TY - JOUR
T1 - The roles of acid strength and pore diffusion in the enhanced cracking activity of steamed Y zeolites
AU - Williams, B. A.
AU - Babitz, S. M.
AU - Miller, J. T.
AU - Snurr, R. Q.
AU - Kung, H. H.
N1 - Funding Information:
The authors would like to acknowledge financial support from the National Science Foundation, Division of Chemical and Thermal Systems, the Engelhard Corporation and the Ashland Petroleum Company.
PY - 1999/2/22
Y1 - 1999/2/22
N2 - Steamed HY or ultrastable Y (H-USY) zeolites are active hydrocarbon cracking catalysts. The high activity of H-USY compared to HY zeolite has been previously explained by the generation of unusually strong and active Brønsted acid sites, or an increase in the number of accessible sites in a micropore diffusion-controlled reaction. However, neither model explains the accumulated literature observations. A model is proposed that incorporates a change in the predominant cracking reaction mechanism as a function of alkane conversion and the very different rates of these mechanisms. Additionally, an oligomeric cracking mechanism is introduced to explicitly account for coking and deactivation of the catalyst. The model is capable of accounting for most literature results. It concludes that the large enhancement in cracking activity by steaming is due to a proportionally smaller increase in external surface area of the zeolite crystals and possibly a small increase in the specific initiation activity of each site. These small changes lead to a much larger overall effect because of the sensitive dependence of oligomeric cracking and, to a lesser extent, bimolecular cracking on the alkene partial pressure.
AB - Steamed HY or ultrastable Y (H-USY) zeolites are active hydrocarbon cracking catalysts. The high activity of H-USY compared to HY zeolite has been previously explained by the generation of unusually strong and active Brønsted acid sites, or an increase in the number of accessible sites in a micropore diffusion-controlled reaction. However, neither model explains the accumulated literature observations. A model is proposed that incorporates a change in the predominant cracking reaction mechanism as a function of alkane conversion and the very different rates of these mechanisms. Additionally, an oligomeric cracking mechanism is introduced to explicitly account for coking and deactivation of the catalyst. The model is capable of accounting for most literature results. It concludes that the large enhancement in cracking activity by steaming is due to a proportionally smaller increase in external surface area of the zeolite crystals and possibly a small increase in the specific initiation activity of each site. These small changes lead to a much larger overall effect because of the sensitive dependence of oligomeric cracking and, to a lesser extent, bimolecular cracking on the alkene partial pressure.
KW - Cracking mechanism on zeolites
KW - Cracking, effect of acid strength
KW - Cracking, effect of diffusion
KW - Cracking, effect of steaming
KW - Diffusion effect on cracking
KW - Hydrocarbon cracking
KW - Zeolite Y
UR - http://www.scopus.com/inward/record.url?scp=0040887840&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0040887840&partnerID=8YFLogxK
U2 - 10.1016/S0926-860X(98)00264-6
DO - 10.1016/S0926-860X(98)00264-6
M3 - Article
AN - SCOPUS:0040887840
VL - 177
SP - 161
EP - 175
JO - Applied Catalysis A: General
JF - Applied Catalysis A: General
SN - 0926-860X
IS - 2
ER -