14256
J. Phys. Chem. 1996, 100, 14256-14264
Flexibility of the Zeolite RHO Framework. In Situ X-ray and Neutron Powder Structural Characterization of Cation-Exchanged BePO and BeAsO RHO Analogs† Tina M. Nenoff,*,‡,§ John B. Parise,| Glover A. Jones,⊥ Laurine G. Galya,⊥ David R. Corbin,*,⊥ and Galen D. Stucky* Department of Chemistry, UniVersity of California, Santa Barbara, California 93106-9510, Department of Earth and Space Sciences, State UniVersity of New York, Stony Brook, New York 11794-2100, and Central Research and DeVelopment Department, DuPont Company, Experimental Station, P.O. Box 80262, Wilmington, Delaware 19880-0262 ReceiVed: February 12, 1996; In Final Form: June 3, 1996X
This is an extensive study of the non-aluminosilicate analogs of the zeolite RHO. This molecular sieve is of great interest commercially because of its catalytic properties. In the absence of rigid supporting structural subunits (smaller cages or channels), the aluminosilicate RHO exhibits atypical framework flexibility with large displacive rearrangements. The beryllophosphate and berylloarsenate analogs are easily synthesized under very mild reaction conditions and therefore may be of interest for inexpensive and rapid commercial production. However, they have decreased thermal stability. In an effort to increase thermal stability and explore framework flexibility, we have synthesized and characterized a series of analogs of the nonaluminosilicate RHO framework. All materials crystallize in the space group I23, ranging from a ) 13.584(2) Å for Li-BePO RHO to a ) 14.224(4) Å for Ba-RbBeAsO RHO for hydrated phases. The extra framework cations are distributed over the double 8-ring, single 8-ring, and two single 6-ring sites. Partially and fully dehydrated phases were also studied for changes in framework stability. Predictive trends based on the type of cation exchanged into the framework were determined by 9Be and 31P MAS NMR.
Introduction Zeolites are crystalline aluminosilicate materials with open framework structures of molecular dimensions. These frameworks are three-dimensional structures with intracrystalline voids such as cages, channels, and/or pore openings. It is precisely this characteristic that enables the zeolites to behave as molecular sieves with shape and size selectivity and to be used as catalysts, as adsorbants, and for the alignment of second harmonic generation (SHG) active molecules.1 The structures of these materials can be characterized in terms of alternating tetrahedrally coordinated atoms (T-atoms) and oxygen atoms which form framework ions, or building blocks. Although an infinite number of 3D nets can be derived from these T-atom and oxygen linkages, a finite number of subunits (and therefore zeolitic frameworks) are actually found in nature. It has also been proven that the type and/or amount of the cation (framework-charge balancing) present directly results in the type of framework formed. It is the limiting geometry of the net linkages which determines the sizes and shapes of the windows into the cages, thereby producing a selectively adsorbing material.2 Many new materials form frameworks that are isostructural with the aluminosilicates and also follow a strict alternation of T-atoms in the framework, yet in general have greatly varying molecular sieving properties and thermal stabilities. With the synthesis of a number of aluminophosphates (AlPO),3 gallosilicates (GaSiO),2,4 and a novel gallophosphate (GaPO)5 phase, the discovery of tiptopite (a BePO4 cancrinite framework),6 * Authors to whom correspondence should be addressed. † DuPont Publication no. 7391. ‡ Current address: Sandia National Laboratories, P.O. Box 5800, MS 0709, Albuquerque, New Mexico 87185-0709. § University of California, Santa Barbara. | State University of New York. ⊥ DuPont Company. X Abstract published in AdVance ACS Abstracts, August 1, 1996.
S0022-3654(96)00429-7 CCC: $12.00
pahasapaite (a natural Be/P analog of zeolite RHO),7 lovdarite (a zeolitic Be/Si containing 3-rings),8 the new Weinebereite (a zeolitic mineral Be/P containing 3-rings),9 and the gallosilicate RHO,10 and the synthesis of several beryllophosphate frameworks11 and the zincosilicates,12 it is apparent that complete framework substitution by elements other than only those in groups 13 and 14 is possible and can produce zeolite structural analogs to zeolites, namely molecular sieves. Furthermore, we have synthesized and characterized a number of zeolitic zincophosphates (ZnPO),13,14 zincoarsenates (ZnAsO),13,14 berylloarsenates (BeAsO),13,15,16 beryllophosphates (BePO),11,15,16 and gallogermanates.16 Molecular sieve frameworks have been known to exhibit small distortions upon sorption of different solvents or as a function of temperature.17-19 However, the RHO molecular sieve framework is considered highly flexible and easily distorted. This flexibility offers an opportunity to introduce a high degree of catalytic selectivity by controlled cation siting at reaction temperatures. Extensive earlier studies have also shown that zeolite RHO exhibits a unique selectivity and activity (in the selective synthesis of dimethylamine from methanol and ammonia) as compared to other zeolites.20-26 The RHO framework is able to distort and relieve the strain imposed by the charge-compensating cations, unlike some zeolites that decompose under mild conditions. For these reasons, much study of the aluminosilicate RHO flexibility has been performed. It has now been extended to the non-aluminosilicate RHO frameworks, in particular, the beryllophosphates (BePO) and berylloarsenates (BeAsO).14 The framework of RHO is composed of a body-centered cubic arrangement of truncated cubo-octahedra (R-cages) linked Via double 8-rings (D8R). Under different conditions of cation exchange,27-34 temperature,28,29,35-38 and degree of hydration,34,35,39 either the centrosymmetric (space group Im3hm) or the noncentrosymmetric (space group I4h3m) form of the AlSiO © 1996 American Chemical Society
Flexibility of the Zeolite RHO Framework
J. Phys. Chem., Vol. 100, No. 33, 1996 14257
Figure 2. Variation of the cubic cell length (Å) Versus the distortion parameter (∆/a).
Figure 1. Cage structure of (a) hydrated RHO and (b) ion-exchanged, distorted RHO (8R outlined in black).
RHO can occur. For the BePO and BeAsO analogs, strict alternation of the cations occurs throughout the framework, thereby removing the mirror plane in (110) and lowering the symmetry to I23. The extra framework cations are distributed over the double 8-ring (D8R), single 8-ring (S8R), and two single 6-ring (S6R) sites. Earlier studies of AlSiO, BeAsO, and BePO RHO framework cation sitings indicate there is a preferred siting of the cations in both the D8R, which is fully occupied at room temperature, and the S6R.40 A simple parameter, ∆, which relates the degree of ellipticity of the single 8-ring (S8R) (see Figure 1) to the unit cell distortion was earlier introduced by Parise et al.28,39 It is defined as the ratio of the x crystallographic parameters of atoms O(2) and O(3) and is related to both the effective cross section of the S8Rs and the size and shape of the openings connecting the R-cages. The size of ∆ can be modified by a variety of experimental conditions, such as high temperatures and large cations exchanged into the cages resulting in lower ∆ and dehydration and replacement of AlSiO T-atoms with smaller atoms (such as Be and P) resulting in higher ∆.34,40 The various cation-exchanged aluminosilicate RHOs exhibit a near linear relationship (for ∆ e 14.7 Å) between the unit cell length a and the corresponding ∆ values. In studying the naturally occurring pahasapaite (BePO RHO) and a few of the monovalently exchanged non-aluminosilicate RHO frameworks, our results did not initially correlate with the AlSiO RHO plot. However, with the introduction of the parameter ∆/a, in order to correct for the differences in mean (Si,Al)-O and (Be,P)-O
bond lengths, a linear correlation between ∆ and a resulted for all combinations of T-atoms in the RHO framework40 (see Figure 2). In order to fully understand the distortion in any RHO material with cubic cell parameters in the range between the small pahasapaite (a ≈ 13 Å) and the aluminosilicates (a ≈ 14-15 Å), we have extended these studies to describe the effect of some other monovalent cation exchanges of the smaller T-atom frameworks of the beryllophosphate and berylloarsenate RHO molecular sieves. These results include structure refinement from neutron diffraction data of two monovalent cation exchanged samples, K-BePO and Tl-BePO. Furthermore, attempts have also been made to increase the thermal stability of the non-aluminosilicate RHOs through cation exchange. In situ X-ray powder diffraction studies were carried out on monovalent/divalent cation exchanged materials. 31P and 9Be MAS NMR data have been collected on all samples in an effort to correlate chemical shifts with dehydrated unit cell size. Experimental Section Although the toxicity of these materials is not known, they were presumed to be noxious and were handled as such. LiBePO RHO Series. Li24Be24P24O96‚40H2O was prepared by following a method similar to that described by Harvey and Meier.11 Our method has been modified by the elimination of tetraethylammonium hydroxide and the reaction at temperatures below 100 °C for 8 h.14 LiBeAsO RHO Series. Li24Be24As24O96‚40H2O was prepared by following the method described by Gier and Stucky.14 The amount of reactants was increased 4-fold, in order to produce enough product for characterization studies. The method involved dissolving 48 mmol of Be(NO3)2 and 56 mmol of H3AsO4 in 48 mL of H2O. After thorough mixing, 152 mmol of LiOH and 40 mL of H2O were added. The resultant gel was shaken, upon which it transformed to a milky solution (pH ) 7.5). The mixture was heated for 8 h at 70 °C, and then the settled microcrystalline powder was recovered by vacuum filtration and air drying. The reaction yielded 5 g of product. RbBeAsO RHO Series. Rb24Be24As24O96‚40H2O was prepared by following the method described by Gier and Stucky.14 This product was synthesized in a manner similar to that of the LiBeAsO analog; however, RbOH was used and the product fully formed after 2 days at 70 °C. Each of the RHO analogs was fully characterized and then ion exchanged with a number of monovalent and divalent cations, X. The cations include Li+, Na+, K+, Rb+, Cs+, Tl+, NH4+, Ca2+, Cd2+, and Ba2+.
14258 J. Phys. Chem., Vol. 100, No. 33, 1996 X-LiBePO RHO. A sample of Li/Be/P was prepared and then exchanged three times (1 h each) in aqueous 10% XNO3 (10 mL/g) at 90 °C to give X-BePO RHO. Chemical analyses gave the following unit cell compositions: X ) Na, Na17.2Li7.2Be24.0P24.0O96; X ) N(H4+), (NH4)15.76Li8.24Be24.8P23.2O96; X ) K, K16.2Na0.07Li7.0Be24P24O96; X ) Cs, Cs12.5Na0.65Li9.7Be24.6P23.3O96; X ) Tl (for NMR), Tl21.0Na0.81Li1.06Be24.5P23.5O96; X ) Ca, Ca9.1Li7.4Be24.1P23.9O96; X ) Cd, Cd9.5Li6.7Be24.5P23.5O96. X-LiBeAsO RHO. A sample of Li/Be/As was prepared and then exchanged three times (1 h each) in aqueous 10% XNO3 (10 mL/g) at 90 °C to give X-BeAsO RHO. Chemical analyses gave the following unit cell compositions: as synthesized Li7.9Be24.9As23.1O96; X ) N(H4+), (NH4)6.7Be26.3As21.7O96; X ) Cd, Cd13.1Be26.9As21.1O96; X ) Ca, Ca7.5Be25.2As22.8O96; X ) Ba, Ba3.4Be29.5As18.5O96. X-RbBeAsO RHO. A sample of Li/Be/P was prepared and then exchanged three times (1 h each) in aqueous 10% XNO3 (10 mL/g) at 90 °C to give X-Rb/BeAsO RHO. Chemical analyses gave the following unit cell compositions: as synthesized, Rb28.8Be24As24O96; X ) N(H4+), (NH4)7.1Rb4.6Be24As24O96; X ) Cd, Cd12.3Rb1.3Be24As24O96; X ) Ca, Ca11.1Rb3.8Be24As24O96; X ) Ba, Ba7.0Rb9.7Be24As24O96. Elemental Analyses. Elemental analyses were performed by inductively coupled plasma spectroscopy. Throughout the structural refinements, the framework compositions were assumed as Be24P24O96 and Be24As24O96, consistent with an ordered arrangement of cations in the space group I23. In situ X-ray Powder Diffraction Studies. The in situ X-ray studies of thermally and chemically induced transformations were monitored with a totally automated diffractometermicroreactor system.41-43 The equipment was designed for in situ structural studies of gas/liquid/solid and solid/solid interactions in the interval of 0-1000 °C. High-temperature X-ray diffraction studies of each stage of desorption and thermally driven phase transitions were conducted on an automated highresolution Rigaku diffractometer, model PMG-VH CN2172/R1. The unit was equipped with a curved crystal monochromator (model ME210ET) and aligned at a radius of 250 mm. A 1400 °C furnace was mounted in horizontal mode for vacuum (
