Bromide Hydrate Research Articles

Investigation on the Adhesion Force between Tetrabutylammonium Bromide Hydrate Particles Using Atomic Force Microscopy.

This work investigates the adhesion force between tetrabutylammonium bromide (TBAB) hydrate particles dispersed in decane at different temperatures and TBAB concentrations using an atomic force microscopy. The thickness of the quasi-liquid layer (QLL) on the surface of the hydrate particles is calculated based on an adhesion force model. The results of force measurements indicate that the adhesion force between the hydrate particles increases with increasing temperature when TBAB concentration is 30 wt %. The increment of adhesion force between particles could be due to the increase in the QLL thickness on the particle surfaces. Furthermore, the force results also reveal that the adhesion force between hydrate particles(ice) at 253 K decreases when TBAB concentration increases from 0 to 30 wt %. The calculation indicates that QLL on the surface of formed hydrate particles becomes thinner at higher TBAB concentrations, which could be due to the conversion rate of water to hydrate within particles. The thickness of QLL is directly influenced by the temperature and TBAB concentration, which contributes to the adhesion force between hydrate particles.

Effect of calcium bromide hydrate on the structural and optical properties of polyethylene glycol/niobium pentoxide polymer layers

The blend polymer layers of polyethylene glycol with various Nb2O5 and CaBr2⋅xH2O concentrations were synthesized using the solution-dropping process. X-ray diffraction spectra revealed polyethylene glycol peaks, which could be attributed to the polymers' inherent semi-crystalline phase. The Raman spectra revealed stretching Nb–O bonds and confirmed the Nb2O5 phase. The stretching modes of the CH, CH2, and CH3 groups are presented for polyethylene glycol. Adding more CaBr2⋅xH2O (0–3 wt%) increases the transmittance values to about 0.64 %, 1.18 %, 2.79 %, and 6.29 %. At the same time, the band gap values of the polymer layers go down from 3.0 eV to 2.06 eV. The refractive index values increased from 6.16 (0 wt% CaBr2⋅xH2O) to 7.46 (3 wt% CaBr2⋅xH2O). The low Urbach energy of 837.26 meV for the 2 g polyethylene glycol/4 wt% Nb2O5/1 wt% CaBr2⋅xH2O sample confirms the appropriate concentration of less disorder in the polymer layer.

Nucleation-promoting effect of mixed Zn and ZnO particles on tetra-n-butylammonium bromide hydrate

Tetra-n-butylammonium bromide (TBAB) hydrate is an ionic semi-clathrate hydrate, and it can be used as an energy-storage and gas-separation medium. Nucleation of TBAB hydrate is unlikely to occur and this characteristic causes problems in the refrigeration system due to the need for excessive cooling and delays in gas separation due to the slow formation. Therefore, a technique for rapid formation of TBAB hydrate is required. In a previous study, it was found that addition of mixed and ground Zn and ZnO particles (Zn–ZnO) to TBAB aqueous solution significantly promoted nucleation of TBAB hydrate. Since the nucleation of TBAB hydrates was effectively promoted by the very simple addition of Zn–ZnO, this technique can be easily applied to conventional systems. However, the nucleation-promoting mechanism have not been clarified. The clarification of the nucleation-promoting effect of Zn–ZnO on TBAB hydrates is important to elucidate the nucleation-promoting mechanism of clathrate hydrates by the addition of substances. Therefore, the objective of the present study was to investigate the nucleation-promoting mechanism of Zn–ZnO on TBAB hydrate. Experimental results showed that immersion of Zn–ZnO in TBAB aqueous solution reduced the nucleation-promoting effect, but a decrease in the nucleation-promoting effect was not observed when Zn–ZnO was immersed in water. Microscopy observation confirmed that immersion of Zn–ZnO in TBAB aqueous solution had little effect on its surface morphology, but X-ray diffraction and X-ray photoelectron spectroscopy indicated formation of solid TBAB on Zn–ZnO. Zn–ZnO did not show a significant nucleation-promoting effect on tetrahydrofuran hydrate, which is a non-ionic clathrate hydrate. This suggest that Zn–ZnO has properties that attract tetra-n-butylammonium and Br ions, and that the proximity of these ions may promote formation of TBAB hydrate. It was also suggested that the mechanism of nucleation promotion by the addition of substances is different for ionic clathrate hydrates and non-ionic clathrate hydrates.

Morphology-dependent wrinkled silica-supported Pd catalysts for hydrogenation of furfural under mild conditions

The production of tetrahydrofurfuryl alcohol (THFAL) from biomass-derived furfural (FF) via one-pot hydrogenation is challenging owing to the high hydrogen pressure and temperature requirements of the system. In this study, a hydrothermal method is used to synthesize a series of wrinkled silica (KCC-1) supports having different morphologies and textural properties by modulating the cetylpyridinium bromide hydrate (CPB)/tetraethyl orthosilicate (TEOS) ratio. Subsequently, Pd-supported KCC-1 catalysts are fabricated along with a Pd/SiO2 catalyst (for comparative studies) via a chemical reduction method. Interestingly, the morphological features of the supports, such as the specific surface area, pore size, and Pd anchoring sites, differed significantly for different CPB/TEOS ratios. Among the synthesized catalysts, Pd/KCC 0.3, with the lowest proportion of Pd nanoparticles deposited on the external porous surface, exhibits the highest catalytic activity, owing to its large specific surface area and moderate pore size. Moreover, it shows the highest FF conversion of 100% and THFAL selectivity of 85% in 6 h at 50 °C under 20 bar H2.

The Adhesion Strength of Semi-Clathrate Hydrate to Different Solid Surfaces

The adhesion between a hydrate and a pipe wall is the main cause of hydrate deposition and blockage. In this study, the adhesion strength of semi-clathrate hydrate (tetrabutylammonium bromide hydrate) to four kinds of solid surfaces (E235B carbon steel, E355CC low alloy steel, SUS304 stainless steel, and polytetrafluoroethylene) was measured. This investigation reveals that the adhesion strength of the hydrate to a solid surface is negatively correlated with the wettability of the solid surface, which suggests that hydrophobic materials effectively reduced the hydrate adhesion to the pipe wall. The surface roughness showed different effects on the adhesion of the hydrate to hydrophilic or hydrophobic surfaces. To be specific, when the surface roughness increased from 3.2 µm to 12.5 µm, the hydrate adhesion strength to the hydrophilic surface of SUS304 increased by 123.6%, whereas the hydrate adhesion strength to the hydrophobic surface of polytetrafluoroethylene only increased by 21.5%. This study shows that low wettability and low surface roughness effectively reduce the critical rate required to remove hydrate deposition, which achieves the self-removal of hydrates. At the same time, it was found that the adhesion strength of the hydrate to surfaces increases with increasing subcooling. This investigation holds significant theoretical implications for designing self-cleaning surfaces for oil and gas pipes.

Crystal growth of tetra-n-butylammonium bromide hydrate nucleated from metallic particles

Nucleation of tetra-n-butylammonium bromide (TBAB) hydrate is unlikely to occur; thus, nucleation promotion is required to nucleate TBAB hydrate. In a previous study, it was found that addition of specific metallic particles effectively promotes nucleation of TBAB hydrate. TBAB hydrate is known to have two stable crystal structures (type A and type B). It is expected that the type of TBAB hydrate crystals that initially grow will differ depending on the molecular structure of the precursor liquid phase influenced by the added metallic particles. In this study, the correlation between the type of added metallic particles and the crystal structure of the formed TBAB hydrate was experimentally investigated. Addition of Ag, Ag–AgBr, Zn, and Zn–ZnO particles promoted nucleation of TBAB hydrate. When Ag or Ag–AgBr was added, type-A TBAB hydrate crystals formed from 10 wt% TBAB aqueous solution, where type-B TBAB hydrate should preferentially be formed. In contrast, when Zn or Zn–ZnO was added, only type-B TBAB hydrate crystals formed from 10 wt% TBAB aqueous solution. These results suggest that Ag and Ag–AgBr affect the molecular structure of the liquid phase, facilitating formation of type-A TBAB hydrate crystals.

Electrical conductivity-based assessment method for semi-clathrate hydrate conversion and phase change characteristics in gas capture and energy storage

In gas capture/storage and energy storage, tetrabutylammonium bromide (TBAB) hydrates have been regarded as superior carriers owing to their unique molecular selectivity and high latent heat. Hydrate conversion amount is a significant parameter in evaluating gas selection and cold storage characteristics. This study proposes a method for calculating hydrate conversion amount proposed based on the correlation between electrical conductivity and solution concentration. The correlation index (R2) between electrical conductivity and TBAB solution concentration was above 0.99 at concentrations less than below 5 wt%. The conversion amounts of different hydrate types were obtained using a unique hybrid calculation method. The differences in hydrate conversion amount, conversion ratio, and memory effect were compared at different mass fractions. The results indicated that the hydrate morphology and accumulation patterns were influenced by the TBAB solution ionization degree. Regular hexagonal TBAB hydrate crystals were observed at a low ionization degree. The maximum conversion ratio of TBAB hydrate reached 96% (1 °C/30 wt%). The variation trend of TBAB hydrate phase equilibrium temperature was consistent with that of the electrical conductivity of the solution. The investigation of the memory effect revealed that the phase change process of TBAB hydrate is more predictable in high-concentration initial solutions.

Energy assessment and thermodynamic evolution of a novel semi-clathrate hydrate cold storage system with internally circulating gas bubble disturbance

Low energy storage density, intermittent phase changes, and heat transfer barriers have posed significant challenges in the implementation of hydrate energy storage systems. Based on the heterogeneous nucleation mechanism for tetrabutylammonium bromide (TBAB) hydrate phase change energy storage, a novel cold storage system with internally circulating gas disturbance was constructed for energy evaluation and thermodynamic evolution. The hydrate cold storage efficiency, response time, dynamic driving force, unique force pattern of the coils and guest molecule diffusion were analyzed for the first time. The bubbles generated by gas internal circulation provided numerous nucleation sites for hydrate formation, efficiently promoting the energy storage process. The disturbances caused by the bubbles also rapidly transmitted the energy generated by the phase change. A hydrate storage density of 57.4 kWh/m3 for this system was the maximum among known hydrate storage systems. Notably, a change in solution concentration (from 10 wt% to 40 wt%) resulted in a 2–3-fold increase in the cold charge capacity. Rapid dilution of local solutions and rapid release of cold were the ways to promote the cold discharge. Less damage to the stress structure of the coil was due to the loose accumulation of hydrate particles under gas disturbance. The microbubbles generated by the gas disturbance enabled heterogeneous nucleation and heat-mass transfer in the hydrate cold storage. The gas disturbance-based hydrate energy storage process holds significant guiding value for various applications such as refrigerated transport, building cooling systems, and grid peak shaving.

Multifunctional Amphiphobic Coating toward Ultralow Interfacial Adhesion of Hydrates.

Hydrate adhesion is a challenging issue in some practical applications. However, most current anti-hydrate coatings fail to maintain their properties when subject to crude oil and corrosive contaminants. In addition, the effect of surface properties on the nucleation of hydrates is still unexplored from a microscopic perspective. In this study, a multifunctional amphiphobic PF/ZSM-5 coating consisting of 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane modified ZSM-5 zeolite (F/ZSM-5) and adhesive polyethersulfone was fabricated by the spraying method. The interfacial nucleation and adhesion of hydrates on substrates were studied from a microscopic perspective. The coating exhibited excellent repellencies to various liquids, including water, edible oil, liquid paraffin, vacuum pump oil, n-hexadecane, and crude oil. The tetrabutylammonium bromide (TBAB) hydrate is readily nucleated on the bare Cu surface. In contrast, the coated substrate effectively inhibited the hydrate nucleation on the surface and even reduced the adhesion force to 0 mN/m. Furthermore, this coating was fouling- and corrosion-resistant and can maintain an ultralow hydrate adhesion force even after immersion in crude oil or TBAB solution for 20 and 300 d, respectively. The durable anti-hydrate performance of the coating was attributed mainly to the unique architecture and excellent amphiphobic properties enabling stable air cushions between the solid-liquid interface.

Interplay of Ionic Species in Salts of Homoleptic Quaternary Phosphonium Cations Bearing Linear Biphenyl Moieties

Quaternary phosphonium salts are popular candidates used in many chemical transformations and synthetic chemistry, notably in catalysis. We have examined the single crystals of two bulky phosphonium compounds, tetra([1,1′-biphenyl]-4-yl) phosphonium dicyanamide (C48H36P+·N(CN)2−, compound 1), and tetra([1,1′-biphenyl]-4-yl) phosphonium bromide hydrate (C48H36P+·Br−, CH3CN, H2O, compound 2), and herein report the structural properties for the compounds with an emphasis on the influence of the ion-ion interaction towards self-assembly; the overall self-assembly for both structures is very similar, with subtle differences in the cell parameters. The symmetrical tetra ([1,1′-biphenyl]-4-yl) phosphonium cations in both compounds self-assembled to form robust stacked columns in the solid-state, with voids occupied by anions or solvent molecules. Quantitative examination of intermolecular interactions using Hirshfeld surface analysis found that classical and non-classical hydrogen bonding appears to be the dominant contributor in stabilizing the self-assembly in both cases. The present work can not only benefit in understanding the mutual interaction between the sterically encumbered tetra ([1,1′-biphenyl]-4-yl) phosphonium cations and between counterions, but also provide insights for the self-assembled arrays in the solid-state.

Energy and exergy efficiency analysis for biogas De-CO2 with tetra-n-butylammonium bromide hydrates

The two-stage hydrate-based biogas De-CO2 process in the presence of TBAB was simulated by ASPEN plus software. Energy consumption, sensitivity analysis of key process parameters and exergy of the whole process were calculated and analyzed. The results show that reduce the hydrate formation pressure from 4 MPa to 1 MPa, the energy consumption of the whole process is reduced by 18.1%. When the gas-liquid ratio is reduced from 100 to 40, the stirring energy consumption increases by 134%, and the whole process energy consumption increases by 33.6%. The decrease of CH4 recovery rate has a significant and negative impact on energy consumption efficiency. The exergy loss of hydrate formation and dissociation unit is more than 73%. After optimizing with utilization of process energy, the power generation of product gas is 6.8 kWh/Nm3, the total energy consumption was reduced by 22.3%, and the energy cost was lowered to 0.451 kWh/kg CO2, which has advantages compared with other conventional carbon capture processes.

Performance characteristics of direct contact refrigeration system based on phase change materials and different refrigerants

• Direct contact refrigeration system is built to improve system efficiency. • The problem of nozzle ice blockage is solved with phase change material. • COP of the system is 6.23 by using 35 wt% TBAB. In conventional ice storage refrigeration systems, the evaporator copper tube is immersed in a cold storage tank. Therefore, the thickness of the ice layer increases, the thermal conductivity of the evaporator decreases, resulting in a decrease in the system efficiency. To improve the efficiency of the refrigeration system, a new type of direct contact refrigeration system is designed and developed in this study. The direct contact between the refrigerant and water causes ice blockage in the nozzles and pipes. This is a common problem in direct contact refrigeration systems, and therefore, it is necessary to solve this problem. Four types of working media (R22, R134a, R410A, and HR223) are used to solve the problem of ice blockage in nozzles, and the impact on the system performance is discussed. The results showed that the working medium possesses a large latent heat of phase change, resulting in high system efficiency, however, and ice blockage still exists in the nozzles; the maximum efficiency of the system is 1.68. Water is replaced by phase change material (PCM), and the material is used for heat exchange with the refrigerant. The PCM is tetrabutylammonium bromide hydrate (TBAB), and the performance of TBAB with five mass fraction (25 wt%, 30 wt%, 35 wt%, 40 wt%, and 45 wt%) in the system is analysed. The results indicated that the refrigeration system with TBAB can solve the problem of ice blockage in nozzles. In addition, based on the experimental results, the coefficient of performance (COP) of the refrigeration system with TBAB concentration of 25 wt%, 30 wt%, 35 wt%, 40 wt%, and 45 wt% is 3.11, 4.55, 6.23, 4.83, and 4.46, respectively. A series of experiments demonstrate that TBAB of 35 wt% exhibited the best performance in the direct contact refrigeration system. This paper presents a novel and innovative approach to solve the problems of ice blockage in direct contact refrigeration systems to improve system efficiency.

Heat transfer, energy conversion, and efficiency during cold discharge of a novel tetrabutylammonium bromide hydrate cold storage system

• External melting by internal heat exchange was used to study hydrate cold discharge. • Thermal buffering occurred due to energy balance caused by heat transfer. • Forced convection from gas disturbance reduced cold discharge time by 3 times. • Cold discharge efficiency can be maintained above 83 % under different conditions. • Time-efficiency number was proved to quantify the total cold discharge efficiency. Cold storage technologies using clathrate hydrates as working media have recently attracted more attention in the energy storage field because they undergo phase transitions with large latent heats and suitable temperatures for cold storage. However, the application of hydrate cold storage technologies is limited by the fact that the cold discharge mechanism is still unclear. Thus, this paper adopted the method of external melting by internal heat exchange to investigate the heat transfer, energy conversion and efficiency of a novel tetrabutylammonium bromide (TBAB) hydrate cold storage system during cold discharge. The system consisted of an internal circulating gas disturbance device and a metal spiral hydrate-on-coil heat exchanger. The heat transfer mechanism and cold discharge capacity, rate, and efficiency for different cold charge temperatures, flow rates and disturbance modes were compared and analyzed. A novel time-efficiency number was introduced to evaluate the comprehensive performance of the time, energy conversion and efficiency of cold discharge. The results showed that the heat exchange balance during the cold charge and discharge stages had a thermal buffering effect on the storage unit. Forced convection caused by gas disturbance resulted in a larger cold discharge capacity and reduced the discharge time by a factor of 3 compared to operation without gas disturbance. In addition, the cold discharge capacity gradually increased with decreasing cold charge temperature and increasing flow rate. The cold discharge efficiency was more than 83% under different conditions. A comparison showed that the time-efficiency number tended to be larger with gas disturbance. This study showed that the time-efficiency number can be used for quantitative comparison of the cold discharge time and efficiency in hydrate cold storage systems.

Surface complexation model theory application in NaOL and CTAB collector adsorption differences of diaspore and kaolinite flotation

Although the relevant direct and revers flotation mechanism has been discussed a lot in diaspore and kaolinite separation, the quantitative analysis of surface-active sites and their reaction descriptions during flotation have not been reported yet. In this study, the hydration and collector adsorption reactions of anionic sodium oleate (NaOL) and cationic hexadecyl trimethyl ammonium bromide (CTAB) onto diaspore and kaolinite were quantized. Surface complexation model (SCM) was referred to obtain intrinsic reaction equilibrium constants. It can be found that for NaOL adsorption, the equilibrium constant of diaspore (1013.55) was obviously bigger than that of kaolinite (1011.76 and 1010.35 for ≡AlOH and ≡SiOH, respectively), corresponding to direct flotation of bauxite. While the interaction (103.90 for ≡XNR) between the permanent negatively charged site ≡X− and CTAB was much stronger than other sites. Besides, the single adsorbed amount in mixed minerals were successfully predicted at ratios of 1:2, 1:1 and 2:1. Finally, the fitted potential curves were also acceptable to zeta potential tests. These conclusions from SCM calculation can provide strong supports for collector adsorption differences and then help for separation much more efficiently and economically through saving preliminary exploration cost.

Synthesis and evaluation of green phase change materials for efficient air conditioning by tetrabutylphosphonium phosphate ionic semiclathrate hydrate

A tetrabutylammonium bromide (TBAB) hydrate is the only ionic semiclathrate hydrate that has been commercialized as a medium for efficient thermal energy storage targeting general air conditioning. However, TBAB is an environmental pollutant, while liquid water and ice that have been widely used as conventional media, also have difficulty in increasing the thermal energy storage density as well as the coefficient of performance (COP) of a refrigerator to form solid. In this study, a tetrabutylphosphonium phosphate ((TBP)3PO4) hydrate was proposed as an alternative green medium. The dissociation heat and the phase equilibrium temperature, which were both crucial factors to evaluate the thermal energy storage density, and COP was experimentally determined. It was found that the maximum dissociation heat was 154.1 ± 2.0 kJ･kg−1 at the mass fraction of 0.309. The highest phase equilibrium temperature was 10.0 °C at the mass fractions of 0.290, 0.299, and 0.309. The thermal energy storage density of (TBP)3PO4 hydrates was 3.7 times larger than that of water. (TBP)3PO4 hydrates increased the COP of a refrigerator to form solid by 42% as compared with ice, indicating the potential size-reduction of conventional devices with the decrease in the power consumption of a refrigerator. The effective balance between environmental protection and the system performance through (TBP)3PO4 hydrates was also discussed.

Key factors influencing the kinetics of tetra-n-butylammonium bromide hydrate formation as a cold storage and transport material

Cold energy storage by semi-clathrate hydrates is highly advantageous as they have large latent heat and appropriate phase change temperatures for cooling applications. The knowledge of semi-clathrate hydrate formation kinetics is essential to the process design and optimization for cold storage and transportation. This work investigated the kinetics of tetra-n-butylammonium bromide (TBAB) hydrate formation as a cold storage material using a quantitative method based on electrical resistance measurement. The effects of the initial TBAB concentration in solution (10 wt%−30 wt%) and mixing rate (250 rpm–550 rpm) were examined. As the initial TBAB concentration was increased from 10 wt% to 30 wt%, the induction time was shortened by 89% and the hydrate growth rate was enhanced by more than 5.6 times. Increase of mixing rate from 250 rpm to 550 rpm showed an 84.9% reduction of induction time and more than 44% increase of average hydrate growth rate. The addition of pre-produced hydrate seed into the subcooled TBAB solution immediately induced hydrate nucleation. The type of TBAB hydrates formed was dependent on the type of pre-produced seed. Type B TBAB hydrate seed is recommended to achieve faster hydrate formation kinetics, higher final hydrate fraction and avoid the structural transition from type A to type B hydrates. Finally, an artificial neural network model was developed for sensitivity analysis and revealed that TBAB concentration, temperature and type of seed were the three major contributors to the kinetic performance of TBAB hydrates formation. These results would provide valuable insights for optimizing the hydrate formation process for cold storage and transport applications.

Cu(C2N4H4)2Br2·2H2O: an antiferromagnetic cyanoguanidine coordination compound and its characterization

Abstract Phase-pure copper(II) cyanoguanidine bromide hydrate, Cu(C2N4H4)2Br2·2H2O (1), was precipitated from aqueous solution and its structure was solved and refined from single-crystal X-ray diffraction data at 100 K. 1 crystallizes in space group P21/n with a = 12.09(3) Å, b = 3.925(9) Å, c = 13.79(3) Å, β = 96.62(6)°, Z = 2, and V = 650(2) Å3. The copper(II) cation is coordinated by two cyanoguanidine molecules adopting the cyanoimine shape and four bromide anions in a Jahn–Teller-distorted motif, forming infinite chains of edge-sharing octahedra along the crystallographic b axis. IR spectroscopic and magnetic susceptibility measurements were carried out in addition to density-functional electronic-structure calculations performed to assess both the magnetic ground state and the exchange interactions. Experiment and theory agree as regards antiferromagnetism and weak magnetic exchange.

Experimental study on the thermodynamic performance of a novel tetrabutylammonium bromide hydrate cold storage system

Clathrate hydrates with huge phase change latent heat are rapidly becoming a key means of improving and replacing current cold storage technologies. Efficient hydrate cold storage systems play a pivotal role in achieving carbon peaks. This study proposes a novel hydrate cold storage system, with a hydrate-on-coil component and an internal circulating gas disturbance device- and investigates the cold storage characteristics. In this study, comparative analyses of the tetrabutylammonium bromide (TBAB) hydrate cooling rate, cooling capacity, and energy storage ratio outside the hydrate-on-coil were conducted for various values of the coolant inlet temperature, flow rate, and gas disturbance, and the in situ formation morphology of TBAB hydrates outside the hydrate-on-coil were recorded. The results indicated that internal circulating gas disturbance achieved the double promotion effect of enhanced heat transfer and promoted hydrate phase change, which led to a significant increase in cooling capacity. Gas disturbance boosted the cooling capacity by approximately 20% to more than 80% of the theoretical cooling capacity. A theoretical cooling capacity of 86% was achieved at a storage temperature of 1 °C. Notably, gas disturbance could reduce the cooling capacity due to the increase in coolant temperature and the decrease in flow rate. The hydrate packing factor reached 79.5%, and the system energy storage efficiency was maintained at over 85%. Furthermore, the form of hydrate accumulation outside the hydrate-on-coil was influenced by natural convection and bubble uplift (forced convection). These two types of heat transfer affected the hydrate accumulation pattern and cold storage process. The insights gained from this study may provide important experimental support and a reference for the application and improvement of cold storage technology.

Experimental and theoretical investigation on hydrate nucleation in TBAB droplets

The efficient and rapid formation of natural gas hydrate is of great significance for its large-scale industrial application, which is inseparable from the intensive study of hydrate formation mechanism. Most of the previous studies focused on the growth of hydrate crystals and the effects of different factors on the morphology. However, the research specifically on the nucleation and theoretical analysis of tetra-n-butylammonium bromide (TBAB) hydrate has not been reported. In this work, the nucleation of TBAB hydrate in 2 μL droplets was studied using high-speed photography. Combined with the classical nucleation theory, a model for calculating the Gibbs free energy required for TBAB hydrate nucleation by fugacity is proposed. Through this model and combined with macro experimental phenomena, it is calculated for the first time that the maximum critical nucleation size of A-type TBAB hydrate is about 29(±6) Å and that of B-type TBAB hydrate is about 15.6(±6) Å. This work has great significance for further study of hydrate formation kinetics.

Efficient homogeneous TEMPO-mediated oxidation of cellulose in lithium bromide hydrates

An appropriate cellulose-dissolving solvent is critical for the homogeneous oxidation of cellulose using TEMPO (2, 2, 6, 6-tetramethylpiperidine-1-oxyl)-mediated system. Herein, TEMPO/NaClO/NaClO2 system in lithium bromide hydrates (LBHs) was developed for the homogeneous selective-oxidation of cellulose, which was two-stage protocol involving cellulose dissolution and homogeneous oxidation. Specifically, cellulose was firstly dissolved in LiBr·3.5H2O and offered the optimal pH (5.6) for the subsequent TEMPO/NaClO/NaClO2 oxidation without precipitation of the cellulose chains. Effect of reaction conditions on cellulose oxidation was investigated. The results showed that high degree of oxidation (DO) and evenly distributed carboxyl could be achieved. The particle size gradually decreased with DO, and oxidized cellulose with high water solubility and amorphous structure could be obtained. Furthermore, FT-IR, 13C NMR and fractionation analysis verified that cellulose was successfully converted and the carboxyl uniformly distributed onto the cellulose chains. This TEMPO-mediated system using LBHs as solvent presented an efficient method on the homogeneous selective oxidation of primary hydroxyl in cellulose.