9924
J. Phys. Chem. 1995,99, 9924-9932
Synthesis and Structure Determination of ECR-10. A Gallosilicate Zeolite with the RHO-Framework J. M. Newsam,*,?D. E. W. Vaughan,’ and K. G. Strohmaieri Biosym Technologies Inc., 9685 Scranton Road, Sun Diego, California 92121-2777, and Exxon Research and Engineering Company, Route 22 East, Annandale, New Jersey 08801 Received: December 6, 1994; In Final Form: March 29, 1995@
Gallosilicate zeolites, termed ECR-10, have been crystallized from gel compositions aCs2O:bNa20:Ga203: cSiO;?:dH20 gels with 0.5 < a < 0.8, 1.0 .c b < 2.0, 2 < c < 2.5, and 50 < d < 100. ECR-10 has a typical composition of Na~.~Cs;?.~Si~.~Ga4.~02~*ltH~O (Si:Ga = 1.4), a cubic unit cell with a, 14.9 A and the same framework topology as that of the aluminosilicate zeolite rho (RHO-framework). The Si:Ga ratios typical of ECR-10 are substantially smaller than the Si:A1 ratios of zeolite rho materials; attempts to synthesize either ECR-10 or rho in overlapping composition domains have not, to date, been successful. Distance least-squares modeling was used to demonstrate the feasibility of accommodating the required number of framework gallium atoms in the unit cell observed for ECR-10. Subsequent refinement of this trial structure for the dehydrated zeolite was achieved by Rietveld analysis of monochromatic powder neutron diffraction data. The derived structural parameters are compared with those of other RHO-framework materials. Framework gallium substitution leads to a substantial distortion of the eight-ring windows, reducing the effective pore dimensions and suggesting the possibility of pore dimension control via partial framework cation substitution.
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Introduction Knowledge of the framework connectivity or topology of a zeolite provides a preliminary basis on which to rationalize its catalytic and sorptive behavior.’-4 The properties that might be expected on the basis of the gross framework topology are sensitive both to the details of the framework geometry and to the nature and location of nonframework species. The zeolite A (LTA5-6) framework, for example, can be tailored by appropriate nonframework cation exchange to provide effective pore cross sections of about 3 8, (K+-form, termed zeolite 3A), 4 8, (Na+-form, 4A), or 5 A (Ca2+/Naf-foxm, 5A) (see ref 1, Chapter 7F of ref 2, and Chapter 2). In these cases the differing sorption properties reflect direct steric constraints imposed by the presence, or absence, of nonframework cations in partially blocking window sites. Generally more subtle effects arise from the manner in which the nonframework cations influence the framework geometry through their differing coordination requirements.’-I0 An alternative route to modified framework chemical characteristics and geometries is by framework cation substitution. Most aluminosilicate zeolites will support a range of Si:Al ratios and certain T-atom replacements (T = tetrahedral species Si, Al, etc.) by, for example, Ga, Ge, B, Fe, etc. The detailed structural effects of many of these types of substitutions remain little explored. Gallosilicate and aluminogallosilicate zeolites are of considerable interest in this context. The similar chemical characters of aluminum and gallium promise nearisomorphous Ga - A1 substitution in a wide range of systems, combined with markedly differing T - 0 bond lengths (expectation distances Si-0 1.61A, A1-0 1.7258,, Ga-0 1.8238,).The structural effects of gallium incorporation into the MAZ, LTL, FAU, ABW, and SOD frameworks (ref 11 and refs cited), zeolite beta (BEA),I2 and ANAI3 and NATI4 framework materials have already been described. In addition to gallosilicate analogs of known aluminosilicate zeolite frameworks, apparently new phases have also been
’Biosym Technologies Inc. @
Exxon Research and Engineering Company. Abstract published in Advance ACS Abstracts, May 15, 1995.
0022-365419512099-9924$09.00/0
RHO
KFI
LTA
Figure 1. A truncated cuboctahedron and the manner in which this cage is interlinked to generate the RHO framework of zeolite rho (top). The related frameworks LTA (of zeolite A) and KFI (of zeolite ZK-5) which also contain this unit are also shown.
observed. In the present report we describe the synthesis and characterization of ECR- 10, a gallosilicate zeolite that was originally thought to possess a novel framework topology (on the basis of framework composition and unit cell volume). ECR10 is, however, shown by a combination of distance leastsquares modeling and Rietveld analyses of powder neutron diffraction data to adopt the same framework topology as Rho is a aluminosilicate zeolite rho (RHO, Figure l).5.6.15 synthetic material that had no known mineral counterpart until the discovery of the isotopological beryllophosphate mineral paha~apaite.’~,~’ Rho was first synthesized by Robson at a Si: A1 ratio of from 2.5 to 3.5;’*structural chara~terization’~ revealed it to adopt a framework topology hypothesized earlier.l 9 The synthetic beryllophosphateZ0and berylloarsenate2’ analogs of rho have also been prepared using lithium and rubidium cations. Zeolite rho itself has been identified as an active and selective catalyst for the synthesis of dimethylamine from ammonia and methanol.22 This has resulted in numerous studies of its cation exchange, sorptive and catalytic properties, and substantial characterization efforts.23 In particular, nonframework cation exchange has been explored as a means of tailoring 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 24, 1995 9925
Synthesis and Structure Determination of ECR- 10 ~.
effective pore aperture d i m e n s i o n ~ , l ~although * ~ ~ ~ ~with ~ the attendant complexity of potential blockage of pore windows by those cations that are distorting the pore aperture geometry. We describe here the contrasting composition ranges and framework geometries of the alumino- and gallosilicate RHOframework zeolites and discuss these data in the context of available structural data on synthetic aluminosilicate rho and on the newly discovered natural beryllophosphate mineral p a h a ~ a p a i t e . ' ~The . ~ ~structures of the latter have recently been compared with one another and with a berylloarsenate i ~ o t y p e . ~A~ theoretical analysis has also rationalized the observed ready deformability of the RHO framework.43
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Synthesis of ECR-10 All the synthesis work reported on zeolite rho indicates the necessity for Cs+ as at least one cation in the gel. To date, ECR- 10 has been observed exclusively in crystallizations from systems containing both cesium and sodium cations. Optimal syntheses are based on seededu or cold aged gel compositions in the range45
P
o.7cs20: 1 .5Na20:[( 1 - x)Ga~A1],0~:(2-2.5)Si0,:70H~0 with x = 0.00-0.02 (1) Typical starting ingredients were 50% CsOH solution, 50% NaOH solution, sodium silicate (28.7% Si02, 8.9% Na20; PQ Corp., N-brand), and Ga2O3 (Ingal). The gallia was dissolved in the combined base solution, then added to the sodium silicate with thorough mixing in a blender. In seeded syntheses, dilute H2SO4 was added, when necessary, to lower the total base concentration. Crystallization times were typically 4-8 h at 80- 100 "C after cold aging at room temperature for 3-5 days. At lower base concentrations than listed in the above formulation, the gallosilicate analogs of zeolites X (FAU) and sodalite (SOD) cocrystallize with ECR-10. At higher silica levels, the gallosilicate analogs of zeolites Y,zeolites CSZ-1 or CSZ-3,a-49 pollucite (ANA),13and F (EDI) are observed. ECR-10 was not observed in syntheses at Si02/Ga203 1 3.0. Synthesis products were analyzed and characterized by inductively coupled plasma emission spectroscopy (ICPES-Jarrel Ash 1loo), powder X-ray diffraction (Siemens D500 and Phillips analog diffractometers, Cu K a radiation), thermogravimetric analysis (TGA-Du Pont 1090/951), and 29Sisolid state NMR with magic angle sample spinning (JEOL FX200 operating at 4.7 T). These synthesis conditions contrast with those optimal for aluminosilicate zeolite namely a gel of approximate composition 0.44Cs20:2.96N~0:Al2O3:11.1Si0,:1 10H20
(2)
which can also yield amounts of zeolites Y, CSZ-1, and CSZ3, particularly when seeded. The principal cocrystallization products observed under other synthesis conditions that yield zeolite rho are zeolites P (GIs), and synthetic pollucite (ANA) and chabazite (CHA). Direct substitution of A1 for Ga in the above formula (1) yielded pure zeolite F. Similarly, direct substitution of gallium into typical rho synthesis compositions does not yield ECR- 10. Crystallizations over extended periods from gel compositions similar to those typical for aluminosilicate rho zeolites, but using Ga for A1 replacement, were examined (Table 1). At Ga/(Ga Al) 2 0.5 CSZ-1 and FAU type materials were made. At Ga/(Ga Al) = 0.25 a mixture of CSZ-1 and rho was produced, and pure rho crystallized from gels having Ga/(Ga Al) 5
+
+
+
ECR- 1 0
c
i
v
ECR- 10
Figure 2. Comparative scanning electron micrographs of representative zeolite rho (upper) and ECR-10 (lower) samples.
0.1. Highly crystalline ANA-framework aluminogallosilicates were recovered after extended crystallization times. l 3 Morphologically, aluminosilicate zeolite rho forms regular 0.5- 1 pm rhombododecahedral crystals, in contrast to the irregular, highly twinned tetrahedra or tristetrahedra of ECR10 (Figure 2). Zeolite rho has an Si:Al ratio of between 2.5 and whereas ECR-10 has a typical Si:Ga ratio of between 1 and 2. Detailed studies of aluminosilicate rho crystallizations covering a range of gel compositions have been reported by Barrer et al.50 Like aluminosilicate zeolite rho, ECR-10 has limited sorption properties in the as synthesized form, typically 5,15350
Newsam et al.
9926 J. Phys. Chem., Vol. 99, No. 24, 1995 TABLE 1: Summaw of ECR-10 and Rho Syntheses expt no. cszo: NazO: Ga203: SiOz: El E2 E3 E4 E5 E6
0.5 0.35 0.7 0.7 0.7 0.7
1.6 1.3 1.5 1.5 1.5 1.5
R1 R2 R3 R4 R5 R6
0.44 0.44 0.44 0.44 0.44
2.96 2.96 2.96 2.96 2.96 2.00
0.30
seeding'
H2O:
ECR-10 Syntheses" 2.0 70 2.0 70 2.0 70 2.0 70 2.0 70 2.0 70
0.98 0.98 0.98 0.00 0.98 1.oo
6h 6.5 h 6.25 h 3.5 h 6.5 h
2%
2% 2%
2% 2%
cold age14 d Effect of Gallium Substitution on Rho Synthesisb 0.00 11.1 110 cold age15 d 0.50 11.1 110 cold age15 d cold age/5 d 1.oo 11.1 110 cold age/5 d 0.25 11.1 110 cold age/5 d 0.10 11.1 110 cold age/5 d 0.00 7.5 75
product
timed
4d
3d 3d 6d 2d 5d 3d
ECR-IO + Ga-sodalite Ga-X + Ga-sodalite ECR-10 F (EDI) ECR-10 ECR-10
rho csz-1 weak CSZ- 113 CSZ-1 rho rho rho s-chabazite
+
+
+
Temp. = 100 "C; sodium silicate (PQ COT., N-brand) as silica source; (AI203 GazO3) = 1.0. Temp. = 85 OC; colloidal silica (DuPont Ludox, HS-40) as silica source; (A1203 Ga203) = 1.0. Seeding composition given as % A1203from nucleant slurry of composition 13.3Na20: AI203:12.5Si02:267H20. h = hours; d = days.
+
TABLE 2: Chemical Analyses of Various ECR-10 Samples" element E3b E6' E6Ad E6L' Si A1
Ga Si:Ga Si:(Ga AI)
+
Na cs
H20,wt ?GJ Ag+ Li+ unit cell (A)
hydrated
12.4 0.68 23.4 1.32 1.22 4.44 24.7* 9.3
60
70
80
90
100
110
120
130
140
ppm vs. TMS
Figure 3. Comparative 29SiNMR spectra of aluminosilicate (lower, Si:Al = 2.7) and gallosilicate (upper, sample E6, Si:Ga = 1.45) RHOframework zeolites (JEOL FX200WB spectrometer operating at 4.7 T using magic angle sample spinning).
sorbing 9 to 10 wt % H20, < 1 wt % N2, and
