Catalytic conversion of biomass

There is a global need to deal with the growing chemical and energy demands without compromising the environment. The conversion of different biomass-derivate feedstock still needs to improve to accomplish a biorefinery able to compete with the conventional refineries. The work presented in this thesis investigates two different catalytic approaches to selectively cleave the C-O bond over the C-C bond and vice versa for bio-derived feedstock molecules. In the industrial biodiesel production, the by-product obtained in large proportion – glycerol –requires to be converted into more valuable products, such as propanediols. Therefore, the design of heterogeneous catalyst for the selective scission of the C-O bond in the presence of the C-C bond of glycerol is one of the objectives of this thesis. Another kind of biomass is lignocellulosic biomass (waste biomass). Depolymerisation of lignin requires the breaking of C-C bond over the C-O bond. In this context, the second objective is to develop a catalytic system that selectively cleaves C-C bond for lignin model compounds, aiming at lignin depolymerisation. The first part of the thesis reports the synthesis, catalytic activity, and characterisation of monometallic Pd, Ru, Pt and bimetallic PdRu, RuPt nanoparticles supported on TiO2 for the hydrogenolysis of glycerol at relatively low temperatures (165 °C) using gaseous H2. All these catalysts were found to be active but showing different products distributions. It was found that the incorporation of a second metal to monometallic Pt and Pd catalysts resulted in a compromise between conversion and selectivity towards C3 products via C-O bond cleavage. Detailed characterisation using XPS, SEM-EDX, STEM, TGA and computational modelling was employed to rationalise the difference in the catalytic properties of monometallic and bimetallic catalysts. The data revealed the important correlation between metal oxidation state, nanostructure and their catalytic properties. Among all the catalysts tested, PdRu over TiO2 catalysts presented good conversion, selectivity and reusability upon glycerol hydrogenolysis. For glycerol hydrogenolysis reaction, the role of support was investigated using the best PdRu bimetallic combination. Several metal oxides and zeolites with diverse framework structure types were employed for the study. The textural properties of these catalysts were analysed (BET surface area and pore size distribution) and their elemental and structural properties were further characterised using XPS, MP-AES, SEM, and TEM. Furthermore, the hydrothermal stability of the zeolite-based catalyst was investigated. Finally, the relative acid site density of the samples was determined by NH3-TPD for all the catalysts. Pyridine DRIFT of the most significant catalysts was also performed. The effect of the catalyst acidity was found to correlate with the bifunctional catalyst activity, showing an optimum value within a volcano plot. In the next part of the thesis, the study of lignin model compounds bearing similar linkages and functionalities, present in native lignin, are used to investigate lignin depolymerisation research. This thesis reports the catalytic bond cleavage of different inter-unit linkages present in lignin model compounds. Ruthenium ion catalysed oxidation, known as RICO reaction, is an effective method to disrupt the most recalcitrant inter-unit linkages in lignin, such as β-5 and 5-5’, at room temperature and atmospheric pressure. Oxidation of simple model compounds and degradation of a polymeric model as β-O-4 polymer and a hexamer model compound, closer to the lignin structure, was accomplished at rapid reaction times. Several techniques, namely, FT-IR, 1H, 13C, HSQC, HMBC and 31P NMR, were used to characterise the materials and monitor the reactions. A detailed description of the methodology employed for the estimation of the potential bond cleavage of the inter-unit linkages is detailed in this thesis. From the data reported, RuO4 could play an important role in the oxidative depolymerisation of technical and native lignin via the opening of aromatic rings to form carboxylic acids and aldehydes. Finally, a summary of all the results and potential ideas for future research in the areas of bimetallic catalysts for glycerol hydrogenolysis and RICO for oxidative depolymerisation of lignin are presented

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

Deactivation studies of bimetallic AuPd nanoparticles supported on MgO during selective aerobic oxidation of alcohols

Here we report the synthesis and characterisation of high surface area MgO prepared via the thermal decomposition of various magnesium precursors (MgCO3, Mg(OH)2 and MgC2O4). Bimetallic gold-palladium nanoalloy particles were supported on these MgO materials and were tested as catalysts for the solvent-free selective aerobic oxidation of benzyl alcohol to benzaldehyde. All these catalysts were found to be active and very selective (>97%) to the desired product (benzaldehyde). However, MgO prepared via the thermal decomposition of magnesium oxalate displayed the highest activity among all the magnesium oxide supports tested. Attempts were made to unravel the reasons for the deactivation of these catalysts using different characterisation techniques namely in situ XRD, XPS, ICP-MS, TEM, and TGA-MS. From the data obtained, it is clear that MgO undergoes phase changes from MgO to Mg(OH)2 and MgCO3 during catalyst preparation as well as during the catalytic reaction. Besides phase changes, strong adsorption of reactants and products on the catalyst surface, during the reaction, were also observed and washing the catalyst with organic solvents did not completely remove them. The phase change and catalyst poisoning were reversed through high temperature heat treatments. However, these processes led to the sintering of the metal nanoparticles. Moreover, substantial leaching of the support material (MgO) was also observed during the reaction. These latter two processes led to the irreversible deactivation of AuPd nanoparticles supported on MgO catalyst during the solvent-free selective aerobic oxidation of alcohols. Among the three different MgO supports studied in this article, an inverse correlation between the catalytic activity and Mg leaching has been observed. This article reports a deeper understanding of the mode of deactivation observed in metal nanoparticles supported on MgO during liquid phase reactions

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 β-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 β-5, 5–5′ and β-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 β-5, 5–5′ and β-O-4 linkages and to identify the products. After 30 min, >90% of the 5–5′ and linkages and >80% of the β-5′ are converted with this methodology. This is the first report on the conversion of the 5–5′ linkage in lignin model hexamer

Review on catalytic cleavage of C-C inter-unit linkages in lignin model compounds: Towards lignin depolymerisation

Lignin depolymerisation has received considerable attention recently due to the pressing need to find sustainable alternatives to fossil fuel feedstock to produce chemicals and fuels. Two types of interunit linkages (C–C and C–O linkages) link several aromatic units in the structure of lignin. Between these two inter-unit linkages, the bond energies of C–C linkages are higher than that of C–O linkages, making them harder to break. However, for an efficient lignin depolymerisation, both types of inter-unit linkages have to be broken. This is more relevant because of the fact that many delignification processes tend to result in the formation of additional C–C inter-unit bonds. Here we review the strategies reported for the cleavage of C–C inter-unit linkages in lignin model compounds and lignin. Although a number of articles are available on the cleavage of C–O inter-unit linkages, reports on the selective cleavage of C–C inter-unit linkages are relatively less. Oxidative cleavage, hydrogenolysis, two-step redox-neutral process, microwave assisted cleavage, biocatalytic and photocatalytic methods have been reported for the breaking of C–C inter-unit linkages in lignin. Here we review all these methods in detail, focused only on the breaking of C–C linkages. The objective of this review is to motivate researchers to design new strategies to break this strong C–C inter-unit bonds to valorise lignins, technical lignins in particular

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