Growth of LTA zeolite nanoclusters using a block copolymer template

Figure 1. SAXS data during the early stage of the LTA zeolite synthesis are described by a Guinier type analysis (for q<0.15 nm-1) as reported in the inset and a core-shell approach (for q>0.3 nm-1).

Figure 2. Sketch of the self-assembly stages involved during the synthesis of block copolymer templated hybrid nano-particles. The initial association of aluminosilicate species into the corona region of the PDMS-PEO copolymer micelles (a) generates primary units with a core-shell morphology (b). Progressive aggregation process among these primary units leads to the formation of extended secondary fractal structures (c), while further cross-linking, fusion and rearrangement of the secondary particles leads to the formation of final submicron aggregates (d). In the inset (e) the time evolution of the SAXS intensity profile (at T=45°C) after the mixing of the main components is presented, while the scanning electron microscopy (SEM) image of the final nano-aggregates is reported in the inset (f).
Employment of highly branched copolymer micelles as core substrate to control the soft interactions at the boundary between organic and inorganic domains provides a steric stabilization that enhances the colloidal stability during LTA zeolite formation. Our results, while indicating an interesting possibility in substitution of more traditional templates for the zeolite synthesis, give important insights to the comprehension of the self-assembly processes involved in the development of organic-inorganic mesoporous nanoparticles and for alternative protocols in porous materials production.

In recent years, special efforts have been devoted to develop strategies for the synthesis of supramolecular organic-inorganic nanostructures based on porous materials. The main goal of the different strategies is to achieve a synergy between the properties originating from the porous inorganic substrate and the properties of the involved organic components. Particularly stimulating is the study of alternative protocols for the assembly mechanism of such materials in which a macromolecular template drives the formation of nanostructures with peculiar final properties. Our investigation, performed at the Austrian SAXS beamline of Elettra, outline the self-assembly processes involved in the formation of sub-micron particles of zeolite LTA grown on a polydimethylsiloxane-b-polyethyleneoxide (PDMS-b-PEO) diblock copolymer used as templating agent. The combination of supramolecular interactions, together with the ability to control both the length scale and the structural morphologies, makes block copolymers particularly attractive templates in the synthesis of nonporous materials with new characteristic and properties.

The early stage of the nanoparticles growth process, restricted to an initial time between 1-3 hours, was characterized by the incorporation of the (LTA zeolite) aluminosilicate components into the surface of the nanotemplate with the formation of primary units with a core-shell morphology, while the presence of aggregation processes among primary units led to the formation of extended secondary fractal structures (Fig. 1). 

Further cross-linking, fusion and rearrangement of the secondary particles leads to the formation of final submicron aggregates as reported in Fig. 2, where the multistep mechanism of formation of the hybrid nano-clusters is proposed. The formation of large supramolecular assemblies at the late stage of the synthesis process was finally confirmed by scanning electron microscopy (SEM) experiments (Fig. 2f), that showed the presence of large spherical nanosized aggregates. The back-scattered SEM image of the system confirmed a condensation of the aluminosilicate components on the aggregates surfaces as proved by the energy dispersive x ray (EDX) microprobe analysis, while x ray diffraction (XRD) experiments indicated the formation of crystalline zeolite LTA, thus confirming the porous nature of the generated particles.

Generally, the driving interaction regulating the structure formation in zeolites are difficult to understand, due also to the difficulty to follow (in situ) the time evolution in a multi-component complex environment. In this respect, the use of high molecular weight copolymer template seems to be an interesting possibility in substitution of more traditional templates, as the presence of micellar block copolymers provides a steric stabilization that induces an enhanced (transient) colloidal stability to the synthesis environment. The obtained results indicate how micellar block copolymers precursors offer, from a molecular point of view, favorable conditions for the self-assembly processes involved in the synthesis of hybrid matrices. The soft interaction involved in the sol–gel process as well as the high adaptability to the reaction conditions reveals the very promising properties that polymer based amphyphylic templates can offer in the design and construction of hybrid inorganic-organic functional materials based on zeolites.

Retrieve article

Self-assembly in poly(dimethylsiloxane)-poly(ethylene oxide) block copolymer template directed synthesis of Linde type A zeolite;
L. Bonaccorsi, P. Calandra, M.A. Kiselev, H. Amenitsch, E. Proverbio and D. Lombardo;
Langmuir 29, 7079 (2013). doi: 10.1103/10.1021/la400951s

Last Updated on Tuesday, 14 May 2019 17:05