(369f) How Microwaves Enhance Interfacial Reactions!

Many reactions may be accelerated by order(s) of magnitude when exposed to microwaves[1,2]. Reaction selectivities are often enhanced. Most recently, these microwave enhancements of catalytic reactions have been demonstrated for the conversion of biological feedstocks to fuels and chemicals[3-6].

Reasons for microwave reaction enhancements are speculative, often conflicting[7]. We have demonstrated that microwaves can change the energies and/or the “effective temperature” of individual species at surfaces[8]. Moreover, variations in microwave exposure in time or space can result in significant rate enhancement.  Such variations may provide unique rate control.

We have demonstrated that microwave exposure can change the “effective temperatures” or, equivalently, the relative energies for individual sorbing and sorbed species [9]. Meta-stable intermediates in a reaction mechanism can be increased significantly, easily by over an order in magnitude, even if the activation energy maxima do not decrease. Adsorbed species are particularly susceptible to this influence. .

As an example, our QUENS show that methanol is heated over 100K above benzene during microwave exposure[8]. Thus, we anticipate that the recent studies(3-6] are the tip of the iceberg in the application of microwaves to enhance the transformation of biologically derived feeds, particularly on heterogeneous catalyst surfaces.

Changes in the intermediate energy states in a reaction sequence are found to change the reaction kinetics significantly at energies far below those required to break specific chemical bonds. Indeed, the ability of microwaves to manipulate intermediate energies within the context of a given reaction sequence could lead to significant rate enhancement without changing the mechanism. However, the relative energy profile does not have to be changed permanently to have a significant effect.

I will focus on the unpublished variation of selective heating in time and space at interfaces naturally leads to rate enhancement for catalytic reactions.  It would be possible to capitalize on this ability to manipulate intermediate reaction state energies in several ways. Periodic variations in microwave power and their influence on chemical reactions could provide knowledge about reaction rate dynamics just as “frequency response” methods are employed in control theory or diffusion analyses. Characteristic reaction times would be evident at the period of the power variations .

References

[1] W. C. Conner and Geoffrey Tompsett, J. Physical Chem. B (2008), 112- 7,  2110-18

[2] G. Tompsett. W.C. Conner*, and K. S. Yngvesson ChemPhysChem 2006. 7(2): p. 296-319

[3] Youyu Wu, Zaihui Fu, Dulin Yin, Qiong Xu, Fenglan Liu, Chunli Lu and Liqiu Mao,

Green Chem., 2010, 12, 696–700

[4] Zhen Fang a, Fan Zhang, Hong-Yan Zeng, Feng Guo, Bioresource Technology 102 (2011) 8017–8021

[5] Aurore Richel, Pascal Laurent, Bernard Wathelet, Jean-Paul Wathelet, Michel Paquot

C. R. Chimie 14 (2011) 224–234.

[6] Chun-Hui Zhou, Xi Xia, Chun-Xiang Lin, Dong-Shen Tong and Jorge Beltramini, Chem. Soc. Rev., 2011, 40, 5588–5617.

 [7] Turner, M. D.; Laurence, R. L.; Conner, W. C.; Yngvesson, K. S. AIChE Journal 2000, 46, 758.

[8] H. Jobic, J. E. Santander, W. C. Conner, Jr., G. Wittaker, G. Giriat, A. Harrison, J. Ollivier, and Scott M. Auerbach, Phys. Rev. Letters 106, 157401 (2011)

 [9] Vallee, S.; Conner, W. C. Journal of Physical Chemistry, B. 2006, 110, 15459.