Anal. Chem. 1987, 59, 2207-2209 2207

CORRESPONDENCE

Retention Properties of a Palladium Chloride/Silica Sorbent for the Liquid Chromatographic Separation of Polycyclic Aromatic Sulfur Heterocycles

Sir: The group isolation of polycyclic aromatic sulfur heterocycles (PASHs) from complex samples such as various petroleum products is a highly desirable goal. None of the procedures suggested in the literature achieves this satisfac- torily. The most recent method developed involves ligand- exchange chromatography on silica containing 5 % palladium chloride deposited on its surface ( I ) . It has been used for isolating the PASHs from the aromatic fraction of a sol- vent-refined coal heavy distillate and a catalytically cracked petroleum bottom ( I ) , a coaT tar and a coal liquid vacuum residue (2), and a catalytically cracked petroleum vacuum residue (3).

The procedure calls for depositing 5% PdC12 on silica gel which afterward is activated at 200 "C. The solution to be chromatographed is the preisolated aromatic fraction of the whole sample. Five grams of the gel is used in an open column with chloroform/hexane (1/1) as eluent. The first 30 mL defines the first fraction containing the polycyclic aromatic hydrocarbons (PAHs), and the next 50 mL collected yields the second fraction containing only PASHs.

Problems reported ( I ) for this chromatographic procedure are as follows: (1) in order to destroy the PdCl,/PASH complexes, which is the form in which the sulfur heterocycles elute from the column, diethylamine has to be added. If this treatment is left out and if hydrogen is used as carrier gas, some compounds are desulfurized in the gas chromatographic injection port on subsequent GC analysis; (2) benzothiophenes partly elute in the PAH fraction; (3) compounds with a ter- minal thiophene ring show low recovery.

In this study those problems were investigated and im- provements of the published procedure worked out. The problems referred to above will be discussed as (point 1) complex destruction, (2) benzothiophene elution, and (3) terminal PASHs, respectively.

EXPERIMENTAL SECTION Silica gel (40-63 fim, Merck) was impregnated with 5% pal-

ladium chloride as described in the original procedure ( I ) . The (aminopropy1)silica column was 300 X 8 mm and packed with 10-pm LiChrosorb NH2.

The gas chromatographic analyses were performed on a PU 4500 instrument equipped with a flame-ionization or a flame- photometric detector. A DB-5 column (30 m X 0.25 mm, 0.25 pm fdm thickness) was used with hydrogen as carrier gas at a linear velocity of about 60 cm/s (at 80 "C). The temperature program was 80 OC/2 min, ramp at 4 "C/min to 270 "C.

Hexane and chloroform were pro analysi quality (Merck). A Knauer liquid chromatographic pump, used at a flow rate of 2.5 mL/min, was combined with an Alltech Associates UV detector operated at 254 nm. Methylene chloride was used to define the dead volume.

The shale oil used comes from Pertisau, Austria, and is in- dustrially obtained by heating the oil shale to about 400 O C .

RESULTS AND DISCUSSION Point 1. Complex Destruction. If a sample eluted from

the PdCl,/silica column is analyzed by GC directly, com- pounds containing a terminal thiophene ring are desulfurized

Table I. Recovery Ratios of Five Pairs of PASH/PAH after Chromatography on 5 % PdCl,/Silica with 25% Chloroform in Hexane as Eluent

peak area ratios (PAR) of PASHIPAH

PAR,,,,/ compounds PAR&art PARehrorn PARchrorn

benzothiophene/naphthalene 1.25 1.22 0.98 naphtho[l,2-b]thiophene/fluor- 1.28 1.25 0.98

4-methyldibenzothiophene/ 1- 1.44 1.49 1.03

benzo[b]naphtho[2,1-d]- 2.60 2.75 1.06

ene

methylphenanthrene

thiophene/chrysene

chrysene phenanthro[ 3,441 thiophene/ 2.75 2.74 1.00

in the GC injection port through the catalytic activity of coeluted PdCl, and the hydrogen carrier gas. For instance, 80% of naphtho[2,3-b]thiophene appeared in the GC trace as 2-ethylnaphthalene (I). The eluted PASH/PdC12 com- plexes could be destroyed through addition of 250 pL of di- ethylamine after reduction of the volume of the PASH fraction to about 500 pL.

If the nitrogen atom in diethylamine coordinates strongly enough with the palladium ion to destroy the PASH/PdC12 complex, the primary amino group in aminopropyl-bonded silica should be able to serve the same purpose. This indeed proved to be true. Either a short column filled with an (am- inopropy1)silica gel coupled in series with a liquid chroma- tographic PdCl,/silica column or a small amount of this material at the exit end of such a PdC12/silica column retained all the PdC1,. All eluting solutions were colorless when chromatographed in this way in contrast to the case when the (aminopropy1)silica was left out; the eluting fractions were then brownish in color. Fractions collected can be analyzed directly by GC without any further treatment.

In order to demonstrate that PASHs can be as easily chromatographed as PAHs on the PdC12/silica phase (with (aminopropy1)silica as the decomplexation agent) without specific losses of any sulfur heterocycles, several pairs of structurally related PASHs/PAHs were chromatographed on a 12-cm column filled with 5% PdC12/silica and some (ami- nopropy1)silica and eluted with 25% chloroform in hexane. Only one fraction was collected and the peak area ratio of PASH/PAH was determined by GC before and after the liquid chromatographic step. The data are presented in Table I as ratios of the peak areas of the two compounds before chromatography, (PASH/PAH),m, and after chromatography, (PASH/PAH)ch,,,. If there are no selective losses of one compound, the ratio of those two ratios should equal 1. The third column of the table shows that this ratio lies between 0.98 and 1.06 for the five compound pairs, which means that PASHs can be chromatographed as quantitatively as PAHs on PdCl,/silica with (aminopropy1)silica retaining the palla- dium chloride.

0003-2700/87/0359-2207$01.50/0 0 1987 American Chemical Society

2208 ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1 , 1987

Table 11. k'Values for Some Aromatic Compounds on 57~ PdClJSilica with Hexane as Eluent

compound

naphthalene 1-methylnaphthalene 2,3-dimethylnaphthalene fluorene phenanthrene 1-methylphenanthrene anthracene chrysene benzothiophene 2-methylbenzothiophene 3-methylbenzothiophene 5-methylbenzothiophene 7-methylbenzothiophene 2,7-dimethylbenzothiophene naphtho[ 1,2-b]thiophene naphtho[ 2 ,141 thiophene dibenzothiophene 2-methyldibenzothiophene phenanthro[3,4-b] thiophene ~henanthrol2.1-bl thioDhene

k'

1.1 1.1 1.4 2.1 2.1 2.3 2.2 4.6 2.1 5.5

15.7 5.3 2.9 7.: 2.0 2.6

37.3 56.3 4.7 6.3

Point 2. Benzothiophene Elution. In order to study the retention behavior of various aromatics, their capacity factors k'were determined. Instead of using an open column with 5 g of sorbent (I), a 12-cm liquid chromatographic column was filled with the PdClzsilica (about 1 g was needed) and a small amount (about 0.1 g) of aminopropyl-bonded silica at the exit end. Thus the effluents could be monitored with a conven- tional UV detector for the presence of solutes. Hexane was used as eluent instead of chloroform/ hexane (1/ l), allowing the weak interactions between the sorbent and benzo- thiophenes to manifest themselves to the largest extent possible. The capacity factors measured are listed in Table 11. Reproducibility of retention times was poorer than usual in LC; therefore the k'values are given to one decimal place only.

As expected, benzothiophene is more strongly retained than naphthalene and has about the same capacity factor as fluorene and phenanthrene. A surprising behavior is shown by alkylated benzothiophenes. Their retention times are longer than those of the parent systems, sometimes dramat- ically so, whereas the effect for alkylated PAHs seems to be much smaller. Several monomethylbenzothiophenes are more strongly retained than chrysene. The effect resembles that found on chromatography on a mercuric acetate substituted phenylsilica phase where the retention volume was found to increase with the number of methyl groups. However, in that case the effect was as pronounced for alkylated naphthalenes as for alkylated benzothiophenes and considerably weaker than for the present sorbent ( 4 ) .

In Figure Ib, the separation of benzothiophene and its 3-methyl and 2,3-dimethyl derivatives can be compared with that of phenanthrene and benzonaphtho[ 1,2-d]thiophene in Figure la. The latter two compounds are those which were used to define the PAH and the PASH fractions, respectively, in the original procedure (1) The chromatograms in the figure demonstrate that if fractiwls are cut as defined by the com- pounds in Figure la, beinzothiophene would coelute with phenanthrene and appear in the PAH fraction while 2,3-di- methylbenzothiophene would coelute with the benzo- naphthothiophene and would thus belong to the PASH fraction. 3-Methylbenzothiophene occupies a middle position and would be collected in both fractions. From this it is clear that many alkylated benzothiophenes elute late enough to be collected together with the PASHs.

Point 3. Terminal PASHs. Another unexpected finding was that those PASHs, apart from benzothiophene, which

b

-7 I ' i ' B I I , , , I I 12 m n I2 ' 16

Flgure 1. Liquid chromatography of h e standard aromatic compounds on a 50 X 4.6 mm column packed with 5% PdCl,lsilica plus a small amount of (aminopropy1)silica. Eluent was 1 % chloroform in hexane at 2.5 mL/min.

contain a terminal thiophene ring (fused on one side only) elute considerably faster than those PASHs whose heterocyclic ring is in an internal position (fused on two or three sides); see Table 11. In fact, contrary to most normal-phase chro- matographic situations, the number of unsaturated carbon atoms is not correlated with the capacity factor. The two naphthothiophenes elute with benzothiophene and consid- erably faster than dibenzothiophene. This is true also for the phenanthrothiophenes which, despite their 16 unsaturated carbon atoms, are about as weakly retained as the methyl- benzothiophenes with half as many unsaturated carbon atoms.

This might explain two results found for samples chro- matographed on PdClz/silica ( I ) . In the first place, recoveries for PASHs with terminal thiophene rings were found to be low and it was suggested that such compounds react with the adsorbent during fractionation. The present results indicate that those compounds might very well have been present in the first fraction which was assumed to contain only PAHs and benzothiophenes. Furthermore, in a catalytically cracked vacuum residue no PASHs with a terminal thiophene ring were found (3). A reasonable explanation is desulfurization in the catalytic cracking process as suggested; nevertheless, the possibility that those compounds appeared in the PAH fraction should be overlooked. This might be further indicated by the fact that the PAH fraction contained 1.4% sulfur. This figure is not expected to come from such benzothiophenes which coelute with the PAHs since benzothiophenes are rather reactive compounds and probably would not survive the catalytic cracking used to produce this sample. Instead the sulfur in the PAH fraction might come from higher PASHs which elute in this fraction.

All of the points discussed above were brought together in the analysis of a shale oil. The preisolated aromatic fraction was separated into two fractions on a 300 X 8 mm colum filled with 5 % PdCl,/slica and some (aminopropy1)silica. Hexane was used as eluent for the first fraction and 20% chloroform in hexane for the second. On subsequent GC analysis the PASHs were visualized through use of a flame-photometric detector in the sulfur mode. Figure 2a shows the first fraction

Anal. Chem. 1987, 59, 2209-2211 2209

5

I

2 3

4 6

I

Flgure 2. Gas chromatograms of a liquid chromatographic fractiiation of the aromatlc portlon of a shale oil: (a) LC column: 300 X 8 mm 5% pdcl,/silica, first fraction, eluent hexane; (b) three-ring fraction of the sample shown in (a) obtained on an (aminopropyl)sili column with hexane as eluent; (c) as in (a) but the second fraction, eluent 20% chloroform in hexane; (d) two-ring fraction from the sample shown in (c) obtained on an (aminopropy1)silica column wlth hexane as eluent. For GC conditions, sea Experimental Section. Compound identification is as follows: 1, benzothiophene (BT); 2, Cl-BT; 3, C,-BT; 4, C,-BT; 5, C,-BT; 6, dibenzothiophene (DBT); 7, C1-DBT; 8, C,-DBT; 9, na- phtho[ 1,241thiophene; 10, naphtho[2,1-b]thiophene; 11, naphtho- [2,3-b ]thiophene.

which obviously contains a large number of PASHs. In order to distinguish between benzothiophenes and such higher PASHs, which according to the previous results ought to come in this fraction owing to the fact that they contain a terminal thiophene ring, a second liquid chromatographic step was performed on (aminopropy1)silica with hexane as eluent. This separates aromatics according to the number of unsaturated carbon atoms. The fraction collected for the three-ring PASHs gave the chromatogram reproduced in Figure 2b. Very prominent peaks are seen for naphtho[ 1,241thiophene and naphtho[2,14]thiophene; a minor one coincides with na- phtho[2,3-b]thiophene. The other peaks are expected mainly

to result from alkylated naphthothiophenes. Those results on a real-world sample thus support the findings in point 3 above.

The second fraction from the PdCl,/silica chromatography showed the chromatogram depicted in Figure 2c. Several peaks are visible a t shorter retention times than dibenzo- thiophene and might thus represent substituted benzo- thiophenes. The same separation on (aminopropy1)silica was performed as above and the gas chromatogram from the two-ring fraction is illustrated in Figure 2d. All the peaks here should be produced by benzothiophenes. This demonstrates the results in point 2 above, namely, that not all benzo- thiophenes are lost with the PAHs but coelute with the higher PASHs.

This experiment on a complex sample thus well reveals several properties of the PdCl,/silica column. It can obviously be used for efficient separation only of PAHs and such PASHs which contain internal thiophene rings or carry several alkyl groups. PASHs with a terminal thiophene ring together with a low degree of alkylation often elute with the PAHs.

ACKNOWLEDGMENT The shale oil was generously provided by the Tiroler

Steinoelwerke, Pertisau, Austria. K. Ballschmiter is thanked for his continued interest in this work.

Registry No. PdCl,, 7647-10-1; SiO,, 7631-86-9; naphthalene, 91-20-3; 1-methylnaphthalene, 90-12-0; 2,3-dimethylnaphthalene, 581-40-8; fluorene, 86-73-7; phenanthrene, 85-01-8; l-methyl- phenanthrene, 832-69-9; anthracene, 120-12-7; chrysene, 218-01-9; benzothiophene, 95-15-8; 2-methylbenzothiophene, 1195-14-8; 3-methylbenzothiophene, 1455-18-1; 5-methylbenzothiophene, 14315-14-1; 7-methylbenzothiophene, 14315-15-2; 2,7-dimethyl- benzothiophene, 16587-40-9; naphtho[ 1,2-b]thiophene, 234-41-3; dibenzothiophene, 132-65-0; phenanthro[3,4-b]thiophene, 195-52-8; naphtho[2,l-b]thiophene, 233-02-3; 2-methyldibenzothiophene, 20928-02-3; phenanthro[2,14]thiophene, 219-25-0; 4-methyldi- benzothiophene, 7372-88-5; benzo[b]naphtho[2,1-d]thiophene, 239-35-0; naphtho[2,3-b]thiophene, 268-77-9.

LITERATURE CITED (1) Nishioka, M.; Campbell, R. M.; Lee, M. L.; Castle, R. N. Fuel 1988, 65,

270-273. (2) Nlshloka, M.; Lee, M. L.; Castle, R. N. Fuel 1986, 65, 390-398. (3) Nishioka, M.; Whiting, 0. G.; Campbell, R. M.; Lee, M. L. Anal. Chern.

1886, 58, 2251-2255. (4) Andersson, J. T. Fresenlus' 2. Anal. Chern. 1987, 326, 425-433.

Jan T. Andersson Department of Analytical Chemistry University of Ulm D-7900 Ulm, Federal Republic of Germany

RECEIVED for review January 27,1987. Accepted May 11,1987. This work was financially supported by the Deutsche For- schungsgemeinschaft.

Self- Induced Chiral Recognition in the Association of Enantiomeric Mixtures on Silica Gel Chromatography

Sir: Achiral chromatography with enantiomerically en- riched mixtures of chiral solutes can provide two split zones differing in enantiomer excess when the enantiomers undergo diastereomeric solute-solute interactions to different extents. Enantiomeric mixtures of N-acetylvaline tert-butyl ester (1) were found to generate the above autoseparation in nona- queous silica gel chromatography. This was observed when

injecting 14C-radiolabeled racemic 1, diluted with unlabeled (S)-1, into a column and detecting the radioactivity. Several investigators have observed the autoseparation of enantiomers but not in all cases have they succeeded in defining the dia- stereomeric association of chiral solutes responsible for this phenomenon (1-3). An explanation for the origin of enan- tiomer recognition has yet to be found. This paper reports

0003-2700/87/0359-2209$01.50/0 0 1987 American Chemical Society