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When the microspheres of the present invention contain more than about 4% by weight rare earth oxides, they have excellent hydrothermal stability. In particular, their deactivated and severely deactivated activities are at least about 1.5 times greater than, and preferably at least about 2.0 times greater than, that of standard commercial catalyst.

Analyses of the products obtained during the ASTM standard procedure described above show that the catalyst of the present invention has good to excellent selectivity characteristics. In particular, the preferred microspheres of the catalyst of the present invention, when they are steam deactivated to provide about 70% measured conversion in the ASTM standard procedure, yield no more than about the amount of coke and most preferably at least about 20% less coke, and no more than about the amount of hydrogen and most preferably at least about 25% less hydrogen, than does standard commercial catalyst which has also been steam deactivated to provide about 70% measured conversion. In the same tests, microspheres of the present invention having a micropore volume of less than about 0.05 cc/g. typically yield somewhat less light cycle oil at 70% measured conversion than does standard commercial catalyst. The procedures by which coke and hydrogen yields at 70% measured conversion may be determined are described in Example 1 below. Hereinafter, hydrogen and coke yields at "70% measured conversion" are referred to as hydrogen and coke yields at "70% conversion".

The activity and selectivity characteristics of the microspheres of the catalyst of the present invention are achieved even though, in some cases, the microspheres have relatively low total porosity, as compared to fluid catalytic cracking catalysts currently on the market. In particular, the microspheres of the present invention, in some cases, have a total porosity of less than about 0.15 cc/g. or even less than about 0.10 cc/g. In comparison, we are unaware of any fluid catalytic cracking catalyst currently on the market that has a total porosity of less than 0.20 cc/g. As used herein, "total porosity" means the volume of pores having diameters in the range of 35-20,000.ANG., as determined by the mercury porosimetry technique described below.

We believe that it is surprising that microspheres having a total porosity of less than about 0.15 cc/g. exhibit the activity and selectivity characteristics of the microspheres of the present invention. For example, such a result is contrary to the prior art disclosure discussed above that low pore volumes "can lead to selectivity losses due to diffusional restrictions."