M. Brotas de Carvalho

Characterisation of clays and aluminium pillared clays by adsorption of probe molecules

Abstract Aluminium pillared clays (PILCs) from three different starting materials namely two Portuguese clays from Benavila (Alentejo) and Porto Santo (Madeira archipelago) and a Wyoming SPV-200 clay were obtained. A characterisation of the textural and some surface chemistry properties of the starting materials and PILCs was made using the adsorption of different probe molecules (water, ethylene glycol, nitrogen, n-hexane, cyclohexane, triethylamine) and also by other routine techniques. The adsorption of probe molecules was also employed to characterise the microporosity of the obtained PILCs. The effect of freeze-drying on PILCs texture was also discussed and related with the size of the clay particles.

Preparation of activated carbons from cork waste by chemical activation with KOH

In the western Mediterranean region, and specially in the final temperature attained values in the range 500–900 8C south of Portugal, cultivation of cork oak, Quercus suber, and was maintained for a time, t (h), from 0 to 16 h. After is an important economic activity, with Portugal being the carbonisation the samples were cooled to room temperaworld’s major producer. Besides the production of stopture under nitrogen flow, washed up to pH 7 and dried at pers, an economically important product, the industry 100 8C. The samples will be designated according to the creates enormous volumes of waste formed by particles expression: ratio KOH:cork /T (8C)/t (h). Adsorption isowith very small dimensions, generally designated cork therms of N at 2196 8C were determined in a convention2 powder. This industrial waste was the raw material used in al volumetric apparatus. Hydrocarbon (n-hexane and this work since it has the essential features of an activated cyclohexane) vapour adsorption was studied at 25 8C by carbon precursor and, according to the results of previous the gravimetric method. The samples were studied in the studies [1,2], allows one to obtain materials with textural form of small wafers made by pressing (without binder) characteristics, e.g. apparent surface areas, comparable the as-obtained form, i.e. finely divided powders. From N2 with those of commercially available carbons. This work adsorption data, the apparent surface area, A , was BET 0 reports the first systematic evaluation of the influence of determined through BET equation (0.05,p /p ,0.15). several experimental parameters (activating agent content Using the Dubinin-Radushkevich (DR) equation, the miand temperature and duration of calcination) on the textural cropore volume, V , the characteristic adsorption energy, DR characteristics of the activated carbons prepared from E , and the mean micropore half width, L , were esti0 0 industrial cork waste using KOH as activating agent. mated. To calculate L , the empirical equation: L 5 0 0 25 3.5 The raw material was a sample of industrial waste, (13.028–1.53310 E ) /E [3] was used. The micropor0 0 previously treated with H SO (10%) at room temperature, osity was also assessed by applying the a method, taking 2 4 S washed up to pH 7, dried at 80 8C and finally sieved to as reference the isotherm reported in Ref. [4]. Hydrocarbon obtain the fraction of cork particles smaller than 0.297 adsorption data were analysed by the DR equation. mm. Cork powder was physically mixed with different The N adsorption isotherms displayed in Fig. 1 are 2 volumes of KOH solution (25%), to obtain mixtures with representative of the curves obtained in this study and weight ratios of KOH:cork between 0:1 and 4:1. After reveal the essential microporous nature of the materials. drying at 80 8C, the mixtures were heated in a horizontal The values of apparent A (Table 1) show that the BET 3 21 furnace under a nitrogen flow (2 cm s ). The temperaexperimental parameters have different effects on the ture was increase in two steps: from room temperature up development of the microporous structure of the samples. 21 to 300 8C (5 8C min ), maintained for 3 h, and afterwards In fact, the increase in the calcination time and in the 21 raised (10 8C min ) up to a higher value, T (8C). The calcination temperature promote a continuous increase in the A values, which is considerably more accentuated BET in the case of the temperature. The increase of the KOH:cork weight ratio, although initially (until * KOH:cork51:1), also has a positive effect on the apparent Corresponding author. Fax: 1351-217-500-088. E-mail address: ana.carvalho@fc.ul.pt (A.P. Carvalho). surface area of the samples, for high contents of KOH, the

Aluminum-pillared clays : Decomposition of the intercalating species and textural properties

Pillared clays (PILCs) are microporous solids obtained by the intercalation of suitable clays with large polyoxicationic species which, in the process of preparation, are thermally decomposed into oxide species forming the pillars. In this work the thermal decomposition of the intercalating species—aluminum polyoxications—is studied by t.g.-d.s.c. and the results are compared with the chemical analysis. Textural properties of PILCs obtained by two methods are studied by nitrogen adsorption and the use of different reference materials, in order to analyze adsorption data, is discussed.

Simultaneous aluminium oxide pillaring and copper(II) Schiff base complexes encapsulation in a montmorillonite

Copper(II) acetylacetonate and two complexes with pentadentate N3O2 Schiff-base ligands derived from acetylacetone–bis(acetylacetonate)-3-amino-bis(propyldiimine), ([Cu(acac)2trien]), and bis(acetylacetonate)-3-methylamino-bis(propyldiimine), [Cu(acac)2Metrien], were encapsulated into a pillared clay (PILC) using a new methodology that involves simultaneous oligomer pillaring and complex encapsulation: in the present work the pillaring oligomer was [Al13O4(OH)24+x(H2O)12−x](7−x)+ and the starting clay was a Portuguese montmorilonite. All the materials were characterised by X-ray photoelectron spectroscopy, powder X-ray diffraction, adsorption of nitrogen at low temperature and infrared spectroscopy and, for copper-based materials, by chemical analysis. Data indicate that all three complexes have been immobilised within the matrix, but only for [Cu(acac)2] and [Cu(acac)2trien], did metal complex immobilisation take place concomitantly with aluminium pillar intercalation. Furthermore, the encapsulation of the latter complexes induced an increase in the PILC basal distance when compared with the pillared clay prepared without complex present. Finally, in all the materials the encapsulated complexes exhibit structural properties that are different from those of the free complexes, a consequence of the steric effects imposed by the porous structure of the PILC.

Carbon dioxide in Y and ZSM-20 zeolites : adsorption and infrared studies

Abstract Adsorption of carbon dioxide was performed on sodic and protonic forms of Y and ZSM-20 zeolites. The adsorption isotherms of carbon dioxide were determined in the temperature range of 293—318 K by the volumetric method. Isosteric heats were obtained by the Clausius-Clapeyron equation. In order to obtain information on the adsorbed molecules at low coverages, the infrared technique was used. It is shown that, in spite of having closely related structures, the Y and ZSM-20 zeolites present structures with different polar sites. This difference in textural properties can influence the adsorption of molecules capable of having specific interactions with surface polar groups.

Adsorption of methane and ethane in zirconium oxide pillared clays

Abstract In zirconium oxide pillared clays ( A BET ∼266 m 2 g −1 ), prepared with clays from different sources, the adsorption of pure methane and ethane, the more abundant hydrocarbon components of natural gas, were determined in the temperature range between 215 and 293 K up to the atmospheric pressure. The Langmuir model and the vacancy solution theory (VST) of Suwanayuen and Danner were used to fit the experimental data. As expected, the VST model leads to a better adjustment. The parameters obtained with this model were used to predict the binary adsorption isotherms. The data predicted for mixtures were consistent with the experimental results obtained in the same volumetric apparatus used for the adsorption of pure components, analysing by gas chromatography the evolution of the fluid phase until the attainment of equilibrium. The results reveal a higher affinity towards ethane. The equilibrium selectivity towards ethane decreases when temperature and pressure increase, ranging from more than 100, at the lowest temperatures and pressures studied, to 3–4 near ambient temperature and atmospheric pressure. Even in the latter, less favourable situation, which in fact corresponds to the more usual working conditions, the phase diagram, predicted by the VST model indicates that the studied zirconium pillared clays are promising materials for ethane/methane separation.

Characterization of the acidity of Al- and Zr-pillared clays

The surface acidic properties of pillared clays (PILCs) with Al or Zr oxide pillars (prepared from a natural Portuguese smectite and a synthetic Laponite), and of a protonic NaHY zeolite, were studied by two methods: pyridine adsorption followed by infrared (IR) spectroscopy, and the catalytic transformation of 1-butene. The results of the pyridine adsorption revealed that all the pillared clays studied have mainly Lewis-type acidity, the exception being the clay pillared with Zr oxide, obtained from natural smectite, which also presents a significant number of Brönsted acid sites. The results of 1-butene transformation showed that the pillared clays exhibit catalytic properties similar to those of the protonic Y zeolite. The acid character of the solids was confirmed by the values of the cis/trans 2-butene isomers ratio. At short reaction times, product distribution showed that the main reaction is oligomerization, followed by cracking. After 15 min the products are mainly the linear isomers, cis and trans-2-butene, showing that the majority of the catalytic active sites, are already poisoned after 15 min of reaction. The particular behaviour of Laponite intercalated with Al oxide pillars is discussed. The IR spectra of the pyridine adsorbed on the fresh samples and after the catalytic assays, revealed that Lewis acid sites behave as active sites for 1-butene catalytic transformation. The consistency between the results of the two complementary techniques used for the characterization of the acidity of the solids was proved.

Dealumination of zeolites: Part 4. Dealumination of offretite through hydrothermal treatment

Abstract Nine samples of dealuminated offretite have been prepared by simple or double treatment, without addition of water, at high temperature (873–1123 K) of a KHOffretite sample (K0.95H3.45Al4.4Si13.6O36). The samples were characterized by X-ray diffraction (unit cell parameters a and c, and “crystallinity” CRX), by 29Si n.m.r. (framework Si Al ratio and distribution of Al in T1 and T2 sites), by 27Al n.m.r. (framework and extraframework Al species), by i.r. spectroscopy (frequencies υ1 and υ2 of the asymmetric and symmetric TO bands), and by X.p.s. ( Si Al ratio of the outer surface). As with Y zeolites, highly dealuminated samples with a high CRX value can be obtained only through successive treatments with, intermediately, exchange of the K cations. The dealumination causes an increase in the Al T1 Al T2 ratio and of υ1 and υ2 but has practically no effect on the c parameter. Parameter a first decreases with the number of framework Al per unit cell (NAl), then remains quasi-constant for NAl ≤ 2.7. Nearly linear relationships are obtained between υ1 and υ2 and NAl, which allows an estimation of NAl to be made by i.r. spectroscopy.

Preparation of dealuminated faujasites for adsorption of volatile organic compounds

A series of dealuminatead faujasitic zeolites, obtained from the ammonium form of Y zeolite, was prepared by two different methodologies: a relatively common self-steaming method, where temperatures in the range 813 to 1093 K were used, and a less well studied solid-state dealumination method using ammonium hexafluorosilicate, which employed a lower temperature (453 K) than those used in the self-steaming method. It was possible to obtain samples by solid-state dealumination with both high levels of dealumination and good crystallinity. Textural and surface chemistry parameters, such as the micropore volume and the hydrophobic character of the surface, respectively, were correlated with the extent of dealumination. The adsorption isotherms of two selected volatile organic compounds (VOCs), one oxygenated (methyl ethyl ketone) and the other chlorinated (1,1,1-trichloroethane), were measured at 298 K by the gravimetric method. For one of the samples obtained by solid-state dealumination in particular, the adsorption isotherms of the studied VOCs presented adequate characteristics for the adsorption of these molecules, namely significant adsorption capacity and a moderately rectangular shape. This latter aspect can be of importance, for instance, in terms of regeneration of the adsorbent.

Water adsorption in aluminium pillared clays and zeolites

Adsorption of water at 298 K has been studied in aluminium pillared clays (PILCs) and in zeolite types A, Y and ZSM-20, and kinetic and equilibrium results obtained. PILCs were prepared by two methods, which differ only in the OH/Al ratio of the oligomer solution, and by using three different parent clay materials. These methods led to PILCs that, when calcined at 623 K, present BET areas ranging from 250 to 350 m 2 g -1 and basal spacings from 1.6 to 1.8 nm. For Y and ZSM-20 zeolites, the sodium and protonated forms were studied. The equilibrium results were compared with literature data corresponding to an ideal isotherm for use in dehumidification and discussed in terms of textural and chemical properties, namely the Si/Al ratio of zeolites and PILCs. For these latter materials an effect of the parent clay on water adsorption was studied, which seems to be related to the degree of substitution in the tetrahedral sheets.