Biomass Photoreforming via "Green" Photocatalytic Routes

Photocatalysis is an attractive approach for utilizing the abundant solar energy for coupled synthesis of ‘green’ fuels and chemicals. The implementation of clean and economically sustainable photocatalytic processes requires the use of low cost, scalable, ‘green’, and Earth abundant materials. Despite recent advances in the field, many photocatalytic systems still use expensive sacrificial electron donors which limit sustainability and create toxic by-products. Lignocellulose is a sustainable alternative to traditional electron donors, which enables the production of ‘green’ H2 and value-added organics via photoreforming. In this work, we have developed sustainable photocatalytic systems which are based upon the metal-free, carbon nitride (CNx) photocatalysts. CMB0.5-CNx and CMB0.05-CNx, synthesized from the calcination of supramolecular assemblies (SA) derived from low-cost organics, cyanuric acid (C), melamine (M), and barbituric acid (B). The benchmark NH2-CNx photocatalyst was also synthesized from melamine for comparative purposes. All photocatalysts underwent characterization by ATR-FTIR, UV/Vis, and XRD, which confirmed their successful preparation, along with relevant information on their structural characteristics (i.e., lattice/interlayer spacings), and band gap energies. In an attempt to overcome the most significant limitation of the traditional biomass photoreforming systems, which use extreme alkaline conditions to digest biomass, this work will employ the enzyme cellulase to digest α-cellulose, as it is a ‘greener’ and more sustainable alternative approach. The capacity of CMB0.5-CNx, CMB0.05-CNx, and NH2-CNx to photoreform biomass substrates, 4-MBA, glucose, cellobiose, α-cellulose, and digested α-cellulose, was evaluated in various aqueous media including phosphate buffer (KPi, pH 4.5 and 7), and NaOH (pH 14 – 15), using Pt (8 wt%) and MoS2 (4 wt%) as the co-catalysts. The best H2 yields for all biomass substrates were obtained with the NH2-CNx/Pt system under alkaline conditions (pH 14 – 15), and the lowest H2 yields in acidic and benign conditions (4.5 – 7). Glucose was proven to the best substrate, and α-cellulose the most challenging to photoreform. In general, NH2-CNx/Pt was the best biomass photoreforming system, followed by CMB0.05-CNx/Pt, whereas CMB0.5-CNx/Pt showed no H2 evolution capacity. CMB0.05-CNx showed twice higher H2 evolution capacity when tested with MoS2 compared to Pt, which is very promising for the development of sustainable photocatalytic processes to couple H2 evolution and biomass reforming