Gas phase dehydration of glycerol to acrolein over tungstated zirconias: key parameters for higher stability and acrolein selectivity
Résumé
Introduction Glycerol is a significant by-product derived from the production of biodiesel. It is predicted that by 2015, 1.54 million tones of glycerol will be generated worldwide [1], all of which should be efficiently processed in order to achieve sustainability of the biodiesel sector. Promising route to glycerol valorization is its catalytic dehydration to produce acrolein, which is an important industrial intermediate for the chemical and agrochemical industries. However, such process is limited by deactivation of catalysts with time on stream by coking and the acrolein selectivity has to be improved. In the present work, key parameters influencing the acrolein selectivity and stability were evidenced correlating catalytic data with textural and acido-base properties. Experimental Different catalysts were prepared from hydrous zirconias doped or not with silica and wet impregnated by ammonium metatungstate or exchanged with peroxotungstates followed by calcination at different temperatures [2]. For comparison, tungstated zirconias were prepared from commercial ZrO2 owning pore size of 10 nm. All the compounds were tested at atmospheric pressure and 300°C. In standard conditions, 20 wt% aqueous glycerol solution was fed (3.8 g.h-1) in a N2 flow leading to glycerol/H2O/N2 composition of 2.3/46.3/51.4 (GHSV=2900 h-1 or GHSV=8700 h-1). Pore sizes and BET surface areas were determined by nitrogen physisorption at liquid nitrogen temperature after heat treatment under vacuum at 300 °C. Structural characterization was achieved crossing XRD patterns and Raman spectra. Acid-base properties were investigated by NH3 and CO2-TPD measurements and FTIR pyridine adsorption. Results and discussion Different preparation parameters such the pH of precipitation of hydrous zirconias, the maturation time, addition of basic reflux, the amounts of tungsten and silica precursors, the calcination temperature were varied in order to determine their influences on the pore size and surface area. In addition to identification of key parameters, it allowed to get catalysts with pore sizes ranging from 4 to 10 nm and then to evidence direct correlation between stability and this parameter. Catalysts prepared from commercial ZrO2 exhibited higher stability than the best catalysts prepared from hydrous zirconias and owning similar pore sizes. This improvement was explained by different distribution of pore size that appeared as the real key parameter. It was obtained in spite of lower selectivity to acrolein (70 instead of 80%) suggesting that this parameter does not significantly influence the stability of catalysts. Stability can also be improved using particular reaction conditions or pre-treatments. Activation of SiO2-doped W/ZrO2 catalyst under H2 at 500°C improved the acrolein selectivity measured after short times on stream and stability (Figure 1). It was explained by conversion of Lewis acid sites to Brönsted acid sites. Even if addition of H2 to the carrier gas used during reaction (N2) also improved stability, the acrolein selectivity was decreased. Comparison of the CO2-TPD curves of undoped and SiO2-doped W/ZrO2 catalysts (Figure 2a) showed that the last ones contain lower amounts of basic sites. As their acrolein selectivities reached 80% whereas it did not exceed 64% for undoped W/ZrO2, it confirmed that SiO2 addition during preparation avoids formation of unselective basic sites of ZrO2 [2,3]. Furthermore, for SiO2-doped W/ZrO2 catalysts that owned equivalent amounts of basic sites, correlation between the selectivity to acrolein and the amount of Brönsted sites was established (Figure 2b). From the IR spectra, It was also inferred that they are located on terminal W=O bonds of supported tungsten species. Concerning the Lewis acid sites arising from both coordinatively unsaturated Zr4+ and W6+ cations, they could favor formation of by-products from acrolein for the first hours on stream damping the acrolein selectivity. Conclusion As conclusion, this work highlighted the need to tailor both the textural and acido-base properties of ZrO2 to improve the acrolein selectivity and stability of tungstated zirconias for gas phase dehydration of glycerol. A high amount of Brönsted acid sites and a low amount of basic sites favor the acrolein selectivity whereas high pore size increases stability. This parameter can also be improved adding H2 to the gas feed. References [1] Global Industry Analysts, http://www.strategyr.com/Glycerin_Market_Report.asp. [2] P. Lauriol-Garbey, S. Loridant, V. Belličre-Baca, P. Rey, J.-M. M. Millet, Catal. Commun. 16 (2011) 170174. [3] P. Lauriol-Garbey, G. Postole, S. Loridant, A. Auroux, V. Belliere-Baca, P. Rey, J.M.M. Millet, Appl. Catal. B: Environ. 106 (2011) 94.