Membrane Medium pore zeolite

Focus in the field of medium pore (ten-membered oxygen ring) zeolite-based membranes has been almost entirely on MFI-type zeolites. This zeolite and its all-silica form. 

For a zeolite T (OFF stmcture, 0.68 nm XRD pore diameter), Tanaka et al. [131], observed that the separation factor of a water/acetic acid (50/50 wt%) measured at 75°C decreased monotonically after the immersion of the membrane into the acetic acid mixmre. Initially, the separation factor and water flux were 182 and 1.46 kg/m h, respectively, and after 32 h these values changed to 86 and 1.77 kg/m h, showing a deterioration of the membrane. Cui et al. [130] also smdied the stability of crystals and membranes of zeolite T in acid medium. The powders were immersed in a 50/50 wt% water/acetic acid mixture for 7 days at 75°C and also in HCl solutions 0.5 and 1 M for 1 h at 50°C. The analysis of the samples after the treatment by ICP and XRD indicated that the sample treated in the acetic acid solution maintained its original Si/Al ratio equal to 4 however, the hydrochloridric acid treatment with the 1 M solution destroyed the zeolite stmcmre and the 0.5 M solution dealuminated the zeolite to a Si/Al equal to 8.9 and the XRD analysis corresponded to zeolite T. The membrane performance, after being used for 1 week at different water/acetic acid concentrations, remains almost unchanged and the separation factor of the membrane treated in HCl dramatically decreased as was expected. 

Microporous membranes in general (pore diameter < 2 nm), and zeolite membranes in particular, have pores whose dimensions are similar to those of many molecules. This means that often molecules cannot pass each other in a restrictive pore medium, and single file diffusion occurs. Such a molecular queuing (see Figure 11.24) may provide a new scenario for avoiding secondary reactions, that is, to increase selectivity in consecutive reaction networks with a valuable intermediate... 

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