Hydrotalcite composition

FIGURE 12.3 Effect of MRP on HRRs of EVA/hydrotalcite composites. (From Du, L. et al., Polym. Degrad. Stabil., 91, 995, 2006. With permission.)... 

Wang, H., Bao, Y.-Z., Huang, Z.-M., and Weng, Z.-X. (2004) Stmeture and properties of poly(vinyl chloride) /dodecyl sulfonate-piUared hydrotalcite composites. Zhongguo Suliao, 18 (11), 54-58. 

Mg4Al2(OH) 2C02 3H20, is commonly written however, these minerals are generally non stoichiometric by nature and can include some amounts of alternative elements in then compositions. They function similarly to the zeoHtes but exist in layered stmctures and have a different trapping mechanism. In addition to then performance enhancement, the hydrotalcite minerals are compatible with PVC and can be used effectively in clear PVC appHcations as well as the pigmented formulations. 

The great importance of minerals in prebiotic chemical reactions is undisputed. Interactions between mineral surfaces and organic molecules, and their influence on self-organisation processes, have been the subject of much study. New results from Szostak and co-workers show that the formation of vesicles is not limited to one type of mineral, but can involve various types of surfaces. Different minerals were studied in order to find out how particle size, particle shape, composition and charge can influence vesicle formation. Thus, for example, montmorillonite (Na and K10), kaolinite, talc, aluminium silicates, quartz, perlite, pyrite, hydrotalcite and Teflon particles were studied. Vesicle formation was catalysed best by aluminium solicate, followed by hydrotalcite, kaolinite and talcum (Hanczyc et al., 2007). 

Theoretical calculations [43] based on first principles molecular dynamics discussed in Sect. 3.2.6 have suggested that Mg Al LDHs are most stable for n = 3 (i.e. x = 0.25) and indeed many minerals, including hydrotalcite itself, have this stoichiometry [4]. It has been reported that the synthesis of LDHs (with benzoate or terephthalate anions in the interlayers) from solutions containing Mg/Al = 2, leads to LDHs having the same composition when the synthesis is carried out at moderate temperatures but LDHs with Mg/Al = 3 (plus AlOOH) when the reaction is carried out under hydrothermal conditions [44]. It was proposed that the latter ratio represents the thermodynamically most favorable product. A similar observation has been reported [45] for solutions with Ni VPe = 2, where hydrothermal preparation led to segregation of an LDH with Ni VPe = 3 and Ni Fe 204. An attempt to synthesize a Co sAl LDH resulted in partial oxidation of the Co and formation of a Co o.yCo o.s LDH with complete migration of Al " from the layers to generate interlayer aluminum oxy-species [46]. 

Fig. 3. Relationship between the rate constant K) for isophorone isomerization and the number of sites for CO2 adsorption on calcined hydrotalcites with various compositions (107).

Table 1. Chemical composition of the fresh hydrotalcite-like compounds.

Corma, A., Fomes, V. and Rey, F. Hydrotalcites as base catalysts influence of the chemical composition and synthesis conditions on the dehydrogenation of isopropanol, J. Catal., 1994, 148, 205-12. 

Valente, J. S., Figueras, F., Gravelle, M., Kumbhar, P., Lopez, J. and Besse, J. P. Basic properties of the mixed oxides obtained by thermal decomposition of hydrotalcites containing different metallic compositions, J. Catal., 2000, 189, 370-381. 

L. Du, B. Qu, and Z. Xu, Flammability characteristics and synergistic effect of hydrotalcite with microencapsulated red phosphorus in halogen-free flame retardant EVA composite, Polym. Degrad. Stabil., 2006,91 995-1001. 

The composition of the nickel-alumina hydro-xycarbonate (NiAlC03-HTlc) may thus vary, but an idealized composition is that of the mineral takovite Ni6Al2(0H)j6C03 4 H2O. Hydrotalcites as catalytic precursors, amongst them NiAlCXh-HTlc, are discussed in an excellent recent review [49]. 

Miyata, S. (1980). Physico-chemical properties of synthetic hydrotalcites in relation to composition. Clays Clay Miner. 28, 50. 

Water retained after D-drying, known as non-evaporable water, has often been wrongly identified with chemically bound water. It excludes much of the interlayer water in C-S-H, AFm and hydrotalcite-type phases and much of the water contained in the crystal structure of AFt phases. It is often used as a measure of the fraction of the cement that has reacted, but can only be approximate in this respect, because the clinker phases react at different rates and yield products containing different amounts of non-evaporable water. Fully hydrated cement pastes typically contain about 23% of non-evaporable water, referred to the ignited weight. Copeland et al. (C38) determined the non-evaporable water contents of a series of mature cement pastes and carried out regression analyses on the cement composition. For pastes of w/c ratio 0.8 and aged 6.5 years, they obtained the approximate expression ... 

Ratios of water/cement/pfa = 0.5 0.72 0.28. pfa glass reacted = 6.0%, CH content = 13%, both referred to the ignited weight of composite cement, pfa HP = in situ product from Pfa. Fe HP = hydrogarnet-type product from ferrite phase. Mg HP = hydrotalcite-type phase, excluding any present in pfa HP. Pfa res. = unreacted non-vitreous material from Pfa. Other components mainly C and PjO,. Other phases mainly insoluble residue, and alkalis present in or adsorbed on products or contained in the pore solution. Discrepancies in totals arise from rounding (T5). 

Thermal treatments induce dehydration, dehydroxylation and loss of the charge-compensating anions, resulting in mixed oxides with the MgO-type structure. Hydrotalcites are consequently a class of precursors useful for the preparation of catalytically active oxides showing basic properties [94], The acid-base properties of Mg-Al mixed oxides are governed by the Mg Al molar ratio, calcination temperature and preparation conditions. The study of the influence of the acid-base properties and chemical composition on the catalytic performance of calcined hydrotalcites is thus of interest. 

Table 6 shows an example of the cyclic steady state performance of the SERP concept using an admixture of a SMR catalyst (noble metal on alumina) and a CO2 chemisorbent (K2CO3 promoted hydrotalcite) in the reactor [20]. The reactor temperature was 490°C. The feed H20 CH4 ratio was 6 1. The concept can directly produce 95% H2 product (dry basis) with a CH4 to H2 conversion of 73%. The trace impurities in the product gas contained less than 40 ppm COx- The corresponding product gas composition (thermodynamic limit) of a SMR reactor operated without the CO2 chemisorbent will be -67.2% H2, 15.7% CH4, 15.9% CO2, and 1-2% CO (dry basis), and the CH4 to H2 conversion will be only 52%. Thus, the SERP concept may be attractive for direct production of a CO free H2 stream for fuel cell applications. 

Advancements in the preparation of new PLS s nearly parallels that of the zeolite and zeolite-like phases. Initially the pillared smectite clays were investigated but the quest for new materials with new properties led to e qiloring the pillaring of other layered phases. These include, most notably, the layered zirconium phosphates, double hydroxides (hydrotalcites), sihcas and metal oxides. The parallel paths of discovery in new material compositions for the layered phases and the microporous (zeoUte) phases are summarized in Table 1. A conq>arison between the pore architectures of the zeohtes and the two dimensional PLS is shown in Table 2. 

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