Effect Of Alkaline Earth Metals Modified HZSM-5 Zeolites On Oil Shale Pyrolysis

The shale oil and combustible gas obtained through pyrolysis of oil shale have great utilization value.In this paper,two alkaline earth metals,Ca and Mg,were used to modified HZSM-5 zeolites,aim at exploring its catalytic behaviour in oil shale pyrolysis.Ca and Mg were supported to HZSM-5 in different load?3%,6%and 9%?by impregnation method.The HZSM-5 zeolites before and after modification were characterized.The results indicated that the load of alkaline earth metal introduced more mesoporous into the HZSM-5.Low loading could increase the number of micropores.With the increase of loading,the micropores decreased in number.The ratio of Lewis and Br?nsted acid sites increased with the increase of load,owing to the metal oxides coverage of Br?nsted acid sites.The weight loss of zeolites was concentrated before 200°C.TG-MS-FTIR was used to investigate the catalytic behavior.The results indicated that the acid sites?mainly Br?nsted acid sites?in zeolites could protonated C-C/C-H to form carbocation intermediates,which decreased the peak temperature of the C1-C4 light aliphatic hydrocarbon's precipitation curves.Ca O's coverage of Br?nsted acid sites weakened the catalytic effect.Meanwhile,Ca O reacted with carboxylic acid in oil shale formed carboxylate,leaded to a better deoxygenation effect.Because Mg O had a weaker alkalinity than Ca O,the Mg-modified HZSM-5 did not show good deoxidation effect.But,it produced the most CH₄.The modified HZSM-5 zeolite did not show catalytic behaviour for H₂ and CO.The activation energy of oil shale pyrolysis in the main stage?360-520°C?was calculated by Coats-Redfern method.The results indicated that modified HZSM-5 could reduce more activation energy.HZSM-5 modified by alkaline earth metals improved the yields of aromatic hydrocarbons in shale oil.Because of excellent decarboxylation performance,shale oilcontained almost no acids.The catalytic performance of Mg O to Fischer-Tropsch leaded to the olefin content was higher inshale oil than catalysed by Ca-modified HZSM-5.However,the modified HZSM-5 also catalysed the secondary reactions of aromatic hydrocarbons,resulted in the increase of phenol content.After catalytic pyrolysis by modified HZSM-5,aliphatic hydrocarbons and carbonates in shale ash were basically disappeared.Due to enhanced aromatization,the polycyclic aromatic compounds could not pass through the pore channel of the zeolites,remained in the shale ash.Br?nsted acid sites had obvious catalytic effect on aliphatic hydrocarbons,while Lewis acid sites were more targeted at the aromatization process of pyrolysis products.