Hexitols including mannitol and sorbitol are important chemicals derived from biomass and are regarded as the optimal platform chemicals for biorefinery applications [1]. Hexitols are extensively used as sweeteners for the food industry. Additionally, they are also widely used as chemicals for the production of fuels and chemicals [2, 3, 4, 5] . Generally, mannitol and sorbitol are produced by the hydrogenation of fructose and glucose using Raney Ni catalysts. Approximately 7 × 105 tons of hexitols are produced annually [6, 7] . Nevertheless, the feedstock of this process is monosugars, which is competing with the food industry. Therefore, it is highly desirable to search for non-food routes for production of hexitols.
In the past decade, catalytic conversion of cellulose into hexitols has attracted significant attention [8, 9, 10] . In 2006, Fukuoka et al. [11] first reported Pt/Al2O3-catalyzed conversion of cellulose into hexitols with yields of 31% at 463 K. Further research by Liu et al. [12] employed hot water to produce protons in situ to obtain increased hexitol yields over a Ru/C catalyst at 518 K. The catalytic conversion of cellulose into hexitols is a cascade reaction, involving the hydrolysis of cellulose and the resultant sugars (glucose and fructose) hydrogenation, which necessitates the employment of bifunctional catalysts. Following the pioneering work of Fukuoka et al. [13, 14, 15, 16, 17, 18, 19, 20, 21] , various bifunctional catalysts have been developed towards the improvement of hexitol yields. Although cellulose has advantages of being a non-edible and abundant resource, there still remains a great challenge for the large-scale production of hexitols from cellulose. First, the task to separate cellulose from raw lignocellulose materials is arduous and the lignin component often significantly hinders the conversion of cellulose [22]. Second, the abundance of hydrogen bonds together with the crystalline nature of cellulose makes it difficult to degrade under milder conditions [23]. Therefore, searching for an alternative to cellulose as a sustainable feedstock for the production of hexitols is highly desired.
Jerusalem artichoke is an important economic energy crop, which is fast-growing and requires relatively low inputs in terms of pesticides, fertilizer and water [24]. Widely cultivated in the north of China, the Jerusalem artichoke is strongly adaptable and yields high productivity. Inulin is the major component of the Jerusalem artichoke tuber (JAT), which is a mixture of linear β-(2-1)-linked fructose chains possessing a terminal glycopyranose unit at the reducing end. In difference to the rigid structure of cellulose, JAT is composed of low degree polymerized amorphous fructose and glucose, which can be converted easily at relatively mild conditions with high efficiency. Such features renders JAT as an ideal feedstock candidate for biorefinery applications in China [25, 26, 27] .
In this work, we developed a bifunctional catalyst, Ru nanoparticles supported on sulfonated carbon, for the efficient conversion of JAT to hexitols. By characterizing the catalyst surface acidity and Ru dispersion, correlation of reactivity to the properties of the catalysts was observed. The catalysts were highly active for the one pot conversion of JAT to hexitols under mild reaction conditions and the stability is dependent on the feedstock purity.
The JAT was received from Wafangdian in Dalian, China. Prior to use, the artichoke was chipped and dried at 60 °C for 12 h. Thereafter the artichoke was milled into powders targeting a mesh size of < 60 mesh.
The reducing sugars in the JAT were analyzed via a 3,5-dinitrosalicylic acid method as described elsewhere [32, 28].
AC-SO3H was prepared according to our previous report [29]. In detail, 1 g activated carbon (AC; Norit) was added to 15 mL concentrated H2SO4 (98%) in an in-house fabricated glass tube prior to the carbon being sulfonated at 523 K for 24 h under N2 (10 mL/min). After cooling, dilution and filtering, the as-prepared AC-SO3H was washed with copious amounts of hot water until no detection of SO42− anions in the Ba(NO3)2 filtrate. Finally, the AC-SO3H was heated at 473 K under N2 for 2 h and then washed thoroughly with water until no SO42− was detected in the filtrate.
The Ru/(AC-SO3H) catalyst was prepared via an impregnation method. Taking 1%Ru/(AC-SO3H) as an example, 1 g AC-SO3H support was impregnated with 3 mL RuCl3 solution containing 0.01 g Ru, followed by drying at 393 K for 12 h. The sample was then reduced under H2 at 473 K for 1 h.
The surface area and pore size distribution of the supports and catalysts were determined by N2 adsorption-desorption at 77 K using a Micromeritics ASAP 2010 apparatus. Prior to measurements, the samples were degassed at 523 K for 3 h.
The contents of Ru and -SO3H in the catalysts were measured with Inductive Couple Plasma Atomic Emission Spectrometer (ICP-AES, Thermal Scientific-IRIS Intrepid ΙΙ XSP). Prior to the measurements, the samples were dissolved with aqua regia in a microwave digestion system at 483 K for 10 min.
The acid density of the catalysts were measured following a titration method [30]. Typically, 0.05 g catalyst was added to 20 mL of NaOH aqueous solution (0.01 mol/L). After stirring the mixture at 298 K for 60 min, the solution was separated from the solid by a 0.45-μm membrane syringe filter. The supernatant solution was titrated with HCl aqueous solution (0.01 mol/L) using phenolphthalein as an indicator.
Transmission electron microscopy (TEM) analysis was performed on a JEM-2000EX (JEOL) microscope. Before analysis, the catalysts were dispersed in an ethanol solution and ultrasonicated before depositing the suspension drop wise onto a copper grid.
The reaction of the catalytic conversion of JAT into hexitols was conducted in a stainless steel autoclave (Parr Instrument Company, 100 mL) at a designated temperature with 6 MPa initial H2 pressure. Before reaction, the polysugar in JAT was extracted with hot water at 363 K for 10 min. Thereafter, the JAT saccharides solution (25 mL), catalyst (0.3 g) and water (25 mL) were charged into the autoclave and stirred (960 r/min). For comparison, fructose and inulin (provided by J&K) were also employed as feedstock for catalytic conversion with similar conditions. After reaction, the liquid-phase products were filtered and analyzed by HPLC (Agilent 1200, Shodex Sugar SC1011 column, differential refractive index detector).
Control experiments were conducted in a 300-mL Parr autoclave. Typically, 100 mL JAT saccharides solution and 1.2 g catalyst were loaded in the autoclave with 100 mL water before charging the autoclave with 6 MPa N2 or H2. Finally, the autoclave was heated to 373 K and maintained for a designed time.
The yields of polyols were calculated as: yield (%) = (mole of carbon in the products) / (mole of carbon in the feedstock) × 100%.
The composition of JAT is highly dependent on numerous factors such as: geographical location where it is farmed, fertilization and the harvest season [31]. As shown in Fig. 1, the major component of JAT is inulin, which accounts for 57.0%. Meanwhile, the existence of monosugars JAT is evident. The content of fructose and glucose are 12.0% and 4.5% respectively. Total sugars in JAT reach 73.5%, including inulin, fructose and glucose. In addition to sugar content, the rest of the make-up consists of 1.7% ashes and 24.8% other components such as cellulose, lignin, protein and amino acid [32, 33] . To avoid the negative effect of lignin [33, 34] , JAT was first extracted with hot water with the extracted liquid—containing inulin, fructose, glucose and some soluble impurities—used as the feedstock for the reaction.
The properties of the bare supports and the final catalysts are summarized in Table 1. The AC has a surface area of 768 m2/g and an acid density of 0.21 mmol/g. The acidity of the AC can be attributed to the surface -OH and -COOH groups. After sulfonation at 523 K, the surface area of the resulting AC-SO3H increased to 1020 m2/g with the microporous surface area being reduced from 477 to 376 m2/g indicating that the sulfonation process not only removed the impurities that block the pores in the AC but further enhanced the accessibility of the pores by oxidizing the surface carbon of AC. Additionally, the acid density of AC-SO3H dramatically increased to 0.68 mmol/g containing 0.10 mmol/g of -SO3H groups. This result suggested that the high temperature sulfonation process anchored -SO3H groups onto the surface of AC and introduced more surface acidic groups such as -COOH and -OH by oxidation reaction.
With the creation of abundant acidic groups by sulfonation, Ru was further loaded on the AC-SO3H support at varying concentrations. The AC support was also loaded with Ru for comparison purposes. As shown in Table 1, the loading of Ru led to a slight decrease in the surface area, both for AC and the AC-SO3H supports. The actual loadings of Ru, determined by ICP-AES, were ~1.5 times higher than the nominated values because of the methanation of carbon supports during the reduction process [36] . Quite different from 1%Ru/AC where the loading of Ru caused the decrease of acid density from 0.21 to 0.10 mmol/g, the loading of Ru on the AC-SO3H support brought about a significant increase in the surface acid density, suggesting there is a synergistic effect between -SO3H groups and Ru. Previously, Fukuoka et al. [37, 38] reported RuO2·2H2O as being the state of Ru on the carbon material after reducing with H2 and subsequent passivation at room temperature and the formed RuO2·2H2O could promote cellulose hydrolysis to generate acidic properties. However, in situ Raman spectra of the 1%Ru/(AC-SO3H) catalyst showed that Ru existed as the metallic state, either for the freshly reduced sample or for passivated samples, thus the contribution from RuO2·2H2O can be safely excluded.
The 1%Ru/AC and 1%Ru/(AC-SO3H) catalysts were further characterized by TEM. As shown in Fig. 2, there are no discernable Ru particles visible under the given magnification of the TEM micrographs, indicating Ru particles were highly dispersed on the two supports. The dispersion of Ru was further measured by CO chemisorption with CO uptake values of 0.05 and 0.18 mmol/gcat, which corresponded to a Ru dispersion of 30.9% and 74.2% for 1%Ru/AC and 1%Ru/(AC-SO3H) catalysts respectively. Evidently, the sulfonation process of the carbon support greatly improved the dispersion of Ru because of the creation of new additional anchoring sites.
The catalytic conversion of JAT into hexitols is a cascade reaction. JAT is firstly hydrolyzed into sugars and then hydrogenated into hexitols. Bifunctional catalysts are therefore required for the high production of hexitols from JAT. The reaction was performed at mild conditions (373 K, 6 MPa H2, 5 h). Table 2 lists the yields of sugars and hexitols from JAT catalyzed with different catalysts. When 1%Ru/AC was employed as the catalyst, the yield of hexitols was 52.7% including 19.2% mannitol and 33.5% sorbitol. In addition, sugars including fructose (14.8%), glucose (6.5%) and sucrose (16.1%) were also produced, indicating that the hydrogenation capability of the 1%Ru/AC catalyst was not sufficiently adequate. In contrast, the 1%Ru/(AC-SO3H) catalyst yielded dramatically reduced sugar production while the hexitol yield significantly increased to 84.1%. This improvement can be attributed to the enhanced acid density and Ru dispersion on the surface of the 1%Ru/(AC-SO3H) catalyst, as shown in Table 1. The high density of acid groups of 1%Ru/(AC-SO3H) catalysts, including -OH, -COOH and -SO3H, promoted JAT hydrolysis while the highly dispersed Ru centers catalyzed the hydrogenation of sugars instantly upon their production, as a result, the hexitol yield was enhanced greatly. Moreover, the hydrogenation capability of Ru/(AC-SO3H) catalysts was further improved as a function of increasing Ru loading. For instance, when the nominal Ru loading was raised from 1% to 2% (corresponding to actual Ru loadings of 1.6% and 3.4% respectively), the hexitol yield increased from 84.1% to 91.3% whilst the total sugar yield decreased to less than 5%. Further increasing the Ru loading to the nominal value of 3% led to a slight increase of the hexitol yield (92.6%). To the best of our knowledge, this hexitol yield is the highest reported hitherto for one pot conversion of JAT to hexitols and comparable to that obtained from inulin conversion previously reported by Peters et al. [39] .
Differences from JAT, when inulin and fructose were used as the feedstock, are observed as higher hexitol yields are obtainable over both Ru/AC and Ru/(AC-SO3H) catalysts. Furthermore, when fructose was employed as the feedstock the two catalysts performed similarly, while 1%Ru/AC was vastly inferior to the 1%Ru/(AC-SO3H) counterpart when inulin was the feedstock. This result suggests that the hydrolysis of inulin and JAT may be the rate-determining step in the whole conversion process, while the acid sites created during the sulfonation process could promote this rate-determining step.
To elucidate the reaction pathway from JAT to hexitols, we monitored the product distribution as a function of reaction time under N2 and H2 atmospheres respectively. As shown in Fig. 3, when the reaction proceeded under N2, fructose and glucose were the major products during the whole reaction process with only a minor sucrose yield. Evidently, under a N2 atmosphere the hydrolysis of JAT was the main reaction pathway. Additionally, fructose achieved its plateau at shorter time-on-stream over the 1%Ru/(AC-SO3H) catalyst than that over 1%Ru/AC, which is consistent with the much higher acid density in the former catalyst. In contrast, when the reaction was conducted under a H2 atmosphere (Fig. 4), sugars (fructose > glucose > sucrose) underwent maxima within 1 h before starting to decrease with time, meanwhile the hydrogenation products (mannitol and sorbitol) increased steadily with time, indicating that sugars are merely intermediates under the H2 atmosphere. Again, the higher acid density and hydrogenation activity of the 1%Ru/(AC-SO3H) catalyst is attributable to the sugars reaching their maxima in a shorter duration time and their rapid consumption. Based on these results, the reaction pathway from JAT to hexitols can be expressed as follows:
To obtain a high hexitol yield, a suitable balance between the acid sites and the hydrogenation sites is required and sulfonation of the carbon support before loading Ru is a crucial prerequisite step to achieve this requirement.
Because Ru/(AC-SO3H) catalysts exhibited superior activity for the one pot production of hexitols form JAT, the 1%Ru/(AC-SO3H) catalyst was further investigated for its reaction stability and recycling performance. As shown in Fig. 5, when JAT was used as feedstock, the hexitol yield gradually decreased from 87% to 55% during the successive four runs, concurrently the yield of sugars increased. ICP analysis of the liquid after the reaction indicated that no leaching of Ru occurred during the reaction. This result suggests that the hydrogenation metal sites are poisoned gradually by the impurities (e.g., protein, inorganic salts, see Fig. 1) in the JAT feedstock [40] . CO chemisorption of the used catalyst after three runs indicated the dispersion of Ru decreased significantly from 74.2% to 17.8%, confirming covering of the Ru active sites by impurities of the JAT feedstock. In contrast, when inulin was used as feedstock, the stability of the catalyst improved greatly; the hexitol yield only marginally decreased after four runs (Fig. 5(b)). The difference between inulin and JAT was the impurity content; the former is the main component of the latter and without any impurities. Therefore, for the 1%Ru/(AC-SO3H) catalyst, the most suitable feedstock is inulin, which is easily obtained from JAT through extraction by hot water or dilute acid.
Bifunctional catalysts Ru/(AC-SO3H) have been developed for the one-pot conversion of JAT into hexitols. In comparison with the Ru/AC catalyst, the sulfonation process introduced an abundance of -OH, -COOH and -SO3H groups on the support surface, which not only provided acidity but also functioned as anchoring sites for Ru nanoparticles. Thus, the dispersion of Ru was significantly enhanced. As a consequence, the hexitol yields dramatically improved. After 5 h of reaction at 373 K, the hexitol yields reached 92.6% over the 3%Ru/(AC-SO3H) catalyst. The stability of the Ru/(AC-SO3H) catalyst was critically dependent on the feedstock purity. The level of impurities in JAT resulted in catalytic poisoning, while using inulin as the feedstock allowed recycling of the catalyst during four successive runs with only a slight decrease in the hexitol yield.
2015, Vol. 36 Issue (10): 1694-1700
PDF (759 KB)
2015, Vol. 36 Issue (10): 1694-1700
PDF (759 KB)