Catalysis by Hydrotalcite Materials

Clays are one of the important natural materials, which have been examined for catalytic applications. Clays are characterized by a layered structure and hence they are slippery when wet. They are broadly classified as cationic or smectite type (having layer lattice structure in which two-dimensional oxyanions are separated by layers of hydrated cations) and anionic or brucite type (in which the charge on the layer and the gallery ion is reversed complimentary to smectite type). Anionic clays are rare in nature but they are relatively easy and inexpensive to prepare in the laboratory.
Hydrotalcite belongs to the class of anionic clays wherein the positively charged contiguous layers formed by the edge sharing of Mg*(OH)6 and Al(OH)6 octahedra are separated by charge balancing anions and water, these occupying the interlayer spaces. A large variety of compounds with hydrotalcite like structure represented by the general formula
[M(II)1-x M(III)x (OH)2] [An+x/n]m.H2O
Where 0Co-AL>Ni-Al
Among the Mg-Al LDHs the order was found to be
Mg-Al CO32->Mg-Al CO32-,SO42->M-Al CO32-,Cl-
Further the selectivity to acrolen was found to be higher on Mg-Al LDH than on MgO. Thi anamalous behaviour is explained by considering the acid- base cooperation – the basic sites facilitating the carbanion formationfrom acetaldehyde and the acid sites increasing the electrophilicity of the carbonyl carbon. Such a coopertive behaviour is further established by the fact theat the Mg-Al LDH calcined at 573 K show higher activity as compared to the 673 K calcined sample. The sample calcined at 573 K has a shigher surface Al content as compared to the sample calcined at 673 K. This behaviour is assuemed to exist in many of the formaldehyde involved condernsations.
Chaudhary et al [] have been successful in carrying out the aldol condensation reactions with uncalcined Mg-Al O TBu catalyst which gave excellent yield of aldols with no further dehydration at a temperature as low as 273 K.
Tichit et al [] have investigated the Claisen-Schmidt condensation of benzaldehyde with acetone over Mg Al carbonate LDH. The initial rate of the reaction was found to depend on the calcination temperature of the sample. Maximum rate was observed for the sample calcined at 823 K. The results obtained are in agreement with the observed decomposition patterns indicating the formation of high surface area mixed oxide and at higher temperatures, an MgAl2O4 spinel []. However, the suggestion that the active sites for the reaction involve hydroxyl groups rather than O2- species seems to be artificial.
The Claisen-Schmidt condensation of benzaldehyde with substituted acetophenones was carried out by Climent et al over a series of basic catalysts like Cs exchanged zeolite and sepiolite and calcined hydrotalcites with varying Mg/Al ratio. Maximum activity was observed for Mg-Al LDH with Al/(Al+Mg) ratio between 0.25 and0.30. The increase in Al content upto certain extent causes the segregation of Al on the surface resulting in the generation of cation defect sites and hence more low coordinated oxide ions[]. A further increase in Al content may result in lowering in strength of these sites due to Al Lewis acidity or in decrease in number of such sites due to the decrease in Alsurface/Albulk ratio. Further, the crystallite size of the catalyst was also found to affect the activity. Samples with smaller crystallites were found to be more active. In such samples, a higher population of lower coordinated oxide ions is expected and hence these sites may be the active sits for the Claisen-Schmidt condensation. To our knowledge, the cannizaro reaction had not been successful with uncalcined or calcined LDHs, which indicate the requirement of stronger basic sites for the reaction.
Hydrogenation-Dehydrogention Reactions
Hydrogenation reactions over solid bases are unique in several aspectsinclding faster hydrogentionof conjugated dienses as compared to monoenes, preference for 1,4 additonover 1,2 additiona nd the retention of molecular identity of hydrogen atoms during the reaction []. Mixed oxides derived from some transition metal-functionlised layered doube hydroxides have been reported to be active in certain commercialy significant hydrogenation reactions.
Cabello et al [] have studied the infuence of various variables like catalyst composition, temperatue and basicity of the catalyts on the nickel catalyzed hydorgenation of acetonitrile. The higher basicity of the Mg-containing nickel catalyst {NiMgAl LDH} over the NiAl LDH was evidenceed by the observed heat of adsorption of monoethylamine (MEA) data for the twocatalysts and an optimum basicity at the Mg/(Mg+Al) ratio of 0.23 was found to be suitable for maximum MEA selectivity. Further, the numbero fby products formed by the transmination reaction increased at higher temperature and acetonitrilte conversion. Moreover, maximum selectiity to MEA [] was observed at a caclcination temperature of 623 K followed by the redution of the Ni2+ in the catalyts to Ni with hydrogen at 723 K.
Castiglioni, Ferrari et al [] have carreid out the selective vapour phase hydrogenation of maleic anhydride to  -butyro lactone with Cu/Zn/Al and Cu/Zn//Ga LDHs and not compred the activity with that of Cu/Zn/Cr catalysts. Better yields and conversion was obtained with the Al contianing catalysts while the activity of Cu/Zn/Ga samples were only comparable with the Al contining samples.
Heterogeneous basic catalysts are known to catalyse the dehydrogenationof alcohols in prefernec to dehydration taking place over solid acid catalysts. Corma et al [] have used the dehydrogenation of isopropanol as a mdel reaction to assess the acid-base properties of Mg-Al LDHs. They have investigaed the infulecne of Al/(Al+MG) ratio on the basic strenght of the catalsyt and found that the ratio of 0.25 was optimum for the basicity requiremnt of the rectin. Further they observed the presence of Td Al species in the calcined samples from the Al MAS NMR data. Similar observation was made by Attila Beres et al []. The presnece of both Td and Oh. Al species in mixed oxides formed on calcination give rise to an unstable inverse spinel type structure. From the XRD patterns of the calcined smaples, it is clear that the mied oxides have an MgO type structure with isomorphous substitution of Al. This leads to the generationof Mg2+ and Al3+ defects in the frame work and thus forming cooordnatively unsaturated oxide ions which act as strong basic sites.
Liquid Phase Oxidations
Oxdationis one of the fundamental reactions in organic synthesis. Catalytic oxidations, especially the metal catalysed oxidaions suing eniviromentally benign and economical oxidants like H2O2 have attracted cosniderable attention int he recent past and a number of homogeneous and heterogeneous catalytic systems have been reported [].
Among these different catalysts the crystalline microporous titanium silicalite molecular sieves (TS-1 and TS-2) have figured preminently in the literature, mainly due to their heterogneous nature, transiton state, size and shape selectivity and the possibility of incorporating various metal ions with redox characteristics as the active sites. These solid acid catalysts have been found to be active in the oxidationof a variety of organic compounds using aqueous hydrogen peroxide as the oxidant, under mild conditions []. The relatively new class of heterogeneous basic catalysts called layered double hydroxides LDHs with wider scope of structural modifications and relatively easier preparation methods have recently been found to give excellent yield and selectivity comparable to or much greater than TS-1 in many oxidative organic transformations [].
Kaizheng Zhu et al [] have carried out the liquid phase phenol hydroxylation using hydrogen peroxide with various ternary LDH systems containing copper and have compared their activity with some binary analogues like Mg-Al, Zn-Al and Co-Al LDHs and also with TS-1. The result show highest phenol conversion (53.5%) and catechol selectivity (59%) with Cu-Al LDH. Under similar conditins, only 27% conversion of phenol with 53% catechol selectivity was observed with TS-1, while the Mg-Al, Zn-Al, Co-Al and Ni-Al LDHs exhinited little activity. Urther, among the Cu-Al LDHs studies, the catalytic activity was found to increase with increase in Cu/Al ratio. Also in the range of reactionconditions investigated, maimum activity was observed with water as the solvent and at 333K and neutral pH. Based on these observations, Cu2+ centres inthe lamellae are suggested to be the active sites for the reaction.
The higher actiity observedin the case of copper containing LDH over TS-1 is presumably due ot the lesser deactivation fo the active sites in the course of the reaction. Such a deactivation has been reported [] with TS-1 catalysed phenol hydroxylation reaction. However, this seems to be less significant in the LDH catalysed reaction, especially at lower temperatures. This view is further substantitated by the fact that the Cu-containing LDHs show little activity in acetone, which is known to be the most efficient solvent for the dissolution of tar species. [].
The lower activity observed with the Cu M(II)AlCO32-+ catalysts at low and high pH of the medium had been correlated witht he dissolution of Cu2+ and the blocking of the Cu2+ centres by OH- respectively. However, the catalyst is reported to show considerably lower activity after calcinatin. This observation coupled ith the poor activity observed int he case of CuSo4 catalyst indicate a possible participation of surface hydroxyl groups in the LDH catalysed reaction. Moreover, the surface model of the CuM(II) al CO32- catalyst showing coordinatively unsaturated Cu2+ as the active sites for the reaction seems to be quite unlikely keeping in mind the preparation and pretreatment conditions reported. Further, the lower activity of the CuM(II)AlCO32- catalyst at high pH ma be due to the loss of lamellar structure by the leaching out of the Al3+ ions into the solution. Consequently a mechanims for the reaction involving the participation of the surface OH groups seems to be more reliable for the reaction than the proposed radical mechanims. Liu Yumin, Liu Shetian et al [] have studies the liquid phase oxidation of p-cresol top-hydroxy benzaldehyde using coblat contianing LDH as the catalyst. The effect of various metal ions in promting the catalytic acitvity was studied and Cu2+ ion was found to show the maximum promoting effect. The results were also compared with a mechanical mixture of Co2O3 CuO and Al2O3. The catalytic activities were found to follow the order CoCuAlCO32- > CoFeCO3 > CoAlCO3 > mechnical mixture. Further,best results were obtained with a Co/Cu/Al ratio of 3:1:1 at the calcination and reaction temperatures of 723 K and 973 K. The decrease in surface area of the catalyst due to sintering and the deactivation of the active sites by tar formation are suggested to be the major reasons for the lower reactivities observed at higher calcination and reaction temperatures. Further, th uncalcined catalyst showed considerably lower activity for the reaction and hence the mixed oxide formed by calcination is supposed to be the catalytically active phase for the reaction.
The oxidation of allylic and benzylic alcohols to the corresponding aldehydes or ketones was achieved with Ru-Mg-Al LDH in the presence of molecular oxygen by Kaneda, Yamashita et al []. Excellent yields were obtained with the carbonate containing LDH. The conversion and yield were poor with the Mg-AlCO3 LDH. Moreover the Ru-Mg-Al CO3 shoed higher heat of adsorption of benzoic acid as compared to the Mg-Al LDH catalyst suggesting Ru_OH sites in the LDH lamellae to be the active centres for the reaction. The natue of the interlayer ions was found to effect the activity of LDH and the catalysts iwth bulkier gallery anions exhibited lesser activity.
A similar observation was made in the Baeyer Villiger oxidaion fo various ketones using a combination of oxidant system ofmoleclar O2 and aldehydes []. These observations indicate that the strongly basic external surface hydroxyl gorups may be the acive sites for these reactions and with increasing basal spacing a protion of the reactant molecules may diffuse into the interlayer spaces making themselves unavailable for the reaction. In the Baeyer Villiger oxidation of ketones the yeild of lactone was found in be influenced by the nature of the aldehyde used, the type of solvent and the ring size of the ketones. Better yeilds were obtained by using benzaldehyde as the aldehyde and CCl4 as the solvent. Furthermore cyclopentanone underoges fasteroxidaitonthan cycloheanone. The latter observation is in sharp contrast to the observed actiity in the absence of LDH and also in organic per acid systems. Further, the authors have compared the activity of the MgAlCO3 LDH with its multimetallic analogues like Mg-Al-Fe CO3 and Mg-Al-Cu-CO3 []. In these oxidations it was found that in most cases the multimetallic LDHs exhibited higher activities and in a few cases quantitative yield of the corresponding lactones was also obtained. In addition the pent catalysts of these systms were reused without apprciable loss of activity. In comparison, the Mg-Al LDHs as such showed considerably lower recyclability. Further, the fact that MgAlCuCo3 LDH was found to oxidise the bicyclic ketones more effectively than the monocyclcic ones is again contrarty to the behaviour shown by thehomogeneous Cu(acetate)2 catalsyts. With these observed features, a cooperative action of transition metal sites and the basic sites is envisaged in the Baeyer-Villiger oxidations catalysed by these multimetallic LDH catalysts anda tentative mechanims was suggested which involves the generation fo perbenzoic acid by the auto-oxidation of benzaldehyde followed by base assited oxygen transfer to ketones.
Fraile, Garcia et al [] have compared the epoxidaiton activities of the oxidant-catalyst systems TBHP-Ti/SiO2 H2O2-Mg-Al LDH and TBHP-KF/Al2O3 towards a relatively stable substrate, - isophorone. Although the TBHP-KF/Al2O3 system yield better selectivity and higher conversions as compared to the the H2O2-Mg-Al LDH catalyst, thelatter uses a more environment friendly and economical oxidant, which cannot be employed with the KF-Al2O3 system due to the solubility of KF. Moreover, the results are compared under varying experimental conditions due to which a definite conclusion on the relative activity of the catalysts seem to be unreliable.
Cativiela et al [] have been successful in achieving the epoxidation of various  , unsaturated cyclci and acyclic ketones with Mg-Al LDH and H2O2. While theopen chain and the  -substituted  ,  unsaturated compounds exhibit stereoselective epoxidations with good yeild, the  substituted cyclic compounds showed lower activity.
Ueno, Yamagchi et al [] have investigated the base catalysed epoxidations of linear and cyclic olefins with Mg-Al LDHs using hydrogen peroxide combination with PhCN. The addition of surfactants like dodecyl sulphate was found to enhance the activity considerably by increasing the contact area between the aquesous and organic phases. The epoxidation reaction is thought to be a two-step process proceeding through an initial base assisted generation of peroxy carboximidic acid by the reaction between the nitrile and hydrogen peroxide andits subequent oxygen transfer to the olefin to yield the corresponding epoxides.
The liquid phase oxidationof several benzylic hydrocarbons using Ni-Al, Mg-Al , Cu-Zn-Al and Zr-Cr LDHs have been reported by chaudhary et al []. The conversion to the corresponding ketones were only modest except with Zn-Cr LDH and in the case of diphenyl methane and flourene. As observed with many LDH catalysed oxidations, the activity and selectivity was maintained to a considerable extent in the repeated cycles.
Ben Sels et al [] have reported a novel biomimetic oxidative bromination route using tungstate exchaged layered doube hydroxide in combination with NH4Br and hydrogen peroxide as an alternative for the conventional bromination reaction which makes use of elemental bromine which is both a pollutant and health hazard. The postively charged LDH lamellae is supposed to facilitate the approach of the bromide anions towards thenegatively charged tungstate species. Both the specific activity and TOF (turn over frequency) for the tungstate exchanged MgAlLDH were found to be considerably higher than many solid acid heterogeneous oxidation catalysts like Ti_MCM 48, Ti-MCM-41 and TS-1, under similar reaction conditions. Moreover,using alkylated olefins as the substrates, exceleent yields of epoxides were obtained via bromohydrin route.