Chromium oxide gel

Plot of pMp° - p) against p/p° (r is expressed in cm (stp)). (1) Unpromoted Fe catalyst (2) AljOj-promoted Fe catalyst (3) AI2O3-KjO-promoted Fe catalyst (4) fused copper catalyst (5) chromium oxide gel (6) silica gel. (Courtesy Brunauer, Emmett and Teller.)... 

It has been known for many years that the adsorptive and catalytic properties of chromium oxide gels are very sensitive to the conditions of preparation, storage and heat treatment (Burwell et al., 1960 Deren et al., 1963 Carruthers and Sing, 1967 Baker etal., 1970,1971). 

Catalytic systems available for the synthesis of pentafluoroethane (CF3CHF2 HFC-125) are essentially similar to those reported for the synthesis of HCFC-123 and HCFC-124. If one starts with PCE, the end point after the initial HF addition is HFC-125. Again, as with HCFC-124, a suitable pentahaloethane such as HCFC-123 or even HCFC-124 can also be employed. Active catalysts seem to contain chrome in some form. Pretreatment of such catalysts have been reported to modify activity. For example. Firth and Foil (15) have claimed that if a chromium hydroxide precursor is treated with steam prior to calcination to chromium(III) oxide, the activity is superior to the one prepared without such a pretreatment. Swamer (66) has disclosed that chromium oxide gel prepared by special techniques involving the addition of alcohol during the preparative procedure is an excellent catalyst for both HF addition as well as halogen exchange. 

These observations, when extended to other oxides or mixtures of oxides, gave in part confirmatory data and, in one case, that of manganese-chromium oxide, data which significantly differed from those obtained with zinc oxide. Thus, with chromium oxide gel with a surface area of 189.5 sq. meters/g. the quantities of gas involved in the desorption-readsorption phenomena between 56 and 302° C. amounted to 9 cc. or nearly 5% of the total surface. With a zinc chromium oxide of 21 sq. meters/g. the area involved in desorption-readsorption phenomena was 13% of the total surface between 0 and 302° C. With manganese-chromium oxide, on the other hand, no readsorption of gas was observed when the sample was cooled in hydrogen from 218 to 0° C. 

When, however, desorption does occur in the ascending portion of the curve BC, there should, on cooling down from C, no longer be a horizontal section CE but continuously increasing values of the adsorption CG, measuring, in excess of the values over the horizontal section, the quantity of gas which was desorbed during the measurements made in passing upwards in temperature from B to C. It is this behavior which is shown by zinc oxide, chromium oxide gel, and zinc-chromium oxide. 

In a study of the effect of gas pressure on the chemisorption of hydrogen on chromium oxide gel, Taylor and Burwell (6f) found it necessary to resort to an arbitrary process of subtraction from experimentally observed amounts of adsorbed gas in order to display a relatively uniform area of gel for hydrogen chemisorption. Within close limits, at temperatures of 457 and 491° K. and at pressures of 1, 0.5 and 0.25 atmospheres, the energy of activation for the adsorption process was nearly independent of the amounts of adsorption (0-35 cc.) and the activation energy was about 21 kcal. In the temperature range 383 to 457° K. no such uniformity was observed. For example, if the amounts adsorbed with time at 405 and 427° K. be used to calculate activation energies the values received increase from 0 to 18.5 kcal. as the amount of gas adsorbed on the surface increases from 1.6 to 8.5 cc. Similar results can be drawn from the earlier measurements of Kohlschutter (17). 

Beebe and Dowden (18) measured the heat of adsorption of hydrogen on chromium oxide gel at liquid air temperatures. They found a heat of adsorption of 5 kcal./mole which is larger than that to be expected for van der Waals adsorption. This gas, however, with the low energy of activation allowable at liquid air temperature cannot be held to the surface in the temperature range 457 to 491° K. where chemisorption with an activation energy of 21.7 kcal. on an area relatively uniform and large in extent is occurring. 

Precipitation with an acid or a base (see hydrated chromium oxide gel, p. 1648 silica gel, p. 1648). 

The effect will be clear from a comparison of the magnetic properties of chromium oxide gel with those of massive crystalline chromic oxide. Chromium oxide gel may be made by precipitation of the hydroxide from a nitrate solution, followed by slow dehydration. Several other processes are available, of which slow reduction from a basic chromate solution is one. On ignition, these gels generally undergo the glow-phenomenon during which they revert to Crystalline chromic oxide. 

Table II shows the magnetic susceptibility of a chromium oxide gel compared with that of crystalline chromic oxide. The gels always contain some water, hence a more striking comparison is made by calculating the susceptibility of the chromium ions in each substance.

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