Removal Of Organic Templates From Porous Materials

Inorganic porous materials with controlled pore-size distribution are veryattractive for wide range of applications such as catalysis, adsorption, and separation,etc. Two main families have been reported: crystalline microporous solids and orderedmesoporous materials. Most of these solids are prepared by hydrothermal orsolvothermal treatment of an inorganic mixture in the presence of an organic templatearound which the inorganic framework forms. After synthesis, the resulting material(as-synthesized material) is a nonporous organic-inorganic hybrid since the organicagent is usually trapped in the cavities of the solids. To obtain a porous solid that isuseful, it is necessary to remove the organic templates from the as-synthesizedmaterials.Generally, the organic templates are removed by calcinations, where thecomposites are treated in air at a temperature higher than 823 K. However, thismethod is not always effective and may damage the porous materials when theinorganic solid is highly temperature-sensitive. Besides, the template is destroyedand can not be recovered in this process, increasing the cost significantly. These urgeus to find other new mild methods for removing organic template from inorganicsolids to obtain genuine porous materials.Firstly, the thermal decomposition mechanism of amine-templatedaluminophosphates was investigated using X-ray powder diffraction (XRD),thermogravimetric analysis (TGA) and temperature-programmed decompositioncoupled with mass spectrometer (TPD-MS), Fourier transform infrared spectroscopy(FTIR), and Solid-state NMR. It is found that the decomposition mechanism of template is related to the pore size and the types of templates. In the case ofAlPO₄-HDA, the hydrogen bond between the template and the inorganic framework isbroken first, and then the template undergoes a Hoffman elimination raction followedby successiveβ-elimination raction. As for AlPO₄-21, the thermal removal of templateand bridging OH groups between two Al atoms is an endothermic process occurringin a single step, which accompanies the structure transformation of AlPO₄-21 toAlPO₄-25. The temperatures to remove different organic amines occluded withinAlPO₄-21 are directly related to the shapes and sizes of the templates, as well as theproton affinities of guest cations. These factors also determine the removalmechanisms of templates from AlPO₄-21. Two different thermal mechanism oftemplate removals are observed. The linear and small templates, such as ethylamine(EA), are mainly liberated from the pores through desorption at relatively lowertemperature. The complex and bulky templates, such as trimethylarnine (TMA), weredecomposed to small moleculars at higher temperature and then relaesed from thepores.Secondly, a novel non-destructive procedure microwave-assisted extraction hasbeen developed for removing organic templates from mesoporous and microporoussolids. It takes less than 10 minutes to completely remove organic templates inMCM-41 and MCM-48 using microwave-assisted extraction with solutions of an acidor salt in ethanol, which is much shorter than the traditional solvent extraction method.Further more there is less solvent consumed and higher template removal efficiency.The resulting materials show higher surface area and larger pores than the calcinedsamples. As for microporous SAPO-5, up to 90% of the templates can be removedusing microwave-assisted extraction with an acid methanol solution. Compared to thecalcinated products, SAPO-5 catalysts resulted from this treating have more Bronstedacid sites which lead to higher activity for isomerization of m-xylene.Finally, two novel mild chemocal methods have been developed for the removalof organic from mesoporous and miceoporous materials. One is the UV/H₂O₂ methodwhich has been developed to remove organic templates from mesoporous silica SBA-15 and microporous zeolite beta. This method combines UV irradiation anddilute H₂O₂ treatment, allowing complete removal of the organic templates in 3-4 h atroom temperature. Effects of some inportant factors, such as pH value, initial H₂O₂concentration and Fe²⁺, have been investigated to search for the optimum condition oftemplate removal. Detailed characterizations of the samples treated by UV/H₂O₂method in comparison with other methods are performed with the aid of powder XRD,TGA, FTIR, solid-state NMR spectra and nitrogen adsorption. The final productresulted from UV/H₂O₂ treatment has well-defined structure with perfect pore quality.This study provides an easy, cheap, mild and useful method for template removalfrom porous materials at room temperature. The other one is called pulsedhigh-voltage discharge plasma method. Complete removal of organic template fromZrSBA-15 was achieved using this method in less than 1.5 h at room temperature.Pulsed high-voltage discharge in water may produce several active species, suchas·OH,·H,·O, H₂O₂, O₃. These oxidants can react with the organic template occludedwithin the pores of the inorganic solids, and degrade them into CO₂ and H₂O, whichwill then be released from porous solids. The resulted materials show higher surfaceareas, larger pore volumes and lower structure shrinkage compared with the calcinedsamples. It provides an alternative mild method for the removal of organic templatefrom porous materials.