Method for producing lactams using oligophosphate catalysts

Details Method for producing lactams using oligophosphate catalysts Method for producing lactams using oligophosphate catalysts

2000-09-14

2002-06-18

Kifle, Bruck (Department: 1624)

Organic compounds -- part of the class 532-570 series

Organic compounds

Unsubstituted hydrocarbyl chain between the ring and the -c-...

Reexamination Certificate

active

06407230

ABSTRACT:

Lactams are versatile compounds. For instance, N-methylbutyrolactam (N-methylpyrrolidone) is a widely used solvent and e-caprolactam is an important monomer for the manufacture of polyamide fibers.
Lactams can be prepared by hydrolytic cyclization of aminonitriles in the gas phase. Catalysts having-dehydrating properties, such as aluminum oxide, silica gel or borophosphoric acid, are used.
EP-A 0 659 741 describes the preparation of lactams from aminonitriles and water by hydrolytic cyclization in the gas phase using metal orthophosphates, especially aliminum, zirconium, niobium and lanthanum orthophosphates, as catalysts. The catalysts may additionally be impregnated with basic alkali or alkaline earth metal compounds, preferably of cesium rubidium and potassium.
However, the selectivity of the prior art catalysts still leaves something to be desired. The formation of by-products makes it difficult to isolate the lactams and may lead to poisoning of the catalysts used.
It is an object of the present invention to provide a catalyst for preparing lactams by hydrolytic cyclization of aminonitriles that is highly selective at high rates of conversion.
We have found that this object is achieved by a process for preparing lactams by hydrolytic cyclization of aminonitriles in the gas phase in the presence of a metal phosphate catalyst, which comprises using a catalyst comprising one or more oligophosphates of the general formula (I)
M(PO
a
)
b
  (I)
where M is a metal of group 3 or 4 of the periodic table, including the lanthanides, a is >2.5 and <4.0, and b is such that electrical neutrality is ensured, or a mixture of one or more oligophosphates of the general formula (I) with one or more further salts of a metal of group 3 or 4 of the periodic table, including the lanthanides, with an inorganic acid.
The catalyst used may comprise one or more oligophosphates of the general formula (I). Said formula (I) must be understood as gross stoichiometric formula and not as the molecular formula of actual existing compounds. Oligophosphates for the purposes of the present invention are phosphates which are formally derived from acids which are obtainable by condensation of orthophosphoric acid with elimination of water. The condensation of orthophosphoric acid H
3
PO
4
with intermolecular elimination of water yields chain-like oligophosphoric acids H
n+2
P
n
O
3n+1
(tri-, tetra-, pentaphosphoric acid etc.; n=3, 4, 5, etc.) or (for large n) polymeric polyphosphoric acids. Triphosphoric and higher acids may also undergo an intramolecular condensation to form ring-shaped metaphosphoric acids H
n
P
n
O
3n
(tri-, tetrametaphosphoric acid etc.; n=3, 4, etc.), and not only a chain-extending but a chain-branching condensation with the formation of branched ultraphosphoric acids (e.g., isotetraphosphoric acid H
6
P
4
O
13
). The formal end product of the condensation is polymeric phosphorus pentaoxide P
2
O
5
. For the oligophosphates of the general formula (I) derived from these acids, a is between the corresponding value for polymeric phosphorus pentaoxide (2.5) and that of orthophosphate (4.0). That is, 2.5 <a<4.0. a is preferably from 2.6 to 3.5, particularly preferably from 3 to 3.5. In particular, a=3.
The choice of b is such as to ensure electrical neutrality. If the phosphorus in the oligophosphates is exclusively pentavalent phosphorus, b is especially (2a−5)/z, where z is the number of charges on the M cations.
M is a metal of group 3 or 4 (=transition group III or IV, respectively) of the periodic table, including the lanthanides, i.e., Sc, Y, Ti, Zr, Hf, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, preferably a lanthanide, particularly preferably La or Ce, very particularly preferably La. The oligophosphates used according to the present invention may contain one or more species of metals M, preferably they contain just a single species of metals M.
The catalyst used may be a specific oligophosphate, preferably trimetaphosphate, or a mixture of a plurality of different oligophosphates of the general formula (I).
A very particularly preferred catalyst is trimeta-phosphate, especially lanthanum trimetaphosphate (LaP
3
O
9
)
The catalysts used may further be mixtures of one or more of the aforementioned oligophosphates of the general formula (I) with one or more further salts of the aforementioned metals of group 3 or 4 of the periodic table, including the lanthanides, with inorganic acids. The metals present in the oligophosphates and in the further salts may be identical or different; they are preferably identical. Preferred further salts are the orthophosphates, sulfates, carbonates, silicates, arsenites, arsenates, antimonites, antimonates and nitrates, particularly preferably the orthophosphates of the metals mentioned.
The oligophosphates of the general formula (I) may be used alone or mixed with the further salts. In general, the ratio of further salts to oligophosphates is up to 50:1, preferably up to 10:1, particularly preferably up to 5:1, very particularly preferably from 0.1:1 to 5:1, and especially from 1:1 to 5:1.
The catalyst is further particularly preferably a mixture of trimetaphosphate and orthophosphate, especially with lanthanum as metal M.
The oligophosphates of the general formula (I) and the further salts may each contain up to 5 molecules of water per unit formula.
The catalyst is generally prepared from the nitrate, nitrite, carbonate, formate, acetate, oxalate or some other salt of an organic acid, but preferably from the nitrate of metal M and ammonium phosphate as preferred precursors. These components are intimately mixed with each other in the desired molar ratio as fine powders. The chosen molar ratio of phosphorus:metal (=b) will be present in the product (the oligophosphate or the mixture of different oligophosphates) after the reaction has taken place. After mixing, the precursors are slowly heated, for example in an open crucible, at temperatures from generally 140 to 200° C., preferably from 150 to 180° C., for generally from 2 to 48 hours, preferably from 8 to 36 hours, to decompose the precursors. This is followed by gradual heating to 250-900° C., preferably 400-650° C., for 1-8 days, preferably 2-5 days, to complete the conversion into the oligophosphate. This method makes it possible to obtain any oligophosphates having a phosphorus:metal ratio=3 as mixtures, but also, in some instances, in pure form. Trimetaphosphate, for example, is thus obtainable in pure form.
The present invention further provides a process for preparing a catalyst, which comprises the steps of:
a) preparing a mixture of ammonium dihydrogen-phosphate and the nitrate of said metal M in the desired molar ratio;
b) heating this mixture in stages to decompose the precursors and form the metal oligophosphate in a solid state reaction.
Metaphosphates are further obtainable by precipitating M as dihydropyrophosphate from a solution of a salt of M with pyrophosphoric acid H
4
P
2
O
7
and calcining the resulting precipitate to form the metaphosphate.
The present invention further provides a process for preparing a catalyst, which comprises the steps of:
a) preparing a solution comprising a salt of said metal M;
b) precipitating said metal M from this solution as dihydropyrophosphate and removing the precipitate comprising the metal dihydropyrophosphate;
c) optionally washing and drying said precipitate;
d) calcining said precipitate.
The precipitating of M as dihydropyrophosphate is preferably carried out at a pH of generally 0.5-4, preferably 0.8-2. To effect precipitation, a base may be added to a solution comprising the salt of metal M and pyrophosphoric acid to establish a certain pH. Preferred bases are ammonia, alkali metal hydroxides, primary, secondary and tertiary amines, particularly preferably ammonia. It is further possible to add a solution comprising the salt of metal M to an aqueous alkaline solution of pyrophosphoric acid. Suitable salts are water-soluble salts of metal M,

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