Quantitative Analysis of Aromatic Hydrocarbons in the 2- to 25-Micron Infrared Region JAMES M. MARTIN, JR., ROBERT W. B. JOHNSTON, and MILBURN J. O'NEAL, JR. Houston Refinery Research Laboratory, Shell Oil Co., Houston, rex. The need for rapid analytical methods for the analysis of individual aromatic compounds through the CPboiling range prompted a study of various spectroscopic techniques. It was discovered that increased u tilization of infrared spectroscopy could be obtained through the combined use of sodium chloride and potassium bromide optics, thus extending the use of the infrared region to 25 microns. The method described herein has made possible the analysis of individual aromatics through 330" F. on two fractions prepared by distillation. Analytical error, expressed as standard deviation, did not exceed zk0.45% for any of the individual components. The increased selectivity, obtained through the combined use of sodium chloride and potassium bromide optics, has resulted in greater scope and improved accuracy for the various components. Applications to products from catalytic reforming processes are shown.
A
region. Also, the most suitable band for the determination of benzene occurs near 14.84 microns which is very close to the limit of sodium chloride optics and results in lese than the desired accuracy for benzene determinations. Undesirable inaccuracies would be predicted for the determination of n-propylbenzene, 4-ethyl-l-methylbenzene, and tert-butylbenzene in the sodium chloride region because of interference from other aromatics in the mixture. All of the above mentioned compounds exhibit better selectivity in the potassium bromide spectral region. The marked increase in selectivity is immediately evident from the comparison shown for ethylbenzene in Table I.
Table I.
Ethylbenzene Isopropylbenzene Toluene 1,3-Dirnethylbenzene Benzene 1,2-DimethyIbeneene 1,4-Dimethylbenzene Relative t o ethylbenzene.
N INCREASED demand for the production of high purity
aromatic concentrates and the increased interest in the catalytic processing of light naphthas have emphasized the need for rapid analytical methods for determining individual aromatic compounds through the CP range. The advantage of spectroscopic techniques, with regard to speed and small sample size, was recognized. Experience in this laboratory and of other workers in the field (5, 6, 8, 9) suggested that mixtures of such complexity could not be successfully handled by ultraviolet or mass spectrometric techniques without tedious and time-consuming separation procedures. Infrared spectroscopy has been successfully employed in analytical methods for the determination of a limited number of individual aromatics ( 4 , 7). Recently, Williams et al., (IO) developed an infrared method, combined with fractional distillation, for the determinat'on of individual aromatics from benzene through the C,O range; however, their work was confined to the 2- to 15-micron spectral region and required analyses to be performed on several distillate fractions. In the present m-ork it has been found that increased selectivity can be achieved by extending the use of the spectral region to 25 microns with the aid of potassium bromide optics. The infrared analytical method described herein has been developed with the aid of two interchangeable monochromators (sodium chloride and potassium bromide optics) which, because of simplicity of instrument design, require no refocusing or readjustment. The use of the two monochromators in effect doubles the spectral field (2 to 25 microns) in the search for suitable analytical wave lengths. As a result, both the scope and accuracy have been improved over that obtained nith techniques normally involving only the sodium chloride monochromator. Samples containing individual aromatics boiling up t o 330" F. have been successfully analyzed in two fractions (initial 295" F. and 295" to 330" F.) with the aid of distillation techniques and mass spectrometry to assess the efficiency of separation a t the 330" F. cutpoint. tert-Butylbenzene and 1,2,4trimethylbenzene have been included in the analytical method for the latter fraction as minor constituents that may be present from higher boiling materials. SPECTRAL BEHAVIOR
Close scrutiny of the spectra of pure compounds ( 1 ) revealed the poor selectivity exhibited by ethylbenzene in the sodium chloride
Sensitivityafor Ethylbenzene in Sodium Chloride and Potassium Bromide Spectral Regions %" NaC1, 14.35 p 100 96.0 66.5 23.1 10.7 0.0 0.0
KBr, 17.97 p 100 36.9 0.0 0.7
*
0.7 0.7 2.0
EXPERIMENTAL
All absorption measurements were made using a Beckman IR-2 infrared spectrophotometer equipped with interchangeable potassium bromide and sodium chloride monochromators. Absorptions a t frequencies below 14.50 microns were obtained using the sodium chloride monochromator, while those above 14.50 microns were obtained on the potassium bromide monochromator. Ap roximatelt. 15 to 20 minutes is required for interchanging t t e two monochromators. The change is not generally made, however, until all samples under investigation a t one time have been measured on one unit, thus making the changeover negligible as far as time per sample is concerned. All absorbances were obtained on the null system of the IR-2 except for 4-ethyl-1-methylbenzene (21.62 microns), Because of thc decrease in incident energy a t the longer wave lengths it was not possible to balance the instrument for 100% transmittance. However, the Brown electronic recorder was used satisfactorily for measuring the detector signal on both the blank cell and the sample. The percentage transmittance was then calculated from these data with accuracy comparable to that obtained on the null system. The turret mechanism of the instrument was used to estsblish fixed analytical wave-length positions. A single salt plate, comparable in thickness to the sample cell, was used to balance the instrument for 100% tranemittance. The sample cell blank R as obtained by measuring cyclohexane (which also served as a diluent for the samples) in the sample cell relative to the 100% transmittance of the salt plate. This value, after conversion to absorbance, was then subtracted from the absorbance of the sample diluted in cyclohexane. Use of the dilution technique is advantageous because of the high absorptivities of the aromatic compounds, and because it minimizes small shifts in absorption bands sometimes observed when changes in relative composition occur in the sample. I t also furnishes a convenient means of nullifying changes in transmission characteristics of the sample cell, which usually occur with age. Ordinarily the cyclohexane blank measurements need to be made only a t the beginning and end of each day's work. All samples were weighed and diluted to the proper volume in cyclohexane, with the concentrations being expressed in grams per liter. A 1-mm. sample cell was used for all absorbance measurements.
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V O L U M E 2 6 , NO. 1 2 , D E C E M B E R 1 9 5 4 Table 11.
Boiling Point and -4rrangement of Compounds Included in Analytical Scheme Compound
Isopropylbenzene
Boiling Point, Initial 295' F.
F. (2)
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butylbenzene offer major interference, they did not produce analytical inaccuracies beyond practical limits. As is the general case with infrared analysis, the absorptivities, for the pure compounds were not constant as the concentration was varied. To simplify calculations a method similar to that used by Brattain et al. (3) was used to establish a semitheoretical absorptivity as a constant, and experimentally determined absorptivities were corrected to conform to the constant value.
306.31
295' to 330' F. 1,2-Dimethylbenzene 291.94 Isopropylbenzene 306.31 n-Propylbenzene 318.59 3-Ethyl-1-methylbenzene 322.35 4-Ethyl-1-methylbenzene 323.58 1,3,5-Trimethylbenzene 328.49 2-Ethyl-1-methylbenzene 3 2 9 . 2 8 C u t point 330' F. tert-Butylbenzene 336.41 1,2,4-Trirnethylbenzene 336,83
CALIBRATION
The analytical scheme for which calibrations were obtained is presented in Table I1 showing boiling point of the compounds considered in each of the fractions to be analyzed. All compounds used in calibrations were API Research standard reference materials having a purity of >99.5 mole %. Several blends were prepared in varying concentration for each Component to be considered in the scheme of analysis. Each blend was measured a t each of the analytical wave lengths under the established instrument and operating conditions shown in Table 111. A comparison of sensitivities for the various compounds is shown in Table IV, expressed relative t o 100% for the major component a t each of the analytical wave lengths. Although isopropylbenzene and tert-
Table V.
Comparison of Absorption Level of Saturates to Aromatics
Absorptivities, Wave Liters per Gram Cm. Length, Microns Component Determined Aromatics Saturates Initial 295' F. 2.910 0.001 14.84 Benzene 2.743 0.047 13.73 Toluene 2,730 0.039 13.49 12-Dimethylbenzene 1.735 0.023 13.03 1,3-Dimethylbenzene 1.888 0.010 12.59 1.4-Dimethvlbenzene 0.152 0.002 17.97 Ethylbenzehe 1.057 0.018 13.17 Isopropylbencene Typical Saturate Fractionso 295O to 330' F. 1.550 0.0061 0.0035 1,3,5-Trimethylbenzene 11.96 0 , 5 5 9 0.0155 0.0196 1-Methyl-3-ethylbenzene 12.80 0.132 0.0011 0.0014 n-Propylbenzene 20.47 0.110 0.0012 0.0015 1-Methyl-4-ethylbenzene 21.62 1.280 0.0038 0.0048 1,2,4-Trimethylbenzene 12.41 0 . 7 2 5 0,0365 0.0519 13.71 1-Methyl-2-ethylbenzene 2 . 6 1 0 0,0259 0.0364 12-Dimethylbenzene 13.49 0 . 2 9 5 0.0003 0 , 0 0 0 9 tert-Butylbenzene 18.38 1 . 0 4 0 0.0154 0,0202 Isopropylbenzene 13.17 0 These saturate fractions from different sources indicate the need for establishing new saturate corrections when sample source is varied.
________ ANALYTICAL PROCEDURE
Analyses have been performed on aromatic concentrates as Table 111. Instrument and Operating Conditions" well as saturate-aromatic mixtures. In the case of aromatic conAnalytical Slit centrates, the samples were weighed and diluted to appropriate Wave Length, Width, Compound Microns Mm. Monochromator concentrations in cyclohexane. Usually a weight concentration of Initial 295' F 10% or less furnished absorbances in the optimum range (0.2 to 14.84 1.50 KBr Benzene 0.6). In practice, the absorbances were measured for both the 13.73 1.70 NaCl Toluene solvent and the diluted sample, with the sample blend undergoing 1,2-Diniethylbensene 13.49 1.20 NaCl 1,3-Dimethylbenzene 13.03 1.50 NaCl a correction for solvent absorption. These corrected absorbances 1.4-Dimethvlbenzene 12.59 1.30 NaCl Ethylbenzene 17.97 1.50 KBr were converted to concentration values in a system of calculations Ieopropylbenzene 13.17 1.50 NaCl discussed in a subsequent section. 295' to 330' F. Additional care was required in the handling of saturate-aro1,2-Diniethylbenzene 13.49 l.fiO NaCl matic mixtures. Generally samples containing greater than 75% Isopropylbenzene 13 17 1.50 NaCl n-Propylbenzene 20.47 2.60 KBr aromatics do not require a saturate correction. h'evertheless, 2-Ethyl-1-methylbenzene 13.71 1,50 NaCl 3-Ethyl-1-methylbenzene 12.80 1.10 NaCl experience has proved that, for highest accuracy and for samples 4-Ethyl-1-methylbenzene 21.82 2.60 KBr containing
