(426h) Understanding the Colloidal Surface Interactions between Solid Acid Catalysts and Cellulose Using DLVO Theory

Successful depolymerization of cellulose to monosaccharides and subsequent recovery are the key steps towards biomass-based renewable energy. Solid acids, such as zeolites and polymers bearing functional groups (e.g. -SO₃, -COOH, etc.), have been widely studied as catalysts to hydrolyze cellulose. Compared to liquid acids, solid acids offer advantages that include ease of separation, reusability, tunability of surface properties, reduced corrosion risk, and less harm to environment. A recurring theme in the literature is that solid acid catalysts exhibiting cellulose hydrolysis activity must be bifunctional, possessing both acid groups for hydrolysis and binding groups for cellulose targeting. To date, solid acid catalyst design has been guided by molecular-level intuition with little thought about interactions at the colloidal level. As a result, the strength of colloidal interactions between solid acid catalysts and cellulose in the presence of water or other solvents remains largely unknown. Understanding the factors such as catalyst surface energy, surface potential, and dissociation constant of the functionalized solid acid are important for designing highly active catalysts. Here, we use existing theories of colloidal interactions to study the effects of surface free energy (), surface potential () and acid dissociation constant (pKa) on cellulose-acid colloidal interactions. Specifically, we have used DLVO theory to determine the effects of van der Waals forces and electrostatic interactions on colloidal stability. We find that van der Waals interactions result in an attractive force, which dominates for particle-particle separation less than about 1 nm. However, electrostatic interactions can either be attractive or repulsive, depending on the relative charge density of the colloidal particles. In fact, the surfaces of native cellulose particles in aqueous mixtures are weakly negatively charged; solid acid catalysts will also be negatively charged, owing to partial dissociation of surface acid groups. Accordingly, the resulting cellulose-catalyst interaction is repulsive at distances greater than approximately 1 nm, with the electrostatic interaction representing a formidable activation energy >10 k_bT. This finding casts doubt on previous studies claiming solid acid catalyst activity for cellulose hydrolysis. In the second part of the work, we examine potential methods to overcome this limitation, including use of non-aqueous solvents, manipulation of charge densities, and utilization of non-DLVO effects, such as hydration forces, hydrophobic effects, and steric effects.

Figure 1 Schematic of DLVO interaction between cellulose and solid acid particle