Lucília S. Ribeiroa,
Juan J. Delgadob,
José J. de Melo Órfãoa and
Manuel Fernando Ribeiro Pereira
*a
aLaboratório de Processos de Separação e Reação – Laboratório de Catálise e Materiais (LSRE-LCM), Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal. E-mail: fpereira@fe.up.pt
bDepartamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias, Universidad de Cádiz, Campus Rio San Pedro, 11510 Puerto Real, Cádiz, Spain
First published on 30th September 2016
An efficient one-pot reaction system for converting hemicellulose (corncob xylan) into xylitol was developed by using a heterogeneous catalyst and water as solvent, without the presence of any acids. A xylitol yield of 46.3% was achieved after 45 min of reaction using Ru/CNT as catalyst, which showed excellent stability after repeated use. Since the conversion of hemicellulose consists of xylan hydrolysis to xylose followed by the subsequent hydrogenation to xylitol, the two steps were then evaluated separately. The effect of the presence of cellulose on the conversion of xylan and distribution of products was also studied and the yield of xylitol was increased up to around 60% in less than 1 h of reaction. Furthermore, a yield of sorbitol over 80% could also be attained in just 2 h of reaction. Being this result one of the best ever reported for the direct conversion of cellulose and hemicellulose using an environmentally friendly approach, the proposed method shows great potential for the optimization of the catalytic production of xylitol and sorbitol.
Dhepe and Sahu reported for the first time a one-pot process for the conversion of solid hemicellulose into xylose, arabinose and furfural using solid acid catalysts in aqueous media.16 Since then, some works have reported the hydrolytic hydrogenation of arabinogalactans,2,11 but only a few have addressed the conversion of xylans.17,23 Inspired by Kobayashi et al. previous work using an alcohol as hydrogen source for the conversion of cellulose into sorbitol and mannitol,24 Yi and Zhang developed a one-pot process for the selective conversion of hemicellulose (xylan) to xylitol via transfer hydrogenation.17 They achieved a xylitol yield of 81.8% after 3 h using isopropanol instead of H2 high-pressure. However, in these conditions, they found out that a small amount of sulphuric acid was crucial for the conversion of hemicellulose into xylitol, attaining a yield of xylitol of only 5.7% in the absence of any acidic additive. More recently, Liu et al. also studied the conversion of xylan into xylitol and obtained a yield of 79% after 12 h of reaction at 200 °C and 60 bar of H2, using a Ir-ReOx/SiO2 catalyst combined with H2SO4.23
To the best of our knowledge, the number of studies dealing with the direct production of xylitol from xylan by heterogeneous catalysis is very limited, and the effect of many variables on the catalytic activity of the catalyst still remains an open question. For these reasons, this paper reports a new approach for xylitol production from hemicellulose (corncob xylan), evaluating the role of the catalyst in terms of xylitol yield from xylose and xylan, and avoiding the use of acids (and the problems associated with their use, such as disposal), with the aim of designing a green catalytic pathway for the development of a sustainable xylitol production.
The carbon nanotubes (CNT) sample shows a N2 adsorption isotherm of type II (Fig. 2), which is typical of non-microporous materials. The BET surface area of the Ru/CNT catalyst decreased slightly compared to the support (Table 1). Therefore, it was assumed that the textural properties of the catalyst are not significantly different from those of the support.
Sample | SBET (m2 g−1) | Smeso (m2 g−1) | Vmicro (cm3 g−1) | dM (nm) [TEM] | D (%) [TEM] |
---|---|---|---|---|---|
CNT | 267 | 267 | 0 | — | — |
Ru/CNT | 245 | 245 | 0 | 1.0 ± 0.1 | 74 |
The average crystallite size (dM) and metal dispersion (D) of Ru/CNT are included in Table 1 (see Fig. S1†). The impregnation method used allows obtaining relatively small metal crystallite sizes with a good metal dispersion throughout the carbon nanotubes.25
Thermogravimetric analysis allowed tracking the mass loss histories of cellulose and hemicellulose (xylan) under different atmospheres as shown in Fig. 3. Cellulose presented a great mass loss between 290 and 360 °C under inert atmosphere (N2), corresponding to a loss around 90% of the total mass (Fig. 3a).26 Under oxidative atmosphere (air), another mass loss was detected for higher temperatures (450–550 °C), due to the oxidation of the char. Thermal analysis of xylan demonstrated a first event in the range of 70–100 °C due to dehydration. The second and most relevant loss of 70% appeared in the range of 220 and 360 °C due to polymer decomposition. Total mass losses close to 100% above 600 °C were achieved under oxidative atmosphere for both materials, so there was almost no residue left at the end. The temperature range that corresponds to the first mass loss did not vary with the surrounding environment. The results obtained are consistent with those reported in literature.27,28
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Fig. 3 Weight loss (in black) and DTG (in blue) curves of cellulose (a) and xylan (b) decomposition under nitrogen or air. |
First derivatives of the thermograms were calculated and are also represented in Fig. 3 to highlight the inflexion points that indicate thermal transitions under the different atmospheres. Xylan starts decomposing at slightly lower temperatures than cellulose, but the corresponding differential thermogravimetry (DTG) curves are similar in both inert and oxidative atmospheres. Cellulose presents a transition peak around 330 °C in both atmospheres. For xylan, the transition peak is around 300 °C and there can also be observed an extra peak which can be attributed to the presence of non-cellulosic impurities.
The elemental analysis of cellulose and hemicellulose revealed carbon, hydrogen and oxygen contents of about 42–44%, 6–7% and 49–52%, respectively (Table 2). The results obtained are compatible with cellulose and hemicellulose's molecular formulas, (C6H10O5)n and (C5H8O4)m, respectively.
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Fig. 4 Conversion of xylan and yields of xylose and xylitol with the reaction time. Reaction conditions: xylan (0.75 g), water (300 mL), 0.4% Ru/CNT (0.3 g), 205 °C, 50 bar H2, 150 rpm. |
The reuse of the catalyst was performed for the one-pot conversion of xylan. After reaction, the catalyst was recovered by filtration of the final reaction mixture, washed with deionized water and dried overnight in an oven. Due to some losses during the filtration, a small amount of fresh catalyst (<5 wt%) was added to the reaction mixture before each run. Four successive tests were carried out under the same conditions and the results are shown in Fig. 5. A conversion of xylan of 100% was achieved after 5 min of reaction in each run and the yield of xylitol after 30 min of reaction was kept almost constant around 43–46%. These results indicate that the Ru/CNT catalyst exhibits almost no deactivation and presents excellent stability.
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Fig. 5 Successive tests of Ru/CNT on the conversion of xylan to xylitol after 30 min of reaction. Reaction conditions: xylan (0.75 g), water (300 mL), 0.4% Ru/CNT (0.3 g), 205 °C, 50 bar H2, 150 rpm. |
Following the same procedure used for the one-pot conversion of cellulose to sugar alcohols,26 the direct conversion of hemicellulose comprises two steps: the hydrolysis of hemicellulose to sugars followed by their hydrogenation to sugar alcohols. Table 3 shows the conversion of xylan and distribution of products with CNT and Ru/CNT as catalysts and also without any catalyst (blank experiment). It can be seen that after 30 min of reaction the conversion of xylan is close to 100% even without any catalyst. However, no sugar alcohols (<10%) could be detected. In this conditions, some reducing sugar (22.5%) could be detected. Using CNT as catalyst, the amount of sugar alcohols detected was still very little but the amount of xylose slightly increased. When Ru/CNT was used as catalyst, the yield of xylitol increased greatly (to 45.4%) and almost no xylose could be detected (2.5%). Also, there was a slight increase in the amount of sorbitol, EG and PG. When the reaction was performed without catalyst or in the presence of CNT, the reaction mixture started to become yellowish and then turned dark brown with the increase of the reaction time (Fig. 6), indicating further degradation of products and formation of humins.2 It has also been reported by Käldström et al. that the unstable sugars degrade (caramelize) under high temperatures.29 At the end of the reaction, part of the humins formed was deposited on the stirrer and the wall of the reactor in both cases. Also, the 3–4% difference from 100% in the conversion of xylan (see Table 3) could probably be attributed to the formation of the solid product. On the opposite, in the presence of the metallic phase (Ru), the resulting mixture was transparent (Fig. 6). As reported in the literature, xylose can be converted to furfural, which gives humins by polymerization.1,2,23 Therefore, the presence of the metallic phase is very important to assure the rapid hydrogenation of xylose to xylitol, suppressing the formation of furfural and subsequent polymerization to humins. These results indicate that the first step of conversion of xylan, which is the hydrolysis to xylose, can be carried out even under the conditions of the blank test and can be slightly enhanced by the presence of CNT. On the other hand, the second step – hydrogenation of xylose to xylitol, can only occur in the presence of the metallic phase (Ru).
Catalyst | Xxylan ± 3 (%) | Product yield ± 0.8 (%) | |||||
---|---|---|---|---|---|---|---|
Xylose | Xylitol | Sorbitol | Formic acid | Ethylene glycol (EG) | Propylene glycol (PG) | ||
a Reaction conditions: xylan (0.75 g), water (300 mL), CNT or 0.4% Ru/CNT (0.3 g), 205 °C, 50 bar H2, 150 rpm. | |||||||
Blank (no catalyst) | 96 | 22.5 | 9.2 | 2.9 | 4.0 | 6.2 | 3.1 |
CNT | 97 | 25.5 | 11.8 | 2.6 | 3.0 | 5.1 | 3.3 |
Ru/CNT | 100 | 2.5 | 45.4 | 5.9 | 3.9 | 6.4 | 4.7 |
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Fig. 6 Evolution of the reaction mixture colour with the reaction time with (CNT or Ru/CNT) or without (blank) catalyst. |
In order to confirm this two-step process, the individual hydrolysis of xylan and hydrogenation of xylose were also tested. First, the hydrolysis of xylan was carried out with CNT as catalyst and the main product observed was xylose with a yield of 42.3% (Fig. 7). Also, a low amount of sugar alcohols was detected (xylitol). When the reaction was started using xylose as substrate (hydrogenation step) and Ru/CNT as catalyst, xylitol was produced with about 33.7% yield, which is close to that of the one-pot process with Ru/CNT as catalyst and xylan as starting material (45.4%). Moreover, the hydrogenation step was also performed using CNT as catalyst and both conversion of xylose (77.8%) and yield of xylitol (10.5%) were much lower indicating that the presence of the metallic phase is essential for the selective conversion of xylose to xylitol. So, these results confirm that the direct conversion of xylan to xylitol is in fact a two-step reaction where CNT promotes the hydrolysis of xylan to xylose and Ru catalyses its consecutive hydrogenation to xylitol. These results are in agreement with those obtained by Kusema et al. for the hydrolytic hydrogenation of arabinogalactan, where Ru demonstrated a high catalytic activity in the hydrogenation of arabinose and galactose, manifested by the high selectivity towards the corresponding polyols (arabitol and galactitol).11 Accordingly, the reductive atmosphere (H2) combined with the metallic phase of the catalyst, seems to be a good strategy to increase the selectivity of the reaction towards the more stable sugar alcohols.
As aforementioned, xylitol and sorbitol are some of the most interesting products to be directly obtained from xylan and cellulose. Therefore, concerning this set of experiments, the formation of these two main products (sorbitol and xylitol) was evaluated. Besides sorbitol and xylitol, glucose, xylose, formic acid, EG and PG were also detected in very small amounts.
Performing the simultaneous hydrolytic hydrogenation of xylan and BM-cellulose, there was an enhancement in the production of the main products. In this case, yields of 48.3% of xylitol and 76.6% of sorbitol were reached in just 3 h of reaction (Fig. 8), corresponding to xylan and BM-cellulose conversions of 100 and 96.1%, respectively. So, an enhancement around 15 and 25% was observed in the yields of xylitol and sorbitol, respectively, in comparison to those observed when separately performing the conversion of xylan (34.2% yield of xylitol after 3 h) and BM-cellulose (50.7% yield of sorbitol after 3 h). To check if the reason was the decrease of the catalyst/substrate ratio, another test was performed using half of the amount of substrate (375 mg of xylan and 375 mg of BM-cellulose) for the same amount of catalyst (300 mg) (reaction denoted as 1/2xylan + 1/2BM-cellulose + Ru/CNT in Fig. 8), in order to maintain the initial substrate/catalyst ratio. Xylitol and sorbitol yields of 46.1 and 71.9% were achieved after 3 h, respectively, indicating that the enhanced yield is not due to the substrate/catalyst ratio, but results from a synergetic effect between the two substrates. Furthermore, this result shows that the amount of substrate can even be reduced, since the yields of xylitol and sorbitol obtained using 375 mg of each substrate are only slightly lower than those reached when using 750 mg of each substrate for the same amount of catalyst. Also, if MC-cellulose was converted simultaneously with xylan (instead of BM-cellulose), an enhancement in the yield of sorbitol from 8.7 to 26.7% after 3 h of reaction could be observed. On the other hand, the yield of xylitol was practically the same as that reached when simultaneously converting BM-cellulose and xylan, indicating that there is no effect of ball-milling of cellulose on the yield of xylitol, but its presence is sufficient. Note that practically no xylitol was observed for the conversion of cellulose (microcrystalline or ball-milled) (Fig. 8a), as well as practically no sorbitol was observed during the conversion of xylan (Fig. 8b).
In a previous work of our group,26 an enhanced yield of sorbitol (close to 70%) was attained by ball-milling cellulose together with the catalyst (Ru/AC) before performing the reaction at the same conditions as those used in the present work. That result was one of the best ever achieved for the one-pot conversion of cellulose by an environmentally friendly approach. Accordingly, in order to try to outperform that result, a new experiment was carried out, where the simultaneous conversion of xylan and cellulose ball-milled together with the catalyst (sample (cellulose + Ru/CNT)_mix) was performed. As depicted in Fig. 9, besides the enhanced yield of sorbitol (81.1% after 2 h), yields of xylitol around 60% were achieved in less than 1 h of reaction, with 100% conversion of both xylan and cellulose, making these results even more outstanding in both fields of direct and environmentally friendly conversion of hemicellulose (xylan) and cellulose.
Furthermore, recycling tests of the xylan and cellulose mixture were performed in order to evaluate the catalyst stability. For these tests, the catalyst was separated from the reaction mixture by filtration, washed with water and dried in an oven overnight. The recovered catalyst was then used in successive tests and the results are presented in Fig. 10. No significant changes in yields of products were observed, indicating that the catalyst can be reused at least up to three cycles without metal leaching into solution and without loss in activity and selectivity.
Although the comparison between the results obtained and those reported in literature can be particularly difficult due to the different conditions used from author to author, to the best of our knowledge, this is one of the best sorbitol and xylitol yields ever reported for the direct catalytic conversion of cellulose and hemicellulose by using an environmentally friendly approach.
Hemicellulose (corncob xylan) was used as received, without any pre-treatment.
Microcrystalline cellulose (MC-cellulose) was ball-milled in a ceramic pot using a Retsch Mixer Mill MM200 apparatus operating at a frequency of 20 Hz for 4 h (see details in Ribeiro et al.26), and denoted as BM-cellulose. MC-cellulose was also ball-milled together with the catalyst under the same conditions (denoted as mix-milling), and the sample was denoted as (cellulose + Ru/CNT)_mix.
Microcrystalline cellulose and hemicellulose were characterized by elemental analysis and thermogravimetry (TG). Elemental analysis was performed on an EA1108 CHNS-O elemental analyser from Carlo Erba Instruments. TG analysis was carried out on a STA 409 PC/4/H Luxx Netzsch thermal analyser, where the samples were heated from 50 to 800 °C (heating rate of 10 °C min−1) under nitrogen or air.
In a typical hydrogenation experiment, xylose was used as substrate instead of xylan and the reaction was stopped after 1 h.
In a typical hydrolysis experiment, the system was not pressurized with hydrogen but kept under nitrogen atmosphere. The reaction was initiated when the desired temperature was achieved (205 °C) and stopped after 5 h.
The conversions of corncob xylan, xylose and glucose were calculated using eqn (1)–(3), respectively:
![]() | (1) |
![]() | (2) |
![]() | (3) |
The conversion of cellulose was determined based on the total organic carbon (TOC) data obtained with a Shimadzu TOC 5000-A according to eqn (4):
![]() | (4) |
Xylan was also converted simultaneously with cellulose, which allowed increasing the yields of xylitol and sorbitol in comparison to those achieved during the separate conversion of xylan and cellulose. As a result, yields of 48% of xylitol and 77% of sorbitol were reached in just 3 h of reaction, corresponding to an increase of the yields around 15 and 25%, respectively. If the cellulose was previously ball-milled together with the catalyst and then simultaneously converted with xylan, the yields of xylitol and sorbitol could be even further increased to 57 and 81%, respectively.
To the best of our knowledge, this article presents one of the best results ever attained for the direct catalytic conversion of xylan and cellulose into xylitol and sorbitol using an environmentally friendly process.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19666g |
This journal is © The Royal Society of Chemistry 2016 |