Combined H2O2/nitrile/bicarbonate system for catalytic Baeyer-Villiger oxidation of cyclohexanone to ε-caprolactone over Mg–Al hydrotalcite catalysts

Article abstract of DOI:10.1016/j.catcom.2019.105821

Magnesium-aluminium hydrotalcite-like compounds (Ht) with molar ratios of Mg/Al = 2, 3, 4 and 6, were synthesized and used as catalysts in the Baeyer-Villiger oxidation of cyclohexanone to ε-caprolactone. Oxidation was carried out in mild conditions (70 °C, atmospheric pressure) either with conventional H₂O₂/acetonitrile oxidizing mixture, or with a newly designed H₂O₂/acetonitrile/bicarbonate system. The presented results show clearly superiority of bicarbonate-containing setup, which provides a significant increase of the ε-caprolactone yield. The neutralizing effect of bicarbonate, strongly limiting leaching of magnesium from Ht, is considered the main cause of the improved catalytic performance.

Full text of DOI:10.1016/j.catcom.2019.105821

CatalysisCommunications132(2019)105821
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Catalysis Communications
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Short communication
Combined H2O2/nitrile/bicarbonate system for catalytic Baeyer-Villiger
oxidation of cyclohexanone to ε-caprolactone over MgeAl hydrotalcite
catalysts
Robert Karcza,⁎, Joanna E. Olszówkaa, Bogna D. Napruszewskaa, Joanna Kryściak-Czerwenkaa,
Ewa M. Serwickaa, Agnieszka Klimekb, Krzysztof Bahranowskib
a Jerzy Haber Institute of Catalysis and Surface Chemistry Polish Academy of Sciences, Niezapominajek 8, 30-239 Krakow, Poland
b Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Krakow, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords:
Magnesium-aluminium hydrotalcite-like compounds (Ht) with molar ratios of Mg/Al = 2, 3, 4 and 6, were
synthesized and used as catalysts in the Baeyer-Villiger oxidation of cyclohexanone to ε-caprolactone. Oxidation
was carried out in mild conditions (70 °C, atmospheric pressure) either with conventional H2O2/acetonitrile
oxidizing mixture, or with a newly designed H2O2/acetonitrile/bicarbonate system. The presented results show
clearly superiority of bicarbonate-containing setup, which provides a significant increase of the ε-caprolactone
yield. The neutralizing effect of bicarbonate, strongly limiting leaching of magnesium from Ht, is considered the
main cause of the improved catalytic performance.
Hydrotalcite catalyst
Baeyer-Villiger oxidation
Hydrogen peroxide
Sodium bicarbonate
1. Introduction
reaction. The role of nitrile compound goes beyond that of a solvent,
because, due to its reaction with perhydroxyl anion species (HOO−), a
Baeyer-Villiger (BV) oxidation of ketones to esters and cyclic ke-
tones to lactones yields products of immense practical importance,
ranging from fine chemicals to intermediates for the high tonnage
commodities [1]. In particular, BV transformation of cyclohexanone
gives ε-caprolactone, a monomer used in manufacturing of biodegrad-
able polymers, finding application in tissue engineering, biocompatible
coatings, or drug delivery systems [2]. Industrially, ε-caprolactone is
obtained with the use of organic peroxy acids [3], which is en-
vironmentally burdensome due to the explosive nature of the oxidants
and generation of acid by-product. For this reason a trend emerged
aiming at substitution of peroxy acids with other, more benign oxi-
dants, for instance H2O2, in combination with an appropriate catalyst
[1], e.g., Sn-beta-zeolite, pioneered by Corma et al. [4]. Another fre-
quently investigated system is based on the use of hydrogen peroxide in
combination with nitrile solvent, in the presence of basic solid catalysts,
peroxycarboximidic anion is generated, the active intermediate oxidant
[15,16]. Although benzonitrile has been shown to ensure best ε-ca-
prolactone yields [8], acetonitrile is significantly cheaper and less toxic
[17]. Therefore, in pursuance of the greener character of the overall
reaction system, in our studies we focus on the use of acetonitrile sol-
vent [11–13]. Reaction of peroxycarboximidic anion formation from
acetonitrile is illustrated in Fig. 1a.
The attractive features of MgeAl Ht catalysts are the relatively easy
and cheap synthetic methods, nontoxicity, facile modification of che-
mical composition and, most importantly, capability of activating ra-
ther inert hydrogen peroxide. However, recent studies from our la-
boratory indicated that the acidic environment of the reaction mixture,
stemming from the presence of hydrogen peroxide, results in leaching
of magnesium from the hydrotalcite catalyst, and causes partial deac-
tivation of the Ht catalyst [11,13]. Although the catalytic activity can
be restored by soaking of the used catalyst in the solution of Mg(OH)2,
the problem of leaching remains non-trivial, especially that it has been
repeatedly demonstrated that catalytic performance of MgeAl Ht is
enhanced by low crystallinity of the catalysts [12,13,18–27]. This ad-
vantage is limited by the fact that small crystallites are more susceptible
to leaching effects [13]. The problem cannot be solved by simple
such as MgeAl hydrotalcites [5–13]. MgeAl hydrotacites are layered
n−
compounds with the general formula [Mg1−xAlx(OH)2]x+(A )·mH2O,
x/n
where “x” is the degree of Al for Mg substitution, An− denotes anion
and “m” indicates the content of water of hydration [14]. The materials
are widely used in base catalysis, since the change of “x” offers means
for tuning their basic character to the requirements of a particular
⁎ Corresponding author.
E-mail address: nckarcz@cyf-kr.edu.pl (R. Karcz).
https://doi.org/10.1016/j.catcom.2019.105821
Received 9 July 2019; Received in revised form 4 September 2019; Accepted 10 September 2019
Availableonline11September2019
1566-7367/©2019ElsevierB.V.Allrightsreserved.

R. Karcz, et al.
CatalysisCommunications132(2019)105821
Fig. 1. Generation of a) peroxycarboximidic anion from acetonitrile, b) peroxymonocarbonate anion in BAP system, c) peroxycarboximidic anion from acetonitrile
and peroxymonocarbonate anion in BAP system.
alkalization of the reaction medium, because hydrogen peroxide is
prone to the alkali-induced decomposition [28]. It is therefore highly
desirable to find means to counteract the negative impact of the acidic
reaction environment, without causing excessive unproductive de-
struction of the H2O2 oxidant.
In search for the solution to this problem we turned our attention to
the joined use of H2O2 and sodium bicarbonate as a reagent active in
epoxidation reactions, discovered by Richardson et al. [29]. The key
aspect of this system, referred to as bicarbonate-activated peroxide
(BAP), is that hydrogen peroxide reacts with bicarbonate to form per-
oxymonocarbonate, which acts in the capacity of the actual oxidant
(Fig. 1b). Of particular importance for the present work is that the BAP
system operates at nearly neutral pH.
3. Results and discussion
In this research note we report on the effect of joined use of H2O2,
nitrile and bicarbonate in the MgeAl hydrotalcite catalyzed reaction of
cyclohexanone lactonization and demonstrate that this novel combi-
nation brings about an improvement of the catalytic performance with
respect to the separate action of either H2O2/nitrile/Ht or BAP systems.
Catalytic activity of MgeAl Ht compounds is known to increase with
the relative Mg content, therefore the catalysts with nominal Mg/Al
ratio = 2, 3, 4, and 6, referred to as Mg2Al, Mg3Al, Mg4Al and Mg6Al
were used in this study. Detailed physico-chemical characterization of
such samples is given elsewhere [30], but the most important feature,
confirming the structural identity of the materials, is that they show
XRD patterns typical of a hydrotalcite structure (Supporting Informa-
tion Fig. S1) [14]. Crystallographic data revealed that both the d003
(related to the interlayer distance) and d110 (related to the mean ca-
tion–cation distance) values increase with decreasing Al content, the
former due to the weakening of electrostatic attraction between the
positively charged layers and interlayer anions, the latter due to the
difference in ionic radii between Mg2+ and Al3+(0.072 and 0.054 nm,
respectively). The actual Mg/Al values, were close to the intended ones,
i.e. 1.9, 2.9, 3.8 and 5.6 for Mg2Al, Mg3Al, Mg4Al, and Mg6Al, respec-
tively. The phase purity of the samples is further confirmed by Raman
spectroscopy, which shows that even in the Mg-rich samples no segre-
gation of other phases, in particular brucite, occurs (Fig. S2, Supporting
Information) [30].
Catalytic activity of the samples was investigated in the Baeyer-
Villiger oxidation of cyclohexanone to ε-caprolactone with H2O2/acet-
onitrile system. The results, presented in Fig. 2a, show that the catalytic
performance of the as received Ht catalysts improves with increasing
Mg content, i.e. with the catalysts basicity, in agreement with the lit-
erature data [7,11]. The increase of the Ht samples basicity with
growing Mg content has been confirmed by measurements of benzoic
acid adsorption and determination of the pH of aqueous suspensions of
the catalysts (Table 1S, Supporting Information). In the absence of any
additive, the best caprolactone yield of 33% is obtained for the Mg6Al
sample.
In view of this we decided to check on the effect of bicarbonate
addition to the cyclohexanone/H2O2/nitrile/Ht system, in expectation
that it might alleviate the effect of hydrotalcite catalyst deactivation,
while simultaneously introducing the BAP-related activity.
2. Experimental
Mg-Al Ht compounds with nominal Mg/Al ratio = 2, 3, 4, 6 were
synthesized by the co-precipitation at pH = 10, described in details
earlier [30] (see Supporting Information). The materials were char-
acterized with ICP OES, PXRD, Raman and FTIR spectroscopies, and
basicity determination by means of benzoic acid adsorption [31] and
measurement of pH of aqueous suspensions [32]. Results and mea-
surement details are provided in the Supporting Information file. The
amounts of metal cations leached to the solution during catalytic ex-
periments were determined with AAS using a Thermo Scientific ICE
3500 AAS spectrometer. Catalytic tests were carried out for 3 h at 70 °C
in a glass reactor nested in a magnetic stirrer equipped with the heat-on
system in the presence of 0.2 g of catalyst. Reaction mixture consisted of
0.025 mol of cyclohexanone, 0.2 mol of 30% hydrogen peroxide and
0.4 mol of acetonitrile. In the experiments with bicarbonate, 4 × 0.05 g
of NaHCO3 was added after 1, 15, 30 and 120 min of reaction duration.
Recovery of catalyst was performed by centrifugation of reaction mix-
ture. Liquid reactants were decanted and catalyst was reintroduced into
the reactor without drying or purification. Centrifugation vessel was
washed with acetonitrile in order to transfer all of the catalyst into the
reactor. After that substrate and hydrogen peroxide were added into the
reactor. Reaction mixture was analyzed by gas chromatography using
Thermo Trace GC Ultra instrument fitted with TR-5 capillary column
with flame ionization detector. Identification of products was per-
formed using GC–MS analysis with Thermo Trace GC Ultra equipped
with TR-5-MS capillary column and DSQ 2 mass detector. Hydrogen
peroxide efficiency was determined on the basis of iodometric titration
of the reaction mixture.
However, when the catalysts were subjected to recycling experi-
ments, in all cases a significant loss of the activity, selectivity and the ε-
caprolactone yield was observed (Fig. 2b). This loss of activity has been
previously attributed to partial leaching of Mg in the slightly acidic
environment of the H2O2-containing reaction mixture [11]. Indeed, the
AAS analysis of the liquid phase separated from the reaction mixture
after the first catalytic run revealed that some magnesium initially
present in the catalysts was transferred to the solution, the amount
growing with the catalyst Mg/Al ratio from ca. 5% for Mg2Al to ca. 10%
for Mg6Al sample. The amount of leached Al was two orders of mag-
nitude lower, indicating that the loss of Mg takes place in a selective
manner. Since the occurrence of a homogeneous catalytic reaction with
participation of leached Mg2+ has been excluded [11], the observed
2

R. Karcz, et al.
CatalysisCommunications132(2019)105821
100
80
60
40
20
0
Conversion
Selectivity
Yield
Conversion
Selectivity
Yield
a)
b)
1st run
2nd run
70
67
64
66
52
52
52
48
43
40
36
33
28
29
14
28
24
25
13
23
21
18
12
9
Mg6Al
Mg6Al
Mg2Al
Mg3Al
Mg4Al
Mg2Al
Mg3Al
Mg4Al
Fig. 2. Catalytic oxidation of cyclohexanone to ε-caprolactone with H2O2/acetonitrile system over MgnAl catalysts: a) first run, b) second run.
leaching-induced loss of Mg is assumed to be the chief cause of dete-
rioration of the catalytic performance. Due to the complex composition
of the reaction medium it was not possible to conclude on the presence
or absence of the leached carbonate species, but in view of the well-
known affinity of carbonate anions to the Ht lattice and difficulty of
their deintercalation/desorption, it seems unlikely that Mg leaching is
accompanied by a meaningful loss of interlayer carbonate. It is tenta-
tively proposed that the Mg-depleted areas resemble nonstoichiometric
Al hydroxycarbonate, known to form easily at the surface of Al(OH)3
[33].
In order to get better insight into the changes of acid-base properties
of the reaction medium during the course of the catalytic process, the
pH value of the reaction mixture was monitored, and, as an example,
the data for the Mg4Al catalyst are shown in Fig. 3.
The reaction mixture without the catalyst was characterized by pH
around 4.5. Addition of the catalyst raised the pH value to ca. 6.7, and,
subsequently, a gradual fall of pH was observed to the final reading of
5.3, thus confirming that the acid-base interaction between the catalyst
and the reaction medium occurred.
observed for NaHCO3 without the catalyst. This confirms that in the
presence of NaHCO3 a more favorable environment for the operation of
a Ht catalyst is created. Indeed, upon addition of NaHCO3 to the re-
action systems containing Ht catalysts a significant improvement of the
catalytic performance is observed, as indicated by the data in Fig. 4.
The efficiency of H2O2 utilization measured as the selectivity towards ε-
caprolactone was 23
2% without bicarbonate, and 19
2% in tests
with NaHCO3 addition.
The relative increase of ε-caprolactone yield with respect to that
observed in the absence of bicarbonate is 205, 188, 179 and 127% over
Mg2Al, Mg3Al, Mg4Al, and Mg6Al, respectively. This shows that the
beneficial influence of NaHCO3 addition is most pronounced in the
catalysts least susceptible to the elution of Mg. Analysis of the liquid
phase after completion of the reaction over Mg4Al showed that the
amount of leached Mg was ca. 1.5% of the total Mg, i.e. much lower
than the 8.5% loss observed without bicarbonate addition. The effect is
understandable, since the equilibrium between Mg bound in the Ht
lattice and Mg leached to the solution is pH dependent, according to the
Eq. (1):
Prior to the tests with the joint use of a Ht catalyst and sodium
bicarbonate, a blank experiment, checking on the NaHCO3 effect on the
reaction mixture in the absence of the catalyst has been carried out. The
result showed that BAP system itself is active in the Baeyer-Villiger
transformation of cyclohexanone, and gives 35% cyclohexanone con-
version. However, the 51% reaction selectivity is less than in any pro-
cess carried out with the use of Ht samples (64–70%). As a result, the
[Mg − OH]+ + H+ ↔ Mg2+ + H2O
(1)
Ht
In consequence, alkalization of the reaction medium by addition of
bicarbonate, evidenced by pH measurements, suppresses the leaching
of Mg. It should be noted that the unique property of bicarbonate is that
it not only ensures pH more favorable for the catalyst stability, but in
combination with H2O2 forms a system capable of cyclohexanone
transformation into ε-caprolactone, the ability which other possible
alkalizing agents (e.g., NaOH) don't have.
observed
order
of
ε-caprolactone
productivity
is:
BAP = Mg2Al < Mg3Al < Mg4Al < Mg6Al. Monitoring of pH during
oxidation with the neat BAP system, i.e. with NaHCO3 but without the
Ht catalyst, (Fig. 3) shows that after initial increase to ca. 7.3, the pH
falls and stabilizes slightly above 6. A similar experiment carried out for
the Mg4Al + NaHCO3 system, showed that, due to the buffering prop-
erties of NaHCO3 solution, the pH changes were quite similar to those
As to the impact of treatment with BAP on the composition of Ht
catalyst interlayer, FTIR spectra show that the main effect is a certain
increase of the carbonate content due to the replacement of nitrate
impurity with carbonate anions from bicarbonate solution, while no
meaningful change in the intensity of bicarbonate-related bands is ob-
served (Fig. S3, Supporting Information).
7.5
The enhanced stability of the Ht catalysts in the bicarbonate-con-
taining reaction medium brought about a spectacular improvement of
the catalysts recyclability. The data in Fig. 4 show that in the second
catalytic run the used Mg2Al, Mg3Al, Mg4Al, and Mg6Al samples per-
form nearly as well as the fresh ones, and the observed ε-caprolactone
yield corresponds to 97, 91, 88 and 83% of the first run values.
Noteworthy, while in the absence of NaHCO3 the best performance
in terms of the ε-caprolactone yield is given by the most basic Mg6Al
catalyst, in the combined H2O2/nitrile/bicarbonate system, the op-
timum catalytic properties are achieved for the Mg4Al sample, which
yields 50% ε-caprolactone. Apparently, in the case of Mg6Al, the neu-
tralizing effect of NaHCO3 addition is not sufficient to enable satisfac-
tory retention of Mg in the catalyst structure. Chemical analysis of Mg
content in the NaHCO3 containing reaction mixture separated from the
Mg6Al catalyst shows 4.5% loss of Mg, which, although significantly
lower than the 10% observed without NaHCO3, is the likely reason for
Mg4Al
7
NaHCO3
6.5
Mg4Al + NaHCO3
6
5.5
5
4.5
0
30
60
90
Time [min]
120
150
180
Fig. 3. Dependence of the pH of the reaction medium on time of the catalytic
process.
3

R. Karcz, et al.
CatalysisCommunications132(2019)105821
100
80
60
40
20
0
Conversion
Selectivity
Yield
Conversion
Selectivity
Yield
1st run
65 65
86
82
81
a)
b)
2nd run
79
72
66
61
57
57
58
56
54
44
50
50
50
45
42
41
37
36
35
Mg6Al
Mg6Al
Mg2Al
Mg3Al
Mg4Al
Mg2Al
Mg3Al
Mg4Al
Fig. 4. Catalytic oxidation of cyclohexanone to ε-caprolactone with H2O2/acetonitrile/bicarbonate system over MgnAl catalysts: a) first run, b) second run.
the poorer performance of Mg6Al.
4. Conclusions
Analysis of the catalytic data indicates that the observed substantial
increase of the ε-caprolactone yield occurs both on the account of
higher conversion and of enhanced selectivity. Variations in selectivity
may be attributed either to changing contribution of consecutive
transformation of ε-caprolactone, such as hydrolysis or polymerization,
and/or to some parallel processes, which consume cyclohexanone.
Reaction mixtures were subjected to GC–MS analysis which identified
the presence of compounds such as acetamide, dimethoxycyclohex-
anone, epoxycyclohexanone, cyclohexanone oxime, nitrocyclohexane.
The presence of acetamide confirms that reaction follows the me-
chanism proposed by Jimenez-Sanchidrian et al. [6] involving forma-
tion of peroxycarboximidic acid and a Criegee adduct, which rearranges
to lactone with release of an amide. This conclusion is further supported
by lack of cyclohexanone conversion when acetonitrile is substituted
with dioxane or methanol (Table S2, Supporting Information). The
occurrence of other nitrogen-containing products suggests that the
nonselective routes also involve participation of the acetonitrile sol-
vent. On the other hand, no evidence of products expected upon hy-
drolytic decomposition of ε-caprolactone have been found, such as 6-
hydroxyhexanoic acid, the product of lactone ring opening upon reac-
tion with water, thus eliminating significant hydrolysis as a possible
cause for the lowering of selectivity. Another possibility is some con-
tribution of the radical reaction pathway, occurring parallel to the de-
sired route which involves consumption of perhydroxyl anions formed
over Mg-OH Brønsted basic sites. To check on this, an experiment was
performed with addition of butylated hydroxytoluene (BHT), a radical
scavenger, to the reaction mixture over Mg4Al catalyst. As a result, the
cyclohexanone conversion fell from 40 to 24%, but the selectivity in-
creased from 70 to 96%, thus confirming the role of parallel radical
reactions in lowering the selectivity to ε-caprolactone. In view of this, it
is suggested that the addition of NaHCO3, by stabilizing the Ht catalyst
surface against leaching of magnesium, preserves Brønsted basic sites
responsible for activation of H2O2 to OOH- – perhydroxyl anions re-
quired for the selective BV process, and promotes its occurrence.
In view of the essential role of acetonitrile in ε-caprolactone
synthesis with use of the Ht-activated H2O2/nitrile system, it was of
interest to check whether there is any involvement of acetonitrile in the
BV oxidation of cyclohexanone with neat BAP system (without the Ht
catalyst). To this end the reaction with BAP was carried out with other
solvents (methanol and dioxane) instead of acetonitrile. The results
(Table S2, Supporting Information) show that BAP was capable of
oxidizing cyclohexanone to ε-caprolactone only in the presence of
acetonitrile as a solvent. This suggests that, similarly as in the case of
Ht-activated H2O2/nitrile system, the oxidizing species is the perox-
ycarboximidic anion, formed by reaction of peroxybicarbonate anion
with acetonitrile (Fig. 1c).
Results presented in this study show clearly the superiority of the
H2O2/acetonitrile/bicarbonate/Ht setup described in this work, over
conventional H2O2/acetonitrile/Ht system. Joint operation of both the
catalytic and the non-catalytic processes brings about two positive ef-
fects: a) significant increase of the lactone yield, and b) catalyst stabi-
lization enabling efficient operation upon recycling. Formation of ε-
caprolactone with the use of BAP system requires the use of a nitrile
solvent, suggesting a mechanism involving peroxycarboximidic anion
as an oxidizing intermediate. Full understanding of the observed phe-
nomena requires further studies, but it appears that the neutralizing
effect of bicarbonate, rendering the Ht catalyst less susceptible to
magnesium leaching, is the main cause of the observed changes. The
proposed novel, experimentally facile reaction setup, represents an at-
tractive option for any catalytic process in which basic catalysts sus-
ceptible to acidic environment are used in combination with H2O2 so-
lution.
Acknowledgements
The work was supported in part by the statutory funding of Jerzy
Haber Institute of Catalysis and Surface Chemistry PAS and by the
National Science Center Poland, project OPUS 2017/27/B/ST5/01834.
K. Bahranowski thanks AGH University of Science and Technology for
financial support (subvention 16.16.140.319).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.catcom.2019.105821.
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