Abstract
M.Tech.
Clinoptilolite possesses acid and base properties, and is hydrothermally stable at
high temperatures. It can be reactivated by simple oxidation, and is highly
selective due to its unique-shape selectivity. It is also not harmful to the
environment and it has shown benefits to soil as well (Mumpton,1977).
An initial study of the reactions of methanol amination was done using both
natural local and synthetic clinoptilolites. Characterization of each was done
before and after modification using XRD, TPD and BET analysis. The BET
analysis showed an increase in the surface area and pore diameter of both the
natural and synthetic types after acid treatment and calcination. Most interesting
of all was the large increase in surface area, where the natural type increased
from 16 to 36.7 m 2/g after acid treatment and the synthetic type increased almost
13 times, from 2 to 27.7 m2/g after acid treatment. This effect had a strong
influence on the selectivity of methylamines by allowing a larger surface area to
be in contact with the feed reagents. The XRD of the treated and untreated
natural and synthetic clinoptilolite were compared and discussed. The catalytic
properties of zeolites depend on the treatment of clinoptilolite, and the
characteristic peaks of natural and the acid treated form can be identified in the
XRD plots.The TPD analyses showed that there were two acid sites associated with the
clinoptilolite; one weak and one strong. The synthetic clinoptilolite weak acid site
also showed a much lower ammonia uptake, than the natural clinoptilolite weak
acid site.
Clinoptilolite was shown to be successful as a catalyst for the production of
dimethylamine with high selectivity. This was attributed to a transition state shape
selectivity associated with the clinoptilolite channels. The natural clinoptilolite displayed good selectivities at 400°C and a 5:1 ratio of
methanol to ammonia, and gave a high conversion of > 90% ammonia to amines.
The synthetic clinoptilolite gave better conversions and lower TMA selectivity
than the natural clinoptilolite over a large range of feed ratios and temperatures.
This included conversions of over 90% for most ratios (at 400 and 450°C). The
natural clinoptilolite gave better results than those obtained using the amorphous
clinoptilolite, which is currently used in industry. For example, low TMA selectivity
was observed when a ratio of 5:1 or greater (methanol to ammonia) was used.
The TMA selectivity is still far lower than the thermodynamic equilibrium
selectivities obtained without a catalyst (62%). The catalyst lifetime has been
shown to deteriorate with time.