Routes of Formation of Bifunctional C6 Carboxylic Acids in the Cyclohexane Oxidation Process

Article abstract of DOI:10.1134/S0965544119060136

Abstract: The buildup kinetics of the main and side products during uncatalyzed and cobalt naphthenate-catalyzed oxidation of cyclohexane at 150°C have been studied. 6-Hydroxyhexanoic, 6-oxohexanoic, and adipic acids are accumulated in parallel during the oxidation of cyclohexane, cyclohexanone, and 2-hydroxycyclohexanone. The critical consideration of their known formation pathways and the reactivity of possible precursors gives evidence for the predominant formation of 6-hydroxyhexanoic acid at the step of cyclohexanol oxidation and adipic acid from 2-hydroxycyclohexanol.

Full text of DOI:10.1134/S0965544119060136

ISSN 0965-5441, Petroleum Chemistry, 2019, Vol. 59, No. 6, pp. 587–595. © Pleiades Publishing, Ltd., 2019.
Routes of Formation of Bifunctional C Carboxylic Acids
6
in the Cyclohexane Oxidation Process
a,
a
A. L. Perkel’ * and S. G. Voronina
a
Kuzbass State Technical University, Institute of Chemical and Oil and Gas Technologies, Kemerovo, 650000 Russia
e-mail: perkel2@rambler.ru
*
Received March 23, 2018; revised February 9, 2019; accepted February 12, 2019
Abstract—The buildup kinetics of the main and side products during uncatalyzed and cobalt naphthenate-
catalyzed oxidation of cyclohexane at 150°C have been studied. 6-Hydroxyhexanoic, 6-oxohexanoic, and
adipic acids are accumulated in parallel during the oxidation of cyclohexane, cyclohexanone, and 2-hydrox-
ycyclohexanone. The critical consideration of their known formation pathways and the reactivity of possible
precursors gives evidence for the predominant formation of 6-hydroxyhexanoic acid at the step of cyclohex-
anol oxidation and adipic acid from 2-hydroxycyclohexanol.
Keywords: cyclohexane, cyclohexanone, 2-hydroxycyclohexanone, liquid-phase oxidation, 6-hydroxyhexa-
noic acid, 6-oxohexanoic acid, adipic acid, formation routes
DOI: 10.1134/S0965544119060136
Bifunctional C6 carboxylic acids (adipic, 6-
This work is devoted to the investigation of the
hydroxyhexanoic, and 6-oxohexanoic) are one of the kinetics of buildup of the desired and main side prod-
ucts in the uncatalyzed and cobalt naphthenate-cata-
lyzed liquid-phase oxidation reaction of cyclohexane
and discussion of possible routes of the formation of
adipic, 6-hydroxyhexanoic, and 6-oxohexanoic acids.
main side products in the process of cyclohexane oxi-
dation to cyclohexanol and cyclohexanone [1, 2]. The
routes of their formation are still under debate. It is
supposed [1, 3–5] that adipic and 6-oxohexanoic
acids are formed from cyclohexanone. By measuring
the activity of side products in experiments on the co-
EXPERIMENTAL
14
oxidation of C -labeled cyclohexane with an admix-
Cyclohexane of the chemically pure grade was
purified according to the procedure given in [11].
Bp 80.5°C (published value, 80.738°C [11]).
ture of unlabeled cyclohexanone on one hand and
14
unlabeled cyclohexane with an admixture of C -
labeled cyclohexanone on the other hand, it was
shown that , 86–95% adipic acid is produced through
cyclohexanone during cyclohexane oxidation [3].
Later, based on the investigation of the kinetics of
buildup of side products at the initial steps of uncata-
lyzed and cobalt(II) and cobalt(III) acetate-catalyzed
oxidation of cyclohexane, Hermans and coworkers
Cyclohexane was oxidized by oxygen in a stainless
steel bubble reactor at 150°C and a pressure of
1
.47 MPa in the absence of a catalyst and with 8.5 ×
–4
1
0
mol/L cobalt(II) naphthenate. The accuracy of
temperature control was ±1°C.
The total concentration of peroxide compounds in
oxidized cyclohexane was determined using iodomet-
ric titration [12, 13], and the other products were
determined by GLC. To prevent the disturbing influ-
ence of peroxide compounds on the results of determi-
nation of nonperoxide products, the samples were
reduced with triphenylphosphine immediately after
sampling. 2-Hydroxycyclohexanone was determined
after acylation with acetic anhydride in a pyridine
medium. Carboxylic acids and monocyclohexyl adi-
pate were transformed to methyl esters by treating with
[
6
6–8] concluded that adipic, 6-hydroxyhexanoic, and
-oxohexanoic acids are predominantly produced
from cyclohexyloxyl radicals. This conclusion contra-
dicts the data on a substantial decrease in the selectiv-
ity of the cyclohexane oxidation process even at a low
conversion as a result of the involvement of the alcohol
and ketone into the radical chain process [1, 9]. The
hypothetic mechanisms proposed in [6–8] include
intramolecular isomerization (chain transfer) through
seven- and eight-membered transition states, which is diazomethane prior to the determination. The sample
unlikely due to the increase in the entropic barrier in preparation procedures, analysis conditions, and
the case of formation of a folded conformation of an metrological characteristic of the methods are pre-
alicyclic molecule [10].
sented in [14] for cyclohexanol, 2-hydroxycyclohexa-
587

5
88
PERKEL’, VORONINA
11
11
10
1
5
3
4
8
6
10
3
1
6
4
2
2
5
2
4
2
6
4
2
0
5
4
3
2
1
1
4
7
0
6
7
5
3
2
1
10
5
9
6
8
8
4
2
2
1
9
10
0
20 40 60 80 100 120 140
0
10
20
30
t, min
40
Fig. 1. Buildup rate curves for the products of uncatalyzed
oxidation of cyclohexane at 150°C: (1) peroxides, (2)
cyclohexanol, (3) cyclohexanone, (4) 2-hydroxycyclohex-
anone and 2-hydroperoxycyclohexanone, (5) 6-hydroxy-
hexanoic acid, (6) 6-oxohexanoic acid, (7) adipic acid, (8)
adipic anhydride, (9) monocyclohexyl adipate, (10) ε-
caprolactone, and (11) total oxidation products.
Fig. 2. Buildup rate curves for the products of cyclohexane
oxidation at 150°C in the presence of cobalt(II) naphthen-
ate. See Fig. 1 for the designation of the curves.
ated branching is compensated by the factors that
decelerate the radical chain process. These include the
change in the composition of peroxyl radicals due
to the involvement of cyclohexanol, cyclohexanone,
none, and compounds with carboxyl groups; [15] for
cyclohexanone; and [16] for adipic anhydride.
2-hydroxycyclohexanone, etc. into the oxidation reac-
tion, as well as a decrease in the activity of peroxyl rad-
icals as a result of their solvation by oxygen-containing
compounds [5, 17].
RESULTS AND DISCUSSION
In accordance with the set task, the product
buildup kinetics in the case of uncatalyzed and cobalt
naphthenate-catalyzed oxidation of cyclohexane at
The pattern of the buildup rate curves of peroxide
compounds and total nonperoxide products
1
50°C have been studied (Figs. 1, 2). The buildup rate
(
Figs. 1, 2) suggests the predominant formation of the
curve for total oxidation products shows the presence
of an induction period in the case of uncatalyzed reac-
tion; after this period, cyclohexane is oxidized at an
almost constant rate (Fig. 1).
latter from the former products. Apparently, the non-
terminating routes of combination of peroxyl radicals
i
i
2
ROO ⎯ ⎯ ⎯ →2RO ,
(1)
−
O2
The shape of a similar curve in the cobalt(II) salt-
catalyzed reaction indicates the absence of the induc- revealed in the case of oxidation of cyclohexanol [18]
tion period; the rate of oxidation is higher than in the or 2-hydroxycyclohexanone [19] are unimportant
uncatalyzed process, but no noticeable autoaccelera- during the oxidation of cyclohexane.
tion is observed in this case as well. The absence of
autoacceleration in both experiments cannot be
explained by the constancy of the rate of reactions of
degenerated branching—homolytic decomposition of
cyclohexyl hydroperoxide.
At the initial steps of uncatalyzed oxidation, adipic,
6
-hydroxyhexanoic, and 6-oxohexanoic acids are
accumulated in parallel and in close amounts (Fig. 1).
In the cobalt naphthenate-catalyzed reaction, the ini-
tial buildup rate of 6-hydroxyhexanoic acid is higher
On the opposite, their role should increase as a by a factor of 1.8 than that of adipic acid (Fig. 2). The
result of the accumulation of oxygen-containing com- formation rate of 6-hydroxyhexanoic acid decreases in
pounds (cyclohexanol, cyclohexanone, 2-hydroxycy- 20 min after the onset of oxidation, whereas that of
clohexanone, etc.), which substantially accelerate the adipic acid increases and it becomes the main carbox-
degradation of cyclohexyl hydroperoxide including ylated product of the reaction. This finding does not
the homolytic decomposition [5, 17]. Apparently, contradict the idea advance by Hermans et al. [6–8]
acceleration due to the increase in the rate of degener- about the formation of 6-hydroxyhexanoic and adipic
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ROUTES OF FORMATION OF BIFUNCTIONAL C CARBOXYLIC ACIDS
589
6
acids from the cyclohexyloxyl radical, nor does it main pathway of the transformation of 1 in the
prove the preferred formation of adipic acid from 6- medium of cyclohexane under oxidation is the dehy-
hydroxyhexanoic acid, since 6-oxohexanoic acid, the drogenation of the substrate yielding cyclohexanol [1,
supposed intermediate product of such a transforma- 4, 5]:
tion, is accumulated in parallel with adipic acid at the
beginning of uncatalyzed and catalyzed oxidation
OOH +Me2+
O
OH
+
−R
RH
(2)
(
Figs. 1, 2).
Me(OH)2+
.
−
It is known that adipic, 6-hydroxyhexanoic, and 6-
(
1)
oxohexanoic acids can be produced not only from the
cyclohexyloxyl radical [6–8], but also at the steps of
oxidation of cyclohexanol [1, 4, 5], cyclohexanone [1,
To a substantially lesser extent 1 is subjected to the
destructive decomposition with the opening of the
cyclohexane ring which leads to 6-oxohexyl radical (2)
4
, 5, 20, 21], and 2-hydroxycyclohexanone [5, 6, 19,
2]. Because of this, it is reasonable to critically con-
2
[
4, 5]. In the case of thermal decomposition of tert-
sider all the alternative routes of formation of these
compounds.
butylcyclohexyl peroxide in cyclohexane at 140°C,
6% cyclohexyloxyl radicals are transformed to cyclo-
hexanol [4].
5
Formation of Bifunctional C Acids
6
It is supposed in [6–8] that in the case of oxidation
of cyclohexane (145°C, conversion of up to 7.5%), the
from Cyclohexyloxyl Radical
The cyclohexyloxyl radical (1) is generated in the side products are predominantly formed as a result of
case of thermal, catalytic, and free radical-induced radical chain transformations of radical 2. According
decomposition of cyclohexyl hydroperoxide [1, 4, 5, to [6], in a medium of cyclohexane under oxidation, 2
1
7]. The homolytic decomposition of the latter is is successively transformed to 6-hydroxyhexanoic
accelerated by oxygen-containing compounds (alco- (reactions (3)), 6-oxohexanoic (reactions (4)), and
hols, ketones, carboxylic acids, etc.) [1, 5, 17]. The adipic (reactions (5)) acids:
+O2
1
H C (CH ) CHO
(CH ) CHO
2
2 4
OO CH2
2 4
2
3
O
HOO CH2 (CH ) C
O
O CH2 (CH2)4C
2 4
(
3)
4)
OH
4
5
O
+RH
HO CH2 (CH ) C
2 4
−R
OH ,
6
O
O
+
RO2
+O2
6 −
HO CH (CH ) C
2 4
OO CH(OH) (CH ) C
2 4
RO2H
OH
O
OH
(
O CH (CH ) C
2
4
−HO2
OH,
7
O
+O2
O
+
RO2
OO C(O) (CH ) C
7
O C (CH ) C
2
4
2 4
−RO2H
OH
OH
(5)
O
O
O
+
RH
O
C (CH ) C
...
C (CH ) C
2 4
2
4
−R
OH .
HOO
OH
HO
The feasibility of transforming primary hydroxy with an increase in the number of units. This limita-
and aldehyde groups into carboxyl groups during tion is associated with an increase in the entropic bar-
cyclohexane oxidation cannot be denied. However, rier for the formation of a folded conformation of an
the likelihood of a number of transformation steps alicyclic molecule [10]. In the case of cyclization of ω-
(
3)–(5) appears to be low. The conversion of 3 to 4 and
bromocarboxylic acids to lactones
Br(CH2)n −2COO−
(CH )
4
to 5 should occur through nine- and seven-mem-
bered transition states. It is known [10] that in the case
of formation of large cycles, the formation of a cyclic
transition state becomes progressively more unlikely
СО
O
(6)
2 n −2
PETROLEUM CHEMISTRY Vol. 59 No. 6 2019

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90
PERKEL’, VORONINA
Table 1. Influence of the size of the cyclic transition state on the directionality of the abstraction of hydrogen atoms by an
ω-formyl radical (·(CH ) CHO) (160°C, dodecane, N ) [23]
2
n
2
Directions of H atom abstraction, %
Number of atoms in
transition state N
intramolecular
from –CHO
other CH– bonds
of the radical
from solvent
n = 4
n = 5
n = 6
N = 6
N = 7
N = 8
74
29
38
12
19
8
14
52
54
the relative rate of reaction (6) at 150°C is as follows of the directions of abstraction of hydrogen atoms by
[
10]:
ω-formyl radicals (Table 1).
The selectivity of the transformation of 2 to
-hydroxyhexanoic, 6-oxohexanoic, and adipic acids
6
Number
of atoms
in the cycle
5
6
7
8
9
10 11
should also be decreased by the side reactions of
decarbonylation (e.g., of radical 2 [5]) and decarbox-
ylation [5], the probability of the occurrence of which
under the conditions of oxidation of cyclohexane is
quite high. The oxidation of 6-oxohexanoic acid by
the CH– bonds in position 5 leads to formic and 5-
6
4
Relative rate 1.5 × 10 1.7 × 10 97.3 1.00 1.12 3.35 8.51
The fact that the intramolecular transformations of
3
to 4 and 4 to 5 are little competitive with the corre-
sponding intermolecular interactions is also con- oxohexanoic acids identified in the composition of the
firmed by the published data [23] obtained in a study cyclohexane oxidation products [24]:
COOH +RO2,O2,RH
CHO
COOH
CHO
COOH
+
HCOOH.
(7)
CHO
OOH
Formation of Bifunctional C Acids from Cyclohexanol
i
2
6
cies are predominantly HO radicals (reaction (9)),
and Cyclohexanone
and there are mobile equilibria between cyclohexa-
Under the conditions of a radical chain process, none and α-hydroxycyclohexylperoxyl radical (8)
cyclohexanol is predominantly oxidized at the α-CH (reaction (8a)) [25] and between cyclohexanone and
bonds (reaction (8)) [1, 5]. The chain propagating spe- α-hydroxycyclohexyl hydroperoxide (9) [5, 17, 26].
OH
O
HO OO
HO OOH
(c)
+
RO2,O2
(a)
HO2
b)
+H2O2
,
9
−
−H2O2
(8)
8
(
+RH
−R
i
2
i
HO + RH → H O + R .
(9)
2
2
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ROUTES OF FORMATION OF BIFUNCTIONAL C CARBOXYLIC ACIDS
591
6
In the presence of hydroperoxides, e.g., cyclohexyl the reaction mixture, they reversibly add to cyclohex-
hydroperoxide or 2-hydroperoxycyclohexanone, in anone to give semiperketal (10) [1, 4, 5, 17]:
OH
OO
O
OOH
+
.
(10)
10
In addition to the dissociation to cyclohexa- compounds 9 and 10 undergo homolysis [4, 5,
none and the corresponding peroxide compound, 17]:
O
HO OOR
HO
O
HO OH
+
−
RH
R
(11)
.
−RO
−H2O
11
At temperatures below 80°C, 1-hydroxycyclohexy- cyclohexanone (reaction (11)). At higher tempera-
loxyl radical (11) predominantly abstracts a hydrogen tures, the ring-opening degradation of 11 leading to
atom from the substrate with the regeneration of ω-carboxyalkyl radical 12 is possible [4, 5]:
O
O
+
O2
11
OO CH (CH ) C
2 2 4
CH (CH ) C
2
2 4
OH
OH
12
(12)
O
O
+
−
RH
R
HOO CH (CH ) C
...
HO CH (CH ) C
2
2 4
2
2 4
.
OH
OH
Radical 12 can either dehydrate the substrate to
The transformation of 13 to 6-oxohexanoic acid
give hexanoic acid or add oxygen to be transformed can occur through dioxetane intermediate (14) [5]:
into ω-carboxyhydroperoxide and then to 6-hydroxy-
hexanoic acid (reaction (12)) [4, 5].
OH
O
O
13
HOOC(CH ) CHO.
2
4
Cyclohexanone undergoes oxidation according to
the free-radical mechanism predominantly at the α-
CH bonds, the reactivity of which is substantially
higher than that of the C–H bonds in cyclohexane
(14)
14
Like cyclohexyl hydroperoxide, 13 can enter inter-
[
27], to form 2-hydroperoxycyclohexanone (13):
molecular interactions with oxygen-containing com-
pounds which will accelerate its homolytic decompo-
sition. In cyclohexanone under oxidation, the reaction
with the substrate yielding semiperketal (15) which
degrades to form 11 and 2-oxocyclohexyloxyl radical
(16) is preferable [1, 4, 5, 17]:
O
O
OOH
+RO2, O2, RH
.
(13)
13
O+ 11.
O
O
OO
1
3 +
(15)
O
HO
15
16
Radical 16 is transformed to 2-hydroxycyclohexa-
none [5, 19, 20, 22, 28, 29]:
There are two possible routes of the transformation
of 2-oxocyclohexyloxyl radical 16 with the degrada-
tion of the carbon chain. Route 17a explains the for-
mation of 6-oxohexanoic, monoperadipic, and adipic
acids in the cyclohexanone oxidation process [1, 4, 5]:
+RH
O
.
(16)
16
−R
OH
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5
92
PERKEL’, VORONINA
O
C=O
CHO
(a)
(b)
16
CHO
(17)
.
CH2
17
Formation of Bifunctional C Acids During Oxidation
i
6
(
18) either eliminates the HO radical to be trans-
2
of 2-Hydroxycyclohexanone
formed into 1,2-cyclohexanedione (19) [5] (reaction
The oxidation of 2-hydroxycyclohexanone occurs (18)) or abstracts hydrogen from the substrate to form
according to the radical chain mechanism by the most 2-hydroxy-2-hydroperoxycyclohexanone (20) (reac-
reactive α-C–H bond [5, 19, 20, 28]. Peroxyl radical tion (19)):
OO
OH
O
+RO2,O2
O
O .
(18)
OH
−HO2
O
18
19
lytic and nonhomolytic mechanisms [5, 28]. The homo-
lytic decomposition of 20 predominantly involves the
degradation of the carbon chain and leads to adipic acid
in the presence of oxygen (through the adipic monoper-
OH
+
RH
OOH
(19)
18
−
R
O .
0
2
Compound 20 reversibly dissociates to 1,2-cyclohex- oxyacid (23) step) [5, 28],with 19 being formed as well,
anedione and H O or undergoes degradation via homo- although in smaller amounts (reaction (20)):
2
2
O
O
C=O
COOH
22
C OOH
C OH
+O2,RH
20
O
−HO
OH
(20)
21
23
O
+RH
+RH
HOOC(CH ) COOH, 21
19
.
2
4
−R , H2O
By comparing the potential yield of the peroxya- dation, but also interacts with peroxide compounds.
cid with the yield of adipic acid in the case of oxida- The effective rate constants of decomposition of tert-
tion of 2-hydroxycyclohexanone at 50°C, it was
found that about 50% of adipic acid was formed
according to reaction (20) [5, 28]. The other half
amount of adipic acid was formed in parallel with
adipic anhydride as a result of the pericyclic rear-
rangement of 20 through dioxetane and oxirane
butyl hydroperoxide at 150°C in the presence of equi-
molar additions of cyclohexanol, cyclohexanone,
4
and 2-hydroxycyclohexanone k × 10 are 0.17, 0.20,
ef
−
1
and 10.0 s , respectively [5, 22]. The interaction can
include the addition of the peroxide compound to the
intermediates, respectively [5, 28]. 2-Hydroxycyclo- carbonyl group of 2-hydroxycyclohexanone and lead
hexanone not only participates in radical chain oxi- to 6-oxohexanoic acid [5, 22, 28]:
OH
O
OH
O
OH
+ROOH
OR
H
COOH
CHO .
OOR
OH
(21)
−ROH
O
24
R = H, alkyl, or acyl
In the case of introduction of methyl ethyl ketone 123°C, almost complete inhibition of the oxidation of
to 2-hydroxycyclohexanone subjected to oxidation at the latter was observed [22]; the oxidation was
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ROUTES OF FORMATION OF BIFUNCTIONAL C CARBOXYLIC ACIDS
593
6
Table 2. Effect of conversion and catalysts on the relative concentration of 6-hydroxyhexanoic (HHA), 6-oxohexanoic
(
OHA), and adipic (AA) acids (mol % of their total amount (ΣRCOOH)) in the case of oxidation of cyclohexane, cyclo-
hexanone, and 2-hydroxycyclohexanone
ΣRCOOH = 0.05 mol/L
ΣRCOOH = 0.1 mol/L
Object and conditions of oxidation
HHA
OHA
AA
HHA
OHA
AA
Cyclohexane, 150°C
Cyclohexane + cobalt(II) naphthenate, 150°C
Cyclohexanone, 120°C, chlorobenzene [21]
35.0
41.1
1.7
23.3
21.0
40.0
41.7
37.9
58.3
30.3
29.3
3.9
27.4
20.9
28.8
42.3
49.8
67.3
Cyclohexanone + cobalt(II) naphthenate, 120°C,
chlorobenzene [20]
Cyclohexanone + chromium(III) naphthenate, 120°C,
chlorobenzene [21]
2
3.8
4.8
71.4
20.0
4.1
8.0
72.0
71.4
2
.0
25.5
13.6
72.5
77.0
24.5
2-Hydroxycyclohexanone, 123°C, methyl ethyl ketone [22]
9.4
a
a
a
21.2
7.3
71.5
a
Taking into account ε-caprolactone formed according to reaction (22).
resumed after the quantitative conversion of 2- (reactions of type (21)), 6-hydroxyhexanoic acid, and
hydroxycyclohexanone to adipic anhydride and adipic ε-caprolactone. The formation of the latter is most
acid (apparently, through 20), 6-oxohexanoic acid likely due to the homolytic decomposition of (24):
OH
O
O
COOH +RH
CHOH
COOH
CH OH
O
24
(22)
−RO
−R
.
2
OH
Therefore, there are several parallel and consecu- destruction of the C–C bond more readily than 1 [4,
tive–parallel routes of the formation of 6-hydroxyhex- 5] and the formation of 6-hydroxyhexanoic acid from
anoic, 6-oxohexanoic, and adipic acids, which are not
always distinguishable. On the basis of the results
shown in Figs. 1 and 2 and published data, Table 2 was
composed to present information about the influence
of the conversion and catalysis nature on relative con-
centrations of the acids under consideration.
it requires smaller number of stages. The increase in
the concentration of 6-hydroxyhexanoic acid in the
case of oxidation of cyclohexanone in the presence of
a cobalt (not chromium) catalyst (Table 2) also
deserves attention. Since the yield of 6-oxohexanoic
acid in the presence of the cobalt catalyst decreases
simultaneously with the growth in the concentration
of 6-hydroxyhexanoic acid, it can be supposed that
cobalt naphthenate not only stimulates the homolytic
decomposition of 10 to give 11 (reaction (12)), but also
promotes the transformation of 6-hydroperoxyhexa-
It is seen from Table 2 that the relative concentra-
tion of 6-hydroxyhexanoic acid substantially decreases
on passing from the cyclohexane oxidation to the
cyclohexanone oxidation experiments. This finding
can support the idea that a significant part of this com-
pound is produced from the cyclohexyloxyl radical
and/or upon the oxidation of cyclohexanol. The sec- noic acid to 6-hydroxyhexanoic rather than 6-oxohex-
ond option is preferable, since 11 decomposes with the anoic acid:
Co2+
Co(OH)2+
O
O
+
HOO CH (CH ) C
O CH (CH ) C
2 2 4
2
2 4
−
OH
OH
(23)
O
+RH
HO CH (CH ) C
−R
2
2 4
OH .
Transformations of type (23) provide a higher yield
In the case of uncatalyzed and cobalt salt-catalyzed
of cyclohexanol in comparison with cyclohexanone in oxidation of cyclohexane a decrease in the relative
the industrial cyclohexane oxidation process [1, 5, 17]. concentration of 6-hydroxyhexanoic acid and a simul-
PETROLEUM CHEMISTRY Vol. 59 No. 6 2019

5
94
PERKEL’, VORONINA
Table 3. Calculated values for the relative rates of reactions of the products of oxidized cyclohexane (150°C, ΔRH =
0.6 mol/L) with a cumyl peroxy radical at 85°C
Without a catalyst
With Co(II) naphthenate
br
k ,
p
Product
L mol−1 s−1
br
C , mol/L
C , mol/L
W W
i ch
i
k ,
i
p
Cyclohexane
Cyclohexanol
Cyclohexanone
0.21 [31]
6.5 [31]
0.58 [31]
41.7 [32]
7.40
0.21
0.34
0.011
1.00
0.88
0.13
0.30
7.40
0.30
0.24
0.010
1.00
1.25
0.09
0.27
2-Hydroxycyclohexanone
Table 4. Calculated values for the relative rates of reactions of the products of oxidized cyclohexane (150°C, ΔRH =
.6 mol/L) with a tert-butyl peroxy radical
0
br
Without a catalyst
With Co(II) naphthenate
k ,
p
Product
T, °C
L mol−1 s−1
C , mol/L
W W
i
C , mol/L
W W
i
ch
i
i
ch
a
Cyclohexane
60
50
0.046
7.4
1
1
7.4
1
1
a
1
2.2 [33]
Cyclohexanol
Cyclohexanone
60
60
60
60
0.43 [27]
0.22 [27]
0.63
2.19
69.2b
0.21
0.34
0.014
0.01
0.26
0.22
0.026
0.064
0.043
0.30
0.24
0.016
0.007
0.38
0.16
0.029
0.046
0.030
6
6
-Hydroxyhexanoic acid
-Oxohexanoic acid
b
150
a
Calculated from the data logA = 10.4 and E = 81.5 kJ/mol [33]. b Calculated from the data logA = 3.7 and E = 15.05 kJ/mol for butanal [34].
br
br
taneous increase in the relative concentration of adipic where k and k are the rate constants of the chain
pi
pch
acid are observed with the increase in the depth of oxi- propagation reaction of the ith compound and cyclo-
−
1 −1
dation (Figs. 1 and 2, Table 2). This does not imply
that the successive transformation of 6-hydroxyhexa-
noic acid to 6-oxohexanoic and adipic acids is of sub-
stantial importance. Oxidized cyclohexane contains
adipic anhydride and monocyclohexyl adipate (Figs. 1
and 2). The formation of the latter is associated excep-
tionally with the alcoholysis of adipic anhydride by
cyclohexanol [29, 30]. Because of this, the concentra-
tion of the ester reflects the amount of adipic anhy-
hexane, L mol s , and C and C are the concentra-
tions of the product and cyclohexane, mol/L, respec-
tively.
i
ch
br
pi
There is information on k for cyclohexane and
some of its oxidation products in reactions with cumyl
and tert-butyl peroxy radicals (Tables 3 and 4). For 6-
br
pi
oxohexanoic acid, k for butanal has been adopted
dride formed. The latter is produced in parallel with [34]. For 6-hydroxyhexanoic acid, there are no data
adipic acid during the radical chain oxidation of 2- with these radicals [33]. It is known that partial rate
hydroxycyclohexanone, with adipic acid being formed constants of the reaction of a secondary hexadecyl
in a larger amount than the anhydride [5, 19, 22, 28]. peroxy radical with the α-CH bonds of 2-octanol and
−1
−1
The buildup kinetics of adipic acid and monocyclo- 1-decanol at 130°C are 20.7 and 15.1 L mol s ,
hexyl adipate (Figs. 1, 2) suggest that a significant respectively [33]. From these data, the ratio of the
amount of adipic acid is produced from 2-hydroxycy- overall rate constants for primary and secondary alco-
clohexanone even at a low conversion of cyclohexane. hols will be 2 × 15.1/1 × 20.7 = 1.46. To evaluate the
br
This assumption is also confirmed by evaluation
calculation of the relative rates for the reaction of the
cyclohexane oxidation products with cumyl peroxy
radicals and tert-butyl peroxy radicals (Tables 3 and 4).
rate of oxidation of 6-hydroxyhexanoic acid, k equal
pi
br
pi
to 1.46k for cyclohexanol was used.
The consideration of the data of Tables 3 and 4
The rate of oxidation of an organic compound is shows that in the case of uncatalyzed and cobalt naph-
br
i
2
thenate-catalyzed oxidation of cyclohexane (ΔRH =
.6 mol/L (∼6 mol %)), 2-hydroxycyclohexanone
present in the reaction medium is oxidized at a rate
ane, by the equation W W = k [C ] k [C ], that is by an order of magnitude higher than that for 6-
defined by the equation W = k [C ][RO ] and the
i
pi
i
0
rate normalized to the rate of oxidation of cyclohex-
br
pi
br
pch
i
ch
i
ich
PETROLEUM CHEMISTRY
Vol. 59
No. 6
2019

ROUTES OF FORMATION OF BIFUNCTIONAL C CARBOXYLIC ACIDS
595
6
hydroxyhexanoic and 6-oxohexanoic acids (Tables 3 14. A. L. Perkel’, B. G. Freidin, and S. G. Voronina, Zh.
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1
5. A. L. Perkel’, B. G. Freidin, S. G. Voronina, and
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R. L. Perkel’, Zh. Anal. Khim. 48, 1399 (1993).
1
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(
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Translated by E. Boltukhina
PETROLEUM CHEMISTRY Vol. 59 No. 6 2019