J. Phys. Chem. 1993,97, 13703-13707
13703
ESCA Studies of Aluminophosphate Molecular Sieves Heyong He, Klaus Alberti, Tery L. Barr,’’+and Jacek Klinowski’ Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1E W,U.K. Received: April 13, 1993; In Final Form: July 28, 1993”
We describe the first detailed X-ray photoelectron spectroscopic (ESCA) examination of the surface and near-surface chemistry of VPI-5, AlP04-8, and AlPO4-11 as well as pseudo-boehmite and several AlP04’s of “compromized integrity”. A distinct set of core level binding energies was determined and compared to related spectra for cloverite (a gallium phosphate-based sieve) and several zeolites. ESCA line shifts are interpreted in terms of the relative degree of ionicity (covalency) of the A1-0 and P-0 bonds in the framework.
Introduction Since 1982 a new family of porous solids has been synthesized. The aluminophosphate molecular sieves, known as AlP04-n, where n is an integer indicating the structure type,’ were the first to be discovered. AlP04-n are the porous analogues of aluminum phosphate and are built from alternating A104- and Po4+ tetrahedra. AlP04 frameworks, unlike those of zeolites, which are built of alternating A104-and S O 4tetrahedra, are electrically neutral. They are synthesized from gels containing sources of aluminum, phosphorus, and at least one organic structuredirecting template. Incorporation of a silicon source into the gel results in the formation of silicoaluminophosphates,SAPOS,and the incorporationof a metal, M (such as Mg, Mn, Fe, Co, or Zn), into AIPO4 and SAPO gives the MeAPO and MeAPSO sieves, respectively. Of the more than 20 AlP04-n prepared so far, some are aluminophosphateanaloguesof known aluminosilicatezeolites, but some have completely novel structures. Many offer exciting possibilities for shape-selective sorption. VIP-52 is the AlPO4 sieve with the widest channels, but cloverite channels are even wider.3 VPI-5 has one-dimensional channels made up of 18membered rings of A1 and P atoms, while in cloverite, a related Gap04 sieve with a unit cell formula [Ga768Pm02~6(OH)192]-192QF,where Q is quinuclidine,some P and Ga atoms are linked to terminal hydroxyl groups and the channels are 20membered. We reported a detailed X-ray photoelectron spectroscopy (variously known as XPS or ESCA) study of aluminophosphate molecular sieves. ESCA has already played a significant role in the characterization of the surfaces of zeolites, where repetitive shifting patterns in the core level4and valence band structures5 have led to the identification of different zeolite type^,^,^ imp~rities,~ degradative byproducts,7qsand variations in acidity? Suib, Winiecki, and Kostapapas’O have recently described an ESCA study of several aluminophosphatesystems not examined in the present investigation, and that work may be consulted for a comparison. Our ESCA procedures with respect to charging, quantification, and binding energy referencing are described in detail e l s e ~ h e r e . ~I > ~ J
Experimeotal Section VPI-5 was synthesized using tetrabutylammonium hydroxide (TBA) and di-n-propylamine(DPA) as templates.2b Conditions of synthesis were the same as those described by Davis et a1.2b except for the different gel composition, TBA:A1203:P205:H20 = 1.12:1.OO:1.00:50,in the synthesis of TBA-VPI-5. The samples are referred to as TBA-VPI-5 and DPA-VPI-5. TBA-AlP04-8 ~~
~
+ Permanentaddreas: Department of Materials and Laboratory for Surface
Studies, University of Wisconsin-Milwaukee, Milwaukee, WI 53201. *Abstract published in Aduance ACS Abstracts, December 1, 1993.
0022-365419312097-13703$04.00/0
and DPA-AlP04-8 were prepared by calcining TBA-VPI-5 and DPA-VPI-5, respectively, at 200 OC for 2 h. AlP04-ll was synthesized by heating at 200 OC for 24 h with a gel composition of DPA:A1203:P205:H20 = 1.OO:l .00:1.o0:4Oo1Pseudo-boehmite, the raw material in the synthesis of AlP04-n, is a commercial product (Catapal B) from the Vista Chemical Co. X-ray diffraction (XRD) shows that the samples of TBA-VPI-5 and AlPO4-11 are pure and DPA-VPI-5 contains minor quantities of AlP04-11, while TBA-AlP04-8 and DPA-AlP04-8 are very similar to one another. ESCAmeasurements were carried out on the V.G. ESCALAB system at the Surface Analysis Facility of the University of Wisconsin-Milwaukee. Prior to being investigated, all of the samples were pressed into an indium foil. Binding energies were all referenced to C( 1s) (set at 284.6 eV) from the modest amount of adventitious carbon adsorbed on the surface of all solids. Via this procedure, the justification for which is given el~ewhere,~JJ~ the moderate charging shifts were removed from all spectra. Qualitative ESCA determination, based on the A1(2p)/P(2p) intensity ratio, showed that the A1f P ratio was ca. 1.O for TBAVPI-5, TBA-A1PO4-8, DPA-VPI-5, DPA-AlP04-8, and AlPO4- 11. ReSUltS
Pseudo-Boehmite. Pseudo-boehmite, a form of AlOOH, is used as a raw material for the synthesis of A1PO4-n. A sample of pseudo-boehmite was therefore also examined as well. We note that, although our group has an extensive history of ESCA studies of various aluminate systems: most previous investigations used different ESCA equipment. This study will therefore address the question of universality of these measurements by, for example, comparing the results directly to those for the various AlP04-n (for which a boehmite-like species is thought to be one of the major byproducts),and also to those for related systems achieved in earlier studies. The Al(2p) and O(1s) binding energies for pseudo-boehmite (Table 1) are both on the high side of the expected range for alumina-type materials as suggested in our previous work. Thus Al(2p) is above 74.0 eV but below 74.5 eV.8J2 The range is partly based on the fact that a-A1203produces an Al(2p) binding energy of ca. 73.8-74.0 eV, and the presence of an -OH group is known to cause a moderate increase in the Al(2p) value.12The resulting Al(2p) and O(1s) line widths for pseudo-boehmite are found to be broader than the corresponding line widths for a-and yA1203, but this is also expected, since the presence of mixed A 1 4 and AI-0-H groups is known to produce a modest, but detectable, spread in the binding energies of the A1 and 0 peaks. TBA-VPI-5. The bulk composition and structure of TBAVPI-5 were found to be unique and uniform. ESCA spectra suggest the same properties for the surface-near-surface: those 0 1993 American Chemical Society
He et al.
13704 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
TABLE 1: Binding Energies and, in Brackets, Corresponding Line Widths (f0.05 eV) for Alp04 Molecular Sieves and Related Materials. ~~~
~~
~
material TBA-VPI-5 TBA-AIPO4-8 DPA-VPI-5 DPA-AIP04-8 AIPO4-11 pseudo-boehmite cloverite y-AI203' P205 zeolite NaAb zeolite NaXb zeolite N a y b zeolite ZSM-Sb
AWP) 75.5 (2.15) 75.45 (2.2) 74.8 (2.8) 75.0 (3.2) 74.9 (2.5) 74.4 (3.0)
POP) 134.85 (2.3) 135.05 (2.4) 134.7 (2.65) 134.55 (2.6) 134.4 (2.6) 134.1 (2.3)
74.0 (2.1) 135.3
73.5 (1.56) 73.95 (1.60) 74.2 (1.55) 74.5 (1.75)
O(1S) 532.6 (2.15) 532.85 (2.2) 532.45 (2.50) 532.3 (2.4) 532.1 (2.4) 531.7 (3.2) 531.9 (2.5) 531.1 (2.0) 533.3 530.5 (1.75) 531.1 (1.65) 531.8 (2.05) 532.45 (1.9)
(d)
-
DPA Alp04
-8
Bindingenergiesarereferencedto C( 1s) = 284.6 eV. Measurements performed with a high-resolutionHewlett-Packard ESCA equipped with a monochromator. h
of a unique, relativelysingular species. Thus the core level binding energies (Table 1) and valence band features from TBA-VPI-5 form a set of peaks and structures which are quite different from those for any other aluminum- or phosphorus-containingsystem yet examined by ESCA (except the TBA-AlP04-8 described below).11J2Further, with one exception, the line widths (Table 1) and line structures (Figures 1 (a), 2 (a), and 3 (a)) produced by the TBA-VPI-5 suggest a singular species." The exception is the presence of a modest shoulder peak in the Al(2p) line structure (Figure 1 (a)). Although obviously present, this secondary shoulder effect is not dramatic enough to create a significant broadening of the total line manifold. In this case there is a slight distortion of the Al(2p) line away from the Gaussian-Lorentzian-type structure which would be produced by a material with a totally singular aluminum-containingspecies. As indicated by Figure 1 (a), the peak distortion seems to occur on the low binding energy side of the VPI-5 AI(2p) line. It is apparently caused by the presence of aluminum-containingspecies with distinctly lower binding energies than the Al(2p) binding energyoftheprincipalcomponentatca. 75.5 eV. Anapproximate deconvolution of the Al(2p) line manifold suggests that the secondary structure may be due to several peaks with binding energy between 74.5 and 73.8 eV. This is the binding energy range for simple aluminum oxides, Le. Alz03,AlOOH, and Al(OH)3 (see Table l ) ? ~The ~ relative ~ amount of these secondary species detected on theTBA-VPI-5 is less than 20%,and perhaps as little as 10%. Their presence in this surface-oriented ESCA examination is expected because of the nature of the materials being investigated. It is known that exposureof most solid surfaces to STP air, handling, glassware, and the vagaries of non-UHV transport will "compromise" the outermost surface of that material. Oxides, particularly those with complex lattice structures, are among the materials whose surfaces are most altered. The thickness of the compromised layer is generally of the order of 5 to at most 20 A. Surface degradation results in the formation of a "crust". When the sample is "well-made", NMR and XRD will be insensitive to surface alterations, as both techniques are notoriously surface-insensitive. On the other hand, the results from extremely surface-sensitive techniques, such as LEED and EELS, will be compromised. ESCA, which is optimized to detect features in materials such as the present oxides in the depth range of ca. 5 to ca. 50 A, will exhibit these surface alterations as a byproduct in a matrix dominated by near-surface features which should reflect almost exclusivelythe bulk chemistry and structure of these molecular sieves.I0 Typically, the compromised outer surface of a mixed oxide such as an AlPO4, adjacent to the structurally sound bulk A1PO4,should favor the retention of simple oxides of the more electropositive element, in the present case AIOOH, and Al(OH), at the expense of phosphorus oxide, formed from the less positive element.13 This is, in fact, what is
$ C
A
a
78
74
70
66
82
Binding energy I eV Figure 1. High-resolution AI(2p) ESCA spectra of aluminophosphate
molecular sieves: (a) TBA-VPI-5, (b) TBA-AIPO4-8, (c) DPA-VPI-5, (d) DPA-AlPO4-8, (e) AIPO4-11. The binding energy scales have been adjusted to C(1s) = 284.6 eV.
detected in the present case (Figure 1 (a)). Thus, the secondary A1 binding energies suggest the presence of small quantities of A1 oxides with little or no corresponding phosphorus-containing species. The structure of the principal Al(2p) peak at 75.5 eV appears to be due to a singular specieswhich, as far as ESCA is concerned, is expected to constitute the entire subsurface of the system being examined, i.e. the A1 signature for VPI-5. The corresponding P(2p) spectrum for this material (Figure 2 (a)) is slightly broader than the Al(2p) spectrum but still within the range expected for a chemically singular oxide. Further, the relative size of this P(2p) peak suggests that the phosphorus is only associated with the predominant AI-containing species. We therefore assign the P(2p) binding energy of 134.85 eV to the phosphorus in VPI-5. Note that this value of the binding energy is distinctly different
ESCA Studies of Aluminophosphate Molecular Sieves
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13705
A
/Ye'
A"
(a) T B A - W I - 5
i',.___ (a) TBA-VPI-5
534
4
136
134
('*'
128
124
120
530
526
522
518
Bindina enerav I eV Figure 2. High-resolution P(2p) and Al(2s) ESCA spectra: (a) TBA-
Binding energy I eV Figure 3. High-resolution O(ls) ESCA spectra: (a) TBA-VPI-5, (b) TBA-AlP04-8, (c) DPA-VPI-5, (d) DPA-AIP04-8, (e) AIPO4-11. The binding energy scales have been adjusted to C(1s) = 284.6 eV.
from the P(2p) for P205120rGaPO4-related cloveriteI4(seeTable l), a feature which is considered below in terms of the relative covalency/ionicity of these materials. The O( 1s) spectrum for this system (Figure 3 (a)) is also reasonably singular, as is the valence band pattern, and both exhibit features that are expected upon the basis of the present interpretation of VPI-5. More details will be given in further papers.I5 TBA-AlPOc8. NMR and XRD confirm that calcination does convert VPI-5 into AlP01-8.1~ The key ESCA results are given in Table 1 and Figures 1 (b), 2 (b), and 3 (b). As in the case of the TBA-VPI-5, TBA-AlP04-8 produces relatively singular ESCA lines (note the line widths), suggesting the presence of almost exclusively one species in the near-surface. Once again, the resulting binding energies are distinctly different from those of most other types of aluminate and phosphate materials, such as A1@3,I0 A1OOH,ll Al(OH)3,11zeolite NaA," PzO5,l2 and
GsP04.I4 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 TBA-AlP04-8 appear to be slightly larger than those for TBA-VPI-5 (Table l), the two sets of peak positions differ only slightly. Thus the binding energies of the Al(2p) lines for the two systems (Figure 1 (a) and (b)) are within experimental error of one another. The only difference between these two AI(2p) lines appears to be in the relative amounts of AlzO3, A100H, and Al(0H)s byproducts, Le. the AI(2p) shoulder, with approximatelyone-half the amount detected on the TBA-AlP048. The P(2p) and O(1s) lines for TBA-AlP04-8 (Figures 2 (b) and 3 (b)) are shifted to higher binding energies compared to the equivalent lines for TBA-VPI-5. The shift is small (a. 0.2 eV) and may be insignificant. However, it may be the photoelectron response to the stages of transformation of the aluminophosphate system from TBA-VPI-5 to TBA-AIP04-8 (see below).
VPI-5, (b) TBA-AlFQ-8, (c) DPA-VPI-5, (d) DPA-AlPO4-8, (e) AlPO~-11.The binding energy scales have been adjusted to C(1s) = 284.6 eV.
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The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
DPA-VPI-5 and DPA-AIPO4-8. Compared to the excellent ESCA characteristics of TBA-VPI-5 and TBA-AIPO4-8 and the XRD characteristics of TBA-VPI-5, DPA-VPI-5 contains AlP04-11 as a minor impurity and has a lower thermal stability than TBA-VPI-5.” It is useful, therefore, to compare the ESCA spectra of the surfaces of DPA-VPI-5 and DPA-A1PO4-8. ESCA analysis of DPA-VPI-5 and DPA-AlP04-8 also provided ample evidence of some type of near-surface imperfection. Thus, all ESCA peaks for the DPA-VPI-5 (Figures 1 (c), 2 (c), and 3 (c)) are substantially broader and shifted in binding energy compared to those of TBA-VPI-5 (see Table 1). The breadth of these lines suggeststhe presence of several relatively independent A1-0 and P-O species. The binding energies suggest that, while some pure VPI-5 may be produced, it is, at best, just one significant component. The other components have Al(2p) and P(2p) binding energies which are substantially lower than those for pure TBA-VPI-5. The non-VPI-5 binding energies indicate the presence of significant quantities of A1-O-A1 and AI-O-H bonded spe~ies,~J* where “proper”AI-0-P bonds have not been formed or are substantially modified, Le., in the place of the intended species are significant amounts of alumina and boehmite-type byproducts (see Table 1). The variation in the P(2p) binding energies (TBA- to DPA-VPI-5) is less substantial than that for the Al(2p) lines. This is not unexpected in view of the relatively small shift found for the P(2p) for P-0-P (P205) compared to P-0-AI (see Table 1). The O(1s) line from DPAVPI-5 (Figure 3(c)) is also broader and noticeably distorted at lower binding energy where a peak would be expected from Al0-H and A1-O-A1 bonds. The near-surfacecharacteristics of DPA-AlP04-8 also suggest some sort of significant compromise, with a very broad Al(2p) (Figure 1 (d)) peak showing the presence of a mixture of Al-0-P and substantial amounts of A1-O-AI and A1-O-H species. The P(2p) peak (Figure 2 (d)) is also shifted to a much lower binding energy than for TBA-AlP04-8. On the other hand, the O(1s) peak (Figure 3 (d)) is relatively singular. Possible reasons for these features are discussed below. Integrity of TBA- and DPA-AlP04-n during W A Analysis. In the course of this study an effort was made both to confirm our results and to obtain spectra of optimum quality. To this end, fresh samples of the same A1P04-n were examined using a much greater number of ESCA scans. We easily reproduced our general results in every case, thus establishingthat ESCA spectra of AlP04-n form a unique set substantially different from the spectra of cloverite and other alumino- and gallophosphates. On the other hand, with an increasing number of scans, the differences between the samples prepared using TBA and DPA (see above) largely disappear as a result of the substantial line broadening, particularly for the TBA samples. The broader lines were easily seen to be a superposition of the spectra of several species rather than just of the AlP04-n under examination. This strongly suggests that these AIP04-n are not surface-stable with respect to extensive exposure to the A1 Ka X-ray beam used, a problem which may be aggravated for the DPA materials. The “short scan” results are therefore more representative of the status of premeasurement AlP04-n species. This effect also throws light on the surface sensitivity of X-ray diffraction measurements, indicatingthat X-ray beams used in XRD must cause much more surface damage than the relatively “soft” XPS source. AlPO4-11. On the basis of the relative quantification and line widths (Table I), the sample of AIPO4-11 appears to be fairly singular, but not as pure as TBA-VPI-5 and TBA-AIPO4-8. The most interesting aspect of AlP04-11 spectra is that the set of binding energies for all species (A1(2p), P(2p), and O( Is), see Figures 1 (e), 2 (e), and 2 (e)) are distinctly different from those for either TBA-VPI-5 or TBA-AlP04-8. These shifts point to a difference in A1-O-P bonding which may be of significance if
one considers that A1PO4-11 is a known impurity in VPI-5 (see above).
Discussion We have earlier presented arguments to explain the complex shifting patterns of various oxide systems similar to the aluminophosphates, such as aluminas,” zeolites? and various silicates.lIJ8 The following were shown: (1) For any M,O, oxide, the more electropositive the M, the lower the O( 1s) binding energy. (2) In the case of mixed oxides, A,M,O, the insertion of an electropositive (A) component into an M,O, lattice makes the corresponding M-O bond more covalent. The converse applies to the A-O bond. As a result, the binding energies of A are higher for A,M,Of than for A,O,, and the binding energy of M is lower. The binding energy of the O( 1s) in A,M,O, lies between those for A,O, and M,O,. (3) As a result, for a related series of oxide compounds (such as zeolites with different framework Si/Al ratios), a progressive changein core level binding energies reflects a regular progression in the degree of covalency/ionicity in the metal (metalloid)oxygen bond.’* These shifts may also be directly related to the changes in the Br~lnstedacidity of these systems? In the present case we find (see Table 1) that the introduction of the moderately ionic A 1 4 units into the P-0-P structure of P205 results in a decrease in the binding energy of the P(2p) due to the increased covalency of the P-O bond in P U A l units. At the same time, theAl(2p) binding energy changesso as to suggest that the A 1 4 bond in Alp04 is much more ionic than the same bond in A1203 or AlOOH (note the results for pseudo-boehmite in Table 1). In addition, we see from Table 1that the Ga-O bond in cloverite is, as expected, much more ionic than the A 1 4 bond in any of the A1PO4’s. Thus, the P(2p) binding energies for both AlP04-8 and VPI-5 are substantially higher than in cloverite, reflecting the greater covalency of the P-O bond in the latter material. The most important feature of this observation obviously rests on the consideration of the relative acidity of the systems. Enhanced covalency of a P-O bond for a P-0-H unit will make any bonded hydrogen more easily removable as H+ (Le. an ionic S H unit). This enhanced P-O covalency is a characteristic of cloverite, but not the AIPOs’s, making the former a potentially stronger Br~lnstedacid. Comparingthe Al(2p) binding energies of AIP04-n and VPI-5 with those for various zeolites (see Table I), one sees that, as expected, the A 1 4 bond for the aluminophosphates is much more ionic, i.e. the AI binding energy is more positive, suggesting the presence of relatively ionic A13+,while in most zeolites all of the aluminum is part of the relatively covalent Al-OSi bonding units. The reason why the AlP04-11 binding energiesare subtantially lower than in AlP04-8 and VPI-5 is not entirely clear. The structures of AlP04-8 and VPI-5 are quite closely related, while the structure of AlP04-11 is distinctly different with a smaller pore size. It may be that the closer proximity of all the Al-0-P bonds in AlP04-11 reduces their inherent ionicity. However, in view of the very limited data, this argument is very tentative. We only note that A1PO4-11 has a distinct set of binding energies. The line widths of the AIP04-11 spectra (Table 1) suggest that it has higher surface purity than the DPA-derived materials, but may be less surface pure than the aluminophosphatesprepared with TBA as the template. In addition to the results discussed here, useful information has been obtained from the O(ls) spectra and the valence bands. We postpone their detailed considerationto another paper,15but note that the results support the arguments given above.
ESCA Studies of Aluminophosphate Molecular Sieves
References and Notes (1) Wilson, S.T.; Lok, B. M.; Flanigen, E. M. US. Patent No.4,310,440,1982. (2) (a) Davis, M. E.; Saldamaga, C.; Montes, C.; Garces, J.; Crowder, C. Nature 1988,331,698. (b) Davis, M. E.; Montes, C.; Hathaway. P. E.; Gar-, J. M. Stud. Surf Sci. Coral. 1989, 49A, 199. (3) Estermann, M.; McCusker, L. B.; BHrlocher, C.; Merrouche, A.; Kwsler, H. Narure 1991,352, 320. (4) Barr, T. L. Appl. Surf Sci. 1983, 15. 1. (5) Barr, T. L.; Chen, L. M.; Mohsenian, M.; Lishka, M. A. J. Am.
Chem. Soc. 1988, 110,7962. (6) Barr, T. L. In PracticalSurface Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; John Wiley: Chichester, U.K., 1990; Chapter 8. (7) Barr, T. L.; Lishka, M. A. J. Am. Chem. Soc. 1986, 108, 3178. (8) Barr, T. L. Crir. Rev. Anal. Chem. 1991, 22, 229.
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13707 (9) Barr, T. L. Zeolites 1990, 10, 760. (10) Suib, S.L.; Winiecki, A. M.; Kmtapapas, A. Longmuir 1987,3,483. (1 1) Barr, T. L. Crit. Reo. Anal. Chem. 1991, 22, 113. (12) Briggs, D., S a h , M. P., Eds.; Practical Surface Analysis, 2nd ed.; Wiley: Chichester, U.K., 1990. (13) Barr, T. L.; Kramer, B.; Shah, S.I.; Ray, M.; Greene, J. E. Mater. Res. Soc. Symp. Proc. 1985,47,205. (14) Barr, T.L.; Klinowski, J.; He,H.; Albcrti, K.; Muller, G.; Lercher, J. A. Nature 1993. 365,429. (15) Barr, T. L.; He, H.;Alberti, K.; Klinowski, J. J. Phys. Chem., submitted. (16) Scirby, K.; Szostak, R.; Ulan, J. G.; Gronsky, R. Carol. Lett. 1990, 6, 209. (17) Davis, M. E.; Montes, C.; Hathaway, P. E.; Arhancet, J. P.; Hasha, D. L.; Garces, J. J. Am. Chem. Soc. 1989,111, 3919. (18) Barr, T. L. J. Yac. Sci. Technol. 1991, A9, 1793.
