Chapter 24
Synthesis of NaX Zeolites with Metallophthalocyanines 1
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Kenneth J. Balkus, Jr., C. Douglas Hargis, and Stanislaw Kowalak Department of Chemistry, University of Texas at Dallas, Richardson, TX 75083-0688
Aluminosilicate molecular sieves with the FAU structure have been crystallized in the presence of several metallophthalocyanines. A percentage of the complexes becomes included into the zeolites. The synthesis of NaX around the metal chelate represents a new method for encapsulating such complexes and modifying zeolite molecular sieves. The entrapped complexes were characterized by XRD, IR and UV-VIS spectroscopy. Preliminary results suggest the metal complexes may function as templates by modifying the gel chemistry.
The application of zeolite encapsulated metal chelate complexes in catalysis is a promising area of research. In particular shape selective oxidations catalyzed by metallophthalocyanines (MPc), shown in Figure 1, included in synthetic faujasite ( F A U ) type zeolites (2-10) appear to be competitive with other molecular sieve based catalysts that may have commercial potential. T h e restricted apertures (-7.4 Â ) to the supercages (12Â) in F A U type zeolites precludes removal of the large M P c complex unless the zeolite lattice is destroyed. Such physically trapped complexes have been termed ship-in-a-bottle complexes as well as zeozymes (to reflect the biomimetic reactivity that is often associated with these catalysts). The various strategies for preparation of zeolite encapsulated phthalocyanine complexes have largely involved the condensation of cticyanobenzene ( D C B ) around an intrazeolite metal ion to form the M P c complex. T h e efficiency of this template synthesis depends on the nature and location of the intrazeolite metal ion to be complexed. F o r example, metals have been introduced to the zeolite by ion exchange (7-13), metal carbonyls (14-19) and metallocene complexes (2-5,19-21) prior to reaction with D C B . Some of the advantages and disadvantages of these methods have been detailed by Jacobs (2). However, there are several problems that are inherent to the template synthesis in general. Often there is incomplete
Current address: A. Mickiewicz University, Faculty of Chemistry, ul Grunwaldzka 6, 60-780 Poznan, Poland
In Supramolecular Architecture; Bein, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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SUPRAMOLECULAR ARCHITECTURE
Figure 1. Metallophthalocyanine (MPc) complexation of metal which can complicate reactivity studies or formation of free base ligand may be observed which may lead to blockage of diffusion pathways. The formation of the phthalocyanine ring requires two reducing equivalents. If this is derived from trace water then protons are formed that can be detrimental to the zeolite lattice. Additionally, the high temperatures and pressures involved in the template synthesis of MPc may be unsuitable for certain metal oxidation states. Furthermore, dicyanobenzene diffusing into the zeolite should first react with metal ions located at the outer portion of the crystal. Therefore, pores quickly become blocked and a heterogeneous distribution of MPc will result. This may explain typical MPc loadings on the order of -10% occupation of the unit cells. Despite these limitations encouraging catalytical results have been reported. We have recently reported an alternative strategy for the preparation of zeolite ship-in-bottle complexes which involves synthesis of the zeolite around a metal complex (22,23). This allows one to prepare well defined intrazeolite metal complexes without uncomplexed metal ions or free ligand complicating characterization or reactivity. This method also provides a mechanism for encapsulating metal complexes inside zeolites with restricted apertures that would not otherwise adsorb the metal complex or ligand precursors. The metal phthalocyanines are attractive for this application because of their size as well as chemical and thermal stability. In this paper we report further results for the synthesis of NaX zeolites in the presence of MPc complexes (M = Fe(II), Co(II), Νΐ(Π)). X R D , IR, and electronic spectra provide evidence for the encapsulation of these complexes in NaX. Additionally, preliminary results suggest the MPc complexes may function as templates by modifying the gel chemistry during synthesis.
In Supramolecular Architecture; Bein, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
24.
BALKUS ET AL.
Synthesis of NaX Zeolites with Metallophthalocyanines
Experimental Aluminum isopropoxide, silica and sodium hydroxide were obtained from Aldrich and used without further purification. Metallophthalocyanines Fe(II), Co(II), Ni(II)) were purchased from Strem Chemical. X-ray diffraction patterns were collected on a Scintag XDS 2000 using fluorite as an internal standard. IR spectra were obtained from K B r pellets using a M E ) A C FT-IR. Electronic spectra were recorded on a Hitachi U2000. Solution S i N M R spectra were obtained on a JOEL 200 using teflon lined 10mm N M R tubes and referenced to TMS.The silicate solution were scanned 500times (15 sec delay) and the aluminosilicate 10,000 times (15 sec delay). Elemental analyses were conducted by Galbraith Laboratories, Knoxville, T N .
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The NaX molecular sieve containing metallophthalocyanines was prepared as follows. Freshly prepared silicate and aluminate solutions were combined with the M P c by adding the complex to the aluminosilicate gel, the aluminate or preferably the silicate solution. The highest levels of inclusion are obtained when the metal complex is mixed with the dry silica followed by NaOH and water. A typical mixture contains 2.0 g silica, 1.6 g NaOH, 0.04 g MPc and 4.0 mL H 2 O . Addition of the aluminate solution (4.5 g Al(iOPr)3, 1.6 g NaOH, 6.0 mL H2O) results in a sticky gel of uniform color. A n additional 18.0 mL of water are added and the gel transferred to a polypropylene bottle. The mixture is aged at room temperature with magnetic stirring for 24 hours, then heated at 90C for 6-10 hours. The resulting solid is washed with copious amounts of water, then dried at 90C for 18 hours. The molecular sieves containing metallophthalocyanines were placed in a vacuum sublimator at 0.170 CoPc > CoPc — 0.022 NiPc 0.084 NiPc > 0.140 NiPc > 0.120 NiPc > — NiPc > — NiPc NiPc — NiPc — NiPc d
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a. per AI2O3 b. determined by X R D and FT-IR c. unknown phase d. MPc added to silica first e. aged 24 hours The zeolites were characterized by X R D , IR and elemental analysis. The MPc complexes are best characterized by U V - V i s spectroscopy. For example Figure 2 shows the electronic spectra for FePc inside the zeolite and in H2SO4 after digestion of the zeolite. The characteristic π*
