The Pentagallium Cation in Zeolite Y. Preparation and Crystal

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The Pentagallium Cation in Zeolite Y. Preparation and Crystal Structure of Ga42Tl9.3-Si121Al71O384 Containing Ga57þ, Gaþ, Ga2þ, Ga3þ, and Tlþ Jong Jin Kim,† Cheol Woong Kim,† Dipak Sen,† Nam Ho Heo,*,† and Karl Seff *,‡ †

Laboratory of Structural Chemistry, Department of Applied Chemistry, Kyungpook National University, Daegu, 702-701 Korea ‡ Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, Hawaii 96822-2275, United States

bS Supporting Information ABSTRACT: The extraframework gallium cations Ga57þ, Gaþ, Ga2þ, and Ga3þ have been introduced into zeolite Y. Ga,Tl-Y (Ga42Tl9.3-Y or |(Ga57þ)5.8Gaþ6.6Ga2þ4.8Ga3þ1.5 Tlþ9.3|[Si121Al71O384]-FAU) was prepared by the reaction of Tl71-Y with Ga0 under anhydrous conditions at 623 K. Its structure was determined by single-crystal crystallography with synchrotron X-radiation, and its composition was confirmed by energy dispersive X-ray analysis. The structure was refined in the space group Fd3m (a = 24.527(1) Å) with all 1126 unique data; the final error index, R1 = 0.074, was calculated using only the 935 reflections with Fo > 4σ(Fo). Ga0 reacted with 87% of the Tlþ ions in the zeolite, mostly to give Ga57þ cations (centered tetrahedral, Ga-Ga = 2.512(3) Å). Ga57þ centers 5.8 of the 8 sodalite cavities per unit cell; each terminal atom (at site I0 ) bonds to three framework oxygen atoms of a double 6-ring (Ga-O = 2.050(5) Å and O-Ga-O = 99.0(2)o). The remaining Ga ions occupy five crystallographically distinct cationic sites: 2.3 Gaþ at site I0 , 4.3 Gaþ at site II, 2.1 Ga2þ at a second site II, 2.7 Ga2þ at site III0 , and 1.5 Ga3þ at a second site I0 . The gallium ions at sites I0 and II bond only to three 6-ring oxygen atoms; those at sites II and III0 extend into the supercage and are easily accessible for sorption and catalysis. All Tlþ ions occupy supercage sites.

1. INTRODUCTION Ga-containing zeolites have attracted enormous attention scientifically and industrially because they enhance the activity and selectivity of many important catalytic reactions.1-3 This has been known for more than two decades. Various methods of incorporating Ga into zeolites have led either to well-defined isomorphously substituted framework species such as those in gallosilicates and/or to various, somewhat poorly defined, extraframework species.3,4 The catalytic properties of the zeolites MFI,1,2,5-8 MOR,5,9 FER,5,10 and FAU5,11,12 containing extraframework Ga species have been extensively studied. Some of the important reactions investigated are the aromatization of light alkanes/alkenes based on the commercial Cyclar process,1,2,8,13 the conversion of methanol to olefins and aromatics,6,7 the alkylation, disproportionation, and isomerization of aromatics,3,14 and the selective catalytic reduction (SCR) of NOx.5,9,10,15 Despite its importance, the precise role of Ga in these reactions has proven difficult to determine,16,17 mainly because the nature of the active Ga species remains unclear. Numerous experimental observations1,2,8,18 and theoretical calculations4,19 have claimed that various gallium species such as Ga2O3, Gaþ, GaOþ, GaH2þ, and GaH2þ are involved in these processes; this remains a matter of active discussion in the literature. r 2011 American Chemical Society

The inclusion of extraframework Ga species into some zeolites with relatively low Al content (and therefore low ion-exchange capacity) has been demonstrated. Various incorporation methods, such as ion-exchange (aqueous14,20 and solid-state ion-exchange (SSIE)21,22), impregnation,18,20,23 and chemical vapor deposition (of Ga(CH3)31,17 and GaCl324) followed by additional treatments, have produced a variety of intrazeolitic Ga species. Although numerous studies on the preparation, characterization, and catalytic applications of Ga-containing zeolites have been carried out, no definitive structural study of extraframework Ga species has been reported.3,25 This could be due to the presence of multiple or complex Ga species2-4 and/or to their presence in relatively small amounts in the zeolites studied. In this work, we attempted to synthesize a high concentration of crystalline extraframework Ga species in zeolite Y (FAU) and to characterize the product crystallographically. Zeolite Y (Si/Al = 1.69)26,27 was selected for study because its transitionmetal exchanged forms show considerable chemical and thermal stability.28 Its moderately high framework Al content requires correspondingly large numbers of extraframework cations for Received: September 28, 2010 Revised: December 23, 2010 Published: January 25, 2011 2750

dx.doi.org/10.1021/jp109298e | J. Phys. Chem. C 2011, 115, 2750–2760

The Journal of Physical Chemistry C

2. EXPERIMENTAL SECTION 2.1. Synthesis. Large single crystals of sodium zeolite Y (|Na71(H2O)x|[Si121Al71O384]-FAU, Na71-Y 3 xH2O, Na71-Y, or Na-Y; Si/Al = 1.69) were prepared by Lim et al.26,27 using the synthetic method of Vaughan et al.37 A single crystal, a colorless octahedron about 0.15 mm in cross-section, was lodged in a fine Pyrex capillary. Fully Tlþ-exchanged zeolite Y (|Tl71(H2O)x|[Si121Al71O384]-FAU, Tl71-Y 3 xH2O, Tl71-Y, or Tl-Y)38,39 was prepared by the dynamic ion-exchange (flow method) of the Na-Y crystal with 0.10 M aqueous (pH = 6.4) thallous acetate (Tl(C2H3O2), 99.99%, Aldrich) at 294 K. This and similar ionexchange procedures had resulted in the complete Tlþ-exchange of zeolites A,31-33 X,34,35 and Y.38-40 The resulting colorless Tl-Y crystal was dehydrated at 673 K and 8 10-7 Torr for 48 h and was then brought into contact with Ga0 (99.999%, Aldrich) in a fine Pyrex capillary. This was achieved in a vessel sealed under vacuum by condensing Ga0 on and near the crystal, whose temperature, 623 K, was 100 K lower than that of the Ga metal, in coaxially connected heating ovens. The reaction that occurred under these conditions, which were maintained for 5 days, is discussed in section 6: it may seem that no reaction should have occurred because of the low vapor pressure (and therefore transport) of Ga0 at 623 K (2.3 10-12 Pa = 1.7 10-14 Torr) and 723 K (2.8 10-9 Pa = 2.1 10-11 Torr).41 Other experimental procedures for the reaction were similar in detail to those previously described for the preparation of In-A,31-33 In-X,34,35 and In-Y.36 Still under vacuum in its capillary after cooling to room temperature, the crystal (Ga, Tl-Y), now black, was sealed off from the Pyrex reaction vessel for diffraction experiments. 2.2. Diffraction. X-ray diffraction data for the Ga,Tl-Y single crystal were collected at 294(1) K on an ADSC Quantum210 detector at Beamline 6B1MXI of The Pohang Light Source. Crystal evaluation and data collection were done with a detector-tocrystal distance of 60 mm. Preliminary cell constants and an orientation matrix were determined from 36 sets of frames collected at scan intervals of 5° with an exposure time of 1 s

Table 1. Experimental Conditions and Crystallographic Data crystal cross-section (mm)

0.15

ion exchange for Tlþ (T (K), days, mL)

294, 9, 10

dehydration of Tl-Y (T (K), days, Torr)

673, 2, 8 10-7

reaction of Tl-Y with Ga (T1 (K), T2 (K), days)a

623, 723, 5

temperature for data collection (T (K)) X-ray source

294(1) PLS(6B1MXI)b

wavelength (Å)

0.90000

space group, no.

Fd3m, 227

unit cell constant, a (Å)

24.527(1)

maximum 2θ for data collection (deg)

80.80

no. of unique reflections measured, m

1126

no. of reflections (Fo > 4σ(Fo))

935

no. of variables, s data/parameter ratio, m/s

70 16.3

weighting parameters: a/b

0.1563/52.377

final error indices R1c

0.0723

R2d

0.2308

GOF (goodness of fit)e

1.130

a

T1 is the temperature of the crystal, and T2 is that of the Ga P metal. b c Beamline P 6B1MXI of the Pohang Light Source, Korea. R1 = |Fo - | Fc / Fo; R1 is P calculated using only reflections for which Fo > P those 2 2 1/2 4σ(Fo). d R2 = [ w(Fo2 - Fc2)2/ w(F is calculated using all o ) ] P unique reflections measured. e GOF = ( w(Fo2 - Fc2)2/(m - s))1/2. )

charge balance. An oxidative SSIE (OSSIE) method, reaction between Ga0 and the Tlþ ions in fully Tlþ-exchanged zeolite Y, would be used to prepare Ga-exchanged zeolite Y under anhydrous conditions at elevated temperature. With a 10.5 kJ/mol difference (ΔHionization) in the first ionization potentials of Ga0 and Tl0 (578.8 and 589.3 kJ/mol, respectively),29 it was hoped that Ga0 þ Tlþ f Gaþ þ Tl0 would occur. It was also hoped that the vapor pressure of Tl(s) at 623 K (1.63 10-4 Pa = 1.22 10-6 Torr),30 would be high enough to allow any elemental thallium produced during the above reaction to distill away from the crystal. The corresponding reaction with In0 had successfully introduced extraframework In species into zeolites A,31-33 X,34,35 and Y.36 Using single-crystal X-ray diffraction (SXRD) techniques with synchrotron X-radiation and scanning electron microscopy with energy dispersive X-ray (SEM-EDX) analysis, the crystal structure and chemical composition of the product would be determined. Thus, the crystalline extraframework Ga species in the zeolite would be identified, including their relative abundances and positioning within the zeolite, their geometry and coordination environments, and their oxidation states. Some of these chemically and thermally stable species could be of catalytic importance.

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per frame. The basic data file was prepared using the program HKL2000.42 The reflections were successfully indexed by the automated indexing routine of the DENZO program.42 About 90 000 reflections were harvested by collecting 72 sets of frames with a 5° scan and an exposure time of 1 s per frame. These highly redundant data sets were corrected for Lorentz and polarization effects; negligible corrections for crystal decay were also applied. The space group Fd3m, standard for zeolite Y, was determined by the program XPREP.43 A summary of the experimental and crystallographic data is presented in Table 1. 2.3. SEM-EDX Analysis. Two additional single crystals of Ga, Tl-Y were prepared using the methods described in section 2.1. One was exposed to the atmosphere and was attached without breaking to a piece of carbon attach tape for SEM-EDX analysis (sample GaTlYa). The other was exposed to the atmosphere and washed with deionized water with the hope of removing any residual Tl or Ga species that might be present on the surface of the crystal as was done in the preparation of In-A,31-33 In-X,34,35 and In-Y.36 The latter crystal, still black after washing, was lodged in a fine Pyrex capillary and dehydrated at 623 K and 7 10-7 Torr for 48 h. It remained black after redehydration like similarly prepared In-X and In-Y (unlike In-A, which was pale yellow or colorless). Still under vacuum in its capillary, it was sealed off from the vacuum line after cooling to room temperature, followed by exposure to the atmosphere for SEM-EDX analysis (sample GaTlYb). It was intentionally broken to expose fresh surface and was attached to a piece of carbon attach tape for analysis. Their SEM-EDX spectra were obtained by using a Koriba EDX-3500 instrument within a Hitachi S-4300 at the Central Laboratory of Kyungpook National University. They show no elements other than Si, Al, O, Ga, and Tl, consistent with the crystallographic characterization of Ga,Tl-Y to be presented in 2751

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section 3. Two SEM-EDX spectra of GaTlYa are shown in Figure 1. A comparison of the compositions as determined by crystallographic and SEM-EDX analysis is presented in Table 2.

The initial refinements with anisotropic thermal parameters converged to the high error indices (defined in a footnote to Table 1) R1 = 0.53 (step 1 of Table 3). Fixed weights were used initially. The final weights were assigned using the formula w = 1/[σ2(Fo2) þ (aP)2 þ bP], where P = [max(Fo2,0) þ 2Fc2]/3, with a and b as refined parameters (see Table 1). The detailed progress of structure determination as subsequent peaks were found on difference Fourier functions and identified as extraframework atoms is given in Table 3. The thermal parameters of Ga31, Tl31, Tl32, and Tl33, all near site III0 (near the three consecutive 4-rings in supercages), were fixed at a reasonable value, 0.05 Å2; otherwise, due to their relatively low occupancies and virtual closeness to one another, they became unrealistically large, and the geometry about those atoms became unreasonable. In the final model, isotropic thermal parameters were used for all gallium and thallium positions except for the high occupancy, strongly scattering Ga11 and Tl2 positions. The low occupancy Ga12 and Ga13 positions have been difficult to refine, perhaps because of their virtual proximity to the high occupancy Ga11 position, and may not be reliable. Finally, the occupancy ratio at Ga11 and GaU, 25.0(3)/5.75(9) = 4.35(9), suggested the presence of (Ga11)4(GaU) clusters in some sodalite cavities. (It is concluded in sections 5.2.2.1 and 5.2.4 that this polyatomic cation is Ga57þ.) Similar ratios, ranging from 3.2 to 4.3, were routinely observed for the isostructural In57þ clusters at the same location in similarly prepared In-A,31-33 In-X,34,35 and In-Y.36 Therefore, in a subsequent least-squares refinement, the Ga11/GaU occupancy ratio was constrained to be 4.0 (Table 3). This refinement quickly converged to R1 = 0.072 and R2 = 0.231 without noticeable changes in the other occupancies except for that at Ga13, which emerged from overlap with Ga11 (Tables 3 and 4). In the final cycle of least-squares refinement, all shifts were less than 0.1% of their corresponding esds. The final structural parameters are presented in Table 4, and selected interatomic distances and angles are given in Table 5. The oxidation-state assignments of extraframework species are summarized in Table 6. Atomic scattering factors for Ga3þ, Ga2þ, Gaþ, Ga0, O-, and (Si,Al)1.82þ were used.26,45,46 The scattering factors of Ga2þ and Gaþ were calculated from those of Ga3þ and Ga0 as follows: (2Ga3þþGa0)/3 and (Ga3þ þ 2Ga0)/3, respectively. The function describing (Si,Al)1.82þ is the weighted mean of the Si4þ, Si0, Al3þ, and Al0 functions (Si/Al = 1.69). All scattering factors were modified to account for anomalous dispersion.47,48 Other experimental and crystallographic details are given in Table 1.

3. STRUCTURE DETERMINATION Trial refinements in the lower space group Fd3 showed insignificantly different Si-O and Al-O distances, supporting the long-range Si/Al disorder expected for zeolite Y; all further refinements were done in space group Fd3m. Full-matrix leastsquares refinements (SHELXL97)44 were done on F2 using all unique reflections measured. They were initiated with the atomic parameters of the framework atoms (T(Si,Al), O1, O2, O3, and O4) in K71-Y.26

Figure 1. SEM-EDX spectrum of GaTlYa: (a) scanned up to 12.5 keV and (b) scanned up to 3.5 keV. The unlabeled peaks are standard for the instrument and procedure used: near 0 keV, instrumental; near 0.3 keV, carbon contamination; near 2.1 keV, platinum.

Table 2. Compositions of Ga,Tl-Y Crystals Determined by Crystallographic and SEM-EDX Analysis Si sample

analysis method c

no.a

Al at. %b

d

no.

O at. %

d

no. d

Ga

total

Tl

at. %

no.

at. %

no.

at. %

at. %

61.2 58.9

41.9

6.7 8.8

9.3

1.5 3.3

100 100

Ga,Tl-Y GaTlYae

SXRD EDXf

GaTlYbg

EDXf

16.4

10.1

65.8

5.8

2.1

100

ave. (EDX)

17.5

10.3

62.3

7.3

2.7

100

121.0

19.3 18.5

71.0

11.3 10.5

384.0

a

Number of atoms per unit cell. bAtomic percent of the element. c Composition determined crystallographically using single-crystal X-ray diffraction. d References 26 and 27. e The outer surface of an unwashed crystal was analyzed. fComposition determined by SEM-EDX analysis. From the variability seen in duplicate analyses, the EDX results appear to be less reliable than those from SXRD. g The inner surface (the crystal was intentionally broken) of a washed and redehydrated crystal was analyzed. 2752



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Table 3. Steps of Structure Determination as Extraframework Atomic Positions Were Found number of ions or atoms per unit cella atom 1

Ga11

Ga12

Ga13

Ga2

Ga21

GaU

Ga31

error indicesb Tl2

Tl31

Tl32

Tl33

c

R1

R2

0.5324

0.8667

0.3832

0.7634

0.2990

0.7053

2

15.7(8)

3

25.5(10)

6.3(3)

4

24.7(3)

5.91(13)

7.23(14)

0.1006

0.3656

5d

24.9(3)

4.6(3)

5.79(11)

7.39(11)

0.0764

0.2682

6e

24.7(4)

4.7(3)

5.72(10)

7.40(11)

0.0855

0.2705

7f

24.7(3)

4.1(3)

5.74(10)

7.61(11)

0.0845

0.2656

8g 9g

24.7(24) 24.88(24)

4.1(3) 4.0(3)

5.73(10) 5.71(10)

7.68(11) 7.70(11)

0.88(10) 0.91(10)

0.56(8)

0.0815 0.0795

0.2504 0.2433

10

24.83(24)

4.0(10)

2.3(3)

5.69(9)

6.7(4)

0.88(10)

0.57(7)

0.0773

0.2389

11g

24.89(23)

3.8(8)

2.4(3)

5.69(9)

2.3(6)

6.8(4)

0.76(10)

0.58(7)

0.0768

0.2383

12g

25.56(22)

4.1(9)

2.1(3)

5.75(9)

2.9(3)

6.7(4)

0.96(10)

0.47(7)

1.15(11)

0.0731

0.2323

13

25.0(3)

1.2(3)

0.52(17)

4.2(8)

2.1(3)

5.75(9)

2.6(3)

6.6(4)

0.97(10)

0.54(7)

1.15(10)

0.0716

0.2286

14h

23.2(3)

1.5(4)

2.3(5)

4.3(9)

2.1(3)

5.79(8)

2.7(3)

6.6(4)

0.92(10)

0.59(7)

1.18(11)

0.0723

0.2308

Numbers in parentheses are the estimated standard deviations in the units of the least significant figure given for the corresponding parameter. Scattering factors used are Ga0 for GaU, Ga2þ for Ga11 and Ga21, Ga3þ for Ga31, and Gaþ or Tlþ for other positions. b Defined in footnotes to Table 1. c The framework atoms were refined anisotropically. d A two-parameter weighting system was applied (Table 1). e An extinction parameter (EXTI) was introduced and refined. f Ga11 and Tl2 were refined anisotropically. g Fixed thermal parameters (0.05 Å2) were used for all site-III0 cations (Ga31, Tl31, Tl32, and Tl33). h The occupancy-ratio constraint Ga11/GaU = 4.0 was introduced. a

Table 4. Positional, Thermal, and Occupancy Parametersa occupancyc atoms Wyckoff position site

x

y

z

U11 or Uisob

T

192(i)

-4986(4)

12460(4)

3702(4)

214(6)

O1 O2

96(h) 96(g)

-9819(13) 11(14)

9819(13) 11(14)

0d 15094(18)

493(16) 479(14)

U22

U33

U23

196(6)

203(6)

-27(4)

U13 -5(4)

fixed

-15(4)

192 96 96

1(17)

96

128(19)

96

O3

96(g)

-7278(13)

-7278(13)

1712(19)

422(13)

422(13) 501(24) -37(13) -37(13)

96(g)

17007(14)

17007(14)

32323(17)

509(15)

509(15) 361(24)

Ga11

32(e)

I0

6588(7)

6588(7)

6588(7)

286(6)

Ga12

32(e)

I0

4349(243)

4349(243)

4349(243)

1270(370)

Ga13

32(e)

I0

7234(171)

7234(171)

7234(171)

954(306)

GaU

8(a)

U

12500d

12500d

12500d

282(9)

Ga2 Ga21

32(e) 32(e)

II II

24897(172) 24897(172) 24897(172) 23362(84) 23362(84) 23362(84)

462(47) 313(63)

Ga31

192(i)

III0

15796(170) 18351(156) 40233(179)

500f

Tl2

32(e)

II

Tl31

192(i)

Tl32 Tl33

26159(37)

varied

493(16) 500(24) -87(13) -87(13) -11(19) 479(14) 453(23) -6(14) -6(14) 85(19)

O4

26159(37)

U12

286(6)

286(6)

16(13)

16(13)

-8(4)

-8(4)

-8(4) 23.2(3)e 1.5(4) 2.3(5) 5.76(8)e 4.3(9) 2.1(3) 2.7(3)

580(18) 580(18) -28(17) -28(17) -28(17)

26159(37)

580(18)

6.6(4)

III0

15217(145) 16969(153) 43173(154)

500f

0.92(10)

96(g)

III

0

13484(110) 13484(110) 43798(167)

500f

0.59(7)

192(i)

III0

17192(124) 19597(118) 43468(130)

500f

1.18(11)

Positional parameters 105 and thermal parameters 104 are given. Numbers in parentheses are the estimated standard deviations in the units of the least significant figure given for the corresponding parameter. b The anisotropic temperature factor is exp[-2π2a-2(U11h2 þ U22k2 þ U33l2 þ 2U12hk þ 2U13hl þ 2U23kl)]. c Occupancy factors are given as the number of atoms or ions per unit cell. d Exactly, by symmetry. e These occupancies were constrained in the final least-squares refinement. f These thermal parameters were fixed in the least-squares refinement. a

4. BRIEF DESCRIPTION OF THE FAU FRAMEWORK AND CATION SITES The framework structure of zeolite Y, a synthetic analogue of the naturally occurring mineral faujasite (FAU), is characterized by the double 6-ring (D6R), the sodalite cavity (a cuboctahedron), and the supercage (Figure 2). Each unit cell has 8 supercages, 8 sodalite cavities, 16 D6Rs, 16 12-rings,

and 32 single 6-rings (S6Rs). The exchangeable cations, which balance the negative charge of the FAU framework,49 usually occupy some or all of the sites shown with roman numerals in Figure 2. The maximum occupancies at the cation sites I, I0 , II, II0 , III, and III0 in FAU are 16, 32, 32, 32, 48, and (in Fd3m) 192, respectively. Further detailed descriptions are available.50,51 2753



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Table 5. Selected Interatomic Distances (Å) and Angles (deg)a distances

Table 6. Assignments of Oxidation States and Charge Budget

angles

sites

atoms

occ.a,b

I0

M-O,c Å

r,d Å

NCe

charge

P

charges

Ga11

23.2(3)

2.050(5)

0.73

4

þ2f

46.4

T-O1

1.628(2)

O1-T-O2

112.9(2)

Ga12

1.5(4)

1.801(5)

0.48

3

þ3

4.5

T-O2 T-O3

1.655(2) 1.741(3)

O1-T-O3 O1-T-O4

109.1(2) 112.0(2)

Ga13

2.3(5)

2.19(4)

0.87

3

þ1

2.3

Ga2

4.3(8)

2.40(4)

1.08

3

þ1

4.3

T-O4

1.637(2)

O2-T-O3

102.6(2)

Ga21

2.1(3)

2.105(13)

0.79

3

þ2

4.2

mean

1.665

O2-T-O4

110.2(2)

Tl2

6.6(4)

2.744(12)

1.42

3

þ1

6.6

O3-T-O4

109.6(2)

Ga31

2.7(3)

1.99(4)

0.67

2

þ2

5.4

mean

109.4

Tl31 Tl32

0.92(10) 0.59(7)

2.70(4) 3.07(4)

1.38 1.75

2 2

þ1 þ1

0.92 0.59

Tl33

1.18(11)

2.81(5)

1.49

GaU P Ga P Tl

5.76(8)

Ga11-O3

2.050(5)

Ga12-O3

1.801(5)

Ga13-O3

2.19(4)

T-O1-T

153.0(3)

2.512(3)

T-O2-T T-O3-T

131.1(3) 121.5(3)

T-O4-T

138.9(3)

mean

136.1

Ga11-GaU Ga2-O2

2.40(4)

Ga21-O2

2.105(13)

Ga31-O4

1.99(4)

Ga31-O1

2.56(4)

Tl2-O2

O3-Ga11-O3

99.04(18)

GaU-Ga11-O3

118.56(14)

O3-Ga12-O3

119.97(19)

O3-Ga13-O3

90.6(22)

2.744(12) Ga11-GaU-Ga11

Tl31-O4

2.70(4)

Tl31-O1

3.14(4)

O2-Ga2-O2 O2-Ga21-O2

109.47b

II

III0

U

41.86g

2

þ1

1.18

4

-1f

-5.76 70.63b,h

g

9.29

a

Occupancy given as the number of ions per unit cell. bActual refined occupancies are used, without rounding off as was done in the text, to essentially the same end. c Shortest Ga-O and Tl-O bond lengths. d Radii of Ga and Tl species obtained by subtracting 1.32 Å (the radius of the oxide ion)29,53 from the shortest Ga-O and Tl-O bond lengths. e Coordination numbers of Ga and Tl ions. f Formal charge. g Numbers of Ga and Tl ions per unit cell. h These 70.63 positive charges closely balance the 71- (best integer) charge of the zeolite framework per unit cell.

91.1(20) 109.2(9)

Tl32-O4

3.07(5)

Tl32-O4

3.40(4)

O1-Ga31-O4

71.7(13)

Tl33-O4

2.81(5)

O2-Tl2-O2

77.4(4)

Tl33-O1

3.09(3) O1-Tl31-O4

54.7(7)

O4-Tl32-O4

57.6(8)

O1-Tl33-O4

54.4(6)

a

The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding value. b The tetrahedral angle by symmetry.

5. RESULTS 5.1. Framework Geometry. The mean T-O bond length (1.665 Å, see Table 5) in Ga,Tl-Y is appropriately between the mean Si-O (1.61 Å) and Al-O (1.74 Å) distances in Ca-LSX (FAU).52 Among the four T-Oi bonds, i = 1-4, the T-O3 bond length, 1.741(3) Å, is substantially longer than the others, 1.628, 1.655, and 1.637 Å (Table 5). This effect has been seen in dehydrated FAU zeolites containing extraframework cations with charges higher than 1þ, such as In2þ and In3þ in In-Y[In4(OH)4] (1.739 Å vs 1.631, 1.654, and 1.639 Å)36 and Ni2þ in Ni34-Y (1.706 Å vs 1.620, 1.658, and 1.617 Å).28 (Similar but smaller differences are commonly seen in nonalkali-metal exchanged forms of FAUs such as In66.8-Y (1.692 vs 1.639, 1.674, and 1.646 Å)36 and In87-X (1.699 vs 1.641, 1.675, and 1.660 Å),34 while it is almost absent in dehydrated K 71-Y (1.665 vs 1.647, 1.660, and 1.647 Å).26) One reason for this is that nearly all Ga2þ ions and all Ga3þ ions in Ga,Tl-Y coordinate primarily to O3 atoms. It is indicative of the degree of distortion that Ga2þ and Ga3þ induce in the zeolite framework as a result of their

Figure 2. Stylized drawing of the framework structure of zeolite Y. Near the center of the each line segment is an oxygen atom. The nonequivalent oxygen atoms are indicated by the numbers 1-4. There is no evidence in this work of any ordering of the silicon and aluminum atoms among the tetrahedral positions, although it is expected that Loewenstein’s rule49 would be obeyed. Extra-framework cation positions are labeled with roman numerals or the letter U.

higher charges, smaller sizes,53,54 and ability to bond to framework oxygens more covalently than Kþ. The difference between the longest and shortest T-O bond lengths, (T-O3) (T-O1) = 0.113 Å, is about the same as that in In-Y[In4(OH)4], (T-O3) - (T-O1) = 0.108 Å, and somewhat larger than that in Ni34-Y,28 (T-O3) - (T-O4) = 0.087 Å. However, it is twice as large as that in In66.8-Y (0.052 Å)36 and 6 times larger than that in K71-Y (0.018 Å).26 Although its mean T-O bond length, 1.665 Å, is close to that in In66.8-Y, 1.663 Å,36 and in K71-Y, 1.655 Å (both with Si/Al = 1.69 as in this work),26 the Ga,Tl-Y framework is much more distorted. The mean T-O-T angle, 136.1°, somewhat smaller than that in In66.8-Y, 139.7°, and much smaller than that in K71-Y, 143.9°, 2754



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Table 7. Displacements of Ions (Å) from 6-Ring (111) Planes at O3 and O2 atoms or ions

Figure 3. Stereoview85 of a representative sodalite cavity (not containing a Ga57þ cluster). The zeolite Y framework is drawn with open bonds between oxygen and (Si,Al) atoms. The coordination about Ga ions is indicated by thin solid lines. Ellipsoids of 50% probability are shown.

again indicates the ability of Ga3þ and Ga2þ to distort the zeolite framework. Among the four T-O-T angles, T-O3-T is the smallest (121.5°), because the Ga2þ and Ga3þ ions (Tables 4 and 5) are predominantly located at sites I0 where they bond primarily to O3, pulling the oxygens inward toward the centers of the D6Rs. The remaining angles are 153.0°, 131.1°, and 138.9° (Table 5). The difference between the largest and smallest T-O-T angles, (T-O1-T) - (T-O3-T) = 31.5°, lies between those in InY[In4(OH)4], (T-O1-T) - (T-O3-T) = 36.8°, and Ni34Y,28 (T-O4-T) - (T-O3-T) = 29.4°. It is, however, 2.5 times larger than that in In66.8-Y, (T-O1-T) - (T-O3-T) = 12.8°, and twice as large as that in K71-Y, (T-O2-T) - (T-O1-T) = 15.1°.26 The mean O-T-O angle (109.4°, essentially the tetrahedral angle) is unaffected; it is about the same as that in dehydrated K-Y (109.5°).26 5.2. Gallium Ions. Most of the 42 Ga atoms per unit cell, all but the 5.8 at GaU, occupy six crystallographically distinct cationic sites (three sites I0 , two sites II, and one site III0 ) and are therefore cations. Considering the negative charge to be balanced per unit cell of the zeolite framework (71-) and the charges of the remaining Tlþ ions (9.3þ), the 42 - 5.8 = 36.2 Ga cations must balance a charge of about (71-) þ (9.3þ) = 61.7-. (“About” is said because the charge at GaU has not yet been assigned.) Many of them must therefore be in oxidation states higher than 1þ. The oxidation states of the gallium ions were assigned primarily on the basis of their ionic radii, obtained by subtracting the ionic radius of O2- from their shortest Ga-O bond lengths. Their coordination numbers and environments were also considered. These assignments were confirmed when charge balance with the negative zeolite framework was seen. 5.2.1. Monopositive Gallium Ions. Per unit cell, 2.3 gallium ions were found at site I0 (Ga13) and 4.3 at site II (Ga2). They lie on 3-fold axes, Ga13 opposite D6Rs in sodalite cavities and Ga2 opposite S6Rs in supercages. Each of these 6.6 ions is 3-coordinate trigonal. 5.2.1.1. Gaþ Ions at Site I0 . Each of the 2.3 gallium ions at Ga13 is 1.26 Å from the (111) plane of three O3s to which it bonds at 2.19(4) Å (Figure 3 and Tables 5 and 7). Two impossibly close contacts with cations at other sites I0 (Ga13-Ga11 = 0.27 Å and Ga13-Ga12 = 1.23 Å) are avoidable: Ga11 and Ga12 approach other D6Rs (sections 5.2.2.1 and 5.2.3). Taking the ionic radius of the framework oxygens (O2-) to be 1.32 Å,29,53 the radius of Ga13 is 2.19 - 1.32 = 0.87 Å. It lies between the scattered values of ionic radii reported for Gaþ, 0.8153 and 1.13 Å,29,55 and is substantially longer than those for

sites

at O3a

at O2b

Ga11

I0

0.98

Ga12 Ga13

I0 I0

0.03 1.26

Ga2

II

1.31

Ga21

II

0.66

Tl2

II

GaU

U

1.84 3.49

a

Displacements into the sodalite unit (from a 6-ring of a D6R) are given as positive. b Displacements into the supercage (from a S6R) are given as positive.

Figure 4. Stereoview85 of a supercage in Ga,Tl-Y showing all extraframework Ga and Tl cations. The coordination about Ga and Tl ions is indicated by thin solid lines. Ellipsoids of 50% probability are shown. See the caption to Figure 3 for other details.

Ga3þ, 0.6153 and 0.62 Å.29,55 If the ionic radius of O2- is taken to be 1.40 Å,54 the radius of Ga13 would be 2.19 - 1.40 = 0.79 Å, very close to the smaller tabulated Gaþ radius. Accordingly, the gallium ions at Ga13 were assigned to be Gaþ. 5.2.1.2. Gaþ Ions at Site II. Each of the 4.3 gallium ions at Ga2 lies 1.31 Å from the (111) plane of three O2s to which it bonds at 2.40(4) Å (Figure 4 and Tables 5 and 7). Again, two impossibly close contacts with cations at other sites II (Ga2-Ga21 = 0.65 Å and Ga2-Tl2 = 0.54 Å) are avoidable; Ga21 and Tl2 approach other S6Rs (sections 5.2.2.2 and 5.3). The ionic radius of Ga2, 2.40 - 1.32 = 1.08 Å, again lies in the range of ionic radii reported for Gaþ, 0.8153 and 1.13 Å,29,55 and is also substantially longer than those for Ga3þ, 0.6153 and 0.62 Å.29,55 When the 1.40 Å54 ionic radius for O2- is used, the Ga2 radius becomes 2.40 - 1.40 = 1.00 Å, again within the range of reported Gaþ radii. Accordingly, the gallium cations at Ga2 also appear to be in their lowest oxidation state, Gaþ. Among the gallium cations in this structure, Ga2 has the largest ionic radius (sections 5.2.2-5.2.4, Table 6). It cannot, however, be mistaken for a Tlþ ion (r = 1.40,55 1.47,53 or 1.49 Å29), the only other elemental extraframework cation in the structure, because its radius is far too short. 5.2.2. Dipositive Gallium Ions. Most of the 42 gallium atoms per unit cell, 28.0, were found at site I0 (Ga11), at a second site II (Ga21), or at site III0 (Ga31). The Ga11 and Ga12 positions are are sharply closer to 6-ring oxygens than Ga13 and Ga2 (see Table 7), indicating an oxidation state higher than 1þ. 5.2.2.1. Ga2þ Ions at Site I0 . Per unit cell, 23.2 gallium ions were found at Ga11. They lie on 3-fold axes opposite D6Rs in sodalite cavities. Each is 0.98 Å from the (111) plane of the three 2755



Figure 5. Stereoview85 of a sodalite cavity with a Ga57þ cluster. Ga-Ga bonds are indicated by thick solid lines. This cluster is held in place and stabilized by 12 Ga2þ-O (Ga11-O3) bonds. See the caption to Figure 3 for other details.

O3 atoms to which it bonds at 2.050(5) Å (Figure 5 and Tables 5 and 7). The resulting ionic radius, 2.05 - 1.32 = 0.73 Å, is sharply less than those of Gaþ (0.86 and 1.07 Å, section 5.1), so their oxidation state must be higher. Although the conventional ionic radius is not commonly tabulated for the Ga2þ ion, this ionic radius lies between those reported for Gaþ (0.8153 and 1.13 Å29,55) and Ga3þ (0.62 Å).53,55 It is also essentially the same as the ionic radii of the two adjacent 2þ cations in the periodic table, Zn2þ (0.73 Å) and Ge2þ (0.74 Å),53 supporting the assigned oxidation state of 2þ at Ga11. The geometry about Ga11 is suggested by the O3-Ga11-O3 angle, 99.1(2)o, which is more like the tetrahedral angle (109.47°) than the trigonal angle (120°), indicating that these Ga2þ ions, each with an additional ligand to be identified, may be tetrahedral. Furthermore, Ga2þ has an odd number of electrons in its outermost shell (4s14p0) and is a less stable oxidation state, so it may require further coordination to be stable. Indeed, the Ga atoms at GaU can complete the tetrahedral geometries of these Ga2þ ions with a reasonable bond distance: Ga11-GaU = 2.512(3) Å. (Ga11 and GaU will be identified as members of the Ga57þ complex cation in section 5.2.4.) 5.2.2.2. Ga2þ Ions at Site II. Per unit cell, 2.1 gallium ions were found at Ga21, on 3-fold axes opposite S6Rs in the supercages. Each is 0.66 Å from the (111) plane of three O2s to which it bonds at 2.11(1) Å (Figure 4 and Tables 5 and 7). The resulting Ga21 radius, 2.11 - 1.32 = 0.79 Å, is very much like those of the Ga2þ ions at Ga11 discussed in section 5.2.2.1 (Tables 5 and 6), indicating that the cations at Ga21 are also Ga2þ. Each Ga2þ ion at Ga21 bonds to only three O2s; it is 3-coordinate trigonal. 5.2.2.3. Ga2þ Ions at Site III0 . Another 2.7 gallium ions per unit cell were found at Ga31 near the three consecutive 4-rings in supercages (Figure 4). Each approaches an O4 framework oxygen atom at 1.99(4) Å, so its ionic radius is 1.99 - 1.32 = 0.67 Å. This is somewhat less than the Ga2þ radii seen at Ga11 (0.73 Å) and Ga21 (0.79 Å) (sections 5.2.2.1 and 5.2.2.2), but longer than that reported for Ga3þ, 0.61 Å.53 Unlike the Ga2þ ions at Ga11 and Ga21, which are 4- and 3-coordinate, respectively (Table 6), Ga31 is only 2-coordinate (Figure 4). Ga31’s smaller ionic radius, 0.67 Å, is consistent with its lower coordination number,54 so it is assigned a 2þ oxidation state. It is surprising that Ga2þ would occupy site III0 ; the 6-ring sites (I, I0 , II0 , and II) would provide better coordination. Perhaps this is the result of a favorable placement of framework aluminum ions. Extensive efforts to locate additional ligands about Ga31

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were unsuccessful, due possibly to their virtually closeness to Tlþ ions at other sites III0 (section 5.3). 5.2.3. Tripositive Gallium Ions. Per unit cell, 1.5 gallium ions were found at Ga12. They lie on 3-fold axes opposite D6Rs in sodalite cavities. Each is 3-coordinate trigonal planar (O3-Ga12-O3 = 120.0(2)o), lying on the (111) plane of three O3s to which it bonds at 1.801(5) Å (Figure 3 and Tables 5 and 7). The resulting ionic radius, 1.80 - 1.32 = 0.48 Å, is substantially less than those observed for the Ga2þ ions at Ga11, Ga21, and Ga31 (0.73, 0.79, and 0.67 Å, respectively) described in section 5.2.2. However, it is somewhat close to those reported for Ga3þ (0.6153 and 0.62 Å29). The difference (0.13 or 0.14 Å) may be attributed to an inaccurate O3 position; O3 is an averaged position; the majority of the O3’s bond to the highly populated Ga11 position, and least-squares refinement is likely to have converged to or near this majority position. The 1.6 gallium ions at Ga12 are likely to have modified the 6-rings they occupy. Because of its very small measured ionic radius, Ga12 is assigned a 3þ oxidation state. 5.2.4. Ga57þ Polyatomic Cations. The 5.8 gallium atoms at GaU, at the very center of the sodalite cavity, do not approach the negative oxygen atoms of the zeolite framework and thus do not appear to be cations. Instead, each bonds to four tetrahedally arranged site-I0 Ga2þ ions (Ga11, section 5.2.2.1) at 2.512(3) Å (Figure 5 and Table 5) forming a centered tetrahedral (GaU)(Ga11)4 = Ga5 cluster (Ga11-GaU-Ga11 = 109.47°, exact value by symmetry). This cluster has a charge of approximately 8þ, only “approximately” because the charge at GaU has not yet been established. This cluster is taken to be Ga57þ, which, like In57þ,31-36 has both the structure and the G. N. Lewis electron dot structure of methane. The formal charges are 2þ for Ga11 and 1- for GaU. The formal charge of 2þ at Ga11 supports the distance and radius arguments used in section 5.2.2.1 to assign it that oxidation state. 5.3. Tlþ Ions. The 9.3 Tlþ ions per unit cell are distributed among four cation sites, one site II (Tl2) and three III0 sites (Tl31, Tl32, and Tl33) (Table 4). Per unit cell, 6.6 Tlþ ions were found at Tl2. They lie on 3-fold axes opposite S6Rs in supercages. Each is 1.84 Å from the plane of the three O2s to which it bonds at 2.74(1) Å (Figure 4 and Tables 5 and 7). This approach distance is very close to those found in the large cavities and supercages of the fully Tlþexchanged zeolites A,31-33 X,34,35 and Y.38,39 These Tlþ ions are also 3-coordinate trigonal. Per unit cell, 2.7 Tlþ ions are distributed among three III0 positions, Tl31, Tl32, and Tl33, all near the three consecutive 4-rings in the supercages (Figure 4). Tlþ ions commonly occupy such multiple III0 positions in zeolites X34,35 and Y.38-40 These Tlþ positions are less precisely determined than the others because of their relatively low occupancies (each ca. one ion per unit cell) and their virtual closeness to one another. This closeness has made it impossible to refine their positions without using fixed values for their thermal parameters (section 3 and Table 4). 5.4. Charge Balance. As summarized in Table 6, the 5.8 Ga57þ clusters per unit cell carry a charge of (5.8) (7þ) = 40.6þ, the 6.6 Gaþ ions, 6.6þ, the 4.8 Ga2þ ions, 9.6þ, the 1.5 Ga3þ ions, 4.5þ, and the 9.3 Tlþ ions, 9.3þ. The sum of these charges, 70.6þ, very closely balances the 71- charge of the zeolite framework, Si121Al71O38471-.26,27 This supports the reliability of the crystallographically determined occupancies and assigned oxidation states. 2756


The Journal of Physical Chemistry C 5.5. Arrangements of Extraframework Species. Of the eight sodalite cavities per unit cell, 5.8 are centered and filled by Ga57þ clusters; they have no other Ganþ nor Tlþ content (Figure 5). Each of the remaining 2.2 sodalite cavities contains, on average, 1.5/2.2 = 0.68 Ga3þ ions opposite D6Rs (Ga12 at the second I0 site) and 2.3/2.2 = 1.05 Gaþ ions (Ga13 at the third I0 site) (Figure 3). The supercages are quite sparsely occupied. On average, 4.3/ 8 = 0.5 Gaþ ions (Ga2), 2.1/8 = 0.26 Ga2þ ions (Ga21), and 6.6/ 8 = 0.8 Tlþ ions (Tl2), a total of 1.6 cations, approach the 32 S6Rs per unit cell (Figure 4). Finally, on average, 2.7/8 = 0.34 Ga2þ ions (Ga31) and 2.7/8 = 0.34 Tlþ ions (Tl31, Tl32, and Tl33) approach the six bands of 4-rings in each supercage. A plausible arrangement of all supercage cation positions is shown in Figure 4. 5.6. Verification of Atomic Composition by SEM-EDX Analysis. Figure 1 shows that Ga,Tl-Y is an aluminosilicate containing both Ga and Tl. Therefore, the reaction between Ga0 and Tlþ did not go to completion. In all EDX spectra recorded, including those shown in Figure 1, the KR lines of Al and Si appeared consistently with the same intensity ratio; this indicates that the composition of the zeolite framework was not modified by the reaction. The average Si/Al ratio (17.5/10.3 = 1.70) is consistent with the previously reported value (1.69)26,27 for the zeolite Y crystals used in this work. However, it may be higher on the outer surface (18.5/10.5 = 1.78 for GaTlYa) than on the inner surface of the intentionally broken crystal (16.4/10.1 = 1.62 for GaTlYb). This may be due to the additional steps of washing and redehydration done to prepare GaTlYb, or the crystals used may have been grown with such a compositional difference. Nonetheless, the average atomic percentage of each component element (Table 2) is reasonably consistent with that determined crystallographically, considering the crystallographic esd’s (Table 4), the acceptable error range of the EDX analysis, and the additional treatments for some samples (two Ga,Tl-Y crystals from two different preparations and further treatments for the third one, sections 2.1 and 2.3). The atomic concentrations of all elements (Al, O, Ga, and Tl) except for Si are only marginally different from those determined crystallographically: only about 1.0 atomic percent (at. %) smaller and within the acceptable error range of SEM-EDX analysis. This further supports the crystallographic characterization of the Ga and Tl species in this work, including their occupancies and assigned oxidation states.

6. DISCUSSION Within the esd’s of this determination, the net reaction that occurred upon exposure of Tl71-Y to Ga0 is Tlþ 71 - Y þ 42 Ga0 f ðGa5 7þ Þ5:8 Gaþ 6:6 Ga2þ 4:8 Ga3þ 1:5 Tlþ 9:3 - Y þ 61:7 Tl0 Of the 71 Tlþ ions per unit cell of the reactant Tl-Y, 61.7 were reduced by gallium and left the zeolite. They were replaced by 42 extraframework gallium ions in various oxidation states at various locations within the zeolite. It is expected that the vapor pressure of the liquid Tl0 product (5.8 10-5 Pa = 4.3 10-7 Torr at 623 K41) was sufficient for it to have migrated within the reaction vessel to form Ga,Tl(l) amalgams on and/or near the crystal.

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Table 8. Number of Cations per Unit Cell

a

Ga,Tl-Ya

In66.8-Yb

M = Ga

M = In

Mþ

6.6

57.4

M57þ

5.76

1.8

M2þ

4.8

0.5

M3þ

1.5

Tlþ

9.29

This work. b Reference 36.

6.1. Gallium Cations. Of the 42 gallium ions per unit cell, 29 are present as 5.8 Ga57þ complex cations. The others are monocationic in all three oxidation states, 6.6 Gaþ, 4.8 Ga2þ, and 1.5 Ga3þ (Table 8). Gaþ and Ga3þ are the stable cationic forms of gallium. Ga2þ is elusive; it has been seen as Ga24þ in compounds like Ga2X4L2 and the [Ga2X6]2- anion.56,57 A stoichiometric crystal structure containing monocationic Ga2þ has not been reported. Gallium and indium are both group IIIA elements. Although Ga3þ ions are present in Ga,Tl-Y, In3þ ions were not found in similarly prepared fully indium-exchanged zeolites A,31-33 X,34,35 and Y,36 nor in their sorption complexes with H2O,34 H2S,32,35 and sulfur.33 Even air-oxidation of In-Y at 623 K oxidized only most of the indium ions to In3þ; oxidation in pure oxygen at 623 K was even less effective.36 Moreover, the distribution of gallium ions among the various oxidation states is quite different from that seen with indium (see Table 8). This is consistent with the general trend in the chemistry of the group IIIA elements (B, Al, Ga, In, Tl): the lighter elements are more electropositive and are more commonly found as M3þ. 6.2. Ga57þ. The tetrahedral Ga57þ complex cation was first seen in {(Et2O)2ClGa}Ga{GaCl2(Et2O)}3, a molecular compound with Ga57þ at its center and seven chloride ions and five oxygen atoms (of diethylether molecules) coordinating to the four terminal Ga ions.58 The four Ga-Ga bond lengths, ranging from 2.417(2) to 2.450(2) Å, are a little shorter than that found in this work, 2.512(3) Å, and Ga57þ is somewhat distorted in its environment, which lacks the symmetry seen in this report. These Ga-Ga bond lengths are similar to those found in Ga2Cl4(dioxane)2 (2.406 Å),56,59 Ga2Cl62- (2.39 Å),56 and Ga2Br62- (2.41 Å).56 They are also similar to those found in many organogallane compounds, from 2.3560 to 2.45 Å61 in digallanes and from 2.53662 to 2.572 Å63 in tetragallanes, with even longer Ga-Ga bonds in some larger organogallanes like [GanRn]x- (x = 0, 1, 2; n = 6, 8, 9; R = SiMe(SiMe3)2, fluorenyl, and/or CMe3).62 The shortest Ga-Ga distance in gallium metal is 2.484 Å.64 In57þ clusters were commonly seen in the sodalite cavities of fully In-exchanged zeolites A,31-33 X,34,35 and Y,36 so it is not surprising that Ga57þ has formed in the same cavities of zeolite Y. The tetrahedral In57þ complex cation was first reported in 1998.65 It had been seen twice before 1998, but it had been assigned the incorrect charges 6þ66 and 8þ.31 Anionic sodalite cavities often host and stabilize cationic clusters like Ga57þ (Figures 5 and 6). Examples are In57þ, 31-33 Cd6S44þ,67 Cd2Na2S4þ,67 Cd2O2þ,67 and Pb7O(OH)39þ 68 in zeolite A (LTA), S44þ,69 In57þ,34,35 Pb8O4nþ,70,71 Na54þ, 72 Ni4AlO43þ,73 Zn7HAlO410þ,74 Cs43þ,75 and (Cd2þZn)816þ 76 in zeolite X (FAU), and Na43þ,77 Zn56þ,78 cyclo-Zn68þ, 78 Cd8O48þ, 79 Pb2S2þ,80 Ni8O4 3 xH2O8þ,28 In57þ, 36 and In4(OH)48þ 36 in zeolite Y 2757



Figure 6. The Ga57þ cation: (a) a space-filling model of Ga57þ in a sodalite cavity;86 and (b) the Ga57þ cluster using ellipsoids of 90% probability.85

(FAU). In addition to being anionic, the sodalite cavity provides appropriate coordination environments for all of the above clusters. Note that In57þ 31-36 and Na54þ 72 are isostructural with Ga57þ. 6.3. Comparisons among Ga57þ, In57þ, and Their Host Sodalite Cavities. Gallium atoms (at. no. 31) are smaller than indium atoms (at. no. 49), and gallium ions are smaller than indium ions of the same charge. It follows that Ga57þ should be smaller than In57þ. Indeed, the Ga-Ga bond length in Ga57þ, 2.512(3) Å, is shorter than the In-In bond in In57þ, 2.68(1) Å, a mean value from several determinations in zeolites X and Y.34-36 Similarly, the Ga-O3 coordination bonds are 2.050(5) Å as compared to 2.170(7) Å in In66-X,35 a reliable comparison because In66-X has full In57þ occupancy. Consistent with the above, all 12 O3 atoms in each Ga57þ-containing sodalite cavity are pulled inward by 0.14 Å as compared to an In57þ-containing cavity.35 In the zeolite framework, this difference can most easily be seen in the T-O3-T angles (Table 5); they are 121.5(3)o in Ga,Tl-Y as compared to 126.6(4)o in In66-X.35 These comparisons are only somewhat quantitative because of the differing M57þ occupancies, the different framework compositions of X and Y, and the presence of Tlþ ions in Ga,Tl-Y. 6.4. Incomplete Reaction of Tl71-Y with Ga0. The redox reactions of In0 with the Tlþ ions in Tl-A,31-33 Tl-X,34,35 and Tl-Y36 all went to completion to give products free of thallium. That did not happen with Ga0 (the present work), although the reaction conditions were the same. This may be attributed to the reduced total number of extraframework cations due to the production of Ga3þ, Ga2þ, and Ga57þ ions in relatively large number (Table 8); the number of extraframework cations in this structure (45.5 per unit cell) is substantially less than those in various indium exchanged zeolites X and Y, typically 58.886.0.34-36 Fewer, more highly charged cations are less able to locally balance the relatively evenly distributed negative charge of the zeolite framework. Indeed, although Ga is most commonly found as Ga3þ in its compounds, there are only 2.7(3) per unit cell in this structure, indicating that additional Ga0 atoms have been accepted into the zeolite to keep the mean positive charge of the gallium cations low. There are additional possible explanations or considerations for the failure of this reaction to go to completion. It could be attributed to the smaller difference in ionization potential between Ga0 and Tlþ ions of Tl-Y (10.5 kJ/mol, section 2.1) as compared to that for In0 and Tlþ (31 kJ/mol).29 It might also be attributed to poor mass transport of Ga to the crystal. Because a substantial amount of Ga was found in the crystal studied, it is clear that its surface was open to Ga atoms during the synthesis procedure. However, if insufficient Ga metal was in contact with the crystal, or (for Ga near but not in contact with the crystal) the lower vapor pressure of Ga0 (2.3 10-12 Pa = 1.7 10-14 Torr)41 as compared to that of In0 (4.7 10-10 Pa = 3.5 10-12

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Torr)41 at 623 K was insufficient (although both seem far too small for effective mass transport), or if trace amounts of a volatile intermediate were absent or ineffective (in In-zeolite chemistry, InOH was suggested to be sufficiently volatile81), the reaction could have failed to go to completion. 6.5. The Four III0 Cation Positions. Ga31, Tl31, Tl32, and Tl33 are 2-coordinate at III0 sites. Tl32 bridges between two O4 oxygen atoms. The remaining three each bond to two oxygen atoms, each of which in turn bonds to a single T (Al or Si) atom (Figure 4). This T atom must be Al because cations must approach centers of negative charge, the AlO2- units. Because Ga31, Tl31, and Tl33 all bond more closely to O4 than to O1 (Table 5), and Tl32 bonds only to two O4 atoms, at least some O4 atoms must offer more negative charge to these cations than O1. Indeed, O1 and O4, the 12-ring oxygens, were identified by Heidler et al.82 to be the two most basic oxygen positions, although the detailed basicity of each individual framework oxygen atom must depend on the nearest Al and exchangeable cation positions.82-84 The number of different III0 positions in Ga,Tl-Y, all virtually close together, may be attributed to somewhat long-range effects: intercationic repulsions and the various positions that the second nearest framework Al ions may occupy. 6.6. Implications for Catalysis by Gallium Cations. The gallium ions in the supercages are all coordinatively unsaturated. The Gaþ ions at Ga2 and the Ga2þ ions at Ga21 are 3-coordinate; the Ga2þ ions at Ga31 are only 2-coordinate. They are all easily accessible for catalysis through the 12-ring channel system. Also, because there are still some empty sites II in the supercages, Ganþ ions at site I0 may become accessible by moving into the supercages to associate with approaching reactants. The presence of gallium ions in various oxidation states may be the underlying basis for the utility of gallium-containing zeolites as active catalysts in many important catalytic processes (section 1). It may be important the redox potentials between these oxidation states are relatively small.

7. SUMMARY The extraframework gallium cations Ga57þ, Gaþ, Ga2þ, and Ga3þ were introduced into zeolite Y. Ga,Tl-Y (Ga42Tl9.1-Y or |(Ga57þ)5.8Gaþ6.6Ga2þ4.8Ga3þ1.5Tlþ9.3|[Si121Al71O384]-FAU) was prepared by the reaction of Tl-Y with Ga0 under anhydrous conditions at 623 K. Its structure was determined by singlecrystal crystallography with synchrotron X-radiation, and its composition was confirmed by energy dispersive X-ray analysis. The structure was refined in the space group Fd3m (a = 24.527(1) Å) with all 1126 unique data. Ga0 reacted with 87% of the Tlþ ions in the zeolite, mostly to give Ga57þ. Tetrahedral polyatomic Ga57þ clusters (isostructural with methane, Ga-Ga = 2.512(3) Å) center and fill 5.8 of the 8 sodalite cavities per unit cell; each terminal atom (at site I0 ) bonds to three framework oxygens of a double 6-ring (Ga-O3 = 2.050(5) Å and O3-Ga-O3 = 99.0(2)o). The remaining Ga atoms occupy four crystallographically distinct cationic sites: 2.3 and 4.3 Gaþ ions at sites I0 and II, respectively, 2.1 and 2.7 Ga2þ at another site II and site III0 , respectively, and 1.5 Ga3þ at another site I0 . The Ganþ ions at sites I0 and II bond only to three 6-ring oxygen atoms; those at sites II and III0 extend into the supercage where they are easily accessible to guest molecules. All Tlþ ions occupy supercage sites. 2758



’ ASSOCIATED CONTENT

bS

Supporting Information. Observed and calculated structure factors for Ga,Tl-Y. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (N.H.H.); seff@hawaii.edu (K.S.).

’ ACKNOWLEDGMENT We gratefully acknowledge Dr. Ghyung Hwa Kim for her assistance and POSTECH’s Pohang Accelerator Laboratory for their diffractometer and computing facilities. The financial support of Kyungpook National University (2009) and the Korea Science & Engineering Foundation (2009-0074154) is also gratefully acknowledged. ’ REFERENCES (1) Rane, N.; Overweg, A. R.; Kazansky, V. B.; van Santen, R. A.; Hensen, E. J. M. J. Catal. 2006, 239, 478. (2) Price, G. L.; Kanazirev, V. I.; Dooley, K. M.; Hart, V. I. J. Catal. 1998, 173, 17. (3) Fricke, R.; Kosslick, H.; Lischke, G.; Richter, M. Chem. Rev. 2000, 100, 2303. (4) Pereira, M. S.; Nascimento, M. A. C. J. Phys. Chem. B 2006, 110, 3231. (5) Yogo, K.; Ihara, M.; Terasaki, I.; Kikuchi, E. Catal. Lett. 1993, 17, 303. (6) Choudhary, V. R.; Kinage, A. K. Zeolites 1985, 15, 732. (7) Romannikov, V. N.; Chumachenko, I. S.; Mastkhin, V. M.; Ione, K. G. React. Kinet. Catal. Lett. 1985, 29, 85. (8) Dooley, K. M.; Price, G. L.; Kanazirev, V. I.; Hart, V. I. Catal. Today 1996, 31, 305. (9) Satsuma, A.; Yamada, K.; Mori, T.; Niwa, M.; Hattori, T.; Murakami, Y. Catal. Lett. 1995, 31, 367. (10) Mecarova, M.; Miller, N. A.; Clark, N. C.; Ott, K. C.; Pietrass, T. Appl. Catal., A 2005, 282, 267. (11) Buckles, G.; Hutchings, G. J.; Graham, J.; Williams, C. D. Catal. Lett. 1991, 8, 115. (12) Mihalyi, R. M.; Beyer, H. K.; Keindl, M. Stud. Surf. Sci. Catal. 2001, 135, 206. (13) Mowry, J. R.; Anderson, R. F.; Johnson, J. A. Oil Gas J. 1985, 83, 1288. (14) Bigey, C.; Su, B.-L. J. Mol. Catal. A 2004, 209, 179. (15) Armor, J. N. Catal. Today 1996, 31, 191. (16) Biscardi, J. A.; Iglesia, E. Catal. Today 1996, 31, 207. (17) El-Malki, E.-M.; van Santen, R. A.; Sachtler, W. M. H. J. Phys. Chem. B 1999, 103, 4611. (18) Rodrigues, V. d. O.; Eon, J.-G.; A. C. Faro, J. J. Phys. Chem. C 2010, 114, 4557. (19) Mikhailov, M. N.; Mishin, I. V.; Kustov, L. M.; Lapidus, A. L. Microporous Mesoporous Mater. 2007, 104, 145. (20) Yakerson, V. I.; Vasina, T. V.; Lafer, L.-I.; Sytnyk, V. P.; Dykh, Z. H. L.; Mokhov, A. V.; Bragin, O. V.; Minachev, K. M. Catal. Lett. 1989, 3, 339. (21) Kanazirev, V.; Price, G. L. Zeolites and Related Materials: State of the Art. In 10th International Zeolite Conference; Weitkamp, J., Karge, H. G., Pferfer, H., Holderich, W., Eds.; Elsevier: Amsterdam, 1994; Vol. 98C, p 989. (22) Karge, H. G. Zeolites and Microporous Crystals. In The International Symposium on Zeolites and Microporous Crystals; Hottori, T., Yashima, T., Eds.; Kodansha Ltd. and Elsevier: Nogoya, Japan, 1994; Vol. 83, p 135.

ARTICLE

(23) Joly, J. F.; Ajopt, A. E.; Merlen, E.; Raatz, F.; Alario, F. Appl. Catal., A 1991, 79, 249. (24) Kwak, B. S.; Sachtler, W. M. H. J. Catal. 1994, 145, 456. (25) Dooley, K. M.; Chang, C.; Price, G. L. Appl. Catal., A 1992, 84, 17. (26) Lim, W. T.; Choi, S. Y.; Choi, J. H.; Kim, Y. H.; Heo, N. H.; Seff, K. Microporous Mesoporous Mater. 2006, 92, 234. (27) Lim, W. T.; Seo, S. M.; Kim, G. H.; Lee, H. S.; Seff, K. J. Phys. Chem. C 2007, 111, 18294. (28) Kim, C. W.; Jung, K. J.; Heo, N. H.; Kim, S. H.; Hong, S. B.; Seff, K. J. Phys. Chem. C 2009, 113, 5164. (29) Emsley, J. The Elements; Clarendon Press: Oxford, 1991. (30) Wade, K.; Banister, A. J. In Comprehensive Inorganic Chemistry; Bailar, J. C., Jr., Ed.; Pergamon Press: Oxford, 1973; Vol. 1, p 997. (31) Heo, N. H.; Choi, H. C.; Jung, S. W.; Park, M.; Seff, K. J. Phys. Chem. B 1997, 101, 5531. (32) Heo, N. H.; Chun, C. W.; Park, J. S.; Lim, W. T.; Park, M.; Li, S. L.; Zhou, L. P.; Seff, K. J. Phys. Chem. B 2002, 106, 4578. (33) Heo, N. H.; Kim, S. H.; Choi, H. C.; Jung, S. W.; Seff, K. J. Phys. Chem. B 1998, 102, 17. (34) Heo, N. H.; Jung, S. W.; Park, S. W.; Park, M.; Lim, W. T.; Seff, K. J. Phys. Chem. B 2000, 104, 8372. (35) Heo, N. H.; Park, J. S.; Kim, Y. J.; Lim, W. T.; Jung, S. W.; Seff, K. J. Phys. Chem. B 2003, 107, 1120. (36) Kim, J. J.; Kim, C. W.; Heo, N. H.; Lim, W. T.; Seff, K. J. Phys. Chem. C 2010, 114, 15741. (37) Ferchiche, S.; Valcheva-Traykova, M.; Vaughan, D. E. W.; Varzywoda, J. J.; Sacco, A. J. J. Cryst. Growth 2001, 222, 801. (38) Jeong, G. H.; Lee, Y. M.; Kim, Y. J.; Vaughan, D. E. W.; Seff, K. Microporous Mesoporous Mater. 2006, 94, 313. (39) Seo, S. M.; Lee, O. S.; Kim, H. S.; Bae, D. H.; Chun, I. J.; Lim, W. T. Bull. Korean Chem. Soc. 2007, 28, 1675. (40) Lim, W. T.; Seo, S. M.; Wang, L.; Lu, G. Q.; Heo, N. H.; Seff, K. Microporous Mesoporous Mater. 2010, 129, 11. (41) Alcock, C. B.; Itkin, V. P.; Horrigan, M. K. Can. Metall. Q. 1984, 23, 309. (42) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307. (43) XPREP, Version 6.12, Program for Automatic Space Group Determination; Bruker AXS Inc.: Madison, WI, 2001. (44) Sheldrick, G. M. SHELXL97, Program for the Refinement of Crystal Structures; University of G€ottingen: G€ottingen, Germany, 1997. (45) Doyle, P. A.; Turner, P. S. Acta Crystallogr., Sect. A 1968, 24, 390. (46) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV, p 71. (47) Cromer, D. T. Acta Crystallogr. 1965, 18, 17. (48) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV, pp 148. (49) Loewenstein, W. Am. Mineral. 1954, 39, 92. (50) Bae, D.; Seff, K. Microporous Mesoporous Mater. 1999, 33, 265. (51) Verberckmoes, A. A.; Weckhuysen, B. M.; Schoonheydt, R. A. Microporous Mesoporous Mater. 1998, 22. (52) Vitale, G.; Bull, L. M.; Morris, R. E.; Cheetham, A. K.; Toby, B. H.; Coe, C. G.; MacDougall, J. E. J. Phys. Chem. 1995, 99, 16087. (53) Handbook of Chemistry and Physics, 64th ed.; Weast, R. C., Ed.; The Chemical Rubber Co.: Cleveland, OH, 1983; p F170. (54) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr., Sect. B 1969, 25, 925. (55) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; John Wiley & Sons, Inc.: New York, 1999; p 176. (56) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; John Wiley & Sons, Inc.: New York, 1999; p 203. (57) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon Press: Oxford, 1984. (58) Loos, D.; Schn€ockel, H.; Fenske, D. Angew. Chem., Int. Ed. Engl. 1993, 32, 1059. (59) Beamish, J. C.; Small, R. W. H.; Worrall, I. J. Inorg. Chem. 1979, 18, 220. 2759



ARTICLE

(60) Pott, T.; Jutzi, P.; Schoeller, W. W.; Stammler, A.; Stammler, H.-G. Organometallics 2001, 20, 5492. (61) Baker, R. J.; Cameron, J.; Kloth, M.; Platts, J. A. Organometallics 2004, 23, 4811. (62) Linti, G.; Seifert, A. Dalton Trans. 2008, 3688. (63) Wiberg, N.; Amelunxen, K.; Lerner, L. W.; N€oth, H.; Ponikwar, W.; Schwenk, H. J. Organomet. Chem. 1999, 574, 246. (64) Sharma, B. D.; Donohue, J. Z. Kristallogr. 1935, 117, 293. (65) Deiseroth, H. J.; Kerber, H.; Hoppe, R. Z. Anorg. Allg. Chem. 1998, 624, 541. (66) Wagner, G.; Hoppe, R. J. Less-Common Met. 1986, 116, 129. (67) Kim, S. H.; Heo, N. H.; Kim, G. H.; Hong, S. B.; Seff, K. J. Phys. Chem. B 2006, 110, 25964. (68) Ronay, C.; Seff, K. Zeolites 1993, 13, 97. (69) Song, M. K.; Kim, Y.; Seff, K. J. Phys. Chem. B 2003, 107, 3117. (70) Yeom, Y. H.; Kim, Y.; Seff, K. J. Phys. Chem. B 1997, 101, 5314. (71) Yeom, Y. H.; Kim, Y.; Seff, K. Microporous Mesoporous Mater. 1999, 28, 103. (72) Kim, Y.; Han, Y. W.; Seff, K. J. Phys. Chem. 1993, 97, 12663. (73) Bae, D.; Seff, K. Microporous Mesoporous Mater. 2000, 40, 219. (74) Bae, D.; Zhen, S.; Seff, K. J. Phys. Chem. B 1999, 103, 5631. (75) Heo, N. H.; Seff, K. J. Am. Chem. Soc. 1987, 109, 7986. (76) Zhen, S.; Seff, K. J. Phys. Chem. B 1999, 103, 6493. (77) Armstrong, A. R.; Anderson, P. A.; Woodall, L. J.; Edwards, P. P. J. Am. Chem. Soc. 1995, 117, 9087. (78) Seff, K. Microporous Mesoporous Mater. 2005, 85, 351. (79) Lee, Y. M.; Jeong, G. H.; Kim, Y.; Seff, K. Microporous Mesoporous Mater. 2006, 88, 105. (80) Sun, T.; Seff, K. J. Phys. Chem. 1993, 97, 7719. (81) Solt, H.; Lonyi, F.; Mihalyi, R. M.; Valyon, J.; Gutierrez, L. B.; Miro, E. E. J. Phys. Chem. C 2008, 112, 19423. (82) Heidler, R.; Janssens, G. O. A.; Mortier, W. J.; Schoonheydt, R. A. J. Phys. Chem. 1996, 100, 19728. (83) Vayssilov, G. N.; R€osch, N. J. Catal. 1999, 186, 423. (84) Barthomeuf, D. J. Phys. Chem. B 2005, 109, 2047. (85) Burnett, M. N.; Johnson, C. K. ORTEP-III, Report ORNL-6895 ed.; Oak Ridge National Laboratory: Oak Ridge, TN, 1996. (86) Palmer, D.; Conley, M. CrystalMaker: a crystal and molecular structures program for Mac and Windows, 2.2.4 ed.; CrystalMaker Software Ltd.: Oxford, England, 2002.

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The Pentagallium Cation in Zeolite Y. Preparation and Crystal

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