Explain why alumina-containing zeolites possess chemically active surfaces
Alumina containing zeolites have composition that can be represented as 

 

where M is metal ion with m charge, and so on, respectively. 
Meaning you have $[SiO_4]^{4-}$ and $[AlO_4]^{5-}$ tetrahedra which make up negatively charged framework. Positive ions compensate for this negative charge. There is a rule from 1954. called Löwenstein rule, which states that x/y has to be less than 1, which in turn implies there are no Al-O-Al bonds, as two tetrahedra have Al atoms in their tetrahedral positions. This gives really great internal specific area and resistance to high temperature, which makes this type of zeolite a great candidate for catalysis (heterogenous).
The following picture nicely represents the active sites in zeolites - they act as molecular sieves, as only the specific molecules fit in the pores: