Page 50 - Principles of Catalyst Development

36 CHAPTER 2
during catalyst regeneration. Furthermore, the phase change occurs slowly
at lower temperatures, which for anticipated lifetimes (3+ years) could
result in significant changes. Incorporation of small amounts of Si02 or
Zr02 into the 'Y-AI20J (only 1-2%) moves the a-AI 20 J phase transition to
higher temperatures. (65) The support is then adequately protected against
major upset and long term changes.
Most often, promoters are added to supports in order to inhibit undesir-
able activity, such as coke formation. Coking originates from cracking on
Bronsted sites followed by acid-catalyzed polymerization to give (CHx)n
species that cover surface sites and ultimately block pores. Removal of the
coke by burning may itself lead to activity loss due to sintering. Acidic
cracking sites are neutralized by bases, most effectively alkalis. (66) Potassium,
added as potash during preparation, lowers the coking tendencies of alumina
supports. Sodium is used also but appears to facilitate sintering and is more
mobile, often poisoning the active component.
When dual-functional activity is needed, as in catalytic reforming, extra
acidity is achieved by adding chloride ions to the surface. (56) Since chlorine
is removed during regeneration, it must be replenished during reactivation
or chlorine compounds must be added to the feed during operation.

TABLE 2.5. Examples of Promoters in Major Processes

Catalyst Promoter Function

AI 20, Si0 2 , ZrO: , P Improves thermal stability
Support and catalyst K,O Poisons coking sites
HCI Increases acidity
MgO Retards sintering of active component
Si0 2·AI 20, PI I ncreases CO oxidation
Cracking catalyst and matrix
Zeolites Rare earth ions Increases acidity and thermal stability
Cracking catalyst Pd Increases hydrogenation
PI/ AI 20, Re Decreases hydrogenolysis and sin·
Catalytic reforming tering
MoO,/AI 20, Ni,Co Increases hydrogenolysis of C-S and
Hydrotreating C-N
P,B Increases MoO, dispersion
Ni/ceramic supports K I ncreases carbon removal
steam reforming
Cu-ZnO-AI 20, ZnO Decreases Cu sintering
Low temperature shift