Abstract
Three nanosized ZSM-5 zeolites were successfully prepared from reactive gels
with the same Si/Al ratios by different synthetic procedures that included the
use of tetrapropylammonium hydroxide or n-butylamine as a template and a seeding
method that did not use an organic additive. The effect of the synthetic method
on the physicochemical properties of the prepared samples was investigated by
XRD, XRF, XPS, N2 physisorption, SEM, TEM,
27Al MAS NMR, NH3-TPD, and
Py-FTIR. The catalytic performance of the
nanosized ZSM-5 zeolites in the alkylation of naphthalene with methanol was
compared. The prepared samples were phase-pure, highly crystalline ZSM-5
zeolites, but they had different bulk and surface Si/Al ratios as well as
textural and acidic properties. The study of the prepared catalysts in
naphthalene methylation revealed that both the acid characteristics of the ZSM-5
nanosized zeolites and their textural properties were responsible for their
activity in the reaction. A difference in the composition of
monomethylnaphthalenes and dimethylnaphthalenes was attributed to the ability of
the catalyst to isomerize the primary reaction products on acid sites located on
the external surface of the zeolite crystals. 2,7-DMN was found to be the
preferred reaction product over 2,6-DMN when formed at pore entrances to ZSM-5
channels due to the differences in their dimensions. In contrast,
2,6-dimethylnaphthalene could be produced on weaker external Brønsted acid
sites, which are hydroxyls attached to octahedral Al atoms. The presented
results show that the method used to synthesize nanoscale ZSM-5 zeolites is a
critical factor that determines the physicochemical properties and catalytic
performance of the resulting crystals.
Modification of nanocrystalline HZSM-5 zeolite with tetrapropylammonium hydroxide and its catalytic performance in methanol to gasoline reaction
