Improved hydrogen production via thermophilic fermentation of corn stover by microwave-assisted acid pretreatment

A microwave-assisted acid pretreatment (MAP) strategy has been developed to enhance hydrogen production via thermophilic fermentation of corn stover. Pretreatment of corn stover by combining microwave irradiation and acidification resulted in the increased release of soluble substances and made the corn stover more accessible to microorganisms when compared to thermal acid pretreatment (TAP). MAP showed obvious advantages in short duration and high efficiency of lignocellulosic hydrolysis. Analysis of the particle size and specific surface area of corn stover as well as observation of its cellular microstructure were used to elucidate the enhancement mechanism of the hydrolysis process by microwave assistance. The cumulative hydrogen volume reached 182.2 ml when corn stover was pretreated by MAP with 0.3 N H₂SO₄ for 45 min, and the corresponding hydrogen yield reached 1.53 mol H₂/mol-glucose equivalents converted to organic end products. The present work demonstrates that MAP has potential to enhance the bioconversion efficiency of lignocellulosic waste to renewable biofuel.     

Hydrothermal pretreatment of switchgrass and corn stover for production of ethanol and carbon microspheres

Pretreatment of biomass is viewed as a critical step to make the cellulose accessible to enzymes and for an adequate yield of fermentable sugars in ethanol production. Recently, hydrothermal pretreatment methods have attracted a great deal of attention because it uses water which is a inherently present in green biomass, non-toxic, environmentally benign, and inexpensive medium. Hydrothermal pretreatment of switchgrass and corn stover was conducted in a flow through reactor to enhance and optimize the enzymatic digestibility. More than 80% of glucan digestibility was achieved by pretreatment at 190 °C. Addition of a small amount of K₂CO₃ (0.45-0.9 wt.%) can enhance the pretreatment and allow use of lower temperatures. Switchgrass pretreated at 190 °C only with water had higher internal surface area than that pretreated in the presence of K₂CO₃, but both the substrates showed similar glucan digestibility. In comparison to switchgrass, corn stover required milder pretreatment conditions. The liquid hydrolyzate generated during pretreatment was converted into carbon microspheres by hydrothermal carbonization, providing a value-added byproduct. The carbonization process was further examined by GC-MS analysis to understand the mechanism of microsphere formation.     

Co-digestion of swine manure and corn stover for bioenergy production in MixAlco™ consolidated bioprocessing

MixAlco™ consolidated bioprocessing (CBP) employs a mixed culture of terrestrial microorganisms to anaerobically ferment waste streams (e.g., animal manure, agriculture residues) into mixed carboxylate salts that can be further chemically converted to commodity chemicals (e.g., acetic acid, acetone) and liquid transportation fuels (e.g., ethanol, mixed alcohols, bio-gasoline). For countercurrent fermentations of 60% swine manure/40% lime-treated corn stover at 55 °C, the highest acid productivity [1.8 g/(L·d)] and highest conversion (73%) in this study occurred at an acid concentration of 25.1 and 17.0 g/L, respectively. The continuum particle distribution model (CPDM) predicted the experimental total acid concentrations and conversions around 11.1% and 17.2%, respectively. The CPDM prediction "map" for MixAlco™ CBP indicates that both high conversions (>79%) and high total acid concentrations (>35 g/L) are possible at industrial scale. The present study shows continuous co-digestion of corn stove and swine manure in the MixAlco™ process has the potential to produce annually 9.3 billion gallons of alcohol fuels (e.g., ethanol and mixed alcohols) in the United States.     

Klebsiella pneumoniae发酵稻草纸浆水解液生产2,3-丁二醇工艺的研究

对Klebsiella pneumoniae发酵稻草纸浆水解液生产2,3-丁二醇的工艺进行了初步的实验研究。考察了温度、时间、底物浓度、pH等不同因素对稻草纸浆酶水解和2,3-丁二醇发酵的影响。结果表明,在纸浆的酶用量为135IU/g、底物浓度为20g/L、50℃、pH4.8的条件下反应20h,还原糖得率最高为68.15%;2,3-丁二醇的最佳发酵条件为pH6.0、葡萄糖初始浓度100g/L、30℃、接种量15%、150r/min、反应72h,2,3-丁二醇的最高转化率为17.92%。     

Enhanced biohydrogen production from corn stover hydrolyzate by pretreatment of two typical seed sludges

Five individual pretreatment methods (heat, ultrasonic, ultraviolet, acid, and base) were performed on two typical seed sludges (river sediments and anaerobic granular sludge) to evaluate their effectiveness on enriching efficient hydrogen (H₂)-producing bacteria and enhancing H₂ production using corn stover hydrolyzate. Results indicated that pretreatment processes caused more remarkable improvements for river sediments than anaerobic granular sludge. Among the five protocols, heat pretreatment reached high H₂ yield for both river sediments (4.17 mmol H₂/g utilized sugar) and anaerobic granular sludge (2.84 mmol H₂/g utilized sugar). Ultraviolet and ultrasonic pretreatments were conditionally effective for river sediments and anaerobic granular sludge, respectively. In most cases, pretreatment processes altered soluble metabolites distribution towards more acetate and less ethanol production. Microbial community analysis indicated that heat and ultrasonic pretreatments can respectively lead to significant and indistinctive change on original microbial community. Besides frequently detected Escherichia spp., Serratia spp., and Klebsiella spp., some species of Clostridium spp. and Bacillus spp. might be efficient H₂ producer responsible for better H₂-producing performances.     

The inhomogeneity of corn stover and its effects on bioconversion

Compared with wood, corn stover (CS) is a low-value raw material. Traditionally, CS is used in whole. To develop a CS high-value utilization process, the inhomogeneity of CS and its effects on bioconversion were investigated. The results showed that the chemical compositions of parts from CS had a remarkable difference. The Standard Deviation of cellulose, hemicellulose, Klason lignin, ash of CS parts (leaf, shell, core, node) were 3.09, 9.29, 3.32 and 4.58, respectively. The percents of fiber cell, parenchyma cell, epidermis cell and vessel cell were 30%, 30%, 10%, 30% (in leaf), 50%, 20%, 25%, 5% (in shell), 30%, 60%, 10% and 0% (in core), respectively. The inhomogeneity of CS chemical compositions and cell types affects its enzymatic hydrolysis and fermentation performances. The cellulose enzymatic hydrolysis ratio of corn core at 48 h was 130% higher than the leaf. After 7 d solid state fermentation, the filter paper activity of shell fiber, leaf fiber, core fiber, shell mixed cells, leaf mixed cells and core mixed cells were 40.6, 62.9, 64.1, 67.3, 194.2 and 154.0 IU g⁻¹ dry medium, respectively. The differences proved that the whole utilization process was unsatisfactory and suggested the potential of CS fractionation. Based the results, a pilot scale CS fractionation process (CS- Steam explosion-Water washing-Mechanical fiber fractionation-fiber cell and miscellaneous cells) was tested and divided corn stover into fiber cell and miscellaneous cells in the ratio of 1:1 approximately. The study showed the essentiality of CS fractionation and feasibility of fractionation by a simple method.     

Composite of g-C3N4 and poly(3-hexylthiophene) prepared by polymerizing thiophene-3-acetic acid treated g-C3N4 and 3-hexylthiophene for enhanced photocatalytic hydrogen production

Composite of g-C₃N₄ and poly(3-hexylthiophene) (P3HT) with enhanced photocatalytic H₂ production activity was prepared by polymerizing 3-hexylthiophene and g-C₃N₄, which was treated with thiophene-3-acetic acid (T3A). The morphology, chemical structure, and light absorption properties of samples were characterized by SEM, TEM, BET, XRD, FT-IR, XPS, UV–visible diffuse reflectance spectra (UV–vis). The migration and separation efficiency of charge carriers were characterized by photoluminescence (PL) emission spectra, Time resolved photoluminescence spectra, transient photocurrent responses, and electrochemical impedance spectroscopy (EIS). The photocatalytic activity of the catalysts were tested as the H₂ evolution rate from water under visible light irradiation in the presence of triethanolamine as sacrifice agent. The results indicated that g-C₃N₄-P3HT composite shows significant enhanced migration and separation efficiency of charge carriers, and photocatalytic H₂ production activity from water. The intrinsic nature causing the significance enhanced photocatalytic performance was discussed. Our findings here may provide a new strategy to design composite photocatalyst with high photocatalytic activity.     

Effects of different pretreatment methods on fermentation types and dominant bacteria for hydrogen production

In order to enrich hydrogen producing bacteria and to establish high-efficient communities of the mixed microbial cultures, inoculum needs to be pretreated before the cultivation. Four pretreatment methods including heat-shock pretreatment, acid pretreatment, alkaline pretreatment and repeated-aeration pretreatment were performed on the seed sludge which was collected from a secondary settling tank of a municipal wastewater treatment plant. In contrast to the control test without any pretreatment, the heat-shock pretreatment, acid pretreatment and repeated-aeration pretreatment completely suppressed the methanogenic activity of the seed sludge, but the alkaline pretreatment did not. Employing different pretreatment methods resulted in the change in fermentation types as butyric-acid type fermentation was achieved by the heat-shock and alkaline pretreatments, mixed-acid type fermentation was achieved by acid pretreatment and the control, and ethanol-type fermentation was observed by repeated-aeration pretreatment. Denaturing gradient gel electrophoresis (DGGE) profiles revealed that pretreatment method substantially affected the species composition of microbial communities. The highest hydrogen yield of 1.96 mol/mol-glucose was observed with the repeated-aeration pretreatment method, while the lowest was obtained as the seed sludge was acidified. It is concluded that the pretreatment methods led to the difference in the initial microbial communities which might be directly responsible for different fermentation types and hydrogen yields.     

Pd electrodeposition on a novel substrate of reduced graphene oxide/ poly(melem-formaldehyde) nanocomposite as an active and stable catalyst for ethanol electrooxidation in alkaline media

Palladium nanoparticles (Pd NPs) were successfully electrodeposited on a reduced graphene oxide/poly(melem-formaldehyde) nanocomposite (rGO/PMF) NC as a catalyst for ethanol electrooxidation in alkaline media; melem was used as a nitrogen-rich source in the substrate structure for the first time. The specific surface area and average pore diameter of (rGO/PMF) NC were 481.61 m² gr^?1 and 10.23 nm, respectively. High nitrogen doping and structural defects improved the dispersion and anchoring of Pd NPs on (rGO/PMF) NC. The onset potential (E_onset) of Pd/(rGO/PMF) NC was shifted negatively to 110 mV, in comparison to Pd/rGO. Also, the current density and electrochemical active surface area (EASA) of Pd/(rGO/PMF) NC were enhanced to 44 mA cm^?2 and 67.58 m² gr^?1, respectively, as compared to Pd/rGO. Furthermore, the stability of Pd/(rGO/PMF)NC was indicated against ethanol oxidation intermediates during 7000 s. This work also produced a superior graphene-based material for direct ethanol fuel cell anode catalysts applications.     

Promotional effect of alkaline earth over Ni-La2O3 catalyst for CO2 reforming of CH4: Role of surface oxygen species on H2 production and carbon suppression

Alkaline earth elements (Mg, Ca and Sr) on Ni-La₂O₃ catalyst have been investigated as promoters for syngas production from dry CO₂ reforming of methane (DRM). The catalysis results of DRM performance at 600 °C show that the Sr-doped Ni-La₂O₃ catalyst not only yields the highest CH₄ and CO₂ conversions (∼78% and ∼60%) and highest H₂ production (∼42% by vol.) but also has the lowest carbon deposition over the catalyst surface. The XPS, O₂-TPD, H₂-TPR and FTIR results show that the excellent performance over the Sr-doped Ni-La₂O₃ catalyst is attributed to the presence of a high amount of lattice oxygen surface species which promotes C-H activation in DRM reaction, resulting in high H₂ production. Moreover, these surface oxygen species on the Ni-SDL catalyst can adsorb CO₂ molecules to form bidentate carbonate species, which can then react with the surface carbon species formed during DRM, resulting in higher CO₂ conversion and lower carbon formation.