Process for producing 3,3',5,5'-tetra-t-butylbiphenol

Abstract: A process for producing 3,3',5,5'-tetra-t-butylbiphenol, which process comprises the oxidative dimerization of 2,6-di-t-butylphenol in the presence of a catalyst, wherein the catalyst is a catalyst mixture comprising an alkali metal catalyst and an alkylphenol or an alkali metal catalyst, an alkylphenol and phenol.

Description: The present invention relates to a process for producing 3,3',5,5'-tetra-t-butylbiphenol. More particularly, the present invention is concerned with a process for producing 3,3',5,5'-tetra-t-butylbiphenol useful as a starting material of, for example, 4,4'-biphenol through an oxidative coupling of purified 2,6-di-t-butylphenol or crude 2,6-di-t-butylphenol. Further, the present invention is concerned with a process for producing 3,3',5,5'-tetra-t-butylbiphenol suitably employed as a starting material of a 4,4'-biphenol of high quality which is highly purified and is free from coloration. BACKGROUND OF THE INVENTION It is common practice to use an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide as a catalyst in the oxidative coupling of 2,6-di-t-butylphenol. Japanese Patent Laid-open Publication No. 5(1993)-97740 discloses that the above alkali metal hydroxide can be used not only in the form of a solution but also in anhydrous or nonsolvent form. However, when the alkali metal catalyst such as an alkali metal hydroxide is dissolved in water to thereby obtain an aqueous solution (e.g., 48% aqueous KOH solution) and used as the catalyst as mentioned above, the alkali metal catalyst falls in the form of solution drops to the bottom of the reaction vessel because the solubility of alkali metal catalyst in 2,6-di-t-butylphenol is low. Thereafter, water vaporization occurs, so that the alkali metal catalyst cannot be homogeneously dispersed in the 2,6-di-t-butylphenol placed in the reaction vessel. Therefore, problems have been encountered such that the oxidative coupling reaction rate is decreased and the amount of impurities is increased to thereby lower the yield of 3,3',5,5'-tetra-t-butylbiphenol and cause an aeration port of the reaction vessel to have an increased likelihood to clog up. The occurrence of the above heterogeneous dispersion of alkali metal catalyst in the reaction system disenables continuous oxidation reaction. A large amount of alkali metal catalyst must be used for preventing the lowering of oxidative coupling reaction rate. However, at that time, it is also inevitable to add a large amount of acid compound for neutralization. With respect to the purified 2,6-di-t-butylphenol, the phenol content thereof is generally not greater than 0.1% by weight and, thus, it substantially does not contain phenol. Further, in the purified 2,6-di-t-butylphenol, the content of o-t-butylphenol is generally not greater than 0.1% by weight and the content of t-butylphenols other than o-t-butylphenol is generally not greater than 0.5% by weight. In the oxidative coupling of the purified 2,6-di-t-butylphenol as a starting material of 3,3',5,5'-tetra-t-butylbiphenol in the presence of an alkali metal catalyst, an increase in the degree of oxidation of 2,6-di-t-butylphenol lowers the selectivity of desired 3,3',5,5'-tetra-t-butylbiphenol and increases the formation of by-products such as 3,3',5,5'-tetra-t-butyldiphenoquinone, 2,6-di-t-butylbenzoquinone and carbon dioxide contained in oxidation waste gas. The above increase in the formation of by-products promotes the conversion of reaction products to a tar with the result that color of oxidation reaction mixture (3,3',5,5'-tetra-t-butylbiphenol containing by-products) is deteriorated and that, further, 3,3',5,5'-tetra-t-5 butylbiphenol is debutylated to thereby deteriorate the quality (purity and color) of obtained 4,4'-biphenol. On the other hand, crude 2,6-di-t-butylphenol obtained by reacting phenol with isobutylene in the presence of an aluminum phenoxide catalyst, for example as disclosed in Euopean Patent Application No. EP 0 421 883, generally contains phenol, o-t-butylphenol, t-butylphenols other than o-t-butylphenol and 2,4,6-tri-t-butylphenol and 2,5-di-t-butylphenol in respective amounts of up to 1% by weight, up to 5% by weight, up to 3% by weight and 73 to 82% by weight of 2,6-di-t-butylphenol. When a large amount of starting material phenol and by-product o-t-butylphenol remain in the phenol butylating reaction mixture, the yield of 2,6-di-t-butylphenol would be decreased. In the oxidative coupling of crude 2,6-di-t-butylphenol is the reserve of an alkali metal catalyst to give 3,3',5,5'-teta-t-butylphenol, for example as described in EP 0 421 883, the alkali metal catalyst B heterogeneongly dispersed in the reaction system. This hinders continuous oxidation and lowers the yield of 3,3',5,5'-tetra-t-butylphenol. Further in such as oxidative coupling of crude 2,6-di-t-butylphenol as in the oxidative coupling of purified 2,6-di-t-butylphenol, an increase in the degree of oxidation of 2,6-di-t-butylphenol lowers the selectivity of desired 3,3',5,5'-tetra-t-butylbiphenol and increases the formation of by-products such as 3,3',5,5'-tetra-t-butyldiphenoquinone, 2,6-di-t-butylbenzoquinone and carbon dioxide gas. Furthermore, the amount of products of oxidative coupling of 2,6-di-t-butylphenol with t-butylphenols mixed in the crude 2,6-di-t-butylphenol is increased. The inventors have made extensive and intensive studies with a view toward solving the above problems. As a result, it has been found that, in the catalytic oxidative coupling reaction of 2,6-di-t-butylphenol, the solubility of alkali metal catalyst in 2,6-di-t-butylphenol is markedly increased by the use of the alkali metal catalyst in combination with alkylphenols such as t-butylphenols or a liquid mixture of alkylphenols such as t-butylphenols and phenol, so that the oxidative reaction is homogeneously advanced to thereby enable producing 3,3',5,5'-tetra-t-butylbiphenol in high oxidation efficiency with the occurrence of by-products such as carbon dioxide gas and 2,6-di-t-butylbenzoquinone reduced. The present invention has been completed on the basis of the above findings. OBJECT OF THE INVENTION The present invention has been made with a view toward solving the above problems of the prior art. Thus, it is an object of the present invention to provide a process for producing 3,3',5,5'-tetra-t-butylbiphenol in high yield through a homogeneous dispersion of an alkali metal catalyst, which process can be conducted in a continuous manner. SUMMARY OF THE INVENTION The process for producing 3,3',5,5'-tetra-t-butylbiphenol according to the present invention comprises adding catalyst to 2,6-di-t-butylphenol and conducting as oxidative dimerization of the 2,6-di-t-butylphenol in the presence of said catalyst to thereby obtain 3,3',5,5'-tetra-t-butylbiphenol, wherein the catalyst is a catalyst mixture (1) composed of an alkali metal catalyst and alkylphenols or a catalyst mixture (2) composed of an alkali metal catalyst, alkylphenols and phenol is used as the alkali metal catalyst for producing 3,3',5,5'-tetra-t-butylbiphenol. It is preferred that the alkylphenols be contained in the catalyst mixture (1) or catalyst mixture (2) in an amount of 1 to 5 times mole of the alkali metal catalyst, and that water be contained in the catalyst mixture (1) or catalyst mixture (2) in an amount of 2 to 20% by weight. The catalyst mixture (2) preferably contains alkylphenols and phenol in a weight ratio of alkylphenols/phenol of at least 70/30. The above catalyst mixture (1) and catalyst mixture (2) can instantaneously be dissolved in the 2,6-di-t-butylphenol reaction mixture. Therefore, the process for producing 3,3',5,5'-tetra-t-butylbiphenol according to the present invention enables continuously feeding the starting material and the catalyst and, hence, is most suitable for a continuous production of 3,3',5,5'-tetra-t-butylbiphenol. DETAILED DESCRIPTION OF THE INVENTION The process for producing 3,3',5,5'-tetra-t-butylbiphenol according to the present invention will be described in detail below. The process for producing 3,3',5,5'-tetra-t-butylbiphenol according to the present invention comprises conducting an oxidative dimerization of purified 2,6-di-t-butylphenol (i) or crude 2,6-di-t-butylphenol (ii) obtained by reacting isobutylene with phenol in the presence of an aluminum catalyst and removing the aluminum catalyst, in the presence of an alkali metal catalyst to thereby obtain 3,3',5,5'-tetra-t-butylbiphenol, wherein a catalyst mixture (1) composed of an alkali metal catalyst and alkylphenols or a catalyst mixture (2) composed of an alkali metal catalyst, alkylphenols and phenol is used as the alkali metal catalyst for producing 3,3',5,5'-tetra-t-butylbiphenol. Starting material The starting material of 3,3',5,5'-tetra-t-butylbiphenol for use in the present invention is purified 2,6-di-t-butylphenol or crude 2,6-di-t-butylphenol. The crude 2,6-di-t-butylphenol is, for example, a 2,6-di-t-butylphenol composition comprising 2,6-di-t-butylphenol and by-products, produced from phenol and isobutylene, as described in Japanese Patent Publication No. 6(1994)-74227. In the process for producing p,p'-biphenol as described in Japanese Patent Publication No. 6(1994)74227, the content of 2,6-di-t-butylphenol and the content of phenol and p-t-butylphenol (PTBP) among byproducts in the crude 2,6-di-t-butylphenol (by-product containing 2,6-di-t-butylphenol composition mentioned above) are 73 to 82% by weight and up to 0.5% by weight, respectively. Alkali metal catalyst Examples of alkali metal catalysts suitably employed in the present invention include a hydroxide, a carbonate and a bicarbonate of an alkali metal. Of these, an alkali metal hydroxide is preferred. Preferred form is a solution of the alkali metal hydroxide. Further, the alkali metal is preferably sodium and potassium, especially potassium. Although the amount of added alkali metal catalyst is not particularly limited, it is generally preferred that the alkali metal catalyst be used in an amount ranging from 0.3 to 1.5 mol% per 100 mol% of charged 2,6-di-t-butylphenol from the viewpoint that too small an amount thereof causes the reaction rate to be low and too large an amount thereof requires a large quantity of acid for neutralization. In the present invention, the alkali metal catalyst is used in the form of a catalyst mixture (1) comprising the alkali metal catalyst mixed with alkylphenols or in the form of a catalyst mixture (2) comprising the alkali metal catalyst mixed with a liquid mixture of alkylphenols and phenol. Examples of alkylphenols suitably employed in the above catalyst mixtures (1) and (2) include t-butylphenols, cresols, xylenols, propylphenols, sec-butylphenols and mixtures thereof. Of these, t-butylphenols are preferred. Specific examples of the t-butylphenols include o-t-butylphenol (OTBP), p-t-butylphenol (PTBP), m-t-butylphenol (MTBP), 2,4-di-t-butylphenol (2,4TBP) and mixtures thereof. Of these, p-t-butylphenol (PTBP) is preferred. Although not particularly limited, the amount of alkylphenols (solvent) such as t-butylphenols added is preferably such that the concentration of the alkali metal catalyst ranges 5 to 15%. When the alkali concentration is too high, a mixture of the alkali metal catalyst and alkylphenols has a less tendency to be liquid at low temperatures. On the other hand, when the alkali concentration is too low, the alkylphenols for catalyst mixing are needed in greater amount, thereby increasing the formation of oxidative coupling by-products from the alkylphenols and 2,6-di-t-butylphenol. Further, it is preferred that the water content of each catalyst mixture be adjusted so as to fall within the range of 2 to 20% by weight. When the water content is too low, the freezing point of the catalyst mixture becomes high and the catalyst mixture has a less tendency to be liquid at low temperatures. On the other hand, when the water content is too high, the solubility of the catalyst mixture in 2,6-di-t-butylphenol is lowered with the result that the catalyst mixture is likely to settle in the form of liquid drops on the bottom of the reaction vessel. The content of alkylphenols in each of the above catalyst mixtures (1) and (2) generally ranges from 1 to 5 mol and preferably from 2 to 4 mol per mol of the alkali metal catalyst. The water content of the catalyst mixture is in the range of 2 to 20% by weight, preferably, 5 to 15% by weight. In the use of the above crude 2,6-di-t-butylphenol composition as a starting material of 3,3',5,5'-tetra-t-butylbiphenol, the amount of by-product alkylphenols such as o-t-butylphenol (OTBP), p-t-butylphenol (PTBP), 2,4-di-t-butylphenol (2,4TBP) and 2,4,6-tri-t-butylphenol (2,4,6TBP) is considered when the proportion of alkylphenols to be mixed with the alkali metal catalyst is determined. In the present invention, a liquid mixture of the alkylphenols and phenol can be used in place of the alkylphenols in order to lower the freezing point of the catalyst mixture (1) composed of the alkali metal catalyst and alkylphenols. That is, the catalyst mixture (2) can be used in place of the catalyst mixture (1). It is preferred that the above liquid mixture consist of p-alkylphenols and phenol, especially, p-t-butylphenol and phenol. The weight ratio of alkylphenols to phenol [alkylphenols/phenol] in the above catalyst mixture (2) is preferably at least 70/30. When the proportion of phenol is too high, the situation is the same as in the use of an aqueous solution of alkali metal hydroxide alone and the catalyst mixture settles in the form of liquid drops on the bottom of the reaction vessel. In the use of the above crude 2,6-dialkylphenol composition as a starting material of 3,3',5,5'-tetra-t-butylbiphenol, the respective amounts of by-product alkylphenols and phenol are considered when the proportions of alkylphenols and phenol to be mixed with the alkali metal catalyst are determined. The method of mixing the alkali metal catalyst, alkylphenols and phenol is not particularly limited. Generally, the alkylphenols are mixed with phenol in a given ratio to thereby obtain a liquid mixture, and the alkali metal catalyst is added to the liquid mixture and mixed. When the alkali metal catalyst is solid, catalyst heating and dissolving operation is required. When the alkali metal catalyst is an aqueous solution of a hydroxide, the aqueous solution of alkali metal hydroxide is mixed with a liquid mixture of alkylphenols and phenol, optionally followed by heating and dehydration, and used in the catalytic reaction. Preferably, heating and dehydration are not conducted. The alkali metals used as a catalyst, alkylphenols and liquid mixture of alkylphenols and phenol are soluble in arbitrary proportions in the purified 2,6-di-t-butylphenol, 2,6-di-t-butylphenol composition and oxidation reaction mixture thereof, so that they can be used in both a batch reaction and a continuous reaction. That is, not only is it feasible to first add the purified 2,6-di-t-butylphenol or crude 2,6-di-t-butylphenol and the catalyst mixture (1) or (2), secondly heat the mixture to a given temperature and thirdly blow air or other oxygen containing gas thereinto to thereby carry out an oxidative reaction but also the purified 2,6-di-t-butylphenol or crude 2,6-di-t-butylphenol together with the catalyst mixture (1) or (2) can continuously be charged while performing an oxidative reaction. The above catalyst mixture (1) or (2) and purified 2,6-di-t-butylphenol or crude 2,6-di-t-butylphenol can separately be introduced in the reaction vessel. Alternatively, the catalyst mixture (1) or (2) and purified 2,6-di-t-butylphenol or crude 2,6-di-t-butylphenol can be mixed together just prior to introduction into the reaction vessel, and thereby introduced into the reaction vessel. The oxidative reaction temperature ranges from 150 to 250°C, preferably, from 180 to 200°C. Although the reaction pressure may be atmospheric or superatmospheric, the oxidative reaction is preferably conducted at 98 to 490 kPa (1 to 5 kg/cm 2 ). Although the oxygen source for use in the oxidative reaction may be selected from among pure oxygen, air and an oxygen containing gas diluted with inert gas, the oxidative reaction is generally preferably conducted with the use of air or air diluted with nitrogen gas. The amount of oxygen absorbed by the 2,6-di-t-butylphenol as a starting material of 3,3',5,5'-cetra-t-butylbiphenol ranges from 10 to 30 mol%, preferably, from 15 to 25 mol% per 100 mol% of charged 2,6-di-t-butylphenol. When the oxygen absorption is low, 2,6-di-t-butylphenol remains unreacted in greater proportion to thereby lower the yield of 3,3',5,5'-tetra-t-butylbiphenol. On the other hand, when the oxygen absorption is too high, a large amount of by-products such as 3,3',5,5'-tetra-t-butyldiphenoquinone (DPQ) occur. The above process for producing 3,3',5,5'-tetra-t-butylbiphenol according to the present invention enables homogeneously performing the oxidative coupling reaction through a homogeneous dispersion of the alkali metal catalyst in the starting material with the result that 3,3',5,5'-tetra-t-butylbiphenol can be obtained in high yield. EFFECT OF THE INVENTION The process for producing 3,3',5,5'-tetra-t-butylbiphenol according to the present invention enables homogeneously performing the oxidative coupling reaction through a homogeneous dispersion of the alkali metal catalyst in the starting material with the result that 3,3',5,5'-tetra-t-butylbiphenol can be obtained in high yield. Further, this process enables producing 3,3',5,5'-tetra-t-butylbiphenol both in batches and continuously. EXAMPLE The present invention will now be illustrated in greater detail with reference to the following Examples. Example 1 Phenol was reacted with isobutylene in the presence of an aluminum phenoxide catalyst by the customary procedure. Thereafter, the aluminum phenoxide catalyst was removed, thereby obtaining crude 2,6-di-t-butylphenol. This crude 2,6-di-t-butylphenol contained, as by-products, 0.5% by weight of phenol, 4.8% by weight of o-t-butylphenol, 0.6% by weight of p-t-butylphenol, 1.2% by weight of 2,4-di-t-butylphenol and 15% by weight of 2,4,6-tri-t-butylphenol and contained, as a principal product, 77.3% by weight of 2,6-di-t-butylphenol. 600 g of the obtained crude 2,6-di-t-butylphenol was charged into a 1 lit. cylindrical glass flask having an inside diameter of 80 mm and a height of 180 mm, purged with nitrogen gas and heated to 195°C to thereby obtain a liquid crude 2,6-di-t-butylphenol. 10 g of an alkali metal catalyst mixture obtained by adding 25 g of a 48% aqueous potassium hydroxide solution to a liquid mixture of 80 g of p-t-butylphenol (PTBP) and 10 g of phenol (PhOH) and conducting a mixing was dropped from the top of the cylindrical glass flask into the above obtained liquid crude 2,6-di-t-butylphenol. The alkali metal catalyst mixture was instantaneously dissolved in the liquid crude 2,6-di-t-butylphenol, thereby obtaining a homogeneous solution. 500 g of the resultant alkali metal catalyst containing crude 2,6-di-t-butylphenol was transferred to a 1 lit. autoclave, and an oxidative coupling was performed without agitation while blowing air thereinto at a rate of 150 ml/min for 6 hr under conditions such that the reaction temperature and reaction pressure were 195°C and 294 kPa (3 kg/cm 2 -G), respectively. A gas chromatography analysis showed that the oxidative reaction mixture comprised 14.3% by weight of unreacted 2,6-di-t-butylphenol and 61.5% by weight of 3,3',5,5'-tetra-t-butylbiphenol. Further, it was shown that the degree of oxidation of 2,6-di-t-butylphenol was 81.8 mol% and that the existence yield of desired 3,3',5,5'-tetra-t-butylbiphenol was 79.4 mol% (based on charged 2,6-di-t-butylphenol). Example 2 In place of the crude 2,6-di-t-butylphenol of Example 1, 600 g of purified 2,6-di-t-butylphenol having a purity of 99.9% was charged into a 1 lit. cylindrical glass flask having an inside diameter of 80 mm and a height of 180 mm, purged with nitrogen gas and heated to 195°C to thereby obtain a liquid purified 2,6-di-t-butylphenol. 10 g of an alkali metal catalyst mixture obtained by adding 25 g of a 48% aqueous potassium hydroxide solution to a liquid mixture of 80 g of p-t-butylphenol (PTBP) and 10 g of phenol (PhOH) and conducting a mixing was dropped from the top of the cylindrical glass flask into the above obtained liquid purified 2,6-di-t-butylphenol. The alkali metal catalyst mixture was instantaneously dissolved in the liquid purified 2,6-di-t-butylphenol as observed in Example 1, thereby obtaining a homogeneous solution. 500 g of the resultant alkali metal catalyst containing purified 2,6-di-t-butylphenol was transferred to a 1 lit. autoclave, and an oxidative coupling was performed without agitation while blowing air thereinto at a rate of 150 ml/min for 8 hr under conditions such that the reaction temperature and reaction pressure were 195°C and 294 kPa (3 kg/cm 2 -G) respectively. A gas chromatography analysis showed that the oxidative reaction mixture comprised 15.4% by weight of unreacted 2,6-di-t-butylphenol and 83.1% by weight of 3,3',5,5'-tetra-t-butylbiphenol. Further, it was shown that the degree of oxidation of 2,6-di-t-butylphenol was 84.6 mol% and that the existence yield of desired 3,3',5,5'-tetra-t-butylbiphenol was 83.6 mol% (based on charged 2,6-di-t-butylphenol). Examples 3 and 4 600 g of crude 2,6-di-t-butylphenol of Example 1 was charged into a 1 lit. cylindrical glass flask having an inside diameter of 80 mm and a height of 180 mm, purged with nitrogen gas and heated to 195°C to thereby obtain a liquid 2,6-di-t-butylphenol. About 10 g of each of alkali metal catalyst mixtures consisting of a 48% aqueous potassium hydroxide solution, p-t-butylphenol (PTBP) and phenol (PhOH), specified in the following Table 1, was prepared and separately dropped from the top of the cylindrical glass flask into the above obtained liquid 2,6-di-t-butylphenol. The mode of dissolution of liquid drops was observed. As a result, both the alkali metal catalyst mixtures specified in Table 1 were almost instantaneously dissolved in the liquid crude 2,6-di-t-butylphenol as observed in Example 1, thereby obtaining homogeneous solutions. (alkali metal catalyst mixture) Example No. TBPs TBPs/PhOH (wt.ratio) TBPs/KOH (mol.ratio) KOH conc. (wt.%) 3 PTBP 80/20 2.5 9.6 4 PTBP 73/27 2.5 8.9 Note TBPs: t-butylphenols, and PTBP: p-t-butylphenol. Examples 5 to 7 100 g of a liquid mixture of t-butylphenols and phenol was charged in a 500 ml four-necked flask. A given amount of a 48% aqueous potassium hydroxide solution was added thereto, heated to 200°C and dehydrated. Thus, alkali metal catalyst mixtures (all solid at room temperature) specified in the following Table 2 were prepared. 600 g of crude 2,6-di-t-butylphenol having the same composition as in Example 1 was charged into the same cylindrical glass flask as in Example 1, purged with nitrogen gas and heated to 195°C. 5 to 10 g of each of the alkali metal catalyst mixtures (cakes) specified in Table 2 was separately dropped from the top of the flask thereinto. The mode of dissolution of the alkali metal catalyst mixture cakes was observed. As a result, all the alkali metal catalyst mixture cakes specified in Table 2 were instantaneously dissolved in the liquid crude 2,6-di-t-butylphenol as observed in Example 1. (alkali metal catalyst mixture) Example No. TBPs TBPs/PhOH (wt.ratio) TBPs/KOH (mol.ratio) Water content (wt.%) 5 OTBP 100/0 2.8 2.0 6 PTBP 90/10 2.3 2.0 7 PTBP 70/30 1.8 2.5 Note TBPs: t-butylphenols, OTBP: o-t-butylphenol, and PTBP: p-t-butylphenol. Comparative Example 1 600 g of crude 2,6-di-t-butylphenol having the same composition as in Example 1 was charged into a 1 lit. cylindrical glass flask having an inside diameter of 80 mm and a height of 180 mm, purged with nitrogen gas and heated to 195°C to thereby obtain a liquid crude 2,6-di-t-butylphenol. 2.6 g of a 48% aqueous potassium hydroxide solution was dropped from the top of the cylindrical glass flask into the liquid crude 2,6-di-t-butylphenol. The aqueous potassium hydroxide solution settled in liquid drops on the bottom of the flask. Water was boiled off with the catalyst KOH left aggregating on the bottom of the flask. 500 g of the supernatant of the crude 2,6-di-t-butylphenol containing the alkali metal catalyst was transferred to a 1 lit. autoclave, and an oxidative coupling was performed without agitation while blowing air thereinto at a rate of 150 ml/min for 6 hr under conditions such that the reaction temperature and reaction pressure were 195°C and 294 kPa (3 kg/cm 2 -G) respectively. The reaction rate was slow and the oxidative reaction stopped prior to completion of the reaction. A gas chromatography analysis showed that the oxidative reaction mixture comprised 52.1% by weight of unreacted 2,6-di-t-butylphenol and 23,7% by weight of 3,3',5,5'-tetra-t-butylbiphenol. Further, it was shown that the degree of oxidation of 2,6-di-t-butylphenol was 33.0 mol% and that the existence yield of desired 3,3',5,5'-tetra-t-butylbiphenol was as low as 30.5 mol% (based on charged 2,6-di-t-butylphenol). Comparative Examples 2 to 4 600 g of crude 2,6-di-t-butylphenol having the same composition as in Example 1 or 600 g of purified 2,6-di-t-butylphenol having a purity of 99.9% was charged into the same cylindrical glass flask as in Example 1, purged with nitrogen gas and heated to 195°C. 5 to 10 g of each of 48%, 24% and 6% aqueous potassium hydroxide solutions was separately dropped from the top of the flask thereinto. The mode of dissolution of liquid drops was observed. As a result, in all the comparative examples specified in the following Table 3, each of the aqueous potassium hydroxide solutions settled in liquid drops on the bottom of the flask and, thereafter, water was boiled off with the catalyst KOH left aggregating on the bottom of the flask, as observed in Comparative Example 1. (aqueous alkali solution) Comp. Ex. No. Starting mat'l 26B Purity of 26B (wt.%) Conc. of aq. KOH soln. (wt.%) 2 crude 77.3 24 3 crude 77.3 6 4 purified 99.9 48 Note 26B: 2,6-di-t-butylphenol. Comparative Examples 5 to 7 600 g of crude 2,6-di-t-butylphenol having the same composition as in Example 1 was charged into the same glass flask as in Examples 1 to 3, purged with nitrogen gas and heated to 195°C to thereby obtain a liquid crude 2,6-di-t-butylphenol. 5 to 10 g of each of the alkali metal catalyst mixtures specified in the following Table 4 was separately dropped from the top of the flask thereinto. The mode of dissolution of liquid drops was observed. As a result, in all the comparative examples specified in the following Table 4, each of the aqueous potassium hydroxide solutions settled in liquid drops on the bottom of the flask and, thereafter, water brought into contact with the flask bottom was boiled off with the catalyst KOH left aggregating on the bottom of the flask, as observed in Comparative Example 1. (alkali metal catalyst mixture) Comp.Ex. No. TBPs TBPs/PhOH (wt. ratio) PhOH/KOH (mol. ratio) Water Content (wt.%) KOH conc. (wt.%) 5 PTBP 67/33 1.5 11.2 10.4 6 - 0/100 2.5 2 21.4 7 PTBP 50/50 2.1 5 16 Note TBPs: t-bucylphenols, and PTBP: p-t-butylphenol.

Claims: A process for producing 3,3',5,5'-tetra-t-butylbiphenol, which process comprises adding catalyst to 2,6-di-t-butylphenol and conducting an oxidative dimerization of the 2,6-di-t-butylphenol in the presence of said catalyst, wherein the catalyst is a catalyst mixture comprising an alkali metal catalyst and an alkylphenol or an alkali metal catalyst, an alkylphenol and phenol. A process according to claim 1, which comprises the oxidative dimerization of purified 2,6-di-t-butylphenol or of crude 2,6-di-t-butylphenol obtainable by reacting isobutylene with phenol in the presence of an aluminum catalyst followed by removal of the aluminum catalyst. A process according to claim 1 or claim 2, wherein the catalyst mixture contains 1 to 5 moles of alkylphenol per mole of alkali metal catalyst, and wherein the catalyst mixture contains 2 to 20% by weight of water. A process according to any one of the preceding claims, wherein the catalyst mixture contains alkylphenol and phenol in a weight ratio of alkylphenol/phenol of at least 70/30. A process according to any one of the preceding claims, wherein the alkylphenol comprises p-t-butylphenol.

Inventors: ISOTA YOICHIRO, KAWAHARA MIKIO, YAO KAZUHIKO, ISOTA, YOICHIRO, KAWAHARA, MIKIO, YAO, KAZUHIKO
Applicants: HONSHU CHEMICAL IND, HONSHU CHEMICAL INDUSTRY CO. LTD.
Application year: 1998
Main classification: C07C37
IPC codes: C07C39, C07C37