Camila P. Ferraza,
Marco Aurélio S. Garciaa,
Érico Teixeira-Netob and
Liane M. Rossi*a
aDepartamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748, 05508-000, São Paulo, SP, Brazil. E-mail: lrossi@iq.usp.br
bLaboratório Nacional de Nanotecnologia, CNPEM/ABTLuS, Caixa Postal 6192, 13083-970, Campinas, SP, Brazil
First published on 1st March 2016
Gold nanoparticles have shown excellent catalytic properties in selective oxidation of alcohols in the presence of base; however, the influence of the nature and concentration of the base on gold catalyst activity and selectivity is not completely understood. We here present a study of the effect of using weak and strong bases on the conversion and selectivity of PVA-stabilized gold nanoparticles supported on titania (AuPVA/TiO2) for benzyl alcohol oxidation. The increase in the concentration of base had little effect on conversion when a weak base was used (K2CO3, Na2B4O7 and Na(CH3COO)), due to the buffer effect, but strongly affected selectivity. The bases with higher pKa values provided higher conversions and increased production of benzoic acid. For the strong base NaOH, benzoic acid was always the major product, although conversion decreases in excess of base. The formation of benzoic acid is avoided by using K2CO3 in non-aqueous media; benzaldehyde is the main product in cyclohexane whereas benzyl benzoate is also formed in significant amounts in solvent-free conditions. The promotion effect observed in the presence of base was discussed in terms of reaction mechanism.
Metal catalysts enable the use of green oxidants, such as molecular oxygen, which generates only water as a byproduct.4,5,14 Considered as catalytically inert, gold, when in nanometric dimensions, presents a number of new properties, including very high catalytic activity in reactions of industrial and environmental interest.3,5–8,15–20 Gold-based catalysts are more stable and selective for the aerobic oxidation of organic compounds in water than the conventional liquid-phase oxidation catalysts based on Pt and Pd, because the Au catalysts can offer better resistance to water and O2.9–11 Thus, gold offers unique opportunities for catalysis, besides an excellent alternative to the development of viable industrial process for the conversion of renewable biomass in aqueous phase.9–11 Previous studies with gold nanocatalysts in oxidation reactions of simple alcohols, such as benzyl alcohol, showed that an excellent selectivity can be achieved under mild conditions.4,5,9,21–40 Understanding the selective oxidation of benzyl alcohol can assist further application of catalytic oxidations for the valorization of renewable biomass substrates, such as glucose, HMF and glycerol.
Oxidation reactions in aqueous phase have been successful, but there is still room for further development, once water plays an important role in the efficiency of gold catalysts.4,9,21,27,28,33–35,38,39,41 The catalytic activity of gold catalysts for CO oxidation16–18 and for benzyl alcohol oxidation3,21 can be promoted significantly by water. Under identical reaction conditions, the activity of Au/TiO2 for benzyl alcohol oxidation in the presence of water is consistently higher than that in ethanol and p-xylene.21 Even in aqueous medium, the reaction rates are quite low.3,9,21,28,39 Acceptable conversions from a practical point of view for benzyl alcohol oxidation using gold catalysts are obtained in the presence of base promoters (aqueous3,4,9,27,33,35,38,39 and non-aqueous media9,22,30,32,37), by using a basic and/or nanosized support (non-aqueous media3,23–25,31,36,40,42) or by adding a second metal, in particular Pd (aqueous43,44 and non-aqueous media45–47). Aqueous oxidation of aromatic and aliphatic alcohols in general uses hydroxides and carbonates as base promoters and the amount of base has been arbitrarily chosen from sub-stoichiometric to equimolar or even large excess (from 0.5 to 3 equivalents). The catalytic activity in base-free conditions is generally an order of magnitude lower than the catalytic activity obtained in high-pH conditions,3,9,28,39 which emphasizes the critical role of the base as a promoter in gold catalysis. At the same time, in alkaline reaction conditions the selectivity is greatly affected towards benzoic acid.3–5,9,39,48,49 In this context, it would be interesting to ascertain how the amount of weak and strong bases affects the activity and selectivity of the catalytic oxidation of alcohols.
Besides the effect of base promoters in aqueous solution, marked examples underline the sensitivity of gold catalysts to the reaction media, that leads to different product distributions.3,5,9,39 As reported for benzyl alcohol oxidations, majority of gold catalysts in non-polar and solvent-free media favor selectivity towards benzaldehyde,3,23–25,36,40,42 while performing the oxidation in alcohols, such as MeOH, the corresponding ester is formed.26,31 In some cases, hydroxides and carbonates have been also applied as promoters;22,29,30,32,37 however, reports on the effect of the amount of base in these media are rather sparse.
Here we report on the effect of using weak and strong bases as promoters in the oxidation of benzyl alcohol catalyzed by AuPVA/TiO2. We evaluated how the nature of base and different [base]:[alcohol] molar ratios affect the reaction conversion and selectivity. The results were also compared to that obtained in non-aqueous media (cyclohexane and solvent-free).
# | Base | pKa | [Base]:[alcohol]([HO−] mol L−1) | Initial pH | Final pH | Conv. (%) | Selectivityb (%) | |
---|---|---|---|---|---|---|---|---|
BnCHO | BnCOOH | |||||||
a Reaction conditions: benzyl alcohol (0.3 mL, 2.9 mmol), catalyst (15 mg, 1.3 μmol Au), O2 (6 bar), 100 °C, 1 h. BnCHO = benzaldehyde, BnCOOH = benzoic acid.b Mass balance > 85%. | ||||||||
1 | No base | 7.00 | 0 | 5.85 | 3.83 | 19 | 92 | 8 |
2 | Na(CH3COO) | 4.87 | 0.1 (1.28 × 10−5) | 9.11 | 5.74 | 12 | 82 | 18 |
3 | 1 (4.05 × 10−5) | 9.61 | 6.26 | 15 | 87 | 13 | ||
4 | 4 (8.10 × 10−5) | 9.91 | 6.57 | 28 | 92 | 8 | ||
5 | Na2B4O7 | 9.23 | 0.1 (2.15 × 10−3) | 11.33 | 8.67 | 31 | 68 | 28 |
6 | 1 (6.97 × 10−3) | 11.84 | 9.35 | 32 | 50 | 47 | ||
7 | 4 (14.02 × 10−3) | 12.15 | 9.84 | 43 | 41 | 59 | ||
8 | K2CO3 | 10.32 | 0.1 (2.35 × 10−3) | 11.37 | 8.54 | 48 | 57 | 38 |
9 | 0.5 (5.38 × 10−3) | 11.73 | 10.29 | 65 | 34 | 59 | ||
10 | 1 (7.65 × 10−3) | 11.88 | 10.37 | 60 | 29 | 63 | ||
11 | 2 (10.86 × 10−3) | 12.04 | 10.39 | 72 | 13 | 87 | ||
12 | 4 (15.40 × 10−3) | 12.19 | 10.77 | 66 | 11 | 88 | ||
13 | NaOH | — | 0.1 (2.89 × 10−2) | 12.46 | 5.56 | 36 | 53 | 47 |
14 | 1 (2.89 × 10−1) | 13.46 | 13.18 | 54 | 5 | 95 | ||
15 | 4 (1.16) | 14.06 | 13.60 | 34 | 0 | 100 |
The presence of hydroxide ions plays an important role during the oxidation reaction. Davis et al. have discussed how the product distribution depends on pH.9,49 In principle, the buffer effect can explain similar initial TOFs for the reactions; however, the increase in the buffer strength represents an increase in the concentration of HO− (6.5 times for [K2CO3]:[alcohol] from 0.1 to 4) and, consequently, can sustain the differences observed in the reaction selectivity. It is well known that aldehydes in basic aqueous solution can be oxidized to carboxylic acid more easily.9,49 The proposed mechanism involves the reversible hydration of the aldehyde to germinal diols, which is accelerated in high pH. The germinal diols will adsorb in the metal surface to form an alkoxide, that later undergoes a β-elimination to form carboxylic acid.9,49 Thus, the combination of high concentration of OH− and longer reaction times favors the oxidation to carboxylic acid.
Recently, Zheng and Stucky37 have shown that the addition of sub-stoichiometric amounts of either acetates, borates or carbonates, can enhance the activity of supported gold nanoparticles for the oxidation of alcohols in solvent-free medium. Considering the ability of these species with different pKa values to form buffered solution, we become interested in investigating their behavior in aqueous media and the consequences on the reaction selectivity.
The aqueous oxidation of benzyl alcohol was studied in the presence of different amounts of Na2B4O7 and Na(CH3COO) and the results are summarized in Table 1 (entries 2–7). The benzyl alcohol conversion after 60 min of reaction follows the same order as the pKa: K2CO3 > Na2B4O7 > Na(CH3COO), most probably associated to the concentration of benzyloxide species that are involved in the first step of the reaction. In fact, a linear correlation was found between the initial reaction pH or the amount of benzyloxide at initial pH and the reaction conversion (Fig. 2a). The conversion was only slightly improved with the increase in the [base]:[alcohol] molar ratio, due to the buffer formation, as observed previously in the experiments with potassium carbonate. That was not the case for the reaction with the strong base NaOH (Table 1, entries 13–15), wherein the conversion was greatly affected by the amount of base and does not follow the previous described tendency in Fig. 2a. For [NaOH]:[alcohol] = 0.1, the conversion was 35% and pH drops from 12.46 to 5.56. As the buffer is not formed, the formation of benzoic acid decreased the pH and then the reaction rate, which results in a lower conversion compared to the reaction with K2CO3. For [NaOH]:[alcohol] = 1 and 4, the final pH was not highly affected by the amount of acid formed in the reaction (13.18 and 13.60, respectively). An improvement in the conversion was observed for 1 equivalent of NaOH, reaching 54%, which was close and comparable to that obtained with K2CO3 (ca. 60%). However, for [NaOH]:[alcohol] = 4, the conversion dropped to 35%. In this condition, the concentration of HO− in the aqueous solution is about 75 times higher than it was produced by a similar amount of K2CO3, which suggests a lack of correlation between the reaction conversion and [HO−] (Fig. 2b). In order to rationalize these results, we should remember that the activation of O2 in the aqueous oxidation of alcohols proceeds through the formation and dissociation of peroxide (OOH*) and hydrogen peroxide (HOOH*) intermediates,9,49 which are necessary species to close the catalytic cycle (removal of excess electrons from the metal surface, oxidize metal-hydride bonds and regenerate hydroxide ions).9,25,27,42,49,53 It has been suggested that the oxidation of alcohols can be less effective in high concentration of NaOH (>0.6 M) due to the faster decomposition of peroxide.27 In order to confirm these assignments, the hydrogen peroxide formation was investigated for the aqueous oxidation reaction using K2CO3 and NaOH (Fig. 2b). When increasing the amount of NaOH from 0.1 to 1 equiv., the increase in the [HO−] favored the formation of H2O2,54 which caused an increase in the reaction conversion. However, for 4 equiv. of NaOH, the amount of H2O2 decreased as well as the reaction conversion. These results suggest that the excess of NaOH ([NaOH]:[alcohol] > 1) is detrimental to the oxidation reaction. Therefore, the fast decomposition of the peroxide in strong alkaline media can provide an explanation for the lack of correlation between the conversion and pH for NaOH.
Fig. 2 (a) Conversion as a function of initial pH (solid symbols) and initial concentration of benzyloxidea ([BnO−]) (empty symbols) for aqueous oxidation of benzyl alcohol catalyzed by AuPVA/TiO2 using K2CO3 (■), NaOH (●), Na2B4O7 (▼) and Na(CH3COO) (▲) as base for 60 min. (b) Conversion (solid symbols) and concentration of H2O2 (empty symbols) as a function of [base]:[alcohol] molar ratio (0.1, 1 and 4) for aqueous oxidation of benzyl alcohol catalyzed by AuPVA/TiO2 using K2CO3 (■) and NaOH (●) as base for 60 min. Reaction conditions: benzyl alcohol (2.9 mmol), catalyst (1.3 μmol Au), O2 (6 bar), 100 °C. aCalculated value using the Henderson Hasselbalch equation at each initial pH, according to ref. 52. |
A general tendency was observed in the selectivity for the reactions performed in the presence of weak bases, i.e., increasing the [HO−] decreases the selectivity to benzaldehyde, while favoring the formation of benzoic acid (Table 1, entries 2–12). Na(CH3COO) produces the lowest [HO−] and the best selectivity to benzaldehyde. In this case, the lower pH delays the further oxidation to acid, but also prevents the reaction to proceed to high conversions. However, K2CO3 was the most effective base to produce benzaldehyde or benzoic acid in aqueous media, producing benzaldehyde (57% selectivity at 48% conversion) in the [base]:[alcohol] = 0.1 and benzoic acid (87% selectivity at 72% conversion) in the [base]:[alcohol] = 2. The increase in the [base]:[alcohol] ratio represents little variation in the [HO−] (ca. 6.5 times), thus benzaldehyde was still observed in all reactions. For the strong base NaOH, the increase in the [base]:[alcohol] ratio represents an increase of up to 40 times in the [HO−], thus benzoic acid is the major product no matter the amount of NaOH used.
Further studies on the oxidation of benzyl alcohol using K2CO3 as a base promoter were conducted in cyclohexane and solvent-free conditions. Conversion and selectivity after 1 h reaction as a function of the amount of added base ([K2CO3]:[alcohol] = 0.1, 1 and 4) are presented in Table 2. Contrary to aqueous phase, only very low conversions were observed in absence of base for non-aqueous media (Table 2, entries 16 and 20). However, the use of K2CO3 as base promoter represented a marked improvement on reaction conversions. When comparing the use of K2CO3 in aqueous and cyclohexane media, the results suggest that the conversion was slightly affected by varying the reaction media, but selectivity has significantly changed towards benzaldehyde. In the solvent-free condition, the addition of base resulted also in high conversions, reaching 90% already with 1 equivalent of base, but selectivity was not as high as in cyclohexane. Differences in reactivity can be rationalized considering the proposed mechanism.9,25,42,49 The oxidation of alcohol proceeds through 3 steps: (1) metal-alkoxide formation; (2) β-elimination; (3) metal-hydride formation and catalyst regeneration. In the aqueous-phase, the basic solution favors the initial deprotonation of the alcohol to form an alkoxy intermediate in the first step and also decreases the energy barrier in step 2.9,49 In the non-aqueous phase, the support or a promoter must play an important role in the initial activation of the alcohol that might occur on the catalyst surface,9,49 just as the dehydration in the step 2. Reasonable conversions and the formation of benzaldehyde were only possible with the addition of K2CO3, indicating that both steps are favored in the presence of a base promoter. Both in aqueous or non-polar organic media, where K2CO3 is acting as a homogeneous base and as a solid base, respectively, similar conversions of benzyl alcohol were observed for the same amount of base (Table 1, entries 8–12 and Table 2, entries 16–19). In the solvent-free condition (Table 2, entries 20–22), when the neat alcohol was used, the conversion was boosted probably due to the high concentration of alcohol. Under this condition, a large majority of the metal sites will be coordinated forming the alkoxide.25
# | Medium | [Base]:[alcohol] | Conv. (%) | Selectivityc (%) | ||
---|---|---|---|---|---|---|
BnCHO | BnCOOH | BnBzO | ||||
a Reaction conditions: benzyl alcohol (0.3 mL, 2.9 mmol), catalyst (15 mg, 1.3 μmol Au), O2 (6 bar), 100 °C, 1 h.b Benzyl alcohol (1 mL, 9.7 mmol), catalyst (52 mg, 4.4 μmol Au), O2 (6 bar), 100 °C, 1 h.c Mass balance > 93%.d [K2CO3]:[alcohol] = 4 was not performed because the mass of base would be higher than the mass of alcohol. This amount of base would not provide a liquid medium for the reaction. | ||||||
16 | Cyclohexanea | 0 | 2 | 100 | 0 | 0 |
17 | 0.1 | 44 | 89 | 0 | 11 | |
18 | 1 | 55 | 87 | 0 | 13 | |
19 | 4 | 72 | 85 | 0 | 15 | |
20 | Solvent-freeb | 0 | 3 | 100 | 0 | 0 |
21 | 0.1 | 61 | 70 | 0 | 30 | |
22 | 1 | 87 | 63 | 0 | 37 | |
23 | 4d | — | — | — | — |
Differently from the aqueous media, in cyclohexane and solvent-free conditions, benzoic acid was not formed. We performed the oxidation of benzaldehyde under the same reaction conditions (see Table S2, ESI†). Benzaldehyde is converted in benzoic acid in the absence of catalyst in non-aqueous, but the formation of benzoic acid is inhibited by AuPVA/TiO2 catalyst. In aqueous media, benzaldehyde is converted in benzoic acid either with or without the catalyst. Thus, the results under non-aqueous media suggest that the formation of benzoic acid is not favorable on the catalyst surface, but its formation can still occur in alkaline aqueous medium once the HO− ions are available.
The only byproduct for the reaction in non-aqueous media is the benzyl benzoate, which is favored by the increase in the amount of K2CO3. As the conversions in water and in cyclohexane are similar, the difference relies on the selectivity: higher base concentration favors the formation of benzoic acid in aqueous media, while in cyclohexane, the ester is preferred with no formation of acid. The formation of benzyl benzoate is minimal in cyclohexane (up to 15% selectivity), but it is appreciable in solvent-free condition. In the presence of [K2CO3]:[alcohol] as low as 0.1, the selectivity for benzyl benzoate is already 30% and increases to 37% for [K2CO3]:[alcohol] = 1. This resulted in moderate selectivity to benzaldehyde (up to 70%). According to proposed mechanism for the oxidation of alcohols, the ester is formed by the attack of the alkoxy species (formed in step 1) into the aldehyde (formed in the step 2).48 Thus, the benzyl benzoate is more likely produced when the rate of formation of benzyloxide is high compared to the rate of formation of benzaldehyde. In the solvent-free medium, the benzyl ester is formed in the highest yields, indicating high rate of formation of benzyloxide, which is boosted by the base.
The stability of the AuPVA/TiO2 catalyst against aggregation after being exposed to different reaction conditions used in this study was investigated by Scanning Transmission Electron Microscopy (STEM). The STEM images (Fig. S1, ESI†) revealed that the gold nanoparticles of the as prepared catalyst (4.4 ± 1.5 nm) only slightly increase in size when used in aqueous solution in the presence of acetate (4.7 ± 1.6 nm), borate (5.2 ± 1.9 nm) and NaOH (5.4 ± 2.4 nm), but significantly increase in size in the presence of carbonate (7.6 ± 2.6 nm). In the solventless reaction, the gold particle size was maintained (5.1 ± 1.8 nm), but in cyclohexane particle's growth was noticed (10.8 ± 8.3 nm). There is not a clear correlation between the reactivity and the particle size changes in the catalytic reaction studied. None of the samples were attacked by the reaction media and all of them are comprised of supported gold nanoparticles after reaction.
In a typical solvent-free reaction, the glass reactor was loaded with benzyl alcohol (1 mL, 9.7 mmol), the AuPVA/TiO2 catalyst (52 mg, 4.4 μmol Au) and the base ([K2CO3]:[alcohol] = 0.1 or 1 (133 mg or 1.33 g)). The reactor was purged five times with O2, leaving the vessel at 6 bar. The temperature was maintained at 100 °C with an oil bath on a hot stirring plate connected to a digital controller (ETS-D5 IKA). The reactions were constantly stirred using Teflon-coated magnetic stir bars for 60 min. To the reaction mixture, 9 mL of CH2Cl2 was added and the catalyst was removed by centrifugation. An aliquot of the solution was removed and analyzed by GC.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01795a |
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