Hydrodeoxygenation of Methyl Heptanoate over Noble Metal Catalysts

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Hydrodeoxygenation of Methyl Heptanoate over Noble Metal Catalysts: Catalyst Screening and Reaction Network Yuwei Bie,* Andrea Gutierrez,† Tuula R. Viljava, 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: Hydrodeoxygenation (HDO) of methyl heptanoate was performed over zirconia-supported mono- and bimetallic Rh and Pt catalysts in a batch reactor. The Rh/ZrO2 catalyst gave the highest conversion and selectivity toward hydrocarbons (dominantly hexane) at 250 °C and 8 MPa of H2. The formation of alkenes was not detected due to high hydrogenation activity of the noble metal catalysts. Furthermore, a complete reaction network for the HDO of methyl heptanoate over Rh/ZrO2 was proposed on the basis of the HDO experiments with reaction intermediates, such as heptanoic acid, heptanal, and heptanol. It was proposed that heptanoic acid originated from the direct hydrogenolysis of methyl heptanoate. The prevailing reaction pathway to hexane was suggested to be decarbonylation reaction from either heptanoic acid or heptanal. Heptanal was highly reactive on Rh catalyst in the presence of H2. The effect of reaction temperature on the HDO of methyl heptanoate over Rh/ZrO2 was also investigated. Increasing temperature significantly improved the conversion of methyl heptanoate and the selectivity toward hydrocarbons.

1. INTRODUCTION With declining petroleum reserves and emerging environmental concerns, biomass-based fuels are increasingly being considered as a replacement for traditional fossil fuels. Vegetable oils, especially nonedible vegetable oils, are a promising renewable resource that can be used to produce biofuel. Biodiesel, known as fatty acid methyl esters, is generally produced by the transesterification process of vegetable oils and fats. However, the high oxygen content of vegetable oils leads to deleterious fuel properties, such as low energy density, high viscosity, chemical instability, and immiscibility with hydrocarbon fuel. Upgrading is thus required to obtain better fuel qualities.1−3 Hydrodeoxygenation (HDO) is a useful method for removing oxygen functionalities from biofuel at a moderate temperature with high-pressure hydrogen in the presence of heterogeneous catalysts.3 Conventional hydrotreating catalysts have been widely investigated in the HDO of real feedstock or model compounds of vegetable oils.4−12 Şenol et al.6,7 performed extensive studies on the HDO of methyl heptanoate using sulfided NiMo/Al2O3 and CoMo/Al2O3 catalysts. A reaction network (see Figure 1) was proposed by Turpeinen8 to describe a series of reaction pathways from methyl heptanoate to the final hydrocarbon products, that is, heptane and hexane. However, the addition of sulfiding agent is necessary to maintain the activity of these catalysts,8,9 which leads to the formation of some sulfurcontaining products. To avoid sulfur contamination in the final products, significant attention has more recently been paid to utilizing nonsulfided catalysts in the deoxygenation of vegetable oils, such as noble metal supported catalysts.13−19 Murzin and co-workers13−16 have reported the deoxygenation of fatty acid ester and fatty acid into diesel-like hydrocarbons over a variety of noble metal catalysts. They found that the beneficial effect of a noble metal in the deoxygenation © XXXX American Chemical Society

Figure 1. Reaction pathways for the HDO of methyl heptanoate over sulfided catalysts.8

reaction of fatty acid in inert gas atmosphere descended in the order of Pd, Pt, Ni, Rh, Ir, Ru, and Os at 300 °C and that carbon was the most effective support as compared to Al2O3, SiO2, and MgO.10 They further reported that fatty acid was converted into hydrocarbons via decarboxylation reaction in an inert gas atmosphere; however, it was converted into hydrocarbons via decarbonylation reaction in a hydrogen atmosphere.16 Resasco et al.17 studied the HDO of methyl octanoate and methyl stearate over Pt/Al2O3 at 330 °C in a semibatch reactor in an inert or H2 atmosphere. The conversion of both methyl esters mainly resulted in hydrocarbons with one carbon shorter than their corresponding fatty acids. They also found that the Received: April 20, 2013 Revised: July 15, 2013 Accepted: July 23, 2013

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dx.doi.org/10.1021/ie4012485 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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this study were ground and screened to obtain a particle size of 0.25−0.42 mm for use. 2.2. Catalyst Characterization. The metal loading of catalysts was determined using an X-ray fluorescence spectrometer (XRF) or inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The N2 physisorption and H2 chemisorption were performed with a Coulter Omnisorp 100 CX to measure the BET surface area and irreversibly chemisorbed H2, respectively. The experimental setup and procedures are reported in detail elsewhere.19,20 The carbon content of the used catalysts was analyzed using a carbon-analyzer (LECO, SC-444 series). 2.3. Activities of Noble Metal Catalyst: Catalyst Screening. The HDO experiments with methyl heptanoate for catalyst screening work were carried out in a batch slurry reactor (40 mL) made of stainless steel at 250 °C and with H2 of 8 MPa of total pressure. Prior to the HDO experiment, 0.3 g of catalysts (calcined at 700 °C) was dried with 2 MPa of N2 at 350 °C for 1 h and subsequently reduced with 2 MPa of H2 at 350 °C for 1 h in situ. After pretreatment, 10 mL of 5 wt % methyl heptanoate (Fluka, >99%) in n-dodecane (Merck, ≥99%) was charged into the reactor. Thereafter, the reactor was heated to 250 °C and then pressurized to 8 MPa with H2. Sampling from the line was not possible during the run. After 90 min, the reactor was cooled to room temperature, and gas and liquid samples were taken for analysis. 2.4. HDO Reaction Network Studies on Rh/ZrO2. To study the HDO reaction network, experiments were carried out in a 50 mL batch reactor, which was equipped with a circular catalyst basket and a mechanical stirrer. The catalyst basket was made from stainless steel screen (100 meshes, Cronvall Oy), and it was located at the bottom of reactor. During all of the experiments, the stirring speed was kept constant at 700 rpm during the reactions. The Rh/ZrO2 catalyst (calcined at 450 °C, 0.4−0.5 g) was placed in the basket and packed into the reactor. First, catalyst was dried at 350 °C with 2 MPa of N2 for 1 h and then reduced at 350 °C with 2 MPa of H2 for 1 h. Thereafter, 30 mL of 5 wt % methyl heptanoate in hexadecane (Sigma-Aldrich, >99.9% anhydrous) was introduced into the reactor at the reaction temperature from a feeding vessel, and subsequently the reactor was pressurized up to 8 MPa with H2. Different reaction temperatures, ranging from 250 to 340 °C, were used for the HDO experiments. Liquid samples were withdrawn from the sampling line for analysis. Fresh H2 was introduced into the reactor again to compensate for the pressure drop due to sampling. 1-Heptanoic acid (Merck, ≥99%), 1-heptanol (Merck, ≥99%), 1-heptanal (Fluka, ≥95%), and heptyl heptanoate (Creative Dynamics Inc., >95%) were also separately tested as starting material under identical HDO conditions to elucidate their role in the reaction network of methyl heptanoate. Moreover, thermal experiment of methyl heptanoate was carried out without a catalyst in the presence of H2 (8 MPa) at 340 °C. Conversion of 8% was obtained after 24 h, and only small amounts of light hydrocarbons were detected, indicating thermal cracking reactions were negligible in this study. The effect of ZrO2 support on the HDO of methyl heptanoate was also investigated in the presence of H2 at 250 and 340 °C, respectively. 2.5. Analysis of Liquid Samples and Gas Samples. Liquid samples were analyzed using a Hewlett-Packard 6890 gas chromatograph (GC) equipped with a capillary column

addition of H2 in the carrier gas suppressed the formation of heavier products, such as diheptyl ketone, n-pentadecane, and octyl octanoate compounds, and promoted the selectivity toward paraffins as compared to the reaction in an inert atmosphere. Valyon et al.18 compared the activity of Pd/C and NiMo/ γ-Al2O3 catalysts in the hydrotreatment of tricaprylin and caprylic acid as a model reaction for biofuel production at 300−400 °C in a flow reactor. They found that the reaction proceeded in consecutive steps: hydrogenolysis of tricaprylin acid into caprylic acid and propane, followed by HDO of the caprylic acid intermediate. The overall reaction rate was governed by the rate of caprylic acid HDO reaction. The prevailing reaction path of caprylic acid HDO was direct decarbonylation over Pd/C, giving mainly C-7 alkane and CO. Nevertheless, the HDO of caprylic acid over nonsulfided NiMo/γ-Al2O3 predominantly increased the selectivity toward C-8 alkene, C-8 alkane, and water via consecutive dehydration and hydrogenation reactions, which is in accordance with the finding of Şenol et al.6,7 ZrO2-supported noble metal catalysts (Rh, Pt, and Pd) have been found to be active and selective in the HDO of 2methoxyphenol (guaiacol), which is used as a model component for the lignin fraction of wood-based pyrolysis oil. A higher selectivity toward benzene and a lower carbon deposition were found with the noble metals in comparison to the sulfided catalyst at 300 °C.19 ZrO2 support is known to decrease carbon deposition because of its lower acidity and higher water resistance than Al2O3.4 The present study aims to provide novel information related to the HDO of vegetable oils over noble metal catalysts by using methyl heptanoate as the model compound. We screened ZrO2 supported mono- and bimetallic Rh and Pt catalysts for HDO activity. The most active catalyst was then used for further studies of reaction pathways for the comparison with sulfided catalyst, which our group had previously studied. A number of reaction intermediates were tested to verify the overall reaction scheme.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Monometallic Rh, Pt, and bimetallic RhPt catalysts were prepared using an incipient wetness impregnation method on ZrO2 (MEL Chemicals EC0100, 0.25−0.42 mm) to screen the most promising catalyst. The precursors of the monometallic catalysts were Rh (NO3)3 (Aldrich, 10 wt % Rh in >5 wt % nitric acid) and Pt(NH3)2(NO2)2 (Aldrich, 3.4 wt % in dilute ammonium hydroxide), respectively. The bimetallic RhPt catalysts were prepared using a coimpregnation method. The detailed preparation procedures have been reported elsewhere.20−22 The theoretical noble metal loading of the mono- and bimetallic catalysts is 0.5 wt %. In the case of bimetallic RhPt/ZrO2, the target metal loading of each metal is 0.25 wt %. After impregnation, all of the catalysts were dried at room temperature for 4 h and then at 100 °C overnight. Thereafter, they were calcined in a synthetic air flow at 700 °C for 1 h. Furthermore, a new batch of Rh/ZrO2 catalyst was prepared using incipient wetness impregnation method in a vacuum system to investigate the HDO reaction network of methyl heptanoate. ZrO2 support was first dried under vacuum conditions at 100 °C for 60 min. Rh(NO3)3 precursor was then introduced to the dried support for vacuum impregnation. Thereafter, the catalyst was dried in vacuum at 40 °C for 1.5 h and then at 60 °C for 30 min, and subsequently calcined in a synthetic air flow at 450 °C for 2 h. All of the catalysts used in B



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(Innowax, 60m × 0.25 mm × 1 μm) and a flame ionization detector. Supplementary qualitative analysis was performed with an Agilent Technologies 5975C mass spectrometer connected to the GC. The products were quantified by the internal standard method with cumene as the internal standard. The experimental setup did not allow for quantitative analysis of the gas phase. The qualitative analysis of methane in the gas phase was occasionally performed by using GC equipped with a TCD detector and a molecular sieve column. For parts of the experiments, the concentrations of CO and CO2 in the gas phase were also monitored with a Dräger tube at the end of the run to get an indication of their relative abundances. The selectivity of product i in this study is calculated as the required reactant (i.e., methyl heptanoate) for yielding 1 mol of desired product i divided by the total moles of reactant converted based on the reaction network. The desired products mainly focused on C-6 and C-7 products (alkanes/alkenes with 6 or 7 carbon atoms) and their reaction intermediates.

Table 1. Conversion and Liquid Product Selectivity in HDO of Methyl Heptanaote over Noble Metal Catalysts after 90 mina

3. RESULTS 3.1. Catalyst Properties. The metal loadings of the fresh noble metal catalysts were measured using XRF and ICP-AES, and the results are presented in Table S1 (Supporting Information). All of the measured results are in moderate agreement with the target value (0.5 wt %), except that the Rh/ZrO2 calcined at 700 °C gave a clearly lower metal loading (0.3 wt %) with the ICP-AES measurement than the target value. It may be because Rh is difficult to dissolve in the acids used during the ICP-AES measurement.21 The N2-physisorption and H2-chemisorption results are also presented in Table S1 (Supporting Information). For the catalysts calcined at 700 °C, monometallic Pt/ZrO2 clearly exhibited the lowest amount of irreversible H2 chemisorption as compared to both Rh/ZrO2 and RhPt/ZrO2 catalysts. Reducible species on Pt/ZrO2 were not observed in the H2-TPR measurements, which is consistent with the lowest irreversible H2 uptakes and can be explained by the fact that platinum remained in the oxidic form. We also noticed that the calcination temperature significantly influenced the metal loading and irreversible H2-chemisorption capacity. When the catalysts were calcined at 450 °C instead of 700 °C, higher rhodium loading amount was obtained, and the irreversible adsorption capacity of H2 on Rh/ZrO2 was increased remarkably (Table S1). The encapsulation, agglomeration, or sintering of noble metal particles on zirconia at higher calcination temperature may be the reason for the low irreversible H2 uptakes.21,22 3.2. Catalyst Screening. The catalyst screening experiments for the HDO of methyl heptanoate were performed at 250 °C at 8 MPa of total pressure. The conversions and product selectivity obtained with all of the catalysts are presented in Table 1. After 90 min, the conversion of methyl heptanoate with the Rh-containing catalysts was the highest (29%), whereas a 20% conversion was achieved with a monometallic Pt catalyst. The higher activity of Rh and RhPt catalysts could be related to the higher active surface area indicated by the amount of irreversible H2 chemisorption. Similar results were also observed in the HDO of guaiacol when using Rh/ZrO2 and Pt/ZrO2 catalysts.19 Regardless of the type of noble metal catalyst used, we detected methanol, hexane, heptane, heptanal, heptanol, heptanoic acid, and heptyl heptanoate in the final liquid product. Hexane and heptane were regarded as the final HDO

products. The formation of olefins such as heptene and hexene was not detected over any of the catalysts, probably due to the stronger hydrogenation activity of noble metal catalysts. The concentration of heptane in the products was considerably lower than that of hexane. In contrast, heptane is the predominating HDO product over sulfided NiMo and CoMo catalysts.6−8 The product distribution was affected by the type of noble metal catalyst. Monometallic Pt/ZrO2 produced the largest amounts of heptyl heptanoate and methanol, whereas Rh-containing catalysts produced more heptanoic acid. In particular, monometallic Rh/ZrO2 gave the highest selectivity toward hexane, followed by bimetallic RhPt/ZrO2, and subsequently by Pt/ZrO2. Therefore, when combining the conversion level and product selectivity toward hydrocarbons, Rh/ZrO2 is the most promising catalyst for the HDO of the methyl ester. 3.3. Catalytic Effect of ZrO2 Support. The effect of plain ZrO2 support on the conversion of methyl heptanoate was studied at 250 and 340 °C, respectively, with H2 at 8 MPa of total pressure. At 250 °C, the conversion of methyl heptanoate over ZrO2 was 6.3% after 24 h, mainly resulting in heptanal, heptanoic acid, and trace amounts of hexane in the final product. However, at 340 °C the conversion of methyl heptanoate over ZrO2 reached 24% after 6 h. A variety of products including hydrocarbons and oxygen-containing compounds were formed (see Table 2). Interestingly, heptane and unsaturated heptene were substantially observed. 3.4. HDO Reaction of Methyl Heptanoate over Rh/ZrO2. HDO experiments of methyl heptanoate using Rh/ZrO2 calcined at 450 °C were carried out at 200, 250, 300, and 340 °C in an H2 atmosphere at 8 MPa of total pressure to investigate the reaction network. The conversion of methyl heptanoate and the selectivity of main liquid products are shown in Table 2. It should be noted that methanol has been excluded from the calculations, because its high degree of volatility may cause inaccuracies in the liquid sampling process. The results shown in Table 2 are in qualitative agreement with those in Table 1 for the Rh catalyst despite the differences in the Rh loading and irreversible H2 chemisorption capacities brought about by the different calcination temperatures. Alkenes were not detected, and hexane was the main hydrocarbon product. Except at 340 °C, the formation of heptane was negligible as compared to formation of hexane. It was also observed that the formation of other cracking products, such as pentane, octane, and nonane, became visible

Rh

Pt

RhPt

conversion (%):

29

20

29

Selectivity, mol % hexane hexene heptane heptene heptanal heptanol heptanoic acid heptyl heptanoate

30.7 0 0.4 0 0.1 6.9 48.3 13.7

2.1 0 0.7 0 0.3 7.1 5.0 84.8

18.3 0 0.8 0 0.2 11.0 18.3 51.5

Catalysts used were calcined at 700 °C, and reaction conditions were 250°C and 8 MPa H2 of total pressure; methanol was detected in the liquid phase with the concentration Pt > RhPt > Rh.

a

C



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Table 2. HDO Reaction of Methyl Heptanoate over Plain ZrO2 and Rh/ZrO2 at 200−340 °C and 8 MPa H2 of Total Pressurea ZrO2

Rh/ZrO2 reaction temp (°C)

Selectivity, mol % hexane hexene heptane heptene heptanal heptanol heptanoic acid heptyl heptanoate cracking products conversion, % carbon balance, %

340 °C

200 °C

250 °C

300 °C

340 °C

5 0.6 22.3 17.6 6.8 5.4 4.9 26.4 12 65 74−89

28 0 trace 0 1.4 14.5 13 41 0 30 88−95

32.0 0 0.36 0 trace 15.9 4.5 47.2 0 42 87−94

81.8 0 1.3 0 trace 2.2 6.2 8.5 trace 78 76−94

88.3 0 3.6 0 trace 0.8 3.6 1.3 2.3 80.8 76−90

a

Methanol was detected in the liquid phase. Carbon balance is presented in the form of range from lowest value to highest value obtained along reaction time.

at 340 °C. Nonetheless, the concentrations of the cracking products were far smaller than those of the HDO products. Temperature influenced the selectivity of products considerably. At low temperatures like 200 and 250 °C, heptyl heptanoate was the main product, followed by hexane and heptanol after 24 h. At 300 and 340 °C, hexane became the exclusively dominating product, whereas the selectivities of oxygen-containing products (heptyl heptanoate, heptanoic acid, and heptanol) were much lower than that of hexane (Table 2). It seems that higher reaction temperatures above 300 °C are favorable for the selective formation of hydrocarbons in the HDO of methyl ester on noble metal catalyst. The conversion as a function of the reaction time at abovementioned temperatures is depicted in Figure 2. A complete

Figure 3 shows the concentrations of the main liquid products as a function of time at the different temperatures tested. The concentration of hexane, the expected final product, increased gradually over time in all of the tests performed. However, temperature influenced the selectivity of products considerably. With respect to oxygen-containing products (heptyl heptanoate, heptanoic acid, and heptanol), it was observed that their concentrations increased constantly throughout the experiment at 200 and 250 °C; however, there are clear maxima in the concentrations of heptanoic acid and heptyl heptanoate at 300 and 340 °C (Figure 3A,B). The heptanol concentration profile also followed a similar trend at 300 and 340 °C. The maxima in the concentrations of oxygen-containing products were not observed in the experiments done at temperatures of 200 and 250 °C, probably due to the low conversion level. The emergence of a maximum concentration of the oxygen-containing compounds confirms their nature as intermediates in the HDO of methyl heptanoate. 3.5. HDO Experiments with Oxygen-Containing Intermediates over Rh/ZrO2. As mentioned above, we identified heptanoic acid, heptanol, and heptyl heptanoate as reaction intermediates in the HDO of methyl heptanoate on Rh and Pt containing catalysts. Therefore, we further studied the HDO reactions of these compounds on Rh/ZrO2 at 250 °C with H2 or N2 at 8 MPa of total pressure. The conversion and corresponding product selectivity after 6 h are listed in Tables 3 and S2 (Supporting Information). 3.5.1. HDO Experiments with Heptanoic Acid. For the HDO of heptanoic acid with H2, hexane (selectivity: 43 mol %) is the main end product and heptyl heptanoate (30 mol %) is the major byproduct. Heptanal was detected in trace amounts similarly as observed in the HDO experiment with methyl heptanoate. A high amount of methyl heptanoate (12.5 mol %) was detected, and its concentration of methyl heptanoate increased during the course of the reaction. The analysis of gas phase in the end of the experiment (Dräger tube) revealed that the CO2 concentration was more than 10 times higher than the CO concentration. However, in the experiment of heptanoic acid under N2, only hydrocarbon products, hexane and hexene, were detected. In view of the conversion values obtained under different gas atmospheres, N2 atmosphere slightly improved the

Figure 2. Conversion of methyl heptanoate on Rh/ZrO2 over time at different temperatures.

conversion was not achieved under any of the applied conditions. We also observed that the conversion rate of methyl heptanoate was high at the beginning of the HDO experiment, and then it leveled off along with the reaction time, especially at higher temperatures. The reaction temperature had a marked effect on the conversion of methyl heptanoate on Rh/ZrO2: the conversion at 250 °C was 41% after 24 h, while it reached 78% at temperature above 300 °C. At the lowest temperature tested (200 °C), the conversion was only 30% after 24 h. D



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Figure 3. Product concentration profile versus reaction time for the HDO of methyl heptanoate at various temperatures.

Table 3. HDO of Heptanoic Acid, Heptanol, and Heptanal over Rh/ZrO2 Catalyst, at 250 °C and 8 MPa of Total Pressure after 6 ha

a

reactant:

heptanoic acid + H2

heptanoic acid + N2

heptanal + H2

heptanol + H2

heptanol + N2

conversion, %:

16

21

80

24

20

Selectivity (mol %) hexane hexene heptane heptene heptanal heptanol heptanoic acid heptyl heptanoate methyl heptanoate

43 0 0.3 0 1.7 12.5 reactant 30 12.5

40 60 0 0 0 0 reactant 0 0

30.1 0.94 0.47 3.77 reactant 52.8 0.47 11.3 0

78.6 0 3.8 0 0.7 reactant 0 0.6 0

43.4 19.2 0.6 0.4 18.2 reactant 0 18.2 0

Reaction conditions: heptanoic acid solution: 5.0 wt %, heptanol solution: 5.0 wt %, heptanal solution: 3.0 wt %; catalyst amount: 0.4 g.

CO2 and CO molar ratio of more than 10:1. In the case of N2, hexane was still the main product, although the selectivity toward hexane was lower than in the case of H2. A significant quantity of hexene (19.2 mol %) was detected due to the lack of H2. An obvious amount of heptanal accumulated in the experiment of heptanol with N2, which implies that effective conversion of heptanal requires the participation of H2. 3.5.4. HDO Reaction of Heptyl Heptanoate. For the HDO experiment with heptyl heptanoate, the molar concentration of heptyl heptanoate was maintained at the same level as methyl heptanoate, and catalyst loading remained the same as well. The products detected during the HDO of heptyl heptanoate (Table S2 in the Supporting Information) were in good accordance with the HDO of methyl heptanoate (Table 2). Given that the conversion of heptyl heptanoate and methyl heptanoate, respectively, reached 5.8% and 18% after 2 h at 250 °C, methyl heptanoate was clearly more reactive than heptyl heptanoate under identical experimental conditions. The product distributions also exhibited an obvious difference at a similar conversion level of 18%. The formation of heptane from heptyl heptanoate was significant (selectivity: 33.6%), whereas it was negligible in the HDO experiment with methyl heptanoate.

deoxygenation of heptanoic acid as compared to H 2 atmosphere. 3.5.2. HDO Experiments with Heptanal. For the HDO experiment of 1-heptanal under H2, the conversion reached 60% after 1 h and 80% after 6 h (Table 3), which indicates that heptanal is very reactive on Rh/ZrO2. This explains why heptanal was detected in trace amounts in the HDO experiment with methyl heptanoate and heptanoic acid. Heptanol was the main product, suggesting that heptanal can easily be hydrogenated in the presence of H2. Both heptyl heptanoate and heptanoic acid were observed in trace amounts, revealing that the disproportionation reaction (Tishchenko reaction) of heptanal to heptyl heptanoate has taken place under the conditions studied. 3.5.3. HDO Experiments with Heptanol. The reactivity of 1-heptanol over Rh/ZrO2 was studied in the presence of H2 and N2, respectively. After 6 h, the conversion of heptanol was 24% with H2 and 20% with N2 (Table 3). Hydrogen atmosphere slightly improved the conversion of heptanol over Rh/ZrO2 as compared to inert atmosphere. In terms of product distribution, in the case of H2, hexane was the dominant product, with a selectivity of 78.6%, and heptane was only produced in small amounts. Heptanal was detected in trace amounts similarly as observed in the HDO experiment of methyl heptanoate and heptanoic acid. Methanol was detected in large quantities (not shown in Table 3). CO2 and CO were detected in the gas phase by Dräger tube, with a

4. DISCUSSION 4.1. Reaction Mechanism for Hydrogenolysis of Methyl Heptanoate. It has been observed that experiments E



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esterification and the transesterification reactions of methyl heptanoate are exothermic. Thus, a higher temperature can inhibit the selective formation of heptyl heptanoate, which may explain why the concentration of heptyl heptanoate was much higher at 250 °C than at 300 °C or above. The determination of CO2 and CO in the gas phase implied that heptanoic acid could be decarboxylated or decarbonylated to form hexane. However, the water gas shift reaction (CO + H2O ↔ CO2 + H2) strongly favors the formation of CO2 at the expense of CO under the studied conditions, as revealed by thermodynamic data estimated with the HSC Chemistry program.24 Thus, the relative abundances of carbon oxides may not reveal a prevalence of decarboxylation over decarbonylation. The exclusive formation of hexene and hexane in the deoxygenation of heptanoic acid under N2 implies that decarboxylation and decarbonylation significantly occurred over Rh/ZrO2. Considering the slightly higher selectivity of hexene than hexane, decarbonylation appears to be the main route as compared to decarboxylation over Rh/ZrO2. Similarly, by measuring the ratio of CO and CO2 in the gas phase on line, Murzin et al.16 concluded that the major route of lauric acid deoxygenation reaction over Pd/C catalyst in the presence of H2 is decarbonylation via aldehyde intermediate rather than decarboxylation reaction. With these facts in mind, it is worth pointing out that decarbonylation of heptanoic acid can proceed in two different ways (reactions 1 and 2):6,13,16 via direct decarbonylation by removal of CO and H2O, or via indirect decarbonylation of heptanal intermediate.

with noble metal catalysts (Tables 1 and 2) resulted in the same products as with sulfided NiMo and CoMo catalysts, except for the sulfur-containing compounds. On the basis of the reaction network in Figure 1,8 heptanoic acid can be produced via the hydrolysis reaction of methyl heptanoate with sulfided NiMo and CoMo catalysts. The formed heptanoic acid can either be reduced into heptanol via the heptanal intermediate or transformed into hexane via consecutive decarbonylation and hydrogenation reactions. Heptanol can be further dehydrated into heptene, which is then hydrogenated into heptane as the main end-product.6 Şenol and co-workers6,7 suggested that the hydrolysis of methyl heptanoate resulting in the formation of heptanoic acid can be catalyzed by acid or base sites on sulfided NiMo/Al2O3 catalysts. However, little evidence was found in this study to support the hydrolysis on Rh/ZrO2 because water was not detected in the liquid products and the feedstock when conducting a nuclear magnetic resonance (NMR) analysis. Methanol was detected, which may be an indication to hydrolysis reaction; however, other reaction routes can also produce methanol. For instance, the hydrogenolysis of methyl heptanoate can directly result in the formation of heptanol and methanol, as reported by Şenol et al.6 This reaction pathway cannot be ruled out with our catalyst, because, on one hand, Rh is catalytically active in hydrogenation reaction; on the other hand, methanol was always detected in the HDO of methyl heptanoate. Thus, the possibility of hydrolysis reaction mechanism for the formation of heptanoic acid over Rh/ZrO2 is ruled out in this study. We propose that heptanoic acid can be produced from the direct hydrogenolysis of methyl heptanoate, giving methane as a byproduct (C6H13COO−CH3 + H2 → C6H13COOH + CH4). The HDO experiment of heptyl heptanoate confirms the pathway of direct hydrogenolysis for fatty acid methyl ester, because the high amount of heptane formed during the experiment can only originate from direct hydrogenolysis of heptyl heptanoate (C6H13COO−C7H15 + H2 → C6H13COOH + C7H16), given that the dehydration reaction of heptanol is being suppressed over Rh/ZrO2. A similar mechanism has also been proposed for the HDO of tricaprylin by Valyon et al.,18 resulting in caprylic acid and propane on Pd/C and NiMo/ Al2O3 catalysts. Therefore, hydrogenolysis seems to be the most likely route for the formation of heptanoic acid. In addition, the acid/base-catalyzed transesterification reaction of methyl heptanoate and heptanol formed is rational, yielding heptyl heptanoate. Transesterification reaction has been reported in the hydrogenation of methyl laurate on Rh− Sn/Al2O3.23 Authors concluded that a large amount of lauryl laurate was produced due to transesterification reaction of methyl laurate and the formed lauryl alcohol, even in the absence of a catalyst.23 4.2. Reaction Pathways of HDO Intermediates. For the HDO of heptanoic acid over Rh/ZrO2, the formation of heptanol can be explained by the sequential hydrogenation of heptanoic acid via heptanal intermediate. Heptanal was reactive to produce heptanol, as observed in the HDO experiment of heptanal. Heptyl heptanoate, the main product in the HDO experiment of heptanoic acid, may result from the esterification reaction of heptanoic acid and the formed heptanol. Moreover, transesterification could contribute to the formation of heptyl heptanoate as discussed previously, because methyl heptanoate was detected during the HDO experiment of heptanoic acid. However, such a mechanistic explanation is still unclear about how methyl heptanoate was initially formed. Both the

R−COOH → R′ + CO + H 2O

(1)

R−COOH + H 2 → R−CHO → R + CO

(2)

Although it is difficult to clarify to which degree the decarbonylation reaction proceeds, respectively, via reaction 1 or reaction 2, it appears that the latter reaction pathway can occur more easily with abundant hydrogen over noble metal catalysts. The dominating product in the HDO reaction of heptanol in the presence of H2 over Rh/ZrO2 was hexane, whereas heptane over sulfided NiMo/Al2O3 catalyst,6 indicating indicating suppressed dehydration of heptanol over noble metal catalyst. Heptanol can be dehydrogenated into heptanal, which is very reactive for further decarbonylation reaction into C-6 hydrocarbons.16 Despite a high level of hydrogen pressure in the gas phase, heptanol dehydrogenation reaction can still take place in the presence of excessive heptanol. Lercher et al.25 also reported that ethane was formed in the aqueous phase HDO of 1-propanol over Pt/Al2O3 via decarbonylation of aldehyde generated by dehydrogenation. For the experiment of heptanol with N2, the detection of unsaturated hexene and the accumulation of heptanal in liquid phase suggest that the decarbonylation of yielded heptanal occurred. Moreover, hexane may also result from direct hydrogenolysis of heptanol through the scission between the α and β carbon atoms of heptanol. The bond strength of R−CH2OH is 356.2 kJ/mol, which is smaller than the strength of RCH2−OH (390 kJ/mol),26 revealing that the direct hydrogenolysis of heptanol into hexane is thermodynamically more favorable than the dehydration of heptanol into heptene. Because the NiMo and CoMo catalysts preferably produce heptane6−8 and the noble metal catalysts produce hexane, the reason must be kinetic rather than thermodynamic. The ratio for the rates of F



Article

possible under the conditions studied here.24 Methane may have been formed in the direct hydrogenolysis of methyl heptanoate to form heptanoic acid. Furthermore, both CO2 and CO can react with H2 to yield methane and water. Water gas shift reaction may also change the relevant amount of CO and CO2. Methanol was detected in the liquid phase in the HDO of methyl heptanoate; it may have suffered a decomposition reaction (eqs 3 and 7). However, the presence of methanol in the liquid phase is just an indication of its real amount in the system due to its high vapor pressure and partial evaporation during the sampling process. The total carbon balance in the liquid product was also addressed, as shown in Table 2. The carbon balance depended to a great extent on the conversion level. Approximately 75% of the total balance was obtained with almost a complete conversion, whereas more than 90% of the total balance was obtained with low the conversion level. Moreover, the carbon balance was affected by the reaction temperature, with it being worst at 340 °C. We attribute this to the increased fraction of heavier organic compounds in the vapor phase, which was caused by vapor−liquid phase equilibrium at elevated temperatures. The carbon balance is also influenced to a minor extent by the exclusion of most C-1 species and carbonaceous deposits on the catalyst (1.0−2.0 wt %). 4.5. HDO Reaction Network of Methyl Heptanoate over Rh/ZrO2. On the basis of the detected products and the discussion above, we propose a new reaction network for the HDO of methyl heptanoate over Rh/ZrO2, as is shown in Figure 4. Although we observed that dehydration of heptanol was severely suppressed over noble metal catalyst, this pathway remained in the network (dotted line).

dehydration and hydrogenolysis must be different over NiMo or CoMo catalysts than that over noble metals. The hypothesized hydrogenolysis of heptanol should produce equimolar amounts of hexane and methanol. We indeed analyzed substantial amounts of methanol in HDO of heptanol on Rh/ZrO2 but less than equimolar amounts, which can be attributed to the competing reaction route via dehydrogenation into heptanal and to the inaccuracy of the methanol analysis. 4.3. Catalytic Effect of the ZrO2 Support. The HDO experiment of methyl heptanoate on plain ZrO2 support indicated marked catalytic activity at a higher temperature (340 °C). C-7 products were much more abundant during the HDO experiment over ZrO2 in comparison to Rh/ZrO2. Apparently, the acid-catalyzed dehydration reaction for the formation of heptane was favored on plain ZrO2. In addition, unlike in any HDO experiment over Rh/ZrO2, the heptanal was accumulated over pure ZrO2, which indicates that the decarbonylation of heptanal was quite slow in the absence of noble metal. Also, the hydrogenation reactions were slow as indicated by the accumulation of alkenes. Thus, we can conclude that reactions on ZrO2 support likely contribute to the overall activity of Rh/ZrO2 in the HDO experiment of methyl heptanoate, and the role of a noble metal is essential for the reductive reaction steps and the decarbonylation step, both of which are slow on plain ZrO2. 4.4. Reactions of the C-1 Species and the Mass Balance. We only analyzed the gas phase (Dräger tube) during certain experiments to identify the carbon oxides (CO2 and CO). Methane was observed to be the main component in the gas phase together with nonreacted hydrogen. Table 4 lists Table 4. Possible Reactions Involving C-1 Species

5. CONCLUSIONS We tested and compared the activity of mono- and bimetallic noble metal catalysts (Rh and Pt) for the HDO of methyl heptanoate. All noble metal catalysts and the plain zirconia support exhibited activity in this reaction. Rh/ZrO2 catalyst gave the highest conversion and selectivity toward hydrocarbons. The noble metal catalysts tested are much more selective to hexane, and the reaction pathway to heptane via dehydration is strongly suppressed. Only trace amounts of alkenes were detected, indicating a high hydrogenation activity of noble metal catalysts under the studied conditions. HDO of

reaction equation

CO2 + 3H 2 ↔ CH4 + H 2O

(3)

CO + 4H 2 ↔ CH4 + 2H 2O

(4)

CO + H 2O ↔ CO2 + H 2 (WGS)

(5)

CH3OH + H 2 → CH4 + H 2O

(6)

CH3OH → CO + 2H 2

(7)

some possible reactions involving C-1 species (light compounds containing only one carbon atom), which are thermodynamically

Figure 4. Proposed reaction network for HDO of methyl heptanoate over Rh/ZrO2. G



Article

(6) Şenol, O. I.̇ ; Ryymin, E. M.; Viljava, T. R.; Krause, A. O. I. Reactions of methyl heptanoate hydrodeoxygenation on sulphided catalysts. J. Mol. Catal. A: Chem. 2007, 268, 1−8. (7) Şenol, O. I.̇ ; Viljava, T. R.; Krause, A. O. I. Effect of sulphiding agents on the hydrodeoxygenation of aliphatic esters on sulphided catalysts. Appl. Catal., A 2007, 326, 236−244. (8) Turpeinen, E. M. Hydrodeoxygenation of methyl heptanoate and phenol over sulphided supported NiMo and CoMo catalysts. Ph.D. Dissertation, Aalto University, Finland, 2011. (9) Ryymin, E. M.; Honkela, M. L.; Viljava, T. R.; Krause, A. O. I. Insight to sulfur species in the hydrodeoxygenation of aliphatic esters over sulfided NiMo/γ-Al2O3 catalyst. Appl. Catal., A 2009, 358, 42−48. (10) Guzman, A.; Torres, J. E.; Prada, L. P.; Nunez, M. L. Hydroprocessing of crude palm oil at pilot plant scale. Catal. Today 2010, 156, 38−43. (11) Hancsók, J.; Kasza, T.; Kovács, S.; Solymosi, P.; Holló, A. Production of bioparaffins by the catalytic hydrogenation of natural triglycerides. J. Cleaner Prod. 2012, 34, 76−81. (12) Mikulec, J.; Cvengroš, J.; Joríková, L.; Banič, M.; Kleinová, A. Second generation diesel fuel from renewable sources. J. Cleaner Prod. 2010, 18, 917−926. (13) Snåre, M.; Kubičková, I.; Mäki-Arvela, P.; Eränen, K.; Murzin, D. Y. Heterogeneous catalytic deoxygenation of stearic acid for production of biodiesel. Ind. Eng. Chem. Res. 2006, 45, 5708−5715. (14) Snåre, M.; Kubičková, I.; Mäki-Arvela, P.; Chichova, D.; Eränen, K.; Murzin, D. Y. Catalytic deoxygenation of unsaturated renewable feedstocks for production of diesel fuel hydrocarbons. Fuel 2008, 87, 933−945. (15) Kubičková, I.; Snåre, M.; Eränen, K.; Mäki-Arvela, P.; Murzin, D. Y. Hydrocarbons for diesel fuel via decarboxylation of vegetable oils. Catal. Today 2005, 106, 197−200. (16) Rozmysłowicz, B.; Mäki-Arvela, P.; Tokarev, A.; Leino, A. R.; Eränen, K.; Murzin, D. Y. Influence of hydrogen in catalytic deoxygenation of fatty acids and their derivatives over Pd/C. Ind. Eng. Chem. Res. 2012, 51, 8922−8927. (17) Phuong, T. D.; Chiappero, M.; Lobban, L. L.; Resasco, D. E. Catalytic deoxygenation of methyl-octanoate and methyl-stearate on Pt/Al2O3. Catal. Lett. 2009, 130, 9−18. (18) Boda, L.; Onyestyák, G.; Solt, H.; Lónyi, F.; Valyon, J.; Thernesz, A. Catalytic hydroconversion of tricaprylin and caprylic acid as model reaction for biofuel production from triglycerides. Appl. Catal., A 2010, 374, 158−169. (19) Gutierrez, A.; Kaila, R. K.; Honkela, M. L.; Slioor, R.; Krause, A. O. I. Hydrodeoxygenation of guaiacol on noble metal catalysts. Catal. Today 2009, 147, 239−246. (20) Kaila, R. K.; Gutierrez, A.; Korhonen, S. T.; Krause, A. O. I. Autothermal reforming of n-dodecane, toluene, and their mixture on mono- and bimetallic noble metal zirconia catalysts. Catal. Lett. 2007, 115, 70−78. (21) Kaila, R. K.; Gutierrez, A.; Slioor, R.; Kemell, M.; Leskela, M.; Krause, A. O. I Zirconia-supported bimetallic RhPt catalysts: Characterization and testing in autothermal reforming of simulated gasoline. Appl. Catal., B 2008, 84, 223−232. (22) Burch, R.; Loader, P. K. An investigation of the use of zirconia as a support for rhodium catalysts. Appl. Catal., A 1996, 143, 317−335. (23) Miyake, T.; Makino, T.; Taniguchi, S.; Watanuki, H.; Niki, T.; Shimizu, S.; Kojima, Y.; Sano, M. Alcohol synthesis by hydrogenation of fatty acid methyl esters on supported Ru-Sn and Rh-Sn catalysts. Appl. Catal., A 2009, 364, 108−112. (24) Roine, A. Outokumpu HSC Chemistry User’s Guide, Version 5.1; Outokumpu Research Oy: Pori, Finland, 2002. (25) Peng, B.; Zhao, C.; Mejía-Centeno, I.; Fuentes, G. A.; Jentys, A.; Lercher, J. A. Comparison of kinetics and reaction pathways for hydrodeoxygenation of C3 alcohols on Pt/Al2O3. Catal. Today 2012, 183, 3−9. (26) Luo, Y. R. Comprehensive Handbook of Chemical Bond Energies; CRC Press: New York, 2007.

methyl heptanoate exhibited different reaction dynamics on noble metal catalysts than on NiMo and CoMo catalysts. A complete reaction network was proposed for the HDO of methyl heptanoate over Rh/ZrO2 on the basis of the results from experiments with reaction intermediates. Heptanoic acid, heptyl heptanoate, heptanal, and heptanol were confirmed to be the reaction intermediates in HDO of methyl heptanoate. Heptanoic acid was proposed to originate from the direct hydrogenolysis of methyl heptanoate, giving methane as a byproduct. The prevailing reaction path to hexane was suggested to be the decarbonylation of heptanoic acid or heptanal. Direct hydrogenolysis of heptanol was also proposed, which results in hexane. Heptanal was found highly reactive on Rh catalyst in the presence of H2, supporting its detection in trace amounts in HDO experiments with methyl heptanoate. The effect of reaction temperature on HDO of methyl heptanoate was investigated over Rh/ZrO2. Increasing the temperature significantly improved the conversion of methyl heptanoate. Moreover, higher temperatures (>300 °C) greatly improved the selectivity toward hydrocarbons (mainly hexane) and suppressed the formation of heptyl heptanoate and heptanoic acid.

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ASSOCIATED CONTENT

* Supporting Information S

Characterization results of catalyst used (Table S1) and the experimental results in the HDO of heptyl heptanoate over Rh/ZrO2 (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: yuwei.bie@aalto.fi. Present Address †

Biofuels R&D, UPM Research center, Paloasemantie 19, FI-53200 Lappeenranta, Finland. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Finnish Graduate School in Chemical Engineering (GSCE) is acknowledged. Dr. Hannu Revitzer, Mr. Esko Ahvenniemi, and Mr. Arto Mäkinen are thanked for the ICP-AES, XRF, and chemisorption determinations, respectively.

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REFERENCES

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Hydrodeoxygenation of Methyl Heptanoate over Noble Metal Catalysts

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