Effect of various catalysts on selectivity of C2~C3 polymers
Ru/IL nanoparticles were prepared through four kinds of ionic liquids. Their properties were all characterized (Supporting Information). Hydrogenolysis reactions were carried out to evaluate the potential for producing polyols over Ru/IL nanoparticles, meanwhile, Ru/C catalysts were chosen to have a comparison. Table 1 shows the effect of various catalysts on selectivity of C2~C3 polymers.
Table 1 Catalytic activity of Ru catalysts on hydrogenolysis conversion of alkaline cellulose a
Entry
|
Catalyst
|
Conv.b
|
Selectivitiy
|
|
|
(%)
|
XL
|
EL
|
GL
|
1,2-PDO
|
EG
|
1
|
Ru/C c
|
21.97
|
7.14
|
2.32
|
0.00
|
0.00
|
0.00
|
2
|
Ru/C
|
59.23
|
4.65
|
4.78
|
1.87
|
18.08
|
6.16
|
3
|
Ru/[Bmim]PF6
|
65.22
|
7.27
|
2.02
|
1.87
|
23.51
|
6.47
|
4
|
Ru/[Bmim]BF4
|
63.78
|
5.90
|
2.42
|
18.37
|
22.55
|
17.99
|
5
|
Ru/[Bmim]Cl
|
56.65
|
4.70
|
2.06
|
1.96
|
13.97
|
1.73
|
6
|
Ru/[HO-emim]Cl
|
61.82
|
3.56
|
1.44
|
1.42
|
11.24
|
1.21
|
a Reaction condition: alkaline cellulose (0.1 g), Ru/C (0.02 g, 5 wt%) or Ru nanoparticles (0.001 mmol), H2O (2 mL), H2 (5 MPa), 433 K and 5h. XL: xylitol; EL: erythritol; GL: glycerol; 1,2-PDO: 1,2-propanediol; EG: ethylene glycol.
b Conversion of alkaline cellulose.
c The substrate was not pre-treated with NaOH solution.
It can be seen from Table 1 that it has the lowest conversion (21.97%) of native cellulose over commercial Ru/C catalyst, and the detected products were mainly xylitol and erythritol, moreover, no C2~C3 polymers were detected. In contrast, the conversion was higher for alkaline cellulose (59.23 %) over the same Ru/C catalyst with 1,2-propanediol in majority. This result suggested that alkaline pretreatment had profound effects to the hydrogenolysis of cellulose to low molecular weight products.
Seen from Table 1(Entry 3-6), Ru/[Bmim]PF6 nanoparticles showed a much higher conversion of alkaline cellulose (65.22 %) than that on Ru/C catalyst (21.97 %). Furthermore, remarkably high selectivity to C2~C3 polyols (58.91 %) was obtained over Ru/[Bmim]BF4 catalysts, while only 26.11 % selectivity of C2~C3 polyols was gained from Ru/C catalyst. This may attribute to the small diameter and the high dispersion of nanoparticles which provided larger amounts of active sites. From the molecular point of view, monodisperse ruthenium nanoparticles will make the trend of entering the internal structure of cellulose in the aqueous solution more obvious, thereby improving the conversion of cellulose. Ionic liquid [Bmim]BF4 has a larger molecular steric hindrance than [Bmim]Cl and [HO-emim]Cl. Meanwhile, considering of anion, it can be seen that the electronegativity of F is greater than that of Cl, so [Bmim]Cl is weaker in the effect of catalytic hydrogenation than that of [Bmim] BF4. Therefore, [Bmim] BF4 displays better selectivity for C2~C3 polymers.
To clarify this point further, Fig. 1 were demonstrated that how does the Ru/IL nanoparticle catalyst have high catalytic performance for alkaline cellulose. The amorphous region due to the cleavage of the alkaline cellulose carbon chain makes the nano-sized catalyst easier to enter the interior of the cellulose for basic catalytic reaction, while the carbon-supported catalyst can only react on alkaline cellulose surface to break the chain. It makes a larger area of the amorphous region for Ru/IL nanoparticles to enter the interior. Thus, Ru/C has a lower catalytic efficiency than the nanoparticle catalyst.
Effect of crystalline structure for hydrogenolysis
To investigate the structural changes after hydrogenolysis reaction over different catalysts, the morphology of untreated cellulose and alkaline cellulose before and after hydrogenation are presented in Fig.2(a) and (c). The untreated microfiber cellulose has fine structure and relatively flat surface, while the alkaline cellulose exposes a kind of semi-dissolved amorphous region which is formed of microfiber and tiny cavities. The chemical reactions between cellulose and NaOH cause the rupture of cellulose chains, which reduced their crystallinity and make them easier to be hydrogenated. For this reason, alkaline cellulose appears to a better catalytic activity. In hydrogenation process, the amorphous region of the alkaline cellulose is the main conversion site and metal Ru is playing the main role of hydrogenolysis.
As Ru/C catalyst, the metal Ru is supported on the C atom. During the reaction, the C atom is too large to penetrate into the interior of the cellulose which may not result in a further degradation. Seen from Fig.2(b), because the surficial structure becomes smooth after degradation reaction, Ru/C catalyst is easily to wash away. It shows that the surface of the alkaline cellulose before the reaction has a large number of amorphous regions, so that it forms many open regions, which provides excellent conversion spots for hydro-conversion. When the hydrogenation reaction over Ru/C catalyst is carried out, the active region is converted, so that the crystallized region having a good degree of polymerization is exposed. Ru/C catalyst can only carry out the hydrolysis reaction of the active site of the alkaline cellulose to obtain an intermediate product such as glucose, and catalyze the reaction with the metal Ru on the surface of the C atom in a liquid environment to complete the process of breaking bonds and hydrogenation to form new polyol products.
(a) untreated cellulose; (b) untreated cellulose after hydrogenation on Ru/C ; (c) alkaline cellulose (d) alkaline cellulose after hydrogenation on Ru/C; (e) alkaline cellulose after hydrogenation on Ru/ [Bmim]BF4 nanoparticles.
Ru nanoparticles has the same active sites as Ru/C catalyst, Nonetheless, due to its minisize that Ru nanoparticles could penetrate into the interior of the alkaline cellulose through the small cavities, depressions or scars on the surface. This permeability leads to a further conversion and degradation in alkaline cellulose. It is interesting to note that, the rinse after the reaction does not entirely take away the nanoparticles hided in the cavies.
Effect of Nickel-doped Ru/IL in hydrogenation
In the same synthetic path, Ni-Ru/ILs was prepared for hydrogenolysis. When Ni was added into Ru nanoparticles as an ameliorant, the catalytic performances were improved (Table 2). The alkaline cellulose conversions over Ni-Ru/IL catalysts were all increased compared to reactions catalyzed with Ru/IL catalysts (Table 1), and the highest conversion was also achieved by the catalyst prepared with [Bmim]PF6 ionic liquid (67.21%). From the comparison of Table 1 and Table 2, the same growth trending of conversion rate proves again that ionic liquid promotes the conversion of cellulose, and also shows that the conversion rate of hydrophobic ionic liquid to cellulose is better than that of hydrophilic ionic liquid. It should be noted that, contrary to expectation, the selectivity to C2~C3 polyols with Ni-Ru/[Bmim]Cl catalysts was much higher than that catalyzed by other Ni-Ru/IL catalysts. Especially, the selectivity to ethylene glycol is growing up to almost eleven times. The maximum selectivity of C2~C3 polymers of 62.66% was observed in the case of using Ni- Ru/[Bmim]Cl catalyst, which was 3.5 times higher than that from Ru/[Bmim]Cl catalyst. Ni-Ru/[HO-emim]Cl still had a poor preforms that the selectivity of C2~C3 polymers was totally 22.58%.
Table 2 Catalytic activity of Ru catalysts on hydrogenolysis conversion of alkaline cellulose a
Entry
|
Catalyst
|
Conv.b
|
Selectivity (%)
|
|
|
(%)
|
XL
|
EL
|
GL
|
1,2-PDO
|
EG
|
1
|
Ni-Ru/[Bmim]PF6
|
67.21
|
11.54
|
3.25
|
5.43
|
27.60
|
7.44
|
2
|
Ni-Ru/[Bmim]BF4
|
65.07
|
3.41
|
3.60
|
12.02
|
24.88
|
11.71
|
3
|
Ni- Ru/[Bmim]Cl
|
59.67
|
7.17
|
3.29
|
19.52
|
24.13
|
19.01
|
4
|
Ni-Ru/[HO-emim]Cl
|
62.41
|
7.07
|
1.93
|
1.53
|
16.55
|
4.50
|
|
|
|
|
|
|
|
|
|
|
a Reaction condition: alkaline cellulose (0.1 g), Ni-Ru/IL nanoparticles (nNi:nRu=2:1, nRu=0.001 mmol ), H2O (2 mL), H2 (5 MPa), 433 K and 5h. XL: xylitol; EL: erythritol; GL: glycerol; 1,2-PDO: 1,2-propanediol; EG: ethylene glycol.
b Conversion of alkaline cellulose.
This is because that nickel is a transition metal and has a d-electron orbital which is not full. Due to the holes of the d orbital, the ability to accept external electrons and adsorbed substances to bond with them is enhanced. The addition of nickel enhances the ability of the active metal to adsorb electrons, and plays a positive role in the fracture of C-C bonds. In nickel-doped Ru/IL catalysts,Ni promotes long-chain molecules to further break C-C bonds, so the selectivity to C2~C3 will increase. Comparing Table 1, it can be seen that the conversion of alkaline cellulose is not obvious, and the selectivity to 1,2-propanediol is not greatly improved, but the selectivity to ethylene glycol is increased several times. Despite, ionic liquid [HO-emim]Cl displays a weakness in generating C2~C3 polymers. In ionic liquid [HO-emim]Cl, the lone pair electrons carried by oxygen in the hydroxyl group may form a repulsive force against the lone pair electrons in the hydroxyl groups of cellulose, thereby causing the separation of the active center germanium atoms and reducing the catalytic selectivity.
Left-land axis: selectivity; right-hand axis: conversion(¡). Reaction condition: alkaline cellulose (0.1 g), H2O (2 mL), H2 (5 MPa), 433 K and 5 h. Ni-Ru/IL catalyst (the amount of Ru is 0.001 mmol): (a) Ni-Ru/[Bmim]BF4; (b) Ni-Ru/[Bmim]PF6; (c) Ni- Ru/[Bmim]Cl; (d) Ni-Ru/[HO-emim]Cl.
Figure 3 illustrates the effect of nickel content for hydrogenlolysis of alkaline cellulose pre-treated with 4% NaOH solution. It was clear that the effect of the addition amount of nickel on the catalyst prepared by different ionic liquids has the same effect on the conversion trend, and all have a good conversion rate when the proportion of nickel is 53.7%. On the other hand, the addition of nickel in the catalyst prepared by different ionic liquids has different effects on the selectivity of the products, but in general, erythritol is the main product when Ni/ILs is used alone, and 1,2-propanediol is the main product on Ru/ILs catalysts, and the selectivity of glycerol is relatively higher when the amount of nickel added is 53.7%.
Effect of reaction time in hydrogenolysis
To further understand the process of selective change in the reaction, the effect of reaction time during hydrogenalysis of alkaline cellulose on 53.7 % Ni-Ru/[Bmim]BF4 catalyst were presented in Figure 4. It is clear that the conversion rate was rapidly increased from 42.9 % to 65.07 %, and barely raised after 5h. The products with selective advantages were mainly 1,2-propanediol and ethylene glycol at 1 h. When the reaction was carried out for 2 h, the selectivity of xylitol was significantly improved from 8.18 % to 12.17 %, and the selectivity of erythritol was slightly decreased, indicating that the cellulose was mostly subjected to hydrolysis to form polyols such as xylitol, and erythritol was catalyzed to degrade that may means a tendency to convert to lower alcohols. When the reaction was carried out for 5 h, the selectivities of glycerol and 1,2-propanediol were significantly improved to 11.71 % and 24.88 %, respectively. The rapid decrease in xylitol selectivity indicated that the polyols such as xylitol were cleaved by C-C bonds and converted to C3. Nevertheless, even the reaction time increased to more than 10 h, the selectivity of C2~C3 polyols decreased slightly. After 24 h, the selectivity to C2~C3 polyol remained unchanged with the slight increase of conversion. Therefore, the study considers that 5 h is the optimum reaction time for alkaline cellulose at 433 K reaction temperature and 5 MPa H2.
Mechanism
The major polyols detected by HPLC and ESI-MS were xylitol, erythritol, glycerol, 1,2-propanediol and ethylene glycol. Other by-products that can be detected but not significant were glucose, sorbitol, formic acid, glyceraldehyde, methanol and ethanol. The hydrogenolysis process of alkaline cellulose on metal nanoparticle catalysts is described in Scheme 1. Cellulose was firstly hydrolyzed into sugars, mainly glucose, and then conversed in the pathway to polyols and aldehyde as intermediates under alkaline conditions (Yin et al. 2011). These intermediates transform through a series of the retro-aldol reaction in the presence of hydroxy ions (Zhao et al., 2010), and finally hydrogenated on metals. In another pathway, the vapors of primary and secondary alcohols can dehydrogenate on the surface of the metal Ni catalyst at high temperature to form aldehydes and ketones, respectively. The intermediates generally contain one or two unsaturated bonds which are too unstable to be hydrogenated in the following steps.
The main pathways to obtain 1,2-propanediol and ethylene glycol are described in scheme 1. Cellulose firstly hydrolysis to glucose, one way is hydrogenation to sorbitol, sorbitol is then hydrocracking to glycerol directly (Pathway III) or dehydrogenated to ketone which will crack and being hydrogenated to ethylene glycol and erythritol (Pathway I) (Knill and Kennedy, 2003). Another straightway is that glucose is hydrocracked to ethylene glycol and erythritol (Pathway IV). The other probable way is that glucose is hydrocracked to xylitol in the first step and then through dehydration of polyols to enols (Montassier et al. 1991). The unstable enol form will change their structures to ketone. In this structure of ketone, the presence of electron-withdrawing group makes adjacent carbon atom become positive, so they are more likely to be dehydrogenated. In this way, during the hydrogenation process, it promotes a retro-aldol reaction, which converts the ketone into 1,2-propanediol and ethylene glycol (Pathway II) (Li and Huber, 2010). Due to the nano-size of metal catalyst which is benefit of increasing the contact area between catalyst and raw material, the conversion of cellulose was promoted. It is well known that Ru is an effective catalyst for hydrogenation, and Ni has a good performance in C-C bond cleavage.
Compared to the monometal ruthenium catalyst, the selectivity of 1,2-propanediol is higher than that of xylitol in the initial stage of the reaction. It indicates that in the presence of metallic nickel, a-H is more likely to undergo an elimination reaction with b-H, which causes a-C to be changed to a methyl group, and then a bond breaking and hydrogenation reaction occurs to increase the yield of 1,2-propanediol, while long-chain polyols are broken and converted to C2~C3. In Scheme 1, likewise, C3-C6 polyols can undergo a reaction similar to pathway I. Therefore, it can be seen that after the addition of nickel, not only the conversion rate of the reaction is improved, but also the selectivity of the C2-C3 polyol such as 1,2-propanediol can be significantly improved.