S. Naz, M. Uroos and N. Muhammad
Journal of Molecular Liquids 334 (2021) 116523
process efficiency and products selectivity by reducing the forma-
tion of humins and other side-products. It enhances the rate of
reaction even for more complex carbohydrates; glucose, cellobiose,
starch and cellulosic polymers.
aq. solution of methanesulfonic acid (Alfa Aesar), taurine (Sigma
Aldrich) and metal chlorides were of analytical grade and used
without any further purification. All the solvents such as n-
hexane, ethyl acetate and HPLC grade acetonitrile were purchased
and distilled before use. (D)(ꢀ)-Fructose (Daejung), D-galaxose and
microcrystalline cellulose were purchased and used as such. Wheat
husk as lignocellulosic feedstock was harvested from a local farm
in Punjab, Pakistan. It was ground and meshed to collect particle
sizes of 100, 250 and 500 mm for investigation. The samples were
dried at 70 °C for 48 h to remove moisture before processing in
ILs. Standard of 5-hydroxymethylfurfural was purchased from
Sigma Aldrich and used as received.
ILs are actually the molten organic salts with less than 100 °C
melting points due to poorly coordinated organic heterocyclic
cations (mostly nitrogen or phosphorus as heteroatom) and an
inorganic anion. The poor coordination is attributed to the steric
hindrance due to large size of cation [7]. These are highly designed
solvents as their features and properties can be adjusted by wise
selection of cation and anion pair. Cations are responsible for vis-
cosity, melting points and electrochemical stability variations.
While anions are accountable for hydrogen bonding and in turn
for miscibility with other solvents or water [8].
2.1. Synthesis of ILs
Conversion efficiencies of ILs as well as product selectivities are
also markedly affected by molecular structure of both anion and
cations. Mostly, the catalytic activity is dependent on anion type
particularly due to the role of hydrogen bonding and acidity in con-
version and product selectivities [9]. From previous reports, it can
be concluded that ILs with chloride anion possess high dissolution
abilities for carbohydrates while switching their anion to tetraflu-
oroborate or hexafluorophosphate improve their conversion effi-
ciency for furan related compounds [10,11]. Dehydration related
mechanism is highly promoted by hydrogen sulfate anion due to
its strong acidic character [12]. But this anion is almost ineffective
for glucose/cellulose to 5-HMF conversion where additional sac-
charification and isomerization steps are involved; suggesting that
it actually inhibits the glucose–fructose inter-conversion [13].
Adding some Lewis acidic character with chloride anion makes it
appropriate for conversion of glucose and cellulose into 5-HMF.
Basically the coordination ability of metal center in Lewis acids
and their additional halide ligands coordinated with IL chloride
anion improves the rate of glucose isomerization [14]. Hence, the
careful selection of anion is most important for desired application
of IL. Talking about cation, both the cation and alkyl chain is impor-
tant for conversion process. Wang et al. reported that the product
2.1.1. Synthesis of 1-butylsulfonic-3-methylpyridinium based ILs
(IL 1–10)
Equimolar 3-methylpyridine and 1,4-butanesultone were taken
in a 100 mL round bottom flask and heated at 50–60 °C with con-
1
tinuous stirring for one hour to form Zwitterionic white solid (C -
ꢀ
+
C SO Py ). This white solid was washed with distilled ethyl acetate
4 3
to remove any unreacted 3-methylpyridine present in it and then
1
analyzed by H NMR (See SI).
White solid, Yield (>95%), 1H NMR (400 MHz, D
O) d (ppm):
), 2.425 (3H,
), 4.49 (2H, t, J = 10 Hz, CH ),
2
2 2
1.621–1.723 (2H, m, CH ), 1.994–2.095 (2H, m, CH
s, CH ), 2.84 (2H, t, J = 10 Hz, CH
3
2
2
7.798 (1H, t, J = 8.4 Hz, Ar-H), 8.236 (1H, d, J = 10.4 Hz, Ar-H),
8.536 (1H, d, J = 8.4, Ar-H), 8.595 (1H, s, Ar-H).
The vacuum-dried white solid was then refluxed at 80 °C with
ꢀ
ꢀ
ꢀ
ꢀ
1 eq. of different HXs (where X = HSO
4
, Cl , CH
3
COO , CF
3
COO ,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
CCl
3
COO , PhSO
3
, CH
3
PhSO
3
2
, NH SO
3
, CH
3
SO
3
) to produce com-
pletely viscous ILs.
4 1
2.1.2. Synthesis of 1-butyl-3-methylpyridinium chloride [C C Py]Cl
In a 100 mL round bottom flask, 1 eq. of 3-methylpyridine and
1.1 eq. of 1-chlorobutane were added and refluxed at 110 °C with
continuous stirring at 120 rpm. Progress of reaction was monitored
via TLC using 50% ethyl acetate: n-hexane solvent system. Reaction
was continued for 35–40 h until the formation of considerable
amount of product. After that, the product was washed several
times with ethyl acetate to ensure the complete removal of unre-
acted 3-methylpyridine. The final single spotted pure IL was stored
5
-HMF exhibits hydrogen bonding with anion of IL, weak interac-
tions with imidazolium cation via C@O functionality and almost
no interaction was detected between 5-HMF and alkyl chain of
the cation [15].
Besides inserting Lewis acidic character in IL, 5-HMF selectivi-
ties can also be enhanced by formation of organic electrolyte solu-
tions (OES). OES is formed by addition of some polar organic co-
solvent in IL to reduce its viscosity and enhance its thermal stabil-
ity [16]. Addition of polar protic or polar aprotic solvent in IL
increases the yield of 5-HMF by suppressing the rate of side reac-
tions and thus increasing the 5-HMF selectivity.
in an oven dried vial and sealed to prevent the moisture.
1
White solid, Yield (85%), H NMR (400 MHz, D
(3H, t, J = 7.6 Hz, CH
2
O) d (ppm): 0.85
), 1.88–1.93 (2H, m,
), 4.83 (2H, t, J = 7.2 Hz, CH ), 7.85 (1H, t,
3 2
), 1.24–1.29 (2H, m, CH
CH ), 2.46 (3H, s, CH
2
3
2
J = 6.8 Hz, Ar-H), 8.27 (1H, d, J = 8 Hz, Ar-H), 8.56 (1H, d, J = 5.6,
In this study, we used an unsymmetrical 3-methyl pyridinium
cation based on its four times less price as compared to the highly
investigated imidazolium ILs [17] and its best efficiencies for disso-
lution and conversion of lignocellulosics [18,19] according to our
previous findings. Quaternization is done by butane sulfonic acid
and 1-chlorobutane based on the best dissolution and conversion
Ar-H), 8.62 (1H, s, Ar-H).
2.2. Synthesis of 5-hydroxymethylfurfural (5-HMF)
IL (400 mg) was taken in an oven-dried vial followed by the
addition of metal salt (10 wt%). It was heated at 80–100 °C with
continuous stirring at 100 rpm until the formation of IL–MCl com-
plex. After that, 10 wt% fructose, glucose, cellulose or wheat husk
was added into it and stirred at specified temperature (80–
130 °C) for specific time (1–7 h). After completion of reaction,
the reaction mixture was cooled and subjected to qualitative and
quantitative analyses.
efficiencies of C
4
side chain [18] and best dehydration efficiencies
of SO H groups [13]. With these cationic cores, ten different anions
3
are tested for their dehydration, saccharification and isomerization
power with/without metal chloride and co-solvents.
2
. Materials and methods
-Methylpyridine (Sigma Aldrich), 1,4-butanesultone (Sigma
2.3. Analysis of products
3
Aldrich), 1-chlorobutane (Fischer Scientific), sulfuric acid (Acros),
hydrochloric acid (Sigma), acetic acid (Sigma), trichloroacetic acid
2.3.1. HPLC analysis
Quantification of produced 5-HMF was performed using D-Star
HPLC instrument with UV–Vis detector equipped with Discovery,
(
(
Aldrich), trifluoroacetic acid (Merck), benzene sulfonic acid
Sigma Aldrich), para-toluene sulfonic acid (Sigma Aldrich), 70%
HS C18 column; 5
l
m particle size (25 cm ꢁ 4.6 mm) using
2
Muhammad, Nawshad
