The encapsulation of metal nanoparticles within zeolitic voids of molecular dimensions protects metal surfaces from contact by reactant or poison species that are too large to enter framework apertures, and confers sinter-stability to confined metal domains via intervening channels that prevent cluster coalescence. Such channels can also stabilize specific transition states or retain undesired products until they fragment into smaller molecules capable of egress by diffusion. These size-selective properties are governed, in each instance, by the size of the microporous channels and voids in a specific framework. The encapsulation of noble metal (e.g., Au, Pd, Pt) clusters within small-pore (8-member ring (8-MR) apertures) and medium-pore (10-MR) zeolites, however, often cannot be achieved through established post-synthetic exchange or impregnation techniques, because solvated metal cations may be too large to enter the apertures of these zeolites. Base metal cations (e.g., Ni2+, Co2+, Fe2+), though generally small enough to enter even small-pore zeolites, tend to form highly refractory complexes when ion-exchanged, thus precluding their conversion into catalytically active metal or metal oxide particles by reductive or oxidative treatments. We have developed synthetic strategies and guiding principles for the successful preparation of Au and bimetallic (AuPd, AuPt, PdPt) nanoparticles within the microporous voids of zeolite or zeotype materials with medium (MFI, TS-1) and small (LTA) sized pores. Synthetic techniques have also been devised for the encapsulation of base metal oxide (NiO, Co3O4, Fe2O3) nanoparticles within large (FAU; 12-MR), medium (MFI), and small-pore (LTA) zeolites.
The encapsulation of Au and bimetallic clusters within LTA and MFI was achieved via hydrothermal self-assembly of crystalline frameworks around ligated metal cation precursors (Au3+, Pd2+, Pt2+). Bifunctional ligands containing a thiol moiety are used to coordinate with and protect metal cations, thus preventing their premature precipitation or reduction in alkaline synthesis gels at elevated temperature. Such ligands also contain alkoxysilane moieties that form covalent linkages with nucleating silicate oligomers to enforce metal uptake into crystallizing frameworks. The controlled deprotection of ligated cations with sequential oxidative and reductive treatments forms encapsulated clusters that are small (<2 nm), uniform in size, and sinter-stable at temperatures as high as 870 K. The simple incorporation of two ligated metal precursors into zeolite synthesis gels leads to the encapsulation of bimetallic clusters that are uniform in size and composition.
The encapsulation of Au clusters within zeotype frameworks via hydrothermal assembly around ligated Au cation precursors poses a greater challenge for TS-1 than MFI or LTA, because the high temperature typically required for the crystallization of TS-1 (448 K; LTA: 373 K; MFI: 393 K) strongly promotes the deprotection of even ligand-protected Au precursors. TS-1 was successfully crystallized at much milder conditions than previously reported in order to prevent decomposition (or reaction with alkanols formed during hydrolysis of Ti or Si precursors) of Au-ligand complexes in the synthesis gels without adversely affecting Ti incorporation into the framework or sample crystallinity. The changes made to the established synthesis procedures for TS-1 include much lower crystallization temperatures (393 K vs. 448 K) compensated by longer crystallization times (120 h vs 48 h). Post-synthetic oxidative treatment procedures were developed in order to remove ligand species and organic structure-directing agents from the crystallized Au/TS-1 solids while minimizing the generation of local exotherms; these procedures led to the formation of small (~3.8 nm in diameter) Au nanoparticles embedded within TS-1 crystals.
Techniques developed for the encapsulation of base metals (Ni, Co, Fe) differ from those used for noble metals in (i) the types of ligands used for metal cation stabilization in synthesis gels (thiol ligands for noble metals; bidentate amine ligands for base metals), (ii) the precise order of reagent addition to zeolite synthesis gels needed to ensure ligand attachment and the prevention of metal precipitation, and (iii) the post-synthetic treatments that are most effective at removing ligand residues and subsequently forming encapsulated nanoparticles. It is critically important to prevent the attachment of base metal cations to zeolite exchange sites during both the zeolite assembly process and post-synthetic treatments, because the conditions required to convert such exchanged cations into clusters (>1100 K in H2) are so prohibitively extreme as to cause degradation of the host zeolite frameworks. Base metals were successfully encapsulated by adding Ni2+, Co2+, or Fe2+ cations protected by chelating amine ligands to LTA, MFI, or FAU synthesis gels. Such ligands protect metal cations from precipitation as metal hydroxides or oxides in the alkaline gels and form siloxane bridges with nucleating aluminosilicate oligomers, thereby enforcing metal uptake into zeolite crystallites during framework assembly. The amine ligands also sterically preclude the direct attachment of base metal cations to zeolite exchange sites, thus allowing the conversion of occluded base metal species into metal or metal oxide clusters at conditions that do not incur damage to the zeolite frameworks. Oxidative treatment of the crystallized zeolites removes ligand species occluded within the zeolite pores and forms encapsulated metal oxide clusters.
The selectivity of metal encapsulation within zeotype crystals was quantified by examining rates of probe reactions involving small molecules (e.g., ethanol, O2) on metal-zeotype samples exposed to bulky organosulfur compounds (e.g., dibenzothiophene) that are too large to enter framework apertures. Such organosulfur compounds selectively titrate and deactivate extracrystalline metal surfaces, leading to probe reaction rates that exclusively or predominantly reflect turnover events occurring on encapsulated metal surfaces. These experiments indicate that >95% of active metal surfaces reside within framework crystals for each metal-zeotype sample, confirming the efficacy of the encapsulation methods and demonstrating the size-selective properties conferred by confinement.
The synthesis strategies presented in this work, which demonstrate the requirements for successful syntheses, provide guiding principles for the selective encapsulation of metal and oxide nanoparticles within a wide variety of crystalline environments through simple one-step hydrothermal assembly routes. Such strategies provide versatile and broadly applicable alternatives to the limited or system-specific schemes for nanoparticle encapsulation that have been developed previously. These encapsulated clusters have potential uses in diverse applications that exploit the catalytic benefits conferred by metal encapsulation within microporous solids, including stability against sintering during reactions, reactant or product shape selectivity, and the protection of catalytic surfaces from contact with poisons that block their active sites.
