Burtron H. Davis

Paraffin dehydrocyclization: VI. The influence of metal and gaseous promoters on the aromatic selectivity

Abstract The addition of tin to a PtAl 2 O 3 catalyst greatly enhanced the activity and/or decreased the rate of deactivation for the dehydrocyclization of alkanes. There appeared to be an optimum ratio of Pt:Sn = 1:4 for a 0.6% Pt catalyst. The addition of Sn, Ag, or Cu to a RhAl 2 O 3 catalyst had a similar effect with Sn showing the greatest improvement. The addition of tin altered the ethylbenzene (EB): ortho -xylene (OX) ratio from 1:1 for pure 0.6% Pt to EB:OX = 1:2 for the PtSn (1:4) catalyst. The addition of thiophene and the dehydrocyclization of the octenes or octynes also changed the EB:OX ratio to favor the OX in a similar manner. The addition of tin or thiophene greatly decreased the second cyclization to the bicyclic aromatic; e.g., the addition of tin produced a tenfold decrease in the indan (bicyclic)/total aromatic ratio. The results are interpreted to be consistent with a selectivity that is determined by an electronic factor.

Conversion of labeled hydrocarbons: VI. n-[1-14C-] and n-[4-14C-] heptane over PtSn- and PtReAl2O3 catalysts☆

The 14C distribution in toluene from the aromatization of n-[1-14C-] and n-[4-14C-] heptane over both PtSn- and PtReAl2O3 catalysts was that expected for a direct 1,6-ring closure. The results using the bimetallic catalysts were in excellent agreement with results previously reported for a metal oxide catalyst (chromia) and a metal catalyst (Pt) suggesting that dehydrocyclization follows a similar mechanism over all three catalyst systems.

Dehydration and dehydrogenation of 2-octanol by thorium oxide

The selectivity of 2-octanol reaction by dehydration to octenes vs. dehydrogenation to 2-octanone has been determined for thorium oxide prepared by well-defined and reproducible procedures. The selectivity was very dependent on the catalyst pretreatment. In general, thoria pretreated at 600 °C with hydrogen was a dehydration catalyst while thoria pretreated at 600 °C with oxygen was a more effective catalyst for dehydrogenation. While the pretreatment was the major factor for determining selectivity, the thoria preparative procedure also played a role. The selectivity for 1-octene formation as compared to 2-octene production was also dependent on the pretreatment and on the thoria preparation procedure. Certain thoria preparations were found to be extremely selective for 1-octene formation while others were not very selective. Over some thoria preparations, a large amount of n-octane was formed at an early time on stream.

Paraffin dehydrocyclization: VII. Comparison of aromatic distribution from n-octane conversion over Pt at atmospheric and 200 psig

A similar C8-aromatic distribution was obtained for the dehydrocyclization of n-octane at atmospheric pressure and at a pressure of 200 psig. Furthermore, the aromatic distribution changed in a similar manner for both pressures to favor ortho-xylene when tin was added to the Pt catalyst. The aromatics were those expected for a direct six-carbon ring formation.

Dehydrocyclization of paraffins. Aromatic distribution from paraffins and naphthenes containing a quaternary carbon

Abstract The aromatic product distribution from the dehydrocyclization of several paraffins that contain a quaternary carbon and at least one six-carbon chain are compared to the aromatics obtained from the conversion of the naphthenes corresponding to direct six-carbon ring formation from the paraffins. The distributions obtained using Pt and chromia supported on “nonacidic” alumina are compared. The geminal -dimethyl naphthenes gave similar results over chromia and a low isomerization Pt catalyst with demethylation at the quaternary carbon the predominate reaction pathway. Comparing the conversion of the paraffin with the corresponding naphthene shows that over Pt dehydrocyclization occurs by direct six-carbon ring formation. For chromia this comparison shows a different distribution for the paraffin and naphthene; this difference probably is due to an isomerization of the paraffin prior to cyclization. The aromatic distribution is very sensitive to the Pt-Al 2 O 3 preparation; different preparations on the same alumina support gave very different aromatic distributions.

14C tracer study of the dehydrocyclization of n-heptane over Pt on nonacidic alumina

The 14C in the methyl position of toluene from the dehydrocyclization of n-heptane labeled in the 1 or 4 position was consistent with a mechanism including a direct six-carbon ring formation. The aromatic ring was degraded, and the 14C distribution in the ring was also in agreement with 80% or more of the aromatic being formed by a direct six-carbon ring formation.

Paraffin dehydrocyclization: V. The influence of Pt loading on the aromatic selectivity

Abstract It was observed that the platinum content of PtAl2O3K and Pt-carbon catalysts determined the aromatic distribution for the dehydrocyclization of n-octane and 3-methylheptane. At higher loading (1.2% Pt) the aromatics approached the distribution predicted for an equal contribution of all the cyclization pathways that lead directly to a six-carbon ring. For the low (0.15% Pt) loading the aromatic distribution appeared to be determined by the relative strength of the carbon-hydrogen bonds required to be broken to form the six-carbon ring; the pathway involving the weaker bonds was favored. The relative amount of hydrogenolysis of the reactant and the aromatic products to form lower carbon number aromatics increased as the platinum loading increased.

Paraffin dehydrocyclization. Competitive conversion of paraffins and naphthenes

Abstract The competitive conversion of several naphthene-paraffin mixtures was studied over supported metal and metal oxide dehydrocyclization catalysts. The conversion of each component was not simply the additive conversion of the pure components. Rather, it was found that for equal molar naphthene and paraffin mixtures the conversion was on a one-to-one basis. A change in paraffin-naphthene mole ratio resulted in a similar change in the relative conversion. The results at atmospheric pressure are consistent with a mechanism in which the adsorption of naphthene and paraffin are equal and the adsorption is the slow step for the reaction. It appears that the relative conversion is dependent on the hydrogen partial pressure; increasing the hydrogen pressure favors an increase in the relative naphthene conversion. The results eliminate the contribution of a gas phase naphthene in the dehydrocyclization mechanism.

Catalytic conversion of alcohols: V. Selectivity properties of Y2O3

Abstract Yttria was active for both dehydrogenation and dehydration of alcohols; it was more selective for dehydration of acyclic alcohols than for cyclic alcohols. The dehydration selectivity increased with increasing temperature and with increasing time-on-stream. The temperature coefficients for dehydration and dehydrogenation for both cyclic and acyclic alcohols were similar to those obtained with an alumina catalyst. Yttria was very selective in forming 1-alkenes from 2-ols. Cis - or trans -2-methylcyclohexanol was isomerized to the equilibrium cis/trans composition more rapidly than the combined dehydration-dehydrogenation reaction. It appears that the major pathway for the elimination of water from 2-methylcyclohexanol was an anti mechanism.

Catalytic conversion of alcohols: VI. Selectivity of indium oxide

India, in contrast to two other oxides of group IIIA metals (alumina and gallia), is active for dehydrogenation as well as dehydration. Furthermore, india is selective for the formation of 1-alkenes from 2-ols. Neither the alkene selectivity nor the dehydrogenation-dehydration selectivity changed uniformly as one progressed through the group IIIA metal oxide catalysts. Primary and secondary alcohols reduced indium oxide to an undetermined oxide composition at the reaction temperatures. The dehydrogenation-dehydration selectivity depended on the degree of reduction of the catalyst.