Catalytic Conversion of Biomass into Chemicals and Fuels over

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Catalytic conversion of biomass into chemicals and fuels over magnetic catalysts Bing Liu, and Zehui Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02094 • Publication Date (Web): 04 Dec 2015 Downloaded from http://pubs.acs.org on December 8, 2015

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165x54mm (300 x 300 DPI)

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Catalytic conversion of biomass into chemicals and fuels over magnetic catalysts Bing Liu, Zehui Zhang* Key Laboratory of Catalysis and Materials Sciences of the State Ethnic Affairs Commission & Ministry of Education, College of Chemistry and Material Science, South-Central University for Nationalities, Wuhan, 430074, P. R. China. ABSTRACT: Biomass has emerged as a potential alternative to the dwindling fossil fuel reserves. Catalytic conversion of biomass has become the main route for the transformation of biomass into a variety of commodity chemicals or liquid fuels. Currently, many reviews have well documented the use of homogeneous and heterogeneous catalysts for the catalytic transformation of biomass into chemicals and fuels, however, comparatively less attention has been paid to the use of magnetic catalysts in biomass transformation. Magnetic nanoparticles are well-designed way to bridge the gap between heterogeneous and homogeneous catalysts. The introduction of magnetic nanoparticles in a variety of solid matrices allows the combination of well-known procedures for catalyst heterogenization with techniques for magnetic separation. The present review will highlight the recent advance in the development of magnetic catalysts for the transformation of carbohydrates or carbohydrates derived chemicals into valuable chemicals and liquid fuels. 1


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KEYWORDS: Biomass conversion, Magnetic particles, Nanocatalyst, Sustainable Chemistry.

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1. INTRODUCTION Majority of essential commodity chemicals and fuels are produced from fossil fuels, which are non-renewable resources such as coal, petroleum and natural gas. The decrease in fossil fuel reserves as well as the resulting high price of petrochemicals have thus imposed the search for renewable resources as alternatives to fossil fuels resources.1-3 Biomass, as one of the most abundant renewable resources, has received a significant attention in the production of fuels, fine chemicals, and commodity materials.4–7 Carbohydrates, the major component of biomass, are considered to be ideal feedstocks for production of both biofuels and platform chemicals. For example, in 2004, the Department of Energy (DOE) of the United States identified 12 kinds of top value added chemicals valuable from carbohydrates.8 For the effective conversion of carbohydrates into chemicals and fuels, the use of an appropriate catalyst plays a crucial role in achieving high conversion and high selectivity.9-10 As far as the catalyst used for biomass conversion, homogeneous and heterogeneous catalysts are the main two types. Homogeneous catalysts, where the active sites are in the same phase as the reactants, can react very fast with the substrates and provide a good conversion rate per molecule of the catalyst, but it is difficult to recycle and reuse of the homogeneous catalysts. Heterogeneous catalysts are in solid phase with reaction occurring on the surface. Thus, heterogeneous catalysts can overcome the difficulty in the recycling of homogeneous catalysts, however, the tedious recovery procedure via filtration or centrifugation and the inevitable loss of solid catalysts in the separation

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process still limited their application.11-16 To overcome these issues of homogenous and heterogeneous catalysts, the use of magnetic catalysts appears to be the most logical solution. Magnetic catalysts can be simply and efficiently removed from reaction mixtures with an external magnetic field in a few minutes, and MNPs have emerged as ideal catalysts or supports.17-20 Many kinds of chemical reactions, especially the liquid organic chemical reactions have been performed over different kinds of magnetic catalysts,21-26 and some reviews have been well documented on the synthesis and applications of magnetic materials.27-30 However, to the best of our knowledge, there is no such a review that documents the use of magnetic nanocatalysts in biorefinery. Herein, the overall objective of this review is to offer an overview of the state of the art of the magnetic nanocatalysts in the biomass transformation into valuable chemicals and fuels from carbohydrates or biomass derived chemicals. We will highlight of the different types of reactions in biorefinery over different magnetic catalysts for the synthesis of value added chemicals and high energy density liquid fuels. 2. MAGNETIC CATALYST PREPARATION Magnetism is a class of physical phenomena that are mediated by magnetic fields. In fact, every material is influenced to some extent by a magnetic field. The most familiar effect is on permanent magnets, which have persistent magnetic moments caused by ferromagnetism. Most materials do not have permanent moments. Some are attracted to a magnetic field (paramagnetism); others are repulsed by a magnetic field

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(diamagnetism); others have a more complex relationship with an applied magnetic field (spin glass behavior and antiferromagnetism). Substances that are negligibly affected by magnetic

fields

are

known

asnon-magnetic substances.

These

include copper, aluminium, gases, and plastic. Pure oxygen exhibits magnetic properties when cooled to a liquid state. In addition, the magnetic state (or magnetic phase) of a material depends on temperature and other variables such as pressure and the applied magnetic field. A material may exhibit more than one form of magnetism as these variables change. Some Fe, Co, Ni based materials show permanent magnets, and can be collected by a permanent magnet. Thus, magnetic catalysts are mainly reported by the use of Fe, Co, Ni based materials, especially Fe based catalysts. Generally, the active sites were immobilized on the magnetic support. There are several methods for the preparation of magnetic supports, such as the co-precipitation method, the micromulsion technique, the sol − gel method, spray and laser pyrolysis, the hydrothermal reaction method and microwave irradiation.31-34 The magnetic supports mainly include metals (Fe, Co, Ni), alloys (FePt, CoPt), iron oxides (FeO, Fe2O3, Fe3O4), or spinel ferrites MFe2O4 (M = Co, Mn, Cu, Zn). Among them, magnetite (Fe3O4) is the ideal and most widely used support in catalysis because of its low cost and easy preparation. However, magnetic particles are not very stable. They are sensitive to oxidation and agglomeration due to their magnetic nature and also exhibit high chemical reactivities as well as strong magnetic dipole interactions.35-36 When oxidized, thin oxide layers generally form on the surface of the

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particles, which greatly changes their properties. Due to their high surface energy and the existence of van der Waals forces, magnetic nanoparticles (MNPs) turn to aggregation to form bulk phase, which also restricts the use of such particles in various applications. To solve this problem, modification of MNPs using suitable stabilizing ligands or coating materials such as silica, polymers, and carbon has been proved to be the best solution to date.37-40 In addition, the coating of an out layer of the magnetic particles sometimes can also give large surface area. More importantly, the surface modification of MNPs provides reaction sites or active groups for covalently or noncovalently grafting the active catalytic units onto the coated MNPs to construct magnetically recoverable catalysts. For example, triethoxysilyl-functionalized molecules such as commercially available NH2-, SH-, and Cl-terminated compounds can be directly coordinate metal catalyst or stabilize the metal nanoparticles. As far as the fields of biomass conversion, magnetic acid catalysts can catalyze the

hydrolysis

of

cellulose

into

glucose,

dehydration

of

fructose

into

5-hydroxymethylfurfural (HMF), the etherification of HMF into 5-Ethoxymethylfurfural (EMF). Magnetic metal catalysts have been used for the chemical oxidation and reduction of biomass-based chemical intermediates. In addition, magnetic base catalysts can be used for the synthesis of biodiesel via transesterification reaction. In the following part, we will highlight the recent progress in the use of magnetic catalysts in the fields of biorefinery for the synthesis of valuable chemicals and high energy density of liquid fuels.

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3.

HYDROLYSIS

OF

CELLULOSE

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AND

LIGNOCELLULOSES

OVER

MAGNETIC CATALYSTS 3.1. Hydrolysis of Cellulose and Lignocelluloses by Magnetic Acid Catalysts Table 1. The results of the hydrolysis of carbohydrates over magnetic catalysts Mol ratio of Entry

Substrate

Solvent

Catalysts

glucose unit to

T (oC)

Glucose

TRS

yield

yield

Time

+

acid (H ) Amorphous

1

(%)

(%)

Ref.

a

H2O

cellulose H2O

Fe3O4–SBA–SO3H

3.7

150

3h

50

-

43

Fe3O4–SBA–SO3H

3.7

150

3h

26

Fe3O4–SBA–SO3H

3.8

150

3h

-

45

43

2

cellulose

43

3

Corn cob

4

Sucrose

H2O

CoFe2O4@SiO2–SO3H

7.8

100

20 min

95

-

45

5

Cellobiose

H2O

CoFe2O4@SiO2–SO3H

7.8

100

20 min

88

-

45

6

Starch

H2O

CoFe2O4@SiO2–SO3H

2.57

130

4h

48

-

45

7

Cellulose

H2O

CoFe2O4@SiO2–SO3H

12.3

150

3h

7.0

32

45

8

Cellulose

H2O

Fe3O4@C–SO3H

1.6

140

12 h

25.3

48.6

46

9

Corn stalk

H2O

sulfonated MCNAs

9.6

150

2h

-

31.0

47

10

Cellulose

H2O

Fe-GO-SO3H

7.7

75

44 h

50

94.3

48

11

Cellulose

[BMIM]Cl

Fe3O4@SiO2–SO3H

4.0

130

8h

51.4

73.2

49

12

Cellulose

[BMIM]Cl

PCM–SO3H

9.6

130

3h

-

68.9

50

13

Cellulose

H2O

PCM–SO3H

9.6

180

9h

-

51

50

14

Rice straw

[BMIM]Cl

PCM–SO3H

9.6

150

3h

-

35.5

50

H2O

a

TRS is the abbreviation of total reducing sugar Lignocelluloses is the most abundant biomass resources, which has been regarded as a

promising alternative to nonrenewable fossil resources, and cellulose represents the major

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component of lignocellulosic biomass. Thus, cellulose has received great attention as a sustainable feedstock for the production of chemicals and fuels. However, cellulose is a linear polymer formed by the repeating connection of glucose units through β-1,4-glycosidic linkages and the strong hydrogen bonds between cellulose chains form a cross-linked structure. Thus, cellulose does not dissolve in water and common organic solvents, which hampers the cellulose utilization. Therefore, hydrolysis of cellulose into glucose is the primary and essential step in the transformation of cellulose into fuels and chemicals. Mineral acids were early used as effective acid catalysts for the hydrolysis of cellulose as the free H+ could easily attack with the β-1,4-glycosidic linkages to promote the cellulose hydrolysis. However, the use of mineral acids demonstrates some distinct drawbacks such as reactor corrosion, waste treatment and poor recyclability. The use of heterogeneous catalysts for cellulose hydrolysis can overcome the above-mentioned drawbacks caused by the use of mineral acids, but the challenge with the cellulose hydrolysis reactions with a solid catalyst is the interaction between two solids.41 Although cellulose can be degraded into soluble sugars, the lignin components of the actual cellulosic biomass as well as the humins are difficult to separate from the heterogeneous catalysts.42 These solid residues might cover on the surface of the active sites of the heterogeneous catalysts, lowering the catalyst activity. The use of magnetic catalysts not only facilitated the recycling of the catalysts, but also benefited the separation of magnetic catalysts from the solid residues formed during the hydrolysis of cellulose.

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In recent years, some magnetic acid catalysts were reported for the hydrolysis of cellulose or lignocelluloses in water or in the ionic liquids. Fu and co-workers studied the hydrolysis of cellulose or lignocelluloses over magnetic sulfonated mesoporous silica (Fe3O4–SBA–SO3H) in water.43-44 The magnetic Fe3O4–SBA–SO3H catalyst was prepared by a surfactant-templated sol–gel method. Fe3O4 magnetic nanoparticles were dispersed in the block copolymer P123 for the co-condensation of tetraethoxysilane (TEOS). After assembly, mercapto groups were introduced to the surface of the magnetic mesoporous silica (SBA-15) by the addition of 3-(mercaptopropyl)trimethoxysilane (MPTMS). The mercapto groups were then oxidized into sulfonic acid groups inside the pores of the mesoporous silica by H2O2. Fe3O4–SBA–SO3H showed higher catalytic activity toward the hydrolysis of cellobiose than H2SO4 with the same amount of H+ under the same reaction conditions. For example, cellobiose conversions were 98% and 54% at 120 oC after 1 h in the presence of Fe3O4-SBA-SO3H and 0.11 mmol mL-1 H2SO4 with the same amount of H+, respectively. A plausible explanation was that the channels in the Fe3O4-SBA-SO3H catalyst contained concentrated acid sites and the uniform channels facilitated the reactant to easily enter and interact with these acid sites. Glucose yields were 50% and 26% from the hydrolysis of amorphous cellulose (pretreated with 1-butyl-3-methylimidazolium chloride) and microcrystalline cellulose at 150 oC for 3 h (Table 1, Entries 1~2). These results indicated that the pretreatment of microcrystalline cellulose disrupted the hydrogen bonds between cellulose chains, making the catalyst contact with the β-1,4-glycosidic linkages easily, thus producing higher glucose yield.

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Fe3O4–SBA–SO3H exhibited higher catalytic activity than AC–SO3H and Amberlyst-15, which both have strong acidity. It is presumably due to its higher surface area of its mesoporous structure of the Fe3O4–SBA–SO3H catalyst. In addition, hydrolysis of corn cob as a model of lignocellulose biomass afforded the total reducing sugar (TRS) yield of 45% at 150 oC for 3 h (Table 1, Entry 3). The Fe3O4–SBA–SO3H catalyst could be easily recycled from the hydrolysis solution by an external magnet and showed no deactivation during the three recycling experiments after the treatment with 1 M H2SO4. In contrast to the incorporation of Fe3O4 in the mesoporous SBA [43-44], the core-shell structured magnetic acid catalysts with a magnetic core have received much more interest in cellulose hydrolysis. Ebitani and co-workers prepared a core-shell structured magnetic acid catalyst for cellulose hydrolysis.45 CoFe2O4 as the magnetic core was embedded with a silica shell to generate CoFe2O4@SiO2, and then functionalized with thiol groups on its surface followed by oxidation with H2O2 into CoFe2O4@SiO2SO3H. The CoFe2O4@SiO2–SO3H catalyst exhibited high catalytic performance for the hydrolysis of the disaccharides of sucrose and cellobiose in water, affording glucose yields in 93% and 88% at 373 K after 20 min, respectively (Table 1, Entries 4 & 5). Similar with the results of Fu and co-workers,43-44 the authors also observed that the magnetic CoFe2O4@SiO2–SO3H catalyst showed much higher catalytic activity than the strongly acidic ion-exchange resin (Amberlyst-15) under the same reaction conditions. Glucose yield was obtained in 93% from sucrose at 373 K after 20 min over CoFe2O4@SiO2–SO3H, while that was 22% over Amberlyst-15 catalysts under the same

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conditions and the same amount of –SO3H group. Hydrolysis of polysaccharides starch over the CoFe2O4@SiO2–SO3H catalyst produced glucose with a yield of 48% at 130 oC for 4 h (Table 1, Entry 6). However, a very low glucose yield of 7% was only obtained at 150 oC for 3 h by the use of cellulose as the feedstock (Table 1, Entry 7). These results indicated that the hydrolysis of cellulose is much more difficult than that of other carbohydrates due to the rigid structure of cellulose. In contrast to the Fe3O4–SBA–SO3H catalyst,43-44 the CoFe2O4@SiO2–SO3H catalyst can be reused and remained its activity without the regeneration in H2SO4 solution. Besides the introduction of the acidic sites into the silica oxide, magnetic carbonaceous acid catalysts were also studied for the hydrolysis of cellulose. He and co-workers prepared core-shell structured Fe3O4@C–SO3H catalyst for the hydrolysis of cellulose in water.46 Fe3O4@C was prepared by the one-step hydrothermolysis of a mixture of FeCl3, glucose and urea in water at 180 oC for 14 h, followed by the sulfonation with sulfuric acid to give Fe3O4@C–SO3H catalyst. Hydrolysis of cellulose over the Fe3O4@C–SO3H catalyst was performed in water at 140 oC for 12 h, affording 48.6 % cellulose conversion with 52.1 % glucose selectivity (Table 1, Entry 8). In addition, sulfuric acid functionalized magnetic carbon nanotube catalyst (sulfonated MCNAs) was also prepared and used for the hydrolysis of crop stalks.47 Magnetic carbon nanotube arrays were synthesized by the chemical vapor deposition, using xylene and ferrocene as C and Fe precursors. Sulfonated magnetic carbon nanotube arrays were prepared by the treatment of magnetic carbon nanotube arrays with sulfuric acid at

11


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250 °C under a flow of N2. The sulfonated MCNAs catalyst was then used for the hydrolysis of corn stalk, and TRS yield was achieved in 31% after 2 h at 150 oC (Table 1, Entry 9). Recycling experiments indicated that catalyst was stable without the loss of its catalytic activity. Besides the magnetic carbon catalysts, a sulfuric acid functionalized graphene oxide in combination with iron nanoparticle (Fe-GO-SO3H) was studied for the hydrolysis of cellulose.48 Hydrolysis of cellulose in water over Fe-GO-SO3H catalyst produced glucose in a yield of 50.0% yield in conjunction with TRS yield of 94.3% at a low reaction temperature of 75 oC, but a long reaction time of 44 h (Table 1, Entry 10).48 The excellent catalytic activity of Fe-GO-SO3H is attributed to the ability of the water soluble nanostructured material with a large concentration of polar groups (-OH, -COOH) which readily adsorb cellulose, while providing a large concentration of acidic functionality to hydrolyze the cellulose. Ionic liquids (IL) are salts in the liquid state, in which the ions are poorly coordinated, resulting in these solvents being liquid below 100 °C, or even at room temperature. Ionic liquids have some unusual physical properties such as high thermal stability, lack of inflammability, low volatility, and chemical stability. Particularly, some ionic liquids showed good ability to dissolve cellulose by the disruption of the hydrogen bonds. Therefore, some methods on the hydrolysis of cellulose were reported with good results. A core-shell structured magnetic acid catalyst (Fe3O4@SiO2–SO3H) was prepared and used for the hydrolysis of cellulose in ionic liquids.49 The procedure of the catalyst preparation is illustrated in Fig. 1. Fe3O4 was used as the magnetic core and it was

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embedded with silica shell to give the core-shell structured Fe3O4@SiO2 nanoparticles. Fe3O4@SiO2 nanoparticles were then treated with chlorosulfonic acid to generate the magnetic Fe3O4@SiO2–SO3H catalyst. The preparative process is simple and the amount of H+ in the Fe3O4@SiO2–SO3H catalyst is up to 2.50 mmol/g. The Fe3O4@SiO2–SO3H catalyst were effective for the hydrolysis of cellulose in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), and 73.2% yield of TRS and 51.4% of glucose yield were obtained after 8 h at 130 oC (Table 1, Entry 11). Compared with the results from the hydrolysis of cellulose in water (Table 1, Entries 7 &8), hydrolysis of cellulose in ionic liquids produced higher TRS and glucose yields (Table 1, Entry 11). The possible reason should be that cellulose completely dissolves in [BMIM]Cl, which makes the acid sites easily contact with β-1,4-glycosidic bonds of cellulose chain, facilitating the hydrolysis of cellulose. In addition, the Fe3O4@SiO2-SO3H catalyst could be reused without the significant loss of its catalytic activity (73.2% in first cycle vs 69.4% in sixth cycle).

Fe (II ) + Fe (III)

NH4OH

Fe3O4

TEOS NH4OH

Fe3O4 ClSO3H OH r.t.

OSO3H + HCl

Fig. 1 Schematic illustration for the synthesis of Fe3O4@SiO2-SO3H. Ref.49

Later, Qi and co-workers prepared a cellulose-derived carbonaceous magnetic acid catalyst (PSM-SO3H) for the hydrolysis of cellulose.50 The procedure of the preparation of PSM-SO3H is shown in Fig. 2. The catalyst was synthesized by the incomplete

13


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hydrothermal carbonization of cellulose followed by Fe3O4 grafting and –SO3H group functionalization. The as-prepared PSM-SO3H catalyst containing –SO3H, –COOH, and phenolic –OH groups, showed a similar catalytic activity as the Fe3O4@SiO2-SO3H49 towards the hydrolysis of cellulose in ionic liquids. TRS yield of 68.9% was obtained within 3 h at 130 °C in [BMIM][Cl] (Table 1, Entry 12). However, the hydrolysis of cellulose in water only produced TRS in a yield of 51.0% even under harsh reaction conditions at 180 °C and long reaction time of 9 h (Table 1, Entry 13). These results once again indicated the ionic liquid [BMIM][Cl] was superior to water for cellulose hydrolysis. Furthermore, hydrolysis of lignocelluloses (rice straw) produced a TRS yield of 35.5% in [BMIM][Cl] at 150 °C in 2 h (Table 1, Entry 14). However, TRS yield decreased from 68% in the first run to 43% in the third run. Although the TRS yield decreased during the recycling runs, the–SO3H groups on the used catalyst (0.64 mmol/g) remained constant compared with the fresh catalyst, indicating that no leaching of the–SO3H groups from the catalyst had occurred. The authors claimed that the decrease in the reducing sugar yield in the recycle experiments was probably be attributed to the mass loss of the catalyst in the washing steps and the blockage of active acid sites by humins products.

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OH O

incompleted hydrothermal carbonization

OH

HO

O O

HO

O

OH

n

OH

COOH O4Fe3 HO

COOH

HO ferric nitrate OH

Fe3O4

Ammonia water O4Fe3

OH

OH

OH

COOH

COOH

SH

PTSA

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O HO

O4Fe3

COOH

SO3H

O4Fe3

COOH

SH

HO HO

Fe3O4

Fe3O4

hygrogen peroxide

O4Fe3

OH

O4Fe3

OH

OH HO3S

COOH

OH

SO3H HS COOH

SH

Fig. 2 Procedure for the preparation of superparamagnetic carbon material solid acid catalyst functionalized with –SO3H groups (PCM-SO3H) from cellulose.Ref.50 3.2. Hydrolysis of Cellulose and Lignocelluloses by Magnetic Biocatalysts Besides magnetic acid catalysts, magnetic enzyme catalysts were also studied for the hydrolysis of cellulose. Hobley and co-workers used non-porous magnetic particles to immobilize whole cellulase mixtures for the hydrolysis of cellulose.51 Silica coated 15


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magnetic particles activated by cyanuric chloride was used to the immobilize cellulase mixtures. Magnetic particles immobilized CellicCTec2 showed the highest activity in the hydrolysis of microcrystalline cellulose, by mass of reducing sugar produced per mass of particles (2.8 g kg-1 min-1). The magnetic CellicCTec2 could also be used for the hydrolysis of pretreated wheat straw biomass, confirming the potential of hydrolyzing real lignocellulosic substrate. Puri and co-workers also prepared magnetic nanoparticles supported cellulases for the hydrolysis of carboxymethyl cellulose (CMC) or hemp hurd biomass.52 Under optimal conditions, hydrolysis of CMC with free and immobilized enzymes was 88% and 81%, respectively. With pretreated hemp hurd biomass, the free and immobilized enzymes resulted in maximum hydrolysis in 48 h of 89% and 93%, respectively. In addition, the immobilized enzyme retained 50% enzyme activity up to five cycles, with thermostability at 80 °C superior to that of the free enzyme. Although the magnetic biocatalysts can promote the cellulose or lignocelluloses under mild condition, these processes generally require long reaction time and suffer the loss of the activity of the enzymes. The chemical hydrolysis of cellulose over magnetic acid catalysts is the main method for the hydrolysis of cellulose or lignocelluloses due to the flexible of catalyst structure and wide reaction conditions. The efficiency of cellulose hydrolysis is affected by both the catalyst and reaction conditions. The catalyst with high surface area and high acidity showed high catalytic performance towards the hydrolysis of cellulose. In addition, the use of ionic liquids also produce high TRS and glucose yields under relative mild conditions than the reaction in water. In many cases, the

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magnetic catalysts encountered the loss of the active sites during the reaction process. Therefore, design of active and stable catalysts together with the selection of a suitable solvent should be crucial for the effective hydrolysis of cellulose with high yields. 4.

SYNTHESIS

OF

5-HYDROXYMETHYLFURFURAL

AND

5-ETHOXYMETHYLFURFURAL OVER MAGNETIC CATALYSTS 4.1. Synthesis of 5-Hydroxymethylfurfural Nowadays, one of the most attractive and promising approaches is to convert C6-based carbohydrates into HMF, which have been recognized as important platform molecule for the synthesis of a broad range of new products as well as for the replacement of fossil resources-derived fuels and chemicals.53-56 There are many reports on the synthesis of HMF from carbohydrates either using homogeneous catalysts or heterogeneous catalysts. In the following part, we will review mainly on the synthesis of HMF over magnetic acid catalysts. Just as the hydrolysis of cellulose, synthesis of HMF from dehydration of fructose is mainly catalyzed over magnetic sulfonic acid catalysts. Wang and co-workers prepared a magnetic acid catalyst with a Fe3O4 core and sulfonic acid functionalized silica shell for the dehydration of fructose into HMF.57 Fe3O4 core was coated with a phenyl modified silica shell nanolayer, followed by the sulfonation of the phenyl groups to generate the magnetic Fe3O4@Si/Ph‐SO3H catalyst (Fig. 3). The as-prepared Fe3O4@Si/Ph‐SO3H catalyst could effectively catalyze the dehydration of fructose into HMF in dimethylsulfoxide (DMSO), affording HMF yield of 82% at fructose conversion of 99%

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at 100 oC after 3 h. However, the conventional Amberlyst-15 catalyst under the same reaction condition with the same amount of H+ only produced HMF in a yield of 16.7% with fructose conversion of 60.1%. The TOF values of the two different catalysts Fe3O4@Si/Ph‐SO3H and SBA-15 were 250.0 and 49.8 h-1. These results indicated that Fe3O4@Si/Ph‐SO3H catalyst showed higher catalytic activity than the Amberlyst-15 catalyst. On the one hand, Fe3O4@Si/Ph‐SO3H had a surface area of 138.1 m2/g, nearly four times greater than that of Amberlyst‐15. On the other hand, Fe3O4@Si/Ph‐SO3H is in nano-size structure with a thin shell, thus the reactions occurred predominantly on the outside surface, which represents a favorable scenario for the substrate and the product, in terms of them being able to readily diffuse to and/or from the active sites. Besides the excellent catalytic activity, the catalyst could be magnetically separated and recycled several times without losing its activity. Similar work was also reported by Martinez and co-workers, in which they also used sulfonated silica coated Fe3O4 as a magnetic acid catalyst for the dehydration of xylose to furfural.58 Later, a magnetic carbonaceous acid catalyst using lignin residue as the carbon resource was prepared by Hu and co-workers, and used for the dehydration of fructose into HMF in DMSO (Fig. 4).59 The magnetic carbonaceous catalyst was prepared using the enzymatic hydrolysis lignin residue (EHL) as a precursor via a simple and inexpensive process including the impregnation of EHL with FeCl3, the subsequent carbonization of FeCl3-load EHL under N2 atmosphere to generate magnetic carbonaceous materials and the final sulfonation process to obtain the magnetic carbonaceous acid catalyst. The resulting catalyst possessed a porous structure

18


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with a surface area of 234.61cm2/g and its surface was enriched with –SO3H, –COOH, and phenolic –OH groups. The magnetic carbonaceous acid catalyst showed a good catalytic activity for the dehydration of fructose into HMF, affording HMF yield of 81.1% with a 100% conversion at 130 oC for 40 min. Other byproducts were mainly the soluble and insoluble humins from the polymerization or cross-polymerization of HMF or reaction intermediates. The results were similar with those reported by Wang and co-worker.57 The magnetic lignin-derived carbonaceous catalyst exhibited an excellent catalytic stability without no obvious decrease of HMF yield in 5 successive runs. In contrast to the use of magnetic Brønsted acid, a magnetic Lewis acid catalyst was used for the dehydration of fructose into HMF.60As shown in Fig. 5, the out surface of γ-Fe2O3 was coated with a layer of hydroxyapatite (HAP) to give the core-shell structured γ-Fe2O3@HAP. The Ca2+ in HAP could be exchanged with Cr3+ generated γ-Fe2O3@HAP-Cr catalyst. The dehydration of fructose in DMSO over the γ-Fe2O3@HAP-Cr catalyst produced HMF with a yield of 88.9% after 240 min at 120 oC. The γ-Fe2O3@HAP-Cr catalyst could be easily separated from the reaction solution by an external magnet, and showed high stability without the loss of its catalytic activity.

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FeCl3 FeSO4

NH3 H2O

TEOS

OA

PTES MNP

H2SO4 XSO3

SO3H SO3H

Fig. 3 Preparation of the magnetic solid nanoparticles with core/shell structure. MNP: Magnetic nanoparticles; TEOS: tetraethoxysilane; PTES: phenyltriethoxysilane.Ref.57 SO3H HO

OH Fe3O4 COOH

HOOC SO3H HO

O OH OH OH HO Fructose

Magnetic lignin-deirved solid acid catalyst DMSO

HO

O O HMF

Fig. 4 Magnetic lignin-derived carbonaceous catalyst for the dehydration of fructose into 5-hydroxymethylfurfural in dimethylsulfoxide.Ref.59

20


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Fe (II ) NH4OH

Page 22 of 51

CrCl3

CaP

+

Fe (III) Fe2O3@HAP Fe2O3@HAP-Cr

Fig. 5 Schematic illustration for the synthesis of γ[email protected] Although HMF can be readily obtained from fructose by acid-catalyzed dehydration, synthesis of HMF from glucose especially cellulose is much more popular, as glucose and cellulose are abundant and cheap. However, the transformation of glucose or cellulose is much more difficult, as it requires multiple reaction steps. For example, the conversion of cellulose requires three consecutive steps, the hydrolysis of cellulose into glucose, the isomerization of glucose into fructose, and the dehydration of fructose into HMF. Recently, Wu and co-workers made a great process using the magnetic biocatalysts in combination with common acid catalysts for the one-pot conversion of cellulose into HMF via three steps.61 Fe3O4-loaded in mesoporous silica nanoparticles (MSN) (Fe3O4@MSN) were used as hosts for the immobilization of cellulase and isomerase, affording two kinds of biocatalysts cellulase-Fe3O4@MSN and isomerase-Fe3O4@MSN, respectively. Cellulase-Fe3O4@MSN and isomerase-Fe3O4@MSN were used for the hydrolysis of cellulose into glucose and the isomerization of glucose into fructose in aqueous solution. After each step, the corresponding biocatalyst could be facilely separated from the reaction mixture by an external magnet. Then the dehydration of fructose into HMF was catalyzed by the acid HSO3-MSN catalyst in DMSO. HMF of

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yield 46.1% was obtained from cellulose. This study indicates that the concept of integrating enzymatic and chemocatalytic biomass processing can be an effective and economically friendly process for various catalytic applications. 4.2. Synthesis of 5-Ethoxymethylfurfural EMF, the etherification product of HMF, has been proposed as a potential liquid biofuel for the future.62 It has a high energy density of 8.7 kWh/L, close to the standard gasoline (8.8 kWh/L).63 Currently, much attention has been paid on the one-pot synthesis of EMF from fructose or inulin (Fig. 6) over various acidic catalysts, which integrates the hydrolysis of inulin to fructose, the dehydration of fructose to HMF and the etherification of HMF into EMF. OH O

HO HO

OH HO

O

H+

O OH

HO

OH

O

CH2 O

O

CH2OH OH C

n

OH OH

CH2OH

H+

O O

OH

O

O

O

H+

OH OH

EtOH

EMF HMF

Fructose CH2OH

Inulin

Fig. 6 Schematic illustration for the synthesis of EMF from fructose and inulin in ethanol.

In 2013, a magnetic acid catalyst (Fe3O4@SiO2-HPW) was prepared and used for the synthesis of EMF either from HMF or fructose.64The catalyst was easily prepared by the impregnation of phosphotungstic acid (HPW) on the surface of silica-coated Fe3O4 (Fe3O4@SiO2) nanoparticles. The etherification of HMF over the Fe3O4@SiO2-HPW 22


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catalyst afforded EMF with a yield of 83.6%. However, the one-pot conversion of fructose into EMF could only give a moderate EMF yield of 54.8%, which was possibly due to the low acidity of the catalyst and the hindrance of the acid sites with the reactant. In order to improve the yield of EMF from fructose, sulfonic acid functionalized magnetic Fe3O4@SiO2-SO3H catalyst with strong acidity was used for this reaction, as it was found to show high catalytic activity towards cellulose hydrolysis.65 The one-pot conversion of fructose over the Fe3O4@SiO2-SO3H catalyst afforded EMF in a yield of 72.5%. More importantly, EMF was also obtained in a satisfactory yield of 63.3% using polysaccharides inulin as the starting material. Later, synthesis of EMF from fructose based carbohydrates was performed over a new magnetic Fe3O4@C-SO3H catalyst.66 Unlike Fe3O4@SiO2-SO3H, the magnetic core of Fe3O4 in the Fe3O4@C-SO3H catalyst was coated with a carbon shell, which was formed from the hydrothermolysis of renewable glucose. Similar with the Fe3O4@SiO2-SO3H catalyst, the Fe3O4@C-SO3H catalyst also showed high activity towards the one-pot conversion of fructose based carbohydrates into EMF. EMF was obtained in a yield of 67.8% from fructose, and that was 58.4% from inulin. Recently, an acidic polyionic liquids grafted on magnetic material was successfully prepared and used for the synthesis of EMF.67 As shown in Fig. 7, mercaptopropyl-modified silica coated Fe3O4 reacted with bis-vinylimidazolium salts through

click

reaction

and

radical

oligomerization

in

the

presence

of

azobisisobutyronitrile AIBN of bis-vinylimidazolium salts, generating the magnetic polyionic liquids. Then bromide ions in the magnetic polyionic liquids were finally

23


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ACS Catalysis

exchanged

with

hydrosulfate

ions,

generating

the

magnetic

acid

catalyst

(Fe3O4@SiO2-SH-Im-HSO4). Under optimal conditions, the one-pot conversion of fructose, sucrose and inulin catalyzed by Fe3O4@SiO2-SH-Im-HSO4 generated EMF with yields of 60.4%, 34.4% and 56.1%, respectively. The catalyst can be recycled without the significant loss of its catalytic activity. Fe (II )

NH4OH

+ Fe (III)

SH

NH4OH Fe3O4

N

+

MPTMS

TEOS

N+ Br

BrN+ N

AIBN s

N

N+ Br

BrN+

N

HSO4-

H2SO4 -HBr

Fe3O4@SiO2-SH

Fe3O4@SiO2

s

N

N+ HSO4-

N+

N

Fig. 7 Scheme of the procedure of the synthesis of magnetic Fe3O4@SiO2-SH-Im-HSO4 acid catalyst.Ref. 67 Based on the above results, synthesis of HMF and EMF is mainly fouced on the use of fructose based carbohydrates as the feedstock over the magnetic acidic catalysts. Much more effort should be devoted to the use of abundant and cheap glucose based carbohydtaes into HMF and EMF. As the isomerization of glucose to fructose is curical step for the sucessful synthesis of HMF and EMF, the design of magnetic catalysts with multiple active sites is important to realize the one-pot conversion of glucose based carbohydates into HMF and EMF. It is reported that the base has the ability to promote

24


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the isomerization of glucose to fructose. Therefore, one possible route is that the introduction of base and acid sites into the magnetic catalysts that can simultaneously isomerize glucose into furctose and the subsequent conversion of fructose into HMF and EMF by acidic sites. Of course, many other types of magnetic catalysts with bifunctional sites can also be designed for the conversion of glucose based carbohydtaes into HMF and EMF. 5. CATALYTIC OXIDATION OF HMF OVER MAGNETIC CATALYSTS From the literature on the production of chemicals from biomass, synthesis of furanic chemicals has become extremely important via directly catalytic oxidation of HMF or carbohydrates by several consecutive steps (Fig. 8). As shown in Fig. 8, several kinds of furanic derivatives can be obtained via catalytic oxidation of HMF, mainly including 2, 5-diformylfuran

(DFF),

5-formyl-2-furancarboxylic

acid

(FFCA),

and

5-hydroxymethyl-2-furancarboxylic acid (HFCA), and 2,5-furandicarboxylic acid (FDCA). Among them DFF and FDCA are found to be useful in many fields. For example, DFF can be used as a versatile precursor for the synthesis of furanic polymers, pharmaceuticals, antifungal agents, nematocides, fluorescent material, and porous organic frameworks. FDCA has been identified as one of the top 12 value-added chemicals from biomass by the U.S. Department of Energy. It has a similar structure with petroleum-derived terephthalic acid, thus it can serve as a substitute for terephthalic acid for the manufacture of poly(ethylene terephthalate) (PET) plastics.68 In the following part,

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we will review mainly on the synthesis of DFF and FDCA from the oxidation of HMF or directly from carbohydrates over magnetic catalysts. Dehydration Carbohydrates

O

OH [O]

O

O

O

O

O

O

[O]

O OH

O

O O

[O]

OH

HO

DFF

HMF

FFCA

[O]

[O]

FDCA

O O HO

OH

HFCA

Fig. 8

Schematically illustration of synthesis of furanic chemicals from carbohydrates.

5.1. Synthesis of 2, 5-Diformylfuran from HMF

H2 N NH2 H2 N Fe (II )

NH4OH

APTES

TEOS

Fe3O4

Fe3O4

+

NH2

NH4OH

Fe (III) Fe3O4@SiO2 MNP 3+

3+

NH2

H 2N

RuH2N NH2Ru3+

NH2

RuH2N

RuCl3 (aq)

Fe3O4 NH2Ru3+

3+

NH2Ru3+

RuH2N NH2Ru3+

Fe3O4@SiO2-NH2-Ru(III)

26


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Fig. 9 Schematic illustration of the preparation of magnetically recoverable Fe3O4@SiO2-NH2-Ru(III).Ref.70 Currently, the oxidation of HMF into DFF is mainly catalyzed by the supported ruthenium (Ru) catalysts.69 In order to facilitate the catalyst recycle, two kinds of magnetic Ru catalysts were used for the oxidation of HMF into DMF, in which the active sites of Ru were not in the form of metallic nanoparticles.70-71 As shown in Fig. 9, amino functionalized silica-coated magnetic support (Fe3O4@SiO2-NH2), which showed a high affinity to anchor Ru3+, generating a magnetic Ru catalyst (Fe3O4@SiO2-NH2-Ru3+).70 The Fe3O4@SiO2-NH2-Ru3+ catalyst showed excellent catalytic activity in the aerobic oxidation of HMF into DFF under mild conditions, affording high HMF conversion of 99.3% and DFF yield of 86.4% after 4 h at 120 °C under 1 atm O2. High catalytic performance was also observed when the oxidation of HMF was carried in the air over Fe3O4@SiO2-NH2-Ru3+ catalyst. After reaction, the Fe3O4@SiO2-NH2-Ru3+ catalyst could be readily recovered from the reaction mixture by a permanent magnet, but it showed a slight decrease in its catalytic activity, possible due to the leaching of Ru3+ into the reaction solution. As the Ca2+ in the magnetic support Fe3O4@HAP can be exchanged with the metal cations,60 magnetic γ-Fe2O3@HAP-Ru catalyst was recently prepared and used for the aerobic oxidation of HMF into DFF.71 The exchange of Ca2+ in HAP with Ru3+ generated the γ-Fe2O3@HAP-Ru catalyst. The γ-Fe2O3@HAP-Ru catalyst showed high catalytic activity, affording full HMF conversion and high DFF yield of 89.1% after 4 h at 90 oC. Furthermore, the one-pot conversion of fructose into DFF via two-step 27


Page 29 of 51

method was further investigated (Fig. 10). The dehydration of fructose into HMF was first performed over the magnetic Fe3O4@SiO2-SO3H acid catalyst, affording HMF in a yield of 90.1%. Then the magnetic Fe3O4@SiO2-SO3H acid catalyst was removed from the reaction solution with a permanent magnet, and HMF in the resulting solution was further oxidized to DFF with a yield of 79.1% over γ-Fe2O3@HAP-Ru catalyst, which was also easily recovered by an external magnet. The γ-Fe2O3@HAP-Ru was stable during the reaction process without the loss of its catalytic activity.

100 HMF yield DFF yield

80 60

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

40 20 0 0

3

6

9

12

15

18

21

24

27

Time (h)

Fig. 10 The results of the one-pot conversion of fructose into DFF via two consecutive steps. Reaction conditions: The first step: Fructose (143 mg, 0.8 mmol) was firstly dissolved in a mixed solvent of DMSO (1 mL) and 4-chlorotoluene (4 mL), then Fe3O4@SiO2–SO3H (150 mg) was added into the mixture, and the reaction was carried out at 110 oC. The second step: γ-Fe2O3@HAP-Ru (150 mg) was added into the reaction

28


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solution and the oxidation reaction was carried out with O2 flow rate at 20 mL min-1.Ref.71 Although Ru catalysts showed high catalytic activity towards the oxidation of HMF into DFF, the cost of Ru catalysts is high, which is not economic for practical production of DFF. Therefore, it is much more attractive to develop the none-noble catalysts for the oxidation of HMF into DFF. Some magnetic catalysts without noble-metal active sites were reported for the oxidation of HMF into DFF. Karimi and co-workers reported a new method for the oxidation of HMF into DFF by the use of magnetic organocatalyst (Fe3O4@SiO2-TEMPO,

MNST),

in

which

2,2,6,6-tetramethylpiperidine-N-oxide

(TEMPO) was grafted on the surface of magnetic Fe3O4@SiO2 support.72 As shown in Fig. 11, this catalytic system required the use of tert-butyl nitrite (t-Bu-ONO) as a co-catalyst and acetic acid as an additive. The mediation of electron transfer from alcohol to O2 by NO/NO2 couples and immobilized nitroxyl radical (MNST) through a combination of two redox cycles is required. DFF was achieved in excellent chemoselectivity (>99%) with full HMF conversion at 50 oC for 10 h under 1 atm O2 pressure. Although a high yield of DFF was obtained, this catalytic system demonstrated two drawbacks. On the one hand, it required additional use of tert-butyl nitrite (t-Bu-ONO) and acetic acid, resulting in a higher production cost and a difficulty in the purification of DFF. On the other hand, the catalyst activity decreased from the first run to five run quickly, with HMF yields of 99% in the first run and 77% in the fifth run. The transition manganese compounds such as KMn8O16.nH2O and Mn3O4 was also found to show high catalytic activity for the oxidation of HMF into DFF.73 However, it is difficult 29


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ACS Catalysis

to recycle the nano-sized manganese compounds. Magnetic Fe3O4 supported Mn3O4 nanoparticles (Fe3O4/Mn3O4) could overcome the tedious separation procedure and showed high catalytic activity towards the oxidation of HMF into DFF, which was prepared by the solvent thermal treatment of Mn(OAc)2 in DMSO in the presence of Fe3O4 nanoparticels.74 The Mn3O4 in the catalyst showed a uniform size with diameters of 6~8 nm. The catalytic oxidation of HMF into DMF in dimethylformamide over Fe3O4/Mn3O4 catalyst produced DFF with a yield of 82.1% and full HMF conversion at 120 oC within 4 h under 1 atm O2. Interesting, it was found that the commercial Mn3O4 showed very low catalytic activity towards HMF oxidation. The reason might be that the prepared Mn3O4 nanoparticles with small size had high surface area to provide a large number of active sites. Fe3O4/Mn3O4 could be readily collected by a permanent magnet, and showed high stability.

SiO2

SiO2

Fe3O4

Fe 3O4

N O

O

O HO O2

NO

N O SiO2

HMF

Fe3O4 O NO2

O O H

H DFF

tBu-ONO

HOAc

HONO

N OH

Fig. 11 Proposed reaction pathway for the aerobic oxidation of HMF into DFF using MNST catalyst.Ref.72 30


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5.2. Synthesis of 2,5-Furandicarboxylic Acid from HMF Compared with DFF, the catalytic synthesis of FDCA has received much more attention. Recently, several kinds of magnetic Pd catalysts were prepared and showed high catalytic performance towards the oxidation of HMF into FDCA.75-77 Followed by our

previous

work,60

the

magnetic

γ-Fe2O3@HAP

supported

Pd

catalysts

(γ-Fe2O3@HAP-Pd) was prepared and used for the aerobic oxidation of HMF into FDCA.75 The catalyst was easily prepared by the exchange of Pd2+ with Ca2+ in γ-Fe2O3@HAP, followed by reduction of the Pd2+ to Pd(0) nanoparticles. The Pd nanoparticles can be stabilized by the phosphorus-oxygen group and homogeneously distributed on the surface of the support with an average size of 2.8 nm. The γ-Fe2O3@HAP-Pd catalyst showed high activity towards the aerobic oxidation of HMF to FDCA, affording HMF conversion of 97% and FDCA yield of 92.9% after 6 h at 110 oC with 0.5 equiv of K2CO3 under atmospheric oxygen pressure. TEM images indicated that the particle size of Pd nanoparticles did not change in the spent catalyst, and the catalyst could be recycled without the obvious loss of its catalytic activity. In parallel, magnetically separable graphene oxide supported Pd nanoparticles (C-Fe3O4-Pd) was prepared for the oxidation of HMF into FDCA, in which Fe3O4 nanoparticels and Pd nanoparticels were simultaneously deposited on graphene oxide by the one-step solvothermal route.76 The oxygen-containing functional groups of graphene oxide can be used to stabilize the Pd nanoparticles and the catalyst showed excellent catalytic performance, giving high HMF conversion (98.1%) and FDCA yield (91.8%) after 4 h at 31


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ACS Catalysis

80 oC with K2CO3/HMF molar ratio of 0.5. Later, with the aim to suit the sustainability chemistry, another kind of magnetic C@Fe3O4 support was used to immobilize Pd nanoparticles for the aerobic oxidation of HMF into FDCA under mild conditions.77 The core–shell structure C@Fe3O4 support was prepared by the in situ carbonization of renewable glucose on the surface of the Fe3O4 microspheres. Compared with the use of γ-Fe2O3@HAP-Pd75 and C-Fe3O4-Pd,76 FDCA was obtained with a slight decrease yield of 87.8% after 6 h at 80 oC at 1 atm O2, but with the same HMF conversion of 100%. The Pd/C@Fe3O4 catalyst also showed good stability in the subsequent recycling experiments, and XPS technology confirmed that the metallic Pd(0) was remained in the surface of the spent catalyst. All of our developed magnetic Pd catalytic systems for the oxidation of HMF into FDCA showed the common advantages: (a) These methods did not require a large amount of base; (b) These methods could be conducted under atmospheric oxygen pressure, not requiring high oxygen pressure; (c) These catalysts could be easily separated by an external magnet and reused without the loss of catalytic activity. In order to decrease the cost of the catalyst, a none-noble magnetic Nano-Fe3O4−CoOx catalyst was prepared and used for the oxidation of HMF into FDCA.78 Nano-Fe3O4−CoOx catalyst showed high catalytic activity towards the oxidation of HMF into FDCA with t-BuOOH as the oxidant. Then two-step strategy was then applied for the synthesis of HMF from fructose. The dehydration of fructose over the Fe3O4@SiO2−SO3H catalyst was performed as the first step to produce HMF in DMSO. Then the magnetic Fe3O4@SiO2−SO3H was easily separated from the reaction system by an external magnet,

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and the remaining reaction solution was oxidized into FDCA with t-BuOOH over nano-Fe3O4−CoOx catalyst. FDCA was obtained in a yield of 59.8% after 15 h based on the starting fructose. Compared with the use of noble metal catalysts for the oxidation of HMF into FDCA, the use of transition metal as the catalyst in our catalytic system makes this method much more economical for the practical synthesis of FDCA from renewable carbohydrates. Although the catalytic synthesis of DFF and FDCA is an important aim in biorefinery, the use of magnetic catalyst has not been well documented in the literature. Further work should be paid on the development of transition metals based magnetic catalyst for the oxidation of HMF into DFF or FDCA. The as-prepared catalysts should show high catalytic activity for the aerobic oxidation of HMF with high selectivity. Meanwhile, the magnetic catalysts also should have the high stability and could be reused without the loss of the catalyst activity. It is preferred to perform the oxidation of HMF without base, as the use of base is much more expensive, less sustainable, and higher difficult to purify the products. More importantly, the design of magnetic catalysts combining acidic and metal sites is also encouraged for the one-pot synthesis of DFF or FDCA from carbohydrates. 6. PRODUCTION OF BIODIESEL OVER MAGNETIC CATALYSTS Biodiesel is a kind of engine fuel, which is biodegradable, non-toxic, and environmentally

friendly. It can be not only used as an alternative fuel directly, but also can be used as an additive for clean burning of diesel fuel. Biodiesel is typically made by transesterification 33


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ACS Catalysis

of vegetable oil, soybean oil or animal fat with an alcohol in the presence of acid, base or enzyme catalysts. Recently, many kinds of magnetic base catalysts were reported to be effective for the synthesis of biodiesel via transesterification reaction. Among the magnetic base catalysts, the use of active compound of CaO has received much more interest in production of biodiesel . Fan and co-workers prepared a series of magnetic CaO/MFe2O4 (M2+) catalysts by hydrothermal method with MFe2O4 as the magnetic core and applied to the transesterification of soybean oil for the production of biodiesel.79 The CaO/CoFe2O4 catalyst showed strong magnetic strength and basic strength. Compared with CaO/ZnFe2O4 and CaO/MnFe2O4, CaO/CoFe2O4 showed better catalytic performance, weaker hydroscopicity and stronger wettability, which indicated that catalytic performance was relative to both basicity of catalyst and the full contact between the catalyst and the reactants, but the latter was a main factor in the catalytic system (Table 2, Entries 1~3). The highest biodiesel yield of 87.4% was obtained over the CaO/CoFe2O4 catalyst at 70 oC after 5 h. Liu and co-worker prepared a simple magnetic

solid

base

catalyst

(CaO/Fe3O4)

by

loading

CaO

on

Fe3O4 with

Na2CO3 and NaOH as precipitator, and studied its catalytic activity for the transesterification reaction.80 The proportion of Ca2+ to Fe3O4 affected the catalytic performance and the highest catalytic activity was obtained with the proportion of Ca2+ to Fe3O4 is 7:1, affording biodiesel yield of 95% within 80 min under the conditions of methanol/oil molar ratio of 15:1, catalyst dosage of 2 wt% and temperature of 70 oC

34


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(Table 2, Entry 4). They also found that the catalytic activity and recovery rate of the nanometer magnetic solid base catalysts are much better than those of CaO. Later, similar work was also by Han and co-workers, in which they used KF modified CaO-Fe3O4 magnetic catalyst (KF/CaO-Fe3O4) for the synthesis of biodiesel.81 KF/CaO-Fe3O4 was prepared by the impregnation of CaO on the surface of magnetic Fe3O4 nanoparticles, following the dip of KF solution. The KF/CaO-Fe3O4 catalyst possessed a unique porous structure with an average particle diameter of ca. 50 nm. The amount of KF loading showed a great effect on the catalytic activity. The highest biodiesel yield of 95% was obtained with KF loading of 25 wt.% (Table 2, Entry 5). When the KF loading is beyond 25 wt.%, however, the activity of catalyst is decreased with the increase of loading of KF. This is probably due to the fact that the excessive KF covers the active sites of catalyst surface, resulting in the decrease of catalytic activity. Lin and co-workers prepared magnetic recycled CaO hollow fibers (CaO/α-Fe) for the transesterification of rapeseed oil with methanol to produce biodiesel.82 Under the optimal conditions, biodiesel yield was achieved in a yield of 95.7% (Table 2, Entry 6). The catalyst could be separated by a magnetic field and recycled 20 times, and 85.2% biodiesel yield was still obtained. Magnetic calcium oxide hollow fibers are active, stable, and easily separated green catalysts that may find practical uses in biodiesel industry. Fang and co-workers prepared a weak magnetic CaFe2O4-Ca2Fe2O5-based catalyst by coprecipitation and calcinations and applied it for the synthesis of biodiesels.83 The magnetic intensity of the CaFe2O4-Ca2Fe2O5-based catalyst was too weak to be attracted by the normal permanent

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magnet. However, the magnetism of the CaFe2O4-Ca2Fe2O5-based catalyst can be enhanced by the reduction of its component of nonmagnetic Fe2O3 to Fe3O4-Fe under H2 atmosphere, and could be collected by the permanent magnet in a few minutes. Both catalysts were used for the catalytic transesterification of soybean and Jatropha oils to biodiesel. The highest biodiesel yields for soybean oil of 85.4% and 83.5% were obtained over the weak and strong magnetic catalysts, respectively under the optimized conditions (373 K, 30 min, 15/1 methanol/oil molar ratio and 4 wt% catalyst) (Table 2, Entries 7 & 8).

Table 2. The results of the synthesis of biodiesel over different magnetic base catalysts. Entry

Temperature

Ref. No

Catalyst

1

soybean oil

CaO/CoFe2O4

70

5h

87.4

79

2

soybean oil

CaO/ZnFe2O4

70

5h

≈ 63

79

3

soybean oil

CaO/MnFe2O4

70

5h

≈13

79

4

soybean oil

CaO/Fe3O4

70

80 min

95

80

5

Stillingia oil

KF/CaO-Fe3O4

70

3h

95%

81

6

Rapeseed oil

CaO/α-Fe

60

2h

95.7

82

7

soybean oil

CaFe2O4-Ca2Fe2O5-Fe2O3

100

30 min

85.4

83

8

soybean oil

CaFe2O4-Ca2Fe2O5-Fe3O4-Fe

100

30 min

83.5

83

Na2O-SiO2/Fe3O4

65

100 min

97%

84

1

9

Cottonseed oil

(oC)

Time

Yield

Substrate

(%)

Besides CaO as the active sites for the synthesis of biodiesels, Huang and co-workers prepared a new magnetic Na2O-SiO2/Fe3O4 base catalyst for the production 36


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of biodiesel in magnetically stabilized fluidized bed reactor (MSFBR).84 Reaction temperature, molar ratio of methanol to oil, and reactant flow rate were considered to be the most important factors that affect the transesterification conversion efficiency over Na2O-SiO2/Fe3O4 catalysts. Among them, methanol to oil molar ratio was found to be the most important determinant of transesterification conversion efficiency. Under the optimum reaction conditions, the conversion efficiency reached 97% in 100 min at 65 oC with methanol to oil molar ratio 8:1, 40 cm3 min-1 reactant flow rate, 225 Oe magnetic field intensity (Table 2, Entry 9). Enzymes immobilized on magnetic supports were also used to catalyze the transesterification oils to biodiesels. Chang and co-workers developed a clean and effective method for biodiesel synthesis by the use of hydrophobic magnetic particles supported lipase.85 Silica layer coated Fe3O4 (Fe3O4@SiO2) was treated with [3-(trimethoxysilyl) propyl] octadecyldimethylammonium chloride to generated an hydrophobic magnetic Fe3O4@SiO2 particles, which then absorbed the Burkholderia lipase to construct the magnetic biocatalyst for the transesterification of olive oil with methanol in the production of biodiesel. The magnetic lipase catalyst could be repeatedly carried out six times without severe activity loss, and have high tolerance with high content of water as compared with chemical catalysts. Similar work was also reported by Chen and co-workers,86 in which they used magnetic chitosan microspheres immobilized Rhizopus oryzae lipase for the production of biodiesel production by the transesterification of soybean oil and with methanol. The immobilized lipase in the reactor showed excellent reusability, retaining 82% productivity even after six batches.

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Although high biodiesel yields could be obtained, there is still room for the improvement the transesterification of vegetable oil, soybean oil or animal fat to produce biodiesels with high quality. Besides the magnetic base catalysts, magnetic acid catalysts should also be tested for the transesterification reaction. The active sites should have high strong base or acidity to effective catalyze the transesterification reaction. In addition, the structure of the active component should also be carefully designed such as with the nano-size and high surface area. In addition, according to the transesterification reaction, the reaction occurred in a hydrophobic environment is also suggested, as the hydrophobic environment benefits the adsorption of the substrate with weak polarity, and the desorption of the byproduct glycerol with a strong polarity. 7. MAGNETIC CATALYSTS USED FOR HYDROGENATION AND PYROLYSIS REACTIONS As discussed above, magnetic catalysts were found to be useful in hydrolysis, dehydration, oxidation of HMF and transesterification reaction in biorefinery. They are also used in other reactions in biomass conversion such as reduction reaction. For most cases, the magnetic catalysts were used the iron oxides mainly Fe3O4 as the magnetic core or support to load active sites for chemical reactions. Other metals such as Ni and Co also showed magnetic property. Some magnetic Ni based catalysts were reported for the reduction in biorefinery, although the magnetic intensity was not strong as the Fe based magnetic catalysts. Wu and coworkers prepared magnetic Ni/Cu/Al/Fe catalysts for the hydrogenation of fructose.87 The magnetic Ni/Cu/Al/Fe catalysts were prepared from

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the reduction of Ni/Cu/Al/Fe hydrotalcite-like compound, which was synthesized by a co-precipitation of the Ni2+, Cu2+, Al3+ and Fe3+ in the solution of NaOH and Na2CO3 at pH = 2. Hydrogenation of fructose generated a ratio of mannitol to sorbitol of 1.2~1.4:1. The desired yields of mannitol and sorbitol were 56.9% and 42.9% at the reaction temperature of 383 K under 3.0 Mba H2 pressure. The catalyst activity could be regenerated by H2 reduction, which possible removed the absorbed substance and regenerated the active sites, and only a slight decrease in the total yields of mannitol and sorbitol. Later, the authors used the same kind of catalyst for the hydrogenation of maltose into sorbitol, affording sorbitol with a yield of 93.1% at 458 K for 3 h.88 Due to the high cost of fructose and maltose, the same authors used cellulose as the starting material for the production of sorbitol over the same kind magnetic catalyst (Ni4.63Cu1Al1.82Fe0.79).89 But a low concentration of phosphoric acid was required to hydrolyze the cellulose into glucose, followed by the hydrogenation of glucose into sorbitol over Ni4.63Cu1Al1.82Fe0.79 catalyst (Fig. 12). A sorbitol yield of 68.07% was obtained at 488 K with a catalyst dosage of 20%. Fukuoka and co-workers developed an efficient method for the conversion of cellulose into sorbitol and mannitol carbon-supported Ni catalyst, affording up to 67% yield of hexitols (sorbitol and mannitol) in the conversion of cellulose at 483 K and 5 MPa H2 pressure.90 Physicochemical analysis indicated that the use of carbon supports had two benefits: no basicity and high water-tolerance. CeO2, ZrO2, γ-Al2O3 and TiO2 cause side-reactions due to basicity, and SiO2, γ-Al2O3 and CeO2 were less stable in hot water. Another important

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factor is high Ni loading as the increase of Ni content from 10 wt% to 70 wt% significantly improved the yield of hexitols and the durability of catalysts. Larger crystalline Ni particles were more resistant to sintering of Ni and surface coverage by Ni oxide species. The carbon supported Ni catalyst was durable in the recycling experiments. The Ni based catalysts could be separated by a permanent magnet only when the weight percentage of Ni was beyond 50%. OH OH HO HO

OH

O HO O O O HO OH OH

OH

Cellulose

OH O HO OH O O OH OH n

H2 O

OH HO HO

H2 O

Glucose

OH

HO OH

OH

OH

OH

H2

OH Sorbitol OH

OH OH

HO OH

OH Mannitol

Fig. 12 Conversion of cellulose to hexitols.Ref.90 Besides the hexitols, other polyol are also found to be useful in chemical industry. Yuan and co-workers studied the aqueous-phase hydrogenolysis of sorbitol to glycols (1,2-propylene glycol and ethylene glycol) over magnetic Ni/Al2O3 catalyst. The authors found that the addition of cerium into the Ni/Al2O3 catalyst improved its catalytic performance.91 The enhanced effect was related to the increase in chemisorption amount of H2 on the Ce-containing catalysts. The 0.5% Ce-20% Ni/Al2O3-CP catalyst with an average particle size of 5 nm showed good activity, high selectivity to glycols and superior recyclability. The sorbitol conversion reached 94% and the overall selectivity to

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glycols was sustained at 55-60% during the eighteen recyclings at 513 K under 7.0 MPa for 12 h. Unlike the reports by Fukuoka and co-workers, in which they reported the Ni catalysts with the weight percentage beyond 50% could be separated by a permanent magnet, the 0.5% Ce-20% Ni/Al2O3-CP catalyst could be fast collected by a permanent magnet in a few minutes, and reused for 11 runs with sorbitol conversion decreasing from 94% to 86%. Besides hydrogenation reaction, magnetic catalysts also applied in other fields. Lu and co-workers prepared magnetic superacid (SO42-/TiO2–Fe3O4) for catalytic fast pyrolysis of cellulose and poplar wood to produce levoglucosenone (LGO).92 The catalytic performance of SO42-/TiO2–Fe3O4 catalyst was a little better than the non-magnetic SO42-/TiO2 and H3PO4, and much better than the H2SO4. The maximal LGO yields reached 15.43 wt% from cellulose and 7.06 wt% from poplar wood at 300 oC with the feedstock/catalyst ratio of 1/1, respectively. 8. CONCLUSION AND PERSPECTIVE As shown here, catalytic conversion of renewable biomass into fuels and chemicals with magnetic nanocatalysts is a rapidly growing field in the context of the high demands for development of sustainable and green chemistry. In order to prevent aggregation and achieve grafting catalyst species on presynthesized MNPs, modification and functionalization of MNPs with coating/encapsulating materials (including silica, polymers, carbon, graphene, carbon nanotubes) are essential. Further covalent or noncovalent binding processes to transition metal catalysts, organocatalysts, and enzymes

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efficiently provides various magnetically recoverable catalysts that can be used in a wide range of reactions. The magnetic catalysts used in biomass transformation can be mainly divided into the following three areas: (1) Immobilization of enzymes on magnetic supports generates the enzymatic catalysts, which are mainly used for the hydrolysis of lignocelluloses, cellulose and polysaccharides into sugars or the transesterification reaction for the production of biodiesels. (2) Magnetic acids catalysts promote the hydrolysis of lignocelluloses, cellulose and polysaccharides into water soluble reducing sugars or the dehydration reaction of fructose based carbohydrates into HMF or EMF in ethanol. (3) Magnetic metal catalysts catalyze the oxidation of HMF into DMF or FDCA, and the hydrogenation and hydrogenolysis reactions for the production of polyols. (4) Magnetic base catalysts promote the production of biodiesels via transesterification reaction reactions. Although remarkable progress has been achieved in biorefinery by the use of magnetic catalyst, further improvements are still necessary in many cases with the aim to realize the commercial application of these process for the production of valuable chemicals through biorefinery. (1) Development of new multifunctionalized materials and useful methods of immobilizing catalysts units are still required in order to overcome these current problems such as the intrinsic instability of metal nanoparticles over a long period of time and the leaching of catalysts under harsh conditions. (2) Extension of the scope of the field by exploring more magnetic catalysts for more biomass transformations is also called for. (3) It is expected that the design novel multi-functional catalysts will allow more substrates as the starting materials to produce chemicals via several steps in

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one-pot reaction, thus avoid the separation and purification of intermediates. (4) The design of mesoporous core-shell magnetic catalyst with large surface area and large pore size to let the reactant freely contact with the active sites. (5) Futher improve acidity or basicity of magnetic catalyst to improve the efficiency of the chemical reactions. (6) The development of an easy method for large scaled and low-cost preparation of magnetic catalysts. (7) The preparation of magnetic catalyst with super magnetic response to further aid separation. (8) The development of none noble based catalysts to replace expansive “noble” metals in oxidation or reduction reactions should be encouraged for increased sustainable processes. (9) It is necessary to get more insights into the mechanism of the transformation reactions, and the structure-property of the catalysts, which is of great significance both scientific and practical viewpoint. Although these processes still need to be improved for the practical use, we believe that the further progress will realize the biorefinery by heterogeneous catalysis to ensure the sustainable development. Acknowledgements This project was supported by the National Natural Science Foundation of China (no. 21203252). AUTHOR INFORMATION Corresponding Author *Tel.: +86-27-67842572. Fax: +86-27-67842572. E-mail: [email protected] Notes 43


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The authors declare no competing financial interest. REFERENCES (1) Kobayashi, H.; Ohta, H.; Fukuoka, A. Catal. Sci. Technol. 2012, 2, 869–883. (2) Lanzafame, P.; Centi, G.; Perathoner, S.; Chem. Soc. Rev. 2014, 43, 7562–7580. (3) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044-4098. (4) Bozell, J. J. Science. 2010, 329, 522-523. (5) Liu, Q. Y.; Bie, Y. W.; Qiu, S. B.; Zhang, Q.; Sainio, J.; Wang, T. J.; Ma, L.L.; Lehtonen, J. Appl. Catal. B-Environ. 2014, 147, 236-245. (6) Osatiashtiani, A.; Lee, A. F.; Granollers, M.; Brown, D. R.; Olivi, L.; Morales, G.; Melero, J. A.; Wilson, K. ACS Catal. 2015, 5, 4345-4352. (7) Wang, T. F. ; Nolte, M. W. ; Shanks, B. H. Green Chem. 2014, 16, 548-572. (8) Werpy, T.; Petersen, G.; Aden, A.; Bozell, J.; Holladay, J.; White, J.; Manheim, A.; Eliot, D.; Lasure, Jones, L. S. U. S. Department of Energy report. Paciﬁc Northwest National Laboratory/U.S. Department of Energy: Oak Ridge, TN, 2004. (9) Dethlefsen, J. R.; Lupp, D.; Teshome, A.; Nielsen, L. B.; Fristrup, P. ACS Catal. 2015, 5, 3638-3647. (10) Shi, D. M.; Vohs, J. M. ACS Catal. 2015, 5, 2177-2183. (11) Fihri, A.; Bouhrara, M.; Nekoueishahraki, B.; Basset,J.M.; Polshettiwar, V. Chem. Soc. Rev. 2011, 40, 5181-5203. (12) Roduner, E. Chem. Soc. Rev. 2006, 35, 583-592. (13) Somorjai, G. A.; Frei, H.; Park, J. Y. J. Am. Chem. Soc. 2009, 131, 16589-16605.

44


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 51

(14) Goesmann, H.; Feldmann, C. Angew. Chem., Int. Ed. 2010, 49, 1362-1395. (15) Arpád, M.; Chem. Rev. 2011, 111 , 2251-2320. (16) Chng, L. L.; Erathodiyil, N.; Ying, J. Y. Acc. Chem. Res. 2013, 46, 1825-1837. (17) Varma, R. S.; Verma, D.; Sinha, A. K.; Jain, S. L. Appl. Catal. A:Gen. 2015, 489, 17-23. (18) Baig, R. B. N.; Varma, R. S. Chem. Commun., 2012, 48, 6220-6222. (19) Polshettiwar, V.; Luque, R.;Fihri, A.; Zhu, H.; Bouhrara, M.; Basset, J. -M. Chem. Rev. 2011, 111, 3036-3075. (20) Baig, R. B. N.; Nadagouda, M. N.; Varma, R. S. Green Chem. 2014, 16, 4333-4338. (21) Varma, R. S.; Baig, R. B. N.; Han, C.; Nadagouda, M. N.; Varma, R. S. Chem. Commun. 2015, 5, 15554-15557. (22) Shylesh, S.; Schünemann, V.; Thiel, W. R. Angew. Chem., Int. Ed. 2010, 49, 3428-3459. (23) Baig, R. B. N.; Varma, R. S. Ind. Eng. Chem. Res. 2014, 53, 18625-18629. (24) Baig, R. B. N.; Varma, R. S. Green Chem. 2012, 14, 625-632. (25) Rossi, L. M.; Garcia, M. A. S.; Vono, L. L. R.; Braz. J. Chem.Soc. 2012, 23, 1959-1971. (26) Baig, R. B. N.; Varma, R. S. Green Chem. 2015, 15, 398-417. (27) Baig, R. B. N.; Nadagouda, M. N.; Varma, R. S. Coordin. Chem. Rev. 2015, 287, 137-156. (28) Dalpozzo, R. Green Chem. 2015, 17, 3671-3686.

45


Page 47 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(29) Baig, R. B. N.; Varma, R. S. Chem. Commun. 2013, 49, 752-770. (30) Varma, R. S. Green Chem. 2014, 16, 2027-2041. (31) Zhang, D.; Zhou, C.; Sun, Z.; Wu, L. Z.; Tung, C. H.; Zhang, T. Nanoscale 2012, 4, 6244-6255. (32) Hyeon, T. Chem. Commun. 2003, 927-934. (33) Yin, M.; Brien, S. O. J. Am. Chem. Soc. 2003, 125, 10180-10181. (34) Laurent, S.; Forge, D.; Port, M.; Roch, A.; Robic, C.; Vander Elst, L.; Muller, R. N. Chem. Rev. 2008, 108, 2064-2110. (35) Robinson, I.; Tung, L. D.; Maenosono, S.; Wälti, C.; Thanh, N. T.; Nanoscale 2010, 2, 2624–2630. (36) Wei, S.; Wang, Q.; Zhu, J.; Sun, L.; Lin, H.; Guo, Z.; Nanoscale 2011, 34474– 4502. (37) Qiu, X. L.; Li, Q. L.; Zhou, Y.; Jin, X. Y.; Qi, A. D.; Yang, Y. W. Chem. Commun. 2015, 51, 4237-4240. (38) Jiang, X.; Yang, X. L.; Zhu, Y. H.; Yao, Y. F.; Zhao, P.; Li, C. Z.; J. Mater. Chem. A. 2015, 3, 2361-2369. (39) Lu, A. -H.; Schmidt, W.; Matoussevitch, N.; Böennemann, H.; Spliethoff, B.; Tesche, B.; Bill, E.; Kiefer, W.; Schüth, F. Angew. Chem. Int. Ed. 2004, 43, 4303-4306. (40) Baig, R. B. N.; Varma, R. S. ACS Sustainable Chem. Eng. 2014, 2, 2155−2158. (41) Wiredu, B.; Amarasekara, A.S. Catal. Commun. 2014, 48, 41–44. (42) Shen, S. G.; Cai, B.; Wang, C.Y.; Li, H. M.; Dai, G.; Qin, H. F. Appl. Catal. A: Gen.

46


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 51

2014, 473, 70-74. (43) Lai, D.; Deng, L.; Guo, Q.; Fu, Y. Energy Environ. Sci. 2011, 4, 3552−3557. (44) Lai, D.; Deng, L.; Li, J.; Liao, B.; Guo. Q.; Fu, Y. ChemSusChem 2011, 4, 55−58. (45) Takagaki, A.; Nishimura, M.; Nishimura, S.; Ebitani, K. Chem. Lett. 2011, 40, 1195−1197. (46) Zhang, C. B.; Wang, H. Y.; Liu, F. D.; Wang, L.; He, H. Cellulose 2012, 20, 127– 134. (47) Liu, Z. W.; Fu, X.; Tang, S. R.; Cheng, Y.; Zhu, L.; Xing, L. L.; Wang, J.; Xue, L. L. Catal. Commun. 2014, 56, 1-4. (48) Verma, D.; Tiwari, R.; Sinha, A. K. RSC Adv. 2013, 3, 13265–13272. (49) Xiong, Y.; Zhang, Z. H.; Wang, X.; Liu, B.; Lin, J. T. Chem. Eng. J. 2014, 235, 349–355. (50) Guo, H. X.; Lian, Y. F.; Yan, L. L.; Qi, X. H.; Smith, R. L. Green Chem. 2013, 15, 2167–2174. (51) Alftren, J.; Hobley, T. J. Biomass Bioenerg. 2014, 65, 72-78. (52) Abraham, R. E.; Verma, M. L.; Barrow, C. J.; Puri, M. Biotechnol Biofuels. 2014, 7, 90-99. (53) Van Putten, R. J.; Van der Waal, J. C.; De Jong, E.; Rasrendra, C. B.; Heeres, H. J.; De Vries, J. G. Chem. Rev. 2013, 113, 1499–1597. (54) Saha, B.; Abu-Omar, M. M. Green Chem. 2014, 16, 24–38. (55) Dutta, S.; De, S.; Saha, B. Biomass Bioenerg. 2013, 55, 355–369.

47


Page 49 of 51

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(56) Rosatella, A. A.; Simeonov, S. P.; Frade, R. F. M.; Afonso, C. A. M Green Chem. 2011, 13, 754-793. (57) Zhang, X. C.; Wang, M.; Wang, Y. H.; Zhang, C. F.; Zhang, Z.; Wang, F.; Xu, J.; Chinese J. Catal. 2014, 35, 703-708. (58) Martinez, J. J.; Nope, E.; Rojas, H.; Cubillos, J.; Sathicq, A. G.; Romanelli, G. P. Catal. Lett. 2014, 144, 1322–1331. (59) Hu, L.; Tang, X.; Wu, Z.; Lin, L.; Xu, J. X.; Xu, N.; Dai, B. L. Chem. Eng. J. 2015, 263, 299-308. (60) Lan, J. F.; Zhang, Z. H. J. Ind. J. Eng. Chem. 2015, 23, 200–205. (61) Lee, Y. C.; Dutta, S.; Wu, K. C. W. ChemSusChem 2014, 7, 3241-3246. (62) Mascal, M.; Nikitin, E. B. Angew. Chem. Int. Ed. 2008, 47, 7924-7926. (63) Mascal, M.; Nikitin, E. B. ChemSusChem 2009, 2, 859-861. (64) Wang, S. G.; Zhang, Z. H.; Liu, B.; Li, J. L. Catal. Sci. Technol. 2013, 3, 2104-2112. (65) Zhang, Z. H.; Wang, Y. M.; Fang, Z. F.; Liu, B. ChemPlusChem 2014, 2, 233-240. (66) Yuan, Z. L.; Zhang, Z. H.; Zheng, J. D.; Lin, J. T. Fuel 2015, 150, 236-242. (67) Yin, S. S.; Sun, J.; Liu,B.; Zhang, Z. H. J. Mater. Chem. A 2015, 3, 4992-4999. (68) Jacquel, N.; Saint-Loup, R.; Pascault, J. P.; Rousseau, A.; Fenouillot, F. Polymer 2015, 59, 234–242. (69) Antonyraj, C. A.; Jeong, J.; Kim, B.; Shin, S.; Kim, S.; Lee, K.-Y.; Cho, J. K. J. Ind. Eng. Chem. 2013, 19, 1056–1059. (70) Wang, S. G.; Zhang, Z. H.; Liu, B.; Li, J. L. Ind. Eng. Chem. Res. 2014, 53, 5820–

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5827. (71) Zhang, Z. H.; Yuan, Z. L.; Tang, D. G.; Ren, Y. S.; Lv, K. L.; Liu, B. ChemSusChem 2014, 7, 3496-3504. (72) Karimi, B.; Mirzaei, H. M.; Farhangi, E. ChemCatChem 2014, 6, 758–762. (73) Yang, Z. Z.; Deng, J.; Pan, T.; Guo, Q. X.; Fu, Y. Green Chem. 2012, 14, 2986– 2989. (74) Liu, B.; Zhang, Z. H.; Lv, K. L.; Deng, K. J.; Duan, H. M. Appl. Catal. A: Gen. 2014, 472, 64–71. (75) Zhang, Z. H.; Zhen, J. D.; Liu, B.; Lv, K. L.; Deng, K. J. Green Chem. 2015, 17, 1308–1317. (76) Mei, N.; Liu, B.; Zhang, Z. H. Catal. Sci. Technol. 2015, 5, 3194-3202 (77) Liu, B.; Ren, Y. S.; Zhang, Z. H. Green Chem. 2015, 17, 1610–1617. (78) Wang, S. G.; Zhang, Z. H.; Liu, B.ACS Sustainable Chem. Eng. 2015, 3,406−412. (79) Zhang, P. B.; Han; Q. J.; Fan, M. M.; Jiang, P. P. Appl. Surf. Sci. 2014, 317 , 1125-1130. (80) Liu, C.; Lv, P. M.; Yuan, Z. H.; Yan, F.; Luo, W. Renew. Energy. 2010, 35, 531– 1536. (81) Hu, S. Y.; Guan, Y. P.; Wang, Y.; Han, H. Y. Appl. Energ. 2011, 88, 2685-2690. (82) Lin, L.; Vittayapadung, S.; Li, X. Q.; Jiang, W. W.; Shen, X. Q. Environ. Prog. Sustain. Energ. 2013, 32, 1255-1261. (83) Xue, B. J.; Luo, J.; Zhang, F.; Fang, Z. Energy 2014, 68, 584-591.

49


Page 51 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(84) Guo, P.

M.; Huang, F.H.; Huang, Q. D.; Zheng, C. J. Am. Oil Chem. Soc. 2012, 89,

497-504. (85) Liu, C. H.; Huang, C. C.; Wang, Y. W.; Lee, D. J.; Chang, J.S. Appl. Energ. 2012, 100 , 41-46. (86) Zhou, G. X.; Chen, G. Y.; Yan, B. B. Biotechnol. Lett. 2014, 36, 63–68. (87) Zhang, J.; Xu, S. H.; Wu, S. B.; Liu, Y. Chem. Eng. Sci. 2013, 99, 171-176. (88) Zhang, J.; Li, J. B.; Wu, S. B.; Liu, Y. Bioresources 2013, 8, 4676-4686. (89) Zhang, J.; Wu, S. B.; Liu, Y. Energ. Fuel 2014, 28, 4242-4246. (90) Kobayashi, H.; Hosaka, Y.; Hara, K.; Feng, B.; Hirosaki, Y. A. Fukuoka, Green Chem. 2014, 16, 637-644. (91) Ye, L. M.; Duan, X. P.; Lin, H. Q.; Yuan, Y. Z. Catal. Today. 2012, 183, 65-71. (92) Lu, Q.; Ye, X. N.; Zhang, Z. B.; Dong, C. Q.; Zhang, Y. Bioresourc. Technol. 2014, 171, 10-15.

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