In recent years, the methanol-to-olefin (MTO) process has become attracted due to its competitive cost in producing ethylene and propylene in China [
1‒
9]. Among the catalysts used in this process, one of the silicoaluminophosphate molecular sieves, marked as SAPO-34, is an efficient one for its suitable channel size [
10‒
15]. Its smaller pore opening than other zeolites like ZSM-5 leads to a higher selectivity of lower olefins [
16‒
19,
6], but its smaller pore also leads to the diffusion difficulty [
20‒
23], which is strongly related to its crystal size. A bigger crystal size may result in a higher diffusion resistance and a higher probability of secondary reactions such as hydrogen transfer reactions and coke deposition reactions. Under this consideration, a detailed measurement of the diffusion of different molecules is essential to MTO study and industrial application. To the best of our knowledge, however, a detailed quantitative study of the diffusion behavior in the SAPO-34 system is lacking [
20]. Herein, we present a high accuracy diffusion test for the SAPO-34 system to determine quantitatively the influence of crystal size on diffusion resistance. It is difficult to measure directly the diffusion of ethylene and propylene because they tend to react in zeolite even at room temperature. Therefore, ethane and propane were selected to replace ethylene and propylene, respectively, for their similar molecular size and lower reactivity, though the diffusion of propylene and propane might be different due to the potential separation effect of 8-membered rings [
24,
25].