US20160024421A1

US20160024421A1 - Ionic liquids containing quaternary phosphonium cations and carboxylate anions, and their use as lubricant additives

Info

Publication number: US20160024421A1
Application number: US14/444,029
Authority: US
Inventors: Jun Qu; Huimin Luo; Yan Zhou; Jeffrey Dyck; Todd Graham
Original assignee: UT Battelle LLC
Current assignee: UT Battelle LLC
Priority date: 2014-07-28
Filing date: 2014-07-28
Publication date: 2016-01-28

Abstract

The invention provides an ionic liquid composition having the following generic structural formula (1):

wherein R¹, R², R³, and R⁴are each independently a hydrocarbon group containing at least 4 carbon atoms, and R⁵is a branched hydrocarbon group containing 4 to 8 carbon atoms atoms. The invention also provides a lubricant composition containing an ionic liquid dissolved in a base oil.

Description

This invention was made with government support under Prime Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to the fields of ionic liquids, and more particularly, to their application as additives in lubricating oils, such as engine and motor oils.

BACKGROUND OF THE INVENTION

Friction and wear account for ˜6% of U.S. GNP. Particularly, in an internal combustion engine, 10-15% energy is lost to parasitic friction. In 2009, it was estimated that 208,000 million liters of fuel was consumed to overcome the friction in passenger cars worldwide. One way to boost the engine efficiency is to use a lower viscosity lubricating oil, which also challenges the wear protection of engine bearing components. Therefore, developing a new class of more effective anti-wear (AW) lubricant additives is of great interest from both fundamental and practical perspectives in energy savings.
Ionic liquids have been explored as lubricant additives for at least the last decade. Ionic liquids are composed of cations and anions. The four commonly used cations are phosphonium, ammonium, pyridinium, and imidazolium; and the forms of anions vary. Ionic liquid lubrication has mainly been explored in neat form or as base stocks. Many ionic liquids possess high thermal stability that allows them to be potentially applied for high-temperature lubrication where traditional hydrocarbon lubricants are unstable.
However, ionic liquids tend to have low solubility in common base oils, which is a significant obstacle to their use since the low concentrations used and/or incomplete miscibility results in substandard or inconsistent wear and friction control. Thus, there is a need for improving the solubility of ionic liquids in various lubricating oils. Moreover, there is a need for new ionic liquid compositions having improved anti-wear and friction reduction properties.

SUMMARY OF THE INVENTION

In one aspect, the invention is directed to an ionic liquid composition having the following generic structural formula (1):
wherein R¹, R², R³, and R⁴are each independently a hydrocarbon group containing at least 4 carbon atoms, and R⁵is a branched hydrocarbon group containing 4 to 8 carbon atoms atoms.
In another aspect, the invention is directed to a lubricant composition comprising (i) an ionic liquid having the following generic structural formula (1):
wherein R¹, R², R³, and R⁴are each independently a hydrocarbon group containing at least 4 carbon atoms, and R⁵is a branched hydrocarbon group containing 4 to 8 carbon atoms; and (ii) a base oil; wherein the ionic liquid is dissolved in the base oil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B: Molecular structures of trihexyltetradecylphosphonium [C₇H₁₅COO] (A) branched and (B) straight chained version.

FIGS. 2A, 2B, 2C: Three different anion alkyls. (A) branched [C₉H₁₉COO]; (B) straight [C₉H₁₉COO]; and (C) straight [C₁₇H₃₅COO].

FIG. 3: Branched C7 phosphonium-carboxylate IL ([P₆₆₆₁₄][C₇H₁₅COO]b) outperforms a commercial amine-phosphate AW additive.

FIGS. 4A, 4B, 4C, 4D, 4E, 4F: TEM (A,B) and EDS (C) examination on the cross-section of the worn surface lubricated by PAO+[P₆₆₆₁₄][C₇H₁₅COO]b. (B) corresponds to the dotted box in (A). The EDS cross-sectional elemental maps show concentrations of iron (D), oxygen (E) and phosphorus (F).

FIGS. 5A, 5B, 5C, 5D, 5E. XPS detailed spectra of (A) Fe2p, (B) O1s, and (C) C1s, and (D) P2p core levels on the worn surface lubricated by PAO+[P₆₆₆₁₄][C₇H₁₅COO]b; (E) composition-depth profile.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “about” generally indicates within ±0.5%, 1%, 2%, 5%, or up to ±10% of the indicated value. For example, the term “about 100° C.” generally indicates, in its broadest sense, 100° C.±2%, which indicates 98-112° C. The term “about” may alternatively indicate a variation or average in a physical characteristic of a group.
In one aspect, the invention is directed to an ionic liquid useful as a lubricant additive or lubricant itself. As understood in the art, the term “ionic liquid compound” or “ionic liquid” is an ionic compound that is, itself, a liquid, i.e., without being dissolved in or solvated with a solvent. The ionic liquid is typically a liquid at room temperature (e.g., 15, 18, 20, 22, 25, or 30° C.) or lower. However, in some embodiments, the ionic liquid may become a liquid at a temperature above 30° C. Thus, in some embodiments, the ionic liquid may have a melting point of up to or less than 100, 90, 80, 70, 60, 50, 40, or 30° C. In other embodiments, the ionic liquid is a liquid at or below 10, 5, 0, −10, −20, −30, or −40° C.
The density of the ionic liquid is typically in the range of 0.6-1.6 g/mL at an operating temperature of interest, and particularly at a temperature within 20-40° C. The viscosity of the ionic liquid is typically no more than 50,000 centipoise (50,000 cP) at an operating temperature of interest, and particularly at a temperature within 20-40° C. In different embodiments, the viscosity of the ionic liquid may be about, up to, less than, at least, or above, for example, 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000, 5000, 10,000, 15,000, 20,000, or 25,000 cP, or a viscosity within a range bounded by any two of these values.
The ionic liquid composition of the invention contains a quaternary phosphonium cation and a carboxylate containing anion. In particular embodiments, the ionic liquid compositions are conveniently described by the following generic structural formula (1):

Quaternary Phosphonium Cation

In Formula (1) above, R¹, R², R³, and R⁴are each independently a hydrocarbon group containing at least 4 carbon atoms. The term “hydrocarbon group” or “hydrocarbon linker” as used herein for R¹, R², R³, and R⁴, designates, in a first embodiment, groups or linkers composed solely of carbon and hydrogen. In different embodiments, one or more of the hydrocarbon groups or linkers can contain precisely, or a minimum of (i.e., at least), or a maximum of (i.e., up to), for example, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, fourteen, fifteen, sixteen, seventeen, eighteen, nineteen, or twenty carbon atoms, or a number of carbon atoms within a particular range bounded by any two of the foregoing carbon numbers. Hydrocarbon groups or linkers in different compounds described herein, or in different parts or positions of a compound, may possess the same or different number (or preferred range thereof) of carbon atoms in order to independently adjust or optimize the activity or other characteristics of the compound, such as its level of hydrophobicity or solubility level in a hydrophobic medium, or its wear-enhancing or friction-reducing ability.
The hydrocarbon groups or linkers in R¹, R², R³, and/or R⁴can be, for example, saturated and straight-chained, i.e., straight-chained alkyl groups or alkylene linkers. Some examples of straight-chained alkyl groups (or alkylene linkers) include n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl, n-octadecyl, n-nonadecyl, and n-eicosyl groups (or their respective linker analogs).
The hydrocarbon groups or linkers as used herein for R¹, R², R³, and/or R⁴can alternatively be saturated and branched, i.e., branched alkyl groups or alkylene linkers. Some examples of branched alkyl groups include isopropyl(2-propyl), isobutyl(2-methylprop-1-yl), sec-butyl(2-butyl), t-butyl, 2-pentyl, 3-pentyl, 2-methylbut-1-yl, isopentyl(3-methylbut-1-yl), 1,2-dimethylprop-1-yl, 1,1-dimethylprop-1-yl, neopentyl(2,2-dimethylprop-1-yl), 2-hexyl, 3-hexyl, 2-methylpent-1-yl, 3-methylpent-1-yl, isohexyl(4-methylpent-1-yl), 1,1-dimethylbut-1-yl, 1,2-dimethylbut-1-yl, 2,2-dimethylbut-1-yl, 2,3-dimethylbut-1-yl, 3,3-dimethylbut-1-yl, 1,1,2-trimethylprop-1-yl, 1,2,2-trimethylprop-1-yl, 2-ethylhexyl, isoheptyl, isooctyl, isononyl, and isodecyl, wherein the “1-yl” suffix represents the point of attachment of the group. Some examples of branched alkylene linkers are those derived by removal of a hydrogen atom from one of the foregoing exemplary branched alkyl groups, e.g., isopropylene (—CH(CH₃)CH₂—).
The hydrocarbon groups or linkers as used herein for R¹, R², R³, and/or R⁴can alternatively be saturated and cyclic, i.e., cycloalkyl groups or cycloalkylene linkers. Some examples of cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl groups. The cycloalkyl group can also be a polycyclic (e.g., bicyclic) group by either possessing a bond between two ring groups (e.g., dicyclohexyl) or a shared (i.e., fused) side, e.g., decalin and norbornane. Some examples of cycloalkylene linkers are those derived by removal of a hydrogen atom from one of the foregoing exemplary cycloalkyl groups.
The hydrocarbon groups or linkers as used herein for R¹, R², R³, and/or R⁴can alternatively be saturated and cyclic, i.e., cycloalkyl groups or cycloalkylene linkers. Some examples of cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl groups. The cycloalkyl group can also be a polycyclic (e.g., bicyclic) group by either possessing a bond between two ring groups (e.g., dicyclohexyl) or a shared (i.e., fused) side, e.g., decalin and norbornane. Some examples of cycloalkylene linkers are those derived by removal of a hydrogen atom from one of the foregoing exemplary cycloalkyl groups.
The hydrocarbon groups or linkers as used herein for R¹, R², R³, and/or R⁴can alternatively be unsaturated and straight-chained, i.e., straight-chained olefinic or alkenyl groups or linkers. The unsaturation occurs by the presence of one or more carbon-carbon double bonds and/or one or more carbon-carbon triple bonds. Some examples of straight-chained olefinic groups include, 2-propen-1-yl(allyl), 3-buten-1-yl(CH₂═CH—CH₂—CH₂—), 2-buten-1-yl(CH₂—CH═CH—CH₂—), butadienyl (e.g., 1,3-butadien-1-yl), 4-penten-1-yl, 3-penten-1-yl, 2-penten-1-yl, 2,4-pentadien-1-yl, 5-hexen-1-yl, 4-hexen-1-yl, 3-hexen-1-yl, 3,5-hexadien-1-yl, 1,3,5-hexatrien-1-yl, 4-hepten-1-yl, 5-hepten-1-yl, 6-hepten-1-yl, 4-octen-1-yl, 5-octen-1-yl, 6-octen-1-yl, 7-octen-1-yl, 2,6-octadien-1-yl, 8-decenyl, 9-decenyl, or 4,8-decadien-1-yl, ethynyl, propargyl(2-propynyl), and the numerous C₇, C₈, C₉, C₁₀, C₁₁, C₁₂, C₁₃, C₁₄, C₁₅, C₁₆, C₁₇, C₁₈, C₁₉, and C₂₀unsaturated and straight-chained hydrocarbon groups. Some examples of straight-chained olefinic linkers are those derived by removal of a hydrogen atom from one of the foregoing exemplary straight-chained olefinic groups.
The hydrocarbon groups or linkers as used herein for R¹, R², R³, and/or R⁴can alternatively be unsaturated and branched, i.e., branched olefinic or alkenyl groups or linkers. Some examples of branched olefinic groups include 1-buten-2-yl(CH₂═C.—CH₂—CH₃), 1-buten-3-yl(CH₂═CH—CH.—CH₃), 1-propen-2-methyl-3-yl(CH₂═C(CH₃)—CH₂.), 1-penten-4-yl, 1-penten-3-yl, 1-penten-2-yl, 2-penten-2-yl, 2-penten-3-yl, 2-penten-4-yl, 1,4-pentadien-3-yl, 2,4-pentadien-3-yl, 3-methyl-2-buten-1-yl, 2,3-dimethyl-2-buten-1-yl, 4-methyl-2-penten-1-yl, 2-hexen-5-yl, and the numerous C₇, C₈, C₉, C₁₀, C₁₁, C₁₂, C₁₃, C₁₄, C₁₅, C₁₆, C₁₇, C₁₈, C₁₉, and C₂₀unsaturated and branched hydrocarbon groups. Some examples of branched olefinic linkers are those derived by removal of a hydrogen atom from one of the foregoing exemplary branched olefinic groups.
The hydrocarbon groups or linkers as used herein for R¹, R², R³, and/or R⁴can alternatively be unsaturated and cyclic (i.e., cycloalkenyl groups or cycloalkenylene linkers). The unsaturated and cyclic group can be aromatic or aliphatic. Some examples of unsaturated and cyclic hydrocarbon groups include cyclopropenyl, cyclobutenyl, cyclopentenyl, cyclopentadienyl, cyclohexenyl, cyclohexadienyl, phenyl, benzyl, cycloheptenyl, cycloheptadienyl, cyclooctenyl, cyclooctadienyl, and cyclooctatetraenyl groups. The unsaturated cyclic hydrocarbon group can also be a polycyclic group (such as a bicyclic or tricyclic polyaromatic group) by either possessing a bond between two of the ring groups (e.g., biphenyl) or a shared (i.e., fused) side, as in naphthalene, anthracene, phenanthrene, phenalene, or indene fused ring systems. Some examples of unsaturated cycloalkenylene linkers are those derived by removal of a hydrogen atom from one of the foregoing exemplary cycloalkenyl groups (e.g., phenylene and biphenylene).
One or more of the hydrocarbon groups or linkers as used herein for R¹, R², R³, and/or R⁴may (i.e., optionally) be substituted with (i.e., include) one or more heteroatoms, which are non-carbon non-hydrogen atoms. Some examples of heteroatoms include oxygen (O), nitrogen (N), sulfur (S), and halogen (halide) atoms, wherein some examples of halogen atoms include fluorine, chlorine, bromine, and iodine. In some embodiments, the heteroatom atom inserts between at least two carbon atoms (as in —C—O—C— ether, —C—N(R)—C— tertiary amine, or —C(═NR)C— imine) or between at least one carbon atom and at least one hydrogen atom (as in —C—OH, —C—SH, —C—NH₂, —C—NH—C—, or —C(═NH)C—), wherein the shown carbon atom in each case can be considered part of a hydrocarbon group described above. In other embodiments, the heteroatom replaces one or more hydrogen atoms and/or one or more carbon atoms in the hydrocarbon group, as in halogen-substituted groups (e.g., as in —CH₂F, —CHF₂, and —CF₃) and carbonyl-substituted groups, such as ketone and aldehyde groups. In the case of nitrogen or sulfur substitution, the nitrogen or sulfur atom may be bonded to a sufficient number of groups to make it positively charged, as in an ammonium group (e.g., —NR′₃ ⁺) or sulfonium group (e.g., —SR′₂ ⁺), in which case the positively charged moiety is necessarily associated with a counteranion, wherein R′ independently represents hydrogen atom or any of the hydrocarbon groups described above. Likewise, a heteroatom may bear a negative charge, as in a deprotonated alkoxide or thio group, in which case the negatively charged moiety is necessarily associated with a countercation.
When two or more same or different heteroatoms are bound to each other or located on the same carbon atom, the resulting group containing the heteroatoms is herein referred to as a “heteroatom-containing group”. Thus, substitution with one or more heteroatoms also includes heteroatom-containing groups, unless otherwise specified. Some examples of heteroatom-containing groups and linkers include carboxy (—C(O)OR′ or —OC(O)R′), carboxamide (—C(O)NR′₂, —C(O)NR′—, or —N(R′)C(O)—), urea (—NR′—C(O)—NR′₂or —NR′—C(O)—NR′—), carbamate (—NR′—C(O)—OR′, —OC(O)—NR′₂, or —NR′—C(O)—O—), nitro (NO₂), nitrile (CN), sulfonyl (—S(O)₂R′ or —S(O)₂—), sulfinyl (i.e., sulfoxide, —S(O)R′ or —S(O)—), disulfide (—C—S—S—C—), sulfonate (—S(O)₂R′), and amine oxide (as typically found in a nitrogen-containing ring), wherein R′ can independently represent, for example, hydrogen atom or any of the hydrocarbon groups described above. For example, —C(O)OR′ includes carboxylic acid (—C(O)OH) and carboxylic ester (—C(O)OR), wherein R can be any of the hydrocarbon groups described above. The heteroatom-containing group may also either insert between carbon atoms or between a carbon atom and hydrogen atom, if applicable, or replace one or more hydrogen and/or carbon atoms.
In some embodiments, the hydrocarbon group or linker as used herein for R¹, R², R³, and/or R⁴is substituted with one or more halogen atoms to result in a partially halogenated or perhalogenated hydrocarbon group. Some examples of partially halogenated hydrocarbon groups include —CHX′₂, —CH₂X′, —CH₂CX′₃, —CH(CX′₃)₂, or a monohalo-, dihalo-, trihalo-, or tetrahalo-substituted phenyl group, wherein X′ represents any of F, Cl, Br, or I, and more commonly, F or Cl. Some examples of perhalogenated hydrocarbon groups include —CX′₃, —CX′₂CX′₃, —CX′₂CX′₂CX′₃, —CX′(CX′₃)₂, or a perhalophenyl group —C₆X′₅.
In particular embodiments, the hydrocarbon group (R¹, R², R³, and/or R⁴) is, or includes, a cyclic or polycyclic (i.e., bicyclic, tricyclic, or higher cyclic) saturated or unsaturated (e.g., aliphatic or aromatic) hydrocarbon group that includes at least one ring heteroatom, such as one, two, three, four, or higher number of ring heteroatoms. Such heteroatom-substituted cyclic hydrocarbon groups are referred to herein as “heterocyclic groups”. As used herein, a “ring heteroatom” is an atom other than carbon and hydrogen (typically, selected from nitrogen, oxygen, and sulfur) that is inserted into or replaces a ring carbon atom in a hydrocarbon ring structure. In some embodiments, the heterocyclic group is saturated, while in other embodiments, the heterocyclic group is unsaturated, i.e., aliphatic or aromatic heterocyclic groups, wherein the aromatic heterocyclic group is also referred to herein as a “heteroaromatic ring”, or a “heteroaromatic fused-ring system” in the case of at least two fused rings, at least one of which contains at least one ring heteroatom.
Some examples of saturated heterocyclic groups containing at least one oxygen atom include oxetane, tetrahydrofuran, tetrahydropyran, 1,4-dioxane, 1,3-dioxane, and 1,3-dioxepane rings. Some examples of saturated heterocyclic groups containing at least one nitrogen atom include pyrrolidine, piperidine, piperazine, imidazolidine, azepane, and decahydroquinoline rings. Some examples of saturated heterocyclic groups containing at least one sulfur atom include tetrahydrothiophene, tetrahydrothiopyran, 1,4-dithiane, 1,3-dithiane, and 1,3-dithiolane rings. Some examples of saturated heterocyclic groups containing at least one oxygen atom and at least one nitrogen atom include morpholine and oxazolidine rings. An example of a saturated heterocyclic group containing at least one oxygen atom and at least one sulfur atom includes 1,4-thioxane. Some examples of saturated heterocyclic groups containing at least one nitrogen atom and at least one sulfur atom include thiazolidine and thiamorpholine rings.
Some examples of unsaturated heterocyclic groups containing at least one oxygen atom include furan, pyran, 1,4-dioxin, benzofuran, dibenzofuran, and dibenzodioxin rings. Some examples of unsaturated heterocyclic groups containing at least one nitrogen atom include pyrrole, imidazole, pyrazole, pyridine, pyrazine, pyrimidine, 1,3,5-triazine, azepine, diazepine, indole, purine, benzimidazole, indazole, 2,2′-bipyridine, quinoline, isoquinoline, phenanthroline, 1,4,5,6-tetrahydropyrimidine, 1,2,3,6-tetrahydropyridine, 1,2,3,4-tetrahydroquinoline, quinoxaline, quinazoline, pyridazine, cinnoline, 5,6,7,8-tetrahydroquinoxaline, 1,8-naphthyridine, and 4-azabenzimidazole rings. Some examples of unsaturated heterocyclic groups containing at least one sulfur atom include thiophene, thianaphthene, and benzothiophene rings. Some examples of unsaturated heterocyclic groups containing at least one oxygen atom and at least one nitrogen atom include oxazole, isoxazole, benzoxazole, benzisoxazole, oxazoline, 1,2,5-oxadiazole (furazan), and 1,3,4-oxadiazole rings. Some examples of unsaturated heterocyclic groups containing at least one nitrogen atom and at least one sulfur atom include thiazole, isothiazole, benzothiazole, benzoisothiazole, thiazoline, and 1,3,4-thiadiazole rings.
The positive (+) charge shown in Formula (1) resides on the phosphorus (P) atom shown in Formula 1. However, one or more additional positive charges may exist elsewhere in the quaternary phosphonium cation, which would add to the overall positive charge of the quaternary phosphonium cation. The quaternary phosphonium cation can be, for example, any of the phosphonium moieties disclosed in U.S. Pat. No. 3,654,342 and U.S. Pat. No. 3,459,795.
In one set of embodiments, the quaternary phosphonium cation is trihexyltetradecylphosphonium [P₆₆₆₁₄], which has the following structural formula (Formula (2)):
In another set of embodiments, the quaternary phosphonium cation is tetraoctylphosphonium [P₈₈₈₈], which has the following structural formula (Formula (3)):
In still another set of embodiments, the quaternary phosphonium cation is tributyltetradecylphosphonium [P₄₄₄₁₄], which has the following structural formula (Formula (4)):
In a further set of embodiments, the quaternary phosphonium cation is tributyltetraoctylphosphonium [P₄₄₄₈], which has the following structural formula (Formula (5)):

Carboxylate Anion

In Formula (1) above, R⁵is a branched hydrocarbon group having 4 to 8 carbon atoms. The hydrocarbon group can be a saturated or unsaturated branched hydrocarbon group. Examples of saturated or unsaturated branched hydrocarbon groups having 4 to 8 carbon atoms include those hydrocarbons described above for R¹, R², R³, and R⁴having 4 to 8 carbon atoms. The branch can occur on any of the carbon atoms in the hydrocarbon group. For example, the branch can occur on the first, second, third, fourth, fifth, sixth or seventh carbon atom of R⁵.
In one set of embodiments, R⁵is branched in an octanoate (C₇H₁₅COO). The branch can occur anywhere in the hydrocarbon moiety. For example, the branch can occur on the first, second, third, fourth, fifth, or sixth carbon atom of R⁵.
In one embodiment, branched C₇H₁₅COO is a 2-ethylhexanonate, as illustrated in Formula (6):
In Formula (6) above, the letter “b” in [C₇H₁₅COO]b denotes that C₇H₁₅COO is in a branched configuration. The branch in Formula (6) occurs on the carbon in the second position of the chain (first position of R⁵).

Ionic Liquid Composition

The ionic liquid composition of the invention includes any of the above cationic phosphonium species (herein identified as L⁺) and any of the above anionic species X⁻, in accordance with Formula (1). The ionic liquid composition can be conveniently expressed by the formula L⁺X⁻, wherein L⁺ is a cationic component of the ionic liquid and X⁻ is an anionic component of the ionic liquid. The formula (L⁺)(X⁻) is meant to encompass a cationic component (L⁺) having any valency of positive charge, and an anionic component (X⁻) having any valency of negative charge, provided that the charge contributions from the cationic portion and anionic portion are counterbalanced in order for charge neutrality to be preserved in the ionic liquid molecule. More specifically, the formula (L⁺)(X⁻) is meant to encompass the more generic formula (L^+a)_y(X^−b)_x, wherein the variables a and b are, independently, non-zero integers, and the subscript variables x and y are, independently, non-zero integers, such that a.y=b.x (wherein the period placed between variables indicates multiplication of the variables). The foregoing generic formula encompasses numerous possible sub-formulas, such as, for example, (L⁺)(X⁻), (L⁺²)(X⁻)₂, (L⁺)₂(X⁻²), (L⁺²)₂(X⁻²)₂, (L⁺³)(X⁻)₃, (L⁺)₃(X⁻³), (L⁺³)₂(L⁻²)₃, and (L⁺²)₃(X⁻³)₂.
The ionic liquid compositions described above can be synthesized by methodologies well known in the art. The methodologies typically involve salt-forming exchange between cationic- and anionic-containing precursor compounds. For example, a phosphonium halide compound of the formula [PR¹R²R³R⁴]⁺[X′]⁻ (where the halide X′ is typically chloride, bromide, or iodide) can be reacted with the acid or salt form of any of the carboxylate-containing anions described above to form an ionic liquid according to Formula (1) above, with concomitant liberation of the corresponding hydrogen halide or halide salt. Such methods are described, for example, in J. Qu, et al., Applied Materials and Interfaces, 4, pp. 997-1002, 2012, which is herein incorporated by reference in its entirety.
The ionic liquid compositions described above possess complete solubility in a base oil when included in the base oil in amounts of at least 0.1, 0.5, 1, 2, 5, 10, or 50 wt % or within a concentration bounded by any two of these concentrations.

Lubricant

In another aspect, the invention is directed to a lubricant composition that includes one or more of the ionic liquid compositions described above dissolved in a base oil. The term “dissolved”, as used herein, indicates complete dissolution of the ionic liquid in the base oil, i.e., the ionic liquid is completely miscible in the base oil. In different embodiments, the ionic liquid is dissolved in the base oil in an amount of at least 0.1, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, or 90 wt % (i.e., weight of ionic liquid by weight of the total of ionic liquid and base oil) or dissolved in the base oil within a range bounded by any two of the foregoing values. Generally, the ionic liquid in the lubricant composition is one, two, or more selected from any of the ionic liquids herein described, in the absence of other ionic liquids that do not possess the features of the instantly described ionic liquids, such as a quaternary phosphonium cation component having at least four hydrocarbon groups or a carboxylate anion containing a hydrocarbon group having 4 to 8 carbon atoms. In some embodiments, the lubricant composition having any of the above concentrations of ionic liquids is used directly as a lubricant without diluting in additional oil or organic solvent. In other embodiments, the lubricant composition having any of the above concentrations of ionic liquid is diluted before use. Thus, any of the above-described lubricant compositions having any of the above concentrations of ionic liquid (particularly those of higher concentration, e.g., at least 10, 20, 30, 40, or 50 wt %) may be stored as a commodity, and optionally diluted, prior to use.
The base oil can be any of the polar or non-polar base oils known in the art useful as mechanical lubricating oils. As well known in the art, the mechanical lubricating oil can be further classified as, for example, an engine (motor) lubricating oil, industrial lubricating oil, or metal working fluid. The classification, uses, and properties of such oils are well known in the art, as provided, for example, by U.S. Pat. No. 8,268,760, the contents of which are herein incorporated by reference in their entirety. In particular, the base oil may belong to any of the well established five categories of hydrocarbon oils (i.e., Groups I, II, III, IV, or V) classified according to the extent of saturates, sulfur, and viscosity index. The base oil can have any of the typical boiling points, e.g., at least 100, 120, 150, 180, or 200° C. and up to 250, 300, 350, 400, 450, or 500° C. In some embodiments, the base oil is a synthetic oil, such as any of the Groups I-V, and may or may not include polyalphaolefins (PAO). Some other synthetic oils include hydrogenated polyolefins, esters, fluorocarbons, and silicones. In other embodiments, the base oil may be natural, such as a mineral oil, vegetable oil, or animal oil. In yet other embodiments, the base oil may have a substantially high enough viscosity to qualify it as a grease, wherein the grease typically lowers in viscosity during use by virtue of heat generated during use.
The lubricant composition may also include any one or more lubricant additives well known in the art. The term “additive”, as used herein, is understood to be a compound or material, or mixture of compounds or materials, that provides an adjunct or auxiliary effect at low concentrations, typically up to or less than 1, 2, 5, 7, or 10 wt % by weight of the lubricant composition. The additive can be, for example, an anti-wear additive (typically metal-containing), extreme pressure additive, metal chelator, ultraviolet stabilizer, radical scavenger, anti-oxidant, corrosion inhibitor, friction modifier, detergent, surfactant, anti-foaming agent, viscosity modifier, or anti-foaming agent, or combination thereof, all of which are well known in the art, as further described in U.S. Pat. Nos. 8,455,407 and 8,268,760, both of which are herein incorporated by reference in their entirety.
In particular embodiments, the lubricating composition described above includes a non-ionic liquid (non-IL) anti-wear additive, such as a metal-containing dithiophosphate, sulfur-containing fatty acid or ester thereof, dialkyl sulfide, dithiocarbamate, polysulfide, or boric acid ester. In further embodiments, the additive is a metal-containing dialkyldithiophosphate or dialkyldithiocarbamate, wherein the metal is typically zinc or molybdenum, as in zinc dialkyldithiophosphate (ZDDP) or molybdenum dialkyldithiocarbamate (MoDTC), and the alkyl groups typically include between 3 and 12 carbon atoms and can be linear or branched. The anti-wear additive can be included in the lubricating composition in any suitable amount typically used in the art, such as between 1 and 15 wt %. In some embodiments, the anti-wear additive is advantageously used in an amount less than typically used in the art, e.g., in an amount of less than 1 wt %, or up to or less than 0.5 or 0.1 wt %, by virtue of the improved properties provided by the instantly described ionic liquids or by a synergistic interaction between the instantly described ionic liquids and the non-IL anti-wear additive.
In one embodiment, the ionic liquid or the lubricating composition is not dissolved, admixed with, or otherwise in contact with a non-ionic liquid organic solvent (i.e., “solvent”). In other embodiments, the ionic liquid is dissolved in, or admixed with, or in contact with one or more organic solvents, either in the absence or presence of a base oil. If the ionic liquid is dissolved in a base oil, then the organic solvent should be completely soluble in the base oil. The organic solvent can be, for example, protic or non-protic and either polar or non-polar. Some examples of protic organic solvents include the alcohols, particularly those more hydrophobic than methanol or ethanol, such as n-propanol, isopropanol, n-butanol, sec-butanol, isobutanol, t-butanol, n-pentanol, isopentanol, 3-pentanol, neopentyl alcohol, n-hexanol, 2-hexanol, 3-hexanol, 3-methyl-1-pentanol, 3,3-dimethyl-1-butanol, isohexanol, and cyclohexanol. Some examples of polar aprotic solvents include ether (e.g., diethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,3-dioxolane, and tetrahydrofuran), ester (e.g., 1,4-butyrolactone, ethylacetate, methylpropionate, and ethylpropionate), nitrile (e.g., acetonitrile, propionitrile, and butyronitrile), sulfoxide (e g, dimethyl sulfoxide, ethyl methyl sulfoxide, diethyl sulfoxide, methyl propyl sulfoxide, and ethyl propyl sulfoxide), and amide solvents (e.g., N,N-dimethylformamide, N,N-diethylformamide, acetamide, and dimethylacetamide). Some examples of non-polar solvents include the liquid hydrocarbons, such as the pentanes, hexanes, heptanes, octanes, pentenes, hexenes, heptenes, octenes, benzene, toluenes, and xylenes.
In another aspect, the invention is directed to methods for using the above-described ionic liquids, either autonomously (i.e., in the absence of a base oil) or within a lubricant composition, for reducing wear and/or reducing friction in a mechanical device for which lubricity is beneficial. The mechanical device may be, for example, a bearing (e.g., a slide bearing, ball bearing, rolling element bearing, or jewel bearing), piston, turbine fan, rotary blade, compressor blade, gear, axle, engine part (e.g., engine valve, piston, cylinder, or transmission), hydraulic system, or metal cutting tool or machine. The parts being lubricated are typically constructed of a metal or metal alloy, which may be or include, for example, steel, iron, aluminum, nickel, titanium, or magnesium, or a composite or alloy thereof. If used autonomously, the ionic liquid is not included in a base oil, but may be combined with any one or more of the additives described above if the ionic liquid and additive are miscible with each other. The ionic liquid or lubricant composition described above can be applied to a mechanical component by any means known in the art. For example, the component may be immersed in the ionic liquid compound, or a coating (film) of the ionic liquid compound may be applied to the component by, e.g., dipping, spraying, painting, or spin-coating.
In some embodiments, a single ionic liquid compound according to Formula (1) is used, while in other embodiments, a combination of two or more ionic liquid compounds according to Formula (1) is used. In a first incarnation, the combination of ionic liquid compounds corresponds to the presence of two or more cationic species of any of those described above in the presence of a single anionic species of any of those described above. In a second incarnation, the combination of ionic liquid compounds corresponds to the presence of a single cationic species in the presence of two or more anionic species. In a third incarnation, the combination of ionic liquid compounds corresponds to the presence of two or more cationic species of any of those described above in the presence of two or more anionic species of any of those described above.
The ionic liquids described above reduce wear and/or friction. In some embodiments, the ionic liquid or lubricating composition in which it is incorporated provides a coefficient of friction (i.e., friction coefficient) of up to or less than, for example, 0.5, 0.4, 0.3, 0.2, 0.1, or 0.05, or a reduction in friction by any of the foregoing values or by at least 10, 20, 30, 40, 50, 60, 70, 80, or 90%. In other embodiments, the ionic liquid or lubricating composition may or may not have an appreciable effect on friction, but may reduce the wear rate, e.g., by at least or greater than 10, 20, 30, 40, or 50%. In yet other embodiments, the ionic liquid or lubricating composition may or may not also improve the corrosion resistance of the treated substrate. The improved corrosion resistance may be evidenced by a resistance to corrosion in air or after treatment in a liquid corrosion test, such as treatment in a salt solution of at least 0.1 M, 0.2 M, 0.5 M, 1.0 M, 1.5 M, or 2.0 M concentration for at least 0.5, 1, 2, 3, 4, 5, 6, 12, 18, 24, 36, or 48 hours. In still other embodiments, the ionic liquids described herein may provide a multiplicity of functions, which can be two or more of, for example, anti-wear, extreme pressure, friction modifier, anti-oxidant, detergent, and anti-corrosion functions.
Examples have been set forth below for the purpose of illustration and to describe certain specific embodiments of the invention. However, the scope of this invention is not to be in any way limited by the examples set forth herein.

EXAMPLES

Example 1

Synthesis of the ionic liquid trihexyltetradecylphosphonium 2-ethylhexanoate ([P₆₆₆₁₄][C₇H₁₅COO]b)

[P₆₆₆₁₄][C₇H₁₅COO]b was synthesized as follows. A 5 L three necked jacketed flask fitted with an addition funnel, condenser, thermal well and nitrogen inlet was charged with [(hexyl)₃P(C₁₄H₂₉)]Cl (1079 g, 2.077 mol) and 2-ethylhexanoic acid (329 g, 2.288 mol) and 390 g of toluene, and the mixture was heated to an internal temperature of ca. 50° C. A solution of NaOH (116 g, 2.900 mol) in 200 g of water was added while maintaining the internal temperature at ca. 50° C. (addition over 1 h). The solution was stirred one hour further and was allowed to phase separate overnight. The aqueous phase was removed and the organic layer was washed with 5×˜500 g water (to pH=7). Vacuum stripping of the organic material to a maximum temperature of 160° C. and vacuum of 0.32 mbarr afforded 1.141 kg of [P₆₆₆₁₄][C₇H₁₅COO]b (88% yield, 99% purity). ¹H NMR (toluene-D₈, ppm): 0.50-0.75 (m, 1811, CH₃); 0.91-1.69 (m, 56H, CH₂); 2.01 (m, 1H, CH); 2.41 (m, 8H, PCH₂). ³¹P NMR (toluene-D₈, ppm): 33.8 (s).

Example 2

Comparison of ionic liquid trihexyltetradecylphosphonium 2-ethylhexanoate ([P₆₆₆₁₄][C₇H₁₅COO]b) with its straight-chain version, trihexyltetradecylphosphonium octanoate ([P₆₆₆₁₄][C₇H₁₅COO]s)

Density, Viscosity, Oil-Solubility, and Thermal Stability:
The oil-solubility in PAO 4 cSt base oil, decomposition temperature, density, and kinematic viscosity of [P₆₆₆₁₄][C₇H₁₅COO]b) (see FIG. 1A) are compared with those of a commercial amine-phosphate anti-wear additive, [P₆₆₆₁₄][C₇H₁₅COO]s, where s denotes a straight chain (see FIG. 1B), and three other phosphonium-carboxylate ionic liquids (ILs) with the same cation but different alkyls on the anion (see FIG. 2).
The solubility of the ILs in the PAO base oil was determined using direct observation after the centrifugation of the blends at 13,000 rpm for 5 min. Among the three straight-chain-alkyl phosphonium-carboxylate ILs, the IL with the long C17-alkyl ([P₆₆₆₁₄][C₁₇H₃₅COO]s) possesses high oil-solubility (>10%) however the other two ILs with C9- and C7-alkyls ([P₆₆₆₁₄][C₉H₁₉COO]s and [P₆₆₆₁₄][C₇H₁₅COO]s) are not soluble in PAO (<1%). In contrast, both the two ILs with branched-chain alkyls ([P₆₆₆₁₄][C₉H₁₉COO]b and [P₆₆₆₁₄][C₇H₁₅COO]b) are soluble in PAO to various extents, 2-5% and >10%, respectively. Evidently, the branched alkyl structure is the game changer. Further, the longer branch (ethyl) of [P₆₆₆₁₄][C₇H₁₅COO]b compared to the methyl branch on [P₆₆₆₁₄][C₉H₁₉COO]b is believed responsible to the higher oil-solubility of [P₆₆₁₄][C₇H₁₅COO]b (though its alkyl has a lower number of carbons overall C7 vs. C9). See Table 1.

TABLE 1

Oil-solubility, decomposition temperature, density, and kinematic viscosity

	Solubility	Hildebrand	Decomp.	Density	Kinematic
	in PAO	solubility	temperature	@ 20° C.	Viscosity (mPa · s)
Lubricant	(wt %)	parameter	(° C.)	(g/mL)	@ 40° C.

PAO 4 cSt base oil	—	—	250	0.80	17.6
Commercial amine-	>10	—	310	0.92	1107
phosphate AW
[P₆₆₆₁₄][C₇H₁₅COO]b	>10	24.9	308	0.88	184.8
[P₆₆₆₁₄][C₇H₁₅COO]s	<1	26.0	261	0.93	122.2
[P₆₆₆₁₄][C₉H₁₉COO]b	>2, <5	24.8	258	0.91	127.1
[P₆₆₆₁₄][C₉H₁₉COO]s	<1	25.5	NM	0.89	solid
[P₆₆₆₁₄][C₁₇H₃₅COO]s	>10	23.5	308	0.88	174.5

Molecular dynamics simulation (Materials Studio 6.0) was used to compute the Hildebrand solubility parameters (δs) for the selected ILs and the PAO oil. The δ values of the five ILs in Table 1 are in the range of 23.5-26.0. The PAO oil molecules were represented using two alkanes, decane and icosane, whose δs are 13.29 and 13.68, respectively.
Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (TGA-2950, TA instruments) at a 10° C./min heating rate in air. All ILs possess higher thermal stability than the base oil. The kinematic viscosities were measured on a Petrolab Minivis II viscometer.

Example 3

Corrosive Testing

Initial test results suggest that none of the phosphonium-carboxylate ILs listed in Table 1 is corrosive to cast iron. A droplet of each IL was placed on the surface of a piece of grey cast iron in the ambient environment for 7 days and no rusting was observed on any sample surface.

Example 4

Superior Anti-Wear and Friction Reduction

The branched C7 phosphonium-carboxylate IL ([P₆₆₆₁₄][C₇H₁₅COO]b) was mixed into the PAO base oil at 1.65 wt % and the wear protection performance was evaluated using a ball-on-flat tribological bench test. Results were benchmarked against the commercial amine-phosphate AW additive at a treat rate of 1.67 wt % in the PAO base oil. Both fluids had similar phosphorus content of ˜800 ppm (meeting the ILSAC GF-5 specifications).
The tribo-tests were carried out on a reciprocating tribometer (Plint TE77, Phoenix Tribology Ltd.) by using a 10 mm AISI 52100 steel ball sliding against a CL35 cast iron flat. The flats were polished by using P1200 SiC abrasive paper in distilled water with random motion. Tests were performed under a 100 N normal load and at a temperature of 100° C. The oscillation frequency used was 10 Hz with a stroke of 10 mm and the sliding distance was 1000 m. 2-3 repeat tests were carried out for each lubricant.
The total wear rates (ball and plate) of the three fluids, neat PAO base oil, PAO treated with the commercial amine-phosphate (PAO+1.67% AP), and PAO treated with [P₆₆₆₁₄][C₇H₁₅COO]b (PAO+1.65% IL) are compared in FIG. 3. The branched C7 phosphonium-carboxylate IL significantly reduced the wear rate when added into PAO and even outperformed the commercial amine-phosphate AW additive by >56% wear reduction.

Example 5

Anti-Wear Tribofilm of [P₆₆₆₁₄][C₇H₁₅COO]b

A tribofilm formed on the worn surface lubricated by PAO+1.65%[P₆₆₆₁₄][C₇H₁₅COO]b, as shown in FIGS. 4 and 5. Relatively low phosphorus concentration was detected in the tribofilm from either cross-section TEM-EDS or top surface XPS chemical analyses, and results suggest that the tribofilm is primarily composed of iron oxides and iron carboxylate complexes.
The cross-section TEM images (FIG. 4) suggest that the tribofilm is 230-370 nm in thickness and contains a large amount of nano-particles embedded in an amorphous matrix. The EDS cross-sectional elemental maps show high concentrations of oxygen and iron and just trace marks of phosphorus.
The XPS spectra of Fe2p, O1s, C1s, and P2p on the surface lubricated by PAO-[P₆₆₆₁₄][C₇H₁₅COO]b are shown in FIGS. 5A-D. Two iron oxide peaks were identified: Fe (II) at 709.0 eV and Fe (III) at 710.9 eV. A satellite peak from Fe (II) was found at 715.5 eV. The oxygen O1s peak was resolved into two separate peaks: iron oxides and others possibly including P═O, C—O, and C═O bonds. Three types of carbon groups were identified: carbon group at 284.9 eV, alcohol group at 286.4 eV, and carboxylate group at 289.4 eV. It is believed that iron carboxylate complexes form when an iron surface interacting with carboxylates. The signal of P2p exhibited two peaks: 2p_3/2and 2p_1/2. Decomposition of a phosphonium cation usually generates phosphine oxides, which might further combined with Fe ions. Figure Se shows the XPS depth-composition profile of the tribofilm enabled by ion-sputtering, and results suggest a phosphorus content of 2-3 at %.

Claims

What is claimed is:

1. An ionic liquid composition having the following generic structural formula:

wherein R¹, R², R³, and R⁴are each independently a hydrocarbon group containing at least 4 carbon atoms, and R⁵is a branched hydrocarbon group containing 4 to 8 carbon atoms atoms.

2. The ionic liquid of claim 1, wherein the ionic liquid is soluble (>0.1% wt.) or fully miscible (>10% wt.) in a non-polar hydrocarbon lubricating oil.

3. The ionic liquid of claim 1, wherein R¹, R², R³, and/or R⁴is a hexyl group.

4. The ionic liquid of claim 1, wherein R¹, R², R³, and/or R⁴is an octyl group.

5. The ionic liquid of claim 1, wherein the quaternary phosphonium is trihexyltetradecylphosphonium [P₆₆₆₁₄].

6. The ionic liquid of claim 1, wherein the quaternary phosphonium is tetraoctylphosphonium [P₈₈₈₈].

7. The ionic liquid of claim 1, wherein the quaternary phosphonium is tributyltetradecylphosphonium P[₄₄₄₁₄].

8. The ionic liquid of claim 1, wherein the quaternary phosphonium is tributyltetraoctylphosphonium [P₄₄₄₈].

9. The ionic liquid composition of claim 1, wherein R⁵is a branched heptyl group.

10. The ionic liquid of claim 1, wherein the carboxylate is a 2-ethylhexanoate (C₇H₁₅COO).

11. A lubricant composition comprising:

(i) an ionic liquid having the following generic structural formula:

wherein R¹, R², R³, and R⁴are each independently a hydrocarbon group containing at least 4 carbon atoms, and R⁵is a branched hydrocarbon group containing 4 to 8 carbon atoms; and

(ii) a base oil;

wherein said ionic liquid is dissolved in said base oil.

12. The lubricant composition of claim 11, wherein said base oil is a mechanical lubricating oil.

13. The lubricant composition of claim 11, wherein said ionic liquid is included in said base oil in an amount of at least 0.1 wt %.

14. The lubricant composition of claim 11, wherein said ionic liquid is included in said base oil in an amount of at least 1 wt %.

15. The lubricant composition of claim 11, wherein said ionic liquid is included in said base oil in an amount of at least 10 wt %.

16. The lubricant composition of claim 11, wherein said ionic liquid is included in said base oil in an amount of at least 50 wt %.

17. The lubricant composition of claim 11, wherein R¹, R², R³, and/or R⁴is a hexyl group.

18. The lubricant composition of claim 11 wherein R¹, R², R³, and/or R⁴is an octyl group.

19. The lubricant composition of claim 11, wherein the quaternary phosphonium is trihexyltetradecylphosphonium [P₆₆₆₁₄].

20. The lubricant composition of claim 11, wherein the quaternary phosphonium is tetraoctylphosphonium [P₈₈₈₈].

21. The lubricant composition of claim 11, wherein the quaternary phosphonium is tributyltetradecylphosphonium [P₄₄₄₁₄].

22. The lubricant composition of claim 11, wherein the quaternary phosphonium is tributyltetraoctylphosphonium [P₄₄₄₈].

23. The lubricant composition of claim 11, wherein R⁵is a branched heptyl group.

24. The ionic liquid of claim 11, wherein the carboxylate is a 2-ethylhexanoate (C₇H₁₅COO).