Buchang Shi

Catalytic Dehydration of Alcohols. Kinetic Isotope Effect for the Dehydration of t-Butanol

The kinetic isotope effects (KIE) for the conversion of an equimolar mixture of C4D9OH and C4H9OH and for the KIE for the intermolecular elimination of water from (CD3)2CH3COH for an alumina-catalyzed reaction were the same, 2.1–2.3. These results eliminate a carbocation intermediate for the dehydration of even a tertiary alcohol. Thus, for an alumina catalyst alcohol dehydration occurs by a concerted elimination mechanism. H/D exchange occurs to different extents prior to or during elimination for zirconia, thoria and magnesia catalysts; however, the data are consistent with a concerted elimination mechanism in combination with H/D exchange.

13C-tracer study of the Fischer–Tropsch synthesis: another interpretation

Abstract The 13C-tracer results from the introduction of 13C2H4 into syngas prior to conversion with a rhodium catalyst have been used to support a surface vinyl mechanism for Fischer–Tropsch synthesis. The results were first interpreted by a mechanism that involved a decrease in 13C species on the surface as the carbon number increased. This model is shown to be incorrect. Considering only the 13C-labeled products, the data are consistent with earlier tracer studies showing that the added 13C2H4 initiates chains.

Alcohol dehydration. Isotope studies of the conversion of 3-pentanol

Three deuterated 3-pentanols and undeuterated 3-pentanol were converted using an alumina catalyst. Similar values of the kinetic isotope effect (KIE; kH/kD) were obtained for the rate of conversion of 3-pentanol and the rate of H(D) removal in forming the alkene products. The data support a catalytic dehydration mechanism that involves a concerted removal of H(D) from the –CH2−xDx group and OH (or O) from the carbinol carbon and with a timing such that the transition state resembles neither a carbonion-like nor carbenium-like species.

Gas chromatographic separation of pairs of isotopic molecules

Abstract Nine pairs of isotopic (hydrogen/deuterium) molecules have been completely separated and quantitatively determined by gas chromatograph using a DB-5 column. All pairs exhibited an inverse isotope effect. The differences of enthalpy, entropy and free energy changes have been calculated for the chromatographic process. The data show that deuterium on the aliphatic part of a molecule makes more of a contribution to the inverse isotope effect than those on aromatic rings.

The kinetic isotope effect for alkane dehydrocyclization

Abstract The dehydrocyclization of a mixture of n -octane-d 18 and n -octane-d 0 are consistent with a mechanism that includes irreversible adsorption of the n-octane and a rate limiting step that involves a kinetic isotope effect of 3.5 ± 0.4 at 482°C. In a competitive conversion of methylcyclohexane and n -octane, where one reactant is the perdeutero isotopomer, the kinetic isotope effect for n -octane is 3.3 ± 0.6 and is 2.9 ± 0.6 for methylcyclohexane. Thus, for dehydrogenation and dehydrocyclization, irreversible chemisorption involves a kinetic isotope effect that is larger than the one that would be expected based only upon the ground state energy differences in the C-H and C-D bonds of the reactants.

D2O tracer studies in Co catalyzed Fischer–Tropsch reaction

Abstract The data show that the deuterium added in water together with synthesis gas provides hydrogen for the Fischer–Tropsch synthesis. In fact, the deuterium initially present in water nearly equilibrates with the hydrogen present in synthesis gas. Thus, water, once formed, is not inert but adsorbs competitively on the cobalt–titania catalyst to activate hydrogen. The data do not permit a definition of whether the exchange occurs on cobalt or the alumina support. The H/D ratio in the paraffin products is 4.4 and is very close to the H/D ratio in the feed of D 2 O/H 2 (4.1).

High-performance liquid chromatographic separation of pairs of isotopic labeled (deuterium/protium) molecules

Abstract The separation of n-octane-d0 and n-octane-d18 has been accomplished using a liquid chromatographic technique; baseline separation was exhibited with the n-octane-d18 eluting first. Thus, the liquid chromatographic separation exhibits an inverse isotope effect just as has been observed for gas chromatographic separations. An inverse isotope effect was observed for isotopomer pairs even when they were not separated sufficiently to exhibit two distinct peaks.

Hydrogen production in the conversion of 2-methylbutane over a series of acid catalysts

Abstract The production of hydrogen from the conversion of 2-methylbutane was studied over a series of acid catalysts in a recirculation reactor system. Conversion of 2-methylbutane over an amorphous silica–alumina catalyst and ZSM-5 zeolite resulted in significant amounts of hydrogen. This supports a carbonium ion mechanism with a penta-coordinated carbonium ion intermediate. The conversion of 2-methylbutane over the USY zeolite and sulfated zirconia did not result in hydrogen being produced thus supporting the bimolecular carbenium ion mechanism.

Gas chromatographic separation of deuterated and optical isomers of di-2-butyl ethers

Abstract By connecting a DB-5 column in series with a Cyclodex-B column, nine of ten isomers of a mixture of di-2-butyl ether-d0, di-2-butyl ether-d5 and di-2-butyl ether-d10 produced during the dehydration of a 1:1 mixture of 2-butanol-d0 and 2-butanol-d5 using an Al2O3 catalyst, were separated and identified. The (R,S) and (S,R) isomers of di-2-butyl ether-d5 could not be separated due to the similarity of their interactions toward the DB-5 and Cyclodex-B columns.