heights for hydrogen and nitrogen carrier (Figure 3) indicates that diffusion in the liquid phase contributes significantly to plate height when 30% liquid is used. The equal minimum plate heights obtained with hydrogen and nitrogen on the 10% column (Figure 4) indicate that this effect has become negligible. Effect of Use of Different Carrier Gases. The data obtained on the 4foot 30% column with nitrogen and hydrogen carriers show t h a t the number of plates, and hence the resolution, is slightly smaller with hydrogen. HoLTever, the increased optimum carrier gas flow rate obtained with this gas reduces the retention time for butadiene by a factor of 2.3. The effect of the carrier gas is more pronounced for the 10% column. Again equivalent resolution is obtained with either carrier, but the time required with hydrogen is only one fourth that required for nitrogen. Performance with hydrogen is superior to that with helium on both the 3oY0and 10% columns. I n each case, the smaller number of plates and higher pressure drop obtained with helium carrier make the value of Ri/ti for helium smaller than for hydrogen. If the column used to obtain run 5 were lengthened to provide the resolution obtained in run 6, the helium column would require 75% more time than the hydrogen column.
Eff ect of Column Length. Figures 5 and 6 show t h a t the values of Ri/ti increase with decreasing retention time (increasing carrier gas flow rate). This indicates t h a t for a required resolution, Ri, a larger value of & / t i may be obtained, in some cases, by operating longer columns a t flow rates well above optimum. Thus the 10% column which will give an optimum value of Ri/tt with a resolution of 10.8 will lie somewhere between the 4-fOOt and 12-foot columns and will operate a t a flow rate greater than 62 ml. per minute. The resolution obtained with the 12foot column, a t optimum flow rate, is 4 3 times that obtained with the 4-foot column, as predicted by Equation 8. The value of Ri/ti decreases in agreement with Equation 10. Comparison of chromatograms B and D is of interest. The time required to obtain the separation is the same in both cases, but the resolution is almost twice as good with the 12-foot column and hydrogen carrier. The combined effects of the quantity of liquid phase and the type of carrier gas are shown in Table 11. The same resolution is obtained from the 4-fOOt 30% column with nitrogen carrier and the 4-foot 10% column with hydrogen carrier, while the ratio of the times required is 9 to 1.
LITERATURE CITED
(1) Bohemen, J., F e l l , J. H., “Gas
Chromatogra hy, D. H. Desty, ed., p. 6, Academic ires,, New York, 1958. (2) Cheshire, J. D., Scott, R. P. W., J. Znst. Petrol. 44, 74 (1958). (3) Deem ter, J. J. van, Zuiderweg, F. J., Klinkenterg, A., Chem. Eng. Sci. 5 , 271 (1956). (4) Desty, D. H., ed., “Gas Chromatography,” p. xi, Academic Press, Xew York, 1958. (5) Desty, D. H., ed., “Vapor Phase Chromatography,” p. xi, Academic Press, Kew York, 1957. ( 6 ) Golay, M. J. E., ASAL. CHEM.29, 928 (1957). (7) Gola;, hf. J. E., “Gas Chromatography, D. H. Desty, ed., p. 36, Academic Press, Kew York, 1958. (8) Golay, M. J. E., Nature 182, 1146 f 1958).
(9) Jones, W. L., Kieselbach, R., ANAL. CHEM.30, 1590 (1958). (10) Karasek, F. W., Ayers, B. O., Instrument Society of America, 5th National Symposium on Instrumental Methods of Analysis, Houston, Tex., I!lay 1959. (11) Keulemans, A. I. bl., Gas Chromatography,” p. 135, Reinhold, New York, 1957. (12) Keulemans, A. I. M., Kwantes, A., “Vapor Phase Chromatography,” D. H. Desty, ed., p. 15, Academic Press, New York. 1957. (13) McWfiliam, I. G., Dewar, R. A., “Gas Chromatography,” D. H. Dest , ed., p, 142, Academic Press, New Yorz, 1958. (14) Porter, P. E., Deal, C. H., Jr., Stross, F. H., J.Am. Chem. SOC.78,2999 (1956). (15) Scott, R. P. W., “Gas Chromatography, D. H. Dest ed., p. 189, Academic Press, New &rk, 1958. for review October 2, 1959. AcRECEIVED cepted February 11, 1960.
Molecular Sieve Adsorption Application to Hydrocarbon Type Ana lysis JOHN G. O’CONNOR and MATTHEW S. NORRIS
Gulf Research & Development Co., Pittsburgh, Pa.
b A method is described for the determination of normal hydrocarbons in 100’ to 650” F. petroleum distillates using powdered Molecular Sieves, Type 5-A, without prior fractionation of the distillates into narrow boiling cuts. The accuracy, calculated as a standard deviation, is to =tO.8% of the normal hydrocarbon content of the sample. The procedure consists of weighing the sample into an adsorption column containing the sieves and eluting the nonadsorbed hydrocarbons with isopentane. The excess eluent is removed by vacuum evaporation at room temperature, and the adsorbed normal hydrocarbon content
determined by weighing. Also described is a procedure for recovering the adsorbed normal hydrocarbons from the sieves by extraction with n-pentane. The mechanism of recovery appears to be a diffusion-controlled process, with the rates of desorption varying inversely with the molecular weight.
P
knowledge of the normal hydrocarbon content of wide boiling range hydrocarbon distillates is important to the petroleum industry. This information aids in the evaluation of various processes, such as the hydroRECISE
isomerization of gasolines or the production of kerosines of low freezing point. Molecular Sieve adsorption has found a convenient application for these purposes. Barrer (I,.%’) reported the use of naturally occurring zeolitic minerals for the adsorption of normal paraffins and investigated conditions for adsorption, the size of the molecules adsorbed, and the relative rates of adsorption on chabazite. When synthetic zeolites became available commercially, Nelson, Grimes, and Heinrich (7) and Schwartz and Brasseaux (9) published methods for measuring the normal hydrocarbon content of petroleum distillates. Both VOL. 32, NO. 6, M A Y 1960
701
methods were rapid and accurate but were limited to narrow boiling fractions. This paper describes a technique which permits an accurate determination of the normal hydrocarbon content of petroleum distillates boiling between 100" and 650' F. without prior fractionation of the distillate into narrow boiling cuts. The method also provides for the recovery of both adsorbed normal hydrocarbons and unadsorbed iso- and cyclic hydrocarbons. The standard deviation calculated for the precision and accuracy is lt0.4 and A 0.8%, respectively.
ISLMPLEJ SEPARATION
7
ANALYTICAL CHEMISTRY
SPECTROMETRY TYPE
5A
I S O M O N O O L E F I NS
General Description. A schematic diagram (Figure 1) illustrates the various steps in the analysis of naphthas, gasolines, jet fuels, kerosines, and light gas oils.
702
AROMATICS
MOLECULAR
SPECTROMETRY
EXPERIMENTAL
The hydrocarbon sample is fractionated on a silica gel column into saturate, olefin, and aromatic types. The saturates and olefins are analyzed by mass spectrometry and Molecular Sieve adsorption to determine the information listed. If desired, the adsorbed normal hydrocarbons and the unadsorbed isoand cyclic hydrocarbons may be recovered for further study. Silica Gel Separation. The silica gel-FIA technique (ASTM D 1319) is used for fractionating the sample into aromatics, olefins, and saturates. Several F I A separations are necessary to obtain sufficient material for the hlolecular Sieve adsorption. Therefore, the FIA procedure was modified to permit a charge of 2.5 ml. instead of the usual 0.75 ml. by enlarging the analyzer section of the column. The modified column used consists of a 150-mm. charger section, 12 mm. in inside diameter, a 220-mm. separator section, 5 mm. in inside diameter, and a 1200mm. analyzer section, 3.5 mm. in inside diameter. The fluorescent dye shows the boundaries of the aromatics, olefins, and saturates which are displaced by isopropyl alcohol. I n recovering each zone from the column, the fraction is cut as close to the interface as possible, the interface material being discarded. Molecular Sieve Adsorption. An adsorption column, sample tube, and reservoir are constructed as shown in Figure 2. A plug of glass wool is placed in the circular indentation a t the bottom of the adsorption column and a n Asco-Seal O-ring standard-taper joint (Arthur F. Smith Co., Rochester, K.Y.) on top of the adsorption column to form a vacuum seal. The normal paraffin content of the saturate fraction and the normal olefin content of the olefin fraction are determined by the following procedure. The adsorption column is packed to vr-ithin 1 inch of the top with about 15 grams of powdered Molecular Sieves, Type 5-A, previously dried by heating for 6 hours a t 450" C., 1 to 5 mm. of
0
v SIEVES TYPE 5A
0
OLEFINS
SATURATES
ISOOIOLEFINS
Figure 1.
NORMAL DIOLEFINS ISOOIOLEFINS
Schematic analysis of petroleum distillates
G
B
#-O
.A
SAMPLE
-G .H
TUBE
RESERVOIR ~
-'I - J
A MO'OR D R V M STIRRING RW 8 GROUNO GL4SS SLEEVE C 24/40 I JOINT 3 24/40 I JC,NT, FI'TED WITH WATER COhDENSER E THERMOMETER WELL, 10/3o I 0b-m F 1 9 m I JOINT G 38 MM 3 D TUBING H
I2 M M 0 0 TUBING
I EXTQACTION CY4MBER. Z O O M L CAPACITY MEDIUM, FR!TTED GLASS DISC X FL4SK. 100 H L
J
A 14/35 $ JOINT B DROPPER, I 5 CM LONG C ORWPER, 2 5 CM LONG D 50 MLCAPACITY E 5 M L CARCITY F MEDIUM FRITTED CLASS DISC C. TEFL08 STOPCOCK, 2 5 MM BORE
CAP4ClTY
H TUBE, I5MM 00. J. CIRCULAR INDENTATION. I M M . DEEP
Figure 3. ' apparatus I111 ADSORPTION
Continuous
extraction
u
COLUMN
lTOTAL LENGTH, 30 C M 1
Figure 2. Adsorption apparatus
mercury pressure, and cooled under vacuum. With practice, the column can be completely filled with sieves, quickly sealed, and mechanically vibrated to pack the sieve particles as tight as possible. After packing, the sealed column is allowed to stand a t room temperature for 30 minutes or until the tare weight is constant. The cap is then replaced by the sample tube and 0.5 to 1.0 gram of the saturate or olefin fraction is placed in the funnel. The absorption column is evacuated to
1 to 5 mm. of mercury pressure and the sample tube stopcock opened to allow all but 2 or 3 drops to run into the column. The drip tip on the sample tube permits the sample to be charged directly to the surface of the sieves. The absorption column is then vented by attaching a drying column (15 mm. in outside diameter X 1 meter), packed with Molecular Sieves, Type 5-A, l / inch pellets a t the bottom stopcock. Thirty minutes are allowed for the column to fill with air. The cap is replaced and the column weighed to determine the sample weight. The reservoir is attached to the column and filled with isopentane (99 mole %), previously purified by percolating 500
~
Table I.
Hydrocarbon Group Type Ca lculations
(Molecular Sieves, FIA,. mass spectrometry) Sa X F s S o r m a l paraffins = ~
Isoparaffins =
100
( M e X Fc)
- ( S a XFs) 100
hlonocycloparaffins
= ___ ilrf X F s
100
M g X Fs
Dicycloparaffins
= ___
100
Sormal mono-olefins (max.) Isomono-olefins (min.)
S o X Fo
= ____
100
=
( J I h X F o ) - (SOX Fu)
100 Cyclomono-olefins Mi X Fo Isodiolefins Tormal diolefins = ____ 100 iicetylenes Cyclodiolefins Figure 4. Desorption of paraffins and olefins using continuous extraction apparatus
=
M J X Fo ~
100
Aromatics = Fa Fa = vol. yo aromatics, FIA Fo = vol. % olefinP, FIA Fs = vol. yo saturates, FIA Sub = wt. 7 0 normal paraffins in saturate fraction, sieves Sob = wt. Yo normal olefins in olefin fraction, sieves Me = vol. % ' normal isoparaffins (tctal paraffins), mass Nf = vol. yomonocycloparaffins, mass M g = vol. yodicyclo araffins, mass i ~ = h vol. % normay isomono-olefins (total olefins), mass Mz = vol. % cyclomono-olefins, normal diolefins, isodiolefins, and acetylenes, mass MJ' = vol. % cyclodiolefins, mass
+
+
Silica gel separation by ASTM D 1319. Weight per cent data from Molecular Sieve adsorption are considered equivalent to volume per cent, as the density ratio of adsorbed phase to unadsorbed phase is approximately unity. a
b
-TIME
Figure 5. Gas chromatographic determination of normal paraffins in fraction 1 , light gas oil
ml. through a column, 2.5 em. X 1 meter, packed with powdered Molecular Sieves, Type 5 4 , a t the rate of 100 ml. of eluent per hour. This purification step is necessary ta remove traces of normal hydrocarbon impurity. The rate of isopentane flow into the adsorption column is adjusted t o about 1 drop every 5 seconds and the elution is continued until 10 ml. of eluate has been collected. The time required for the elution of unadsorbed hydrocarbons is about 2 hours. The cap is replaced on the adsorption column and the system is evacuated through the bottom stop-
cock a t approximately 300 mm. of mercury pressure for 20 minutes or until the column appears dry. The column is further evacuated at 1 t o 5 mm. of mercury pressure for 30 minutes to remove the final traces of eluent. The system is vented through the drying column for 30 minutes. The normal paraffin or normal olefin content of the sample is then determined by weighing the adsorption column and calculating the percentage of the sample that has been absorbed on the sieves. The weight per cent of unadsorbed hydrocarbons is determined by calculat-
ing the difference between the adsorbed material and the charge. M a s s Spectrometry Analysis. As shown in Figure 1, t h e saturate fraction is analyzed by mass spectrometry (3) t o determine total paraffins, monocycloparaffins, and dicycloparaffins. The olefin fraction is also analyzed (6) to determine total mono-olefins, cyclomono-olefins, normal diolefins, isodiolefins, and cyclodiolefins. These data may then be incorporated \\-it11 data. from the silica gel separation and Xolecular Sieve adsorption techniques to determine the hydrocarbon types. Calculations. T h e calculations necessary for obtaining the various hydrocarbon types are shown in Table I. The symbols and groups are selfVOL. 32, NO. 6 . MAY 1960
703
Table It.
Adsorption of Hydrocarbon Standards by Molecular Sieves Weight yo Weight % Adsorbed Desorbed Compound Adsorbed Desorbed Compound 0.7 99.3 98.9 n-Pentane 1.1 1-Hexene 99.2 0.8 100.3 0.0 n-Decane 0.5 99.5 trans-3-Hexene 100.2 0.0 99.0 1.0 n-Tridecane 99.6 0.4 cis-3-Hexene 100.0 0.0 99.3 0.7 I-Hexadecene 99.9 0.1 n-Tetradecane 99.7 0.3 100.2 0.0 99.8 0.2 2,3,3-Trimethyl100.6 0.0 0.0 100.4 1-butene n-Eicosane 100.2 0.0 0.8 99.2 Cyclohexene 0.2 99.8 n-Octacosane 99.9 0.1 0.9 99.1 2-Methyldecane 0.4 99.6 0.1 99.9 1,bHexadiene 1.4 98.6 0.6 99.4 ZMeth 1-1,s 0.9 99.1 buta&ene 0 100. 1 0.2 99.8 Benzene 0.9 99.1 1.9 98.1 n-Decylcyclo1.2 98.8 n-Decylbenzene 1.9 98.1 hexane 1.6 98.4
explanatorj.’ The normal mono-olefins determined by Molecular Sieves may also contain normal diolefins; this may lead to a low value for the isomonoolefins when the sieve data are subtracted from the mass analysis. Therefore, the normal mono-olefins are calculated as a maximum and isomonoolefins as a minimum value. If the diolefin content is less than 2 weight %, the error produced is within the experimental error of the method and can be neglected. If the diolefin content is higher than 2%, the unadsorbecl phase should be recovered from the hIolecular Sieve adsorption for subsequent analysis by mass spectrometry, and the normal diolefin content determined by the difference between the two mass spectrometric determinations. Recovery of Normal Hydrocarbons. T h e adsorbed normal hydrocarbons may be recovered from t h e sieves by extracting with n-pentane (99mole %) using the extraction apparatus shown in Figure 3. The sieves are easily transferred into the extraction apparatus by ently tapping the adsorption column. %bout 30 nil. of n-pentane is placed in the flask and 150 ml. in the extraction chamber. Heat is applied and npentane is continuously refluxed throu h the sieves with stirring to increase titi flow rate of the solvent through the system. The progress of the extraction may be checked periodically by exchanging flasks containing about 30 ml. of new solvent and determining the weight per cent hydrocarbon desorbed. The n-pentane is separated from the 704
ANALYTICAL CHEMISTRY
extracted hydrocarbons by distilling off the pentane in a simple distillation apparatus. When the extraction is completed, the weight of extracted hydrocarbons should check with the wei h t of the adsorbed phase determined by holecular Sieve absorption.
OL
4b
do 60 TOTAL HOURS EXTRACTION
sb
I
0
Figure 6. Desorption of light g a s oil normal paraffins from 5-A Molecular Sieves
the cavities of the sieves before adsorption occurs. Cis, trans, and terminal normal monoolefins were adsorbed. Difficulty was anticipated with the cis isomer because the restricted rotation of the methylene groups adjacent to the double bonded Recovery of Iso- and Cyclic Hydrocarbons gives the molecule a larger carbons. T h e unadsorbed hydrocarcritical cross-sectional diameter than the bons are also recovered from the trans isomer. The larger diameter Molecular Sieve adsorption eluate by tends to slow the rate of adsorption; distilling off the isopentane in a simple however, data presented for cis-3-hexene distillation apparatus. If the lowest show that conditions selected for this boiling member is a Cloor higher, the isostudy allowed ample time for complete pentane may be removed by heating the adsorption. sample to 50’ C. in a 25-ml. Erlenmeyer Adsorption studies of diolefins were flask fitted with an air condenser. The beyond the scope of this method because weight of residue should check to a conthey are present only in small amounts stant weight with the weight of unadin most petroleum distillates and besorbed hydrocarbon calculated precause it is not possible t o separate dioleviously. fins quantitatively from mono-olefins and aromatics on a silica gel column. DISCUSSION However, to ensure that normal diolefins are adsorbed and iso- and cyclodiolefins are unadsorbed, two diolefins typifying Molecular Sieve Adsorption Studthese classes were used as standards. ies. Quantitative d a t a were obtained 1,5-Hexadiene is quantitatively adfor a number of pure hydrocarbon sorbed, while 2-methyl-1,3-butadiene is standards by the method outlined not adsorbed. above. These data (Table 11) show that Three synthetic blends were prepared normal hydrocarbons, C5 through C Z ~ , and analyzed to test the accuracy and are quantitatively adsorbed, while isoreproducibility of the Molecular Sieve and cyclic hydrocarbons are unadsorbed. adsorption (Table 111). The first synData are presented which show that thetic blend was composed of approxicyclic hydrocarbons possessing long mately equal amounts of normal parafnormal alkyl substitution (n-decylfins to make up the adsorbed phase, and cyclohexane and n-decylbenzene) and equal amounts of iso- and cycloparafnormal hydrocarbons with terminal fins for the unadsorbed phase. The branching (Zmethyldecane) are unadcarbon number distribution of the blend sorbed. Apparently, the entire moleranged from Csto Clr, which simulates cule must enter the apertures guarding
the range of a saturate fraction from a heavy gasoline. Duplicate determinations of normal paraffins were in good agreement with the blended values. Synthetic blend 2 consisted of normal paraffins, normal olefins, and various isoand cyclic hydrocarbons. The analysis of the blend shows that aromatics and olefins do not interfere with the adsorption of normal hydrocarbons and that a hydrocarbon sample boiling between 100' and 650' F. (Ca to C20) may be accurately analyzed. These data further illustrate t h a t only two groups of information may be obtained from Molecular Sieve adsorption-via., the adsorbed normal hydrocarbon and the unadsorbed iso- and cyclic hydrocarbon content. The final synthetic blend was composed of a normal paraffin, cycloparaffin, normal mono-olefin, isomono-olefin, and an aromatic. This blend illustrates how it is possible to obtain additional information by combining a silica gel separation with the Molecular Sieve adsorption. By combining these techniques, it is possible to determine separately the normal paraffin and normal olefin contents, and the unadsorbed paraffins, and olefins. The aromatic concentration is determined by the FIAsilica gel technique. The examination of three petroleum distillates, following the scheme suggested in Figure 1, shows the information that may be obtained by using a combination of silica gel fractionation; Molecular Sieve adsorption, and mass spectrometry (Table IV). The molecular weight distribution of the samples varied from Ca to Ce for the light gasoline, Ce to C1l for the heavy gasoline, and Clz to C23 for the light gas oil. Each distillate was fractionated on silica gel. These data show that petroleum distillates up to a t least a light gas oil may be accurately analyzed for normal hydrocarbons using Molecular Sieves and that additional information may be obtained using silica gel separations and mass spectrometry. It is expected that the method can be applied to material of higher molecular weight.
~~
Analysis of Synthetic Blends
Table 111.
Weight, % Blended
2,4,4-Trimethyl entane 2,2,5-Trimethylgexane C clohexane d t h y l c yclohexane Ethylcyclo hexane n-Prop lcyclohexane n-But yrcyclohexane n-Decylcyclohexane
Detd.
30.4
30.7
30.1
69.6
69.3
69.9
100.0
100.0
100.0
46.4
47.1
46.0
63.6
52.9
54.0
100.0
100.0
100.0
>
Volume, % Blended Detd. Blend 3 n-Tridecane n-Butylc yclohexane 1-Octene 2-Methylpentene Xylene
20.0 20.0 20.0 20.0
apparatus shown in Figure 3. The adsorbed hydrocarbon, after diffusing into the solvent, flows through the sinteredglass disk and is concentrated in the flask. I n this way fresh solvent is continuously in contact with the sieves and carrying the desorption to completion. The desorption of two paraffins and two mono-olefins is shown in Figure 4.
Table IV.
20.4 19.5 20.3 19.8
20.0 100.0
Total
20.0 100.0
The plot shows that tridecane was recovered in about 6 hours, while nearly 30 hours were necessary to desorb eicosane. These results are analogous to rates of adsorption, which increase with increasing molecular weight (1). The 1-nonene was completely extracted after 8 hours, while only 85% of the 1-hexadecene was recovered after 27 hours. The extraction was terminated,
Analysis of Petroleum Distillates
Found, Volume 70 Heav) gasoline Boiling Range, O F. 106-300 282-410 3.9 3.9 3.6 3.3 21.7 21.7 12.9 13.2 70.4 .2 07.2 .4
Desorption of Normal Hydrocarbons. T h e authors have reported (8) t h a t adsorbed normal paraffins (chain length from Ce to Clo) can be desorbed from a column of Molecular Sieves, Type 5 4 , by elution with n-pentane. Apparently, the pentane enters the cavities of the sieves and forms a continuous phase with the external area; thus, it permits the adsorbed hydrocarbons to diffuse continuously into the external solution until an equilibrium concentration of desorbed hydrocarbon is obtained. Because diffusion equilibrium seems to govern the desorption of hydrocarbons, the desorption process w carried out in the continuous extraction
~
Light gasoline
Normal paraffins Isoparaffins Monocycloparaffiris Dicycloparaffins
;:;
Normal mono-olefins (max.) Isomono-olefins (min.) Cyclomono-olefins Normal diolefins Isodiolefhs Cyclodiolefins Triolefins Aromatics
Total
Ligbt
gas
oil
485-620 9.7 9.6 11.4 11.5 10.0 10.0
14.5 31.4 13.8
14.4 31.5 13.8
3.4 15.1 10.9
15.5 10.9
3.0
0.4 1.7 3.3
0.4 1.7 3.3
0.7
0.7
0.6
0.6
1.9
1.9
6.4 4.62 6.4 46.2 61.6 61.6 100.0 100.0 100.0 100.0 100.0 100.0
VOL. 32, NO. 6, MAY 1960
705
because it would have taken a t least 50 hours t o recover all of the C I S olefin. These data show that olefins are more difficult t o recover than paraffins of equal molecular weight, and that hydrocarbons of lower molecular weight are more easily desorbed than higher weight material. Thus, when petroleum distillates are processed, it is necessary to desorb the adsorbed phase completely to obtain a fraction representative of the normal hydrocarbons in the original sample. dpproximately 0.6 gram of norinal paraffins was adsorbed from thc saturate fraction of a light gas oil (C12 to '223). The Molecular Sieves were then charged to the estraction apparatus and the straight-chain paraffins recovered with n-pentane. Yearly 100 hours of coiitinuous extraction were required for 100% recovery of the paraffins. To determine how the different components of the light gas oil were desorbed, fractions were taken during the extraction and each cut was eyamined by gas chromatography for the carbon number distribution. The chromatographic column was 20 feet long and packed with 35- to 60-mesh conimercial detergent. The column vias operated a t 230" C. with liclium as the currier gas. The pressure of the carrier pas was adjusted to give a flow rate of about 50 cc. per minute. A typical chromatogram for this separation (Figure 5) shows how normal paraffins a t each molecular u-eight are clearly distinguished. The areas under each of the peaks represented by the gas chroinatographic separation were calculated in terms of the cumulative per cent of c : d i paraffin recovered according to ciirbon number per unit of time. Some of these data presented graphically in Figure 6 illustrate the general pattern of desorption for a light gas oil. Initially, lower molecular weight material tended to diffuse out of the sieves more rapidly than higher weight material. But as the extraction progressed, the lower treight material tailed off and the higher carbon number material became dominant. The curves presented show that the rate of desorption is a function of chain length. Figure 4 shows that eicosane was desorbed in about 30 hours. However, the Czohydrocarbon from the light gas oil (Figure 6) was not
706
ANALYTICAL CHEMISTRY
coinpletely recovered until the total sample had been desorbed. Apparently, the total time required to desorb a complex mixture cannot be accurately predicted from the desorption rates of pure components because of the competition of the various molecules to diffuse into the solvent.
Table V. Isomerization of Monoolefins with Molecular Sieves, Type 5 - A
Olefiti Type
R;C=CR,
Heptene Charge Stock, Ultraviolet Spectrosco y -inalpis, l\loPe % Before -4fter 27.9
0.0
23.3
26.3
30.9 17.9
34.0 39.7
Isomerization of Olefins. Cheniically, Molecular Sieves, Type &A, are calcium aluminosilicates. Because silicates are known to isomerize olefins (d), the first two fractions from the extraction of 1-hexadecene (Figure 4) were blended and analyzed by mass and infrared spectrometrj.. Mass spectrometry showed no evidence for polymerization or catalj.tic cracking. Infrared, holyever, showed that isomerization of the terminal olefin had occurred and that the terminal double bond had shifted to a n internal position. d p proximately 65 mole % isomerized to a trans isomer, 15% to a cis isomer, and 20y0 did not isomerize. Because internal olefins are more stable than terminal olefins, these data indicate that the sieves catalyze the isomerization of the terminal olefin to a molecular configuration of greater stability. It was not possible to determine whether the cis or trans isomer was the more stable compound, because the desorption rates of the two isomers differ and tlie olefin was not completely recovered from the sieves. Thus, the fact that more trans isomer was present in fraction 1 does not indicate that it is more stable. A heptene distillate nhich consisted entirely of isomono-olefins was charged
to a column oi llolecular Sieves and eluted with isopentane. -4lthough the isomono-olefins were not adsorbed, they were in contact n-ith tlie surface of the sieves for 2 hours while they irere being eluted from the column. The desorbed heptenes were examined by ultraviolet spectroscopy, using the charge transfer spectrum of the pi complexes formed between olefins and iodine (6). The data presented in Table T' show thnt terminal isoniono-olefins had isomerized mostly to tetrasubstituted olefins, indicating that the double bond had shifted to a position adjacent to an alkyl substitution. Olefins n-ith double bonds adjacent t o alkyl groups are usually more stable than olefins with terminal unsaturation. The isomerization of normal monoolefins and isomono-olefins by Molecular Sieves makes it impractical to recover olefins for molecular type studies, because the structure of the olefin is changed. 13ut this does not prevent a determination of normal olefins and iso-olefins by Molecular Sieves, as no evidence was obtained to indicate that normal mono-olefins are converted to isomono-olefins. ACKNOWLEDGMENT
The authors are grateful to N. D. Coggeshall and R. E. Snyder for many helpful suggestions, to P. C. Talarico for performing some of tlie experiments, to D. H. Lichtenfcls anti F. H, Buron. for gas chromatographic data. mid to G. F. Crable and G. L. Kearns for maw spectrometric data. LITERATURE CITED
Barrer, R. If., Ibbiteon, D. A,, Trans. Faraday SOC. 40, 195-2Oti
(1)
11944'1. (2) I b i d : , pp. 206-16. (3) Crable, G. F., Coggeshall, S . D., ANAL.CHEX 30, 310-13 (1958).
(4) Dunning, H. Pi., Znd. Eng. Chem.
45, 551-64 (1553). (5) Long, D. R., Seuzil, R. W., ASAL. CHEJI. 27, 1110-14 (1955). (6) Lumpkin, H. E., Ibid., 30, 321-5 (19:s). (7) helson I
