Elsevier

Catalysis Today

Volume 289, 1 July 2017, Pages 20-28
Catalysis Today

The multiple benefits of glycerol conversion to acrolein and acrylic acid catalyzed by vanadium oxides supported on micro-mesoporous MFI zeolites

https://doi.org/10.1016/j.cattod.2016.08.005Get rights and content

Highlights

  • The alkaline treatment of zeolite was effective for a micro-mesoporous formation.

  • The removal of extra framework aluminum was achieved by subsequent acid treatment.

  • The dispersion of V2O5 increased the quantity of acid sites on the zeolites.

  • The beneficial effects of micro-mesoporosity and V2O5 dispersion was perceived in glycerol conversion.

Abstract

The ZSM-5 zeolite (MFI structure, Si/Al = 40) was treated using NaOH and either oxalic acid or HCl to obtain hierarchical materials with different characteristics, followed by impregnation with vanadium oxides (V2O5) to generate redox-active sites. The impact of the multiple treatments on the efficiency and stability of the catalysts in the conversion of glycerol to acrolein and acrylic acid was investigated and correlated with catalyst porosity, acidity, and chemical composition. The treated and impregnated V2O5 catalysts were subjected to XRD, 27Al NMR, nitrogen physisorption, TPD-NH3, TG, and UV–Vis analyses, in order to associate the properties of the catalysts with their activities. The studies showed that the catalytic performance of the materials depended on the acidic and textural properties of the zeolites, which influenced both the dispersion of V2O5 and its interaction with the acid sites of the supporting zeolites. All the catalysts provided conversion values exceeding 65%, even after 6 h on glycerol stream. The distribution of products strongly reflected the effects of pore formation, acid treatment with oxalic acid or HCl, and the presence of vanadium oxide. The effects of these modifications resulted in higher selectivity to acrolein and acrylic acid, a reduced rate of coke accumulation in the zeolite, and a longer catalyst lifetime.

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).

References (57)

  • C.A.G. Quispe et al.

    Renew. Sustainable Energy Rev.

    (2013)
  • M.A. Dasari et al.

    Appl.Catal. A-Gen.

    (2005)
  • T. Miyazawa et al.

    J. Catal.

    (2006)
  • P.K. Vanama et al.

    Catal. Today

    (2015)
  • H. Atia et al.

    J. Catal.

    (2008)
  • F. Wang et al.

    Appl. Catal. A-Gen.

    (2010)
  • F. Cavani et al.

    Appl. Catal. B-Environ.

    (2010)
  • J. Deleplanque et al.

    Catal. Today

    (2010)
  • Y.T. Kim et al.

    Appl. Catal. A-Gen.

    (2011)
  • C.S. Carriço et al.

    J. Catal.

    (2016)
  • J.P. Lourenco et al.

    Catal. Commun.

    (2012)
  • J.A. Cecilia et al.

    Appl. Catal. A-Gen.

    (2016)
  • A.S. Paula et al.

    Microporous Mesoporous Mater.

    (2016)
  • L. Shen et al.

    Chem. Eng. J.

    (2012)
  • L. Shen et al.

    J. Ind. Eng. Chem.

    (2014)
  • A. Chieregato et al.

    Catal. Today

    (2012)
  • A. Chieregato et al.

    Appl. Catal. B-Environ.

    (2014)
  • L. Shen et al.

    Chem. Eng. J.

    (2014)
  • L.G. Possato et al.

    Appl. Catal. A: Gen.

    (2015)
  • M.V. Rodrigues et al.

    Appl. Catal. A: Gen.

    (2015)
  • H. Decolatti et al.

    Microporous Mesoporous Mat.

    (2015)
  • L.G. Possato et al.

    J. Catal.

    (2013)
  • J.C. Groen et al.

    Microporous Mesoporous Mat.

    (2004)
  • P. Mars et al.

    Chem. Eng. Sci.

    (1954)
  • V. Paixao et al.

    Appl. Catal. A-Gen.

    (2011)
  • R. Giudici et al.

    Appl. Catal. A-Gen.

    (2000)
  • Z.M. Yan et al.

    J. Mol. Catal. A-Chem.

    (2003)
  • S. Storck et al.

    Appl. Catal. A-Gen.

    (1998)
  • Cited by (34)

    • Unveiling the contribution of Mo, V and W oxides to coking in catalytic glycerol oxidehydration

      2021, Molecular Catalysis
      Citation Excerpt :

      For example, Mo-V mixed oxides catalysts that Rasteiro et al. synthesized presented 33.5% selectivity to acrylic acid [23]. Possato et al.'s [24] work showed Mo-V-O-based catalysts for the glycerol oxidehydration to acrylic acid with 32% of selectivity to acrylic acid. However, as the reaction went on, the catalyst deactivated rapidly due to coke formation.

    • A review on one-pot synthesis of acrylic acid from glycerol on bi-functional catalysts

      2021, Journal of Industrial and Engineering Chemistry
      Citation Excerpt :

      However, excessive catalyst modification could cause over-oxidation or pore filling of the actives sites which consequently led to poorer acrylic acid selectivities [43,44]. The sustainability if this synthesis approach is also subjects to the drawbacks of feedstock price hike and insufficient supply of propylene in the coming decades due to other competing processes [49]. Fig. 2 illustrates the proposed oxidation pathway of propane over the Te-VPO catalyst to acrylic acid [50].

    • Controlling the porosity and crystallinity of MgO catalysts by addition of surfactant in the sol-gel synthesis

      2020, Catalysis Today
      Citation Excerpt :

      The development of porous materials is of great interest in many areas of scientific research, including catalysis, adsorption processes and medicine [1–7].

    • Activation of Mo and V oxides supported on ZSM-5 zeolite catalysts followed by in situ XAS and XRD and their uses in oxydehydration of glycerol

      2020, Molecular Catalysis
      Citation Excerpt :

      In order for these reactions to occur in a single catalytic bed, it is necessary to use bifunctional catalysts that possess acid and oxidative properties [12–14]. For instance, vanadium oxides supported on highly acidic zeolites, or bare mixed oxides (of Co, Fe, V, Mo, or W) possessing both acid and oxidative sites [15–19], have been successfully used in gas phase reactions for glycerol conversion [20–24]. In the case of W-Nb-O catalysts, only acrolein is produced, while acrylic acid is also produced when W-Nb-V-O catalysts are used [25].

    View all citing articles on Scopus
    View full text