Craqueamento de hidrocarbonetos naftênicos sobre zeólitas HZSM-12 modificadas – rendimento e seletividade a olefinas leves
Abstract
It was studied the catalytic performance of ZSM-12 zeolites modified by desilication in the
cracking of naphthenic hydrocarbons. ZSM-12 zeolites with nominal a SiO2/Al2O3 ratio were
synthesized during 96 or 144 h under hydrothermal conditions and the obtained zeolites were
treated with NaOH solutions under different conditions. The samples were characterized by
thermogravimetric analysis (TA), X-ray diffraction (XRD), scanning electron microscopy
(SEM), N2 physisorption, temperature programmed desorption of ammonia (NH3-TPD), 27Al
MAS NMR spectroscopy, energy-dispersive X-ray spectroscopy (EDX) and infrared
spectroscopy (Py-FTIR). The treatment with NaOH solutions under mild conditions was more
effective for the ZSM-12 zeolite synthesized during 96 h. For the HZ12-96-0,2 sample,
obtained by treatment at 35 °C with 0.2 mol.L-1 NaOH solution during 15 min, it was verified
an increase in the external surface area and the formation of mesopores in the range of 2 to 14
nm. On that zeolite occurred a higher yields to light olefins during the cracking of
cyclohexane at 400 °C and also in the cracking of methyl and ethylcyclohexane at 450 °C.
This result was mainly related to the higher density of acid sites exhibited by the HZ12-96-0,2
zeolite compared with the parent ZSM-12 one, as consequence of the applied alkaline
treatment. It was also verified that the ZSM-12 zeolites modified by more severe alkaline
treatment (0.5 or 1.0 mol.L-1 NaOH solution at 80 °C for 30 min) presented significant
increase of the external surface area and mesopores volume. The catalytic cracking of
cyclohexane, methylcyclohexane and ethylcyclohexane at 500 °C, as well as the physicochemical
characteristics of the MZ12-96-0,5 zeolite enhanced the formation of light olefins.
The highest yield to light olefins was obtained on that zeolite during the cracking of
ethylcyclohexane, which increased 9% when compared with the yield obtained on the not
modified HZSM-12 zeolite. The selectivity to light olefins on the studied HZSM-12 zeolites
was strongly influenced by presence of a side chain in the naphthenic ring (methyl or ethyl),
as well as by the employed cracking operating conditions.