ZSM-5分子筛上轻烃裂解性能:晶粒尺寸的影响

设计合成了纳米、亚微米和微米级晶粒尺寸ZSM-5分子筛,并研究了其在两种反应温度下(510,650℃)对正庚烷催化制低碳烯烃反应行为。结果表明,在反应初始阶段,两种反应温度下晶粒尺寸对正庚烷转化率和产物选择性影响较小。但随着反应进行,纳米和亚微米ZSM-5在510℃下反应性能(低碳烯烃选择性及反应活性稳定性)相近且均高于微米ZSM-5;而650℃下,具有更短扩散路径和更大外表面积的纳米ZSM-5则体现出更高的反应性能。微米ZSM-5在两种温度下虽具有相对较高的低碳烯烃选择性,但其活性稳定性最低。进一步研究晶粒尺寸对费-托过程中石脑油催化裂解性能的影响发现,亚微米ZSM-5表现出最高的催化反应性能,这可能与反应原料的组成及相关反应途径变化有关。     

NO oxidation kinetics on iron zeolites: influence of framework type and iron speciation

Topics in Catalysis - Zeolites having MFI, FER and *BEA topology were loaded with iron using solid state cation exchange method. The Fe:Al atomic ratio was 1:4. The zeolites were characterized...     

The influence of stress and attrition on crystal size distribution

An important parameter influencing the crystal size distribution in mass crystallization from solution is the mechanical stress exerted on crystals in the crystallizer. This contribution presents the study of the influence of mechanical stress and attrition of the system potassium nitrate-water in an FC-crystallizer and in various draft-tube crystallizers, fitted with different types of impellers. The intensity of stress is a newly defined variable which is used to describe the level of stress in crystallizers. The reduction of crystal size by attrition is described by the linear attrition rate. The influence of impeller design and crystal hold-up on crystal size distribution and scale-up rules is discussed.     

Influence of crystal size and probe molecule on diffusion in hierarchical ZSM-5 zeolites prepared by desilication

The improvement of molecular transport properties of hierarchical H-ZSM-5 obtained by desilication was evidenced by studying the desorption of o-xylene and isooctane by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This technique enabled monitoring simultaneously bands associated with the molecular probes and the zeolite, using powdered sample masses as low as 1 mg. Two H-ZSM-5 samples with markedly different crystal sizes were investigated. The first sample was commercial and consisted of small crystallites (ca. 250 nm). The second sample were laboratory-made large crystals with coffin-like shape (ca. 17 × 4 × 4 μm³). The hierarchical derivatives of the small and large zeolite crystals displayed 250 and 120 m² g⁻¹ of mesopore surface area, respectively, in contrast to the 62 and 5 m² g⁻¹ of the parent counterparts. The data based on o-xylene desorption were partly disguised by site-desorption limitations. Desorption experiments using isooctane evidenced a 4-fold reduction in the characteristic diffusion path length on both mesoporous small and large zeolites with respect to their purely microporous analogues. These results confirm the substantial potential for improvement of commercial nanocrystalline zeolites in diffusion-limited reactions upon the introduction of intra-crystalline mesoporosity by post-synthesis modification.     

Effect of crystal size and surface modification of ZSM-5 zeolites on conversion of ethanol to propylene

The conversion of ethanol to propylene was carried out over ZSM-5 zeolites (Si/Al ratio ≈ 20) with a small crystal size of ca. 30 nm. Catalyst deactivation was significantly suppressed over the nanometer-sized ZSM-5 zeolite, indicating that the small crystal was more tolerant to coking. On the other hand, the selectivity of this zeolite to propylene was lower than that of conventional ZSM-5 zeolites (ca. 2 μm). It was suggested that the large external surface area of the nanometer-sized ZSM-5 zeolite catalyzed undesired reactions. To elucidate the reason for the decreased selectivity, the external surfaces of the nanometer-sized crystals were covered with a very thin pure-silica ZSM-5 layer by a hydrothermal synthesis. The obtained crystal maintained the same crystal size and had a silica-rich surface (Si/Al ratio ≈ 50). After the surface modification, the selectivity to propylene was improved without any decrease in the catalyst life.     

Antiferroelectrics: History,fundamentals, crystal chemistry,crystal structures,size effects,and applications

Antiferroelectric (AFE) materials are of great interest owing to their scientific richness and their utility in high-energy density capacitors. Here, the history of AFEs is reviewed, and the characteristics of antiferroelectricity and the phase transition of an AFE material are described. AFEs are energetically close to ferroelectric (FE) phases, and thus both the electric field strength and applied stress (pressure) influence the nature of the transition. With the comparable energetics between the AFE and FE phases, there can be a competition and frustration of these phases, and either incommensurate and/or a glassy (relaxor) structures may be observed. The phase transition in AFEs can also be influenced by the crystal/grain size, particularly at nanometric dimensions, and may be tuned through the formation of solid solutions. There have been extensive studies on the perovskite family of AFE materials, but many other crystal structures host AFE behavior, such as CuBiP₂Se₆. AFE applications include DC-link capacitors for power electronics, defibrillator capacitors, pulse power devices, and electromechanical actuators. The paper concludes with a perspective on the future needs and opportunities with respect to discovery, science, and applications of AFE.     

Influence of the Si/Al ratio and crystal size on the acidity and activity of HBEA zeolites

Three HBEA samples were prepared through ion exchange followed by calcination under dry air flow at 550 °C from parent samples synthesized with different total Si/Al ratios or crystal sizes and characterized by various techniques. The denomination, Si/Al ratio and crystal size were the following—Cal: 12.5, ∼20 nm; LC: 14.5, 3000–10,000 nm; RAl: 7.0, ∼50 nm. Pyridine adsorption followed by FTIR shows very different values of the Lewis/Brønsted site concentration ratio: 1.2 with Cal, 0.2 with LC and 0.6 with RAl, which indicates a predominant positive effect of the crystal size on the framework stability. Methylcyclohexane transformation at 450 °C was found to occur through both protolysis and the carbenium ion chain mechanism. Protolysis plays a significant role on Cal only. This very difficult reaction which requires very strong protonic sites, most likely sites resulting from interaction of bridging OH groups with neighbouring Lewis sites was 3 and 12 times faster on Cal than on RAl and LC samples, respectively. Furthermore, from the low turnover frequency value of the protonic sites of LC, severe diffusion limitations were demonstrated to occur on this large crystal size sample.     

Mesoporous zeolite single crystal catalysts: Diffusion and catalysis in hierarchical zeolites

During the last years, several new routes to produce zeolites with controlled mesoporosity have appeared. Moreover, an improved catalytic performance of the resulting mesoporous zeolites over conventional zeolites has been demonstrated in several reactions. In most cases, the mesoporous zeolites exhibit higher catalytic activity, but in some cases also improved selectivity and longer catalyst lifetime has been reported. The beneficial effects of introducing mesopores into the zeolites has in most instances been attributed to improved mass transport to and from the active sites located in the zeolite micropores. Here, we briefly discuss the most important ways of introducing mesopores into zeolites and, for the first time, we show experimentally that the presence of mesopores dramatically increases the rate of diffusion in zeolite catalysts. This is done by studying the elution of iso-butane from packed beds of conventional and mesoporous zeolite catalysts. Moreover, we discuss in detail the recent observation of improved activity and selectivity in the alkylation of benzene with ethene using mesoporous zeolite single crystal catalysts. For this reaction, we show by calculation of the Thiele modulus that this improved performance can be mainly attributed to a diffusional limitation of ethylbenzene in the zeolite pores. This is verified in new ethylbenzene dealkylation experiments where mesoporous zeolite catalysts show significantly improved activity over conventional zeolite catalysts.     

高温氧化铝原晶粒度与其需水性的关系

运用原晶粒度的概念并通过对原晶粒度、流动度的检测 ,对高温氧化铝的需水性进行了研究和探讨。研究认为 :高温氧化铝粉的原晶粒度在本质上影响着其使用时的需水性 ,原晶粒度大的 ,所制备料浆的流动性也大 ,即需水性低 ;而其原晶粒度又受煅烧温度、矿化剂等工艺条件的影响。因此 ,制备低需水性优质高温氧化铝产品的关键是选择适宜的工艺条件来控制产品的原晶粒度     

The influence of zeolite type A on metal concentrations in water and waste water

Investigations were made to determine the removals of calcium, copper, chromium, nickel, lead and zinc from water and waste water samples by different concentrations of zeolite type A and with zeolite type A forms where sodium had been partially exchanged for calcium. Metal concentrations were determined after a range of different treatments such as filtration, sedimentation in columns and centrifugation. The results indicated that the rate of removal of calcium hardness from samples was a function of the calcium hardness present in the samples. High concentrations of zeolite removed all the metals except chromium from water, but only lead, zinc and cadmium from synthetic waste water samples. In water samples containing cadmium, nickel, copper and zinc, an increase in the calcium exchanged for sodium in the zeolite resulted in a decrease in the percentage of nickel removed from suspension on mixing. At lower environmental concentrations, zeolite type A had no effect on the removal of metals from raw waste water samples.     

Kinetic studies on sorption of n-hexane in vanadium substituted MFI type zeolites of various crystal morphologies

The MFI type materials isomorphously substituted with vanadium form crystals of two morphology types. Investigations of sorption kinetics for n-hexane indicated for both morphologies a non-typical increase in the value of corrected transport diffusion coefficient with the crystals dimensions. An increase in the D₀ values with the vanadium content of the crystals has also been found, although it is not so well expressed as that with the dimensions. The increase in the D₀ values is from 1.1 × 10⁻¹¹ to 1.1 × 10⁻¹⁰ m²/s and may be a consequence of an additional system of larger pores, which is not reflected in the adsorption isotherms due to common occurrence of these pores in all crystals. It is also possible that vanadium causes a superior structure ordering and a decrease in/weakening of diffusion barriers.     

反应料浆含水量对重钙生产的影响及其对策

对料浆法重钙生产中反应料浆含水量偏高的原因及其对生产的影响进行了分析 ,并针对带入水分的不同环节 ,提出了相应的整改措施。其中 ,根据贵州瓮福磷矿的反应活性 ,提出取消二次磨矿的可能性 ,对改善反应料浆的含水量具有重要意义。通过整改 ,使重钙产量由 2 2 .7t/ h提高到 5 5 .7t/ h     

Catalyzed gasification of active carbon by oxygen: influence of catalyst type,temperature, oxygen partial pressure and particle size

A thermogravimetric study of the oxygen gasification of a commercial active carbon using Co, Ni and Cu as catalysts has been carried out. This follows a previous study of the influence of the particle size, gas volumetric flow rate, initial sample weight temperature and oxygen partial pressure an non‐catalytic gasification. The results show that for T < 500 °C; P₀₂ < 1 atm; D _p < 0.4 mm, Q _v > 50 cm³ min⁻¹ and M _o < 75 mg the gasification of the active carbon with oxygen is controlled by the chemical reaction. The presence of a catalyst increases in all cases the reaction rate, both temperature and catalyst concentration showing a very positive influence. The activity of the different catalysts follows the order: Co > Cu > Ni. A kinetic study applied to both types of gasification has allowed the activation energy and the reaction order with respect to oxygen and the catalyst to be determined. In all cases both the activation energy and the Arrhenius pre‐exponential factor are lower in the catalytic gasification. This brings about the existence of an isokinetic point for the catalytic and the non‐catalytic processes, which were also determined. © 2000 Society of Chemical Industry     

The influence of size and healing content on the performance of extrinsic self-healing coatings

Among the several approaches for the protection of metallic structures from corrosion, covering with a polymeric coating has attracted more attention due to their convenient application, cost-effective price, and the relatively benign environmental impact. However, the polymeric coatings are sensitive to mechanical/thermal shocks and aggressive environments, leading to damages in the coatings that affect their barrier performance. Self-healing polymeric coatings have introduced remarkable development by extending the service life and reducing maintenance costs, leading to a significant boost in the reliability and durability of the conventional polymeric coatings. Among the different strategies to develop self-polymeric coatings, encapsulating healing agent within micro/nanocapsules, micro/nanofibers, and microvascular systems and incorporating them within the conventional coatings have been widely acknowledged as the most applicable approach. However, several factors, such as the effect of the healing system's size and content, have a significant influence on healing performance. Therefore, this review aims to reveal the effects of healing system size and healing content on the self-healing performance in polymeric coatings through the analysis of recently published articles.     

High niobium oxide content in germanate glasses: Thermal,structural, and optical properties

Niobium alkali germanate glasses were synthesized by the melt‐quenching technique. The ternary system (90‐x)GeO₂–xNb₂O₅–10K₂O forms homogeneous glasses with x ranging from 0 to 20 mol%. Samples were investigated by DSC and XRD analysis, FTIR and Raman spectroscopy, and optical absorption. Structural and physical features are discussed in terms of Nb₂O₅ content. The niobium content increase in the glass network strongly modifies the thermal, structural and optical properties of alkali germanate glasses. DSC, Raman and FTIR analysis suggest niobium addition promotes NbO₆ groups insertion close to GeO₄ units of the glass network. XRD analysis also pointed out that samples containing high niobium oxide contents exhibit preferential niobium oxide‐rich phase after crystallization after heat treatment, which is similar to orthorhombic Nb₂O₅. Absorption spectra revealed high transmission range between 400 nm to 6.2 μm, added to a considerably decreased hydroxyl group content as the addition of niobium in the alkali germanate network. The niobium oxide‐rich phase crystallization process was studied and activation energy was determined, as well as nucleation and crystal growth temperatures and time for obtaining transparent glass‐ceramics.     

Grain size influence on the flexibility and luminous intensity of inorganic CaTiO3:Pr3+ crystal nanofibers

Traditional inorganic materials exhibit rigidity owing to the lack of polymer chains in polymer materials or atom slipping in metals. However, nanometerization has been recently proposed for the conversion of inorganic oxide materials into flexible materials. Herein, the flexible inorganic luminescent material, CaTiO₃:0.2%Pr³⁺, was synthesized through electrospinning, and the macroscopic flexibility of pure inorganic CaTiO₃:0.2%Pr³⁺ was achieved. The flexible membrane was characterized via X-ray diffraction, thermogravimetry, scanning electron microscopy (SEM), and photoluminescence analyses. The grain size was analyzed at various calcination temperatures via SEM, and the results suggested that the increase in the calcination temperature resulted in the growth of crystal grains. Studies have reported that the growth of crystal grains is beneficial for improving the luminescence performance; however, to obtain better flexibility, smaller crystal grains are required. This study provides an important reference for the design of flexible inorganic materials.     

Agglomeration in suspension of salicylic acid fine particles: influence of some process parameters on kinetics and agglomerate final size

Agglomeration in suspension is a size-enlargement method that facilitates operations of solid processing (filtration, transport, galenic) and preserves the solubilization properties of fine particles. It consists in adding to a suspension of microparticles a small quantity of a second liquid acting as an interparticle binder; in a suitably agitated equipment with a critical quantity of binder, spherical and dense agglomerates of a few millimeters in diameter may be formed. This paper presents a new methodology to study the agglomeration process. The system [salicylic acid/aqueous solution/chloroform] is chosen as a model system. To follow in situ the agglomerate formation and growth, an original device based on image acquisition and analysis is developed; agglomerate porosity and compressive strength are also measured. These measurements allow us to identify the influence of the process parameters on the agglomeration kinetics, the size and the compressive strength of the final agglomerates. They also give interesting insights into the mechanisms.