Process for preparing pyrimidine

Alkylpyrimidines are dealkylated to the corresponding pyrimidines by contacting with an oxidation catalyst containing one or more oxides of at least one of the elements of the group consisting of Bi, Mo and V in the presence of water and oxygen in the gas phase at a temperature of 300.degree.-450.degree. C.

The invention relates to a process for preparing pyrimidine. Pyrimidine is 
used, among other things, as raw materials for preparing crop protection 
chemicals. In Chemische Berichte 91 (1958), pages 2832-2849, a laboratory 
synthesis for pyrimidine is described. 1-methoxy-1,3,3-triethoxypropane is 
reacted at 208.degree. C. with a 6 to 10 fold excess of formamide. The 
yield of pyrimidine then formed is 72 mole % calculated in respect of 
moles 1-methoxy-1,3,3-triethoxypropane. The disadvantage of such a 
synthesis lies in expensive raw materials, the necessity of a large excess 
of formamide and a relatively low yield of 28 wt % when calculated as a 
percentage of the weight of 1-methoxy-1,3,3-triethoxypropane. 
The object of the invention is a commercially attractive process for 
preparing pyrimidine. 
According to the present invention pyrimidine is prepared from an 
alkyl-substituted pyrimidine by contacting the latter in the gas phase at 
a temperature of 300.degree.-450.degree. C. with an oxidation catalyst 
containing one or more oxides of at least one of the elements of the group 
consisting of Bi, Mo and V, and in the presence of water and of a gas 
containing molecular oxygen. Pyrimidine is recovered from the reaction 
products mixture at high conversion levels. 
This invention provides a completely new type of reaction. Unexpectedly, 
the use of water in the reaction system at the high temperatures used does 
not preclude the synthesis of pyrimidine. 
Oxidation and ammoxidation represent a large class of reactions, in which a 
large group of raw materials can be employed. It is, however, always 
unexpected whether the use of a new type of starting compound will thereby 
give rise to a useful, easy obtainable end-product. 
For instance pyridines are very different from pyrimidines in stability 
with respect to water or heat. Even within the class of pyridines, 
problems arise when dealkylating groups are substituted at different 
places on the ring. 
This new process now makes it possible for pyrimidine to be prepared in a 
practical manner, starting from cheap raw materials which are available on 
a large scale. Thus, for instance, in the prior art the preparation of 
2-methylpyrimidine in a high yield has been described using as starting 
materials 1,3-diaminopropane and acetaldehyde, see Yakugaku Zasshi 96, 
pages 1005-1012 (1976); further the preparation of 2-ethylpyrimidine is 
described by, inter alia, the dehydrocyclodimerisation of 
1,3-diaminopropane as described in Yakugaku Zasshi 96, pages 801-809 
(1976). 
In the process according to the present invention alkyl-pyrimidines with 
substituents preferably in the 2 and/or 4 (6) positions are used as 
starting materials. The alkyl group(s) preferably contain(s) 1-4 C atoms 
such as, for instance, 4-methylpyrimidine and 2-ethylpyrimidine, and 
particularly 2-methylpyrimidine. 
The catalyst used in this invention is the same type of catalyst already 
known for selective oxidation or ammoxidation of hydrocarbons and which 
contains one or more oxides of one or more of the following elements: Bi, 
Mo and V. Because the oxidation state of the metals changes during the 
oxidation reactions, it is not significant to specify the same. 
The metal oxide catalyst can be used on a carrier material, known per se. 
Such carriers may contain, for instance, silicon oxide and/or aluminium 
oxide. Catalysts on a carrier normally contain between 1 and 40% by weight 
of the metal, preferably between 4 and 25% by weight. 
The process according to the invention can be carried out at an elevated 
temperature in the range of 200.degree.-550.degree. C., with preference 
given to a temperature of 300.degree.-450.degree. C., advantageously from 
320.degree. to 400.degree. C. 
The process according to the invention, in which the alkyl substituent(s) 
is (are) oxidized, is carried out in the presence of water and a gas 
containing molecular oxygen. The object of the presence of water is to 
confine the oxidation to the alkyl substituent(s). Generally at least 5 
moles water are used per mole substituted pyrimidine, advantageously more 
than 10 moles of water. 
For economic reasons the gas containing molecular oxygen is preferably air. 
For the oxidation reaction of 2-methylpyrimidine a stoichiometric amount 
of 1.5 moles, preferably 3 moles, oxygen is required per mole of 
substituted pyrimidine. If air is used as a gas containing molecular 
oxygen, these amounts correspond with 7.5 and 15 moles air, respectively. 
In what follows hereafter 1 mole oxygen has been taken, for reasons of 
simplicity, to equal 5 moles air. If the substituted pyrimidine contains 
more than one methyl substituent, or if the alkyl group(s) contain(s) more 
than one C atom, a corresponding amount of extra oxygen may be required 
for carrying out the process. However, less oxygen may also be used, but 
then the conversion will be reduced. 
The upper limit of the excess of water is not critical and is determined 
mainly by the contents of the reactor and by the desired period of 
contact. Neither does a maximum exist for the upper limit of the quantity 
of air, but here should be remembered that too large a quantity of air may 
result in a lower selectivity. Preferably at least 15 moles water and at 
least 20 moles air are used per mole alkyl-substituted pyrimidine. 
For the practical realization of the process according to the invention, 
the techniques already known per se for the realization of gas phase 
reactions may be used. For instance the gaseous starting mixture may be 
passed over the catalyst in the form of a fixed bed or a so-called 
fluidized bed. The space velocity can be varied, for instance between 
0.001 and 2 g starting compound per milliliter catalyst material (bulk 
volume) per hours. The pressure at which the reaction takes place in the 
gas phase is not important as such; accordingly the reaction is generally 
carried out at autogenous pressure. 
The further processing and recovery of the pyrimidine product obtained in 
the reaction mixture can be effected in a manner known per se by cooling 
and by subsequently carrying out, for instance, a distillation, an 
extraction or a crystallization process.

The invention is further elucidated in the following examples. 
EXAMPLE I 
Through a vertical tubular reactor (diameter 17 mm, length 400 mm) 
containing a zone of 10 ml (bulk volume) catalyst and provided with a 
heating jacket, a gaseous mixture of 2-methylpyrimidine, water and air was 
passed from top to bottom. Per mole 2-methylpyrimidine 17 moles water and 
45 moles air were used. The catalyst used was V.sub.2 O.sub.5 on a carrier 
of .alpha.-alumina with a specific surface of 25-35 m.sup.2 /g (10% (wt) V 
calculated as metal in respect of the total quantity of catalyst). Per ml 
(bulk volume) catalyst 0.18 g 2-methylpyrimidine was passed through per 
hour. The temperature of the heating jacket was varied (see Table 1). The 
heat developed during the exothermic reaction caused a higher temperature 
in the zone containing the catalyst. 
After an operating period of 2 hours the reaction conditions were kept 
constant for 1 hour and the reaction mixture obtained during this period 
was cooled to 12.degree. C. The composition of the then condensed product 
was determined by gas chromatography. From this determination and from the 
weight of the quantity of 2-methylpyrimidine that had been passed over in 
the said period of 1 hour, the conversion of 2-methylpyrimidine and the 
selectivity to pyrimidine could be calculated. 
The conversion is understood to mean the amount of 2-methylpyrimidine 
converted (amount of 2-methylpyrimidine passed over less the amount of 
2-methylpyrimidine in the condensed product), expressed in percentages of 
the amount of 2-methylpyrimidine passed over. The selectivity to 
pyrimidine is understood to mean the amount of pyrimidine that can 
theoretically be formed from the amount of 2-methylpyrimidine converted. 
The results are mentioned in Table 1. 
EXAMPLE II-VII 
The process according to Example I was followed with the understanding 
that, instead of an operating period of 2 hours, an operating period of 
respectively 4, 5.5, 7, 9, 11 and 14 hours was applied, before the 1 hour 
measurement was started. In scrutinizing these experiments, it must be 
borne in mind, that the results are virtually not influenced by the period 
of time that the catalyst was used. 
The results of Examples II-VII are summarized in Table 1. 
TABLE 1 
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Example I II III IV V VI VII 
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operating period 
2 4 5.5 7 9 11 14 
in hours 
temperature of the 
410 380 360 340 320 300 260 
heating jacket (.degree.C.) 
temperature of the 
447 404 383 359 333 309 262 
catalyst zone (.degree.C.) 
conversion 94 84 81 75 69 44 21 
2-methylpyrimidine 
(%) 
selectivity 32 46 53 55 50 56 33 
pyrimidine (%) 
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As is clear from these examples, a higher temperature gives rise to a 
higher conversion although at the expense of selectivity. Under the here 
performed conditions, a temperature range of 320-380 appears to be 
optimal. 
EXAMPLES VIII-X 
In the manner described in Example I, 2-methylpyrimidine was dealkylated at 
a heating jacket temperature of 380.degree. C., using 17 moles of water 
per mole of 2-methylpyrimidine, while the amount of air supplied was 
varied. The results are summarized in Table 2. 
TABLE 2 
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Example VIII IX X 
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operating period in hours 
7 19 21 
air/2-methylpyrimidine ratio 
21 30 42 
(mole/mole) 
temperature of the catalyst 
390 395 405 
zone (.degree.C.) 
conversion 2-methylpyrimidine (%) 
23 50 89 
selectivity pyrimidine (%) 
76 81 39 
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From these experiments, it is clear that a lower air supply gives rise to a 
lower conversion; a very high supply however results in a lower 
selectivity. 
EXAMPLES XI-XIII 
In the manner described in Example I, 2-methylpyrimidine was dealkylated at 
a heating jacket temperature of 380.degree. C., using 45 moles of air per 
mole of 2-methylpyrimidine, while an amount of water supplied was varied. 
At the same time extra nitrogen was supplied in order to keep the period of 
contact constant. 
The results are summarized in Table 3, showing good results at all 
supplying ratios. 
TABLE 3 
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Example XI XII XIII 
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operating period in hours 
4 6 11 
water/2-methylpyrimidine ratio 
10 15 25 
(mole/mole) 
nitrogen/2-methylpyrimidine ratio 
9 4 0 
(mole/mole) 
conversion 2-methylpyrimidine (%) 
70 31 50 
selectivity pyrimidine (%) 
61 89 87 
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EXAMPLES XIV-XVIII 
In the manner described in Example I, 2-methylpyrimidine was dealkylated, 
while the catalyst used was V.sub.2 O.sub.5 with KHSO.sub.4 on 
.alpha.-alumina with a specific surface of 12.+-.4 m.sup.2 /g (5% (wt) V 
and 6.7% (wt) KHSO.sub.4). Per ml (bulk volume) catalyst 0.19 g 
2-methylpyrimidine was used. Per mole 2-methylpyrimidine 19 moles water 
and 41 moles air were supplied. The temperature was varied as indicated. 
The results are summarized in Table 4. Again, the results are not 
dependent on the operating period. It is shown, that with this V.sub.2 
O.sub.5 catalyst also, good results are obtained. 
TABLE 4 
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Example XIV XV XVI XVII XVIII 
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operating period 
11.5 14.5 20.5 24 26 
in hours 
heating jacket 
325 350 375 385 400 
temperature (.degree.C.) 
conversion 52 69 79 79 81 
2-methylpyrimidine 
(%) 
selectivity 74 71 58 57 52 
pyrimidine (%) 
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EXAMPLES XIX-XXIII 
In the manner described in Example I, 2-methylpyrimidine was dealkylated, 
while 10 ml of a catalyst was used containing the elements Bi and Mo (4.4 
wt % Bi and 20.4 wt % Mo calculated as metals in respect of the total 
amount of catalyst) on an SiO.sub.2 carrier. This catalyst was prepared by 
mixing 22.4 g phosphomolybdic acid. H.sub.3 PO.sub.4.12MoO.sub.3.24H.sub.2 
O with 2.47 g 85% (wt) orthophosphoric acid, 5.7 g bismuth nitrate 
Bi(NO.sub.3).sub.3.5H.sub.2 O and 38 g SiO.sub.2 (the trade product 
aerosil). The thus resulting mixture was calcined at 660.degree. C. 
The temperature of the heating jacket and the amount of air supplied were 
varied. The results are summarized in Table 5. Obviously, experiment XXII 
shows the best performance with this catalyst, indicating, that a higher 
amount of air has a positive effect on the yield obtained. 
TABLE 5 
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Example XIX XX XXI XXII XXIII 
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operating period 
1 2.5 5 9 11 
in hours 
heating jacket 
380 380 380 380 400 
temperature (.degree.C.) 
temperature catalyst 
398 401 403 405 416 
zone (.degree.C.) 
air/2-methylpyrimidine 
23 30 35 45 23 
ratio (mole/mole) 
conversion 2-methyl- 
48 62 66 97 41 
pyrimidine (%) 
selectivity 93 87 72 79 81 
pyrimidine (%) 
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EXAMPLES XXIV-XXVII 
With the catalyst as used in Examples XIX-XII, a gaseous mixture of 
2-ethylpyrimidine, water and air was dealkylated. Per ml (bulk volume) 
catalyst 0.13 g 2-ethylpyrimidine was passed through per hour. Per mole 
2-ethylpyrimidine 27 moles water were used. The heating jacket temperature 
was 380.degree. C. The amount of air supplied was varied. 
The results, summarized in Table 6, show that this alkylpyrimidine is also 
dealkylated with this process in a high yield, and that a high ratio of 
air to 2-ethylpyrimidine favors a higher conversion. 
TABLE 6 
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Example XXIV XXV XXVI XXVII 
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operating period in hours 
1 2.5 4 9 
air/2-ethylpyrimidine ratio 
30 35 40 45 
(mole/mole) 
conversion 2-ethylpyrimidine % 
50 66 85 97 
selectivity pyrimidine % 
85 72 71 79 
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EXAMPLE XXVIII 
With the V.sub.2 O.sub.5 catalyst as used in Examples XIV-XVIII, a gaseous 
mixture of 4-methylpyrimidine, water and air was dealkylated. Per ml (bulk 
volume) 0.18 g 4-methylpyrimidine was passed through per hour. Per mole 
4-methylpyrimidine 17 moles water and 40 moles air were used. The 
temperature of the heating jacket was 380.degree. C. After an operating 
period of 4 hours the resulting conversion of 2-methylpyrimidine was 92% 
and the selectivity to pyrimidine 39%. 
EXAMPLE XXIX 
In the manner described in Example I, a gaseous mixture of 
2-methylpyrimidine, water and air was converted. However per mole (bulk 
volume) catalyst 0.095 g 2-methylpyrimidine was passed through per hour. 
The temperature of the heating jacket was 380.degree. C. After an 
operating period of 1.5 hours the resulting conversion of 
2-methylpyrimidine was 81% and the selectivity to pyrimidine 66%. 
As the foregoing examples demonstrate, the novel process of this invention 
provides for the economic production of pyrimidine from a relatively 
inexpensive starting material, alkyl-substituted pyrimidines, and it is 
operative under a wide variety of conditions with desirably high ultimate 
yields.