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Clays and Clay Minerals

, Volume 17, Issue 5, pp 279–287 | Cite as

Structural Organization in Amorphous Silico-Aluminas

  • P. Cloos
  • A. J. Léonard
  • J. P. Moreau
  • A. Herbillon
  • J. J. Fripiat
Article

Abstract

A structure model for amorphous hydrated or dehydrated silico-aluminas with composition varying between 0 < Al: Al + Si < 1 is presented. A central core made from a tetrahedral network in which silicon is partially substituted by aluminium carries a net negative electrical charge. This charge is balanced by more or less polymerized hydroxyaluminium cations forming a coating around the core.

As Al: Al + Si increases, the number of substitutions in the core increases as well as the complexity of the hydroxyaluminium cations in the coating.

For Al: Al + Si ≳ 0·8, a demixing is observed, leading to the formation of a crystalline pseudo-boehmite and bayerite.

Upon heating, the coating as well as the demixed phases are transformed into a spinel structure containing tetrahedral aluminium, while the core structure remains unaffected.

This model could explain the solubility features, the phosphate reaction and the catalytic properties of amorphous silico-aluminas.

Résumé

In modèle présente de structure pour des silico-alumines amorphes hydratés ou deshydratés, dont la composition varie entre O < Al: Al + Si < 1. Un noyau central construit à partir d’un réseau tetraédrique dans lequel la silice est substituée en partie par l’aluminium, porte une charge électrique négative. Cette charge est équilibrée par des cations hydroxy-aluminium plus ou moins polymérisés formant une couche autour du noyau. Tandis que Al:Al + Si augmentent, le nombre de substitutions dans le noyau s’accroit ainsi que la complexité des cations hydroxy-aluminium dans le revêtement. Pour Al:Al + Si ≳ 0,8, on observe un démixage qui conduit à la formation d’un pseudo-boehmite cristallin et bayerite. Par l’action du chauffage, le revêtement ainsi que les phases de démixage sont transformés en une structure spinelle contenant de l’aluminium tétrahydrique, tandis que la structure du noyau rest inchangée. Ce modèle pourrait expliquer les caractéristiques de solubilité, la réaction du phosphate et les propriétés catalytiques des silico-alumines amorphes.

Kurzreferat

Ein Strukturmodell für amorphe hydrierte bzw. dehydrierte Silica-Tonerden mit einer Zusammensetzung zwischen 0< Al: Al + Si < 1 wird erörtert. Ein zentraler Kern, der aus einem tetrahedralen Netz, in dem Silizium teilweise durch Aluminium substituiert ist, besteht, trägt eine Nettoladung negativer Elektrizität. Diese Ladung wird durch mehr oder weniger polymer-isierte Hydroxy-Aluminium-Kationen ausgeglichen, die eine Schale rings um den Kern bilden. Mit zunehmenden Al:Al + Si nimmt auch die Anzahl der Substituierungen in dem Kern zu, wie auch die Kompliziertheit der Hydroxy-Aluminium-Kationen in der Schale. Bei Al:Al + Si≥0,8 beobachtet man eine Entmischung, die zur Bildung von kristallischem Pseudoböhmit und Bayerit führt. Nach der Erwärmung werden die Schale und die entmischten Phasen in eine Spinellstruktur umgewandelt, die tetrahedrales Aluminium enthält, während die Kernstruktur unberührt bleibt. Dieses Modell könnte die Löslichkeitsmerkmale. die Phosphatreaktion und die katalytischen Eigenschaften amorphe-Silica-Tonerden erklären.

Резюме

Предложена структурная модель для аморфной гидратированной или дегидратиро¬ванной смеси кремнезема и глинозема, состав которой изменяется в пределах: О<А1: А1 + Si < 1. Внутреннее ядро образовано тетраэдрической сеткой, в которой кремний частью замещен алюминием; сетка имеет отрицательный заряд. Этот заряд компенсируется более или менее полимеризованными гидроксиалюминиевыми катионами, образующими оболочку вокруг ядра.

С возрастанием отношения А1: А1 + Si число замещений в ядре увеличивается, а компле¬ксность гидроксиалюминиевых катионов в оболочках усиливается.

При отношении А1: А1 + Si ≳0,8 наблюдалось обособление глинозема с образованием кристаллических псевдобёмита и байерита. После нагревания как оболочки так и обособившие¬ся кристаллические фазы дали вещество со шпинелевой структурой, содержащее алюминий в тетраэдрической координации; при этом структура ядра не изменилась.

Предложенная модель может объяснить особенность растворения, фосфатную реакцию и каталитические свойства аморфных алюмокремниевых смесей.

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Copyright information

© Clay Minerals Society 1969

Authors and Affiliations

  • P. Cloos
    • 1
    • 2
  • A. J. Léonard
    • 1
    • 2
  • J. P. Moreau
    • 1
    • 3
  • A. Herbillon
    • 1
    • 4
  • J. J. Fripiat
    • 1
    • 4
  1. 1.Laboratoire de Physico-Chimie MinéraleInstitut des Sciences de la TerreHeverlee-LouvainBelgium
  2. 2.The University of LouvainBelgium
  3. 3.France
  4. 4.The University of Louvain and M.R.A.C. (Tervuren)Belgium

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