Thiophenes in Shale-Oil Naphtha

Mountains near. Wallgran, Germany, d. Shale from Seefeld (Tyrol), Aus- tria. e. Russian shale. Seventeen thiophenes were identified in Colorado shale-...
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Thiophenes in Shale-Oil Naphtha I. W. KINNEY, JR.1, J. R. SMITH, AND JOHN S. BALL Petroleum and Oil-Shale Experiment Station, Bureau of Mines, Laramie, Wyo. Almost 80%ofthe sulfur inshale-oil naphtha occursin thiophenic compounds. Isolation and identification of individual members of this group of compounds provide fundamental data for solvingprocessingproblems caused by them. A method was developed for the isolation and identification of thiophenes. Seventeen thiophenes were identified from a naphtha distilled from Colorado shale oil. Quantitative estimates were made of the contents of most of these. The 17 thiophenes represent about two thirds of the

T

HE presence of thiophenes in shale-oil naphtha was first

established by Scheibler ( I S ) in 1916. Since that time fourteen different thiophenes have been identified by various workers ( 3 , 4 , 8 , 1 1 , 1 2 , 1 5 , 1 6 ) . The principal effort to make quantitative estimates is the scheme of Pison (IO) for the determination of thiophene molecular-weight groups. The present paper describes the techniques used for the isolation and identification of 17 thiophenes from a shale-oil naphtha. A quantitative determination of the individual thiophenes through a molecular weight of 112 ( C , thiophenes) was made. Estimates were made of the quantities of some of the individual thiophenes of higher molecular weight.

thiophenic sulfur in the naphtha. Isolation of the thiophenes was accomplished by fractional distillation, adsorption, and derivative formation. Their identification was based on a combination of mass spectral, mercuric acetate derivative, methylation, and hydrogenation data. The quantitative identification of this large group of thiophenes from a single source permits establishing a pattern of their occurrence. From these data, mechanisms for the formation of thiophenes may be postulated.

The sulfur compounds are predominantly thiophenes with smaller amounts of thiols, sulfides, and disulfides. Thiols

Table I.

0.8077 43.7

Recoiery Residue Loss Nitrogen, wt. % Sulfur, wt. %

ISOLATION OF THIOPHENES

A method for the isolation of thiophenes in shale-oil naphtha was devised. This method, which is shown schematically, consists of the removal of polar compounds from the naphtha, fractional distillation, preparation of a n aromatic-thiophene concentrate by adsorption, and removal of the thiophenes from the concentrate by reaction with mercuric acetate. The thiophenes are regenerated by refluxing the mercuric acetate derivatives with dilute acid. S C H E M E FOR S E P A R A T I O N A N D I D E N T I F I C A T I O N OF T H I O P H E N E S IN S H A L E - O I L N A P H T H A SHALE-OIL NAPHTHL

I TAR 0ASES

THIOLS

LROHITICS

b THIOPHENES

TAR At105

d SULFIDES

SITURATEC 8 OLEFINS

" C I C Y I I C .orl&TT

I"E.I*C*T

I

I

bN ESSENTIILLY PURE THIOPHENE

AROMATICS

I

S I D E 'CHAIN IDENTIFICATION

SIDE 'CHIIN LOCATION

CONFIRMATION OF STRUCTURE

Shale-Oil Naphtha. The naphtha used in this investigation was obtained from operations a t the Bureau of Mines Oil-Shale Demonstration Plant, Rifle, Colo. The naphtha resulted from a topping distillation of a crude shale oil produced in an Y-T-U type ( 6 ) retort. Inspection data for this naphtha are shown in Table I. The naphtha consists chiefly of saturated, olefinic, and aromatic hydrocarbons with small amounts of sulfur, nitrogen, and oxygen compounds (see Table 11). 1

Present address, I. W. Kinney a n d Co., Champaign, Ill.

Inspection Data on N-T-U Naphtha

67 141 167 189 2 13 98.0 0.4 1.6 1.21 1.24

Table 11. Composition of Shale-Oil Naphtha Type of Compound Saturates Olefins

Volume % 29.1 40.9

were shown to be present by a reduction in sulfur content when the naphtha was treated with 10% sodium hydroxide. Sulfides were shown to be present by a similar reduction after powdered mercurous nitrate treatment. Peaks attributable to disulfides were found in the mass spectral patterns of several aromatic concentrates. Removal of Polar Compounds. The polar compounds of shaleoil naphtha (tar acids and tar bases) were removed by treatment with 10% sodium hydroxide followed by treatment with lOyo sulfuric acid. The dilute base removes most of the thiols, carboxylic acids, and phenols. The dilute acid removes most of the pyridines and a part of the pyrroles. The resulting neutral naphtha was washed thoroughly with water, then dried. Distillation. About 6 liters of neutral shale-oil naphtha were distilled in a fractionating column having 6 feet of Stedmanscreen packing. This column has an efficiency of 60 theoretical plates at total reflux a t a pressure of 760 mm. of mercury. The distillation was conducted a t 560-mm. pressure to a vapor temperature of 138" C. The pressure was reduced to 200 mm. and the distillation continued to a vapor temperature of 142' C. The pressure was then reduced to 100 mm. for the remainder of the distillation. These pressure reductions were made t o keep the pot temperature below 175" C., thus minimizing decomposition of the sample. Approximately 0 . 5 7 , cuts (30 ml.) were obtained a t a reflux ratio of about 75. These cuts were combined on the basis of 1749

ANALYTICAL CHEMISTRY

1750

boiling point and sulfur content in such a way as to separate thiophenes of different molecular weights and a t least partially the isomers within a molecular-weight group. The combined fractions are shown in Table 111. Adsorption. The composite fractions were treated with 0.05 S silver nitrate to remove any remaining thiols and with powdered mercurous nitrate ( I ) to remove sulfides. The sulfur remaining in the fractions was primarily in the form of thiophenes. A concentrate of aromatics and thiophenes was prepared from the fractions by adsorption. Florisil, activated for 4 hours a t 150" C., was used as the adsorbent, as this material gave sharp separations of the thiophenes from interfering substances such as olefins and nitrogen compounds. Elution techniques were used employing pentane, an adsorbent to sample ratio of 30, and a column having a height t o diameter ratio of a t least 50. The first eluate, containing saturates and olefins, was discarded. The succeeding eluate containing thiophenes and aromatics was stripped of solvent by distillation. The resulting aromatic concentrates normally contained over 95% of the thiophenes and were contaminated with only trace amounts of olefins.

eight peaks in the spectra of thiophene homologs. The four correlations applicable to this paper are summarized briefly as follows:

For most thiophene homologs the base or highest peak results from the breaking of a @-bond (C-C or C-H bond once removed from the ring). For polymethyl thiophenes having the 2- and 3-positions occupied, the base peak results from the breaking of an a-bond (C-C bond next to the ring). The parent peak (equivalent t o molecular weight of compound) is greater than 50% of the base peak when only methyl groups are on the ring and less than 50% of the base peak for all other types of substitution. The parent peak less mass 1 is greater than 50% of the base peak when only methyl groups are on the ring, is 2 to 50% of the base peak when methyl and ethyl groups or ethyl groups are on the ring, and is less than 2% of the base peak when a single alkyl c r o w other than methyl or ethvl is on the rine. " TLe 59+ peak is greater th"an 15% of t h z base peak when methyl groups are in the 2,5-positions, is 5 to 15% of the base peak when a methyl group is in the 2-position, and is 2 to 5 % of the base peak when the 2- and 5-positions are unoccupied or occupied by an) alkyl group other than methyl.

Table 111. Sulfur Distribution in Shale-Oil Naphtha

Another technique for limiting the possible structures of an unknown thiophene is to hydrogenate it to the alkane. The Raney nickel hydrogenation procedure of Blicke and Sheets ( 2 )is used. The resulting alkane may be identified by comparing its mass spectrum with reference standards. SIDECHAII~ LOCATION.The number of side chains and their relative positions on the ring are determined by mercuric acetate derivative formation, methylation, and hydrogenation when necessary. Mercuric acetate derivatives are prepared by Kinney and Cook'smodification (7)of Steinkopf's niethod(I4). Under the conditions specified mercuric acetate reacts with thiophenes a t 25' C. to replace the 2,Shydrogens present. As an example, P-methylthiophene forms a mono- derivative, whereas 3-methylthiophene forms a di- derivative. Derivatives are differentiated by either sulfur or mercury determinations. If there are no 2,5-hydrogens, mercuric acetate a t 25" C. replaces the 3,4hydrogens present. iit 60' C. mercuric acetate reacts to replace all of the ring hydroDerivative Formation. Thiophenes were separated from the gens. By derivative formation a t the two temperatures the aromatic concentrate by preparation of their mercuric acetate unknown thiophene may then be classified in one of the following derivatives. The aromatic concentrates were made to react with groups: 2-substituted; %substituted; 2,s- or 3,4-disubstituted; mercuric acetate in 50% acetic acid a t 60" C. Thiophenes having 2,3- or 2,4disubstituted; and 2,3,4- or 2,3,5-trisubstituted. ring hydrogens react with mercuric acetate, so that the hydrogens are re laced. The mercuric acetate derivatives were filMethylation provides a means for differentiating a 2,5- from tered from t i e reaction mixture, washed, and dried. Thiophenes a 3,4disubstituted thiophene and a 2,3,4- from a 2,3,5-trisubwere regenerated from these derivatives by refluxing with 10% stituted thiophene. hydrochloric acid. The recovered thiophenes were reacted with The methylated thiophene is obtained by reduction of the mercuric acetate as before. Thiophenes regenerated from these second derivatives usually contained less than 10% hydrocarbon aldehyde prepared by the procedure of King and Nord (6). If impurities, mainly aromatics. the 59 peak of the mass spectrum of the thiophene after methylation is more than 1.5 times the 59+ peak of the original thioQUALITATIVE IDENTIFICATION OF THIOPHENES phene, the methyl group entered the 2- or 5-position. This correlation assumes that the methyl group will enter the 2- or 5The isolation procedure sometimes yields an individual thioposition if it is open. phene, but usually gives a group of thiophene isomers, of which Neither of the above methods of locating side chains will one is predominant. If an individual thiophene is obtained, differentiate a 2,3- from a 2,4-disubstituted thiophene. Up to conclusive identification is based on a direct comparison between molecular weight of 154 this distinction can be made in most properties of the isolated compound and an authentic sample. cases by hydrogenating the unknown thiophene and identifying If a mixture of isomers is obtained, one or more of the individual the resulting alkane by mass spectra. components may be identified with reasonable certainty by a Isolated thiophenes consisting of a mixture of certain isomers suitable combination of data obtainable from mass spectra, mercan sometimes be separated by taking advantage of the difference curic acetate derivatives, methylation, and hydrogenation. In in solubility of mono- and dimercuric chloride derivatives (8, 9). this paper direct comparison of mass spectra has been used for The mixed mercuric acetate derivatives obtained from the isolathe identification of compounds for which reference spectra are tion procedure are refluxed 8 hours nith 10% sodium chloride available. For other compounds the particular combination of to form the corresponding mercuric chloride derivatives. These identification techniques used depended on the isomers under are dried and extracted 8 hours with hot ethyl alcohol. The exconsideration. traction removes the soluble mono- and leaves the insoluble Techniques Used for Identification. SIDECHAINIDENTIFICAdimercuric chloride derivatives. The thiophenes may be reTION. When a thiophene cannot be identified directly, its generated by refluxing with dilute hydrochloric acid. structure can be limited to a few possibilities using the mass specApplication of the Methods. THIOPHENE. The thiophene tral correlations developed by Kinney and Cook ('7). These isolated from the fraction boiling from 55' to 92" C. vias identicorrelations of mass spectra with molecular structure are for Sulfur Boilin: Weigh; Fraction Jtange, C. 70 0.20 1 55- 92 1.36 92-118 2 1.22 118-134 3 1.04 134-135 4 2.08 135-139 5 139-145 6 0.66 7 1% 145-1.62 0 44 ~~. 2 78 152-159 126 1 69 159-161 126 9 2 05 161-167 126 10 0 94 167-172 140 11 0 89 172-175 140 12 1 33 175-180 140 13 1 52 180-187 140 14 0 86 187-194 154 15 0 37 194-200 154 16 1 16 200-205 154 17 2 30 154a 205-2 10 18 a This fraction also can contain benzothiophene, which has a molecular weight of 134. Mol. wt. Group 84 98 112 112 112 112

s

~~~

~~~

+

1751

V O L U M E 24, NO. 11, N O V E M B E R 1 9 5 2 fied by mass spectrometer analysis. The close agreement of the spectrum of the isolated thiophene with that of pure thiophene i p ehon-n belo~v: Relative Ion Intensities Thiophene from shale-oil naphtha Thiophene 4 5 4.4 100 0 100 0 6 6 6.1 8 0 7 1 65.4 175 4

1 on

++ + 5" + 86 84

83 o8

+

C j THIOPHESES (92" to 118' (3,). The thiophenes isolat'ed from this fraction were analyzed by mass spectrometer. Methylthiophenes were shown to he present by comparison of mass spectra : _________Relative Ion Intensities ( 'b

tliii>pheiie>

Ivn

++ + 38 +

2-lIethyltlliophene

3-Methylthiophene

135139' C. thiophenes 89 4 100.0 92 6 4 6 14 9

Ion 112f

98 97

n9

lll+ 9i

The intensities of the Cj thiophene peaks fall between those of the corresponding peaks of 2-methyl- and 3-methylthiophene. The presence of both methylthiophenes is indicated with 2met,hylthiophene predominating. C6 THIOPHENES (118' to 134" (3.). The thiophenes isolated from this fraction were analyzed by mass spectra. Comparison of the spectra of the isolated thiophenes with spectra of thiophenes which would be expected in a fraction of this boiling range is given below. For ease of comparison all of the spectra are calculated with the base or reference peak a t the same ion (97+).

Ion 112+

These results show the presence of the three thiophenes. 2Lthylthiophene predominates over 3-ethylthiophene, u hich i q a distribution similar to that found for the methylthiophenes. Cg THIOPHENES (134' to 135' C.). The presence of 2,5dimethylthiophene in this fraction x a s indicated by a high 59+ peak in the mass spectrum of the thiophenes. Confirmation of the presence of 2,5-dimethylthiophene was made using derivatives. A purified mercuric acetate derivative prepared from the ii,icition decomposed a t 295" t o 300" C. compared t o 295' to 300" C. for the corresponding derivative of 2,5-dimethylthiophene. Cg THIOPHESES (135' to 139" C.) Comparison of the spectra of the isolated thiophenes with those of the three isomers considered probable from boiling point data is given below:

118-134' C. thiophenes 91 2 98 9

111+ 97 65

++

100 3 1 23

84 T 59

+

0 1 9 4

Relative Ion Intensities %Ethyl3-Ethylt h i o ~ h e n e . thiouhene.

6.p.

-132' C. 38 5 7 4

100 0 2 1

2 1 1 4

6.p.

-135'

38 7 100 2 4

2,5-DiinethyC thiouhene,

C. 9 5 0 6 0

1 4

b p.

-1136' 13,5 173 100 3

C

.5 9

0

8 1 4 37 4

The high intensities of the 112+, 111+, and 59+ peaks show that the isolated thiophenes contain a large amount of 2,5dimethylthiophene mixed with one or both of the ethylthiophenes. The spectra of the two ethylthiophenes differ significantly only with respect to the 84+ peak. The value for this peak in the spectrum of the isolated thiophenes indicates the presence of 2-ethylthiophene with the possibility that some 3-ethylthiophene is present. In order to establish the presence of Sethylthiophene, the mercuric chloride derivatives of the thiophenes were prepared. The soluble derivative formed by the Zethylthiophene was leached out, leaving the insoluble derivatives of the other two compounds. Thiophenes regenerated from the insoluble derivatives gave the following mass spectrum:

Ion 112+ 111+ 97 85 84 59

+ +++

Relative Ion Intensities of Thiophenes Regenerated from Insoluble Mercuric Chloride Derivatives 103 4 120 1 100 0 3 5 2 4 28 5

For ease of comparison I\ ith previous results the spectrum is given with the 9 i f peak as reference, although this peak is no longer the highest. The increase in relative concentration of the 2,5-dimethylthiophene, resulting from the removal of 2-ethylthiophene, is shown by the increase in the intensities of the 112+, I l l + , and 59+ peaks. With the three compounds under consideration, the increase in the 84+ peak can be accounted for only by the presence of 3-ethylthiophene.

85 59

++ +

Relative Inn Intensities 2,s2,4DimethylDimethylthiophene, thiophene, b.p. b.p. -136' C. -137'C. 72 8 77 9 100 0 100 0 57 5 48 5 2 2 2 0 21 5 7 0

2.3Diniethylthiophene, b.p. -140' C. 100.2 100.0 120 2 5 5 16 6

The relativelj high intrnsitie- of the 85+, 97+, and 112+ p e a k show that 2,3-(iimeth\Ithiophene is the predominant compound. C, THIOPHENES (139' to 145' C.). The spectrum of the thiophene concentrate from this fraction is compared to the spectra of the two highest-boiling Cg thiophenes: Relative I o n Intensities ~2,33 4DimethylDim'sthyl139thiophene, thiophene, 145" C . b.p. b.p. thiophenes - 140' C. 146' C. 69.9 80 2 100.2 100.0 100.0 100.0 6j.6 120.2 48 8 8 3 16.6 3 2 7.7 13 1 2 9 ~

-

1i,n 112+ Ill+ 97 59 58+

++

From the above comparison, it is seen that the isolated thiophenes have values intermediate betu een 2,3- and 3,4-dimethylthiophene. Both of these thiophenes appear to be present. Additional evidence of the presence of 3,4-dimethylthiophene v a s obtained by preparing the dimercuric chloride derivative as previously described in order to separate the two compounds, The dimercuric acetate derivative was prepared from the thiophene regenerated from the insoluble dimercuric chloride derivative. The acetate derivative melted and decomposed at 261' to 264" C. as compared to 262" to 264" C. for the dimercuric acetate derivative of 3,4dirnethylthiophene. C, THIOPHESES (145" to 152' C.). -1thiophene i~olatedfrom this fraction was identified as P-isopiop>Ithiophene. Thr close agreement of the mass spectrum of the isolated material it i t h that of 2-iqopropylthiophene is shon.11 brlon . Relative Ion Intensities 2-Isopropyl3-Isopropylthiophene, thiophene, 1529 c. b.p. b.p. thiophenes, -133" C. - 157' C. 27.9 27 7 34.4 1.7 1 9 1 7 100.0 100.0 100.0 6.4 6.0 6.6 2 .5 2.2 2.1 145-

Ion 126f 125+ lll+

86 59

++

C7 THIOPHESES (152" to 159" C.). The thiophene isolated from this fraction was identified using mass spectral correlations, mercuric acetate derivatives, and hydrogenation. Direct comparison of the mass spectrum shoivn below n i t h those given previously for isopropylthiophenes indicates that the bulk of the thiophenes are not isopropylthiophenes. Ion

+

126 125+

lll+

97+ 59

+

Relative Ion Intensitiea of 152-159' C. Thiophenes 39.0 5.4

100.0 10.0 8.7

1752

ANALYTICAL CHEMISTRY

The principal unknown thiophene is not one of the propylthiophenes, as the latter have base peaks a t 97+ instead of 111+ (base peak correlation). It is not a trimethylthiophene because the 126+ peak is less than 50% of the base peak (parent peak correlation), and the 125+ peak is less than 50% of the base peak (parent peak less mass 1 correlation). The predominant compound must be a methylethylthiophene. Since the 59+ peak is 8.7% or between 5 and 15% of the base peak, a methyl group should be in a 2,5-position (59 peak correlation). The location of the ethyl group was determined by the use of mercuric acetate derivatives. A dimercuric acetate derivative formed a t both 25' and 60' C., indicating either a 2,5- or a 3,4disubstituted thiophene. With the methyl group already located in the 2-position by mass spectral correlations, the compound is identified as 2-methyl-5-ethylthiophene. The principal alkane resulting from hydrogenation was identified by mass spectrum as n-heptane. This supports the identification of 2-methyl-5-ethylthiophene, as this compound gives n-heptane on hydrogenation. C I THIOPHENES (159' to 161' C.). The thiophenes isolated from this fraction were a complex mixture from which no compounds were positively identified in addition to those discussed under other fractions. C? THIOPHEXES (161' to 167" C.). The predominant thiophene in the 161" to 167" C. fraction was identified as 2,3,5trimethylthiophene. The identification was made using massspectral and mercuric acetate derivative data. Confirmation was obtained by a direct comparison of the mass spectra of isolated and synthesized 2,3,5-trimethylthiophene. Details of this identification are given in the mass spectral correlations paper of Kinney and Cook ( 7 ) . C?ASD C8 THIOPHENES (167' to 172" C.). This fraction was a transition fraction containing both Cr and Cg thiophenes. Mass spectral data indicated that the predominant compounds were probably 2,3,5-trimethylthiopheneand 2-methyl-5-isopropylthiophene. Cs THIOPHENES (172' to 175' C.), The predominant thiophene in the 172' to 175' C. fraction was identified as 2-methyl5-isopropylthiophene. Mass spectral correlations, mercuric acetate derivative data, and hydrogenation were used to identify this compound. Confirmation of the identification was made by comparison of the mass spectra of the isolated and synthesized 2-methyl-5-isopropylthiophene. Details of the identification are given in the paper of Kinney and Cook ( 7 ) . CSTHIOPHENES (175' to 180" C.). Mass spectiometer analysis of the isolated thiophenes indicated a mixture of CS thiophenes with a preponderance of one isomer. The base peak was a t 111+, showing the loss of an ethyl group (140 less 29). Of the Cs thiophenes only sec-butylthiophenes and methyl-npropylthiophenes lose an ethyl group to form the base peak (base peak correlation). The 59+ peak of the spectrum was 10.07, of the base peak showing the presence of a methyl group in the 2-position (59+ peak correlation). Hydrogenation of the thiophenes yielded a mivture of octanes, the preponderant one being n-octane. Neither of the sec-butylthiophenes can form n-octane on hydrogenation. The only methyl-n-propylthiophene which yields n-octane on hydrogenation is 2-methyl-5-npropylthiophene. Mass spectral data had already shown the presence of a methyl group in a 2-position. The predominant compound was identified as 2-methyl-5-n-propylthiophene. Cs THIOPHENES (180' to 187" C.). Mass spectral correlations applied to the thiophenes isolated from this fraction indicated that a methyl group was being lost from a CSthiophene to form the base peak a t 125+ (140 less 15). Mercuric acetate derivatives showed trisubstitution, since only mono- derivatives formed a t 25" and 60' C. The only possible trisubstituted CS thiophenes are dimethylethylthiophenes. Hydrogenation of the isolated thiophenes yielded primarily a mixture of 3-methyland 4methylheptane. The only trisubstituted thiophenes

+

forming these two octanes on hydrogenation are 2,3-dimethyl5-ethylthiophene and 2,4-dimethyI-5-ethylthiophene. The isolated thiophenes were thus identified as a mixture of these two compounds. Cg THIOPHEXES (187" to 194' C.). The thiophene concentrate isolated from the 187" to 194" C. fraction was found to be a mixture of CSthiophenes, according to mass spectral data. Application of the correlations to the mass spectral data gave the follo~inginformation: A methyl group is in the 2-position, as the 59+ peak is betiveen 5 and 15% of the base peak (59+ peak correlation). As the presence of a methyl group has been shown, the compound must contain a n-propyl or sec-butyl group to form the base peak a t 125f (154 less 29). The identification was completed by preparing mercuric acetate derivatives a t 25" and 60" C. Dimercuric acetate derivatives formed a t both temperatures, showing either 2,s- or 3,4substitution. As a methyl group has already been located in the 2-position, the thiophene is identified as 2-methyl-5-sec-butylthiophene. The 2,5-disubstitution eliminated the possibility of a n-propyl group being present. HIGHERBOILINGTHIOPHENES (194' to 210' C.). On the basis of sulfur content, three composite fractions were originally prepared from the material in this boiling range. However, the only individual compound isolated was 2,3-benzothiophene which was predominantly in the fraction boiling a t 205" to 210' C. The identification was made using both ultraviolet and mass spectral data. .ittempts to find tetrasubstituted thiophenes were unsuccessful. Ultraviolet absorption peaks of standard 2,3-benzothiophene are compared with corresponding peaks of the isolated compound: Ultraviolet Absorption Peaks, A. Isolated 2,3-Benzothiophene 2972 2905 2880 2862

Standard 2,3-Benaothiophene 2971 2903 2883 2864

Selected peaks in the mass spectra of the standard and isolated compounds are compared below: Ion 135-k 134f 108t 90 f 67f

Relative Intensities Isolated Standard 2,3-benaothiophene 2,3-benzothiophene 8.0 9.1 100.0 lo?.? J.J 3.7 8.1 8.9 9.2 9.9

The slightly higher values for the isolated material are due to small amounts of impurities. QUANTITATIVE ESTIMATION OF THIOPHENES

I n addition to qualitative identification, some estimate of abundance is necessary in order to evaluate the importance of any given compound or to establish substitution patterns. .is mass spectra are available for all the thiophenes through the Ce thiophenes, the isolated materials in this range can be analyzed quantitatively if the mixture is not too complex and if the patterns of the isomers present are significantly different. The results of such analyses on aromatic concentrates from the first six composite fractions are given in Table IV. Only a few of the mass spectra are available for thiophenes of molecular weight above the Cg thiophenes, so complete quantitative analyses cannot be made. However, the composite fractions from the distillation were prepared on the basis of peaks in the sulfur concentration and it is found that in most instances these peaks contain a single isomer in predominant quantities. Hence, some estimate as to relative abundance can be made based on sulfur content and distillation data. An estimate of the quantity of the most abundant compound in some of the fractions is sholvn in Table T'. Of the three compounds for which

1753

V O L U M E 24, NO. 11, N O V E M B E R 1 9 5 2 Table IV.

Thiophene Analysis through Cg Thiophenes

Compound Thiophene 2-Methylthiophene 3-Methylthiophene 2-Ethylthiophene 3-Ethylthiophene 2,s-Dimethylthiophene 2,4-Dimethylthiophene 2,3-Dimethylthiophene 3,4-Dimethylthiophene

Table V.

WJ.7cof

1101. Wt. Group

Leutral Saphtha 0.011 0.135 0.018

84

98 98 112 112 112 112 112 112

0.050

0.013 0.114

o:ioo 0,019

% of Total Sulfur in Neutral Saphtha 0.35 3.70 0.49 1.21 0.30 2.73 2:41 0.46

Approximate Thiophene Analysis for C7 through CSThiophenes

Compound 2-Methyl-5-ethylthiophene 2 , 33-Trimethylthiophene 2-Isopropylthiophene 2-Methyl-5-n-propylthiophene 2-Methyl-%isopropylthiophene 2,4-Diniethyl-5-ethylthiophene 2,3-Dimethyl-5-ethylthiophene 2-Methyl-5-sec-hutylthiophene 2,3-Benzothiophene

Wt. % of Yeutral Naphtha 0.5 0.3 0:3

0.4

..

0:;s 0.3

%of Total Sulfur in Neutral Saphtha 10

..66 7

.. ..

4

quantitative data are not shown, the 2-isopropylthiophene is present in very small quantity, whereas the other two are present in substantial quantities.

presupposes the existence of a thiophene nucleus in the large molecules. No data are available concerning the presence of the thiophene nucleus in kerogen or in other naturally occurring substances. A third possibility is that the products of retorting may have been subjected to sufficiently high temperatures for adequate periods of time during retorting of the shale for thermodynamic equilibrium to govern the type and abundance of the isomers of thiophenes. Thermodynamic data are not available to compare the relative abundance of the isomers with that calculated for thermodynamic equilibrium. SU>ViVlARY

Techniques are described for the isolation and identification of thiophenes in shale-oil naphtha. The basic method of isolation of the thiophenes is the removal of polar compounds from the naphtha, preparation of an aromatic concentrate, and removal of the thiophenes from the aromatic concentrate by reaction with mercuric acetate. Regeneration of the thiophenes is accomplished by refluxing the mercuric acetate derivatives with dilute acid. Identification methods include mass spectrometer analysis, mass spectral correlations, mercuric acetate derivative formation, methylation, and hydrogenation.

Table VI.

Thiophenes Identified from Shale Oil Sourcea

DISCUSSION

.Ipproximately 80% of the total sulfur in a 210” C. end-point Colorado shale-oil naphtha appears to be thiophenic. About two thirds of the total thiophenic sulfur is represented by the 17 thiophenes reported in this paper. These thiophenes together n-ith the thiophenes previously identified in shale oil or related product, are given in Table TI. -411 but four of the thiophenes previously identified have been found in Colorado shale-oil naphtha: 2-n-propylthiophene, 3-npropylthiophene, 3-isopropylthiophene, and 2-n-butylthiophene. However, some evidence has been found for the presence of all hut one of these four (3-isopropylthiophene), but positive identifir itions have not been made. Positive identifications have been niitle for seven thiophenes not previously reported. The 17 thiophenes identified represent only 14% of the 122 thiophenes t h it could theoretically be present in a naphtha of this boiling imge. The compounds identified indicate a definite substitution pattern: 2-monosubstituted predominate over 3-monosubstituted thiophenes; 2,5-disubstituted predominate over the other disubstituted thiophenes; and 2,3,5-trisubstituted predominate over 2,3,4trisubstituted thiophenes. I n a homologous series such as 2-monoalkylthiophenes, the quantity of the compound present decreases as the length of the alkyl substituent increases. 2Nethylthiophene, 2,5-dimethylthiophene, 2-methyl-5-ethylthiophene, and 2-methyl-5-isopropylthiophene are the predominant isomers in their respective molecular n-eight groups. The relative abundance of these compounds indicates a tendency for the predominant isomer in a molecular weight group to have a methyl plus another group of increasing size rather than1a single group or two groups of equal size-for example, 2-methyl-5isopropylthiophene rather than diethylthiophene or butylthiophene. Hydrogenation of 15 of the 17 compounds isolated results in normal alkanes or monomethyl alkanes, hydrocarbons which are abundant in shale oil. This suggests that thiophenes in shale oil may have been formed by a reversal of this mechanism -that is, dehydrogenation of the alkanes followed by cyclization in the presence of sulfur. Another possibility for the origin of the thiophenes is the simple degradation of larger molecules present in the kerogen. This

Literature Reference

This Paper

Thiophene a , b, e ( 3 . 4, 11) X 2-Methylthiophene a , b, c, e (3, 4, 11, 15, 16) x 3-M ethylthiophene C (16) X 2,3-Dimethylthiophene a , h, c , e (3, 4, 11, 15) X 2,5-Dimethylthiophene (16) X 3,4-Dimethylthiophene c (15) X 2-Ethylthiophene x ( 3 , 11, 16, i6) a, c , e 3-Ethylthiophene c (15) X 2-n-Propylthiophene (8) 3-n-Propylthiophene d e (11, 1 8 ) 2-Isopropylthiophene d: e ( 8 . 11, 18) X 3-Isopropylthiophene (8) 2-1\f ethyl-5-ethylthiophene x 2,3,5-Trimethylthiophene X 2-n -Butylthiophene e (6, 18) 2-Methyl-5-isopropylthiophene X 2-Methyl-5-n-propylthiophene X 2,3-Dimethyl-5-ethylthiophene X 2,4-Dimethyl-5-ethylthiophene Y 2-I\fethyl-5-sec-butylthiophene X 2,3-Bensothiophene a, c ( 3 , 16) Y a Source. a. Kimmeridge shale, Dorset, England. h. Shale from Kashpirshi mines near Sysran, Russia. c . Shale from Bavarian Karwendel Mountains near Wallgran, Germany. d. Shale from Seefeld (Tyrol), Austria. e. Russian shale.

Seventeen thiophenes were identified in Colorado shale-oil naphthas: thiophene, Zmethylthiophene, 3-methyIthiophene, 2-ethylthiophene, 3-ethylthiophene, 2,3-dimethylthiophene, 2,5dimethylthiophene, 3,4-dimethylthiophene, 2-isopropylthiophene, 2-methyl-5-ethylthiophene, 2,3,5-trimethylthiophene, 2-methyl5-isopropylthiophene, 2-methyl-5-n-propylthiophene, 2,3-di2,4-dimethylðyl-thiophene, 2methyl-5-ethylthiophene, methyl-5-sec-butylthiophene, and 2,3-benzothiophene. Thiophenes through the CSthiophenes were analyzed quantitatively, using mass spectral data. Estimates were made of the abundance of thiophenes of higher molecular weight. ACKROWLEDG-VENT

This project was part of the Synthetic Fuels Program of the Bureau of Mines and was performed a t the Petroleum and OilShale Experiment Station under the general direction of H. P. Rue and H. M. Thorne. Thanks are due Shirley Nix, G. L. Cook, C. W. Bailey, and G. U. Dinneen for some of the analytical r o r k and review of the manuscript. Several of the thiophenes were made available through American Petroleum Institute Project 48 on “Synthesis, Properties and Identification of Sulfur Compounds in Petroleum.” The work was done under a cooperative

ANALYTICAL CHEMISTRY

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agreement between the University of Wyoming and the Department of Interior, Bureau of Mines. LITERATURE CITED

(1) Ball. J. S.. U. S. Bur. Mines, Rept. Inuest. 3591 (1941). (2) Blioke. F. F., and Sheets. D. G., J . Am. Chem. Soc., 71, 4010 (1949). (3) Challenger, F.,Haslam, J., and Brambdl, R. J., J . Insl. Pel~oleumTechnol., 12, 106 (1926). (4) Dodonov, J.. and Soshestrenska. E..B e . , 59B. 2202 (1926). (5) Gavin. M. J., and Desmond. J. S.. U. S. Bur. Mines, Bull. 315 (1930).

(6) King, W. .I. and , Nod. P. F., J . Ow. Chem.. 13, 635 (1948). (7) Kinney. I. W..and Cook, G. L., ANAL.CHEM..24, 1391 (1952).

(8) Lecl6rre and LeclBre, Compt. rend.. 194,286 (1932). (9) MeKittrick, D.5.. Ind. Eng. C h m . . 21. 585 (1929). (10) Pison, M., Bu.11. 8 0 ~chim. . F~ance.58,296 (1949). (11) Rakovskii, E. V.. "Bituminous Shale and Its Technical Utilization, p. 121.Leningrad. Lenkhimsektor, 1932. (12) Scheibler, H., and Rettig, F., Ber.. 59B, 1198 (1926). (13) Scheibler, Z.B., Ihid., 48. 1815 (1915). (14)Steinkopf, W.,and Killingstad, A,, Ann.. 532,288 (1937). (15) Steinkopf,W., and Nitschke, IT., A & Pharm..278,360 (1940). (16) Tehoubar, B.,I d . parfum. 2. 193 (1947). R E C F ~ Vfor E ~review June 16, 1923. Aeocpted Seoternber 4, 1952.

ADDlication of ter Meulen Nitrogen Method to Petroleum Fractions HOLOWCHAK, G. E. C. WEAK, AND E. L. BALDESCHWIEL tories, Research Diuision, Standard Oil Development Co., Linden

The petroleum industry has long been concerned with the accurate determination of small amounts of nitrogen i n petroleum feed stocks and products. The conventional Dumas method is unsatisfactory because of the low levels of nitrogen content encountered, while substituted pyridines, quinolines, pyrroles, and other ring-type nitrogen compounds whioh are present i n petroleum and ita produots are particularly refractory to Kjeldahl digestion. The fer Meulen method has heen sueeessfully applied to these types of materials with a n accuracy of *lvo of the amount of nitrogen present. The procedure consists of thermal decomposition of the sample in an atmosphere of hydrogen and conversion of nitrogen to ammonia by passage of the pyrolysis products over a niol