UTFacultiesTNWEventsFULLY DIGITAL (UNTIL FURTHER NOTICE) : PhD Defence Mariël Elshof | Advances in thin-film composite membranes for demanding industrial conditions

FULLY DIGITAL (UNTIL FURTHER NOTICE) : PhD Defence Mariël Elshof | Advances in thin-film composite membranes for demanding industrial conditions

Advances in thin-film composite membranes for demanding industrial conditions

Due to the COVID-19 crisis the PhD defence of Mariël Elshof will take place online (until further notice).

The PhD defence can be followed by a live stream.

Mariël Elshof is a PhD student in the research group Inorganic Membranes (IM). Her supervisor is prof.dr.ir. N.E. Benes from the Faculty of Science and Technology (TNW).

Membrane technology is a well-developed technology and membranes are used in a wide range of applications, such as drinking water production, wastewater treatment, and fractionation of products in the food industry. However, the application of membranes under more demanding industrial conditions is more challenging,  mainly because of the limited stability of commercial membranes. Therefore, research has focused on the development of new types of membranes that have improved stability, and that can expand the range of membrane applications. This thesis focuses on the synthesis of such new types of membranes, for two types of demanding applications. Firstly, nanofiltration membranes that can be used under extreme pH conditions. Secondly, membranes that can be used for hot gas separation applications.

Chapter 1 provides a brief introduction to the field of membrane technology and explains the different preparation techniques that are used in this thesis. Furthermore, it provides the challenges for both pH stable nanofiltration membranes and thermally stable gas separation membranes, and gives an overview of relevant previous research on these topics.

Chapter 2 describes the synthesis of poly(aryl cyanurate) thin-film composite nanofiltration membranes, via interfacial polymerization of 1,1,1-tris(hydroxy phenyl)ethane and cyanuric chloride on top of a polyethersulfone support. Cyanuric chloride has gained interest as a monomer in interfacial polymerization because it allows for formation of membranes that do not contain the hydrolysis-susceptible amide bond; this bond inherently limits the stability of traditional polyamide membranes. The membranes display a typical nanofiltration behavior with a pure water permeance (PWP) of 1.77 L·m-2h-1bar-1 and molecular weight cut-off (MWCO) of 400 g·mol-1. A negative surface charge is apparent from the retentions of  Na2SO4  (97.1%) > MgSO4 (92.8%) > NaCl (51.3%) > MgCl2 (32.1%). The pH stability is assessed by comparing the performance before and after exposure to 0.1 M HNO3 and NaOH. Degradation of the membranes is observed that can be attributed to the aryl cyanurate bond, that acts as an ester like bond, and is hydrolyzed when exposed to extreme pH.

In Chapter 3, 1,3,5-tris(bromomethyl)benzene is presented as an alternative monomer for the preparation of pH stable nanofiltration membranes. Polyamine thin-film composite nanofiltration membranes are prepared by interfacial polymerization between p-phenylenediamine and 1,3,5-tris(bromomethyl)benzene. Similar to when using cyanuric chloride, the absence of amide bonds results in a pH stable membrane. The membranes show a pure water permeance of 0.28 L·m-2h-1bar-1 and MWCO of 820 g·mol-1. By exchanging p-phenylenediamine for m-phenylenediamine, the performance could be enhanced to a permeance of 1.3 L·m-2h-1bar-1 and MWCO of 566 g·mol-1.

An alternative approach to prepare thin-film composite membranes is via the layer-by-layer coating of polyelectrolytes. In Chapter 4 we provide a systematic study on the long-term pH stability of four different polyelectrolyte multilayer nanofiltration membranes. The four different systems represent combinations of both strong polyelectrolytes, that are charged over the full pH regime, and weak polyelectrolytes, whose charge is dependent on the pH. The pH stability is evaluated by comparing the performance before and after exposure to 1 M HNO3 and 1 M NaOH. Poly(diallyldimethylammonium chloride) (PDADMAC)/poly(styrenesulfonate) (PSS) nanofiltration membranes show excellent stability under both extreme acidic and basic conditions for more than 2 months (10.7 L·m-2h-1bar-1 PWP, 95.5% MgSO4 retention, 279 g·mol-1 MWCO). The high stability is attributed to the use of strong polyelectrolytes. Poly(allylamine hydrochloride) (PAH)/PSS membranes show to be stable when exposed to extreme acidic conditions (9.7 L·m-2h-1bar-1 PWP, 97.5% MgSO4 retention, 249 g·mol-1 MWCO). This is explained by the fact that the weak polyelectrolyte, PAH, remains charged under these conditions and therefore results in a stable multilayer. The results indicate that polyelectrolyte multilayer nanofiltration membranes, and specifically PDADMAC/PSS membranes, have a great potential for use under extreme pH conditions.

 

The previous three chapters all focus on the preparation of pH stable nanofiltration membranes. In the next chapter, another demanding industrial condition is assessed, namely membranes for use in hot gas separation applications. Chapter 5 reports on the preparation of hybrid polyphosphazene thin-film composite membranes by the interfacial polymerization of poly octahedral oligomeric silsesquioxane (POSS) with hexachlorocyclotriphosphazene (HCCP) atop of a ceramic support. By the combination of polyphosphazenes, which are known for their thermal stability and hybrid nature, with octa aminopropyl POSS, a network is obtained with moderated polymer dynamics that makes effective molecular sieving possible. Single gas permeation experiments reveal that the membranes have permselectivities as high as 130 for H2/CH4 at 50 ºC. At the higher temperatures of 250 ºC permselectivities persist; H2/N2 (40), H2/CH4 (31), H2/CO2 (7), and CO2/CH4 (4) respectively, while also maintaining permeances in the order of 10-7 – 10-8 mol·m-2s-1Pa-1. Compared to other polymer-based membranes, especially the H2/N2 and H2/CH4 permselectivities are high, which makes them potentially interesting for use in high-temperature gas separation applications.

The last chapter of this thesis, Chapter 6, reflects on the obtained results in the different chapters. Furthermore, the so-called “challenges” as faced during this thesis are also discussed. Finally, some perspectives for future research are given.