Hydrogen-deuterium equilibration

An equimolar mixture of hydrogen and deuterium was passed at a total flow rate of 256 mmoles per hour per gram of Cr203 over a fresh sample of chromia. After flushing for 3 hours at room temperature, the temperature was raised at the rate of 50° per hour and the effluent stream was analyzed periodically for HD by mass spectroscopy. The first sign of conversion was observed at 194°, conversion was about 80% of the equilibrium conversion at 248°, and essentially 100% at 273°. When the catalyst was cooled to 110°, conversion was 86% of the equilibrium conversion. A sample activated above 300° gave an equilibrium conversion at 25°. 

Processes in the Hydrogenation of Hexenes and in Isotopic Exchange 

Sections VII, B, D, E, and F establish that a rather large number of processes occur during passage of hexenes plus deuterium over chromia catalysts. These are as follows  

The effect of temperature of activation and of crystallization has been reasonably well characterized for most of these processes. As the activation temperature increases all processes marked with an asterisk decline relatively. Processes (6) and (9) become small on amorphous 

In most aspects, these studies are much more detailed and they supercede our earlier studies on olefin hydrogenation (12, 21, 46, 52). In particular, we had not earlier separated and examined the unreacted and the isomerized olefin from experiments employing deuterium nor had we run more than a few experiments on chromias activated at less than 470°. However, certain aspects of previous work were not reexamined and we will mention important conclusions from that work below. 

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Scholten and Konvalinka (9) in 1966 published the results of their studies on the kinetics and the mechanism of (a) the conversion of para-hydrogen and ortho-deuterium and (b) hydrogen-deuterium equilibration. At first the a-phase of the Pd-H system was used as catalyst, and then the results were compared with those obtained when the palladium had previously been transformed into its /3-hydride phase. 

It has been known since the early work of Farkas and Sachsse (6) that the ortho-para conversion may be catalyzed by paramagnetic species. That the mechanism for this kind of conversion is nondissociative is shown by the absence of hydrogen-deuterium equilibration at a comparable rate under similar conditions. But proof that the nondissociative... 

Turkevich, J., Laroche, J. Catalytic Activity of a Graded Set of Charcoals for the Hydrogen-Deuterium Equilibration and the Ortho-Para Hydrogen Conversion and Electron Spin Resonance. Z. Phys. Chem. N.F. 15, 399 (1958). 

The reactions with which we will be mainly concerned in this section are the hydrogen-deuterium equilibration... 

Hydrogen-deuterium equilibration took place at around 473 K with Au/Si02, Au/MgO and A11/AI2O3 with particle sizes between 6 and 9nm rate constants and activation energies (75-116 kJ mol-1) were independent of dispersion. Rates were slower with larger particles (>16.5 nm) and the... 

Not all reactions take place on the Bronsted acid sites. The hydrogen-deuterium equilibration takes place on the Lewis acid sites (Figure 1). 

Ortho-ZParahydrogen Conversion and Hydrogen-Deuterium Equilibration over Carbon Surfaces, Y. Ishikawa, L G. Austin, D. E. Brown, and P. L. Walker, Jr. 

Run 247 exhibits a higher fraction of exchange into methyl than the other runs and its distribution pattern approaches that of run 10. The catalyst of run 247 was not activated in the usual way. In the course of hydrogen-deuterium equilibration reactions described in the next section, the catalyst had been held in the vicinity of 273° for 15 hours. 

Fig. 5. Comparison of the number of active sites for cumene cracking as determined by poisoning experiments, with the activity for hydrogen-deuterium equilibration as a function of zeolite pretreatment.

Is the shifting hydrogen or deuterium isolated or protected from the surface hydrogen-deuterium pool And how fast does the hydrogen-deuterium pool equilibrate over the surface To partly answer these questions we consider apopinene. 

Deuterium exchange within saturated products occurs by dissociative adsorption followed by multiple stepwise migrations as described in assumption 15 followed by the addition of deuterium and/or hydrogen from the equilibrated surface hydrogen-deuterium pool. 

It has been observed [23,91,92] that when an unsaturated hydrocarbon is reacted with (a) equilibrated and (b) non-equilibrated hydrogen—deuterium mixtures, the deuteroalkane distributions are identical. Such observations indicate that the direct addition of a hydrogen molecule across the olefinic bond does not occur, and provides strong evidence for the formation of a half-hydrogenated state , that is, an adsorbed alkyl radical, first suggested by Horiuti and Polanyi [81], as a relatively stable reaction intermediate. The process of hydrogenation may thus be represented as... 

It is generally agreed that the kinetics and the distributions of deuter-ated products from the reactions of alkynes or alkadienes with deuterium are satisfactorily interpreted in terms of the consecutive addition of two hydrogen atoms, of unspecified origin, to the adsorbed hydrocarbon to yield the monoolefin. The identity of the distributions of deuteroethyl-enes from the reaction of acetylene with equilibrated and non-equil-ibrated hydrogen—deuterium mixtures also provides strong evidence for such a mechanism [91]. 

Process (6). This proce.ss might be supposed nonexistent and all 2-hexcne-do to have been equilibrated with a surface hydrogen-deuterium pool of low (D ). This would require (D ) steadily to decrease... 

The catalytic work on the zeolites has been carried out using the pulse microreactor technique (4) on the following reactions cracking of cumene, isomerization of 1-butene to 2-butene, polymerization of ethylene, equilibration of hydrogen-deuterium gas, and the ortho-para hydrogen conversion. These reactions were studied as a function of replacement of sodium by ammonium ion and subsequent heat treatment of the material (3). Furthermore, in some cases a surface titration of the catalytic sites was used to determine not only the number of sites but also the activity per site. Measurements at different temperatures permitted the determination of the absolute rate at each temperature with subsequent calculation of the activation energy and the entropy factor. For cumene cracking, the number of active sites was found to be equal to the number of sodium ions replaced in the catalyst synthesis by ammonium ions up to about 50% replacement. This proved that the active sites were either Bronsted or Lewis acid sites or both. Physical defects such as strains in the crystals were thus eliminated and the... 

An indication of the importance of the steric inhibition of resonance in aromatic substitution reactions has been gained by a study of the exchange reaction of dialkylanilines with deuterium oxide.21 With di-methylaniline, for example, the ortho and para hydrogen atoms equilibrate readily with deuterium oxide. This reaction, like most aromatic substitution reactions (see Chapter 13), is believed to proceed by the attack of a positively charged fragment (in this case a hydrogen or deuterium ion) at a point of high electron density. Consequently, the ease with which equilibration occurs can be taken as an approximate measure of the contribution of forms such as XLV or XLVI to the structure of the... 

There is much evidence for the belief that the transformation of an adsorbed olefin into a saturated hydrocarbon occurs principally in two stages, and that a half-hydrogenated state, i.e., an adsorbed alkyl radical, is a relatively stable intermediate (2). The most impressive evidence for this view is the identity of products obtained when equilbrated and non-equilibrated hydrogen-deuterium mixtures are employed (25-27). This is not to deny that a part of the reaction may proceed through the addition of a molecule of hydrogen essentially in one step (28). The paraffin once formed is quite unreactive, and its readsorption may be neglected. 

At 90° on a moderately active catalyst of nickel wire in the absence of ethylene the hydrogen-deuterium reaction is complete within 3 hrs. The presence of ethylene markedly retards the rate of this reaction as the following experiment showed 12 mm. of C2H4,9.6 mm. D2, and 10.1 mm. of H2 were contacted with the nickel wire for 4 hrs. At the end of this period, when 10% addition to the double bond took place, there were 6.1 mm. of D2,8.4 mm. H2, and 4.5 mm. HD. If equilibrium had been attained, the composition would be 4.0 mm. D2,6.0 mm. Hg, and 9.2 mm. HD, indicating that the ethylene had suppressed the equilibration of the hydrc en isotopes. 

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