Role of pentahedrally coordinated titanium in titanium silicalite-1 in propene epoxidation

Abstract

Role of pentahedrally coordinated titanium in titanium silicalite-1 in propene epoxidationA new pentahedrally coordinated Ti was discovered in TS-1 and showed a higher catalytic activity than the tetrahedrally coordinated one.Two titanium silicalite-1 samples with different crystal sizes were synthesized in the tetrapropylammonium bromide (TPABr) and tetrapropylammonium hydroxide (TPAOH) hydrothermal systems. The small-crystal TS-1 with a size of 600 nm was then treated with different organic bases. These TS-1 samples were evaluated in the epoxidation of propene, and characterized by ultraviolet-visible diffuse reflectance (UV-vis), X-ray absorption near edge structure (XANES) and Raman spectroscopies. The Ti L-edge absorption spectra show that a new Ti species, pentahedrally coordinated Ti, appears in some of the samples. This pentahedrally coordinated Ti species is correlated with the catalytic oxidation activity of TS-1, closely. Tetrahedrally coordinated Ti in TS-1 is the primary active center for selective oxidation reactions, but the existence of a small amount of pentahedrally coordinated Ti can further improve the catalytic activity. A high molar ratio of Si/Ti (n(Si/Ti)) in the synthesis process (n(Si/Ti)=92.78) was beneficial for the generation of pentahedrally coordinated Ti. The improved catalytic activity of the TPAOH treated TS-1 is mainly due to the increasing amount of pentahedrally coordinated Ti, besides the elimination of diffusion limitation. Slowing down the crystallization rate can also increase the content of pentahedrally coordinated Ti.

The effects of impregnation of precious metals on the catalytic activity of titanium silicate (TS-1) in epoxidation of propene using hydrogen peroxide

“…In several liquid-phase epoxidation routes to convert propene into propene oxide, much attention has been directed to heterogeneous catalytic oxidation using hydrogen peroxide as a more environmentally benign and profitable chemical process [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Titanium silicalite (TS-1) catalyst has been extensively investigated for this route because propene oxide can be produced with a high selectivity in methanol under mild conditions [4,5,[10][11][12][13][14][15][16][17][18][19][20][21][22][23]. The direct epoxidation process based on hydrogen peroxide (oxidant) and TS-1 (catalyst) was commercialized and referred to as "hydrogen peroxide-to-propene oxide" (HPPO) process.…”

Enhanced Catalytic Performance of Titanium Silicalite‐1 in Tuning the Crystal Size in the Range 1200–200 nm in a Tetrapropylammonium Bromide System

“…attributed it to isolated Ti species that coordinate with two water molecules 41. We also studied the assignment of this band of Ti in our previous work,30, 42 and found that at least two kinds of Ti species existed in this region. We conclude that the band at λ =250–280 nm in the samples in this work can be attributed to Ti coordinated octahedrally formed by one Ti coordinated tetrahedrally that bonds with two water molecules.…”

“…The other important factor that influences the catalytic performance of TS‐1 is the coordination state of Ti and the diffusion properties. Ti coordinated tetrahedrally is the primary active center for selective oxidation, and we found that Ti coordinated pentahedrally showed a higher activity than tetrahedral Ti 30. However, the synthesis of TS‐1 with a high content of pentahedral Ti is difficult.…”

Au/TS‐1 catalyst for propene epoxidation with H2/O2: A novel strategy to enhance stability by tuning charging sequence

“…The dominant band centered at 210 nm is assigned to the Ti 4+ O 2− to Ti 3+ O − ligand‐to‐metal charge transfer, and is characteristic of isolated tetrahedrally coordinated Ti atoms in framework . No adsorption bands attributed to octahedrally coordinated Ti containing Ti‐O‐Ti bond or isolated Ti species coordinated water molecules are observed at 240–290 nm. In addition, there is no adsorption band at 330 nm, indicating the absence of extra framework anatase TiO 2 .…”