GB2308849A

GB2308849A - Anti-knock additive

Info

Publication number: GB2308849A
Application number: GB9600284A
Authority: GB
Inventors: Miltos J Papachristos; Jonathan S Evans
Original assignee: Associated Octel Co Ltd
Current assignee: Innospec Ltd
Priority date: 1996-01-08
Filing date: 1996-01-08
Publication date: 1997-07-09
Anticipated expiration: 2016-01-08
Also published as: GB2308849B; GB9600284D0

Abstract

Anti-knock additive compositions comprise a dihydro benzoxazine as anti-knock agent in a gasoline miscible carrier or diluent. The agent is typically of formula where R is -H or a hydrocarbyl group and R 1 to R 4 are -H, hydrocarbyloxy or hydrocarbyl groups.

Description

Gasoline Compositions This invention relates to improved fuels for internal combustion engines. More particularly, it relates to improving the anti-knock properties of the fuel by the addition of novel metal-free ashless anti-knock agents.

The petroleum industry has long recognized the need for greater fuel economy and efficiency in the operation of gasoline powered spark-ignition engines. It is also recognized that the most economical burning of the fule is obtained at the higher compression ratios, and so to obatain high performance in high compression ratio engines without the risk of knock damage, fuels must possess a high octane number and thus good anti-knock properties.

Whilst octane ratings of fuels can be improved by additional refining operations, the additional refinery processes necessary are extremely costly. Refiners have therefore employed anti-knock additives as a cost effective means to increase the octane number of the fuel.

Numerous compounds have been suggested as anti-knock additives for fuel compositions. Of the anti-knock agents that have been used most are those of the organometallic type, and the most successful of these have been the organo-lead agents. However, their use is now severely limited by incompatibility with exhaust catalysts, and by environmental concerns which may mean that their use may be prohibited in future. Numerous non-lead, anti-knock compounds have been suggested as alternatives, including organo-iron derivaltives such as iron pentacarbonyl and ferrocene, and organo-manganese products such as methylcyclopentadienyl manganese tricarbonyl (MMT). However, these additives have their own associated problems when blended into fuels for internal combustion engines, such as increased engine wear (ferrocene) and alleged toxic emissions into the environment (MMT).

Alkali metal salts have also been claimed as anti-knock agents on a number of occasions. US-A-3,367,759 claimed the use of alkali metal oxides of hydroxy-ethers as anti-knock agents, US-A-3,771,979 claimed the use of alkali metal salts of diarylcarbamates, and US-A3,770,397 claimed the use of alkali metal salts of Nalkyl and N,N-dialkylaminoalkylphenols of the general formula

(where R1 is alkyl, R2 is alkyl or hydrogen, and M is an alkali metal) as effective anti-knock agents.

The art has for some time now sought anti-knock agents of the non-metallic type, which will not pollute the environment with toxic metals following their combustion.

The most widely known organic metal-free anti-knock agents that have been proposed include aniline and alkyl derivatives thereof such as N-methylaniline and Nethylaniline, phenylenediamines, nitriles such as proprionitrile, alkyl carbonates and malonates, tertiary alkyl ethers, and alcohols such as ethanol and tertiary butanol.

o-Azidoanilines were claimed as anti-knock agents in US A-4,266,947, while US-A-3,685,976 claimed the use of 1,3-diimino-2-hydroxypropanes of the following general formula:

(wherein R and R1 are the same or different and are alkyl, cycloalkyl, aromatic or substituted aromatic groups, and the R2, R3, R4 and R5 groups are hydrogen or methyl groups) . An example of such a compound is N,N'disalicylidine-2-hydroxy-1,3-propane

Benzylic amine compounds such as N-methylbenzylamine and dibenzylamine were claimed as ashless anti-knock agents in US-A-4,321,063.

2-Alkylamino- and dialkylaminomethyl phenol derivatives have been claimed as ashless anti-knock agents on a number of occasions. The use of N-substituted amine derivatives of 3-hydroxypyridine compounds was claimed in US-A-4,295,861, and the use of 2 (dialkylaminomethyl)phenols, hereafter referred to as DMAMP, substituted in the 4-position with fluoride and methoxy groups, of the following general formula, was claimed in US-A-4,378,231:

(where R is a fluoro- or methoxy- group). The use of these compounds to improve octane quality was described more fully by L Burns in Chem. Tech., Dec. 1984, pp.

1782-7, where it was disclosed that DMAMP was more effective as an anti-knock agent than the sum of the effectiveness observed for phenol and N,Ndimethylbenzylamine when the Research octane number (RON) gain was measured at a concentration of 0.1 mol/l:

ARON 2. 7 0.7 0.6 Tertiary amines have more commonly been found to be proknock, and hence the anti-knock effect of DMAMP was surprising. Burns proposed a mechanism to explain the effectiveness of DMAMP involving the pyrolytic loss of dimethylamine on introduction of DMAMP to the engine cylinder:

ARON 2. 7 0.7 2.0 The elimination was facilitated by a low bond dissociation energy for the O-H group and a quasi-sixelectron arrangement. When dimethylamine was introduced into the engine cylinder at 0.1 mol/l with the fuel, an increase of 2.0 RON was observed. Further evidence to support the mechanism came from a pyrolysis experiment, where dimethylamine was trapped in 60% yield conversion from DMAMP.

N-Alkylaminomethylphenol derivatives of the following formula were subsequently investigated by Papachristos, Swithenbank et al. (J. Inst. Energy, June 1991, 63, pp.

113-123):

(where R is alkyl typically containing 1 to 10 carbon atoms, and Rl is hydrogen or alkyl). These aminated phenols, synthesised from phenol, formaldehyde and the corresponding primary amine RNH2 by the Mannich reaction, are known as Mannich base phenols.

Products of the above formula I have also found use as dispersants, detergents, stability improvers, anti-icing additives, anti-rust addititves and anti-oxidants for liquid fuels. For instance, products derived from a long chain substituted phenol, formaldehyde, and a polyamine of the following formula:

(in which R is a long chain alkyl group containing from about 8 to about 40 carbon atoms (preferably n-dodecyl), and x is from 1 to about 8) have been claimed as dispersant additives in US-A-4,090,854, 4,938,880, 5,026,495, and 5,306,313.

Products derived from higher alkyl (typically 40-250 carbon atoms) phenols, i.e. polymer substituted phenols, formaldehyde, and a polyamine, have been claimed as detergent and/or anti-rust additives for liquid fuels in US-A-3,649,229, 4,054,422, and 4,083,699. Mannich products of a 4-alkylphenol, formaldehyde, and a bis(polyisobutenyl succinimide) of a polyamine of the following type:

(in which R is alkyl, x and y are integers between 0 and 10) were claimed as diesel fuel detergents in US-A5,030,249, 5,039,307, and 5,122,161, while Mannich products of polyisobutenyl succinimide polyamine, formaldehyde, and a 4-substituted phenol were claimed as stability improving additives for liquid fuels in US-A4,501,595. Additives to stabilise fuels comprising a mixture of a polyamine with a Mannich base product derived from a phenol, typically 4-n-dodecylphenol, formaldehyde, and a primary amine or secondary amine, such as methylamine, were claimed in US-A-4,166,726.

Mannich base reaction products derived from an Nhydrocarbyl- substituted phenylene diamine or an N,N'dihydrocarbyl- substituted phenylene diamine, an aldehyde or ketone, and a hindered phenol such as 2,6di-tert-butylphenol of the following formula:

(where R2, Rs and R6 are hydrocarbyl, and R1, R3 and R4 are hydrogen or hydrocarbyl) were claimed as antioxidants for lubricating oil, grease or liquid fuels in US-A-5,312,461.

Bis(disubstituted aminomethyl)phenols of the following general formulae:

(where R is hydrogen or an alkyl group having 1 to 6 carbon atoms, and R1 and R2 are alkyl groups with 1 to 6 carbon atoms, or taken together can be alkylene or alkylene ether) were claimed as antioxidant additives for hydrocarbon fuels and lubrication compositions in US-A-4,322,304.

It has now been found that 3,4-dihydro-1,3-(2H)benzoxazines are useful anti-knock agents and have the added advantage of being soluble in gasolines.

Thus viewed from one aspect the present invention provides anti-knock additive compositions comprising as anti-knock agent one or more 3-4-dihydro-1,3-(2H)benzoxazines, for example a 3,4-dihydro-1,3-(2H)benzoxazine represented by the formula:

(wherein R is a hydrogen atom or a hydrocarbyl group containing up to 20 carbon atoms, and Rl, R2, R3 and R4 are hydrogen, hydrocarbyl, or hydrocarbyloxy groups) together with a gasoline miscible liquid carrier or diluent.

Preferably the anti-knock additive composition contains on a volume basis 30-70t of the anti-knock agent.

Viewed from a further aspect the present invention provides a gasoline comprising a minor amount of an anti-knock agent as hereinbefore defined.

The said anti-knock agent is preferably present in the gasoline in an amount up to 20% by volume based on the gasoline volume, more preferably an amount in the range 0.1 to 15%, especially preferably an amount in the range 0.2 to 5%.

The invention also provides a method of improving the anti-knock rating of a gasoline comprising adding to the gasoline an anti-knock agent as hereinbefore defined.

As used herein, hydrocarbyl may denote a straight or branched chain alkyl, alkenyl, cycloalkyl (including combinations such as alkylcycloalkyl, cycloalkylalkyl, etc.), aryl (including mono- and fused ring structures e.g. naphthyl, and alkyl-substituted aryl, e.g. xylyl, tolyl, etc.) and aralkyl, e.g. benzyl (including alkylsubstituted aralkyl). The preferred hydrocarbon values are straight or branched Cl-Cl0 alkyl, especially Cl-C6.

The preferred hydrocarbon values for Rl, R2 and R3 are straight or branched chain Cl-ClO alkyl, especially C1-C6, and phenyl. In a preferred embodiment of the invention, Rl, R3 and R4 are hydrogen, R is C1-C6 alkyl, preferably branched rather than straight chain, particularly preferably t-butyl, and R2 is Cl-C6 alkyl, preferably branched rather than straight chain, or alkoxy.

The gasoline miscible liquid carrier or diluent in the compositions according to the invention may be a liquid hydrocarbon, alcohol or ether, or mixture of two or more thereof, or may also be itself a gasoline blend, or base oil stock. The additive compositions according to the invention may of course additionally comprise one or more gasoline additives for example antioxidants, rust prevention agents and detergents.

As used herein, 'gasoline' refers to fuels meeting ASTM standard D-439, and includes blends of distillate hydrocarbon fuels with other additives such as antioxidants, detergents, demulsifiers, corrosion inhibitors, metal de-activators, dyes, deposit modifiers, anti-knock additives (e.g., tetraethyl lead and MMT), oxygenates such as MTBE and alcohols (e.g., ethanol, TBA) and the like.

Anti-knock characteristics of an additive are typically evidenced by an increase in the Research and Motor octane numbers of the fuel when the additive is admixed therewith. The Research (RON) and Motor (MON) octane numbers of fuel compositions are typically measured by ASTM D 2699 and ASTM D 2700 respectively. While RON and MON are by themselves good indicators of the anti-knock characteristics of an additised fuel, another measure of the anti-knock characteristics of a fuel is the average of the two numbers (RON + MON) /2, which provides a fairly good approximation of the octane number required by engines under typical driving conditions. This average is often quoted as the typical rating used for commercial products.

The compounds useful in this invention can be prepared by methods well known in the art. They can be made by reacting the appropriate primary amine with the appropriate phenol and two moles of formaldehyde, as described, for instance by WJ Burke et al. {J. Amer.

Chem. Soc., 74, 602 (1952); ibid., 76, 1677 (1954)}. The formaldehyde can be employed as an aqueous solution (typically about 40%), in the gaseous state passed through solution, or as paraformaldehyde.

It has been found that when the phenol, formaldehyde, and the amine are reacted typically in mole ratios of 1.0:0.8-1.0:0.8-1.0 respectively at temperatures between about -20 C and 10"C, then the aminomethylated phenol derivatives described in prior art by Papachristos, Swithenbank et al. (J. Inst. Energy, June 1991, 64, pp.

113-123) can be isolated:

However, when the phenol, formaldehyde, and the amine are reacted typically in mole ratios of 1.0:2.0-3.0:1.0-2.0 at temperatures above 20"C, then the 3,4-dihydro-1,3-(2H)benzoxazine can be isolated:

It should be noted that the dimethylaminomethyl phenol (DMAMP) derivatives described as anti-knock agents by Burns cannot form this type of derivative due to the absence of a hydrogen atom on the nitrogen. This hydrogen is needed to allow the formation of the second methylene bridge between the phenol oxygen and the nitrogen atom.

The finding that the 3,4-dihydro-1,3-(2H)-benzoxazines are anti-knock agents, is a surprising one considering Burns' proposed mechanism to explain the effectiveness of DMAMP derivatives as anti-knock agents, involving pyrolytic loss of dimethylamine from the phenol:

It can be seen that one requirement for the DMAMP derivative to be effective by this mechanism is the presence of a hydrogen atom bound to the phenol oxygen.

The 3,4-dihydro-1,3-(2H)benzoxazines of this invention have no such hydrogen atom. It is not possible for the 3,4-dihydro-1,3-(2H)benzoxazine to liberate amine RNH2 on pyrolysis, as can occur with N-alkylaminomethyl phenol derivatives. If a pyrolytic decomposition were to occur in the case of the 3,4-dihydro-1,3-(2H)benzoxazine, the expected product would be an imine, R-N=CH2; however, these products are not known for their anti-knock effectiveness:

It has also been found that the 3,4-dihydro-1,3 (2H)benzoxazines have the advantage of being more soluble in gasolines than the corresponding Nalkylaminomethyl phenol derivatives.

The following examples and comparative data will serve to illustrate the preparation of the anti-knock compounds useful in the present invention and their efficacy in improving the octane number of liquid hydrocarbon fuels. It will be understood, however, that it is not intended that the invention be limited to these particular anti-knock compounds. Various modifications of these additives can be effectively employed, as will be readily apparent to those skilled in the art.

EXAMPLE 1 2-N,N-dimethylaminomethyl-4-cresol This example describes the general method of preparing known disubstituted aminomethyl phenols used for comparative purposes. Into a 11 round bottom flask equipped with a stirrer, dropping funnel and thermometer was charged 108g 4-cresol and 140ml 40% aqueous dimethylamine. To this was added 62ml 40% wt. aqueous formaldehyde solution keeping the temperature below 30"C over a period of about an hour. The solution was then stirred for about two hours.

The products were then extracted three times with 200ml diethyl ether. The combined ether extracts were then washed twice with water to remove excess formaldehyde and amine, and then extracted with two aliquots of 250ml 15% aqueous hydrochloric acid. The acid extracts were then washed with three 200ml aliquots of ether to remove unreacted 4-cresol, and then basified above pH 10 with 50% NaOH. Extraction of the aqueous solution with ether or chloroform, drying over Na2SO4, filtration, and removal of solvent gave 149g (90%) 2-N,Ndimethylaminomethyl-4-cresol of the following formula, whose identity was confirmed by GC/MS:

EXAMPLE 2 2-N-tert-butylaminomethyl-4-cresol This example describes the preparation of known monosubstituted aminomethyl phenols used for comparative purposes. Into a 11 round bottom flask equipped with a stirrer, dropping funnel and thermometer was charged 108g 4-cresol and 91.3g tert-butylamine. To this was added 62ml 40% wt. aqueous formaldehyde solution keeping the temperature below -10 C over a period of about an hour. The solution was then stirred at this temperature for about two hours. Work-up and isolation of the product in the same manner as in example 1 afforded 134g (75%) 2-N-tert-butylaminomethyl-4-cresol, of the following formula:

EXAMPLE 3 N-Methyl-6-methyl-3,4-dihydro-1,3-(2H)benzoxazine 120ml 40% Aqueous formaldehyde was added to a solution of 108g 4-cresol and 93ml 40% aqueous methylamine dropwise at 20-30"C with vigorous stirring. The mixture was then heated to about 45"C for a period of about 2 hours.

The products were then extracted three times into 200ml diethylether. The combined ether extracts were then washed twice with water to remove excess formaldehyde and amine, and then extracted with two aliquots of 250ml 15% aqueous hydrochloric acid. The acid extracts were then washed with three 200ml aliquots of ether to remove unreacted 4-cresol, and then basified above pH 10 with 50% NaOH. Extraction of the aqueous solution with ether or chloroform, drying over Na2SO4, filtration, and removal of solvent gave 114g (70%) N-methyl-6-methyl 3,4-dihydro-1,3-(2H)benzoxazine:

EXAMPLE 4 N-sec-Butyl-6-methyl-3,4-dihydro-l, 3- (2H)benzoxazine 120ml Aqueous formaldehyde, 108g 4-cresol and 87.6g 2aminobutane were reacted and worked up as in Example 3, to afford 185g (90t) N-sec-butyl-6-methyl-3,4-dihydro1,3-(2H)benzoxazine with the following structure:

EXAMPLE 5 N-tert-Butyl-6-methyl-3,4-dihydro-1,3-(2H)benzoxazine 120ml Aqueous formaldehyde, 108g 4-cresol and 87.6g 2amino-2-methylpropane were reacted and worked up as in Example 3, to afford 160g (78%) N-tert-butyl-6-methyl3,4-dihydro-1,3-(2H)benzoxazine with the following structure:

EXAMPLE 6 N-2-Ethylhexyl-6-methyl-3,4-dihydro-1,3-(2H)benzoxazine 120ml Aqueous formaldehyde, 108g 4-cresol and 154.8g 1amino-2-ethylhexane were reacted in 100ml tetrahydrofuran solution, and worked up as in Example 3, to afford 104g (40%) N-ethylhexyl-6-methyl-3,4-dihydro1,3-(2H)benzoxazine with the following structure:

EXAMPLE 7 N-sec-Butyl-6-tert-butyl-3,4-dihydro-1,3-(2H)benzoxazine 120ml Aqueous formaldehyde, 150g 4-tert-butylphenol and 87.6g 2-aminobutane were reacted and worked up as in Example 3, to afford 222g (90%) N-sec-butyl-6-tert butyl-3,4-dihydro-1,3- (2H)benzoxazine with the following structure:

EXAMPLE 8 N-tert-Butyl-6-tert-butyl-3,4-dihydro-l,3- (2H)benzoxazine 120ml Aqueous formaldehyde, 150g 4-tert-butylphenol and 87.6g 2-amino-2-methylpropane were reacted and worked up as in Example 3, to afford 148g (60%) N-tert-butyl-6 tert-butyl-3,4-dihydro-1,3-(2H)benzoxazine with the following structure:

EXAMPLE 9 N-tert-Butyl-6-methoxy-3,4-dihydro-1, 3- (2H)benzoxazine 120ml Aqueous formaldehyde, 124g 4-methoxyphenol and 87.6g 2-amino-2-methylpropane were reacted and worked up as in Example 3, to afford 181g (82%) N-tert-butyl-6methoxy-3,4-dihydro-1,3-(2H)benzoxazine with the following structure:

EXAMPLE 10 N-tert-Butyl-7-methyl-3,4-dihydro-1,3-(2H)benzoxazine 120ml Aqueous formaldehyde, 108g 3-cresol and 87.6g 2amino-2-methylpropane were reacted and worked up as in Example 3, to afford 168g (82%) N-tert-butyl-7-methyl3,4-dihydro-1,3-(2H)benzoxazine with the following structure:

EXAMPLE 11 N-tert-Butyl-7-tert-butyl-3,4-dihydro-1,3 (2H)benzoxazine 120ml Aqueous formaldehyde, 150g 3-tert-butylphenol and 87.6g 2-amino-2-methylpropane were reacted and worked up as in Example 3, to afford 143g (58%) N-tert-butyl-7 tert-butyl-3,4-dihydro-1,3- (2H)benzoxazine with the following structure:

EXAMPLE 12 N-tert-Butyl-7-methoxy-3,4-dihydro-1,3-(2H)benzoxazine 120ml Aqueous formaldehyde, 124g 3-methoxyphenol and 87.6g 2-amino-2-methylpropane were reacted and worked up as in Example 3, to afford 168g (76t) N-tert-butyl-7methoxy-3,4-dihydro-1,3-(2H)benzoxazine with the following structure:

TEST DATA The anti-knock performance of several gasoline samples was determined by measuring the research Qctane number (RON) and the motor Qctane number (MON) of each sample.

The samples tested contained the compositions according to the invention and the conventional octane enhancers MTBE and N-methylaniline for the purpose of comparisons The base gasoline used in these tests had the following properties: Inspection Property Mean Value Density &commat; 15 C, g/ml 0.7239 Reid Vapour Pressure, mbar 341 FIA, % volume Saturates 82.0 Olefins 0.2 Aromatics 17.8 Distillation &commat;, "C IBP 43.8 2 t 60.8 5 % 69.4 10 t 76.0 20 % 84.7 30 % 91.8 40 % 95.8 50 W 97.4 60 % 98.4 70 t 99.4 80 % 100.4 90 W 101.1 95 t 102.4 FBP 109.6 vol recovery 98.8 vol residue 0.6 W vol loss 0.6 The results of these tests are shown in Table 1 below.

Run Additive Molar Treat RON aRON MON aMON (RON+ (RON+ Conc" Rate MON)/2 LMON)/2 (mol/L) (%wt/vol) Run 1-4 none 0 () 0 92.2 87.2 89.7 89.95 (a) Prior art 5 Example 1 0.025 ().415 93.0 0.8 87.5 0.3 90.25 0.55 6 Example I ().125 2.413 95.5 3.3 88.1 ().9 91.8 2.1 7 Example 1 0.225 3.735 97.5 5.3 88.6 1.4 93.05 3.35 8 Example 2 0.025 0.483 92.6 0.4 87.2 () 91.1 ().2 9 Example 2 ().125 2.413 94.5 2.3 10 Example 2 ().225 4.343 96.0 3.8 11 2-N-sec-butylarnino- 0.125 2.413 93.8 1.6 88.0 0.8 90.9 1.2 12 methyl-4-cresol 0.225 4.343 95.0 2.8 89.3 2.1 92.15 2.45 13 2-N-tert-butylamino- 0.025 0.584 92.6 0.4 87.4 ().2 90.0 ().3 14 methyl-Scresol- 0.125 2.413 93.3 1.1 87.7 ().5 90.5 (1.8 15 2-N-tert-bu'ylamino- ().125 2.938 94.0 1.8 16 methyl-5-tert-butylphenol 0.225 5.558 94.9 2.7 (b) Invention 17 Example 4 0.025 0.513 92.6 0.4 87.6 0.4 90.1 0.4 18 Example 4 0.125 2.563 94.4 2.2 88.4 1.2 91.4 1.8 19 Example 4 0.225 4.613 95.9 3.7 20 Example 5 0.025 0.513 92.7 0.5 87.7 0.5 90.2 ().5 21 Example 5 0.125 2.563 94.9 2.7 88.9 1.7 91.9 2.2 22 Example 5 0.225 4.613 96.7 4.5 89.8 2.6 93.25 3.55 23 Example 7 0.025 0.618 92.6 0.4 87.4 0.2 90.0 0.3 24 Example 7 0.125 3.088 94.0 1.8 25 Example 7 0.225 5.558 94.9 2.7 26 Example 10 0.025 0.513 92.6 0.4 87.3 0.1 89.95 ().25 27 Example 10 0.125 2.563 93.5 1.3 87.9 0.7 90.7 1.0 28 Example 10 0.225 4.613 94.5 2.3 88.9 1.7 91.7 2.() 29 Example 11 0.025 0.618 92.6 0.4 87.7 ().5 9().15 ().45 30 Example 11 ().125 3.088 94.0 1.8 88.4 1.2 91.2 1.5 31 Example 11 ().225 5.558 94.9 2.7 88.8 1.6 91.85 2.15 (c) Prior art 32 none 0 91.9 86.9 89.4 33 N-methylaniline 0.27 93.5 1.6 87.6 0.7 90.55 1.15 34 do. 0.5 94.2 2.3 88.2 1.3 91.2 1.70 35 do. 1.0 96.2 4.3 89.1 2.2 92.65 3.25 36 none 0 92.1 86.8 89.45 37 MTBE 37e vol 93.0 0.9 87.6 0.8 90.3 0.85 38 do. 55E vol 93.9 1.8 88.1 1.3 91.0 1.65 39 do. 10% vol 95.4 3.3 89.1 2.3 92.25 2.80 The data in Table 1 show that 3,4-dihydro-1,3 (2H)benzoxazines of this invention are effective octane enhancers. In particular, additives described in examples 4 and 5 show significant octane enhancement properties relative to conventional oxygenates such as MTBE.

Claims

1. An anti-knock additive composition comprising as anti-knock agent one or more 3,4-dihydro-1,3-(2H)benzoxazines together with a gasoline miscible liquid carrier or diluent.

2. An anti-knock additive composition according to claim 1 comprising one or more 3,4-dihydro-1,3 (2H)benzoxazines of formula:

(where R is hydrogen or a hydrocarbyl group containing up to 20 carbon atoms, and, R1, R2, R3 and R4 are independently selected from hydrogen, hydrocarbyloxy groups or hydrocarbyl groups), in admixture with a gasoline miscible liquid carrier or diluent.

3. An additive composition according to claim 1 or 2, containing on a volume basis, 30-70t of the anti-knock agent.

4. An additive composition according to any preceding claim, wherein the carrier or diluent is a liquid hydrocarbon, alcohol or ether or mixture of two or more thereof.

5. An additive composition according to any preceding claim, wherein the carrier or diluent is itself a gasoline blend, or an aromatic hydrocarbon.

6. A gasoline comprising a minor amount of an antiknock agent as defined in claim 1.

7. A gasoline according to claim 6 wherein the said anti-knock agent is present in an amount up to 20 vol. % based on the volume of the gasoline.

8. A gasoline according to either of claims 6 and 7 wherein the said anti-knock agent is present in an amount from 0.2 to 5%.

9. A method of improving the anti-knock rating of a gasoline, said method comprising adding to the blend an anti-knock agent as defined in claim 1.

10. A method according to claim 9, wherein the said anti-knock agent is incorporated into the blend in an amount of up to 20% by volume.

11. A method according to claim 9 or 10, wherein said anti-knock agent is incorporated into the blend as a preformed additive composition as claimed in any one of claims 1 to 5.