Improvement On Mass-Transfer Properties Of Zeolites Synthesis And Post-Treatment

Zeolites represent an important class of materials exhibiting a well-defined hydrothermally stable framework with pores in molecular dimension(0.3-1.5 nm). One of the major drawbacks arising in catalysis and/or separation applications concerns the limited molecular transport in these micropores,and it has been repeatedly demonstrated that mass transfer limitations restrict the extent of catalytic conversions over zeolites.For example,ZSM-5 and mordenite zeolites,which are widely used as catalysts in the fluidized catalytic cracking(FCC) of petroleum and isomerization reactions for converting n-paraffins into iso-paraffins,respectively, exhibit severe mass transfer limitations during catalytic reaction processes.In order to circumvent the mass transfer limitations,materials exhibiting a hierarchical architecture of porosity have widely attracted attention of scientists and engineers.The larger pores facilitate physical transport whereas each micropore acts as a nano-reactor providing active sites and shape selectivity.Hydrothermal treatment and/or acid leaching are classical post-synthesis approaches in research labs as well as in the chemical industry to induce mesoporosity in acidic zeolites.However,the removed aluminum species after acid leaching stay in ZSM-5 zeolite channels and restrict their accessibility.An innovation methodology is desilication,a post-synthesis modification treatment,in which,contrarily to the well-known dealumination treatment,silicon is selectively extracted from the zeolite framework.The substantial mesoporosity and in particular its high accessibility to micropores make desilication an advantageous approach to manufacture hierarchically structured micro- and mesoporous zeolites for improved transport properties as compared to the classical dealumination post-treatment.In addition,the alkaline treatment does not change the micropore properties of zeolites in terms of adsorption,diffusion,and catalysis.The current work is focused on improving the mass-transfer properties of zeolites via post-treatment.The zeolites mordenite and ZSM-5 are chosen for synthesis and post-treatment in order to improve their mass-transfer properties, because they are widely applied in the chemical industry,in which their mass-transfer limitations are available to a great extent.The objectives and main results of the present study are summarized as follows:1.Large mordenite crystals were synthesized hydrothermally using silicic acid powder as a silica source and the synthesized crystals were characterized by the XRD and SEM techniques.The effects of several synthetic parameters,such as the heat pretreatment of silicic acid,the particle size of silicic acid powder,the Si/Al ratio,and alkalinity in the synthesis solution,on the formation of the mordenite crystals were systematically investigated.The results show that reducing the particle size of silicic acid powder,increasing the Si/Al ratio,and lowing the alkalinity in the synthesis solution can prevent the formation of hybrid phases such as the zeolites ANA and PHI and inherently pure MOR crystals with uniform shape and large size can be synthesized.The synthesized mordenite crystals often have a lower accessibility than expected,which is experimentally proven that Na⁺ cations and some amorphous aluminum species could be localized in the channels,resulting in a lower accessibility.The characterization with different techniques such as N₂ adsorption at 77 K and ICP-AES shows that the oxalic acid treatment can effectively remove the extra-framework aluminium species,resulting in an improved accessibility.2.Different post-treatment strategies have been used to create mesopores in the synthesized mordenite crystals.The results show that the direct base leaching of the synthesized zeolite crystals cannot create mesopores due to their low Si/Al ratios in the crystals,while when first the zeolite crystals are treated with acid leaching,which can increase the Si/Al ratio in the zeolite,and are followed by base leaching,mesopores can be created in the mordenite crystals.This is confirmed by the characterization with the XRD,N₂ adsorption at 77 K,and NH₃-TPD techniques,indicating that the meso-zeolite still remains the crystalline and microporous characteristics of mordenite,and however,its acidity becomes stronger.3.MFI-type zeolites were synthesized in the TPABr system containing fluoride and in alkaline media with 1,6-hexanediol as the template,respectively.The synthesized silicatite-1 and ZSM-5 crystals have uniform shapes and large sizes that can be controlled,which are confirmed by observation with the XRD and SEM techniques.This provides the base for fundamental research on MFI-type zeolites in terms of adsorption and diffusion.