High viscosity index synthetic lubricant compositions

This application is a continuation-in-part of U.S. patent application Ser. No. 147,064 filed Jan. 22, 1988 which is a continuation of application Ser. No. 946,226 filed Dec. 24, 1986, both now abandoned. This invention relates to novel lubricant compositions. The invention, more particularly, relates to novel synthetic lubricant compositions prepared from alpha-olefins, or 1-alkenes. The invention specifically relates to novel synthetic lubricant compositions from 1-alkenes exhibiting superior viscosity indices and other improved characteristics essential to useful lubricating oils. BACKGROUND OF THE INVENTION Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for at least fifty years and have led to the relatively recent market introduction of a number of superior polyalpha-olefin synthetic lubricants, primarily based on the oligomerization of alpha-olefins or 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved viscosity index, while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These new synthetic lubricants lower friction and hence increase mechanical efficiency across the full spectrum of mechanical loads from worm gears to traction drives and do so over a wider range of operating conditions than mineral oil lubricants. The chemical focus of the research effort in synthetic lubricants has been on the polymerization of 1-alkenes. Well known structure/property relationships for high polymers as contained in the various disciplines of polymer chemistry have pointed the way to 1-alkenes as a fruitful field of investigation for the synthesis of oligomers with the structure thought to be needed to confer improved lubricant properties thereon. Due largely to studies on the polymerization of propene and vinyl monomers, the mechanism of the polymerization of 1-alkene and the effect of that mechanism on polymer structure is reasonably well understood, providing a strong resource for targeting on potentially useful oligomerization methods and oligomer structures. Building on that resource, in the prior art oligomers of 1-alkenes from C.sub.6 to C.sub.20 have been prepared with commercially useful synthetic lubricants from 1-decene oligomerization yielding a distinctly superior lubricant product via either cationic or Ziegler catalyzed polymerization. Theoretically, the oligomerization of 1-decene, for example, to lubricant oligomers in the C.sub.30 and C.sub.40 range can result in a very large number of structural isomers. Henze and Blair, J.A.C.S. 54,1538, calculate over 60.times.10.sup.12 isomers for C.sub.30 -C.sub.40. Discovering exactly those isomers, and the associated oligomerization process, that produce a preferred and superior synthetic lubricant meeting the specification requirements of wide-temperature fluidity while maintaining low pour point represents a prodigious challenge to the workers in the field. Brennan, Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 2-6, cites 1-decene trimer as an example of a structure compatible with structures associated with superior low temperature fluidity wherein the concentration of atoms is very close to the center of a chain of carbon atoms. Also described therein is the apparent dependency of properties of the oligomer on the oligomerization process, i.e., cationic polymerization or Ziegler-type catalyst, known and practiced in the art. One characteristic of the molecular structure of 1-alkene oligomers that has been found to correlate very well with improved lubricant properties in commercial synthetic lubricants is the ratio of methyl to methylene groups in the oligomer. The ratio is called the branch ratio and is calculated from infra red data as discussed in "Standard Hydrocarbons of High Molecular Weight", Analytical Chemistry, Vol.25, no.10, p.1466 (1953). Viscosity index has been found to increase with lower branch ratio. Heretofore, oligomeric liquid lubricants exhibiting very low branch ratios have not been synthesized from 1-alkenes. For instance, oligomers prepared from 1-decene by either cationic polymerization or Ziegler catalyst polymerization have branch ratios of greater than 0.20. Shubkin, Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 15-19, provides an explanation for the apparently limiting value for branch ratio based on a cationic polymerization reaction mechanism involving rearrangement to produce branching. Other explanations suggest isomerization of the olefinic group in the one position to produce an internal olefin as the cause for branching. Whether by rearrangement, isomerization or a yet to be elucidated mechanism it is clear that in the art of 1-alkene oligomerization to produce synthetic lubricants as practiced to-date excessive branching occurs and constrains the limits of achievable lubricant properties, particularly with respect to viscosity index. Obviously, increased branching increases the number of isomers in the oligomer mixture, orienting the composition away from the structure which would be preferred from a consideration of the theoretical concepts discussed above. U.S. Pat. No. 4,282,392 to Cupples et al. discloses an alpha-olefin oligomer synthetic lubricant having an improved viscosity-volatility relationship and containing a high proportion of tetramer and pentamer via a hydrogenation process that effects skeletal rearrangement and isomeric composition. The composition claimed is a trimer to tetramer ratio no higher than one to one. The branch ratio is not disclosed. A process using coordination catalysts to prepare high polymers from 1-alkenes, especially chromium catalyst on a silica support, is described by Weiss et al. in Jour. Catalysis 88, 424-430 (1984) and in Offen. DE 3,427,319. The process and products therefrom are discussed in more detail hereinafter in comparison with the process and products of the instant invention. It is an object of the present invention to provide a novel synthetic liquid lubricant composition having superior lubricant properties based on oligomerized alpha-olefins. It is another object of the instant invention to provide a novel synthetic liquid lubricant having a low branch ratio, high viscosity index and low pour point. Yet another object of the invention is to provide a hydrogenated polyalpha-olefin synthetic liquid lubricant having a high viscosity index and low pour point. SUMMARY OF THE INVENTION Liquid hydrocarbon lubricant compositions have been discovered from C.sub.6 -C.sub.20 1-alkene oligomerization that exhibit surprisingly high viscosity index (VI) while, equally surprisingly, exhibit very low pour points. The compositions comprise C.sub.30 -C.sub.1300 hydrocarbons, said compositions having a branch ratio of less than 0.19; weight average molecular weight between 300 and 45,000; number average molecular weight between 300 and 18,000; molecular weight distribution between 1 and 5 and pour point below -15.degree. C. Further, a novel composition has been discovered comprising 11-octyldocosane having the structure ##STR1## The foregoing composition has been found to exhibit superior lubricant properties either alone or in a mixture with 9-methyl,11-octylheneicosane. Surprisingly, the mixture has a viscosity index of greater than 130 while maintaining a pour point less than -15.degree. C. These compositions are representative of the instant invention comprising C.sub.30 H.sub.62 alkanes having a branch ratio, or CH.sub.3 /CH.sub.2 ratio, of less than 0.19. These low branch ratios and pour points characterize the compositions of the invention, referred to herein as polyalpha-olefin or HVI-PAO, conferring upon the compositions especially high viscosity indices in comparison to commercially available polyalpha-olefin (PAO) synthetic lubricants. Unique lubricant oligomers of the instant invention can also be made in a wide range of molecular weights and viscosities comprising C.sub.30 to C.sub.1000 hydrocarbons having a branch ratio of less than 0.19 and molecular weight distribution of about 1.05 to 2.5. The oligomers can be mixed with conventional mineral oils or greases of other properties to provide compositions also possessing outstanding lubricant properties. Compositions of the present invention can be prepared by the oligomerization of alpha-olefins such as 1-decene under oligomerization conditions in contact with a supported and reduced valence state metal oxide catalyst from Group VIB of the IUPAC Periodic Table. Chromium oxide is the preferred metal oxide.

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DESCRIPTION OF THE FIGURES

FIG. 1 is a comparison of PAO and HVI-PAO syntheses.

FIG. 2 compares VI for PAO and HVI-PAO.

FIG. 3 shows pour points for PAO and HVI-PAO.

FIG. 4 shows C-13 NMR spectra for HVI-PAO from 1-hexene.

FIG. 5 shows C-13 NMR spectra of 5 cs HVI-PAO from 1-decene.

FIG. 6 shows C-13 NMR spectra of 50 cs HVI-PAO from 1-decene.

FIG. 7 shows C-13 NMR spectra of 145 cs HVI-PAO from 1-decene.

FIG. 8 shows the gas chromatograph of HVI-PAO 1-decene trimer.

FIG. 9 shows C-13 NMR of HVI-PAO trimer of 1-decene.

FIG. 10 shows C-13 NMR calculated vs. observed chemical shifts for HVI-PAO 1-decene trimer components.

DETAIL DESCRIPTION OF THE INVENTION

In the following description, unless otherwise stated, all references to HVI-PAO oligomers or lubricants refer to hydrogenated oligomers and lubricants in keeping with the practice well known to those skilled in the art of lubricant production. As oligomerized, HVI-PAO oligomers are mixtures of dialkyl vinyledenic and 1,2 dialkyl or trialkyl mono-olefins. Lower molecular weight unsaturated oligomers are preferably hydrogenated to produce thermally and oxidatively stable, useful lubricants. Higher molecular weight unsaturated HVI-PAO oligomers are sufficiently thermally stable to be utilized without hydrogenation and, optionally, may be so employed. Both unsaturated and hydrogenated HVI-PAO of lower or higher molecular exhibit viscosity indices of at least 130 and pour point below-15.degree. C.

Referring to FIG. 1, the novel oligomers of the invention, or high viscosity index polyalphaolefins (HVI-PAO) are described in an illustration comparing them with conventional polyalphaolefins (PAO) from 1-decene. Polymerization with the novel reduced chromium catalyst described hereinafter leads to an oligomer substantially free of double bond isomerization. Conventional PAO, on the other hand, promoted by BF.sub.3 or ALCl3 forms a carbonium ion which, in turn, promotes isomerization of the olefinic bond and the formation of multiple isomers. The HVI-PAO produced in the present invention has a structure with a CH.sub.3 /CH.sub.2 ratio <0.19 compared to a ratio of >0.20 for PAO.

FIG. 2 compares the viscosity index versus viscosity relationship for HVI-PAO and PAO lubricants, showing that HVI-PAO is distinctly superior to PAO at all viscosities tested. Remarkably, despite the more regular structure of the HVI-PAO oligomers as shown by branch ratio that results in improved viscosity index (VI), they show pour points superior to PAO. Conceivably, oligomers of regular structure containing fewer isomers would be expected to have higher solidification temperatures and higher pour points, reducing their utility as lubricants. But, surprisingly, such is not the case for HVI-PAO of the present invention. FIGS. 2 and 3 illustrate superiority of HVI-PAO in terms of both pour point and VI.

It has been found that the process described herein to produce the novel HVI-PAO oligomers can be controlled to yield oligomers having weight average molecular weight between 300 and 45,000 and number average molecular weight between 300 and 18,000. Measured in carbon numbers, molecular weights range from C.sub.30 to C.sub.1300 and viscosity up to 750 cs at 100.degree. C., with a preferred range of C.sub.30 to C.sub.1000 and a viscosity of up to 500 cs at 100.degree. C. Molecular weight distributions (MWD), defined as the ratio of weight average molecular to number average molecular weight, range from 1.00 to 5, with a preferred range of 1.01 to 3 and a more preferred MWD of about 1.05 to 2.5. Compared to conventional PAO derived from BF.sub.3 or AlCl.sub.3 catalyzed polymerization of 1-alkene, HVI-PAO of the present invention has been found to have a higher proportion of higher molecular weight polymer molecules in the product.

Viscosities of the novel HVI-PAO oligomers measured at 100.degree. C. range from 3 cs to 5000 cs. The viscosity index for the new polyalpha-olefins is approximately described by the following equation:

VI=129.8+4.58.times.(V.sub.100 C)0.5,

where V.sub.100 .degree. C. is kinematic viscosity in centistokes measured at 100.degree. C.

The novel oligomer compositions disclosed herein have been examined to define their unique structure beyond the important characteristics of branch ratio and molecular weight already noted. Dimer and trimer fractions have been separated by distillation and components thereof further separated by gas chromatography. These lower oligomers and components along with complete reaction mixtures of HVI-PAO oligomers have been studied using infra-red spectroscopy and C-13 NMR. The studies have confirmed the highly uniform structural composition of the products of the invention, particularly when compared to conventional polyalphaolefins produced by BF.sub.3, AlCl.sub.3 or Ziegler-type catalysis. The unique capability of C-13 NMR to identify structural isomers has led to the identification of distinctive compounds in lower oligomeric fractions and served to confirm the more uniform isomeric mix present in higher molecular weight oligomers compatible with the finding of low branch ratios and superior viscosity indices.

1-hexene HVI-PAO oligomers of the present invention have been shown to have a very uniform linear C.sub.4 branch and contain regular head-to-tail connections. In addition to the structures from the regular head-to-tail connections, the backbone structures have some head-to-head connection, indicative of the following structure as confirmed by NMR: ##STR2##

The NMR poly(1-hexene) spectra are shown in FIG. 4.

The oligomerization of 1-decene by reduced valence state, supported chromium also yields a HVI-PAO with a structure analogous to that of 1-hexene oligomer The lubricant products after distillation to remove light fractions and hydrogenation have characteristic C-13 NMR spectra. FIGS. 5, 6 and 7 are the C-13 NMR spectra of typical HVI-PAO lube products with viscosities of 5 cs, 50 cs and 145 cs at 100.degree. C.

In the following tables, Table A presents the NMR data for FIG. 5, Table B presents the NMR data for FIG. 6 and Table C presents the NMR data for FIG. 7.

TABLE A ______________________________________ (FIG. 5) Point Shift (ppm) Intensity Width (Hz) ______________________________________ 1 79.096 138841. 2.74 2 74.855 130653. 4.52 3 42.394 148620. 6.68 4 40.639 133441. 37.6 5 40.298 163678. 32.4 6 40.054 176339. 31.2 7 39.420 134904. 37.4 8 37.714 445452. 7.38 9 37.373 227254. 157 10 37.081 145467. 186 11 36.788 153096. 184 12 36.593 145681. 186 13 36.447 132292. 189 14 36.057 152778. 184 15 35.619 206141. 184 16 35.082 505413. 26.8 17 34.351 741424. 14.3 18 34.059 1265077. 7.65 19 32.207 5351568. 1.48 20 30.403 3563751. 4.34 21 29.965 8294773. 2.56 22 29.623 4714955. 3.67 23 28.356 369728. 10.4 24 28.161 305878. 13.2 25 26.991 1481260. 4.88 26 22.897 4548162. 1.76 27 20.265 227694. 1.99 28 14.221 4592991. 1.62 ______________________________________

TABLE B ______________________________________ (FIG. 6) No. Freq (Hz) PPM Int % ______________________________________ 1 1198.98 79.147 1056 2 1157.95 77.004 1040 3 1126.46 74.910 1025 4 559.57 37.211 491 5 526.61 35.019 805 6 514.89 34.240 1298 7 509.76 33.899 1140 8 491.45 32.681 897 9 482.66 32.097 9279 10 456.29 30.344 4972 11 448.24 29.808 9711 12 444.58 29.564 7463 13 426.26 28.347 1025 14 401.36 26.691 1690 15 342.77 22.794 9782 16 212.40 14.124 8634 17 0.00 0.000 315 ______________________________________

TABLE C ______________________________________ (FIG. 7) Point Shift (ppm) Intensity Width (Hz) ______________________________________ l 76.903 627426. 2.92 2 40.811 901505. 22.8 3 40.568 865686. 23.1 4 40.324 823178. 19.5 5 37.158 677621. 183. 6 36.915 705894. 181. 7 36.720 669037. 183. 8 36.428 691870. 183. 9 36.233 696323. 181. 10 35.259 1315574. 155. 11 35.015 1471226. 152. 12 34.333 1901096. 121. 13 32.726 1990364. 120. 14 32.141 20319110. 2.81 15 31.362 1661594. 148. 16 30.388 9516199. 19.6 17 29.901 17778892. 9.64 18 29.609 18706236. 9.17 19 28.391 1869681. 122. 20 27.514 1117864. 173. 21 26.735 2954012. 14.0 22 22.839 20895526. 2.17 23 14.169 16670130. 2.06 ______________________________________

In general, the novel oligomers have the following regular head-to-tail structure where n can be 3 to 17: ##STR3## with some head-to-head connections.

The trimer of 1-decene HVI-PAO oligomer is separated from the oligomerization mixture by distillation from a 20 cs as-synthesized HVI-PAO in a short-path apparatus in the range of 165.degree.-210.degree. C. at 0.1-0.2 torr. The unhydrogenated trimer exhibited the following viscometric properties:

V@40 C.=14.88 cs; V@100.degree. C.=3.67 cs; VI=137

The trimer is hydrogenated at 235.degree. C. and 4200 kPa H.sub.2 with Ni on kieselguhr hydrogenation catalyst to give a hydrogenated HVI-PAO trimer with the following properties:

V@40.degree. C.=16.66 cs; V@100.degree. C.=3.91 cs; VI=133

Pour Point=less than -45.degree. C.;

Gas chromatographic analysis of the trimer reveals that it is composed of essentially two components having retention times of 1810 seconds and 1878 seconds under the following conditions:

G. C. column-60 meter capillary column, 0.32 mmid, coated with stationary phase SPB-1 with film thickness 0.25 .mu.m, available from Supelco chromatography supplies, catalog no. 2-4046.

Separation Conditions--Varian Gas chromatograph, model no. 3700, equipped with a flame ionization detector and capillary injector port with split ratio of about 50. N.sub.2 carrier gas flow rate is 2.5 cc/minute. Injector port temperature 300.degree. C.; detector port temperature 330.degree. C., column temperature is set initially at 45.degree. C. for 6 minutes, programmed heating at 15.degree. C./minute to 300.degree. C. final temperature and holding at final temperature for 60 minutes. Sample injection size is 1 microliter. Under these conditions, the retention time of a g.c. standard, n-dodecane, is 968 seconds.

A typical chromatograph is shown in FIG. 8.

The C-13 NMR spectra, (FIG. 9), of the distilled C30 product confirm the chemical structures. Table D lists C-13 NMR data for FIG. 9.

TABLE D ______________________________________ (FIG. (9) Point Shift (ppm) Intensity Width (Hz) ______________________________________ 1 55.987 11080. 2.30 2 42.632 13367. 140. 3 42.388 16612. 263. 4 37.807 40273. 5.90 5 37.319 12257. 16.2 6 36.539 11374. 12.1 7 35.418 11631. 35.3 8 35.126 33099. 3.14 9 34.638 39277. 14.6 10 34.054 110899. 3.32 11 33.615 12544. 34.9 12 33.469 13698. 34.2 13 32.981 11278. 5.69 14 32.835 13785. 57.4 15 32.201 256181. 1.41 16 31.811 17867. 24.6 17 31.470 13327. 57.4 18 30.398 261859. 3.36 19 29.959 543993. 1.89 20 29.618 317314. 1.19 21 28.838 11325. 15.1 22 28.351 24926. 12.4 23 28.156 29663. 6.17 24 27.230 44024. 11.7 25 26.986 125437. -0.261 26 22.892 271278. 1.15 27 20.260 17578. -22.1 28 14.167 201979. 2.01 ______________________________________

The individual peak assignment of the C-13 spectra are shown in FIG. 9. Based on these structures, the calculated chemical shifts, as shown in FIG. 10, matched closely with the observed chemical shifts. The calculation of chemical shifts of hydrocarbons is carried out as described is "Carbon-13 NMR for Organic Chemists" by G. C. Levy and G. L. Nelson, 1972, by John Wiley & Sons, Inc., Chapter 3, p 38-41. The components were identified as 9-methyl,11-octylheneicosane and 11-octyldocosane by infrared and C-13 NMR analysis and were found to be present in a ratio between 1:10 and 10:1 heneicosane to docosane. The hydrogenated 1-decene trimer produced by the process of this invention has an index of refraction at 60.degree. C. of 1.4396.

The process of the present invention produces a surprisingly simpler and useful dimer compared to the dimer produced by 1-alkene oligomerization with BF.sub.3 or AlCl.sub.3 as commercially practiced. Typically, in the present invention it has been found that a significant proportion of unhydrogenated dimerized 1-alkene has a vinylidenyl structure as follows:

CH.sub.2 .dbd.CR.sub.1 R.sub.2

where R.sub.1 and R.sub.2 are alkyl groups representing the residue from the head-to-tail addition of 1-alkene molecules. For example, 1-decene dimer of the invention has been found to contain only three major components, as determined by GC. Based on C.sup.13 NMR analysis, the unhydrogenated components were found to be 8-eicosene, 9-eicosene, 2-octyldodecene and 9-methyl-8 or 9-methyl-9-nonadecene. The hydrogenated dimer components were found to be n-eicosane and 9-methylnonacosane.

Olefins suitable for use as starting material in the invention include those olefins containing from 2 to about 20 carbon atoms such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene and 1-tetradecene and branched chain isomers such as 4-methyl-1-pentene. Also suitable for use are olefin-containing refinery feedstocks or effluents. However, the olefins used in this invention are preferably alpha olefinic as for example 1-heptene to 1-hexadecene and more preferably 1-octene to 1-tetradecene, or mixtures of such olefins.

Oligomers of alpha-olefins in accordance with the invention have a low branch ratio of less than 0.19 and superior lubricating properties compared to the alpha-olefin oligomers with a high branch ratio, as produced in all known commercial methods.

This new class of alpha-olefin oligomers are prepared by oligomerization reactions in which a major proportion of the double bonds of the alphaolefins are not isomerized. These reactions include alpha-olefin oligomerization by supported metal oxide catalysts, such as Cr compounds on silica or other supported IUPAC Periodic Table Group VIB compounds. The catalyst most preferred is a lower valence Group VIB metal oxide on an inert support. Preferred supports include silica, alumina, titania, silica alumina, magnesia and the like. The support material binds the metal oxide catalyst. Those porous substrates having a pore opening of at least 40 angstroms are preferred.

The support material usually has high surface area and large pore volumes with average pore size of 40 to about 350 angstroms. The high surface area are beneficial for supporting large amount of highly dispersive, active chromium metal centers and to give maximum efficiency of metal usage, resulting in very high activity catalyst. The support should have large average pore openings of at least 40 angstroms, with an average pore opening of >60 to 300 angstroms preferred. This large pore opening will not impose any diffusional restriction of the reactant and product to and away from the active catalytic metal centers, thus further optimizing the catalyst productivity. Also, for this catalyst to be used in fixed bed or slurry reactor and to be recycled and regenerated many times, a silica support with good physical strength is preferred to prevent catalyst particle attrition or disintegration during handling or reaction.

The supported metal oxide catalysts are preferably prepared by impregnating metal salts in water or organic solvents onto the support. Any suitable organic solvent known to the art may be used, for example, ethanol, methanol, o