Acidity modifications of nanozeolite-Y for enhanced selectivity to olefins from the steam catalytic cracking of dodecane

Steam catalytic cracking and lump kinetics of naphtha to light olefins over nanocrystalline ZSM-5 zeolite

This study investigates the reaction pathways and kinetics to comprehend the catalytic cracking of dodecane, a heavy naphtha model compound, over the nanocrystalline ZSM-5 catalyst in the presence and absence of steam with the aim of increasing olefin production. The nanocrystalline zeolite was characterized using XRD and BET, and the surface acidity was measured by NH3-TPD and Py-FTIR. The steam treated ZSM-5 contributed to an increase in pore volume with extra-framework alumina, resulting in highly catalytic active sites and hence higher olefin selectivity. The high conversion of dodecane (>90%) was achieved during catalytic cracking in the presence and absence of steam. In the presence of steam, the short pores of nano ZSM-5 led to an increase in the naphtha-to-olefin conversion with lesser dry gas and coke formation. The activation energies of primary cracking in the presence and absence of steam were slightly different. Lower activation energies through secondary cracking routes and higher reaction rate constants were obtained via assisted-steam catalytic cracking, promoted the selectivity towards light olefin products. Meanwhile the hydrogenation and alkylation reactions toward LPG and C5+ were favored in the absence of steam. Moreover, the ZSM-5 nano zeolite pores promoted more β-scission reactions, resulting in higher selectivity towards ethylene and dry gas.

Steam Catalytic Cracking of n-Dodecane to Light Olefins over Phosphorous- and Metal-Modified Nanozeolite Y

Nanozeolite Y was synthesized without a template and modified with phosphorous (P) and metals. P was introduced via impregnation with different weight loadings (0.5, 1, and 2 wt %), while ion exchange was developed to introduce zirconium (Zr) and cobalt (Co). The physicochemical properties of the catalysts were characterized with X-ray diffraction (XRD), N₂ adsorption-desorption, temperature-programmed desorption of ammonia (NH₃-TPD), and ²⁷Al and ³¹P solid-state nuclear magnetic resonance (NMR). The parent nanozeolite Y showed an identical XRD pattern to that of a previous study, and the modified nanozeolite Y showed a lower crystallinity. The introduction of P altered tetrahedral Al to an octahedral coordination, which affected the catalyst acidity. Then, the catalyst was evaluated to produce olefins from n-dodecane at 550, 575, and 600 °C. The conversion, gas yield, and olefin yield increased with increasing temperature. The maximum olefin yield (63%) was achieved with the introduction of 1 wt % P with the highest selectivity to ethylene. The Co-modified nanozeolite altered the zeolite structure and exhibited similar activity to the P-modified one. Meanwhile, Zr-modified nanozeolite Y caused excessive metal distribution, blocked the porous structure of the zeolite, and then reduced the catalytic activity.

Catalytic conversion of heavy naphtha to reformate over the phosphorus-ZSM-5 catalyst at a lower reforming temperature

Naphtha reforming to aromatics, naphthenes, and iso-paraffins is an essential process to increase the octane number of gasoline through the utilization of middle naphtha (whole). A ZSM-5 zeolite catalyst with modified medium pores was developed to comprehend the existing limitation of catalytic reforming to the unutilized refinery feedstock of heavy naphtha. The study applied a lower reforming conversion temperature (350 °C) than a conventional reformer without noble metal addition in an effort to lower the carbon footprint of the process and catalyst cost. The modified zeolite catalyst was impregnated with phosphorus oxide and spray-dried, followed by a hydrothermal treatment with steam. The parent and modified catalysts were characterized by NH₃-TPD, SEM, XRD, NMR, FTIR, and N₂ physisorption. Steam treatment was conducted to reduce the original zeolite acidity, mainly in the form of Brønsted acid sites, which resulted in the formation of phosphorus–aluminum species in the framework. The modified catalyst consisting of 40% ZSM-5 and 60% binder delivered high conversion of dodecane, and the reforming reaction selectivity favored the formation of carbonium ions through β-scission. Therefore, monomolecular cracking took place, resulting in the production of olefins and paraffin alongside iso-paraffins, aromatics, and naphthenes, which are associated with the bimolecular pathway. The reforming of heavy naphtha was different; the free radicals from β-scission were affected by the surrounding molecules of feedstock, and the bimolecular reactions were more dominant through zeolite pores. The study demonstrated that the addition of 10% steam during the reaction of heavy naphtha suppressed coke formation. Furthermore, high conversion and steady selectivity were maintained during the reaction, which resulted in gasoline reformate with a high research octane number (RON).

Catalytic conversion of heavy naphtha to reformate product over the phosphorus-ZSM-5 catalyst at a lower reforming temperature.