Studies of Brønsted/Lewis Acid-Catalyzed Dehydration of Xylose to Furfural and Simultaneous Separation of Furfural by Pervaporatio

Pervaporation is a membrane-based process in which a liquid mixture is placed in contact with the feed side of the membrane while a vapor is located on the permeate side. A vacuum is used to reduce the partial pressure, and therefore the fugacity, of components in the permeate, which provides the driving force for mass transfer. Pervaporation is most often used to separate water from concentrated ethanol solutions, but may also be used to remove organics selectively, e.g. furfural, from aqueous solutions. Membranes used for such applications are typically made of polydimethylsiloxane (PDMS), but researchers have also used the PDMS-containing triblock copolymer poly(styrene-block-dimethylsiloxane-block-styrene) (SDS). Pervaporation with a furfural-selective membrane may be used to extract furfural as it is produced and concentrate it, rather than dilute it as steam stripping and LLE do.;The feasibility of pervaporation as a means for in situ furfural extraction was studied in comparison to LLE and a reaction without extraction during batch-mode furfural production. Both LLE and pervaporation with a commercial PDMS membrane were found to improve furfural yield over the reaction without extraction, but pervaporation with PDMS yielded a product phase that was 6.6x as concentrated as that obtained with LLE. Additionally, switching the PDMS membrane with an SDS membrane resulted in similar furfural yields, but the product with SDS was 10x as concentrated as the LLE product. Furthermore, the amount of furfural extracted was qualitatively different for LLE- and pervaporation-assisted reactions: LLE was limited to 85%, the equilibrium distribution of furfural among the organic and aqueous, whereas the amount of furfural extracted by pervaporation increased monotonically over time, reaching as high as 67% during experiments. The reaction/pervaporation system was simulated in order to identify the full extent of the benefits of reaction with pervaporation. In the simulations, water lost from the reactor due to removal by pervaporation was replenished at the equivalent rate. The simulations revealed that as the reaction approached complete xylose conversion, both the PDMS and SDS membranes led to product concentrations greater than was possible with LLE, while extracting nearly all (>98%) of the furfural formed. Ultimately, pervaporation with the SDS membrane could produce a product phase with 33% greater furfural yield than that achievable by LLE.;The membrane-reactor design was revised to permit continuous, pervaporation-assisted reaction in both batch- and continuous-mode operation, with both reaction and pervaporation occurring at the same temperature. Batch-mode reactions were fed water, while continuous-mode reactions were fed an aqueous xylose solution. The reactions took place at a relatively low temperature of 90 °C, catalyzed by chromium (III) chloride (CrCl3), which contributed both Bronsted and Lewis acidity. Batch-mode reactions with varying rates of pervaporation revealed that furfural extraction had no effect on furfural yield under these conditions, but a moderate pervaporation rate did lead to an order-of-magnitude increase in furfural concentration relative to that obtained without pervaporation. Pervaporation was also found to retain all of the CrCl3 inside the reactor, demonstrating a simple way to separate product from homogeneous catalyst. This enabled continuous furfural production with only an initial charge of catalyst, in which an aqueous xylose solution was fed to the reactor while a furfural/water vapor was permeated from the reactor. The furfural permeability of the SDS membrane decreased over time during the course of reactions carried out at 90 °C due likely to interactions of soluble humins with the membrane. Experiments with the cross-linked PDMS membrane demonstrated that cross-linking of the membrane can inhibit this behavior and result in a much more stable furfural permeability. Additionally, cross-linking could lead to greater membrane thermal stability, permitting the pervaporation-assisted reaction at higher temperatures, which would benefit the chemistry by allowing extraction to have an impact on furfural yield.;Pervaporation-assisted furfural production with CrCl3 and sulfuric acid at 130 °C was then simulated. Reaction rate constants were measured at this temperature but in the absence of pervaporation. Pervaporation data collected at lower temperatures were extrapolated to represent a hypothetical membrane that could operate at 130 °C. Simulations of batch-mode reactions demonstrated that increasing the membrane-area-to-reactor-volume ratio, a, would lead to higher furfural yield and more furfural extracted, but also reduce the permeate furfural concentration, demonstrating a tradeoff between furfural production and concentration. Simulations of continuous-mode reactions showed that furfural concentration and selectivity were maximized at an intermediate value of a = 0.17 cm-1. Conversely, furfural production rate increased nearly linearly with a, indicating that the optimal value of a depends on process economics and not just technical considerations.