J. Phys. Chem. B 1998, 102, 4301-4309
4301
Encapsulation, Stabilization, and Catalytic Properties of Flexible Metal Porphyrin Complexes in MCM-41 with Minimal Electronic Perturbation by the Environment Brian T. Holland, Chad Walkup, and Andreas Stein* Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455 ReceiVed: February 24, 1998; In Final Form: April 1, 1998
meso-Tetrakis(5-trimethylammoniopentyl)porphyrin (TMAP) was incorporated in MCM-41 directly during a hydrothermal synthesis or by a surfactant T porphyrin ion-exchange reaction with as-synthesized MCM-41. Both methods permitted encapsulation of the porphyrin within the mesoporous channel system. UV-vis absorption spectra indicated that porphyrin molecules dimerized or formed larger agglomerates in the ionexchanged samples, while TMAP molecules remained isolated in hydrothermally prepared samples. Spectra of the latter samples closely resembled those of TMAP in solution with no significant broadening of the Soret and Q-band absorptions. Acid extraction of the surfactant converted the encapsulated free-base porphyrin to the dication TMAP-H22+, which could be further metalated with Cu2+, Ni2+, or Fe2+ without leaching porphyrin from the mesoporous sieve. The catalytic activity of the copper porphyrins was evaluated in the oxidative bleaching reaction of the azo dye β-naphthol violet. During this reaction, TMAP-Cu2+ also degraded when it was free in solution or incorporated in MCM-41 by ion exchange. However, in samples where the porphyrin was directly incorporated during the hydrothermal synthesis, TMAP-Cu2+ was stabilized and exhibited greater catalytic activity for longer time periods. It is suggested that isolation of the porphyrin molecules within the MCM-41 channels prevented their mutual oxidation.
Introduction Porphyrins and phthalocyanines are well-known for their biological, catalytic, conductive, and photoactive properties.1 They have also been investigated extensively in an effort to mimic enzymatic systems, especially those of the cytochrome P-450 family.2,3 Many studies of synthetic and natural encapsulated metal complexes have demonstrated that steric effects, as well as specific binding at the active metal site within the enzyme, play an important role in substrate selectivity.4-6 In an ideal enzymatic model system a host material should provide steric selectivity for the substrate, while the active center of the encapsulated metal porphyrin complex should remain essentially unaffected by the host. Because of their cavity or gallery structures, zeolitic and intercalated clay-type materials have been used as models to mimic the enzymatic protein cavities.7-11 Intercalation of porphyrins within clay-type materials results in swelling of the layers, permitting access to the porphyrin for chemical reactions. However, the clay typically affects the electronics of the porphyrin, as manifested in significant peak broadening and peak shifts in diffuse reflectance (DR) UV-vis spectra upon intercalation. In the case of uncharged porphyrins, only a small fraction remains bound to the silicate surface after metalation of the porphyrin.10,12 Zeolites, with their ordered arrays of channels and cavities, are also suitable hosts for metal porphyrin complexes. The zeolite cavities provide space for the active metalloporphyrin and allow diffusion of small substrates toward these centers while inducing shape and size selectivity.7 The metal complexes sometimes retain certain solution properties in these hybrid heterogeneous catalysts. However, as in clays, the electronic structure of the occluded metal complexes is affected by the zeolite walls, as evidenced by changes in peak positions and peak widths in the DR UV-vis spectra of the solids compared
to solution UV-vis spectra of dissolved metal complexes. Due to a tight fit and high electric field gradients, zeolite cavities can have a large influence on the metal centers, possibly inhibiting the full potential of the metal porphyrin complexes. In addition, the angstrom-sized zeolite channels and cavities, while providing geometric selectivity, can sometimes become clogged with products as a reaction proceeds, leading to low turnovers.13 The interaction between the support and the active center may be reduced by using a support with slightly larger pores that permits some spatial separation between the porphyrin center and the cavity wall. Mesoporous silicate sieves of the type MCM-41 provide this additional space, possessing narrow pore size distributions in the low nanometer range and extremely high surface areas in excess of 1000 m2/g. We chose an MCM41 system with 22-24 Å channel openings (interchannel spacing based on d100 reflections: 33-38 Å) and selected the guest, meso-tetrakis(5-trimethylammoniopentyl)porphyrin (TMAP), a porphyrin originally synthesized by Onaka et al. for clay studies.14 This molecule consists of a rigid porphine core and four flexible alkyl side chains each containing a trimethylammonium group (Figure 1). Fully extended TMAP has an effective diameter of ca. 23 Å and a porphine ring diameter of ca. 10 Å. This molecule is therefore suitable for incorporation into MCM-41 with 20+ Å channel diameters. It was anticipated that the alkyl groups provide enough flexibility to allow movement of the porphyrin through the channels while maintaining some isolation of the porphine center from the host support. In addition, the cationic alkyl arms induce water solubility and compatibility with the hydrothermal synthesis conditions. Porphyrins with neutral phenyl groups at the mesopositions have recently been studied in MCM-41,15,16 and phthalocyanines have been incorporated in various oxide mesophases.17,18
S1089-5647(98)01273-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/07/1998
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Figure 1. Structures of (a) TCPP, (b) the TMAP free base, (c) the TMAP-H22+ dication, and (d) the metal-exchanged porphyrin (TMAP-M2+), M2+ ) Cu2+, Ni2+, and Fe2+.
It has been shown previously that water soluble porphyrins and phthalocyanines can be incorporated directly into the galleries of clays by in situ crystallization of synthetic clay layers.11,19 In the present study, TMAP was incorporated in MCM-41 directly during the surfactant-templated hydrothermal synthesis. Mixtures of the chloride salt, TMAP-Cl, with the surfactant dodecyltrimethylammonium bromide (DTABr) were used, as this surfactant typically templates mesoporous channels in the desired pore diameter range. Low concentrations of TMAP produced ordered MCM-41, while higher concentrations rendered less ordered mesoporous materials. TMAP could also be incorporated by ion exchange with the cationic surfactant in MCM-41, leading to greater uptake of TMAP compared to encapsulation during the synthesis. The surfactant could be extracted from directly synthesized or ion-exchanged TMAPMCM-41 with minimal removal of TMAP. Various transition metal atoms, such as Cu2+, Ni2+ and Fe2+, were introduced into the porphyrin cores. Optical spectra of MCM-41 loaded with TMAP during the hydrothermal synthesis and spectra of the corresponding dications or metal-exchanged products showed sharp absorption bands similar to solution phase spectra of the alkyl porphyrin. These data indicated that TMAP complexes remained isolated from each other within the channels, and the porphine cores displayed little interaction with the support. A catalytic study of the copper-loaded porphyrins (TMAP-Cu2+) in the oxidative bleaching reaction of the azo dye β-naphthol violet supported this conclusion. TMAP-Cu2+ was stabilized when the porphyrin was directly incorporated during the hydrothermal synthesis, compared to either ion-exchanged porphyrins or porphyrins in solution.
Experimental Section Synthesis of meso-Tetrakis(5-trimethylammoniopentyl)porphyrin Chloride Salt (TMAP-Cl). The synthesis of TMAP-Cl followed a three-step procedure. First, 6-chlorohexanal was synthesized according to a modified literature procedure.20 A slurry of 21.56 g (100 mmol) pyridinium chlorochromate (Aldrich) in 25 mL of chloroform (Aldrich) was prepared in a round-bottom flask equipped with a magnetic stirrer. To this a solution of 3.42 g (25 mmol) 6-chlorohexanol (Aldrich) in 25 mL of chloroform was added. The round-bottom flask was covered with aluminum foil to exclude light. The reaction was stirred for 1 h at room temperature, and the progress of the oxidation was followed using thin-layer chromotography. Upon completion of the oxidation, 200 mL of anhydrous ether (Mallinckrodt) was added while stirring for another 10 min. The resultant supernatant was filtered three times through a short column containing 60-200 mesh silica gel (Aldrich). The silica gel was washed with 2 × 100 mL portions of anhydrous ether. Ether was removed from the combined aliquots by rotary evaporation to obtain the pure aldehyde. The identity of the desired 6-chlorohexanal was confirmed by 1H NMR: (CDCl3) δ 9.8 (s, aldehyde), 3.5 (t, CH2Cl), 2.5 (t, CH2COH), 1.8 (q, CH2), 1.7 (q, CH2), 1.5 (q, CH2). In the second step, meso-tetrakis(5-chloropentyl)porphyrin (TCPP) was synthesized according to the literature.14 Montmorillonite K10 (Aldrich) (15.0 g) was predried at 120 °C and 0.1 Torr for 3 h and then placed in a 2000 mL Erlenmeyer flask. Methylene chloride (EM Science, dried over molecular sieve 4A) (1450 mL) was added to the flask with a solution of 2.03 g (15 mmol) of 6-chlorohexanal in 50 mL of methylene chloride, and 1.00 g (15 mmol) of neat pyrrole (Aldrich) was added
Flexible Metal Porphyrin Complexes
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TABLE 1: TMAP Content and Nitrogen Adsorption Data of Surfactant-Extracted Samples TMAP-Cl/SiO2 BET surface pore (mg/g in synthesis TMAP-Cl/MCM-41 BJH pore or ion exchange) (mg/g in product) volume (cm3/g) area (m2/g) diameter (Å)
sample
description
MCM-41-1 MCM-A1 MCM-A1a MCM-I1 MCM-41-2 MCM-C2 MCM-I2 MCM-J2
synthesis procedure 1 TMAP-Cl incorporated during synthesis TMAP-Cl incorporated during synthesis TMAP added by ion exchange of MCM-41-1 synthesis procedure 2 TMAP-Cl incorporated during synthesis TMAP added by ion exchange of MCM-41-2 TMAP added by ion exchange of extracted MCM-41-2 TMAP added by ion exchange of extracted MCM-41-2
MCM-J2a
0 51 188 39 0 18 68 52
0 3 7 35 0 11 56 51
10
2
1.27 0.91 0.83 0.42 0.63 0.50
1520 1230 1070 560 1120 1020
24 22 23 19 (broad) 22 21
TABLE 2: UV-Vis Absorption Maxima of Soret Band and Q-Bands wavelengths of absorption peaks/nma
sample
description
TMAP-Cl TMAP-Cl/DTABr TMAP-Cl/DTABr TMAP-Cl/DTABr TMAP-H22+ TMAP-H22+/DTABr TMAP-Cu2+ TCPP
aqueous solution aqueous solution, porphyrin/surfactant ratio of 2.4 × 10-3 aqueous solution, porphyrin/surfactant ratio of 1.1 × 10-4 33% of water removed from above solution aqueous solution aqueous solution, porphyrin/surfactant ratio of 2.4 × 10-3 aqueous solution solution in chloroform, porphyrin without trimethylammonium groups TMAP-Cl incorporated during synthesis by procedure 2 MCM-C2 converted to dication by acid extraction of surfactant MCM-H2 washed with excess of 0.2 M aq. NaOH to regenerate free base TMAP added by ion exchange of MCM-41-2, higher TMAP loading TMAP added by ion exchange of MCM-41-2, lower TMAP loading TMAP added by ion exchange of extracted MCM-41-2, higher TMAP loading TMAP added by ion exchange of extracted MCM-41-2, lower TMAP loading back-exchange of DTABr into MCM-J2a, ca. 60% removal of TMAP surfactant removed by acid extraction of MCM-I2 TCPP added to extracted MCM-41-2 Cu2+ exchanged into MCM-H2 Cu2+ exchanged into MCM-I2X Ni2+ exchanged into MCM-H2 Fe2+ exchanged into MCM-H2
MCM-C2 MCM-H2 MCM-H2B MCM-I2 MCM-I2a MCM-J2 MCM-J2a MCM-S2 MCM-I2X MCM-41-2X/TCPP MCM-Cu2 MCM-I2XCu MCM-Ni2 MCM-Fe2
color
412, 520, 558, 588, 642 412, 520, 558, 592, 642 414, 520, 556, 590, 646 416, 518, 556, 592, 628, 652 418, 580, 626 418, 578, 626 412, 542, 580 420, 520, 556, 600, 656
f f f f d d m f
maroon maroon maroon maroon green green rose purple
418, 520, 554, 598, 656 420, 580, 628 414, 524, 558, 592, 650
f d f
brown green brown
416 (br), 520, 558, 596, 654 416, 520, 554, 598, 656 414 (br), 520, 556, 596, 650
f f f
brown brown brown
415, 520, 556, 592, 650
f
brown
418, 520, 554, 598, 656
f
brown
420, 582, 628 418 (br), 582 408, 542, 580 408, 542, 580 412, 532, 580 412, 516, 582
d f m m m m
green maroon pink pink brown orange
a UV-vis transmission spectra for solutions, diffuse reflectance spectra for solid samples. Notations “f”, “d”, and “m” refer to the free base, dication, and metal-exchanged porphyrins, respectively.
successively. The suspended mixture was stirred for 1 h. Solid tetrachloro-1,4-benzoquinone (Aldrich) (2.83 g (11.5 mmol)) was then added, and the mixture was refluxed at 50 °C for 1 h. The solution was filtered to remove solid materials and washed with 400 mL of ethyl acetate (Fisher Scientific). The combined filtrate was condensed by rotary evaporation and adsorbed on 10 g of Florosil (Aldrich). The adsorbate was placed on top of a 300 g alumina column (aluminum oxide, activated, basic, Brockmann I (Aldrich)) and developed with a hexane (EM Science)/methylene chloride (1:1) mixture. This purification step was repeated. UV-vis and 1H NMR spectra agreed with literature values. 1H NMR (CDCl3): δ -2.70 (s, 2H, NH), 1.95 (m, 16H), 2.54 (m, 8H), 3.57 (t, 8H), 4.94 (t, 8H), 9.47 (s, 8H). The UV-vis absorption peaks are listed in Table 2. The third step involved substitution of alkylammonium groups at the halide positions of meso-tetrakis(5-chloropentyl)porphyrin.14 A mixture of 0.80 g (1.1 mmol) meso-tetrakis(5chloropentyl)porphyrin, 10 mL of toluene (Aldrich), and 15 mL (59 mmol) of a 25% aqueous solution of trimethylamine (Aldrich) was stirred in an autoclave at 90 °C for 2 days. Excess trimethylamine was removed by evaporation. The aqueous phase was then washed with methylene chloride to remove any
excess unreacted porphyrin. The crude TMAP-Cl was recrystallized by dissolving it in 1 mL of methanol (Aldrich) for every 100 mg of product and then adding 15 mL of diethyl ether for every 1 mL of methanol. UV-vis and 1H NMR spectra agreed with literature values. 1H NMR (DMSO-d6): δ 1.82 (m, 16H), 2.48 (m, 8H), 3.03 (s, 36H), 3.36 (br), 5.02 (br, 8H), 9.74 (m, 8H). The UV-vis absorption peaks are listed in Table 2. Synthesis of MCM-41 and Encapsulated TMAP. MCM41 was prepared by two different methods, using either sodium silicate or tetraethyl orthosilicate (TEOS) as precursors. Procedure 1. In a typical synthesis by the first method (samples MCM-41-1, MCM-A1, MCM-A1a),21 1.29 g of dodecyltrimethylammonium bromide (DTABr, TCI) was dissolved in 8.8 mL of deionized water in a polyethylene bottle, along with a given amount of TMAP-Cl (27-100 mg). A 1.85 g sample of sodium silicate (N-brand, PQ Corp., SiO2 ) 28.74 wt %) and 0.50 g of 10 wt % H2SO4 (Mallinckrodt) were added to this solution. After 30 min of stirring, the pH was lowered to 10 using 50 wt % H2SO4 (ca. 0.020 g). The final composition of the synthesis mixture was 2.12 SiO2:1.0 DTABr:0.00670.0250 TMAP-Cl:1.27 Na+:138 H2O. The mixture was heated in an autoclave at 100 °C for 2 days. The resulting brown
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precipitate was filtered, washed thoroughly with deionized water, and air dried. Procedure 2. In a typical synthesis by the second method (samples MCM-41-2, MCM-C2), 2.78 g (9.0 mmol) of DTABr and 0.0220 g of TMAP-Cl were dissolved in 40 mL of deionized water in a polyethylene bottle. Subsequently, 2.0 g of a 1 M tetrapropylammonium (TPA+) hydroxide solution (Aldrich) and 4.17 g (20 mmol) of TEOS (Aldrich) were added, and the resultant solution was stirred at 60-70 °C (final composition of synthesis mixture: 2.22 SiO2:1.0 DTABr:0.0025 TMAP-Cl: 0.22 TPA+:259 H2O). A brown precipitate formed in ca. 10 min. After 3 h of reaction, the resultant material was filtered, washed thoroughly with deionized water, and air dried. Extraction of Surfactant. Surfactant molecules were removed from the mesoporous materials by H+ extraction using 50 mL of a 0.4 wt % HCl/ethanol solution for every 1.0 g of material. The resulting suspension was refluxed for 24 h, filtered, and then air dried. All extracted materials turned green, typical of the dication form TMAP-H22+. TMAP Ion Exchange in MCM-41. Surfactant T TMAP ion exchanges were carried out with varying ratios of TMAPCl to nonextracted or extracted MCM-41. In a typical procedure, a suspension containing 0.737 g of MCM-41 (synthesized as above, but without porphyrin) and 50 mg (5.2 × 10-5 mol) TMAP-Cl in 20 mL of deionized water was stirred for 12 h at room temperature, then filtered, and washed well with deionized water. Exchange conditions for other samples are listed below: sample
mass MCM-41/g
extracted
mass TMAP-Cl/mg
exchange time/h
MCM-I1 MCM-I2 MCM-J2 MCM-I2a MCM-J2a
0.517 0.737 0.116 0.209 0.203
no no yes no yes
20 50 6 1 2
12 12 11 14.5 3
Back-Exchange of Surfactant. The feasibility of backextracting TMAP-Cl with cationic surfactant was examined by stirring 0.14 g of sample MCM-J2a in a solution containing 0.20 g of DTABr in 20 mL of water for 15 h at room temperature. The solid was collected by filtration and washed well with deionized water. Sixty percent of the TMAP-Cl was removed by exchange with DTABr (sample MCM-S2). Concentration Dependence Studies. The effects of TMAPCl concentration and surfactant-TMAP-Cl interactions on optical spectra were studied using a solution that originally contained 2.84 g (9.2 mmol) of DTABr and 1 mg (1.0 × 10-6 mol) of TMAP-Cl in 40 mL of deionized water. UV-vis spectra were obtained from this solution and after 33%, 60%, and 100% removal of water by evaporation at 70 °C. Metal Exchange. Metal ions were introduced into the TMAP-H22+-modified (surfactant-extracted) MCM-41 according to typical literature procedures for porphyrins.22 A mole ratio of ca. 30:1 metal acetate to porphyrin was used. The metal acetate was dissolved in a minimal amount of aqueous acetic acid (4:1 (v:v) ratio of acetic acid to water) required for complete dissolution. TMAP-H22+/MCM-41 material was then added and the suspension refluxed for 10 min with Ni(OAc)2 or Cu(OAc)2, or for 15 min with Fe(OAc)2. The suspension was filtered, washed with deionized water, and dried before catalysis experiments. Completeness of exchange was confirmed by the absence of UV-vis absorptions associated with TMAP-H22+. Chemical Analysis. The amount of porphyrin inside MCM41 was determined by dissolving 15-25 mg of material in an aqueous solution containing 36.3 wt % HCl and 0.9 wt % HF, and comparing the optical absorbance at λmax ) 420 nm to a
calibration curve of standards containing TMAP-Cl in the same acid mixture. Copper analyses were carried out by the Geochemical Lab, University of Minnesota, Minneapolis, MN. Catalysis. The oxidation of the azo dye β-naphthol violet (4-NO2C6H4N:N-4-C10H4-3-OH-2,7-(SO3Na)2) was carried out in an aqueous solution containing 8 mL of 30% H2O2, (1.62.8) × 10-6 mol β-naphthol violet, and the mesoporous sieve containing (0-2.8) × 10-7 mol TMAP-Cu2+ in a total volume of 80.0 mL. A TMAP-free MCM-41-2 sample (0.126 g) treated with 0.040 g Cu(OAc)2, as described above, was used as the catalyst in a control experiment. The mixture was stirred at 66 °C, and absorbance changes were monitored by UV-vis spectroscopy. Product Characterization. X-ray powder diffraction patterns were obtained using the Siemens D5005 wide angle XRD with Cu KR radiation. N2 adsorption measurements were carried out with a Micromeritics ASAP 2000 sorption analyzer. Solution 1H NMR was performed on a Varian VXR-300 spectrometer (300 MHz). Chemical shifts are given in ppm downfield referenced to tetramethylsilane. 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 KubelkaMunk equation. Results Porphyrin Incorporation. The meso-substituted porphyrin with four cationic tetraalkyl sidearms had been selected for its solubility in water during a hydrothermal synthesis, its comparable size to mesoporous channel systems, and the partial isolation that the sidearms provide between the porphyrin ring and adjacent species, such as a silicate wall. Cationic, anionic, and nonionic surfactants have previously been used to solubilize porphyrins in water. The porphyrins can be dispersed molecularly by intercalation in the micelle.23-25 In studies of bilayered cationic surfactant systems mixed with porphyrins it has been found that a single positive charge on a porphyrin sidearm is sufficient to move the porphin ring toward the aqueous phase, while hydrophobic alkyl side chains remain situated within the surfactant bilayers.26 By analogy one can expect intercalation of TMAP within cylindrical DTABr micelles and an interaction of the four cationic sidearms (as well as the cationic surfactant headgroups) with the silicate precursors during the hydrothermal synthesis. Table 1 lists the amounts of TMAP-Cl used in the MCM-41 synthesis and the amounts present in the final product after acid extraction of the surfactant. The relative porphyrin loadings in the products are comparable to those in other studies involving clays or MCM-41 as hosts.15,27 In synthesis procedure 1, based on sodium silicate precursors, only a small fraction of the porphyrin was included in the mesoporous sieve during the hydrothermal synthesis. The large excess of surfactant in the competition between DTA+ and TMAP for anionic silicate species lowered the amount of TMAP that was incorporated. The loading could be increased slightly by increasing the porphyrin concentration in the synthesis mixture. A larger fraction of porphyrin was incorporated in the mesoporous sieve by synthesis procedure 2, which was based on TEOS as a precursor and involved a much shorter synthesis time. In contrast, the direct encapsulation of a neutral porphyrin in MCM-41 in a previous study was unsuccessful.16
Flexible Metal Porphyrin Complexes
Figure 2. Powder X-ray diffraction patterns of (a) MCM-41-1, (b) MCM-A1, (c) MCM-A1a, (d) MCM-41-2, and (e) MCM-C2.
Incorporation of the porphyrin was also possible by ionexchange reactions. When TMAP-Cl was added to the assynthesized MCM-41 or to the acid-extracted MCM-41, the cationically substituted porphyrin displaced the cationic surfactant molecules (MCM-I2, MCM-I2a) or protons (MCM-J2, MCM-J2a). In these processes, most of the porphyrin present was taken up by the mesoporous sieve, resulting in larger porphyrin loadings than in the direct synthesis. To maintain charge balance, each TMAP molecule could displace four DTABr molecules or protons. The exchange occurred despite the relatively large size of these molecules. A reintroduction of surfactant to the ion-exchanged samples led to a 60% decrease in the TMAP content. Acid extraction, on the other hand, resulted in removal of surfactant but not any significant amount of TMAP (based on UV-vis analysis of the extract). Powder XRD Data. Figure 2 shows powder XRD patterns of the extracted mesoporous sieves with various loadings of TMAP-Cl incorporated during the hydrothermal synthesis. In comparison to pure MCM-41 prepared by synthesis procedure 1, the low-angle diffraction lines were broadened when TMAPCl was present. It was demonstrated previously that product powder XRD peaks broaden when organic auxiliary components, such as normal alkanes or 1,3,5-trimethylbenzene, are added to surfactant molecules in the synthesis of MCM-41.28,29 At a low porphyrin loading (MCM-A1), d100, d110, and d200 reflections were still resolved at d spacings of 34.8, 20.0, and 17.3 Å, respectively. As the porphyrin concentration increased (MCM-A1a), the d110 and d200 lines merged into one broader peak. In contrast, when TMAP-Cl was added to MCM-41 by ion exchange (sample MCM-I1), the powder XRD pattern was nearly identical to that of the MCM-41 precursor, showing three distinct d100, d110, and d200 peaks. The d100 peak became slightly less intense with little change in the rest of the powder pattern. Synthesis by the much shorter procedure 2 typically resulted in a lower degree of hexagonal order. A significantly broadened d100 peak together with a weak and broad peak at a slightly higher angle were observed. The appearance of the powder pattern did not change upon incorporation of the porphyrin. Nitrogen Adsorption Analyses. Nitrogen adsorption measurements of the acid-extracted samples (porphyrin in the TMAP-H22+ form; see below) yielded type IV isotherms, typical
J. Phys. Chem. B, Vol. 102, No. 22, 1998 4305 of mesoporous materials, even when TMAP was included during the synthesis. As the loading of porphyrin increased, both the pore volumes and BET surface areas decreased (Table 1). These data are consistent with inclusion of the porphyrin within the channels. The acid-extracted, TMAP-ion-exchanged MCM-41 (sample MCM-I1) produced a type I isotherm, which is associated with narrower pores.30 The pore volume and surface area of this sample decreased to ca. one-third of their values in the starting material. Even though adsorption of the porphyrin on the external surface of the mesoporous support could not be precluded, the dramatic decrease in pore volume indicated that a large amount of TMAP was present in the pores of this sample. If TMAP had been attached only on the outside of MCM-41, one would expect to retain a similar pore volume and type IV isotherm as observed for the unexchanged MCM-41. The isotherm showed no indication of bottlenecks due to partially covered or plugged pore openings. UV-Vis Analyses. Porphyrins exhibit characteristic UVvis spectra whose features have been studied extensively.31 The absorptions arise from π f π* transitions in the conjugated π-electron system of the porphyrin ring. Typical UV-vis spectra consist of two regions, the Soret band in the UV range from ca. 400-450 nm and the less intense Q-bands in the visible range between 450 and 700 nm. Depending on the symmetry of the porphyrin, the intense Soret band encompasses one or more allowed electronic transitions from the nearly degenerate a1u(π) and a2u(π) molecular orbitals to the lowest empty eg*(π*) orbital.32 These transitions are often not resolved. The Q-bands follow a “ladder” pattern, their relative intensities depending on the substitution and symmetry of the porphyrin ring, as well as on the presence of metal ions in the ring. The band positions and intensities can exhibit a weak dependence on the environment (solvent, host matrix) of the porphyrin. Absorption maxima for all samples in this study are listed in Table 2. The solution UV-vis spectra of the meso-substituted porphyrins, TCPP in chloroform, as well as TMAP-Cl, TMAPH22+, and TMAP-Cu2+ in water are consistent with literature reports.14,33 The maroon-colored free-base TMAP has D2h symmetry and is characterized by four quasi-allowed absorptions in the Q-band region. In order of increasing wavelength, these have been assigned as bands IV, III, II, and I. Bands I and III correspond to individual electronic transitions (A1g f B3u, A1g f B2u); bands II and IV are the vibronic overtones. The order of relative intensities (IV > III > I > II) is common in porphyrins with alkyl group substitution at the methine positions.34 The free base is converted to the green-colored dication TMAP-H22+ by acid treatment. After the increase in symmetry to D4h only two peaks are observed in the Q-band region. The absorption at shorter wavelength (II) corresponds to the vibronic transitions IV and II under D2h symmetry, while the absorption at longer wavelength (I) has been assigned to the A1g f Eu transition, related to bands III and I under D2h. Discussion Direct Occlusion of Free-Base TMAP during MCM-41 Synthesis. Figure 3 shows a comparison of UV-vis spectra for TMAP-Cl in aqueous solution and sample MCM-C2 in which TMAP was incorporated during the synthesis. (Spectra for MCM-A1 and MCM-A1a were identical to the spectrum of MCM-C2.) The Soret band in MCM-C2 underwent a 6 nm redshift (an energy difference of 350 cm-1), and the splittings between bands III and I, as well as between IV and II, increased by small amounts. However, the relative intensity patterns and in particular the peak widths remained nearly unchanged
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Figure 3. (a) UV-vis spectrum of an aqueous solution of TMAP and (b) DR UV-vis spectrum of MCM-C2. Insets: absorbance axis expanded 5-fold.
compared to the solution spectrum. Previous studies of porphyrins trapped between clay layers or within zeolite cages showed significant increases in the width of the Soret band absorption.10,11,13 The sharpness of the Soret band in MCMC2 suggests that optical electrons in the porphyrin ring experience little perturbation by the surroundings. The porphyrin ring appears to be more isolated within the structural support of the mesoporous sieve than porphyrins that have been described previously in clays, zeolites, and MCM-41. Band broadening and a decrease in the intensity of the Soret band can be caused by several factors. Inhomogeneous broadening can arise from variations in the microenvironment if a strong field is present or if sufficient electronic interactions exist between porphyrin and surroundings (solvent, solid host). Confinement in an inorganic host material can change the electron density of the conjugated π-orbitals of the central ring structure. Distortions would lower the ring symmetry and result in greater splitting of the excitations that contribute to the Soret band. All of these sources are likely to contribute to the width of Soret bands observed for clay-intercalated porphyrins or for porphyrins confined in zeolite cages.9,26,35,36 Dimerization or association of larger porphyrin aggregates can increase the number of electronic transitions related to the Soret band. This factor would play a role for porphyrins adsorbed on silica or MCM-41, especially in a polar solvent, where association is more likely. At the higher temperature of the hydrothermal synthesis, monomers are expected to dominate, especially with the large cationic substituents in TMAP.37 To probe the environment and structural arrangement of the porphyrin in our mesoporous sieves further, we studied the peak broadening under a variety of variables designed to examine interactions with the silicate wall, with surfactant molecules, with solvent molecules, and with other porphyrins. Interactions with the Silicate Wall. Based on the nitrogen adsorption data for MCM-A1, MCM-A1a, and MCM-C2, TMAP molecules were present within the channels. It was expected that the alkyl arms could maintain a reasonable separation between the porphin ring and the silicate walls in the 2 nm wide channels, minimizing electronic interactions of the porphine center with the walls. To examine such electronic interactions, we compared optical spectra of MCM-C2 with a system where spatial proximity between silicate and the porphyrin ring was more likely, i.e., where TMAP was trapped in denser, amorphous silica. TMAP-Cl was added to an aqueous mixture of TEOS and TPAOH similar to synthesis procedure
Figure 4. DR UV-vis spectra of (a) MCM-I2, (b) MCM-I2a, (c) MCM-J2, (d) MCM-J2a, and (e) MCM-S2. Insets: absorbance axis expanded 5-fold.
2, but without any surfactant template. Protonation of the free base in this product to form the dication was unsuccessful, indicating that the porphine center was inaccessible to protons. The UV-vis spectrum of the amorphous product exhibited absorptions in the same positions as for MCM-C2; however, the Soret band was significantly weaker and broadened. These data suggest that spatial separation between the ring and the silica wall can affect the width of the Soret band. Interactions with Surfactant Molecules. The synthesis of MCM-41 type mesoporous sieves involves the cooperative assembly of silicate building blocks with surfactant micelles. The peripheral structure of the substituted porphyrin TMAP resembles the cross section of a surfactant micelle to some degree; cationic alkylammonium groups form a hydrophilic environment around a hydrophobic core. The diameters of TMAP and DTABr micelles are also similar. One could therefore envision the porphyrin as being in a confined liquid crystalline state, as it is incorporated within the micelle structure during the formation of MCM-C2. To investigate the effect of the surfactant on the optical spectra of TMAP, we studied spectra of an aqueous solution with similar surfactant and TMAP-Cl concentrations as those used in the hydrothermal synthesis procedure 2. The original solution displayed a slightly broadened Soret band that was red-shifted to 414 nm compared to pure TMAP-Cl in solution (412 nm). The Q-bands showed little change except for a red-shift of band I from 642 nm in pure water to 646 nm. The Soret band shifted progressively to the red as the surfactant and porphyrin concentrations were increased by water removal. The position of the Soret band after complete removal of water coincided with that in MCMC2 (418 nm), but the band was much weaker and broader than in the mesoporous sieve. Q-bands II and I were also shifted compared to MCM-C2. These data indicated that the surfactant had a small influence on the porphyrin spectrum, and it did not prevent broadening of the Soret band. The low porphyrin concentrations initially favored the porphyrin monomer. However, as solvent was removed from the TMAP-Cl/DTABr mixture, dimerization and further aggregation of porphyrin
Flexible Metal Porphyrin Complexes molecules was likely and could contribute to the broadening effect. Aggregation of porphyrins results in strong excitation coupling and causes a spectral shift of the Soret band.38 Interactions with Other Porphyrin Molecules. As previously noted, larger amounts of TMAP could be introduced into the mesoporous sieve by ion exchange than by the direct synthesis. For these samples, it was possible that some porphyrin covered the external surface of the mesoporous sieves, although nitrogen adsorption data indicated that a significant amount also filled the channel space. Since water was used as a solvent, association of porphyrin molecules at the silicate surface was very likely. Figure 4 shows UV-vis absorption spectra of the ion-exchanged materials. The peak positions in the Q-band region were similar to those of the directly synthesized MCMC2. (Band I appeared at 656 nm if surfactant was present and at 650 nm without surfactant.) The maximum of the Soret band depended on the presence of surfactant. In samples that had been extracted before the ion exchange the maximum occurred closer to that of TMAP in solution (412 nm); when surfactant templates were present, it shifted toward the red (up to 418 nm). The interaction between TMAP and the MCM-41 support was surprisingly strong, even when the porphyrin was added by ion exchange. Unlike in clays, Cu2+ addition was possible without loss of the metalloporphyrin. However, back-exchange of surfactant into the template-free materials removed 60% of the TMAP and also resulted in a red-shift to 418 nm (Figure 4e). At this stage the spectrum resembled that of MCM-C2 containing TMAP and surfactant from the original synthesis. These observations are consistent with our TMAP/surfactant studies described above. At low TMAP concentrations the Soret bands were only slightly broader than in MCM-C2. They broadened significantly as the porphyrin loading increased, particularly on the high-energy sides of the bands, and the peak intensity was reduced. Similarly broad Soret bands were observed in MCM-41 loaded with meso-tetraphenylporphyrin by adsorption from a chloroform solution15 and in clays containing microcrystalline aggregates of metalated porphyrins.12 These results suggest a direct relationship between porphyrin concentration (and probably dimerization or aggregation)37 and the width of the Soret band. The UV-vis data therefore indicate that most of the TMAP molecules were isolated from each other within the channels of sample MCM-C2. At the temperature of the hydrothermal synthesis of MCM-C2, porphyrin molecules were expected to be present as monomers, even if they formed dimers or aggregates before the reaction.34 The Dication TMAP-H22+. Extraction of the surfactant templates was carried out using an HCl/ethanol mixture. While the surfactant was removed by this procedure (based on FT-IR and 13C solid-state NMR analyses), very little porphyrin was lost to the extract (UV-vis analysis). Under these acidic conditions the free-base porphyrin in MCM-C2 was completely converted to the green dication TMAP-H22+. The UV-vis spectrum of the product (MCM-H2) closely resembled the solution spectrum, except for a slight red-shift of the Soret band (from 418 to 420 nm) and of the lowest wavelength Q-band I (from 626 to 628 nm), Figure 5. Because of the increase in symmetry from D2h to D4h, only two absorptions occur in the visible region. The free-base spectrum (unperturbed by surfactant) could be regenerated by washing MCM-H2 with dilute NaOH solution. The UV-vis spectrum of a sample formed by acid extraction of TMAP-ion-exchanged MCM-41 was similar to that of MCM-H2, with only a slight decrease in intensity of the Soret band and little broadening. Electrostatic
J. Phys. Chem. B, Vol. 102, No. 22, 1998 4307
Figure 5. UV-vis spectrum of an aqueous solution of (a) TMAPH22+, and DR-UV spectra of (b) MCM-H2 and (c) MCM-I2X.
repulsions between the charged cores prevent agglomeration of dications and of metal-exchanged porphyrins.37 Metal-Exchanged Porphyrins. Metalloporphyrins and related metal-substituted species play an important role as catalysts in biological and other reactions. In this study, treatment of MCM-H2 with the acetates of Cu2+, Ni2+, and Fe2+ resulted in occluded metalloporphyrins within the mesoporous silicate structure. In other studies involving intercalation of porphyrins in clays or MCM-41, the porphyrin leached out of the support after metal incorporation.9 Exchange of free-base porphyrins into transition metal ion-exchanged clays typically resulted in the formation of dications, rather than metalloporphyrins on the clay surface.9 Chemical analysis of the TMAP-containing mesoporous sieves showed that the metals did not displace the porphyrins from the support. A strong affinity of the porphyrins for the silica support was observed when TMAP was included in the hydrothermal reactions, as well as when it was introduced by ion exchange. The UV-vis spectrum of MCM-Cu2 (Figure 6) was nearly identical to that of TMAP-Cu2+ in solution. The relative intensities of Q-bands II and I were switched compared to the protonated dication, consistent with literature observations for solution spectra.33 In the series MCM-Cu2, MCM-Ni2, MCM-Fe2 the splitting between Q-bands increased from 1209 cm-1 to ca. 2200 cm-1 (peak II shifting to the blue) and all absorptions broadened, especially for the iron compound (see Figure 6). In the case of the iron porphyrin, the broadening was not caused by porphyrin association but by a distortion of the porphyrin ring and further lowering of symmetry upon incorporation of iron. A broadening and decrease in intensity of the Soret band has also been observed for iron porphyrin in solution. It has been suggested that the large size of the iron does not allow the metal to reside in the center of the porphin ring, causing broadening in the spectrum.33 Catalysis. The catalytic activity of copper-substituted TMAP was studied in the peroxidation reaction of the azo dye β-naphthol violet. In previous homogeneous catalysis reactions, metal porphyrin complexes were found to be active for oxidation reactions involving H2O2 as the oxygen donor.39 While the mechanism of oxidation of azo dyes is complex and not well understood,40 the reaction provides useful insight into the stabilization of TMAP by the mesoporous sieve. The reaction was followed spectroscopically by monitoring a decay of the
4308 J. Phys. Chem. B, Vol. 102, No. 22, 1998
Figure 6. UV-vis spectrum of an aqueous solution of (a) TMAPCu2+ and DR-UV spectra of (b) MCM-Cu2, (c) MCM-Ni2, and (d) MCM-Fe2. Inset: absorbance axis expanded 5-fold.
Figure 7. Temporal decay of the β-naphthol violet absorption at 510 nm in catalytic oxidation reactions. All solutions contained 2.8 µmol β-naphthol violet. The total content of TMAP-Cu2+ was 0.28 µmol for each run, except for the blank (9). The amounts of solid added were (9) 0.093 g MCM-41-2; (0) 0.116 g MCM-41-2, treated with Cu(OAc)2; (b) 0.010 g MCM-I2XCu (28.1 µmol TMAP-Cu2+/g MCM41); (O) 0.096 g of MCM-Cu2 (2.9 µmol TMAP-Cu2+/g MCM-41); (() aqueous TMAP-Cu2+ solution, no solid added. Other conditions are listed in the Experimental Section.
dye absorption at 510 nm, as the dye was cleaved at the azo group. Figure 7 shows the corresponding kinetic plots of absorbance versus time for mixtures containing 2.8 × 10-6 mol β-naphthol violet and 2.8 × 10-7 mol TMAP-Cu2+. The results followed similar patterns at other reagent and catalyst concentrations. In a blank run with MCM-41, H2O2, and dye, the absorbance remained nearly constant over a 2 h period. For the homogeneous solution reaction of unsupported TMAP-Cu2+ a large initial decrease was observed for the dye absorption at 520 nm within the first two minutes. Thereafter the decay continued gradually at a first-order rate with a half-life of 248 min. It should be noted that the porphyrin itself also decomposed during this reaction, following approximately a first-order decay (a good fit was also obtained for second order) with a half-life of 109 min. The faster rate, compared to dye decomposition, indicated that mutual oxidation must have occurred between TMAP-Cu2+ molecules. Destruction of a porphyrin catalyst via autoxidation has been reported in other solution studies involving bleaching of dyes.41 Results for an MCM-41 sample in which TMAP-Cu2+ was incorporated by ion exchange were similar, exhibiting a slightly lower conversion. After an initial rapid drop in absorbance, the rate decreased significantly. The lower catalytic activity compared to the porphyrin in solution may be due to poorer
Holland et al. mixing of dye and porphyrin molecules in the solid sample, as well as faster degradation of the porphyrin in the solid, where porphyrin molecules are closer to each other and can autoxidize more readily. Similar oxidative degradation has also been observed for MCM-41-immobilized metal-phthalocyanine complexes used in the oxidation of n-hexane with tert-butylhydroperoxide.18 In the present study, the porphyrin-exchanged MCM-41 continued to catalyze oxidation of the azo dye over a period of ca. 17 h, resulting in complete bleaching of the dye. These results indicate that diffusion of the dye to trapped porphyrin molecules in the mesoporous channels may be very slow in these highly loaded samples. Nitrogen adsorption measurements had suggested a large decrease in pore volume in these samples compared to the pure mesoporous support. Dye decomposition products may have blocked the pores further. Samples of the type MCM-Cu2, obtained from a mesoporous sieve in which TMAP was incorporated during the synthesis, behaved very differently from both the ion-exchanged porphyrinloaded sieves and the porphyrin solutions. In the presence of these samples, over 90% of the dye was oxidized within less than 2 h, even with a 10-fold excess of dye compared to the catalyst. The decay of the 510 nm dye absorption followed a first-order rate law with a half-life of 34 min (even shorter in other runs). The sample was reused in an additional catalysis run and continued to perform better than the unsupported catalyst in solution. Based on a UV-vis spectrum, no TMAP was observed in solution after the catalysis run. The copper concentration in solution was measured to be 165 ppb. To ensure that the uncomplexed copper was not responsible for the good catalytic performance, a control experiment was carried out with a copper-treated MCM-41 sample lacking the porphyrin. No significant activity for bleaching was observed in the control run (see Figure 7). These results demonstrate that the copper porphyrin remains active as a peroxidation catalyst longer when it is supported and stabilized by the mesoporous sieve. The stabilization may in part be due to isolation of the porphyrins from each other, in particular at the low loading (2.8 mg TMAP-Cl/g MCM-41) used in the catalysis samples. The separation of porphyrin molecules prevented mutual oxidation. Another result of low loading was that channels were sufficiently open to permit diffusion of the dye molecules (critical dimension ca. 12 Å) and their oxidation products. In a study involving ruthenium porphyrin encapsulated in MCM-41 a higher catalytic activity was also observed at low ruthenium content, which was ascribed to efficient site isolation and diffusion pathways.16 Conclusion This study has shown that the meso-tetraalkylammoniumsubstituted porphyrin TMAP could be incorporated in MCM41 during a direct synthesis, occupying the channels as well as the external surface of the support. The charge on the sidearms appeared to be important to permit a strong association of the porphyrin with the mesoporous support. Che et al. recently reported the stabilization of a ruthenium porphyrin in MCM41.16 In that study a direct encapsulation of the porphyrin with chlorophenyl arms was unsuccessful, and the porphyrin was attached to the aminosiloxane-modified silica surface via Ruamine ligand bonding. In the case of TMAP, the porphyrin/ MCM-41 interaction was strong enough that the porphyrin remained in the sieve even after surfactant extraction or metalation. While hindered diffusion of the large porphyrin through the channel may account for the strong association, it is interesting to note that even samples prepared by ion-exchange
Flexible Metal Porphyrin Complexes loading of TMAP did not lose significant amounts of porphyrin after metalation. In those samples one would expect a large fraction of TMAP to occupy the external surface of the MCM41 particles. The UV-vis absorption studies indicated that in samples where TMAP was incorporated in MCM-41 during the hydrothermal synthesis, the porphyrin molecules remained relatively isolated from each other. The spectral intensities and peak widths resembled those found in dilute porphyrin solutions, showing little perturbation by the host or by neighboring porphyrin molecules. The catalytic studies suggest that the porphyrin is most stabilized by the mesoporous support and exhibits the largest catalytic activity when TMAP molecules are present at low concentration. Under those conditions mutual oxidation is avoided and diffusion of reactants or products through the channels is more facile. Thus the TMAP/MCM41 system is a good biomimetic starting model for biological metalloporphyrin structures where active porphyrin sites are often isolated. Acknowledgment is made to 3M, Dupont, the David and Lucille Packard Foundation, the donors of the Petroleum Research Fund administered by the American Chemical Society, the National Science Foundation (DMR-9701507), and the Office of the Vice President for Research and Dean of the Graduate School of the University of Minnesota (U of M) for support of this research. B.H. thanks the Center for Interfacial Engineering at the U of M for a CIE-NSF graduate fellowship. C.W. was supported by the U of M NSF-REU and Lando summer research programs. References and Notes (1) Ogawa, M.; Kuroda, K. Chem. ReV. 1995, 95, 399-438. (2) White, R. E.; Coon, M. J. Annu. ReV. Biochem. 1980, 49, 315356. (3) Gunsalus, I. C.; Sligar, S. G. AdV. Enzymol. 1978, 47, 1-44. (4) Mansuy, D.; Battioni, P. In Bioinorganic Catalysis; Reedijk, J., Ed.; Marcel Dekker: New York, 1993; p 395. (5) Shilov, A. E. J. Mol. Catal. 1988, 47, 351-362. (6) Cook, B. R.; Reinert, T. J.; Suslick, K. S. J. Am. Chem. Soc. 1986, 108, 7281. (7) Herron, N.; Stucky, G. D.; Tolman, C. A. J. Chem. Soc., Chem. Commun. 1986, 1521-1522. (8) Balkus, K. J., Jr.; Gabrielov, A. G.; Bell, S. L.; Bedioui, F.; Roue´, L.; Devynck, J. Inorg. Chem. 1994, 33, 67-72. (9) Carrado, K. A.; Winans, R. E. Chem. Mater. 1990, 2, 328-335. (10) Cady, S. S.; Pinnavaia, T. J. Inorg. Chem. 1978, 17, 1501-1507. (11) Carrado, K. A.; Thiyagarajan, P.; Winans, R. E.; Botto, R. E. Inorg. Chem. 1991, 30, 794-799. (12) Bergaya, F.; van Damme, H. Geochim. Cosmochim. Acta 1982, 46, 349-360. (13) Nakamura, M.; Tatsumi, T.; Tominaga, H. Bull. Chem. Soc. Jpn. 1990, 63, 3334-3336.
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