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Catulysia Today, 9 (1991) 255-283 Eleevier Science Publishers
B.V., Am&.&am
255
NITRIC ACID REACTION OF CYCLOHEXANOL TO ADIPIC ACID
A. CASTELLAN, J.C.J. BART, and S. CAVALLARO
Dipartimento di Chimica Industriale, Universita degli Studi di
Messina, 98010 Sant'Agata di Messina (Italy)
SUMMARY
The reaction chemistry of the oxidation of cyclohexanol to
adipic acid under the influence of nitric acid has been critically
reviewed. With the traditional process maximum selectivity towards
adipic acid is reached at a high degree of HN03 consumption. Only
under particular reaction conditions can the nitric acid
consumption be reduced without affecting the selectivity; certain
conditions even lead to improved selectivity.
CONTENTS
1
2
3
3.1
3.2
4
4.1
4.2
4.3
4.4
4.5
5
5.1
5.2
6
6.1
Introduction
Chemical properties of nitric acid
First steps of the oxidation of cyclohexanol
Oxidative dehydrogenation of cyclohexanol
Nitrosation of cyclohexanone or cyclohexylnitrite
Reactions of a-nitrosocyclohexanone
Acid-catalysed isomerisation
Hydrolysis to 1,2-cyclohexanedione
Oxidation of a-diketone
Preparation and properties of cyclohexanedione hemihydrate
Formation of a-nitrocyclohexanone
Reactions of a-nitrocyclohexanone
Isomerisation of a-nitrocyclohexanone
Formation of adipic acid from a-nitrocyclohexanone
Adipomononitrolic acid
Formation of 1,1-nitronitrosocyclohexanone and hydrolysis
to AMNA
6.2 Kinetics of the formation of AMNA
6.3 Reactions of AMNA
6.3.1 Oxidative hydrolysis to adipic acid
6.3.2 Isomerisation
6.3.3 Oxidation
256
257
260
260
261
261
261
263
263
264
265
265
265
266
268
268
268
269
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0920~5861/91/$10.15 0 lCf91 Eleevier Science Publishers B.V.
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6.3.4 Oimerisation
7 The effect of the temperature on the oxidation
7.1 Intermediates formed in oxidation at low temperatures
a The effect of V5+ and Cu2+ on the oxidation 9 Reaction
stoichiometry, composition of the off-gas and
nitric acid consumption
10 Summary of the proposed routes to adipic acid
11 Conclusions
References
272
273
273
274
277
279
279
281
1 INTRODUCTION
Adipic acid (AAO), or 1,6-hexanedioic acid, HOOC-(CH2)4-COOH, is
used
industrially to produce nylon 6.6 (in a 1 : 1 ratio with
hexamethylenediamine,
HMO) and many synthetic resins. The product is obtained almost
exclusively via
the nitric acid oxidation of cyclohexanol (cyl) or of a mixture
of cyclohexanol
and cyclohexanone (olone) (ref. 1). The reaction is usually
effected at a tem-
perature between 55 and 100 "6, using 50 - 60 wt.% HN03
solutions containing a
copper-vanadium-based catalyst. The process is characterized by
the following
three major steps: a) the oxidation reaction; b) the recovery
and purification
of AAO; c) the recovery and recycling of the oxidant
mixture.
This paper concentrates on the first step, in particular the
reaction che-
mistry of the oxidation - a topic that has been studied in great
depth with a
view to determining the reaction conditions leading to the
highest selectivity
towards AA0 at the lowest possible HNO3 consumption.
The complex reaction mechanism can be represented as follows
(refs. 2 - 5):
OH
HNOB
COOH
In the case of route [l], AA0 is formed via the direct reaction
of two moles
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257
of HN03 with one mole of cyl. The alternative route, according
to which the
oxidation of a-diketone proceeds via the stoichiometric reaction
of V5+,
involves only one molecule of HN03. In the first process HN03 is
reduced
completely to N20, whereas in the second NO and NO2 are also
formed (ref. 4).
The reaction mechanism explains the various degrees of reduction
of the nitro-
gen oxides that are formed during the process under certain
reaction conditions
but does not sufficiently reveal the complexity of the process.
At a certain
oxidant concentration (oxidant/cyl ratio > 9) and
temperature, the complete
reaction takes no longer than about 90 minutes.
It was decided to carry out a thorough study of all of the
information
published on this subject in the (patent) literature in the hope
that this
would lead to a better understanding of the process and would
reveal any con-
nections between reaction mechanisms and reaction parameters. A
data-base
search (Chem. Abstracts) revealed that about 70 % of the total
of 182 publica-
tions on this subject written between 1967 and 1987 concerns
patents (25 % of
Soviet/Eastern bloc countries, 39 % USA/European patents, 36 %
Japanese
patents). The scientific literature on this subject is not
easily accessible
either, 63 % having been published in Soviet/Eastern bloc
journals, 22 % in
USA/European journals, 11 % in Japanese and 4 % in Indian
journals. Reviews on
the oxidative preparation of adipic acid are scarce and not of
recent date
(refs. 6 - 8).
2 CHEMICAL PROPERTIES OF NITRIC ACID
Before studying the various reactions that may take place in the
process of
the oxidation of cyclohexanol and cyclohexanol/cyclohexanone, we
must first
consider some chemical properties of HN03 solutions. Nitric acid
shows a very
complex activity; in the first place it can have numerous
different effects
(acidifying, nitrating, nitrosating and oxidizing) and,
secondly, its activity
is greatly dependent on any impurities it may contain (such as
HN02, NO2, N2O3,
N2O5, etc.). The main features of reactions with HN03 are (ref.
9): a) oxida-
tions require the presence of an initiator, such as HN02 and/or
N02; b) reac-
tions with HN03 are often autocatalytic; c) oxidations are
usually catalysed
with the aid of mineral acids; d) in nitric acid oxidation
either one or two
nitrogen atoms are incorporated in the substrate, dependent on
the nature of
the reactant or the reaction conditions. As to the role of HN02,
consider that
HN03
2 HNO2 * NO2 + NO + H20 and HO-NO -' NO + H20
after which radical reactions may take place.
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268
TABLE 1
Variation in nitric acid structure with concentrationa
HN03 Mole H20/ Pseudo- Non-dissociated Ionized concentration
mole HN03 acid acid acid (%1 (%I (%I (%I
77.3
:+5
70 25 5
48.3 7:5 50 32 31.6 2 60 ::
a From ref. 10, p. 88.
TABLE 2
Some physico-chemical properties of nitric-acid solutionsa
concentration
HOb Density (20 "C)
wt.% mole/l
-3.07 + + 0.98 0.21 i.015
17:o l?Z
- 0.18 1.035 - * 0.67 1.02 1.065 1.097
22.3 - 1.32 1.130 27.0 - 1.57 1.160 31.7 - 1.79 1.192 36.4 -
1.99 1.227
a From refs. 63, 64. b Ho = acidity index on Hammett scale.
The physicochemical properties of HN03 in aqueous solution vary
in
dance with the concentration (Table 1). In fact, we can
distinguish a
acidic form:
O=N- OH
d I
accor-
pseudo-
and a true acidic form: H+ NOg-. Only the pseudo-acidic form has
nitrating and
esteri_fying properties. Table 2 lists some physicochemical
properties of HN03
solutions.
In the oxidation of cyl to AAD, the selectivity of which depends
greatly on
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259
the HN03 concentration, nitric acid can serve as:
a) a mineral acid, by supplying hydrogen:
HN03 + Hz0 + NOg- + H30"
b) a C-nitrosating or C-nitrating agent, by introducing one or
more NO and/or
NO2 groups into the organic substrate:
RH + HONO -+ RN02 + Hz0
c) an esterification agent (O-nitration):
ROH + HONO + RON02 + Hz0
d) an oxidant, by supplying one or more oxygen atoms:
RCHO + HN03 -+ RCOOH + HN02
According to Topchiev (ref. lo), HN03 acts as an oxidant in
secondary reac-
tions only, which can only take place in the presence of
previously nitrosated
compounds. It is possible to greatly reduce the oxidizing action
of HN03 and to
increase the yield of nitro-compounds by varying the HN03
concentration. The
nitration rate depends mainly on the temperature, pressure and
acid con-
centration (via nitronium ions in highly concentrated HN03 or
via hydrated
nitronium ions at high water concentrations). The reaction route
and the nature
of the product depend largely on the HN03 concentration and the
residence time
(tr) chosen. Prolonged boiling with a substantial excess of HN03
(high ox/cyl
ratio) leads mainly to oxidized products, whereas short heating
times and rela-
tively low HN03 concentrations lead to high nitro-derivative
yields.
Therefore, the interaction of HN03 with saturated hydrocarbons
initially
leads exclusively to isonitric compounds (ref. 10). But as they
are unstable
under acidic conditions they are then either transformed into
stable nitro-
compounds or are oxidized, first of all to aldehydes and
ketones, and then to
the corresponding acids:
I-> R$HNO2
R$H2 + 0 = N = 0 + R$ = N = 0 H+
II
[31
H b H -L > R2COOH Labile isonitric compound
Usually, the substitution of a hydrogen atom bound to an
aliphatic carbon
atom by a nitro(so) group requires the presence of adjacent
electron-attracting
groups. Keto groups in particular have a strong activating
effect on the
-
nitr(os)a of an adjacent carbon atom. Alcohols and esters are
usually oxi-
dized to aldehydes or ketones, which are in turn transformed
into the
corresponding carboxylic acids. Cycloalcohol homologues can be
transformed into
the corresponding bicarboxylic acids: cyclododecanol for example
yields dodeca-
nedioic acid (ref. 11). Glycols also lead to the corresponding
bicarboxylic
acids in the presence of V205, at 50 - 70 "C; ethylene glycol
treated with HN03
and H2S04 yields oxalic acid (> 93 %) (ref. 12).
The kinetics and the mechanism of the oxidation of aliphatic
alcohols have
been studied for the nitric acid reaction of G-methoxyethanol in
the presence
of H2S04. It has been found (ref. 13) that in the presence of
excess HN03 the
reaction rate varies according to v = k [Alcohol]. The lg k - Ho
(Hammett's
acidity function) diagram is a straight-line relationship with a
gradient equal
to unity, i.e. the reaction velocity is directly proportional to
the activity
of the H+ ions in the reaction medium.
3 THE FIRST STEPS OF THE OXIDATION OF CYCLOHEXANOL
The reaction of cyclohexanol is usually schematically outlined
as follows:
OH HNO,
(NO,) 0”” ;i : a:0 + H2o
/I/ 141
3.1 Step I: Oxidative dehydrogenation of cyclohexanol
Cyclohexanol can be oxidized by HN03 at very low temperatures,
as opposed
to cyclohexanone, which requires temperatures of above about 200
OC (ref. 14).
The nitric acid oxidation of alcohols generally starts with the
abstraction of
a hydride ion from the carbon atom at the a-position to the
hydroxyl (ref. 15).
In the case of cyclohexanol, the hydrogen of the hydroxyl group
must be mobile
(ref. 16). Cyclohexyl acetate does not react with HN03 at low
temperatures.
Kinetic studies of nitric acid oxidations have shown that the
active reactant
is not HN03 as such but nitrous acid, HN02 (ref. 17). or
dissolved NO2 and N2O4
(ref. 18). Therefore, the first oxidation product of
cyclohexanol must be
cyclohexyl nitrite rather than cyclohexanone (refs. 18, 19),
according to the
following equation:
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261
C6"llO" + "NO2 i C6"llONO + Hz0
It has been demonstrated that cyclohexyl nitrite is always
present together
with cyclohexanone (ref. 20). Pure nitric acid is only capable
of esterifying
cyclohexanol (ref. 18) because of its properties as a
medium-strong acid (ref.
15):
HC104 > H2SO4 > ClS03H > HBr > HN03 > H3P04
3.2 Step II: Nitrosation of cyclohexanone or cyclohexyl
nitrite
In the presence of HNO2 (especially in statu nascendi, e.g. when
formed from
nitrite with the aid of mineral acids (ref. 18)), the
cyclohexanone inter-
mediate is nitrosated in the a-position to the carbonyl group
with the for-
mation of nitrosoketone. This reaction is very slow at low
temperatures (< 30
"C) (ref. 20) and leads to the accumulation of some
cyclohexanone in solution
(ref. 21). In the hydrolysis of cyclohexyl nitrite nitrogen
oxides are formed,
which attack cyclohexanone and lead to the formation of
isonitrosocyclohexanone
(ref. 5). If the reaction medium contains a compound which traps
HN02, such as
urea, the nitric acid oxidation of cyclohexanol is blocked
completely (ref.
16). This shows that HN02 is not only necessary in the first
step of the reac-
tion but is also required to ensure the continuation of the
oxidation.
4 REACTIONS OF a-NITROSOCYCLOHEXANONE
a-Nitrosocyclohexanone is a key intermediate in the oxidation of
cyclohexa-
nol. This product can react in many ways and can therefore
greatly affect the
selectivity and HN03 consumption of the overall process. It
undergoes preva-
lently isomerisation or hydrolysis (see sections 4.1 and 4.2) or
it is nitrated
further (section 4.5). Accordingly, two main routes (indicated
as I and V in
Fig. 1) lead to the formation of AAD. Let us first see how AAD
is obtained via
the direct reaction of one mole of cyclohexanol with one mole of
HN03.
4.1 Acid-catalysed isomerization
a-Nitrosocyclohexanone isomerizes in the presence of an acid
(ref. 18). The
oxidative process may start from any isomeric form.
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262
C COOH G--O2
-\a 0 NOH HsO+ t
0
/
kOH HNO:, \HsO+
b COOH
C COOH C COOH’ + 2N0 COOH COOH + H,NOH (11) (1)
Fig. 1. Overview of acid. Pathway, reference: V (ref. 4).
H+ c
/3/
C COOH C=N -0 HsO+
I
C COOH COOH + NH,OH i
HNO,
(111) NO2
proposed routes to oxidation of cyclohexanol to adipic
I (ref. 26); II (ref. 41); III (ref. 31); IV (ref. 2);
[53
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263
4.2 Hydrolysis by HN02 or HN03 to 1,2-cyclohexanedione
In the presence-of HN02 (a weak, non-oxidizing, acid) a-diketone
and
hydroxylamine are formed from a-nitrosocyclohexanone
(Claisen-Manasse reaction)
(ref. 18):
a 0 HNO, 0 + H,O- NO a + H,NOH 0 /l/ /4/
PaI
The nitric acid oxidation of an oxime with the aid of HN02 leads
to
carbonyl-containing compounds and gaseous products, such as N20,
NO and/or N2,
dependent on the acidity of the medium (ref. 22): in the
presence of strong
mineral acids the reaction leads to 78 % N20 and 20 % N2, but in
their absence
67 % NO and 22 % N20 are formed. In the presence of HN03, the
oxidative hydro-
lysis of the monoxime /3/ leads to NO2 instead of hydroxylamine
(ref. 23):
0 + 2N0 + H,O WI
0
Of course, the oxidizing action depends on the HN03
concentration.
4.3 Oxidation of a-diketone
a-Diketone and/or the monoxime, formed in reactions [7] and [6],
respec-
tively, can be oxidized to form acids according to a reaction
mechanism which
is still not fully understood. According to some authors (refs.
5, 18), the a-
diketone is responsible for the formation of succinic and oxalic
acid according
to the following reaction:
0 a + 2HNO,- HOOC-_(CH,),-COOH + HOOC-COOH 0 /4/ WI
Others (ref. 23) claim that 38% glutaric acid is formed in the
presence of V6+
(0.01 mol.%). However it may be, when the a-diketone is oxidized
in the pre-
sence of suitable V6+ concentrations, AAD is formed with a
selectivity of over
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264
9.2 % (refs. 24, 25). Therefore, it is likely that the selective
oxidation of a-
diketone and/or the monoxime is based on a well-balanced
reaction of HN03 and
v5+, in such a way that V5+ directly oxidizes the cyclic
compound, while HN03
rapidly reoxidizes the resultant V4+:
+ V(OH): H+/H,O
e [complex] +C COOH 4 + vo*+ -H+ COOH /4/ AAD r91
VO2+ + HN03 + VO2+ + NO2 + H+ [lOI
and/or
3VO*+ + HN03 + H20 + 3VO2+ + NO + 3H+ El11
Note that the formation of AA0 is here accompanied by the
reduction of HN03
to the regenerable oxides NO and N02, whereas the common
production process
based on the use of adipomononitrolic acid leads to N20 which
cannot be reoxi-
dized (see below).
4.4 Preparation and properties of cyclohexanedione
hemihydrate
The nitric acid oxidation of cyclohexanol at low temperatures
(10 - 15 "C)
leads to the formation of adipomononitrolic acid (AMNA) and the
hemihydrate of
cyclohexanedione (predione) rather than the a-diketone (ref.
26):
0 8$3 0 0 Predione /6/ OH
The predione/AMNA ratio increases with the HNO2 concentration
(ref. 2). At
a HE103 concentration of 50 %, 57 % predione is formed at 10 -
15 "C, in the
presence of NaN02 (refs. 27, 28). The mechanism of this reaction
is not yet
understood though. The predione does not appear to be formed
simply through
dimerisation of 1,2-cyclohexanedione in HN03, nor via the
reaction of
2-hydroxy-cyclohexanone or of a mixture of 1,2-cyclohexanedione
and cyclohexa-
none (ref. 27). Moreover, this compound has never been observed
in the nitric
acid oxidation of cyclohexene (ref. 23). The stable,
crystalline, compound
-
releases H20 at 150 OC and thus forms 1,2-cyclohexanedione (ref.
27).
In the past, predione was considered the key intermediate in the
formation
of the lower glutaric and succinic acids. However, it has
recently been shown
(ref. 2) that predione can be oxidized to AA0 with a good
selectivity (of about
90 %) in the presence of metavanadate. Moreover, it has also
been shown that
the transformation into AA0 does not proceed via AMNA with
additional HN03
consumption (ref. 3).
The great interest in this compound (as is apparent from the
abundant patent
literature (ref. 29)) is due to the fact that it presents an
alternative route
for the synthesis of AA0 from cyclohexanol, which presents the
advantages of
good selectivity and a low HN03 consumption and does not involve
the formation
of AMNA.
4.5 Formation of a-nitrocyclohexanone
Instead of undergoing hydrolysis (as discussed above),
a-nitrosocyclo-
hexanone yields mainly a-nitrocyclohexanone (refs. 20, 21, 30)
in strongly oxi-
dizing solutions:
0 a 0 NO + HN03 - a + HNO;! NO2 H H
/I/ /7/
[W
Dependent on the reaction conditions, the compound thus obtained
may be
hydrolysed to AA0 or isomerized, which leads to undesired
byproducts (such as
picric acids).
5 REACTIONS OF a-NITROCYCLOHEXANONE
5.1 Isomerisation of a-nitrocyclohexanone
In ketonic form, a-nitrocyclohexanone isomerizes under mild
conditions to
form a mixture of tautomers (refs. 31 - 34). The ease at which
it isomerizes
can be taken as an indication of the high instability of the
compound. It can
act as an oxidative dehydrogenation catalyst which, via
nitration, leads to the
formation of picric acids according to eq. [13]. The reaction
can take place in
the absence of N2O4 (ref. 18).
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266
0
cx
OH Oxidative dehydrogenation
NO*H-==- CC0 - a,,,
L Picric acids nitration
H 2 181 /7/ 191 [I31
5.2 Formation of adipic acid from a-nitrocyclohexanone
It is known that primary nitroparaffins are hydrolysed in hot
mineral acids
to form carboxylic acids and hydroxylamine according to:
+ H+ R - CH3 - NO2 --> R - C = NOH --> RCOOH + HENOH
[I41
HZ0 b
“~0 H
When HN03 is used, NO2 is formed rather than hydroxylamine,
probably
through oxidation of the latter by excess acid (ref. 35). A
similar hydrolysis
reaction has also been proposed for a-nitrocyclohexanone in the
presence of 96
% HzS04, 36 % HCl or HNO3, the AAD yield amounting to 95.3 %
(refs. 31 - 34):
H*o : c COOH HNO, COOH +NOe II151 171 AAD
In this case too hydrolysis with HNO3 finally leads to NO2
instead of hydroxyl-
amine. The mechanism proposed for this reaction is very similar
to that of the
formation of cyclic imides from oximes (refs. 31 - 35):
Ring closure at the nitrogen atom (as indicated) of /ll/ leads
to the formation
of a cyclic imide, while further hydrolysis results in
bicarboxylic acid and
hydroxylamine (or NOp).
Reaction [15] is clearly a direct method for synthesizing AAD
that results
in good yields and makes economic use of HN03. The fact that
a-nitrocyclo-
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267
hexanone undergoes further nitrosation to AMNA, as discussed in
section 6,
instead of being hydrolysed, explains the formation of N20 under
normal reac-
tion conditions (route I in Fig. 1). To avoid the risk of
overreduction with
formation of N20, reaction conditions will have to be found
under which hydro-
lysis of the nitro-compound prevails over nitrosation.
Procedures [15] and [16] for the formation of AA0 (route III of
Fig. 1) appear to be even more interesting if one considers that
a-nitrocyclohexanone
can be prepared directly from cyclohexanone and HN03 (ref.
30):
0
+ HN03 20-40°C/10 min. 0
(99- 100%) CCI,
-a + H,O
NO2
/7/ [I71
Direct nitration of cyclohexanone has also been advanced by
Lubyanitskii et
al. (36), namely through the nitric acid oxidation of
cyclohexanol at a high
temperature (75 "C), via a radical intermediate:
OH HNO,
NO
Its presence in the reaction medium can be detected
polarographically, because
the ketonic form /7/ is reducible at Et = -0.65 V, whereas the
enolic form is
reduced at Et = -0.26 V (ref. 36).
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268
6 ADIPOMONONITROLIC ACID
6.1 Formation of 1,1-nitronitrosocyclohexanone and hydrolysis to
AMNA
a-Nitrosocyclohexanone shows a strong tendency towards
hydrolysis and oxida-
tion, but also towards nitration (ref. 36):
0
CT 0
NO + H-0-N02-- a NO + H,O WI
H NO2
/I/ /2/
Of course, the same product can be obtained by nitrosation of
a-
nitrocyclohexanone with the aid of HNO7 (ref. 30):
0 a + H-O-NO - + H,O [201 H
NO2
/7/ /2/
Hydrolysis of 1,1-nitronitrosocyclohexanone, obtained according
to reactions
[19] and [ZO], leads to adipomononitrolic acid (refs. 20,
21):
’ a H+/ H,O +C COOH NO NO2
$402
NOH
[211
/2/ AMNA
This is one of the most stable intermediates formed in the
oxidation of
cyclohexanol and it is very selective towards AAD formation
(ref. 26).
6.2 Kinetics of the formation of AMNA
According to Van Asselt et al. (ref. Z), the decomposition of
AMNA in HN03
proceeds at a lower rate than its formation from cyclohexanol.
This makes it
possible to study the two reaction rates separately (ref. 21).
Refs. 21, 37 and
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269
38 discuss the rate of AMNA formation as a function of a number
of reaction
parameters. On the whole, the maximum concentration that can be
obtained in
solution depends on the oxidation temperature, the contact time
T, the HN03
concentration and the type of catalyst used. The highest
concentration (40 %
of the theoretical value) was obtained in oxidation at 50 'C for
T = 2.25 min,
using 57 % HN03 (starting concentration) (ref. 39). As already
reported, it is
clear that not all of the cyclohexanol is transformed into AMNA:
a portion is
converted into other intermediate products.
6.3 Reactions of AMNA
6.3.1 Oxidative hydrolysis to adipic acid -------- _--------
The formation of AA0 from AMNA takes place via the intermediate
adipomono-
hydroxamic acid, as isolated by Hanin (ref. 40):
HOOC - (CH2)4 - C = NOH
b H /12/
The following reaction mechanism (cf. route I of Fig. 1) is
proposed (ref. 40):
+ H?O HOOC - (CH2)4 - C = NOH
I HOOC - (CH2{4 - ; = NOH + yN02
ItO2 AMNA
/12/ I I hi HOOC -
HOOC -
The formation of adipomonohydroxamic acid
reaction with Fe3+, which leads to an intense
j Fe3+ + HOOC - (CH2)4 - C - NHOH --> HOOC -
(CH,2)4 - C - NHOH
I
1
t
N20 + H20
H+ H20
(CH2)4 - C - OH + H2NOH
II 0
AAD [22J
/12/ can be demonstrated via the
red-violet chelate:
(CH2)4 - C - NH + H+ [23]
B d
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270
The hydrolysis of AMNA is acid-catalysed and can be accelerated
by removing
HN02, which leads to a shift in the reaction equilibrium:
HCl HOOC - (CH2)4 - C = NOH + HOOC - (CH2)4 -
I
= NOH + HNO2 --+ AAD +
NO2 H NOCl + NH2OH
AMNA fI2/ 1241
When use is made of highly concentrated HN03 or HCl, which react
very
rapidly with HN02 and remove it from the equilibrium reaction,
the hydrolysis
rate is much higher than with H2S04 (refs. 40, 41). In the case
of HCl, decom-
position of AMNA may lead to nitrosylchloride and hydroxylamine
as byproducts
(ref. 42).
Another interesting oxidative hydrolysis reaction was developed
by White
(ref. 43):
AMNA /I2/ 1251
This reaction leads to the formation of AAD and
cyclohexanonoxime, which is
used as an intermediate in the production of caprolactam.
Adipomonohydroxamic
acid can indeed react with cyclohexanone according to the
following reaction
scheme:
C ;OH
+ o”“- (2::: + CL,,, AWNA [=I
Nitric oxidation of cyclohexylnitrite (formed in reaction [25])
at 20-40 “C
results in AMNA:
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271
ON0 H2O COOH
+ 2 HNO + HNO, + H,O
3 20-40 “C F
-NO,
AMNA r271
This series of reactions does not only lead to the production of
both AAD
and caprolactam under suitable reaction conditions but also
enables an almost
total recovery of HN03, because in none of the reactions is
nitrogen reduced
to a valency of below 2+.
6.3.2 Isomerisation -_-_--_
It is very difficult to explain the quantitative formation of
N20 via hydro-
lysis of AMNA without assuming that the nitrolic acid undergoes
isomerisation
first (ref. 44). In fact, reaction [22] provides more insight
into the simulta-
neous production of HN02 and hydroxylamine than into the
formation of N20.
Moreover, in view of the thermal and chemical stability of the
nitric group and
of hydroxylamine, hydrolysis is not likely to take place at a
low temperature.
A number of Russian researchers have studied the isomerisation
of AMNA (refs.
19, 44, 45). They give the following equation for the formation
of an
electrochemically active compound, nitroamide:
R - C = NOH + R - C = NO2H --> RC - N = N02H + RC - NH - NO2
[=I I I
b I II
NO2 NO 0
Hydrolysis of the,nitroamide explains the formation of N20 in
stoichiometric
terms:
0 r > H20 HOOC - (CH2)4 - 1! - NHN02 + H20 HOOC - (CH2)4 -
COOH + NH2N02 - rw L> N20
The isomerisation rate of AMNA increases with the dielectric
constant of the
solution; H+ and OH- ions catalyse the reaction (ref. 45).
-
212
6.3.3 Oxidation -----
The reaction rate of the hydrolysis of AMNA and the amount of
NO, formed
instead of N20 increase with the HN03 concentration (refs. 40,
41). This means
that HNO3 has a dual effect on AMNA, namely that of a
proton-donor and of an
oxidant, and can thus lead to two different reactions (I and II
of Fig. 1):
C COOH YH -NO,
C
COOH C COOH C-NO, COOH iOH
AMNA
/
I AAD
C COOH C=NOH dH
1141 1301
The formation of /13/ explains the formation of the more
desirable NO instead
of N20; for thermodynamic reasons the protonated complex /14/
can only exist
at low temperatures, in the presence of a substantial excess of
acid (ref. 41).
6.3.4 Dimerisation _-----
If present in sufficiently high concentrations,
adipomononitrolic acid can
also dimerise. It is known that nitric oxidation of ketones can
lead to the
formation of furazane derivatives (ref. 9). The proposed
mechanism for this
process envisages the formation of an intermediate nitrolic
acid:
A dimer. R- C- C=NOH --> R-C- C=N+O --> R - CO - C - C -
COR
1 I HNO2 II NO2 0
1 II
\ilo
[3Il
-
273
Low temperatures favour the formation of these products (ref.
9). A similar
reaction, also starting from AMNA, may explain the formation of
the undesired
coloured byproducts which are sometimes present in crude AAD
(industrial speci-
fications usually require s 5 "Hazen). Zaitseva et al. (ref. 46)
have discussed
organic trace impurities in adipic acid.
7 THE EFFECT OF THE TEMPERATURE ON THE OXIDATION
It is possible to favour certain oxidation routes of
cyclohexanol by
choosing the right reaction conditions (temperature, [HN03],
HN03/cyl molar
ratio, and the type and concentration of the metal catalyst).
The temperature
at which an oxidation process is effected can have a
considerable effect on the
reaction rate. Lubyanitskii et al. (ref. 36) have shown that the
composition of
the oxidation mixture is strongly dependent on the temperature.
At 20 'C only
two polarographically active intermediate compounds are formed,
whereas four
such compounds are found at 60 "C (refs. 21, 36). At low
temperatures the oxi-
dation proceeds mainly via an ionic-molecular mechanism, whereas
it proceeds
via a radical molecular mechanism at high temperatures.
7.1 Intermediates formed in oxidation at low temperatures
Following Godt and Quinn (ref. 26), we have recently carried out
experiments
at very low temperatures (< 20 "C) to slow down the various
process steps as
much as possible in order to be able to study the nature of the
intermediate
products (refs. 47, 48). At low temperatures (11 - 13 “C), the
progress of the
reaction can be followed visually and three distinct steps can
be
distinguished, namely:
a) the formation of one or more brightly coloured products
(emerald green),
which are insoluble in the aqueous solution;
b) the homogenization of the phases and the formation of
nitrogen-containing
gases (due to the hydrolysis of some nitroso-derivatives);
c) the precipitation of white crystalline (needle-shaped)
compounds
(hemihydrate of cyclohexanedione, AMNA and AAD) accompanied by
gas formation
(Np, N20, NO, NO2. CO2).
The higher the HN03 concentration, HN03/cyl ratio and reaction
temperature,
the faster the separate steps succeed one another. (Remember
that at low tem-
peratures some NaN02 must be added to the solution to avoid the
build-up of
dangerous explosive reaction mixtures). At a HN03 concentration
of < 40 % and a
TR of < 15 'C the reaction did not proceed beyond the first
step for several
-
2'74
hours. In order to better define the role of the various
reaction conditions,
the products were analysed that were obtained after a particular
reaction time,
using different HN03 concentrations, oxidant/cyl ratios and
different types
of catalyst, in varying concentrations. These products were
found to be AAD,
AMNA and the hemihydrate of cyclohexanedione (the latter product
being formed
together with or instead of a-diketone or a-hydroxycyclohexanone
in the hydro-
lysis of the nitroso-derivative). The other compounds formed
(characterized by
a very bright yellow-orange colour) were not all extractable
with ether. Under
these conditions, the formation of AMNA appears to be favoured
at high HN03
concentrations and HN03/cyl ratios, whereas concentrations of
about 50 % were
found to lead to the formation of maximum amounts of
cyclohexanedione hemi-
hydrate and of other hydrolysis products (refs. 47, 48).
8 THE EFFECT OF V5+ AND Cu2+ ON THE OXIDATION
The role of vanadium in the nitric acid oxidation of
cyclohexanol has never
been fully explained. However, it has been experimentally
determined that its
presence results in increased selectivity towards AAD,
especially when it is
combined with copper (refs. 37, 49). The effect has been studied
for the
various steps in the oxidation of cyclohexanol and cyclohexanone
to bicar-
boxylic acids. The results lead to the following
conclusions:
a) vanadium has a positive effect on the oxidation kinetics of
cyclohexanol
at low temperatures (< 15 "C) (refs. 2, 50);
b) vanadium accelerates the formation of AMNA up to 30 "C,
whereas the pro-
cess is slowed down at higher temperatures (refs. 2, 38);
c) vanadium has a specific effect on the conversion of
cyclohexanedione
hemihydrate to adipic acid (ref. 2).
The presence of vanadium does not seem to affect the
transformation of AMNA to
AAD (refs. 3, 40, 41). This is not in accordance with the data
presented by
Russian authors though (ref. 51): they report the catalytic
effect of metavana-
date on the hydrolysis kinetics of AMNA in the presence of 43.3
% aq. H2SO4.
This effect implies a complex of the type
//NOH -- - 0,/H
Hooc-(CH2)4-c~~0 __ .HO’ ‘OSO;!
0
-
275
It is also known that vanadium has a catalytic effect on the
selectivity of
the oxidation of cyclohexanone to AAD with the aid of perchloric
(ref. 52) or
persulphuric acid (ref. 53), as well as on the oxidation of
cyclohexanol/
cyclohexanone with the aid of N204 or HN03 at low temperatures
(refs. 29, 54)
and on the nitric acid oxidation of cyclohexene (ref. 23). The
mechanism pro-
posed for the last reaction is as follows:
O-NO2 -cc vo; - AAD + VO2+ 0 1321
The VO2+ ion is then reoxidized by HN03 according to eqs. [lo]
and [ll] (ref.
55).
On the basis of this scheme the main function of vanadium would
be to favour
the rupture of C-C bonds, which leads to ring opening in
one-electron oxida-
tion. Such an effect has been observed for several compounds
which may be con-
sidered possible intermediates in the oxidation process, such as
a-ketols and
cyclic a-glycols. Nitric acid oxidation of these compounds in
the absence of
vanadium leads exclusively to glutaric and succinic acid,
whereas in the pre-
sence of 0.01 at.% V5+ the selectivity towards AAD exceeds 92 %
(refs. 56, 57).
A similar catalytic effect has been assumed in the case of the
nitric acid
oxidation of cyclohexanol (refs. 23, 37, 58). Ring opening
favoured by vanadium
is not likely to be followed by nitrosation and/or nitration at
positions adja-
cent to carbonyl. In this way the formation of products such as
2,6-dinitro-
2,6-dinitrosocyclohexanone, which are responsible for the
degradation of the
organic chain, is totally or partially suppressed. The proposed
reaction scheme
is:
C6HllOH + V(OH)4+ + NO+aq -> C6HlDO + NOgas + 3H20 + V02+
c331
with AGO298 = -21.3 kcal/mole.
The reaction mechanism implies the transfer of an a-hydrogen to
the metal
coordination sphere (ref. 13):
-
276
OH+ V02++2H20 E341
The carbene radical can be oxidized to form a ketone with the
aid of another
oxidant. Vanadium also catalyses the effect of HN03 on mixtures
of cyclohexanol
and cyclohexanone (refs. 56, 57). Interaction of V5+ with
cyclohexanone could
also take place directly in the ketonic rather than in the
enolic form (ref.
59):
OH
0= 0 4 VO; + H,O+ =
P
'2t )4V-OH-
0;
o+v4+
OH l
H
The rate of the V5*-catalysed oxidation of cyclohexanone in HCl
can be
increased by adding Cu2+ (ref. 60). Cy cl o hexanone probably
plays a part in the
formation of a Cu2+ adduct, which is more easily oxidized by V5+
than pure
cyclohexanone.
It should also be noted that vanadium could be partly
responsible for the
formation of glutaric acid (ref. 57); the following reaction is
thermo-
dynamically possible:
VO2+ + NOz(g) + 2H20 -> V(OH)4+ + NO+(aq) AGo2gg = -2.6
kcal/mole
[361
In the event of a considerable excess of NO* more nitrosated
products can be
formed, such as 2,6-dinitro-2,6-dinitroso-cyclohexanone.
Glutaric acid is then
formed through thermal degradation (see eq. (421).
In a series of nitric acid oxidations of cyclohexanol at low
temperatures
(11 - 13 "C) it was finally noted that (under otherwise the same
reaction con-
ditions) higher V5+ concentrations lead to an increase in the
amount of adipic
acid formed at the expense of (pre)dione and, to a lesser
extent, AMNA; the
-
amounts of the other reaction intermediates did not appear to be
affected
(refs. 47, 48). These results can be interpreted as an
indication that V5+
accelerates the oxidation of predione to AAD, as proposed by Van
Asselt et al.
(refs. 2, 3), or prevents the formation of cyclohexanedione
hemihydrate and
thus favours the direct transformation of its precursors
(a-diketone, a-
hydroxyketone or a-dihydroxycyclohexane) to AAD (ref. 25). The
presence of Cu2+
has no effect on the process at low temperatures. In addition,
Cu*+ appears to
act as a catalyst at high HN03 concentrations only.
g REACTION STOICHIOMETRY, COMPOSITION OF THE OFF-GAS AND NITRIC
ACID
CONSUMPTION
The data reported in the literature on the composition of the
nitrogen-
containing off-gas mixtures formed in the oxidation of
cyclohexanol or in the
hydrolysis of AMNA vary considerably, not only with regard to
the composition
of the gas itself, but also with regard to the derived reaction
stoichiometry.
In some cases (refs. 3, 40) very low N2 fractions are given; in
others (ref.
41) they are about as high as those given for N20. On the whole,
the reaction
mechanisms appear to lead to the formation of more or less
strongly oxidized
nitrogen compounds.
Russian authors (ref. 61) have tried to express the HN03
consumption (the
number of moles of HN03 transformed into N2 and N20) as a
function of the
selectivity with the aid of a simple empirical equation of the
type:
AHN03 = 3 - 1.2 NAMNA [371
where AHNO is the amount of HNO3 consumed and N is the fraction
of cyl trans-
formed into AMNA. The equation assumes the following oxidation
steps:
1381
-
278
2 HOOC - (CH2)4 - C = NOH + H20 --> 2 HOOC - (CH2)4 - COOH +
N20 + H2N202 I NO2 1391
In the presence of HN03, hyponitrous acid formed is dismutated
into N2 and
nitric acid:
H2N202 --> 0.4 HN03 + 0.8 N2 + 0.8 H20
The overall stoichiometry of the reaction is therefore:
1401
CbHIIOH + 1.8 HN03 --> AA0 + 0.5 N20 + 0.4 N2 + 1.9 H20
1411
As shown in Fig. 1, AAD can be obtained from AMNA via a route
that involves
the formation of NO from HN03. Therefore, the usefulness of eq.
[37] is
limited. In general terms, however, it can be stated that the
AMNA route (I and
II in Fig. 1) leads to high yields of AAD, whereas the
alternative AAD produc-
tion routes (routes III-V in Fig. 1) permit the recovery of
larger amounts of
HN03 (as NO and N02), though often at the expense of the
selectivity (the
decrease in selectivity being dependent on the reaction
conditions). The pre-
vious equations lead to a specific consumption of HN03 of 0.777
kg/kg of AAD
for complete conversion of cyclohexanol to AMNA. However, if a
certain amount
of glutaric and succinic acid is formed in addition to AAD, the
consumption
increases, because the formation of these acids implies further
nitrations
(refs. 19, 20). This is illustrated for glutaric acid as
follows:
NO2 NO
0
HNO,--+ 0
e NO2
NO
CO2 + HOOC-( CH&-COOH + 2 N,O 1421
In the case of succinic and oxalic acid, which are formed
simultaneously, in
equimolar amounts, from 1,2-cyclohexanedione, three moles are
used, as
expressed by the following set of equations (refs. 5, 18):
OH
+ HN03 -
0 a + 2HNO,- 0 HOOC-(CH2)2-COOH + HOOC-COOH 1431
-
10 SUMMARY OF THE PROPOSED ROUTES TO ADIPIC ACID
Figure 1 presents a schematic outline of the various proposed
preparation
methods for AAD via nitric acid oxidation of
cyclohexanol/cyclohexanone. Each
route shows a different selectivity and HNOS consumption. In
view of the fact
279
that at least four oxygen atoms are necessary to oxidize cyl to
AAO:
OH
C COOH + 4 [o] - COOH + H,O c441 it is clear that these atoms
must come from different HNOS molecules, which
means that nitrogen is released at different oxidation levels,
i.e. in dif-
ferent reoxidizable states. Consequently, if the object is to
increase the AA0
yield and to reduce HNOS consumption it is a matter of choosing
the right reac-
tion conditions to favour the desired reaction routes.
11 CONCLUSIONS
The aim of our research, which was based on a study of the
information
available in the literature, was to obtain a better
understanding of the pro-
cess and to determine whether it is possible to obtain high
adipic acid yields
(> 95 %) at a low HN02 consumption (< 0.86 kg HNOS/kg
AAD). The reported data
were found to be partly conflicting and were not all completely
convincing. For
this reason the authors carried out additional experimental work
to clarify and
confirm some of the data given in the literature. The
laboratory-scale work,
which was carried out at low temperatures in order to slow down
the various
oxidation steps so as to be able to determine the presence of
intermediates
(ref. 47), has in the meantime been extended to a pilot plant
simulating
industrial process conditions (ref. 48). The information
contained in the rele-
vant (patent) literature combined with the results of our own
experiments led
to the following conclusions. The nitric acid oxidation of
cyclohexanol to AA0
can proceed via several different routes, involving different
intermediates,
dependent on the applied reaction conditions. Most authors,
having studied the
industrial process effected under standard reaction conditions,
i.e. a high
temperature in the oxidation steps (60 - 100 "C) and a high HN02
concentration
(> 55 %), agree on the issue of the required high selectivity
(2 95 %). How-
ever, they also report an undesirably high HNOS consumption due
to the for-
mation of N20 (route I of Fig. 1). These findings correspond to
a mechanism
-
involving the AMNA intermediate, in which V5+ does not play a
specific role as
it affects neither the reaction rate nor the selectivity.
Instead, Cu2+ plays
an important part in this reaction mechanism as it blocks the
formation of
radicals, which lead to degradation.
The reaction can also proceed via different routes and numerous
different
oxidation intermediates, dependent on the reaction conditions
chosen. The
number of moles of HN03 which interact with the organic reagent
varies in
accordance with the chemical properties of the nitric acid
solutions (concen-
tration effect); in the absence of a catalyst, the direct
reaction of the
acid leads mainly to the formation of nitrogen compounds that
cannot be re-
dxidized (N2 and N20), whereas V 5+-catalysed oxidation can lead
to useful
(regenerable) nitrogen oxides.
Whereas the recovery of useful nitrogen-containing products (NO,
N02) is
virtually nil in the case of the normal AMNA route (I of Fig.
l), a significant
amount of NO, can be regenerated in routes involving alternative
or additional
intermediates. This is expressed by the following two limiting
(theoretical)
equations:
OH
+ 2HN0, AMNA +- C
COOH
COOH +N20+2H,0
[451
OH various COOH
+ 2.7 HN03 vs+
C COOH + 2.7 NO+2.4 H,O
1461
In eq. [45] the consumption is 2 moles of HN03/mole of AAD (no
NO, recovery),
based on a specific consumption of 0.863 kg of HN03/kg of AAD
for 100 % selec-
tivity. In eq. [46] the specific HN03 consumption is zero. Such
extreme pro-
cesses are not described in the literature. In practice, the
reaction proceeds
at intermediate selectivity and HNO3 consumption values.
In process II of Fig. 1 the formation of AAD from AMNA is
accompanied by the
formation of the more desirable NO but this is a difficult route
in technical
terms because it means maintaining high HN03 concentrations
throughout the pro-
cess. Economic use of HNO3 is also realized in route III; this
route, which is
based on the nitrating effect of the acid, requires a 100 % HN03
concentration.
-
From a technical point of view routes IV and V seem to yield a
good compro-
mise: the first step, consisting of non-catalysed
low-temperature oxidation
(up to the diketone), requires a high HN03 concentration, while
the second,
which is effected at a higher temperature (in the presence of
diluted HN03), is
catalysed by V5+. The amount of V5+ required must be accurately
determined
because V5+ serves as an oxidant, minimizes AMNA formation and
favours ring
opening, the selectivity and the kinetics of the reaction. The
temperature and
HN03 concentration are critical reaction parameters.
The procedure of ref. 62 specifies conditions that lead to the
lowest
possible consumption at maximum selectivity. Yields of 96 - 97 %
can be
realized at a consumption of 0.6 - 0.65 kg of HN03/kg of AAD.
These results
show that even in this case AAD is formed partly via the
(pre)dione/a-diketone
and partly via the AMNA intermediate.
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