Chem. Eng. Technol. 2006, 29, No.

10, 12271231

Holger Becker1
Herbert Vogel1
1

Department of Chemistry,
Ernst-Berl-Institute for
Technical Chemistry and
Macromolecular Science,
University of Technology
Darmstadt (TUD), Germany.

1227

Research Article

The Role of Hydroquinone Monomethyl Ether

in the Stabilization of Acrylic Acid
Stabilizers prevent the inadvertent polymerization of acrylic acid; they work as
scavengers. In practice, hydroquinone monomethyl ether (p-methoxyphenol,
MeHQ), among others, has established itself as a standard stabilizer. Under the
process conditions of acrylic acid production, MeHQ is required at high temperatures and in the presence of dissolved atmospheric oxygen for polymerization inhibition. Our investigations in air and nitrogen atmospheres confirm that MeHQ
is only effective as a stabilizer in the presence of dissolved molecular oxygen. Up
to temperatures of 60 C, MeHQ consumption in acrylic acid is practically negligible. At temperatures above 60 C, the stabilizer concentration decreases almost
linearly with time. At temperatures above 80 C, degradation accelerates. The
consumption ratio of O2 (in number of moles) to MeHQ (in number of moles) is
about 6:1. Based on the degradation kinetics of O2 und MeHQ that are determined, the amounts of MeHQ required for a reasonable stabilisation can now be
calculated.
Keywords: Acrylic monomers, Oxygen, Polymerization
Received: December 13, 2005; accepted: May 29, 2006
DOI: 10.1002/ceat.200500401

Introduction

1.1

Motivation and Objective

Acrylic acid (AA) is an intermediate product which is produced on an industrial scale. The world annual production
amounts to ca. 3.4 million t a1 (in 1994, ca. 2 million t a1) [1,
2]. In the last few years, the consumption of acrylic acid has
increased immensely, mainly due to the great increase in demand for super absorber polymers.
The inadvertent polymerization of acrylic acid is caused by
radicals (impurities, UV radiation, cosmic radiation etc.). The
strongly exothermic polymerization (DHR = 76 kJ mol1)
constitutes a significant safety risk during production, workup, as well as in storage and transportation. On the one hand,
the released heat can cause deflagration and explosions. On
the other hand, the polymers can lead to blockages and breakdowns in parts of the production plant, leading to losses in
production and higher maintenance costs. Therefore, acrylic
acids, e.g., phenothiazin, hydroquinone monomethyl ether
(MeHQ), and/or molecular oxygen (O2) are added as poly-

Correspondence: Prof. Dr.-Ing. H. Vogel (vogel@ct.chemie.tu-darmstadt.de), Department of Chemistry, Ernst-Berl-Institute for Technical
Chemistry and Macromolecular Science, Darmstadt University of
Technology (TUD), Petersenstrae 20, D-64287 Darmstadt, Germany.

2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

merization inhibitors [3]. Under storage and process conditions, MeHQ is introduced in combination with dissolved O2.
Here, the MeHQ does not react directly with the primary radicals (R.); the primary inhibitor is the dissolved molecular oxygen. The underlying mechanisms of how the O2/MeHQ stabilizer system acts, is as follows: The primary radicals react with
O2 to form peroxide radicals (RO2.). These are then be
trapped up by the reaction with MeHQ [4] (see Scheme 1).
Based on the consumption of the oxygen and MeHQ, conclusions can be drawn on the stability of the monomers. Thus,
it was an objective of this work to study the kinetics of the O2
and MeHQ consumption during the inhibition period in
acrylic acid. In order to judge the role of the acrylic acid, as a
comparison, measurements were made in acetic acid (HAc),
which has similar solvent properties to acrylic acid but cannot
+ O2

RO2

+ HO

RO2

OCH3

RO2H +

OCH3

OOR
RO2

OCH3

O
OCH3

Scheme 1. Reaction mechanism.

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Chem. Eng. Technol. 2006, 29, No. 10, 12271231

H. Becker et al.

polymerize naturally. From the determined consumption kinetics, an improvement in the prediction of the stability of
acrylic acid should be possible.

1.2

State of the Art

The inadvertent radical polymerization is initiated by primary

radicals (R.) which can, amongst others, form via impurities,
light, and cosmic radiation. In the presence of O2, these primary radicals form peroxide radicals (RO2.) [5]. In the unstable monomer (M), they react with another monomer unit
(rate determining step). Thus, a copolymer of M-O2-units
forms. Polymerization (M-M-) and copolymerization (M-O2M-O2-) are competitive reactions in the presence of dissolved
oxygen. The solubility of O2 in acrylic acid under normal conditions and air coverage amounts to 56 ppm (g/g). The polymerization is inhibited in the presence of O2 because the copolymerization proceeds faster than the polymerization [6]. The
dissolved oxygen is consumed by the formation of copolymers.
With strict alternation of the chain reactions, a linear O2 consumption results [7] with the following kinetics:
rtotal

dO2

dt

dM
k4
dt

with : I
2RO2

k6 =Lmol 1 s

M
RO2

k1 =s

s
2 k1
I M const:
k6

MeHQ Analysis

For the quantitative determination of MeHQ and its degradation products, a HPLC device (Model HP 1090 Series L Liquid
Chromatograph) with a UV-VIS detector (Filter Photometric
Detector) was used. A modified RP18 column [NC-03 (250
3.0 mm) PRONTOSIL 120-3-C18-AQ 3.0-956m] was used.
The standard mobile phase was a mixture of 40 % (L/L) acetonitrile and 60 % (L/L) water (Flow rate: 0.5 mL min1). The
UV detector was operated at 280 nm (absorption maximum of
MeHQ). The column was operated at 50 C and the injection
volume was varied according to the MeHQ concentration.

2.2.1 Chemicals and Equipment Used

The investigations were carried out using pure acrylic acid stabilized with 200 ppm MeHQ supplied by BASF AG, Ludwigshafen. For the purification and the elimination of the storage
stabilizer, the monomer was only re-crystallized once. After
the cleaning process, the desired MeHQ concentration was adjusted and controlled via the HPLC. The monomer was either
directly used or stored at 20 C to avoid side reactions such as
the formation of diacrylic acid. The chemicals and equipment
used are listed in Tabs. 1 and 2.

! 2R

! RO2 O2 R

k4 =Lmol 1 s

2.2

Table 1. Equipment and their manufacturer(s).

Equipment

Producer

O2-sensor
(electrochemical)

Aero2-Mat 4125, FA. Syland Scientific GmbH,

Heppenheim, Germany

Pumps

PTFE-Minidosierer BF 411, (060 mL/min),

Fa. Telab, Dosiertechnik & Handelsgesellschaft
GmbH, Duisburg, Germany

AD-transducer

Airflow Memory AM-2, Fa. Airflow

Lufttechnik GmbH, Rheinbach, Germany

Thermostats

Julabo HC 5, Fa. Julabo Labortechnik GmbH,

Seelbach, Germany

! RO2 M

Thus, the degradation kinetics of MeHQ are directly

coupled with the consumption of O2, since MeHQ can only react with peroxide radicals that form from primary radicals and
O2. Since one MeHQ molecule is capable of taking up two peroxide radicals, in theory, a consumption ratio of O2 to MeHQ
of 2:1 (mol/mol) is expected. The inhibition reaction (R-O2MeHQ) and the copolymerization (R-O2-M-O2-) of the peroxide radicals, however, represent competitive reactions.
Therefore, with insufficient concentrations of MeHQ, the
reaction rate of the copolymerization can be greater than the
inhibition reaction. This means a change from O2/MeHQ stabilization to pure O2 stabilization.

Experimental

2.1

Apparatus and Chemicals

The apparatus and devices used for the O2/MeHQ consumption measurements are described extensively in the literature
[4, 8]. The chemicals used and their pre-treatment are also to
be found there, and are stated in more detail.

2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Mass Flow Controller MFC 5850 TR, N2 von 05 L/min, Fa. Brooks
Instrument B. V., Veenendaal, The Netherlands
HPLC system

HP 1090 Series L Liquid Chromatograph

HPLC software

Varian Star 5.3

Results and Discussion

3.1

MeHQ Consumption

Fig. 1 shows a typical HPLC-chromatogram. The peak with a

retention time of 3.06 min was caused by acrylic acid and is
not quantitatively meaningful for the analysis. The measurements show that during the reaction of MeHQ in acrylic acid,
three degradation products, that have not been studied further,
are formed: A1-MeHQ, A2-MeHQ, and A3-MeHQ.

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Chem. Eng. Technol. 2006, 29, No. 10, 12271231

Acrylic acid

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Table 2. Chemicals used.

Chemical

Purity

Company

Acrylic acid Purum > 99.5 % stabilized BASF AG Ludwigshafen,

with 200 ppm (g/g) MeHQ Germany
Acetic acid

99100 %

Riedle-de Haen AG, Seelze,

Germany

MeHQ

> 98 %

Fluka Chemie AG, Buchs,

Switzerland

PTZ

> 99 %

Acros, Neuss, Germany

Nitrogen

99.999 % (N2 5.0)

Linde AG, Wiesbaden,

Germany

Acetonitrile 99.99 %

Fisher Chemicals, Germany

Water

TUD, Darmstadt, Germany

bidistilled

110
100

A1-MeHQ

60

A3-MeHQ

70

A2-MeHQ

80

MeHQ

mVolts

90

50
44

2.5

5.0

7.5

10.0

12.5

Minutes

Figure 1. Chromatogram of MeHQ in AA under air atmosphere

(90 C) at the beginning (grey) and after 9 hours (black) (40 %
acetonitrile, 60 % H2O, 280 nm, 0.5mL min1, T(HPLC) = 50 C).

The experiments on MeHQ consumption were carried out in

acrylic acid and acetic acid, respectively. Acetic acid serves as a
reference medium for acrylic acid as it possesses similar solvent
properties but is unable to form primary radicals. In acrylic
acid, the consumption kinetics were determined in air atmosphere (21 vol-% O2) and in acetic acid, the measurements were
also carried out in a nitrogen atmosphere (0 vol-% O2). In the
experiments, a linear decrease in MeHQ as a function of retention time was confirmed up to ca. 70 C (see Fig. 2).
From approximately 80 C, an acceleration in the consumption of MeHQ occurs at the end of the inhibition period. The
consumption rates kges were calculated from the MeHQ-concentration/time curves. This asserts that the consumption of
MeHQ in acrylic acid is a multiple larger than in acetic acid.
Thus, the ratio of the initial consumption in acrylic acid to
acetic acid rises from 58 at 80 C, to 134 at 100 C (see Fig. 3).
The degradation speed of MeHQ in acetic acid in the presence of nitrogen, is only slightly smaller than that in air (see
Fig. 4), which suggests that MeHQ is relatively resistant to oxidation under the chosen conditions.

2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 2. Decline of the MeHQ concentration as a function of

the retention time at different temperatures. In acrylic acid with
air (a), in acetic acid with air (b).

Figure 3. MeHQ consumption rates in acrylic acid in air (),

and the relationship of the MeHQ consumption rates in acrylic
acid and acetic acid in an air atmosphere (D), respectively.

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Chem. Eng. Technol. 2006, 29, No. 10, 12271231

H. Becker et al.

3.2

Figure 4. MeHQ consumption rates in acetic acid as a function

of temperature in air () and N2 () atmospheres.

Correlation of O2 and MeHQ Consumption

Experiments in a continuously stirred tank reactor, in which

the O2 and MeHQ consumptions are measured simultaneously, should deliver more precise insight into the inhibitions
processes. Theoretically, two molecules of oxygen should be
consumed per molecule of MeHQ. Examination of this relationship provides information on the effectiveness of MeHQ
as a peroxide radical scavenger. With 200 ppm (g/g) of MeHQ
in acrylic acid, MeHQ consumption is only measurable above
70 C within the constraints of the chosen timeframe. MeHQ
and O2 consumption, respectively, increase exponentially with
temperature (see Fig. 6). The outcome is an O2 to MeHQ relationship of approximately six (80 C).
This means that the peroxide radicals react with other
monomers and oxygen and form short-chained copolymers
before the reaction with MeHQ, which on average consist of
two monomer units (M), one primary radical unit (R) and
three oxygen units (O2). Only then are two of the mix polymer
peroxide radicals deactivated by the inhibitor, the first via
H-transfer and the second via radical recombination of the
MeHQ radicals (see Fig. 7).

Figure 5. Arrhenius plot of the MeHQ consumption rates in

acrylic acid in air (), and in acetic acid in air () and N2 ()
atmospheres, respectively.

Examination of the Arrhenius plot shows that the consumption of MeHQ in acetic acid is thus well described. In acrylic
acid, however, anomalies arise at higher temperatures (see
Fig. 5).
This can be explained as change in the mechanisms of the
radical formation, since at higher temperatures more radicals
can be generated through the decomposition of the copolymers formed from O2 and acrylic acid (-O2-AA-O2-AA-). This
causes a higher consumption of MeHQ. From a temperature
of 80 C, the gradient (which corresponds to a decrease in EA)
of the Arrhenius curve flattens. This flattening serves as an indication of the change in the governing mechanisms.
With the help of the Arrhenius plots, the rate constants
kges.(MeHQAA) can be estimated at temperatures higher than
90 C by extrapolation; the MeHQ consumption under the
conditions of the acrylic acid workup can be calculated (100
130 C). Thus, along with the knowledge of the O2 consumption, one possesses another parameter for monitoring the production process.

2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 6. O2 () and MeHQ () consumption rates in a continuously operated stirred tank reactor as a function of temperature (VR = 5079 mL, VWZ = 820 h, T = 4090 C).

Figure 7. The peroxide radicals formed from primary radicals

and O2, pass through, on average, two copolymer growth steps
before deactivation with MeHQ is initiated.

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Chem. Eng. Technol. 2006, 29, No. 10, 12271231

Conclusions

The measurements of the O2 and MeHQ consumptions show

that the peroxide radicals can only be absorbed after copolymer formation from acrylic acid and O2. This is crucial for the
progression of the inhibition period since the decomposition
of the copolymers produces new radicals. These radicals are
then responsible for the acceleration of the MeHQ/O2 consumption. The effectiveness of MeHQ as a scavenger as well as
a stabilizer can therefore be described as moderate.
The MeHQ consumption as the O2 consumption can be
described with a kinetic of zeroth order. With the appropriate
Arrhenius parameters, the MeHQ consumption can be extrapolated to higher operating temperatures, which e.g., must be
considered for acrylic acid rectification for temperatures up to
130 C. Thus, the required amounts of MeHQ can be adjusted
during the acrylic acid work-up and predictions can be made
on the actual acrylic acid stability.

2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Acrylic acid

1231

Acknowledgements
We would like to thank BASF AG, Ludwigshafen, Germany, for
financial support.

References
[1] Markets & Economics, Acrylic Acid, Chemical Week, 2001,
January.
[2] H. Vogel, CHEManager 2001, 34.
[3] W. Kurze, F. Raschig, Ullmans Enzyklopdie der Technischen
Chemie, Bd. 8, Antioxidantien, Verlag Chemie, Weinheim
1975, 19.
[4] S. Schulze, H. Vogel, Chem. Eng. Technol. 1998, 21 (10), 829.
[5] P. Gladyshev, D. K. Kitaeva, V. A. Popov, E. I. Penkov, Proc.
Acad. Sci. USSR 1974, 215, 354.
[6] J. J. Kurland, J. Polym, Polym. Sci. 1980, 18, 1139.
[7] H. Becker, H. Vogel, Chem. Eng. Technol. 2002, 25 (5), 547.
[8] H. Becker, H. Vogel, Chem. Eng. Technol. 2004, 27 (10), 1122.
DOI: 10.1002/ceat.200302114

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