-MOFs might be more suitable candidates for this reaction. Indeed, MOF-808 had an edge over the other two MOFs. The
To overcome the use of high reaction temperature was another reason is quite clear, its greater surface area provides more active
issue behind this objective. For this purpose, two alternative Zr- sites and its bigger pore size provides easy access to active sites.
MOFs (MOF-801 and MOF-808), which possesses metal center UiO-66(Zr) gives higher conversion (43.3%) than MOF-801 (28.1%)
6 4 4
(Zr O (OH) ) in their framework like UiO-66(Zr) were selected for despite having the same window size (6 Å). This was probably due
2
this reaction. Table 2 represents the porous properties of the to higher external surface area (390 m /g for UiO-66(Zr) and 168
2
selected Zr-MOFs along with linker and molecular formulas. m /g for MOF-801) and pore volume. We have compared our
Representative structures of the two Zr-MOFs are shown in Fig. S4.
2 2
results with ZrO , ZrO(OH) and other Zr based catalysts reported in
UiO-66(Zr) and MOF-801 have the same metal center to linker co- the literatures, studied under comparable reaction conditions
ordination number (12), and almost similar porous properties.
6
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ARTICLE
Table 2 Linker, and molecular formulas and porous properties of Zr-MOFs.
a
BET
b
c
Molecular
S
PV
PD
MOF
Linker
2
3
formula
(OH) (fumarate)
(OH) (BDC)
(m /g)
(cm /g)
(Å)
MOF-801
UiO-66(Zr)
Fumaric acid
Zr
6
O
4
4
6
990
0.44
6
1
,4 Benzenedicar-
Zr
6
O
4
4
6
1046
1450
1.65
0.8
6
boxylic acid (BDC)
1
,3, 5 Benzenetri-
MOF-808
Zr
6
O
4
(OH)
4
(BTC)
2
(HCOO)
6
7.4, 12.5
carboxylic acid (BTC)
a
S
b
c
BET =BET surface area as a nominal surface area, PV= Pore Volume, PD= Pore Diameter
(entry 6 -8). MOF-808 was found superior in terms of GVL formation of the catalysts make homogeneous systems less efficient.
rate compared to all other catalysts mentioned in Table 3 under Therefore, it is highly desirable to use heterogeneous catalyst to
specific reaction conditions. Amorphous Zr-complexes were also overcome the difficulties associated with homogeneous catalytic
75-77
showed good performance at lower temperature (entry 6, and 7), systems.
In return, catalyst must be active and selective toward
but they lagged behind MOF-808 in terms of GVL formation rate. targeted compounds under moderate reaction conditions. This is
This was ascribed to their low crystallinity or poorly ordered because pressurized reactions are out of consideration in organic
structure, which resulted to smaller surface area compared to synthesis due to the hazardous nature of several chemicals and
highly ordered, crystalline MOF-808. Nevertheless, at higher related safety concerns. MOF-808 showed good performance in
temperature all the compared catalysts showed quantitative open system MPV reduction of EL at its boiling point, producing 75%
product yield. The porous properties of the catalysts mentioned in GVL yield in 18 h. It should be noted that MOF-808 was still active in
table 3 are reported in the supplementary information Table S2.
open system at milder reaction conditions and could be utilized in
With such exceptional catalytic activity of MOF-808, we organic synthesis where most of the reactions are performed in
extended our approach to carry out this reaction in an open system open system at moderate reaction conditions.
using the solvent reflux method (entry 10). In organic synthesis,
However, when LA was used as precursor for the GVL
several reduction reactions have been practiced using production, MOF-808 has some limitations, which showed only 4.3%
7
2-74
homogeneous catalytic systems.
The moisture sensitivity, GVL yield due to strong binding ability of carboxylic group in LA to
the basic metal sites of MOF-808 confirmed by FTIR and TG analysis
stoichiometric use and difficulties in the separation and reusability
Table 3 CTH of EL catalyzed by various catalysts at moderate reaction temperature
f
TOF
Reference
Temp.
Time
Conv.
(%)
Yield
(%)
Mass ratio
catalyst :ester
GVL FR
-
1
Entry
Catalyst
O
(h )
(
h)
(μmol/g/min)
(
C)
1
2
3
MOF-808
UiO-66(Zr)
MOF-801
ZrO2
130
130
130
3
3
3
100
43.3
28.1
85.0
8.0
1:2.9
1:2.9
1:2.9
94.4
8.9
7.7
0.8
1.1
0.2
This study
This study
This study
This study
12.2
13.6
4
130
3
36.5
21.9
1:2.9
24.3
5
6
7
ZrO(OH)
2
130
120
130
118
82
3
4
8
50.5
82.1
98.9
88
33.7
50.1
95.4
83
1:2.9
1:0.7
1:0.7
1:1.2
1:0.6
1:0.7
1:0.7
37.4
10.4
9.9
0.3
0.1
0.7
3.7
0.1
1.1
0.1
This study
Zr-HBA
34
39
Zr-PhyA
Zr-Beta
b
8
22
18
18
18
6.3
42
c
9
Zr-Beta
5.6
4.0
0.2
42
d
1
1
0
1
MOF-808
MOF-808
82
100
61.7
75
3.5
This study
This study
e
82
4.3
0.2
a
b, c
Reaction conditions: EL 4 mmol; isopropanol 400 mmol; catalyst 0.2 g; naphthalene 0.24 g;
Reactant was LA and 2-pentanol as
c
d,e
hydrogen donor, for Open system solvent reflux method was used;
Reaction carried out in open system (solvent reflux method)
e
f
catalyst- 0.8g; Reactant was LA; GVL FR (formation rate) = (mole of GVL)/(amount of catalyst X reaction time). Turn-over frequency
(
TOF)= (mole of GVL)/(mole of active sites of catalyst X time)
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of fresh and spent catalysts (Fig S6). FTIR shows extra peaks (C=O
According to previous studies, the amphoteric nature of the
-1
-1
stretch at 1715 cm and C-O stretch at 1165 cm ) in spent catalyst, catalysts was the major cause for their high activity in MPV
35, 39
TGA shows higher weight loss of spent catalyst in the temperature reduction of EL to GVL.
Therefore, Zr-MOFs (MOF-808 and UiO-
range of 350 C to 550 C attributed to LA bonding with MOF-808. 66(Zr)) were subjected to NH -TPD and CO -TPD to determine their
Acid-base sites of Zr-MOFs are required for MPV reduction acid and base properties, respectively (Fig S10). The obtained
explained thoroughly in mechanism part). Blocking these basic results shows that both the Zr-MOFs possess acidic and basic sites;
O
O
3
2
(
sites by strong adsorption of LA would be thus reflected as a decline and that moreover, the quantity of these sites was much greater in
in catalytic activity. This kind of inhibiting effect of LA in CTH MOF-808 than UiO-66(Zr). The origin of the acid-base sites was
3
5
reaction over ZrO
Effect of reaction temperature on catalytic activity of MOF-808 clusters of the Zr-MOFs, similar to the bonding observed in ZrO
was studied at moderate temperatures (see Fig.S7). It was found Zr(OH) . Considering all the above mentioned points, a plausible
that, at lower reaction temperature 110 C catalyst produces reaction mechanism for this reaction was proposed in Scheme 2.
2
catalyst was reported previously.
development from Zr-O-Zr or Zr-OH-Zr bonding present in the metal
2
and
4
O
4
+
2-
around 64% GVL yield with IPL as major side product. MOF-808 The combination of acid and base sites (Zr and O ) from the metal
O
showed its best performance at 130 C with 85% GVL yield.
cluster interacted with both isopropanol and EL to form a six
35, 36, 37, 81
Furthermore, MOF-808 was recycled five times in a batch-type membered ring transition state.
At this stage,
pressurized reaction system without notable change in catalytic corresponding alkoxide generated from the dissociation of
activity Fig S8. Recycled catalyst was characterized in detail, using X- isopropanol, readily transfered hydride ions to attack the carbonyl
ray diffraction, N physisorption, TGA, FTIR, and SEM analysis as group of EL to yield 4-hydroxypentanoate (4-HPE). At the same time,
2
shown in Fig S9. XRD patterns displayed no change in the crystal isopropanol was converted into acetone with the loss of two
structure of used catalyst; all the peaks retained the same intensity. hydrogen atoms. No detection of 4-HPE by GC analysis supported
The BET surface area of the used catalyst decreased from the initial the fact that fast intramolecular transesterification of 4-HPE into
2
35, 37, 42
1
450 to 900 m /g. TGA and FTIR patterns further validated the GVL was accompanied by equimolar production of ethanol.
structural changes that occurred in the catalyst after the fifth run. Liu et.al. thoroughly studied the pathway and mechanism for this
o
37
Weight loss in the temperature range of 375 to 700 C assigned to reaction using ZrO(OH)2. Only two other major products were
the BTC ligand was almost identical in fresh and used catalyst, detected by GC-MS besides GVL. The first was IPL, a by-product
which confirmed no leaching of the BTC ligand from the structure. formed by transesterification of EL, which underwent further CTH
However, the main difference in weight loss occurred in the middle (like EL) to yield GVL. Another observed by-product was ethyl 4-
o
temperature range (175 to 375 C) associated with OH and remnant isopropoxypentanoate
(E4iPOPA),
probably
formed
by
formate group in the MOF-808 structure. Substantial reduction in etherification of 4-HPE with isopropanol. Formation of E4iPOPA
OH stretching frequency in the FTIR pattern of used catalyst could be considered another indirect evidence for the formation of
indicates the deviations in the OH group present in MOF-808. 4-HPE. Several attempts have been made to produce these kinds of
Additionally, no leaching of metal from MOF structure was bio-oxygenates from GVL which considered as more suitable
confirmed by ICP analyses (Table S1). Hitherto, unlike UiO-66(Zr), it candidates for gasoline additives and diesel components than GVL
8
2, 83
is difficult to propose the probable formula for MOF-808 using TG itself.
and elemental analysis due to overlapping of the degradation
temperature range of OH and formate groups in the thermo-
gravimetric analysis. In previous reports, formate groups were
easily replaced by other groups and were considered the main sites
7
8, 79
for activation and functionalization of MOF-808.
Morphological
and X-ray diffraction study suggested that no phase transition or
structure collapse occurred even after five recycle tests, and this
observation is in line with the high chemical stability reported for
8
0
MOF-808 in previous studies. Further research is in progress to
understand the structural changes after post synthetic modification
of MOF-808. Once again, as with UiO-66(Zr), structural changes in
MOF-808 did not affect the catalytic activity or product selectivity.
3
.5 Reaction Mechanism
Earlier in this paper, we proved that ligand functionality (acid or
base) was not really effective in selective transformation of alkyl
levulinate into GVL, and instead decreased the catalytic activity.
Furthermore, the results obtained from characterization of fresh
and used catalyst also supported the fact that metal
clusters/centers of the Zr-MOF framework were the active sites
producing GVL from EL via CTH.
Scheme 2 Plausible reaction mechanism for CTH of EL to GVL using
Zr-MOFs
8
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Finally, it has been proved that metal cluster in Zr-MOFs are the
active sites in this reaction and that the unique ability of the Zr-
MOFs to undergo structural rearrangement to stabilize its structure
helped to maintain the catalyst activity and selectivity toward GVL.
Higher surface area, pore size of Zr-MOFs and the accessibility of
the reactant molecule to the metal center of MOFs play pivotal
roles in achieving high GVL formation rate and turnover frequency
under moderate reaction conditions.
4
A. Osatiashtiani, A. F. Lee, M. Granollers, D. R. Brown, L.
Olivi, G. Morales, J. A. Melero and K. Wilson, ACS Catal.,
2
015,
R. Weingarten, W. C. Conner and G. W. Huber, Energy
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5, 4345.
5
6
5
P. P. Upare, J.-W. Yoon, M. Y. Kim, H.-Y. Kang, D. W. Hwang,
Y. K. Hwang, H. H. Kung and J.-S. Chang, Green Chem., 2013,
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M. S. Holm, S. Saravanamurugan and E. Taarning, Science,
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2
8
9
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1
1
0 D. M. Alonso, S. G. Wettstein and J. A. Dumesic, Green
Chem., 2013, 15, 584.
1 W. Luo, M. Sankar, A. M. Beale, Q. He, C. J. Kiely, P. C.
In summary,
a series of crystalline porous Zr-MOFs were
synthesized and tested for CTH of EL via MPV reduction using
isopropanol as hydrogen donor. The roles of the metal center and
ligand functionality of UiO-66(Zr) were determined in order to
Bruijnincx and B. M. Weckhuysen, Nat. Commun., 2015,
540.
2 J. P. Lange, R. Price, P. M. Ayoub, J. Louis, L. Petrus, L. Clarke
6,
6
1
understand the origin of the active sites. UiO-66(Zr) showed high
and H. Gosselink, Angew. Chem. In. Ed., 2010, 49, 4479.
o
chemical stability and catalytic activity at high temperature (200 C). 13 J. Q. Bond, A. A. Upadhye, H. Olcay, G. A. Tompsett, J. Jae, R.
Xing, D. M. Alonso, D. Wang, T. Zhang, R. Kumar, A. Foster, S.
Therefore, it appears a good potential catalyst for high temperature
M. Sen, C. T. Maravelias, R. Malina, S. R. H. Barrett, R. Lobo,
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2011, 4, 1749.
1
1
(
open system using the solvent reflux method. Both UiO-66(Zr) and
MOF-808 catalysts could be recycled at least five times under the
specified reaction conditions without loss of catalytic activity. The
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1
1
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transfer hydrogenation of EL at high and low temperature will pave
the way for a detailed exploration of these catalysts for the MPV
reduction of various substrates in biorefinery and organic synthesis.
2
2
2
2
,
3
Acknowledgements
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