J. Phys. Chem. 1996, 100, 15947-15953
15947
Reducibility, Location, and Adsorbate Interactions of Ni(I) Ions in Ni(II)-Exchanged Silicoaluminophosphate Type 41 Studied by Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopies A. M. Prakash, Tomasz Wasowicz, and Larry Kevan* Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641 ReceiVed: May 29, 1996X
Medium-pore silicoaluminophosphate type 41 (SAPO-41) molecular sieve has been synthesized by hydrothermal methods. The reducibility of Ni(II) ions ion-exchanged into SAPO-41 (NiH-SAPO-41) by various reduction methods was studied by electron spin resonance spectroscopy. Dehydration of NiH-SAPO-41 at temperatures above 573 K produces an isolated Ni(I) species with g| ) 2.498 and g⊥ ) 2.115. When dehydrated and oxidized NiH-SAPO-41 is treated with hydrogen, more than one isolated Ni(I) species are observed depending on the reduction temperature. When NiH-SAPO-41 is γ-irradiated at 77 K, Ni(I) is also formed. Adsorption of D2O into dehydrated NiH-SAPO-41 forms Ni(I)-(O2)n, indicating water decomposition by Ni(I). Adsorption of ammonia on reduced NiH-SAPO-41 leads to Ni(I)-(ND3)n with similar ESR characteristics as that of the Ni(I)-(O2)n complex produced by water adsorption. The similar ESR spectra of these two species suggest that both species are localized at the same site in the molecular sieve structure and have similar symmetry with two or three molecular ligands. Methanol adsorption on NiH-SAPO-41 forms Ni(I)(CH3OD)1 with axially symmetric g values. Adsorption of ethylene on NiH-SAPO-41 reduces most Ni(I) ions formed during hydrogen reduction to Ni(0). The location of Ni(I) in NiH-SAPO-41 was determined by 31P electron spin echo modulation and is located in the main 10-ring channel near a six-ring window.
Introduction Since their first synthesis in 1984, crystalline silicoaluminophosphate (SAPO) molecular sieves have generated considerable interest for catalysis.1 The modification of these materials by incorporation of transition metals in either framework or extraframework positions is of potential significance for specific catalytic reactions.2 The SAPO molecular sieves have a net negative framework charge which introduces some cation exchange capacity by which transition-metal ions can be introduced into these materials. Various techniques such as electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies and X-ray absorption fine structure (EXAFS) have been effectively employed to investigate the physical and chemical nature of incorporated metal species in SAPO materials.3,4 SAPO materials such as SAPO-5, SAPO11, and SAPO-34 with various transition metal ions in both framework and extraframework positions have been investigated.4-10 To a lesser extent similar studies have been reported on other SAPO structures such as SAPO-37,11 SAPO-42,12 and SAPO-18.13 However, there are other important SAPO materials which have not yet been studied for the incorporation of transition metals into the structure mainly because of the difficulty in preparing these materials in pure form. Several large- and medium-pore molecular sieves such as SAPO-40, SAPO-41, and SAPO-46 belong in this group. SAPO-41 is a novel medium-pore molecular sieve with adjacent 10-ring channels. The structure is analogous to AlPO441.14 The framework topology consists of elliptical onedimensional 10-ring channels that are slightly larger than the 10-ring channels in AlPO4-11. Substitution of silicon into an AlPO4 framework can be visualized in terms of silicon substituting only aluminum (mechanism 1), only phosphorus (mechanism 2), or a phosphorus-aluminum pair (mechanism 3). It has been generally accepted that mechanisms 2 and 3 X
Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01532-8 CCC: $12.00
are responsible for the formation of SAPO structures.15 The extent of silicon substitution depends on the topology of the framework and on the method of synthesis. The mechanism of silicon substitution in all the three known medium-pore SAPO structures, i.e., SAPO-11, SAPO-31, and SAPO-41, has been investigated recently.16 While silicon substitution mostly proceeds via mechanism 3 in SAPO-11 and SAPO-31, mechanism 2 was found to be prominent in SAPO-41. It should be noted that while mechanism 2 produces Bro¨nsted acid sites, mechanism 3 does not. The framework negative charge is normally balanced by protonated amine in the as-synthesized form and by H+ ions in the calcined form. The catalytic activity for toluene methylation is high for SAPO-41. This has been interpreted as due to the relatively large amount of Bro¨nsted acid sites of SAPO-41 created by the substitution of silicon at framework phosphorus sites. The H+ ions of the calcined SAPO materials can be exchanged to some extent by transition-metal ions. Thus, it is of considerable interest to see whether the high H+ ion concentration in SAPO-41 facilitates exchange of transition-metal ions. It has been shown that Ni(I) ions can be stabilized in zeolites and SAPO materials.17-19 Supported Ni(I) ions can be active sites for acetylene cyclomerization as well as ethylene and propylene oligomerization.20,21 The catalytic activities of such materials depend upon the valence state, location, and dispersion of the metal ion. Successful incorporation of Ni(II) into both framework and extraframework positions of SAPO-5 and SAPO-11 has been reported.22,23 Ethylene dimerization activity and selectivity on these materials have very recently been studied.24 Several methods have been applied to reduce Ni(II) ions in zeolites and other molecular sieves, including thermal reduction in H2 at 473-573 K, photoreduction by ultraviolet (UV) light irradiation in H2 at 77 K, or γ-irradiation in H2 at 77 K.25,26 The photoreduction method is very inefficient, and more than 10 h of UV irradiation from a 900 W high-pressure mercury © 1996 American Chemical Society
15948 J. Phys. Chem., Vol. 100, No. 39, 1996
Figure 1. SAPO-41 structure showing possible cation positions. See text for description of the cation positions.
lamp is necessary to reach a Ni(I) concentration sufficient for electron spin resonance. Thermal reduction in H2 produces adequate Ni(I) ions on certain materials, but the conditions must be carefully controlled to suppress the formation of metallic nickel. Although γ-irradiation at 77 K produces sufficient Ni(I) ions, other species like trapped holes on oxygen bonded to aluminum or silicon are also formed.26 By analogy with the designation of cation sites in SAPO5,22 SAPO-11,19 and zeolite X,27 possible cation sites in SAPO41 are shown in Figure 1. Site I (S I) is in the center of a hexagonal prism (double six-ring straight channels). Site II (S II) is at the center of a six-ring window that constitutes part of a side of a 10-ring straight channel. Site II* (S II*) is displaced from site SII toward the 10-ring channel, and site II′ (S II′) corresponds to displacement of site II away from the 10-ring channel into a double six-ring. There are several reports about the change of nickel ion location in Ni-faujasite,28 NiNa-Y,29 and NiCa-X30 zeolites. More detailed information on the location of Ni ions in SAPO-5 and SAPO-11 has been reported.19,22 In this study we have synthesized SAPO-41 hydrothermally using di-n-propylamine. NiH-SAPO-41 has prepared by introducing Ni(II) ions into extraframework sites of SAPO-41 by solid state ion exchange. The reducibility of divalent nickel in NiH-SAPO-41 by various methods of reduction has been studied. The probable location of Ni(I) ions in SAPO-41 has been identified. Additionally, Ni(I) complexes with various molecules have been studied in order to develop a better understanding of Ni(I) interactions with various adsorbates of catalytic interest. Experimental Section Preparation. SAPO-41 was prepared hydrothermally using di-n-propylamine as a nominal template. A sample of AlPO441 was also prepared. The following chemicals were used without further purification: orthophosphoric acid (85%, Mallinckrodt), pseudoboehmite (Catapal-B, Vista), fumed silica (Sigma), di-n-propylamine (98%, Aldrich), and NiCl2‚H2O (Aldrich). Syntheses were carried out in 100 cm3 stainless steel reactors lined with Teflon at autogenous pressure without agitation. The molar composition of the reaction mixture for the preparation of SAPO-41 was 1.0 Al2O3:1.2 P2O5:0.1 SiO2:4.0 dipropylamine:55 H2O. In a typical synthesis, 5.83 g of pseudoboehmite was slurried in 20 g of H2O and stirred for 2 h. Then 9.22 g of phosphoric acid was added to this slurry dropwise followed by 10 g of H2O. The mixture was stirred for 2.5 h. Fumed silica was then added slowly, and the mixture was stirred for 1 h. Finally, 16.19 g of dipropylamine was added dropwise, and the mixture was further stirred for 1 h. The pH of the gel
Prakash et al. was adjusted to 7.7 by adding 0.65 g of phosphoric acid diluted with 6.46 g of H2O. The final mixture was stirred for another 1 h before putting it into an autoclave and heating to 453 K for 264 h. After crystallization the product was separated from the mother liquor, washed with water, and dried at 373 K overnight. H-SAPO-41 was prepared by heating the as-synthesized material slowly to 823 K in O2 and maintaining this temperature for 12 h for complete removal of the organic matter. NiH-SAPO-41, where Ni(II) ions exist in extraframework positions, was prepared by solid-state ion exchange with NiCl2‚ 6H2O and H-SAPO-41 at 823 K for 16 h. Before and after solid state ion exchange, the color of the sample remained white. The idealized chemical composition of this sample was Ni0.002H0.028(Si0.032Al0.510P0.458)O2 based on electron probe microanalysis. Sample Treatment and Measurements. X-ray powder diffraction patterns were recorded on a Philips PW1840 diffractometer using Cu KR radiation. Thermogravimetric (TG) analysis were carried out in O2 on a Dupont 957 thermal analyzer at a heating rate of 10 K/min. Chemical analyses of the samples were carried out by electron microprobe analysis on a Jeol JXA-8600 spectrometer. For ESR and ESEM measurements, calcined and hydrated samples were loaded into 3 mm o.d. by 2 mm i.d. Suprasil quartz tubes and gradually heated in vacuum (
