Hydrogen Exchange between Deuterium Oxide an ydrocarbons J
ON SILICA-ALUMINA CATALYST R . C. HANSFORD',P. G. WALDO, L. C. DRAKE, AND R . E. HONTG2 Socony- Vacuum Laboratories, Research and Development Department, Paiclsboro, .V. J .
I
N A previous publication (6) it was shown that hydrocarbons
undergo hydrogen exchange with water adsorbed on a typical silica-alumina cracking catalyst, such exchange being detected by means of deuterium oxide. The exchange takes place at temperatures much below those at which measurable cracking occurs, and the lowest temperature a t which it can be detected after a 1-hour contact depends on the structure of the hydrocarbon. The qualitative data previously presented suggested that the interaction of a hydrocarbon with a cracking catalyst involves the formation of an ionic complex, the reactions of whichcan account for the products of catalytic cracking (6, 6). The present paper deals with a more quantitative study of the exchange reaction, and relates the mechanism of the reaction t o the current generally accept'ed ionic theory of catalytic cracking ( 4 , 6 , 1 4 ) . FXPERIM ENTA L PROCEDURE
The apparatus consisted of a simple vacuum manifold system tm which were attached, through standard taper or ball-andsocket joints, the component parts. The latter consisted of a 34nil. catalyst chamber of 96% silica glass; two small diameter, sealed end tub- graduated to 0.01 ml. for measuring the quantity of liquids (deuterium oxide and liquid hydrocarbons) vaporized into t'he catalyst bulb; gas and deuterium oxide storage bulbs; gas purification vessels for freezing and degassing light hydrocarbons; and a mercury manometer. A vacuum system, consist,ing of a single-stage mercury diffusion pump backed by a mechanical oil pump, was sealed to one end of the manifold. The catalyst used in all of this work was from a typical commercial batch of Socony-Vacuum bead cracking catalyst (10, I S ) . The following analyses are for the particular commercial batch from which the catalyst used in this study was taken. The chemical analysis was based on 760' C., dry weight, and the catalyst was heated in air at 760' C. for 10 hours for the physical analysis. Chemical Analysis Silicon dioxide Aluminum oxide Sodium ion Iron oxide
Per Cent 89.4 10.2 0.2 0.02
Physical Analysis Surface area Pore volume Real density
420 sq. m./g. 0.46 cc./g. 2 . 3 3 g./ml.
T h e hydrocarbons investigated were from the following sources and with the exceptions noted were used without further purification: benzene, General Chemical Co., reagent grade; cyclohexane, Eastman, reagent grade; methylcyclohexane, Eastman, practical grade, redistilled through a 30-plate column, 100.0" to 100.7' C. cut, naDO 1.42305; neopentane, isobutane, n-butane, and 2butene, Phillips Petroleum Co., research grade; n-heptane, Phillips Petroleum Co., pure grade; isobutylene, Matheson, C.P. grade; and Zmethylpropane-24, laboratory preparation from 1 2
Present addreas, Union Oil Co., Research Center, Brea, Calif, Present address, RC-4 Laboratories, Princeton, N. J.
tert-butylmagnesium bromide in n-butyl ether by hydrolysis with 91).8YGdeuterium oxide, folloned by low temperature fractional condensation. The deuterium oxide used in the exchange experiments had a purity of 96r0 as DzO. The catalyst (22 grams) was conditioned by heating to 730" to 760" C. (1350" to 1400' F.) under high vacuum for 1 hour. Retween successive experiments it was freed from any carbonaceous deposits by contacting with oxygen at 760" C. before evacuation. After cooling to room temperature, the catalyst was hydrated by allowing it to absorb deut'erium oxide vapor from a calibrated buret containing liquid deuterium oxide. The quantity of deuterium oxide allowed to be adsorbed from the buret was 0.25% by weight of the catalyst, except in the experiments designed to determine the effect of the degree of deuteration on the exchange reaction with isobutane. Evacuation of the catalyst at 760' C. for 1 hour removed most of the adsorbed water, but it was impractical to remove all of it. Complete dehydration would have involved collapse of the catalyst structure, which had water of constitution associated with it as hydroxyl groups. The data reported were obtained on a catalyst which had been contacted with deuterium oxide many times in previous experiments and which was reproducible with respect to deuterium content. After this catalyst pretreatment and hydration with deuterium oxide, the catalyst bulb was heated to the desired temperature. Thermostating to 10.2' C. was accomplished by means of an electrically heated oil bath in the temperature range up to 250" C.; above this, temperatures constant to f1' C. were obtained with an electric air furnace. I n heating the catalyst to reaction temperature, negligible amounts of deuterium oxide were desorbed. For example, after hydration at room temperature with o.2570 deuterium oxide and raising the temperature to 308' C., the p r e s sure rose to only 16 mm. of mercury, corresponding to a desorption of 0.01 millimole of deuterium oxide out of 2.8 millimoles initially adsorbed. Before admission to the catalyst, the hydrocarbon charge waa frozen in a bulb in liquid nitrogen and permanent gases were pumped out. This process was repeated with alternate thawing and freezing until the sample was free of air. The amount of gaseous hydrocarbon admitted to the catalyst was measured from the pressure in the known volume of the system external to the catalyst bulb. The quantity of vapor from liquid hydrocarbons was measured by the decrease in volume of liquid contained in the buret. The total amount of gas entering the catalyst chamber varied with the temperature and the adsorption characteristics of the hydrocarbon. After the catalyst had reached reaction temperature, the hydrocarbon to b e studied was admitted as a gas or vapor to the catalyst. Following the reaction period, a sample of reaction product was removed by condensation in a bulb cooled in liquid nitrogen, after the manifolding system had been evacuated of gas from the initial charging step. Except as otherwise noted, the reaction time was 1 hour.
1108
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1952
Gas samples obtained from the exchange experiments were analyzed on a commercial mass spectrometer (Consolidated Engineering Corp., Model 21-lOl), with particular emphasis on the isotopic distribution of product molecules. Details of the special techniques employed are discussed elsewhere (9).
TABLE11. DEUTERATION OF HYDROCARBONS BY DEUTERIUM OXIDEON SILICA-ALUMINA CATALYST
Hydrocarbon Neopentane
DISCUSSION OF EXPERIMENTAL RESULTS
The experimental data are summarized in Figures 1, 2, and 3; further details are presented in the tables. Figure 1 is essentially a plot of the logarithm of the rate of deuterium substitution versua the reciprocal of the absolute temperature, for eight hydrocarbons and for hydrogen. Actually, the reaction time was held constant a t 1 hour, and hence the rate is the number of moles of reactant converted during the first hour. The conversion is taken as the mole sum of all deuterated products, irrespective of the d e g e e of substitution. Since the total molar conversion is generally less than 10% and since equilibrium concentrations are much higher than this, the observed mole per cent of conversion was taken as proportional t o the reaction velocity constant of the exchange reaction. Table I presents data which show that this assumption is approximately true, a t least for isobutane.
n-Butane n-Heptane Isobutane
Isobutane and 0.5 mole % ’ isobutylene Cyclohexane
TABLE I. D E ~ R A T I O OFNISOBUTANE us. TIMEAT 100” C. Time, Hr. 0.5 1.0 2.0 4.0
Pressure. Mm. Hp 110 110 110 110
Charae in CataSyst Chamber, Millimolea CdHio 1.11 1.11 1.11 1.11
Conversion, Mole % 0.66 1.61 2.75
6.11
Conversion per Hour, Mole % ’ 1.32 1.61 1.37 1.52
Methylcyclohexane
2-Butene *
Benzene
The principal significance of the data presented in Figure 1 and Table I1 is a more quantitative evaluation of the temperature level a t which hydrogen exchange occurs between catalyst and hydrocarbons of different structures and molecular weights. The previously reported “threshold” exchange temperatures (6)for several hydrooarbons are of limited significance since they were based on a less sensitive technique for measuring the extent of exchange, and the effect of other factors, such as impurities, was not then appreciated. It is of interest to uote that the data plotted
I100
1109
Hydrogen
Temp., Prassure, O C. Mm. Hg 280 356 300 386 320 405 340 431 200 286 240 345 385 280 200 37 240 42 48 280 60 50 75 80 100 110 150 120 132 120 135 120 134 120 40 22 60 45 74 80 210 70 229 66 234 65 240 60 260 80 280 60 308 80 5 90 100 5 9 120 130 10 20
