Hydrolysis kinetics of benzyl phenyl ether in high temperature liquid water

Article abstract of DOI:10.1016/j.cclet.2010.12.026

The application of high temperature liquid water (HTLW) to decomposition of lignin as efficient and green solution for phenolic compounds recovery was studied. Benzyl phenyl ether (BPE), the lignin model compound, was treated at temperatures ranging from

Full text of DOI:10.1016/j.cclet.2010.12.026

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 733–737
www.elsevier.com/locate/cclet
Hydrolysis kinetics of benzyl phenyl ether in high
temperature liquid water
*
Xiao Yu Wu, Xiu Yang Lu¨
Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China
Received 20 September 2010
Abstract
The application of high temperature liquid water (HTLW) to decomposition of lignin as efficient and green solution for phenolic
compounds recovery was studied. Benzyl phenyl ether (BPE), the lignin model compound, was treated at temperatures ranging
from 220 to 250 8C. BPE undergo hydrolysis in HTLW, and main products were phenol and benzyl alcohol with the minimum
selectivities of 75.7% and 82.8%, respectively. Lower temperature led to higher selectivity in 220–250 8C temperature range. The
kinetics on BPE hydrolysis was studied and the activation energy was determined as 150.3 Æ 12.5 kJ/mol with the first-order kinetic
equations. Based on products distribution, the reaction mechanism for decomposition of benzyl phenyl ether was proposed. The
investigated process provides insights into the design of a commercial method for utilization of lignin.
# 2010 Xiu Yang Lu¨. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Hydrolysis; Kinetics; Benzyl phenyl ether; High temperature liquid water; Lignin model compound
In response to depletion of fossil resources, the potential for using the biomass as a source of both commodity and
specialty chemicals attracts more attentions nowadays. Ether, especially aromatic ether, prevails in lignin, which is a
good source of phenolic monomers [1]. Various approaches have been attempted to cleave the ether in lignin, such as
pyrolysis [2], acid hydrolysis [3], supercritical fluid treatment [4], and hydrogenation [1]. However, these methods
have the disadvantages such as low selectivity, serious pollution, difficult recovery, and undesired by-products in
abundance. An efficient and green technology was needed.
High temperature liquid water (HTLW), defined as hot water between 100 and 374 8C, is a useful medium for
chemical reactions. Properties of HTLW are temperature-dependent and can be manipulated to optimize the reaction
environment, for example, the ion product increases by 3–4 orders of magnitude from its value at ambient conditions
its maximum value at around 250 8C, which enhances acidity/basicity in favor of acid- or base-catalyzed hydrolysis
[5]. These properties make HTLW a very promising technology for biomass refinery [6,7].
Few studies have been performed on the decomposition of lignin in HTLW. Wahyudiono et al. [8] carried out the
decomposition of lignin to phenolic compounds in near- and supercritical water. The results showed that higher
temperatures promoted the conversion of lignin into its derivate compounds. Pourali et al. [9] carried out the
hydrolysis of rice bran to produce valuable materials in HTLW. Phenolic compounds were mainly produced from
* Corresponding author.
E-mail address: luxiuyang@zju.edu.cn (X.Y. Lu¨).
1001-8417/$ – see front matter # 2010 Xiu Yang Lu¨. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.12.026

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X.Y. Wu, X.Y. Lu / Chinese Chemical Letters 22 (2011) 733–737
734
decomposition of lignin, carbohydrate, and phenolic compounds. For the purpose of further understanding the
mechanism and kinetics of the lignin hydrolysis in HTLW, benzyl phenyl ether (BPE) was selected as model
compound for lignin. The behavior of lignin model in HTLW was the first to be investigated.
1. Experimental
A carefully measured amount of BPE was added into the batch reactors consisted of 316 L stainless steel tubes with
volume of around 14.5 mL, and 10 mL deionized water was loaded. Thus, these experiments took place essentially
under water the saturation vapor pressure at each reaction temperature. After a desired time, not including the heat up
time of ca. 20 min, the reactor was removed from the furnace and placed into water to quench the reaction. Samples in
the reactor were discharged into a 50 mL volumetric flask. The reactor was then washed with methanol five times, with
each washing being added to the flask. Then the volumetric flask was filled with methanol. The standard deviations
were determined from replicate experiments.
The liquid products was identified by fragmentation patterns from GC–MS (Agilent 6890GC/5973 MSD) and by
matching HPLC retention times with known standards and quantitatively analyzed by HPLC (Agilent 1100).
2. Results and discussion
Fig. 1 shows the effect of initial concentration on BPE hydrolysis at 240 8C. It shows conversion rates of BPE were
similar at 0.01 and 0.02 g/mL. The conversions being invariant under nearly concentration signal first-order kinetics
for the hydrolysis reaction. Fig. 2 shows the effect of temperature on the BPE hydrolysis. Reactions were conducted at
220, 230, 240 and 250 8C with an initial BPE concentration of 0.02 g/mL. The BPE conversion was affected by both
temperature and time. BPE conversions were essentially 100% for 3 h at 250 8C, and 26.6% for 3 h at 220 8C.
The selectivities of phenol and benzyl alcohol from BPE at 220, 230, 240 and 250 8C are shown in Figs. 3 and 4.
The reaction temperature had a significant effect on reaction selectivities. Compared to phenol selectivity of about
102% at 220 8C for 4 h, the selectivity at 250 8C was 76.1% for 4 h. At the same condition, the selectivity of benzyl
alcohol was from 99.2% (220 8C) to 83.9%(250 8C). In addition, decrease of benzyl alcohol selectivity as a function of
time was much slighter than the phenol. Selectivities of benzyl alcohol were 93.4% at 240 8C for 0.5 h and 90.1% at
240 8C for 4 h, with only 3.4% decrease. However, the phenol decreased by 12.2%. The decline of selectivities on
phenol and benzyl alcohol indicated that higher temperature promoted the conversion of BPE into its derivative
compounds. BPE sample at 240 8C for 4 h was analyzed by GC–MS. Besides the phenol and benzyl alcohol, other
derived products such as benzylphenol (2- or 4-), dibenzylphenol and dibenzyl ether were found in MS fraction.
Benzyl cation (5) from benzyl alcohol caused alkylation of phenol to yield benzylphenol (6, 7) and dibenzylphenol (8),
and interacted with benzyl alcohol to yield dibenzyl ether (9) [10], and the increase in temperature enhanced the
alkylation reaction.
[()TD$FIG]
120
100
80
60
40
20
0
0.01g/mL
0.02g/mL
0
1
2
3
4
5
time/h
Fig. 1. Effect of initial BPE concentration on BPE conversion.

[()TD$FIG]
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X.Y. Wu, X.Y. Lu / Chinese Chemical Letters 22 (2011) 733–737
735
120
100
80
60
40
20
0
250°C
240°C
230°C
220°C
0
1
2
3
4
5
6
7
time/h
Fig. 2. Effect of different temperatures on BPE conversion.
[()TD$FIG]
120
100
80
60
40
20
0
250°C
240°C
230°C
220°C
0
1
2
3
4
5
6
time/h
Fig. 3. Effect of different temperatures on the selectivity of phenol from BPE conversion.
Fig. 5 shows the hydrothermal stability of benzyl alcohol and phenol at reaction temperature of 240 8C with the
initial concentration of 0.02 g/mL. Benzyl alcohol was quite stable at these reaction conditions, and its conversion was
only 5.2% for 4 h. However, phenol was not stable at 240 8C. The conversions were around 25% within 4 h reaction
time.
It is well-established that the formation of phenoxyl radical (10) is thermodynamically favored [11], which could
cause condensation polymerization. The by-products of 4-phenoxyphenol (11), diphenoxyphenol (12) from these
[()TD$FIG]
120
100
80
60
40
20
0
250°C
240°C
230°C
220°C
0
1
2
3
4
5
6
time/h
Fig. 4. Effect of different temperatures on the selectivity of benzyl alcohol from BPE conversion.

[()TD$FIG]
736
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X.Y. Wu, X.Y. Lu / Chinese Chemical Letters 22 (2011) 733–737
100
80
60
40
20
0
benzyl alcohol
phenol
0
1
2
3
4
5
time/h
Fig. 5. Hydrothermal stability of phenol and benzyl alcohol.
reactions by GC–MS analysis contributed to the instability of phenol. Moreover, part of phenol would be quickly
oxidized with a small amount of air in the tube at high temperature [12], which also led to the instability of phenol. The
main oxidation intermediate products were identified by the GC–MS as maleic acid (13), acrylic acid (14) and so on.
Through GC–MS analysis of benzyl alcohol sample, we identified low concentrations of components which were
benzaldehyde (15) and dibenzyl ether (9). The reaction of benzyl alcohol under hydrothermal conditions without the
presence of air was discussed by Hirashita [13], where small amount of benzaldehyde was also yielded. Therefore,
benzaldehyde in our experiments was probably not generated from the reaction with oxygen. One is forced to provide
for a pathway similar to that proposed by Martino et al. [14]: benzaldehyde may be formed by b-scission of the oxy
radical (16) from the decomposition of benzyl alcohol. It was short-lived and rare (see Fig. 6).
With BPE conversion in Fig. 2, we calculated pseudo-first-order of BPE hydrolysis. The values were
0.1209 Æ 0.0084 hÀ1, 0.3106 Æ 0.0342 hÀ1, 0.5913 Æ 0.0203 hÀ1, and 1.0028 Æ 0.1025 hÀ1 at 220 8C, 230 8C,
240 8C, and 250 8C, respectively. The Arrhenius parameters was determined by linear regression of Àln k vs 1000/RT,
which were log10A(hÀ1) = 34.6 Æ 2.9 and Ea = 150.3 Æ 12.5 kJ/mol. These kinetics results are the first to be reported
[()TD$FIG]
K
W
Ph
OH
+
H O
2
Ph
H
H
+
O
H
1
Ph
fast
O
Ph
Ph
Ph
Ph
+
H O
OH
+
Ph
O
Ph
Ph
2
O
H
1
O
H
PhCH
3
+
Ph
OH
2
2
OH
PhCH
Ph
Ph
+
H O
2
H
+
2
4
OH
O
OH
O
Ph
OH
Ph
Ph
Ph
O
O
10
11
12
O
HOOC
Ph
COOH
O
Ph
OH
+
2
+
OH
13
14
OH
Ph
CH
2
5
OH
7
OH
5
5
5
Ph
dibenzylphenol
8
Ph
Ph
OH
OH
Ph
+
6
O
9
Ph
Ph
O
¦Â-scission
CH
15
OH
Ph
O
PhCH
2
Ph
16
Fig. 6. Postulated reaction mechanism for hydrolysis of BPE in HTLW.

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X.Y. Wu, X.Y. Lu / Chinese Chemical Letters 22 (2011) 733–737
737
for the hydrolysis of BPE. The curves in Fig. 2 show that the conversion profiles calculated by the model do a very
good job of representing the experimental data at all temperatures. This data fitted provided estimation for the rate
constants at each temperature.
The hydrolysis of ethers in HTLW is postulated to be cleaved by an ionic SN1 mechanism due to the stable and
charged carbocation intermediates (1) [5], formed from protonation of ether by water and by H+ generated from self-
dissociation of water. The intermediate then decomposed unimolecularly in the rate-determining step to form an
alcohol (2) and a carbon cation (3). After that, the carbon cation reacted with water to generate another alcohol (4) and
H+, recycled to ether. The thermodynamic stability of radicals, providing a quantitative basis for rational design of
radical reactions in organic synthesis, can be predicted by radical stabilization energy (RSE). More negative was the
RSE of radical, more stable was the radical [15]. The RSE of the benzyl radical (À50.4 kJ/mol) was lower than
benzene radical (+10.3 kJ/mol), indicating the benzyl radical was more stable than benzene group. Combined with
previous sections, ionic mechanism of hydrolysis of BPE in HTLW was shown in Fig. 6.
3. Conclusions
Benzyl phenyl ether undergoes hydrolysis on the time scale of hours in liquid water at temperatures around 240 8C.
Phenol and benzyl alcohol are main products. Oxygen presence and high temperature affected the reaction selectivities
of phenol and benzyl alcohol and the distribution of products. BPE exhibited first-order kinetics and the activation
energy was determined as 150.3 Æ 12.5 kJ/mol. Based on these results, HTLW could be a promising medium in the
decomposition of lignin ethers and phenolic compounds recovery.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 20976160), and Zhejiang
Provincial Natural Science Foundation of China (No. R4080110).
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