Because of the increasing consumption of limited fossil resources and the associated environmental problems caused by this process,more attention is being given to processes that use biomass as a feedstock to prepare fuels and chemicals. This is an important method of decreasing carbon dioxide emissions [1, 2, 3]. Carbohydrates are the most abundant biomass resource. The transformation of carbohydrates into important platform chemicals is significant for biomassconversion. Carbohydrates can be dehydrated to 5-hydroxymethylfurfural (HMF). HMF is a crucial platform chemical. It can be converted into a variety of chemicals and liquid fuels through hydrogenation,oxidation,hydrolysis,and etherification [4, 5, 6, 7]. Recently,we developed methods to convert HMF to monomers such as maleic anhydride,furan-2,5-dicarbaldehyde,furan-2,5-dicarboxylic acid,and their polymers. On the other hand,we have developed methods to convert HMF to liquid fuels such as 5- (ethoxymethyl)furan-2-carbaldehyde [8, 9, 10, 11, 12, 13, 14, 15, 16]. The development of a simple and practical method for the preparation of HMF is important for this application.
It is relatively easy for fructose to be converted into HMF. Much research has been conducted in this field [17, 18, 19, 20]. However,the potential application of fructose as a feedstock is limited because of its high cost. Compared with fructose,glucose is a better feedstock for the preparation of HMF because of its lower cost and because it is a richer resource. However,it is very difficult for glucose to be converted to HMF. On the one hand,compared with the five-membered furan ring of fructose,glucose with its six-membered pyran ring is more stable and inactive [21]. On the other hand,HMF is active and easily hydrolyzed as well as easily polymerized,and thus the yield of HMF is not very high. In ionic liquids,when using chromium or other metal salts as catalysts,glucose can be converted to HMF [22, 23, 24, 25]. However,the application of this method has been limited because of the high cost of ionic liquids,the toxicity of chromium salts,and their impact on the environment. Non- toxic or low toxicity catalysts were also used to convert glucose to HMF in common solvents [26, 27, 28]. In previous research,we found that in N,N-dimethylacetamide (DMAC),AlCl3 could catalyze the conversion of glucose to HMF [29]. In this paper,we investigated the promotion effect of alkali metal halides in the AlCl3-catalyzed conversion of glucose to HMF in DMAC,and found that NaI promoted the reaction significantly.
Glucose,fructose,sucrose,and ethyl acetate were purchased from Tianjin Kermel in China. Mannose,maltose,and cellobiose were purchased from Aladdin Reagent Co.,Ltd. in China. Inulin and methyl benzoate (99%) were purchased from Alfa Aesar. HMF (99%) was purchased from Sigma-Aldrich. DMAC,AlCl3,NaF,NaCl,NaBr,and NaI were purchased from the Shanghai Chemical Reagent Company. DMAC was distilled under reduced pressure before use. Unless otherwise noted,all reagents were analytical grade or better and were used as received without further purification.
To a 50 ml two-necked flask equipped with a reflux condenser,0.5 mmol (90 mg) glucose or 0.5 mmol monosaccharide units of carbohydrate,0.05 mmol AlCl3,90 mg alkali metal halide,and 2 ml DMAC were added under N2. The flask was then placed in an oil bath,which had been heated to a predetermined temperature. After a predetermined reaction time under magnetic stirring,the flask was cooled in an ice-water bath. The reaction mixture was then filtered. After the addition of a specific amount of methyl benzoate as an internal standard,the filtrate was diluted to 10 ml with ethyl acetate for the analysis of HMF.
HMF was analyzed qualitatively using gas chromatography-mass spectrometry (Agilent 6890 N GC/5973 MS,30 m × 0.25 mm × 0.3 μm HP-5 column),1H NMR (Bruker DRX-400,400 MHz),and 13C NMR (Bruker DRX-400,100 MHz). HMF was analyzed quantitatively by gas chromatography (Agilent 4890D,30 m × 0.32 mm × 0.4 μm FFAP column,hydrogen flame ionization detector). From the area ratio of the HMF peak compared with the internal standard peak,the HMF yield was determined by the internal standard curve method.
The carbohydrates were analyzed using high performance liquid chromatography (Waters 6895,4.6 mm × 250 mm high performance carbohydrate column,differential refractive index detector). From the peak area of the carbohydrate in the reaction mixture,the concentration of unconverted carbohydrate and the conversion of carbohydrate were determined using an external standard curve.
In a 50 ml two-necked flask,2.5 mmol (450 mg) glucose,0.25 mmol AlCl3,450 mg NaI,and 10 ml DMAC were mixed evenly. Five 0.5 ml aliquots of the mixture were added to five NMR tubes (5 mm,Wilmad). After reacting at 130 °C for 2,5,10,20,and 30 min,the mixtures were placed in an ice bath. Then they were analyzed by 13C NMR (Bruker DRX-400,100 MHz,DMSO-d6 was used as the external standard).
The promotion effect of alkali metal halides on the AlCl3-catalyzed conversion of glucose to HMF in DMAC was investigated. The results are shown in Fig. 1. The reaction was conducted at 130 °C for 15 min. Without any additives the glucose conversion was 71% and the HMF yield was 36%. When NaF was used as an additive the reaction was inhibited significantly. Under the same conditions,the glucose conversion was reduced to 40% and the HMF yield decreased sharply to 8%. Using NaCl as the additive had no obvious influence on the HMF yield. However,when NaBr was used as the additive,the glucose conversion increased to 86% and the HMF yield increased to 55%. When NaI was used as the additive,compared with NaBr,no obvious change was observed for the glucose conversion. However,the HMF yield increased to 62%. In summary,NaF significantly inhibited the AlCl3-catalyzed conversion of glucose to HMF. However,NaI and NaBr significantly promoted the reaction,but NaI promoted the reaction more obviously. To verify whether alkali metal halides catalyzed the conversion of glucose to HMF,AlCl3 blank experiments were undertaken. When neither AlCl3 nor NaI were added only a very small amount of HMF was detected. When only NaI was added without AlCl3,no obvious change was found for the HMF yield under the same conditions. These results show that NaI cannot catalyze the conversion of glucose to HMF.
It is generally accepted that the conversion of glucose to HMF involves the mutarotation of α-glucopyranose to β- glucopyranose,the isomerization of glucose to fructose,and the dehydration of fructose to HMF. When metal compounds are used as catalysts to convert glucose to HMF,the metal compounds are coordinated with glucose. This results in the isomerization of glucose to fructose [22,30,31]. In previous research,we found that AlCl3 could catalyze the mutarotation of α- glucopyranose to β-glucopyranose and the isomerization of glucose to fructose [29]. The mutarotation of α-glucopyranose to β- glucopyranose can be attributed to the interaction between halides and the hydrogen atoms of the hydroxyl groups of glucose [24,25]. Additionally,the isomerization of glucose to fructose might contribute to the coordination of glucose with aluminum. It is well known that halides can serve as ligands for metals,and the catalytic activity of metal catalysts can be influenced by the halide ligand because of its electronic properties and nucleophilicity [32]. In this work,F-,Br-,and I- could coordinate with AlCl3. This influences the catalytic isomerization activity of AlCl3 and thus influences the conversion of glucose to HMF. The fact that NaF significantly inhibited the AlCl3-catalyzed conversion of glucose to HMF might be due to the strong ability of F- to coordinate with aluminum [33]. This hinders the coordination of glucose with aluminum and thus hinders the isomerization of glucose to fructose. On the other hand,Binder et al. [30] reported that halogen ions could promote the dehydration of fructose to HMF by nucleophilic addition and elimination. In our research,the fact that NaBr and NaI promoted the AlCl3- catalyzed conversion of glucose to HMF might partially be because NaBr and NaI promoted the dehydration of fructose to HMF after fructose was formed by the AlCl3-catalyzed isomerization of glucose. Both the nucleophilic ability and the elimination ability of I- are stronger than those of Br-,and NaI thus has a better ability to promote the dehydration of fructose to HMF.
With AlCl3 as a catalyst,DMAC as a solvent,and NaI as an additive,we investigated the effect of temperature on the conversion of glucose to HMF over 15 min. The results are shown in Fig. 2. From 80 to 130 °C,a higher glucose conversion and HMF yield were obtained at the higher temperature. When the reaction was conducted at 80 °C for 15 min,the glucose conversion was only 45%,and the HMF yield was less than 10%. When the temperature was increased to 130 °C within the same reaction time,the glucose conversion increased to 86%,and the HMF yield increased to 62%. Upon increasing the temperature to 140 °C,glucose was converted completely. However,no obvious change was found for the yield of HMF. During the reaction a black substance was obtained,especially when the reaction was conducted at 140 °C. This might be humin [22]. To reduce the formation of the black substance,the optimum reaction temperature was set to 130 °C.
With AlCl3 as a catalyst,DMAC as a solvent,NaI as an additive,and at 130 °C the effect of reaction time on the conversion of glucose to HMF was investigated. The results are shown in Fig. 3. When the reaction was conducted within 4 min,the glucose conversion was 67%,and the HMF yield was 34%. With a longer reaction time,the glucose conversion increased gradually,and glucose was converted completely within 30 min. However,the HMF yield reached a maximum within 15 min. Extending the reaction time led to a slight decrease in the HMF yield. This might be because more humin was formed. In this reaction,fructose was detected,which means that glucose was isomerized to fructose during the reaction.
The reaction was monitored using 13C NMR. The results are shown in Fig. 4. The peaks between δ = 100 and 60 are from the carbons of glucose. The peak at δ = 97.94 is from the C1β of β-glucopyranose. The peak at δ = 93.26 is from the C1α of α-glucopyranose [29,34,35]. The peaks at δ = 178.30,163.16,152.84,125.00,110.12,and 56.91 are from the six carbons of HMF [36]. For the reaction conducted within 2 min,a mixture of β-glucopyranose and α-glucopyranose was apparent,and a peak was present at δ = 178.30. This comes from the aldehyde group carbon of HMF,indicating that HMF was formed. Figure 4 shows that as the reaction proceeds from 2 to 30 min,the peaks between 100 and 60 weaken gradually and finally disappear,indicating that the glucose concentration decreases until glucose is converted completely. The HMF signal peaks increased gradually. At δ = 170.00,a carbon chemical shift from DMAC was present. The signal exceeded the scale.
The AlCl3-NaI-DMAC system was also used for the catalytic conversion of other carbohydrates such as fructose,mannose,sucrose,maltose,cellobiose,and inulin. The results are shown in Fig. 5. When the catalytic system was used to convert mannose and fructose,the isomers of glucose,41% and 67% HMF yields were obtained,respectively. The catalytic system was also used to convert disaccharides including sucrose (a disaccharide of glucose and fructose),cellobiose (a dimer of glucose linked by a β-1,4 glycosidic bond),and maltose (a dimer of glucose linked by a α-1,4 glycosidic bond),and 63%,46%,and 25% HMF yields were obtained,respectively. The catalytic system was also used to convert the fructan inulin,and a 48%HMF yield was obtained. When the disaccharides and inulin were used as the feedstock,they might have been hydrolyzed to a monosaccharide catalyzed by the Lewis acid AlCl3 at first. The monosaccharides were then converted to HMF.
NaF inhibited the AlCl3-catalyzed conversion of glucose to HMF. However,NaI and NaBr promoted the reaction significantly,and the promotion effect of NaI was more obvious than that of NaBr. In DMAC,when NaI was used as an additive for the AlCl3-catalyzed conversion of glucose at 130 °C for 15 min,the glucose conversion increased from 71% to 86%,and the HMF yield increased from 36% to 62%. In this reaction,glucose was isomerized to fructose,and then fructose was dehydrated to HMF. The AlCl3-NaI-DMAC system was also used for the catalytic conversion of other carbohydrates such as fructose,mannose,sucrose,maltose,cellobiose,and inulin. When sucrose was used as the feedstock,a 63% yield of HMF was achieved.
2014, Vol. 35 Issue (4): 496-500
PDF (617KB)
2014, Vol. 35 Issue (4): 496-500
PDF (617KB)