Silica Gel and Ion Exchange Chromatography Separation of Polar Fractions

Background

The separation and study of polar compounds in crude oil and bitumens is of considerable importance in the fields of petroleum recovery and processing. Numerous methods for separation of polar compounds

Figure 9 Construction of a coated-rod TLC scanner, latroscan TH-10 Mk III. (Modified from Mukherjee, 1991.)

from crude oil and bitumens have been developed. Among these, the most widely used is the method developed by the Bureau of Mines and American Petroleum Institute. The acid and base fractions of heavy petroleum distillates or residues are isolated by ion exchange chromatography, the neutral nitrogen compounds by complexation chromatography on ferric chloride and saturates, and mono-, di- and polyaromatics by adsorption chromatography on activated alumina or on a combined alumina-silica gel column. The definition of acids or bases by ion exchange methods is based on the hydrogen donating or accepting tendency of the molecules. In as much as many polar compounds are amphoteric, their definition as acids or bases depends on the analytical sequence employed. Although this method is rather complex and tedious, it is used to separate the polar fractions from crude oil and bitumens.

Separation Procedure

There are two stages in the separation: firstly, silica gel chromatography and secondly, ion exchange chromatography. The first stage separates the crude oil or bitumen into four fractions (I-IV: Figure 10). The polar fraction obtained from the first stage is further split into subfractions by the second-stage chromatography.

Silica gel chromatography Fractionation of crude oil or bitumen is carried out by solvents with increasing polarity through a column using Baker reagent grade, 60-200 mesh silica gel. The silica gel is thermally activated before use. The sample (10-20 g) is dissolved in 200 mL tetrahydrofuran (THF) and then filtered. The volumetric ratio of the sample to the adsorbent is about 1:35. The columns are eluted with «-hexane, toluene, 4:1 (v/v) toluene-methanol and 2:1 (v/v) toluene-methanol, to obtain fractions I, II, III and IV, respectively. Figure 10 shows the overall separation scheme. All samples must be evaporated to a constant weight by (rotary) evaporation and using a vacuum oven. Recovery of fraction IV is generally lower than 5% and, therefore, this fraction has not been considered for further study. The most polar

Figure 10 Solvent fractionation scheme using a silica gel column for crude oil and shale oil.
Figure 11 Separation of fraction III (see Figure 10) to subfractions by ion exchange chromatography: A, acid; B, base; and N, neutral.

groups are present in fraction III because of the high interfacial tension compared with those for fractions I and II. Fraction III is further fractionated by ion exchange chromatography.

Ion exchange chromatography The polar material (fraction III) obtained using the silica gel column is mixed with cyclohexane. There are three further stages of separation: anion exchange, cation exchange and clay-FeCl3 complexation. In order to obtain acidic fractions from the polar material, a strongly basic anion exchange resin is used and an eluting scheme is employed, based on increasing polarity of the solvent. This scheme includes four elution steps: cyclohexane; toluene; a 3:2 mixture of toluene-meth-anol; and a 3:2 mixture of toluene-methanol saturated with CO2. The first fraction eluted by cyclo-hexane from the anion exchange resin is further fractionated by passing it through a strongly acidic cation exchange resin. The same elution scheme as in the first elution step is used, with a slight modification for the cation exchange elution in that the fourth step uses a 5.4: 3.6:1.0 mixture of toluene-methanol-isopropylamine. Three basic fractions result from these fractionations.

A column packed with clay-FeCl3 is employed to separate further the material obtained from the cyclohexane elution in the second stage. The solvents used to obtain the neutral fractions are cyclohexane and dichloromethane. The volumetric ratio of sample to adsorbent is around 1:10 for the three adsorbents used. Two neutral fractions can be obtained at this last stage of separation process. A detailed diagram of the scheme is given in Figure 11.

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