Controlling the dispersion of supported polyoxometalate heterogeneous catalysts: impact of hybridization and the role of hydrophilicity–hydrophobicity balance and supramolecularity

Abstract

SummaryThe hybridization of polyoxometalates (POMs) through an organic–inorganic association offers several processing advantages in the design of heterogeneous catalysts. A clear understanding of the organization of these hybrid materials on solid surfaces is necessary to optimise their properties. Herein, we report for the first time the organization of Keggin phosphotungstic [PW12O40]3− and Wells–Dawson (WD) phosphomolybdic [P2Mo18O62]6− anions deposited on mica (hydrophilic), and highly oriented pyrolytic graphite (HOPG) (hydrophobic) surfaces. Next, the supramolecular organization of the organic–inorganic hybrid materials formed from the association of POM anions and dimethyldioctadecylammonium bromide (DODA) is investigated as a function of the hydrophilic or hydrophobic nature of the surfaces. The height of the Keggin-POM anions, measured with tapping mode (TM-AFM) is always in good agreement with the molecular dimension of symmetric Keggin-POM anions (ca. 1 nm). However, the asymmetric WD-POM anions form monolayer assemblies on the surfaces with the orientation of their long molecular axis (ca. 1.6 nm) depending on the hydrophilic or hydrophobic properties of the substrate. Namely, the long axis is parallel on mica, and perpendicular on HOPG. When hybridized with DODA, the organization of the hybrid material is dictated by the interaction of the alkyl side chains of DODA with the substrate surface. On HOPG, the DODA–POM hybrid forms small domains of epitaxially arranged straight nanorod structures with their orientation parallel to each other. Conversely, randomly distributed nanospheres are formed when the hybrid material is deposited on freshly cleaved mica. Finally, a UV–ozone treatment of the hybrid material allows one to obtain highly dispersed isolated POM entities on both hydrophilic and hydrophobic surfaces. The hybridization strategy to prevent the clustering of POMs on various supports would enable to develop highly dispersed POM-based heterogeneous catalysts with enhanced functionalities.

Direct Observation of Asphaltene Nanoparticles on Model Mineral Substrates

“…The reduced height is attributed to flattening of the aggregates due to attraction between HOPG and the hydrocarbon moieties on the asphaltene molecules. Similar epitaxial interaction of carbon atoms of HOPG with alkyl chains of adsorbed species was reported for other systems with hydrocarbon chains. , …”

“…Similar epitaxial interaction of carbon atoms of HOPG with alkyl chains of adsorbed species was reported for other systems with hydrocarbon chains. 49,50 The results described above clearly show that surface properties (hydrophobicity, and affinity toward polycyclic aromatic rings) determine the morphology and size of asphaltene aggregates. Dramatic differences in the height and size of asphaltene nanoaggregates are observed for two extreme surfaces, namely mica and HOPG.…”

“…The typical height of alkyl chains adsorbed on HOPG is ∼0.35 nm. 50 Thus, the protruded structures could be two or more strands of alkyl chains stacked together in the aggregate. On the other hand, on mica, the hydrophobic alkyl chains may tilt away from the hydrophilic surface or are buried within the aggregates.…”

Permeability of Dawson–type polyoxometalates through vertically oriented nanoporous silica membranes on electrode: Effect of pore size and probe charge

“…5A). This can be attributed to a slower mass transport of [P2W18O62] 6-through the nanochannels due to its obviously bigger size compared to [SiW12O40] 4-; for [SiW12O40] 4a hydrodynamic radius has been reported (RH = 0.52 nm in H2O + 250 mM NaCl [55]), corresponding to a sphere with diameter of 1.04 nm (consistent with its molecular size determined by X-ray crystallography [56][57][58]) and, even if no RH data is available for [P2W18O62] 6-, it should be significantly larger on the basis of crystallographic information (rugby ball-like shape with a molecular size of ca. 1.1×1.45 nm for [P2W18O62] 6-[58-60]).…”

Local Structure of Supported Keggin and Wells–Dawson Heteropolyacids and Its Influence on the Catalytic Activity

“…The primary structure corresponds to those in Figure 1, i.e. the heteropolyanion itself; the secondary structure is assigned to a three-dimensional arrangement including counter cations, in our case H + and water molecules [3,[7][8][9][10]. In addition, the arrangement of the particles, that is dependent on their morphology and size, generates the tertiary structure that is responsible for the surface area of HPAs and for the accessibility of substrates to the HPAs acid sites.…”