Zelinsky's scientific activity was very versatile: his works on the chemistry of thiophene and the stereochemistry of organic dibasic acids are widely known. In the summer of 1891, Zelinsky participated in an expedition to survey the waters of the Black Sea and the Odessa estuaries on the gunboat Zaporozhets, where he proved for the first time that the hydrogen sulfide contained in the water was of bacterial origin. During the period of life and work in Odessa, Nikolai Zelinsky wrote 40 scientific papers.
A number of his works were also devoted to electrical conductivity in non-aqueous solutions and to the chemistry of amino acids, but his main works were related to the chemistry of hydrocarbons and organic catalysis.
In 1895-1907 he was the first to synthesize a number of cyclopentane and cyclohexane hydrocarbons, which served as standards for studying the chemical composition and the basis for artificial modeling of oil and oil fractions.
In 1910 he discovered the phenomenon of dehydrogenation catalysis, which consists in the exclusively selective action of platinum and palladium on cyclohexane and aromatic hydrocarbons and in the ideal reversibility of hydro- and dehydrogenation reactions only depending on temperature. In 1911 he carried out a smooth dehydrogenation of cyclohexane and its homologues into aromatic hydrocarbons in the presence of platinum and palladium catalysts; he widely used this reaction to determine the content of cyclohexane hydrocarbons in gasoline and kerosene fractions of oil (1920-1930), and also as an industrial method for obtaining aromatic hydrocarbons from oil. These Zelinsky’s studies underlie the modern processes of catalytic reforming of petroleum fractions.
Subsequent research led Zelinsky and his students to the discovery of the reaction of hydrogenolysis of cyclopentane hydrocarbons with their transformation into alkanes in the presence of platinized coal and excess hydrogen in 1934.
In 1915, Zelinsky successfully used oxide catalysts for oil cracking, which led to a decrease in the process temperature and an increase in the yield of aromatic hydrocarbons. In 1918-1919, he developed a method for producing gasoline by solar oil and petroleum cracking in the presence of aluminum chloride and aluminum bromide; the implementation of this method on an industrial scale played an important role in providing gasoline to the Soviet state. Zelinsky improved the reaction of catalytic conversion of acetylene into benzene by suggesting the use of activated carbon as a catalyst. Zelinsky and his students also studied the dehydrogenation of paraffins and olefins in the presence of oxide catalysts.
Being a supporter of the theory of the organic origin of oil, Zelinsky conducted a series of studies to connect its genesis with sapropels, oil shale and other natural and synthetic organic substances.
Zelinsky and his students proved the intermediate formation of methylene radicals in many heterogeneous catalytic reactions: in the decomposition of cyclohexane, in the synthesis of hydrocarbons from carbon monoxide and hydrogen on a cobalt catalyst, in the reactions of hydrocondensation of olefins with carbon monoxide and hydropolymerization of olefins in the presence of small amounts of oxide carbon which were discovered by him.
The works of Zelinsky and his scientific team on the adsorption of gases on activated carbons were important for the countrys defense ability, the creation of a coal gas mask in cooperation with Kumant (1915) and its adoption during the First World War in the Russian and allied armies were significant for the countrys defense ability.