GRAPHENE OXIDE-BASED MEMBRANE FOR LIQUID AND GAS SEPARATION A Dissertation Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Han Lin August 2020 GRAPHENE OXIDE-BASED MEMBRANE FOR LIQUID AND GAS SEPARATION Han Lin Dissertation Approved: Accepted: Advisor Department Chair Dr. Jiahua Zhu Dr. Michael Cheung Committee member Interim Dean of the College Dr. George Chase Dr. Craig Menzemer Committee member Dean of the Graduate School Dr. Jie Zheng Committee member Date Dr. Siamak Farhad Committee member Dr. Xiong Gong ii Dedicated to My beloved parents and family members who helped me all things great and small. Without your endless love and support, none of my success would be possible. iii ACKNOWLEDGEMENTS I would like to express my special appreciation and deepest thanks to my advisor Dr. Jiahua Zhu, for the patient guidance, encouragement and advice he has provided in the past four years. Your advice on both research as well as on my career have been invaluable. I would also like to thank my committee members, Dr. George Chase, Dr. Jie Zheng, Dr. Siamak Farhad, and Dr. Xiong Gong for their valuable comments and suggestions that improve the quality of the work. I am grateful to all of labmates, Dr. Liwen Mu, Dr. Long Chen, Dr. Tuo Ji, Dr. Nitin Mehar, Dr. Marjanalsadat Kashfipour, and Mr. Yifan Li for their help during my struggling time. I really enjoyed time we worked together, as well as had coffee together. I would like to thank my best friends, Jiahui Wang, Chengwei Polly Zhou, and Hao Zhang for our unbelievable friendship for past 12 years. I will never forget the most precious time that we studied together at the library. I would also like to thank Dr. Tao Liu for the great suggestions and timely encouragement on both research and daily life. Last but not least, I would like to express my vehement protestations of gratitude to my parents, Jianyu Lin and Chengxia Wu, for their endless love and unconditional support. Without their sacrifices, I could never receive the best education and finish this dissertation. iv ABSTRACT Graphene oxide (GO), a 2-dimensional material, has attracted great attention in membrane research in the past decades due to its intrinsic physicochemical properties, such as the tunable surface functionality, excellent chemical inertness, and thermal stability. With a similar 2-D structure as graphene, the rich surface functional groups at both edge area and basal plane enable a vast opportunity to design the membrane functionality. However, there are still a few key challenges need to be addressed before GO based membranes can be practically used including but not limited to weak mechanical strength at nanoscale thickness, low membrane integrity due to swelling and limited approaches to tune d-spacing. Although it has been demonstrated that d- spacing, the interlayer distance of GO nanosheets, is a very important parameters that influence the separation performance of GO based membranes, the d-spacing, surface charge, wrinkling effect, and size of GO nanosheets are also important factors for excellent separation performance. Here, we presented three different approaches, that were utilized for fabricating stable GO based membranes with excellent gas and liquid separation performance. First, epoxide ring-opening reaction and subsequent modification with oxalic acid (OA) were proceeded to enrich the functional groups on the basal plane of GO sheets. By facilitating the plane-plane connection of GO sheets with activated in-plane groups, better packing (reduced wrinkling) of GO membrane would be expected and thus superior separation performance. GO membranes were fabricated by pressure-assisted v filtration method on porous polymer support. Membrane thickness was simply controlled by the amount of filtration solution. Cross-linking was introduced in the membrane to improve the mechanical strength as well as reduce swelling in aqueous media. The GO modification, cross-linking reaction, microstructure and wrinkle structure of GO membrane were systematically characterized. The prepared membranes were tested in various separation applications including H2/CO2 separation, organic dye separation from water, and desalination. The prepared membrane showed good permselective H2/CO2 separation with a separation factor of 16.14 and H2 permeance of 13.6 × 10-9 mol.m-2s-1Pa-1. In MB separation tests, the rejection rates of 70 ppm MB solution of OAGO/EDA-1 and OAGO/EDA-2 membranes achieved nearly 100% under + + - 2- 2+ 300 psi. The overall high ions (Na , K , Cl , and SO4 and Mg ) rejection rates of > 98.1% were also obtained in a pervaporation desalination system. This work offers a new strategy to fabricate modified GO membrane with strong plane-plane interaction and better ordered microstructure. Secondly, a controlled pre-cross-linking method is developed to address the swelling issue and d-spacing control simultaneously. Cross-linking and GO reduction are coupled in the pre-cross-linking reaction. Hybrid GO/rGO membranes with well- patterned layer stacking structure is fabricated by vacuum filtration method. The cross- linking reaction and microstructure evolution of the GO membranes are systematically characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscope, and X-ray diffraction. The properties of the prepared membranes are tested in both gas and liquid separation systems. The H2/CO2 separation is measured with a flow-work gas separation system, and desalination test was carried out in a pervaporation system. The hybrid membrane shows excellent permselective H2/CO2 vi - separation with a separation factor of 22.93±1.57 and H2 permeance of 2.46±0.01× 10 8 -2 -1 -1 + + 2+ - 2− mol.m s Pa . Extremely high ion (Na , K , Mg , Cl , and SO4 ) rejection rate of >99% is also obtained in a pervaporation desalination system. This work offers a new strategy to fabricate hybrid GO/rGO membrane with small d-spacing and excellent stability particularly in aqueous environment. Thirdly, amino boron nitride (a-BN) nanosheets were prepared from bulk boron nitride (BN) by ball milling method with urea as agent. By cross-linking GO nanosheets with a-BN nanosheets, the a-BNGO hybrid membrane with narrow d-spacing and long- term stability in liquid separation applications would be expected and thus advanced methylene blue (MB) separation performance. The a-BNGO hybrid membranes were fabricated by pressurization filtration method on porous polyethersulfone (PES) substrate with polydopamine (PDA) coating. Membrane thickness can be simply controlled by the amount of GO and a-BN nanosheets. A two-step cross-linking process was processed to ensure the covalent bonds connection, one was applied in a-BNGO mixture solution before membrane fabrication and another was applied after membrane was formed. The a-BN modification, cross-link reaction, microstructure and surface morphology and properties were characterized systematically. The prepared a-BNGO membranes were tested in MB separation from water under different pH conditions. The a-BNGO membrane showed the best result with rejection rates of 99.98% and water flux of 4.15 LMH within first 3-hour period and only slight drops were observed for the following second and third 3-hour periods. The a-BNGO membranes also showed a wide pH range of operation, from 4.0 to 10.0, without sacrificing any performance. This work provides a new choice of cross-linker to fabricate non-swelling GO membranes. vii At last, the effect of GO nanosheet lateral size on membrane microstructure and liquid separation performance was investigated systematically. The EDA cross-linked GO membranes were fabricated by three groups of GO nanosheets with different lateral sizes, which was simply separated by centrifugation method. The differently sized GO nanosheets form unique stacking patterns that leads to varied separation performance, especially in different pH conditions. All the membranes showed excellent long-term (24 hours) stability in neutral (pH=7), acidic (pH=4) and basic (pH=10) conditions. MB separation performance has been found closely related to GO lateral size at specific pH environments. Specifically, the highest rejection rate in GO/EDA-S, GO/EDA-M, and GO/EDA-L membranes has been found at pH=4, 7 and 10 conditions, respectively. For MO, the separation is less effective relatively as compared to MB, while rejection rate of > 96.0% can be still achieved on all three membranes. Rejection rate of > 99.0% for Cr (VI) was accomplished on GO/EDA-S and GO/EDA-M membranes taking advantage of narrowed d-spacing and enlarged ion size in acidic environment. viii TABLE OF CONTENT LIST OF TABLES ........................................................................................................ xi I. INTRODUCTION ...................................................................................................... 1 II. BACKGROUND ....................................................................................................... 4 2.1. GO synthesis and its chemical property.......................................................... 4 2.2. Separation mechanism of GO-based membranes ........................................... 5 2.3. Fabrication of GO based membranes .............................................................. 7 2.3.1. Filtration method .................................................................................. 7 2.3.2. Layer-by-layer assembling method...................................................