Hydrotreatment of sunflower oil using supported molybdenum carbide

Abstract

Pure sunflower oil was hydrotreated (T = 633 K, P = 5 MPa) aiming the production of a biofuel in the diesel range using β-Mo₂C/Al₂O₃ as catalyst. The catalyst was synthesized in situ using the temperature-programmed carburization (TPC) methodology with a 20% (v/v) CH₄/H₂ gas mixture and 923 K/2 h as synthesis temperature. The catalytic evaluation results indicate that for the employed conditions n-C₁₈ was the major product. The association of the results of the experiments without (blank) and with catalyst suggests that the overall triglyceride transformation into linear alkanes proceeds in two steps: (i) thermal cracking of the triglyceride forming free fatty acids and (ii) hydrogenation of the double bonds and of the carboxylic group of the free fatty acid forming n-alkanes. No CO and/or CO₂ formation were detected implying that decarbonylation and/or decarboxylation routes do not play an important role when molybdenum carbide is used, contrarily to what is commonly observed when supported Co–Mo or Ni–Mo sulfides are employed as catalysts.

Introduction

In addition to the growing concerns regarding the increased consumption of oil products, natural gas and coal to generate all of the energy required primarily by the industrial and transportation sectors, additional concerns have arisen with respect to environmental issues, which has resulted in the creation of stricter laws worldwide. The increase in CO₂ emissions from the burning of oil and coal is currently the major international concern because it allegedly contributes significantly to the greenhouse effect. In this sense, the search for alternative sources of renewable energy with minimum emissions is necessary.

Biodiesel is produced from the process of transesterification of triglycerides (vegetable oils or fats) and has been used in several countries in its pure form or mixed with petroleum diesel. The major disadvantage of this process is related to the production of large amounts of glycerol, which has significantly impacted the market of this product. If this process is effectively implemented in large countries, such as Brazil, the United States and China, or in economic regions, such as the European Community, the supply of glycerol will be so high that experts predict that there will be a collapse in the market for this product.

Several studies available in the literature show that the hydrotreating (HDT) process employed in refineries around the world can be used to obtain hydrocarbons in the range of diesel and gasoline from pure vegetable oils or from mixtures of vegetable oil and diesel using the conventional catalytic HDT technology and commercially available catalysts [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11].

The vast majority of studies dealing with the processing of vegetable oils by hydrotreating have utilized the NiMo/Al₂O₃ sulfide catalyst [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. However, studies exploring the use of CoMo/Al₂O₃ [11], [12], [13], [14], Pt deposited on various supports [15], [16], [17], [18], Pd/SAPO-31 [19], commercial hydrocracking catalysts [15], [17], [19], [20], NiMoW/Al₂O₃ [21], [22] as well as the use of CoMo and NiMo sulfide phases on supports such as SiO₂ [14], [18], MCM-41 [4], [23] and zeolites [7], [9], [11], [13], [14], [18] have also been reported.

If from one side the used of sulfide catalysts in the co-processing of vegetable oils and refinery streams is advantageous because the existing infrastructure can be used without too many modifications, from the other it is not convenient for the processing of streams of pure vegetable oil unless some sulfur is added to them in order to keep the catalysts in its active form. In fact Kubička and Horáček [14] have shown that when a CoMo/Al₂O₃ sulfide catalyst was used for prolonged periods of time it underwent deactivation due to the loss of sulfur from the active phase. Therefore, the addition of a sulfiding agent to the pure vegetable oil to be processed, such as CS₂ or dimethyl disulfide (DMDS), is necessary to minimize catalyst deactivation. However, the addition of sulfiding agents to vegetable oil eliminates the advantage of obtaining sulfur-free biofuels because, depending on the operating conditions employed, sulfur compounds may be present in the final product [14].

Quite recently other phases than sulfides or noble metals have been explored as catalysts for the hydrodeoxygenation of vegetable oils. Amongst these new phases there are nitrides of molybdenum, vanadium and tungsten supported on γ-Al₂O₃ [24], molybdenum carbide supported on carbon [25], and nickel phosphide supported on SBA-15 [26]. There are several advantages on the use of these phases, the most important ones being those related to stability (no deactivation with time on stream), sulfur tolerance (allowing the processing of mixed phases of vegetable oil and diesel), and in the case of Ni₂P/SiO₂ a lower decarboxylation/decarbonylation ration was observed as shown by Yang et al. [26]. The lower CO₂ and CO production could be an advantage for the refiners, as they are not used to deal with these gases during normal hydrotreating of oil fractions otherwise modifications in the process have to be implemented [27].

It is well known from the literature that transition metal carbides, especially those of molybdenum and tungsten, are excellent hydrotreating catalysts [28] but so far these phases have been little explored in the HDT of vegetable oils or free fatty acids (FFA). For this reason, the main objective of this work was to evaluate Mo₂C/Al₂O₃ as catalysts in the HDT of pure commercial sunflower oil using the same reaction conditions (T, P, and WHSV) as those employed by Huber et al. [2] who have used a commercial sulfided NiMo/Al₂O₃ catalyst, in order to verify not only if there is any differences in conversion levels and product distribution but also to evaluate the possibility of employing molybdenum carbide as catalyst in the hydrotreating of pure vegetable oils.