Pd on the Composite of Perlite and Allylamine- N -isopropylacrylamide Copolymer: A Thermo-Responsive Catalyst for Hydrogenation of Nitroarenes under Mild Reaction Condition

: A novel thermo-responsive catalyst for the hydrogenation of nitroarenes under mild reaction condition was devised. To prepare the catalyst, a thermo-responsive polymer was ﬁrst synthesized through the co-polymerization of N -isopropylacrylamide and allylamine and then covalently grafted on the Cl-functionalized perlite. The resulting composite was subsequently utilized as a support for the stabilization of Pd nanoparticles. Investigation of the catalytic activity of the catalyst approved its high catalytic activity at a temperature above the lower critical solution temperature. More precisely, 0.03 g of the catalyst can promote the reaction of 1 mmol of nitro-compounds in H 2 O/EtOH (1:1) at 45 ◦ C to furnish the corresponding products in 70–100% yields. This issue was assigned to the collapse of the polymeric component and formation of a hydrophobic environment that was beneﬁcial for the mass-transfer of the hydrophobic nitroarenes. Notably, the catalytic activity of the catalyst was higher than that of palladated perlite and thermos-responsive polymer due to the synergistic effects between the perlite and polymeric moiety. Furthermore, the study of the substrate scope conﬁrmed that a wide range of substrates with different steric and electronic properties could tolerate hydrogenation reaction. Moreover, the catalyst was highly selective toward hydrogenation of the nitro group and could be recycled up to seven runs with insigniﬁcant Pd leaching and loss of catalytic activity. The hot ﬁltration test also conﬁrmed the heterogeneous nature of the catalysis. Author Contributions: Conceptualization, S.S. and M.M.H.; Methodology, N.A.-D.; Validation, S.S.; Formal analysis, N.A.-D.; Investigation, resources, S.S. and M.M.H.; Data curation, N.A.-D.; Writing— original preparation, S.S.; Writing—review and editing S.S.; Visualization, N.A.-D.; Supervision, S.S. and M.M.H.; Project administration, S.S. and M.M.H.; Funding S.S.


Introduction
Devising cost-effective heterogeneous catalysts is an important subject in the field of catalysis. In this context, use of natural compounds for the immobilization of catalytic species has attracted particular attention. Perlite (Per) is an amorphous volcanic glass that is generated through rapid cooling of a viscous lava [1][2][3][4][5][6][7][8]. This low-cost volcanic rock, which contains~70-75% of SiO 2 and 12-18% of Al 2 O 3 is highly stable, none-corrosive, nontoxic, and easy to handle [1,2,[9][10][11][12]. Thanks to the presence of siloxane groups, Per can be chemically modified by organic moieties. To date, various catalysts based on functionalized Per have been reported [1]. As an example, TiO 2 nanoparticles have been stabilized on Per granules and successfully used for the photocatalytic degradation of ammonia in wastewater [13]. Moreover, synthesis of Pd/Per nanocomposite and its catalytic activity have been reported [14].
One of the most widely utilized thermo-responsive polymers is poly(Nisopropylacrylamide) (PNIPAM). This polymer displays thermo-responsive behavior that produces a phase-separation (i.e., coil-to-globule transition) in the aqueous solution at a lower critical solution temperature (LCST) [24]. Such a dehydration and phase separation is caused by altering the hydrogen bonds between the PNIPAM network and H 2 O molecules below and above LCST [24,25]. More recently, copolymers of PNIPAM have been extensively studied. As an example, allylamine-N-isopropylacrylamide copolymer with LCST about 34 • C have been used to design smart catalysts [26].
Among the various types of hydrogenation reactions [27,28], hydrogenation of nitrocompounds is of great importance [29][30][31]. This class of hydrogenation reaction not only can be applied for the synthesis of fine chemicals, but is also applicable for environmental remediation. Similar to other hydrogenation reactions, reduction in nitro-compounds proceeds with the aid of a hydrogen source and a catalytic species [32,33]. The conventional catalyst for this process is supported Pd species. Among various supporting materials, natural compounds that benefit from low-cost, availability, low-toxicity, and biocompatibility are of priority. To date, various natural compounds such as clays [34] and carbohydrates [35] have been utilized for the immobilization of Pd nanoparticles.
In the continuation of our research on using natural compounds for the design of heterogeneous catalysts [36,37], in this article, we wish to report the synthesis of a thermoresponsive catalyst, Pd@Per-P. To prepare the catalyst, Per was covalently functionalized with a thermo-responsive polymer and then utilized as a support for the immobilization of Pd nanoparticles (Figure 1). The activity and recyclability of the catalyst were appraised for the hydrogenation of nitroarenes under a mild reaction condition.

Catalyst Characterization
The SEM image of Per and the as-prepared catalyst are depicted in Figure 2. As illustrated, Per possesses a sheet-like morphology. In the case of the SEM image of the     The TEM image of the catalyst is shown in Figure 5. In this image, apart from Per (dark area), the polymeric sheet can also be detected. Moreover, it can be observed that Pd nanoparticles (with average diameter of 4 ± 0.2 nm) were dispersed on the composite homogeneously.
XRD analysis was used to examine the crystal phases of Per and Pd@Per-P ( Figure 6). As shown, Per has an amorphous mineral phase and showed a broad band in the range of 2θ = 10-34 • [38]. In the XRD pattern of Pd@Per-P, the characteristic band of Per (i.e., the broad band at 2θ = 10-34 • ) as well as small bands at 2θ = 40.3 • and 45.7 • could be detected. These peaks can be assigned to Pd nanoparticles. Notably, other characteristic bands of Pd nanoparticles are not detectable due to the their low intensities.  FTIR spectra of Per, Per-Cl, P, and Pd@Per-P are depicted in Figure 7. In the spectrum of Per, the absorbance band at 3407 cm −1 is related to the -OH group [39], while the band at 460 cm −1 is due to the rocking motion of the bridging oxygen atom perpendicular to the Si-O-Si plane. Furthermore, the band at 1059 cm −1 can be ascribed to the stretching vibration of Si-O-Si or Si-O-Al. The absorption band at 792 cm −1 is due to the Si-O stretching vibration of Si-O-Al [12]. Comparison of the FTIR spectrum of Per and Per-Cl indicated that the FTIR spectrum of Per-Cl showed two additional bands at 2926 cm −1 and 582 cm −1 , which are illustrative of -CH 2 and -C-Cl stretching vibration, respectively, confirming conjugation of CPTMC [40]. In the FTIR spectrum of P, the bands located at 1654 cm −1 and 1545 cm −1 were assigned to the amide group, while those at 1461 cm −1 , 1386 cm −1 , and 1367 cm −1 were assigned to the stretching vibrations of the isopropyl group [26]. Moreover, the bands located at 2876-2973 cm −1 were illustrative of stretching vibrations of aliphatic -C-H. The bands located at 3069 cm −1 and 3283 cm −1 were assigned to -N-H of amine and amide groups. In the Pd@Per-P spectrum, the characteristic bands of Per and thermo-responsive P could be seen, which is an indication of the catalyst formation. amide groups. In the Pd@Per-P spectrum, the characteristic bands of Per and thermo-responsive P could be seen, which is an indication of the catalyst formation. Thermal stability of Pd@Per-P was studied by TGA. The thermograms of Per and Pd@Per-P are illustrated in Figure 8. The results showed that Per had a weight loss below 120 °C, which is due to the evaporation of physically adsorbed water. Moreover, a steady weight loss at 570 °C can be attributed to the dihydroxylation of Per [1]. Pd@Per-P displayed three weight loss steps. The first weight loss was related to the loss of water, the weight loss in the range of 300-400 °C (20 wt.%) could be due to the decomposition of polymeric component (P) [41,42] and the third weight loss was due to the dihydroxylation of Per. The textural properties of Pd@Per-P have also been studied via BET. The results showed that the specific surface area of the catalyst was 2.41 m 2 .g −1 . Additionally, the N2 Thermal stability of Pd@Per-P was studied by TGA. The thermograms of Per and Pd@Per-P are illustrated in Figure 8. The results showed that Per had a weight loss below 120 • C, which is due to the evaporation of physically adsorbed water. Moreover, a steady weight loss at 570 • C can be attributed to the dihydroxylation of Per [1]. Pd@Per-P displayed three weight loss steps. The first weight loss was related to the loss of water, the weight loss in the range of 300-400 • C (20 wt.%) could be due to the decomposition of polymeric component (P) [41,42] and the third weight loss was due to the dihydroxylation of Per. The textural properties of Pd@Per-P have also been studied via BET. The results showed that the specific surface area of the catalyst was 2.41 m 2 ·g −1 . Additionally, the N 2 adsorption-desorption isotherm of the catalyst, illustrated in Figure 9, was of type II, implying that Pd@Per-P contains a macroporous or non-porous nature.

Activity
The catalytic activity of Pd@Per-P as a thermo-responsive catalyst was assessed for the hydrogenation of nitro-aromatic compounds (Figure 10b). Initially, synthesis of aniline from hydrogenation of nitrobenzene was selected as a model reaction. To achieve an environmentally benign procedure, water was selected as the solvent and hydrogen gas (1 atm) was used as the reducing agent. According to the literature [24,26], it was expected that at a temperature above the LCST (34 • C), the polymeric component on the structure of the catalyst collapses to generate a hydrophobic environment that is beneficial for the masstransfer of hydrophobic substrates (Figure 10a). To verify this issue, the model reaction was carried out in the presence of 30 mg of Pd@Per-P at three different temperatures (25, 45 and 50 • C). The results (Table 1) indicated that at a temperature below LCST (25 • C), the yield of the reaction was lower than the temperature above LCST. According to these results, 45 • C was selected as the best reaction temperature. To further improve the yield of the model reaction, the effect of the reaction solvent was investigated. The results in Table 1 indicate that the mixture of H 2 O:EtOH (1:1) was more efficient than water and the other examined solvents and performing the reaction in the presence of 30 mg of Pd@Per-P in H 2 O:EtOH (1:1) at 45 • C led to the formation of aniline in 98% yield. Finally, to appraise the effect of the loading of Pd@Per-P, the model reaction was repeated under the aforementioned condition in the presence of various amounts of the catalyst. The results implied that 30 mg was the optimum amount of the catalyst and use of a lower content of the catalyst resulted in a lower yield of the model product.

Comparison with Control Catalysts
One of the most important issues in catalysis is the simplicity of the catalysts. Therefore, the catalysts that can be prepared easily through facile synthetic procedures and cost-effective raw materials are of great interest. On the other hand, the catalytic performance is a crucial factor. In this study, it was assumed that hybridization of Per, which is a low-cost support with a thermo-responsive polymer, could enhance the activity of the resultant catalyst. To validate this assumption and to appraise whether the presence of the thermo-responsive polymer was necessary for achieving high catalytic activity, a control catalyst, Pd@Per, was prepared through palladation of Per via the same procedure used for the palladation of Per-P and its catalytic activity for the model hydrogenation reaction under the optimized condition was examined. Interestingly, it was found that this control catalyst exhibited low catalytic activity (20%). On the other hand, a second control catalyst, Pd@P, was prepared through a similar procedure used for the synthesis of the catalyst, except that P was applied as a support instead of Per-P. Studying the activity of this control catalyst showed that this catalyst led to the formation of aniline in 56%. These results indicate that immobilization of Pd nanoparticles on the composite of P and Per significantly improved the activity of the resulting catalyst. This may be due to the synergistic effect between P and Per.

Substrate Scope
To confirm the generality of this methodology, hydrogenation of various nitroarenes was carried out under the optimum reaction condition. As listed in Table 2, nitroarenes with electron-donating and electron-withdrawing groups can undergo hydrogenation reaction to furnish the corresponding products in high yields. Furthermore, hydrogenation of 1-nitro naphthalene, which is a sterically demanding substrate, proceeded efficiently to give the desired product in excellent yield. Notably, the required reaction time for this nitro-compound was longer than the other examined nitroarenes. In the case of 1,3-dinitrobenzene, which contains two nitro-functionalities, both nitro groups were hydrogenized to furnish 1,3-diaminobenzene in high yield.

Selectivity of Pd@Per-P
To investigate the selectivity of Pd@Per-P toward the hydrogenation of nitro groups, two substrates (i.e., 4-nitroacetophenone and 4-nitrobenzaldehyde) that contain competing functionalities were hydrogenized under Pd@Per-P catalysis. The results (Table 2) imply that both of the aforementioned substrates were selectively hydrogenated to furnish 4aminoacetophenone and 4-aminobenzaldehyde as sole products in high yields. This experiment approved high selectivity of Pd@Per-P toward the hydrogenation of the nitro group.

Comparative Study
Although the main target of this research was the development of a thermo-responsive catalyst based on Per and poly(NIPAM-co-AAm) and the hydrogenation of nitroarenes was selected only as a model chemical transformation to examine the catalyst performance, a comparative study was conducted to appraise whether the activity of Pd@Per-P was comparable with other reported catalytic systems. To this purpose, the activity of the catalyst for the model reaction was compared with some other catalysts that have been reported for this reaction by using hydrogen gas as a reducing agent and Pd-based catalysts. In Table 3, the comparison of the reaction time, temperature, solvent, hydrogen pressure, and yield of the model product for each catalytic system has been reported. As shown, some of the reported methodologies utilized high hydrogen pressure that is neither safe nor economic. In one procedure, a hazardous solvent was applied that is not environmentally benign. Comparing the reaction conditions and yields of other protocols, it can be concluded that Pd@Per-P exhibited high activity, which is comparable or even higher than some of the previously reported catalysts.

Recyclability
To evaluate the recyclability of Pd@Per-P, the recycled catalyst from hydrogenation of nitroarene was washed with toluene, dried, and applied for the next run of the same reaction under similar conditions. Recovery and recycling were repeated for seven consecutive runs and the yield of aniline for each run was obtained ( Figure 11). Comparison of the obtained yields confirmed that the activity of Pd@Per-P decreased slightly upon each recycling run. Figure 11. The yields of the model product for seven reaction runs by using the recovered Pd@Per-P under optimum reaction condition.
To investigate the origin of the loss of the activity of Pd@Per-P upon recycling, the FTIR spectrum of the recycled catalyst (after seven runs) was compared with that of the fresh catalyst. As shown in Figure 12, the FTIR spectra of the reused and fresh Pd@Per-P were very similar. In fact, all of the absorbance bands of fresh Pd@Per-P were detectable in the FTIR spectrum of the reused catalyst, indicating the stability of Pd@Per-P upon recycling. Next, to elucidate whether recycling can induce Pd leaching, the recycled Pd@Per-P after the seventh run was analyzed via ICP analysis. Comparison of the Pd content of the fresh and recycled Pd@Per-P (recovered from the seventh run) implied that recycling can result in slight Pd leaching (1.2 wt.% of the initial loading). This issue can justify the decrement of the activity of the catalyst.
In fact, visual comparison of the color of the fresh and reused Pd@Per-P ( Figure 13) affirmed that even after seven reaction runs, the catalyst preserved its grayish color. This issue can indicate that the Pd leaching was low. To study the effect of recycling on the catalyst morphology, TEM images of the recycled catalyst (after seventh run) were obtained ( Figure 14). As shown in Figure 14, in the recycled catalyst, the dispersion of Pd nanoparticles was almost homogeneous and no aggregation was detected, indicating that recycling did not induce significant morphological change and Pd aggregation.

Hot Filtration Test
In the final part of this research, a hot filtration test was conducted to appraise the nature of the catalysis. In this context, the model hydrogenation reaction was halted after a short period of time and the reaction yield was estimated. Then, Pd@Per-P was removed from the reaction media and the reaction was continued with no catalyst. The progress of the hydrogenation reaction was monitored to check whether the reaction proceeded in the absence of Pd@Per-P. It was found that after removal of the catalyst, the hydrogenation yield did not increase. This result ruled out the possibility of Pd leaching and re-deposition in the course of the reaction and confirmed the heterogeneous nature of the catalysis.
Pd@Per-P formation was affirmed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), elemental mapping, and Brunauer-Emmett-Teller (BET). The technical data of the used apparatus are as follows: A BRUKER TENSOR 35 spectrophotometer 65 (Germany, Berlin) with a scan time of 1 s and spectral resolution of 2 cm −1 by using a KBr pellet was used to conduct FTIR spectroscopy. The KBr pellets were fabricated using 1 wt.% of the samples. Room temperature powder XRD patterns were recorded using a Rigaku Ultima IV (Japan, Tokyo) with Cu Kα radiation from a sealed tube. TGA was carried out using a Mettler Toledo instrument (model Leicester, Leicester, UK) with a heating rate of 10 • C·min −1 from 50 to 800 • C under O 2 atmosphere. TEM images were recorded on a Phillips EM 208S microscope at 100 kV (USA, Beaverton). EDS and elemental mapping analyses were carried out by Tescan Mira II (Czech Republic, Kohoutovice). BET analysis was conducted using a Belsorp Mini II instrument (BEL Japan, Inc., Osaka, Japan). Pd@Per-P was degassed by heating at 150 • C for 3 h.

Synthesis of Poly(NIPAM-co-AAm): P
To synthesize the polymer, NIPAM (1 g) and AAm (1 g) were dissolved in 30 mL EtOH and mixed under an Ar atmosphere. Then, the temperature was increased to 70 • C and a solution of AIBN (0.4 g in EtOH (10 mL)) was added in a dropwise manner. After stirring for 24 h, the reaction was stopped, the reaction solvent was evaporated and the polymer was collected.

Chemical Modification of Per: Synthesis of Per-Cl
In order to functionalize Per surface, Per (2 g) was dispersed in dry toluene (40 mL) with the aid of ultrasonic irradiation (160 W for 30 min). Then, CPTMS (2.30 mL) was added to the mixture and the resultant suspension was refluxed for 24 h. Ultimately, the product, Per-Cl, was collected, washed with toluene, and dried in an oven at 50 • C.

Formation of Per-P Nanocomposite
To graft the as-prepared copolymer, P, on Per-Cl, Et 3 N (2.43 mL) was dissolved in EtOH (50 mL) and then added to the mixture of P (2 g) and Per-Cl (2 g). Subsequently, the mixture was refluxed under an Ar atmosphere for 24 h. At the end of the reaction, the solvent was evaporated and the solid product was collected, rinsed with distilled water to remove Et 3 N + Cl − salt and then dried in an oven at 50 • C.

Immobilization of Pd Nanoparticles on Per-P Nanocomposite: Pd@Per-P
In order to prepare Pd@Per-P, Per-P (1 g) was stirred in toluene (20 mL) in a 2-neck round flask under an Ar atmosphere at room temperature for 30 min. Subsequently, a solution of Pd(OAc) 2 (0.02 g) in toluene (15 mL) was added in a dropwise manner to the aforementioned suspension and stirring was continued for 1 h. Then, a solution of NaBH 4 (0.2 N) in MeOH (20 mL) was slowly added to the mixture to reduce Pd(II) to Pd(0). After stirring for 2 h, the as-prepared Pd@Per-P was filtered, washed with toluene, and dried in an oven at 50 • C. The synthetic method for the preparation of Pd@Per-P is illustrated in Figure 1. The ICP analysis of the catalyst established that the content of Pd was 1 wt.%.

Typical Procedure for Hydrogenation of Nitroarenes
In order to hydrogenize the nitroarenes, a nitro aromatic compound (1 mmol) was dissolved in H 2 O/EtOH (1:1) and mixed with the catalyst (30 mg, containing 0.3 mg Pd). Then, the reaction mixture was heated to 45 • C and H 2 gas (1 atm) was purged into the reactor. The progress of the hydrogenation reaction was monitored by TLC and at the end of the reaction, the reaction mixture was cooled down and the catalyst was filtered. The recovered catalyst was washed repeatedly with toluene and dried at 50 • C in an oven. Then, it was used for the second run of the reaction. The aromatic amine was achieved after evaporation of the solvent. The reaction yield was estimated via GC analysis.

Conclusions
Cl-functionalized Per was reacted with a thermo-responsive polymer, prepared from co-polymerization of NIPAM and AAm, to give a thermo-responsive composite, Per-P. Then, Pd nanoparticles were stabilized on the as-prepared composite to furnish an efficient catalyst for the hydrogenation of nitroarenes under mild reaction conditions. Study of the reaction temperature indicated that the catalytic activity of the catalyst, Pd@Per-P, was higher at a temperature above LCST. This issue was attributed to the collapse of the polymeric component and formation of a hydrophobic environment that was beneficial for the mass-transfer of the hydrophobic substrates. Using this protocol, various substrates with different steric and electronic properties could be hydrogenated to furnish the corresponding products in high yields. Furthermore, the catalyst was highly selective toward the hydrogenation of nitro functionalities. The recycling test also affirmed high recyclability of the catalyst and low Pd leaching. Regarding the nature of the catalysis, the hot filtration test affirmed that the catalysis was heterogeneous. It was also found that the catalytic activity of the catalyst was higher than that of Pd@Per and Pd@P. This issue was attributed to the possible synergistic effects between two components. Funding: This research received no external funding.

Data Availability Statement:
The data presented in this study are available in the article.