Self-Oxidation of Lignin to Aromatic Acids with High Selectivity

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Self-Oxidation of Lignin to Aromatic Acids with High Selectivity Catalyzed by Designed Acidic Mesoporous Molecular Sieves Incorporating Heteroatoms Lu Li, Shitao Yu, Congxia Xie, Fusheng Liu, Shiwei Liu, Kun Li, and Zhiyong Ding ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01758 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 13, 2016

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ACS Sustainable Chemistry & Engineering

CLE

Self-Oxidation of Lignin to Aromatic Acids with High Selectivity Catalyzed by Designed Acidic Mesoporous Molecular Sieves Incorporating Heteroatoms Lu Li† *, Shitao Yu † *, Congxia Xie ‡, Fusheng Liu†, Shiwei Liu†, Kun Li†, Zhiyong Dong†

†

College of Chemical Engineering, Qingdao University of Science and Technology, 53

Zhengzhou Road, Qingdao 266042, P. R. China. Fax: +86 0532 84022719; Tel: +86 0532 84022864; E-mail: [email protected]; [email protected]

‡

Key Laboratory of Eco-Chemical Engineering, Ministry of Education, College of Chemistry

and Molecular Engineering, Qingdao University of Science and Technology, 53 Zhengzhou Road, Qingdao 266042, P. R. China

KEYWORDS: ionic liquids, biomass, catalysis, lignin, self-oxidation

ABSTRACT: An economic method for the synthesis of crystalline mesoporous sieves and the self-oxidation of alkali lignin (INDULIN AT) to aromatic acids is reported herein. Crystalline mesoporous molecular sieves ILM-Al have been prepared in strongly acidic ionic liquid (IL) media, and used as catalysts for the self-oxidation of alkali lignin to aromatic acids. In the

ACS Paragon Plus Environment


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preparation of the new type of mesoporous molecular sieves, the imidazolium-based acidic functional IL system served as both solvent and structure-directing agent. The ILM-Al exhibited higher catalytic performance and selectivity for aromatic acids in the self-oxidation of lignin than in that of fossil fuels, and the main products were homoveratric acid and homovanillic acid. The catalyst could be recycled and reused with negligible loss in activity over five cycles, and the IL could also be recycled for further use. Moreover, Zr and Sn were also successfully incorporated into the framework of ordered crystalline mesoporous materials by the same simple method. The obtained ILM-Zr and ILM-Sn also showed excellent catalytic activity for the self-oxidation of alkali lignin.

INTRODUCTION

Aromatic acids have been reported to exert multiple biological and pharmacological activities, owing to their role as antioxidants and their implication in the prevention of pathologies such as cardiovascular disease, cancer, and inflammatory disorders.1,2 These aromatic acids are usually synthesized by direct oxidation of coal tar using KMnO4, CrO3, Na2Cr2O7, PbO2, FeCl3, etc.3,4 The traditional operation has many disadvantages, such as high requirement of the amount of catalyst, serious corrosion to equipment, complicated separation process, environment problems, and other by-products. In view of these factors, there has long been a need to develop an environmental benign process for the preparation of aromatic acids. Lignin, a main constituent of


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lignocellulosic biomass, has been proven to have the potential to produce aromatic chemicals and biofuels by means of various strategies, such as hydrogenation, oxidation, and pyrolysis.5-7 Moreover, lignin is an unstable mixture of oxygenated aromatic molecules, and its pyrolysis should release abundant oxygen element.8 If the freed oxygen could be used to self-oxidize the fraction of lignin pyrolysis, aromatic acids would be generated during the process. Not only does this process not require extra oxygen, but aromatic acids would be produced in one step during the pyrolysis. To achieve this aim, an effective catalyst must be obtained.

Porous materials are of great interest because of their specific structures and applications.9-11 This is especially true for mesoporous materials, which offer an opportunity to exploit silicate as a versatile catalyst and catalyst support for conversion of large molecules, which has stemmed from the discovery of MCM-41.12 Porous materials are also utilized for the pyrolysis of lignin.13,14 However, the poor hydrothermal stability and low catalytic activity of these materials have greatly limited their extensive use in the oxidation of the fraction from lignin pyrolysis.15 There is no doubt that the development of new protocols to synthesize novel mesoporous materials with high hydrothermal stability and catalytic activity is highly desirable, but remain challenging.16-19 In traditional hydrothermal systems, the hydrolysis rate of Si-O bonds cannot match those of heteroatoms that provide the active sites in molecular sieves, and as a result it is difficult to introduce heteroatoms into the skeletons of mesoporous materials.20 On the other hand, work on the assembly of nanoclusters has greatly progressed in recent years21,22 and



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several types of aluminosilicate nanoclusters (zeolite primary structure units and aggregates) have been reported.23-26 This method can be used to introduce heteroatoms into the mesoporous framework, create crystalline pore walls, and improve the hydrothermal stability of the material obtained. However, the method is rather tedious, especially when starting from zeolite seeds, and requires expensive tetraethylammonium hydroxide (TEAOH). In addition, the synthetic process inevitably requires the use of an autoclave and is difficult to control, which greatly limits its application. Therefore, it is essential to devise a new system for the preparation of highly efficient and hydrothermally stable crystalline mesoporous materials incorporating heteroatoms.

Ionic liquids (ILs) are a unique kind of solvents that have virtually no vapor pressure and possess versatile solvent properties.27-32 Recently, they have been effectively demonstrated as superior solvents for conducting many organic reactions.33-36 In 2004, Cooper first prepared aluminophosphate zeolite analogues by using an imidazolium-based ionic liquid that served as both solvent and template, and thereby obtained four zeotype frameworks under different experimental conditions.37 More recently, ionic liquids have attracted great attention for their broad applications in the synthesis of mesoporous materials incorporating heteroatoms.38-43 The results suggested that ionic liquids could provide an excellent environment to promote the selfassembly of heteroatom-containing units to form mesoporous materials. However, the synthesis of silicon and heteroatom-based mesoporous materials was rarely studied in ionic liquids.


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In this work, we demonstrate that in a strong acidic ionic liquid system, Al, as a heteroatom, could be effectively introduced into ordered mesoporous silica, giving a new material that we designate as ILM-Al. Besides easy to synthesize, the mesostructure is mostly built-up of zeolitelike sites, which imparts the material with very high catalytic activity and excellent hydrothermal stability. The material proved to be very active, selective, and stable for the self-oxidation of lignin to produce aromatic acids due to its specific structure and morphology. In addition, active metals, such as Sn or Zr, could also be incorporated into the new ordered mesoporous silica materials. The catalytic activities of the resulting materials have also been investigated. Furthermore, the new type of catalysts also show excellent catalytic performance in the selfoxidation of raw lignins, such as nut, pine, willow, and bamboo lignins, which were obtained by self-extraction.

EXPERIMENTAL

Materials

Alkali lignin (INDULIN AT) was purchased from Sigma-Aldrich China (Shanghai, China, CAS No. 8068-05-1) as a free-flowing brown powder and its molecular weight (Mw) distribution was determined by Gel-permeation chromatography (GPC) (see the ESI†). Elemental analysis showed it have a composition of 47.6% C, 4.8% H, 24.5% O, 0.9% Na, 0.1% N, 4.0% S, and 18.1% ash. Nut lignin (67.1% C, 5.7% H, 27.1% O), pine lignin (63.6% C, 9.0% H, 27.4% O),



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willow lignin (62.7% C, 10.0% H, 27.3% O), and bamboo lignin (62.8% C, 7.5% H, 29.7% O) were obtained by self-extraction according to the related enzyme method.21 PEO20PPO70PEO20 (Pluronic P123, (Polythylene oxide)20(Polypropylene oxide)70(Polythylene oxide)20), Na2SiO3, Na2AlO2, ZrOCl2, SnCl4, TEAOH aqueous solution (20%), fumed silica, and HCl were all purchased from Aldrich and directly used without further purification. ZSM-5 (molar ratio Si/Al = 60) and SBA-15 (molar ratio Si/Al > 1000) were purchased from Nanjing JC NANO Tech Co., Ltd. 1-Methyl-3-(3-sulfopropyl)imidazolium hydrogensulfate ([HSO3-pmim][HSO4], purity >95%), 1-methyl-3-(3-sulfopropyl)imidazolium dihydrogenphosphate ([HSO3pmim][H2PO4], purity >95%), 1-methyl-3-(3-sulfopropyl)imidazolium acetate ([HSO3pmim][CH3COO], purity >95%), and MAS-7 were prepared in our laboratory (see the ESI†).

Typical synthesis of ILM-Al

NaAlO2 (0.30 g) and fumed silica (4.8 g) were added to [HSO3-pmim]A (10 g) (A = [HSO4], [H2PO4], [CH3COO]) under stirring (Al2O3/SiO2/Na2O/[HSO3-pmim]A molar ratio of 1.0/60/1.0/18). The mixture was then transferred to a round-bottomed flask for 4 h at 140 ℃ to obtain a clear solution. P123 (0.8 g) was then added to the solution obtained. The mixture was stirred at 40℃ for 20 h, and then transferred to a round-bottomed flask for additional reaction at 100 ℃for 24 h. The mixture was filtered to separate the ILM-Al from the liquid phase. The ILMAl was washed three times with deionized water, dried in an oven at 60℃ overnight, and


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calcined at 550℃ for 5 h to remove the template before being used as a catalyst in the selfoxidation of lignin. The liquid fraction was collected and distilled to recover the ionic liquid.

Typical synthesis of ILM-Zr, ILM-Sn or ILM-Si

ILM-Zr and ILM-Sn were prepared under the same conditions as ILM-Al, except that NaAlO2 was replaced by ZrOCl2, SnCl4 or Na2SiO3, respectively. The molar ratio was the same as in the [HSO3-pmim][HSO4] system.

Characterization

X-ray diffraction patterns were obtained with a Siemens D5005 diffractometer using Cu-Kα radiation. Transmission electron microscope (TEM) images were obtained on a Philips CM200FEG with an acceleration voltage of 200 kV. Nitrogen adsorption isotherms were measured at the temperature of liquid nitrogen using a Micromeritics ASAP 2010 system. The samples were outgassed for 10 h at 300 ℃ before the measurements. The pore-size distribution was calculated using the Barrett−Joyner−Halenda (BJH) model. 27Al and 29Si NMR spectra were recorded on a Bruker MSL-300WB spectrometer, and chemical shifts were referenced to Al(H2O)63+ or (SiO4)4-. Scanning electron microscope (SEM) images were recorded on a JEOL S-4800 with an acceleration voltage of 200 kV. Energy-dispersive spectrometry (EDS) images and data were recorded with a JEOL JSM-6010. NH3-TPD data were obtained with a DLUT-1 automatic temperature-programmed desorption apparatus. Before adsorption, the sample was



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thoroughly dried. After adsorption, it was purged with Ar at 100℃ to desorb any physisorbed ammonia. ICP-MS was performed with an Agilent 7700 series ICP mass spectrometer (Agilent Technologies, Palo Alto, CA, USA). A 1.5 mm diameter injector and a 0.3 mm diameter tube line were applied. The refrigerated section was kept at -5 ℃ (G1879B heat exchanger). Oxygen was added at the injector entrance to minimize carbon formation in the plasma. The system was cleaned overnight by a flow of pure THF. Heteronuclear single quantum correlation 2D NMR spectra were recorded on a Bruker AV 500 MHz spectrometer at 303 K with an inverse gated decoupling pulse sequence. The products prepared for 2D NMR analysis is as follows, approximately 80 mg of the sample and 0.8 mg internal standard of 1,3,5-trioxane were dissolved in 600 µL of dimethyl sulfoxide (DMSO).

Recycling of ionic liquids

After the sieve materials had been filtered off, the filtrate was centrifuged to remove any remaining small solid particles. Water was removed from the ionic liquid by rotary evaporation. The recycled ionic liquid was reused directly without further purification.

Self-oxidation of lignin

Self-oxidation of lignin(Scheme 1) was carried out under the protection of N2 (0.3 cd3/s) at 160 ℃ in a 50 mL four-necked flask equipped with a thermometer, a reflux condenser, a mechanical stirrer, and a nitrogen port. The reactor was charged with lignin (W1), solvent (W2),


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and catalyst (W3) and purged with a continuous stream of N2. The mixture was then heated to 160 ℃ by an external electrical resistance and self-oxidation of lignin was allowed to proceed for 1 h. When the reaction was complete, the flask was cooled to ambient temperature. The mixture in the flask was poured out, and the flask was washed with ethanol three times. The combined reaction mixture and washings were filtered to separate the solid residue, consisting of coke, oligomers, and catalyst, from the liquid phase. The solid residue was washed with ethanol three times, and then dried in an oven at 60 ℃ overnight before weighing for conversion calculation; the weight of the residue was designated as W4. The liquid fractions were collected and analyzed by GC-MS (Agilent 7890A-Agilent 5975C VL MSD, HP-5, helium as carrier gas, flow rate 1.8 mL/min; analysis conditions: column held at 100 ℃ for 1 min and then heated to 240 ℃ at a rate of 20 ℃/min; injector temperature 250℃) using the internal standard method (αpinene as internal standard). The process is as follows: 1mL α-pinene was mixed with 5mL products, and then 5µL mixture was injected into GC-MS to test the preliminary structure. Based on the results of GC-MS, the related standard samples will be used to test, thus the main product can be further identified. Because the gaseous products were too little to be cooled and collected, the relevant conversion could only be obtained by mass balance (see the ESI†). The pyrolysis conversion (wt %) for lignin was expressed as follows:

Conversion

/% =

W1 + W 3 − W W1

4

× 100 %


（1）


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The yield of the main liquid product was calculated according to the GC results and the conversion. The reaction was performed in triplicate to determine the accuracy of the results.

Scheme 1. Self-oxidation of lignin.

Stability measurement

The reusability of the catalyst was studied by using the recovered catalyst in consecutive reaction cycles. After each use, the catalyst was washed with ethanol and calcined in a muffle furnace at 550 ℃ for 3 h.

RESULTS AND DISCUSSION

Composition of products

Alkali lignin (INDULIN AT) was oxidized in the presence of the new catalyst ILM-Al (reaction conditions: 12 mL ethylene glycol as solvent, lignin 1 g, catalyst 0.1 g, reaction temperature 160℃, reaction time 1 h).

Lignin

has different C-O linkages, specifically β-O-4’, β-5, 4-O-5, dibenzodioxocin, and β-β linkages.


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More than half of the linkages in various kinds of lignin species are β-O-4’ linkages, cleavage of which will lead to low molecular weight aromatic products, and the relevant structural units are depicted in the Supporting Information (ESI†, Fig. S2).44, 45 To analyze the main reaction products, GC-MS was used. The GC-MS trace in Figure 1 shows that more than 20 liquid products were generated. To assess the mechanism, we focused on the principal products. Three major products were identified, namely homoveratric acid (retention time 12.269 min), homovanillic acid (retention time 14.071 min), and guaiacol (retention time 6.060 min). Furthermore, the contents of the two aromatic acids (Scheme 2) were seen to be greater than that of guaiacol. According to the mechanism of the biodegradation of lignin under acidic conditions, aromatic aldehydes should be the major products.46, 47 However; aromatic acids were the major products in the liquefaction reaction catalyzed by ILM-Al under N2 protection. It is well known that lignin is an unstable mixture of oxygenated aromatic molecules.8 Therefore; its pyrolysis under acidic conditions should release abundant oxygen element, which can then oxidize aromatic aldehydes to aromatic acids. To verify the decomposition reactions of the dominant linkages, such as β-O-4’ and 4-O-5, 2D HSQC NMR was used to test the raw lignin and liquid products. Lignin spectra can be considered as being composed of three parts, namely aromatic, aliphatic, and side-chain unit regions.48 Since the major products were aromatic derivatives, we focused on the aromatic and side-chain unit regions of alkali lignin and the liquid products, and the results are presented in Figure 2. It can be seen that most of the correlation signals for the



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alkali lignin structure (A-α, δ70/δ4.7 ppm; A-β, δ82/δ4.5 ppm; B-α, δ86/δ4.1 ppm; C-α, δ85/δ4.8 ppm) could not be shown, which meant that the alkali lignin structure were broken and the strength of correlation signals were weak. 45 The correlation of β-O-4’ appeared at δ60/δ4.6 ppm and that of C-β at δ57/δ3.4 ppm; the intensities of these signals were clearly diminished in the liquid product compared to the alkali lignin (Figure 2, A and C). This suggested the cleavage of ether bonds, resulting in low molecular weight of the liquid product.49 For the aromatic regions in the spectra of the raw lignin and liquid product (Figure 2, B and D), there is a distinct difference in the peak positions. Typically, the peaks due to aromatic protons appear at about δ136/δ7.7 ppm in the product spectrum, which are not seen in the alkali lignin spectrum, which means that new species are generated during the reaction.45 From the above results, we can deduce the reaction process. Under strongly acidic conditions, lignin (an SEM image of raw lignin is shown in the ESI†, Fig. S3) is firstly depolymerized to low molecular weight aromatic derivatives, whereupon its linkages, such as β-O-4’ or C-β, are cleaved with the release of oxygen element (Scheme 1). The low molecular weight aromatic derivatives can enter the pores of ILM-Al, whereupon free oxygen element could oxidize them to produce aromatic acids according to the good shape selectivity imposed by the well-ordered hexagonal arrays of the catalyst (Figure 7). Therefore, lignin can be self-oxidized to aromatic acids by the acid catalyst.


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Figure 1. GC-MS of liquid product.

Figure 2. 2D HSQC NMR HSQC of alkali lignin (A and B) and liquid product (C and D).

A

B

Scheme 2. Structures of homoveratric acid (A) and homovanillic acid (B).

Choice of catalyst



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The activities of different catalysts were investigated in the self-oxidation of lignin. The performance of the catalysts was evaluated in terms of the conversion of lignin and the yield of the main product. The detailed results are collected in Table 1. From Table 1, it can be seen that the novel mesoporous molecular sieve-based ILM-Al showed excellent catalytic activity and the conversion of lignin reached 60%, which was not only higher than that achieved with traditional catalysts, such as H2SO4 or ZSM-5, but also higher than that typically achieved with mesoporous molecular sieves, such as SBA-15 and MAS-7. Moreover, ILM-Al gave excellent yields of aromatic acids, amounting to more than 40%, most notably homoveratric acid and homovanillic acid, especially the former. Table 1. Self-oxidation of lignin by different catalysts.

Reaction conditions: 12 mL ethylene glycol as solvent, lignin 1 g, catalyst 0.1 g, reaction temperature 160℃, reaction time 1 h; the Si/Al molar ratios of ZSM-5, SBA-15, MAS-7, and ILM-Al are 60, >1000, 30, and 30, respectively.

For the self-oxidation of lignin, the acidity of the catalyst is an important influencing factor. Therefore, we used 27Al NMR to detect the state of the Al atoms in ILM-Al. The 27Al NMR spectrum of ILM-Al (Figure 3) features a signal at δ = 58 ppm attributable to four-coordinated Al. The results show that almost all Al species are incorporated in the framework of ILM-Al. Notably, the peak due to tetrahedral aluminum in ILM-Al (calcined) appears at δ = 58 ppm,


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similar to that in zeolite beta, δ ≈ 60 ppm,16,17 which suggests that the environment of aluminum in ILM-Al is very similar to that in the beta crystal. According to the EDS spectrum of ILM-Al (Figure 3), the amount of Al therein was about 4.89% (4.78% by ICP-MS), which further confirmed that Al had been introduced into the new material. 29Si NMR (Figure 4) was also used to detect the coordination state of Si species in the new material. From Figure 4, it can be seen that the spectrum featured a single strong and broad signal at δ=–86.4 ppm, which can be assigned to Si(ⅣAl), indicating incorporation of Al into the pore walls50. The above results show that the new approach efficiently introduced heteroatoms into the framework of the mesoporous material. Additionally, the NH3-TPD curves on MAS-7 and ILM-Al (Figure 5), with similar Si/Al ratios in the raw materials, suggest that the number of acid sites on ILM-Al is much higher than that of MAS-7, especially the moderately and strongly acidic sites, as well as the total number of acidic sites.51 The results to some extent imply that there is a higher content of Al in ILM-Al than that in MAS-7.52-53 Under the same conditions, the TOF of ILM-Al (5.2 × 10-4 s-1) is not significantly different from that of MAS-7 (6.2 × 10-4 s-1);23-25 however, in terms of the yield of target product, ILM-Al is better than MAS-7. These results suggest that the activity of the catalyst was not only determined by the acid, but also by the structure of the framework.



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Figure 3. 27Al NMR spectrum (A) and EDS (B) of calcined ILM-Al.

Figure 4. 29Si NMR spectrum of calcined ILM-Al.

Figure 5. NH3-TPD curves of different samples.


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Figure 6. XRD of ILM-Al.

Figure 7. SEM image (A) and TEM image (B) of ILM-Al.

On the other hand, the structure of ILM-Al is also an important factor for its catalytic properties. Our study by X-ray diffraction (XRD) analysis (Figure 6), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) (Figure 7) indicated that in the strongly acidic ionic liquid [HSO3-pmim][HSO4] (1-methyl-3-(3-sulfopropyl)imidazolium hydrogensulfate) medium (pH < 0), Al heteroatoms were successfully introduced into the



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mesoporous structure to produce a new crystalline mesoporous material incorporating heteroatoms. The processes are as follows. First, an Al source, such as NaAlO2, and a silica source, such as fumed silica, are combined in [HSO3-pmim][HSO4] in a suitable ratio. The aim of this process is to obtain zeolite nanoclusters. [HSO3-pmim][HSO4] is not a solvent, but displays a significantly stronger tendency for self-aggregation and supramolecular templating, owing to the distinct polarizability of its head groups and the special high-concentration phases of such ILs.38-40 After a period of crystallization, the triblock copolymer P123 was introduced into the [HSO3-pmim][HSO4] medium to afford the ILM-Al. The XRD patterns in the low-angle region (0.4−60 )and the high-angle region (10−500 (Figure 3))clearly indicate that ILM-Al has well-ordered hexagonal arrays of mesopores of uniform size,12 and the distinct peak in the wideangle region of the XRD pattern (10−500 )for ILM-Al suggests that a five-membered ring zeolite structure was formed in the [HSO3-pmim][HSO4] acidic ionic liquid system. Owing to its ionic character, it is a polar solvent suitable for dissolution of the inorganic precursors; likewise, it can effectively adjust the hydrolysis and polycondensation of Si and other heteroatoms, stabilize a particular oxidation state, and enhance the synergistic interaction between inorganic sources, thereby improving the formation of zeolite structure.52,53 At the same time, the ionic liquid can disperse and stabilize aluminosilicate nanoclusters, thereby increasing the interaction between the inorganic source and the organic template. It is very interesting to note that the d (100) value and wall thickness of ILM-Al are larger than those of traditional mesoporous materials such as


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SBA-15 and MAS-7 prepared under hydrothermal conditions.8 Comparing the results of the N2 adsorption isotherms (ESI†, Fig. S4), it is clear that the zeolite primary structure units have greater rigidity and larger volume in the new, strongly acidic ionic liquid system as compared to those in conventional hydrothermal systems.23 The features of ILM-Al were observed by SEM (A) and TEM (B) (Figure 7). The SEM image shows that the sample was of spherical structure and dispersion. The morphology of the material was directly related to its internal microstructure and the formation of its ordered mesoporous structure, such that the regular spherical structure reflects the ordered internal structure of the new material.54 From the TEM image, ILM-Al is seen to consist of well-ordered hexagonal arrays of mesopores of uniform size. The results further confirmed that a new mesoporous material with a crystalline structure had been synthesized in the [HSO3-pmim] [HSO4] acidic ionic liquid system. The well-ordered hexagonal arrays of mesopores and crystalline structure of ILM-Al seemingly improved its selectivity for large molecular substrates.

Choice of solvent

Table 2. The effect of solvent on the self-oxidation of lignin.



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Reaction conditions: solvent 12 mL, lignin 1 g, ILM-Al 0.1 g, reaction temperature 160 oC, reaction time 1 h.

For the self-oxidation of lignin, the solvent is also an important factor in determining the outcome of the reaction. Therefore, the effects of six different solvents were studied and the results are collected in Table 2. From Table 2, it can be seen that the selectivity in favour of the target product was almost the same in the different solvents, implying that the selectivity for the product was mainly determined by the catalyst. However, for the conversion of lignin, the nature of the solvent was an important factor. When ethylene glycol was used as the solvent, the conversion of lignin reached 60%, suggesting that ethylene glycol can provide a proton to accelerate cleavage of the ether bonds.55 Therefore; we chose ethylene glycol as the solvent in our subsequent research.

Synthetic conditions for obtaining ILM-Al

The synthetic conditions directly affect the structure and acidity of the designed mesoporous molecular sieves. Therefore, different types of ionic liquids, varying in the anion, were studied for the production of ILM-Al and the results are collected in Table 3. From Table 3, it can be seen that the conversion of lignin and selectivity for homoveratric acid varied with different


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anions of the ILs (entries 1−3). With [HSO4] as the anion, the obtained mesoporous catalyst ILM-Al showed the highest catalytic activity, giving a lignin conversion of 60% and a selectivity for homoveratric acid of 45.7% (entry 1). With [H2PO4] or [CH3COO] as the anion, inferior results were obtained, not only in terms of the conversion of lignin, but more especially in the selectivity for homoveratric acid (entries 2 and 3). The type of anion may have affected the structure or acidity of the obtained mesoporous catalysts, and these were probed by XRD and NH3-TPD analyses. From the XRD patterns (Figure 8), the obtained materials were all found to be mesoporous.12 However, according to the intensity and width of the diffraction peak at about 2θ=2°, the nature of the anions greatly affected the crystallinity and long-range order.12 With [HSO4] as the anion, the diffraction peak of the synthetic mesoporous ILM-Al was the strongest and sharpest, showing this ILM-Al to have the best crystallinity and long-range order.12 Furthermore, ILM-Al ([HSO4] as anion) showed excellent shape selectivity for large molecular substrates, consistent with the results of the reaction. The number of acid sites and acid strength distribution were obtained from the NH3-TPD traces shown in Figure 9. The ILM-Al from ILs with different anions exhibited low acid strength. Among the respective ILM-Al samples, the number of acid sites on ILM-Al ([HSO4] as anion) was the highest. This result was consistent with the high activity of this ILM-Al. These results indicate that the type of anion of the IL plays a key role in the crystallite formation and the synergistic interaction between inorganic sources, which would affect the catalytic activity of the ILM-Al. In addition, TEAOH, a traditional



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template for the synthesis of microporous molecular sieves under hydrothermal conditions, was used as a template to prepare ILM-Al in this system, and the XRD pattern of the material in the low-angle region (0.4−10o) can be found in the ESI†, Fig. S5. From the pattern in Fig. S5, it can be seen that the material obtained using TEAOH as microporous template had the same typical mesoporous structure as the material prepared without it. The catalytic properties of the material obtained using TEAOH as template were also studied (Table 3, entry 4), and proved to be inferior, not only in terms of the conversion of lignin, but also in the selectivity for homoveratic acid. To further probe this issue, NH3-TPD of the catalysts prepared with and without TEAOH was studied, and the results are presented in Figure 10. According to Figure 10, the number of acid sites on ILM-Al prepared without TEAOH was obviously greater than that on ILM-Al prepared with TEAOH; the TOF of the former was 5.2 × 10-4 s-1, as compared to just 2.9 × 10-4s-1 for the latter. It may be that the addition of TEAOH as a strong base to the strongly acidic preparation system of mesoporous molecular sieves affects the hydrolysis rate of the metal heteroatoms, making them less likely to enter the framework of the obtained mesoporous material.52-53 Therefore, we selected ILM-Al from the [HSO3-pmim][HSO4] IL system for our subsequent research. Table 3. Effect of the synthetic system on the efficacy of the obtained ILM-Al in the selfoxidation of lignin.


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Reaction conditions: 12 mL ethylene glycol as solvent, lignin 1 g, ILM-Al 0.1 g, reaction temperature 160oC, reaction time 1 h. [a] ILM-Al was synthesized in the [HSO3-pmim][HSO4] system using 20 mL TEAOH (25 wt%) as a template.

Figure 8. XRD patterns of ILM-Al samples from different IL systems.

Figure 9. NH3-TPD traces of ILM-Al samples from different IL systems.

Figure 10. NH3-TPD traces of ILM-Al samples obtained under different synthetic conditions. Catalytic stability of ILM-Al



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Figure 11. Reusability of the ILM-Al catalyst. Reaction conditions: 12 mL ethylene glycol as solvent, lignin 1 g, ILM-Al 0.1 g, reaction temperature 160oC, reaction time 1 h.

Figure 12. XRD pattern of an ILM-Al sample after its fivefold use as a catalyst.

Figure 13. NH3-TPD trace of an ILM-Al sample after its fivefold use as a catalyst. Recycling of the catalyst is a crucial aspect as it brings down the cost of self-oxidation. Thus, the stability of the catalyst was studied and the results are shown in Figure 11. The catalyst could be reused without further treatment after each reaction. The conversion of lignin still reached


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60% and the selectivity for homoveratric acid remained at 45% even after five successive cycles of reuse. Moreover, after the fifth use, the ILM-Al still retained an excellent mesoporous structure (see Figure 12)12 and a large number of acid sites (see Figure 13),56 implying that atomic Al still existed in its framework. Compared with the fresh ILM-Al, the used ILM-Al had more strongly acidic sites and fewer moderately acidic sites. This phenomenon may be attributed to the recycling methodology. After reaction, the catalyst was recycled by calcination, which may have improved its surface activity.56 Thus; the catalyst could be recycled and reused with negligible loss of activity. From the above analysis, a new type of highly stable mesoporous material with a zeolite structure and a high content of heteroatoms was synthesized in the [HSO3pmim][HSO4] acidic ionic liquid system.

Importantly, after the ILM-Al had been filtered off, about 90% of the mass of ionic liquid used in the preparation could be recovered, and then successfully used in the preparation of further ILM-Al, which was characterized by XRD (see the ESI†, Fig. S6). The results indicated that the ionic liquid used in the preparation process could be easily recycled. Therefore, the new mesoporous material is not only highly stable, but also inexpensive.

Catalytic activity of ILM-Al towards different lignins

Table 4. Self-oxidation of different lignins by ILM-Al.



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Reaction conditions: 12 mL ethylene glycol as solvent, lignin 1 g, catalyst 0.1 g, reaction temperature 160oC,reaction time1h. Ⅳ

Natural lignin samples were self-oxidized by ILM-Al, as indicated in Table 4. Natural lignins from nut, pine, willow, and bamboo were extracted in our laboratory. In this self-oxidation reaction system, ILM-Al displayed excellent catalytic activity. From Table 4, it can be seen that the conversions of all lignin samples exceeded 55%, and the yields of liquid products exceeded 30%. Therefore, our system represents a low-cost and practical platform for the efficient utilization of natural lignins.

Catalytic activities of different metal-doped catalysts

Table 5. Self-oxidation of lignin by different metal-doped mesoporous materials.

Reaction conditions: 12 mL ethylene glycol as solvent, lignin 1 g, catalyst 0.1 g, reaction temperature 160 oC, reaction time 1 h. [a] The molar ratios Si/A (A=Al, Zr, Sn, Si) were the same when the materials were prepared, but the actual molar ratios in the obtained materials were different, as detected by EDS (ESI†, Figs. S8 and S10; Tables S1 and S2).

By the same synthetic method, Zr and Sn were successfully introduced into the framework of the new mesoporous material, and the products were designated as ILM-Zr and ILM-Sn,


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respectively. ILM-Zr and ILM-Sn retained the mesoporous structure, as characterized by XRD (ESI†, Fig. S7 and S9) 9. Although Zr and Sn were also introduced into the new ordered mesoporous material, the different atomic diameters of Al (0.53 Å), Zr (0.72 Å), and Sn (0.69 Å) resulted in different amounts of the respective atoms being incorporated in the framework. According to the EDS results (ESI†, Figs. S8 and S10, Tables S1 and S2), the molar ratio of Si/Zr was 39.6 and that of Si/Sn was 27.3, as compared to 6.3 for Si/Al. ICP-MS was also used to detect the contents of Al, Zr, and Sn, and gave molar ratios of 38.1, 28.4, and 6.8 for Si/Zr, Si/Sn, and Si/Al, respectively, in good agreement with those obtained by EDS. It is suggested that because the atomic diameter of Al (0.53 Å) is similar to that of Si (0.4 Å), Al can be easily introduced into the framework of the new mesoporous material.15 The catalytic activities of these materials were also investigated in the self-oxidation of lignin, and the results are shown in Table 5. Fortunately, ILM-Zr and ILM-Sn showed the same excellent catalytic activity as ILMAl, even though the contents of Zr or Sn were lower. To explain the phenomena, the acidity of these catalysts was tested by NH3-TPD (see Figure 14). From the NH3-TPD traces shown in Figure 14, it can be seen that the amounts of weakly, moderately, and strongly acidic sites on the three catalysts (ILM-Zr, ILM-Sn, and ILM-Al) were slightly different, but the total numbers of acidic sites were similar.33 The results indicate that the type of active metal plays a key role in the catalytic activity through its synergistic action. To further understand this question, the pure silicon ILM-Si characterized by XRD (ESI†,Fig. S11) was also suggested. From the results



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(Table5 and Figure 14), we can see that ILM-Si without active metal atom is of low acidity, therefore ILM-Si is of poor catalytic activity.

Figure 14. NH3-TPD traces for ILM-Al, ILM-Zr, ILM-Sn and ILM-Si. CONCLUSIONS

In summary, the results presented in this work have shown that aromatic acids, especially homoveratric acid, could be obtained by the self-oxidation of lignin using ILM-Al (Sn, Zr) crystalline mesoporous molecular sieves incorporating heteroatoms as a catalyst. Importantly, the new catalysts obtained in strongly acidic ionic liquid media consist of hexagonal arrays of mesopores and five-membered rings of zeolite structure, have a high content of heteroatoms, and can be easily separated from the product mixture. They can also be reused at least five times. Thus, ILM-Al (Sn, Zr) shows a higher catalytic performance for the self-oxidation of lignin compared to other traditional catalysts. Further studies of this self-oxidation reaction, such as on its mechanism and on the properties of the new catalysts are still needed, and are currently under investigation in our research group.


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ASSOCIATED CONTENT

○ S Supporting Information

Synthesis of ionic liquids[HSO3-pmim][HSO4], [HSO3-pmim][H2PO4], [HSO3pmim][CH3COO]; Calculation on the pyrolysis conversion of lignin by the mass balance ; Main classical and acylated substructures ; GPC and SEM of raw lignin; Synthesis of MAS-7; N2 ad/de of ILM-Al; XRD of ILM-Al using TEAOH; XRD of ILM-Al by used ILs; Characterization of ILM-Zr, ILM-Sn and ILM-Si.This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]. Tel.: +86 532 84022719. Fax: +86 532 84022719. *E-mail:

[email protected]. Tel.: +86 532 84022864. Fax: +86 532 84022719

ACKNOWLEDGEMENTS

The work described was supported by a 973 pre-basic research project (SQ2015CB040439), the Taishan Scholar Program of Shandong (ts201511033), the Natural Science Foundation of China (31570573 and 21176131), the National Training Programs of Innovation and Entrepreneurship for Undergraduates (201510426015), the Colleges and Universities in Shandong Province



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Science and Technology Plan Projects (J11LB05), and the Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (2011-18).

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CLE

TOC Self-Oxidation of Lignin to Aromatic Acids with High Selectivity Catalyzed by Designed Acidic Mesoporous Molecular Sieves Incorporating Heteroatoms

Lu Li†*, Shitao Yu †*, Congxia Xie ‡, Fusheng Liu†, Shiwei Liu†, Kun Li†, Zhiyong Dong†

The self-oxidation of alkali lignin to aromatic acids using novel acidic crystalline mesoporous sieves as catalysts.


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Self-Oxidation of Lignin to Aromatic Acids with High Selectivity

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