Lately,
to considerably reduce the diffusion constraints in the
catalysis performed by zeolites, there has been significant interest
in syntheses of partially formed zeolites with extremely accessible
active sites, named embryonic zeolites. Their preparation relies on
stopping the crystallization of conventional micron-sized zeolites
before the zeolites reach full-crystallization, as detected by X-ray
diffraction. In this current study, ZSM-5 zeolites were synthesized
by using the same starting batch without an organic template and different
times, obtaining amorphous, ill-crystallized, and fully grown structures
that were characterized by various physicochemical methods and used
in two catalytic test reactions. Using this synthesis strategy, two
significant features of zeolites in catalysis are highlighted: (1)
the accessibility of their inner pores for reactants and (2) the confinement
effect, that is, the ability of zeolites to stabilize transition states
in some catalytic reactions. As the ZSM-5 was being formed, it had
a decreased activity for glycerol condensation with acetone, indicating
steric restrictions to the bulky product that has a kinetic diameter
(0.63 nm) considerably higher than the zeolite micropores (0.51 ×
0.54 nm). On the other hand, the catalytic activity for methanol dehydration
to dimethyl ether was proportional to crystallinity, as dimethyl ether
intermediates adsorb within the zeolite cavities that tend to reciprocally
enhance interactions giving an optimal effect in the catalytic property.
The [CTA]-MCM-41 hybrid silica is a useful and simply prepared heterogeneous basic catalyst for the transesterification reaction. Here, the effect of hydrothermal treatment during catalyst preparation was investigated, with the aim of improving the structural stability of this catalyst during the reaction. It was observed that the hydrothermal step led to the formation of a material with a higher degree of organization and a greater wall thickness, which improved its structural stability. However, the catalyst prepared using this treatment presented lower catalytic activity, due to the presence of fewer active sites.
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