Zeolite synthesis using a semicontinuous reactor,Part 2: Synthesis at high nucleation rates

Following an earlier study of controlled monolithic crystal growth in a semicontinuous reactor system, the present report is concerned with the effect of allowing the nucleation rate to rise. The initial result is a broadening of the crystal size distribution; however, under suitable conditions at highly enhanced nucleation rates a product morphology of high surface area polycrystalline aggregates can be sustained. The trends observed can be rationalized in terms of (1) the rate of nutrient supply; (2) the crystal surface area available for mass deposition; and (3) the chemical limitations to growth at the crystal surface. The M,TPA-ZSM-5 synthesis system has been used as a model for these investigations. However, the same principles will apply in other cases provided that reaction parameters can be suitably chosen and the product is stable to phase transformation under the synthesis conditions.     

Zeolite ZSM-20: synthesis and characterization through XRD,SEM and thermal analysis

Zeolite ZSM-20 was synthesized without contaminations of the co-crystallizing zeolite beta. Phase purity was checked by X-ray powder diffraction and scanning electron microscopy. Thermal analysis (t.g./d.t.g./d.t.a.) using nitrogen or air flow gave valuable information about the state of the organic template in the zeolite framework and guidelines for its removal from the zeolite pores prior to the use of ZSM-20 as a catalyst.     

Temperature-programmed desorption of hydrocarbons from ZSM-5, ZSM-11, and THETA-1: Part II. p-xylene and benzene

p-Xylene and benzene have been adsorbed at room temperature and relative pressure $\text{p}\text{p}{}_0\text{∼ 0.5}$ on ZSM-5, ZSM-11, and THETA-1 with low $\text{Si}\text{Al}$ ratios of 15, 15, and 32, respectively. The desorption of these sorbates was then followed by temperature-programmed desorption (t.d.p.). The t.p.d. profiles were analyzed using an on-line microcomputer. Both single-heating rate-variable coverage and variable-heating rate-constant coverage methods of analyses have been used. The former method gave more reasonable activation energies and entropies of desorption as a function of coverage. These parameters have been compared with the corresponding values obtained in Part I for n-hexane and n-octane. Desorption of p-xylene from ZSM-5 and ZSM-11 zeolites of varying $\text{Si}\text{Al}$ ratio has also been studied. It was observed that as the $\text{Si}\text{Al}$ ratio increased the amount adsorbed at saturation increased but the activation energy decreased.     

Controlled nucleation and growth kinetics of lead nitrate phosphate in silica hydrogel and characterization of grown crystals

Lead nitrate phosphate (LNP) single crystals were grown in silica hydrogel at an ambient temperature employing various nucleation control methods. The effect of concentration of feed solution, gel density, gel pH, ageing of gels and temperature on reduction of nucleation centres is reported. Kinetic studies of the nucleation and growth of LNP crystals and dentrites were studied. The crystals are characterized by X-ray diffraction, density measurements, magnetic susceptibility, mechanical strength measurements and dissolution behaviour.     

On the crystallization mechanism of zeolite ZSM-5: Part 1. Kinetic compensation effect for the synthesis with some diamines

Zeolite ZSM-5 was synthesized from the initial gel: 33 Na₂O: 44R: Al₂O₃: 100 SiO₂: 4000 H₂O: 25 H₂SO₄, where R is tetrapropylammonium bromide (TPA) and C₂, C₃, C₄, and C₆ diamines are in the temperature range 423–453 K. The kinetic analysis of the crystallization of ZSM-5 permits us to follow the role of the used templates during the consecutive steps of the process. The diminishing of the crystallization rate with the increasing of the linear dimension of the molecule is proved. A kinetic compensation effect (KCE) was established between activation energy and the preexponential factor. The obtained KCE is proof for the uniform mechanism of crystallization of zeolite with C₂, C₃, C₄, and C₆ diamines and for a different one for the synthesis with TPA.     

一步合成核壳结构ZSM-5/EU-1复合分子筛及其表征

在添加适量柠檬酸的条件下采用双模板剂一步法, 一次晶化合成ZSM-5/EU-1核壳共生复合分子筛, 对其物理化学性质进行表征, 结果显示: 核壳分子筛中存在ZSM-5和EU-1两种分子筛晶相, 且EU-1分子筛在ZSM-5分子筛表面缺陷处外延生长; 复合以后核壳分子筛为微孔-介孔多级孔道结构, 且其微孔孔径达到0.94 nm; 相比于单一分子筛, 核壳分子筛的强酸量相对提高, 酸性质有所改善。通过对柠檬酸加入量、晶化时间影响因素的分析, 得到合成核壳共生结构的最佳条件为: n(citrate)/n(Al₂O₃)=0.8~1.2, 晶化时间为72~96 h, 并推测核壳分子筛可能的生长过程。     

双模板剂B-EU-1/ZSM-5复合分子筛的快速合成及催化应用

以不同氢氧化物做碱源合成B-EU-1/ZSM-5复合分子筛, 并比较各平衡阳离子的结构导向作用及其形貌特征, 发现钠离子的导向作用最强, 且合成复合分子筛的表面较光滑、无明显晶面界限。对合成样品进行了TG-DTG、N₂吸附-脱附和NH₃-TPD分析。结果表明: 双模板剂一步法合成的复合分子筛模板剂脱除的失重率为5.67%, 小于机械复合分子筛的失重率7.31%, 复合分子筛的比表面积、孔容和微孔平均孔径均有所增大, 同时其酸强度、酸量都增大, 有利于二甲苯优先从复合分子筛的孔道结构中扩散出来。将分子筛应用于甲醇转化制二甲苯催化反应, 结果显示, 双模板剂一步法合成的复合分子筛催化产物油相中芳烃的选择性最高达到66.72%, 芳烃中二甲苯的含量为46.15%, 二甲苯中对二甲苯的含量达到30.75%。这是由于B-EU-1/ZSM-5催化剂的特殊孔道结构具备择形效应, 使得分子动力学直径较小的二甲苯分子优先从其孔道中扩散出来。     

甲醇定向转化制二甲苯的复合分子筛ZSM-5/EU-1的合成及其应用

以硅溶胶、硫酸铝、氢氧化钠、四丙基溴化铵以及硼酸和九水硝酸铁为原料, 采用EU-1晶种, 通过水热法合成了2种含杂原子的微孔结构ZSM-5/EU-1复合分子筛。采用XRD、SEM、N2吸附–脱附和NH3-TPD对合成样品进行了表征, 并在连续流动固定床反应器上考察了该分子筛催化剂对甲醇制二甲苯反应的催化性能。结果表明: 含杂原子微孔结构的ZSM-5/EU-1复合分子筛同时具有ZSM-5和EU-1的特征衍射峰, 结晶度较好, 平均孔径明显增大, 当催化剂的酸强度和酸量增大时, 有利于甲醇芳构化趋势以及二甲苯优先扩散。B-ZSM-5/EU-1催化产物中芳烃在油相中的选择性达到84.70%; B-ZSM-5/EU-1和Fe-ZSM-5/EU-1两种分子筛催化产物的芳香烃中二甲苯的含量最高, 分别为41.32%~45.88%和33.88%~39.16%。由于B-ZSM-5/EU-1较高的酸性、酸量和相对较小的孔道内径(0.8060 nm), 较Fe-ZSM-5/EU-1更有利于产物中二甲苯的生成; 不仅如此, B-ZSM-5/EU-1复合分子筛催化产物中对二甲苯在二甲苯中选择性范围为29.75%~47.47%; 不过, Fe-ZSM-5/EU-1产物中对二甲苯在二甲苯中的含量最高可达53.75%, 这是由于Fe-ZSM-5/EU-1复合分子筛的粒径较大, 为催化反应提供了较长的孔道结构, 易使产物中邻、间二甲苯在扩散过程中异构化转化为对二甲苯的缘故。     

Facile and selective approach towards synthesis of a series ZSM-5/ZSM-12 catalysts for methanol to hydrocarbons reactions: Applying different synthesis driving force and conditions

The use of ultrasonic waves was developed for synthesis of zeolites at shorter crystallization time with improving their desirable properties. A series of nanostructured ZSM-5/ZSM-12 composite zeolites with different Si/Al ratios and alkalinity using organic templates were prepared by hydrothermal and sonochemical synthesis methods. The physicochemical properties of synthesized nanocatalysts such as structure, morphology, textural, and acidity were characterized via XRD, FESEM, N₂ physisorption, FTIR, TPD-NH₃, TGA-DTG techniques. The results revealed that increasing the Si/Al ratio and alkalinity in the hydrothermal samples enhanced the crystallization, formation of amorphous microcrystals, and dominant phase of MFI with decreasing MTW competitive phase. Zeolites synthesized by high-temperature and short-time sonochemical method had higher crystallinity, less dominant phase of ZSM-5, smaller crystals, greater surface areas, higher concentration of Brønsted acid sites, and stronger strength of moderate/strong acid sites. The catalytic performance of the zeolites for MTH conversion was evaluated under a reaction temperature of 450 °C at different times on stream. The results showed that the sonochemical zeolite had a higher methanol conversion (100%), higher selectivity toward olefins (28% vs. 19%) with more C₃⁼/C₂⁼ ratio (0.79 vs. 0.58), and lower alkanes selectivity (66% vs. 72%) after 240 min TOS.     

Enhanced nucleation and growth of diamond thin films using a nanodiamond monolayer

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Mixed templates application in ZSM-5 nanoparticles synthesis: Effect on the size,crystallinity, and surface area

ZSM-5 nanoparticles of 50–100 nm size were synthesized by simultaneously adding tetrapropylammonium hydroxide and bromide using the following stoichiometry: NaAlO₂: 25 SiO₂: x TPAOH: y TPABr: 0.033 Na₂O: 1000 H₂O in which x + y is equal to 9.The synthesized samples were characterized by X-ray diffraction, field emission scanning electron microscope, transmission electron microscopy and Brunauer–Emmett–Teller surface area analysis. Impacts of various ratios of tetrapropylammonium hydroxide to tetrapropylammonium bromide, on size, crystallinity and surface were examined and it was found that in 6 tetrapropylammonium bromide to 3 tetrapropylammonium hydroxide mixed templates optimized ratio, the smallest size with the biggest surface and high crystallinity was achieved. Using tetrapropylammonium bromide template only for synthesis of ZSM-5 in high molar stoichiometry ratio (9 tetrapropylammonium bromide) in synthesis gel lead to get amorphous particles because of the synthesis gel sharp pH drop during crystallization.     

The on-line synthesis of enzyme functionalized silica nanoparticles in a microfluidic reactor using polyethylenimine polymer and R5 peptide

A simple microfluidic reactor system is described for the effective synthesis of enzyme functionalized nanoparticles which offers many advantages over batch reactions, including excellent enzyme efficiencies. Better control of the process parameters in the microfluidic reactor system over batch based methodology enables the production of silica nanoparticles with the optimum size for efficient enzyme immobilization with long-term stability. The synthetic approach is demonstrated with glucose oxidase (GOD) and two different nucleation catalysts of similar molecular mass: the natural R5 peptide, and polyethylenimine (PEI) polymer. Near-quantitative immobilization of GOD in the nanoparticles is obtained using PEI; the immobilization is attributed to electrostatic interaction between PEI and GOD. This interaction, however, limits the mobility of the immobilized enzyme, producing orientation hindrance of the enzyme's active sites as compared to free GOD in solution. In contrast, when the GOD is immobilized inside the silica nanoparticles using R5, lower enzyme immobilization efficiencies are obtained compared to using PEI polymers; however, similar Michaelis-Menten kinetic parameters (i.e.?Michaelis constant and turnover number) to those of free GOD are observed. Reactions were monitored in situ using simple, rapid, separation-free amperometric detection.     

Controlled synthesis and growth of perfect platinum nanocubes using a pair of low-resistivity fastened silicon wafers and their electrocatalytic properties

Perfect platinum (Pt) nanocubes with high density have been synthesized by controlled reduction of hexachloroplatinic acid in the presence of H₂SO₄ and HCl, employing a pair of low-resistivity fastened silicon (FS) wafers at room temperature. The presence of the additive charges (induced by prior etching of the silicon surface with HF to remove any SiO₂ layer) between the interfaces of the FS surface results in a high charge density and facilitates fast deposition of Pt nanoparticles via electroless plating. The charge density, stirring time, and homogeneity of the aqueous solution influenced the geometrical shapes of the Pt nanoparticles. The parameters were finely tuned in order to control the nucleation and growth rates and obtain perfect Pt nanocubes. The perfect Pt nanocubes were single crystalline with exposed {100} facets. Per equivalent Pt surface areas, the perfect Pt nanocubes showed enhanced catalytic activity relative to truncated Pt nanocubes or spherical Pt nanoparticles for the electrooxidation of liquid feed fuels such as methanol and ethanol. Moreover, there a strong correlation was observed between the optical, electrical, thermal, magnetic, and catalytic properties of the perfect Pt nanocubes which should lead to a variety of technological applications of these materials.     

Controlled nucleation and growth of surface-confined gold nanoparticles on a (3-aminopropyl)trimethoxysilane-modified glass slide: a strategy for SPR substrates

The thickness of the gold film and its morphology, including the surface roughness, are very important for getting a good, reproducible response in the SPR technique. Here, we report a novel alternative approach for preparing SPR-active substrates that is completely solution-based. Our strategy is based on self-assembly of the gold colloid monolayer on a (3-aminopropyl)trimethoxysilane-modified glass slide, followed by electroless gold plating. Using this method, the thickness of films can be easily controlled at the nanometer scale by setting the plating time in the same conditions. Surface roughness and morphology of gold films can be modified by both tuning the size of gold nanoparticles and agitation during the plating. Surface evolution of the Au film was followed in real time by UV-vis spectroscopy and in situ SPRS. To assess the surface roughness and electrochemical stability of the Au films, atomic force microscopy and cyclic voltammetry were used. In addition, the stability of the gold adhesion is demonstrated by three methods. The as-prepared Au films on substrates are reproducible and stable, which allows them to be used as electrodes for electrochemical experiments and as platforms for studying SAMs.     

Sharp cubic and octahedral morphologies of poly(vinylpyrrolidone)-stabilised platinum nanoparticles by polyol method in ethylene glycol: their nucleation,growth and formation mechanisms

Pt nanoparticles (NPs) were synthesised by a modified polyol method with the addition of silver nitrate. The results showed that the specific shapes of Pt NPs were influenced by the relevant factors, which are the contents of silver nitrate, synthetic time and temperature. A small content of silver nitrate has played an important role in determining their final shapes of platinum NPs. We observed that Pt NPs in the forms of very sharp shapes such as Pt cubes, octahedrons, cuboctahedrons and tetrahedrons have been obtained. In addition, the shape growth mechanisms and formation of Pt NPs have been studied. They exist in both cubic and octahedral shapes. Importantly, Pt nanocrystals can grow into main cubic and octahedral shapes for a short time less than 15?min. Moreover, Pt nanocrystals can also grow into different shapes from cubic and octahedral into spherical ones for several hours. Especially, they exhibited interesting shapes of multiple-branched Pt nanostructures because of their overgrowth and aggregations. Clearly, large cubic and octahedral Pt NPs of 160?nm diameter were observed. The growth and formation of large cubic and octahedral Pt NPs were due to the aggregation of Pt clusters or initial Pt seeds, even small Pt nanocrystals.     

Unified model of nucleation and growth of fatigue cracks. Part 1. Use of force parameters of fracture mechanics of materials in the stage of crack nucleation

According to the phenomenological model of nucleation of a fatigue macrocrack, the process is considered as a two-parameter process. The process is described by the local stress or strain range and a certain linear parameter of the material. We propose the corresponding parameters, namely, the local stress range Δσ _y ^* and the characteristic sized ^* of the prefracture zone. The formation of this zone is caused by the anomaly of the yield strength of the material in subsurface layers, the microstructure, the loading amplitude, the cyclic strain hardening, and the environment. The quantityd ^* is a constant of the material, which is independent of the geometry of notch and specimen. the boundary of the prefracture zone is considered as a macrobarrier that determines the growth of microstructurally short and physically small cracks. The moment when a physically small crack oversteps the boundary of the prefracture zone is defined by the quantitative criterion (a ₀=d ^*) of the initial sizea ₀ of a macrocrack in the material. The proposed dependences of (Δσ _y ^* ,N _i ),N _i ) and (d ^*,N _i ) can be regarded as a basis for the determination of characteristics of resistance of the material to the nucleation of a fatigue macrocrack. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 1, pp. 7–21, January–February, 1998.     

Controlled synthesis and optical properties of BaFBr:Eu2+ crystals via ethanol/water solutions

BaFBr:Eu²⁺ crystals with different structures were successfully fabricated via a simple precipitation method using ethanol/water mixtures as solvents. The amount of ethanol in the solvent mixtures played a significant role in the formation of final products, enabling the well-controlled growth of the BaFBr crystals. A possible formation mechanism was proposed based on the results of controlled experiments. The phases and morphologies of the resulting samples were systematically investigated by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED) and elementary analysis. The optical properties of the annealed BaFBr:Eu²⁺ nano-cuboids were investigated using photoluminescence (PL), photo-stimulated luminescence spectroscopy (PSL) and kinetic decays. Faster decay behaviors demonstrate that these BaFBr:Eu²⁺ phosphors are promising materials for applications in optical storage fields. Furthermore, it is envisaged that this environmentally benign method can be extended to prepare other fluoride halides.