Ejax. Vandesandt et al., SELECTION OF ACTIVATED CARBON FOR THE SELECTIVE HYDROGENOLYSIS OF CCL2F2 (CFC-12) INTO CH2F2 (HFC-32) OVER PALLADIUM-SUPPORTED CATALYSTS, Applied catalysis. A, General, 173(2), 1998, pp. 161-173
In this study, a selection of the support for palladium in the hydroge
nolysis of CCl2F2 into CH2F2 has been made. Activated carbon has been
selected because of its inertness against HF and HCl, co-products form
ed under reaction conditions. Different types of activated carbon have
been applied in the investigation. These include peat-based, coconut-
shell based, and synthetic carbons, which are either steam or chemical
ly activated. The carbons have been purified by different methods, suc
h as washing with aqueous sodium hydroxide and aqueous hydrochloric ac
id or boiling in concentrated hydrochloric acid or nitric acid or by i
mmersing the carbon in concentrated hydrofluoric acid. A peat-based, s
team-activated carbon resulted in the optimal performance for the pall
adium catalyst. Before impregnation with the palladium precursor, it i
s necessary to purify this support by a washing procedure to remove im
purities present in this carbon. This procedure consists of washing wi
th aqueous sodium hydroxide, aqueous hydrochloric acid, and water. For
characterization of the catalysts several methods have been applied,
like X-ray fluorescence, CO-chemisorption, and N-2-adsorption. The pre
pared catalyst shows a selectivity of 65-85 mol% for the desired produ
ct Ch(2)F(2) at all conversion levels of CCl2F2. It is observed that w
hen too many impurities are removed, the performance of the catalyst i
s not as good. Therefore, it is concluded that the impurities play an
important role in its performance. This is in harmony with the perform
ance of the catalysts prepared from very pure, synthetic carbons which
also do not show a good performance. It is postulated that the impuri
ties play a role as either a promoter or anchoring site, which influen
ces the Pd-dispersion. (C) 1998 Elsevier Science B.V. All rights reser
ved.