COKE FORMATION AND ITS EFFECTS ON SHAPE-SELECTIVE ADSORPTIVE AND CATALYTIC PROPERTIES OF FERRIERITE

Channels or cavities of ferrierite are blocked by carbonaceous deposits (coke) which are formed during butene treatments. The pore blocking inside ferrierite/alumina catalysts affects the yield and selectivity to isobutylene in the catalytic reaction of butene isomerization. Pore size distribution experiments show that the blocking of 10-member ring channels (4.2 x 5.4 Angstrom) and 8-member ring channels (3.5 x 4.8 Angstrom) of ferrierite by coke reduces the channel size smaller than that of the nitrogen molecule (4.09 Angstrom). TPD data show that ammonia uptake for coked samples of different times on stream is decreased from about 62% to 35% of that fbr the fresh sample. This suggests that channels in coked ferrierites are at least larger than the size of the ammonia molecule (2 Angstrom). Uptakes for more bulky molecules such as 1-butene (2.99 X 4.71 Angstrom) and isobutylene (3.28 x 4.14 Angstrom) are severely reduced by coke formation (< 9% of 1-butene and isobutylene uptakes for the fresh sample). About 76.9% of the coke formed after 18 h is deposited inside the micropores (< 10 Angstrom) of ferrierite/alumina. Modification of pore shapes through such coke deposition favors reactions involving small molecules, such as butene isomerization to isobutylene. In addition, kinetics studies of coke formation with microbalance methods reveal that the amount of coke deposit is limited to less than 11.1 wt % by the micropore volume of ferrierite. Such coke is aromatic in nature, and its hydrogen to carbon ratio decreases with time on stream. Acid sites on ferrierite having different size constraints are classified as H+(XL), H+(L), and H+(S) (XL = extra large, L = large, S = small) from a structural point of view. These three kinds of acid sites were successfully probed with ammonia, 1-butene, and isobutylene. These experiments suggest that the adsorption of probe molecules on ferrierite is also a shape selective process.