Direct and indirect observation of coke deposits on HZSM-5 by 1H NMR imaging

The location of coke deposits in industrial HZSM-5 pellets contained in a fixed-bed reactor was revealed in two different ways by H-1 NMR imaging (MRI). In the first way, coke depositions in the reactor were detected directly by using the SPRITE technique, a special MRI sequence for detecting materials with short T-2 relaxation times. In the second, indirect way, carbonaceous residues were visualized by applying spin-echo sequences to a coked catalyst sample loaded with propane. Because the behavior of adsorbed gases depends on their interaction with the catalyst it was possible to distinguish between fresh and coked zeolitic areas within the bed. The application of MRI for coke-concentration profiling in fixed-bed reactors and catalyst pellets is discussed. (C) 2002 Elsevier Science (USA).