Modified HZSM-5 as FCC additive for enhancing light olefins yield from catalytic cracking of VGO

• Mn/HZSM-5 as an FCC additive for boosting light olefins yield.
• Alkaline treated HZSM-5 of micro–meso pore architecture as an FCC additive.
• Mn/HZSM-5 and mesoHZSM-5 exhibited comparable enhancement to light olefins.
• Mn/HZSM-5 exhibited higher density of Lewis sites compared with parent HZSM-5.
• Alkaline treated HZSM-5 showed high density of Lewis sites and improved diffusion.

HZSM-5 with varying Si/Al2 ratios were modified with Mn and alkaline treatment and tested as FCC catalyst additives to enhance the yield of light olefins (in particular, propylene) from catalytic cracking of hydrotreated Arab Light vacuum gas oil (VGO). The performance of the Mn/HZSM-5 and alkaline treated HZSM-5 additives were assessed using a commercial equilibrium USY FCC catalyst (E-Cat) in a fixed-bed micro-activity test unit at 550 °C and various catalyst/oil ratios. The yield of light olefins over E-Cat containing parent HZSM-5 increased to 18.1, 25.4 and 27.4 wt% for Si/Al2 of 30, 80 and 280, respectively, compared with 15.4 wt% over E-Cat (at 75 wt% conversion). Further increase in light olefins yield was achieved upon using Mn modified HZSM-5 as an additive. The highest light olefins yield of 29.2 wt% was achieved over E-Cat-Mn/HZSM-5 (Si/Al2 = 80, 2.0 wt% Mn). The enhanced production of light olefins over the Mn/HZSM-5 additive is mainly attributed to the decrease in the acid density and the amount of strong acid sites, and (possibly) the partial narrowing of ZSM-5 micropore. Alkaline treated HZSM-5 also exhibited high light olefins yield compared with parent HZSM-5, due to the formation of hierarchical micro-meso topology. The highest light olefins yield of ∼28.3 wt% was obtained over E-Cat-containing alkaline treated HZSM-5 (Si/Al2 = 80–280, using the appropriate desilication conditions).

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