UC Santa Barbara

The cleavage of chemical bonds has a number of applications including the refinement of natural feedstocks and the controlled delivery of specific chemicals. This work describes two such examples, unified in that the cleavage is facilitated by inorganic, heterogenous catalysts. The first study describes the efforts to understand the reactivity of copper doped porous metal oxides (CuPMO), a promising lignocellulose disassembly catalyst, which cleanly converts lignin as well as lignocellulose composites, such as sawdust, to organic liquids with little or no formation of tars or chars. The second part examines the photo-oxidation of dithioxalates to produce carbon disulfide, a potentially therapeutic small molecule, in a controlled manner. In both cases, product analysis was a key component of the study and underscored the potential in each system.

Despite its ability to cleanly disassemble lignocellulose in supercritical methanol, CuPMO

also catalyzes less desirable aromatic ring hydrogenations and various methylations that dramatically increase the complexity of the product stream. In order to determine the reaction rates relevant to the reductive disassembly of lignin by this catalyst system, a quantitative experimental evaluation of model reactions was performed. These helped to quantitatively assess the rates of desirable and less desirable chemical steps that define the overall product selectivities. Global fitting analysis methods were used to map the temporal evolution of key intermediates and products and to elucidate networks that provide guidelines regarding the eventual fates of reactive intermediates in this catalysis system.

Phenolic compounds were shown to display multiple reaction pathways, but substrates such as benzene, toluene, and alkyl- and alkoxy-substituted aromatics were considerably more stable under these conditions. These results indicate that modifying this catalytic system in a way that controls and channels the reactivity of phenolic intermediates should improve selectivity toward producing valuable aromatic chemicals from biomass-derived lignin. To this end we demonstrated that the O-methylating agent dimethyl carbonate can intercept the phenol intermediate formed from hydrogenolysis of the model compound benzyl phenyl ether. Trapping the phenol as anisole thus gave much higher selectivity towards aromatic products.

Following up on the reactivity study was an investigation into the dependencies of the temporal product evolution for model compounds over copper doped porous metal oxides in supercritical-methanol. These investigated 1-phenylethanol (PPE), benzyl phenyl ether (BPE), dihydrobenzofuran (DHBF), and phenol for operating temperature ranges from 280 – 330 °C. The first three model compounds represent the β-O-4 and α-O-4 linkages in lignin as well as the furan group commonly found in the β-5 linkage. Phenol was investigated due to its key role in product proliferation. In general, the apparent activation energies (Ea’s) for ether hydrogenolysis proved to be much smaller than that for the hydrogenation of phenol, a major side reaction leading product proliferation. Thus, temperature tuning may be a useful strategy to preserve product aromaticity as demonstrated by the more selective conversion of BPE and PPE at the lower temperatures. Rates of methanol reforming over this catalyst system were also studied over the temperature range 280 – 320 °C, since MeOH reforming to produce reducing equivalents is an integral component of this catalytic system. In the absence of substrate, the gaseous products H2, CO, and CO2 were formed in ratios stoichiometrically consistent with catalyzed methanol reformation and water gas shift reactions. The latter studies suggest that the H2 production ceases to be rate limiting early in batch reactor experiments, but also suggest that overproduction may also lead to product proliferation.

The second part of this work describes the successful attempt to photochemically release carbon disulfide motivated by the potentiality of its therapeutic effects. In this study carbon disulfide is generated via oxidative cleavage of 1,1dithiooxalate (DTO) photosensitized by CdSe quantum dots (QDs). Irradiation of DTO−QD conjugates leads to λirr independent photooxidation with a quantum yield of ∼4% in aerated pH 9 buffer solution that drops sharply in deaerated solution. Excess DTO is similarly decomposed, indicating labile exchange at the QD surfaces and a photocatalytic cycle. Analogous photoreaction occurs with the O-tert-butyl ester tBuDTO in nonaqueous media. It is proposed that oxidation is initiated by hole transfer from photoexcited QD to surface DTO and that these substrates are a promising class of photocleavable ligands for modifying QD surface coordination.