Oxidation of amines over alumina based catalysts

Article abstract of DOI:10.1016/j.apcata.2010.01.040

Amines were oxidized by molecular oxygen in the vapor phase at atmospheric pressure over alumina and silicotungstic acid/alumina catalysts. The study is focused on the influence of structure of amine and catalyst properties on the composition of the main reaction products and byproducts. Coating of γ-Al₂O₃ with silicotungstic acid or its semisalt can significantly enhance its catalytic activity in amine oxidation. The adsorption of amine on weak acidic sites of catalyst is essential for its oxidation to main reaction products. Cycloalkylamines are oxidized mainly to cyclic oximes (selectivity up to 64%) and Schiff bases of appropriate cycloalkanone and cycloalkylamine (selectivity up to 38%). Mainly nitriles (selectivity up to 55%) and appropriate Schiff bases (selectivity up to 54%) were observed in the oxidation products of primary alkylamines. Their molar ratio depends on the catalyst acidity and reaction conditions. 1,6-Hexanediamine is oxidized mainly to caprolactam (yield 48%) and other cyclic lactames and Schiff bases as well as to dinitrile (yield 13%).

Full text of DOI:10.1016/j.apcata.2010.01.040

Applied Catalysis A: General 378 (2010) 33–41
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Oxidation of amines over alumina based catalysts
*
´ˇ
Karol Rakottyay , Alexander Kaszonyi, Stanislav Vajıcek
Department of Organic Technology, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinske´ho 9, 81237 Bratislava, Slovakia
A R T I C L E I N F O
A B S T R A C T
Article history:
Amines were oxidized by molecular oxygen in the vapor phase at atmospheric pressure over alumina
and silicotungstic acid/alumina catalysts. The study is focused on the influence of structure of amine and
Received 26 November 2009
Received in revised form 26 January 2010
Accepted 28 January 2010
Available online 6 February 2010
catalyst properties on the composition of the main reaction products and byproducts. Coating of g-Al2O3
with silicotungstic acid or its semisalt can significantly enhance its catalytic activity in amine oxidation.
The adsorption of amine on weak acidic sites of catalyst is essential for its oxidation to main reaction
products. Cycloalkylamines are oxidized mainly to cyclic oximes (selectivity up to 64%) and Schiff bases
of appropriate cycloalkanone and cycloalkylamine (selectivity up to 38%). Mainly nitriles (selectivity up
to 55%) and appropriate Schiff bases (selectivity up to 54%) were observed in the oxidation products of
primary alkylamines. Their molar ratio depends on the catalyst acidity and reaction conditions. 1,6-
Hexanediamine is oxidized mainly to caprolactam (yield 48%) and other cyclic lactames and Schiff bases
as well as to dinitrile (yield 13%).
Keywords:
Alumina
Cycloalkylamine
Primary aliphatic amine
Isoalkylamine
Oxidation
Acidic sites determination
NH3-TPD
ß 2010 Elsevier B.V. All rights reserved.
FT-IR
Catalyst titration
Caprolactam
Oximes
Nitriles
Schiff bases
Carbonyl compounds
Condensation reactions
Reaction pathway
1. Introduction
primary amines to nitroso derivatives. Very high amine conversion
(up to 100%) and selectivity for nitroso compounds (99%) have
The oxidation of amines is a fundamental reaction for the
synthesis of o-containing amine derivatives and both industry and
academia have paid considerable attention to them. Depending on
amine, catalytic system, reaction conditions and oxidizing agent
different oxidation products are formed. Catalytic system for
selective aerobic oxidation of amines to imines or nitriles involves
copper(I) or copper(II) chloride as catalyst, toluene as solvent, and
molecular sieve 3A as dehydrating agent under atmospheric
pressure of oxygen [1]. Using this system variety of amines can be
oxidized by molecular oxygen to corresponding nitriles (up to 97%
yield; TON up to 60 from primary amines) or to imines (up to 90%
yield; TON up to 45 from secondary amines) [1]. Lu and Xi oxidized
primary aromatic amines by air to azo derivatives, anils, and/or
quinone anils over Cu-chloride catalyst [2].
been obtained using 30% hydrogen peroxide as an oxidant. The
oxo–peroxo Mo(VI) complex was also very active in the oxidation
of various substituted primary aromatic amines with electron
donating as well as electron withdrawing substituents on the
aromatic ring [3].
Bifunctional gold–titania catalytic system was reported by
Klitgaard et al. [4] to facilitate the oxidation of amines by molecular
oxygen into amides with high selectivity. High catalytic activity of
Ru/Al2O3 catalyst was observed at the oxidation of amines by
molecular oxygen in liquid phase at atmospheric conditions [5].
Catalysis with heteropoly compounds has been widely studied
in recent decades. For their acidic and redox properties they can be
used as catalysts in gas–solid as well as in liquid–solid reactions.
The oxidations of anilines with 30% H2O2 catalyzed by Preyssler
catalysts (H14[NaP5W30O110]) in various solvents with different
molar ratio of amine to oxidant, have been investigated by
Bamoharram et al. [6]. It has been recognized that a major
drawback of used heteropolyacid is its low surface area (1–10 m2/
g), separation problem from the reaction mixture and low thermal
stability when applied at high temperature [7]. Although the
The molybdenum acetylide oxo–peroxo complex obtained in
situ was used as an efficient catalyst for selective N-oxidation of
* Corresponding author.
E-mail address: karol.rakottyay@stuba.sk (K. Rakottyay).
0926-860X/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.01.040

34
K. Rakottyay et al. / Applied Catalysis A: General 378 (2010) 33–41
thermal stability depends on the structure and composition of
heteropolyacid [8], heteropolyacids can get high surface area and
also thermal stability by supporting them onto suitable support.
Recently a facile, efficient and substrate-selective oxidation of
Modified titration method [14] was also used to compare the
acidity of solid catalyst. In a blank experiment 50 ml of acetonitrile
was titrated with titrating agent (0.1 M ethylamine in acetonitrile)
to record the baseline of titration. In normal titrations 1 g of
catalyst sample and 0.05 ml of titrating agent were added to 50 ml
of acetonitrile. After agitation for 0.5 h and stabilization of
apparent pH the suspension was titrated with mentioned solution
of ethylamine. The titrating agent was added regularly every 5 min
in 0.5 ml doses and the pH was recorded just before adding next
dose, i.e. at the end of time interval. The change in pH during last
minute of time intervals was less than Æ0.01 pH. Hannah HI223 pH
meter and combined glass/SCE electrode were used for measuring pH.
Before each titration the pH meter was adjusted by buffer solution
(pH 7.04) and after adjustment the electrode was rinsed by distilled
water.
benzylamine with NaClO as oxidant catalyzed by
-CD) was reported [9]. Primary amines, which could form host-
guest complexes with -cyclodextrin were oxidized to nitriles
b-cyclodextrin
(
b
b
with excellent yields (98%) at ambient temperature (50 8C).
The oxidation of cyclohexylamine by H2O2 or molecular oxygen
produces mainly cyclohexanone oxime and N-cyclohexylidene-
cyclohexylamine, the condensation product of cyclohexanone and
cyclohexylamine [10,11]. Cyclohexanone oxime is also formed by
ammoxidation of cyclohexanone by H2O2 [12]. In both oxidations
the catalytic system contains alumina or silica supported hetero-
polymetalates of tungsten.
In the present work we have investigated vapor phase oxidation
of amines by molecular oxygen (air) as an oxidizing agent at
atmospheric pressure over alumina and silicotungstic acid
containing catalysts. This process with cheap oxidizing agent
without solvent and inorganic byproducts allows simple separa-
tion of the used heterogeneous catalyst and unused components of
oxidizing agent after condensation of reaction products. The
influence of silicotungstic acid (STA) and its sodium salt on
catalytic activity and selectivity of alumina in the oxidation of
various amines with molecular oxygen was also evaluated and
compared.
Shimadzu IRAffinity-1 FT-IR equipment with Specac Golden
Gate ATR accessory was used to obtain the FT-IR spectra of
catalysts in a wave number range of 4000–600 cmÀ1
.
2.3. Catalytic test
All chemicals were purchased from commercial source (Fluka)
and used without further purification.
The oxidation process of amines was performed in a fixed bed
glass reactor with 1.6 cm inner diameter which contains 2 g of
catalyst. The space velocity of amines was 0.7 hÀ1. The gas leaving
the reactor was passed through a cool trap in order to condense the
formed products. The oxidation was carried out by air with
increased content of oxygen (32 vol.%) at atmospheric pressure
and at temperature range of 180–185 8C. The flow rate of air was
2. Experimental
2.1. Catalysts preparation
24 cm3 minÀ1
.
All catalysts were made from
g
-Al2O3 (Alfa Aesar,
The reactions were monitored during first 6 h of time on stream
of a given catalyst. The samples of the reaction mixture were
withdrawn from the reactor periodically in 1 h intervals and were
analyzed on Shimadzu GC 2014 equipped with FID and CP-SIL 5CB
SBET = 211 m2 gÀ1, Vp = 0.6 cm3 gÀ1, acidity = 0.7 mmol gÀ1). 0.3 g
of polyoxomatelate (NanH4Àn[Si(W3O10)4], n = 0 for silicotungstic
acid and n = 2 for its sodium salt, Lachema Brno) was dissolved in
100 ml of deionised water. Then 2 g of alumina was added and
mixed for 1 h at ambient temperature. After impregnation all
catalysts were dried at 110 8C for 8 h and calcined with a ramp 4 8C/
min for 2 h, then kept at constant temperature 500 8C for 1 h.
capillary column (25 m  0.53 mm  1.0
mm). The used tempera-
ture program was: 60 8C (3 min), then ramp 9 8C minÀ1 up to
240 8C. For quantitative analysis the method of the external
standard was used.
The reaction products were identified on GC/MS QP5000
(Shimadzu) with EI and capillary column (HP-1, 50 m  0.2 mm
2.2. Catalysts characterization
 0.33
mm), carrier gas was helium (1 ml/min); temperature
The BET surface areas and pore volumes were determined from
nitrogen adsorption–desorption isotherm data obtained at
À196 8C on a Micromeritics ASAP 2020 apparatus. The pore
volumes were determined at relative pressure (p/p0) of 0.99. The
samples were degassed at p = 2 Pa, T = 350 8C for 12 h before
measurement. The pore size distributions in samples were
determined by BJH method, calculated assuming cylindrical pore
model [13].
program: from 60 8C with ramp 9 8C minÀ1 up to 240 8C.
3. Results and discussion
3.1. Properties of modified alumina
The textural properties of commercial
g-Al2O3, alumina
The total amount of acidic sites of the catalyst was determined
by temperature programmed desorption of ammonia (NH3-TPD).
The eventually adsorbed compounds were desorbed or decom-
posed from the surface of all catalysts in He just before saturation
by NH3 with the same temperature program as during NH3-TPD.
Thus about 0.1 g of the sample was initially flushed with He (flow
rate 24 cm3 minÀ1) at temperature 600 8C for 1 h to eliminate
adsorbed impurities. Then it was cooled to 100 8C under He and
saturated with NH3. After NH3 exposure the sample was purged
with He during 12 h at 100 8C to remove the not chemisorbed
initial excess of NH3. The sample with stabilized amount of
modified by 15 wt% of silicotungstic heteropolyacid or its sodium
salt (catalysts: C, CH and CHN, respectively) are shown in Table 1.
The surface area, pore volume and pore diameter of the calcinÀe1d
3
alumina are found to be 211 m2 gÀ1, 84 A and 0.6 cm g
,
˚
respectively. After loading of 15 wt% of heteropolyacid (HPA) on
the surface of alumina these properties moderately decrease. There
is a further very mild decrease of mentioned properties after partial
neutralization of silicotungstic acid by NaOH.
The pore size distribution curves of all catalysts are narrow,
symmetric and centered around 4.1 nm (Fig. 1). From the
mentioned results follows that the pores of alumina are not
clogged by HPA or its Na salt. Moreover, the contribution of HPA to
BET surface and pore volume is positive, because the content of
alumina in alumina-supported HPA is reduced by 15 wt%,
however, the BET surface and pore volume only by 5.7% and
6.7%, in the case of HPA salt by 7.5% and 10%, respectively.
ammonia was heated from 100 8C to 600 8C by
a ramp
10 8C minÀ1and kept at 600 8C for 1 h. To monitor the effluent
content of ammonia thermal conductivity detector was
employed. The measured values were recorded as a function of
temperature.

K. Rakottyay et al. / Applied Catalysis A: General 378 (2010) 33–41
35
Table 1
Textural properties and acidity of tested catalysts.
Catalyst
BET surface
area (m2 gÀ1
Pore
Pore
Amount of
)
diameter
volume
(cm3 gÀ1
adsorbed NH3
(mmol gÀ1
)
˚
(A)
)
C
211
199
195
82
84
70
0.60
0.56
0.54
0.71
1.18
0.75
CH
CHN
TPD-NH3 curves of tested catalysts are shown in Fig. 2 and the
total amounts of chemisorbed ammonia (acidity of catalyst)
derived from them are in Table 1. For commercial alumina three
desorption peaks of ammonia centered at 223 8C, 355 8C and 551 8C
were found. In the case of catalyst containing 15 wt% of HPA
sodium salt one broad desorption peak of ammonia centered at
229 8C followed by a small peak at 600 8C was observed. The
highest peaks were found in the case of alumina-supported HPA.
The first peak is centered at 227 8C and the second broad one
around 300 8C. In the TPD-NH3 curves of tested catalysts the first
desorption peak around 220 8C belongs to sites with the weakest
acidity on the boundary of physisorbed and chemisorbed
ammonia. HPA, supported on alumina increased the intensity of
the first peak and of broad peak between 250 8C and 450 8C. Partial
neutralization of HPA reduced this broad peak, which belongs to
sites with mild acidity.
Fig. 3. Titration curves of catalysts by 0.1 M ethylamine in acetonitrile: (C)
alumina (Alfa Aesar); (CH) -alumina-supported HPA (15 wt% H4[Si(W3O10)4]);
(CHN) -alumina-supported neutralized HPA (15 wt% Na2H2[Si(W3O10)4]).
g-
g
g
the equilibrium between the hydrated surface of glass electrode
and amount of ethylamine was checked and recorded as a
dependence of apparent pH on the amount of added ethylamine
to 50 ml of acetonitrile. Acetonitrile was chosen as a solvent,
because solvents with active proton can react with ethylamine or
surface of alumina. For example, at the presence of water
gelatinous alumina is formed after addition of ethylamine at
higher pH. Before each titration the used glass electrode was rinsed
by distilled water for cleaning and rehydration of its surface. After
addition of 1 g of a given catalyst increased acidity of the formed
The dependence of pH on the amount of titrant (ethylamine) in
the suspension of catalysts is shown in Fig. 3. In the blank titration
suspension (containing also 5
mmol of ethylamine in 50 ml of
acetonitrile) was observed. The measured apparent pH decreased
as a result of new protolytic equilibrium between acidic centers of
catalyst, ethylamine and surface of glass electrode. In this initial
step of the titration of catalysts C, CH and CHN apparent initial pH
values equal to 7.7, 6.7 and 6.1, respectively, were measured. Thus
the highest initial acidity was achieved by partially neutralized
heteropolyacid. It is known, that in same partially neutralized HPA
the acidic strength of protons can be higher as in HPA [7,15]. To get
the same apparent pH value as in the blank titration at the
presence of
5 mmol of ethylamine in 50 ml of acetonitrile
(0.1 mmol(EA) dmÀ3, app. pH 9.55) it was necessary to add to
the suspension of catalyst C, CH and CHN 0.67 mmol, 1.6 mmol and
1.44 mmol of extra ethylamine, respectively. The first derivative of
titration curves of catalyst shows maximum (equivalent point of
titration) at pH around 10. The apparent pH 10 in the blank
titration was achieved after addition of 0.2 mmol ethylamine
(4 mmol(EA) dmÀ3). To get the same pH in the titration of catalysts
C, CH and CHN an extra ethylamine is necessary in the amount of
0.85 mmol, 2.95 mmol and 2.15 mmol, respectively. The above
mentioned amounts of extra ethylamine may be interpreted as its
adsorbed amount on the surface of 1 g of catalyst C, CH and CHN at
concentration of free ethylamine 0.1 mmol dmÀ3 (apparent pH
9.55) or at 4 mmol dmÀ3 (apparent pH 10), respectively. The
presence of sites with very low acidity (equivalent point around
apparent pH 10) is the reason, why in the TPD-NH3 experiment is
not possible to distinguish between physisorbed and chemisorbed
ammonia. The total amount of H+ in catalyst CH and CHN
originated from the used amount of HPA should be 0.2 mmol and
0.1 mmol, respectively. Because the differences between adsorbed
amounts of ethylamine by catalyst C, CH and CHN are higher, HPA
is decomposed on the surface of alumina to components with
weaker acidity which can adsorb more ethylamine (have higher
total acidity).
Fig. 1. Pore size distributions determined by BJH method: (C)
Aesar); (CH) -alumina-supported HPA (15 wt% H4[Si(W3O10)4]); (CHN)
alumina-supported neutralized HPA (15 wt% Na2H2[Si(W3O10)4]).
g-alumina (Alfa
g
g-
Fig. 2. NH3-TPD profiles of
g
-alumina: (a) without heteropolyacid; (b)
g-alumina-
Fig. 4 shows the FT-IR spectra of non-calcined and calcined
alumina-supported silicotungstic acid. The tungsten oxygen bonds
supported neutralized HPA (15 wt% Na2H2[Si(W3O10)4]); (c)
HPA (15 wt% H4[Si(W3O10)4]).
g-alumina-supported

36
K. Rakottyay et al. / Applied Catalysis A: General 378 (2010) 33–41
Table 2
Conversion of cycloalkylamines and selectivity of formation of appropriate oxime
and Schiff base over -alumina: (C) without heteropolyacid (HPA); (CH) -alumina-
supported HPA (15 wt% H4[Si(W3O10)4]); (CHN) -alumina-supported neutralized
g
g
g
HPA (15 wt% Na2H2[Si(W3O10)4]). Reaction conditions: T = 180 8C, atmospheric
pressure, results at 2 h time on stream.
Amine
Catalyst
Conversion (%)
Selectivity (%)
Oxime
Schiff base
Cyclopentylamine
C
36
42
54
21
30
37
35
32
38
CH
CHN
Cyclohexylamine
Cyclooctylamine
C
21
26
30
54
64
63
30
20
23
CH
CHN
C
19
23
58
64
12
13
CH
both amines bands around 2920, 2850, 1450, 1260, 840 and
780 cmÀ1 are assigned to –CH2– groups. The IR spectrum of N-
octylamine contains also bands assigned to –CH3 group and the
spectrum of cyclohexylamine bands assigned to –CH– group and
ring deformation. The same bands nearly at the same wave
numbers were found in the spectra of used catalysts, which beside
mentioned bands and broad bands of alumina contain also
unknown bands. In the region, where N–H bonds of amines
absorb, the spectra of deposits and amines are different. In the
spectra of both amines, bands around 3330, 3250, 1575 and
1035 cmÀ1 assigned to –NH2 group and –C–N bond were found.
The bands around 3300 cmÀ1 in the spectrum of catalyst used for
cyclohexylamine oxidation are overlapped by a broad band
assigned to the amine adsorbed on acidic centers by different
acidity. This broad band in the spectrum of catalyst used for N-
octylamine oxidation is superimposed by band with wave number
3298 cmÀ1, which probably belong to secondary amine (for
example, di-octylamine). We could not recognize differences in
the FT-IR spectra of used alumina and used alumina-supported STA
catalyst.
Fig. 4. FT-IR spectra of (a)
g-alumina (Alfa Aeser); other spectra: g-alumina-
supported HPA calcined at 500 8C, HPA content: (b) 5 wt%; (c) 15 wt%; (d) 23 wt%;
(f) 33 wt%; (g) 40 wt%; (i) 50 wt%; spectra measured before calcination; HPA
content: (e) 33 wt%; (h) 40 wt%; (j) 50 wt%; (k) HPA mechanically mixed with
alumina (1:2).
g-
(W–Oe–W, W–Oc–W, W55Ot), and Si–O bond for pure hetero-
polyacid or mechanical mixture with alumina appear at 730, 906,
976 and 1016 cmÀ1, respectively, which are the fingerprints of STA
in the region between 500 and 1600 cmÀ1 [14–16]. These bands are
recognizable only before calcination and at concentration of STA
over 30% (lines e, h and j, Fig. 4). The intensity of peaks is very low
in comparison with peaks in the mechanical mixture containing
33% of STA (line k, Fig. 4). According to Mizuno and Misono,
alumina tends to decompose STA [15]. Thus, the bonds W–O–W,
W55O and Si–O are reorganized, but part of silicotungstic acid at
very high concentrations retains the Keggin structure before
calcination. During calcination new bonds are formed between
alumina and tungsten which are not optically active at the
measured region of FT-IR (4000–600 cmÀ1). The formed surface
compounds significantly increased the amount of active sites with
mild acidity. FT-IR spectra of original alumina and of alumina-
supported STA with content of STA 5, 15 and 23 wt% were the same
before and after calcination, i.e. without characteristic bands of
STA.
Next minor byproducts of studied oxidation are specific for
oxidized cycloalkylamine. The oxidation of cyclopentylamine
3.2. Oxidation of cycloalkylamines
The oxidation of amines was carried out at temperature 180–
185 8C, which was found as optimal in our previous works [10,11]
for the oxidation of cyclohexylamine. The main products of the
oxidation of cycloalkylamines (C5, C6 and C8) were oximes and N-
cycloalkylidene-cycloalkylamines (Table 2). The last compound is
a condensation product (Schiff base) of cycloalkanones and
cycloalkylamines. Between minor byproducts we found appropri-
ate nitrocycloalkane, bis-cycloalkyl-amine and cycloalkene. On the
surface of all used catalyst carbonaceous deposits were formed
with initial dark yellow color, which was gradually changed by
time on stream through dark brown up to black. The rate of
deposits formation was increasing by increasing molecular weight
of oxidized amine. Part of deposits was soluble in alcohols. All
deposits could be removed in the presence of molecular oxygen at
temperatures over 500 8C.
Fig. 5. FT-IR spectra of (a)
from oxidation of N-octylamine, (c) pure N-octylamine, (d) pure cyclohexylamine
and (e) used -alumina (Alfa Aesar) from oxidation of cyclohexylamine.
g-alumina (Alfa Aesar), (b) used g-alumina (Alfa Aesar)
The FT-IR spectra of catalyst with mentioned deposits (Fig. 5)
contain bands which are typical for alkyl- and cycloalkylamines. In
g

K. Rakottyay et al. / Applied Catalysis A: General 378 (2010) 33–41
37
Scheme 1. Products of cyclopentylamine oxidation over catalyst C, CH and CHN.
Fig. 7. Selectivity of cyclopentanone oxime formation during oxidation of
cyclopentylamine over -alumina: (C) without heteropolyacid; (CH) -alumina-
supported HPA (15 wt% H4[Si(W3O10)4]); (CHN) -alumina-supported neutralized
g
g
g
HPA (15 wt% Na2H2[Si(W3O10)4]) [T = 180 8C at atmospheric pressure].
produced a small amount of pyridine (up to 7 wt%). This product of
ring isomerization and aromatization was not found in the
oxidation products of other amines. Benzene and N-cyclohexylani-
line were found in the oxidation product of cyclohexylamine
(yields <0.5%, respectively, <2%), i.e. cyclohexene and N-cyclo-
hexylidene-cyclohexylamine were partially dehydrogenated and
aromatized. The driving force of formation of mentioned aromatic
compounds is most probably the energy of aromatization. In the
reaction products of cyclooctylamine we did not found products of
Fig. 6. Conversion of cyclopentylamine during its oxidation over
without heteropolyacid; (CH) -alumina-supported HPA (15 wt% H4[Si(W3O10)4]);
(CHN) -alumina-supported neutralized HPA (15 wt% Na2H2[Si(W3O10)4])
[T = 180 8C at atmospheric pressure].
g-alumina: (C)
g
g
Fig. 8. Conversion of N-alkylamines and selectivity of formation of their main reaction products over
g-alumina (Alfa Aeser): (C) without heteropolyacid; (CH) g-alumina-
supported HPA (15 wt% H4[Si(W3O10)4]); (CHN) -alumina-supported neutralized HPA (15 wt% Na2H2[Si(W3O10)4]) [T = 180 8C at atmospheric pressure].
g

38
K. Rakottyay et al. / Applied Catalysis A: General 378 (2010) 33–41
Fig. 9. Selectivity of formation of main products of oxidation of N-alkylamines and isopentylamine over
g-alumina (Alfa Aesar): (C) without heteropolyacid; (CH) g-alumina-
supported HPA (15 wt% H4[Si(W3O10)4]); (CHN) -alumina-supported neutralized HPA (15 wt% Na2H2[Si(W3O10)4]) [T = 180 8C at atmospheric pressure].
g
aromatization. On the other side, we found products of potential
hydrogenation, for example, in the case of cyclohexylamine we
found cyclohexanol [11]. In all cases we found bis-cycloalkyl-
amine (yields <1%), which can be formed by hydrogenation of
appropriate Schiff base or by alkylation of amine by cycloalkene
(Scheme 1).
Generally the conversion of cycloalkylamines during first 2 h
of time on stream was decreasing by increasing molecular
weight. In the same direction the selectivity of oxime formation
increased and the selectivity of Schiff base formation decreased
(Table 2). After impregnation of alumina by HPA or its
semisodiumsalt the conversion of cycloalkylamines and selec-
tivity of oxime formation mostly increased or stayed on the
same level within experimental error. Carbonaceous deposits
formed during first 2–3 h of time on stream deactivate the
catalyst and the observed conversions of cycloalkylamines after
a maximum began to decrease by increasing time on stream
(Figs. 6 and 7).
Schiff bases and amides with original length of alkyl chain, also
series of Schiff bases and amides with shortened length of alkyl
chain were found in low concentrations. The concentration of
mentioned Schiff bases and amides gradually decreases by
decreasing length of alkyl chain. However, during oxidation of
N-octylamine almost the same amount of N-hexylideneoctylamine
and N-octylideneoctylamine was formed. The selectivity of N-
octylideneoctylamine was significantly lower in comparison with
full length Schiff bases formed from N-pentyl-, isopentyl- and N-
hexylamine (Fig. 9). The alkyl chains were shortened only in the
part of molecules originated from the aldehydes and acids, i.e. the
shortening of alkyl chain was a result of oxidative decomposition
of aldehydes. In these cyclic reactions aldehyde is oxidized to acid.
The acid is decarboxylated to alkyl radical with shortened alkyl
chain, which is quickly oxidized by molecular oxygen to aldehyde
(Scheme 2).
The influence of STA and its semisalt on the conversion of
pentylamine is the same as in the case of cycloakylamines. STA and
its semisalt have practically no influence on the conversion of
hexylamine. In the case of octylamine STA significantly enhanced
its conversion. The influence of semisalt was mild.
3.3. Oxidation of N-alkylamines
During oxidation of linear aliphatic amines (N-pentyl, N-hexyl,
and N-octylamine) over catalysts C, CH and CHN in the gas phase
the main oxidation products were nitriles (selectivity up to 55%)
and Schiff bases (selectivity up to 56%, Figs. 8 and 9).
Oximes, aldehydes, nitro compounds and alkyl amides were
also observed in the reaction products, as minor byproducts
(selectivity <5%). In the reaction products besides dominating
The formation of nitriles is preferred and the formation of Schiff
bases was suppressed by semisalt of STA at all linear amines. The
influence of STA on the formation of nitriles is not so clear. In the
case of N-pentylamine had negative; in the case of N-hexyl- and N-
octylamine had mostly positive effect. STA and its semisalt have
positive effect on the formation of Schiff base in the case of
isopentylamine.

K. Rakottyay et al. / Applied Catalysis A: General 378 (2010) 33–41
39
Scheme 2. Products of N-hexylamine oxidation over catalyst C, CH and CHN.
Fig. 11. Results of oxidation of 1,6-hexanediamine over
g
-alumina at different
temperatures: conversion of 1,6-hexanediamine, selectivity of (1) e-caprolactam
formation and (2) adiponitrile formation, respectively. Reaction condition:
atmospheric pressure, results at 3 h time on stream.
oxidation process the amount of adiponitrile is higher, however, by
Fig. 10. Conversion of isopentylamine during its oxidation over
Aesar): (C) without heteropolyacid; (CH) -alumina-supported HPA (15 wt%
H4[Si(W3O10)4]); (CHN) -alumina-supported neutralized HPA (15 wt%
g-alumina (Alfa
increasing of time on stream the selectivity to
e-caprolactam
g
g
increases. After 6 h of time on stream the activity of catalyst
decreased and the conversion reached only 10%. One of the reasons
can be the enhancement of carbonaceous tar formation on the
Na2H2[Si(W3O10)4]) [T = 180 8C at atmospheric pressure].
catalyst surface. Besides main reaction products (e-caprolactam and
Oxime was found only in the case of N-pentylamine and N-
hexylamine on the levels up to 5 wt%. The influence of STA and its
semisalt in this case is mostly in the range of experimental error.
hexanedinitrile), several minor reaction byproducts were found:
butyrolactam, valerolactam, cyclicSchiffbaseswith6, 5and 4carbon
in the ring, pentanedinitrile, pyridine, tetrahydropyridine and 5-
cyano-valeric acid methyl ester (methyl-5-cyanopentanoate).
3.4. Oxidation of isoalkylamines
The oxidation of isobutyl- and isopentylamine also produces
high amount of Schiff base (selectivity up to 65%). In the reaction
products N-isopentylformamide and N-isopentylacetamide were
observed as a minor byproduct, too. As shown in Fig. 10, conversion
of isopentylamine was the highest on alumina (maximum at 30%).
Modification by STA and its semisalt had negative effect on the
formation of Schiff base.
The isobutylamine was more resistant to the selective oxidation
than isopentylamine. Reaction products contained only traces of
nitriles and imines.
3.5. Oxidation of 1,6-hexanediamine
From the group of diamino-compounds we oxidized 1,6-
hexanediamine. Specially, we studied the possibility of
tam production in the gas phase. Klitgaard et al. [4] reported that
e-caprolac-
e
-
caprolactam is formed by oxidation of 1,6-hexanediamine by
molecular oxygen in a liquid phase over gold-titania catalyst. For
the reaction we used 1 M of 1,6-hexanediamine in methanol, which
isliquidandwewereabletooxidizeitatthesameconditionsasother
amines, i.e. at atmospheric pressure and at different temperatures
(170–230 8C). The results of these experiments are shown in Figs. 11
and 12. It is seen that by increasing temperature the conversion of
1,6-hexanediamine increases of up to 85%, but the formation of
Fig. 12. Results of oxidation of 1,6-hexanediamine over catalysts
Aesar): (C) without heteropolyacid; (CH) -alumina-supported HPA (15 wt%
H4[Si(W3O10)4]); (CHN) -alumina-supported neutralized HPA (15 wt%
Na2H2[Si(W3O10)4]). Conversion of 1,6-hexanediamine, selectivity of
g-alumina (Alfa
g
g
byproducts also increases. The highest selectivity to e-caprolactam
e-
(45%) was observed at lower temperatures (170–180 8C). Moreover,
the same trend was observed in the case of the selectivity to
adiponitrile, which is the main byproduct. At the beginning of the
caprolactam formation (1) and selectivity of adiponitrile formation (2),
respectively. Reaction conditions: atmospheric pressure, temperature range 180–
185 8C, results at 3 h time on stream.

40
K. Rakottyay et al. / Applied Catalysis A: General 378 (2010) 33–41
Scheme 3. Reaction pathways of amines oxidation.
TheresultsoftheinfluenceofcatalystacidityareshowninFig. 12.
1,6-hexanediamine decreased to 32%. On the other hand the
selectivity of adiponitrile formation increased three times (up to
23%) in comparison to pure alumina.
The acidity of catalyst in the oxidation of amine plays an important
role [10,11]. The best results of 1,6-hexanediamine oxidation were
reached over pure alumina. Conversion of 1,6-hexanediamine
reached 75%. The observed selectivity of
increased up to 52%, but the selectivity of adiponitrile stayed at low
level (7%). The alumina-supported HPA (15 wt%) had negative effect
e
-caprolactam formation
3.6. Reaction pathway of oxidation of amines in gas phase over
alumina and alumina partially covered by STA or its semisalt
to both, conversion of 1,6-hexanediamine and selectivity of
caprolactam formation. Moreover, the alumina-supported HPA
semisalt (15 wt%) had the worst catalytic activity. The conversion of
e
-
From the composition of oxidation products of different amines
and acidity measurements of catalyst follows the next set of
processes and reactions:

K. Rakottyay et al. / Applied Catalysis A: General 378 (2010) 33–41
41
1. Amine should be adsorbed on the acidic surface before
oxidation. (Amine was not oxidized in the gas phase and
overneutralized or basic alumina, where amine was practically
not adsorbed [11].) The acidity of protons of adsorbed amine
was higher in comparison with free amines, i.e. the autopro-
tolytic equilibrium of amines at the presence of acidic sites is
shifted to the right side (reactions (1)–(3)) (Scheme 3).
the oxidation products of practically all amines including
cycloalkylamines. In parallel by the formation of amides with
shortened alkyl chain also Schiff bases are formed by the
reaction of intermediate aldehydes with shortened alkyl chain
and original amine.
4. Alkenes as minor byproducts were formed by deamination. On
the strong acidic sites these alkenes could form dialkylamines by
the reaction with amines. These dialkylamines were found in the
reaction mixture of some amines. The formation of some minor
aromatic compounds (pyridine, cyclohexylaniline and benzene)
was powered by the energy of aromatization. Because this
aromatization was connected by dehydrogenation of interme-
diate products, the formed hydrogen could be consumed by
hydrogenation of other compounds, for example, hydrogenation
of cyclohexanone to cyclohexanol, Schiff base to dialkylamine,
cyclohexene to cyclohexane. These compounds were recognized
in some reaction products at low concentrations.
+
The increased concentration of R–NH3 was detected as
increased acidity of the solution of amine at the presence of
catalyst during titration of its slurry by amine in comparison with
acidity of blank solutions. If Ac (acidic centers) is a strongly acidic
proton, then the last step of equilibrium represents only an
exchange of proton between molecules of amine, practically
without further significant change. However, if amine is adsorbed
on a weak acid site, after exchange of proton the electron density
onnitrogenora-carbonofamineisincreasedtothelevelbetween
density of electrons on mentioned atoms in original amine and
RNHÀ ion. By increasing electron density the sensitivity of amine
to oxidation increases. On the other side, the formed R–NH3+ with
4. Conclusion
significantly lowered electron density on nitrogen and a-carbon
is practically insensitive to oxidation. Thus the protonized part of
amine isblockedagainstoxidation.However, anequivalentpartis
oxidized more easily. This effect, which depends on the acidity of
site, can increase the rate of oxidation of amine, i.e. to catalyze it.
It was found that modification of alumina by polyoxometalate
(NanH4Àn[Si(W3O10)4]ÁxH2O (n = 0 or 2)) had only little effect to the
specific surface and also the pore volume size of catalyst, but the
total acidity of catalyst markedly increased. This modification of
alumina markedly changes its activity and selectivity of formation
of main and byproducts in the oxidation of amines.
In the oxidation of cycloalkylamines the main products are
appropriate oximes and Schiff bases (formed by condensation of
cycloalkylamine and cycloalkanone). Between minor byproducts
appropriate nitrocycloalkane, bis-cycloalkyl-amine and cycloalk-
ene were found. Products of ring decomposition were not observed
in the reaction products.
During oxidation of aliphatic primary amines the main oxidation
products were nitriles and Schiff bases. Between minor byproducts
oximes, alkylamines and Schiff bases with shortened alkyl chain
were found. Isobutylamine was more resistant to the oxidation than
isopentylamine or linear amines. In the oxidation products of 1,6-
hexanediamine were identified the similar compounds, but in the
cyclic form, the aminic and aldehydic or acidic ends of intermediate
products form cyclic lactames or Schiff bases.
2. The autooxidation of amines in liquid phase is initiated on the a-
C–H group or N–H group [17]. We suppose that in the initiation
step of oxidation of adsorbed amine also two radicals are formed
(reaction (4)).
The formed radicals by the reaction with molecular oxygen
and extraction of hydrogen can form hydroperoxides (reactions
(5) and (6)). When instead of substance R–H the originally
formed radicals react, imines are formed, which can by easily
overoxidized to nitroso and nitro compounds. The nitroso
derivative is mostly isomerized to oxime. When R1 or R2 is
hydrogen, the imines are oxidized to nitriles (reactions (7) and
(8)). Oximes and carbonyl compounds can be also formed by
direct decomposition of hydroperoxides ((9) and (10)).
From the known experimental data is not clear, what is the
main route of the oxime formation. The most probable way is
through imine, because in the case of alkylamines, which have
two hydrogen atoms on the
product. Onthe otherside, inthecaseofcycloalkylaminewithone
hydrogen atom on -carbon, oxime belongs to the main products,
a-carbon, nitriles belong to the main
Acknowledgement
a
Financial support from the Slovak Grant Agency VEGA 1/0768/
08 is gratefully acknowledged.
because the formation of nitrile from imine is not possible. The
formation of oxime by the reaction of carbonyl compounds and
hydroxylamine is less probable because of high concentration of
free amine in the system, which by the reaction with carbonyl
compounds easily forms Schiff bases (reaction (11)). Schiff bases
belong to the main reaction products of all amines. An addition of
water to amine shifts this equilibrium to the left, but does not
change the total amount of formed carbonyl compound [10], i.e.
the eventual formation of carbonyl compounds by hydrolysis of
imines is not significant. In the case of diamines volatile cyclic
Schiff bases are formed. Probably the linear Schiff bases form part
of carbonaceous deposits on the surface of catalyst by oligomeri-
zation of double bond or in the case of diamines by repeated
condensation of carbonyl and –NH2 group.
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