~~~
~~~
~~
~
Reaction Kinetics as a Diagnostic Tool for the Gas Chromatographic Column SIR: In the course of our recent study of reaction kinetics in a packed gas-liquid chromatographic column (1, 2), we found the surface of the support to be frequently efficacious in catalyzing several reactions. Particularly noteworthy, was the pseudo first order depolymerization of trioxane and paraldehyde to give formaldehyde and acetaldehyde, respectively, which are considerably more volatile than the parent trimers. Such reactions, and similar ones, are of special interest at this time because they provide a reaction kinetic approach, contrasting with the previously used equilibrium phenomena (3, 4, for characterizing the gas chromatographic support and column under some conditions. In a period of expanding interest in more elaborate and preparative columns, the kinetic approach has the advantage of providing a means of characterizing the column throughout its length. Potentially, it is possible to study the surface treatment of the solid support, temperature variation in the column, interaction between the liquid phase and solid support, and mass transfer properties of the liquid phase-solid support combination. Illustration of the first two applica tions can be seen from Figure 1. Chromatogram (a) was obtained when trioxane was chromatographed on a 64 X I / h inch packed column containing silicone D.C. 550 on 60-80 mesh commercial acid treated trimethylsilanized brick. The elution curve preceding the trioxane peak is indicative of formaldehyde formed continuously on passage through the column. A chromatogram obtained using a similar column with commercially available Gas Chrom Q (Applied Science Laboratories) as a support showed no depolymerization of the trioxane which emerged as a single conventional peak. Chromatogram (b) was obtained from the same column as (a) except that a 4-inch heated zone 21 “C higher than ambient was built into the column with center approximately 31 inches from the column ourlet. A maximum in reaction rate, or product production, is clearly evident and it appears that reaction kinetics might be used to find temperature gradients of this type in an operating column. The location of the maximum or center of the heated zone is readily calculable from VRT = (1 -
where x
X)(VRT),
a y
u1-z
+
X(y,T)?q
(1)
UZ
fraction of column distance from column outlet (0 to 1) a = average gas velocity for the column = average gas velocity over x iiz = average gas velocity over 1 - x VRT = observed retention volume of maximum conversion (here 48.5 cc) =
(1) J. Y. Yurchak, M.S. Thesis, University of Wisconsin, Madison,
Figure 1. Chromatograms for 0.5 p1 of trioxane on D.C. 550-silanized brick (15/85 w/w). Helium at 62 cc/min. Po = 742, Pi/P, = 1.53 (a) Normal operation at 135 “C (b) 4-inch heated zone present in the column. Region 1 is reaction product, peak 2 is trioxane
(v,’), = retention volume of pure reactant (trioxane = 72.8 cc here) (VRT)2= retention volume of pure product (formaldehyde = 18.3 cc here)
If ii = uoj; ii, = u,jz
(2)
where u, is gas velocity at the column outlet and j is the appropriate pressure correction factor (5, 6)over the fraction of column indicated, then (3) For the chromatogram shown, Equation 3 was solved by an iterative procedure, adjusting j z for each value of x ; after one iteration, x was found to be 0.496 or the center of the hot spot was calculated to be 31.7 inches from the outlet in good agree-
Wis., 1966.
(2) S. H. Langer and J. Y. Yurchak, 153rd National Meeting, ACS,
Miami Beach, Fla., April 1967. (3) J. Bohemen, S. H. Langer, R. H. Perrett, and J. H. Purneil, J. Chem. SOC.,1960,2444. (4) D. T. Sawyer and J. K. Barr, ANAL.CHEM.,34, 1518 (1962).
( 5 ) A. T. James and A. J. P. Martin, Biochem. J.,50, 679 (1952).
(6) A. B. Littlewood, “Gas Chromatography,” Academic Press, New York, 1962, pp 24-8. VOL. 40, NO. 1 1 , SEPTEMBER 1968
1747
ment with the true location. Such good agreement may not occur under less favorable conditions. The reaction temperature sensitivity can be estimated from the typical Arrhenius equation and sensitivity of VR to temperature as shown in Equations 4 and 5, respectively. k =
(4)
where B is a constant for a given solute and column, and AH8 is the differential molar heat of vaporization of the reactant from solution. Since the conventional steady state flow reactor equation applies to a non-steady state pulse situation (7) for the special case of a first order reaction, (VRT)l ln(1 - fi) = k= AB F F
e(AHs-Ea)/RT
mV
(6)
wherefi is fractional conversion of the reactant and F is the corrected flow rate of carrier gas through the column. It is evident then that the change in conversion in a column due to the presence of a “hot spot” of the type with which we are concerned will depend on the difference between AH, and E A , the energy of activation for reaction. The greater the difference, the greater the sensitivity. Where these quantities are approximately equal, the increase in reaction rate during residence at the higher temperature in the column will compensate for the decrease in retention time due to presence of the higher temperature zone. The location of the “hot spot” is best estimated when retention volumes of product and reactant are significantly different and the difference between AHs and E A is favorable for indicating a temperature difference. In Figure 2, chromatogram (a) was obtained when paraldehyde and reference material ethylbenzene were chromatographed on D.C. Silicone 550 on Gas Chrom Q but with 2 inches of acid-treated trimethylsilylated support-D.C. 550 combination inserted 4 to 6 inches after the inlet of the column, From the characteristic retention volume of acetaldehyde and paraldehyde the location of the poorly treated section of the column could be ascertained readily. Chromatogram (b) of Figure 2 was obtained from the same column after treatment with a commercial trimethylsilylating agent recommended for silylation of the column in situ ( 5 , 10 pl injections at 175 “C). The effectiveness of the treatment is clearly evident from the disappearance of product acetaldehyde in the elution chromatogram. The rate of depolymerization of paraldehyde is about an order of magnitude greater than the rate of depolymerization of trioxane over the temperature range of 70-150 “C. Therefore, the behavior of paraldehyde at a given temperature and on a given column is a good indication of what trioxane will do at a higher temperature and vice versa. This fact, coupled with the ability to vary the carrier gas flow rate over a range greater than an order of magnitude gives the method, employing these reactions, a wide range of sensitivity. Surface catalyzed depolymerization of paraldehyde is quite evident at temperatures as low as 60 “C with liquid phases such as silicone D.C. 550, squalane, and six-ring polyphenyl ether. The reaction is also acid and metal-catalyzed so care must be taken with phases containing metal ions, impurities, or acids. Even with untreated supports, the depolymerization reaction does not take place with a basic liquid phase such as 4,4-
Figure 2. Paraldehyde (0.5 pl) chromatographed on D.C. 550-Gas Chrom Q (lSj85 w/w). Helium flow = 30 cc/min PJPo = 1.19 Two inches of commercial brick-D.C. 550 inserted near entrance of column (6) After trimethylsilylation. Peak 1, reaction product; Peak 2, paraldehyde; Peak 3, ethylbenzene (a)
dimethylenedianiline at 100 “C. Reaction also does not take place under our chromatographic conditions with polyethylene glycol 1000. Presumably hydrogen bonding to surface hydroxyl sites can quench the sites and prevent reaction as it prevents surface adsorption (3). Heretofore, adsorption and desorption behavior and associated tailing of certain compounds have been customarily used to investigate and classify the surface properties of solid supports (3, 4). Occasionally, the less well understood decomposition and isomerization reactions of organic materials such as terpenes (8) or steroids (9) also have been used for qualitative evaluation of solid supports. While we have discussed only one general reaction, it is apparent that a clean-cut reaction taking place in a gas chromatographic column is an additional means for characterizing the solid support and it can serve as a general diagnostic tool. Future developments with this and other viable reactions can be expected. STANLEY H. LANGER JOANNE Y . YURCHAK M. SHAUGHNESSY CHRISTOPHER Department of Chemical Engineering University of Wisconsin Madison, Wis. 53706 RECEIVED for review May 23, 1968. Accepted June 26, 1968. Work supported by the Petroleum Research Fund and the Wisconsin Alumni Research Foundation. (8) J. Jadak and H. Cvrkol, “Gas Chromatography 1958,” by D. H.
Desty, Ed., Butterworths, London, 1958. (7) D. W. Bassett and H. W. Habgood, J. Phys. Chem., 64, 769 (1960).
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ANALYTICAL CHEMISTRY
(9) W. R. Supina, R. S. Henly, and R. F. Kruppa, J. Amer. Oil Chem. SOC.,43, No. 5 , 202A (1966).
