Role of Micronized Silica Gel Texture in Template-Assisted and Template-Free Synthesis of Pentasil-Family Zeolites (ZSM-5 and ZSM-11/ZSM-5)

ZSM-5 zeolites (TPAOH as the template) and mixed ZSM-11/ZSM-5 zeolites (TBAOH as the template) were synthesized hydrothermally under static conditions from two Russian-made micronized silica gels (RusSilica) in the presence of a structure-directing agent. ZSM-5 zeolites were also prepared by a template-free method. The samples were characterized by elemental analysis, X-ray diffraction, optical microscopy, dynamic light scattering, low-temperature nitrogen adsorption, and temperature-programmed NH3 desorption. The properties of the zeolites synthesized in the presence of a template using the two types of silica gel were comparable: all samples had similar Si/Al ratios (≈25), textural characteristics (specific surface area SBET = 424–454 m2/g, micropore volume Vmicro = 0.13–0.15 cm3/g, total pore volume VΣ = 0.23–0.25 cm3/g), and acidity (concentration of high-temperature sites 642–752 μmol/g) and differed only in crystal size (0.42–0.64 μm). In template-free synthesis, the choice of silica gel was critical. The silica gel with a smaller pore size and specific pore volume, higher bulk density, and lower content of water-soluble substances (VΣ = 1.24 cm3/g, Dpore = 16.1 nm, ρbulk = 254 g/L, water-soluble substances 2.0%) yielded highly crystalline ZSM-5 zeolite (Si/Al ≈ 16.5, SBET = 376 m2/g, Vmicro = 0.14 cm3/g, VΣ = 0.16 cm3/g) with the highest concentration of high-temperature sites (975 μmol/g) and the largest particle size (~9.8 μm) among all the samples. The more porous silica gel with a lower bulk density and higher content of water-soluble substances (VΣ = 1.62 cm3/g, Dpore = 23.8 nm, ρbulk = 154 g/L, water-soluble substances 10.6%) yielded a poorly crystallized product containing a residual amorphous phase and encapsulated sodium (2.5%), with lower textural characteristics (SBET = 99 m2/g, Vmicro = 0.03 cm3/g, VΣ = 0.07 cm3/g) and acidity (486 μmol/g).