ELSEVIER Microporous Materials 6 (1996) 195-204

MICROPOROUS MATERIALS

Isomorphous substitution in the microporous titanosilicate ETS-10

M.W. Anderson a,., j. Rocha b, Z. Lin b, A. Philippou a, I. Orion b, A. Ferreira b a Department of Chemistry, UMIST, P. 0. Box 88, Manchester M60 1QD, UK

b Department of Chemistry, University of Aveiro, 3800 Aveiro, Portugal

Received 8 September 1995; accepted 28 November 1995

Abstract

Silicon has been isomorphously substituted by aluminium and gallium in the microporous titanosilicate ETS-10 to produce ETAS-10 and ETGS-10 respectively. For aluminium substitution a series of samples have been prepared with AI/Ti ratios ranging from 0.10 to 0.48. Both aluminium and gallium substitute exclusively in tetrahedral silicon sites in a manner which avoids Ti-O-A1 and Ti-O-Ga linkages. Based upon detailed analysis of solid-state NMR spectra, models for the ordering of aluminium in ETAS-10 are discussed.

Keywords: Zeolite; Titanosilicate; ETS-10; Solid-state NMR; Isomorphous substitution

1. Introduction

ETS-10 (Engelhard titanosilicate structure 10) is potentially a very important new microporous inorganic titanosilicate framework material. The structure which was first synthesized by Engelhard [1,2] and solved by our group [3-5], consists of corner-sharing octahedral titanium(IV) and tetra- hedral silicon. ETS-10 is one of a small group of zeolite or zeotype materials which contain a three- dimensional 12-ring pore system (the only other materials are zeolite Y (FAU), hexagonal poly- morph of zeolite Y (EMT), zeolite beta (BEA) and cloverite (CLO) which has a three-dimensional 20-ring pore system but is very unstable). Furthermore, ETS-10 exhibits good thermal sta- bility up to ca. 650°C in air. Owing to the high

* Corresponding author.

framework charge associated with the octahedral Ti, ETS-10 also has a very high cation exchange capacity. Every titanium in the framework has an associated two minus charge which gives ETS-10 a similar ion-exchange capacity to zeolite Y. The basic anhydrous formula of ETS-10 is

r a - I - • • M2/mTISIsO13 where M is a cation of charge m (3/2 Na + and 1/2 K ÷ in the as-synthesised sample). Further interest resides in the fact that, like zeolite beta, ETS-10 is highly disordered being composed of a random intergrowth of two end- member polymorphs. Polymorph A belongs to a chiral space group and, like zeolite beta, possesses a spiral channel. Consequently, there is interest to synthesise pure polymorph A - perhaps by the use of a chiral organic template - in order to make a potential chiral separator or catalyst.

Recently, we have reported that it is possible to incorporate both aluminium [6] and gallium [7]

SSDI 0927-6513(95)00098-4

196 M. W. Anderson et al./Microporous Materials 6 (1996) 195-204

into ETS-10 to produce ETAS-10 and ETGS-10 respectively. It was demonstrated by 29Si solid- state NMR that both A1 and Ga isomorphously replace Si in tetrahedral sites in a manner which avoids A1-O-Ti and Ga-O-Ti linkages. In this work we investigate this isomorphous substitution more closely and examine models of A1 and Ga ordering in the respective materials.

night at 120°C. The dried powder (5.3 g) was then treated with 10.0 g NaOH 5 wt% solution for 24 h at 200°C, after which it was cooled down, filtered, washed and dried as previously. The final product was calcined in air for 2 h at 350°C (71Ga NMR shows that this treatment does not extract gallium from the framework).

2.2. NMR experimental

2. Experimental

2.1. Synthetic methods

The following preparations give the best condi- tions to prepare pure ETAS-10 and ETGS-10 samples.

Synthesis of ETAS-10 with Si/AI =20. An alka- line solution was made by mixing 11.78 g sodium silicate solution (Na20 8 wt%, SiO2 27 wt%); 2.5g H20, 0.48g NaOH; 0.45g KF. A titanium/aluminium solution was made by adding 5.72g TiCI3 (1.9M solution of TiCla in 2.0M HCI) to a solution made from 5.08 g H20, 0.48 g NaOH; 0.46 g KF; 0.49 g KC1, 0.36 g NaAIO2. The alkaline silicate solution and the titanium/aluminium solution were combined with thorough stirring. 0.1 g seed of ETS-10 was added to the resulting gel. The gel, with a Ti/AI mole ratio of 2, was autoclaved under autogenous pres- sure for 6 days at 200°C after which the crystalline product was filtered, washed with distilled water at room temperature and dried at 120°C.

Synthesis of ETGS-10 with Si/Ga = 13. An alka- line solution was made by mixing 11.43 g sodium silicate (Na20 8 wt%, SiO2 27 wt%), 7.56 g H20, 1.68 g NaOH, 0.94 g KF. A titanium/gallium solu- tion was made by adding 5.70g TiCIa (1.9M solution of TiC13 in 2.0 M HC1) to 0.78 g GaCI3. The alkaline silicate solution and the titanium/gallium solution were combined with thorough stirring. 0.12 g seed ETS-10 was added to the resulting gel. This gel, with a composition

4.58Na20:0.93K20:5.91SiO2:TiO2:0.25Ga203: 126H20, was autoclaved under autogenous pres- sure for 5 days at 200°C. The resulting product was cooled down to room temperature, filtered and washed with distilled water and dried over-

All solid-state NMR spectra were recorded on a Bruker MSL-400 solid-state spectrometer operat- ing at 79.494 MHz, 122.028 MHz and 104.262 MHz for 29Si, 71Ga and 27A1 respectively. Samples were spun at the magic-angle in a double- bearing zirconia rotor at 5000 Hz for 29Si and 15000Hz for 27A1 and 71Ga. All spectra shown are single-pulse and the interpulse delays were 30 s, 0.2 s and 0.2 s for 29Si, 71Ga and 27A1 respec- tively. For 29Si spectra a 3.5/~s 45 ° pulse was used and for 27A1 and 7~Ga a 0.6 #s 1r/18 pulse was used. 27A1 satellite transition MAS NMR (SATRAS) spectra were recorded close to (on-) resonance and 500 kHz off-resonance. Due to the relatively low AI contents of the samples about 150,000 transients were accumulated per spectrum. To measure t2 relaxation times, the Carr-Purcell-Meiboom-Gill (CPMG) sequence (90x-z-180Facquire) was used.

2.3. XRD experimental

The X-ray diffraction patterns were recorded on a Rigaku diffractometer (CuK~ filtered by Ni) from 3 ° to 50 ° 20 (step size 0.05 ° ) or from 23 ° to 29 ° 20 (step size 0.01°). Silicon powder was used as an internal standard in these latter measurements.

3. Results and discussion

Fig. 1 shows a series of 29Si MAS NMR spectra for ETAS-10 with a range of isomorphous substitu- tion. Fig. 2 shows a 29Si MAS NMR spectrum for ETGS-10 in comparison with that for ETAS-10 and ETS- 10. Assignment of chemical environments is also given on these figures in accordance with

M. W. Anderson et aL /Microporous Materials 6 (1996) 195-204 197

Si(3SI ,1TI)

f

AI /T i Si(4SI)

I I I I I I

- 8 5 - 9 5 - 1 0 5

8 Fig. 1.29Si MAS NMR spectra of ETAS-10 with varying AI/Ti ratios as indicated. Notice the broadening of the peak at ca. - 104 ppm.

previous work [6, 7]. For both types of substitution only three silicon chemical environments are observed at low to medium substitution levels which have been attributed to AI, Ti or Ga, Ti avoidance. Most importantly, the absence of a resonance corresponding to Si(3Si, IA1) or Si(3Si, lGa) is interpreted by the following scheme (similar scheme for gallium substitution).

Si Si Si

E T S - 1 0 " 1 3 - S i - S i - - Si--Ti

Si

two chemical environments

Si Si Si Si Si Si

E T A S - 1 0 ' l ' i - - S i ~ A l ~ S i - - T i T i - - A l - - S i - - S i - - T i

i Si Si ~t S i ~ $ i

k a l lowed not allowed ~k

resonaace not ob|¢rvefl resonance observed

which is direct proof of an avoidance principle. The position for aluminium or gallium substitution is shown schematically in the diagram in Fig. 3.

Sl(3$i,lTI)A

ET

i i i

-9o 1~oo 111o

Fig. 2. 298i MAS NMR spectra of ETS-10, ETAS-10 and ETGS-10. The insert shows a deconvolution of the spectrum for ETGS-10.

Substitution of both aluminium and gallium almost exclusively into tetrahedral sites is further substantiated by the 27A1 and 7tGa MAS NMR spectra respectively (see Fig. 4). In ETAS-10 there is an indication of a small (less than 3 mol%) incorporation of aluminium in octahedral environ- ment (signal near 0 ppm), however, we have no proof whether this aluminium isomorphously replaces titanium or resides in extra-framework sites.

Nitrogen adsorption isotherms (not shown) on ETS-10, ETAS-10 and ETGS-10 are all rectilinear, characteristic of microporous materials, with sim- ilar maximum uptakes (estimated from the iso- therm plateau) of ca. 0.12 gnitrogen/gadsorbant" Both ETAS-10 and ETGS-10 exhibit X-ray diffraction patterns that are very similar to ETS-10 (see Fig. 5). There are some extra weak reflections and we are in the process of trying to simulate these patterns using similar methods to those used for ETS-10 [3,4], however, it is too early to say

198 M. I~. Anderson et aL /Microporous Materials 6 (1996} 195-204

f /

Fig. 3. Framework connectivity in ETS-10. The local silicon environment is identical for both polymorphs A and B as well as the real disordered material. Silicon is white and titanium is black. Oxygen atoms not shown lie approximately halfway along each bond. One aluminium atom is shown (shaded) incorporated at a site which avoids neighbouring titanium. Gallium is incorporated at a similar site to aluminium.

T

75

27AI

i

5'0 2 5 () 8

I

' 460 ' 6 -4(30

Fig. 4. 27A1 MAS NMR spectrum of ETAS-I0 (A1/Ti=0.24) and 71Ga MAS NMR spectrum of ETGS-10 (Ga/Ti=0.37).

ETGS-10 I i i

I TS- 1 0

4 14 24 34

20/degrees

Fig. 5. Powder XRD patterns of ETS-10, ETAS-10 (AI/Ti= 0.24) and ETGS-10 (Ga/Ti = 0.37). Diamonds and solid circles depict faint ETS-4 and unidentified reflections respectively. The peak given by the internal silicon powder standard is denoted 'Si'.

hi. IV.. Anderson et aL/Microporous Materials 6 (1996) 195-204 199

whether these extra reflections are due to a super- lattice or to small concentrations of impurity phase. It is clear, however, that there is a small expansion of the unit cell, indicated by a shift in all reflections. This is illustrated in Fig. 6 for the reflection at ca. 24.5 ° 20 which shows a progressive shift to higher d-spacing with increasing aluminium content. A similar expansion of the unit cell is always observed in zeolites as the framework alu- minium content increases. [8]

Increasing the aluminium content in ETAS-10 also has a strong broadening influence on the linewidths of resonances in the 295i MAS NMR spectrum. In particular the line at ca. - 104.5 ppm, see Fig. 1, becomes very broad increasing from 40 Hz in ETS-10 to 200 Hz in ETAS-10 with a AI/Ti = 0.48. There is also a significant shift in the position of this resonance from ETS-10 ( - 103.5 ppm) to ETAS- 10 A1/Ti = 0.48 ( - 104.6 ppm). In contrast, the lines at -94 .8 and -96 .7 ppm remain constant in ETAS-10 relative to ETS-10. In this respect the line at - 104.5 ppm (Si(4Si)) is the most sensitive to aluminium substi- tution. In fact there is an almost perfectly linear correlation, see Fig. 9, between the F W H M of the resonance at -104.5 ppm and the A1/Ti ratio (AI/Ti ratio is equivalent to aluminium content as aluminium substitutes for Si not Ti). It is interes- ting to discuss why this resonance is so sensitive to aluminium incorporation. In the case ofzeolites, it has been shown that the broadening of spectral lines is often due to an increase in the t2 relaxation

24,44 24.58 Z4.63

ETS-IO

ETAS-1 0 (0.24)

r . . . . . i

23.7 24.7 25.7 26.7

20 /deg rees

Fig. 6. Powder XRD patterns of ETS-10, ETAS-10 (A1/Ti= 0.24) and ETAS-10 (A1/Ti=0.48) in the region 20=23.7 ° to 26.7 ° showing progressive shift of reflection to higher d-spacing with increased aluminium content.

200

100 / /

0.1 0.2 0.3 0.4

AI/Ti

Fig. 7.27A1 SAT RAS spectra of two ETAS-10 samples recorded on resonance using spinning rates of 14.5 (bottom) and 15 kHz (top).

brought about by 29Si-27A1 dipolar interactions [9]. To assess this possibility, we have measured spin-spin relaxation times of a selected ETAS-10 sample (Si/AI=0.24) using a CPMG pulse train (see Fig. 7). The two resonances at high frequency have short t2 times (about 5 ms), confirming their assignment to silicons with A1 neighbours; the signals attributed to Si(3Si, lTi) and Si(4Si) envi- ronments have similar, much longer relaxation times (35-49 ms). Therefore, the broadening of the Si(4Si) resonance cannot be explained by the above mentioned dipolar mechanism. Fig. 3 illustrates that the Si(4Si) site remains at least 3 T-sites remote from the position of aluminium insertion. The 29Si-27A1 dipolar interaction falls off with 1/r 3 and is expected to be fairly weak across the twelve ring. We conclude that the most likely explanation for the linebroadening of the Si(4Si) signal is a distortion of the lattice upon A1 incorporation. ETS-10 can be considered to be composed of rods assembled in an orthogonal manner. The centre of these rods are the • . .Ti-O-Ti-O.. . chains. These rods are very rigid units which are connected together through the Si(4Si) sites which by comparison are somewhat 'floppy'. Aluminium incorporation results in a distortion of the lattice which is manifested most significantly at the closest Si(4Si) site thereby broadening the29Si NMR resonance.

In order to study the local distortion of the A1 sites as a function of the degree of A1 incorporation we have recorded 2TA1 SATRAS spectra [10,11]. Selected examples of on-resonance spectra are

200 M. W. Anderson et aL /Microporous Materials 6 (1996) 195-204

AI /T i=0.48

500 300 100

kHz

I I t I I

-85 -95 -105

ppm from TMS 1

Fig. 8. Variation of the full-width-at-half-maximum of the - 104.5 ppm peak in the 29Si MAS NMR spectrum of ETAS-10 as a function of A1/Ti ratio (this is equivalent to aluminium content as aluminium substitutes for Si and not Ti).

Fig. 9. 29Si MAS N M R spectra (absolute intensity) recorded using the CPMG pulse sequence and the echo delays depicted. The arrows show the lines assigned to Si sites with AI neigh- bours which have short t2. The exact values measured for tz of the different 29Si resonances are: - 9 4 . 8 p p m t2=49 ms; -96 .7 ppm t 2 =35 ms; - 104.7 ppm t2 =43 ms.

shown in Fig. 8. We were very careful in comparing spectra of samples with a range of A1/Ti ratios, prepared using the same synthesis route and with similar crystallinity (as ascertained by powder XRD). The inner (m= + 1 / 2 ~ m = +3/2) sate- llite transition (ST) sidebands spread out over a frequency range 2v o=(3/10)Co, where C o (quadrupolar coupling constant) is a measure of the distortion of the coordination polyhedra. Fig. 8 clearly shows that the envelope of the ST sidebands changes with increasing A1 content; i.e. the envelope corresponding to the sample with less aluminium (AI/Ti=0.1) is Gaussian-like display- ing no singularities. This effect is usually observed in the 27A1 SATRAS spectra of glasses and disor- dered materials; it is due to the fact that varying bond angles and distances cause distributions not only of the chemical shifts but especially of the electric field gradient (EFG) tensors [12]. In

ETAS-10 the dispersion of EFG probably arises as a result of lattice distortion upon AI insertion. An average C o of 3.5+0.5 Mhz was estimated from the 500kHz off-resonance (not shown). Detailed inspection of SATRAS spectra suggest that the average C o increases slightly with increas- ing A1 content, indicating that the AI tetrahedra become more distorted as more aluminium is incor- porated. However, calculation of accurate average C o values requires simulation of the ST sideband pattern using a model which takes into account the distribution of the EFG tensors [12]. Work along this line is in progress.

3.1. Predicting framework compositions and ordering from 29Si MAS N M R

One important outcome of the presence of these two avoidance rules (AI, Ti and Ga, Ti) is that the framework Si/AI, Si/Ti and Si/Ga ratios can

M. W. Anderson et al./Microporous Materials 6 (1996) 195-204 201

easily be calculated from the 298i MAS N M R spectrum in a similar fashion to zeolites.

First consider ETAS-10 where, because A1-O-Ti linkages are not allowed all aluminium environ- ments are

$i I o

I S~O.---AI--.-O---Si

I 0 I Si

and all Ti environments are

Si

o /

Si.-.-O-..~'~O---Si

Si

The only difference being the factor m in the divisor of Eq. 4. In ETGS-10 Eq. 4 still holds and the corresponding equation for Si/Ga ratio is

4 - m 4 - n

L 2 Isi(nGa,mTi) Si/Ga = .=o m=O 4 - m 4 - n ( 5 )

0 .25 2 Z nlsi(nGa,mTi) n=O m=0

Specifically for ETAS-10 there are 3 chemical environments

Si(1Al,lTi) = x

Si(0Al,lTi) --y

Si (0A1,0Ti) = z

Therefore, using Eqs. 3 and 4

Si 4(x + y + z) - ( 6 )

A1 x

Consequently, both aluminium and titanium neighbour four silicon atoms. The relative amount of silicon in the sample can simply be determined from the total area under the 29Si N M R spectrum

4 - m 4 - n

Si oc 2 2 Isi(nAl,mTi) ( 1 ) n=O ra=O

Then the amount of aluminium is given by multiplying the area of each resonance by the number of neighbouring a luminum atoms, nIsitnAl,mTi), and summing this for every peak. A factor of one quarter must be applied as each aluminium will be counted four times in this calculation

4 - m 4 - n

' Aloc0.25 ~ ~ l'lIsi(nAl,mTi ) (2) n=O m=O

4--m 4 - n

L L Isi(nAl,mTi) => Si/A1 = . = o ,. = o 4-,, 4 - . (3)

0.25 2 ~2 nlsi(nAl,mTi) n=0 m=0

A similar argument for titanium gives

4 - m 4--n

L 2 Isi(nAl, mTi) Si/Ti = . = o m = o 4 - m 4 - . ( 4 )

0.25 ~ ~ mlsi(nAl,mTi ) n=0 m=0

Si 4(x + y + z) and - (7)

Ti x + y

A1 x - ( 8 )

Ti x + y

Further, for ETAS-10 from stoichiometry

Si + AI T ~ - 5 (9)

substituting Eqs. 7 and 8 in Eq. 9 gives

Y - 4 = constant (10)

Z

therefore also

X X - = 4 - ( 1 1 ) z y

Eqs. 10 and 11 can be used to check the validity of the model and some comparisons with experi- ment are shown in Table 1. Further comparisons with chemical analyses are illustrated in Tables 2 and 3. All these findings indicate that the model works well at low to medium aluminium incorpo- ration, but begins to break down at higher levels of aluminium incorporation.

In order to investigate whether this break-down is due to a break down of the model or due to

202 M. W. Anderson et aL /Microporous Materials 6 (1996) 195-204

Table 1 Comparison of theoretical and experimental data

Sample y/z a x/z 4x/y A1/Ti Si/Ti Si/A1 Si(0AI,1Ti)/Si(0A1, 0Ti) Si(1AI, 1Ti)/Si(0A1,0Ti) 4Si(1AI, 1Ti)/Si(0A1,1Ti)

1 73.0/18.6=3.92 8.5/18.6=0.46 4 x 8.5/73.0=0.47 0.10 4.9 47.1 2 63.5/16.7 = 3.80 19.7/16.7 = 1.12 4 x 19.7/63.5 = 1.24 0.26 4.8 18.7 3 54.4/16.2=3.36 29.3/16.2=1.81 4 x 29.3/54.4=2.15 0.35 4.8 13.6

Should equal 4. b x/z should be equal to 4x/y.

Table 2 Analytical data

Sample A1/Ti Si/Ti Si/AI

NMR Chem. anal. a NMR Chem. anal. NMR Chem. anal.

1 0.1 0.12 4.9 5.02 47.1 41.7 2 0.26 0.32 4.8 4.7 18.7 14.1 3 0,35 0.36 4.8 4.5 13.6 12.6

Chemical analysis on sample 1 was by ICP-AES and for samples 2, 3 and 4 (see Table 3) using energy dispersive adsorption of X-rays (EDAX). A bulk analysis was not appropriate for these last 3 samples as small impurity phases were present. Therefore, EDAX was selectively performed on the ETAS-10 or ETGS-10 phase,

Table 3 Analytical data

Sample Ga/Ti Si/Ti Si/Ga

NMR Chem. anal. ~ NMR Chem. anal. NMR Chem. anal.

4 0.37 0.41 4.7 5.0 12,7 12.1

Chemical analysis performed by EDAX. See footnote to Table 2 for explanation.

inaccuracies in the determination of areas in the 298i MAS N M R spectra it is necessary to examine how these areas are influenced by aluminium content. In this respect two different extreme models have been investigated: first, strict A1, Ti avoidance; second, random aluminium distribution.

Fig. 10 shows relative signal intensities expected for different silicon environments for the two situa- tions (a) AI, Ti avoidance and (b) random distribu- tion of aluminium. The latter spectral intensities are calculated assuming a binomial distribution of

silicon environments [13,14] given by:

%Isi~oAl,~i~ = (1 _p)3 x 80 = ~ x 80

%Isi(1gl.lTi) = 3 X ( 1 _p)2p X 80 ( 71 = 3 x ~ x80

%/Si(2AI,1Ti) = 3 x ( 1 _p)p2 x 80 ( 71 =3x ~-~ x80

M. W. Anderson et aL/Microporous Materials 6 (1996) 195-204 203

.g g

(a)

so~ 70 ' l .

60

50 •

40

30 "

2olo -- -~" ~,¢J~- 19-0 +

0 " / '~

o

- - - - - S i ( 0 A I , 1T i ) - - - S i ( 1 A I , 1T i ) . . . . . Si(,0AI, 0T i ) - - S i (1AI , OTi) J

J

J

J

. /

% /

?oV ~ •

0.2 0.4 0.6 0.8 Number of A1/5T sites

g 0J

t ~

- - - - - S i ( 0 A I , 1T i ) ] - - - S i ( I A I , I T i )

I . . . . . S i ( O A I , 0 T i ) 80 . . - . " . - - - ] - - S i ( I A I , OTi)

] - - - S i ( 2 A I , I T i ) 7o - " . . - . - - . . . . . Si (2A l , OTi)

30 ~ ~ - o ~

20 -e . - . Do- . . ~ /

0 -

0 0.2 0.4 0.6 0.8 (h) Number of A1/5T sites

Fig. 10. Plots showing intensity of 29Si MAS NMR resonances for ETAS-10 as a function of aluminium incorporation (data for three samples given as circles, squares and diamonds). + 2% error shown on intensity measurement. For a model to be valid all intensities for an individual sample should yield the same aluminium concentration. This is true for the samples labelled with circles and squares for (a) A1, Ti avoidance model, and vertical lines showing aluminium concentration have been drawn accordingly. (b) Is for random incorporation of alumin- ium and the model fits so poorly that confidence limits have been excluded for clarity.

%Isi(3Al, lTi) =p3 × 8 0 = ~ × 80

% I s i ( 0 A 1 , 0 T i ) = ( 1 - - p ) 4 × 2 0 = ( ~ + I ) 4 X 2 0

%Isi(1A1,0Ti) = 4 × ( 1 _p)ap × 20

= 4 x ~ x20

%/Si(2AI,0Ti) = 6 x (1 _p)ZpZ x 20

= 6 x ~5 x20

%Isi(3AI,0Ti) = 4 x ( 1 _p)p3 x 20

= 4 x ~3 x20

%/Si(4A1,0Ti) = p 4 X 20 = ~ ~'~ X 20

where p is the probability that a neighbouring T site is aluminium and R is the Si/AI ratio [p is related to R by p = 1/(R+ 1)]. The factors, 80 and 20, originate from the fact that four sites neighbour titanium for every one site that does not neighbour titanium.

Fig. 10 illustrates that the Si(1A1,0Ti) intensity is always below ca. 8% for a random aluminium distribution even at high aluminium concen- trations. Consequently, it is important not to base all arguments exclusively upon the presence or absence of this resonance. To this end the intensity of all resonances for three different samples have been superimposed upon Fig. 10. A confidence limit of +2% on the integrated area of NMR resonances has been imposed. It can be seen that for low to medium aluminium concentration (Si/AI>20 or up to ca. 25% of Si (4Si) sites occupied by aluminium) an excellent fit is found for obeyance of the A1, Ti avoidance model. At higher concentrations neither model fits, although the avoidance model is much better. As a random model is clearly unacceptable, ordering of the aluminium must be invoked. This is most probably due to a coupling of a small break-down in the AI, Ti avoidance but with Lowensteinian A1, A1 avoidance operating.

3.2. Repercussions of aluminium ordering on acidity in ETS-IO

ETS-10 has a very high ion exchange capacity as every framework titanium atom has two associ- ated monovalent cations. In the as-synthesised form 3/4 of these cations are Na ÷ and 1/4 are K ÷ and these are readily exchanged for other

204 M. W. Anderson et aL / Microporous Materials 6 (1996) 195-204

cations. Indeed it has been reported that ETS-10 is highly selective for Pb 2÷ uptake [15]. In prin- ciple it should also be possible to incorporate Bronsted acidity into ETS-10 possible via ion exchange with ammonium ions followed by calcination in a similar manner to zeolites. However the bridging acid sites formed would be of the form SiO(H)Ti which initial calculations suggest would be rather weak compared to zeolites. Isomorphous substitution gives the possibility to form acid sites of a more conventional zeolite nature, i.e. SiO(H)A1. In zeolites there is a strong correlation between aluminium content and acidity - the acidity decreases markedly at high aluminium content. These weak acid sites are usually ascribed to the closeness of aluminium in the framework, i.e. the presence of A1-O-Si-O-AI linkages (Lowenstein's rule precludes A1-O-AI linkages). Consequently, it is impossible to achieve a very high acid site density whilst retaining very high acidity. In ETAS-10, however, because of alumin- ium ordering through Ti, AI avoidance the closest aluminium linkage is AI-O-Si-O-Si-O-AI even at Si/AI=4. In theory, therefore it should be possible to generate both a high acid site density and high acidity. Initial experiments to investigate this phenomenon have indeed revealed 1H chemi- cal shifts indicative of very high acidity. This work will be reported separately.

A c k n o w l e d g e m e n t

We would like to thank the European Commission through Joule II for the funding of

AF, to Shell for partial support for AP and to EPSRC.

R e f e r e n c e s

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[4] M.W. Anderson, O. Terasaki, T. Ohsuna, P.J. O'Malley, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha and S. Lidin, Philos. Mag., 71 (1995) 813.

[5] T. Ohsuna, O. Terasaki, D. Watanabe, M.W. Anderson and S. Lidin, Stud. Surf. Sci. Catal., 84 (1994) 413.

[6] M.W. Anderson, A. Philippou, Z. Lin, A. Ferreira and J. Rocha, Angew. Chem., Int. Ed. Engl., 34 (1995) 1003.

[7] J. Rocha, Z. Lin, A. Ferreira and M.W. Anderson, J. Chem. Soc., Chem. Commun., (1995) 867.

[8] E. Dempsey, G.H. KOhl and D.H. Olson, J. Phys. Chem., 73 (1969) 387.

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[14] G. Engelhardt, U. Lohse, E. Lippmaa, M. Tarmak and M. M~igi, Z. Anorg. Allg. Chem., 482 (1981) 49.

[15] S.M. Kuznicki and K.A. Thrush, US Pat. 4,994,191 (1991).