Preparation of porous metakaolin-based alkali-activated materials by microwave curing as supports for non-noble metal catalysts

Abstract

Generally, alkali activated materials (AAMs) are prepared by curing an alkaline paste via conventional methods such as at room temperature for weeks, even months, or in oven for days to achieve the required properties. In this paper, fast electromagnetic radiation (microwaves) was investigated in detail as an alternative to conventional methods to produce environmentally friendly porous metakaolin-based AAMs as supports for catalysts (MK-AAMs). Hydrogen peroxide was used as a foaming agent to achieve porosity of the porous MK-AAMs. The non-porous MK-AAMs without a foaming agent were used as reference for the porous MK-AAMs. Curing of the MK-AAMs were conducted at room temperature (CU1), in an oven (CU2), and by microwaves (CU3). The specific surface area (SSA) of porous and non-porous MK-AAMs cured via CU1–3 was 36, 49, 90 m2/g and 9.0, 23, 24 m2/g, respectively. The average compressive strength (MPa) was 3.8, 2.7, 2.1 and 8.9, 6.4, 5.2, respectively. The catalytical performance of porous and non- porous MK-AAMs with Cu-catalyst (i.e., Cu-composites) were tested with an aqueous solution of bisphenol A (BPA) in catalytic wet air oxidation (CWAO). After 5 h of oxidation, the highest BPA conversion of 97 % was achieved with the porous Cu-composites. A slight mass loss of 1.5 wt % was observed in the Cu composites due to erosion caused by strong fluid flow in the CWAO experiments. Based on the various analyzes performed, no significant structural or chemical changes were observed in porous and non-porous MK-AAMs regardless of the curing method of CU1–3, which also confirmed that MK-AAMs can be produced by microwaves without losing the desired properties of the materials, such as porosity and mechanical durability.