ao9b03645_si_001.pdf (1.36 MB)
Impact of Titanium in Controlling Silver Particle Size on Enhancement of Catalytic Performance of AgMoO3/Ti-HMS for Direct Epoxidation of Propylene
journal contribution
posted on 2020-02-25, 09:29 authored by Hany M. AbdelDayem, Shar S. Al-Shihry, Salah A. HassanIn this work, direct gas-phase epoxidation
of propylene (DPO) to
propylene oxide by molecular oxygen has been studied by using Ag–MoO3 supported on titanium-containing hexagonal mesoporous silica
(Ti-HMSn) of different Si/Ti molar ratios.
The promotion effect of NaCl on the synthesized catalysts has also
been investigated. Among the studied supports, the hexagonal mesoporous
silica (HMS) with a Si/Ti ratio of 10 was the most suitable one for
production of propylene oxide (PO). The optimal performance of the
AgMo/Ti-HMS10 catalyst in DPO exhibited a selectivity to
PO of 43.2% with a propylene conversion of 14.1%, at 400 °C,
0.1 MPa, and a space velocity of 12,000 h–1. The
catalyst verified good stability over at least 20 h on stream. Only
2.7% PO selectivity with a propylene conversion of 10.1% was achieved
over the AgMo/HMS sample. The incorporation of Ti into the HMS frame
could optimize the particle size distribution of Ag, producing Ag
nanoparticles with an average size of 6.8 nm compared with that of
Ag/HMS (24.3 nm). The in situ Raman spectrum of AgMo/Ti-HMS10 heated in a stream of C3H6/He at 400 °C
showed new bands at 616, 390, and 210 cm–1, characteristic
of the AgxMoyOz intermediate phase. The obtained results
suggested that this formed AgMo/Ti-HMS10 phase could most
likely be relevant for selective epoxidation of propylene. However,
during the reaction of C3H6 with AgMo/HMS, the
formation of this intermediate was hardly detected. On the other hand,
the hydrogen temperature-programmed reduction measurements indicated
improved reducibility of MoO3 in the AgMo/Ti-HMS10 catalyst, which acknowledged the role of Mo6+ in gaining
electrons from silver to form positively charged Ag. This could reduce
the effective charge of the adsorbed oxygen on silver sites and in
turn favor the epoxidation path of propylene rather than the combustion
route. Also, during the reaction of C3H6 with
NaAgMo/Ti-HMS10 at 400 °C, two new Raman bands were
detected at 277 and 350 cm–1, characteristic of
Ag2MoO4.