Catalytic conversion of furfuryl alcohol or levulinic acid into alkyl levulinates using a sulfonic acid-functionalized hafnium-based MOF

Article abstract of DOI:10.1016/j.catcom.2019.03.003

Biomass conversion using reusable solid acid catalysts are highly desirable to comply with the principles of green chemistry. Here, we report a sulfonic acid-functionalized hafnium-based metal-organic framework (MOF), UiO-66(Hf)-SO₃H, as an efficientsolid acidcatalyst for the alcoholysis of furfuryl alcohol (FA) andesterification of levulinic acid (LA) affording alkyl levulinates (ALs). Among the as prepared UiO-66 based MOFs(UiO-66(Hf), UiO-66(Hf)-NH₂, UiO-66(Hf)-SO₃H and UiO-66(Zr)-SO₃H), UiO-66(Hf)-SO₃H holds highest Br?nsted acidity and therefore exhibits excellent catalytic activity towards production of ALs. The highest Br?nsted acidity in UiO-66(Hf)-SO₃His the result of the covalently bound sulfonic acid groups present inorganic linkers along with the ligated hydroxyl groups (Hf-μ₃-OH) to the Hf metal clusters.

Full text of DOI:10.1016/j.catcom.2019.03.003

Accepted Manuscript
Catalytic conversion of furfuryl alcohol or levulinic acid into alkyl
levulinates using a sulfonic acid-functionalized hafnium-based
MOF
Shyam Sunder R. Gupta, Mannepalli Lakshmi Kantam
PII:
S1566-7367(19)30070-6
DOI:
https://doi.org/10.1016/j.catcom.2019.03.003
CATCOM 5641
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
27 November 2018
26 February 2019
4 March 2019
Please cite this article as: S.S.R. Gupta and M.L. Kantam, Catalytic conversion of furfuryl
alcohol or levulinic acid into alkyl levulinates using a sulfonic acid-functionalized
hafnium-based
MOF,
Catalysis
Communications,
https://doi.org/10.1016/
j.catcom.2019.03.003
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ACCEPTED MANUSCRIPT
Catalytic conversion of furfuryl alcohol or levulinic acid into alkyl
levulinates using a sulfonic acid-functionalized hafnium-based MOF
*
Shyam Sunder R. Gupta and Mannepalli Lakshmi Kantam lk.mannepalli@ictmumbai.edu.in
Department of Chemical Engineering, Institute of Chemical Technology, Matunga (E),
Mumbai - 400019, Maharashtra, India.
*
Corresponding author.

ACCEPTED MANUSCRIPT
Abstract
Biomass conversion using reusable solid acid catalysts are highly desirable to comply with
the principles of green chemistry. Here, we report a sulfonic acid-functionalized hafnium-
based metal-organic framework (MOF), UiO-66(Hf)-SO H, as an efficientsolid acidcatalyst
3
for the alcoholysis of furfuryl alcohol (FA) andesterification of levulinic acid (LA) affording
alkyl levulinates (ALs). Among the as prepared UiO-66 based MOFs(UiO-66(Hf), UiO-
6
6(Hf)-NH , UiO-66(Hf)-SO H and UiO-66(Zr)-SO H), UiO-66(Hf)-SO H holds highest
2 3 3 3
Brønsted acidity and therefore exhibits excellent catalytic activity towards production of ALs.
The highest Brønsted acidity in UiO-66(Hf)-SO His the result of the covalently bound
3
sulfonic acid groups present inorganic linkers along with the ligated hydroxyl groups (Hf-μ3-
OH) to the Hf metal clusters.
Keywords: Furfuryl alcohol; Alkyl levulinates; Levulinic acid; Hafnium-based; metal-organic
framework.

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1
.Introduction
Rapid consumption of fossil resources and environmental concerns over carbon footprint
have triggered global research initiatives in the field of catalytic conversion of biomass into
chemicals[1]. Development of highly efficient and cost-effective catalytic processes for
biomass conversions are desirable in order to replace fossil-based fuels and chemicals with
biomass derived fuels and chemicals[2,3]. Alkyl levulinates (ALs) have been attractive due
to their versatile applicationssuch as a greener solvents, fuel & food additives and renewable
starting materials[4]. The unique properties of ALs such as high octane number, flash point
stability and lower toxicity make them ideal oxygenates for blending in gasoline[5].Along
withhigh octane number,the greater solubility of levulinate ester in gasoline or diesel is
desirable as less soluble alkyl levulinatescan separate from gasoline or diesel at cold
temperatures. Levulinate esters of higher molecular-weight alcohols have better solubility in
gasoline and diesel[4,5]. In this regard, isopropyl levulinate (IPL) having highoctane number
and high solubility in gasoline and dieselbecomes the choice of material to be used as fuel
additive.ALs can be obtained from various biomass-based compounds (Fig. S1 ESI). In
general, acid catalyzed esterification of LAproduces ALs [6]. However, considering the high
cost of LA, commercial production of ALs utilizing cheaply available starting materialssuch
as cellulose [7] and FA [8]is desirable.
Earlier research for the production of ALs involved the use of strong mineral acids such
as HCl and H SO [9],however, the associated problems such as reactor corrosion, catalyst
2
4
separation and neutralization limit their applications. Considering the environmental aspects,
solid Brønsted acid catalyzed ring opening reaction of FA in alcohols has become the
meaningfull approach to produce ALs. In this view,various solid acid catalysts such as
zeolites [10], metal supported silicates [11], metal oxides [12], heteropoly acids [13],
sulphated zirconia [14] and sulfonic acid functionalized materials including graphene [15],
silica[16] and carbon [17]have been developed reporting moderate to excellent catalytic
activity in production of ALs.
The unique properties of MOFssuch as exceptional porosity, high thermal and chemical
stability andability to undergo post synthetic modificationsmake them promising materials for
heterogeneous catalysis [18,19]. Brønsted acidity in MOFs can be introduced by various
methods[20]. The use of sulfonated ligands has been the most popular approach to introduce
the -SO H group in MOFs. One of the most successful examples is MIL-101-SO H,
3
3

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demonstrated for cellulose hydrolysis [21], and alcoholysis of FA[22]. However, the harsh
synthetic conditions and severe toxicity of chromium reagent greatly limit their
applications.Hence, -SO H functionalized Zr-based MOFshave received considerable
3
attention in biomass conversions [23]. Hf and Zr being chemical twins, share similar
properties. The, Hf-based MOFs exhibit exceptional thermal and chemical stability [24,25].
Brønsted acidity in Hf-based MOFs can be incorporated using sulfonated ligands during their
synthesis and the presence of Hf-μ -OH groups provides additional Brønsted acidity due to
3
the higher dissociation energy of Hf-O bond (802 kJ/mol) as compared to Zr-O bond (766
kJ/mol) [26]. Recently, Hu et al. [27] have reported the Hf-based MOF for the conversion of
fructose into 5-hydroxymethylfurfural (HMF).
Herein, for the first time we report direct hydrothermally prepared sulfonic acid-
functionalized UiO-66(Hf)-SO H, as an efficient catalyst for the conversion of FA or LA into
3
ALs. Due to the presence of both organic and inorganic Brønsted acidic sites, UiO-66(Hf)-
SO H shows superior performance than that of UiO-66(Zr)-SO H towards the production of
3
3
ALs.Reaction conditions and activity of the present catalytic system have been compared
with other reported catalysts for alcoholysis of FA and the results are summarized in Table
S1 (ESI).
2
. Experimental
The detailed information regarding materials, methods, catalyst preparations and
procedures for alcoholysis of FA and esterification of LA are provided in ESI. The graphical
representation of the hydrothermal synthesis of UiO-66(Hf)-SO H is shown in Fig. 1.
3
Fig. 1. Hydrothermal synthesis of UiO-66(Hf)-SO H.
3

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3
. Results and Discussion
3
.1 Catalyst characterization
The crystallinity of the as prepared MOFs was initially examined by powder XRD. The
PXRD patterns (Fig. S2, ESI) of UiO-66(Hf), UiO-66(Hf)-NH , UiO-66(Hf)-SO H and UiO-
2
3
6
6(Zr)-SO H bear close similarity to the earlier reports [25,27]. The sharp XRD peaks at 2θ
3
°
°
°
°
value of 3.34 , 5.54 , 7.72 and 8.88 (Fig. S2 d) suggest for the cubic crystal structure of
UiO-66(Hf)-SO H. As inthe earlier report [27], UiO-66(Hf)-SO H and UiO-66(Zr)-SO H are
3
3
3
isostructural
having
molecular
formula
of
C O S M H {[M O (OH) L] .xH O},where M is either Hf or Zr. Thus, XRD results
120 291 15 21 386
6
4
8
3.5
2
confirm the successful formation of highly crystalline MOFs.
The chemical analysis of UiO-66(Hf)-SO H was studied by XPS (Fig. S3).The surveys
3
scan (Fig. S3a) containing peaks corresponding to Hf, S, C and O element only, indicates the
high purity of UiO-66(Hf)-SO H. Absence of peak corresponding to Na elementsignifies the
3
complete protonation of the ligand during the synthesis of MOF. The formation of Hf-O
chemical bond between Hf metal clusters and organic linkers was confirmed fromthe peaks
presentat 225 eV and 214.5 eV (Fig. S3b) assigned to the Hf 4d5/2 and Hf 4d3/2 [28]. The Hf
4
f peaks appeared at 20.7 eV and 19.4 eV and were assigned to Hf 4f5/2 and Hf 4f7/2 (Fig.
S3 c). The Hf 4f peaks in UiO-66(Hf)-SO H appeared at higher binding energies (Fig. S3c)
3
as compared to the Hf 4f peaks in HfO (Hf 4f5/2 at 18.5 eV and Hf 4d7/2 at 16.9 eV),
2
indicating thestrong Lewis acidity of UiO-66(Hf)-SO H [29]. The S 2p high-resolution scan
3
(Fig. S3d) shows two peaks at 170.5 eV (S 2p1/2) and 169.6 eV (S 2p3/2) consistent with the
presence of -SO H groups in UiO-66(Hf)-SO H.
3
3
The presence of completely protonated -SO H groups in UiO-66(Hf)-SO H was further
3
3
-1
-1
confirmed by FTIR analysis (Fig. S4). The FTIR bands at 1230 cm and 1180 cm (Fig.
S4c,d) can be attributed to the symmetric and asymmetric stretching vibrations of O=S=O,
respectively [30].
The FE-SEM images (Fig. S5) and TEM images (Fig. S6a,b) reveal the polyhedral
morphology of UiO-66(Hf)-SO Hhaving average particle size of 1 μm. HR-TEM analysis
3
(Fig. S6c,d) indicate the high crystallinity of the material with lattice spacing of ~0.3 nm
representing the (110) crystal plane of UiO-66(Hf)-SO H.
3

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Elemental composition and distribution of the as-synthesized UiO-66(Hf)-SO were
3
studied by HAADF-STEM.. The mapping result (Fig. S7 and Fig. S8, ESI) confirmed the
uniform distribution of elements throughout the crystal.
The N physical adsorption/desorption isotherms (Fig. S9, ESI) indicate the high porosity
2
2
-1
of UiO-66(Hf)-SO H, possessing a BET surface area of 510 m g . The total pore volume of
3
-3 -1
the material was 0.64 cm g .
The high thermal stability of UiO-66(Hf)-SO H was confirmed byTG analysis. TGA
3
result (Fig. S10, ESI) shows weight loss of the material in two steps. The first weight loss by
°
about 10% in the temperature range of 50-250 C could be possibly due to the removal of
residual solvents and Hf-μ -OH groups from the MOF. The second weight loss by about 25%
3
°
above 400 C can be assigned to the collapse of framework structure due to the degradation of
organic matter.
The Brønsted acid concentrations in various as synthesised MOFs were obtained by acid-
base titration experiment and results are summarised in Table 1. Saturated aqueous solution
of NaCl was used for ion-exchange to release the acidic protons from MOFs in the
solution,which was subsequently titrated with 0.1 M NaOH to quantify the acidic protons
exchanged out in the previous step. Among the as synthesised MOFs, UiO-66(Hf)-SO H
3
exhibits highest Brønsted acidity due to the higher concentration of acidic protons (14.3
mol)per mole of MOF.
3
.2 Catalytic conversion of furfuryl alcohol into alkyl levulinates
.2.1 Screening of the as-prepared catalysts
3
The presence of completely protonated -SO H groups suggest that UiO-66(Zr)-SO H and
3
3
UiO-66(Hf)-SO H should be the efficient solid acid catalyst for alcoholysis of FA (Scheme
3
S1). The catalytic activity of as-prepared UiO-66-based MOFs was evaluated towards the
alcoholysis of FA to isopropyl levulinate (IPL) using isopropyl alcohol, and the obtained
results are given in Table 1. Initially, UiO-66(Hf) was used as a catalyst which afforded 8%
yield of IPL (Table 1, entry 1), whereas no formation of IPL was observed when UiO-66(Hf)-
NH was used as a catalyst (Table 1, entry 2). These results clearly indicate the catalytic
2
inactivity of Lewis acidic as well as Lewis basic sites towards the alcoholysis of FA. It was
interesting to observe that the use of UiO-66(Zr)-SO H as a catalyst provided 52% yield of
3

ACCEPTED MANUSCRIPT
IPL with 77% conversion of FA (Table 1, entry 4). This is because of the Brønsted acidity in
UiO-66(Zr)-SO H arising from the dangling -SO H groups. The reaction afforded 88% yield
3
3
of IPL with quantitative conversion of FA when UiO-66(Hf)-SO H was used as the catalyst.
3
This can be attributed to the presence of stronger Brønsted acidity in UiO-66(Hf)-SO H,
3
which is also confirmed by the acid-base titration experiment. The acid-base titration results
(
Table 1) reveal the higher concentration of acidic protons in UiO-66(Hf)-SO H (14.3 mol) as
3
compared to the UiO-66(Zr)-SO H (13.1 mol) per mole of the MOFs. The higher
3
concentration of Brønsted acidity in UiO-66(Hf)-SO H might be contributed tothe ligated
3
hydroxyl groups (Hf-μ -OH) besides the dangling -SO H groups inthe organic ligands [27].
3
3
Another reaction without any catalyst under similar conditions provided negligible yield of
IPL (Table 1, entry 5), indicating the role of the catalyst. Hence, it is concluded that UiO-
6
6(Hf)-SO H with highest Brønsted acidity among the as-prepared MOFs is the most active
3
catalyst for the alcoholysis of FA.
Table 1.Screening of various UiO-66-based catalysts for the conversion of FA into isopropyl
a
levulinate.
b
c
d
Entry
Catalyst
Mole of acidic proton
per mole of MOF
FA
IPL
FE
Conversion
Yield
Yield
+
-1
[
H ]/mol mol
(%)
13
5
(%)
8
(%)
1
2
3
4
5
UiO-66(Hf)
0
0
-
-
UiO-66(Hf)-NH2
-
UiO-66(Hf)-SO H
14.3
13.1
-
100
77
6
88
52
-
-
3
UiO-66(Zr)-SO H
23
-
3
-
a
Reaction conditions: FA, 1 mmol; catalyst, 100 mg; isopropyl alcohol, 20 mL; temperature,
20 C; reaction time, 4h.
°
1
b
c
d
FA, furfuryl alcohol. IPL, isopropyl levulinate. FE, furfuryl ether.
3
.2.2 Effect of catalyst loading
The effect of UiO-66(Hf)-SO H loading was studied and results are presented in Fig.
3
2
.By decreasing the loading of UiO-66(Hf)-SO H to 0.4 mol %resulted in lowering the
3

ACCEPTED MANUSCRIPT
conversion of FA (87%) and IPL yield (47%),possibly due to the reduced catalytic sites
availablefor the reaction. Increased catalyst loading afforded higher yield of the product, and
maximum yield of IPL (88%) was obtained using 0.8 mol % of catalyst. Further increasing
the catalyst loading to 1.0 mol % resulted into a slightly decrease in the yield of IPL (83%)
due to the side reactions.
Fig. 2. Effect of catalyst loading on alcoholysis of furfuryl alcohol. Reaction Conditions: FA,
°
1
mmol; UiO-66(Hf)-SO H, 0.4 – 1.0 mol %; isopropyl alcohol, 20 mL; temperature, 120 C;
3
reaction time, 4h.
3
.2.3 Effect of reaction temperature
The reaction temperature (Fig. 3) has significant role inthe alcoholysis of FA. At the
°
lowest temperature studied, that is 90 C,a 92% conversion of FA was achieved
corresponding to a 65% yield of IPL along with 25% yield of FE. Increasing the reaction
°
temperature to 100 C, complete conversion of FA was seen with 73% yield of IPL, whereas
the yield of FE was dropped to 12%, result which indicates the consumption of FE during the
progress of reaction. Further increase in temperature resulted into an increased yield of IPL
and afurther decrease in the yield of FE. The maximum yield of IPL (88%) was achieved at
°
1
20 C with negligible amount of FE indicating that the FE is the dominant intermediate in
alcoholysis of FA in our case and elsewhere[22,31].

ACCEPTED MANUSCRIPT
Fig. 3. Effect of reaction temperature on alcoholysis of furfuryl alcohol. Reaction Conditions:
FA, 1 mmol; UiO-66(Hf)-SO H, 0.8 mol %; isopropyl alcohol, 20 mL; temperature, 90-130
3
°
C; reaction time, 4h.
3
.2.4 Effect of reaction time
As summarized in Fig. S11, reaction monitoring after 1h shows 85% conversion of FA
with 12% of IPL and 64% of FE yields. Continuing the reaction for another 1h provided 92%
conversion of FA, the yield of IPL was increased to 47%, whereas the yield of FE was
decreased significantly to 40%, indicating the consumption of FE during the course of the
reaction. The complete conversion of FA was achieved after 3h providing 72% yield of IPL
along with 23% yield of FE. The reaction after 4h revealed complete consumption of FE
affording the maximum yield of IPL,ca. 88%. Hence, these results suggest the conversion of
FA into FE as an intermediate, which further gets converted into IPL successively as reaction
time increases.
3
.2.5 Preparation of different alkyl levulinates
To establish the general applicability of UiO-66(Hf)-SO H as an efficient solid acid
3
catalyst for the production of ALs, a range of different ALs have been prepared fromthe

ACCEPTED MANUSCRIPT
alcoholysis of FA using various aliphatic alcohols under optimized reaction conditions. As
presented in Table 2, alcoholysis of FA in methanol as well as in ethanol produced the
corresponding ALs in lower yields, 48% and 62%, respectively, due to the formation of large
amount of intermediates (B and C in Fig. S14),which slowlygets converted into product. The
alcoholysis of FA in propanol resulted in suppressed formation of intermediates, affording
7
6% and 88% yield of desired products in 1-propanol and 2-propanol, respectively. It was
observed that alcohols having longer carbon chain lengths (4, 5 and 6 carbons) provided a
lower yield of the corresponding products. The presence of large carbon chain might have
created the steric crowding near the hydroxyl group of alcohol, which probably reduced their
efficient nucleophilic attack during the progress of reaction.
a
Table 2.Conversion of furfuryl alcohol into different alkyl levulinates.
b
Entry
Alcohol
methanol
ethanol
Product
Yield (%)
1
2
3
4
5
6
7
8
methyl levulinate
ethyl levulinate
48
62
76
88
72
66
64
57
1-propanol
2-propanol
1-butanol
2-butanol
1-pentanol
1-hexanol
1-propyl levulinate
2-propyl levulinate
1-butyl levulinate
2-butyl levulinate
1- pentyl levulinate
1- hexyl levulinate
a
Reaction conditions: FA, 1 mmol; UiO-66(Hf)-SO H, 0.8 mol %; alcohol, 20 mL;
3
°
temperature, 120 C; reaction time, 4h.
b
Yield was calculated by GC-MS.
3
.3 Catalytic conversion of levulinic acid into alkyl levulinates
The catalytic activity of UiO-66(Hf)-SO H was also studied towards the esterification of
3
LA (Scheme S2, ESI). Optimization of various reaction parameters provided 86% yield of the
desired product with 93% conversion of LA at 120 °C in 2h using 0.8 mol % of the catalyst
(Table 3, entry 2). As summarized in Table 3, different alkyl levulinates were obtained in
good yields from the esterification of LA in various alcohols using UiO-66(Hf)-SO H as a
3

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catalyst. These results indicate the wide applicability of UiO-66(Hf)-SO H as an efficient
3
solid Brønsted acid catalyst.
a
Table 3. Catalytic conversion of levulinic acid into alkyl levulinates.
b
Entry
Solvent
Temperature
°C)
Time
(h)
2
LA Conversion
AL Yield
(%)
65
(
(%)
78
93
93
86
94
90
87
88
1
2
3
4
5
6
7
8
methanol
methanol
methanol
methanol
methanol
ethanol
100
120
140
120
120
120
120
120
2
86
2
87
1.5
2.5
2
73
85
85
1-propanol
2-propanol
2
78
2
80
a
Reaction conditions: LA, 1 mmol; UiO-66(Hf)-SO H, 0.8 mol %; alcohol, 20 mL;
3
°
temperature, 120 C; reaction time, 2h.
b
Yield was calculated by GC-MS.
3
.4 Catalyst reusability
Besides the good catalytic activity, reusability is also an important factor considered
while catalyst selection, especially in industry. The reusability of UiO-66(Hf)-SO H was
3
demonstrated by recycling the catalyst for four times in the production of IPL from the
alcoholysis of FA. After each catalytic run, the catalyst was isolated by centrifugation and
°
washed three times with isopropyl alcohol and dried under vacuum at 100 C for 12h and then
used for the next run. Isolation of the catalyst from reaction mixture by centrifugation
provided the maximum recovery of the catalyst (90 mg) with 10 mg loss of the catalyst after
recycling for four times. The reusability of UiO-66(Hf)-SO H was also studied towards
3
esterification of LA. As represented in Fig. 4, the yield of IPL during alcoholysis of FA was
decreased from 88% to 73% after the fourth recycle of UiO-66(Hf)-SO H. The slight loss in
3
catalytic activity can be attributed to the adsorption of some oligomerized products on the

ACCEPTED MANUSCRIPT
catalyst surface [31,32]. The catalyst UiO-66(Hf)-SO H exhibited excellent catalytic
3
recyclability towards esterification of LA (Fig. 4).The XRD (Fig. S12, ESI) and FEG-SEM
analysis (Fig. S13, ESI) of UiO-66(Hf)-SO H catalyst recycled for four times shows retention
3
of its crystallinity and polyhedral morphology, respectively. These results suggest high
stability of the UiO-66(Hf)-SO H catalyst during recycling towards alcoholysis of FA.
3
Fig. 4. Reusability of UiO-66(Hf)-SO H.
3
3
.5 Plausible reaction mechanism
On the basis of the identified intermediates and earlier reports [13,17,31], a plausible
reaction mechanism for the alcoholysis of FA using UiO-66(Hf)-SO H as a catalyst has been
3
proposed and shown in Fig. S14. The detailed discussions have been provided in ESI.
4
. Conclusions
The two sulfonic acid-functionalized MOFs, UiO-66(Hf)-SO H and UiO-66(Hf)-SO H
3
3
wereprepared by direct hydrothermal synthesis and used for the alcoholysis of FA to produce
ALs. Among the as-prepared sulfonic acid-functionalized MOFs, UiO-66(Hf)-SO H
3
exhibited excellent catalytic performance providing 88% of IPL with quantitative conversion
of FA at 120 °C in 4h. The superior catalytic activity of UiO-66(Hf)-SO H was attributed to
3
its stronger Brønsted acidity resulting from the ligated hydroxyl groups of hafnium metal
clusters besides the presence of sulfonic acid groups. Furthermore, the UiO-66(Hf)-SO H can
3
be used as an efficient Brønsted acid catalyst for esterification of LA to afford ALs. Our

ACCEPTED MANUSCRIPT
results have demonstrated the efficient production of alkyl levulinates from biomass derived
renewable and abundant molecules.
Acknowledgements
Authorsare greatly thankful to GCPL and DST, Government of India for providing the
financial support.

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Highlights




Efficient production of alkyl levulinates using a reusable solid acid catalyst.
Sulfonic acid-functionalized MOF, UiO-66(Hf)-SO H holds strong Brønsted acidity.
3
Alcoholysis of furfuryl alcohol and esterification of levulinic acid.
UiO-66(Hf)-SO H shows excellent catalytic activity and recyclability.
3