13C n.m.r. quantitative analysis of catalyst carbon deposits

Abstract

An assessment has been made of several techniques for the quantitative analysis of carbonaceous compounds by MAS 13C n.m.r. The best results were obtained using the proton gated decoupling pulse sequence coupled with depth pulses. Calibration was performed with external model compounds. From comparative studies using model compounds and samples of real catalyst carbon deposits (coke), it is concluded that only ∼ 50% of the carbon atoms in coke are ‘n.m.r.-visible’. The ‘invisible’ carbon atoms are either graphite-like or different kinds close to the paramagnetic centres, e.g. The Ni 2+ species. This conclusion affects previous n.m.r. results and will make n.m.r. data collected in the future more reliable so that they can provide valuable support for other techniques used to characterize carbonaceous deposits. From the T 2 ∗ curves, the transverse relaxation rates of the different carbons of the cokes can be estimated. Even if the carbons in the coke are relaxed by the Haldor Topsøe TK551 catalyst and paramagnetic metals present in it, the transverse relaxation rates are higher for the aliphatic carbons than for the aromatic carbons.