Role of alkali in catalytic oxidation of p-cresols

Article abstract of DOI:10.1016/j.molcata.2016.04.003

The investigation of alkali acting as the key component during the liquid-phase catalytic oxidation of p-cresols to p-hydroxyl aromatic aldehydes is far from satisfied, thus a detailed exploration is presented. It was observed that the p-cresols were activated by alkali to form phenolate salt to ensure mild reaction conditions demonstrated by comparing the oxidation of p-cresol and its phenolate sodium. It was also found that excess alkali improved the selectivity of aldehyde by inhibiting the formation of dimeric side products in view of an analysis of the side products distribution. The alkaline alcoholic anion facilitates the 1,6-addition reaction between p-benzoquinone methide and solvent. Thus, more alkali was needed in solvents with high pKa values and high water content. The detection of trace acid during the oxidation of p-cresols suggested the phenoxy radical mechanism rather than the classic benzyl radical mechanism proposed for interpreting the oxidation of non-hydroxyl aromatic hydrocarbons. These conclusions contribute significantly to both the understanding of role of alkali during the reaction process and the mechanism study for the catalytic oxidation of cresols and non-hydroxyl aromatic hydrocarbons.

Full text of DOI:10.1016/j.molcata.2016.04.003

Journal of Molecular Catalysis A: Chemical 420 (2016) 45–49
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Role of alkali in catalytic oxidation of p-cresols
Qiyi Maa, Cheng Lianga, Kexian Chenb, Kejing Liuc, Jianyong Maod, Zhirong Chena,
Haoran Lia,c,∗
a College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, PR China, PR China
b College of Food and Biology Engineering, Zhejiang Gongshang University, Hangzhou, 310018, PR China
c Department of Chemistry, ZJU-NHU United R&D Center Zhejiang University, Hangzhou 310027, PR China
d Zhejiang NHU Company Ltd., Xinchang 312500, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The investigation of alkali acting as the key component during the liquid-phase catalytic oxidation of
p-cresols to p-hydroxyl aromatic aldehydes is far from satisfied, thus a detailed exploration is presented.
It was observed that the p-cresols were activated by alkali to form phenolate salt to ensure mild reaction
conditions demonstrated by comparing the oxidation of p-cresol and its phenolate sodium. It was also
found that excess alkali improved the selectivity of aldehyde by inhibiting the formation of dimeric side
products in view of an analysis of the side products distribution. The alkaline alcoholic anion facilitates
the 1,6-addition reaction between p-benzoquinone methide and solvent. Thus, more alkali was needed
in solvents with high pKa values and high water content. The detection of trace acid during the oxidation
of p-cresols suggested the phenoxy radical mechanism rather than the classic benzyl radical mecha-
nism proposed for interpreting the oxidation of non-hydroxyl aromatic hydrocarbons. These conclusions
contribute significantly to both the understanding of role of alkali during the reaction process and the
mechanism study for the catalytic oxidation of cresols and non-hydroxyl aromatic hydrocarbons.
© 2016 Elsevier B.V. All rights reserved.
Received 6 February 2016
Received in revised form 1 April 2016
Accepted 2 April 2016
Available online 9 April 2016
Keywords:
Aldehyde
Alkali
p-Cresols
Water
1. Introduction
NaOH: (i) facilitating phenoxy radical formation via phenolate
anion; (ii) inhibiting free radical-based coupling side-reactions; (iii)
p-Hydroxyl aldehyde is an important intermediate for the indus-
trial synthesis of a wide variety of special chemicals, such as
pharmaceutical, perfumes, dyes, and polymers [1–13]. Among
many synthetic methods, liquid-phase catalytic oxidation of p-
cresols with molecular oxygen could be most promising [1–13].
Alkali was normally needed during the liquid-phase oxidation
of p-cresol and mono-substituted p-cresols [1–3,14–21]. To the
best of our knowledge, no reports on the liquid-phase oxidation
of p-cresols to p-hydroxyl aromatic aldehydes under base-free
conditions have been presented. It is also interesting that trace
p-hydroxyl aromatic acids are detected during the oxidation of p-
cresols, which is very different from that of non-hydroxyl aromatic
hydrocarbons [1–3]. The alkali was used to improve the selectivity
of aldehyde by inhibiting the side reactions, which were mentioned
by many references without in-depth interpretations [2,3,16–22].
For example, Ji and co-workers proposed three possible roles of
protecting in situ the formed aldehyde group via dearomatization-
enolization [3]. Unfortunately, no solid evidence was shown in this
reference. Our preliminary experiments also showed that no vanil-
lic acid was detected when 3-methoxyl-4-hydroxyl-benzaldehyde
(vanillin) was oxidized under base-free conditions, which are con-
tradictory with the above explanation (iii). Despite Xu et al. believed
that sodium hydroxide is essential for preventing oxidation on the
benzene ring and benefiting for selective oxidation [17] and Chan-
dalia et al. also held that a large amount of alkali was necessary
to ensure a high selectivity [22], the origin of role of alkali has not
yet been clearly understood. Moreover, the amount of alkali used
are also varied in different alcoholic solvents [1–6,14–22], and the
reason of which is still unknown. Therefore, a systematic study on
the important roles of alkali is urgently needed.
The mechanism for the liquid-phase catalytic oxidation of
p-cresols has been discussed for several decades [1–30]. The for-
mation of p-benzoquinone methide through phenoxy radical is
considered as the main viewpoint [1–30]. Although the mecha-
nism is still under discussion, many evidences have been provided.
In our previous studies, p-benzoquinone methide, aromatic ether,
and dimers were found during the catalytic oxidation of 4-methyl
∗
Corresponding author.
E-mail addresses: maqy@zju.edu.cn (Q. Ma), rebeccalc@163.com (C. Liang),
ckx chem@zju.edu.cn (K. Chen), jackielkj@163.com (K. Liu), chemmjy@zju.edu.cn
(J. Mao), chenzr@zju.edu.cn (Z. Chen), lihr@zju.edu.cn (H. Li).
http://dx.doi.org/10.1016/j.molcata.2016.04.003
1381-1169/© 2016 Elsevier B.V. All rights reserved.

46
Q. Ma et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 45–49
Table 1
guaiacol to vanillin. Therefore, the evidence above shows that phe-
noxy radical mechanism would be reasonable for oxidation of
p-cresols. Traditional classic benzyl radical mechanism is usually
applied into non-hydroxyl aromatic hydrocarbons, such as toluene,
p-chlorotoluene, and p-methoxytoluene [31–35].
The main goal of this work is to study the roles of alkali during
the liquid-phase oxidation of p-cresols. The necessity of alkali to
activate the substrate is demonstrated. The role of alkali to improve
the selectivity of aldehydes was explained in terms of the mecha-
nistic study and experimental results. It was also found that the
amount of alkali needed is closely related with the water content
and the pKa values of solvents. The mechanistic explanations are
given to explain why p-hydroxyl aromatic aldehydes are hard to be
oxidized to form the corresponding acids.
The liquid-phase oxidation of p-cresols under base-free/base conditions.a
Entry
Substrate
Time/h
Conv./%
TOFb
1
2
3
4
5
p-cresol
2-bromo-cresol
4-methyl guaiacol
Vanillyl alcohol
4-methyl guaiacol sodium salt
8
8
8
8
8
0 (99.9)
0 (99.9)
0 (99.9)
0 (99.9)
99.9
0 (30.2)
0 (30.2)
0 (30.2)
0 (30.2)
30.2
a
Reaction conditions: temp, 353.2 K; substrate, 50 mmol; catalyst, 0.15 g; sol-
vent, EGME 20.5 g; 500 rpm; oxygen, 25 ml min−1. The values in parentheses were
obtained by addition of 50 mmol NaOH.
b
Turnover frequency (TOF) = number of moles of substrate converted per mole of
catalyst per hour.
Table 2
The liquid-phase oxidation of 4-methyl guaiacol under different base/substrate
ratio.a
2. Materials and methods
Entry
Base/Substrate (molar ratio)
Time/h
Conv./%
Sel./%
TOF
2.1. Materials
1
2
3
4
1.0
2.0
2.7
4.0
0
8
8
8
8
8
8
8
99.9
99.9
99.9
99.9
99.9
99.9
99.9
5.0
81.1
90.0
90.8
4.4
30.2
30.2
30.2
30.2
30.2
30.2
30.2
Reagent-grade 4-methyl guaiacol, p-cresol, 2-bromo-4-cresol,
and vanillyl alcohol with mass fraction purity ≥99.0% were
obtained from Sahn Chemical Technology Co., Ltd and used with-
out further purification. The ethylene glycol monomethyl ether
(EGME), employed as the solvent with mass fraction purity ≥99.5%,
was obtained from Sinopharm Chemical Technology Co., Ltd. The
vanillin and vanillyl alcohol used as external standard in HPLC with
mass fraction purity ≥99.5% were also obtained from Sinopharm
Chemical Technology Co., Ltd. The O2 with purity of ≥99.99 mol%
and N2 with purity of ≥99.999 mol% were used in the reaction.
5b
6b
7b
1.7
3.0
90.2
89.5
a
Reaction conditions: temp, 353.2 K; 4-methyl guaiacol, 50 mmol; NaOH; cata-
lyst, 0.15 g; solvent, EGME 20.5 g; 500 rpm; oxygen, 25 ml min−1
.
b
4-methyl guaiacol sodium salt was used as the substrate.
in alcohol-water solution (25 vol% enthaol and 75 vol% distilled
water) under 278.15 K for 5 h. The mixtures of EGME and water
were separated by rectification.
N,Nꢀ-ethylenebis(acetylacetoniminato)-cobalt(II)
hexafluo-
rophosphoric pyridinium (Co-[Salen-Py][PF6]2) was used as
the catalyst in the experiment. Detailed information about the
synthesis route is shown in the reference [2].
3. Results and discussion
3.1. The substrates were activated by NaOH
2.1.1. Experimental and analytical procedure
The importance of NaOH to activate the substrate was confirmed
by control experiments. No reaction occurred when no alkali was
added (Entries 1–4 in Table 1). However, when equivalent molar
of NaOH was added, the reaction proceeded smoothly. The phe-
nomenon was reinforced when the 4-methyl guaiacol sodium salt
was used as the substrate. We concluded that the p-cresol sodium
salt was much easier to be oxidized than 4-methyl guaiacol, which
could be explained by the higher electron-donating ability of the
phenolate anion than that of hydroxyl group. Namely, the abstrac-
tion of proton from the hydroxyl group by alkali was a key step
during the oxidation of p-cresols.
The experimental apparatus employed is the agitator bubbling
reactor comprising a 100 ml volume, four-neck, and round-bottom
reactor with a stirring device above. The reactor was placed in
a water bath providing isothermal operating conditions with an
accuracy of 0.1 K. The gas reactant flowed into the reactor through
an inlet needle. The reagent NaOH, solvent, substrate and cata-
lyst were fed into the reactor in sequence. The temperature of the
water bath was set at 353.2 K before the reaction occurred. Oxygen
was introduced into the reactor continuously after the reactants
dissolved under nitrogen atmosphere. The reactant solution was
acidified by aqueous HCl before being measured.
The substrate (4-methyl guaiacol), intermediates (vanillyl alco-
hol, ether intermediate) and desired products (vanillin, vanillic
acid) were confirmed by comparing with standard chemicals by
HPLC and HPLC–MS. Specifically, the mobile phase consisted of
three eluents: buffer liquid (1.2 vol% phosphoric acid), 30 vol%
methanol and 70 vol% distilled water. The reactant was diluted
by solutions consisting of 50 vol% methanol and 50 vol% distilled
water. The wave number of UV detector of HPLC was set as 279 nm
and the flow rate of mobile phase was 1 mL min−1 with a C18 rever-
sal pillar (size: 150 × 4.6 mm).
3.2. The promotion effect of NaOH on the selectivity of vanillin
The addition of alkali improved the selectivity of aldehyde dur-
ing the oxidation of 4-methyl guaiacol, which was also confirmed
by many studies [2–6,16,22]. As shown in Table 2, the selectivity
of vanillin could be greatly improved by increasing the amount of
base. Only selectivity of 5% was obtained when 1.0 molar equiva-
lent of base was added. However, selectivity of 90.0% was achieved
when 2.7 molar equivalents of NaOH were used. More excess
alkali has minor effect on the reactions (Table 2). The same result
was also found when 4-methyl guaiacol sodium salt was used as
the substrate. This experiment also demonstrated that 1.0 molar
equivalent of NaOH was utilized to activate the substrate. The
side products under insufficient alkaline conditions were analyzed
(see supporting information). Analysis indicated that the dimers of
vanillin and 4-methyl guaiacol were the main side products.
To explain the above results, the mechanism was proposed. The
detailed reaction scheme for the liquid-phase oxidation of 4-methyl
To isolate the pure products, the reaction mixtures were stirred
for 0.5 h in an open system with H2O (20 ml) added, and then con-
centrated to recover the mixtures of EGME and water. MTBE (50 ml)
was added to the residue and diluted hydrochloric acid (1.0 M) was
used to adjust the pH to the range of 2.0–3.0. Furthermore, the solu-
tion was partitioned into two layers, and the aqueous phase was
extracted with MTBE by three times. The combined organic layers
were dried over anhydrous Na2SO4, and concentrated in vacuo to
give a solid. The purified vanillin was obtained by recrystallization

Q. Ma et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 45–49
47
Scheme 1. The proposed mechanism for the liquid-phase oxidation of 4-methyl guaiacol to vanillin.
Table 3
guaiacol to vanillin was also explored based on a series of con-
Effect of water on the molar of alkali used.a
trol experiments and careful analysis of the reaction processes (see
supporting information). Additionally, many sound evidences were
also shown in the literatures [1–3,7–29]. Therefore, a proposed
mechanism was presented (Scheme 1). The 4-methyl guaiacol was
activated by alkali to form 4-methyl guaiacol salt. Then the phe-
noxy radical was formed by oxidation catalyzed of LCo3+, which
was demonstrated by dimeric side products. The highly reactive
p-benzoquinone methide was produced during the disproportion-
ated step [3,27,36]. A large amount of alkali was needed during this
step to ensure a good selectivity [3]. The p-benzoquinone methide
was also detected during our reaction (see supporting information).
Then the aromatic ether was formed by 1,6-addition reaction. Trace
vanillin was detected when non-alcoholic solvent was applied,
demonstrating that the ether was an important intermediate to
be further oxidized to vanillin. Oxidation of aromatic ether was
explained by the same way mentioned above.
Entry
Base/Substrate (molar ratio)
Time/h
Conv./%
Sel./%
TOF
1
2.7
2.7
2.0.
2.7
5.0
2.4
2.4
8
8
8
8
8
8
8
99.9
99.9
99.9
99.9
99.9
99.9
99.9
90.0
90.2
89.8
56.7
84.4
85.0
89.7
30.2
30.2
30.2
30.2
30.2
30.2
30.2
2b
3b
4c
5c
6
7d
a
Reaction conditions: temp, 353.2 K; 4-methyl guaiacol, 50 mmol; NaOH; cata-
lyst, 0.15 g, solvent, EGME 20.5 g; 500 rpm; oxygen, 25 ml min−1
.
b
Sodium salt of ethylene glycol monomethyl ether was used to replace NaOH.
c5.0 g water was added.
d
2.0 g water was removed before the reaction.
3.3. The relationship between the amount of NaOH and water
content/solvents
The relationship between the amount of NaOH and water con-
tent was obtained by exploring the influence of water content.
Water is the main side product during the oxidation reactions;
hence, it is very necessary to have a research on it in depth
[37]. In our case, a part of water was produced before the oxi-
dation occurred due to the acid-base reactions between NaOH
and alcoholic solvent/4-methyl guaiacol. The control experiments
showed that the water-free conditions had minor effect on the yield
(Entries 1–2, Table 3). However, the amount of NaOH needed can be
efficiently decreased from 2.7 molar equivalents to 2.0 molar equiv-
alents under water-free conditions, which will be a great strategy to
decrease the amount of NaOH used. To investigate the relationship
between the amount of NaOH and water content, a series of exper-
imental results were presented. On the one hand, only 2.4 molar
equivalents of NaOH were needed when 2.0 g of water was removed
by azeotropic distillation before reaction (Entries 6–7, Table 3). On
the other hand, the selectivity will be decreased from 90.0 to 56.7%
if 5.0 g of water was added (Entry 4, Table 3). Additionally, the selec-
tivity was increased again when another 2.3 molar equivalents of
NaOH were added. Thus, more NaOH was needed in case the water
content is high.
Fig. 1. The yield of vanillin versus base/substrate molar ratio under different sol-
vents. Reaction conditions: temp, 353.2 K; 4-methyl guaiacol, 50 mmol; NaOH;
catalyst, 0.15 g; solvent, 20.5 g; 500 rpm; oxygen, 25 ml min−1
.
acid-base reactions occurred between NaOH and alcoholic solvent,
a set of experiments concerning different alcoholic solvents were
thus carried out to investigate the influence of pKa values. Obser-
vation indicated that more NaOH was used to ensure an optimum
yield in case solvents have high pKa values, such as 1-propanol,
and 1-butanol (Fig. 1). The alcoholic solvents react with NaOH to
form RONa (Eq. (1)). Obviously, more anion RO− will be formed in
solvents with relatively low pKa values, such as EGME (pKa = 14.8,
298.15 K). This phenomenon was also found in the electrocatalytic
oxidation of alcohols [38]. The rate of 1,6-addition reaction between
RO− and p-benzoquinone methide is faster when compared with
The results in the references showed that the amount of alkali
needed also varied in different solvents [1–6,14–22]. Due to the

48
Q. Ma et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 45–49
Table 4
The liquid-phase oxidation of vanillin under various solvent.a
Entry
Solvent
Time/h
Conv./%
TOF
Dimers + Tars
Acid
1
EGME
EGME
EGME
EGME
DMSO
Acetic acid
7
7
8
8
8
7
7.8
7.1
20.4
22.2
4.3
2.7
2.5
6.2
6.7
1.3
3.1
7.8
7.1
20.4
22.2
4.3
trace
trace
trace
trace
trace
trace
2b
3c
4d
5
6
8.9
8.9
a
Reaction conditions: vanillin, 1.0 g; temp, 353.2 K; NaOH, 5.4 g; solvent: 50.0 g,
500 rpm, oxygen 25 ml min−1
.
b
No NaOH was added.
CuO was used as the catalyst.
Fe/CN was used as the catalyst.
c
d
A serial of experiments were performed with vanillin used as
the substrate. As Table 4 shows, trace amount of vanillic acid was
detected no matter under base conditions or not, demonstrating
that alkali is unnecessary to inhibit the further oxidation of vanillin.
The effects of solvent and catalysts were also investigated as shown
in Table 4. All the experiments showed that vanillin could be oxi-
dized while the products are mainly dimers and tars. Therefore, a
mechanism for the liquid-phase oxidation of vanillin was proposed
(Scheme 4). As Scheme 4 shows, the vanillin was oxidized due to the
oxidation of hydroxyl group to phenoxy radical. However, it is diffi-
cult to form p-benzoquinone methide for vanillin (Scheme 4). Thus,
dimers were the mainly products detected (See supporting infor-
mation). The traditional oxidation mechanism with respect to the
synthesis of non-hydroxyl aromatic acid was normally explained
by the abstraction of hydrogen from the aldehyde group of non-
hydroxyl aromatic aldehyde [31,41,42]. Thus, we concluded that
the oxidation of p-hydroxyl group to form phenoxy radical rather
than oxidation of methyl group to form benzyl radical involved dur-
ing the oxidation of p-cresols. That is to say, different mechanism
leads to different products.
Scheme 2. The oxidation of aromatic aldehyde to aromatic acid.
ROH indicated from Scheme 1. Thus, more NaOH was needed to
ensure enough concentration of RO− for 1-propanol, and 1-butanol
(pKa = 16-17, 298.15 K). Additionally, the water also has a negative
effect on the formation of RO− (Eq. (1)). Therefore, more NaOH was
needed in case additional water was added into the reactor (Entry 5,
Table 3). To demonstrate this point, experiment with half of EGME
was replaced by DMSO was carried out. The result showed that only
2.2 molar equivalents of NaOH were needed to obtain a high yield
of 89.0% due to the water-absorbing ability of DMSO.
Therefore, the amount of NaOH affected by the water content
and solvents was interpreted.
ROH + NaOH ꢀ RONa + H2O
(1)
3.4. Further oxidation of aromatic aldehyde containing
p-hydroxyl group
4. Conclusions
The difficulty for the oxidation of aromatic aldehyde to acid
was investigated in view of the experimental results and proposed
mechanism. Contrary to the general belief, trace amount of aro-
matic acid was detected during the oxidation of p-cresol, which
was also mentioned in other references [2,3]. However, the aro-
matic aldehyde with no p-hydroxyl group could be easily oxidized
to acid (Scheme 2) [31,39,40]. Normally, we even tried to protect
p-hydroxyl group firstly, and then carried out the oxidation reac-
tions in order to synthesize p-hydroxyl aromatic acid (Scheme 3)
[31,40].
The alkali plays important roles during the liquid-phase catalytic
oxidation of p-cresols. Firstly, it activated the substrate by reaction
with p-hydroxyl group, and then enabled the oxidation to happen
under mild conditions. A large amount of alkali improved the selec-
tivity of aldehyde products by favoring the disproportionated step
via p-benzoquinone methide. The mechanism was proposed based
on the analysis of side products. The amount of NaOH needed to
ensure high selectivity is affected by water content and the pKa
values of solvents. The alkali reacted with the alcoholic solvent to
form anion to facilitate the 1,6-addition reaction. The utilization
Scheme 3. The liquid-phase oxidation synthesis of p-hydroxyl aromatic acid.

Q. Ma et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 45–49
49
Scheme 4. The proposed mechanism for the liquid-phase oxidation of vanillin to dimers.
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was undertaken. The experimental results and proposed mecha-
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radical rather than oxidation of methyl group to form benzyl rad-
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can not only to help us to understand the roles of alkali during
the liquid-phase oxidation of p-cresols, but also to differentiate
the mechanisms between the oxidation of p-cresols and aromatic
hydrocarbons with no hydroxyl group. Moreover, the results can
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Acknowledgements
Financial support of this work from the Program for Zhejiang
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