Alexandra Chaumonnot

Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations

Despite the widespread use of amorphous aluminosilicates (ASA) in various industrial catalysts, the nature of the interface between silica and alumina and the atomic structure of the catalytically active sites are still subject to debate. Here, by the use of dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) and density functional theory (DFT) calculations, we show that on silica and alumina surfaces, molecular aluminum and silicon precursors are, respectively, preferentially grafted on sites that enable the formation of Al(IV) and Si(IV) interfacial sites. We also link the genesis of Brønsted acidity to the surface coverage of aluminum and silicon on silica and alumina, respectively.

Direct Aerosol Synthesis of Large‐Pore Amorphous Mesostructured Aluminosilicates with Superior Acid‐Catalytic Properties

An old dream comes true: A direct and environmentally benign synthetic strategy was developed for the aerosol-based mass production of large-pore mesostructured aluminosilicate powders (see TEM image). Although amorphous, some powders exhibit higher activity towards m-xylene isomerization and lower coke formation than a Y-zeolite based industrial reference catalyst.

Quantification of Brønsted Acid Sites of Grafted Amorphous Silica–Alumina Compounds and their Turnover Frequency in m‐Xylene Isomerization

The number and types of acid sites and the catalytic activity of amorphous silica–alumina, obtained by grafting silicon species to the surface of γ‐alumina, varies with the synthesis conditions and the amount of grafted silicon. IR spectroscopy of the OH region proves that deposition occurs selectively, first on the (1 0 0) and then on the (1 1 0) facets. Grafting onto the (1 0 0) surface lowers the number of active sites in ethanol dehydration. Grafting onto the (1 1 0) surface yields Brønsted acid sites that are active in the dehydration of ethanol and that catalyze the isomerization of m‐xylene. Strong Lewis acid sites, or “defect sites”, as detected by CO adsorption, also appear, although they are absent on the parent alumina. The stoichiometric dehydration of ethanol on Brønsted sites, monitored by using thermogravimetric analysis, enables the calculation of the turnover frequency of these sites in m‐xylene isomerization (1.4×10−3 s−1 site−1 at 350 °C, atmospheric pressure, 0.6 cm3 h−1 of m‐xylene, and 0.5 g of catalyst). This number is 22 times lower than on a USY zeolite without extraframework aluminum.

Creation of Bronsted Acidity by Grafting Aluminum Isopropoxide on Silica under Controlled Conditions: Determination of the Number of Bronsted Sites and their Turnover Frequency for m-Xylene Isomerization

There is not a unique Brønsted acid site for aluminosilicates (ASAs). IR spectroscopy following CO adsorption proves the creation of Brønsted acid sites on Al/SiO2 ASAs, which are synthesized by the deposition of aluminum species on hydroxylated silica. These sites are active for ethanol dehydration and m‐xylene isomerization. Controlled deposition under anhydrous conditions optimizes the number of sites, whereas the presence of water leads to alumina agglomerates with no Brønsted acidity. The turnover frequency for m‐xylene isomerization (4.3×10−4 s−1 site−1 at 350 °C, atmospheric pressure, and 0.6 cm3 h−1 of m‐xylene) is approximately 3 times lower than that of the Brønsted acid sites of Si/Al2O3 and 75 times lower than that of an ultrastable Y‐type zeolite without extra‐framework aluminum.

Towards the Improvement of Hydrotreatment Catalysts through the Encapsulation of Heteropolyoxometalates inside the Framework of SBA‐15 Silica

In the context of the search for higher performance hydrodesulfurization catalysts, materials made by a novel method of immobilizing heteropolyoxometalates on mesoporous oxides were compared to those derived by incipient wetness impregnation for the hydrogenation of toluene. The catalysts prepared by the one‐pot encapsulation of HPMo into the walls of SBA‐15 silica were found to be significantly more active for the reaction.

Behaviour of Amorphous Aluminosilicates towards Ageing by High-Temperature Steaming

The structure of amorphous aluminosilicates obtained by grafting silicon on alumina (Si/Al2O3) mostly remains the same after ageing by steaming. The rearrangement of surface silicon species on the facets of γ‐alumina slightly decreases the number of Brønsted acid sites, which is determined through ethanol adsorption followed by thermogravimetry as well as m‐xylene isomerisation. Al/SiO2 prepared by grafting aluminium on silica undergoes sintering of the silica support and the agglomeration of the surface aluminium species during steaming. This process leads to a sharp decrease in the number of Brønsted acid sites, which are associated with the dispersed aluminium species, and dehydroxylation of the silica surface.

Tuning pore size and acidity of mesostructured aluminosilicates made by spray drying: design of new catalysts

Abstract This work describes the unique preparation and the characterization of strongly acidic mesostructured aluminosilicate powders made by spray-drying. We followed here a global approach, taking into account at the same time the synthesis process industrial suitability and the tunability of both the mesostructure and the acidity of the aluminosilicates obtained. Particles are submicrometric spheres with tunable and uniform pore size from 5 to 16 nm and thick walls of 5 to 10 nm. For the first time, such large porosity mesostructured powders could be structured by a [PEO] m -[PPO] n -[PEO] m block copolymer in alkaline media (pH~11) by coupling it with tetrapropylammonium hydroxide as both alkaline source and co-structuring agent. Materials with controlled Si/Al molar ratios from 50 to 6 and high specific surface areas were synthesized. The aluminium incorporation in the oxide network through those synthesis conditions led to very acidic catalysts compared with standard amorphous aluminosilicates, mesostructured or not. Indeed, the iso-surface activity of 8.77 mmol.h -1 .m -2 exhibited in m-xylene isomerization by a Si/Al = 12 material reported here is equivalent to the activity of the Y-zeolite based catalytic reference (8.64 mmol.h -1 .m -2 ).

Thermal behavior of Pd@SiO2 nanostructures in various gas environments: a combined 3D and in situ TEM approach

The thermal stability of core-shell Pd@SiO2 nanostructures was for the first time monitored by using in situ Environmental Transmission Electron Microscopy (E-TEM) at atmospheric pressure coupled with Electron Tomography (ET) on the same particles. The core Pd particles, with octahedral or icosahedral original shapes, were followed during thermal heating under gas at atmospheric pressure. In the first step, their morphology/faceting evolution was investigated in a reductive H2 environment up to 400 °C by electron tomography performed on the same particles before and after the in situ treatment. As a result, we observed the formation of small Pd particles inside the silica shell due to the thermally activated diffusion from the core particle. A strong dependence of the shape and faceting transformations on the initial structure of the particles was evidenced. The octahedral monocrystalline NPs were found to be less stable than the icosahedral ones; in the first case, the Pd diffusion from the core towards the silica external surface led to a progressive decrease of the particle size. The icosahedral polycrystalline NPs do not exhibit a morphology/faceting change, as in this case the atom diffusion within the particle is favored against diffusion towards the silica shell, due to a high amount of crystallographic defects in the particles. In the second part, the Pd@SiO2 NPs behavior at high temperatures (up to 1000 °C) was investigated under reductive or oxidative conditions; it was found to be strongly related to the thermal evolution of the silica shell: (1) under H2, the silica is densified and loses its porous structure leading to a final state with Pd core NPs encapsulated in the shell; (2) under air, the silica porosity is maintained and the increase of the temperature leads to an enhancement of the diffusion mechanism from the core towards the external surface of the silica; as a result, at 850 °C all the Pd atoms are expelled outside the silica shell.

Three-Dimensional Analytical Surface Quantification of Heterogeneous Silica-Alumina Catalyst Supports

The ability of energy‐filtered transmission electron microscopy (EFTEM) tomography to provide 3 D chemical maps at the nanoscale opens a new way to analyse heterogeneous materials quantitatively. In association with other techniques, EFTEM tomography has been employed in the study of amorphous silica‐alumina catalyst supports. Two types of samples prepared either by mechanical mixing (MM) or by the precipitation of silica on boehmite (PSB) that have similar proportions of silica and alumina were analysed. The sample synthesised by the PSB method shows a smaller degree of heterogeneity than the sample obtained by MM. For both types of samples, a higher concentration of alumina was found at the surface, whereas silica mostly constituted the core of the sample. A thermal treatment in a humid atmosphere was shown to redistribute the silica inside the sample as well as on its surface, which decreased the specific surface area at the same time. The acid sites localisation was defined as a specific curve at the interface between the two components upon reaching the surface of the support. The length of this curve, the “alumina–silica boundary line”, was estimated by using EFTEM tomography and discussed qualitatively with the chemical inter‐mixing information deduced from additional techniques such as FTIR and NMR spectroscopy.