catalysts Article
Unprecedented Proline-Based Heterogeneous Organocatalyst for Selective Production of Vanillin Farveh Saberi 1,2 , Daily Rodriguez-Padrón 2 , Araceli Garcia 2 Rafael Luque 2,3, * ID 1 2 3
*
ID
, Hamid Reza Shaterian 1, * and
Department of Chemistry, Faculty of Sciences, University of Sistan and Baluchestan, Zahedan 98135-674, Iran;
[email protected] Departamento de Quimica Organica Universidad de Cordoba Campus de Rabanales, Edificio Marie Curie, Ctra N. IV-A, Km 396, 14014 Cordoba, Spain;
[email protected] (D.R.-P.);
[email protected] (A.G.) Peoples Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya str., 117198 Moscow, Russia Correspondence:
[email protected] (H.R.S.);
[email protected] (R.L.)
Received: 3 April 2018; Accepted: 17 April 2018; Published: 20 April 2018
Abstract: An organocatalytic system based on an unprecedented proline analogue and iron oxide magnetic nanoparticles (Prn/Fe2 O3 @SiO2 ) was designed and employed in vanillin production from isoeugenol and vanillyl alcohol. Full characterization of the obtained catalyst revealed the successful functionalization of the nanoparticle surface with the organic moieties. The activity of the magnetic bifunctional material was compared with its proton-unexchanged counterpart. Interestingly, the oxidation of isoeugenol resulted in being highly dependent on the acidic functionalities of the organocatalyst. Nonetheless, the catalytic performance of the proton-unexchanged catalyst suggested that the acidic and basic sites of the Prn/Fe2 O3 @SiO2 exhibited a synergic effect, giving rise to higher conversion and selectivity. The presence of bifunctional groups in the proline analogue, together with the magnetic properties of the iron oxide nanoparticles, could lead to high efficiency, versatility, recoverability, and reusability. Keywords: vanillyl alcohol; isoeugenol; organocatalyst; vanillin; magnetic core–shell
1. Introduction Valorization of lignocellulosic biomass has become an attractive approach for the production of added value chemicals and fuels, decreasing as well, environmental problems related to agricultural residues. Lignocellulosic biomass is typically composed by three main constituents: cellulose, hemicellulose, and lignin. In particular, lignin is a highly functionalized aromatic compound, which could lead to the design of outstanding chemical platforms. In this sense, vanillin is one of the most preeminent molecules, which can be obtained from lignin derived compounds such as eugenol, isoeugenol, and ferulic acid via oxidation pathways [1]. Synthetic vanillin is commonly used as flavoring agent in food, cosmetic, pharmaceutical, and fine chemical industries. Nowadays, this molecule is mainly produced from petro-based intermediates, predominantly glyoxylic acid and guaiacol, employing non-sustainable synthetic methodologies such as Riedel process [2]. Although synthesis of vanillin has been explored for many years, much more effort should still be devoted to excavating facile an environmentally-friendly protocols for the catalytic oxidation of lignin model compounds using cost effective organo-catalytic based technologies. Vanillin synthesis from isoeugenol and vanillyl alcohol should be further investigated in order to find new alternatives to the conventional oxidation methods that require stoichiometric amounts of inorganic oxidants such as potassium permanganate, which are highly toxic and polluting [3]. Catalysts 2018, 8, 167; doi:10.3390/catal8040167
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In order to minimize chemical waste, the scientific community is moving towards the use of green oxidants including, H2 O2 [4,5] and molecular oxygen [6]. In addition, the inherent arduous separation and recovery of homogeneous catalysts have led to the development of heterogeneous systems, as a priority of research activity in the green chemistry field. Thus, the use of clean oxidants and Catalysts 2018, 8, x FOR PEER REVIEW 2 of 11 heterogeneous catalytic systems, open new possibilities to further develop environmentally friendly catalyzed processes [7,8]. In this regard, supported iron oxide nanoparticles has been reported to oxidants including, H2O2 [4,5] and molecular oxygen [6]. In addition, the inherent arduous convertseparation the isoeugenol to vanillin using H2 Ocatalysts 2 [9]. and recovery of homogeneous have led to the development of heterogeneous Moreover, thea priority use of heterogeneous catalytic systems usually requires a filtration oroxidants centrifugation systems, as of research activity in the green chemistry field. Thus, the use of clean and heterogeneous catalytic new possibilities to further develop environmentally step to recover the catalyst. In thissystems, regard, open magnetic nanoparticles (MNPs) has attracted a great interest friendly catalyzed processes [7,8]. Inthe thisreaction regard, supported iron oxidea nanoparticles has been since they can be easily separated from mixture by using magnetic external field and reported to convert the isoeugenol to vanillin using H2O2 [9]. reused for several cycles [10–12]. Nonetheless, maintaining the stability of these particles for a long Moreover, the use of heterogeneous catalytic systems usually requires a filtration or time without agglomeration or precipitation challenge [13,14]. Aiming to stabilize centrifugation step to recover the catalyst. isInstill this aregard, magnetic nanoparticles (MNPs) has MNPs, several attracted strategies have been proposed, such as coating or encapsulation in the form of a great interest since they can be easily separated from the reaction mixture by usingcore–shell a magnetic external field and[15]. reused several cycles [10–12]. Nonetheless, maintaining theofstability structures or nanocomposites Infor particular, core–shell frameworks composed γ-Fe2 O3 and of these particles for a long time without agglomeration or precipitation is still a challenge [13,14]. silica (γ-Fe 2 O3 @SiO2 ) are one of the most stable and efficient reported nanostructure for catalysis. Aiming to stabilize MNPs, several strategies have been proposed, such as coating or encapsulation Silica surface can be easily post-modified with a wide variety of catalytic species by using different in the form of core–shell structures or nanocomposites [15]. In particular, core–shell frameworks organosiloxane precursors [16,17]. composed of γ-Fe2O3 and silica (γ-Fe2O3@SiO2) are one of the most stable and efficient reported Functionalization of γ-Fe2Silica O3 @SiO organocatalysts could significant progress 2 with nanostructure for catalysis. surface can be easily post-modified withrepresent a wide variety of catalytic in the field of by catalysis [18–20]. So far, many organocatalysts have been developed based on their species using different organosiloxane precursors [16,17]. Functionalization of γ-Fe2O3@SiO 2 with organocatalysts could role represent in [21–23]. carboxylic and amine functionalities, which could play a crucial in thesignificant catalyticprogress reaction the field of catalysis [18–20]. So far, many organocatalysts have been developed based on their Particularly, proline is an amino acid which contains α-amino and α-carboxylic acid groups [24,25]. carboxylic and amine functionalities, which could play a crucial role in the catalytic reaction [21–23]. Proline and its derivatives have been used as asymmetric catalysts in proline organocatalysis Particularly, proline is an amino acid which contains α-amino and α-carboxylic acid groups [24,25]. reactions [26], and including aldol reaction [27], and multicomponent reaction [29]. Proline its derivatives have been usedKnoevenagel as asymmetric[28], catalysts in proline organocatalysis To the reactions far of our preparation of proline solid-supported organocatalysts [26],knowledge, including aldolthe reaction [27], Knoevenagel [28], based and multicomponent reaction [29]. To the requires far of our knowledge, thesteps preparation of proline based solid-supported organocatalysts commonly several reaction and non-environmentally friendly procedures [30]. Although commonlyhave requires several reaction explored, steps and much non-environmentally proceduresespecially [30]. organocatalysis been extensively remains to befriendly accomplished, in Although organocatalysis have been extensively explored, much remains to be accomplished, the context of a truly sustainable protocols. Herein, we have developed an innovative strategy to especially in the context of a truly sustainable protocols. Herein, we have developed an innovative produce a magnetic supported organocatalyst. This methodology includes the functionalization of strategy to produce a magnetic supported organocatalyst. This methodology includes the γ-Fe2 O3functionalization @SiO2 with 3-aminopropyltriethoxysilane (APTES), aiming(APTES), to obtain γ-Fe2toO3obtain @SiO2 -NH2 . of γ-Fe2O3@SiO2 with 3-aminopropyltriethoxysilane aiming Subsequently, a proline wasa synthetized using one-step using procedure by post-modifying the γ-Fe2O3@SiO 2-NH2.analogue Subsequently, proline analogue was asynthetized a one-step procedure by the γ-Fe2O 3@SiO 2-NH2 surface penicillin which finally led toof formation of an γ-Fe2 O3post-modifying @SiO2 -NH2 surface with penicillin G, with which finallyG,led to formation an unprecedented proline analogue organocatalyst (Prn/Fe2O3@SiO2). In particular, this work has proline unprecedented analogue organocatalyst (Prn/Fe 2 O3 @SiO2 ). In particular, this work has focused on penicillin focused on penicillin G but it could be in principle extended to other proline derivatives. The G but it could be in principle extended to other proline derivatives. The obtained material was applied obtained material was applied to the catalytic oxidation of isoeugenol and vanillyl alcohol (Scheme to the catalytic oxidation of isoeugenol and vanillyl alcohol (Scheme 1). 1).
Scheme 1. Schematic depiction of vanillin production from the different reactants in this work.
Scheme 1. Schematic depiction of vanillin production from the different reactants in this work.
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2. Results Resultsand andDiscussion Discussion 2. A magnetic-separable magnetic-separable proline-based proline-based heterogeneous heterogeneous organocatalyst organocatalyst was was successfully successfully prepared prepared A following the protocol depicted in Figure 1. The unique properties of MNPs, which simplify the following the protocol depicted in Figure 1. The unique properties of MNPs, which simplify the recovery of of the the material, material, together together with with the the outstanding outstanding functionalities functionalities provides provides by by the the proline proline recovery analogue, make Prn/Fe O @SiO a potential candidate as catalyst in a broad range of reactions. 2 3 2 analogue, make Prn/Fe2O3@SiO2 a potential candidate as catalyst in a broad range of reactions. An An innovative protocol designed for the in situ formation a proline analogue on the surface innovative protocol was was designed for the in situ formation of aofproline analogue on the surface of of the aminofunctionalized γ-Fe O @SiO by reaction with penicillin G. The latest molecule contains 2 3 2 the aminofunctionalized γ-Fe2O3@SiO2 by reaction with penicillin G. The latest molecule contains a a four-membered β-lactam ringwhich whichcan canreact reactwith with the the amine amine group group of -NH22. . four-membered β-lactam ring of γ-Fe γ-Fe22O O33@SiO @SiO22-NH Consequently, a secondary amine adjacent to the carboxylate group was formed, corresponding Consequently, a secondary amine adjacent to the carboxylate group was formed, corresponding to to the proline analogue. Furthermore, carboxylic acid groupis isformed formedfrom fromcarboxylate carboxylateto to by by the proline analogue. Furthermore, thethe carboxylic acid group proton exchange. proton exchange.
Figure @SiO2 2magnetic magneticorganocatalyst. organocatalyst. Figure 1. 1. Overview of the preparation preparation of of Prn/Fe Prn/Fe22OO33@SiO
SEM Prn/Fe2OO3@SiO 2 exhibited a homogeneous distribution with the formation of SEM image image of of Prn/Fe 2 3 @SiO2 exhibited a homogeneous distribution with the formation of nanospherical nanospherical particles particles (Figure (Figure 2a). 2a). This Thisanalysis analysisdisplayed displayedaacertain certaintendency tendencyto toform formagglomerates, agglomerates, mostly associated with the magnetic properties of the obtained nanostructures. SEM analysis mostly associated with the magnetic properties of the obtained nanostructures. SEM analysis revealed revealed a particle average 14.5 nm. As expected, EDS spectrum 2b) of a particle size average size of 14.5 nm. As of expected, EDS spectrum (Figure 2b) of Prn/Fe2 O(Figure @SiO 3 2 showed Prn/Fe 2O3@SiO2 showed the presence of Si, O, N, C, S, and Fe, which is in accordance with the presence of Si, O, N, C, S, and Fe, which is in accordance with SEM-mapping results (Figure 2e–j). SEM-mapping results (Figuretechniques 2e–j). Both EDS and the SEM-mapping techniques corroborates the Both EDS and SEM-mapping corroborates effective functionalization of the iron oxide effective functionalization of the iron oxide G MNPs and therefore that group penicillin G MNPs and therefore confirmed that penicillin successfully reacted confirmed with the amine on the successfully reacted with the amine group on the γ-Fe 2O3@SiO2-NH2 surface, giving rise to the γ-Fe2 O3 @SiO2 -NH2 surface, giving rise to the desired proline analogue. TEM analysis also confirmed desired proline analogue. analysis also confirmed the nanometric structure of Prn/Fe2O3@SiO2 the nanometric structure TEM of Prn/Fe 2 O3 @SiO2 γ (Figure 2c), in good agreement with SEM results. γSelected (Figurearea 2c), in good agreement SEM results. area electron diffraction (SAED) pattern electron diffractionwith (SAED) pattern ofSelected Prn/Fe2 O 3 @SiO2 (Figure 2d) revealed clear rings of Prn/Fe 2O3@SiO2 (Figure 2d) revealed clear rings in accordance with the formation of crystalline in accordance with the formation of crystalline gamma iron oxide [31]. gamma iron oxide [31].
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Figure 2. 2. (a) image, (b) (b) EDS EDS spectrum, spectrum,(c) (c)TEM TEMimage, image,and and(d) (d)SAED SAEDpattern patternofofPrn/Fe Prn/Fe2O O3@SiO2, Figure (a) SEM SEM image, 2 3 @SiO2 , (e–j) SEM SEM elemental elemental mapping mapping of of Prn/Fe Prn/Fe2OO3@SiO (e–j) @SiO2. . 2
3
2
Thermal stability of Prn/Fe2O3@SiO2 was investigated by thermogravimetric analysis (Figure Thermal stability of Prn/Fe2 O3 @SiO2 was investigated by thermogravimetric analysis (Figure 3a). 3a). A progressive weight loss of 5 wt % was observed from 100 °C to 420 °C owing to the presence A progressive weight loss of 5 wt % was observed from 100 ◦ C to 420 ◦ C owing to the presence of of unbounded/physisorbed water and solvents, and the possible dehydration of some silanol unbounded/physisorbed water and solvents, and the possible dehydration of some silanol groups [32]. groups [32]. In addition, Figure 3a displayed a drastically drop of weight (12 wt %) at 420 °C. This In addition, Figure 3a displayed a drastically drop of weight (12 wt %) at 420 ◦ C. This decrease can decrease can be attributed to the degradation of organic moieties in the Prn/Fe2O3@SiO2 material. be attributed to the degradation of organic moieties in the Prn/Fe2 O3 @SiO2 material. Moreover, Moreover, DTA measurements displayed an exothermic band at 490 °C associated to the DTA measurements displayed an exothermic band at 490 ◦ C associated to the decomposition of the decomposition of the proline analogue. proline analogue. The magnetic properties of the synthesized materials were investigated by VSM analysis The magnetic properties of the synthesized materials were investigated by VSM analysis (Figure 3b). The magnetization analysis of both, γ-Fe2O3 and Prn/Fe2O3@SiO2 samples confirmed (Figure 3b). The magnetization analysis of both, γ-Fe2 O3 and Prn/Fe2 O3 @SiO2 samples confirmed their remarkably magnetic properties. The saturation magnetizations of γ-Fe2O3 and their remarkably magnetic properties. The saturation magnetizations of γ-Fe2 O3 and Prn/Fe2 O3 @SiO2 , Prn/Fe2O3@SiO2, resulted in similar values, 65 and 40 emu/g, respectively [33]. The slightly lower resulted in similar values, 65 and 40 emu/g, respectively [33]. The slightly lower magnetization of magnetization of the functionalized γ-Fe2O3 could be attributed to the proline analogue loading the functionalized γ-Fe2 O3 could be attributed to the proline analogue loading after functionalization. after functionalization. In any case, without a considerably loss of magnetism after the preparation In any case, without a considerably loss of magnetism after the preparation process, Prn/Fe2 O3 @SiO2 process, Prn/Fe2O3@SiO2 features interesting properties of magnetic separation and manipulation in features interesting properties of magnetic separation and manipulation in view of their potential view of their potential catalytic applications. catalytic applications. X-ray diffraction analysis of γ-Fe2O3, γ-Fe2O3@SiO2-NH2, and Prn/Fe2O3@SiO2 materials was X-ray diffraction analysis of γ-Fe2 O3 , γ-Fe2 O3 @SiO2 -NH2 , and Prn/Fe2 O3 @SiO2 materials was carried out and is reported in Figure 3c. The X-ray diffraction patterns of the three samples carried out and is reported in Figure 3c. The X-ray diffraction patterns of the three samples exhibited exhibited characteristic peaks, which matched well with standard γ-Fe2O3 reflections and revealed characteristic peaks, which matched well with standard γ-Fe2 O3 reflections and revealed their highly their highly crystalline nature [34]. After introducing amorphous SiO2, a small and expectable crystalline nature [34]. After introducing amorphous SiO2 , a small and expectable intensity decrease intensity decrease was observed. Nonetheless, these results suggest that after the functionalization was observed. Nonetheless, these results suggest that after the functionalization process there is process there is not considerable loss of crystallinity. Moreover, BET analysis (Figure 3d) showed a not considerable loss of crystallinity. Moreover, BET analysis (Figure 3d) showed a surface area of surface area of 75.2 m2·g−1 and 53.1 m2·g−1 for the γ-Fe2O3 and Prn/Fe2O3@SiO2 materials, 75.2 m2 ·g−1 and 53.1 m2 ·g−1 for the γ-Fe2 O3 and Prn/Fe2 O3 @SiO2 materials, respectively. Both respectively. Both products present a mesoporous structure, with a pore size of around 15 nm products present a mesoporous structure, with a pore size of around 15 nm (Table 1). (Table 1).
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Figure 3. 3. (a) (a) Thermogravimetric (TG) andand differential thermogravimetric (DTA) analyses of of Figure Thermogravimetric (TG) differential thermogravimetric (DTA) analyses Prn/Fe @SiO γ-Fe22OO3 3 and andPrn/Fe Prn/Fe O3 @SiO (c) XRD pattern of2O3, 3@SiO (b) VSM VSM diagram diagram of γ-Fe 2O32@SiO 2, (c) pattern of γ-Fe Prn/Fe 2 O32O 2 , 2,(b) 2 , XRD γ-Feγ-Fe γ-Fe @SiO and Prn/Fe physisorption isotherms and 3@SiO 2 2 -NH and 2 Prn/ Fe2O3@SiO 2, (d) NN 2 2physisorption isotherms ofofγ-Fe γ-Fe 3 and 2 O32,O 2 O23-NH 2 O3 @SiO 2 , (d) 2 O32O Prn/Fe O @SiO . Prn/Fe 2 32O3@SiO 2 2. Table 1. Textural properties. Table 1. Textural properties. Material
[a] (m2 g−1) Material [a] 2SBET SBET (m g−1 ) γ-Fe2O3 75 75 Prn/Fe2O3@SiO53 2 53
DBJH [b][b](nm) VBJH [c] (cm3 g−1)[c] DBJH (nm) VBJH (cm3 g−1 ) 14.5 0.27 14.5 0.27 15.2 0.2 15.2 0.2
γ-Fe2 O3 Prn/Fe2 O3 @SiO2 [a] SBET: specific surface area was calculated by the Brunauer–Emmett–Teller (BET) equation. [b] DBJH: [a] S [b] D BET : specific surface area was calculated by the Brunauer–Emmett–Teller (BET) equation. BJH : mean pore [c] VBJH: pore pore diameter was calculated by the Barret–Joyner–Halenda (BJH) equation. size mean diameter wassize calculated by the Barret–Joyner–Halenda (BJH) equation. [c] VBJH : pore volumes were calculated by the Barret–Joyner–Halenda (BJH) equation. volumes were calculated by the Barret–Joyner–Halenda (BJH) equation.
effective functionalization γ-Fe22Osurface 3@SiO2 with surface with penicillin-derived proline TheThe effective functionalization of γ-Fe2of O3 @SiO penicillin-derived proline analogue analogue was corroborated by FT-IR (Figure 4). This conclusion can be inferred from was corroborated by FT-IR (Figure 4). This conclusion can be inferred from the appearance of a peak the −1, related to the amide group formed by the reaction between appearance of a peak at amide 1644 cm at 1644 cm−1 , related to the group formed by the reaction between penicillin G and amino penicillin and2 O amino groups on the γ-Fe2O3@SiO2 surface. In addition, the band associated with groups on theGγ-Fe 3 @SiO2 surface. In addition, the band associated with carbonyl group in the carbonyl group the lactam ring (1174 cm−1) [35] was absent in Prn/Fe2O3@SiO2 lactam ring of purein penicillin (1174 cmof−1pure ) [35]penicillin was absent in Prn/Fe 2 O3 @SiO2 spectrum, pointing −1 −1 was spectrum, pointing to ring opening upon functionalization. Furthermore, a strong band at 1080 to ring opening upon functionalization. Furthermore, a strong band at 1080 cm observed forcm was observed for both γ-Fe2O3@SiO2 and Prn/Fe2O3@SiO2 materials, corresponding to Si-O-Si both γ-Fe 2 O3 @SiO2 and Prn/Fe2 O3 @SiO2 materials, corresponding to Si-O-Si stretching vibration. −1 wavenumber −1 wavenumber stretching FT-IR spectra also severalcm peaks in the 2900–3000 FT-IR spectra vibration. also showed several peaks inshowed the 2900–3000 range,cm which can be range, which can be assigned to C-H bonds in the aliphatic chains. Besides, characteristic assigned to C-H bonds in the aliphatic chains. Besides, characteristic transmission bands at 625 cm−1 −1 bands at 625 cm cm−1[36]. were associated with iron oxide [36]. andtransmission 550 cm−1 were associated withand iron550 oxide
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Figure4.4.FT-IR FT-IRspectra spectraofof(a)(a)γ-Fe γ-Fe 2O 3@SiO -NH22,, (b) (b) Prn/Fe Prn/Fe22O Figure O33@SiO @SiO2,2and , and(c) (c)penicillin penicillinG.G. 2O 3 @SiO 22-NH
The catalytic catalytic activity activity of of Prn/Fe Prn/Fe2O 3@SiO2 was investigated in two reactions: (1) oxidation of The 2 O3 @SiO2 was investigated in two reactions: (1) oxidation of isoeugenol and (2) vanillyl alcohol to vanillin. the amount amount of of catalyst catalyst was was optimized optimized to to 20 20 mg mg isoeugenol and (2) vanillyl alcohol to vanillin. Firstly, Firstly, the of catalyst catalyst per per each each mmol mmol of of starting starting material. material. Subsequently, both reactions reactions were were studied studied using using four four of Subsequently, both oxidant agents: (a) hydrogen peroxide, (b) urea hydrogen peroxide (UHP), (c) tert-butyl oxidant agents: (a) hydrogen peroxide, (b) urea hydrogen peroxide (UHP), (c) tert-butyl hydroperoxide hydroperoxide (tBHP), and (d) molecular oxygen. Although, latest did not displayed satisfactory (tBHP), and (d) molecular oxygen. Although, the latest did notthe displayed satisfactory results, the other results, the other three oxidant agents resulted to be effective in this reaction, in particular, the best three oxidant agents resulted to be effective in this reaction, in particular, the best results were achieved results were achieved using hydrogen peroxide. Moreover, the influence of the solvent on the using hydrogen peroxide. Moreover, the influence of the solvent on the effectiveness of the catalytic effectiveness of the catalytic reaction was investigated using acetonitrile and toluene, the first one reaction was investigated using acetonitrile and toluene, the first one being the most appropriated. being mosthand, appropriated. the other hand, the effect by of temperature wasreaction settled by On thethe other the effect On of temperature was settled performing the at performing 80, 90, and the reaction at 80, 90, and 100 °C, establishing 90 °C as the most suitable temperature for the2).catalytic ◦ ◦ 100 C, establishing 90 C as the most suitable temperature for the catalytic reaction (Table reaction (Table Under the 2). model conditions, we monitored the reaction progress every 30 min. After 4 h, the Under the model conditions,conversion we monitored reaction progresstowards every 30vanillin min. After 4 h, the reaction achieved the maximum withthe a stable selectivity production. reaction achieved the maximum conversion with a stable selectivity towards vanillin production. Compared to γ-Fe2 O3 and catalyst-free reaction, Prn/Fe2 O3 @SiO2 showed higher conversion and Compared under to γ-Fesimilar 2O3 and catalyst-free reaction, Prn/Fe2O3@SiO2 showed higher conversion and selectivity conditions (Tables 3 and 4). The catalyst was also compared with its selectivity under similar conditions and 4). in The catalyst was alsolatest compared with its proton-unexchanged counterpart, for (Tables which a3decrease conversion for the material could proton-unexchanged counterpart, for which a decrease in conversion for the latest material could be be observed. These results indicated that the acidic nature, provided by the carboxylic group seems observed. These results indicated that the acidic nature, provided by the carboxylic group seems to to have a crucial effect on the catalytic performance. In absence of the carboxylic acid, the amine have a crucial effect on the catalytic performance. In absence of the carboxylic acid, the amine group group itself could promote the reaction. However, the oxidation reaction of isoeugenol, using the itself could promote the reaction. However, the oxidation reaction of isoeugenol, using the proton-unexchanged catalyst did not achieve a good catalytic activity towards vanillin production proton-unexchanged catalyst did not achieve a good catalytic activity towards vanillin production (Table 3). The presence of acidic species may be decisive in the catalytic conversion of isoeugenol (Table 3). The presence of acidic species may be decisive in the catalytic conversion of isoeugenol to to vanillin. vanillin.
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Table 2018, 2. Oxidation of vanillyl alcohol (A) and isoeugenol (B) to vanillin. (In bold: conditions for the Catalysts 8, x FOR PEER REVIEWalcohol 7 of 11 Table 2. Oxidation of vanillyl (A) and isoeugenol (B) to vanillin. (In bold: conditions for the Table 2. Oxidation of vanillyl alcohol (A) and isoeugenol (B) to vanillin. (In bold: conditions for the best catalytic formance). best catalytic formance). best catalytic formance). Table 2. Oxidation of vanillyl alcohol (A) and isoeugenol (B) to vanillin. (In bold: conditions for the best catalytic formance).
EntryEntrySolvent Oxidant Time (h) TT(°C) Solvent Oxidant Time (h) (°C) Entry Solvent Oxidant Time (h) T (◦ C) 1 CH 3CN 15 90 1Entry CH 3CN -Oxidant 15 Solvent Time (h) 90 T (°C) 21 2 CH CN 9090 CH33CH CN 151515 90 3CN - O2 O2 3CN 15 90 CN tBHP 9090 32 13 CH CH3CH CN 90 3CN O 3CH 2 tBHP 154 4 3CN O 2 15 90 CH3CH CN tBHP 4 90 CN UHP 4 90 43 24 CH 3 CN UHP 4 90 CH 3 3CN tBHP 90 CH3CH CN 90 3CN UHP H22O2 4 4 44 CN H2 O 9090 54 35 CH 3CH 3CN UHP 90 4 CHCH 90 Toluene H222O2 4 10410 65 6 Toluene H22O O 9090 3 CN H CH3CN H OH2O2 10 4 6 5 Toluene 90 90 2 2 6
H2 O 2
10
90
88
Selectivity A
Selectivity A Selectivity (%) (%) A (%) A Selectivity 92 - (%) 92 -88 88 92 92 88 86 86 88 86 92 92 86 92 88 88 92 88 88
Conversion B
Conversion B Conversion (%) (%) B (%) B Conversion traces traces (%) traces 1313 traces 13 3333 1352 33 52 3383 52 83 5283 60 60 8360 60
Table 3. Catalytic oxidationofofisoeugenol isoeugenol [a].. (In system). Table 3. Catalytic oxidation (In bold: bold:best bestcatalytic catalytic system). [a] Table 3. Catalytic oxidation of isoeugenol . (In bold: best catalytic system). [a] Table 3. Catalytic oxidation of isoeugenol . (In bold: best catalytic system). [a]
Selectivity B
Selectivity B Selectivity B (%)(%) (%) B Selectivity (%) 32 -32 - 32 43 43 3243 48 48 4348 51 51 4851 42 42 5142 42
Selectivity (mol %) Conversion Time (h) Conversion (mol %)Others [b] (mol %) Vanillin Selectivity Diphenyl ether Selectivity (mol %) [b] Conversion (mol %) Vanillin Diphenyl ether Selectivity (mol %)
