Efficient oxidation of 2,3,6-trimethyl phenol using non- exchanged and H+ exchanged manganese oxide octahedral molecular sieves (K-OMS-2 and H-K-OMS-2) as catalysts

Article abstract of DOI:10.1007/s10562-012-0779-3

A new and efficient oxidation process of 2,3,6- trimethyl phenol to 2,3,6-trimethyl benzoquinone (TMQ) is reported forthwith using non-exchanged and H⁺-exchanged manganese oxide octahedral molecular sieves (K-OMS-2 and H-K-OMS-2) as benign catalysts. The oxidation reaction is efficiently carried out using TBHP as oxidant and with catalytic amounts of OMS-2 achieving >95% conversion with excellent selectivity (～99%) to TMQ in 30 min. Springer Science+Business Media, LLC 2012.

Full text of DOI:10.1007/s10562-012-0779-3

Catal Lett (2012) 142:427–432
DOI 10.1007/s10562-012-0779-3
Efficient Oxidation of 2,3,6-Trimethyl Phenol using Non-
Exchanged and H Exchanged Manganese Oxide Octahedral
Molecular Sieves (K-OMS-2 and H–K-OMS-2) as Catalysts
+
Naftali N. Opembe
Steven L. Suib
•
Cecil K. King’ondu
•
Received: 9 November 2011 / Accepted: 28 January 2012 / Published online: 22 February 2012
Ó Springer Science+Business Media, LLC 2012
Abstract A new and efficient oxidation process of 2,3,6-
trimethyl phenol to 2,3,6-trimethyl benzoquinone (TMQ) is
Current industrial production of vitamin E involves
condensation of isophytol (IP) with 2,3,6-trimethyl-1,4-
hydroquinone (TMHQ) [3] obtained by hydrogenating
TMQ as shown schematically in Fig. 1. The industrial
production is carried out via oxidation of TMP with
molecular oxygen or air in the presence of copper halides as
catalysts [4–6]. Generally, TMQ is obtained from oxidation
of TMP [7, 8]. The order of reactions involves para-sulfo-
nation of TMP followed by its oxidation using stoichiome-
tric amounts of commercial manganese dioxide. Several
researchers have studied the oxidation step of the reaction
scheme with the aim of reducing the overall steps involved
as well as moving toward processes that are more environ-
mentally friendly. One-step reactions have been explored
employing different oxidants (molecular oxygen (O2),
hydrogen peroxide (H O ) or tertiary butyl hydroperoxides
?
reported forthwith using non-exchanged and H -exchan-
ged manganese oxide octahedral molecular sieves
(
K-OMS-2 and H–K-OMS-2) as benign catalysts. The
oxidation reaction is efficiently carried out using TBHP as
oxidant and with catalytic amounts of OMS-2 achieving
[95% conversion with excellent selectivity (*99%) to
TMQ in 30 min.
Keywords TMP oxidation Á Heterogeneous catalysis Á
Efficient oxidation
1
Introduction
2
2
Vitamin E finds extensive use in food, medicine, and
healthcare products. a-tocophenol is the form of vitamin E
known to meet human needs [1] and plays a vital role in
protecting human cells from the damaging effects of free
radicals thus acting to prevent various diseases, for
instance, cardiovascular diseases and cancer [2]. In addi-
tion to its antioxidant role, vitamin E also plays an
important role in the regulation of gene expression, cell
signaling, and other metabolic processes [1].
(TBHP) [9-13]. Sun et al. [9, 10] describe an approach
utilizing copper (II) chloride (CuCl ) as homogenous cata-
2
lysts in ionic liquid medium of 1-butyl-3-methylimidazo-
lium chloride (BMIM-Cl) with n-butanol as a co-solvent. In
that study, the yield of TMQ reached 98% with use of higher
amounts of CuCl gradually decreasing with reduction in the
2
catalyst amount. Although molecular oxygen was utilized,
this method required closed vessel, pressurized conditions
(up-to 10 bars) to achieve the reported results. Baiker
and coworkers [11] utilized CuCl2 in the presence of
NH OHÁHCl co-catalyst with TBHP as the oxidant in place
2
of molecular oxygen. In this approach the amount of CuCl
2
could substantially be reduced without substantial reduction
in product yield, however, catalyst recovery is still a chal-
lenge. Interestingly, the solubility of CuCl in the CuCl
N. N. Opembe Á C. K. King’ondu Á S. L. Suib
Department of Chemistry, University of Connecticut,
Storrs, CT 06269-3060, USA
2
2
system is not mentioned. CuCl is highly soluble in aqueous
2
S. L. Suib (&)
Institute of Materials Science, University of Connecticut,
Storrs, CT 06269-3136, USA
e-mail: steven.suib@uconn.edu
media and has been reported elsewhere to have a solubility
of 164 mg/100 g solution in a system involving 1-butyl-
3-methylimidazolium hexafluorophosphate—[BMIM] PF6
123

4
28
N. N. Opembe et al.
3
Experimental
3.1 Reagents
Potassium permanganate (KMnO ), manganese sulfate
4
monohydrate (MnSO ÁH O), 2,3,6-trimethyl phenol, ace-
4
2
tonitrile (MeCN), and tert-butyl hydroperoxide (70% in
water) were purchased from Sigma-Aldrich while con-
centrated nitric acid (HNO ) was obtained from Alfa Aesar.
3
All reagents were used without any further purification.
3.2 Catalyst Syntheses
Potassium containing OMS-2 (K-OMS-2) was synthesized
using a variation of earlier reported procedures [26–28]. In a
typical reaction, 42 mmol (6.65 g) KMnO was added to
4
1
00 mL distilled-deionized water (DDW) to make mixture
Fig. 1 Scheme showing steps involved in the current industrial
production of vitamin E viz: (i) oxidation of TMP to TMQ and its
subsequent reduction to TMHQ, and (ii) condensation of TMHQ with
isophytol to form vitamin E
A. In another flask, 59 mmol (9.9 g) of MnSO ÁH O was
4
2
added to 33 mL DDW to make mixture B. Both A and B were
stirred separately until complete dissolution of the reagents.
To B, 3.4 mL of concentrated HNO was added and further
3
stirred. Solution A was transferred to a dropping funnel and
drop-wise added to B under vigorous stirring. The resultant
mixture was refluxed for 24 h in an oil bath maintained at
[
12]. These solubility data imply that a similar trend is
possible in the reported catalytic syntheses utilizing
BMIM]Cl, and thus homogeneous as opposed to hetero-
[
110 °C upon which the product was filtered, washed until
geneous species could be the active components of the
afore-mentioned systems.
neutral, dried in air at 120 °C for 12 h, and ground to fine
powder. This powder (5 g) was ion-exchanged for H by
?
On the other hand, the use of molecular oxygen (under
pressure) resulted in comparatively poor yields. Fenton’s
dispersing in 100 mL of 1 M HNO and stirring at 70 °C for
3
6
h followed by a similar washing and drying procedure to
reagent [13] (FeSO /5% H O ) system afforded 100%
4 2 2
form a strongly protonated H–K-OMS-2.
conversion with 99.9% selectivity to TMQ after a 3 h
reaction period. Ti-based catalysts have also been studied
for the synthesis of TMQ in a one-step reaction with high
conversion and selectivities after a 30 min reaction period
3
.3 Catalytic Reactions
Catalytic oxidation reactions were performed by adding
[
14–19].
Herein, we report the benign oxidation of TMP to TMQ
1
5
mmol of TMP to 10 mL of acetonitrile that contained
0 mg catalyst and a given amount of TBHP as oxidant.
using K-OMS-2 and H–K-OMS-2 catalysts. These mate-
rials have previously been studied as selective catalysts for
the oxidation of different organic compounds [20–25].
These materials showed excellent selectivity in the alcohol
oxidation as well as in other catalytic systems.
Reactions were carried out at different temperatures for
0 min and samples withdrawn using a syringe fitted with
3
an inline (0.45 lm) filter. The composition was analyzed
and quantified using GC–MS based on their relative peak
areas. Time and temperature studies were performed by
withdrawing samples for analysis at different time intervals
at given set temperatures.
2
Aim of Study
3.4 Catalyst Leaching Test
Given that K-OMS-2 materials have performed exempl-
arily in the oxidation of reported organic molecules, their
performance in the oxidation of this important industrial
chemical and the determination of the best reaction con-
ditions are the driving forces of our current study. These
aims are successfully realized, thus extending the appli-
cations of OMS-2 to include this important oxidation
reaction.
To test whether the reaction was truly heterogeneous we
performed the hot filtration test [29]. This test was per-
formed at 65 °C with all the other reaction conditions
similar to those of catalytic reactions tests as reported
above. At exactly 5 min after start of reaction, all the liquid
was drawn into a syringe and then released into a clean flask
that had been maintained in a drying oven. An in-line filter
1
23

Efficient Oxidation of 2,3,6-Trimethyl Phenol
429
was used to exclude solid materials and the hot filtrate
reacted for a further 30 min at the same reaction conditions.
crystallinity. Figure 2 shows the results from these diffraction
studies. The XRD peaks could be indexed to the Q-phase of
Cryptomelane-type manganese oxide (JCPDS card no.
29-1020) for all of the three samples. Electron microscopy
4
Characterization of the Catalysts
experiments were performed on K-OMS-2 and are shown in
Fig. 3 while nitrogen sorption isotherms and BET surface
areas of K-OMS-2 are shown in Fig. 4. FE-SEM micrographs
revealed a fibrous morphology while the BET surface area
The catalysts were characterized using: X-ray diffraction
XRD), scanning electron microscope (FE-SEM), and
(
2
Brunauer-Emmett-Teller (BET) surface area and pore
size distribution measurements. Results of these charac-
terizations were compared to those in literature reports.
was 98 m /g well in line with previously reported results.
5.2 Catalytic Results
4
.1 X-Ray Diffraction
5.2.1 Effects of the Type of Catalyst and Oxidant
on Oxidation of TMP
Powder XRD studies were performed using a Scintag XDS-
000 diffractometer having Cu Ka (k = 0.15406 nm) radi-
ation and operating at a beam voltage of 45 kV and a current
of 40 mA. XRD data were collected continuously in the 2h
range of 5-75° at a scan rate of 1.0°/min and the crystalline
phase identified using a JCPDS database card number
2
The effect of the type of catalyst was studied by comparing
a similar amount of H–K-OMS-2 to K-OMS-2 in the
2
9-1020. XRD studies were performed on three catalyst
?
materials: freshly synthesized K-OMS-2, H -exchanged
H–K-OMS-2, and recycled K-OMS-2 that had previously been
used in one reaction after a washing and drying procedure.
4
.2 Scanning Electron Microscope
FE-SEM was performed on a Zeiss DSM 982 Gemini
instrument with a Schottky emitter at an accelerating
voltage of 2 kV and a beam current of 1 mA. The carbon
tape mount method was used in sample mounting. Powder
samples were finely ground and dispersed in absolute
ethanol in a glass vial then ultra-sonicated prior to being
dispersed on Au–Pd-coated silicon glass chips previously
mounted onto aluminum stubs with a two-sided carbon
tape and dried by vacuum desiccation prior to SEM studies.
Fig. 2 X-ray diffraction patterns of as synthesized K-OMS-2 (bot-
tom), H–K-OMS-2 (middle), and regenerated K-OMS-2 (top). All
materials were indexed to the Q-phase of Cryptomelane (JCPDS card
no. 29-1020)
4
.3 BET Surface Area and Pore Size Distribution
These were performed using nitrogen sorption on a
Micrometrics ASAP 2010 accelerated surface area system.
Samples were degassed at 200 °C for 12 h prior to pore
size distribution experiments which were carried out at
7
7 K. The specific surface area of the material was deter-
mined by the BET method.
5
Results
5
.1 Catalyst Characterization
K-OMS-2, H–K-OMS-2, and regenerated K-OMS-2 were
subjected to XRD studies to identify their crystal structure and
Fig. 3 FE-SEM micrograph images of the as-synthesized K-OMS-2
catalyst
123

4
30
N. N. Opembe et al.
Fig. 4 Nitrogen sorption
isotherms of the as-synthesized
K-OMS-2 materials. Horvath–
Kawazoe pore size distribution
curve (inset). Corresponding
2
BET surface area was 98 m /g
oxidation reaction. The effect of oxidant type was studied
by comparing the results obtained by using molecular
oxygen (present in air) or TBHP as oxidants. Also studied
were: the effect of performing the catalytic tests under
nitrogen atmosphere. The effect of the amount of TBHP
was also studied. The results of these studies are summa-
rized in Table 1.
30 min. Different temperatures (room temperature, 40, 65,
and 82 °C) were picked and the reactions monitored by
analyzing the composition of samples withdrawn at short
intervals over a 30 min duration using a syringe with an
inline filter. The results of these studies are shown graph-
ically in Figs. 5, 6, 7, 8.
5.2.3 Leaching Test Results
5
.2.2 Temperature and Reaction Time Effect
The results from the leaching test are shown in Table 1,
entry 11. GC results of first filtrate had a conversion of 57%
and after a 30 min filtrate reactions, stayed at 59% which is
close to the first results and so can be assumed to be a
constant.
Temperature effect on the oxidation of TMP to TMQ was
studied by performing the reactions at the best conditions
experimentally determined of: 1 mmol substrate, 2 mmol
TBHP in 10 mL of acetonitrile and 50 mg K-OMS-2 for
Table 1 Effects of selected parameters on the oxidation of TMP
Entry
Catalyst loading (mg)
Oxidant amount (mmol)
TMP amount (mmol)
Temp. (°C)
Conv. (%)
Sele. (%)
TON
1
2
3
4
5
6
7
8
9
None
None
50
None
(
2.00
1
1
1
1
1
1
1
1
1
1
1
65
65
65
65
65
65
65
65
65
65
65
0
0
0
a)
Trace
6
–
–
Nitrogen
Air
0
1
50
15
0
2
50
0.25
0.50
1.00
2.00
2.00
2.00
2.00
46 (35)
53
42 (45)
48
7 (6)
9
50
50
70
56
11
16
16
11
9 (9)
50
99
99
(
b)
50
40
50
98
99
1
1
0
1
70
99
(
c)
57 (59)
85 (87)
Percentage quantification (conversion and selectivity) was based on relative GC–MS peak areas (averaged for three injections) and confirmed
using toluene as an internal standard to around 3% difference. Entry 1 corresponds to ‘‘blank’’ i.e. performed in the absence of catalyst. (–) was
not determined due to the product peak being below the quantification range. All reactions were performed for 30 min except (a) for 60 min and
(
c) initially for 5 min. Similarly, all reactions were performed in the presence K-OMS-2 except (b) which was performed in the presence of H–K-
OMS-2. Entry 5 (0.25 equivalent TBHP), a reaction under nitrogen purge gave results shown in the parenthesis. Hot catalyst filtration test results
are shown in entry 11 with the final reaction results in parenthesis. Turnover number (TON) is the ratio of moles of substrate converted to moles
of catalyst used. Unless noted, the oxidant used was TBHP
1
23

Efficient Oxidation of 2,3,6-Trimethyl Phenol
431
Fig. 5 Oxidation of TMP (1 mmol) at room temperature (25 °C)
with 2 mmol TBHP in 10 mL acetonitrile and 50 mg of K-OMS-2
Fig. 8 Oxidation of TMP (1 mmol) at 82 °C with 2 mmol TBHP in
1
0 mL acetonitrile and 50 mg of K-OMS-2
?
H
ions. A regenerated K-OMS-2 retained the crystalline
structure albeit with decreased peak intensities. Nitrogen
sorption experiments reveal type-II isotherms due to an
indefinite multilayer formation at higher relative pressure
after complete formation of monolayer at lower relative
pressure. Pore size distribution curves obtained using the
Horvath–Kawazoe method indicate that the pore sizes are
narrowly distributed at about 3.7 nm.
6.2 Effect of Catalyst Nature on Oxidation of TMP
Fig. 6 Oxidation of TMP (1 mmol) at 40 °C with 2 mmol TBHP in
1
0 mL acetonitrile and 50 mg of K-OMS-2
There is no significant difference between the performances
of K-OMS-2 and protonated H–K-OMS-2 in the oxidation
process, thus Br o¨ nsted acidity alone has no significant
contribution to this oxidation reaction (Table 1, entries 8, 9)
but rather, the active sites (manganese in different oxidation
states) present on the catalyst and the oxidizing species
(radical species from TBHP) present in solution play a more
significant role. No oxidation occurred in the absence of the
catalyst lending credence to the fact that this is indeed a
reaction facilitated by the presence of K-OMS-2 (Table 1,
entries 1 and 2). Furthermore, hot filtration of the catalyst
and reacting the filtrate alone did not significantly change
the conversion (Table 1, entry 11). Similar leaching studies
using this catalyst for other oxidation studies have been
reported [25]. The amount of K-OMS-2 used (50 mg,
0.0625 mmol) when compared to 1 mmol of substrate also
suggests that this reaction is not stoichiometric either.
Calculated turnover numbers (TON) are also above the
threshold of 1 in stoichiometric reactions.
Fig. 7 Oxidation of TMP (1 mmol) at 65 °C with 2 mmol TBHP in
0 mL acetonitrile and 50 mg of K-OMS-2
1
6
Discussion
6
.1 Catalyst Characterization
Pure, crystalline K-OMS-2 materials were successfully
synthesized as evidenced by the XRD patterns that were
indexed to the Q-phase of Cryptomelane (Fig. 2) and
FE-SEM micrographs (Fig. 3). Ion-exchanging to prepare
H–K-OMS-2 materials appears not to have a significant
effect on the crystal structure (change in d-spacing or in the
unit cell size) as identified from XRD patterns whose peak
positions remain unchanged (not shifted) from those of
K-OMS-2. This may indicate a lack of framework substi-
6.3 Role of Oxidant on Oxidation of TMP
The presence of oxidant plays a crucial role. In the absence
of the oxidant, the reaction does not occur (Table 1, entry
1). Use of inert atmosphere (nitrogen) gave a conversion of
6% (Table 1, entry 3) while use of air gave a slightly
higher conversion of 15% (entry 4). When TBHP was used,
conversions of up-to 99% were obtained (Table 1, entries
5–9). At levels of 0.25–1 equivalent TBHP the conversion
?
tution and a possible exchange of the tunnel K cations for
123

4
32
N. N. Opembe et al.
was low (46% no purge, 35% under nitrogen purge) and
only reached the highest (99%) at two equivalent (Table 1,
entries 5–8). This implies that dissolved oxygen also plays
a role in the oxidation process. Conversion and selectivity
obtained were highest when 50 mg of catalyst is used, two
equivalent TBHP and the reaction carried out for 30 min.
Thus the foregoing are the best-determined reaction con-
ditions for this system. Recycled K-OMS-2 achieved a
conversion of 70% when just 40 mg of the recycled cata-
lyst is used in reaction (Table 1, entry 10). The reuse was
achieved by scrapping the filtered catalyst off the filter
paper, washing with ethanol and water then drying over-
night at 120 °C.
Acknowledgment We thank the Department of Energy, Office of
Basic Energy Sciences, Division of Chemical Science, Biological
Sciences and Geosciences under Grant DE-FG02-86ER13622.A000
for support of this research. We also acknowledge the assistance from
Dr. Hui Huang.
References
1
2
. Traber MG (2007) Annu Rev Nutr 27:347
. Verhagen H, Buijsse B, Jansen E, Bueno-de-Mesquita B (2006)
Nutr Today 41:244
3. Bonrath W, Dittel C, Giraudi L, Netscher T, Pabst T (2007) Catal
Today 121:65
4
. Rucker RB, Suttie JW, McCormick DB (2001) Handbook of
vitamins. Mercel Dekker, New York
. Mercier C, Charbardes P (1994) Pure Appl Chem 66:1509
5
6
.4 Temperature and Reaction Time Effect
on Oxidation of TMP
6. Bonrath W, Netscher T (2005) Appl Catal A 280:55
7
. Kholdeeva OA, Golovin AV, Maksimovskaya RI, Kozhevnikov
IV (1992) J Mol Catal 75:235
8
. Meng X, Sun Z, Lin S, Yang M, Yang X, Sun J, Jiang D, Xiao
F-S, Chen S (2002) Appl Catal A 236:17
Conversion increased when temperature was increased from
room temperature to 82 °C (Figs. 5, 6, 7, 8). As reaction
time was increased at any given temperature, the conversion
also increased. Time increase was limited to the first 30 min
only. At room temperature, conversion increases from
slightly below 5% at the start of the reaction to about 33%
after 30 min (Fig. 5). At this temperature selectivity towards
TMQ is poor. The other two products had a molecular mass
of 270 but occurred at different retention times. 2,6-Dime-
thyl-4-[(2,4,6-trimethylphenoxy)methyl]phenol was a given
possibility (from the database). At 40 °C, conversions
steadily increase from 5 to 80% in the first 15 min reaching a
high of 95% after 30 min (Fig. 6). At 65 °C, in as little as
9. Sun H, Harms K, Sundermeyer J (2004) J Am Chem Soc
26:9550
0. Sun H, Li X, Sundermeyer J (2005) J Mol Catal A Chem 240:119
1. Bodnar Z, Mallat T, Baiker A (1996) J Mol Catal A Chem 110:55
2. Yu L, Sun H, He J, Wang D, Jin X, Hu X, Chen GZ (2007)
Electrochem Commun 9:1374
3. Li Y, Zhang P, Wu M, Liu W, Yi Z, Yang M, Zhang J, Zhang G,
Bai Z (2009) Chem Eng J 146:270
4. Kholdeeva OA, Ivanchikova ID, Guidotti M, Pirovano C, Rava-
sio N, Barmatova MV, Chesalov YA (2009) Adv Synth Catal
351:1877
5. Kholdeeva OA, Invanchikova ID, Guidotti M, Ravasio N, Sgobba
M, Barmatova MV (2009) Catal Today 141:330
6. Kholdeeva OA, Ivanchikova ID, Guidotti M, Ravasio N (2007)
Green Chem 9:731
17. Kholdeeva OA, Trukhan NN, Vanina MP, Romannikov VN,
Parmon VN, Mrowiec-Bialon J, Jarz e˛ bski AB (2002) Catal
Today 75:203
8. Trukhan NN, Romannikov VN, Paukshtis EA, Shmakov AN,
Kholdeeva OA (2001) J Catal 202:110
19. Trubitsyna T, Kholdeeva OA (2008) Kinet Catal 49:371
0. Trukhan NN, Kholdeeva OA (2003) Kinet Catal 44:347
1. Kumar R, Sithambaram S, Suib SL (2009) J Catal 262:304
2. Sithambaram S, Kumar R, Son Y-C, Suib SL (2008) J Catal
1
1
1
1
1
1
1
1
1
0 min, the conversions reached over 95% and increased to
9
9% in 30 min (Fig. 7). At this temperature, selectivity rose
to over 90% in the first 10 min. At 82 °C, the conversion was
at 97% in 10 min and reached 99% after 30 min (Fig. 8).
However, unlike at 65 °C where conversions were at 99%, at
1
2
2
2
8
2 °C the conversion or selectivity is constant at 90% by the
end of the reaction.
2
53:269
2
3. Malinger KA, Ding Y-S, Sithambaram S, Espinal L, Gomez S,
Suib SL (2006) J Catal 239:290
7
Conclusion
2
2
4. Chen X, Shen Y-F, Suib SL, O’Young CL (2001) J Catal 197:292
5. Son Y-C, Makwana VD, Howell AR, Suib SL (2001) Angew
Chem Int Ed 40:4280
From the foregoing, 65 °C is the optimum temperature for
carrying out the oxidation of 2,3,6-trimethyl phenol using
K-OMS-2 as benign catalysts. This temperature gave
conversions of 99% and selectivity of 99% in 30 min of
reaction time. Other optimum conditions are: 50 mg
K-OMS-2, and 2 mmol TBHP for 1 mmol of TMP. These
results are comparable to or better than reported [9–13, 30]
results and are achieved using much milder and relatively
inexpensive catalyst materials compared to literature
reports. The heterogeneity of the catalyst towards this
reaction was also confirmed to be truly heterogeneous.
26. Opembe NN, Son Y-C, Sriskandakumar T, Suib SL (2008)
ChemSusChem 1:182
2
2
2
3
7. Shen Y-F, Zerger RP, Suib SL, McCurdy L, Potter DI, O’Young
C-L (1992) J Chem Soc Chem Commun 17:1213–1214
8. Opembe NN, King’ondu CK, Espinal AE, Chen C-H, Nyutu EK,
Crisostomo VM, Suib SL (2010) J Phys Chem C 114:14417
9. Sheldon RA, Wallau M, Arends IWCE, Schuchardt U (1998) Acc
Chem Res 31:485
0. Zhou J, Hua Z, Cui X, Ye Z, Cui F, Shi J (2010) Chem Commun
4
6:4994
1
23