Selective monoalkylation of hydroquinone in the presence of SO3H-functionalized ionic liquids as catalysts

Article abstract of DOI:10.1007/s11696-020-01230-1

A process aiming at O-alkylation of hydroquinone (HQ), where ionic liquids (ILs) act as catalyst is objectively described. Five SO₃H-functionalized ILs having different cations were prepared and characterized by NMR and FTIR techniques. The acidity and thermal stability of ILs were determined by Hammett function and thermogravimetric analysis (TGA), respectively. The catalytic activity of these ILs were tested for O-alkylation of HQ with methanol in 4-methoxyphenol (4MP) in the presence of small amount of benzoquinone (BQ). The effect of reaction parameters such as temperature, time, catalyst loading and substrate concentration on the conversion of HQ and product distribution was examined and optimized to maximize the yield of 4MP using 1,3-disulfonic acid imidazolium hydrogen sulfate (IL2) catalyst. Maximum yield of desired product 4MP 93.79percent was obtained at 338?K temperature, 5.45 × 10^–2?mol HQ, 8.33 × 10^–3?mol BQ, and 10.37?molpercent catalyst loading in 120?min reaction time. Single-product formation was observed up to 338?K temperature but higher temperature (above 338?K) and longer reaction time resulted in the formation of 2,4-dimethoxyphenol (24DMP) as a by-product. Catalyst recyclability was also established up to the fifth run which showed no declination in its activity.

Full text of DOI:10.1007/s11696-020-01230-1

Chemical Papers
https://doi.org/10.1007/s11696-020-01230-1
ORIGINAL PAPER
Selective monoalkylation of hydroquinone in the presence
of SO3H‑functionalized ionic liquids as catalysts
Priyanka V. Bhongale1,2 · Sunil S. Joshi1,2 · Nilesh A. Mali1,2
Received: 24 February 2020 / Accepted: 26 May 2020
© Institute of Chemistry, Slovak Academy of Sciences 2020
Abstract
A process aiming at O-alkylation of hydroquinone (HQ), where ionic liquids (ILs) act as catalyst is objectively described.
Five SO3H-functionalized ILs having diferent cations were prepared and characterized by NMR and FTIR techniques. The
acidity and thermal stability of ILs were determined by Hammett function and thermogravimetric analysis (TGA), respec-
tively. The catalytic activity of these ILs were tested for O-alkylation of HQ with methanol in 4-methoxyphenol (4MP) in
the presence of small amount of benzoquinone (BQ). The efect of reaction parameters such as temperature, time, catalyst
loading and substrate concentration on the conversion of HQ and product distribution was examined and optimized to maxi-
mize the yield of 4MP using 1,3-disulfonic acid imidazolium hydrogen sulfate (IL2) catalyst. Maximum yield of desired
product 4MP 93.79% was obtained at 338 K temperature, 5.45×10–2 mol HQ, 8.33×10–3 mol BQ, and 10.37 mol% catalyst
loading in 120 min reaction time. Single-product formation was observed up to 338 K temperature but higher temperature
(above 338 K) and longer reaction time resulted in the formation of 2,4-dimethoxyphenol (24DMP) as a by-product. Catalyst
recyclability was also established up to the ffth run which showed no declination in its activity.
Keywords O-alkylation · Ionic liquids · 4-Methoxyphenol · Hydroquinone · Benzoquinone
Abbreviations
IL2
1,3-Disulfonic acid imidazolium hydrogen
HQ
Hydroquinone
sulfate
BQ
Benzoquinone
IL3
3-Methyl-1-sulfonic acid-imidazolium hydro-
gen sulfate
4MP
24DMP
IL
4-Methoxyphenol
2,4-Dimethoxyphenol
Ionic liquid
IL4
1-(4-Sulfonic acid) butylpyridinium hydrogen
sulfate
IL1
1-(4-Sulfonic acid) butyl-3-methylimidazo-
lium hydrogen sulfate
IL5
N-(4-Sulfonic acid) butyl triethylammonium
hydrogen sulfate
H2SO4
Sulfuric acid
DMSO-d6 Deuterated dimethyl sulfoxide
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-020-01230-1) contains
supplementary material, which is available to authorized users.
FTIR
NMR
TGA
KBr
H0
Fourier-transform infrared
Nuclear magnetic resonance
Thermogravimetric analysis
Potassium bromide
Hammett acidity
Minute
* Sunil S. Joshi
ss.joshi@ncl.res.in
Priyanka V. Bhongale
pv.bhongale@ncl.res.in
min
mg
Milligram
Nilesh A. Mali
K
Kelvin
na.mali@ncl.res.in
mM
µM
Millimolar
1
Micromolar
Chemical Engineering and Process Development Division,
CSIR-National Chemical Laboratory, Dr. Homi Bhabha
Road, Pune 411008, India
2
Academy of Scientifc and Innovative Research (AcSIR),
CSIR-Human Resource Development Centre, Campus Postal
Staf College Area, Ghaziabad 201002, India
Vol.:(0123456789)
1 3

Chemical Papers
in patents (Suzuki and Sasaki 2001; Peter et al. 1972;
Ashtekar et al. 2006). Monoetherifcation of phenolic com-
pounds mainly HQ and its derivatives is reported using
alcohols and ester along with triethyl amine and its salt
(McCloud et al. 1975), alcohols with strong acids (Cor-
reale et al. 1990), halogen or hydrohalic acid and hydrogen
peroxide, and alcohol in the presence of transition metal
salts (Rivetti et al. 1984). Synthesis of 4MP using BQ in
the presence of methanol was reported by Rina Mondal
and co-researchers in 2014 (Mondal et al. 2014). Various
patents were published on the etherifcation or alkylation
of phenol and its derivative (Pansegrau and Munson 1998;
Torii et al. 1986; Satoh et al. 1995).
Introduction
Selective synthesis of O-alkylated products of phenol and
dihydric phenol such as anisole and methoxyphenols is a
subject of high industrial interest because these products
play a prominent role in the production of useful chemi-
cals such as butylated hydroxyanisoles and β-cyclodextrin
4-methoxyphenol (Abedi et al. 2013; Newman and Cella
1974; Yadav et al. 2005). The monomethyl ether of hyd-
roquinone, commonly named mequinol, is an important
synthetic intermediate used in the production of fne and
speciality chemicals (Bal et al. 2003). It is also used as
an antioxidant, polymerization inhibitor, stabilizer, lubri-
cating additive and key intermediate in many pharmaco-
logical applications (Yadav et al. 2005; Bal et al. 2003;
Gambarotti et al. 2013).
Several eforts are being made by various researchers
towards the application of ILs in catalysis which leads to
greener catalytic processes (Titze-Frech et al. 2013; Zhang
et al. 2011; Shirini and Khaligh 2013; Joseph et al. 2005;
Gui et al. 2004). It is essential to consider environmen-
tal and economic aspects while developing a new process
for aromatic alkylation and related process optimization.
Researchers are enhancing atom efciency of the process
by improving the selectivity of the desired product and
analyzing the efect of substrate variation and catalyst
activity (Titze-Frech et al. 2013). Sulfuric acid, p-tolue-
nesulfonic acid, and ion exchange resins were reported as
catalysts under mild reaction conditions. The present work
describes O-methylation of HQ in the presence of IL as
catalyst for the frst time and mainly focuses on attaining
high conversion and selective formation of product with
good catalyst recyclability.
Traditional method of synthesizing 4MP involves the
methylation of HQ with dimethyl sulfate or methyl halide
in the presence of sodium hydroxide (Newman and Cella
1974; Yadav et al. 2005; Bal et al. 2003; Gambarotti et al.
2013; Park et al. 2009). These processes are hazardous
and highly polluting and there has been a high demand to
reduce their negative environmental impacts and to dis-
cover clean and economical process based on green cata-
lytic system (Bal and Sivasanker 2003; Sad et al. 2010).
O-alkylation of phenol and its derivatives was studied
using homogeneous (sulfuric acid, perchloric acid, hydro-
halic acid) and heterogeneous (γ-alumina, zeolites, ion
exchange resins) catalysts at high pressure and high tem-
perature (Yadav et al. 2005; Gambarotti et al. 2013; Cor-
reale et al. 1990; Sad et al. 2014). Mono- and di-alkylation
of dihydroxybenzenes were studied in a vertical down-
fow glass reactor by Bal et al. (2003), Bal and Sivasanker
(2003) and Bal et al. (2002). Yadav et al. reported selec-
tive formation of hydroquinone monomethyl ether in the
presence of BQ over heteropolyacids supported on clays,
in which a detailed reaction kinetics was investigated and
corresponding reaction mechanism was described (Yadav
et al. 2005). Vijayaraj et al. found K-MgO to be an efcient
catalyst for O-methylation of HQ with dimethyl carbonate
at 583 K reaction temperature in a fow reactor (Vijayaraj
and Gopinath 2006). Synthesis of monoalkylated prod-
ucts of dihydroxybenzene using diferent solvents, such as
trichloroethylene, benzene, nitrobenzene and anisole, has
been patented in 1966 (Levy et al. 1966). Recently, Gam-
barotti et al. studied efect of catalytic amount of NaNO2
on the formation of mequinol with excellent isolated yield
with more than 99% selectivity and proposed a reaction
mechanism based on semi-quinone intermediate formation
(Gambarotti et al. 2013). Process for producing alkyl-sub-
stituted HQ, and monoalkylation of unsubstituted dihydric
phenols and dihydroxy aromatic compounds were reported
Experimental section
Chemicals
HQ and imidazole were procured from Loba Chemie Pvt.
Ltd., and BQ from Avra synthesis Pvt. Ltd. Methanol, sul-
furic acid, chlorosulfonic acid, dichloromethane, and 4MP
were purchased from Merck Millipore Pvt. Ltd. 1-Methyl
imidazole, 1,4-butane sultone, pyridine, and triethylamine
were obtained from Sigma-Aldrich Pvt. Ltd. 24DMP was
acquired from TCI chemicals.
Analysis
Reaction samples were analyzed by gas chromatography
equipped with fame ionization detector and HP-5 capil-
lary column. The individual compounds were identifed by
overlaying the curves with respective standards.
1 3

Chemical Papers
IL4 and IL5: IL4 and IL5 were prepared by following
the same procedure as described for IL1 with pyridine and
triethyl amine, respectively, instead of 1-methylimidazole.
Procedure
O-methylation of quinol using methanol was conducted
in a batch reactor. For performing reactions up to 338 K,
an assembly comprising a round-bottomed fask equipped
with a refux condenser and a temperature sensor to monitor
and control the reaction temperature was used. Reaction at
363 K was carried out in a Parr autoclave reactor in which
the desired temperature of the reaction mixture was main-
tained by an electric heating mantle and after completion
of reaction, chilled water was passed through the internal
cooling coil of the reactor to cool the reaction mixture to the
ambient temperature. A typical reaction mixture comprised
of HQ and BQ in excess of methanol. An initial sample was
withdrawn for analysis from the reaction mixture to confrm
the quantities of reactants taken, and the required catalyst
loading was added into the mixture. The reaction assembly
was maintained at a desired temperature and at a stirring
speed of 500 rpm. Further samples were withdrawn peri-
odically and analyzed by gas chromatography. The experi-
ments were carried out by varying the temperature in the
range of 303–363 K, time 0–180 min, HQ concentration
0–7.27 × 10–2 mol, BQ concentration 0–1.11 × 10–2 mol,
catalyst loading 0–12.45 mol% (mol of catalyst×100/ mol
of HQ+BQ), and 1.25 mol of methanol was used. Selectiv-
ity of 4MP was estimated by considering consumed moles
of HQ and BQ.
Catalyst characterization
The 1H and 13C nuclear magnetic resonance (NMR) study
was carried out on Avance 400 NMR spectrometer under
ambient conditions. Fourier transform infrared (FTIR) anal-
ysis of ILs was carried out in the range of 400–3500 cm−1
on Perkin Elmer spectrum using KBr pellets. The thermal
stability of ILs was measured using Perkin Elmer Pyris TGA
apparatus under nitrogen atmosphere at 25 ml/min fowrate,
and TGA was performed on 4–5 mg of catalyst in a platinum
pan with 5 K/min heating rate. Thermal stability of ILs was
determined using a dynamic method, by heating the sample
from 313 to 773 K temperature. Acidity of ILs was estimated
using the Hammett acidity function.
Results and discussion
The infuence of various reaction parameters, such as reac-
tion time, temperature, catalyst concentration, and substrate
concentration, on O-alkylation of HQ with methanol over
IL was investigated. Excess of methanol which serves as a
solvent as well as a methylating agent was used to perform
the reaction. The unreacted methanol can be easily recovered
and recycled using distillation. Consumption of HQ and BQ
was not observed in the absence of catalyst and thus catalyst
is found to be necessary to enhance the rate of reaction. The
role of catalyst is briefed through the reaction mechanisms
represented in Schemes 2, 3, 4, and 5. O-alkylation reaction
of HQ and BQ is depicted in Scheme 1.
Preparation of ionic liquids (ILs)
The Bronsted acidic ILs were prepared by the procedure
reported in the literature which is as illustrated below
(Shirini and Khaligh 2013; Gui et al. 2004; Shirini et al.
2012; Khaligh 2011).
4MP is formed by two routes:
IL1: 1-Methylimidazole (0.1 mol) and 1, 4-butane sultone
(0.1 mol) were added to a round-bottomed fask and the mix-
ture was kept under stirring at 313 K for 720 min. Further,
the white solid formed was washed repeatedly with ether and
dried under vacuum. After drying, an equivalent amount of
sulfuric acid (98%, 0.1 mol) was added dropwise and the
mixture was stirred for 360 min at 353 K to form IL1.
IL2: Chlorosulfonic acid (0.2 mol) was added dropwise to
a stirred solution of imidazole (0.1 mol) and dichlorometh-
ane (dry) at 273 K and kept for 180 min. Further, sulfuric
acid (98%, 0.1 mol) was added dropwise to the mixture over
a period of 10 min at room temperature and the mixture
was stirred overnight under nitrogen to remove HCl gas.
The mixture was refuxed for 180 min, dichloromethane was
decanted, and residue was repeatedly washed with dichlo-
romethane followed by drying under vacuum to give IL2.
IL3: For the preparation of IL3, 1-methyl imidazole was
used instead of imidazole and the same procedure was fol-
lowed as that mentioned for IL2.
1. HQ reacts with methanol in the presence of BQ to give
4MP and water.
2. Reaction of BQ with methanol to produce 4MP in which
the reducing property of alcohol is observed.
Parallelly, BQ reacts with two moles of methanol which
undergoes monoalkylation and methoxylation to yield
24DMP along with water. Role of BQ to initiate reaction
between HQ and methanol is well explained through the
reaction mechanism in Scheme 2.
BQ (2) interacts with acidic catalyst to give protonated
BQ (I2). Delocalization of I2 is expressed in Scheme 3. Fur-
ther reaction of I2 and protonated methanol (I3) generates an
intermediate I4 and I5. Water (5) is formed by the deprotona-
tion of I5. Similarly, HQ (1) in the presence of acidic catalyst
produces protonated HQ (I1). An intermediate I4 is reduced
with I1 to produce a protonated form of desired product 4MP
1 3

Chemical Papers
Scheme 1 O-alkylation of
hydroquinone in the presence of
benzoquinone over acid catalyst
Scheme 2 Reaction pathway
for the O-methylation of HQ in
presence of BQ (Yadav et al.
2005)
1 3

Chemical Papers
Scheme 3 Delocalization of
protonated BQ
(I6) along with the regeneration of I2. Here, BQ acts as a co-
catalyst due to its regeneration tendency and thus no forma-
tion of 4MP is possible in the absence of BQ. In next step,
the deprotonation of I7 resulted in product 4MP (4).
reduction in the concentration of HQ and BQ was inspected
with a corresponding selective rise in 4MP when the tem-
perature was increased from 303 to 363 K (Fig. 2). Above
338 K, change in the catalytic behavior was noticed with a
decrement in the yield of 4MP from 93.79 to 83.82%, and
enhancement in the undesired product 24 DMP with 12.22%
yield. Byproduct formation was observed after an increase
in the temperature above 338 K and thus this temperature
was chosen as an optimized parameter to study further the
efect of other reaction parameters on the rate of reaction.
The efect of contact time on alkylated product composition
over IL2 was studied by carrying out reactions in the range
of 0–180 min at 338 K. Sampling was done periodically
up to 180 min at 338 K and it showed a rapid decrease in
the concentration of HQ as compared to BQ (Fig. 3). As
mentioned in Scheme 2, protonated BQ is regenerated and
thus the rate of consumption of HQ was found to be higher
as compared to BQ. The conversion of HQ is much faster
during the initial 60 min, and further drop in the rate was
observed up to 180 min. After 120 min of reaction time,
slight advancement in the yield of 24DMP was detected.
The reaction pathway for the formation of 24DMP follows
Michael substitution reaction (Scheme 5). According to this
mechanism, BQ reacts with protonated methanol and inter-
mediate I11 is formed with the removal of water. Further
reaction between I11 and methanol follows Michael substitu-
tion mechanism through path I12 →I13. The product 24DMP
is formed by the deprotonation of intermediate I13.
Efect of diferent catalysts
In the present work, initially, the efect of diferent ILs
such as IL1, IL2, IL3, IL4, and IL5 (Fig. 1) on the conver-
sion of reactants and selectivity of 4MP was explored. The
acid–base property of ILs was determined by the cation and
anion present in the IL. The variation in cation resulted in
a deviation in the acidity of IL which directly afects the
activity of the catalyst (Tao et al. 2014). The performance of
sulfuric acid catalyst was investigated under identical con-
ditions which produce 4 MP with 94.98% selectivity and
remaining 5.02% 24 DMP. Maximum yield of the desired
monoalkylated product was obtained in case of IL2 with
100.00% selectivity. Conversion of HQ is directly associated
with the acidity of the IL and thus the catalytic activity is
in the order of IL2>IL3>IL4>IL5>IL1>H2SO4. Hence,
among the catalysts examined listed in Table 1, IL2 catalyst
was found to be the most efective for O-alkylation of HQ
into 4MP and thus chosen for further optimization study.
Yadav et al. investigated the O-alkylation of HQ in
the presence of BQ as co-catalyst at 483 K and reported
100% selectivity of 4MP and also confrmed the absence of
1,4-dimethoxybenzene which might be due to the presence
of excess of methanol. There were no data reported regard-
ing the conversion of BQ (Yadav et al. 2005). In the present
work, consumption of BQ 80–92% was noticed which fol-
lows the reaction pathway depicted in Scheme 4. Carboca-
tion form of protonated BQ (I2) reacts with methanol to give
oxonium intermediate I7 followed by the transfer of proton
resulting in I8. On dehydration of I8, an intermediate I9 is
produced. Methanol acts as a reducing agent which helps
reduce an intermediate I9 and proceeds to yield I10. Further,
the deprotonation of I10 leads to the formation of product
4MP (4).
Prolonged reaction time adversely afects the yield of the
desired product and hence optimum reaction time was set to
be 120 min in these experiments. An increasing trend for the
conversion of the substrate with temperature was observed
experimentally, which suggests that the reaction is intrinsi-
cally kinetically controlled.
No dialkylated product, 1,4-dimethoxybenzene, was
observed while optimizing reaction parameters which is in
good agreement with previously reported work by Yadav
et al. (2005) and Gambarotti et al. (2013). The substan-
tial difference in the acidity of monoalkylated product
4MP (pKa-10.21) and protonated benzoquinone (pKa-4)
(Hasegawa et al. 2017) causes 4MP not to react with metha-
nol. The dissociation energies of O–H bond in 4MP and pro-
tonated BQ are 349 (Evgeny and Taisa 2011) and 226 kJ/mol
(Gambarotti et al. 2013), respectively, which led to a higher
rate of alkylation of protonated BQ instead of 4MP at mild
Efect of time and temperature
The catalytic performance of IL2 towards the O-alkylation
of HQ in the presence of BQ was evaluated as a function of
temperature varying in the range of 303–363 K. A signifcant
1 3

Chemical Papers
Scheme 4 Reaction pathway for
the formation of 4MP from BQ
(Mondal et al. 2014)
Scheme 5 Reaction mechanism
for the formation of 24 DMP
(Gambarotti et al. 2013)
Fig. 1 Structure of ionic liquids
1 3

Chemical Papers
HQ
4MP
BQ
24DMP
Table 1 Catalyst
screening
(Reaction
conditions:
HQ:
100
80
60
40
20
0
5.45 × 10−2 mol, BQ: 8.3 × 10−3 mol, methanol: 1.25 mol; catalyst:
10.37 mol%, temperature: 338 K, and time: 120 min. IL1: 1-(4-Sul-
fonic acid) butyl-3-methylimidazolium hydrogen sulfate, IL2:
1,3-disulfonic acid imidazolium hydrogen sulfate, IL3: 3-methyl-
1-sulfonic acid-imidazolium hydrogen sulfate, IL4: 1-(4-sulfonic
acid) butylpyridinium hydrogen sulfate, and IL5: N-(4-sulfonic acid)
butyl triethylammonium hydrogen sulfate)
Catalyst
Conversion (%)
HQ BQ
0.00
70.00
94.03
90.58
82.00
75.00
88.68
Selectivity (%)
4MP 24DMP
0.00
No catalyst
IL1
IL2
IL3
IL4
IL5
Sulfuric acid
0.00
85.00
92.23
80.54
81.20
82.00
87.23
0.00
3.00
0.00
97.00
100.00
88.01
89.85
95.00
94.98
0
30
60
90
120
150
180
Time (min)
11.99
10.15
5.00
Fig. 3 Efect of reaction time on the methylation of HQ at HQ:
5.45×10–2 mol, BQ: 8.3×10–3 mol, methanol: 1.25 mol, IL2:
10.37 mol%, and temperature: 338 K
5.02
100
80
60
40
20
0
HQ
4MP
BQ
24DMP
100
80
60
40
20
HQ
4MP
BQ
24DMP
0
2
4
6
8
10
12
14
Catalyst loading (mol%)
0
293
313
333
353
373
Temperature (K)
Fig. 4 Efect of catalyst loading on the methylation of HQ at HQ:
5.45×10–2 mol, BQ: 8.3×10–3 mol, methanol: 1.25 mol, catalyst:
IL2, temperature: 338 K, and time: 120 min
Fig. 2 Efect of temperature on the methylation of HQ at HQ:
5.45×10–2 mol, BQ: 8.3×10–3 mol, methanol: 1.25 mol, IL2:
10.37 mol%, and time: 120 min
Selectively single-product formation was observed with
catalyst loading up to 10.37 mol%. Above 10.37 mol% cata-
lyst loading, reaction favors to follow Michael substitution
path (Scheme 5) with a rise in the yield of 24DMP up to
4.68%. Acidic sites play a signifcant role in the determina-
tion of conversion and yield of a product. A slight rise in
the catalyst loading from 10.37 to 12.45 mol% resulted in
the reduction of concentration of 4MP, and 19:1 mol ratio
of 4MP:24DMP was noticed.
reaction condition. According to the literature, dialkylation
of hydroquinone can occur at higher temperature (above
473 K) and in the presence of basic catalytic system (Bal
et al. 2003; Sang et al. 2002). Thus, alkylation of free OH
group does not occur, and the lack of formation 1,4-dimeth-
oxybenzene was observed.
Efect of catalyst loading
Efect of substrate concentration
Catalyst loading (mol%) was calculated as moles of cata-
lyst per combined moles of HQ and BQ, and its infuence
on the rate of reaction was studied by varying the catalyst
loading from 0 to 12.45 mol%. The results of the same
are depicted in Fig. 4. The concentration of HQ and BQ
showed a decreasing trend with increase in catalyst loading.
Experiments were performed with diferent substrate (HQ
and BQ) concentrations and their efect on the rate of reac-
tion is summarized in Figs. 5 and 6. One reaction was con-
ducted in the absence of HQ and results showed 29.75%
yield for 4MP and 68.32% yield for 24DMP. For this,
1 3

Chemical Papers
100
80
60
40
20
0
drastic deviation in molar ratio of 4MP to 24DMP from 0.43
to 11.5. Similarly, reactions were carried out by altering the
initial concentration of BQ from 0 to 1.11×10–2 mol and it
revealed a remarkable change in the reaction rate towards
monoalkylation. Methanol to BQ mole ratio was also found
to signifcantly afect the rate of reaction.
HQ
4MP
BQ
24DMP
When methanol was used in higher molar ratio with BQ,
the reaction redirected towards 24DMP formation in accord-
ance with Scheme 5.
Catalyst recycle study
0
2
4
6
8
Reusability of IL2 as catalyst was examined by conducting
experiments up to the ffth recycle run, and the results are
summarized in Table 2. Initially methanol was completely
recovered by vacuum distillation and further excess of
dichloromethane was added into the residue to solubilize all
the organics into it. This mixture was separated by centrifu-
gation, and residue was again washed with dichlorometh-
ane followed by centrifugation. The catalyst–water (water
formed during the reaction) residue obtained was heated to
373 K for 120 min under vacuum to remove water and then
reused. It was observed that the activity of catalyst does not
show any signifcant changes and thus conversion of HQ and
BQ was reduced from 94.03 to 91.38% and 92.23 to 90.09%,
respectively, from the fresh catalyst run to the ffth run, with
100.00% selectivity of 4MP. There is no formation of side
product even after the ffth recycle run. The recycled catalyst
was characterized by NMR and FTIR technique. Thus, the
reduction in conversion is marginal and it suggests that the
catalyst is stable and reusable without signifcant loss in its
activity.
Initial HQ × 102 (mol)
Fig. 5 Efect of HQ concentration on the methylation of HQ at BQ:
8.3×10–3 mol, methanol: 1.25 mol, IL2: 10.37 mol%, temperature:
338 K, and time: 120 min
100
HQ
4MP
BQ
24DMP
80
60
40
20
0
0
2
4
6
8
10
12
Initial BQ × 103(mol)
Catalyst characterization
Fig. 6 Efect of BQ concentration on the methylation of HQ at HQ:
5.45×10–2 mol, methanol: 1.25 mol, IL2:10.37 mol%, temperature:
338 K, and time: 120 min
1H NMR and 13C NMR analysis: For NMR analysis,
approximately 20 to 30 mg IL was added to solvent
DMSO-d6, and NMR was measured on Avance 400 NMR
reaction follows the mechanism as mentioned in Schemes
4 and 5. Similarly, another reaction in the absence of BQ
resulted in no conversion of HQ and thus BQ was found
to be necessary to initiate the reaction. The dissociation
energy of O–H bond in HQ is 339 kJ/mol (Chandra and
Uchimaru 2002), and reaction at mild conditions is unable
to acquire such high energy and thus no methylation of HQ
was observed in the presence of HQ only as substrate.
Variation in initial concentration of HQ showed sig-
nifcant change in the product distribution; for instance, an
increase in the concentration of HQ from 0 to 7.27×10–2 mol
led to signifcant growth in the yield of 4MP and corre-
sponding decay in the 24 DMP concentration. When the
reaction was performed with BQ as substrate, it resulted in
lower mole ratio of 4MP to 24DMP. Change in the initial
concentration of HQ from 0 to 4.09× 10–2 mol showed a
Table 2 Catalyst recycle study (Reaction conditions: HQ:
5.45 × 10−2 mol, BQ: 8.3 × 10−3 mol, methanol: 1.25 mol, IL2:
10.37 mol%, temperature: 338 K, and time: 120 min)
Recycle run
Conversion (%)
HQ
Selectivity (%)
4MP
BQ
Fresh
94.03
93.82
93.06
92.75
91.98
91.38
92.23
92.17
91.86
91.19
90.64
90.09
100.00
100.00
100.00
100.00
100.00
100.00
1
2
3
4
5
1 3

Chemical Papers
spectrometer and corresponding graphs are added in the
Supporting Information (Figs. S1–S12).
1H NMR (400 MHz, DMSO-d6)
IL1
IL2
IL3
IL4
IL5
0
20
IL1: δ = 1.50, 1.83, 2.46, 3.81, 4.14, 7.67, 7.73, 9.11.
IL2: δ = 7.63, 9.02, 14.23.
IL3: δ = 3.82, 7.59, 7.63, 8.15, 8.99, 14.15.
IL4: δ = 1.54, 1.95, 2.46, 3.12, 4.59, 8.10, 8.57, 9.06.
IL5: δ = 1.12,1.62, 2.46, 3.12, 3.17, 4.42, 5.16.
Recycled IL2: δ = 7.70, 9.09, 14.28.
13C NMR (400 MHz, DMSO-d6)
40
60
IL1: δ = 22.12, 29.06, 49.01, 50.93, 122.84, 124.14,
137.11.
80
IL2: δ = 119.85, 134.73.
100
IL3: δ = 35.90, 120.24, 123.69, 136.32.
IL4: δ = 21.94, 30.35, 50.82, 60.93, 128.65, 145.30,
146.05.
300
400
500
600
700
800
Temperature(K)
IL5: δ = 7.65, 20.38, 22.53, 50.70, 52.50, 56.30.
Recycled IL2: δ = 119.85, 134.85.
Fig. 7 TGA of ILs
FTIR (400–3500 cm−1
)
FTIR analysis results are mentioned below and the
spectra of each IL are mentioned in the Supporting Infor-
mation in Figs. S13–S18:
Hammett acidity
The acidity of all ILs and sulfuric acid was examined by
Hammett function using a UV–visible spectroscopy (Ela-
varasan et al. 2009).
IL1: 3115, 2941, 1698, 1574, 1457, 1233.
IL2: 2800–3200, 1684, 1233, 1062, 874, 570.
IL3: 2874, 1691, 1574, 1296, 1170, 1072, 1008, 883.
IL4: 3081, 2946, 1636, 1493, 1161, 1035.
IL5: 2937, 1484, 1395, 1161, 1000.
Comparison of Hammett acidity function of ILs and
sulfuric acid in water with 4-nitroaniline as an indicator is
shown in Fig. 8. It can be inferred from the fgure that the
acidity follows the order of acidity as IL2 > IL3 > IL4 > I
L5 > IL1 > H2SO4. The values of Hammett function (H0)
of 4000 µM IL are mentioned in Table 3.
Recycled IL2: 2800–3200, 1726, 1242, 1062, 883, 596.
TGA
The thermal stability of all ILs was tested in the range of
313–773 K and results of the same are listed in Table 3.
An overlay of weight % versus temperature for all ILs is
as shown in Fig. 7.
3.0
IL1
2.8
2.6
2.4
2.2
2.0
1.8
1.6
IL2
IL3
IL4
IL5
H
SO
2
4
Table 3 Hammett acidity and thermal stability of ILs
Catalyst
H0 of 4000 µM IL
Thermal
stability up
to (K)
IL1
IL2
IL3
IL4
IL5
H2SO4
2.3247
1.8580
1.9445
2.0721
2.2409
2.4098
651
538
528
618
606
–
0
2000
4000
6000
8000
10000
12000
Concentration (µM)
Fig. 8 Hammett acidity of ILs in water with 4-nitroaniline as indica-
tor
1 3

Chemical Papers
Chandra AK, Uchimaru T (2002) The O–H bond dissociation energies
of substituted phenols and proton afnities of substituted phe-
noxide ions: a DFT study. Int J Mol Sci 3:407–422. https://doi.
org/10.3390/i3040407
Conclusions
O-methylation of hydroquinone using catalytic amount of
ionic liquid resulted in high substrate conversion and high
product selectivity. Among all investigated ILs, 1,3-disul-
fonic acid imidazolium hydrogen sulfate was found to be the
most efcient towards mono-O-methylation of hydroquinone
into 4-methoxyphenol. The reaction gave 93.79% yield for
the desired product using IL2 under mild reaction condi-
tions, refux temperature, and atmospheric pressure. Also, no
byproducts formation were observed up to 338 K. The efect
of reaction parameters on the reaction rate was examined,
and optimized parameters were obtained through experimen-
tal results. No dialkylated product was observed. Reaction
mechanism governing the formation of 4–methoxyphenol
via two diferent routes was reported. Efective recyclability
of IL2 was confrmed up to the ffth cycle without signifcant
loss in its activity. Excellent reaction rate with selective sin-
gle-product formation, ease of separation and recyclability
of the catalyst, and reactions under mild conditions are the
advantages of this method which shows a potential applica-
tion in industry.
Correale M, Panseri P, Romano U, Minisci F (1990) Process for
the preparation of mono-ethers of hydroquinones. US Patent
4,933,504. Enichem Synthesis S.p.A., Palermo, Italy, Appl.No.
275,968
Elavarasan P, Kondamudi K, Upadhyayula S (2009) Relative acidity
measurement of Bronsted acid functional ionic liquids by UV-
spectroscopy. Bull Cataly Soc India 8:107–113
Evgeny D, Taisa D (2011) Dissociation energies of O-H bonds of phe-
nols and hydroperoxides. In: Application of thermodynamics to
biological and materials science, ISBN: 978-953-307-980-6. https
://doi.org/10.5772/13290
Gambarotti C, Melone L, Punta C, Shisodia SU (2013) Selective
monoetherifcation of 1,4-hydroquinone promoted by NaNO2.
Curr Organ Chem 17:1108–1113. https://doi.org/10.2174/13852
72811317100011
Gradef PS, Bertrand C (1972) Monoalkylation of unsubstituted dihy-
dric phenols with lower alkyl chlorides using alkali metal carbon-
ated or bicarbonated as a condensing agent. US Patent 3,689,570,
Appl.No.2,951
Gui J, Cong X, Liu D et al (2004) Novel Bronsted acidic ionic liq-
uid as efcient and reusable catalyst system for esterifcation.
Cataly Commun 5:473–477. https://doi.org/10.1016/j.catco
m.2004.06.004
Hasegawa R, Saito K, Takaoka T, Ishikita H (2017) pKa of ubiquinone,
menaquinone, phylloquinone, plastoquinone, and rhodoquinone
in aqueous solution. Photosyn Res 133:297–304. https://doi.
org/10.1007/s11120-017-0382-y
Acknowledgements Priyanka Bhongale acknowledges the support
from Council of Scientifc and Industrial Research (CSIR), India, for
providing Senior Research Fellowship (SRF). The authors thank Acad-
emy of Scientifc & Innovative Research (AcSIR) and CSIR-National
Chemical Laboratory for additional support. We thank Dr. Subhash
Chavan for valuable help.
Joseph T, Sahoo S, Halligudi SB (2005) Bronsted acidic ionic liquids: a
green, efcient and reusable catalyst system and reaction medium
for Fischer esterifcation. J Mol Cataly A Chem 234:107–110.
https://doi.org/10.1016/j.molcata.2005.03.005
Khaligh NG (2011) Preparation, characterization and use of 3-methyl-
1-sulfonic acid imidazolium hydrogen sulfate as an eco-benign,
efcient and reusable ionic liquid catalyst for the chemoselec-
tive trimethylsilyl protection of hydroxyl groups. J Mol Cataly A
Chem 349:63–70. https://doi.org/10.1016/j.molcata.2011.08.021
Levy J, Paramus NJ, Friedman A (1966) Method for producing mono-
alkyl ethers of dihydric phenols. US Patent 3,274,260. Universal
Oil Products Company
Compliance with ethical standards
Conflict of interest On behalf of all authors, the corresponding author
states that there is no confict of interest.
McCloud GT, Brimer MR, Gibson CL (1975) Etherifcation of phe-
nolic compounds. US Patent 3,911,022. Eastman Kodak Com-
pany, Rochester, NY, Appl.No.367,038
References
Mondal R, Guha C, Mallik AK (2014) Facile conversion of para-
benzoquinones to para-alkoxyphenols with primary/secondary
alcohols and amberlyst-15: a process showing novel reducing
property of such alcohols. Tetrahedron Lett 55:86–89. https://
doi.org/10.1016/j.tetlet.2013.10.119
Abedi JA, Abbasali K, Yadollah M (2013) Vapor-phase selective
O-alkylation of catechol with methanol over lanthanum phosphate
and its modifed catalysts with Ti and Cs. J Molec Cataly A Chem
372:79–83. https://doi.org/10.1016/j.molcata.2013.02.011
Ashtekar S, Kumbhar P, Mahalingam RJ, Thampi J (2006) Process for
the monoalkylation of dihydroxy aromatic compounds. US Patent
7,087,705 B2. General Electric Company, Schenectady, NY (US),
Appl.No.10/815,879
Newman MS, Cella JA (1974) Studies on the Monoalkylation of Hyd-
roquinone. J Org Chem 39:214–215. https://doi.org/10.1021/jo009
16a020
Pansegrau PD, Munson BP (1998) Method for the production of
alkoxy- and aryloxy-phenols. US Patent 5,840,997. Dakota Gasi-
fcation Company, Beulah, N. Dak., Appl.No.831,474
Park JJ, Lee SC, Lee SS et al (2009) The efect of phase transition of
methanol on the reaction rate in the alkylation of hydroquinone.
Korean J Chem Eng 26:649–653. https://doi.org/10.1007/s1181
4-009-0108-8
Bal R, Mayadevi S, Sivasanker S (2003) O-methylation of dihydroxy-
benzenes with methanol in the vapour phase over alkali-loaded
SiO2 catalysts: a kinetic analysis. Org Process Res Dev 7:17–21.
https://doi.org/10.1021/op020054o
Bal R, Sivasanker S (2003) Vapour phase selective O-alkylation of
phenol over alkali loaded silica. Appl Cataly A Gen 246:373–382.
https://doi.org/10.1016/S0926-860X(03)00082-6
Rivetti F, ROmano U, Muzio ND (1984) Process for preparing mon-
oalkylethers of hydroquinone and its derivatives. US Patent
4,469,897. Anic S.p.A., Palermo, Italy, Appl.No.313,677
Sad ME, Padro CL, Apesteguia CR (2010) Study of the phenol meth-
ylation mechanism on zeolites HBEA, HZSM5 and HMCM22. J
Bal R, Tope BB, Sivasanker S (2002) Vapour phase O-methylation
of dihydroxy benzenes with methanol over cesium-loaded silica,
a solid base. J Mol Cataly A Chem 181:161–171. https://doi.
org/10.1016/S1381-1169(01)00361-2
1 3

Chemical Papers
Mol Cataly A Chem 327:63–72. https://doi.org/10.1016/j.molca
Tao D, Wu J, Wang Z et al (2014) SO3H-functionalized Bronsted acidic
ionic liquids as efcient catalysts for the synthesis of isoamyl
salicylate. RSC Adv 4:1–7. https://doi.org/10.1039/c3ra45921g
Titze-Frech K, Ignatiev N, Uerdingen M et al (2013) Highly selec-
tive aromatic alkylation of phenol and anisole by using recyclable
Bronsted acidic ionic liquid systems. Eur J Org Chem. https://doi.
org/10.1002/ejoc.201300579
ta.2010.05.014
Sad ME, Padro CL, Apesteguia CR (2014) Phenol methylation on acid
catalysts: study of the catalyst deactivation kinetics and mecha-
nism. Appl Cataly A Gen 475:305–313. https://doi.org/10.1016/j.
apcata.2014.01.039
Sang SL, Soo C, Lee JCK (2002) Selective O-alkylation reaction of
hydroquinone with methanol over Cs Ion-exchanged zeolites.
Korean J Chem Eng 19:406–410. https://doi.org/10.11405/nissh
oshi1964.62.850
Torii S, Tanaka H, Akada M (1986) Process for preparing alkoxyphe-
nols. US Patent 4,588,835. Otsuka Kagaku Yakuhin Kabushiki
Kaisha, Osaka, Japan, Appl.No.662,542
Shirini F, Khaligh NG (2013) 1,3-Disulfonic acid imidazolium hydro-
gen sulfate as an efcient and reusable ionic liquid catalyst for the
N-Boc protection of amines. J Mol Liq 177:386–393. https://doi.
org/10.1016/j.molliq.2012.10.022
Vijayaraj M, Gopinath CS (2006) Selective production of methoxyphe-
nols from dihydroxybenzenes on alkali metal ion-loaded MgO. J
Cataly 243:376–388. https://doi.org/10.1016/j.jcat.2006.08.009
Yadav GD, Deshmukh SARK, Asthana NS (2005) Synthesis of hydro-
quinone monomethyl ether from hydroquinone and methanol over
heteropolyacids supported on Clay: Kinetics and mechanism. Ind
Eng Chem Res 44:7969–7977. https://doi.org/10.1021/ie050519+
Zhang Q, Zhang S, Deng Y (2011) Recent advances in ionic liquid
catalysis. Green Chem 13:2619–2637. https://doi.org/10.1039/
c1gc15334j
Shirini F, Khaligh NG, Akbari-Dadamahaleh S (2012) Preparation,
characterization and use of 1,3-disulfonic acid imidazolium
hydrogen sulfate as an efcient, halogen-free and reusable ionic
liquid catalyst for the trimethylsilyl protection of hydroxyl groups
and deprotection of the obtained trimethylsilane. J Mol Cataly A
Chem 365:15–23. https://doi.org/10.1016/j.molcata.2012.08.002
Satoh T, Matsumoto H, Niiro Y (1995) Hydroquinone derivatives.
US Patent 5,416,242. Nippon Hypox Laboratories Incorporated,
Tokyo, Japan, Appl.No.283,713
Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional afliations.
Suzuki T, Sasaki T (2001) Process for producing alkyl substituted
hydroquinones. US Patent 6,303,801 B1. Sumitomo Chemical
Company, Limited, Osaka (JP), Appl.No.09/517,058
1 3