10216
J. Phys. Chem. 1994, 98, 10216-10224
SAPO-18 Catalysts and Their Brdnsted Acid Sites Jiesheng Chen, Paul A. Wright, John Meurig Thomas,' Srinivasan Natarajan, Leonard0 Marchese, Susan M. Bradley, Gopinathan Sankar, and C. Richard A. Catlow Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London WIX 4BS. U.K.
Pratibha L. Gai-Boyes Central Research & Development, E. I. Du Pont de Nemours Experimental Station, Wilmington, Delaware 19880-0356
Rodney P. Townsend and C. Martin Lok Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral L63 3JW, U.K, Received: April 21, 1994; In Final Form: June 22, 1994@
The incorporation of silicon via direct synthesis into AlPO4-18 ( M I ) , which has a framework structure related to, but crystallographically distinct from, that of the well-known solid acid catalyst SAPO-34, was investigated by a range of techniques. Unlike the Si/(Si A1 P) ratio (typically about 0.10) in SAPO-34, which can be varied only within a very narrow range under normal synthetic conditions, the Si/(Si A1 P) ratio in SAPO-18 is tunable from 0 to 0.10 by varying the silicon content in the synthetic gel. 29Si MAS NMR spectroscopy reveals that silicon substitutes for both phosphorus and aluminum in SAPO-18, whereas in SAPO-34, silicon substitutes only for phosphorus. Infrared and 'H MAS NMR spectroscopies and temperatureprogrammed desorption (TPD)of ammonia were used to examine the Bransted acid sites borne by SAPO-18 samples. As expected, the concentration of Bransted acid sites in all SAPO-18 samples is much less than that in SAPO-34. In contrast to the essentially neutral AlPO4-18, which catalyzes methanol conversion only to dimethyl ether, SAPO-18 catalytically converts methanol to light olefins with high activity and selectivity. The maximum conversion of methanol to ethene and propene reaches 80% with a 100% conversion of methanol to hydrocarbons. With an optimum framework composition, SAPO-18 retains its catalytic activity and selectivity longer than SAPO-34.
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Introduction Crystalline microporous aluminophosphates (originally designated AlPO4-n) composed of corner-sharing tetrahedral A104 and PO4 units are a new family of molecular sieves1,*which have adsorption capacities similar to the well-known zeolitic aluminosilicates. Since an aluminosilicate framework is negatively charged, Bransted acidity can be produced in a zeolite by introducing charge-balancingprotons, and as a consequence, zeolites are powerful solid acid catalyst^.^,^ Similarly, incorporation of heteroatoms into otherwise neutral microporous aluminophosphate~~-~ produces solid acids of variable acid strength: substitution of Si for P or divalent metals for A1 confers negative charges upon the framework, so that, as in zeolites, Bransted acidity can be introduced. Silicon- and metalsubstituted aluminophosphates are designated SAPO-n6 and MeAPO-n,7 respectively, where n represents various structure types. Nevertheless, the Bransted acidity of substituted aluminophosphates varies greatly depending on the particular structure type. Among the silicon-containing aluminophosphates, SAPO34 and its analogues (MeAPSO-34, SAPO-44, and MeAPSO47), which have a framework structureg identical to that of chabazite (therefore, the structure code for these materials is CHA), have been extensively investigated for they bear strong Bransted acid and are good shape-selective catalysts for the conversion of methanol to light hydrocarbon^.^*-^^
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, September 1, 1994.
0022-365419412098-10216$04.5010
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AlPO4-18 (structure code AEI)is a crystallographically novel but chabazite-related aluminophosphate.16 Prepared,'s2 like SAPO-34, in the presence of tetraethylammonium hydroxide as a structure-directing template, the shape of the cavities and the size of the micropores (free diameters 3.8 x 3.8 A) in AlP04-18 are closely similar to but crystallographicallydistinct from those in SAPO-34. The main structural difference between AlP04-18 and SAPO-34 lies in the orientation of the double six-membered-ringunits of which both are built up. In AlP0418 alternate layers of double six-membered rings parallel to the ab plane are related by a c glide and thereby possess different orientations. In SAPO-34, they are related by a simple translation and therefore have the same orientation. Because of the structural similarity between AlPo4-18 and SAPO-34, a silicon-substituted AlP04- 18 would be expected, like SAPO34, to possess strong Bransted acidity and to act as a good solid catalyst. However, previous work revealed* that SAPO-18could not be obtained by using tetraethylammonium hydroxide as a template since in the presence of silicon this led to the formation of SAPO-34 rather than SAPO-18. In an attempt to synthesize new aluminophosphate-based molecular sieves, we found that N,N-diisopropylethylamine (CgHlgN), which, to our knowledge, has not been used previously as a template in the preparation of zeolitic materials, readily directed" the formation of AlP04-18, SAPO-18,18 and MeAPO-18.19 Unlike SAPO-34 and its analogue SAPO-44, which have so far only been prepared with uniformly high silicon contents (Si/(Si A1 P) ratios around or higher than 0.lo), SAPO- 18 may be synthesized with variably-low silicon
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0 1994 American Chemical Society
SAPO-18 Catalysts and Their Bronsted Acid Sites
J. Phys. Chem., Vol. 98, No. 40, 1994 10217
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TABLE 1: Synthetic Gel Compositions and Product Si/(Si AI P) Ratios for AlPO4-18, SAPO-18, and SAPO-34 and Unit Cell Types of the Corresponding As-synthesized (as), Calcined, Dehydrated (cd), and Calcined, Rehydrated (cr) Samples for the 18 Series unit cell tvDe sample gel composition Si/(Si A1 P) in product as cd cr AlP04-18 1 .80CsHi~N:A1~03:P~O5:50HzO 0 I* IIb III' SAPO-18( 1) 1.60CsH19N:O.1OSi0~:A1~0~:0.95P~O~:50H~0 0.028 I I1 I1 I1 + I11 SAPO- 18(2) 1.60C~H~~N:0.20Si0~:Alz0~:0.95PzO~:50H~0 0.047 I + I1 11 11 + I11 SAPO-18(3) 1.60C~H~~N:0.40Si0~:Al~0~:0.90P~0~:50H~00.087 I + I1 I1 I1 SAPO- 18(4) 1.60C~H~~N:0.60Si0~:Al~0~:0.90P~O~:50H~00.095 I+II I1 I1 SAPO-34 2.00Et~N:O.50Si0~:A1~0~:0.90P~0~:60H~0 0.100 Unit cell I: space group C2/c, a = 13.512,b = 12.624,c = 18.439A, # =l 95.55' (parameters from ref 16). Unit cell 11: space group C2/c, a = 13.711,b = 12.731,c = 18.570 A, B = 90.01' (parameters from ref 16,see also the parameters in Table 2). 'Unit cell 111: triclinic, space group and cell parameters unknown.
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contents, and the acidities and catalytic properties can be followed with increasing silicon. Experimental Section Synthesis of Materials. As a structure-directing template, both tetraethylammonium hydroxide and triethylamine (and a few other N-containing molecules) can lead to the formation of SAPO-34, and the samples directed by these different molecules have silicon contents similar to one another. The SAPO-34 sample under present investigation was prepared using triethylamine as a template.1° All AEI-type samples were prepared hydrothermally by adopting the procedures described elsewhere."J8 For AlP04-18, to a solution of phosphoric acid in water was added aluminum hydroxide hydrate, and the mixture was stirred until homogeneous. To this mixture N,Ndiisopropylethylamine (C8H19N) was added, and a gel was formed by stirring the final reaction mixture vigorously. This gel was sealed in a Teflon-lined stainless steel autoclave and heated at 160-180 "C under autogeneous pressure for 8 days. The solid product was recovered by filtration, washed with distilled water, and dried in air at about 50 "C. SAPO-18 samples were prepared similarly except that various amounts of silica were added to the reaction mixtures as the silicon source, and the relative ratios of the starting materials were adapted accordingly. The empirical gel compositions for the preparation of all the AlP04-18 and SAPO-18 samples are presented in Table 1. The recovered solid SAPO-18 samples are designated SAPO-18( l), SAPO-18(2), SAPO-18(3), and SAPO-18(4), respectively, in the order of increasing Si02 in the reaction mixture. The AlPO4-18, SAPO-18(1), SAPO-18(2), and SAPO-18(3) samples were each obtained as a single crystalline phase, but the as-synthesized SAPO-18(4) was found to be accompanied by an amorphous gel after crystallization. Nevertheless, this amorphous material, which was believed to be unreacted silica, could easily be removed from the crystalline material since it existed as a separate phase on the top of the latter. The unreacted silica in the SAPO-18(4) product suggests a limit to the amount of silicon capable of being incorporated into the framework structure. All as-synthesized crystalline samples appear to be phase pure on the basis of their electron microscopic images. The particle size of the crystals is around 2 pm and less. Energy dispersive X-ray (EDX) emission analysis (taken over a number of particles) gives rise to Si/(Si A1 P) ratios shown in Table 1. To obtain the template-free samples, the as-synthesized materials were respectively calcined in a stream of dry oxygen at 550 "C for 5-10 h without disruption of the framework structure. Characterization. X-ray powder diffraction (XRD) was performed on a Siemens diffractometer fitted with a St& rotating
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copper anode using a nickel filter. The diffractometer was fitted with a heatable stage to permit patterns to be taken at different sample temperatures. For a more accurate measurement of structural and lattice parameters, calcined, dehydrated samples of AlP04-18 and SAPO-18(4) were examined with the highresolution powder diffractometer at Station 2.3 of the Daresbury Synchrotron Laboratory in flat plate geometry. A speciallydesigned water-tight sample holder, loaded in a dry box, was used. The wavelength of the X-rays was calibrated using a silicon standard. Lattice parameters were refined during Rietveld refinement of the structure using the GSAS program.20 A Philips CM20 Ultratwin electron microscope operating at 200 kV was used for transmission electron microscopy and electron diffraction experiments on SAPO- 18(4). Although the sample was very beam sensitive, losing crystallinity in seconds, medium- and high-resolution micrographs were obtained. Scanning electron microscopy with energy dispersive X-ray emission analysis was performed on a JEOL 2 kV electron microscope fitted with a LINK analytical system. In this way an average composition was measured over a large number of particles and can be taken as approximating the bulk composition. 29Si, 27Al, and 'H magic-angle spinning nuclear magnetic resonance (MAS Nh4R) spectra were recorded on a Bruker MSL 400 spectrometer at room temperature. All of the 29Sispectra were collected for as-synthesized samples, while 27Al spectra were collected on calcined samples that had been allowed to rehydrate in moist air. 'H spectra were collected only for anhydrous template-free samples. For the 29Si and 27Al measurements, the sample was directly pressed into a 4 mm zirconia rotor, whereas for each 'H measurement, the sample was pretreated in a glass ampule in vacuum at 400 "C to remove the adsorbed water and sealed. The ampule was then inserted in a 7 mm zirconia rotor. 29Sispectra (at 59.614 MHz) were collected using high-power proton decoupling and a spinning rate of 4.8 H z . The chemical shift was set to the tetramethylsilane scale. Spectra were run over a 47.6 kHz spectral width using 3 ps pulses, 12 ps acquisition times, and pulse delays of 30 s, and for sufficient time to obtain an acceptable signal-tonoise ratio. 27Alspectra (at 104.262 MHz) were collected using a single pulse and acquire sequence with high-power proton decoupling. Pulse times of 0.6 ps ( d 1 2 flip angles) and repetition times of 0.5 s were employed with a MAS rate of 14 kHz, and 1000 scans were recorded. Chemical shifts are with respect to Al(H20)63+. The proton spectra were measured using a Hahn echo sequence21 with a Chemagnetics probe. The background spectrum was obtained by recording the NMR signal of the rotor assemblage and an empty glass ampule under the same conditions. Infrared spectra of pelletized self-supported wafers were recorded on a Perkin-Elmer 1725X FTIR spectrometer-fitted with a quartz cell with KT3r windows. The pellets were mounted
Chen et al.
10218 J. Phys. Chem., Vol. 98, No. 40,1994
I
I
5
10
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20
25
30
35
40
5
Two-Theta (Degrees)
Figure 1. X-ray powder diffraction patterns of as-synthesized (a)
SAPO patterns (asterisked) are due to silicon-rich regions of the structure that have a different unit cell from the as-synthesized m o d 18 when hydrated.
(a) 15
20
25
30
20
25
30
35
40
Figure 3. X-ray powder diffraction pattems of rehydrated templatefree (a) A1P04-18, (b) SAPO-18(2), and (c) SAPO-18(4). TABLE 2: Comparison of Unit Cell Parameters between Calcined, Dehydrated AlP04-18 and SAPO-18(4) unit cell parameters sample a (A) b 6) c (4 B (")AlP04- 18 SAPO-18(4)
10
15
Two-Theta (Degrees)
AlP04-18, (b) SAPO-18(2), and (c) SAP0-18(4). Extra peaks in the
5
10
35
40
Two-Theta (Degrees)
Figure 2. X-ray powder diffraction patterns of calcined (b) AlPO418, (c) SAPO-18(2), and (d) SAPO-18(4); pattern (a) is simulated on the basis of the m O 4 - 1 8 framework structure reported in the literature (ref 16). on a gold foil sample holder within the cell and heated in vacuum at 550 "C for 30 min to remove adsorbed water. Catalytic Testing. The catalytic performance for methanol conversion was tested in a tubular quartz reactor 10 mm in internal diameter; 0.2 g of template-free sample was pelletized, and then crushed to particles 1-3 mm in diameter. The sample was activated at 400 "C for 30 min before methanol in a stream of saturated nitrogen was admitted. The total reaction pressure was approximately 1 atm, of which the fractional pressure for methanol was 0.2 and that for nitrogen 0.8. The weight hourly space velocity (WHSV, in g reactant-(g catalyst)-'*h-') was 2.5 h-l. Reaction products were analyzed on line gas chromatographically using a Porapak-N column. Results and Discussion X-ray Powder Diffraction. The framework of AlP04-18 is very flexible, and it is known16s22that the unit cell dimensions and interaxial angles show considerable changes upon going from the as-synthesized form to the calcined, dehydrated and to the calcined, rehydrated forms. These unit cell changes are reflected by large changes in the diffraction pattems (Figures la, 2b, and 3a, respectively). For the as-synthesized samples, peaks in the SAPO-18 patterns are observed in addition to those of as-synthesized
13.716(1) 13.725(1)
12.734(1) 12.745(1)
18.572(1) 18.596(1)
90.01 89.93
AlP04-18 (for example, at 28 = 16.0°, d spacing = 5.6 A). Upon gentle dehydration at a temperature at which the template remains intact (e.g., 125 "C),the corresponding SAPO-18 patterns change to closely resemble that of the monophasic calcined, dehydrated series of AlP04-18 and SAPO- 18 samples (Figure 2). Upon rehydration in moist air, the original patterns of the as-synthesized SAPO-18 samples are restored. Closer inspection reveals that an as-synthesized SAPO- 18 pattern may be considered as two superimposed patterns, which correspond closely to those for the first two unit cells given in Table 1 (exemplified by as-synthesized AlP04- 18 and calcined, dehydrated AlP04-18). The amount of the crystallographic form exhibiting a unit cell similar to the calcined, dehydrated AlPO418 increases with the amount of silicon in the structure. That two different crystallographic forms are present in one SAPO18 sample suggests inhomogeneity in the distribution of silicon within a single framework structure. Only minor variations are observed amongst the calcined, dehydrated samples (Figure 2b-d) and all closely represent the pattern (Figure 2a) simulated from the structure parameters reported in the literature.16 From data collected using synchrotron radiation, the unit cell of SAPO-18(4) is found to be larger (by 0.27%)than that of the pure AlPO4-18 (Table 2). This is believed to arise from substitution of silicon for the smaller phosphorus atom. The synchrotron experiments also reveal a peak broadening (Figure 4) in the diffraction peaks more clearly than the XRD pattems collected on the Siemens diffractometer (the synchrotron diffractometer has very narrow instrumental broadening, whereas the laboratory diffractometer fitted with a rotating anode and with no secondary monochromator has more instrumental broadening). The SAPO-18 broadening is not attributable to crystallite size, but to loss of order due to silicon substitution for phosphorus. Moreover, high resolution electron microscopic imaging, as seen later on, indicates the presence of microstructural modulations around 65 8,in dimension which also contributes. Rehydration of calcined AlP04-18 causes a much larger change in the XRD pattern than rehydration of calcined SAPO18 samples (Figure 3), indicating that adsorption of water has a much greater effect on the unit cell of the silicon-poor SAPO-
J. Phys. Chem., Vol. 98, No. 40, 1994 10219
SAPO-18 Catalysts and Their Bransted Acid Sites
--
0.30
-
Q)
1
2
3
4
5
6
7
8
9
d-spacing (Angstroms) Figure 4. Variation of peak width with d spacing observed using synchrotron X-ray radiation for calcined, dehydrated APO4-18 (+) and
SAP0-18(4) (O),compared with that observed for silicon (V)on the same instrument.
the structure has good crystallinity, without the presence of twinning or stacking faults, but does exhibit microstructural modulations along (1 10) with well-defined periodicity (ca. 65 A). The precise nature and cause of these modulations is the object of further studies. High-resolution images (Figure 7) of the structure clearly show the arrangement of the main, eight-membered-ring channels as well as additional detail. We are currently matching these experimental images with images computed25taking into account sample thickness and tilt as well as objective lens defocus and beam damage. MAS NMR Spectroscopy.26-2*In principle, the framework structure of AlP04-18 is neutral, and therefore, it contains no bridging Si-OH-A1 groups or Bransted acid sites. As expected, very little ‘H signal intensity is seen from the AlP0418 ‘H MAS NMR spectrum (Figure 8). The ’H spectrum of SAPO-34 is essentially the same as that reported earlier,” the maximum of the signal being located at a chemical shift of 3.8 ppm relative to TMS. This signal corresponds to the protons of the bridging Si-OH-A1 groups present in the SAPO-34 structure. A similar spectrum is observed for SAPO-18(3), but the signal intensity (normalized against sample mass) is only around 0.6 times that of SAPO-34. Obviously, the concentration of the Bransted acid sites in SAPO-18(3) is considerably less than that in SAPO-34, although the respective Si/(Si A1 P) ratios for these two samples are not very different. The reason for the lower efficiency of Bransted site generation by silicon in SAPO-18 than in SAPO-34 becomes clear from Figure 9, where 29SiMAS NMR spectra of SAPO-18(2) and SAPO-18(3) are compared with that of SAPO-34. SAPO-34 exhibits a single resonance at a chemical shift of -92 ppm, as reported p r e v i o u ~ l y . ~This ~ , ~resonance ~ was assigned to silicon surrounded by four A104 neighbors. Apparently, in SAPO-34, silicon substitutes only for phosphorus under the preparation conditions (we designate the corresponding substitution mechanism Si P as mechanism I). On the other hand, the two similar SAPO-18 spectra each possess two well-defined signals at -92 and - 111 ppm, respectively, as well as three less intense peaks between them at chemical shifts of -96, -100, and -105 ppm, respectively. The first signal is again attributed to isolated silicon atoms substituting for phosphorus. The other peaks can be assigned to silicon surrounded by (3.4104 and lSi04), (2A104 and 2Si04), ( lA104and 3sio4), and (4sio4), respectively, in a fashion similar to that for aluminosilicate zeolites.29 The peak areas ratios of the five signals are 1(-92):0.28(-96):0.24(100):0.34(-105):0.45(- 111) (chemical shift values in parentheses are given in ppm) (see Figure 9 inset). The presence of the additional peaks in SAPO-18 indicates that silicon is present not only in the phosphorus position surrounded by four A104 units but also in island^"^^-^^ where the mechanism of substitution more closely resembles 2Si A1 P (so-called mechanism 11). Whereas for each silicon substituting for phosphorus alone, a Bransted site must result, silica “islands” will be largely neutral, only being able to generate acidity at their edges where silicon atoms respectively surrounded by (3A104 and 1SiO4), @A104 and 2sio4), and (1.4104 and 3sio4) exist. The acid sites will therefore be less abundant in SAPO18 than in SAPO-34, as already shown. While silicon surrounded by four A104 neighbors is observed, there is no evidence that Si surrounded by four PO4 units can exist in SAPO-18, since such an environment is expected to give a signal at -127(&10) ppm on the basis of the assumption that a linear deshielding correlation for tetrahedral silicon is similar to that observed for tetrahedral aluminum in similar environments.
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150
100
50
0
-50
-100
Chemical shift (ppm)
Figure 5. *’A1 MAS NMR spectra of calcined, rehydrated samples of (a) AlP04-18, (b) SAPO-18(1), (c) SAPO-18(2), and (d) SAPO-18(3). Peaks at around 40 ppm are attributed to tetrahedral aluminum, and those at around -15 ppm are attributed to octahedral aluminum. The
minor peak at aluminum.
ca. 15
ppm might be attributable to pentacoordinated
18 than on that of the silicon-rich SAPO-18. The cause of this lack of distortion for silicon-rich SAPO-18 is probably the absence of long-range order in the water-framework interaction resulting from the substitution of silicon into the framework. 27Al MAS NMR of calcined, dehydrated and calcined, rehydrated m04-18** shows that rehydration causes more than onehalf of the aluminum to increase its coordination from tetrahedral to octahedral. Similar changes in aluminum coordination were observed for the SAPO-18 samples (Figure 5 ) , suggesting that it is not the tetrahedral-octahedral transition alone that is responsible for the unit cell change upon hydration of the calcined AlP04-18. When hydrated samples of calcined AlP04-18 and SAPO18 are re-dehydrated, their XRD patterns revert to those observed for the corresponding calcined samples, indicating that these materials undergo reversible hydratioddehydration in a way similar to that observed previously for SAP0-34.11$23,24 Electron Microscopy. Medium resolution microscopy and associated diffraction down the [Ool] axis (Figure 6 ) reveal that
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Chen et al
10220 J. Phys. Chem, Vol. 98, No. 40, 1994
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0
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Chemical shin (wm)
Figure 8. Normalized 'H MAS NMR specha for anhydrous templateh e (a) AIFQb-18, (b) SAFQ-18(3), and (c) SAPO-34 after backgmund subtraction, with cenWL peak and spinning sidebands.
-1
nm
Figure 7. HREM images of SAPO-IX(4) taken down L0011. The strucbure projection is given for comparison.
Infrared Spectroscopy. The different silicon-substitution mechanisms for SAPO-I8 and SAPO-34 are further evident from infrared spectroscopic studies. The infrared spectra of template-free A1P04-18 and SAPO-18 samples within the hydroxyl stretching absorption region are shown in Figure 10A. AIPO4-18 has one main absorption band at 3676 cm-I,
attributable to P-OH groups and observed for almost all aluminophosphate-basedmolecular sieves. These groups probably originate from defects present in the structure although the contribution of terminal P-OH groups on the external surface cannot be excluded. For AIPO4-18, besides the main P-OH absorption, there are two bands of low intensity attributed to AI-OH groups, one at 3768 cm-' and the other at 3793 m-l. In addition to the'P-OH and AI-OH absorptions observed for AIPO4-18, template-free SAPO-18 samples possess two other distinct absorption peaks at 3600 and 3626 cm-' and one less intense band at 3743 cm-I. The 3743 cm-' band can be assigned to Si-OH groups in the structure. No significant variation in the frequency of any of these absorptions was observed between samples. The 3600 and 3626 cm-' peaks have also been observed for SAPO-34 previously:lo.11.33,34 the higher frequency absorption is thought to belong to hydroxyls located in the large cages and the lower frequency one to hydroxyls associated with the double six-membered rings. These hydroxyls result from the
J. Phys. Chem., Vol. 98, No. 40, 1994 10221
SAPO-18 Catalysts and Their Bransted Acid Sites
c 0
1.2-
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-60
-80
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-120
-140
-160
3700
3500
3600
3400
Wavenumbers (cm-l) 5-
-180
.-B
Chemical shift (ppm)
5
Figure 9. 29Si NMR spectra for as-synthesized (a) SAPO-34, (b) SAPO-18(2), and (c) SAPO-18(3). Inset: decomposition of 29SiNMR
2
. 4-
-0-
SAPO-18 SAPO-34
.
signals for SAPO-18(3).
attachment of charge-balancing protons to two types of crystallographically different framework 0 atoms bridging between Si and A1 and infer Br#nsted acidity. Both types of hydroxyls are accessible for molecules as large as methanol, and their local environments are closely similar to each other since only one 'H NMR signal is observed as seen earlier. From SAPO-18(1) through to SAPO-18(3), the overall intensity of the two bridging hydroxyl peaks (as calculated from their areas) increases with the amount of silicon in the structure (Figure 10B). However, on going from SAPO-18(3) to SAPO18(4), the intensity decreases slightly, although the framework Si/(Si A1 P) ratio increases. SAPO-34, in which the Si/ (Si A1 P) ratio (0.10) is similar to that in SAPO-18(4), has a much higher concentration of bridging OH groups than all of the SAPO-18 samples. When the stoichiometry of the parenf aluminophosphate framework, AlP04, is considered, each silicon substituting for phosphorus alone (mechanism I as mentioned earlier) results in the production of one net negative charge and hence one Bransted acid site. The coupled substitution (2Si A1 P, mechanism 11) retains framework neutrality and therefore introduces no Bransted acidity. For SAPO-34, the substitution mechanism has been shown to be exclusively of the former type as revealed by 29Si MAS NMR spectroscopy, so that one Bransted acid site is produced for every silicon incorporated in the framework. For the SAPO-18 samples, however, this is not so, and on the basis of the IR data, the fraction of Si substituting via mechanism I decreases from SAPO-18(1) to SAPO-18(4), going from 0.86 to 0.70 to 0.62 to 0.5; the remaining Si for each sample substituting via mechanism II. Indeed, the slight decrease in Bransted acid sites upon going from SAPO-18(3) with a Si/(Si A1 P) ratio of 0.087 to SAPO-18(4) with one of 0.095 indicates a limit to mechanism I. Temperature-Programmed Desorption (TPD) of NH3. Ammonia has been extensively used as a probe molecule to measure the amount and strength of acid sites in solid^^^,^^ and is particularly applicable to small pore zeolites and alumino-
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0.04
0.06
0.08
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Si/(Si+AI+P)
Figure 10. (A) Infrared spectra (normalized against mass) of (a) AlPO418, (b) SAP0-18(1), (c) SAPO-18(2), (d) SAPO-18(3), (e) SAPO-18(4), and (0SAPO-34. Peaks at 3626 and 3600 cm-' are those attributed to bridging hydroxyls (Br~nstedacid sites). (B) IR absorption band area of the bridging hydroxyls versus framework Si/(Si A1 P) for the SAPO-34 and the SAPO-18 samples.
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phosphate-based molecular sieves such as those described here. Figure 11A compares the TPD curves for the calcined AlP0418 and SAPO-18 samples with those for SAPO-34 and HZSM-5 (SUA1 = 23). The AP04-18 curve exhibits only one weak desorption peak at around 155 "C which is probably due to weak Lewis acid sites and/or P-OH groups since AlPO4-18 contains no Bransted acid sites. It is reasonable to attribute the higher temperature (HT) desorption peaks for the SAPO samples to Bransted acid sites of the kind observed by IH NMR and IR. The area-based intensity of the HT peak for SAPO-34 is much higher than those for the SAPO-18 samples (also see Figure 11B). The intensity of the lower temperature (LT) peak of the TPD curve for each SAPO-18 sample is proportional to that of the HT one, suggesting the presence of ammonia molecules in the cavities hydrogen bonded to those directly attached to the Bransted acid sites. From SAPO-18(1) through to SAP0-18(3), the HT peak increases in intensity, but from SAPO-18(3) to SAPO-18(4) the peak intensity remains more or less the same. Similar to the IR results, the TPD intensities from SAPO-18(1) through to SAPO-18(4) are not linearly proportional to the silicon contents (Figure 11B). For instance, the Si/(% A1 P) in SAPOl(3) is 3.1 times as large as that in SAPO-18(1), but the TPD intensity for the former is just about 2.1 times that for the latter. Again, this observation suggests that not all the Si atoms incorporated in SAPO-18 substitute for P atoms via mechanism
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10222 J. Phys. Chem., Vol. 98, No. 40, 1994
Chen et al.
Bransted acidity that will permit tuning of its activity and deactivation behavior. Table 3 summarizes the respective activities and selectivities of A1P04-18 and all the SAPO-18 samples alongside SAPO34. As expected, AIpO4- 18 catalyzes methanol only to dimethyl ether: even at elevated temperatures, the amount of hydrocarbons in the reaction product is negligible. A small amount of silicon in the structure (SAPO-18(1)) improves its catalytic performance dramatically. At 400 "C and above, the conversion of methanol over SAPO-18(1) reaches 100% and the percentage of carbon in the product in the form of hydrocarbons (rather than dimethyl ether) is over 95%, ethylene and propylene 100 200 300 400 500 600 making up ca. 75% of this. At this reaction temperature, SAPOTemperature ('C) 18(1) lasts for at least 2 h without diminution in the extent of methanol conversion. -- Y . At 300 "C, the conversions for SAPO-18(3) and SAPO-18---t SAPO-18 (4) are similar to that for the SAPO-34 which has a Si/(Si A1 P) ratio of 0.10 but more than twice as high as those for SAPO-18(1) and SAPO-18(2). The selectivities for ethene and propene in the hydrocarbon products for all the SAPO catalysts are similar. At higher temperatures, all the samples have more or less the same activities and selectivities, suggesting that the amount and strength of the Bransted acid sites in the structure are not crucial at elevated temperatures for a short reaction time 0.00 0.02 0.04 0.06 0.08 0.10 0.12 on stream. Si/(Si+AI+P) For the first 2 h of reaction time on stream, the change in Figure 11. (A) NH3 TPD profiles recorded by mass spectroscopy ( d e activity and selectivity for all samples remains within a relatively = 17): (a) AlPO4-18, (b) SAPO-I8(1), (c) SAP0-18(2), (d) SAPOsmall range. From 2 to 3 h, SAPO-18(1) drops dramatically in 18(3),(e) SAPO-18(4), (f) SAPO-34, and ( g ) HZSM-5 with SdAl = activity (Figure 12). The following activity drop with reaction 23. (B) TPD desorption peak area of Bransted acid site versus time for this sample is reasonably slow. Only at longer reaction framework Si/(Si + Al P) for the SAPO-34 and the SAPO-I8 times does SAPO-18(2) decrease its activity significantly, samples. whereas SAPO-18(3) and SAPO-18(4) retain relatively high activity even after 8 h of reaction time on stream. On the other I. The marked TPD intensity difference between SAPO-18 and hand, the activity of SAPO-34 drops after about 3 h of time on SAPO-34 also indicates that for SAPO-18, Si substitutes for stream and is the lowest of all the SAPO samples after 8 h. It both A1 and P via mechanism 11as well as via mechanism 1. It seems that, for SAPO-18, with the increase of silicon content, is estimated that of the total incorporated Si atoms, those not only is the activity at lower temperature enhanced but the substituting for P only via mechanism I are around 70, 64,48, deactivation rate is also decreased; SAPO-18(3) as well as and 42% for SAPO-18(1)-SAPO-l8(4), respectively, and SAPO-18(4), which have the highest Si(Si A1 P) ratios although the precise numbers differ, the trend parallels that among the SAPO-18 samples, are the best from the point of observed by IR. view of deactivation. In SAPO-34, all the silicon substitutes Generally, the strength of an acid site is proportional to the for phosphorus and the number of Bransted acid sites is much corresponding desorption temperature of ammonia, although higher than that in SAPO-18 samples. The higher the number crystal size and amount of Bransted acid sites also affect the of acid sites, the higher the rate of coke formation in the desorption temperat~re.~'Since the temperature ramping rate c a v i t i e ~ ,and ~ ~ the , ~ rapid deactivation after 3 h may be due to for the TPD measurements is rather small (5 "C/min), we assume formation of coke in the pores and consequential prevention of that the desorption temperature for these SAPO samples is the access of reactant to the active sites. Although the initial mainly determined by the strength of the Bransted acid sites. It activity is proportional to the number and strength of the acid is seen that for SAPO-18 and SAPO-34, the more Bransted acid sites, it is not true that the higher the number and strength of sites, the higher the temperature at which the maximum of the the acid sites, the better under the same reaction conditions, HT desorption peak is located (Figure 11A), suggesting that a because of faster deactivation. There seems to be an optimum SAPO with a high acid content may contain more strong acid number (and strength) of acid sites which is lower than that for sites than a structurally similar SAPO with a low acid content. SAPO-34, and SAPO-18(4) can be a candidate bearing this Two TPD peaks are also observed for HZSM-5. Although the optimum number of acid sites. area-based intensity of the HT peak for HZSM-5 is much lower After recalcination, the activity of the completely deactivated than that for SAPO-34, its maximum is at a temperature higher SAPO-18 can be recovered. At 400 "C and above, the than that for any of the SAPO samples examined. Apparently, conversion of methanol to hydrocarbons over the recalcined the Bronsted acid sites in all these SAPO samples are, on SAPO-18(3) is higher than 98% and the selectivity for ethene average, weaker than those in HZSM-5. and propene is essentially the same as that over the fresh sample. Catalytic Performance for Methanol Conversion. The conversion of methanol (directly or indirectly via dimethyl ether) SAPO-44 was f o ~ n d ' to ~ ,be ~ ~active as well for methanol to hydrocarbons, either to gasoline (MTG) or light olefins conversion to light olefins. Since its framework composition (MTO) is of continuing industrial and academic i n t e r e ~ t . ~ ~ - ~is~close to that of SAPO-34, it has a catalytic lifetime similar SAPO-18 has considerable promise as a catalyst for the MTO to that of SAPO-34. Various modifications, including dilution reaction, since it has a small pore structure that is likely to show of methanol feed with ~ a t e r ' and ~ , ~steaming ~ removal46 of shape selectivity for ethene and propene preparation and variable excess Bransted acid sites from the catalysts, have previously
+
+
+
+ +
SAPO-18 Catalysts and Their Bransted Acid Sites
J. Phys. Chem., Vol. 98, No. 40, 1994 10223
TABLE 3: Catalytic Performance of AlP04-18, SAPO-18, and SAPO-34 for Methanol Conversion product distribution (exclusive of water, %) sample temp ("C) time on stream (min) methanol conv (%) (CH3)zO C& CZ& CzH6 C3H6 AlP04- 18
SAPO-18(1)
SAPO-18(2)
SAPO-18(3)
SAPO-18(4)
SAPO-34
200 300 400 500 200 300 400 500 400 400 400 200 300 400 500 400 400 400 200 300 400 500 400 400 400 200 300 400 500 400 400 400 200 300 400 500 400 400 400
10 10 10 10 10 10
10 10 60 120 180 10 10 10 10 60 120 180 10 10 10 10 60 120 180 10 10 10 10 60 120 180 10 10 10 10 60 120 180
0
0
20.2 72.6 78.0 47.0 86.1 100 100 100 100 93.5 53.3 86.5 100 100 100 100 100 55.5 100 100 100 100 100 100 62.0 100 100 100 100 100 100 66.9 100 100 100 100 100 100
100 100 97.5 100 67.9 1.5 0 13.9 16.5 59.8 100 63.7
120
-
SAPO-18(1) SAPO-1812) 100
0
gm ?e
80
c
O B
c .P
60
23
40
r ?0e
8s
20
0 0 4.7 9.1 19.4 100 10.2 2.6 1.5 6.0 11.8 20.7 100 9.3
0 0 1.5 4.3 6.4 100 11.3
0 0 3.6 13.0 31.4
0 0 0 1.1 0 0.7 0.2 5.2 0.5 0.6 0.7 0 0.9 0.3 2.7 0.4 0.5 0.6 0 1.o 0.6 5.6 0.7 0.7 0.5
0 1.3 0.3 3.2 0.6 0.6 0.7
0 1.8 0.5 1.6 0.4 0.3 0.3
0 0 0 0.3 0 3.4 22.2 43.0 24.5 23.7 12.3
0 3.5 21.7 39.9 25.8 23.6 21.3
0 8.6 29.4 47.2 26.5 25.1 21.9
0 8.7 21.9 39.1 26.1 25.2 25.5
0 17.2 28.1 45.8 29.1 25.2 19.2
0 0 0
0 0 0 0 0
0 0
0.4 0 20.0 53.0 38.5 43.5 40.9 20.1
0 0 0.5 0
0 0 0 0 0.2 0.5 0 0.2
0 0 0
0.3 0.6 0.2 0 0.2
0 0
0 11.0 25.1 15.0 21.5 23.8 21.6 0 34.5 18.1 9.5 18.8 18.5 18.7 0 34.5 28.0 16.7 23.9 24.0 22.1 0 27.0 21.9 14.5 21.4 22.0 18.7
0 42.5 49.3 37.4 45.0 38.9 29.4
0.4 0.5 0.2 0.3 0.2
8.1 23.1 12.2 17.3 18.2 6.9
0
0
0 0 0
0 0 0
21.4 52.0 41.8 47.1 42.5 37.9 45.4 49.0 34.6 47.8 43.9 37.8 0 46.3 49.5 40.0 47.5 45.7 45.0
0.2 0.8 0.2 0
~~
Cd+
via direct hydrothermal preparation Unlike SAPO-34, which usually has- to be prepared-in the presence of a considerable amount of Si and as a result contains a high concentration of Si in its framework, SAPO-18 may be prepared with very low amounts of silicon. In SAPO-18, the incorporated silicon substitutes for aluminum as well as phosphorus (the amount of substituted phosphorus is larger than that of the substituted aluminum) whereas in SAPO-34, the incorporated silicon substitutes only for phosphorus. These two factors render SAPO-18 a solid acid containing fewer Bransted acid sites than SAPO-34. The number of Bransted acid sites in SAPO-18 increases with the amount of silicon incorporated in the structure. Nevertheless, this increase reaches a maximum at a Si/(Si A1 P) ratio of about 0.09. SAPO-18 is much superior to AlP04-18 for the conversion of methanol to light hydrocarbons. The activities and selectivities for all SAPO-18 samples under investigation are comparable to those for SAPO-34: the conversion of methanol to hydrocarbons can reach 100% and the optimum distribution of ethene and propene in the hydrocarbon product is 80% at this conversion. SAPO-18 materials with a high Si/(Si A1 P) ratio (typically SAPO-18(4)) retain their catalytic activity for a longer reaction time on stream than those with a low Si/(Si A1 P) ratio (typically SAPO-18(1)). On the other hand, the catalytic activity of SAPO-34, which has twice as many Brensted acid sites as the most acidic (in the sense of both number and strength of acid sites) SAPO-18 sample, drops off more rapidly than
+
+
0 0
2
4
6
8
10
Time on stream (Hours) Figure 12. Dependence of activity on reaction time on stream for methanol conversion over SAPO-18(1), SAPO-18(2), SAP0-18(3),
SAPO-18(4), and SAPO-34.
been carried out to improve the catalytic longevity of the CHAtype catalysts. Whether or not these modifications improve the performance of the AEI-type catalyst SAPO-18 remains a matter of continuing study. Conclusions By adjusting the silicon content in the synthetic gel, SAPO18 samples with various amounts of silicon can be obtained
+
+ + +
10224 J. Phys. Chem., Vol. 98, No. 40, 1994
the latter. Therefore, a SAPO-18 with an optimum framework composition is a superior catalyst to SAPO-34 in terms of lifetime for the MTO reaction. Acknowledgment. We are grateful to SERC for general support of this work and one of us (J.C.) is indebted to Unilever Plc for financial support. We also thank JEOL Plc (UK) for the EDX analysis and Dr. M. W. Anderson and B. Gore (SERC NMR service, UMIST, UK) for performing the NMR measurements. Dr. D. Laundy (Daresbury Laboratory, UK) is thanked for his help and the instrumental broadening curve of Station 2.3 at Daresbury. References and Notes (1) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. SOC. 1982, 104, 1146. (2) Wilson, S. T.; Lok, B. M.; Flanigen, E. M. U S . Patent 4,310,440, 1982. (3) Holderich, W.; Hesse, M.; Naumann, F. Angew. Chem., Int. Ed. Engl. 1988, 27, 226. (4) Thomas, J. M. Sci. Am. 1992, 266, 82. (5) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannon, T. R.; Flanigen, E. M. J. Am. Chem. SOC.1984, 106, 6092. (6) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannon, T. R.; Flanigen, E. M. U.S. Patent 4,440,871, 1984. (7) Wilson, S. T.; Flanigen, E. M. US. Patent 4,567,029, 1986. (8) Flanigen, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S. T. Pure Appl. Chem. 1986, 58, 1351. (9) Ito, M.; Shimoyama, Y.; Saito, Y. Acta Crystallogr. 1985, C41, 1698. (10) Marchese, L.; Chen, J.; Wright, P. A.; Thomas, J. M. J. Phys. Chem. 1993, 97, 8109. (11) Zibrowius, B.; Loeffler, E.; Hunger, M. Zeolites 1992, 12, 167. (12) Liang, J.; Li, H.; Zhao, S.; Guo, W.; Wang, R.; Ying, M. Appl. Catal. 1990, 64, 31. (13) Inui, T.; Phatanasri, S.; Matsuda, H. J. Chem. Soc., Chem. Commun. 1990, 205. (14) Thomas, J. M.; Xu,Y.; Catlow, C. R. A.; Couves, J. W. Chem. Mater. 1991, 3, 667. (15) Chen, J.; Thomas, J. M. Catal. Lett. 1991, 11, 199. (16) Simmen, A.; McCusker, L. B.; Baerlocher, Ch.; Meier, W. M. Zeolites 1991, 11, 654. (17) Chen, J.; Thomas, J. M.; Townsend, R. P.; Lok, C. M. UK Patent Application 9318644.3, 1993. (18) Chen, J.; Thomas, J. M.; Wright, P. A.; Townsend, R. P. Submitted for publication.
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