Effect of hydrothermal treatment on properties of Ni-Al layered double hydroxides and related mixed oxides

The Ni-Al layered double hydroxides (LDHs) with Ni/Al molar ratio of 2, 3, and 4 were prepared by coprecipitation and treated under hydrothermal conditions at 180 °C for times up to 20 h. Thermal decomposition of the prepared samples was studied using thermal analysis and high-temperature X-ray diffraction. Hydrothermal treatment increased significantly the crystallite size of coprecipitated samples. The characteristic LDH diffraction lines disappeared completely at ca. 350 °C and a gradual crystallization of NiO-like mixed oxide was observed at higher temperatures. Hydrothermal treatment improved thermal stability of the Ni2Al and Ni3Al LDHs but only a slight effect of hydrothermal treatment was observed with the Ni4Al sample. The Rietveld refinement of powder XRD patterns of calcination products obtained at 450 °C showed a formation of Al-containing NiO-like oxide and a presence of a considerable amount of Al-rich amorphous component. Hydrothermal aging of the LDHs resulted in decreasing content of the amorphous component and enhanced substitution of Al cations into NiO-like structure. The hydrothermally treated samples also exhibited a worse reducibility of Ni²⁺ components. The NiAl₂O₄ spinel and NiO still containing a marked part of Al in the cationic sublattice were detected in the samples calcined at 900 °C. The Ni2Al LDHs hydrothermally treated for various times and related mixed oxides obtained at 450 °C showed an increase in pore size with increasing time of hydrothermal aging. The hydrothermal treatment of LDH precursor considerably improved the catalytic activity of Ni2Al mixed oxides in N₂O decomposition, which can be explained by suppressing internal diffusion effect in catalysts grains.     

Cavitation and pore blocking in nanoporous glasses

In gas adsorption studies, porous glasses are frequently referred to as model materials for highly disordered mesopore systems. Numerous works suggest that an accurate interpretation of physisorption isotherms requires a complete understanding of network effects upon adsorption and desorption, respectively. The present article deals with nitrogen and argon adsorption at different temperatures (77 and 87 K) performed on a series of novel nanoporous glasses (NPG) with different mean pore widths. NPG samples contain smaller mesopores and significantly higher microporosity than porous Vycor glass or controlled pore glass. Since the mean pore width of NPG can be tuned sensitively, the evolution of adsorption characteristics with respect to a broadening pore network can be investigated starting from the narrowest nanopore width. With an increasing mean pore width, a H2-type hysteresis develops gradually which finally transforms into a H1-type. In this connection, a transition from a cavitation-induced desorption toward desorption controlled by pore blocking can be observed. Furthermore, we find concrete hints for a pore size dependence of the relative pressure of cavitation in highly disordered pore systems. By comparing nitrogen and argon adsorption, a comprehensive insight into adsorption mechanisms in novel disordered materials is provided.     

Adsorption properties of ordered mesoporous silicas synthesized in the presence of block copolymer Pluronic F127 under microwave irradiation

Ordered mesoporous silicas (OMSs) were prepared at different temperatures by using tetraethyl orthosilicate (TEOS) as a silica source, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic F127) as a structure directing agent and sodium chloride as an additive under acidic conditions and microwave irradiation. The small angle X-ray diffraction patterns of these samples indicate the presence of ordered mesopores, while adsorption studies show that they possess high volumes of pores, bimodal pore size distributions and large pore sizes. There is an interesting change in the hysteresis loop of nitrogen adsorption isotherms with increasing temperature of hydrothermal treatment; a delayed desorption characteristic for cage-like mesostructures is observed for the OMS samples treated at 100 and 120?°C, while the hydrothermal treatment at 140 and 160?°C leads to the samples having hysteresis loops characteristic for channel-like materials.     

On the hysteresis loop and equilibrium transition in slit-shaped ink-bottle pores

Bin grand canonical Monte Carlo simulations have been carried out to study adsorption–desorption of argon at 87.3 K in a model ink-bottle mesoporous solid in order to investigate the interplay between the pore blocking process, controlled by the evaporation through the pore mouth via the meniscus separating the adsorbate and the bulk gas surroundings, and the cavitation process, governed by the instability of the stretched fluid (with a decrease in pressure) in the cavity. The evaporation mechanism switches from pore blocking to cavitation when the size of the pore neck is decreased, and is relatively insensitive to the neck length under conditions where cavitation is the controlling mechanism. We have applied the recently-developed Mid-Density scheme to determine the equilibrium branch of the hysteresis loop, and have found that, unlike ideal simple pores of constant size and infinite length, where the equilibrium transition is vertical, the equilibrium branch of an ink-bottle pore has three distinct sub-branches within the hysteresis loop. The first sub-branch is steep but continuous and is close to the desorption branch (which is typical for a pore with two open ends); this is associated with the equilibrium state in the neck. The third sub-branch is much steeper and is close to the adsorption branch (which is typical for either a pore with one end closed or a closed pore), and is associated with the equilibrium state in the cavity. The second sub-branch, connecting the other two sub-branches, has a more gradual slope. The domains of these three sub-branches depend on the relative sizes of the cavity and the neck, and their respective lengths. Our investigation of the effects of changing neck length clearly demonstrates that cavitation depends, not only on fluid properties, as frequently stated, but also on pore geometry.     

Modification of titanium phosphate precipitated from titanylsulfate

The titanium phosphate-synthesized sol–gel method was subjected to the mechanochemical treatment in a planetary ball mill Pulverisette 6 in air or water environment or the hydrothermal treatment at 873 K or 1073 K. The properties of such modified/treated samples were examined using the XRF, XRD, TGA, static light scattering, FTIR Raman and adsorption/desorption of N₂ methods. The mechanochemical treatment of xerogel samples leads to a small decrease in water contents in the sample, but the structure remains amorphous. Additionally, during milling of the sample aggregation of titanium phosphate particles and a decrease in specific surface area were observed. The hydrothermal treatment of the xerogel sample leads to a crystal product composed of titanium oxide phosphate hydrate, titanium oxide phosphate and titanium dioxide. The TGA analysis confirms the presence of titanium oxide phosphate hydrate. The hydrothermally treated titanium phosphate samples are monodispersed and have a larger specific surface area than the initial sample.

     

Effect of hydrothermal treatment on the structure, stability and acidity of Al containing MCM-41 and MCM-48 synthesised at room temperature

The effect of hydrothermal treatment of the synthesis gel on the structure, hydrothermal and mechanical stabilities and acidity of MCM-41 and MCM-48 aluminosilicates synthesised at room temperature has been investigated by X-ray diffraction, nitrogen adsorption at 77 K and DRIFTS with pyridine as probe molecule. The influence of the Al content and pore size on the structure of the resulting treated Al-MCM-41 materials has also been studied. For all samples improvement of the structural ordering and increase of the pore size, was observed, with pore wall thickness remaining practically unchanged. For Al-MCM-48 an improvement of the pore size uniformity occurs during the treatment. Only a small loss of pore size uniformity occurred for Al-MCM-41 prepared with hexadecyltrimethylammonium bromide, but with samples prepared with tetra and octadecyltrimethylammonium bromide the treatment generated a bimodal pore size distribution. The pore volume increased (17%) in the case of Al-MCM-48 but decreased (5.5–14%) for Al-MCM-41, suggesting a decrease in surface roughness resulting from increase of the degree of condensation of the pore walls. Both treated and untreated samples presented relatively strong Brønsted sites and increase of the Lewis acidity was found to occur upon treatment. Treated samples were found to be more resistant to refluxing in boiling water and mechanical compaction, which was attributed to more polymerised pore walls, with Al-MCM-41 samples tested demonstrating higher stability than Al-MCM-48. However, the differences in stability of samples prepared with or without hydrothermal treatment were not significant. Both treated and untreated samples presented high hydrothermal stability. Although refluxing in boiling water lead to some loss of structural ordering, only a small decrease of pore volume (3–5.5% for Al-MCM-41 and 8-14% for Al-MCM-48) occurred, with practically no alterations in pore size and wall thickness. Ordered mesopore structure, with narrower pores and thicker walls, was still observed after compression at 590 MPa for most of the samples tested.     

Systematic Experimental Study on Quantum Sieving of Hydrogen Isotopes in Metal-Amide-Imidazolate Frameworks with narrow 1-D Channels

Quantum sieving of hydrogen isotopes is experimentally studied in isostructural hexagonal metal-organic frameworks having 1-D channels, named IFP-1, −3, −4 and −7. Inside the channels, different molecules or atoms restrict the channel diameter periodically with apertures larger (4.2 Å for IFP-1, 3.1 Å for IFP-3) and smaller (2.1 Å for IFP-7, 1.7 Å for IFP-4) than the kinetic diameter of hydrogen isotopes. From a geometrical point of view, no gas should penetrate into IFP-7 and IFP-4, but due to the thermally induced flexibility, so-called gate-opening effect of the apertures, penetration becomes possible with increasing temperature. Thermal desorption spectroscopy (TDS) measurements with pure H₂ or D₂ have been applied to study isotope adsorption. Further TDS experiments after exposure to an equimolar H₂/D₂ mixture allow to determine directly the selectivity of isotope separation by quantum sieving. IFP-7 shows a very low selectivity not higher than S=2. The selectivity of the materials with the smallest pore aperture IFP-4 has a constant value of S≈2 for different exposure times and pressures, which can be explained by the 1-D channel structure. Due to the relatively small cavities between the apertures of IFP-4 and IFP-7, molecules in the channels cannot pass each other, which leads to a single-file filling. Therefore, no time dependence is observed, since the quantum sieving effect occurs only at the outermost pore aperture, resulting in a low separation selectivity.     

Ordered mesoporous silica with large cage-like pores: structural identification and pore connectivity design by controlling the synthesis temperature and time

FDU-1 silicas with large cage-like pores (diameter about 10 nm) were synthesized under acidic conditions from tetraethyl orthosilicate in the presence of a poly(ethylene oxide)-poly(butylene oxide)-poly(ethylene oxide) triblock copolymer template B50-6600 (EO(39)BO(47)EO(39)). High-resolution transmission electron microscopy and small-angle X-ray scattering provided strong evidence that FDU-1 silica synthesized under typical conditions is a face-centered cubic Fm3m structure with 3-dimensional hexagonal intergrowth and is not a body-centered cubic Im3m structure, as originally reported. Samples synthesized in a wide range of conditions (initial temperatures from 298 to 353 K; hydrothermal treatment at 333-393 K) exhibited similar XRD patterns and their nitrogen adsorption isotherms indicated a good-quality cage-like pore structure. The examination of low-pressure nitrogen adsorption isotherms for FDU-1 samples, whose pore entrance diameters were evaluated using an independent method, allowed us to conclude that low-pressure adsorption was appreciably stronger for samples with smaller pore entrance sizes. This prompted us to examine low-pressure adsorption isotherms for a wide range of samples and led us to a conclusion that the FDU-1 pore entrance size can be systematically enlarged from about 1.3 nm (perhaps even lower) to at least 2.4 nm without an appreciable loss of uniformity by increasing the temperature of the hydrothermal treatment or the initial synthesis. Further enlargement of pore entrance size was achieved for sufficiently long hydrothermal treatment times at temperatures of 373 K or higher, as seen from the shape of nitrogen desorption isotherms. This allowed us to obtain samples with uniform pore sizes, high adsorption capacity, and with pore entrances enlarged so much that their size was similar to the size of the pore itself, resulting in a highly open porous structure. However, in the latter case, there was evidence that the pore entrance size distribution was quite broad.     

晶化温度对SAPO-34结构稳定性的影响

用二乙胺结构导向剂, 分别在473和573 K水热晶化得到两种SAPO-34分子筛材料. SEM和XRD结果表明, 晶化温度升高, SAPO-34晶粒增大, 结晶度提高, 比表面积和孔径增大. 升高晶化温度, 提高了样品的最高焙烧和水蒸汽热处理承受温度.     

水热处理对白石湖煤热解过程中钠钙迁移行为的影响

采用高压釜对富镜质组白石湖煤进行了水热处理,通过傅里叶红外光谱(FT-IR)分峰拟合方法分析了煤中含氧官能团变化。在固定床中开展分离废液和未分离废液水热处理煤样的热解实验,利用原子吸收分光光度计(AAS)测定了热解产物中钠钙含量。结果表明,白石湖煤经水热处理后水分、挥发分、氧含量以及Cl、Na和Ca等无机元素含量明显降低。水热处理过程中芳香醚水解和羧酸盐发生离子交换反应使得氢含量和H/C原子比增加,促进固定床热解焦油产率升高。300℃下水热处理脱除了部分有机形式Ca后随水废液被分离;废液中钠钙等无机元素的催化作用导致未分离废液较分离废液的样品具有更高热解气产率和更低焦油产率。白石湖原煤及其水热处理样品热解产物中Na含量和分布由高到低顺序均为:热解焦热解水焦油热解气,Ca含量和分布顺序为:热解焦焦油热解水热解气。水热处理温度越高,热解过程钠和钙释放率越低,释放的Na主要进入热解水,其次焦油;而释放的Ca则主要分布于焦油中,其次热解水。     

High-temperature synthesis of highly hydrothermal stable mesoporous silica and Fe-SiO2 using ionic liquid as a template

Mesoporous silicas and Fe-SiO₂ with worm-like structures have been synthesized using a room temperature ionic liquid, 1-hexadecane-3-methylimidazolium bromide, as a template at a high aging temperature (150-190 °C) with the assistance of NaF. The hydrothermal stability of mesoporous silica was effectively improved by increasing the aging temperature and adding NaF to the synthesis gel. High hydrothermally stable mesoporous silica was obtained after being aged at 190 °C in the presence of NaF, which endured the hydrothermal treatment in boiling water at least for 10 d or steam treatment at 600 °C for 6 h. The ultra hydrothermal stability could be attributed to its high degree of polymerization of silicate. Furthermore, highly hydrothermal stable mesoporous Fe-SiO₂ has been synthesized, which still remained its mesostructure after being hydrothermally treated at 100 °C for 12 d or steam-treated at 600 °C for 6 h.     

The properties of gold catalysts precursors adsorbed on the MCM-41 materials modified with Mn and Fe oxides

Silica mesoporous materials modified with manganese and iron were obtained by the hydrothermal method. Gold was introduced to pure and modified silica materials by the direct hydrothermal and impregnation methods. Nitrogen adsorption/desorption studies evidenced formation of the materials with large total surface area and mesoporous structure. Unmodified silica materials showed regular pore arrangement. The uniform porous structure was distorted in the iron or manganese containing samples. XRD, UV-Vis/DRS spectroscopy and temperature programmed reduction studies revealed changes of the nature of transition metal oxide and gold species on the different preparation stages. The oxide species after drying were strongly dispersed and partially incorporated to the silica framework. High temperature treatment led to the formation of extraframework Mn and Fe oxide species. Complex processes of gold deposition were observed during hydrothermal synthesis and impregnation of modified silica materials. The increase of the size of gold species was observed during calcination. The presence of transition metal oxides decreased sintering of gold crystallites.     

分子在ITQ-3分子筛窄孔道内扩散的过渡态理论模型

建立了一个基于过渡态理论的分子在ITQ-3窄孔道方向扩散的模型. 该模型中, 由于分子从空腔中的一个吸附位越过势垒到相邻的另一个空腔中的吸附位需要克服很大的势垒能, 因而分子在势垒处可以简化处理为只存在排斥势, 可得到扩散系数依赖温度和Lennard-Jones作用参数的解析关系. 用分子动力学方法对CF₄在ITQ-3上扩散进行了模拟并验证了解析关系的合理性. 分子动力学模拟计算得到的扩散活化能、势垒能和吸附位势能与实际值相吻合. 模拟结果也显示了扩散系数依赖于附载量, 表现为先增大后减小的变化模式. 扩散活化能的计算证实了这一变化机理, 即当附载量增加时, 由于处于空腔中的分子彼此抬高了势能, 降低了扩散活化能, 使得扩散系数随附载量的增加而增大, 之后由于堵塞效应, 扩散系数随附载量的增加而逐步减小.     

水热提质对褐煤理化结构及复吸水特性的影响

选取云南昭通褐煤进行了水热提质处理,依托傅里叶变换红外光谱技术和氮气等温吸附模型分析研究了不同提质温度(150-300℃)下煤样表面含氧官能团和孔隙结构的演化行为,考察了复吸水特性的变化规律。结果表明,随着提质温度的不断升高,主要亲水性含氧官能团(-OH、-COOH、C=O)不断减少;煤样的孔隙结构发生显著改变,比表面积和总孔容积先增后减,在250-300℃水热条件下有焦油产生;受理化结构协同影响的复吸水特性得到了有效遏制,复吸水率随着提质温度的升高不断下降;对提质褐煤进行储存时,环境湿度是影响褐煤复吸水的关键因素,高温水热、低温低湿度储存更能提高褐煤的提质效果。     

One-step synthesis of hydrothermally stable mesoporous aluminosilicates with strong acidity

Using tetraethylorthosilicate (TEOS), polymethylhydrosiloxane (PMHS) and aluminium isopropoxide (AIP) as the reactants, through a one-step nonsurfactant route based on PMHS-TEOS-AIP co-polycondensation, hydrothermally stable mesoporous aluminosilicates with different Si/Al molar ratios were successfully prepared. All samples exclusively showed narrow pore size distribution centered at 3.6 nm. To assess the hydrothermal stability, samples were subjected to 100 °C distilled water for 300 h. The boiled mesoporous aluminosilicates have nearly the same N₂ adsorption-desorption isotherms and the same pore size distributions as those newly synthesized ones, indicating excellent hydrothermal stability. The ²⁹Si MAS NMR spectra confirmed that PMHS and TEOS have jointly condensed and CH₃ groups have been introduced into the materials. The ²⁷Al MAS NMR spectra indicated that Al atoms have been incorporated in the mesopore frameworks. The NH₃ temperature-programmed desorption showed strong acidity. Due to the existence of large amount of CH₃ groups, the mesoporous aluminosilicates obtained good hydrophobicity. Owing to the relatively large pore and the strong acidity provided by the uniform four-coordinated Al atoms, the excellent catalytic performance for 1,3,5-triisopropylbenzene cracking was acquired easily. The materials may be a profitable complement for the synthesis of solid acid catalysts.     

Effect of Single and Dual Hydrothermal Treatments on the Resistant Starch Content and Physicochemical Properties of Lotus Rhizome Starches

Heat-moisture treatment (HMT) changed the morphology and the degree of molecular ordering in lotus rhizome (Nelumbo nucifera Gaertn.) starch granules slightly, leading to some detectable cavities or holes near hilum, weaker birefringence and granule agglomeration, accompanied with modified XRD pattern from C- to A-type starch and lower relative crystallinity, particularly for high moisture HMT modification. In contrast, annealing (ANN) showed less impact on granule morphology, XRD pattern and relative crystallinity. All hydrothermal treatment decreased the resistant starch (from about 27.7–35.4% to 2.7–20%), increased the damage starch (from about 0.5–1.6% to 2.4–23.6%) and modified the functional and pasting properties of lotus rhizome starch pronouncedly. An increase in gelatinization temperature but a decrease in transition enthalpy occurred after hydrothermal modification, particularly for hydrothermal modification involved with HMT. HMT-modified starch also showed higher pasting temperature, less pronounced peak viscosity, leading to less significant thixotropic behavior and retrogradation during pasting-gelation process. However, single ANN treatment imparts a higher tendency of retrogradation as compared to native starch. For dual hydrothermally modified samples, the functional properties generally resembled to the behavior of single HMT-modified samples, indicating the pre- or post-ANN modification had less impact on the properties HMT modified lotus rhizome starch.     

以水热法合成的ZrO_2负载Au催化剂上的低温水煤气变换反应

采用一种简便的水热法合成了一系列ZrO2,并采用沉积-沉淀法制得相应1.0%Au/ZrO2催化剂,在模拟甲醇重整气气氛下评价了它们的低温水煤气变换(WGS)反应催化性能.结果发现,于150oC水热合成的ZrO2负载的Au催化剂活性最佳,240oC反应时CO转化率达87%,明显高于相同反应条件下Au负载量较高的Au/Fe2O3,Au/CeO2及Au/CeZrO4催化剂.采用X射线衍射、原子吸收光谱、N2物理吸脱附及扫描电子显微镜等手段对样品进行了表征.结果表明,Au/ZrO2催化剂的总孔体积及平均孔径越大、圆形片状形貌越规整,其低温WGS催化活性就越高.     

Temperature dependence of the diffusion coefficient of copper(II) chloride in porous-glass membranes

Diffusion transport of a 0.5 M aqueous solution of CuCl₂ was studied in a set of porous-glass membranes with predominant pore radii of 4.5 to 70 nm in the temperature range 25–70°C. Raising the temperature causes a rise in diffusion coefficients and their gradual equalization, which eliminates their dimension dependence on the radius of membrane channels.     

Adsorption/desorption hysteresis in inkbottle pores: a density functional theory and Monte Carlo simulation study

The mechanisms of adsorption and desorption in inkbottle-shaped pores are considered for lattice models using grand canonical mean field density functional theory and Monte Carlo simulation. We find that they depend significantly on the particular pore geometry, the nature of the fluid-solid interaction, and the temperature. We find two mechanisms for desorption. One mechanism involves the emptying of the main cavity even as the density of fluid in the necks remains high, a mechanism observed recently in studies of an off-lattice model of an inkbottle. The other is a simultaneous desorption from the entire pore space, behavior that is more closely related to the traditional picture of pore blocking in the inkbottle system.