Utilization of renewable resources: Investigation on role of active sites in zeolite catalyst for transformation of furfuryl alcohol into alkyl levulinate

Article abstract of DOI:10.1016/j.mcat.2020.111361

A bio-derived furfuryl alcohol transformation into various high-value chemicals is a growing field of interest among researchers. This study reports an exclusive investigation of the porosity and active sites responsible for the efficient alcoholysis of furfuryl alcohol to alkyl levulinate by the aid of zeolite catalyst. Alkyl levulinate is a promising platform chemical potentially used as a fuel additive and also for the production of chemicals. A detailed study using well-characterized HZSM-5 catalyst on the influence of acidity and post synthesis modification like desilication, dealumination, metal ion exchange and phosphate modification revealed the most desired type of acid sites required to catalyze this reaction. Among the HZSM-5 catalysts tested, HZSM-5 (SAR 95) showed the best performance of ≥ 99 % furfuryl alcohol conversion and 85 % butyl levulinate selectivity under optimum conditions. The catalyst exhibited good recyclability additionally addressing all the challenges reported in the previous literature fulfilling the green chemistry principles.

Full text of DOI:10.1016/j.mcat.2020.111361

Molecular Catalysis 502 (2021) 111361
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Utilization of renewable resources: Investigation on role of active sites in
zeolite catalyst for transformation of furfuryl alcohol into alkyl levulinate
a
,
b
a
a
a
B.J. Vaishnavi , S. Sujith , Nagendra Kulal , Pandian Manjunathan ,
a
,
Ganapati V. Shanbhag *
a
Materials Science and Catalysis Division, Poornaprajna Institute of Scientific Research, Bidalur, Devanahalli, Bengaluru, 562 164, India
b
Graduate Studies, Manipal Academy of Higher Education (MAHE), Manipal, 576104, Karnataka, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
A bio-derived furfuryl alcohol transformation into various high-value chemicals is a growing field of interest
among researchers. This study reports an exclusive investigation of the porosity and active sites responsible for
the efficient alcoholysis of furfuryl alcohol to alkyl levulinate by the aid of zeolite catalyst. Alkyl levulinate is a
promising platform chemical potentially used as a fuel additive and also for the production of chemicals. A
detailed study using well-characterized HZSM-5 catalyst on the influence of acidity and post synthesis modifi-
cation like desilication, dealumination, metal ion exchange and phosphate modification revealed the most
desired type of acid sites required to catalyze this reaction. Among the HZSM-5 catalysts tested, HZSM-5 (SAR
Biomass
Furfuryl alcohol
Alcoholysis
Br o¨ nsted acidic catalyst
ZSM-5
Alkyl levulinate
9
5) showed the best performance of ≥ 99 % furfuryl alcohol conversion and 85 % butyl levulinate selectivity
under optimum conditions. The catalyst exhibited good recyclability additionally addressing all the challenges
reported in the previous literature fulfilling the green chemistry principles.
1
. Introduction
The present era of globalization is leading to the exhaustion of the
biomass for manufacturing commodity chemicals, fuels and fuel addi-
tives has gained a lot of consideration in recent years. Among the various
chemicals obtained from lignocellulosic biomass, such as., glycerol,
lactic acid, serine, aspartic acid, threonine, sorbitol, levoglucosan,
xylitol, arabinitol, etc., furfural is projected as one of the 30 potential
candidates as building blocks that would transform into multiple func-
tionality chemicals [1].
conventional fossil fuels which would diminish the chance of meeting
the energy demands for the future. The impact of the oil on mankind and
its disastrous effects such as an increase in oil price, carbon footprint,
foreign oil dependency, and disturbing world peace is evident. Despite
its applications in daily life, extraction by mining and drilling itself has a
lot of detrimental effects on our ecosystem such as landslides, flash
floods, water pollution and greenhouse gas emissions. Due to the
decrease in the reserves of these non-renewable resources, there is a
serious necessity to switch into renewable resources which are abun-
dant, carbon-neutral and environmentally friendly. Apparently, as the
oil well drains up, not only the production of transportation fuels is
affected, but also the petrochemicals manufactured from crude oil and
its derivatives. There are excellent alternatives for fuels and energy
production which can replace fossil fuels, for instance, solar energy,
wind energy, nuclear energy, biomass, etc. However, these resources
chosen should also effectively compete with the existing technology to
produce chemicals. Apart from fossil fuels, the only two sustainable,
Furfuryl alcohol produced by chemoselective hydrogenation of
furfural is an abundant, C-5 sustainable platform chemical used for the
synthesis of high-value products like lysine, methyl furan, ascorbic acid,
levulinate esters, angelica lactones, γ-valerolactone, lubricants, resins,
plasticizers, fragrances and adhesives [2,3]. Annually 62 % of the
globally produced furfural (approximately 200 000 tonnes) is consumed
for the synthesis of furfuryl alcohol due to its expanding application
profile and increasing market value. Alcoholysis of furfuryl alcohol
yields alkyl levulinate (Scheme 1) that has potential applications as
biofuels and, additives in flavor and fragrance products [4]. As alkyl
levulinates are traditionally produced by the esterification of levulinic
acid, an expensive chemical, alcoholysis of furfuryl alcohol serves as an
inexpensive route which also promotes the bio-based economy. This
reaction utilizing furfuryl alcohol is challenging as the intermediate
2
renewable carbon sources are biomass and CO . Therefore, exploiting
*
Corresponding author.
E-mail address: shanbhag@poornaprajna.org (G.V. Shanbhag).
https://doi.org/10.1016/j.mcat.2020.111361
Received 27 August 2020; Received in revised form 9 December 2020; Accepted 16 December 2020
Available online 11 January 2021
2
468-8231/© 2020 Elsevier B.V. All rights reserved.

B.J. Vaishnavi et al.
Molecular Catalysis 502 (2021) 111361
2
-(alkoxy methyl) furan (AMF), an ether, formed is to be successfully
series of conventional solid acid catalysts were tested which included
ordered mesoporous aluminosilicate, ion-exchange resin, silicoalumi-
nophosphate, medium and large pore zeolites. Zeolite ZSM-5 was stud-
ied in detail to understand the relation between its physicochemical
properties and the alcoholysis reaction. To discover the desired type of
acid site for this reaction, post-modification of ZSM-5 was performed by
desilication, dealumination, phosphate modification and metal ion ex-
change. The physicochemical properties of all the catalysts were inves-
tigated using various characterizations such as XRF, PXRD, nitrogen
converted into alkyl levulinate (keto ester) which is difficult compared
to carboxylic acid route (levulinic acid). Hence alcoholysis of furfuryl
alcohol is a greener approach to yield alkyl levulinate which helps to
understand and explore the reaction.
For the liquid phase butanolysis of furfuryl alcohol to butyl levuli-
nate, various solid acid catalysts have been reported which includes,
graphite oxide (GO) and reduced graphite oxide (rGO) catalysts [5],
lignin‑based carbonaceous acid [6] modified SBA-15 [7–9], titanium
exchanged mesoporous silica [10], modified ionic liquids [11,12], ionic
liquids [13], zinc exchanged heteropoly tungstate supported on niobia
3
adsorption-desorption isotherms, NH -TPD and SEM. Reaction param-
eters such as catalyst concentration, reactants mole ratio and tempera-
ture were studied using the catalyst with the best performance. To
evaluate the reusability of the potential catalyst, the material was
screened for multiple cycles at the optimized reaction conditions.
[
14], hematite [15], functionalized fibrous silica, [16], sulfonic acid
functionalized TiO nanotubes [17], tin exchanged tungstophosphoric
2
acid and tin phosphate [18,19]. From these reports, it is understood that
the Br o¨ nsted acid site plays a major role in the selective synthesis of alkyl
levulinate. However, many of these reported catalysts have not been
studied reusability or failed to retain their catalytic activity upon recy-
cling [5,9,12,13,15,17,6]. The catalyst deactivation is mainly because of
the accumulation of oligomeric products formed due to the polymeri-
zation of furfuryl alcohol on the active sites of the catalyst. Hence, high
thermal stability should be one of the important virtues of the desired
catalyst for this reaction as the catalyst regeneration at high tempera-
tures is easily achievable. Some catalysts also reportedly gave good
catalytic performance only in the presence of excessive usage of catalyst
or reactants. [7,10–12,14,15,17,18,19] One promising catalyst that can
overcome all these drawbacks is zeolite owing to its high surface area,
strong Br o¨ nsted acidity, ordered microporosity, high thermal stability
and better recyclability.
2. Experimental section
2.1. Materials
ZSM-5 with different SiO to Al O ratio (SAR), mordenite (SAR20),
2
2 3
H-Beta (SAR25), were obtained from Nankai University Catalyst Co.
China. ZSM-5 (SAR22) and Y-Zeolite (SAR 5.1) were procured from
Zeolyst International. Furfuryl alcohol, ammonium dihydrogen phos-
phate (NH H PO ), copper nitrate trihydrate, zinc nitrate hexahydrate,
4
2
4
tetraethyl orthosilicate, concentrated HCl, concentrated H SO ,
2
4
concentrated H PO ammonium acetate, aqueous ammonium hydrox-
3
4,
ide, Al(NO3)3, NaOH, 1-butanol, 1-propanol and methanol were pur-
chased from Merck India Pvt. Ltd. Amphiphilic triblock co-polymer
poly-(ethylene glycol)-block poly-(propylene glycol)-block poly-
(ethylene glycol), ludox, fumed silica and morpholine were purchased
from Sigma-Aldrich. Tetrapropyl ammonium bromide, zirconium oxy-
chloride octahydrate were purchased from Loba Chemie. Citric acid and
ethanol were procured from Otto biochemical reagents and CSS
respectively. Amberlyst-15 was obtained from Alfa-Aesar. Plural SB
(pseudoboehmite) was procured from Sasol.
There are a few reports, where zeolite based catalysts have been
investigated for this transformation. Lange et al. explored a range of acid
catalysts such as H
2
SO
4
, ion-exchanged resins and zeolites in a semi-
◦
batch semi-continuous mode at varied temperatures (125 and 225 C)
for the ethanolysis of furfuryl alcohol. The ZSM-5 (SAR30) was reported
to give 65 mol% ethyl levulinate yield which is the best among the other
zeolites tested (ZSM-5, ZSM-12, ZSM-23, H-Beta, mordenite). However,
zeolites as such were ranked low among the other catalysts [20]. In
another study by Yao-Bing Huang et al., HZSM-5 and H-Beta catalysts
were compared with various metal salt catalysts under microwave
2
.2. Catalyst synthesis
2 4 2
irradiation. The metal salt Al (SO ) , showed 80 % yield for methyl
HZSM-5 catalyst was synthesized from the procedure similar to the
levulinate, whereas HZSM-5 and H-Beta showed 4.9 and 1% yield
respectively [21]. A study on zeolite-based material was reported over
H-ZSM-5, hierarchical zeolite, H-Beta and USY for ethanolysis of furfuryl
alcohol by Nandiwale et al. [22]. The trend for ethyl levulinate yield was
reported one [26]. In a typical synthesis, the required quantity of NaOH,
Al(NO (aluminium source), ludox (silicon source) were added to the
3 3
)
distilled water and stirred well. To this, a template tetra propyl ammo-
nium bromide was added. The solution was transferred to a teflon lined
5
%, 8%, 13 % and 19 % for USY, H-Beta, HZSM-5, hierarchical zeolite
◦
autoclave and placed in an oven for 24 h at 180 C. The material is
respectively. Under optimized reaction conditions, hierarchical zeolite
was found to be the most active catalyst with ethyl levulinate yield of 73
◦
washed filtered, dried and calcined at 550 C. To obtain the ammonium
form of the synthesized and commercially obtained ZSM-5, the material
was subjected to three-fold ammonium exchange using 0.5 M ammo-
%
. Additionally, other literatures are focusing on various catalysts by
comparing their catalytic performance with zeolites [8–10,12,16,
3–25].
From the careful literature survey, we found that zeolite, in general,
◦
◦
nium acetate for 4 h at 80 C. The material was finally calcined at 550 C
2
◦
ꢀ 1
for 5 h at a heating rate of 5 C min to yield the protonic form of the
material [27]. The obtained materials were labelled as HZSM-5.
Other catalysts such as sulfated zirconia, SAPO-34 and Al-SBA-15
were synthesized from the reported literature [28–30]. Post modifica-
tion procedures such as desilication, dealumination, phosphate modifi-
cation, metal ion exchange are provided in ESI Section 2.2.
can be a potential catalyst for alcoholysis of furfuryl alcohol, but a
detailed study over the zeolite to explore its properties to achieve
enhanced performance has not been conducted so far. Moreover, a
thorough study on the zeolite catalyst for this reaction is important to
understand the intricacies of how the structural and textural properties
of the catalyst influence the catalytic reaction. Hence, in this work, a
For recyclability study, the spent catalyst was washed with meth-
◦
anol, filtered, dried and finally calcined at 550 C for 5 h after each
Scheme 1. Conversion of furfuryl alcohol to alkyl levulinate.
2

B.J. Vaishnavi et al.
Molecular Catalysis 502 (2021) 111361
catalyst recycle.
C
BL
S
BL(%) = C
× 100
FA(i) ꢀ CFA(f )
2
.3. Catalyst characterization
Where XFA and SBL are furfuryl alcohol conversion and butyl levulinate
selectivity respectively. CFA(i) and CFA(f) correspond to initial and final
molar concentrations of furfuryl alcohol respectively. CBL is the molar
concentration of butyl levulinate formed in the reaction.
To determine the silica to alumina ratio (SAR) quantitatively, the
catalysts were analyzed using X-ray fluorescence spectroscopy with the
consideration of loss on ignition (LOI) using S4 Pioneer sequential
wavelength-dispersive X-ray spectrometer (Bruker).
PXRD patterns of all the catalysts were recorded using Bruker D2
3
. Results and discussion
phaser X-ray diffractometer with Cu K
α
radiation as a source and
◦
wavelength of radiation λ = 1.542 A with high-resolution Lynxeye de-
3
.1. Characterization of the catalysts
tector. Diffraction patterns were recorded by scanning all the catalysts
◦
◦
ꢀ 1
through the 2θ range of 5–60 (step size of 0.02 s ).
XRF analysis provided the silica to alumina ratio (SAR) of all the
Scanning electron microscope (SEM) micrographs of HZSM-5 were
recorded using Hitachi SU instrument to explore the morphology and
particle size of the catalysts.
commercial and synthesized zeolite catalysts tested for this trans-
formation. The ZSM-5 zeolites with a wide range of SAR from 22 to 160
were chosen for this study.
The nitrogen adsorption-desorption isotherms were obtained using
Belsorp-mini II instrument (Bel, Japan) at liquid nitrogen temperature
All the diffractograms of HZSM-5 with varied SARs reflect the
◦
◦
characteristic pattern of MFI topology with main peaks at 2θ 13 , 23
◦
◦
(
ꢀ 196 C). All the catalysts were degassed at 300 C for 4 h prior to the
◦
and 45 [ESI Fig. S1 (a)] [31]. XRD patterns after the modifications such
as desilication, dealumination, phosphate modification and metal ion
exchange showed no change in the structure [ESI Fig. S1 (b–e)]. Also,
the XRD patterns of SAPO-34, sulfated zirconia, Al-SBA-15, mordenite,
H-Beta, Y-zeolite matched well with the literature. [ESI Fig. S2 (a–f)].
SEM analysis of HZSM-5 (SAR95) was performed to investigate the
morphology and the particle size. The micrographs disclosed that the
material contained spherical morphology with an average particle size
analysis. The specific surface area of the catalysts was determined using
the Brunauer-Emmett-Teller (BET) equation. The pore volume of the
catalysts was obtained using the MP plot.
The acidity of the modified and parent HZSM-5 catalysts was
analyzed using the Belcat II instrument (Bel, Japan) by employing
3
temperature-programmed desorption. NH was used as a probe mole-
cule and the instrument was equipped with a thermal conductivity de-
◦
tector. All the catalysts were pretreated at 550 C for 1 h with the carrier
of 0.5
μ
m (Fig. 1).
◦
gas, helium and cooled to 50 C. The materials are then saturated with
The BET surface area of unmodified HZSM-5 with varied SARs was
ammonia/helium stream for 30 min and are purged with helium for 15
min to remove the physically adsorbed ammonia. The ammonia
desorption was recorded with a temperature program ranging from 50 to
2
ꢀ 1
around 400 m g which confirms its high surface area (Table 1). The
post-modification, by desilication, metal ion exchange, phosphate
modification, led to the decrease in surface area as shown in the Table1.
But in the case of dealumination, there was a negligible increase in
surface area. Also, there was a marginal difference in pore volume
among the catalysts. Sorption studies were performed for all the cata-
lysts tested which showed comparable results with the literature (ESI
Fig. S3, Fig. S4). [28–30]
◦
◦
ꢀ 1
5
50 C with a heating rate of 10 C min . The deconvoluted data for all
the conventional catalysts, ZSM-5 with varied SARs and post modified
ZSM-5 were obtained. The strength of acidity viz. weak, moderate and
◦
◦
strong was calculated at temperatures, <200 C, 200ꢀ 350 C and >350
◦
C respectively.
The Br o¨ nsted and Lewis acid sites for all the post modified ZSM-5
catalysts were investigated by the aid of pyridine adsorption experi-
ments using FTIR instrument (Bruker Alpha-T). The catalyst in the form
The total acidity of HZSM-5 from NH
3
-TPD decreased from 1.06 to
ꢀ 1
0
.28 mmol g with an increase in SAR from 22 to 160 as expected
[
Fig. 2 (a), Table 1]. The strengths of acidity of all the unmodified
◦
of the self-supported wafer was calcined at 550 C and saturated with
HZSM-5 catalysts were dominated by weak and moderate acidic sites.
Among them, SAR 57 and 95 possessed a good number of strong acidic
sites. Sulfated zirconia and Al-SBA-15 possessed acid strength in the
◦
pyridine at room temperature. It was then heated at 150 C for an hour
to remove the physisorbed pyridine. The subtraction of the FTIR spec-
trum after pyridine adsorption with that of untreated catalyst in the
◦
broad range of 50–550 C. Y-zeolite contained a similar number of weak
ꢀ
1
wavenumber range of 1410 –1560 cm gives the peaks only due to
pyridine–acid site interaction.
and moderate acid sites, whereas H-Beta possessed predominantly
strong acid sites. SAPO-34 and Y-zeolite contained mainly weak and
moderate acid sites. [Table 1, ESI Fig. S5 (b–g).]
2
.4. Catalytic activity studies
The treatment of HZSM-5 with 0.1 M NaOH decreased the acidity
ꢀ
1
drastically from 0.59 to 0.29 mmol g which might be due to the
removal of extra framework aluminium along with the desired silicon
atoms. For 0.2 M NaOH treatment, the acidity of the catalyst decreased
In a typical procedure, the alcoholysis of furfuryl alcohol was per-
formed in a liquid phase batch reaction under magnetic stirring at the
desired temperature. The required molar composition of furfuryl alcohol
and butanol, and the catalyst (with respect to the total reactants) were
taken in a 25 mL round bottom flask connected to a condenser. The
reaction of furfuryl alcohol was also conducted with other alcohols with
a similar procedure under reflux conditions. The samples of the reaction
mixture were periodically collected and quantitatively analyzed by gas
chromatography (GC) (Agilent Technologies 7820A) equipped with HP-
ꢀ 1
to 0.38 mmol g but it was still higher than 0.1 M NaOH treated
catalyst. It is because the efficiency of desilication increased with in-
crease in molar concentration of NaOH to 0.2 M, thereby increasing the
acidity by removing more silicon atoms compared to the 0.1 M NaOH.
However, a subsequent increase in the concentration of NaOH treatment
might remove more number of silicon atoms but at the expense of the
framework stability. [32] Hence, the NaOH treatment was stopped to
5
capillary column (0.25 mm I.D., 30 m length) coupled with the flame
0
.2 M concentration. In both the treatments of NaOH, the structural
ionization detector. The identity of the products was confirmed by Gas
chromatography mass spectrometry (GCMS). The furfuryl alcohol con-
version and butyl levulinate selectivity were determined using the
external standard method in the GC.
integrity of HZSM-5 was found to be retained by PXRD indicating that
the zeolite framework was still stable after the removal of silicon atoms.
Upon desilication, the B/L ratio of HZSM-5 decreased from 3.9 to 2.1
and 2.0 for 0.1 M NaOH and 0.2 M NaOH treatment respectively
(Table 1, ESI Fig. S6). This is due to the generation of extra framework
aluminium and also reincorporation of leached Al species resulting in an
increase in Lewis acidity.
FA(%) = CFA(i)
ꢀ CFA(f )
X
× 100
C
FA(i)
3

B.J. Vaishnavi et al.
Molecular Catalysis 502 (2021) 111361
Fig. 1. SEM images of HZSM-5 (SAR95).
Table 1
Physicochemical properties of the catalysts.
2
ꢀ 1 [a]
3
ꢀ 1
[d]
Catalyst
Surface area (m
g
)
Pore Volume (cm
g
)
Acidity
B/L
ꢀ
1 [b]
(
mmol g
)
Weak
Moderate
Strong
Total
Sulfated Zirconia
SAPO-34
78.2
0.12
0.28
0.27
0.88
0.36
0.16
0.21
0.17
0.18
0.19
0.18
0.19
0.18
0.18
0.19
0.19
0.18
0.17
0.15
0.18
0.19
0.03
0.89
–
0.10
0.55
–
0.11
0.24
–
0.25
1.69
–
666.6
40.3
–
[
c]
Amberlyst-15
4.7
–
Al-SBA-15 (SAR35)
Y-Zeolite (SAR5.1)
H-Beta (SAR25)
Mordenite (SAR20)
HZSM-5 (SAR22)
HZSM-5 (SAR57)
HZSM-5 (SAR95)
HZSM-5 (SAR117)
HZSM-5 (SAR160)
DS (0.1 M NaOH)
DS (0.2 M NaOH)
DA (HCl)
677.4
809.5
592.1
498.0
403.7
422.1
418.9
404.6
426.9
364.6
404.8
423.5
421.8
403.1
373.4
323.7
411.6
417.1
0.07
0.67
0.20
0.63
0.53
0.29
0.19
0.20
0.10
0.12
0.16
0.05
0.18
0.17
0.18
0.16
0.11
0.14
0.05
0.66
0.28
0.22
0.13
0.10
0.18
0.08
0.03
0.07
0.10
0.10
0.17
0.14
0.12
0.13
0.18
0.17
0.06
0.26
0.43
0.48
0.39
0.42
0.21
0.16
0.14
0.09
0.12
0.07
0.18
0.11
0.10
0.08
0.16
0.17
0.19
1.6
–
–
0.92
1.34
1.06
0.82
0.59
0.45
0.28
0.29
0.38
0.24
0.53
0.42
0.40
0.37
0.45
0.48
–
–
–
–
3.9
–
–
2.1
2.0
1.9
4.6
4.3
6.8
5.3
0.8
1.2
DA (Citric acid)
1
3
5
% P-ZSM-5
% P-ZSM-5
% P-ZSM-5
Cu-ZSM-5
Zn-ZSM-5
[
3
a] BET surface area, [b] NH -TPD [c] Acid-base titration [d] Pyridine-FTIR.
Dealumination was performed using HCl and citric acid as these
creating more than one P OH sites per Al site. This type of interaction
–
agents act differently in the matrix. The HCl, being a strong acid and
small molecule, caused a decrease in acidity to a higher extent than the
mild citric acid. The acidity drastically decreased from 0.59 to 0.24
of the phosphate group is more predominant for zeolite with closely
positioned acid sites (lower SAR). [35–38]
The B/L ratio increased from 3.9 (HZSM-5) to 4.3 and 6.8 upon
treatment with 1% and 3%P respectively. This shows that Al acid sites
are converted into weaker -Pꢀ OH type acid sites. Further increase in
phosphate to 5% decreased the B/L ratio which could be attributed to
pore narrowing of the zeolite leading to blocking of accessibility to the
pyridine probe molecule. (Table 1, ESI Fig. S6) [27,39]
ꢀ
1
mmolg
for HCl due to its access towards more aluminium sites
throughout the matrix. The B/L ratio decreased from 3.9 to 1.9 due to
the removal of framework aluminium mainly decreasing the Br o¨ nsted
acidity. In the case of citric acid, due to its mild acidity, extra framework
aluminium (Lewis acid site) could be predominantly dealuminated. The
-
1
+
total acidity decreased marginally from 0.59 to 0.53 mmolg , whereas
B/L ratio increased from 3.9 to 4.6 (Table 1, ESI Fig. S6). [33,34]
Phosphate treatment resulted in a decrease in total acidity affecting
Metal ion was exchanged with H of HZSM-5 to induce Lewis acidity
in the zeolite matrix. When copper and zinc ions were exchanged with
the Br o¨ nsted acidic protons of ZSM-5, the Lewis acidity is expected to
◦
ꢀ 1
primarily the strong Br o¨ nsted acid sites (>350 C). Upon an increase in
increase [40]. But the total acidity of HZSM-5 (0.59 mmol g ) was
ꢀ 1
the concentration of phosphate treatment from 1 to 5%, the acidity
found to decrease to 0.48 and 0.45mmolg upon modification with
ꢀ 1
2+
2+
marginally decreased from 0.42 to 0.37 mmolg (Table 1) due to the
generation of weaker phosphate type of acid sites. [27] The strong
Br o¨ nsted acidity decreased from 0.22 to 0.08 mmolg-1 for HZSM-5 upon
an increase in the phosphate treatment from 0 to 5%. The phosphate
Zn and Cu respectively due to the lowering of Br o¨ nsted acidity [ESI
Fig. S5 (a)]. The B/L ratio decreased from 3.9 (HZSM-5) to 1.2 and 0.8
for Zn-ZSM-5 and Cu-ZSM-5 respectively as expected due to the incor-
poration of Lewis acidic metal centers (Table 1, ESI Fig. S6).
treatment creates low strength P
interactions with framework Al sites. Certain interactions can cause the
replacement of two Al acid sites by one P
H Br o¨ nsted acid site
–
O
–
H acidic sites by different types of
In the case of all the modifications on HZSM-5 (SAR95), it is observed
that the strong acid sites were affected the most as there is a shift in the
desorption peak towards lower temperature which is evident from the
O
– –
leading to a decrease in total acidity. For ZSM-5 with higher SAR, the
TPD profiles and acidity values (Table 1). NH
3
-TPD studies were per-
total acidity might increase due to the enhanced acid site spacing, thus
formed for all the catalysts tested which showed comparable results with
4

B.J. Vaishnavi et al.
Molecular Catalysis 502 (2021) 111361
Fig. 2. Temperature programmed desorption profiles of (a) HZSM-5 with varied SAR [a-SAR22, b-SAR57 c-SAR95 d-SAR117 e-SAR160], (b) Desilicated ZSM-5 [a-
HZSM-5(SAR95) b-DS (0.2 M NaOH) c-DS (0.1 M NaOH)] (c) Dealuminated ZSM-5 [a-HZSM-5(SAR95) b-DA (Citric acid), c-DA (HCl)] (d) Phosphate modified ZSM-5
[
a-HZSM-5(SAR95), b-1%P-ZSM-5, c-3%P-ZSM-5, d-5%P-ZSM-5].
the literature [ESI Fig. S5 (b–g)] [28–30].
taken in this study. The HZSM-5 with its unique uniform medium-sized
micropore structure as well as its strong Br o¨ nsted acid character might
have helped in getting good performance for this reaction. The rest of
the catalysts exhibited lower catalytic activity (< 20 %) as shown in
Fig. 3. Mordenite having two channels (channel dimension: 0.70*0.65
and 0.57*0.26) behaves as a one-dimensional pore system as one of the
pores restricts the movement of molecules, lowering its catalytic per-
formance [41]. The low performances of the H-Beta and Y-zeolite show
that the large pore structure may not be suitable for this reaction. H-Beta
gave ≥ 99 % furfuryl alcohol conversion, but the intermediate butoxy
methyl furan (hereafter BMF) conversion into butyl levulinate was low.
The presence of strong acidity in H-beta helped to achieve high con-
version but resulted in high selectivity towards side products; BMF and
5,5-dibutoxy-2-pentanone. On the other hand, Y-zeolite exhibited poor
conversion of furfuryl alcohol itself which may be due to the dominated
presence of weak and moderate number of acid sites. Though SAPO-34
contained mainly weak and moderate acid sites with good surface area,
the presence of weaker phosphate type groups could be the reason for its
low furfuryl alcohol conversion (12 % at 6 h). Al-SBA-15 possessing
3
3
.2. Catalytic activity studies
.2.1. Catalyst screening and comparison
From the prior knowledge on the alcoholysis reaction, it is under-
stood to be purely an acid catalyzed reaction and the Br o¨ nsted acid sites
specifically play a major role in the selective synthesis of alkyl levuli-
nate. Hence, various conventional solid acid catalysts like zeolites,
mesoporous aluminosilicates, silicoaluminophosphate, cation exchange
resin and sulfated zirconia were screened for this reaction. To know the
essential qualities that are required in a catalyst to get maximum ac-
tivity, the catalytic conversion and selectivity were correlated with their
physicochemical properties.
Butanolysis of furfuryl alcohol to yield butyl levulinate was carried
over various solid acid catalysts such as HZSM-5, Y-zeolite, H-Beta,
mordenite, amberlyst-15, sulfated zirconia, SAPO-34 and Al-SBA-15.
Prior to this study, a blank reaction was performed in the absence of
the catalyst which gave a trace level conversion.
Amberlyst-15, a pure Br o¨ nsted acidic catalyst, gave the highest
selectivity (92 %) for butyl levulinate followed by HZSM-5 (66 %) with
conversions for both the catalysts near to completion. The better cata-
lytic performance of amberlyst-15 catalyst can be attributed to the high
number of Br o¨ nsted acidity present in it compared to any other catalyst
◦
mesoporosity, acidity in the broad range of 50ꢀ 550 C and high surface
area gave ≥ 99 % furfuryl alcohol conversion but its low selectivity
towards butyl levulinate could be because of the low conversion of the
intermediate BMF due to the weaker Al acid sites. Though sulfated zir-
conia possessed acid strength in the broad range of 50ꢀ 550 ◦C,
5

B.J. Vaishnavi et al.
Molecular Catalysis 502 (2021) 111361
◦
Fig. 3. Catalyst screening. Reaction conditions: catalyst concentration-3 wt%, temperature-110 C, mole ratio- 1:10 (furfuryl alcohol: butanol), reaction time- 6 h.
Fig. 4. (a) Effect of silica to alumina ratio (SAR) of HZSM-5 (b) Correlation of butyl levulinate selectivity with acidity. Reaction conditions: Catalyst concentration- 3
wt%, temperature- 110 ◦C, mole ratio- 1:10 (furfuryl alcohol: butanol), reaction time- 6 h. For all the catalysts, furfuryl alcohol conversion ≥ 99 %.
6

B.J. Vaishnavi et al.
Molecular Catalysis 502 (2021) 111361
conversion of furfuryl alcohol was low (13 % at 6 h) which could be due
to its smaller surface area and lower number of acidity compared to
HZSM-5 (Table 1). Both Al-SBA-15 and sulfated zirconia gave a
considerably high amount of 5,5-dibutoxy-2-pentanone side product
with 11 and 5% selectivity respectively. Hence, we can conclude, in
general, that pore size, strength and number of acid sites are largely
responsible for the catalyst efficiency for this two-step tandem reaction.
Amberlyst-15 gave almost complete conversion and high selectivity
for butyl levulinate. However, being an organic cation-exchange resin, it
is always suspected about its performance in successive catalyst cycles.
should be marked that the high amount of acidity had less influence on
the selectivity though the trend of the catalytic performance was a
volcanic peak [Fig. 4(b)]. The order of the catalytic performance of the
SARs in HZSM-5 with respect to selectivity (%) is SAR95 > SAR117 >
SAR160 > SAR57 > SAR22. This trend may be due to the competing
reactions leading to pore/ active site blockage making the reaction
slower with time for HZSM-5 with lower SAR. Oligomerization is more
prone in lower SAR catalysts due to the higher Br o¨ nsted acidity. In the
case of higher SARs, low acid site density leads to low catalytic activity.
A similar trend was reported for the different acid-catalyzed reaction
using HZSM-5 [42]. Therefore, SAR95 seems to be the optimal catalyst
that favors efficient production of butyl levulinate compared to the rest.
To explore the nature and to understand the active sites responsible for
the transformation, the catalyst with the highest catalytic selectivity of
66 %, HZSM-5 (SAR95) was post modified by desilication, deal-
umination, metal-ion exchange and phosphate modification were
screened for this reaction.
◦
Due to its low thermal stability (120 C), the calcination cannot be
employed for the removal of the adsorbed substrate/product species
which block the active sites. Therefore, recycling by repeated methanol
◦
washing and drying at 110 C for 12 h was employed which, however,
did not help in retaining its activity. It is found that there was a drastic
decrease in activity from 92 to 22 % during the recyclability test (ESI
Fig. S7). Hence, HZSM-5 with high thermal stability was opted for
further studies which gave almost complete conversion and fairly high
selectivity of 66 % for butyl levulinate.
3.2.3. Catalytic activity studies for post synthetically modified HZSM-5
The butanolysis reaction of furfuryl alcohol with butanol was con-
ducted using post synthetically modified HZSM-5 (SAR95) by desilica-
tion and dealumination using 0.1 M, 0.2 M NaOH and citric acid/ HCl
respectively. The alkali treatment was restricted to 0.2 M NaOH con-
centration as it lowers the crystallinity of the material upon an increase
in the concentration ≥ 0.5 M which would lead to the destruction of the
framework resulting in the partial/complete collapse of the zeolite
structure [32]. These modifications resulted in a decrease of butyl
3
.2.2. Effect of SAR
As the change in SAR in zeolites results in the variation of number
and strength of acidity, HZSM-5 with different SAR was investigated for
alcoholysis of furfuryl alcohol with butanol. The conversion of all the
HZSM-5 catalysts with the SAR ranging from 22 to 160, was above 95 %
◦
at 110 C [Fig. 4(a)]. Interestingly, the selectivity towards butyl levu-
linate did not show any linearity with respect to the SARs. However, it
Fig. 5. Catalyst performance of HZSM-5 (SAR 95) upon post modification (a) Desilication (b) Dealumination (c) Phosphate modification (d) Metal ion exchange.
◦
Reaction conditions: Catalyst concentration- 3 wt%, temperature- 110 C, mole ratio- 1:10 (furfuryl alcohol: butanol), reaction time- 6 h. Furfuryl alcohol conversions
for all the reactions ≥ 99 %.
7

B.J. Vaishnavi et al.
Molecular Catalysis 502 (2021) 111361
levulinate selectivity in the range of 55‒42 % compared with parent
HZSM-5 (SAR95) (66.6 %) which could be due to the lowering of
number and strength of acidity [Fig. 5 (a) and 5(b)]. As the conversion
level after modifications were almost similar (>99 %), it implies that the
conversion of intermediate BMF to butyl levulinate is mainly affected by
these modifications [ESI Fig. S8 (a, b)].
was studied with HZSM-5 (SAR95) as shown in Fig. 6 (a). It is observed
that the variation in catalyst loading had a pronounced effect on the
butyl levulinate selectivity. The selectivity increased substantially from
25.3–66.6 % with an increase in the catalyst loading from 0.75 to 3 wt%.
At a lower catalyst loading, though the conversion of furfuryl alcohol
was not affected (≥ 99 % at 6 h), it is observed that the BMF (inter-
mediate) conversion was lower, thereby affecting the butyl levulinate
selectivity. Hence, the first transformation of furfuryl alcohol to inter-
mediate BMF occurs easily with a low amount of the catalyst, whereas
the second transformation of the intermediate to butyl levulinate re-
quires higher catalyst amount [ESI Fig. S9 (a)]. With further increase in
the catalyst loading to 3.75 wt%, the selectivity of butyl levulinate
decreased as the product concentration was distributed mainly over
BMF and other side product, 5,5-dibutoxy-2-pentanone. As the reaction
demands a minimum of 3 wt % catalyst loading, which is less compared
with that used in most of the previous reports, the catalyst concentration
of HZSM-5 (SAR95) was fixed to 3 wt% for further studies.
Upon treatment of varied concentration of phosphate, the catalysts
were tested to explore the role of weaker acid sites on the reaction. It was
found that with an increase in the phosphate treatment the selectivity
towards butyl levulinate drastically decreased from 53.2–30.9 % [Fig. 5
(
c)]. This is due to the decrease in the concentration of strong Br o¨ nsted
acidity which was replaced by weaker acid sites upon phosphate treat-
ment as expressed in the TPD profile [27]. In the TPD profile of the
highest phosphate modification (5% P-ZSM-5), strong Br o¨ nsted acid
sites diminished after the treatment which explains its poor performance
[
Fig. 2(d)]. This proves the necessity of a sufficient number of strong
Br o¨ nsted acidity for this transformation.
The Zn2 and Cu ions were exchanged with HZSM-5 (SAR95) to
understand the role of Lewis acidity on the reaction. While the two
modified catalysts retained almost complete conversion, butyl levuli-
nate selectivity decreased with respect to the parent HZSM-5. The initial
formation of butyl levulinate of Cu-ZSM-5 and Zn-ZSM-5 till 2 h was
close to that of parent HZSM-5 after which the gap between them
increased indicating the poorer conversion of BMF intermediate to the
product at higher reaction time compared to parent HZSM-5 [ESI Fig. S8
+
2+
3.2.4.2. Effect of reactants mole ratio. Since this reaction involves sub-
strate, furfuryl alcohol which is prone to polymerization, the other
substrate concentration (butanol) becomes important. Butanol not only
takes part in the alcoholysis but, at the same time, also reduces the
formation of polymeric side products by acting as a solvent or a diluent.
Thus, the optimized mole ratio (furfuryl alcohol: butanol) helps in
reducing the furfuryl alcohol side reactions thereby affectively avoiding
catalyst poisoning. Though the conversion of furfuryl alcohol was not
altered (≥ 99 %) by varying the mole ratio, butyl levulinate selectivity
was mainly influenced by it. At lower molar ratios from furfuryl alcohol:
butanol, 1:2.5 to 1:7.5, the butyl levulinate selectivity was not much
changed (45–47 %) [Fig. 6(b)]. However, upon increasing the mole ratio
from 1:7.5 to 1:10, the selectivity of butyl levulinate was enhanced by 20
% (66.6 %). Further increase in the mole ratio to 1: 12.5 and 1:15, the
selectivity considerably decreased (51ꢀ 48 %) which might be due to the
partial blockage of active sites for the intermediate BMF to react further
by excess butanol that predominantly resides on the active sites being
more polar compared to the ether (BMF) [ESI Fig. S9 (b)]. Hence this
study confirms that furfuryl alcohol to butanol ratio of 1:10 is the
(
d)]. The overall decrease in the selectivity of parent HZSM-5 from
6.6–50.6 % (Zn-ZSM-5) and 49.1 % (Cu-ZSM-5) could be due to
decrease in number and strength of acid sites [Table 1, Fig. 5(d)].
6
3
.2.4. Influence of reaction conditions
Among all the well-known solid acids and acidity-structure modified
ZSM-5 catalysts, HZSM-5 (SAR95) that showed the highest efficiency
was selected to study the influence reaction parameters such as catalyst
concentration, reactants mole ratio and temperature.
3
.2.4.1. Effect of catalyst concentration. The effect of the catalyst
loading ranging from 0.75 to 3.75 wt% (with respect to total reactants)
◦
Fig. 6. Influence of reaction conditions. (a) Effect of catalyst loading : Reaction conditions: Temperature- 110 C, mole ratio- 1:10 (furfuryl alcohol: butanol),
◦
reaction time- 6 h. (b) Effect of reactants mole ratio: Reaction conditions: Catalyst concentration- 200 mg, temperature- 110 C, reaction time- 6 h. (c) Effect of
reaction temperature: Reaction conditions: Catalyst concentration- 3 wt%, mole ratio- 1:10 (furfuryl alcohol: butanol), reaction time- 6 h. Furfuryl alcohol con-
versions for all the reactions ≥ 99 %.
8

B.J. Vaishnavi et al.
Molecular Catalysis 502 (2021) 111361
optimal stoichiometric mole ratio for this reaction.
3
.2.4.3. Effect of reaction temperature. Effect of temperature on the
alcoholysis of furfuryl alcohol was investigated in the temperature range
of 90ꢀ 117 ◦C in the batch mode while keeping other parameters con-
stant [Fig. 6 (c)]. Though the temperature change did not affect the
furfuryl alcohol conversion (≥ 99 % at all temperatures), there was a
visible impact on the selectivity of butyl levulinate. In the case of tem-
◦
perature 90 and 100 C, though the intermediate BMF formed was
◦
considerably high in concentration (73 %) compared to 110 C (67 %),
the conversion of BMF to butyl levulinate over the time was slower (48
◦
◦
◦
%
for 90 C and 53 % for 100 C). At temperature 110 C, the selectivity
towards butyl levulinate improved to 66.6 % due to an appreciable
decrease in BMF concentration from 67 to 20 % [ESI Fig. S9 (c)]. Further
◦
increase in temperature to 117 C, decreased the butyl levulinate
◦
selectivity by 8.9 % (to 57.7 %). Hence at 110 C, the reaction was
suitable for higher butyl levulinate selectivity. Therefore, after the
complete investigation on the reaction parameters, the optimized tem-
perature, the mole ratio of furfuryl alcohol to butanol and catalyst
Fig. 8. Time resolved study. Reaction conditions: Catalyst concentration- 3 wt
◦
%, temperature- 110 C, mole ratio- 1:10 (furfuryl alcohol: butanol), reaction
time- 24 h.
◦
concentration are 110 C, 1:10, and 3 wt% respectively.
also demonstrates the progressive performance of the catalyst with time
by efficiently converting the intermediate BMF to the desired product
which proves that this is a consecutive reaction.
3
.2.5. Catalyst recyclability study
To investigate the catalyst stability towards multiple cycles, experi-
ments were conducted by regenerating the catalyst in between the four
cycles under the optimized conditions (Fig. 7). During each cycle, the
catalyst demonstrated almost the same catalytic activity as that of the
fresh catalyst. This implies that there was no loss of any active sites
during reaction or in the regeneration process which was reflected at the
spent catalyst characterization. This proves HZSM-5 (SAR95) to be a
highly potential catalyst for alcoholysis reaction as it addresses all the
drawbacks of the reported catalysts such as poor recyclability, the need
of high catalyst loading and butanol requirement, high cost of the
catalyst and difficult synthesis procedures.
Under the optimized reaction conditions, the HZSM-5 (SAR 95) gave
99 % conversion and 85 % selectivity in 24 h. This catalyst activity was
compared with other reported catalysts from the literature. The catalyst
performed well under milder reaction conditions (lesser catalyst loading
and a lower mole ratio of the reactants) and excellent recyclability in
comparison with most of the reported catalysts (ESI Table S2).
3.2.7. Plausible reaction mechanism
The plausible mechanistic pathway of alcoholysis of furfuryl alcohol
to form alkyl levulinate is proposed in Scheme 2. The reaction proceeds
by the formation of an intermediate alkoxy methyl furan which then gets
transformed to 2-alkoxy-5-methylene-2,5-dihydrofuran by 1,4 addition
of the alcohol. Ring-opening occurs by subsequent production of alkyl
levulinate [43]
3
.2.6. Time resolved study
Time resolved study was carried out for the alcoholysis of furfuryl
alcohol for 24 h using HZSM-5 (SAR95) catalyst under the optimized
conditions. The catalyst showed ≥ 99 % furfuryl alcohol conversion
within an hour and the selectivity towards the intermediate BMF
decreased from 67 (15 min) to 1.9 % with an increase in time to 24 h.
During the reaction, the butyl levulinate selectivity reached 24 % in 15
min and then increased rapidly to 66 % in 6 h, but then took more time
to finally attain 85 % in 24 h (Fig. 8). This suggests that there was a slow
conversion of the intermediate, BMF to butyl levulinate after 6 h which
may be attributed to the active site saturation with products in the
course of time. The increase in the butyl levulinate selectivity with time
3.2.8. Substrate scope study
Due to the vast applications of various alkyl levulinates, it is inter-
esting to explore the catalytic performance of HZSM-5 (SAR95) catalyst
using various alkyl alcohols. Hence, the effect of the substrate was
evaluated for alcoholysis of furfuryl alcohol to yield methyl levulinate,
ethyl levulinate, propyl levulinate and butyl levulinate at the reflux
temperature. Under optimized conditions, the conversion of furfuryl
alcohol, irrespective of the alcohols used, was >95 % and the selectivity
◦
Fig. 7. Catalyst recyclability study. Reaction Conditions: temperature: 110 C, catalyst concentration- 3 wt%, mole ratio- 1:10 (furfuryl alcohol: butanol), reaction
time- 6 h.
9

B.J. Vaishnavi et al.
Molecular Catalysis 502 (2021) 111361
Scheme 2. Plausible mechanistic pathway for the alcoholysis of furfuryl alcohol to yield alkyl levulinate.
towards the product increased with an increase in the alkyl chain of the
alcohol in the following order methanol < ethanol < propanol < butanol
(
Fig. 9). At its reflux temperature, when methanol was employed, the
selectivity towards the intermediate methoxy methyl furan (MMF) and
other side products were 50.2 and 35.5 % respectively. This clearly
explains the low selectivity of methyl levulinate (14.3 %) during the
reaction. In the case of ethanolysis of furfuryl alcohol, the selectivity for
ethyl levulinate was 20.7 % for 6 h reaction. The ethoxy methyl furan
(
EMF) and the other side products were formed with a selectivity of 55.6
and 23.7 % respectively. Reactions for propyl levulinate and butyl lev-
ulinate synthesis resulted in 50.3 and 66.6 % selectivities for these
products respectively under reflux temperature. This trend could be due
to the + I effect of the alkyl group guiding the conversion of the inter-
mediate (alkoxy methyl furan) to its respective products (ESI Fig. S10).
Therefore, as the alkyl chain in the alcohol is increased, the conversion
of the intermediate is enhanced, thereby increasing the selectivity to-
wards the alkyl levulinate.
Fig. 9. Effect of alkyl chain on alcoholysis. Reaction Conditions: Catalyst-
HZSM-5 (SAR95), temperature: Reflux condition, catalyst concentration-200
mg, mole ratio- 1:10 (furfuryl alcohol: alcohol), reaction time- 6 h. The fur-
furyl alcohol conversion of all the substrates >95 %.
4
. Conclusions
Zeolites being one of the classic materials are still startling the sci-
entists by its flexible material properties and promising catalytic activ-
ities in many important and challenging transformations. In alcoholysis
of furfuryl alcohol which was efficiently converted into butyl levulinate
with the aid of HZSM-5 (SAR95) in a batch regime proved the exclusive
requirement of the uniform medium size micropore structure, high
surface area, strong Br o¨ nsted acidity, optimal number of acid sites and
good thermal stability for this reaction. The combination of the prop-
erties inherited by the HZSM-5 with the tuning of acidity makes it
different from the other catalysts. Considering the green chemistry
principles such as moderate experimental conditions, excellent reus-
ability, low reactants mole ratio and catalyst concentration, HZSM-5
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
B.J.V. thanks Admar Mutt Education Foundation (AMEF) for
providing a research fellowship. Authors thank AMEF for providing fa-
cilities to carry out research and Vision Group on Science and Tech-
nology (VGST), Govt. of Karnataka (CESEM, GRD No. 307) for the
analytical facility for BET surface area analysis.
(
SAR95) proves to be the best candidate for this transformation as it
addresses all the drawbacks of the reported catalysts.
1
0

B.J. Vaishnavi et al.
Molecular Catalysis 502 (2021) 111361
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