Chromia on alumina

CATOFIN [CATalytic OleFIN] A version of the Houdry process for converting mixtures of C3 - C5 saturated hydrocarbons into olefins by catalytic dehydrogenation. The catalyst is chromia on alumina in a fixed bed. Developed by Air Products Chemicals owned by United Catalysts, which makes the catalyst, and licensed through ABB Lummus Crest. Nineteen plants were operating worldwide in 1991. In 1994, seven units were used for converting isobutane to isobutylene for making methyl /-butyl ether for use as a gasoline additive. 

At K-band only a /3-phase type resonance was detected from the reduced chromia-on-alumina samples and was unchanged in width. No change in the character of the resonances from the reduced chromia-on-alumina samples was observed at —196 . 

The peak in the derivative of the 5-phase resonance does not change drastically in intensity upon oxidation. The /8-phase resonance, however, does change considerably in intensity in the intermediate concentration range (0.5-3.6 wt. %) upon oxidation. Upon exposure of reduced chromia-on-alumina samples of low concentration to air at room temperature, a rapid color change from blue to green occurs and the EPR characteristic of the 7-phase immediately appears, although reduced in intensity in comparison to the 7-phase resonance intensity after oxidation at 600°. 

Fig. 31. Concentration of unpaired electrons in 7-phase resonance of chromia-on-alumina versus weight per cent chromium (17S).

The relatively sharp y-phase resonance which appears upon oxidation of the reduced chromia-on-alumina is possibly due to single electrons trapped on Cr + ions, i.e., Cr + ions. 

One would expect the presence of trapped electrons in the oxidized samples to give rise to n-type conductivity, conduction possibly taking place by jump migration of the odd electron in the lattice of Cr + ions in a somewhat similar manner to the mechanism discussed by Heikes 174) for the migration of Ni + holes in lithia-doped NiO. The observed p-type conductivity of chromia in an oxygen atmosphere is presumably due to electron holes in a solid which is predominantly CraOs for the low concentrations of chromia-on-alumina where the 7-phase resonance intensity is maximum, the chromium is predominantly in the d-6 valence state 167). 

Eischens and Selwood 176) have made a study of the activity of reduced chromia-on-alumina catalysts for the dehydrocyclization of n-heptane. The activity per unit weight of chromium was found to increase sharply at concentrations below about 5 wt. % Cr activities were measured down to 1.9 wt. % Cr concentration at which point the highest activity was observed. Selwood and Eischens concluded that this effect is due to the fact that the chromia is most dispersed at these low concentrations, in agreement with the present EPR data. However, if a two-site mechanism 177) is necessary for dehydrocyclization, the activity may drop at even lower chromium concentrations due to isolation of individual chiomium spins. 

In summary, catalytic C-H transformations in small unfunctionalized alkanes is a technically very important family of reactions and processes leading to small olefins or to aromatic compounds. The prototypical catalysts are chromia on alumina or vanadium oxides on basic oxide supports and platinum on alumina. Reaction conditions are harsh with a typical minimum temperature of 673 K at atmospheric pressure and often the presence of excess steam. A consistent view of the reaction pathway in the literature is the assumption that the first C-H abstraction should be the most difficult reaction step. It is noted that other than intuitive plausibility there is little direct evidence in heterogeneous reactions that this assumption is correct. From the fact that many of these reactions are highly selective toward aromatic compounds or olefins it must be concluded that later events in the sequence of elementary steps are possibly more likely candidates for the rate-determining step that controls the overall selectivity. A detailed description of the individual reactions of C2-C4 alkanes can be found in a comprehensive review [59]. 

DETOL [DE-alkylation of TOLuene] A process for making benzene by de-alkylating toluene and other aromatic hydrocarbons. Developed by the Houdry Process and Chemical Company, and generally similar to its Litol process for the same purpose. The catalyst is chromia on alumina. Licensed by ABB Lummus Global. Twelve plants, had been licensed in 1987. 

FBD [Fluidized Bed Dehydrogenation] A catalytic process for converting alkanes to alkenes. Two fluidized beds are used a reactor and a regenerator. The catalyst beads are recirculated from the regenerator to the reactor, providing heat to the reactor. The catalyst is chromia on alumina. 

A comparison of the UV Raman spectrum measured for coke deposited during the MTH reaction with that deposited during butane dehydrogenation catalyzed by chromia on alumina (66) shows clear differences in the spectral intensity distribution (Fig. 11). In particular, the intensity of the features in the regions 1340-1440cm and 1560 1630 cm are nearly equal for the MTH reaction. 

Sullivan, V.S., Jackson, S.D. and Stair, P.C. (2005) In situ ultraviolet Raman spectroscopy of the reduction of chromia on alumina catalysts. Journal of Physical Chemistry B, 109 (1), 352-6. 

A detailed description of a chromia-on-alumina catalyst prepared by impregnation has been given elsewhere . Another supported nonmetallic catalyst widely used commercially is cobalt molybdate-on-alumina. The preparation of this catalyst using an alumina support with controlled pore-size distribution is as follows. Silica-stabilized alumina, with greater than 50% of its surface area in 3-8 nm pores and at least 3% of the total pore volume in pores greater than 200 nm in diameter, is impregnated with an aqueous solution of cobalt and molybdenum. The finished oxysulfide catalyst was tested for hydrodesulfurization of petroleum residuum at 370°C and 100 atm for 28 days and compared with a convential cobalt-molybdate catalyst having a major portion of the surface area in 3-7 nm pores. The latter catalyst and controlled pore catalyst maintained 57 and 80% activity, respectively. 

Another type of selectivity effect that can become important during the deactivation of oxidation catalysts for halogenated hydrocarbons arises from the inherent selectivity of the catalyst. Ramanathan and Spivey 0 studied the catalytic oxidation of dichloroethane and of trichloroethane over chromia on alumina catalyst. The selectivity observed is summarized in Figure 9, below ... 

The dehydrogenation process was first commercialized in the 1940 s (Houdry s CATADIENE). The initial catalyst used was chromia on alumina. Most commercial process still use this catalyst today. However, two recent processes use Pt on alumina catalyst (UOP s OLEFLEX) (Pujado, 1990) or other noble metals Phillip s STAR) (Brinkmeyer et al, 1983). 

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