Article pubs.acs.org/EF
Direct Conversion of Cellulose into Sorbitol over a Magnetic Catalyst in an Extremely Low Concentration Acid System Jun Zhang, Shu-bin Wu,* and Ying Liu* State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China ABSTRACT: Cellulose could be efficiently converted into sorbitol in the presence of hydrogen and an extremely low phosphoric acid concentration over a magnetic catalyst. Effects of various process conditions toward the conversion efficiency were discussed in greater detail. X-ray powder diffraction analysis indicated the essential properties of prepared catalysts. A sorbitol yield of 68.07% was obtained at 488 K with a catalyst dosage of 20%. The experiments showed that Ni4.63Cu1Al1.82Fe0.79 sustained better activity after being reused 3 or 4 times. was filtered, washed, and dried at 353 K for 24 h. The precursor was required to be activated at 923 K under a H2 atmosphere for 3 h before being used. The used catalyst was reduced again before the next use. Inductively coupled plasma−atomic emission spectrometry (ICP− AES) analysis was introduced to identify the chemical composition of the precursor, and the resulting catalyst was named Ni4.63Cu1Al1.82Fe0.79. The method described above was applied to synthesize the Cu/Al/Fe catalyst, and the obtained sample was denoted as Cu1Al1.71Fe0.72. 2.2. Catalytic Reaction. The conversion of cellulose (crystallinity of 60%) to sorbitol was conducted in a high-pressure reactor (PARR 5500). In a typical run, cellulose, catalysts, and 0.08 wt % H3PO4 were intrduced into the autoclave. Afterward, the reactor was purged with H2 3 times to remove the residual air, pressurized with H2 to 4 MPa, and then heated to certain temperatures with a stirring rate of 600 rpm. After experiments, the furnace was cooled to ambient temperature rapidly with an ice−water mixture and then the products were collected by filtration and stored in the refrigerator prior to analysis. For the heating−stirring pretreatment, the cellulose material in acidic solution was heated to 358 K under a high agitation speed (∼750 rpm) for 3 h. Then, the obtained mixtures were used for sorbitol production under the catalyzing of magnetic catalysts. Liquid samples were identified from their retention times as measured by ion chromatography (IC) analysis on a Dionex ICS-3000 instrument furnished with a CarboPac PA10 column at a column temperature of 303 K. The eluent was 40 mM NaOH solution with a flow rate of 0.25 mL/min. The quantitative determination of byproducts was detected by high-performance liquid chromatography (HPLC, Waters 1525, Milford, MA) using a Platisil ODS C18 column and a refractive index detector (RID, Waters 2414, Milford, MA) at 298 K. 2.3. Characterizations of the Catalyst and Pretreated Samples. Chemical analysis was carried out on a Thermo Elemental ICP−AES spectrometer after dissolution of the sample in a HNO3 solution. X-ray powder diffraction (XRD) was recorded on a Bruker D8 Advance diffractometer using Cu Kα radiation, operated at 40 kV and 40 mA at a scan rate of 2°/min. The crystallite size of the catalyst was calculated by XRD line broadening according to the Scherrer
1. INTRODUCTION Cellulose is the most abundant biopolymer on earth, and it holds great potential as an alternative source of valuable chemicals and fuels, especially because the gradual depletion of fossil fuels is now becoming of worldwide concern. Besides, emissions of greenhouse gases associated with the use of fossil fuels can be improved through using biomass as a carbon and energy source. These aspects have aroused a great deal of interesting research into the use of biomass as a renewable source of fuels and chemicals.1,2 The resistance of cellulose to hydrolysis is well-known. However, cellulose can be evidently degraded with liquid acids,3 bases,4 enzymes,5 and hot-compressed water.6 Currently, catalytic conversion of cellulose into sorbitol has attracted a lot of attention in recent research.7−9 The valuable polylols can be obtained through the hydrogenolysis of sorbitol over metal catalysts. More importantly, light paraffins and H2 will be produced through aqueous-phase reforming of sorbitol.10 Usually noble metal catalysts show high activity in the hydrolytic hydrogenation of cellulose, such as Ru/C11 and Pt/ C.12 However, the preparation costs for these catalysts are too high that will limit the industrial application. Herein, the cheap Ni/Cu/Al/Fe hydrotalcite was synthesized and served as a precursor for preparing the hydrogenation catalyst, which also can be easily separated from reaction mixtures in a magnetic field. In this paper, the magnetic catalyst was used for cellulose conversion in an extremely low concentration acid, which is environmentally friendly as well as economical. Notably, the simple heating−stirring pretreatment was advanced to facilitate the conversion of cellulose into sorbitol. Effects of various processing conditions on cellulose conversion are investigated to achieve the optimal sorbitol yield. 2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The Ni/Cu/Al/Fe hydrotalcite precursor was prepared using a co-precipitation method.13 A solution of 4.53 g of Cu(NO3)2·6H2O, 27.26 g of Ni(NO3)2·6H2O, 3.75 g of Fe(NO3)3·9H2O, and 10.55 g of Al(NO3)3·9H2O with a cation concentration of 1 mol/L was added dropwise into 150 mL of NaOH (12.0 g) and Na2CO3 (1.99 g) solutions at a constant pH of around 9.0. The slurry thus obtained was kept at 333 K for 10 h. The sample © 2014 American Chemical Society
Special Issue: International Biorefinery Conference Received: January 6, 2014 Revised: April 16, 2014 Published: April 16, 2014 4242
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equation.14 The structures of pretreated samples were analyzed on a scanning electron microscope (SEM) S-3700N operating at 15 kV.
3. RESULTS AND DISCUSSION 3.1. XRD Test of the Catalyst. The XRD pattern of assynthesized hydrotalcite-like material was shown in the inset of Figure 1. The XRD pattern of the precursor revealed the peaks
Figure 1. XRD patterns of prepared catalysts: (a) fresh, (b) used once, and (c) used twice. (Inset) XRD pattern of the catalyst precursor.
characteristic of layered double hydrotalcite (LDH) phases with the presence of sharp and symmetrical lines for (003), (006), and (018) planes and broad and asymmetric lines for (012), (015), and (113) planes.15 The (003), (006), (012), (015), (018), and (113) diffractions of the hydrotalcite-like compound (HTlc) were ascribed to the 2θ of 11.7, 23.3, 35.0, 39.0, 47.2, and 61.6° respectively (Figure 1). The obtained results demonstrated that Ni and Cu centers were homogeneously merged into the matrixes of Ni4.63Cu1Al1.82Fe0.79 HTlc, as compared with previous work,16−19 because no obvious peaks representing individual Ni, Cu, Al, and Fe hydroxides were found. The XRD diagram of the reduced sample is given in Figure 1. It was found that all of the hydrotalcite structure was destroyed and some new crystal phases of active metals were formed after reduction. According to the Joint Committee on Powder Diffraction System 4-836 (JCPDS 4-836), peaks corresponding to planes (200) at 2θ of 51.7° indicated the presence of Cu0 in the face-center-cubic structure. Furthermore, peaks at 2θ of 44.2° and 75.8° revealed the formation of a Ni-rich alloy (NiCu), which were assigned to the planes (111) and (220), respectively. These results are in agreement with that described by Rao et al.20 and Wu et al.,21,22 in relation to the peak positions in the XRD diagram of the Cu−Ni alloy. Meanwhile, the crystallite sizes of catalysts increased apparently during the recycling experiments (as given in the table in Figure 1), which might be caused by particle sintering at a high reduction temperature. It was also worth noting that no obvious phase change was observed in the recycled catalysts. 3.2. SEM Analysis of Pretreated Cellulose Samples. To obtain information on a possible morphology evolution, we performed SEM analysis of the samples before and after the heating−stirring pretreatment. The pictures in Figure 2 indicate that the average particle size decreased significantly and the cellulose particles were widely dispersed after 3 h of pretreatment. Furthermore, many particles below 10 μm were
Figure 2. SEM analysis of cellulose samples under various conditions: (A) untreated, (B) 0.1% H3PO4, (C) 0.08% H3PO4, (D) 0.06% H3PO4, (E) 0.1% H2SO4, (F) 0.08% H2SO4, and (G) 0.06% H2SO4.
found in the pretreated samples, as shown in panels B−G of Figure 2. When the samples was treated in low acid concentrations (0.06% H3PO4 and 0.06% H2SO4), some large particles were found to be apparently formed in the resulting mixtures. Higher acid concentrations led to the formation of fine granules, which might be caused by the deep hydrolysis of cellulose materials. For the untreated sample (Figure 2A), most likely, these larger particles may correspond to agglomerations of the primary cellulose crystals because the crystallinity index was as high as around 60%. The results demonstrated that the cellulose structure had been evidently destroyed to some extent. Therefore, a change in the morphology of the cellulose particles occurred during the pretreatment. 3.3. Catalytic Performance for the Conversion of Cellulose. 3.3.1. Effect of the Reaction Temperature. The influence of the reaction temperature on the conversion of cellulose into sorbitol was discussed, and the experiments were carried out at 478, 488, and 498 K within a given time. As seen in Figure 3, the reaction temperature played a major role in the conversion efficiency. First, the sorbitol yield grew significantly with increasing the temperature from 478 to 488 K in the same reaction time. However, the obtained sorbitol would be further degraded into lower alcohols at a higher temperature in a 4243
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byproducts, such as glycol, glycerol, and 1,2-propylene glycol, were formed. 3.3.3. Catalyst Recycling. Long-term reusability of the heterogeneous catalyst is an extremely key point for future industrial application to lower practical cost. After reaction, the catalysts separated from the liquid products were reactivated at 923 K for 3 h in a hydrogen atmosphere before next use. As seen from Figure 5, the sorbitol yield decreased from 42.91 to
Figure 3. Effect of the reaction temperature on the yield of sorbitol with the reaction time. Reaction conditions: 0.5 g of cellulose, 50 mL of 0.08 wt % H3PO4, 0.1 g of catalyst, and 4 MPa H2.
hydrogen atmosphere, thus leading to an obvious decrease in the yield of sorbitol. At the temperatures of 488 and 498 K, much sorbitol was produced in the initial phase of the reaction and then the sorbitol yield decreased obviously with increasing the reaction time because of the hydrogenolysis of produced sorbitol. The glycerol yield was raised up to 20.04% with increasing time to 3 h at 478 K. The optimum yield was attained at the reaction time of 3 h. At 488 K, the yield of sorbitol increased smoothly with the prolonging of the reaction time, which may need a longer reaction time to reach the equilibrium conversion. 3.3.2. Effect of the Catalyst Dosage. Figure 4 shows the influence of the catalyst dosage on the yield of sorbitol, and the
Figure 5. Sorbitol yield as a function of the recycling times of the catalyst. Reaction conditions: 0.5 g of cellulose, 50 mL of 0.08 wt % H3PO4, 0.1 g of catalyst, 488 K, 4 MPa H2, and 3 h.
40.96% in the second run. In the following two runs, the catalytic activity of the catalyst had not been completely recovered after reduction and the sorbitol yield was around 35%, which reached about 81% of that obtained for the fresh catalyst. This may be caused by partial loss of active sites in the catalyst during catalysis. It can be seen from Table 1 that active Table 1. Amount of Leached Cu and Ni in the Fresh Catalyst recycling time (h)
Cu content (wt %)
Ni content (wt %)
1 2 3
0 0 0
0.043 0.067 0.095
Ni species of magnetic catalysts were clearly leached into the reaction solution during the recycling experiments, which might be due to the chelation between the hydroxyl group from sugar alcohol compounds and active Ni. Meanwhile, the deposited humins on the surface of the used catalyst could not be easily removed during reduction treatment; therefore, the activity of the recycled catalyst could be partly improved after reduction. Accordingly, how to improve the long-term stability of the reduced Cu/Ni/Al/Fe hydrotalcite-like catalyst remained as one of the topics for future exploration. On the other hand, the leaching of active Ni was a big problem for the practical application, because the final solutions would be contaminated to a certain extent. The production cost was also raised significantly for removing residual Ni. 3.3.4. Effect of the Heating−Stirring Pretreatment on Cellulose Conversion. Table 2 shows the influences of different pretreatment conditions on the conversion of cellulose into sorbitol. It can be seen that much produced sorbitol and mannitol would be further converted into lower molecular weight alcohols and other unidentified byproducts in the acid
Figure 4. Effect of the catalyst dosage on the yield of sorbitol with the reaction time. Reaction conditions: 0.5 g of cellulose, 50 mL of 0.08 wt % H3PO4, 498 K, and 4 MPa H2.
specific experiments were carried out at three different catalyst loadings (20, 30 and 40%) as a function of the reaction time. With increasing the amount of magnetic catalyst, the sorbitol yield decreased significantly in the initial stage at a temperature of 498 K, especially for the catalyst dosage of 40%. It was inferred that much of the produced sorbitol was converted into lower alcohols under such a high temperature at hydrogen atmosphere. For a catalyst dosage of 20%, the sorbitol yield began to increase with the prolonging of the reaction time that reached the highest level of 40.32% after a reaction of 1.5 h. Further increasing the reaction time was unfavorable for the direct conversion of cellulose into sorbitol, and many 4244
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Table 2. Effect of Pretreatment Conditions on the Conversion of Cellulosea yield (%) conditions untreated 0.1% H2SO4 0.08% H2SO4 0.06% H2SO4 0.1% H3PO4 0.08% H3PO4 0.06% H3PO4 0.06% H3PO4b
sorbitol 42.91 11.72 14.15 35.31 21.20 42.07 68.07 28.69
± ± ± ± ± ± ± ±
0.39 0.23 0.49 0.22 0.38 0.24 0.41 0.32
mannitol 9.68 3.61 3.99 8.87 3.79 7.00 11.96 5.37
± ± ± ± ± ± ± ±
glycerol
0.19 0.08 0.05 0.10 0.06 0.04 0.34 0.09
10.98 15.30 14.15 18.41 9.16 1.20 3.25 2.31
± ± ± ± ± ± ± ±
glycol
0.15 0.21 0.08 0.17 0.31 0.04 0.03 0.05
2.41 4.49 4.32 5.04 4.72 4.03 4.21 3.35
± ± ± ± ± ± ± ±
0.07 0.09 0.12 0.07 0.04 0.03 0.06 0.11
propanediol 1.47 5.79 4.24 2.41 1.96 0.34 0.87 0.82
± ± ± ± ± ± ± ±
0.11 0.05 0.11 0.09 0.04 0.06 0.03 0.07
a Reaction conditions: 0.5 g of cellulose, 0.1 g of catalyst (Ni4.63Cu1Al1.82Fe0.79), 488 K, 4 MPa H2, and 3 h. bA total of 0.1 g of catalyst (Cu1Al1.71Fe0.72).
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concentration of 0.1%. The structures of cellulose crystals were greatly destroyed under higher concentrations, which would facilitate the hydrolytic hydrogenation and hydrogenolysis of pretreated samples. As a comparison, samples treated by H2SO4 solutions could be deeper degraded than those treated by H3PO4 solutions, thus leading to an obvious decrease in the yield of sorbitol. Because the acid strength of H2SO4 was much higher than that of the same concentration of H3PO4, the glucose obtained from cellulose hydrolysis might be further converted into other chemicals under the catalyzing of inorganic acids, such as 5-hydroxymethylfurfural and levulinic acid. In comparison to untreated cellulose, the 0.08% H3PO4pretreated sample did not show better activity toward sorbitol production, which might be caused by the excess degradation of glucose during the process of the heating−stirring pretreatment. For H3PO4 pretreatment, a lower acid concentration favored the formation of sorbitol and mannitol, with the yields reaching up to 68.07 and 11.96%, respectively. Therefore, a proper heating−stirring pretreatment of cellulose samples in 0.06% H3PO4 could facilitate the proceeding of hydrolysis− hydrogenation reactions. To verify the specific active sites, the prepared Cu1Al1.71Fe0.72 was used for the conversion of 0.06% H3PO4-pretreated cellulose. Under the catalysis of Cu1Al1.71Fe0.72, the sorbitol yield was around 29%. However, a sorbitol yield of 68.07% was attained when catalyzed by Ni4.63Cu1Al1.82Fe0.79 under the same conditions. The catalyst activity had been greatly improved after the modification of the Ni element. Therefore, Cu and Ni/ Cu were the active sites for the hydrogenation reaction combined with XRD analysis, and the modification of Ni accelerated the proceeding of the catalytic reaction. There might be a synergistic effect between active Cu and Ni/Cu during the reaction.
AUTHOR INFORMATION
Corresponding Authors
*Telephone/Fax: +86-20-22236808. E-mail: shubinwu@scut. edu.cn. *Telephone/Fax: +86-20-22236808. E-mail:
[email protected]. cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the National Key Basic Research Program of China (2013CB228101) and the National Natural Science Foundation of China (31270635). Finally, the authors are grateful to the kind support from the Committee of the 4th International Conference on Biorefinerytowards Bioenergy (ICBB 2013) in Xiamen, China.
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4. CONCLUSION This work presented a feasible method for efficient conversion of cellulose into sorbitol with the presence of a magnetic catalyst and an extremely low phosphoric acid concentration. A simple and efficient preprocessing method had been developed. The structures of cellulose samples could be obviously destroyed on the basis of the SEM analysis. The desired sorbitol yield could reach 68.07% at 488 K for 3 h under 4 MPa H2. The XRD test of used catalysts indicated that a high crystallite size of the catalyst was not conducive to the formation of sorbitol. Ni4.63Cu1Al1.82Fe0.79 could sustain better catalytic activity during catalyst recycling. 4245
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