Performance of ZSM-5 as a Fluid Catalytic Cracking Catalyst Additive:  Effect of the Total Number of Acid Sites and Particle Size

Abstract

The performance of ZSM-5 as a fluid catalytic cracking (FCC) catalyst additive has been tested in a microactivity test unit for the cracking of gas oil. Laboratory-synthesized ZSM-5 samples, with various Si/Al ratios, fresh or hydrothermally dealuminated were tested. A 2 wt % ZSM-5, on total catalyst weight, found by previous investigators to be the optimum additive concentration, was used for all of the experiments. It has been found that a direct and smooth correlation exists between the product yields and the total number of ZSM-5 acid sites measured by ammonia temperature-programmed desorption tests. This is a unique function of the aluminum content of fresh samples as well as the temperature of the hydrothermal deactivation of steamed ZSM-5 zeolite samples. Previous works have been qualitatively compared to the results of this work and have been classified on the basis of different total acidity regions of the ZSM-5 used in each case. Higher total acidity results in gasoline loss, liquified petroleum gases (LPG) and ethylene increases, and research octane number gain. The C5 aliphatics are increased in the low acidity region because of cracking and isomerization of larger alkenes, while an increase of the total ZSM-5 acidity resulted in a monotonic decrease of all of the gasoline range hydrocarbons, except of n-C5 alkane, C6−C7 aromatics, and C5 naphthenes. The branched/linear (B/L) ratios for C5−C6 alkenes were found to increase with acidity, while the B/L ratios for C5−C9 alkanes decrease. The number and strength of the additive's acid sites control the contribution of the different reaction paths, through which cracking, isomerization, and aromatization occur, to the final product distribution. The sizes of the ZSM-5 particles have a strong effect on the changes in product yields and gas and gasoline compositions. Smaller particles favor a decrease in gasoline and an increase in LPG yield more than larger particles. The effect is more pronounced in the high acidity region where the decrease in the yields of C7+ isoalkanes, naphthenes, and aromatics is favored by small particle additives, while the effect of particle size on gasoline range hydrocarbons is clearly evident on the yields of gaseous products.

Catalytic pyrolysis of polyethylene: A comparison between pillared and restructured clays

“…As expected, even a low-acidity H-ZSM5 (Si/Al = 235) was a more active catalyst, being its acidity much higher than that of the clay-based catalysts. On the other hand, the stronger and more numerous zeolitic acid sites [34][35][36][37] must have accelerated the initial breakdown of the original macromolecules and, thereby, the overall degradation reaction. This higher acidity leads to higher production of heavily cracked products, which, in turn, produces smaller molecules, a lower liquid fraction and higher coke formation as occurs in USY as well as in H-ZSM5 [28].…”

Maximizing propylene production via FCC technology

“…In ZSM-5 there exists a remarkable molecular sieving effect for light hydrocarbons and this has been widely used as shape selective catalysts in various hydrocarbon processes; however, because the crystal sizes of ZSM-5 are usually much larger than the sizes of the micropores, the rate limiting step of the reaction tends to be the diffusion of the reactant/product molecules within the micropores [40,54,101]. Moreover, carbon solid (coke) readily forms near the external surface of the crystal under diffusion controlled conditions, thereby, rapidly plugging the pores, leading to a short catalyst lifetime.…”

Solid-state MAS NMR studies on the hydrothermal stability of the zeolite catalysts for residual oil selective catalytic cracking

“…Therefore, the preservation of good activity and selectivity in the highly severe reaction and regeneration processes of RSCC requires the catalysts to maintain good thermal and hydrothermal stabilities. The HZSM-5 zeolite shows unique shape-selective properties and moderate acidity, and is usually used as catalyst additives in gas oil cracking, and plays a key role in enhancing the gasoline octane rating [7]. It has rather small (intermediate size) pore openings (5.5 Å), which restricts the access of branched and cyclic hydrocarbons from the matrix of the catalysts into its pores where the active sites are located, but allows straight chain and monomethyl paraffins to enter and be preferentially cracked to lighter products [6].…”