Aspects of the deactivation of H-ZSM-5 zeolite in the conversion of methanol to hydrocarbons

Abstract

An investigation of both the deactivation by coke formation and regeneration of the methanol conversion catalyst H-ZSM-5 is presented. Infrared (FTIR), Raman and ultraviolet-visible (TUVS) spectroscopies and temperature-programmed desorption-mass spectrometry (TPD-MS) were employed in both static and in situ studies of: (i) coke composition, (ii) coke formation chemistry, (iii) its influence on the acidity of H-ZSM-5, and (iv) regeneration chemistry (coke removal). These studies were conducted for three different preparations of the zeolite ZSM-5. The acidity of this zeolite was evaluated by infrared observations of chemisorbed ammonia and pyridine and of the acidic hydroxyl groups, and the temperature-programmed desorption of ammonia. Coke was discovered to primarily consist of two components, a molecular polyaromatic coke formed within the channels of ZSM-5 and a graphitic coke on the exterior surfaces of this zeolite, which is the major component at high coke levels. This description is in agreement with that of Bibby et al., J. Catal., 97, 493 (1986). Methylaromatics are the precursors to coke, these yield polyaromatic compounds and are formed via a facile carbenium ion pathway. Pyrolysis of coke results mainly in the evolution of methane and the carbon oxides and is related to the loss of alkyl (chiefly methyl) and carbonyl groups. Graphitisation of coke remaining in the channels of the zeolite was indicated by the evolution of hydrogen above 500°C. The Brønsted acid sites are where coke formation originates in ZSM-5. Differences in the amount of coke deposited per acid site between the three preparations of ZSM-5 during the initial deactivation stage are attributed to differences in zeolite aluminium distribution. Two-stage deactivation of the acid sites is consistent with internal coke formation occurring before external coke formation. Nonidentical chemisorption of ammonia and pyridine at high coke levels in two of the preparations of ZSM-5 is indicative of topological channel blocking by coke. Air regeneration above 500°C was required to remove the majority of coke from the zeolite and to recover most of the original acid site concentration. The appearance of an IR band at 1780 cm-1 was established by 18O oxidation as pertaining to cyclic anhydride and other carbonyl groups on coke. Temperature-programmed oxidation shows these decompose to the carbon oxides at higher temperatures.