Enhancement of Sorption Processes in the Zeolite H‐ZSM5 by Postsynthetic Surface Modification

Diffusion in Zeolites – Impact on Catalysis

“…From this point of view, molecules do not enter the pores of the zeolite directly but a weak surface adsorption step is involved in the adsorption process [60] as already proposed by Barrer [61]. The influence of this type of transport barrier has been reduced by Reitmeier et al [62] by coating an amorphous silica layer (∼1-1.5 nm) on top of ZSM-5 crystals. With this coating, the local adsorption at the pore entrance and hence the uptake rate into the zeolite was increased.…”

“…As previously noted, Reitmeijer et al [62] succeeded by coating a thin top layer of amorphous silica on top of the crystals. Tzoulaki et al [59] obtained surface barrier free silicalite-1 crystallites by an alkaline treatment, removing a layer of amorphous silica that was the suspected surface barrier.…”

Probing the Nature of Surface Barriers on ZSM‐5 by Surface Modification

“…Moreover, desorbing molecules from external surface to gas phase is relatively easy, and the sticking coefficients are generally very low (10 -6~1 0 -7 ) [11,12,28,38]. Therefore, hopping out of a pore mouth (step 2) should be the rate determining step of surface transport; the surface properties affect this step and, subsequently, the surface barriers.…”

“…The molecule needs to overcome this high energy barrier before adsorbing on the external surface, which can cause surface barriers even for zeolites free of structural imperfections. The amorphous SiO 2 near the surface pores can reduce this energy barrier, although it could also narrow or even block the surface pores, according to the work by Lercher et al [28,38,39]. The probe molecule, n-heptane, is relatively small compared to the micropore sizes of ZSM-5 and, therefore, the change in energy barrier is more important in affecting surface barriers.…”

“…Chmelik et al [36] and Gueudré et al [37], however, reported that HF etching of silicalite-1 crystals did not change the sorption rates of isobutene and cyclohexane, respectively. Lercher et al [28,38,39] deposited a layer of amorphous SiO 2 on ZSM-5 using the CLD method, and found that the sorption rate can be enhanced or decreased depending on the structure of the probe molecules and the surface properties of zeolites. Since different batches of the same zeolite can have different properties, one may reach improper conclusions on the nature of surface barriers by simply comparing the effects of surface modification for different zeolite samples.…”