Abstract

Transition metals supported on al-pilcs as catalysts for C₆H₅Cl oxidation

L.C.A.Oliveira^I; R.M.Lago^I; J.D.Fabris^I; C.Solar^II; K.Sapag^II

^IDepartamento de Química, Universidade Federal de Minas Gerais UFMG, Belo Horizonte, MG, Brazil

^IILaboratorio de Cs. de Superficies y Medios Porosos, Chacabuco 917, Universidad Nacional de San Luis, San Luis, Argentina

^{Address to correspondence} Address to correspondence K.Sapag E-mail: sapag@unsl.edu.ar

ABSTRACT

In the present work, clays pillared with aluminium and impregnated with transition metals (Fe, Co and Cr) were prepared, characterised and studied as catalysts in the oxidation of chlorobenzene. The pillared clay was synthesised using a natural montmorillonite from San Juan (Argentina) as the starting material and an aluminium polycation solution. The catalysts were prepared by impregnating the pillared clay and then calcinating at 500^oC. The catalysts were characterised by XRD, temperature-programmed reduction (TPR) and nitrogen adsorption isotherms. The samples were tested in the deep oxidation of chlorobenzene in some cases reaching more than 90% of total conversion.

Keywords: montmorillonite, pillared clays, chlorobenzene oxidation, CVOCs, environmental catalysis.

INTRODUCTION

The control of pollution is one of the aims of the scientific community and environmentalists. Pollutants include organic and inorganic effluents, volatile organic compounds (VOCs) and chlorinated organic compounds (CVOCs) from a variety of commercial, industrial and community activities. CVOCs are among the most important pollutants, and consequently, stringent regulations have been imposed by many countries to reduce their emission levels. Thermal combustion is a current alternative for their destruction, but it is not a feasible process for their abatement, because it generally operates at a high temperature (>1000 ^oC) and the process is not cheap. One of the ways to reduce CVOCs, as described by Spivey (1987), is catalytic oxidation, which produces less harmful products such as CO₂ and H₂O by the oxidation of organic components and the conversion of clorine present in the organic molecule in HCl.

It has been argued that the catalytic combustion of chlorinated organics requires both acidic and redox centres. Indeed, the catalytic systems that are active in this process include supported noble metals, bulk or supported metal oxides, mixed oxides with a perovskite or spinel structure, as well as classic acid catalysts such as zeolites and Al₂O₃ (Noordally et al., 1993; Green et al., 1995 and Lintz and Wittstock, 1996). The high cost of noble metals has increased interest in their substitution; transition metal oxides in particular may fulfill the requirements. Some authors demonstrated that metal oxides can sometimes have higher levels of activity than noble metal catalysts (Larsson et al., 1996 and Pope et al., 1978). When the active phase is known or chosen, the support plays an important role. Furthermore, if the active phase has redox properties, it is important for the support to have acidic characteristics.

Pillared clays, PILCs, are one of the most widely studied families among the new groups of microporous materials developed in molecular engineering. These solids are obtained by exchanging the interlayered cations of layered clays with bulky inorganic polyoxocations, and later by calcining. The intercalated polycations increase the basal space of the clays, and after heating, they are converted into metal oxide clusters by dehydration and dehydroxylation. These metal oxide clusters, referred to as "pillars" are inserted between the clay layers, yielding temperature-stable oxide pillars that permanently keep the layers apart, preventing their collapse. As a result, an interlayer space of molecular dimensions, a two-dimensional porous network, is generated. After pillaring, the presence of this new porous structure and the incorporation of new active sites suggest several potential applications for these materials. In the last several years, the suitability of pillared clays as catalyst supports has been explored (Figueras, 1988 and Gil et al., 2000), specially because of the textural properties of these solids, given by their specific area and their porosity (Sapag et al., 2001). Since the preliminary work of Vaughan et al. (1973), many articles have been published (Burch, 1988) concerning the synthesis of this kind of materials with different oligocations, specially metallic ions from water-soluble salts. Of these metallic ions aluminium and zircon were the most extensively studied (Pinnavaia, 1983) with different source materials, using either natural or synthetic clays. The catalytic properties of a pillared clay strongly depend on the chemical composition of the pillars and the nature of the cations present in the interlayer space. In aluminium pillared clays both, acidity sites, the Bronsted and the more important Lewis type have been found (Sapag, 1997); therefore they are candidates for catalytic support. Keeping these results in view, in the present work, a natural clay pillared with aluminium and impregnated with transition metals (Fe, Co and Cr) was prepared, characterised and tested as catalysts in the combustion/decomposition of chlorobenzene.

EXPERIMENTAL

Methods and Techniques

The previously disintegrated sample was analysed by Atomic Absorption Spectrophotometry (AAS) with Metrolab 315 aa/ae equipment.

The changes produced in the structural and mineral composition were studied by the X-ray diffraction technique (XRD), referred to as the dust method, with a Rigaku Geigerflex diffractometer with a Cu anode and a Ni filter (l =1,54056 Å). The diffractograms were obtained from q :2q with steps of 0.02º and a retention time of 1 second in each angle.

The BET method was used to analyse the texture of the samples. Nitrogen isotherms were obtained with a Micromeritics^® ASAP-2000 nitrogen sorptometer. The data corresponding to the microporous region were calculated with the Harkins and Jura equation and the total pore volume according to the Gursvitch rule (applied at p/p^o = 0.98). The samples (0.3 to 0.8 g) had been previously degassed at 300° C during approximately 18 hours and then studied at -196° C.

The TPR analysis was made with ChemBET 3000 Quantachrome equipment with a total gas flow of 80 cm³/min of 5% H₂ into N₂.

The catalytic reaction was held in a quartz fixed-bed reactor at atmospheric pressure. The sample weighing 30 mg in a flux of air (30 ml/min) was used in each experiment. The vapour of chlorobenzene was incorporated into the air flux by a saturator system at 0^oC. The products were analysed on-line in a Shimadzu GC17A chromatographic system with a capillar column (Alltech Econo-CapSE) and a FID detector. The conversion data were calculated from the decomposition of the VOC.

Synthesis of the Support

As the former material, a natural clay called SGB from the Santa Gema mine, San Juan, Argentina, with the following characteristics was selected: specific area = 40m²/g; micropore volume = 0.006137cc/g; porous total volume = 0.091cc/g; basal distance = 12.4Å and approximate mineralogical composition = montmorillonite (85%), quartz (10%) and feldspar. After purification by the Stokes method, this material remains free of quartz and has an approximate structural formulae determined by AAS:

(Al_2,680 + Fe_0,48 + Mg_0,820 + Mn_0,02)_VI + (Al_0,22 + Si_7,78)_IV O₂₀ (OH)₄ M_1,3

where VI and IV are referred to as the coordination numbers in the octahedrical and tetrahedrical positions, respectively, and M corresponds to interchange cations.

The pillaring oligocation was prepared from 0.2M of AlCl₃6H₂O solution and 0.5M of NaOH solution with a basicity ratio of OH/Al = 2. The sodium hydroxide solution was incorporated drop by drop into the aluminium chloride solution, which was maintained under stirring at 60° C. The polymeric solution was incorporated drop by drop into a suspension made of 5% clay in deionized water. The amount of the incorporated oligocation had a ratio of 20 meq of Al per gram of clay. The sample was washed in a dialysis membrane, dried at 60^oC and calcinated at 500ºC for 1 hour. The resulting material was denominated PILC.

Preparation of Catalysts

Fe, Co and Cr catalysts were prepared by impregnation with solutions of Co(NO₃)₂ 6H₂O, FeN₃O₉ 9H₂O and Cr(NO₃)₃ 9H₂O with the PILCs as supports. The same ratio of impregnating solution per gram of support was used in the preparation. After drying in an oven at 60° C, the samples were calcinated at 500° C in air, ensuring the presence of oxides. The metal content was 5% in weight and the catalysts were labelled Fe-Imp, Co-Imp and Co-Imp.

RESULTS AND DISCUSSION

In Figure 1 the diffractograms obtained from the natural clay, the PILC and the samples after incorporation of the active phase into the PILCs, the catalysts, (SGB, PILC, Co-Imp, Cr-Imp and Fe-Imp) are shown. From this figure, it is deduced that the incorporation of Al produced a large increase in the distance between the 001 (d₀₀₁) planes, from 12.4 to 18 Å, whereas the rest of the structure was not appreciably affected.

For the catalysts, incorporation of the metals causes a decrease in the basal distance (d₀₀₁) of the supports as well as in the intensity of the first peak. This effect is probably a consequence of incorporation of active phases between the layers, causing the decrease in plane distance in some cases and blockage of the pores between the layers, in others. Moreover, some peaks that appear correspond to the oxidised states of active phases, indicating the presence of the active phase on the surface of the catalysts. For Co these peaks appear at 2.85, 2.43, 2 and 1.6 Å, corresponding to a mixture of CoO and Co₂O₃ (ASTM Card, 1990). For Fe they appear at 2.7, 2.5, 1.8 and 1.7Å corresponding to Fe₂O₃ (ASTM Card, 1991). For Cr the peaks appear at 2.67, 2.48 and 1.67 Å, characteristics of Cr₂O₃ (ASTM Card, 1997). The rest of the structure is not much affected, which was foreseeable due to the small amount of metal incorporated.

In Figure 2 the N₂ adsorption-desorption isotherms of these samples, which resemble a type IV isotherm in the BDDT classification, and hysteresis of type H4 from IUPAC are shown. An increase in surface area is obtained by means of the pillaring process, thus emphasising that the pillared material keeps the same layered structure as the source clay (Figure 2). For the catalysts, a decrease in adsorption in the microporous region of the catalyst when compared with that in the support is observed. Special emphasis is given to the variation in surface area after incorporation of the active phase. The Fe sample is the most affected, the Cr sample less and the Co catalyst still a bit less, probably according to the acidity of the solutions, whit the Fe solution being the most acidic. The mesoporous region is not much affected by incorporation of the active phase, where the decrease in the hysteresis loop can be associated with the presence of metal oxides as the union between the layers of the PILC.

In Table 1, the numerical results obtained by characterisation of catalysts are shown. The variation in the basal distance (d₀₀₁), surface area (S_BET) and microporous (Vm _p) and total (V_total) volumes for the different samples are emphasised. It is noticeable that if compared with the former SGB, the pillaring process produced an increase of almost five times in surface area and up to ten times in microporous volume. According to the incorporation of the metals, it is seen that their presence in the PILC decreases its microporosity and its basal space.

In Figure 3 the curves of the catalysts obtained by TPR are shown. For the PILC, a reduction appears between 600 and 800^ºC, probably due to the presence of Fe detected in the structure of the clay. For the catalysts, the profiles show different oxidation states. From the calculations made with these data we can conclude that stoichiometric reduction is not achieved in any of the samples, assuming that their initial state is the most oxidised. As a consequence, a mixture of metal oxides appears in the samples in different positions, with different accessibilities for their reduction. Probably a fraction of them could appear as extralayered oxide; an other fraction could continue forming intralayered oxide, which is less oxidised probably due to diffusional facts. Additionally, dispersed phases could appear, probably due to the oxide incorporated into the Al pillars, both on the pillar surface and forming the corresponding spinel, given by the wide peak at higher temperatures.

In Figure 4, catalyst activity in the oxidation of chlorobenzene is shown. In this figure, decomposition of chlorobenzene vs. temperature is plotted (because oxidation was total without other organic products). The pillared clay has a slight conversion at temperatures up to 600^ºC. The Fe catalyst shows a little conversion of chlorobenzene starting at 200^ºC and approaching 20% at 600^ºC. The Co catalyst has a higher level of activity, up to 65% conversion at 600^ºC, and the Cr catalyst has the highest level of activity, 90% conversion at 500^ºC and 95% at 600^ºC.

CONCLUSIONS

Starting from a natural clay we synthesised a porous material, PILC, with good characteristics for catalysis, basically due to its surface and porosity. This material was used as the support of the metal transition catalysts, showing different behaviours the impregnation process. For Fe, incorporation seriously affects the PILC, destroying some pillars and blocking some porous. For Co and Cr, the incidence is less important; however the original texture of the support is also affected. In the catalyst tests, all the samples show activity, with the Cr catalyst having the highest level of activity, which reaches more than 95% conversion. More studies are necessary to compare the quality of these catalysts in the oxidation of CVOCs with that of others. In this work we conclude that, we obtained a good catalyst for the destruction of chlorobenzene from a natural clay.

Received: March 5, 2002

Accepted: August 28, 2002

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