1
The local structure of aluminium sites in zeolites

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

2
The long range ordering and thus the average structure of crystalline zeolites can be determined by various diffraction and spectroscopic techniques.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

3
It has, however, proved difficult to establish the local structure surrounding these aluminium sites by diffraction methods.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

4
The most useful information has come from theoretical studies (M. Brandle et al., J. Chem. Phys., 1998, 109, 10379; U. Eichler et al., J. Phys. Chem. B, 1997, 101, 10035) which suggest that the Al–O distance associated with the proton is longer than other Al–O interatomic distances.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

5
Employing in situ X-ray absorption fine structure spectroscopy (EXAFS) of the aluminium edge at 1565.6 eV, we report individual bond lengths angles for the local aluminium environment of neutral and acidic zeolites.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

6
For two acidic zeolites we find that there is indeed one Al–O distance that is significantly longer than those in a neutral material.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

7
We also show that for the average T-atom positions our EXAFS results are consistent with X-ray diffraction measurements, to an accuracy of ca. 0.01 Å.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

8
Changes in bond angles show how the zeolite structure distorts to accommodate Brønsted acidity.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

Introduction

9
Structural studies of zeolites indicate that they are highly ordered materials with pore networks in up to three dimensions.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

10
Yet there is local disorder in these frameworks, that appears wherever an aluminium atom is present.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

11
This disorder is not evident from the results of techniques such as electron microscopy or X-ray diffraction.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

12
Yet these local disorders have immense chemical significance, and so need to be understood.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot2

13
In particular, the structural changes associated with the introduction of Brønsted acidity are of interest.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot2

14
One reason for the introduction of local disorder with aluminium is its generally greater size, compared to the silicon atom.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

15
In quartz (SiO2) and in aluminium-free ZSM-5 and faujasite, the Si–O interatomic distances are very constant at ca. 1.605 ± 0.002 Å.3,4

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

16
In tetrahedral environments such as berlinite (AlPO4) and in the crystalline aluminophosphates the Al–O interatomic distance is longer, in the range 1.70–1.80 Å.5

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

17
It is therefore to be expected that Al–O distances in zeolites are longer than those found when silicon occupies a similar site.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

18
Indeed it is well-known that increasing the aluminium content of faujasite increases the unit cell size.6

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

19
For each aluminium zeolite framework atom, there is a need for an additional positive charge, provided by charge balancing cations, which also introduce local disorder.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

20
The location of the cation depends on a number of factors, including its size and charge, and the state of hydration of the zeolite.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

21
The cation is normally considered to be part of the local structure near to aluminium and a proton is typically represented as Al–OH–Si.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

22
Good theoretical calculations suggest that the Al–O distance in Al–OH is longer again than Al–O–Si distances not associated with the acid site.2

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

23
Structural determination methods provide little detailed information on the local environment surrounding aluminium in zeolites.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met1

24
Aluminium and silicon are very close in X-ray scattering power, so XRD gives only the weighted average of the Si–O and Al–O interatomic distances present at each distinct site.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met1

25
Some attempts to probe structural variations have been made by MASNMR spectroscopy.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

26
In faujasite (FAU) the presence of more than one aluminium site has been suggested by 27Al MASNMR.7

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

27
27Al NMR has also been used to estimate the aluminium–hydrogen distance in Al–O(H) entities.8

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

28
More recently in a very interesting combination between MASNMR studies and ab initio calculations, Al–O bond lengths have been inferred.9

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met2

29
The most interesting and detailed picture of the likely local structure around aluminium in acidic zeolites, however, probably comes from theoretical studies.1

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met3

30
These suggest that the Al–O distance associated with the acidic proton is significantly longer than the other Al–O distances, by up to 0.2 Å.2,9

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

31
Extended X-ray absorption fine structure (EXAFS) of the aluminium K edge at ca. 1560 eV should be a fruitful way to probe the local structure around aluminium in zeolites.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met4

32
Although the EXAFS information is less accurate than the averages from XRD, its ability to probe the local structure around a selected element is key, as is the (relative) ease of in situ measurement.

Type: Method | Advantage: Yes | Novelty: New | ConceptID: Met4

33
In an early X-ray absorption studies of the aluminium environment in zeolites, Koningsberger and Miller10 studied three hydrated faujasite samples, Na–Y, NH4–Y and H–Y.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

34
In each case they concluded that each aluminium had four oxygen neighbours, with the four Al–O bonds of the same length, 1.62 Å for Na–Y, 1.64 Å for NH4–Y and a longer value of 1.70 Å for their acidic sample, H–Y.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

35
Hydration is expected to lead to a more ordered tetrahedral site, so the observation of constant Al–O distances is not unexpected.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

36
The Koningsberger group has also been very successful in extracting information from studies of the aluminium X-ray absorption near edge structure (XANES) in zeolites.11,12

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met5

37
XANES however does not directly yield interatomic distances.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met5

38
In a preliminary study13 we have reported some structural data on H-ZSM-5 and Na-FAU.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

39
Subsequent to this study, van Bokhoven and Prins have reported a preliminary study of HY and Na–Y and the interaction of the faujasite with hydrocarbons.14

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

40
The two studies are in general agreement.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

41
In this work we have extended our studies to H-FAU and also include NMR and infrared evidence to attest to the integrity of our materials.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa2

42
We have analysed our results using the multiple scattering capabilities of the ab initio code EXCURV98, which allows the Al–O–Si angles to be studied, and which also allows determination of second nearest neighbour distances—information that is not available from the empirical analysis approach of van Bokhoven and Prins.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

43
We present results first for the non-acidic zeolite Na-FAU, followed by those for the weakly acidic H-FAU and then for the strongly acidic H-ZSM-5.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

44
Importantly, we show that our SOXAFS results are consistent with X-ray diffraction studies of similar materials and we also consider how the variation in acidity between H-FAU and H-ZSM-5 is mirrored in their structures.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa3

Results and discussion

45
Fig. 1a shows experimental and best fit calculated EXAFS spectra for Na-FAU (Si/Al ratio = 2.5).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

46
The experimental measurements were made in vacuo and in situ, after the sample had been heated to 640 K for 1 h, to remove residual water and cooled to 423 K.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

47
Eight spectra were collected over 6 h, and averaged.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

48
The quality of fit obtained is very good, with the R factor of 40.9 largely reflecting the noise level of the experiment.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

49
The parameters used in the calculation are listed in Table 1, and a representation of the local structure is given in Fig. 2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

50
Although the range of the spectrum is limited by the presence of the Si K edge at 1839 eV, we are able to draw statistically significant conclusions15 about aluminium–oxygen and aluminium–silicon nearest neighbour distances, and about the associated angles.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

51
EXAFS indicates that each aluminium atom in our Na-FAU sample has exactly four oxygen nearest neighbours.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

52
Further confirmation of this is provided by analysis of the XANES spectra, using the approach of van Bokhoven et al.,12 which shows the absence of detectable quantities of octahedral aluminium.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

53
The Al–O interatomic distances in Na-FAU are all very similar, at 1.74 ± 0.01 Å.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

54
Our fit also includes four Al–O–Si distances, with an average length of 3.12 Å.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

55
Only a single crystallographically distinct T site, where T represents aluminium or silicon is available for the FAU framework.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

56
Note however, that due to perturbation of the bridging protons with neighbouring oxygen atoms two distinct OH frequencies and NMR signals can be found.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

57
These are attributed to OH groups pointing into supercages and β cages, respectively.16

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

58
Each T–O bond has a slightly different T–O distance, with an average of 1.74 Å, so agreement with our EXAFS conclusions is good.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

59
Lastly we find that introduction of a sodium atom gives a significant improvement in the fit.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

60
The angular information obtained shows that the tetrahedral environment of aluminium in Na-FAU is rather regular.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

61
The average O–Al–O angle is 109.3°, compared to the ideal tetrahedral value of 109°.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

62
However, as we will see, the standard deviation of the six O–Al–O angles in the tetrahedron provides a better measure of structural distortion.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

63
The standard deviation of these angles for Na-FAU is small, at 6°.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

64
We have also studied the acidic materials H-FAU and H-ZSM-5, again making in situ measurements after dehydration in vacuo, and collecting data over 11 h (H-FAU) and 18 h (H-ZSM-5).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

65
The EXAFS spectra are shown in Figs. 1b and c, and the parameters used in the calculations are in Table 1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

66
Again there are exactly four oxygen nearest neighbours, and the XANES spectra show no evidence of the presence of octahedral aluminium.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

67
For each zeolite, three similar but not identical Al–O distances are found, together with one Al–O distance which is significantly longer.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

68
For H-ZSM5 the three Al–O distances are a little shorter than in Na-FAU, with an average length of 1.70 Å, while the longer distance is 1.98 ± 0.01 Å.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

69
Although hydrogen is a poor scatterer of X-rays and so makes no contribution to the EXAFS spectra, we take our cue from the theorists, and associate this longer bond with the Al–O(H) distance.2,9

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

70
This elongated distance has also been suggested for Fe–silicalite the iron containing analogue to ZSM-.517

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

71
The aluminium–silicon distances in the EXAFS calculations are encouragingly close to the crystallographic values.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

72
In contrast to the earlier results for hydrated samples,10,11 we observe significant differences in the different bonds of aluminium to the surrounding four oxygen atoms.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

73
Mainly on the basis of NMR evidence, it was suggested that distortion of tetrahedral aluminium occurs in protonic zeolites upon dehydration.18

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

74
Both dehydrated Brønsted acidic zeolites (H-ZSM-5 and H-FAU) exhibited a single lengthened Al–O bond, well in line with the distortion suggested by Fajula et al.18,19

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

75
Interesting changes are also noted in the angles.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

76
For H-ZSM-5 there is a decrease in the three (H)O–Al–O angles, from the average value of 109° to 97 ± 6° (standard deviation) and an increase in the other three O–Al–O angles, to 119 ± 6°.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

77
These changes provide an insight into how the zeolite structure adapts to the longer Al–O(H) distance in the acidic materials.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

78
Fig. 2 shows that the change is accommodated by movement both of the acidic oxygen and the aluminium atom.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

79
The distance of the other oxygen atoms from aluminium changes little, but the aluminium atom moves much closer to the plane that they inhabit.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

80
To test their significance, all of the above EXAFS derived conclusions have been subjected to detailed statistical analysis (see ESI).

Type: Method | Advantage: None | Novelty: New | ConceptID: Met7

81
We consider it important to find ways to test for consistency between the local structural information provided by EXAFS and the results obtained from X-ray diffraction.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot3

82
This can be done by recognising that silicon–oxygen interatomic distances are almost invariant in different materials, as is to be expected for a strong covalent bond.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp2

83
In α-quartz the average Si–O distance is 1.609 Å, in highly siliceous FAU it is 1.606 Å, and in silicalite it is 1.594 Å.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

84
For comparison with EXAFS results, accurate to ±0.01 Å, we assume that the Si–O bond length is invariant on the introduction of aluminium, at 1.602 Å and 1.594 Å for FAU and ZSM-5, respectively.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp2

85
We can then calculate that average T–O distance that we expect from an X-ray diffraction experiment from:(T–O)average/Å = [Si–Osilic.* Φ/(Φ + 1)] + Σ [0.25.di / (Φ + 1)],where Φ is the Si/Al ratio of the zeolite, Si–Osilic is the Si–O distance of the siliceous ZSM-5 and FAU, respectively and the summation is over the four Al–O distances, di, obtained from EXAFS.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

86
Table 2 shows the results of applying this formula to a number of materials, and assuming that the Al–O distances stay constant as the Si/Al ratio changes.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

87
The agreement between the X-ray diffraction results and the calculations is remarkably good, and suggests that the EXAFS results may be accepted with a high degree of confidence.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

88
It is tempting to speculate that the increase in length of this bond may be related to acid strength.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp3

89
Brønsted acid strength in zeolites is associated with the barrier for removal of the acidic proton.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

90
In general, methods measure the ease of this transfer by the energy released upon interaction with bases.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

91
This energy depends, besides other factors, on the charge density at the proton as well as the charge distribution and total structure associated with the oxygen bonded to the proton and the nearby aluminium atom.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp4

92
The charge distribution in turn is influenced by structural parameters as well as the aluminium content of the zeolite.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp4

93
What is very clear from our data is that the Al–O(H) bond length varies with the zeolite structure.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

94
The shortest bond length (1.75 Å) of the elongated bond is found for Na-FAU and the longest for H-ZSM-5 (1.98 Å) H-FAU has an intermediate bond length of 1.87 Å.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

95
Na-FAU shows at most very weak Lewis acidity, and the environment around each aluminium atom is an almost perfect tetrahedral.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

96
It is generally accepted that H-ZSM-5 shows significantly higher acid strength than H-FAU.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp5

97
Calorimetric measurements for ammonia adsorption show an adsorption energy constant with loading at ca. 150 kJ mol−1,20 whereas for H-FAU21 values in the range 110–130 kJ mol−1 are reported.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

98
Increasing the Al–O bond length will result in variation of the charge distribution between the oxygen, proton and aluminium atoms which in turn is expected to influence the acidity of the protonic zeolite.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp5

99
In addition the distortion of the tetrahedron increases for the H-ZSM-5 compared to H-FAU, resulting in the highest net energy gain by relaxation upon removal of the proton.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp5

100
This effect is of course also influenced by the long range order, structure type and flexibility of the zeolite structure.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp6

101
It is important to note that other properties such as the bond angles, and confinement effects are also expected to play a significant role in the acidity of zeolite materials.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp6

102
For example, it was suggested that the interaction of bases with the framework oxygen play an important role for the interaction energies observed.22

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

103
Other structural parameters, sorbate–sorbate interactions and ordering also contribute to the interaction energies of e.g., alkanes23 with the solid acid.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

Material and methods

104
The zeolites were obtained from Sűd Chemie, Munich (ZSM-5) and Zeolyst (FAU).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

105
EXAFS experiments were performed on Station 3.4 at the Daresbury SRS, using a YB66 monochromator.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

106
Thin disc samples were held in a cell designed by van der Eerden et al.,24 in which the sample could be heated to >500 °C.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

107
Within the beamline hardware the cell achieved a vacuum of ca. 10−5 mbar.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

108
FAU samples were heated at about 5 K min−1 in vacuum to 650 K, while ZSM-5 was heated to 725 K.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

109
Samples were kept at the activation temperature for 1 h and cooled to 423 K.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

110
Spectra were then collected for the appropriate time at 423 K.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

111
EXAFS data analysis used the standard suite of Daresbury programs, including EXCURV98.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

112
Statistical significances and error bars were determined by standard methods.25

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met8

113
Our data analysis approach and phase shifts were validated by studies of an AlPO4 material of known structure, in which the Al–O environment is a regular tetrahedron.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met9

114
Since EXAFS averages over the whole range of various aluminium sites, it is very important to demonstrate the structural integrity of the zeolite samples studied.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot4

115
In particular, if octahedral aluminium were present in the samples it would be difficult to separate the contributions of different sites to the EXAFS spectrum, and the structural conclusions could be compromised.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp7

116
We have used two experimental methods to examine the extent to which the aluminium in our key H-ZSM-5 sample is in a tetrahedral environment, infrared spectroscopy and 27Al magic angle spinning nuclear magnetic resonance (MASNMR).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

117
Fig. 3 shows an infrared spectrum of H-ZSM-5 after activation in a vacuum for an hour at 823 K.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

118
Bands at 3716 cm−1 and at 3600 cm−1 are attributed to terminal SiOH and bridging OH bands, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

119
Fig. 4 shows the same sample after adsorption of ammonia at 423 K and 10−2 mbar pressure.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

120
N–H stretching vibration bands can be observed in the region 3350–2800 cm−1, characteristic of ammonia adsorbed on acidic zeolites.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

121
More informative is the N–H deformation region, where there is an intense band at 1454 cm−1, attributed to the formation of an ammonium ion, by interaction of ammonia with Brønsted acid sites.26

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

122
The material shows very little Lewis acidity, as no significant band at ca. 1620 cm−1 is observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

123
Lewis acidity in this material would be indicative of extra-framework aluminium, probably in an octahedral environment.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp8

124
The absence of any additional band in the region of 3600–3700 cm−1 indicates the absence of extra-framework AlOOH species.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

125
Fig. 5 shows the 27Al MASNMR spectrum of the same H-ZSM-5 zeolite material.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

126
Aluminium MASNMR spectra of dehydrated zeolite H-ZSM-5 and H-FAU samples are known to exhibit quadrupolar line broadening to such an extent that the signal may become invisible.19

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac18

127
This artefact can be eliminated by hydration, so vapour water was adsorbed on our sample in order to ensure that all of the aluminium present was detected.27

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac18

128
The major band in Fig. 5 is observed at a chemical shift of 54.5 ppm, and is due to tetrahedral aluminium.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

129
The band at 0.15 ppm is attributed to octahedral aluminium, but comprises <3% of the total intensity.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

130
Fig. 6 shows the infrared spectra for H-FAU together with the spectrum of the same material in contact with 10−1 mbar ammonia.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

131
The three bands observed above 3500 cm−1 are typical, and are assigned to terminal OH groups (3735 cm−1), high frequency bridging OH groups (3640 cm−1) and low frequency bridging OH groups (3534 cm−1).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

132
On ammonia adsorption (Fig. 6) the main new band observed is at 1425 cm−1, typical of Brønsted acidity; the bands corresponding to bridging hydroxyl groups disappear, and that due to terminal OH groups is unperturbed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

133
There is no indication of Lewis acidity, which would result from the presence of octahedral alumina in our H-FAU sample.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

134
Fig. 7 shows the 27Al MASNMR spectrum for H-FAU.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

135
The large band at 58.3 ppm, corresponding to ca. 86% of the aluminium in our sample, is attributed to aluminium in a tetrahedral environment.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

136
The band at −1 ppm is attributed to octahedral aluminium, but we believe that it results from a reversible transformation of part of the tetrahedral framework aluminium into an octahedral structure28 upon hydration of the zeolite.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

137
In clean phases and in contact with ammonia, we do not observe any evidence for the formation of extra-framework phases.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

138
We have included a detailed description and statistical analysis of the EXAFS fitting and analysis procedure as well as a colour copy of Fig. 2 as ESI.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16