A novel aluminophosphate precursor that transforms to AlPO4-5 molecular sieve at high temperature

摘要

50 45 40 35 30 25 20 15 10 5 2q / deg; Intensity (a.u.) AIPO4-5 UHM-5 ( b) ( a) dagger; * * A novel aluminophosphate precursor that transforms to AlPO4-5 molecular sieve at high temperature A. M. Prakash,a Martin Hartmannb and Larry Kevan*a a Department of Chemistry, University of Houston, Houston, Texas 77204, USA b Institute f�ur Chemische Technologie, Universit�at Stuttgart, 70550 Stuttgart, Germany A novel aluminophosphate microporous compound, denoted UHM-5, synthesized from an aqueous aluminophosphate gel at 150 deg;C in the presence of cyclohexylamine as the template, transforms upon calcination above 300 deg;C into AlPO4-5 molecular sieve.Hydrothermal synthesis of microporous materials using organic structure directing templates normally leads to a crystalline solid (precursor) that possesses a three dimensional framework of TO4 tetrahedra, where T is a tetrahedral atom, and the subsequent elimination of the template by calcination occurs without significant structural variation.Surprisingly, this is not the case for the precursor reported in this work. However, partial removal of the organic species from the precursor by calcination above 300 deg;C leads to the AFI type structure. A similar behavior has been reported recently in the structural transformation of JDF-20, an extra large pore molecular sieve with a 20-membered ring, to AlPO4-5.2 JDF-20 has been reported to be crystallized from a predominantly non-aqueous synthesis medium using triethylamine which is also a template for AlPO4-5.Although both UHM-5 and JDF-20 exhibit similar transformation to AlPO4-5, these materials differ significantly in their structural and compositional details. From various characterization results on UHM-5, we believe that this is a new member of the extra-large pore molecular sieve family. Synthesis of UHM-5 was achieved from an aqueous aluminophosphate gel using cyclohexylamine (CHA) as the structure directing template.UHM-5 was prepared by a modification of a synthesis procedure which would otherwise yield AlPO4-5 molecular sieve. Typically the molar composition of the gel was: 1 CHA : 1 Al2O3 : 1 P2O5 : 50 H2O. In a typical synthesis procedure aluminium isopropoxide (Aldrich, 98+) was slurried in deionized water. While stirring, orthophosphoric acid (Merck, 85) diluted with water was added. After stirring for 10 min, cyclohexylamine (Aldrich, 99) was added slowly. The mixture was then heated at 70 deg;C for 30 min using a water-bath.The final gel was crystallized at 150 deg;C for 74 h. Fig. 1 shows the X-ray powder diffraction patterns of a sample in the as-synthesized form and after heating this sample in O2 at 300 deg;C. The observed crystalline pattern for the assynthesized material is novel. The very low 2q value (d = 17.96 Aring;) of the most intense line in the XRD pattern is indicative of an extra-large pore molecular sieve. The weak line at 2q 7.57deg; is probably due to a very small amount of AlPO4-5 which can probably crystallize along with UHM-5.After heating the sample at 300 deg;C, the structure changes to that of well crystallized AlPO4-5 as also shown in Fig. 1. This structural transformation is brought about by the partial removal of the organic template. There is an extra line at 2q 21.80deg; in the XRD pattern which does not belong to AlPO4-5. Note that a line at 2q 21.85deg; is also present in the XRD pattern of the as-synthesized sample.Thus, we believe that this extra line is possibly due to a small amount of a dense impurity phase present in the sample. By considering the first 15 lines after excluding the lines due to AlPO4-5 (2q 7.57deg;) and the impurity phase (2q 21.85deg;) the powder pattern of UHM-5 has been indexed as a tetragonal unit cell with lattice parameters of a = 18.15, c = 20.59 Aring; and U = 6786.3 Aring;3. To remove the organic template without effecting structural transformation, ion-exchange with NH4Cl was attempted.As in the case of thermal treatment, this procedure also leads to transformation of UHM-5 to AlPO4-5 with significant loss in crystallinity. This suggests strong interaction between cyclohexylamine and the framework of UHM-5. The interaction between template and framework is further evidenced from thermal analysis results. Thermogravimetry (TG) and differential thermal analysis (DTA) of the material (Fig. 2) show two exothermic mass losses; a sharp loss at ca. 300 deg;C and a gradual loss between 300 to 500 deg;C due to the desorption and decomposition of the organic template. The total mass loss of 34 mass observed on UHM-5 is significantly higher than the mass loss reported for any of the extra large pore molecular sieve materials and indicates the very high internal volume confined by the aluminophosphate framework. For example, both JDF-20 and VPI-5 show about 25 mass losses.2,3 Interestingly, the material contains a very low amount of water as only 1 of the total mass loss is accounted for by water desorption.The product composition was estimated from electron microprobe analysis and TG results as 0.36 CHA (Al0.5P0.5)O2middot;x H2O. The observed Al/P ratio of unity in UHM- 5 is characteristic of three dimensional microporous solids with Fig. 1 X-Ray powder diffraction patterns of (a) as-synthesized UHM-5 and (b) AlPO4-5 obtained after calcination of UHM-5 at 300 deg;C in oxygen. In the as-synthesized sample the line marked with dagger; is due to a small amount of AlPO4-5 phase present. In both the as-synthesized and calcined samples, the lines marked by * are due to an impurity phase.Chem. Commun., 1997 22210 ndash;2 ndash;4 ndash;6 ndash;8 800 600 400 200 0 T / deg;C Loss in mass / mg ndash;4 ndash;2 0 2 4 6 8 D T / deg;C DTA TGA ndash;130 ndash;90 ndash;50 ndash;10 30 70 ndash;80 ndash;40 0 40 80 ndash;130 ndash;90 ndash;50 ndash;10 30 70 ndash;80 ndash;40 0 40 80 d d 27Al MAS NMR after transformation 27Al MAS NMR of UHM-5 31P MAS NMR after transformation 31P MAS NMR of UHM-5 ( a) ( b) alternate Al and P in the framework and is not observed in layered aluminophosphate materials.Previously synthesized layered aluminophosphates have a framework Al/P ratio of 0.75 and consist of vertices sharing AlO4 and PO4 tetrahedra with all of the vertices of the AlO4 units being shared but only threequarters of the PO4 units being shared thus leaving formally a PNO group. It has been shown that these structure types can be related to two dimensional nets with the inclusion of a capping PO4 32 group.4 Fig. 3 shows the 27Al and 31P NMR spectra recorded on UHM-5 and after heating at 300 deg;C. The as-synthesized UHM-5 exhibits a single asymmetrical resonance in the 27Al MAS NMR spectrum at d 41.2 (reference: aqueous aluminium nitrate) characteristic of tetrahedral aluminium.5 After the solid state transformation an additional peak is detected at d 36.8. Assynthesized and calcined AlPO4-5 is reported to exhibit a single resonance centered at d ca. 35.5 The fact that the line at d 41.2 did not completely disappear during the transformation indicates that only a fraction of the Al sites undergo changes in environment during the transformation. The 31P MAS NMR spectrum of as-synthesized UHM-5 exhibits a single sharp line at d 24.6 and an additional broad line at d 230 (reference 85 H3PO4). After the transformation, the line at d 24.6 completely disappears while the line at d230 increases substantially. A 31P line at d 228.1 has been determined for AlPO4-5.6 Tetrahedral phosphorus in microporous materials generally exhibits resonances in the range d 225 to 235 range characteristic of P(OAl)4 environments.5 The resonance at d ca. 230 in UHM-5 is clearly due to tetrahedral phosphorus as found in several other microporous materials. The fraction of AlPO4-5 present in the original as-synthesized sample is very small. Thus, the relatively strong NMR signal at d ca. 230 in an as-synthesized sample cannot be fully accounted for by an AlPO4-5 phase, although a small contribution of this phase to the observed signal is obvious.In AlPO4-based materials, the interaction of the framework phosphorus with extraframework species such as water or organic moles can modify or shift the resonance signal considerably.7 A wide range of chemical shifts spanning from d 30 to 285 has been reported for five-coordinate phosphorus compounds.8 A framework P/Al ratio of unity in UHM-5 implies an alternate arrangement of Al and P units in the framework. The observed resonance at d 24.6 could be due to five-coordinate phosphorus of the form P(OAl)4(OH) formed by coordinating with four framework Al and an extraframework species such as a hydroxyl group.The presence of hydroxyl groups was observed in UHM-5 by FTIR measurements which show an intense sharp band at 3650 cm21 normally assigned to hydroxyl stretching vibrations.9 The disappearance of the resonance at d 24.6 after heating above 300 deg;C can be viewed as a consequence of phase transformation where the fivecoordinated phosphorus transforms to tetrahedral to form AlPO4-5 molecular sieve.Associated with each P(OAl)4(OH) unit one would expect a protonated template species. AlPO4 units in which both Al and P are in tetrahedral coordination are neutral and do not require amine to balance any framework charge. From the observed chemical composition of UHM-5, it can be seen that for every ten P atoms there are about seven CHA species.This suggests that out of ten P atoms of the framework, roughly seven are in five-coordination and three in tetrahedral coordination consistent with the two 31P NMR signals. The observations made above suggest that the framework of UHM-5 probably consists of both AlPO4 and (AlPO4)OH units. The framework structure of UHM-5 is stable in the presence of the organic species and transforms upon heating to the AlPO4-5 structure.UHM-5 crystallizes in small 4ndash;6 mm platelets bunched together to form large clusters of ca. 100 mm as revealed by scanning electron microscopy. Because of the small crystal size, single crystal analysis would be difficult. All the characterization results so far obtained on UHM-5 indicate an extra large pore material. Additional experiments are necessary for a better understanding of the transformation mechanism and work is in progress to determine the structure of UHM-5. This work was supported by the Robert A.Welch Foundation and the National Science Foundation. Footnote and References * E-mail: kevan@uh.edu 1 S. T. Wilson, B. M. Lok, C. A. Messina, T. R. Cannan and E. M. Flanigen, J. Am. Chem. Soc., 1982, 104, 1146. 2 Q. Huo, R. Xu, S. Li, Y. Xu, Z. Ma, Y. Yue and L. Li, in Proceedings from the Ninth International Zeolite Conference, ed. R. von Ballmoos, J. B. Higgins and M. J. M. Treacy, Butterworth-Heinemann, Boston, 1993, pp. 279ndash;286. 3 M. E. Davis, C. Montes, P. E. Hathaway, J. P. Arhancet, D. L. Hasha and J. M. Garces, J. Am. Chem. Soc., 1989, 111, 3919. 4 P. A. Barrett and R. H. Jones, J. Chem. Soc., Chem. Commun., 1995, 1979. 5 C. S. Blackwell and R. L. Patton, J. Phys. Chem., 1984, 88, 6137. 6 D. M�uller, E. Jahn, B. Fahlke, G. Ladwig and U. Haubenreiser, Zeolites, 1985, 5, 53. 7 M. Goepper, F. Guth, L. Delmotte, J. L. Guth and H. Kessler, in Zeolites: Facts, Figures, Future, ed. P. A. Jacobs and R. A. van Santen, Elsevier, Amsterdam, 1989, pp. 857ndash;866 (Stud. Surf. Sci. Catal., 1989, 49, 857). 8 L. W. Dennis, V. J. Bartuska and G. E. Maciel, J. Am. Chem. Soc., 1982, 104, 230. 9 U. Lohse, E. L�offler, K. Kosche, J. J�anchen and B. Parlitz, Zeolites, 1993, 13, 549. Received in Columbia, MO, USA, 10th July 1997; 7/04602B Fig. 2 Thermogravimetry (TG) and differential thermal analysis (DTA) results on UHM-5. The data were recorded with a heating rate of 5 deg;C min21 in O2 atmosphere. The initial mass of the sample was 17.418 mg. Fig. 3 27Al and 31P MAS NMR spectra recorded (a) for as-synthesized UHM-5 and (b) after heating UHM-5 at 300 deg;C in oxygen 2222 Chem. Commun.