The Role of Biomass in a Future World without Fossil Fuels

Concatenating Microbial, Enzymatic, and Organometallic Catalysis for Integrated Conversion of Renewable Carbon Sources

The chemical industry can now seize the opportunity to improve the sustainability of its processes by replacing fossil carbon sources with renewable alternatives such as CO2, biomass, and plastics, thereby thinking ahead and having a look into the future. For their conversion to intermediate and final products, different types of catalysts-microbial, enzymatic, and organometallic-can be applied. The first part of this review shows how these catalysts can work separately in parallel, each route with unique requirements and advantages. While the different types of catalysts are often seen as competitive approaches, an increasing number of examples highlight, how combinations and concatenations of catalysts of the complete spectrum can open new roads to new products. Therefore, the second part focuses on the different catalysts either in one-step, one-pot transformations or in reaction cascades. In the former, the reaction conditions must be conflated but purification steps are minimized. In the latter, each catalyst can work under optimal conditions and the "hand-over points" should be chosen according to defined criteria like minimal energy usage during separation procedures. The examples are discussed in the context of the contributions of catalysis to the envisaged (bio)economy.

Toward optimal use of biomass as carbon source for chemical bioproduction

Biomass is widely identified as a promising, renewable replacement for fossil feedstocks in the production of energy, fuels, and chemicals. However, the sustainable supply of biomass is limited. Economic and ecological criteria support prioritization of biomass as a carbon source for organic chemicals; however, utilization for energy currently dominates. Therefore, to optimize the use of available biomass feedstock, biorefining development must focus on high carbon efficiencies and enabling the conversion of all biomass fractions, including lignin and fermentation-derived CO₂. Additionally, novel technological platforms should allow the incorporation of nontraditional, currently underutilized carbon feedstocks (e.g. manure) into biorefining processes. To this end, funneling of waste feedstocks to a single product (e.g. methane) and subsequent conversion to chemicals is a promising approach.

Design new organic material based on triphenylamine (TPA) with D-π-A-π-D structure used as an electron donor for organic solar cells: A DFT approach

Because of the increasing scarcity of fossil fuels and the growing need for energy, it has become necessary to research new renewable energy resources. In this study, five new high-performance materials (TP-FA₁F-TP - TP-FA₅F-TP) of the D-π-A-π-D configuration based on triphenylamine (TPA) were theoretically investigated by applying DFT and TD-DFT methods for future application as heterojunction organic solar cells (BHJ). The influence of the modification of the acceptor (A) of the parent molecule TP-FTzF-TP on the structural, electronic, photovoltaic and optical properties of the TP-FA1F-TP - TP-FA5F-TP organic molecules was investigated in detail. TP-FA₁F-TP - TP-FA₅F-TP showed E_gap in the interval of 1.44-2.01 eV with λ_abs in the range of 536-774 nm, open-circuit voltage (V_oc) values varied between 0.3 and 0.56 V and power conversion efficiencies (PCE) ranging from (3-6) %. Our results also show that the donor molecules suggested in this research exhibit an improved performance compared to the recently synthesized TP-FTzF-TP, such as a lowest HOMO energy, a smaller E_gap, and a greater absorption spectrum, and can lead to higher performance. Indeed, this theoretical research could lead to the future synthesis of better compounds as active substances used in BHJ.

Citrulline deiminase pathway provides ATP and boosts growth of Clostridium carboxidivorans P7

BACKGROUND

Clostridium carboxidivorans P7 is capable of producing ethanol and butanol from inexpensive and non-food feedstock, such as syngas. Achieving improved ethanol and butanol production in the strain for industrial application depends on the energetics and biomass, especially ATP availability.

RESULTS

This study found that exogenous addition of citrulline promoted accumulation of ATP, increased specific growth rate, and reduced the doubling time of C. carboxidivorans P7. In heterotrophic fermentation experiments, the addition of citrulline increased intracellular ATP by 3.39-fold, significantly enhancing the production of total alcohol (ethanol + butanol) by 20%. Moreover, in the syngas fermentation experiments, the addition of citrulline improved the level of intracellular ATP and the biomass by 80.5% and 31.6%, respectively, resulting in an 18.6% and 60.3% increase in ethanol and the alcohol/acid production ratio, respectively.

CONCLUSIONS

This is the first report that citrulline could promote the growth of C. carboxidivorans P7 and increase the level of intracellular ATP, which is of great significance for the use of C. carboxidivorans P7 to synthesize biofuels.

Heterologous expression of Phanerochaete chrysosporium cellobiose dehydrogenase in Trichoderma reesei

Background

Cellobiose dehydrogenase from Phanerochaete chrysosporium (PcCDH) is a key enzyme in lignocellulose depolymerization, biosensors and biofuel cells. For these applications, it should retain important molecular and catalytic properties when recombinantly expressed. While homologous expression is time-consuming and the prokaryote Escherichia coli is not suitable for expression of the two-domain flavocytochrome, the yeast Pichia pastoris is hyperglycosylating the enzyme. Fungal expression hosts like Aspergillus niger and Trichoderma reesei were successfully used to express CDH from the ascomycete Corynascus thermophilus. This study describes the expression of basidiomycetes PcCDH in T. reesei (PcCDH_Tr) and the detailed comparison of its molecular, catalytic and electrochemical properties in comparison with PcCDH expressed by P. chrysosporium and P. pastoris (PcCDH_Pp).

Results

PcCDH_Tr was recombinantly produced with a yield of 600 U L⁻¹ after 4 days, which is fast compared to the secretion of the enzyme by P. chrysosporium. PcCDH_Tr and PcCDH were purified to homogeneity by two chromatographic steps. Both enzymes were comparatively characterized in terms of molecular and catalytic properties. The pH optima for electron acceptors are identical for PcCDH_Tr and PcCDH. The determined FAD cofactor occupancy of 70% for PcCDH_Tr is higher than for other recombinantly produced CDHs and its catalytic constants are in good accordance with those of PcCDH. Mass spectrometry showed high mannose-type N-glycans on PcCDH, but only single N-acetyl-d-glucosamine additions at the six potential N-glycosylation sites of PcCDH_Tr, which indicates the presence of an endo-N-acetyl-β-d-glucosaminidase in the supernatant.

Conclusions

Heterologous production of PcCDH_Tr is faster and the yield higher than secretion by P. chrysosporium. It also does not need a cellulose-based medium that impedes efficient production and purification of CDH by binding to the polysaccharide. The obtained high uniformity of PcCDH_Tr glycoforms will be very useful to investigate electron transfer characteristics in biosensors and biofuel cells, which are depending on the spatial restrictions inflicted by high-mannose N-glycan trees. The determined catalytic and electrochemical properties of PcCDH_Tr are very similar to those of PcCDH and the FAD cofactor occupancy is good, which advocates T. reesei as expression host for engineered PcCDH for biosensors and biofuel cells.

Side-by-Side Comparison of Clean and Biomass-Derived, Impurity-Containing Syngas as Substrate for Acetogenic Fermentation with Clostridium ljungdahlii

Syngas, the product of biomass gasification, can play an important role in moving towards the production of renewable chemical commodities, by using acetogenic bacteria to ferment those gaseous mixtures. Due to the complex and changing nature of biomass, the composition and the impurities present in the final biomass-derived syngas will vary. Because of this, it is important to assess the impact of these factors on the fermentation outcome, in terms of yields, productivity, and product formation and ratio. In this study, Clostridium ljungdahlii was used in a fed-batch fermentation system to analyze the effect of three different biomass-derived syngases, and to compare them to equivalent, clean syngas mixtures. Additionally, four other clean syngas mixtures were used, and the effects on product ratio, productivity, yield, and growth were documented. All biomass-derived syngases were suitable to be used as substrates, without experiencing any complete inhibitory effects. From the obtained results, it is clear that the type of syngas, biomass-derived or clean, had the greatest impact on product formation ratios, with all biomass-derived syngases producing more ethanol, albeit with lesser total productivity.

Enhanced Direct Dimethyl Ether Synthesis from CO2-Rich Syngas with Cu/ZnO/ZrO2 Catalysts Prepared by Continuous Co-Precipitation

The manufacturing of technical catalysts generally involves a sequence of different process steps, of which co-precipitation is one of the most important. In this study, we investigate how continuous co-precipitation influences the properties of Cu/ZnO/ZrO2 (CZZ) catalysts and their application in the direct synthesis of dimethyl ether (DME) from CO2/CO/H2 feeds. We compare material characteristics investigated by means of XRF, XRD, N2 physisorption, H2-TPR, N2O-RFC, TEM and EDXS as well as the catalytic properties to those of CZZ catalysts prepared by a semi-batch co-precipitation method. Ultra-fast mixing in continuous co-precipitation results in high BET and copper surface areas as well as in improved metal dispersion. DME synthesis performed in combination with a ferrierite-type co-catalyst shows correspondingly improved productivity for CZZ catalysts prepared by the continuous co-precipitation method, using CO2-rich as well as CO-rich syngas feeds. Our continuous co-precipitation approach allows for improved material homogeneity due to faster and more homogeneous solid formation. The so-called “chemical memory” stamped during initial co-precipitation is kept through all process steps and is reflected in the final catalytic properties. Furthermore, our continuous co-precipitation approach may be easily scaled-up to industrial production rates by numbering-up. Hence, we believe that our approach represents a promising contribution to improve catalysts for direct DME synthesis.

The Influence of the Gold Particle Size on the Catalytic Oxidation of 5-(Hydroxymethyl)furfural

For the production of chemicals from biomass, new selective processes are required. The selective oxidation of 5-(Hydroxymethyl)furfural (HMF), a promising platform molecule in fine chemistry, to 2,5-furandicarboxylic acid (FDCA) is considered a promising approach and requires the oxidation of two functional groups. In this study, Au/ZrO2 catalysts with different mean particle sizes were prepared by a chemical reduction method using tetrakis(hydroxymethyl)phosphonium chloride (THPC) and tested in HMF oxidation. The catalyst with the smallest mean particle size (2.1 nm) and the narrowest particle size distribution was highly active in the oxidation of the aldehyde moiety of HMF, but less active in alcohol oxidation. On the other hand, increased activity in FDCA synthesis up to 92% yield was observed over catalysts with a larger mean particle size (2.7 nm), which had a large fraction of small and some larger particles. A decreasing FDCA yield over the catalyst with the largest mean particle size (2.9 nm) indicates that the oxidation of both functional groups require different particle sizes and hint at the presence of an optimal particle size for both oxidation steps. The activity of Au particles seems to be influenced by surface steps and H bonding strength, the latter particularly in aldehyde oxidation. Therefore, the presence of both small and some larger Au particles seem to give catalysts with the highest catalytic activity.

Mechanistic insights into the selective oxidation of 5-(hydroxymethyl)furfural over silver-based catalysts

Silver-catalyzed oxidation of 5-(hydroxymethyl)furfural (HMF) to 5-hydroxymethyl-2-furancarboxylic acid (HFCA) was investigated using in situ X-ray absorption spectroscopy under reaction conditions over Ag/ZrO₂ and Ag/TiO₂ catalysts.

Direct Catalytic Route to Biomass-Derived 2,5-Furandicarboxylic Acid and Its Use as Monomer in a Multicomponent Polymerization

Efficient synthesis of valuable platform chemicals from renewable feedstock is a challenging, yet essential strategy for developing technologies that are both economical and sustainable. In the present study, we investigated the synthesis of 2,5-furandicarboxylic acid (FDCA) in a two-step catalytic process starting from sucrose as largely available biomass feedstock. In the first step, 5-(hydroxymethyl)furfural (HMF) was synthesized by hydrolysis and dehydration of sucrose using sulfuric acid in a continuous reactor in 34% yield. In a second step, the resulting reaction solution was directly oxidized to FDCA without further purification over a Au/ZrO₂ catalyst with 84% yield (87% selectivity, batch process), corresponding to 29% overall yield with respect to sucrose. This two-step process could afford the production of pure FDCA after the respective extraction/crystallization despite the impure intermediate HMF solution. To demonstrate the direct application of the biomass-derived FDCA as monomer, the isolated product was used for Ugi-multicomponent polymerizations, establishing a new application possibility for FDCA. In the future, this efficient two-step process strategy toward FDCA should be extended to further renewable feedstock.

Recent Advances in the Development of 5-Hydroxymethylfurfural Oxidation with Base (Nonprecious)-Metal-Containing Catalysts

5-Hydroxymethylfurfural (HMF) is one of the versatile platform molecules that can be derived from biomass, and a promising starting substrate for producing 2,5-diformylfuran (DFF) and 2,5-furandicarboxylic acid (FDCA). DFF is a platform chemical with applications in pharmaceuticals, macrocyclic ligands, and functional polymeric materials. Importantly, FDCA is being considered as a potential alternative to replace terephthalic acid for producing the bioplastic polyethylene furanoate, instead of polyethylene terephthalate, by blending with ethylene glycol. A significant number of studies have focused on the oxidation of HMF to FDCA with metal-containing heterogeneous catalysts in both aqueous and organic media in the presence of peroxides/air/molecular oxygen as the oxidant. In this regard, articles have recently been published related to HMF oxidation with base (nonprecious)-metal-containing catalysts that exhibit appealing activity towards DFF or FDCA in terms of yield. Thus, this Minireview focuses on recent developments in efficient transformations of HMF to DFF and FDCA with base-metal-containing heterogeneous catalysts in aqueous and organic media. This review further focuses on the direct transformation of glucose/fructose to DFF and/or FDCA with nonprecious-metal-containing catalysts in various solvents. Photocatalytic approaches for HMF oxidation with nonprecious metal- containing catalysts are also briefly discussed.

Selective Hydrodeoxygenation of Alkyl Lactates to Alkyl Propionates with Fe-based Bimetallic Supported Catalysts

Hydrodeoxygenation (HDO) of methyl lactate (ML) to methyl propionate (MP) was performed with various base-metal supported catalysts. A high yield of 77 % MP was obtained with bimetallic Fe-Ni/ZrO₂ in methanol at 220 °C and 50 bar H₂ . A synergistic effect of Ni increased the yield of MP significantly when using Fe-Ni/ZrO₂ instead of Fe/ZrO₂ alone. Moreover, the ZrO₂ support contributed to improve the yield as a phase transition of ZrO₂ from tetragonal to monoclinic occurred after metal doping giving rise to fine dispersion of the Fe and Ni on the ZrO₂ , resulting in a higher catalytic activity of the material. Interestingly, it was observed that Fe-Ni/ZrO₂ also effectively catalyzed methanol reforming to produce H₂ in situ, followed by HDO of ML, yielding 60 % MP at 220 °C with 50 bar N₂ instead of H₂ . Fe-Ni/ZrO₂ also catalyzed HDO of other short-chain alkyl lactates to the corresponding alkyl propionates in high yields around 70 %. No loss of activity of Fe-Ni/ZrO₂ occurred in five consecutive reaction runs demonstrating the high durability of the catalyst system.