Combining Solar Steam Processing and Solar Distillation for Fully Off-Grid Production of Cellulosic Bioethanol Oara Neumann, †,#,∇ Albert D. Neumann, §,∇ Shu Tian, §,#,∇ Christyn Thibodeaux, †,# Shobhit Shubhankar, †,# Julius Mü ller, ∥,# Edgar Silva, ⊥ Alessandro Alabastri, †,# Sandra W. Bishnoi, †,# Peter Nordlander, ‡,# and Naomi J. Halas* ,†,‡,∥,# † Department of Electrical and Computer Engineering, ‡ Department of Physics and Astronomy, § Department of Civil Engineering, ∥ Department of Chemistry, ⊥ Department of Mechanical Engineering, # Laboratory for Nanophotonics and the Smalley-Curl Institute, Rice University, 6100 Main Street, Houston, Texas 77005, United States * S Supporting Information ABSTRACT: Conventional bioethanol for transportation fuel typically consumes agricultural feedstocks also suitable for human consumption and requires large amounts of energy for conversion of feedstock to fuel. Alternative feedstocks, optimally those not also in demand for human consumption, and off-grid energy sources for processing would both contribute to making bioethanol far more sustainable than current practices. Cellulosic bioethanol production involves three steps: the extraction of sugars from cellulosic feedstock, the fermentation of sugars to produce ethanol, and the purification of ethanol through distillation. Traditional production methods for extraction and distillation are energy intensive and therefore costly, limiting the advancement of this approach. Here we report an initial demonstration of the conversion of cellulosic feedstock into ethanol by completely off-grid solar processing steps. Our approach is based on nanoparticle-enabled solar steam generation, in which high-efficiency steam can be produced by illuminating light-absorbing nanoparticles dispersed in H 2 O with sunlight. We used solar-generated steam to successfully hydrolyze feedstock into sugars; we then used solar steam- distillation to purify ethanol in the final processing step. Coastal hay, a grass grown for livestock feed across the southern United States, and sugar cane as a control are successfully converted to ethanol in this proof-of-concept study. This entirely off-grid solar production method has the potential to realize the long-dreamed-of goal of sustainable cellulosic bioethanol production. B ioethanol is an alternative fuel candidate that has long been advocated for its potential to considerably reduce our need for fossil fuels. Bioethanol is currently produced using agricultural products such as corn and sugar cane, utilizing valuable agricultural land that could be utilized for human food production. In contrast, the long-term goal of achieving bioethanol using cellulosic feedstock, such as agricultural residue, native grasses such as switchgrass, livestock feed such as coastal hay, or woody biomass, has yet to be realized in a cost-effective manner. 1,2 To convert cellulosic feedstock into bioethanol requires pretreatment to extract the polysaccharides from plant matter and convert them into monosaccharides for bacterial conversion into ethanol. Current methods used for pretreatment typically require the input of energy in the form of heat and the use of acids, bases, or enzymes to degrade plant cell walls. 1−6 This pretreatment adds a substantial energy cost to bioethanol production. This is in addition to the already energy-intensive and costly distillation of the final ethanol product, which has been estimated to constitute 70−85% of the total energy costs of bioethanol production. Sustainable, low-cost methods for conversion of cellulosic feedstock into bioethanol are critically needed to make this approach a practical and sustainable reality. Degradation of the hemicellulose and lignin constituents of plant walls is essential to reach the cellulose core and release the available stored sugars. Cellulose is a polymer composed of D-glucose monomers linked by a β-1,4 glycoside bond. Cellulose polymers have an amorphous−crystalline structure due to hydrogen bonds between the hydroxyl groups of glucose; these bonds pack the glucose polymeric chains tightly, making hydrolysis difficult. Because glucose converts most efficiently relative to other constituent monosaccharides or disaccharides in the production of bioethanol, a primary challenge is the dissociation of cellulose into glucose Received: October 10, 2016 Accepted: November 21, 2016 Published: November 21, 2016 Letter http://pubs.acs.org/journal/aelccp © 2016 American Chemical Society 8 DOI: 10.1021/acsenergylett.6b00520 ACS Energy Lett. 2017, 2, 8−13