Abstract
A new route to obtain pillared, disordered or desilicated MWW-type zeolites was developed assisted by quaternary ammonium surfactants with different hydrocarbon tail size acting as swelling agents (C12TA+, C16TA+, C18TA+) and TPA+ both exchanged to their hydroxide forms instead of only one swelling agent (CnTA+ or TPA+) in hydroxide form. Effect of surfactant concentration and swelling conditions were determinant to obtain MWW-type zeolites with different lamellar organization and spatial distribution of individual zeolitic layers. Specifically, soft swelling at 25 °C with C12TA+ preserved layer structure reaching a final disordered/pillared structure while pillared structures are obtained in the case of materials swollen with C16TA+ and C18TA+. Aggressive swelling processes at 80 °C favored desilication, damaging the layers structure in case of C12TA+ while pillared materials are obtained after swollen with C16TA+ and C18TA+ surfactants. It was proved that both swelling agents in hydroxide forms combining with swelling and pillaring procedure influenced on physico-chemical and morphological nature of MWW-type materials due to the particular conditions used. The obtained derivative MWW zeolites with different morphology, order and accessibility levels were firstly evaluated by catalytic dehydration of fructose to 5-hydroxymethylfurfural (5-HMF) showing superior activity compared to beta zeolites reported in literature.
Highlights
Zeolites are microporous crystalline aluminosilicates composed of tetrahedral bonds TO4 (T= Si or Al usually) coordinated by oxygen atoms
This behavior can be monitored by X-ray diffraction (XRD) with the increase of basal spacing which is directly related with the thickness of MWW layer, the length of swelling agent and its height of head-group and tail-group
The soft swelling treatment using C16TA+ resulted in the overlapping of the (101) and (102) diffraction bands, yielding only a broad peak located between 8° and 10° 2ɵ range
Summary
Zeolites are microporous crystalline aluminosilicates composed of tetrahedral bonds TO4 (T= Si or Al usually) coordinated by oxygen atoms. Zeolites with a hierarchical pore system have received growing attention in recent times.[4,6,7,8] The strategy based on generating secondary mesoporosity regions in zeolites emerges as a way to facilitate molecular diffusion and increase the variety of reagents capable to achieve and react with internal active sites.[6] Among the strategies "bottom-up" to hierarchy zeolites, pillaring process is able to separate individual layers of two-dimensional (2D) zeolitic precursors This procedure creates mesoporous regions by inserting organic or inorganic species as pillars, located in the interlayer space, together with intrinsic microporosity present in the zeolitic layers.[9, 10] Considering the best examples in the state-of-art of successful hierarchical pillared zeolites, we found MCM-36 (MWW topology), ITQ-36 (FER topology) and recently, nanosheet pillared and self-pillared MFI-type zeolites.[11,12,13,14]
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