The multiple benefits of glycerol conversion to acrolein and acrylic acid catalyzed by vanadium oxides supported on micro-mesoporous MFI zeolites
Graphical abstract
Introduction
The growth of greenhouse gas emissions and the limited reserves of easily extracted fossil fuels have led researchers and industry to pursue new alternatives to replace, even partially, the use of fossil fuels. As a result, the use of compounds derived from biomass, such as biogas, ethanol from sugarcane, and biodiesel has increased in recent years. In the case of biodiesel, production has increased and consequently there has been an increase in the concomitant formation of coproduced glycerol [1], [2], [3].
Glycerol is a focus of green catalytic processes, because this molecule offers interesting chemical versatility that can be exploited for the formation of compounds that are currently provided by the petrochemical industry. An example of glycerol valorization is the synthesis of 1,2-propanediol and 1,3-propanediol, which are used as antifreeze fluids in automobiles. In industry, both compounds are obtained from the hydration of propene. However, a high conversion and selectivity to glycols has been obtained by glycerol hydrogenolysis using catalysts based on metallic Ni, Ru, and Cu [4], [5], [6], [7].
Another important conversion of glycerol into petrochemical-type compounds is the formation of acrolein and acrylic acid, which are used in the manufacture of resins. Acrolein can be obtained by gas phase glycerol dehydration on acid catalysts such as heteropolyacids [8], impregnated phosphate groups on metal oxides [9], sulfated zirconia [10], Nb2O5 [11], mixed oxides [12], zeolites [13], [14], [15], functionalized mesoporous silica [16], [17], and vanadium-silicates [18]. The glycerol can also be dehydrated in liquid phase [19], [20]. Acrylic acid is obtained from the oxidation of acrolein. The one-step conversion of glycerol into acrylic acid using bifunctional catalysts with acid and redox sites occurs according to Scheme 1 [21], [22], [23], [24], [25], [26]. An interesting aspect of coupling the two reactions is the mutually supporting endothermic dehydration of glycerol (ΔH0 = 3.04 kcal/mol) and exothermic oxidation of acrolein (ΔH0 = − 61.02 kcal/mol).
For glycerol dehydration using zeolites, maximum performance of the catalyst is achieved by combining Brønsted acid sites of medium strength (strong acid sites lead to severe coke formation, while weak acid sites are less capable of converting glycerol) [27], porosity (which enhances the diffusion of glycerol and acrolein), and high specific area (which increases access to catalytic sites). For instance, members of the lamellar MWW zeolite family, which includes microporous MCM-22, pillared MCM-36, and delaminated ITQ-2, offer advantageous characteristics for glycerol dehydration [22]. Following pillarization and delamination of the MWW structure, the strengths of acid sites decrease, but the increases in mesopores and specific area raise the overall performance of the catalyst [28].
Despite the attraction of lamellar zeolites for use in glycerol dehydration, the laborious multiple steps and the expense associated with catalyst preparation are notable disadvantages. Alternatively, the desilication of commercially available zeolites by treatment with sodium hydroxide solution seems to be more practical [29], [30]. The alkaline process is simple, with hydroxyl groups attacking and removing silicon atoms from the zeolite structure, creating randomly distributed pores in the zeolite crystals. The diameter and volume of the pores can be tuned by adjusting the concentration of the alkaline solution and by varying the exposure time of the zeolite (usually a few minutes) and the desilication temperature (which normally ranges from room temperature to a few tens of degrees Celsius) [31], [32], [33], [34], [35], [36], [37]. The broad distribution of mesopore families results in catalytic performance in glycerol dehydration similar to that of the MWW zeolites.
A disadvantage of the desilication method is that during the zeolite treatment process, aluminum atoms are removed as well as silicon atoms. Silicon species are mostly found in the alkaline liquid phase, but aluminum tends to form insoluble oligomeric species that can precipitate on the catalyst surface as extra-framework aluminum atoms (EFA). Consequently, the mesopores created are obstructed due to an alkali-induced alumination of the external surfaces of the crystals, and the nature of the acid sites of the zeolite shifts from Brønsted to Lewis acid sites. This catalytic acid behavior must be considered in the design of catalysts by desilication, because EFA sites are selective in converting glycerol into undesirable byproducts. However, the EFA can be removed from the zeolite by acid leaching; as a result, the selectivity to acrolein is enhanced and the diffusion of chemicals through the pores is increased due to the removal of aluminum species.
In the second step of glycerol conversion to acrylic acid (Scheme 1), redox active sites are required. Vanadium oxides are strong candidates for this purpose because they possess a very important redox characteristic, namely the capacity to adopt multiple oxidation states. On these catalysts, acrolein is oxidized by removing a surface oxygen atom from V2O5, giving rise to acrylic acid and an oxygen vacancy in V2O5-x. In a subsequent step, the catalytic site is oxidized and reestablished by feeding an excess of molecular O2 in the stream (V2O5-x +1/2O2 → V2O5). This redox mechanism and the changes in V5+/V4+ oxidation states during the catalytic reaction are known as the Mars-Van Krevelen mechanism [38].
In a recent publication, we described additional useful features of the V2O5/zeolite catalytic system [20]. Besides the advantages mentioned above, vanadium oxides supported on zeolites were much less susceptible to deactivation, compared to the bare zeolites. Several parallel and unknown reactions occur simultaneously with glycerol dehydration to acrolein. Byproducts include acetaldehyde, acetol, and acetic acid, as well as very harmful and deactivating coke molecules. After catalytic experiments with bare zeolites, the polymerization of bulky molecules on the surfaces of the catalysts led to coke formation and a characteristic black appearance. However, in the previous work it was found that when a V2O5/zeolite catalyst was used, the coke was continuously oxidized due to the presence of well-dispersed vanadium oxides on the zeolite surface, which maintained the catalytic sites active for longer periods.
The aim of the present work was to explore further the multiple benefits of porous V2O5/MFI catalysts in the one-step glycerol conversion to acrylic acid. The work focused on zeolite supports prepared by sequential processes of desilication (in NaOH solution) and dealumination (in HCl or oxalic acid solutions) in order to tailor the pores and the quality of acid sites derived from either aluminum in tetrahedral coordination in the zeolite or from EFA. Improved transformation of glycerol was achieved on the micro-mesoporous V2O5/MFI zeolites, due to higher catalytic conversion, improved selectivity to acrolein and acrylic acid, extended catalyst stability, and decreased coke formation.
Section snippets
Preparation of zeolite supports
Zeolite of MFI structure (Si/Al mole ratio of 40) was kindly provided by Zeolyst (USA). The sample was submitted to alkaline treatment at 60 °C for 1 h using an aqueous solution of NaOH (0.6 mol/L). Detailed information concerning the desilication procedure is provided elsewhere [30], [39]. The desilicated zeolite was then submitted to two different acid treatments using aqueous solutions of hydrochloric or oxalic acids. The acidic treatments were performed under reflux using 0.1 mol/L acid
Characterization of the zeolite supports
The XRD patterns of the MFI zeolites before treatment (sample A in Fig. 1), after the alkaline treatment (sample B), and after the acidic treatments (samples C and D) revealed the presence of the main reflections related to the MFI structure: (011), (200), and (051). The samples submitted to the treatments showed decreases in peak intensity, consistent with the reduction of long-range order caused by the extraction of silicon and aluminum atoms from the MFI structure [35]. The oxalic acid and
Conclusions
The alkaline treatment of ZSM-5 zeolite with NaOH was effective for obtaining a micro-mesoporous zeolite by partial disruption of the crystalline framework due to silicon removal. A portion of the framework aluminum atoms in tetrahedral coordination was also removed, but due to their low solubility, these atoms remained on the zeolite as EFA (extra-framework aluminum). The removal of EFA was achieved by subsequent acid treatment using either hydrochloric acid or oxalic acid. The latter was more
Acknowledgements
This work was supported by the Brazilian agencies CNPq (grants 473456/2012-5 and 401679/2013-6) and FAPESP (grants 2013/10204-2, 2013/50023-7 and 2014/11952-5). The authors also thank the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas for use of the XPD beamline (proposal XPD-17839).
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