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Assessing the acid properties of desilicated ZSM-5 by FTIR using CO and 2,4,6-trimethylpyridine (collidine) as molecular probes

Paper ID Volume ID Publish Year Pages File Format Full-Text
42643 45935 2009 8 PDF Available
Title
Assessing the acid properties of desilicated ZSM-5 by FTIR using CO and 2,4,6-trimethylpyridine (collidine) as molecular probes
Abstract

A series of desilicated ZSM-5 catalysts previously shown to have improved catalytic performance in the MTG (methanol-to-gasoline) reaction [M. Bjørgen, F. Joensen, M.S. Holm, U. Olsbye, K.-P. Lillerud, S. Svelle, Appl. Catal. A 345 (2008) 43] was subjected to thorough examination using FTIR. Clearly, defects represented by internal Si-OH sites are removed upon NaOH treatment. In a parallel manner, free Si-OH sites increase in concentration and the results point to a selective mechanism for formation of mesopores as the framework dissolution preferentially takes place at defective sites in the crystallites. The acid properties of the desilicated materials were investigated by applying CO and collidine (2,4,6-trimethylpyridine) as molecular probes. Monitoring the induced frequency shifts upon CO adsorption at liquid N2 temperature revealed that the desilication procedure did not alter the acid strength of the Brønsted sites significantly. This weakens the explanation linking improved catalytic behaviour, such as increase in both catalyst activity and hydrogen transfer activity, to modified Brønsted acid strength. Simultaneous to the mesopore formation resulting from the desilication, strong Lewis acid sites were generated, presumably from dislodged framework aluminium. Collidine, which is too bulky to enter the micropore system of ZSM-5, could access Lewis acidity, suggesting that these sites were predominantly generated on the external surface or in the newly created mesopores. Additionally, by first saturating the zeolite surface with collidine and subsequently adsorbing CO, we show that barely any Lewis acidity was uncoordinated post-collidine saturation while the Brønsted acidity continuously was protected behind the micropore system. It is hypothesized from the present study that the desilication procedure can lead to a slight dealumination of the samples while forming Lewis acidity preferentially at the crystal surface.

Graphical abstractThis FTIR study shows that zeolite defects (internal Si-OH sites) are removed upon NaOH treatment. Free Si-OH sites in the mesopores increase in concentration. Some mesopores are protected by the microporous framework. Mesopore formation appears to take place at defective sites. The acid strength of Brønsted sites is unaffected, but Lewis acidity on the external crystal surface is formed.Figure optionsDownload full-size imageDownload as PowerPoint slide

Keywords
MFI; MTG; MTH; NaOH treatment; Mesoporous; Lewis acidity
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Assessing the acid properties of desilicated ZSM-5 by FTIR using CO and 2,4,6-trimethylpyridine (collidine) as molecular probes
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Publisher
Database: Elsevier - ScienceDirect
Journal: Applied Catalysis A: General - Volume 356, Issue 1, 1 March 2009, Pages 23–30
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
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Any Questions? feel free to contact us