Defence of doctoral thesis: Abdolrahim Abbaszad Rafi – Sustainable Engineering, Click Chemistry and Catalysis: Modification, Fabrication and Application of Cellulosic Materials

Due to challenges such as sustainability and increasing carbon footprint, there is a growing demand to replace fossil-based materials with green sustainable alternatives like cellulosic materials. However, unmodified cellulosic materials often encounter issues like high wettability and low mechanical strength that limit their applicability. To overcome these drawbacks, functionalization and modification are crucial and inevitable. Reported methods often involve toxic/harsh conditions or reagents, and multi-step processes. The focus of this thesis is on the fabrication, functionalization, and modification of cellulosic materials through facile and eco-friendly approaches to enhance their properties and broaden their potential applications.

We started with immobilizing copper nanoparticles on controlled pore glass substrate and used it as a recyclable heterogenous catalyst for the copper-catalyzed alkyne-azide cycloaddition (CuAAC). Focusing on sustainability, we also employed cellulosic materials as catalyst supports. First, cellulose was functionalized using a mild organocatalytic approach. Then, copper or palladium nanoparticles were immobilized onto the functionalized cellulose and used as effective recyclable heterogeneous catalysts in different reactions.

Direct esterification of CNC materials with thioglycolic acid was performed enabling us to introduce thiol groups onto CNC materials. The reaction occurred under mild conditions using natural nontoxic organic acid as an organocatalyst. The method was applied on different CNC materials, producing the corresponding thiol-functionalized CNC materials. The thiol-functionalized CNC was used as a heterogeneous recyclable reducing agent to reduce Cu(II) to Cu(I), which is the active form of copper in CuAAC. The prepared thiol-functionalized CNC materials further functionalized with attaching UV active molecules via thiol-ene click chemistry.

Lactic acid (LA) functionalized CNFs were prepared by using an ecofriendly one-step reaction method in high yields. This was achieved by converting pulp fibers into nanofibrillated cellulose lactate under mild conditions, using LA as both reaction media and catalyst. The process was concurrent and involved an autocatalytic esterification reaction without using metal-based or harsh acid catalysts. Moreover, the LA media were recycled and reused in multiple reaction cycles.

In the fourth study, strong hydrophobic cellulosic materials were prepared via a facile, scalable and eco-friendly method. The method involves a betulin treatment and hot-pressing processes. First, a water-based betulin formulation was developed and used for the treatment of cellulosic materials. The betulin-treated samples were then hot-pressed. Hot-pressing altered the morphologies and led to dense structures. Moreover, it caused a polymorphic transformation of the betulin particles. Water contact angle and tensile tests revealed that the applied betulin/hot-pressing treatment method noticeably enhanced the samples’ hydrophobicities as well as their tensile strengths. Furthermore, a synergistic effect was noticed between the hot-pressing, betulin treatment, and sulfonation during the pulping process.

Densified and strong large veneers were fabricated via a facile and scalable method. The method involves a combination of chemical modifications of aspen veneers followed by hot-pressing. The study showed that hot-pressing enhanced the tensile strengths. The chemical modifications further improved the efficiency of the hot-pressing, resulting in higher tensile strengths. The chemical modifications changed the wood’s composition promoting wood softening and increasing the bonding. Since the method uses convenient and mild treatments combined with continuous hot-pressing, it enables the processing of large samples. It can also lower time/energy consumption, production costs and the environmental impact.