4. Conclusion
The chemical treatment of kaolinite with H2O2 resulted in partial exfoliation/ delamination of kaolinite, decreasing the amount of acidic sites by increasing their strength. Calcination has no effect on the acidity amount, decreased their strength and enlarged the particle size. Ball milling decreased the particle size, increased the amount of acidic sites with the same strength of KT sample resulted in increasing the catalytic activity at low temperature but decreased at high temperature. Calcination and ball milling create active Al–OH and Si–OH on the external surfaces (basal planes and edges) which interacted with CaSO4 in KB-Ca. This interaction resulted in a creation of more acidic sites with slightly lower strength relative to KB. So the catalytic activity decreased at lower temperatures but increased at higher temperatures up to 400 °C then decreased with rising temperature. The catalytic activity increased by increasing the maximum temperature of acidic sites. The chemical and mechanical treatments resulted in complete selectivity to DME up to 400 °C and increased the thermal stability up to 500 °C except with CaSO4 which gives the maximum selectivity to DME (84%) at 400 °C.