How to reach 100 % selectivity in H₂O₂-based oxidation of 2,3,6-trimethylphenol to trimethyl-p

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• 作者：Oxana A. Kholdeeva ; Irina D. Ivanchikova ; Matteo Guidotti ; Nicoletta Ravasio ; Maila Sgobba ; Marina V. Barmatova
• 关键词：Selective oxidation ; 2 ; 3 ; 6-Trimethylphenol ; Trimethylbenzoquinone ; Titanium-silicate catalysts ; Hydrogen peroxide
• 刊名：Catalysis Today
• 出版年：2009
• 出版时间：30 March 2009
• 年：2009
• 卷：141
• 期：3-4
• 页码：330-336
• 全文大小：519 K

文摘

A comprehensive study of structure/activity/selectivity relationships and mechanistic aspects of the oxidation of 2,3,6-trimethylphenol (TMP) with aqueous H₂O₂ over a wide variety of titanium-silicate catalysts allowed us to infer requirements to an optimal catalyst and optimal reaction conditions for this reaction and to produce 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, Vitamin E precursor) with nearly 100 % selectivity at 100 % substrate conversion. The main by-products in the TMP oxidation are C–C and C–O dimers, formed by coupling of intermediate phenoxyl radicals. The formation of TMBQ is favoured by (1) a poor coordinating solvent (MeCN), (2) elevated temperature (80 °C), (3) low TMP concentration (not higher than 0.1 M), (4) high H₂O₂/TMP molar ratio (ca. 3.5), and (5) low TMP/Ti ratio (<10–20). The crucial factors which determine the selectivity of Ti,Si-catalysts in TMP oxidation to TMBQ are mesoporosity and an optimal surface concentration (ca. 0.7–1.0 Ti atoms/nm²) of accessible highly dispersed, probably, dimeric Ti(IV) species. The catalysts prepared by a simple, affordable and cheap synthesis methodology via grafting titanium(IV) precursors onto the surface of commercial mesoporous silica completely fulfil these requirements and thus can be viewed as promising catalysts for environmentally benign TMBQ production.