Thermogravimetric study of some dichromates and molybdates of organic bases

Thermochimica Acta. 43 (1981) 221-227 Elsevier Scientific Publishing Company, Amsterdam - Printed in Belgium

THERMQGRAVIMETRIC STUDY OF SOME DICHROMATES MOLYBDATES OF ORGANIC BASES

221

AND

P. GILI Departamento

de Quimica Inorga’nica, Universidad

C. SANTIAGO,

Departamento

P. ROMAN

de Quimica,

de la Laguna, Tenerife

(Spain)

and L. LORENTE

Universidad

de Bilbao (Spain)

(Received 7 July 1980)

ABSTRACT A comparative study is made of the thermal and kinetic stabilities of the thermal decomposition of dichromates and molybdates of organic bases with formulae (BH)2Crz0, and (BH)4Mos026, respectively. The compounds show several steps of decomposition controlled by a random nucleation mechanism.

INTRODUCTION

In previous works [l-6] the kinetics of the thermal decomposition of some molybdates of organic bases have been investigated, relations being obtamed between the pK, of the bases, the initial temperatures of decomposition and the activation energies obtained by dynamic methods. A connection was also found between the initial decomposition temperatures and the symmetry of steric effects of the organic bases. Continuing a systematic investigation of oxoanions of organic bases, the kinetics of the thermal decomposition of organic based dichromates {(BH),Cr,O,} and molybdates of the same bases {(BH)4M08026) studied by thermogravimetry are reported here. The bases are: Z-methylquinoline (C-l for the dichromate and M-l for’the molybdate); 4-methylquinoline (C-2 for the dichromate and M-2 for the molybdate); 6-methylquinohne (C-3 for the dichromate and M-3 for the molybdate); and 8-methylquinoline (C-4 for the dichromate and M-4 for the molybdate). The kinetic study-has been carried out in a dynamic regime following the theoretical model of Satava [ 131.

EXPERIMENTAL

The compounds were prepared for the first time in our laboratory, the X-ray and IR characterization being described in refs. 3 and ‘7. The molybdates proved to have a low solubility in water, being soluble only in acids 0040-6031/81/0000-0000/$02.50

0 1981 Elsevier Scientific Publishing Company

222

and concentrated inorganic bases, with very low solubility in the usual organic solvents. The dichromates, on the contrary, are soluble in cold water, slightly soluble in ethanol and acetone and only very slightly in ether and benzene. They are sensitive to sunlight and humidity. With aqueous solutions as starting material, crystals were obtained in both the molybdates and dichromates. By II?, spectroscopic methods and V-UV, evidence is obtained for the existence of the protonated base in all the compounds as also for the Cr20$’ anion. The determination of the structure by X-ray in a monocrystal of a compound similar to the molybdates under study shows that it is an octomolybdate (BH)4M~8026 [S] and not a tetramolybdate as described in ref. 3, since the studies were carried out on polycrystalline samples and were represented by the simplest formulae. Both the equipment and the experimental conditions have been described refs. 9 and 10. Several runs were made on each sample with good reproducibility.

RESULTS

4ND

DISCUSSION

The stability interval was obtained for each compound studied from thermogravimetric curves (Table 1). Decomposition of the dichromates begins in the 355-362 K interval and occurs in a violent way, so the work had to be carried out at a heating rate of 1.25” C min-‘. However, the molybdates present a higher interval in the initial temperature of decomposition, 417474 K, and are less energetic than the dichromates. All the dichromates present three steps in the thermal decomposition, ‘except compound C-3 which shows five. In accordance with the weight loss, the following decomposition equations may be given for compounds C-l and C-2 1st step: (BH)&&O, +B+H?_O+B-2CrOa 2nd step: B - 2 CrOB + 2 Cr02 + B + O2 3rd step: 2 CrO, + Cr,o, + l/2 02 Compounds C-3 and C-4 follow the same behaviour pattern for the first step but differ thereafter. However, the global process is identical for all (BH)&JrzO, + Cr*O, + Hz0 + 2 B +3/2 O2 The Cr,O, is confirmed by X-ray, allowing the quantitative determination of the chrome content of the compounds to be established with great precision. CrO, - B addition compounds are well known 111,123, as is the ferromagnetic and metallic conductor CrOz. The chrome passes from an initial state (VIj to a final state (III). As for the molybdates, compounds M-l and M-4 present three decomposition steps while M-2 and M-3 show only two. In accordance with weight losses, the following model of decomposition may be given for compounds M-l and M-4 1st step: (BH)4MosOz6 + 8 Moo3 - 5/2B + 3/2 B + 2 Hz0 2nd step: 8 Moo3 - 5/2B+SMoO,-B+3/2B 3rdstep: 8MoO,.B+ 8Mo03+B

223

0 r-,

p,

° 0~ 0

0

e~

° 0~

e-, o0~

0

r~

0 0

l

|

j

m

m

w

|

w

224

For compounds M-3 and M-2, we have 1st step: (BH)4M~g026 * 8 Moos - 4/5 B + 16/5 B + 2 Hz0

2nd step: 8 Moo3 .4/5B+8MoOj+4/5B According to the experimental results the global process is (BH)3Mos026 + 8 Moo3 + 4 B + 2 Hz0 Addition compounds of general formula MoOsnB as described previously [3-53 are also presented here, but there is no reduction in the Mo(V1) in the final product, which is not the case of the dichromates. The ivfu& was confirmed by means of X-ray diffraction. Selected a vs. T. curves for the dichromates are presented in Figs. 1 and 2. The curves obtained show a reaction with an initial acceleratory period followed by a deceleratory one. In some cases such as the second step of com-

pound C-3, for example, the behaviour is more complex. The QI vs. T curves are shown for the molybdates in Figs. 3 and 4. The shape of the curves in the first step is the same for all the compounds , being of the type previously described for the dichromates. Compounds M-l and M-4 present curves in the second step of decomposition, in which there scarcely exists an accelerator-y period. Finally, in the last step the a! vs. T curves again become similar t_o those of the first step. Figures 5 and 6 show a SatavQ analysis for the first steps of the decomposition for both the dichromates and the molybdates. Similar plottings were observed for all other compounds and for all decomposition steps examined. It is observed that the straight line criterion is better fulfilled by the equation corresponding to a random nucleation: g(cx) = {-ln(l-ar)} 1’3. The ratedetermining process must be related to the structure of the compounds. In the case of the dichromates, there are two CrOz- ions of tetrahedral symmetry bonded by a vertex, while in the molybdates there are groups of 8 Moo6 octahedra which join to form polymeric chains. Although the thermal

T(K)

Fig. 1. Fraction compound C-l.

of

decomposed

T(K)

material

(0~) vs. temperature

for the first step of

Fig. 2. Fraction of decomposed material ((Y) vs. temperature for the second step of compound C-2.

225

530

500

T(K)

T(K)

Fig.

3. Fraction

of

decomposed

material

temperature for the first step of com-

(CY) vs.

pound M-l. Fig. 4. Fraction of decomposed material (Q) vs. temperature for the second step of compound M-4.

r

--3

--2

,

-ii

H --1

0

I

250

2.65

260

2.55 (l/T)

x103

(K-l)

Fig. 5.8atavG representations for the first step of compound C-l. a, D1; X, D2; 0, DJ;~, Dq;g,F1;I,Az;*,Aj;o,Rz;+,Rg 1133.

I

149

1

I

155 (i/T)xlO'

161

I

(Ki)

Fig. 6. Satavi repre-.rntations for the second step of compound the same meaning as in Fig. 5.

M-3. The symbols have

decomposition kinetics of dichromates and molybdates follow the same random nucleation process, they present different behaviour patterns regarding the activation energy values. In general, the values of this parameter are greater in the dichromates than in the molybdates. AIthough the structure of the former is simpler, the decomposition brings about changes in the states of oxidation of the metal, which must imply greater activation energies. The structure of the CrzO;- ion is also very different from that of the Cr203. Contrarily, the molybdates present no changes in the states of oxidation and no changes exist in the coordination of the metal when passing from (BH),Mo~O~~ to the Mo03. Tne values of the initial temperatures of decomposition of the dichromates indicate that the order of stability is: C3 < C2 < Cl cv C4. According to the position of the methyl group in the molecule of quinoline, steric effects should be produced and the incorporation of the methyl group in position 6 with regard to the nitrogen of the quinoline is what brings about the lesser thermal stability for the dichromate. In the case of the molybdates, the order followed with regard to thermal

227

s t a b i l i t y is: Ma ~ M4 ( M2 -~ M~. It is also o b s e r v e d h e r e t h a t t h e 6 - m e t h y l q u i n o l i n e c o m p o u n d is t h e least stmble. H o w e v e r , t h e order o f s/mbility is m o d i f i e d w i t h regard t o t h e d i c h r o m a t e s . In s t u d i e s carried o u t o n tetram o l y b d a t e s o f r n e t h y l a t e d , e t h y l a t e d a n d a m i n a t e d derivatives o f p y r i d i n e at p o s i t i o n s 2, 3 a n d 4 [ 1 - - 5 ] it is o b s e r v e d t h a t t h e p o s i t i o n o f t h e a l k y l g r o u p or t h e n m i n e g r o u p p r o d u c e s a s t a b l i z a t i o n u p o n e m i g r a t i o n o f t h e said g r o u p f r o m p o s i t i o n 2 to 4. T h e s e c o m p o u n d s h a v e d i f f e r e n t bases f r o m t h o s e s t u d i e d in t h e p r e s e n t w o r k a n d a s y m m e t r y o b s e r v e d in t h e m is n o t to be f o u n d in t h e bases u n d e r s t u d y here. R e l a t i o n s h a v e n o t b e e n o b s e r v e d b e t w e e n t h e pKa a c t i v a t i o n e n e r g y a n d initial t e m p e r a t u r e s o f d e c o m p o s i t i o n as in ref. 5.

REFERENCES 1 P. Gili, L. Lorente, P. R o m a n , M. Tello and E. Hern~ndez, An. Quim., 73 (1977) 349. 2 P. Gili, L. Lorente, P. R o m a n , C. Santiago and A.R. Arnaiz, An. Quire., 74 (1978) 731. 3 P. GUi, L. Lorente, P. R o m a n , C. Santiago and A.R. Arnaiz, An. Qulm., 74 (1978) 738. 4 M. Tello, E. Hern~ndez, P. Gill, L. Lorente and P. R o m a n , Thermochim. Acta, 12 ( 1 9 7 5 ) 65. 5 M. Tello, L. L o r e n t e , P. Romfin, P. Gill and C. Santiago, T h e r m o c h i m . Acta, 21 ( 1 9 7 7 ) 135. 6 P. Gili, P. R o m a n and I. Katrine, P o l y m . Bull., 1 ( 1 9 7 9 ) 509. 7 P. Gill, C. Santiago, L. L o r e n t e and P. R o m a n , An. Quim., in p r e s s . 8 P. R o m ~ u , J. J a ud and J. Galy, Z. Kristallogr., in p r e s s . 9 M. Tello, E. Hernfindez, M.A. Arrlandlaga and H. Arend, T h e r m o e h i m . Aeta, 11 ( 1 9 7 5 ) 96. 10 M. Alvarez, J. Icaza, E. H e r n~ nde z and M. Tello, T h e r m o c h i m . Acta, 12 ( 1 9 7 5 ) 117. 11 H. Sisler, W.C. L o h , E. M et t er and F.R. H url ey, J. Am. Chem. Soc., 75 (1953) 446. 12 M. Camelot, Rev. Chim. Miner., 6 ( 1 9 6 9 ) 853. 13 V. Satav~i, T h e r m o c h i m . Acta, 2 ( 1 9 7 1 ) 423.