Acidic Properties of Various Silica Catalysts Doped with Chromium for the Oxidative Dehydrogenation of Isobutane to Isobutene

Abstract

Although previous researchers have found that FSM-16 (#16 Folded Sheet Mesoporous material) doped with chromium and related Cr-doped silica catalysts has shown great activity for the oxidative dehydrogenation of isobutane to isobutene, information on the nature of these catalysts is insufficient. For this study, three types of Crdoped silica catalysts were prepared by applying the template ion exchange method. CrOx/FSM-16 and CrOx/SiO 2 were used as references. These catalysts were used for oxidative dehydrogenation, which was then characterized via various techniques. The most active catalyst was Cr-doped silica, which did not have the hexagonal structure that is characteristic of mesoporous FSM-16. Various characterizations showed that the catalytic activity of the Cr-species, stemmed from a weak acidic nature and a redox nature that originated from the combination of silicate and a Cr cation, as opposed to the hexagonal structure and strong acidic nature of FSM-16.

Effect of the template ion exchange behaviors of chromium into FSM-16 on the oxidative dehydrogenation of isobutane

“…In our previous paper, the redox property was detected using H 2 -TPR since the loading of the Cr-species was as low as 0.3%. In the present study, a similar redox property reported in our previous paper 7) was directly detected in the XPS for Cr species due to the greater loading of the Cr species (1.44%). Therefore, this again confirmed that a Cr species with a redox nature contributes to catalytic activity.…”

“…7) Upon the addition of the Cr species into FSM-16 using a TIE time of between 0 and 1 h (0 and 1 h Cr-FSM-16), both the conversion and the selectivity were improved, resulting in an enhancement in the yield of isobutene of as much as 2%, as shown in our previous study. 7) Further extension of the TIE time to 10 h resulted in an evident improvement (up to 24%) in the selectivity to isobutene from the initial time-on-stream of 3.25 h for the yield of isobutene (5 and 10 h Cr-FSM-16). Unfortunately, an evident deactivation was detected on both catalysts at 4.5 h on-stream and longer.…”

“…In our previous paper, 7) when 0.3% Cr-FSM-16, Cr-species with weak acidic properties and redox nature contributed to the catalytic activity. Therefore, NH 3 -TPD was used to examine the acidic properties of FSM-16 and Cr-FSM-16.…”

“…4) In contrast, when Ni 2+ is substituted for the Si 4+ in a MCM-41 framework, it results in a tetragonal coordination of the structure, such as with NiO 4 . 5) In our previous studies, 6),7) the incorporation of 0.3 wt % chromium cations into another mesoporous silica, FSM-16 (#16 Folded Sheets Mesoporous Materials), via the TIE procedure showed great activity for the oxidative dehydrogenation of isobutane to isobutene. FSM-16, which was first reported by Inagaki et al in 1993, 8) featured characteristics such as a hexagonal structure and rather strong acidic properties that are typical of mesoporous silica, MCM-41.…”

“…8, NH 3 -desorption peaks were detected at temperatures between 373 and 673 K as reported in our previous paper. 7) In Table 1, the acidic ratio for each form of Cr-FSM-16 against that of FSM-16 are described as estimated from the data in Fig. 8, indicating that the acid amount tended to increase with the TIE time and the Cr loading.…”