Studies on dispersion and reactivity of vanadium oxides deposited on lamellar ferrierite zeolites for condensation of glycerol into bulky products
Graphical abstract
Introduction
Massive amounts of cheap glycerol have become commercially available, due to its formation as a co-product in the biodiesel industry [1]. Biodiesel consists of fatty acid methyl esters that are normally obtained by transesterification of vegetable oils and methanol in the presence of an alkaline catalyst [2]. Approximately 10 wt.% of crude glycerol is generated during this process, and due to the growing interest in biodiesel as an alternative to fossil fuels, the production of 37 million metric tons is expected in 2020. Consequently, the amount of glycerol generated cannot be absorbed by traditional consumer industries, such as the cosmetics, pharmaceutical, and tobacco sectors, hence depressing its price and creating opportunities to find new applications for this compound [3]. Chemical transformations that are being explored in order to add value to glycerol include dehydration [4], [5], [6], [7], [8], [9], oxidative dehydration [10], [11], [12], ammoxidation [13], [14], hydrogenolysis [15], [16], [17], [18], etherification [19], [20], [21], esterification [22], [23], selective oxidation [24], [25], and acetalization [26], [27], [28], [29].
The acetalization of aldehydes and ketones with glycerol has been extensively studied using molecules such as formaldehyde [30], acetaldehyde [31], benzaldehyde [32], butyraldehyde [33], [34], and acetone [35]. The acetalization of acetone with glycerol over acid catalysts (Scheme 1) can be used to produce cyclic acetals such as solketal. These compounds are especially interesting because their use as fuel additives reduces particulate emissions and improves the cold-flow properties [1], [36]. The addition of 1–5 vol.% of solketal to gasoline has been found to significantly decrease gum formation caused by fuel degradation, while improving the octane number [37].
Conventionally, this reaction can be performed using heterogeneous or homogeneous catalysts, but for economic and environmental reasons, heterogeneous catalysts are normally preferred. Most studies of acetone acetalization using heterogeneous catalysts have focused on determining the effects of the molar ratio of reactants, catalyst loading, and time and temperature of the reaction. Brønsted acid catalysts including zeolites [35], Amberlyst resins [35], [38], and supported heteropolyacids [39] are commonly used, but good results have also been obtained using oxides of transition metals such as Hf and Zr supported on silica frameworks, which provide Lewis acid sites [40]. A promising transition metal for use in heterogeneous catalysis is vanadium, which has not yet been studied in this reaction. Vanadium-based catalysts have well known uses in oxidative processes such as oxidative dehydrogenation [41], oxidative dehydration [11], and selective oxidation reactions [42]. In addition to their use in oxidation reactions, this class of catalysts also exhibits moderate acidity, combining Lewis and weak Brønsted acid sites [43], [44]. Essentially three types of vanadium oxide species can be found at the surfaces of the catalysts: monomeric VO4 and oligomeric VOx species, with tetrahedral coordination, and V2O5 crystallites. [45] The formation of these species is influenced by multiple factors including the synthesis and impregnation methods, the vanadium precursors employed, and the textural and chemical properties of the support.
The structural properties of the catalyst are critical in this reaction, since diffusion of the reactants and products at the catalyst in liquid phase reactions is favored by open-framework structures, where the active sites are more dispersed and are easily accessible. Catalysts based on zeolites offer good performance due to their high porosity and surface area. Ferrierite zeolites (FER structure) have shown remarkable catalytic performance in various processes. The discovery of the FER structure lamellar precursor called PreFER [46] has made it possible to develop different types of materials derived from this structure. Purely microporous FER zeolite can be obtained by direct calcination of the precursor, while the micro/mesoporous ITQ-6 zeolite is produced by expansion of the precursor layers with CTABr, sonication, and calcination (Scheme 2). The mesoporosity in ITQ-6 is generated by spaces between disorganized layers. In addition to the higher surface area of this material, compared to other mesoporous materials such as MCM-41, it was found to be structurally more stable (the structure did not collapse), even after impregnation using high metal loadings [41].
In the present work, the silicalite forms of purely microporous (FER) and micro/mesoporous (ITQ-6) zeolites were synthesized using the same precursors and were employed as supports for vanadium oxide impregnation. The resulting catalysts were characterized by X-ray diffraction, nitrogen physisorption, diffuse reflectance UV–vis spectroscopy, X-ray absorption (XANES and EXAFS), and temperature-programmed desorption of ammonia. The catalysts were applied in acetone acetalization with glycerol. The preparation of these two supports with the same structure but with different porosity helped to understand the ways in which porosity influences the dispersion of vanadium at catalyst surfaces, as well as the diffusion of reagents and products to and from the active sites.
Section snippets
Si-PreFER lamellar precursor
An aqueous solution (22.9 g of distilled water) containing 7.5 g of ammonium fluoride, 1.0 g of hydrofluoric acid, 19.9 g of 4-amino-2,2,6,6-tetramethylpiperidine (R), and 7.7 g of fumed silica (molar composition: 25 SiO2: 10 HF: 40 NH4F: 25 R: 250 H2O) was prepared and stirred for 180 min at 30 °C, followed by heat treatment in a stainless steel autoclave for 120 h at 150 °C. The solid product was filtered and washed with distilled water.
Si-FER microporous support
The microporous support was obtained by calcination of the
Results and discussion
The X-ray diffraction patterns (Fig. 1A) confirmed formation of the crystalline structures of the lamellar precursor, Si-PreFER, and the microporous support, Si-FER [46]. During the process of exfoliation and formation of the micro/mesoporous support, Si-ITQ-6, the diffraction peak intensity was suppressed, due to the structural disorganization of the layers [41], [47]. The X-ray diffraction patterns of the catalysts after vanadium impregnation of Si-FER (Fig. 1B) and Si-ITQ-6 (Fig. 1C) showed
Conclusions
The lamellar precursor Si-PreFER and the supports Si-FER and Si-ITQ-6 were successfully synthesized. After impregnation, the supports showed high structural stability, even with high vanadium loadings. Three main vanadium species were identified on the catalyst surfaces: VO4 monomers, VOx oligomers, and V2O5 agglomerates. When Si-FER was used as the support, the formation of V2O5 agglomerates was observed on the [5V]Si-FER sample, while the Si-ITQ-6 support only showed the presence of V2O5 on
Acknowledgements
This work was supported by the Brazilian agencies FAPESP (grants #2013/10204-2, #2014/20116-6 and #2016/10597-2) and CNPq (grant #304698/2014-8). The authors also thank the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas for use of the XPD (proposals XPD-18893 and XPD-20150244) and XAFS 1 (proposal 20150135) beamlines.
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