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Simultaneous harvesting and cell disruption of microalgae using ozone bubbles: optimization and characterization study for biodiesel production
Wan N. A. Kadir, Man K. Lam, Yoshimitsu Uemura, Jun W. Lim, Peck L. Kiew, Steven Lim, Siti S. Rosli, Chung Y. Wong, Pau L. Show, Keat T. Lee
Front. Chem. Sci. Eng..  https://doi.org/10.1007/s11705-020-2015-9
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In the present study, ozone was introduced as an alternative approach to harvest and disrupt microalgae cells (Chlorella vulgaris) simultaneously for biodiesel production. At the optimum ozonation conditions (6.14 g·h–1 ozone concentration, 30 min ozonation time, 1 L·min–1 of ozone flowrate at medium pH of 10 and temperature of 30 °C), the sedimentation efficiency of microalgae cells increased significantly from 12.56% to 68.62%. It was observed that the microalgae cells aggregated to form flocs after pre-treated with ozone due to the increment of surface charge from –20 to –6.59 mV. Besides, ozone had successfully disrupted the microalgae cells and resulted in efficient lipid extraction, which was 1.9 times higher than the control sample. The extracted microalgae lipid was mainly consisted of methyl palmitate (C16:0), methyl oleate (C18:1) and methyl linolenate (C18:3), making it suitable for biodiesel production. Finally, utilization of recycled culture media after ozonation pre-treatment showed robust growth of microalgae, in which the biomass yield was maintained in the range of 0.796 to 0.879 g·h–1 for 5 cycles of cultivation.

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Recovery of free volume in PIM-1 membranes through alcohol vapor treatment
Faiz Almansour, Monica Alberto, Rupesh S. Bhavsar, Xiaolei Fan, Peter M. Budd, Patricia Gorgojo
Front. Chem. Sci. Eng..  https://doi.org/10.1007/s11705-020-2001-2
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Physical aging is currently a major obstacle for the commercialization of PIM-1 membranes for gas separation applications. A well-known approach to reversing physical aging effects of PIM-1 membranes at laboratory scale is soaking them in lower alcohols, such as methanol and ethanol. However, this procedure does not seem applicable at industrial level, and other strategies must be investigated. In this work, a regeneration method with alcohol vapors (ethanol or methanol) was developed to recover permeability of aged PIM-1 membranes, in comparison with the conventional soaking-in-liquid approach. The gas permeability and separation performance, before and post the regeneration methods, were assessed using a binary mixture of CO2 and CH4 (1:1, v:v). Our results show that an 8-hour methanol vapor treatment was sufficient to recover the original gas permeability, reaching a CO2 permeability>7000 barrer.

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The prior rules of designing Ti3C2Tx MXene-based gas sensors
Yingying Jian, Danyao Qu, Lihao Guo, Yujin Zhu, Chen Su, Huanran Feng, Guangjian Zhang, Jia Zhang, Weiwei Wu, Ming-Shui Yao
Front. Chem. Sci. Eng..  https://doi.org/10.1007/s11705-020-2013-y
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Working temperature, sensitivity, and selectivity are some of the characteristics of the applied gas sensors. How to design and fabricate an ideal gas sensor working at room temperature is still challenging and attracting lots of interest. Two-dimensional (2D) materials with ultra-thin structure have been demonstrated as a family of ideal candidates to achieve this goal. Among them, Ti3C2Tx MXene, a kind of layered sheet synthesized by selectively etching MAX phases materials, shows remarkable potential to be the sensitive materials solely or in a composite. However, their designing rules are still lacking critical thinking from the viewpoint of the intrinsic property of Ti3C2Tx MXene based materials. In this article, two critical features, i.e., the thickness of the sensitive materials, and the scope of the analytes, are elaborated towards Ti3C2Tx MXene based gas sensors after characterizing the performance of sensing reducing gases (NH3 and CO) and oxidizing gas (NO2). First, the thinner the Ti3C2Tx MXene sensitive layer, the better the sensitivity. Second, the Ti3C2Tx MXene based gas sensor is not suitable for strong and moderate oxidation gas due to its ease of oxidation. These two rules are demonstrated, and could be considered with priority both in the future researches and practical applications.

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A DNA sensor based on upconversion nanoparticles and two-dimensional dichalcogenide materials
Konstantina Alexaki, Davide Giust, Maria-Eleni Kyriazi, Afaf H. El-Sagheer, Tom Brown, Otto L. Muskens, Antonios G. Kanaras
Front. Chem. Sci. Eng..  https://doi.org/10.1007/s11705-020-2023-9
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We demonstrate the fabrication of a new DNA sensor that is based on the optical interactions occurring between oligonucleotide-coated NaYF4: Yb3+; Er3+ upconversion nanoparticles and the two-dimensional dichalcogenide materials, MoS2 and WS2. Monodisperse upconversion nanoparticles were functionalized with single-stranded DNA endowing the nanoparticles with the ability to interact with the surface of the two-dimensional materials via van der Waals interactions leading to subsequent quenching of the upconversion fluorescence. By contrast, in the presence of a complementary oligonucleotide target and the formation of double-stranded DNA, the upconversion nanoparticles could not interact with MoS2 and WS2, thus retaining their inherent fluorescence properties. Utilizing this sensor we were able to detect target oligonucleotides with high sensitivity and specificity whilst reaching a concentration detection limit as low as 5 fmol·L–1, within minutes.

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Efficient photodegradation of phenol assisted by persulfate under visible light irradiation via a nitrogen-doped titanium-carbon composite
Yan Cui, Zequan Zeng, Jianfeng Zheng, Zhanggen Huang, Jieyang Yang
Front. Chem. Sci. Eng..  https://doi.org/10.1007/s11705-020-2012-z
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To realize the utilization of visible light and improve the photocatalytic efficiency of organic pollutant degradation in wastewater, a nitrogen-doped titanium-carbon composite (N-TiO2/AC) prepared by sol-gel methods was applied in the photodegradation of phenol assisted by persulfate under visible light irradiation (named N-TiO2/AC/PS/VIS). The results show that a synergistic effect exists between visible-light photocatalysis and persulfate activation. Compared with TiO2/PS/VIS, the phenol degradation rate was found to be observably improved by 65% in the N-TiO2/AC/PS/VIS system. This significant increase in degradation rate was mainly attributed to the following two factors: 1) The N and C doping can change the crystal structure of TiO2, which extends the TiO2 absorption wavelength range to the visible light region. 2) As an electron acceptor, PS can not only prevent electrons and holes from recombining with each other but can also generate strong oxidizing radicals such as ∙SO4 and ∙OH to accelerate the reaction dynamics. The process of phenol degradation was found to be consistent with the Langmuir pseudo-first-order kinetic model with an apparent rate constant k of 1.73 min–1. The N-TiO2/AC/PS/VIS process was proven to be a facile method for pollutant degradation with high pH adaptability, excellent visible-light utilization and good application prospects.

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Probing the catalytic activity of M-N4xOx embedded graphene for the oxygen reduction reaction by density functional theory
Fan Ge, Qingan Qiao, Xin Chen, You Wu
Front. Chem. Sci. Eng..  https://doi.org/10.1007/s11705-020-2017-7
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In this work, the detailed oxygen reduction reaction (ORR) catalytic performance of M-N4xOx (M= Fe, Co, and Ni; x = 1–4) has been explored via the detailed density functional theory method. The results suggest that the formation energy of M-N4xOx shows a good linear relationship with the number of doped O atoms. The adsorption manner of O2 on M-N4xOx changed from end-on (x = 1 and 2) to side-on (x = 3 and 4), and the adsorption strength gradually increased. Based on the results for binding strength of ORR intermediates and the Gibbs free energy of ORR steps on the studied catalysts, we screened out two highly active ORR catalysts, namely Co-N3O1 and Ni-N2O2, which possess very small overpotentials of 0.27 and 0.32 V, respectively. Such activities are higher than the precious Pt catalyst. Electronic structure analysis reveals one of the reasons for the higher activity of Co-N3O1 and Ni-N2O2 is that they have small energy gaps and moderate highest occupied molecular orbital energy levels. Furthermore, the results of the density of states reveal that the O doping can improve the electronic structure of the original catalyst to tune the adsorption of the ORR intermediates.

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Kinetics and mechanism of nitrobenzene degradation by hydroxyl radicals-based ozonation process enhanced by high gravity technology
Weizhou Jiao, Shengjuan Shao, Peizhen Yang, Kechang Gao, Youzhi Liu
Front. Chem. Sci. Eng..  https://doi.org/10.1007/s11705-020-1998-6
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This study investigated the indirect oxidation of nitrobenzene (NB) by hydroxyl radicals (·OH) in a rotating packed bed (RPB) using competitive kinetics method with p-nitrochlorobenzene as a reference compound. The rate constants of NB with ·OH are calculated to be between (1.465±0.113) × 109 L/(mol·s) and (2.497±0.192) × 109 L/(mol·s). The experimental data are fitted by the modified Arrhenius equation, where the activation energy is 4877.74 J/mol, the order of NB concentration, rotation speed, and initial pH is 0.2425, 0.1400 and 0.0167, respectively. The ozonation process of NB could be enhanced by RPB, which is especially effective for highly concentrated NB-containing wastewater under alkaline conditions. The high gravity technology can accelerate ozone mass transfer and self-decomposition of ozone to produce more ·OH, resulting in an increase in the indirect oxidation rate of NB by ·OH and consequently effective degradation of NB in wastewater.

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Recent progress in the design and fabrication of MXene-based membranes
Kai Qu, Kang Huang, Zhi Xu
Front. Chem. Sci. Eng..  https://doi.org/10.1007/s11705-020-1997-7
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Two-dimensional membranes have attracted significant attention due to their superior characteristics, and their ability to boost both flux and selectivity have led to their reputation as potential next-generation separation membranes. Among them, emerging MXene-based membranes play significant roles in the competitive membrane-separation field. In this mini-review, we systematically discuss the assembly and separation mechanisms of these membranes. Moreover, we highlight strategies based on the crosslinking of MXene nanosheets and the construction of additional nanochannels that further enhance the permeabilities and anti-swelling properties of MXene-based membranes and meet the requirements of practical applications, such as gas-molecule sieving, ion sieving, and other small-molecule sieving. MXene nanosheets can also be used as additives that introduce specific functionalities into hybrid membranes. In addition, extended applications that use MXenes as scaffolds are also discussed.

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Modified iron-molybdate catalysts with various metal oxides by a mechanochemical method: enhanced formaldehyde yield in methanol partial oxidation
Xue Liu, Lingtao Kong, Shengtao Xu, Chaofan Liu, Fengyun Ma
Front. Chem. Sci. Eng..  https://doi.org/10.1007/s11705-020-2008-8
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A mechanochemical method was employed to prepare modified iron molybdate catalysts with various metal salts as precursors. The physicochemical properties of the iron molybdate catalysts were characterized, and their performances in catalyzing the reaction from methanol to formaldehyde (HCHO) were evaluated. Iron molybdate catalysts doped with Co(NO3)2·6H2O and Al(NO3)3·9H2O resulted in high HCHO yields. Compared with a commercial catalyst, the HCHO yields in the reaction with the modified catalyst at an optimal Co/Mo molar ratio reached 97.37%. According to chemical state analysis, the formation of CoO and the efficient decrease in the MoO3 sublimation rate could be important factors enhancing the HCHO yield in reactions catalyzed with iron molybdate doped with different Co/Mo mole ratios.

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Block copolymers as efficient cathode interlayer materials for organic solar cells
Dingqin Hu, Jiehao Fu, Shanshan Chen, Jun Li, Qianguang Yang, Jie Gao, Hua Tang, Zhipeng Kan, Tainan Duan, Shirong Lu, Kuan Sun, Zeyun Xiao
Front. Chem. Sci. Eng..  https://doi.org/10.1007/s11705-020-2010-1
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Emerging needs for the large-scale industrialization of organic solar cells require high performance cathode interlayers to facilitate the charge extraction from organic semiconductors. In addition to improving the efficiency, stability and processability issues are major challenges. Herein, we design block copolymers with well controlled chemical composition and molecular weight for cathode interlayer applications. The block copolymer coated cathodes display high optical transmittance and low work function. Conductivity studies reveal that the block copolymer thin film has abundant conductive channels and excellent longitudinal electron conductivity due to the interpenetrating networks formed by the polymer blocks. Applications of the cathode interlayers in organic solar cells provide higher power conversion efficiency and better stability compared to the most widely-applied ZnO counterparts. Furthermore, no post-treatment is needed which enables excellent processability of the block copolymer based cathode interlayer.

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