Synthesis of hierarchical ZSM-5 by silanization and alkoxylation of protozeolitic units

Hierarchical ZSM-5 zeolite has been synthesized by means of a method involving a precrystallization stage to form the protozeolitic units, the addition and subsequent grafting of both silanization and alkoxylation agents, and a final hydrothermal crystallization. The influence of the alkoxylation with different alcohols (methanol, ethanol, 2-propanol and n-butanol) on the properties of the final hierarchical ZSM-5 samples has been investigated. In every case, the alcohol addition increased the incorporation of the seed silanization agent as it decreases the gel viscosity. In addition, the presence of alcohols deeply affects the physicochemical properties of the final materials. The samples prepared with 2-propanol and methanol were highly crystalline and presented improved textural properties with regard to the reference h-ZSM-5 and n-ZSM-5. In contrast, the samples obtained with ethanol and n-butanol were partially and totally X-ray amorphous, respectively. 1H and 13C-CP solid state MAS NMR spectra proved the alkoxylation of the external surface of the protozeolitic units. Catalytic cracking of LDPE pointed out the higher TOF values obtained over the hierarchical samples prepared with methanol and 2-propanol due to a right combination of accessibility and crystallinity in these materials. The differences observed among the samples prepared with alcohols were ascribed to the strong interaction produced between the silanization agent and the linear alcohols on the surface of the protozeolitic nanounits, which form a very stable protective layer, hindering their aggregation and subsequent crystallization.

The incorporation of alcohols during the preparation of hierarchical ZSM-5 zeolite through the seed silanization method affects deeply the physicochemical properties and catalytic activities of the final materials. The alcohols, added in the silanization step, not only increase the degree of the organosilane grafting onto the external surface of protozeolitic units, but they play also a complementary role as they are anchored to the latter by alkoxylation reactions. These facts results in an enhancement of the contribution of the secondary mesoporosity in hierarchical ZSM-5. The best results are obtained with the samples prepared with 2-propanol and methanol as they combine a high crsytallinity and accessibility, showing improved activity in polyolefin cracking.Figure optionsDownload high-quality image (278 K)Download as PowerPoint slide