Chemistry of Gum Formation in Cracked Gasoline E. L. WALTERS, H. B. iMINOR, AND D. L. YABROFF Shell Development Compuny, Emeryuille, Culif. Studies were made on the stability of cracked gasolines and pure hydrocarbons by measuring the rates of gum and peroxide formation on aging. Gasoline gum appears to originate from oxidation of both reactive hydrocarbons and gasoline impurities (nonhydrocarbons). Products forming gum were found to be more difficult to inhibit against oxidation than those forming only peroxides. The gum stability of cracked gasoline was improved by treatment, inhibition, or dilution with straight-run components; boiling range was found to be important. Metal catalysts reduced gum stability but were found capable of complete inactivation by metal deactivators. Factors influencing antioxidant and deactivator behavior are discussed. Activation energies of gum-formation reactions were altered by antioxidants and metal catalysts.
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TABILITY of a gasoline is commonly understood tu be the length of time a gasoline will remain stable in storage before i t forms objectionable amounts of deterioration products such as gum and peroxides. These products of deterioration are objectionable because of their interference with the normal operation of automotive engines. Gasoline gum has been found ( 7 , 9) to contribute materially to deposits in automotive induction systems, while the presence of peroxides (Figure 1) leads to a depreciation of antiknock quality (12, 16). The permissible level of over-all oxidation of gasoline is low, about one tenth of t h a t reported for lubricating oils (11). Recently, methods have been presented (20, 21) which permit direct measurement of gasoline deterioration products (gum or peroxides) during accelerated aging and the correlation of these with similar measurements in normal storage. The present paper indicates the application of these methods to the study of stability factors such as composition, treatment, inhibition, and catalysis. The results serve to clarify and supplement numerous previous studies (4, 6, 8 ) based primarily on indirect accelerated stability measuremrnts such as the induction period.
The present studies were made on west coast thermally cracked and straight-run products, typical properties of which are shown in Table I. Results have since been confirmed on similar stocks from other sources and may prove applicable in part at least t o catalytically cracked fuels. MECHANISM OF GUM FORMATION
GENERAL ASPECTS. The initial and final oxidation products of gasoline during normal storage are considered to be peroxides and gum, respectively (11, 12). Gum comprises 15 to 50% of t h e total oxidation products, higher percentages being found in less refined products (17). Gum contains up to 20% oxygen and is rich in acids and saponifiable matter; an ester or lactone structure is postulated (13, 18). Gum has been found (16) to be similar in elementary chemical composition to piston lacquer in gasoline engines, as shown in Table 11. For clarity, the term gum as used herein refers to the existent gum present a t the time of test as determined by A.S.T.M. air jet method D 381-44 ( 2 ) .
TABLE 11. COMPOSITION OF GASOLINE GUMAND ENGINE LACQUER Analyses C, % weight HZ % weight 0 2 ' % weight S,'% weight N, % weight C : H*ratio
(Ash-free bases Gasoline G u m
Engine Lacquer
During normal oxidation, gum or peroxide formation is generally autocatalytic in nature and proceeds a t a simple exponential rate. Typical gum curves during controlled storage trsts are shown in Figure 2. Typical gum, oxygen pressure (induction period), and temperature curves during accelerated aging are shown in Figure 3. Conventional accelerated test conditions arc such (1 ) t h a t the heat from oxidation is excessive and inadequately dissipated near or beyond the end of the induction period, and this tends to increase the TABLEI. GENERALPROPERTIES OF TYPICAL STRAIGHT-RUN rate of gum formaAKD THERMALLY CRACKED GASOLINES tion over the simple Acid Treated a n d Rerun, exponential r a t e . Untreated, Full-Range, Full-Range, Thermally This effect usually Property of Gasoline Straight-Run Cracked occurs above the A.S.T.M. distillation gum level, 7 nig. Initial boiling point, O F. 107 103 Final boiling point, F. 384 380 per 100 ml. (6, 7 ) . 10% volume 158 141 50% volume 267 270 I n closed sys90% volume 3 59 340 tems, the maximuin PEROXIDES Gravity, A.P.I. 57 5 55 3 gum level attainBronune No. 1 60-70 Total sulfur % weight 0 04 0 15-0 2 able in a given fuel =I0 A S T M induction eriod minutes > 1000 200-300 is proportional t o A:S:T:M: gum, mg.fiOO mi 0- 1 1-2 the amount b f air Estimated hydrocarbon composition, % volume = o 4 8 12 (oxygen) available DROP IN F-2 OCTMIE NUMBER Paraffins 33 20 t o react. Gum (or Naphthenes 58 20 Figure 1. Relation between Octane Aromatics 9 8 p e r o x i d e ) 1e v e 1 s Olefins Number Drop and Formation of
