Abstract
Selective heating of microwave-absorbing solid catalysts in a heterogeneous medium may affect a chemical reaction; such selectivity cannot be achieved by conventional oil-bath or steam heating methods. Moreover, microwave methods are often misunderstood with respect to equipment and temperature measurements, so that additional experimentation is necessary. In this regard, the present study intended to clarify the effect of microwave selective heating on acid hydrolytic processes using a sulfonated activated carbon catalyst (AC-SO3H). The model reaction chosen was the acid hydrolysis of cellulose carried out in a Pyrex glass microwave reactor, with the process being monitored by examining the quantity of total sugar, reducing sugar, and glucose produced. Heat transfer from the catalyst to the aqueous solution through absorption of microwaves by the catalyst occurred as predicted from a simulation of heat transfer processes. The resulting experimental consequences are compared with those from the more uniform microwave conduction heating method by also performing the reaction in a SiC microwave reactor wherein microwaves are absorbed by SiC. Some inferences of the influence of microwave selective heating of carbon-based catalyst particles are reported. Under selective heating conditions (Pyrex glass reactor), the yield of glucose from the acid hydrolysis of cellulose was 56% upon microwave heating at 200 °C, nearly identical with the yield (55%) when the hydrolytic process was performed under mainly conventional heating conditions in the SiC reactor. Although the beneficial effect of catalyst selective heating was not reflected in the reaction efficiency, there were substantial changes in the state of adsorption of cellulose on the catalyst surface.
Highlights
The last two decades have witnessed the emergence of microwave radiation as a new heat source with which to carry out chemical reactions
The current study examined the formation of glucose from the acid hydrolysis of cellulose in the presence of sulfonated activated carbon-based heterogeneous catalyst particulates (AC-SO3 H), a reaction of significant relevance and of important consequence to Green Chemistry
[17] inferredwith that the the biomass, enhanced rate of hydrolysis of cellulose is likely to a of notable interaction of microwaves with the and further demonstrated that the selectivity anddue yield glucose depend on the microwave power biomass, and further demonstrated that the selectivity and yield of glucose depend on the microwave density and distribution; the latter demonstration was outside the scope of the present study
Summary
The last two decades have witnessed the emergence of microwave radiation as a new heat source with which to carry out chemical reactions. Combining heterogeneous catalysts and microwave radiation presents several advantages [5] in that some solid catalyst supports (e.g., activated carbon [6]) are strong microwave absorbers and can function as an internal heat source within the solution. The use of catalysts in green chemistry is an important practice specified in the 12 Principles of Green Chemistry [8], with regard to microwave-induced heating of solid catalysts that can increase the green extent and enhance reaction efficiency, not to mention the significant energy savings in chemical processes. The effects of microwave-induced selective heating of microwave-absorbing heterogeneous catalysts (MAHCs) on a chemical reaction that originates at the catalyst/solution interface can be substantial. Catalysts 2017, 7, and production of231 renewable materials could be achieved
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