The effect of acid treatment on the active sites and reaction intermediates of the low-cost TS-1 in propylene epoxidation

Abstract

This study investigated the roles of the different titanium species of low-cost TS-1 in propylene epoxidation and the influence of acid treatments. The low-cost TS-1 zeolites with different titanium species were synthesized using the hydrothermal method and characterized by ultraviolet (UV)-Raman, X-Ray Diffraction (XRD), ultraviolet visible diffuse reflectance spectroscopy (UV-Vis), N 2 physical absorption and NH 3 temperature programmed desorption (NH 3 -TPD) techniques. The roles of the different titanium species in the low-cost TS-1 samples were investigated by gas chromatography-Raman spectrometry (GC-Raman) during the propylene epoxidation process. The framework titanium species was found to be active for propylene epoxidation, while the extra-framework Ti species were found to be harmful for propylene epoxidation due to their acidity. The extra-framework Ti species can be removed by acid treatments. It was found that the acid treatment of the as-synthesized TS-1 is more effective since the amorphous TiO 2 was not transformed into anatase TiO 2 upon calcination. Strong acids, such as HCl, HNO 3 and H 2 SO 4 , are more efficient for the elimination of the extra-framework TiO 2 , but do not significantly affect the framework titanium species. After treatment with an acid of a suitable concentration, the amount of the active intermediate species Ti-OOH(h 2 ) can be increased, and a higher conversion of propylene and yield of PO can be achieved.

Inter-connected and open pore hierarchical TS-1 with controlled framework titanium for catalytic cyclohexene epoxidation

“…Typically, the framework titanium is the active site for epoxide production during epoxidation reactions, whereas the extra-framework titanium species are responsible for secondary and often unwanted reactions 8 . The extra-framework titanium species include anatase and rutile-type titanium dioxide (TiO2) and amorphous titanium species, which will decompose H2O2 and lead to side reactions, such as hydrolysis and ring opening of the epoxide [9][10][11] . The traditional synthesis method of TS-1 inevitably produces extra-framework titanium, particularly when synthesizing TS-1 with low silicon to titanium ratios [12][13][14] .…”

Carboxylic acids to butyl esters over dealuminated–realuminated beta zeolites for removing organic acids from bio-oils

“…a novel method to nely tune the acidity of H-beta zeolites. It has been found that dealumination of beta zeolites by steaming, 17 leaching with mineral acids such as HCl, 18 and treatments with chemicals such as ammonium hexauorosilicate, 19 and organic acids 20,21 are effective for this purpose. Among these approaches, acid leaching is considered to be the most feasible route from the point of view of environmental protection and industrial application, but the negative effects of dealumination by acid leaching are structural changes in zeolite crystals and uncontrollable loss of framework aluminum atoms that decreases active sites.…”

Synthesis of Hierarchical Titanium‐Rich Titanium Silicalite‐1 Zeolites and the Highly Efficient Catalytic Performance for Hydroxylation of Phenol

“…The adsorption peak at 270 nm (amorphous Ti species) and 330 nm were appeared when the (NH 4 ) 2 CO 3 /SiO 2 molar ratio was 0.3. The amorphous Ti species and anatase TiO 2 on the surface of s‐TS‐1(0.3) zeolite can be easily washed away by 1 mol/L muriatic acid before calcination (Figure (C) lines e‐f). But the anatase TiO 2 on the surface of conventional TS‐1 zeolite just can be washed away by 3 mol/L muriatic acid (Figure (C) lines a‐d).…”

“…The Si/Ti ratio of s‐TS‐1(0.3) is 31.12, after washed with 1 mol/L muriatic acid the Si/Ti ratio of s‐TS‐1(0.3) is 34.79. The increased Si/Ti ratios of TS‐1‐m and s‐TS‐1‐m(0.3) indicated that the extraframework Ti species can be washed away by muriatic acid, which agrees with the UV/vis spectra (Figure C).…”

“…After acid washing of s‐TS‐1(0.3) zeolite, the peaks at 144 cm −1 , 637 cm −1 and 700 cm −1 were disappeared (line b), indicating that the anatase TiO 2 and amorphous Ti species were washed away from s‐TS‐1‐m (0.3). The characteristic peaks at 380 and 817 cm −1 illustrate that both conventional TS‐1 zeolite and Ti‐rich TS‐1 zeolites are MFI structure . The character peaks at 475 cm −1 , 960 cm −1 and 1125 cm −1 refer to the asymmetric stretching vibration and the symmetric stretching vibration of Ti‐O−Si, demonstrated the exist of framework Ti species in the conventional TS‐1 zeolite and Ti‐rich TS‐1 zeolites ,.…”