C-13 NMR QUANTITATIVE-ANALYSIS OF CATALYST CARBON DEPOSITS

An assessment has been made of several techniques for the quantitative analysis of carbonaceous compounds by MAS C-13 n.m.r. The best result s were obtained using the proton gated decoupling pulse sequence coupl ed with depth pulses. Calibration was performed with external model co mpounds. From comparative studies using model compounds and samples of real catalyst carbon deposits (coke), it is concluded that only simil ar to 50% of the carbon atoms in coke are 'n.m.r.-visible'. The 'invis ible' carbon atoms are either graphite-like or different kinds close t o the paramagnetic centres, e.g. The Ni2+ species. This conclusion aff ects previous n.m.r. results and will make n.m.r, data collected in th e future more reliable so that they can provide valuable support for o ther techniques used to characterize carbonaceous deposits. From the T -2 curves, the transverse relaxation rates of the different carbons o f the cokes can be estimated. Even if the carbons in the coke are rela xed by the Haldor Topsoe TK551 catalyst and paramagnetic metals presen t in it, the transverse relaxation rates are higher for the aliphatic carbons than for the aromatic carbons. Copyright (C) 1996 Elsevier Sci ence Ltd.