“…Even zeolite Y has problems of access by molecules in the higher boiling range [157,158], and it is for this reason that sometimes an ASA is added [159]: it is much less acidic but it does not induce any diffusion limitations, thus serving to convert the species that the zeolite cannot cope with [160] (see Figures 18.16 and 18.17). Zeolite Y can be modified by combinations of steaming and/or calcination, ion-exchange, leaching, and various chemical treatments [161,162], with the aim of controlling acid-site density, nonframework aluminum species and mesoporosity, while preserving a high crystallinity [99,163,164]. The experimental space set up by these techniques (see Figure 18.18) is so enormous that to this day new, more effective modifications of Y continue to be found.…”