Adsorption of Methanol on ZSM-5 Zeolites

Abstract

The interaction of methanol with a NaZSM-5 and a HZSM-5 zeolite was investigated by means of temperature-programmed desorption (TPD) and adsorption measurements. For both zeolites the desorption curves are structured, indicating different adsorption sites. In the case of HZSM-5 the desorption at higher temperatures is accompanied with a partial conversion of methanol. It was investigated how the course of the reaction depends on the initial adsorbed amount of methanol. Using a regularization method, desorption energy distribution functions have been calculated. The energy range of these distributions correlates well with the heats of adsorption in the literature. The desorption energy distributions between 50 and 65 kJ mol -1 , which can be attributed to a nonspecific interaction, reflect the formation of stronger hydrogen bonds in the methanol clusters at higher loading on HZSM-5 than on NaZSM-5. The shape of the desorption energy distribution of NaZSM-5 in the range 65-100 kJ mol -1 , specific for the interaction of methanol with Na + cations, shows two clearly distinguished maxima, indicating two kinds of Na + cations. The obtained results for the HZSM-5 zeolite reflect the energetic heterogeneity of the bridging Si-OH-Al groups. Both, the shape and wideness of adsorption energy distributions extracted from measured adsorption isotherms agree well with the determined desorption energy distributions.

Mechanistic Insights into the Desorption of Methanol and Dimethyl Ether Over ZSM-5 Catalysts

“…Higher values of activation energies of desorption of DME than methanol are generally in accordance with previous studies [40,[52][53][54][55] but in contrast to values obtained by Pope [56] through calorimetric methods (Table 5). …”

A Kinetic Study of Methane Partial Oxidation over Fe‐ZSM‐5 Using N2O as an Oxidant

“…Indeed, Hunger et al. observed that methanol has a stronger affinity for the ZSM‐5 surface than water, citing the presence of the CH 3 group as a possible reason for this . The CH 3 OH that is retained forms coke more efficiently and lowers the mass balance of the reaction.…”

The Role of Non-Framework Lewis Acidic Al Species of Alkali-Treated HZSM-5 in Methanol Aromatization

“…The desorption of hydrogen, methane, ethylene, methanol, C3 + hydrocarbons, and DME were monitored by m/e = 2, 16, 27, 31, 43, and 45, respectively. Desorbed methanol was detected mostly below 200 °C, indicating an operating condition in which methanol conversion had not completed and side reaction should be suppressed [32]. DME was observed in the range from 150 °C to 300 °C, a so-called pre-inition phase of methanol conversion [33], which can be used to interpret the bonding strength of the Brønsted acid [32].…”

“…Thus, aromatic molecules are subsequently formed via the hydride transfer of olefin precursors [36]; therefore, the trend of hydride mobility may be considered as the indication of the aromatization Desorbed methanol was detected mostly below 200 • C, indicating an operating condition in which methanol conversion had not completed and side reaction should be suppressed [32]. DME was observed in the range from 150 • C to 300 • C, a so-called pre-inition phase of methanol conversion [33], which can be used to interpret the bonding strength of the Brønsted acid [32]. The desorption peak of DME increased with the following trend: HZ-D (230 • C) < HZ (249 • C) < HZ-DA (256 • C), in line with the Brønsted acidity observed from NH 3 -TPD.…”