Synthesis and Characterization of Solid Catalysts

Abstract

Amorphous silica-alumina catalysts having different silicon aluminum ratios were prepared. The Brönsted surface acidity of these catalyst samples was measured by the base exchange method using aqueous CHCOONH solution and the results were compared with those measured by the base exchange method using NH4OH and KOH as adsorbates from aqueous solution. The amount of NH4' in CH3COONH4 and NH4OH and K in KOH adsorbed was determined by spectrophotometrically and titrimetrically respectively by measuring the concentration of NH4 and K in the solution before and after adsorption. Variation of surface acidity of catalyst samples-A (synthesized without pore regulating agent), sample-B (synthesized with tetramethylammonium bromide as a pore regulating agent) and sample-C (synthesized with pore regulating agent and base exchanged with NILCI solution to obtain H' form by calculation) have been studied using different concentrations of ammonium acetate, ammonium hydroxide and potassium hydroxide solutions. It has been observed that the surface acidity of a given size catalyst increases with the increasing of concentration of CH3COONH, NH4OH and KOH but dilute solutions of CHCOONH, NH4OH and KOH are sufficient to saturate the catalyst surface. It has been observed that the surface acidity of the catalyst samples found by using CHCOONI, is greater than that of NH4OH and KOH: Of the two bases NH4OH gives slightly greater surface acidity than that of given by KOH. At a given concentration of CH3COONH, NH4OH and KOH solutions the surface acidity was more pronounced when ammonium acetate solution was used as an adsorbate. A plausible explanation for this discrepancy is that with the use of NH4OH and KOII as bases considerable dealumination takes place with a consequent decrease of surface acidity of the catalyst samples. In all cases, the surface acidity was found to increase with the decrease of sample size. Surface acidity was also found to increase as the Al content of the catalyst was increased. The surface acidity of the catalyst samples-13 and C are greater than that of sample-A. But this increase of surface acidity is more noticeable in case of sample-C. Iron(III) dispersed on silica was prepared by the incipient wetness method and its surface acidity measured by CH3COONH, solutions was found negligibly small and was comparable to that of chromatographic silica and alumina. Finally, the Brönsted surface acidity of the catalyst sample-A measured by the base exchanged method using ammonium acetate has been found to be well correlated with the equilibrium pH of CH3COONH4 solution. This may therefore, be concluded that surface acidity of silica-alumina catalyst depends on its mode of preparation, sample size, ratio of Si:Al and concentration of titrants although very dilute solutions of titrants are sufficient to saturate the catalyst surface. A weak base like CH3COONH4 gives comparatively high values of surface acidity of all the silica-alumina catalyst samples where dealumination from the silica-alumina catalyst samples are supposed to be negligible. The total surface acidity of the silica-alumina catalyst sample was also determined by the Tamele method (Amine titration method). Considering the carcinogenic effect of benzene and toxicity of n-butylamine substitutes of benzene as a solvent and n- butylamine as a titrant have been sought. With this end in view attempts have been made to modify the Amine titration method by replacing benzene with cyclohexane or n-hexane or n-heptane and replacing n-butylamine by di-n-butylamine or di-iso- butylamine or sec-butylamine. For this parameters varied were concentration of di-n- butylamine, di-iso-butylamine, n-butylamine and sec-butylamine solutions, sample size of catalyst and types of catalyst samples. The results in the present investigation show that surface acidity of catalyst samples tends to increase with the decrease of catalyst sample size. Unlike Brönsted acidity for a given size of sample the total surface acidity tends to increase with the silicon content of the catalyst sample. For bases, the total surface acidity of various catalyst samples increase in the order of di-iso- butylamine<di-n-butylamine<sec-butylamine<n-butylamine. A concentration of 0.5N solutions of organic bases is sufficient to saturate the catalyst surface. Since the total surface acidity of all the catalyst samples are greater when cyclohexane is used as solvent compared to that when n-hexane and n-heptane are used as solvents a modified Tamele method has been suggested replacing benzene with cyclohexane as a solvent. It has been found that using dilute solutions of di-n-butylamine, di-iso-butylamine, n- butylamine and sec-butylamine in cyclohexane (or n-hexane or n-heptane) total surface acidity of silica-alumina catalyst can successfully be measured. Preferential heat of adsorption of pyridine in n-heptane solution on silica-alumina amorphous catalyst and Fe-SiO2 samples have been measured by a flow micro- calorimeter. For comparison preferential heats of adsorption of pyridine on a standard sample of ZSM-5 and SiO2 and Al2O3 have also been measured. The total surface acidity measured by the n-butylamine titration method of all the catalyst samples have been correlated with the integral heat of adsorption. A good correlation of surface acidity with the integral heat indicates that the flow micro-calorimeter method may be used as a standard technique for characterizing solid catalysts. Both in the base exchange method and amine titration method increase of concentration bases give increased values of surface acidity with surface acidity versus concentration of bases plots resembling BET type-1 (Langmuir type- monolayer) adsorption isotherms. Similarly, plots of integral heat of adsorption versus concentration of ammonium acetate for ZSM-5, Ferrisilicate-Fe-SiO2 and Aluminosilicate with Si:Al ratio of 90:10 resemble BET type-1 isotherms but aluminosilicate samples with Si:Al ratio of 70:30 and 50:50 give integral heat of adsorption versus concentration of ammonium acetate solution plots resembling BET type-II (multilayer) adsorption isotherms. This may, therefore, be concluded that the nature of interaction of the inorganic bases with the Brönsted acid sites remain the same for all types of catalyst. But organic bases like n-butylamine and pyridine are able to interact with both the Brönsted and the Lewis acid sites and their nature and extent of interaction with the catalyst sample varies with the type of catalysts. It may finally be concluded that for the measurement of Brönsted acidity ammonium acetate, CH3COONH4 as a base occupy a favorably distinct position so also for the amine titration method n-butylamine occupy a favorably distinct position. This is further concluded that the carcinogenic compound, benzene, may successfully be replaced by cyclohexane as solvent in the amine titration method (Tamele method).