Synthesis, Modification And Catalytic Performance In Propylene Epoxidation Of Titanium Silicalite-1

Titanium silicalite-1(TS-1) with a small crystal size (small-crystal TS-1) was synthesized in a TPABr-ethylamine hydrothermal system. The as-synthesized small-crystal TS-1was modified with different organic bases to further improve its catalytic activity. By investigating the influence of acid/base on the catalytic performance of propylene epoxidation, a new reaction mechanism is presented. According to the experiment in laboratory, a pilot plant of propylene epoxidation was carried out. The deactivated catalysts were characterized to study the reason for the deactivation and were regenerated under different conditions. The main results are shown as follows:1. Small-crystal TS-1with a crystal size of about600nm×400nm×250nm, was synthesized in a TPABr-ethylamine hydrothermal system using the mother liquor of nano-sized TS-1as seeds. The synthesis conditions were systematically studied. When the molar ratio of Si/Ti was50, the weight ratio of seed/SiO2was0.06, using precipating separation for three times and the gel was crystallized for48h, the highest catalytic properties were obtained. The X(H2O2) and S(PO) were92%and98%, respectively, for the epoxidation of propylene, and the XPHE) was22%for the hydroxylation of phenol. The reasons for its high catalytic properties were considered as follows:(1) Larger surface area and rougher surface provided more active sites for the reactions, so that the reactant conversions were higher.(2) Smaller crystal size and larger pore diameter than micro-sized TS-1decreased the internal diffusion limitation, so that the selectivity of the main product in propylene epoxidation increased.(3) A small amount of sodium ions could improve the selectivity of propylene oxide by covering the acid centers on the TS-1.2. The reaction conditions of propylene epoxidation over small-crystal TS-1extrudates were optimized in a fixed-bed reator. A long term test was performed under the optimal conditions (40℃,3.0MPa, n(C3H6)/n(H2O2)=3, WHSVs of propylene, methanol and H2O2were0.6,2.5and0.16h-1, and the NH3·H2O concentration was1.2mmol·L-1). The X(H2O2) and S(PO) were both more than95%after1000h reaction.3. Different organic bases were used to modify the small-crystal TS-1. The direction of recrystallization depended on the kind of bases. TPA+and TBA+show a protection effect on the framework Ti in the crystals. When0.06mol·L-1TPAOH solution was adopted to treat the TS-1for72h, the highest conversion of phenol was obtained in phenol hydroxylation.4. The influences of different organic base additives on the propylene epoxidation were investigated. A new mechanism of propylene epoxidation is presented, according to the experimental data, characterization and DFT calculations. Acid centers were considered to be promotors for the epoxidation reactions.5. The process and results of the HPPO pilot plant over EPO-08were dicussed. The EPO-08showed excellent catalytic performances both in acetone/methanol mixed solvent and in pure methanol solvent.6. The EPO-08deactivated partly after1700h pilot plant. The activity decreased from the entrance of the pilot-plant reactor to the exit. The main reason for the deactivation was blocking of pores by propylene oxide oligomers. The deactivated EPO-08could be regenerated by external and in-situ regeneration, while the in-situ regeneration was preferred in industry.