Abstract
Hierarchical ZSM-5 zeolites were prepared by alkali treatment (desilication) at two temperatures, two reaction times, and two NaOH concentrations. A 23 factorial design was used to study the effect of these variables on the crystallinity, microporosity, and mesoporosity of the zeolite due to the desilication treatment. The factorial design analysis showed that the temperature, reaction time and NaOH concentration and the second order interaction between temperature and time have statistically significant effect on the micropore volume. On mesopore volumes (Vmeso) and areas (Smeso), the reaction time and NaOH concentration factors have statistically significant effects. In the case of mesopore volumes, the second order reaction time and NaOH concentration interaction factor are also significant. On the other hand, only the alkali concentration affected, negatively, the relative crystallinity. Two hierarchical ZSM-5 zeolites with the highest relative crystallinity and mesoporosity were selected and further characterized by inductively coupled plasma (ICP) analysis, 29Si nuclear magnetic resonance (NMR), 27Al NMR, Fourier transform infrared (FTIR) spectroscopy in the OH stretch region and pyridine adsorbed FTIR spectroscopy. NMR and FTIR results showed that the alkali treatment selectively removed silica from the zeolite framework, decreasing the SAR (silica-to-alumina ratio) values, while decreasing Brønsted acid site concentrations and increasing Lewis acid sites concentrations.
Highlights
The most important zeolite property explored in catalytic processes in the chemical industry is their shape selectivity, or, in other words, their ability to favor certain reaction pathways based on the size relationship between their cavities and molecules when reactions take place in a restricted environment.[16,17]
A 23 complete factorial design was used to study how the microporosity (Vmicro), mesoporosity (Vmeso and Smeso), and crystallinity (CXRD) of the HZSM-5 zeolite depend on the temperature (1), reaction time (2) and alkali concentration (3) used in the treatment, for which the design table was already shown in the Experimental section (Table 1)
The factorial design results showed that all principal factors and the second-order interaction between temperature and reaction time have statistical significance to the micropore volumes
Summary
One of the drawbacks of this sterically hindered set of channels is the restriction of molecular diffusion inside the zeolite crystal, decreasing the apparent reaction rates in these kinetic systems, an effect known as internal diffusional limitations.[20] Many strategies have been used in the literature to reduce these internal diffusional limitations, one of the most popular approaches being the introduction of a second set of pores inside the ZSM-5 structure, but with diameters in the order of magnitude of mesopores. In this new set of mesopores, molecules can rapidly diffuse through the structure, improving the contact between catalytic active sites and fluid phase.[20,21] These zeolites are said to have a hierarchical pore structure
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