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Highly efficient production of 2,3,5-trimethyl-1,4-benzoquinone using aqueous H2O2 and grafted Ti(IV)/SiO2 catalyst Oxana A. Kholdeeva,*a Irina D. Ivanchikova,a Matteo Guidottib and Nicoletta Ravasiob Received 23rd November 2006, Accepted 15th February 2007 First published as an Advance Article on the web 27th February 2007 DOI: 10.1039/b617162a
The oxidation of 2,3,6-trimethylphenol with aqueous H2O2 over titanium(IV) grafted on commercial mesoporous silica produces 2,3,5-trimethyl-1,4-benzoquinone, with nearly quantitative yield. The vitamin E precursor, 2,3,5-trimethyl-1,4-benzoquinone (TMBQ), is currently produced in industry via oxidation of 2,3,6-trimethylphenol (TMP) with molecular oxygen in the presence of a copper chloride catalyst used in amounts close to stoichiometric.1 The evident shortcomings of the CuCl2/O2 homogeneous system are corrosiveness and product contamination with transition metal (Cu) and chlorine-containing compounds, even if the process is operated under two-phase conditions. The development of green alternatives for the production of TMBQ is thus a challenging goal of fine chemistry. Recently, some of us have suggested the production of TMBQ via oxidation of TMP with aqueous H2O2 over a mesoporous titanium-silicate catalyst (Scheme 1).2 Both well-ordered mesostructured materials and amorphous TiO2–SiO2 mixed oxides were found to operate as true heterogeneous catalysts in this reaction when acetonitrile was used as a solvent.3 The highest yield of TMBQ (up to 98%) was attained over TiO2–SiO2 aerogels with titanium loading in the range of 1.7–6.5 wt%. However, despite the fact that the catalysts did not suffer from titanium leaching into solution, the catalytic activity dramatically decreased after the first run because of the hydrolytic instability of the porous structure and irreversible titanium oligomerization on the surface.3 The progress in solving the problem of the hydrothermal instability of mesoporous titaniumsilicates is related to the synthesis of mesoporous TS-1,4 Ti-MMM-15 and Ti-MMM-26 materials. Meanwhile, the selectivity to TMBQ attained with Ti-MMM-2 does not exceed 80%,6 which is not enough to make this process commercially attractive. Furthermore, the synthesis of the catalyst requires the use of the
Scheme 1
a
Boreskov Institute of Catalysis, Pr. Ac. Lavrentieva 5, Novosibirsk, 630090, Russia. E-mail:
[email protected]; Fax: +73833309573; Tel: +73833309573 b CNR-ISTM, Centro CIMAINA and Dip. Chimica IMA, via G. Venezian 21, Milano, 20133, Italy. E-mail:
[email protected]; Fax: +39 02 50314405; Tel: +39 02 50314428
This journal is ß The Royal Society of Chemistry 2007
expensive and toxic ionic surfactant, cetyltrimethylammonium bromide. Tuel and Hubert-Pfalzgraf have reported that nanometric monodispersed titanium oxide particles supported on mesostructured silicates, such as SBA-15, MCM-41 and HMS, using the hexanuclear cluster [Ti6(m3-O)6(m-O2CC6H4OPh)6(OEt)6] are highly active and selective in the H2O2-based TMP oxidation to TMBQ.7 Unfortunately, the catalyst stability and recycling tests were not provided. Some of us showed that titanium-grafted materials obtained from non-ordered commercially available silicas are efficient catalysts for the epoxidation of unsaturated terpenes and fatty acid derivatives.8 These systems can be prepared by a less expensive and time-consuming method than the ordered template-based molecular sieves and can provide comparable, or even superior, performances. In this work we demonstrate that titanium catalysts (Ti/SiO2) prepared by grafting titanocene dichloride onto the surface of commercial silicas, adapting the procedure developed by Maschmeyer et al.,9 are highly efficient in TMBQ production. The catalytic performance of titanium grafted onto non-ordered mesoporous (A) and pyrogenic non-porous (B) silica supports was assessed in TMP oxidation with H2O2 in acetonitrile under optimal reaction conditions determined previously.3 The results of the catalytic tests are presented in Table 1. The results obtained with the mesostructured Ti-MMM-2 catalyst prepared by hydrothermal synthesis6 are given for comparison. The non-porous pyrogenic silica-grafted catalyst shows a rather low activity compared to the mesoporous materials, and TMBQ TMP oxidation with H2O2 over Ti,Si-catalystsa
Table 1
Catalyst (Ti, wt%)
TMP conversion (%)
TMBQ selectivityc (%)
TOFd/ min21
Ti/SiO2 (A)e (1.97) Ti/SiO2 (A)e (0.92) Ti/SiO2 (B)g (1.78) Ti-MMM-2h (2.00)
100 100 85 98
96 (100)f 79 47 76
2.0 1.4 0.7 1.3
b
(2.0) (2.0) (1.0) (1.9)
a Reaction conditions: TMP, 0.1 M; H2O2, 0.4 M; catalyst, 0.006 mmol of Ti; MeCN, 1 mL, 80 uC, 30 min. b All the catalysts were calcined at 560 uC for 5 h in air directly before experiments. c GC yield based on TMP consumed. d TOF = (moles of TMBQ formed)/(moles of Ti) 6 (time), determined from the initial rates of TMBQ formation; in parentheses, (moles of TMP consumed)/(moles of Ti) 6 (time), determined from the initial rates of TMP consumption. e Surface area of the support, A, 290 m2 g21, volume of mesopores, V, 1.48 cm3 g21, mean pore diameter, d, 20.4 nm. f The catalyst amount was increased twice. g Non-porous, A 268 m2 g21. h A 1120 m2 g21, V 0.57 cm3 g21, d 3.2 nm.
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yield does not exceed 40% based on the initial substrate. The activities of the Ti/SiO2 catalysts grafted onto the non-ordered mesoporous silica and of Ti-MMM-2 expressed in TOF values determined from the initial rates of TMP consumption are similar, and 98–100% TMP conversion is reached after 15–20 min. Acetonitrile is the solvent of choice for this reaction. In methanol or iso-propanol, the reaction rate is several times slower and TMBQ selectivity significantly decreases. It is worth noting that Ti/SiO2 containing about 2 wt% of Ti reveals superior selectivity in TMBQ formation, TMBQ yields as high as 96–100% being attained (Table 1). Both Ti-MMM-2 and Ti/SiO2 with a lower titanium loading (0.92 wt%) have a higher degree of isolation of Ti(IV) sites, which is clearly indicated10 by the higher energy position of the band in the DR-UV spectra (Fig. 1). Meanwhile, these two catalysts are less selective, and only 76–79% yield of TMBQ is achieved (Table 1). Comparing the TOF values determined from the initial rates of TMBQ production and TMP consumption, which are given in Table 1, we may conclude that titanium site isolation is crucial for the activity of the Ti,Sicatalysts but not for the high selectivity of the TMBQ formation. Oppositely, a high surface concentration of well-dispersed, probably dimeric,11 titanium species, which is manifested by a broad maximum of the band in the DR-UV spectrum in the range of 220–270 nm (Fig. 1), favours the formation of the monoquinone at the expense of the typical by-product of TMP oxidation, biphenol. This conclusion is in agreement with the results reported earlier by Tuel and Hubert-Pfalzgraf7 and by some of us.3 Importantly, no bands in the range of 300–330 nm are present in the DR-UV spectra of all the titanium catalysts studied in this work, indicating the lack of anatase-like oligomeric species, which are known to be responsible for a low activity of Ti,Si-catalysts in selective oxidations with H2O2.10 To better fulfil the green chemistry guidelines and to assess the propensity of the materials to be recovered and reused, the Ti/SiO2 catalyst was recycled in two consecutive catalytic runs. In contrast to the TiO2–SiO2 aerogels, the grafted Ti/SiO2 catalyst is considerably more stable and can be used repeatedly without a significant loss in both the activity and selectivity (Fig. 2).
Fig. 2 Recycling of Ti/SiO2 (A) (Ti 1.97 wt%) in TMP oxidation with H2O2. Reaction conditions: TMP, 0.1 M; H2O2, 0.4 M; catalyst, 0.013 mmol of Ti; MeCN, 5 mL, 80 uC, 30 min.
Fig. 3 Oxidation of TMP with H2O2 over Ti/SiO2 (A) (Ti 1.97 wt.%). Reaction conditions as in Table 1.
Typically for Ti,Si-catalysts,3 no further TMP conversion is observed in the filtrate after fast hot catalyst filtration at about 50% of TMP conversion, indicating a true heterogeneous nature of catalysis over Ti/SiO2 (Fig. 3). Elemental analysis data confirm that no titanium leaching from the solid catalyst occurs under the conditions of TMP oxidation. The catalysts prepared by grafting titanium onto non-ordered commercial mesoporous silica combine excellent activity, very high selectivity to the desired product and good recyclability in the TMP oxidation by aqueous H2O2. In addition, these solids are prepared by a simple, affordable and cheap synthesis methodology. All these facts allow us to view them as prospective heterogeneous catalysts for the clean and sustainable synthesis of TMBQ.
Acknowledgements Fig. 1 DR-UV spectrum of Ti/SiO2 (A) (Ti 1.97 wt%) (1), Ti/SiO2 (B) (Ti 1.78 wt%) (2), Ti/SiO2 (A) (Ti 0.92 wt%) (3), Ti/SiO2 (Ti-MMM-2) (Ti 2.00 wt%) (4).
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We thank M. P. Vanina for preliminary results and T.V. Larina for DRS-UV measurements. OAK thanks CNR for the Short Term Mobility Program grant (N 30212, year 2006) This journal is ß The Royal Society of Chemistry 2007
Notes and references 1 W. Bonrath and T. Netscher, Appl. Catal., A, 2005, 280, 55. 2 O. A. Kholdeeva, V. N. Romannikov, N. N. Trukhan, V. N Parmon, RU Patent 2196764, 2001. 3 N. N. Trukhan, V. N. Romannikov, E. A. Paukshtis, A. N. Shmakov and O. A. Kholdeeva, J. Catal., 2001, 202, 110; O. A. Kholdeeva, N. N. Trukhan, M. P. Vanina, V. N. Romannikov, V. N. Parmon, J. Mrowiec-Bialon and A. B. Jarzebski, Catal. Today, 2002, 75, 203. 4 I. Schmidt, A. Krogh, K. Wienberg, A. Carlsson, M. Brorson and C. J. H. Jacobsen, Chem. Commun., 2000, 2157. 5 R. H. P. R. Poladi and C. C. Landry, Microporous Mesoporous Mater., 2002, 52, 11. 6 O. A. Kholdeeva, M. S. Melgunov, A. N. Shmakov, N. N. Trukhan, V. V. Kriventsov, V. I. Zaikovskii and V. N. Romannikov, Catal. Today, 2004, 91–92, 205.
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