Conversion of Furfuryl Alcohol to Levulinic Acid in Aqueous Solution

Subscriber access provided by READING UNIV

Article

Conversion of Furfuryl Alcohol to Levulinic Acid in Aqueous Solution Catalyzed by Shell Thickness-Controlled Arenesulfonic Acid Functionalized Ethyl-Bridged Organosilica Hollow Nanospheres Sai An, Daiyu Song, Yingnan Sun, Qingqing Zhang, Panpan Zhang, and Yihang Guo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03133 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39 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 Sustainable Chemistry & Engineering

Conversion of Furfuryl Alcohol to Levulinic Acid in Aqueous Solution Catalyzed by Shell ThicknessControlled Arenesulfonic Acid Functionalized Ethyl-Bridged Organosilica Hollow Nanospheres Sai Ana,b, Daiyu Songa, Yingnan Suna, Qingqing Zhanga, Panpan Zhanga, and Yihang Guoa,* a

School of Environment, Northeast Normal University, 2555 Jingyue Street, Changchun

130117, P.R. China b

State Key Laboratory of Chemical Resource Engineering, 15 North Third Ring Road, Beijing

University of Chemical Technology, Beijing 100029, P.R. China

ACS Paragon Plus Environment

1

ACS Sustainable Chemistry & Engineering 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 2 of 39

ABSTRACT: A series of arenesulfonic acid functionalized ethyl-bridged organosilica hollow nanospheres, ArSO3H-Et-HNS, with different shell thicknesses (2−6 nm) were successfully fabricated by P123 and/or F127-directed sol-gel co-condensation route and carefully controlled molar composition of the starting materials in the synthetic gel. The ArSO3H-Et-HNS were applied in the synthesis of levulinic acid (LA) from the hydrolysis of furfuryl alcohol (FAL), and the influence of experimental parameters including solvent nature, volume ratio of FAL-tosolvent-to-water, shell thickness of hollow nanospheres and reaction temperature were considered. The hydrolysis activity of the ArSO3H-Et-HNS outperformed commercially available HY-zeolite, Amberlyst-15 and p-toluenesulfonic acid, regardless of the shell thickness of the hollow nanospheres; additionally, P123-directed ArSO3H-Et-HNS with the thinnest shell (2 nm) and the largest BET surface area (529 m2 g–1) exhibited the highest hydrolysis activity among various ArSO3H-Et-HNS nanohybrids, and under the conditions of 0.72 mol L−1 FAL in 4:1 acetone-H2O, 3 wt% catalyst and 120 oC, the yield of LA reached 83.1% after the reaction proceeded for 120 min. The excellent hydrolysis activity of the ArSO3H-Et-HNS was explained in terms of the strong Brønsted acid nature, unique hollow spherical nanostructures with opening shell and hydrophobic surface. Based on the catalytic test results and the identified intermediates, the reaction mechanism of the ArSO3H-Et-HNS-catalyzed hydrolysis of FAL to LA was tentatively revealed. Finally, the reusability of the ArSO3H-Et-HNS was studied through three consecutive catalytic cycles.

KEYWORDS: Biomass, Solid acid, Hollow nanosphere, Furfuryl alcohol, Levulinic acid


2

Page 3 of 39 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


INTRODUCTION Population growth and industrialization have brought concerns regarding the depletion of fossil fuel reserves and the increase of anthropogenic CO2 emissions. Facing to these problems, efforts to search for the alternative resources are necessary. Renewable, abundant and low-cost lignocellulosic biomass are the promising alternative feedstocks, and acid catalyzed-biomass conversion to carbon-neutral transportation fuels and versatile chemical building blocks have therefore attracted worldwide attention.1–3 Levulinic acid (LA), one of the top 12 most attractive and important biomass-derived chemical building blocks, can be used as a versatile intermediate to produce a broad range of chemicals (e.g., diphenolic acid4 and δ-aminolevulinic acid5), fuel additives (e.g., alkyl levulinates6,7 and γ-valerolactone8–10) and liquid hydrocarbon fuels.6,10 The synthesis of LA from lignocellulosic biomass is accomplished by acid-catalyzed hydrolysis of C6-carbohydrates in cellulose or C5-carbohydrates in hemicellulose.9,11–15 For the first route, C6carbohydrates suffer from dehydration in acidic media to form hydroxymethylfurfural, and which undergo hydration to produce an equi-molar mixture of LA and formic acid.16 In the case of the second route, dehydration of C5-carbohydrates form furfural firstly, and furfuryl alcohol (FAL) is formed after the hydrogenation of furfural. Further hydrolysis of FAL can produce LA.17 In this route, conversion of C5-carbohydrates to LA can improve the carbon utilization effectively and energy efficiency, and it is worthy nothing that the hydrolysis of FAL is of fundamental importance in the synthesis of LA.16,18 Homogeneous strong Brønsted acids such as H2SO4, HCl or p-toluenesulfonic acid can catalyze the above process, however, one of the major challenges of producing LA from FAL is the high catalytic activity of acid-catalyzed polymerization of FAL into unwanted oligomeric products or humins, resulting in poor selectivity to LA; meanwhile, the use of homogeneous acids suffers from drawbacks including


3


Page 4 of 39

risk in handling, corrosive to equipment, pollution to environment and difficulty in their separation, recovery and reuse. Driven by environmental and economic considerations as well as safety concerns, the replacement of hazardous and corrosive liquid acids by environmentally friendly solid acids is one of the important tasks for green and sustainable production of LA from lignocellulosic biomass. For this purpose, some solid acids such as zeolites,16,19,20 heteropoly acids,11 sulfonated zirconia21 and ion exchange resins16,17 have been studied in the LA production from the hydrolysis of lignocellulosic biomass/derivatives. One of the major challenges in the above process is FAL polymerization and carbonization, which significantly affects the yield of LA and the reusability of the solid acids. Therefore, rational catalyst design and optimized the experimental parameters (e.g., solvent nature, concentrations of the substrates and temperature) are essential to reach a high LA yield. Aiming at the development of novel solid acid catalysts for efficient hydrolysis of FAL to LA, in the present work, a series of arenesulfonic acid-functionalized ethyl-bridged organosilica hollow nanospheres (ArSO3H-Et-HNS) with controllable shell thicknesses are successfully fabricated by a copolymer surfactant-templated sol-gel co-condensation strategy. Alkyl-bridged organosilicas are the novel and robust high-surface-area supports with well-defined mesoporosity, tunable morphological characteristics and surface hydrophilicity/hydrophobicity; moreover, they can be easily functionalized by various active components.22–25 Functionalization of organosilicas by sulfonic acid groups can give rise to strong Brønsted solid acids that have found wide applications in various acid-catalyzed biomass conversion processes;24–30 moreover, the heterogeneous acid catalytic activity of the −SO3H-functionalized organosilicas can be further improved

by

tuning

their

morphological

characteristics

and

surface

hydrophilicity/hydrophobicity, which can influence the substrate accessibility to the acid sites.31–


4

Page 5 of 39 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


35

As-prepared ArSO3H-Et-HNS nanohybrids (particle size of ca. 20 nm) with the outstanding

advantages including hollow interiors, permeable and thin shells (shell thickness of 2−6 nm), shortened diffusion distance and excellent porosity properties can serve as the nanoreactors for FAL transformations. Additionally, the ArSO3H-Et-HNS nanohybrids have hydrophobic surface owing to the presence of bridging ethyl groups in silica/carbon framework, which is expected to inhibit FAL polymerization in some extent, ensuring the production of LA with high yield. During the catalytic studies, the influence of various important experimental parameters including solvent nature, volume ratio of FAL-to-solvent-to-water, shell thickness of hollow nanospheres and reaction temperature are considered. Since the shell thickness can influence the diffusion and mass transfer of the reactants and products and thereby the activity, we pay special attention to evaluate the influence of the shell thickness on the catalytic activity of the ArSO3HEt-HNS. The hydrolysis activity of the ArSO3H-Et-HNS is also compared with that of the commercially available reference acid catalysts including HY-zeolite, Amberlyst-15 and ptoluenesulfonic acid. Based on the catalytic test results and the identified intermediates, the reaction mechanism of the ArSO3H-Et-HNS-catalyzed hydrolysis of FAL to LA is tentatively revealed.

EXPERIMENTAL SECTION Catalyst Preparation P123 ArSO3H-Et-HNSx. Typically, P123 (EO20PO70EO20, EO = CH2CH2O, PO = CH2(CH3)CHO, 0.5 g) was dissolved in a mixture of water (12.7 mL), HCl (12 mol L−1, 2.4 mL) and 1,3,5-trimethylbenzene (TMB, 1.3 mL) under stirring at room temperature for 1 h. Subsequently,

1,2-bis(trimethoxysilyl)ethane

(BTMSE,

0.6

mL)

and

2-(4-

chlorosulfonylphenyl)ethyl trimethoxysilane (CSPTMS, 50% in dichloromethane, 0.22 mL)


5


Page 6 of 39

were added successively to the above clear solution at the interval of 45 min. The molar composition

of

the

starting

materials

in

the

synthetic

gel

was

P123:H2O:HCl:TMB:BTMSE:CSPTMS = 0.86:7056:282:93:23.4:5.2. The resulting suspension was stirred at 40 oC for 24 h, and then it was transferred to an autoclave followed by heating at 100 oC with a heating rate of 2 oC min−1 for additional 24 h. Subsequently, the product was dried at 100 oC for 12 h. P123 in the product was removed completely by boiling ethanol washing. The surfactant-free product was air dried at 100 oC overnight, and it is denoted as P123 ArSO3H-EtHNSx, where x represents the loading of −SO3H group in the product. F127 ArSO3H-Et-HNSx. The preparation process is the same as that described in P123 ArSO3H-Et-HNSx but replacement of F127 (EO106PO70EO106, 0.175 g) for P123, accordingly, the concentrations of the other reactants in the synthetic gel were different. The initial molar composition was F127:H2O:HCl:TMB:BTMSE:CSPTMS = 0.14:7167:221:14.6:27.2:5.6. P123-F127 ArSO3H-Et-HNSx. The preparation process is the same as that described in P123 ArSO3H-Et-HNSx but replacement of the mixture of P123 and F127 for P123. Based on the different concentrations of the initial reactants, the products are denoted as P123-F127 ArSO3HEt-HNSx(1)

and

P123-F127

ArSO3H-Et-HNSx(2),

respectively.

The

initial

P123:F127:H2O:HCl:TMB:BTMSE:CSPTMS molar composition for the preparation of the P123-F127 ArSO3H-Et-HNS7.2(1) was 0.3:0.14:7167:221:14.6:27.2:5.6, and the molar composition was 0.3:0.14:1006:316:50.7:36.4:7.4 for the preparation of the P123-F127 A rSO3H-Et-HNS6.4(2). Catalyst Characterization Nitrogen gas porosimetry measurements were performed on a Micromeritics ASAP 2020M surface area and porosity analyzer after the samples were outgassed under vacuum at 363 K for 1


6

Page 7 of 39 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


h and 373 K for another 12 h. TEM observations were performed on a JEM-2100F high resolution transmission electron microscope at an accelerating voltage of 200 kV.

13

C cross

polarization-magic angle spinning (CP-MAS) NMR and 29Si MAS NMR spectra were recorded on a Bruker AVANCE III 400 WB spectrometer equipped with a 4 mm standard bore CP MAS probe head. The dried and finely powdered samples were packed in the ZrO2 rotor closed with Ke–F cap which were spun at 12 KHz rate. Chemical shifts for

13

C CP-MAS NMR and

29

Si

MAS NMR spectra were referenced to the signal of C10H16 standard (δCH2 = 38.5) and 3(trimethylsilyl)-1-propanesulfonic acid sodium salt standard (δ = 0.0), respectively. XPS was performed on an Axis Ultra DLD instrument with a monochromated Al-Kα source at a residual gas pressure of below 10−8 Pa. All the binding energies were referenced to the C 1s peak at 285 eV of the surface adventitious carbon. Thermogravimetric analysis (TGA) was conducted on a PerkinElmer Pyris Diamond TG/DTA instrument under air atmosphere. The Brønsted acid site densities (A, µeq(H+) g−1) of as-prepared ArSO3H-Et-HNS catalysts were determined by an ICAP6300-Thermoscientific inductively coupled plasma-optical emission spectroscopy (ICP-OES). Specifically, the sulfur contents (wt.%) of the ArSO3H-Et-HNS catalysts were firstly determined by ICP-OES after the samples were digested by 2 mol L−1 NaOH solution and then diluted with deionized water to suitable volumes. Afterwards, the ArSO3H loadings or H+ contents of the catalysts were obtained by calculation. The Brønsted acid strength of the catalysts was measured by a WDDY-2008J microcomputer automatic potentiometric titration instrument, and the procedure followed by the reported method.36,37 The sample (50 mg) was suspended in acetonitrile and stirred for 3 h, and then the suspension was titrated with 0.1 mol L‒1 n-butylamine/acetonitrile solution. Catalytic Tests


7


Page 8 of 39

The catalysts were dried at 120 oC in vacuum for 2 h before the tests. Typically, hydrolysis of FAL to produce LA was performed under the conditions of 0.72 mol L−1 FAL in 4:1 acetoneH2O, 3 wt% catalyst and 120 oC. After the reaction, 0.1 mL of the separated reaction liquid was withdrawn and dried at room temperature overnight to remove acetone. After being diluted with acetonitrile, the clear solution was analyzed by an Agilent Technologies 1200HPLC to obtain the concentration of the yielded LA. The HPLC was equipped with a VisionHT C18 column (film thickness 5 µm, i.d. 4.6 mm, length 250 mm) and an UV detector. The catalytic activity was evaluated quantitatively by the yield of LA (Y, %), and Y (%) = (MD/MT) × 100 with MD and MT representing the number of moles of LA produced and theoretically calculated. The possible intermediates and byproducts yielded during the process of FAL hydrolysis were identified by both mass spectrometry equipped with high performance liquid chromatography (Waters Acquity UPLC-Quattro Premier XE) and an UV detector, and mass spectrometry coupled with gas chromatography (HP6890GC-5973MSD).

RESULTS AND DISCUSSION

Preparation and Characterization of the ArSO3H-Et-HNS Nanohybrids. Catalyst Preparation. The ArSO3H-Et-HNS nanohybrids are prepared by P123 and/or F127directed sol-gel route, and the process includes co-hydrolysis and -condensation of bridging organosilane BTMSE and CSPTMS in the presence of micelle-expanding agent (TMB) under acidic environment. In order to obtain the hybrids with hollow spherical nanostructures, the molar composition of the starting materials (P123/F127:H2O:HCl:TMB:BTMSE:CSPTMS) in the synthetic gel should be adjusted carefully, which affects the rates of hydrolysis and condensation of silica precursors and thereby the morphologies of lyotropic liquid crystal


8

Page 9 of 39 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


phases.38–40 After formation of the ArSO3H-functionalized silica/carbon framework, it suffers from hydrothermal treatment at 100 oC to further fasten the linkage of ArSO3H groups with the framework. The hollow spherical nanostructures are finally constructed after removal of P123 and/or F127 by boiling ethanol washing (Scheme 1).

Scheme 1. Illustration of the Preparation of Arenesulfonic Acid Functionalized Ethyl-Bridged Organosilica Hollow Nanospheres.

In comparison of the post grafting preparation route with multiple step process, the sol-gel strategy is simple and time-saving, and the product exhibits well-defined mesoporosity; meanwhile, the in-situ incorporated ArSO3H groups with controllable loadings are distributed uniformly throughout the silica/carbon framework. Most importantly, the ArSO3H groups can bond to the silica/carbon framework covalently through a ‒Si‒O1.5‒Si‒CH2‒CH2‒Si‒O1.5‒Si‒ CH2CH2‒C6H4‒SO3H covalent linkage during the co-hydrolysis and -condensation process (Scheme 1), which can effectively restrict the leaching of the ArSO3H groups from the organosilica supports and maintain high catalytic stability of the ArSO3H-Et-HNS nanohybrids even under harsh reaction conditions.


9


Page 10 of 39

In the above preparation system, the shell thickness of the ArSO3H-Et-HNS nanohybrids can be tuned by changing the molar composition of the starting materials in the synthetic gel. It is found that: i) with the molar ratio of the F127-to-H2O-to-HCl-to-TMB-to-BTMSE-to-CSPTMS remaining unchangable, the addition of P123 in the preparation system can make the shell thickness of the ArSO3H-Et-HNS decrease in some extent, and the P123 ArSO3H-Et-HNS6.7 obtained in a F127-free system had the thinnest shell. The shell thickness of P123 ArSO3H-EtHNS6.7, P123-F127 ArSO3H-Et-HNS7.2(1), P123-F127 ArSO3H-Et-HNS6.4(2) and F127 ArSO3H-Et-HNS6.9 was 2, 3, 4 and 6 nm, respectively (see Figure 1 and Table 1). Amphiphilic copolymer surfactants such as P123 and F127 with both hydrophilic −CH2CH2O (EO) and hydrophobic −CH2(CH3)CHO (PO) blocks can readily self-assemble into micelles through hydrogen bonding and hydrophobic/hydrophilic interactions under acidic conditions. The micelles can further aggregate to form spherical lyotropic liquid crystal structures under current preparation conditions. In this liquid-crystal structure, the −CH2(CH3)CHO blocks formed the core of the spherical micelles, while the −CH2CH2O blocks formed a hydrated corona around the core. At the same time, the hydrophobic TMB molecules as a swelling agent are dissolved in the core of P123 and/or F127 micelles to expand micelles. With the penetration of TMB molecules into the micelles, the charge density on the surface of micelles further decreases, leading to welldispersed hollow nanospheres. On the basis of the above discussion, the shell thickness of the nanospheres depended on the polymerization degree of −CH2CH2O blocks. The structure of P123 and F127 is EO20PO70EO20 and EO106PO60EO106, obviously, the polymerization degree of −CH2CH2O blocks in P123 is much lower than that in F127. Hence, the shell thickness of the hollow nanospheres is clearly influenced by the concentration of P123 in the preparation system;


10

Page 11 of 39 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


and ii) under the same P123 and F127 concentration, the H2O-to-HCl-to-TMB-to-BTMSE-toCSPTMS molar ratio also influenced the shell thickness of the ArSO3H-Et-HNS nanohybrids.

Figure 1. TEM images of the P123 ArSO3H-Et-HNS6.7 (a); P123-F127 ArSO3H-Et-HNS7.2(1) (b); P123-F127 ArSO3H-Et-HNS6.4(2) (c) and F127 ArSO3H-Et-HNS6.9 (d).

Morphological Characteristics and Textural Properties. TEM technique is applied to reveal the morphology of as-prepared ArSO3H-Et-HNS nanohybrids. As presented in Figure 1, four ArSO3H-Et-HNS nanohybrids are all composed of small and uniform spherical nanostructures with hollow interior and similar particle size (18‒21 nm, Table 1). However, their shell thickness is different (Table 1). The P123 ArSO3H-Et-HNS6.7 (2 nm, Figure 1a) and F127 ArSO3H-EtHNS6.9 (6 nm, Figure 1d) has the thinnest and thickest shell. As for the P123-F127 ArSO3H-EtHNS7.2(1) and P123-F127 ArSO3H-Et-HNS6.4(2) prepared at the same concentration of P123


11


Page 12 of 39

and F127 but different H2O-to-HCl-to-TMB-to-BTMSE-to-CSPTMS molar ratios, their shell thickness is 3 (Figure 1b) and 4 nm (Figure 1c), respectively. HRTEM observation is applied to study the opening of the shell by selecting the F127 ArSO3H-Et-HNS6.9 as the representative sample. As shown in Figure S1, the shell of the F127 ArSO3H-Et-HNS6.9 nanohybrids is opening with uneven distributed micropores (< 2 nm). Table 1. Textural Parameters, Particle Size, Shell Thickness, Initial Electrode Potential and Acid Density of Various ArSO3H-Et-HNS Nanohybrids. SBETa

Dpb

Vp c

Sd

δd

Ei e

Af

(m2 g–1)

(nm)

(cm3 g–1)

(nm)

(nm) (mV) (µeq(H+) g‒1)

529

12

1.0

19

2

550

832

P123-F127 ArSO3H-Et-HNS7.2(1) 378

9/30

1.0

18

3

567

892

P123-F127 ArSO3H-Et-HNS6.4(2) 499

9/25

1.2

20

4

546

853

F127 ArSO3H-Et-HNS6.9

8

0.5

21

6

570

796

Catalyst

P123 ArSO3H-Et-HNS6.7

a

448

Surface area (SBET) was calculated using Brunauer–Emmett–Teller (BET) equation. bPore diameter

(Dp) was estimated from BJH adsorption determination. cPore volume (Vp) was estimated from the pore volume determination using the adsorption branch of the N2 isotherm curve at P/P0 = 0.99 single point. d e

Particle size (S) and shell thickness (δ) of hollow nanospheres were estimated from TEM images.

Initial electrode potential (Ei) was measured by nonaqueous potentiometric titration with n-butylamine

in acetonitrile. fAcid density (A) was calculated by the sulfur contents determined by ICP-OES method.

As-prepared ArSO3H-Et-HNS nanohybrids exhibit interesting hierarchical pore structure with both micro and mesoporosity, which are confirmed by nitrogen gas porosimetry measurements. At first, the mesoporosity of the samples are studied by using micro-meso porous program, and the resulting nitrogen gas adsorption-desorption isotherms and BJH pore size distribution curves


12

Page 13 of 39 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


are shown in Figure 2a and b. All tested ArSO3H-Et-HNS nanohybrids exhibit type IV isotherm, suggesting their mesoporosity; additionally, they all exhibit two hysteresis loops. The capillary condensation steps occurring at P/P0 = 0.45−0.90 and 0.90‒0.99, respectively, for the thinnest ArSO3H-Et-HNS, while the condensation steps appear at P/P0 = 0.40‒0.80 and 0.85‒0.99, respectively, for the other three nanohybrids. Additionally, with the shell gradually thickening, the first hysteresis loop of the ArSO3H-Et-HNS becomes small gradually, and the P123 ArSO3HEt-HNS6.7 with the thinnest shell possesses the largest hysteresis loop at lower P/P0 region. The aforementioned interesting textural properties of the nanohybrids are reflected in their corresponding BJH pore size distribution curves; additionally, the results obtained are in agreement with the TEM observations. The P123-F127 ArSO3H-Et-HNS7.2(1) and P123-F127 ArSO3H-Et-HNS6.4(2) exhibit double peaks, implying the bimodal porous structure of the nanohybrids (Figure 2b). The narrower peak centered at 9 nm corresponds to the uniform hollow interior of the nanospheres, while the wider peak positioned at 30 or 25 nm (Table 1) is attributed to the void space between the loosely packed spherical nanoparticles. As for the P123 ArSO3HEt-HNS6.7 and F127 ArSO3H-Et-HNS6.9 nanohybrids, only one type of pore centered at 12 or 8 nm was found, attributing to the hollow interior of the nanospheres. It is inferred that the unfound larger pore may appear beyond the testing range (larger than 50 nm). The microporosity of the ArSO3H-Et-HNS nanohybrids is subsequently studied by the nitrogen gas porosimetry measurement under microporous program, and the F127 ArSO3H-EtHNS6.9 (shell thickness of 6 nm, Table 1) is chosen as the representative sample. As shown in Figure 2c, the adsorption isotherm of the F127 ArSO3H-Et-HNS6.9 exhibits a rapid increase in the amount of nitrogen adsorbed at P/P0 lower than 0.1, and the amount of nitrogen adsorbed reaches 114 cm3 g‒1 at P/P0 = 0.099. The result suggests that the F127 ArSO3H-Et-HNS6.9


13


Page 14 of 39

nanohybrids also possess the microporosity. From the pore size distribution curve shown in inset of Figure 2c it is found that the micropores are mainly centering at 0.8, 1.5 and 2.0 nm, respectively. The t-plot obtained from the low P/P0 portion of the isotherm using an empirical Harkins-Jura equation can further evidence the microposity of the sample (Figure 2d). The plot shows the linearly increased amount of the nitrogen adsorbed from the thickness-Harkins and Jura of 0.26 to 0.37 nm, and the increase of the adsorbed nitrogen amount becomes slow gradually with further increasing the thickness. This is the characteristic of the materials with microporosity. Combination of the results of nitrogen gas porosimetry measurement, TEM and HRTEM it is concluded that as-prepared ArSO3H-Et-HNS nanohybrids possess both micropores and mesopores. The micropores of the nanohybrids come from their shell, while the mesopores originate from both the hollow interior of the nanospheres and void space between the loosely packed spherical nanoparticles. The calculated textural parameters are summarized in Table 1. It shows that four ArSO3H-EtHNS nanohybrids with similar ArSO3H loading (6.4‒7.2%) possess considerably large BET surface area (378‒529 m2 g−1) and high pore volume (0.5‒1.0 cm3 g−1); additionally, the BET surface area and pore volume of hollow spherical nanohybrids with different shell thicknesses are different. The P123 ArSO3H-Et-HNS6.7 nanohybrids possess the largest BET surface area (529 m2 g−1) and the highest pore volume (1.0 cm3 g−1); as for the P123-F127 ArSO3H-EtHNS7.2(1) and P123-F127 ArSO3H-Et-HNS6.4(2), they possessed the same BET surface area (378 m2 g−1), and the latter (0.9 cm3 g−1) with the thicker shell had the lower pore volume with respect to the former (1.0 cm3 g−1). The F127 ArSO3H-Et-HNS6.9 nanohybrids with the thickest


14

Page 15 of 39

shell exhibit the lowest pore volume (0.5 cm3 g−1), further confirming the smallest hollow interior of it.

P123-F127 ArSO3H-Et-HNS7.2 (1)


1600


P123-F127 ArSO3H-Et-HNS6.4 (2) F127 ArSO3H-Et-HNS6.9

dV/dlog(D)

Vads/cm3g-1

1.5


1200

800

400

F127 ArSO3H-Et-HNS6.9 P123 ArSO3H-Et-HNS6.7

1.0

0.5

0

a 0.0

0.2

0.4 0.6 P/P0

0.8

b

0.0

1.0

10

100

Dp/nm 150

3

Volume Adsorbed (cm /g)

120

0.75

-1

Pore Volume (cm3/g)

80 3

Vads/cm g

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


40

0.50

0.25

0.00 0

1

2

3

Dp (nm)

0 0.00

0.02

0.04

P/P0

0.06

0.08

c 0.10

140 130 120 110 100

d

90 0.2

0.3

0.4

0.5

0.6

0.7

0.8

Thickness-Harkins and Jura (nm)

Figure 2. Nitrogen gas adsorption-desorption isotherms (a) and BJH pore size distribution profiles (b) of various ArSO3H-Et-HNS nanohybrids determined by micro-meso porous program. Nitrogen gas adsorption isotherm (c), pore size distribution curve (inset of Figure 2c) and t-plot (d) of the F127 ArSO3H-Et-HNS6.9 nanohybrids determined by microporous program.


15


Structural Information. The structural integrity of the silica/carbon framework and the incorporated ArSO3H groups in as-prepared ArSO3H-Et-HNS nanohybrids are confirmed by S 2p XPS (Figure 3), 13C CP-MAS NMR (Figure 4a) and 29Si MAS NMR (Figure 4b) by choosing the P123 ArSO3H-Et-HNS6.7, P123-F127 ArSO3H-Et-HNS7.2(1) and F127 ArSO3H-Et-HNS6.9 as the representative samples. The S 2p XP spectrum of the P123 ArSO3H-Et-HNS6.7 nanohybrids is deconvoluted into two signals centered at 168.4 and 170.0 eV (Figure 3), originating from S 2p3/2 and S 2p1/2 spin-orbit components correspondingly. Similarly, these two signals are also found in the P123-F127 ArSO3H-Et-HNS7.2(1) and F127 ArSO3H-Et-HNS6.9 nanohybrids. Therefore, it is confirmed that the ArSO3H groups are successfully incorporated into the silica/carbon framework of the nanohybrids. S 2p3/2 S 2p1/2 P123 ArSO3H-Et-HNS6.7

S 2p Intensity (a.u.)

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 16 of 39

S 2p3/2

S 2p1/2 P123-F127 ArSO3H-Et-HNS7.2 (1)

S 2p3/2

S 2p1/2 F127 ArSO3H-Et-HNS6.9

176

172

168

164

160

Binding energy (eV)

Figure 3. S 2p XP spectra of the P123 ArSO3H-Et-HNS6.7, P123-F127 ArSO3H-Et-HNS7.2(1) and F127 ArSO3H-Et-HNS6.9.


16

Page 17 of 39 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


Figure 4.

13

C CP-MAS NMR (a) and

29

Si MAS (b) spectra of the P123 ArSO3H-Et-HNS6.7,

P123-F127 ArSO3H-Et-HNS7.2(1) and F127 ArSO3H-Et-H NS6.9 nanohybrids.

Five signals centered at δ = 15.8, 27.9, 127.6, 139.8, and 148.6 ppm in the 13C CP-MAS NMR spectra of the representative samples correspond to the carbon species of the ArSO3H groups (C1‒C5 in Figure 4a) from the ArSO3H-Et-HNS nanohybrids, while the strongest signal positioned at 5.3 ppm is attributed to the carbon species of bridging ethyl groups of the silica/carbon framework (C6 in Figure 4a). In addition, the other two peaks in the 13C CP-MAS NMR spectra positioned in the range of 50‒75 ppm originates from the carbon species of the residual P123. In the

29

Si MAS NMR spectra, the characteristic resonance at ‒66.0 ppm is

assigned to the organosiloxane species of T3 [‒CH2CH2Si(OSi)3] within the ethyl-bridged organosilica framework (Figure 4b). Additionally, the resonance signals corresponding to inorganic SiO4 species such as Q3 [Si(OSi)3(OH), δ = −90 ppm] and Q4 [Si(OSi)4, δ = −120 ppm]33,41–43 are not found. Combination of 29Si MAS NMR and 13C CP-MAS NMR spectra it is evidenced that the framework of ArSO3H functionalized ethyl-bridged organosilica is


17


successfully fabricated during the process of P123 and/or F127-directed sol-gel co-condensation followed by hydrothermal treatment, and the formed framework constructed the shell structure of the ArSO3H-Et-HNS nanohybrids without the cleavage of Si−C bond.

100

90

Weight (%)

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 18 of 39

o

191.7 C

80 o

166.1 C

o

598.5 C

o

456.9 C

70

o

o

168.8 C

o

456.9 C o

471.6 C

598.5 C o

581.6 C

60 200

400

600

800

1000

o

Temperature ( C)

Figure 5. TG-DTA profiles of P123 ArSO3H-Et-HNS6.7 (red), P123-F127 ArSO3H-EtHNS7.2(1) (blue) and F127 ArSO3H-Et-HNS6.9 (black).

Thermal Properties. The thermal stability of the ArSO3H-Et-HNS nanohybrids is investigated by TGA in the temperature range of 30 to 1000 oC and air atomsphere, and the P123 ArSO3H-EtHNS6.7, P123-F127 ArSO3H-Et-HNS7.2(1) and F127 ArSO3H-Et-HNS6.9 are selected as the representative samples. As shown in Figure 5, TGA plots of the tested nanohybrids show three weight loss steps. The first weight loss step takes place in the range of 30 to 100 oC, attributing to the loss of water adsorbed on the catalyst surface. The second weight loss occurs in the range


18

Page 19 of 39 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


of 150‒400 oC, which is mainly originated from the decomposition of the residual surfactant (P123 and/or F127). Significant weight loss happens in the range of 400‒700 oC, which is due to the decomposition of ArSO3H groups and collapse of the silica/carbon framework of the nanohybrids. The total weight loss is 39.4% for P123 ArSO3H-Et-HNS6.7, 35.7% for P123-F127 ArSO3H-Et-HNS7.2(1) and 30.8% for F127 ArSO3H-Et-HNS6.9. Although the determined total weight loss is slightly higher than the theoretical weight loss (31.5% for P123 ArSO3H-EtHNS6.7 and 32.1% for P123-F127 ArSO3H-Et-HNS7.2(1)), the result is reasonable, and the weight loss derived from the adsorbed water and residual P123 and/or F127 during the sample heating process should be considered. As for the F127 ArSO3H-Et-HNS6.9, its determined total weight loss is slightly lower than the theoretical weight loss (31.8%), which may be due to the testing error. As a consequence, the ArSO3H-Et-HNS nanohybrids are thermally stable below 400 oC. Brønsted Acid Nature. The Brønsted acid nature of the materials includes the total number and strength of Brønsted acid sites. Here, the total number of the Brønsted acid sites of the ArSO3H-Et-HNS nanohybrids is calculated by the determined sulfur contents using ICP-OES method, while the strength of the Brønsted acid sites is measured by nonaqueous potentiometric titration with n-butylamine in an acetonitrile solution. In this method, the initial electrode potential (Ei) indicates the maximum acid strength of the surface site, and the catalysts are defined as super strong solid acids when its Ei values are higher than 100 mV.37 From the result summarized in Table 1 it is found that the Ei values of the four tested hollow spherical nanohybrids are in the range of 534 to 570 mV, and the determined total number of Brønsted acid sites is in the range of 796 to 892 µeq(H+) g‒1. The results powerfully demonstrate the super


19


Page 20 of 39

strong Brønsted acid nature of the ArSO3H-Et-HNS nanohybrids, regardless of the shell thickness of the hollow nanospheres. Catalytic Tests The heterogeneous acid catalytic performance of the ArSO3H-Et-HNS nanohybrids is tested in the synthesis of LA from the hydrolysis of FAL under the conditions of 3 wt% catalyst and 120 o

C. Generally, the acid-catalyzed hydrolysis reaction of biomass/derivatives is more difficult to

obtain high product yield in comparison of other biomass conversion processes. For the ArSO3HEt-HNS-catalyzed hydrolysis of FAL to LA, the reactivity is influenced by not only the Brønsted acid nature of the nanohybrids but also the reaction parameters. In order to obtain the highest LA yield in current catalytic system, influence of the important reaction parameters including solvent nature, volume ratio of FAL-to-solvent-to-water, shell thickness and reaction temperature on the hydrolysis activity of the ArSO3H-Et-HNS nanohybrids is considered firstly. Influence of Solvent Nature. Generally, in an acidic medium, FAL can polymerize to form alternative oligomeric byproducts during its hydrolysis process, which causes not only low LA yield but also generates some operational problems; moreover, FAL polymerization becomes predominant at high FAL concentrations.16,19,44 An alternative to avoiding or limiting the undesired FAL polymerization reaction is to control the concentration of FAL in the reaction media. Accordingly, suitable organic cosolvent should be added to the reaction media, which plays a dominant role to obtain high LA yield by restricting the formation of oligomeric byproducts. Generally, alcohols are green solvents used in various catalytic processes; however, alcohols are not suitable for the hydrolysis of FAL to LA because they can react with FAL to produce the corresponding levulinate esters. Additionally, FAL can dissolve in both aqueous and organic phases but mostly locate in organic phase, and suitable solvent in current catalytic


20

Page 21 of 39 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


system should be inert and miscible with both FAL and water. Consequently, acetone, acetonitrile and tetrahydrofuran (THF) are selected, and the influence of solvent nature on the yield of LA is studied under the reaction conditions of 1:3 FAL-to-water volume ratio and 120 o

C by selecting the P123 ArSO3H-Et-HNS6.7 as the representative catalyst. As shown in Figure

6a, the lowest hydrolysis activity is obtained in 4:1 THF-to-water volume ratio system, and the yield of LA is 22.8% after the P123 ArSO3H-Et-HNS6.7-catalyzed FAL hydrolysis reaction proceeds for 60 min. Under the same conditions, the LA yield is 31% in solvent-free system. Slightly increased hydrolysis activity with LA yield of 37.9% is found in 4:1 acetonitrile-to-H2O volume ratio system. The highest hydrolysis activity is observed in 4:1 acetone-to-H2O solvent system, and the yield of LA reaches 56.3% over period of 60 min. This is due to the fact that the miscibility of acetone with FAL or water is the strongest among the selected solvents; meanwhile, acetone could act as an inhibitor to limit the polymerization of FAL,19 ensuring the hydrolysis reaction proceeding at higher LA yield; on the other hand, more exposed rather than covered ArSO3H sites can participate in FAL hydrolysis reaction. Hence, acetone was chosen as the optimum solvent for subsequent catalytic tests. Influence of Acetone-to-Water Volume Ratio. Influence of acetone-to-water volume ratio on the catalytic activity of the representative catalyst, P123 ArSO3H-Et-HNS6.7, is studied to find out the optimum acetone amount. As shown in Figure 6b, after the reaction proceeds for 60 min the yield of LA continuously increases from 39.4, 43.3, 50.6 to 56.3% as increasing acetone-towater volume ratio from 1:1, 2:1, 3:1 to 4:1. However, further increasing the volume ratio to 5:1 and 6:1, the yield of LA decreases to 37.5 and 32.4%. The above result suggests that suitable amount of acetone can promote FAL hydrolysis by inhibit the polymerization, but much more acetone may dilute the reactants (FAL and water), leading to the decreased LA yield.


21


Page 22 of 39

Figure 6. Influence of solvent (a), volume ratio of acetone-to-water (b) and volume ratio of FAL-to-water (c) on the catalytic activity of the P123 ArSO3H-Et-HNS6.7 nanohybrids. 3 wt% catalyst; 120 oC; 60 min. Volume ratio of FAL-to-water = 1:3 for (a) and (b); volume ratio of acetone-to-water = 4:1 for (c).


22

Page 23 of 39 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


Influence of FAL-to-Water Volume Ratio. FAL-to-water volume ratio is one of the most important experimental parameters that affect the hydrolysis activity. Herein, the influence of FAL-to-water volume ratio (3:1, 1:1, 1:3 and 1:5) on the yield of LA is studied by selecting the P123 ArSO3H-Et-HNS6.7 as the representative catalyst, and acetone-to-water volume ratio is controlled at 4:1. From the result presented in Figure 6c it is observed that the lowest LA yield is obtained (8.2%, 60 min) at the highest FAL-to-water volume ratio (3:1 or FAL concentration of 4.32 mol L−1), and the LA yield (30.7%, 60 min) increases rapidly as decreasing the FAL-towater volume ratio to 1:1 (FAL concentration of 1.92 mol L−1). The highest LA yield is obtained (56.3%, 60 min) with continuous reducing the volume ratio to 1:3 (FAL concentration of 0.72 mol L−1). However, further decreasing the volume ratio to 1:5 (FAL concentration of 0.44 mol L−1), the LA yield decreases to 25.3%. The above result suggests that FAL-to-water volume ratio affects the yield of LA by changing the initial concentration of FAL. Much higher FAL-to-water volume ratio or FAL concentration can significantly promote FAL polymerization reactions, leading to the decreased LA yield accompanying with the production of humins with high concentrations. Through LC-MS analysis, the brownish-black soluble humins are mainly composed of furan resins including 2,2’-difurfuryl ether, 2,2’-difurfurylmethane and 4-hidroxy2-cyclopenteone (Figure S2 and Scheme S1). However, too lower FAL-to-water volume ratio may reduce the FAL hydrolysis rate, resulting in low LA yield. The optimum FAL-to-water volume ratio of 1:3 is therefore chosen for subsequent catalytic test. Influence of Reaction Temperature. It is well-known that FAL polymerization reactions are promoted at higher temperature,45 and therefore, the reaction temperature is also one of the key experimental parameters that affect the yield of LA. Herein, influence of the reaction temperature (100, 120 and 140 oC) on the hydrolysis activity of the P123 ArSO3H-Et-HNS6.7 is


23


Page 24 of 39

studied, and the result is displayed in Figure 7. When FAL hydrolysis reaction performs at 100 o

C, the yield of LA is 15.1% after the reaction proceeds for 60 min. At the same reaction time,

the yield of LA reaches to 56.3% as increasing the temperature to 120 oC. However, the yield of LA drops to 44.3% with the temperature further rising to 140 oC. At this temperature, more humins are observed in reaction media, and therefore, lower LA yield at higher temperature is due to the formation of more byproducts from the carbonization or polymerization of FAL. The reaction temperature is therefore controlled at 120 oC in current catalytic test.

Figure 7. Influence of the reaction temperature on the hydrolysis activity of the P123 ArSO3HEt-HNS6.7 nanohybrids. 0.72 mol L−1 FAL in 4:1 acetone-H2O; 3 wt% catalyst; 60 min.

Influence of Shell Thickness of Hollow Nanospheres. As shown in Figure 8, four hollow spherical ArSO3H-Et-HNS nanohybrids with different shell thicknesses follow the hydrolysis activity order P123 ArSO3H-Et-HNS6.7 > P123-F127 ArSO3H-Et-HNS7.2(1) > P123-F127 ArSO3H-Et-HNS6.4(2) > F127 ArSO3H-Et-HNS6.9, and the corresponding LA yield is 83.1, 73.6, 69.1 and 50.5%, respectively, after the reaction proceeds for 120 min. The result indicates


24

Page 25 of 39

that the hydrolysis activity of the ArSO3H-Et-HNS nanohybrids is influenced by the shell thickness in some extent, and at similar ArSO3H loading the hollow ArSO3H-Et-HNS nanospheres with the thinnest shell thickness exhibits the highest activity. This is due to the fact that the ArSO3H-Et-HNS nanohybrids with enough thin shell (e.g., 2 nm) possess excellent permeability, which can decrease the mass transfer resistance and improve the accessibility of FAL or water molecules to the ArSO3H sites, resulting in it higher hydrolysis activity. 100 P123 ArSO3H-Et-HNS6.7

60

a

b

0

0 0

20

40 60 80 Reaction time (min)

100

120

p-toluenesulfonic acid

20

Amberlyst-15

20

HY-zeolite

-1

TOF (h )

40


40

Amberlyst-15 HY-zeolite

60



p-toluenesulfonic acid


P123-F127 ArSO3H-Et-HNS6.4(2)

80


P123-F127 ArSO3H-Et-HNS7.2(1)

Yield of LA (%)

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


Catalysts

Figure 8. Catalytic activity comparison of various ArSO3H-Et-HNS nanohybrids and reference solid acids revealed by (a) yield and (b) TOF (h−1) in the hydrolysis of FAL to LA. 0.72 mol L−1 FAL in 4:1 acetone-H2O; 3 wt% catalyst; 120 oC.

Comparison with the Reference Solid Acids. The catalytic activity of the most active P123 ArSO3H-Et-HNS6.7 is further compared with p-toluenesulfonic acid, Amberlyst-15 and HYzeolite. As shown in Figure 8a, the LA yield rapidly increases at the beginning of ptoluenesulfonic acid-catalyzed FAL hydrolysis reaction, and the yield reaches to 31.5% after the


25


Page 26 of 39

reaction proceeds for 30 min. However, the increase of the LA yield becomes gradually slow as further increasing the reaction time, and the yield is 46.0% over period of 120 min. As for the Amberlyst-15-catalyzed FAL hydrolysis reaction, the LA yield increases to 33.4% sharply over period of 30 min, and then the hydrolysis reaction rate becomes slow obviously. The LA yield reaches to the maximum value (44.3%) at 60 min, afterwards, the yield gradually reduces, for example, over period of 120 min, the LA yield reduces to 28.2%. Both p-toluenesulfonic acid and Amberlyst-15 are super strong Brønsted acid with extremely high acid site density (6846 and 4800 µeq g−1)46, which can ensure them significantly high reaction rate at the initial stage of FAL hydrolysis to LA. However, their super strong Brønsted acid nature can also facilitate FAL polymerization in current reaction system, leading to unwanted brownish-black oligomers or humins (Figures S2 and S3, and Scheme 1). Accordingly, the selectivity of p-toluenesulfonic acid or Amberlyst-15 to LA decreases as the reaction goes on, accompanying with the slowly increased or even decreased LA yield. HY-zeolite with much smaller pore (lower than 2 nm)47,48 exhibits the lowest FAL hydrolysis activity among the tested acid catalysts. Considering the difference of the acid site densities among the aforementioned acid catalysts, their hydrolysis activity is further compared by TOF values. Herein, TOF values are calculated from the linear portion of the initial reaction rate profile for LA yields, which are normalized by the acid site densities (A, Table 1). As shown in Figure 8b, the catalysts follow the TOF value (h−1) order P123 ArSO3H-Et-HNS6.7 (60.5) > P123-F127 ArSO3H-Et-HNS7.2(1) (58.8) > P123F127 ArSO3H-Et-HNS6.4(2) (43.8) > F127 ArSO3H-Et-HNS6.9 (26.6) > HY-zeolite (17.5) > Amberlyst-15 (4.22) > p-toluenesulfonic acid (1.0). Namely, TOF values of four as-prepared ArSO3H-Et-HNS are closely related to their shell thickness, consistence with the activity


26

Page 27 of 39 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


revealed by the yields of LA (Figure 8a); the TOF value of HY-zeolite, Amberlyst-15 or ptoluenesulfonic acid is much lower than that of the ArSO3H-Et-HNS. Discussion. The excellent hydrolysis activity of the ArSO3H-Et-HNS significantly depends on their inherent strong Brønsted acid nature. In order to better understand the contribution of the Brønsted acid nature to the hydrolysis activity of the ArSO3H-Et-HNS, the reaction mechanism of the ArSO3H-Et-HNS-catalyzed hydrolysis of FAL to produce LA is studied by the identified intermediates by LC-MS (Figure S2) and GC-MS (Figure S3), and they are 5-methylene-2,5dihydrofuran-2-phenol (Compound B), 5-methylfuran-2(3H)-one (Compound D), 5-methylfuran2(5H)-one (Compound E), 4,5,5-trihydroxypentan-2-one (Compound H) and 5,5-trihydroxypent4-penten-2-one (Compound I). In the initial stage of the ArSO3H-Et-HNS-catalyzed FAL hydrolysis reaction, hydrogen ion on the –OH group of FAL is protonated by the ArSO3H sites in aqueous media, accompanying with the release of water molecule and formation of carbocation intermediate (species A in Scheme S1). Subsequently, 5-methylene-2,5dihydrofuran-2-phenol is formed after the addition of species A with water molecule, and which suffers from ring opening to form species C, and then it undergoes hydrogen shift to form 5methylfuran-2(3H)-one and 5-methylfuran-2(5H)-one. Afterwards, they successively undergo hydration and hydrogen shift to generate the important intermediate, 4,5,5-trihydroxypentan-2one. The target product LA is finally produced via the release of water molecule to 5,5trihydroxypent-4-penten-2-one followed by tautomerization. Based on the above discussion it is found that the strong Brønsted acid nature of the ArSO3H-Et-HNS can significantly facilitate the conversion of FAL to LA. However, carbonaceous byproducts also produce inevitably during the above process, giving rise to the decreased selecivity of the catalyst to LA. Formation of the byproducts can be visibly observed by the color changes of the reaction media from light-yellow


27


Page 28 of 39

to brownish-black; meanwile, these byproducts are identified by LC-MS, and they are furan resins, 2,2’-difurfuryl ether, 2,2’-difurfurylmethane and 4-hidroxy-2-cyclopenteone (Figure S2 and Scheme S1). On the other hand, the morphological characteristics and porosity properties of the ArSO3HEt-HNS nanohybrids influence the hydrolysis activity in some extent. The opening shell of the hollow spherical ArSO3H-Et-HNS nanohybrids allow the guest molecules (e.g., FAL) to diffuse into the interior of the hollow nanospheres, and the nanoshperes serve as the nanoreactores to provide acid sites-rich confined space for the FAL hydrolysis reaction. As a consequence, the diffusion pathway of the reactants and products are shortened, leading to efficient mass transfer and improved substrate accessibility to the ArSO3H sites. Additionally, the hollow nanostructures of the ArSO3H-Et-HNS can provide plentiful external and internal surface, resulting in them large BET surface areas (378 to 529 m2 g–1) and thereby high population of the ArSO3H sites. Both of the advantages paly the important role to the excellent hydrolysis activity of the ArSO3H-Et-HNS nanohybrids. Additionally, the hydrophobic surface of as-prepared ArSO3H-Et-HNS nanohybrids is benefit to hold a lower water concentration within its internal structure of catalysts, ensuring higher concentration of FAL near the acid sites and thereby higher LA yield. Also, the ArSO3H-Et-HNS nanohybrids with the hydrophobic surface can inhibit FAL polymerization in some extent, ensuring the production of LA with considerably high yield. Among four tested ArSO3H-Et-HNS nanohybrids, their hydrolysis activity difference is mainly determined by their different BET surface areas, pore sizes, pore volumes as well as shell thicknesses. P123 ArSO3H-Et-HNS6.7, P123-F127 ArSO3H-Et-HNS7.2(1) and P123-F127 ArSO3H-Et-HNS6.4(2) nanohybrids possess similar pore volumes, but the BET surface area of


28

Page 29 of 39 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


the P123 ArSO3H-Et-HNS6.7 is larger than P123-F127 ArSO3H-Et-HNS7.2(1) or P123-F127 ArSO3H-Et-HNS6.4(2); meanwhile, its shell is thinner than that of the P123-F127 ArSO3H-EtHNS7.2(1) or P123-F127 ArSO3H-Et-HNS6.4(2). Therefore, the P123 ArSO3H-Et-HNS6.7 exhibits higher hydrolysis activity than the P123-F127 ArSO3H-Et-HNS7.2(1) or P123-F127 ArSO3H-Et-HNS6.4(2) because the accessibility to the acid sites can be improved and the mass transfer resistance from external surface of the hollow nanospheres to the interior is reduced significantly. The lowest hydrolysis activity of the F127 ArSO3H-Et-HNS6.9 nanohybrids is owing to the cooperation of the poorest porosity properties and the thickest shell. Regeneration and Reusability. It is an important issue to evaluate the reusability of the supported catalysts because the leaching of the anchored active sites generally occurs during the catalytic process, especially at higher reaction temperature; additionally, the adsorption of carbonaceous byproducts on the catalyst surface may severely deteriorate the catalytic activity. Herein, the reusability of the ArSO3H-Et-HNS nanohybrids is evaluated by the representative catalyst, P123 ArSO3H-Et-HNS6.7, and it catalyzes FAL hydrolysis reaction is repeated for three times under the conditions of 0.72 mol L−1 FAL in 4:1 acetone-H2O solution. After each catalytic cycle, the catalyst is separated and then washed completely with ethanol and acetone, and the recovered catalyst is applied to the second and third cycles. As shown in Figure 9a, the yield of LA is 83.1, 77.4 and 60.1%, respectively, for the first, second and third cycle. The reasons of the activity drop are therefore studied. At first, in order to estimate the leaching of the acid sites, sulfur content in catalyst-free reaction solution is determined by ICP-OES, showing that the concentration of sulfur in the system is below the detect limit. Therefore, the leaching of the anchored ‒SO3H groups hardly occurs under current reaction conditions.


29


100

a

80 Yield of levulinic acid (%)

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 30 of 39

60

40

20

0 0

60

120 60 120 Reaction time (min)

60

120

Figure 9 (a) Reusability of the P123 ArSO3H-Et-HNS6.7 in hydrolysis of FAL to yield LA. Volume ratio of FAL-to-water = 1:3; 3 mL acetone; 3 wt% catalyst; 120 oC; (b) 13C CP-MAS NMR spectrum of the third time used P123 ArSO3H-Et-HNS6.7. Subsequently, the structure of the third time used ArSO3H-Et-HNS is further analyzed by 13C CP-MAS NMR, showing that all characteristic signals concerning about carbon species of arenesulfonic acid groups (C1−C5 in Figure 9b, 17.4, 27.7, 127.5, 142.0, and 149.6 ppm) and ethyl-bridged organosilica moieties (C6 in Figure 9b, 5.7 ppm) are found, implying that arenesulfonic acid functionalized ethyl-bridged organosilica framework remains intact after catalytic cycle, which is attributed to the covalently anchoring arenesulfonic acid groups into the silica/carbon framework. However, other new signals centered at 13.0, 38.4, 108.0 and 121.2 ppm in comparison of the

13

C CP-MAS NMR spectrum of fresh P123 ArSO3H-Et-HNS6.7

(Figure 4a) are found. Therefore, it is confirmed that some humins or carbonaceous residues still adsorb on the catalyst surface after washing with ethanol and acetone, which can limit the


30

Page 31 of 39 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


accessibility of the reactant molecules to the ArSO3H sites. This is the main reason of the activity drop of the catalyst during the recyclying utilization.

CONCLUSIONS Arenesulfonic acid functionalized ethyl-bridged organosilica hollow nanospheres with controllable shell thicknesses are successfully fabricated by a P123- and/or F127-directed sol-gel co-condensation route, and they exhibit shell thickness-dependent catalytic activity in conversion of FAL to LA in aqueous solution. The P123-directed ArSO3H-Et-HNS with the thinnest shell and the largest BET surface area possess the highest hydrolysis activity among various ArSO3HEt-HNS nanohybrids, which is explained by the improved accessibility to the acid sites and the reduced mass transfer resistance from external surface of the hollow nanospheres to the interior. It is also showed that the main experimental parameters such as solvent nature, volume ratio of FAL-to-solvent-to-water and reaction temperature significantly affect the yield of LA, and at the optimal reaction conditions (0.72 mol L−1 FAL in 4:1 acetone-H2O and 120 oC) polymerization of FAL into unwanted oligomeric products or humins can be inhibited obviously, resulting in considerably high LA yield. The reusability testing result indicates that although leaching of the anchored ArSO3H groups in the ArSO3H-Et-HNS hardly occurs under current reaction conditions, strong adsorption of humins or carbonaceous residues on the surface of the ArSO3HEt-HNS can lead to somewhat activity drop after three consecutive catalytic runs via limiting the accessibility of the reactant molecules to the ArSO3H sites.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge.


31


Page 32 of 39

HRTEM images of F127 ArSO3H-Et-HNS6.9 nanohybrids; LC-MS and GC-MS analysis results of hydrolysis of FAL to produce LA catalyzed by ArSO3H-Et-HNS after the reaction proceeded for 30 min and 60 min; possible reaction mechanism for the synthesis of LA from hydrolysis of FAL

catalyzed

by

the

ArSO3H-Et-HNS

nanohybrids.

(PDF)

AUTHOR INFORMATION Corresponding Author *Prof. Yihang Guo. Tel.: +86 431 89162626. Fax: +86 431 89165626. E-mail address: [email protected].

ACKNOWLEDGMENTS This work was supported by the Natural Science Fund Council of China (21573038 and 21703030).

REFERENCES

(1) Xu, J. M.; Xie, X. F.; Wang, J. X.; Jiang, J. C., Directional liquefaction coupling fractionation of lignocellulosic biomass for platform chemicals. Green Chem. 2016, 18 (10), 3124–3138. (2) Zhang, X. S.; Lei, H. W.; Zhu, L.; Wu, J.; Chen, S. L., From lignocellulosic biomass to renewable cycloalkanes for jet fuels. Green Chem. 2015, 17 (10), 4736–4747. (3) Isikgor, F. H.; Becer, C. R., Lignocellulosic biomass: a sustainable platform for the production of bio-based chemicals and polymers. Polym. Chem. 2015, 6 (25), 4497–4559.


32

Page 33 of 39 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


(4) Guo, Y. H.; Li, K. X.; Yu, X. D.; Clark, J. H., Mesoporous H3PW12O40-silica composite: efficient and reusable solid acid catalyst for the synthesis of diphenolic acid from levulinic acid. Appl. Catal. B: Environ. 2008, 81 (3), 182–191. (5) Flannelly, T.; Lopes, M.; Kupiainen, L.; Dooley, S.; Leahy, J. J.; Non-stoichiometric formation of formic and levulinic acids from the hydrolysis of biomass derived hexose carbohydrates. RSC Adv. 2016, 6 (7), 5797–5804. (6) Climent, M. J.; Corma, A.; Iborra, S., Conversion of biomass platform molecules into fuel additives and liquid hydrocarbon fuels. Green Chem. 2014, 16 (2), 516–547. (7) Ciptonugroho, W.; Al-Shaal, M. G.; J. Mensah, B.; Palkovits, R., One pot synthesis of WOx/mesoporous-ZrO2 catalysts for the production of levulinic-acid esters. J. Catal. 2016, 340, 17–29. (8) Kuwahara, Y.; Kaburagi, W.; Osada, Y.; Fujitani, T.; Yamashita, H.; Catalytic transfer hydrogenation of biomass-derived levulinic acid and its esters to γ-valerolactone over ZrO2 catalyst supported on SBA-15 silica. Catal. Today 2017, 281 (3), 418–428. (9) Weingarten, R.; Conner, W. C.; G. Huber, W., Production of levulinic acid from cellulose by hydrothermal decomposition combined with aqueous phase dehydration with a solid acid catalyst. Energy Environ. Sci. 2012, 5 (6), 7559–7574. (10)

Braden, D. J.; Henao, C. A.; Heltzel, J.; Maravelias, C. C.; Dumesic, J. A., Production of

liquid hydrocarbon fuels by catalytic conversion of biomass-derived levulinic acid. Green Chem. 2011, 13 (7), 1755–1765. (11)

Kumar, V. B.; Pulidindi, I. N.; Mishra, R. K.; Gedanken, A., Development of Ga salt of

molybdophosphoric acid for biomass conversion to levulinic acid. Energy Fuels 2016, 30 (12), 10583−10591.


33


(12)

Page 34 of 39

Rackemann, D. W.; Bartley, J. P.; Harrison, M. D.; Doherty, W. O. S., The effect of

pretreatment on methanesulfonic acid-catalyzed hydrolysis of bagasse to levulinic acid, formic acid, and furfural. RSC Adv. 2016, 6 (78), 74525–74535. (13)

Yan, L.; Greenwood, A. A.; Hossain, A.; Yang, B., A comprehensive mechanistic kinetic

model for dilute acid hydrolysis of switchgrass cellulose to glucose, 5-HMF and levulinic acid. RSC Adv. 2014, 4 (45), 23492–23504. (14)

Hu, X.; Song, Y.; Wu, L. P.; Gholizadeh, M.; Li, C. Z., One-pot synthesis of levulinic

acid/ester from C5 carbohydrates in a methanol medium. ACS Sustainable Chem. Eng. 2013, 1 (12), 1593−1599. (15)

Li, J.; Ding, D. J.; Xu, L. J.; Guo, Q. X.; Fu, Y., The breakdown of reticent biomass to

soluble components and their conversion to levulinic acid as a fuel precursor. RSC Adv. 2014, 4 (29), 14985–14992. (16)

Mellmer, M. A.; Gallo, J. M. R.; Alonso, D. M.; Dumesic, J. A., Selective production of

levulinic acid from furfuryl alcohol in THF solvent systems over H-ZSM-5. ACS Catal. 2015, 5 (6), 3354−3359. (17)

Maldonado, G. M. G.; Assary, R. S.; Dumesic, J.; Curtiss, L. A., Experimental and

theoretical studies of the acid-catalyzed conversion of furfuryl alcohol to levulinic acid in aqueous solution. Energy Environ. Sci. 2012, 5 (5), 6981–6989. (18)

Hronec, M.; Fulajtárová, K.; Soták, T., Kinetics of high temperature conversion of

furfuryl alcohol in water. J. Ind. Eng. Chem. 2014, 20 (2), 650–655. (19)

Guzmán, I.; Heras, A.; Güemez, M. B.; Iriondo, A.; Cambra, J. F.; Requies, J., Levulinic

acid production using solid-acid catalysis. Ind. Eng. Chem. Res. 2016, 55 (18), 5139−5144.


34

Page 35 of 39 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


(20)

Gürbüz, E. I.; Wettstein, S. G.; Dumesic, J. A., Conversion of hemicellulose to furfural

and levulinic acid using biphasic reactors with alkylphenol solvents. ChemSusChem 2012, 5 (2), 383–387. (21)

Gürbüz, E. I.; Gallo, J. M. R.; Alonso, D. M.; Wettstein, S. G.; Lim, W. Y.; Dumesic, J.

A., Conversion of hemicellulose into furfural using solid acid catalysts in γ-valerolactone. Angew. Chem. Int. Ed. 2013, 52 (4), 1270–1274. (22)

Ide, A.; Scholz, G.; Thomas, A., Tunable porosity in bridged organosilicas using self-

organizing precursors. Langmuir 2008, 24 (21), 12539–12546. (23)

Ciriminna, R.; Fidalgo, A.; Pandarus, V.; Béland, F.; Ilharco, L. M.; Pagliaro, M., The

sol-gel route to advanced silica-based materials and recent applications. Chem. Rev. 2013, 113 (8), 6592−6620. (24)

Clippel, F. D.; Dusselier, M.; Vyver, S. V. D.; Peng, L.; Jacobsa, P. A.; Sels, B. F.,

Tailoring nanohybrids and nanocomposites for catalytic applications. Green Chem. 2013, 15 (6), 1398–1430. (25)

Ferré, M.; Pleixats, R.; Man, M. W. C.; Cattoën, X., Recyclable organocatalysts based on

hybrid silicas. Green Chem. 2016, 18 (4), 881–922. (26)

Yang, Q. H.; Kapoor, M. P.; Shirokura, N.; Ohashi, M.; Inagaki, S.; Kondoc, J. N.;

Domen, K., Ethane-bridged hybrid mesoporous functionalized organosilicas with terminal sulfonic groups and their catalytic applications. J. Mater. Chem. 2005, 15 (6), 666–673. (27)

Wang, L.; Xiao, F. S., Nanoporous catalysts for biomass conversion. Green Chem. 2015,

17 (1), 24–39.


35


(28)

Page 36 of 39

Ahmad, E.; Alam, M. I.; Pant, K. K.; Ali. Haider, M., Catalytic and mechanistic insights

into the production of ethyl levulinate from biorenewable feedstocks. Green Chem. 2016, 18 (18), 4804–4823. (29)

Enumula, S. S.; Koppadi, K. S.; Gurram, V. R. B.; Burri, D. R.; Kamaraju, S. R. R.,

Conversion of furfuryl alcohol to alkyl levulinate fuel additives over Al2O3/SBA-15 catalyst. Sustainable Energy Fuels 2017, 1, 644–651. (30)

Pirez, C.; Lee, A. F.; Jones, C.; Wilson, K., Can surface energy measurements predict the

impact of catalyst hydrophobicity upon fatty acid esterification over sulfonic acid functionalised periodic mesoporous organosilicas? Catal. Today 2014, 234, 167–173. (31)

Zhang, X. H.; Su, F.; Song, D. Y.; An, S.; Lu, B.; Guo, Y. H., Preparation of efficient and

recoverable organosulfonic acid functionalized alkyl-bridged organosilica nanotubes for esterification and transesterification. Appl. Catal. B: Environ. 2015, 163, 50–62. (32)

Lu, B.; An, S.; Song, D. Y.; Su, F.; Yang, X.; Guo, Y. H., Design of organosulfonic acid

functionalized organosilica hollow nanospheres for efficient conversion of furfural alcohol to ethyl levulinate. Green Chem. 2015, 17 (3), 1767–1778. (33)

An, S.; Song, D. Y.; Lu, B.; Yang, X.; Guo, Y. H., Morphology tailoring of sulfonic acid

functionalized organosilica nanohybrids for the synthesis of biomass-derived alkyl levulinates. Chem. Eur. J. 2015, 21 (30), 10786–10798. (34)

An, S.; Sun, Y. N.; Song, D. Y.; Zhang, Q. Q.; Guo, Y. H.; Shang, Q. K., Arenesulfonic

acid-functionalized alkyl-bridged organosilica hollow nanospheres for selective esterification of glycerol with lauric acid to glycerol mono-and dilaurate. J. Catal. 2016, 342, 40–54.


36

Page 37 of 39 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


(35)

Pirez, C.; Caderon, J. M.; Dacquin, J. P.; Lee, A. F.; Wilson, K., Tunable KIT-6

mesoporous sulfonic acid catalysts for fatty acid esterification. ACS Catal. 2012, 2 (8), 1607– 1614. (36)

Rao, K. N.; Reddy, K. M.; Lingaiah, N.; Suryanarayana, I.; Prasad, P.S. S., Structure and

reactivity of zirconium oxide-supported ammonium salt of 12-molybdophosphoric acid catalysts. Appl. Catal. A: Gen. 2006, 300 (2), 139–146. (37)

Altass, H. M.; Khder, A. E. R. S., Surface and catalytic properties of triflic acid supported

zirconia: Effect of zirconia tetragonal phase. J. Mol. Catal. A: Chem. 2016, 411, 138–145. (38)

Liu, J.; Bai, S. Y.; Zhong, H.; Li, C.; Yang, Q. H., Tunable assembly of organosilica

hollow nanospheres. J. Phys. Chem. C 2010, 114 (2), 953–961. (39)

Voort, P. V. D.; Esquivel, D.; Canck, E. D.; Goethals, F.; Driessche, I. V.; Romero-

Salguero, F. J., Periodic mesoporous organosilicas: from simple to complex bridges; a comprehensive overview of functions, morphologies and applications. Chem. Soc. Rev. 2013, 42 (9), 3913–3955. (40)

Mizoshita, N.; Tani, T.; Inagaki, S., Syntheses, properties and applications of periodic

mesoporous organosilicas prepared from bridged organosilane precursors. Chem. Soc. Rev. 2011, 40 (2), 789–800. (41)

An, J. Z.; Cheng, T. Y.; Xiong, X.; Wu, L.; Han, B.; Liu, G. H., Yolk-shell-structured

mesoporous silica: a bifunctional catalyst for nitroaldol-Michael one-pot cascade reaction. Catal. Sci. Technol. 2016, 6 (14), 5714–5720. (42)

Huybrechts, W.; Lauwaert, J.; Vylder, A. D.; Mertens, M.; Mali, G.; Thybaut, J. W.;

Voort, P. V. D.; Cool, P., Synthesis of L-serine modified benzene bridged periodic


37


Page 38 of 39

mesoporous organosilica and its catalytic performance towards aldol condensations. Microporous Mesoporous Mater. 2017, 251, 1–8. (43)

Wang, J. Q.; Zhou, M.; Gu, C. Q.; Zhang, W. Q.; Lv, M. Y.; Guo, C.; Sun, L. B.; Zhang,

H. L., Architecture of novel periodic mesoporous organosilicas based on the flexible skeleton of aspartic acid-bridged organosilane. Mater. Lett. 2017, 193, 299–304. (44)

Kim, T.; Assary, R. S.; Pauls, R. E.; Marshall, C. L.; Curtiss, L. A.; Stair, P. C.,

Thermodynamics and reaction pathways of furfuryl alcohol oligomer formation. Catal. Commun. 2014, 46, 66–70. (45)

Kim, T.; Assary, R. S.; Marshall, C. L.; Gosztola, D. J.; Curtiss, L. A.; Stair, P. C., Acid-

catalyzed furfuryl alcohol polymerization: characterizations of molecular structure and thermodynamic properties. ChemCatChem 2011, 3 (9), 1451−1458. (46)

Sun, Y. N.; Hu, J. L.; An, S.; Zhang, Q. Q.; Guo, Y. H.; Song, D. Y.; Shang, Q. K.,

Selective esterification of glycerol with acetic acid or lauric acid over rod-like carbon-based sulfonic acid functionalized ionic liquids. Fuel 2017, 207, 136–145. (47)

Ramli, N. A. S.; Amin, N. A. S., Fe/HY zeolite as an effective catalyst for levulinic acid

production from glucose: characterization and catalytic performance. Appl. Catal. B: Environ. 2015, 163, 487–498. (48)

Chen, L. H.; Li, X. Y.; Rooke, J. C.; Zhang, Y. H.; Yang, X. Y.; Tang, Y.; Xiao, F. S.;

Su, B. L., Hierarchically structured zeolites: synthesis, mass transport properties and applications. J. Mater. Chem. 2012, 22 (34), 17381–17403.


38

Page 39 of 39 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


Abstract Graphic

O

OHHO OH Si O Si CH2 OH H2C O S CH2 H2C HO O Si OH HO HO Si O O Si Si OH OH OH HO O O HO Si Si CH2 O S HO H C HO O 2 CH2 H2C Si O Si OH HO OH HO

OH

HO

O OH

O

Synopsis: Hollow spherical arenesulfonic acid functionalized ethyl-bridged organosilica nanospheres exhibited shell thickness-dependent catalytic activity in the synthesis of levulinic acid from the hydrolysis of furfuryl alcohol.


39

Conversion of Furfuryl Alcohol to Levulinic Acid in Aqueous Solution

Recommend Documents