Oxidation of benzyl alcohol catalyzed by gold nanoparticles under alkaline conditions: Weak vs. strong bases

Article abstract of DOI:10.1039/c6ra01795a

Gold nanoparticles have shown excellent catalytic properties in selective oxidation of alcohols in the presence of base; however, the influence of the nature and concentration of the base on gold catalyst activity and selectivity is not completely underst

Full text of DOI:10.1039/c6ra01795a

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Oxidation of benzyl alcohol catalyzed by gold
nanoparticles under alkaline conditions: weak vs.
strong bases†
Cite this: RSC Adv., 2016, 6, 25279
a
a
b
a
*
Camila P. Ferraz, Marco Aurelio S. Garcia, E´rico Teixeira-Neto and Liane M. Rossi
´
Gold nanoparticles have shown excellent catalytic properties in selective oxidation of alcohols in the
presence of base; however, the influence of the nature and concentration of the base on gold catalyst
activity and selectivity is not completely understood. We here present a study of the effect of using weak
and strong bases on the conversion and selectivity of PVA-stabilized gold nanoparticles supported on
titania (AuPVA/TiO2) for benzyl alcohol oxidation. The increase in the concentration of base had little
effect on conversion when a weak base was used (K2CO3, Na2B4O7 and Na(CH3COO)), due to the buffer
effect, but strongly affected selectivity. The bases with higher pKa values provided higher conversions
and increased production of benzoic acid. For the strong base NaOH, benzoic acid was always the major
product, although conversion decreases in excess of base. The formation of benzoic acid is avoided by
using K2CO3 in non-aqueous media; benzaldehyde is the main product in cyclohexane whereas benzyl
benzoate is also formed in significant amounts in solvent-free conditions. The promotion effect
observed in the presence of base was discussed in terms of reaction mechanism.
Received 20th January 2016
Accepted 28th February 2016
DOI: 10.1039/c6ra01795a
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dimensions, presents a number of new properties, including very
high catalytic activity in reactions of industrial and environ-
mental interest.3,5–8,15–20 Gold-based catalysts are more stable and
Introduction
Selective oxidation is one of the most important functional
group transformations in organic chemistry, being crucial in
industrial processes.1–5 Classes of organic compounds con-
taining oxygen, such as epoxides, ketones, aldehydes, alcohols
and acids serve as intermediates in the manufacture of plastics,
detergents, paints, cosmetics, and food additives.1–4 Efficient
industrial processes for commercial applications involving
these chemicals have been pursued.6–8 Currently, special
attention has been given to oxidation of substrates derived from
biomass, owing to their potential to replace petroleum-derived
chemicals.9–13 Due to their high extent of functionalities and
hydrophilic properties, substrates derived from biomass should
be processed in liquid-phase, typically in aqueous phase.12,13
Thus, the development of efficient liquid-phase technologies
are welcome, as well as improved catalysts to work under mild
reaction conditions.12,13
selective for the aerobic oxidation of organic compounds in water
than the conventional liquid-phase oxidation catalysts based on
Pt and Pd, because the Au catalysts can offer better resistance to
water and O2.9–11 Thus, gold offers unique opportunities for
catalysis, besides an excellent alternative to the development of
viable industrial process for the conversion of renewable
biomass in aqueous phase.9–11 Previous studies with gold nano-
catalysts in oxidation reactions of simple alcohols, such as benzyl
alcohol, showed that an excellent selectivity can be achieved
under mild conditions.4,5,9,21–40 Understanding the selective
oxidation of benzyl alcohol can assist further application of
catalytic oxidations for the valorization of renewable biomass
substrates, such as glucose, HMF and glycerol.
Oxidation reactions in aqueous phase have been successful,
but there is still room for further development, once water
plays an important role in the efficiency of gold cata-
lysts.4,9,21,27,28,33–35,38,39,41 The catalytic activity of gold catalysts for
CO oxidation16–18 and for benzyl alcohol oxidation3,21 can be
promoted signicantly by water. Under identical reaction
conditions, the activity of Au/TiO2 for benzyl alcohol oxidation
in the presence of water is consistently higher than that in
ethanol and p-xylene.21 Even in aqueous medium, the reaction
rates are quite low.3,9,21,28,39 Acceptable conversions from a prac-
tical point of view for benzyl alcohol oxidation using gold
catalysts are obtained in the presence of base promoters
(aqueous3,4,9,27,33,35,38,39 and non-aqueous media9,22,30,32,37), by
Metal catalysts enable the use of green oxidants, such as
molecular oxygen, which generates only water as a byproduct.4,5,14
Considered as catalytically inert, gold, when in nanometric
a
´
´
˜
Departamento de Quımica Fundamental, Instituto de Quımica, Universidade de Sao
˜
Paulo, Av. Prof. Lineu Prestes, 748, 05508-000, Sao Paulo, SP, Brazil. E-mail:
lrossi@iq.usp.br
b
´
Laboratorio Nacional de Nanotecnologia, CNPEM/ABTLuS, Caixa Postal 6192,
13083-970, Campinas, SP, Brazil
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra01795a
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using basic and/or nanosized support (non-aqueous
Paper
a
media3,23–25,31,36,40,42) or by adding a second metal, in particular
Pd (aqueous43,44 and non-aqueous media45–47). Aqueous oxida-
tion of aromatic and aliphatic alcohols in general uses
hydroxides and carbonates as base promoters and the amount
of base has been arbitrarily chosen from sub-stoichiometric to
equimolar or even large excess (from 0.5 to 3 equivalents). The
catalytic activity in base-free conditions is generally an order of
magnitude lower than the catalytic activity obtained in high-pH
conditions,3,9,28,39 which emphasizes the critical role of the base
as a promoter in gold catalysis. At the same time, in alkaline
reaction conditions the selectivity is greatly affected towards
benzoic acid.3–5,9,39,48,49 In this context, it would be interesting to
ascertain how the amount of weak and strong bases affects the
activity and selectivity of the catalytic oxidation of alcohols.
Besides the effect of base promoters in aqueous solution,
marked examples underline the sensitivity of gold catalysts to
the reaction media, that leads to different product distribu-
tions.3,5,9,39 As reported for benzyl alcohol oxidations, majority of
gold catalysts in non-polar and solvent-free media favor selec-
tivity towards benzaldehyde,3,23–25,36,40,42 while performing the
oxidation in alcohols, such as MeOH, the corresponding ester is
formed.26,31 In some cases, hydroxides and carbonates have
been also applied as promoters;22,29,30,32,37 however, reports on
the effect of the amount of base in these media are rather
sparse.
Here we report on the effect of using weak and strong bases
as promoters in the oxidation of benzyl alcohol catalyzed by
AuPVA/TiO2. We evaluated how the nature of base and different
[base] : [alcohol] molar ratios affect the reaction conversion and
selectivity. The results were also compared to that obtained in
non-aqueous media (cyclohexane and solvent-free).
Fig. 1 Aqueous oxidation of benzyl alcohol catalyzed by AuPVA/TiO2:
(a) conversion as a function of time for ([K2CO3] : [alcohol] molar ratio
¼ 0 (+), 0.1 (-), 0.5 (C), 1 (:), 2 (;) and 4 (A)); (b) evolution of pH as
a function of time for ([K2CO3] : [alcohol] molar ratio ¼ 0.1, 0.5, 1, 2 and
4); (c) turnover frequencies (hꢀ1); (d–h) selectivity (benzaldehyde (,),
benzoic acid (B) and benzyl benzoate (O)) and mass balance (>) as
a function of time for [K2CO3] : [alcohol] molar ratio ¼ (d) 0.1, (e) 0.5, (f)
1, (g) 2 and (h) 4. Reaction conditions: benzyl alcohol (2.9 mmol),
catalyst (1.3 mmol Au), O2 (6 bar), 100 ꢁC.
Results and discussion
Gold supported on titania (AuPVA/TiO2) catalyst was prepared
using the well-known sol immobilization method with polyvinyl
alcohol (PVA) stabilized Au NPs.40,50,51 PVA is a versatile stabi-
lizing agent used to obtain colloidal metal dispersions. The
presence of PVA has been demonstrated to affect the activity of
AuPVA/TiO2 catalyst;51,52 but also to increase the catalyst stability.
This method was chosen because of its high reproducibility.
The catalyst was rst tested in the aqueous oxidation of benzyl
alcohol in the absence and presence of carbonate (K2CO3) as
a base promoter. The conversion was monitored as a function of
time, as shown in Fig. 1a. The catalyst exhibits some activity
(33% conversion in 2.5 h) in the absence of a base promoter,
because O2 adsorption on TiO2 could be facilitated in water.21
However, much higher conversions were achieved in the pres-
ence of K2CO3. The inuence of the amount of base on the
catalytic activity of AuPVA/TiO2 for benzyl alcohol oxidation was
determined for a series of ve different [K2CO3] : [alcohol]
molar ratios (0.1, 0.5, 1, 2 and 4). In the aqueous oxidation of
alcohols, the deprotonation of the alcohol to form an alkoxy
intermediate is the rst step and should be facilitated in alka-
line media. Thus, the extent of the reaction should be directly
affected by the amount of base, although the initial activation of
the alcohol has also been suggested to occur on the catalyst
surface.9,49 The results shown in Fig. 1a suggest that the
conversion was little affected by varying the amount of K2CO3
from 0.5 to 4 equivalents, reaching 100% conversion within 2.5
h. The reaction performed with 0.1 equivalents of K2CO3
reached a maximum conversion of 70%. To analyze the effect of
K2CO3 in the reaction, it must be considered the formation of
a carbonate/bicarbonate buffer in the initial reaction solutions,
producing HOꢀ. Changes in pH values were monitored as
a function of reaction time, as shown in Fig. 1b. Selected results
are presented in Table 1 (entries 1, 8–12) and detailed data can
be obtained in Table S1 (ESI†). The initial pH values of all
reaction solutions were quite similar due to the buffer forma-
tion. The increase in the amount of base represents an increase
in the buffer strength of the initial solutions. For [K2CO3] :
[alcohol] > 0.5, the benzoic acid concentration increased along
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Table 1 Aqueous oxidation of benzyl alcohol catalyzed by AuPVA/TiO2 in the presence of weak and strong basesa
Selectivityb (%)
#
Base
pKa
[Base] : [alcohol]([HOꢀ] mol Lꢀ1
0
)
Initial pH
Final pH
Conv. (%)
BnCHO
BnCOOH
1
2
3
4
5
6
7
8
No base
Na(CH3COO)
7.00
4.87
5.85
9.11
9.61
3.83
5.74
6.26
6.57
8.67
9.35
9.84
8.54
10.29
10.37
10.39
10.77
5.56
19
12
15
28
31
32
43
48
65
60
72
66
36
54
34
92
82
87
92
68
50
41
57
34
29
13
11
53
5
8
18
13
8
0.1 (1.28 ꢂ 10ꢀ5
)
1 (4.05 ꢂ 10ꢀ5
4 (8.10 ꢂ 10ꢀ5
)
)
9.91
Na2B4O7
K2CO3
9.23
0.1 (2.15 ꢂ 10ꢀ3
)
11.33
11.84
12.15
11.37
11.73
11.88
12.04
12.19
12.46
13.46
14.06
28
47
59
38
59
63
87
88
47
95
100
1 (6.97 ꢂ 10ꢀ3
)
4 (14.02 ꢂ 10ꢀ3
0.1 (2.35 ꢂ 10ꢀ3
0.5 (5.38 ꢂ 10ꢀ3
)
)
)
10.32
9
10
11
12
13
14
15
1 (7.65 ꢂ 10ꢀ3
)
2 (10.86 ꢂ 10ꢀ3
4 (15.40 ꢂ 10ꢀ3
)
)
)
NaOH
—
0.1 (2.89 ꢂ 10ꢀ2
1 (2.89 ꢂ 10ꢀ1
)
13.18
13.60
4 (1.16)
0
a
Reaction conditions: benzyl alcohol (0.3 mL, 2.9 mmol), catalyst (15 mg, 1.3 mmol Au), O2 (6 bar), 100 ꢁC, 1 h. BnCHO ¼ benzaldehyde, BnCOOH ¼
benzoic acid. b Mass balance > 85%.
the reaction, but the nal pH is little affected. However, for involves the reversible hydration of the aldehyde to germinal
[K2CO3] : [alcohol] ¼ 0.1, the formation of benzoic acid over the diols, which is accelerated in high pH. The germinal diols will
reaction time breaks the buffer solution. The decrease in the pH adsorb in the metal surface to form an alkoxide, that later
might be the reason for the lower conversion observed at long undergoes a b-elimination to form carboxylic acid.9,49 Thus, the
reaction times. In addition, the benzoic acid can adsorb on the combination of high concentration of OHꢀ and longer reaction
catalyst surface, causing poisoning and reducing the reaction times favors the oxidation to carboxylic acid.
rates.22,25 The turnover frequencies at initial reaction times
Recently, Zheng and Stucky37 have shown that the addition of
(TOF, hꢀ1) were only slightly affected by the amount of K2CO3 sub-stoichiometric amounts of either acetates, borates or
(base to alcohol molar ratio) used with values ranging from 2400 carbonates, can enhance the activity of supported gold nano-
to 3200 hꢀ1 (Fig. 1c), which is an evidence of pH control under particles for the oxidation of alcohols in solvent-free medium.
buffer conditions. The selectivity was strongly inuenced by the Considering the ability of these species with different pKa values
amount of K2CO3 used (Fig. 1d–h). The conversion of benzyl to form buffered solution, we become interested in investi-
alcohol into benzaldehyde is favored in the presence of sub- gating their behavior in aqueous media and the consequences
stoichiometric amounts of base (Fig. 1d), while benzoic acid on the reaction selectivity.
is produced in higher yields in excess of base, reaching 99%
The aqueous oxidation of benzyl alcohol was studied in the
selectivity (Fig. 1g and h). The formation of benzoic acid was presence of different amounts of Na2B4O7 and Na(CH3COO) and
observed since the beginning of reaction, in all amounts of base the results are summarized in Table 1 (entries 2–7). The benzyl
studied, and the selectivity to acid increased within the reaction alcohol conversion aer 60 min of reaction follows the same
time, while decreasing the selectivity to benzaldehyde. In excess order as the pKa: K2CO3 > Na2B4O7 > Na(CH3COO), most prob-
of base ([K2CO3] : [alcohol] ¼ 2 and 4), benzoic acid is always the ably associated to the concentration of benzyloxide species that
major product. Aer 1 h of reaction, the selectivity to acid is are involved in the rst step of the reaction. In fact, a linear
already >90%. The formation of benzyl benzoate was minimal correlation was found between the initial reaction pH or the
in all cases, not exceeding 8% selectivity. Moreover, the mass amount of benzyloxide at initial pH and the reaction conversion
balance was very unfavorable for long reactions times (150 min) (Fig. 2a). The conversion was only slightly improved with the
in all reaction conditions studied (Fig. 1d–h).
increase in the [base] : [alcohol] molar ratio, due to the buffer
The presence of hydroxide ions plays an important role formation, as observed previously in the experiments with
during the oxidation reaction. Davis et al. have discussed how potassium carbonate. That was not the case for the reaction
the product distribution depends on pH.9,49 In principle, the with the strong base NaOH (Table 1, entries 13–15), wherein the
buffer effect can explain similar initial TOFs for the reactions; conversion was greatly affected by the amount of base and does
however, the increase in the buffer strength represents an not follow the previous described tendency in Fig. 2a. For
increase in the concentration of HOꢀ (6.5 times for [K2CO3] : [NaOH] : [alcohol] ¼ 0.1, the conversion was 35% and pH drops
[alcohol] from 0.1 to 4) and, consequently, can sustain the from 12.46 to 5.56. As the buffer is not formed, the formation of
differences observed in the reaction selectivity. It is well known benzoic acid decreased the pH and then the reaction rate, which
that aldehydes in basic aqueous solution can be oxidized to results in a lower conversion compared to the reaction with
carboxylic acid more easily.9,49 The proposed mechanism K2CO3. For [NaOH] : [alcohol] ¼ 1 and 4, the nal pH was not
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H2O2,54 which caused an increase in the reaction conversion.
However, for 4 equiv. of NaOH, the amount of H2O2 decreased
as well as the reaction conversion. These results suggest that the
excess of NaOH ([NaOH] : [alcohol] > 1) is detrimental to the
oxidation reaction. Therefore, the fast decomposition of the
peroxide in strong alkaline media can provide an explanation
for the lack of correlation between the conversion and pH for
NaOH.
A general tendency was observed in the selectivity for the
reactions performed in the presence of weak bases, i.e.,
increasing the [HOꢀ] decreases the selectivity to benzaldehyde,
while favoring the formation of benzoic acid (Table 1, entries 2–
12). Na(CH3COO) produces the lowest [HOꢀ] and the best
selectivity to benzaldehyde. In this case, the lower pH delays the
further oxidation to acid, but also prevents the reaction to
proceed to high conversions. However, K2CO3 was the most
effective base to produce benzaldehyde or benzoic acid in
aqueous media, producing benzaldehyde (57% selectivity at
48% conversion) in the [base] : [alcohol] ¼ 0.1 and benzoic acid
(87% selectivity at 72% conversion) in the [base] : [alcohol] ¼ 2.
The increase in the [base] : [alcohol] ratio represents little
variation in the [HOꢀ] (ca. 6.5 times), thus benzaldehyde was
still observed in all reactions. For the strong base NaOH, the
increase in the [base] : [alcohol] ratio represents an increase of
up to 40 times in the [HOꢀ], thus benzoic acid is the major
product no matter the amount of NaOH used.
Further studies on the oxidation of benzyl alcohol using
K2CO3 as a base promoter were conducted in cyclohexane and
solvent-free conditions. Conversion and selectivity aer 1 h
reaction as a function of the amount of added base ([K2CO3] :
[alcohol] ¼ 0.1, 1 and 4) are presented in Table 2. Contrary to
aqueous phase, only very low conversions were observed in
absence of base for non-aqueous media (Table 2, entries 16 and
20). However, the use of K2CO3 as base promoter represented
Fig. 2 (a) Conversion as a function of initial pH (solid symbols) and
initial concentration of benzyloxidea ([BnOꢀ]) (empty symbols) for
aqueous oxidation of benzyl alcohol catalyzed by AuPVA/TiO2 using
K2CO3 (-), NaOH (C), Na2B4O7 (;) and Na(CH3COO) (:) as base for
60 min. (b) Conversion (solid symbols) and concentration of H2O2
(empty symbols) as a function of [base] : [alcohol] molar ratio (0.1, 1
and 4) for aqueous oxidation of benzyl alcohol catalyzed by AuPVA
/
TiO2 using K2CO3 (-) and NaOH (C) as base for 60 min. Reaction
conditions: benzyl alcohol (2.9 mmol), catalyst (1.3 mmol Au), O2 (6
bar), 100 ꢁC. aCalculated value using the Henderson Hasselbalch
equation at each initial pH, according to ref. 52.
highly affected by the amount of acid formed in the reaction
(13.18 and 13.60, respectively). An improvement in the conver-
sion was observed for 1 equivalent of NaOH, reaching 54%,
which was close and comparable to that obtained with K2CO3
(ca. 60%). However, for [NaOH] : [alcohol] ¼ 4, the conversion
dropped to 35%. In this condition, the concentration of HOꢀ in
the aqueous solution is about 75 times higher than it was
produced by a similar amount of K2CO3, which suggests a lack
of correlation between the reaction conversion and [HOꢀ]
(Fig. 2b). In order to rationalize these results, we should
remember that the activation of O2 in the aqueous oxidation of
alcohols proceeds through the formation and dissociation of
peroxide (OOH*) and hydrogen peroxide (HOOH*) intermedi-
ates,9,49 which are necessary species to close the catalytic cycle
(removal of excess electrons from the metal surface, oxidize
metal-hydride bonds and regenerate hydroxide ions).9,25,27,42,49,53
It has been suggested that the oxidation of alcohols can be less
effective in high concentration of NaOH (>0.6 M) due to the
faster decomposition of peroxide.27 In order to conrm these
assignments, the hydrogen peroxide formation was investigated
for the aqueous oxidation reaction using K2CO3 and NaOH
(Fig. 2b). When increasing the amount of NaOH from 0.1 to 1
equiv., the increase in the [HOꢀ] favored the formation of
a
marked improvement on reaction conversions. When
comparing the use of K2CO3 in aqueous and cyclohexane media,
the results suggest that the conversion was slightly affected by
varying the reaction media, but selectivity has signicantly
changed towards benzaldehyde. In the solvent-free condition,
the addition of base resulted also in high conversions, reaching
90% already with 1 equivalent of base, but selectivity was not as
high as in cyclohexane. Differences in reactivity can be ratio-
nalized considering the proposed mechanism.9,25,42,49 The
oxidation of alcohol proceeds through 3 steps: (1) metal-
alkoxide formation; (2) b-elimination; (3) metal-hydride
formation and catalyst regeneration. In the aqueous-phase,
the basic solution favors the initial deprotonation of the
alcohol to form an alkoxy intermediate in the rst step and also
decreases the energy barrier in step 2.9,49 In the non-aqueous
phase, the support or a promoter must play an important role
in the initial activation of the alcohol that might occur on the
catalyst surface,9,49 just as the dehydration in the step 2.
Reasonable conversions and the formation of benzaldehyde
were only possible with the addition of K2CO3, indicating that
both steps are favored in the presence of a base promoter. Both
in aqueous or non-polar organic media, where K2CO3 is acting
as a homogeneous base and as a solid base, respectively, similar
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Table 2 Oxidation of benzyl alcohol catalyzed by AuPVA/TiO2 in the presence of K2CO3 performed in non-aqueous media
Selectivityc (%)
#
Medium
[Base] : [alcohol]
Conv. (%)
BnCHO
BnCOOH
BnBzO
16
17
18
19
20
21
22
23
Cyclohexanea
0
0.1
1
4
0
2
44
55
72
3
61
87
—
100
89
87
85
100
70
0
0
0
0
0
0
0
—
0
11
13
15
0
30
37
—
Solvent-freeb
0.1
1
63
—
4d
a
Reaction conditions: benzyl alcohol (0.3 mL, 2.9 mmol), catalyst (15 mg, 1.3 mmol Au), O2 (6 bar), 100 ꢁC, 1 h. b Benzyl alcohol (1 mL, 9.7 mmol),
catalyst (52 mg, 4.4 mmol Au), O2 (6 bar), 100 ꢁC, 1 h. c Mass balance > 93%. d [K2CO3] : [alcohol] ¼ 4 was not performed because the mass of base
would be higher than the mass of alcohol. This amount of base would not provide a liquid medium for the reaction.
conversions of benzyl alcohol were observed for the same study was investigated by Scanning Transmission Electron
amount of base (Table 1, entries 8–12 and Table 2, entries 16– Microscopy (STEM). The STEM images (Fig. S1, ESI†) revealed
19). In the solvent-free condition (Table 2, entries 20–22), when that the gold nanoparticles of the as prepared catalyst
the neat alcohol was used, the conversion was boosted probably (4.4 ꢃ 1.5 nm) only slightly increase in size when used in
due to the high concentration of alcohol. Under this condition, aqueous solution in the presence of acetate (4.7 ꢃ 1.6 nm),
a large majority of the metal sites will be coordinated forming borate (5.2 ꢃ 1.9 nm) and NaOH (5.4 ꢃ 2.4 nm), but signicantly
the alkoxide.25
increase in size in the presence of carbonate (7.6 ꢃ 2.6 nm). In
Differently from the aqueous media, in cyclohexane and the solventless reaction, the gold particle size was maintained
solvent-free conditions, benzoic acid was not formed. We per- (5.1 ꢃ 1.8 nm), but in cyclohexane particle's growth was noticed
formed the oxidation of benzaldehyde under the same reaction (10.8 ꢃ 8.3 nm). There is not a clear correlation between the
conditions (see Table S2, ESI†). Benzaldehyde is converted in reactivity and the particle size changes in the catalytic reaction
benzoic acid in the absence of catalyst in non-aqueous, but the studied. None of the samples were attacked by the reaction
formation of benzoic acid is inhibited by AuPVA/TiO2 catalyst. In media and all of them are comprised of supported gold nano-
aqueous media, benzaldehyde is converted in benzoic acid particles aer reaction.
either with or without the catalyst. Thus, the results under non-
aqueous media suggest that the formation of benzoic acid is not
Conclusions
favorable on the catalyst surface, but its formation can still
occur in alkaline aqueous medium once the HOꢀ ions are
The comparative study of weak and strong bases in the aqueous
available.
The only byproduct for the reaction in non-aqueous media is
oxidation of benzyl alcohol by PVA-stabilized supported gold
nanoparticles showed that conversion is related to the initial pH
of the reaction, since the bases with higher pKa values (greater
initial pH values) provided higher conversions, regardless the
amount of base used. However, the increase in the [base] :
[alcohol] molar ratio affects the reaction selectivity, favoring the
oxidation of the aldehyde to benzoic acid. For the strong base,
low amounts of base are not enough to keep the high pH during
the reaction. The increase in the amount of base is benecial for
the catalyst activity until [base] : [alcohol] molar ratio is 1. In
excess of base, the intermediate (H2O2) is degraded decreasing
the conversion. These results suggest that weak bases are the
most effective promoters for the selective oxidation of alcohols
in aqueous media. The buffer formation: (1) allows better pH
control over reaction; (2) provides lower [HOꢀ] and conse-
quently better selectivity to aldehyde, in sub-stoichiometric
amount of base; (3) provides higher [HOꢀ] and consequently
better selectivity to benzoic acid, in excess of base; K2CO3 is
a very effective base to produce benzaldehyde or benzoic acid in
aqueous media, producing preferentially benzaldehyde in the
[base] : [alcohol] ¼ 0.1 and benzoic acid in the [base] : [alcohol]
¼ 4. The study in non-aqueous media revealed that the presence
of the base promoter increased conversion and prevented
the benzyl benzoate, which is favored by the increase in the
amount of K2CO3. As the conversions in water and in cyclo-
hexane are similar, the difference relies on the selectivity:
higher base concentration favors the formation of benzoic acid
in aqueous media, while in cyclohexane, the ester is preferred
with no formation of acid. The formation of benzyl benzoate is
minimal in cyclohexane (up to 15% selectivity), but it is
appreciable in solvent-free condition. In the presence of [K2-
CO3] : [alcohol] as low as 0.1, the selectivity for benzyl benzoate
is already 30% and increases to 37% for [K2CO3] : [alcohol] ¼ 1.
This resulted in moderate selectivity to benzaldehyde (up to
70%). According to proposed mechanism for the oxidation of
alcohols, the ester is formed by the attack of the alkoxy species
(formed in step 1) into the aldehyde (formed in the step 2).48
Thus, the benzyl benzoate is more likely produced when the rate
of formation of benzyloxide is high compared to the rate of
formation of benzaldehyde. In the solvent-free medium, the
benzyl ester is formed in the highest yields, indicating high rate
of formation of benzyloxide, which is boosted by the base.
The stability of the AuPVA/TiO2 catalyst against aggregation
aer being exposed to different reaction conditions used in this
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completely the formation of benzoic acid. Mainly in the solvent- (0.3 mL, 2.9 mmol), the AuPVA/TiO2 catalyst (15.6 mg, 1.3 mmol
free condition, the increase in the amount of base favored the Au) and the base ([base] : [alcohol] ¼ 0.1, 0.5, 1, 2 or 4); where the
formation of benzyl benzoate. In summary, benzaldehyde is bases are: K2CO3, NaOH, Na(CH3COO) and Na2B4O7. The reactor
produced with the best selectivity in cyclohexane at low amount was purged ve times with O2, leaving the vessel at 6 bar. The
of base, benzoic acid in water at high amount of base and benzyl temperature was maintained at 100 ꢁC with an oil bath on a hot
benzoate in solvent-free condition also at high amount of base. stirring plate connected to a digital controller (ETS-D5 IKA). The
These observations are quite important when the selective reactions were constantly stirred using Teon-coated magnetic
oxidation of substrates derived from biomass, such as HMF, stir bars for the desired amount of time (15, 30, 45, 60, 75, 150
glycerol and glucose are considered. These reactions are min). The reaction mixture was collected and the catalyst was
generally performed in water, so the nature and concentration removed by centrifugation. The pH of the resulting solution was
of the base promoters evaluated here are of great importance for measured and adjusted to 5 and the products extracted with
further developments in this eld.
CH2Cl2. The aqueous phase was collected and the pH adjusted to
2. The products were extracted with CH2Cl2 and the organic
phases were combined and analyzed by GC.
Experimental
General remarks
In a typical solvent-free reaction, the glass reactor was loaded
with benzyl alcohol (1 mL, 9.7 mmol), the AuPVA/TiO2 catalyst
(52 mg, 4.4 mmol Au) and the base ([K2CO3] : [alcohol] ¼ 0.1 or 1
(133 mg or 1.33 g)). The reactor was purged ve times with O2,
leaving the vessel at 6 bar. The temperature was maintained at
100 ꢁC with an oil bath on a hot stirring plate connected to
a digital controller (ETS-D5 IKA). The reactions were constantly
stirred using Teon-coated magnetic stir bars for 60 min. To the
reaction mixture, 9 mL of CH2Cl2 was added and the catalyst
was removed by centrifugation. An aliquot of the solution was
removed and analyzed by GC.
HAuCl4, NaBH4, TiO2 (anatase) and polyvinyl alcohol (PVA, MW
ꢄ 9000–10000 g molꢀ1, 80% hydrolyzed) were purchased from
Sigma-Aldrich; NaOH, Na(CH3COO), K2CO3 and Na2B4O7 were
purchased from Merck; benzyl alcohol was purchased from
J.T. Baker and used as received. Standards for GC analysis
(benzaldehyde, benzoic acid and benzyl benzoate) were ob-
tained from Sigma-Aldrich.
Instrumentation
FAAS analysis was performed on a Shimadzu AA-6300 spectro-
photometer using an Au hollow cathode lamp (Photron). TEM
analyses were performed at the LNNano Laboratory (CNPEM,
Campinas, Brazil) using a JEOL-JEM 2100F. Catalyst samples for
TEM were prepared by sonicating the catalyst powder in iso-
propanol. A drop of the resulting dispersion was placed on
a thin carbon lm, which was deposited on standard 400-mesh
TEM copper grids and air-dried. GC analyses were carried out
with a Shimadzu GC-2010 equipped with a RTx-Wax column (30
m ꢂ 0.25 mm ꢂ 0.25 mm) and a FID detector. Method: Ti ¼ 50 ꢁC,
Tf ¼ 200 ꢁC, 30 min, T FID and SPLIT ¼ 200 ꢁC. Internal standard:
biphenyl.
Quantication of hydrogen peroxide
The oxidation reactions were performed for 1 h and aer cool-
ing to room temperature, the catalyst was removed. Hydrogen
peroxide was determined spectrophotometrically by the perox-
idase assay.55 To estimate the amount of H2O2 present in the
different conditions, an aliquot from the reaction (10 mL) was
tested for the formation of triiodide (3 ¼ 25 500, l ¼ 350 nm)
using the HRP enzyme (horseradish peroxidase). In a UV cell
containing 2990 mL of KI in acetate buffer (pH ¼ 3.8) and 10 mL
of HRP enzyme solution (1.0 g mLꢀ1), the aliquot from reaction
was added and measured at 353 nm in duplicate. The blank was
measured in a UV cell containing 2990 mL of KI in acetate buffer
(pH ¼ 3.0) and 10 mL of HRP enzyme solution (1.0 g mLꢀ1).
Synthesis of AuPVA/TiO2
The Au nanoparticles were prepared based on literature.40,50 A 2%
solution of PVA was added to an aqueous HAuCl4 solution (5.08 ꢂ
10ꢀ4 M) with vigorous stirring (PVA/Au (w/w) ¼ 1.2); a 0.1 M
freshly prepared solution of NaBH4 (NaBH4/Au (mol/mol) ¼ 5) was
then added to form a metallic sol; the color of the sol was light-red
(Au). Aer 30 min of sol generation, the colloid was immobilized
by adding commercial titania (anatase) under vigorous stirring.
The amount of support was calculated to give a total nal metal
loading of 2 wt% (nominal). Aer 2 h the slurry was ltered, the
solid washed with hot water (2 ꢂ 25 mL) and ethanol (2 ꢂ 25 mL)
and dried at 100 ꢁC for 1 h. The metal content in the AuPVA/TiO2
catalyst determined by FAAS was 1.7 wt% Au.
Acknowledgements
The authors acknowledge support from FAPESP (Fellowship
Process No. 2014/10824-3; Grant Process 2014/15159-8), CNPq
´
and INCT-Catalise. We also thank LNNano-CNPEM (Campinas,
´
Brazil) for the use of TEM facilities and the Laboratorio de
ˆ
ˆ
Quimiluminescencia Organica (IQ-USP, Brazil) for the hydrogen
peroxide quantication assay.
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25284 | RSC Adv., 2016, 6, 25279–25285
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