Understanding the effects of water in catalysis by solid acid and bases has attracted increasing interest in fundamental and applied heterogeneous catalysis research. Three model reactions, namely the Brønsted-acidity-specific 3,3-dimethylbut-1-ene (33DMB) skeletal isomerization for evaluating medium to strong Brønsted acidity and methylenecyclohexane (MECH) isomerization for relatively week Brønsted acidity, and the dehydration of 2-propanol (2PO) for overall (Lewis and Brønsted) acidity at the catalyst surface, are employed in this study to uncover the effect of water on the surface acidic property of typical solid acid catalysts (Brønsted acidic HZSM-5, Lewis acidic γ-Al2O3, Brønsted and Lewis acidic Nb2O5 and SiO2-Al2O3). The reactions were conducted in a fixed-bed differential flow reactor at 150 or 200 °C at atmospheric pressure by under varying partial pressures of water (H2O). The presence of water produces little effect on the Brønsted acidity of HZSM-5 but results in lowering of the overall Lewis acidity on HZSM-5 and γ-Al2O without generation of Brønsted acidity. On the Nb2O5 and SiO2-Al2O3 catalysts, the presence of water leads to significant enhancement in the Brønsted acidity but decline in the Lewis acidity. And, those ‘newly generated’ Brønsted sites are active over Nb2O5 for catalyzing both isomerization reactions but are only active over SiO2-Al2O3 for catalyzing the isomerization reaction of MECH. Also, water presence on SiO2-Al2O3 could turn a part of the 'originally enough strong' Brønsted acidic sites (on the 'dry' surface), which are responsible for catalyzing the skeletal isomerization of 33DMB, into the relatively weak Brønsted acidic sites capable for catalyzing the isomerization of MECH. Three Nb2O5 samples with varying calcination temperatures are included in this study. The reaction data gained over these samples will be presented to show that the effects of water on the surface acidity of Nb2O5 would be sensitive to the sample calcination temperature. Discussion will be made to demonstrate that the model reaction approaches can be exploited as in situ probes for gaining comprehensive insights into the effects of water presence on the acidic property of solid catalysts, with an emphasis on highlighting the uniqueness of Nb2O5 (niobic acid) for Brønsted acid catalysis in water-containing reaction systems.

Probing by model reactions the effects of water on the surface acidity of some typical solid acids

2019 Fall National Meeting

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