An object of the present invention is to provide a method for hydrogenating a biodiesel fuel for producing a biodiesel fuel having an excellent oxidation stability and cold flow property. The present invention serves to produce a biodiesel fuel excellent in oxidation stability, by hydrogenating a biodiesel fuel with oxygen present in the reaction system in the presence of a catalyst containing at least one type of noble metal selected from metals of Groups 8 to 10 in the periodic table.

A neutral or cationic mononuclear ruthenium divalent complex represented by formula (1) can actualize exceptional catalytic activity in at least one reaction among a hydrosilylation reaction, hydrogenation reaction, and carbonyl compound reduction reaction.

##STR00001##

(In the formula, R.sup.1-R.sup.6 each independently represent a hydrogen atom or an alkyl group, aryl group, aralkyl group, organooxy group, monoorganoamino group, diorganoamino group, monoorganophosphino group, diorganophosphino group, monoorganosilyl group, diorganosilyl group, triorganosilyl group, or organothio group optionally substituted by X; at least one pair comprising any of R.sup.1-R.sup.3 and any of R.sup.4-R.sup.6 together represents a crosslinkable substituent; X represents a halogen atom, organooxy group, monoorganoamino group, diorganoamino group, or organothio group; L each independently represent a two-electron ligand other than CO and thiourea ligands; two L may bond to each other; and m represents an integer of 3 or 4.)

A neutral or cationic mononuclear ruthenium divalent complex represented by formula (1) can actualize exceptional catalytic activity in at least one reaction among a hydrosilylation reaction, hydrogenation reaction, and carbonyl compound reduction reaction.

##STR00001##

(In the formula, R.sup.1-R.sup.6 each independently represent a hydrogen atom or an alkyl group, aryl group, aralkyl group, organooxy group, monoorganoamino group, diorganoamino group, monoorganophosphino group, diorganophosphino group, monoorganosilyl group, diorganosilyl group, triorganosilyl group, or organothio group optionally substituted by X; at least one pair comprising any of R.sup.1-R.sup.3 and any of R.sup.4-R.sup.6 together represents a crosslinkable substituent; X represents a halogen atom, organooxy group, monoorganoamino group, diorganoamino group, or organothio group; L each independently represent a two-electron ligand other than CO and thiourea ligands; two L may bond to each other; and m represents an integer of 3 or 4.)

An object of the present invention is to provide a method for hydrogenating a biodiesel fuel for producing a biodiesel fuel having an excellent oxidation stability and cold flow property. The present invention serves to produce a biodiesel fuel excellent in oxidation stability, by hydrogenating a biodiesel fuel with oxygen present in the reaction system in the presence of a catalyst containing at least one type of noble metal selected from metals of Groups 8 to 10 in the periodic table.

An object of the present invention is to provide a method for hydrogenating a biodiesel fuel for producing a biodiesel fuel having an excellent oxidation stability and cold flow property. The present invention serves to produce a biodiesel fuel excellent in oxidation stability, by hydrogenating a biodiesel fuel with oxygen present in the reaction system in the presence of a catalyst containing at least one type of noble metal selected from metals of Groups 8 to 10 in the periodic table.