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Silica promoted self-assembled mesoporous aluminas. Impact of the silica precursor on the structural, textural and acidic properties

Paper ID Volume ID Publish Year Pages File Format Full-Text
53923 46989 2015 8 PDF Available
Title
Silica promoted self-assembled mesoporous aluminas. Impact of the silica precursor on the structural, textural and acidic properties
Abstract

•Adding colloidal silica, or TEOS by co-hydrolysis, enhances the ordering and surface area of these aluminas.•The crystallization temperature into γ-alumina for the TEOS-based mesophases was suppressed by 50–100 °C.•The activated SiO2-based aluminas display a lower acid density, and reduced fraction of stronger sites.•Migration of SiO2 to the surface is suggested to block part of the acidity.

This paper investigates the effect of silica addition on the structural, textural and acidic properties of an evaporation induced self-assembled (EISA) mesoporous alumina. Two silica addition protocols were applied while maintaining the EISA synthesis route. The first route is based on the addition of a Na-free colloidal silica suspension (Ludox®), and the second method consists of the co-hydrolysis of tetraethyl orthosilicate (TEOS) with aluminium tri-sec-butoxide, to favour a more intimate mixing of the Al- and Si-hydrolysed species. The properties of the so derived materials were compared to the SiO2-free counterpart. The SiO2 addition was always beneficial from a structural and textural standpoint. TEOS appears to have a truly promoting effect; the ordering, surface area and pore volume are all improved. For Ludox®, the enhancement comes from the formation of smaller pores by a densification of the structure. The crystallization of γ-alumina depends on the interaction between the Al- and Si-species in the mesophase. Ludox®-based materials achieved crystallization at 750 °C but the intimate mixing in the TEOS-based mesophases shows a suppression of the phase transformation by 50–100 °C, with respect to the SiO2-free counterpart. This reduces the textural features substantially. For all SiO2-modified materials, the enhancement in the surface area is not accompanied by a concomitant improvement of total acidity, and the formation of weak Lewis acid sites was promoted. These effects were ascribed to SiO2 migration to the surface that blocks part of the acidity.

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Keywords
Gamma alumina; Sol–gel; Evaporation-induced self assembly; Silica promotion
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Silica promoted self-assembled mesoporous aluminas. Impact of the silica precursor on the structural, textural and acidic properties
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Publisher
Database: Elsevier - ScienceDirect
Journal: Catalysis Today - Volume 250, 15 July 2015, Pages 115–122
Authors
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Subjects
Physical Sciences and Engineering Chemical Engineering Catalysis
Get Full-Text Now
Don't Miss Today's Special Offer
Price was $35.95
You save - $31
Price after discount Only $4.95
100% Money Back Guarantee
Full-text PDF Download
Online Support
Any Questions? feel free to contact us