The Application Of A Novel Ti-MWW Titanosilicate In Environmental Friendly Chemical Processes

Green Chemistry or environmentally benign chemistry is the design of chemical products and processes that reduce of the use and generation of hazardous substances that harm either our health or the environment. In 1991, Trost, the famous chemist in America, propose the conception of atomic-economy reaction. After this, Sheldon, a Netherlands Professor, use the E-factor(the weight radio of byproduct and product) to divide the chemical industry. The E-factor of bulk chemicals is 1-5, and that of fine chemicals is 5-50. Thus, the fine chemicals industry has a great pressure to reduce the pollution of the environment. Thus, in order to obtain a cleaner environment, to R&D of new processes that can save energy or raw materials and low the byproduct is essential to both chemists and chemical engineers. Catalysis can play an important role in these new cleaner processes. Cyclohexanone oxime, propylene oxide and Methyl ethyl ketone oxime are important chemicals that have many uses, the conventional manufacture processes of all these compounds are involved of producing a large amount of byproducts, bringing about environmental problems, having high energy-cost and not atomic-economy reactions.Structurally analogous to well-known alluminosilicate of MCM-22, Ti-MWW possesses a pore system consisting of 12-membered ring (MR) side cups and two independent interlayer and intralayer 10-MR channels, one of which contains the supercages of 0.7 × 0.7 × 1.8 nm dimension. This unique pore structure leads to a superior catalytic activity and product selectivity in the epoxidation of various alkenes with H₂O₂ in comparison to conventional TS-1. As Ti-MWW is synthesized from inorganic silica source and inexpensive structure direction agent (SDA) of piperidine or hexametheleneimine, it has also advantage in the manufacture cost of the catalyst when competing with TS-1.In view of the higher catalytic activity and product selectivity of Ti-MWW in epoxidation of various alkenes and lower product cost. The thesis investigates the use of Ti-MWW in the above-mentioned processes. The thesis mainly discusses the following three sections:Section 1 Liquid-phase ammoximation of cyclohexanone over Ti-MWW catalystBy investigation the reaction parameters, we found Ti-MWW proves to be a highly active, selective and reusable catalyst for the synthesis of cyclohexanone with ammonia and hydrogen peroxide to oxime in the presence of water. In comparison to other titanosilicates, Ti-MWW shows much higher catalytic activity even superior to TS-1. Ti-MWW is capable of giving a conversion of cyclohexanone and an oxime selectivity both over 99% under optimum reaction conditions. The catalytic performance of Ti-MWW greatly depends on the operating conditions of the reaction, especially the adding method of substrates. The reason why the adding method of substrates plays important effects on the ammoximation of ketones or aldehydes over Ti-MWW has been interpreted after a detailed investigation into the reaction mechanism. The ammoximation over Ti-MWW proceeds through the oxidation of ammonia by hydrogen peroxide to an intermediate of hydroxylamine, and then the oximation of ketone with hydroxylamine to oxime, and the non-catalytic oximation of which with ketone is the rate-determining step. Moreover, Ti-MWW showed much higher oxidation activity for hydroxylamine than TS-1. Thus, to achieve high conversion of ketone to oxime, it is necessary to avoid the presence of free H₂O₂ and the extensive oxidation of hydroxylamine on the Ti species of Ti-MWW with extremely high oxidation ability.Section 2 New process for synthesis of methyl ethyl ketone oxime through ammoximation over Ti-MWWCompared with other titanosilicates, Ti-MWW produced MEKO more efficiently and selectively, achieving a conversion and selectivity both over 99% under optimized conditions, using water as solvent and lower NH3/MEK radio. Ti-MWW was particularly superior to TS-1 in MEKO selectivity as TS-1 co-produced easily 2-nitrobutane byproduct unless the ammonia/MEK molar ratio was increased to 4. However, linear ketone like MEK because both the substrate and oxime molecules of linear types are adsorbed easily into the pores and then reach the Ti active sites readily. Deep oxidation thus takes place inside the pores to yield unnecessary byproducts. When an excess of NH₃ molecules were present inside the pores, they would compete with MEKO to adsorb to and cover the Ti sites. This then is helpful to avoid further oxidation of MEKO.We found the catalyst deactivation during the reuse in the MEK ammoximation, there are considerable reasons such as the deposition and pore blocking with heavy products, the leaching of active sites, the change of coordination states of Ti species and the structural degradation of titanosilicate crystals. By these, we found several better catalyst regeneration methods: high-temperature calcination, framework rearrangement method, acid treatment method and Si addition. Although these methods all can improve the lifetime of catalyst to some extent, however, from the commercial view, the fore-three method isn't suitable to a practical process. By adding a controlled amount of silica source into the reaction mixture is a practical method that suitable to a continuous process, the deactivation of Ti-MWW catalyst was suppressed and then the lifetime was prolonged effectively.Section 3 Highly efficient epoxidation of propylene over a novel Ti-MWW catalystBy investigation the reaction parameters, we found Ti-MWW showed a superior catalytic activity to other conventional titanosilicates, even TS-1. The most suitable solvent for propylene epoxidation is CH₃CN and methanol for Ti-MWW and TS-1, respectively. The aprotic solvent of CH3CN improves the selectivity of PO. More important, the CH3CN and PO don't form azeotropic compounds. However, since their boiling points are very near, methanol and PO are considered easily to form azeotropic compounds, which induce difficulties in separation by routine method. Moreover, methanol also leads to readily the ring-open reactions of PO to produce glycol ethers, decreasing the selectivity of PO. Thus, Ti-MWW shows the advantages by favoring the aprotic solvent of CH3CN. Due to the pore-jamming by high boiling substances predominantly accounted for the loss of activity, the catalyst deactivation during the reuse for both Ti-MWW and TS-1, by the calcination at 823 K, the activity was recovered to the same level of the original one. TS-1 lose activity more quickly than Ti-MWW.Moreover, we also found Ti-MWW after trimethylsilylation can improve the PO yield and utility of H₂O₂ in CH₃CN for liquid epoxidation of propylene...