Chemical Weathering of Alumina in Aqueous Suspension at Ambient Pressure: A Mechanistic Study

Abstract

Transition aluminas rank among the main supports used for heterogeneous catalysis. Their stability in the aqueous phase is a key issue for catalytic processes, as their hydration can be strongly detrimental to their physicochemical and mechanical properties. As a consequence, the design of more stable alumina‐based supports relies on a better understanding of the mechanisms leading to their chemical and physical degradation. It is shown here that if suspended in water at atmospheric pressure and at temperatures up to 70 °C, all transition aluminas (from γ to θ‐Al2O3) transform into Al(OH)3 polymorphs (bayerite, gibbsite, and nordstrandite), although to different extents. A quantitative study of the aluminum concentration in solution and of the amount of hydroxides demonstrates that Al2O3 hydration occurs through a two‐step dissolution/heterogeneous precipitation process, with nucleation of Al(OH)3 on the surface of the alumina grains followed by particle growth. The grains become more fragile because of chemical weathering; the ensuing mechanical degradation by attrition, in turn, brings the weathering process to completion. The nature of the main hydroxide polymorph is a function of aluminum concentration and ageing time: first the kinetic product, bayerite, then nordstrandite and eventually gibbsite, the most thermodynamically stable hydroxide. Increasing crystallinity and decreasing specific surface area of alumina leads to a reduced amount in hydroxide formation.

Selective Carbon Deposition on γ-Alumina Acid Sites: toward the Design of Catalyst Supports with Improved Hydrothermal Stability in Aqueous Media

“…26,27 The mechanism of this transformation has already been discussed by Carrier and al. 4 and Abi Aad et al 3,11 It was proposed to be a dissolution/recrystallization process, which is in good agreement with our observations. Other textural properties were also modified during the HT treatment.…”

“…For examγ-alumina, a widely used industrial catalyst support, undergoes irreversible transformation into various aluminum hydroxides by a dissolution/re-precipitation process when placed in a water-rich medium at moderate temperature, resulting in strong modification of its intrinsic properties. 3,4,[5][6][7][8] The stability of alumina materials in water has been a matter of great interest during the last years and many strategies have been proposed to prevent or at least minimize their transformation into hydroxides. The approach generally consists in modifying the alumina surface in order to make it less sensitive to the modifications produced by water under hydrothermal conditions.…”

Interfacial coordination chemistry for catalyst preparation

“…and/or oxy-hydroxides (AlOOH) along with a profound structural change of the initial support [147]. Weathering was demonstrated to occur through a dissolution/heterogeneous precipitation of the hydroxide on the initial γ-Al 2 O 3 particles [147,148]. Mesoporous aluminas are especially sensitive to this phenomenon of dissolution and conversion to aluminum hydroxide [149,150].…”

Reactivity of shape-controlled crystals and metadynamics simulations locate the weak spots of alumina in water

“…While its polymorph α-Al 2 O 3 is stable, γ-Al 2 O 3 indeed transforms into various sorts of bulk hydroxides (AlO x H y ) thereby seriously damaging the catalyst (decrease of surface area, sintering and encapsulation of catalytic particles) 23–25 . Albeit not fully described at the atomic scale yet, the decomposition of γ-Al 2 O 3 was shown to proceed, at the macroscopic scale, through a sequential mechanism involving first dissolution of Al atoms, and then precipitation thereof into the undesired hydroxides 16,24 . These structural changes can be retarded by tuning the surface chemical composition: silica deposition 26 , presence of metallic nanoparticles 12 , or impregnation with metal ions 27 .…”

“…Being able to understand and control their surface stability and desired properties is therefore at the heart of a variety of research fields: kinetics of drug release 1 , corrosion of metals and alloys 2 , lithium batteries 3 , geochemistry 4 with in particular the re-equilibration of solid phases in presence of a liquid 5 , water treatment 6 , heterogeneous catalysis 7,8 , from the preparation 9,10 , the utilization to the degradation of the catalyst 11,12 , etc. Several parameters have been identified as key in all these fields: the solid phase (which polymorph) 13 , the nature of the surface exposed (kink, rugosity) 4 , and the species in solutions (additives, ions, pH) 14–16 .…”