Metalloporphyrin covalently bound to silica. Preparation, characterization and catalytic activity in oxidation of ethyl benzene

Article abstract of DOI:10.1016/j.catcom.2010.01.023

Manganese(III) porphyrin complexes [Mn(TMCPP)] [TMCPP: 5, 10, 15, 20-tetrakis (4-methoxy carbonyl phenyl) porphyrin] were grafted on the organo-functionalized silica gel. This new catalyst was characterized by UV-Vis, FTIR, surface area measurements and elemental analysis, and it was used as catalyst in the oxidation of ethyl benzene by different oxidants such as hydrogen peroxide. The manganese porphyrin catalyst proved to be efficient and selective for synthesis of acetophenone. This catalyst showed a reasonable conversion (48%), and excellent selectivity (96.6%), when tert-butyl hydroperoxide in organic solvent was used as oxidant.

Full text of DOI:10.1016/j.catcom.2010.01.023

Catalysis Communications 11 (2010) 694–699
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Metalloporphyrin covalently bound to silica. Preparation, characterization and
catalytic activity in oxidation of ethyl benzene
*
Mehran Ghiaci , Fatemeh Molaie, Mohammad E. Sedaghat, Nasim Dorostkar
Department of Chemistry, Isfahan University of Technology, Isfahan 8415683111, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Manganese(III) porphyrin complexes [Mn(TMCPP)] [TMCPP: 5, 10, 15, 20-tetrakis (4-methoxy carbonyl
phenyl) porphyrin] were grafted on the organo-functionalized silica gel. This new catalyst was character-
ized by UV–Vis, FTIR, surface area measurements and elemental analysis, and it was used as catalyst in
the oxidation of ethyl benzene by different oxidants such as hydrogen peroxide. The manganese porphy-
rin catalyst proved to be efficient and selective for synthesis of acetophenone. This catalyst showed a rea-
sonable conversion (48%), and excellent selectivity (96.6%), when tert-butyl hydroperoxide in organic
solvent was used as oxidant.
Received 22 November 2009
Received in revised form 24 January 2010
Accepted 28 January 2010
Available online 4 February 2010
Keywords:
Manganese porphyrin
Immobilization
Ó 2010 Elsevier B.V. All rights reserved.
Ethyl benzene
tert-Butyl hydroperoxide
Hydrogen peroxide
1. Introduction
ural enzymes, along with the high reactivity and selectivity of
the latter species toward oxidation of organic substrates, have
Aromatic ketones are important chemical intermediates for the
synthesis of a various perfumes, drugs, and pharmaceuticals. Pro-
duction of these ketones by Friedel–Crafts acylation of aromatics
by acyl halide or acid anhydride, using stoichiometric quantities
of homogeneous acid, leads to the formation of a large volume of
toxic and corrosive wastes [1]. Previously, efforts have been made
to produce benzylic ketones by oxidizing the methylene group of
alkylaromatics using stoichiometric quantities of KMnO4 as an oxi-
dizing agent [2]. However, in the above stoichiometric oxidation,
the volume of waste produced is also very large, and separating
reactants and products from the reaction mixture is difficult.
Recently, there has been increased interest in developing clean,
economical catalytic processes for synthesizing value-added prod-
uct ketones by benzylic oxidation of alkyl aromatics [3–13]. The
use of heterogeneous catalysts in liquid-phase oxidation, particu-
larly of neat substrate, is a subject of considerable interest from
the standpoint of environmental consciousness, because losses of
both solvent and catalyst on separation can lead to unacceptable
levels of waste.
stimulated the use of manganese porphyrins as models of lignin
peroxidase and horseradish peroxidase [5]. The use of metallopor-
phyrins substituted with electron-withdrawing groups and their
immobilization on inorganic supports has resulted in efficient
and selective catalysts for oxidation of hydrocarbons, because the
matrix support can impose shape selectivity and promote a special
environment favoring the approach of the substrate to the active
species [6,7]. In addition to this, the immobilization may prevent
molecular aggregation or bimolecular self-destruction reactions,
which lead to deactivation of catalytic metalloporphyrin species.
Furthermore, the use of supported metalloporphyrins provides an
easy way to recover and recycle the catalyst from the reaction
media, a fact of great importance for the development of functional
degradation routines. In the case of inorganic matrices, the use of
silica gel (or substances produced by its chemical modification) ap-
pears as an interesting alternative, mainly on account of its high
stability [9].
In this work, we have studied the mimetic–enzymatic activity
of the manganese(III) porphyrin complex [Mn(TMCPP)] [TMCPP:
5, 10, 15, 20-tetrakis (4-methoxy carbonyl phenyl) porphyrin]
(Scheme 1) immobilized on silica gel modified with aminopropyl-
trimethoxysilane groups (SF-3-APTS). The synthesized catalyst was
characterized by N2 adsorption, TGA, AAS, UV–Vis and FTIR spec-
troscopy. Our data have demonstrated the selective oxidation of
benzylic C–H bond of ethyl benzene (which is difficult to oxidize
due to the absence of an electron-donating aromatic ring-activat-
ing group) in the liquid phase using tert-butyl hydroperoxide
A combination of environmental interest and chemist curiosity
in regard to natural phenomena, lead him/her to biomimetic cata-
lysts such as immobilized metalloporphyrins, as cytochrome P-450
models, and their potential for substrate oxidation [2]. The similar-
ity between metalloporphyrins and the active center of many nat-
* Corresponding author. Tel.: +98 311 3913254; fax: +98 311 3912350.
E-mail address: mghiaci@cc.iut.ac.ir (M. Ghiaci).
1566-7367/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.01.023

M. Ghiaci et al. / Catalysis Communications 11 (2010) 694–699
695
EtO
EtO
EtO
O
OH
- 3 EtOH
Si(CH2)3NH2
Si(CH2)3NH2
O
OH
O
OH
Surface silanol
groups of silica
APTMS
SiO2-Pr-NH2
O
O
1) NaH
O
SiO2-Pr-NH2
O
Si
N
N
N
NH
O
O
2) TMCPP, THF, reflux, 72h, N 2
3) MnCl2.4H2O, DMF, 140 oC, 4h, N2
O
Mn
O
Cl
N
O
O
Scheme 1.
(Fluka) 80% in water:di-tert-butoxide (1:1) (TBHP) as the oxidant.
2.4. Immobilization of free ligand (TMCPP) on silica SF-3-APTS
The results are discussed in the following sections.
In a round bottom flask, 4.5 g of SF-3-APTS was suspended in
50 ml of dry tetrahydrofuran (THF) under nitrogen [15]. The por-
phyrin (TMCPP) (0.63 mmol) was dissolved in 5 ml of THF and
added drop wise to the silica gel suspension. Then 0.32 g sodium
hydride was added to the flask. The mixture was refluxed during
72 h under nitrogen. After reaction, the solid was isolated and neu-
tralized by acetic acid solution (30%) and exhaustively washed in a
Soxhlet extractor with CH2Cl2 for removing porphyrin bonded
physically to silica SF-3-APTS. The excess of solvent was removed
by maintaining the mixture at 80 °C during 24 h. The final material
(SF-ATPS-TMCPP) was characterized by UV–Vis spectroscopy and
elemental analysis (C: 10.5%, H: 2.2%, N: 1.9%). The mean composi-
2. Experimental
2.1. Materials
All chemicals were purchased from Aldrich, Sigma, Fluka, or
Merck and solvents were obtained from Merck with the maximum
purity degree, and used as received. Toluene was distilled and
dried with sodium. Silica gel (Merck, 60/70–230) was activated
at 500 °C. 3-aminopropyltrimethoxysilane (3-APTS) (Aldrich) was
manipulated under nitrogen atmosphere without previous treat-
ments. The 5, 10, 15, 20-tetrakis (4-methoxy carbonyl phenyl)por-
phyrin free base was prepared according to published procedures
[7]. Ethyl benzene (Merck) was used in the oxidation reactions as
substrate.
tion was calculated as 270
silica.
lmol of TMCPP groups per gram of
2.5. Preparation of SF-ATPS-Mn(III)TMCPP
SF-ATPS-TMCPP (4.5 g) was suspended in 50 ml of DMF in a
round bottom flask followed by addition of MnCl2Á4H2O (0.75 g,
4.0 mmol). The system was heated at 140 °C during 4 h under
magnetically stirring and nitrogen atmosphere. The supernatant
was analyzed by UV–Vis spectroscopy. The silica SF-ATPS-
Mn(III)TMCPP was washed with deionized water until pH 7. The
purified silica SF-ATPS-Mn(III)TMCPP was dried at 80 °C for 24 h.
The manganese porphyrin loading was obtained by elemental anal-
ysis, and atomic absorption. The loading determined by these pro-
2.2. Characterization techniques
The catalyst was characterized using atomic absorption
spectroscopy (AAS), elemental analyses (C, H, N, Carlo-Erba 1106
analyzer), surface area analysis (BET; NOVA 1200 Quanta Chrome
instrument), thermal analysis (Seiko DTA-TG 320), FTIR spectros-
copy (JASCO) and diffuse-reflectance UV–Vis spectroscopy
(DRUV–Visible; Shimadzu UV-2500 PC) techniques. The reaction
products were analyzed by gas chromatography using an Agilent,
6890 Series GC instrument, equipped with a wide bone OV-17
(30 m) capillary column and a FID detector.
cedures was 85 lmol Mn(III)TCPP per gram of silica.
2.6. Reaction procedure
2.3. Functionalization of silica gel with aminopropyltrimethoxisilane
groups
The liquid-phase oxidation of ethyl benzene was carried out in
the liquid-phase at 150 °C in a sealed reactor immersed in a ther-
mo-stated bath and a magnetic stirrer. A typical reaction run was
as follows: 0.05 g catalyst, and 9.3 mmol of tert-butyl hydroperox-
ide (80%) [TBHP (80%)] was suspended in a solution of 9.3 mmol of
ethyl benzene in solvent free condition. The reaction mixture was
put in a Teflon lined stainless steel hydrothermal bomb, and heated
at the desired temperature and stirred for 24 h. When the reaction
The modified silica (SF-3-APTS) was prepared according to pro-
cedures reported in the literature [10,14]. The material was charac-
terized by elemental analysis (CHN) (C: 6.2%, H: 1.7%, N: 2.0%).
According to these results the mean composition was calculated
as 1.30 mmol of 3-APTS groups per gram of silica.

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M. Ghiaci et al. / Catalysis Communications 11 (2010) 694–699
was completed, the reaction mixture was filtered and the reaction
products were analyzed by GC.
3. Result and discussion
3.1. Preparation and characterization of immobilized Mn-porphyrin
In the preparation of the catalyst, the surface silanol (Si–OH)
groups of silica were first condensed with 3-aminopropyltrime-
thoxysilane (APTMS) yielding SiO2–pr–NH2. 5, 10, 15, 20-Tetrakis
(4-methoxy carbonyl phenyl) porphyrin (TMCPP) was then immo-
bilized on the functionalized surface. Sodium hydride was used to
catalyze the formation of the amide bond between COOMe groups
of porphorin and NH2 groups of silica gel SF-3-APTS. The resulting
solid was Soxhlet extracted with methanol, to remove the TMCPP
that was physically adsorbed on the support. In this way, only
the TMCPP covalently bound to the support remained on the solid
catalyst. The amount of the organic functional groups and the
immobilized TMCPP was estimated by chemical analyses
(C: 10.5%, H: 2.2%, N: 1.9%). The mean composition was calculated
as 270 lmol of TMCPP groups per gram of silica. Diffuse-reflec-
tance UV–Vis spectroscopic studies of the SF-3-APTS-porphyrin
showed bands centered at 518, 522, 584, 630 nm and the sort band
at 424 nm, similar to the spectrum of H2(TMCPP) in solution,
suggesting that the porphyrin ring was not significantly affected
by immobilization on the support (Fig. 1). Finally, the solid
SF-Mn(TMCPP) was obtained by metallation of the free basic por-
phyrin grafted previously onto aminopropylated silica (SF-
TMCCPP). The influence of organo-functionalization and grafting
on the catalytic properties of the Mn-complex has been investi-
gated. ‘‘Neat” Mn(TMCPP)Cl also showed weakly intense d–d bands
at 420, 487, 564 and 723 nm. These bands for Mn(TMCPP)Cl immo-
bilized on propylamine-functionalized silica gel appeared at 483
and 570 nm.
The FTIR spectra of the prepared samples are given in Fig. 2. The
strong absorption bands observed in the spectrum of silica (Fig. 2a)
at 2900–3800, 1040–1260, 820 and 500 cmÀ1 are due to O–H of the
silanols, adsorbed water molecules and Si–O–Si stretching vibra-
tions, respectively. After immobilization of 3-aminopropyltrieth-
oxysilane on silica new absorption bands appear at about
1547 cmÀ1 due to the N–H bonds in the amine group and 2928
2852 cmÀ1 due to C–H stretching modes of the propyl spacer
Fig. 2. FTIR spectra of (a) silica gel, (b) SF-3-APTS, (c) SF-ATPS-TMCPP and (d)
SF-ATPS-Mn(III)TMCPP.
(Fig. 2b). The NH2 stretching vibrations should be hidden under
the O–H vibration bands. Infrared spectra of SF-3-APTS-porphyrin
and the SF-Mn(TMCPP) complex display the characteristic bands
associated with the N–H and C@O bonds of the amide functionality
present in the immobilized porphyrin and its metal complex.
Each material surface area was determined by the nitrogen
adsorption–desorption isotherms of previously degassed solids,
at 120 °C. Measurements were carried out at liquid nitrogen boil-
ing point, in a volumetric apparatus, using nitrogen as probe. All
the samples have surface area lying between 349 and 480 m2/g
(Table 1).
Thermogravimetric analyses of the SF-Mn(TMCPP) complex
shows distinct weight losses that depend, in part, on framework
composition. Three distinct steps can be distinguished at 50–150,
Table 1
BET surface area of immobilized Mn-porphyrin complex.
Compound
BET (m2/g)
Silica gel
SF-3-APTS
SF-ATPS-Mn(III)TMCPP
480
375
349
Fig. 1. Diffuse-reflectance UV–Vis spectra of (a) SF-3-APTS, (b) SF-ATPS-TMCPP and
(c) SF-ATPS-Mn(III)TMCPP.

M. Ghiaci et al. / Catalysis Communications 11 (2010) 694–699
697
Fig. 3. TGA of SF-ATPS-Mn(III)TMCPP: 0–220 °C, loss of H2O and aminopropyl group; 250–1000 °C, loss of supported porphyrin.
200–400 and above 400 °C (Fig. 3). The weight loss at 50–150 °C
can be due to desorption of water. The weight loss in the range
at 200–400 °C can be ascribed to the decomposition of the organic
linker. The weight loss above 400 °C can be assigned to the decom-
position of porphyrin and calcination of coke and the loss of silanol
groups.
The oxidation reaction is expected to involve a hydrogen
abstraction from benzylic carbon probably in the rate-determining
step of ethyl benzene oxidation. Concerning the intimate mecha-
nism of carbon–hydrogen bond breaking, conclusive results are
not available. The mechanistic picture summarized in Scheme 3,
though reasonable, is indeed a speculative one which needs cor-
roboration by more experimental data, particularly as far as the
intimate process of the oxygen transfer from the oxo-derivative
to the aliphatic C–H bond is concerned. According to the proposed
mechanism, the oxo-manganese species is formed in the rate-
determining step of the catalytic process. It is initiated by the
direct interaction of the hydrocarbon with oxygen, leading to the
formation of an intermediate hydroperoxide species, which subse-
quently decomposed to the corresponding carbonyl compound and
water. The presence of hydroperoxide in the reaction product was,
however, not observed, most probably because of its fast catalytic
decomposition; the catalyst has very high peroxide decomposition
activity.
3.2. Catalytic studies
3.2.1. Oxidation of ethyl benzene by different oxidants
Results of the oxidation of ethyl benzene (Scheme 2) that
contains a methylene group by different oxidants over the immo-
bilized Mn(TMCPP)Cl catalyst are presented in Table 2. A compar-
ison of the oxidation of the ethyl benzene indicates that, the
reactivity of different oxidants is similar. However, the order for
TOF in the oxidation was TBHP (80%) > TBHP (70%) > H2O2. From
the results of Table 2, one could judge that the immobilized
Mn(TMCPP)Cl catalyst showed high selectivity (above 95%) for oxi-
dation of ethyl benzene to acetophenone by H2O2; clearly, the
activity increases when using TBHP as the oxidant. However,
hydrogen peroxide offers many advantages as oxidant, such as
low cost, generation of water as the only by-product, and commer-
cial availability. For all these reasons, it is considered a clean oxi-
dant that can be potentially employed in metalloporphyrin-
catalyzed oxidation reactions. However, H2O2 poses limitations to
catalytic activity because it may decompose into water and oxygen
via a reaction catalyzed by the metalloporphyrin. Another draw-
back is that this peroxide may undergo hemolytic cleavage of the
O–O bond, triggering radical reactions that may destroy the cata-
lyst and/or the support. Therefore, we chose TBHP as the oxidant
in this study.
3.2.2. Solvent effect
In order to probe the role of solvents, a series of solvent like
CH3CN, (CH3)2CO and CH3CN:pyridine (1:1) was used in this study
(Table 3). Interestingly, the conversion of ethyl benzene and selec-
tivity to acetophenone was highest under solvent free conditions.
Although it is known that the catalytic activity of manganese por-
phyrins is significantly improved in the presence of axial ligands
like nitrogen bases [16], but in this study the best reaction condi-
tion was under solvent free condition. Hence, we believe that the
hydrophilic nature of the support may attract the polar solvent
molecules and leave less empty room for the substrates. Therefore,
this might be true that solvent had negative impact over the per-
formance of the catalyst, which may possibly arise from the block-
ing of the active catalytic sites by the solvent molecules.
O
3.2.3. Effect of temperature
The influence of reaction temperature on conversion and
selectivity was studied at 298, 353, 423 and 453 K. The results
are presented in Table 4. The major products were found to be ace-
tophenone, benzaldehyde and benzoic acid. Ethyl benzene conver-
sion increased with increase in temperature from 298 to 423 K and
decrease afterwards. The selectivity to acetophenone showed a
similar trend. At 453 K conversion and selectivity to acetophenone
catalyst
Scheme 2.

698
M. Ghiaci et al. / Catalysis Communications 11 (2010) 694–699
Table 2
Table 3
Effect of different oxidants on the conversion and selectivity of the reaction.
Effect of solvent on conversion and selectivity of the oxidation reaction.
Entry Oxidant
Conversion Selectivity
Benzaldehyde Acetophenone Benzoic
Entry Solvent
Conversion Selectivity
Benzaldehyde Acetophenone Benzoic
acid
acid
1
2
3
TBHP
(80%)
TBHP
(70%)
H2O2
40.8
39.1
30.3
2.2
3.9
3.2
96.6
94.8
95.1
1.2
1
2
3
4
Solvent free 40.8
2.2
6.7
6.7
4.2
96.6
91.2
91.2
95.1
1.2
2.1
2.1
0.7
Acetone
Acetonitrile
34.5
34.9
1.3
1.7
Acetonitrile: 32.9
pyridine
(1:1)
Reaction conditions: ethyl benzene (1 g, 9.3 mmol); oxidant 9.3 mmol; cata-
lyst = 0.05 g; T = 423 K; reaction time = 24 h. (absence of solvent).
Reaction conditions: ethyl benzene 1 g (9.3 mmol); TBHP (80%) 1 mL (9.3 mmol);
catalyst = 0.05 g; reaction time = 24 h; temperature = 423 K; solvent 3 mL.
decreased that might be due to breakdown and demolition of the
oxidant at this temperature.
are presented in Table 5. The reaction displays more conversion
at the EB:TBHP molar ratio of 1:3, due to the presence of excess
tert-butyl hydroperoxide coordinated with manganese. At this mo-
lar ratio, the selectivity to benzaldehyde and benzoic acid de-
creases, while the selectivity to the acetophenone increased. The
selectivity to acetophenone increases with increase in time since
3.2.4. Effect of molar ratio of EB:TBHP (80%)
The oxidation of ethyl benzene with tert-butyl hydroperoxide
over SF-Mn(TMCPP) catalyst has been examined at 423 K with
the molar ratios of EB:TBHP from 1:1 to 1:5. The results obtained
Ot-Bu
Cl
O
t-BuOOH
MnIII
MnIII
Ot-Bu
O
O
v
MnIII
Mn / t-BuO
O
OH
v
Mn / t-BuO
+ Mn
OH
OO
OH
MnIII
+ MnII
O
2
O
O
O
+ O
2
2
2
OH
O
O
2
+
OH
OO
O
O
t-BuOOH
OH
O
PMnII
PMnIII
-OH
Scheme 3.

M. Ghiaci et al. / Catalysis Communications 11 (2010) 694–699
699
Table 4
3.2.5. Catalyst reusability
Effect of temperature on conversion and selectivity of the oxidation reaction.
In order to study the catalyst deactivation occurring during the
oxidation of ethyl benzene, we carried out this reaction several
times with modified SF-ATPS-Mn(III)TMCPP. After the reaction
was completed, the catalyst sample was filtered, washed with ace-
tone and diethyl ether, and was used again in a new experiment.
The results were shown in Table 7. The catalyst showed excellent
reusability in the oxidation reaction. Interestingly, the catalytic
activity was found to increase after its use in oxidation reaction.
This may be due to the removal of strongly adsorbed water from
the catalyst during the first and subsequent use of the catalyst.
The selectivity also remained high (>96%) in the reuse of the cata-
lyst. As a final point it should be stated that these observations re-
veal that the oxidation reaction catalyzed by the solid catalyst is
truly heterogeneous, and no leaching of Mn(TMCPP)Cl from the
support was observed in the reaction.
Entry Temperature Conversion Selectivity (%)
(K)
(%)
Benzaldehyde Acetophenone Benzoic
acid
1
2
3
4
298
353
423
453
18.5
43.9
40.8
38.2
9.3
15.7
2.2
71.6
58.7
96.6
94.1
19.1
25.6
1.2
0.1
3.8
Reaction conditions: ethyl benzene 1 g (9.3 mmol); TBHP (80%) = 1 mL (9.3 mmol);
catalyst = 0.05 g; reaction time = 24 h (absence of solvent).
Table 5
Effect of molar ratio of EB:TBHP (80%) on conversion and selectivity to products.
Entry
EB:TBHP (80%)
Conversion
Selectivity
a
b
c
4. Conclusion
1
2
3
4
5
1:1
1:2
1:3
1:4
1:5
41.4
65.7
91.4
80.9
81.1
2.2
0.9
Trace
1
96.6
98.4
99.9
97.9
96.4
1.2
0.7
0.1
1.1
0.3
A new immobilized manganese prophyrin complex has been
synthesized. The presence of manganese in the immobilized por-
phyrin is evident through elemental analysis. The catalytic activity
of SF-ATPS-Mn(III)TMCPP has been examined for liquid-phase oxi-
dation of ethyl benzene using tert-butyl hydroperoxide as the best
oxidant. At 423 K, the study indicates a high conversion and selec-
tivity for the prepared catalyst. Both C–H and C–C bond cleavage of
ethyl benzene are observed to be acted upon by activated tert-bu-
tyl hydroperoxide. Acetophenone was observed as the major prod-
uct. The other products obtained were benzaldehyde and benzoic
acid. So this study reveals that SF-ATPS-Mn(III)TMCPP can be a
convenient eco-friendly substitute for hazardous stoichiometric
oxidants.
3.2
Reaction conditions: ethyl benzene (1 g, 9.3 mmol); oxidant, TBHP (80%); cata-
lyst = 0.05 g; T = 423 K; reaction time = 6 h (absence of solvent).
Table 6
Effect of reaction time on conversion and selectivity to products.
Entry Time
(h)
Conversion Selectivity
Benzaldehyde Acetophenone Benzoic
acid
1
2
3
4
4
6
12
24
38.3
41.4
38.0
40.8
2.5
0.5
2.2
2.2
95.2
98.6
94
1.6
0.9
3.8
1.2
Acknowledgements
96.6
Thanks are due to the Research Council of Isfahan University of
Technology and Center of Excellence in the Chemistry Department
of Isfahan University of Technology for supporting of this work.
Reaction conditions: ethyl benzene (1 g, 9.3 mmol); TBHP (80%) 1 mL (9.3 mmol),
EB:TBHP = 1:1; catalyst = 0.05 g; T = 423 K (absence of solvent).
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Table 7
Catalyst reusability.
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acetophenone is not a precursor of any daughter product (Table 6).
However, by increasing the time, the amount of benzaldehyde and
benzoic acid increase; these product probably produced through
C–C bond cleavage which it has a much higher activation energy.