Development of ’Lignin-First’ Approaches for the Valorization of Lignocellulosic Biomass

Currently, valorization of lignocellulosic biomass almost exclusively focuses on the production of pulp, paper, and bioethanol from its holocellulose constituent, while the remaining lignin part that comprises the highest carbon content, is burned and treated as waste. Lignin has a complex structure built up from propylphenolic subunits; therefore, its valorization to value-added products (aromatics, phenolics, biogasoline, etc.) is highly desirable. However, during the pulping processes, the original structure of native lignin changes to technical lignin. Due to this extensive structural modification, involving the cleavage of the β-O-4 moieties and the formation of recalcitrant C-C bonds, its catalytic depolymerization requires harsh reaction conditions. In order to apply mild conditions and to gain fewer and uniform products, a new strategy has emerged in the past few years, named ‘lignin-first’ or ‘reductive catalytic fractionation’ (RCF). This signifies lignin disassembly prior to carbohydrate valorization. The aim of the present work is to follow historically, year-by-year, the development of ‘lignin-first’ approach. A compact summary of reached achievements, future perspectives and remaining challenges is also given at the end of the review

Catalytic Depolymerization of Lignin and Woody Biomass in Supercritical Ethanol:Influence of Reaction Temperature and Feedstock

The one-step ethanolysis approach to upgrade lignin to monomeric aromatics using a CuMgAl mixed oxide catalyst is studied in detail. The influence of reaction temperature (200-420 °C) on the product distribution is investigated. At low temperature (200-250 °C), recondensation is dominant, while char-forming reactions become significant at high reaction temperature (&gt;380 °C). At preferred intermediate temperatures (300-340 °C), char-forming reactions are effectively suppressed by alkylation and Guerbet and esterification reactions. This shifts the reaction toward depolymerization, explaining high monomeric aromatics yield. Carbon-14 dating analysis of the lignin residue revealed that a substantial amount of the carbon in the lignin residue originates from reactions of lignin with ethanol. Recycling tests show that the activity of the regenerated catalyst was strongly decreased due to a loss of basic sites due to hydrolysis of the MgO function and a loss of surface area due to spinel oxide formation of the Cu and Al components. The utility of this one-step approach for upgrading woody biomass was also demonstrated. An important observation is that conversion of the native lignin contained in the lignocellulosic matrix is much easier than the conversion of technical lignin.</p

Preparative Aspects of Supported Ni2P Catalysts for Reductive Upgrading of Technical Lignin to Aromatics

Supported Ni2P was evaluated as a hydrodeoxygenation (HDO) catalyst in the reductive upgrading of a soda lignin in supercritical ethanol by a hydrotalcite-derived mixed Cu-Mg-Al oxide (CuMgAlOx) catalyst. Various Ni2P catalysts were prepared by different approaches on silica, γ-alumina and a siliceous amorphous silica-alumina (ASA) supports. Calcined NiO/SiO2 precursors were impregnated with phosphate, phosphite and hypophosphite followed by reduction. With γ-alumina, the desired Ni2P could not be obtained, presumably due to the reaction of the P-source with alumina. NiO on ASA could be converted to Ni2P by addition of phosphite, preferably at a P/Ni ratio of 1. Low P/Ni ratio avoids blockage of the pores by P-oxide species remaining after reduction. By further comparison to a sol–gel prepared NiO/SiO2 and co-impregnated silica, it was established that the most active Ni2P catalyst was obtained by impregnation of NiO/SiO2 with phosphate at P/Ni = 1 and reduction at 620 °C. In combination with CuMgAlOx, more than half of soda lignin can be converted to aromatics monomers with a relatively high degree of deoxygenation and limited degree of ring hydrogenation. The co-catalyst system is more active than the separate catalysts

Transition metal (Ti, Mo, Nb, W) nitride catalysts for lignin depolymerisation

Metal nitrides are promising catalysts for depolymerisation of lignin in supercritical ethanol; cheap and abundant titanium nitride affords an aromatic monomer yield of 19 wt% from soda lignin. The reaction mechanism is discussed on the basis of the products and a guaiacol model compound study

Recent Advances in Methane Pyrolysis: Turquoise Hydrogen with Solid Carbon Production

Beside steam reforming, methane pyrolysis is an alternative method for hydrogen production. ‘Turquoise’ hydrogen with solid carbon is formed in the pyrolysis process, contrary to ‘grey’ or ‘blue’ hydrogen via steam methane reforming, where waste carbon dioxide is produced. Thermal pyrolysis is conducted at higher temperatures, but catalytic decomposition of methane (CDM) is a promising route for sustainable hydrogen production. CDM is generally carried out over four types of catalyst: nickel, carbon, noble metal and iron. The applied reactors can be fixed bed, fluidized bed, plasma bed or molten-metal reactors. Two main advantages of CDM are that (i) carbon-oxide free hydrogen, ideal for fuel cell applications, is formed and (ii) the by-product can be tailored into carbon with advanced morphology (e.g., nanofibers, nanotubes). The aim of this review is to reveal the very recent research advances of the last two years achieved in the field of this promising prospective technology

Methane pyrolysis on NiMo/MgO catalysts: The significance of equimolar NiMo alloy resisting nanosize segregation during the reaction

As a perspective catalyst composition for methane non-oxidative decomposition/methane pyrolysis (CH4⇌C+2 H2) yielding clean hydrogen and only solid carbon, the combination of nickel and molybdenum on MgO support was investigated. With deliberately low Ni content and strong metal-support interaction, 7%Ni4% Mo/MgO and 7%Ni12%Mo/MgO catalysts and the monometallic references were prepared. Structural analysis was performed using TPR, XRD, TEM, XPS and Raman spectroscopy in reduced state and after methane decomposition test. Catalytic performance was investigated i) in a highly diluted CH4 flow in a fixed bed reactor under temperature ramp and ii) in 50% CH4/Ar using a horizontal reactor at 800 ◦C. Synergetic interaction of Mo and Ni was observed under both conditions. The results revealed that the deactivation was coupled with alloy segregation for the low Mo loading, while the more stable, non-segregating Mo/Ni~1 composition of the individual metal particles in 7%Ni12%Mo/MgO sample resulted in good activity and high carbon nanotube yield

Adsorption and diffusion of selenite on Boda Claystone Formation

This study provides adsorption and diffusion data of selenite on Boda Claystone Formation (BCF) which is a potential host rock of a deep geological disposal of high-level radioactive waste. The experiments were per- formed on two diverse core samples: one albitic claystone sample characteristic for the entire BCF and one pyrite containing sample sparsely occurring in BCF. The experiments were carried out under atmospheric conditions. Batch experiments were carried out to study the kinetics of adsorption at a high initial concentration (1.2 × 10 3 M), the adsorption isotherms and reversibility were investigated in the 10 10–10 3 M concentration range. Adsorption onto petrographic thin sections was done to study the elemental distribution on the microscale and the oxidation state of selenium. The maximum of the distribution coefficient was found as Kd ≈ 200 L/kg and a decrease was experienced around 10 6–10 7 M equilibrium concentration, which showed similarities to other argillaceous rocks. Isotopic exchange experiments revealed reversibility of selenite adsorption. Diffusion was studied with through-diffusion and in-diffusion experiments. Using X-ray fluorescence, despite a low initial concentration of 2.3 × 10 5 M in the in-diffusion experiment, a meaningful diffusion profile of selenium could be obtained, from which the selenite apparent diffusion coefficient Dapp selenite = (1.5–4.3) × 10 14 m2/s and the selenite rock capacity factor αselenite = 1.4–2.2 were determined. As selenium species are redox sensitive the oxidation state of adsorbed species was studied with X-ray absorption near edge structure spectroscopy on Se–K edge. Adsorbed selenium remained in +IV oxidation state, however reduction was experienced on the pyritic sample