74 research outputs found
Sn-Beta zeolites with borate salts catalyse the epimerization of carbohydrates via an intramolecular carbon shift
Carbohydrate epimerization is an essential technology for the widespread production of rare sugars. In contrast to other enzymes, most epimerases are only active on sugars substituted with phosphate or nucleotide groups, thus drastically restricting their use. Here we show that Sn-Beta zeolite in the presence of sodium tetraborate catalyses the selective epimerization of aldoses in aqueous media. Specifically, a 5 wt% aldose (for example, glucose, xylose or arabinose) solution with a 4:1 aldose:sodium tetraborate molar ratio reacted with catalytic amounts of Sn-Beta yields near-equilibrium epimerization product distributions. The reaction proceeds by way of a 1,2 carbon shift wherein the bond between C-2 and C-3 is cleaved and a new bond between C-1 and C-3 is formed, with C-1 moving to the C-2 position with an inverted configuration. This work provides a general method of performing carbohydrate epimerizations that surmounts the main disadvantages of current enzymatic and inorganic processes.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Award DMR-0819762)DuPont MIT Alliance (Graduate Research Fellowship)National Institutes of Health (U.S.) (Grant EB-001960)National Institutes of Health (U.S.) (Grant EB-002026)National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant 1122374
Ternary Ag/MgO-SiO2 Catalysts for the Conversion of Ethanol into Butadiene
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Impartial Selection with Additive Approximation Guarantees
Impartial selection has recently received much attention within the
multi-agent systems community. The task is, given a directed graph representing
nominations to the members of a community by other members, to select the
member with the highest number of nominations. This seemingly trivial goal
becomes challenging when there is an additional impartiality constraint,
requiring that no single member can influence her chance of being selected.
Recent progress has identified impartial selection rules with optimal
approximation ratios. Moreover, it was noted that worst-case instances are
graphs with few vertices. Motivated by this fact, we propose the study of {\em
additive approximation}, the difference between the highest number of
nominations and the number of nominations of the selected member, as an
alternative measure of the quality of impartial selection.
Our positive results include two randomized impartial selection mechanisms
which have additive approximation guarantees of and
for the two most studied models in the literature,
where denotes the community size. We complement our positive results by
providing negative results for various cases. First, we provide a
characterization for the interesting class of strong sample mechanisms, which
allows us to obtain lower bounds of , and of for their
deterministic and randomized variants respectively. Finally, we present a
general lower bound of for all deterministic impartial mechanisms.Comment: SAGT 201
An eco-friendly soft template synthesis of mesostructured silica-carbon nanocomposites for acid catalysis
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