Recent Advances in the Synthesis of MXene Quantum Dots.

Quantum dots (QDs) with ultrahigh surface-to-volume ratio, abundant edge active sites, forceful quantum confinement and other remarkable physio-chemical properties, have garnered considerable research interest. MXene QDs, as an emerging member of them, have also attracted wide attention in the last six years, and shown great achievements in many fields. This critical review systematically summarizes the various methods for synthesizing MXene QDs. The characteristics and corresponding applications of various MXene QDs are also presented. The advantages and disadvantages of various synthetic methods, and the limitations of corresponding MXene QDs are compared and highlighted. Finally, the challenges and perspectives of synthesizing MXene QDs are proposed. We hope this review will enlighten researchers to the fabrication of more advancing and promising MXene-based QDs with proprietary properties in diverse applications.

Efficient degradation of hydroquinone by a metabolically engineered Pseudarthrobacter sulfonivorans strain.

Pseudarthrobacter sulfonivorans strain Ar51 can degrade crude oil and multi-substituted benzene compounds efficiently at low temperatures. However, it cannot degrade hydroquinone, which is a key intermediate in the degradation of several other compounds of environmental importance, such as 4-nitrophenol, g-hexachlorocyclohexane, 4-hydroxyacetophenone and 4-aminophenol. Here we co-expressed the two subunits of hydroquinone dioxygenase from Sphingomonas sp. strain TTNP3 with different promoters in the strain Ar51. The strain with 2 hdnO promoters exhibited the strongest hydroquinone catabolic activity. However, in the absence of antibiotic selection this ability to degrade hydroquinone was lost due to plasmid instability. Consequently, we constructed a hisD knockout strain, which was unable to synthesise histidine. By introducing the hisD gene onto the plasmid, the ability to degrade hydroquinone in the absence of antibiotic selection was stabilised. In addition, to make the strain more stable for industrial applications, we knocked out the recA gene and integrated the hydroquinone dioxygenase genes at this chromosomal locus. This strain exhibited the strongest activity in catabolizing hydroquinone, up to 470 mg/L in 16 h without antibiotic selection. In addition, this activity was shown to be stable when the strain has cultured in medium without antibiotic selection after 20 passages.

Influence of Biochar Addition on Nitrogen Transformation during Copyrolysis of Algae and Lignocellulosic Biomass.

Algae are extremely promising sustainable feedstock for fuels and chemicals, while they contain high nitrogen content, which may cause some serious nitrogen emission during algae pyrolysis utilization. In this study, we proposed a feasible method to control the nitrogen emission during algae pyrolysis by introducing lignocellulosic biomass and biochar addition. Nitrogen transformation mechanism was investigated through quantitative analysis of N-species in the pyrolysis products. Results showed that copyrolysis of algae and lignocellulosic biomass greatly increased nitrogen in solid char with large amount of NH3 and HCN releasing (∼20 wt %), while nitrogen in bio-oil decreased largely. With biochar addition, NH3, HCN, and N-containing intermediates (amines/amides and nitriles) reacted with higher active O-species (O-C═O, -OH, and -COOH) in biochar addition, and formed large amounts of amine/amide-N, pyridinic-N, pyrrolic-N, and quaternary-N on the surface of biochar addition, which led to most nitrogen being enriched in char product and biochar addition (over 75 wt %) at the expense of amines/amides, nitriles, and N-containing gas (only 3 wt % NH3 and HCN emission; decrease of 85%). These results demonstrated that biochar addition showed a positive influence on fixation of N-species during thermochemical conversion of algae and could convert nitrogen to valuable N-doped biochar materials.

Transformation of Nitrogen and Evolution of N-Containing Species during Algae Pyrolysis.

Transformation and evolution mechanisms of nitrogen during algae pyrolysis were investigated in depth with exploration of N-containing products under variant temperature. Results indicated nitrogen in algae is mainly in the form of protein-N (∼90%) with some inorganic-N. At 400-600 °C, protein-N in algae cracked first with algae pyrolysis and formed pyridinic-N, pyrrolic-N, and quaternary-N in char. The content of protein-N decreased significantly, while that of pyrrolic-N and quaternary-N increased gradually with temperature increasing. Pyridinic-N and pyrrolic-N formation was due to deamination or dehydrogenation of amino acids; subsequently, some pyridinic-N converted to quaternary-N. Increasing temperature decreased amides content greatly while increased that of nitriles and N-heterocyclic compounds (pyridines, pyrroles, and indoles) in bio-oil. Amides were formed through NH3 reacting with fatty acids, that underwent dehydration to form nitriles. Besides, NH3 and HCN yields increased gradually. NH3 resulted from ammonia-N, labile amino acids and amides decomposition, while HCN came from nitrile decomposition. At 700-800 °C, evolution trend of N-containing products was similar to that at 400-600 °C. While N-heterocyclic compounds in bio-oil mainly came from pyrifinic-N, pyrrolic-N, and quaternary-N decomposition. Moreover, cracking of pyridinic-N and pyrrolic-N produced HCN and NH3. A mechanism of nitrogen transformation during algae pyrolysis is proposed based on amino acids decomposition.

Hydrogen-rich syngas production from biomass gasification using biochar-based nanocatalysts.

Nanocatalysts are beneficial for tar elimination and syngas production during biomass gasification. In this study, novel biochar-based nanocatalysts loaded with Ni/Ca/Fe nanoparticles was prepared by one-step impregnation method for catalytic steam gasification of biomass. Results showed that the metal particles were evenly distributed with the particle size of less than 20 nm. With the introduction of nanoparticles, H2 yield and tar conversion were obviously increased. Ni and Fe particles help to maintain the stability of the carrier microporous structure. Fe loaded biochar showed the best catalytic gasification performance, with 87% tar conversion and 42.46 mmol/g H2 production. The catalytic effect of Fe was also higher than that of Ni and Ca if deducting the influence of carrier consumption. It demonstrated that Fe-loaded biochar was a promising catalyst candidate for hydrogen-rich syngas production from biomass gasification.

Preparation of carbon nanotubes by catalytic pyrolysis of dechlorinated PVC.

Plastic waste is attracting growing interest for its utilization potential as a valuable resource. However, conventional thermochemical methods can hardly achieve high-value utilization of certain plastics, such as polyvinyl chloride (PVC) characterized with high chlorine content. Here, a low-temperature aerobic pretreatment method was introduced to realize high-efficiency dechlorination of PVC, and then the dechlorinated PVC was used to prepare carbon nanotubes (CNTs) by a catalytic pyrolysis. The results demonstrate that oxygen can significantly promote the HCl release in a pretty low-temperature range (260-340 °C). Chlorine was almost completely eliminated at 280 °C under 20 % oxygen concentration. Compared to untreated PVC, using the dechlorinated PVC as raw material, higher carbon deposition was obtained and over 60 % CNTs could be collected from the carbon deposition. This study provides a high-value utilization way for the production of CNTs from waste PVC.

Coeffect of pyrolysis temperature and potassium phosphate impregnation on characteristics, stability, and adsorption mechanism of phosphorus-enriched biochar.

Potassium phosphate (K3PO4)-impregnated bamboo was pyrolyzed at temperatures ranging from 350 to 950 °C to explore the coeffect of pyrolysis temperature and K3PO4 impregnation on biochar's characteristics and adsorption behavior. The degree of aromatization and graphitization in phosphorus-enriched biochars (PRBCs) rose as temperature increased, whereas H/C and O/C ratios, pH value, and O-containing group content decreased. The pre-aging impact of K3PO4 impregnation results in increased stability and adsorption performance of PRBCs. Adsorption mechanism of PRBCs to heavy metal varies from pyrolysis temperature. Micropores dominate medium-temperature PRBCs (prepared at 550 âˆ¼ 750 °C), possessing the highest P-containing group content (116 % that of PRBC-350) and maximal adsorption capacity (greater than289 mg/g). The medium-temperature PRBCs adsorb Cd (II) via the role of O-containing groups, PO43-, and P2O74-, mainly by reactions of organic complexation, precipitation and inorganic complexation, respectively. 550 °C is the optimal pyrolysis temperature for both energy saving and heavy metal adsorption.

Pyrolysis of boron-crosslinked lignin: Influence on lignin softening and product properties.

In this study, ammonium borate was used as an additive to inhibit lignin softening during the pyrolysis process, and the influence on the pyrolysis process and product characteristics were investigated with potential mechanism being explored in depth. Results showed that with boron addition, glassy transition temperature and thermal stability of lignin increased, and the yield of gas and liquid decreased, while the content of CO, CO2 and H2 increased. Simultaneously, liquid oil showed higher content of simple phenols, especially the diphenols which the maximum reached 80% with 3%BN at 650 ℃, while the yield of heavy components (300 âˆ¼ 400 Da) decreased. With regard to B-doped char, oxygenic groups and specific surface area (509 m2/g of 5%BN at 650 ℃) increased greatly. Increasing temperature promoted the transformation of B doping form from BC2O to BCO2.

Effect of support type and crystal form of support in the catalytic gasification of old corrugated containers using Fe-based catalysts.

Catalytic gasification of old corrugated containers with Fe-based catalysts is a promising way to produce renewable H2 along with the utilization of solid waste. In this study, the effect of support type and crystal form of support in Fe-based catalysts on the catalytic gasification of old corrugated containers was systematically investigated. The results show that, the introduction of Fe/γ-Al2O3, Fe/TiO2, Fe/SiO2, and Fe/ZSM5-30 promote H2 production. Among them, Fe/TiO2 has the highest catalytic activity on H2 yield (25.10 mmol/g) related to the formation of Fe2TiO5 solid-melt material. Fe/γ-Al2O3 shows the best H2 selectivity (46.34 %) and good H2 yield (24.19 mmol/g) due to good dispersity of Fe. Further, the order of catalytic effect on H2 selectivity is Fe/amorphous Al2O3 (51.46 %) > Fe/α-Al2O3 (46.98 %) > Fe/γ-Al2O3 (46.34 %). With the increase in cycle index, Fe/amorphous Al2O3 shows the best catalytic effect on H2 yield (25.56 mmol/g) after 11 indexes due to the formation of Al2FeO4. Fe/γ-Al2O3 shows the best stability on H2 selectivity (∼43 %) after 11 indexes.

Biomass hydrothermal conversion under CO2 atmosphere: A way to improve the regulation of hydrothermal products.

In this study, batched hydrothermal experiments on corn stalk were conducted at 240-330 °C under CO2 or inert (N2) atmosphere. The distribution and characteristics of gaseous, solid, and liquid products were analyzed in detail to comprehensively investigate the effects of CO2 on the hydrothermal conversion of biomass, especially on the cellulose and lignin in biomass. The results demonstrate that compared with N2, CO2 slightly increased the liquid and gas yields and significantly improved the control effect of temperature on bio-oil components. Under CO2 atmosphere, bio-oil achieved effective enrichment of ketones and phenols at 240 °C and 300 °C, respectively, and their highest relative contents reached 44.8% and 62.0%, respectively. In addition, the hydrochar obtained under CO2 atmosphere showed higher crystallinity, which is conducive to its subsequent utilization. This study explored the feasibility of introducing CO2 into the biomass hydrothermal process to realize the high-value utilization of biomass waste and the reuse of CO2.

Temperature-dependent magnesium citrate modified formation of MgO nanoparticles biochar composites with efficient phosphate removal.

Nano-MgO biochar composites (nMBCs) have been considered as potential adsorbents for phosphate removal from aqueous solution. It is an effective strategy to improve P removal efficiency that adjustment of the size, distribution and crystallinity of MgO particles embedded into the carbon matrix. Herein, we prepared a highly efficient phosphate adsorbent by co-pyrolysis of lotus seedpod and magnesium citrate and studied its adsorption mechanisms. Results showed that the uniformly dispersed MgO nanoparticle was formed on the surface of nMBCs with the temperature increasing, with the particles size ranging from 3 to 10 nm. Furthermore, high temperature promoted the formation of a large amount of reactive lattice oxygen, which was demonstrated to be the main active adsorption site, thus the phosphate immobilization capacity of nMBCs was greatly improved with the pyrolysis temperature increasing from 450 °C to 750 °C. Besides, some stable CO bonds were formed due to the catalysis of Mg2+, which could bond to HPO42-/H2PO4- by hydrogen bond, enhancing the adsorption performance. The isotherm adsorption experiment showed that MBC-750 achieved an excellent phosphorus adsorption amount of 452.752 mg-P/g. The effectiveness of nMBCs is enhanced and a method for producing an effective nanocomposite adsorbent material for removing phosphate from wastewater is provided.

Machine learning prediction of pyrolytic gas yield and compositions with feature reduction methods: Effects of pyrolysis conditions and biomass characteristics.

This study aimed to utilize machine learning algorithems combined with feature reduction for predicting pyrolytic gas yield and compositions based on pyrolysis conditions and biomass characteristics. To this end, random forest (RF) and support vector machine (SVM) was introduced and compared. The results suggested that six features were adequate to accurately forecast (R2 > 0.85, RMSE < 5.7%) the yield while the compositions only required three. Moreover, the profound information behind the models was extracted. The relative contribution of pyrolysis conditions was higher than that of biomass characteristics for yield (55%), CO2 (73%), and H2 (81%), which was inverse for CO (12%) and CH4 (38%). Furthermore, partial dependence analysis quantified the effects of both reduced features and their interactions exerted on pyrolysis process. This study provided references for pyrolytic gas production and upgrading in a more convenient manner with fewer features and extended the knowledge into the biomass pyrolysis process.

High-value products from ex-situ catalytic pyrolysis of polypropylene waste using iron-based catalysts: the influence of support materials.

Catalytic pyrolysis is considered a promising strategy for the utilisation of plastic waste from the economic and environmental perspectives. As such, the supporting materials play a critical role in the properties of the catalyst. This study clarified this influence on the dispersion of the iron (Fe) within an experimental context. Four different types of typical supports with different physical structures were introduced and explored in a two-stage fixed-bed reactor; these included metallic oxides (Al2O3, TiO2), a non-metallic oxide (SiO2), and molecular sieves (ZSM-5). The results show that the liquid products were converted into carbon deposits and lighter gaseous products, such as hydrogen. The Al2O3-supported catalyst with a relatively moderate specific surface areas and average pore diameter exhibited improved metal distribution with higher catalytic activity. In comparison, the relatively low specific surface areas of TiO2 and small average pore diameters of ZSM-5 had a negative impact on metal distribution and the subsequent catalytic reformation process; this was because of the inadequate reaction during the catalytic process. The Fe/Al2O3 catalyst produced a higher yield of carbon deposits (30.2 wt%), including over 65% high-value carbon nanotubes (CNTs) and hydrogen content (58.7 vol%). Additionally, more dispersive and uniform CNTs were obtained from the Fe/SiO2 catalyst. The Fe/TiO2 catalyst promoted the formation of carbon fibre twisted like fried dough twist. Notably, there was interesting correspondence between the size of the reduced Fe nanoparticles and the product distribution. Within certain limits, the smaller Fe particle size facilitates the catalytic activity. The smaller and better dispersed Fe particles over the support materials were observed to be essential for hydrocarbon cracking and the subsequent formation of carbon deposits. The findings from this study may provide specific guidance for the preparation of different forms of carbon materials.

Comparative pyrolysis behaviors of stalk, wood and shell biomass: Correlation of cellulose crystallinity and reaction kinetics.

In this study, the pyrolysis process of 20 kinds of biomass samples in 3 types (stalk-type, wood-type and shell-type) was investigated with thermogravimetric analyzer, and the correlation of biomass pyrolysis property with biomass chemical structure was put forward. The results showed that the pyrolysis of the 20 kinds of biomass can be classified by types as the pyrolysis of stalk-type biomass had an overlapping decomposition peak of hemicellulose and cellulose at 317 °C. However, the pyrolysis of wood-type and shell-type biomass showed two separated peaks at low temperature and the cellulose peak was higher in wood-type biomass (365 °C) compared to shell-type biomass (348 °C). The different pyrolysis process mentioned above could be due to the positive correlation between cellulose crystallinity and the decomposition temperature of cellulose as well as the activation energy at the decomposition stage of cellulose.

Co-pyrolysis of microalgae with low-density polyethylene (LDPE) for deoxygenation and denitrification.

To upgrade the algae pyrolytic oil, the influence of algae components on co-pyrolysis with LDPE were studied, with Spirulina platensis (SP), Nannochloropsis sp. (NS) and Enteromorpha Prolifera (EP) as typical algae samples, as they are enriched with proteins, lipids and carbohydrate, respectively, especially, the N and O transformation behavior during the co-pyrolysis was studied in depth. During co-pyrolysis, the interaction on products depended on the components of algae. EP and SP were prior to form CO2, rather than CO. For pyrolytic oil, co-pyrolysis effectively inhibited the formation of N- and O-compounds, but promoted the generation of long-chain alcohol and formic/acetic ester. And the obvious decrease of N and O content in co-pyrolytic oil was observed. However, the rich lipids in NS resulted in the improvement of N yield in pyrolytic oil during co-pyrolysis.

Synergetic effect of magnesium citrate and temperature on the product characteristics of waste lotus seedpod pyrolysis.

To understand the synergetic effect of magnesium citrate (MC) and temperature on biomass pyrolysis, co-pyrolysis of lotus seedpod (LS) and MC was carried out in a fixed bed reactor. With the addition of MC, CO2 become the dominate composition in gas (55.83-90.75 vol%). And with temperature increasing, the main components in bio-oil converted from carboxylic acid to phenols and aromatics. Meanwhile, the mesoporous carbon was formed, with the BET specific surface area up to 514.66 m2/g, and pore diameter mainly focused at 3-8 nm. For the catalytic effect, the secondary cracking of pyrolytic volatiles (acetic acid and anhydrosugar) was inhibited, therefore the gas releasing was inhibited below 550 °C. However, at higher temperature, MgO catalysts favored the reduction of acids and deoxygenation via ketonization and aldol condensation reactions. The formed MgO as a template and the catalysis of MgO during co-pyrolysis contributed to the mesoporous structure of solid char.

Synthesis and characterization of magnesium oxide nanoparticle-containing biochar composites for efficient phosphorus removal from aqueous solution.

The effective removal and recovery of phosphorus from aquatic environments are highly important for successful eutrophication control and phosphorus recycling. Herein, we prepared biochar containing MgO nanoparticles (MgO-biochar) by fast pyrolysis of MgCl2-impregnated corn stalks, probed its phosphate adsorption performance. Through the fast pyrolysis, the MgCl2 promoted the formation of micropores and mesoporous, and decomposed into MgO nanoparticles with the size smaller than 100 nm. The adsorption experiments showed that the adsorption property increased with the increase of Mg content, and had a strong correlation with the external surface area. And the phosphate adsorption was well described by the Langmuir-Freundlich model (maximum adsorption capacity was determined as 60.95 mg P/g). Kinetic analysis and characterization analysis of MgO-biochar for different adsorption time indicated that phosphate adsorption onto MgO-biochar was mainly controlled by rapid binding to the external surface (about 75% of the equilibrium adsorption amount), and the uptake rate was limited by the slow diffusion of phosphate into the biochar interior (about 25% of the equilibrium adsorption amount). The results suggested that the synthesized MgO-biochar with enough MgO active site dispersed on a higher external surface can be used as a potential adsorbent for phosphate removal and recovery from aqueous solution.

Bimetallic carbon nanotube encapsulated Fe-Ni catalysts from fast pyrolysis of waste plastics and their oxygen reduction properties.

Carbon-based bimetallic electrocatalysts were obtained by catalytic pyrolysis of waste plastics with Fe-Ni-based catalysts and were used as efficient oxygen reduction reaction (ORR) catalysts in this study. The prepared iron-nickel alloy nanoparticles encapsulated in oxidized carbon nanotubes (FeNi-OCNTs) are solid products with a unique structure. Moreover, the chemical composition and structural features of FeNi-OCNTs were determined. The iron-nickel alloy nanoparticles were wrapped in carbon layers, and the carbon nanotubes had an outer diameter of 20-50 nm and micron-scale lengths. FeNi-OCNT with a Fe/Ni ratio of 1:2 (FeNi-OCNT12) exhibited remarkable electrochemical performance as an ORR catalyst with a positive onset potential of 1.01 V (vs. RHE) and a half-wave potential of 0.87 V (vs. RHE), which were comparable to those of a commercial 20% Pt/C catalyst. Furthermore, FeNi-OCNT12 exhibited promising long-term stability and higher tolerance to methanol than the commercial 20% Pt/C catalyst in an alkaline medium. These properties were attributable to the protective OCNT coating of the iron-nickel alloy nanoparticles.

Production of furfural and levoglucosan from typical agricultural wastes via pyrolysis coupled with hydrothermal conversion: Influence of temperature and raw materials.

The liquid product from biomass direct pyrolysis is usually complex and difficult to effectively utilize. By combining hydrothermal conversion and low-temperature pyrolysis, the hemicellulose and cellulose of biomass can be transformed into value-added furfural and levoglucosan (LG), respectively. The effects of temperature during hydrothermal treatment (160-240 °C) and subsequent pyrolysis (340-400 °C) on the production of furfural and LG were investigated by using three typical agricultural wastes, namely corn stalk, peanut shells, and rice stalk. The maximum furfural yield of 4.2% was achieved upon hydrolysis of peanut shells at 200 °C. The hydrochar produced from peanut shells presented the highest LG yield of 7.3% (based on original biomass weight) for a pyrolysis temperature of 360 °C. Under this optimal condition, the total revenue from various products of the hybrid thermochemical process was estimated at $0.362 per kilogram of peanut shells, whereas furfural and LG account for 90% of the revenue.

Effect of deashing on activation process and lead adsorption capacities of sludge-based biochar.

To explore the effect of inorganic minerals on activation process and lead adsorption of sludge-based biochar, sludge-based biochar was pre-deashed using hydrochloric acid or hydrofluoric acid followed by potassium acetate activation. The results indicate that hydrochloric or hydrofluoric acid deashing can improve the pore parameters of sludge-based biochars and promote subsequent activation effect of potassium acetate. The specific surface area of biochar activated by potassium acetate after hydrochloric acid and hydrofluoric acid pretreatment increased from 583.36 m2/g to 718.70 m2/g and 991.55 m2/g, respectively. The enhancement of pore structure is conducive to enhancing the physical adsorption of lead on sludge-based biochar, while the chemical adsorption is not significantly affected at the same time. Thereby, the biochar and activated biochar pretreated with hydrofluoric acid showed better lead adsorption capacities (16.70 and 49.47 mg/g) than untreated biochar (7.56 and 38.49 mg/g).