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In situ pH and Attenuated Total Reflection (ATR) infrared techniques have been successfully applied in order to gain insights into the dissolution process connected to mesopore formation occurring upon alkaline treatment of ZSM-5 zeolites. Online pH measurements reveal a similar consumption of OH(-) ions in the initial stage of the reaction independent of the Si/Al ratio of the zeolite. In view of the greatly different mesoporosity development, the extraction of polymeric silica entities is anticipated, its structure depending on the framework Si/Al ratio. In agreement, ATR-IR experiments have confirmed dissolution of polymeric silicon-containing species that in the course of the alkaline treatment disintegrate into smaller entities. A direct relation between the type of porosity developed and the process of silicon extraction as measured in the liquid phase cannot be drawn.

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The appearance of catalyst-reactant interactions observed in on-line ATR FT-IR spectra of Nafion/silica catalysed esterification and etherification reactions of 1-octanol was investigated. It was assessed by variation of catalyst and solvent that the catalyst-reactant band is a result of the reaction of silica with 1-octanol, yields Si-O-R functionalities. Based on off-line TPD-MS and TGA characterisation of the used solid catalyst powder, the formation of Si-O-R linkages on the SiO 2 surface was confirmed. This demonstrates conclusively that in specific liquid-phase reactions, on-line analysis of intermediate species adsorbed on heterogeneous catalysts is possible. Using on-line particle analyzers, it was assessed that the absorptions were not the result of severe attrition of the catalyst particles in the course of the reaction, but apparently the result of apolar interactions of the particles with the diamond crystal.

This study evaluates the applicability of fiber-optic-based Raman probes for on-line reaction monitoring of high-pressure catalytic hydrogenation reactions in batch autoclaves. First, based on trends in the strong intensity of the 945 cm(-1) C-O-C vibration of 1,3-dioxolane, the effect of various experimental parameters on sensitivity was evaluated and can be summarized as follows: (1) above 500 rpm a linear increase in stirring speed induces a linear decrease in Raman intensity; (2) a linear increase in hydrogen pressure also leads to a linear decrease of the Raman signal; (3) linear temperature elevation exponentially decreases the Raman intensity; and (4) increasing the catalyst particle concentration results in a steep nonlinear decrease of the Raman signal. Light scattering by gas bubbles, or combined scattering and absorption by (black) catalyst particles, reducing the amount of light collected by the optical fiber probe, explain the observed experimental trends. Second, the sensitivity of Raman spectroscopy was directly compared with attenuated total reflection-Fourier transform infrared (ATR-FT-IR) spectroscopy in the analysis of three different hydrogenation reactions over a Cu-ZnO catalyst. From the applied target molecules, diethyl maleate hydrogenation could be very well analyzed by Raman spectroscopy due to the high Raman scattering efficiency of the C=C bond, while for analysis of the hydrogenation of gamma-butyrolactone or 1-butanal, ATR-FT-IR is the technique of choice.

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