Hydrothermal synthesis and characterization of vanadyl alkylphosphonates VORPO3 · H2O

Materials Chem&y

and Physics, 35 (1993) 199-204

199

Hydrothermal synthesis and characterization alkylphosphonates VORP03 HZ0

of vanadyl

l

G. Huan*, J.W. Johnson, J.F. Brody and D.P. Goshorn ExxonResearch and Engineering Company, Annandale, NJ 08801 (USA) A.J. Jacobson Department of Chemist?y, University of Houston, Houston, TX 77204 (USA)

Abstract P03).yH,0 @=1.5, l4 have compositions and structures identical to those of the compounds prepared via water treatment of the which were reported previously. benzyl alcohol intercalation compounds VO(C,H%+, P03).H,0.C&CH20H,

Introduction The vanadium organophosphonates are members of a large class of metal organophosphonates and phosphates that have attracted attention because of their interesting structural chemistry, sorption and catalytic properties [l-lo]. In general, these compounds form layered structures with alternating organic and inorganic layers, which in some instances can show further interlayer reactivity, including ion exchange reactions and intercalation of neutral molecules. The composition and structure of a specific metal organophosphonate are controlled both by the size of the organic group and by the synthesis method used. Molecular species present in the reaction mixture can act as templates and direct the synthesis toward a specific structure type. In a previous paper [9b], we described the synthesis of a series of vanadyl alkylphosphonates with the general composition VO(C, H, + 1PO,). H,O . C,H,CH,OH (n = 1 to 12, 18). The compounds were synthesized by reaction of V,05 with the corresponding alkylphosphonic acid in benzyl alcohol. The products of the reaction contain one molecule of benzyl alcohol per formula unit intercalated between the vanadium organophosphonate layers. The presence of benzyl alcohol during the reaction directs the synthesis toward the formation of a layered structure with the same connectivity as that found in the mineral newberyite *Present address: Carus Chemical Co., 1001 Boyce Memorial Drive, Ottawa, IL 61350, USA.

(MgHP0,*3H20) [ll]. The intercalated benzyl alcohol can be removed from the structure by heating at 140 “C without any structural rearrangement other than a contraction of the interlayer separation. In contrast, we have found that when the intercalated benzyl alcohol is removed by treating the compounds with a large excess of water, a structural rearrangement occurs. The final composition, VORP03 +HzO, obtained by water washing is identical to that obtained by thermal treatment. The rearrangement is observed for the alkyl compounds, but not for the aryl derivatives, which lose benzyl alcohol without a structural rearrangement of the layers upon water treatment [9a]. In order to further investigate the transformation and the relationship between the two structures, we have synthesized the compounds VO(C,H, + ,PO,) . yH,O under hydrothermal conditions. The results of the two synthetic approaches are compared in this paper.

Experimental ThecompoundsVO(C,H,+,PO,).yH,Ofor 1
200

and Vz03 (0.300 g, 4 mmol) were transferred to the autoclave. Distilled water was added to bring the volume to 70% ‘of the total. The autoclave was sealed and heated for two days at 200 “C. The autoclave was then cooled to room temperature and the reaction product recovered by filtration, washed with distilled water and dried in air. For compounds with IZ< 3 blue crystals in the form of thin plates were recovered; when IZd 4 the product was microcrystalline, but with much smaller crystal size, Solid yields were typically in the range 60-70% based on vanadium. The compositions of the products were determined by elemental analysis (Galbraith Laboratories) and by the~ogra~met~c analysis in flowing helium at a rate of 10 “C mind1 using a DuPont 900 thermal analyzer. The analytical results are given in the Appendix and in Table 1. Powder X-ray diffraction measurements were made using a Siemens D500 diffractometer with a diffracted beam mon~hromator. Magnetic susceptibility measurements were made from 5 to 300 K using either a Quantum Design model MPMS SQUID magnetometer or a George Associates Faraday magnetometer. Attempts were made to grow single crystals of the alkyl compounds by varying the synthesis conditions. Only for the methyl compound were we able to obtain crystals of sufficient quality to attempt to solve the structure by single-crystal X-ray diffraction. After evaluation of many crystals, a data set was collected by Molecular Structure Corporation on a turquoise plateshaped crystal of VO(CH,PO,) - lSH,O with dimensions 0.50 x 0.30 x0.02 mm. Measurements were made on a Rigaku AFC6R diffractometer with MO ISo radiation and a 12 kW rotating anode generator. Although a data set for structure solution and refinement was obtained, the structure could be refined only to an R factor of 11% because of the poor quality of the crystal. TABLE n

1. VO(C,,Hz,+,P03)-yHr0 Weight

loss”

13.90 13.41 12.67 8.15 7.55 7.23 6.99 6.30

Y

390 300 220 280 320 300 310 310

Results Elemental analysis of the samples prepared hydrothermally confirmed that the compositions correspond analto VQ(C,Hz,+r PO,) *yH,O. Thermogravimetric ysis in helium indicated two different types of behavior. For the compounds with n ~3, the first weight loss begins at 50 “C and ends below 400 “C. A second step occurs at higher temperature and corresponds to the decomposition of the alkyl chain. The first weight loss apparently occurs in two steps, indicating two types of bound water molecules. Results for the methyl compound are shown in Fig. 1. The overall weight loss corresponds closely to 1.5 H,O per formula unit for the n < 3 compounds. In contrast, the compounds with IZ> 4 show a weight loss at higher temperatures (150-320 “C) corresponding to the loss of one water molecule per formula unit. The ~om~si~ons determined by thermogravimetric analysis are given in Table 1. The

compounds

(%) 1 2 3 4 5 6 7 8

The thermal parameters of the non-hydrogen atoms varied over a wide range and the structure could not be refined anisotropically. The partial structure solution was sufficient to reveal the connectivity of the V-P-O layers and gave reasonable values for the bond lengths. A complete solution, however, will require a single crystal of better quality. A second set of samples was prepared by treatment of the benzyl alcohol intercalation compounds VO(C,H %+ ,PO,) . H,O . C6HsCH,0H [9b] with a large excess of water at 60 “C, filtering, and then vacuum drying at room temperature. Thermogravimetric analysis confirmed the removal of the intercalated benzyl alcohol and the presence of one water molecule per formula unit. The interlayer spacings were determined by powder X-ray diffraction.

1.45 1.51 1.52 1.08 0.99 1.01 1.02 0.97

‘I0’ db

d'

(A)

(4

8.304 9.70 11.10 15.48 17.77 19.50 21.64 23.72

11.26 13.77 15.58 17.89 19.45 21.66 23.79

aWeight loss below T(“C). Decomposition of the alkylphosphonate begins above T (“C). bLayer spacings for compounds prepared by hydrotherma1 synthesis. ‘Layer spacings for compounds prepared by water treatment of VO(C,H,+,PO,) .H,O.C&iJCHZOH phases.

100

90 %

%

60

70

60”““” 0

260

400 Tempotature

600

600

1000

f’C)

Fig. 1. Thermogravimetric analysis data for VO(CH3POS) and VO(CSHllP03)~ l.OH,O.

1l.SH,O

201

thermogravimetric analysis data for the C, compound, which are typical of the II a4 compounds, are also shown in Fig. 1. Powder X-ray diffraction data for all samples show strong 001 series of reflections characteristic of the interlayer separation. In some cases mixed reflections were observed, but in general it was not possible to one case, determine unit cells. In VO(GH,PO,) . 1.5H20, a reasonable fit to the data was obtained with a unit cell related to that of VO(HPO,),.0.5H,O and of VO(CH,PO, .1.5H,O (see below), with cell constants a = 22.209(5) B , b = 7.459(2) 8, and c = 9.399(4) A. The interlayer separations versus carbon number (n) are given in Table 1 and Fig. 2 for the complete series of compounds. The data show a pronounced discontinuity between n = 3 and n = 4. For n =4 and above, the data are fit by the relationship d (A) = 7.412+ 2.035n, where d is the interlayer separation. The corresponding relationship when n ~3 is d (A) = 6.905 + 1.398~ The layer spacings for the second set of samples prepared from the benzyl alcohol intercalation compounds are also shown in Fig. 2 and given in Table 1. The relationship between the layer spacing d and n for this set of samples is given by d (A) = 7.407+ 2.044n, but the data do not show a discontinuity at n = 3 as is observed in the first series. The magnetic susceptibility data also show distinct differences between the C-C, and C,-C, compounds. Results for x versus T are shown in Fig. 3 for the compounds with n = 2, 3, 5 and 7. The data for n =2 and 3 show a maximum in the susceptibility versus T characteristic of antiferromagnetically exchange-coupled dimers. The data for these samples were consequently analyzed as described previously using the expression

Ood

01 (4

T

(K)

I

I

67

E

.i~.~

300

(K)

T

/

300

200

O

\ k

=‘-

IK)

100

r

I

’

200 T

I

I

I

/4

3.0_

m

0,

/

2.0

-;j/(, 0 @)

50

100 T

,; ; 150

200

250

300

(K)

Fig. 3. Magnetic susceptibility data compounds with (a) n =2, VO(C,H, + IPO,).yHZO bottom; (b) n = 5, top, n = 7, bottom.

X”XO+cil(

Variation of the interlayer separation for Fig. 2. compounds prepared by hydroVO(C. Hz~+ 1P03) .Y Hz0 synthesis (0) and by water treatment of thermal VO(C,H2n+,P03) .HZO.C,H,CH,OH phases (Cl).

T - 6) + 4C,/{T[3 +

exp( - 21/k,

for the top, n = 3,

7+)]>

For the phases with IZ= 2 and 3, the low-temperature data were insufficient to obtain a reliable value of 8, and consequently 0 was assumed to be zero. The final fits to the data for the compounds with n =2 and 3 are shown in Fig. 3(a). The magnetic susceptibility data for the compounds with n =7 and 5 are similar and

202

were fitted to the Curie-Weiss expression, x=x0 + C/ (T- 0), for temperatures greater than 40 K. Evidence for weak antiferromagnetic coupling is observed in the departure from Curie-Weiss behavior below 40 K (see Fig. 3(b)). The n =5 sample was prepared by the hydrothermal method and the n = 7 compound via the benzyl alcohol intercalation compound. The results of the fits to the magnetic data are given in Table 2 together with the data for VO(CH,PO,). 1.5H,O reported previously. As described above, the single-crystal data from VO(CH,P03) - 1.5H,O could not be refined sufficiently well to warrant a detailed description of the structure. More than twenty crystals were surveyed from several different syntheses in an attempt to find a better crystal and improve the quality of the structure solution. All of the crystals were extremely thin plates, often intergrown, and so far we have been unable to find a synthesis route through which the crystal quality can be improved. Nevertheless, in spite of some uncertainties we believe that the basic structure is correct. The partial structure is important in providing a description of the connectivity of the vanadium, phosphorus and oxygen atoms in the layers. The unit cell is apparently monoclinic, space group P2,/u, though a higher-symmetry orthorhombic C-centered space group seems metrically correct and could not be completely ruled out. The monoclinic unit cell parameters are a = 17.281(5) A, b =7.499(6) A, c=9.415(4) A and /3 = 106.04”. The structure of VO(CH,P03) - 1.5H,O consists of layers of VO, octahedra connected through comers by PO& tetrahedra stacked along the a direction. Methyl groups extend out from both sides of the layer, resulting in a layer repeat distance of 8.30(2) A. The layer is formed with face-shared V,0s(H20) dimers connected through comers by PO$ tetrahedra. The overall structure of VO(CH,PO,). 1.5H,O desimilar to that of scribed above is very VO(HOP03) .0.5Hz0 [12], with the methyl groups of the methylphosphonate compound replacing the OH groups of the hydrogen phosphate and one extra water molecule per formula unit occupying interlayer sites in the structure of the methylphosphonate. The in-plane

TABLE ?I

1” 2 3 5 7

2. Magnetic susceptibility data for VO(C,,Hzn+IPOs)*yH20

unit cell parameters 6 and c of VO(CH,PO,) - 1.5H,O (7.499 and 9.415 A) are very similar to the corresponding a and b parameters of VO(HOP0,) .0,5H,O (7.420 and 9.609 A), as is expected. The magnetic properties of the two compounds are nearly identical, providing additional confirmation of the general features of the structure of vanadyl methylphosphonate.

Discussion

A series of layered compounds of general composition 0)=1.5, 102~3; y=l.O, VO(C,H,+, PO,).yH,O 4 Q n G 8) has been synthesized by hydrothermal reaction of V,O, and the corresponding phosphonic acid in water at 200 “C. Oxidation of vanadium from V(II1) to V(IV) occurs during reaction, presumably by dissolved oxygen. The same products can be obtained using V(IV) starting materials (e.g., VOSO,), but the product crystallinity is lower than when the less soluble V,O, is used. The products are obtained as microcrystalline solids with individual crystallites in the form of very thin plates. It has not proved possible to grow good single crystals for diffraction studies, despite many attempts. Consequently, a description of the structures of the compounds rests largely on indirect measurements, the partial structure of VO(CH,PO,) .1.5H,O, and analogy with the known structure of the hydrogen phosphate, VO(HPO,)*0.5H,O [12]. The layer spacings shown in Fig. 2 indicate that the compounds fail into two classes: (i) phases with 4 in Q 8, where the interlayer spacing increases by 2.04 8, per CH, unit in the alkyl chain and with an intercept of 7.41 A at II = 0, and (ii) phases with 1 Qn Q 3, where the increase in interlayer spacin is 1.40 8, per CH, unit and the intercept is 6.905 x . The larger lattice constant increase observed for the first class is consistent with the formation of a bilayer of alkyl chains in the interlayer region with the chains inclined at an angle of 53.4” to the layers. This chain angle implies that the P-C bond is nearly perpendicular to the layer. Thermogravimetric analysis shows the presence of one water molecule per vanadium atom. The water molecule

compounds

G (cm’ K g-‘)

Ufka

ci

(K)

(cm3 K g-l)

2.00x 10-3 1.89 x lo-’ 1.75 x 1o-3 1.604x 1O-3 1.288x lo-’

-43.8 -51.6 - 52.0 -

5.26 x lo-’ 2.82x lo-’ 2.40 x lo’-’ -

“Data from ref. 9(e).

e

-

5.3” 0 0 - 11.0” -5.6”

x0

Peff

(cm3 K g-‘)

(&I)

-4.44x -9.60x -7.86x -5.32x -2.69x

1.75 1.75 1.74 1.74 1.65

lo-’ lo-’ lo-’ lo-’ lo-’

203

is relatively strongly bound and is not released until 150-320 “C in the~o~avime~ic experiments at a heating rate of 10 “C min-‘. The magnetic data for the n =5 compound show the presence of weak antiferromagnetic interactions, but do not indicate the formation of the vanadium dimers observed in both VO(HP0,) -0.5H,O and VO(CH,PO,) .1.5H,O. For impounds with 4
structures is apparently easier for the alkyl than for the aryl compounds. For n d 3, the VO(HP0,) +OSH,O structure is apparently the more stable, even though compounds with n =2 or 3 can be made with either structure type. The present results show that the specific compositions and structures obtained for the vanadium alkylphosphonates are sensitive both to the intralayer V-P-O bonding and to the interactions between the organic groups in the interlayer. The balance between the covalent intralayer bonding and the van der Waals interactions between the organic groups results in a diverse chemistry for the vanadium organophosphonates and for other similar classes of layered organic-inorganic com~unds.

Acknowledgements One of us (A.J.J.) wishes to thank the Robert A. Welch Foundation for partial support of this work.

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Appendix Analytical data n=2: C, 11.52%; H, 4.23%; V, 26.63%; P, 15.53%; calculated for C&H,VPO,,: C, 11.89%; H, 3.99%; V, 25.22%; P, 15.33%; n=3: C, 16.05%; H, 4.98%; V, 24.52%; P, 14.49%; calculated for ~H,,VPO,,s: C, 16.68%; H, 4.67%; V, 23.58%; P, 14.34%; n=4: C, 21.57%; H, 5.24%; V, 23.87%; P, 14.26%; calculated for C4HllVP05: C, 21.74%; H, 5.02%; V, 23.05%; P, 14.01%; n=6: C, 28.72%; H, 6.31%; V, 19.50%; P, 11.98%; calculated for C&,,VPO,: C, 28.93%; H. 6.07%; V, 20.45%; P, 12.43%; n=7: C, 32.14%; H, 6.81%; V, 19.45%; P, 11.83%; calculated for cH17VP05: C, 31.95%; H, ,6.51%; V, 19.36%; P, 11.77%; n=8: C, 36.03%; H, 7.30%; V, 17.68%; P, 11.88%; calculated for C8H19VP05: C, 34.67%; H, 6.91%; V, 18.38%; P, 11.18%.