CONSTITUTION OF A S P H A L T I C BITUMEN Characterization of Bitumens b y a Combination of @rolysis, Hydrogenation, and Gas -Liquid Chromatography J A N KNOTNERUS
Konink[ijke/Shell-Laboratorium,Amsterdam, Holland
Downloaded by UNIV OF SUSSEX on September 2, 2015 | http://pubs.acs.org Publication Date: March 1, 1967 | doi: 10.1021/i360021a008
The pyrolysis, hydrogenation, and gas-liquid chromatography techniques were applied to bitumens and bitumen constituents in order to enlarge our knowledge of their chemical constitution. The pyrograms could b e satisfactorily interpreted on the basis of data obtained with model compounds. Bitumens can b e rapidly characterized as to their cyclic or paraffinic nature by the chemical constitution of their pyrolyzates. Side chains with a t least 10 carbon atoms occur in “normal” bitumens which, on the average, contain about three branched side chains and at least one isolated, noncondensed ring per molecule.
HE chemical constitution of bitumen has a strong influence Ton its rheological behavior, solubility, volatility, and attachment to other materials. For a better understanding of these and other factor:, connected with origin or processing conditions, a more detailed knowledge of this constitution would be welcome. Several, mainly physical, methods of analysis and determinations of elemental composition have provided valuable information on the presence of aromatic and other cyclic structures, but our picture of the average ‘.bitumen molecule” or, more specifically, of the average asphaltene and the average inaltene molecule, is far from complete. A simple technique that may help to fill in some details of this picture, in particular as regards the structure of the aliphatic parts of the molecules, is the combination of pyrolysis and gasliquid chromatography (GLC). I n its most convenient form, this technique involves hydrogenation of the pyrolysis products before their identification. V a n Schooten and E,venhuis (72) developed the technique in this form and applied it successfully in their studies of the structure of poly-ct-olefins. Review articles give more detailed information on its application in polymer analysis (3,8, 74, 76), and M’all ( 7 5 ) has published a more theoretical article. Several authors have studied the pyrolysis of pure organic compounds by this and related methods (2, 4, 6 , 73) and K a r r (70) used pyrolysis/GLC to obtain information on the chemical structure of coal tar pitches. Perry ( 7 7 ) applied the technique to bitumen analysis. Three asphaltic bitumens were pyrolyzed and differences were found in structures coni aining benzene rings. Ariet and Schweyer (7) also pyrolyzed asphaltic materials. They found that differences in the pyrograms are very real, but stated, “the general similarity of the results for widely different bitumens and fractions separatedthere from raises the question as to the feasibility of this technique in studying bitumens a t this time.” I n view of these published results it seemed worthwhile to adapt the combined pyrolysis/GLC technique to the study of bitumens and their constituents, even though their structure, like that of coal tar pitches, lacks the regularity of polymer structure and is far more complicated than those of pure organic compounds. T h e technique may be used for fingerprinting and characterization of bitumens or for obtaining more insight into their chemical structure. I n particular we hoped to get
more reliable information about the number and lengths of paraffinic side chains, as these influence the solubility and mixing properties of bitumens and bitumen components considerably. Numerous experiments with model compounds were indispensable, before we could hope to analyze the bitumen components successfully. This publication describes these model experiments and the application of the results in the structural analysis of a number of bitumens, bitumen fractions, and related compounds. Experimental
Apparatus and Procedure. T h e apparatus used initially was essentially identical with that described by van Schooten and Evenhuis (72). A submilligram sample is heated for a few seconds on a coiled Nichrome filament a t about 600’ C. in hydrogen as a carrier gas and the decomposition products are identified, after hydrogenation, by temperature-programmed GLC with flame-ionization detection. A hydrogenation step is highly desirable, as then the number of compounds of a pyrolyzate is reduced, and isomerization of olefins is largely prevented. The only modification was replacement of the Nichrome wire by a platinum one, because trace metals always present in bitumens caused heavy corrosion of the Nichrome. The GLC column was heated from 50’ to 170’ C. a t a rate of 1.50’ C. per minute (instead of from 90’ to 170’ C . a t a rate of 0.75’ C. per minute). This variation improved the resolution of the chromatograms of the low-molecular-weight products of pyrolysis, including compounds with one to 12 carbon atoms per molecule. One experiment takes about 1‘/zhours. Flash pyrolysis of rather low-molecular-weight compounds placed directly on the platinum wire appeared to yield hardly any products of decomposition; on heating, the material evaporated or spattered from the wire before cracking could occur to any extent. Yields of pyrolyzate increased considerably when the sample was adsorbed before pyrolysis on a small cylindrical grain of charcoal (2 X 4 mm.), which was placed inside the coiled platinum wire. Molecular-sieve pellets were also tried out as a n adsorbent, but this resulted in the secondary isomerization and cyclization reactions that are normal in catalytic cracking. aAlumina or electrode coal gave results comparable with those obtained without any adsorbent. With charcoal no isomerization was observed, but cyclization could not be fully suppressed, as this inevitably accompanies free-radical reactions. Temperature and Time of Reaction. T o measure the temperature and the duration of the pyrolysis reaction more exactly, a Chromel-Alumel thermocouple was introduced into a VOL. 6
NO. 1 M A R C H 1 9 6 7
43
small hole (diameter 0.5 mm.) in a grain of charcoal. This grain was placed inside the platinum coil, and coil and charcoal were preheated to about 100' C. in a stream of hydrogen as is usual in pyrolysis experiments. Then the rate of temperature increase with time was measured for various values of the electrical current.
Downloaded by UNIV OF SUSSEX on September 2, 2015 | http://pubs.acs.org Publication Date: March 1, 1967 | doi: 10.1021/i360021a008
T h e results are shown in Figure 1. For the normally applied current of 4 amperes, a temperature of 600' C. is reached within 5 seconds. At this temperature pyrolysis of the sample is presumably complete. I n practice, the current is switched off after 10 to 12 seconds, when the temperature has risen to about 675' C. Samples. T h e complex nature of bitumens and consequently of their pyrograms made experiments with model compounds indispensable, the more so as very few data on model compounds are mentioned in the literature. O n the other hand, the experience with, and published information on, polymers and resins proved highly valuable. Pyrolysis experiments were carried out with various types of model compounds : normal paraffins, isoparaffins, monocyclic naphthenes with long side chains, aromatics with short
Calibration of Apparatus. T h e peaks in the pyrograms were identified with the aid of the relevant model compounds from the laboratory's collection of hydrocarbons, and data
Table 1.
3 5 Amp. a I
.
3 Amp.
W
9
5I-
OJ
I
I
I
I
I
I
I
Retention Times as Distances from Methane Peak, in Millimeters on Recording Chart
Speed. 900 mm. per hour GLC column. 7 meters X 3 mm. 16.7y0 Apiezon L on firebrick 50-80 mesh Column temperature. 50" to 170" C. Increase of temperature. 1.50' C. per minute Hydrogen. 3.9 liters per hour C,. Saturated n-membered straight carbon chain c y . Cyclo i. Is0 m. Methyl
E. Ethyl P. Propyl B. Butyl Pent. Pentyl H. Hexyl c1
cz
C8 iCa
c4
ic5 c 5
2mC5 3mCh cyc5
ce
2,2mC5 2,4mCs mCyC5 2,2,3mC4 3,3mCj 2rnC6 2,3mC5 3mCs cyc6 1,lmCy Cs
44
0 3 10 22 34 74 94 168 189 178 211 247 253 272 27 5 313 318 333 342 344 350
Cl 2,5mCe 2,4mCs mCyC6 ECyCj 1,1,3mCyCs 2mC7 4mG 3mC1 2,2,4mC~ C8 1-truns-4mCyCs 1,lmCyCe 2,2mG 2,4mC1 1-cis-4mCyC6 l-cis-2mCyCs 1-trans-2mCyCe iPCyC.5 nPCyCj ECyG
I&EC P R O D U C T RESEARCH A N D D E V E L O P M E N T
385 429 440 467 480 480 513 521 532 568 585 594 598 610 614 638 642 642 652 683 705
2,4,6mC7 2mC8 3mC8 c 9
2,4mCs
1-E-truns-4mCyCe
2,7mC8
1-E-cis-4mCyCe
iPCyC6 nPCyCs 2mCg 4,6mCg ClO 2,4,6mC8 nBCyCa GI1
2mC~ nPentCyCe c12
nHCyCe
708 710 727 787 806 826 835 858 884 896 908 978 982 1062 1090
1165 1197 1275 1339 1450
1
alumina, or silica give a lower yield of cyclanes, but also yield less pyrolyzate. T h e formation of cyclanes appears to depend on the severity and the rate of the decomposition reaction (Table 11). Platinum also appears to promote cyclization. T h e extent of formation of cyclanes appeared to depend on the length of the paraffinic carbon chain. This was established with a series of normal paraffins. The total number of fiveand six-membered rings increased from 1.25 to 2.2 per 100 carbon atoms of pyrolyzate for paraffins in the series from decane to pentacosane. With paraffins of higher molecular weight the number remained fairly constant a t about 2.2 Table I11 and Figure 3). When the number of rings formed is calculated per molecule of base material, it correlates very well with the number of carbon atoms of the base material. This is also shown by Table I11 and Figure 3. O n extrapolation, the straight line obtained intersects with the abscissa at carbon number 6; this signifies that no cyclanes would be formed on pyrolysis of hexane and lower paraffins, if the method were applicable to them (these hydrocarbons are too volatile; they evaporate before pyrolysis can occur). The degradation mechanism of hydrocarbon polymers has been thoroughly discussed by Wall (75) and others (72). For initiation and depropagation reactions, and for inter-
pyrolysis u p to hydrocarbons containing 12 carbon atoms per molecule. When a flame-ionization detector is used, the height and the surface area of each peak are proportional to the concentration of the compound and depend directly on the amount of carbon in the molecule. T o determine the yield of pyrolyzate and of its constituents quantitatively, the total peak area per milligram of carbon had to be known. For this purpose 0.1- to 0.7-mg. samples of a few volatile hydrocarbons were introduced directly into the GLC column. T h e peak surface was determined in each case, and the area per milligram of carbon could be derived by extrapolation (Figure 2) to be 28 sq. cm. h 3% under the prevailing conditions.
Downloaded by UNIV OF SUSSEX on September 2, 2015 | http://pubs.acs.org Publication Date: March 1, 1967 | doi: 10.1021/i360021a008
Pyrolysis of Model Compounds
Normal Paraffins. Normal paraffins are decomposed to a large extent, the products of decomposition being principally normal paraffins of lower molecular weight. Because of the presence of the hydrogenation section immediately beyond the platinum wire, olefins are not found, and isomerization can be totally neglected. Cyclization does occur, however, as noted above. T h e charcoal, which always contains trace metals, may have enhanced cyclization : T h e adsorbents electrode coal, CY-
Table II. Formation of Cyclanes on Pyrolysis of Hexatriacontane under Various Conditions
(Sample size about 0.3 mg.) Expt.
No. 1 2 3 4 5 6 7 a
Adsorbent
Charcoal Charcoal Silica Electrode coal a-Alumina Directly on Pt wire Charcoal 2y0 Pt
T i m e of Reaction,a Sec. 3
Yield of Pyrolyzate
