J. Phys. Chem. 1981, 85, 1930-1933
1930
other hand, a recent X-ray determination on noncrystalline MOSS has shown the presence of molybdenum both in tetragonal pyramidal and in tetrahedral symmetry.24 Above -300 "C the symmetry of the MoS3+species may be determined by the MoS2 structure that becomes the prevalent phase. The fact that gllfor signal Bzis -2.0 and greater than g, suggests a large rearrangement of the structural parameters. We suggest that in this case the ground state could be lAl), i.e., largely a d,z Mo atomic orbital, as expected for gll 2.0. The observation that signal Bzbecomes more resolved with increasing temperature of treatment is also in agreement with the increased structural order a shown by X-ray data. It should be noted that the discontinous trend in the signal intensity in the range 300-350 "C corresponds to the temperature range in which the decomposition MOSS MoSz is known to take place in the largest extent. Signal C. The three-g-value signal (gl = 2.048, g, = 2.034, g3 = 2.004) must be attributed to an unsymmetrical and strongly immobilized species since paramagnetic species in rotational and/or librational motions adsorbed on a surface should give partially averaged values of the anisotropic parameters. This is also proved by the fact that the g values and the overall line shape do not depend on the registration temperature from 293 to 77 K. The observed values of the g tensor components and the spectrum shape (relatively sharp line even at 293 K) rule out their attribution to Mo in lower oxidation degrees. Spectra similar to that here reported have been previously observed in various sulfur-containing systems (Table 11). For example, Seshadri et a1.20have observed on sulfided molybdema-alumina catalysts a triplet centered around g = 2.0 that was assigned to sulfur atom chains adsorbed on the surface. Lojacono et al.,25in studying the ESR properties of sulfided molybdenum-cobalt-alumina catalysts, have assigned the observed three-g-value signal to short sulfur chains strongly attached to the surface. These
-
(24)E.Diemann, 2.Anorg. Allg. Chem., 432,127 (1977). (25)M.Lojacono, J. L. Verbeek, and G. C. A. Schuit, Proc. Int. Congr. 5th, 1972,1409 (1973).
authors did not exclude the possible existence of Sy. This latter species has been elegantly identified on the surface of partially hydroxylated MgO treated with elemental sulfur at 400 "C, and the g values together with the 33S hyperfine structure have indicated that the S3- ions are librating in the plane of the three sulfur atoms.26 Furthermore, Dudzik and Preston2' have reported a three-gvalue signal that was assigned to long sulfur chains in the lattice of zeolites impregnated with sulfur vapor at 400 "C. By analogy we assign signal C to sulfur atom chains long enough to preclude the interaction of the unpaired electrons located on the extremities. Such a species is probably not only adsorbed on the surface because the intensity of signal C is practically insensitive to oxygen adsorption, washing with CCll and treatment for 2 h in flowing helium. It disappears only when the samples are used in catalytic hydrotreating,22but under these conditions the A and B signals are also largely removed. On this basis we assume that the sulfur radical chains in our system are largely "hidden" in the MoS3-MoSz bulk. Again the intensity variations of signal C with the decomposition temperature can be used as a diagnostic method for following the irreversible thermal decomposition MOSS MoSz. Thus in the range 250-300 "C, where MOSSbegins to decompose, the signal intensity is a t its maximum, and then it drastically drops in the range 300-350 "C and tends to stabilize above 450-500 "C,where MoS3 is presumed to be totally decomposed to MoS2,as indicated by the constant values of the Mo/S ratio in the latter range of temperatures (Table I).
-
Acknowledgment. Thanks are due to Dr. F. Pincolini for the sample preparation. The National Council of Research (CNR) and the Minister0 della Pubblica Istruzione have provided partial support for this study. (26)J. H.Lunsford and D. P. Johnson, J. Chem. Phys., 58, 2079 (1973). (27)Z.Dudzik and K. F. Preston, J. Colloid Interface Sci.,26, 374 (1968). (28)J. R. Morton, Proc. Colloq. Ampere, 15, 299 (1969);J. Chem. Phys., 43,3418 (1965). 1~~
~~I
Phase Transitions in the Layer Structure Compounds Zr(HPO,)(n-C, Hzn+,PO4) Shoji Yamanaka,
Karunori Sakamoto, and Makoto Hattori
Department of Applied Chemistry, Faculty of Engineering, Hlroshima University, Hiroshima 730, Japan (Recelved: November 5, 1980; In Final Form: March 11, 198 1)
The thermal behavior of the layer structure compounds Zr(HP0)4(n-C,Hzn+lP04) (n = 1-18) has been studied by X-ray diffraction and calorimetric measurements. Compounds with n 2 7 have structural phase transitions in the temperature range from 45 to 95 "C accompanied by an increase in basal spacing. The transition temperature and the transition entropy increase linearly with the number of carbon atoms (n)in the alkyl chain. The entropy change would be mainly due to the increase in conformational freedom of the alkyl chains. The increase in the entropy change per CH2group is 0.66R (R is the gas constant), indicating that the alkyl bilayers sandwiched by zirconium phosphate layers are in a quasi liquid state in the high temperature phases.
Introduction Zirconium bis(mon0hydrogen orthophosphate) is an insoluble ion exchanger with a layer structure. The monoand dihydrate, Zr(HP0.J2.Hz0 and Zr(HP04)2.2Hz0,are known and are called a-and y-zirconium phosphate, re-
spectively. The crystal structure of the a form was analYzed by Clearfield and Smith.' Each Phosphate layer consists of Z r 0 6 octahedra lying on a Plane and POdOH) (1)A. Clearfield and G. D. Smith, Inorg. Chem., 8, 431 (1969).
0022-3654/81/2085-1930$01.25/00 1981 American Chemical Society
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tetrahedra situated above and below the octahedral plane. Every P03(OH) tetrahedron shares its three oxygen corners with three different Zr06 octahedra. The unshared fourth corner of the tetrahedron, which bears the exchangeable hydrogen ion, points toward the adjacent phosphate layer. Water molecules reside between the layers. It has been shown that the layer structure of the y form is analogous to that of the a f ~ r m . ~ e Recently, we have shown that the interlayer PO,(OH) groups are exchangeable with various phosphoric ester groups of the type PO,(OR) where R is an organic group:'$
e
Zr(HP04)2.2H20+ x R O P O ~ ~ -
Zr(ROP03)x(HP04)2-,.yH20 + x H O P O ~ ~(1) The rigid layer framework of y-zirconium phosphate remains unchanged by the exchange reaction. In the resulting compounds the organic bilayers are sandwiched by the zirconium phosphate layers, the organic groups being linked to the inorganic layer by P-0-C ester bonds. Several inorganic layer compounds other than y-zirconium phosphate are known to accomodate alkyl bimolecular layers between the inorganic layers such as mica-type layer silicates,6 niobates,' vanadates,6 transition metal disulfide^,^ and divalent metal phosphosu1fides.l0 The alkyl chains between those layers are in the forms of alkanols, alkylamines, and alkylammonium ions. Much interest has been paid to the structural transitions of the sandwiched alkyl layers on heating, because the transitions have parallelism with the behavior of the alkyl chains in polyethylene," smectic liquid crystals, and lipid membranes. In a previous study, a homologous series of Zr(HP04)(n-C,H,+,P04) compounds have been prepared from y-zirconium phosphate by the above exchange method and a structural model for the arrangement of the alkyl chains has been proposed.12 The present study is concerned with the structural transitions of these compounds which include bimolecular layers of a new type in the sense that one end of every alkyl chain is linked by the ester bond to the inorganic layer.
Experimental Section The layer compounds Zr(HP04)(n-C,Hz,+lP04).yH20 ( n = 1-18; 0.4 < y < 0.8) used in this study were identical with those prepared in a previous study.12 Analytical results indicated that half of the monohydrogen orthophosphate groups of y-zirconium phosphate Zr(HP0J2. 2H20 were exchanged with alkyl phosphate groups. The water molecules b H 2 0 ) were removed either by heating or by evacuation and were readily readsorbed when the compounds were allowed to stand in air. It seems likely that the water molecules are taken up near the unexchanged phosphate groups by forming hydrogen bonds of (2) A. Clearfield, R. H. Blessing, and J. A. Stynes, J. Znorg. Nucl. Chem., 30, 2249 (1968). (3) S. Yamanaka and M. Tanaka, J.Inorg. Nucl. Chem., 41,45 (1979). (4) S. Yamanaka and M. Hattori, Chem. Lett., 1073 (1979). (5) S. Yamanka, M. Mataunaga, K. Yamasaka, and M. Hattori, submitted to Angew. Chem. (6)G. Lagaly, Angew. Chem., Int. Edit. Engl., 15, 575 (1976). (7) G. Lagaly and K. Beneke, J.Inorg. Nucl. Chem., 38, 1513 (1976). (8) A. Weiss and K. J. Hilke, Angew. Chem., Int. Edit. Engl., 4, 353 (1965). (9)G. V. Subba Rao and M. W. Shafer in "Intercalated Layer Materials", F. A. LCvy, Ed., Reidel, Dordrecht, 1979, p 99. (10)S. Yamanaka, H. Kobayashi, and M. Tanaka, Chem. Lett., 329 (1976). (11) W. Pechhold, E. Liska, H. P. Grossmann, and P. C. HHgele, Pure Appl. Chem., 46, 127 (1976). (12) S. Yamanaka, M. Matsunaga, and M. Hattori, J. Inorg. Nucl. Chem., in press.
uu 10
50
100
200
150
Temperature ("C)
Flgure 1. Basal spacings of a homologous series of Zr(HP0,KnC,H,,+,P04)yH20 compounds as a f u n d i of temperature on heating (0). Basal spacings on cooling (0)are shown for the representative compounds with n = 12 and 14.
3 n Figure 2. Basal spacings of Zr(HP04)(n-C,H,,+1P04).yH20 at room temperature (0)and after the transition (0)vs. the number of carbon atoms ( n ) .
+P-OH.-OH, type. X-ray examination was performed on the powder sample spread on glass slides by using Ni-filtered Cu K a radiation. The slides were heated stepwise to 200 OC and cooled similarly. The diffraction pattern was recorded a t each step with a temperature control of f2 "C. Since the layer structured samples preferred to orient with the basal plane parallel to the plane of the glass slide, an integral series of (001) reflections was observed up to high orders. The basal spacings were calculated by using the reflection eaks up to the fourth order with an error of about 0.1 Calorimetric measurements were carried out on a Sinku-Riko differential scanning calorimeter (DSC) with a heating rate of 5 "C/min. Transition enthalpies were determined with reference to the enthalpy of fusion of indium metal.
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The Journal of Physical Chemistry, Vol. 85, No. 13, 1981
Yamanaka et al. . ..
.......
(a)
i loss 2 . 6 4
Wt.
T
TI
150
IO0
50
Temoerature ( " t ) PhOSphOrUS
Q Hydroxyl
0
Oxygen
0 Carbon
Figure 3. Structural model of the arrangement of the alkyl chains between the phosphate layers. The inclined alkyl chains in the region designated by I are closely packed.
Results and Discussion The temperature dependence of the basal spacing of Zr(HP04)(n-C,,H2n+1P04)-yH20 on heating is shown in Figure 1. For samples with n 1 7, transitions accompanied by an increase in basal spacing are observed. The transition temperature was shifted toward higher temperatures with an increase in the number of carbon atoms of the alkyl chain. The spacing reverted to the initial value, but with hysteresis, on cooling. Figure 2 shows the basal spacings of the homologous series of the compounds at room temperature (25 "C) and after the transitions on heating. Since the dehydration of the adsorbed water begins at about 60 "C, the compounds are in anhydrous forms after the transition. It was found that the amount of water adsorbed did not affect the basal spacing values. The values of the anhydrous forms a t room temperature were essentially similar to those of the hydrated forms. In a previous study,12 a structural model for the arrangement of the alkyl chains between the phosphate layers was proposed to explain the relations between the basal spacing and the number of carbon atoms. The slopes of the linear relations in Fi e 2 for the room temperature phases are 2.43 and 1.42 per CH2 for n I4 and n I5 respectively. The structural model of the compound with long alkyl chains is redrawn in Figure 3. The alkyl chains shown in figure have bends; the part of each alkyl chain linked directly to the phosphate layer is oriented almost perpendicular to the phosphate layer and the other part of the alkyl chain is inclined to the phosphate layer a t an angle 34O in all-trans configuration. The alkyl chains can be packed more closely than those in the perpendicular orientation by inclining them to the phosphate layers. the packing density calculated for the inclined alkyl chain layers is close to that of polyethylene.12 After the structural transition, the basal spacing with n 1 7 increases linearly with a new slope of 1.67 A per CH2. This corresponds to the apparent increase of the inclination angle of the alkyl chains from 34 to 41O. Electron microscope examination was carried out on the compound with n = 12 to see the change in the (hkO) diffraction pattern. The microscope was equipped with specimen heating facilities, and an electron beam of 100 keV was radiated perpendicular to the basal plane of the thin crystals (5 X 0.5 pm2). The electron diffraction photographs taken before and after the structural transition essentially coincided with each other. The lattice constants calculated on the basis of the rectangular a*-b*
Y
Figure 4. Thermal analysis curves for Zr(HPO4)(n-C,,H,PO4).yH2O: (a) weight loss, (b) DSC cwve for the hydrated form (the first run), and (c) DSC curve for the anhydrous form (the second run).
TABLE I: Transition Temperatures, Enthalpies, and Entropies for Zr(n-C, H,,, PO,)( HPO,) heating
n
T/K
7 8 9 10 12 14 16 18
318 318 333 338 348 353 360 370
AS/cal AH/kcal mol-' mol'] K-I
0.28 0.66 1.17 1.44 2.30 3.90 4.36 5.64
0.87 2.06 3.52 4.26 6.60 11.04 12.11 15.23
cooling AH/kcal mol-'
2.42 3.64 4.45 6.33
AS/cal mol-' K-I
6.95 10.30 12.35 17.10
lattice plane were l / a * = 5.35 8,and l/b* = 6.42 A which are very close to the lattice constants, a = 5.376 and b = 6.636 A, of y-zirconium phosphate3 This finding indicates that the layer framework of y-zirconium phosphate remains unchanged on transition. Figure 4 shows typical DSC and weight loss curves of the compound with n = 18. The weight loss curve (Figure 4a) indicates that the adsorbed water was gradually desorbed in the temperature range from 60 to 180 "C. A broad endothermic background in the DSC curve (Figure 4b) would be due to this desorption. A sharp endothermic peak distinguishable from the background was shifted toward higher temperatures with an increase in the number of carbon atoms, which can be attributed to the structural transitions. After the DSC measurement was carried out up to 150 OC, the sample was evacuated for 1 h in the calorimeter to ensure complete dehydration followed by cooling under a dry nitrogen atmosphere. On the second run, the foregoing broad endothermic peak disappeared and only a sharp endothermic peak was observed as shown in Figure 4c. The areas under the two sharp peaks in Figure 4, b and c, are very similar, although the peak of the second run was shifted toward a lower temperature. The enthalpy change at the transition was measured as a function of the carbon atoms in the alkyl chain in both heating and cooling runs. For compound with n I10, the exothermic peak in the cooling curve became too broad to estimate the enthalpy change. The enthalpy and entropy changes together with the transition temperature are listed in Table I. The transition temperatures are taken from the X-ray data shown in Figure 1. The transition entropy begins to increase at n = 6 with the increase in n as shown in Figure 5. The average increase in the transition entroy per CH2,hereafter referred to as AScH2,is of the order of
Thermal Behavior of Layer Structure Compounds
,
The Journal of Physical Chemistry, Vol. 85,No. 13, 1981 1833
, A
'
0 0
"
'
I
'
5
10
15
20
n
Flgwe 5. Transition entropy and transition temperature vs. the number of carbon atoms (n). The values ( 0 )and (A)were obtained from the peaks during heating and cooling scans, respectively.
1.32 cal mol-' deg-' = 0.66R (R is the gas constant). The bimolecular layers of alkyl chains formed between mica-type silicate layers5 and of compounds of the type (n-C,H2,+1NH3)2MC14(M = Cu, Mn, Cd, Fe, Pd, Co, and Zn) have been reported to have similar structural transitions a t elevated temperatures.13-17 Those are orderdisorder transitions. In the room temperature phases, the alkyl chains form ordered bimolecular layers in all-trans configuration between the inorganic layers while, in the high temperature phases, special sequences of trans-gauche configuration gtg (so-called kinks) are formed in the alkyl chain. The alkyl layers are in liquidlike state; the kinks move up and down in the alkyl chains. For the order-disorder transitions, the transition entropies lie between the two limiting values.13 The higher limiting value is the order of ASCH,= R which is close to A S C Hfor ~ the melting of n-paraffins ( A S C H = ~ 1.18-1.33R) and polyethylene (ASCH, = 1.18R).18 The lower limiting (13) R. Kind, S. PlBsko, H. Arend, R. Blinc, B. h k i , J. Seliger, B. Lo*, J. Slak, A. Levstic, C. FilipiE, V. h g a r , G. Lahajnar, F. Milia, and G. Chapuis, J. Chern. Phys., 71,2118 (1979). (14)C. Socias, M.A. Arriandiage, M. J. Tello, J. FernBndez, and P. Gili, Phys. Status Solidi, A57,405 (1980). (15)E.Landi and M. Vacatello, Therrnochim. Acta, 13, 441 (1975). (16)V. Salerno, E.Landi, and M. Vacatello, Therrnochirn. Acta, 20, 407 (1977). (17)E.Landi, and M. Vacatello, Therrnochirn. Acta, 12,141 (1975). (18)A. Bondi, Chern. Rev., 67, 565 (1967).
value is ASCH, = 0.69R = R In 2, indicating that each C-C group can be interpreted as an independent pseudospin with two equivalent sites.13 The higher limiting value can be interpreted in terms of the conformational entropy for free rotation around each C-C axis. The value obtained in this study is in agreement with the lower limiting value. This suggests that some degree of conformational ordering persists in the alkyl bilayers of the high temperature phases. The increase in conformational freedom would result in the expansion of the alkyl bilayers, which in turn causes the increase in basal spacing. As mentioned in the foregoing paragraph, the apparent inclination angle of the alkyl chains to the phosphate layers increases from 34 to 41'. This corresponds to the expansion of the inclined alkyl layers by 15%. On the other hand, the basal spacings of the compounds with n 5 4 in Figure 1 decrease gradually with temperature. The alkyl chains are oriented almost perpendicular to the phosphate layers in those compounds, forming bimolecular layers with methyl groups in headto-head arrangement between the phosphate layers at room temperature. The packing density of the alkyl chain of this perpendicular region is half as much as the value of the inclined region designated by I in Figure 3, and there are vacant sites above the unexchanged phosphate groups. Probably, a t elevated temperatures one side of the alkyl layer forming the bimolecular layer can be keyed into the other side of the layer. The partial collapse in the basal spacing observed for the compounds with n 5 4 would be caused by the keying. Even in the compounds with long alkyl chains, a part of each alkyl chain, which is located close to the phosphate layer, is not involved in the phase transition. This can be attributed to the restriction in the movement by the ester bonding to the phosphate layer. As described above, the other part of the alkyl chain which is located apart from the phosphate layer (region I of Figure 3) has a phase transition similar to the alkyl chains of the bimolecular layer between mica-type layer silicates and of the compounds of the type (CnH2n+lNH3)2MC14 reported so far. Acknowledgment. The authors thank professor Yuroku Yamamoto and Dr. Terubumi Fujiwara for the use of the differential scanning calorimeter and for valuable discussions. Thanks are also due to J. Shitaoka for the electron microscopic examination. This study has been supported in part by a grant in aid of special research project from The Ministry of Education of Japan.
