Amino-modified silica as effective support of the palladium catalyst of hydrogenation

The article describes synthesis of aminoorgano-functionalized silica as a perspective material for catalysis application. The amino groups have electron donor properties valuable for metal chemical state of palladium. So presence of electron donor groups is important for increasing of catalysts stability. The research is devoted to investigation of silica amino-modified support influence on activity and stability of palladium species in 4-nitroaniline hydrogenation process. A series of catalysts with different supports such as SiO 2 , SiO 2 -C 3 H 6 -NH 2 (amino-functionalized silica), γ-Al 2 O 3 and activated carbon were studied. The catalytic activity was studied in the hydrogenation of 4-nitroaniline to 1,4-phenylenediamine. The catalysts were characterized by scanning electron microscopy, transmission electron microscopy, x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and pulse chemisorption of hydrogen. The 5 wt. % Pd/SiO 2 -C 3 H 6 -NH 2 catalyst exhibited the highest catalytic activity for 4-nitroaniline hydrogenation with 100% conversion and 99% selectivity with respect to 1,4-phenylenediamine.

Possible ways to increase catalytic activity and selectivity include development of catalysts synthesis methods, optimization of reaction conditions, catalyst modification by different chemical elements and compounds, application of supports and modification of catalyst surface [4,[11][12][13][14][16][17][18][19][20]. Modification of catalyst support surface is possible solution to increase catalysts activity and stability. Presence of functional groups on support surface has great influence on electronic state of active metal and metal dispersion [21]. The aminopropyl groups presence a unique possibility for further surface modification. Amino groups modified supports are very useful for further surface nucleophilic substitution and obtaining the highest metal dispersion and study of this effect is sufficiently novel trend in catalysis [4,19,20,[22][23][24][25]. A lot of articles are dedicated to application of amino functionalized mesoporous silica in medicine, pharmaceutics and smart materials, however publications on their influence on active metals catalytic properties are fragmental [26][27][28][29][30][31]. Present study reports synthesis of aminoorgano-functionalized silica immobilized by palladium nanoparticles using sol-gel method. This way allows to synthesize a various hybrid composition materials by one-pot method.
For comparison amorphous silica, activated carbon and gamma-alumina were used as supports for palladium particles.
Catalytic activity of synthesized catalysts was tested in 4-nitroaniline hydrogenation ( Figure 1). Liquid phase hydrogenation of substituted nitrobenzenes is essential technology for the production of various aromatic amineskey intermediates for manufacturing agrochemicals, isocyanates, pharmaceuticals and dyes [32][33][34][35]. The most commonly used catalysts for different hydrogenation processes contains platinum group metals (palladium, platinum, iridium etc.) attached on different supports [33][34][35][36][37][38]. Palladium nanoparticles were found to be the most active and have high potential for their catalytic application of essential chemical products [33][34][35]  Such compounds as 4-nitroaniline are using as industrial raw material to produce of agricultural chemicals, rubber compounding agents, synthetic resin additives, polyamides, pharmaceuticals and dyes. The production of conductive polyamines applicable in electronics [39], also as antioxidants, preservatives [40][41][42].
The polyamines can find wide application in films materials and membranes due to the biodegradable and electro conductive properties.

FTIR spectroscopy of silica supports
The silica modification with aminopropyl groups was studied by Fourier transform infrared spectroscopy (Figure 2). The analysis of FTIR spectra of modified and pure supports revealed the broad bands centered around 3441 cm -1 that corresponds to O-H groups vibrations. On the other hand, the spectrum of amino-functionalized silica has peaks at 2940 cm -1 and 2886 cm -1 assigned to the C-H stretching vibrations of CH-and -CH 2 -groups, that can be attributed to the incorporation of the amino group [22,43]. The peak of C-N stretching vibration at  defects. At the 467 cm -1 , the plane stretching vibrations of Si-C observed [43].
However, the use of tetraethoxysilane as silica precursor led to the formation of surface methyl functionalized silica. This was certified by broad peak in the range 1338-1500 cm -1 that can be assigned to the C-H deformation vibrations of aliphatic bands [43].

The pulse chemisorption
The hydrogen pulse chemisorption data is presented in the Table 1.

XPS of catalysts before the reaction
The surface chemical composition of silica supports determined by X-ray photoelectron spectroscopy. High resolution XPS spectra of C 1s (Figure 3   The bonds -C-ONa is belong to molecules of sodium carbonate ( Figure 3) [43,44].
The Figure 10 shows the model decomposition of XPS high-resolution spectra of Pd 3d energy core-level of catalysts before the reaction. The atomic concentration of palladium compounds were gave in  An interesting observation was a shift of the binding energy by ~0.5 eV of palladium metal in the 5 wt. % Pd/SiO 2 -C 3 H 6 -NH 2 catalyst. There was an assumption that this may be the result of the displacement of the electron cloud of amino groups to palladium atoms.

XPS of used catalysts
The atomic surface concentrations of palladium decrease in 5 wt. % Pd/γ-Al 2 O 3 , 5 wt. % Pd/SiO 2 and 5 wt. % Pd/C catalysts (Tables 2, 3.). That can be attributed to metal particles diffusion into matrix of support [50]. While for amino modified sample 5 wt. % Pd/SiO 2 -C 3 H 6 -NH 2 the change of active metal surface concentration wasn't observed. Quantity of zero valence metal in 5 wt. % Pd/SiO 2 and 5 wt. % Pd/SiO 2 -C 3 H 6 -NH 2 samples before and after reaction did not significantly changed ( Table 3). The change of Pd O surface concentrations in 5 wt.
% Pd/C and Pd/γ-Al 2 O 3 catalysts were the most noticeable. In case of 5 wt. % Pd/C catalyst the quantity of PdO was decreased on 0.7 At.% and content of PdO 2 was increased on 0.7 At.% (Tables 2, 3). Also, the contents of PdO and Pd o in 5 wt. % Pd/SiO 2 and 5 wt. % Pd/γ-Al 2 O 3 catalysts were decreased.

2 3
Analysis of the conversion rate of 4-nitroaniline and the formation of 1,4phenylenediamine showed a similar tendency to decrease in the values of the rates, as according to the data on the consumption of hydrogen ( Table 4).
The second catalyst with a lower content of amino groups was obtained to confirm the effect of the amount of amino groups on the surface.  Figure 10. The higher the concentration of amino groups on the matrix surface was fixed, the more active was the palladium deposited on the silica matrix [24].
According to gas chromatography analysis the conversion of 4-nitroaniline and the yield of 1.4-phenylenediamine also amounted to 100%.

Chemicals and materials
Tetraethoxysilane (

Synthesis of amino-functionalized silica
The first step of reaction procedure was to prepare oil/water emulsion. For this 104 ml of distillated water and 16 ml of pure ethanol were mixed at stirrer with mixing rate 1500 rpm. Then 2 ml of cyclohexane was added as co-template. After

Fourier transform infrared spectroscopy (FTIR)
The diffuse reflectance IR spectroscopy was carried out using the FTIR spectrometer IRPrestige-21 (Shimadzu, Japan) equipped with a diffuse reflection attachment DRS-8000 was used for the qualitative composition of catalyst surface.
The resolution of all spectra was 4 cm -1 and spectra were registered in the wave number range 490-4000 cm -1 .

Hydrogen pulse chemisorption
The metal dispersion and the chemically active surface area were determined due to pulse chemisorption analysis by applying a pulsed titration of the catalyst with a hydrogen. The spectra were registered on Automatic analyzer of chemosorption AutoChem HP 2950 (Micromeritics, USA).

XPS
X-ray photoelectron spectroscopy (XPS) data were obtained using ES-2403 spectrometer (manufacturer: Institute for Analytic Instrumentation of RAS, St. Petersburg, Russia) with anode Mg Kα (hν = 1253.6 eV), energy analyzer PHOIBOS 100-MCD5 (SPECS, Germany) and X-Ray source XR-50 (SPECS, Germany). All the data were acquired at X-ray power of 250 W. Survey spectra were recorded at an energy step of 0.5 eV with the analyzer pass energy 40 eV, and high resolution spectra were recorded at an energy step of 0.05 eV with the analyzer pass energy 7 eV. Samples were degassed within 180 min before analysis and were stable during the treatment.

SEM
The morphological characteristics of the amino-functionalized mesoporous silica and catalyst were examined by scanning electron microscopy (SEM, TESCAN, Vega-LSU) equipped with X-ray microanalysis (OXFORD INCA PentaFETx3). Scanning electron microscope images were acquired at a magnification of 66.1 kX at 20 kV with SE detector.

TEM
The

Catalysts activity experiments description
The stirred reactor with the temperature-control was used to carry out of 4- The test for contribution of homogeneous catalysis to the reaction rate was performed by Sheldon's filtration test methodology. After partial conversion of 4nitroaniline, the reaction stops, the catalyst was extracted by filtration. Further the reaction was continued without catalyst, but the chemical transformation wasn't observed. Thus, the hydrogenation catalyzing only by heterogeneous catalysts.
Chromatographic analysis was performed using a gas chromatograph (Crystal, manufacturer Chromatek). The reaction rate was controlled through the 4-nitroaniline conversion. After the process completion the catalyst was separated from the reaction mixture by centrifugation.

Deactivation experiments using recovered catalysts
Deactivation of catalysts was study in condition under the hydrogenation of 4-nitroaniline to 1,4-phenylenediamine in water solution of 2-propanol with repeat injecting of 4-nitroaniline at the end of reaction. The five repeated injections of 4nitroaniline were carried out on each catalyst.

Conclusions
The differences in quantity of hydrogen uptake in hydrogenation on various This is follows from the data of scanning, transmission electron microscopy.
The studying the effect of small metal particles in catalysts on differences in the values of metal dispersion, metal surface area and concentration of active centers on activity is an interesting task.
In total the concentration of active center is correlated with amount of hydrogen adsorbed under normal conditions. The only exception is 5% palladium on silica. Although a modification of the catalyst synthesis technique can correct the result. Obviously that, the best ability of hydrogen sorption was observed in case of 5 wt. % Pd/SiO 2 -C 3 H 6 -NH 2 .
According to XPS data, the main role in catalysis belongs to metallic palladium. The greater the proportion of metallic palladium, the higher the catalytic stability of palladium. The more faulted or developed the surface structure of the carrier, the greater was the concentration of palladium on the surface and the greater activity.
In case of 5 wt. % Pd/SiO 2 -C 3 H 6 -(30%)NH 2 and 5 wt. % Pd/SiO 2 -C 3 H 6 -(10%)NH 2 were found the higher the concentration of amino groups on the matrix surface was fixed, the more active was the palladium deposited on the silica matrix. This confirms the hypothesis that the electron-donating properties of the amino group have a positive effect on the catalytic activity of the supported metal.