Propriedades ácidas e texturais de zeólitas ZSM-5 dessilicalizadas ou desaluminizadas – análise do rendimento e seletividade a olefinas leves durante a transformação de cicloexano e metilcicloexano
Abstract
Nowadays, the Brazilian petroleum is extracted from very deep fields and possesses a high
naphthenic hydrocarbons composition, which imposes new challenges to refineries and
specially to the catalytic cracking process. In that process, the catalyst must act maximizing
the production of the highly demanded gasoline, diesel and light olefins from heavy fractions.
Taking into consideration the above discussed context, this work aimed to evaluate the effect
of basic or acid treatments applied on ZSM-5 zeolites (Si/Al=12 or 23) in the activity to
cyclohexane or methylcyclohexane transformation. XRD and 27Al-NMR showed that the dealuminated zeolites presented an increase in their crystallinity due to the extra-framework
aluminum lixiviation. On the other hand, in the desilicated zeolites occurred a decrease in
their crystallinity as a consequence of the extra-framework aluminum generation. MEV
images do not evidence any morphological change that could have been produced by the acid or basic treatments, however, the desilicated ZSM-5 zeolites treated under harder conditions
presented significant textural modifications. As expected, the chemical ICP analyses showed a
decrease in the Si/Al ratio in the desilicated zeolites and an increase of that ratio for those
dealuminated ones, being the last variation more significative in the external surface of the
zeolite crystals, as was evidenced by XPS analyses. Data from NH3-TPD showed that the
acid treatment resulted in a higher ratio of strong acid sites, which suffered more deactivation
during reaction. N2 fisisorption analyses of the ZSM-5 zeolites, showed that the desilication
done at higher temperature was more efficient to mesopore generation. In the cyclohexane
and methylcyclohexane transformation, the dealuminated zeolites were less active due to their lower aluminum content, nevertheless were more stable and presented a small increase to
light olefins selectivity. The desilicated ZSM-5 zeolites presented higher activity and higher
yield to light olefins that were supported by their lower Si/Al ratio and mainly by the presence
of mesoporosity that enhanced the reagents and products internal diffusivity.