Combination of Gas Chromatography with Fluorescence and Phosphorescence in Analysis of Petroleum Fractions H. V. DRUSHEL and A. L. SOMMERS Esso Research laboratories, Humble Oil & Refining Co., Baton Rouge, l a .
b
Luminescence characteristics of numerous aromatic sulfur and nitrogen compounds were determined. Recorded spectra were corrected for the spectral response of the phototube by a computer program previously reported. Examples of the use of luminescence spectra to characterize selected nitrogen-rich petroleum fractions are presented. Phosphorescence, including decay characteristics, provided more information than fluorescence in the study of the nitrogen compounds isolated from petroleum. The sensitivity of these techniques made them particularly suitable for the study of fractions trapped from gas chromatographs.
A
techniques of obtaining luminescence spectra have been used by physical chemists for many years, they have found little use in analytical chemistry until recently. One reason for this slow rate of application has been the lack of commercially available instrumentation. However, instruments are now available and the photoelectric recording of fluorescence, phosphorescence, and excitation spectra is within the scope of most well-equipped analytical laboratories. Fluorometry has been used by analytical chemists for some time but only recently has spectrofluorometry with choice of excitation wavelengths been used for analysis and characterization of organic materials. Parker and Rees (15) have used the analogy that fluorometry is related to spectrofluorometry as filter photometry is related to absorption spectrophotometry. Spectrofluorometry offers a t least two distinct advantages over absorption spectrophotometry, uiz., greater sensitivity and several spectra (excitation and emission) as criteria for identification. However, not all substances which absorb radiation exhibit fluorescence. Spectrophosphorimetry, which is now beginning to be applied in analytical chemistry, offers even more information than spectrofluorometry for the characterization and identificaLTHOUGH
10
ANALYTICAL CHEMISTRY
70827
tion of organic materials. I n addition to the advantages cited for fluorescence, the lifetime (7) for the time-delayed emission (phosphorescence) of substances becomes a criterion for identification and purity. Several different spectrometric techniques have been used for the characterization of sulfur and nitrogen compounds isolated or concentrated from petroleum in projects associated with the American Petroleum Institute (2) or in petroleum company laboratories (9). These techniques have usually been limited to three of the established spectrometric disciplines (mass, infrared, and ultraviolet), Because of the greater sensitivity and specificity of the luminescence techniques over absorption spectrophotometry, they should find considerable use in characterization of small amounts of material following complex micro separation techniques such as chromatography in its various ramifications. I n this paper, the luminescence properties of a variety of sulfur and nitrogen compounds are presented. In addition, the application of fluorescence and phosphorescence to the characterization of several nitrogen compound concentrates from petroleum middle distillates is demonstrated. Combination of luminescence techniques with gas chromatography is stressed. EXPERIMENTAL
Instruments. Fluorescence, phosphorescence, and excitation spectra were obtained with an Aminco-Kiers spectrophosphorimeter with fluorescence attachment. Spectra were recorded with a Moseley Model 1 Autograph X-Y Recorder. Decay curves for phosphorescence were obtained by photographing the oscilloscopic trace of emission intensity us. time presented on a Hewlett-Packard oscilloscope with built-in time base sweep. Ultraviolet absorption spectra were determined with a Cary Model 14M spectrophotometer. Gas chromatographic separations were performed with a programmed-temperature Model 500 F&M chromatograph using a 17foot SE-30 column. Solvents. Spectro-grsde isooctane and carbon tetrachloride were used
in obtaining ultraviolet, fluorescence, and infrared spectra. For phosphorescence spectra, a 50:50 (by volume) mixture of methylcyclopentane and methylcyclohexane (hereafter referred to as PH) was used which formed a clear transparent glass a t liquid nitrogen temperatures. Both of these hydrocarbons were purified from Philips pure grade materials by percolation over silica gel. Compounds not soluble in PH were studied in the common solvent mixture EPA (ethyl ether, isopentane, ethyl alcohol; 5:5:2 by volume) which was prepared from reagent grade ether and alcohol and Philips pure grade isopentane. Reference Compounds. Sulfur and nitrogen compounds were obtained from various sources. Most of the compounds were purchased from commercial supply houses. Some were made available by the American Petroleum Institute through the small-sample program of Research Projects 48 and 52. The source of each compound studied is shown in Table I. Purities were not known. However, in most cases, identity was confirmed and purities were estimated to be 98-99% by nuclear magnetic resonance spectrometry using a Varian -4-60 NMR spectrometer. Because the quantum efficiencies for most of the compounds were high, contributions from luminescence by impurities were assumed to be relatively minor. Calibration. Spectral response characteristics of the emission monochromator with a lP28 phototube were determined with a tungsten lamp and a Hanovia quartz mercuryvapor arc lamp (lamp code SH). Spectral energy distribution data for the emission from the tungsten lamp was calculated from Wien’s law. Relative spectral line intensities for the mercury lamp were supplied by the manufacturer. Values for the phototube response were compared with the known relative intensities from these lamps and a curve was constructed for use in correcting recorded spectra. An IBM 1620 computer was used to correct recorded spectra (intensity from photomultiplier photometer us. wavelength) to give relative intensity in units of energy us. frequency. Since an X-Y plotter was not available, the spectral information was converted to punched cards and a standard IBM printer was used to display graphically
the resultant spectra. Details of the correction and presentation of spectra using the computer approach have been presented ( 5 ) . A mercury arc lamp was used to calibrate the wavelength axis of the recorded spectra. With this calibration, reported peak wavelengths are probably reliable to =k2 mp, the estimated precision of measurement.
Table II. Excitation and Emission Wavelengths and Corrected Fluorescence Intensities (Relative to Carbazole) for Some Aromatic Compounds Containing Sulfur, Nitrogen, or Oxygen Atoms
Solvent: Spectro-Grade Iso-octane ExcitaPredomtion Fluorescence inant waveWaveCorrected vibrational lengths, length, rel. spacing, Compound m/r mP intensity cm.-l Diphenylamine 267 333 0,353 ... e-Naphthoquinoline 236 346 0.472 Table 1. Source of Sulfur and Nitrogen 256 364 0.386 1400 Compounds 320 384 0,0954 1450 335 406 0.0178 1400 API Research Projects 48 & 52, Small 5,7-Dimethylbenzacridine 253 389 0.182 Sample Program 287 410 0,135 1300 1-Thiaindane 338 434 0.0346 1350 2-Thiaindane 355 460 0.0088 1300 Acridane 375 Carbazole [2-Benzo(b)thienyl] henylthiamethane Diphenyl ether 225 296 0.0501 ... 1,2,3,4TetrahydrotRianaphtheno2,3-Benzodiphenylene oxide 248 354 2.26 (2,3-b)thianaphthene 308 372 1.50 2,3-3',2 '-Dithianaphthene 1350 3-hIethylbenzo(b )thiophene 392 0.553 1370 Benzo ( 1,2-b,5,Pb)dithiophene 414 0.129 1380 Dithieno (2,3-b,5,4-b) thiophene 352 3,PBenzoquinoline 265 0.025 L. Light & Co., Ltd. 363sh. 0.0186 1,2-Dimethylindole 343 313 0,00226 3-Methylisoquinoline Dimethylthiant hrene 232 350sh. 1.25 1-Methyl-2-phenylindole 5,7-Dimethylbenzacridine 291 364 1.48 ... 2,2'-Dinaphthyl ether 1-Azapyrene 380 1.08 ... o-Dianisidine 231 2,2'-Dinaphthyl ether 335 0.198 2-hIethylacridine 257 349 0.158 1200 Diphenyl ether Diphenylamine 218 302 Diphenylene oxide 0.709 a-Naphthoquinoline 237 312 0.955 ... 1,2-Benzocarbazole 275 323sh. 0.480 ... 1-Xethyl-2-phenylindole 3,PBenzoquinoline 275 355 0.00465 Diphenylene oxide 320 369 0.00374 1080 2,3-Benzodiphenylene oxide 335 385sh. 0.00144 1120 2-Methylindole ' Phenyl sulfide 263 310 0,00451" ... Eastman 263 2-Methylindole 0,605 302 ... 2-Amino-Pmethylpyridine 2-Amino-4methylpyridine 230 324 0,0907 Diphenyl sulfide 278 Dibenzothiophene Di-p-tolyl sulfone o-Dianisidine 298 363 1.16 ... h-aphthalene Quinaldine 317 367 0.00184 ... Triphenylmethane Thianthrene Dimethylthianthrene 252 435 0.00180" ... Indole 1,2-Dimethylindole 275 310 0.948 Methyl p-tolyl sulfide Ethyl p-tolyl sulfide Tetrahydrocarbazole 271 318 0.465 ... Di-p-tolyl disulfide Dibenzothiophene 5-dioxide 300 360 0.0180 ... Diphenyl disulfide 330 Dibenzyl disulfide 8-Methylquinoline Thianthrene 330 430 Not detnd. ... Isoquinoline 1,2,3,4-TetrahydrothiaQuinoline naphtheno-( 2,3-b) 2,6-Dimethylquinoline thianaphthene ... Weak fluor. 2,2 '-Bip yridine Di-p-tolyl sulfone [2-Benzo (b) thienyll2,4,6-Trimethylpyridine phenylthiamethane ... Weak fluor. ... ... 1-Thiaindane Rutgerswerke-Aktiengesellschaft ... Weak fluor. ... ... 3-Methylbenzo (ti)thiophene Tetrahydrocarbazole 268 307 0.0505 ... Quinaldine Acridane 280 338 1.86 ... 7,8-Benzoquinoline 2,3-3',2'-Dithianaphthene 3-hIethylisoquinoline 255 339 0.255 ... 2-Methylpyridine 300 350 0.248 ... 4bIethylpyridine 320 Synthesized by N. L. Cull, Esso Research Benzo (1,2-b,5,4-b) dithioLabs. phene 250 332 0.143 ... 287 Dibenzothiophene dioxide I Carbazole 235 331 1.00 ... Matheson Coleman & Bell 282 346 0.862 *.. Phenothioxin 10-dioxide Acetophenone a Emission increases with excitation time indicating secondary reaction. ~
~
I
VOL. 38, NO. 1, JANUARY 1966
0
11
Figure 1 , Typical recorded excitation, fluorescence, and phosphorescence spectra Compound: 1, 2-dimethylindole
Fluorescence
Phosphorescence
1 .O pg./ml. Isooctane Curve 4 Curve 2
Concn. Solvent Excitation Emission
0.58 mg./ml. EPA Curve 3 Curve 1
carbazole. These relative intensity data are not very precise because of the long period of time over which data were accumulated and the wide variety of slit combinations used. The main purpose in accumulating the luminescence data for reference compounds was to provide a basis for the qualitative identification of compound types in trapped gas chromatographic fractions. For this reason the vibrational fine structure and the corrected relative intensities within a given spectrum are important. The ~~
~~~
~
~~
~
~
RESULTS AND DISCUSSION
Luminescence Characteristics of Reference Compounds. Typical ex-
citation, fluorescence, and phosphorescence spectra for one of the compounds as recorded are reproduced in Figure 1. Data on relative intensities, which are affected by the choice of slit combination and other instrumental parameters, will be presented below. Data from the recorded spectra were corrected and converted to relative intensity us. frequency (micron-') by means of the developed computer program. The luminescence properties of many of the compounds studied have not been reported previously in the literature. Several oxygen-containing analogs were included to determine the effects of other heteroatoms. Corrected relative fluorescence intensities are listed in Table 11. Intensities were converted to values equivalent to a single arbitrary set of experimental conditions (photomultiplier setting, sensitivity, slit combination, and concentration). Solutions were degassed and blanketed with high-purity dry nitrogen to eliminate the possibility of oxygen quenching. Most compounds were studied a t concentrations of 1 to 10 pg./ml. to reduce effects of concentration quenching or other phenomena which normally give rise to nonlinearity with concentration. All relative peak heights were corrected for spectral response of the phototube by use of the technique described earlier (5)* Although different wavelengths were used for excitation, no correction was made for differences in excitation energy a t different wavelengths. Rather than use arbitrary units, intensities were calculated relative to the intensity of
12
ANALYTICAL CHEMISTRY
Table 111. Excitation and Emission Wavelengths and Phosphorescence Intensities (Relative to Carbazole) for Some Aromatic ComPounds Containina Sulfur, Nitrogen, or Oxygen Atoms
-
Compound Tetrahydrocarbazole
Solvent EPA
1,2-Dimethylindole
EPA
Dimethylthianthrene
EPA
Quinaldine
EPA
Excitation wavePhosphorescence lengths. Wavelength. Corrected mp mp rel. intensity 292 411 0.36 224 439 0.53 0.44 460sh. 0.28 500b.sh. 414 288 0.31 441 0.44 454sh. 0.34 464sh. 0.33 285 422? ... 482 0.14 46 1 300 0.086 471sh. 0.059 0.13 493 0.11 529 0,042 568 0.0080 332 590 Y
5,7-Dimethylbenzacridine
EPA
2,2'-Dinaphthyl ether
EPA
279 315
1-Asapyrene
EPA
368
,
370
485
515 -
542sh. 524
561 -
o-Dianisidine
EPA
305
2-Amino4methylpyridine 2-Methylacridine
EPA EPA
302 292
Diphenyl sulfide Diphenyl ether
EPA EPA
Diphenylamine
EPA
285 234 264 305
597 498 524 439 437 451sh. 464 416 404 40 1 417 427
0.16 0.27 0.20 0.00053 0,00055 0.00041 0.053 0.078 1.02 0.0012 0,0013 0.0014
Predominant vibrational spacing, cm.-l
... 1580
... ... ... 1450 ... ...
... ... ...
1400 1400 1500
...
... ...
1210
iib 1090
... ... ... ... ...
1300
1.4
...
0.44
...
3.6 2.8 2.5
... ... ... (Continued)
Table 111. Excitation and Emission Wavelengths and Phosphorescence Intensities (Relative to Carbazole) for Some Aromatic Compounds Containing Sulfur, Nitrogen, or Oxygen Atoms (Confinued)
-
Compound Dithieiio (2,3-b,5,4b)thiophene B ~ ~.( lJ2-b,5,4-b)z o dithiophene
Solvent PH PH
Excitation wavePhosphorescence lengths, Wavelength. Corrected rel. intensity mw mp 272 287
3-Methylbenzo ( b ) thiophene
PH
280
1-Thiaindane 2-Thiaindane [2-Benzo ( b ) thienyllphenylthiamethane
PH PH PH
293 280 300
1,2,3,PTetrahydrothianaphtheno-(2,3-b)thianaphthene Acridane
PH
297
EPA
308
465 459 466 476 494 ash. 511sh. 527sh. 440sh. 450sh. 466
497
PH
295
318
Carbazole
a-Naphthoquinoline
7,8-Benzoquinoline
1,2-Benzocarbazole
2-Methylindole
EPA
EPA
EPA
EPA
EPA
1-Methyl-2-phenylindole
EPA
314-Benzoquinoline
EPA
Diphenylene oxide
EPA
285 325 315
332 -
282 317 329 317
338 285
315 315
330 -
284
440 435 460sh. 485 515sh. 474 414 __ 425 437 479 492 501 518 531 54lsh. 559 574 410
426 439 456 465sh. 466 475 501 538 575sh. 468 477 504 514sh. 540 580sh. 483 497 519 553sh. 408 420sh. 434 446sh. 458 500 531 568sh. 455 460sh. 488 523 560sh. 414 435sh. 444
461 469 479
0.0031
Predominant vibrational spacing, cm.-l
...
1.1
1.1 1.0 1.1 1.0 0.97 0.88 0.145 0.16 0.28 0.27 0.069
300 430 1530
0.0001
...
... ... ... ... ... ...
...
...
1.6 2.8 2.8 0.62
... ... ... ...
1.5 1.3
...
1.2
0.31 0.28 0.20 0.44 0.38 0.27 0.27 0.19 1.0 0.55 0.99 0.55
0.52 0.047 0.036 0.064 0,045 0.045 0.19 0,090 0.23
...
0.17
0.034
0.031 0.063 0.042 0.38 0.27 0.44 0.30 0.28 0.011 0.019 0.012 0.12 0.11 0.20 0.17 0.078 0.75 0.50
0.81 0.45 0.44 0.42
600 680
... 500 ...
1550 480
...
... ... ...
... 1600
... ... ... ...
1500
... ... ...
1600
... ... ... ... ...
1500
... ...
1’500
...
... ... ... ... ... ...
1400
... ... ... ...
1650
...
... (Cmkked)
spectral sensitivity of the phototube is thus taken into account. The evaluation of absolute intensities or of quantum efficiencies ( 5 ) is beyond the scope of the present application. The listing of the corrected intensities relative to carbazole provides some idea of the sensitivity of the technique in qualitative identification. Because of the many variables involved (slit combinations] photomultiplier settings, excitation wavelengths, etc.) the listed data should not be used for quantitative calculations. Relative phosphorescence intensities were more difficult to evaluate because of slight variations related to positioning of the small sample tube in the quartz Dewar flask. ;ilso, since phosphorescence intensities are much lower than fluorescence intensities] spectra were obtained a t high concentrations which may have produced concentration quenching effects. However, some approximate relative phosphorescence intensities were calculated as shown in Table 111. Again, as with fluorescence, the intensity data correspond to a single set of experimental conditions (at 1 mg./ml.) relative to the intensity for carbazole. Band positions for excitation are also included. The same limiting factors as discussed under fluorescence intensities apply to the phosphorescence data in Table 111. Values of the half life for phosphorescence are presented in Table IV. Most values were determined by manual rotation of the shutter. For very short half lives the shutter was rotated a t an appropriate speed by the shutter motor. Some information on the luminescence of complex compounds of carbon, hydrogen, and sulfur or nitrogen has been published (these data are collected in Table V for convenience). The fluorescence characteristics of aniline, methylaniline, dimethylaniline, indole, skatole, and 2-methylindole were compiled by the American Instrument Co. (20). Several nitrogen heterocyclic compounds were included in the report of the fluorescence spectra of some aromatic compounds by the Chicago Medical School (13). A bibliography of phosphorescent molecules compiled by Chen for the American Instrument Co. lists several nitrogen compounds (4). Russian workers recently reported on the luminescence of a series of nitrogen compounds (6) polarization of the fluorescence of porphyrins (8),and the luminescence of acridine (17) and quinoline ( 7 ) . Van Duuren studied the fluorescence of various heterocyclic aromatic compounds ($1). Two dibenaacridines were also studied by Schoental and Scott (16). The fluorescence of a series of unusual azaindoles was presented by Adler (1). Heckman (10) studied the phosphorescence spectra of indole, thianaphthene (benzothiophene), VOL. 38, NO. 1, JANUARY 1966
13
Table Ill. Excitation and Emission Wavelengths and Phosphorescence Intensities (Relative to Carbazole) for Some Aromatic Compounds Containing Sulfur, Nitrogen, or Oxygen Atoms (Continued)
PredomExcitainant tion vibrawavePhosphorescence tional lengths, Wavelength, Corrected spacing, Compound Solvent mfi mr rel. intensity cm. -1 2,3-Benzodiphenylene oxide EPA 318 520 0.094 ... 337 528sh. 0.058 ... 542 0,063 ... 563 0.099 1460 588sh. 0.060 ... 3-Methylisoquinoline EPA 282 495sh. ... 310 515 o.ooi2 ... Wsh. ... ... Older data (different 1P28 phototube) Dibenzothiophene 5-dioxide EPA 285 445 0.30 ... 315 478 0.45 ... 510 0.39 ... EPA 312 425 Dibenzothiophene 1.5 ... 439 1.3 ... 456 1.5 ... EPA 270 370sh. Not detnd. p-Tolyl sulfone . .. 385 Not detnd. ... EPA 285 364 0.99 Phenothioxin-10-dioxide 382 1.2 iioo 400 0.87 ... 475 PH 290 0,019 Naphthalene ... 487 0.012 . . 510 0,027 1450 549 0,019 ... PH 280 ... 354sh. Triphenylmethane ... 373 0.27 ... 386 0.28 ... 405sh. 0.19 ... EPA 285 480 0.13 Thianthrene ... 410 EPA 283 0.070 Indole ... 437 0.097 1550 450sh. 0,052 ... 414 EPA 285 0.27 Methyl p-tolyl sulfide *.. 285 420 0.34 Ethyl p-tolyl sulfide ... Very weak phosphorescence EPA Di-p-tolyl disulfide ... Very weak phosphorescence EPA Diphenyl disulfide ... Very weak phosphorescence EPA Dibenzyl disulfide ... 303 EPA 8-Meth ylquinoline 474 0.0069 ,.. 486 0.0045 ... 0.0097 1400 508 546 0.0069 ... EPA 308 472 0.0014 Isoquinoline 550 484 0.0016 350 493 0.0025 500 506 0.0031 519 550 0.0030 0,0039 529 350 543 0.0030 500 458 0.028 EPA Quinoline 298 ... 470 0.020 550 490 0.036 1450 502sh. 0.025 ... 526 0.027 ... 0.025 EPA 310 473 2,6-Dimethylquinoline ... 0.022 482sh. ... 508 0.044 1450 544 0,033 ... 429 2,2 ’-Bipyridine EPA 295 0.025 ... 449 ... 0.025 1600 460 0.036 0.031 480 ... 0.063 PH 240 389 Acetophenone ..* 1650 416 0.095 278 1750 320 448 0.070 0.041 1600 483
14
ANALYTICAL CHEMISTRY
carbazole, dibenzofuran, and dibenzothiophene. He found that there was no phosphorescence for pyrrole, furan, and thiophene. Yoshida (23) obtained the fluorescence spectra of a variety of aromatic compounds with -NHz groups. Van Duuren (82) has also studied the fluorescence of various substituted indoles. Shimada (18, 19) carried out a vibrational analysis of the phosphorescence spectra of pyrazine and pyrimidine.
Table IV. Lifetime for Decay of Phosphorescence Emission for Some Selected Compounds
Compound Aromatic hydrocarbons Acenaphthene Indene 1-Methylnaphthalene 2,7-Dimethvlnaphthalkne Oxygen compounds Diphenyl ether Dibenzofuran(diphenylene oxide) 2,3-Benxodiphenylene oxide Nitrogen compounds Diphenylamine Quinoline Quinaldine 2-Methylacridine Tetrahydrocarbazole 1,2-Dimethylindole 5,7-Dimethylbenzacridine 3,CBenzoquinoline 7,8-Benzoquinoline 2-Methylpyridine CMethylpyridine 2,4,6-Trimethylpyridine 2,2’-Bipyridine Acridane Carbazole
Mean lifetime,” Sol- secvent onds PH PH PH
2.8 0.5 2.0
PH
1.8
EPA 0.45 EPA 6 . 1 EPA 1 . 5
EPA PH EPA EPA EPA EPA
1.9 0.8 1.7 3.8 4.8 5.2
EPA EPA EPA PH PH PH PH EPA EPA
0.26 1.2 2.1 1.3 1.7 1.8
1.0 3.0
8.1
Sulfur compounds Di-p-tolyl sulfone EPA 1 . 4 Phenothioxin 10dioxide EPA 0.75 Diphenyl sulfide EPA 0.034 Dimethylthianthrene EPA 0.072 EPA 0.063 Methyl p-tolyl sulfide Thianthrene EPA 0.040 [2-Benzo ( b ) thienyl] phenylthiamethane PH 0.055 1-Thiaindane PH 0.045 3-Methylbenzo ( b ) thiophene PH 0.22 2,3-3’,2’-Dithianaphthene P H 0.034 Benzo ( 1,2-b15,4b)dithiophene PH 0.26 1,2,3,PTetrahydrothianaphthenoP H 0.074 (2,3-b) thianaphthene PH 0.074 a Time required to reach 36.8y0 of initial intensity.
Theory, Interpretation, and Discussion of Spectra. I n the fluorescence process, the molecule is raised from a singlet ground state ( 8 ) to an or vibrational excited singlet state (S*) level thereof, followed by vibrational deactivation to the zero vibrational level of S*, thence via fluorescent emission to S or vibrational level thereof. Since the vibrational levels of S and S* are similar, the emission spectrum is usually an approximate mirror image of the absorption spectrum (12). Thus, where fine structure may be observed, something of the vibrational frequencies in the molecules may be determined and used as a means of characterization or identification. Phosphorescence involves a metahaving a longer stable triplet state (T*) lifetime and a lower energy than the singlet excited state. Electronic transitions between two states involving a change in multiplicity, as in the T* + S phosphorescent emission, are normally forbidden. The probability of transitions between these states, however, is governed by the degree of spin-orbital interaction (magnetic interaction between the spin and orbital motions of an electron). I n atoms of low atomic number the spin-orbital coupling is small, while for heavier atoms it is larger, The constant in the expression describing this magnetic interaction is proportional to the fourth power of the effective nuclear charge. McClure (14) found that substituting heavy atoms in organic molecules produced the same effect as observed in atoms. Although this substitution causes little change in the wavelength and intensity of the absorption spectra, increased probability of T*-+ S transitions decreases the lifetime in the T* state. For the halogenated naphthalenes substituted in the a-position, McClure (14) found the following lifetimes for phosphorescence: F = 1.5, C1 = 0.30, Br = 0.018, and I = 0.0025 second. Nitrogen and oxygen analogs of aromatic hydrocarbons should exhibit phosphorescence lifetimes equivalent to their hydrocarbon analogs (Tables IV and V). Sulfur compounds show much shorter lifetimes and higher intensities than the hydrocarbon or oxygen analogs because of increased spin-orbital interaction (Table IV). The phosphorescence intensities of many of the oxygen, nitrogen, and sulfur compounds shown in Table I11 are high compared to their fluorescence intensities (Table 11). This is contrasted to the behavior of the hydrocarbon analogs which show weak phosphorescence. It is for this reason that spectrophosphorimetry becomes useful in the characterization of sulfur and nitrogen compounds, particularly when micro separation techniques are used and sensitivity is needed.
Table V.
Some Luminescence Characteristics of Pertinent Nitrogen Compounds Compiled from the Literature
Excitation max ,, Emission max., mp Phos. Compound mr Fluor. 280 340 ... Aniline ... 290 360 Methylaniline ... 255 370 Dimethylaniline ... 280 350 Indole ... 290 370 Skatole ... 355 280 2-Methylindole ... . . . 404 Benz(c)acridine . . . 406 ... 11H-Benzo(a)carbazole ... 11H-Benzo(b)carbazole ... 408 396 ... 7H-Benzo(c)carbazole . . . . . . 372' * Aniline ... . . . 408, 438 Carbazole ... . . . 865 Chlorophyll-b ... . . . 493 Dimethyl-a-naDhthvl- amine 2,4-Dimethylquinoline ... . . . 458 . . . . . . 420 p-Dimethyltoluidine ... . . . 395, 412, Diphenylamine 423 ... . . . 680-870 Metal-etioporphyrin11 complexes 400-600 ... ... Indole 462 ... Quinoline 400-600 323 ... Isoquinoline 660-760 ,.. hletal-mesoporphyrinIX . . . complexes 470 ... ... 8-Meth ylquinoline a-Naphthylamine ... . . . 498 . . . . . . 498 p-Naphthylamine 502 ... ... Di-@-naphthylamine . . . . . . 494, 540, a-Naphthonitrile 587 ... . . . 484 p-Naphthonitrile . . . 495 Phenyl-p-naphthylamine . . . 335 350 ... Phenanthridine 367 385 400 300 ... 395 Dibenz(a,h)acridine 403 418 445 470 300 ... 395 Dibenx(a,j)acridine 400 416 440 465 280 348 ... Indole 256 322 ... Isoindazole 290 250 ... 310 Benzimidazole 275 -
Hal flife of phos., seconds
... ... ... ... ... ... ... *..
... ...
Long Long 10-4 Long Long
...
Long 10-4
...
Long
5 X'io-4
~
4-Axaindole 5-Azaindole 6Axaindole 7-Axaindole Benztriazole 3,4-Diaxindole 3,5-Diazaindole Purine Aniline N,N-Dimethylaniline N,N-Diethylaniline Diphenylamine Triphenylamine
Long Long Long Long Long Long Long
...
...
...
... ...
2.5gb 3.80b
...
4.45b
417 404
...
*..
... ...
1.096 0.276
392
...
...
2.16b
391 350 320
...
...
*..
... ... ...
0.026 4,466
... ... ... ...
355 370 33OC 336O 336c 345c
... ...
... ...
0.286 0. 14b
...
35OC
299 270 282 266 294 293 270 255 285 280 281
...
...
408 417C 412c 398" 415" 422c 410~ 427" 435c
0.3gb
4.2 2.4 2.0 1.9
0.70
(6)
(Continued) VOL. 38, NO. 1, JANUARY 1966
15
To ble V.
Some Luminescence Characteristics of Pertinent Nitrogen Compounds Compiled from the Literature (Continued)
Compound 9,10-Dihydroacridine Indole Carbazole Indole Carbazole 5H-Benzo(b)carbazole 7H-Dibenzo(a,g)carbazole 7H-Dibenzo(c,g) carbazole
Excitation max., Emission max., mru mp Fluor. Phos. , , . 347~ 412O . .. 439c 297c 403c . . . 43lC 455c . . . 342< 406c 357c 41SC . . . 435c 280 297 ... 32 1 335 ... 348 330 387 ... 411 440 340 367 386 408 430 350 363 ... 382 400 424 330 ... 317 400 360 ... 297 285 306 280 315 280 291 315 ... 282 320 ... 290 337 306
Half life
of phos.,
seconds 1.6
Ref.
0.36
(6)
0.55
(6)
Remarks
(6)
. .. ...
(21) (21)
...
(21)
IN A
LATE.
...
(21)
* .. (21) Quino1ine ... (21) Acridine Indole 2-Methylindole 3-Methylindole 1,2-Dimethylindole 2,3-Dimethylindole ... (82) 2-( 3-Indolyl)-2,3dihydroindole (indole dimer) 360 .. (22) 1-Methyl-2-phenylindole 310 .. (22) 332 290 Carbazole 348 318 330 Ap roximate limit of detection in pg./ml. b RePative intensity in water at p~ of maximum emission. c Values read from reproduced spectra (relative intensity us. frequency). d Relative intensity in cyclohexane (uncorrected).
55
50
45
35
40
30
6.0d 8.2d 64. Od 5.4d
25
TIME (MINUTES)
Figure 2. Gas chromatogram of a basic nitrogen extract from a petroleum fraction Column Programming rate Initial temp. Sample size (Fraction cut-points as shown)
16
ANALYTICAL CHEMISTRY
The vibrational fine structure corresponds to vibrational frequencies (as observed in infrared and Raman spectra) representing fundamental skeletal or C-C, C-0, C-N, or C-S vibrations. Some approximate vibrational frequencies are calculated for several of the compounds showing fine structure and the values are included in Tables I1 and 111. In most cases, the values seem to correspond to the heavy atom-carbon stretching frequencies or the skeletal vibrations in aromatic hydrocarbons. It is this vibrational fine structure which becomes useful in the identification of certain molecular types by luminescence. Application of Luminescence to Characterization of Petroleum Fractions. BASICKITROGEXCOMPOUXDS
430-650" F.
17-Foot SE-30 on Chromasorb W C./minute
4'
150' C. 30 pl.
STRAIGHT-RUN MIDDLEDISTILX 430-650" F. distillation
cut of a crude petroleum was extracted with dilute (1:1) hydrochloric acid to concentrate the basic nitrogen compounds. The acid extract was neutralized with sodium hydroxide and the released nitrogen compounds were extracted into hexane. After thoroughly washing the hexane with water, the solution was evaporated in a nitrogen atmosphere to yield the basic nitrogen concentrate. Components of the basic nitrogen extract were partially separated by gas chromatography on an SE30 column (Figure 2). The small amount of material represented by each trapped fraction did not permit the use of infrared spectrophotometry for characterization. However, even upon diluting to several milliliters, there was sufficient material in each fraction to obtain ultraviolet spectra (Figure 3). Although evidence for quinolines is apparent, it would be difficult to detect the presence of substituted pyridines or tetrahydroquinolines in this fraction on the basis of the ultraviolet absorption bands. Phosphorescence spectra provided information not readily obtained from the ultraviolet spectra. By changing the excitation wavelength, it was possible to observe both pyridine (or tetrahydroquinoline) and quinoline species in the mixture of compounds represented in each trapped fraction (Figure 4). Pyridines and tetrahydroquinolines were excited a t 260 mp while quinolines were excited at 295 mp with several characteristic emission bands between 460 and 550 mp. Further evidence for the identity of the molecular types present was provided by the half life for decay of phosphorescence. The decay characteristics for the compounds in the gas chromatographic fractions are listed in Table VI. A gradual increase in the half life was observed with increase in cut number (increase in boiling point or molecular weight). This was expected on the
1.0
0.8
0.6
W
Y m < 0.4
IIAF"WE
(Mp)
Figure 4. Phosphorescence and excitation spectra of gas chromatographic fraction No. 4 of a basic nitrogen extract of a 430650" F. petroleum fraction Curve Curve Curve Curve
0.2
1. 2. 3. 4.
Excitation spectrum (emission set at 4 0 5 mp) Excitation spectrum (emission set at 4 9 5 mp) Phosphorescence spectrum (excitation set at 260 m p ) Phosphorescence spectrum (excitation set at 2 9 5 mp)
0
Figure 3. Ultraviolet spectrum of gas chromatographic fraction No. 4 of a basic nitrogen extract of a 430-650" F. petroleum fraction
basis of the half life values determined for the reference compounds (Table IV). Although the object of this study was not a quantitative analysis of this fraction, some idea of the relative amounts of pyridines or tetrahydroquinolines and quinolines present in each fraction was obtained from the ratio of emission intensities a t 400 and 500 mp with excitation at 265 and 300 mp, respectively (Table VII). These data showed that the pyridines and tetrahydroquinolines are concentrated in the first three fractions and that as the molecular weight increases, the quinoline types predominate. The high intensities at 400 mp for the first three fractions may also be derived from the presence of some anilines or other related aromatic amines. The fluorescence of pyridines and quinolines is weak as in the case of their hydrocarbon analogs. However, acridines and benzacridines fluoresce strongly with well-resolved vibrational fine structure. Examination of the gas chromatographic fractions showed an emission very characteristic of acridine or benzacridine (Figure 5 ) in all
\ \
'-v
I
I
400
300
:\,
2(
WAVELENGTH (H?)
Figure 5. Fluorescence and excitation spectra of gas chromatographic fraction No. 4 of a basic nitrogen extract of a 430-650" F. petroleum fraction
fractions. These compounds were expected in only the last several fractions. Because this fluorescence occurred in all fractions at about the same intensity, it was concluded that the acridines were bleeding from the column as a result of the sample having been run several times before trapping was done. These data emphasize the fact that high-boiling
polar compounds tend to tail or be retained in a gas chromatographic column causing contamination of subsequent trapped fractions. The retention and subsequent bleeding by high-boiling polar compounds may not be as serious a problem when using less sensitive techniques such as infrared spectrophotometry. VOL. 38, NO. 1, JANUARY 1966
17
OC PEAK 2
GC PEAK 7
----wc. -EXC.
a10ql
OC PEAKS 9 & 10
A -EXC.
---EXC. 250 288
298W
WAVELENWCK @)
Figure 7. Phosphorescence spectra of trapped gas chrornatographic fractions of product from catalytic hydrogenation of quinoline
TIME (MIN.) Figure 6. Gas chromatogram of nitrogen compounds from catalytic hydrogenationof quinoline Trapped fractions are indicated by number
Infrared and NMR spectra of this basic nitrogen fraction showed a relatively high methyl rather than methylene group content, indicating a high degree of short cha.in or methyl group substitution. The infrared spectrum, in the hydrogen out-of-plane bending region, suggested very little substitution on the nonheterocyclic rings. The XMR spectrum indicated almost complete substitution of the a-hydrogen by alkyl groups (or fused naphthene rings). NITROGENCOMPOUNDS FROM CATALYTIC HYDROGENATION OF QUINOLINE. Phosphorescence proved invaluable in
Table VI. Lifetime for Phosphorescence of Trapped Fractions
Basic Nitrogen Extra.& of 430-650' F. Fraction Mean lifetime for GC phosphorescence, seconds fraction Excitation: 265 mp 290 mp No. Emission: 4 0 5 m ~ 500mp 0.8
1.2 1.0 1.1 1.4
1 2 3 4
0.9
0.8 1.1
1.3 1.3 1.4
5 6 7 8 9
1.2 1.2
1.5 1.2 1.4
13 14 15
1.2
12
Total sample
1.2
1.4
1.3
1.3
Reference compounds Quinoline No.emis2-Methylpyridine
sion 1. 3
PMethylpyridine
1.7
2,4,6-Trimethylpyridine
1.8
18
ANALYTICAL CHEMISTRY
n.6
0.3
13
0.6 1.0 15 0.7 Intensity of emission from pyridines and tetrahydroquinolines. b Intensity of emission from quinolines, 14
gen compounds is presented in Table From this information, the reaction paths below were deduced,
VIII.
I
catalytic alkyl chain transfer
catalytic
J. Zmethylquinoline 2-ethylquinoline Zisopropylquinoline I
catalyst
0.8
No. emission No.emission No.emission ~
11 12
hydrogen (by transfer)
1.2
1.3 1.2 1.1 1.2
Basic Nitrogen Extract of 430-650' F. Petroleum Fraction Corrected rel. intensity ratio GC 405 m p (Exc. at 265 mpc)" fraction No. 500 mp (Exc. at 302 mp)* 1 31.8 2 17.6 3 19.9 4 3.2 5 1.2 6 1.0 7 0.9 8 0.9 9 1.1 10 0.8
uinoline
1.2 1.2 1.0 1.1
1.4 1.2
10 11
characterizing a nitrogen compound concentrate from catalytic hydrogenation of quinoline. This mixture was complex as shown by the chromatogram (Figure 6) obtained on a '/cinch S.E. 30 column using a small sample (injected 1 pl.). Larger samples (ca. 25 pl.) and larger columns (0.25-inch) were required for infrared spectral study of trapped fractions. Under these conditions, the polar nitrogen compounds were poorly resolved, particularly the smaller components appearing near the larger ones. The greater sensitivity of the phosphorescence technique permitted the use of small samples (5 pl.) for improved resolution during trapping from the 0.25inch column. Compound type identification was easily accomplished from the phosphorescence spectra. Choice of excitation wavelengths permitted identification of several compound types even when they appeared in the same trapped fraction. Certain components too small to be detected by infrared spectrophotometry were observed by phosphorescence. By combining the phosphorescence and infrared spectral data with relative retention data (expressed
Table VII. Intensities for the Tetra. hydroquinolines or Pyridines Relative to the Quinolines in Trapped Fractions
as boiling points), the majority of the compounds were identified. Some typical phosphorescence spectra exemplifying the compound types found are combined in Figure 7. The auantitative distribution o?. the identified nitro-
ACKNOWLEDGMENT
The authors thank Research Laboratories, €hmble oil &z Refining CO., for permission to publish this material.
The assistance of R. C. Cox in writing the computer program, W. L. Senn, Jr., in obtaining the NMR spectra, and J. S. Table VIII. Nitrogen Compounds Identified in Product from Catalytic Hydrogenation of Quinoline by Hydrogen Transfer from Diluent
GC peak No. 1
2 3 4 5 6 7
8 9 10 11 12
Compound identified Wt., yo Aniline 12.3 o-Toluidine 5.3 &Ethylaniline 1.2 N-Ethyl-o-toluidine 0.3 o-Propylaniline (and other 4.2 compounds) Quinoline 47.8 2-Methylquinoline and 14.1 1,2,3,4tetrahydroquinoline 3-Methylquinoline 1.3 Indole 4.4 2-Ethylquinoline 4.4 2-Isopropylquinoline 2.6 Dimers and codimers of 2.1 partially hydrogenated quinoline, alkylquinolines and indoles 100.0
Ellerbe and J. A. Hodgeson in obtaining some of the luminescence spectra is gratefully acknowledged. We also thank J. B. Zachry for supplying the sample from denitrogenation studies and T. P. Hawes for assistance in obtaining some gas chromatograms. LITERATURE CITED
(1) Adler, T. K., ANAL. CHEM.34, 685 (1962). (2) Ball, J. S., Rall, H. T., Proc. A m . Petrol. Inst. Sect. 111 42, 128 (1962). (3) Becker, R. S., Ph.D. thesis, Florida State University, Tallahassee, Fla., 1955. (4) Chen, M., “Bibliography of Phosphorescent Molecules,” American Instrument Co., Inc., 1960. (5) Drushel, H. V., Sommers, A. L., Cox, R. C., ANAL. CHEM.35, 2166 (1963). (6) Ermolaev,V. L., Opt. Spectry. (USSR) English Transl.) 11, 266 (1961). (7) Ermolaev, V. L., Kotlyar, I. P., Ibid., 9, 183 (1960). (8) Gurinovich, G. P., Sevchenko, A. N., Solov’ev, K. N., Ibid., 10, 396 (1961). (9) Hartung, G. K., Jewell, D. M., Anal. Chim. Acta 26, 514 (1962). (10) Heckman, R. C., J . Mol. Spectry. 2, 27 (1958). \ - - - - I
(11) Lewis, G. N., Kasha, M., J . A m . Chem. SOC.66, 2100 (1944). (12) Lewschin, W. L., 2. Physik 27, 368, 382 (1931). (13) Lijinsky, W., Chestnut, A., Raha, C. R., Chicago Med. School Quart. 21, 49 (1960). (14) McClure, D. S., J . Chem. Phys. 17, 905 (1949). (15) Parker, C. A., Rees, W. T., Analyst 87, 83 (1962). (16) Schoental, R., Scott, E. J. Y., J. Chem. Soc. (London)1949,1683. (17) Shablya, A. V., Terenin, A. N., Opt. Spectry. 10,324 (1961). (18) Shimada, R., Spectrochzm. Acta 17, 14 (1961). \----,.
(lG)Ibid., p. 30. (20) “Supplemental DBta Sheet to Aminco Bulletin No. 2278, American Instrument Co., Inc., Silver Spring, Md., Atxi1 1959. (21)’Van Duuren, B. L., ANAL. CHEM. 32, 1436 (1960). (22) Van Duuren, B. L., J . Org. Chem. 26, 2954 (1961). (23) Yoshida, T., Igaku Kenkyu 27, 443 (1957); C . A . 52, 10721i (1958).
RECEIVED for review February 18, 1965. Accepted October 7, 1965. Presented at the Symposium on Sulfur, Nitrogen, and Oxygen Compounds, Division of Petroleum Chemistry, 149th Meeting, ACS, Detroit, April 1965.
Isolation and Identification of Nitrogen Compounds in Petroleum H. V. DRUSHEL and A.
L. SOMMERS
Esso Research laboratories, Humble Oil & Refining Co., Baton Rouge, l a .
b A separation scheme for the isolation of nitrogen compounds from petroleum was devised which provides fractions of specific chemical classes. Separation was achieved by use of solid-liquid chromatography, chemical extraction, and gas chromatography, Included among the extractants were sodium aminoethoxide in ethanolamine and 7270 perchloric acid which were used to isolate a variety of weakly acidic compounds. Characterization of the fractions from gas chromatography relied upon use of the usual infrared, ultraviolet, and mass spectral methods as well as sensitive fluorescence and phosphorescence techniques. Application of this scheme to a light, catalytic cycle oil resulted in identification of pyridines, quinolines, pyrindines, cyclopentaquinolines, indoles, carbazoles, pyrroloquinones, phenols, and other hydroxy compounds. Many of these compounds are believed to be responsible for the color, odor, and gum-forming characteristics of heating oils and other related petroleum fractions.
N
70821
ITROGEN COMPOUNDS in petroleum
adversely affect many of the important refining processes. They are believed to reduce the activity of cracking or hydrocracking catalysts because of their polarity and basicity. It has also been suspected that nitrogen compounds are to a great extent involved in gum formation, color formation, odor, and the poor storage properties of fuels. To overcome the deleterious effects of the nitrogen compounds it is helpful to know the various types of compounds that exist in petroleum throughout different phases of refining. Numerous studies have been made on the nature of nitrogen compounds in petroleum and several separation and identification schemes have been reported. Hartung and Jewell (6) used alumina adsorption, perchloric acid extraction, and spectrophotometric examination to identify indoles, carbazoles, phenazines, and dibenzofuran in a hydrogenated, catalytically-cracked furnace oil. The same authors also reported (7) the use of zinc chloride and ferric chloride as complexing agents for
the isolation of nitriles from a hydrogenated furnace oil. Carbazole was identified in Wilmington, Calif., petroleum by Helm and coworkers (9) using distillation, adsorption, chemical treatment, gas chromatography, and spectrometry. As a means of identifying nitrogen compounds in gas chromatographic effluents Thompson and coworkers (18) developed a catalytic hydrogenation micromethod in which the nitrogen free products are identified. Snyder and Buell (16, 17) have used linear elution adsorption chromatography and ion exchange resins to isolate basic nitrogen compounds, carbazoles, and other nitrogen compounds from gasolines through cracked gas oils. They used ultraviolet spectrophotometry to estimate indoles, carbazoles, and benzcarbazoles in the isolated fractions. Sauer and coworkers (15) applied mass spectrometry to estimate carbazoles, indoles, pyrroles, pyridines, and quinolines in concentrates isolated from heating oils. La Lau (12) used mass spectrometry to identify and estimate the quantity of pyridines, quinolines, VOL. 38, NO. 1, JANUARY 1966
19
