A study has been made of the activity recovery of H-ZSM5 zeolite
based catalysts, which have
been used in reaction−regeneration cycles in the transformation of
methanol into gasoline.
Catalyst stability is sensitive to catalyst calcination conditions
(temperature and time) and to
zeolite Si/Al ratio. From experiments carried out on automated
reaction−regeneration equipment
provided with an isothermal fixed-bed reactor, optimum catalyst
equilibration conditions have
been determined, in order to minimize the irreversible deactivation in
the regeneration step.
The effect of this step on the catalyst acid structure has been
studied.
In this work, the effect of HZSM-5 zeolite acidity on hydroconversion of methylcyclohexane and toluene has
been studied. These are test reactions for the second step and the single step of aromatic valorization process,
respectively, with the aim of obtaining C2+
n-alkanes and isoalkanes. Monofunctional HZSM-5 zeolite catalysts
(Si:Al ratio between 15 and 140) have been studied in methylcyclohexane ring opening while bifunctional
catalysts (hybrid Pt/Al2O3-HZSM-5, same zeolites) have been used in the hydrocracking of toluene. Runs
have been carried out in a fixed bed reactor under 250−450 °C and 20−80 bar. A positive effect of HZSM-5
zeolite acidity on methylcyclohexane conversion and C2+
n-alkane selectivity is evident at certain conditions,
whereas the maximum selectivity to isoalkanes requires an intermediate value of acidity. On the basis of the
relationship between conversion and the Si:Al ratio of the HZSM-5 zeolite, the hydrogenolytic cracking of
methylcyclohexane is proposed as a test reaction to determine the Si:Al ratio. Acidity has a highly favorable
effect in the hydrocracking of toluene given that it avoids the thermodynamic restrictions for toluene
hydrogenation and enhancing all the cracking steps during methylcyclohexane (MCH) transformation, which
increases selectivity to C2+
n-alkanes and isoalkanes.
A kinetic model based on the postulates of
Langmuir−Hinshelwood has been proposed for the
catalyst deactivation by coke in heterogeneous polymerizations.
The originality of the model
lies in the fact that deactivation is also taken into account during
the initiation period by defining
the activity as the useful fraction of active sites throughout the
polymerization. The validity of
the proposed deactivation kinetic model has been shown for the
polymerization of gaseous benzyl
alcohol in an ample range of operating conditions: catalysts
(silica/aluminas and a HY zeolite)
with different acidity and different porous structure, temperature, and
monomer concentration.
The results are compared with those of the previously defined
model, where activity referred to
the maximum polymerization rate.
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