Cross-linked chitosan aerogel modified with Pd(II)/phthalocyanine: Synthesis, characterization, and catalytic application

Palladium(II) phthalocyanine (PdPc) tetrasulfonate was chemically bonded to an amine moiety of chitosan aerogel. The reaction was promoted by the transformation of sulfonic acid groups of PdPc to sulfonyl chloride, which is highly active for amination. The porous composite showed good catalytic activity in the oxidation reaction of some alkylarenes, aliphatic and benzylic alcohols, and cyclohexanol. High conversions and excellent selectivities were obtained for the solvent-free reactions under aerobic conditions at 80 °C during 24 h. While many oxidation reactions have been reported catalysed with palladium phthalocyanine, this is the first reported oxidation of alkylarenes via this catalyst. The organometallic compound is applicable as a heterogeneous catalyst having high chemical stability with recyclability up to six times.

The harmful effects of hazardous materials encourage organic researchers to develop new procedures using biocompatible catalysts to supress pollutant effects of chemical processes. Accordingly, biodegradable materials are increasingly utilized in organic transformations. For example, many heterogeneous catalysts have been synthesized using biocompatible materials such as cellulose and chitosan 1,2 . Chitosan, as a chiral polyamine, has attracted the researcher's interest due to its important role in many applications [3][4][5][6] . This naturally-based polymer shows good flexibility, insolubility in many solvents, and affinity for most metal ions 7,8 . Recently, the porous form of chitosan, known as chitosan aerogel (CA), has been used extensively in biomedical applications, catalysis, removal of pollutants, and insulating materials due to its high aspect ratio [9][10][11][12][13][14][15][16] . In addition, CA can be used as a support for catalysts because the amine groups on the backbone of the polymer allow to easy chemical modifications, and the high surface area of CA affords a high number of available sites for reactions [17][18][19][20] .
Oxidation reactions of organic substrates are considered one of the main organic transmutations and are powerful strategies for achieving valuable compounds from natural materials. Many studies have been carried out to promote them according to green chemistry principles. Aerobic catalytic oxidations have gained green chemistry credentials because they are free from toxic oxidizing agents which avoid from the hazardous waste formation 21,22 . Metalophthalocyanines (MPcs) are known, as potent catalysts for oxidation reactions, obtained industrial interest arising from their high performance, easy preparation, inherent nontoxicity, recovery possibility, and recyclability 23 . Pcs of transition metals such as Co(II), Fe(II), and Cu(II) have been employed in the oxidation reactions of various organic substrates. While Pd(II)-Pc is an efficient catalyst for the C-C coupling in Suzuki reaction and reduction of nitroarenes 24 , its catalytic activity for oxidation reactions is not well investigated. Oxidations of trichlorophenol 25 , 1,3-diphenylisobenzofuran 26 , 4-nitrophenol 27 , and chlorophenol 28 are some examples of reactions promoted using PdPc. To the best of our knowledge, PdPc catalytic activity has not been extended to the oxidation of alkylarenes.
In continuation of our efforts to modify biopolymers such as chitosan with Pd-D-penicillamine and Au(III)-dimercaprol 29,30 , herein we modified CA with PdPc tetrasulfonate to achieve a biocompatible catalyst for the oxidation of alkylarenes. Loading PdPc on CA has at least the following two advantages: (1) distribution of = .
= . PdPc@CA demonstrates a porous network attributed to the CA. The procedure employed for the preparation of CA was solving the chitosan to obtain a gel and then removing the solvent to generate fine particles of chitosan in a porous network. As seen in Fig. 5, a decrease in the particle size has been performed successfully, which is more clearly observable in the comparison SEM image of PdPc@CA with chitosan.
Brunauer-Emmett-Teller (BET) analyses of chitosan and PdPc@CA. BET analysis of PdPc@ CA with nitrogen (N 2 ) adsorption/desorption at 77 K shows that the surface area for PdPc@CA is 52.14 m 2 /g  www.nature.com/scientificreports www.nature.com/scientificreports/  www.nature.com/scientificreports www.nature.com/scientificreports/ ( Fig. 6). A significant difference was observed between the surface area of PdPc@CA and employed chitosan with 0.53 m 2 /g surface area. Increasing the aspect ratio of PdPc@CA compared to chitosan let us to have a catalyst with highly available reaction sites. Nitrogen sorption analysis of PdPc@CA revealed the presence of macropores, which is also shown by a pore size diagram with a maximum size of 67 nm.
Thermal gravimetric analysis (TGA) of PdPc@CA. The thermal stability of PdPc@CA was investigated with TGA in the air. Good thermal stability was observed with decomposition above 274 °C due to the degradation of the polymeric structure into small units with some degasification (Fig. 7) 37 . The second decomposition point was observed above 485 °C. This result is related to the decomposition of new small units formed at the previous decomposition temperature 37 .
Catalytic examination of PdPc@CA. The catalytic activity of PdPc@CA was investigated in the oxidation reaction of alkylarenes. At first, the oxidation of ethylbenzene was selected as the model reaction, with an O 2 balloon as the oxidant in the presence of PdPc@CA. The reaction needed 0.036 mmol (0.2 g) of PdPc@CA under solvent-free conditions at 80 °C to produce high yields (Table 1). Gas Chromatography (GC) analysis of the product showed acetophenone as the product with 73% conversion and 100% selectivity. Decrease in the catalyst  www.nature.com/scientificreports www.nature.com/scientificreports/ amount from 0.036 to 0.018 mmol declined the conversion from 73% to 66% and its increase to 0.054 mmol did not improve the conversion sophistically ( Table 1, entries 1-3). The reaction gave 73% conversion at 80 °C and it was decreased significantly at 70 °C to 58%, while increasing temperature to 90 °C did not afford higher yields compared to 80 °C. A control experiment under N 2 atmosphere indicated that the reaction needs O 2 for performing (Table 1, entry 6). Among various solvents screened for the oxidation reaction of ethylbenzene, acetonitrile produced the highest yields compared to other solvents. However, solvent-free conditions were the best for the oxidation reaction because they preserved the green chemistry credentials.
The oxidation reaction was developed using various alkylarenes, such as propylbenzene, 4-ethylphenol, 1,2,3,4-tetrahydronaphtalene, and diphenylmethane ( Table 2). High conversions and excellent selectivities were obtained for the oxidation reactions. The reaction also was examined for divers' aliphatic and aromatic alcohols. PdPc@CA transformed liquid aromatic alcohols to the corresponding aldehydes in high yields under solvent-free conditions in 24 h at 80 °C. For solid alcohols such as 4-nitrobenzyl alcohol and 4-chlorobenzyl alcohol, acetonitrile has been used as the solvent under reflux conditions to produce high yields. Selectivities for aromatic alcohols reached 92-98% with benzoic acid derivatives as the byproducts. PdPc oxidized aliphatic alcohols and cyclohexanol to the corresponding aldehydes or ketones effectively with 100% selectivity for the secondary alcohols. The oxidation reaction for allylalcohol was failed because of missing large amounts of the substrate during the reaction. This problem is attributed to the low boiling point of allylalcohol.
proposed mechanism. The mechanism of ethylbenzene oxidation using PdPc@CA catalyst is conceivable as Fig. 8  PdPc@CA leaching study. Potential PdPc leaching into the mixture of the benzyl alcohol oxidation reaction was also analysed with FAAS. For this purpose, the sample was taken through a syringe filter during the heterogeneous oxidation reaction of benzyl alcohol and was dissolved in HNO 3 for 1 h. The FAAS analysis of the sample showed that the Pd concentration in the reaction mixture was less than the detection limit (less than 9 μg/L). This result indicates that virtually no Pd leaches from PdPc@CA into the mixture. This procedure was also performed for the benzyl alcohol oxidation reaction in the presence of CA mixed with PdPc (0.036 mmol), obtaining a Pd concentration of 337 μg/L. Therefore, PdPc@CA has great chemical stability compared to PdPc mixed with CA.
Recyclability of PdPc@CA. The recyclability of PdPc@CA was investigated in the oxidation reaction of benzyl alcohol. After carrying out the reaction, the catalyst was separated via filtration as a brown solid, washed with acetone (2 × 5 ml), and reused. Only a minor decrease in the reaction yield was observed after six repetitive cycles for this reaction (Fig. 9). To investigate the catalyst stability, the amount of Pd on the catalyst was determined before use and after six runs, obtaining 0.18 mmol per 1 g catalyst for both of them.
Comparison of the results. A comparison was performed between the results of benzyl alcohol oxidation with PdPc@CA and some previously supported metallophthalocyanines such as FePc 38 , CoPc 32 , and CuPc (Table 3) 39 . FePc and CoPc transformed benzyl alcohol to benzaldehyde under milder reaction conditions in a shorter time compared to PdPc@CA. The oxidation reaction with CuPc gave high selectivity in a short reaction time, similar to the reaction with CoPc. However, PdPc@CA utility in the reaction under green conditions such as aerobic oxidation through a solvent-free approach showed both high yield and selectivity. While the oxidation of phenol has been reported with PdPc 27 , this work extended the application of PdPc to the oxidation of various alcohols and alkylarenes. Recyclability of the catalyst is another important characteristic of PdPc@CA which led to the minimizing waste. The catalytic activity of PdPc@CA was compared with that of PdPc and PdPc@CA. PdPc as the catalyst (0.036 mmol) gave much lower conversions than PdPc@CA, yet with higher selectivity. This result www.nature.com/scientificreports www.nature.com/scientificreports/ shows that the reaction can be performed with a low catalyst amount when the catalyst is dispersed on a support. Moreover, PdPc@C afforded low yields compared to PdPc@CA because the high surface area of CA provided more available reaction sites.

conclusion
In conclusion, PdPc was immobilized on chitosan aerogel by chemical bonding. This organometallic compound was applied as a heterogeneous catalyst for the oxidation of alkylarenes, aliphatic alcohols, benzyl alcohols, and cyclohexanol with high yields and excellent selectivities. All the reactions gave the corresponding aldehydes or ketones under aerobic oxidation with O 2 . Most of the reactions were solvent free. The catalyst proved recyclable and no catalyst leaching to the reaction mixture was observed. This is evidence that the new catalyst is chemically stable.

Materials and Methods
All reagents were purchased from Merck or Aldrich and used without further purification. Chitosan (80-90% deacetylated, 2000 MW) was obtained from Golden-shell Biochemical Co., Ltd. (Zhejiang, China). GC High Resolution Gas Chromatograph Mass Spectrometer was carried out using Thermo Scientific. SEM images were prepared with a JEOL JSM 7001F. Pd determination was carried out on a FAAS (Shimadzu 105 model AA-680 atomic absorption spectrometer) with a hollow cathode lamp. The elemental analysis was performed with an Elementar Analysensysteme GmbH VarioEL. BET surface area was measured by nitrogen (N 2 ) adsorption/desorption at 77 K using a QuadraSorb SI surface area analyzer after degassing the samples at 100 °C for 10 h. High resolution was carried out using a Thermo Scientific Mass Spectrometer. XRD pattern was obtained using a Bruker D8 ADVANCE X-ray diffractometer with a Cu-K α radiation source (λ = 1.5406 Å) operating at 40 kV, 40 mA, and a scanning range of 10-80° 2θ, with a 2θ scan step of 0.015° and a step time of 0.2 second. Fourier transform infrared spectroscopy (FTIR) was used to characterize different functional groups of the composite using a Jasco 6300 FTIR instrument in the range of 600-4000 cm −1 . XPS spectra were recorded on a Thermo ESCALAB 250 Xi using monochromatic Al K α radiation (1486.6 eV) with a spot size of 850 µm. The spectra acquisition and  www.nature.com/scientificreports www.nature.com/scientificreports/ processing were carried out using Thermo Avantage software. The sample was stuck on the sample holder using a double-sided carbon tape and then introduced into the preparation chamber and was degassed until the proper vacuum was achieved. Then it was transferred into the analysis chamber, where the vacuum was 9-10 mbar. The analysis was carried out using the following parameters: pass energy of 20 eV, dwell time of 50 ms, and step size of 0.1 eV. Selectivity was calculated as: (peak area of the desired product/sum of the peak areas for desired and by-product) × 100.
Preparation of PdPc@CA. Synthesis of CA. Chitosan (2.00 g) was dissolved in an acetic acid solution (50 ml, 1.00 vol %) containing glutaraldehyde (2.00 ml). The mixture was stirred vigorously for 2 min to produce    Table 3. Comparison of the results for the oxidation of benzyl alcohol.