J . Phys. Chem. 1994,98, 8175-8119
8775
Structural Defects Cause Different Rates of Phase Transformation of the Molecular Sieves TBA-VPI-5 and DPA-VPI-5 into AlP04-8 Heyong He, Tery L. Barr,+ and Jacek Klinowski' Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 lEW, U.K. Received: March 21, 1994; In Final Form: June 12, 1994"
Chemical analysis, IR, and ESCA reveal that the framework of the aluminophosphate molecular sieve DPAVPI-5 contains randomly distributed phosphorus defects, while the structure of TBA-VPI-5 is relatively perfect. The microstructural difference is responsible for the different rates of phase transformation of DPA-VPI-5 and TBA-VPI-5 to AlP04-8. The defects enhance the phase transformation of DPA-VPI-5 to AlP04-8 in the absence of adsorbed water.
Introduction VPI-5 is a hydrophilic crystalline molecular sieve containing 18-membered rings of P and A1 atoms and with the chemical formula AlP04.1 The large channel diameter of ca. 12 8, gives the material potential for the separation of large molecules and for catalytic cracking of heavy fractions of petroleum. At relatively low temperatures (80-1 20 "C) VIP-5 transforms2-14 into AlP04-8, another aluminophosphate molecular sieve with a one-dimensional 14-membered ring structure,l5J6 via the bond rearrangement shown in Figure 1.2910 When the 18-membered ring channels are blocked by large guest molecules, such as C60,17 the transformation cannot occur because there is insufficient space for the structural rearrangement. The extent of the transformation depends on the rate of heating,2J the manner of sample w a ~ h i n g , ~ Jthe ~ J ~dehydration-rehydration procedure,IO the method of preparation of the precursor12 and the nature of the organic temp1ate.M TBA-VPI-5 and DPA-VPI-5 are prepared using tetrabutylammonium hydroxide (TBA) and di-n-propylamine (DPA) as templates, respectively.I8 Davis et al. reported that TBA-VPI-5 has a much higher thermal stability than DPA-VPI-5.19 Under the same conditions of treatment, TBA-VPI-5 undergoes a partial transformation and DPA-VPI-5 a complete transformation into A1P04-8.5 We have confirmed that the thermal stabilities of TBA-VPI-5 and DPA-VPI-5 are different. On the other hand, it is known that the Al/P ratios of the two materials are ca. 1.0 and 1.1,respectively.l3J9 Although no detailed study is available, it seems likely that microstructural differences, probably related to the presence of the framework defects, are the key factors responsible for the different thermal stabilities. We have studied the microstructure and thermal stability of TBA-VPI-5 and DPAVPI-5 using chemical analysis, FT-IR and ESCA. Although ESCA has played a significant part in the characterization of zeolites,20q21 it has only recently been applied to aluminophosphate molecular s i e v e ~ . ~ ~Samples , ~ 3 of AlP04-8 prepared from TBAVPI-5 and DPA-VPI-5 were also studied by ESCA. All DPA samples show the presence of P defects in the framework. Experimental Section TBA-VPI-5 and DPA-VPI-5 were synthesized according to ref 18, except that, to obtain pure TBA-VPI-5, the composition of the synthesis gel was 1.1 2 TBA:1 .OO A1203:l .OO P205:50 H20.24 AlP04-8 was prepared by calcination at 200 "C for 2 h and is referred to as TBA-AlPO4-8 and DPA-AlP04-8, respectively. X-ray diffraction (XRD) shows that theoverall structureof TBAt Permanent address: Department of Materials and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, WI 53201. Abstract published in Aduance ACS Abstracts, August 1, 1994.
0022-3654/94/2098-8775$04.50/0
Figure 1. Structural transformation of VPI-5 into AIPOd-8,
AlP04-8 and DPA-AlPO4-8 is very similar. Dehydrated VPI-5 was prepared by evacuation at 10-6 Torr for 2 days. The XRD patterns were recorded using a Philips 1710 powder diffractometer with Cu K a radiation (40 kV, 40 mA), 0.025O step size and 1 s step time. 27Al magic-angle-spinning (MAS) N M R spectra were measured at 104.3 MHz with very short, 0.6 ps (less than lo"), radiofrequency pulses and 0.3 s recycle delays. 31PMAS spectra were recorded a t 162.0 MHz with 30" pulses and 120 s recycl delays. These conditions are sufficient to obtain a quantitative intensity ratio of the three 3lP peaks of VPI-5 given that their TIrelaxation times are as short as ca. 26 s.25 27Al and 3IP chemical shifts are quoted in ppm from external Al(H20)63+and 85% H3P04, respectively. ESCA measurements were carried out on the V.G. ESCALAB system at the Surface Analysis Facility of the University of Wisconsin-Milwaukee. The samples were pressed into an indium foil. Binding energies were all referenced to the C(1s) peak (at 284.6 eV) from the small amount of adventitious carbon on the surface of all solids. Via this procedure, the charging shifts were removed from all spectra. Infrared spectra were recorded on a Nicolet 5DX ET-IR spectrometer using KBr wafers. X-ray fluorescence analysis (XRF) was performed on a Zeiss VRA-20 instrument, and plasma analysis (ICP) on an A.S.U. JattellAASH series 800 Mark-I1 ICAPspectrometer. Energy-dispersive X-ray analysis (EDX) was done on a JEOL-200 CX electron microscope equipped with an X-ray analysis attachment. Results Chemical Analysis and IR. Elemental analysis using XRF, ICP, and EDX gives the P/A1 ratios of 0.84 (XRF), 0.87 (ICP), and 0.87 (EDX) for DPA-VPI-5 and 1.OO (XRF, ICP, and EDX) for TBA-VPI-5. To confirm the reality of the low P/Al ratio of DPA-VPI-5, samples from separate preparations were examined with the same results.l3J9 Elemental analysis for C, H, and N was also performed. P defects and consequent AI-OH nests consistent withP/Al= 0.9 are present in DPA-VPI-5 (composition A1203:0.9 P205:0.04 DPA), while the framework of TBA-VPI-5 is structurally perfect (with composition A1203:P205:0.006TBA and P/Al = 1.00). 0 1994 American Chemical Society
He et al.
8776 The Journal of Physical Chemistry, Vol. 98, No. 35, 1994 (a)
1500
(b)
1‘00
850
I
650
Wavenumbers (cm-‘)
Figure 2. Infrared spectra of DPA-VPI-5 and TBA-VPI-5 in the (a) framework vibration region and (b) hydroxyl vibration region.
Infrared spectra of DPA-VPI-5 and TBA-VPI-5 in the framework vibration region shown in Figure 2a are similar, except for the relative intensities of the bands. This is consistent with the same overall framework structure of both materials. The framework hydroxyl is detected more sensitively in the range above 3000 cm-1 than in the framework range provided that the water molecules are removed from VPI-5 channels. However, infrared spectra in the O H vibration region of samples pretreated at 100 OC under vacuum for 4 h (see Figure 2b) show significant differences. Thus DPA-VPI-5 gives four bands from O H groups at 3650, 3625, 3547, and 3461 cm-I. Absorption is so intense that the bands can be observed using the standard KBr wafer technique. By contrast, the spectrum of TBA-VPI-5 does not show resolved OH bands in the same region. The O H peaks in DPA-VPI-5 do not come from pseudo-boehmite or AlP04-11, both possible impurities in DPA-VIP-5, since the O H bands in pseudo-boehmite are found at ca. 3300 and 3090 cm-I 26 and those of AIPO4-11 at 3675 and 3679 ~ m - 1 . 1 Considering ~ the deficiency of P atoms, the O H groups must be associated with AI but not P atoms, and the spectrum is comparable to that of bayerite and g i b b ~ i t e .This ~ ~ indicates that, unlike TBA-VPI-5, DPA-VPI-5 contains AI-OH groups. Evacuation and heat treatment of VPI-5 should remove water molecules, completely, given that the loss of water in TGA begins at ca. 60 OC. However, in view of the presence of intense O H absorption bands, it is clear that the amount of residual strongly chemisorbed water cannot be significant. If strong interactions were to occur between the O H groups and the water molecules, the resolution of the corresponding vibrations would be reduced and the bands would be shifted towards lower wavenumbers.27 ESCA. It is known that exposure of most solid surfaces to STP air and handling “compromises” the outermost surface, and the thickness of the affected layer is between 5 and at most 20 A. When the sample is “well made”, N M R and XRD are insensitive to surface alterations, as both techniques are surfaceinsensitive. ESCA, which is optimized to detect features in the depth rangeof ca. 5-50 A, detects these alterations as a byproduct in a matrix dominated by near-surface features which reflect almost exclusively the bulk chemistry and structure.22 Typically, the compromised outer surface of a mixed oxide such as AIP04 adjacent to the structurally sound bulk Alp04 should favor the retention of simple oxides of the more electropositive element, in
78
74
70
136
136
128
534
530
525
Binding energy (eV)
Figure 3. High-resolution A1(2p), P(2p), and O(1s) ESCA spectra of (a) TBA-VPI-5, (b) DPA-VPI-5, (c) TBA-AlPO4-8, and (d) DPA-AlP048. The binding energy scales have been adjusted to C(1s) = 284.6 eV.
TABLE 1: Binding Energies and, in Brackets, Corresponding Linewidths (k0.05eV) for VPI-5 and AIPOd-8* material AU2P) WP) O( 1s) TBA-VPI-5 DPA-VPI-5 TBA-AIP04-8 DPA-AlP04-8
75.5 (2.15) 74.8 (2.8) 75.45 (2.2) 75.0 (3.2)
134.85 (2.3) 134.7 (2.65) 135.05 (2.4) 134.55 (2.6)
532.6 (2.15) 532.45 (2.50) 532.85 (2.2) 532.3 (2.4)
Binding energies are referenced to C ( 1s) = 284.6 eV.
this case A1203, AlOOH, and Al(OH)3, at the expense of phosphorus oxide formed from the less positive element.28 The ESCA linewidths (Table 1) and line structures (Figure 3) from TBA-VPI-5 indicate the presence of a singular species.20 The exception is the shoulder peak on the low binding energy side in the AI(2p) line structure (Figure 3). In this case there is a slight distortion of the AI(2p) line away from the Gaussian-
Molecular Sieves TBA-VPI-5 and DPA-VPI-5 Lorentzian-type structure which would be produced by a material with a totally singular aluminum-containing species. This is caused by the presence of aluminum-containing species with distinctly lower binding energies than the Al(2p) binding energy of the principal component at ca. 75.5 eV. A deconvolution of the Al(2p) line manifold (not shown) suggests that the secondary structure is due to several peaks with binding energies between 74.5 and 73.8 eV: the binding energy range for simple aluminum oxides, such as A1203, AIOOH, and A1(OH)3.29 The relative amount of these secondary species in TBA-VPI-5 is ca. 10%. Their presence is expected because of the nature of the surface degradation and those additional A1 oxides with little or no phosphorus-containing species. Thus the structure of the principal Al(2p) peak at 75.5 eV appears to be due to a singular species which, as far as ESCA is concerned, is expected to constitute the entire subsurface of the system, the A1 signature for TBA-VPI-5. The corresponding P(2p) spectrum is slightly broader than the Al(2p) but still within the range expected for a chemically singular oxide. The relative intensity of this P(2p) peak suggests that the phosphorus is associated only with thedominant aluminous species. The P(2p) binding energy of 134.85 eV is therefore assigned to the phosphorus in TBA-VPI-5. The O(1s) spectrum for this material is also reasonably singular and entirely consistent with the above interpretation of the structure of TBA-VPI-5. ESCA analysis of DPA-VPI-5 provides ample evidence for the presence of near-surface imperfections. Thus, all ESCA peaks for the DPA-VPI-5 (Figure 3 and Table 1) are broader, particularly the A1(2p), and shifted in binding energy to lower range compared with TBA-VPI-5 (see Table 1). The very broad Al(2p) line from DPA-VPI-5 indicates the presence of significant quantities of A1-0-H groups.29 Thevariation in theP(2p) binding energies (TBA-VPI-5 to DPA-VPI-5) is smaller than that for Al(2p). This is not unexpected for the reasons discussed in connection with TBA-VPI-5, and is probably due to the presence of AI-0-H groups in DPA-VPI-5. The O( 1s) line from DPAVPI-5 is also broader and noticeably distorted at a lower binding energy where a peak would be expected from AI-0-H bonds. Like TBA-VPI-5, TBA-A1P04-8 produces relatively singular ESCA lines (note the linewidths), suggesting the presence of almost exclusively one species in the near-surface. In this case, however, there is a close analogy with the binding energies for pure TBA-VPI-5. Thus, although the line widths for the TBAAlP04-8 are somewhat larger than those for TBA-VPI-5 (see Table l), the two sets of peak positions differ only slightly. The binding energies of the Al(2p) lines for the two systems are within experimental error of one another. The only difference concerns the relative amount of A1203,AlOOH, AI(OH)3 byproducts, Le., the Al(2p) shoulder, with approximately one-half the amount detected on the TBA-A1P04-8. The P(2p) and O(1s) lines for TBA-AlP04-8 (Figure 3) are shifted to higher binding energies compared with TBA-VPI-5. However the shift is small (ca. 0.2 eV), and may be insignificant. The near-surface characteristics of DPA-AlP04-8 also suggest a significant compromise, with a very broad Al(2p) peak showing the presence of a mixture of AI-0-P and substantial amounts of AI-0-H groups. However, the P(2p) and O(1s) peaks are relatively singular.
Discussion The P and A1 atoms in aluminophosphate molecular sieves occupy alternate framework positions, with P/Al = 1 for a perfect framework. The extra 0.1 A1 atoms in DPA-VPI-5 may come from impurities or from framework AI-OH groups. The XRD pattern indicates that DPA-VPI-5 contains little crystalline impurity. Significant quantities of amorphous impurities can also be ruled out, since 3lP and Z7AlMAS N M R spectra in Figure 4 show no signals a t -1 0 to -1 8 and ca. 6 ppm from P-OH and 6-coordinated A1 in an amorphous phase, respectively. Therefore, the PIA1 ratio of 0.9 in DPA-VPI-5 can only be explained by the
The Journal of Physical Chemistry, Vol. 98, No. 35, I994 8777 31P MAS NMR
20
10
-io
-10
-0
-io
-30
-60
-50
-io
ppm from 85% H3P04
2 7 ~ MAS 1 NMR
6-4
50
40
30
20
10
0
-10
-20
-30
ppm from AI ( H ~ O ) ~ ~ +
Figure 4. (a) 31P and (b) Z7Al MAS N M R spectra of DPA-VPI-5.
Asterisks denote spinning sidebands. presence of AI-OH groups resulting from P defects. Because such A1 is four-coordinated, its 27Al signal overlaps with the ”normal” four-coordinated A1 signal. The OH groups in the IR spectrum of DPA-VPI-5 must also come from the AI-OH groups. The weak peak at 3675 cm-1 19 in TBA-VPI-5 which comes from hydroxyl groups on the outer surface of the crystals is absent from our spectrum probably because of the low content of the sample in the KBr disk. The A1 species associated with AI-OH groups are found in DPA-VPI-5 and DPA-AlP04-8 ESCA spectra. Although surface degradation usually occurs in ESCA experiments, the same experimental conditions used for all samples indicate that AI-OH groups come from DPA-VPI-5 and DPAAlP04-8 frameworks. Given the substantial changes of linewidth and bonding energy of the Al(2p) line, the changes of the P(2p) spectrum between TBA and DPA samples may be neglected. The changes of linewidths and bonding energies of O( 1s) in all samples are less sensitive than those of Al(2p). We conclude that DPA-VPI-5 and DPA-AlPO4-8 contain AI-OH groups brought about by the presence of P defects, while the frameworks of TBA-VPI-5 and TBA-AlP04-8 are relatively perfect. This result has not been reported, although the phosphorus deficiency (P/Al = 0.9) in DPA-VPI-5 is well-known. The 31P and Z7Al MAS N M R spectra of TBA-VPI-5 and DPAVPI-5 are very similar. Both materials give three 31PN M R lines in the intensity ratio of 1.OO:1.03: 1.OO for DPA-VPI-5 and 1 .OO: 1.03:0.99forTBA-VPI-5. Thenearly 1:1:1 intensityratiosuggests that the distribution of P defects over the three crystallographic sites in DPA-VPI-5 is random. Microstructural differences between TBA-VPI-5 and DPAVPI-5 are related to their different thermal stabilities. SR-EDD experiments show that TBA-VPI-5 and DPA-VPI-5 phase transform to A1P04-8 at different rates.30 Between 135 and 155 OC, the rate of phase transformation of DPA-VPI-5 increases faster than that of TBA-VPI-5, although the former rate is lower
He et al.
8778 The Journal of Physical Chemistry, Vol. 98, No. 35, 1994
DPA-VPI
-5
*
Alp04
-8
r+1,AI-@-/
3
a
3
ia
23
>a
33
38
& ) A \ ! :
2 0 (degrees)
Figure 5. The XRD pattern of DPA-VPI-5 after dehydration and calcination at 350 OC for 1 h. The peaks marked with asterisks are from AIP04-8. The pattern of AIP04-8 is shown for comparison.
at both temperatures. Dehydrated VPI-5 is reported to be stable at high temperaturese6 However, after dehydration and calcination at 350 O C , the XRD pattern of TBA-VPI-5 shows a pure phase, while DPA-VPI-5 always contains a small amount of AlP04-8 (see Figure 5). We have found that pure DPA-VPI-5 had the same thermal stability as DPA-VPI-5 containing the usual very small amount of AlP04- 1 1. This means that impurities are not a factor in the phase transformation. Although we have found that TBA-VPI-5 and DPA-VPI-5 contain different amounts of the respective templates, this is unlikely to be the reason for their different thermal stability. The reason must be the presence of P defects in DPA-VPI-5. We should, however, reconsider the reasons why the rates of transformation are different.30 The reaction is generally controlled by kinetic factors at low temperatures and by thermodynamic factors at high temperatures. As shown in Figure 6, the structural rearrangement in the defective DPA-VPI-5 framework and the prefect TBA-VPI-5 framework may proceed via the breaking of two A1-0 bonds (each corresponding to energy of ca. 122 kcal mol-’) instead of breakon one P-0 bond (ca. 143 kcal mol-’) and one 0-H bond (ca. 102 kcal mol-’). The slowness of the phase transformation of DPA-VPI-5 at low temperatures is caused by the size of the AI-OH groups, the consequent distortion of the framework (not detected by NMR) and the lower rate of bond rearrangement. In TBA-VPI-5 this problem does not arise. Thermal vibration of atoms is much more pronounced at high temperatures, and the energy of bond breaking becomes a major factor. As a result, the rate of phase transformation of DPAVPI-5 and TBA-VPI-5 is similar a t high temperatures, as the energy of bond breaking is the same. In the dehydrated form, the presence of A1-OH nests may also cause the low thermal stability of DPA-VPI-5. As shown in Figure 7, when two defects are on adjacent 4-4 sites, the rearrangement requires only a rotation, but not breaking, of chemical bonds. In this case phase transformation is much easier than in a perfect framework.
Figure6. Structural transformation of (a) perfect TBA-VPI-5 framework and (b) defective DPA-VPI-5 framework into AIP04-8 via a rearrangement involving the breaking of two AI-0 bonds. Chemically bonded water moelcules are omitted for clarity.
D Figure 7. Structural transformation of defective VPI-5 into AIP04-8. When two P defects are on adjacent 4-4 site positions, the rearrangement involves bond rotation instead of bond breaking. The large and small solid circles represent AI atoms and OH groups, respectively.
However, the population of the “twin” defects required cannot be large because the defects are randomly distributed throughout the framework. This could be the reason why dehydrated and calcined DPA-VPI-5 usually contains a small amount of AlP048.
References and Notes ( 1 ) Davis, M . E.; Saldarriaga, C.; Montes, C.; Garces, J.; Crowder, C. Nature 1988,331, 698. (2) Vogt, E. T.C.; Richardson, J. W. J . Solidstate Chem. 1990,87,469. (3) Prasad, S.;Balakrishnan, I. Inorg. Chem. 1990,29, 4830.
Molecular Sieves TBA-VPI-5and DPA-VPI-5 (4) Ssrby, K.; Szotsak, R.; Ulan, J. G.; Gronsky, R. Coral. Lett. 1990, 6, 209. ( 5 ) Vinje, K.; Ulan, J. G.;Szostak, R.; Gronsky, R. Appl. Caral. 1991, 72,361. (6) Annen, M. J.; Young, D.; Davis, M. E.; Cavin, 0. B.; Hubbard, C. R. J. Phys. Chem. 1991, 95, 1380. (7) Potvin, C.; Manoli, J. M.; Brienda, M.; Barthomeuf, D. Catal. Lett. 1991, 10, 225. ( 8 ) Martens, J. A.; Feijen, E.; Lievens, J.; Grobet, P. J.; Jacobs, P. A. J. Phys. Chem. 1991, 95, 10025. (9) Davis, M. E.; Young, D. Stud. Surf.Sci. Caral. 1991, 60, 53. (10) Maistriau, L.; Gabelica, Z.; Derouane, E. G.; Vogt, E. T. C.; van Oene, J. Zeolites 1991, 11, 583. (1 1) Maistriau, L.; Gabelica, Z.; Derouane, E. G.ADLJI. . _ Coral. 1991.67. L11. (12) Maistriau, L.; Gabelica, Z.; Derouane, E. G.Appl. Catal. 1992,A81, 67. (13) Clearfield, A.; Perez, J. 0. Synrhesis of Microporous Materials, Occelli, M. L., Robson, H., Eds.; Van Nostrand Reinhold: New York, 1992; Vol. 1: Molecular Sieves, p 266. (14) Liu, X.;He, H.; Klinowski, J. J . Phys. Chem. 1991, 95, 9924. (15) Dessau, R. M.; Schlenker, J. L.; Higgins, J. B. Zeolites 1990,10,522. (16) Richardson, J. W., Jr.; Vogt, E. T. C. Zeolites 1992, 12, 13. (17) Anderson, M. W.; Shi, J.; Leigh, D. A.; Moody, A. E.; Wade, F. W.; Hamilton, B.; Carr, S.W. J. Chem. SOC.,Chem. Commun. 1993, 533.
The Journal of Physical Chemistry, Vol. 98, No. 35, 1994 8779 (18) Davis, M. E.; Montes, C.; Hathaway, P. E.; Garces, J. M. Srud.Surf. Sci. Carol. 1989, 49A, 199. (19) Davis, M. E.; Montes, C.; Hathaway, P. E.; Arhancet, J. P.; Hasha, D. L.; Garces, J. M. J. Am. Chem. Soc. 1989, 111, 3919. (20) Barr, T. L. Crit. Rev. Anal. Chem. 1991, 22, 113. (21) Barr, T. L. Crit. Rev. Anal. Chem. 1991, 22, 229. (22) Suib, S.L.; Winiecki, A. M.; Kostapapas, A. Lmgmuir 1987,3,483. (23) He, H.; Alberti, K.; Barr, T. L.; Klinowski, J. J. Phys. Chem. 1993, 97,13703. (24) He, H.; Klinowski, J. J . Phys. Chem. 1994, 98, 1192. (25) Rccha, J.; Kolodziejski,W.; He, H.; Klinowski, J. J . Am. Chem. SOC. 1992, 114, 4884. (26) Russell, J. D.; Farmer, V. C.; Lewis, D. G.Spectrochim. Acta 1978, 34A, 1151. (27) Farmer, V. C.; Palmieri, F. Soil Components; Gieseking, J. E., Ed.; Springer-Verlag: New York, 1975; Vol. 2, Inorganic Components, p 573. (28) Barr, T. L.; Kramer, B.; Shah, S.I.; Ray, M.; Greene, J. E. Mater. Res. SOC.Proc. 1985, 47,205. (29) Briggs, D., Seah, M. P., Eds.; Practical Surface Analysis, 2nd Ed.; John Wiley: Chichester, 1990. (30) He, H.; Barnes, P.; Mum, J.; Turrillas, X . ; Klinowski, J. Chem. Phys. Lett. 1992, 196, 267.
