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APPLIED CHEMISTRY Tautomerism and Octane Quality in Carbonyl-Containing Oxygenates Michael Golombok† Shell Research and Technology Centre, Badhuisweg 3, 1031 CM Amsterdam, The Netherlands
A subclass of the oxygenates for potential use in octane boosting of gasoline are molecular species containing carbonyl groups. We consider the kinetics of the radicals formed from carbonyl species and how they affect the overall radical propagation and termination during the combustion process. Using previous work on olefins, we deduce those chemical characteristics of carbonyl molecules which will make them good octane boosters. We identify the CH bond on the R carbon to be crucial to radical branching in the kinetic scheme. By a consideration of the molecular phenomena which affect the bond strength, we identify the gas-phase keto-enol tautomerism constant as a determining factor in radical propagation as well as the number of allylic hydrogens formed on the most stable enol dimer. We demonstrate a correlation for a limited data set between the experimental blending octane number and the independently measured gas-phase equilibrium constant for tautomerism and give a mechanistic explanation of the effect. Introduction Oxygenates are used to boost oxygen content and octane number in reformulated gasoline. Testing a candidate oxygenate involves blending in a base fuel and measurement of the resulting enhancement in the octane number or exhaust gas properties. Vast ranges of such oxygenate blends have been tested, and some promising substances have been identified.1 The problem is that the choice of compounds is largely empirical or based on qualitative criteria such as chemical intuition. Lack of a precise knowledge of the mechanism by which oxygenates boost the octane number inhibits quantitative optimization of the choice of oxygenate. In this paper we attempt to start to tackle the problem of relating the chemical characteristics of a subclass of oxygenates to their octane blending performance. This is developed by analogy to previous work on models for blending octane performance. The role of oxygenates as octane boosters can be understood from the fact that they are related to the intermediates formed during the combustion of hydrocarbons.2 Thus, the oxidation of alkanes is normally taken to involve the successive addition of O2 to alkyl radicals with intermediate internal H transfer, leading to the formation of radical precursors known as branching agents. These are normally assumed to be carbonyl peroxides from which OH is abstracted to yield the propagating radical. Species that contain carbonyl groups and/or peroxides are therefore expected in a chemically qualitative way to assist in the oxidation process. Peroxides are too reactive to include directly as blending components; however, directly reduced precursors such as alcohols and ethers are well-known †
Currently with Shell International Chemicals at the same location. E-mail:
[email protected]. Phone: 31 20 630 2794. Fax: 31 20 630 3085.
to provide significant octane boosting performance when blended in gasoline.1 The carbonyl compounds also show promising potential although the problem here is that many of the compounds containing CdO groups such as esters or carboxylates are quite acidic or are acidic precursors and probably unsuitable for use in engines. A number of criteria are valid when candidate oxygenates are chosen such as production costs, boiling point, and RVP (Reid vapor pressure). However, there is a lack of any criterion for choosing compounds that would make promising high octane additives without actually measuring these blending properties themselves. The oxidation of even simple saturated hydrocarbons consists of a sequence of mechanisms so that prediction from parent structures can be a rather uncertain and unreliable process. Some progress has been made in relating the octane number to chemical kinetic parameters for alkanes.2 We have extended this work in a semiquantitative manner to the assessment of alkenes, where we have examined the role of hydrogen abstraction and how it relates to the ignition quality and thus the octane number of a molecular species.3 Furthermore, we have shown that oxygenate structure is crucial in determining the potential for octane boostingsa nonlinear blending property.4 This is in line with other parallel efforts to expand the range of candidate oxygenates for blending into the mogas pool.5 Such options include the creation of various oxygencontaining moieties on hydrocarbon molecules such as the etherification of catalytically cracked straight run fractions.6 These are all options aimed at overcoming increasing aromatic restrictions in gasoline.7 In this paper we are concerned with one particular class of oxygenatessthose containing carbonyl groups. We briefly recapitulate the theory of blending octane numbers and develop it to establish a systematic measure of blending nonlinearity. We then consider the
10.1021/ie990164s CCC: $18.00 © 1999 American Chemical Society Published on Web 08/27/1999
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Figure 2. Keto-enol tautomerism.
Figure 1. (a, left) Nonlinear octane blending of a fuel component in a base fuel with geometric interpretation of nonlinearity. The solid line ONref to ONf represents the experimentally observed octane number. The dotted line ONref to ONf is the linear interpolation. The dashed line ONref to BON represents linear extrapolation via the observed value of ONbl at fraction f, to the theoretical value at 100% pure component, to define the blending octane number BON in eq 1. (b, right) Associated triangles used to define BON in eq 1 and blending octane ratio in eq 2.
kinetics of combustion and analyze the mechanisms which we relate to gas-phase tautomerism. Finally we correlate that data with blending octane properties. Theory An octane boosting species of octane number ONf blended into a reference fuel of octane number ONref usually blends nonlinearly (see Figure 1a). In general for a volume fraction f, we can define the blending octane number BON by the extrapolated value of a line from zero concentration through the octane number of the blend (ONbl). Thus
BON ) ONref + (1/f)(ONbl - ONref)
(1)
(We have used “ON” here as a general symbol for octane number. All the arguments presented here can apply equally to research or motor octane numbers, RON and MON, respectively, or to their blending analogues defined in eq 1sBRON and BMON.) Since we are interested in using oxygenates as blending components (as opposed to pure fuels), we need to take into account the nonlinear boosting effect which they have on the octane performance. This can be measured using a geometric interpretation of blending nonlinearity based on the definitions of eq 1 in Figure 1. We look at the relative boosting of the oxygenate with respect to the reference fuel compared to the pure oxygenate with respect to the reference fuel. From Figure 1 this is best represented by the blending octane ratio
r)
BON - ONref ONf - ONref
(2)
This factor represents the ratio of the two opposite sides of the ONf and BON triangle in Figure 1bsor alternatively the ratio of the sines of the representative angles for the blended and pure components. In kinetic terms, high octane blending components effectively interfere with the combustion kinetics to delay ignition.2 Thus octane boosters are effectively radical chain terminators. For alkenes, we have previously shown that the number of allylic hydrogens on a molecule was a good indication of this radical termination rate and thus the blending octane number.3 This was ascribed to the fact that allylic CsH bonds are weak and will be the major route for termination of the radical
species involved in combustion. The weakness of CsH bonds in hydroperoxide radical isomerization is also important during alkane combustion in determining how easily hydrogen transfer occurs to form branching agents. In both the cases of the CdC in alkenes and the CdO in carbonyl-containing species, the π electron cloud below and above the σ bond framework withdraws electrons from adjacent bonds, leading to weakening of the CsH bonds on the carbon (i.e., the R carbon) adjacent to the π-bonded carbon. It is thus a logical step to consider the role of weak CsH bonds adjacent to carbonyl groups in oxygenates. A measure of the weakness of the a CsH bond in carbonyl-containing groups such as ketones is the equilibrium constant for the process of tautomerism.8 This is the process whereby internal 1,3 hydrogen transfer turns a ketone (or other CdO-containing compound) into an enol (Figure 2). The usual intermediate is taken to be an enolate anion because the interest of organic chemistry in this process is almost exclusively in the liquid phase in solution where the process is acid or base catalyzed. In general, it is the ease of breaking the CsH bond which is important in this process. For example, ketones with at least one R hydrogen are relatively easy to oxidize because of the resulting enolic CdC double bond. We might therefore expect the considerations previously stated for olefinic double bonds to be relevant (such as for radical termination) to the enol species, the ease of formation of which depends on the equilibrium constant for tautomerism K ) [enol]/[carbonyl]. Most data on tautomerism deal with liquid-phase equilibria (e.g., in a process analogous to combustion, tautomerism plays a key role in the oxidative decomposition of glucose to CO2 via enol formation9). For its role in the kinetics of combustion we are interested in the gas-phase oxidation process, so it is gas-phase measurements to which we turn, as solvation conditions will drastically alter the behavior of the species of interest. We therefore turn to the much newer area of gas-phase tautomerism. Two characteristics are very noticeable from a study of the relevant literature.10-12 First, there have been only very few studies and then only to demonstrate increased sensitivity of a measurement technique, such as ion cyclotron resonance, mass spectroscopy, or NMR. Second, it has clearly been identified that in the gas phase there is a generally stronger weighting of the equilibrium value toward the enol side of the equilibrium than is observed in the corresponding liquid phase. This last observation indicates further support for the proposed importance of enol formation during oxidation. Test There is a considerable amount of data on blending oxygenates in base fuels; however, this needs to be treated with care due to variations in the base fuel ONref and indeed the volume fraction f itself.13 The most uniform studies have been with an f ) 20% blend in a
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Figure 3. Blending octane nonlinearity as a function of the product of the number of enol allylic hydrogens n and the equilibrium constant pK: (1) 4-methylpentanone; (2) acetone; (3) cyclohexanone; (4) 2-butanone; (5) cyclopentanone.
reference fuel consisting of 60% isooctane and 40% n-heptanesa so-called RON 60 reference fuel. The tautomeric equilibrium constants (K) were separately obtained from molecular orbital calculations12 of relative free energy, where we have used the relationship
pK ) ∆G/2.3RT
(3)
The appropriate temperature to use is that associated with cool flames at around 650 K. The free energy calculations are believed to be systematically overevaluated, but the relative relationships are considered to be good. When the calculated values are normalized to a known experimental value, good agreement with experiment is found. It is unclear whether the rate-determining step is the conversion of ketone to enol, or rather the abstraction of the allylic hydrogen from the enol. Since the likelihood of abstraction depends on the number of allylic hydrogens n available, these were evaluated for the most stable enol tautomer. The product n(pK) was then calculated. Figure 3 shows a plot of the blending octane ratio against our tautomeric factor n(pK). High octane nonlinearity would appear to be associated with relatively little conversion to the enol form. That is, high r (in eq 2) is associated with higher values of pK, that is, with very small concentrations of enol relative to ketone. This is in fact the opposite of what we might have expected. We started our study by noticing that, in olefins, the allylic hydrogen correlated to high blending octane number and thus high r values. We then postulated a similar behavior for the enol tautomer, so that high r values would result from a high occurrence of enols, which in turn would be associated with low pK values. The trend observed actually suggests that the keto form is associated with the termination step; i.e., conversion to the enol form actually decreases the r value. This could be associated with the breaking of the CsH hydrogen bond on the R carbon of the ketone during tautomeric interconversion. Thus, a CsH bond might play a role exactly analogous to the allylic hydrogen abstracted during the termination step in olefins. (Such an R CsH bond typically has a bond strength of 411 kJ/mol in ketones whereas allylic CsH bonds on olefinic species are somewhat weaker at 361 kJ/mol.) Assuming that protonation of the carbonyl oxygen actually occurs first, then the R CsH could actually be associated with
two competing processes: hydrogen abstraction associated with radical termination and normal gas-phase tautomeric conversion. If the tautomeric route were favored, then conversion to the enol would increase, and pK would be lowered. Correspondingly, radical termination would be less likely to occur. This would explain the observed results. The relative outlier in Figure 3 is acetone, and this may be due to the fact that acetone represents a rare case where (using the definitions in eq 1 above) RON > BRONsa quite unusual phenomenon associated with negative deviations in the octane number of the blends from the linear extrapolation. A more extended study would be required to conclusively establish a connection between tendency to form enols and the octane quality of a carbonyl-containing oxygenate when blended in a primary reference fuel. Until recently, determination of gas-phase tautomeric equilibria was difficult; however, it is now possible to measure values derived from gas-phase NMR studies. We note that our considerations are only applicable to carbonyl-containing groups. Should the proposed paths prove profitable, the results might be generalized by examining gas-phase CsH detachability as measured, for example, by Hammet acidity constants, which are closely related in the gas phase to the tautomeric equilibrium. Such molecular kinetic mechanisms for octane boosting are a fruitful route for further exploration by our academic colleagues. Literature Cited (1) Spindelbalker, C.; Schmidt, A. Oxygen-containing fuel extenders. Erdoel Erdgas Kohle 1986, 102 (10), 469. (2) Morley, C. A fundamentally based correlation between alkane structure and octane number. Combust. Sci. Technol. 1987, 55, 115. (3) Golombok, M.; de Bruijn, J. N. H.; Morley, C. Kinetically based NMR method of measuring blending octane number of olefins. Chem. Eng. Res. Des. 1995, 73A, 849. (4) Golombok, M.; Tierney, S. Effect of oxygenates on water uptake in hydrocarbon fuels. Ind. Eng. Chem. Res. 1997, 36 (11), 5023. (5) Jakkula, J. J.; Ignatius, J.; Jaervelin, H. Increase oxygenates and lower olefins in gasoline. Produce TAME and heavier etherssEuropean developments. Fuel Reformulation 1995, 5 (1), 46. (6) Trotta, R. Deeply etherify FCC light cracked naphtha. Fuel Technol. Manage. 1996, 6 (2), 65. (7) Hancsok, J.; Hollo, A.; Gergely, J.; Perger, J. Environmentally friendly possibilities to compensate octane deficiency resulting from benzene content reduction of motor gasolines. Pet. Coal. 1998, 40 (1), 33. (8) Lowry, T.; Richardson, K. Mechanism in Organic Chemistry; Harper & Row: New York, 1976. (9) Fessenden, R. J.; Fessenden, J. S. Organic Chemistry; Brooks: Monterey, 1986. (10) Lias, S. G.; Liebman, J. F.; Levin, R. D. Evaluated gasphase basicities and proton affinities of molecules; heats of formation of protonated molecules. J. Phys. Chem. Ref. Data 1984, 13 (3), 695. (11) Toullec, J. Enolization of simple carbonyl compounds and related reactions. In Advances in Organic Chemistry; Gold, V., Bethell, D., Eds.; Academic Press: London, 1982; Vol. 18. (12) Toullec, J. Keto-enol equilibrium constants. In The Chemistry of Enols; Rappaport, Z., Ed.; Wiley: New York, 1990. (13) Knocking characteristics of pure hydrocarbons; API Research Project 45, ASTMS no. 225; API: Philadelphia, 1958.
Received for review March 4, 1999 Revised manuscript received June 10, 1999 Accepted July 5, 1999 IE990164S
