Conversion of Biodiesel-Derived Crude Glycerol into Useful

Article pubs.acs.org/IECR

Conversion of Biodiesel-Derived Crude Glycerol into Useful Chemicals over a Zirconia−Iron Oxide Catalyst Aya Konaka, Teruoki Tago,* Takuya Yoshikawa, Hirofumi Shitara, Yuta Nakasaka, and Takao Masuda Research Group of Chemical Engineering, Division of Chemical Process Engineering, Faculty of Engineering, Hokkaido University, N13W8 Kita-ku, Sapporo, Hokkaido, Japan 060-8628 ABSTRACT: In this study, catalytic conversion of crude glycerol was performed over ZrO2−FeOx catalysts for the production of useful chemicals. A crude glycerol solution obtained from biodiesel production was used as a feedstock, and a catalytic reaction was carried out in a fixed-bed flow reactor at 623 K under atmospheric pressure. The reaction was found to involve a series of consecutive reactions in which allyl alcohol, propylene, hydroxyacetone, carboxylic acids, acrolein, and ketones are included as intermediate and terminated products. Although the crude glycerol contained methanol, free-fatty acids, and potassium as impurities, using a ZrO2−FeOx catalyst, the glycerol in the feedstock converted into these useful chemicals, the composition of which were identical regardless of the crude glycerol concentration. Moreover, an increase in the W/F (weight ratio of catalyst to feedstock) value allowed the consecutive reactions to progress, and the products were summed up in 24 mol % C of propylene and 25 mol % C of ketones.

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allyl alcohol20−22 and propylene (pathway I), and another involves the dehydration of glycerol to produce hydroxyacetone and acrolein (pathway II).23−27 Moreover, hydroxyacetone was easily converted into carboxylic acids (acetic and propionic acids), which are then further reacted to form ketones (acetone, methyl ethyl ketone, and pentanone) via a ketonization,17,21 as shown in Figure 1.

INTRODUCTION The development of renewable and alternative fuels such as biodiesel is becoming increasingly important because of the depletion of fossil fuels and global warming.1−3 Biodiesel is currently produced by transesterification of triglycerides, such as vegetable oils and animal fats, and methanol,4,5 in which glycerol is produced as a main byproduct (crude glycerol); thus, the development of an effective utilization for glycerol is needed. Glycerol, propan-1,2,3-triol, is expected as one of the major platform chemicals in a biorefinery, and there are a large number of research works on the catalytic conversion of glycerol into useful chemicals such as acrolein, hydroxyacetone, or 1,2- and 1,3-propanediol. Acrolein6,7 and hydroxyacetone8,9 are produced through gas-phase dehydration of glycerol over acidic catalysts, including Nb2O5, heteropolyacids, and zeolites, and copper metal catalysts, respectively. Moreover, glycerol is hydrogenolyzed to produce 1,2- and 1,3-propanediol over supported ruthenium, rhodium, and platinum catalysts.10−12 On the other hand, crude glycerol obtained in biodiesel fuel production contains impurities such as fatty acids and potassium ions;13 therefore, catalysts used for the catalytic conversion of crude glycerol require the following properties: they show an activity for hydroxyl groups of alcohols and have resistance to fatty acids and alkali metals. We have developed a zirconia−iron oxide catalyst, ZrO2− FeOx, for the catalytic conversion of biomass resources. While an actual biomass contains organic and inorganic impurities such as tar-like hydrocarbon, alkaline metal, and ash, in using ZrO2−FeOx, we have succeeded in carrying out the selective production of phenol and ketones from tar derived from wood biomass,14−17 sewage sludge,18 and fermentation residue.19 Moreover, we have succeeded in producing useful chemicals, including propylene, allyl alcohol, carboxylic acids, and ketones, from reagent glycerol using ZrO2−FeOx.20,21 These useful chemicals are expected to be produced from glycerol through two main pathways: one pathway involves the production of © XXXX American Chemical Society

Figure 1. Expected reaction pathways for glycerol over ZrO2−FeOx.

In this study, ZrO2−FeOx was used for the catalytic conversion of crude glycerol, which is the waste solution obtained from the production of biodiesel. The main objective of this study is conversion of crude glycerol to yield useful chemicals. The effects of the ZrO2 content in the catalyst and of W/F values (weight ratio of catalyst to feedstock) on the yields of useful chemicals were investigated. Special Issue: NASCRE 3 Received: March 3, 2013 Revised: June 19, 2013 Accepted: June 19, 2013

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dx.doi.org/10.1021/ie4006645 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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EXPERIMENTAL SECTION Catalyst Preparation and Characterization. All reagents were purchased from Wako Pure Chemical Industries, Japan. ZrO2−FeOx was prepared by a coprecipitation method using Fe(NO3)3·9H2O and ZrO(NO3)2 aqueous solutions and an ammonia solution. A 10 wt % ammonia solution was added by a micropump to the mixture of Fe(NO3)3·9H2O and ZrO(NO3)2 aqueous solution under continuous stirring until the pH of the solution reached pH 7. The solution was stirred for 1 h and subsequently filtered. The obtained precipitates were dried at 383 K for 24 h and calcined at 773 K for 2 h in an air atmosphere. The ZrO2 content in ZrO2−FeOx, varying in the range of 0−65 wt %, was analyzed by an X-ray fluorescence analyzer (XRF Supermini; Rigaku Co. Ltd.). The catalysts are denoted hereafter as ZrO2(Y)−FeOx, where Y is the weight percent of the supporting ZrO2. The FeOx catalyst without ZrO2 and the ZrO2 catalyst without FeOx were also prepared by the same method for comparison. The crystallinity of the catalysts was analyzed by an X-ray diffractometer (JDX-8020; JEOL). The Brunauer−Emmett−Teller (BET) surface area of the catalysts was calculated using N2 adsorption isotherms (Belsorp mini; Bel Japan). It was considered that potassium ions in the crude glycerol were deposited on the catalyst surface during crude glycerol conversion. In order to examine the effect of potassium on the catalyst during the conversion of crude glycerol, glycerol conversion was carried out using a potassium-supported ZrO2− FeOx catalyst (K/ZrO2−FeOx). The K/ZrO2−FeOx catalyst was prepared by an impregnation method. Briefly, an aqueous solution of KNO3 was mixed with distilled water, which was sufficient to fully wet the ZrO2−FeOx precipitate. The ZrO2− FeOx precipitate and the KNO3 aqueous solution were mixed, stirred, and subsequently dried in a vacuum at 313 K for 2 h and 333 K for 2 h. The obtained precipitate was dried at 383 K for 24 h and finally calcined at 773 K for 2 h in an air atmosphere. The amount of potassium on the catalyst was 1.8 wt %. Catalytic Reactions over ZrO2−FeOx. The total amount of carbon in crude glycerol was measured by a CHN elemental analyzer (Costech, ECS 4010). The concentrations of glycerol, methanol, and free fatty acids were measured by the same gas chromatography (GC) system as that used for analysis of the liquid products, and the concentration of potassium was evaluated by X-ray fluorescence (XRF). Moreover, components in the crude glycerol were also measured by high-performance liquid chromatography (JEOL, HPLC LC-2000) with KC-811 and HG-5 columns. Low-molecular-weight components in the crude glycerol identified by HPLC were methanol and glycerol. Heavymolecular-weight components were composed of palmitic acid, oleic acid, linoleic acid, stearic acid, and their methyl esters. On the other hand, undefined components with heavy molecular weight existed in HPLC analysis. The weight percent of carbon in crude glycerol measured by an elemental analyzer was 54 wt %, composed of glycerol, methanol, free fatty acids, and others. The weight percent of glycerol was 37 wt %, and methanol (8 wt %), free fatty acids (25 wt %), and potassium (4 wt %) were also present, based on GC and XRF analyses. To be noted, the concentration of free fatty acids was evaluated by assuming that all contained free fatty acids are palmitic acid. The weight ratio of methanol to glycerol measured by HPLC was 4.7, which was almost the

same value as the ratio measured by GC (4.6). Undetectable hydrocarbons (26 wt %) in GC analysis were speculated to be composed of polymeric glycerol, other free fatty acids, their methyl esters, and acyl glycerol. The catalytic reaction using ZrO2−FeOx was carried out in a fixed-bed flow reactor at a reaction temperature of 623 K under atmospheric pressure. ZrO2−FeOx was pelletized and formulated into a particle size of 300−850 μm. Nitrogen gas was introduced as a carrier gas at a flow rate of 20 cm3/min. The value of W/F, where W is the amount of catalyst/g and F is the flow rate of the feedstock/g·h, was varied in the range of 0−10 h. Crude glycerol obtained from biodiesel production was diluted by 10, 30, and 50 wt % with distilled water, and aqueous crude glycerol was used as a feedstock and fed to the reactor with a syringe pump. During the reaction over the catalyst, oxidation of organic chemicals in the crude glycerol uses lattice oxygen of iron oxide in the ZrO2−FeOx catalyst. The occupied lattice oxygen by organic chemicals leads to oxygen defects in the catalyst. The oxygen defects will regenerate by oxygen active species that are produced from the decomposition of water molecules over ZrO2. Therefore, water in the feed was required. In order to inspect the stability of the ZrO2−FeOx catalyst, the effects of the crude glycerol concentration on the product yields and the crystallinity of the catalyst were examined. The liquid and gas products were collected with an ice trap and a gas bag, respectively. The liquid products were analyzed by a gas chromatograph equipped with a flame ionization detector (GC-2014; Shimadzu Co. Ltd.) and a gas chromatograph mass (GC-17A GCMS-QP5050) equipped with a DBWax capillary column. The gas products were analyzed by gas chromatographs with thermal conductivity (TCD) and flame ionization (FID) detectors (GC-8A; Shimadzu Co. Ltd.) equipped with activated charcoal and Porapak Q columns, respectively. Because nitrogen used as the carrier gas was collected in the gas bag, nitrogen was used as an internal standard in GC analysis with the TCD, where CO, CO2, CH4, and H2 produced during glycerol conversion were quantified. CH4, propylene, and other hydrocarbon gases were also quantified by GC with the FID using standard curves. Moreover, liquid products were identified by GC with the FID, where 2-propanol and 1,2-butanediol were used as an internal standard. The product yields were calculated based on the amount of glycerol fed to the reactor.

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RESULTS AND DISCUSSION Effect of the Catalyst Composition for the Conversion of Crude Glycerol. ZrO2−FeOx catalysts with different ZrO2 concentrations were prepared by the coprecipitation method. ZrO2 and FeOx were also prepared for comparison. Figure 2a shows the X-ray diffraction (XRD) patterns of these catalysts. The crystal size of iron oxide (with hematite structure) and the BET surface area of the catalysts are listed in Table 1. While ZrO2 and FeOx show the peaks corresponding to tetragonal ZrO2 and α-Fe2O3 (hematite), respectively, ZrO2−FeOx catalysts showed only the peaks corresponding to hematite in low ZrO2 concentration ranging from 0 to 7 wt %. The domain sizes of FeOx in the catalysts, which were calculated by Scherrer’s equation from the patterns, were 32 and 16 nm at ZrO2 concentrations of 0 and 7 wt %, respectively. As the concentration of ZrO2 increased above 27 wt %, only the slight peaks corresponding to hematite could be observed. These results indicated that ZrO2 particles were highly dispersed in B



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In order to investigate the effect of the ZrO2 concentration in the ZrO2−FeOx catalyst on the catalytic conversion of crude glycerol, a 10 wt % crude glycerol solution was reacted for 2 h over ZrO2, FeOx, and ZrO2−FeOx at W/F of 5 h. The solution was also reacted without a catalyst for comparison. The product yields of each are shown in Figure 4.

Figure 2. XRD patterns of the catalysts before (a) and after (b) the reaction of a 10 wt % crude glycerol solution. Reaction conditions: ZrO2(Y)−FeOx (Y = 0−65 wt %); W/F = 5 h; reaction time = 2 h.

Table 1. BET Surface Area and Crystal Size of FeOx of ZrO2−FeOx Catalysts with Different ZrO2 Concentrations ZrO2 concentration in the ZrO2−FeOx catalyst (wt %) BET surface area (m2 g−1) crystal size of FeOx (hematite) (nm)

0.0

7.0

27

50

65

100

14 32

56 16

124

143

149

88

iron oxide, which suppressed the crystal growth of iron oxide during calcination. Hence, the BET surface area of the catalyst was increased with increasing the ZrO2 concentration and exhibited the maximum value of 140−150 m2/g at the ZrO2 concentration of 50−65 wt %. Figure 3 shows the pore-size distributions of ZrO2−FeOx catalysts calculated by the Barrett−Joyner−Halenda method

Figure 4. Effect of the ZrO2 content in ZrO2−FeOx on the product yields. Reaction conditions: 10 wt % crude glycerol solution; ZrO2(Y)−FeOx (Y = 0−65 wt %); W/F = 5 h; reaction time = 2 h.

When a crude glycerol solution was reacted without catalyst, glycerol conversion was only 14 mol % C and useful chemicals were not produced successfully. On the other hand, when ZrO2, FeOx, and ZrO2−FeOx were used as catalysts, complete glycerol conversion was attained. ZrO2 mainly produced the products from reaction pathway II, which were assumed to be generated by dehydration of glycerol, whereas FeOx and ZrO2− FeOx gave the products from reaction pathway I, in addition to the products from pathway II. On the basis of the results obtained, the main reaction site for reaction pathway I was expected to be on FeOx. Moreover, adding a small amount of ZrO2 in FeOx resulted in higher product yields of useful chemicals. When 7 wt % ZrO2 was added to FeOx, the total product yields of reaction pathways I and II marked the highest value of 60 mol % C; however, adding 50 wt % ZrO2 caused a decrease in the product yields of reaction pathways I and II. When the reaction was performed using ZrO2(65)−FeOx, while the catalyst showed the largest surface area, the total product yields of reaction pathways I and II decreased to 45 mol % C. The decrease in their total product yields was expected to be caused by a decrease in the proportion of FeOx in ZrO2−FeOx, the main reaction site for reaction pathway I. Major gaseous products were propylene and CO2, and the amount of CH4 produced from crude glycerol conversion was very few, about one hundredth of the amount of CO2. Moreover, there was almost no CO production. CO2 was mainly produced from ketonization of carboxylic acid in the conversion of glycerol, in which two molecules of carboxylic acids react to produce one molecule of ketone and CO2. In addition, the oxidative decomposition of other hydrocarbons such as free fatty acids led to CO2 generation. Meanwhile, H2

Figure 3. Pore-size distribution of the ZrO2−FeOx catalyst with different ZrO2 contents.

using the N2 adsorption isotherm. In FeOx without ZrO2, the average pore radius was 12 nm, whereas the pore sizes were drastically decreased below approximately 5 nm and the pore volumes were increased with increasing ZrO2 content. The changes in the pore size and volume resulted from the suppression behavior of ZrO2 against iron oxide sintering. As discussed above, the ZrO2−FeOx catalyst was composed of small crystalline iron oxide on which ZrO2 were highly dispersed. As the ZrO2 content increased, the crystal size of iron oxide decreased, leading to the formation of mesopores among the FeOx particles. C



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generation was observed during crude glycerol conversion, where the production of H2 is ascribed to the decomposition of formic acid28 produced by decomposition of hydroxyacetone29 and 3-hydroxypropionaldehyde23 (unstable intermediate), discussed in our previous research.21 A portion of the produced hydrogen atoms on the catalyst surface reacted with glycerol to produce allyl alcohol, and the rest was generated as H2 gas, the amount of which was almost the same as that of CO2. Figure 2b shows the XRD patterns of catalysts after the conversion of glycerol. The ZrO2−FeOx catalysts with ZrO2 of 0, 27, and 50 wt % showed diffraction peaks due to hematite (α-Fe2O3) in addition to magnetite (Fe3O4). We previously reported that the organics adsorbed on the ZrO2−FeOx surface were oxidatively decomposed by consuming lattice oxygen in FeOx, which was regenerated by active oxygen species produced from H2O over ZrO2.30,31 Because a feedstock used in this study contained glycerol as well as impurity hydrocarbons such as methanol and free fatty acids, the oxidation of these impurity hydrocarbons simultaneously occurred over the catalyst using the lattice oxygen of iron oxide during the conversion of glycerol. In the catalyst with a low surface area (Fe2O3 without ZrO2) and a low FeOx concentration (ZrO2 concentrations of 27 and 50 wt %), it was considered that the excessive consumption of the lattice oxygen in the ZrO2−FeOx catalysts led to the reduction and/or structural change of FeOx from hematite to magnetite. In contrast, the ZrO2(7)−FeOx catalyst maintained the crystallinity of the hematite structure during the conversion of crude glycerol because the lack of the lattice oxygen in FeOx could be regenerated by oxygen active species produced from a water molecule over ZrO2.30,31 Moreover, as shown in Figure 3, as compared with ZrO2(27)−FeOx and ZrO2(50)−FeOx, the pore size of ZrO2(7)−FeOx was large enough for diffusion of the reactant molecules such as glycerol and free fatty acids. From the results above, although crude glycerol used as a feedstock contained impurities such as methanol, fatty acids, and potassium ions, a crude glycerol solution was successfully converted into useful chemicals using ZrO 2 −FeO x in accordance with the reaction scheme shown in Figure 1; therefore, the suitable ZrO2 content was concluded to be 7 wt %. Stability and Durability of ZrO2−FeOx during the Catalytic Conversion of Crude Glycerol. The catalytic reaction of a 10 wt % crude glycerol solution was performed for 6 h and W/F of 5 h to examine the stability and durability of ZrO2(7)−FeOx. The liquid and gas products were collected every 2 h; thus, results showed the average product yields for each 2 h. As shown in Figure 5, the yields of allyl alcohol and carboxylic acids increased with increasing reaction time possibly because of suppression of the consecutive reactions in pathways I and II, which were propylene production from allyl alcohol and ketone production from carboxylic acid, respectively. The amount of potassium deposited on ZrO2(7)−FeOx after 6 h of reaction was analyzed by XRF. The weight ratio of potassium on the catalyst was about 3.1 wt %. The percentage of deposited potassium on the catalyst was calculated as the percentage of potassium contained in the feedstock and was set to be 100 wt %, and approximately 62 wt % potassium in the feedstock deposited on the catalyst after 6 h of reaction, which was ascribed to suppression of the consecutive reactions. In order to examine the effect of potassium on the catalyst performance during the conversion of crude glycerol in detail, glycerol conversion was carried out using the potassium-

Figure 5. Changes in the product yields with time on stream during conversion of a 10 wt % crude glycerol solution over the ZrO2(7)− FeOx catalyst (W/F = 5 h).

supported ZrO2−FeOx (K/ZrO2−FeOx) catalyst, which was prepared by an impregnation method of KNO3 on ZrO2−FeOx for 12 h and W/F of 5 h. The amount of potassium on the catalyst was 1.8 wt %, corresponding to the used catalyst after reaction for approximately 3.5 h when using 10 wt % crude glycerol as the feedstock (Figure 5). Figure 6 shows the changes in the product yields belonging to reaction pathways I and II with time on stream.

Figure 6. Changes in the product yields with time on stream during the conversion of a 10 wt % crude glycerol solution over the K/ ZrO2(7)−FeOx catalyst (W/F = 5 h).

As shown in Figure 6, the yields of propylene and ketones were lower than those in Figure 5. This resulted from the difference in the potassium concentration on the catalyst surface. While potassium oxide was uniformly dispersed on the catalyst surface in K/ZrO2−FeOx (Figure 6), potassium oxide on the ZrO2−FeOx catalyst (Figure 5) was mainly deposited on the catalyst surface at the inlet side of the reactor, indicating that the potassium concentration on the ZrO2−FeOx surface was low at the outlet side of the reactor. As crude glycerol conversion progressed, the acidity of the catalyst was expected to decrease with an increase in the D



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amount of potassium on the catalyst, thereby suppressing the dehydration of glycerol in reaction pathway II. In our previous research,21 the effect of potassium on the catalyst on the product yields and catalytic stability was examined. Up to 3 wt % potassium, the product yield of reaction pathway I increased and the catalytic activity was maintained, and then these values were slightly decreased with potassium loading at 6 wt %. However, the total amount of chemicals related to these pathways were almost unchanged, indicating that ZrO2(7)− FeOx showed stable activity for the conversion of crude glycerol. In the study above, a 10 wt % crude glycerol solution was successfully converted into useful chemicals over ZrO2(7)− FeOx. In order to save energy consumption in the processes, using a high concentration of a crude glycerol solution as a feedstock is desirable. Figure 7 shows the effect of the crude

Figure 8. XRD patterns of ZrO2(7)−FeOx before and after the reaction of 10, 30, and 50 wt % crude glycerol solutions. Reaction conditions: ZrO2(7)−FeOx; W/F = 3 h; reaction time = 2 h.

maintained the hematite structure, despite the concentrations of crude glycerol solutions. These results concluded that the conversion of crude glycerol over ZrO2(7)−FeOx had little dependence on the concentration of crude glycerol. Effect of the W/F Value on the Product Yields. Figure 9 shows the effect of the W/F value (weight of the catalyst/feed

Figure 7. Effect of the crude glycerol concentration on the product yields. Reaction conditions: ZrO2(7)−FeOx; W/F = 3 h; reaction time = 2 h.

glycerol concentration on the product yields. The reaction was performed using 10, 30, and 50 wt % crude glycerol solutions (glycerol concentrations: 3.73, 11.19, and 18.65 wt %, respectively) over ZrO2(7)−FeOx for 2 h and W/F of 3 h. Complete glycerol conversion was achieved for any concentrations of solutions. On the other hand, the yields of “others” and “undetectable compounds” increased with crude glycerol concentrations, leading to a slight decrease in the total yields of the useful chemicals such as allyl alcohol and acrolein. High concentrations of allyl alcohol and acrolein were expected to induce a polymerization reaction. The XRD patterns of ZrO2(7)−FeOx before and after the reaction of each concentration of crude glycerol were analyzed. As shown in Figure 8, the crystallinity of ZrO2(7)−FeOx before the reaction was the hematite structure. After the reactions of 10, 30, and 50 wt % crude glycerol solutions, the intensity of the peak around 35.5°, corresponding to the overlapping peak composed of hematite and magnetite, slightly increased, indicating that oxidation of free fatty acids and other heavy components induced consumption of the lattice oxygen of iron oxide. In contrast, because the consumed lattice oxygen was regenerated by oxygen active species produced from decomposition of H2O over ZrO2,30,31 the crystallinity of the catalysts

Figure 9. Effect of the W/F value on the yield of major products. Reaction conditions: 10 wt % crude glycerol solution; ZrO2(7)−FeOx; reaction time = 2 h.

rate) on the yield of major products after reaction of the 10 wt % crude glycerol solution over ZrO2(7)−FeOx for 2 h. The product yields are listed in Table 2 in detail. The compositions of aldehyde, carboxylic acids, and ketones are also listed in Table 3. An increase in the W/F value allowed the consecutive reactions shown in Figure 1 to proceed. The yields of intermediates such as aldehyde, carboxylic acids, and allyl alcohol increased with the W/F value and reached maximum values around W/F of 2 h. Carboxylic acids were composed of acetic, propionic, and butyric acids. Acetic and propionic acids were directly produced from glycerol. On the other hand, it was speculated that butyric acid was produced by the reaction of propionic acid and a methyl radical, which was produced in ketonization. In W/F values above 5 h, the yield of the intermediates decreased and the products were summed as propylene and ketones, which were the terminated chemicals in E



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Table 2. Effect of the W/F Value on the Product Yield of Glycerol Conversion over the ZrO2−FeOx Catalyst product yields (mol % C)

a

W/F (h)

allyl alcohol

propylene

hydroxyacetone

aldehyde and carboxylic acids

ketones

acrolein

othersa

0.0 1.5 2.0 3.0 5.0 7.0 10

0.09 12.5 15.9 12.0 7.12 0.23 1.03

0.00 0.00 0.00 14.2 20.3 23.4 24.1

1.03 1.46 0.30 0.27 0.00 0.00 0.00

1.87 9.11 11.4 10.9 10.6 2.22 0.37

0.30 8.46 10.6 13.5 17.4 25.3 25.4

0.00 1.88 4.61 4.08 3.54 0.38 0.08

13.0 27.1 19.4 10.8 6.83 10.4 15.5

Unidentified hydrocarbons detectable by GC.

Table 3. Compositions of Produced Aldehyde, Carboxylic Acids, and Ketones composition (mol % C) aldehyde and carboxylic acids

ketones

W/F (h)

acetaldehyde

acetic acid

propionic acid

isobutylic acid butyric acid

acetone

methyl ethyl ketone

pentanone

0.0 1.5 2.0 3.0 5.0 7.0 10

66.3 28.4 25.9 29.2 31.4 48.2 100

0 16.3 16.8 0 13.6 0 0

33.7 40.6 40.0 56.3 44.2 51.8 0

0 14.7 17.3 14.5 10.8 0 0

100 39.7 45.3 41.4 48.1 40.7 44.8

0 52.0 45.4 52.3 46.6 52.7 47.8

0 8.3 9.3 6.3 5.3 6.6 7.4

the series of reactions. With W/F of 10 h, 24 mol % C of propylene and 25 mol % C of ketones were obtained. In this experiment, the major components of produced ketone were acetone (C3 ketone) and methyl ethyl ketone (C4 ketone), and only a slight production of pentanone (C5 ketone) was observed. Acetone is produced from two molecules of acetic acid in accordance with ketonization,17 and methyl ethyl ketone and pentanone are produced from ketonization of “acetic acid and propionic acid” and “two molecules of propionic acid”, respectively.19 Because acetic acid and propionic acid were produced from acetol that was an intermediate of glycerol conversion, these ketones were expected to be produced from glycerol. On the other hand, although ketones produced form carboxylic acids derived from glycerol and free fatty acetic acid were also possible products, unfortunately, the ketones derived from free fatty acids could not be detected by our GC system. In biomass-refinery processes, produced chemicals from biomass are sometimes obtained as water−organic mixtures, for instance, an ethanol solution;32 thus, separation and/or purification of the obtained chemicals from water is necessary. Purification of glycerol from crude glycerol is quite difficult because of the impurities contained in the solution and arbitrary solubility of glycerol in water.33 In the case of catalytic conversion using iron oxide catalysts, the main products are summarized into propylene and ketones by an increase in the amount of the catalyst. The former product is simply separated from the product system as a gas, and the latter one is easily separated by distillation. Therefore, the conversion of crude glycerol over iron oxide catalysts is effective considering the separation of produced chemicals.

alcohol, carboxylic acids, and ketones. The catalytic activity and stability were improved by adding ZrO2 to FeOx, and the suitable ZrO2 content was found to be 7−27 wt %. An increase in the W/F value allowed the consecutive reactions to proceed, and the products were summed up in propylene and ketones. ZrO2−FeOx showed stable activity for conversion of a 10 wt % crude glycerol solution for 6 h. It also successfully converted 30 and 50 wt % crude glycerol solutions for useful chemicals and showed little dependence on the concentration of crude glycerol.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-11-706-6551. Fax: +81-11-706-6552. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan soap and detergent association and the Industrial Technology Research Grant Program in 2008, 08B36001c from the New Energy and Industrial Technology Development Organization of Japan.

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REFERENCES

(1) Ma, F.; Hanna, M. A. Biodiesel production: a review. Bioresour. Technol. 1999, 70, 1−15. (2) Fukuda, H.; Kondo, A.; Noda, H. Biodiesel fuel production by transesterification of oils. J. Biosci. Bioeng. 2001, 92, 405−416. (3) Meher, L. C.; Sagar, D. V.; Naik, S. N. Technical aspects of biodiesel production by transesterificationa review. Renew. Sust. Energy Rev. 2006, 10, 248−268. (4) Fukuda, H.; Kondo, A.; Noda, H. Biodiesel fuel production by tranesterification of oils. J. Biosci. Bioeng. 2001, 92, 405−416. (5) Behr, A.; Eilting, J.; Irawadi, K.; Leschinski, J.; Lindner, F. Improved utilisation of renewable resources: new important derivatives of glycerol. Green Chem. 2008, 10, 13−30.

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CONCLUSIONS In this work, the catalytic conversion of crude glycerol over ZrO2−FeOx was studied. ZrO2−FeOx successfully converted crude glycerol into useful chemicals such as propylene, allyl F



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Conversion of Biodiesel-Derived Crude Glycerol into Useful

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