Alumina Based Composite Material for Catalytic Purposes

Alumina based composite material for catalytic purposes

Alumina based composite material for catalytic purposes

Ana-Maria Kaszoni Pricop1, Elisabeth-Jeanne Popovici1,

Doru Roiban1 , Traian Ursaleş1, Rodica Grecu1 and Emil Indrea2

1“Raluca Ripan“ Institute for Research in Chemistry, 30 Fantanele, 3400 Cluj-Napoca, Romania; e-mail:

2 National Institute for R&D of Isotopic & Molecular Technology 3400 – Cluj-Napoca, P.O. Box 700, Romania

Abstract

Alumina based composite material could be used as porous absorbing layer for coatings on ceramic or metallic substrates in order to manufacture the catalytic supports. The paper presents some of our results referring to the preparation and characterisation of alumina hydrate (boehmite) that are to be used as binder for the preparation of alumina coatings on metallic substrates. Thermal analysis, X-ray diffraction and FTIR investigation were used to characterise the prepared alumina samples.

Key words: boehmite, alumina, catalysts, support.

1. INTRODUCTION

Alumina-based composite materials are generally used as catalytic substrate in order to increase the surface area and the mechanical resistance of catalysts. Alumina-based compositions could be also used as coatings on ceramic or metallic substrate for the manufacture of honeycomb type-catalysts that are largely used for medium protection.

In order to manufacture a catalytic support with metallic skeleton (CSMS), a study was initiated referring to the formation of alumina coatings on metallic substrate (ACOMS). ACOMS could be prepared from suspensions containing alumina (porous material), boehmite (binder) and water (dispersing medium). Crystalline structure, particle morphology and size and hydratation degrees could influence the binding ability of boehmite and as the result, the ACOMS adherence. The boehmite quality is extremely sensitive to the synthesis conditions [1-3].

The paper presents some of our results referring to the preparation and characterisation of boehmite (alumina hydrate) that is to be used for the manufacture of CSMS.

2. Experimental part

Samples of alumina hydrates were prepared by adding diluted nitric acid to sodium aluminate solutions obtained by the reaction of either aluminium powder or aluminium nitrate with sodium hydroxide solution. The nitric acid solution was slowly added, under continuous agitation, until the medium pH was 9. The separated gel was centrifuged, washed and dried at 1100C for 24 hours. The dried gel was washed with ammoniac aqueous solution, dried and then dispersed into iso-propanol.

Alumina hydrate samples were investigated by thermal analysis (Paulik-Erdely OD-102 Derivatograff; heating rate = 60C/min), X-ray diffraction (DRON 3M Diffractiometer; CuKradiation) and FT-IR spectroscopy (JASCO-610 Spectrophotometer; KBr disks).

3. Results and discussion

Two series of alumina hydrate samples (Al2O3 xH2O) were prepared by the acidulation of some diluted sodium aluminate solutions. The aluminate solutions were prepared either from aluminium powder, or from aluminium nitrate. The two synthesis routes that are schematically presented bellow, generate two alumina hydrates types possessing different physical characteristics.

• Sample code B2

/ Al(NO3)3 9H2O → Na[Al(OH)4] → Al2O3 xH2O

• Sample code B3

/ Al → Na[Al(OH)4] → Al2O3 xH2O

The thermal analysis illustrates the thermal behaviour of the two-alumina hydrate series. The thermogravimetric (TG), differential thermogravimetric (DTG) and differential thermal analysis (DTA) curves of the samples are depicted in figure 1.

The experimental thermal weight losses of the two samples, 33.5 % for B2 and 31.2 % for B3 are quite equal and almost twice the theoretical value (15%) of the anhydrous boehmite AlO(OH). Visible differences do appear between the thermal curve shape and the position of the characteristic endothermic peaks. The B2 sample shows a strong endothermic peak at about 3000C whereas the B3 sample presents a relatively weaker one at about 1600C. The observed endothermic effect and the corresponding weight variation are correlated with the remove of chemical and physical bonded water, respectively. One can presume that B2correspond to aluminium hydroxide (considered alumina trihydrate Al2O33H2O) whereas B3is boehmite (considered alumina monoydrate Al2O3H2O). Large amounts of adsorbed water are present in both sample series.

The FTIR spectra of the two series of alumina hydrate put also in evidences the different physical characteristics of the samples (figure 2).

The FTIR spectra suggest that B2 sample is well-crystallised as compared with B3 sample. The structure of the spectrum in the high frequencies domain suggests the simultaneous presence of adsorbed water, crystallisation water and free OH groups. For the B2 series, the pair of bands at 980 and 1020 cm-1 could be associated with the characteristic vibrations of the Al-OH bonding. The wide absorption band with maximum at 3470.28 cm-1 is due to the vibration of H2O molecules, which take part in hydrogen bonding with Al2O3 surface [4].

Figure 1. The thermal analysis curves of the two type of alumina hydrate namely

B2 series (left) and B3series (right)
Figure 2. FTIR spectra of the two type of alumina hydrate namely
B2 (up) and B3(down)

In the case of the B3, an intense broad band at 3426.4 cm-1 was observed. This suggests a high content of unstable H2O [4]. Al-O-H bending mode is expected in the region 900-1100 cm-1 [5].

The FTIR analysis proved the existence of two types of alumina hydrates. B2sample seems to possess an Al2O3·3H2O structure whereas B3 sample an AlO(OH) (boehmite) structure. This supposition is confirmed by the XRD patterns (figure 3). Sample B2 possesses high cristallinity degree and the spectrum corresponds to aluminium hydroxide with monoclinic structure of bayerite type. In spite of the fact that B3 show very low cristallinity, the bands of aluminium oxy-hydroxide with orthorhombic structure of boehmite type could be noticed. In fact, the reflection lines at 13.6 (6.5 Å) and 14.5 (6.1 Å) degree suggest that the material is a mixture of boehmite and pseudoboehmite. The latter one is a water-rich variety of boehmite that can be XRD recognised from the shift of the (020) reflection from 6.1 Å towards higher values [1, 2].

Figure 3. XRD spectra of the two type of alumina hydrate namely
B2 (left) and B3 (right)

4. Conclusions

The alumina hydrate samples could be classified in two basic categories that are easily recognised by their FTIR spectra, thermal analysis curves and XRD patterns. In our experimental conditions, the aluminium sodium aluminate  alumina hydrate synthesis route proved to be appropriate for the synthesis of boehmite for the formation of alumina coatings on metallic substrate (ACOMS).

Acknowledgements:

The work was supported by MATNANTECH National Research Program, under contract no. 109/2002.

References:

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