I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
November. 1929
I087
Action of Sulfuric Acid on Mercaptans' S. F. Birch and W. S. Norris THEANGLO-PERSIAN OIL
COMPANY,
URING the refining of certain light petroleum distillates-e. g., gasoline, white spirit (naphtha), kerosene, etc.-by means of sulfuric acid, it has been found that a minute quantity is produced of some substance which is soluble in the petroleum distillate and which renders it a t the boiling point corrosive to copper. -4 clrnn strip of the metal becomes coated with black, flsky copper sulfide. As shown brlow, the substance is not elementary sulfur, although sulfur is obviously formed during its dccomposition. The form:ttion of the substance cannot be avoided by washing the distilhte, prior to the acid treatment, with caustic soda solution (which removes hydrogen sulfide and partly rpmoves mercaptans) ; neither is the corrosive substance removed by soda-washing or even by boiling with alcoholic soda after the acid wash. Moreover. a sample of petroleum which docs not, initially react with copper may become corrosive when treated with concentrated or fuming acid. The corrosive substance is not soluble in concentrated sulfuric acid. This has b w n shown by extracting the used acid, either diluted or not, with a further quanti1,y of sitlfurfree, non-corrosive petroleum, which after soda-washing was found not to have dissolved any of the corrosive: substance. Moreover, the first quant,ity of petroleum, rendered corrosive by the acid treatment, could not be frecd from the corrosive substance by repeated extraction with further quantities of acid. The material which corrodes copper does not react in the cold with metallic mercury; this distinguishes it from elementary sulfur. I n fact, it may be recognized by shaking with mercury to exhaustion and boiling thi. filtered petroleum with coppcr; if the substance is present the copper will be attacked. Under these conditions, if a petroleum solution of sulfur is taken, the reaction with mercury is complete, irrespective of the initial concentration of sulfur, and the copper remains quite bright and clean. Actually, petroleum containing certain derivatives of sulfur is found to contain elementary sulfur after treatment with acid (as is shown below), and this must be eliminated b y means of mercury before testing for the corrosive material by the copper test. A rough measure of spontaneous decomposition in the absence of copper may be obtained by keeping the solution, lreviously freed from elementary sulfur, under the desired conditions and then shaking with mercury in the cold. A t room temperature spontaneous deconiposition is very gradual, and this is particularly true if the solution is very dilute and is kept in the dark; this accords with the very slow tarnishing of copper in the cold and with the somewhat obvious hypothesis that decomposition precedes the action on the metal. At more elevated temperat,ures-e. g., in boiling naphtha solution-decomposition in the absence of copper is more rapid, but nevertheless appears to become comparatively slow as the concentration of the corrosive substance falls. It seemed improbable that a substance having these properties could be produced from sulfuric acid and the hydrocarbon constituents of light petroleum; a reaction between the acid and some organic derivative (or derivatives) of sulfur appeared a more likely source.
D
Previous Investigations
Wood. Lo-, and Faragher (IO) have studied the action of varioua refining agents upon petroleum solution of individual I
Received August 6, 1929.
LTD.,SUNBURY-ON-TH.421lES, EXGLAND
sulfur compounds. They find that, whereas organic sulfides, sulfoxides, or sulfones are removed from petroleum by the salient action of sulfuric wid. disulfides are less readily remoT ed, some oxidation occurring as shown by the evolution of sulfur dioxide, n-hile mercaptans ( 4 ) and hydrogen sulfide are readily osidizcd to disulfides and elementary sulfur, resprctively. Wood, Lo~vy,and Farngher note that when the quantity of acid employed in treating a merraptan solution is I ery small, the sulfur content of the petroleurn, after washing to remove inorganic acids, is higher than that of the origiml mercaptnn solution. They ascribe this t o the formation of the alkyl thio- or dithiosulfate. This work was later extended by Rood, Sheely, and Trusty (12) and by Youtz and Perkins ( 1 3 , but without any new fact of importance being elicited. The greater ease of removal of compounds of lower molecular weight, and the much greater solvent power of mure concentrated acid-e. g., fuming acid-arc recorded. Thierry ( 8 ) has shown that a series of organic sulfidrs presumably present in a Pcrsian distillate can be isolated by dilution of the acid tar formed during refining-a method used by Mnbery and his collaborators ( 6 ) for investigating Ohio and Canadian petroleums. Kcither Thierry nor hlabery semis to have contemplated the possible presence of disulf i d ~ or s the possibility that the comporinds origindly in the distillate might undergo change during treatment with sulfuric acid. Bennett and Story (1) find that, when n-butyl mercnptan in petroleum solution is treated with concentrated sulfiiric acid, n-butyl, n-butylthiosulfonate (di-n-butyl disulfoxide) may be isolated from the acid layer by dilution and extraction with a n organic solvent. Wood, Sheely, and Trusty (11) have examined the corrosive action on many metals, including copper, of petroleum solutions of a variety of sulfur compounds. Since many such solutions could be formed as intermediate or final products in the action of sulfuric acid on sulfurous petroleum distillates, the results of these investigators have been considered, but none of the substances dealt with by them appears to correspond in general as ~vellas in corrosive properties with the material now under investigation. The latter appears, in fact, t o be some product that has hitherto escaped observation. Action of Sulfuric Acid on Organic Sulfur Compounds
A systematic study was therefore carried out on the action of concentrated and fuming sulfuric acid on organic compounds of sulfur in naphtha solution. The naphtha was prepared from a commercial painter's naphtha containing less than 0.01 per cent of sulfur by distilling in R current of steam leaving a 5 per cent residue. The distillate, which was sulfurfree, commenced to boil a t 130" C., 90 per cent distilling between 150" and 180" C.; i t had a density at 15.5" C. of 0.777. When this solvent was distilled to dryness in the presence of dean polished copper, the copper remained untarnished. The treated solutions were always immediately washed with dilute aqueous alkali to remove inorganic acids. The solutions were tested before and after the acid treatment b y distillation from a n Engler flask containing a clean polished strip of copper. The results set forth in the table reveal the striking fact that only when the original solution contains a mercaptan is the final solution corrosive to copper at the boiling point. The properties of the corrosive substance so formed are identical with those of the corrosive substance found in
INDUSTRIAL AND ENGINEERING CHEMISTRY
1088
acid-treated commercial distillates. The sulfoxide and the disulfoxide (the latter is identical structurally with Bennett and Story's alkyl alkylthiosulfonate) certainly attack copper. Neither of these can be the substance under investigation because they are soluble in sulfuric acid, their solutions in naphtha being desulfurized and rendered non-corrosive by treatment with acid, while the corrosive material from a mercaptan or from an industrial distillste is not removed b y acid treatment. Although the disulfides of the lower alkyl groups are quite stable a t such comparatively low temperatures, as the boiling point of naphtha, it was a t one time suspected that the corrosive properties of acid-treated commercial
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spent acid, followed b y partial solution b y the alkali of t h e mercaptan (if any) which escaped the original oxidation. B u t the maximum sulfur content reached with an intermediate quantity of acid is greater than the sulfur content of the original mercaptan solution. Hence there is a t least one other prtroleum-soluble product the sulfur content of which is derived a t least partly from the sulfuric acid. This confirms the observation of Wood, Lowy, and Faragher that thio- or dithiosulfuric esters may be formed. Probability is lent to their hypothesis by the observation now made that if a naphtha solution of a mercaptan is trcated with acid and with soda and is then distilled to a small residue using super-
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