Investigation of water effects on heterogeneous acid catalysts: application to bio-alcohols dehydration into olefins
Résumé
1. Introduction
Bio-alcohols dehydration is of prominent relevance because it represents a sustainable alternative route for olefins production. However, water present in bio-alcohols and generated by their dehydration often gives rise to structural, textural and acidity modifications of heterogeneous catalysts [1]. A full understanding of each effect is required to synthesize efficient catalysts in the presence of water. Nevertheless, contradictions can be found in the literature because most of studies on water effects are achieved ex situ. In this work, a method based on in situ techniques has been developed to understand the water effects on heterogeneous catalysts used for conversion of isobutanol into linear butenes. In particular, the improvement of catalytic activity when water was added to the feed was explained by in situ acidity measurements achieved in flowing water vapor at the reaction temperatures.
2. Experimental
Catalysts preparation: TiO2/SiO2 oxide catalysts (named Ti/SiO2) were prepared by a grafting method using Ti(OiPr)2(OC5H10) as precursor and H4SiW12O40/SiO2 (named HSiW/SiO2) were synthetized by wetness impregnation with H4SiW12O40 aqueous solutions, followed by drying and thermal treatment in flowing air at 698 K and 573 K respectively. Both oxides were supported on a pure silica provided by Evonik (Grace, BET surface area 557 m²/g, BJH pores volume 0.82 cm3/g).
Characterizations: The catalysts structures were determined by XRD, Raman spectroscopy and TEM, their textures by BET, BJH measurements and their acidity properties by FTIR-pyridine, acetonitrile and CO adsorption. Furthermore, in situ-FTIR acidity measurements were achieved on self-supporting disks in a specially design short path length (~2.2 mm) IR cell reactor [2]. In brief, it allowed relevant data in the temperature range 293-800 K under controlled gas mixture flow rate (without and with water up to 3 %) to be obtained. Heats of adsorption for NH3 and H2O adsorbed species were determined using the AEIR method [3].
Catalytic measurements: Catalytic tests were conducted in a fixed-bed reactor at atmospheric pressure between 450 and 573K. Conversions and products yields were determined with both on-line and off-line gas chromatographs equipped with FID detectors. Reaction parameters such as the contact time, the time on stream and feed water addition were investigated.
3. Results and discussion
For each catalytic system, the metal loading was maximized whilst amorphous and well dispersed supported oxides were maintained. After checking the absence of acidity on the support by FTIR spectra of adsorbed probes, it was shown that Ti/SiO2 catalysts possess mostly moderate Lewis acid sites whereas strong Brønsted acid sites mainly compose HSiW/SiO2 catalysts (Table 1).
Table 1. BET surfaces areas of oxides catalysts and acidity densities determined by FTIR spectra of adsorbed pyridine.
CatalystS BET
[m²/g]Brønsted density
[µmol/m²]Lewis density
[µmol/m²]Lewis/Brønsted
21.4%TiO2/SiO23817.6x10-31.5x10-120.6
20.9%H4SiW12O40/SiO23759.1x10-24.1x10-2 0.4
The catalytic activity and selectivity to linear butenes were better on HSiW/SiO2 than on Ti/SiO2 confirming that Brønsted acid are more active in isobutanol dehydration and that high strength is required for isomerization. Moreover, C5+ secondary products were formed on HSiW/SiO2 which was not the case on Ti/SiO2. Both catalysts deactivated with time on stream but Ti/SiO2 deactivated more slowly. This was shown to arise from coke formation but not from structural or textural modifications. For Ti/SiO2 catalysts, addition of water to the feed significantly increased the conversion especially at high contact time (Fig. 1a) whereas the selectivity was unchanged. The comparison of apparent activation energies revealed that the strength of the acid sites and the mechanisms involved in the reaction are unchanged by water addition. Similar positive effects of water on conversion were obtained for HSiW/SiO2 catalyst but to a lesser extent.
In parallel, in situ FTIR acidity measurements were achieved by NH3 adsorption in presence of water from room temperature to 720 K. For Ti/SiO2 catalyst, spectra have revealed that the number of Brønsted acid sites increased by a factor of 4 when 3% H2O was added to the NH3/He gas mixture in the temperature range (473-573 K) while the Lewis sites remained unchanged (Fig.1b). It clearly showed that the increase in catalytic activity does not arise from conversion of Lewis acid sites into Brønsted acid sites as frequently mentioned in the literature. Furthermore, heats of adsorption were determined according to the AEIR method. On Ti/SiO2 catalyst (Fig.1c), it has been shown that the heats of adsorption of (a) the NH3 adsorbed species on the Lewis acid sites (IR band at 1606 cm-1) and (b) the NH4+ species protonated by Brønsted sites (IR band at 1442 cm-1) follow the Temkin adsorption model with a linear decrease from (E(0)) to (E(1)) at low and high coverages. Results have showed the presence of two Lewis species and two Brønsted species with different strength. Focusing on adsorbed species in the reaction temperature range (shaded area on the Fig.1c), the values comparison clearly demonstrated that NH3 interaction with Lewis and Brønsted sites was not hindered by water, water having much weaker heat of adsorption. Finally, as bridged OH groups were shown to be formed under water at the reaction temperatures and to correspond to Brønsted sites, it was concluded that the activity improvement arises from such bridged OH sites. For HSiW/SiO2 catalysts, the number of Brønsted sites also increased under water vapor. In this case, the improvement of Brønsted sites density could be induced by a redistribution of the H4SiW12O40 units on the surface under water.
Figure 1. a) Conversion evolution at 523 K of Ti/SiO2 catalyst versus contact time, with and without additional water in the feed, b) Evolution with water partial pressure (0-3%) of the NH3 IR bands (2%) adsorbed on Lewis sites and NH4+ formed on Brønsted sites of Ti/SiO2 catalyst, T 523 K c) Experimental coverage evolutions of adsorbed species: NH3-L (□), NH4+-B (Δ) and H2O (○) as a function of temperature, PNH3 = 1 kPa and PH2O = 3 kPa; and their associated theoretical curves (1, 2, 3) obtained from the Temkin model.
4. Conclusions
TiO2 and H4SiW12O40 oxides highly dispersed over SiO2 have been synthesized and tested in conversion of isobutanol into linear butenes. Brønsted acid sites are active for the dehydration step whereas strong sites are required for isomerization. For both oxides, the catalytic activity was enhanced by the addition of water into the feed and this feature was not due to change in the structural and textural properties of the solid. In situ FTIR measurements under flowing NH3 and H2O at the reaction temperatures have showed that it arises from a large increase in the density of Brønsted acid sites whereas their strength was unmodified. Moreover, the addition of H2O does not change the amount of Lewis sites. The heats of adsorption of the NH3 and H2O adsorbed species revealed that H2O does not compete with NH3. The effect of water on isobutanol adsorption and butenes desorption is under investigation by in situ and operando FTIR experiments and the mechanism of Brønsted acid sites formation is also being considered.
Acknowledgments
This work has been cofounded by CNRS and IFPen.
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