Date of Award

2022

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Forest Science (PhD)

Administrative Home Department

College of Forest Resources and Environmental Science

Advisor 1

Xinfeng Xie

Committee Member 1

Alper Kiziltas

Committee Member 2

David R. Shonnard

Committee Member 3

David DeVallance

Committee Member 4

Rebecca G. Ong

Abstract

Lignin is the second abundant natural polymer and has been highlighted as a potential substitute for fossil-based raw materials. However, the inherent molecular heterogeneity and the complex recovery processes result in the challenge of controlling the molecular properties and value-added applications of lignin in large scale. To address those issues, a novel acid-liquefaction process was developed in this study to recover Kraft lignin with improved molecular homogeneity directly from black liquor.

In the first study, the liquefaction parameters were screened based on yield and molecular weight properties of the recovered lignin. Then, the recovered lignin samples were used to replace 20 wt% of the fossil-based polyols to prepare flexible polyurethane foams. It was found that most of the recovered lignin had improved molecular uniformity (polydispersity (PDI) value < 2) than the traditional acid-precipitated lignin (PDI = 2.2~5.4). Also, the recovered lignin with the Mw value of 1600 Da and the PDI value of 1.8 could maintain the major properties of the flexible PU foams.

In the second study, the Box-Behnken response surface methodology (RSM) was employed to investigate the effects of the liquefaction parameters (pH, reaction temperature, and reaction time) on the yield, molecular weights, polydispersity, and quantities of different types of hydroxyl groups of the lignin. Computational models were developed and refined to establish the relationships between the liquefaction parameters and the lignin properties. The yield, molecular weight, and polydispersity of the lignin could be predicted by the optimized models with high R2(pred) values of 87.5-91.5%.

An iron-based desulfurization process was developed in third study to remove covalently bonded sulfur in the lignin. The effects of the desulfurization parameters, including reaction temperature, time, and amount of iron, on the sulfur content and desulfurization rate of the lignin were studied. It was found that the highest desulfurization rate was 39.3% at 90 ℃, 16 h, and 0.3 g iron.

A life cycle assessment on the lignin production processes and its comparison to the fossil-based polyols were demonstrated in the last study. The lignin produced from the liquefaction process with 140 ℃, pH = 7, and 9 min was more suitable for replacing the fossil-based polyols due to lower CO2 emissions and energy consumptions.

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