(425d) Advanced Characterization of Hierarchical Zeolites for Optimal Xylene Separation

The efficiency of xylene separation is related to both
transport and selectivity properties of the used faujasitic adsorbents. In an
attempt to improve the transport properties, the use of hierarchical FAU-type
zeolites has been proposed. While the generation of new porous domains with
meso or macro-pores is expected to enhance the access to micropores, its impact
on the selectivity still remains unknown. In particular, as pointed out by
Bellat et al. [1], the polarity difference between xylene isomers may be at the
origin of a change in surface selectivity (from pX to mX e.g). Here, two types
of NaX hierarchical zeolites are considered to explore different types of porous
networks. Both types are shown in Figure1(A): Agglomerated Crystals (AC) and
Layer-Like zeolite (LL) [2]. For AC zeolites, samples with different external
surface area are also tested. Considering the key role of the  zeolite surface
for xylene selectivity, an accurate characterization of the quantity and
quality of the external surface is required.

The textural characterization of hierarchical
zeolites  was made through  novel models denominated “direct identification”
[3] and “superimposition principle” [4]. These methods were applied as an
alternative to the conventional t-plot method, which has been shown to
overestimate the external surface area and underestimate the micropore volume
[5]. Both the direct identification and superimposition methods, which allow
obtaining a model with an excellent fit of the N2 isotherm, rely on the
decomposition of the adsorption isotherm as the sum of the contribution of the
microporous adsorption  and the adsorption at the external surface/mesopores
[4]. We also used the recent method proposed by Kenvin and coworkers in which
unified pore size distributions are obtained through the combination of Hg
porosimetry and N2 adsorption experiments [6].

The surface chemistry of the different hierarchical
zeolites, which is expected to significantly influence xylene selectivity, was
also investigated. Given that hydroxyl groups present in the external surface
must have an impact on its polarity, IR analysis was performed in the OH zone
(3000-4000 cm-1). Such characterization allowed us to quantify the silanol
groups on the external surface appearing in a band at 3744 cm-1(Fig. 1(B)). A
linear correlation between the respective IR band area and the external surface
area of the samples is observed. This correlation suggests that the different
hierarchical zeolites exhibits the same OH density on their external surface.

Figure 1. Morphological, textural and surface
characterization of different hierarchical zeolites used for xylene separation.

Finally, the adsorption and diffusion of xylene
molecules were investigated through batch experiments. It was possible to
detect a loss in the para-xylene selectivity linked to the external surface
area of the hierarchical zeolites and its quality (amount of –SiOH groups). Regarding the diffusion properties, the effective
diffusion time was reduced in about 90% by using the hierarchical zeolites, in
comparison to a conventional NaX zeolite.

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J.-P. Bellat, E. Pilverdier, M.-H. Simonot-Grange, and S. Jullian. Microporous
Materials, 9, (1997).

[2]
A. Inayat, C. Schneider  and W. Schwieger. Chem. Commun., 51, (2015).

[3]
M. J, Remy, and G. J., Poncelet, J.Phys. Chem. 99, (1995).

[4] L. Deliere, F.
Villemot, D. Farrusseng, A. Galarneau, S. Topin, B. Coasne Microp. Mesop. Mater., 229, (2016).

[5]
A.
Galarneau, F. Villemot, J. Rodriguez,  F. Fajula and B. Coasne Langmuir,30,
(2014).

[6] J. Kenvin, J. Jagiello, S. Mitchell and J.
Pérez-Ramírez.
Langmuir,
31,  (2015).