Layered hafnium phosphates. Synthesis, characterization, crystalline structure and intercalation behaviour1

MOLSTR 10367

Journal of Molecular Structure 470 (1998) 105–119

Layered hafnium phosphates. Synthesis, characterization, crystalline structure and intercalation behaviour1 M. Sua´rez*, L.M. Barcina, R. Llavona, J. Rodrı´guez Departamento de Quı´mica Orga´nica e Inorga´nica, Facultad de Quı´mica, Universidad de Oviedo, 33071 Oviedo, Spain Received 22 October 1997; accepted 1 December 1997

Abstract This review compiles the recent investigations on layered hafnium phosphates in the a and g varieties. Their preparation, properties and crystalline structures are described. The behaviour of both compounds during the intercalation of n-alkylamines (n = 1–6) and the cyclic amines aniline, benzylamine, cyclohexylamine, piperidine, pyridine, n-methylpiperidine and 4-methylpiperidine, in the vapour and liquid phases, is reported. The formula of the intercalates and the estimation of the guests arrangement in the interlayer space is discussed. 䉷 1998 Elsevier Science B.V. All rights reserved. Keywords: Hafnium phosphates; Amine intercalation; Crystal structures; Layered compounds

1. Introduction Many layered phosphates of tetravalent metals have been synthesized and characterized, and two different structure types, of the layers referred to as a and g, exist. A large number of phases can be obtained by the exchange of the protons with other cations, as well as by the intercalation of polar molecules in the interlayer region. Extensive information has been compiled in recent reviews and books [1–5]. The majority of investigations on the chemistry of layered acid salts have been performed with a- and g-zirconium and titanium hydrogen * Corresponding author. Tel: 00 34 85103 499; Fax: 00 34 85103 446; e-mail: [email protected] 1 Dedicated to Professor Abraham Clearfield for the occasion of his 70th birthday.

phosphates [6–8]. Layered hafnium phosphates are isomorphous with zirconium phosphates, and since the replacement of the tetravalent atom by a different tetravalent metal does not appreciably modify the acid strength of the phosphate group, both compounds should exhibit very similar properties. However, differences in the values of interlayer distances, free area and unit cell volumes, may result in differences in the ion-exchange and intercalation properties. This review summarizes the recent investigations on the layered hafnium phosphates in both varieties: Hf(HPO 4) 2·H 2O (a-HfP) and Hf(PO 4)(H 2PO 4)·2H 2O (g-HfP). Table 1 shows some characteristics of a- and g-HfP. The g-variety could be prepared and its crystalline structure solved by X-ray powder diffraction. Intercalation reactions in both compounds were carried out with several linear and cyclic amines from the gas and liquid phases.

0022-2860/98/$ - see front matter 䉷 1998 Elsevier Science B.V. All rights reserved. PII S 00 22 - 28 6 0( 9 8) 0 04 7 4- 8

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Table 1 Some characteristics of a- and g-HfP

a (pm) b (pm) c (pm) b (deg.) Interlayer distance (pm) Free area (10 4 pm 2) I-Exch. cap. (mmol H + g −1) V (A) 3 a

a-Hf(HPO 4) 2·H 2O a

g-Hf(PO 4)(H 2PO 4)·2H 2O b

901.42(1) 525.66(5) 1547.68(2) 101.64 760.0 23.7 5.15 713.37(8)

534.99(3) 659.49(4) 1239.39(8) 98.594 1210.0 17.64 4.92 432.37

Data taken from Ref. [11]. Data taken from Ref. [35].

b

2. a-hafnium phosphate 2.1. Synthesis and crystalline structure a-HfP was first prepared by Clearfield et al. following the HF and reflux procedures earlier used in the preparation of a-zirconium hydrogen phosphate (a-ZrP) [9]. Later, Tomita et al. prepared a-HfP by refluxing newly formed amorphous hafnium phosphate in 11–12 mol dm −3 of H 3PO 4 at boiling point, for more than 210 h [10]. In our experiments we have followed this method with a shorter reflux time (we proved that 48 h was adequate). Crystal data for a-HfP was obtained by the Rietveld analysis of the X-ray powder diffraction patterns [11].

The crystals are monoclinic with a = 901.42(1), b = 525.66(5) and c = 1547.68(2) pm, and b = 101.64⬚, space group P2 1/c. The layers consist of metal atoms lying slightly above and below the mean plane and bridged by phosphate groups from above and below. The structure determination allows the evaluation of some parameters of interest, for interpreting the data on the intercalation of neutral and charged species into the interlayer space of a-HfP. The interlayer distance is 760 pm and the thickness of the layer, calculated as the shorter distance between the baricentre of the oxygen of P–OH groups present in the opposite sides of one layer, is 630 pm. The free area associated with each P–OH group is 23.7 × 10 4 pm 2 [5].

Fig. 1. Neutron diffraction pattern of a-HfP.

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A more complete crystal structure of a-HfP including H-atoms positions, was recently determined by Rietveld refinement and Fourier synthesis, using constant-wavelength neutron diffraction data with an intense source [12]. Details of the refinement parameters are given in Table 2. The final difference plots of the neutron Rietveld refinement are shown in Fig. 1, while the atomic parameters are contained in Table 3. All thermal parameters range within normal values. It is interesting to observe the forms of the zeolitic cavities, since the active centres (H-atoms) of the material are located in the cavities. Fig. 2 illustrates Table 2 Structural parameters for a-HfP HfP 2O 9H 4 388.46 Monoclinic 8.9955(5) 5.2439(3) 16.224(1) 111.234(4) 713.37 (8) 4 P2 1/c 71 1.66 0.90 4.38 3.43

Empirical formula Formula weight Cell setting ˚) a (A ˚) b (A ˚) c (A b (deg.) V (A 3) Z Space group No. of parameters R awp R bexp R cF x2d

Table 3 ˚) Hydrogen bond geometry (A a-Hfp O8[O4]–H1 O8…9 H1…O9 O8[O4]–H1…O9 O9–H3 O9…O4[O8] H3…O4[O8] O9–H3…O4[O8] O4[O8]–H2 O4[O8]…O9 H2…O9 O4[O8]–H2…O9 O9–H4 H3–O9–H4

1.13(3) 2.67(2) 1.69(3) 143.0(2) 0.90(2) 2.79(2) 1.90(2) 170.0(2) 1.08(2) 3.14(2) 2.06(2) 178.0(2) 1.11(2) 107.0(2)

the geometry of the cavities in the a-HfP structure and Table 4 contains the geometry of the water molecule and the hydrogen bonds. Although a-HfP was refined from a-TiP because it was assumed that they are isostructural, small displacements of O4 and O8 atoms make similar cavities translated in the structure. In fact, when we tried to refine H-atom positions from a-TiP in the structure of a-HfP, the refinement did not progress correctly. Further, for similar cavities the orientation of the water molecule is different in both compounds. In spite of this effect, the topological distribution of hydrogen bonding is similar in both cases.

a

R wp =

8 9 2 1=2 < ∑ wi (yi, obs − yi, calc ) = i

:

∑ wi (yi, obs )2

;

:

i

b

RF =

1=2 1=2 ∑ (1K⬘, obs⬘ ) − (1K, calc ) K ∑ (1K, ⬘obs⬘ )1=2

:

K

c

R exp =

R wp (x2 )1=2

:

d

x2 = ∑ i

wi (yi, obs − yi, calc )2 : (Nobs − Nvar )

where N obs and N var = the number of observations and variables.

Fig. 2. Cavities in the structure of a-HfP.

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Table 4 ˚ 2) for a-HfP Fractional atomic coordinates and isotropic displacement parameters (A x Hfl P1 P2 O1 O2 O3 O4 O5 O6 O7 O8 O9 H1 H2 H3 H4

0.7373(8) −0.0238(11) 0.4976(12) 0.1231(12) −0.0830(11) −0.1661(12) 0.0120(9) 0.3639(12) 0.4287(14) 0.5981(10) 0.6004(10) 0.2370(15) −0.1000(2) 0.659(2) 0.294(3) 0.212(2)

y 0.2519(19) 0.753(3) 0.234(2) 0.813(2) 0.472(2) 0.939(2) 0.746(2) 0.447(2) −0.017(2) 0.303(2) 0.259(2) 0.222(2) 0.811(4) 0.076(4) 0.079(4) 0.221(5)

2.2. Ion-exchange reactions Ion-exchange M +/H + (M = Li, Na and K) on a-hafnium phosphate was reported by Tomita et al. [10,13,14]. The experiments were carried out by the batch method. The crystalline phases obtained and their interlayer distances are compiled in Table 5. Li +/H + ion-exchange proceeds in a single step, without the formation of any intermediate Li + ionexchange phases. In contrast, the K +/H + and Na +/H + ion-exchange proceeds in two steps, and intermediate phases were observed. 2.3. Amine intercalation We have studied the intercalation behaviour in a-HfP of n-alkylamines and cyclic amines (aniline, benzylamine, cyclohexylamine, piperidine and pyridine) from the gas and the liquid phases [15– 17], and the stepwise evolution of the a-HfP from the formation of intermediate unsaturated phases [18,19]. 2.3.1. n-alkylamines When the intercalation process of n-alkylamines in a-HfP is carried out in the vapour phase [15], the intercalation compounds were obtained by placing the a-HfP in an atmosphere saturated with n-alkylamine vapour at room temperature from 6 h

z

U iso

0.4853(4) 0.6095(5) 0.6129(5) 0.5968(6) 0.5713(6) 0.5610(7) 0.7067(5) 0.5901(7) 0.5955(7) 0.5571(6) 0.7110(5) 0.7406(7) 0.7167(15) 0.7282(16) 0.763(2) 0.6681(10)

0.0021(11) 0.0078(12) 0.0078(12) 0.0122(6) 0.0122(6) 0.0122(6). 0.0122(6) 0.0122(6) 0.0122(6) 0.0121(6) 0.0122(6) 0.029(2) 0.052(3) 0.052(3) 0.052(3) 0.052(3)

to 15 days. The intercalation involves all active centres of the material and the process may be represented by: a − Hf (HOPO3 )2 ·H2 O + 2RNH2 → a − Hf (OPO3 )2 ·2RNH3 ·H2 O, except for methylamine, in which case the amine retention overcomes the value of 2 mol/mol a-HfP, and the intercalation process may be represented by: a − Hf (HOPO3 )2 ·H2 O → a − Hf (OPO3 )2 ·2CH3 NH3 ·H2 O → a − Hf (OPO3 )2 ·2CH3 NH3 ·xCH3 NH2 ·nH2 O: Table 5 Interlayer distances of the M +/H + (M = Li, Na, K) ion-exchange in a-HfP. Phases

˚) d (A

Lithium

Hf(LiPO 4) 2·H 2O Hf(LiPO 4) 2·4H 2O

7.9 10.1

Sodium

Hf(NaPO 4)(HPO 4)·5H 2O Hf(NaPO 4) 2·3H 2O

11.9 9.8

Potassium

Hf(KPO 4)(HPO 4)·H 2O Hf(KPO 4)(HPO 4) Hf(KPO 4) 2·3H 2O Hf(KPO 4) 2·H 2O

8.1 7.6 10.8 9.2

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Fig. 3 shows the X-ray diffraction patterns of the intercalated samples. It can be seen that the intercalates maintain their crystallinity. Reflections corresponding to the 002 and higher harmonics (004, 006) are observed. Table 6 lists the interlayer distances of a-HfP/n-alkylamine intercalates and analogous compounds obtained by using an a-TiP and a-ZrP layered acid host. The interlayer distances of the a-Hf(OPO 3) 2· 2RNH 3·H 2O solids increase linearly with the number of carbon atoms in the alkyl chain. The straight line follows the equation: d 002 = 10.26 + 2.15n c. Since the increment of the alkyl chain, in the trans– ˚ for each trans conformation is estimated to be 1.27 A additional carbon atom [20], it is reasonable to assume that the amine is present in the a-HfP as a bimolecular

Table 6 ˚ ) for monohydrate intercalation compounds Interlayer distances (A of a-TiP, a-ZrP and a-HfP containing two molecules of n-alkylamine for unit formula Intercalated amine a-TiP a

a-ZrP b

a-HfP c

Methylanune Ethylamine Propylamine Butylamine Pentylamine Hexylamine

13.4 14.7 17.3 18.6 21.2 23.3

12.8 14.0 17.1 18.7 21.0 23.2

13.1 14.3 16.9 18.8 21.1 23.1

a

Data taken from Ref. [20]. Data taken from Ref. [21]. c Data taken from Ref. [15]. b

layer of extended molecules, since the slope of the straight line defining the interlayer distances is higher ˚ . The average inclination angle of the than 1.27 A molecules with respect to the sheet is a = sin −1 (2.15/2.54) = 57.8⬚. This bilayered arrangement can be explained, since there is just room for one alkyl chain in the upright position for every P–OH group (the cross-sectional area of a trans–trans alkyl chain, 18.6 × 10 4 pm 22, is comparable with the free area surrounding each phosphate group, 23.7 × 10 4 pm 2), intercalated amines cannot interpenetrate the film already present and an ordered bilayer is obtained [23]. The values of the packing parameters for a-HfP/nalkylamine intercalates are given in Table 7. Except in the methylamine case, V p always takes values of higher than 0.87, indicating that the packing density is not very different to that present in crystalline n-paraffin. Intercalation from the liquid phase [16] was carried out by placing the a-HfP in contact with the pure amine and shaking for 48 h at 25⬚C. The characterization of these phases indicated that they are similar to those obtained by the vapour phase process: all the crystal sites available for intercalation are occupied and phases of composition a-HfP · 2RNH 2·H 2O are formed, consistent with a bilayered disposition of the amine molecules between the a-HfP layers. As in the earlier case, methylamine is an exception and a phase with three amine molecules is formed (interlayer ˚ ). The process can be represented as: distance 15.3 A a − Hf (HPO4 )2 ·H2 O + 3CH3 NH2

Fig. 3. X-ray diffraction patterns of a-Hf(OPO 3) 2·2C nH 2n+1NH 3· H 2O (n = 2–6) compounds.

→ a − Hf (PO4 )2 ·2CH3 NH3 ·CH3 NH2 ·H2 O:

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Table 7 Packing parameters (V p) for monohydrate intercalation compounds of a-HfP containing two molecules of n-alkylamine for unit formula Intercalated amine

a-Hfp

Methylamine Ethylamine Propylamine Butylamine Pentylamine Hexylamine

0.78 1.07 0.87 0.94 0.93 0.92

The stepwise intercalation of linear amines [17] was carried out by equilibrating the host compound with aqueous C nH 2n+1NH 2 from 0.5 to 10 mmol amine/g a-HfP at 25⬚C, for 5 days, following the batch method. Isotherms representing pH and millimole of amine intercalated per gram of a-HfP, against the amine added, are presented in Figs. 4 and 5. It can be observed that the process is almost quantitative until the retention of 1 mol of amine (2.57 mmol amine/g a-HfP), which is consistent with the almost constant pH in this range. At higher loadings the process becomes more difficult and the pH increased in a marked way. The saturation is reached for additions of 6 (ethylamine), 8 (methylamine and propylamine) and 10 mmol amine/g a-HfP (the remaining amines), corresponding to the intercalation of 2 mol of amine molecules. Thermal analysis of the saturated phases showed that the products contained 2 mol of amine and 1 mol of water, which is in good agreement with the data obtained from elemental analysis. X-ray diffraction of methyl, ethyl and propylamine intercalates presented a different behaviour toward the formation of partial substitution phases. In the case of amines with n = 1–3, the intercalation process occurs with the formation of an intermediate phase of composition, a-HfP·amine·H 2O, that only could be isolated as a pure phase in the methylamine case. In the amines with n = 4–6, phases of intermediate composition were never detected, and the full saturated phase is present from the beginning of the process. Table 8 presents the d 002 values of the phases obtained. As in the a-ZrP case [21], the amines with small carbon chains initially form a phase of intermediate composition, probably with the chain backbone lying paralell to the phosphate layers, while longer-chain amines form the saturated phase even from the start

Fig. 4. Titration curves (W) and intercalation isotherms (A) of a-HfP with: (a) methylamine, (b) ethylamine and (c) propylamine. Table 8 ˚ ) for intercalation compounds of a-HfP Interlayer distances (A containing one (a) or two (b) molecules of n-alkylamine per unit formula Intercalated amine

(a)

(b)

Methylamine Ethylamine Propylamine Butylamine Pentylamine Hexylamine

9.4 10.5 13.8 — — —

12.8 14.5 18.0 18.7 21.0 23.2

M. Sua´rez et al./Journal of Molecular Structure 470 (1998) 105–119

111

Fig. 5. Titration curves (W) and intercalation isotherms (A) of a-HfP with: (a) butylamine, (b) pentylamine and (c) hexylamine.

of the process. Nevertheless, unlike the a-ZrP [21] and a-TiP behaviour [24], the formation of amorphous materials in our case was never detected along the intercalation process, and the intermediate solids always presented a high degree of crystallinity, the half intercalation phase for methylamine being isolated. 2.3.2. Cyclic amines The amines were chosen to represent a class of: (i) primary cyclic amines (cyclohexylamine), (ii)

Fig. 6. X-ray diffraction patterns of a-HfP and their intercalation compounds with cyclic amines specified in the figure.

secondary cyclic amines (piperidine), (iii) primary aromatic amines (aniline and benzylamine), and (iv) ternary amines (pyridine). The experiments were made in both vapour and liquid phases. When the a-HfP is exposed to the vapours of the amines from 8 to 60 days, only in the benzylamine and piperidine cases, were pure

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Table 9 Interlayer distances and composition of the intercalates with the formula X[Hf(HPO 4) 2·y amine·H 2O]·(1 − x) a-HfP Amine

˚) d 002(A

x

y

Aniline Benzylamine Cyclohexylamine Piperidine Pyridine

18.2 18.7 18.0 13.1 11.1

0.1 1.0 0.2 1.0 0.7

2.0 2.0 2.0 1.0 1.0

intercalation phases obtained, the rest of the solids maintaining a certain percentage of the initial phase. Fig. 6 shows the X-ray diffraction patterns of a-HfP and the amine intercalates. The corresponding d 002 values are indicated in Table 9. The estimation of the relative intensity of the 002 reflections of the initial and intercalated phases and elemental (C, N, H) and thermal analysis, allows the establishment of the composition of the solids obtained in the intercalation process (Table 10). All the intercalates obtained are crystalline solids which maintain the lamellar structure of the hosts and are stable in water. The interlayer distances are always ˚ ). higher than that of the starting a-phosphates (7.6 A In some cases, the characteristic reflections of a-HfP remains, indicating that the intercalation does not go to completion as a result of kinetic factors. The intercalation rate is probably very slow for the low pressure of these cyclic amines at room temperature, and very long times of reaction could be necessary to reach a true equilibrium. This is in agreement with the fact that the amount of derivative formed (Table 9) decreases with the increasing boiling points: aniline

185⬚C, cyclohexylamine 134.8⬚C, pyridine 114⬚C, and piperidine 106.4⬚C. For each intercalate, the free space between the layers can be estimated by susbtracting the thickness of the a-HfP layers fom the d 002 value and taking into account the amine size, we can assign a bilamellar arrangement in the aniline, benzylamine and cyclohexylamine cases, and monolamellar in the piperidine and pyridine cases. Intercalation from the liquid phase [16] was carried out by placing the a-HfP in contact with the pure amine and shaking for 48 h at 25⬚C. The results obtained are different than those in the vapour phase, since pure phases are reached, except for aniline where the initial phase remains in a marked way. Moreover, in the piperidine case, a different ˚ is formed, corresponding phase with d 002 = 15.4 A to the intercalation of 2 amine moles. These facts are in agreement with the behaviour reported previously [5] for intercalates of this type: when the guest molecule contains an NH 2 group (aniline, benzylamine, cyclohexylamine and piperidine) which upon protonation gives rise to an sp 3-hybridized NH+3 group, giving a good fit to the hexagonal arrangement of the P–OH on the layer, the intercalation compounds contain 2 mol of amine/mol Hf and the guests are arranged in a bilayered fashion, while molecules containing an sp 2-hybridized N (pyridine) give rise to compounds containing 1 mol of amine/mol Hf. The good behaviour of piperidine as the intercalation guest in a-HfP [16,17] led us to select two isomeric forms of methylpiperidines [4-C 6H 12NH

Table 10 Microanalytical data (C, N) and experimental weight loss at 800⬚C of the intercalation compounds with cyclic amines Solid 0.1[Hf(HPO 4) 2·2C 6H 5NH 2·H 2O]·0.9a-HfP 0.4[Zr(HPO 4) 2·2C 6H 5NH 2·H 2O]·0.6a-ZrP Hf(HPO 4) 2·2C 7H 7NH 2·H 2O 0.8[Zr(HPO 4) 2·2C 7H 7NH 2·H 2O]·0.2a-ZrP 0.2[Hf(HPO 4) 2·2C 6H 11NH 2·H 2O]·0.8a-HfP 0.6[Zr(HPO 4) 2·2C 6H 11NH 2·H 2O]·0.4a-ZrP Hf(HPO 4) 2·C 5H 10NH·H 2O Zr(HPO 4) 2·C 5H 10NH·H 2O 0.7[Hf(HPO 4) 2·C 5H 5N 2·H 2O]·0.3a-HfP Zr(HPO 4) 2·C 5H 5N·H 2O

Experimental C (%) N (%)

Calculated Weight loss (%) C (%)

3.36 15.51 27.80 28.67 6.69 20.69 12.62 15.61 9.57 15.76

13.62 29.72 41.83 43.97 17.81 39.01 25.63 31.45 20.70 30.42

0.71 2.89 4.62 4.69 1.38 3.94 2.91 3.57 2.19 3.54

3.54 15.34 27.88 28.45 6.73 20.57 12.67 15.54 9.46 15.78

N (%) 0.69 2.98 4.65 4.74 1.31 4.00 2.96 3.62 2.21 3.68

Weight loss (%) 13.41 29.40 41.50 43.86 17.67 38.86 25.56 31.33 20.58 30.25

M. Sua´rez et al./Journal of Molecular Structure 470 (1998) 105–119

(4-MP) and n-C 6H 13N (n-MP)], in order to study the influence of the basicity and the position of the methyl group in the molecule on the intercalation process. The stepwise intercalation procedure used was to equilibrate the a-HfP with aqueous solutions of 4- and n-MP containing from 0.5 to 30 mmol amine/g a-HfP, at 25⬚C during 6 days following the batch method [18]. New phases of composition Hf(HPO 4) 2C 6H 12·NH· ˚ (n-MP) y 14.2 A ˚ H 2O and interlayer distances 13.08 A (4-MP) were obtained. Their diffraction patterns (for additions of 3 mmol amine/g a-HfP (4-MP) and 6 mmol amine/g a-HfP (n-MP) are presented in Fig. 7. The material reached a hydrolysis degree of 5.9% (4-MP) y 39.8% (n-MP). The intercalation of 1 mol of amine takes place in an almost quantitative way without the degradation of a-HfP. The best intercalation behaviour of 4-MP with respect to that of n-MP, can be related to steric factors and to the stronger basicity of this amine. 4-MP shows a higher affinity toward the acid sites of the layer as a result of its basic strength [pK b (4-MP) = 1.78; pK b (n-MP) = 3.92]. n-MP, in addition to its weaker basicity, presents steric hindrances as a result of the presence of the methyl group bonded to the nitrogen atom. Thus, the intercalation in this case is not quantitative, and high hydrolysis levels are produced.

Fig. 7. X-ray diffraction patterns of a-HfP with: (a) n-MP and (b) 4-MP.

113

3. g-hafnium phosphate 3.1. Synthesis Some attempts to prepare g-HfP following the method used by Alberti in the preparation of g-TiP [25], and several experiments of synthesis carried out hydrothermally with different concentrations (from 12 to 17 M H 3PO 4), temperatures (from 120 to 200⬚C) and reaction times (from 6 to 24 h), were not successful, and the a-variety which is thermodynamically the more stable phase was always obtained. The synthesis of g-HfP was made following the reflux method used by Clearfield [26] in obtaining g-ZrP: sodium dihydrogen phosphate monohydrate was mixed with 2 M phosphoric acid (⬃7 M in NaH 2PO 4·H 2O). This mixture was heated to boiling with stirring and then 30 ml of a 1 M HfOCl 2·8H 2O (twice recrystallized from HCl) solution was added dropwise. Boiling under reflux was continued for 3 weeks. The solid was separated by centrifugation and washed with 4 M HCl (until Na + ions were eliminated), with 0.2 M H 3PO 4 (until free of chlorides), and with deionized water (until pH = 4), and finally dried in air. A well-crystallized product was obtained from the earlier mentioned procedure. Elemental analysis of the solid indicated 15.32% P and 43.62% Hf, which is in good agreement with the theoretical values (15.24% P and 43.91% Hf) deduced from the formula Hf(PO 4)(H 2PO 4)·2H 2O. The X-ray diffraction pattern (Fig. 8) shows that the new g-HfP phase presents an interlayer distance ˚ , close to that of the other g-phosphates of 12.1 A ˚ and g-TiP: of tetravalent metals (g-ZrP: 12.2 A ˚ ). 11.6 A The TG curve (Fig. 9) clearly shows three mass loss steps. The first step takes place between room temperature and 100⬚C (8.92% weight loss) and corresponds to the loss of crystallization water molecules (8.86% theoretical weight loss). The two following steps occur at 350⬚C and 750⬚C (4.34% weight loss) and were a result of the condensation of the hydrogenphosphate groups to a-HfP 2O 7 (4.43% theoretical weight loss). The process occurs in two steps due to the formation of an intermediate phosphate–pyrophosphate phase in a similar way as

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Fig. 8. X-ray pattern of g-HfP.

in the g-TiP case, explaining the high temperature at which the condensation process takes place [27]. Thermal treatment of the g-HfP phase leads to ˚; the formation of three phases: b-HfP (d = 9.3 A heating temperature = 200⬚C), a hafnium phosphate– pyrophosphate phase with lamellar structure (d = ˚ ; 450⬚C), and hafnium pyrophosphate (800⬚C). 8.2 A This is in good agreement with the results deduced from the TG curve. 3.2. Crystalline structure Step-scanned X-ray powder data were collected by means of a Philips computer automated diffractometer operating at 40 kV and 30 mA with a copper target and graphite-monochromated radiation. Data were collected in the range of 3⬚ ⬍ 2v ⬍ 110⬚ with a step size of 0.02⬚ and a count time of 10 s per step. The position of 15 low-angle unambiguosly characterized reflections were extracted from the data and indexed by trial and error methods implemented in the

program TREOR [28], with a monoclinic cell with figures of merit M 15 = 36 [29] and F 15 = 26(0.0102, 57) [30]. Crystal data of g-HfP are compared with those of other g-phosphates in Table 11. As a result of the similarities of the crystal data, the structural parameters of g-ZrP [31] were used as the initial values for the Rietveld refinement using the program FULLPROF [32]. After the initial refinement of the scale, background, profile and cell parameters, the atomic positions were refined using soft constraints only for the P–O distances [1.53 (1)]. All the atoms were refined with a common isotropic temperature factor. No corrections were made for anomalous dispersion or absorption. A correction for preferred orientation along [001] was included [33]. A summary of the crystallographic data is given in Table 12, final positional parameters in Table 13, and bond lengths and angles in Table 14. Final Rietveld refinement plot is shown in Fig. 10. A plot of the structure is shown in Fig. 11.

Table 11 Crystal data for g-phases

Space group ˚) a (A ˚) b (A ˚) c (A b (deg.) ˚ 3) V (A

g-HfP a

g-ZrP b

g-TiP c

P2 1 5.3499(3) 6.5949(4) 12.3939(8) 98.594(5) 432.37

P2 1 5.3825(2) 6.6337(1) 12.4102(4) 98.687(2) 438.03

P2 1 5.186(1) 6.3505(8) 11.865(3) 102.52(3) 381.47

a

Data taken from Ref. [35]. Data taken from Ref. [31]. c Data taken from Ref. [36]. b

Fig. 9. TG curve of g-HfP.

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M. Sua´rez et al./Journal of Molecular Structure 470 (1998) 105–119 Table 14 ˚ ) and angles (deg.) for g-HfP Bond lenghts (A ˚) Bond lengths (A

Fig. 10. Rietveld refinement pattern of g-HfP. Points correspond to observed data; the solid line is the calculated profile. Tick marks show the positions of allowed reflections, and a difference curve, in the same scale, is plotted at the botton of the pattern. Table 12 Crystallographic data for g-HfP HfP 2O 10H 6 406.48 monoclinic 3.12 2 P2 1 596 (K a doublets) 8 54 0.054 0.031 0.033 3.11

Empirical formula Formula wt. Crystal system r calc Z Space group No. contributing reflections No. geometric constraints No. parameters R wp R exp RF x2 For formulae see Table 2. Table 13 Atomic positional parameters for g-HfP Atom

x

y

z

Hf P1 P2 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10

0.8026(4) 0.244(1) 0.370(1) 0.054(3) 0.201(9) 0.21(1) 0.520(2) 0.089(2) 0.543(4) 0.368(5) 0.398(4) 0.185(3) 0.123(4)

0.25000 0.240(1) 0.187(1) 0.216(5) 0.059(4) 0.434(3) 0.228(7) 0.225(7) 0.255(7) 0.952(1) 0.318(3) 0.190(3) 0.524(3)

0.1258(2) 0.9395(5) 0.3089(5) 0.018(1) 0.863(3) 0.873(3) 0.993(1) 0.258(1) 0.227(1) 0.315(2) 0.4127(9) 0.641(1) 0.539(2)

Hf–O1 Hf–O2 Hf–O3 Hf–O4 Hf–O5 HP–O6 P1–O1 P1–O2 P1–O3 P1–O4 P2–O5 P2–O6 P2–O7 P2–O8 O9…O8⬘ O9…O8⬙ O9…O10 O9…O10⬘ O9…O7 O10…O7 O10…O8 O10…O8⬘ O1–Hf–O6 O1–Hf–O5 O1–Hf–O4 O1–Hf–O3 O1–Hf–O2 O2–Hf–O6 O2–HP–O5 O2–Hf–O4 O2–Hf–O3 O3–HP–O6 O3–Hf–O5 O3–HP–O4 O4–Hf–O6 O4–Hf–O5 O5–Hf–O6 O2–P1–O4 O2–P1–O3 O3–P1–O4 O1–P1–O4 O1–P1–O3 O1–P1–O2 O7–P2–O8 O6–P2–O8 O6–P2–O7 O5–P2–O8 O5–P2–O7 O5–P2–O6 Hf–O1–P1 Hf–O2–P1 HP–O3–P1 Hf–O4–P1 Hf–O5–P2 Hf–O6–P2

2.04(2) 2.04(3) 2.08(2) 2.06(1) 2.07(1) 2.01(2) 1.52(2) 1.52(3) 1.53(3) 1.53(1) 1.56(1) 1.54(2) 1.55(1) 1.54(2) 3.32(2) 3.45(3) 2.54(3) 2.79(3) 2.93(3) 3.07(3) 3.20(3) 2.67(3) Bond angles (deg) 174.2(6) 91.3(6) 87.1(6) 85.0(1) 100.0(1) 86.0(2) 92.0(2) 96.0(2) 175.0(1) 90.0(2) 86.0(2) 86.0(1) 90.3(6) 171.5(5) 90.5(6) 106.0(2) 109.0(1) 109.0(2) 114.1(8) 113.0(2) 106.0(2) 121.5(9) 113.0(9) 109.0(1) 103.0(1) 100.0(1) 108.7(9) 168.0(1) 138.0(2) 147.0(2) 152.7(8) 152.6(8) 161.0(1)

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M. Sua´rez et al./Journal of Molecular Structure 470 (1998) 105–119

Fig. 11. Perspective plot of the g-HfP crystal structure down the a axis. The c-axis is vertical and the b-axis is horizontal. For clarity, atoms are labelled in different asymmetric units than those in Table 13.

Initially a pseudo-Voigt profile shape function was used in the refinement, although the refinement progressed well, the peak shape was not correctly described. A close examination of peak width showed that an anisotropic broadening along the c-axis was present. In order to account for this effect a Thompson– Cox–Hastings pseudo-Voigt function [34] was used. A Lorenttzian broadening along [001] was chosen. This leads to an improvement of the reliability factors. The quality of the refinement is clearly limited in this case by the presence of a severe peak broadening, due to strain/size effects. The unit dimension cell of g-HfP are close to those of g-ZrP, being sligthly contracted in the three directions. Accordingly, the interlayer distance was found to be approximately equal in the two compounds.

g-HfP and g-ZrP are isostructural, then g-HfP has a layered structure built from PO 4 tetrahedra and HfO 6 octahedra stacked along the [001] direction. The bond distances between the metal atoms and oxygen in the ˚ ) are similar to structure of g-HfP (mean = 2.05 A the corresponding distances in a-HfP. The water molecules are hydrogen bonded to each other, forming a zig-zag chain between layers along the b-axis. On the other hand, water molecules are connected with the terminal hydroxyl groups. O9 only has a possible hydrogen contact to O7. O10 is close to ˚ ] and O8 three hydroxyl groups, O7 [3.07(3) A ˚ [3.20(3) A] in the same layer and a symmetry related ˚ ] in the other layer. By comparison with O8 [2.67(3) A g-ZrP, in this case O10 seems to link adjacent layers through hydrogen bonding to the oxygen atoms O7

M. Sua´rez et al./Journal of Molecular Structure 470 (1998) 105–119

and O8, instead of O8 and O8. However, a complete description of hydrogen bond networking will be possible only when a neutron diffraction study becomes available for these compounds. 3.3. Amine intercalation The intercalation of n-alkylamines (n = 1–6) and cyclic amines (aniline, benzylamine, cyclohexylamine, piperidine and pyridine) in g-HfP was studied in order

117

to confirm its lamellar structure and verify its behaviour as an intercalation host [35]. X-ray patterns of the intercalates show the formation of new phases, with interlayer distances of higher than that of the starting compound (Table 15) and the total disappearance of the reflections corresponding to g-HfP (Fig. 12). The formula of the intercalates was determined from elemental and thermal analysis. From the total weight loss deduced from TG curves, the

Fig. 12. X-ray diffraction patterns of (a) g-HfP and g-HfP intercalates with (b) methylamine, (c) ethylamine, (d) propylamine, (e) butylamine, (f) pentylamine, (g) hexylamine, (h) aniline, (i) benzylamine, (j) cyclohexylamine, (k) piperidine and (l) pyridine.

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M. Sua´rez et al./Journal of Molecular Structure 470 (1998) 105–119

Table 15 d-spacings, micro-analytical data (C,N) and experimental weight loss at 800⬚C of g-hafnium phosphate amine intercalates and their tentative formulae Amine

˚) d (A

%C

%N

Formula

Methylamine Ethylamine Propylamine Butylamine Pentylamine Hexylamine Aniline Benzylamine Cyclohexylamine Piperydine Pyridine

14.22 16.2 19.4 20.8 21.7 23.9 19.8 23.2 20.8 18.0 20.3

5.06 7.46 11.55 14.01 16.72 18.41 14.37 22.20 14.21 12.20 12.34

6.03 4.47 4.40 4.03 3.90 3.55 2.82 3.71 3.76 2.85 2.89

Hf(PO 4)(H 2PO 4)·2CH 5N·2H 2O Hf(PO 4)(H 2PO 4)·1.5C 2H 7N·2H 2O Hf(PO 4)(H 2PO 4)·1.6C 3H 9N·2H 2O Hf(PO 4)(H 2PO 4)·1.5C 4H 11N·2H 2O Hf(PO 4)(H 2PO 4)·1.5C 5H 13N·2H 2O Hf(PO 4)(H 2PO 4)·1.4C 6H 15N·2H 2O Hf(PO 4)(H 2PO 4)·C 6H 7N·2H 2O Hf(PO 4)(H 2PO 4)·1.5C 7H 9N·2H 2O Hf(PO 4)(H 2PO 4)·C 6H 13N·2H 2O Hf(PO 4)(H 2PO 4)·C 5H 11N·2H 2O Hf(PO 4)(H 2PO 4)·C 5H 5N·2H 2O

number of water molecules of each intercalate was obtained. The phases have a composition Hf(PO 4)(H 2PO 4)·xamine·H 2O, where x varies from 1.4 to 2 for n-alkylamines and 0.8 to 1.5 for cyclic amines (Table 15). g-HfP reacts easily with pure solutions of different amines, giving rise to new pure phases with expanded interlayer distances, which for n-alkylamines, increase with the number of the C atoms of the alkyl chain. The most striking feature of these data is that in none of the cases are the amines able to saturate all the acidic groups of the exchanger, except for methylamine which intercalates 2 mol of amine/ mol g-HfP, reaching the true saturation point. This behaviour as intercalation host makes the g-HfP an adequate precursor for the insertion of large species into the interlayer space.

Acknowledgements We wish to gratefully acknowledge the financial support of II P.R.I. Asturias (Spain), Research Project no. PB-MAT97-05.

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