Study On Catalytic Cracking Performance Of Naphthenic Aromatic Hydrocarbons On Different Zeolites

Catalytic cracking light cycle oil(LCO)has high contents of polycyclic aromatics,and difficulty in hydrogenation upgrading.LCO can't be directly used for the blending component of vehicle diesel.At the same time,with the change of fuel demand,the consumption of diesel to gasoline is declining year by year,so the way of LCO is facing severe challenges.The LCO hydrogenation-catalytic cracking combined process is one of the ideal ways to solve the problem of LCO excess.The difficulty of this process is how to convert the hydrogenated LCO to the desired product such as light aromatics and light olefin.The naphthenic aromatic hydrocarbon model compound tetralin was used as a probe molecule to study its catalytic cracking performance on different zeolites to lay the foundation for optimizing the catalytic cracking catalyst of naphthenic aromatics.Firstly,using the fixed bed micro-reactor experiment device,the catalytic cracking performance of tetralin on USY,Beta and ZSM-5 zeolites was investigated.The conversion of tetralin and the selectivity of the target product on the three zeolites were largely different.The pore structure had a significant effect on the catalytic cracking of tetralin.Under the same conditions,the conversion rates of tetralin on ZSM-5 and Beta zeolites with similar acid amounts were 11.5 wt.% and 81.3 wt.%,respectively.Beta zeolite had higher isomerization selectivity and some naphthenic rings couldn't be opened.USY zeolite contained super-cage structure and large acid density,so the hydrogen transfer reaction was severe.When the conversion of tetralin was about 80 wt.%,the yield of dehydrogenation products of USY zeolite could reach 40.4 wt.%,and the Beta zeolite was only 29.7 wt.%.At low conversion,the three zeolites all tended to undergo ring-opening reaction.Secondly,the effects of olefins,alkanes and polycyclic aromatics on the catalytic cracking performance of tetralin on USY and Beta zeolites were investigated.The hydrogen supply capacity of each hydrocarbon was as follows: methylcyclohexane >n-hexadecane >1-hexene>methylnaphthalene.As the hydrogen supply capacity of other hydrocarbons was weakened,the ring opening selectivity of tetralin decreased,and the dehydrogenation selectivity increased on USY zeolite.1-hexene could reduce the ring opening selectivity of tetralin on USY zeolite by 14.2 percentage points while the hydrogen selectivity of tetralin on Beta zeolite was only 13.2 % due to the limit of pore structure.Methylcyclohexane promoted the ring opening reaction of tetralin on both zeolites.Under the similar conversion,methylcyclohexane could increase the ring-opening selectivity of tetralin by 17.7 percentage points on USY zeolite while 9.9 percentage points on Beta zeolite.N-hexadecane had a certain promoting effect on ring opening reaction of tetralin on both zeolites.Under the similar conversion,n-hexadecane could increase the ring-opening selectivity of tetralin by 7.6percentage points on USY zeolite while 19.2 percentage points on Beta zeolite.Methylnaphthalene significantly reduced the ring opening selectivity of tetralin on both zeolites.Under the similar conversion,methylnaphthalene could increase the hydrogenation selectivity of tetralin by 18.3 percentage points on USY zeolite while 21.9 percentage points on Beta zeolite.Under the same reaction conditions,Beta zeolite always appeared higher isomerization selectivity and lower conversion than USY zeolite.Finally,the catalytic cracking performance of tetralin on different modified USY zeolites was studied.It was favorable to the ring-opening of naphthenic aromatics with high Br?nsted acid content,low Lewis acid content,large B/L value,high acid strength and large acid amount.After hydrothermal treatment,the Br?nsted acid content of USY zeolite decreased,so the conversion and ring opening selectivity of tetralin decreased.Under the same conditions,the conversion of tetralin on USY zeolite after aging at 720 °C for 4 h was 18.6 percentage points lower than before.At the same conversion,the dehydrogenation selectivity increased by 7.0percentage points.Phosphorus modification could reduce the Lewis acid content.At a phosphorus content of 2.0 wt.%,the conversion and ring opening selectivity of tetralin reached the highest,7.8 and 1.4 percentage points higher than before,respectively.When the phosphorus content was high,it could destroy the crystal structure,block the pores and reduce the conversion and ring opening selectivity.When the USY zeolite was treated with citric acid solution,the Br?nsted acid content and B/L value were increased,and the zeolite pores were more smooth.When the concentration of citric acid solution was 0.3 mol/L,the conversion and the ring-opening selectivity of tetralin on USY zeolite could be increased by 5.4 and 7.8percentage points respectively.