Optimization of nanopores and acidity of USY zeolite by citric modification

To meet the challenges of the crude oil becoming heavier and poorer quality , and the tendency of the increasing liquefied petroleum gas (LPG) and dies el in fluid catalytic cracking (FCC) process, the ultrastable Y (USY) zeoli tes were further modified by citric acid to optimize their nanopores and ac id sites. The nanopores and acid sites of modified zeolites were extensivel y investigated by N-2 adsorption, XRD and in situ IR methods. The micro-act ivities of modified samples were also evaluated. The results showed that th e nanopore volume of the modified samples is rather more developed than tha t of the parent zeolite. It indicated that micropores and mesopores might b e generated by skeletal reconstruction. The dealumination process might be occurred gradually from the upmost surface to subsurface, then to bulk site s of the USY framework because of the larger volume of citric acid molecule s. It resulted in the gradient distribution of nanopores and acid sites in modified zeolites. The hydroxyls with IR bands of 3610 and 3575 cm(-1) in t he micropores or small cages bears the stronger acidity and much more sites than those in mesopores or larger cavities. This gradient acid distributio n was rather preferable to produce preferably LPG and diesel in FCC process . MAT evaluations indicated that the LPG and diesel yields of citric modifi ed samples were effectively improved. Optimization of nanopores and acid si tes of ultrastabilized USY zeolites modified with citric acid in unbuffered system might result in an industrial process to design novel FCC catalysts . (C) 2001 Elsevier Science B.V. All rights reserved.