Biomass is a natural resource that is both abundant and sustainable. Its efficient utilization has long been the focus of research and development efforts with the aim to substitute it for fossil-based feedstock. In addition to the production of biofuels (e.g., ethanol) from biomass, which has been to some degree successful, its conversion to high value-added chemicals is equally important. Among various biomass conversion pathways, catalytic conversion is usually preferred, as it provides a cost-effective and eco-benign route to the desired products with high selectivities. The research of this thesis is focused on the conversion of biomass to various chemicals of commercial interest by selective catalytic oxidation. Molecular oxygen is chosen as the oxidant considering its low cost and environment friendly features in comparison with commonly used hydrogen peroxide. However, the activation of molecular oxygen usually requires high reaction temperatures, leading to over oxidation and thus lower selectivities. Therefore, it is highly desirable to develop effective catalysts for such conversion systems. We use kegging-type heteropoly acids (HPAs) as a platform for catalysts design because of their high catalytic activities and ease of medication. Using HPA catalysts allows the conversion taking place at relatively low temperature, which is beneficial to saving production cost as well as to improving the reaction selectivity. The strong acidity of HPA promotes the hydrolysis of biomass of giant molecules (e.g. cellulose), which is the first as well as the most difficult step in the conversion process. Under certain circumstances, a HPA combines the merits of homogeneous and heterogeneous catalysts, acting as an efficient homogeneous catalyst during the reaction while being easily separated as a heterogeneous catalyst after the reaction. We have successfully applied HPAs in several biomass conversion systems. Specially, we prepared a HPA-based bi-functional catalyst (Au/Cs2HPW12O40) that enabled the selective conversion of cellobiose to gluconic acid with a very high yield of 96.4% (Chapter II); we realized a direct oxidative conversion of cellulose to glycolic acid (yield of 49.3 %) in a water medium for the first time, by using a phosphomolybdic acid catalyst (chapter III); we found that a vanadium-substituted phosphomolybdic acid catalyst (H4PVMo11O40) is capable of converting various biomass-derived substrates to formic acid and acetic acid with high selectivity, and under optimized reaction conditions, high yield of formic acid (67.8%) can be obtained from cellulose (chapter IV); we discovered that the vanadium-substituted phosphomolybdic acids can also selectively oxidize glycerol, a low-cost by-product of biodiesel, to formic acid, and interestingly this conversion can be performed in highly concentration aqueous solution (glycerol: water = 50: 50), giving rise to exceptionally high conversion efficiency (chapter V). These results reveal that HPAs are useful and suitable catalysts for selective oxidation of biomass, and that the reaction pathway is largely determined by the type of addenda atom in the HPA catalyst. The optimization of the reaction conditions and processes in these systems are also discussed in this thesis.

Due to the water pollution problems created by MTBE, significant research was focused on the production of alternative oxygenates, such as ethyl tert-butyl ether (ETBE), tert-amyl-methyl-ether (TAME) and tert-amyl-ethyl-ether (TAEE) as octane enhancing gasoline blending components. These oxygenates are expected to improve the burning characteristics of gasoline and reduce exhaust emissions of CO and hydrocarbons. Generally, macroreticular acidic resin catalysts (Amberlyst-15) are used for the etherification reactions between C5 iso-olefins (2M1B/2M2B) and alcohols (ethanol/methanol). But in recent years, heteropoly acid compounds are being used in the production of tert-ethers to replace those macroreticular acidic resin catalysts. HPAs are known to be active catalysts for many of homogeneous and heterogeneous acid catalyzed reactions.These compounds have high acidity, high catalytic activity but they are highly soluble in polar solvents such as water,alcohol when they are used in bulk form. In this research, applicability of bulk heteropoly acid (HPA) and its supported form, to the gas-phase etherification reaction of iso-olefin (2-methyl- 2-butene) with ethyl alcohol in a continuous differential reactor was investigated. The heteropoly acid (H3PW12O40.xH2O) was supported on activated carbon, at two different loading levels, by aqueous impregnation technique. After catalyst characterization, kinetic experiments were done in a temperature range between 80&deg / C-97&deg / C with a feed concentration of 30 vol.%2M2B+70 vol.% ethanol. Supported HPA catalysts yielded lower conversion and rate of reaction than the bulk HPA. After that, to make a comparison, same experiments have been carried out with Amberlyst-15 and a different HPA (H3PMo12O40.xH2O) at 90oC. The results showed that, at this temperature, bulk tungstophosphoric acid (H3PW12O40.xH2O) was highly active among the other catalysts. Moreover, the deactivation of bulk and supported HPA were investigated and found that partial deactivation occurred when they were reused, without any treatment. In the final part of the study, the activity of alcohol-treated supported HPA catalyst and formation of side products, dimethyl or diethyl ether, at 90&deg / C were investigated. When the supported catalyst was treated with alcohol, before utilizing in the experiments, lower conversion was obtained with respect to the conversion value obtained in the presence of fresh catalyst. The studies done on the formation of side product showed that, no side product was formed at this working temperature.

Fast depletion of oil reserves necessitates the development of novel alternative motor vehicle fuels. Global warming problems also initiated new research to develop new fuels creating less CO2 emission. Nowadays, dimethyl ether (DME) and diethyl ether (DEE) are considered as important alternative clean energy sources. These valuable ethers are produced by the dehydration reaction of methanol and ethanol, respectively, in the presence of acidic catalysts. Besides DEE, ethylene which is very important in petrochemical industry, can also be produced by ethanol dehydration reaction. In the first part of this study, the catalytic activity of tungstophosphoric acid (TPA), silicotungstic acid (STA) and molybdophosphoric acid (MPA), which are well-known heteropolyacids were tested in ethanol dehydration reaction. The activities of other solid acid catalysts, such as Nafion and mesoporous aluminosilicate, were also tested in the dehydration reaction of ethanol. In the case of DME production by dehydration of methanol, activities of STA, TPA and aluminosilicate catalysts were tested. Among the heteropolyacid catalysts, STA showed the highest activity in both ethanol and methanol dehydration reactions. With an increase of temperature from 180oC to 250oC, Ethylene selectivities increased while DEE selectivities decreased. Ethylene yield values over 0.70 were obtained at 250oC. The presence of water in the feed stream caused some reduction in the activity of TPA catalyst. Very high DME yields were obtained using mesoporous aluminosilicate catalyst at about 450oC. The surface area of heteropolyacids are very low and they are soluble in polar solvents such as water and alcohols. Considering these drawbacks of heteropolyacid catalysts, novel mesoporous STA based high surface area catalysts were synthesized following a hydrothermal synthesis route. These novel catalysts were highly stable and they did not dissolve in polar solvents. The catalysts containing W/Si ratios of 0.19 (STA62(550)) and 0.34 (STA82(550)) have BJH surface area values of 481 m2/g and 210 m2/g, respectively, with pore size distributions ranging in between 2-15 nm. These catalysts were characterized by XRD, EDS, SEM, TGA, DTA, DSC, FTIR and Nitrogen Adsorption techniques and their activities were tested in ethanol dehydration reaction. Calcination temperature of the catalysts was shown to be a very important parameter for the activities of these catalysts. Considering the partial decomposition and proton lost of the catalysts over 375oC, they are calcined at 350oC and 550oC before testing them in ethanol dehydration reaction. The catalysts calcined at 350oC showed much higher activity at temperature as low as 180oC. However, the catalysts calcined at 550oC showed activity over 280oC. Ethylene yield values approaching to 0.90 were obtained at about 350oC with catalysts calcined at 350oC. DEE yield past through a maximum with an increase in temperature indicating its decomposition to Ethylene at higher temperatures. However, at lower temperatures (&lt / 300oC) Ethylene and DEE were concluded to be formed through parallel routes. Formation of some acetaldehyde at lower temperatures indicated a possible reaction path through acetaldehyde in the formation of DEE. DRIFTS results also proved the presence of ethoxy, acetate and ethyl like species in addition to adsorbed ethanol molecules on the catalyst surface and gave additional information related to the mechanism.