Characterisation and evaluation of hypothetical zeolite frameworks

1222

Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.

CHARACTERISATION AND EVALUATION OF HYPOTHETICAL ZEOLITE FRAMEWORKS Bell, R . G . \ Foster, M . D . \ Simperler, A.^ and Klinowski, J.^ ^Davy Faraday Research Laboratory, Royal Institution of Great Britain, 21 Albemarle Street, London WIS 4BS, UK. E-mail: rob(g)ri.ac.uk ^Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 lEW, UK.

ABSTRACT A series of hypothetical zeolites, derived from the results of tiling theory, has been evaluated using computational chemistry techniques. Simulated heats of formation (i.e. the lattice energy with respect to a-quartz or a-berlinite for silica polymorphs and AIPO4 polymorphs, respectively) are used as an initial criterion for the chemical "feasibility" of these structures. This data is further correlated with various structural and topological properties, such as framework density, coordination sequences, accessible volume and internal surface. Uninodal frameworks have been treated both as silica and AIPO4 polymorphs, and comparisons made between the two compositions. Finally, we discuss three hypothetical structures with respect to their feasibility, structural properties and secondary/periodic building units.

INTRODUCTION The systematic enumeration of all possible networks of atoms in inorganic structures is a matter of considerable interest, not least in the field of zeolite science with the vast application potential of materials possessing varied microporous architectures. In the past, new 4-connected networks have been generated by empirical methods, either by linking together structural subunits in new ways [1] or by computer search algorithms accompanied by the use of energetic cost functions [2-3]. Recently a complete solution to the problem has been reported based on recent advances in mathematical tiling theory [4]. So far 164, 117 and 926 topological types of 4-connected uninodal, binodal and trinodal networks, respectively, have been enumerated. In order to assess the nets thoroughly in terms of solid state chemistry, we have optimised them as inorganic solids using a suite of computer simulation methods, and evaluated the structures against a set of criteria, including measures of thermodynamic stability, density and porosity. The data from the known zeolite structures are used as a "control set" with which to compare the hypothetical frameworks. We present overall trends of the relative lattice energies of hypothetical uni-, bi-, and trinodal hypothetical zeolites versus their framework density, which is the first step in screening the pool for chemically feasible structures. Roughly, all structures which are < 30kJ/mol less stable than quartz can be considered as feasible. The set of structures is further characterised in terms of accessible volume and internal surface area, important descriptors of porosity and molecular sieve function. We discuss briefly the differences and/or similarities between uninodal AIPO4S and their corresponding silica polymorphs. The question we address is whether it is sufficient to consider only siliceous frameworks or whether different compositions may give qualitatively different sets of results. Three representative hypothetical structures are introduced; their structural components (i.e. secondary or periodic building units) are elucidated and their properties are described. METHOD The 4-connected nets generated by tiling theory were converted into model silicate materials by inserting silicon atoms at the net vertices and oxygens at points midway between the vertices. Each structure was optimised first by DLS [5] and subsequently by energy minimisation with GULP [6] using the ionic potential model of Sanders et al.[7]. This forcefield is known to give a fairly accurate correlation with experimental heats of formation for silica polymorphs. The minimised structures have been characterised by their lattice energies, crystallographic data, framework densities, internal volume and surface area. In this work we focus on an overview of all hypothetical zeolite structures with a relative lattice energy < 30 kJ/mol. The energy data and other quantities have been obtained from the GULP output, whilst the connectivity was obtained

1223 from zeoTsites (version 1.2)[8].To calculate the accessible volume and surface area, we Volume module of the Cerius^ package [9] which applies the Connolly method [10] In addition to the Si02 composition, uninodal structures have also been considered as alternating Al and P atoms within the structures, where possible (i.e. only those structures odd-membered rings were treated as ALPO4S). Thus, 126 ALPO4S were handled in the same above and the Gale and Henson potentials [10] were used for GULP calculations.

used the Free ALPO4S, with possessing no way described

RESULTS Hypothetical silica polymorphs A plot of lattice energy (relative to a-quartz) of the hypothetical silica polymorphs with Eiatt,si02< 200 kJ/mol, against framework density is given in Figure 1. The vast majority are more than 30 kJ/mol less stable than quartz, the range into which all known siliceous zeolites (marked by black squares boxes in Figure 1) fall, as already demonstrated by Akporiaye and Price [11]. 200

150

m Known

o 0*00

^ 100 LU

o Trinodals

4-

a Binodals A Uninodals

i5 p

'-^'M^n^lr '

50

0

5

10

15

20

25

30

35

40

FD(T/1000A^ Figure 1. Relative lattice energy, Eiatt,si02, vs. framework density, FD, for hypothetical silica polymorphs. On this thermodynamic basis, we can exclude a large number of structures as being probably not readily synthesised by hydrothermal methods, i.e. mainly those with Eiatt,si02> 30 kJ/mol. Thus, we study the next correlations only for the structures we consider as thermodynamically feasible (i.e. 94 uninodals, 36 binodals, and 113 trinodals). A quantity similar to the FD is the coordination sequence of the 4^*" shell that can be used as a secondary criterion to screen for feasible structures. Both FD and the coordination sequence of the 4^^ shell show a clear linear correlation for the known structures [11]. Hence, besides having a reasonably low relative lattice energy, a hypothetical zeolite should be found within this region or very close to it, to be considered as chemically feasible. The graph in Figure 2 shows the correlation of Eiatt,si02 and the coordination sequence of the 4* shell. Three of the known zeolites deviate quite obviously from the linear region, i.e. OSO (Berylllosilicate), CZP (ZnPO), and WEI (BePO), respectively, due to their different chemical nature. We omit these from our graphs for the sake of clarity. The uninodal structures are more commonly found among the more stable structures, with many being also dense, non-porous structures. Some of binodal and trinodal structures show low FD and also a relative lattice energy in the feasibility region. To be of major interest as a catalyst or molecular sieve, a zeolite must provide accessible volume and a large internal surface area. In addition to the thermodynamic feasibility we also evaluate their

1224 properties in these respects by calculating accessible volumes and internal surface areas.

30 -

\

* * < . ^IS.

'3

•^ i

^ 2 5 -

2 S 20 -

* Knowi * Trinodals

iJ5

^^ Binodal

lll5-

i Uninodal

4S>

^^

^iT*-^

10 -

i^

^ 5 -

&

i.

n -

10

20

30

40

50

CoondinadonSequence -4

60

shell

Figure 2. Relative lattice energy, Eiatt,si02, vs. average coordination sequence of the 4* shell for hypothetical silica polymorphs. 35 30 A =r o ^ 25 J &

3

30

* , '^ ^ ' ^ *

;*

"

o

€25

^* ^^

^ KnovMi

S 20 LD

> Trinodals ^^Binodals

15-

i Uninodals

2 p; 20 'v>

^ ,^

LU 15 QJ

^t^ ^

"cS 10 Oi

10

5 0 -

1

1

1

1

20 30 40 10 accessible volume/T [A /T-site]

50

surface area/T [A /T-site]

Figure 3. Relative lattice energy, Eiatt,si02, vs. accessible volume (plot to the left) and relative lattice energy, Eiatt,si02, vs. surface area for hypothetical silica polymorphs. The majority of structures with large accessible volumes are binodal or trinodal frameworks, whilst the uninodal frameworks represent the majority of structures with little or no accessible volume, and therefore, no internal surface area. A1P04 vs. silica polymorphs A comparison between hypothetical uninodal aluminium phosphates and their isostructural silica polymorphs reveals more similarities than differences between the two groups of structures. The most significant is that calculations of relative stabilities showed the AIPO4S to be more stable relative to berlinite than the silica polymorphs are with respect to quartz. However, with only a few exceptions, the stability sequences of AIPO4S and silica polymorphs show similar trends for the 80 most stable structures as can be seen in Figure 4.

1225 100

1 o hyp-AIP04s

^p' a

a hyp-ZEOs

o F — ^)

OD

80 Dd

D

n a n n

60

CM

u

^> '^

(/) CD c

40

>-

of a

s

a

• * —

CO

^^

20

LU

<

L DaarmB#nDniiSl»%ii^^

0 hyp-AIP04S in order of increasing AEiatt,beriinite and their isostructural hyp-ZEOs

Figure 4. Calculated relative lattice energies of the hypothetical AIPO4S in order of increasing energies and the corresponding data for their isostructural hypothetical silica polymorphs ("ZEOs"). With more dense structures, the Gale and Henson potential overestimates the strength of the A l - 0 bonds, and, thus, the stability of the structure. All the other important correlations, as shown in Figures 1-3 give similar trends for both hypothetical uninodal AIPO4S and their silica polymorphs (so we do not show such plots). An attempt to find a linear dependence between average geometries and stability failed, as we could find none at all. However, a study of the amount of distortion of the TO4 units of AIPO4S and ZEOs (silica polymorphs) revealed a significant difference between these two families of zeotypes. To examine the magnitude of distortion of TO4 units, we calculated the difference between the largest and smallest tetrahedral angles, Aa(O-T-O), as can be seen in Figure 5. 80 -

1

80

60 •

S 40-

%

<^

at

-1

40

n

20-

20

' known-ZEOs ^ hvp-ZEOs

* 0 -

I 60

-1

. ^

n

E 60

i''"

1

80

, 20

1

40

Aa.(;0-Si-0)fl

t
1

0 +-

60

0

Z 40 5. m 20

l
o420

40

Aa(0-AI-0)rt

D

20

40

60

AaCO-P-0)f]

Figure 5. From left to right: Relative lattice energy, AEiatt,si02, vs. Aa(O-Si-O), and relative lattice energy, AEiatt,beriinite, VS. Aa(O-Al-O) and Aa(O-P-O) for known and hypothetical uninodal silica polymorphs and AIPO4S. We find in ZEOs that values for Aa(O-Si-O) range between 0 and 7°. In AIPO4S the values for AIO4 units are between 2 and 11° and in PO4 units between 0 and 7°, for all structures with AEiatt < 20 kJ/mol. Structures with AEiatt > 20 kJ/mol have quite distorted TO4 units; thus, Aa(O-Si-O) can be up to 32°, . Aa(O-P-O) ranges up to 30°, whereas Aa(O-Al-O) ranges up to 55. The differences in the a(O-T-O) angles become clear when we consider the ionicity [12] of the AIPO4 framework, which allows greater distortion.

1226 Three trinodal zeolites with D4R and low framework density We selected 3 trinodal zeolites, which contain D4R (double 4-membered rings) and have low framework densities. Zeolite 3 4 2 8 (Figure 6a) has an FD of 13.05 and the accessible volume is 29.41 AVX-site. The structure is 27.8 kJ/mol less stable than quartz. The structure is tetragonal (space group I4/mmm), and is composed of chains of D8Rs, which are separated from each other via D4Rs (Figure 6b). Alternate D8Rs are twisted by 90°. These chains, which run in the a and b directions, define [4^^8^10^] cages, which are separated from each other by D8Rs, and may additionally be accessed through 10-rings (Figure 6c).

(a)

(b)

(c) Figure 6. (a) Zeolite 3428 and (b-c) its structural details.

Zeolite 3 7 7 7 (Figure 7a) has an FD of 15.84 and its accessible volume is 16.84 AVT-site. The structure is 21.7 kJ/mol less stable than quartz. The structure is tetragonal (P4/mmm). The main building unit is the [4^6^] sodalite cage (Figure 7b, marked green). In the a and b directions they are linked via D4R units, but in the c they are directly fused via a common 6-ring. Larger [8^6^^4^] cages are thus defined (figure 7c), which present an 8-ring channel in one direction (c) only.

1227

(a)

(b)

(c)

Figure 7. (a) Zeolite 3777 and (b-c) its structural details. Zeolite 3_835 (Figure 8a) has an FD of 15.43 its accessible volume is 18.33 AVX-site. The structure is 17.34 kJ/mol less stable than quartz. The tetragonal (I4/mmm) structure can be thought of being built up from layers of D4Rs (Figure 8b, marked green) and [4^5^] units (Figure 8b, marked red). Eight of the [4^5^] units surround each D4R in the way pictured in Figure 8b. Other recognisable subunits are the [4^^6^8^] cages (Figure 8c, marked blue), which define the 8-membered ring channels by sharing 8-membered rings with each other, and [4^5^6^8^] units (Figure 8c, marked yellow), which share a 4-membered ring with a D4R and an 8-membered ring window with a [4^^6^8^] cage.

1228

(a)

4' •.'.'^ "'

(b)

(c)

Figure 8. (a) Zeolite 3 8 3 5 and (b-c) its structural details.

CONCLUSIONS Purely mathematical models have been used to generate uninodal, binodal, and trinodal zeolitic structures. Atomistic calculations with the use of appropriate forcefields reveal the relative stability of silica and AIPO4 polymorphs with respect to quartz and berlinite, and other structural parameters of interest. A screening procedure has been developed by relating the data of the hypothetical structures to those of the already known frameworks. The most decisive feature we find is a region of linear dependency between stability and framework density. If hypothetical structures with data within this linear region or close by are found, they can be considered as chemically feasible. These selected structures are characterised according to their accessible volume and the internal surface area to filter potential candidates for catalytic reactions and/or sorption. As well as considering purely siliceous frameworks, we also address the question of whether our study is applicable to frameworks of all possible compositions. This led to the comparative study of uninodal hypothefical ZEOs and AIPO4S. It is clear that the use of different potentials predicted the AIPO4S to be more stable than their corresponding silica counterparts, though the trends were unmistakeably the same. Also other correlations turned out to be the same for both structure types. However, the Aa(O-T-O) values, i.e. the distortion of the TO4 units, revealed clearly the chemical differences betweeb the ZEOs, with a more covalent character, compared to the ionicity of the AIPO4S, where aluminium exists as Al^^ cation, surrounded by four P04^' anions. The outcome of this study is that calculations for the silica polymorphs are sufficient as a rough guide to stability for both the hypothetical AIPO4 and ZEO structures, but detailed differences of AlPOs and ZEOs can be only revealed by simulating both the zeotype families with appropriate forcefields.

1229 ACKNOWLEDGEMENTS We are grateful to the EPSRC and the Leverhulme Trust for funding. REFERENCES 1. Smith, J.V., Chem. Rev., 88 (1988), 149-182. 2. Treacy, M.M.J., Randall, K.H., Rao, S., Perry, J.A., Chadi, D.J., Z. Krist, 212 (1997), 768-791. 3. Boisen, M.B., Gibbs, G.V., O'Keeffe, M., Bartelmehs, K.L, Microporous Mesoporous Mater., 29 (1999), 219-. 4. Delgado Friedrichs, O., Dress, A.W.M., Huson, D.H., Klinowski, J., Mackay, A.L., Nature, 400 (1999), 644-647. 5. Baerlocher, C , Hepp, A., Meier, W. M. DLS-76. A Program for the Simulation of Crystal Structures by Geometric Refinement.; Institut fur Kristallographie und Petrographie, ETH: Zurich, Switzerland (1977). 6. Gale, J. D. J. Chem. Soc, Faraday Trans. 93 (1997), 629-. 7. Sanders, M. J., Leslie, M., Catlow, C. R. A., J. Chem. Soc, Chem. Commun., 19 (1984), 1271. 8. Sastre, G., Gale, J. D. Micropor. Mesopor. Mat, 43 (2001), 27-. 9. Cerius2 v. 4.0; Molecular Simulations Inc.: San Diego (1999). 10. Connolly, M. L., J. Am. Chem. Soc, 107(1985), 1118-1124. 11. Akporiaye, D.E., Price G.D., Zeolites, 9 (1989), 321-328. 12. Cora, F., Catlow, C. R. A. J. Phys. Chem., 105B (2001), 10278-10281.