Gas phase glycerol oxidative dehydration over bifunctional V/H-zeolite catalysts with different zeolite topologies
Graphical abstract
Introduction
The conversion of glycerol into valuable products has been investigated as a strategy to attain a sustainable biodiesel production chain [1], [2], [3]. Gas-phase glycerol dehydration to acrolein is one of the most promising route, because acrolein is an important intermediary in the production of acrylic acid, superabsorbent polymers, pharmaceuticals and plasticizers. Many acid catalysts were investigated in the literature, such as heteropolyacids, mixed oxides, phosphates and zeolites, with satisfactory glycerol conversion and acrolein selectivity [4], [5]. However, rapid deactivation by coke formation is still the main limitation in their use for industrial purposes and a lot of effort has been payed to solve this problem. Among the investigated strategies, one could mention the use of moving bed reactors [6], alternating cycles of reaction and coke burning [7] or oxygen co-feeding [8]. This last option seems to be the most adequate for practical purposes, since oxidative conditions favors desorption of the oxygenate oligomer compounds that would act as coke precursors, maintaining the surface acid sites available for further catalytic cycles of reaction and improving catalyst lifetime [9].
The acrolein, produced in a first bed containing an acid catalyst (usually a zeolite or a heteropolycompound), can be further oxidized in a consecutive bed containing a redox catalyst, such as V-W-Nb mixed oxides [10], [11], resulting in high yields of acrylic acid. On the other hand, the use of catalysts containing both acid sites and redox species has been envisaged as a way to produce acrylic acid in a one-step oxy-dehydration process [1]. In most cases, it is observed an acrolein yield higher than to acrylic acid [1], [12], [13]. Recently, catalysts based on phosphoric acid modified W-V-Nb catalysts have shown acrylic acid yields around 60% in a short time run [14]. There is no mention to long time stability.
Redox molecular sieves, such as zeolites containing transition metal oxides, are promising alternative catalysts for this purpose, but there are few studies in the literature. Pestana et al. [1] have evaluated the zeolite-β (BEA) containing 5 wt.% or 10% of vanadium, prepared by impregnation or physical mixture, in the glycerol oxidative dehydration at 275 °C, observing a selectivity to acrylic acid of 25% and 12%, respectively, at a glycerol conversion of ca. 75%. Acrolein and acetol were the main products formed. Other oxygenates, such as acetaldehyde and acetic acid were observed as minor byproducts, but a significant amount of unidentified products with high boiling points were also formed during the reaction. Based on XPS measurements, the authors correlated the catalytic performances to the vanadium dispersion in the zeolite pores.
In a recent publication, Possato et al. [12] have studied the use of V2O5/H-ZSM-5 (MFI topology) prepared by using vanadyl sulphate (VOSO4) or ammonium metavanadate (NH4VO3) as vanadium precursors. At 350 °C, glycerol conversions up to 97% and selectivity to acrylic acid of 17% were attained for the catalyst prepared by wet impregnation with VOSO4 [12]. According to the authors, the presence of vanadium improved the catalyst lifetime, because catalyzes both the oxidation of acrolein to acrylic acid and the oxidation of coke precursors. The formation of acrylic acid depends on the dispersion of vanadium oxide, which facilitates the redox cycle V4+/V5+, as suggested by XPS and DTA analyses.
In this work, a series of catalysts were prepared by wet impregnation with 5 wt.% of vanadium using NH4VO3 and evaluated in the gas phase oxidative dehydration of glycerol, in order to evaluate the effect of zeolite topology on the nature, dispersion and reducibility of vanadium species in the selectivity to acrylic acid.
Section snippets
Preparation of the catalysts
Zeolite ZSM-5 (MFI), Beta (BEA), ferrierite (FER), zeolite Y (FAU), offretite (OFF) and mordenite (MOR) were synthesized according to the IZA methods [15]. The zeolite ZSM-11 (MEL) was synthesized by the method proposed by Gonzales et al. [16]. Zeolite MCM-22 (MWW) was synthesized with molar ratio SiO2/Al2O3 = 30 by the method proposed by Carriço et al. [7]. The calcined materials were ion exchanged with a solution with 0.1 mol L−1 NH4NO3 and further calcined to obtain the acid form of the
Catalyst characterization
The X-ray diffraction patterns of calcined V/H-zeolites samples with different topologies are shown in Fig. 1. The X-ray diffraction patterns confirmed the formation of a crystalline structure of the desired topologies as compared with the respective IZA standard [17]. No peaks of vanadium oxides were observed in the powder diffraction patterns, suggesting that they are very well dispersed in the pore systems of the zeolites.
The as-synthesizes ferrierite presented low crystallinity, even before
Conclusions
Bifunctional catalysts V/H-zeolite, prepared by impregnation of V2O5 on the acid zeolites of different topologies, have shown to be very active in the gas-phase oxidative dehydration of glycerol, achieving high glycerol conversions and producing mainly acrolein and acrylic acid. Among these catalysts, those with MWW and BEA topologies resulted in better catalytic performances, with selectivities to acrylic acid of about 20%, but a significant coke deposition was observed for these more open
Acknowledgments
T. Q. Silva thanks to National Council of Technological and Scientific Development (CNPq) and Institutional Programme of Scientific Initiation (PIBIC/UFBA) for the scholarship.
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