Catalytic Properties in Polyolefin Cracking of Hierarchical Nanocrystalline HZSM-5 Samples Prepared According to Different Strategies
Two series of hierarchical nanocrystalline ZSM-5 zeolites prepared by different synthesis strategies (at low temperature and from silanized seeds) and with external surface areas ranging from 150 to 250 m2 g-1 were tested in the cracking of pure LDPE and HDPE at 340ºC and of waste polyethylene at 360ºC. Hierarchical zeolites showed quite higher activity, with values even 6 times higher than a standard nanocrystalline sample used as reference (n-HZSM-5). The activity values decreased from LDPE to HDPE due to the occurrence of some degree of branching in the former polymer, which act as preferential cracking sites. The major products were C1 - C4 hydrocarbons (in the range 30 - 70%, mostly C3 - C4 olefins) and C5 - C12 hydrocarbons (20 - 60%), whose share depends on both the polyolefin and the catalyst. The amount of C13 - C40 hydrocarbons was practically negligible (< 1%) due to the high acid strength of the zeolites which promotes end-chain cracking reactions. Likewise, hierarchical nanocrystalline HZSM-5 zeolites prepared from silanized protozeolitic units showed higher activities than the hierarchical nanocrystalline HZSM-5 samples synthesized at low temperature and atmospheric pressure. The differences were especially remarkable in the case of waste polyethylene cracking. These results were ascribed to the stronger acidity of the hierarchical zeolite samples prepared from silanized seeds.
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