Calculating the Concentration of SiOHAl Species in Ferrierites by 29Si

Dec 29, 2008 - The 27Al and 29Si NMR spectral resolutions of two NH4-FER samples with ... Two possible PerBU models and a dealumination route were ...
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J. Phys. Chem. C 2009, 113, 907–913

907

Calculating the Concentration of SiOHAl Species in Ferrierites by 29Si and 27Al NMR Spectroscopy Tama´s I. Kora´nyi*,†,‡ and Ja´nos B.Nagy‡ Department of Molecular Spectroscopy, Institute of Structural Chemistry, Chemical Research Center of the Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary, and Laboratoire de R.M.N, Facultes UniVersitaires Notre Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium ReceiVed: October 2, 2008; ReVised Manuscript ReceiVed: NoVember 13, 2008

Two NH4-ferrierites (NH4-FER) and their calcined H-FER forms with Si/Al ratios of 10 and 27.5 were characterized by 27Al and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The 27Al and 29Si NMR spectral resolutions of two NH4-FER samples with Si/Al ) 8.4 and 30.0 were taken from the literature. The quantitative contributions of the Al sites, defect-free Si(nAl), and silanol groups to the different periodical building units (PerBUs) of the FER zeolite framework were calculated from the various Si/Al ratios and relative 27Al and 29Si NMR signal intensities. Two possible PerBU models and a dealumination route were suggested by the FER structure, and the distribution of PerBUs followed the composition of the framework with appropriate precision in the studied composition range (Si/Al ) 8.4-30.0). The independently measured [Fourier transform infrared (FTIR) spectroscopy, NH3 temperature-programmed desorption (TPD)] extraframework aluminum (EFAl) concentrations and those measured by 27Al NMR spectroscopy were in a good agreement. The concentration of strong Brønsted acidic SiOHAl groups calculated with the SiOHAl PerBU model revealed that one-quarter to one-third of all OH groups were present in these SiOHAl sites in the ferrierites with Si/Al ) 10 and one-half of them were in these sites in the NH4-FER sample with Si/Al ) 27.5. The former zeolites were dealuminated at a higher degree than the latter. Tetrahedral aluminum species occupied the SiOHAl sites, and both deformed and octahedral Al species were present in the EFAl positions. Introduction Ferrierite (FER) is a naturally occurring aluminosilicate zeolite mineral. The remarkable catalytic properties of protonexchanged H-ferrierites for isomerization of linear butenes to isobutene have a high commercial importance in the petrochemical industry.1 A thorough understanding of the structure and properties of ferrierite is important because it is a valuable catalyst. The frameworks of zeolite structures are built from periodic, structurally invariant periodic building units (PerBUs). The PerBU of FER can be considered as a strongly corrugated layer of (fused) six-ring boats with “handles” of three additional tetrahedral (T) atoms. The neighboring PerBUs in FER are connected through six-, eight-, and 10-ring channels.2 The space group Immm has been assigned to siliceous ferrierite with four different T sites,2 but because of a 170° (instead of 180°) Si-O-Si bond angle in the structure, it strictly has a lower Pnnm symmetry with five nonequivalent T sites.3 The Pnnm space group requires intensity ratios of 1:2:2:2:2, whereas the Immm model corresponds to intensity ratios of 1:2: 2:4, but the experimental 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectrum of siliceous ferrierite does not completely separate into five distinctive signals.3,4 Recently, it was shown that siliceous FER undergoes a phase transition around 410 K from the low-temperature Pnnm phase to the high-temperature Immm phase of the material.5 * To whom correspondence should be addressed. Tel.: +3614381100. Fax: +3614381143. E-mail: [email protected]. † Chemical Research Center of the Hungarian Academy of Sciences. ‡ Facultes Universitaires Notre Dame de la Paix.

The presence and distribution of aluminum in zeolites has significance in terms of the degree of cation exchange available and the strength of acidity in their hydrogen form. It is wellknown that aluminum in most cases is nonuniformly distributed in zeolites. An ab initio molecular orbital calculation on model clusters of ferrierite clarified that Al atoms are located and paired preferentially in the six-membered rings in para positions (i.e., diagonal pairing is favored).6 Other studies have also suggested that aluminum atoms are exclusively located in the sixmembered rings, but in triple meta positions (three Al atoms in the ring).7 Two-dimensional 27Al multiquantum (5Q) MAS NMR spectra clarified that two distinct Al sites exist in NH4FER samples with an average ratio of 4:5 and that, at higher Si/Al ratios, aluminum tends to form aluminum-rich zones, leaving some unit cells without Al atoms.8 We developed a new method to evaluate the distribution of aluminum in different PerBUs of mordenite (MOR),9-11 beta (BEA),9,10,12 and faujasite (FAU)13 zeolites. In addition to two (diagonal PerBUs) or no (allSi PerBUs) Al atoms siting in the four-membered rings of PerBUs of MOR zeolites,14 we assumed that lone (lone PerBUs) Al atoms could also be situated in these rings of MOR and BEA.9,10 The presence of highly symmetric hydrated octahedral (BEA)9,10 and deformed tetrahedral (MOR)11 framework-related aluminum species was revealed by 27Al NMR spectroscopy. The aim of this study is to extend the application of our PerBU model9-13 to FER zeolites based on our own 27Al and 29 Si MAS NMR measurements and results taken from the literature.8 Assuming that Al atoms are located exclusively in the six-membered rings,6,7 the six-ring boats with the adjacent

10.1021/jp808727s CCC: $40.75  2009 American Chemical Society Published on Web 12/29/2008

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12 T atoms are regarded as the PerBUs of ferrierites in this work (Scheme 1). Experimental Section Two synthetic NH4-ferrierites with different crystallite sizes, bulk Si/Al ratios (10 and 27.5, designated as NH4F10 and NH4F28, respectively), and ammonium ion and extra-framework aluminum (EFAl) contents were obtained from ZEOLYST International, Valley Forge, PA. The hydrogen form of the catalysts was obtained from the NH4 forms by in situ thermal deammoniation (designated as HFER10 and HFER28, respectively). More details on the preparation and characterization of the HFER10 and HFER28 samples are available in refs 15 and 16. The silicon and aluminum NMR spectra were recorded on a Bruker Avance 500 spectrometer. For 29Si (79.4 MHz) spectra, a 2-µs (Θ ) π/6) pulse was used with a repetition time of 6 s, and for the cross-polarized (CP) experiments, a 6-µs (Θ ) π/2) pulse with a contact time of 5 ms was applied with a repetition time of 5 s. For 27Al (130.3 MHz) spectra, a 1-µs (Θ ) π/12) pulse was used with a repetition time of 0.1 s. Chemical shifts were referenced to tetramethylsilane (Si) and Al(NO3)3 (Al). The decomposition of the 29Si and 27Al NMR spectra was carried out with a precision of about 5%. The effect of this fitting error on the proposed PerBUs and Al and Si site contributions was also about 5%. Results The 29Si NMR spectra of the four ferrierites show five (Si/ Al ) 10) or four (Si/Al ) 28) resonances, which can be ascribed to Si(2Al), Si(1Al)A, Si(1Al)B, Si(0Al)A, and Si(0Al)B sites (Figure 1 and Table 1). The 29Si NMR spectral resolutions of NH4F8 and NH4F30 NH4-ferrierites (Table 1) were taken from the literature.8 The signal at -98.8 ppm was assigned to Si(2Al), and that at -101.8 ppm was assigned to Si(1Al) sites in the spectrum of NH4F8.8 The 1H-29Si cross-polarized (CP) NMR spectra of NH4F10 and HFER10 zeolites confirm the presence of Si(OH)x groups in the bands assigned to Si(2Al) and Si(1Al) configurations, as the signal intensities compared to those of Si(0Al) bands are higher in the CP spectra than in the standard spectra (compare Figure 1b,d with Figure 1a,c, respectively, and compare the 2Si(OH)2 + SiOH concentrations in Table 1). The ratio of tetrahedral silicon to aluminum in the zeolite framework (Si/AlNMR) can be directly calculated from the line intensities in a 29Si MAS NMR spectrum (ISi(nAl), where n is the number of connecting Al atoms) by the following equation, assuming that the Al-O-Al avoidance rule of Loewenstein17 is obeyed and that Si(OH)x signals are not included in the bands18

4

4

n)0

n)0

Si/AlNMR ) ∑ ISi(nX)/ ∑ (n/4)ISi(nX)

(1)

The resulting Si/AlNMR ratio underestimates the bulk Si/Al ratio in all except NH4F30 samples (Table 1), which means that defect silanol sites [Si(OH)x groups] are present in these zeolite frameworks. It is possible to calculate the concentration of Si(OH)x sites by substituting the defect-free line intensity values (ISi(nAl) ) Iline(n) - ISi(OH)n, where n ) 2 or 1) and the framework Si/Alfram ratios into eq 1. Then, the following 2ISi(OH)2 + ISiOH concentrations (Table 1) can be calculated from eq 29-13

Si/Alfram)Si/Albulk/AlT

or

Si/Albulk/(AlT+AlD)

Si/Alfram)4/[2(Iline2-ISi(OH)2) + (Iline1 - ISiOH)]

(if ∑ ISi(nAl) ) 1)

2ISi(OH)2+ISiOH)2Iline2+Iline1-4/(Si/Alfram) (2) The small negative value of 2ISi(OH)2 + ISiOH (-1.3%) for the NH4F30 literature sample (Table 1) is within the precision limits of our method, and it means that this ferrierite does not contain defect silanol groups. The 27Al NMR spectra of NH4F10, NH4F28, and HFER10 ferrierites exhibit signals for tetrahedral aluminum (AlT) at around 54-61 ppm, a broad shoulder for deformed tetrahedral (or pentacoordinated) aluminum (AlD) at 44-45 ppm, and a small line for octahedral aluminum (AlO) at 0 ppm, which are assigned generally to framework, deformed lattice, and extraframework aluminum (EFAl) species, respectively (Figure 2). The 27Al NMR spectrum of HFER28 ferrierite presents tetrahedral (AlT) and some octahedral (AlO) aluminum species, but no AlD sites (Figure 2). The 27Al 5Q NMR spectra of the NH4F8 and NH4H30 samples showed two distinct types of AlT sites (Table 1).8 Discussion Using 2D 27Al 5QMAS NMR spectroscopy, two distinct types of tetrahedral aluminum sites were identified in the NH4F8 and NH4F30 zeolites with an average ratio of 4:5 (Table 1), which fits the Pnmm space group.8 The PerBU model of ferrierite (Scheme 1a) predicts a 1:2 intensity ratio of atoms sitting on T4 and T2 and on T3 and T1 sites, assuming that the Immm space group is valid. This prediction is confirmed by our preliminary multiquantum (MQ) MAS results on the four samples, which gave a 1:2 intensity ratio for the two tetrahedral aluminum sites; therefore, these sites should correspond to T4 and T2 aluminum sites, respectively (Table 1). The Si NMR peak intensities of the NH4F28 sample were calculated with the measured AlT4 and AlT1 line intensities (Table 1); accordingly, the Si NMR signals designated as Si(nAl)A and Si(nAl)B (n ) 0, 1) originate from

SCHEME 1: (a) Periodic Building Unit (PerBU) of Ferrierite with 18 (8 T1, 4 T2, 4 T3, and 2 T4) T Sites in the Immm Space Group and (b) 2-4-2 Triple Al and (c) 4-4 Diagonal Al PerBUs

SiOHAl in Ferrierites by NMR Spectroscopy

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Figure 1. Normal and CP 29Si NMR spectra, respectively, of (a,b) NH4F10, (c,d) HFER10, (e,f) NH4F28, and (g,h) HFER28 zeolites.

TABLE 1: Tetrahedral (AlT), Deformed (AlD) and Octahedral (AlO) Aluminum Fractions (%), Framework and NMR Si/Al Ratios, Relative Si(nAl) (n ) 2, 1, 0) Coordinations (%), and Calculated 2Si(OH)2 + SiOH Contents (%) of FER Zeolites Calculated from 27Al and 29Si NMR Spectra 27

Al NMR

FER zeolite AlT4 AlT2 AlD AlO AlT AlT + NH4F10 HFER10 NH4F28 HFER28 NH4F8a NH4F30a a

61.9 34.4 3.7 16.16 58.3 33.4 8.3 17.16 18.1 63.1 10.5 8.2 33.83 26.4 67.7 5.9 29.23 48 52 8.40 47 53 30.00

Si NMR

AlD Si/AlNMRb

10.38 10.91 29.95

2Si(OH)2 + SiOHc

29

Si/Al framework 8.78 9.47 26.85 23.45 7.92 33.33

Si(2Al) Si(1Al)A(T1) Si(1Al)B(T2-4) Si(0Al)A(T1) Si(0Al)B(T2-4) 2.9 4.4 2.2

21.2 20.2 7.2 9.4 21.8 5.6

18.7 13.2 7.8 7.7 24.3 6.4

25.5 31.5 40.5 36.8 23.6 39.9

31.7 30.7 44.6 46.1 28.1 48.1

AlT

AlT + AlD

20.8 18.9 3.1 3.4 2.9 -1.3

7.0 5.5 1.5

Data taken from ref 8. b Calculated by eq 1. c Calculated by eq 2.

silicon atoms sitting in T1 and T2 + T3 + T4 sites, respectively, in the PerBU unit (Scheme 1a). Suppose that our PerBU model9-13 is also applicable for FER zeolites. In this case, its structure is composed of triple (Scheme 1b), diagonal (Scheme 1c), lone, and allSi building units. To simplify the first model, all concentrations of defect silanol groups are taken into allSi PerBUs, and aluminum species are not distinguished; their sum is taken as the Al contribution. To separate the concentrations of SiOH and Si(OH)2 defect sites from the value for the sum (2ISi(OH)2 + ISiOH) calculated by eq 2, some assumptions must be made. The Si NMR signal intensity of line 2 (Iline2) includes the intensities of Si(2Al) and Si(OH)2

species. As defect sites originate during the dealumination process, two alternative assumptions were taken: the ratio of defect Si(OH)2 silanol groups to Iline2 was assumed to be equal to the ratio of either AlD + AlO or AlO to the sum of all Al species (∑Al) calculated by eqs 3a and 3b, respectively

Si(OH)2 ) Iline2(AlD+AlO)/∑Al

(3a)

Si(OH)2 ) Iline2AlO/∑Al

(3b)

The PerBU composition of this so-called simplified model is shown in Table 2. The contributions of the different building units (Table 3) were calculated from the 27Al and 29Si NMR

910 J. Phys. Chem. C, Vol. 113, No. 3, 2009

Figure 2.

Kora´nyi and B.Nagy

27

Al NMR spectra of (a) NH4F10, (b) NH4F28, (c) HFER10, and (d) HFER28.

TABLE 2: Compositions of Possible FER PerBUs with 18 T Sites in the Simplified, FER10, and FER28 SiOHAl Modelsa PerBU b

AlT

triple diagonalb loneb allSib-d

3 2 1

tripSiOHAlc trip1Xdeal trip2Xdealc trip-dealc diagSiOHAlc diag1Xdealc diag-dealc

3 2 1

loneSiOHAld lone-deald

1

AlD+AlO

SiOHAl

Si(2Al)

Si(OH)2

3

Si(1Al)

2 1

1 2

3 1

3 1 1 3

6 4 2

2 1

8 4

1

4

1

Si(0Al)

Si/Al

ye

6 8 13 18 - x - ye

5 8 17 ∞

4 8

6 6 6 6 8 8 8

5 5 5 5 8 8 8

4

13 13

17 17

6 8 4 xe

1 2 3

SiOH

4 6 6

a Normative concentrations are marked in bold. b Simplified model. c FER10 model. d FER28 model. e x ) Si(OH)2 contribution, y ) SiOH contribution.

spectral resolutions (Table 1), corrected with Eqs 2, 3a, and 3b, modified with the PerBU compositions (Table 2), finally normalized to 1. E.g. the values in the first row of Table 3 originate from the Si(2Al) normative concentration (1.6%, assuming that the sum of all Al and all Si species is 100%). Based on the triple PerBU composition (Table 2) Al has the same content as Si(2Al), and the Si(1Al) and Si(0Al) species have double concentrations compared to the Si(2Al) composition. Finally the sum of all PerBUs is normalized to 1, which results in the 1.5, 1.5, 3.0, and 3.0 contributions (%) for AlT, Si(2Al), Si(1Al), and Si(0Al) species, respectively (Table 3). The simplified model fits for all four samples and for the ferrierites published in the literature.8 Naturally, this model is oversimplified, but we have demonstrated in this way that the PerBU model is also applicable for ferrierites and that, in addition to the allSi units, triple and diagonal building units and lone building units must be taken into account for high aluminum contents (NH4F8, NH4F10, and HFER10) and low Al contents, respectively. (The combinations of triple plus lone building units and diagonal plus allSi building units result in negative values; therefore, together, they are impossible contributions.) The ratio of triple units to the sum of triple and

diagonal units is either around 13% (NH4F8 and NH4F10 if the assumption in footnote a to Table 3 is correct) or 20% (NH4F10 if the assumption in footnote b to Table 3 is correct) and increases in both possible cases to higher ratios as a result of calcination (HFER10). The contributions of diagonal (47-78%) and allSi (10-43%) units change in a relatively wide range, whereas those of lone units are the most stable (57-63%, Table 3). The next step is taking into account the different aluminum species. The main problem is whether the AlD species can be assigned to either framework or extra-framework (EF) aluminum. If these species are situated in lattice positions (AlT + AlD columns in Table 1), the framework Si/Al ratios and 2ISi(OH)2 + ISiOH concentrations calculated by eq 2 are much lower than in the opposite case (AlT columns in Table 1). A high amount of defect sites suggests AlD sites in EFAl positions. The CP 29 Si NMR spectra (Figure 1) provide qualitative information on the concentration of Si(OH)x sites, but the wide signals in the spectra of the NH4F10 and HFER10 samples support the previously mentioned assumption. The extra-framework aluminum contents of NH4F10 and NH4F28 samples are 0.64 and 0.15 mmol/g, respectively (Table 4).15,16 These EFAl contents

SiOHAl in Ferrierites by NMR Spectroscopy

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TABLE 3: FER Zeolite PerBU Units, ∑Al, Si(nAl) and Si(OH)x Contributions (%), and Triple/(Triple + Diagonal) PerBU Ratios (%) in the Simplified Model FER NH4F10 NH4F10 HFER10 HFER10 NH4F28 NH4F28 HFER28 NH4F8 NH4F30 a

PerBU a

triple diagonala allSia tripleb diagonalb allSib triplea diagonala allSia tripleb diagonalb allSib lonea allSia loneb allSib lone allSi triple diagonal allSi lone allSi

∑atoms

∑Al

Si(2Al)

Si(1Al)

Si(0Al)

8.9 62.5 28.6 14.9 59.1 26.0 12.8 55.5 31.8 21.6 47.4 30.9 61.0 39.0 62.1 37.9 62.9 37.1 11.8 78.2 10.0 56.8 43.2

1.5 6.9

1.5

3.0 27.8

2.5 6.6

2.5

5.0 26.3

2.1 6.2

2.1

4.3 24.7

3.6 5.3

3.6

7.2 21.1

3.0 27.8 17.5 5.0 26.3 20.6 4.3 24.7 22.6 7.2 21.1 26.9 45.6 36.5 45.6 36.5 45.6 34.4 3.9 34.7 7.6 41.9 43.2

3.5

11.9

3.5

13.0

3.5

13.7

2.0 8.7

2.0

3.9 34.7

3.2

11.6

Si(OH)2

SiOH

tr/(tr + diag) 12.5

0.9

10.2 20.2

0.1

5.3 18.7

1.5

7.6 31.3

0.3

3.7 2.5 1.4 2.7 13.1 2.4

Calculated from Si/Alfram ) Si/Albulk/AlT (eq 2) and eq 3a. b Calculated from Si/Alfram ) Si/Albulk/(AlT + AlD) (eq 2) and eq 3b.

TABLE 4: Extra-Framework Aluminum (EFAl) Contents (mmol/g and mol %) Determined from Released NH3,15,16 Ratios of Deformed (AlD) Plus Octahedral (AlO) Aluminum Sites to All Aluminum Sites Measured by 27Al NMR Spectroscopy, OH Contents (mmol/g) from Fourier Transform Infrared (FTIR) Spectroscopy and NH3 Temperature-Programmed Desorption (TPD),8 and OH/Al and SiOHAl/(∑OH) Ratios Calculated from the 27Al and 29Si NMR Spectra of FER Zeolites FER zeolite NH4F10 HFER10 NH4F28 HFER28 NH4F8 NH4F30 a

∑Al (mmol/g) a

EFAl (mmol/g)

EFAl/Al

1.56

a

0.64

0.41

0.55a

0.15a

0.27

Al NMR (AlD + AlO)/Al

27

OH (mmol/g)

0.38 0.42 0.19 0.06 1.75b 0.49b

OH/Al

SiOHAl/∑OH

1.42 1.28 0.70 0.77 0.23

0.29 0.26 0.53 0.80

Data taken from refs 15 and 16. b Data taken from ref 8.

represent 41% and 27% of the total aluminum contents in NH4F10 and NH4F28 zeolites, respectively (Table 4). Relative to the total aluminum contributions, the sums of the broad AlD and AlO signals (Figure 2) represent 38% and 42% for the NH4F10 and HFER10 samples, as well as 19% and 6% for the NH4F28 and HFER28 samples, respectively (Table 4). The values for NH4F10 and HFER10 are very close to the EFAl content of NH4F10 ferrierite (41%, see Table 4). The ratio of octahedral AlO species is below 10% of the sum of all Al species (Table 1). The ratios of triple units to the sum of triple and diagonal units are very similar in NH4F8 (13.1%) and NH4F10 (12.5%) if the assumption in footnote a to Table 3 is correct. Based on these different and independent results, we suggest that the sum of AlD and AlO species must be assigned to extraframework aluminum species. In other words, the assumption in footnote a to Table 3 is correct. In the third and final step, we attempt to calculate the concentrations of strong Brønsted acidic bridging SiOHAl groups and defect Si(OH)x silanol sites in these ferrierites by our PerBU method. Unfortunately, the OH concentrations of only NH4F8 and NH4F30 samples were determined from Fourier transform infrared (FTIR) and NH3 temperature-programmed desorption (TPD) measurements (Table 4). The maximum values of OH concentrations assuming complete NH4+-to-H+ ion exchange can be 1 relative to all Al atoms. The all OH/all Al

ratios calculated from the 27Al and 29Si NMR spectra vary from 0.23 to 1.42 (Table 4). Nevertheless, the defect silanol groups are also counted in the values that are greater than 1, so the calculated OH/Al ratios can be considered acceptable. For example, nonacidic SiOH silanol groups are present in the NH4F8 sample (2.9% in Table 1 and 0.23 OH/all Al ratio in Table 4). As a compromise, we assume that all AlT species are SiOHAl species; therefore, this model is called the SiOHAl model. The initial PerBU units are called triple SiOHAl (tripSiOHAl), diagonal SiOHAl (diagSiOHAl), and lone SiOHAl (loneSiOHAl), and they decompose during the dealumination process through various PerBUs shown in Scheme 2. (The decomposition step of loneSiOHAl is not shown because it is similar to the first step of both of the other routes.) The PerBU composition of this SiOHAl model is reported in Table 2, and the contributions of the different building units are listed in Table 5. The contributions of the trip1Xdeal, trip2Xdeal, and diag1Xdeal units could not be calculated for arithmetic reasons [too many parameters to be calculated from too few normative concentrations (bold entries in Table 2), which is not possible], so only the concentrations of tripSiOHAl, trip-deal, diagSiOHAl, and diag-deal units were taken into account. The higher the SiOHAl/(SiOHAl + dealuminated) ratios (SiOHAl/all column in Table 5), the lower the degree of dealumination in the different PerBUs. These values are

912 J. Phys. Chem. C, Vol. 113, No. 3, 2009

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SCHEME 2: Suggested Dealumination Routes of (Top) tripSiOHAl and (Bottom) diagSiOHAl PerBUs

TABLE 5: FER Zeolite PerBU Units; Al, Si(nAl), Si(OH)x, and SiOHAl Contributions (%); and SiOHAl/(SiOHAl + Dealuminated) [SiOHAl/All] PerBU Ratios (%) in the SiOHAl Model FER

PerBU

∑atoms

AlT

NH4F10

tripSiOHAl trip-deal diagSiOHAl diag-deal allSi tripSiOHAl trip-deal diagSiOHAl diag-deal allSi loneSiOHAl lone-deal allSi loneSiOHAl lone-deal allSi

11.2 5.9 40.0 22.2 20.8 15.8 9.7 28.7 18.5 27.3 53.9 11.5 34.6 63.3 3.7 33.1

1.6

HFER10

NH4F28 HFER28

AlD + AlO

SiOHAl

Si(2Al)

1.6

1.6

Si(OH)2

1.0 4.0

Si(1Al)

1.0 4.0

2.0 16.0

2.5 2.3

9.8 0 2.3

2.3

1.6 2.9

4.5 1.6

2.9

3.2 11.5

2.1 2.8

8.2 0 2.8

11.6

0.6 3.3

SiOH

3.2

2.5 0 3.3

13.5

0.2

surprisingly uniform for the FER10 samples (61-65%). They do not depend on either the cationic form (NH4+ or H+) or the kind of the PerBU unit (triple or diagonal) of the zeolite. We emphasize that these four very similar SiOHAl/all ratios were all calculated independently from one another. The PerBU compositions were calculated from the normative concentrations marked in bold in Table 2. Accordingly, the contributions of triple SiOHAl species originate from the Si(2Al) or Si(OH)2 concentrations measured by 29Si NMR spectroscopy, and those of diagonal SiOHAl species come from the aluminum concentrations determined by 27Al NMR spectroscopy. Because of the high SiOHAl/all ratios of FER28 samples (82% and 94% in Table 5), their degree of dealumination is lower than the dealumination level of FER10 zeolites (61-65% in Table 5). According to the SiOHAl model, Si(OH)x defect silanol groups are generated in parallel with the formation of AlD + AlO species. The SiOHAl/∑OH ratios for all four dealuminated ferrierites are included in Table 4. The SiOHAl contribution to all OH groups is 26-29% for the FER10 sample and 53-80% for the FER28 sample. These ratios are in accordance with the literature as the proportions of SiOHAl groups determined by NH3 TPD were 36%, 46%, and 63% for HFER10, HFER19, and HFER32 samples, respectively.19

0.8 0

Si(0Al)

SiOHAl/all

3.2 2.0 16.0 9.8 20.8 4.5 3.2 11.5 8.2 27.3 36.8 8.3 34.6 43.2 2.6 33.1

65.5 64.4 62.0 60.8 82.5 94.5

Conclusions Two different periodic building unit (PerBU) models were considered. The simplified model demonstrates the applicability of the PerBU model to FER zeolites. The independently measured (FTIR spectroscopy, NH3 TPD) extraframework aluminum (EFAl) concentrations and those measured by 27Al NMR spectroscopy are in a good agreement. The presence of tetrahedral AlT species in the strong Brønsted acidic SiOHAl groups is suggested in the SiOHAl PerBU model, and a possible dealumination route is proposed. The three-Al-atom-containing tripSiOHAl PerBU and the two-Al-atom-containing diagSiOHAl PerBU decompose at the same relatively high level, but the oneAl-atom-containing loneSiOHAl PerBU disintegrates to the lowest degree during the dealumination process. This is explained by the model, as one-quarter to one-third of all OH groups are strong Brønsted acidic SiOHAl sites in the NH4F10 and HFER10 ferrierites whereas one-half of all OH groups are SiOHAl sites in the NH4F28 zeolite. This is the first study to compute the concentrations of SiOHAl sites in ferrierites by 29 Si and 27Al NMR methods. Acknowledgment. T.I.K. is indebted to FNRS (Belgium) for financial support.

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