Direct Determination of Oxygen in Organic Compounds Containing Sulfur ter Meulen Method W. WALKER RUSSELL
AND
MAURICE E. MARKS, Metcalf Laboratory, Brown University, Providence, R. I.
I
The Catalyst
N PREVIOUS studies (2, 6, 7 ) the authors found that,
when certain modifications were made, the ter Meulen method (3, 4, 5 ) for the direct determination of oxygen in organic compounds by catalytic hydrogenation gave satisfactory results. Thus when a very active thoria-promoted nickel catalyst was used, all oxygen was quantitatively converted to water in the analysis of compounds containing only carbon, hydrogen, and oxygen (6), and this was equally true when nitrogen (7) or small amounts of sulfur (9) were also present. The last work (W),which dealt primarily with the direct determination of total oxygen in oils whose sulfur and nitrogen content was below 0.1 per cent, appeared to justify further work with organic sulfur compounds. I n an endeavor to make the method more generally applicable, the behavior of compounds containing considerable amounts of sulfur and various types of sulfur linkage has been studied. Even though sulfur is recognized as a serious poison for nickel catalysts, a method has been developed which has given satisfactory successive analyses with the several types of organic sulfur compounds studied.
Unsupported and quartz-supported nickel catalysts, both unpromoted and thoria-promoted, have been investigated. Ordinarily any of these catalysts entirely sorbed any sulfur compounds not held back by the platinized quartz cracking surface. I n fact, hydrogen sulfide was not obtained from any active catalyst under conditions of analysis, or even at 450" C., unless uncracked benzene passed through the system. When this occurred hydrogen sulfide was evolved and the catalyst became completely poisoned. Considerable amounts of sulfur could be present in the system before the catalysts failed in analysis. Thus with aliphatic compounds, in whose analysis the catalyst was the limiting factor, from eight to thirteen runs could be made, thereby introducing some 200 to 400 mg. of sulfur, before the 5 to 10 grams of unsupported promoted catalyst failed to give quantitative conversion of oxygen to water. The smaller nickel content of the supported promoted catalysts allowed about four successive runs on 20 grams of this material. Certain samples of unsupported catalyst, both unpromoted and thoria-promoted, exhibited a marked and persistent increase in blanks immediately following an analysis-for example, an initial blank of 0.5 mg. per half hour might increase to 2 t o 3 mg. directly after a run. However, after passing hydrogen for a few hours, the blanks decreased and finally became constant below their initial values. Observations of this phenomenon led to the belief that hydrogen sulfide was in some way acting upon the catalyst. This contention was strengthened by the results of experiments in which hydrogen sulfide was passed through the tube under the conditions of analysis. I n the cases of both promoted and unpromoted catalysts, the blanks were considerably increased thereby. It seems necessary to assume, therefore, that hydrogen sulfide, and perhaps other sulfur compounds which may exist a t this stage in the analysis, are capable of accelerating the reduction of nickel oxide which escaped reduction by hydrogen alone during the preparation of the catalyst. Small amounts of such oxide are known to exist within catalyst granules even after prolonged reduction. It is interesting to note that the increased blanks persisted for some hours after the passage of hydrogen sulfide had been discontinued. This may signify a slow reaction with adsorbed hydrogen sulfide or sulfur, or a specific catalytic effect. Although the phenomenon of increased blanks was not observed when reduction was prolonged for 2 weeks, it was shown by certain samples of unsupported catalyst, among a number which were believed to be identical in composition and mode of preparation, but which were reduced for 24 hours only. Some factor not under control, therefore, appears to have been responsible for a variation in the amounts of residual oxide present in the more rapidly reduced catalysts. A simple and very effective solution of the problem was found to lie in the use of granular quartz-supported nickel catalysts. The nickel was now present only in very thin layers which were readily freed from all but negligible amounts of oxide by reduction in hydrogen overnight a t 500" C. It is recommended, therefore, that the supported, promoted nickel be used in all analyses of sulfur compounds as catalyst, but not as cracking surf ace.
The Cracking Surface Cracking surfaces composed of platinized quartz granules, and also of nickelieed quartz granules, both with and without thoria, were investigated. While the nickel-coated surfaces appeared slightly more active, they proved unsatisfactory because of large blanks. The platinized quartz cracking surface (6) proved very efficient for most organic sulfur compounds studied. More or less sulfur from the compound was always retained by this cracking surface. Because the aliphatic compounds cracked easily to gaseous products between 600" and 800" C., depositing little carbon, the efficiency of the cracking surface was maintained, and the capacity of the catalyst to resist sulfur poisoning became the limiting factor in determining the number of successive runs possible with one tube-filling. Aromatic compounds which may require cracking temperatures up to 1100" C. deposited considerable carbon which diminished the activity of the cracking surface as successive runs were made. Thus the amount (about 20 cc.) of platinized quartz employed allowed about five successive analyses to be made of either p-xylenesulfonic acid or di-p-tolyl sulfoxide, about three of sulfobenzoic anhydride, or about two of diphenyl sulfone. Failure occurred when the cracking surface was no longer able to prevent easily condensable decomposition products from passing through. A larger amount of cracking surface should make possible an increase in the number of successive analyses of aromatic sulfur compounds. Of the aromatic compounds studied, diphenyl sulfone required the highest temperature for cracking-i. e., about 1100" C.-while for the other aromatics temperatures down to 800" C. sufficed. That the number of successive analyses possible with a given aromatic compound offered a criterion of its cracking characteristics f o l l o ~ from s observations that the rate of vaporization of the sample had to be decreased in the above sequence in order to obtain satisfactory cracking. Thus the full extension of the analysis time which has been recommended for organic sulfur compounds was necessary only in the cases of the compounds most difficult t o crack. 453
VOL. 8, NO. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
454
I n the analysis of compounds containing only hydrogen, carbon, oxygen, and nitrogen (6, 7 ) regeneration of the catalyst was possible by oxidizing and then reducing with hydrogen. However, when sulfur was held by the catalyst, oxidation formed nickel sulfate which proved impossible to reduce. Heating the used catalysts to red heat in a stream of hydrogen, without preliminary oxidation, also was unsuccessful because of the slowness of sulfide decomposition and the danger of injuring the quartz tube by contact with melted sulfide. The small amount of trouble and expense involved in replacing a poisoned catalyst with a fresh one is more than offsekby the knowledge that a fully active catalyst is in the tube, an especially important consideration when working with new substances.
Apparatus, Materials, and Method The apparatus used in the major part of the work was essentially that already described (6), with the following exceptions: To eliminate sources of water within the system, the liquid used in the safety tube and in the flowmeter was purified mineral oil. Either platinized asbestos or reduced nickel was used to remove traces of oxygen from the electrolytic hydrogen employed. By placing the flowmeter before the purification train, it was possible to have the metered hydrogen come into contact only with glass surfaces, except at the short thick rubber connection joining the purification system t o the quartz tube. The'hydrogen was first dried by calcium chloride and then by Drierite. The Drierite was regenerated in place by an electric furnace heated to 230" t o 250"9. Eachof the.Schwartz tubes in the absorption train also contained Drierite (2). The cracking surface was heated by a heavy-duty electric furnace which was automatically turned on by a simple, improvised, time-power switch. The unsupported nickel catalysts were prepared as previously described (6), except that final reduction was carried out at 450' C. From 5 to 10 grams of the ignited oxides were used t o charge the tube. The su ported catalysts were prepared by thoroughly mixing the desire$ amount of 20-mesh granulated quartz with 5 per cent of its weight of nickel in the form of pure nickel nitrate hexahydrate, and 0.1 per cent of its (quartz) weight of thoria as pure thorium nitrate, dissolved together in just enough hot water to coat the quartz granules uniformly. The mixture was then heated in a casserole over a Bunsen flame until the nitrates were completely decomposed to oxide. About 20 grams of the oxidecoated quartz granules were used t o charge the tube. After reduction at 500' C. the granules of quartz were uniformly covered with a very thin deposit of nickel. The platinized uartz cracking surface was prepared as before (6),while the nic%eliaed cracking surface was prepared in the same manner as the supported catalysts, except that reduction was finally completed at 1000° C. The method of analysis was in general similar t o that described elsewhere (69,except that the rate of hydrogen flow was reduced to 40 cc. er minute, although the catalyst temperature remained at 350.' The time of analysis may well be extended to 1.5 hours in the case of aromatic sulfur compounds. After reducing the catalysts for 24 hours, the blank on the system was between 0.5 and 1 mg. per half hour, The blank gradually diminished during successive analyses.
8.
Discussion of Results The results of analyses made by the foregoing method upon high-grade organic chenlicals containing sulfur are given in Table I. The accuracy of the analyses is in general better than 1 per cent relative error, which is about the order of accuracy obtained in previous studies (6, '7) of this method. The behavior in analysis of the sulfur compounds studied divided them into two groups-aromatic and aliphaticprimarily on the basis of their cracking characteristics. The number of successive analyses possible with one tube-filling depended in the former group upon the capacity of cracking surface present, and in the latter group, upon the amount of catalyst used. Although the aromatics as a group were more d a c u l t to crack, they showed definite differences among themselves in this respect. Thus these compounds decreased
TABLEI. RESULTSOF ANALYSESBY MODIFIEDTER MEULEN METHOD Substance
Quality
Weight Bample of Sulfur' Calcd. Gram
U.S.P.
Trional
0.2103 0.1848 0.1024 0,1909 0.1147 0.1228 Ethyl sulfite Xahlbaum 0.1758 0.1184 CzHsO 0.1168 0.1219 0.1040 CoHsO 0.1130 Thiaoetic aoid Eastman 0.1030 highest 0.1135 CHBCOSH purity 0.1198 0.1091 p-Xylenesulfonic Eastman 0.1172 acid (dihydrate) highest 0.1143 puritya 0 , 1 0 2 0 0.1035 fiiOaH.2HzO 0.1189
Oxygen Relative Found Calod. Error
%
%
%
%
26.47
26.29 26.36 26.39 26.33 26.58 26.49 34.89 34.69 34.61 34.85 34.87 34.61 21.32 21.09 21.07 21.18 36.06 36.85 36.08 36.12 35.92
26.43
-0.5 -0.3 -0.2 -0.4 $0.6 $0.2 $0.3 -0.2 -0.6 $0.2 +0.3 -0.5 4-1.3 $0.2 $0. 1 $0.7 +0.1 -0.5 co.1 4-0.3 -0.3
23.21
>,,
42.14
14.43
34.77
21.04
36.03
v
CHs Di-p-tolyl sulfoxide
Eastman highest purity
0.2626 0.2682 0.2689 0.2222
13.93
Eastman highest purity
0.1549 0.1241 0.1260 0.1019 0,1032 0.1135
17.41
6.94 6.93 6.94 6.92
6.96
-0.3 -0.4 -0.3 -0.6
cHaa)so
C
H
B
O
Sulfobenzoic anhydride
V\CO' Diphenyl sulfone
Epstmsn
34.83 34.77 34.81 35.13 34.98 34.63 35.05
-0.4 $0.8
14.79 14.42 14.88 14.46 14.69 14.68
-1.8 +1.2 -1.6 $0. 1 0.0
14.68
$0.2 $0.1 +1.0
+0.6
+0.7
I
a
Recrystallized from water.
in ease of cracking in the following order: p-xylenesulfonic acid, di-p-tolyl sulfoxide, sulfobenzoic anhydride, di-phenyl sulfone. This is the order which would be predicted from the work of Egloff and co-workers ( I ) . This work emphasized the function of side chains in facilitating the cracking of aromatic compounds by furnishing gaseous olefins. I n this way the number of methyl groups would explain the relative positions of p-xylenesulfonic acid and di-p-tolyl sulfoxide, while absence of side chains would account for the end position of diphenyl sulfone. The intermediate position of sulfobenzoic anhydride may be attributed possibly to a direct formation of anthracene, which is known to be more readily decomposed than benzene. On the other hand the fact that the aliphatic compounds which contained such varied sulfur linkages as
C--8-H,
c)S=
