Research Article pubs.acs.org/journal/ascecg
Direct Catalytic Hydrogenolysis of Kraft Lignin to Phenols in CholineDerived Ionic Liquids Fei Liu,† Qiaoyun Liu,†,‡ Aiqin Wang,*,† and Tao Zhang*,† †
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China University of Chinese Academy of Sciences, Beijing 100049, P. R. China
‡
ABSTRACT: A new dual catalyst system composed of choline-derived ionic liquids (ILs) and Pd/C was developed for the selective hydrogenolysis of Kraft lignin to monophenols. Among a series of investigated choline-derived ILs, [Ch][MeSO3] displayed a strong acidity, good thermal stability, and excellent lignin solubility. Under the reaction conditions of the mass ratio of [Ch][MeSO3] to Pd/C being 1, the Pd/C loading of 3.5 wt %, H2 pressure of 2.0 MPa, reaction time of 5 h, and temperature of 200 °C, the conversion of Kraft lignin and the selectivity to phenol (PL) and catechol (COL) reached 20.3%, 18.4%, and 18.1%, respectively. In order to rationalize the formation of PL and COL in our [Ch][MeSO3]-Pd/C system, the hydrogenolysis of a suitable lignin model compound (guaiacylglycerol-β-guaiacyl ether) was studied under the same condition for Kraft lignin. The results suggested that the mechanism involved fragmentation of lignin catalyzed by both acid and Pd/C, followed by acid-catalyzed C−O and C−C cleavage of the fragmented compounds resulting in the formation of PL and COL. KEYWORDS: Kraft lignin, Hydrogenolysis, Hydrolysis, Choline-derived ionic liquids, Phenols
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INTRODUCTION Phenolic compounds are widely used in the chemical industry as key precursors of plastics/cosmetics/pharmaceutics.1 Nowadays, all the industrial methods to obtain phenolic compounds rely heavily on the use of petrochemicals as starting materials. However, the predicted depletion of fossil resources together with the concern about global warming forces us to find an alternative and sustainable resource to produce these important chemicals.2 Lignin is being considered to be an ideal choice as it is a massive biomass raw material and capable of producing phenolic compounds.3−5 In fact, the conversion of lignin to valuable fine chemicals has become a hot topic in the field of green chemistry. Various methods including oxidation, hydrogenation, and hydrogenolysis have been developed.6−8 Nevertheless, most of the previous systems suffered from poor selectivity toward one or two specific compounds due to the complicated chemistry involved in the lignin conversion. Phenol (or catechol) is the simplest compound incorporating an aromatic ring and a hydroxyl group. These groups are involved in the basic structural units of lignin.9 Therefore, direct catalytic production of phenol or catechol from lignin should be theoretically possible. However, until now, the yield of phenol or catechol from lignin was pretty low, and the maximum value reached only 0.5 wt %.10 Therefore, development of an innovative method that can selectively convert lignin to phenol or catechol is still a pending challenge in this area. On the other hand, valorization of lignin in the liquid phase with heterogeneous catalyst is plagued by an insufficient interaction between insoluble lignin and heterogeneous © 2016 American Chemical Society
catalysts. To overcome this problem, a reaction medium that is capable of dissolving lignin is appealingly needed. Ionic liquids have been recently demonstrated to be effective solvents for lignin.11 In particular, Brønsted acidic ionic liquids that incorporate imidazolium cations, such as [BMIm]+, [MMIm]+, and [EMIm]+, together with different acidic anions, have displayed a promising promoting effect in the valorization of lignin.12 These acidic ionic liquids are not only able to dissolve lignin in decent amount,11,12 but also capable of activating the ether moieties in the lignin, resulting in a partial cleavage of C− O bond.13−15 Furthermore, concerted functions of acid and redox active sites can facilitate the selective hydrogenolysis reaction of lignin, which involves cleavage of both C−O−C (approximately two-thirds of all the linkages) and C−C (approximately one-third of all the linkages) bonds.16−18 Taking a cue from the previous reports, we envisage that, by combining acidic ionic liquid together with a Pd catalyst, one may be able to establish an effective method for depolymerizing lignin, with which some valuable compounds could be prepared as well. Nowadays, although a large number of ILs have been synthesized with high quality and yield, most of them are toxic19−23 and costly due to an energy-consuming downstream purification process, which defeated to some extent the sustainability of the process. Therefore, searching for nontoxic, Received: March 28, 2016 Revised: May 24, 2016 Published: May 26, 2016 3850
DOI: 10.1021/acssuschemeng.6b00620 ACS Sustainable Chem. Eng. 2016, 4, 3850−3856
Research Article
ACS Sustainable Chemistry & Engineering
water was attained, and then separated by centrifugation (4000 rpm, 20 min) and dried at 80 °C overnight before use. Typical Procedure for Hydrogenolysis of Lignin. The treated Kraft lignin (4 g), [Ch][MeSO3] (4 g), and Pd/C catalyst (0.14 g) were placed in a 75 mL autoclave reactor. The reactor was sealed and purged with H2. The reaction was conducted at 200 °C at a stirring speed of 800 rpm. After completion of the reaction, the reactor was cooled to ambient temperature. The liquid product was washed with water, then filtered and extracted with AcOEt, and concentrated in vacuo. Products were analyzed using an Agilent 7890A gas chromatograph (GC) equipped with an HP-5 capillary column. The internal standard is p-xylene.
biodegradable, cheap, and environmentally benign ILs that are capable of dissolving lignin is an urgent task for current chemists. Choline chloride (ChCl) is a cheap (2 €/kg), easily available, and biodegradable solid chemical (melting point = 302 °C) that has been widely used as a key component of deep eutectic solvents (DESs), which displayed many unique properties close to that of imidazolium-based ionic liquids.24 Choline-derived ionic liquids have been used either as excellent solvents or as important components of catalysts in catalytic valorization of biomass.25 Pereira et al. reported that cholinium alkanoates were capable of dissolving cork at 100 °C with solubilities ranging from 40 to 65 wt %.26 Zong et al. found that cholinium-based ILs with amino acid anions displayed a good solubility of lignin, up to 220 mg g−1.27 Because of all these unique properties of choline-derived ILs and also good compatibility of these ILs with metal catalysts, it is conceivable that these ILs should have great potential in valorization of lignin, particularly in metal-catalyzed hydrogenolysis of lignin. In continuation of our research to develop efficient catalyst systems for the valorization of biomass,28−30 in this work, we established a dual acid/redox catalyst system for the hydrogenolysis of Kraft lignin by using a choline-derived ionic liquid as both a solvent and an acid catalyst and Pd/C as a metal catalyst, which was able to produce phenol and catechol in high selectivity (Figure 1). In particular, the reaction was performed with an acceptable mass ratio of Kraft lignin to IL, which ensures the catalytic system a good practicability.
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RESULTS AND DISCUSSION Structures and abbreviations of the choline-derived ionic liquids are shown in Figure 2.
Figure 2. Chemical structures and abbreviations of choline-derived ILs.
Some of the obtained choline-derived ILs have not been reported yet. Therefore, their physicochemical properties, including thermal stability, acidity, and dissolution ability, etc., were investigated, and the results are shown in Table 1. [Ch][Ace] and [Ch][CF3CO2] are liquids at room temperature, and their melting points (Tm) are −11 and −15 °C, respectively. The rest are all solids, and analysis with differential scanning calorimetry (DSC) revealed that the glass transition temperature (Tg) of [Ch][H2PO4], [Ch][Lev], and [Ch][MeSO3] are 48, 56, and 96 °C, respectively.32 The structures of choline-derived ILs have significant impact on their thermal stability and acidity.33 Thermal gravimetric analysis (TGA) (Figure 3) was used to determine the decomposition temperature (Td), and the results show that the choline-derived IL with [MeSO3] anion has higher thermal stability than all the others, and its decomposition occurs at 267 °C. Acidities of the choline-derived ILs were then examined by pH meters. It was found that the acidities of these ILs decreased in the following sequence: [Ch][MeSO3] > [Ch][CF3CO2] > [Ch][H2PO4] > [Ch][Lev] > [Ch][Ace]. The capacity of solvents to dissolve lignin was considered to be a key factor in determining its efficiency in the depolymerization of lignin.34 We therefore examined the solubilities of Kraft lignin that was isolated from Kraft pulp in the choline-derived ILs. In this study, [Ch][MeSO3] clearly stood out, dissolving lignin with the highest amount at 120 °C (ca. 80 mg g−1), with [Ch][Lev] in a distant second place (ca. 64 mg g−1). The above results manifested that strong acidity, good thermal stability, and excellent lignin solubility make [Ch][MeSO3] an ideal choice of solvent for the catalytic conversion of lignin. The choline-derived ILs were then used together with Pd/C to catalyze the hydrogenolysis of lignin. Because industrial lignin contains some metal species that may interfere with our investigation, we therefore treated the lignin by acid before use.35 The reaction for lignin conversion was performed at 200 °C, with 2 MPa H2, and with 3.5 wt % of Pd/C catalyst. The mass ratio of lignin to ILs was fixed tentatively at 1:1. A gas−
Figure 1. One-pot production of phenols from lignin.
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EXPERIMENTAL SECTION
Materials. All commercial chemicals were analytical reagents and were used without further purification. Methanesulfonic acid, phosphoric acid, trifluoroacetic acid, acetic acid, levulinic acid, and AcOEt were purchased from Tianjin Kemel Chemical Reagent Co., Ltd. Choline chloride was purchased from Acros Organic. Pd/C was purchased from Energy Chemical, and it contains 5 wt % of Pd. All choline-derived ionic liquids were synthesized according to previous literature reports.31 For example, [Ch][MeSO3] was prepared by reaction of choline hydroxide with an equimolar amount of methanesulfonic acid. To keep the water content of the IL as low as possible, the IL was kept under vacuum at 100 °C for 4 h before testing and using it. Thermal Stability of the Ch-Derived ILs. A TA Instruments SDT Q600 DSC/TGA system was used to collect TG scans. The instrument was calibrated in the temperature range 30−400 °C. Experiments were carried out in alumina crucibles at the temperature range from 30 to 400 °C with heating rate of 10 °C/min under an argon atmosphere (100 mL/min). Solubility of Kraft Lignin. To determine lignin solubility, a 10 mg sample was added to a glass vial containing 3 g of IL at 120 °C with stirring, and its solubility was visually checked. If the solution was clear, 10 or 15 mg of sample was added. The solubility was calculated when the solution remained heterogeneous within 12 h. Typical Procedure for Acid-Treated Lignin. Industrial lignin in powder form from Shandong Longlive Biotechnology Co., Ltd. (Shandong, China) was used. Demineralization was carried out by treating the industrial lignin with acidified water (pH = 1) for 2 h, in a proportion of 20 mL of solution per gram of sample, followed by washing extensively with deionized water until neutral pH in washing 3851
DOI: 10.1021/acssuschemeng.6b00620 ACS Sustainable Chem. Eng. 2016, 4, 3850−3856
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Main Physicochemical Properties, Acidities, and Dissolution Abilities of Choline-Derived ILs
a
entry
Ch-derived ILs
Td (°C)
1 2 3 4 5
[Ch][CFCO2] [Ch][H2PO4] [Ch][Ace] [Ch][Lev] [Ch][MeSO3]
161 201 176 199 267
Tg (°C)
Tm (°C)
pHa (0.1 g/mL)
dissolved ligninb (mg g−1)
−15
1.34 3.40 5.45 3.73 0.95
13 2), the yield to monophenols decreased significantly although the starting material, lignin, was consumed up to 50%; concurrently, more gas phase products were formed due to overhydrogenolysis. On the contrary, a smaller amount of IL (the mass ratio
