Al-OH Groups Research Articles

Exploiting Lignin Structure and Reactivity to Design Vitrimers with Controlled Ratio of Dynamic to Non-Dynamic Bonds.

Lignin is an abundant biobased feedstock, representing the first source of renewable aromatic structures. Thanks to its high functionality in aliphatic hydroxyls (Al-OH), phenolic hydroxyls (Ph-OH) and carboxylic acids (COOH), lignin is an attractive precursor to crosslinked polymer materials. Different biobased macromolecular architectures can be designed from lignins, whose end-of-life should also be considered in the context of a circular bioeconomy. To enhance the recyclability of crosslinked polymer networks, the introduction of dynamic linkages to design vitrimers is a promising strategy. In this study, Kraft lignin was chemically modified with succinic anhydride, to prepare a series of modified lignins with a controlled COOH/Ph-OH ratio, exploiting the difference in reactivity between Al-OH and Ph-OH groups. Upon crosslinking with a diepoxy, mixed vitrimer networks with variable ratios between dynamic ester bonds and non-dynamic ether bonds were synthesized. The analysis of their properties evidenced the impact of the non-dynamic linkages on the materials behaviors, including their dynamicity and reprocessing ability. Although the activation energy for bond exchange is increased, non-dynamic linkages do not hinder the reprocessability of these adaptable materials, and provide them high creep resistance. The controlled introduction of non-dynamic linkages appears as a promising strategy to enhance the properties of lignin-based vitrimers.

The influence of multi-step modification method on the adsorption effect of alkali metal adsorbent in high temperature corrosive environment

In order to mitigate the serious high-temperature corrosion problem caused by gaseous alkali metal salts on the heating surface of boilers, the adsorption effect of kaolinite for alkali metal vapor stood out among many silicon-aluminum-based adsorbents. To improve the adsorption capacity of kaolinite for alkali metal vapor, a novel multi-step modification method was proposed in this reported work. The multi-step modification method consisted of the following four steps, which were pre-intercalation, replacement intercalation, multiple intercalation, and exfoliation, and it was found that the multi-step modification method resulted in the increase of the particle size, the expansion of the interlayer spacing, and the expansion of the porosity structure of kaolinite. The physicochemical analyses showed that the multi-step modification method made it easier to dehydrate kaolinite to produce metakaolinite. In addition, it was found that the multi-step modification method led to the generation of a new crystalline phase of kaolinite, which was formed by the dehydration and condensation of methanol and AlOH groups. It also led to the pre-removal of some of the hydroxyl groups in kaolinite. The insoluble sodium loading capacity of the multi-step modified kaolinite was enhanced by 64.47% and the insoluble potassium loading capacity was enhanced by 29.68% under the adsorption condition of 850 °C for 45 min. Therefore, the multi-step modification method enhanced the alkali metal vapor adsorption capacity of raw kaolinite under high-temperature environment, which laid a certain foundation for subsequent research in this field.

Basic Sites on Alumina with Preadsorbed Ethanol and Ammonia-An IR Study.

The adsorption of ethanol and ammonia changes the basic properties of alumina, and new basic sites are created. Ethanol reacts with surface Al-OH groups, forming ethoxy group Al-O-C2H5. The substitution of Al-OH by Al-O-C2H5 increases the negative charge of neighbouring oxygen atoms, and they became sufficiently basic to react with adsorbed CO2 forming carbonate species CO32-. These carbonates were found to be monodentate and bidentate species. Preadsorption of ammonia also increases the basicity of alumina, but the mechanism is different than for ethanol adsorption. Adsorbed ammonia interacts with surface Lewis acid sites being three-coordinated aluminium atoms. This interaction is accompanied by an electron transfer from ammonia molecules to surface sites, and increases the basicity of the neighbouring oxygens, which can react with the absorbed CO2. The carbonate species formed are polydentate ones.

Precise Structural and Dynamical Details in Zeolites Revealed by Coupling-Edited 1H-17O Double Resonance NMR Spectroscopy.

Despite the extensive industrial and research interests in zeolites, their intrinsic catalytic nature is not fully understood due to the complexity of the hydroxyl-aluminum moieties. 17O NMR would provide irreplaceable opportunities for much-needed fine structural determination given the ubiquitous presence of oxygen atoms in nearly all species; however, the low sensitivity and quadrupolar nature of oxygen-17 make its NMR spectroscopic elucidation challenging. Here, we show that state-of-the-art double resonance solid-state NMR techniques have been combined with spectral editing methods based on scalar (through-bond) and dipolar (through-space) couplings, which allowed us to address the subtle protonic structures in zeolites. Notably, the often-neglected and undesired second-order quadrupolar-dipolar cross-term interaction ("2nd-QD interaction") can actually be exploited and can help gain invaluable information. Eventually, a comprehensive set of 1H-17O/1H-27Al double resonance NMR with J-/D-coupling spectral editing techniques have been designed in this work and enabled us to reveal atomic-scale precise structural and dynamical details in zeolites including: 1) The jump rate of the bridging acid site (BAS) proton is relatively low, i.e., far less than 100 s-1 at room temperature. 2) The Al-OH groups with 1H chemical shift at 2.6-2.8 ppm, at least for nonseverely dealuminated H-ZSM-5 catalysts, exhibit a rigid bridging environment similar to that of BAS. 3) The Si-OH groups at 2.0 ppm are not hydrogen bonded and undergo fast cone-rotational motion. The results in this study predict the 2nd-QD interaction to be universal for any rigid -17O-H environment, such as those in metal oxide surfaces or biomaterials.

Sulfate ion induced formation of unsaturated coordination of AlF3 nanocluster catalyst confined in carbon framework for catalytic dehydrofluorination

The gas-phase catalytic dehydrofluorination of hydrofluorocarbons (HFCs) to produce hydrofluoroolefins is an important strategy for the resource utilization of HFCs. AlF3 is the most suitable and efficient catalyst for dehydrofluorination of HFCs. However, sintering and preservation of coordinatively unsaturated aluminum centers pose significant challenges for AlF3 catalysts. Herein, we prepared carbon-confined AlF3 nanocluster catalysts with aluminum fumarate metal organic frameworks (AlFu-MOF) as the precursors, followed by carbonization and fluorination treatments. During catalyst preparation, anion ligand in the aluminum salt plays a major role in the formation of coordinatively unsaturated Al centers. Compared with Al(NO3)3·9H2O and AlCl3·6H2O, AlFu-MOF (named AlFu-S) prepared with Al2(SO4)3·18H2O exhibits a higher concentration of Al-OH species. Additionally, the residual SO42- ions can indeed retard the crystallization of AlF3 at elevated temperatures. Hence, the decomposition of Al-OH groups and the presence of residual SO42- ions in AlFu-S contribute together to the abundance of unsaturated coordinated Al centers in the finally carbon-confined AlF3 nanocluster catalyst (named AlFu-S-550-F). Also, the confinement effect of the carbon material formed by the carbonization of AlFu-S effectively prevent AlF3 nanocluster from sintering and phase transition, hence, in the dehydrofluorination of HFC-134a, AlFu-S-550-F demonstrates excellent stability compared to AlF3 catalysts prepared by precipitation methods. In summary, this study provides a strategy for the preservation of coordinatively unsaturated aluminum centers in AlF3 catalysts due to sintering and crystal structure transformation. Furthermore, the methodology proposed in this work is also applicable to the preparation of other coordinatively unsaturated metal catalysts.

Selective oxidation of methanol to green oxygenates on vanadium-aluminum phosphate based catalysts

The methanol transformation reaction in the presence of oxygen was carried out in the temperature range of 210–310 ºC, over vanadium catalysts based on vanadium oxide (V2O5) and on vanadium-phosphorus oxide (VPO) calcined at 450 ºC in air. Specifically, the vanadium oxide systems with V2O5 loadings below, above or corresponding to the theoretical monolayer (1–34 wt% of V2O5) were obtained by the impregnation of a mesoporous amorphous aluminum phosphate, AlPO4, calcined at different temperatures (from 350 ºC to 650 ºC) with vanadyl oxalate. Additionally, mechanical mixtures of either VPO or V2O5 with AlPO4 were also employed. Under the experimental conditions employed, with a methanol enriched feed (O2/methanol molar ratio=0.6), formaldehyde and dimethoxymethane were the main products obtained with all the catalysts studied. Vanadyl phosphate (VPO) exhibited slightly lower activity than vanadium oxide, formaldehyde being the main product (SFA ≥ 95%). The conversion of both vanadium oxide and vanadyl phosphate appreciably increased when mixed with the AlPO4, also promoting the formation of dimethoxymethane. The interaction of the vanadium species with the aluminum phosphate surface took place preferentially through the surface Al-OH groups, the vanadium species with less development being the most active in the formation of formaldehyde and dimethoxymethane, per at-g of vanadium.

Dynamic evolution of HZSM-5 zeolite framework under steam treatment

Understanding the dynamic evolution of zeolite framework structures and the interactions between various hydroxyl groups or aluminum species under different steam conditions at the atomic scale is extremely crucial. Herein, using a series of characterization methods, the framework structures of HZSM-5 zeolites (Si/Al = 36) following exposure to steam in the temperature range of 100 to 500 °C are investigated. Under mild steam conditions (T ≤ 200 °C), dealumination is nearly absent, and the silanol nests directly condense to create new framework Si-O-Si bonds. Conversely, under severe steam conditions (T ≥ 300 °C), the framework tetrahedral aluminum atoms Al(IV)-1 can be sequentially converted to partially coordinated framework aluminum Al(IV)-2 and extra-framework aluminum (EFAL) through partial and complete hydrolysis, which cause an increase in the framework Si/Al ratio and a decrease in crystallinity. Al(IV)-2 is recognized as a significant intermediate species for framework complete dealumination. The Brønsted acid sites on Al(IV)-2 can be perturbed by the framework Al-OH groups due to hydrogen bonding interactions, leading to a shift in 1H chemical shifts to lower fields, appearing at 6.0-9.0 ppm and 12.0-15.0 ppm. The newly generated EFAL and silanol nests further evolve through condensation as well. Meanwhile, during dealumination, the spatial correlations (or interactions) of various hydroxyl groups on structurally distinct aluminum species [Al(IV)-1, Al(IV)-2, and EFAL] and aluminum species become extremely intricate. Based on these findings, the dynamic evolution path of HZSM-5 zeolite framework structures under mild and severe steam conditions is proposed.

Effective Synergistic Hafnium-Aluminum Bimetallic Oxides catalysts for the Synthesis of 5-Hydroxymethylfurfural from Glucose and Fructose

5-Hydroxymethylfurfural (5-HMF) is considered as a bridge connecting biomass and petrochemical products with excellent application potential and market prospects. In this work, a series of hafnium-aluminum bimetallic oxide (Al-HfOx) catalysts were synthesized with the coprecipitation method, possessing outstanding catalytic effect in the conversion of glucose or fructose to 5-HMF. 5-HMF yield of 49.31% was acquired from glucose in a methyl isobutyl ketone (MIBK)/saturated salt water (SSW) biphasic solvent at 170 ℃ after 2 h. When fructose was the substrate, 5-HMF yield was 74.85% in DMSO at 150 ℃ after 75 min. In addition, the reaction mechanism was speculated that the isomerization process could be promoted by the synergistic effects of aluminum and hafnium species, in which Hf-OH and Al-OH groups, and Hf active sites served as Brønsted and Lewis acidic sites, respectively. After at least four cycles, there was no remarkable wastage in the catalytic activity of Al-HfOx. This paper offers a prospective catalytic strategy for heterogeneous bimetallic oxide catalysts used in the effective conversion of carbohydrates to 5-HMF.

Tuning Al sites in Y-zeolite for selective production of ϒ-valerolactone from levulinic acid

Converting biomass derived levulinic acid (LA) to γ-valerolactone (GVL) through catalytic transfer hydrogenation, using Lewis acidic catalysts and alcohols as hydrogen donors, is an important route for biomass valorization. Zeolites containing Lewis acidic metal ions have been widely studied for catalytic transfer hydrogenation of LA. On the other hand, zeolites themselves have Al-Lewis acid sites, inherent to their aluminosilicate framework, and these sites can be generated and tuned through simple post-synthesis methods. The present study shows that properly tuned Al sites in Y zeolites can catalyze LA to GVL conversion, and the key requirement for this conversion is creating more penta-coordinated Al sites in the catalyst. In order to establish this, a series of thermally-treated (500–800 °C) and steam-treated (500 and 700 °C) Y zeolites were prepared, characterized by X-ray diffraction, N2 adsorption, temperature programmed desorption of NH3, 27Al MAS NMR spectroscopy and pyridine adsorption – FTIR spectroscopy and then tested for LA to GVL conversion. Thermal dealumination of NH4Y zeolite at 700 °C (TY700) gave the highest percentage (21%) of penta-coordinated Al and it was found to be more selective (∼94%) in forming GVL than other catalysts studied under the optimized conditions. The turnover frequency of formation GVL, (TOFGVL) from isopropyl levulinate (IPL) on TY700 was three times higher than that of TY500 (NH4Y thermally treated at 500 °C). During the reaction, acid sites of the zeolite catalyze esterification of LA with isopropyl alcohol and hence LA is readily converted to isopropyl levulinate and then Meerwein–Ponndorf–Verley (MPV) reduction of IPL leads to the formation of isopropyl 4-hydroxy pentanoate, which undergoes lactonization to form GVL. Based on the experimental results, a plausible reaction mechanism involving the penta-coordinated Al Lewis acid sites and -Al-OH group was proposed.

Synthesis of bifunctional nanostructured adsorbents based on anionic/cationic clays: effect of arrangement on simultaneous adsorption of cadmium and arsenate.

The development of bifunctional hybrid materials based on natural clays and layered double hydroxide (LDH) and their application on the simultaneous adsorption of Cd(II) and As(V) was investigated in this work. Two different synthesis routes, in situ and assembly, were employed to obtain the hybrid materials. Three types of natural clays, namely bentonite (B), halloysite (H), and sepiolite (S), were used in the study. These clays are characterized by a laminar, tubular, and fibrous structural arrangement, respectively. The physicochemical characterization results indicate that the hybrid materials were formed through interactions between the Al-OH and Si-OH groups present in the natural clays, and the Mg-OH and Al-OH groups present in the LDH for both synthesis routes. However, the "in situ" route yields a more homogenous material because the LDH formation is performed on the natural clay surface. The hybrid materials showed an anion and cation exchange capacity up to 200.7meq/100g and an isoelectric point near 7. The arrangement of natural clay has no impact on the properties of hybrid material but influences the adsorption capacity. The adsorption of Cd(II) onto hybrid materials was enhanced in comparison with natural clays, obtaining adsorption capacities of 80, 74, 65, and 30mg/g for 15:1 (LDH:H)INSITU, 1:1 (LDH:S)INSITU, 1:1 (LDH:B)INSITU, and 1:1 (LDH:H)INSITU, respectively. The adsorption capacities of hybrid materials to adsorb As(V) were between 20 and 60μg/g. The 15:1 (LDH:H)INSITU sample showed the best adsorption capacity being ten folds greater than halloysite and LDH. In all cases, the hybrid materials showed a synergistic effect for Cd(II) and As(V) adsorption. The adsorption study of Cd(II) onto hybrid materials showed that the primary adsorption mechanism is cation exchange between the interlayer cations in natural clay and Cd(II) in the aqueous solution. The adsorption of As(V) showed that the adsorption mechanism is attributed to anion exchange between CO23- in the interlayer space of LDH and H2ASO4- in the solution. The simultaneous adsorption of As (V) and Cd (II) shows that, during the As(V) adsorption, there is no competition by the adsorption sites. Still, the adsorption capacity towards Cd(II) was enhanced 1.2-folds. This study ultimately revealed that the arrangement of clay has a significant influence on the adsorption capacity of the hybrid material. This can be attributed to the similar morphology between the hybrid material and natural clays, as well as the important diffusion effects observed in the system.

Physicochemical Properties and Phosphorus Adsorption Capacity of Ceramsite Made from Alum Sludge

Alum sludge is an inevitable by-product from the water purification process, which had been applied as substrates in some constructed wetlands with good performance, especially for phosphorus (P) adsorption. The raw alum sludge is similar to a clay lump with an irregular shape, and there is a concern of it leaching into water. For better reuse, herein, some sludge was fired to produce alum sludge ceramsite (ASC) with a uniform spherical shape via a four-step process of kneading the sludge ball, air drying, preheating at 400 °C for 10 min, and firing at 600 °C for 5 min. Meanwhile, an air-dried alum sludge ball (adASB) was manufactured for comparison. The physicochemical properties and P adsorption ability of ceramsite were investigated subsequently. Through XRD and FT-IR tests, there was no obvious difference between ASC and adASB on the phase structure, but there was a certain amount of Al-OH group loss on the surface of ASC. The structure of ASC was still amorphous, similar to adASB, while ASC possessed more micropore structure and a bigger specific surface area than adASB. Adsorption experiments showed the P adsorption behaviors of ASC and adASB were much similar, and their adsorption kinetics were in accordance with the two-step adsorption kinetics rate equation and pseudo-second-order kinetics equation. The maximum adsorption capacities of ASC and adASB fitted by the Langmuir model were 1.66 mg/g and 1.89 mg/g, respectively. It should be pointed out that, compared with other adsorbents, the ASC produced in this study still had a greater ability to adsorb P. Therefore, ASC should have a great application potential for P removal in wastewater treatment in China.

Unraveling the capture mechanism of gaseous As2O3 over H-ZSM-5 zeolite from coal-fired flue gas: Experimental and theoretical insights

Gaseous As2O3 discharged from coal-fired power plants results in severe detriments to the ecological environment. It is of great urgency to develop highly efficient As2O3 capture technology for reducing atmospheric arsenic contamination. Utilizing solid sorbents for gaseous As2O3 capture is a promising treatment for As2O3 capture. The zeolite of H-ZSM-5 was applied for As2O3 capture at high temperatures of 500–900 °C. Special attention was paid to clarifying its capture mechanism and identifying the influence of flue gas components via density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. Results revealed that due to high thermal stability with large specific areas, H-ZSM-5 demonstrated excellent arsenic capture at 500–900 °C. The captured arsenic consisted of As3+ and As5+ speciations, ascribed to As2O3 adsorption and oxidation. Moreover, As3+ and As5+ compounds were both through physisorption or chemisorption at 500–600 °C while dominant chemisorption at 700–900 °C. In particular, As3+ compounds were much more steadily fixed in products at all operating temperatures. Combining the characterization analysis and DFT calculations, it further verified that both Si–OH–Al groups and external Al species of H-ZSM-5 could chemisorb As2O3, and the latter exhibited much stronger affinities via orbital hybridization and electron transfer. The introduced O2 could facilitate As2O3 oxidation and fixation in H-ZSM-5, especially at a lower concentration of 2%. Additionally, H-ZSM-5 possessed great acid gas resistance for As2O3 capture under the concentration of NO or SO2 less than 500 ppm. AIMD simulations further identified that compared to NO and SO2, As2O3 was far more competitive and occupied the active sites of the Si–OH–Al groups and external Al species of H-ZSM-5. Overall, it demonstrated that H-ZSM-5 is a promising sorbent for As2O3 capture from coal-fired flue gas.

Green and sustainable nanoformulation of tebuconazole based on capsules of bionanocomposites halloysite/alginate

In this work, the synthesis of bionanocomposites based on halloysite/alginate with the fungicide tebuconazole (Tbz), as an active ingredient and its application as nanoformulation, was studied. The resultant nanoformulations were friendly to the environment and can be an alternative to increase the yiel of agricultural fields. To achieve this, the surface of the halloysite (Hal) was modified by the adsorption of five surfactants (one cationic and four nonionic) to obtain an organophilic organohalloysites (OHal). Adsorption isotherms and FTIR characterization demonstrated that OHal was obtained due to hydrogen bonding interactions between the nonionic surfactants and the Si-OH and Al-OH groups of the Hal, as well as due to cation exchange with the cationic surfactant. The Tbz was encapsulated in the OHal by adsorption. The mass of Tbz adsorbed was 6.73, 2.92, 4.41, 2.19, and 1.86 mg/g for the OHal with Span 20, Span 80, Tween 80, Tween 85, and HDTMA, respectively. This demonstrated that the surfactants with the highest uptake capacity in OHal have the lowest HLB (hydrophilic–lipophilic balance). The release kinetics of Tbz in the OHal occurred rapidly (approximately in 2 days), regardless of the surfactant type. Once encapsulated the Tbz with the alginate, the release time occurred between 5 and 15 days, decreasing the released quantity between 50 and 75%, evidencing a slow release and greater kinetics control. The confrontation of nanoformulations vs. Fusarium spp. delayed the fungus growth, like commercial tebuconazole does, corroborating the antifungal effect of the material, even though the nanopesticide synthesized in this work has 20 times less Tbz.

Surface Properties and Microstructures of Al-SBA-15 under Different Temperature Calcinations in Relation to Adsorption Performance

In this study, the calcination temperature was adjusted to influence the properties, including surface alumina types, microstructure, and surface acidity, and then to explore their association with the performance of As(V) adsorption after preparing aluminum-modified SBA-15 (Al-SBA-15x) by the incipient wetness impregnation method. The results showed that calcination promoted the formation of aluminum oxides with different coordination environments on SBA-15 carriers, resulting in different kinds of acid sites and Al–OH groups, which were the adsorption sites for As(V). However, with the increase in the calcination temperature, the total surface acidity on the Al-SBA-15x samples gradually decreased, and the dehydration and dehydroxylation reactions of aluminum oxides occurred, which reduced the Al–OH groups and changed the acid–base environment on the samples. Besides, high-temperature calcination caused a partial collapse of mesoporous channels, which was not conducive to dispersing the active adsorption centers on the surface and further reduced the effective utilization of active adsorption centers. Therefore, the adsorption capacity of an Al-SBA-15x adsorbent for As(V) decreased with the increase of the calcination temperature. Furthermore, kinetic models were used to investigate the relationship between the microstructure and adsorption processes. Finally, recycling tests were performed to demonstrate the reusability of Al-SBA-15x samples.

Cobalt–aluminum oxide clusters-embedded γ-Al2O3 nanosheets for peroxymonosulfate activation: Interfacial pH-buffering property to eliminate cobalt leaching and boost the catalytic activity

Cobalt–aluminum oxide clusters-embedded γ-Al2O3 nanosheets (CAO@AO) were facilely fabricated and developed for peroxymonosulfate (PMS) activation. Interestingly, the Al–OH-rich amphiphilic surface of CAO@AO endowed the catalyst with unique interfacial pH-buffering property, which stabilized the reaction media at nearly neural level and thus prevented the cobalt sites from leaching. Moreover, the presence of Al–OH groups around the Co−Al oxide clusters facilitated the activation of PMS and the redox cycle of Co2+/Co3+, which resulted in a boosted catalytic activity. Consequently, the CAO@AO exhibited zero leaching of cobalt and high efficacy for PMS activation, enabling complete and ultrafast removal of metronidazole with a significantly higher turnover frequency (TOF, 1.83 min−1) than that of Co3O4 (0.004 min−1). It was also demonstrated that introducing suitable foreign metal into CAO@AO could further enhance the catalytic activity (e.g., TOF of Mn(40%)-CAO@AO: 5.80 min−1). Furthermore, Mn(40%)-CAO@AO-decorated granular support was synthesized that dominated a continuous-flow degradation in column reaction system.

Formation conditions of quartz deposits in the Southern Urals: Fluid inclusion data and IR spectroscopy

Research subject. Quartz veins of the Svetlorechenskoye, Karayan, Gora Khrustalnaya and Tolstikha quartz deposits in the Southern Urals.Methods. An optical study of quartz was performed using an Olympus BX51 optical microscope. A fluid inclusion study was performed using a thermostage TMS-600 (Linkam) equipped with the LinkSys V-2.39 software and an Olympus BX51 optical microscope (South Urals State University, Miass). The fluid composition in the inclusions was estimated from eutectic temperatures. Fluid salinity was calculated based on the melting temperatures of crystalline pha ses. Registration of infrared spectra was carried out using an infrared Fourier spectrometer Nicolet-6700 Thermo Scientific (SU FRC MG UB RAS, Miass). The spectra were processed using the OMNIC Thermo Nicolet software package and the Peakfit program. The extinction coefficients for calculating the concentration of molecular water and OH-groups were used from.Results. The veins are composed of coarse-grained milky-white quartz. The fluid inclusion data shows that the quartz veins were deposited from similar in composition NaCl-H2O fluids with salinities of 3–9 wt % NaCl eq. and at temperatures ranging from 100 to 280°C. Quartz in the Tolstikha deposit was deposited at the highest temperatures. According to IR spectroscopy data, quartz in the studied deposits is characterized by high contents of molecular water and average concentrations of Al-OH groups. According to the content of OH-groups, quartz in the Tolstikha deposit approaches industrial granulated quartz used in the production of high-purity quartz concentrates.Conclusions. Quartz veins in the studied deposits formed at temperatures ranging from 100 to 280°C. The salinity of inclusions in quartz ranged from 10 to 3.5 wt % NaCl eq. Mineral-forming fluids were of Na-chloride or Na-K-chloride composition, which indicates quartz crystallization during the post-diagenetic (metamorphic) transformation of rocks. Quartz in the studied deposits is characterized by a specific ratio of water and Al-OH, which is associated with the conditions of deposit formation and incompleteness of quartz metamorphism processes therein.

Spatial Proximities between Brønsted Acid Sites, AlOH Groups, and Residual NH4+ Cations in Zeolites Mordenite and Ferrierite

The interaction between Brønsted acid sites (BAS), AlOH groups, and residual NH4+ cations in zeolites mordenite and ferrierite was investigated by solid-state 1H magic-angle spinning and 1H–27Al rotational-echo adiabatic-passage double-resonance (REAPDOR) nuclear magnetic resonance (NMR) techniques in combination with density functional theory (DFT) cluster calculations. Higher temperatures are needed for the full elimination of NH4+ (853 K instead of 723 K compared to other high-silica zeolites). NMR data reveal that residual NH4+ cations are primarily in the vicinity of BAS and other NH4+ in the case of mordenite and AlOH groups in ferrierite materials. DFT calculations for a variety of cluster models with BAS and NH4+ ions embedded in their zeolite environment rationalize the experimental data. It is suggested that NH4+ ions are preferably stabilized within 8-ring pores, explaining the higher temperature required to decompose them.

Understanding Cd2+ Adsorption Mechanism on Montmorillonite Surfaces by Combining DFT and MD

The adsorption mechanism of Cd2+ on different cleavage planes of montmorillonite was investigated using density functional theory (DFT) calculations and molecular dynamics (MD) simulations. The most stable adsorption energies of Cd2+ on the (001) and (010) surfaces were −88.74 kJ/mol and −283.55 kJ/mol, respectively. On the (001) surface, Cd2+ was adsorbed on the centre of the silicon–oxygen ring by electrostatic interactions, whereas on the (010) surface, Cd2+ was adsorbed between two ≡Al–OH groups and formed two covalent bonds with O, which was mainly due to the interaction between the Cd s and O p orbitals. Upon the partial substitution of Na+ by Cd2+, Cd2+ was adsorbed on the (001) surface as inner-sphere surface complexes, with a hydration number of 5.01 and a diffusion coefficient of 0 m2/s. Whereas, when Cd2+ completely replaced Na+, part of the Cd2+ moved from the inner-sphere surface complexes to the outer-sphere surface complexes owing to its competitive adsorption. In this case, its hydration number became 6.05, and the diffusion coefficient increased to 1.83 × 10−10 m2/s. This study provides the theoretical background necessary for the development of montmorillonite-based adsorbents.

Transition metal oxy/hydroxides functionalized flexible halloysite nanotubes for hydrogen evolution reaction

The hierarchical halloysite nanotubes (HNT) have alumina containing positive Al-OH groups on its inner surface and silica-containing negative siloxane groups of Si–O–Si on its outer surface. The silicate laminate consists of silicon-oxygen at tetrahedral sites and aluminum-oxygen at octahedral sites. Since HNT has an abundant hydroxyl group on the surface with exceptional cation/anion exchange capacity, the surface-functionalized HNT could boost electrocatalytic activity. Hence, we have synthesized Ni, Co, and Cu metal oxy/hydroxides functionalized HNT by a facile hydrothermal method for HER. Among them, Co(OH)2@HNT on flexible carbon cloth displays an ultra-low overpotential of 65 mV at 10 mA cm−2 current density and Tafel slope of 181 mV dec-1 and also exhibited a larger exchange current density of 3.98 mA cm−2 in alkaline 1 M KOH electrolyte due to superior electrostatic affinity between OH– and Co2+. The electrolyzers with anion exchange membrane consisting of RuO2||Co(OH)2@HNT show remarkable stability of over 50 h at 10 mA cm−2 in alkaline electrolyte. The post stability sample retains the same surface oxidation state which confirms the robustness of the electrocatalyst. The reported results are far better than many of the transition metal oxides/chalcogenides electrocatalysts and hence it is expected that HNT could act as a potential alternative candidate to replace the benchmark platinum catalyst.

Probing Acid Sites in MOR Zeolite Using Low-Temperature 13C Solid-State NMR Spectroscopy of Adsorbed Carbon Monoxide

Understanding the structure of Lewis and Brønsted acid sites in zeolites is a key step toward unraveling mechanisms of catalytic reactions, establishing molecular-level structure–activity relationships and thereby rational improvement of their performance. Acid sites are typically studied indirectly, using probe molecules, whose spectroscopic signatures are linked to the nature of specific sites. In this context, carbon monoxide and pyridine (Py) are the most frequently used probe molecules due to their sensitivity toward the nature and strength of surface acid sites. Although infrared and solid-state NMR spectroscopies of adsorbed Py are now widely used, the utilization of carbon monoxide to study surface sites has been mainly limited to infrared spectroscopy. Considering that NMR spectroscopy parameters, such as isotropic chemical shift (δiso) and chemical shift anisotropy (CSA), provide detailed structural information, we have explored the capabilities of low-temperature carbon-13 NMR spectroscopy of adsorbed 13CO on dehydrated mordenite. One-dimensional (1D) and two-dimensional (2D) 13C low-temperature magic-angle spinning NMR spectra show that carbon monoxide interacting with Lewis (LAS) and Brønsted (BAS) acid sites displays distinct 13C NMR signatures, in line with density functional theory (DFT) calculations. Furthermore, {1H}13C HETCOR NMR allows the observation of surface OH groups interacting with CO and points to the spatial proximity (within 1 nm) of BAS to both Si-OH and Al-OH groups, while LAS predominantly comprises the Al-OH group. These observations suggest the formation of defective aluminum sites with one and/or two OH groups bound to aluminum, upon calcination of a NH4-MOR with Si/Al = 6 at high temperature (580 °C). The NMR protocol discussed is perfectly suited to explore the surface sites of other solid materials.