Hydrothermal synthesis of Mo-V mixed oxides possessing several crystalline phases and their performance in the catalytic oxydehydration of glycerol to acrylic acid

Highlights

• Acrylic acid was the main condensed reaction product.

• Mo_4.65V_0.35O₁₄ crystallographic phase enhanced catalytic performance.

• Catalytic reactions under pure O₂ showed the best results.

• V⁴⁺/V⁵⁺ equilibrium was crucial for maximizing selectivity towards acrylic acid formation.

Abstract

The one-step oxydehydration of glycerol to acrylic acid over molybdenum and vanadium mixed oxides was investigated. The Mo-V oxide catalysts were prepared by a simple hydrothermal method under different synthesis and calcination atmospheres and were characterized by in situ XRD, TPD-NH₃, N₂ adsorption/desorption, X-ray absorption near vanadium K-edge spectroscopy and thermogravimetry. The catalytic performance of the samples at different temperatures (290, 320 and 350 °C) and under different gas flow compositions (20% O₂ in N₂, 100% O₂, or 100% N₂) revealed that the arrangement of the crystallographic structures of the active phases directly influenced the catalytic performance. It was found that the catalysts heat-treated in oxidizing atmosphere gave superior catalytic results comparing with the catalysts heat-treated in inert atmosphere due to the equilibrium between the crystalline phases MoVO₅ and Mo_4.65V_0.35O₁₄ that contains V⁺⁴ and V⁺⁵. Catalytic oxydehydration at 320 °C under a flow of 100% O₂ gave the best performance, achieving selectivity of 33.5% towards acrylic acid and 100% conversion of glycerol.

Introduction

The depletion of fossil fuel reserves and the need for cleaner energy sources have intensified the search for alternative fuels that are cleaner and less harmful to the environment [1], [2], [3]. Biodiesel has proved to be a good option and its production involves a simple process that essentially occurs by the transesterification of oils and fats derived from plant and animal sources with short chain alcohols, typically methanol, in the presence of catalysts [4], [5]. Glycerol is a major co-product of this process, equivalent to 10 wt.% of the biodiesel production. The use of biodiesel increases every year, and it is forecast that in 2020 biodiesel production will be nearly 36.9 million metric tons, 10 million metric tons more than the current production. Therefore, for the success of the biodiesel production chain, it is essential to find pathways to use or consume glycerol [6], [7]. Due to its non-toxicity, glycerol has many commercial applications, such as in the manufacture of medicines, foods, beverages, and textiles [8]. Beyond these applications, other pathways are being studied at the laboratory scale, and this is the case of glycerol dehydration and oxidation to produce value-added products such as acrolein and acrylic acid [7].

Acrylic acid is an important industrial bulk chemical used mainly to produce superabsorbent polymers, due to its high liquid absorbance capacity [9]. It can be produced by glycerol dehydration to form acrolein, followed by oxidation of acrolein to acrylic acid, by means of a two-step process occurring in a single or double catalytic bed [10], [11]. In 2011 Dubois et al. [12], [13] patented a method for producing acrylic acid from glycerol (US 7,910,771 B2 and US 8,212,070 B2), but using a double bed reactor with two catalysts. Although presenting a high selectivity, a high catalyst mass in relation to the glycerol feed is used. Comparing the productivity of acrylic acid per gram of catalyst per hour, a bifunctional catalyst is more valuable.

In order to perform the glycerol conversion to acrylic acid using a single catalysts, this catalyst has to be bifunctional, because acid and redox active sites are needed for the dehydration and oxidation steps, respectively [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. The use of a bifunctional catalyst to convert glycerol not only has engineering and economic advantages, but also minimizes the use of petrochemical resources as raw materials, hence providing a competitive route for the production of acrylic acid [12], [13], [14].

Approximately 75% of applications in the chemical industry use mixed oxide catalysts [22], which have been studied for many gas phase reactions including ethane oxidation [23], [24], ethane oxidative dehydrogenation [25], selective oxidation of light alkanes [26], propane oxidation [27], [28], glycerol dehydration to acrolein [29], and more recently the conversion of glycerol to acrylic acid [10], [20], [30]. Some works have also reported the transformation of glycerol to acrolein by a liquid phase reaction, giving a great catalytic activity [31], [32]. Although materials such as W-Mo-V oxides [17], [18], W-V oxides [20], [30], Mo₃VO_x/H₄SiW₁₂O₄₀/Al₂O₃ [16], and W-V-Nb oxides [14] have shown high capacity to convert acrolein to acrylic acid, only a few studies have reported good selectivity [14], [33], [34], Omata et al. [34] was one of the few that achieved a high selectivity to acrylic acid using W-V-Nb mixed oxides (59.2%), but with the modification of the surface with phosphoric acid. One of the first studies of mixed oxides containing vanadium and molybdenum catalysts occurred in 1978, when Thorsteinyon et al. [35] produced a mixed oxide of vanadium, molybdenum, and niobium to perform the oxidative dehydrogenation of ethane. More recently, Mo_xV_yO_z catalysts have gained attention, because these catalysts not only possess the acid sites that are necessary for the dehydration of glycerol, but also present oxidative potential, which is essential for the oxidation step [17], [31], [32], [33]. Furthermore, only a few studies have used molybdenum and vanadium as the only metals in catalysts for the oxydehydration of glycerol to acrylic acid [19], [30], [36], so there is still great potential for exploration of the use of Mo_xV_yO_z catalysts in this catalytic reaction.

In this study, we synthesized mixed Mo-V oxides possessing several crystalline phases and assessed their performance in the catalytic oxydehydration of glycerol to acrylic acid. The influence of the synthesis procedure on formation of the active catalyst components and, consequently, on the oxydehydration reaction, was explored using a combination of several techniques: in situ X-ray diffraction (XRD), ammonia temperature programmed desorption (NH₃-TPD), elemental chemical analysis, thermogravimetry, and X-ray absorption near vanadium K-edge spectroscopy.