In order to investigate the relative rates of hydrolysis and dehydration, experiments were conducted on xylose, xylan, and xylobiose using a small, glass reactor that was heated with microwave energy [CEM-Discover]. In these experiments, products were measured using high pressure liquid chromatography (HPLC). The microwave reactor system con­sists of an 8 mL glass tube enclosed inside the cavity of a microwave heating unit. Ex­periments were conducted with 2 mL of solution containing the substrate in an aqueous solution of sulfuric acid (1.2% by weight). The tube was fitted with a Teflon-coated cap and a pressure sensor, which are designed to contain pressures up to 250 psi. The tubes contained a Teflon-coated stir bar and the temperature was measured with an optical py­rometer. Batch experiments were conducted in which the samples were heated to a fixed temperature.

Hydrolyzates were subjected to chemical analysis using HPLC to determine the con­centrations of xylose, xylobiose, and xylose degradation products present in the reaction solutions. The solutions were analyzed using an Agilent 1100 series HPLC with an HPX — 87H column and a precolumn (Bio-Rad Laboratories) operated at 65°C. The eluant was 0.01N H2SO4 flowing at 0.6 mL/min. Samples and standards were injected (10 ^L) onto the column after filtering through a 0.45 ^m nylon membrane filter (Pall, Acrodisc Sy­ringe filter). Solute concentrations were measured with an Agilent 1100 refractive index detector controlled to 45° C and a diode array detector. The detectors were calibrated with a set of four standards for all solutes except xylobiose, which had a single-point calibra­tion. The HPLC was controlled and data was analyzed using Agilent Chemstation software (rev A.09.03).

Before the microwave heating system was used for kinetic measurements of xylobiose and xylan decomposition, an accurate temperature in the reactor was obtained. The provided optical pyrometer measures the infrared light emitted from the reactor and could provide an inaccurate temperature if the walls of the reactor were cooler than the solution. A more accurate technique for measuring the temperature would be to use a chemical reaction with known activation energy (chemical thermometer). In this study, we used the thermal decomposition of xylose in acid solutions as our chemical thermometer. We measured the decomposition of xylose at a fixed nominal temperature and compared the measured rate constant to the values reported in the literature to extract an effective temperature. The relationship between the rate constant for the decomposition of xylose and the temperature has been reported (19) to be

k = 0.0453a 8ухСце-3570( т ) (Equation I)

where a is the ratio of the density of xylose solution to that of the solution without xylose (a = 1), 8 is the specific gravity of water at a given temperature relative to the specific gravity at 30°C, yx is a correlation constant that was empirically determined (yx = 0.95), Cn is the acid concentration, and т is the absolute temperature.

These data were then used in Equation I to determine the actual reactor temperatures, which are shown in Table 9.2. As Table 9.2 shows, the effective temperature is typically slightly higher than the nominal temperature measured by the optical pyrometer. Since the microwaves heat the solution directly, it is reasonable that the glass reactor tube would have a lower temperature than the solution. The optical pyrometer measures the temperature on the surface of the glass and it is not surprising that the nominal temperature is lower than the effective temperature in the solution.