Article pubs.acs.org/IECR
Hydrodeoxygenation of Methyl Heptanoate over Rh/ZrO2 Catalyst as a Model Reaction for Biofuel Production: Kinetic Modeling Based On Reaction Mechanism Yuwei Bie,* Jaana M. Kanervo, and Juha Lehtonen Industrial Chemistry Research Group, Department of Biotechnology and Chemical Technology, Aalto University, P.O. BOX 16100, FI-00076 Aalto, Espoo, Finland S Supporting Information *
ABSTRACT: The kinetic experiments for the hydrodeoxygenation (HDO) of methyl heptanoate were studied over Rh/ZrO2 catalyst in a three-phase batch reactor in the temperature range 250−330 °C and in the pressure range 60−100 bar with H2. The complex HDO reaction network of methyl heptanoate was simplified and modeled using both empirical power-law models and mechanistic models. Surface reaction mechanism was for the first time applied to develop a mechanistic model for the key reaction pathways in the HDO reaction network. Two types of active sites were assumed for the adsorption of oxygenated components (ester, fatty acid, and alcohol) and H2, respectively. Both the power-law model and the mechanistic model adequately described the features of HDO kinetics, while the mechanistic model outperformed the power-law model. Kinetic modeling was performed rigorously by taking into account the vapor−liquid equilibrium and thermodynamic nonideality in the three-phase batch reactor model. however, need to be sulfided to maintain catalytic activity.4−7 To meet the increasingly stringent sulfur emission limitations, more focus has been placed on the development of nonsulfided transition metal or noble metal catalysts, such as Pt, Rh, Pd, Ni, Co supported on C, or various oxides (SiO2, Al2O3, ZrO2, CeO2).3,8−14 Gosselink et al. have recently reviewed the reaction pathways for the HDO of vegetable oils and related model compounds over various catalysts and different process conditions (feeds, temperature, hydrogen partial pressure, etc.). Different catalysts appear to favor distinct reaction pathways, indicating that precise mechanisms of HDO reaction steps, which depend on the nature of the catalysts employed, still need further research.3 ZrO2 supported metal catalysts have been recently found to be promising catalyst materials for HDO applications.8,11 In our recent work, a Rh/ZrO2 catalyst was tested for HDO of methyl heptanoate, which was used as a model compound for triglyceride, and a comprehensive reaction network was proposed (Figure 1).8 The HDO reaction was found to proceed initially via hydrogenolysis of methyl heptanoate to heptanoic acid intermediate, which was further deoxygenated into hydrocarbons via the formation of aldehyde intermediate. ZrO2 supported noble metal catalysts were found to favor the formation of hydrocarbon product (mainly hexane) with one carbon atom shorter than the original fatty acid chain via decarbonylation/decarboxylation reaction. The dehydration reaction of heptanol intermediate for the heptane formation was suppressed, which, however, is a significant end product over sulfided NiMo/CoMo supported on Al2O3 catalysts.6 Peng
1. INTRODUCTION With increasing concerns about fossil fuel shortages and environmental challenges, the sustainable production of biofuels from renewable feedstocks for the transportation sector has become a very important subject. Triglyceride-based feedstocks, such as vegetable oils (especially nonedible), animal fats, and algae oils, are promising for that purpose. Catalytic transesterification has been widely used to transform vegetable oils into biodiesel (fatty acid methyl esters) which can be blended with conventional diesel fuel. However, due to the relatively high oxygen content, biodiesel on its own still possesses some poor fuel properties, such as low heating value, chemical instability, high viscosity, and poor cold weather performance. In recent years, hydrodeoxygenation (HDO), applying conventional hydroprocessing technology, has attracted increasing attention for directly transforming triglyceride-based feedstock into diesel-like hydrocarbon fuels. From a viewpoint of economics, one of the most important advantages of the HDO process is that the capital cost can be minimized by taking advantage of existing plants and equipment.1−3 The HDO process to convert triglycerides into hydrocarbon products typically involves complex catalytic reaction pathways such as hydrogenolysis, hydrogenation, decarboxylation, decarbonylation, dehydration, water gas shift (WGS), methanation, etc. A variety of oxygenated intermediates (fatty acids, alcohols, and aldehydes) and other light compounds (CO2, CO, CH4 and H2O) are formed as well.3 Catalysts are essential for HDO reactions, and a significant amount of research has been dedicated to developing catalysts and to understanding the HDO reaction chemistry by using model compounds of real oil/fat feedstocks. The most selected representative model compounds are fatty acid methyl esters or triglycerides.3 The conventional hydrotreating catalysts (NiMo or CoMo/Al2O3) have been widely studied for the HDO reactions, which, © 2015 American Chemical Society
Received: Revised: Accepted: Published: 11986
September 2, 2015 November 12, 2015 November 16, 2015 November 16, 2015 DOI: 10.1021/acs.iecr.5b03232 Ind. Eng. Chem. Res. 2015, 54, 11986−11996
Article
Industrial & Engineering Chemistry Research
results can be found in our previous study.8 In a typical HDO experiment, Rh/ZrO2 (0.44 g, 0.25−0.42 mm) was placed into the spinning catalyst basket and reduced in situ with 20 bar of static hydrogen pressure at 350 °C for 1 h. After the reduction, 30 mL of 5 wt % methyl heptanoate (Fluka, >99%) diluted in hexadecane (solvent, Sigma-Aldrich, >99.9%) was introduced into the reactor at the room temperature. The reactor was heated to the target temperature with minor stirring under a low hydrogen pressure (30 bar) to minimize the progress of reaction during the heating. After having reached the target temperature, the reactor was pressurized to the targeted H2 pressure and stirring (700 rpm) was switched on, marking the start of the experiment. In each HDO experiment, four to six liquid samples (about 0.6 mL for each) were collected from sampling tube for the quantitative analysis by gas chromatography (GC) and the gas phase was qualitatively analyzed by GC at the end of experiment. The pressure drop caused by the sampling was compensated by recharging H2 to restore the original target pressure. Table 1 shows the range of experimental conditions employed to collect kinetic data. For HDO of methyl
Figure 1. Reaction scheme in HDO of methyl heptanoate over Rh/ ZrO2 catalyst. Adapted from ref 8. Copyright 2013 American Chemical Society.
et al. have performed a systematic investigation on the HDO of palmitic acid as a model compound and real microalgae oil feed by using Ni/ZrO2 catalyst and have gained valuable mechanistic insights based on in situ IR spectroscopy technique. They found that hydrodeoxygenation of fatty acid can be catalyzed either solely by Ni or synergistically by Ni and the ZrO2 support.11 In spite of extensive studies on HDO reactions of triglyceride-based feeds, kinetic study and more detailed modeling have been scarcely reported in the open literature. The development of a suitable kinetic model is very important for process design and for fundamental understanding of reaction chemistry. In the limited number of kinetic reports on HDO reactions of triglycerides and their derivatives, empirical power-law models have been employed to correlate the experimental data and to obtain kinetic parameters, such as activation energies and reaction rate constants.15,16 To the best of our knowledge, a mechanistic model for HDO of triglyceride-like feeds has not yet been reported, though some groups have developed mechanism-based models to describe the decarboxylation/decarbonylation of fatty acids or esters into hydrocarbon compounds based on the assumption of a conventional Langmuir−Hinshelwood-type mechanism.17,18 The aim of this work is to carry out kinetic modeling for the HDO reaction pathways of methyl heptanoate over Rh/ZrO2 catalyst. Both empirical power-law models and mechanistic models will be used for kinetic modeling. A surface reaction mechanism for the key HDO reaction pathways will be proposed in order to derive the mechanistic rate expressions. Vapor−liquid equilibrium and thermodynamic nonideality in the multiphase reactor system will be taken into account in the dynamic batch reactor model used for modeling.
Table 1. Reaction Conditions for Kinetic Experiments
a
process variable
value
catalyst mass methyl heptanoate heptanoic acida heptanola solvent (hexadecane) temperature range pressure range reaction time
0.44 g (constant) 5 or 7 wt % 4.5 wt % 4.5 wt % 21 g 250−330 °C 60−100 bar (H2) 3−8 h
Used as reactants in separate HDO experiments.
heptanoate, four temperatures (250, 275, 300, and 330 °C) were tested under 80 bar total pressure with H2 using 5 wt % methyl heptanoate. The influence of H2 pressure was tested by using total pressures of 60, 80, and 100 bar at 300 °C. Two initial concentrations of methyl heptanoate were tested at 300 °C under 80 bar H2 pressure. The HDO of reaction intermediates such as 1-heptanoic acid (Merck, ≥99%) and 1-heptanol (Merck, ≥99%) as reactants were separately tested at 270 °C under 80 bar total pressure with H2.
3. RESULTS OF KINETIC EXPERIMENTS A reaction network (Figure 1) has been previously proposed to describe the HDO reaction pathways of methyl heptanoate based on the comprehensive analysis of liquid phase products.8 In this current study, a modified reactor system equipped with a spinning catalyst basket was applied to collect a consistent set of kinetic data. Even though the spinning basket reactor may have improved the hydrodynamics and mass transport compared to the previously used stationary basket reactor, however, the qualitative observations of products were in agreement with the earlier findings by using an identical Rh/ ZrO2 catalyst. In the HDO of methyl heptanoate over Rh/ZrO2 catalyst under H2 atmosphere, hexane was the dominant deoxygenated product while the formation of heptane was negligible (yield
