Scientific Bases for Preparation of Oxide Supports and Catalysts Via Sol–Gel Methods

9 1998Elsevier ScienceB.V. All rights reserved. Preparation of CatalystsVII B. Delmonet al., editors.

593

S C I E N T I F I C BASES FOR PREPARATION OF OXIDE S U P P O R T S AND CATALYSTS VIA SOL-GEL METHODS

Oleg P. Krivoruchko B o r e s k o v Institute o f Catalysis, Pr. A k a d . L a v r e n t i e v a 5, N o v o s i b i r s k 630090, R u s s i a 1. I N T R O D U C T I O N Poorly soluble hydroxides are widely used to prepare catalysts, supports, adsorbents and other materials. An understanding of the mechanism of hydroxide formation at the level of sol-gel processes and gel evolution on its ageing in mother solutions permits one to control their properties such as phase composition, dispersion, and porous structure that are important for catalysis. Before the publication of our works, the mechanism of hydroxide formation was usually treated from two standpoints. Regarding the first one, hydrolysis of aqua cations in solutions is a step-wise process and "monomer molecules" of hydroxides form at the initial stage. When the rate of such "molecules" formation is high, they produce hydroxide precipitates in the amorphous state/phase. The second view holds that hydroxides form via a permanent growth of inorganic polymer chains to the point when their size and molecular weight range up to that of colloid particles. The latter approach suggests a wide range of polymer complexes in the solution at every instant. It is well known that hydrolysis is always complicated by formation of polynuclear hydroxo complexes (PHC) on the interaction between aqua cations (III, IV) and bases. For a long time, complex hydrolytic reactions occurring in solutions and the mechanism of hydroxide formation were studied in parallel as two independent problems. We have combined both studies from a conceptual point of view, since the results of these studies exhibit the regulations of separate stages of a single process. We have established [1] a new, not known earlier mechanism of formation of poorly soluble hydroxides and formulated the theoretical concepts, providing an explanation for relations and dependences between the stages of genesis of hydroxides and oxides on catalyst synthesis. Thus, we state that PHC of precipitated metals act as intermediate materials on hydroxide formation. As was shown, a limited number of complexes forms on polycondensation. In the course of mutual conversions, "key" polynuclear hydroxo complexes (KPHC) are formed and accumulated in solutions to produce sol particles. Consequently, the "key" complexes are not engaged in the formation of complexes with a different nuclearity and structure at the following steps of the hydroxide formation. In this report, we present the data related to formation of individual and binary hydroxides of AI(III), Fe(III), and Cu(II) using the sol-gel methods.

594 2. F O R M A T I O N PROCESSES

OF INDIVIDUAL

HYDROXIDES

VIA SOL-GEL

2.1. Hydroxides of AI(III) Aluminium hydroxides are extensively used to manufacture supports and catalysts. For this reason, they are the subject for numerous studies, primarily, of empirical nature. Nevertheless, the mechanism of formation and the structure of amorphous AI(III) hydroxides are not sufficiently elucidated. For any method and precipitation conditions, the interaction between a precipitated metal salt and an alkaline, the system passes through a number a states which are characterized by various a~ = [OH-]/[AP+]. A particular distribution of AI(III) complexes with a different nuclearity corresponds to every aa. 27A1 N M R permits one to follow separately the complexes containing 1, 2, and 13 atoms of aluminium (All, Alz, and Al~3, respectively). The structure of complexes AI2 and AI~3 have been determined elsewhere [2,3]. ZVAI N M R lines were attributed in [4]. On polycondensation of concentrated solutions of AI(III) salts, Alp forms which is not observed in the 27A1 N M R spectra. We suggest the following experimentally substantiated structural and crystallochemical model of this complex in solution: AlyO2(OH)14(H20)103+ ( denoted as AIT). The complex contains 1 atom of aluminium in a tetrahedral coordination and 6 atoms in an octahedral coordination with respect to oxygen [5]. For diluted salt solutions ([AI(III) < 0.3 mol/l), as aa increases, a fraction of (x (All + A12) decreases and a fraction of the key complex ix(All3) increases. The limiting value is 2.3. When a~ is higher than the limiting value, sol particles (SP) of hydroxide begin to form from the key PHC of AI(III). This is evidenced by an increase in the relation between longitudinal and cross times of nuclear relaxation of water deuterium (T1/T2), a broadening of the 3sC1NMR line and by a decrease in the fraction of complex Al~3 in solutions (Fig. 1). The products of polycondensation of aqua ions of AI(III) are different in concentrated solutions. Here, AI7 is the key complex. However, polycondensation processes proceed in a similar way [6]. Af,

I ~~-"~'-~-~ ~ "-vTi - - - v of.,

1

"-5

"-"

i

+------% 1

,

0

'

.

2.,..,.f-

Hz

v,

I

.

"

,

1

.

i

I9

!

.Ip

Tq, T 2 ,ITIS 50O

1 Ill I/

.

.

.

,

2

ee

Figure 1. Polycondensation in solutions at [A1C13] = 0.2 mol/l and parameters of N M R spectra as a function of a~ - [OH-]/[Al(III)]: 1. tx (Air + A12); 2. ct(Alt3); 3. Af(35C1); 4,5. 2D(T~, T2). pH = 3.0-3.7 (I), 3.7-4.0 (II) and 4.0-4.2 (III).

595 When a base is added to an aluminium salt solution, polycondensation may follow the routes (1) Al(H20)63+ ~ Alz(OH)2 (H20)84+ +--~A1,304(OH)24(H20)i27+ --+ sol particles (SP~) and (2) AI(H20)63+ ~ A12(OH)2(H20)84+ 4+ A1702(OH)14(H20)lo 3+ --+ sol particles (SP2). Schemes (1) and (2) exhibit the limiting cases for diluted and concentrated solutions of AI(III) salts. In the general case, both KPHC of AI(III) exist on polycondensation in solutions. The key complexes of AI(III) form true solutions. At the stage preceding the formation of sol particles, the key complexes of AI(III) strongly bonded with anions of the mother salt pass together into a volume of SP. These anions are structural elements of SP and can not be removed by sediment washing. The anions and SP surface form a weakly bonded layer of counter ions, which is in the state of quick exchange with solution anions (a broadening of line Af(35C1) in Fig. 1) and can be easily removed on precipitate washing. It was found that the chemical nature of the initial salt anions affects essentially both the direction of polycondensation processes of aqua cations and the whole ensuing evolution of hydroxides on their ageing. Thus, sulfate ions hinder formation of Al13 complexes, and can completely suppress it when the concentration is sufficiently high. Anions that can enter the first coordination sphere of the aqua cation considered affect most essentially the direction of polycondensation processes and the structure of PHC formed. The effect of anions on the polycondensation of aqua ions of AI(III) and the properties of gels decreases in a series: SO42- >> CI- = NO3- > C104-. Using the RDA technique, calculations of simulated models [5] and SAXS [7], we have shown that AI~3 and AI7 pass from the solution into gels of AI(III) amorphous hydroxides with no detectable changes in their structure and size (18 and 15 .~, respectively) at pH = 4-10 and T < 50 ~ Hence it follows that (1) a minimum, theoretically attainable size of sol particles is determined by sizes of key complexes of precipitated metals, and (2) gels of AI(III) hydroxides "inherit" the structure of key complexes of AI(III). Note that an interaction between several key complexes yielding particles of AI(III) hydroxide sols 30-50 A in size is more typical. In later works, the authors used [8-10] our methodology to study experimentally polycondensation of aqua ions of AI(III) and reproduced a number of our results. At the close of gel formation at pH - 7-10, T > 60 0C, a series of complex physicochemical processes begins spontaneously. Crystallization of AI(III) hydroxides is the most important one. The process is accompanied by significant changes in the chemical and phase compositions, particle dispersion, specific surface, and porous structure of hydroxides. We have established the mechanism of crystallization of poorly soluble hydroxides. Crystallization proceeds in a volume of every primary particle via the rearrangement of its polymer structure and growth of secondary crystals and by oriented accretion of the primary, partially crystallized particles over the similar faces of the other particles [1], rather than via dissolution of highly disperse primary particles. The phase composition of crystallization products is determined by the nature of key complexes yielding an amorphous hydroxide gel. Thus, the gels obtained from Al~3 or A17 transform into bayerite and pseudoboehmite, respectively, during ageing, the other conditions being the same [11].

596

2.2. Hydroxides of Fe(llI) The main stages of formation of gels of amorphous hydroxides of Fe(III) are (3) Fe(H20)63§ ~ Fe2(OH)2(H20)84§ ++ Fep --> sol particles (primary particles, PP) --+ disordered aggregates of PP (gel). The key complexes of Fep composed of double chains of octahedrons [FeO6] are joint at the edge (Fig. 2). According to the number of oxo and hydroxo groups, and coordinately bonded water per 1 atom of iron, the composition of Fep complexes is FeOOH 9H20. Antiferromagnetism of PHC of Fep indicates a high ordering in the state of Fe(III) cations in complexes. When polynuclear hydroxo complexes Fep transfer to the volume of sol particles, their structure and composition do not practically change. Gels of amorphous hydroxides of Fe(III) are composed of disordered aggregates of PP with a narrow size distribution (35-45/~). The size of PP remain almost constant (40 A) under a wide range of precipitation conditions: pH = 4-12, T = 25-100~ and [Fe(III)] = 0.01-2.0 g-ion/1. The mechanism of formation of amorphous hydroxides of Fe(III) from PHC of Fep permits one to predict that the volume of primary particles, visually observed as a unit, contains some substructure. Analyzing the curves of radial atom distribution, and using the bright image and dark image of TEM techniques, we have discovered local, structurally ordered regions 10/k in size. The atom distribution in such regions is characteristic of cz-FeOOH (goethite) [12]. These ordered regions are not oriented relative to each other in the volume of primary particles, which predetermines the possibility of micropore formation on dehydration of Fe(III) hydroxides.

A

OH2

I~o-

A

i I

~o

H

I I

OH2

~''" ' ~ o

H

OH2

o

1

H

_o....

-~I" i ~ o / .

H O

i H~O--F9

1 I

l i I

H

OH2

-- i

0 -- ~ ~c~O

p

.

H

OH2

H

OH2

6 H

Figure 2. A structure fragment of key complex Fep. A: the projection of complex Fep along its axis. As in the case of Al(III) hydroxides, the anions of the initial salt participate in polycondensation of aqua ions of Fe(III) and SP formation yielding strong and weak bonds. The anions strongly bonded with the key Fep complexes pass to a SP volume, and weakly bonded anions form a layer of counter ions on the SP surface. On coagulation of sol particles, the latter carry away such anions to the gel volume. The nearest order of atom distribution in the structure of key Fep complexes is similar to that of goethite crystals. So, we have predicted and experimentally proved that on ageing of such gels, precisely hetite forms but not some other structural

597 modification of Fe(III) hydroxides [12,13]. On the basis of the theoretical concept on the mechanism of crystallization of poorly soluble hydroxides, we have developed an approximate kinetic model: do~/dt = K(1 - c~)2 (c~ is a fraction of the crystal goethite phase). This model adequately describes all experimental data on gel ageing in alkaline media at 25-90 ~ Parameters Ko = 7.085 10 ~ and E = 76.8 + 2.6 kJ/rnol were determined elsewhere [13]. After drying at 110 ~ gels of Fe(III) hydroxides (xerogels) composed only of disordered PP aggregates have a surface of 400-500 mZ/g and micropores 10-20 A in size. In the course of ageing, rough secondary crystals (SC) are formed and accumulated in gels, which leads to appearance of meso- and macropores in xerogels and a decrease in their surface area down to 20-50 m2/g. The surface area of such samples is proportional to the content of highly disperse PP of amorphous Fe(III) hydroxide. Thus, a knowledge of the mechanism and kinetics of crystallization of amorphous Fe(III) hydroxides permits one to manufacture (using one technological scheme) a wide range of hydroxides and even oxides with the following parameters: S = 20-500 m2/g, the overall pore volume is 0.1-1.0 cm3/g, size of pores ranging from 10 to thousands A. 3. F O R M A T I O N PROCESSES

OF BINARY HYDROXIDES

USING SOL-GEL

3.1. Hydroxides of AIOII) - Fe(IH) The results above indicate that the mechanism of formation of amorphous poorly soluble hydroxides of AI(III) and Fe(III) has much in common, although the composition and structure of key complexes have a number of distinctions, resulting from a difference in the chemical properties of the precipitated aqua cations. Both facts are important for understanding the mechanism of formation of hydroxides from mixed solutions of initial salts of AI(III) and Fe(III). Because the PHC formed on separate polycondensations of aqua ions of AI(III) and Fe(III) differ essentially in their composition and structure, the primary particles comprising both cations should not form in mixed solutions. The data on the NMR studies, NM relaxation and magnetic susceptibility (7;) show (Table I) that polycondensation of aqua ions of Fe(III) begins first in mixed solutions of Al(III) and Fe(III) salts. It proceeds up to formation of sol particles of Fe(III) hydroxide. Then polycondensation of aqua ions of AI(III) begins. This is proved by the facts such as decrease in the relative magnetic susceptibility (1), increase in a line width Af (3sC1) and the T~/T2 ratio for ~H (2), decrease in a relative intensity (I) of a~ 35C1 N M R line (3), and absence of polycondensation processes of AI(III) aqua ions (4), with a ranging from 0 to 0.9. As in the case of individual solutions, the polycondensation of every component of mixed solutions proceeds via the same stages and intermediate complexes. Sol particles of Fe(III) hydroxide contain no detectable amount of AI(III) ions and vice verse. When a molar ratio is AI/Fe > 1, hydroxo complexes of AI(III) stabilize sol particles of Fe(III) hydroxide . The latter begin to coagulate when the larger part of AI(III) particles converts to sol particles of AI(III) hydroxide. At AI/Fe <_ 1, a sol of Fe(III) hydroxide is not stabilized and begins to precipitate even before the polycondensation of aqua ions of AI(III). Thus, the interaction of precipitated components in the AI(III)-Fe(III) system is at the level of primary particles of the individual hydroxides of AI(III) and Fe(III) within the

598 framework of mixed aggregates of the primary particles. For A1/Fe > 1, the composition of mixed aggregates (forming gels) corresponds to a ratio of precipitated metals in the initial solutions. When AI/Fe <_ 1, two types of aggregates are precipitated. The first is enriched with PP of Fe(III) hydroxide, the other - with PP of AI(III) hydroxide [14]. On ageing, gels of AI(III)-Fe(III) hydroxides undergo the processes as follows: (1) dehydration, (2) partial crystallization in the volume of PP, (3) decomposition of the mixed aggregates of primary particles, yielding segregation and interaction of particles with a similar chemical nature and structure, (4) formation and growth of secondary crystals via the mechanism of oriented accretion. Table 1. Properties of chloride solutions with A1 : Fe = 3:1 ratio regarding aa = [OH-]/[AI(III)] + [Fe(III)]. [AI(III)] + [Fe(III)] = 0.5 g-ion/1 a?

pH

~

NMR27AI oc(Allq-A12) oc(All3)

NMR3SCI

IH- R,ms

I

Af, Hz

T~

T2

0

1.60

1.00

0.95

0

1.00

240

1.30

0.5

0.3

1.70

0.70

1.00

0

0.80

560

2.55

0.8

0.6

2.20

0.35

1.00

0

0.50

785

29.5

5.5

0.9

3.30

0.30

0.80

0

0.50

940

260

18.5

1.2

3.40

0.30

0.75

0

0.55

790

295

13.5

1.5

3.55

0.30

0.65

0.10

0.50

940

330

15.5

1.8

3.65

0.30

0.35

0.15

0.45

1165

285

14.5

2.0

3.70

0.30

0.25

0.30

0.50

1285

270

11.0

2.2

3.85

0.30

0.15

0.35

0.50

1450

240

10.5

2.4

gel

0.30

0.05

0.25

0.40

1300

160

5.5

3.2. Hydroxides of Al0II)-Cu(II) Poorly soluble binary hydroxides of two- and three-valence metals (M(II)M(III)) are very interesting as precursors for synthesis of complex oxide supports and catalysts which have, in particular, a spinel-like structure. To elucidate the mechanism of such systems formation, one should study the polycondensation of aqua cations in mixed solutions of their salts. It is well known that mononuclear hydro• complexes and dimers are dominating on hydrolysis of individual salts of M(II). For polycondensation of M(III), formation of multinuclear complexes is typical. This difference should be also observed for polycondensation of mixed solutions of salts of two- and three valence metals, and consequently, at the stage of formation of gels of coprecipitated hydroxides. Consider sol-gel processes proceeding on polycondensation of mixed solutions of salts AI(III)-Cu(II) (Table 2). At the initial stage of polycondensation, aqua cations form labile aluminium-copper complexes with an existence of about ms. This is

599 indicated by a five-fold increase in the line width Af(All) at m = 0-0.5, which is associated with a chemical exchange of 27A1nuclei between the states: (4) AI(OH) 2+ + Cu(OH)l++-> AI(OH)2Cu 3+ Further the nuclei rearrange themselves into some other complexes. At the stages preceding formation of gels of binary hydroxides, the key complexes A113 and AI7 , and heteropolynuclear hydroxocomplexes (HPHC) Alp-Cu(II) are formed. It is interesting that at pH ranging from 3.0 to 5.0, the gels of such hydroxides are formed by aggregation of the above key complexes, bypassing the stage of sol particle formation. This is evidenced by constant Af(14N) and T1/T2 at ~ = 0 to 2.7 (Table 2). On ageing of gels at pH = 6.0-11.0 and T = 25-90 ~ All3, AI7, and Alp-Cu(II) complexes produce bayerite, pseudoboehmite, and hydroxoaluminate of Cu(II), respectively, with a structure of hydrotalcite. Table 2. N M R and ESR spectra of mixed salt solutions at [A10NIO3)3] = 0.3 and [Cu(NO3)2]= 0.025 mol/l regarding ee = [OH-]/[AI(III)]. a~

pH

NMR 14N, Af, Hz

NMR27AI

EPR Cu(II), I

oc(Al,+Alz) oc(Alt3) Af(All),Hz

2D- R, ms

Tt

T2

0

2.80

1.00

0

56

19

1.00

308

315

0.5

3.55

0.85

0

276

17

1.20

-

325

1.0

3.65

0.75

0.05

318

-

0.50

324

290

1.5

3.75

0.60

0.20

366

-

0.65

340

340

2.0

3.90

0.30

0.45

418

17

0.70

-

325

2.2

3.90

0.30

0.40

452

-

0.70

350

340

2.5

4.05

0.05

0.70

500

-

0.55

360

355

2.7

4.10

0.01

0.80

753

21

0.55

376

345

4. C O N C L U S I O N In this report, we have considered the sol-gel processes occurring on the formation of individual and binary, poorly soluble hydroxides, materials-precursors of supports and catalysts, which contain AI(III), Fe(III) and Cu(II). Sol particles and hydroxide gels form from the key complexes of the precipitated metals. We have established a very important phenomenon of "inheritance" of the structure of key polynuclear hydroxocomplexes by hydroxides, which determines a genetic relation of the nearest order between gels of amorphous hydroxides and the structure of the corresponding PHC. The structure, chemical composition, and arrangement of the functional atom groups of the key PHC carry an information about

600 the evolution of amorphous hydroxides on ageing up to the point when they transform to the crystal state. The number of crystal phases formed on ageing corresponds to the number of structurally and chemically detectable versions of amorphous hydroxides in precipitates. When one knows the structure of the key PHC and their concentration in the parental solution, it is possible to predict quantitatively a phase composition of the ageing products.

References 1. 2. 3. 4.

R.A.Buyanov and O.P.Krivoruchko, React. Kinet. Catal. Lett.,35(1987)293. G.Johanson, Acta Chem. Scand., 16(1962)403. G.Johanson, Arkiv. Kemi, 30(1963)321. J.W. Akitt, N.N. Greenwood, B.L. Khandelwal and G.D. Lester, J. Chem. Soc. Dalton Trans.,No. 5(1972)604. 5. T.A. Kriger, O.P. Krivoruchko, L.M. Plyasova and R.A. Buyanov, Izv. SO AN SSSR, Set. khim. nauk, No. 7(1979) 126 (in Russian). 6. M.A. Fedotov, O.P. Krivoruchko and R.A. Buyanov, Zh. Neorg. Khim. 23(1978)2326 (in Russian). 7. O.P. Krivoruchko, V.N. Kolomiichuk and R.A. Buyanov, Zh. Neorg. Khim., 30(1985)306 (in Russian). 8. J.Y. Bottero, J.M. Gases, F. Fiessinger and J.E. Pokier, J. Phys. Chem., 84(1980)2933. 9. D. Muller, W.Gessner, S.Schonherr and H. Gorz, Z. anorg, und allg. Chem.,483(1981)153. 10. R. Bertman, W. Gessner, D. Muller, H. Gorz and S. Schonherr, Z.anorg. und allg. Chem., 525(1985)14. 11. O.P. Krivoruchko, R.A. Buyanov, M.A. Fedotov and L.M. Plyasova, Zh. Neorg. Khim.,23 (1978) 1798. 12. T.A. Kriger, O.P. Krivoruchko and R.A. Buyanov, React. Kinet. Catal. Lett., 24(1984)401. 13. O.P. Krivoruchko, V.V. Malahov, A. Ermakova, R.A. Buyanov and L.F. Lokotko, Kinetika i Kataliz, 28(1987)442 (in Russian). 14. M.A. Fedotov, O.P. Krivoruchko, A.V. Golovin and R.A. Buyanov, Izv. AN SSSR. Ser. khim., No.2(1977)473 (in Russian).