Solar Photocatalytic Phenol Polymerization and Hydrogen Generation for Flocculation of Wastewater Impurities

Abstract

Achieving global sustainability will require balancing encroaching climate changes while maintaining existing quality of life. Using sunlight to purify wastewater while simultaneously generating usable fuels is an opportunity to approach both targets in a cost-efficient manner. In addition, converting biomass products to usable polymers is a sustainable approach for potentially replacing polystyrene or other petroleum derived polymers. Phenols from medical, manufacturing, and agricultural waste are commonly found in many water sources, and they are known to foul common reverse osmosis membranes. Here, we show oxidative polymerization of guaiacol, an aromatic compound derived from biomass, with concurrent hydrogen gas generation by using platinum-seeded cadmium sulfide nanorods (Pt@CdS) as photocatalysts. Rather than forming short oligomers as typically made by enzymes such as laccase and peroxidase, the resulting polymers show higher molecular weights that can more easily flocculate out of water. By comparing guaiacol conversion to molecular weight and dispersity, the guaiacol was found to polymerize via a chain-growth process. We also show that Pt@CdS can polymerize other phenols as well by testing the monomers phenol, 2,6-dihydroxybenzoic acid, gallic acid, and vanillin. Lastly, because the aqueous solubility of these aromatic polymers decreases dramatically with molecular weight, polymerization reactions were also tested in biphasic solutions to determine if chain growth could propagate in the oil phase. We show that the Pt@CdS nanoparticles can form stable Pickering emulsions in various biphasic combinations, and that both H₂ formation and polymer molecular weight correlated with the partition coefficient of guaiacol into the oil phase as well as the solubility of the growing polymer chains. These combined studies demonstrate the possibility of using nanoscale photocatalysts to oxidatively polymerize phenolic substrates via a chain-growth mechanism, thereby providing a path for pretreating water by flocculating out contaminants with concurrent generation of hydrogen.

Keywords: Biphasic Systems; Green Chemistry; Phenol Polymerization; Photocatalysis; Wastewater purification.

Figure 1:

(a) Transmission electron microscopy (TEM) image of Pt-decorated CdS nanorods (Pt@CdS). Scale bar = 100nm. (b) Ultraviolet-visible light (UV-Vis) spectrum of Pt@CdS.

Figure 2:

(a) Production of H₂ after 3 h irradiation of varying amounts of guaiacol with 50 nM of Pt@CdS. (b) Images of a typical reaction solution before irradiation (left) and after 6 h irradiation (right). (c) Percent conversion of guaiacol monomer via colorimetric assay after an irradiation time of 6 h. Error bars represent one standard deviation from triplicate measurements.

Figure 3:

Gel permeation chromatography (GPC) spectra of guaiacol polymerization products after 6 h reaction with Pt@CdS and irradiation (a) or with the enzyme laccase and oxygen (b).

Figure 4:

(a) Compilation of ¹H NMR spectra of guaiacol and products formed after reacting guaiacol with either laccase or Pt@CdS (b) Compilation of ATR-FTIR spectra of guaiacol and products formed after reacting guaiacol with either laccase or Pt@CdS

Figure 5:

(a) Average molecular weight values and (b) dispersity of the molecular weight values plotted against the % conversion values of the polymerization of guaiacol catalyzed by Pt@CdS. M_n (black squares) is the number average molecular weight, M_w is the weight average molecular weight (red circles) and the hollow symbols represent the guaiacol monomer.

Figure 6:

(a) Conversions measured by PB assay and (b) GPC spectra for reactions of Pt@CdS with various phenolic substrates after 6 h. Error bars represent one standard deviation from triplicate measurements.

Figure 7:

(a) Production of H₂ after 3 h irradiation for biphasic tests with 50nM Pt@CdS and 60mM guaiacol (b) GPC spectra for biphasic reactions after 6 h for all products from organic and aqueous phases (c) Partition coefficients, guaiacol concentrations, and polymer distribution data for the biphasic tests. Error bars represent one standard deviation from triplicate measurements.

Scheme 1:

Reaction scheme for oxidative polymerization of guaiacol and concurrent hydrogen evolution on Pt-seeded cadmium sulfide nanorods (Pt@CdS).