ZrO2 Catalyst as a

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Hydrodeoxygenation of methyl heptanoate over Rh/ ZrO2 catalyst as a model reaction for biofuel production: kinetic modeling based on reaction mechanism Yuwei Bie, Jaana Kanervo, and Juha Lehtonen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03232 • Publication Date (Web): 16 Nov 2015 Downloaded from http://pubs.acs.org on November 21, 2015

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Industrial & Engineering Chemistry Research

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, Juha Lehtonen Industrial Chemistry Research Group, Department of Biotechnology and Chemical Technology, Aalto University, PO BOX 16100, FI-00076, Aalto, Espoo, Finland

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 of 250 °C-330 °C and in the pressure range of 60 bar-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 power-law model and mechanistic model adequately described the features of HDO kinetics, while mechanistic model outperformed the power-law

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model. Kinetic modeling was performed rigorously by taking into account the vapor-liquid equilibrium and thermodynamic non-ideality in the three-phase batch reactor model. 1. Introduction With the increasing concern about fossil fuel shortage and environmental challenge, the sustainable production of biofuels from renewable feedstocks for transportation sector has become a very important subject. Triglyceride-based feedstock, such as vegetable oils (especially non-edible), animal fats, algae oils, are promising for that purpose. Catalytic transesterification has been widely used to transform vegetable oils into biodiesel (fatty acid methyl ester) which can be blended with the 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 HDO process is that the capital cost can be minimized by taking advantage of existing plants and equipment. 1-3 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) and methanation, etc. A variety of oxygenated intermediates (fatty acid, alcohol and aldehyde) 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


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the HDO reaction chemistry by using model compounds of real oils/fats feedstock. 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, however, need to be sulfided to maintain the catalytic activity.

4-7

To

meet the increasingly stringent sulfur emission limitations, more focus has been placed on the development of non-sulfided 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. 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 et al. have performed systematic investigation


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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

Figure 1. Reaction scheme in HDO of methyl heptanoate over Rh/ZrO2 catalyst. Adapted with permission from reference 8. Copyright 2013 American Chemical Society. 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, mechanistic model for HDO of triglyceride-like feeds has not yet been reported, though some groups have developed mechanism-based models


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to describe the decarboxylation/decarbonylation of fatty acids or esters into hydrocarbon compounds

based

on

the

assumption

of

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. 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 non-ideality in the multiphase reactor system will be taken into account in the dynamic batch reactor model used for modeling. 2. Experimental Kinetic experiments for the HDO of methyl heptanoate were executed in a 50 ml stainless steel batch reactor (Parr). The reactor is equipped with a spinning catalyst basket as an impeller and a sampling tube. Rh/ZrO2 catalyst used for HDO reaction was prepared by incipient impregnation method. ZrO2 support (MEL chemicals, ECO100) was calcined at 700 °C for 10 h before impregnation and Rh/ZrO2 catalyst was calcined at 450 °C for 2 h. The obtained Rh loading is 0.4 wt.% and the measured surface area of Rh/ZrO2 catalyst is 24 m2/g. More detailed preparation procedures and characterization 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. Reactor was heated up to the target temperature with a minor stirring under a low hydrogen pressure (30 bar)


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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, 4 to 6 liquid samples (about 0.6 ml for each) were collected from sampling tube for the quantitative analysis by 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 re-charging H2 to restore the original target pressure. Table 1 shows the range of experimental conditions employed to collect kinetic data. For HDO of methyl heptanoate, four temperatures (250°C, 275°C, 300°C and 330°C) were tested under 80 bar total pressure with H2 using 5 wt% methyl heptanoate. Influence of H2 pressure was tested by using total pressure of 60 bar, 80 bar and 100 bar at 300°C. Two initial concentrations of methyl heptanoate were tested at 300 °C under 80 bar H2 pressure. HDO of reaction intermediates such as 1-heptanoic acid (Merck, ≥99%), 1-heptanol (Merck, ≥99%) as reactants were separately tested at 270 °C under 80 bar total pressure with H2. Table 1. Reaction conditions for kinetic experiments.

a

Process variables

Value

Catalyst mass

0.44 g (constant)

Methyl heptanoate

5 wt% or 7 wt%

Heptanoic acida

4.5 wt%

Heptanola

4.5 wt%

Solvent (hexadecane)

21 g

Temperature range

250 °C-330 °C

Pressure range

60-100 bar (H2)

Reaction time

3-8 h

: used as reactants in separate HDO experiments


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3. Results of kinetic experiments 3.1. Reaction pathways. A reaction network (Figure 1) has been previously proposed to describe the HDO reaction pathways of methyl heptanoate based on 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 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 < 1.0%). In the course of HDO reaction, oxygenated intermediates, such as heptanoic acid, heptanol and heptyl heptanoate were observed in liquid phase. Figure 2 depicts the concentration profile of the reactant, intermediates and final product along with reaction time in a typical run. The conversion was not zero in the beginning of the experiment, which indicates a slight occurrence of HDO reactions during the heating-up process. According to the HDO reaction network, heptanoic acid was produced from the hydrogenolysis of methyl heptanoate, with CH4 as byproduct (Figure 1). Subsequently, heptanoic acid could be transformed into hexane or hexene via two reaction pathways: either consecutive hydrogenationdecarbonylation or direct decarbonylation/decarboxylation. The former reaction pathway, forming heptanal as intermediate, is typically regarded to be dominant in the presence of excessive H2. However, heptanal at the most was detected in trace amounts due to its high reactivity over the noble metal catalyst.8 In addition, heptanal can be hydrogenated into heptanol,


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which can be reversibly dehydrogenated and further decarbonylated into hexane. The HDO reaction pathways of heptanoic acid and heptanol were also confirmed in their separate HDO experiments.8 Gaseous compounds such as CO, CO2 and CH4 were detected in the gas phase. CO and CO2 can be formed respectively via decarbonylation and decarboxylation reactions during the HDO of heptanoic acid. Moreover, it is worth mentioning that side reactions, such as water-gas-shift reaction and methanation reaction, can influence the composition of gas components. In this study, gaseous components were not quantitatively measured for kinetic analysis.

Figure 2. Typical concentration profiles in the HDO of methyl heptanoate. (Reaction conditions: 7.0 wt% methyl heptanoate initial concentration, 330°C, 80bar with H2, 0.4 g Rh/ZrO2) The effect of temperature on the HDO of methyl heptanoate was investigated in the temperature range of 250 °C-330 °C. As is shown in Figure 3, the conversion of methyl heptanoate was considerably enhanced with increasing the reaction temperature. Moreover, reaction temperature


8

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was found to influence the product selectivity clearly (Figure S1, supporting information). Increasing temperature enhanced the formation of heptanoic acid, whereas suppressed the formation of heptanol and heptyl heptanoate at a specific methyl heptanoate conversion. The selectivity of heptanol and heptyl heptanoate were much smaller than that of heptanoic acid under all the temperatures, indicating that heptanoic acid was the most abundant intermediate.

Figure 3. Effect of reaction temperature on the conversion of methyl heptanoate under 80 bar pressure with H2. (Reaction conditions: 5 wt% methyl heptanoate; 0.4 g Rh/ZrO2. Solid curve: modelled results based on mechanistic models; Points: experimental results) As a minor side product, heptyl heptanoate can be formed via reversible esterification reaction of heptanoic acid and heptanol. It can be further reacted via a series of reactions such as hydrolysis, hydrogenolysis and transesterification reaction, etc8. In this work, only hydrolysis of heptyl heptanoate was considered to account for the conversion of heptyl heptanoate for kinetic study. Other reaction pathways associated with heptyl heptanoate were excluded in Figure 1, which is slightly different from the previous version8.


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The hydrogenation of heptanal towards heptanol and the esterification reaction towards heptyl heptanoate were both reversible and exothermic based on the thermodynamic calculations. Thereby, the suppressed effect of temperature on their formation could be attributed to the thermodynamic limitation. For example, the reaction enthalpy (∆H) of heptanal hydrogenation calculated based on HSC software19 is around -90 kJ/mol at 260°C, and the enthalpy change of esterification reaction is around -9.45 kJ/mol at 298°C, calculated by using Joback method20. The influence of hydrogen pressure on methyl heptanoate conversion is shown in Figure 4. The influence was much less as compared to the temperature effect presented before. Increasing the pressure from 60 bar to 100 bar with H2 at 300°C slightly promoted the conversion rate of methyl ester. The influence became clearer in the higher conversion range. Increasing the pressure of H2 slightly suppressed the accumulation of heptanoic acid and enhanced the formation of heptanol. This can be attributed to the fact that the increase of H2 pressure favored the hydrogenation rate of heptanoic acid towards heptanol. More detailed selectivity data under various pressures are given in Figure S2 (Supporting information).


10

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Figure 4. Effect of hydrogen pressure on the conversion of methyl heptanoate at 300°C. (Reaction conditions: 5 wt% methyl heptanoate; 0.4 g Rh/ZrO2. Solid curve: modelled results based on mechanistic models; Points: experimental results) 4. Kinetic modeling 4.1. Reaction network simplification for kinetic modeling. As HDO is a complex process that involves several kinetically relevant components and multiple consecutive/parallel reaction steps, only key reaction steps were considered for kinetic modeling. Some minor components and reactions were neglected and some consecutive reaction steps were lumped together. Figure 5 below presents the simplified kinetic scheme, consisting of 6 key reaction steps.

Figure 5. Simplified kinetic scheme network for kinetic modeling. Reaction step 1 refers to the initial hydrogenolysis of methyl heptanoate to heptanoic acid. Reaction step 2 lumps the consecutive hydrogenation of heptanoic acid to heptanol via heptanal intermediate. Reaction step 3 lumps the consecutive hydrogenation-decarbonylation of heptanoic acid into hexane via heptanal intermediate. Here, lumping of two consecutive reaction steps is justified by the kinetic irrelevance of heptanal because it was hardly detectable experimentally


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owing to its high reactivity. After the aforementioned lumping, heptanal intermediate, which is involved in multiple steps (i.e. reversible hydrogenation and decarbonylation reactions), was not considered for kinetic modeling. Formation of hexane and hexene were also lumped together since

alkenes

were

not

detected

under

the

H2-rich

conditions.

Direct

decarbonylation/decarboxylation of heptanoic acid into hexane (Figure 1) was assumed to be minor compared to hydrogenation reaction in the presence of H28 and thus was not included in the modeling. Reaction step 4 lumps together the dehydrogenation-decarbonylation of heptanol via heptanal intermediate. Reaction step 5 refers to the reversible esterification reaction accounting for the formation and decomposition of heptyl heptanoate. Reaction step 6 refers to the hydrogenolysis reaction of methyl heptanoate into heptanol and methanol, although this route is expected to be minor8. It should be mentioned that reaction pathways in Figures 1 and 5 are associated with the liquid phase products. Thus, the estimation of kinetic parameters in this work are based on the composition of liquid phase products. Water-gas-shift reaction and methanation reaction8 that are associated with gaseous products were not considered for kinetic modeling due to the lack of quantitative measurement. 4.2. Empirical power-law models. Although power-law models cannot provide fundamental information on reaction mechanism, it is often very useful from an engineering standpoint for evaluating the effect of the operation variables on the reactor performance. The 6 reaction steps in Figure 5 can be expressed by the following power-law rate equations:

(1)

(2)

=

=


12

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(3)

(4)

= = = −

(5)

.

=

(6)

where are the rate constants for reaction step j (j = 1, 2 …6). Pseudo-first order with respect to oxygenated components was assumed for all the reaction steps. The reaction order with respect to H2 (denoted by superscript m) was estimated to indicate the effect of hydrogen pressure. The reversible reaction of esterification ( ) was expressed by equilibrium constant !. , which is estimated by heats of formation (∆Hform), Gibbs energies of formation (∆Gform) and heat capacities (CP) estimated by using Joback method20. For modeling purpose, the temperature dependency of equilibrium constant !. was fitted into the following eq 7 based on the Van’t Hoff equation, where R is the universal gas constant, and T is the reaction temperature.

!.

= 0.338 ∙ & /(∙)

(7)

4.3. Surface reaction mechanism and mechanistic models. In this section, surface reaction mechanism for the key reaction steps (shown in Figure 5) will be proposed according to the available literature reports and mechanistic models will be derived based on the proposed surface reaction steps. Reaction step 1: hydrogenolysis of methyl heptanoate It has been widely reported that hydrogenolysis reaction of aliphatic ester on supported metal or metal oxide catalysts proceeds via dissociative adsorption of the ester on both metal sites and metal oxide sites (i.e. Al2O3, ZrO2, CeO2 and Cr2O3) 21-23. This reveals that methyl heptanoate in this study is not adsorbed on Rh/ZrO2 catalyst in a molecular form. By analogy, hydrogenolysis


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of methyl heptanoate could occur via the cleavage of C-O bond within the methoxy group (C6H13COO−CH3), producing a heptanoate (C6H13COO*) and methyl (CH3*) surface species on both Rh nanoparticle and ZrO2. For the latter, oxygen vacancies are reported to be active sites for the selective adsorption of carboxylate (i.e. C6H13COO*).24-26 Here, symbol * denotes surface species to differentiate from the bulk liquid phase component. In this work, we assume a single active site (denoted by S1) for the adsorption of heptanoate species on both Rh nanoparticle and the oxygen vacancies from ZrO2. Hydrogen was assumed to be dissociatively adsorbed on Rh sites, whereas the contribution of ZrO2 on hydrogen dissociation is considered to be very minor, because catalytic activity of plain ZrO2 for HDO of methyl heptanoate HDO was found to be negligible without the addition of Rh metal8. The adsorbed H2 on Rh nanoparticles may spillover onto ZrO2 sites to facilitate the HDO reactions27. The active site for H2 adsorption is denoted by S2. Foraita et al. proposed a similar reaction mechanism for the hydrogenation of stearic acid over Ni/ZrO2 catalyst and plain ZrO2. 28 They gave a plausible explanation that the addition of Ni metal onto ZrO2 aided the dissociation of H2 and therefore the formation rate of oxygen vacancy sites on the ZrO2 was enhanced due to the reaction of adsorbed surface carboxylate and hydrogen28 Based on the discussion above, surface reaction steps for reaction step 1 are proposed and presented in Table 2 (eqs 8-13). The dissociation of methyl heptanoate on site S1 (eq 8) is assumed to be rate determining step (RDS). The formed heptanoate species can easily react with surface H* to form heptanoic acid in the bulk phase. This reaction is reversible and assumed to be in fast quasi-equilibrium (eq 10). The backward reaction accounts for the dissociation adsorption of heptanoic acid. Methyl species (CH3*) can rapidly react with surface H* to for methane in the gas phase (eq 11). Active site for CH3* is denoted by S3, which is a distinct site


14

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from both S1 and S2. Kinetic model equation is presented in Table 2. Detailed procedures for model derivation are provided in supporting information.

Table 2. Surface reaction mechanism for reaction steps 1. Surface reaction steps for C6H13COOCH3 + S1 + S3 H2 + 2S2

2 HS2

C6H13COOS1 + HS2 CH3S3 + HS2 R1 = R1 =

C6H13COOS2 + CH3S3

C6H13COOH + S2 + S1

Note (8)

RDS，k1

(9)

Quasi-equilibrium

(10) Quasi-equilibrium (11) Fast

CH4 + S2 + S3 k1 ⋅ c A

(1 + K B c B / K H c H + K C cC / K H c H )

(12) Competitive adsorption of heptanoate and heptoxy

k1 ⋅ c A (1 + K B c B / K H c H )

(13) Non-competitive adsorption of heptanoate and heptoxy

Note: B: heptanoate; C: heptoxy; H: dissociative H atom. Reaction step 2: hydrogenation of heptanoic acid into heptanol and reaction step 3: hydrogenation-decarbonylation of heptanoic acid into hexane Hydrogenation mechanism of heptanoic acid over Rh/ZrO2 can be referred to the finding of Peng et al.11 They proposed that hydrogenation of stearic acid over Ni/ZrO2 can be catalyzed synergistically by metal (Ni) and ZrO2 or solely by metal (Ni). This implies multiple active sites for the HDO of fatty acid over bi-functional ZrO2 supported metal catalyst. It was also proposed that oxygen vacancy sites from ZrO2 could be the active sites for the consecutive hydrogenation of stearic acid into aldehyde surface intermediate via forming ketene (C=C=O) surface species.11 Based on the discussion above, surface reaction steps for reaction steps 2 and 3 are speculated and presented in Table 3. The same site S1 is assumed for the dissociation adsorption of


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heptanoic acid (eq 14), consistent with eq 10. Hydrogenation step of heptanoic acid into heptanal via a ketene intermediate (eqs 17 and 20) was assumed to be rate-determining step (RDS) for R2 and R3, respectively. In the case of reaction step 2, heptanal intermediate is hydrogenated into heptanol via forming a heptoxy surface species (C6H13CH2O*). These surface steps (eqs 18 and 19) are reversible and are assumed to be in quasi-equilibrium. In the case of reaction step 3, heptanal intermediate is rapidly decarbonylated into hexane (eq 21). . Table 3. Surface reaction mechanism for reaction steps 2 and 3. Surface reaction steps for and C6H13COOH + S2 + S1 C6H13COOS1

C6H13COOS1 + HS2

C5H11C=C=OS1 + H2O

C5H11C=C=OS1 + 2HS2 C6H13C=OS1 + HS2

C6H13C=OS1 + 2S2 C6H13CHOS1 + S2

Note (14) Quasi-equilibrium, and (15) Quasi-equilibrium, and (16) Quasi-equilibrium, and (17) RDS of , k2,

C6H13CHOS1 + HS2

C6H13CH2OS1 + S2

(18) Quasi-equilibrium,

C6H13CH2OS1 + HS2

C6H13CH2OH + S2 + S1

(19) Quasi-equilibrium,

C6H13C=OS1 + HS2 C6H13CHOS1 R2 = R2 = R3 = R3 =

C6H13CHOS1 + S2

C6H14 + CO + S1 k 2 ⋅ K B c B ⋅ ( K H cH )

(1 + K B cB / K H cH + KC cC / K H cH ) ⋅ (1 + K H cH ) k2 ⋅ KBcB ⋅ (KH cH ) (1 + KBcB / KH cH ) ⋅ (1 + KH cH ) k3 ⋅ K B c B ⋅ ( K H c H ) (1 + K B cB / K H cH + KC cC / K H cH ) ⋅ (1 + K H cH ) k3 ⋅ KB cB ⋅ (KH cH ) (1 + KB cB / KH cH ) ⋅ (1 + KH cH )

(20) RDS of , k3, (21) Fast, Competitive adsorption of (22) heptanoate and heptoxy Non-competitive adsorption of (23) heptanoate and heptoxy Competitive adsorption of (24) heptanoate and heptoxy Non-competitive adsorption of (25) heptanoate and heptoxy

Note: B: heptanoate; C: heptoxy; H: dissociative H atom Reaction step 4: dehydrogenation-decarbonylation of heptanol into hexane


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Similar as heptanoic acid, HDO of heptanol was assumed to occur via dissociative adsorption mechanism. Surface reaction steps regarding the dehydrogenation-decarbonylation of heptanol are given in Table 4. Consistent with eqs 18 and 19, heptanol is dehydrogenated into heptanal via heptoxy species (quasi-equilibrium). The following step is the rapid decarbonylation of heptanal intermediate. We assume that adsorption of heptoxy species is occurring on S2, either competitively (mechanistic model I) or non-competitively (mechanistic model II) against heptanoate species. Accordingly, two types of mechanistic models (I and II) were respectively derived for the reaction steps 1-4, as shown in Tables 2-4. In both mechanistic models, oxygenated organic compounds and H2 were assumed to be adsorbed on different active sites. The esterification of heptanoic acid and heptanol into heptyl heptanoate (reaction step 5) is described with the empirical power-law model (eq 5), because the discussion of esterification reaction mechanism is out of the scope of this work. Hydrogenolysis of methyl heptanoate into heptanol and methanol (reaction step 6 in Figure 5) was found to be very minor based on the results with power-law model (see section 5.0) and thus this reaction was not further considered in the mechanistic models. Table 4. Surface reaction mechanism for reaction steps 4. Surface reaction steps for C6H13CH2OH + S1 + S2 C6H13CH2OS1 + S2 C6H13CHOS1

C6H13CH2OS1 + HS2 C6H13CHOS1 + HS2

C6H14 + CO + S1

Note (26) Quasi-equilibrium (27) Quasi-equilibrium (28) RDS, k4

R4=

k 4 ⋅ K C cC (1 + K B c B / K H c H + K C cC / K H c H ) ⋅ ( K H c H )

(29)

R4=

k 4 ⋅ K C cC (1 + K C cC / K H c H ) ⋅ ( K H c H )

(30)

Competitive adsorption of heptanoate and heptoxy Non-competitive adsorption of heptanoate and heptoxy

Note: B: heptanoate; C: heptoxy; H: dissociative H atom.


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In the mechanistic models (I and II), the considered surface species were heptanoate, heptoxy and hydrogen atom, which come from the dissociation of their original moleculars: heptanoic acid, heptanol and hydrogen, respectively. Surface coverage of heptanal was neglected in the kinetic model as a result of its high reactivity and absence in the liquid phase product. Hexane and other light components, such as H2O, CO, CO2 and CH4 were all assumed to desorb rapidly from the active sites and thus are neglected in the kinetic models. 4.4. Reactor mass balance and vapor-liquid equilibrium. HDO reactions of methyl heptanoate were occurring in a three-phase batch reactor that was operated isothermally. Internal and external mass transfer limitation were tested by studying the effect of different catalyst particle sizes and stirring speeds on the initial HDO reaction rate and product selectivity. It was found that external and intra-particular diffusion resistance did not affect the HDO reaction kinetics and could be neglected (data seen in Figures S3 and S4, supporting information). Thus, a pseudo-homogeneous reactor model could be applied. Moreover, vapor-liquid equilibrium was presumed at the gas-liquid interface due to fast gas-liquid mass transfer. Material balance for all the components in the gas phase and liquid phase can be described by the dynamic batch reactor model (eqs 31 and 32): ./01 .2 ./0; .2

= −3R ∙5GL, i ∙:GL

(31)

=

ZrO2 Catalyst as a

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