Lignocellulosic biomasses have a tremendous potential to cover the future demand of bio-based chemicals and materials, breaking down our historical dependence on petroleum resources. The development ...of green chemical technologies, together with the appropriate eco-politics, can make a decisive contribution to a cheap and effective conversion of lignocellulosic feedstocks into sustainable and renewable chemical building blocks. In this regard, the use of an indirect H-source for reducing the oxygen content in lignocellulosic biomasses and in their derived platform molecules is receiving increasing attention. In this contribution we highlight recent advances in the transfer hydrogenolysis of cellulose, hemicellulose, lignin, and of their derived model molecules promoted by heterogeneous catalysts for the sustainable production of biofuels and biochemicals.
This review provides an overview of heterogeneous bimetallic Pd-Fe catalysts in the C–C and C–O cleavage of platform molecules such as C2–C6 polyols, furfural, phenol derivatives and aromatic ethers ...that are all easily obtainable from renewable cellulose, hemicellulose and lignin (the major components of lignocellulosic biomasses). The interaction between palladium and iron affords bimetallic Pd-Fe sites (ensemble or alloy) that were found to be very active in several sustainable reactions including hydrogenolysis, catalytic transfer hydrogenolysis (CTH) and aqueous phase reforming (APR) that will be highlighted. This contribution concentrates also on the different synthetic strategies (incipient wetness impregnation, deposition-precipitaion, co-precipitaion) adopted for the preparation of heterogeneous Pd-Fe systems as well as on the main characterization techniques used (XRD, TEM, H2-TPR, XPS and EXAFS) in order to elucidate the key factors that influence the unique catalytic performances observed.
The direct conversion of untreated microcrystalline cellulose into C2–C3 alcohols, through a one-pot process promoted by the heterogeneous bimetallic Pd/Fe3O4 catalyst, is presented. The process is ...selfsustainable without the addition of external molecular hydrogen or acid/basic promoters and is mainly selective toward ethanol. At 240 °C, a complete cellulose conversion was reached after 12 h with an ethanol molar selectivity of 51% among liquid products. The synergistic effect played by water (which aids in the chemical pretreatment means of cellulose through the hydrolysis process) and the Pd/Fe3O4 catalyst (which catalyzes the hydrogenolysis reaction driving the pattern of obtained products) is elucidated.
High chemical stability, surface area and redox strength are basic requirements of effective heterogeneous CWAO catalyst design. Display omitted
•The mechanisms of the catalytic wet air oxidation ...(CWAO) of organic pollutants are assessed.•The heterogeneous CWAO proceeds via a surface Langmuir–Hinshelwood reaction path.•Surface oxidation is the rate determining step of the CWAO process.•High chemical stability, surface area and redox strength are basic catalyst requirements.
The pressing need to prevent further damages to environment and deterioration of natural resources urges a global effort to shift from the current money-making industrial policies to a sustainable development pattern. This requires an easy availability of handy and economical technologies for depollution of gas and liquid wastes, “green” industrial manufacturing processes and “zero-emission” energy supply systems. In this context the purification and reuse of industrial wastewaters represents a very critical environmental issue because of the global freshwater shortage, the continuing water resources depletion, and the increasing pollutants release. In this context, the heterogeneous catalytic wet air oxidation (CWAO) is nowadays considered the most promising technology for large-scale application to detoxification of noxious wastewaters, provided that robust, effective and cost-effective catalysts are available. Then, this work provides an overview of mechanistic and kinetic issues of the homogeneous and heterogeneous CWAO of several classes of industrial pollutants, at basis of recent advances on design and development of supported noble metal and oxide catalysts.
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•Mg/Ga oxides catalyze phenol methylation to trimethylphenols.•The fresh catalyst shows excellent yield to 2,4,6-trimethylphenol.•Ga oxide enhances MgO dehydrogenating properties and ...increases catalyst acidity.•Water plays an important role and affects products distribution.
In this paper, we report on a new catalyst type based on Mg/Ga/O for the gas-phase methylation of phenol with the aim of obtaining the synthesis of 2,4,6-trimethylphenol. This catalyst showed an outstanding performance compared to similar catalysts reported in the literature, with high yield to the desired product. Reasons for this excellent performance were related to the high activity in methanol dehydrogenation to formaldehyde, which is the rate-limiting step in the multi-step (but one-pot) process, as well as to the moderate acidic features due to Ga sites, which enhanced the intramolecular rearrangement of O-alkylated compounds. The surprising role for co-fed steam, which greatly enhanced the selectivity to 2,4,6-trimethylphenol, was also investigated, combining reactivity experiments, in-situ FT-IR spectroscopy, and DFT calculations. It was found that the co-adsorption of water significantly affects the chemo-selectivity of the reaction, thus decreasing the formation of O-alkylated compounds in favour of the desired C-alkylated compounds. Reasons for catalyst deactivation were also studied.
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•The hydrogenolysis of C6-C3 polyols can be effectively promoted by the heterogeneous Pd/Fe3O4, catalyst at low H2 pressure.•At 240 °C, sorbitol (C6 polyol) is fully converted into ...lower alcohols with ethanol being the main reaction product in liquid phase.•The gas phase analysis reveals the excellent performance of the Pd/Fe3O4 catalyst in promoting the water-gas-shift reaction.•The high activity shown by the Pd/Fe3O4 catalyst has to be attributed to the strong interaction between palladium and Fe3O4 support.
The hydrogenolysis of sorbitol and various C5-C3 polyols (xylitol; erythritol; 1,2- 1,4- and 2,3-butandiol; 1,2-propandiol; glycerol) have been investigated at low molecular hydrogen pressure (5 bar) by using Pd/Fe3O4, as heterogeneous catalyst and water as the reaction medium. Catalytic experiments show that the carbon chain of polyols is initially shortened through dehydrogenation/decarbonylation and dehydrogenation/retro-aldol mechanisms followed by a series of cascade reactions that include dehydrogenation/decarbonylation and dehydration/hydrogenation processes. At 240 °C, sorbitol is fully converted into lower alcohols with ethanol being the main reaction product in liquid phase.
The hydrogenolysis and the aqueous phase reforming of glycerol have been investigated under mild reaction conditions, using water as the reaction medium and Pd/Fe as the catalyst. The experiments, in ...the presence of added H 2 or under inert atmosphere, clearly show that the dehydration/hydrogenation route is the key step in the case of C–O bond cleavage (hydrogenolysis) while dehydrogenation is a prerequisite for C–C bond breaking (APR), with the latter favoured at higher reaction temperatures. The temperature dependence of the C–C and C–O bond rupture is discussed by taking into account the bond energies involved in the competitive hydrogenolysis and APR reactions. Finally, the Pd/Fe catalyst was also tested in the hydrogenolysis and APR of ethylene glycol in the same temperature range, with the aim of clarifying the selective cleavage of C–O and C–C bonds in biomass derived C 2 –C 3 polyols.
The direct conversion of untreated microcrystalline cellulose into C
-C
alcohols, through a one-pot process promoted by the heterogeneous bimetallic Pd/Fe
O
catalyst, is presented. The process is ...selfsustainable without the addition of external molecular hydrogen or acid/basic promoters and is mainly selective toward ethanol. At 240 °C, a complete cellulose conversion was reached after 12 h with an ethanol molar selectivity of 51% among liquid products. The synergistic effect played by water (which aids in the chemical pretreatment means of cellulose through the hydrolysis process) and the Pd/Fe
O
catalyst (which catalyzes the hydrogenolysis reaction driving the pattern of obtained products) is elucidated.
The hydrogenolysis and the aqueous phase reforming of glycerol have been investigated under mild reaction conditions, using water as the reaction medium and Pd/Fe as the catalyst. The experiments, in ...the presence of added H
2
or under inert atmosphere, clearly show that the dehydration/hydrogenation route is the key step in the case of C-O bond cleavage (hydrogenolysis) while dehydrogenation is a prerequisite for C-C bond breaking (APR), with the latter favoured at higher reaction temperatures. The temperature dependence of the C-C and C-O bond rupture is discussed by taking into account the bond energies involved in the competitive hydrogenolysis and APR reactions. Finally, the Pd/Fe catalyst was also tested in the hydrogenolysis and APR of ethylene glycol in the same temperature range, with the aim of clarifying the selective cleavage of C-O and C-C bonds in biomass derived C
2
-C
3
polyols.
The hydrogenolysis and the aqueous phase reforming of glycerol have been investigated using Pd/Fe as the catalyst. At 180 °C, the C-O bond is preferentially cleaved while C-C bond breaking is favoured at higher reaction temperatures.
The hydrogenolysis and the aqueous phase reforming of glycerol have been investigated under mild reaction conditions, using water as the reaction medium and Pd/Fe as the catalyst. The experiments, in ...the presence of added H sub(2) or under inert atmosphere, clearly show that the dehydration/hydrogenation route is the key step in the case of C-O bond cleavage (hydrogenolysis) while dehydrogenation is a prerequisite for C-C bond breaking (APR), with the latter favoured at higher reaction temperatures. The temperature dependence of the C-C and C-O bond rupture is discussed by taking into account the bond energies involved in the competitive hydrogenolysis and APR reactions. Finally, the Pd/Fe catalyst was also tested in the hydrogenolysis and APR of ethylene glycol in the same temperature range, with the aim of clarifying the selective cleavage of C-O and C-C bonds in biomass derived C sub(2)-C sub(3) polyols.
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