Elsevier

Catalysis Today

Volume 344, 15 March 2020, Pages 102-107
Catalysis Today

Nucleation and crystallization of the MWW-type lamellar zeolitic precursor

https://doi.org/10.1016/j.cattod.2018.10.033Get rights and content

Highlights

  • The MWW-type precursor was monitored by different techniques, such as X-ray diffraction (XRD) and Small-Angle X-ray Scattering (SAXS).

  • The MWW-type precursor reflections were identified by XRD from 48 h of synthesis.

  • The MWW-type precursor was formed by thin platelet-like particles, observing its growth by Scanning Electron Microscopy (SEM).

  • A primary population of nanoparticles was identified in the first stage of reaction by SAXS, followed by the transformation to two-dimensional objects.

Abstract

The nucleation and crystallization of the layered zeolitic precursor (P)MCM-22 with Si/Al gel ratio = 25 was studied by several characterization techniques such as X-ray Diffraction (XRD), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Infrared Spectroscopy (IR), Thermogravimetric Analysis (TGA) and Small-Angle X-ray Scattering (SAXS). The crystallization kinetics of the samples was firstly monitored by XRD identifying reflections from the sample obtained at 48 h. The ICP-OES analysis concluded that the Si/Al ratio was 23, which is very similar to the initial gel one. SEM analyses corroborated the XRD results observing the formation of smooth particles from 48 h of crystallization and, after 168 h, the formation of thin platelet-like particles. The IR results showed an increase of external silanol groups when the crystallization time increases, due to terminating silanol groups between zeolitic crystals, whereas the bands attributed to the structure directing agent- hexamethyleneimine (HMI) − increase up to 168 h. Finally, the SAXS results indicated the existence of primary nanoparticles population in the first stages of the reaction, allowing to follow their growth and transformation from spherical morphology to two-dimensional objects forming aggregates.

Introduction

Zeolites are microporous crystalline materials used in various processes e.g., adsorption, molecular sieving, ion exchangers and catalysis [1], [2], [3], [4]. This versatility is due to their crystalline structures based on tetrahedral (SiO4 and AlO4, mainly) which are coordinated by oxygen atoms. The organization between these units generates microporous materials with different cavities and channels with shape-selectivity [5].

According to the International Zeolite Association (IZA), there are more than 230 zeolite topologies; however, less than 20 have a lamellar zeolitic precursor (LZP) [6]. The MWW family comprises zeolites such as PSH-3, SSZ-25, MCM-22, MCM-49 (obtained by direct crystallization), MCM-56 (partial delaminated), EMM-10 (disordered stacking of lamellae) and ITQ-1 (pure silica) [7], [8], [9], [10], [11], [12], [13]. Among them, the LZP of the MCM-22 zeolite [(P)MCM-22] is the most remarkable source of innovation to obtain new open pore architectures [14]. With the LZP of the MCM-22 it is possible to obtain expanded (IEZ-MWW), delaminated (ITQ-2), pillared (MCM-36), desilicated and hybrid lamellar-type zeolites with tuneable accessibility and acidity [15], [16], [17], [18], [19]. In this way, zeolites with lamellar structure are considered as potential candidates to replace commercial catalysts [20].

The tridimensional form of MCM-22 zeolite (Fig. 1) is obtained after the calcination of the LZP. The MCM-22 zeolite has two independent pore systems, both accessed by 10-ring elliptical windows. One pore system is defined by sinusoidal (0.40 × 0.50 nm) and bidirectional 10-ring (0.40 × 0.55 nm) channels whereas the other pore system consists of 12-ring supercavities with a free internal diameter of 0.71 nm and internal height of 1.82 nm. These supercavities are interconnected through double 6-membered rings [10]. This tridimensional zeolite is the result of the condensation of silanol groups of the surface concomitant to the removal of the HMI molecules used as structural directing agent were removed after calcination.

Several studies have been carried out to obtain separated MWW monolayers by direct synthesis using special structure directing agents (linked with long alkyl chain lengths) or organosilanes and cationic polymers to obtain nanosized crystals [23], [24], [25], [26]. However, little attention has been devoted to understanding the formation of the lamellar precursors [20].

Small-Angle X-ray Scattering (SAXS) is a powerful characterization technique widely used in studies of nucleation and growth mechanisms due to the length scales probed, ranging from the nanometer to the microscale [27]. SAXS data enable to extract information such as particle size, polydispersity and morphology with time resolution of several seconds. In the case of zeolites, a little proportion of the existent topologies has been studied using this technique, for example the LTA and MFI zeolites [28], [29]. Therefore, it is important to keep searching the other topologies to define their mechanism of formation to be able to choose the synthesis conditions for a predefined material with the selected characteristics.

In this context, the present research work is focused on understanding the nucleation and crystallization processes of the MCM-22 precursor, carrying out an ex situ study of its synthesis using X-ray diffraction (XRD), infrared spectroscopy (IR), SAXS and electron microscopy.

Section snippets

Synthesis of MCM-22 precursor

The synthesis of the precursor was carried out with a Si/Al = 25 molar ratio similar to the one reported in the literature [30]. For the synthesis, 0.75 g of sodium hydroxide (NaOH, Sigma Aldrich) and 0.75 g of sodium aluminate (NaAlO2, Riedel de-Haën), were mixed in 162 g of deionized water under magnetic stirring. Consequently, 9.96 g of hexamethyleneimine (HMI, Sigma Aldrich) and 12 g of fumed silica (SiO2, Aerosil 200, Degussa) were added to the mixture and stirred for 2 h. The resulting

Results and discussion

Fig. 2 shows the X-ray diffractograms of the precursor at different times of synthesis, 24, 48, 72, 96 and 168 h. After 24 h, only a broadband between 2ɵ = 15–30° characteristic of amorphous silica was observed. The XRD pattern of the P22-48 sample showed the intralamellar (310) diffraction peak which emerged from 48 h of synthesis.

The crystallization and growth were observed after 48 h where the decrease of the broadband at 2ɵ = 15 − 30° indicated that the amorphous silica has been consumed.

Conclusions

In this work, we studied the nucleation and crystalline growth of the lamellar MCM-22 precursor by several characterization techniques. In accordance with the results, we propose the following nucleation and growth mechanism for (P)MCM-22. Initially, several amorphous spherical nanoparticles of 9 nm radius, detected by electron microscopy and SAXS, are formed and grow by silanol condensation, as observed in the infrared spectra, over time while forming new amorphous spherical nanoparticles with

Acknowledgements

The authors thank LABPEMOL, UFRN and Petrobras for the Project entitled “Aperfeiçoamento da síntese da zeólita ITQ-2 e aplicação no craqueamento de frações pesadas” (n° 0050.0085290.13.9). We are also grateful to the LNLS/CNPEM and LNNano/CNPEM for providing time on SAXS1 beamline (approved proposal 20160104) and electron microscopy (approved proposal “SEM-23412 − Zeolites nucleation and growth”). A. J. Schwanke is grateful to ITQ for the IR analyses. F. Meneau and P. Vinaches acknowledge CAPES

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