The Use of Diffuse Reflectance Infrared Spectroscopy for Studies of Molecular Sieve Catalysis

Abstract

Diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) in conjunction with gas chromatography has been used to investigate the interaction of light hydrocarbons with the molecular sieve catalyst H-ZSM-5. Experiments were conducted in a controlled-environment (temperature, gas composition) diffuse reflectance cell interfaced with an on-line gas Chromatograph. The reactivity of CH3OH, CH3CH2OH, and CH2=CH2 on H-ZSM-5 has been investigated by observing the changes in intensity of the framework hydroxyl stretching vibrations as a function of the type of reactant, temperature, and product mix. C-H stretching vibrations of the organic fragments that form in the molecular sieve channels and pores are also observed in these experiments, and they too are correlated with reactant, temperature, and product mix with the use of spectral subtraction techniques. Important functional features of molecular sieve catalysis detectable by the DRIFT method include (1) the creation of protonated sites during calcining of the synthesis template (the process by which the catalytically active form of the sieve material is produced); (2) the labile nature of these protons as a function of reactant and temperature; and (3) the onset of coking of the molecular sieve as a result of extended use or use at higher temperatures.