Understanding the molecular basis for the controlled design of ruthenium nanoparticles in microporous aluminophosphates

Abstract

Controling the structural properties of nanoparticle catalysts within a microporous framework is a major challenge. Using in situ X-ray Absorption Fine Structure (XAFS) Spectroscopy we detail the influence of activation parameters on the nature of ruthenium particles that are located within the confines of a nanoporous aluminophosphate (RuAlPO-5) architecture. These in situ studies confirm that controlled annealing conditions can tailor the formation of specific ruthenium species, which alter the catalytic performance towards the oxidation of cyclohexane to KA oil (a 1:1 mixture of cyclohexanone and cyclohexanol), the precursor for Nylon-6 and Nylon-6,6. IntroductionGiven the chemical industry's dependence on crude oil, the activation of inert hydrocarbons is of fundamental interest. [1][2][3][4] Once oxidized these molecules gain significant value as precursors in the fine-chemical and polymer industries. Heterogenized ruthenium species have been used for a variety of sustainable oxidation processes, [5] as the wide-range of available oxidation states makes ruthenium highly versatile for performing transformations of activated functional groups, e.g. alcohols to ketones and amines to nitriles. [6][7][8] Many approaches have been used to heterogenize ruthenium onto porous supports, thereby combining the selectivity control of a nanoporous material (commonly oxidic or carbon-based) with the catalytic potential of the metallic species. [9][10][11] A variety of metal precursors and post-synthesis thermal treatments have been used, which have proved effective in enhancing the catalytic activity of Ru. [12][13][14][15][16][17] In order to optimize catalytic performance many synthesis procedures have been developed with a view to creating uniform ruthenium sites. Ionic liquids are increasingly employed to generate nanoparticles (NPs) with a narrow size distribution, as the ionic liquid hinders particle agglomeration. [18][19][20] Similarly polymers such as poly(vinylpyrrolidone) are used as capping agents to restrict the size of the NPs formed via a micellar method, [21] before they are bound to a surface. The size and ensuing stability of ruthenium NPs has also been modified by inorganic additives, whereby a secondary metal can coat ruthenium NPs, maintaining metallic state and hindering subsequent formation of oxidic species. [22] A range of ruthenium-containing complexes have also been developed, aiming to preserve the integrity and isolated (< 10 atoms) nature of the precursor. Arguably, the most common of these are multimetallic nanoclusters, which form defined nanoparticles with distinct stoichiometry, concordant with the original cluster. [23,24] However, these strategies are commonly hindered by complex and sensitive precursors, such as multimetallic metal clusters, or precisely tailored organometallic species. [23,24] The use of isomorphously substituted metal atoms into a microporous framework (as outlined in Scheme S1, where small quantities of dopant transition metals can be used to replace Al(III) and P(V...

Exploring the origins of crystallisation kinetics in hierarchical materials using in situ X-ray diffraction and pair distribution function analysis

“…Co, Ru or Zn) can be readily distinguished from the lighter atoms in the gel. 20,23,24 This has allowed groups to observe the progression from the metal salt precursor, to tetrahedral framework species, coinciding with the framework formation. Al XAS studies have also been reported; however these were limited to XANES analysis, due to the lower energy required.…”

The influence of porosity on nanoparticle formation in hierarchical aluminophosphates

“…However, due to the limited pore aperture and channels of these systems, even sub-nanometre particles can block the framework and hinder activity, thereby preventing reagents from accessing the internal active sites. In contrast, mesoporous species (pores greater than 2 nm) maintain a large portion of their porosity when hosting metal nanoparticles, although they lack the more subtle ability to control the space around the active site [13]. In our previous work we have shown that inclusion of a micellular agent, i.e., dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOD) in an AlPO synthesis, alongside a microporous template, allows silanol-lined mesopores to form simultaneously with the microporous network, yielding a hierarchically porous (HP) system [14].…”

The influence of porosity on nanoparticle formation in hierarchical aluminophosphates

“…In contrast, mesoporous species (pores > 2 nm) maintain a large portion of their porosity when hosting metal nanoparticles, though lack the more subtle ability to control the space around the active site. [13] In our previous work we have shown that inclusion of a micellular agent; DMOD (Dimethyloctadecyl [3-(trimethoxysilyl)propyl] Ammonium Chloride) in an AlPO synthesis, alongside a microporous template, allows silanol-lined mesopores, to form simultaneously, alongside with the microporous network, yielding a hierarchically porous (HP) system. [14] In this work we utilise both a HP silicon-doped aluminophosphate (SAPO) system and a traditional microporous SAPO-5 species, to demonstrate the advantages of hierarchical systems for nanoparticle deposition.…”