A renewable route to p-xylene from biomass-derived dimethylfuran and ethylene is investigated with zeolite catalysts. Cycloaddition of ethylene and 2,5-dimethylfuran and subsequent dehydration to p-xylene has been achieved with 75% selectivity using H–Y zeolite and an aliphatic solvent at 300 °C. Competitive side reactions include hydrolysis of dimethylfuran to 2,5-hexanedione, alkylation of p-xylene, and polymerization of 2,5-hexanedione. The observed reaction rates and computed energy barriers are consistent with a two-step reaction that proceeds through a bicyclic adduct prior to dehydration to p-xylene. Cycloaddition of ethylene and dimethylfuran occurs without a catalytic active site, but the reaction is promoted by confinement within microporous materials. The presence of Brønsted acid sites catalyzes dehydration of the Diels–Alder cycloadduct (to produce p-xylene and water), and this ultimately causes the rate-determining step to be the initial cycloaddition.

Pyrolytic biofuels have technical advantages over conventional biological conversion processes since the entire plant can be used as the feedstock (rather than only simple sugars) and the conversion process occurs in only a few seconds (rather than hours or days). Despite decades of study, the fundamental science of biomass pyrolysis is still lacking and detailed models capable of describing the chemistry and transport in real-world reactors is unavailable. Developing these descriptions is a challenge because of the complexity of feedstocks and the multiphase nature of the conversion process. Here, we identify ten fundamental research challenges that, if overcome, would facilitate commercialization of pyrolytic biofuels. In particular, developing fundamental descriptions for condensed-phase pyrolysis chemistry (i.e., elementary reaction mechanisms) are needed since they would allow for accurate process optimization as well as feedstock flexibility, both of which are critical to any modern high-throughput process. Despite the benefits to pyrolysis commercialization, detailed chemical mechanisms are not available today, even for major products such as levoglucosan and hydroxymethylfurfural (HMF). Additionally, accurate estimates for heat and mass transfer parameters (e.g., thermal conductivity, diffusivity) are lacking despite the fact that biomass conversion in commercial pyrolysis reactors is controlled by transport. Finally, we examine methods for improving pyrolysis particle models, which connect fundamental chemical and transport descriptions to real-world pyrolysis reactors. Each of the ten challenges is presented with a brief review of relevant literature followed by future directions which can ultimately lead to technological breakthroughs that would facilitate commercialization of pyrolytic biofuels. Paper bodyAs the world population grows, there is a need for new energy technologies that are domestic and sustainable. Achieving both objectives requires improving existing energy systems as well as utilizing renewable feedstocks, such as biomass. In addition to supporting agricultural economies, biomass is the only renewable source for liquid fuels and chemicals. 1,2 For this reason, the U.S. Department of Energy has made it a goal to replace 30% of all transportation fuels with biofuels. 3 The 2005 'Billion-Ton Study' (BTS) sponsored by the U.S. Department of Energy employed conservative assumptions to determine that more than a billion tons of biomass (unrestricted by price) is available annually for biofuels. This amount of biomass is capable of displacing 30% of U.S. petroleum consumption, as put forth in the government targets. 3 In 2011, an update to the BTS revisited the resource availability and confirmed the findings of the 2005 study. 4 Both government-

Biomass pyrolysis utilizes high temperatures to produce an economically renewable intermediate (pyrolysis oil) that can be integrated with the existing petroleum infrastructure to produce biofuels. The initial chemical reactions in pyrolysis convert solid biopolymers, such as cellulose (up to 60% of biomass), to a short-lived (less than 0.1 s) liquid phase, which subsequently reacts to produce volatile products. In this work, we develop a novel thin-film pyrolysis technique to overcome typical experimental limitations in biopolymer pyrolysis and identify a-cyclodextrin as an appropriate smallmolecule surrogate of cellulose. Ab initio molecular dynamics simulations are performed with this surrogate to reveal the long-debated pathways of cellulose pyrolysis and indicate homolytic cleavage of glycosidic linkages and furan formation directly from cellulose without any small-molecule (e.g., glucose) intermediates. Our strategy combines novel experiments and first-principles simulations to allow detailed chemical mechanisms to be constructed for biomass pyrolysis and enable the optimization of next-generation biorefineries.

p-Xylene, the precursor for PET bottles, was synthesized at 90% yield by [4 + 2] cycloaddition of biomass-derived ethylene and dimethylfuran followed by subsequent dehydration with Beta zeolite. Scheme 1 Diels-Alder cycloaddition of dimethylfuran [1] and ethylene produces an oxa-norbornene cycloadduct [2] which dehydrates to p-xylene [3]. Water hydrolyzes dimethylfuran [1] to 2,5-hexanedione [4] in equilibrium. † Electronic supplementary information (ESI) available. See

Fast pyrolysis of biomass is a next-generation biofuels production process that is capable of converting solid lignocellulosic materials (in their raw form) to a transportable liquid (bio-oil) which can be catalytically hydrogenated to fuels and chemicals. While biomass fast pyrolysis has enormous potential to produce renewable fuels, an understanding of the fundamental chemistry that converts biomass components, such as cellulose, to bio-oil is not available in the literature. In this work, we use thin-film pyrolysis to reveal the effect of temperature under transport-free reaction conditions and then evaluate the effect of sample dimension (i.e., characteristic length scale) by comparing product distributions of conventional powders and thin films. In the first part of the work, we show that the yield of total furan rings (i.e., all products containing a five-membered furan ring) does not change significantly with increased reaction temperature compared to other pyrolysis products, such as light oxygenates and anhydrosugars. However, we find that the functional groups bound to the furan ring (e.g., alcohols and aldehydes) are easily cleaved to produce smaller furans. In the second part of the work, we show that sample dimension is a key descriptor for product yields. For example, levoglucosan (the most abundant product of cellulose pyrolysis) yield differs significantly between conventional powder (millimeter-sized samples which are transport-limited) pyrolysis and thin-film (micrometer-scale thin-films which are isothermal) pyrolysis (49% for powder; 27% for thin-film at 500 °C).