Preparation and characterization of terbium palygorskite clay as acid catalyst

Palygorskite clays containing 5% and 10% terbium (W/W) were prepared by ion exchange from acid-leached natural clay. Samples were calcined at 523, 573, 623 and 673 K to verify the e(cid:128)ect of the temperature over the physical–chemical properties. Characterization was using X-ray di(cid:128)raction, infrared spectroscopy ( n -butylamine adsorption), UV–visible di(cid:128)use reﬂectance spectroscopy and N 2 adsorption. Isopropyl alcohol (IPA) transformation was also performed in order to improve the acid behavior. TB 3 (cid:135) introduction does not change the palygorskite structure, albeit leading to an increase in the surface area and generation of Lewis acid sites. Results from the DRS spectra along with a decrease in the catalytic activity suggested the formation of terbium oxide for relatively high TB 3 (cid:135) contents. (cid:211) 2000 Elsevier Science B.V. All rights reserved.


Introduction
Palygorskite clays are potential candidates for a number of processes in heterogeneous catalysis.This class of clay minerals is characterized by porous crystalline structures containing tetrahedral layers alloyed together by longitudinal sideline chains.A typical unit cell consists of (Mg,Al) 5 Si 8 O 20 (OH) 2 (H 2 O) 4 Á4H 2 O, with Mg preferentially located in octahedral sites.These mineral clays possess Mg-and Al-type cations that can be easily exchanged [1,10].
Mineral clays, either pillared or not, have been extensively used as catalysts to crack heavy fractions of oil, support for some metals or oxide phases, selective adsorbents and ion exchangers [2,3].The acidity of these materials is associated with the presence of the structural hydroxyl groups in the layers (Br onsted sites) as well as to interlayer species, such as cations (Lewis sites) [4].In particular, palygorskite clays depict a high susceptibility to ion exchange, large speci®c surface area, considerable porosity and thermal stability.Acid palygorskite clays can be obtained by acid leaching or ion exchange driven by acid cations.Rare earth Microporous and Mesoporous Materials 38 (2000) 345±349 www.elsevier.nl/locate/micromesocations (La 3 , Ce 3 , Tb 3 etc.), for instance, play a double role improving both the activity and stability of dierent materials (zeolites [6], mineral clays [1±4] and alumina [5]) leading to very ecient acid catalysts.Several chemical reactions catalyzed by the acid solids can evaluate the acid activity of palygorskite clays.The decomposition of isopropyl alcohol (IPA) especially, is extensively used for estimating the acid±basic or redox characteristics [7].Whereas, acetone is preferentially formed as a dehydrogenation product on basic sites, propene is essentially formed on acid sites [8].
The proposal of this work was to verify the in¯uence of Tb 3 on the physical±chemical and acid catalytic properties of palygorskite clays using the transformation of IPA as test reaction.

Experimental
The starting material used herein consisted of a natural palygorskite obtained from the Brazilian state of Piaui, located in the northeastern region of the country.It was dried at 383 K for 2 h, powdered and sieved down to 200 mesh.It was then calcined at 773 K to eliminate organic compounds (sample labeled PQ).Subsequently, the sample was leached with a 2 M HCl solution to improve proton acidity, rinsed with deionized water until full chloride elimination and calcined under a ¯ow of N 2 /O 2 at 523 K (sample labeled PQHCl).The Tb 3 cations were introduced in acid-leached palygorskite by ion exchange carried out in a water suspension containing 10% solids and terbium nitrate: Tb(NO 3 ) 3 Á7H 2 O in concentrations of 5% and 10% W/W Tb 3 /palygorskite, resulting in the samples labeled PQTb5 and PQTb10, respectively.The mixture was stirred at 353 K for ®ve days, ®ltered, dried out and calcined at 523, 573, 623 and 673 K.The main physical±chemical properties of the resulting samples were investigated by X-ray diraction (XRD), atomic absorption spectroscopy (AAS), infrared spectroscopy with n-butylamine adsorbed (IR), UV±visible diuse re¯ectance spectroscopy (DRS) and N 2 adsorption (ASAP-2000).
The IPA conversion experiments were performed in a tubular glass ¯ow microreactor (1.5 mm ID).The catalysts were activated in situ at the reaction temperature for 2 h under a N 2 ¯ow of 80 ml min À1 .The space velocity was set to 42.3 h À1 and the reaction test temperatures were 523, 573 and 623 K.The products were analyzed online using a gas chromatograph unit equipped with a Porapaq±Q packed column (working at 523 K) and a ¯ame ionization detector (FID±GC±Varian).

Results and discussion
Magnesium cations were exchanged for terbium cations to generate palygorskite clays containing 5% (PQTb5) and 10% Tb 3 (PQTb10) in the initial reactional mixture.Atomic absorption spectroscopy (AAS) on acid leached palygorskite revealed Mg, Ca and Na contents lower than 0.1% W/W.In addition, the samples PQTb5 and PQTb10 contained 1.6% and 3.1% Tb 3 , respectively, indicating an ion exchange yield of around 30%.A and was attributed to the basal space of the palygorskite framework [9].The peaks at 2h 13X9°Y 16X5°Y 19X9°and 20.9°represent the Si±O±Si crystalline layers in the clay [9].The peak at 2h 12X4°is due to a hydrated oxide with sodium and magnesium cations present between layers.Finally, the peak scanned at 2h 26X7°was attributed to the quartz impurities (d 101 3X34 # A).After acid leaching, there was a considerable loss of crystallinity.This was most likely due to a decrease in the interlayer spacing of the structure resulting from the octahedral magnesium leaching.Fig. 1(b) shows two additional peaks located at 19.9°and 20.9°.They were probably related to the Si±O±Si crystalline arrangements, which could not be eliminated by acid leaching, as was the case for quartz impurities (2h 26X7°).
Fig. 2 shows the XRD patterns obtained from samples PQTb5 and PQTb10, after calcination at 673 K.They did not present any considerable dierences from the samples calcined at other temperatures (523, 573 and 673 K).The patterns only presented the main peaks corresponding to the palygorskite framework (2h 8X7°, 19.9°and 20.9°) and quartz impurities (2h 26X7°).The reappearance of the basal distance represented by d 110 10X4 # A (2h 8X7°) con®rmed the introduction of terbium at interlayer positions.Nevertheless, the d 110 basal distance increased to 15±16 # A for temperatures above 573 K. Apparently, the relatively high terbium contents contributed to the development of the oxide formed upon calcination.
In order to verify the surface and porous characteristics of the terbium palygorskite clays, samples were evaluated by N 2 adsorption.The surface area and the micropores volume of the original sample were determined to be 136.9m 2 /g and 0.03 cm 3 /g, respectively.Table 1 summarizes the surface area values for samples with terbium as a function of the calcination temperature.No signi®cant eect of the calcination temperature on the surface characteristics was observed.Conversely, the surface area increased with increasing terbium contents, since Tb 3 is smaller than Mg 2 cations and three Mg 2 cations were exchanged for two Tb 3 cations.Fig. 3 shows the 4000±400 cm À1 range of the infrared (IR) spectra for samples PQ, PQTb5 and PQTb10.The bands at 1083 and 465 cm À1 are Fig. 2. XRD patterns of palygorskite with 5% and 10% terbium (PQTb5 and PQTb10) after calcination at 673 K. ascribed to the Si±O±Si bonds in the layers [10].
The band near 800 cm À1 may correspond to the Al±O±Si bonds [10].Both PQTb5 and PQTb10 samples exhibited typical bands that could be assigned to nitrogen (from n-butylamine) bonded to acid sites: 1500 cm À1 ± Br onsted site [11] and 1638 cm À1 ± Lewis site [12].One can observe that the Lewis sites band (1638 cm À1 ) is more intensive for larger terbium contents, indicating that the terbium cations are responsible for those kind of active sites.
Terbium palygorskite samples were analyzed by DRS (UV±visible diuse re¯ectance spectroscopy).This method is particularly useful in gathering additional information on the electronic transition of adsorbed species as well as on the oxidation states.The DRS spectra (Fig. 4) suggested that terbium was not susceptible to changes due to dierent calcination temperatures.On the other hand, more intense peaks were detected at 260 nm as the terbium contents increased, indicating an additional association of atoms.This could be associated to the formation of an oxide phase, which con®rmed the results obtained from XRD.
The IPA transformation was carried out for PQ, PQTb5 and PQTb10 samples calcined at 573 K in order to verify the terbium eect on the palygorskite catalytic properties.The original palygorskite sample (PQ) did not show any signs of acid activity.On the other hand, propene was invariably obtained as the main dehydration product from both PQTb5 and PQTb10 samples, which is characteristic of the acid behavior of terbium palygorskites [7,8,11].Although the formation of di-isopropyl ether by IPA dehydration may take place by an intermolecular mechanism, involving nearby acid sites [11], this was not the case.Fig. 5 displays IPA conversion data as a function of the reaction temperature.The IPA conversion improved as the temperature increased from 523 K to 573 K and reached a nearly constant value at higher temperatures.One can observe a smaller IPA conversion for higher Tb 3 contents, albeit the surface area increases.It seems plausible that high Tb 3 contents lead to terbium oxide, according to the DRS results (Fig. 4), which could be the cause for the reduction in the acid activity of terbium palygorskite.

Conclusions
Natural palygorskite clays showed highly crystalline layers and quartz impurities.This material could be modi®ed to produce acid catalysts or to be used as catalyst supports, with a reasonable ion exchange capacity.Acid treatment (octahedral magnesium leaching) resulted in a considerable reduction in the basal distance, later restored by the introduction of terbium between clay layers.The presence of terbium also leads to an increase in the surface area and the generation of Lewis acid sites.The terbium palygorskite clays showed very good activity for alcohol dehydration.Nevertheless, relatively high terbium contents may result in the development of an oxide, decreasing the catalytic activity of the palygorskite.

Fig. 1 (
a) presents the XRD patterns of the original (PQ) and acid leached (PQHCl) samples.The original palygorskite is a very crystalline clay.Firstly, the peak at 2h 8X6°had an interlayer distance d 10X4 #