Liquid-phase synthesis of cyanuric acid from urea

Article abstract of DOI:10.3390/molecules15031898

The focus of this paper was to identify a cheaper solvent from among diesel fuel, kerosene, sulfolane or a mixture of sulfolane and cyclohexanol for the preparation of cyanuric acid heterocyclization of urea. To obtain a higher yield, the effects of catalyst (sodium, ammonium, calcium and zinc salts) and temperature (160 °C to 220 °C) on the trimerization of urea were also carefully studied. We established the optimal reaction conditions and further validated them in our scale-up experiments.

Full text of DOI:10.3390/molecules15031898

Molecules 2010, 15, 1898-1902; doi:10.3390/molecules15031898
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Liquid-Phase Synthesis of Cyanuric Acid from Urea
Dong-Mei She 1,2, Hai-Lin Yu 2, Qi-Liang Huang 2, Fen-Ming Li 2 and Chun-Jiu Li 1,*
1
College of Science, China Agricultural University, Beijing 100193, China;
E-Mail: cjiuli@cau.edu.com (C-J.L.)
2
Plant Protection Institute of the Chinese Academy of Agricultural Sciences, Beijing 100193, China;
E-Mails: yuhaili6258@163.com (H.-L.Y.); qlhuang@ippcaas.cn (Q.-L.H.)
* Author to whom correspondence should be addressed; E-Mail: dmshe@ippcaas.cn;
Tel.: +86 1062890876; Fax: +86 1062816909.
Received: 30 January 2010; in revised form: 24 February 2010 / Accepted: 8 March 2010 /
Published: 16 March 2010
Abstract: The focus of this paper was to identify a cheaper solvent from among diesel fuel,
kerosene, sulfolane or a mixture of sulfolane and cyclohexanol for the preparation of
cyanuric acid heterocyclization of urea. To obtain a higher yield, the effects of catalyst
(sodium, ammonium, calcium and zinc salts) and temperature (160 °C to 220 °C) on the
trimerization of urea were also carefully studied. We established the optimal reaction
conditions and further validated them in our scale-up experiments.
Keywords: cyanuric acid; urea; liquid-phase; yield
Introduction
Cyanuric acid (2,4,6-trichloro-1,3,5-triazine or isocyanuric acid) is a key intermediate for the
synthesis of a variety of organic compounds, include epoxy resins, chlorinated derivatives, detergents,
antioxidants, dyes, pesticides, antitumour agents and so on. It is also the raw material for many new
products such as trichloroiminocyanuric acid (TCCA), 1,3,5-tri-2–propenyl-1,3,5–triazine-2,4,6-
(1H,3H,5H)-trione (TAIC), triglycidyl isocyanurate (TGIC) and tricarboxylethyl isocyanurate (TCIC).
These new products are well known and widely used in chemical engineering, light manufacturing,
electrical appliances, mechanical industry, spinning and agriculture [1–4].

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Cyanuric acid is a white crystalline solid, with a melting point of 145–148 °C and a boiling point of
190 °C. Various methods have been reported for the synthesis of cyanuric acid. Those methods can be
divided into two types. One involves production of cyanuric acid from urea as starting material and the
other involves polymerization of cyanogen chloride, or treatment of HCN and Cl in the presence of a
catalyst [5]. Of the two routes, the use of urea is more traditional, economical and convenient as one
can typically obtain higher quality products in better yield. Moreover, it is easier to scale-up for
manufacturing with ordinary equipment.
There are two ways for preparing cyanuric acid from urea. One is by heating urea at high
temperature (≥ 250 °C) without solvent, and the other is the trimerization of urea in a solvent, with or
without a catalyst [6–9]. The first method has the disadvantage that since the reaction product changes
from a solid phase to a liquid phase and back again to the solid phase, so it may be agglutinated or
solidified and adhere to the heating surface or appropriate heat transfer surface, with the result that the
product quality is lowered and the reaction equipment can be damaged [4]. At the same time, the
product can’t be used directly in the chemical industry without refining. Comparatively, the second
method is environmentally benign, safe, low cost and the product can be used unrefined, but this
method has one weak point, in the high boiling point solvent must be regenerated after being used a
few times, and some of the potentially expensive solvents used like sulfolane, tetraethylene glycol
dimethyl ether, butyl carbitol and cyclohexanol may be wasted.
In our study, the procedure for the trimerization of urea in a solvent was further developed
(Scheme 1). We tried to identify a cheaper solvent like diesel or kerosene for urea trimerization and
cyanuric acid was obtained from those high boiling point solutions. To our attempts to improve the
yield by investigation of the influence of catalyst, we obtained different results under our study
condition from those obtained by Kawahara and Tejima [7] in which cyanuric acid was prepared by
heating urea in the presence of alkali metal hydrides, alkali metal amides, and alkali metal salts in
organic solvents. For example, urea was treated with NaH in tetraethylene glycol methyl ether at
150 °C for 5 hours to give 99% cyanuric acid, whereas 0% cyanuric acid was obtained without NaH
[7]. In our study, the catalysts we have selected for the reaction did not improve the yield much. The
influence of temperature on the result was also carefully studied, and the optimal reaction conditions
established were validated in our subsequent larger scale experiments.
Scheme 1. The synthesis of cyanuric acid.
H
O
O
N
a
3NH2CONH2
+
HN
NH
3NH3
b
O
a. Cat.; b. Solvent
Results and Discussion
Considering that the trimerization of urea should take place at a high temperature [11,12], by
heating urea or biuret in an inert solvent (sulfolane, 4-chloro-m-cresol, 3-methylsulfolane,
2-methoxyethyl ether), and better at temperatures above 180 °C [11], our initial efforts focused on the

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solvent to be used. Three kinds of single solvent: kerosene (low odor paraffinic solvent, CAS number
64742-47-8, boiling point 220~270 °C), diesel (light diesel oil, CAS number 68334-30-5, boiling point
250~400 °C) and sulfolane, and one mixture of solvents - sulfolane and cyclohexanol - were tested
first (Table 1). Cyanuric acid was obtained in a good yield (88.7% and 80.5%, respectively) in the
single solvents kerosene and diesel at 180 °C (Table 1). Maybe it was propitious to form the cyanuric
acid in a solvent of lower polarity, so the nonpolar solvents kerosene and diesel are better than the
polar solvents sulfolane and cyclohexanol. Kerosene and diesel are also cheap and can probably
be recycled.
Table 1. Effect of different kinds of solvent on experimental results.
Entry
Solvent
Yield (%)
88.7
1
2
3
4
kerosene
diesel
sulfolane
sulfolane: cyclohexanol = 3:1
80.5
54.8
64.6
Conditions: 180 °C, solvent (v)/urea (q) = 2:1.
Next we examined the effects of the temperature of reaction (Table 2). The experimental results
show that the highest yield (88.9.0%) was obtained at 190 °C. The temperatures from 180 °C to
210 °C did produce different yields of cyanuric acid, but the reaction completed faster at 210 °C. The
process appeared to be accelerated at higher temperatures while the yield did not increase
correspondingly. At reaction temperatures higher than 210 °C, urea could be lost when the reaction
was under vacuum, and the yield was decreased. To confirm this a cold trap was placed between the
reactor and the pump and white urea was found in the trap. An interesting phenomenon was observed
in that the solution appeared turbid at the beginning of the reaction process due to the presence of
undissolved starting materials, while the reaction mixture gradually became clear in the middle of the
reaction time and turbid again in the end as a result of the appearance of the insoluble product.
Table 2. Effect of reaction temperature on experimental results.
Entry
Temperature (°C)
Yield (%)
1
2
3
5
5
180
190
200
210
220
85.7
88.9
87.4
80.4
67.5
Conditions: kerosene(v)/urea (q) = 2:1.
In our research the yield changed a little with the catalyst species tested (Table 3). The highest yield
(82.4%) was obtained with (NH4)2SO4. Different catalysts resulted in small changes in yield. This
result differs for that seen when urea is heated directly without solvent, in which many kinds of salts
can accelerate the reactions and increase the cyanuric acid yield. In our experiments when cyanuric
acid is prepared in an organic solvent, the catalyst does not promote the yield of cyanuric acid much

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nor accelerate the reaction either. Perhaps the salts make the system melt at a lower temperature and
facilitate cyclization of the urea in the solid state reaction without solvent. There are maybe other
reasons for the negligible influence of the tested catalysts on cyanuric acid yield, so this needs
further exploration.
Table 3. Effect of catalyst on experimental results.
Entry
Catalyst
(NH4)2SO4
CaCl2
Yield (%)
82.4
1
2
77.6
3
NH4Cl
80.9
4
ZnCl2
79.4
5
KNO3
82.4
6
NaCl
76.3
7
8
NH4NO3
CaSO4
80.6
75.3
9
10
11
Ca(NO3)2
ZnSO4
ZnNO3
77.3
81.2
82.0
Conditions: kerosene, solvent (v)/urea (q) = 2:1; temperature: 190 °C.
Experimental
General
1
13
All chemicals were of reagent grade and used as purchased. H-NMR (300 MHz) and C-NMR
(75 MHz) spectra were recorded on a Bruker AC-P300 spectrometer using CDCl3 as the solvent; TMS
was the internal standard. IR spectra were determined on an FTS-185 instrument as neat films. All
reactions were carried out in pre-dried glassware (150 °C, 4 h) cooled under a stream of dry air.
Synthesis of cyanuric acid
The synthesis of cyanuric acid is depicted in Scheme 1. Urea (20 g) with (or without) catalyst was
added to solvent (40 mL) in a round-bottom flask. The mixture was stirred at 150 °C, placed under
vacuum (10 mm Hg) and then heated at 160~220 °C. The reaction was monitored by pH test paper to
test emission of NH3..When the pH test paper did not change color anymore the reaction was
considered complete. The reaction mixture was cooled to 80 °C, put into water (15 mL) and stirred for
1 hour to precipitate the product. The obtained dry solid was heated to 150 °C for 2 hours to remove
the crystallization water and afford pure cyanuric acid. The structure of the product was confirmed by
m.p., IR, 1H-NMR and 13C-NMR spectra.
Conclusions
In conclusion, liquid-phase synthesis of cyanuric acid from urea in a stable high yield, convenient
operation and low cost, is possible using kerosene as solvent.

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Acknowledgements
This research was supported by the National Science & Technology Pillar Program during the
Eleventh Five-year Plan Period (Project No. 2006BAF07B03).
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