Hydrolysis reactions of inverted 1:1 and layered 2:1 silicates.

摘要

Crystalline porous materials find wide-spread use as catalysts, ion-exchangers and adsorbents. They also are potentially useful as composites and materials for the design of electronic, optical or magnetic devices. The nanoporous regime (1-10 nm) spans the mid-micropore region characteristic of traditional crystalline porous materials ({dollar}<{dollar}2.0 nm) and the lower mesopore size range (2.0-50 nm) typical of amorphous oxides. Regularly ordered nanoporous materials would represent new arenas for chemistry in constrained environments.; The present work reports a new approach for synthesizing nanoporous materials. The approach makes use of a layered nonporous material as a template for the formation of a new nanoporous derivative that can not be obtained by using direct crystallization. The viability of this concept is demonstrated for the topochemical acid hydrolysis of antigorite, a silicate with an inverted wave structure. Approximately 70% of the octahedral Mg can be depleted by acid hydrolysis without dramatically changing the crystallographic order of the antigorite. A BET surface area of more than 300 m{dollar}sp2{dollar} g{dollar}sp{lcub}-1{rcub}{dollar} was obtained. This is a very large increase compared to 6 m{dollar}sp2{dollar} g{dollar}sp{lcub}-1{rcub}{dollar} for the starting clay. The mechanism for acid hydrolysis of antigorite is considered to have three main steps: (i) Initial acid attack of the octahedral Mg sheet through the eight membered rings of the basal plane of antigorite; (ii) Secondary lateral hydrolysis of the octahedral Mg of already hydrolyzed 001 planes; (iii) A relatively slow, compared to the steps i and ii, edge hydrolysis process. A regular nanoporous magnesium silicate was synthesized by topochemical hydrolysis of antigorite. The nanopore size varied from a diameter {dollar}sim{dollar}8 A to {dollar}sim{dollar}39 A depending on the Mg{dollar}sp{lcub}2+{rcub}{dollar} depletion and the rearrangement of the SiO{dollar}sb2{dollar} sheet.; The acid hydrolysis reactions of kaolinite, phlogophite and fluorohectorite also were studied. Kaolinite with aluminum in octahedral sites is not a good candidate for acid hydrolysis reactions as judged by the insignificant change in surface area upon hydrolysis. Acid hydrolysis of phlogophite increases the surface area from 2 m{dollar}sp2{dollar} g{dollar}sp{lcub}-1{rcub}{dollar} to a maximum value of 77 m{dollar}sp2{dollar} g{dollar}sp{lcub}-1{rcub}{dollar} at 87% Mg depletion. The acid hydrolysis of phlogophite was found to involve an edge hydrolysis diffusional mechanism. Fluorohectorite is more sensitive towards acid hydrolysis than all other silicates studied. The final product exhibits a substantially higher surface area (208 m{dollar}sp2{dollar} g{dollar}sp{lcub}-1{rcub}{dollar}) compared to the starting surface area (3 m{dollar}sp2{dollar} g{dollar}sp{lcub}-1{rcub}{dollar}). Acid attack most likely occurs both through the basal surface (hexagonal cavities) and the edge sites of the layer. Depletion of octahedral Mg occurs starting from edges of the clay particles as it would be expected to occur in talc and mica clays. But fluorohectorite, unlike mica or talc, affords an exceptionally high surface area comparable to those of acid hydrolyzed palygorskite, sepiolite and other smectites.; According to the present work and the early work done in area of acid hydrolysis of clay minerals, three different categories of minerals can be identified depending on their behavior towards the acids: (1) Swellable clay minerals including vermiculite give high BET surface areas upon acid treatment; (2) Nonswellable clays afford little or no surface area increase upon acid treatment (e.g. talc, phlogophite); (3) Nonswellable clays with special structural features give high surface areas (e.g. sepiolite, palygorskite, antigorite).