Phenylboronic Acid Functionalized SBA-15 for Sugar Capture

Published: October 24, 2011

r 2011 American Chemical Society 14554 dx.doi.org/10.1021/la203121u | Langmuir 2011, 27, 14554–14562

ARTICLE

pubs.acs.org/Langmuir

Phenylboronic Acid Functionalized SBA-15 for Sugar CaptureYong-Hong Zhao and Daniel F. Shantz*

Department of Chemical Engineering, Texas A&M University, 3122 TAMU, College Station, Texas, 77843-3122, United States

bS Supporting Information

’ INTRODUCTION

Due to the impending decline of fossil fuel reserves andpotential deleterious effects of climate change, the progressivechangeover of the chemical industry to renewable feedstocks forits raw materials is emerging as a critical issue.1 While manysolutions are being explored, fuels and chemicals from renewablebiomass sources have emerged as one solution with both short-and long-term potential.2,3 Generating fuel and chemicals frombiomass carbohydrates, such as lignocelluloses, is very attractiveas it represents a shift away from importing energy and is inprinciple carbon neutral.4�6

The structure of lignocelluloses is totally different from that ofpresent fuels and chemicals. It needs to be depolymerized ordeoxygenated to be suitable for these applications. A variety ofchemistries and processes, such as hydrolysis, pyrolysis, gasifica-tion, and so forth, have been applied to convert lignocellulosicbiomass to valuable chemicals or intermediates.3,7,8 One ofthe most important products in the conversion of lignocellulosesare sugars which can be subsequently converted to a variety ofderivatives through biological or chemical conversions. Sugarsare the main feedstocks for bioethanol production throughfermentation. Bioethanol has been touted as a promising biofuelthat has the potential to be a valuable substitute for, or comple-ment to, gasoline.9�11 Numerous important chemical buildingblocks, such as lactic acid, succinic acid, 3-hydroxy propinoic acid,itaconic acid, and so forth, can also be obtained by fermentation

of sugars.12 Chemical technologies also offer a variety of routes toupgrade sugars. For example, sugars can be hydrogenated toC5�6 polyols such as xylitol, mannitol, and sorbitol,13,14 hydro-genolyzed to C2�3 glycols,

15 or further upgraded via oxidation orhalogenation reactions.16

One clear need in these processes is the ability to effectivelyseparate sugars from mixtures of processed biomass. For in-stance, in the acid-hydrolysis fermentation conversion of ligno-cellulosic biomass, the sugars obtained from hydrolysis need tobe processed before they are used in fermentation media. Ingeneral, the following operations are needed: concentration ofsugars by evaporation; detoxification by active carbon adsorp-tion; neutralization of acids; and removal of the insoluble saltsformed during the neutralization of acids by filtration.17 Theseoperations are usually costly and time-consuming, which haslimited the current economic impact of fermentation products.Thus, substantial improvements in the existing separation tech-nology are needed in order to allow the biofuel and the chemicalbuilding blocks from fermentation to penetrate further into theorganic chemical industry.12

Organic�inorganic hybrid materials have long attractedinterest from the scientific and engineering communities.18�21

Received: August 10, 2011Revised: October 24, 2011

ABSTRACT: The synthesis and characterization of organic�inorganic hybridmaterials that selectively capture sugars from model biomass hydrolysis mixturesare reported. 3-Aminophenylboronic acid (PBA) groups that can reversibly formcyclic esters with 1,2-diols, and 1,3-diols including sugars are attached tomesoporousSBA-15 via different synthetic protocols. In the first route, a coupling agent is used tolink PBA and SBA-15, while in the second route poly(acrylic acid) brushes arefirst grafted from the surface of SBA-15 by surface-initiated atom transfer radicalpolymerization and PBA is then immobilized. The changes in pore structure,porosity, and pore size due to the loading of organic content are measured bypowder X-ray diffraction and nitrogen porosimetry. The increase in organic contentafter each synthesis step is monitored by thermal gravimetric analysis. Fouriertransform infrared spectroscopy and elemental analysis are used to characterize thechemical compositions of the hybrid materials synthesized. D-(+)-Glucose andD-(+)-xylose, being the most commonly present sugars in biomass, are chosen to evaluate the sugar adsorption capacity of thehybrid materials. It is found that the sugar adsorption capacity is determined by the loading of boronic acid groups on the hybridmaterials, and the hybrid material synthesized via route two is much better than that through route one for sugar adsorption.Mathematical modeling of the adsorption data indicates that the Langmuir model best describes the sugar adsorption behavior of thehybrid material synthesized through route one, while the Freundlich model fits the data most satisfactorily for the hybrid materialprepared via route two. The adsorption kinetics, reusability, and selectivity toward some typical chemicals in cellulose acidichydrolysis mixtures are also investigated.

14555 dx.doi.org/10.1021/la203121u |Langmuir 2011, 27, 14554–14562

Langmuir ARTICLE

Thesematerials hold great promise in areas including sensing, separa-tions, optics, and catalysis, given that they combine the features ofboth organic and inorganic materials. Ordered mesoporoussilicas (OMS)22�28 have been studied extensively as supportsfor hybrid materials given that they possess intrinsic high specificsurface areas, regular and tunable pore sizes, large pore volumes,as well as stable and interconnected frameworks with active poresurfaces for modification or functionalization. Such features meetthe requirements as excellent adsorbents, not only providing alarge interface and space capable of accommodating capaciousguest species, but also enabling the possibility of specific binding,enrichment, and separation.29 Within the context of the currentwork, it should be noted that many functional groups have beenattached to ordered mesoporous silica,30,31 and in fact the use ofOMS as a support for atom transfer radical polymerization (ATRP)catalysts has been reported previously.32,33 There have also beenreports of the use of SBA-15 as a support to form polymer brushesvia ATRP.34�37 On the other hand, the adsorption of sugarsusing zeolites in aqueous solutions has been reported38 and therecognition of saccharides by boronic acids is an intriguingsubject in supramolecular chemistry.39�41 There is generalagreement that boronic acids covalently react with 1,2-diolsand 1,3-diols including sugars by reversibly forming cyclic esterswith five or six member cyclic structures.42,43 This interaction hasbeen extensively exploited, with a particular emphasis onseparation,42 transport,44 and detection of sugars.45 In order topromote solid phase separation and reusability of boronic acids, itis usually important to immobilize the boronic acids on insolublesolid supports.

In our previous work, we synthesized organic�inorganichybrid membranes that could efficiently separate the valuablelow molecular weight chemicals, such as sugars, aldehydes andorganic acids, etc., from cellulose acid hydrolysis mixtures.46

In this report, we describe the synthesis of hybrid materials thatcan selectively capture sugars. A widely studied OMS material,SBA-15, was selected as the inorganic support. Boronic acidswere then immobilized to SBA-15 surface via two methods

(Scheme 1). In the first route, a short coupling agent was usedto link the 3-aminophenylboronic acid (PBA) to the surface ofSBA-15, which should lead to amonolayer of boronic acid groupson the pore surface. In the other one, poly(acrylic acid) (PAA)polymer brushes were first grafted to the SBA-15 surface viaATRP, and the abundant carboxyl groups in the grafted PAAbrushes provided multiple sites for the subsequent immobiliza-tion of boronic acids, which should result in higher loading ofboronic acids. Two sugars, D-(+)-glucose and D-(+)-xylose,being most commonly present in lignocelluloses, were used toevaluate the sugar adsorption capacities of the hybrid materi-als synthesized via the above two methods. The adsorptionkinetics, reusability, and selectivity toward some typicalchemicals in cellulose acidic hydrolysis mixtures were alsoinvestigated. The current work represents a route to selec-tively isolate sugars from aqueous mixtures generated frombiomass conversion and thus a potentially enabling route toexpanding the scope of biofuel production.

’EXPERIMENTAL SECTION

Materials. Tetraethoxysilane (TEOS, g 99%) was purchased fromFluka. Pluronic P123 (EO20PO70EO20,Mw = 5800) was obtained fromBASF. 3-Aminopropyltriethoxysilane (APTES, 99%), 2,20-bipyridyl(Bpy, 99%), 2-bromoisobutyryl bromide (BIBB, 98%), CuBr (98%),CuBr2 (99%), succinic anhydride (SA, g99%), N,N0-dicyclohexylcar-bodiimide (DCC, g 99%), sodium acrylate (NaAc, 97%), D-(+)-xylose(99%), D-(+)-glucose (99.5%), acetic acid (g99%), levulinic acid(98%), furfural (98%), sodium hydroxide (NaOH, 98%), and hydro-chloric acid (HCl, 37%) were purchased from Aldrich and used asreceived. 3-Aminophenylboronic acid monohydrate (PBA, 98%) wasobtained fromAlfa and used as received. Triethylamine (TEA, 99%) andN,N-dimethylformamide (DMF, 99.8%) from Aldrich were purified bydistillation and stored over 4 Å molecular sieves. Ethanol, methanol,toluene, and tetrahydrofuran (THF) (all ACS reagent grade) werepurchased from VWR. Toluene and THF were dried using a MBRAUNMB-SPS solvent drying system; all other solvents were used as received.

Scheme 1. Synthetic Protocols for PBA-Functionalized SBA-15


Langmuir ARTICLE

Deionized (DI) water (18 MΩ) purified with a Milli-Q system(Millipore) was used in all syntheses and measurements.Synthesis of SBA-15. SBA-15 was synthesized using a method

comparable to that reported previously.47 Pluronic P123 (4.0 g) wasdissolved in 60 mL of 4 MHCl and 85 mL of DI water by stirring for 5 hat room temperature. Then, 8.50 g of TEOS was added to that solutionand stirred for 24 h at 35 �C. Themixture was then aged at 80 �C for 24 hunder static conditions. The solid product was filtered, washed withcopious quantities of DI water, and air-dried overnight. The solidproduct was calcined to remove the Pluronic. The calcination procedurewas as follows: the sample was heated from room temperature to 100 �Cat a rate of 1 �C min�1; held at 100 �C for 1 h; increased from 100 to500 �C at a rate of 1 �C min�1; and then held at 500 �C for 5 h.Hybrid SBA-15 Materials. Two synthetic protocols were used in

this work to functionalize the SBA-15, as depicted in Scheme 1. For bothprotocols, the SBA-15 was first functionalized with amine groups bygrafting of APTES. 0.22 g (1.0 mmol, for route one) or 0.33 g(1.5 mmol, for route two) of APTES was added to 1 g of calcinedSBA-15 in 100mL of anhydrous toluene under nitrogen protection. Thismixture was stirred overnight in a closed container at room temperature.The product was collected by filtration, washed with copious DI water,and dried at 60 �C under vacuum overnight. The amine-functionalizedSBA-15 is denoted as SBA-15-NH2. In route two, 1 g of SBA-15-NH2

was dispersed in 25 mL of anhydrous DMF and then added dropwise toa flask containing 0.20 g (2 mmol) of SA and 0.02 g (0.1 mmol) of DCCin 25 mL of anhydrous DMF under rigorous stirring. The mixture wasstirred for 24 h, and the resultant carboxyl-functionalized SBA-15(denoted as SBA-15-COOH) was washed with DMF, ethanol, and DIwater sequentially and then dried at 60 �Cunder vacuum overnight. PBAwas then grafted to the carboxyl-functionalized SBA-15. Next, 1 g ofSBA-15-COOH was dispersed in 50 mL of anhydrous DMF, and then0.32 g (2 mmol) of PBA and 0.02 g (0.1 mmol) of DCC were added.This mixture was stirred for 24 h, and the resultant PBA-functionalizedSBA-15 (denoted as SBA-15-g-PBA) was collected by filtration, washedsequentially with DMF, ethanol, and DI water, and air-dried.

In route two, to immobilize PBA on SBA-15, PAA brushes were firstgrafted to SBA-15 using ATRP. To achieve this, the amine-functional-ized SBA-15 was first reacted with BIBB at room temperature in thepresence of TEA and under nitrogen protection, using anhydrous THFas solvent. The product (denoted as SBA-15-Br) was collected byfiltration, washed sequentially with methanol�DI water�methanol,and air-dried. Then, 0.14 g (1 mmol) of CuBr, 0.06 g (0.25 mmol) ofCuBr2, and 1 g of SBA-15-Br were placed into a 100 mL three-neckedflask under nitrogen protection. The NaAc monomer (0.28 g, 3 mmol)and Bpy (0.31 g, 2 mmol) were dissolved in a 50 mLDI water/methanol(1/1, v/v) mixture, and then the solution was transferred to the three-necked flask after purging with nitrogen for 30 min. The mixture wasstirred at room temperature for 24 h, and the ATRP was terminated byexposure to air. After being thoroughly washed with methanol and DIwater, the grafted poly(sodium acrylate) on SBA-15 by ATRP wasconverted to poly(acrylic acid) by washing with a pH 5 HCl solution,followed by washing with copious DI water again. The product (denotedas SBA-15-g-PAA) was then dried at 60 �C under vacuum overnight.Finally, PBA was grafted to the PAA grafted SBA-15. One gram of SBA-15-g-PAA was dispersed in 50 mL of anhydrous DMF, and then 0.47 g(3mmol) of PBA and 0.02 g (0.1mmol) ofDCCwere added. Thismixturewas stirred for 24 h, and the product (denoted as SBA-15-g-PPBA) wascollected by filtration, washed sequentially with DMF, ethanol, and DIwater, and air-dried.Analytical. Fourier transform infrared spectroscopy (FTIR) mea-

surements were carried out on a ThermoNicolet Nexus 670 instrument.A total of 64 scans were acquired at a resolution of 4 cm�1. Powder X-raydiffraction (PXRD)measurements were performed using a Bruker-AXSD8powder diffractometer with Cu Kα radiation over a range of 0.5�5� 2θ.

Peak intensities and 2θ values were determined using the Brukerprogram EVA. Nitrogen adsorption experiments were performed on aMicromeritics ASAP 2010 micropore system using approximately 0.6 gof sample. The samples were degassed under vacuum at room tempera-ture for 2 h, then at 40 �C for 4 h, and then at 60 �C for 6 h beforeanalysis. The micropore and mesopore volumes were determined usingthe αs-method.48 The mesopore size distributions were calculated fromthe adsorption branch of the isotherm using the Barret�Joyner�Halenda (BJH) method49 with a modified equation50 for the statisticalfilm thickness. Thermal gravimetric analyses (TGA) were performedusing a TG 209C Iris instrument fromNetzsch over a temperature rangeof 25�600 �Cwith oxygen and nitrogen as carrier gases and temperatureramping rate of 5 �C min�1. Elemental analysis (Si, N, C, and B) wasperformed by the Galbraith Laboratories.Solution Sugar Capture. The PBA-functionalized SBA-15 was

tested for adsorption of D-(+)-xylose, D-(+)-glucose, acetic acid, levulinicacid, and furfural. Sugar solutions of different concentrations wereprepared by dissolving the sugars in DI water, of which the pH valueswere adjusted to 8.7 using a 0.001 M NaOH solution. The acids andaldehyde solutions of different concentrations were prepared by dissol-ving the acids or the aldehyde in DI water. Adsorption experiments wereperformed as follows. 0.1 g of PBA-functionalized SBA-15 was added to2 mL of a solution containing the target compounds for capture at adesired concentration. The mixture was shaken for varying predeter-mined times, and then the PBA-functionalized SBA-15 was removed bycentrifuging at 6000 rpm for 10 min. Solution concentrations weredetermined with high performance liquid chromatography (HPLC,Agilent 1120) using an Agilent ZORBAX Eclipse Plus C18 columnand an Agilent 1260 Infinity refractive index detector (RID). Themobilephase was degassed DI water fed at a rate of 1.5 mL min�1, and thetemperature of the column was at 25 �C. The amount of adsorbedsubstrate per unit mass of the PBA-functionalized SBA-15 at equilibriumconcentration, qe (in g g�1), was calculated from the mass balanceequation

qe ¼ C0 � Ce

m=Vð1Þ

where C0 and Ce are the initial and equilibrium substrate concentrations(in g L�1), respectively, V is the volume of the liquid phase (in L), andm is the mass of the PBA-functionalized SBA-15 (in g).

The reusability of the PBA-functionalized SBA-15 in sugar capturewas investigated. An amount of 0.1 g of the PBA-functionalized SBA-15was added to 2 mL of 30 g L�1 sugar solution. The mixture was shakenfor 6 h, and then the solids were separated by centrifugation at 6000 rpmfor 10 min. The supernatant liquid was removed, and the sugar con-centration determined by HPLC. Afterward, 10 mL of acidic solution(pH 5) was added to the separated solid, the mixture was shaken for 2 h,and then the solid was separated again by centrifugation. This processwas repeated three times. Finally, the solid was washed once by DI waterusing the same process. The solid was dried at 60 �C under vacuumovernight and then used for the sugar adsorption again. The aboveprocedure was repeated five times.

’RESULTS AND DISCUSSION

Hybrid Material Characterization. Powder X-ray diffractionwas used to investigate the mesostructure of the parent and theboronic acid functionalized SBA-15 materials. The PXRD pat-terns are shown in Figure 1. The parent SBA-15 (Figure 1a)shows three well-defined peaks at 2θ values between 0.8 and 5�that can be indexed as the (100), (110), and (200) Bragg peaks,typical of hexagonal (p6mm) SBA-15.51 For the hybrid materials,the intensity of the reflections decreases with increasing organic


Langmuir ARTICLE

content; the intensity of the (100) peak of the hybrid materialssynthesized through route two (Figure 1c) is lower than thatsynthesized through route one (Figure 1b). These are consistentwith our previous findings that the intensities of the reflectionsbecome weaker as the organic content of the hybrid materialincreases and no noticeable change in the peak positions isobserved.Nitrogen adsorption was used to quantify the change in

porosity of the hybrid materials. The adsorption isotherms ofthe parent and the PBA functionalized SBA-15 materials areshown in Figure 2, and the data for all the samples are sum-marized in Table 1. The surface areas and pore volumes usingthe αs method follow the anticipated trends. The values obtainedfor the bare silica are consistent with previous literature values.The SBA-15-g-PBA material has a noticeable reduction in porevolume and surface area, consistent with grafting of the ligand inthe pore. That the pore size is slightly larger than the SBA-15 issurprising, but the values are comparable and one interpretationis that presence of the ligand on the OMS surface slightly shifts

the pressure where capillary condensation is observed. Given thecomplexities of these hybrid materials, one must exercise cautionin overinterpreting the pore size dstribution obtained given theassumptions made in the various models may or may not beexactly valid for the samples under investigation. By contrast, theSBA-15-g-PPBA material is essentially nonporous, indicating amuch higher amount of organic incorporation in the mesoporesystem. It is also worth noting that for the parent SBA-15 andthe SBA-15-g-PBA there is a small amount of microporosity(∼0.05 cm3 g�1), consistent with literature reports. The trendsin the adsorption data are consistent with the chemistry in Scheme 1and indicate that a significant amount of the organic materialdeposited is in fact in the mesopores.The increase of organic content after each step during the

syntheses was monitored by TGA. The TGA curves and the totalweight losses of all the samples are presented in Figure 3. Basedon the increase of the organic content, the mole amount of thechemicals grafted to the hybrid material in each step is calculated.The calculation method and the results are given in the Support-ing Information. In route one, the approximate mole amount ofAPTES, SA, and PBA grafted to 1 g of parent SBA-15 is 0.63,0.41, and 0.32 mmol, respectively. Thus, the conversion of theamine-functionalized SBA-15 (SBA-15-APTES) to the carboxyl-functionalized SBA-15 (SBA-15-COOH) was not quantitativebecause only about two-thirds of the amine groups have beenconverted. Similar phenomena have been reported previously,and one possible explanation consistent with our prior work isthat some of the amines are preferentially grafted into themicropores of SBA-15 and thus are inaccessible for furtherfunctionalization.52 The TGA data also indicates that only someof the carboxyl groups on SBA-15-COOH have reacted withPBA. This is likely due to steric effects in that not all of thecarboxyl acid groups are accessible due to the steric bulk ofadjacent PBAgroups. In route two, the approximatemole amount of

Figure 2. Nitrogen adsorption isotherms for (a) SBA-15, (b) SBA-15-g-PBA, and (c) SBA-15-g-PPBA.

Table 1. Summary of Adsorption Data of Pure and PBAFunctionalized SBA-15 Samples

sample S(αs) (m2 g�1) νmeso(αs) (cm

3 g�1) dBJH (nm)

SBA-15 940 0.98 7.3

SBA-15-g-PBA 515 0.64 7.5

SBA-15-g-PPBA ∼10 ∼0

Figure 3. TGA traces of samples prepared through (a) route one and(b) route two.

Figure 1. PXRD patterns of (a) SBA-15, (b) SBA-15-g-PBA, and (c)SBA-15-g-PPBA.


Langmuir ARTICLE

APTES, BIBB, acrylic acid (AA), and PBA grafted to 1 g of parentSBA-15 based on the total weight loss is 0.71, 0.29, 3.3, and3.0 mmol, respectively. Because the ATRP initiator (BIBB) isdifficult to immobilize on SBA-15 directly, SBA-15 was func-tionalized with APTES first. A higher concentration of APTESwas used in the first step in route two to increase the loading ofamine groups and subsequently the density of the ATRPinitiator. Not surprisingly, the conversion of SBA-15-APTESto SBA-15-Br in route two was also incomplete and less thanhalf of the amine groups on SBA-15-APTES have reacted withBIBB. However, after the ATRP, PAA brushes with abundantcarboxyl groups were grafted to SBA-15-Br. This providesmultiple sites for the final immobilization of PBA. It isnoteworthy that most of the carboxyl groups in route twohave been converted to boronic acid groups according to themole amounts of AA and PBA calculated from the TGA results.This is probably because the comblike structure of the PAAbrushes provides adequate steric spaces for the grafting of PBA.As anticipated, the above TGA results demonstrate that theATRP method in route two leads to higher total organic contentand higher PBA loading, consistent with the nitrogen adsorptionresults.The bulk chemical composition of the PBA functionalized

SBA-15 via different synthetic protocols was also characterizedby elemental analysis. Table 2 lists the weight percentages ofthe major elements in the PBA functionalized SBA-15 samples.By comparing the weight percentages of elements in the twosamples, we can see that route two resulted inmuchmore organicmatter in the hybridmaterials. Particularly, the weight percentageof boron in SBA-15-g-PPBA is almost five times that in SBA-15-g-PBA. The results shown here reinforce the conclusions fromTGA analysis and further confirm that the ATRP method inroute two leads to higher total organic content and higher PBAloading.Figure 4 shows the IR spectra of the parent and the function-

alized SBA-15. Figure 4a shows the IR spectra of the originalSBA-15, and Figure 4b the APTES functionalized SBA-15. Anambiguous peak at approximately 3000�2800 cm�1 that isascribed to the C�H stretch appeared in Figure 4b. After theimmobilization of the ATRP initiator, a new peak at 1540 cm�1

was observed in Figure 4c, which should be due to the amidegroups formed by the reaction of amine groups and acyl bromidegroups. In Figure 4d, the appearance of the characteristic peak forcarboxyl groups at 1740 cm�1 and the more obvious peak ataround 3000�2800 cm�1 for the C�H stretch demonstrate thesuccessful surface-initiated polymerization of the sodium acrylate(NaAc) from SBA-15-Br in the ATRP process. Figure 4e showsthe spectrum of the PBA functionalized SBA-15 through routetwo and Figure 4f is the spectrum of the pure PBA. In Figure 4f,the two sharp peaks at 3470 and 3390 cm�1 represent theprimary amine groups in PBA; the other two adjacent peaks at1450 and 1360 cm�1 are induced by the benzene ring; and thepeak at around 700 cm�1 should be ascribed to the boronic acid

groups. By comparing Figure 4e and f, it can be seen that thecharacteristic peaks for the primary amine groups disappearedand a new peak at approximately 3370 cm�1 appeared instead,consistent with amide formation after the grafting of PBA to thePAA brushes. Also, the appearance of the characteristic peaks forbenzene ring and the boronic acid groups in Figure 4e indicatesthe successful immobilization of the PBA on the SBA-15 surface.Figure 4g and h shows the spectra of the samples prepared inroute one. After the reaction between the succinic anhydride andthe amine groups, the surface of the SBA-15 was functionalizedwith carboxyl groups, which led to the appearance of the peak at1740 cm�1 in Figure 4g. In Figure 4h, the characteristic peaks forthe PBA appeared, but the intensity of the peaks is much weakerthan that of those in Figure 4e. This is because the method inroute two led to a higher loading of the PBA on SBA-15, as theTGA and elemental analysis results have shown. The IR resultsindicate that, albeit qualitatively, the protocols depicted inScheme 1 are feasible.Sugar Capture. Two types of sugars are present in biomass:

hexoses (six-carbon sugars), of which glucose is the most common,and pentoses (five-carbon sugars), of which xylose is the mostcommon. In this work, D-(+)-glucose and D-(+)-xylose werechosen as the model sugars to investigate the sugar capturecapacity of the boronic acid functionalized hybrid materials. Theadsorption properties of the two synthesized hybrid materialswere evaluated with aqueous solutions (pH = 8.7) containingsugars of different concentrations. As a control, the parent SBA-15 was used to adsorb sugars under the same conditions, but noadsorption was observed. Data obtained from adsorption iso-therms were fitted to the Langmuir and Freundlich adsorption

Table 2. Summary of Elemental Analysis of PBA Function-alized SBA-15 Samples

element (wt %)

sample Si C N B

SBA-15-g-PBA 36.3 8.37 1.23 0.261

SBA-15-g-PPBA 23.1 21.57 3.03 1.29

Figure 4. IR spectra of (a) SBA-15, (b) SBA-15-APTES, (c) SBA-15-Br,(d) SBA-15-g-PAA, (e) SBA-15-g-PPBA, (f) pure PBA, (g) SBA-15-COOH, and (h) SBA-15-g-PBA.


Langmuir ARTICLE

models. The Langmuir isotherm is given by the followingequation:53

qe ¼ qmKLCe

1 þ KLCeð2Þ

where qm is the maximum adsorption capacity (g g�1) andKL is the equilibrium adsorption constant related to the adsorp-tion energy (L g�1). The Freundlich isotherm is given by thefollowing equation:54

qe ¼ KFðCeÞn ð3Þwhere KF and n are the Freundlich constants related to theadsorption capacity and intensity, respectively. The equilibriumconstants for these models were determined using linear regres-sion analysis.Figures 5 and 6 show the adsorption isotherms of sugars and

corresponding Langmuir and Freundlich plots. The obtainedfitting parameters together with the correlation coefficients (R2)are listed in Table 3. For the hybrid material prepared through

route one (i.e., SBA-15-g-PBA), the Langmuir model providesmore satisfactory fits than the Freundlich model for both D-(+)-glucose and D-(+)-xylose adsorption isotherms based on theR2 values. One could imagine that the chemistries used in route oneshould lead to a monolayer of boronic acid groups on the poresurface of SBA-15, and this would favor monolayer adsorptiondescribed by the Langmuir model. For the hybrid materialprepared through route two (i.e., SBA-15-g-PPBA), becausethe PAA brushes grafted onto the pore surface of SBA-15 byATRP provided multiple sites for the immobilization of PBA,multilayer boronic acid groups should present on the poresurface of SBA-15, which would lead to multilayer adsorptionof sugars. This was in fact observed in the adsorption data forSBA-15-g-PPBA. FromTable 3, it can be seen that the Freundlichmodel gives a better description of the adsorption mechanismthan the Langmuir model for both D-(+)-glucose and D-(+)-xylose adsorption isotherms. The good fit to the Freundlichmodel for SBA-15-g-PPBA confirms that the multilayer adsorption

Figure 5. (a) Adsorption isotherms of sugars on SBA-15-g-PBA, (b)analysis of data in (a) using the Langmuir isotherm model, and (c)analysis using the Freundlich isotherm model.

Figure 6. (a) Adsorption isotherms of sugars on SBA-15-g-PPBA, (b)analysis of data in (a) using the Langmuir isotherm model, and (c)analysis using the Freundlich isotherm model.


Langmuir ARTICLE

(or heterogeneous site adsorption) has actually occurred. Also,the small values of exponent (n < 1) in the Freundlich modelsuggest the favored adsorption of sugars by SBA-15-g-PPBA.Each boronic acid group will capture one sugar molecule

by forming a cyclic ester.39 To better understand the sugar adsorp-tion behavior of the synthesized hybrid materials, we comparedthe experimental sugar adsorption data with the calculatedmaximum sugar adsorption data. Table 4 gives the PBA loadingand the theoretical maximum sugar adsorption capacity (assumingone boronic acid group capture one sugar molecule) of thehybrid materials calculated from the TGA data. In Figure 5a, thehighest sugar adsorption for SBA-15-g-PBA observed in theisotherms is 0.039 g g�1 for D-(+)-glucose and 0.027 g g�1 forD-(+)-xylose; in Figure 6a, the highest sugar adsorption for SBA-15-g-PPBA is 0.208 g g�1 for D-(+)-glucose and 0.171 g g�1 forD-(+)-xylose with a sugar concentration of 30 g L�1. Theseresults are very close to the calculated data in Table 4. Alsonoteworthy is that the calculated theoretical maximum sugaradsorption for SBA-15-g-PBA in Table 4 agrees well with the

maximum sugar adsorption predicted by the Langmuir model inTable 3. The above results demonstrate that the maximum sugaradsorption capacity of the hybrid materials is determined by theloading of the boronic acid groups, which indicates the methodproposed in route two is much better than that in route one forincreasing the sugar adsorption capacity.Sugar Uptake Kinetics/Recyclability. SBA-15-g-PPBA was

chosen to analyze the adsorption rates of sugars, and the resultsare shown in Figure 7. Two stages of adsorption can be observedfor both sugars. There is a period where sugar uptake is relativelyfast, and then the adsorption leveled off, reaching the saturationadsorption capacity of the hybrid material. It also can be seen thatthe adsorption of D-(+)-xylose reached the plateau about 2 hearlier than that of D-(+)-glucose. This is presumably because the�CH2OH group linked to the fifth carbon of D-(+)-glucoseincreased the migrating hindrance through the polymer brushesduring the adsorption. The higher saturation adsorption capacityof the hybrid material for D-(+)-glucose than that for D-(+)-xylose should be due to the higher molecular weight of D-(+)-glucose, since the maximum sugar adsorption capacity of thehybrid materials is determined by the loading of the boronic acidgroups as discussed above.The ability to regenerate and recycle the hybrid adsorbent was

also explored. The sugar saturated hybrid material (SBA-15-g-PPBA) was regenerated by rinsing with a HCl solution of pH 5that is well below the pKa value of phenyl boronic acid (pKa =8.7�8.9)55 and reused for sugar capture. As shown in Figure 8,no dramatic loss of sugar adsorption capacity was observed afterfive adsorption/regeneration cycles. These results show that thehybrid materials are stable under conditions employed for sugarcapture.

Table 4. PBA Loading and Theoretical Maximum SugarAdsorption Capacity Calculated from TGA Data

theoretical maximum sugar

adsorption (qmax.theor., g g�1)b

sample

PBA loading

(mmol g�1)a D-(+)-xylose D-(+)-glucose

SBA-15-g-PBA 0.26 0.039 0.047

SBA-15-g-PPBA 1.60 0.240 0.288aThe calculationmethod is given in the Supporting Information. b qmax.theor. =PBA loading � molecular weight of sugars.

Table 3. Langmuir and Freundlich Isotherm Adsorption Constants for Sugars on PBA Functionalized SBA-15

Langmuir model Freundlich model

sample sugar KL (L/g) qm (g/g) R2 KF n R2

SBA-15-g-PBA D-(+)-xylose 0.237 0.032 0.990 0.010 0.461 0.967

D-(+)-glucose 0.216 0.045 0.992 0.006 0.513 0.879

SBA-15-g-PPBA D-(+)-xylose 0.042 0.372 0.783 0.025 0.816 0.961

D-(+)-glucose 0.072 0.381 0.881 0.015 0.862 0.963

Figure 7. Transient uptake curves for SBA-15-g-PPBA.

Figure 8. Equilibrium uptake of SBA-15-g-PPBA as a function ofadsorption cycle.


Langmuir ARTICLE

One goal of this work is to synthesize novel materials that canselectively capture sugars from cellulose hydrolysis mixtures.Therefore, we chose some of the major chemicals besides thesugars produced in cellulose hydrolysis, that is, acetic acid, furfural,and levulinic acid, to evaluate the selective adsorption of thehybrid material synthesized toward sugars. Figure 9 shows theselectivity of SBA-15-g-PPBA toward the five chosen chemicals.It was found that the hybrid material showed almost no adsorp-tion affinity to acetic acid and levulinic acid, though it couldcapture a small amount of furfural. Although the data in Figure 9represents adsorption from single-component solutions, theresults demonstrate that the hybrid material synthesized haspotential for the selectively capturing sugars from the cellulosehydrolysis mixture.

’CONCLUSIONS

In summary, organic�inorganic hybridmaterials, phenylboro-nic acid functionalized SBA-15, were successfully synthesized viavarying chemistries. These hybrid materials showed good ad-sorption capacity for D-(+)-glucose and D-(+)-xylose, but almostno adsorption for some organic acids or aldehydes, such as aceticacid, levulinic acid, and furfural, which is a proof of concept thatsugars can be selectively separated from biomass (i.e., cellulose)hydrolysis mixtures. The hybrid materials can also be easilyregenerated, showing good durability and reusability that areimportant in practical applications. The current work demon-strates that it is possible to utilize chemistries, such as ATRP, onhigh surface area supports to make hybrid materials that can betailored to selectively capture compounds from solution. Thesehybrid materials represent one route to achieve separations ofcomplex mixtures produced by, for instance, biomass hydrolysisin aqueous solutions.

’ASSOCIATED CONTENT

bS Supporting Information. Total weight loss after eachstep during the syntheses measured by TGA; calculation methodfor the mole amount of the chemicals grafted to the hybridmaterial in each step based on the TGA results; pH values ofacids and aldehydes solutions. This material is available free ofcharge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

’ACKNOWLEDGMENT

The authors acknowledge financial support from the NationalScience Foundation (CBET-0957943).

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Figure 9. Selective adsorption of SBA-15-g-PPBA.


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Phenylboronic Acid Functionalized SBA-15 for Sugar Capture