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Langmuir 1999, 15, 8300-8308
Anion Exchange Properties of a Mesoporous Aluminophosphate Dean A. Kron, Brian T. Holland, Robert Wipson, Caroline Maleke, and Andreas Stein* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Received May 6, 1999. In Final Form: July 6, 1999
A mesoporous aluminophosphate support, meso-AlPO, was synthesized by phosphate treatment of an AlO4Al12(OH)24(H2O)7+ cluster/dodecyl sulfate salt, followed by extraction of the surfactant with an acetate/ methanol solution. The meso-AlPO product was shown to be an effective anion exchange material. Anion exchange capacities for chromate and several monoanionic and dianionic organic dyes fell in the range from approximately 1.3-1.6 meq/g. Control experiments with the neutral support MCM-41 and a nonionic dye indicated that anion adsorption in meso-AlPO was caused predominantly by electrostatic interactions rather than pure physisorption. Multiple exchanges were possible if the support was regenerated by acid treatment at pH 4.3. While the material exhibited size-selectivity for anionic dyes, the average pore sizes increased during anion exchanges in aqueous solutions. The mesopore structure of meso-AlPO was thermally stable up to 200 °C, and anion exchanges at 100 °C under aqueous reflux resulted in similar exchange capacities as room-temperature exchanges.
Introduction Groundwater contamination by metals and organic dyes has become a major industrial problem.1-3 A substantial portion of the effluents consists of anionic species. For instance, aromatic sulfonates are widely used in industrial processes, as well as in consumer products; 2.4 million tons of linear alkylbenzenesulfonates were consumed worldwide in 1992.4 One approach to reducing or eliminating industrial wastewater discharge is based on ion exchange materials.5 Today most anion exchangers are organically based, such as anion exchange resins, which typically have high exchange capacities (3-4 meq/g) but limited thermal stability.6 While a large number of inorganic cation exchange materials (such as zeolites) are available, few inorganic anion exchangers exist; hydrotalcite clays form one such class of materials.7 Compared to the organically based anion exchangers, inorganic anion exchange materials can possess several potential advantages, such as resistance to higher temperatures and radiation, high ion selectivity in certain separations, and increased durability with organic solvents and oxidizing agents.8 Potential advantages of inorganic anion exchange materials containing mesopores with diameters of a few nanometers include higher surface areas compared with clay materials and the possibility of size/shape selectivity for the anionic species. Recently in our group, a new class of inorganic anion exchange materials has been synthesized by Holland et (1) Dunaway, C. S.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1998, 14, 1002-1012. (2) Altenbach, B.; Giger, W. Anal. Chem. 1995, 67, 2325-2333. (3) Reichle, W. T. Chemtech 1986, 1, 58. (4) de Almeida, J. L. G.; Dufaux, M.; Ben Taarit, Y.; Naccache, C. J. Am. Oil Chem. Soc. 1994, 71, 675-694. (5) Clearfield, A. Ind. Eng. Chem. Res. 1995, 34, 2865-2872. (6) Harland, C. E. Ion Exchange: Theory and Practice, 2nd ed.; The Royal Society of Chemistry: Cambridge, UK, 1994. (7) Cavani, F.; Trifiro`, F.; Vaccari, A. Catal. Today 1991, 11, 171291. (8) Lieser, K. H. In Ion Exchangers; Dorfner, K., Ed.; Walter de Gruyter: Berlin, 1991; pp 519-546.
al.9 The ordered mesoporous aluminophosphates (mesoAlPO) exhibit high surface areas in excess of 600 m2/g and pore volumes of ca. 0.4-0.5 cm3/g. The pore size distribution is narrow with pore diameters peaking at 17-21 Å, depending on the sample, with no evidence for larger pores.9 The ion exchange properties of meso-AlPO are noteworthy. The material possesses anionic and cationic charge balancing groups, both of which may be exchangeable. The cation exchange sites involve Na+ ions or protons that balance the single remaining charge on phosphate groups linking aluminum species. The anion exchange sites result from the high positive charges on the Kegginlike cluster precursors, AlO4Al12(OH)24(H2O)7+ (Al13), which are originally balanced by the anionic surfactant template. The focus of this work is to evaluate the anion exchange properties of the new mesoporous aluminophosphate and determine the material’s limitations by investigating the exchange isotherms, Gaines-Thomas equilibrium constants, size selectivity, possible structural changes during anion exchange, and the thermal stability of meso-AlPO. Experimental Section Materials. Deionized water was used in all preparations. Dowex 21K Cl anion exchange resin, 16-30 mesh (Dowex), was purchased from Aldrich. The mesoporous aluminophosphates were prepared by treating a salt composed of Al13 polyoxometalate clusters and an anionic surfactant (sodium dodecyl sulfate, SDS) with a phosphate buffer via the method described previously.9 Analysis of a typical nonextracted aluminophosphate sample: 18.85 wt % Al2O3, 22.85 wt % P2O5, 0.74 wt % Na2O, 25.69 wt % C, 5.80 wt % H, and 5.69 wt % S (Al/P ) 1.15). The surfactant (SDS) was removed by stirring 0.5 g of the aluminophosphate mesophase in 40 mL of 0.05 M sodium acetate/methanol solution at room temperature for 20 min. The product was filtered, washed with excess methanol, and dried in air. The mesostructure of the sample was confirmed by powder X-ray diffraction (PXRD), with a typical d100 spacing of 36.0 ( 3 Å. Analysis of a typical extracted (9) Holland, B. T.; Isbester, P. K.; Blanford, C. F.; Munson, E. J.; Stein, A. J. Am. Chem. Soc. 1997, 119, 6796-6803.
10.1021/la990553r CCC: $18.00 © 1999 American Chemical Society Published on Web 09/14/1999
Anion Exchange in Mesoporous Aluminophosphate
Langmuir, Vol. 15, No. 23, 1999 8301
Table 1. List of Dyes and Anions Investigated in the Ion Exchange Studya compound
abbreviation
λmax (nm)
length (Å)
width (Å)
alizarin yellow GG
AZ
352
14.6
8.7
brilliant yellow
BY
518
26.9
9.6
fast garnet GBC base
FGGB
380
14.0
8.2
methyl red, sodium salt
MR
434
16.1
8.8
mordant yellow 10
MY10
354
16.1
7.8
naphthol blue black
NBB
618
22.5
12.2
naphthol yellow S
NYS
434
12.8
8.2
potassium chromate
CrO42-
376
5.7
5.8
structure
a
Dye concentrations in the supernatant solutions were determined from the absorption values at λmax listed. The dimensions of the dye molecules were estimated using structures optimized by the CS Chem3D Pro program. Van der Waals radii were included in these dimensions.
aluminophosphate sample (meso-AlPO): 25.30 wt % Al2O3, 28.60 wt % P2O5, 3.30 wt % Na2O, 4.32 wt % C, 2.34 wt % H, and 0.0 wt % S (Al/P ) 1.23). Nitrogen adsorption measurements indicated a Brunauer-Emmett-Teller (BET) surface area of 615 m2/g, a cumulative adsorption pore volume of 0.48 cm3/g (Barrett-Joyner-Halenda (BJH)), and a narrow pore size distribution peaking at 20 Å. A nontemplated aluminophosphate was prepared in a similar manner to the mesoporous material. The Al13 clusters were precipitated with a Na2HPO4/NaH2PO4 buffer solution (Al/P ) 1; total [P] ) 2.5 M) originally buffered at a pH of 4.25. The white precipitate, which formed immediately, was allowed to age for 56 h. The filtered white precipitate was washed with deionized water and air-dried. PXRD showed no low angle features. Nitrogen adsorption measurements indicated a BET surface area of 110 m2/g. Analysis of the nontemplated aluminophosphate material (NT-AlPO): 32.64 wt % Al2O3, 43.02 wt % P2O5, and 5.06 wt % Na2O (Al/P ) 1.06). Siliceous MCM-41 samples were prepared by the method of Corma et al.10 An aqueous solution containing 9.86% cetyltrimethylammonium bromide (CTMABr) was added to an aqueous solution of tetramethylammonium silicate obtained from the reaction between silica (Cab-osil M-5, Fluka) and tetramethylammonium hydroxide (TMAOH) solution (25% TMAOH, 10% SiO2). Amorphous silica was then added under continuous stirring for 2 h. The gel (pH > 11) was transferred to Teflon-lined stainless steel autoclaves and heated at 170 °C under static conditions for 18 h. The occluded organic was removed by heating the sample at 550 °C under a continuous flow of N2 for 2 h, then under a flow of air at 550 °C for 10 h. The PXRD d100 spacing was 60.0 ( 1 Å. Determination of Anion Exchange Capacity and Isotherms at Room Temperature. Aqueous solutions of the following salts or dyes were prepared: K2CrO4 (Baker, 0.1 M); Methyl Red sodium salt (MR), Mordant Yellow 10 (MY10), Brilliant Yellow (BY), Naphthol Blue Black (NBB) (all 0.01M); and Alizarin Yellow GG (AZ) (0.001M). A 0.01 M solution of the nonionic dye Fast Garnet GBC Base (FGGB) was prepared in methanol. All dyes were purchased from Aldrich and used without further purification. The dye solution, together with enough solid (ca. 0.2 g) estimated to absorb approximately half of the dye, was (10) Corma, A.; Kan, Q.; Navarro, M. T.; Pe´rez-Pariente, J.; Rey, F. Chem. Mater. 1997, 9, 2123-2126.
added to a 100 mL volumetric flask with a ground glass stopper. The suspension was stirred vigorously for 24 h at room temperature. Upon addition of meso-AlPO, the pH values of the suspensions decreased from the following initial (anion solution) to final values (anion-meso-AlPO suspension): CrO42- (pH 9.5 f 8.2), MR (pH 9.8 f 7.9), MY10 (pH 8.4 f 5.5), BY (pH 5.5 f 4.9), NBB (pH 9.3 f 6.4), AZ (pH 9.9 f 6.2), water only (pH 6.8 f 5.6). The contents of the flask were filtered and the equilibrium concentrations of dye in the supernatant were measured spectrophotometrically by comparing the absorption peak of the supernatant solution at each particular λmax value, (see Table 1) to a series of standards. The standards were prepared in the same manner as the dye-solid support suspensions, but excluding the solid support. The standard solutions were stirred vigorously for 24 h at room temperature to correct for any physisorption of the anions by the volumetric flask and stir bar. The amount of the dye bound by the solid support was calculated from the difference between the initial and final (equilibrated) dye concentrations. Determination of Anion Exchange Capacity under Reflux Conditions. An aqueous solution of the methyl red sodium salt (0.01 M) together with enough meso-AlPO estimated to adsorb approximately half of the dye, was added to a 250 mL round-bottom flask. The mixture was refluxed for 24 h in an oil bath at 110-115 °C. The dye concentration in the supernatant was determined spectrophotometrically as described above. Extraction of Dye from Dye-Exchanged meso-AlPO. The anionic dye in the meso-AlPO-dye material was replaced by acetate ions by stirring in a 0.01 M solution of sodium acetate/ methanol (0.1 g of meso-AlPO-dye: 0.5 g of sodium acetate) for 12 h. The Fourier transform infrared spectrum (FTIR) of the product (meso-AlPO-dye-Ac) confirmed the replacement of the dye molecule with the acetate (Ac) ion. Regeneration of meso-AlPO. The re-extracted material (meso-AlPO-dye-Ac) was protonated by stirring 0.3 g of the material in 50 mL of an aqueous 0.1 M acetic acid/sodium acetate buffer solution (pH ∼ 4.3) for 30 min at room temperature. The product was filtered, washed with deionized water, and dried in air. Product Characterization. Chemical analyses for C, H, N, and S were carried out by Atlantic Microlab Inc., Norcross, GA. Al, Na, and P analyses were carried out by the Geochemical Lab, University of Minnesota, Minneapolis, MN. PXRD patterns were
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Figure 1. Room-temperature ion exchange isotherms for mesoAlPO-Ac-CrO42j (4), 0.1 M total anion concentration; mesoAlPO-Ac-MY102j (n), meso-AlPO-Ac-MRj (0), 0.01 M total anion concentration; and meso-AlPO-Ac-AZj ()), 0.001 M total anion concentration. All lines in Figures 1-5 are intended as guides to the eye. obtained using the Siemens D-5005 wide-angle X-ray diffractometer with Cu KR radiation. Solution UV-vis spectra were obtained on a Hewlett-Packard 8254A diode array spectrophotometer and diffuse reflectance UV-vis spectra on the same instrument with a Labsphere RSA-HP-84 reflectance spectroscopy accessory; the instrument has a resolution of 2 nm. Reflectance data were converted to f(R∞) values, which are directly proportional to absorbance, using the Kubelka-Munk equation. N2 adsorption measurements were carried out with a Micromeritics ASAP 2000 V3.00 sorption analyzer. The samples were dried under vacuum for 3-5 h at 70 °C and a final pressure of 0.003 mmHg. Based on the PXRD intensity of the d100 peak, the sample structure and order was maintained during the drying process. The BET method was used to derive surface areas and pore volumes from the adsorption isotherms. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TGA-7 thermal analyzer attached to a PC via a TAC7/DX thermal controller. The samples were heated under flowing nitrogen from 30 to 800 °C at 15 °C/min. The FTIR spectra were collected on a Nicolet Magna-IR 760 spectrometer, using KBr pellets. Solid-state 27Al MAS NMR spectra (single pulse, 104.27 MHz, pulse width 30 µs, pulse delay 1.5 s, 2560 transients) were collected on a Chemagnetics CMX-400 Infinity spectrometer with a 7.5 mm zirconia rotor spinning at 4 kHz. 1 M Al(H2O)63+ was used as an external chemical shift standard.
Kron et al.
Figure 2. Selectivity coefficient for the meso-AlPO-Ac-CrO42j system as a function of meso-AlPO composition.
Figure 3. Selectivity coefficient for the meso-AlPO-Ac-MY10 system as a function of meso-AlPO composition.
Results Ion Exchange Isotherms. The ion exchange isotherms for meso-AlPO in Ac-CrO42j solution (total anion concentration: 0.1M), Ac-MRj, Ac-MY102j solutions (total anion concentration: 0.01M), and Ac-AZj solution (total anion concentration: 0.001 M) are shown in Figure 1. In the ion exchange isotherms, SA on the abscissa represents the equivalent fraction of the dye anion A in aqueous solution compared to the total anion concentration in solution, and (meso-AlPO)A on the ordinate represents the equivalent fraction of the dye anion A exchanged onto meso-AlPO at equilibrium. Langmuir type ion exchange isotherms11 are observed for meso-AlPO-CrO42j, mesoAlPO-MRj, and meso-AlPO-MY102j, indicating a higher affinity for the entering anionic dye compared to acetate. The isotherm for meso-AlPO-AZj is characteristic of incomplete exchange. Determination of Selectivity Coefficients. To estimate the number of anionic exchange sites available on the extracted inorganic support it was assumed that one dodecyl sulfate molecule was present at each exchange (11) Abe, M. In Ion Exchange Processes: Advances and Applications; Dyer, A., Hudson, M. J., Williams, P. A., Eds.; The Royal Society of Chemistry: Cambridge, UK, 1993; pp 199-203.
Figure 4. Selectivity coefficient for the meso-AlPO-Ac-MR system as a function of meso-AlPO composition.
site of the nonextracted sample. The loading of dodecyl sulfate was determined from the sulfur analysis of the nonextracted sample. The selectivity coefficients, KAAc, of the anion A with respect to acetate ion were calculated from the ion exchange isotherms in Figure 1 in accordance with the following equations:12 -
A when z ) 1, KAc - )
2-
A when z ) 2, KAc - )
(meso-AlPO)ASAc (meso-AlPO)AcSA (meso-AlPO)0.5 A SAc (meso-AlPO)0.5 Ac SA
where z ) the charge on the anion A. Figures 2-5 show the logarithm of selectivity coefficients as a function of the mole fraction of anion A on meso-AlPO. The equilibrium constants (Ke) were estimated (12) Miyata, S. Clays Clay Miner. 1983, 31, 305-311.
Anion Exchange in Mesoporous Aluminophosphate
Langmuir, Vol. 15, No. 23, 1999 8303 Table 3. Milliequivalents of Anion (Anj) Exchanged per Gram of Solid Support (X ) meso-AlPO, MCM-41, Dowex 21K Cl Anion Exchange Resin, or Nontemplated NT-AlPO)a
Figure 5. Selectivity coefficient for the meso-AlPO-Ac-AZ system as a function of meso-AlPO composition. Table 2. Gaines-Thomas Equilibrium Constants for AlPO4-Ac-A Compounds anion (Anj)
log Ke
MRj AZj CrO42j MY102j
0.64(3) 0.12(3) 0.60(3) 0.52(3)
by using the method of Gaines and Thomas13 from the following equation:
log Kc )
(meso-AlPO) )1 A log KAc ∫(meso-AlPO) )0 A
z-
A
d(meso-AlPO)A
where (meso-AlPO)A is the mole fraction of anion A adsorbed onto meso-AlPO. The integral in the above equation was evaluated graphically by calculating the area under the curve in Figures 2-5. The graphs of log KAAc versus (meso-AlPO)A were extrapolated to the limits of 0 and 1 to satisfy the integral. The calculated Gaines and Thomas equilibrium constants are listed in Table 2. The equilibrium constants indicate that the ion selectivity of meso-AlPO for the anions tested follows the order MRj g CrO42j > MY102j > AZj. The equilibrium constants are similar to reported values for hydrotalcite clays for several anions.12 While hydrotalcites have been shown to exhibit greater affinity toward divalent anions,14-16 the mesoAlPO material did not follow the same trend. The highest selectivity was exhibited for the monovalent dye MRj, although the selectivities were nearly the same for MRj and the divalent anions CrO42j and MY102j. Anion Exchange Capacities. Anion exchange capacities of meso-AlPO for acetate, chromate, and a number of dyes are listed in Table 3. The amount of acetate taken up by the meso-AlPO sample (1.80 meq/g) corresponds to the amount of dodecyl sulfate originally present (5.69 wt % ) 1.77 meq/g). Except for some special cases, which are discussed below, the values for most anionic dyes fall in the range from 1.3 to 1.6 meq/g. Control experiments were carried out to compare the performance of meso-AlPO with a commercial anion resin, as well as to investigate the contributions of physisorption and “true” anion exchange to the uptake. Table 3 shows results from the control experiments carried out with other supports (Dowex 21 K Cl anion exchange resin, MCM-41, a nontemplated aluminophosphate, and pretreated meso-AlPO), and with a nonionic dye, FGGB. For all dye solutions a pH drop was measured as meso-AlPO was stirred in the solutions (13) Gaines, G. F.; Thomas, H. C. J. Chem. Phys. 1953, 21, 714-718. (14) Chaˆtelet, L.; Bottero, J. Y.; Yvon, J.; Bouchelaghem, A. Colloids Surf. A 1996, 111, 167-175. (15) Miyata, S. Clays Clay Miner. 1980, 28, 50. (16) Sato, T.; Wakabayashi, T.; Shimada, M. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 89.
X-Anj a
meq Anj/g exchanged
meq OH-/g exchanged
meso-AlPO-Acmeso-AlPO-MRmeso-AlPO-MR- (100 °C, H2O) meso-AlPO-MR--Ac--MRmeso-AlPO-MR--Ac--H+-MRmeso-AlPO-MR- (100 °C, N2) meso-AlPO-MR- (200 °C, N2) MCM-41-MRDowex-MRNT-AlPO-MRmeso-AlPO-AZMCM-41-AZDowex-AZmeso-AlPO-CrO42j meso-AlPO-MY102meso-AlPO-NBB2meso-AlPO-NBB2--Ac--H+-NBB2meso-AlPO-NBB2--Ac--H+-MRNT-AlPO-NBB2meso-AlPO-BY2meso-AlPO-NYS2meso-AlPO-FGGB MCM-41-FGGB
1.80b 1.48 1.49 0.10 0.72 0.84 0.48 0.10 1.36 0.77 1.27 0.21 0.95 1.49 1.52 0.07 0.12 0.56 0.31 0.11 1.6c 0.13 0.01
0.001 0.31
0.40 0.15 0.01 0.10
0.00
a The values of “meq OH-/g exchanged” were estimated from the pH change measured as meso-AlPO was added to the anion solution. All exchanges were carried out on unheated and unused supports, except for the following samples. meso-AlPO-MR (100 °C, H2O): exchange at 100 °C under reflux in water. meso-AlPO-MR--Ac-MR-: second exchange with MR after dye extraction of meso-AlPOMR with acetate ions. meso-AlPO-MR--Ac--H+-MR-: second exchange with MR after dye extraction of meso-AlPO-MR in an acetate/methanol solution followed by protonation in an acetic acid/ sodium acetate buffer. meso-AlPO-MR (100 °C, N2): exchange with meso-AlPO heated in nitrogen at 100 °C for 1 h. meso-AlPO-MR (200 °C, N2): exchange with meso-AlPO heated in nitrogen at 200 °C for 1 h. meso-AlPO-NBB2--Ac--H+-NBB2-: second exchange with NBB after dye extraction of meso-AlPO-NBB in an acetate/ methanol solution and protonation in an acetic acid/sodium acetate buffer. meso-AlPO-NBB-Ac-H+-MR: second exchange with MR after dye extraction of meso-AlPO-NBB in an acetate/methanol solution and protonation in an acetic acid/sodium acetate buffer. b Determined from the acetate concentration in the solid. c Result reported in Holland et al.9
(see Experimental section). The change in [OH-], which can involve competitive uptake of OH- or deprotonation of framework water in meso-AlPO, is also listed in Table 3. Discussion Physisorption versus True Anion Exchange. Two control experiments were performed to distinguish between physisorption effects and a true anion exchange process. These experiments were carried out by the same procedure employed for the meso-AlPO material. (1) The exchange was carried out with two anionic dyes on the high surface area support MCM-41. Except for possible defect sites, the walls of siliceous MCM-41 are not charged. Hence uptake of the azo dyes occurs in MCM41 mostly by physisorption. In the absence of electrostatic interactions, the dye molecules were not strongly retained within the channel structure, even though the surface area of MCM-41 was approximately twice as large as that of meso-AlPO. The exchange capacities for MCM-41-AZ (0.21 meq/g) and MCM-41-MR (0.10 meq/g) were considerably lower than those for meso-AlPO-AZ (1.27 meq/ g) and meso-AlPO-MR (1.48 meq/g). These results suggest
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Kron et al.
Table 4. Results from Nitrogen Adsorption Experiments for meso-AlPO Samples before and after Anion Exchange with NBB, MR, or BY dye (meso-AlPO-NBB, meso-AlPO-MR, meso-AlPO-BY), the Dye-Exchanged Samples after Re-extraction with Acetate (meso-AlPO-NBB-Ac, meso-AlPO-MR-Ac), a Sample Stirred in Water for 24 h (meso-AlPO-H2O), and a Nontemplated AlPO Sample (NT-AlPO)a sample
BET surface area (m2/g)
BJH cumulative adsorption pore volume (cm3/g)
meso-AlPO meso-AlPO-NBB meso-AlPO-NBB-Ac meso-AlPO-MR meso-AlPO-MR-Ac NT-AlPO meso-AlPO(2) meso-AlPO(2)-H2O meso-AlPO(2)-BY
615 308 291 195 355 110 762 310 316
0.48 0.46 0.46 0.22 0.52 0.86b 0.50 0.54 0.42
peak pore diameter (Å) adsorption desorption 20 46-53 (br) 53 (br) 20,31 (br, weak) 53 (br) 700 (br) 20 37-41 25 (br)
21 36 37 37 36 400 (br) 20 37 40
a The pore diameters refer to peak maxima in a BJH pore size distribution plot (br ) broad). Samples labelled with (2) were prepared from a second batch of meso-AlPO. b Pore volume for pores with diameters below 200 Å: 0.09 cm3/g.
that physisorption played only a small role in the uptake of anions in the porous meso-AlPO material and that a true anion exchange process predominated. (2) Both the meso-AlPO material and the MCM-41 support were exchanged with the nonionic azo dye FGGB. The azobenzene backbone in FGGB is similar to the other dyes used in the study, and the molecule has comparable dimensions to the anionic dyes that exhibited good uptake. Due to the low solubility of FGGB dye in water, a methanol solution was used rather than the aqueous solution employed in the previous exchanges. Exchange reactions in methanol were possible with the meso-AlPO system. The original anion exchange of the surfactant SDS with acetate ions was carried out in methanol and resulted in complete removal of the surfactant, based on the sulfur analysis. The exchange capacity of the meso-AlPO-FGGB system (0.13 meq/g) was considerably lower than the capacity for the other anionic species and was similar to the MCM-41-FGGB, MCM-41-MR, and MCM-41-AZ exchanges. The low amount of adsorption further confirmed the importance of electrostatic interactions, and provided additional evidence that dye uptake by mesoAlPO occurred by anion exchange and not simply by physisorption. Size Selectivity of the meso-AlPO Material. As previously reported, the meso-AlPO structure consists of pseudohexagonal arrays of channels with an average pore diameter of ca. 20 Å and a narrow pore size distribution.9 Therefore, one can expect selective uptake of guest molecules based on their size and shape. The possibility of size exclusion was investigated using the large dianionic dye NBB, which contained structural components and functional groups present in many of the other dyes studied (see Table 1). The largest dimensions of the NBB dianion were approximately 22.5 Å × 12.2 Å, about 5-8 Å larger in each dimension than the other anionic dyes which exhibited uptakes greater than 1.3 meq/g of meso-AlPO. The ion exchange capacity for NBB was only 0.07 meq/g of meso-AlPO. A similarly low uptake of 0.11 meq/g was observed for another large dianionic dye, BY. In a competitive uptake experiment involving a solution containing equimolar amounts of MR and NBB, the uptake of the smaller red dye MR (0.32 meq/g) was approximately twice that of the larger blue dye NBB (0.15 meq/g). The solid product was dark red and the supernatant solution was blue-green after this exchange. We hypothesize that the bulkier dye molecules either could not enter the pores of the solid support or plugged the pore entrances, thus preventing additional dye molecules from entering. To investigate the location of the NBB dye, nitrogen adsorption measurements were performed on a series of
samples, including the original extracted meso-AlPO support, a meso-AlPO sample exchanged with NBB dye (meso-AlPO-NBB) and a sample exchanged with MR dye (meso-AlPO-MR). The results are reported in Table 4. As the NBB dye was exchanged onto the solid support, the BJH cumulative pore volume decreased by only 0.02 cm3/ g. In contrast, after an exchange with MR dye, the volume decreased by more than half the original value (i.e., by 0.26 cm3/g). This significant decrease in pore volume was indicative of dye encapsulation within the pore channels. Conversely, the nearly unchanged pore volume in the meso-AlPO-NBB system indicated that the NBB dye was adsorbed mostly on the outer surface of the solid particles or near the pore openings, but not within the pore channels. The measured changes in pore volumes were approximately equal to the difference expected from replacement of acetate by MR or NBB, based on molecular volume estimates and the anion exchange capacities. The calculated changes in volume were 0.05 cm3/g for acetate T NBB exchange and 0.30 cm3/g for acetate T MR exchange. The slightly larger values compared to nitrogen sorption data, if significant, could be due to molecules adsorbed on the external surface of the support. These results provided strong evidence that the majority of anion exchange sites were contained within the channel structure. The size exclusion effect led to the markedly lower anion exchange capacity for NBB and BY, as solid-dye interactions were possible only on the outer surface. The size selective properties of the meso-AlPO material could permit separation of anions by size. Multiple Exchanges with the meso-AlPO Material. The ability of meso-AlPO to undergo multiple anion exchanges was studied with the meso-AlPO-MR-Ac system and with a protonated meso-AlPO-MR-Ac-H+ material. The bound MR dye on the meso-AlPO-MR material was removed by extraction with an acetate/ methanol solution. The meso-AlPO-MR-Ac material was then reexchanged with the MR dye. This time, the exchange capacity was very low (0.10 meq/g), which indicated a loss of positively charged surface sites on the support during the multiple treatments with dye and acetate solutions. The ion exchange with MR had been carried out at pH 7.9, where abstraction of surface protons or even formation of Al(OH)4- species was possible.17 It is known that the exchange capacity for anionic dyes on alumina surfaces is pH dependent, decreasing sharply at increasing pH values between 4.5 and 7.5.18 Upon repro(17) Baes, C. F., Jr.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley and Sons: New York, 1976. (18) Tiwari, M. P.; Tiwari, J. S.; Mundhara, G. L. J. Indian Chem. Soc. 1979, LVΙ, 798-801.
Anion Exchange in Mesoporous Aluminophosphate
Figure 6. Powder X-ray diffraction patterns of (a) SDSextracted meso-AlPO, (b) meso-AlPO stirred in ethanol for 24 h at room temperature, (c) meso-AlPO after anion exchange with MR dye for 1 h, (d) for 24 h (meso-AlPO-MR), (e) mesoAlPO-MR after further ion exchange with acetate ion (mesoAlPO-MR-Ac). The powder X-ray diffraction patterns are all shown on the same intensity scale.
tonation of the meso-AlPO-MR-Ac material in an acetic acid/sodium acetate buffer (pH ∼ 4.3), the protonated support exhibited an exchange capability of 0.72 meq/g, considerably higher than the unprotonated meso-AlPOMR-Ac sample. Thus multiple anion exchanges are possible, if the pH of the environment remains in a range coinciding with the stability of the Al13 ion. Additional evidence for size selectivity in the uptake of anionic dye molecules was provided by multiple ion exchanges with the sample meso-AlPO-NBB after reextraction of the dye with acetate ions, followed by reprotonation. When this material was stirred again in an NBB solution (meso-AlPO-NBB2--Ac--H+-NBB2-), the uptake of NBB changed only slightly, from 0.07 to 0.12 meq/g. The small increase in uptake may be due to an expansion in pore diameter (see below). However, when the reprotonated material was stirred in an MR solution (meso-AlPO-NBB2--Ac--H+-MR-), 0.56 meq/g of the smaller dye were incorporated. Structural Changes of meso-AlPO upon Anion Exchange. Changes in the channel structure of mesoAlPO during the ion exchange processes were investigated by nitrogen adsorption measurements, PXRD, and FTIR spectroscopy. The PXRD pattern of meso-AlPO (Figure 6a) exhibited one low-angle diffraction line corresponding to d100 ) 34.5 Å in the semi-ordered mesopore structure, typical for mesoporous solids with uniform channels that are only weakly ordered. Upon interaction with the MR dye solution, the d100 peak decreased in intensity after 1 h (Figure 6c) and nearly vanished after 24 h of ion exchange (Figure 6d). Such a decrease in intensity of PXRD diffraction lines can imply loss of structural order; it can also be observed in mesoporous materials if the contrast in electron density between walls and channel spaces is reduced by the presence of guest molecules, even if the framework order does not change. In fact, after subsequent extraction of the dye with an acetate solution, some intensity was regained at the original d-spacing (Figure 6e), although the PXRD pattern of the original meso-AlPO
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Figure 7. Adsorption pore volume plots of (a) SDS-extracted meso-AlPO, (b) meso-AlPO after anion exchange with MR dye (meso-AlPO-MR), (c) meso-AlPO-MR after further ion exchange with acetate ion (meso-AlPO-MR-Ac).
material was not obtained. Nitrogen adsorption measurements provided further insight into changes in the pore structure (see Table 4, Figure 7). For meso-AlPO, the BJH pore size distribution plots were nearly identical for the adsorption and desorption isotherms; a relatively narrow peak around 20-21 Å was consistent with uniform cylindrical pores. After anion exchange with the dye MR followed by extraction of the dye with an acetate/methanol solution (meso-AlPO-MR-Ac) the pore size distribution broadened significantly with a peak in the adsorption isotherm centered around 53 Å, which is indicative of a larger void space (Figure 7). Similar pore size increases and decreases in PXRD intensity were observed after hydrothermal treatment of mesoporous silicates.19 It is notable that the desorption isotherm for meso-AlPO-MRAc remained relatively sharp, peaking at 36 Å. This smaller dimension corresponds to the size of the channel opening. The difference in peak positions for the adsorption and desorption pore size distributions implies that bottlenecks are present at the pore openings. A decrease in the BET surface areas from meso-AlPO to meso-AlPO-MR-Ac is consistent with the larger pore sizes. An additional factor contributing to the smaller surface area may be the presence of residual dye after the acetate washing. Based on the disappearance of IR absorptions of the MR dye and the appearance of the acetate νCdO stretching vibrations at 1471 and 1426 cm-1, most of the dye was replaced by the smaller acetate ions. However, a residual amount of dye (not detected by FTIR) was still present after the extraction, indicated by a faint red color. The relative amount of acetate ion reintroduced into the material was less compared to the original mesoAlPO material, based on relative IR intensities. In the meso-AlPO-NBB system, a similar decrease in BET surface area and increase in average pore size distribution to ca. 46-53 Å with 36-37 Å bottlenecks was observed, even though the larger NBB dye did not enter the pore spaces. The PXRD patterns were similar to those of the meso-AlPO-MR system showing a reduction in the (19) Khushalani, D.; Kuperman, A.; Ozin, G. A.; Tanaka, K.; Garce´s, J.; Olken, M. M.; Coombs, N. Adv. Mater. 1995, 7, 842-846.
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original d100 peak to a broad hump with low intensity. The nitrogen adsorption data for meso-AlPO-NBB underwent little change after extraction of the dye. These observations indicate that the original pore structure was transformed during the dye exchange processes for both MR and NBB. As NBB dye molecules did not appear to have access to the original channel system, we believe that the change in pore structure was not caused by swelling in the presence of dye molecules, but rather by hydrolysis with water. An analogous increase in pore size and degradation of the PXRD pattern was observed for meso-AlPO-H2O, a sample that was stirred in water for 24 h at room temperature. In this sample the P/Al ratio decreased by 7% after treatment with water. In microporous aluminophosphates it has been shown that structural degradation can occur by the hydrolysis of -P-O-Al- bonds according to the reaction: -P-O-Al- + H2O S -P-OH + HO-Al-.20 This reaction is also possible in meso-AlPO and can lead to the structural rearrangement, as well as to partial loss of phosphorus. Further support for the involvement of water in the structural changes comes from the observation that a strong d100 peak was maintained in the PXRD pattern of a meso-AlPO sample that was stirred in ethanol for 24 h at room temperature (Figure 6b). An additional factor in the structural rearrangement could be reaction with hydroxyl groups present in the dye solutions. The pH of all anion solutions examined in this study decreased after addition of meso-AlPO. The relative amounts of OH- involved are listed in Table 3. Hydroxyl groups can be competitively exchanged into the support or they can cause deprotonation of the support. Both pathways would reduce the ion exchange capacity of the support, as was observed (see also the section on Multiple Exchanges). Upon deprotonation of terminal water groups, the remaining terminal OH groups at one Al center may become linked to another Al atom in a condensation reaction resulting in the structural rearrangement.21 27Al MAS NMR spectra of meso-AlPO and meso-AlPO-MRAc (Figure 8) show similar features to those described previously,9 with a strong resonance due to octahedral Al (-8 to -10 ppm), a weak resonance assigned to tetrahedral Al (61-62 ppm), and an intermediate resonance (19-20 ppm). Consistent with the proposed mechanism of Al condensation, the ratio of AlOh:AlTd increases after the anion exchange in water. Given the fact that the window openings of meso-AlPO increased from 20 to 36 Å and the pore diameters from 20 to ca. 53 Å during treatment with an NBB solution, one might ask how the material could continue to exclude anionic dye molecules with dimensions less than 27 Å. Even after regeneration of the support and a second anion exchange the uptake of NBB increased from 0.07 only to 0.12 meq/g (see section on Multiple Exchanges). A reasonable explanation involves partial blockage of the channels by dye molecules that are adsorbed near the channel windows, as suggested by the nitrogen adsorption isotherms (Figure 9). The type IV adsorption isotherms for meso-AlPO and meso-AlPO-NBB exhibited two knees, corresponding to the formation of a monolayer and to the filling of mesopores. The second knee occurred at a significantly higher relative pressure (ca. 0.67) in the mesoAlPO-NBB sample than in the original meso-AlPO material (ca. 0.35). The increased pressure would be a result of partial channel blocking by NBB molecules that (20) Parlitz, B.; Lohse, U.; Schreier, E. Microporous Mater. 1994, 2, 223-228. (21) Furrer, G.; Ludwig, C.; Schindler, P. W. Journal Colloid Interface Sci. 1992, 149, 56-67.
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Figure 8. 27Al solid-state NMR spectra of (a) meso-AlPO and (b) meso-AlPO-MR-Ac. The asterisk denotes a spinning sideband.
Figure 9. Nitrogen adsorption isotherms of (a) meso-AlPO (solid line), (b) meso-AlPO-NBB (dashed line), (c) meso-AlPOMR (dotted line) materials.
were adsorbed near the opening of the channels, hindering entry of nitrogen. However, at high enough pressure, nitrogen was forced into the channels, which were mostly unoccupied. Comparison with a Nontemplated Aluminophosphate. The importance of mesopores in the sample was examined by comparing the ion exchange capacities of meso-AlPO with a nontemplated aluminophosphate material (NT-AlPO) that was synthesized from Al13 clusters and a phosphate buffer solution in the absence of the surfactant template. Even though the average particle size of NT-AlPO was smaller than that of the templated material (2-15 µm agglomerates for NT-AlPO, compared to 5-40 µm agglomerates for meso-AlPO), the BET surface area of NT-AlPO (110 m2/g) was significantly lower. No micro- or mesopores were detected in this material. A relatively large pore volume (0.86 cm3/g) was mostly due to macropores between particles. Remarkably, the NTAlPO material exhibited an anion exchange capacity with MR dye of 0.77 meq/g, compared to the 1.48 meq/g of the
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meso-AlPO material. As most of the surface area of NTAlPO was external, it was accessible even to the larger dye, NBB, which was adsorbed to the extent of 0.31 meq/ g, more than four times greater than for meso-AlPO. Electrostatic interactions could occur at protonated surface hydroxyl sites on the aluminophosphate support. Such sites are known to exist on acid-treated hydrous aluminum oxides18 and could be formed similarly on both the NTAlPO and the meso-AlPO materials during the acidification process in the synthesis.
Al-OH + H+ h Al-OH2+ Al-OH2+ + Cl- h Al-OH2+ClAl-OH2+Cl- + A- h Al-OH2+A- + ClThe anion exchange capabilities of the NT-AlPO material suggested that meso-AlPO should continue to exhibit anion exchange properties upon partial loss of order during an initial exchange, as a positive framework charge, a high surface area and some size selectivity could be maintained for the meso-AlPO. Comparison of Exchange Capacity and Thermal Stability with an Anion Exchange Resin. The exchange capacity of the meso-AlPO material was compared to a commercially available anion exchange resin. The Dowex 21K Cl anion exchange resin, 16-30 mesh, was exchanged with MR and AZ dyes in a manner consistent with the procedure employed for the meso-AlPO material. The resin has a particle size range of 0.6-1.2 mm, with a minimum total wet capacity of 1.2 equiv/L.22 In this study, the measured exchange capacities of meso-AlPO and Dowex were comparable with slightly higher dye uptake by the meso-AlPO system in the aqueous environment employed (meso-AlPO-(MR, AZ) (1.48, 1.27 mequiv/ g), Dowex -(MR, AZ) (1.36, 0.95 mequiv/g)). The Dowex resin is thermally stable in an aqueous environment up to approximately 60 °C.23 To investigate the thermal stability of the meso-AlPO material, PXRD patterns were obtained for a series of heated samples. The samples were heated in nitrogen for 1 h at 100, 200, 300, and 400 °C (Figure 10). The low angle d100 peak broadened and became less intense as the sample was heated. The d spacing shifted from 34.5 Å in the unheated sample to about 29.9 Å in the sample heated to 200 °C. At 300 °C, the low-angle peak was reduced to a broad hump with a shoulder near 29 Å. No low-angle features were present at 400 °C. Thermogravimetric analysis (TGA) showed weight losses at ca. 100 °C, due to losely bound water, and between 280 and 480 °C (Figure 11). The second loss may be associated with the water present at octahedral aluminum sites.9 For comparison, in basic aluminum sulfate the Al13 tridecamer units gradually lose their water and hydroxyl groups over the range from 80 to 360 °C.24 The loss of water in the range from 280 to 480 °C resulted in lower order in the mesoAlPO material, indicated by the PXRD patterns. Concomitant with the structural changes and the water loss, the anion exchange capacity of the thermally treated materials decreased from 1.48 meq/g for an unheated sample, to 0.84 meq/g for a meso-AlPO sample heated to 100 °C (meso-AlPO-MR(5)), and to 0.48 meq/g for a mesoAlPO sample heated to 200 °C (meso-AlPO-MR(6)). (22) Dorfner, K. Ion Exchangers; Walter de Gruyter: Berlin, 1991. (23) Personal communication with Dow Chemical Company sales representative, July 1998. (24) Johansson, G. Acta Chem. Scand. 1962, 16, 403-420.
Figure 10. Powder X-ray diffraction patterns of meso-AlPO thermally treated under nitrogen for 1 h; (a) unheated sample, (b) heated at 100 °C, (c) heated at 200 °C, (d) heated at 300 °C, (e) heated at 400 °C. The powder X-ray diffraction patterns are all shown on the same intensity scale.
Figure 11. TGA and derivative TGA curves for the mesoAlPO material.
However, when the exchange was carried out in an aqueous reflux environment (ca. 100 °C), the exchange capacity was maintained at 1.49 mequiv/g. This thermal stability may permit ion exchange applications in aqueous environments at elevated temperatures. Conclusion The inorganic mesoporous support, meso-AlPO, was shown to be an effective anion exchange material for of a number of anionic dyes. The ion exchange capacities for dyes that could enter the mesopores fell in the range from approximately 1.3-1.6 meq/g. These exchange capacities were comparable to a commercially available resin. Langmuir type ion exchange isotherms were observed for meso-AlPO-Ac-(CrO42j, MY122j, MRj), while mesoAlPO-Ac-AZj exhibited incomplete exchange. Control experiments with MCM-41 and the nonionic dye FGGB indicated that anion adsorption in meso-AlPO was caused
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predominantly by electrostatic interactions rather than pure physisorption. The material exhibited size-selective uptake of anionic dyes. Multiple exchanges were possible if the support was regenerated by acid treatment at pH 4.3. Hydrolysis with water during ion exchange resulted in an increase in the average pore sizes. The mesopore structure of meso-AlPO was maintained during thermal treatment up to 200 °C; however, loss of water from the support led to a reduced exchange capacity. In boiling
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water, the anion exchange capacity was comparable to room-temperature exchanges. Acknowledgment is made to the National Science Foundation (DMR-9701507), The David and Lucile Packard Foundation, Dupont, and 3M for support of the research. LA990553R
