Temperature Programmed Surface Reaction Studies of the Methanol to Gasoline (MTG) Conversion over ZSM‐5

Abstract

The~ethanol to gasoline conversion over ZSM-5 has been studied by the Temperature Programmed Surface Reaction (TP~R) technique. The technique is able to delineate the two steps in the process: the dehydration of methanol to dimethyl ether and the subsequent conversion of dimethyl ether to hydrocarbons. The activation barr~ers associated with each step was evaluated from the TPSR profiles and are 108 kJ/mol and 195 kJ/mol respectively, The methanol desorption profile shows considerable change with the amount of methanol molecules adsorbed per Bronsted site of the zeolite. The energy associated with the desorption process (CH OH) H + -ZSM-.-H 30H)n_t H -ZSM-5 + CH 30H, shows a spectrum of values depending on n. The spectrum is not connnuous and has a gap for n = 1. The trend is similar to that for the corresponding gas-phase reaction, (CH30H)nH+-(CH30H)n_IH+ +CH 30H. The present investigation highlights the importance of the prot~nated methanol cluster, (CH30H)nH + -ZSM-5, in the primary step of the MTG process, the formation of dimethyl ether. The reaction is compared with the corresponding gas phase ion-molecule reaction of CH 30H{ .

Adsorption of Methanol on ZSM-5 Zeolites

“…Further, in the last case nearly no desorption of methanol is observed. Similar findings were reported by Jayamurthy et al 20 No experiment has shown any formation of aromatics. This is in accordance with the results of Ono et al 4 for small methanol concentrations.…”

“…Haase and Sauer 15 compared their result with an experimental value of 110−120 kJ mol -1 , given only in a personal communication. This high value seems to agree with a heat of desorption of 120 kJ mol -1 determined for methanol desorption at higher temperarures . In contrast, a relatively low value of 62 kJ mol -1 , determined some years ago by Messow et al., is often used for comparison (see e.g., refs and ).…”

“…Up to 500 K only dimethyl ether and water are desorbed, an observation in good agreement with the results of other authors. 6,7,10,12,20 This behavior should correspond to the so-called "pre-inition" phase of the methanol conversion. 6 Dimethyl ether shows a striking desorption behavior in dependence on the initial methanol coverage.…”

“…This high value seems to agree with a heat of desorption of 120 kJ mol -1 determined for methanol desorption at higher temperarures. 20 In contrast, a relatively low value of 62 kJ mol -1 , determined some years ago by Messow et al, 21 is often used for comparison (see e.g., refs 16 and 17). But, this value was deduced from experimental heats of immersion of liquid methanol, and therefore it represents a mean interaction of methanol with the zeolite at high coverage, which means it is not very specific for the interaction with the acid OH groups.…”

Inhomogeneity in the interaction between methanol molecules and Brønsted acid sites of H-ZSM-5 directly detected by 2D CPMAS 13C NMR spectroscopy

“…1 There are sites in the two kinds of channel and at their intersections in H-ZSM-5, and the local electric field must be reflected in the states of the molecules adsorbed. 3 There have been extensive experimental studies on the state of MeOH using in situ IR analysis, [4][5][6][7] temperature programmed desorption, 8,9 and solid-state NMR analysis, 5,[10][11][12][13][14][15] as well as theoretical calculations. 1,[16][17][18] High-resolution solid-state 1 H NMR spectroscopy has recently shown that MeOH molecules form 'clusters' on the Brønsted acid sites of H-ZSM-5 at high coverages at ambient temperature.…”