J. Chern. SOC., Faraday Trans. I, 1989, 85(6), 1327-1335 Reactivity of A1P04-5 and the Origin of its Hydrophilic Property Akira Endoh, Kiyoshi Mizoe, Kazuo Tsutsumi* and Tetsuo Takaishi Toyohashi University of Technology, Toyohashi, 440 Japan The reactivity of A1P04-5 was measured through the isotope exchange reaction of "0 between P O , and the framework oxygen, and its hydrophilicity was investigated by measuring the heat of adsorption of water. About 25% of the framework oxygen atoms were exchanged, and one quarter of them (ca. 6 YO of the total oxygen atoms) was highly reactive. The differential molar heat of adsorption, calorimetrically determined, was ca. 86 kJ mol-' at the initial stage and suddenly dropped to ca. 50 kJ mo1-'. This tendency is characteristic of hydrophobic zeolites, and indicates the existence of special hydrophilic sites. It is concluded, through quantitative analyses, that the highly reactive oxygen atoms and hydrophilic sites originate from defects, and that the concentrations of the defects, estimated from each of the above two, are in good agreement. Modern zeolite science started from studies of the molecular sieving action of zeolites controlled by the regular crystal structure, and defects in zeolites were not a subject of main concern, in contrast to those in semiconductors. Catalytic properties of zeolites, in particular acidic properties,' have focused our attention on defects in zeolites, but the study of defects is standing still at an elementary stage compared with the semiconductor case.The elucidation of defects is an important issue for the characterization of zeolites and other molecular sieves.Effects of defects must be investigated over a wide temperature range, since molecular sieves are used, in science and industry, at a variety of temperatures in the range 4-800 K. The representative techniques for the study of defects, available at present, are adsorption of gases,2 infrared spectroscopy3 and ,'Si magic-angle-spinning n.m.r. spectroscopy* at low and moderate temperatures, and the 180-exchange between oxygen-containing gaseous molecules and the framework of molecular sieves at higher temperatures5, For the clear determination of the properties of defects, combined use of these techniques is essential. In a series of studies on defects, we first investigate A1P04-5.Aluminophosphate compound contains no exchangeable cation to act as an active site for water adsorption, and might be predicted to be hydrophobic. Contradicting this expectation, alumino- phosphate molecular sieves are hydrophilic to some extent.' We anticipate that the hydrophilicity stems from defects in the crystal, and study the relation between the concentrations of the defects and hydrophilic points. The concentration of the hydrophilic sites is determined from the adsorption calorimetry, and that of defects is determined from the exchange reactivity of l80 between Cl80, and the alumino- phosphate crystal. Experimental Materials Crystals of A1P04-5, supplied by Dr A. W. Chester, Mobil Research and Development, had a composition of (AlP0,),,.8.3 H,O per unit cell, with no residual template molecule.They had a high crystallinity and purity, according to powder X-ray diffraction, 31P and 27Al magic-angle-spinning n.m.r. and scanning electron-microscopy. 1327 45-21328 Hydrophilic Property of A1P04-5 - - - r, n 5 8 50 c) 0 0 - 0 50 100 reaction time/h 150 Fig. 1. Temperature dependence of l8O exchange reaction. Each run contains different amounts of oxygen atoms in gas and solid phases. n,, amount (moles) of oxygen in gas; n,, amount (moles) of oxygen in solid. 0, 973 K, n, = 13.74 x ng = 2.07 x a, 1023 K, n, = 14.53 x ng = 2.21 x @, 1073 K, n, = 15.35 x ng = 2.11 x lop4; 0, 1123 K, n, = 1 5 . 9 0 ~ ng = 2.11 x 10-4. Prior to measurements, the sample was calcined in air at 773 K to ensure the elimination of template molecules.Carbon dioxide (supplied from Prochem Ltd) contained 99 % enriched l80. Apparatus The apparatus used in the 180-exchange reaction was described in a previous paper.6 The calorimeter used was of the twin conduction type, equipped with sensors of semiconductive (Sb, Bi)-telluride, and had a sensitivity of 0.12 ,uV/pW. About 1 g of zeolite sample was used after baking-out in the sample cell at 723 K under a vacuum of Pa for 5 h or longer. The adsorbed amount of gas was determined by the volumetric method with an apparatus combined with the calorimeter. The pressure was measured with a diaphragm gauge (MKS Baratron, type 315BHS and 227HS). Further details were described in a previous paper.8 Results and Analysis Isotope Exchange Reaction It is known that A1P04-5 has a high stability and that no deterioration of its crystallinity takes place up to 1273 K.ll2 About 60 mg of A1P04-5 was installed in the reaction vessel and completely dehydrated at 1073 K for 20 h under a vacuum of Pa; cn. 1850 Pa of Cl80, was introduced in the system, and the "0-exchange reaction was measured at temperatures in the range 373-1 123 K.At first, the effect of temperature on the reaction rate was surveyed roughly, and then the more detailed data shown in fig. 1 were collected.A . Endoh, K. Mizoe, K. Tsutsumi and T. Takaishi 1329 From fig. 1, it is concluded that only about 25 O h of the framework oxygen atoms are exchanged after a long reaction time of 150 h. Since A1P04-5 has four crystallo- graphically different kinds of the framework oxygen atoms in equal amounts, it is considered that only one kind of them is reactive and that the other three kinds of them with low reactivities do not participate in the present reaction.Let us divide the exchangeable oxygen atoms into two groups, one (specified by the subscript 1) lying on normal lattice sites, and another being related to defects with an abnormally high exchange reactivity. There may be various kinds of defects having different reactivities, and we sub-divide oxygen atoms related to defects into two sub- groups, less reactive and more reactive ones specified by subscripts 2 and 3, respectively. Denoting by y the fraction of "0 in a given kind of oxygen atom, and by the subscript g the gas-phase carbon dioxide, we have the equation of the exchange reaction,6 n d-l?g = - n g y , C k , ( l - y , ) n , + n , ( l - y , ) 2 kiyini (1) dt i = 1 , 2 , 3 i = 1 , 2 , 3 where n denotes the number of moles of oxygen, and k the rate constant.The condition of material balance gives an auxiliary equation, n,(Y:-Yg> = c niyi i = 1 , 2 , 3 in which the amount of adsorbed CO, was small and has been neglected. oxygen atoms related to defects, i.e. If there are large differences between the reactivities of the normal lattice oxygen and k , 4 k , 4 k3 (3) then in the latter stage of the reaction, the second and the third kinds of oxygen are in equilibrium with the gas phase, and the rate of exchange is determined by the first kind of oxygen. This situation is expressed by Yg = Y2 = Y3 # Y l (4) by which eqn (1) and (2) reduce to ngY:-(ng+n,+n,)Yg = nlY1.(6) These equations are solved with the condition that eqn (4) must be satisfied even at the beginning of the reaction. This means that yg effectively has an initial value of ngy:/(n,+n3+ng) instead of yi, since only the former value satisfies eqn ( 5 ) at t = 0 and y , = 0. The solution becomes n, +- 22, n g +n3 + ng Y:] On the other hand, in the intermediate stage of the reaction the third kind of oxygen is in equilibrium with the gas phase, and the first kind does not participate in the exchange; hence the rate of reaction is determined by the second kind of oxygen. This situation is expressed by y 3 = y g and y , = 0 (8)1330 E:! Hydrophilic Property of AlPO,-5 f -1.0 s o P + - E: reaction time/h Fig.2. Plots of the degree of exchange us. time, based on eqn (7). (a) 973 K; (b) 1023 K ; (c) 1073 K; ( d ) 1123 K. by which eqn (1) and (2) reduce to with, ngY;-(ng+n3)Y, = n,y,* (10) These equations are solved with the condition that eqn (8) must be satisfied, i.e. y , effectively has an initial value of ng yg/(n3 + ng) instead of y:. The solution becomes which has a similar form to eqn (7). the reaction, and one has approximately, In the initial stage of the exchange, only the first kind of oxygen atom takes part in (12) Yl = Y2 = 0 by which eqn (1) and (2) reduce to, with Solving these equations, one has ngy' = n3Y3 + ngYg' n n y :] = In [ 3 y:] - k3(n3 + ng> t. n3 + n,A . Endoh, K. Mizoe, K. Tsutsumi and T. Takaishi 1331 0 5 1 0 0 S 10 1s reaction timefh Fig.3. Plots of the degree of exchange us. time, based on eqn (11). (a) 973 K; (b) 1023 K; ( c ) 1073 K; ( d ) 1123 K. If the correct value of (n, + n2 + n,) is introduced into eqn (7), the plot of against t becomes linear, apart from its initial portion near t = 0. The best-fit value for (n, + n, + n,) was chosen by iteration, and results are shown in fig. 2. The intersect of the tangent to the curve with the ordinate gives the value for In In, n,Y:/(n,+ n2 + n g ) (n, + n2 + n3 + ng>l and hence values for (n,+n,) and n,. The value of (n,+n,) thus determined was introduced into eqn (1 l), and then plots of In b, - n g Y : m 2 + n3 + ng)l against t become linear, apart from its initial portion near t = 0, as shown in fig. 3. The intersect of the tangent of the curve with the ordinate gives the value for In [a, ngu3(fl, + n g ) (n, + n, + ng)l and hence values for n2 and n3.If the value of n, thus determined is introduced into eqn (15), then k, can be calculated. However, the initial change took place too fast for its profile to be followed with the present measuring system. The values of n,, n2 and n3 at various temperatures are summarized in table 1, where they are shown to be self- consistent. Plots of In k, and In k2 against l/T in fig. 4 give values of 1.38 eV and 1.63 eV for the respective activation energies of the reactions. The activation energy is higher for the reactive kind of oxygen, but the frequency factor is much larger resulting in a higher exchange rate.1332 Hydrophilic Property of A1P04-5 Table 1.Amounts of n,, n2 and n3 determined" ~~ 973 24.3 4.4 1.5 1023 22.6 4.8 1.4 1073 20.0 4.2 1.6 1123 23.3 4.4 1.4 a Concentrations expressed as percentage of the total framework oxygen atoms. T/"C 0s 0.9 1D 1.1 lo3 KIT Fig, 4. Temperature dependence of the rate constants of 180-exchange in A1P04-5. (a) k , ; (b) k,. Heat of Adsorption The calorimetrically determined differential molar heats of adsorption of water at 303 K are plotted against the adsorbed amount in fig. 5 . Details of initial regions of adsorption are inserted in the figure. The calorimetric heat curve reflects the energy distribution of adsorption site' and becomes an indication of hydrophilic sites in the case of water adsorption. The heat curve shows a very high value at an extremely low coverage, say 86 kJ mol-l, which is similar to heats of adsorption on Na-type faujasite.1° The heat value decreases very sharply down to the heat of liquefaction, and then remains almost constant up to theA .Endoh, K. Mizoe, K. Tsutsumi and T. Takaishi 1333 H20 adsorbed/mmol g-' 2 4 6 8 I 1 - 1 I I I I 90 I 00 I 90 1 0 1 2 3 4 H 2 0 molecules adsorbed/u.c. 0 0 0 0 0 0 O 0 0 I I I I I 0 2 4 6 8 10 12 H20 molecules adsorbed/u.c. Fig. 5. Differential molar heats of adsorption of water on A1P04-5 at 303 K. adsorption saturation. The hydrophilicity of aluminosilicate zeolites is clearly ascribed to the interaction of the water dipole with an electrostatic field on a zeolitic surface, the field being produced by cations and negative framework charges. However, in the case of aluminophosphate, there is neither cation nor framework charge.Therefore, the initial high-heat value must be attributed to some lattice defects, which are expected to present to a small extent. Discussion The exchange of oxygen atoms takes place rather easily between CO, and aluminosilicate zeolites; namely, it occurs at much lower temperatures than 773 K, and all framework oxygen atoms are l1 AlPO,-5 is far less reactive than aluminosilicates, and only a limited portion, ca. 25 YO, of the framework oxygen atoms can be exchanged even at a high temperature such as 1123 K. It is proved in zeolite A that the activated complex contains, as its constituents, exchangeable cations in the zeolite, and hence the poor reactivity of A1P04-5 may be attributed partly to the absence of such cations.There remains, however, the question of why only one of the four kinds of specified framework oxygens can be exchanged. Fig. 6 shows the wall of the cylindrical pore channel of A1P04-5 developed on a sheet. There are, on the wall, three kinds of oxygen, crystallographically denoted as 01, 011, and OIII.12'The fourth kind of oxygen OIV is located under the surface of the wall and not exposed to a visiting ClSO, molecule. It is evident that the reactive oxygen atom belongs to a given species of these three on the wall, but a further detailed assignment requires some speculation. 01 and 0111 are located in similar geometrical environments, different to 011, and thereby 01 and 0111 may have similar exchange reactivities.It is thus highly probable that 011 is the unique species with the high exchange reactivity. The large divergence in the strengths of A1-0-P bonds is an important and interesting problem in the crystal chemistry of aluminophosphate molecular sieves, and quantum- chemical studies are awaited.1334 Hydrophilic Property of AlPO,-5 (4 (b) - -60 0 60 + e caxis - 60 0 60 Fig. 6. Configuration of 01, 011, 0111 and OIV on the wall of main channel in A1P04-5. The cylindrical wall is developed on a sheet. (a) Vacancy of T-site atom. (b) Vacancy of P-O-A1 complex. 0, ., vacant site; 0, direct neighbour oxygen atom; a, next-neighbour oxygen atom. We now consider the problem of defects. If one of the A1 or P atoms is lost from a normal site, oxygen atoms around that vacant site may become reactive for the l 8 0 - exchange.The three nearest-neighbour oxygen atoms become most reactive, and the six next-nearest neighbours might also become reactive. If a complex P-0-A1 is lost and two OIVs are exposed as shown in fig. 6(6), then six direct neighbours and eight next- neighbours become reactive. Thus, the number of the reactive oxygen atoms introduced by a defect may range between three and 14. If a defect was formed through either mechanism mentioned above, T-OH linkage may appear in the framework to retain electrical neutrality. In fact the stretching vibration of hydroxyl groups was detected at 3695 cm-l in i.r. spectra of dehydrated A1P04-5. Such linkage becomes a hydrophilic site, similar to the case of Si-OH or ME-OH in aluminosilicates.Water molecules may interact with several, say one to four, T-OH to form a hydrogen bond, e.g. / H 'H T a H . . . . . . . . . . 0 or depending on the population of T-OH groups.A . Endoh, K . Mizoe, K. Tsutsumi and T. Takaishi 1335 The calorimetric result indicates that ca. 0.6 water molecules per unit cell interact with hydrophilic points. On the other hand, the highly reactive oxygen atoms, n, and n3, amount to 2.1 and 0.7 per unit cell, respectively. Thus 0.6 hydrophilic points contain 2.8 (= n, +n,) easily exchangeable oxygen atoms. If we assume that one defect produces one hydrophilic point, the number of easily exchangeable oxygen atoms per defect, v, amounts to 4.7(= 2.8/0.6). This value satisfies the structure condition 3 < v < 14 given in the above.At the present, we cannot discuss further details of the defect. Only a small number of defects, amounting to lo-'' part of a host crystal, play decisive roles in commercial semiconductors, while a much larger number is required to influence chemical properties of oxides. Defects in the semiconductors are easily detected by electric measurements, but the techniques cannot be applied to oxides in spite of an overwhelmingly large concentration of defects. Twin-boundaries in zeolites are observed with an image-electron-microscope13 but point defects are not. If the concentration of defects in zeolites is of the order of lo-,, they can be detected by the ,'Si magic-angle- spinning n.m.r. spectr~scopy.'~~ l5 The concentrations of various defects thus determined can explain well the observed acid strength distribution of (H, K)-L zeolites.16 These techniques, however, cannot be applied to aluminophosphate compounds, and so the "0-exchange technique is the only means, available at present, to measure the concentration of defects within these compounds.Recently, Gelsthorpe and Theocharis presented a speculation, for N, adsorption isotherms on AlPO,-5, that there exist point defects on the wall of the channel pore. Our present results quantitatively support their view.17 The consistency between "0-exchange and H,O-adsorption data encourages us to apply these techniques to various problems or to characterizations of molecular sieves. An example is the application to ZSM-5 and -1 1.18 We sincerely thank Dr A.W. Chester, Mobil Research and Development, for the supply of AlPO,-5. The present work was supported by a Grant-in-Aid for Scientific Research of the Ministry of Education of the Japanese Government, contract no. 6260200 1 . References 1 D. Barthomeuf, Zeolites: Science and Technology (NATO AS1 Ser., 1984), p. 317. 2 K. Tsutsumi, Y. Mitani and H. Takahashi, Colloid Polymer Sci., 1985, 263, 838. 3 E. M. Flanigen, Zeolite Chemistry and Catalysis (ACS Monograph 171, 1979), p. 80. 4 J. Klinowski, Prog. Nucl. Magn. Reson. Spectrosc., 1984, 16, 237. 5 R. V. Ballmoos, "0-Exchange Methods in Zeolite Chemistry (Saller and Sauerlander, 1981). 6 T. Takaishi and A. Endoh, J. Chem. Soc., Faraday Trans. I , 1987, 83, 411. 7 S. T. Wilson, B. M. Lok, C. A. Messina, T. R. Cannan and E. M. Flanigen, J . Am. Chem. Soc., 1982, 8 K. Tsutsumi, H. Q. Koh, S. Hagiwara and H. Takahashi, Bull. Chem. Soc. Jpn, 1975, 48, 3576. 9 K. Tsutsumi, Y. Mitani and H. Takahashi, Colloid Polymer Sci., 1986, 264, 445. 104, 1146; ACS Symp. Ser., 1983, 218, 79. 10 0. M. Dzhigit, A. V. Kiselev, K. N. Mikos, G. G. Muttik and T. A. Rahmanova, Trans. Faraday Soc., 11 C. Gensse, T. F. Anderson and J. J. Fripiat, J . Phys. Chem., 1980, 84, 3562. 12 J. M. Bennett, J. P. Cohen, E. M. Flanigen, J. J. Pluth and J. V. Smith, ACS Symp. Ser., 1983, 218, 13 J. M. Thomas, G. R. Milward and S. Ramadas, ACS Symp. Ser., 1983, 218, 181. 14 T. Takaishi, J. Chem. Soc., Faraday Trans. I , 1987, 83, 2681. 15 T. Takaishi, J. Chem. Soc., Faraday Trans. I , 1988, 84, in press. 16 K. Tsutsumi, K. Nishimiya, A. Shiraishi and T. Takaishi, to be published shortly. 17 M. R. Gelsthorpe and C. R. Theocharis, Catal. Today, 1988, 2, 613. 18 A. Endoh, K. Nishimiya, K. Tsutsumi and T. Takaishi, Proc. Inter. Symp. Zeolite, Wurzburg, 1988. 1971, 67, 458. 109. Paper 8/01 I80J; Receiued 18th October, 1988