NH3-TPD was used to characterize the acidic properties of the samples. NH3-TPD profiles of SAPO-34, Zn-SAPO-34 and Mn-SAPO- 34 are shown in Fig. 5. The desorption temperature indicates the acid strength of sample. For Mn-SAPO-34 or Zn- SAPO-34, two distinctly resolved maxima NH3 desorption peaks appearing from 120 8C to 400 8C or 400 8C to 800 8C, which were indication of the presence of acid sites in two different strengths, were observed. The desorption peaks at low and high temperature corresponded to the weak and moderately strong acid sites in the molecular sieve framework, respectively. The acid amounts corresponding to the amounts of adsorbed NH3 were estimated from the areas underlying the NH3-TPD curves. The relative distribution of these two types of acid sites 1:3, suggesting the presence of a substantial amount of moderately strong acid sites in the Mn-SAPO-34 or Zn- SAPO-34. Compared with SAPO-34, modified samples exhibited higher desorption temperature of weak acid sites and possessed more weak and strong acid sites due to incorporation of Mn2+ or Zn2+ into the SAPO-34. Additionally, Mn-SAPO-34 sample possessed the most amounts of weak acid sites while Zn-SAPO-34 possessed the most amounts of strong acid sites.