TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The metal salt or alkali is ferric chloride, nickel chloride, zinc chloride, ferrous chloride, magnesium chloride, aluminum chloride, potassium hydroxide, sodium hydroxide and/or calcium hydroxide. The ionic liquid catalyst, in which if there are two kinds of metal salt or alkali, then they are mixed in a molar ratio of 1:0.05-2 to form a mixed solution. Preferred method: either the ionic liquid catalyst is formed by mixing an acidic ionic liquid with alkali composed of potassium hydroxide and calcium hydroxide in a molar ratio of 1:0.05-2, where the pyrolysis product is a liquid-phase low molecular weight substance; or the ionic liquid catalyst is formed by mixing an ionic liquid (having high ability of dissolving the metal salt) with metal salt composed of Fe3+ and/or Ni3+ in a ratio of 1:0.05-3, where the pyrolysis product is gas-phase low molecular weight substance.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The ionic liquid is an acidic ionic liquid or an ionic liquid having higher ability of dissolving the metal salt, where the acidic ionic liquid is sulfonic acid-functional butyl pyridine ionic liquid ((HSO3-bpy)CF3SO3), 1-vinyl-3-sulfobutyl imidazole hydrosulfate ((HSO3-bvim)HSO3), N,N,N-trimethyl-N-sulfobutyl hydrogen sulfate ((HSO3-b-N(CH3)3)HSO3) or 1-(3-sulfopropyl)-3-(3-(3-(3-sulfopropyl)-1H-imidazol-3-ium-1-yl)propyl)-1H-imidazol-3-ium hydrogen sulfate of formula (I) (preferred), and the ionic liquid having higher ability of dissolving the metal salt is 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium bromide, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium chloride, 1-hexyl-3-methylimidazolium chloride, mono(1-methyl-3-(3-(3-methyl-1H-imidazol-3-ium-1-yl)propyl)-1H-imidazol-3-ium) chloride of formula (II), 1-methyl-3-(3-(trimethylammonio)propyl)-1H-imidazol-3-ium chloride of formula (III), 1-methyl-3-(3-(triethylammonio)propyl)-1H-imidazol-3-ium chloride of formula (IV) (all are preferred), 1-butyl-3-methylimidazolium terafluoroborate or 1-methyl-3-(3-(tributylphosphonio)propyl)-1H-imidazol-3-ium chloride of formula (V).n = 2-10.