Ruthenium ion catalysed C-C bond activation in lignin model compounds - towards lignin depolymerisation.

Lignin is the most abundant renewable feedstock to produce aromatic chemicals, however its depolymerisation involves the breaking of several C-O and C-C inter-unit linkages that connect smaller aromatic units that are present in lignin. Several strategies have been reported for the cleavage of the C-O inter-unit linkages in lignin. However, till today, only a few methodologies have been reported for the effective breaking or the conversion of the recalcitrant C-C inter unit linkages in lignin. Here we report the ruthenium ion catalysed oxidative methodology as an effective system to activate or convert the most recalcitrant inter unit linkages such as ß-5 and 5-5' present in lignin. Initially, we used biphenyl as a model compound to study the effectiveness of the RICO methodology to activate the 5-5' C-C linkage. After 4 h reaction at 22 °C, we achieved a 30% conversion with 75% selectivity towards benzoic acid and phenyl glyoxal as the minor product. To the best of our knowledge this is the first ever oxidative activation of the C-C bond that connects the two phenyl rings in biphenyl. DFT calculation revealed that the RuO4 forms a [3 + 2] adduct with one of the aromatic C-C bonds resulting in the opening of the phenyl ring. Biphenyl conversion could be increased by increasing the amount of oxidant; however, this is accompanied by a reduction in the carbon balance because of the formation of CO2 and other unknown products. We extended this RICO methodology for the oxidative depolymerisation of lignin model hexamer containing ß-5, 5-5' and ß-O-4 linkages. Qualitative and quantitative analyses of the reaction mixture were done using 1H, 13C NMR spectroscopy methods along with GC-MS and Gel Permeation Chromatographic (GPC) methods. Advanced 2D NMR spectroscopic methods such as HSQC, HMBC and 31P NMR spectroscopy after phosphitylation of the mixture were employed to quantitatively analyse the conversion of the ß-5, 5-5' and ß-O-4 linkages and to identify the products. After 30 min, >90% of the 5-5' and linkages and >80% of the ß-5' are converted with this methodology. This is the first report on the conversion of the 5-5' linkage in lignin model hexamer.

Effect of support acidity during selective hydrogenolysis of glycerol over supported palladium-ruthenium catalysts.

We report the role of the acidity of support during the selectivity hydrogenolysis of glycerol over supported bimetallic palladium-ruthenium (PdRu) catalysts. The PdRu nanoparticles were supported on a series of metal oxides and zeolitic supports via the modified impregnation method and tested for the liquid-phase hydrogenolysis of glycerol using gaseous hydrogen. The relative acid site densities of selected catalysts were determined by ammonia temperature-programmed desorption and pyridine desorption experiments. Based on these studies, we report a direct correlation between the catalytic activity (conversion and 1,2 propane diol yield) and two different acid sites (strong acid sites and very strong acid sites). Besides zeolite-supported catalysts, TiO2 supported PdRu nanoparticles exhibit moderate catalytic activity; however, this catalyst shows high selectivity for the desired C-O bond cleavage to produce C3 products over the undesired C-C bond cleavage to produce < C3 products. This article is part of a discussion meeting issue 'Science to enable the circular economy'.