Vapor transport, catalyst loss

Loss of Catalyst by Vapor Transport. The direct volatilization of catalytic metals is generally not a factor in catalytic processes, but catalytic metal can be lost through formation of metal carbonyl oxides, sulfides, and halides in environments containing CO, NO, 02 and H2S, and halogens (24). 

The anomalous behavior of the sulfated zirconia catalysts is due to the loss of sulfuric acid at elevated temperatures. The catalyst prepared fi-om calcined zirconia looses its sulfuric acid at 673 K and, consequently, is not active at this temperature. As a result, decreasing the temperature of the catalyst to 473 K does not restore the activity. Also the more highly porous catalyst prepared from zirconium hydroxide releases sulfuric acid, but in narrow pores some sulfuric acid is left. The loss of sulfuric acid at 673 K is obviously irreversible. When the catalyst prepared from zirconium hydroxide is, however, kept at 473 K, the transport of water out of the porous structure is thus low that a stable activity is exhibited. Pre-hydration of the bulk anhydrous zirconium sulfate does not provide an active catalyst. That no catalytic activity is induced in this case is due to the fact that bulk anhydrous zirconium sulfete readily reacts to a stable tetrahydrate, viz., Zr(S04)2.4H20 [1]. As a result the hydrolysis of the sulfate by water vapor is suppressed. 

The crystallites sinter (i.e., aggregate to form larger crystallites) in operation by processes that include migration on the support and vapor-phase transport of volatile compounds of the metal. Since sintering implies loss of surface area and, therefore, loss of catalytic activity, supported metal catalysts are typically redispersed (regenerated),... 

However, if a catalyst is used in the liquefaction cycle, para-hydrogen can be produced directly with minimal vaporization loss from self-generated heat of conversion in the liquid storage tank. For this reason, para-hydrogen is the preferred form for storage and transport of the liquid, with 95 percent para being the usual specification. 

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