Probing the Surface Acidity of Supported Aluminum Bromide Catalysts

Md Ashraful Abedin,Zi Wang,James J Spivey,Kunlun Ding,Graham Hutchings,Swarom Kanitkar,Nitin Kumar

doi:10.3390/catal10080869

Abstract

Solid acid catalysis is an important class of reactions. The principal advantages of solid acid catalysts as compared to their corresponding fluid acids include minimal waste and ease of product separation. One type of these catalysts is based on aluminum bromide (Al2Br6), which is a stronger Lewis acid than Al2Cl6. In this report, Al2Br6 is grafted on commercial mesoporous silica (CMS), SBA-15 and silica gel to create a solid catalyst similar to the silica-supported Al2Cl6 superacid. These supported Al2Br6 catalysts were characterized by NH3-Temperature Programmed Desorption (TPD), pyridine Diffuse Reflectance for Infrared Fourier Transform Spectroscopy (DRIFTS) and Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). Formation of acid sites was confirmed and quantified with NH3-TPD. Both Lewis and Brønsted sites were observed with DRIFTS using pyridine as a probe molecule. In addition, thermal stability of acid sites was also studied using DRIFTS. 27Al MAS NMR analysis showed tetrahedral, pentahedral and octahedral co-ordination of Al, confirming that Al2Br6 reacted with –OH groups on silica surface. Performance of these catalysts was evaluated using acid-catalyzed 1-butene isomerization. Conversion above 80% was observed at 200 °C, corresponding to thermodynamic equilibrium.

Highlights

Acid catalysis is one of the most important area of modern catalysis and is a key step in a number of chemical processes [1]
While it was sold as MCM-41, the structural order was found to be poor and not expected for MCM-41, as so it was addressed as commercial mesoporous silica (CMS)
This work includes the study of Si/Al oxide based Aluminum Bromide solid acid catalysts based on their surface acidity

Summary

Introduction

Acid catalysis is one of the most important area of modern catalysis and is a key step in a number of chemical processes [1]. These catalysts are utilized in solid, liquid or gaseous form. Liquid and gas phase acid catalysts often involve inorganic or mineral acids such as H2 SO4 , HBr or HF. Very effective, these catalysts suffer from several drawbacks—(1) difficult separation of products from the catalyst, (2) corrosiveness of the system and (3) significant quantities of waste. One important class of solid acids involve group IIIA halides—in particular aluminum halides [7,8,9]

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