Abstract: To investigate the causes of coloration in 1-methylnaphthalene reagent and its effect on the evaluation of the hydrotreating performance over NiMo/Al₂O₃ catalyst, characterization was conducted using multiple instrumental analytical methods, including inductively coupled plasma mass spectrometry, gas chromatography with sulfur and nitrogen chemiluminescence detectors. The results demonstrated that compared to colorless 1-methylnaphthalene reagent, the brownish-yellow 1-methylnaphthalene reagent contained sulfur-containing compounds such as C₁-benzothiophene, as well as nitrogen-containing compounds including quinoline, isoquinoline, C₁-quinoline, and others. The coexistence of these sulfur- and nitrogen-containing compounds with 1-methylnaphthalene resulted in its brownish-yellow coloration. When evaluating hydrotreating performance of NiMo/Al₂O₃ catalyst using reaction oil formulated with brownish-yellow 1-methylnaphthalene reagent, the conversion rate of 1-methylnaphthalene significantly decreased due to competitive adsorption by sulfur- and nitrogen-containing compounds, particularly the strong inhibitory effect of nitrogen-containing compounds on the hydrogenation of 1-methylnaphthalene. As the reaction temperature increased, the product color gradually transitioned to light brown, whereas the samples derived from reaction oil formulated with colorless 1-methylnaphthalene remained colorless at all tested temperatures. It indicates that the coexistence of sulfur- and nitrogen-containing compounds with 1-methylnaphthalene leads to brownish-yellow coloration as their concentrations increase. Therefore, it is imperative to thoroughly account for color variations in chemical reagents and the uncertainties they might introduce to catalyst performance evaluations.

Key words: 1-methylnaphthalene, coloration causes, sulfur-containing compounds, nitrogen-containing compounds, inhibition, hydrogenation