A novel synthesis of tolunitriles by selective ammoxidation

Article abstract of DOI:10.1007/s11164-010-0209-3

A new approach to synthesize tolunitriles is reported. Tolunitriles can be prepared by selective ammoxidation of methylbenzyl chlorides prepared by chloromethylation of toluene. The total yields can reach 83% and the selectivity of tolunitriles is almost 100%. This approach provides a new path for preparing alkylbenzonitriles and other aromatic nitriles.

Full text of DOI:10.1007/s11164-010-0209-3

Res Chem Intermed (2010) 36:969–973
DOI 10.1007/s11164-010-0209-3
A novel synthesis of tolunitriles by selective
ammoxidation
•
Guangyong Xie Aiqing Zhang Chi Huang
•
Received: 24 July 2010 / Accepted: 22 October 2010 / Published online: 18 November 2010
Ó Springer Science+Business Media B.V. 2010
Abstract A new approach to synthesize tolunitriles is reported. Tolunitriles can be
prepared by selective ammoxidation of methylbenzyl chlorides prepared by chlo-
romethylation of toluene. The total yields can reach 83% and the selectivity of
tolunitriles is almost 100%. This approach provides a new path for preparing
alkylbenzonitriles and other aromatic nitriles.
Keywords Selective ammoxidation Á Tolunitriles Á Methylbenzyl chlorides Á
Chloromethylation
Introduction
Aromatic nitriles are important industrial chemicals and valuable synthetic
intermediates; the nitrile function can easily be converted into a large variety of
other functional groups such as amides, acids, esters, amines, etc., and can also
be used for further synthesis of many useful products [1–3]. Tolunitriles are an
important type of aromatic nitriles, from which many pharmaceuticals, dyes,
fluorescence whitener and other fine chemicals can be produced [4, 5].
Tolunitriles are traditionally synthesized from the dehydration of amide [6] or
aldoxime [7], cyanation of aryl chloride [8, 9] or of toluic acid [10, 11] and
diazotization of toluidine [12], etc. However, these methods usually require
G. Xie (&) Á A. Zhang
Key Laboratory of Catalysis and Materials Science of Hubei Province,
College of Chemistry and Materials Science, South-Central University for Nationalities,
Wuhan 430074, People’s Republic of China
e-mail: xxgyy@hotmail.com
C. Huang
College of Chemistry & Molecular Sciences, Wuhan University, Wuhan 430072,
People’s Republic of China
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G. Xie et al.
expensive starting materials or lead to serious pollution and are not suitable for
industrial production on a large scale. The heterogeneous catalytic ammoxidation of
methyl aromatics is a preferred method for the synthesis of some aromatic nitriles
[1–3, 13]. Tolunitriles can also be prepared by catalytic gas phase ammoxidation of
xylenes, but the yield and selectivity of tolunitriles are very low [14–17]. Angstadt
[14, 15] developed a type of new catalyst for the ammoxidation of p-xylene over
400 °C with an improved selectivity of p-tolunitrile to 64%; however, the
conversion of p-xylene was only 42% and the portion of dinitrile in the products
could reach 31%. In addition, the mononitrile and dinitrile are difficult to separate.
Tolunitriles can also be synthesized from liquid phase ammoxidation of xylenes.
The selectivity of tolunitriles was increased with a lower reaction temperature, but
the yields are very low and do not satisfy the required application [4, 5, 18].
It is well known that, in gas-phase ammoxidation of methylaromatics, the first
step, i.e., proton extraction from the methyl group to generate the benzyl species, is
the rate-determining step of the reaction. Because two methyl groups in xylenes are
identical, it is difficult for only one methyl to be converted to the cyano group at ca.
400 °C temperature. Alkylaromatics can also be ammoxidized to aromatic nitriles,
and two different alkyl groups existing on one benzene ring would have different
reactivity. For example, in the ammoxidation of cymene, the methyl and isopropyl
groups are all ammoxidized to the cyano groups generally, but due to the lower
dissociation energy of the hydrogen in a-C of the isopropyl group, the isopropyl
group is more active and would thus be more easily ammoxidized to the cyano
group first. So except for dinitriles, considerable tolunitriles would exist in products
[19]. We now report a new route to prepare tolunitriles from inexpensive toluene by
chloromethylation to methylbenzyl chlorides, which could then be ammoxidized
selectively to tolunitriles at a relatively low temperature, as shown in Scheme 1.
Chloromethylation of toluene gave o-methylbenzyl chloride and p-methylbenzyl
chloride (gas chromatography), which, because of the very close boiling point and
similar reactivity under the ammoxidation conditions, could not be separated, but
reacted as a mixture to afford the tolunitrile isomers, which can be separated by
fractional distillation. The higher reactivity of the chloromethyl group made it easier
to cleave [20] at relatively low temperatures leading to the high selectivity of
tolunitriles. The ammoxidation temperature of methylbenzyl chlorides is more than
100 °C lower compared with xylenes, and the selectivity for mononitriles can be
increased remarkably.
The most active and selective catalysts used for ammoxidation of methylaro-
matics and propane or propene are generally based on vanadium-containing
mixed oxides; for example, vanadium–phosphorus oxides (V–P–O) [21],
CH3
CH3
CH3
(HCHO)n, HCl
Catalyst
NH3, O2
Catalyst
CH2Cl
CN
Scheme 1 Synthesis of tolunitriles by selective ammoxidation
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A novel synthesis of tolunitriles
971
Table 1 The results of ammoxidation of methylbenzyl chlorides on different catalysts
Entry
Catalysts
Molar ratio of
metal atoms
Conversion
(%)
Yield of
tolunitriles (%)
Selectivity (%)
1
2
3
4
5
6
7
VO/SiO2
97
96
94
86
98
98
96
82
86
89
76
90
97
76
85
90
95
88
92
99
79
VPO/SiO2
V:P = 1:0.5
V:P = 1:1
V:P = 1:2
VCrO/SiO2
V:Cr = 1:0.5
V:Cr = 1:1
V:Cr = 1:2
Reaction conditions: temperature 220 °C, MBCs/Air/NH3 = 1/20/5(mol)
vanadium–titanium oxides (V–Ti–O) [22], vanadium–chromium oxides (V–Cr–O)
[23, 24], vanadium–molybdenum oxides (V–Mo–O) [25], vanadium–antimony
oxides (V–Sb–O) [26], vanadium–zirconium oxides (V–Zr–O) [27] or multi-
component mixed oxides [28]. We found that silica-supported vanadium–phospho-
rus [29] and vanadium–chromium [23, 24] mixed oxides show higher catalytic
activity and selectivity at a lower reaction temperature than the corresponding
unsupported vanadium-containing oxides for ammoxidation of methylaromatics.
They could also be used for ammoxidation of methylbenzyl chlorides at very low
temperatures as shown in Table 1. The silica-supported vanadium-containing oxides
were prepared according to our previous reports [23, 24, 29]. The loading contents
of V–P or V–Cr in catalysts are 10 wt%, in which the catalytic activity and
selectivity for ammoxidation of methylaromatics are highest [23, 29]. Different
catalysts have different catalytic behaviors. VCrO/SiO2 catalysts have better
catalytic performances than VPO/SiO2 or VO/SiO2 catalysts. When the molar ratio
of V and Cr is 1, the VCrO/SiO2 catalyst shows the best catalytic performance with
97% yield and 99% selectivity, respectively. By XPS, it was found that vanadium
exists mainly as pentavalent and chromium as trivalent in the V–Cr–O/SiO2
catalysts, and the X-ray diffraction (XRD) characterization showed that vanadium
and chromium form composite oxides and exist as amorphous phase in 10 wt.%
loadings.
Experimental
General
Reagents from which catalysts were prepared and toluene were analytically or
1
1
chemically pure. Products were characterized by H NMR and IR spectra. The H
NMR spectra were recorded on a Bruker AM-300 MHz spectrometer with
tetramethylsilane as an internal standard. The IR spectrum was measured on a
Nicolet Nexus 470 FT-IR spectrometer. The melting points were determined on a
X6 microscopic warming apparatus and are uncorrected. The purity of products was
determined with Shimadzu GC-17A gas chromatograph.
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Preparation of catalysts
The catalysts, vanadium-containing oxides supported on silica (u0.3–0.45 mm,
BET 210 m2/g), were prepared according to the literature [23, 29]. First, the support
S
was impregnated with an aqueous solution of mixed metal salts or oxides, and then,
after drying overnight, the precursors were heated at 280 °C for 2 h and calcinated
at 500 °C for 5 h to afford the catalysts. The loading of metals is 10 wt % and the
mole ratios of metal atoms are listed in Table 1.
Chloromethylation of toluene [30]
A mixture of toluene, paraformaldehyde, concentrated hydrochloric acid and
anhydrous zinc chloride in athree-necked flask was stirred at 60 °C, with hydrogen
chloride gas being introduced for 6 h. The mixture was cooled to room temperature.
The organic layer was washed twice with 10% sodium carbonate aqueous solution
and three times with water, then dried over Na2SO4, filtered, and concentrated to
remove excess toluene under reduced pressure. The mixture of products was
obtained by vacuum distillation in 86% yields, bp. 74–77 °C/10 mm Hg.
Selective ammoxidation of methylbenzyl chlorides
The selective ammoxidation of methylbenzyl chlorides was carried out in a 30-mm
inside-diameter quartz tube fixed-bed reactor [23, 29] loaded with 10 g catalyst. The
methylbenzyl chlorides were fed by a micropump, vaporized and mixed in a
preheated vessel with ammonia and air after having passed gas flowmeters with
suitable molar ratios. The preheated gas flow was then fed directly into the reactor.
The temperature was maintained at 210 2 °C. Methylbenzyl chlorides (MBCs)
were entered 0.6 ml/h, and the molar ratios of air/MBCs and NH3/MBCs were 20
and 5, respectively. After the reaction, the outlet stream was cooled and the products
were condensed in a condensing apparatus in 97% yields. The reaction can be
carried out continuously. The condensing apparatus was replaced by another one
every 8 h, and the test lasted nearly 80 h. Pure o-tolunitrile and p-tolunitrile could
be obtained by fractional distillation with 47% (bp. 80–82 °C/10 mm Hg) and 50%
(bp. 91–93 °C/10 mm Hg) yields, respectively, and the purities were 97 and 98% by
1
GC. The structures of the products were confirmed by IR and H NMR compared
with the Sadtler Standard Spectra.
Conclusions
In conclusion, tolunitriles were synthesized by selective ammoxidation of meth-
ylbenzyl chlorides prepared by chloromethylation of toluene in high yields and
nearly 100% selectivity at low temperature. This approach provides a new path to
alkylbenzonitriles and other aromatic nitriles.
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A novel synthesis of tolunitriles
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Acknowledgments The authors are grateful for the financial support from the Natural Sciences
Foundation of China (40830852), the Natural Sciences Foundation of Hubei Province (2008CDZ038) and
the Natural Sciences Foundation of South-Central University for Nationalities (YZZ10005).
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