Reducing friction is one of the best ways to lower energy consumption and make processes more environmentally friendly. Because of the growing interest in green lubricants, we investigated the ...synergetic effects of graphene quantum dots (GQDs) combined with aqueous glycerol to improve the lubrication performance of self-mated steel contacts in reciprocating sliding motion. As a comparison, the lubrication performance of some other two-dimensional (2D) graphitic materials (graphite, graphene oxide, and graphene nanoplatelets) was also studied. The results demonstrate that the GQDs-based nano-lubricant reduces the running-in period and provides super-low friction at a high contact pressure in the boundary-lubrication regime, with 72% and 53% improvements in anti-friction and anti-wear performance compared with aqueous glycerol. On the other hand, the 2D graphitic materials provide super-low friction in the mixed-lubrication regime due to the dominant polishing effect of the hydroxyl groups. The surface Raman mapping indicates that the 2D graphitic materials were severely damaged by the continuous reciprocating motion owing to their inferior crack resistance. In contrast, the superior deformation resistance of the GQDs helps to develop a tough tribofilm. This lubrication mechanism suggests that internal shearing of graphene layers inside the GQDs significantly reduces the wear and friction during the running-in period, while the in-situ formation of strongly adhered tough tribofilm with more surface coverage contributed to the realization of super-low friction under a high contact pressure. This study demonstrated that GQDs-based green nano-lubricants could provide super-low friction at a high contact pressure in the boundary-lubrication regime, which was so far characteristic mainly for conventional environment-polluting lubricants.
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•Synergism of GQDs and aqeous glycerol is investigated between steel/steel contact.•GQDs provide super-low friction in boundary lubrication regime at a high contact pressure.•GQDs promotes the formations of tough tribofilm with more surface coverage.•2D graphitic materials provide super-low friction in mixed lubrication regime.•Boundary lubrication mechanism of GQDs with aqueous glycerol is proposed.
•Extraction of polyphenols from grapefruit peels using green solvents.•Intensification of extraction using High Voltage Electrical discharges (HVED).•Deep eutectic solvents and aqueous glycerol ...enhanced the extraction of polyphenols.•The addition of glycerol has reduced the energy of the pre-treatment by 6 times.•Solubility of naringin was estimated through a modelling of its Hansen parameters.
Deep eutectic solvents (DES) and aqueous glycerol were proposed as green alternatives to conventional solvents for the extraction of polyphenols from grapefruit peels. In order to increase the extraction kinetics and yields of polyphenols, high voltage electrical discharges (HVED) were used as a pre-treatment technology (energy varied between 7.27 and 218 kJ/kg). Results showed that the HVED energy input can be reduced, when the subsequent solid-liquid extraction was performed in 20% (w/v) aqueous glycerol or in DES (lactic acid: glucose) instead of water. The addition of glycerol has reduced the energy of the pre-treatment by 6 times. The same diffusivity of polyphenols (4 × 10−11 m2/s) was obtained in water from HVED pre-treated peels at 218 kJ/kg and in aqueous glycerol from pre-treated peels at 36 kJ/kg. The solubility of naringin, the main flavonoid compound of grapefruit peels in the solvents, was investigated through a theoretical modelling of its Hansen solubility parameters.
Achieving macroscale superlubricity on engineering steel by utilizing aqueous green lubricants has gained growing interest, given its substantial potential to reduce energy consumption and carbon ...footprint. However, maintaining superlubricity under diverse sliding conditions over a prolonged duration is a major obstacle for real-scale applications. Herein, we report that a robust and durable tribofilm enabled by a unique lubrication mechanism based on carboxylated graphene quantum dots (CGQDs) in aqueous glycerol triggers macroscale superlubricity in self-mated steel contacts. A dedicated intermittent test was designed to show the superlubricity's robustness and the ability of the tribofilm to adapt to a variety of relevant sliding conditions. Moreover, the boundary film provides an average coefficient of friction of around 0.007 and up to 69 % wear reduction (compared to the base lubricant), resulting in the maintenance of superlubricity at a real final contact pressure of 123 MPa, which increases the upper limit of the contact pressure compared to current aqueous-lubricated steel contacts. The new superlubricity mechanism was enabled by the chemical adsorption of the CGQDs onto the worn metal surface, coupled with the tribo-induced structural degradation and transformation of the CGQDs into layered graphitic structures that generate an adaptable low-shear interface. This work provides new insights into the role of chemical adsorption and structural transformation of CGQDs in achieving superlubricity and is an important step forward for implementing energy-efficient and green lubrication technologies for industrial applications.
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Abstract Aqueous glycerol is a proposed green extractant for anthocyanins and polyphenols as an alternative to conventional solvents. The aim of this study was to investigate the potential use of ...aqueous glycerol to extract anthocyanins from Syzygium cumini fruit pulp with high yields. The ultrasound-assisted extraction process was also examined to enhance the extraction yield. The application of ultrasound-assisted extraction along with glycerol as a modifier remarkably enhanced the extraction of anthocyanins compared to the conventional extraction. Aqueous glycerol (70%) was screened against conventional solvents (ethanol, methanol and water), where glycerol showed promising outcomes. The optimised ultrasonication time and extraction temperature (25 °C for 5 min) were selected based on our previous study of NADESs. The results showed that glycerol gave the highest amount of anthocyanin content (201.05 mg C3G/100 g fresh weight) compared to conventional solvents for retrieving anthocyanins from S. cumini fruit pulp. Besides the anthocyanin content, the extract yield, phenolic content, and antioxidant activities using DPPH and FRAP were also determined. Glycerol showed a higher phenolic recovery (0.9 mg GAE/g dry sample), resulting in higher antioxidant activity (DPPH activity-73.35% inhibition). Therefore, the application of UAE with aqueous glycerol provides accessibility and enhanced anthocyanin extraction efficiency, thus fulfilling the green and sustainable approach to anthocyanin extraction.
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•Graphene quantum dots (GQDs) nano-additives in aqueous glycerol provide super-low friction (µ ≈ 0.012) between self-mated steel contacts.•The super-low friction is observed under a ...contact pressure as high as 316.5 MPa in the boundary lubrication regime (rarely observed for steel/steel contacts).•A 98% improvement in wear performance is observed compared to pure aqueous glycerol without GQDs nano-additives.•A more realistic lubrication mechanism is proposed based on the friction-induced structural degradation of GQDs.
Reducing friction is a promising strategy to decrease material losses and energy consumption in industrial systems. However, in aqueous-lubricated steel contacts, the contact pressure rarely exceeds 50 MPa during super-low friction due to excessive wear. This work demonstrates that even in steel/steel contacts, by combining graphene quantum dots (GQDs) with aqueous glycerol, it is possible to maintain super-low friction (µ ≈ 0.012) under a contact pressure as high as 316.5 MPa. Moreover, the use of GQDs improved the wear performance by 98 % compared to pure aqueous glycerol due to the formation of a tribochemical film, resulting from the electrostatic adsorption of GQDs on the positively charged sites on the worn surface. In particular, the exfoliation of graphene sheets within GQDs, the shearing of graphene layers inside the GQDs, and the OH–OH repulsion between the asperities shortens the running-in period and consequently reduces the friction and wear. At the same time, the formation of a chemically adsorbed tribofilm containing friction-induced structurally degraded GQDs protects the surface from wear and facilitates the maintenance of super-low friction at high contact pressures by improving the load-carrying capacity. This study suggests that green nano-lubricants based on GQDs have immense potential in sustainable engineering.
Use of aqueous glycerol solutions provides a convenient approach for investigation of a variety of aspects of two phase flow phenomena in porous media concerned with viscosity. These include the ...scaling of spontaneous imbibition and the forced displacement of one phase by another such as in waterflooding. Glycerol/water solutions provide variation of aqueous phase viscosity by over three orders of magnitude. Physical properties, commonly needed in studies of multiphase flow in porous media, are reported for aqueous solutions of glycerol. They include density, surface tension (against air), interfacial tension (against three types of refined oil), contact angle (against n-decane), and viscosity. Experimental results were obtained at six temperatures (20, 30, 40, 50, 65 and 80°C) and five levels of ionic strength of the brine used for adjusting the glycerol/brine mixture. Contact angle measurements against n-decane were made at 20 and 60°C. Glycerol is miscible with water at all investigated salinities and temperature levels. Density and surface and interfacial tensions against refined oils are very close-to-linear with respect to weight fraction of glycerol. Aqueous glycerol solutions on quartz surfaces exhibited zero contact angle (perfect wetting) against both air and refined oil. Increase in viscosity of glycerol–water mixtures with addition of glycerol is highly non-linear. All viscosity data were fitted exceptionally well by an equation originally developed for colloidal dispersions. Literature data on vapor pressure, refractive index and viscosity are also presented.
►Properties of glycerol/brine mixtures relevant to mobility in multi-phase flow. ►Densities, refractive indexes, vapor pressures. ►Surface and interfacial tensions, contact angle. ►Viscosities of aqueous liquids for variation in temperature and salinity. ►Mathematical model of the effect of composition on aqueous phase viscosity.
In this study, the Fe
3
O
4
@PEG nanocomposites with core–shell structure were fabricated by a co-precipitation method and characterized by scanning electron microscopy, transmission electron ...microscopy, Fourier transform infrared spectroscopy, X-ray diffractometer and thermal gravimetric. The physicochemical and tribological properties of the Fe
3
O
4
@PEG nanocomposites in aqueous glycerol were systematically evaluated on steel/steel contact for the first time. The aqueous glycerol solution added with 1.0 wt% Fe
3
O
4
@PEG showed no obvious precipitation after standing for 72 h and had good dispersion stability. Compared with the base aqueous glycerol, the Fe
3
O
4
@PEG nanocomposites exhibited excellent friction-reducing and anti-wear properties at an optimal concentration of 1.0 wt%, with a 41.4% decrease in friction coefficient and a 24.1% reduction in wear volume. Also, the Fe
3
O
4
@PEG nanocomposites in aqueous glycerol exhibited superior lubricating performance than unmodified Fe
3
O
4.
The boundary lubrication films formed on the surface were attributed to be the main factor in reducing friction and wear, and a possible lubrication mechanism was suggested.
A fractionation scheme for lignocellulosic biomass where major constituent biopolymers are recovered in applicable forms is essential for integrated biorefinery. In this study, a sequential two-step ...fractionation of sugarcane bagasse using liquid hot water followed an organic acid-catalyzed aqueous glycerol-based organosolv process was developed. Under optimal conditions, 66.1% of the hemicellulose was recovered as saccharides by the acid-catalyzed hydrothermal step. Formic acid was superior to H
2
SO
4
and HNO
3
as a catalyst for the subsequent organosolv step using aqueous glycerol in terms of delignification efficiency and selectivity, resulting in a maximum of 78.3% lignin recovery after acetone extraction with > 90% purity and better conservation of aromatic subunit structure according to 2D-NMR. The majority of cellulose (86.4%) was retained as a solid with high purity (74.8%) and enzymatic digestibility, resulting in 78.2% glucose recovery. The formic acid-aqueous glycerol-spent liquor could be recycled for at least four consecutive batches with no significant decrease in product yield or selectivity. The work represents an efficient sequential process for fractionation of lignocellulose-derived components which allowed solvent recycling and recovery of high-purity lignin with conserved subunit structure. The separated product streams are suitable for valorization in biorefinery.
Graphical abstract
A carbon supported PtMo aqueous phase reforming catalyst for producing hydrogen from glycerol was characterized by analysis of the reaction products and pathway, TEM, XPS and XAS spectroscopy.
...Operando
X-ray absorption spectroscopy (XAS) indicates the catalyst consists of bimetallic nano-particles with a Pt rich core and a Mo rich surface. XAS of adsorbed CO indicates that approximately 25% of the surface atoms are Pt. X-ray photoelectron spectroscopy indicates that there is unreduced and partially reduced Mo oxide (MoO
3
and MoO
2
), and Pt-rich PtMo bimetallic nano-particles. The average size measured by transmission electron microscopy of the fresh PtMo nano-particles is about 2 nm, which increases in size to 5 nm after 30 days of glycerol reforming at 31 bar and 503 K. The catalyst structure differs from the most energetically stable structure predicted by density functional theory (DFT) calculations for metallic Pt and Mo atoms. However, DFT indicates that for nano-particles composed of metallic Pt and Mo oxide, the Mo oxide is at the particle surface. Subsequent reduction would lead to the experimentally observed structure. The aqueous phase reforming reaction products and intermediates are consistent with both C–C and C–OH bond cleavage to generate H
2
/CO
2
or the side product CH
4
. While the H
2
selectivity at low conversion is about 75%, cleavage of C–OH bonds leads to liquid products with saturated carbon atoms. At high conversions (to gas), these will produced additional CH
4
reducing the H
2
yield and selectivity.
A facile and practical approach for the preparation of 3-indole derivatives was performed by the Yonemitsu condensation of indoles with Meldrum’s acid and aldehydes in a mixture of solvent from ...glycerol and water under ultrasonic irradiation at room temperature without catalyst. A series of aromatic aldehydes representative of various electronic and steric conditions were employed to examine the scope of substrates for this protocol in moderate to good yields. No catalyst, clean reaction conditions, tolerating the substrates with diverse functional groups and an environmentally acceptable medium are the best features in this process.
Graphical Abstract
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