Synthetic  zeolites  are  one  of  the  most  important  solid  acids as  catalyst.  They  are  used  for  a  variety  of  industrial  processes,  especially in the field of petroleum refining and the syntheses of petrochemicals. The first academic paper on catalysis by synthetic zeolites  appeared  in  1960  [2,  3].  In  the  following  years,  Y-type zeolites  completely  replaced  the  position  of  amorphous  silicaalumina in fluid catalytic cracking (FCC). This is due to the very high activity  of  the  zeolite  compared  with  amorphous  silica-alumina though  the  composition  and  the  local  structure  of  acid  sites  are seemingly very similar. As shown in Table 4.1, the catalytic activity of hydrogen  form of Y-zeolites  in hexane cracking  is about 30,000 times  higher  than  that  of  amorphous  silica-alumina  [4].  The  significant properties of zeolites and the related materials as solid acids can be summarized as follows:  (1)  The  catalytic  activities  of  zeolites  are  very  high  compared with that of amorphous silica-alumina, as mentioned above. The amount of acid sites is directly related to the framework aluminum  content,  and,  thus,  can  be  adjusted  through  the framework Al content.  (2)  Isomorphous  replacement  of  especially  Al  (e.g.,  by  Ga,  Fe, B, Ti)  in zeolites  is well established. The  intrinsic activity of 

acidic sites can be modified by the isomorphous substitution. The activity decreases in the order Al > Ga > Fe > B.  (3)  Zeolites,  in particular,  the  silica-rich materials,  often exhibit a high  thermal and hydrothermal stability. This allows  their use  as  catalysts  at  high  temperatures,  and  their  frequent oxidative  regeneration even  in  the presence of  some steam. This  is  a  sharp  contrast with  catalysts with  organic  groups, such  as  ion  exchange  resins.  The  thermal  stability  is  much higher than that of heterpolyacids.  (4)  Zeolites  work  as  shape-selective  catalysts  because  of  welldefined pore sizes and cages inside the structures.  (5)  Zeolites  allows  a  relatively  high  concentration  of  reactant molecules inside the zeolite cages; thus, in the presence of a zeolite,  the reaction will proceed as  if  it were carried out at higher partial pressure.  (6)  Zeolites are non-corrosive and non-toxic.