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The benefits of increasing the number of surface hydroxyls on TiO2 nanoparticles (NPs) are known for environmental and energy applications; however, the roles of the hydroxyl groups have not been characterized and distinguished. Herein, TiO2 NPs with abundant surface hydroxyl groups were prepared using commercial titanium dioxide (ST-01) powder pretreated with alkaline hydrogen peroxide. Through this simple treatment, the pure anatase phase was retained with an average crystallite size of 5 nm and the surface hydroxyl group density was enhanced to 12.0 OH/nm2, estimated by thermogravimetric analysis, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Especially, this treatment increased the amounts of terminal hydroxyls five- to six-fold, which could raise the isoelectric point and the positive charges on the TiO2 surface in water. The photocatalytic efficiency of the obtained TiO2 NPs was investigated by the photodegradation of sulforhodamine B under visible light irradiation as a function of TiO2 content, pH of solution, and initial dye concentration. The high surface hydroxyl group density of TiO2 NPs can not only enhance water-dispersibility but also promote dye sensitization by generating more hydroxyl radicals.

High-silica ZSM-5 zeolite with less internal silicon hydroxyl groups was directly synthesized through a one-step hydrothermal crystallization method. The physicochemical properties of high-silica ZSM-5 zeolite with less internal silicon hydroxyl groups together with its catalytic capability for methanol to propene reaction were roundly compared with that of commercial ZSM-5 zeolite and another high-silica ZSM-5 zeolite with similar acid distribution but different hydroxyl distribution to it. The results show that high-silica ZSM-5 zeolite with less internal silicon hydroxyl groups has good crystallinity, uniform crystal size and moderate acidity, which is a highly stable catalyst for methanol to propene reaction. Based on the characterization and reaction results, the high stability of high-silica ZSM-5 zeolite with less internal silicon hydroxyl groups in methanol to propene reaction proves that the distribution of silicon hydroxyl groups plays a significant role in the deactivation of ZSM-5 zeolite.Graphic Abstract

Technical lignins are globally available and a sustainable feedstock. The unique properties of technical lignins suggest that these materials should have several industrial applications. The main proposal of this study is to evaluate the relationship between the structure and properties of two technical lignins. Morphological, chemical, physical, and thermal properties of sodium lignosulfonate (LGNa) and magnesium lignosulfonate (LGMg) were investigated. The results showed that a higher formation of intramolecular hydrogen bonds may occur in lignins with a higher content of phenolic hydroxyl groups, such as LGMg. As a result, an increase in the energy of hydrogen bonds in the lignosulfonate structure was observed, without significant change in the hydrogen bond distances. In addition, higher content of phenolic hydroxyl groups might also reduce lignosulfonates thermal stability. The combustion index value was three times higher for LGMg than for LGNa. The characterization study also revealed that phenolic hydroxyl groups influence the main properties of technical lignins and can be a determining factor when these lignosulfonates are used in industrial applications.

The rheological behavior of shear thickening fluid suspensions synthesized using three dispersion liquids, namely polyethylene glycol, glycerin, and diethylene glycol, having different numbers of hydroxyl groups at the ends of the chain and distinct chain lengths, was researched. The primary objective of this study is to investigate the effects of the length of the chain molecular, the number of –OH groups at the ends of the chain, and the density of dispersion liquids on the rheological behavior. Evaluations were made by taking into account the thickening ratio which expresses the maximum change in the viscosity of the fluid relative to the initial viscosity and the thickening period which states the difference between the shear rate at which the maximum viscosity is obtained and the critical shear rate. As a result of the evaluation made by considering these parameters, the rheological performance of shear thickening fluid suspensions synthesized with liquids having longer molecular chain lengths, higher –OH number, and higher density came to the fore. Samples synthesized with glycerin, which have more hydroxyl groups at the molecular chain ends, provided a more stable distribution by making stronger hydrogen bonds with silica. This situation significantly reduced the thinning behavior in the first region of the rheology curves and provided a stable and continuous thickening behavior after the critical shear rate. In addition, with the increase in the silica ratio, the thickening situation changed from continuous to discontinuous. Increment of silica also decreased the critical shear rate while increasing the initial and maximum viscosity. Increasing the silica content from 22 to 26% resulted in the thickening ratio increasing by 686% from 6.6 to 45 in the samples synthesized with polyethylene glycol while decreasing the thickening period from 559 to 41.2. Similar situations are observed in the samples synthesized with glycerin and diethylene glycol. All of the samples obtained exhibited a reversible behavior rheologically. When the applied shear rate was removed, the sample returned to its former fluid state. Moreover, suspensions synthesized by mixing dispersion liquids showed superior performance compared to single-liquid samples. It is thought that the dispersion liquids interact to form a branched network by making more bonds both with each other and with the silica particles, and it provides an increase in the resistance of the fluid against deformation under high shear stress.

Cu/SiO2 catalyst prepared by the ammonia-evaporation (AE) method is the potential preferred catalyst for hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG). Although significant advancements have been obtained in the confirmation and influence factors of active species in the hydrogenation process, the relationship between the catalytic activity and the sensitive factors in the preparation or pretreatment process of the catalyst is still uncertain. In this paper, Cu/ZSM-5 catalysts for DMO hydrogenation to EG were prepared by the AE method using ZSM-5 molecular sieve with high silicon-alumina ratio (1500) as a support. The ZSM-5 support was pretreated by drying at 393 K for different hours and it was found that the surface hydroxyl group of the support had significant influence on the structure and catalytic hydrogenation performance of the prepared Cu/ZSM-5 catalyst. The distribution of surface hydroxyl groups could be significantly changed by pre-drying the carrier, which further resulted in the change of the copper dispersion and surface properties of subsequent copper-supported catalysts. With the decrease of hydroxyl content on the surface of the ZSM-5 support, the prepared reduced Cu/ZSM-5 catalyst possessed smaller Cu0 particles size, higher copper dispersion, higher surface area of Cu0 and Cu+ species, but weakened surface acidity of the catalyst, which resulted in the great improvement of the catalytic activity. The DMO conversion and EG selectivity could reach 100% and 93% even under the low reaction temperature at 448 K over the Cu/ZSM-5-24 catalyst (based on the ZSM-5 support pretreated by drying for 24 h). In addition, the catalytic activity did not show obvious change after 300 h of reaction, probably due to the low temperature reaction and suitable surface properties of the catalyst.

Most of our knowledge on kraft pulping comes from studies on dissolved lignin in the freely drainable black liquor and isolated residual lignin in pulp. However, entrapped liquor in the delignified chips has been shown to differ significantly from the free liquor. The present study has compared three liquor fractions: free, lumen and fiber wall liquor. The free liquor was obtained by draining the delignified chips, the lumen liquor was separated by centrifugation and the fiber wall liquor by subsequent leaching. The liquor in the fiber wall had the lowest concentration of lignin and hydrosulfide ions and the highest concentration of monovalent cations. The dissolved lignin in the fiber wall liquor had the highest molar mass and the highest content of xylan. The highest concentration of dissolved lignin was in the liquor filling the lumen cavities. The lignin in the free liquor had the lowest molar mass and the lowest content of lignin structures containing β-O-4 linkages and aliphatic hydroxyl groups. The lowest mass transfer rate of dissolved lignin was from the lumen liquor to the free liquor probably restricted by the tortuosity of the chip.

PurposePhyllosilicate minerals are the important components in soils and an important source of activated aluminum (Al) during soil acidification. However, the mechanisms for Al activation in phyllosilicate minerals were not understood well. This study was designed to investigate the role of hydroxyl groups on the surface of montmorillonite in acidification-induced activation of solid aluminum.Materials and methodsMontmorillonite was treated at high temperatures to remove the hydroxyl groups from its surface, and then XRD, FTIR, and 27Al NMR of montmorillonite before and after high-temperature treatment were determined. After that, Al activation during acidification of montmorillonite treated at different temperatures was investigated using the constant pH titration method.Results and discussionThe results showed that hydroxyl groups on the surface of montmorillonite can effectively be removed when the temperature was raised to 670 °C. After high-temperature treatment, the crystallinity of the montmorillonite was reduced. The montmorillonite layer space was reduced, the pH and cation exchange capacity (CEC) of the mineral were decreased, and consequently the amount of acid consumed during acidification was significantly reduced. The decrease of CEC and surface hydroxyl groups in the mineral was responsible for the reduction in acid consumption during mineral acidification. After high-temperature treatment, the Al coordination structure of montmorillonite was transformed from six-coordination to four-coordination. The removal of surface hydroxyl groups inhibited the activation of Al during montmorillonite acidification, and the inhibition effect was increased with increasing temperatures below the complete dehydroxylation point.ConclusionsThe activation of Al during montmorillonite acidification was inhibited, suggesting that the hydroxyl groups at the edge of montmorillonite played an important role in Al activation process.

The aromatic biopolymer lignin, present in lignocellulosic material, can act as an antioxidant and has antimicrobial and UV blocking properties due to the presence of aromatic ring and phenolic hydroxyl groups with aliphatic hydroxyl, carboxyl, methoxy groups. It is produced in huge amounts as a by-product during the delignification process in the paper and pulp industries. Lignin-based polymer composites with advantageous properties can fulfill the growing demand for lightweight polymer composites. Incorporating lignin as reinforcement in polymer composite can make it more environmentally friendly. Among four technical lignin, kraft lignin and lignosulfonate have a unique molecular structure that includes sulfur and can be extracted from black liquor and spent liquor, respectively. Moreover, the negatively charged sulfite groups in lignosulfonate can have an electrostatic attraction to a wide range of positively charged polymers and materials, making it an ideal additive for the adsorbent material. The main intention of this review is to increase the knowledge on the use of cheap and widely available kraft lignin and lignosulfonate as reinforcement in polymer composites and to find the shortcomings to advance further research on the application of technical lignin. The article mainly focuses on advancements in kraft lignin and lignosulfonate polymer composites.

PFRs were produced on biochar during Cr(VI) decontamination. PFRs formation on biochar was owing to the oxidization of phenolic-OH by Cr(VI). Appearance of excessive oxidant led to the consumption of PFRs on biochar. Biochar charred at high temperature possessed great performance to Cr(VI) removal. This study investigated the facilitation of Cr(VI) decontamination to the formation of persistent free radicals (PFRs) on rice husk derived biochar. It was found that Cr(VI) remediation by biochar facilitated the production of PFRs, which increased with the concentration of treated Cr(VI). However, excessive Cr(VI) would induce their decay. Biochar with high pyrolysis temperature possessed great performance to Cr(VI) removal, which was mainly originated from its reduction by biochar from Inductively Coupled Plasma Optical Emission Spectroscopy and X-ray Photoelectron Spectroscopy. And the corresponding generation of PFRs on biochar was primarily ascribed to the oxidization of phenolic hydroxyl groups by Cr(VI) from Fourier Transform Infrared Spectroscopy analysis, which was further verified by the H 2O 2 treatment experiments. The findings of this study will help to illustrate the transformation of reactive functional groups on biochar and provide a new insight into the role of biochar in environmental remediation.

In this work, density functional theory (DFT) calculations and x-ray photoelectron spectroscopy (XPS) were carried out to investigate the hydroxyl groups on a wurtzite GaN (0001) surface. Surface treatments of GaN with piranha and HCl-based solutions were studied via XPS, and peak shifts in the Ga 2p and O 1s XPS spectra were caused by the signal change resulting from surface hydroxyl groups. Further DFT study revealed that the adsorption of hydroxyl groups is more favourable near the centre location than near gallium atoms. To investigate the thermodynamic stability of hydroxyl groups under different coverages, a surface phase diagram of hydroxyl group adsorption on the GaN (0001) surface was constructed over a coverage range of 1/6–1 monolayer (ML). The results showed that a high hydroxyl group coverage is more likely to be present on the GaN surface. The energy barrier for split hydroxyl groups is 1.41 eV. Therefore, the hydroxyl groups can be stable at room temperature. These results provide a systematic explanation of the adsorption between the hydroxyl groups and the GaN (0001) surface.