Structure Design Of MeAPO-11 Molecular Sieve And The Effect On The Performance Of Fluid Catalytic Cracking Catalyst

In order to meet the demand of National V emission standard,gasoline hydrofining is inevitable to decrease the concentration of sulfide.However,olefin saturation occurs and it causes the decrease of research octane number（RON）value.It is more serious for FCC cracking gasoline,which has a high concentration of olefin.To sovle the contradiction between using hydrofining process for clean production of gasoline and the loss of gasoline RON caused by olefin saturation,the performance of FCC catalyst is regulated by adding RON additives to increase the yield of iso-paraffin.The previous research has proved that SAPO-11 molecular sieve can increase the yield of iso-hydrocarbon of gasoline in the cracking reaction of vacuum gas oil.SAPO-11 molecular sieve shows the potential to be the novel RON additive.Therefore,MeAPO-11 molecular sieve was selected as the additive in the present research.Properties can be tuned by optimizing synthetic conditions and framework compositions.The synergistic effect of Y and additive zeolites is investigated via n-dodecane cracking reaction,which can guide the design of additive and provide the theoretical basis for developing novel RON additive.It is available to regulate acidity,morphology,and particle size of SAPO-11 by combining the conventional dry gel conversion method with microwave aging treatment and freeze drying techniques.At a lower temperature of microwave irradiation（40°C）,polymerization of silicon species increases.Most silicon atoms incorporate into framework via SM II mechanism.An increase of temperature to 90 ^oC,the breakage of Si-O bond occurs and the concentration of soluble silicon species increases in sol gel.The substitution mechanism changes from SM II to SM III to form more“silica island”structure（i.e.,Si（4Si））.The Br?nsted acid sites on the edge of silica island are stronger.Besides,a transition of morphology from irregular bulks to nano-sized stripe-like particles（200 nm in length）is observed after freeze drying.These nano-sized particles,possessing hierarchical pore structure,have narrow grain and particle distributions due to the electrostatic repulsion of surface hydroxyl groups.Preparing crystalline precursor by grinding assisted method can increase the concentration of crystal nucleus.The strong interaction among raw material species enhance diffusion rate.Therefore,using grinding assisted method achieve rapid synthesis of SAPO-11 with fine particle size.The crystal size is tunable by chaning amount of silicon atoms.As the silicon atoms increase,the crystal size decreases.It is available to regulate the coordination and electronic cloud of Al and P atoms by incorporating different metal atoms with active valence electrons.Among these MeAPO-11molecular sieves,LnAPO-11(Ln=La³⁺and Ce³⁺)molecular sieves have a superior hydrothermal stability.It can preserve the crystal structure after hydrothermal treatment at800^oC for 4 hours.The coordination of rare earth ions is investigated and the mechanism of enhancing MeAPO-11’s hydrothermal stability by rare earth ions is proposed.Ln（III）ions can replace Al（III）and P（V）sites.It is preferable to replace P（V）sites in thermodynamics.The substitution causes the increase of T-O bond length and framework distortion.When P（V）sites are substituted,framework negative charge attracts H⁺and T-O defects form,which are the source of Br?nsted and Lewis acid sites,respectively.As more Ln（III）ions are added,the incorporation is inhibited.Most ions exist as extra-framework ions.Besides,role of rare earth ions in enhancing hydrothermal stability lists as follows:（1）rare earth ions regulate coordination of silicon atoms and form more Si（4Al）and Si（3Al）species,which are more stable in hydrothermal atmosphere;（2）rare earth ions react with phosphate anions to form LnPO₄,whose phosphate anions participate the construction of zeolitic framework.It inhibits framework from attacking by water molecules;（3）Electrostatic potential energy decreases and bond strength of Al-O increases after adding rare earth ions.Superior hydrothermal stable MeAPO-11 molecular sieve can be employed as additives on fluid catalytic cracking conditions.Strong Br?nsted acid sites on Y zeolite play a key role in chain initiation of paraffin molecules.A proportion of paraffin can be cracked on additives.The formed paraffin carbenium ions are isomerized on additives.That is a main source of iso-paraffin based on the synergistic effect of Y and additive zeolites.Compared with SAPO-11,the acidity of LnAPO-11 molecular sieve is less and weak,which can meet the demand of isomerization reaction and has less contributions to the cracking of iso-paraffin.Extra-framework metallic sites polarize the C-H bond to form carbenium ions for isomerization reaction.It increases the conversion to 87.83%with a high yield of iso-paraffin（10.21%）.This research can provide technical support for developing novel RON additives.