1

The Influence of Catalyst Porosity on Catalyst and Polymer Performance

By Max McDaniel

Not only the activity of Cr/silica catalyst, but the MW and LCB level of the polymer is controlled by the porosity and structue of the silica support.

2

Low PorosityHigh Porosity

Pore Volume = 2.5 cc/gSurface Area = 650 m2/g

Pore Volume = 1.0 cc/gSurface Area = 250 m2/g

3

Influence of Catalyst Pore VolumeHigh PV Cr/silica-titania catalyst was compacted in a press at up to 70,000 psig to produce catalysts of differing pore volume, but constant surface area.

4

Low PV Introduces a High-MW Tail

5

The Pore Volume of the Catalyst also Controls the Elasticity of the Polymer

6

Influence of Catalyst Surface Area on Polymer Elasticity

Commercial Silicas Of Varying Surface AreaCatalysts made from high-surface area hydrogel, alkaline aged to varying extents.

Low Surface Area Introduces a High MW Tail

Coalescence of silica gel through Oswald ripening during alkaline aging, results in a loss of surface area, expanding pore size, and a change from convex to concave pore surface.

Varying Surface Area by Alkaline Aging

Catalysts Made from Colloidal Silicas

MW, LCB and activity (/m2) depend on PV but not on surface area.

11

Pyrogenic Silicas

Higher flame temperature and longer residence time produces lower surface area from fusion and more knitting between primary particles.

LCB rises as surface area drops.

12

Precipitated Silicas

PrecipitateColloid Dried

Produce a bimodal PV distribution. Although the total PV may be high, most of the surface area is inside the small pores, which dominate catalyst character.

Preciptated silicas are also sometimes reinforced by secondary SiO2 deposition that lower surface area.

Both factors produce very high LCB.

Bimodal volume and surface distribution of precipitated silicas. The polymer properties tend to be dominated by the small pores, where most of the area is concentrated.

Precipitated Silicas

14

Precipitated & Reinforced Silica CatalystsProduce very high LCB, which varies with surface area.

MW distributions from precipitated and reinforced silicas show the same high-MW shoulder.

17

Surface area of a highly porous silica can be lowered by coalescence or reinforcement of the structure. LCB increases.

Surface area can also be lowered by gelation of larger colloidal particles. No influence on LCB.

Pore volume determines the number of contacts between primary particles, which controls the strength of the catalyst. LCB increases with lower PV.

LCB Increases with Silica Strength

Polymer made within the interior of a fragment has a higher LCB content from that made on the exterior of a fragment.Weak silicas yield smaller fragments, and proportionally more exterior surface.

Conclusions

Catalyst porosity provides a powerful tool for influencing polymer properties (MW, MWD, die swell, melt strength, orientation, etc).

This is possibly because catalyst porosity determines the strength of the silica structure and the degree to which it resists fragmentation during polymerization.