Article pubs.acs.org/EF
Novel Process for the Extraction of Ethyl Levulinate by Toluene with Less Humins from the Ethanolysis Products of Carbohydrates Xing Tang,† Yong Sun,† Xianhai Zeng,*,† Weiwei Hao,† Lu Lin,*,† and Shijie Liu‡ †
College of Energy, Xiamen University, Xiamen 361005, People’s Republic of China College of Environmental Science and Forestry, State University of New York, 1 Forestry Drive, Syracuse, New York 13210, United States
‡
S Supporting Information *
ABSTRACT: A series of organic solvents were screened for the extraction of ethyl levulinate (EL) from the concentrated ethanolysis products of sucrose. Among these organic solvents, toluene was confirmed as an outstanding extracting agent for separating EL with less humins from the concentrated reaction products. The EL extraction rate was strongly related to the temperature and duration applied in the vacuum concentration; a maximum EL recovery of 96.7% can be achieved through the extraction by toluene from the ethanolysis products. Herein, the catalytic transfer hydrogenation (CTH) of biomass-derived EL to γ-valerolactone (GVL) was also performed over inexpensive metal oxides using ethanol as a hydrogen donor. On the basis of these scenarios, a novel process for the production and separation of EL and GVL from carbohydrates was proposed to integrate the ethanolysis of carbohydrates and the CTH of EL.
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INTRODUCTION The diminishing availability of non-renewable fossil resources alongside the increasing energy demand make people pay intensive attention to the use of renewable resources for sustainable energy production. Biomass is the only renewable resource on the earth that can be converted to liquid fuels and chemicals, which are traditionally produced from fossil resources. Biomass-derived carbohydrates, the most abundant biomass resource, can be converted into various bioplatform molecules,1 including 5- hydroxymethylfurfural (HMF),2,3 levulinic acid (LA) and its esters,4−6 and γ-valerolactone (GVL),7,8 which are envisioned as versatile building blocks for the production of biochemicals and biofuels.9−11 Of several platform molecules mentioned, ethyl levulinate (EL) has extensive applications in the flavoring and fragrance industries because of its comfortable odor.12 EL is also suitable to be used as the biodiesel additive to improve the cold flow property of biodiesel.13 In addition, EL (one of ketoester) is an ideal candidate as the precursor for the production of synthetic materials, such as resins, through condensation and addition reactions occurring at the keto and/or ester functional groups. Recently, our group has reported an extremely low sulfuric acid catalyst system for synthesis of levulinates from carbohydrates via alcoholysis.14 An advantage of this catalytic strategy is that negligible undesired ethers formed from the dehydration of alcohol, the equipment corrosion was slight, and less spent acid needed to be addressed after the reaction. In the present paper, this extremely low sulfuric acid catalytic process was also employed to produce EL from sucrose.
systems have recently been developed to produce levulinates from biomass-derived carbohydrates.12,14−22 However, there have been surprisingly few studies of separating levulinates from the complicated reactant products originating from the alcoholysis of biomass-derived carbohydrates. Undoubtedly, the separation issue is of crucial importance to produce levulinates (such as EL) from biomass in a commercial scale. In comparison to LA, EL is easily separable from ethanolysis products derived from carbohydrates via vacuum distillation because it is acid-free with high volatility.23 Recently, Mao et al. have declared that a one-pot system for the direct acidcatalyzed conversion of cellulosic biomass into EL was achieved, and then the desired product was extracted by ndodecane.24 Nevertheless, EL cannot be dissolved in ndodecane; consequently, the so-called product extraction mentioned by Mao is not an accurate definition. Small amounts of EL would be easily acquired off the complicated reactant products via vacuum distillation in the presence of excessive ndodecane gas, owing to the close boiling point between ndodecane (215−217 °C) and EL (205−206 °C). Paraffin (ndodecane) is unnecessary when adequate amounts of EL exist in the reaction products. It is noted that humins formed from acid-catalyzed conversion of carbohydrates are the authentic troublemaker for the separation of EL.25,26 Humins probably further polymerize to form coke, which will deposit at the inwall and bottom of the reactor during concentration and distillation at an elevated temperature. The resulting coke could result in pipeline plugging and uneven heating, greatly decreasing the service life of the equipment. Humins thus
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RESULTS AND DISCUSSION Industrially, EL is largely produced via esterification of pure LA with ethanol in the presence of sulfuric acid. Therefore, a high EL yield is easily achieved, and pure EL is readily separated from the reaction products by distillation. Multiple catalytic © 2014 American Chemical Society
Special Issue: International Biorefinery Conference Received: January 7, 2014 Revised: March 1, 2014 Published: March 5, 2014 4251
dx.doi.org/10.1021/ef5000497 | Energy Fuels 2014, 28, 4251−4255
Energy & Fuels
Article
It is a simple and reliable approach to rate visually extraction behaviors by the observation of the color of extract and the formation of insoluble solids (humins). Patil et al. have recently concluded that humins, the dark-colored solids, consisted of a furan-rich polymer network containing carbonyl groups conjugated with carbon−carbon double bonds.26−28 Therefore, a black and muddy solution with a significant amount of humins was obtained from the ethanolysis of sucrose in ethanol, whereas there was a light-colored solution after extraction by toluene from the concentrated products because of the poor solubility of humins in toluene, leaving dark-colored sticky solids (toluene-insoluble humins) (Figure 1). A majority of humin components dissolved in ethanol cannot be detected by gas chromatography−mass spectrometry (GC−MS) analysis because of its low volatility; however, we observed distinctly that a fraction of heavy components of the concentrated products was eliminated in the toluene extract according to the results of gas chromatography−flame ionization detector (GC− FID) analysis (see Figure S1 of the Supporting Information). The GC−FID trace of untreated ethanolysis products disclosed that EL was the dominant product as well as a small amount of 5-ethoxymethylfurfural (EMF) (see Figure S1a of the Supporting Information). EMF is confirmed as the intermediate for the EL production by ethanolysis12 and has currently served as a potential diesel additive because of its high miscibility in diesel fuel and high energy density of 30.3 MJ/ L.3,29 It is noted that a broad signal peak appeared at around 16 min in the GC−FID trace of untreated ethanolysis products (see Figure S1a of the Supporting Information) and should be attributed to compounds formed through the condensation between glucose, fructose, and ethanol on the basis of the results of GC−MS analysis. Surprisingly, the signal peak of these compounds disappeared in the concentrated toluene extract and reappeared in the ethanol solution of tolueneinsoluble humins (see panels b and c of Figure S1 of the Supporting Information). These compounds probably further polymerize to humins or coke during the separation of EL by rectification at an elevated temperature. One can thus infer that the desired product EL, excluding awkward humins, was selectively extracted from the concentrated products by toluene. Upon the screening of the extracting agent, the influence of the temperature and duration applied in the concentration stage on the extraction rate of EL was also investigated in this paper. A typical extraction test for EL separation using toluene was detailedly described in the Supporting Information. The extraction rate (ER) of EL is defined here as the ratio of the molar quantities of EL extracted by toluene to that of EL in the initial ethanolysis products. The loss of EL consists of two parts: residual EL in toluene-insoluble humins (RTI, %) and EL taken away by ethanol vapor during the vacuum concentration (RRE, %). Herein, concentration temperatures in the range from 70 to 90 °C were selected to remove low-boiling products (mainly ethanol) by vacuum concentration because these temperatures are close to the boiling point of ethanol (78 °C) and are sufficient to facilitate the removal of ethanol in vacuum. The value of RTI is equal to zero, indicating that no tolueneinsoluble solids formed in the extraction phase. Namely, humins also totally dissolved in the extracting agent because of a considerable amount of ethanol retained in the concentrated products, when ethanolysis products were concentrated under a relatively low temperature and/or short time (entries 1 and 5 in Table 1). Raising the temperature and/or time employed in the
must be removed from the ethanolysis products before the purification of EL via distillation. To eliminate these chewy humins, a visual evaluation for extraction behaviors using various organic solvents from concentrated reaction products derived from ethanolysis of sucrose was performed. The experiment of ethanolysis of sucrose was detailedly described in the Supporting Information. Almost all products, including humins formed from the ethanolysis of sucrose, can be dissolved in ethanol. For the purpose of selective extraction of EL without humins, less polar solvents must be employed as extracting agents. However, it is impossible to directly extract EL by other less polar solvents without humins from the untreated ethanolysis products (containing excessive ethanol), because the miscibility of ethanol with other less polar solvents significantly promotes the solubility of humins to these mixture solvents. Therefore, low boiling components, mainly including ethanol, in the enthanolysis products should be removed through the vacuum concentration before extraction with other less polar solvents. In each extraction run, ethanolysis products (100 mL) were concentrated in vacuum (
