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In order to find an optimum calcination temperature

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Unformatted text preview: ed catalyst, Ni free as well as NiO containing. In order to find an optimum calcination temperature experiments catalytic tests were performed with catalysts calcined at various temperatures. And finally to optimise the reaction conditions the influence of changes in the gas hourly space velocity (GHSV) and the effect of variations of the O2/C3H8 ratio of the feed were investigated. 2. Experimental 51 2. Experimental 2.1. Synthesis of the hydrotalcite-type solids Ni2+ is like Mg2+ and Al3+ a suitable metal ion for the preparation of HT materials. In fact the naturally occurring mineral takovite Ni6Al2(OH)16(CO3)· 4H2O, is isomorphous to hydrotalcite (Mg6Al2(OH)16(CO3)· 4H2O). The successful synthesis of Ni/Al HTs was reported for Ni/Al ratios of 2/1 – 3/1 (Ni0.67Al0.33 – Ni0.75Al0.25). At higher Ni contents the formation of free Ni(OH)2 next to the HT phase was observed, at lower Ni contents free Al(OH)3 next to a HT phase was formed [4, 6, 7]. No limitations for the Ni/Mg ratio for the synthesis of Ni/Mg/Al HTs were reported in the literature [1, 9, 42, 43, 46]. Synthetic Ni containing HTs are generally prepared under the conditions of low supersaturation at pH 8 ± 0.1 to ± 0.2 [1, 7, 8, 9, 43, 46, 53, 60]. A pH value ≥ 10 during the precipitation is not recommendable, because of the possible dissolving of Al3+ ions and the difficult removal of sodium salts from HTs prepared at pH ≥ 10 [4, 66]. Basic alkali containing solutions are generally used in the preparation of HTs, frequently also basic ammonium containing solutions. Metal nitrates are almost exclusively used as metal precursors and only rarely metal chlorides. The preparation was carried out at low supersaturation at pH 8 ± 0.2 at 60-70°C. Metal nitrates were chosen as metal salt precursors and NaOH and NaHCO3 as bases. In order to be able to tolerate small derivations in the (Ni2++Mg2+)/Al3+ ratio, a ratio of 2.57 corresponding to the formula NixMg72-xAl28 was chosen. To improve the degree of crystallinity the coprecipitated solids were hyd...
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