Abstract
A geopolymer with high specific surface area was synthesized from metakaolin, amorphous silica and KOH, in the absence of foaming or saponification agents. The mixture modelling technique was selected for this study among the Design of Experiments tools, using surface area and total pores volume as response variables. The Si/K = 2.46 and Si/Al = 1.37 ratios lead to the optimal experimental conditions, allowing the formation of a geopolymer having a specific surface area of 75 m2/g and total pores volume of 0.28 cm3/g. The pores had a bimodal pore size distribution (7 and 20 nm). In spite of its amorphous nature, this structure is similar to zeolites in terms of ion exchange and metal ions accommodation. Therefore, this study envisages its application as a catalyst support.
Highlights
Geopolymers are inorganic materials produced by the activation of aluminium silicate in a high pH medium, which exhibit morphological and physicochemical properties suitable to catalytic support applications
The solid grains of the aluminium silicate source are solubilised in the alkaline solution, yielding aluminate and silicate species, most probably in monomeric form
All the geopolymers samples were synthetized keeping the water content constant (30 wt%) while a different amount of Si was added in order to vary the Si/Al molar ratio
Summary
Geopolymers are inorganic materials produced by the activation of aluminium silicate in a high pH medium, which exhibit morphological and physicochemical properties suitable to catalytic support applications. It is important to emphasize that the support may act solely as a physical frame to the active agent in the reaction, i.e., the catalyst, or it can play an active role in the process[15,16] In both cases, the support must exhibit some characteristics to make it useful as a structure to the active phase, such as: (i) to yield a high exposed surface for the active agent; (ii) to enhance the stability of the catalytic material, by keeping the active phase particles highly dispersed; (iii) to increase the activity of the catalyst by providing a porous structure, allowing the access of the reactants to the active sites inside the porous channel and diffusion of the products from them; (iv) to dissipate promptly the heat from the reaction sites, avoiding the formation of hot spots and possible sintering of the active phase[11,15,16]. Among the Design of Experiments (DoE) set of tools, the mixture approach was selected due to the nature of the problem under scrutiny, i.e., the controllable variables are not independent
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