Adsorption of Pyruvic and Succinic Acid by Amine ... - ACS Publications

13076

J. Phys. Chem. C 2007, 111, 13076-13086

Adsorption of Pyruvic and Succinic Acid by Amine-Functionalized SBA-15 for the Purification of Succinic Acid from Fermentation Broth Young-Si Jun,† Yun Suk Huh,† Ho Seok Park,† Arne Thomas,‡ Sang Jun Jeon,† Eun Zoo Lee,† Hyo Jin Won,† Won Hi Hong,*,† Sang Yup Lee,† and Yeon Ki Hong§ Department of Chemical and Biomolecular Engineering, Korea AdVanced Institute of Science and Technology, Daejeon, 305-701, Korea, Max-Planck-Institute of Colloids and Interfaces, Department of Colloid Chemistry, Research Campus Golm, D-14424, Potsdam, Germany, and Department of Chemical and Biological Engineering, Chungju National UniVersity, Chungbuk, 380-702, Korea ReceiVed: April 3, 2007; In Final Form: June 13, 2007

In this study, mesoporous silica SBA-15 was functionalized with primary, secondary, and tertiary aminofunctional silanes onto the channel walls using a postsynthesis method as a first attempt to purify succinic acid from a fermentation broth. Ordered mesostructures of pristine and functionalized SBA-15 were evaluated using small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and N2 adsorption/ desorption isotherms. 13C and 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) with 1H cross-polarization (CP-MAS) and thermogravimetric analysis (TGA) revealed that amino-functional silanes were covalently bound to the active layer of pore walls. The distribution and accessibility of amine groups were characterized by scanning transmission electron microscopy (STEM), elemental analysis, and conductivity measurements. Adsorption isotherms were analyzed using the Sips model, simultaneously obtaining the temperature dependence of isotherms derived from the isosteric heats of adsorption. Pyruvic acid had higher adsorption capacities than succinic acid on amine-functionalized SBA-15, resulting in the selective adsorption of pyruvic acid from binary acid solution. In particular, SBA-15 functionalized with primary amino silane obtained higher selectivity on pyruvic acid compared to that of other amine-functionalized SBA-15. The adsorption capacities of pyruvic acid at equilibrium are dependent on the basicity and distribution of amino silanes. The isosteric heats between 10 and 100 kJ/mol and desorption energy between 1 and 10 kJ/mol revealed that the adsorption of pyruvic and succinic acid originated from the formation of an acid-amine complex via hydrogen bonding. It is proposed that the amine functionalization of ordered mesoporous solids provides a simple and effective method of separating or purifying useful carboxylic acids.

Introduction Over the past decade, ordered mesoporous materials such as MCM-n, HMS-n, and SBA-n have been a primary focus in material science because of their regularly ordered pore arrangement, narrow pore-size distribution, and high specific surface area.1 These ordered mesoporous materials offer advantages over other porous solids in terms of good textural properties,2a hydrothermal stability,2b and tunability of pore size ranging from 5 to 30 nm2c in applications that involve adsorption,3 catalysis,4 and use as a host material.5 To combine these attractive properties with a specific chemical reactivity in a single solid, organofunctional groups such as amine, thiol, carboxylic, alkyl chloride, and aromatic have been incorporated into the structure of mesoporous material via postsynthesis methods using active silanol groups on the surface of the material6a or co-condensation with an organosiloxane during the preparation of the material,6b making them an ideal host material for, for example, biomolecules.7 These organic-inorganic host materials immobilize the biomolecules using methods such as chemical bonding,8 physical interactions,9 and layer-by-layer * To whom correspondence should be addressed. E-mail: whhong@ kaist.ac.kr. † Korea Advanced Institute of Science and Technology. ‡ Max-Planck-Institute of Colloids and Interfaces. § Chungju National University.

encapsulation by a polymer electrolyte.10 The immobilized biomolecules have enhanced thermal and pH stability in aqueous solutions and organic solvents.11 They retain their characteristic functionality after immobilization, as demonstrated by their potential for the biocatalytic process.12 In addition, a suitable choice of grafting agent and support material makes it possible to control both the adsorption and release of biomolecules from the inorganic host material, which is important in the fields of sensing13 and drug delivery.14 These hybrid materials are also strong candidates for the adsorption of heavy metal ions,15a dyes,15b and toxic oxyanions15c from aqueous solutions; however, despite this wide range of applications, there are few reports, to the best of our knowledge, on the separation and purification of carboxylic acids using mesoporous material as a potential adsorbent. Succinic acid, a four-carbon dicarboxylic acid produced as a metabolite of the tricarboxylic acid cycle and also as one of the fermentation products of anaerobic metabolism,16 has attracted great interest because of its applications ranging from pharmaceuticals to food processing, cosmetics, biodegradable polymers, and synthetic resins.17 Although succinic acid is mainly produced from petrochemical hydrocarbons, fermentative production using ruminal bacterial species such as Anaerobiospirillum succiniciproducens, Actinobacillus succinogenes, and Mannheimia succiniciproducens is considered as a prospective

10.1021/jp072606g CCC: $37.00 © 2007 American Chemical Society Published on Web 08/15/2007

SBA-15 and the Purification of Succinic Acid

J. Phys. Chem. C, Vol. 111, No. 35, 2007 13077

Figure 2. X-ray diffraction patterns of SBA-15 prior to and following functionalization with primary-amino silane.

potential for the purification of succinic acid from aqueous solutions.22 The strong basicity of these materials makes it possible to sustain their separation capacity at high pH values, thereby reducing the consumption of energy and chemicals during separation; however, the extraction selectivity, textural properties, toxicological safety, and thermal and chemical stability of these materials are somewhat unsatisfying.23 It is therefore necessary to develop new adsorbents for the efficient adsorption of contaminant acids from aqueous solutions. Here, we report on the first attempt to apply SBA-15 functionalized with primary, secondary, and tertiary amine groups as a potential adsorbent in the separation and purification processes of succinic acid. The morphology and textural and physical properties of amine-functional groups of modified SBA15 were characterized using transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM), small-angle X-ray scattering (SAXS), N2 adsorption/desorption isotherms, elemental analysis, conductivity measurement, and nuclear magnetic resonance (NMR); subsequently, the adsorption isotherm and separation mechanism of pyruvic and succinic acids were analyzed using the Sips model, isosteric heats of adsorption, and thermogravimetric analysis (TGA). Materials and Methods

Figure 1. TEM micrographs of pristine SBA-15 mesoporous silica imaged in side view (a) and top view (b).

alternative in line with the global demand for sustainable development.18 Biochemical production generally requires a separation process for the recovery of succinic acid from the fermentation broth because contaminant acids such as acetic, fumaric, and pyruvic acids are also produced, in addition to impurities such as proteins, metabolites, and media components.19 Contaminant acids normally induce product inhibition and have an adverse effect on the recovery of succinic acid from the fermentation broth.20 Many separation techniques are currently available for the recovery of succinic acid from aqueous solutions, including precipitation, distillation, solvent extraction, membrane process, and electrodialysis;21 however, most of these techniques are inappropriate for practical processes as they consume large amounts of energy and chemicals and produce waste during the regeneration of separation agents. Recently, polymeric sorbents, which separate carboxylic acids using reversible complexation with the amine group, have shown

Chemical Reagents. HCl (Aldrich, 37%), toluene (SigmaAldrich, 99%), succinic acid (Sigma-Aldrich, 99.9%), pyruvic acid (Across, 98%), phosphoric acid (Junsei), tetraethyl orthosilicate (TEOS, Aldrich, 98%), 3-aminopropyltriethoxysilane (Aldrich, 99%), N-methylaminopropyltrimethoxysilane (Gelest), 3-(N,N-dimethylaminopropyl) trimethoxysilane (Aldrich, 96%), Dowex MWA-1 (Dow Chemical Co.), and Amberlite IRA-400 (Rohm and Haas Co.) were used without further purification. Pluronic P123 (EO20PO70EO20) was purchased from Aldrich, and 0.2 µm PVDF syringe filter was obtained from Whatman. Distilled water (18 Ω) was produced by Q tech system. Synthesis of SBA-15. Mesoporous silica SBA-15 was synthesized with TEOS as a silica source and P123 as a structure-directing agent according to a previously reported method.24 TEOS and P123 were mixed in 1.6 M HCl solution and were stirred at 308 K for 24 h. After filtration and drying at 373 K, the resulting powder was calcined at 673 K for 4 h. Functionalization of SBA-15. SBA-15 was functionalized with primary-, secondary-, and tertiary-amino silanes by postsynthesis methods.24 Briefly, SBA-15 was treated with 3-aminopropyltriethoxysilane (primary), N-methylaminopropyltrimethox-

13078 J. Phys. Chem. C, Vol. 111, No. 35, 2007

Jun et al.

Figure 3. (a) Nitrogen adsorption-desorption isotherms and (b) pore-size distribution prior to and following functionalization with primary-amino silane.

TABLE 1: Textural and Physicochemical Properties of Amino-SBA-15 material

surface area SBET (m2/g)

pore volume (cm3/g)

pore size dBJH (nm)

d100 (Å)

wall thickness (nm)

N content (mmol/g)a

N content (mmol/g)b

SBA-15 primary secondary tertiary

838.52 318.66 283.28 332.76

0.94 0.47 0.41 0.46

5.45 4.79 4.34 4.53

89.61 91.47 90.54 91.00

4.90 5.77 6.12 5.98

2.18 2.31 1.72

1.82 1.20 0.90

a

N content determined by elemental analysis. b N content determined by conductivity measurement.

ysilane (secondary), and 3-(N,N-dimethylaminopropyl) trimethoxysilane (tertiary) solution in dry toluene. The mixture was heated under reflux for 15 h. The amine-functionalized SBA15 was filtered, was washed with acetone, and was dried at 353 K under vacuum. The products were named primary-, secondary-, and tertiary-SBA-15. Batch Adsorption of Carboxylic Acids. Amino-SBA-15 or polymeric sorbent (0.1 g) was mixed with carboxylic acid solutions (10 mL) in 20 mL glass scintillation vials with Teflonlined screw caps. All experiments were conducted at controlled temperatures for 24 h, which is sufficient for reaching a steady equilibrium concentration. After equilibration, 1 mL samples

were withdrawn from the mixtures, were centrifuged at 6000 rpm for 10 min, and were filtered with a 0.2 µm PVDF syringe filter. The concentration of carboxylic acid was determined by high-performance liquid chromatography (HPLC) with an ion exchange column (Supelcogel C-610H, 300 × 7.8 mm, SUPELCO) using 0.1 vol % H3PO4 aqueous solution as a mobile phase. The flow rate of the mobile phase was 0.6 mL/min, and the absorbance was measured using a UV-vis detector (Waters 2487). Characterization of Amino-SBA-15. TEM images were obtained using a JEOL 200CX field emission transmission electron microscope (FE TEM, 200 kV). Finely ground powders



Figure 4. (a) STEM image of secondary-SBA-15 loaded with HAuCl4 and mass mapping of (b) Si, (c) Au, and (d) Cl.

TABLE 2: Peak Assignments for 29Si MAS NMR Spectra of Amino-SBA-15 sample T2 (int%)

a

sample

T2 (int%)

T3 (int%)

Q2 (int%)

Q3 (int%)

Q4 (int%)

R contenta (mol % Si)

primary secondary tertiary

-57.8 (11.4) -52.7 (2.4) -57.2 (18.8)

-67.6 (14.1) -65.0 (27.8) -75.0 (5.0)

-89.1 (5.9)

-100.6 (16.2) -101.5 (21.0) -100.8 (14.7)

-109.8 (52.5) -110.8 (48.8) -110.3 (56.5)

25.5 30.2 23.9

-91.2 (4.9)

T/(T + Q) × 100.

were suspended in acetone and were deposited on Formvarcoated copper grids. STEM was operated with a probe focused to 0.2 nm and camera length of 20 cm. The scan raster was 512 × 512 points, with a dwell time of 8.5 s per scan. X-ray diffraction patterns were recorded using a Rigaku D/max-2500 (18 kW) with an image plate system equipped with a Cu KR radiation generator. Diffraction data were plotted in the range of 0.25-10° at intervals of 0.01. The interplanar spacing of the SBA-15 structure (d100) was calculated from the first-order Bragg reflections using PW 1877 automated powderdiffraction software. Elemental analysis was performed using a Vario EL 3 elemental analyzer manufactured by Elementar. Thermogravimetric analysis (TGA) was carried out using an SDT-Q600 thermal analyzer produced by TA Instrument.

Temperatures were increased in a linear ramp from 50 to 1000 °C at heating rates ranging from 10 to 40 °C/min under a nitrogen atmosphere. All of the amino-SBA-15 samples were characterized by nitrogen gas adsorption/desorption isotherms at 77 K measured using a Micromeritics Tristar 3000 physisorption surface area and pore size analyzer. Samples were pretreated by heating at 200 °C for 2 h under vacuum. The average pore size was determined using the Barrett-Joyner-Halenda (BJH) method using the desorption branch of the isotherm;25 the specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method.26 13C and 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra with 1H cross-polarization (CP) were collected on a solid-state FT-NMR spectrometer (Bruker, DSX


Jun et al.

Figure 5. (a) 13C and (b) 29Si CP/MAS NMR spectra of amino-SBA15.

TABLE 3: Optimal Parameters of the Sips Equation for Pyruvic and Succinic Acid Adsorption adsorbate

adsorbent

qm (mmol/g)

b (L/mmol)

n

pyruvic acid


1.58 0.56 0.85

0.09 0.11 0.09

0.27 0.35 0.18

succinic acid


0.51 0.41 0.55

0.14 0.07 0.10

0.36 0.32 0.37

400 MHz) with a 4 mm MAS probe. Both 13C and 29Si were taken at a spinning speed of 6 kHz. Electrical conductivity was measured using an Accumet AR50 conductivity meter to measure the amount of amine group attached to the surface of SBA-15. Results and Discussion Characterization of Amino-Functional SBA-15 Mesoporous Silica. SBA-15 and amino-SBA-15 possess 2-D hexagonal arrays of silica pore channels with uniform diameter and wall thickness, as shown in Figure 1 and S1. TEM images reveal that the mesostructure of SBA-15 was intact after chemical modification of amino-functional silanes. These observations in small domains are in agreement with the small-angle X-ray diffraction patterns shown in Figure 2 and S2. Three well-

resolved peaks can be attributed to (100), (110), and (200) reflections related to the hexagonal symmetry.27 The reduction in the intensity of the (100), (110), and (200) reflections for surface-modified SBA-15 is assigned to the change in homogeneity and space correlation of pores caused by the functionalized monolayer of amino silanes or the partial collapse of the ordered structure because of the local hydrolysis by amine group with water molecules.28 The nitrogen adsorption/desorption isotherms in Figure 3a and S3a and c at 77 K show type IV with H1-type hysteresis, which is a typical result for hexagonal mesoporous SBA-15.29 After the chemical modification of SBA-15, a decrease in surface area, pore volume, and pore diameter was observed compared to the original SBA-15, as shown in Table 1;30 however, it retains the mesoporosity derived from the presence of capillary condensation with the hysteresis of open-pore and narrow-pore size distribution, as shown in Figure 3b and S3b and d. Table 1 lists the textural properties of amino-SBA-15 calculated from the nitrogen physisorption and X-ray diffraction (XRD). The quantification of amine groups on SBA-15 was conducted using elemental analysis. The concentration of amino silanes on SBA-15 was calculated from the mass fraction of nitrogen. Since not all of the amine groups are included in the interaction between carboxylic acids and amine groups because of reasons such as steric hindrance, pore blockage, and inhomogeneous distribution of amino silanes,7 the concentration of active sites for carboxylic acid was evaluated by the protonation of surface amine group, which is considered as a suitable method to analyze the accessibility of adsorbate to binding sites in the mesoporous material.31b Taking into consideration the fact that protons of sulfuric acid solutions are consumed by the protonation of amine group on SBA-15,31 the concentration of active amine group was derived from the difference in electrical conductivity. As shown in Table 1, about 80% of surface amine groups act as active sites for primary-SBA-15, while about 50% act as sites for secondary- and tertiary-SBA-15. As previously reported,31b the inhibition of diffusion of target material (i.e., H+, SO42- in this case) is one of the main factors affecting the accessibility to surface amine group in mesoporous system during the protonation. At a certain level of adsorption, the local electric field induced from the accumulated counteranion (SO42-), which is introduced into the mesopore to counterbalance the propylammonium ion, limits the diffusion of ion species to the amine groups deep inside the cylindrical mesopores. Especially in this case, amino-SBA-15s had great charge effect compared to previous results for the same system because of the larger counteranion of sulfuric acid than that of hydrochloric acid. The value of 50% for secondary- and tertiary-SBA-15 means there was also inhibition from the steric hindrance of methyl side group and inhomogeneous distribution of surface amino silane. Further discussion about inhomogeneous distribution will be given in 29Si MAS NMR. The distribution of active amine groups was analyzed to assess their effect on adsorption, as discussed in previous studies.15c Steric congestion of amino silanes was observed by mass mapping of Au and Cl atoms in n STEM image. HAuCl4 was used to stain the amine groups as an indirect means of examining the distribution of silanes over the SBA-15 channel.32 Mass mapping of Au and Cl atoms confirmed that primary amino silanes had a generally uniform distribution on the surface of SBA-15 without significant steric congestion (Figure 4). In addition, amine-functionalized SBA-15s were investigated by 13C and 29Si CP/MAS and 29Si MAS NMR.33 Figure 5a shows



Figure 6. Adsorption isotherms of (a) succinic and (b) pyruvic acid on SBA-15 and amino-SBA-15. 13C

CP/MAS NMR spectra of the organic monolayer. Each carbon atom of the propylene group in amino silanes was detected and assigned to a specific peak. The presence of new peaks at 52.2 ppm for secondary-SBA-15 and at 60.1 ppm for tertiary-SBA-15 was attributed to methylene in the amine group, indicating that three kinds of amine groups had interacted chemically with the surface silanol group of SBA-15. Figure 5b shows 29Si CP/MAS NMR spectra of amino-functional silanes. Peaks at -90, -100, and -110 ppm from SBA-15 were assigned to Si(OH)2(SiO)2(Q2), Si(OH)(SiO)3(Q3), and Si(SiO)4(Q4), respectively. Three additional peaks that appeared after modification were attributed to Si(OH)R(OCH3)(OSi) (T1), Si(OH)R(SiO)2(T2), and SiR(SiO)3(T3), resulting from the anhydrous deposition of silanes on SBA-15. The intensities of the Q2 and Q3 peaks were reduced with the consumption of silanols because of the postfunctionalization, whereas the intensity of the Tn peaks increased during this process. Secondary-SBA-15 had more closely packed monolayers than primary- and tertiarySBA-15, as indicated by the intensity of the T3 peak; this result was in agreement with the results of elemental analysis and 29Si MAS NMR. The assignments of peaks and relative intensities for 29Si MAS NMR are listed in Table 2.

Adsorption Isotherms of Pyruvic and Succinic Acid. Figure 6 shows the adsorption isotherms of pyruvic and succinic acids on primary-, secondary-, and tertiary-SBA-15 at 25 °C. The isotherms were in good agreement with the Sips equation,34 which has the following form:

(b‚C)1/n q ) qm 1 + (b‚C)1/n

(1)

where b is the equilibrium constant (L/mmol), qm is the maximum adsorption capacity of acid (mmol/g), q is the adsorption capacity of acid at equilibrium (mmol/g), C is the initial concentration of acid in the aqueous solution (mmol/L), and n is the exponential constant that describes the heterogeneity of the system. The values of b, qm, and n, which were estimated according to the least-squares method, are listed in Table 3. It appears that the amounts of adsorbed acid increased with increasing concentration of acid in the aqueous solution, becoming saturated at 20 mmol/L. In all cases, functionalized mesoporous adsorbents had better adsorption capacity of pyruvic acid than succinic acid, reflecting the difference in acidity. Primary-SBA-15 showed the best adsorption capacity in terms


Jun et al.

TABLE 4: Adsorption of Pyruvic Acid on Amino-SBA-15 concentration of PA adsorbent primary secondary

tertiary

TABLE 6: Optical Parameters for the Temperature-dependent Sips Equation

specific ads.

initial (mmol/L)

final (mmol/L)

(mmol/g)

coverage

Kda

2.49 13.73 19.04 3.16 6.74 12.97 18.49 4.00 10.03 13.50 17.48

2.27 2.37 4.18 3.01 5.06 9.06 13.43 3.56 7.57 7.16 9.95

0.02 1.14 1.49 0.01 0.17 0.39 0.51 0.04 0.25 0.63 0.75

0.01 0.72 0.94 0.03 0.30 0.70 0.90 0.05 0.29 0.75 0.89

9.75 478.52 355.22 4.81 33.08 43.09 37.68 12.40 32.43 88.59 75.76

qm,0 (mmol/g) b0 (L/mmol) Q/RgT0 n0 R χ

Kd ) the adsorbed amount of acid (mmol/g)/[the final aqueous phase concentration of acid (mmol/L)] × 1000. a

concentration of SA adsorbent primary secondary tertiary

specific ads.

initial (mmol/L)

final (mmol/L)

(mmol/g)

coverage

Kd

11.17 16.11 5.68 21.16 31.22 5.48 10.61 15.51 21.05

5.29 9.17 4.80 17.79 25.69 4.12 6.04 9.23 13.63

0.39 0.46 0.06 0.22 0.37 0.09 0.30 0.42 0.49

0.77 0.91 0.14 0.55 0.90 0.16 0.55 0.76 0.90

73.99 50.45 12.09 12.64 14.37 21.93 50.47 45.31 36.30

of the adsorption of pyruvic acid, which exceeded the adsorption of succinic acid by a factor of 3. Secondary- and tertiary-SBA15 also showed increased adsorption capacity in the adsorption of pyruvic acid, being approximately 1.5 times that of succinic acid. SBA-15 showed negligible selectivity on pyruvic acid as a consequence of only minor differences in the adsorption capacity of the two acids. To compare the adsorption capability of amino-SBA-15s, distribution coefficients were calculated at various surface coverages, as shown in Tables 4 and 5. Primary-SBA-15 exhibited the best distribution of pyruvic and succinic acid at coverages of 0.5 and 0.7, thereby demonstrating an enhancement of 6.5, relative to succinic acid, in the distribution of pyruvic acid at a coverage of 0.7. In contrast, secondary-SBA-15 showed an enhancement of 3.8 at a coverage of 0.5, while the figure for tertiary-SBA-15 was 2. These results indicate that primarySBA-15 is an adsorbent of good capability for pyruvic and succinic acid, while pyruvic acid generally showed a better distribution than succinic acid in functionalized mesoporous adsorbents. Effect of Temperature on the Adsorption of Carboxylic Acid. Isosteric Heat of Adsorption. The isosteric heat of adsorption (-∆Hads) is an important thermodynamic variable in the design of practical adsorption processes. The adsorption process is governed by the molecular cooling of the adsorbate and heat transfer from or through the adsorbent during the adsorption process; this governs the local adsorption of adsorbate molecules and has an influence on the equilibria and kinetics of adsorption.35 The isosteric heat of adsorption, which is derived from the van’t Hoff equation, is given by

(-∆Hads) ) Q - (R‚Rg‚T0)n2 ln

(

q qm - q

)

(2)

secondary

tertiary

1.54 0.09 9.23 0.27 11.26 -8.57

0.58 0.11 15.86 0.38 19.23 -9.30

0.51 0.07 10.03 0.23 55.75 -8.06

TABLE 7: Kinetic Parameters Determined from TGA Analysis

a

TABLE 5: Adsorption of Succinic Acid on Amino-SBA-15

primary

adsorbent

Tmaxa (°C)

Ed (J/mol)

A (S-1)


182.94 215.69 200.13

8660 3180 510

863.91 61.74 5.49

Tmax at heating rate of 20 (°C/min).

where (-∆Hads) represents the isosteric heat of adsorption (kJ/ mol), Q is the measurable adsorption heat (kJ/mol), R is constant parameter, Rg is the ideal gas constant (8.3145 J/mol‚K), and T0 is the reference temperature (K). These parameters were obtained from the temperature-dependence form of adsorption isotherms for the Sips model, which are described by the following equations:34

b ) b0 exp

[ ( )] Q T0 -1 RgT0 T

( ) [ ( )]

T0 1 1 ) +R 1n n0 T qm ) qm,0 exp χ 1 -

T T0

(3)

(4)

(5)

where b0 represents the equilibrium constant at reference temperature T0 (L/mmol), n0 is the exponential constant at reference T0, qm,0 is the maximum adsorption capacity at reference temperature T0, and χ is a constant parameter. The adsorption of pyruvic acid on amino-SBA-15 was conducted at temperatures ranging from 298 to 318 K; the derived parameters for the Sips equation are listed in Table 6. Figure 7 shows plots of the isosteric heats of adsorption on primary-, secondary-, and tertiary-SBA-15 with adsorbed amounts of pyruvic acid. Surface heterogeneity is observed from the decrease in the heats of adsorption with increasing amounts of pyruvic acid adsorbed, simultaneously demonstrating that surface amine groups acted as specific adsorption sites for pyruvic acids in the aqueous solutions.36 The observed negative enthalpies support the fact that the adsorption of pyruvic acid on amino-functional SBA-15 is an exothermic process. The values between 10 and 100 kJ/mol furthermore support the interpretation that the interaction between amino-functional SBA-15 and pyruvic acid predominantly originates from hydrogen bonding between amine and carboxylic groups, as reported previously.37 The heats of adsorption are higher for secondary-SBA-15 than primary- and tertiary-SBA-15 at lower loadings, indicating that in comparison with other functionalized SBA-15, secondarySBA-15 interacted more strongly with pyruvic acid. According to the hard-soft acid base (HSAB) theory, the recovery of carboxylic acid from aqueous solution depends on the basicity of the adsorbent.22 In the case of amino silanes with identical silane groups, the relative basicity of the amine group depends



Figure 7. Isosteric heats of adsorption for pyruvic acid on amino-SBA-15 at 25 °C.

TABLE 8: Competitive Adsorption of Fumaric and Succinic Acid on Amino-SBA-15 concentration of FA

specific ads.

concentration of SA

specific ads.

adsorbent

initial (mmol/L)

final (mmol/L)

(mmol/g)

Kd

initial (mmol/L)

final (mmol/L)

(mmol/g)

Kd


12.42 12.50 12.50

11.29 11.53 11.73

0.11 0.10 0.08

10.01 8.42 6.57

337.55 339.80 340.23

336.34 339.73 340.22

0.12 0.01 0.00a

0.36 0.02 0.00b

a

9.32 × 10

-4. b

2.73 × 10

-3.

TABLE 9: Competitive Adsorption of Pyruvic and Succinic Acid on Amino-SBA-15 concentration of PA

specific ads.

adsorbent

initial (mmol/L)

final (mmol/L)

(mmol/g)


26.92 26.92 26.06

13.94 22.22 23.23

1.30 0.47 0.28

on the organic substituents attached to the nitrogen. The pKa values for ammonium ions are as follows: methylamine (10.66), diethylamine (10.73), and trimethylamine (9.81).38 Therefore, primary- and secondary-SBA-15 should have a stronger interaction with pyruvic acid than tertiary-SBA-15; however, the result for primary-SBA-15 was not consistent with this reasoning. This outcome can be explained by steric hindrance originating from the more closely packed arrays of the monolayer, as shown already in Figure 5b and Table 2. The minor decline in the isosteric heat of primary-SBA-15 also supports this explanation. TGA Data. Thermogravimetric analysis is a useful method for describing the characteristic state of pyruvic acid adsorbed onto the mesoporous adsorbent. Assuming a first-order desorption process, as already used in studies of the adsorption of alkanes, alcohols, and aromatic compounds,39 the activation energy of desorption and preexponential coefficients was obtained using a simple model that relates the heating rate of TGA to the temperature (Tmax) at which the desorption rate is maximized:

(

Ed A β ) exp 2 E R RgTmax d gTmax

)

(6)

where β is the heating rate of TGA (K/min), A is the preexponential coefficient (min-1), and Ed is the activation

concentration of SA

specific ads.

Kd

initial (mmol/L)

final (mmol/L)

(mmol/g)

Kd

93.09 21.14 12.20

361.01 361.01 353.12

360.67 351.80 351.24

0.03 0.92 0.19

0.09 2.62 0.53

energy of desorption (kJ/mol). The values of the activation energy of desorption, Ed, and preexponential coefficient, A, are listed in Table 7. Figure 8 shows derivative TGA plots of tertiary-SBA-15 with pyruvic acid at various heating rates. Peaks derived from adsorbed pyruvic acid exist in the range of 170-220 °C, which is higher than the normal boiling point of pyruvic acid (165 °C) and which shifts toward the higher temperature region as the heating rate is increased. Hydrogen bonding between pyruvic acid and amino silanes reduced the volatility of pyruvic acid adsorbed on amino-SBA-15. The values of the activation energy (1-10 kJ/mol) of desorption for all adsorbents support the fact that the primary mechanism of adsorption is hydrogen bonding between the carboxylic and amine groups. The value of the preexponential coefficient in the range of 1013 (s-1) indicates that the thermal desorption of pyruvic acid from aminoSBA-15 involves elementary reactions without readsorption.39 Effect of pH on the Adsorption of Carboxylic Acid. Figure 9 shows the pH dependence of the adsorption process. The initial concentration of the acid solutions was 20 mmol/L, and the pH values of the pyruvic and succinic acid solutions were 2.43 and 2.95, respectively. The concentrations of undissociated and dissociated succinic and pyruvic acid at various pH values were calculated from the definition of the apparent equilibrium constant via eqs (7-9):40


Jun et al.

Figure 8. Derivative thermogravimetry of tertiary-SBA-15 saturated with pyruvic acid.

Figure 9. pH dependence of adsorption of (a) pyruvic acid on amino-SBA-15 and concentration of pyruvic acid and pyruvate anion and (b) succinic acid on amino-SBA-15 and concentration of succinic acid and bisuccinate anion at various pH values.



Figure 10. Selectivity coefficient for competitive adsorption of fumaric/succinic acid and pyruvic/succinic acid on amino-SBA-15s and polymeric sorbents; some data for amino-SBA-15 are multiplied by 0.1; initial concentrations of each species are [FA]0 ) 12.50 (mmol/L) and [SA]0 ) 340.23 (mmol/L) for fumaric/succinic acid system and [PA]0 ) 26.01 (mmol/L) and [SA]0 ) 361.01 (mmol/L) for pyruvic/succinic acid system.

[SA] )

[BA] )

[SA]t 1 + 10pH-pKA1 + 102pH-pKA1-pKA2 [SA]t‚10pH-pKA1 1 + 10

pH-pKA1

[PA] )

+ 10

2pH-pKA1-pKA2

[PA]t 1 + 10pH-pKA

ki/j )

(7)

(8)

(9)

where SA is succinic acid, BA is bisuccinate anion, and PA is pyruvic acid. The uptake of pyruvic and succinic acid was significantly affected by varying the pH value. The similar profile of the uptake to that of the concentration of undissociated carboxylic acid confirms the adsorption results from the hydrogen bonds between the amine and carboxylic groups. At pH values higher than pKa, carboxylic acid exists as a carboxylate anion, resulting in a significant reduction in the uptake by the amine group. Although there are still bisuccinate anions with one carboxylic group at pH values higher than 6, the uptake of succinic acid is close to zero in these regions. The same trend was observed in the adsorption of pyruvic acid; however, the uptake did not fall to zero at pH values higher than pKa in the case of primaryand tertiary-SBA-15. This can be explained by the hydrogen bond between the oxygen of the carbonyl group and the hydrogen of the amine group. Low uptake at pH values below the ranges of pKa is explained by the consumption of adsorption sites that accompanies the formation of ammonium salt with H+ from HCl. Competitive Adsorption of Contaminant and Succinic Acid. The competitive adsorption of contaminant and succinic acid was conducted on the basis of the composition of the fermentation broth. Initial concentration ratios of succinic to contaminant acids were about 27 for fumaric acid and 13 for pyruvic acid as shown in Tables 8 and 9. The degree of selectivity on contaminant acids over succinic acid was quantified by the selectivity coefficient for adsorption, k, which is defined as

Kd,i Kd,j

(10)

where Kd,i is the distribution coefficient of species i in the mesoporous adsorbent. Despite recording a value more than 10fold the amount of succinic acid, secondary- and tertiary-SBA15 exhibited selectivities of 429.94 and 2399.54 for fumaric acid, respectively, while primary-SBA-15 recorded a selectivity of 988.59 for pyruvic acid, as shown in Figure 10. This means that secondary- and tertiary-SBA-15 showed preferential adsorption of fumaric acid and that primary-SBA-15 showed preferential adsorption of pyruvic acid. The adsorption selectivity of mesoporous adsorbents on contaminant acids was much higher than that of polymeric sorbents such as Dowex MWA-1 and Amberlite IRA-400. As previsouly reported,40 these polymeric sorbents with strong basicity showed selectivity only in the region between 1 and 20 for adsorption of succinic acid over lactic acid and formic acid over acetic acid in equal molarity solution. In particular, the enhanced selectivity is obtained at low pH values, while the maximum selectivity of polymeric sorbents occurs at a pH of about 6 because of the difference in dissociation of acid with different pKa values; this arises because the pH of the aqueous solution is maintained at pH values between 2 and 3 after the primary purification step that uses reactive extraction for the effective separation of succinic acid from the fermentation broth.42 The selectivity of the primary-, secondary-, and tertiary-SBA-15 for contaminant acids at low pH values should be exploited during the secondary purification or polishing step22 and makes amino-SBA-15 attractive as adsorbents for the effective recovery of succinic acid from fermentation broth with greater than 99% purity. Conclusions We investigated the adsorption of pyruvic and succinic acid on SBA-15 functionalized with primary-, secondary-, and tertiary-amino silane. It appears that the adsorption originated from the formation of acid-amine complexes via hydrogen bonding, as indicated by the analyzed isosteric heats of

13086 J. Phys. Chem. C, Vol. 111, No. 35, 2007 adsorption and thermogravimetric analysis. Primary- and tertiarySBA-15 showed the maximum adsorption capacity for pyruvic acid (1.58 mmol/g) and succinic acid (0.55 mmol/g), respectively. The amine-functionalized SBA-15 adsorbed more pyruvic acid than succinic acid at the same coverage. The presence of inactive sites on the adsorbents was identified from conductivity measurements, which were in good agreement with the result from the adsorption of pyruvic acid. Adsorption was also affected by the basicity and distribution of amino silanes; therefore, it is believed that the isolated amino silanes are effective in the adsorption of carboxylic acid. Competitive adsorption revealed that amine-functionalized SBA-15 is suitable for the removal of contaminant acids from fermentation broth. Acknowledgment. The authors are grateful to the Center for Advanced Bioseparation Technology Research (BSEP, KOSEF) and Brain Korea 21 (BK21) for the funding. Supporting Information Available: TEM micrographs, X-ray diffraction patterns, and nitrogen adsorption-desorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hartmann, M. Chem. Mater. 2005, 17, 4577. (2) (a) Zhao, D.; Sun, J.; Stucky, G. D. Chem. Mater. 2000, 12, 275. (b) Choi, M.; Heo, W.; Klietz, F.; Ryoo, R. Chem. Commun. 2003, 1340. (c) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (3) (a) Kang, T.; Park, Y.; Choi, K; Lee, J. S.; Yi, J. J. Mater. Chem. 2004, 14 (6), 1043. (b) Katiyar, A.; Yadav, S.; Smirniotis, P. G.; Pinto, N. G. J. Chromatogr., A 2006, 1122, 13. (c) Liu, X.; Zhou, L.; Huang, D.; Zhou, Y. Chem. Phys. Lett. 2005, 415, 198. (4) (a) Yang, L.; Qi, Y.; Yuan, X.; Shen, J.; Kim, J. Mol. Catal. A 2005, 229, 199. (b) Byambajav, E.; Ohtsuka, Y. Appl. Catal., A 2003, 252 (1), 193. (5) (a) Wang, S.; Choi, D. G.; Yang, S. M. AdV. Mater. 2002, 14 (18), 1311. (b) Lee, J.; Jin, S.; Hwang, Y.; Park, J. G.; Park, H. M.; Hyeon, T. Carbon 2005, 43 (12), 2536. (c) Zhang, L.-X.; Shi, J.-L.; Yu, J.; Hua, Z.L.; Zhao, X.-G.; Ruan, M.-L. AdV. Mater. 2002, 14 (20), 1510. (6) (a) Feng, X.; Fryxell, G. E.; Wang, S.-Q.; Kin, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923. (b) Voss, R.; Thomas, A.; Antonietti, M.; Ozin, G. A. J. Mater. Chem. 2005, 15, 4010. (7) Han, Y. J.; Stucky, G. D.; Butler, A. J. Am. Chem. Soc. 1999, 121, 9897. (8) Yiu, H. H. P.; Wright, P. A.; Botting, N. P. J. Mol. Catal. B 2001, 15, 81. (9) Avnir, D.; Braun, S.; Lev, O.; Ottolenghi, M. Chem. Mater. 1994, 6, 1605. (10) Wang, Y.; Caruso, F. Chem. Mater. 2005, 17, 953. (11) (a) Wang, P.; Dai, S.; Waezsada, S. D.; Tsao, A. Y.; Davison, b. H. Biotechnol. Bioeng. 2001, 74, 249. (b) Pandya, P. H.; Jasra, R. V.; Newalkar, B. L.; Bhatt, P. N. Microporous Mesoporous Mater. 2005, 77, 67. (12) Wang, Y.; Caruso, F. Chem. Mater. 2005, 17, 953. (13) Martin, B. D.; Gaber, B. P.; Patterson, C. H.; Turner, D. C. Langmuir 1998, 14, 3971. (14) Inglis, W.; Sanders, G. H.; Williamsan, P. M.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B. Langmuir 2001, 17, 7402. (15) (a) Wu, G.; Wang, Z.; Wang, J.; He, C. Anal. Chim. Acta 2007, 582 (2), 304. (b) Dhodapkar, R.; Rao, N. N.; Pande, S. P.; Kaul, S. N.

Jun et al. Bioresour. Technol. 2006, 97, 877. (c) Yoshitake, H.; Yokoi, T.; Tatusmi, T. Chem. Mater. 2002, 14, 4603. (d) Lei, C.; Shin, Y. S.; Liu, J.; Ackerman, E. J. J. Am. Chem. Soc. 2002, 124, 11242. (16) Kwon, Y. D.; Lee, S. Y.; Kim, P. J. Microbiol. Biotechnol. 2006, 16 (9), 1448. (17) Zeikus, J. G.; Jain, M. K.; Elankovan, P. Appl. Microbiol. Biotechnol. 1999, 51, 545. (18) (a) Hong, S. H.; Kim, J. S.; Lee, S. Y.; In, Y. H.; Choi, S. S.; Rih, J. K.; Kim, C. H.; Jeong, H.; Hur, C. G.; Kim, J. J. Nat. Biotechnol. 2004, 22, 1275. (b) Samuelov, N. S.; Lamed, R.; Lowe, S.; Zeikus, J. G. Appl. EnViron. Microbiol. 1991, 57, 3013. (19) Davison, B. H.; Nghiem, N. P.; Richardson, L. R. Appl. Biochem. Biotechnol. 2004, 113, 653. (20) (a) Hong, Y. K.; Hong, W. H.; Chang, H. N. Biotechnol. Lett. 2000, 22, 871. (b) Davison, B. H.; Nghiem, N. P.; Richardson, L. R. Appl. Biochem. Biotechnol. 2004, 113, 653. (21) (a) Bessling, B.; Loening, J. M.; Ohligschlaeger, A.; Schembecker, G.; Sundmacher, L. Chem. Eng. Technol. 1998, 21, 393. (b) Han, D. H.; Hong, W. H. Sep. Sci. Technol. 1996, 31, 1123. (c) Zeikus, J. G.; Elankovan, P.; Grethlein, A. Chem. Proc. 1995, 58 (7), 71. (d) Choi, J.; Hong, W. H. J. Chem. Eng. Jpn. 1999, 32 (2), 184. (22) (a) Tung, L. A.; King, C. J. Ind. Eng. Chem. Res. 1994, 33, 3217. (b) Garcia, A. A. Biotechnol. Prog. 1991, 7, 33. (23) Ju, Y.; Webb, O.; Dai, S.; Lin, J.; Barnes, C. Ind. Eng. Chem. Res. 2000, 39, 550. (24) Salis, A.; Meloni, D.; Ligas, S.; Casula, M. F.; Monduzzi, M.; Solinas, V.; Dumitriu, E. Langmuir 2005, 21, 5511. (25) Linsen, B. G.; van den Heuvel, A. The Solid Gas Interface; Mercel Dekker; New York, 1967. (26) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (27) Beck, J. S.; Vartuli, C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (28) (a) Zhao, X. S.; Lu, G. Q. J. Phys. Chem. B 1998, 102, 1556. (b) Evans, J.; Zaki, A. B.; El-Sheikh, M. Y.; El-Safty, S. A. J Phys. Chem. B 2000, 104, 10271. (29) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemienewska, T. Pure Appl. Chem. 1985, 57, 603. (30) (a) Yiu, H.; Wright, P. A.; Botting, N. P. J. Mol. Catal. B 2001, 15, 81. (b) Antochshuk, V.; Jaroniec, M. Chem. Mater. 2000, 12, 2496. (31) (a) Yoon, J. Y.; Park, H. Y.; Kim, J. H.; Kim, W. S. J. Colloid Interface Sci. 1996, 177, 613. (b) Walcarius, A.; Etienne, M.; Lebeau, B. Chem. Mater. 2003, 15, 2161. (c) Walcarius, A.; Etienne, M.; Bessiere, J. Chem. Mater. 2002, 14, 2757. (d) Yang, J. J.; El-Nahhal, I. M.; Chuang, I.-S.; Maciel, G. E. J. Non-Cryst. Solids 1997, 209, 19. (32) (a) Gu, J. L.; Shi, J. L.; You, G. J.; Xiong, L. M.; Qian, S. X.; Hua, Z. L.; Chen, H. R. AdV. Mater. 2005, 17 (5), 557. (b) Engel, A.; Colliex, C. Curr. Opin. Biotechnol. 1993, 4, 403. (33) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923. (34) Duong, D. D. Adsorption Analysis, Equilibria and Kinetics; Imperial College Press: London, 1998. (35) (a) Chakraborty, A.; Saha, B. B.; Koyama, S. Appl. Phys. Lett. 2006, 89, 171901. (b) Sircar, S.; Cao, D. V. Chem. Eng. Technol. 2002, 25, 10. (36) Chin, D.; King, C. J. Ind. Eng. Chem. Res. 1999, 38, 3738. (37) Liu, B. J.; Ren, Q. L. J. Colloid Interface Sci. 2006, 294, 281. (38) (a) McMurry, J. Organic Chemistry; Thomson Learning; Brooks/ Cole: Pacific Grove, CA, 2000. (b) Yoshida, H.; Kishimoto, N. Chem. Eng. Sci. 1995, 50, 2203. (39) (a) Senf, L.; Frank, H. J. Chromatogr. 1990, 520, 131. (b) Suzuki, M.; Misic, D. M.; Joyama, O.; Kawazoe, K. Chem. Eng. Sci. 1978, 33, 271. (40) Husson, S. M.; King, C. J. Ind. Eng. Chem. Res. 1999, 38, 502. (41) Jun, Y. S.; Huh, Y. S.; Hong, W. H.; Hong, Y. K. Biotechnol. Prog. 2005, 21, 1673.

Adsorption of Pyruvic and Succinic Acid by Amine ... - ACS Publications

Recommend Documents