Pd on cyclotriphosphazen-hexa imine decorated boehmite as an efficient catalyst for hydrogenation of nitro arenes under mild reaction condition

Cyclotriphosphazen-hexa imine ligand was prepared through successive reactions of phosphonitrilchloride trimer with 4-hydroxybenzaldehyde and (3-aminopropyl) triethoxy silane. The as-prepared ligand was then covalently grafted on boehmite to furnish an efficient support for the immobilization of Pd nanoparticles and synthesis of a novel heterogeneous catalyst for hydrogenation of nitro arenes. The catalytic tests revealed that the catalyst had excellent catalytic activity for hydrogenation of various nitro arenes with different steric and electronic features under mild reaction condition in aqueous media. Noteworthy, the catalyst was highly selective and in the substrates with ketone or aldehyde functionalities, reduction of nitro group was only observed. The catalyst was also recyclable and only slight loss of activity was detected after each recycling run. Hot filtration test also approved true heterogeneous nature of catalysis.

Cyclotriphosphazen-hexa imine ligand was prepared through successive reactions of phosphonitrilchloride trimer with 4-hydroxybenzaldehyde and (3-aminopropyl) triethoxy silane. The as-prepared ligand was then covalently grafted on boehmite to furnish an efficient support for the immobilization of Pd nanoparticles and synthesis of a novel heterogeneous catalyst for hydrogenation of nitro arenes. The catalytic tests revealed that the catalyst had excellent catalytic activity for hydrogenation of various nitro arenes with different steric and electronic features under mild reaction condition in aqueous media. Noteworthy, the catalyst was highly selective and in the substrates with ketone or aldehyde functionalities, reduction of nitro group was only observed. The catalyst was also recyclable and only slight loss of activity was detected after each recycling run. Hot filtration test also approved true heterogeneous nature of catalysis.
Natural compounds are interesting candidates for industrial applications due to their availability, bio-degradability, bio-compatibility and relatively low-cost 1,2 . These compounds have also attracted considerable attention 3,4 in catalysis and up to now, myriad catalysts for diverse range of chemical transformations have been developed using various types of natural compounds ranging from carbohydrates to clays 5,6 . Some of the reported naturalbased catalysts exhibited comparable or even superior activity compared to their synthetic counterparts. Natural compounds can be utilized either as the main catalytic species or as catalyst supports 7,8 . Moreover, they can be applied as potent reducing or capping agents for the synthesis of nanoparticles (NPs) 9 . To improve the performance of natural compounds, it is necessary to modify them chemically or physically prior to use. Using these modifications, the textural properties, as well as the surface chemistry of natural compounds can be tuned. One of the natural compounds that showed promising results for the catalytic applications is boehmite (γ-AlOOH) 10 that is a non-toxic and thermally and chemically stable compound and can be found in nature in large quantity 10,11 . Boehmite that possesses multiple -OH groups on its surface can also be chemically modified by introduction of various chemical functionalities via covalent or non-covalent approaches 12 . Hence, it is possible to adjust its properties by using proper functional groups. Functionalization of boehmite is more highlighted when immobilization of metallic nanoparticles is targeted. In this case, use of efficient organic ligands 13 , such as phosphonitrilchloride trimer that contains various heteroatoms and can efficiently immobilize NPs and provides high dispersion of metallic NPs is of great importance 14 . Specially, in catalysts, in which precious metals, such as Pd nanoparticles are utilized, introduction of functional groups on the supporting materials for effective anchoring of metallic particles and suppressing their leaching is remarkably important from economic and environmental points of view. As an example, in hydrogenation catalysts that mostly contain precious metals 15,16 , supporting materials are mostly functionalized by multi-heteroatom based ligands to suppress leaching of the immobilized precious metal. Among various types of hydrogenation reactions, hydrogenation of nitro arenes has received immense attention due to the wide range of utility of aniline derivatives 17 . Considering these issues and following our attempts to devise efficient catalytic supports using natural compounds [18][19][20]   To study the possible morphological changes upon introduction of CP-HIm and Pd NPs, SEM images of boehmite and Pd/B-CP-HIm were compared, Fig. 3. As presented, both boehmite and Pd/B-CP-HIm showed orthorhombic cubic morphology, indicating that chemical modification did not affect the morphology of the catalyst remarkably.
To further shed light onto the morphology of Pd/B-CP-HIm, its TEM images were recorded. As illustrated in one of the selected TEM images, Fig. 4, Pd NPs were well-distributed on the as-synthesized B-CP-HIm support with no aggregation. Moreover, the average particle size of Pd NPs was estimated to be 3.50 ± 0.42 nm.
To confirm conjugation of the as-synthesized HIm ligand and also homogeneous dispersion of Pd NPs, EDS and elemental mapping analyses were accomplished, Figs. 5 and 6. As displayed, C, N, O, Al, Si, P and Pd atoms were detected in EDS analysis, among which, Al and O are the elements present in the boehmite structure and     TGA not only can show the thermal stability of Pd/B-CP-HIm, but also can be used to approve grafting of CP-HIm ligand on boehmite. To this purpose, the thermograms of boehmite and Pd/B-CP-HIm were compared, Fig. 7. In boehmite thermogram, loss of structural water at low temperature (~ 150 °C) and decomposition at elevated temperature (~ 450 °C) were detected. In the Pd/B-CP-HIm thermogram, however, three weight loss steps were observed, two of them were related to the loss of water and boehmite decomposition and one of them (detected at 380 °C, 14 wt.%) was due to the decomposition of CP-HIm. In fact, this additional weight loss step approves conjugation of the organic ligand on boehmite.
Formation of Pd/B-CP-HIm was also verified via FTIR spectroscopy, Fig. 8. To have a better understanding, FTIR spectrum of Pd/B-CP-HIm was compared with that of boehmite, PNC, CP-HAL, CP-HIm and Pd/B-CP-HIm. As seen, FTIR spectrum of boehmite is in good accordance with the literature and showed the absorbance   25 . In the case of PNC, the vibration of band in the range of 1190 cm −1 to 1213 cm −1 is ascribed to the asymmetric vibration of (P = N-P) 26,27 and the absorbance bands at 520 cm −1 and 601 cm −1 are attributed to P-Cl bands 28 in the framework of PNC 29 . In the FTIR spectrum of CP-HAL, the stretching vibration around 1700 cm −1 is attributed to -C = O bond. The peak around 1500 cm −1 is related to the benzene ring vibration (Ph-O) and the peak at ~ 970 cm −1 is related to the stretching vibration of (P-O-C) that approves replacement of the phenoxy group with chlorine atoms 28 . Also, the aromatic C-H bands were detected in 3000-3100 cm −127 . In the FTIR spectrum of CP-HIm, the peak at 1700 cm −1 disappeared and the bands at 1600-1650 cm −1 appeared, indicating the formation of imine bond 27  Catalyst activity. As Pd/B-CP-HIm contains Pd NPs in its structure, it can be considered as a versatile catalyst for promoting various chemical transformations, such as hydrogenation. As a model Pd-catalyzed reaction, hydrogenation of nitro-compounds to the corresponding aniline derivatives has been targeted. As some of the parameters, such as Pd/B-CP-HIm dosage, reaction solvent and temperature can affect the yield of the reaction, optimized values of these parameters were first found by repeating a model reaction (hydrogenation of nitro benzene) under different reaction conditions, Table 1. As presented, the first test was performed in water as solvent at 40 °C in the presence of 10 mg Pd/B-CP-HIm. The result showed that under the aforesaid condition, moderate yield of aniline was furnished. To enhance the yield of aniline, the reaction temperature was elevated from 40 to 45 °C. As shown, upon increase of the reaction temperature, the yield of aniline increased. To further elucidate the role of reaction temperature in the reaction yield, the model reaction was repeated at 50 and 55 °C. As tabulated, the reaction yield increased by elevating temperature to 50 °C. However, aniline yield remained constant upon further increase of the reaction temperature. Therefore, the optimum value for reaction temperature was considered as 50 °C. Next, the effect of reaction solvent was studied by performing the model reaction in EtOH As discussed above, Pd/B-CP-HIm exhibited excellent activity for the hydrogenation of nitrobenzene. To assay whether this methodology can be generalized to other nitro-compounds, a variety of nitroarene derivatives with electron donating and electron withdrawing groups were subjected to hydrogenation reaction under the optimized reaction condition. Gratifyingly, the results, Table 2, confirmed that various nitroarenes with different electronic and steric properties can be hydrogenized to give the corresponding products in excellent yields. Notably, sterically demanding derivatives and less active nitroarene derivatives, i.e. the derivatives with electron donating groups, led to slightly lower yields. It is worth mentioning that Pd/B-CP-HIm was selective towards hydrogenation of -NO 2 functionality and in the substrates with -C = O groups ( Table 2,   www.nature.com/scientificreports/ HIm was reused for five successive hydrogenation runs. In Fig. 9, the yields of aniline using fresh and recycled Pd/B-CP-HIm are illustrated. According to the results, Pd/B-CP-HIm can be considered as a recyclable catalyst, with only negligible loss of activity upon each run. The reused Pd/B-CP-HIm (the catalyst recovered after the www.nature.com/scientificreports/ last reaction run) was characterized via FTIR spectroscopy and ICP to assay its stability and Pd leaching respectively. Gratifyingly, the ICP result indicated that Pd leaching that is the origin of loss of the catalytic activity was very scant (~ 1.4 wt.% initial loading of Pd NPs in the fresh Pd/B-CP-HIm). Furthermore, FTIR spectroscopy, Fig. 10, confirmed that both fresh and reused Pd/B-CP-HIm showed similar characteristic absorbance bands in the FTIR spectrum, confirming the chemical stability of Pd/B-CP-HIm upon reusing. Next, the effect of recycling on the aggregation of Pd NPs was investigated by recording the TEM images of the recycled catalyst after last run of the reaction. As shown in Fig. 11, upon successive recycling and reuse of the catalyst, very slight aggregation of Pd NPs was observed and the average Pd particle size reached to 3.55 ± 0.4 nm.
Hot filtration. One of the important factors in heterogeneous catalysis is stability of the supported catalytic species on the supporting material in the course of the reaction. To examine this issue for Pd/B-CP-HIm catalysis, hot filtration test was applied, in which the catalyst was removed from the reaction vessel after a short time period and the reaction was continued and monitored in the absence of the catalyst to appraise whether any reaction progress was observed in the absence of the catalyst. According to this test, Fig. 12, hydrogenation of nitrobenzene (the model reaction) did not proceed after Pd/B-CP-HIm removal, establishing that Pd NPs were remained stabilized on B-CP-Him in the course of hydrogenation reaction.

Comparison of the activity of Pd/B-CP-HIm with some other catalysts. Nitro-arene hydrogena-
tion is a fundamental reaction for the synthesis of aniline derivatives. Hence many research groups reported catalytic systems for promoting this reaction under mild reaction condition. As the reaction condition and the nature of the catalyst for each protocol were different, accurate comparison of the catalytic activity of the catalyst is impossible. However, the data in Table 3 indicate that some protocols needed high hydrogen pressure or hazardous solvents that rendered those methodologies less favorable from environmental point of view. On the other hand, for the synthesis of some of the catalytic systems, multi-step synthetic procedures as well as    www.nature.com/scientificreports/ costly raw materials were applied that made the catalyst less economically attractive. Pd/B-CP-HIm, however, is prepared from naturally occurring boehmite and can promote the hydrogenation reaction under mild reaction condition to furnish the desired product in excellent yield. Hence, it can be concluded that Pd/B-CP-HIm can be recognized as a promising catalyst with comparable or superior activity compared to some reported catalysts.

Experimental section
Materials and instruments. The developed catalyst in this research, Pd/B-CP-HIm, has been prepared by using the following chemicals and solvents: boehmite, phosphonitrilchloride trimer (PNCT), 4-hydroxybenzaldehyde, (3-aminopropyl) triethoxy silane (APTES), trimethylamine, palladium (Π) acetate, sodium borohydride, tetrahydrofuran (THF), toluene, dimethyl sulfoxide (DMSO), ethyl acetate and methanol (MeOH). Nitro-aromatic compounds and ethanol (EtOH) were also used for performing the hydrogenation reaction. All of the above-mentioned compounds were purchased from Sigma-Aldrich. Catalyst synthetic procedure. Synthesis of the catalyst was fulfilled via a three-step procedure. First, a ligand, CP-HIm, was prepared and then conjugated on boehmite covalently to furnish a functionalized support for the immobilization of Pd NPs. In the following, each step is discussed in detail.

Synthesis of CP-HIm. Conjugating of 4-hydroxybenzaldehyde to PNCT: synthesis of cyclotriphosphazen-hexa
aldehyde (CP-HAL). CP-HIm was prepared via a two-step procedure. First, 4-hydroxybenzaldehyde was conjugated to PNCT via a known procedure 38 . Briefly, 4-hydroxybenzaldehyde (3.68 g, 30.16 mmol) was dissolved in THF (50 mL) and transferred into a three-necked-flask equipped with a reflux condenser and an Ar inlet. Subsequently, triethylamine (3.05 g, 30.16 mmol) and a solution of PNCT (1.5 g, 4.31 mmol in THF (40 mL)) were added to the flask and Ar gas was purged to the reaction vessel. Then, the reaction temperature was elevated and the mixture was refluxed for 24 h. At the end, the produced tri-ethylamine hydrochloride salt was filtered and the residual solution was concentrated by evaporating the solvent. The main product (CP-HAL) was obtained by adding large amount of distilled water, followed by recrystallization by ethyl acetate 29 .
Synthesis of cyclotriphosphazen-hexa imine: CP-HIm. Typically, the as-synthesized CP-HAL (1.5 g, 1.74 mmol) was dissolved in THF (30 mL) and then reacted with APTES (2.44 mL, 10.45 mmol) under stirring condition at room temperature for 12 h. The color of the solution was changed and an orange solid was precipitated, which was then filtered, rinsed with EtOH and dried under vacuum.

Conjugation of CP-HIm on boehmite: synthesis of B-CP-HIm.
To covalently graft the as-prepared ligand, CP-HIm, on boehmite, boehmite (2 g) was dispersed in DMSO (20 mL) in a round bottle flask and mixed with a solution of CP-HIm (1.55 g in DMSO (30 mL)) at 100 °C for 24 h. Afterward, the product (B-CP-HIm) was filtered, washed twice with DMSO and THF and dried at ambient temperature overnight.

Immobilization of Pd NPs on B-CP-HIm: synthesis of Pd/B-CP-HIm.
In order to prepare Pd/B-CP-HIm, B-CP-HIm (1.5 g) was dispersed in toluene (20 mL) in a 2-neck round flask under N 2 atmosphere at ambient temperature and mixed for 30 min. Meanwhile, Pd(OAc) 2 (0.03 g) was dissolved in toluene (3 mL) and gradually introduced to the B-CP-HIm suspension. After 2 h, a fresh solution of NaBH 4 (0.2 N in 20 mL MeOH) was dropped to the aforementioned mixture and stirring was continued for 6 h. Finally, the catalyst (Pd/B-CP-HIm) was achieved through filtration and rinsing with toluene, Fig. 1.

Procedure for the hydrogenation of nitroarenes.
To hydrogenize nitro aromatic compounds, Pd/B-CP-HIm (0.03 g) was added to a solution of nitro aromatic compound in 1: 1 mixture of H 2 O/EtOH (7 mL) and hydrogen gas, provided by a hydrogen generator, was purged (P = 1 atm) into the reaction vessel. The reaction temperature was then elevated to 50 °C and the mixture was stirred under the aforesaid condition. Using TLC technique, the progress of hydrogenation reaction was monitored and at the end of the reaction, the mixture was cooled and Pd/B-CP-HIm was separated via centrifugation. To recover Pd/B-CP-HIm, it was rinsed with EtOH three times and dried at 50 ºC overnight. GC analysis was exploited to calculate the yield of the obtained aromatic amine.

Conclusion
Pd/B-CP-HIm catalyst was fabricated through covalent grafting of the as-prepared CP-HIm ligand on boehmite, followed by palladation via wet impregnation method. The catalyst showed excellent activity and selectivity for hydrogenation of nitroarenes under mild reaction condition in aqueous media. The generality study confirmed that Pd/B-CP-HIm could catalyze hydrogenation of various nitro arenes with electron donating and withdrawing groups. On the other hands, in the case of substrates with -C = O functionality, only reduction of nitro groups was observed, confirming high selectivity of the catalyst. Pd/B-CP-HIm was also recyclable and only negligible