| This dissertation focused on the syntheses, characterizations and catalytic performances of Co-containing molecular sieves in the epoxidation of trans-stilbene with molecular O2, and is composed of three parts.In the first part, a series of Co/M-SBA-15(M = Ca, Mg, Sr, Ba) catalysts prepared by the impregnation method were investigated for the epoxidation of trans-stilbene with molecular O2 as the oxidant. The activity in a sequence of decrease order was found to be followed as: Sr > Ca > Mg > Ba. A dramatic improvement of dispersion and catalytic activity of supported Co species was found when adding a small amount of Sr, whereas a large amount of Sr would result in a remarkable decrease in its specific surface area as well as its catalytic activity. The results of XRD, DRS and H2-TPR suggested that highly dispersed Co species existed mainly as isolated Co(II) could be obtained with a cobalt content up to ca 1.0 %, whereas large amount of Co resulted in aggregated CoOx crystalline clusters. Highly dispersed Co species mainly as isolated Co(II) located in the channels of SBA-15 were suggested to be the highly efficient activated component for the oxidation. The 1.0Co/0.5Sr-SBA-15 showed the highest catalytic activity among the prepared samples. Under reaction conditions of 0.1 g of catalyst, 1.0 mmol of trans-stilbene, 10 mL of DMF, 0.1 MPa of O2 pressure, 100 oC and 6 h, trans-stilbene conversion was ca. 40.1 % and epoxide selectivity was ca. 96.3 %. The catalyst could be recycled for several times without appreciable loss of its activity.In the second part, Co(acac)2 was covalently bonded to silica surface of SBA-15 by multi-step grafting using propyltrimethoxysilane as coupling agent. The prepared Co(acac)2-APTS-SBA-15 catalysts were investigated for the epoxidation of trans-stilbene with molecular O2 as the oxidant. The immobilized Co(acac)2-APTS-SBA-15 showed higher activity than the homogeneous counterpart Co(acac)2 in the oxidation. An increase in trans-stilbene conversion with increasing immobilized Co was found when the immobilized cobalt was lower than ca. 0.7 %, but further increases in Co content would result in a much low activated catalyst. The amount of immobilized Co increased with increasing APTS used and leaded to an improvement in the conversion of trans-stilbene at the same time. However, large amount of APTS would result in a remarkable decrease in its specific surface area and pore volume as well as the conversion of trans-stilbene. The best performances were obtained over the catalyst with ca. 0.7 % of cobalt and 1.0 mmol of APTS.In the third part, a series of Co-TUD-1 catalysts (Co: ca. 0.47-12.9 wt.% ) were successfully synthesized by the atomic layer deposition (ALD) method using Co(acac)2 as the Co precursor. The characterizations of XRD, DRS and H2-TPR suggested that the Co species in TUD-1 existed mainly as highly dispersed isolated Co(II) when the Co content was lower than ca. 4.0 %, whereas a high Co content would result in large amount of CoOx clusters located outside the channels of TUD-1. As compared with the impregnated samples at the same Co content, Co-TUD-1 by ALD method exhibited higher activity in the epoxidation of tran-stilbene. The Co species in Co-TUD-1 by ALD method were mainly high dispersed isolated Co(II), whereas the impregnated samples showed large amount of CoOx clusters located outside the channels of TUD-1. The isolated Co(II) species highly dispersed on TUD-1 were suggested to account for the high activity for the epoxidation of trans-stilbene with molecular O2. |
PDF Full Text Request