(V)/Hydrotalcite, (V)/Al2O3, (V)/TiO2 and (V)/SBA-15 catalysts for the partial oxidation of ethanol to acetaldehyde

Highlights

• High activity found for V/TiO₂ catalyst in the partial oxidation of Ethanol.

• First use of a (Vanadium)/Hydrotalcite catalyst for the partial oxidation of Ethanol.

• Novel 1,3,5 Trioxane production from the ethanol oxidation.

Abstract

Vanadium-based catalysts have been investigated in the partial oxidation of ethanol to acetaldehyde with the aim of understanding relationship between vanadium structure and acetaldehyde productivity. Hydrotalcite, Al₂O₃, TiO₂ and SBA-15 with and without a 5% of vanadium content were prepared to study the oxidative dehydrogenation of ethanol. They were characterized by XRF, TPR (H₂), NH₃-TPD, CO₂-TPD, RAMAN, UV-vis, Nitrogen physisorption, XRD and SEM. The most easily reducible catalysts (as determined by TPR) were the most active ones. In the low temperature region (150 °C), the most active catalyst was the V/TiO₂ which presented stable activity in the production of acetaldehyde up to TOS = 200 h. On the contrary, in the high temperature region (250 °C), the most active catalyst was the V/Al₂O₃catalyst. The most promising result was obtained over V/TiO₂ catalyst that afforded a total ethanol conversion of 60.4%wt. and a selectivity to acetaldehyde of 76.2%wt. at TOS = 164 h and T = 150 °C. Also, hydrotalcite was tested for the first time for this type of reaction providing a conversion lower than 7%wt. with a selectivity of 100%wt. to acetaldehyde at T = 150–225 °C.

Introduction

Green production of chemicals is in the spotlight both academic and industrial research worldwide with the aim to decrease the fossil carbon footprint. As a result, platform chemicals from biomass have attracted significant attention in the recent years. Ethanol (bio-ethanol) produced by fermentation of sugars is one of the most important platform chemicals. As a result of the legislation-driven increase in demand for biofuels, the production of bioethanol has significantly increased (from ca. 10 billion US gallons in 2005 to ca. 15 billion US gallons in 2007 with predicted increase to 30 billion US gallons in 2016 [1]) over the past years resulting in a drop in its price and increased availability. Currently, application of bio-ethanol as a fuel accounts for around 75% of the total amount of bio-ethanol produced [2] but the decrease in price makes the more extensive use of bio-ethanol for the production of the higher-value bulk chemicals very attractive. The use of bio-ethanol could lead also to higher CO₂ savings than its use as a fuel [3].

Acetaldehyde is an important intermediate used for the production of various bulk chemicals, including acetic acid, acetic anhydride, ethyl acetate, per acetic acid, butanol, 2-ethylhexanol, pentaerythritol, chlorinated acetaldehydes, glyoxal, alkyl amines, pyridens and others [4]. In 2012 the global acetaldehyde consumption was divided mainly between the production of pyridine and pyridine bases (16% of total production), pentaerythritol (16% of total production), acetate esters (14% of total production), the remainder being used for the synthesis of 1,3-butylene glycol, croton aldehyde and glyoxal, along with some smaller-volume derivatives [4], [5]. Recently, new applications for acetaldehyde have been proposed. As acetaldehyde can be synthesized from various stating materials the choice of it depends on its price and its availability. Today, mainly ethylene is used due to its wide availability and low price for synthesis of acetaldehyde by direct oxidation, while ethanol and acetylene are used only to a small extent. Ethylene oxidation to acetaldehyde, so called Wacker process, developed in late 1950s, consists of oxidation of ethylene using aqueous PdCl₂ and CuCl₂ as catalysts. Although this reaction is characterized by advantages such as small amount of PdCl₂ required for the reaction and regeneration of the catalyst, there are some drawbacks which make this reaction economically quite demanding. Among them belongs i) the need of using corrosive resistant materials together with expensive titanium reactor tubing or ii) need of purification of waste air and treatment of wastewater, in order to remove acetaldehyde, unconverted ethylene and mainly chlorinated hydrocarbons, which are highly toxic and show antimicrobial activity. Therefore, they must be treated before entering the wastewater plant to render them biologically degradable [4], [6], [7]. Thus, the acetaldehyde production via the oxidative dehydrogenation (ODH) of ethanol could be a promising alternative to the Wacker process, occurring more simply in a single step and in tubular reactors, if high activities and selectivities can be achieved under mild conditions.

Supported vanadium oxide catalysts have been studied by different authors with the objective of elucidating their performance in the oxidation of ethanol [7], [8], [9]. The oxidation of ethanol to acetaldehyde is currently investigated with the aim of replacing the conventional processes based on hazardous agents such as chromate or permanganate [10]. Vanadium-based catalysts have been also used and studied for the partial oxidation or oxidative dehydrogenation reactions [3], [5]. Besides, the dispersion and nature of the vanadium over the adequate support is an important issue to obtain a high catalytic activity [11]. Several catalysts with some vanadium content were characterized with the aim of studying the type of vanadium present on their surface and its role in the partial oxidation of ethanol using high space velocity. Here we compare the performance of some vanadium oxides supported on alumina, titania, hydrotalcite and SBA-15 (SiO₂) supports.

Different authors used vanadium catalysts for the oxidation of ethanol. Chimentao et al. [12] studied the oxidation of ethanol to acetaldehyde over Na-promoted vanadium oxide catalysts obtaining an ethanol conversion of 20% with a selectivity to acetaldehyde of 95% at 250 °C (the ethanol was vaporized before feeding it to the reactor). Hsiu-Mei Lina et al. [13] used V₂O₅/TiO₂/MCM-41 catalysts for the catalytic oxidation of ethanol at 300 °C obtaining a conversion of 60% and a selectivity to acetaldehyde of 70%. In this case, a mixture of ethanol and air was used with WHSV = 2.5 h⁻¹. Kannan et al. [14] oxidized ethanol over microporous vanadium silicate molecular sieves with MEL structure at 300 °C and WHSV = 2.6 h⁻¹ obtaining a total conversion of 55% with a selectivity to acetaldehyde of 67%. The published work from Tóth et al. [15] was an example of using different metal oxides for the oxidation of ethanol. Nevertheless, in this work the metal used was the Rh and not the V. The total conversion in this case for the most active catalyst was 97% and the selectivity to acetaldehyde 13.8%. SBA-15 materials are being used mainly for other purposes different than the partial oxidation of ethanol. Nevertheless, some works were published such as the research from Gayoung Lee et al. [16] who used V₂O₅/SBA-15 materials for the oxidation of ethanol with the aim of producing hydrogen gas. Guoan Du et al. [17] used vanadium grafted SBA-15 for the oxidation of methanol but not for ethanol and Li et al. [18] used Ni/SBA-15 for the steam reforming of ethanol.

Many works informed about the characterization of the vanadium supported over different metal oxides [18], [19], [20], [21], [22], [23], [24], [25], [26]. These studies were a source of information which was used for the characterization of the catalysts tested in this work. An aim of this work was the characterization and obtaining a highly active and stable catalyst for the partial ethanol oxidation to acetaldehyde. In the Scheme 1 are represented the possible products which could be obtained from the oxidation of ethanol [9], [27], [28], [29]. The acetaldehyde is the suitable product in this case. Nevertheless, the acetaldehyde diethyl acetal could be produced by the reaction between the ethanol and the acetaldehyde as described by He et al. [29]. Other possible products could be the acetic acid as consequence of the oxidation of the acetaldehyde [27] or the diethyl ether by the dehydration reaction of ethanol.