Lijun
Zhang
a,
Guangzhi
Hu
b,
Song
Hu
c,
Jun
Xiang
c,
Xun
Hu
*a,
Yi
Wang
*c and
Dongsheng
Geng
*d
aSchool of Material Science and Engineering, University of Jinan, Jinan, 250022, P. R. China. E-mail: xun.hu@outlook.com; Fax: +86-531-89736201; Tel: +86-531-89736201
bKey Laboratory of Chemistry of Plant Resources in Arid Regions, State Key Laboratory Basis of Xinjiang Indigenous Medicinal Plants Resource Utilization, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, China
cSchool of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China. E-mail: alenwang@hust.edu.cn
dCenter for Green Innovation, Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing, 100083, P. R. China. E-mail: dgeng@ustb.edu.cn
First published on 23rd January 2018
Phenolic compounds are an important component of pyrolysis oil (bio-oil). Understanding their reaction behaviours during hydrogenation helps to clarify the reaction network during hydrotreatment of bio-oil. This study investigated the hydrogenation of typical phenolic compounds in bio-oil in water, which is an important fraction of bio-oil, and in methanol, which is used to esterify bio-oil. The results indicated that hydrogenation of the phenolics is more efficient in water than in methanol. With water medium, both the unsaturated oxygen-containing functionalities and the benzene ring of the phenolics can be effectively hydrogenated. In comparison, with methanol medium, only the unsaturated oxygen-containing functionality can be hydrogenated, while the hydrogenation of the benzene ring is rather difficult. Stereoselectivity was also observed in the hydrogenation of phenolics with three or more hydroxyl groups/methoxyl groups. For phenolics with two hydroxyl groups/methoxyl groups, the hydrogenation produces both cis- and trans-configurations.
Hydrogenation of the phenolics or furans derived from bio-oil has been performed in many studies with a particular focus on the development of catalysts.11–18 Apart from catalysts, the reaction medium also plays important roles in hydrogenation reactions. In our previous study,19 we have observed the distinct effects of water and methanol media on the conversion of sugars such as levoglucosan and glucose in acid-catalysed reactions. In this study, we thus made an effort to focus particularly on the effect of water and methanol on the conversion of typical phenolics present in bio-oil in hydrogenation reactions. The reason for selecting water as the reaction medium is that the concentration of water in bio-oil can be up to 40 wt%.20 Furthermore, water under subcritical/critical conditions behaves like an acid catalyst,21,22 potentially affecting the hydrogenation of the phenolics. Methanol is often used to esterify bio-oil and is thus a major component of esterified bio-oil.23–25 Water and methanol may impact hydrogenation of the phenolics in bio-oil in various ways.
To gain insight into this, hydrogenation of bio-oil and 14 phenolic compounds with varied oxygen-containing functionalities was conducted over a Pd/C catalyst in water and in methanol medium, respectively. Our results demonstrated that hydrogenation of the phenolics in bio-oil was more efficient in water than in methanol. Further to this, stereoselectivity for the hydrogenation of phenolics with three oxygen-containing functionalities (–OCH3 or –OH but not –CO) was observed.
Phenolics in bio-oil have distinct oxygen-containing functionalities. It has not yet been made clear how methanol and water media affect the transformation of the oxygen-containing functionalities. Hydrogenation of the phenolics with distinct functionalities in water and in methanol was subsequently conducted.
During the hydrogenation of the benzene ring, C4 was the main intermediate. Hydrogenation of the carbon–carbon double bond (from C1 to C5 in water) proceeded in a stepwise manner. During this process, the carbon–carbon double bond together with the adjacent hydroxyl group underwent tautomerization to form ketones. In addition, the formation of stereoisomers confirmed the hydrogenation of the benzene ring in the phenolics in a stepwise manner, as evidenced by the formation of four stereoisomers of compound C5. The effect of vanillin loading on the hydrogenation reaction was also investigated and the results are presented in Fig. S3.† The varied vanillin loading did not impact much the distribution of the products. The difference in reactivity towards hydrogenation in water and in methanol was further confirmed through the hydrogenation of other phenolics.
Fig. 3 Distribution of the products in the hydrogenation of guaiacol in methanol and in water. The experimental conditions were the same as those in Fig. 2. |
Fig. 4 Constant energy (−2800 cm−1) synchronous spectra of the products after hydrogenation of guaiacol in water, in methanol and in methanol/water. |
Hydrogenation of guaiacol was further investigated in methanol/water (Vmethanol/Vwater = 1) to understand the reaction behaviors in the mixed reaction medium. As expected, hydrogenation of the benzene ring of guaiacol was more efficient in methanol/water than in methanol, while being less efficient than that in water. C10 was a major product produced from partial hydrogenation of the benzene ring and the tautomerization of the enol form of the intermediate. C10 was not detected in the products from the hydrogenation in water. It is probable that hydrogenation of the benzene ring in guaiacol was very quick in water and thus there was little chance for tautomerization to take place.
Water is more acidic than methanol, which possibly accounts for the high activity for the hydrogenation reactions. To confirm this hypothesis, hydrogenation of guaiacol in the presence of Amberlyst 70 (A70), a strong solid acid catalyst, was performed in methanol, as shown in Fig. S4 in the ESI.† The conversion of guaiacol increased but that increase did not match the conversion in water. The different acidities of water and methanol were thus not the main reason for the distinct impact of water and methanol on the hydrogenation of guaiacol and vanillin. In addition, the added acid catalyst (A70 catalyst) mainly impacted the acid-catalyzed reactions such as the etherification of compound C16 to C17 (Fig. S4†). The above results clearly indicated that the hydrogenation of the benzene ring with the oxygen-containing functionalities was difficult in methanol. But what would happen if there were no oxygen-containing functionalities attached on the benzene ring of the compound?
Fig. 5 Distribution of the products in the hydrogenation of allylbenzene and eugenol. The experimental conditions were the same as those in Fig. 2. The numbers in red colour are the yields of the product. |
Fig. 6 Constant energy (−2800 cm−1) synchronous spectra of the products after hydrogenation of eugenol in water and methanol. |
In water, however, the benzene ring of eugenol could be effectively hydrogenated. The structural difference of water and methanol must be responsible. Water and methanol can form hydrogen bonds with the phenolic compounds. π–n electron conjugation between π-bonds in the benzene ring and the isolated electron pair (n-electrons) in the oxygen atom in water might exist. The conjugation effect may increase the electron density of the benzene ring and favor the hydrogenation reaction. Methanol could have the same effect, but methanol molecules are much bigger than water molecules in terms of size. The resulting steric hindrance makes the conjugation less effective; this, however, needs further verification.
In addition, both methanol and water can be strongly adsorbed on activated carbon, via the formation of hydrogen bonds with the carboxylic group and the hydroxyl groups on activated carbon.26,27 This will inevitably affect the adsorption/activation of the phenolics and hydrogen. However, Lee et al. reported that methanol is less competitive than water for the adsorption of H2 on Pd/SiO2.28 Nevertheless, hydrogenation of the phenolics was more effective in water than in methanol, and hence competitive adsorption was not the main reason for the distinct reactivity of the phenolics in methanol and in water.
The adsorption of water on the surface of the activated carbon was stronger, due to its high capacity to form hydrogen bonds with functionalities of activated carbon.29 This would make the surface of the catalyst more hydrophilic, facilitating adsorption of the polar phenolics, which might aid the hydrogenation. In order to prove this, a Pd/SiO2 catalyst was prepared via an impregnation method and the temperature-programmed reduction profiles are shown in Fig. S5.† The results for the hydrogenation of vanillin are shown in Table S3.† The results were similar to those over the Pd/C catalyst. The hydrogenation in methanol could not effectively hydrogenate the benzene ring while in water it could. The result obtained herein further confirmed that the carbon support did not play an essential role in the hydrogenation of the benzene ring in the phenolics while the solvent (water or methanol) did. In addition, hydrogenation of the benzene ring possibly needs the co-operation of several metal sites. The higher affinity of the phenolics for the catalyst in water medium would facilitate the hydrogenation of the benzene ring.
In water, steric hindrance was not an issue. Even phenolics such as 3,4,5-trimethylphenol with four substituted functionalities could be effectively hydrogenated (Fig. S3†). Other bigger phenolics such as 4-ethyl guaiacol, 2-methoxyl-4-propyl phenol, and 4-sec-butylphenol could also be fully converted via hydrogenation. However, stereoselectivity was encountered in the hydrogenation of other phenolics.
Similar to the case of vanillin, the carbonyl group in 3,4,5-trimethoxybenzaldehyde could be completely hydrogenated. Nevertheless, hydrogenation of the benzene ring was difficult in methanol (Fig. 7). However, one remarkable difference was that the hydrogenation of C63 produced C64 with only a cis-configuration. No other isomers of C64 were detected although subsequent products such as C65 and C66 still had isomers. The hydrogenation of syringaldehyde, 1,2,4-trimethoxybenzene and 3,4,5-trimethoxy toluene in either water or in methanol initially produced the fully hydrogenated product with only a cis-configuration. The common feature of these “heavier” phenolics is that they all have three oxygen-containing functionalities in the form of either –OH or –OCH3, which is the intrinsic reason for the stereoselectivity. The adsorption–hydrogenation–desorption of the reactants was probably finalized in one step, leaving no chance for isomer formation.
Fig. 7 Distribution of the products in the hydrogenation of the phenolics in methanol and in water. The experimental conditions were the same as those in Fig. 2. |
If the molecules contained one or two oxygen-containing functionalities, isomers would be formed even if the molecules had three to four substituted functionalities (such as 4-ethyl guaiacol and 3,4,5-trimethylphenol). It needs to be noted here that some compounds such as vanillin also contain three oxygen-containing functionalities (hydroxyl group, methoxy group, and carbonyl group). Nevertheless, the carbonyl group in vanillin could be readily hydrogenated and eliminated either in methanol or in water medium. The resulting product, 2-methoxy-4-methyl-phenol, still has two oxygen-containing groups and its hydrogenation produced isomers (Fig. 2).
Activated carbon is the support for the Pd/C catalyst, and has many surface functionalities such as hydroxyl groups and carboxyl groups.30 The oxygen-containing functionalities of the phenolics can form hydrogen bonds with the hydroxyl groups and/or carboxyl groups on the activated carbon surface, as shown in Fig. 8. If the phenolics contain two oxygen-containing functionalities, and they form two hydrogen bonds on the catalyst surface (anchor sites), the benzene ring could still rotate backwards and forwards. This means that the compound could access the metal sites from two directions (Fig. 8). However, if three oxygen-containing functionalities could bind to the surface of the activated carbon via hydrogen bonds, the force of the hydrogen bonds would be tripled. More importantly, the molecule would not be able to rotate easily. The benzene ring could then access the active sites from only one orientation, producing the product with only one cis-configuration. Thus, stereoselectivity is an important factor to be considered when manufacturing fine chemicals via hydrogenation of phenolic compounds with multiple hydroxyl groups/methoxy groups.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8se00006a |
This journal is © The Royal Society of Chemistry 2018 |