Silica poisoning, effect reduction

Partial reduction and poisoning experiments were performed with a Cr/silica-titania activated at 600 °C. The polymerization activity is shown in the upper part of Figure 20 as a function of the reduction temperature in CO. CO treatment at 25 °C had no effect no reduction occurred and full activity was observed. The catalyst remained unchanged from its original orange color. However, when the catalyst was treated with CO at 300 °C,... 

The metathesis of propene on WOs/Si02 is speeded up by pretreatment of the catalyst with HCl (Pennella 1974 Aliev 1977) but the product but-2-ene undergoes considerable isomerization to but-l-ene (Aliev 1978). The inclusion of 1% cycloocta-1,5-diene (COD) in the propene stream also increases the rate of metathesis and reduces the break-in time from 20 min to less than 5 min at 500°C. The latter effect disappears when the additive is removed, so it is not due to reduction of the catalyst. The effects of both HCl and COD have been attributed to a favourable modification in the metal d-orbital levels as the result of the presence of new ligands (Pennella 1973, 1974). Pretreatment with hexamethyldisilazane (HMDS) at 250°C also has a remarkable effect on the activity, increasing it as much as 140 times for the metathesis of propene at 427°C. The same treatment of silica alone completely eliminates its capacity to isomerize rra/j5-but-2-ene at 427°C, so it is concluded that the Bronsted acidic hydroxyl groups, poisoned by HMDS, are not likely to be the precursors for the active sites in propene metathesis over W03/Si02 (van Roosmalen 1980a, 1982). 

Silica often acts as a type of resin poison, but its effect is more mechanical than chemical. Dehydration of the silica, or its production from dissolved silicic acid, leads to its deposition upon the resin beads and to some extent in the interior of the beads. This occurs particularly near the point of entry of the feed solution to the column. This may result in restriction of flow rate or even a complete obstruction to flow, if the voids between beads become filled. In addition, the silica coating upon the resin beads causes the diffusion rate to be reduced. This gives an apparent reduction in the resin capacity. Some minerals give sulphuric acid leach liquors with a higher concentration of silica than others, e.g. the ore from the Mary Kathleen plant in Australia. The reason for this has been stated to be the high degree of silica breakdown which has occurred in the mineral owing to the action of a-radiation. ... 

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