Production of phosphate esters

Details Production of phosphate esters Production of phosphate esters

1999-11-26

2001-05-15

Ambrose, Michael G. (Department: 1613)

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

Organic compounds

Phosphorus esters

C558S092000, C568S785000, C568S789000, C568S790000

Reexamination Certificate

active

06232485

ABSTRACT:

This invention relates to the production of liquid triaryl phosphate esters. More particularly, this invention relates to the production of liquid meta/para isomer triaryl phosphate esters containing a low concentration of triphenyl phosphate and having good liquid properties.
Mixed synthetic triaryl phosphate esters are prepared by alkylating phenol with alkenes such as propylene and butylene to produce a mixture of phenol and alkyl substituted phenols which is commonly referred to as alkylated phenol. This alkylated phenol may be reacted directly with a phosphorylating agent such as phosphorus oxychloride to produce a mixed triaryl phosphate ester. Such processes are widely practised and well known in the art. They are described for example in U.S. Pat. No. 4,093,680.
The composition of the triaryl phosphate ester product is related to the composition of the alkylated phenol feedstock. If the phenol is completely alkylated the alkylate is free from phenol and the phosphate is free from triphenyl phosphate. However, such a product would be very viscous or solid and thereby unsuited for many of the applications in which triaryl phosphate esters find use. A general principle, well known to those skilled in the art, is that the liquid properties, as exemplified by viscosity, may be modified by varying the amounts of the different components of the phenolic feedstock. In certain applications, it is important to have good liquid properties (ie low viscosity) but at the same time not have a high concentration of triphenyl phosphate in the final phosphate ester mixture. There are a number of reasons why a low triphenyl phosphate content (less than 2%) may be desirable. These include improved thermal stability; improved hydrolytic stability; reduced stress cracking in plastics; lower volatile loss and reduced volatility, in particular reduced tendency for flashing in high temperature applications (where flashing is a generation of flame). U.S. Pat. No. 5,206,404 describes processes for the production of liquid triaryl phosphate esters comprising less than 2% triphenyl phosphate by a process of distilling a typical mixed triaryl phosphate ester under particular conditions so as to remove a volatile fraction and leave a liquid residue which is the product. Whilst U.S. Pat. No. 5,206,404 states that it is impossible to produce a liquid phosphate of this type by direct phosphorylation of an alkylated phenol one object of the present invention is to provide a process of producing a liquid phosphate ester having a low content of triphenyl phosphate and good liquid properties by the direct phosphorylation of an alkylated phenol.
Using a standard raw material feedstock mixture, eg phenol/t-butyl phenol, a triaryl phosphate of low triphenyl phosphate content is obtained by reducing the proportion of phenol in the feedstock, eg to 20:80. However, the resulting phosphate can be very viscous or even solid and thus unsuitable for many applications. In order to improve the physical properties of liquid phosphate esters of low triphenyl phosphate content, the composition of the feedstock can be modified such that there is an increase in the amount of meta substituted alkylated phenols present in the feedstock. U.S. Pat. No. 4,414,161 describes a process for the production of low temperature stable liquid phosphate esters which comprises the steps of butylating phenol to produce a p-tertiary butyl phenol, catalytically isomerising said p-tertiary butyl phenol to produce a mixture of para and meta tertiary butyl phenols (which isomerisation also dealkylates a proportion of the tertiary butyl phenol); realkylating the product to produce a mixed meta and para tertiary butyl phenol which is then phosphorylated to produce a triaryl phosphate ester. The processes described in U.S. Pat. No. 4,414,161 produce no meta tertiary butyl phenol in the first stage of the reaction, a product comprising 19.11% of meta tertiary butyl phenol, 30.52% of para tertiary butyl phenol and 43.32% of phenol at the end of the isomerisation step and 16.18% meta, 45.24% para and 30.34% phenol at the end of the realkylation step.
As indicated above, the process for preparing the alkylated phenol feedstock of U.S. Pat. No. 4,414,161 requires three steps, namely alkylation, isomerisation (or transalkylation) and realkylation. A further object of the present invention is to provide a process for the production of liquid phosphate esters which does not require a realkylation step in the preparation of the alkylated phenol.
Furthermore, the alkylated phenol product obtained by the process of U.S. Pat. No. 4,414,161 contains greater than 30% phenol and yet only has around 16% of meta alkylated phenol. In order to generate a phosphate ester containing less than 2% triphenyl phosphate and with good liquid properties, the alkylated phenol feedstock must contain less than 22% phenol and greater than 20% meta-t-butyl phenol. 30% phenol is considered too high a content for obtaining a phosphate ester with acceptable triphenyl phosphate content. Yet another object of the present invention is to provide a process for the production of liquid phosphate esters in which the phenol content and meta isomer content of the alkylated phenol feedstock is such as to obtain a phosphate ester containing less than 2% triphenyl phosphate.
U.S. Pat. No. 4,492,660, U.S. Pat. No. 3,014,079 and GP 1060156 also describe the use of catalysts to obtain meta-alkylated phenols.
According to the present invention there is provided a process for producing a liquid traryl phosphate ester of low triphenyl phosphate content and low viscosity comprising (a) an alkylation stage wherein a phenol is reacted with an olefin having 2 to 12 carbon atoms in the presence of a strong acid catalyst to give a reaction product comprising a mixture of meta and para alkylated phenols; and (b) a transalkylation stage wherein the mixture of alkylated phenols from the alkylabon stage is heated in the presence of a strong acid catalyst to increase the meta isomer content of the mixture to at least 20% whilst maintaining a phenol level below 22%; and (c) a phosphorylation stage wherein the mixture of alkylated phenols from the transalkylation stage is reacted with a phosphorylating agent; and wherein the strong acid catalyst used in stages (a) and (b) is a Bronsted acid having an acid strength of less than zero.
The use of selected acidic catalysts leads to the formation of an alkylated phenol containing a high proportion of the meta alkylated phenol. As stated above such acids are Bronsted acids having an acid strength of less than zero. Acid strength Ho is defined by the equation
Ho
=
pK
a
+
log
⁢
 
⁢
[
B
]
[
BH
+
]
where [B] and [BH
+
] are the concentrations of the neutral base and its conjugate acid respectively. The acid strength may be measured by the use of suitable indicators.
The acid strength is an expression of the ability of an acid to charge a neutral organic base. This charge can occur by transfer of a proton from a Bronsted acid or by transfer of an electron pair from a Lewis acid. Where the catalyst is a solid with a distribution of sites having different acid strengths those useful in this invention have at least some Bronsted acid sites having an acid strength of less than zero.
The preferred olefins for use in the processes of this invention are propylene, n-butylene and isobutylene. The most preferred olefin is isobutylene.
The nature of the catalyst plays a crucial role in determining the composition of the alkylated phenol product. The catalysts conventionally used are Lewis acids such as aluminium chloride and magnesium chloride which lead to the formation of little or no meta alkylated phenol. U.S. Pat. No. 4,492,660 describes the use of trifluoromethane sulphonic acid which is a Bronsted acid having an acid strength of less than zero to catalyse the reaction between phenol and isobutylene. However this reaction produces no meta alkyl phenol under the reaction conditions which are disclosed. We have found that the Bronsted acid c

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