Abstract
Catalytic cracking of n-hexane to selectively produce propylene on Beta zeolite was carried out. The H-Beta (HB) (Si/Al = 77) zeolite showed higher catalytic stability and propylene selectivity than the Al-rich HB (Si/Al = 12), due to its smaller number of acid sites, especially Lewis acid sites (LAS). However, catalytic stability and propylene selectivity in high n-hexane conversions were still not satisfactory. After dealumination with HNO3 treatment, catalytic stability was improved and propylene selectivity during high n-hexane conversions was increased. On the other hand, catalytic stability was not improved after desilication with NaOH treatment, although mesopores were formed. This may be related to the partially destroyed structure. However, propylene selectivity in high n-hexane conversions was increased after alkali treatment. We successfully found that the catalytic stability was improved and the propylene selectivity in high n-hexane conversions was further increased after the NaOH treatment followed by HNO3 treatment. This is due to the decrease in the number of acid sites and the increase in mesopores which are beneficial to the diffusion of coke precursor.
Highlights
Light alkenes, especially propylene which is mainly supplied as a by-product of ethylene production through thermal cracking of naphtha, are gaining more and more significance in the chemical industry
Among the zeolite catalysts examined for catalytic cracking, ZSM-5 (MFI-type) zeolite with three dimensional 10-membered ring (10-MR) channels has been recognized as a prime candidate because of its high thermal and hydrothermal stabilities and its considerable resistance to deactivation caused by coking as well as its strong acidity [4]
Yoshimura et al reported that La2 O3 /P/ZSM-5 zeolite (Si/Al = 100) exhibited a high naphtha catalytic cracking activity and a high yield of ca. 60% to light olefins at 923 K, and the propylene/ethylene ratio was approximately 0.7 [1]
Summary
Especially propylene which is mainly supplied as a by-product of ethylene production through thermal cracking of naphtha, are gaining more and more significance in the chemical industry. Dealuminated MCM-68 (MSE-type) zeolite (Si/Al = 51) with multidimensional 10-MR or 12-MR micropores has been reported to exhibit a high propylene selectivity of ca. Dealumination of zeolite catalyst leads to an increase in propylene selectivity and the improvement of catalytic stability. We have reported that the dealuminated organic structure-directing agent (OSDA) free Beta zeolite showed a better catalytic performance in n-hexane cracking [17]. We found that the catalytic stability in n-hexane cracking can be improved by alkali treatment of ZSM-5 due to the formation of mesopores, which facilitates the diffusion of the coke precursor [20]. The effects of dealumination and desilication on the physicochemical properties (including porosity and acidity) and catalytic properties (including activity, selectivity and catalytic life) of HB zeolites in the cracking of n-hexane as a model compound of naphtha were investigated. As a control, the physicochemical properties and catalytic properties of Al-rich HB were investigated
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