Modification of the Porosity of Pillared Clays by Carbon Deposition II. Hydrocarbon Cracking

To enhance the adsorption selectivity of pillared clays an attempt is made to modify the initial pore size by a controlled deposition of carbon. Cracking of hydrocarbons in the pores of the pillared clay results in coke deposits which can alter the pore size.Based on the evaluation of the amount of coke, the coke density, the decrease in micropore volume, changes in the micropore size distribution and the acidity it was possible to distinguish between pore-blocking, pore-filling and pore-narrowing effects.The modification mechanisms strongly depend on the initial porous structure, the acidity (Brönsted/Lewis), the cracking conditions (static or flow) and the hydrocarbons used.The carbon deposition results in a decrease in pore volume due to pore-filling (Ti-PILC) and pore-blocking (Al-PILC) without achieving a controlled pore-narrowing but some indications for pore-entrance narrowing were found.     

Porosity and Thermal Stability of Montmorillonite Pillared with Mixed Oxides of Lanthanum, Calcium and Aluminium

Montmorillonite intercalated with mixed pillars of lantana and alumina was prepared using a refluxed solution of aluminium chlorohydrate and lanthanum chloride. It is found that there are two main groups of pores in the pillared clays so prepared, one of which is around 8 Å, being close to that in alumina pillared clay (Al-PILC) and the other is about 14 Å. In contrast to the alumina pillared clays, a significantly larger specific surface area and micropore volume remained in LaAl-PILC after heating at high temperatures (700°C and 800°C). It seems that lanthanum ions react with the clay sheets during heating at high temperatures, resulting in a new porous solid phase which is thermally stable but has poor crystallinity. Calcium cations were introduced into the clays during the pillaring process. The calcium, distributed in the micropores of the product, LaAlCa-PILCs, further enhances the resistance to heating at high temperatures. After heating at 800°C, the BET surface area and micropore volume of a LaAlCa-PILC sample remained at about 138 m²/g and 0.05 cc/g, respectively, with a narrow pore size distribution in the micropore region. The influence of lanthanum and calcium on the pore structure as well as the evolution of pore structure in the PILCs upon heating at high temperatures are discussed in detail.     

Pore Structure Tailoring of Pillared Clays with Cation Doping Techniques

Techniques and mechanism of doping controlled amounts of various cations into pillared clays without causing precipitation or damages to the pillared layered structures are reviewed and discussed. Transition metals of great interest in catalysis can be doped in the micropores of pillared clay in ionic forms by a two-step process. The micropore structures and surface nature of pillared clays are altered by the introduced cations, and this results in a significant improvement in adsorption properties of the clays. Adsorption of water, air components and organic vapors on cation-doped pillared clays were studied. The effects of the amount and species of cations on the pore structure and adsorption behavior are discussed. It is demonstrated that the presence of doped Ca²⁺ ions can effectively aides the control of modification of the pillared clays of large pore openings. Controlled cation doping is a simple and powerful tool for improving the adsorption properties of pillared clay.     

Recent Advances in the Control and Characterization of the Porous Structure of Pillared Clay Catalysts

Several methods have been developed and applied in recent years in the characterization of the structure of microporous materials, in general, and of pillared clays, in particular. In the present article, the latest results obtained in the control of the microstructure developed in alumina‐pillared clays and various approaches used to the evaluation of the porosity of these materials are reviewed. These include the control of the microstructure developed in pillared clays by adjusting the several parameters involved in the synthesis process, and the evaluation of the porosity of pillared clays. Emphasis is given to the methods to evaluate the pore size and pore size distribution (PSD) of these materials. Two separate sections are devoted to review mathematical modeling studies and ¹²⁹Xe NMR spectroscopy of physisorbed Xe to investigate the microporous structural properties of these materials.     

Recent Advances in the Control and Characterization of the Porous Structure of Pillared Clay Catalysts

Several methods have been developed and applied in recent years in the characterization of the structure of microporous materials, in general, and of pillared clays, in particular. In the present article, the latest results obtained in the control of the microstructure developed in alumina-pillared clays and various approaches used to the evaluation of the porosity of these materials are reviewed. These include the control of the microstructure developed in pillared clays by adjusting the several parameters involved in the synthesis process, and the evaluation of the porosity of pillared clays. Emphasis is given to the methods to evaluate the pore size and pore size distribution (PSD) of these materials. Two separate sections are devoted to review mathematical modeling studies and ¹²⁹Xe NMR spectroscopy of physisorbed Xe to investigate the microporous structural properties of these materials.     

多孔碳柱撑膨润土的制备与表征

用十六烷基三甲基溴化铵和苄基三甲基氯化铵改性膨润土制备了多孔碳柱撑膨润土.用Fourier变换红外光谱仪、扫描电子显微镜和氮气吸附-脱附、热分析系统地研究了有机改性剂对多孔材料性能的影响.结果表明:膨润土微粒与有机改性剂以共价键和离子嵌入2种形式结合;多孔碳柱撑膨润土的微观形貌呈针片状,最可几孔径分布大约在1.7nm;多孔材料的主要结构是由碳化的大粒子柱撑而构建的二维孔径,烧结后的有机黏土热稳定性大大提高.     

Influence of Heat Treatments on the Pore and Adsorption Characteristics of Sodium Doped Alumina Pillared Bentonite

Various amounts of Na+ ions were exchanged into alumina pillared bentonite (Al-PILB) sample, by controlling the pH of the dispersion of Al-PILB and sodium chloride solution. The Na+ doped pillared clays were calcined at elevated temperatures and adsorption of nitrogen at –196°C, cyclohexane and water at ambient temperature (21 ± 1°C) by the calcined samples were conducted. The results revealed a wide size distribution of the micropores in the pillared clay. Introduction of sodium ions converted the surface of the pore walls from hydrophobic to hydrophilic and blocks some micropores, enhancing water adsorption but reducing nitrogen and cyclohexane adsorption. Existence of Na+ ions in the pores did not improve the thermal stability of the pillared clay. Calcination at high temperatures resulted in a decrease in adsorption capacity. After calcination at 700°C, cyclohexane was inaccessible to the remaining micropores in the Na+ doped pillared clays. The adsorption behavior was clearly related to the cation content as well as the calcination temperature. These results may be useful in developing desiccants and adsorbents from pillared clays for dehumidification and adsorptive cooling applications.     

Pillared clays : preparation and characterization of zirconium pillared montmorillonite

Thermally stable zirconium pillared clays and sulfate modified zirconium pillared clays have been synthetized. Precise control of the preparation parameters allows preparation of solids with controlled morphology, texture and acid properties. The preparation of a new type of split clay is discussed. Introduction of sulfate in the pillar structure produces strongly acid pillared clays. The origin of the acidity is discussed in each case.     

Ion-exchanged pillared clays for selective catalytic reduction of NO by ethylene in the presence of oxygen

Ion-exchanged pillared clays (PILCs) were studied as catalysts for selective catalytic reduction (SCR) of NO by ethylene. Three most important pillared clays, Al₂O₃-PILC (or Al-PILC), ZrO₂-PILC (or Zr-PILC) and TiO₂-PILC (or Ti-PILC), were synthesized. Cation exchanges were performed to prepare the following catalysts: Cu–Ti-PILC, Cu–Al-PILC, Cu–Zr-PILC, Cu–Al–Laponite, Fe–Ti-PILC, Ce–Ti-PILC, Ce–Ti-PILC, Co–Ti-PILC, Ag–Ti-PILC and Ga–Ti-PILC. Cu–Ti-PILC showed the highest activities at temperatures below 370°C, while Cu–Al-PILC was most active at above 370°C, and both catalysts were substantially more active than Cu-ZSM-5. No detectable N₂O was formed by all of these catalysts. H₂O and SO₂ only slightly deactivated the SCR activity of Cu–Ti-PILC, whereas severe deactivation was observed for Cu-ZSM-5. The catalytic activity of Cu–Ti-PILC was found to depend on the method and amount of copper loading. The catalytic activity increased with copper content until it reached 245% ion-exchange. The doping of 0.5 wt% Ce₂O₃ on Cu–Ti-PILC increased the activities from 10% to 30% while 1.0 wt% of Ce₂O₃ decreased the activity of Cu–Ti-PILC due to pore plugging. Cu–Ti-PILC was found to be an excellent catalyst for NO SCR by NH₃, but inactive when CH₄ was used as the reducing agent. Subjecting the Cu–Ti-PILC catalyst to 5% H₂0 and 50 ppm SO₂ at 700°C for 2 h only slightly decreased its activity. TPR results showed that the overexchanged (245%) PILC sample contained Cu²⁺, Cu⁺ and CuO. The TPR temperatures for the Cu–Ti-PILC were substantially lower than that for Cu-ZSM-5, indicating easier redox on the PILC catalyst and hence higher SCR activity.     

Shape selective cracking ofn-octane and 2,2,4-trimethylpentane over an alumina-pillared clay

A mixture ofn-octane (nC₈) and 2, 2, 4-trimethylpentane (224-TMP) was cracked over an alumina-pillared montmorillonite (Al-PILC) acid catalyst as a means of characterising its pore structure and shape selectivity. The shape selectivity of the catalyst was quantified by a time dependent parameter analogous to the constraint index, called the selectivity ratio (SR), and defined as log (fraction of nC₈ remaining)/log (fraction of 224-TMP remaining). The SR was measured over Al-PILC, a 0.16 wt% Pt loaded Al-PILC, a Y zeolite, and an amorphous silicaalumina. The SR was greater over Al-PILC compared with Y-zeolite even though the pore openings of the Al-PILC were of similar or larger size than those in the Y-zeolite. Doping AlPILC with Pt caused a large increase in the SR. As found with constraint index measurements, internal pore dimensions and reaction mechanisms appear to be the determinants of SR. However, SR is better capable of discriminating larger pore microporous materials.     

Poly(l-lactic acid) short fibers prepared by solvent evaporation using sodium tripolyphosphate

A porous material consisting of biodegradable polymer fibers may be one of the best candidates for implants used in the regeneration of damaged tissue, because it has a continuous pore structure that would allow ingrowth of nutriments, tissues, blood vessels or cells. In the present work, short fibers of biodegradable poly(l-lactic acid) (PLLA) were successfully prepared by the dropwise addition of PLLA dissolved in methylene chloride to a poly(vinyl alcohol) (PVA) solution containing sodium tripolyphosphate with stirring. It was suggested that droplets of the PLLA solution form spheres coated with PVA, which are then deformed into fibrous shapes due to stirring. The length of fibers was 200-800 μm and was controlled by the stirring rate, the PLLA concentration of the droplets and the PVA concentration. A PLLA porous block could be easily prepared by sintering the PLLA fibers at 173 °C for 10 min. The material had a continuous pore structure with the average pore size of approximately 40 μm and porosity of about 80%.     

碳化物衍生碳及其在吸附领域中的应用研究进展

碳化物衍生碳（CDC）是近年来开发的一种具有纳米结构的新型多孔碳材料。该材料是将碳化物前体通过物理（如热分解等）或化学（如卤化等）过程,由表及里逐层刻蚀掉非碳原子得到的,可以实现原子水平上的调控。本文介绍了CDC的制备工艺、性质（孔和形貌）及其在吸附领域中的应用。结果表明,可以通过控制刻蚀温度、碳化物前体、后处理等合成条件实现CDC比表面积大范围可调、孔径及孔径分布等微观结构精确可控,得到从无定形碳到高度有序的石墨、碳纳米管或石墨烯等多种结构。最后指出了解制备过程-结构-性质之间的关系有利于精确调控CDC以满足特定应用的要求。     

Chlorophyll adsorption by alumina-pillared acid-activated clays

Clays, acid-activated to an optimum level, have been pillared with alumina to give semi-crystalline expanded materials with surface acidities, pore volumes and average pore diameters generally higher than those of the corresponding pillared materials derived from a clay matrix not previously acid-activated. The chlorophyll adsorption capacity of the pillared acid-activated materials is significantly greater than that of pillared, nonactivated clays. The procedures used in the preparation of these pillared acid-activated clays (i.e., temperature of pillaring, method of drying and calcination temperature) have a significant influence on chlorophyll adsorption capacity because they influence both the physical and the chemical properties of the final pillared material. This variation provides a useful means of relating the various properties of the pillared materials to the chlorophyll adsorption capacity. As a result, a correlation has been demonstrated between adsorption capacity and a combination of pore volume and number of strong acid sites (of strength pK_a < −1.5) present in the pillared material. Optimal adsorbents were obtained from freeze-dried samples prepared by exchange at 20°C and calcination in air at 500°C.     

Synthesis of Smectites and Porous Pillared Clay Catalysts: A Review

This paper reviews the synthesis of smectites and porous pillared clay catalysts. Synthetic as well as natural smectites serve as precursors for the synthesis of Al, Zr, Ti, Fe, Cr, Ga, V, Si, and other pillared clays as well as mixed Fe/Al, Ga/Al, Si/Al, Zr/Al and other mixed metal/Al pillared clays. The use of these pillared clays in some catalytic reactions is also briefly reviewed.     

Immersion Drying of Wheat Using Al-PILC,Zeolite, Clay,and Sand as Particulate Media

Wheat (Triticum durum) was dried by immersion in a particulate medium grain dryer using natural clay, pillared aluminum clay (Al-PILC), zeolite 13X, and sand as particulate medium at different initial bed temperatures and residence times in the dryer. Results showed that zeolite caused the highest grain moisture loss for a given drying time, followed by Al-PILC, natural clay, and, finally, the sand. The drying capacity of the Al-PILC was similar to that of the zeolite. The Al-PILC transferred an amount of heat equivalent to 85–94% of the heat transferred by zeolite and evaporated 83–93% of the moisture removed by zeolite. Results also showed that the zeolite and the Al-PILC had the best heat and mass transfer properties of the four particulate materials used and that the Al-PILC can be used as an alternative of the zeolite in particulate medium immersion grain drying.     

Immersion Drying of Wheat Using Al-PILC, Zeolite, Clay, and Sand as Particulate Media

Wheat (Triticum durum) was dried by immersion in a particulate medium grain dryer using natural clay, pillared aluminum clay (Al-PILC), zeolite 13X, and sand as particulate medium at different initial bed temperatures and residence times in the dryer. Results showed that zeolite caused the highest grain moisture loss for a given drying time, followed by Al-PILC, natural clay, and, finally, the sand. The drying capacity of the Al-PILC was similar to that of the zeolite. The Al-PILC transferred an amount of heat equivalent to 85-94% of the heat transferred by zeolite and evaporated 83-93% of the moisture removed by zeolite. Results also showed that the zeolite and the Al-PILC had the best heat and mass transfer properties of the four particulate materials used and that the Al-PILC can be used as an alternative of the zeolite in particulate medium immersion grain drying.     

多孔柱撑粘土及其汽油同步脱硫脱氮性能

以钙型蒙脱土为原料,以ZrOCl2×8H2O和Ce(NO3)3为交联改性液的前驱体,制备了锆交联粘土和铈改性锆交联粘土,采用吸附法同步脱除汽油中的含硫和含氮物质,从而得到低硫、低氮清洁汽油. 采用XRD和BET孔结构分析、SEM等方法对制备的多孔柱撑粘土结构进行了表征,并提出了粘土交联改性过程中结构变化的模型. 结果表明,交联后粘土层间距d001值可达2.38 nm,比原土的1.54 nm有显著增大,比表面积和孔容明显增大,对汽油中含氮、含硫化合物的同步脱除率分别达72.8%和58.7%.     

Preparation and Adsorption Properties of Mesoporous Pillared Clays with Silica Sol

New mesoporous silica-pillared montmorillonites were prepared by the post-intercalation of bulky organic cations into silica sol exchanged montmorillonites. Pillared clays with SiO₂-Fe₂O₃ sol were first prepared, followed by the dissolution of iron oxide (or hydroxide) from the pillars with hydrochloric acid. A portion of the interlayered sol particles were then replaced with organic cations which led to a rearrangement of silica sol particles between the silicate layers. The removal of the organics by burning resulted in mesoporous sol pillared clays with an average pore size of more than 20 Å in diameter. The BET specific surface area increased to as high as 950 m²/g in these mesoporous pillared clays.     

Optical and electrochemical properties of titania pillared clays

The silicate layers of clay were pillared with TiO₂ sol particles. The sol particles were stabilized between the layers against aggregation on heating up to 500°C, and exhibited a wide optical absorption edge tailing to shorter wavelengths compared with bulk anatase-type TiO₂. This indicates the existence of small TiO₂ sol particles with size quantization effects. The silicate layer used as host matrix was almost transparent in UV-visible region. The TiO₂ sol particles between the silicate layers were electrochemically lithiated, and the pillared clay was used as cathode for a rechargeable lithium cell. Transmission electron micrograph of the section perpendicular to the basal planes showed that the pillared clays have a wide distribution in the interlayer spacing. This wide distribution in interlayer spacings apparently leads to the poor crystallinity and the wide observed pore size distribution of the pillared clays.     

Novel preparative method for porous hydrogels using overrun process

Porous poly(vinyl alcohol) (PVA) hydrogels were prepared using the overrun process which is usually used in manufacturing ice cream. The pores in the hydrogel formed exhibit dual‐pore structure due to the injection of air bubbles and ice recrystallization. Morphological investigation revealed that overrun‐processed hydrogels had closed pore structures and that their pore size and size distribution had been influenced by the impeller rate and concentration of polymer solution. The closed‐pore structure was reformed into interconnected open‐pore structure at lower concentrations of the solution. The freeze–thawing process, which takes place in PVA cross‐linking, has no effect on the bubble structure, but removes the small pores formed during ice recrystallization. Besides the swelling ratio of overrun‐processed PVA hydrogels is increased tenfold in comparison with non‐porous hydrogels. Overrun‐processed hydrogels showed more rapid swelling kinetics than freeze‐dried and film‐like hydrogels due to their larger surface area. In the future, the overrun process can be applied to prepare porous scaffolds containing proteins, such as growth factors and other cytokines, without denaturation, because it operates at a low temperature. Copyright © 2004 Society of Chemical Industry