TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The metal-based catalyst comprises a metal selected from scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, lanthanum, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, and mercury (preferably iron, palladium, ruthenium, nickel, rhodium, and iridium, especially ruthenium, nickel, and rhodium, particularly ruthenium). The ligand is a phosphine ligand (preferably (9,9-dimethylxanthene-4,5-diyl)bis(diphenylphosphine). The nickel precursor is selected Ni(COD)2, Ni(acac)2, NiCl2, NiBr2, Ni(OAc)2, Ni(OH)2, and/or NiCO3.2Ni(OH)2 (nickel carbonate basic) (preferably Ni(COD)2, and/or Ni(acac)2). The metal precursor comprises nickel and the ligand is a phosphine or carbene ligand. The metal-based catalyst is a pre-formed catalyst. The pre-formed catalyst comprises ruthenium and a phosphine ligand; vanadium and a phenol-imine or bis-phenol-imine ligand; and nickel and a phosphine or carbene ligand.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The carbon-oxygen bond is C2 and O2 of a glycerol beta -arylether unit of lignin of formula (I). The cleaving forms at least one product comprises Ar4OH. The carbon-carbon bond between C1 and C2 and/or C2 and C3 of the glycerol beta -arylether unit of lignin (I) and the carbon-oxygen bond between C2 and O2 of the glycerol beta -arylether unit of lignin (I) are cleaved. The cleaving occurs via tandem dehydrogenation and retro-aldol reactions and/or tandem dehydrogenation and carbon-oxygen bond cleavage reactions. The cleaving is catalyzed by a metal-based catalyst, an organometallic catalyst, a catalyst comprises a hydride and carbonyl ligand, and a catalyst formed from a metal precursor and optionally a ligand under the reaction conditions. The cleaving is carried out at a reaction temperature of 80-250 degrees C in the presence of hydrogen, silane or acid. The method (M1) further involves hydrodeoxygenating the reaction products; cracking the reaction products; and hydrogenating the reaction products. The glycerol beta -arylether unit is oxidized prior to the cleaving step. The depolymerizing (M2) lignin involves cleaving a carbon-oxygen bond or a carbon-carbon bond of a phenylpropanoid unit of lignin in a catalytic disproportionation reaction where the cleaving is catalyzed by a metal-based catalyst comprising vanadium. In the method (M2), the cleaving occurs via a disproportionation-elimination reaction; the metal-based catalyst comprises vanadium is formed from a vanadium precursor and optionally a ligand under the reaction conditions; the cleaving is catalyzed by a pre-formed phenol-imine or bis-phenol-imine vanadium catalyst. The depolymerizing (M3) lignin involves cleaving a carbon-oxygen bond of lignin in a catalytic reduction reaction, by contacting lignin with a catalyst and a hydrogen atom source. In the method (M3), the carbon-oxygen bond comprises a diaryl, alkyl aryl, or benzyl alkyl, or benzyl aryl ether linkage; the cleaving is catalyzed by a metal-based catalyst comprising nickel is formed from a nickel precursor and optionally a ligand under the reaction conditions; the ligand:nickel precursor ratio is 2:1; the ligand is an N-heterocyclic carbene ligand or a phosphine ligand (preferably N-heterocyclic carbene ligand); the cleaving is catalyzed by a pre-formed N-heterocyclic carbene nickel catalyst, in the presence of a hydrogen atom source, and in the presence of a base; and the cleaving is carried out at a reaction temperature of 80-250 degrees C; the cleaving has higher selectivity for aryl-carbon-oxygen bonds over alkyl-carbon oxygen bonds in lignin. The method (M3) further involves hydrodeoxygenating the reaction products, cracking the reaction products, and hydrogenating the reaction products. The method (M4) to cleave a diaryl ether linkage involves contacting a diaryl ether with a nickel catalyst and a hydrogen donor in the presence of a base.Ar3 and Ar4=aryl (optionally substituted);andR and R'=H or bond to a phenylpropanoid unit of lignin.Preferred Components: The metal precursor is selected from (Ru3(CO)12), ((Ru(cymene)Cl2)2), ((PPh3)4RuCl2), (Ru(PPh3MCO)(OTf)2(MeOH)), (RuH2CO(PPh3)3), (Ru(TFA)2(CO)(PPh3)2), (Ru(TFA)(PPh3MCO)H), (RuH(TFA)(CO)(PPh3)3), (Ni(cod)2), and (RhCl(coe)2)2 (preferably (RuH2CO(PPh3)3), (Ru(TFA)2(CO)(PPh3)2), (Ru(TFA)(PPh3)2(CO)H), and (RuH(TFA)(CO)(PPh3)3). The acid is selected from AlX3; TiX4; BX'3; organic acids X''-CO2H; and sulfonic acids X'''-SO3H. The vanadium precursor is selected from (VOSO4.xH2O), (VO(acac)2), and/or (VO(OZ-Pr)3). The ligand is a phenol-imine or bis-phenol-imine ligand. The preformed phenol-imine or bis-phenol-imine vanadium catalyst is 4 catalysts as given in specification e.g. compound of formula (II). The N-heterocyclic carbene ligand is selected from 18 ligand as given in specification e.g. compound of formula (III) and (IV). The hydrogen atom source selected from hydrogen, a silane, diisobutylalumnum hydride (DIBAL), and/or lithium tri-tert-butoxyalumnum hydride (preferably dihydrogen). The silane is triethylsilane or tert-butyldimethylsilane. The base is selected from sodium tert-butoxide, sodium tert-pentoxide, sodium iso-propoxide, lithium tert-butoxide, sodium methoxide, potassium tert-butoxide, cesium fluoride, and/or cesium carbonate (preferably sodium tert-butoxide, sodium tert-pentoxide, sodium iso-propoxide). The carbene ligand is an N-heterocyclic carbene ligand.X=tertiary alkoxide, phenoxide, or halogen;X'=F or Br;X''=CF3, CH3, aryl;andX'''=Me or aryl. EXAMPLE - No suitable example given.