- Email: [email protected]
Immobilization of metalloporphyrin complexes in molecular sieves and their catalytic activity
Catalysis Communications 6 (2005) 531–538 www.elsevier.com/locate/catcom
Immobilization of metalloporphyrin complexes in molecular sieves and their catalytic activity V. Radha Rani, M. Radha Kishan, S.J. Kulkarni *, K.V. Raghavan Catalysis Division, Indian Institute of Chemical Technology, Hyderabad 500007, India Received 4 December 2003; accepted 11 April 2005 Available online 20 June 2005
Abstract A series of Fe–porphyrin complexes have been prepared inside the pores of NaY and Al-MCM-41 by various synthesis procedures. The complexes were characterized by XRD, diffuse reflectance UV–vis spectroscopy, IR spectra and thermal analysis. The catalytic performance of the synthesized material was tested by carrying out the epoxidation of cyclohexene at room temperature. tert-Butyl hydroperoxide (TBHP) was found to be the most convenient oxidizing reagent. No leaching of the metalporphyrin or uncomplexed metal ions was observed. 2005 Elsevier B.V. All rights reserved.
1. Introduction The immobilization of homogeneous catalyst systems is an attractive challenge because it opens access to the preparation of new, environmentally friendly means of chemical synthesis. Although homogeneous catalysts show remarkable performance for a large variety of reactions, problems such as separation, recovery and recyclization of the soluble catalysts create the demand for heterogeneous catalysts with comparable performance. Various attempts toward the immobilization of complexes have been investigated previously [1] such as attaching to supporting materials by chemisorption [2,3], immobilization by steric hindrance in zeolitic microor mesopores [4–8] or supported liquid-phase catalysts [9]. HY and Al-MCM-41 type molecular sieves are suitable as carriers for transition metal complexes, offering new opportunities for the encapsulation of large catalyst species and for the catalytic conversion of substrates much larger than in common zeolites [10]. The applica*
Corresponding author. E-mail address: [email protected] (S.J. Kulkarni).
1566-7367/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.04.009
tion of such host/guest compounds as catalysts for oxidative reactions has been successfully tested [11–17]. In this paper, we have attempted to prepare a family of hybrid organic–inorganic catalysts by various methods, which feature dispersed catalytic sites inside ordered pore system having high surface area and larger pore diameters. Catalytic studies were carried out to test the activity of synthesized materials in the oxidation of cyclohexene with TBHP at room temperature. 2. Experimental All experiments were performed under inert gas atmosphere. Due to the high light sensitivity of the porphyrin complexes combined with the high surface area of the heterogeneous catalysts, the best results could be achieved only with freshly prepared materials. The HY zeolite (SiO2/Al2O3 = 5.2) was obtained from PQ Corporation, USA. 2.1. Synthesis of Al-MCM-41 For the synthesis of Al-MCM-41, the quaternary ammonium surfactant (C14H29)N(CH3)Br was added
532
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
under constant stirring to a solution of tetraethyl ammoniumhydroxide (TEAOH) and NaOH at ambient temperature. After 4 h colloidal silica was added dropwise over a period of 1 h, followed by vigorous stirring for 4 h. The Si/Al ratio of the gel was 40. Crystallization took place over 24 h at 105 C. The resulting white solid was washed with water and dried at 120 C. The template was removed by heating the material to 500 C for about 12 h. The Si/Al ratio of the Al-MCM-1 was 15. 2.2. Synthesis of meso-tetraphenyl porphyrin, tetrapyridyl porphyrin and other meso-substituted porphyrins As in general, the synthesis of porphyrins from pyrrole and benzaldehyde need acidic catalysts, we synthesized these tetrapyrrolic macrocycles over acidic zeolite molecular sieves under microwave irradiation in dry media. The reaction was carried out in a pyrex bottle, in which equimolar ratio of pyrrole and aldehyde were mixed with 0.5 g zeolite molecular sieves in appropriate solvent which was then evaporated. The bottle was closed with cotton plug. The mixture was then subjected to microwave irradiation for 12 min with intervals, in BPL domestic microwave oven, with microwave frequency of 2450 MHz and 1.2 kW. After the reaction, +ve catalyst was separated by filtration and washed thoroughly with 100 mL (5 · 20 mL) chloroform. Then, the solvent was removed under vacuum to have a viscous residue. Products were separated by column chromatography using silica (100–200 mesh size) with n-hexane as eluent. Thus, obtained porphyrin was characterized by UV–vis spectrometer, NMR and mass spectrometry. Quantification was done by CAMAG HPTLC system and compared with isolated yields. Commercially available meso-tetraphenyl porphyrin (TPP), meso-tetraphenyl porphyrin iron(III) complex (TPP-Fe), meso-tetrapyridyl porphyrin (TpyP), mesotetrakis(pentafluorophenyl) porphyrin iron(III) complex (TPFP-Fe) were obtained from Aldrich, USA.
5. Anchoring or grafting of complexes in the mesoporous zeolites. (1) Impregnation method. First, an ion exchange of commercially available HY and synthesized Al-MCM41 was done in aqueous solution of ferric nitrate, to have 1 wt% FeY and FeMCM-41. The mixture was stirred over for 4 h. Then, it was filtered and washed with distilled water until no color was found in the mother liquor. The reddish-brown solid (Fe) was first dried at room temperature and then in an oven at 100 C, for 12 h. (2) Flexible ligand method. Using the principle of the diffusion of ligands into an already metal exchanged zeolite pores, meso-tetraphenyl porphyrin (TPP) and 5,10,15,20-tetra-(4-pyridyl) porphyrin (TPyP) ligands which are sufficiently volatile and stable during the adsorption were employed as ligands and incorporated in FeY and FeMCM-41 molecular sieves. For this, the above-mentioned 1 wt% Fe exchanged molecular sieves were stirred with both the ligands in N2 atmosphere for 24 h under reflux conditions. The solvent employed was dichloromethane. The round bottom flask was covered with Al foil to exclude light. The resultant supernatant liquid was filtered and thoroughly washed with dichloromethane. The excess ligand and metal complexes present on the external surface was removed by soxhlet extraction with various solvents so as to avoid the possibility of diffusional constrains to the reactant molecules. (3) Template synthesis method (ship-in-bottle). When the ligand molecular dimensions are more than that of the pore sizes of the zeolites like HY, they cannot diffuse into the pores of zeolite. In such cases, template synthesis method is used in which the ligand itself is constructed inside the zeolite matrix. The molecules that constitute the ligand species (pyrrole and benzaldehyde) are then adsorbed into the FeY and FeMCM-41 zeolite matrix in inert atmosphere. The molecules form the ligands of interest, which then complexes with the metal ions present in the zeolite. The excess ligand precursors, the ligand present on the external surfaces and the com-
2.3. Immobilization of Fe–porphyrin complexes The basic approaches as well as the recently developed methods of entrapping and stabilizing the complexes inside the zeolites were used to prepare various catalysts, in order to screen the best possible catalytic system. Among the various methods employed to incorporate metal complexes inside the pores or cavities of zeolite are 1. 2. 3. 4.
Impregnation method. Flexible ligand method. Template synthesis method. Zeolite synthesis method.
Table 1 Synthesis of meso-tetraphenyl porphyrin (TPP) (1) over various molecular sieves under microwave irradiation S. no.
Catalyst
% Yield of m-TPP
Soret band (nm)
Q bands
1 2 3 4 5
Al-MCM-41 HY SAPO-5 HZSM-5(30) SiO2/Al2O2
23.5 4.99 0.5 28.0 1.54
417 418 418 416 417
515, 592 512, 512, 512,
548, 597, 645 592, 642 545, 592, 640 593
Catalyst weight, 0.5 g; microwave power, H1; time, 12 min; molar ratio of pyrrole: aldehyde, 1:1. Yield and selectivity are based on pyrrole. The catalyst was prepared by the template synthesis method.
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
533
Table 2 Synthesis of porphyrins over Al-MCM-41 molecular sieves: reactant variation S. no.
Reactants pyrrole+
% Conversion of pyrrole
% Yield of the product
Soret band
Q bands
1 2 3 4 5 6
Benzaldehyde Anisaldehyde Tolaldehyde 3,4,5-Trimethoxy benzaldehyde m-Nitro benzaldehyde 4-Pyridine carboxaldehyde
97.9 68.5 66.4 55.4 92.8 23.2
23.5 12.6 33.4 11.6 6.4 11.7
417 419 419 423 420 416
515, 519, 535, 602 601 512,
(1) (2) (3) (4) (5) (6)
548, 597, 645 598, 642 592
590
Catalyst, Al-MCM-41 (0.5 g); microwave power, H1; time, 12 min; molar ratio of pyrrole: aldehyde, 1:1. Yield is based on pyrrole. H-Al-MCM-41 was prepared by the template synthesis method. R
N R
N H
R
N H
N
N
N
N
N
H N
H N
R 1: R = H 4: R = 3,4,5 (OCH3) 2: R = p-OCH3 5: R = m-NO2 3: R = p-CH3
plex present on the external surface were removed by the soxhlet extraction. (4) Zeolite synthesis method. The excess ligand and uncomplexed metal ions are the major disadvantage of
N
N 6
the first three methods. In order to avoid this, the metal complexes as meso-tetraphenylporphyrin iron(III) chloride and 5,10,15,20-tetrakis(pentafluorophenyl) porphyrin iron(III) chloride complexes were encapsulated during
Fig. 1. (A) XRD patterns: (a) NaY, (b) 1 wt% FeY, (c) Fe–Porphyrin Y. (B) XRD patterns: (a) Al-MCM-41, (b) 1 wt% FeMCM-41, (c) Fe– Porphyrin MCM-41, (d) Fe–Porphyrin (anchored) MCM-41.
534
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
the synthesis of zeolite crystallization. The thermal analysis of these catalysts is given in Figs. 2(A) and (B). (5) Anchoring. In case of anchoring method, there is no need to construct metal complex in the cages like in flexible ligand or template synthesis method as mesopor˚ , direct ous materials having pore size more than 20 A encapsulation of metal complexes inside the mesoporous materials can be achieved. 2.4. Characterization X-ray diffraction patterns of powdered samples were obtained using diffractometer equipped with a rotating anode and Cu Ka radiation.
Chemical analysis was performed with inductively coupled plasma atomic emission spectroscopy (ICPAES). The thermogravimetric analysis was carried out in inert atmosphere with the heating rate was 10 C min 1, and a-Al2O3 was used as reference material. FTIR and DRS UV–vis spectrum was recorded using KBr pellets. A typical oxidation reaction involved the following procedure: to 25 ml of solvent 250 mg of catalyst was added followed by the addition of the cyclohexene. Then, the oxidant was added dropwise to the reaction mixture. The reaction mixture was stirred under N2 at room temperature and the catalytic products were analysed using a gas chromatograph equipped with SE-30 column NMR and mass spectra.
Fig. 2. (A) TGA-DTA: (a) Al-MCM-41, (b) Porphyrin complex, (c) Fe–Porphyrin–MCM-41 (flexible method), (d) 1 wt% FeMCM-41, (e) Fe– Porphyrin–MCM-41 (template method), (f) Fe–Porphyrin (during synthesis) MCM-41, (g) Fe–Porphyrin–MCM-41 (by anchoring method). (B) TGA-DTA: (a) NaY, (b) Porphyrin complex, (c) Porphyrin–Y, (d) 1 wt% FeY, (e) Fe–Porphyrin Y, (f) Fe–Porphyrin (during synthesis) Y.
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
535
Fig. 2 (continued)
3. Results and discussion In order to get a first impression of whether a particular zeolite type can be chosen as carrier system that would be a suitable host for encapsulating homogeneously active porphyrin complex, these macromolecules were synthesized over various types of zeolites and as can be seen from the Table 1. Among the various zeolites used, the porphyrin molecule could be obtained selectively over HY and Al-MCM-41. The presences of Soret band at 419 nm in the UV–vis spectra confirmed the formation of the macrocycle inside these two types of molecular sieves. Further, we have attempted to various substitute these porphyrin ligands at both meso- and b-positions, as a step towards stabilizing them from self-oxidation. Table 2 shows that meso-tetrakis(4-pyridyl) porphyrin and meso-tetrakis(4-methylphenyl) porphyrin could be obtained in high yields and selectivity over Al-MCM41. The immobilization of iron porphyrin complexes inside Al-MCM-41 and HY leads to strong interaction
of the complex in the mesoporous system of the carrier material. The carrier material turned greenish, indicating the homogeneous catalyst was loaded onto the support. It is assumed that the complex is adsorbed on the inner and outer surfaces of the Al-MCM-41 structure. The binding energy of this adsorption varies on the different reactive sites like Bro¨nsted and Lewis acid sites and silanol groups. Upon the extraction with methanol, which in contrast to nonpolar dichloromethane adsorbs strongly on the zeolite surface, the complex desorbs from silanol groups due to a competitive reaction with the polar alcohol. The amount of complex immobilized on the carrier was determined by elemental analysis. Iron contents vary between 0.02 and 0.07 mmol per g of zeolite depending on the amount of complex encapsulated. The X-ray diffraction pattern of the isolated materials showed a strong characteristic peaks corresponding to both Al-MCM-41 and Y-zeolite, indicating that the crystallographic structure of the carrier material remains unchanged during the immobilization procedure, Figs. 1(A) and (B).
536
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
Quantitative loading of the complex was demonstrated by thermal analysis. Thermogravimetric and differential scanning calorimetric (DSC) measurements show that the immobilized complex is stable up to 450 C when anchored in Al-MCM-41 whereas decomposes at around 350 C when encapsulated in HY zeolite (Figs. 2(A) and (B)). The first decomposition occurs at 100–130 C and is slightly endothermic. Oxidation decomposition of the fixed complex took place in two steps at 350 and 450 C. The loss of weight of 4.5 wt% caused by the burning of the complex is consistent with the content determined by chemical analysis. The infrared spectra shows no change of wave number but a decrease of intensity for the signal at 3740 cm 1 which is assigned to the stretching vibration of terminal silanol groups. The vibration bands of immobilized complex are similar to those in the solution of dichloromethane. However, these signals resulting from organic compounds are very weak and not characteristic enough to surely identify or resolve a structure. The diffuse reflectance spectra show that the complex when anchored in Al-MCM-41 is most stable as no
additional peaks at higher wavelength appear. A strong characteristic peak corresponding to Al-MCM-41 indicates that the structure of the carrier material remains unchanged during the immobilization procedure. All other methods of immobilization in HY zeolite basically resulted in some distortion of the complex, as seen in Figs. 3(A) and (B). Among the catalysts presented here, it could be seen that several forces could be involved in the bonding of the complex on the carrier material. Electrostatic interaction of the cationic complexes occurs with the anionic framework of the Al-MCM-41 structure. Direct bridging of the iron to surface oxygen of the zeolite walls has also been observed and could occur after cleavage of the complex during the reaction. 3.1. Catalytic tests Several ligands have been applied and the corresponding complexes have been tested for the immobilization. As a test reaction for the catalytic activity the epoxidation of cyclohexene was employed, where the
Fig. 3. (A) DRS UV-Vis: (a) NaY, (b) 1 wt% FeY, (c) Fe–Porphyrin Y (flexible method), (d) Fe–Porphyrin (template method) Y, (e) Fe–Porphyrin (during synthesis) Y. (B) DRS UV-Vis: (a) Al-MCM-41, (b) 1 wt% FeMCM-41, (c) Fe–Porphyrin–MCM-41 (flexible method), (d) Fe–Porphyrin– MCM-41 (template method), (e) Fe–Porphyrin (during synthesis) MCM-41, (f) Fe–Porphyrin–MCM-41 (modified with anchoring agent), (g) Fe– Porphyrin–MCM-41 (by anchoring method).
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
537
Table 3 Encapsulation of porphyrin complex in Y and Al-MCM-41: catalytic activity towards oxidation of cyclohexene S. no.
Catalyst
% Conversion of cyclohexene
Liquid product distribution (%) Epoxide
Cyclohexanol
Cyclohexenone
Cyclohexenol
Diol
Dione
Others
1
FeY FeMCM-41 FeY-A FeMCM-41-A FeY-B FeMCM-41-B FeY-C FeMCM-41-C MCM-41-D Y-E MCM-41-E MCM-41-F MCM-41-G
4.7 3.7 11 6.9 24.3 42.5 6.7 10.7 21.09 2.5 32.9 43.2 98.8
0.15 0.22 0.015 0.04 0.15 0.8 0.062 0.16 0.20 0.18 0.22 0.35 –
0.49 – 0.49 – – – 0.02 0.05 0.025 – 0.02 – –
1.53 1.59 2.46 3.27 5.6 10.6 2.14 2.44 10.2 0.76 18.0 10.0 59.4
0.6 0.04 0.07 0.08 0.3 0.04 0.08 0.06 0.08 0.03 0.01 – 6.1
0.21 0.25 – 0.12 0.24 3.7 0.54 – 0.76 0.29 0.49 1.3 0.16
1.71 1.56 7.9 3.35 10.6 26.6 3.78 7.87 9.63 1.23 13.8 20.38 24.4
0.01 0.04 0.07 0.04 7.41 0.76 0.08 0.12 0.195 0.01 0.36 11.17 8.74
2 3 4 5 6 7 8
A, meso-tetraphenyl porphyrin; B, 5,10,15,20-tetrapyridylporphyrin; C, pyrrole + benzaldehyde; D, meso-tetraphenylporphyrin iron(III) chloride; E, 5,10,15,20-tetrakis(pentafluorophenyl); F, 3-aminopropyl trimethoxysilane (APTMS) + meso-tetraphenylporphyrin iron(III) chloride; G, 3-aminopropyl trimethoxysilane (APTMS) + 5,10,15,20-tetrakis(pentafluorophenyl) porphyrin iron(III) chloride.
catalytic run was performed with cyclohexene in dichloromethane with 1:4 molar ratio of TBHP at room temperature for 8 h. No reaction took place in the blank test when the carrier Y and Al-MCM-41 itself was used as catalyst. The catalytic results of the immobilized iron complexes are depicted in Table 3. The complex when anchored on Al-MCM-41 (Entry 12 and 13) shows the best results with high selectivity of cyclohexenone, at around 99% conversion of cyclohexene. These catalysts can easily be recovered and reused without further treatment. The supported catalyst was recycled four times. After these consecutive runs a decrease in catalytic performance was observed. This phenomenon goes along with the formation of lumps of the catalyst. In order to prove that the reaction is catalyzed heterogeneously and to exclude the possibility of leaching and homogeneous catalysis, the reaction mixture was separated from the catalyst before complete conversion occurs. Oxidation of the reaction solution following filtration after 3 h does not give any further reaction. After 8 h the conversion of the filtered sample remains at around 26% whereas the original batch with anchored catalyst goes to complete conversion of cyclohexene. This test proves that no homogeneous catalysis took place. ICP-AES analysis of the filtered reaction solution showed traces of iron, silicon and aluminium. The relative amounts of this analysis correspond to the composition of the heterogeneous catalyst used. This indicates that this loss occurs by attrition of the AlMCM-41 and not leaching of the complex. 4. Conclusion Homogeneous iron porphyrin catalyst was heterogenized best in Al-MCM-41. The bonding forces could
be due to the ionic interaction of the cationic complex with the anionic host framework. A reduction of the weak acidic sites of Al-MCM-41 has also been observed. After heterogenation also the iron porphyrin complex were suitable for the oxidation of olefins. The complexes remain stable within the mesopores of the carrier under the reaction conditions. The catalyst can be recycled by filtration and no leaching of the homogeneous complex was observed.
Acknowledgment One of the authors, M.R.K. is thankful to CSIR, New Delhi for Senior Research Fellowship.
References [1] (a) A. Baiker, H.U. Blaser, in: G. Erstl, H. Kno¨zinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, vol. 5, VCH, Weinheim, 1997, p. 2422; (b) N. Herron, G.D. Stucky, C.A. Tolman, J. Chem. Soc. Chem. Commun. (1986) 1521. [2] A. Corma, M. Iglesias, F. Mohino, F. Sanchez, J. Organomet. Chem. 544 (1997) 147. [3] B. Pugin, J. Mol. Catal. A107 (1996) 273. [4] C. Heinrichs, W.F. Ho¨lderich, Catal. Lett. 58 (1999) 75. [5] W. Kahlen, H.H. Wagner, W.F. Ho¨lderich, Catal. Lett. 54 (1998) 58. [6] S.b. Ogunwumi, T. Bein, Chem. Commun. (1997) 901. [7] M.J. Sabater, A. Corma, A. Domenech, V. Fornes, H. Garcia, Chem. Commun. (1997) 1285. [8] C. Schuster, W.F. Ho¨lderich, Catal. Today 60 (2000) 193. [9] K.T. Wan, M.E. Davis, Nature 370 (1994) 449. [10] C. Huber, K. Moller, T. Bein, J. Chem. Soc., Chem. Commun. (1994) 2619. [11] E. Armengol, A. Corma, F. Vicente, H. Garcia, J. Primo, Appl. Catal. A 181 (1999) 305.
538
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
[12] S. Ernst, M. Selle, Microporous Mater. 27 (1999) 355. [13] L. Frunza, H. Kosslick, H. Lanesser, E. Ho¨ft, R. Fricke, J. Mol. Catal. A 123 (1997) 79. [14] G.-L. Kim, S.-H. Kim, Catal. Lett. 57 (1999) 139.
[15] S.-S. Kim, W. Zhang, T.J. Pinnavaia, Catal. Lett. 43 (1997) 149. [16] P. Piaggio, P. McMorn, C. Langham, D. Bethell, P.C. BulmanPage, F.E. Hancock, G.J. Hutchings, New J. Chem. (1998) 1167. [17] F. Bedioui, Coord. Chem. Rev. 144 (1995) 37.
Immobilization of metalloporphyrin complexes in molecular sieves and their catalytic activity V. Radha Rani, M. Radha Kishan, S.J. Kulkarni *, K.V. Raghavan Catalysis Division, Indian Institute of Chemical Technology, Hyderabad 500007, India Received 4 December 2003; accepted 11 April 2005 Available online 20 June 2005
Abstract A series of Fe–porphyrin complexes have been prepared inside the pores of NaY and Al-MCM-41 by various synthesis procedures. The complexes were characterized by XRD, diffuse reflectance UV–vis spectroscopy, IR spectra and thermal analysis. The catalytic performance of the synthesized material was tested by carrying out the epoxidation of cyclohexene at room temperature. tert-Butyl hydroperoxide (TBHP) was found to be the most convenient oxidizing reagent. No leaching of the metalporphyrin or uncomplexed metal ions was observed. 2005 Elsevier B.V. All rights reserved.
1. Introduction The immobilization of homogeneous catalyst systems is an attractive challenge because it opens access to the preparation of new, environmentally friendly means of chemical synthesis. Although homogeneous catalysts show remarkable performance for a large variety of reactions, problems such as separation, recovery and recyclization of the soluble catalysts create the demand for heterogeneous catalysts with comparable performance. Various attempts toward the immobilization of complexes have been investigated previously [1] such as attaching to supporting materials by chemisorption [2,3], immobilization by steric hindrance in zeolitic microor mesopores [4–8] or supported liquid-phase catalysts [9]. HY and Al-MCM-41 type molecular sieves are suitable as carriers for transition metal complexes, offering new opportunities for the encapsulation of large catalyst species and for the catalytic conversion of substrates much larger than in common zeolites [10]. The applica*
Corresponding author. E-mail address: [email protected] (S.J. Kulkarni).
1566-7367/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.04.009
tion of such host/guest compounds as catalysts for oxidative reactions has been successfully tested [11–17]. In this paper, we have attempted to prepare a family of hybrid organic–inorganic catalysts by various methods, which feature dispersed catalytic sites inside ordered pore system having high surface area and larger pore diameters. Catalytic studies were carried out to test the activity of synthesized materials in the oxidation of cyclohexene with TBHP at room temperature. 2. Experimental All experiments were performed under inert gas atmosphere. Due to the high light sensitivity of the porphyrin complexes combined with the high surface area of the heterogeneous catalysts, the best results could be achieved only with freshly prepared materials. The HY zeolite (SiO2/Al2O3 = 5.2) was obtained from PQ Corporation, USA. 2.1. Synthesis of Al-MCM-41 For the synthesis of Al-MCM-41, the quaternary ammonium surfactant (C14H29)N(CH3)Br was added
532
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
under constant stirring to a solution of tetraethyl ammoniumhydroxide (TEAOH) and NaOH at ambient temperature. After 4 h colloidal silica was added dropwise over a period of 1 h, followed by vigorous stirring for 4 h. The Si/Al ratio of the gel was 40. Crystallization took place over 24 h at 105 C. The resulting white solid was washed with water and dried at 120 C. The template was removed by heating the material to 500 C for about 12 h. The Si/Al ratio of the Al-MCM-1 was 15. 2.2. Synthesis of meso-tetraphenyl porphyrin, tetrapyridyl porphyrin and other meso-substituted porphyrins As in general, the synthesis of porphyrins from pyrrole and benzaldehyde need acidic catalysts, we synthesized these tetrapyrrolic macrocycles over acidic zeolite molecular sieves under microwave irradiation in dry media. The reaction was carried out in a pyrex bottle, in which equimolar ratio of pyrrole and aldehyde were mixed with 0.5 g zeolite molecular sieves in appropriate solvent which was then evaporated. The bottle was closed with cotton plug. The mixture was then subjected to microwave irradiation for 12 min with intervals, in BPL domestic microwave oven, with microwave frequency of 2450 MHz and 1.2 kW. After the reaction, +ve catalyst was separated by filtration and washed thoroughly with 100 mL (5 · 20 mL) chloroform. Then, the solvent was removed under vacuum to have a viscous residue. Products were separated by column chromatography using silica (100–200 mesh size) with n-hexane as eluent. Thus, obtained porphyrin was characterized by UV–vis spectrometer, NMR and mass spectrometry. Quantification was done by CAMAG HPTLC system and compared with isolated yields. Commercially available meso-tetraphenyl porphyrin (TPP), meso-tetraphenyl porphyrin iron(III) complex (TPP-Fe), meso-tetrapyridyl porphyrin (TpyP), mesotetrakis(pentafluorophenyl) porphyrin iron(III) complex (TPFP-Fe) were obtained from Aldrich, USA.
5. Anchoring or grafting of complexes in the mesoporous zeolites. (1) Impregnation method. First, an ion exchange of commercially available HY and synthesized Al-MCM41 was done in aqueous solution of ferric nitrate, to have 1 wt% FeY and FeMCM-41. The mixture was stirred over for 4 h. Then, it was filtered and washed with distilled water until no color was found in the mother liquor. The reddish-brown solid (Fe) was first dried at room temperature and then in an oven at 100 C, for 12 h. (2) Flexible ligand method. Using the principle of the diffusion of ligands into an already metal exchanged zeolite pores, meso-tetraphenyl porphyrin (TPP) and 5,10,15,20-tetra-(4-pyridyl) porphyrin (TPyP) ligands which are sufficiently volatile and stable during the adsorption were employed as ligands and incorporated in FeY and FeMCM-41 molecular sieves. For this, the above-mentioned 1 wt% Fe exchanged molecular sieves were stirred with both the ligands in N2 atmosphere for 24 h under reflux conditions. The solvent employed was dichloromethane. The round bottom flask was covered with Al foil to exclude light. The resultant supernatant liquid was filtered and thoroughly washed with dichloromethane. The excess ligand and metal complexes present on the external surface was removed by soxhlet extraction with various solvents so as to avoid the possibility of diffusional constrains to the reactant molecules. (3) Template synthesis method (ship-in-bottle). When the ligand molecular dimensions are more than that of the pore sizes of the zeolites like HY, they cannot diffuse into the pores of zeolite. In such cases, template synthesis method is used in which the ligand itself is constructed inside the zeolite matrix. The molecules that constitute the ligand species (pyrrole and benzaldehyde) are then adsorbed into the FeY and FeMCM-41 zeolite matrix in inert atmosphere. The molecules form the ligands of interest, which then complexes with the metal ions present in the zeolite. The excess ligand precursors, the ligand present on the external surfaces and the com-
2.3. Immobilization of Fe–porphyrin complexes The basic approaches as well as the recently developed methods of entrapping and stabilizing the complexes inside the zeolites were used to prepare various catalysts, in order to screen the best possible catalytic system. Among the various methods employed to incorporate metal complexes inside the pores or cavities of zeolite are 1. 2. 3. 4.
Impregnation method. Flexible ligand method. Template synthesis method. Zeolite synthesis method.
Table 1 Synthesis of meso-tetraphenyl porphyrin (TPP) (1) over various molecular sieves under microwave irradiation S. no.
Catalyst
% Yield of m-TPP
Soret band (nm)
Q bands
1 2 3 4 5
Al-MCM-41 HY SAPO-5 HZSM-5(30) SiO2/Al2O2
23.5 4.99 0.5 28.0 1.54
417 418 418 416 417
515, 592 512, 512, 512,
548, 597, 645 592, 642 545, 592, 640 593
Catalyst weight, 0.5 g; microwave power, H1; time, 12 min; molar ratio of pyrrole: aldehyde, 1:1. Yield and selectivity are based on pyrrole. The catalyst was prepared by the template synthesis method.
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
533
Table 2 Synthesis of porphyrins over Al-MCM-41 molecular sieves: reactant variation S. no.
Reactants pyrrole+
% Conversion of pyrrole
% Yield of the product
Soret band
Q bands
1 2 3 4 5 6
Benzaldehyde Anisaldehyde Tolaldehyde 3,4,5-Trimethoxy benzaldehyde m-Nitro benzaldehyde 4-Pyridine carboxaldehyde
97.9 68.5 66.4 55.4 92.8 23.2
23.5 12.6 33.4 11.6 6.4 11.7
417 419 419 423 420 416
515, 519, 535, 602 601 512,
(1) (2) (3) (4) (5) (6)
548, 597, 645 598, 642 592
590
Catalyst, Al-MCM-41 (0.5 g); microwave power, H1; time, 12 min; molar ratio of pyrrole: aldehyde, 1:1. Yield is based on pyrrole. H-Al-MCM-41 was prepared by the template synthesis method. R
N R
N H
R
N H
N
N
N
N
N
H N
H N
R 1: R = H 4: R = 3,4,5 (OCH3) 2: R = p-OCH3 5: R = m-NO2 3: R = p-CH3
plex present on the external surface were removed by the soxhlet extraction. (4) Zeolite synthesis method. The excess ligand and uncomplexed metal ions are the major disadvantage of
N
N 6
the first three methods. In order to avoid this, the metal complexes as meso-tetraphenylporphyrin iron(III) chloride and 5,10,15,20-tetrakis(pentafluorophenyl) porphyrin iron(III) chloride complexes were encapsulated during
Fig. 1. (A) XRD patterns: (a) NaY, (b) 1 wt% FeY, (c) Fe–Porphyrin Y. (B) XRD patterns: (a) Al-MCM-41, (b) 1 wt% FeMCM-41, (c) Fe– Porphyrin MCM-41, (d) Fe–Porphyrin (anchored) MCM-41.
534
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
the synthesis of zeolite crystallization. The thermal analysis of these catalysts is given in Figs. 2(A) and (B). (5) Anchoring. In case of anchoring method, there is no need to construct metal complex in the cages like in flexible ligand or template synthesis method as mesopor˚ , direct ous materials having pore size more than 20 A encapsulation of metal complexes inside the mesoporous materials can be achieved. 2.4. Characterization X-ray diffraction patterns of powdered samples were obtained using diffractometer equipped with a rotating anode and Cu Ka radiation.
Chemical analysis was performed with inductively coupled plasma atomic emission spectroscopy (ICPAES). The thermogravimetric analysis was carried out in inert atmosphere with the heating rate was 10 C min 1, and a-Al2O3 was used as reference material. FTIR and DRS UV–vis spectrum was recorded using KBr pellets. A typical oxidation reaction involved the following procedure: to 25 ml of solvent 250 mg of catalyst was added followed by the addition of the cyclohexene. Then, the oxidant was added dropwise to the reaction mixture. The reaction mixture was stirred under N2 at room temperature and the catalytic products were analysed using a gas chromatograph equipped with SE-30 column NMR and mass spectra.
Fig. 2. (A) TGA-DTA: (a) Al-MCM-41, (b) Porphyrin complex, (c) Fe–Porphyrin–MCM-41 (flexible method), (d) 1 wt% FeMCM-41, (e) Fe– Porphyrin–MCM-41 (template method), (f) Fe–Porphyrin (during synthesis) MCM-41, (g) Fe–Porphyrin–MCM-41 (by anchoring method). (B) TGA-DTA: (a) NaY, (b) Porphyrin complex, (c) Porphyrin–Y, (d) 1 wt% FeY, (e) Fe–Porphyrin Y, (f) Fe–Porphyrin (during synthesis) Y.
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
535
Fig. 2 (continued)
3. Results and discussion In order to get a first impression of whether a particular zeolite type can be chosen as carrier system that would be a suitable host for encapsulating homogeneously active porphyrin complex, these macromolecules were synthesized over various types of zeolites and as can be seen from the Table 1. Among the various zeolites used, the porphyrin molecule could be obtained selectively over HY and Al-MCM-41. The presences of Soret band at 419 nm in the UV–vis spectra confirmed the formation of the macrocycle inside these two types of molecular sieves. Further, we have attempted to various substitute these porphyrin ligands at both meso- and b-positions, as a step towards stabilizing them from self-oxidation. Table 2 shows that meso-tetrakis(4-pyridyl) porphyrin and meso-tetrakis(4-methylphenyl) porphyrin could be obtained in high yields and selectivity over Al-MCM41. The immobilization of iron porphyrin complexes inside Al-MCM-41 and HY leads to strong interaction
of the complex in the mesoporous system of the carrier material. The carrier material turned greenish, indicating the homogeneous catalyst was loaded onto the support. It is assumed that the complex is adsorbed on the inner and outer surfaces of the Al-MCM-41 structure. The binding energy of this adsorption varies on the different reactive sites like Bro¨nsted and Lewis acid sites and silanol groups. Upon the extraction with methanol, which in contrast to nonpolar dichloromethane adsorbs strongly on the zeolite surface, the complex desorbs from silanol groups due to a competitive reaction with the polar alcohol. The amount of complex immobilized on the carrier was determined by elemental analysis. Iron contents vary between 0.02 and 0.07 mmol per g of zeolite depending on the amount of complex encapsulated. The X-ray diffraction pattern of the isolated materials showed a strong characteristic peaks corresponding to both Al-MCM-41 and Y-zeolite, indicating that the crystallographic structure of the carrier material remains unchanged during the immobilization procedure, Figs. 1(A) and (B).
536
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
Quantitative loading of the complex was demonstrated by thermal analysis. Thermogravimetric and differential scanning calorimetric (DSC) measurements show that the immobilized complex is stable up to 450 C when anchored in Al-MCM-41 whereas decomposes at around 350 C when encapsulated in HY zeolite (Figs. 2(A) and (B)). The first decomposition occurs at 100–130 C and is slightly endothermic. Oxidation decomposition of the fixed complex took place in two steps at 350 and 450 C. The loss of weight of 4.5 wt% caused by the burning of the complex is consistent with the content determined by chemical analysis. The infrared spectra shows no change of wave number but a decrease of intensity for the signal at 3740 cm 1 which is assigned to the stretching vibration of terminal silanol groups. The vibration bands of immobilized complex are similar to those in the solution of dichloromethane. However, these signals resulting from organic compounds are very weak and not characteristic enough to surely identify or resolve a structure. The diffuse reflectance spectra show that the complex when anchored in Al-MCM-41 is most stable as no
additional peaks at higher wavelength appear. A strong characteristic peak corresponding to Al-MCM-41 indicates that the structure of the carrier material remains unchanged during the immobilization procedure. All other methods of immobilization in HY zeolite basically resulted in some distortion of the complex, as seen in Figs. 3(A) and (B). Among the catalysts presented here, it could be seen that several forces could be involved in the bonding of the complex on the carrier material. Electrostatic interaction of the cationic complexes occurs with the anionic framework of the Al-MCM-41 structure. Direct bridging of the iron to surface oxygen of the zeolite walls has also been observed and could occur after cleavage of the complex during the reaction. 3.1. Catalytic tests Several ligands have been applied and the corresponding complexes have been tested for the immobilization. As a test reaction for the catalytic activity the epoxidation of cyclohexene was employed, where the
Fig. 3. (A) DRS UV-Vis: (a) NaY, (b) 1 wt% FeY, (c) Fe–Porphyrin Y (flexible method), (d) Fe–Porphyrin (template method) Y, (e) Fe–Porphyrin (during synthesis) Y. (B) DRS UV-Vis: (a) Al-MCM-41, (b) 1 wt% FeMCM-41, (c) Fe–Porphyrin–MCM-41 (flexible method), (d) Fe–Porphyrin– MCM-41 (template method), (e) Fe–Porphyrin (during synthesis) MCM-41, (f) Fe–Porphyrin–MCM-41 (modified with anchoring agent), (g) Fe– Porphyrin–MCM-41 (by anchoring method).
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
537
Table 3 Encapsulation of porphyrin complex in Y and Al-MCM-41: catalytic activity towards oxidation of cyclohexene S. no.
Catalyst
% Conversion of cyclohexene
Liquid product distribution (%) Epoxide
Cyclohexanol
Cyclohexenone
Cyclohexenol
Diol
Dione
Others
1
FeY FeMCM-41 FeY-A FeMCM-41-A FeY-B FeMCM-41-B FeY-C FeMCM-41-C MCM-41-D Y-E MCM-41-E MCM-41-F MCM-41-G
4.7 3.7 11 6.9 24.3 42.5 6.7 10.7 21.09 2.5 32.9 43.2 98.8
0.15 0.22 0.015 0.04 0.15 0.8 0.062 0.16 0.20 0.18 0.22 0.35 –
0.49 – 0.49 – – – 0.02 0.05 0.025 – 0.02 – –
1.53 1.59 2.46 3.27 5.6 10.6 2.14 2.44 10.2 0.76 18.0 10.0 59.4
0.6 0.04 0.07 0.08 0.3 0.04 0.08 0.06 0.08 0.03 0.01 – 6.1
0.21 0.25 – 0.12 0.24 3.7 0.54 – 0.76 0.29 0.49 1.3 0.16
1.71 1.56 7.9 3.35 10.6 26.6 3.78 7.87 9.63 1.23 13.8 20.38 24.4
0.01 0.04 0.07 0.04 7.41 0.76 0.08 0.12 0.195 0.01 0.36 11.17 8.74
2 3 4 5 6 7 8
A, meso-tetraphenyl porphyrin; B, 5,10,15,20-tetrapyridylporphyrin; C, pyrrole + benzaldehyde; D, meso-tetraphenylporphyrin iron(III) chloride; E, 5,10,15,20-tetrakis(pentafluorophenyl); F, 3-aminopropyl trimethoxysilane (APTMS) + meso-tetraphenylporphyrin iron(III) chloride; G, 3-aminopropyl trimethoxysilane (APTMS) + 5,10,15,20-tetrakis(pentafluorophenyl) porphyrin iron(III) chloride.
catalytic run was performed with cyclohexene in dichloromethane with 1:4 molar ratio of TBHP at room temperature for 8 h. No reaction took place in the blank test when the carrier Y and Al-MCM-41 itself was used as catalyst. The catalytic results of the immobilized iron complexes are depicted in Table 3. The complex when anchored on Al-MCM-41 (Entry 12 and 13) shows the best results with high selectivity of cyclohexenone, at around 99% conversion of cyclohexene. These catalysts can easily be recovered and reused without further treatment. The supported catalyst was recycled four times. After these consecutive runs a decrease in catalytic performance was observed. This phenomenon goes along with the formation of lumps of the catalyst. In order to prove that the reaction is catalyzed heterogeneously and to exclude the possibility of leaching and homogeneous catalysis, the reaction mixture was separated from the catalyst before complete conversion occurs. Oxidation of the reaction solution following filtration after 3 h does not give any further reaction. After 8 h the conversion of the filtered sample remains at around 26% whereas the original batch with anchored catalyst goes to complete conversion of cyclohexene. This test proves that no homogeneous catalysis took place. ICP-AES analysis of the filtered reaction solution showed traces of iron, silicon and aluminium. The relative amounts of this analysis correspond to the composition of the heterogeneous catalyst used. This indicates that this loss occurs by attrition of the AlMCM-41 and not leaching of the complex. 4. Conclusion Homogeneous iron porphyrin catalyst was heterogenized best in Al-MCM-41. The bonding forces could
be due to the ionic interaction of the cationic complex with the anionic host framework. A reduction of the weak acidic sites of Al-MCM-41 has also been observed. After heterogenation also the iron porphyrin complex were suitable for the oxidation of olefins. The complexes remain stable within the mesopores of the carrier under the reaction conditions. The catalyst can be recycled by filtration and no leaching of the homogeneous complex was observed.
Acknowledgment One of the authors, M.R.K. is thankful to CSIR, New Delhi for Senior Research Fellowship.
References [1] (a) A. Baiker, H.U. Blaser, in: G. Erstl, H. Kno¨zinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, vol. 5, VCH, Weinheim, 1997, p. 2422; (b) N. Herron, G.D. Stucky, C.A. Tolman, J. Chem. Soc. Chem. Commun. (1986) 1521. [2] A. Corma, M. Iglesias, F. Mohino, F. Sanchez, J. Organomet. Chem. 544 (1997) 147. [3] B. Pugin, J. Mol. Catal. A107 (1996) 273. [4] C. Heinrichs, W.F. Ho¨lderich, Catal. Lett. 58 (1999) 75. [5] W. Kahlen, H.H. Wagner, W.F. Ho¨lderich, Catal. Lett. 54 (1998) 58. [6] S.b. Ogunwumi, T. Bein, Chem. Commun. (1997) 901. [7] M.J. Sabater, A. Corma, A. Domenech, V. Fornes, H. Garcia, Chem. Commun. (1997) 1285. [8] C. Schuster, W.F. Ho¨lderich, Catal. Today 60 (2000) 193. [9] K.T. Wan, M.E. Davis, Nature 370 (1994) 449. [10] C. Huber, K. Moller, T. Bein, J. Chem. Soc., Chem. Commun. (1994) 2619. [11] E. Armengol, A. Corma, F. Vicente, H. Garcia, J. Primo, Appl. Catal. A 181 (1999) 305.
538
V. Radha Rani et al. / Catalysis Communications 6 (2005) 531–538
[12] S. Ernst, M. Selle, Microporous Mater. 27 (1999) 355. [13] L. Frunza, H. Kosslick, H. Lanesser, E. Ho¨ft, R. Fricke, J. Mol. Catal. A 123 (1997) 79. [14] G.-L. Kim, S.-H. Kim, Catal. Lett. 57 (1999) 139.
[15] S.-S. Kim, W. Zhang, T.J. Pinnavaia, Catal. Lett. 43 (1997) 149. [16] P. Piaggio, P. McMorn, C. Langham, D. Bethell, P.C. BulmanPage, F.E. Hancock, G.J. Hutchings, New J. Chem. (1998) 1167. [17] F. Bedioui, Coord. Chem. Rev. 144 (1995) 37.