Effects of steam on toluene hydrogenation over a Ni catalyst

The catalytic toluene hydrogenation over Ni/SiO₂ was carried out using H₂ or a H₂/H₂O mixture. The toluene conversion and MCH selectivity were evaluated under partial steam pressures 0−10 kPa, at H₂/toluene molar ratios 1–5, and at temperatures 393−453 K. In the absence of steam, toluene conversion increased with increasing H₂ partial pressure, and the MCH selectivity decreased with increasing temperature. In the presence of steam, the toluene conversion showed near-constant activities over the entire range of steam partial pressures, and the MCH selectivity increased with increasing steam partial pressure. This result indicated that the addition of steam inhibited byproduct formation. To clarify the effects of steam on the side-reaction, the byproducts composition was analyzed using a gas chromatograph. The results showed that the addition of steam inhibited demethylation, the ring-opening reaction of MCH, the disproportionation of toluene, and isomerization to convert six-membered ring compounds into five-membered ring compounds.