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Subcritical water has potential as an environmentally friendly solvent for applications including hydrolysis, liquefaction, extraction, and carbonization. Here, we report hydrolysis of sugarcane straw, an abundant byproduct of sugar production, in a semi-continuous reactor at reaction temperatures ranging from 190 to 260°C and at operating pressures of 9 and 16MPa. The target hydrolysis products were total reducing sugars. The main products of sugarcane straw hydrolysis were glucose, xylose, arabinose, and galactose in addition to 5- hydroxymethylfurfural and furfural as minor byproducts. Fourier transform infrared spectroscopy and thermogravimetric analysis provided additional information on the surface and bulk composition of the residual biomass. Char was present on samples treated at temperatures equal to and greater than 190°C. Samples treated at 260°C contained approximately 20wt% char, yet retained substantial hemicellulose and cellulose content. Hydrolysis temperature of 200°C provided the greatest TRS yield while minimizing char formation.

Dodecane cracking and aromatization over ZSM‐5 was studied in the presence and absence of supercritical water (SCW). A group‐type model was used to determine five best‐fit rate constants to describe yields to aliphatics, aromatics, coke, and gases. SCW accelerated gas formation while suppressing coke formation. CO and CO2 were formed in the presence of SCW, but not in its absence; a new, low‐temperature coke gasification pathway was suggested to account for this observation. Similarly, a low‐temperature alkane reforming pathway was hypothesized to explain the increased relative rate constant for production of gases in the presence of SCW compared with its absence. Additional tests and analysis indicated that these effects could not be ascribed solely to zeolite degradation in the presence of SCW, implying that water directly influences the reaction mechanism. These results provide new insights into the role(s) of water during oil cracking under supercritical conditions.

Increasing the stability of zeolites in liquid, near-critical, and supercritical water at temperatures greater than 200 °C is required for many important chemical processes, notably including hydrothermal liquefaction. Extending ZSM-5 stability under these conditions was studied using silylation, hydrothermal carbonization, and sequential hydrothermal carbonization and pyrolysis treatments. Coatings resulting from silylation and hydrothermal carbonization were not hydrothermally stable at temperatures greater than 300 °C. In comparison, the coating formed by sequential hydrothermal carbonization and pyrolysis was thermally stable at temperatures as great as 550 °C and hydrothermally stable in the presence of a supercritical water phase at 400 °C. The carbon-coated ZSM-5 (termed C-ZSM-5) was studied for stability and activity for dodecane cracking in supercritical water. C-ZSM-5 exhibited similar cracking activity compared with ZSM-5 while retaining more crystallinity and Brønsted acid sites (BAS) density. The C-ZSM-5 failure mechanism is the disruption of the carbon–zeolite interface during use. C-ZSM-5 has the potential for extending zeolite stability in liquid water at more severe conditions than previously examined.

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