Selective removal of external Ni nanoparticles on Ni@silicalite-1 single crystal nanoboxes: Application to size-selective arene hydrogenation
Introduction
Size-selective membranes and catalysts find important applications in separation, sensing and catalytic technologies. Catalytic hydrogenation is an ubiquitous reaction and is used for instance in the synthesis of fine chemicals and fuel upgrading [1], [2], [3], [4], [5], with substrate sizes that are similar to that of zeolite pores. The possibility to prepare nanoparticles of metals embedded inside zeolite single crystal nanoboxes was recently reported by our group [6], [7], [8], [9], [10], [11], [12] and others [13], [14], [15]. The total size exclusion of mesitylene (kinetic diameter = 0.87 nm) from the silicalite-1 (purely siliceous zeolite with MFI topology) hollow nanocrystals was demonstrated and led to a Pt-based catalyst that was highly active for the hydrogenation of toluene (kinetic diameter = 0.58 nm) and totally inactive for that of mesitylene [6]. In addition, we also have reported that these nanoboxes act as nanoreactors [8], [9] in which the nanoparticles are encapsulated and protected from sintering by coalescence [10]. We recently reported during an investigation of the effect of tars on methane steam reforming that these silicalite-1-based nanoboxes can retain their structural integrity up to 900 °C [16].
The use of noble metals typically led to single nanoparticles with a well-defined size embedded in silicalite-1 nanoboxes [11], [12]. In contrast, the method applied to base metals led in most cases to a large number of nanoparticles present in each nanobox [7]. In the present work dealing with Ni, we show that metal nanoparticles can also be formed outside of the silicalite-1 nanoboxes when attempting to prepare high metal loadings, leading to less size-selective catalysts. The presence of metal nanoparticles outside the nanoboxes can be particularly detrimental if the reactants are converted into deactivating molecules that can poison or block the nanobox surface or pores, as when dealing with reforming or dehydrogenation reactions.
Obuchi and co-workers have previously reported a method to selectively remove Pt nanoparticles located outside ZSM-5 (aluminosilicate zeolite with MFI-type framework) crystals based on a complex procedure involving the use of tetraethylammonium halide salts and halogens [17]. We demonstrate here that a simple citric acid-based treatment can selectively leach out most external Ni nanoparticles, while leaving a significant amount of nanoparticles located inside the nanoboxes. The leached sample exhibited an improved selectivity in the hydrogenation of toluene as compared to that of the bulkier mesitylene.
Section snippets
Sample synthesis
Silicalite-1 was prepared using tetraethylorthosilicate (TEOS, Aldrich, 98%) and 1 M TPAOH (tetrapropyl ammonium hydroxide) solutions obtained from aqueous TPABr (tetrapropyl ammonium bromide) by exchange with Ag2O. The gel of composition SiO2-0.4TPAOH-35H2O was stirred at room temperature overnight to fully hydrolyze the TEOS, then transferred into a Teflon-lined autoclave and heated at 170 °C under static conditions for 3 days. The autoclave was then cooled to room temperature and the solid was
Sample characterizations
The exact Ni loadings measured by ICP on the 5%Ni/sil-1 and 5%Ni@sil-1 were close to the nominal values (Table 1). The loss of Ni due to the citric acid treatment was extensive, resulting in the Ni loading dropping from 6.2 down to ca. 1.5 wt.%. The N2 isotherms of the various samples are shown in Fig. 2 and the textural properties are summarized in Table 2.
The 5%Ni/sil-1 sample exhibited a type-I isotherm (Fig. 2, top, left), which is a fingerprint of purely microporous compounds. The micropore
Conclusions
The present work based on TEM and catalytic data demonstrates that it is possible to use citric acid to selectively leach out most of the external Ni nanoparticles from a Ni@silicalite-1 material. The leached sample exhibited an improved selectivity in the hydrogenation of toluene as compared to that of the bulkier mesitylene.
Acknowledgment
This work was partly supported by the European Union Seventh Framework Programme FP7-NMP-2013, under Grant Agreement number 604277 (acronym FASTCARD).
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