Alumina Adsorbents

Alumina adsorbents in capillary columns were introduced as long ago as 1963 and were made commercially available in fused silica capillary columns in 1981. Alumina has a very high activity and will retain components as light as ethane. To make the highly active alumina work as stationary phase in GC, it has to be deactivated. Deactivation can be done in many ways, but the most practical and reproducible method is deactivation with inorganic salts.

Alumina deactivated with potassium chloride (KCl) provides a general nonpolar adsorbent that is widely used. Such a column will elute acetylene before butane (Figure 3). The alumina surface can be made more polar by deactivation with sodium sulfate instead of KCl. The resulting alumina layer will elute acetylene after the butane peaks, indicating the higher polarity.

The selectivity of alumina for hydrocarbons is very high. All C C4 hydrocarbons can be baseline separated. The resolution between the different hydrocarbons is sufficient to be able to measure many trace amounts of Q-C4 hydrocarbons in a main stream of any one of these hydrocarbons (Figures 4-6). For this reason alumina is one of the most widely used columns in petrochemistry for analysing hydrocarbon impurities in ethylene, propylene (Figure 5) and 1,3-butadience (Figure 6).

Although alumina has unique separation characteristics, it also has limitations. The activity of the adsorbent is such that it will adsorb any moisture, carbon dioxide or other polar impurity in the sample. When

Figure 3 C,-C6 hydrocarbons on an Al2O3, KCl PLOT column. Experimental details: column, 50 mx 0.32 mm fused silica, Al2O3/KCl, df = 5 |im; oven, 70°C (4 min) p 180°C, 10°Cmin~1; carrier gas, helium; injection, split; detection, flame ionization detection (FID). Peaks: 1, methane; 2, ethane; 3, ethylene; 4, propane; 5, cyclopropane; 6, propylene; 7, acetylene; 8, isobutane, 9, propadiene; 10, butane; 11, trans-2-butene; 12, 1-butene; 13, isobutene; 14, c/s-2-butene; 15, isopentane; 16, methylacetylene; 17, pentane; 18, 1,3-butadiene; 19, ethylacety-lene; 20, hexane.

Figure 3 C,-C6 hydrocarbons on an Al2O3, KCl PLOT column. Experimental details: column, 50 mx 0.32 mm fused silica, Al2O3/KCl, df = 5 |im; oven, 70°C (4 min) p 180°C, 10°Cmin~1; carrier gas, helium; injection, split; detection, flame ionization detection (FID). Peaks: 1, methane; 2, ethane; 3, ethylene; 4, propane; 5, cyclopropane; 6, propylene; 7, acetylene; 8, isobutane, 9, propadiene; 10, butane; 11, trans-2-butene; 12, 1-butene; 13, isobutene; 14, c/s-2-butene; 15, isopentane; 16, methylacetylene; 17, pentane; 18, 1,3-butadiene; 19, ethylacety-lene; 20, hexane.

Figure 4 Impurities in trans-2-butene on an Al2O3/Na2SO4 adsorbent PLOT column. Experimental details: column, 50 mx 0.32 mm fused silica, Al2O3/Na2SO4, df = 5 |im; oven, 110°C; carrier gas, nitrogen; injection, split; detection, FID. Peaks: 1, methane; 2, ethane; 3, ethylene; 4, propane; 5, propylene; 6, isobutane; 7, butane; 8, cyclobutane; 9, trans-2-butene; 10,1-butene; 11, isobutene; 12, cis-2-butene; 13, 1,3-butadiene.

Figure 5 Impurities in propylene on an Al2O3/KCl PLOT column. Experimental details: column, 50 mx 0.53 mm fused silica, Al2O3/KCl df=10 |im; oven, 40°C (10 min) p 160°C, 5°Cmin~1; carrier gas, helium; injection, split; detection, FID. Peaks: 1, methane; 2, ethane; 3, ethylene; 4, propane; 5, propy-lene; 6, isobutane; 7, acetylene; 8, butane; 9, propadiene; 10, trans-2-butene; 11,1-butene; 12, isobutene; 13, cis-2-butene; 14, isopentane; 15, methylacetylene; 16, pentane; 17,1,3-butadiene.

Figure 4 Impurities in trans-2-butene on an Al2O3/Na2SO4 adsorbent PLOT column. Experimental details: column, 50 mx 0.32 mm fused silica, Al2O3/Na2SO4, df = 5 |im; oven, 110°C; carrier gas, nitrogen; injection, split; detection, FID. Peaks: 1, methane; 2, ethane; 3, ethylene; 4, propane; 5, propylene; 6, isobutane; 7, butane; 8, cyclobutane; 9, trans-2-butene; 10,1-butene; 11, isobutene; 12, cis-2-butene; 13, 1,3-butadiene.

alumina is exposed to water, the retention times for hydrocarbons are altered and resolution is degraded. The water will deactivate the surface and, as a consequence, the column will have lower retention. The water can simply be removed and the column performance regenerated by heating to 200°C for a short time. If an isothermal setup is required, a polar pre-column that retains the water can be employed. A polyethylene glycol-coated column with a 1.2 |im film works very well as the C1-C6 hydrocarbons will elute from this column before the water and by simple backflushing or a vent switch the water peak can be removed. For more polar impurities, regeneration may take longer, but it is possible. Alumina columns are difficult to destroy and oxygen in the carrier gas will not harm the column. It has been found that sulfur impurities present up to 2000 ppm do not seriously interfere with the retention characteristics of alumina. The maximum temperature of alumina columns should not exceed 200°C; above this temperature recrystallization of the deactivating compound will occur, causing changes in selectivity. Alumina layers coated in Ultimetal tubing are very stable and find wide application in hostile environments.

Figure 5 Impurities in propylene on an Al2O3/KCl PLOT column. Experimental details: column, 50 mx 0.53 mm fused silica, Al2O3/KCl df=10 |im; oven, 40°C (10 min) p 160°C, 5°Cmin~1; carrier gas, helium; injection, split; detection, FID. Peaks: 1, methane; 2, ethane; 3, ethylene; 4, propane; 5, propy-lene; 6, isobutane; 7, acetylene; 8, butane; 9, propadiene; 10, trans-2-butene; 11,1-butene; 12, isobutene; 13, cis-2-butene; 14, isopentane; 15, methylacetylene; 16, pentane; 17,1,3-butadiene.

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