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Scientific bases for the preparation of new cement containing catalysts
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
879
SCIEN'r~-IC BASES F O R T I ~ PREPARATION OF NEW CEMENT. C O N T A I N I N G CATALYSTS
V.I. Yakersona and E.Z. Golosmanb a N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, Moscow 117913, Russia b Institute of Nitrogen Industry, Novomoskovsk 301670, Russia The aspects of the preparation of different cement-containing catalysts are considered. The general trends in the mechanism of formation for nickel-, copper-, and zinc-containing catalysts based on calcivm a l u m i n a t e and high alumina cements are presented. The processes of cement hydration as well as the i n t e r a c t i o n of cements with hydroxocarbonates of active m e t a l s are outlined. It is shown t h a t the cement-containing catalysts combine a high activity with an n c r e a s e d t h e r m a l stability and e n h a n c e d m e c h a n i c a l strength. The high efficiency of cement-like compositions containing metal oxides in hydrogenation of CO, CO2 and 02, in decomposition of ammonia and hydrogenation of butyric aldehyde is clearly established. 1. I N T R O D U C T I O N Progress in modern chemical industry calls for the availability of active and selective catalysts, having favourable t i m e - o n - s t r e a m p a r a m e t e r s , an increased mechanical strength and a reasonable durability. Very significant for the design of the new commercial processes is the application of waste-free or low waste technologies for the synthesis and formation of catalysts. New promising practical possibilities in this area offer the development of catalysts containing either cements or calcium aluminates, which are known to be i m p o r t a n t cement components [1]. These t h e r m o s t a b l e s u b s t a n c e s with
excellent mechanical characteristics can be successfully applied as catalysts for low-and high temperature reactions, in particular for exothermic conversion. Moreover, the cement based catalysts have low coking tendency. Finally, high hydration activity and reactivity of cement and their ingredients provide the basis for the development of novel waste-free and ecologicallypure technologies for the preparation of cement-containing catalysts. METHODS The catalysts were prepared by mixing hydroxocarbonates of metals (MHC) with calcium m o n o a l u m i n a t e (CaA1204), calcium d i a l u m i n a t e (CaA1407) or high a l u m i n a cement (CaA1204 + CaA1407) in aqueous or ammonia-aqueous medium. Phase composition and dispersity of the catalysts at different steps of the p r e p a r a t i o n were determined by X-ray diffraction
880 technique (XRD). Specific surface area was estimated by using the adsorption of benzene and nitrogen and the pore size distribution was evaluated from the benzene hysteresis loop. Adsorption of oxygen was applied to determine the surface area of the metallic phase. IR spectra in the range of 400-4000 cm -1 were recorded on KBr pellets. The coordination state of a l u m i n i u m with respect to the nearest oxygen atoms was identified with 27A1 NMR spectroscopy. For electron microscope examination, u l t r a - t h i n slides or suspension of the catalysts were used. The mechanical s t r e n g t h of the catalysts was determined by the crushing pellet technique (P). Catalytic activity was measured in a flow circulation unit. 3, RESULTS AND DISCUSSION Cements based on calcium aluminates are normally used as hydraulic additives to enhance the mechanical strength and the stability of the catalysts. We succeeded to prepare calcium aluminate with well developed surface area which favourably combines many important properties required from the supports for effective catalysts. Hydration changes the mechanical and structural properties of calcium aluminates and high alumina cements. The range of the variations depends on temperature and duration of the treatment, dispersity of the components, liquid/solid ratio and phase composition of the cements. The t r e a t m e n t in w a t e r or aqueous a m m o n i a solution with subsequent calcination at 400-600~ results in an increase of the surface area (S) from 2 to 200 m2.g-1. After additional acid treatments, the surface area can reach the value of 300-400 m2.g -1. The first step of this process is hydration followed by the formation of species with a l u m i n i u m in octahedral coordination to oxygen. As evidenced by XRD-, IR-, and 27A1 NMR-data [2], the driving force of cement hydration is the unusual tetrahedral position of a l u m i n i u m and constrained Ca-O structures. The h e a t t r e a t m e n t and decomposition of calcium h y d r o a l u m i n a t e s favour the t r a n s f e r from octahedral to tetrahedral aluminium with respect to oxygen. An essentially linear relationship was established between the value of S and the degree of hydration for the starting calcium aluminates and cements. The mechanical strength of the cement based supports increases with increasing hydration degree and reaches values of 30-50 MPa. Another way to prepare calcium aluminate supports and sorbents is based on the interaction of Ca(OH)2 with AI(OH)3 followed by heat treatment of the resulting Ca3[Al(OH)6]2. This last compound can be thermolyzed to yield Ca12Al14033, in which the zeolite type structure was clearly established. IR spectroscopic investigation provided evidence for the bifunctional nature of the active sites on the surface of calcium aluminate based supports and sorbents. The presence of acidic and basic sites on the surface makes the use of these solids as catalysts for acid-base tranformations of organic substances promising. Nickel cement-containing catalysts were prepared by mixing nickel hydroxycarbonate (NiHC) with cements. In the course of mixing, exchange reaction proceeds to form hydroxyaluminate and hydroxycarboaluminate of nickel, CaCO3, AI(OH)3, and Ni(OH)2. The phase composition of the catalyst depends on the ratio of the starting components. The precursor of the active component exists primarily as nickel hydroxyaluminate (NiHA). Thermolysis of this precursor produces firsta poorly crystalline,disordered NiO-Al203 solid solution and then NiO. Heat treatment of nickel calcium aluminate catalysts
881 at 400-1000~ is accompanied by the interaction of A1203 with CaO rather than with NiO. This suppresses the formation of NiAI204 spinel. The presence of hardly reducible substances in nickel cement-like catalysts (NiO-A1203 solid solutions) preserves the high dispersity of the metallic nickel phase. This, in turn, makes nickel cement-like materials valuable catalysts for hydrogenation of CO and CO2 as well as for ammonia decomposition. The interaction of nickel salts with cements is associated with the formation of a mechanically stable polyphase system and concomitant enhancement of the surface area, from 2 to 200 m2.g -1. The activity of these catalysts in the purification of industrial gases from CO and CO2 [3] was found to be 160-170~ when expressed in terms of temperature for break-through of CO and CO2 to the level of <10 ppm. A more complete reduction of the solid solution increases the activity of the catalyst. Nickel catalysts show very high activity in the decomposition of ammonia : the remaining content of NH3 does not exceed 0.05-0.1% at 650-950~ Copper cement-like compositions are promising active catalysts for water shift of CO [4], synthesis of methanol, and oxygen hydrogenation [5]. Interaction of cements with copper hydroxycarbonate (CuHC) involves, as expected, the formation of copper hydroxyaluminate (CuriA). The activity of the catalyst in the water shift of CO correlates with the amounts of CuriA. The samples with the highest activity show an increased amount of CuriA, an enhanced concentration of strongly bound CuO, and a high dispersity of both CuO and Cu. The thermostable porous structure of disordered Cu-A1 spinel exerts a stabilising effect of the particles of CuO and Cu. The formation of CuO-ZnO solid solutions favours the catalytic activity of Cu, Zn-calcium aluminate system. The consequence of the introduction of copper and zinc components in the cements is also of importance. Cu, Zn-cement catalysts are very efficient in the hydrogenation of butyl aldehyde, whereas Cu, Ni-catalysts are excellent in oxygen hydrogenation. Thus, cement-containing catalysts can be used for a variety of reactions. The high activity of these catalysts is fortunately associated with high mechanical strength and thermostability. Another advantage of the cement-containing catalysts lies in the possibility to control their properties by varying the conditions of formation. 4. APPENDIX I Mechanism of formation of cemen~contalning catalysts. The reaction between cements (calcium aluminates) and the active component in aqueous medium takes place at room t e m p e r a t u r e and is accompanied by marked changes in texture and phase composition. This leads to reexamine the view t h a t calcium aluminates and cements are merely hydraulic binding agents. Metal hydroxycarbonate (MHC) reacts with calcium a l u m i n a t e s in aqueous or ammonia-aqueous medium to give a multiphasic system. The composition of the reaction products indicates both hydration of cements and exchange reaction between the calcium aluminates and MHC. Depending on the conditions, the reaction gives the metal hydroxyaluminates (MHA), C a C 0 3 , AI(OH)3, M(OH)2, Ca3[AI(OH)6]2, and Ca(OH)2. Since calcium aluminates, on dissolving in water, give Ca 2+ and AI(OH)4-, the reaction of MHC with alumina cements can be represented as follows : Ca 2+ ions react with CO~ ions in the labile structure of MHC to give insoluble CaC03, whose
882
formation displaces the reaction to the right. To preserve the electrical neutrality of M H C , the place of one 0032 ion carrying two negative charges must be taken by two AI(OH)4- ions, each one carrying only one negative charge: mMCO3.nM(OH)2 (MCH) + Ca 2+ + AI(OH)4- --~ m'M(OH)2.n'AI(OH)3 (MHA) + CaC03 + AI(OH)3 + Ca3[AI(OH)6]2 + Ca(OH)2 (1)
Since M H A has a layered structure, insertion of M(OH)2 into it is possible, so that the ratio M/AI will increase. In addition to metal hydroxyaluminates, hydroxycarboaluminates m a y also be formed. The reaction of M H C with calcium mono- and dialuminates is described by the equation : 3 CaAI204 + 12 H 20 + a[mMCO3.nM(OH)2] (MHC) = am CaC03 + (3-am)/3 Ca3[AI(OH)6]2 + (an+am)/3 M3~2(OH)12(MHA) + (12-2an)/3 AI(OH)3 (2) 3 CRA1407 + 21 H20 + a[mMCO3.nM(OH)2] -- (an+am)/3 M3A12(OH)2 + amCaC03 + (3-am)/3 Ca3[AI(OH)6]2 + (30-2an)/3 AI(OH)3
(3)
The composition of the reaction products depends on the numerical values of the coefficients"a", "m", and "n" (if M=Ni, re=l, and n=l; if M=Zn, m = l ou 2, and n=3; if M=Cu, m=n=l). The phase composition of cement-containing catalysts is described by general equations, based on the reaction between M H C and CaA1204 (CaAl407). The thermal decomposition of the compound formed by the chemical reaction and containing the active components can be described by the equation
m~ ~ MO-A1203 (solid solution) M3A12(OH)12 --~ 3MO-AI203 (solid solution) --, MO/MO-AI203
MO/MO-A120 3
(4)
If two metal hydroxocarbonates take part simultaneously in the reaction (copper and zinc, cobalt and copper, nickel and copper), the first stage of the reaction gives a mixed hydroxocarbonate, from which mixed hydroxoaluminates are formed, and converted into mixed solid solutions : m'M'CO3.n'M'(OH)2 + m"M"CO3.n"M"(OH)2 + CaAI204(CaA1407) + aq --~ M'M"A12(OH)12 --# M'O-M"O-A1203 (solid solution) --, M'O-M"O/M'O-M"O-A1203
(5)
Solid solutions of m i x e d metal hydroxoaluminates and hydroxycarboaluminates give anion-modified solid solutions. The reaction of metal hydroxocarbonate with Ca 2+ and AI(OH)4- ions can be represented as : mMCO3.nM(OH)2 + mCa 2+ + 2reAl(OH)4- --~ m{M[AI(OH)4]2}.nM(OH)2 + mC aC 0 3
(6)
883 The hydroxoaluminates, m{M[AI(OH)4]2}.nM(OH)2, can be represented as Mm+nAl2m(OH)8m+2n. Thus, depending on the composition of the MHC, the products are different hydroxoaluminates with different MO/AI203 ratios, which determine the composition of the products resulting from thermal decomposition (see Table 1):
Table 1. Composition of metal-hydroxocarbonates ' ( M H C ) and metalhydroxoaluminates (MHA), MO/AI203 ratio in the metal-hydroxoaluminates and composition of the products after thermal decomposition Of metalhydroxoaluminates.
mMCO3.nM(OH)2 Mm+nA12m(OH)Sm+ (MHC) 2n (MHA)
MO -4-1203
MHA~nMO + mMA1204 n/m
M = Ni, m=l, n=2 M=Co, m=1, n=2 M=Cu, m=l, n=l M=Zri; m=2, n=3 M=Zn, m=l, n=3
Ni3A12(OH)12 Co3~2(0H)12 Cu2AI2(OH)12 Zn5A14(OH)22 Zn4A12(OH)14
3:1 3:1 2:1 5:2 4:1
2:1
2:1 1:1 3:2 3:1
Table I shows that: a) the composition of the MHA depends markedly on the composition of MHC; b) the MO/AI203 ratio in the MHA is always greater than 1; c) the MO/MA]204 molar ratio of the thermally decomposed products of the MHA, that is the ratio of the free metal oxide to spinel, is exactly equal to n/re. 5. APPENDIX H Carriers and adsorbents based on cements. A source of calcium-aluminium carriers and adsorbents is provided by calcium mono- and di-aluminates, CaAI204 (CA) and CaA1407 (C.~2), which a r e the principal phases in high-alumina cements. The first stage in the preparation of these carriers is hydration which, at elevated temperatures, gives mainly Ca3[AI(OH)6]2 (C3AH6) 9 3CaA1204(CA) + 12 H20 ffi Ca3[AI(OH)6]2 + 4AI(OH)3 3CAA1407 (CA2) + 21 H20 = Ca3[AI(OH)6]2 + 10 AI(OH)3
(7) (8) l-
At lower temperatures, the less stable products C2AH8 and CAHlo are formed (CfCaO, A=AI203, H=H20). The thermal decomposition of hydrated calcium aluminates at different temperatures gives C12A7, A1203, CaO, (CaO)a.(Al203b.(H20)c. A method has been developed for obtaining carriers and adsorbents ("galyumin") with high surface areas based on calcium aluminates (cements). The specific surface area increases from 2-4 m2.g-1 to 400 m2.g-1, i.e., by two orders of magnitude. A study of the infrared spectra of a number of calcium aluminates (CA, CA2, C12A7) and catalysts and adsorbents based on them, with different S (30400 m2.g-1) showed that the bands in the range 400-900 cm -1 are due to the
884 vibrations of aluminium-oxygen tetrahedra in the framework of zeolite-like structure. The calcium ions apparently act as compensation cations. Depending on the preparation method, the value of S and the concentration of CaO, the aluminium-calcium adsorbents and catalysts can be divided into two groups : a) those with a zeolite-likestructure; b) those consisting chiefly of 7A1203 modified by calcium. The water in the aluminium-calcium adsorbents and catalysts is present in different states : it m a y be adsorbed, form surface O H groups, and be present as Ca(OH)2, AI(OH)3, and hydrated calcium aluminates. The fact that the hydration product of A1203 is formed in the hydration of aluminium-calci,m cements provides the starting point for the production and study of two-component systems, namely T-AI203-CA(CaAI204) and 7AI203-CA2(CaAI407). Their mechanical strength reaches 1450 kg.cm -2 and 1000 kg.cm -2 for the composition 40% CA-AI203 and 20% CA-AI203, respectively. The processes taking place in the production of catalysts based on calcium aluminates have been studied by high-resolution 27AI N M R . The structure of anhydrous calci~_!m aluminates (CA, CA2, C3A, C12A7) contains aluminium-oxygen tetrahedra. The alumini~m atoms in the tetrahedra are of different types and their non-equivalence increases, going from highly basic to weakly basic calcium aluminates. Upon hydration, which is one of the steps of the production of aluminium-calcium catalysts and carriers, the process [A104]--, [A106] takes place, and the process [A106]-, [A104] takes place on thermal decomposition. The presence of framework aluminium-oxygen [AIO4] tetrahedra in anhydrous calcium alumlnates makes their structure similar to that of zeolites. For zeolites, Loewenstein's rule applies : according to this rule, two aluminium atoms in tetrahedral coordination should not have a shared oxygen atom. The structure of calcium aluminates clearly indicates that Loewenstein's rule is not universal. R~'~CI~ 1. V.I. Yakerson and E.Z, Golosman, Catalysts and Cements, Khimiya, Moscow (1992). 2. V.I. Yakerson, V.D. Nissenvaum, E.Z. Golosman, V.M. Mastikhin, Kinetics and Catalysis, 27 (1986), 1231. 3. Z.A. Ibraeva, N.V. Nekrasov, V.I. Yakerson et al., Kinetics and Catalysis, 28 (1987), 339. 4. A.L. Turcheninov, N.V. Nekrasov, N.A. Gaidai et al., Kinetics and Catalysis, 28 (1987), 322. 5. B.N. Kuznetsov, V.N. Efremov, M.G. Chudinov et al., Kinetics and Catalysis, 33 (1992), 118.
879
SCIEN'r~-IC BASES F O R T I ~ PREPARATION OF NEW CEMENT. C O N T A I N I N G CATALYSTS
V.I. Yakersona and E.Z. Golosmanb a N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, Moscow 117913, Russia b Institute of Nitrogen Industry, Novomoskovsk 301670, Russia The aspects of the preparation of different cement-containing catalysts are considered. The general trends in the mechanism of formation for nickel-, copper-, and zinc-containing catalysts based on calcivm a l u m i n a t e and high alumina cements are presented. The processes of cement hydration as well as the i n t e r a c t i o n of cements with hydroxocarbonates of active m e t a l s are outlined. It is shown t h a t the cement-containing catalysts combine a high activity with an n c r e a s e d t h e r m a l stability and e n h a n c e d m e c h a n i c a l strength. The high efficiency of cement-like compositions containing metal oxides in hydrogenation of CO, CO2 and 02, in decomposition of ammonia and hydrogenation of butyric aldehyde is clearly established. 1. I N T R O D U C T I O N Progress in modern chemical industry calls for the availability of active and selective catalysts, having favourable t i m e - o n - s t r e a m p a r a m e t e r s , an increased mechanical strength and a reasonable durability. Very significant for the design of the new commercial processes is the application of waste-free or low waste technologies for the synthesis and formation of catalysts. New promising practical possibilities in this area offer the development of catalysts containing either cements or calcium aluminates, which are known to be i m p o r t a n t cement components [1]. These t h e r m o s t a b l e s u b s t a n c e s with
excellent mechanical characteristics can be successfully applied as catalysts for low-and high temperature reactions, in particular for exothermic conversion. Moreover, the cement based catalysts have low coking tendency. Finally, high hydration activity and reactivity of cement and their ingredients provide the basis for the development of novel waste-free and ecologicallypure technologies for the preparation of cement-containing catalysts. METHODS The catalysts were prepared by mixing hydroxocarbonates of metals (MHC) with calcium m o n o a l u m i n a t e (CaA1204), calcium d i a l u m i n a t e (CaA1407) or high a l u m i n a cement (CaA1204 + CaA1407) in aqueous or ammonia-aqueous medium. Phase composition and dispersity of the catalysts at different steps of the p r e p a r a t i o n were determined by X-ray diffraction
880 technique (XRD). Specific surface area was estimated by using the adsorption of benzene and nitrogen and the pore size distribution was evaluated from the benzene hysteresis loop. Adsorption of oxygen was applied to determine the surface area of the metallic phase. IR spectra in the range of 400-4000 cm -1 were recorded on KBr pellets. The coordination state of a l u m i n i u m with respect to the nearest oxygen atoms was identified with 27A1 NMR spectroscopy. For electron microscope examination, u l t r a - t h i n slides or suspension of the catalysts were used. The mechanical s t r e n g t h of the catalysts was determined by the crushing pellet technique (P). Catalytic activity was measured in a flow circulation unit. 3, RESULTS AND DISCUSSION Cements based on calcium aluminates are normally used as hydraulic additives to enhance the mechanical strength and the stability of the catalysts. We succeeded to prepare calcium aluminate with well developed surface area which favourably combines many important properties required from the supports for effective catalysts. Hydration changes the mechanical and structural properties of calcium aluminates and high alumina cements. The range of the variations depends on temperature and duration of the treatment, dispersity of the components, liquid/solid ratio and phase composition of the cements. The t r e a t m e n t in w a t e r or aqueous a m m o n i a solution with subsequent calcination at 400-600~ results in an increase of the surface area (S) from 2 to 200 m2.g-1. After additional acid treatments, the surface area can reach the value of 300-400 m2.g -1. The first step of this process is hydration followed by the formation of species with a l u m i n i u m in octahedral coordination to oxygen. As evidenced by XRD-, IR-, and 27A1 NMR-data [2], the driving force of cement hydration is the unusual tetrahedral position of a l u m i n i u m and constrained Ca-O structures. The h e a t t r e a t m e n t and decomposition of calcium h y d r o a l u m i n a t e s favour the t r a n s f e r from octahedral to tetrahedral aluminium with respect to oxygen. An essentially linear relationship was established between the value of S and the degree of hydration for the starting calcium aluminates and cements. The mechanical strength of the cement based supports increases with increasing hydration degree and reaches values of 30-50 MPa. Another way to prepare calcium aluminate supports and sorbents is based on the interaction of Ca(OH)2 with AI(OH)3 followed by heat treatment of the resulting Ca3[Al(OH)6]2. This last compound can be thermolyzed to yield Ca12Al14033, in which the zeolite type structure was clearly established. IR spectroscopic investigation provided evidence for the bifunctional nature of the active sites on the surface of calcium aluminate based supports and sorbents. The presence of acidic and basic sites on the surface makes the use of these solids as catalysts for acid-base tranformations of organic substances promising. Nickel cement-containing catalysts were prepared by mixing nickel hydroxycarbonate (NiHC) with cements. In the course of mixing, exchange reaction proceeds to form hydroxyaluminate and hydroxycarboaluminate of nickel, CaCO3, AI(OH)3, and Ni(OH)2. The phase composition of the catalyst depends on the ratio of the starting components. The precursor of the active component exists primarily as nickel hydroxyaluminate (NiHA). Thermolysis of this precursor produces firsta poorly crystalline,disordered NiO-Al203 solid solution and then NiO. Heat treatment of nickel calcium aluminate catalysts
881 at 400-1000~ is accompanied by the interaction of A1203 with CaO rather than with NiO. This suppresses the formation of NiAI204 spinel. The presence of hardly reducible substances in nickel cement-like catalysts (NiO-A1203 solid solutions) preserves the high dispersity of the metallic nickel phase. This, in turn, makes nickel cement-like materials valuable catalysts for hydrogenation of CO and CO2 as well as for ammonia decomposition. The interaction of nickel salts with cements is associated with the formation of a mechanically stable polyphase system and concomitant enhancement of the surface area, from 2 to 200 m2.g -1. The activity of these catalysts in the purification of industrial gases from CO and CO2 [3] was found to be 160-170~ when expressed in terms of temperature for break-through of CO and CO2 to the level of <10 ppm. A more complete reduction of the solid solution increases the activity of the catalyst. Nickel catalysts show very high activity in the decomposition of ammonia : the remaining content of NH3 does not exceed 0.05-0.1% at 650-950~ Copper cement-like compositions are promising active catalysts for water shift of CO [4], synthesis of methanol, and oxygen hydrogenation [5]. Interaction of cements with copper hydroxycarbonate (CuHC) involves, as expected, the formation of copper hydroxyaluminate (CuriA). The activity of the catalyst in the water shift of CO correlates with the amounts of CuriA. The samples with the highest activity show an increased amount of CuriA, an enhanced concentration of strongly bound CuO, and a high dispersity of both CuO and Cu. The thermostable porous structure of disordered Cu-A1 spinel exerts a stabilising effect of the particles of CuO and Cu. The formation of CuO-ZnO solid solutions favours the catalytic activity of Cu, Zn-calcium aluminate system. The consequence of the introduction of copper and zinc components in the cements is also of importance. Cu, Zn-cement catalysts are very efficient in the hydrogenation of butyl aldehyde, whereas Cu, Ni-catalysts are excellent in oxygen hydrogenation. Thus, cement-containing catalysts can be used for a variety of reactions. The high activity of these catalysts is fortunately associated with high mechanical strength and thermostability. Another advantage of the cement-containing catalysts lies in the possibility to control their properties by varying the conditions of formation. 4. APPENDIX I Mechanism of formation of cemen~contalning catalysts. The reaction between cements (calcium aluminates) and the active component in aqueous medium takes place at room t e m p e r a t u r e and is accompanied by marked changes in texture and phase composition. This leads to reexamine the view t h a t calcium aluminates and cements are merely hydraulic binding agents. Metal hydroxycarbonate (MHC) reacts with calcium a l u m i n a t e s in aqueous or ammonia-aqueous medium to give a multiphasic system. The composition of the reaction products indicates both hydration of cements and exchange reaction between the calcium aluminates and MHC. Depending on the conditions, the reaction gives the metal hydroxyaluminates (MHA), C a C 0 3 , AI(OH)3, M(OH)2, Ca3[AI(OH)6]2, and Ca(OH)2. Since calcium aluminates, on dissolving in water, give Ca 2+ and AI(OH)4-, the reaction of MHC with alumina cements can be represented as follows : Ca 2+ ions react with CO~ ions in the labile structure of MHC to give insoluble CaC03, whose
882
formation displaces the reaction to the right. To preserve the electrical neutrality of M H C , the place of one 0032 ion carrying two negative charges must be taken by two AI(OH)4- ions, each one carrying only one negative charge: mMCO3.nM(OH)2 (MCH) + Ca 2+ + AI(OH)4- --~ m'M(OH)2.n'AI(OH)3 (MHA) + CaC03 + AI(OH)3 + Ca3[AI(OH)6]2 + Ca(OH)2 (1)
Since M H A has a layered structure, insertion of M(OH)2 into it is possible, so that the ratio M/AI will increase. In addition to metal hydroxyaluminates, hydroxycarboaluminates m a y also be formed. The reaction of M H C with calcium mono- and dialuminates is described by the equation : 3 CaAI204 + 12 H 20 + a[mMCO3.nM(OH)2] (MHC) = am CaC03 + (3-am)/3 Ca3[AI(OH)6]2 + (an+am)/3 M3~2(OH)12(MHA) + (12-2an)/3 AI(OH)3 (2) 3 CRA1407 + 21 H20 + a[mMCO3.nM(OH)2] -- (an+am)/3 M3A12(OH)2 + amCaC03 + (3-am)/3 Ca3[AI(OH)6]2 + (30-2an)/3 AI(OH)3
(3)
The composition of the reaction products depends on the numerical values of the coefficients"a", "m", and "n" (if M=Ni, re=l, and n=l; if M=Zn, m = l ou 2, and n=3; if M=Cu, m=n=l). The phase composition of cement-containing catalysts is described by general equations, based on the reaction between M H C and CaA1204 (CaAl407). The thermal decomposition of the compound formed by the chemical reaction and containing the active components can be described by the equation
m~ ~ MO-A1203 (solid solution) M3A12(OH)12 --~ 3MO-AI203 (solid solution) --, MO/MO-AI203
MO/MO-A120 3
(4)
If two metal hydroxocarbonates take part simultaneously in the reaction (copper and zinc, cobalt and copper, nickel and copper), the first stage of the reaction gives a mixed hydroxocarbonate, from which mixed hydroxoaluminates are formed, and converted into mixed solid solutions : m'M'CO3.n'M'(OH)2 + m"M"CO3.n"M"(OH)2 + CaAI204(CaA1407) + aq --~ M'M"A12(OH)12 --# M'O-M"O-A1203 (solid solution) --, M'O-M"O/M'O-M"O-A1203
(5)
Solid solutions of m i x e d metal hydroxoaluminates and hydroxycarboaluminates give anion-modified solid solutions. The reaction of metal hydroxocarbonate with Ca 2+ and AI(OH)4- ions can be represented as : mMCO3.nM(OH)2 + mCa 2+ + 2reAl(OH)4- --~ m{M[AI(OH)4]2}.nM(OH)2 + mC aC 0 3
(6)
883 The hydroxoaluminates, m{M[AI(OH)4]2}.nM(OH)2, can be represented as Mm+nAl2m(OH)8m+2n. Thus, depending on the composition of the MHC, the products are different hydroxoaluminates with different MO/AI203 ratios, which determine the composition of the products resulting from thermal decomposition (see Table 1):
Table 1. Composition of metal-hydroxocarbonates ' ( M H C ) and metalhydroxoaluminates (MHA), MO/AI203 ratio in the metal-hydroxoaluminates and composition of the products after thermal decomposition Of metalhydroxoaluminates.
mMCO3.nM(OH)2 Mm+nA12m(OH)Sm+ (MHC) 2n (MHA)
MO -4-1203
MHA~nMO + mMA1204 n/m
M = Ni, m=l, n=2 M=Co, m=1, n=2 M=Cu, m=l, n=l M=Zri; m=2, n=3 M=Zn, m=l, n=3
Ni3A12(OH)12 Co3~2(0H)12 Cu2AI2(OH)12 Zn5A14(OH)22 Zn4A12(OH)14
3:1 3:1 2:1 5:2 4:1
2:1
2:1 1:1 3:2 3:1
Table I shows that: a) the composition of the MHA depends markedly on the composition of MHC; b) the MO/AI203 ratio in the MHA is always greater than 1; c) the MO/MA]204 molar ratio of the thermally decomposed products of the MHA, that is the ratio of the free metal oxide to spinel, is exactly equal to n/re. 5. APPENDIX H Carriers and adsorbents based on cements. A source of calcium-aluminium carriers and adsorbents is provided by calcium mono- and di-aluminates, CaAI204 (CA) and CaA1407 (C.~2), which a r e the principal phases in high-alumina cements. The first stage in the preparation of these carriers is hydration which, at elevated temperatures, gives mainly Ca3[AI(OH)6]2 (C3AH6) 9 3CaA1204(CA) + 12 H20 ffi Ca3[AI(OH)6]2 + 4AI(OH)3 3CAA1407 (CA2) + 21 H20 = Ca3[AI(OH)6]2 + 10 AI(OH)3
(7) (8) l-
At lower temperatures, the less stable products C2AH8 and CAHlo are formed (CfCaO, A=AI203, H=H20). The thermal decomposition of hydrated calcium aluminates at different temperatures gives C12A7, A1203, CaO, (CaO)a.(Al203b.(H20)c. A method has been developed for obtaining carriers and adsorbents ("galyumin") with high surface areas based on calcium aluminates (cements). The specific surface area increases from 2-4 m2.g-1 to 400 m2.g-1, i.e., by two orders of magnitude. A study of the infrared spectra of a number of calcium aluminates (CA, CA2, C12A7) and catalysts and adsorbents based on them, with different S (30400 m2.g-1) showed that the bands in the range 400-900 cm -1 are due to the
884 vibrations of aluminium-oxygen tetrahedra in the framework of zeolite-like structure. The calcium ions apparently act as compensation cations. Depending on the preparation method, the value of S and the concentration of CaO, the aluminium-calcium adsorbents and catalysts can be divided into two groups : a) those with a zeolite-likestructure; b) those consisting chiefly of 7A1203 modified by calcium. The water in the aluminium-calcium adsorbents and catalysts is present in different states : it m a y be adsorbed, form surface O H groups, and be present as Ca(OH)2, AI(OH)3, and hydrated calcium aluminates. The fact that the hydration product of A1203 is formed in the hydration of aluminium-calci,m cements provides the starting point for the production and study of two-component systems, namely T-AI203-CA(CaAI204) and 7AI203-CA2(CaAI407). Their mechanical strength reaches 1450 kg.cm -2 and 1000 kg.cm -2 for the composition 40% CA-AI203 and 20% CA-AI203, respectively. The processes taking place in the production of catalysts based on calcium aluminates have been studied by high-resolution 27AI N M R . The structure of anhydrous calci~_!m aluminates (CA, CA2, C3A, C12A7) contains aluminium-oxygen tetrahedra. The alumini~m atoms in the tetrahedra are of different types and their non-equivalence increases, going from highly basic to weakly basic calcium aluminates. Upon hydration, which is one of the steps of the production of aluminium-calcium catalysts and carriers, the process [A104]--, [A106] takes place, and the process [A106]-, [A104] takes place on thermal decomposition. The presence of framework aluminium-oxygen [AIO4] tetrahedra in anhydrous calcium alumlnates makes their structure similar to that of zeolites. For zeolites, Loewenstein's rule applies : according to this rule, two aluminium atoms in tetrahedral coordination should not have a shared oxygen atom. The structure of calcium aluminates clearly indicates that Loewenstein's rule is not universal. R~'~CI~ 1. V.I. Yakerson and E.Z, Golosman, Catalysts and Cements, Khimiya, Moscow (1992). 2. V.I. Yakerson, V.D. Nissenvaum, E.Z. Golosman, V.M. Mastikhin, Kinetics and Catalysis, 27 (1986), 1231. 3. Z.A. Ibraeva, N.V. Nekrasov, V.I. Yakerson et al., Kinetics and Catalysis, 28 (1987), 339. 4. A.L. Turcheninov, N.V. Nekrasov, N.A. Gaidai et al., Kinetics and Catalysis, 28 (1987), 322. 5. B.N. Kuznetsov, V.N. Efremov, M.G. Chudinov et al., Kinetics and Catalysis, 33 (1992), 118.