ARTICLE pubs.acs.org/Langmuir
Phenylboronic Acid Functionalized SBA-15 for Sugar Capture Yong-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 ABSTRACT: The synthesis and characterization of organicinorganic hybrid materials that selectively capture sugars from model biomass hydrolysis mixtures are reported. 3-Aminophenylboronic acid (PBA) groups that can reversibly form cyclic esters with 1,2-diols, and 1,3-diols including sugars are attached to mesoporous SBA-15 via different synthetic protocols. In the first route, a coupling agent is used to link PBA and SBA-15, while in the second route poly(acrylic acid) brushes are first grafted from the surface of SBA-15 by surface-initiated atom transfer radical polymerization and PBA is then immobilized. The changes in pore structure, porosity, and pore size due to the loading of organic content are measured by powder X-ray diffraction and nitrogen porosimetry. The increase in organic content after each synthesis step is monitored by thermal gravimetric analysis. Fourier transform infrared spectroscopy and elemental analysis are used to characterize the chemical compositions of the hybrid materials synthesized. D-(+)-Glucose and D-(+)-xylose, being the most commonly present sugars in biomass, are chosen to evaluate the sugar adsorption capacity of the hybrid materials. It is found that the sugar adsorption capacity is determined by the loading of boronic acid groups on the hybrid materials, 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 the hybrid material synthesized through route one, while the Freundlich model fits the data most satisfactorily for the hybrid material prepared via route two. The adsorption kinetics, reusability, and selectivity toward some typical chemicals in cellulose acidic hydrolysis mixtures are also investigated.
’ INTRODUCTION Due to the impending decline of fossil fuel reserves and potential deleterious effects of climate change, the progressive changeover of the chemical industry to renewable feedstocks for its raw materials is emerging as a critical issue.1 While many solutions are being explored, fuels and chemicals from renewable biomass sources have emerged as one solution with both shortand long-term potential.2,3 Generating fuel and chemicals from biomass carbohydrates, such as lignocelluloses, is very attractive as it represents a shift away from importing energy and is in principle carbon neutral.46 The structure of lignocelluloses is totally different from that of present fuels and chemicals. It needs to be depolymerized or deoxygenated to be suitable for these applications. A variety of chemistries and processes, such as hydrolysis, pyrolysis, gasification, and so forth, have been applied to convert lignocellulosic biomass to valuable chemicals or intermediates.3,7,8 One of the most important products in the conversion of lignocelluloses are sugars which can be subsequently converted to a variety of derivatives through biological or chemical conversions. Sugars are the main feedstocks for bioethanol production through fermentation. Bioethanol has been touted as a promising biofuel that has the potential to be a valuable substitute for, or complement to, gasoline.911 Numerous important chemical building blocks, such as lactic acid, succinic acid, 3-hydroxy propinoic acid, itaconic acid, and so forth, can also be obtained by fermentation r 2011 American Chemical Society
of sugars.12 Chemical technologies also offer a variety of routes to upgrade sugars. For example, sugars can be hydrogenated to C56 polyols such as xylitol, mannitol, and sorbitol,13,14 hydrogenolyzed to C23 glycols,15 or further upgraded via oxidation or halogenation reactions.16 One clear need in these processes is the ability to effectively separate sugars from mixtures of processed biomass. For instance, in the acid-hydrolysis fermentation conversion of lignocellulosic biomass, the sugars obtained from hydrolysis need to be processed before they are used in fermentation media. In general, the following operations are needed: concentration of sugars by evaporation; detoxification by active carbon adsorption; neutralization of acids; and removal of the insoluble salts formed during the neutralization of acids by filtration.17 These operations are usually costly and time-consuming, which has limited the current economic impact of fermentation products. Thus, substantial improvements in the existing separation technology are needed in order to allow the biofuel and the chemical building blocks from fermentation to penetrate further into the organic chemical industry.12 Organicinorganic hybrid materials have long attracted interest from the scientific and engineering communities.1821 Received: August 10, 2011 Revised: October 24, 2011 Published: October 24, 2011 14554
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Scheme 1. Synthetic Protocols for PBA-Functionalized SBA-15
These materials hold great promise in areas including sensing, separations, optics, and catalysis, given that they combine the features of both organic and inorganic materials. Ordered mesoporous silicas (OMS)2228 have been studied extensively as supports for hybrid materials given that they possess intrinsic high specific surface areas, regular and tunable pore sizes, large pore volumes, as well as stable and interconnected frameworks with active pore surfaces for modification or functionalization. Such features meet the requirements as excellent adsorbents, not only providing a large interface and space capable of accommodating capacious guest species, but also enabling the possibility of specific binding, enrichment, and separation.29 Within the context of the current work, it should be noted that many functional groups have been attached to ordered mesoporous silica,30,31 and in fact the use of OMS as a support for atom transfer radical polymerization (ATRP) catalysts has been reported previously.32,33 There have also been reports of the use of SBA-15 as a support to form polymer brushes via ATRP.3437 On the other hand, the adsorption of sugars using zeolites in aqueous solutions has been reported38 and the recognition of saccharides by boronic acids is an intriguing subject in supramolecular chemistry.3941 There is general agreement that boronic acids covalently react with 1,2-diols and 1,3-diols including sugars by reversibly forming cyclic esters with five or six member cyclic structures.42,43 This interaction has been extensively exploited, with a particular emphasis on separation,42 transport,44 and detection of sugars.45 In order to promote solid phase separation and reusability of boronic acids, it is usually important to immobilize the boronic acids on insoluble solid supports. In our previous work, we synthesized organicinorganic hybrid membranes that could efficiently separate the valuable low molecular weight chemicals, such as sugars, aldehydes and organic acids, etc., from cellulose acid hydrolysis mixtures.46 In this report, we describe the synthesis of hybrid materials that can selectively capture sugars. A widely studied OMS material, SBA-15, was selected as the inorganic support. Boronic acids were then immobilized to SBA-15 surface via two methods
(Scheme 1). In the first route, a short coupling agent was used to link the 3-aminophenylboronic acid (PBA) to the surface of SBA-15, which should lead to a monolayer of boronic acid groups on the pore surface. In the other one, poly(acrylic acid) (PAA) polymer brushes were first grafted to the SBA-15 surface via ATRP, and the abundant carboxyl groups in the grafted PAA brushes provided multiple sites for the subsequent immobilization of boronic acids, which should result in higher loading of boronic acids. Two sugars, D-(+)-glucose and D-(+)-xylose, being most commonly present in lignocelluloses, were used to evaluate the sugar adsorption capacities of the hybrid materials synthesized via the above two methods. The adsorption kinetics, reusability, and selectivity toward some typical chemicals in cellulose acidic hydrolysis mixtures were also investigated. The current work represents a route to selectively isolate sugars from aqueous mixtures generated from biomass conversion and thus a potentially enabling route to expanding the scope of biofuel production.
’ EXPERIMENTAL SECTION Materials. Tetraethoxysilane (TEOS, g 99%) was purchased from Fluka. Pluronic P123 (EO20PO70EO20, Mw = 5800) was obtained from BASF. 3-Aminopropyltriethoxysilane (APTES, 99%), 2,20 -bipyridyl (Bpy, 99%), 2-bromoisobutyryl bromide (BIBB, 98%), CuBr (98%), CuBr2 (99%), succinic anhydride (SA, g99%), N,N0 -dicyclohexylcarbodiimide (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 hydrochloric acid (HCl, 37%) were purchased from Aldrich and used as received. 3-Aminophenylboronic acid monohydrate (PBA, 98%) was obtained from Alfa and used as received. Triethylamine (TEA, 99%) and N,N-dimethylformamide (DMF, 99.8%) from Aldrich were purified by distillation and stored over 4 Å molecular sieves. Ethanol, methanol, toluene, and tetrahydrofuran (THF) (all ACS reagent grade) were purchased from VWR. Toluene and THF were dried using a MBRAUN MB-SPS solvent drying system; all other solvents were used as received. 14555
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Langmuir 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) was dissolved in 60 mL of 4 M HCl and 85 mL of DI water by stirring for 5 h at room temperature. Then, 8.50 g of TEOS was added to that solution and stirred for 24 h at 35 °C. The mixture was then aged at 80 °C for 24 h under static conditions. The solid product was filtered, washed with copious quantities of DI water, and air-dried overnight. The solid product was calcined to remove the Pluronic. The calcination procedure was as follows: the sample was heated from room temperature to 100 °C at a rate of 1 °C min1; held at 100 °C for 1 h; increased from 100 to 500 °C at a rate of 1 °C min1; 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 both protocols, the SBA-15 was first functionalized with amine groups by grafting 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 calcined SBA-15 in 100 mL of anhydrous toluene under nitrogen protection. This mixture 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-functionalized SBA-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 to a flask containing 0.20 g (2 mmol) of SA and 0.02 g (0.1 mmol) of DCC in 25 mL of anhydrous DMF under rigorous stirring. The mixture was stirred for 24 h, and the resultant carboxyl-functionalized SBA-15 (denoted as SBA-15-COOH) was washed with DMF, ethanol, and DI water sequentially and then dried at 60 °C under vacuum overnight. PBA was then grafted to the carboxyl-functionalized SBA-15. Next, 1 g of SBA-15-COOH was dispersed in 50 mL of anhydrous DMF, and then 0.32 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-functionalized SBA-15 (denoted as SBA-15-g-PBA) was collected by filtration, washed sequentially with DMF, ethanol, and DI water, and air-dried. In route two, to immobilize PBA on SBA-15, PAA brushes were first grafted to SBA-15 using ATRP. To achieve this, the amine-functionalized SBA-15 was first reacted with BIBB at room temperature in the presence of TEA and under nitrogen protection, using anhydrous THF as solvent. The product (denoted as SBA-15-Br) was collected by filtration, washed sequentially with methanolDI watermethanol, and air-dried. Then, 0.14 g (1 mmol) of CuBr, 0.06 g (0.25 mmol) of CuBr2, and 1 g of SBA-15-Br were placed into a 100 mL three-necked flask under nitrogen protection. The NaAc monomer (0.28 g, 3 mmol) and Bpy (0.31 g, 2 mmol) were dissolved in a 50 mL DI water/methanol (1/1, v/v) mixture, and then the solution was transferred to the threenecked flask after purging with nitrogen for 30 min. The mixture was stirred at room temperature for 24 h, and the ATRP was terminated by exposure to air. After being thoroughly washed with methanol and DI water, the grafted poly(sodium acrylate) on SBA-15 by ATRP was converted to poly(acrylic acid) by washing with a pH 5 HCl solution, followed by washing with copious DI water again. The product (denoted as 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 SBA15-g-PAA was dispersed in 50 mL of anhydrous DMF, and then 0.47 g (3 mmol) of PBA and 0.02 g (0.1 mmol) of DCC were added. This mixture was stirred for 24 h, and the product (denoted as SBA-15-g-PPBA) was collected by filtration, washed sequentially with DMF, ethanol, and DI water, and air-dried. Analytical. Fourier transform infrared spectroscopy (FTIR) measurements were carried out on a Thermo Nicolet Nexus 670 instrument. A total of 64 scans were acquired at a resolution of 4 cm1. Powder X-ray diffraction (PXRD) measurements were performed using a Bruker-AXS D8 powder diffractometer with Cu Kα radiation over a range of 0.55° 2θ.
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Peak intensities and 2θ values were determined using the Bruker program EVA. Nitrogen adsorption experiments were performed on a Micromeritics ASAP 2010 micropore system using approximately 0.6 g of sample. The samples were degassed under vacuum at room temperature for 2 h, then at 40 °C for 4 h, and then at 60 °C for 6 h before analysis. The micropore and mesopore volumes were determined using the αs-method.48 The mesopore size distributions were calculated from the adsorption branch of the isotherm using the BarretJoyner Halenda (BJH) method49 with a modified equation50 for the statistical film thickness. Thermal gravimetric analyses (TGA) were performed using a TG 209C Iris instrument from Netzsch over a temperature range of 25600 °C with oxygen and nitrogen as carrier gases and temperature ramping rate of 5 °C min1. Elemental analysis (Si, N, C, and B) was performed by the Galbraith Laboratories. Solution Sugar Capture. The PBA-functionalized SBA-15 was tested for adsorption of D-(+)-xylose, D-(+)-glucose, acetic acid, levulinic acid, and furfural. Sugar solutions of different concentrations were prepared by dissolving the sugars in DI water, of which the pH values were adjusted to 8.7 using a 0.001 M NaOH solution. The acids and aldehyde solutions of different concentrations were prepared by dissolving the acids or the aldehyde in DI water. Adsorption experiments were performed as follows. 0.1 g of PBA-functionalized SBA-15 was added to 2 mL of a solution containing the target compounds for capture at a desired concentration. The mixture was shaken for varying predetermined times, and then the PBA-functionalized SBA-15 was removed by centrifuging at 6000 rpm for 10 min. Solution concentrations were determined with high performance liquid chromatography (HPLC, Agilent 1120) using an Agilent ZORBAX Eclipse Plus C18 column and an Agilent 1260 Infinity refractive index detector (RID). The mobile phase was degassed DI water fed at a rate of 1.5 mL min1, and the temperature of the column was at 25 °C. The amount of adsorbed substrate per unit mass of the PBA-functionalized SBA-15 at equilibrium concentration, qe (in g g1), was calculated from the mass balance equation qe ¼
C0 Ce m=V
ð1Þ
where C0 and Ce are the initial and equilibrium substrate concentrations (in g L1), respectively, V is the volume of the liquid phase (in L), and m is the mass of the PBA-functionalized SBA-15 (in g). The reusability of the PBA-functionalized SBA-15 in sugar capture was investigated. An amount of 0.1 g of the PBA-functionalized SBA-15 was added to 2 mL of 30 g L1 sugar solution. The mixture was shaken for 6 h, and then the solids were separated by centrifugation at 6000 rpm for 10 min. The supernatant liquid was removed, and the sugar concentration 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 process was repeated three times. Finally, the solid was washed once by DI water using the same process. The solid was dried at 60 °C under vacuum overnight and then used for the sugar adsorption again. The above procedure was repeated five times.
’ RESULTS AND DISCUSSION Hybrid Material Characterization. Powder X-ray diffraction was used to investigate the mesostructure of the parent and the boronic acid functionalized SBA-15 materials. The PXRD patterns 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 14556
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Figure 1. PXRD patterns of (a) SBA-15, (b) SBA-15-g-PBA, and (c) SBA-15-g-PPBA.
Figure 3. TGA traces of samples prepared through (a) route one and (b) route two.
Figure 2. Nitrogen adsorption isotherms for (a) SBA-15, (b) SBA-15g-PBA, and (c) SBA-15-g-PPBA.
Table 1. Summary of Adsorption Data of Pure and PBA Functionalized SBA-15 Samples sample
S(αs) (m2 g1)
νmeso(αs) (cm3 g1)
dBJH (nm)
SBA-15
940
0.98
7.3
SBA-15-g-PBA
515
0.64
7.5
SBA-15-g-PPBA
∼10
∼0
content; the intensity of the (100) peak of the hybrid materials synthesized through route two (Figure 1c) is lower than that synthesized through route one (Figure 1b). These are consistent with our previous findings that the intensities of the reflections become weaker as the organic content of the hybrid material increases and no noticeable change in the peak positions is observed. Nitrogen adsorption was used to quantify the change in porosity of the hybrid materials. The adsorption isotherms of the parent and the PBA functionalized SBA-15 materials are shown in Figure 2, and the data for all the samples are summarized in Table 1. The surface areas and pore volumes using the αs method follow the anticipated trends. The values obtained for the bare silica are consistent with previous literature values. The SBA-15-g-PBA material has a noticeable reduction in pore volume and surface area, consistent with grafting of the ligand in the pore. That the pore size is slightly larger than the SBA-15 is surprising, but the values are comparable and one interpretation is that presence of the ligand on the OMS surface slightly shifts
the pressure where capillary condensation is observed. Given the complexities of these hybrid materials, one must exercise caution in overinterpreting the pore size dstribution obtained given the assumptions made in the various models may or may not be exactly valid for the samples under investigation. By contrast, the SBA-15-g-PPBA material is essentially nonporous, indicating a much higher amount of organic incorporation in the mesopore system. It is also worth noting that for the parent SBA-15 and the SBA-15-g-PBA there is a small amount of microporosity (∼0.05 cm3 g1), consistent with literature reports. The trends in the adsorption data are consistent with the chemistry in Scheme 1 and indicate that a significant amount of the organic material deposited 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 total weight losses of all the samples are presented in Figure 3. Based on the increase of the organic content, the mole amount of the chemicals grafted to the hybrid material in each step is calculated. The calculation method and the results are given in the Supporting Information. In route one, the approximate mole amount of APTES, 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 the amine-functionalized SBA-15 (SBA-15-APTES) to the carboxylfunctionalized SBA-15 (SBA-15-COOH) was not quantitative because only about two-thirds of the amine groups have been converted. Similar phenomena have been reported previously, and one possible explanation consistent with our prior work is that some of the amines are preferentially grafted into the micropores of SBA-15 and thus are inaccessible for further functionalization.52 The TGA data also indicates that only some of the carboxyl groups on SBA-15-COOH have reacted with PBA. This is likely due to steric effects in that not all of the carboxyl acid groups are accessible due to the steric bulk of adjacent PBA groups. In route two, the approximate mole amount of 14557
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Table 2. Summary of Elemental Analysis of PBA Functionalized SBA-15 Samples element (wt %) sample
Si
C
N
B
SBA-15-g-PBA SBA-15-g-PPBA
36.3 23.1
8.37 21.57
1.23 3.03
0.261 1.29
APTES, BIBB, acrylic acid (AA), and PBA grafted to 1 g of parent SBA-15 based on the total weight loss is 0.71, 0.29, 3.3, and 3.0 mmol, respectively. Because the ATRP initiator (BIBB) is difficult to immobilize on SBA-15 directly, SBA-15 was functionalized with APTES first. A higher concentration of APTES was used in the first step in route two to increase the loading of amine groups and subsequently the density of the ATRP initiator. Not surprisingly, the conversion of SBA-15-APTES to SBA-15-Br in route two was also incomplete and less than half of the amine groups on SBA-15-APTES have reacted with BIBB. However, after the ATRP, PAA brushes with abundant carboxyl groups were grafted to SBA-15-Br. This provides multiple sites for the final immobilization of PBA. It is noteworthy that most of the carboxyl groups in route two have been converted to boronic acid groups according to the mole amounts of AA and PBA calculated from the TGA results. This is probably because the comblike structure of the PAA brushes provides adequate steric spaces for the grafting of PBA. As anticipated, the above TGA results demonstrate that the ATRP method in route two leads to higher total organic content and higher PBA loading, consistent with the nitrogen adsorption results. The bulk chemical composition of the PBA functionalized SBA-15 via different synthetic protocols was also characterized by elemental analysis. Table 2 lists the weight percentages of the major elements in the PBA functionalized SBA-15 samples. By comparing the weight percentages of elements in the two samples, we can see that route two resulted in much more organic matter in the hybrid materials. Particularly, the weight percentage of boron in SBA-15-g-PPBA is almost five times that in SBA-15-gPBA. The results shown here reinforce the conclusions from TGA analysis and further confirm that the ATRP method in route two leads to higher total organic content and higher PBA loading. Figure 4 shows the IR spectra of the parent and the functionalized SBA-15. Figure 4a shows the IR spectra of the original SBA-15, and Figure 4b the APTES functionalized SBA-15. An ambiguous peak at approximately 30002800 cm1 that is ascribed to the CH stretch appeared in Figure 4b. After the immobilization of the ATRP initiator, a new peak at 1540 cm1 was observed in Figure 4c, which should be due to the amide groups formed by the reaction of amine groups and acyl bromide groups. In Figure 4d, the appearance of the characteristic peak for carboxyl groups at 1740 cm1 and the more obvious peak at around 30002800 cm1 for the CH stretch demonstrate the successful surface-initiated polymerization of the sodium acrylate (NaAc) from SBA-15-Br in the ATRP process. Figure 4e shows the spectrum of the PBA functionalized SBA-15 through route two and Figure 4f is the spectrum of the pure PBA. In Figure 4f, the two sharp peaks at 3470 and 3390 cm1 represent the primary amine groups in PBA; the other two adjacent peaks at 1450 and 1360 cm1 are induced by the benzene ring; and the peak at around 700 cm1 should be ascribed to the boronic acid
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) SBA15-COOH, and (h) SBA-15-g-PBA.
groups. By comparing Figure 4e and f, it can be seen that the characteristic peaks for the primary amine groups disappeared and a new peak at approximately 3370 cm1 appeared instead, consistent with amide formation after the grafting of PBA to the PAA brushes. Also, the appearance of the characteristic peaks for benzene ring and the boronic acid groups in Figure 4e indicates the successful immobilization of the PBA on the SBA-15 surface. Figure 4g and h shows the spectra of the samples prepared in route one. After the reaction between the succinic anhydride and the amine groups, the surface of the SBA-15 was functionalized with carboxyl groups, which led to the appearance of the peak at 1740 cm1 in Figure 4g. In Figure 4h, the characteristic peaks for the PBA appeared, but the intensity of the peaks is much weaker than that of those in Figure 4e. This is because the method in route two led to a higher loading of the PBA on SBA-15, as the TGA and elemental analysis results have shown. The IR results indicate that, albeit qualitatively, the protocols depicted in Scheme 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 most common. In this work, D-(+)-glucose and D-(+)-xylose were chosen as the model sugars to investigate the sugar capture capacity of the boronic acid functionalized hybrid materials. The adsorption properties of the two synthesized hybrid materials were evaluated with aqueous solutions (pH = 8.7) containing sugars of different concentrations. As a control, the parent SBA15 was used to adsorb sugars under the same conditions, but no adsorption was observed. Data obtained from adsorption isotherms were fitted to the Langmuir and Freundlich adsorption 14558
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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.
models. The Langmuir isotherm is given by the following equation:53 q m KL C e qe ¼ 1 þ K L Ce
ð2Þ
where qm is the maximum adsorption capacity (g g1) and KL is the equilibrium adsorption constant related to the adsorption energy (L g1). The Freundlich isotherm is given by the following equation:54 qe ¼ KF ðCe Þn
ð3Þ
where KF and n are the Freundlich constants related to the adsorption capacity and intensity, respectively. The equilibrium constants for these models were determined using linear regression analysis. Figures 5 and 6 show the adsorption isotherms of sugars and corresponding Langmuir and Freundlich plots. The obtained fitting parameters together with the correlation coefficients (R2) are listed in Table 3. For the hybrid material prepared through
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.
route one (i.e., SBA-15-g-PBA), the Langmuir model provides more satisfactory fits than the Freundlich model for both D-(+)glucose and D-(+)-xylose adsorption isotherms based on the R2 values. One could imagine that the chemistries used in route one should lead to a monolayer of boronic acid groups on the pore surface of SBA-15, and this would favor monolayer adsorption described by the Langmuir model. For the hybrid material prepared through route two (i.e., SBA-15-g-PPBA), because the PAA brushes grafted onto the pore surface of SBA-15 by ATRP provided multiple sites for the immobilization of PBA, multilayer boronic acid groups should present on the pore surface of SBA-15, which would lead to multilayer adsorption of sugars. This was in fact observed in the adsorption data for SBA-15-g-PPBA. From Table 3, it can be seen that the Freundlich model gives a better description of the adsorption mechanism than the Langmuir model for both D-(+)-glucose and D-(+)xylose adsorption isotherms. The good fit to the Freundlich model for SBA-15-g-PPBA confirms that the multilayer adsorption 14559
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Table 3. Langmuir and Freundlich Isotherm Adsorption Constants for Sugars on PBA Functionalized SBA-15 Langmuir model KL (L/g)
qm (g/g)
R2
KF
n
R2
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
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
sample SBA-15-g-PBA
SBA-15-g-PPBA
Freundlich model
sugar
Table 4. PBA Loading and Theoretical Maximum Sugar Adsorption Capacity Calculated from TGA Data theoretical maximum sugar adsorption (qmax.theor., g g1)b PBA loading sample
(mmol g1)a
D-(+)-xylose
D-(+)-glucose
SBA-15-g-PBA
0.26
0.039
0.047
SBA-15-g-PPBA
1.60
0.240
0.288
a
The calculation method is given in the Supporting Information. b qmax.theor. = PBA loading molecular weight of sugars.
Figure 8. Equilibrium uptake of SBA-15-g-PPBA as a function of adsorption cycle.
Figure 7. Transient uptake curves for SBA-15-g-PPBA.
(or heterogeneous site adsorption) has actually occurred. Also, the small values of exponent (n < 1) in the Freundlich model suggest 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 adsorption behavior of the synthesized hybrid materials, we compared the experimental sugar adsorption data with the calculated maximum sugar adsorption data. Table 4 gives the PBA loading and the theoretical maximum sugar adsorption capacity (assuming one boronic acid group capture one sugar molecule) of the hybrid materials calculated from the TGA data. In Figure 5a, the highest sugar adsorption for SBA-15-g-PBA observed in the isotherms is 0.039 g g1 for D-(+)-glucose and 0.027 g g1 for D-(+)-xylose; in Figure 6a, the highest sugar adsorption for SBA15-g-PPBA is 0.208 g g1 for D-(+)-glucose and 0.171 g g1 for 1 D-(+)-xylose with a sugar concentration of 30 g L . These results are very close to the calculated data in Table 4. Also noteworthy is that the calculated theoretical maximum sugar adsorption for SBA-15-g-PBA in Table 4 agrees well with the
maximum sugar adsorption predicted by the Langmuir model in Table 3. The above results demonstrate that the maximum sugar adsorption capacity of the hybrid materials is determined by the loading of the boronic acid groups, which indicates the method proposed in route two is much better than that in route one for increasing the sugar adsorption capacity. Sugar Uptake Kinetics/Recyclability. SBA-15-g-PPBA was chosen to analyze the adsorption rates of sugars, and the results are shown in Figure 7. Two stages of adsorption can be observed for both sugars. There is a period where sugar uptake is relatively fast, and then the adsorption leveled off, reaching the saturation adsorption capacity of the hybrid material. It also can be seen that the adsorption of D-(+)-xylose reached the plateau about 2 h earlier than that of D-(+)-glucose. This is presumably because the CH2OH group linked to the fifth carbon of D-(+)-glucose increased the migrating hindrance through the polymer brushes during the adsorption. The higher saturation adsorption capacity of 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 the hybrid materials is determined by the loading of the boronic acid groups as discussed above. The ability to regenerate and recycle the hybrid adsorbent was also explored. The sugar saturated hybrid material (SBA-15g-PPBA) was regenerated by rinsing with a HCl solution of pH 5 that is well below the pKa value of phenyl boronic acid (pKa = 8.78.9)55 and reused for sugar capture. As shown in Figure 8, no dramatic loss of sugar adsorption capacity was observed after five adsorption/regeneration cycles. These results show that the hybrid materials are stable under conditions employed for sugar capture. 14560
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’ ACKNOWLEDGMENT The authors acknowledge financial support from the National Science Foundation (CBET-0957943). ’ REFERENCES
Figure 9. Selective adsorption of SBA-15-g-PPBA.
One goal of this work is to synthesize novel materials that can selectively capture sugars from cellulose hydrolysis mixtures. Therefore, we chose some of the major chemicals besides the sugars produced in cellulose hydrolysis, that is, acetic acid, furfural, and levulinic acid, to evaluate the selective adsorption of the hybrid material synthesized toward sugars. Figure 9 shows the selectivity of SBA-15-g-PPBA toward the five chosen chemicals. It was found that the hybrid material showed almost no adsorption affinity to acetic acid and levulinic acid, though it could capture a small amount of furfural. Although the data in Figure 9 represents adsorption from single-component solutions, the results demonstrate that the hybrid material synthesized has potential for the selectively capturing sugars from the cellulose hydrolysis mixture.
’ CONCLUSIONS In summary, organicinorganic hybrid materials, phenylboronic acid functionalized SBA-15, were successfully synthesized via varying chemistries. These hybrid materials showed good adsorption capacity for D-(+)-glucose and D-(+)-xylose, but almost no adsorption for some organic acids or aldehydes, such as acetic acid, levulinic acid, and furfural, which is a proof of concept that sugars can be selectively separated from biomass (i.e., cellulose) hydrolysis mixtures. The hybrid materials can also be easily regenerated, showing good durability and reusability that are important in practical applications. The current work demonstrates that it is possible to utilize chemistries, such as ATRP, on high surface area supports to make hybrid materials that can be tailored to selectively capture compounds from solution. These hybrid materials represent one route to achieve separations of complex mixtures produced by, for instance, biomass hydrolysis in aqueous solutions. ’ ASSOCIATED CONTENT
bS
Supporting Information. Total weight loss after each step during the syntheses measured by TGA; calculation method for the mole amount of the chemicals grafted to the hybrid material in each step based on the TGA results; pH values of acids and aldehydes solutions. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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