A non-nitric acid method of adipic acid synthesis: Organic solvent- and promoter-free oxidation of cyclohexanone with oxygen over hollow-structured Mn/TS-1 catalysts

Article abstract of DOI:10.1039/c4gc02333a

A novel hollow-structured Mn/TS-1 catalyst has been reported as a non-nitric acid route for adipic acid production from oxidative cleavage of cyclohexanone. The method generates adipic acid in high yields with molecular oxygen under organic solvent- and promoter-free conditions. The hollow nature of the catalyst with large intra-particle voids facilitates the diffusion of bulky molecules to the internal catalytic site and increases the catalyst activity. The advantage of this catalyst is its reusability with almost consistent reactivity, thereby making it viable for industrial applications. This journal is

Full text of DOI:10.1039/c4gc02333a

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A non-nitric acid method of adipic acid synthesis:
organic solvent- and promoter-free oxidation
of cyclohexanone with oxygen over
Cite this: DOI: 10.1039/c4gc02333a
hollow-structured Mn/TS-1 catalysts†
Guoqiang Zou, Wenzhou Zhong,* Liqiu Mao, Qiong Xu, Jiafu Xiao, Dulin Yin,*
Zisheng Xiao, Steven Robert Kirk and Tao Shu
A novel hollow-structured Mn/TS-1 catalyst has been reported as a non-nitric acid route for adipic acid
production from oxidative cleavage of cyclohexanone. The method generates adipic acid in high yields
with molecular oxygen under organic solvent- and promoter-free conditions. The hollow nature of the
catalyst with large intra-particle voids facilitates the diffusion of bulky molecules to the internal catalytic
site and increases the catalyst activity. The advantage of this catalyst is its reusability with almost consist-
ent reactivity, thereby making it viable for industrial applications.
Received 26th November 2014,
Accepted 21st January 2015
DOI: 10.1039/c4gc02333a
www.rsc.org/greenchem
1
0
about 78% yield. Holmberg and coworkers oxidized cyclo-
hexene with 30% H to AA over WO -containing mesoporous
1
. Introduction
2
O
2
3
1
1
The development of a new and efficient methodology for the oxide. Recently, Thomas et al. have reported a single-step,
production of bulk chemicals, in particular one that takes liquid-phase, aerial oxidation of cyclohexane to AA (selectivity
account of effects on the environment, is becoming more and of AA = 32.2% at cyclohexane conversions of 19.8% and
1
12
more important in industrial chemistry worldwide. Adipic 1.5 MPa). While some positive outcomes were obtained, the
acid (AA) is an important raw chemical for the production of price of the oxidants such as H O and tert-butylhydroper-
2
2
2
,3
nylon-6,6, fibers, plasticizers, food additives, etc. The world oxide, the drastic reaction conditions, additional organic sol-
production capacity of AA was around 3 million metric tons in vents, and the low yields of AA restricted the industrial
2
006 and the demand for it has increased every year in the last application of these new developments. Thus, an environmen-
4
decade. The traditional industrial process utilizes the nitric tally friendly, yet practical route to AA from cyclohexanone
acid oxidation of cyclohexanone and/or cyclohexanol (KA oil) using cheaper oxidants such as O2 without any solvents is
which are derived from benzene (Fig. 1). The main drawback highly desirable. The challenge, however, is the development
5
,6
of this processing is the emission of large amounts of the of an efficient and stable catalyst that would make this new
strong greenhouse gas (N O), an inevitable waste causing route competitive compared with the nitric acid route.
2
global warming and ozone depletion as well as acid rain and
smog.
Titanosilicate molecular sieves TS-1, with MFI cages con-
nected by 10-ring pore openings, have been known for their
7
,8
Many efforts have been made to improve or even get rid of remarkable selective oxidation activity for organic molecules in
1
3
this conventional process. Syntheses starting from cyclohexa- the chemical industry. The diffusion and transport of large
none, cyclohexanol, cyclohexene and cyclohexane were actually molecules within such microporous channels would, however,
9
–12
widely developed.
Chavan et al. have used a Co/Mn cluster
complex as a catalyst to convert cyclohexanone to AA in acetic
9
acid medium with a selectivity of 86%. Sato et al. oxidized
cyclohexanol with 30% H O over H WO , which achieved
2
2
2
4
National & Local United Engineering Laboratory for New Petrochemical Materials &
Fine Utilization of Resources, Hunan Normal University, Changsha 410081,
P. R. China. E-mail: zwenz79@163.com, dulinyin@126.com; Tel: +86-731-88872576;
Fax: +86-731-88872531
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4gc02333a
Fig. 1 Current industrial processes.
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be greatly limited by the small pore openings, resulting in low 40H
2
O and was transferred to a Teflon-lined stainless steel
catalytic activity in the oxidation of bulky substrates. In con- autoclave. Crystallization was carried out by placing the sealed
trast, large-pore Ti-Beta and Ti-MCM were anticipated to solve autoclave in an air oven maintained at 443 K for 3 days. The
the diffusion and transport problems encountered in the oxi- product obtained was filtered, washed with distilled water,
dation of bulky molecules, but they have not been able to sub- dried at 393 K and finally calcined at 793 K for 10 h. The HTS
2
3
stitute for traditional TS-1 as catalysts in industrial chemistry was synthesized according to a literature method. The pre-
mainly due to the presence of Brønsted acid sites associated viously calcined TS-1 was treated with H SO solution with a
with the aluminum framework or their low hydrothermal weight ratio of the TS-1 : H SO : water = 10 : 1.0 : 140 for 3 h at
2
4
2
4
stability resulting from the absence of a zeolite-like crystalline room temperature. The acid-treated TS-1 was dispersed in
1
4–16
framework.
Therefore, a novel large-pore structure (or one TPAOPH solution in a weight ratio of 10 TS-1 : 1.5 TPAOPH :
with large intra-particle voids) which does not have the afore- 125 H O and was recrystallized at 413 K for 3 days under static
2
mentioned disadvantages is urgently desired. In particular, conditions. The solid product was filtered, washed, dried and
large intra-particle voids in a structure can greatly favor the finally calcined at 793 K for 10 h.
mass transport ability of a guest molecule to internal catalytic
The catalyst Mn-modified HTS was prepared by the incipi-
1
7
sites. In the past few decades, various types of Ti-containing ent wetness impregnation of HTS with an EtOH and water
zeolites have been prepared and some encouraging results (1 : 1) solution of manganese acetate to give 5 wt% of manga-
have been reported by the ‘destructive’ and the ‘constructive’ nese in the final product. The volume of the solution used was
1
8–21
synthesis methods.
Among the Ti-containing zeolites, a sufficient to completely wet the HTS sample. The impregnated
kind of hollow-structured TS-1 catalyst (HTS) has received sample was slowly allowed to evaporate at 373 K till dryness,
much attention in the selective oxidation of bulky molecules followed by calcination at 773 K in air for 3 h (with a heating
such as cyclohexane and shown to be more active than TS-1 rate of 5 °C min− ). The investigated samples were referred to
1
2
1
with H
2
O
2
as the oxidant.
2 2
H O is, however, an expensive as xMn-HTS, where x denotes the metal wt%.
oxidant, and its extensive application will be seriously limited.
This arouses our research interest in using other cheap oxi-
2.2. Catalyst characterization
dants such as molecular oxygen to carry out the oxidation of X-ray powder diffraction patterns (XRD) were recorded on a
bulky molecules. Bruker Advanced D8 diffractometer using CuKα radiation (λ =
In a previous paper, we reported on the catalytic behaviour 1.542 Å). Size and particle morphology of the samples
of Mn-doped titanosilicate with a hollow structure in the were determined using scans obtained using a Hitachi
aerobic oxidation of cyclohexane to AA conducted in a free- S-4800 microscope, while the TEM images were taken by using
2
2
solvent. It was found that the hollow structure and Ti sites in a JEOL-JEM-2100 microscope. The textural properties of the
the internal surface are more adapted for the production of samples were measured by N adsorption at 77.4 K using a
2
bulky AA compared with cyclohexanol or cyclohexanone. Tristar 3000 sorptometer. Prior to the tests, the samples were
However, the cyclohexane conversion and AA yield achieved outgassed at 473 K under a reduced pressure of 10− Torr for
were relatively low in industrial chemistry. In this paper, we 3 h. The surface area was determined by the Brunauer–
report the aerial oxidation of cyclohexanone to AA using this Emmett–Teller (BET) equation, while the total pore volume
5
0
hollow-structured TS-1 catalyst. To our knowledge this is a was measured at p/p = 0.99. The pore size distributions were
novel non-HNO , aerial oxidation route for the preparation of determined from the adsorption branch of the isotherms
3
AA from cyclohexanone, without the addition of an additional using the BJH method. The UV-visible diffuse reflectance
solvent or initiator, wherein the yields of AA are comparable to spectra of the samples were recorded from 190 to 500 nm with
those obtained in current, commercial practice. In addition, a Varian-Cary 5000 spectrometer using BaSO as a reference.
4
this solvent-free/dioxygen system can suppress the leaching of X-ray Photoelectron Spectra (XPS) of samples were recorded
active metal ions during heterogeneous reaction. This method with an Imaging Photoelectron Spectrometer (Axis Ultra,
would provide new possibilities for applying titanosilicates in Kratos Analytical Ltd) using Al Kα radiation (1486.7 eV). The
the petrochemical and fine chemical industries.
energy was calibrated with a C 1s peak (BE = 284.8 eV). Fit XPS
software was used for curve fitting. Pyridine-adsorbed FT-IR
spectra were recorded using a PerkinElmer Spectrum 100
spectrometer. The samples were subjected to a conditioning
2
. Experimental
treatment in situ in advance, which involved outgassing at
2
.1. Synthesis of the catalytic materials
−2
4
73 K for 2 h (residual pressure at 1.33 × 10 Pa) and then
TS-1 was synthesized hydrothermally using tetraethyl orthosili- cooling down to room temperature and equilibrating with
cate (TEOS) and titanium butoxide (TBOT) as the sources of Si pyridine for 15 min. After heating at 423 K for 0.5 h, record-
and Ti, respectively, whereas alkali-free tetrapropyl ammonium ings of the FT-IR spectra were collected.
hydroxide (TPAOH) and anhydrous isopropanol were used as
2.3. Cyclohexanone oxidation reaction
the template and mobilizing agent for the formation of micro-
pores, respectively. The starting gel was formed using the The catalytic oxidation of cyclohexanone was carried out in
3
following molar compositions: SiO : 0.4TPAOPH : 0.01TiO : a Teflon-lined stainless steel autoclave of 100 cm capacity,
2
2
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equipped with a magnetic stirrer. In a typical experiment,
cyclohexanone (40 g) was mixed with 0.4 g of the catalyst and
then heated to 363 K. Oxygen was then charged into the
reactor to the desired pressure and the reaction was started by
adjusting the stirrer speed to 450 rpm. After the reaction, the
mixture was cooled down, dissolved in ethanol and filtered.
The reaction mixtures were analyzed by using an Agilent 1100
series high-performance liquid chromatography instrument
equipped with a UV detector (Agilent G1365B MWB), using a
column of Zorbax Eclipse XDB C18 (150 mm × 4.6 mm i.d.).
The compounds of the reaction mixture were quantified by
HPLC using heptane diacid as the internal standard and
identified by HPLC-MS. The quantitative analyses of unreacted
cyclohexanone were carried out by gas chromatography using
chlorobenzene as the internal standard. Mass balances were
verified.
Fig. 3 (A) N
2
adsorption–desorption isotherms and (B) pore diameter
distribution of the parent HTS and the xMn-HTS samples.
3
. Results and discussion
3
.1. Structural analysis
Fig. 4 (A) UV-Vis and (B) pyridine-adsorbed FT-IR spectra of the parent
The XRD patterns of samples shown in Fig. S1 in the ESI† indi- HTS and the xMn-HTS samples.
cate that all the prepared HTS and 5% Mn-HTS samples retain
the MFI-type phase structure of zeolite TS-1 and no diffraction
structured zeolite surface. Furthermore, a crystalline material
containing no amorphous phase could also be clearly found,
indicating their high hydrothermal stability during the reac-
peaks corresponding to crystalline MnO clusters could be
x
2
4
detected.
It is, therefore, believed that the manganese
species have homogeneously dispersed on the surface of the
crystals. The crystallinity and morphology of HTS synthesized
by a dissolution–recrystallization process were investigated by
TEM. The particles of HTS and 5% Mn-HTS showed
large intra-particle voids with an average particle size of
2
tion. N adsorption/desorption isotherms and pore size distri-
butions for samples of the hollow-structured zeolite HTS and
the prepared Mn-HTS are shown in Fig. 3. The presence of
intra-crystalline voids accessible only via entrances smaller
than 4 nm is evident from abrupt closure at p/p = 0.42 on the
0
1
50–250 nm, and the large voids were located exclusively in
desorption branch and differ from those of normal TS-1, as
the inner part of the crystals and never communicating directly
with the surface (see Fig. 2A and C). This kind of hollow
nature is essential to decrease pore diffusion limitations and
facilitate bulky molecular transport to catalytic sites and
increase the activity of the catalyst, which is similarly observed
2
3
evidenced by TEM images. After loading with Mn species,
the samples showed similar N sorption isotherms with a H
2
2
hysteresis to that of matrix HTS, indicating the presence of a
similar structure with intra-crystalline voids. The Ti coordi-
nation states of HTS after calcination were studied by UV-
visible spectroscopy (Fig. 4A). One band centered at 220 nm
could be observed for HTS, which can be attributed to the
2
5
in earlier reports. The TEM results in Fig. 2C and D con-
firmed that manganese species were dispersed onto the hollow
2
−
4+
charge transfer from O to Ti and is characteristic of tetra-
hedrally coordinated Ti highly dispersed in hollow crystals, as
2
6
they are in the original TS-1 zeolite. Another band centered
at 330 nm also appeared, however, most likely reflecting
the presence of ex-framework Ti species (e.g., octahedral Ti
2
7
sites). The big difference from the original TS-1 strongly sup-
ports the conclusion that the partial dissolution and recrystal-
lization of the zeolite can modify the repartition of Ti species
and enrich the surface of hollow crystals with the catalytic
2
3
species. Compared with the reference HTS, the prepared
2
−
3+
Mn-HTS displayed an absorption band at 310 nm (O to Mn
electron transition) which might be somewhat hampered by
2
8
overlap of the second assignments in the matrix HTS. The
change of the absorption band at 310 nm, however, clearly
supports the assertion that manganese species are present on
the surface of the catalyst.
Fig. 2 TEM images of the parent HTS (A and B) and 5% Mn-HTS
samples (C and D).
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a
2
Table 1 Cyclohexanone oxidation with O over HTS catalysts
c
Selectivity (%)
b
Entry
Catalyst
Conversion (%)
AA
GA
SA
Others
1
2
3
4
5
6
7
8
9
Blank
Silicon-1
TS
1.8
1.9
6.7
93.9
93.4
94.6
95.5
93.1
78.6
78.8
67.4
71.6
65.1
92.8
95.4
85.2
85.5
87.8
—
6.1
6.3
5.4
4.5
6.9
11.6
13.2
29.0
28.4
21.2
4.3
—
—
—
—
—
0.3
—
—
—
5.1
3.9
1.1
—
7.3
—
—
—
0.5
—
—
HTS
27.3
68.4
22.4
36.9
28.4
30.6
25.0
11.2
2.4
53.6
61.4
57.9
—
Mn-HTS
Co-HTS
Fe-HTS
Cr-HTS
Cu-HTS
V-HTS
Mn-TS
Mn-HTS
Mn-HTS
Mn-HTS
Mn-HTS
Mn-HTS
—
Fig. 5 (A) Ti(2p) core level XPS, (B) Mn(2p) core level XPS of the parent
HTS and the xMn-HTS samples.
4.7
4.1
2.5
—
6.4
2.9
—
2.5
2.4
1.8
—
1
1
1
0
1
2
The X-ray transmission spectroscopy (XPS) results in Fig. 5A
showed that the sample of HTS possessed binding energy
d
e
f
4.6
3
/2
1/2
levels of Ti 2p and Ti 2p at around 459 eV and 464 eV, 13
12.3
11.6
10.4
—
14
15
16
respectively. Interestingly, the curves for Mn-HTS shifted to
larger binding energy, indicating that the Ti–O bonding
strength was changed by the introduction of manganese
g
h
a
2
9
Reaction conditions: cyclohexanone (40 g), catalyst (0.4 g, 5 wt% of
b
ions. For comparison, the Gaussian values were used in the
metal), O
2
(0.6 MPa), 363 K, 9 h. Cyclohexanone conversion based on
c
3
/2
curve resolution of the Ti 2p
peaks. Two spectra were
cyclohexanone consumed. Product selectivity
=
content of this
observed: the first peak at ca. 458.5 eV was assigned to the product/the cyclohexanone consumed, AA: adipic acid; GA: glutarate
d
4
+
acid; SA: succinate acid; others: probably CO and CO. Mn-HTS
2
octahedrally coordinated Ti ion and the second peak at
e
was treated with trimethylamine. Mn-HTS was treated with
triphenylamine. Addition of acetic acid (0.2 g), 6 h. Addition of
within the silica framework. The relative area ratio of the peak adipic acid (0.2 g), 6 h. Addition of 2,6-di-tert-butyl-p-cresol (5 mol%
4
+
ca. 460.0 eV was assigned to Ti in the tetrahedral coordination
f
g
h
cyclohexanone), 20 h.
at 460.0 eV assigned to tetrahedral Ti species, however,
decreased with the loading of Mn, suggesting a change in the
coordination number of titanium by the formation of a Ti–O–
3
0
3/2
Mn bond. In addition, the Mn 2p signal can be deconvo-
lyst in comparison with Mn-HTS is presumably due to the
solid structure of the Mn-TS catalyst (Table 1, entry 11).
δ+
luted to distinguish the states of Mn and the results are
recorded in Fig. 5B. The binding energies (BE) of Mn 2p in
3
/2
2
According to the TEM and N sorption isotherms results that
the intervals 639.9–640.8 eV, 640.5–642.3 eV and 642.5–643.2
show large intra-crystalline voids of Mn-HTS (see Fig. 2 and 3),
we may capitalize here on ‘product shape-selectivity’. Consider-
ing the large intra-crystalline voids of Mn-HTS, substrate con-
version likely occurs inside the intra-crystalline voids of the
Mn-HTS catalyst. Only those products with appropriate mole-
cular dimensions may diffuse easily out of the intra-crystalline
voids via the pore, whereas larger ones formed in the course of
the reaction are trapped inside on account of their signifi-
cantly retarded diffusion. Qualitatively, the diameter of cyclo-
hexanone is 0.51 nm (the distance between oxygen and the
most distant hydrogen is 0.50 nm; the van der Waals radius of
hydrogen is 0.10 nm) and that of the products cyclohexyl
hydroperoxide (CHHP) and AA should be 0.69 and 1.0 nm,
respectively; the average pore diameter of HTS-1 is 0.55 nm.
2
+
3+
4+
eV can be attributed to the presence of Mn , Mn and Mn
3
1
ions on the surface of Mn-HTS, respectively.
The surface acidities of samples were characterized by the
pyridine-adsorbed FT-IR spectra of HTS and 5% Mn-HTS (see
−
1
−1
Fig. 4B). The two bands at 1450 cm and 1540 cm corres-
ponded to Lewis acid sites and Brønsted acid sites, respecti-
3
2
vely. It is apparent that both the catalysts possess many
more Lewis acid sites than Brønsted acid sites. The introduc-
tion of Mn species leads to an increase in the number of Lewis
acid sites, possibly because a few percent of the Ti species are
coordinated by high valence Mn species.
3
.2 Activities of the catalysts
Table 1 shows the activities of the different metal-modified We therefore envisage that molecular cyclohexanone can enter
HTS catalysts in the selective oxidation of cyclohexanone in the intra-crystalline voids of the Mn-HTS catalyst via the pore.
the liquid phase using oxygen as an oxidant. The hollow-struc- Reaction intermediates (cyclohexyl hydroperoxide, ketonyl
tured Mn-HTS catalyst showed 68.4% cyclohexanone conver- radical complex) formed within the intra-crystalline voids,
sion with 93.1% AA selectivity (Table 1, entry 5); the main by- however, will be held in the vicinity of the active centres, until
product was glutaric acid (GA). Compared with the Mn-HTS oxidation proceeds further to yield the more mobile and
catalyst, some other metal-modified HTS catalysts such as Co- desired linear product AA.
HTS, Fe-HTS, Cr-HTS, Cu-HTS, or V-HTS (Table 1, entries
–10) were much less effective for the catalytic oxidation of (Table 1, entries 2–4) with silicate-1 and TS-1 leave no doubt
cyclohexanone into AA. It is clear that the oxidation activity is that the active sites in our highly active catalyst are enriched Ti
With respect to active centres, detailed comparisons
6
3
3
dependent on the nature of the metal used. In addition, the species on the surface of intra-crystalline voids. One of the key
lower activity observed in the reaction using Mn-TS as the cata- features of our approach to the design of the necessary cata-
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3
5
lysts is to implant transition metal Mn ions in high oxidation termination effect. Moreover, it cannot be excluded that a
states onto the surface of the crystals by calcination. The redox-type mechanism may prevail over autoxidation under
resulting catalyst, with a modified active site, exhibits an conditions of a high catalyst-to-cyclohexanone mass ratio
3
4
activity for the oxidation of cyclohexanone that surpasses that based on the distribution of products. These results indicate
of the original catalyst (Table 1, entries 4 and 5). There is a direct participation of the catalyst in the rate-determining
clearly a favourable electronic influence in proceeding from a step of the process, and suggest that the Mn-HTS has a cata-
3 2
(SiO) Ti–OH to a (SiO) (MnO)Ti–OH, as demonstrated by lytic effect on the process rate. The conversion of cyclohexanone
DR-UV and XPS (see Fig. 4A and 5A). Furthermore, the location and AA selectivity has a complicated dependence on the mole-
of the active Ti sites in Mn-HTS was studied by using position- cular oxygen pressure as shown in Fig. S2 in the ESI.† The
selective poisoning reagents. We selected two molecular maximum yield of AA was observed at 0.6 MPa of molecular
probes: (1) tripropylamine (TPA), which is expected to enter oxygen pressure. At lower or higher pressures, selectivity of AA
the intra-crystalline voids of the catalyst and consequently decreased significantly.
poison the Ti sites, and (2) the bulky triphenylamine (TPhA),
which is too large to enter the intra-crystalline voids of the
3.4 Reuse of the Mn-HTS catalyst
catalyst via entrances and thus should selectively poison the From the context of a ‘green’ approach, the catalytic oxidation
active Ti sites located at the surface of intra-crystalline voids. of cyclohexanone was performed with a reused Mn-HTS cata-
In the case of TPA, after the reaction, the cyclohexanone con- lyst under the same reaction conditions. The catalytic activity
version showed a dramatic yet comparable decrease (Table 1, of the recovered catalyst after 15 consecutive experiments did
entries 12 and 13), suggesting that most of the Ti sites partici- not lead to any significant decline in its efficiency in terms of
pating in the oxidation of cyclohexanone with oxygen are conversion and selectivity (Fig. 7A). After completion of the
located within intra-crystalline voids at the surface of the reaction, the solid catalyst was removed from the reaction
HTS-1 particles.
mixture by filtration during the hot conditions and the oxi-
dation was allowed to proceed with the mother liquor (filtrate)
under the same conditions. The conversion did continue to
3
.3 Effect of various parameters
In order to further understand the product distribution in the increase, albeit at a much lower rate after the removal of the
cyclohexanone oxidation, we studied the effect of reaction catalyst (Fig. 7B). The persistence of a low activity could be
parameters. The catalyst shows a low activity with 100% AA attributed to the thermal autoxidation, initiated by the oxyge-
3
6
selectivity at 353 K but with increasing temperature the conver- nated products, present in the mother liquor. The leaching
sion of cyclohexanone increased. At 363 K, we found that the test was carried out for Ti and Mn by ICP-AES analysis using
formation of AA was optimum. At a higher temperature the filtrate and it was found that no Ti or Mn ions were
(393 K), the yield of adipic acid dropped significantly (Fig. 6A). present in the filtrate. We also observed that the amount of Ti
Most probably the product AA could be continuously oxidized and Mn present in the reused catalyst after 15 cycles of reuse
into by-products under the elevated temperatures. Fig. 6B is the same as that of the fresh catalyst as estimated by
shows the effect of reaction time on both cyclohexanone con- ICP-AES. TEM images of the reused catalyst showed an almost
version and adipic acid selectivity. It is well observed that with similar morphology to that of the fresh catalyst (Fig. S3 in the
increasing time, initially the conversion of cyclohexanone (till ESI†). The retention of the Mn-HTS structure was further con-
3
h) increases slightly, but after 3 h the conversion of cyclo- firmed by comparison of the XRD (Fig. S1 in the ESI†), FT-IR
hexanone increases very rapidly. In the meantime, it was also (Fig. S4 in the ESI†) and UV-Vis (Fig. 4A) spectra of the fresh
found that the AA selectivity decreased slightly. A possible and the 15 times reused catalyst. These results support the be-
explanation for this phenomenon is a radical-type autoxidation havior of Mn-HTS as a truly heterogeneous catalyst during
process. In order to find out the reaction intermediates, we cyclohexanone oxidation, which proves the efficacy of the cata-
took aliquots of the sample at regular intervals and analysed lyst in industry.
them by iodometric titration. No hydroperoxide was detected,
3
.5 The formation route of AA from cyclohexanone
which indicates that these possible reaction intermediates are
3
4
quickly transformed into more stable diacids. The amount of An important issue then arose: the formation route of AA from
Mn loading was found to be very important, since lower Mn cyclohexanone. Generally, cyclic ketones tend to form corres-
loading decreased the reaction rate without affecting the AA ponding diacids with molecular oxygen via a radical autoxida-
3
4
selectivity significantly (Fig. 6C). We have also explored the tion mechanism or
a
redox-type mechanism.
The
effect of the catalyst-to-cyclohexanone mass ratio on catalytic simultaneous C–C and O–O cleavage in the metal cyclohexyl-
behaviour (Fig. 6D). There was an almost proportional effect of peroxo complex seemed to be the breakthrough point to form
3
7
the catalyst amount on cyclohexanone conversion (with a cata- AA via a redox-type pathway. If this hypothesis was right, the
−
2
lyst-to-cyclohexanone mass ratio as low as 1 × 10 ), whereas complex should be easily converted to 6-hydroxyhexanoic acid
the selectivity to the products was not affected much by the under the same conditions. Unexpectedly, no 6-hydroxyhexa-
conversion. After the initial rapid increase, a further addition noic acid was detected by GC-MS. Furthermore when a small
of a catalyst led to a negative effect on the conversion and amount of a radical inhibitor (5 mol% 2,6-di-tert-butyl-p-
AA selectivity, which might be due to the well-known chain- cresol) was added, cyclohexanone conversion was nil (Table 1,
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Fig. 6 Effect of the reaction temperature (A), reaction time (B), Mn loading amount (C) and catalyst amount (D) on the conversion of cyclohexanone
and AA selectivity over the synthesized 5% Mn-HTS (reaction conditions: 40 g cyclohexanone; GA + SA: glutarate acid and succinate acid; others
2
include CO and CO).
2
Fig. 7 Results of hot-separation (A) and recycle (B) test of the 5% Mn-HTS catalyst at 363 K (reaction conditions: 0.6 MPa O , cyclohexanone 40 g,
catalyst 0.4 g).
entry 16), suggesting that the main reaction course might be a however, separate tests of acetic acid oxidation with oxygen
free-radical one. From the effects of reaction parameters, no and the Mn-HTS catalyst led us to exclude the formation of
lighter diacids derived from 2-hydroxylcyclohexanone were any peracid, as inferred by iodometric titration. A similar accel-
detected at first (1–3 h). All these results above indicated that eration effect, although has been already reported in the aerial
the Mn-HTS catalyzed oxidation of cyclohexanone was quite cyclohexanone or cyclohexane using Mn and Mn/Co catalysts
4
1,42
different from those using a Co/Mn cluster, H
5
PMo10
V
2
O
40
,
in the past decade, have almost never been considered.
9
,38–40
H
7
PMo 40 or Mn(OAc) -based catalytic systems.
8
V
4
O
2
In Based on the different configurational isomers of the reactant
those examples, AA was often generated via a redox-type reac- cyclohexanone, in this work we have imagined two possible
tion with electron transfer from the substrate to the catalyst, as isomers: keto-form and enol-form. The optimized structures of
proposed by the authors.
the cyclohexanone isomers were investigated by the DFT
In contrast, the use of acetic acid as the additive greatly method with the B3LYP correlation functional and are shown
enhanced the reaction rate, as shown in Table 1 (entry 14). An in Fig. 8. Calculation results indicate that in both the isomers,
3
explanation is the in situ formation of CH COOOH, which one of the C–H bonds [C(2)–H(7)] connecting the carbonyl or
might eventually act as an oxidant on cyclohexanone oxidation; hydroxyl group is longer than [C(1)–H(9)] near the methylene
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Fig. 8 Calculated bond lengths of cyclohexanone including keto-form and enol-form.
Scheme 1 The activation of cyclohexanone via protonation and tautomerisation.
Scheme 2 Proposed mechanism for the Mn-HTS catalyzed formation of AA.
group; thus it is easily broken from C(2)–H(7) to produce a cyclohexanone itself to produce radical species (Scheme 1).
radical. From a different standpoint, the carbon–hydrogen The use of AA as the additive instead of acetic acid also shows
bond [C(5)–H(13) or C(2)–H(7)] of the enol-form connects a outstanding effects in the acceleration of the reaction rate,
double bond and the corresponding bond distance [C(5)– which confirms the acceleration of the tautomerism between
H(13)] is larger than that of the [C(2)–H(7)] bond. As a result, it cyclohexanone and the corresponding enolic form under
was more easy for homolytic cleavage of the longer [C(5)– acidic conditions. It is evident that the oxidation occurs easily
H(13)] bond to produce the radical for enol-form isomers. upon the Mn-modified HTS. According to the pyridine-
Therefore, a possible explanation for this effect is the accelera- adsorbed FT-IR spectra results, Mn-HTS catalysts possess more
tion of the tautomerism between cyclohexanone and the acid sites, which favor the formation of enolate from the keto-
corresponding enolic form by the addition of acetic acid, form of cyclohexanone in the oxidation process. Based on the
which is more easily activated by radical H abstraction than experiments and DFT theoretical calculation mentioned above,
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we have proposed a pathway for Mn-HTS catalyzed oxidation of
cyclohexanone with molecular oxygen (Scheme 2). The
H-abstraction at the carbon adjacent to the oxygen in cyclohexa-
none (or adjacent to a double bond in enol-form isomers by
tautomerism) can most likely be promoted in the presence of
Ti species, and the production of radical at this position
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12, 1861–1869.
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. Conclusion
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