Small Pore Aluminum Phosphate Molecular Sieves with Chabazite

Publication Date (Web): February 29, 1996 ... The nature of the silicon site depended on the manganese content of the sample as seen from 29Si MASNMR...
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J. Phys. Chem. 1996, 100, 3665-3670

3665

Small Pore Aluminum Phosphate Molecular Sieves with Chabazite Structure: Incorporation of Manganese in the Structures -34 and -44 Sunil Ashtekar,† Satyanarayana V. V. Chilukuri,‡ A. M. Prakash,† and Dipak K. Chakrabarty*,† Solid State Laboratory, Department of Chemistry and Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Bombay 400076, India ReceiVed: June 28, 1995; In Final Form: NoVember 7, 1995X

MnAPO and MnSAPO (APO ) aluminum phosphate; SAPO ) silicoaluminophosphate) with -34 and -44 framework structures have been synthesized. Rapid synthesis of MnAPO-34 could be achieved using morpholine as the template that subsequently changes to MnAPO-20 during synthesis. The presence of manganese in the framework has been established by a number of experimental techniques. ESR spectra of the low manganese-containing samples were better resolved, and by comparison of these with the ESR of Mn2+-exchanged samples, it was possible to conclude that Mn2+ was present in the framework of the MnSAPOs. The nature of the silicon site depended on the manganese content of the sample as seen from 29Si MASNMR. With higher manganese content, more silica-rich regions next to SAPO domains in the framework were formed. In MnAPO-44, a complete ordering of the T atoms appeared to have been achieved with every P atom being surrounded by three Al and one Mn. DTA-TG, infrared, and UV-visible spectra of the samples will be discussed.

Introduction

TABLE 1: Gel Composition and Synthesis Conditions

The silicoaluminophosphates SAPO-34 and SAPO-44 and their metal incorporated structural analogues are similar to the naturally occuring zeolite chabazite. They have been found active as catalysts in the conversion of methanol to lower olefins.1,2 MeSAPO-34 and MeSAPO-44 (Me ) Co, Mn) showed better activity for methanol conversion than the respective SAPOs.3 Substitution of Mn2+ in the AlPO4 or SAPO molecular sieve framework has been attempted by several groups. Thus, Levi et al.4 found that, at low concentration, Mn2+ enters T sites in MnAPO-5, but when the manganese concentration in the gel was higher, the majority of it remains outside the framework. Similar conclusions were arrived at in the case of MnSAPO-44 by Olender et al.5 On the basis of electronic spectra, Rajic et al.6 claimed that Mn2+ substituted in SAPO-34 can be oxidized to Mn3+ and reduced back without its leaving the T site. Parrillo et al.,7 on the other hand, concluded that the changes in the electronic spectra of MnSAPOs are not due to the change in the oxidation state of manganese. Lee et al8. compared MnSAPO-11 samples with SAPO-11 ion-exchanged with Mn2+ and showed that the local environment of Mn2+ in the two samples were different. It may be said that substitution of at least small amounts of Mn2+ in the various MnSAPOs and MnAPOs has been confirmed. Attempts to confirm the substitution of manganese in the AlPO4 molecular sieves in samples with larger amounts of manganese have not been very successful. In view of our success in incorporating fairly large amounts of Co2+ in the -34 and -44 framework,9 we have chosen these two structures for the substitution of manganese. The amount of metal ion that can be substituted in an AlPO4 framework can be rather large, as has been shown recently by a report on the synthesis of a CoPO4 molecular sieve.10 †

Department of Chemistry. Regional Sophisticated Instrumentation Center. X Abstract published in AdVance ACS Abstracts, January 15, 1996. ‡

0022-3654/96/20100-3665$12.00/0

reaction condition sample MnAPO-34 MnAPO-20 MnSAPO-34/1 MnSAPO-34/2 MnAPO-44 MnSAPO-44/1 MnSAPO-44/2 a

gel composition MnO Al2O3 P2O5 SiO2 Ta 0.4 0.4 0.03 0.1 0.4 0.03 0.1

0.8 0.8 0.985 0.95 0.8 0.985 0.95

1.0 1.0 1.0 1.0 1.0 1.0 1.0

0.6 0.6 0.6 0.6

2.7 2.7 2.7 2.7 1.9 1.9 1.9

H2O 60 60 60 60 60 60 60

temp time (°C) (h) 200 200 200 200 180 190 190

3 12 48 48 72 48 48

T ) template.

Experimental Section Morpholine (AR grade, SRL, Bombay) and cyclohexylamine (AR grade, SRL, Bombay) have been used for the synthesis of the manganese-containing samples with structures -34 and -44, respectively. MnAPOs were synthesized by the following method. Pseudoboehmite (Catapal-B, Vista) and orthophosphoric acid (85%, AR grade, E. Merck) were dissolved in water to which manganese sulfate (MnSO4‚H2O, AR grade, BDH) solution was added with vigorous stirring. This was followed by the addition of the template. In the case of the MnSAPOs, a solution of fumed silica (Aerosil-200, Degussa) in the template was added instead of the template alone. The resulting gels were heated in a stainless steel autoclave under autogeneous pressure. The gel compositions and synthesis conditions are given in Table 1. In the case of MnAPO synthesis using morpholine as the template, the metal aluminophosphate gel was divided and charged into several autoclaves and heated for 3, 6, 12, 18, 24, and 30 h, respectively. MnSAPOs of two different compositions each were prepared. The crystals were filtered, washed with water, and dried at 110 °C for 12 h. The ion-exchanged samples MnSAPO-34 and MnSAPO-44 were prepared by adding 60 mL of 10-4 M Mn(CH3 COO)2‚4H2 O solution to 0.5 g of calcined HSAPO-34 or HSAPO-44 samples © 1996 American Chemical Society

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TABLE 2: Chemical Composition and Acidity of the Samples from Temperature Programmed Desorption of Ammonia sample

Mn

Al

P

MnAPO-34 MnAPO-20 MnSAPO-34/1 MnSAPO-34/2 MnAPO-44 MnSAPO-44/1 MnSAPO-44/2 Mn-SAPO-34 Mn-SAPO-44

0.13 0.10 0.007 0.028 0.14 0.0075 0.028 0.0016 0.0028

0.42 0.40 0.49 0.46 0.36 0.48 0.45 0.48 0.48

0.45 0.50 0.37 0.38 0.50 0.38 0.40 0.35 0.36

followed by stirring overnight at 70 °C. The sample was then filtered, washed, and dried at 110 °C for 5 h. X-ray powder diffraction patterns were recorded on a JEOL/ JSM-8040 instrument using Cu KR radiation with a nickel filter. Crystals were observed under Reichert MeF3 A optical microscope. Scanning electron microscopy was carried out using a Cameca-SU-SEM-PROBE analytical scanning electron microscope. Differential thermal (DTA) and thermogravimetric (TGA) analyses were carried out in air on a Dupont 9900 thermal analyzer at a heating rate of 10 °C/minute. Infrared spectra as KBr disks were recorded using a Nicolet 170 SX FT-IR spectrometer. MAS NMR spectra were recorded on a Varian VXR-300S spectrometer with a Doty scientific CP-MAS probe. The frequencies were 78.15, 121.41, and 59.59 MHz for 27Al, 31P, and 29Si, respectively. Pulses of 45° were used for all measurements with repetition times of 3 s for 27Al and 10 s for 31P and 29Si. Data were acquired at a MAS speed of 4.5 KHz. Aluminum nitrate in water, 85% phosphoric acid, and tetramethylsilane were employed as references. EPR spectra were recorded on a Varian X-band Superheterodyne spectrometer at room temperature. Dehydration was accomplished by heating the sample in vacuum at 400 °C. The samples were left at the dehydration temperature for 5-6 h and sealed under vacuum. UV-visible diffused reflectance spectra of the solid samples were recorded on a Shimadzu-260 UV-vis spectrophotometer. Chemical analysis of the samples was carried out after calcination at 500 °C. The samples were dissolved in aqua regia for analyzing aluminum and phosphorus. The undissolved portion was fused with lithium metaborate and subsequently dissolved in dilute nitric acid. Analysis was done on an atomic emission spectrometer with ICP source (Labtam Plasma Lab 8440). Acidities of the calcined samples were determined by temperature programmed desorption of ammonia at a heating rate of 10 °C/minute. The as-synthesized samples were first calcined by raising the temperature at 1 °C/minute up to 450 °C, and the sample was held at this temperature for 12 h, after which it was cooled and ammonia adsorption was carried out at 100 °C. Results and Discussion Synthesis. The MnAPO-44 could be crystallized in pure form only at 180 °C. At lower temperature, the pure phase could not be obtained even after prolonged heating for 7 days. X-ray powder diffraction patterns (XRD) of MnSAPO-34 and MnAPSO-44 were very similar to those of SAPO-34 and SAPO44, respectively.11 MnAPO-34 and MnAPO-44 showed diffraction lines with 2θ values similar to those of the respective SAPOs; however, the intensity of these lines in the case of MnAPO-44 were very close to those of CoAPO-44, showing that the (101) peak had intensity far greater than the rest. This would mean that in MnAPO-44, manganese atoms were

Si

0.14 0.13 0.13 0.12 0.16 0.15

(P + Si)/ (Al + Mn)

acidity (mmol/g) moderate + strong

0.82 1.0 1.02 1.04 1.0 1.05 1.09 1.06 1.06

0.66 1.94 1.75 1.39 1.86 1.53

occupying the same sites as cobalt did in CoAPO-44. It was a little surprising to see that the intensity of the peaks for MnAPO34 was similar to those of SAPO-34 and MnSAPO-34 rather than that of CoAPO-34. However, this appears to be due to low substitution of Mn2+ at the T sites, as will be discussed later. The composition of the as-synthesized samples based on chemical analysis are given in Table 2. In all of the MnSAPOs the ratio (Si + P)/(Al + Mn) is greater than or equal to unity. In principle, silicon can substitute for aluminum (mechanism 1) or phosphorus (mechanism 2) or two silicon atoms can replace a pair of aluminum and phosphorus atoms (mechanism 3). However, the actually known SAPO compositions could be understood if one assumed that substitution occurred according to mechanism 2 along with the formation of silica-rich regions in the structure. It has been shown from 29Si MASNMR results that, in SAPO-34 and SAPO-44 structures, most of the silicon atoms go to the phosphorus sites (mechanism 2), although a small portion of silicon atoms form silica islands, that prevent the formation of Si-O-P linkages.11 One can not also rule out the possibility of some silicon atoms in the silica-rich region being substituted by aluminum. In the case of metal atoms, it has been shown that the Me2+ ions almost exclusively go to the aluminum sites in the framework.12 On the basis of this assumption, the ratio (Si + P)/(Mn + Al) in the MnSAPOs should be unity, if silica islands were not formed. The fact that all the samples of MnSAPO-34 and MnSAPO-44 had this ratio greater than 1 is an indication that at least a part of the silicon has formed silica-rich regions. Such silica patches are more prominent in the samples with higher amount of manganese. This will be discussed further along with the MAS NMR results. The samples without silicon, MnAPO-20 and MnAPO-44, had a (Mn + Al):P ratio equal to 1, while this value is greater than 1 for MnAPO-34 suggesting that the latter has extraframework manganese. Since pure AlPO4-34 and AlPO4-44 could not be synthesized so far, the formation of the -34 and -44 structures with manganese in the absence of silicon would suggest that manganese is indeed occupying a T site in order to stabilize the structure. Crystallization behavior of MnAPO-34 from a gel containing morpholine is interesting. No crystallization of MnAPO-34 was noticed at temperatures below 200 °C. At 200 °C, pure MnAPO-34 crystallized within 3 h. Use of tetramethylammonium hydroxide has been reported to take 7 days or more to crystallize this structure.13 Thus, morpholine can be used for rapid synthesis of MnAPO-34. Figure 1 shows the XRD patterns of the samples obtained by heating the gel at 200 °C for different times. Heating beyond 6 h resulted in the transformation of MnAPO-34 to MnAPO-20. The latter is isostructural with the very small pore zeolite-sodalite. After 12 h of heating, the product obtained was entirely MnAPO-20. Scanning electron micrographs (Figure 2b) clearly show the appearance of the sodalite phase in the reaction product that after 3 h of heating had only MnAPO-34 present (Figure 2a).

APO Molecular Sieves with Chabazite Structure

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Figure 1. X-ray powder diffraction patterns of the products crystallized after various durations: (a) 3, (b) 6, (c) 12, (d) 18, (e) 24, and (f) 30 h.

The change from chabazite topology to that of sodalite is in accord with Ostwald’s rule of succesive phase transformation from a more porous to a less porous structure.14 A phase transformation can occur either by a solution-assisted dissolution followed by recrystallization or by solid-solid transformation.15 Since we noticed this transformation in the presence of the mother gel, the former mechanism is more likely. Interestingly, MnAPO-20 was also unstable in the mother gel, and a third phase began to crystallize after 24 h of heating. After heating for 30 h, the product had only this third phase (Figure 1f). SEM showed octagonal platelike morphology (Figure 2c). XRD of this new phase did not match with any of the known AlPO4type structures. Thermal Analysis. DTA/TGA of the MnSAPO-34 and MnSAPO-44 were similar to the respective SAPOs reported earlier.11 Thermograms of MnAPO-34 and MnAPO-44 are shown in Figure 3a,b, respectively. The DTA and TGA of MnAPO-44 were almost identical with those of CoAPO-44 reported by us earlier.9 The mass loss occurred in two steps with DTG peaks at 440 and 550 °C whereas exothermic DTA peaks appeared at 443 and 547 °C. Even a small exotherm beyond 600 °C is similar to the one from CoAPO-44. This again emphasizes the close resemblence between MnAPO-44 and CoAPO-44 already seen in the XRD. The two large exotherms were due to removal of the template, and the last one is due to structural collapse. The total mass loss (20 wt %) is also identical with that of its cobalt analogue. Although the appearance of the DTA/TGA of MnAPO-34 is similar to

Figure 2. Scanning electron micrographs: (a, bottom) MnAPO-34; (b, middle) MnAPO-20; (c, top) unknown phase.

that of CoAPO-34, its initial mass loss is 9.8 wt % as compared to 4.5 wt % for the latter (Table 3). The greater mass loss in the former case may be due to the large amount of nonframework manganese possibly present as Mn(H2O)62+. The loss of template in the case of MnAPO-34 was 13.2 wt % as compared to 17 wt % for the MnSAPO-34 samples. Since the former contained nonframework manganese and hence had a lower framework charge, it could include only a smaller amount of the protonated template. This suggestion is supported by the chemical composition of the sample (Table 2). The number of template molecules per unit cell for the rest of the samples was similar to the respective SAPOs11 (Table 3). ESR. ESR spectra of MnSAPO-34/1 and MnSAPO-44/1 are shown in Figure 4. The MnSAPO-34/1 sample had a low manganese content (0.7% of the total T site atoms). It showed a six-line hyperfine split spectrum (g ) 2.001; A ) 86 G) typical of Mn2+. The calcined sample after dehydration at 400 °C under vacuum showed a better resolved spectrum with a small change in the hyperfine parameter (A ) 94G). Hydration of the calcined

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Figure 5. ESR spectra of the (a) exchanged MnSAPO-44 and (b) calcined MnSAPO-44 sample.

Figure 3. TGA, DTG, and DTA curves of the as-synthesized samples: (a) MnAPO-34; (b) MnAPO-44.

Figure 4. ESR spectra of MnSAPO-34/1 (a) as-synthesized, (b) calcined, and (c) dehydrated at 400 °C and MnSAPO-44/1 (d) as-synthesized, (e) calcined, and (f) dehydrated at 400 °C.

TABLE 3: Results of Thermal Analysis sample

water loss (wt %)

template loss (wt %)

no. of template molecules/unit cell

MnAPO-34 MnSAPO-34/1 MnSAPO-34/2 MnAPO-44 MnSAPO-44/1 MnSAPO-44/2

9.8 5.5 5.5 0.8 2.0 2.25

13.2 16.5 17.5 18.7 19.5 19.3

4.57 5.33 5.77 5.48 5.49 5.50

sample restored the original spectrum. Similar behavior was shown by MnSAPO-44/1 which had nearly the same Mn2+ content. An A value of 65 G was assigned to Mn2+ in tetrahedral coordination in the framework.5 On the other hand Mn2+ in an octahedral coordination normally gives an A value

around 90 G.5 Thus, the observed value of A ) 86 G in the present case suggests that manganese is most likely present in an octahedral environment. The occurrence of the hyperfine structure in the ESR of the as-synthesized sample does not prove whether Mn2+ is in the framework. However, extraframework Mn2+ on calcination (removal of water) is likely to migrate, and this may be reflected in their ESR spectra. To test this, a calcined SAPO-44 sample was exchanged with Mn2+, keeping the manganese content at less than 1% of the total T sites. The spectrum (Figure 5) shows the hyperfine lines. On calcination, the hyperfine structure disappears showing that the extraframwork Mn2+ has undergone migration on calcination. Such migration of exchanged manganese toward each other leading to the loss of the hyperfine structure due to spin-spin interaction was also noticed in MnAlPO-5 by Levi et al.4 Since MnSAPO-34/1 and MnSAPO44/1 spectra did not show such changes on calcination, it is reasonable to assume that manganese in these samples is in the framework, possibly with extra coordination by water molecules. ESR spectra of MnAPO-11 with varying amounts of Mn2+ were studied by Brouet et al.16 They found that, at low manganese concentration, only one type of Mn2+ was present (g ) 2.01; A) 87.7 G), which they attributed to Mn2+ in the framework. Our results on MnSAPO-34/1 and MnSAPO-44/1 are in good agreement with these authors. ESR was not helpful for samples containing higher amounts of manganese that gave a single broad band with g ) 2.01. Diffuse Reflectence Spectra (DRS). Diffuse reflectance spectra (DRS) of MnSAPO-34 and MnSAPO-44 samples showed an absorption with a peak at about 320-335 nm. After calcination, a second broad, intense band appeared at 480 nm. The appearance of the second peak was interpreted by Rajic et al.6 as being due to oxidation of Mn2+ to Mn3+. Parrillo et al.,7 on the other hand, interpreted this change in the spectrum of MnAPO-5 as being due to a change in the environment of Mn2+ rather than in its oxidation state as their adsorptiondesorption experiments did not show any evidence of a change in the oxidation state of manganese. We did not notice any change in the intensity of the ESR signal of Mn2+ after calcination. If Mn2+ changed to Mn3+ on calcination, there would be a decrease in the intensity of this ESR signal. Although our DRS results were similar to those of Rajic et al.,6 we do not have any firm evidence to support the oxidation of Mn2+ to Mn3+. FTIR. The framework vibration regions of the IR spectra of MnSAPO-34 and MnSAPO-44 samples were similar to the spectra of the respective SAPOs.11 For MnAPOs, the band near 730 cm-1 assigned to the T-O symmetric stretch was a doublet (Figure 6) unlike a single peak in the case of SAPOs. Similar splitting of this band was observed for CoAPO-34 and CoAPO-

APO Molecular Sieves with Chabazite Structure

Figure 6. IR spectra of as-synthesized samples: (a) MnAPO-34; (b) MnAPO-44.

J. Phys. Chem., Vol. 100, No. 9, 1996 3669

Figure 8. 27Al and 31P MAS NMR spectra of (a) MnSAPO-34/1, (b) MnSAPO-34/2, and (c) MnAPO-34 (*, the spinning side bands).

Figure 9. 27Al and 31P MAS NMR spectra of (a) MnSAPO-44/1, (b) MnSAPO-44/2, and (c) MnAPO-44 (*, spinning side bands).

Figure 7. Temperature programmed desorption spectra of ammonia: (a) MnAPO-34; (b) MnSAPO-34/1; (c) MnSAPO-34/2; (d) MnAPO44; (e) MnSAPO-44/1; (f) MnSAPO-44/2.

44 in which the concentration of this transition element at T sites was large.9 This may be taken as evidence of Mn2+ substitution. In the MnSAPOs, the concentration of manganese was too small to observe such splitting. Acidity. Figure 7 shows the temperature programmed desorption profiles of the calcined samples. The first peak at 160 °C was due to ammonia associated with the surface hydroxyl groups. In the MnSAPOs, peaks appearing near 440 °C are related to structural acidity. These patterns were essentially similar to those of SAPO-34 and -44 and their cobalt analogues reported earlier.9,11 These curves were resolved into three clear desorption peaks similar to those of SAPO-34 and -44.11 The last two peaks could be assigned to structural acidity. Since the amount of substituted manganese was very small, the number of acid sites were essentially controlled by the amount of silicon, which was not much different from one sample to another (Table 2). The number of acid sites calculated from TPD was 5-15% higher than the acidity calculated on the basis of the substituted silicon and manganese. This may be due to some ammonia molecules coordinated to manganese, but even then the agreement is quite good. TPD profiles of MnAPO-34 and -44 were different from MnSAPOs. These samples showed peaks at 150, 200, and 330 °C. The last two peaks are possibly arising from structural acidity. Chen and Thomas17 had reported an ammonia desorp-

tion peak for CoAPO-18 at 430 °C. Their studies on DRIFT spectra showed that Bronsted acidity of this material is related to hydroxyls bridging at phosphorus and a metal atom. It is possible to assume a similar hydroxyl bridge between phosphorus and manganese in these samples accounting for their acidity. The total acidity of MnAPO-34 was lower than that of MnAPO-44. This is not unexpected since the amount of manganese substitution in the former was lower as discussed earlier. MAS NMR. 27Al and 31P MAS NMR spectra of the various samples with structures type -34 and -44 are shown in Figures 8 and 9, respectively. All of the spectra showed spinning side bands due to the presence of paramagnetic Mn2+, the intensity of which increased with the Mn2+ content. MnAPO-34 and MnAPO-44 showed 27Al signals at 39.5 and 35.9 ppm, respectively, that can be assigned to tetrahedral aluminum18. Chemical shifts observed in MnAPO samples showed greater deshielding as compared to the corresponding SAPO samples.11 The spectra of MnSAPO-34 and MnSAPO-44 were similar to those of the respective SAPOs.11 These samples had a low amount of manganese, and hence their properties are dominated by the substituted silicon. They showed additional peaks at about 10 ppm due to extra coordination of aluminum.18 31P spectra of MnAPO-34 showed a central band at -26.5 ppm which can be easily identified with tetrahedral phosphorus. In this sample, most of the manganese was outside the framework and the spectrum is similar to those of the MnSAPO34 samples, except that the side bands were more intense. However, in MnAPO-44 which appeared to have a large amount of manganese in the framework, the central peak was shifted

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Ashtekar et al. and aluminum sites, leading to more silica-rich regions. The change in the 29Si MAS NMR spectra of the MnSAPO samples with the increase of manganese content can thus be rationalized. Conclusions

Figure 10. 29Si MAS NMR spectra of (a) MnSAPO-34/1, (b) MnSAPO-34/2, (c) MnSAPO-44/1, and (d) MnSAPO-44/2.

MnAPO-34 and MnAPO-44 were prepared from respective gels. MnAPO-34 crystallized rapidly in the presence of morpholine but changed to MnAPO-20 on further heating. In MnAPO-44, a complete ordering of the framework atoms is seen with each phosphorus atom being surrounded by three aluminum and one manganese, leading to a distortion of the framework. MnSAPO-34 and MnSAPO-44 with different manganese contents have been synthesized. In samples with low manganese content, ESR spectra suggested that Mn2+ was in framework position with extra coordination. The local environment of silicon was found to be influenced by the amount of manganese. With the increase in manganese content, more silica-rich regions were formed in addition to the SAPO domains. Acknowledgment. This work has been funded by a research grant from CSIR, New Delhi. S.A. and A.M.P. are grateful to the CSIR for the award of research fellowships.

Figure 11. Deconvoluted 29Si MAS NMR spectra of MnSAPO-44/2.

References and Notes

TABLE 4: Distribution (%) of Silicon Environments Obtained from Deconvoluted Silicon MAS NMR Spectra

(1) Inui, T.; Phatansri, S.; Matsuda, H. J. Chem. Soc., Chem. Commun. 1990, 205. (2) Chen, J.; Thomas, J. M. Catal. Lett. 1991, 11, 199. (3) Hocevar, S.; Batista, J. In AdVances in Catalysts Design; Graziani, M., Rao, C. N. R., Eds.; World Scientific; Singapore, 1991; p 137. (4) Levi, Z.; Raitsimring, A. M.; Goldfarb, D. J. Phys. Chem. 1991, 95, 3830. (5) Olender, Z.; Goldfarb, D.; Batista, J. J. Am. Chem. Soc. 1993, 115, 1106. (6) Rajic, N.; Stojakovic, D.; Hocevar, S.; Kaucic, V. Zeolites 1993, 13, 384. (7) Parrillo, D. J.; Pereira, C.; Kokotailo, G. T.; Gorte, R. J. J. Catal. 1992, 138, 377. (8) Lee, C. W.; Chen, X.; Brouet, G.; Kevan, L. J. Phys. Chem. 1992, 96, 3110. (9) Ashtekar, S.; Chilukuri, S. V. V.; Prakash, A. M.; Harendranath, C. S.; Chakrabarty, D. K. J. Phys. Chem. 1995, 99, 6937. (10) Chen, J.; Jones, R. H.; Natarajan, S.; Hursthouse, M. B.; Thomas, J. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 639. (11) Ashtekar, S.; Chilukuri, S. V. V.; Chakrabarty, D. K. J. Phys. Chem. 1994, 98, 4778. (12) Flanigen, E. M.; Patton, R. L.; Wilson, S. T. Innovation in Zeolite Materials Science; Grobet, P. J. et al., Eds. Stud. Surf. Sci. Catal. 1988, 37, 13. (13) Wilson, S. T.; Flanigen, E. M. In Zeolite Synthesis; Occelli, M. L., Robson, H. E., Eds.; American Chemical Society: Washington, DC, 1989; p 329. (14) Szostak, R. Molecular Sieves: Principles of Synthesis and Identification; van Nostrand Reinhold: New York, 1989; p 53. (15) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974; Chapter 4. (16) Brouet. G.; Chen, X.; Lee, C. W.; Kevan, L. J. Am. Chem. Soc. 1992, 114, 3720. (17) Chen, J.; Thomas, J. M. J. Chem. Soc., Chem. Commun. 1994, 603. (18) Blackwell, C. S.; Patton, R. L. J. Phys. Chem. 1984, 88, 6135. (19) Grimmer, A. R. Spectrochim. Acta, Part A 1978, 34, 941. (20) Muller, D.; Jahn, E.; Ladwig, G.; Haubenreisser, U. Chem. Phys. Lett. 1984, 109, 332. (21) Cheetham, A. K.; Clayden, N. J.; Dobson, C. M.; Jakeman, R. J. B. J. Chem. Soc., Chem. Commun. 1986, 195. (22) Jakeman, R. J. B.; Cheetham, A. K.; Clayden, N. J.; Dobson, C. M. J. Am. Chem. Soc. 1985, 105, 6249. (23) Hasha, D.; Saldarriaga, L. S.; Saldarriaga, C.; Hathaway, P. E.; Cox, D. F.; Davis, M. E. J. Am. Chem. Soc. 1988, 110, 2127. (24) Martens, J. A.; Grobet, P. J.; Jacobs, P. A. J. Catal. 1990, 126, 299.

sample

Si(4 Al)

Si(3 Al)

Si(2 Al)

Si(1 Al)

Si(0 Al)

MnSAPO-34/1 MnSAPO-34/2 MnSAPO-44/1 MnSAPO-44/2

70.2 50.7 70.2 38.4

13.7 15.7 3.6 19.6

8.7 15.3 12.6 12.4

1.5 4.9 0.3 6.0

5.9 13.4 13.3 23.5

to -4.1 ppm. This unambiguous but unusually large shift we assign to P(Mn, 3 Al) environment. If there is to be a complete ordering of this type throughout the structure, the composition should have a Mn:Al ratio of 1:3. Within the error of chemical ananlysis, this was indeed so. 31P chemical shifts have been correlated with P--O bond length,19 Al-O-P bond angle,20 or to P-O bond strength.21 Muller et al.20 showed an average chemical shift of 0.51 ppm/ deg for some dense aluminophosphates. It is possible that the presence of manganese next to phosphorus atoms changes both the T-O-P angle and the T-O-P distance, leading to a large chemical shift as seen in MnAPO-44. Large chemical shifts for the 31P peak was also noticed in Zn3-x Mgx PO4 phase.22 Figure 10 shows the 29Si spectra of the MnSAPO samples. There was an intense peak at -90.8 ppm and weak resonances at -94, -99, and -109 ppm for MnSAPO-34/1. MnSAPO34/2 which contained more silicon, showed an additional peak at -105 ppm. These peaks suggested multiple silicon environments. The peak at -90.4 is due to a Si(4 Al) environment, and the other peaks are due to Si(n Al, 4 - n Si) environments formed because of the presence of silica-rich regions in the structure.11,23,24 The spectra of MnSAPO-44 samples were similar to those of MnSAPO-34. Figure 11 shows the deconvoluted spectra of MnSAPO-44/2. It can be seen that there was a significant decrease in the intensity of the Si(4 Al) peak in this sample as compared to MnSAPO-44/1 (Table 4). This means that the increase in the amount of manganese gave rise to more silica-rich regions as seen from the 29Si spectra. Mn2+ (r ) 0.66 Å) is larger than Al3+ (r ) 0.43 Å). If one assumes that Mn2+ replaces Al3+, Si4+ (r ) 0.33 Å) being larger than P5+ (0.25 Å) does not prefer isolated substitution (mechanism 2) nearer to manganese, thereby restricting the formation of SiO-Mn bonding. Thus, silicon will occupy both phosphorus

JP951800C