ARTICLE pubs.acs.org/JPCC
Acidic Properties of Cage-Based, Small-Pore Zeolites with Different Framework Topologies and Their Silicoaluminophosphate Analogues Naonobu Katada,*,† Kazuma Nouno,† Jun Kyu Lee,‡ Jiho Shin,‡ Suk Bong Hong,‡ and Miki Niwa§ †
Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan ‡ Department of Chemical Engineering and School of Environmental Science and Engineering, POSTECH, Pohang 790-784, Korea § Nagoya Industrial Science Research Institute, 2F Noah Yotsuya Building, 1-13 Yotsuya-dori, Chikusa-ku, Nagoya 464-0819, Japan
bS Supporting Information ABSTRACT: Acidic properties of cage-based, small-pore aluminosilicate zeolites with CHA, AFX, RHO, LEV, ERI, and LTA topologies and their silicoaluminophosphate (SAPO) analogues were measured by means of an ammonia IRMS (infrared/mass spectroscopy)-TPD (temperature-programmed desorption) method. All SAPO molecular sieves studied here showed weaker Brønsted acid strength (1126 kJ mol1 lower in the heat of ammonia desorption) than their aluminosilicate counterparts. The density functional theory (DFT) calculations of the ammonia desorption energy were in good agreement with experiments; the difference in the energy of ammonia desorption was less than 10 kJ mol1. DFT also showed that the introduction of Al into the SiO2 framework to form aluminosilicate zeolites resulted in large changes to the distance between atoms close to the acid site, while Si substitution into the AlPO4 framework to form SAPO materials predominantly modified the angles between atoms relatively far from the acid site. The introduction of Al into SiO2 frameworks causes higher compression of the AlOHSi bridge, inducing strongly acidic behavior, while the more flexible AlOP bond relaxed the compression in SAPO frameworks.
1. INTRODUCTION Conventional zeolites have silicate frameworks, in which a fraction of the Si4+ atoms are substituted by Al3+, resulting in the generation of Brønsted acidity and various catalytic functions. Aluminophosphate (AlPO4) analogues with zeolitic structures in which a pair of Si4+ are substituted by Al3+ and P5+ have been known. In addition to this, a fraction of P5+ can be substituted by Si4+. Hence, the resulting materials, known as silicoaluminophosphate (SAPO) molecular sieves, also have Brønsted acidity due to charge compensation.1 The Brønsted acid sites in SAPO materials are believed to be weaker than those in aluminosilicate (hereafter AS) zeolites as suggested by the low catalytic activity of SAPO for a wide range of reactions. This is a strong motivator for the application of SAPO analogues to the catalytic reactions in which a low or moderate acid strength is required for realizing a desired selectivity, for example, the use of SAPO-34 with the CHA framework topology as a catalyst for the methanol-to-olefin (MTO) reaction.2 The Brønsted acid strengths of SAPO and AS materials have, however, been compared only within a limited range of species.36 Driven by industrial importance and the ease of theoretical study, a relatively large number of investigations have been carried out, examining the acidic properties of AS and SAPO molecular sieves with the CHA topology, that is, SSZ-13 and SAPO-34, respectively. However, exceptionally strong acid sites found in the SAPO analogue6 complicated the interpretation in some cases; r 2011 American Chemical Society
these sites were presumably due to the unexpected SiO2 structures (hereafter denoted SiO2 islands)7 within the solid. Quantitative and precise analysis of the Brønsted acid strength of SAPO molecular sieves must be necessary for wide application of these materials to acid-catalyzed reactions. The acidic properties, especially the Brønsted acid strength, of SAPO molecular sieves are here analyzed by means of an ammonia IRMS (infrared/mass spectroscopy)-TPD (temperature-programmed desorption) method, which was developed by Niwa et al.8,9 Several kinds of AS and SAPO were systematically prepared, and, in some cases, a charge density mismatch (CDM) approach developed by researchers at UOP10 was applied to the synthesis of high-quality samples.11 Theoretical studies have been carried out for evaluation of the acidic properties of AS and SAPO.12,13 Sastre and Lewis found that the insertion of Al (a larger atom than Si) into SiO2 frameworks had a more significant impact on the structural strain, inducing stronger acidity, than Si inserted into AlPO4 frameworks with the corresponding topologies; the flexibility of the AlOP bonds relaxed structural strain around the SiOHAl bridge in SAPO.13 In the present study, the density functional theory (DFT) is applied to calculate the ammonia Received: August 17, 2011 Revised: October 7, 2011 Published: October 10, 2011 22505
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Table 1. Synthesis Conditions and Characterization Data for Small-Pore Molecular Sieves Employed in This Study T/tc material SSZ-13
IZA code CHA
compositiona AS
gel compositionb 2.0(TMAda)2O 3 15Na2O 3 1.0Al2O3 3 32SiO2 3 1100H2O 4.4K2O 3 1.0Al2O3 3 5.3SiO2 3 245H2O
Si/Al in
crystal shape and
BET SAe
(K/days)
ref
the productd
average size (μm)
(m2 g1)
423/5
15
5.1
heavily overlapped cuboids,
860
0.51 368/4 f
16
3.7
not measured
646
0.5TEA2O 3 1.6DPA 3 1.0Al2O3 3 1.0P2O5
448/6 f
17
0.12
cuboids, 26
530
SAPO-34(B)
3 0.3SiO2 3 50H2O 1.0TEA2O 3 1.0Al2O3 3 1.0P2O5
473/3
18
0.2
not measured
631
SSZ-16
AS
3 0.4SiO2 3 100H2O 4.5Et6-diquat-5 3 15Na2O 3 0.5Al2O3
433/7
19
6.1
heavily overlapped platelets,
650
SAPO
3 30SiO2 3 1200H2O 2.0TMHD 3 1.0Al2O3 3 1.0P2O5
473/4
20
0.21
round platelets, 80 20
440
AS
3 0.6SiO2 3 40H2O 0.5(18-crown-6) 3 1.8Na2O 3 0.3Cs2O
373/8 f
21
3.9
spherulites, 1
850
443/1
22
0.27
rhombic dodecahedron,
650
7.9
68 overlapped rhombohedra,
610
chabazite SAPO-34(A)
SAPO
AFX
SAPO-56 Rho
RHO
81
3 1.0Al2O3 3 10SiO2 3 100H2O SAPO-Rho levyne
LEV
SAPO
2.0DEA 3 1.0Al2O3 3 0.8P2O5 3 0.2CTAB
AS
3 0.4SiO2 3 100H2O 4.5DMP 3 17Na2O 3 0.5Al2O3
433/2
3 30SiO2 3 1200H2O SAPO-35
SAPO
23
0.63.0 473/1f
1.5HMI 3 1.0Al2O3 3 1.0P2O5
24
0.19
3 0.25SiO2 3 55H2O UZM-12
ERI
AS
UZM-9
LTA
6.5TEA2O 3 2HMBr2 3 0.75K2O
373/14
25
5.7
rice-grains, 0.2 0.5
450
SAPO
1.0CH 3 1.0Al2O3 3 1.0P2O5
473/2 f
26
0.04
needles, 0.5 3
610
AS
3 0.1SiO2 3 61H2O 4.0TEA2O 3 0.25TMA2O 3 0.25Na2O
373/14
27
2.6
cuboids, 11.5
780
423/1f
28
0.06
cuboids, 1020
550
3 0.5Al2O3 3 8SiO2 3 240H2O SAPO-42
570
3040
3 0.5Al2O3 3 16SiO2 3 480H2O SAPO-17
overlapped rhombohedra,
SAPO
a
0.017TMA2O 3 0.87DEA 3 0.17HF 3 0.5Al2O3 3 0.5P2O5 3 0.17SiO2 3 46H2O
AS = aluminosilicate. b Fumed silica (Aerosil 200, Degussa) or colloidal silica (Ludox AS-40, DuPont) was used as the SiO2 starting material. Na2O 3 Al2O3 3 H2O (Strem), Al(OH)3 3 H2O (Aldrich), Al(NO3)3 3 9H2O (98%, Junsei), or pseudoboehmite (Catapal B, Vista) and o-H3PO4 (85%, Merck) were used as Al and P sources, respectively. Other starting materials including organic SDAs are the same as those described in the corresponding references. c Crystallization was performed under rotation (60 rpm), unless otherwise stated. d Determined by elemental analysis. e Surface area determined according to BET equation from N2 adsorption data for the proton form of each material. f Crystallized under static conditions.
desorption energies (Edes) of some models of Brønsted acid sites in the AS and SAPO materials. The calculated energies are compared to the experimentally determined values, and local geometric properties are used to illustrate the source of the differences in their acid strengths. The Brønsted acid strengths of AS and SAPO molecular sieves with six different framework topologies (CHA, AFX, RHO, LEV, ERI, and LTA) are quantitatively measured by means of the ammonia IRMS-TPD method in the present study. Theoretical investigation of the origin of difference in acid strength is carried out based on the DFT calculations. We have previously presented the acidic properties of chabazite14 and SAPO-34.7 We extend this work to a wide variety of different framework topologies to gain a general rule.
2. EXPERIMENTAL SECTION Synthesis and Structural Characterization. SAPO and AS samples were synthesized in Teflon-lined 45 cm3 autoclaves according to procedures described in the literature.1528 Their preparation conditions and structural characteristics are given in Table 1. The materials prepared using organic structure-directing
agents (SDA) were calcined in air at 823 K for 8 h to remove the occluded organic species. The proton form SAPO samples were thus obtained, whereas the calcined alkaline form of AS was converted into its proton form by refluxing twice in a 1.0 mol dm3 NH4NO3 solution (1.0 g of solid per 100 cm3 of solution) for 6 h followed by calcination at 823 K for 4 h. Powder X-ray diffraction (XRD) patterns were measured on a PANalytical X’Pert diffractometer with Cu Kα radiation. Elemental analysis for Si, Al, and alkali metal was carried out using a Jarrell-Ash Polyscan 61E inductively coupled plasma (ICP) spectrometer in combination with a Perkin-Elmer 5000 atomic absorption (AA) spectrophotometer. Crystal morphology and average size were determined by a JEOL JSM-6510 scanning electron microscope (SEM). The N2 sorption experiments were performed at 77 K on a Mirae SI nanoPorosity-XQ analyzer. Magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of 29 Si, 27Al, and 31 P were recorded on a JEOL ECP-300 spectrometer. The 29Si, 27Al, and 31P chemical shifts are reported relative to TMS [Si(CH3 )4 ], Al(H2O)6 3+, and o-H3PO4, respectively. Ammonia IRMS-TPD. Ammonia IRMS-TPD was measured as described in our previous paper.8 About 7 mg of sample was 22506
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Table 2. Lattice Parameters Used for DFT Calculations Å
RHO
ERI
c
α
β
γ
a
b
AS
9.388
9.388
9.388 94.2 94.2
94.2
SAPO (1)
9.388
9.388
9.388 94.2 94.2
94.2
SAPO (2)
9.305
9.305
9.305 94.2 94.2
framework type composition CHA
deg
94.2
AS
15.19
15.19
15.19
90.0 90.0
90.0
SAPO (1)
15.19
15.19
15.19
90.0 90.0
90.0
SAPO (2)
14.99
14.99
14.99
90.0 90.0
90.0
AS
13.21
13.21
14.79
90.0 90.0 120.0
SAPO (1) SAPO (2)
13.21 13.12
13.21 13.12
14.79 14.86
90.0 90.0 120.0 90.0 90.0 120.0
Figure 2. 31P MAS NMR spectra of SAPO-34(A) and SAPO-34(B). “*” denotes spinning side bands.
Figure 1. 29Si MAS NMR spectra of SAPO-34(A) and SAPO-34(B). The experimentally observed spectrum is shown by a black line, while the deconvoluted components (I)(III) are shown by blue lines, and the sum of them is shown by a red line.
compressed to form a self-supporting disk with a diameter of 1 cm. The disk was evacuated in an in situ IR cell at 773 K for 1 h, then cooled, and again heated under flowing He (8.2 105 mol s1, 3.3 kPa) from 373 to 773 at 10 K min1. IR spectra were recorded with a Perkin-Elmer Spectrum One infrared spectrometer at 1 min intervals (i.e., every 10 K). Following these measurements, ammonia (13 kPa) was admitted to the sample cell for 30 min at 373 K. The cell was then evacuated for 30 min, and the disk was again heated from 373 to 773 K with IR measurements as before. Simultaneously, a mass spectrum (MS) was recorded using a Pfeiffer Vacuum QMS200M2 mass spectrometer connected to the outlet of the IR cell. DFT Calculations. DFT calculations in periodic boundary conditions were performed using Dmol3 software (Accelrys Inc.)29 based on a generalized gradient approximation (GGA) level using BeckeLeeYangParr (BLYP) exchange and correlation functional.30 All calculations were performed using a double numerical polarization (DNP) basis set. The convergence criteria (energy, force, and displacement) were set to 2 105 Ha, 4 103 Ha Å1, and 5 103 Å, respectively. The
Figure 3. 27Al MAS NMR spectra of SAPO-34(A) and SAPO-34(B). “*” denotes spinning side bands.
calculations were carried out for periodic models with protons on the O1 and O3 oxygen sites of CHA (hereafter O1H and O3H, respectively), where On designates the crystallographically specific O site given by the Structure Commission of the International Zeolite Association (IZA).31 For AS with the CHA framework (SSZ-13), several theoretical studies have suggested preferential siting of a proton on O1,3234 while neutron diffraction studies indicated preferential occupancy on the O1 and O3 sites.35 Proton positions were selected in accordance with these studies. On the other hand, we selected O2H for RHO and O1H for ERI without a specific reason. The initial coordinates of CHA, RHO, and ERI frameworks were obtained from the IZA structure database.31 The lattice parameters (a, b, and c) were, however, adjusted as follows. For AS, the parameters obtained from powder XRD data were used. For SAPO, two cases were assumed, and calculations were carried out for both assumptions: (1) the lattice parameters were fixed to those of AS, or (2) the lattice parameters were taken from the XRD of the SAPO samples. Table 2 lists the parameters assumed as above. One Al or Si atom was introduced to substitute 22507
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Table 3. Chemical Shifts and Relative Intensities of the 29Si MAS NMR Resonances of SAPO-34(A) and SAPO-34(B) relative intensity/% SAPO-34
SAPO-34
assigned species
(A)
(B)
I
85 Si bonded to OH groups
13
52
II
90 isolated Si(OAl)4 in SAPO region
53
9
34
38
resonance δ/ppm
III
110
Si(OSi)4 in SiO2 islands
Figure 4. IR spectra of (a) SSZ-13 and (b) SAPO-34(A) with the CHA structure. The difference spectra, (spectra after adsorption of ammonia) (spectra before adsorption of ammonia), measured with heating are given.
one Si or P atom per unit cell of AS or SAPO, respectively, and one proton was added. Geometry optimization was performed for pure SiO2, H-AS, NH4-AS, AlPO4, H-SAPO, and NH4-SAPO materials with CHA, RHO, and ERI framework topologies. The inner energy gain of desorption, Edes, was calculated using Edes = EH‑Z + ENH3 ENH4‑Z, where EH‑Z, ENH3, and ENH4‑Z are the binding energies of all electrons in the proton form of AS or SAPO materials, gaseous ammonia, and NH4-zeolite, respectively.
3. RESULTS Structural Characterization. All small-pore materials prepared in this work were found to be highly crystalline, which can be further supported by the N2 sorption data in Table 1. Also, no
Figure 5. Ammonia IRMS-TPD spectra of SSZ-13 (a), SAPO-34(A) (b), and SAPO-34(B) (c) with the CHA structure.
X-ray peaks other than those of the corresponding structures are observed (Figure S1). Figure 1 shows the 29Si MAS NMR spectra of SAPO-34(A) and SAPO-34(B). The spectrum of SAPO-34(A) showed resonances around 90 and 110 ppm (resonances II and III, respectively). The former is attributed to isolated Si, that is, a SiO4 tetrahedron substituting a PO4 tetrahedron surrounded by four Al atoms in the AlPO4 framework [hereafter Si(OAl)4]. On the other hand, the latter peak around 110 ppm should be from Si in SiO2 islands, that is, a SiO4 tetrahedron surrounded by four Si atoms [Si(OSi)4]. SAPO-34(B) exhibits a broad peak around 85 ppm (resonance I), as well as resonances II and III. Our previous study using a 22508
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Figure 6. Ammonia IRMS-TPD spectra of SSZ-16 (a) and SAPO-56 (b) with the AFX structure. 1
H cross-polarization (CP) technique has clarified that the peak around 85 ppm in the 29Si NMR spectrum of SAPO-34(B) was ascribed to a Si species bonded to OH groups.7 Figure 2 shows the 31 P NMR spectra of SAPO-34(A) and SAPO-34(B). The former sample showed one sharp resonance at 28 ppm, attributed to P(OAl)4 species. SAPO-34(B) showed an additional peak at 10 to 20 ppm, which has been assigned to partially hydrated structure [P(OAl)x(OH)y] by Watanabe et al.36 The 27Al NMR spectrum (Figure 3) again indicates the partially hydrated structure generating hexa-coordinated Al (AlO6, δ = 10 ppm) in SAPO-34(B), as reported by Watanabe et al.36 On the contrary, SAPO-34(A) showed a single peak of AlO4, assignable to Al(OP)4, at 40 ppm. These NMR spectra tell us that SAPO-34(B) possessed hydrated and distorted portions enriched with OH groups, while SAPO-34(A) consisted of a well-connected AlPO4 framework. The substitution of one Si by Al and H gives +6 and +10 ppm of peak shifts, respectively, in the 29Si NMR.37 The broad shoulder around 85 ppm (resonance I) on SAPO-34(B) is assignable to such hydrated species as, most likely, Si(OSi)1(OAl)1(OH)2 or Si(OSi)1(OAl)2(OH)1 from the above principle. This species should exist in a SiO2 island, because it has a SiOSi bond; it contains OH groups presumably forming a defect, and it also contains an Al atom isolated in the SiO2 island or on a boundary between SiO2 and AlPO4 regions. The former should be located in a region with a distorted structure consisting of OH defects. Table 3 presents the relative populations of different Si species determined from the deconvolution of 29Si NMR spectra as
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Figure 7. Ammonia IRMS-TPD spectra of Rho (a) and SAPO-Rho (b) with the RHO structure.
shown in Figure 1. About one-half of Si atoms in SAPO-34(B) are attributed to Si bonded to OH groups (resonance I), while the content of isolated Si was only 9%. On the contrary, SAPO34(A) has a larger content (53%) of the isolated Si. Thus, Si in SAPO-34(A) is distributed in an isolated manner as compared to SAPO-34(B), which has a distorted structure with SiO2 islands and/or defects. SAPO-34(B) was synthesized according to an old recipe,18 and the structural features are typical of those observed in early studies,36 while SAPO-34(A) was synthesized in optimized conditions using multiple kinds of organic SDAs.17 Ammonia IRMS-TPD. The OH stretching (35003800 cm1) and skeletal (mainly 1200, 1650, and 1900 cm1) vibrations were generally observed in the IR spectra of AS and SAPO samples, which were evacuated at 773 K (Figure S2). The stretching of acidic OH was diminished or suppressed by the adsorption of ammonia, generating a negative band in the difference spectrum at 373 K, as shown in Figure 4. In addition, the NH stretching (multiple bands, 25003300 cm1) and bending of NH4 (1430 cm1) and/or NH3 (1300 and in some cases 1620 cm1) were found to generate positive bands in the difference spectrum. The intensities of representative bands (peak areas), which correspond to NH4+ bound to Brønsted acid sites (1430 cm1) and NH3 coordinated to Lewis acid sites (1300 cm1), were differentiated by temperature. The obtained differentials dINH4/ dT and dINH3/dT, where Ix is the peak area of the IR band of adsorbed species x, as a function of temperature T (K), give the 22509
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Figure 9. Ammonia IRMS-TPD spectra of UZM-12 (a) and SAPO-17 (b) with the ERI structure. Figure 8. Ammonia IRMS-TPD spectra of levyne (a) and SAPO-35 (b) with the LEV structure.
TPD of ammonia from Brønsted and Lewis acid sites, respectively. On the other hand, the gaseous concentration of ammonia, Cg (the so-called TPD spectrum, mol m3), was recorded as a function of T by MS. On the basis of the material balance, eqs 13 are derived. πD2 β dINH4 CgB ¼ ð1Þ 4FεNH4 dT CgL ¼
πD2 β 4FεNH3
dINH3 dT
Cg ¼ CgB þ CgL
ð2Þ ð3Þ
3
where CgB and CgL (mol m ) are the gaseous concentrations of ammonia desorbed from Brønsted and Lewis acid sites, respectively; D, β, and F are the parameters determined by TPD experimental conditions, that is, the diameter of sample disk (102 m), ramp rate (0.167 K s1), and flow rate of carrier gas (6.1 105 m3 s1), respectively; and εx is the molar extinction coefficient in the LambertBeer equation of species x. From the fitting of experimentally determined curves, Cg, dINH4/dT, and dINH3/dT, over the experiment temperature range, εNH4 and
εNH3 were determined and CgB and CgL were obtained for each experiment. Figures 510 show the TPD spectra of Brønsted (CgB) and Lewis (CgL) acid sites of cage-based, small-pore zeolites and their SAPO analogues studied here. The numbers of Brønsted and Lewis acid sites were determined from the peak areas of these spectra. The enthalpy of ammonia desorption (ΔH) was determined from the peak area, position, and shape according to our theory38 to be used as an index of the acid strength; the applicability of the theory has been observed widely for ammonia TPD over zeolites and nonzeolitic solid acids.9 The acidic properties determined are summarized in Table 4. In most cases, the number of Lewis acid sites was not large, indicating that the contribution from extraframework species was limited. Hereafter, the strength of Brønsted acid site will be discussed. The Brønsted acid strength (ΔH) of SAPO materials was in all cases clearly weaker than that of their AS analogues. This tells us that SAPO has intrinsically weaker acid sites than AS, due to its chemical composition. The difference in ΔH between the AS and SAPO analogues of a particular structure type was 1126 kJ mol1. For the study of SAPO materials with the CHA structure, two SAPO-34 samples with different acid strengths were employed. SAPO-34(A), which had a large content of the isolated Si atoms, showed weaker Brønsted acidity than SAPO-34(B), which had a distorted structure containing the SiO2 islands and/or defects. DFT Calculations. Table 5 summarizes the Edes values for the models of AS and SAPO calculated by DFT. For AS with CHA 22510
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Table 4. Acidic Properties Determined by Ammonia IRMSTPD Brønsted acid site IZA material SSZ-13
number/
CHA
SAPO-34(A)
AS
0.63
149
0.40
134
136
0.40b
105
0.86
121
0.01
106
0.37
130
0.08
116
AS
0.33
144
0.03
146
SAPO
0.73
130
0.02
128
AS
1.69
143
0.03
125
SAPO
1.60
128
0.00
a
SAPO AFX
SAPO-56 RHO
SAPO-Rho levyne
LEV
SAPO-35 UZM-12
ERI
SAPO-17 UZM-9
ΔH/
1.67
SAPO-34(B)
Rho
number/
code composition mol kg1 kJ mol1 mol kg1 kJ mol1
chabazite
SSZ-16
ΔH/
Lewis acid site
LTA
SAPO-42
AS
0.31
142
0.03
145
SAPO
0.67
123
0.00
a
AS
0.48
139
0.03
138
SAPO
0.41
113
0.00
a
AS
0.37
128
0.05
132
SAPO
0.08
117
0.12
116
a
Not determined because of an extremely small amount. b Number of O4H that gives only NH3 but not NH4 because of steric hindrance, as analyzed in our previous paper.14
Table 5. Edes Values Calculated by DFT and Measured by Ammonia IRMS-TPD on AS and SAPO Matrials with CHA, RHO, and ERI Topologies Edes/kJ mol1 topology
Figure 10. Ammonia IRMS-TPD spectra of UZM-9 (a) and SAPO-42 (b) with the LTA structure.
CHA
1
structure, Edes was similar (134 or 136 kJ mol ) in the models of Al1O1HSi1 and Al1O3HSi1, where protons were attached to different oxygens. From the observed values of ΔH (136 149 kJ mol1) for AS with CHA structure, that is, chabazite and SSZ-13, Edes (inner energy gain by desorption) can be calculated using the following equation ΔH RTm, where R and Tm are the gas constant and peak temperature of TPD, respectively. These experimentally determined Edes values were found to be 131144 kJ mol1 as shown in Table 5. This range is in agreement with the value based on DFT, supporting the validity of the calculation. The calculated Edes value for AS was also in good agreement with the observed values for RHO and ERI structures. For SAPO, the lattice parameters were assumed to be same as those of AS [assumption 1] or the experimentally observed value [assumption 2]. The changing parameters had little effect on Edes in ether case. This suggests that both models have representative geometry and energy of SAPO Brønsted acid sites. Further analysis was carried out on the basis of assumption 2, which was probably closer to the experimental system. The Edes values calculated for SAPO materials with CHA, RHO, and ERI topologies were again in good agreement with the experimentally observed ones. Some of the interatomic distances and angles in the optimized structures are listed in Table S1; the symbols used in this table are shown in Figure 11. In their theoretical study, Sastre and Lewis focused on the change in TsTn distance caused by the introduction of Al into the SiO2 frameworks to form AS or that of Si into
composition AS SAPO (1) SAPO (2)
RHO
ERI
position
calculated
Al1O1HSi1
134
Al1O3HSi1
136
Al1O1HSi1
118
Al1O3HSi1
125
Al1O1HSi1
117
Al1O3HSi1
121
measured 131a144b 116c
AS
Al1O2HSi1
134
138
SAPO (1) SAPO (2)
Al1O2HSi1 Al1O2HSi1
119 114
123
AS
Al1O1HSi1
129
134
SAPO (1)
Al1O1HSi1
116
108
SAPO (2)
Al1O1HSi1
111
a
Observed on chabazite with a high Al content. b Observed on SSZ-13 with a low Al content. c Observed on SAPO-34(A).
AlPO4 frameworks to form SAPO. They found a trend whereby the former case (introduction of Al into SiO2) resulted in a larger change than the latter, when considering the CHA and AFI topologies.13 Here, we focus on the changes in TsTn distance along with some other characteristic distances and angles. Noteworthy changes are listed in Table 6. In all cases, the OaTs distance was more strongly affected by the introduction of Al into the SiO2 framework to form AS materials than by the introduction of Si into the AlPO4 framework to form SAPO molecular sieves. The TsTn distance was also more strongly altered by the introduction of Al into the SiO2 framework in most cases. By contrast, the TnObTc angle was more significantly affected by 22511
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the introduction of Si into the AlPO4 framework in CHA, RHO, and ERI frameworks.
Brønsted acid sites in SAPO-34 was comparable to those in SSZ13.5 Martins et al. clearly indicated the presence of strong acid sites on the border or inside of SiO2 islands in SAPO-34.6 We have calculated by means of DFT that a strong acid site could be generated on the border of a SiO2 island in SAPO-347 with Edes in the range 113152 kJ mol1 (mean value = 125 kJ mol1), which is not significantly lower than those on AS. Such strong acid sites have been observed in SAPO, and the weaker acidity of SAPO than that of AS has not sufficiently been characterized. In the present study, a comparison of Brønsted acid strengths between SAPO-34(A) and SAPO-34(B) clearly indicates that the origin of weaker Brønsted acid sites is the isolated Si, while stronger acid sites are generated by the distorted structure including SiO2 islands or defects. DFT calculations gave Edes values in good agreement with experimental observations for both AS and SAPO materials, and the lower Brønsted acid strength of SAPO molecular sieve was well supported theoretically. Table 6 shows that the introduction of Al into SiO2 strongly affected the distances between atoms very close to the Brønsted acid sites (OaTs and TsTn), while the influence was small on angles among atoms relatively far from the acid site (TnObTc). The change in interatomic distances among atoms close to the acid site should show a hidden force field around the acid site. This suggests that the insertion of a large Al atom into the SiO2 framework directly increases compression from both sides of the AlOHSi bridge, as proposed by Sastre and Lewis.13 As Katada et al. have analyzed,39 the fundamental origin of Brønsted acidity of zeolites could be speculated to be the withdrawing of electrons of the SiOH group by Lewis acidic Al. Stronger compression should result in a larger extent of withdrawal and a higher Brønsted acid strength of OH. If this principle can be applied to the current work, the larger degree of expansion of the OaTs and TsTn distances in AS than in SAPO should be the origin of the stronger Brønsted acid sites of AS. Interatomic angles relatively far from the acid site
4. DISCUSSION It is clear from Table 4 that SAPO molecular sieves show weaker Brønsted acid strength (ΔH) than their AS analogues when they possess the same framework structure. We observed this principle without exception, indicating that the composition (AS or SAPO) of zeolitic materials has a profound effect upon their Brønsted acid strength. Yuen et al. found by means of calorimetry that most of the acid sites in SAPO-5 with the AFI topology were weaker than those in SSZ-24 (AS) with the same topology, but the strength of a small fraction of acid sites in SAPO-5 was comparable to sites in SSZ24.3 The lower acid strength of SAPO-34 as compared to SSZ-13 has also been shown.4,5 Yet again, the strength of some of the
Figure 11. Symbols of atoms in the calculated model.
Table 6. Selected Geometrical Parameters after Optimization Å topology CHA
RHO
ERI
composition
position
deg
OaTsa
ΔOaTsb
TsTnc
ΔTsTnd
0.301
3.154
0.204
SiO2
Si1O1Si1
1.640
AS
Al1O1HSi1
1.941
AlPO4 SAPO
Al1O1P1 Al1O1HSi1
1.544 1.813
0.269
3.282 3.314
0.032
SiO2
Si1O3Si1
1.628
0.267
3.145
0.184
AS
Al1O3HSi1
1.895
3.358
AlPO4
Al1O3P1
1.546
Al1O3HSi1
1.769
SiO2
Si1O2Si1
1.645
AS
Al1O2HSi1
1.956
AlPO4 SAPO
Al1O2P1 Al1O2HSi1
1.544 1.804
0.260 0.295
SiO2
Si1O1Si1
1.630
AS
Al1O1HSi1
1.925
AlPO4
Al1O1Si1
1.551
SAPO
Al1O1HSi1
1.799
0.223
3.102 3.126
0.152 0.278 0.110
3.048
0.213
3.261 3.197
12.6
146.7
0.3
146.6
1.9
147.5
2.7
150.2
3.185 3.295
2.983
136.7 149.3
144.7
3.404
0.248
5.4
147.0
3.254 0.311
147.3
ΔTnObTcf
152.7
3.329
SAPO
TnObTce
140.5 144.5
3.9
145.6
0.6
145.0 0.214
139.0
4.4
143.4
Distance of OaSis in SiO2, OaAls in AS, OaPs in AlPO4, or OaSis in SAPO. b Change of OaTs by introduction of Al into SiO2 or Si into AlPO4. Distance of SisSin in SiO2, AlsSin in AS, PsAln in AlPO4, or SisAln in SAPO. d Change of TsTn by introduction of Al into SiO2 or Si into AlPO4. e Average of the angles of three SinObSic in SiO2 and AS, or AlnObPc in AlPO4 and SAPO. f Change of average TnObTc angle by introduction of Al into SiO2 or Si into AlPO4. a c
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The Journal of Physical Chemistry C (TnObTc) were more affected by the introduction of Si into AlPO4 frameworks to form SAPO materials. This tells us that compression due to the substitution of Si in place of P was somewhat relaxed due to the AlOP bond angle flexibility as compared to the SiOSi bond angle. We thus speculate that the weak Brønsted acidity of SAPO is due to the flexible AlOP bond angle, as proposed by Sastre and Lewis.13
5. CONCLUSIONS SAPO molecular sieves were found to have generally weaker Brønsted acidity (1126 kJ mol1 lower in the energy or enthalpy of ammonia desorption) than their aluminosilicate analogues when they are characterized by the identical framework structure. The overall results of this study strongly suggest that the weaker acidity is due to the Si atoms substituted into the AlPO4 framework in an isolated manner. The more flexible AlOP bond than the SiOSi bond appears to be the origin of the weaker acidity found in SAPO materials. ’ ASSOCIATED CONTENT
bS
Supporting Information. Table showing detailed geometrical parameters obtained by DFT, and figures showing XRD and IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone/fax: +81-857-31-5684. E-mail:
[email protected]. ac.jp.
’ ACKNOWLEDGMENT This work was partly supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (B, 21360396; and C, 20560721) and the National Research Foundation of Korea (R0A-2007-00020050-0). ’ REFERENCES (1) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1984, 106, 6092–6093. (2) Wilson, S.; Barger, P. Microporous Mesporous Mater. 1999, 29, 117–126. (3) Yuen, L.-T.; Zones, S. I.; Harris, T. V.; Gallegos, E. J.; Auroux, A. Microporous Mater. 1994, 2, 105–117. (4) Bordiga, S.; Regli, L.; Cocina, D.; Lamberti, C.; Bjorgen, M.; Lillerud, K. P. J. Phys. Chem. B 2005, 109, 2779–2784. (5) Bleken, F.; Bjørgen, M.; Palumbo, L.; Bordiga, S.; Svelle, S.; Lillerud, K.-P.; Olsbye, U. Top. Catal. 2009, 52, 218–228. (6) Martins, G. A. V.; Berlier, G.; Colucia, S.; Pastore, H. O.; Superti, G. B.; Gatti, G.; Marchese, L. J. Phys. Chem. C 2007, 111, 330–339. (7) Suzuki, K.; Nishio, T.; Katada, N.; Sastre, G.; Niwa, M. Phys. Chem. Chem. Phys. 2011, 13, 3311–3318. (8) Niwa, M.; Suzuki, K.; Katada, N.; Kanougi, T.; Atoguchi, T. J. Phys. Chem. B 2005, 109, 18749–18757. (9) Niwa, M.; Katada, N.; Okumura, K. Characterization and Design of Zeolite Catalysts: Solid Acidity, Shape Selectivity and Loading Properties; Springer: Berlin, Heidelberg, Dordrecht, and New York, 2010. (10) Lewis, G. J.; Miller, M. A.; Moscoso, J. G.; Wilson, B. A.; Knight, L. M.; Wilson, S. T. Stud. Surf. Sci. Catal. 2004, 154, 364–372.
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