Selective oxidation of cyclohexane over Co-APO-5:Effects of solvent and modification method on the catalytic performance
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摘要: 通过合成改性制备了系列Co-APO-5分子筛催化剂，用于环己烷选择氧化，研究了溶剂和改性方法对其催化性能的影响。结果表明，含有π键的极性溶剂对环己烷选择氧化反应有利，环己烷转化率随着π键极性的增加而提高。引入Si和F降低了Co-APO-5分子筛骨架中四配位钴的含量；引入F后Co-APO-5分子筛的结晶度有所提高，而引入Si则能改善Co物种的氧化和还原性，提升其催化反应活性。同时，Co-APO-5的催化活性与其骨架中四配位Co（Ⅱ）的含量相关，说明骨架Co（Ⅱ）是环己烷选择氧化的催化活性中心。Abstract: A series of Co-APO-5 molecular sieves were prepared and used as the catalysts in the selective oxidation of cyclohexane; the effects of solvent and modification method on the catalytic performance of Co-APO-5 were investigated. The results illustrated that polar solvents containing π-bonds are favorable for the oxidation reaction and the conversion of cyclohexane increases with the increase of solvent polarity. The introduction of Si and F leads to a decrease of the tetrahedral cobalt content in the Co-APO-5 framework. However, introducing F can improve the crystallinity of Co-APO-5, whereas adding Si may promote the oxidation and reduction of cobalt species and then enhance the catalytic activity in oxidation. In general, the activity of Co-APO-5 catalysts is related to the content of tetrahedral cobalt in the framework, suggesting that the framework Co(Ⅱ) is probably the catalytically active species for the oxidation of cyclohexane.
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Key words:
- Co-APO-5 /
- cyclohexane /
- selective oxidation /
- cyclohexanol /
- cyclohexanone /
- solvent effect /
- modification
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Figure 1 XRD patterns of various calcined Co-APO-5 molecular sieves
a: Co-APO-5(23) ; b: Co-APO-5(11.5) ; c: Co-APO-5(7.3) ; d: Co-APSO-5(7.3) ; e: Co-APO-5(7.3)-F; f: Co-APO-5(23)-HAc
Figure 2 Pore distribution of various Co-APO-5 molecular sieves determined by DFT method from nitrogen sorption
Figure 3 SEM images of various Co-APO-5 molecular sieves
(a): Co-APO-5(23) ; (b): Co-APO-5(7.3)-F; (c): Co-APSO-5(7.3) ; (d): Co-APO-5(23)-HAcalt
Figure 4 DR UV-vis spectra of various Co-APO-5 molecular sieves
a: Co-APO-5(11.5) ; b: Co-APO-5(7.3) ; c: Co-APO-5(23) ; d: Co-APSO-5(7.3) ; e: Co-APO-5(23)-HAc; f: Co-APO-5(7.3)-F; g: Co-APO-5(23)-F/HAc
Figure 5 H2-TPR profiles of various Co-APO-5 molecular sieves
a: Co-APSO-5(7.3) ; b: Co-APO-5(7.3) ; c: Co-APO-5(11.5) ; d: Co-APO-5(23) ; e: Co-APO-5(7.3)-F
Figure 6 NH3-TPD profiles of various Co-APO-5 molecular sieves
a: Co-APO-5(23) ; b: Co-APO-5(23)-HAc; c: Co-APO-5(7.3) ; d: Co-APO-5(7.3)-F; e: Co-APSO-5(7.3)
Figure 7 Cyclohexane conversion versus solvent polarity (a) and solvent solubility (b) for the oxidation of cyclohexane over Co-APO-5(23) in the presence of various solvents
a: none; b: acetic acid; c: ethyl acetate; d: acetonitrile; e: acetone; f: methyl ethyl ketone; g: nitrobenzene; h: dimethyl sulfoxide; i: methanol; j: benzene; k: carbon tetrachloride
Figure 8 Adsorption isotherms of cyclohexane on various Co-APO-5 molecular sieves at 25 ℃
Figure 10 Relationship between the framework (a) and extra framework Co content (b) in the Co-APO-5 molecular sieves and their catalytic activity
a: Co-APO-5(23) ; b: Co-APO-5(11.5) ; c: Co-APO-5(7.3) ; d: Co-APO-5(23)-HAc; e: Co-APO-5(23)-F/HAc; f: Co-APSO-5(7.3) ; g: Co-APO-5(7.3)-F
Table 1 Relative crystallinity (XRD), surface area (nitrogen sorption) and elemental content (ICP-AES) of various calcined Co-APO-5 molecular sieves
Sample Co in gel w/% Surface area A/(m2·g-1) Crystallinity /% Elemental contents w/% Al/Co(mol ratio) (Al+Co)/P (mol ratio) total (BET) micro (t-plot) Al Co P Co-APO-5(23) 1.96 304 258 100 0.483 0.022 0.496 22.0 1.02 Co-APO-5(11.5) 4.10 294 258 89 0.461 0.045 0.493 10.2 1.03 Co-APO-5(7.3) 4.82 273 236 81 0.441 0.053 0.505 8.3 0.98 Co-APSO-5(7.3) 4.54 168 138 68 0.430 0.050 0.478 8.7 - Co-APO-5(7.3)-F 4.65 243 220 93 0.469 0.051 0.481 9.2 1.08 Co-APO-5(23)-HAc 2.00 264 212 97 0.490 0.022 0.488 22.4 1.05 Co-APO-5(23)-F/HAc 1.88 - 0.486 0.020 0.494 24.3 1.02 note: Si content in Co-APSO-5(7.3) is 1.83% 
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Table 2 Oxidation of cyclohexane over Co-APO-5(23) in the presence of various solventsa
Solvent Cyclohexane conversion x/% Product selectivity s/% cyclohexanone cyclohexanol others b None 5.2 76.1 5.2 18.7 Acetic acid (HAc) c 58.5 71.6 25.1 3.3 n-butyric acid 18.7 82.3 13.2 4.5 Ethyl acetate 19.8 82.0 15.8 2.2 Acetonitrile 43.7 81.0 15.9 3.1 Acetone 20.1 78.6 19.3 2.1 Methyl ethyl ketone (MEK) 27.2 84.0 13.6 2.4 Nitrobenzene 19.0 88.5 9.7 1.8 HAc + MEK (no TBHP) c, d 30.0 74.6 23.2 2.2 Dimethyl sulfoxide 0.0 - - - Methanol 0.5 100 0 0 t-butanol 11.2 87.7 9.8 2.5 Benzene 0.0 - - - o-dichlorobenzene 5.7 85.3 10.0 4.7 p-dichlorobenzene 9.4 83.7 12.1 4.2 Carbon tetrachloride 2.0 100 0 0 note: a reaction conditions: 50 mg Co-APO-5(23) , 0.5 g cyclohexane (containing 0.1% TBHP), 2.5 mL solvent, 120 ℃, 1.5 MPa O2, 12 h; b others include mainly diacids; c in the presence of HAC, the selectivity to cyclohexanol includes that to cyclohexyl acetate; d 2.5 mL HAc + 1.2 mmol MEK, without TBHP
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Table 3 Oxidation of cyclohexane over various Co-APO-5 molecular sieves with different Al/Co mol ratios or modified by different methodsa
Catalyst Cyclohexane conversion x/% Product selectivity s/% cyclohexanone cyclohexanol othersb Co-APO-5(23) 19.8 82.0 15.8 2.2 Co-APO-5(11.5) 27.2 81.9 11.3 6.8 Co-APO-5(7.3) 24.8 81.3 9.4 9.3 Co-APSO-5(7.3) 27.5 81.4 8.1 10.5 Co-APO-5(7.3)-F 23.0 81.7 9.7 8.6 Co-APO-5(23)-HAc 18.3 82.5 15.9 1.6 Co-APO-5(23)-F/HAc 17.1 82.4 16.1 1.5 note: a reaction conditions: 50 mg catalyst, 0.5 g cyclohexane (containing 0.1% TBHP), 2.5 mL ethyl acetate, 120 ℃, 1.5 MPa O2, 12 h; b others include mainly diacids
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