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Aldehydes are widely recognized as important chemicals in the pharmaceutical, agricultural and fragrance industries1)~3). We previously established a synthesis method for aldehydes by the direct hydrogenation of the corresponding monobasic carboxylic acids in the vapor phase catalyzed by ZrO2 or Cr2O3. Separately, we reported further application of the hydrogenation process for the production of various dialdehydes from dibasic carboxylic acid diesters under almost the same conditions4). The reactivity of the three isomers of dimethyl benzenedicarboxylic acids toward the hydrogenation are quite different. Hydrogenation of dimethyl terephthalate gave monoand dialdehydes with high selectivity. However, hydrogenation of dimethyl phthalate gave no aldehydic products. Our attempts to explain this reactivity with the pKa values or Hammet’s σ value, which represents the degree of polarization at the carboxyl group of the isomers, have failed (for example, pKa1 = 3.54, pKa2 = 4.46, Hammet’s σ value = 0.45 for terephthalic acid, pKa1 = 3.70, pKa2 = 4.60, σ value = 0.37 for isophthalic acid). The adsorption species on metal oxide catalysts in the hydrogenation of monocarboxylic acid have been characterized with fourier transform infrared spectrometer (FT-IR) spectroscopy5)~8). We found that the adsorbed species on ZrO2 catalyst in the hydrogenation of benzoic acid is the benzoate species, which weakly interacts with the catalyst surface, and catalytic hydrogenation of adsorbed benzoate liberates benzaldehyde6). Therefore, the absorbed benzoate species is a key intermediate in the hydrogenation. Few spectroscopic investigations of the catalyst surface in the hydrogenation of dibasic carboxylic acids or diesters are available, although understanding the reaction mechanism will provide useful information for the development of an effective hydrogenation catalyst. Here we report the spectroscopic analysis of the adsorption species in the hydrogenation of dimethyl benzenedicarboxylates as well as benzoic acid on the ZrO2 or Cr_ZrO2 catalyst using diffuse reflectance FTIR and CP/MAS 13C-NMR (nuclear magnetic resonance) spectroscopy, which provide more accurate characterization of organic species adsorbed on a solid catalyst9)~15). A reaction mechanism for the hydrogenation of dimethyl benzenedicarboxylates is also proposed. Hydrogenation of Dicarboxylic Acid Diesters to Corresponding Dialdehydes over ZrO2 Based Catalysts (Part 2) Analysis of Catalyst Surface Using Diffuse Reflectance FT-IR and CP/MAS 13C-NMR Spectroscopy

journal of the japan petroleum institute

Hydrogenation of Dicarboxylic Acid Diesters to Corresponding Dialdehydes over ZrO&lt;sub&gt;2&lt;/sub&gt; Based Catalysts (Part 2) Analysis of Catalyst Surface Using Diffuse Reflectance FT-IR and CP/MAS &lt;sup&gt;13&lt;/sup&gt;C-NMR Spectroscopy