Method of depolymerizing lignin

A method of depolymerizing lignin to molecules containing 1 or 2 aromatic rings, comprising selectively cleaving of the sp3 carbon-oxygen bond of the alkaryl ethers of the β-O-4, α-O-4, β-5, β-1, and β-β type present in lignin, wherein

• a lignin with a level of sulfur below 1.5 wt %, relative to the total weight of the lignin, is reacted, in the presence of a catalyst, with
• a silane compound of formula (I)
• in which
• R1, R2 and R3 represent, independently of one another, a hydrogen atom, a halogen atom, a hydroxyl group, an alkyl group, an alkenyl group, an alkynyl group, an alkoxy group, an aryloxy group, a silylated group, a siloxy group, an aryl group, an amino group, or combinations thereof, said alkyl, alkenyl, alkynyl, alkoxy, silylated, siloxy, aryl, and amino groups optionally being substituted, or
• R3 is as defined above and R1 and R2, taken together with the silicon atom to which they are bound, form a silylated heterocycle, optionally substituted,
• wherein the molecules containing 1 or 2 aromatic rings have an average molar mass by weight below 1500 g/mol for the molecules in silylated form, and an average molar mass by weight less than or equal to 450 g/mol for the non-silyated form, and
• wherein the catalyst is
• an organic catalyst selected from the group consisting of:
• carbocations selected from the group consisting of trityl cation ((C6H5)3C+);
• tropilium (C7H7)+;
• benzyl cation (C6H5CH2+);
• allyl cation (CH3—CH+—CH═CH2);
• methylium (CH3+);
• cyclopropylium (C3H5+);
• cyclopropyl carbocation selected from dimethyl cyclopropyl carbocation and dicyclopropyl carbocation;
• acylium (R1—C═O)+ with R1 selected from methyl, propyl, and benzyl;
• the benzenium cation (C6H5)+;
• and the norbornyl cation (C7H11)+;
• oxoniums selected from the group consisting of (CH3)3O+BF4− and (CH3CH2)3O+BF4−;
• a silylium ion (R1)3Si+, selected from the group consisting of Et3Si+ and Me3Si+;
• disilyl cations having a bridging hydride selected from the group consisting of the formulas shown below
• with the counterion of said silylium ion, of said carbocations and of said disilyl cations being
• a halide selected from the group consisting of F−, Cl−, Br− and I−;
• or
• an anion selected from the group consisting of BF4−, SbF6−, B(C6F5)4−, B(C6H5)4−, and CF3SO3−, PF6;
• an organometallic catalyst selected from the group consisting of:
• the iridium complexes of formula (III)
• in which
• R6 represents an alkyl or aryl group;
• R7 represents a hydrogen atom or an alkyl group;
• X2 represents a —CH2— group or an oxygen atom;
• and
• Y represents a counterion selected from the group consisting of B(C6F5)4 and B(C6H5)4;
• S represents a molecule of solvent, coordinated to the complex, selected from the group consisting of dimethylsulfoxide (DMSO), acetonitrile (CH3CN), and acetone (CH3COCH3);
• and
• the ruthenium complexes of formula (V)
• in which
• R12 represents a hydrogen atom or an alkyl group;
• R13 represents an aryl or an alkyl group, said aryl and alkyl groups optionally being substituted;
• Z represents a —CH2— group, an oxygen atom or a sulfur atom;
• and
• A− represents a counterion selected from the group consisting of B(C6F5)4− and [CHB11H5Cl6]−;
• or
• a Lewis acid type catalyst selected from the group consisting of:
• boron compounds selected from the group consisting of BF3, BF3(Et2O), BCl3, BBr3, triphenyl hydroborane, tricyclohexyl hydroborane, B(C6F5)3, B-methoxy-9-borabicyclo[3.3.1]nonane (B-methoxy-9-BBN), and B-benzyl-9-borabicyclo[3.3.1]nonane (B-benzyl-9-BBN);
• borenium compounds Me-TBD-BBN+, the borenium ferrocene derivatives corresponding to formula
• in which R1 is a phenyl group and R3 is 3,5-dimethylpyridyl;
• aluminum compounds selected from the group consisting of AlCl3, AlBr3, aluminum isopropoxide Al(O-i-Pr)3, aluminum ethanoate (Al(C2H3O2)), Krossing's salt [Ag(CH2Cl2)]{Al[OC(CF3)3]4}, Li{Al[OC(CF3)3]4}, and Et2Al+;
• indium compounds selected from the group consisting of InCl3 and In(OTf)3;
• iron compounds selected from the group consisting of FeCl3 and Fe(OTf)3;
• tin compounds selected from the group consisting of SnCl4 and Sn(OTf)2;
• phosphorus compounds;
• trifluoromethanesulfonate or triflate compounds (CF3SO3−) of transition metals and lanthanides selected from the group consisting of scandium triflate, ytterbium triflate, yttrium triflate, cerium triflate, samarium triflate, and neodymium triflate.