Abstract
The Methanol-to-Olefins (MTO) process for the production of polymer-grade olefins is a possible step
in the upgrading of natural gas. The preferred MTO catalyst is the microporous silicoaluminophosphate
H-SAPO-34 (CHA topology). In the present work, H-SAPO-34 is compared with H-SSZ-13
which has the same topology and density of acid sites, but is slightly more acidic due to the framework
composition. This is a one-parameter study where the effect of acidity on the MTO-process is
investigated. Both materials are very similar as catalysts for the conversion of methanol, the effluent
and retained hydrocarbons contain mainly the same compounds. It is found that H-SSZ-13 has a
lower total conversion capacity than H-SAPO-34. Both catalysts displayed an optimum temperature,
but under the same conditions (Weight Hourly Space Velocity=6.2 hr-1) the optimum temperature
of H-SSZ-13 (350° C) was 50°C below that of H-SAPO-34 (400° C). The production of retained
hydrocarbons was faster in H-SSZ-13, which also produced larger ring compounds than H-SAPO-34
at the same conditions. Upon flushing with He it was possible to discriminate between the active and
non-active compounds in the retained material. The more methylated benzenes were the most reactive
ones in both catalysts. In H-SAPO-34 at 350° C only penta- and hexamethylbenzene were active,
while also tetramethylbenzene was active in H-SSZ-13. When decreasing the temperature to 300°C,
tetramethylbenzene was no longer protonated in H-SSZ-13, so it appears that higher acid strength
has an effect comparable to increasing the temperature. Changing WHSV had a great impact on the
catalyst properties, such as optimum temperature and conversion capacity. Halving WHSV increased
the optimum temperature of H-SAPO-34 to 450° C.
A theoretical study of the adsorption of methanol and CO on the acid sites of the two catalysts is
also presented in this thesis. Frequency calculations for the adsorption of CO showed that H-SSZ-13
was slightly more acidic than H-SAPO-34, in good agreement with experimental results reported in
the literature. The calculated methanol adsorption energies were higher than reported in previous
studies, Furthermore, the calculations indicated that methanol tends to point into the cage rather than
the 8-rings of the structure. This is not in agreement with studies reported earlier.