Bases conceptuales

CHA, AEI, SAV and the novel zeotype reported here with the KFI topology are all built up from D6Rs (Fig. 5.1), and their topological similarities lead to the hypothesis of the D6R as a building unit block.

Fig. 5.1A double six ring (D6R), vertices represent the alternating tetrahedrally coordinated aluminium and phosphorus and lines represent the oxygen bridges.

In the case of CHA, the D6Rs in each layer are tilted in the same way and each layer is stacked along the z direction in AAA fashion. The same type of D6R layer appears in AEI but stacked in ABA fashion, so each layer has an opposite tilt to the next along thez

direction. For SAV the D6R have opposite tilts along x and y but each layer of D6Rs along the z direction is identical. For KFI the tilts of D6Rs alternate in orientation along each of the three crystallographic directions (Fig. 5.2).

CHA and AEI have only one type of cage, but SAV and KFI contain two types of cages, so the co-templating approach is an attractive strategy to move from one structure to the other. 31P MAS NMR in STA-7 and STA-14 give similar results, suggesting that the incorporation of magnesium proceeds by the same general mechanism.

Fig. 5.2The D6R materials with the stacking sequence layer by layer.

STA-7 and STA-14 are built up only from D6Rs, with a different stacking arrangement. Furthermore, the <001> surfaces of the SAV structure are topologically identical to the <100> surfaces of KFI. Related D6R materials AlPO-18 (AEI) and SAPO-34 (CHA) have been also produced during this work using the cyclam/TEA pair. Elemental analysis show that the TEA cation dominates cyclam in the case of AlPO-18 (C/N = 5.6), corroborating the information found in the literature that TEA cation stabilises the AlPO- 18 structure.1For AlPO(F)-34, the elemental analysis shows only the presence of cyclam (C/N = 2.3). STA-7 was not obtained in the AlPO regime because cyclam/TEA pair could not act together. The same scenario appears in the STA-14 system, where in the AlPO composition only K222 is occluded within the framework leading to AlPO-42, the AlPO

A

A

A

A

B

A

A

A

A

A

B

A

version of zeolite A.2The following table summarises the products in these systems using two co-bases.

The co-templating role of TEA cation is proved by its change of conformation from tg.tg to tt.tt when moving from the cage A (STA-7) to the MER cage (STA-14). Such organic- inorganic interaction leading to a structure-directed aluminophosphate crystallisation has been reported for the case of MAPO-34 (M = Zn, Co, Mn) and AlPO-5, with TEA in tg.tg and tt.tt conformations, respectively.3

Table 5.1Summary of syntheses using cyclam/TEA and K222/TEA with typical products. Syntheses of only TEAOH in SAPO, MAPO or MAPSO conditions were not conducted.

Template Co-template Gel Composition Product (by XRD)

TEAOH AlPO with HF AlPO-34 (CHA)

only cyclam occluded AlPO (no HF) aging conditions* AlPO-18 (AEI)

mainly TEA occluded AlPO (no HF) aging conditions** AlPO-5 (AFI)

Cyclam

MgAPO Mg/P = 0.5 intergrowthAEI-CHA

(by SXRD) MgAPO Mg/P < 0.5 CoAPO Co/P = 0.2 SAPO Si/P = 0.1 to 0.4 MgSAPO MnSAPO STA-7 (SAV) cyclam and TEA

occluded

K222 TEAOH AlPO AlPO-42 (LTA)

only K222 occluded MgAPO Mg/P = 0.2 to 0.5 CoAPO Co/P = 0.2 SAPO Si/P = 0.1 to 0.3 MgSAPO STA-14 (KFI) K222 and TEA occluded

These results show the synthetic connections between the D6R materials. Possible intergrowths due to potential structural relationships are summarised as follows and described schematically in figure 5.3:

 MgAPO(CHA) and MgAPO(AEI) have been observed to intergrow. Structurally this is possible because they share a face in common, and it is the way in which subsequent layers are orientated that determines which structure forms. The resulting cages have slightly different shapes.

 The AEI and SAV structures could intergrow, because they share a face in common. In fact, some XRD patterns of STA-7 show an extra peak at 20.5º 2θ that may be attributed to the SAPO/MgAPO/MgAPSO-18 phase. This is explained later when discussing AFM results, and is thought to occur at the later stages of crystallisation as the cyclam becomes exhausted and TEA cation dominates.

 The relationship of the SAV and KFI topologies would permit intergrowth via a common plane, (001)SAV= {100}KFI.

Fig. 5.3 Schematic description of the D6R structure types CHA, AEI, SAV and KFI (top) and their interconnections by face sharing (bottom): CHA/AEI, AEI/SAV and SAV/KFI from the left to the right.

CHA and AEI structures in the SAPO form are active and selective for MTO reaction (Table 5.2).4 , 5 Therefore the research of the related structures SAV and KFI was of interest in understanding the effect of pore geometry on the performance in adsorption and the MTO reaction, as covered in Chapter 7.

CHA AEI SAV KFI

Table 5.2D6Rs materials in the SAPO form and their catalytic applications.

Material Discovery Year Reference Applications

SAPO-34 (CHA) 19846 Flanigenet al. MTO7

SAPO-18 (AEI) 19948 Chenet al. MTO

SAPO STA-7 (SAV) 20039 Wrightet al. MTO reported here SAPO STA-14 (KFI) 200710 Castroet al. MTO reported here