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Hydrogenolysis of sorbitol to glycols over carbon nanofibers-supported ruthenium catalyst: The role of base promoter
Chinese Journal of Catalysis 35 (2014) 692–702
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Article (Special Issue on the 2nd International Congress on Catalysis for Biorefineries (CatBior 2013))
Hydrogenolysis of sorbitol to glycols over carbon nanofibers‐ supported ruthenium catalyst: The role of base promoter Jinghong Zhou *, Guocai Liu, Zhijun Sui, Xinggui Zhou, Weikang Yuan State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 March 2014 Accepted 19 March 2014 Published 20 May 2014
Keywords: Sorbitol Hydrogenolysis Base promoter Glycol Ruthenium catalyst
Sorbitol hydrogenolysis over carbon nanofibers‐supported Ru (Ru/CNFs) was carried out with different bases (NaOH, KOH, Mg(OH)2, Ba(OH)2, and CaO) to investigate the role of base promoter. The results indicated that all the bases used significantly enhanced the sorbitol conversion while the glycol selectivities varied with the base type and amount. CaO was the best base in terms of glycol selectivity for two reasons. CaO provided OH− for the base‐promoted cleavage of C–C bonds, while it also supplied Ca2+ for complexation with the intermediate aldehydes, thus affecting the reaction pathways. We identified an optimum ratio among sorbitol concentration, Ru/CNFs catalyst, and CaO to achieve favorable glycol selectivities in sorbitol hydrogenolysis. Reaction pathways for sorbitol hydrogenolysis into glycols in aqueous solution in the presence of CaO have been proposed based on the mechanistic study. © 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction Conversion of renewable biomass to commodity chemicals has received extensive attention as the depletion of fossil fuel reservoirs has continued over the past two decades. Sugars and sugar alcohols, which can be derived from renewable lignocellulosic biomass instead of fossil resources, are considered as potential feedstocks for the future biorefinery to produce useful chemical products through hydrogenolysis [1,2]. Along this line, sorbitol hydrogenolysis is a promising “green process” that can produce important chemicals such as ethylene glycol (EG), propylene glycol (PG), and glycerol (GL), which are widely used in the manufacture of polyesters, sur‐ factants, pharmaceuticals, and functional fluids [3]. Sorbitol hydrogenolysis was first conducted in a study where sugars were submitted to the action of hydrogen [4].
Clark [5] then reported this process for the purpose of biomass conversion to lower carbon glycols. Since then, sorbitol hydro‐ genolysis to glycols has received much attention in industry circles and a raft of patents on the topic have been issued [6–11]. Most of these patents describe sorbitol hydrogenolysis occurring in the presence of a metal catalyst and base, at a temperature between 180 and 275 °C, and with an elevated H2 partial pressure of 3.4–48.3 MPa. Some academic studies on this process have been reported, mainly focusing on the mech‐ anism of bond cleavage in polyol hydrogenolysis and the de‐ velopment of new catalyst systems. For example, Chen et al. [3] studied sorbitol hydrogenolysis over Ni–MgO catalyst and found that the activity depended strongly on the basicity of catalyst. Banu et al. [12,13] conducted the sorbitol hydrogenol‐ ysis over NaY zeolite‐based catalysts and discussed the effect of Ca(OH)2 on the conversion and selectivity. Sohounloue et al.
* Corresponding author. Tel: +86‐21‐64252169; Fax: +86‐21‐64253528; E‐mail: [email protected] This work was supported by the National Basic Research Program of China (973 Program, 2014CB239702) and the National Natural Science Founda‐ tion of China (21106047). DOI: 10.1016/S1872‐2067(14)60083‐8 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 35, No. 5, May 2014
Jinghong Zhou et al. / Chinese Journal of Catalysis 35 (2014) 692–702
[14] investigated the hydrogenolysis of sorbitol over Ru/SiO2 in a basic medium and discussed the dependence of bond cleav‐ age on temperature with respect to the retro‐aldol condensa‐ tion mechanism. Montassier et al. [15] conducted a study on the modification of Ru catalyst with sulfur and proposed that the bulk reaction causing C–C bond cleavage in sorbitol hydro‐ genolysis was a retro‐Michael reaction under the action of ab‐ sorbed nucleophilic species OH−. Using 1,3‐diols as model compounds, Wang et al. [16] proposed a bond cleavage mecha‐ nism (Scheme 1) for polyol hydrogenolysis; retro‐aldol con‐ densation was proposed as the dominant mechanism for C–C bond cleavage by thermodynamic and kinetic considerations. The initial dehydrogenation step is thought to occur on the transition metal catalyst. Previous research in our laboratory [17–20] has shown that Ru catalyst over carbon nanofibers (CNFs) exhibited high catalytic activity in sorbitol hydrogenolysis to EG and PG owing to the mesoporosity and surface chemistry of the CNFs. The Ru catalysts supported on powdered and structured CNFs showed high performance in a batch reactor (slurry) and in a flow‐through continuous reactor, respectively. Moreover, it is generally acknowledged that the use of base can significantly affect the activity and selectivity of this reaction; our prelimi‐ nary results demonstrated that addition of CaO enhanced the conversion of sorbitol [17]. The use of base in sorbitol hydro‐ genolysis was first thought to prevent leaching of the metal from the catalyst [7, 8], but further work has suggested a more significant role. Banu et al. [13] found that addition of Ca(OH)2 as a promoter to both Ni and Pt catalysts increased the sorbitol conversion significantly without any significant effect on selec‐ tivity. However, studies on the effect of base on glycerol hy‐ drogenolysis found different results that suggested the base aided initial dehydrogenation of glycerol to glyceraldehyde and promoted the dehydration of glyceraldehyde to 2‐ hydroxy‐ acrolein; hence, the selectivities to different products varied significantly [21–23]. Sun et al. [24] investigated the selective hydrogenolysis of xylitol to EG and PG over different catalysts in the presence of Ca(OH)2. They revealed that xylitol hydro‐ genolysis involved the dehydrogenation of xylitol to xylose on the metal surface and subsequent base‐catalyzed retro‐aldol condensation of xylose to glycolaldehyde and glyceraldehyde. This was followed by direct glycolaldehyde hydrogenation to EG and sequential glyceraldehyde dehydration and hydrogena‐
O
O
RCCH2OH + HCR'
Hydrogenation
OH RCHCH2OH + HOCH2R'
OH Retro-aldol Aldol reaction condensation condensation OH
OH
RCHCHCHR' OH
Dehydrogenation Hydrogenation
O
OH
RCCHCHR' OH Dehydration O RCC OH
CHR'
Hydrogenation
OH RCHCHCH2R' OH
Scheme 1. Mechanism of polyol hydrogenolysis to lower polyols [16].
tion to PG. Rass et al. [25] studied the influence of base in the selective oxidation of 5‐hydroxymethylfurfural to 2,5‐furandi‐ carboxylic acid over Pd/C catalyst, and demonstrated that the cation of the base had no effect while the carbonate base facili‐ tated hydration of the aldehyde. Little effort has been made to explain the mechnism of this enhancement by base in sorbitol hydrogenolysis, although al‐ most all previous research has used base additives for this re‐ action. Given that few details are known about how these base additives affect the reaction pathways, the objective of this study was to explore the role of base in sorbitol hydrogenolysis and to optimize the reaction conditions for higher glycol selectivities. Based on our previous work, the effects of the base type and amount on sorbitol hydrogenolysis were systemati‐ cally investigated over Ru/CNFs in a batch reactor. The role of base is discussed in combination with the reaction mechanism.
2. Experimental 2.1. Catalyst preparation CNF catalyst support was synthesized and purified as de‐ scribed by Zhao et al. [17]. 3.0 wt% Ru/CNFs catalyst was pre‐ pared by incipient wetness impregnation using RuCl3·3H2O (Heraeus) as precursor [17]. NaOH, KOH, Mg(OH)2, Ba(OH)2, CaO, and sorbitol were purchased from Alfa Aesar China and used as received. 2.2. Hydrogenolysis reaction of sorbitol Sorbitol hydrogenolysis was carried out in a 500‐mL stain‐ less steel autoclave (Parr 4575A, USA) with magnetic stirring. In a typical run, 330 mL (20 wt% unless specified otherwise) sorbitol aqueous solution, a measured amount of base, and activated Ru/CNFs catalyst were added to the autoclave. After sealing, the autoclave was purged with N2 for 30 min and then H2 for 30 min at a stirring rate of 200 r/min at room tempera‐ ture. After purging, the autoclave was filled with H2 to 2.0 MPa and then heated to 220 °C by an electrical heater with stirring at 800 r/min. H2 was continuously fed into the reactor to maintain the pressure at 8 MPa. After reaction for 4 h, the auto‐ clave was cooled with tap water flowing through the cooling coil. The product mixture was then collected, filtered, and ana‐ lyzed. The products in liquid phase were analyzed qualitatively and quantitatively by high‐performance liquid chromatography (HPLC; HP1100, Agilent, USA) equipped with a refractive index detector. A Platisil ODS C18 AQ column was used at 25 °C to separate the five main products of sorbitol, EG, PG, GL, and an unknown product. Redistilled water (0.6 mL/min) was used as mobile phase. Trace products like ethanol, methanol, and me‐ thane were detected in the gas phase in the reactor, but selec‐ tivity to gaseous products was less than 1%. The liquid prod‐ ucts except the unknown substance were quantified using an external standard method. The selectivity to a specific product was expressed as the ratio of sorbitol converted into product to the total sorbitol converted.
Jinghong Zhou et al. / Chinese Journal of Catalysis 35 (2014) 692–702
3. Result and discussion 3.1. The role of base in hydrogenolysis To better understand the role of base in sorbitol hydrogen‐ olysis, control experiments were conducted under the same conditions, but contained only the sorbitol substrate and one of: CNF support, CaO, Ru/CNFs, or Ru/CNFs + CaO. The product distributions of controlled sorbitol hydrogenolysis are listed in Table 1. Both the CNF support and CaO showed no catalytic effect for sorbitol hydrogenolysis when they were used alone in the reaction. Sorbitol conversion of 17.5% was achieved with more than 90% total selectivity to glycols and glycerol when 0.5 g Ru/CNFs was used. The addition of CaO led to a significant increase in sorbitol conversion to 43.4% with a slight increase in the glycol selectivity and a decrease in glycerol selectivity. This result was similar to that observed by Banu et al. [13] where the addition of Ca(OH)2 increased the conversion signif‐ icantly without any effect on selectivity. The difference in the effect of base promoter on the selectivity to glycerol may be caused by reaction conditions and catalyst applied. It is obvious from Table 1 that without Ru/CNFs, the CaO could not catalyze sorbitol hydrogenolysis. Therefore, CaO is more a promoter than a co‐catalyst for sorbitol hydrogenolysis, although it did greatly enhance the sorbitol conversion. Ac‐ cording to the mechanism proposed by Wang et al. [16] in Scheme 1, the promotion effect is caused by OH− (released when CaO reacts with water), which could catalyze the ret‐ ro‐aldol condensation of the intermediate aldehyde to generate glycols. However, retro‐aldol condensation is the second step in sorbitol hydrogenolysis and only occurs after the sorbitol de‐ hydrogenates to some intermediate aldehyde over Ru catalyst. Thus, only by combining with a metal catalyst, such as Ru/CNFs, can CaO promote sorbitol hydrogenolysis. The effect of base type on sorbitol hydrogenolysis was in‐ vestigated by using five bases of equivalent theoretical OH− but with different metal cations. The results (Table 2) showed that all the bases substantially enhanced sorbitol conversion and deteriorated the selectivity to glycols except CaO. CaO was the best base promoter for sorbitol hydrogenolysis by significantly increasing sorbitol conversion with only a slight effect on the glycol selectivity; hence, glycol yields were greatly enhanced. KOH, NaOH, and Ba(OH)2 added into reactions have equiva‐ lent OH− and completely dissolved in the solution under the reaction conditions. As a result, the hydrogenolysis was sup‐ posed to take place at a same pH when these three bases were used as promoters. However, they affected the product distri‐
Table 2 Effect of base type on sorbitol hydrogenolysis. Product selectivity (%) Base amount * Sorbitol (g) conversion (%) EG PG GL NaOH 10.0 70.6 14.6 17.1 6.5 KOH 14.0 59.9 15.2 20.3 9.2 21.4 52.9 20.6 21.7 10.4 Ba(OH)2 Mg(OH)2 7.3 30.4 24.4 30.7 19.5 CaO 7.0 44.3 26.4 33.7 16.2 Reaction conditions: 20 wt% sorbitol aqueous solution, 3.0 wt% Ru/CNFs 0.5 g, 220 °C, 8.0 MPa. * Theoretical OH− amount 0.25 mol. Base
bution in the order of NaOH> KOH > Ba(OH)2, indicating that besides OH− in the base, the metal cations might play a role during hydrogenolysis. Moreover, it seemed that the more the base enhanced sorbitol conversion, the more it lowered the selectivity to glycols. Actually, more side products other than EG, PG, and GL were observed when these three bases were used as promoters. This could be explained according to the mechanism of polyol hydrogenolysis [16]. The cleavage of C–C bonds basically relies on a retro‐aldol reaction, which is pro‐ moted by OH−. However, an overly high OH− concentration also enhances reversible and scrambling aldol reactions, resulting in a decline in selectivities to desired EG and PG and a rise in yield of other undesirable hydrocracked products [26]. Mg(OH)2 and CaO, which have low solubilities in aqueous solution, showed slightly lower enhancement in sorbitol con‐ version, but only slightly affected the selectivity to glycols. The low solubilities of Mg(OH)2 and CaO limited the concentration of OH− in aqueous solution, although it was still sufficient to accelerate the retro‐aldol reaction of intermediates to glycols and glycerol. Excess OH¯ when combined with Ru catalyst would catalyze further degradation of glycol and glycerol and decrease the glycol yields [22]. Therefore, the addition of Mg(OH)2 and CaO not only enhanced sorbitol conversion, but also maintained high selectivity to glycol. The metal cation of the base promoters, particularly in the case of Ca2+, was probably involved in the reaction. Both sorbi‐ tol and the intermediate aldehyde formed after sorbitol dehy‐ drogenation have chelation ability and can form complexes with divalent metal ions such as Ca2+. Kenner et al. [27] studied the degradation of carbohydrates by alkali and proposed that the cation of the alkali played a role in the complexation of al‐ dehydes, leading to the formation of saccharinic acid. This cat‐ ionic effect of the reacting bases has been also verified in the reaction of d‐glycerose‐3‐C14 with alkali by Sowden et al. [28]. In our case for hydrogenolysis of sorbitol, complexation be‐ tween the intermediates and Ca2+ was also observed. Figure 1
Table 1 Sorbitol hydrogenolysis under control conditions. Catalyst amount Base amount Sorbitol conversion (g) (g) (%) Sorbitol + CNFs — — 0 Sorbitol + Ru/CNFs 0.5 — 17.5 Sorbitol + CaO — 5.0 0 Sorbitol + Ru/CNFs + CaO 0.5 5.0 43.4 EG: ethylene glycol; PG: propylene glycol; GL: glycerol; CNFs: carbon nanofibers. Reaction conditions: 20 wt% sorbitol aqueous solution, 3 wt% Ru/CNFs, 220 °C, 8.0 MPa. Substrate
Product selectivity (%) EG PG GL 0 0 0 25.4 33.5 32.4 0 0 0 27.1 35.6 18.5
Jinghong Zhou et al. / Chinese Journal of Catalysis 35 (2014) 692–702
1 2 3 4 1'
Sorbitol GL EG PG Unknow product
Table 3 Effect of CaO amount on sorbitol hydrogenolysis.
1 1 1
Intensity
23
4
3
4
2 0
2
4 6 8 Resident time (min)
Product selectivity (%) CaO amount Sorbitol conversion (g) (%) EG PG GL 0.5 18.5 31.2 41.2 22.6 1.0 19.6 32.8 34.8 21.4 3.0 29.0 27.7 34.4 19.6 5.0 43.4 27.8 35.6 18.5 7.0 44.3 26.4 33.7 16.2 9.0 48.6 20.5 27.8 14.4 15.0 58.7 15.0 22.7 10.6 Reaction conditions: 20 wt% sorbitol aqueous solution, 3.0 wt% Ru/CNFs 0.5 g, 220 °C, 8.0 MPa.
0.50 g
4
23
1'
1.00 g
3.00 g 10
12
Fig. 1. HPLC chromatograms for hydrogenolysis products with different amounts of CaO.
shows a comparison of HPLC chromatograms for the hydro‐ genolysis products when different amounts of CaO were em‐ ployed in the reaction. It is clear that the intensity of peak 1 for the unknown product increased with the increasing CaO amount. When oxalic acid was added into the liquid product mixture, the unknown substance (peak 1) declined while the concentrations of the other four products did not alter. Simul‐ taneously, a white precipitate was formed and was shown to be calcium oxalate by X‐ray diffraction and Fourier transform in‐ frared spectroscopy [29]. Moreover, the unknown substance was the only product that could be detected by ultraviolet de‐ tection, indicating the existence of aldehyde or a carbonyl group. Therefore, peak 1 was assigned as a complex between Ca2+ and intermediates, although the specific structural formula of this complex has not yet been determined after considerable investigation, including analysis by HPLC‐MASS. It is likely that the complexation of Ca2+ with the intermediate would affect the cleavage of C–C bonds and, subsequently, the selectivities to glycols. Table 3 shows the effect of CaO amount in the reaction sys‐ tem on sorbitol hydrogenolysis. With increased CaO, the con‐ version of sorbitol gradually increased, while the selectivities to glycols and glycerol gradually decreased. CaO reacts with H2O to give Ca(OH)2, but the solubility of Ca(OH)2 in aqueous solu‐ tion is very low and decreases with increasing temperature (0.16 g/100 g H2O at 20 °C and 0.07 g/100 g H2O at 100 °C [30]). In the hydrogenolysis reaction, if Ca2+ was present mere‐ ly in the form of the free ion rather than participating in the reaction by complexing with the intermediate, most of the CaO would exist as undissolved solid because of its low solubility. In such a case, increased CaO would have no effect on the reaction. However, the results in Table 3 show that the CaO amount greatly affected the hydrogenolysis product distribution. This is further evidence that Ca2+ participated in the hydrogenolysis reaction and had an important effect on the selectivities to EG and PG.
Our results suggest that CaO served a bifunctional role in sorbitol hydrogenolysis. As a base additive, it provided a mod‐ erate basic medium for accelerating a retro‐aldol reaction for C–C cleavage and so enhanced the sorbitol conversion. It also provided the metal center for complexation with intermediate aldehyde and released more OH−, further enhancing the ret‐ ro‐aldol condensation. Because the retro‐aldol condensation and the complexation are both reversible reactions under the hydrogenolysis conditions, CaO should reach an equilibrium point between its two roles. As shown in Table 3, when the CaO amount was increased to 5.0 g, the highest yields of glycols were achieved within the investigated range. Further increase in CaO led to severe decline in glycol yields. This is because excess OH− caused the reversible aldol scrambling problem and resulted in a decline in selectivity to glycols, just as in the cases when NaOH and KOH were used as promoters. Therefore, there exists a preferred CaO concentration for the system under which the best glycol yield can be achieved. 3.2. Reaction pathways and optimization of reaction conditions Based on the above discussion and previous mechanistic detail reported in the literature, reaction pathways for hydro‐ genolysis of sorbitol to glycols in the presence of CaO are tenta‐ tively proposed in Scheme 2. As shown in Scheme 2, Ru/CNFs catalyst is necessary for the initial dehydrogenation of sorbitol to intermediate with alde‐ hyde or carbonyl groups, which leads to the cleavage of C–H bonds by dehydrogenation and the subsequent C–C bond cleavage by retro‐aldol condensation. The retro‐aldol reaction is enhanced under basic conditions because it is catalyzed by adsorbed OH− [16]. A probable mechanistic pathway for sorbi‐ tol hydrogenolysis is as follows. The first step is dehydrogena‐ tion of sorbitol on the Ru particle surface to the corresponding intermediates with a sugar carbonyl group. Subsequently, the intermediates undergo either the base‐involved retro‐aldol condensation or complex with Ca2+. The retro‐aldol condensa‐ tion of intermediate aldehyde generates glycolaldehyde and the latter is subsequently hydrogenated to EG. Meanwhile, the ret‐ ro‐aldol condensation of intermediates with a carbonyl group generates glyceraldehyde or dihydroxyacetone, which are then subsequently hydrogenated to GL. GL is further dehydrogenat‐ ed, dehydrated, and then hydrogenated over Ru particle surface to give PG. The complexation between Ca2+ and intermediates
Jinghong Zhou et al. / Chinese Journal of Catalysis 35 (2014) 692–702
OH
OH
OH
OH
OH
OH
Dehydrogenation O
OH
OH
OH
OH
OH
O
OH
OH
HO
OH
Complextion with Ca2+
OH + HO
OH + HO
O
OH
OH
OH
O
Hydrogenation
Retro-aldol condensation O
OH
Dehydration
OH
+
Dehydrogenation
O
OH
OH
OH
HO
Retro-aldol condensation
OH
Hydrogenation OH
OH
HO
O
O
OH
-
-
Retro-aldol condensation
OH
Dehydrogenation
HO OH O
HO
OH
OH
Hydrogenation OH OH
Hydrogenation
HO
Scheme 2. Reaction pathways for sorbitol hydrogenolysis to glycols in aqueous solution in the presence of CaO.
helps to release more OH− and accelerate the retro‐aldol con‐ densation to give glyceraldehyde or glycolaldehyde, and these are further hydrogenated on the Ru surface to the desired EG and PG. During this process, CaO acts as base and provides the complexation cation. The amount of added CaO affects the competition between the retro‐aldol condensation and com‐ plexation with the dehydrogenated intermediates, and dictates the reaction pathways as a consequence. As discussed above, both the dehydrogenation over Ru and the base‐involved retro‐aldol condensation dictate the terms of the hydrogenolysis and the eventual generation of the desired products (EG, PG, and GL). Therefore, there should exist a bal‐ ance between the Ru/CNFs and the CaO promoter to achieve the most favored product distribution. A series of experiments were conducted under different reaction conditions to identify the optimum conditions in terms of the most favored glycol yield. The results showing sorbitol conversion and glycol selec‐ tivities are listed in Table 4. It is generally accepted that increasing the amount of cata‐ lyst in batch processing increases the conversion, but its effect on selectivity depends on the nature of the reaction. As shown in Table 4 (entries 2–4), the sorbitol conversion was remarka‐ bly increased when more catalyst was used while the other conditions remained the same. The selectivities to EG and GL
were slightly decreased, but the selectivity to PG was slightly increased. In addition, it is interesting to note that the change in catalyst concentration imposed inverse effects on the selectivi‐ ties to PG and GL and the sum of the selectivities to PG and GL was kept about the same with increased catalyst amount. Simi‐ lar results were also obtained by Casale et al. [6] when Pt/C catalyst was used in sorbitol hydrogenolysis, and suggests that PG is derived from GL as illustrated in the reaction pathways in Scheme 2. The transformation of GL to PG can be rationalized by considering the adsorption ability of polyols on Ru active sites. It has been proposed that PG has a much lower affinity for Ru active sites than GL or EG, while the latter two have rela‐ tively equal chemisorption ability [31]. When the amount of catalyst was increased, the higher conversion led to a relatively lower sorbitol concentration in the reaction solution. Mean‐ while, more Ru active sites were available for GL to be ad‐ sorbed and the adsorbed molecules might be further dehydro‐ genated and subsequently dehydrated to give PG under the hydrogenolysis conditions. The effect of initial sorbitol concentration on the hydrogen‐ olysis was also investigated at a fixed ratio of Ru/sorbitol and a specified CaO amount (7 g). The results shown in Table 4 (en‐ tries 1, 3, 5, and 7) indicate that the sorbitol conversion gradu‐ ally decreased, while the selectivities to EG and PG reached a
Jinghong Zhou et al. / Chinese Journal of Catalysis 35 (2014) 692–702
Table 4 Effect of reaction parameters on sorbitol hydrogenolysis. Entry 1 2 3 4 5 6 7 8 9
Sorbitol concentration (%) 10 20 20 20 30 40 40 40 40
Ru/CNF amount (g) 0.25 0.25 0.5 1.0 0.75 1.0 1.0 1.0 1.0
CaO amount (g) 7.0 7.0 7.0 7.0 7.0 3.0 7.0 10.0 14.0
Sorbitol conversion (%) 47.0 19.6 44.3 74.9 41.5 30.3 33.9 35.6 47.9
Product selectivity (%) EG PG GL 25.5 29.2 20.4 28.8 27.7 21.3 26.4 33.7 16.2 24.6 35.2 14.2 26.8 29.3 12.8 20.3 27.1 9.1 22.9 22.7 8.3 23.1 16.9 7.8 24.8 18.6 6.9
Reaction conditions: 220 °C, 8.0 MPa.
maximum when the initial sorbitol concentration was 20%. In addition, the selectivity to GL monotonically decreased with increasing initial sorbitol concentration. This result can be ex‐ plained from two aspects. First, the reaction was conducted at a fixed Ru/sorbitol ratio, which means that the number of Ru active sites for each sorbitol molecule was the same. The pro‐ moter CaO, however, also played an important role in deter‐ mining the sorbitol conversion and the selectivities to glycols. When the initial sorbitol concentration increased, the amount of CaO was insufficient for complexation with the dehydrogen‐ ated intermediates and resulted in lower concentration of OH−. As a result, enhancement for the retro‐aldol condensation was not as significant as it was at lower initial sorbitol concentra‐ tion. Another possible reason for the decrease in sorbitol con‐ version is that the reaction order of the sorbitol dehydrogena‐ tion is less than 1, which means the higher the initial concen‐ tration, the conversion would be lower after the same reaction time. According to the proposed reaction pathways in Scheme 2, it is likely that Ru/CNFs loading, sorbitol concentration, and the amount of CaO need to be finely balanced to achieve a favorable product distribution. Ru/CNFs catalyzed the dehydrogenation and hydrogenation during hydrogenolysis, while CaO provided OH− for the retro‐aldol condensation and dehydration and Ca2+ for complexation with the dehydrogenated intermediates. The compromise between these two processes ultimately deter‐ mines the product distribution. Concentrated sorbitol solution (40%) was employed to demonstrate the synergy between the actions of Ru/CNFs and CaO. As can be seen in Table 4 (entries 6–9), when 40% sorbitol solution was used as the initial reac‐ tant, although sorbitol conversion increased with increasing CaO amount, the enhancement was not as significant as it was when 20% sorbitol solution was used (Table 3). Notably, the selectivity to EG increased with increasing CaO amount. This is because more sorbitol reactant and more Ru/CNFs catalyst in these reactions led to more complexation, which favors the production of EG. In summary, fine tuning of the reaction parameters allowed sorbitol to be hydrogenolyzed to EG and PG at a total glycol selectivity of 60.1% with a 75% sorbitol conversion. The opti‐ mum conditions were: 220 °C, hydrogen pressure 8 MPa, sor‐ bitol solution concentration 20 wt%, 3 wt% Ru/CNFs catalyst 1.0 g, CaO base 7.0 g.
4. Conclusions The five bases of NaOH, KOH, Mg(OH)2, Ba(OH)2, and CaO were used to investigate the effect of base on sorbitol hydro‐ genolysis. The results indicated that all the base promoters significantly enhanced the sorbitol conversion, while the glycol selectivities varied with the base type. CaO was the best base promoter in terms of glycol yields. It served a bifunctional role in sorbitol hydrogenolysis as a source of OH− and metal ions for complexation, enhancing the retro‐aldol condensation and dic‐ tating the product selectivities. The mechanistic study for sor‐ bitol hydrogenolysis indicated that Ru/CNFs catalyst was nec‐ essary for the initial dehydrogenation of sorbitol to intermedi‐ ates. The intermediates then undergo C–C bond cleavage by retro‐aldol condensation, which is enhanced under basic condi‐ tions. Pathways for sorbitol hydrogenolysis to EG, PG, and GL in the presence of CaO have been tentatively proposed. We found that there was an optimum ratio among sorbitol concentration, Ru/CNFs catalyst, and CaO to achieve favorable glycol selectivi‐ ties in sorbitol hydrogenolysis. These findings provide a fun‐ damental basis for the rational development of nonbiological approaches for biomass conversion to commodity chemicals. References [1] Chheda J N, Huber G W, Dumesic J A. Angew Chem Int Ed, 2007, 46:
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Jinghong Zhou et al. / Chinese Journal of Catalysis 35 (2014) 692–702
Graphical Abstract Chin. J. Catal., 2014, 35: 692–702 doi: 10.1016/S1872‐2067(14)60083‐8 Hydrogenolysis of sorbitol to glycols over carbon nanofibers‐ supported ruthenium catalyst: The role of base promoter
OH
OH
Jinghong Zhou *, Guocai Liu, Zhijun Sui, Xinggui Zhou, Weikang Yuan East China University of Science and Technology
Dehydrogenation O
OH
HO
OH
Retro-aldol condensation
HO
OH
OH HO
OH
Ru
Ru
OH
Dehydrogenation
OH
OH
O
OH
OH
CaO
-
Ru/CNF catalyst and CaO dictated the reaction pathways in sorbitol hydrogenolysis to glycols, where CaO served as both base and metal ion source for complexation.
OH
OH
OH
Complextion with Ca 2+
OH
HO
OH
OH -
OH
Retro-aldol condensation
HO
OH2+
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130: 261 Sun J Y, Liu H C. Green Chem, 2011, 13: 135 Rass H A, Essayem N, Besson M. Green Chem, 2013, 15: 2240 Andrews M A, Klaeren S A. J Am Chem Soc, 1989, 111: 4131 Kenner J, Richards G N. J Chem Soc, 1957: 3019 Sowden J C, Pohlen E K. J Am Chem Soc, 1958, 80: 242 Zhao L. [PhD Dissertation]. Shanghai: East China Univ Sci Technol, 2010 [30] Haslam R T, Calingaert G, Taylor C M. In: Solubilities of Inorganic and Organic Compounds. Volume 1, Part one, Pergamon Press, 1963. 243 [31] Lahr D G, Shanks B H. Ind Eng Chem Res, 2003, 42: 5467 [24] [25] [26] [27] [28] [29]
available at www.sciencedirect.com
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Article (Special Issue on the 2nd International Congress on Catalysis for Biorefineries (CatBior 2013))
Hydrogenolysis of sorbitol to glycols over carbon nanofibers‐ supported ruthenium catalyst: The role of base promoter Jinghong Zhou *, Guocai Liu, Zhijun Sui, Xinggui Zhou, Weikang Yuan State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 March 2014 Accepted 19 March 2014 Published 20 May 2014
Keywords: Sorbitol Hydrogenolysis Base promoter Glycol Ruthenium catalyst
Sorbitol hydrogenolysis over carbon nanofibers‐supported Ru (Ru/CNFs) was carried out with different bases (NaOH, KOH, Mg(OH)2, Ba(OH)2, and CaO) to investigate the role of base promoter. The results indicated that all the bases used significantly enhanced the sorbitol conversion while the glycol selectivities varied with the base type and amount. CaO was the best base in terms of glycol selectivity for two reasons. CaO provided OH− for the base‐promoted cleavage of C–C bonds, while it also supplied Ca2+ for complexation with the intermediate aldehydes, thus affecting the reaction pathways. We identified an optimum ratio among sorbitol concentration, Ru/CNFs catalyst, and CaO to achieve favorable glycol selectivities in sorbitol hydrogenolysis. Reaction pathways for sorbitol hydrogenolysis into glycols in aqueous solution in the presence of CaO have been proposed based on the mechanistic study. © 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction Conversion of renewable biomass to commodity chemicals has received extensive attention as the depletion of fossil fuel reservoirs has continued over the past two decades. Sugars and sugar alcohols, which can be derived from renewable lignocellulosic biomass instead of fossil resources, are considered as potential feedstocks for the future biorefinery to produce useful chemical products through hydrogenolysis [1,2]. Along this line, sorbitol hydrogenolysis is a promising “green process” that can produce important chemicals such as ethylene glycol (EG), propylene glycol (PG), and glycerol (GL), which are widely used in the manufacture of polyesters, sur‐ factants, pharmaceuticals, and functional fluids [3]. Sorbitol hydrogenolysis was first conducted in a study where sugars were submitted to the action of hydrogen [4].
Clark [5] then reported this process for the purpose of biomass conversion to lower carbon glycols. Since then, sorbitol hydro‐ genolysis to glycols has received much attention in industry circles and a raft of patents on the topic have been issued [6–11]. Most of these patents describe sorbitol hydrogenolysis occurring in the presence of a metal catalyst and base, at a temperature between 180 and 275 °C, and with an elevated H2 partial pressure of 3.4–48.3 MPa. Some academic studies on this process have been reported, mainly focusing on the mech‐ anism of bond cleavage in polyol hydrogenolysis and the de‐ velopment of new catalyst systems. For example, Chen et al. [3] studied sorbitol hydrogenolysis over Ni–MgO catalyst and found that the activity depended strongly on the basicity of catalyst. Banu et al. [12,13] conducted the sorbitol hydrogenol‐ ysis over NaY zeolite‐based catalysts and discussed the effect of Ca(OH)2 on the conversion and selectivity. Sohounloue et al.
* Corresponding author. Tel: +86‐21‐64252169; Fax: +86‐21‐64253528; E‐mail: [email protected] This work was supported by the National Basic Research Program of China (973 Program, 2014CB239702) and the National Natural Science Founda‐ tion of China (21106047). DOI: 10.1016/S1872‐2067(14)60083‐8 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 35, No. 5, May 2014
Jinghong Zhou et al. / Chinese Journal of Catalysis 35 (2014) 692–702
[14] investigated the hydrogenolysis of sorbitol over Ru/SiO2 in a basic medium and discussed the dependence of bond cleav‐ age on temperature with respect to the retro‐aldol condensa‐ tion mechanism. Montassier et al. [15] conducted a study on the modification of Ru catalyst with sulfur and proposed that the bulk reaction causing C–C bond cleavage in sorbitol hydro‐ genolysis was a retro‐Michael reaction under the action of ab‐ sorbed nucleophilic species OH−. Using 1,3‐diols as model compounds, Wang et al. [16] proposed a bond cleavage mecha‐ nism (Scheme 1) for polyol hydrogenolysis; retro‐aldol con‐ densation was proposed as the dominant mechanism for C–C bond cleavage by thermodynamic and kinetic considerations. The initial dehydrogenation step is thought to occur on the transition metal catalyst. Previous research in our laboratory [17–20] has shown that Ru catalyst over carbon nanofibers (CNFs) exhibited high catalytic activity in sorbitol hydrogenolysis to EG and PG owing to the mesoporosity and surface chemistry of the CNFs. The Ru catalysts supported on powdered and structured CNFs showed high performance in a batch reactor (slurry) and in a flow‐through continuous reactor, respectively. Moreover, it is generally acknowledged that the use of base can significantly affect the activity and selectivity of this reaction; our prelimi‐ nary results demonstrated that addition of CaO enhanced the conversion of sorbitol [17]. The use of base in sorbitol hydro‐ genolysis was first thought to prevent leaching of the metal from the catalyst [7, 8], but further work has suggested a more significant role. Banu et al. [13] found that addition of Ca(OH)2 as a promoter to both Ni and Pt catalysts increased the sorbitol conversion significantly without any significant effect on selec‐ tivity. However, studies on the effect of base on glycerol hy‐ drogenolysis found different results that suggested the base aided initial dehydrogenation of glycerol to glyceraldehyde and promoted the dehydration of glyceraldehyde to 2‐ hydroxy‐ acrolein; hence, the selectivities to different products varied significantly [21–23]. Sun et al. [24] investigated the selective hydrogenolysis of xylitol to EG and PG over different catalysts in the presence of Ca(OH)2. They revealed that xylitol hydro‐ genolysis involved the dehydrogenation of xylitol to xylose on the metal surface and subsequent base‐catalyzed retro‐aldol condensation of xylose to glycolaldehyde and glyceraldehyde. This was followed by direct glycolaldehyde hydrogenation to EG and sequential glyceraldehyde dehydration and hydrogena‐
O
O
RCCH2OH + HCR'
Hydrogenation
OH RCHCH2OH + HOCH2R'
OH Retro-aldol Aldol reaction condensation condensation OH
OH
RCHCHCHR' OH
Dehydrogenation Hydrogenation
O
OH
RCCHCHR' OH Dehydration O RCC OH
CHR'
Hydrogenation
OH RCHCHCH2R' OH
Scheme 1. Mechanism of polyol hydrogenolysis to lower polyols [16].
tion to PG. Rass et al. [25] studied the influence of base in the selective oxidation of 5‐hydroxymethylfurfural to 2,5‐furandi‐ carboxylic acid over Pd/C catalyst, and demonstrated that the cation of the base had no effect while the carbonate base facili‐ tated hydration of the aldehyde. Little effort has been made to explain the mechnism of this enhancement by base in sorbitol hydrogenolysis, although al‐ most all previous research has used base additives for this re‐ action. Given that few details are known about how these base additives affect the reaction pathways, the objective of this study was to explore the role of base in sorbitol hydrogenolysis and to optimize the reaction conditions for higher glycol selectivities. Based on our previous work, the effects of the base type and amount on sorbitol hydrogenolysis were systemati‐ cally investigated over Ru/CNFs in a batch reactor. The role of base is discussed in combination with the reaction mechanism.
2. Experimental 2.1. Catalyst preparation CNF catalyst support was synthesized and purified as de‐ scribed by Zhao et al. [17]. 3.0 wt% Ru/CNFs catalyst was pre‐ pared by incipient wetness impregnation using RuCl3·3H2O (Heraeus) as precursor [17]. NaOH, KOH, Mg(OH)2, Ba(OH)2, CaO, and sorbitol were purchased from Alfa Aesar China and used as received. 2.2. Hydrogenolysis reaction of sorbitol Sorbitol hydrogenolysis was carried out in a 500‐mL stain‐ less steel autoclave (Parr 4575A, USA) with magnetic stirring. In a typical run, 330 mL (20 wt% unless specified otherwise) sorbitol aqueous solution, a measured amount of base, and activated Ru/CNFs catalyst were added to the autoclave. After sealing, the autoclave was purged with N2 for 30 min and then H2 for 30 min at a stirring rate of 200 r/min at room tempera‐ ture. After purging, the autoclave was filled with H2 to 2.0 MPa and then heated to 220 °C by an electrical heater with stirring at 800 r/min. H2 was continuously fed into the reactor to maintain the pressure at 8 MPa. After reaction for 4 h, the auto‐ clave was cooled with tap water flowing through the cooling coil. The product mixture was then collected, filtered, and ana‐ lyzed. The products in liquid phase were analyzed qualitatively and quantitatively by high‐performance liquid chromatography (HPLC; HP1100, Agilent, USA) equipped with a refractive index detector. A Platisil ODS C18 AQ column was used at 25 °C to separate the five main products of sorbitol, EG, PG, GL, and an unknown product. Redistilled water (0.6 mL/min) was used as mobile phase. Trace products like ethanol, methanol, and me‐ thane were detected in the gas phase in the reactor, but selec‐ tivity to gaseous products was less than 1%. The liquid prod‐ ucts except the unknown substance were quantified using an external standard method. The selectivity to a specific product was expressed as the ratio of sorbitol converted into product to the total sorbitol converted.
Jinghong Zhou et al. / Chinese Journal of Catalysis 35 (2014) 692–702
3. Result and discussion 3.1. The role of base in hydrogenolysis To better understand the role of base in sorbitol hydrogen‐ olysis, control experiments were conducted under the same conditions, but contained only the sorbitol substrate and one of: CNF support, CaO, Ru/CNFs, or Ru/CNFs + CaO. The product distributions of controlled sorbitol hydrogenolysis are listed in Table 1. Both the CNF support and CaO showed no catalytic effect for sorbitol hydrogenolysis when they were used alone in the reaction. Sorbitol conversion of 17.5% was achieved with more than 90% total selectivity to glycols and glycerol when 0.5 g Ru/CNFs was used. The addition of CaO led to a significant increase in sorbitol conversion to 43.4% with a slight increase in the glycol selectivity and a decrease in glycerol selectivity. This result was similar to that observed by Banu et al. [13] where the addition of Ca(OH)2 increased the conversion signif‐ icantly without any effect on selectivity. The difference in the effect of base promoter on the selectivity to glycerol may be caused by reaction conditions and catalyst applied. It is obvious from Table 1 that without Ru/CNFs, the CaO could not catalyze sorbitol hydrogenolysis. Therefore, CaO is more a promoter than a co‐catalyst for sorbitol hydrogenolysis, although it did greatly enhance the sorbitol conversion. Ac‐ cording to the mechanism proposed by Wang et al. [16] in Scheme 1, the promotion effect is caused by OH− (released when CaO reacts with water), which could catalyze the ret‐ ro‐aldol condensation of the intermediate aldehyde to generate glycols. However, retro‐aldol condensation is the second step in sorbitol hydrogenolysis and only occurs after the sorbitol de‐ hydrogenates to some intermediate aldehyde over Ru catalyst. Thus, only by combining with a metal catalyst, such as Ru/CNFs, can CaO promote sorbitol hydrogenolysis. The effect of base type on sorbitol hydrogenolysis was in‐ vestigated by using five bases of equivalent theoretical OH− but with different metal cations. The results (Table 2) showed that all the bases substantially enhanced sorbitol conversion and deteriorated the selectivity to glycols except CaO. CaO was the best base promoter for sorbitol hydrogenolysis by significantly increasing sorbitol conversion with only a slight effect on the glycol selectivity; hence, glycol yields were greatly enhanced. KOH, NaOH, and Ba(OH)2 added into reactions have equiva‐ lent OH− and completely dissolved in the solution under the reaction conditions. As a result, the hydrogenolysis was sup‐ posed to take place at a same pH when these three bases were used as promoters. However, they affected the product distri‐
Table 2 Effect of base type on sorbitol hydrogenolysis. Product selectivity (%) Base amount * Sorbitol (g) conversion (%) EG PG GL NaOH 10.0 70.6 14.6 17.1 6.5 KOH 14.0 59.9 15.2 20.3 9.2 21.4 52.9 20.6 21.7 10.4 Ba(OH)2 Mg(OH)2 7.3 30.4 24.4 30.7 19.5 CaO 7.0 44.3 26.4 33.7 16.2 Reaction conditions: 20 wt% sorbitol aqueous solution, 3.0 wt% Ru/CNFs 0.5 g, 220 °C, 8.0 MPa. * Theoretical OH− amount 0.25 mol. Base
bution in the order of NaOH> KOH > Ba(OH)2, indicating that besides OH− in the base, the metal cations might play a role during hydrogenolysis. Moreover, it seemed that the more the base enhanced sorbitol conversion, the more it lowered the selectivity to glycols. Actually, more side products other than EG, PG, and GL were observed when these three bases were used as promoters. This could be explained according to the mechanism of polyol hydrogenolysis [16]. The cleavage of C–C bonds basically relies on a retro‐aldol reaction, which is pro‐ moted by OH−. However, an overly high OH− concentration also enhances reversible and scrambling aldol reactions, resulting in a decline in selectivities to desired EG and PG and a rise in yield of other undesirable hydrocracked products [26]. Mg(OH)2 and CaO, which have low solubilities in aqueous solution, showed slightly lower enhancement in sorbitol con‐ version, but only slightly affected the selectivity to glycols. The low solubilities of Mg(OH)2 and CaO limited the concentration of OH− in aqueous solution, although it was still sufficient to accelerate the retro‐aldol reaction of intermediates to glycols and glycerol. Excess OH¯ when combined with Ru catalyst would catalyze further degradation of glycol and glycerol and decrease the glycol yields [22]. Therefore, the addition of Mg(OH)2 and CaO not only enhanced sorbitol conversion, but also maintained high selectivity to glycol. The metal cation of the base promoters, particularly in the case of Ca2+, was probably involved in the reaction. Both sorbi‐ tol and the intermediate aldehyde formed after sorbitol dehy‐ drogenation have chelation ability and can form complexes with divalent metal ions such as Ca2+. Kenner et al. [27] studied the degradation of carbohydrates by alkali and proposed that the cation of the alkali played a role in the complexation of al‐ dehydes, leading to the formation of saccharinic acid. This cat‐ ionic effect of the reacting bases has been also verified in the reaction of d‐glycerose‐3‐C14 with alkali by Sowden et al. [28]. In our case for hydrogenolysis of sorbitol, complexation be‐ tween the intermediates and Ca2+ was also observed. Figure 1
Table 1 Sorbitol hydrogenolysis under control conditions. Catalyst amount Base amount Sorbitol conversion (g) (g) (%) Sorbitol + CNFs — — 0 Sorbitol + Ru/CNFs 0.5 — 17.5 Sorbitol + CaO — 5.0 0 Sorbitol + Ru/CNFs + CaO 0.5 5.0 43.4 EG: ethylene glycol; PG: propylene glycol; GL: glycerol; CNFs: carbon nanofibers. Reaction conditions: 20 wt% sorbitol aqueous solution, 3 wt% Ru/CNFs, 220 °C, 8.0 MPa. Substrate
Product selectivity (%) EG PG GL 0 0 0 25.4 33.5 32.4 0 0 0 27.1 35.6 18.5
Jinghong Zhou et al. / Chinese Journal of Catalysis 35 (2014) 692–702
1 2 3 4 1'
Sorbitol GL EG PG Unknow product
Table 3 Effect of CaO amount on sorbitol hydrogenolysis.
1 1 1
Intensity
23
4
3
4
2 0
2
4 6 8 Resident time (min)
Product selectivity (%) CaO amount Sorbitol conversion (g) (%) EG PG GL 0.5 18.5 31.2 41.2 22.6 1.0 19.6 32.8 34.8 21.4 3.0 29.0 27.7 34.4 19.6 5.0 43.4 27.8 35.6 18.5 7.0 44.3 26.4 33.7 16.2 9.0 48.6 20.5 27.8 14.4 15.0 58.7 15.0 22.7 10.6 Reaction conditions: 20 wt% sorbitol aqueous solution, 3.0 wt% Ru/CNFs 0.5 g, 220 °C, 8.0 MPa.
0.50 g
4
23
1'
1.00 g
3.00 g 10
12
Fig. 1. HPLC chromatograms for hydrogenolysis products with different amounts of CaO.
shows a comparison of HPLC chromatograms for the hydro‐ genolysis products when different amounts of CaO were em‐ ployed in the reaction. It is clear that the intensity of peak 1 for the unknown product increased with the increasing CaO amount. When oxalic acid was added into the liquid product mixture, the unknown substance (peak 1) declined while the concentrations of the other four products did not alter. Simul‐ taneously, a white precipitate was formed and was shown to be calcium oxalate by X‐ray diffraction and Fourier transform in‐ frared spectroscopy [29]. Moreover, the unknown substance was the only product that could be detected by ultraviolet de‐ tection, indicating the existence of aldehyde or a carbonyl group. Therefore, peak 1 was assigned as a complex between Ca2+ and intermediates, although the specific structural formula of this complex has not yet been determined after considerable investigation, including analysis by HPLC‐MASS. It is likely that the complexation of Ca2+ with the intermediate would affect the cleavage of C–C bonds and, subsequently, the selectivities to glycols. Table 3 shows the effect of CaO amount in the reaction sys‐ tem on sorbitol hydrogenolysis. With increased CaO, the con‐ version of sorbitol gradually increased, while the selectivities to glycols and glycerol gradually decreased. CaO reacts with H2O to give Ca(OH)2, but the solubility of Ca(OH)2 in aqueous solu‐ tion is very low and decreases with increasing temperature (0.16 g/100 g H2O at 20 °C and 0.07 g/100 g H2O at 100 °C [30]). In the hydrogenolysis reaction, if Ca2+ was present mere‐ ly in the form of the free ion rather than participating in the reaction by complexing with the intermediate, most of the CaO would exist as undissolved solid because of its low solubility. In such a case, increased CaO would have no effect on the reaction. However, the results in Table 3 show that the CaO amount greatly affected the hydrogenolysis product distribution. This is further evidence that Ca2+ participated in the hydrogenolysis reaction and had an important effect on the selectivities to EG and PG.
Our results suggest that CaO served a bifunctional role in sorbitol hydrogenolysis. As a base additive, it provided a mod‐ erate basic medium for accelerating a retro‐aldol reaction for C–C cleavage and so enhanced the sorbitol conversion. It also provided the metal center for complexation with intermediate aldehyde and released more OH−, further enhancing the ret‐ ro‐aldol condensation. Because the retro‐aldol condensation and the complexation are both reversible reactions under the hydrogenolysis conditions, CaO should reach an equilibrium point between its two roles. As shown in Table 3, when the CaO amount was increased to 5.0 g, the highest yields of glycols were achieved within the investigated range. Further increase in CaO led to severe decline in glycol yields. This is because excess OH− caused the reversible aldol scrambling problem and resulted in a decline in selectivity to glycols, just as in the cases when NaOH and KOH were used as promoters. Therefore, there exists a preferred CaO concentration for the system under which the best glycol yield can be achieved. 3.2. Reaction pathways and optimization of reaction conditions Based on the above discussion and previous mechanistic detail reported in the literature, reaction pathways for hydro‐ genolysis of sorbitol to glycols in the presence of CaO are tenta‐ tively proposed in Scheme 2. As shown in Scheme 2, Ru/CNFs catalyst is necessary for the initial dehydrogenation of sorbitol to intermediate with alde‐ hyde or carbonyl groups, which leads to the cleavage of C–H bonds by dehydrogenation and the subsequent C–C bond cleavage by retro‐aldol condensation. The retro‐aldol reaction is enhanced under basic conditions because it is catalyzed by adsorbed OH− [16]. A probable mechanistic pathway for sorbi‐ tol hydrogenolysis is as follows. The first step is dehydrogena‐ tion of sorbitol on the Ru particle surface to the corresponding intermediates with a sugar carbonyl group. Subsequently, the intermediates undergo either the base‐involved retro‐aldol condensation or complex with Ca2+. The retro‐aldol condensa‐ tion of intermediate aldehyde generates glycolaldehyde and the latter is subsequently hydrogenated to EG. Meanwhile, the ret‐ ro‐aldol condensation of intermediates with a carbonyl group generates glyceraldehyde or dihydroxyacetone, which are then subsequently hydrogenated to GL. GL is further dehydrogenat‐ ed, dehydrated, and then hydrogenated over Ru particle surface to give PG. The complexation between Ca2+ and intermediates
Jinghong Zhou et al. / Chinese Journal of Catalysis 35 (2014) 692–702
OH
OH
OH
OH
OH
OH
Dehydrogenation O
OH
OH
OH
OH
OH
O
OH
OH
HO
OH
Complextion with Ca2+
OH + HO
OH + HO
O
OH
OH
OH
O
Hydrogenation
Retro-aldol condensation O
OH
Dehydration
OH
+
Dehydrogenation
O
OH
OH
OH
HO
Retro-aldol condensation
OH
Hydrogenation OH
OH
HO
O
O
OH
-
-
Retro-aldol condensation
OH
Dehydrogenation
HO OH O
HO
OH
OH
Hydrogenation OH OH
Hydrogenation
HO
Scheme 2. Reaction pathways for sorbitol hydrogenolysis to glycols in aqueous solution in the presence of CaO.
helps to release more OH− and accelerate the retro‐aldol con‐ densation to give glyceraldehyde or glycolaldehyde, and these are further hydrogenated on the Ru surface to the desired EG and PG. During this process, CaO acts as base and provides the complexation cation. The amount of added CaO affects the competition between the retro‐aldol condensation and com‐ plexation with the dehydrogenated intermediates, and dictates the reaction pathways as a consequence. As discussed above, both the dehydrogenation over Ru and the base‐involved retro‐aldol condensation dictate the terms of the hydrogenolysis and the eventual generation of the desired products (EG, PG, and GL). Therefore, there should exist a bal‐ ance between the Ru/CNFs and the CaO promoter to achieve the most favored product distribution. A series of experiments were conducted under different reaction conditions to identify the optimum conditions in terms of the most favored glycol yield. The results showing sorbitol conversion and glycol selec‐ tivities are listed in Table 4. It is generally accepted that increasing the amount of cata‐ lyst in batch processing increases the conversion, but its effect on selectivity depends on the nature of the reaction. As shown in Table 4 (entries 2–4), the sorbitol conversion was remarka‐ bly increased when more catalyst was used while the other conditions remained the same. The selectivities to EG and GL
were slightly decreased, but the selectivity to PG was slightly increased. In addition, it is interesting to note that the change in catalyst concentration imposed inverse effects on the selectivi‐ ties to PG and GL and the sum of the selectivities to PG and GL was kept about the same with increased catalyst amount. Simi‐ lar results were also obtained by Casale et al. [6] when Pt/C catalyst was used in sorbitol hydrogenolysis, and suggests that PG is derived from GL as illustrated in the reaction pathways in Scheme 2. The transformation of GL to PG can be rationalized by considering the adsorption ability of polyols on Ru active sites. It has been proposed that PG has a much lower affinity for Ru active sites than GL or EG, while the latter two have rela‐ tively equal chemisorption ability [31]. When the amount of catalyst was increased, the higher conversion led to a relatively lower sorbitol concentration in the reaction solution. Mean‐ while, more Ru active sites were available for GL to be ad‐ sorbed and the adsorbed molecules might be further dehydro‐ genated and subsequently dehydrated to give PG under the hydrogenolysis conditions. The effect of initial sorbitol concentration on the hydrogen‐ olysis was also investigated at a fixed ratio of Ru/sorbitol and a specified CaO amount (7 g). The results shown in Table 4 (en‐ tries 1, 3, 5, and 7) indicate that the sorbitol conversion gradu‐ ally decreased, while the selectivities to EG and PG reached a
Jinghong Zhou et al. / Chinese Journal of Catalysis 35 (2014) 692–702
Table 4 Effect of reaction parameters on sorbitol hydrogenolysis. Entry 1 2 3 4 5 6 7 8 9
Sorbitol concentration (%) 10 20 20 20 30 40 40 40 40
Ru/CNF amount (g) 0.25 0.25 0.5 1.0 0.75 1.0 1.0 1.0 1.0
CaO amount (g) 7.0 7.0 7.0 7.0 7.0 3.0 7.0 10.0 14.0
Sorbitol conversion (%) 47.0 19.6 44.3 74.9 41.5 30.3 33.9 35.6 47.9
Product selectivity (%) EG PG GL 25.5 29.2 20.4 28.8 27.7 21.3 26.4 33.7 16.2 24.6 35.2 14.2 26.8 29.3 12.8 20.3 27.1 9.1 22.9 22.7 8.3 23.1 16.9 7.8 24.8 18.6 6.9
Reaction conditions: 220 °C, 8.0 MPa.
maximum when the initial sorbitol concentration was 20%. In addition, the selectivity to GL monotonically decreased with increasing initial sorbitol concentration. This result can be ex‐ plained from two aspects. First, the reaction was conducted at a fixed Ru/sorbitol ratio, which means that the number of Ru active sites for each sorbitol molecule was the same. The pro‐ moter CaO, however, also played an important role in deter‐ mining the sorbitol conversion and the selectivities to glycols. When the initial sorbitol concentration increased, the amount of CaO was insufficient for complexation with the dehydrogen‐ ated intermediates and resulted in lower concentration of OH−. As a result, enhancement for the retro‐aldol condensation was not as significant as it was at lower initial sorbitol concentra‐ tion. Another possible reason for the decrease in sorbitol con‐ version is that the reaction order of the sorbitol dehydrogena‐ tion is less than 1, which means the higher the initial concen‐ tration, the conversion would be lower after the same reaction time. According to the proposed reaction pathways in Scheme 2, it is likely that Ru/CNFs loading, sorbitol concentration, and the amount of CaO need to be finely balanced to achieve a favorable product distribution. Ru/CNFs catalyzed the dehydrogenation and hydrogenation during hydrogenolysis, while CaO provided OH− for the retro‐aldol condensation and dehydration and Ca2+ for complexation with the dehydrogenated intermediates. The compromise between these two processes ultimately deter‐ mines the product distribution. Concentrated sorbitol solution (40%) was employed to demonstrate the synergy between the actions of Ru/CNFs and CaO. As can be seen in Table 4 (entries 6–9), when 40% sorbitol solution was used as the initial reac‐ tant, although sorbitol conversion increased with increasing CaO amount, the enhancement was not as significant as it was when 20% sorbitol solution was used (Table 3). Notably, the selectivity to EG increased with increasing CaO amount. This is because more sorbitol reactant and more Ru/CNFs catalyst in these reactions led to more complexation, which favors the production of EG. In summary, fine tuning of the reaction parameters allowed sorbitol to be hydrogenolyzed to EG and PG at a total glycol selectivity of 60.1% with a 75% sorbitol conversion. The opti‐ mum conditions were: 220 °C, hydrogen pressure 8 MPa, sor‐ bitol solution concentration 20 wt%, 3 wt% Ru/CNFs catalyst 1.0 g, CaO base 7.0 g.
4. Conclusions The five bases of NaOH, KOH, Mg(OH)2, Ba(OH)2, and CaO were used to investigate the effect of base on sorbitol hydro‐ genolysis. The results indicated that all the base promoters significantly enhanced the sorbitol conversion, while the glycol selectivities varied with the base type. CaO was the best base promoter in terms of glycol yields. It served a bifunctional role in sorbitol hydrogenolysis as a source of OH− and metal ions for complexation, enhancing the retro‐aldol condensation and dic‐ tating the product selectivities. The mechanistic study for sor‐ bitol hydrogenolysis indicated that Ru/CNFs catalyst was nec‐ essary for the initial dehydrogenation of sorbitol to intermedi‐ ates. The intermediates then undergo C–C bond cleavage by retro‐aldol condensation, which is enhanced under basic condi‐ tions. Pathways for sorbitol hydrogenolysis to EG, PG, and GL in the presence of CaO have been tentatively proposed. We found that there was an optimum ratio among sorbitol concentration, Ru/CNFs catalyst, and CaO to achieve favorable glycol selectivi‐ ties in sorbitol hydrogenolysis. These findings provide a fun‐ damental basis for the rational development of nonbiological approaches for biomass conversion to commodity chemicals. References [1] Chheda J N, Huber G W, Dumesic J A. Angew Chem Int Ed, 2007, 46:
7164 [2] Ruppert A M, Weinberg K, Palkovits R. Angew Chem Int Ed, 2012, [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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Jinghong Zhou et al. / Chinese Journal of Catalysis 35 (2014) 692–702
Graphical Abstract Chin. J. Catal., 2014, 35: 692–702 doi: 10.1016/S1872‐2067(14)60083‐8 Hydrogenolysis of sorbitol to glycols over carbon nanofibers‐ supported ruthenium catalyst: The role of base promoter
OH
OH
Jinghong Zhou *, Guocai Liu, Zhijun Sui, Xinggui Zhou, Weikang Yuan East China University of Science and Technology
Dehydrogenation O
OH
HO
OH
Retro-aldol condensation
HO
OH
OH HO
OH
Ru
Ru
OH
Dehydrogenation
OH
OH
O
OH
OH
CaO
-
Ru/CNF catalyst and CaO dictated the reaction pathways in sorbitol hydrogenolysis to glycols, where CaO served as both base and metal ion source for complexation.
OH
OH
OH
Complextion with Ca 2+
OH
HO
OH
OH -
OH
Retro-aldol condensation
HO
OH2+
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