Short communicationEthanol dehydration on silica-aluminas: Active sites and ethylene/diethyl ether selectivities
Introduction
Silica-aluminas are widely used as solid acid catalysts, catalyst supports and binders in various refining and petrochemical industrial processes, including hydrotreating, mild cracking, isomerization, oligomerization and alkylation [1], [2], [3], [4]. Silica-aluminas are actually different materials depending on the preparation method, Si/Al ratio and pretreatments. Amorphous silica-aluminas (ASA) are silica-rich materials prepared by co-precipitation or co-gelling from mixed Si and Al precursors. Their most typical composition is with a SiO2/Al2O3 molar ratio 10–12 corresponding to an alumina content of 12–15 wt.% [5], [6]. In recent years, a number of materials belonging to this system with relevant mesoporosity have been prepared and developed at the industrial level [7]. Similar amorphous materials can be prepared by grafting moderate amounts of Al compounds on high surface area amorphous silica followed by calcination [8], [9].
Another family of materials are prepared by grafting silica precursors on alumina or boehmite. This is the case in particular of the Siralox family of Sasol (previously Condea), whose preparation and characterization has been reported in some detail [10].
Silica-aluminas are usually reported possessing Brønsted acid sites together with Lewis acid sites, and have been characterized since early times by different methods [11], [12], [13], [14], [15]. However, still the nature of Brønsted acid sites in silica-alumina is not fully established. Several authors propose that Brønsted acidity of silica-alumina is due to small amounts of “zeolitic” bridging Si–OH–Al groups [16], [17], [18]. Other authors instead do not find in these samples the spectroscopic features typical of zeolitic OH's (IR bands at 3650–3500 cm− 1, 1H NMR bands at 3.8–5.2 ppm) but only those typical of terminal silanols ≡ Si–OH (IR band at ca. 3745 cm− 1 and 1H peak at 1.7–1.8 ppm) [8], [19], [20], [21], whose acidity may be enhanced by nearest aluminum ions. In agreement, theoretical studies suggest the presence of “pseudobridging” OH's (i.e. terminal silanols prone to bridge over Al ions when they interact with a basic molecule) as the active sites [22], [23]. A very recent study by Caillot et al. suggests that the nature of the active site may depend on the preparation procedure [24].
We recently investigated the activity of several acid catalysts in converting ethanol to ethylene [25], [26], [27]. Although zeolites are more active than silica-alumina, the key question is stability on time on stream, with reduced coking. Silica-aluminas have been used in the past and found sufficiently stable for this reaction [28]. Over all catalysts, at low temperature and conversion, diethyl ether (DEE) is found as the main product, while, at higher temperature and conversion, ethylene becomes the main product. The mechanisms of these reactions are also the object of studies and some disagreement [29], [30], [31]. In this communication we report on our study of ethanol conversion on silica-alumina materials. The obtained data will allow us to propose some conclusions on both ethanol dehydration mechanism and on the surface acidity of silica-aluminas.
Section snippets
Experimental
The properties of the catalysts investigated and their notations are summarized in Table 1.
Catalytic experiments were performed at atmospheric pressure in a tubular flow reactor feeding 7.9% v/v ethanol in nitrogen at 298 K (total flow rate of 80 cm3/min) as described in our previous studies [25], [26], [27].
IR spectra were recorded using Nicolet 380 FT-IR spectrometers, with KBr disks for skeletal studies and pure powders pressed disks for adsorption studies, performed as described elsewhere [25]
Results
XRD analysis of the samples under study shows complete amorphicity of the SiO2 (S) and the SA87 samples. XRD analysis of A and SA5 shows the patterns of a spinel-type alumina phases, slightly different (γ-Al2O3 for A and γ,δ-Al2O3 for SA5), while that of SA30 shows the same features, weak and broad. The skeletal IR spectra of the catalysts are presented in Fig. 1. Two very broad and poorly resolved bands in the medium IR region in the spectrum of A at ca. 550 and 830 cm− 1 are typical for γ-Al2O3
Discussion
The data reported here confirm that silica-aluminas are active catalysts in converting ethanol to DEE and ethylene. However, they are less active than both zeolites and γ-alumina. Surface characterization confirms that silica-aluminas present strong Lewis acidity and also sufficient Brønsted acidity to protonate in part pyridine. IR spectra confirm that terminal silanol groups are the largely predominant surface hydroxyl groups on silica-aluminas. A detailed analysis of the spectra reveals some
Conclusions
The conclusions of this paper are the following:
- 1.
Amorphous silica-alumina and silica-alumina prepared by deposition of silicic acid on boehmite precursor (Siralox) have similar surface structures and behavior.
- 2.
In all cases the materials present strong Lewis acidity together with Brønsted sites able to protonate pyridine.
- 3.
Both these materials do not show any evidence of “zeolitic” bridging OH's, while they do show significant heterogeneity of terminal silanol groups, part of which are likely
Acknowledgments
TKP acknowledges funding by EMMA (Erasmus Mundus Mobility with Asia) in the framework of the EU Erasmus Mundus Action 2.
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