JultikaUniversity of Oulu repository

Abstract

This thesis examines the novel approach of synthesizing mesoporous geopolymer by hard templating technique using cellulose nanocrystals (CNCs) as the templating agent. Geopolymers have the advantage of tailoring their porosity to form hierarchical porous structure which makes them a promising material in the field of heterogeneous catalysis. Hard templating technique is one of the options through which the porosity of the geopolymer can be tailored in more controllable manner compared to post-synthetic treatment approaches. Thus synthesizing geopolymer through hard templating technique using CNCs as template has the potential of producing mesoporous geopolymer with high surface area and high pore volume which was the prime objective of this thesis. Moreover, the synthesized geopolymer-CNC composites are also environmental friendly and cost-effective as the raw materials for geopolymers are obtained from naturally occurring materials and CNCs are obtained mainly from the trees and plants which also make them abundantly available.

This study presents the synthesis and characteristics of the synthesized geopolymer- CNC composites and investigates the reasons for not having the desired surface area and pore volume in any of the samples. A number of trials were made in the whole work by modifying the chemical composition of geopolymer-CNC composites and also by modifying the synthesis procedure. In all the trials, the molar ratio for SiO₂/Al₂O₃ for normal geopolymer was ~3.5 and for the one synthesized with high silicon content was ~6.5. Different weight percentage of CNCs was added to geopolymer in different attempt of this work. Two forms of CNC were applied; one was aqueous suspension form used as received from the supplier and another was freeze dried form. CNC template was removed from the geopolymer by calcination in air. Ion-exchange of the calcined powder was performed to generate the acidic sites into the geopolymer framework. To induce secondary mesoporosity into geopolymer structure acid wash followed by ion-exchange was also performed in one of the trials.

As the main goal of this work to synthesize mesoporous geopolymer with high surface area and pore volume, most of prepared samples underwent only N2 adsorption-desorption technique in most of the trials. Because of the low surface area and pore volume, only the samples prepared in the final attempt of this thesis underwent further analysis through XRD, FT-IR spectrometry, Raman spectrometry, TGA-MS, SEM, TEM alongside N₂ adsorption-desorption technique to investigate the reason for not getting the desired outcome. From the obtained results of different analysis, some promising results were found from the SEM micrographs of the geopolymer that was prepared with 10wt% aqueous suspension of CNCs and cured at 40° C overnight. Round shaped elements were found which were well dispersed on the geopolymer surface. These elements were assumed to be rod-shaped cellulose nanocrystals which were embedded in the geopolymer particles. The presence of these elements was found even in the samples that were thermally treated at 650° C for longer period of times.

Assuming the obtained particles in geopolymer matrix are CNCs, some modifications can be made in the future endeavors which may include acid hydrolysis of the geopolymer-CNC composite to decompose cellulose nanocrystals or thermal decomposition at a different temperature with different heating rate. If decomposed successfully, the synthesized mesoporous geopolymer may contain promising potential to be used in various heterogeneous catalysis applications.