Abstract:

Catalytic cracking is a process of mass transfer and diffusion of heavy oil macromolecules in the catalyst.It is also a process of acid catalytic reaction.Therefore,the well-developed pore structure and appropriate acidity are important research directions in the design and development of catalytic cracking catalyst.Y zeolite is an important component of FCC catalyst and an important acid supplier.For Y zeolite,there are many ways to adjust its acidity,such as metal modification,hydrothermal super stable acid treatment,etc.Hydrothermal super stable method is often used in industry.Through hydrothermal super stable method,part of framework aluminum of molecular sieve is removed and Al in framework is replaced by Si.As a result,the density of acid center of active component Y zeolite decreases,the acid strength increases and the secondary pores are more abundant.However,the framework of the zeolite will be destroyed significantly after hydrothermal treatment and a large amount of non-framework aluminum will block the pores,which is more likely to coke on the surface of the catalyst and form carbon deposition affecting the distribution of reaction products.Using acid to control the acidity of catalyst has the advantages of flexible control and mild reaction.In this paper,the specific surface,pore volume and acidity of the in-situ crystallized super stable catalyst with different concentrations of citric acid were studied.The characterization results showed that with the increase of concentration of citric acid,the specific surface and pore volume increased and the subsequent change was not obvious.After modification with citric acid,the total acid content of the catalyst decreased,but the medium strong acid content increased.The results of ACE reaction evaluation showed that the cracking enhanced.The conversion yield increased by more than 3.23%,the slurry yield decreased by more than 1.13%,the LPG increased by more than 2.7%,the diesel decreased by more than 1.94% and the coke selectivity improved.It was indicated that citric acid modification could significantly improve in-situ crystallization.The acidity of the catalyst was approachable to improve the selectivity of the reaction.The best reaction result was obtained when the concentration of citric acid was 0.5%.Using phenanthrene as probe molecule,the mass transfer and diffusion rate of samples modified with different concentrations of citric acid were tested.The test results showed that the mass transfer and diffusion rate of probe molecule in the catalyst was faster than that of the contrast agent,which indicated that the citric acid modified catalyst cleaned up the non-framework aluminum deposited on the surface of the catalyst and made the pore channel of the catalyst more stable.The test result was in good correspondence with the response evaluation result.

Key words: citric acid, in situ crystallization catalyst, acidity, non-framework aluminium