Liquid alkylated diphenylamine antioxidant

Abstract: A process for monoalkylating diphenylamine using a clay catalyst is disclosed which results in a reaction product having substantial amounts of desirable monoalkylated diphenylamine and minimal amounts of less desirable disubstituted diphenylamine and unsubstituted diphenylamine. The disclosed process uses clay catalysts which favor monoalkylation over dialkylation and specific conditions such as reaction temperature, mole ratios of alkylating olefin to diphenylamine, reaction times, and catalyst amounts. Preferred olefins are linear alpha olefins having from 6 or 8 to 18 carbon atoms. When more than 1 wt. % residual unreacted diphenylamine is present it may be converted to alkylated diphenylamine by reacting with styrene, alpha-methylstyrene or isobutylene.

Description: FIELD OF THE INVENTION A process for alkylating diphenylamine which results in high amounts of desirable monosubstituted diphenylamine antioxidants is described. These monoalkylated diphenylamines have good antioxidant activity in various lubricating oils and polymeric molding compositions. These antioxidants are low in yellow color, resist further yellowing and are liquid at 25°C. BACKGROUND OF THE INVENTION U.S.-A- 2,943,112 ('112) teaches that alkylated diphenylamines are useful as antioxidants (AO). Therein it describes the alkylation of diphenylamine with relatively unreactive olefins, such as secondary alkenes (column 4, line 9-23), followed by an alkylation reaction with more reactive olefins to scavenge the unreacted diphenylamine to a concentration of less than 3%. This avoided the necessity of distilling out the undesirable unsubstituted diphenylamine. The reference mentions clay and other alkylation catalysts. Although clay offers some other processing advantages (e.g. easier separation of the catalyst from the.product) the reference doesn't differentiate between the alkylation catalysts based on the relative proportions of mono and disubstituted diphenylamine in the reaction products. As disclosed in U.S.-A-2,943,112 column 2, lines 26-29, monoalkylated diphenylamine.is believed to be more reactive than unsubstituted diphenylamine in alkylation reactions. It was difficult to get high amounts of monoalkylated. diphenylamine because as soon as the diphenylamine was monoalkyled it soon thereafter was dialkylated minimizing the amount of monoalkylated diphenylamine. The use of clay as catalyst in the alkylation of diphenylamine is disclosed in U.S.-A-3,452,056 which describes the alkylation of diphenylamine with alpha-methyl styrene and related olefins with clay as the catalyst. In the prior art, clay was mentioned as having the advantage of giving lighter colored product and being easy to remove by filtration after the reaction. As a catalyst, clay and other Lewis Acids, such as AlCl 3 or BF 3 (column 4, line 57-69) are generally taught as being interchangeable. U.S.-A-4,824,601 ('601) teaches that liquid antioxidants are desirably (column 1, line 26-33) prepared from diphenylamine and diisobutylene at reaction temperatures above 160°C (abstract). At the high temperatures, the octyl group was cracked to give a butyl group and perhaps unsubstituted diphenylamine itself (column 2, line 66-68). The high temperature alkylation was continued until less than 25% dioctyl-diphenylamine (DOD) was present. When diisobutylene is the alkylating olefin and the temperature is above 160°C substantial amounts of dioctyldiphenylamine are produced. Dioctyldiphenylamine is a liquid at the alkylation temperature, but when present above 20 or 25 wt. % in the product at room temperature, it results in a solid product which is more difficult to handle or transport since it cannot be pumped from the reactor. Unsubstituted diphenylamine is less desirable as an antioxidant because it sensitizes human skin to other irritants and tends to yellow in the composition in which it is used. High vacuum distillation can separate unsubstituted, monosubstituted, and polysubstituted diphenylamines but this is an expensive and time consuming process step. SUMMARY OF INVENTION A process for alkylating unsubstituted diphenylamine is disclosed which selectively causes a higher proportion of monoalkylsubstitution and produces lower amounts of less desirable unsubstituted diphenylamine and/or disubstituted or polysubstituted diphenylamines than the prior art. The process uses a clay catalyst (which has greater selectivity in alkylation reactions than other alkylation catalysts) and generally uses milder conditions (e.g. lower temperatures or shorter times). Disclosed olefins that result in higher proportions of monoalkylsubstitution are C 6 -C 18 linear olefins with unsaturation between the first and second carbon atom. In particular, the present invention relates to a process for alkylating diphenylamine, comprising: reacting unsubstituted diphenylamine with one or more linear olefins having from 6 to 18 carbon atoms and unsaturation between the first and second carbon atoms in the presence of clay catalysts at a temperature from 130°C to 200°C for at least one hour with a mole ratio of said linear olefin:diphenylamine of from 1:0.556 to 1:0.909 and wherein said unsubstituted diphenylamine contains less than 20 wt.% of monosubstituted diphenylamine and disubstituted diphenylamine, and forming a mixture of diphenylamine and alkylated diphenylamines having at least 50 wt. % monoalkyl substituted diphenylamine, less than 50 wt. % of di and polyalkylsubstituted diphenylamine, and from 0.01 to 5 wt. % of unsubstituted diphenylamine, and wherein said weight percents are based on said mixture of diphenylamine and alkylated diphenylamine. DETAILED DESCRIPTION A process is described for making liquid and highly active antioxidants (AOs) from the alkylation of diphenylamine (DPA) with commonly known and inexpensive classes of olefins, namely the linear alpha-olefins. Commercially available diphenylamine is generally free of alkyl substituents. However alkyl substituents do not interfere with subsequent alkylation so the source of diphenylamine may contain small amounts (e.g. up to 2, 5, 10 or 20 wt. %) of alkylated diphenylamine. Alternatively, when alkylating with the relatively unreactive linear alpha-olefins (a secondary alkene with terminal unsaturation as defined in U.S. -A-2,943,112, column 4, line 9-13), one can either make high amounts of dialkyl DPA along with low amounts of unreacted DPA by using large excess of linear alphaolefin, or one can make high amounts of monoalkyl DPA and high amounts of unreacted DPA by using deficient amount of olefin. Monoalkylated diphenylamines are very desirable for use as antioxidants. As disclosed in U.S.-A-2,943,112 column 1, lines 28-34, if the alkyl group has six or more carbon atoms the monoalkylated diphenylamine will be low in yellow color and will resist yellowing. Di or polysubstituted diphenylamines are less effective than monosubstituted diphenylamines on a weight basis because additional alkylation significantly reduces the number of moles of diphenylamine per gram. For example diphenylamine weighs 169 g/mole, monooctyldiphenylamine weighs 281 g/mole and dioctyldiphenylamine weighs 393 g/mole. As set forth in the background it is desirable to remove most of the unreacted (non alkylated) diphenylamines before the reaction product is used as an antioxidant. As set forth later the unreacted diphenylamine may be reacted with more reactive olefin(s) (isobutylene, styrene and/or alpha-methylstyrene) to convert residual unreacted diphenylamine to alkylated diphenylamine. Clay usually results in a lower degree of yellow color in the alkylated product than other alkylation catalysts because clay preferentially absorbs colored species. The clay is preferably an acid activated bentonite clay. Clay is thought to require higher temperatures and/or longer reaction times than AlCl 3 and related catalysts. It has unexpectantly been found that clay (e.g. acid activated bentonite clay) when used as a catalyst for alkylating diphenylamine results in proportionally more monoalkylated diphenylamine.than the other alkylation catalysts such as AlCl 3 , BF 3 , Et 2 O, and SbCl 3 . When the particular olefin used and the other reaction conditions are. optimized the amount of desirable monoalkylated diphenylamine can be substantial and the amounts of less desirable unsubstituted and polysubstituted diphenylamine can be kept low. These desirable percentages of products are a result of the clay catalysts preferentially catalyzing the alkylation reaction of the unsubstituted diphenylamine rather than the further alkylation of monoalkyl-diphehylamine. The tetrahedral and octahedral layers of clay specifically and precisely repeated are believed to offer less access to the monoalkyldiphenylamine molecule with its bulky tertiary octyl groups than the unsubstituted diphenylamine molecule to the reactive sites in the catalysts. The monoalkylated diphenylamine is formed is converted to 'dialkylated or polyalkylated diphenylamine at a slower rate with clay catalyst than with other catalysts allowing the concentration of mono-alkylated diphenylamine to increase in the reaction product . Note that by specifying clay catalyst the use of amounts of AlCl 3 , ZnCl 3 , SnCl 4 , H 3 PO 4 , BF 3 or other alkylation catalysts other than acidified clay is restricted to those amounts that would be ineffective to cause 10 percent or more of the total alkylation under the conditions specified. The clays useful in alkylation reaction of diphenylamines are those used for bleaching oils and waxes. These are often referred to as acid activated clays. Preferred clays are sub-bentonites or bentonites which are characterized by rapid slaking when it is in the air dried state and only a slight swelling when placed in water. They consist predominantly of the clay mineral montmorillonite. The clay can be used in alkylation reactions in amounts from 0.5 or 1 wt. % to 60 percent and, and in one embodiment more desirably from 2 to 20 wt. % based on the amount of unsubstituted diphenylamine used as a reactant. In another embodiment the clay is from 0.5 to 4 or 5 weight %, more desirably from 1 to about 3 weight percent based upon the amount of unsubstituted diphenylamine used as a reactant. Commercially available clay catalysts include Filtrol™ and Retrol™ available from Engelhard; Fulcat™ 14, Fulmont™ 700C, Fulmont™ 237, and Fulcat™ 22B available from Laporte Industries; and Katalysator™ K10 available from Sud-Chemie. These clays may include acid activated or acid leached clays. Acid activated clays are preferred. The clay catalysts may contain some water as received. Removal of the water prior to use results in a lighter colored reaction product. Therefore it is desirable to use a low water content clay or to remove the water by heating the clay with a nitrogen sweep or with vacuum stripping. Clays are aluminosilicates. The aluminum III cations are bonded to an octahedral arrangement of oxygen anions. Repetition of these AlO 6 units in two dimensions formed an octahedral layer. Likewise a tetrahedral layer is formed from SiO 4 silicate units. Clays are classified according to the relative. number of tetrahedral and octahedral layers. Montmorillonite clays, which have been used in organic chemical applications, have a octahedral layer sandwiched between two tetrahedral layers. The linear alpha olefins are reacted with the solution of unsubstituted diphenylamine in the presence of clay catalysts to produce a diphenylamine alkylated with linear alpha olefins. This reaction yields increased amounts of monoalkylated diphenylamine due to the use of clay catalysts. The linear alpha olefin can have from 6 or 8 to 18 carbon atoms and has a carbon to carbon double bond between the first and second carbon atoms of the molecule. Desirably the linear olefin has only one carbon to carbon double bond. It may be a single alpha olefin or a mixture of alpha olefins. The reaction temperature when using alpha olefins is desirable from 130 to 200°C, more desirably from 140 to 190°C, and preferably from 160 to 185°C for at least one hour, more desirably from 2 hours to 10 hours, and preferably from 4 to 8 hours. In the alkylation reaction of diphenylamine with linear alpha olefins in the presence of clay the mole ratio of linear alpha olefin:diphenylamine is from 1:0.909 to 1:0.556. Most desirably it is from 1:0.833 to 1:0.667. The reaction of linear alpha olefins with diphenylamine in the presence of clay catalyst desirably results in at least 50 wt. % (e.g. 50 to 95 wt. %), more desirably at least 60 wt. % and preferably at least 65 wt. % of monoalkyldiphenylamine based on the total diphenylamine and alkylated diphenylamine in the reaction product. Desirably it results in less than 50 wt . % (e.g. 5 to 50 wt . %), more desirably less than 40 wt. %, and preferably less than 35 wt . % di and/or polysubstituted diphenylamines. Desirably the amount mole ratio of monoalkylated DPA:dialkylated DPA is greater than 1:1. Desirably it results in less than 5 wt. %, e.g. from 0.01 to 5 wt. % , more desirably from 0.01 to 2 wt. %, and preferably from 0.01 to 1 wt. % unsubstituted diphenylamine. Unlike dioctyldiphenylamine the disubstituted diphenylamines.resulting from alkylation with linear α-olefins do not cause solidification of the alkylation reaction product when present at concentrations above 25 wt. %. The unsubstituted diphenylamine or solution of diphenylamines used as a reactant is as previously described. In a preferred embodiment the relative amount of unsubstituted diphenylamine remaining after alkylation with linear α-olefins is low (lower than in the reaction using diisobutylene as the alkylating agent of diphenylamine). This low amount of unsubstituted diphenylamine is accomplished by creating more di-substituted diphenylamine which (as specified above) does not cause solidification in this alkylation reaction product. The addition of linear alpha olefins to the reaction is preferably batch but may be metered, sequential, or another addition method. If greater than 1, 2, or 3 weight percent of unreacted diphenylamine remains in the reaction product of the linear alphaolefin(s) and diphenylamine, that reaction product may be further reacted with the more reactive olefin(s) (e.g., isobutylene, styrene and/or alphamethylstyrene) as previously set forth for the reaction product of diisobutylene with diphenylamine. Sufficient amounts of the more reactive olefin(s) and suitable reaction time and temperatures are used to result in less than 3, 2, or 1 wt. % unreacted diphenylamine. Although solvents have been used in alkylation reactions it is preferred in this process to alkylate with minimal solvent (e.g. less than 5 wt.% solvent based on the mixture of. olefin, diphenylamine, and clay) or no solvent at all. The unreacted olefins may be removed from the reaction product by distillation. Similarly the unreacted diphenylamine may be removed by processes such as fractional distillation or vacuum distillation if necessary. The amount of unreacted diphenylamine is desirably less than 2 or 3 wt. % in the final product. The clay can be removed by filtration or other known separation methods. The alkylation reaction can be carried out in an autoclave if high pressures due to the vapor pressure of the olefin are anticipated. The pressure used for the reaction is primarily controlled by the olefin used and the reaction temperature. As the product is always liquid, the reactants and products may be pumped into and out of the reactor. The alkylated diphenylamine antioxidants of this disclosure are useful to stabilize natural source and synthetic source oils and polymers from oxidative degradation during processing reactions and in their final use as lubricants or articles. They may be used in combination with other antioxidants and additives. The following examples show the alkylation reaction of diphenylamine with linear alpha olefins. Example DPA (0.1 mole, 16.9g) was reacted with 1-tetradecene (0.133 mole, 26.1g) in the presence of a catalyst (Retrol™ clay or AlCl 3 ) to form unsubstituted diphenylamine (DPA) monoalkylated diphenylamine (M14D) and dialkylated diphenylamine (D14D) where the alkyl group was tetradecane. PRODUCT CATALYST & REACTION CONDITIONS DPA M14D D14D 3.0g Retrol™, 175-180°C, 6 hrs < 1 WT % 69.9 WT % 30.1 WT % 1.3g AlCl 3 , 150°C, 8 hrs 12.9 45.9 41.2 1.3g AlCl 3 , 175-180°C, 4.5 hrs 8.4 50.7 40.9 This example shows that Retrol™, a clay, selectively forms more monoalkylated diphenylamine than other alkylation catalysts (e.g. AlCl 3 ). This example also shows that other alkylation catalysts form more of the less desirable disubstituted diphenylamines (having low activity on a weight basis) and leave more unsubstituted diphenylamine in the final product than clay. The unsubstituted diphenylamine is costly to remove or further react. The process minimizes or eliminates olefin cracking, minimizes the amount of olefins used, results in lightly colored products, uses mild conditions, may recycle or further use the clay catalyst, and provides more control over the final composition of the substituted DPA product. The process also provides a reaction product with higher antioxidant efficiency on a weight basis because the product has less unnecessary dialkyl substituents on the diphenylamine. The antioxidants of this disclosure are useful in lubricating oils, polymers, and other hydrocarbon materials subject to oxidative degradation. Lubricating oils are well known and include mineral oils, such as those made from petroleum distillates, arid synthetic ester oils, such as made from mono, di and polycarboxylic acids reacted with mono, di, or polyols in . ratios such that low volatility solutions with desirable viscosities are obtained.

Claims: A process for alkylating diphenylamine, comprising: reacting unsubstituted.diphenylamine with one or more linear olefins having from 6 to 18 carbon atoms and unsaturation between the first and second carbon atoms in the presence of clay catalysts at a temperature from 130°C to 200°C for at least one hour with a mole ratio of said linear olefin:diphenylamine of from 1:0.556 to 1:0.909 and wherein said unsubstituted diphenylamine contains less than 20 wt.% of monosubstituted diphenylamine and disubstituted diphenylamine, and forming a mixture of diphenylamine and alkylated diphenylamines having at least 50 wt. % monoalkyl substituted diphenylamine,. less than 50 wt. % of di. and polyalkylsubstituted diphenylamine, and from 0.01 to 5 wt. % of unsubstituted diphenylamine, and wherein said weight percents are based on said mixture of diphenylamine and alkylated diphenylamine. A process according to Claim 1 , wherein said temperature is from 140°C to 190°C. A process according to Claim 1, wherein said clay is an acid activated bentonite clay. A process according to Claim 1, wherein said monoalkyl substituted diphenylamine is at least 60 wt. %, and said di and polysubstituted diphenylamine is less than 40 wt. %, said unsubstituted diphenylamine is from 0.01 to 2 wt. %.

Inventors: FILLA DEBORAH S, LAI JOHN T, FILLA, DEBORAH S., LAI, JOHN T.
Applicants: NOVEON IP HOLDINGS CORP, NOVEON IP HOLDINGS CORP.
Application year: 1997
Main classification: C07C209
IPC codes: C09K15