Zeolites are strongly acidic compared to silico aluminophosphates. I like to know the reason.
Actually Zeolites are aluminosilicates, where Si/Al is greater than 100. So the Al ions are randomly distributed in silica framework. Each Al incorporation create one Bronsted acid site. So random distribution of Al in Si framework give stronger acidity. In case of Aluinophosphate, aluminium and phosphorous are in alternative in their oxide framework. When we incorporate Si in this we will get silicoaluminophosphate (SAPO). Si may replace phosphorous alone or 2 Si for Al and P both. The maximum possibility of alternative Al exist. Alternative Al will reduce negative charge around oxygen. So ion exchange capacity reduced. Acidity is less.
Zeolites are alumino silicates it is acidic nature. When introduced to phophates its acidic nature increases due to strong interaction of zeolites and phosphate group its acidic nature increases. In your reaction alumino silico phosphates are more suitable.
One way to explain is by choosing Electronegatvity difference between Silicon (E.N=1.9) and Aluminium (E.N=1.6).
given that they follow lowenstein rule,
In case of Zeolites, An aluminium atom is surrounded by four high electronegative Si(1.9) atoms. Hence the electoneutrality of the network is disturbed to a maximum leading to negative framework. the protons that compensate to restore the electroneutrality will give strong Brønsted acidity
Whereas, in AlPOs, an isolated Silicon is surrounded by four Al atoms, which have low E.N of Aluminium(1.6). Hence, the acid site generated by Si atom midst of Aluminium island is comparatively weak or mild.
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