Mesoporous porphyrin-silica nanocomposite as solid acid catalyst for high yield synthesis of HMF in water

Article abstract of DOI:10.3390/molecules26092519

Solid acid catalysts occupy a special class in heterogeneous catalysis for their efficiency in eco-friendly conversion of biomass into demanding chemicals. We synthesized porphyrin containing porous organic polymers (PorPOPs) using colloidal silica as a support. Post-modification with chloro-sulfonic acid enabled sulfonic acid functionalization, and the resulting material (PorPOPS) showed excellent activity and durability for the conversion of fructose to 5-hydroxymethyl furfural (HMF) in green solvent water. PorPOPS composite was characterized by N₂ sorption, FTIR, TGA, CHNS, FESEM, TEM and XPS techniques, justifying the successful synthesis of organic networks and the grafting of sulfonic acid sites (5 wt%). Furthermore, a high surface area (260 m²/g) and the presence of distinct mesopores of ~15 nm were distinctly different from the porphyrin containing sulfonated porous organic polymer (FePOP-1S). Surprisingly the hybrid PorPOPS showed an excellent yield of HMF (85%) and high selectivity (>90%) in water as compared to microporous pristine-FePOP-1S (yield of HMF = 35%). This research demonstrates the requirement of organic modification on silica surfaces to tailor the activity and selectivity of the catalysts. We foresee that this research may inspire further applications of biomass conversion in water in future environmental research.

Full text of DOI:10.3390/molecules26092519

molecules
Article
Mesoporous Porphyrin-Silica Nanocomposite as Solid Acid
Catalyst for High Yield Synthesis of HMF in Water
Arindam Modak 1,† , Akshay R. Mankar 1,†, Kamal Kishore Pant 1,
*
and Asim Bhaumik 2,
*
1
Catalytic Reaction Engineering Lab, Department of Chemical Engineering, Indian Institute of Technology
Delhi, Delhi 110016, India; arindam_modak_2006@yahoo.co.in (A.M.); akshaymankar79@gmail.com (A.R.M.)
School of Materials Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick
Road, Jadavpur, Kolkata 700032, India
2
*
†
Correspondence: kkpant@chemical.iitd.ac.in (K.K.P.); msab@iacs.res.in (A.B.)
These authors contributed equally to this work.
Abstract: Solid acid catalysts occupy a special class in heterogeneous catalysis for their efficiency in
eco-friendly conversion of biomass into demanding chemicals. We synthesized porphyrin containing
porous organic polymers (PorPOPs) using colloidal silica as a support. Post-modification with chloro-
sulfonic acid enabled sulfonic acid functionalization, and the resulting material (PorPOPS) showed
excellent activity and durability for the conversion of fructose to 5-hydroxymethyl furfural (HMF)
in green solvent water. PorPOPS composite was characterized by N2 sorption, FTIR, TGA, CHNS,
FESEM, TEM and XPS techniques, justifying the successful synthesis of organic networks and the
grafting of sulfonic acid sites (5 wt%). Furthermore, a high surface area (260 m2/g) and the presence
of distinct mesopores of ~15 nm were distinctly different from the porphyrin containing sulfonated
porous organic polymer (FePOP-1S). Surprisingly the hybrid PorPOPS showed an excellent yield
of HMF (85%) and high selectivity (>90%) in water as compared to microporous pristine-FePOP-1S
(yield of HMF = 35%). This research demonstrates the requirement of organic modification on silica
surfaces to tailor the activity and selectivity of the catalysts. We foresee that this research may inspire
further applications of biomass conversion in water in future environmental research.
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Citation: Modak, A.; Mankar, A.R.;
Pant, K.K.; Bhaumik, A. Mesoporous
Porphyrin-Silica Nanocomposite as
Solid Acid Catalyst for High Yield
Synthesis of HMF in Water. Molecules
2021, 26, 2519. https://doi.org/
10.3390/molecules26092519
Keywords: mesoporous porphyrin polymer; solid acid catalyst; fructose conversion; microwave
synthesis in water; 5-hydroxymethyl furfural (HMF)
Academic Editor: Himanshu
Sekhar Jena
1. Introduction
Received: 29 March 2021
Accepted: 23 April 2021
Published: 26 April 2021
The depletion of non-renewable fossil-derived resources is a prime concern for our
society. It would be a major breakthrough if we could achieve considerable replacement
of fossil fuels by renewable biomass-based feedstock as green fuel [1]. Among biomass-
derived platform chemicals, 5-hydroxymethyl furfural (HMF) is very demanding and
versatile, since it can be transformed into numerous important chemicals, such as 2,5-
furandicarboxylic acid, 2,5-diformylfuran, levulinic acid and 2,5-dimethyl furan, which
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iations.
are precursors for polymers and fuels [2]. HMF was initially produced by dehydration of
monosaccharides using homogeneous mineral acids like HCl, H2SO4 and HNO3. How-
ever, their high corrosive properties as well as toxicity to the environment has motivated
researchers to invent more user-friendly catalytic reactions. Moreover, soluble acids may
create side reactions, decrease selectivity of the product, and be difficult to separate from
the product mixtures. Commercial Amberlyst-15 shows 80% HMF yield from fructose in
DMSO and ionic liquid mixtures [3,4]. Though ionic liquids are explored in the synthesis
of HMF from cellulose, their application using a biphasic solvent system, water-methyl
isobutyl ketone, water-butanol, and water-THF is disadvantageous due to the issues of
Copyright:
© 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
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4.0/).
recyclability and the toxicity of applying organic solvents [
catalysis for the synthesis of HMF is quite challenging under environmentally benign reac-
tion conditions. High-density solid acids covalently embedded on a recyclable support [
5,6]. Thus, solid-supported
7]
Molecules 2021, 26, 2519. https://doi.org/10.3390/molecules26092519
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 2519
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may catalyze this conversion processes very efficiently. HMF produced through solid acid
catalysts, e.g., AlCl3, SnCl4, polyoxometalates, WO3/SO3H-containing zirconia and cation
exchange resins, show relatively lower product yield and poor selectivity in water [8–10].
Early research carried out with FeCl3, either in a batch reaction or using a continuous
flow micro-reactor, shows promising advantages for fructose conversion to HMF in high
yields [11]. Studies also report that sulfated TiO2-ZrO2 is an excellent dehydration catalyst
that converts glucose or fructose to HMF [12]. Although solid acid catalysts are explored
for the dehydration of biomass, high surface area, poor dispersibility and stability, and
inadequate covalency between the active sites within the support may be problematic for
consecutive recycling purposes. Although ordered mesoporous silica are well explored
for one-pot conversion of biomass to HMF, post-functionalization of active sites on the
silica support using organosilane are disadvantageous due to self-polymerization among
silanes. Furthermore, accessible pores can also be blocked with this approach. In addition,
reaction in neat water may be problematic due to poor solubility of HMF and low stability
of hydrophilic silica. Under such circumstances, hydrophobic modification is required to
tailor the activity and stability of silica supports [13].
Functional porous organic polymers (POPs) have a huge scope in solid catalysis be-
cause of their stability and scalability in synthesis as well as their tailorability in porous
structures [14–16]. POPs possesses abundant surface-exposed aromatic sites for easy func-
tionalization with surface bound Brønsted acids, which facilitates strong interaction with
substrates [17]. However, controlling activity and selectivity with polymer-based catalysts
remains a significant challenge due to microporosity and inadequate interaction with sub-
strates. Hybrid catalysts are appealing, where inorganic components serve as support, and
the organic modification may control diffusion and the product selectivity [18]. Rational
design of hybrid catalysts thus remains challenging, particularly while accomplishing the
reactions in water [19]. Herein, we report that POPs tethered on colloidal silica enriched
with sulfonic acid facilitates fructose conversion in water, while the sulfonated porphyrin
polymer marginally produces HMF in water under similar reaction conditions. Although
related polymeric solid acids have been recently explored for one-pot conversion of fructose
to HMF, poor yields were observed in water [20]. Thus, this research can offer signifi-
cant opportunities for sustainable development of green catalysts for biomass conversion
in water.
2. Results
Synthesis and Characterization of PorPOPS
Hybrid silica-porphyrin composite (PorPOPS) was prepared through a one-pot bottom-
up approach using solvothermal condensation polymerization of pyrrole and terephtha-
laldehyde [21,22]. To functionalize with sulfonic acids, post-functionalization was done
with chlorosulfonic acid (ClSO3H), as shown in Scheme 1. Initial characterization by FE-
SEM, TEM, N2 sorption, and XPS analysis confirms that porphyrin polymers are indeed
functionalized by sulfonic acids and are coated with silica nanoparticles. Furthermore, we
compared PorPOPS with un-functionalized PorPOP as well as FePOP-1. Porosity and pore
size distribution was obtained from N2 sorption isotherms, as shown in Figure 1a–d. Both
PorPOPS and PorPOP exhibited N2 uptake at low P/P0, followed by flat interpolation
within (0.2–0.8) P/P0 and steep uptake beyond 0.8 P/P0, corresponding to the mesoporous
materials [23]. In contrast, FePOP-1S showed microporosity, with steep uptake of N2 at
low P/P0 (<0.1) and almost linear extrapolation in the entire P/P0 region (0.1–0.99). These
characteristics indicate that incorporation of silica during synthesis prevents the formation
of microporous polymers and introduces extended inter-particle mesoporous materials.

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Scheme 1. Schematic representation for the synthesis of PorPOPS.

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Figure 1.
(a,b) N2-sorption isotherm; (c,d) pore size distribution; (e) FTIR-spectra; (f) TGA (air) analysis of Fe-POP-1S,
PorPOP and PorPOPS.
The Brunauer-Emmett-Teller (BET) surface area was calculated to be 260 and 280 m2/g
for PorPOPS and un-functionalized PorPOP, respectively, which is lower compared to
the pristine porphyrin polymer (FePOP-1) (860 m2/g) and even the sulfonated FePOP-
1S (589 m2/g) because of the inclusion of silica nanoparticles that prevents accessible
pores [22]. Pore size distribution of PorPOPS and PorPOP from Figure 1c,d clearly signify
that the hybrid nanostructure contains distinct mesopores of ~15 nm, which is attributed
primarily to the interparticle mesopores. In contrast, sulfonated FePOP-1S possesses
small micropores (0.5–1.5 nm). It is noteworthy that, unlike microporous porphyrin,
mesoporous hybrid PorPOPS could be advantageous as it obviates the diffusion limitation
of substrates imposed by micropores [24]. Thus, with regard to the conversion of large
biomass molecules like fructose, the developed organic–inorganic hybrid catalyst demands
merit. Nevertheless, pore size has a profound influence on the yield as well as the selectivity
of HMF, as shown by Du et al. [25]. Total pore volume was calculated from N2 sorption
isotherm, and it was found to be 0.98 cm3/g. This large pore volume is attributed to the
presence of inter-particle mesopores. The FT-IR spectrum is shown in Figure 1e, where a
sharp and high intense peak appears at ~3374 cm−1, corresponding to the –OH of –SO3H
and silica particles in PorPOPS.
The existence of a porphyrin ring in PorPOPS is attributed to the presence of bands
at 1267 and 1137 cm−1, corresponding to the C–N stretching of pyrrole. Subsequently,
other bands at 1668 and 1430 cm−1 are due to phenyl C=C stretching in the porphyrin
ring [26]. Symmetric and asymmetric stretching of C–S and C–SO3H were observed at 471,
−1
1075 and 1350 cm
[27]. Such characteristics are assigned the confirmation of sulfonic acid
in PorPOPS, whereas these bands are absent in un-functionalized PorPOP. A TGA plot is
shown in Figure 1f; it suggests a stable framework up to 400 ◦C (weight loss ca. ~10%).
Total weight loss of ~37% up to 1000 ◦C suggests that PorPOPS and PorPOP consists of
~27% organic groups (porphyrin functionality). The sulfonated organic polymer, FePOP-1S,
is stable up to 280 ◦C (weight loss ca. ~10%) and completely collapses above 400 ◦C. This
result suggests that PorPOPS and PorPOP, composed of a hybrid composite containing
both porphyrin and silica functionality, have considerably high thermal stability over the
porphyrin polymer alone.

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Morphology was confirmed from TEM and FESEM image analyses; it suggests an
agglomerated nanostructure of PorPOPS and PorPOP, containing silica nanoparticles. It
shows that 150–250 nm porphyrin polymers are surrounded by smaller silica spheres of
about 30 nm (Figures 2 and 3). The spherical shape of pristine (FePOP-1) and the sulfonated
porphyrin nanoparticles (FePOP-1S) are evidenced from both TEM and SEM images, with
a wider distribution of particles (150–300 nm).
Chemical composition of the surface was assigned through X-ray photoelectron spec-
troscopic (XPS) analysis (Figure 4). Deconvoluted N1s spectrum in PorPOPS confirms the
existence of pyrrolic and pyridinic moieties, centered at 399.8 and 398.9 eV, respectively [28].
However, XPS shows no existence of either oxidized nitrogen or graphitic nitrogen beyond
400 eV [29]. Thus, XPS infers the appearance of a large pyrrolic band corresponding to the
porphyrin moiety of PorPOPS. Hence, this suggests that PorPOPS is stable under strong
acid treatment. Interestingly, intensity of the pyridinic band is suppressed in PorPOPS in
contrast to un-functionalized PorPOP, which might be because of the protonated porphyrin
in PorPOPS. Successful grafting of sulfonic acid is further confirmed from the deconvo-
luted S2p spectrum. A fitted doublet appears at 167 and 168.5 eV and accompanies a
broad shoulder at 162.2 eV. The strong band at 168.5 eV resembles a sulfonate band, while
another band appearing at 167 eV could be attributed to oxidized sulfone (
−
SO2 ). On
−
the other hand, the broad shoulder may originate from organic disulfide-type residues [30].
Considering elemental analysis from XPS, higher pyrrolic (70%) to pyridinic (30%) ratio
and a better sulfone (58%) to sulfonate (52%) ratio can be predicted. Furthermore, CHNS
analysis reveals the possibility of ~5.0 wt% sulfonic acid sites in PorPOPS. Thus, the above
results clearly demonstrate that a sulfonic-acid-rich porphyrin network is successfully
formed in PorPOPS.
Figure 2. TEM (a) and FESEM (b) image of pristine porphyrin polymer (FePOP-1); (c,d) TEM image
of PorPOPS.

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Figure 3. FESEM images of (a,b) sulfonated porphyrin silica composite, PorPOPS, (c) FePOP-1S, (d) un-functionalized PorPOP.
Figure 4. XPS spectra of (a) N1s of PorPOP and (b–d) PorPOPS containing N1s, O1s and S2p, respectively.

Molecules 2021, 26, 2519
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3. Discussion
A high surface area and the presence of accessible sulfonic acids in PorPOPS motivated
us to explore its catalytic activity towards fructose dehydration reactions (Scheme 2).
Scheme 2. One-pot green conversion of fructose to HMF using PorPOPS.
Initial screening of the reaction in a microwave reactor was done at different temper-
atures and times using various amounts of catalyst to optimize the reaction conditions.
Contrary to the literature, we obtained the highest HMF yield in water; thus, all the re-
actions were carried out using water only as a solvent. The effect of temperature on the
fructose conversion and the yield of HMF was thoroughly studied within 130 to 180 ◦C
(Figure 5a). Initially, at 130 ◦C, only 15% fructose was converted to 13% HMF (yield). Fur-
◦
ther increases in temperature (from 130 to 160 C) led to a significant improvement in both
◦
the conversion and yield, where highest HMF yield (85%) was observed at 160 C. Although
◦
fructose conversion (>95%) progressively increased at higher temperatures (>160 C), HMF
yield was reduced from 85% (160 ◦C) to 39% (170 ◦C) and 13% (180 ◦C), which indicates
fructose dehydration is highly sensitive to temperature, and an optimum temperature is
necessary to maintain high HMF selectivity, as other side products (levulinic acid, formic
acid) are commonly observed beyond 170 ◦C [31]. Again, polycondensation of fructose
◦
◦
may also reduce HMF selectivity at 170 C [32]. Thus, kinetic study was done at 160 C, as
shown in Figure 5b.
At 0.5 h, lower conversion (46%) and yield (47%) were observed. Further prolonging
the reaction to 2 h, HMF yield was improved to 85%, but decreased to 63% at 2.5 h
(Figure 5b), possibly because of humin formation from fructose decomposition [33,34].
Thus, the controlled experiments suggest that 160 ◦C/2 h is the most suitable condition to
achieve the highest HMF yield over PorPOPS. Since fructose dehydration is dependent
on the total Brønsted acidity, variation of the catalyst dosage may change the reaction
rate. For this, catalyst dosage was varied from 0.005 g (0.1 mmol/g sulfonic acid) to 0.02 g
(0.4 mmol/g sulfonic acid); the highest yield of HMF occurred when 10 mg catalyst was
used, which was subsequently reduced at higher loading, as shown in Figure 5c. Thus, a
higher catalyst amount results in decomposition or degradation of fructose to other by-
products. Recyclability of PorPOPS was tested up to five cycles; HMF yield was reduced to
~70% after the 5th cycle, in contrast to the fresh catalyst (85%) (Figure 6a). Additionally,
the sulfonated porphyrin FePOP-1S was sustained up to five recycle runs, and ~14% loss
of HMF yield was observed at the end of the 5th cycle. This result shows that sulfonated
porphyrin with/without functionalization by silica nanoparticles is advantageous as a solid
catalyst in water. Nevertheless, the un-functionalized PorPOP and FePOP-1 which do not
have any sulfonic acid sites, cannot exhibit HMF yield (Table 1, Rows 13–14). However, due
to feeble acidity of silica hydroxyl sites, only 5% fructose is converted with PorPOP. This
result suggests that a cooperative interaction between porphyrin, silica and sulfonic acids
in PorPOPS is essential for maximum conversion of fructose to HMF. A comparative study
with other contemporary catalysts, e.g., porous carbon, zeolite, metal-organic frameworks
(MOF) and metal oxides, is shown in Table 1.

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Figure 5. Effect of temperature (
2 mL water.
a), time (b) and catalyst dosage (c) for PorPOPS catalysed conversion of fructose to HMF in
Figure 6. Recyclability study of (
reaction to HMF (reaction conditions: 0.01 g fructose (0.04 mmol/g sulfonic acid), 2 mL water, 160 C, 2 h).
a
) PorPOPS and (
b
) sulfonated porphyrin polymers (FePOP-1S) for fructose dehydration
◦

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Table 1. Comparative study of PorPOPS in fructose conversion to HMF in water.
Reaction
Conditions
Conversion of
Fructose (%)
Entry
Catalyst
Yield of HMF (%)
Reference
Phosphorylated mesoporous
carbon
◦
1
120 C, 16 h
78
41.3
[35]
[36]
[37]
◦
2
3
4
100 C, 3 h
Niobium phosphate
Niobium phosphate
Zirconium phosphate
51.2
86.5
96.3
30.4
33.9
39.4
◦
180 C, 10 min
◦
190 C, 8 min
Phosphoric acid
functionalized carbon catalyst
◦
5
170 C, 4.5 h
100
59.9
[38]
◦
6
7
120 C, 24 h
Amberlyst-15
TiO2-SO3H
57
99
15
50
2.3
15
30
[39]
[40]
◦
165 C, 3 h
◦
8
140 C, 5 h
NU-1000
60
[41]
◦
9
140 C, 5 h
PO4/NU(half)
SBA-15-SO3H
50
◦
10
120 C, 1 h
100
[42]
Sulfonated porphyrin
polymer (FePOP-1S)
◦
11
160 C, 2 h
95
35
This study
◦
12
13
14
160 C, 2 h
PorPOPS
95
<5
85
traces (<1)
No
This study
This study
This study
◦
160 C, 2 h
Un-functionalized PorPOP
Un-functionalized FePOP-1
◦
160 C, 2 h
No
Interestingly, PorPOPS shows highest HMF yield among the contemporary solid acid
catalysts. It is noteworthy that stability could be an issue for MOFs in water, as shown
by Corma and Boronat. Importantly, it was shown that high HMF yield in water can be
achieved under anaerobic conditions in nitrogen (1 bar) atmosphere [43]. Interestingly,
the effect of silica on PorPOPS is significant, where microporous sulfonated porphyrin
polymer synthesized in the absence of colloidal silica showed lower HMF yield (35%)
under similar conditions. Unlike sulfonated porphyrin POP, unique mesoporosity, together
with hydrophilic coating of silica on the hydrophobic surface, may be advantageous for
the activation and dehydration of fructose to HMF. Thus, for fructose conversion, PorPOPS
is advantageous in water, possibly due to its high surface area and good accessibility of
aromatic sulfonic acids towards substrates, as previously observed by Schüth et al. [44].
4. Materials and Methods
4.1. Chemicals
Pyrrole (
(99%), terephthalaldehyde (99%), anhydrous 1,2-dichloroethane (99.8%), fructose (
5-hydroxymethylfurfural ( 99%), sulfuric acid (99%), colloidal silica (Ludox, AS-40) and
≥
98%), anhydrous FeCl3 (99%), glacial acetic acid (
≥
99%), chlorosulfonic acid
≥
99%),
≥
HPLC-grade water were purchased from Sigma Aldrich, St. Louis, MO, USA. All chemicals
were used as received, without any further purification.
4.2. Instrumentation
◦
N2 adsorption-desorption isotherms were measured at
−
196 C using a Quantachrome
Instruments (A brand of Anton Paar, Boynton Beach, Florida, USA) Autosorb 1C surface
area analyzer. Prior to the analysis, samples were degassed at 150 ◦C for 5 h under high
vacuum. Pore size was analyzed with a density functional theory (DFT) model using N2 at
77 K on carbon cylindrical pores. The thermal stability of the samples was measured by
thermogravimetry analysis (TGA) using a NETZSCH STA 449F3 analyser (NETZSCH, Selb,
◦
◦
Germany) within the temperature range 25–1000 C at a heating rate of 10 C/min in air. X-
ray photoelectron spectroscopy (XPS) was recorded by VG ESCALAB MK2 (ThermoFisher

Molecules 2021, 26, 2519
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Scientific, Waltham, MA, USA) using AlK
α
(hλ
= 1486.6 eV) as the excitation source.
Fourier transform infrared spectra (FT-IR) were documented on a Nicolet Nexus 470 IR
spectrometer (ThermoFisher Scientific, Waltham, MA, USA). Field-emission scanning
electron microscopy (JEOL JEM 6700F FE SEM, JEOL, Tokyo, Japan) was used to examine
the morphology of powder samples. Transmission electron microscopy (TEM) experiments
were executed using HITACHI HT7700 (HITACHI, Tokyo, Japan) at an acceleration voltage
of 100 kV. Carbon, hydrogen and nitrogen contents were determined using a Perkin Elmer
2400 Series II CHN analyzer (Perkin Elmer, Waltham, MA, USA).
4.3. Synthesis of Organic-Inorganic Hybrid Silica-Porphyrin Conjugated Polymers (PorPOPs)
A porphyrin polymer (FePOP-1) was reproduced from our previous report after slight
modification of the procedure [22]. Initially, terephthalaldehyde (0.2 g, 1.5 mmol) was
taken in a 100 mL well dried beaker and was dissolved in 15 mL of glacial acetic acid under
constant stirring for 10 min. Next, freshly distilled pyrrole (0.1 g; 1.5 mmol) was added to
the solution and stirred for 20 min with the addition of 5 mL colloidal silica. Anhydrous
FeCl3 (0.32 g; 2 mmol) was added to the solution, and stirring was continued to 12 h under
N2 atmosphere. The brownish mixture was transferred to a Teflon-lined autoclave and
was subjected to hydrothermal treatment at 180 ◦C for 48 h under a static condition. The
dark brownish precipitate was then filtered and washed thoroughly with distilled water,
methanol and THF to remove all iron-based impurities and then dried in an oven at 80 ◦C
for 24 h (named as PorPOP). The yield of polymer was 25% based on terephthalaldehyde,
and the elemental analysis (wt%) was C, 80.0; H, 4.1; N, 8.5.
4.4. Synthesis of PorPOPS Through Post-Functionalization with Chlorosulfonic Acid
A total of 0.3 g PorPOP was dispersed in 10 mL anhydrous 1,2-dichloromethane
through sonication for 5 min. The slurry mixture was cooled in an ice bath, and chlorosul-
◦
fonic acid (2 mL) was then added slowly to the solution and stirred for three days at 25 C.
After completion, the dark brownish solution was poured on ice, centrifuged, and washed
thoroughly with water until pH was neutral. Then, the catalyst was dried in an oven
overnight and named PorPOPS. Elemental analysis (wt%) was as follows: C, 30; H, 3.1; N,
5.2; S, 5. The total acid strength of PorPOPS was measured through acid-base titration and
was 1.96 mmol/g.
Un-functionalized porphyrin polymer (FePOP-1) in absence of silica was reproduced
from a previous report [22] and was post-functionalized with sulfonic acid following the
procedure mentioned above. The sulfonic acid functionalized organic polymer was named
Fe-POP-1S.
4.5. One-Pot Conversion of Fructose to HMF
All fructose dehydration reactions were performed using a microwave reactor (Anton
◦
Paar: Monowave 300, Anton Paar, Graz, Austria) with operation limits of 300 C and 30 bar.
In a typical reaction, 0.02 g fructose, PorPOPS (0.005 to 0.02 g) and 2 mL water was fed
in the borosilicate glass vial. A magnetic bead was placed in the vial for stirring and the
sealed vial was placed in the reactor ca◦vity. Then, a series of reactions were performed
under different temperatures (130 to 180 C) for varying reaction times (0.5 h to 2.5 h) using
the as-fast-as-possible heating mode of the microwave reactor [45]. After the completion of
the reaction, the reactor was cooled down to room temperature using the cooling system
provided with the reactor. All the reactions were performed at least twice, and the average
of the results were reported.
4.6. Product Analysis
After the reaction, the catalyst was separated from the liquid product mixture using
filtration. Then, the liquid samples were passed through a micro-syringe filter before
analysing the samples using high-performance liquid chromatography (HPLC). HPLC
(Agilent 1200 infinity series (Agilent Technologies, Santa Clara, CA, USA) equipped with

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an autosampler, refractive index (RI) detector, ultraviolet (UV) detector, and Aminex
HPX-87H column (Bio-Rad Laboratories, Irvine, CA, USA) (300
the quantification of fructose and HMF. An injection volume of 2
×
7.8mm) was used for
L and flow rate of
µ
0.6 mL/min were fixed, and sulphuric acid (5 mM) was used as the mobile phase. The
◦
column temperature was maintained at 50 C, and the UV detector was set at a wavelength
of 284 nm for the detection of HMF. All the samples were injected twice, and the average
readings were used for the calculation of the fructose conversion and the calculation of
HMF yield, using the following equations (Equations (1) and (2)), based on their mass in
the feed:
initial amount of fructose fed − amount of unreacted fructose
% Conversion of fructose =
× 100
(1)
initial amount of fructose fed
amount of HMF produced
initial amount of fructose fed
% Yield of HMF =
× 100
(2)
5. Conclusions
Experimental results shown herein suggest that sulfonated porous porphyrin polymer
impregnated with silica nanoparticles is an efficient solid acid catalyst for one-pot conver-
sion of biomass-derived fructose to HMF. Remarkably, HMF yield (85%) in water is higher
than that reported over related solid acid catalysts. This could be due to its high porosity
accompanied by high loading of accessible surface sulfonic acid groups at the catalyst
surface. Although aromatic sulfonates are demanding in acid catalysis, desulfonation
and humin formation is a problematic issue for biomass conversion at high temperature
reactions. Sulfonic acid functionalized POPs with silica support can afford excellent yield
of HMF from biomass-derived carbohydrates, and there is a huge opportunity to scale up
this conversion beyond the laboratory.
6. Patents
An Indian Patent for the work reported in this manuscript has been filed (Application
No.: 202111012815).
Author Contributions: A.M. and A.R.M. designed the experiments, synthesized the catalysts and
performed the chemical reactions. A.M. wrote the manuscript with the help of A.R.M., K.K.P. and
A.B. K.K.P. and A.B. revised the manuscript and supervised the project. All authors have read and
agreed to the published version of the manuscript.
Funding: A.M., A.R.M. and K.K.P. gratefully acknowledge DST/SERB (CRG/2019/005613) for
providing the funding. K.K.P. would like to thank the Petrotech Society of India for the Petrotech
Chair Professorship grant.
Data Availability Statement: Data are available from the authors on request.
Acknowledgments: The authors kindly acknowledge IIT-Delhi for providing support and infrastructure.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are available from the authors.
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