Sample Vacancy Chromatography and Catalysis C. S. G. Phillips and C. R. Mcllwrick Inorganic Chemistry Laboratory, Oxford University, England
The technique of vacancy chromatography and particularly a n e w variation on this, sample vacancy chromatography, are found to be especially convenient for the study of reactions carried out in columns. Gas-chromatographic examples are given of their application to catalytic reactions and of their relation to stopped-flow reaction chromatography. Sample vacancy chromatography is applicable to continuous flow reactors and measures small changes (0.1% or better) in the mass balance across t h e reactor. As a differential method it also eliminates the effects of non-reacting impurities. Its sensitivity is limited by the constancy of t h e sample feed.
Vacancy Chromatography. In the simple analytical application of vacancy chromatography (1, 2), the sample to be analyzed is passed through a gas-chromatographic column. After a steady state has been established, so that the composition of the exit gas-plus-sample is the same as that of the gas-plus-sample entering the column, a pulse of pure carrier gas is injected at the beginning of the column. The resulting chromatogram, e.g., Figure 1, is then a series of negatiue peaks, one for each component of the mixture. So long as the sorption isotherms are linear, the vacancy chromatogram is inverted but otherwise identical with the normal chromatogram which would be obtained by injection of a suitably-sized sample into the column through which pure carrier gas is flowing. For non-linear isotherms the retention times will depend on the slope of the isotherm (mixed isotherm in the case of more than one component) at the appropriate concentration: the technique thus provides a further way of determining isotherms, essentially equivalent to the method devised by Conder (3), which he calls “elution on a plateau.” In this paper we shall be concerned only with linear-isotherm situations. The mechanism of formation of the vacancy chromatogram may be understood by recognizing that the front of a vacancy peak is effectively the back of a “square wave” introduction of sample (or a reversed frontal analysis), while the back of the vacancy peak is effectively the front of a second “square wave” of sample introduced after the gap produced by the injection of pure carrier gas. If the carrier gas sample is large enough, then the vacancy peak assumes a square “bottom,” which will occur a t the base line (Le., pure carrier gas passing through the detector) in the case of one component. Figure 2 (left-hand side) illustrates such a squared vacancy chromatogram for a 3-component mixture, together with (right-hand side) the reversed frontal analvsis obtained on the same column. In these circumstances the peak “depths” correspond exactly with the heights Of the steps in the reversed sis. The inverted mirror images of such squared-off peaks (1) A . A. Z‘hukovitski and N. M. Turkel’taub, Dokl. Akad. Nauk USSR, 143,646 (1961) (2) A . A. Zhukovitski, “Gas Chromatography 1964.” A. Goldup, Ed. Institute of Petroleum, London 1965, p 161 (3) J. R. Conder, “Progress in Gas Chrornatograohy,”J . t i Purnell, E d . , Interscience, New York, N . Y.. 1968, p 209. 782
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
(which so far as the authors can envisage have no obvious analytical application) may be produced by introducing a large square wave of sample (e.g., from a large sample loop) into a chromatograph otherwise operated in the normal mode with pure carrier gas passing through continuously, see Figure 3. As an example of the use of vacancy chromatography, Figure 4 illustrates some results obtained in this way for the diffusion of cyclopentene, cyclohexene, and cycloheptene through a silicone-rubber plug. The carrier gas passed continuously over one end of the plug, the other end of which was wetted with a liquid mixture of the three cycloolefins, and then into a gas-liquid (fl,fl’-oxydipropionitrile) chromatographic column. Vacancy chromatograms, as Figure 1, were produced by injections of pure carrier gas between the plug and the beginning of the GLC column. Catalysis under Continuous-Flow Conditions. Sample-Vacancy Chromatography. In this section we shall consider the application of vacancy chromatography and, in particular, a new technique, sample-vacancy chromatography, to the study of catalytic reactions under conditions of continuous feed of reactants at constant composition. The same techniques could be applied to many noncatalytic reactions without modification.
EXPERIMENTAL Figure 5 illustrates in outline the general experimental set-up which we have employed in catalytic studies. In all experiments, carrier gas passes through a catalyst column to a detector, but may also pass successively through one or more of the following devices: Precolumns to purify carrier gas and reactant and/or to introduce into the carrier gas a constant feed of reactant. The sensitivity achieved in sample vacancy chromatography is limited by fluctuations in the reactant (sample) feed rate. These fluctuations are most marked with adsorbable materials and result mainly from small temperature fluctuations in the reactor (catalyst) column, analytical column, and in connection tubing. However, even with a diffusion feed, a standard commercial oven and unlagged connection tubing, these fluctuations can be reduced to 0.05% over time intervals much longer than any chromatographic peak widths. A six-port sample valve ( e . g . , Part No 792082, Pye Unicam, Cambridge, England). The carrier gas and sample pass through a sample loop which can be switched from the front to the end of the catalytic column to generate the sample vacancy chromatogram in the GLC (or GSC) column. For frequent analyses it is convenient to use a splitter with a small percentage of carrier gas plus sample going to waste through a suitable restriction via the sample loop: this avoids the small disturbance which Otherwise follows naturally when the sample loop-now containing products-is switched back from the end to the front of the catalyst A tap to stop the gas flow for defined intervals so as to uncouple the catalytic and chromatographic processes occuring within column, the Provision for injection of pure carrier gas or sample (reactant plus carrier gas), The former produces a vacancy chromatogram, the latter a sample vacancy chromatogram in A normal GLC (or GSC) column. For vacancy reaction chromatography, carrier gas is injected immediately before the catalyst column.
i
c ciohexen
/ -a C
5 I
100 minutes 150 0 50 Figure 4. Rates of diffusion of three cycloolefins through silicone rubber as measured by normal vacancy chromatography (see Figure 1 )
Figure 1. A normal vacancy chromatogram. A continuous stream of cyclopentene, cyclohexene, and cycloheptene in carrier gas (nitrogen, 30 mi min-') was passed through a GLC (5foot, 4-mm i.d., P,P'-oxydipropionitrile, 20% on Chromosorb P at 59 "C) column into a detector(F1D). The vacancy chromatogram was imposed on the otherwise constant standing signal by injection of ( 1 mi) pure carrier gas at time V. (Last peak emerges after 2 minutes)
centages of heptane and the heptene isomers were formed. These products appear in the appropriate peaks of the vacancy chromatogram, but these peaks also include the traces of the same hydrocarbons which were present in the original hept-1-ene sample (the reaction of the other heptene isomers was sufficiently small to be neglected to a first approximation). Furthermore, there are other peaks I
Time 4
Figure 2. Square-wave vacancy chromatogram obtained by injection of a large sample (40 mi) of carrier gas at time V into a column (5 foot Phasepak Q at 190 "C)through which a constant Stream of n-pentane, n-hexane, and n-heptane (total concentration 120 micrograms min-' equivalent to l vapor part per 1000) was being passed in nitrogen carrier gas (40 ml m i n - l ) . The mean width of each vacancy "well" is thus 40/40 = 1 min. At time A , this stream was switched to pure carrier gas to generate the reversed frontal analysis. Note that the step heights of the right-hand side correspond to the well depths of the left-hand side TAP FOR STOPPED-FLOW CHROMATOGRAPHY
CATALYST COLUMN
Figure 3. Square-wave chromatogram obtained by injection of a constant mixture (1 vapor part per 1000) of n-pentane, n-hexane, and n-heptane over a period of time (80 sec). Flow rate of carrier gas, 40 mi min-', Column 5-fOOt Phasepak Q at 190 "C
u ACAKV
RESULTS AND DISCUSSION The lower chromatogram of Figure 6 is a vacancy chromatogram obtained by starting with a stream of hept-lene in hydrogen, which was passed onto a Rh catalyst, followed by an analytical GLC column (AgNOa/p,p'-oxydipropionitrile). In order to emphasize certain advantageous features of the sample-vacancy technique, the catalyst was deliberately deactivated so as to keep the overall reaction yields low. Under these circumstances, small per-
DETECTOR
Figure 5. Schematic outline of apparatus used for gas-chromatographic studies of catalysis (marked with arrows in Figure 6) corresponding to other trace impurities. These trace impurities may of course be eliminated by very careful gas-chromatographic purification of the original sample or their effects can be allowed for using the gas-chromatographic analysis of the sample ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
703
I
SAMPLE VqCANCY VACANCY
1
TOTAL S EONVERS ION YIELD
\J 0.89
0.14 2.76
5.54
5.54
007 1.73
5.59
1
VACANCY
Figure 6. A comparison of normal vacancy chromatography (lower chromatogram) with sample vacancy chromatography (upper Chromatogram). In each case, a sample of hept-1-ene (analysis given in Table I ) was passed in hydrogen through a deactivated R h catalyst ( R h metal deposited on Chromosorb P with 20% of diphenylacetonitrile at 98 "C) and on to a GLC analytical column (14 foot of 1.5M AgN03 in P,P'-oxydipropionitrile, 20% on Chromosorb P at 23 "C) and an FID detector. Hydrogen flow rate 30 ml min-'. Heptene feed 130 micrograms min-'. Hept-1-ene peak emerges after 7 minutes
Table I. Impurities (%) in Hept-1-ene Sample (Peak-Area Measurements) By direct gas chromatography
Heptane trans-Hept-3-ene trans-Hept-2-ene cis-Hept-2-ene cis-Hept-3-ene
1.21 0.41 0.67 0.38
