Study on Novel Pd-impregnated Catalyst for Synthesis of 2-Arylpropanoic Acids by Hydrocarboxylation

In the hydrocarboxylation of the Pd-based catalyst, the heterogeneity of homogeneous processes was a reliable means to improve the recyclability of the catalyst and to inhibit the loss of noble metals. Herein, a novel heterogeneous Pd@DT017 catalyst prepared from Pd(II) salt impregnated in the acidic resin of DT017 had been studied, which revealed high performance. Correcting characterization showed that Pd(II) was successfully grafted to -SO3 - on the surface of DT017, and the Pd element was highly dispersed in DT017 with the macroporous structure. The conversion was 99.9%, and the yield of isomeric acid was 91.6% on Pd@DT017. The stability test found that Pd@DT017 had higher cycle stability.


Introduction
2-Arylpropionic acid, an important class of non-steroidal anti-inflammatory drugs (NSAIDs), had garnered significant attention [1] .Catalytic carbonylation of olefins or alcohols by the Pd-based catalyst was a common strategy for the synthesis of 2-arylpropionic acid [2][3] .Huang [4] developed the first example of the olefin carbonylation catalyzed by the Fe(Ⅲ) salt additive in the PdCl 2 -PPh 3 catalyst system to efficiently control the regioselectivity of product acid.It was found that Lewis acid was used instead of proton acid as a cocatalyst to enhance the regioselectivity of isomeric acid (iso-acid) and to improve the disadvantage of equipment corrosion.However, produced Pd black by agglomeration of Pd(0) species in a homogeneous PdCl 2 -PPh 3 -FeCl 3 catalytic system lead to the cycle performance of a Pd-based homogeneous catalyst not ideal.Inhibiting the formation of Pd(0) black and improving the recyclability of the catalyst were among the primary research objectives of Pd catalysts utilized in the heterogeneous hydrocarboxylation reaction at present.
In this paper, a novel heterogeneous Pd@DT017 catalyst with a macroporous structure was prepared and employed for the synthesis of 2-arylpropanoic acids from styrene.The structural properties, thermal stability, and binding mode of Pd species were analyzed by related characterization methods (e.g.SEM, TGA, XPS).The catalytic performance of Pd@DT017 with synergy in PPh 3 and FeCl 3 for hydrocarboxylation was investigated.

Materials
The acidic resin of DT017 was provided by Dandong Pearl Special Resin Co., Ltd.CO with a purity of 99.9% was provided by Qingdao De Yi Gas Co., Ltd.Palladium chloride (PdCl 2 ), 1,4-dioxane and hydrochloric acid (37%, HCl) was purchased from Sinopharm Chemical Reagent Ltd.

Catalyst Preparation
2.5 g of DT017 acidic ion-exchange resin was soaked in 10% HCl solution for 6 h, centrifuged, and washed twice with 2×10 mL of deionized water to remove organic impurities on the surface of the resin.The washed resin was dried in an oven at 60 °C, and then sealed and stored.We weigh 0.017 g of PdCl 2 (0.10 mmol) and 1.5 g of DT017 resin in a 100 mL beaker with an electronic balance, and take 10% HCl equal to the volume of resin in the beaker, so that PdCl 2 can be dissolved in hydrochloric acid solution faster under ultrasonic conditions.After 6 h of impregnation, the dry Pd@DT017 catalyst was obtained by dryness under vacuum at 80 °C.Scheme 1.The fabrication process of Pd@DT017 catalyst.

Characterization
The morphological images of the samples were obtained by the scanning electron microscope (SEM, Hitachi S4800).Fourier transforms infrared (FT-IR, Nicolet IS10) spectroscopy was employed to research the chemical structure.Thermogravimetric analysis (TGA, Q500 V20.13) was used to analyze the structural stability of the materials.The X-ray photoelectron spectroscopy (XPS, ESCALAB 250) measurements were conducted on the spectrometer.Catalytic performance was performed by gas chromatography (GC, GC7809).

Performance Testing
1 g of styrene (9.62 mmol), 0.036 g of FeCl 3 •6H 2 O (0.14 mmol), 1 g of catalyst of Pd@DT017, a certain amount of the PPh 3 and H 2 O, and 10 mL of 1,4-dioxane were added to the Teflon-lined and ultrasonically dissolved.Subsequently, the Teflon-lined containing the reaction solution was transferred to the autoclave, replaced with 0.1 MPa of CO three times, and then charged with 1-5 MPa.The reaction was tested at 60-120 ℃ with stirring at 300 rpm for 6 h.After the reaction, the supernatant was collected by centrifugation.The composition of products was characterized by GC analysis to calculate further reaction performance.

Characterization
Figure 1.The apparent morphology of a DT017 resin and d Pd@DT017 catalyst.
Pore morphology of b-c DT017 resin and e-f Pd@DT017 catalyst.g Element distribution of Pd@DT017: C, O, S, and Pd.
Figure 1 showed the SEM images of DT017 resin and Pd@DT017 catalyst.The DT017 resin was found to be about 750 μm of spherical particles in Figure 1a.It was found in Figure 1b that the pore structure in DT017 resin was composed of the accumulation of many spherical particles.In Figure 1c, it was found that some obvious 100-200 nm of macroporous structure existed in the pores of DT017 resin, which were conducive to the mass transfer of molecules and the catalytic reaction.After Pd salt loading, no obvious morphological changes were found in the microscopic images of DT017.The pore morphology of the resin was not damaged in the strongly acidic loading environment, which indirectly reflected the strong acid resistance of the resin skeleton.The EDS-mapping image showed the dense distribution of S and O elements (Figure 1g), which indicated the existence of relatively rich active sites (-SO 3 -) in the resin, promoting the high dispersion of Pd species within the resin.FT-IR spectra of the Pd@DT017 catalyst before and after Pd loading and reaction were shown in Figure 2a.There was a strong absorption peak of 3459 cm -1 , which was matched with the adsorbed bonded water and S-OH on the resin surface.The absorption peaks at 2959 and 1446 cm -1 belonged to the antisymmetric stretching and deformation vibration of the -CH 2 group, and the peaks at 1674 and 857 cm -1 corresponded to the stretching vibration of C=C and C-H bond on the benzene ring skeleton, respectively.These indirectly reflected the characteristic structure of the DT017 resin skeleton [5] .The absorption peaks at 1180 cm -1 , 1073 cm -1 , 1036 cm -1 , and 621 cm -1 correspond to the structure of S=O and S-O bonds in -SO 3 H, respectively [6] .When resin was loaded with Pd salt, the absorption peaks of -SO 3 H in DT017 were generally weakly enhanced, which was caused by the interaction between Pd 2+ and -SO 3 -.After the reaction, the peak of -SO 3 -in the Pd@DT017 catalyst generally decreased, which may be caused by the shedding of Pd salt during the reaction or the change of valence state (+2→0) [6] .The absence of a resin skeleton characteristic peak was not found in the reaction, which indirectly proved the Pd@DT017 catalyst can be used as a stable catalyst.
The TGA-DTG curve of Pd@DT017 from 28 to 800 °C was shown in Figure 2b.According to the two obvious weight loss peaks in the DTG curve, the whole process of weight loss was divided into three stages.The ΔW 1 was 14.1% at 20-192.2 ℃, which was mainly the bonded water adsorbed on the resin surface.The ΔW 2 was 34.5% at 192.2-429.6 ℃, which was mainly the gradual dissociation of -SO 3 H on the resin skeleton.The ΔW 3 was 9.7 wt% at 429.6-800 °C, which was the decomposition and collapse of the resin skeleton caused by high temperature.The final residual material may be an amount of palladium oxide and an incomplete decomposed resin (about 41.3% by mass).TGA-DTG curve analysis indicated that the Pd@DT017 catalyst was thermally stable in the temperature range of hydrocarboxylation (<150 °C) [7] .Figure 3a showed four distinct characteristic peaks in the Pd 3d spectrum on the Pd@DT017 catalyst, which represented the presence of Pd(0) and Pd(II) species, respectively.The Pd(0) species existed in no reduction of Pd@DT017 catalyst, which could be caused by resin in the benzene ring of conjugate  interaction between electron and Pd species improved higher electron density of active Pd atom to produce trace amounts of Pd(0) [9][10] .The spectra of O 1s and S 2p showed two characteristic peaks (S=O and S-O) and four groups of characteristic peaks (S=O, S-O, S 2p 1/2 , and S 2p 3/2 ), respectively.Compared to the Pd salt loaded, the position deviation of the S=O peak in O 1s was slightly higher than that of S-O, which meant that Pd salt had a stronger binding ability with the O atom in S-O than S=O.The difference in binding energy shift indirectly proved that the binding site with Pd 2+ was S-O in -SO 3 H [8] .

Catalytic Evaluation
Figure 4a showed the effect of the Pd load on the catalytic activity in Pd@DT017.As the load chloride in resin gradually increased from 0.1 wt%, the conversion of styrene and the yield of iso-acid increased rapidly, because the increase of Pd content increased the catalytic active site in the catalyst.When the Pd load was 0.7-1.3wt%, styrene was completely transformed and the yield of iso-acid reached the highest (91.6%%).As the Pd load was 1.3-2.7 wt%, the conversion decreased slightly, which may be excessive Pd(Ⅱ) produced to grow into palladium black without catalytic activity, which further encased active Pd particles to reduce the catalytic activity of the system.Figure 4b showed the effect of H 2 O/styrene on the performance.As H 2 O/styrene gradually increased to 3:1, the conversion and the yield of iso-acid reached the maximum.This was because the increase in water content promoted the equilibrium conversion of styrene.As H 2 O/styrene increased, the conversion decreased sharply until completely deactivated, and styrene polymerization occurred after the reaction.It was speculated that the excess water caused some styrene to polymerize, resulting in a decrease in catalytic activity.The effect of temperature on the reaction was shown in Figure 4c.When the temperature was up to 100 °C, the catalytic activity reached the highest.With the increase in temperature, the yield of iso-acid started to decrease gradually.In addition, a small amount of polymer began to appear in the reaction solution after the reaction.That was caused by the higher reaction temperature accelerated the reaction speed of styrene polymerization.With the increase in CO pressure, the related catalytic activity began to increase gradually.This was because the increase of CO pressure of the reactant was conducive to improving the conversion of the styrene.The iso-acid reached the highest in 3 MPa.When CO pressure exceeded 3 MPa, the yield of iso-acid began to decrease.Higher CO pressure improved the formation of other product acids.
Under the optimal reaction conditions (Pd load=0.7 wt%, H 2 O/styrene=3:1, T=100 °C, and P CO =2 MPa), higher catalytic activity (conversion=99.9%,the yield of iso-acid=91.6%) of Pd@DT017 could be maintained in reused five times.The high catalytic activity and recycling ability of the Pd@DT017 catalyst effectively reduced the defect of noble metal loss.

Conclusion
A novel Pd@DT017 macroporous catalyst was prepared and used for the hydrocarboxylation of styrene.It was found that macroporous structure and abundant active site (-SO 3 -) provided highly dispersed conditions for the Pd species.The Pd@DT017 heterogeneous catalyst obtained a higher catalytic performance of 99.9% conversion and 91.6% yield of iso-acid under the reaction conditions by performance optimization.The efficient catalytic activity and cycling performance improved the defect of noble metal loss in the original system.This Pd-based catalyst can further improve the catalytic performance in optimizing the type of graft groups.

Figure 2 .
Figure 2. a) The FT-IR spectra of DT017 and Pd@DT017 before and after the reaction; b) TGA-DTG spectra of Pd@DT017.

Figure 3 .
Figure 3. XPS spectra of a Pd 3d in Pd@DT017, XPS spectra of b S 2p and c O 1s in DT017 and Pd@DT017.

Figure 4 .
Figure 4. Catalytic performance of Pd@DT017 in styrene.Conversion and yields of iso-acid with different a Pd load, b H 2 O/styrene, c temperature, and d CO pressure.