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Acid Site Characterization Using the AutoChem

Course Name: Advanced Course:
Characterization of Acid Sites Using Temperature-Programmed Desorption
Availability: Open
Course Date: 1/26/2012  - 1/27/2012
Class Highlights:

Who should attend?

Users that characterize solid acids and routinely use a mass spectrometer coupled to the AutoChem

Curriculum
• AutoChem use with a Mass Spectrometer
• Temperature-programmed reactions with a focus on acid site characterization
• Mass spectrometer calibrations
• Quantification of acid sites


Background

Temperature-programmed desorption (TPD) is one of the most widely used and versatile techniques for characterizing the acid sites on oxide surfaces1-5. Determining the quantity and strength of the acid sites on alumina, amorphous silica-alumina, and zeolites is crucial to understanding and predicting the performance of a catalyst. The activity depends on many factors, but the Brønsted-acid site density is usually one of the most crucial parameters.

There are three types of molecular probes commonly used for characterizing acid sites using TPD: ammonia, non-reactive vapors, and reactive vapors. TPD of ammonia is a widely used method for characterization of site densities in solid acids due to the simplicity of the technique. Ammonia often overestimates the quantity of acid sites. Its small molecular size allows ammonia to penetrate into all pores of the solid where larger molecules commonly found in cracking and hydrocracking reactions only have access to large micropores and mesopores. Also, ammonia is a very basic molecule which is capable of titrating weak acid sites which may not contribute to the activity of catalysts. The strongly polar adsorbed ammonia is also capable of adsorbing additional ammonia from the gas phase.

The most commonly used reactive probes are the propyl amines. These amines are reactive and decompose to propylene and ammonia over Brønsted-acid sites. The temperature-programmed decomposition of amines is the most modern technique for measuring Brønsted-acid site concentrations. The method is based on the formation of alkylammonium ions (from adsorbed alkyl amines that are protonated by Brønsted sites) that decompose to ammonia and olefins in a well-defined temperature range via a reaction similar to the Hofmann-elimination reaction.

 

 

Training Fax Number: (770) 662-3696