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Palladium nanoparticles onto ethylenediamine functionalized silica–cellulose substrates [Pd(0)-EDA/SCs]: An efficient and sustainable approach for hydrogenation of nitroarenes and carbonyl compounds under mild conditions
⁎Corresponding author. paul7@rediffmail.com (Satya Paul)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
This paper aimed to report a green and efficient approach for the Pd(0)-EDA/SCs catalyzed hydrogenation of nitroarenes and carbonyl compounds using environment friendly molecular hydrogen at room temperature under benign reaction media. Three different catalysts, Pd(0)-EDA/SC-1, Pd(0)-EDA/SC-2 and Pd(0)-EDA/SC-3 based on immobilization of palladium nanoparticles onto ethylene diamine functionalized silica–cellulose substrates were prepared. Among various catalysts tested, Pd(0)-EDA/SC-2 showed superior catalytic activity. Further, excellent yield of the products, recyclability and the facile work-up make the catalyst more versatile, eco-friendly and economical to perform the desired organic transformations.
Keywords
Pd(0) nanoparticles
Amine functionalization
Silica/cellulose substrate
Hydrogenations
Heterogeneous catalyst
Recyclability
1 Introduction
Catalytic hydrogenation is one of the most significant and broadly used transformations in organic synthesis (Zang et al., 2015; Johan et al., 2014; Stolle et al., 2013). Hydrogenation of organic substrates such as nitroarenes and carbonyl compounds to corresponding amines and alcohols is of supreme interest in chemistry since it is a key operation in the laboratory as well as in chemical industry, and its wide spectrum of applications in the synthesis of biologically active compounds (Zeng et al., 2013; Shokouhimehr et al., 2013; Liu et al., 2015). Some of the well known protocols developed for such conversions involve the use of stoichiometric chemical reagents, such as LiAlH4, NaBH4 or the use of corrosive acetic acid which poses serious limitations both economically and environmentally (Zaccheria et al., 2005; Handique et al., 2001; Wang et al., 2010; Zhang et al., 2008; Bae et al., 2000; Lamm et al., 2013). Further, selective reduction of nitroarenes and carbonyl compounds in the presence of other competitively reducible or degradable functional groups is a challenging task. For example, reduction of nitroarenes often stops at an intermediate stage, leading to hydroxylamines, hydrazines and azoarenes as side products (Yu et al., 2001). In addition, selective reduction of aldehydes and ketones is often complicated by various side reactions such as hydrogenation of aromatic ring as well as hydrogenolysis of the produced alcohol. Thus, selective reduction of nitroarenes and carbonyl compounds by employing environment friendly molecular hydrogen and recyclable catalytic systems is often the most desired pathways in organic synthesis.
Recently, Pd(0) nanoparticles have arisen as a competitive and sustainable option to traditional Pd based catalysts for the hydrogenation of organic substrates (Ley et al., 2006; Hemantha and Sureshbabu, 2011; Arai et al., 2015). Their high surface area to volume ratio, heightens their activity and selectivity by preserving their inherent characteristics as heterogeneous catalyst. But, the catalytic performance of these nanoparticles highly depends on the solid supports used for their immobilization and stabilization techniques to avoid their aggregation during the various steps of their preparation and application (Campelo et al., 2009). Hence, immobilization of metal nanoparticles onto a suitable rigid matrix for their higher stability and dispersion is highly desirable (Nezhad and Panahi, 2012). Recently, organic/inorganic hybrids which represent the new generation of biocomposites, and comprise the combination of biopolymers and an inorganic material have been emerged as viable supports for the immobilization and stabilization of metal nanoparticles (Siracusa et al., 2008; Karim et al., 2008). Functionalized biopolymers such as cellulose are renewable, biodegradable and have broad chemical-modifying capacity (Fischer et al., 2008). Cellulose, the oxygen-rich carbohydrate consists of anhydrous glucose units connected through oxygen linkage of β-1,4-glucosidic bond to form a molecular chain. Intra chain hydrogen bonding between hydroxyl groups and oxygen of the adjoining ring molecules stabilizes the linkage and results in the linear configuration of the cellulose chain. This biomaterial also contains microfibrils with up to 30 nm width that are three dimensionally connected to each other. The metal nanoparticles can be stabilized in the cavities of these microfibrils via oxygen–metal electrostatic interaction, thus adding to the usefulness of cellulose as an efficient support for the metal nanoparticle synthesis (Habibi, 2014). Moreover, inorganic supports such as silica, when bind with the organic support further increase the stability of the composite and are responsible for the properties such as temperature, mechanical resistance and porosity. However, an appropriate surface modification for these inorganic/organic composites with amine functionalization may be done to prevent the aggregation of metal nanoparticles, leading to stable, finely dispersed active species (Sodhi et al., 2014; Kumbhar et al., 2013; Veisi et al., 2015). Furthermore, the catalytic activity of metal nanoparticles highly depends on the nature of the functional groups present on the surface of the support, which can affect the particle size dispersion and chemical state of metal nanoparticles (Wang et al., 2011; Armbruster et al., 2012). Amine functionalization of the support has recently fascinated much research interest because they remarkably increase the performance of the catalytic supports (Xu et al., 2012; Jagadeesh et al., 2013; Li and Antonietti, 2013). The presence of basic nitrogen sites could transform the surface structure of support materials, with increased π-binding ability and improved basicity (Paraknowitsch et al., 2010; Gao et al., 2013). Considering this strategy, we had introduced efficient heterogeneous catalysts for the C—C and C—S couplings based on the immobilization of palladium nanoparticles onto ethylene diamine functionalized silica–cellulose substrates, Pd(0)-EDA/SCs (Bhardwaj et al., 2015). In the present work, we would like to report the synthetic usefulness of Pd(0)-EDA/SCs for the hydrogenation of nitroarenes and carbonyl compounds using environment friendly molecular hydrogen at room temperature under benign reaction media.
2 Experimental
2.1 General procedure for the Pd(0)-EDA/SCs catalyzed hydrogenation of nitroarenes, aldehydes and ketones
To a mixture of nitroarene, aldehyde or ketone (1 mmol) and Pd(0)-EDA/SC-2 (0.1 g, 1.25 mol% Pd) in a round bottom flask (25 mL), water (5 mL for nitroarenes) or water/ethanol (3:1, 5 mL for aldehydes and ketones) were added and the reaction mixture was stirred at room temperature using hydrogen filled balloon for an appropriate time (Tables 3 and 4). After completion of the reaction, the reaction mixture was diluted with ethyl acetate and filtered. The organic layer was washed with water and dried over anhydrous Na2SO4. Finally, the product was obtained after removal of the solvent under reduced pressure followed by passing through column of silica gel and elution with EtOAc-pet. ether. The recovered catalyst was washed with EtOAc (3 × 5 mL) followed by double distilled water (3 × 10 mL). It was dried and then reused for subsequent reactions.
The structures of the products were confirmed by 1H, 13C NMR, mass spectral data and comparison with authentic samples obtained commercially or prepared according to the literature methods.
3 Results and discussion
3.1 Characterization of Pd(0) nanoparticles onto ethylene diamine functionalized silica–cellulose substrates [Pd(0)-EDA/SCs]
Synthesis of Pd(0)-EDA/SCs was achieved via a reported procedure (Bhardwaj et al., 2015) depicted in Scheme 1. (For detailed experimental procedure see ESI.) In this process, the silica chloride and 6-(2′-aminoethylamino)-6-deoxy-cellulose (EDACel) were taken in three different ratios of 2:1, 1:1 and 1:2 to prepare different EDA/SCs. Pd(0) NPs were then immobilized on EDA/SCs to get Pd(0)-EDA/SC-1 (prepared from 2:1 ratio of silica chloride and EDACel), Pd(0)-EDA/SC-2 (prepared from 1:1 ratio of silica chloride and EDACel), and Pd(0)-EDA/SC-3 (prepared from 1:2 ratio of silica chloride and EDACel), with a view to select the most active and selective heterogeneous catalyst for the hydrogenation of nitroarenes and carbonyl compounds. Among various catalysts screened, it was found that Pd(0)-EDA/SC-2 (prepared from 1:1 ratio of silica chloride and EDACel) was found to be most active and selective. The Pd(0)-EDA/SCs have been fully characterized using different spectroscopic techniques (Bhardwaj et al., 2015).
3.2 Catalytic testing for the hydrogenation of nitroarenes and carbonyl compounds
To obtain the most appropriate conditions for the hydrogenation of nitroarenes and carbonyl compounds, we choose nitrobenzene and benzaldehyde as the model substrates. Initially, we examined the catalytic activity of Pd(0)-EDA/SCs by carrying out the reaction with model substrates using Pd(0)-EDA/SC-1, Pd(0)-EDA/SC-2 and Pd(0)-EDA/SC-3 under H2 atmosphere at room temperature. The results are presented in Table 1, which indicated that among the different catalysts screened, Pd(0)-EDA/SC-2 provided the best results in terms of reaction time and yield. Thus, Pd(0)-EDA/SC-2 was selected to carry out the hydrogenation of other nitroarenes and carbonyl compounds respectively.
Next, to study the effect of catalyst amount, the model reaction was performed using variable catalyst amounts i.e. 0.02 g (0.25 mol% Pd), 0.05 g (0.62 mol% Pd), 0.1 g (1.25 mol% Pd) and 0.12 g (1.5 mol% Pd) and best results were obtained with 0.1 g (1.25 mol% Pd) of catalyst. Therefore, 0.1 g of Pd(0)-EDA/SC-2 (1.25 mol% Pd) was herein selected as the optimal amount for carrying out the hydrogenation reactions. With increasing concern for the environment, chemists are persistently looking for ways to lessen the impact of chemical processes. One way in which this can be done, in addition to controlling the side products produced, is by avoiding the use of toxic organic solvents and instead carrying out the reactions in environmentally benign solvents such as water and ethanol. So, further optimization was done with respect to different solvents and the results are presented in Table 2. From the results, it is observed that nature of the solvent affected the conversion of the hydrogenation reactions. For the reduction of nitrobenzene, the reaction was carried out in solvents such as CH2Cl2, MeOH, EtOH and H2O, but the best results were obtained with water in terms of time and yield (entry 4, Table 2). However, the reduction of benzaldehyde, which is poorly soluble in water, took longer reaction time for its completion, while water/ethanol mixture (3:1) reduced the reaction time and improved the yield (entry 5, Table 2). This may be attributed to the beneficial effect of the co-solvent which resulted in good solubility of the organic substrate. Thus, screening of various solvent systems using Pd(0)-EDA/SC-2 showed that water and water/ethanol mixture are the most suitable solvents for the reduction of nitro and carbonyl groups respectively.
To check the compatibility of this reaction in the presence of other functional groups, we examined an array of synthetically valuable aromatic nitroarenes using Pd(0)-EDA/SC-2 under optimized reaction conditions (Scheme 2, Table 3). The reaction worked selectively and efficiently with a wide range of nitro substituted aromatic compounds under hydrogen atmosphere at room temperature. Even in the presence of electron-donating groups (entries 2–4, Table 3), the reaction proceeded efficiently to afford the products in quantitative yields. It is noteworthy to mention that the azoxy, azo and hydrazo compounds as the usual side products of reduction of nitroarenes were not observed in this method. Further, no dehalogenated product was obtained during the hydrogenation of halogen substituted nitroarenes (entries 5–7, Table 3), furnishing the halogenated anilines in excellent yields. 4-Nitrophenol was also selectively reduced to 4-aminophenol in good yield (entry 8, Table 3). Remarkably, 1,4-dinitrobenzene and 1-nitronaphthalene were also completely hydrogenated to 1,4-phenylenediamine and 1-aminonaphthalene in quantitative yields (entries 9 and 10, Table 3). Further, 4-nitrobenzoic acid and 3-nitrobenzoic acid have also been reduced to corresponding 4-aminobenzoic acid and 3-aminobenzoic acid, selectivity (entries 11 and 12, Table 3). When the reduction of 4-aminobenzoic acid and 3-aminobenzoic acid was carried out for longer reaction times i.e. 20 min. in each case, no significant changes from the earlier results have been observed. Furthermore, the longer reaction time did not affect the —COOH functionality.
The mechanism of hydrogenation of nitrobenzene was proposed by Fritz Haber in the late 1890s (Haber, 1898), (Scheme 3). In this mechanism, initially, the nitro group is reduced to a nitroso group followed by the formation of hydroxylamine in very fast consecutive steps. Finally, the hydroxylamine is reduced to aniline in the slow rate determining step.
In another possible mechanism proposed by Gelder et al. (2005), the hydrogenation of nitrobenzene takes place via the formation of nitrosobenzene as an intermediate (Scheme 4), where [C6H5NOH(ads)] is the adsorbed species.
As can be observed from the above schemes, phenylhydroxylamine is the common intermediate in both mechanisms. It has been observed that the biggest dare in the hydrogenation of nitrobenzene is to avoid the accumulation of intermediates, such as hydroxylamine because the accumulation of this hazardous intermediate may lead to rapid exothermic decomposition and formation of condensation products (Layek et al., 2012; Mahata et al., 2008). Further, it has also been reported that the basic nitrogen species present in the support can improve the catalytic activity and selectivity toward the hydrogenation reactions (Armbruster et al., 2012; Li et al., 2013). In view of the above mentioned consideration, a plausible reaction mechanism for the reduction of nitrobenzene to aniline over the surface of Pd(0)-EDA/SC-2 involving phenylhydroxyl amine as intermediate has been proposed and depicted in Scheme 5. In this mechanism, the basic nitrogen species present in the support may account for two roles: (1) possibly the intermediate, hydroxylamine can interact with the surface through the hydroxyl group to form —OH···N or OH··· π interactions and thus, the adsorption of the hydroxylamine molecule on the surface of the support get facilitated by the existence of basic nitrogen species, and (2) the nitrogen species may act as electron donors and change the electron density of the modified support, which thereby increases the electron density of palladium nanoparticles.
To widen the scope of the catalytic system, various aldehydes and ketones were subjected to reduction to their corresponding alcohols (Scheme 6, Table 4) under the optimized reaction conditions. Aromatic aldehydes bearing electron-donating groups or electron-withdrawing groups were reduced to corresponding alcohols in excellent yields at room temperature. Notably, when we carried out the reaction using 4-nitrobenzaldehyde, the aldehyde group reduced first and gave the corresponding alcohol in 1 h (entry 7, Table 4), but when we continued the hydrogenation of 4-nitrobenzaldehyde for more than 1 h, the NO2 group was also reduced giving 4-aminobenzylalcohol in quantitative yield (entry 8, Table 4). Further, reduction of ketones with electron withdrawing as well as donating groups also underwent smoothly and gave corresponding secondary alcohols in satisfactory yields (entries 9–13, Table 4).
| Entry | Substrate | Product | Time (h) | Yield (%)b |
|---|---|---|---|---|
| 1. |
|
|
1 | 95 |
| 2. |
|
|
1.5 | 93 |
| 3. |
|
|
1 | 95 |
| 4. |
|
|
1.5 | 94 |
| 5. |
|
|
2 | 92 |
| 6. |
|
|
2 | 91 |
| 7. |
|
|
1 | 93 |
| 8. |
|
|
1.5 | 94 |
| 9. |
|
|
2 | 92 |
| 10. |
|
|
1.5 | 92 |
| 11. |
|
|
2 | 91 |
| 12. |
|
|
2.5 | 90 |
| 13. |
|
|
2.5 | 88 |
Mechanistically, the surface nitrogen basic sites, in the presence of palladium nanoparticles, may interact with the π∗ acceptor orbital of the C⚌O group, and thus improve the reactivity of the carbonyl group (Denmark and Beutner, 2008). Therefore, a plausible reaction mechanism involving the synergistic effect of the surface palladium(0) nanoparticles and basic nitrogen sites over the surface of Pd(0)-EDA/SC-2 is proposed for the hydrogenation of benzaldehyde (Scheme 7). In this mechanism, the C⚌O group of the benzaldehyde may form a zwitter ionic tetrahedral intermediate via oxygen atom with the basic nitrogen sites on the surface of the catalyst. As a result, the π-bond of C⚌O group gets activated by basic nitrogen sites and may become easy to be attacked by hydrogen on the surface of palladium(0) nanoparticles. Thus, the C⚌O group gets two H atoms from palladium(0) nanoparticles and departs from the surface basic nitrogen sites to give the corresponding alcohol.
3.3 Comparison of Pd(0)-EDA/SC-2 with other catalyst systems
In order to demonstrate the role of Pd(0)-EDA/SC-2 as heterogeneous catalyst, we performed a set of individual experiments by employing various homogeneous and heterogeneous catalysts as well as in the absence of any catalyst under the same set of conditions. The experiments were done in case of entry 1, Table 3 and entry 1, Table 4 and the results are summarized in Table 5. As can be seen from the results, hydrogenation reaction did not proceed in the absence of catalyst (entry 1, Table 5). Further, the results obtained with silica, cellulose and silica/EDACel were also inefficient (entries 2–4, Table 5). The reaction with homogeneous unrecoverable catalysts such as Pd(acac)2, Pd(OAc)2 and PdCl2 occurred but with less efficiency (entries 5–7, Table 5). Although, the use of commercially available Pd/C catalyst increased the yield of expected products it took longer time for completion as compared to our catalyst (Table 5, entry 8). Remarkably, the use of nano Pd(0) based heterogeneous catalysts having large surface to volume ratio led to the increase in the efficiency of the desired reaction (entries 9–11, Table 5). Due to the increased surface area of the Pd(0) NPs, the contact between the reactant molecules and catalyst appreciably increases. This enhanced interaction facilitates the heterogeneous catalytic system and helps to achieve a better reaction rate. But, again, these catalysts (entries 9–11, Table 5) were less effective than our catalyst, Pd(0)-EDA/SC-2 because the average particle size of active palladium nanoparticles is 3.5 nm (Bhardwaj et al., 2015), and at these dimensions they show very high catalytic activities. These results clearly demonstrate the superiority of the present catalytic system, Pd(0)-EDA/SC-2 which played a significant role in catalyzing the hydrogenation reactions more efficiently in terms of selectivity, reaction time and yield (entry 12, Table 5).
| Entry | Catalyst | Nitrobenzene | Benzaldehyde | ||
|---|---|---|---|---|---|
| Time (min) | Yield (%)b | Time (h) | Yield (%)b | ||
| 1 | – | 60 | N.R. | 3 | N.R. |
| 2 | Silica | 60 | Traces | 3 | N.R. |
| 3 | Cellulose | 60 | Traces | 3 | N.R. |
| 4 | Silica/EDACel | 60 | Traces | 3 | N.R. |
| 5 | Pd(acac)2 | 45 | 60 | 2.5 | 55 |
| 6 | Pd(OAc)2 | 40 | 75 | 2.5 | 61 |
| 7 | PdCl2 | 40 | 78 | 2.5 | 66 |
| 8 | Pd/C | 35 | 80 | 2 | 75 |
| 9 | Cellulose-Pd(0) | 25 | 85 | 2 | 78 |
| 10 | Silica/starch-Pd(0) | 20 | 90 | 1.5 | 85 |
| 11 | Silica/cellulose-Pd(0) | 20 | 93 | 1.5 | 90 |
| 12 | Pd(0)-EDA/SC-2 | 5 | 97 | 1 | 95 |
Bold values indicates selected reaction conditions.
4 Recyclability and heterogeneity
In sustainable organic synthesis, the recovery and reusability of the catalyst is of prime importance. Thus, to assess the long-term stability and reusability of Pd(0)-EDA/SC-2, we carried out a set of experiments in case of entry 1, Table 3 and entry 1, Table 4 using Pd(0)-EDA/SC-2. After completion of the reaction, the catalyst was readily recovered by filtration followed by washing and drying and then reused for further reactions by adding new substrates under similar reaction conditions. Pd(0)-EDA/SC-2 could be used at least six times without any significant change in activity and selectivity. The results are summarized in Table 6. The high stability of Pd(0)-EDA/SC-2 may be due to the ethylene diamine functionalized silica/cellulose substrate which stabilizes the Pd nanoparticles through their basic nitrogen sites and thus prevents their leaching from the surface of the catalyst during the several runs of the reactions. In addition, EDAC also acted as a surfactant which restricted the agglomeration and/or growth of the nanoparticles.
| Catalytic runs | Nitrobenzene | Benzaldehyde |
|---|---|---|
| Yield (%)b | Yield (%)b | |
| 1 | 97 | 95 |
| 2 | 97 | 95 |
| 3 | 95 | 92 |
| 4 | 94 | 91 |
| 5 | 93 | 91 |
| 6 | 93 | 90 |
Further, to rule out the contribution of homogeneous catalysis, a reaction (entry 1, Table 3) was carried out until the conversion was 40% (2 min) and at that point the catalyst was filtered off at the reaction temperature. The liquid phase was then transferred to another flask and again allowed to react, but no further significant conversion was observed. This strongly suggests that the catalyst is heterogeneous in nature. The ICP-AES analysis of the used Pd(0)-EDA/SC-2 after the 6th run was performed to calculate the Pd content. The initial content (6.67 wt% of Pd) before the reaction was reduced to 6.60 wt% after the 6th run. FTIR and TGA studies showed that the spent catalyst had no observable structural change relative to the fresh catalyst. The heterogeneity of Pd(0)-EDA/SC-2 was further confirmed by Hg poisoning test. To carry out the Hg poisoning test, the reaction (entry 1, Table 3) was carried out in the presence of Hg and Pd(0)-EDA/SC-2 (Hg:Pd; 100:1) for 20 min. It has been found that the reaction did not stop and almost quantitative yield of the product was obtained which clearly shows that the Hg does not act as a poison for the reaction. This may be due to the presence of basic sites (—NH and —OH) which stabilized the palladium nanoparticles. If palladium has not been stabilized by basic sites, it could form amalgam with Hg and hence poison the catalyst (Phan et al., 2006). This further suggests that the process is truly heterogeneous in nature.
5 Conclusion
In conclusion, we have reported an efficient and green catalytic process based on amine functionalized inorganic/organic biocomposites for the hydrogenation of nitroarenes and carbonyl compounds using environment friendly hydrogen at room temperature in benign reaction media. The operational simplicity, stability, reusability of catalyst and above all, the yield of amines and alcohols with high selectivity in short reaction times and with zero effluent discharge make it an environmentally acceptable and greener alternative for the reduction of nitroarenes and carbonyl compounds.
Acknowledgements
Financial assistance from UGC, New Delhi (SRF to one of the authors, MB and major research project, F 41-281/2012 SR) is gratefully acknowledged.
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Appendix A
Supplementary material
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.arabjc.2016.05.008.
Appendix A
Supplementary material
Supplementary data 1
Supplementary data 1