Synthesis of Pt and Pd Supported Nanocatalysts. Study of Kaolinite Layered Structure Effect on Their Catalytic Activity

doi:10.21203/rs.3.rs-5092668/v1

In recent years kaolinite-supported nanoscale catalytic systems have been widely used in the mining industry owing to their high efficiency and selectivity. In this article, we present a synthesis and catalytic activity study (on n-heptane hydrocracking and hydroreforming processes) of kaolinite-, Al₂O₃- and silica gel- supported Pt/Pd catalysts. The structural peculiarities of these nanocatalysts were characterized using FTIR, SEM, X-ray powder diffraction and BET techniques. The obtained results show that the catalytic activity of Pt catalysts is higher compared to corresponding Pd catalysts, and the n-heptane conversion on Pt/SiO₂ and Pd/SiO₂ catalysts are weaker than corresponding catalysts deposited on Al₂O₃. Moreover, it has been discovered that swelling kaolinite with polar aprotic solvents, such as DMSO, and impregnating with Pt/Pd salts, results in formation of nanosized metal particles with high catalytic activity. The presented results also show that Pt/kaolinite catalysts were weaker in their activity to Pt/kaolinite/DMSO catalyst.

Pt

Pd

Synthesis

Nanocatalyst

Impregnation

Reforming

The dispersion and aggregation of metal nanoparticles are two main factors to be considered for the synthesis of high-performance catalytic system [1–3]. It should be noted that the activity of the catalysts depends on the morphology and composition of the metal-based nanoparticles and supports [4, 5–7]. Hence, the support materials play significant role in the preparation of high-performance catalytic systems. Because carbon materials, due to environmental and economic reasons, are not advisable for practical use, it is important to find new materials as catalyst supports. Kaolin, a clay mineral widely spread in nature, is known as thermally high-resistance, chemically and mechanically stable, and effectively adsorbing material [8, 9]. Veisi et al. studied kaolin-composite materials for catalytic application [10–12]. Kaolin is a layered silicate mineral.

The unit structure of kaolin contains one tetrahedral sheet (SiO₄) of silica linked through oxygen atoms to one octahedral sheet of alumina (AlO₆) [13–17].

Hydrogenolysis reactions are involved in many industrial processes, such as reforming, hydrocracking [18]. Hydrocracking processes occur in the presence of hydrogen at the temperature of 573–723 K and at a hydrogen pressure of 35–200 atm. Hydrocracking reaction of hydrocarbons is accelerated by metallic catalysts, such as Pt/Pd, leading to cyclization and isomerization reactions. In the presence of hydrogen hydrocracking reaction can be presented by this equation:

$$\:CX+\:{H}_{2}=CH+HX,$$

where X can be O, N, C, S or a halogen atom. Aribike et al. studied the kinetics and mechanism of thermal cracking of n-heptane [18]. In general, cracking reactions result in higher yield of n-butane and propane. At low pressures (2–10 atm) 2,3-dimethylpentane and 3-methylhexane were formed as a result of cracking processes. At 20–40 atm pressures 2-methylhexane and 3-methylhexane form, cyclic compounds; such as ethylcyclopentane, methylcyclohexane, toluene and 2-dimethylcyclopentane.

It should be noted that platinum-based catalysts used for catalytic reforming processes are known since the late 1940s. Olafadehan described kinetic model of n-heptane reforming on Pt/Al₂O₃ catalyst based on experimental data from Susu [19]. The objective of this work is to develop the synthesis of Pt/Pd nanocatalysts supported on kaolinite, Al₂O₃ and silica gel, obtained from serpentines, on n-heptane hydrocracking and hydroreforming processes. FTIR spectroscopy, scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and BET techniques are used for structural and morphological characterization of synthesized catalysts.

2.1 Materials

A kaolinite sample with the following chemical composition: SiO₂ − 45.7%; Al₂O₃ − 36.4%; Fe₂O₃ − 3.0%; CaO − 1.2%; MgO - < 0.1%; and dry residue − 14.4%, was obtained from Tandzut region (Armenia) and used as a catalyst support in the synthesis of Pt/Pd nanocatalysts. SiO₂ was obtained from serpentine with the following chemical composition: MgO − 39.1%; Fe₂O₃ − 9.5%; SiO₂ − 36.3%; volatile compounds − 14.7%, from Shorja region (Armenia). Serpentinite samples were preliminarily grinded and sieved through the sieve (100x40 mm, mesh size 250 µm) to achieve a distribution of particles of about 0.1–0.2 µm, with a maximum grain size of about 0.25 µm. Samples were annealed at 750 ^oC for 10 min. The heated serpentinite was leached by hydrochloric acid (HCl) during 10 min, and then, the formed sludge was decanted and filtered [20]. The residue remained after filtration was undergone additional acid treatment by concentrated HCl for the removal of remained ions of magnesium and iron (II). As a result, a high purity (of 98%) amorphous silica with a specific structure and specific surface area of 230–280 m²/g was obtained [21, 22]. Analytical grade chloroplatinic acid hexahydrate H₂PtCl₆.6H₂O, palladium (II) chloride (PdCl₂ ≥ 99.0%), HNO₃ (Sigma-Aldrich Ltd, 65% analytical reagent) and γ-Al₂O₃ (≥ 99.0%) were obtained from Sigma-Aldrich Ltd (Germany). γ-Al₂O₃ specific surface area is 250 m²/g with average particle size of 20 nm. Ethanol, DMSO and other chemicals were obtained from Carl Roth Gmbh (Germany). Deionized water was used for all experiments. The kaolinite, γ-Al₂O₃ and SiO₂ were used as catalyst support.

2.1.1 Preparation of nanaocatalysts

2.1.1.1 Preparation of supported Pt/Pd nanaocatalysts

The studied catalysts were prepared by impregnation method. In this study, Pt/Pd were deposited over kaolinite, Al₂O₃ and SiO₂ surfaces, then were dried, dehydrated and calcined. γ-Al₂O₃, kaolinite (200 mg) and SiO₂ were dried at 383 K for 90 min under a dry air flow with a flow rate of 50 cm³/min, then the catalyst supports were calcined under a dry air flow at 873 K for 120 min. To obtain Pt/Pd (1 wt.%) catalyst, respectively, at impregnation step catalyst supports, containing 2 mL H₂PtCl₆.6H₂O (20 mmol) and 4.75 mL 0.01 М palladium (II) chloride (Na₂PdCl₄) after impregnation were immersed into the solution and dried at 423K for 12 hours and calcined at 873K for 120 min.

2.1.1.2 Preparation of Pt/kaolinite/DMSO nanaocatalyst

Pt/kaolinite/DMSO nanocatalyst is prepared by two-step procedure. The first step includes treatment of the suspension, containing 20 mL water, 10 g of kaolinite in 130 mL DMSO at 70 ^oC with stirring for 2 hours. The obtained solution was filtered, washed by 15 mL ethanol and dried at 70 ^oC for 48 hours. The second step includes immersion of dry residue into the solution containing 2mL H₂PtCl₆.6H₂O. After impregnation step, Pt/kaolinite/DMSO nanocatalyst was dried at 423 K for 12 hours and calcined at 873 K for 120 min.

2.1.2 Catalytic activity test

The activity of Pt/Pd nanaocatalysts was studied using a continuous fixed-bed microreactor packed with 0.2 g of catalyst. The prepared catalysts were loaded into a cylinder with a 400 mm length and a 4 mm ID. n-Heptane with concentration of 1000 mg/L was loaded into the 5.024 cm³ volume reactor with a flow rate of 100 mL/min, temperatures varied from 373 to 553 K. n-Heptane concentrations were detected using gas chromatograph 7890A (an Agilent Technologies), equipped with a flame ionization detector. To calculate n-heptane conversion the following equation was used:

$$\:\eta\:=\frac{{C}_{1}-{C}_{2}}{{C}_{1}}\times\:100\%\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:$$

where η is the n-heptane conversion rate, C₁ and C₂ are the n-heptane initial and final concentrations.

2.1.3 Characterization of nanocatalysts

The Pt/Pd nanocatalysts were characterized by XRD, FTIR, SEM and N₂ adsorption-desorption methods. The X-ray phase analysis was carried out using Dron-3 diffractometer (Russia Manufacturer) under the following conditions: CuK_α - radiation; angular range 2θ = 8- 80^o and power supply 25 kV/10 mA. The weight of samples subjected to X-ray phase analysis was 500 mg. The samples were subjected to FTIR analysis using “Nexus” spectrometer of the Thermo Nicolet Corporation (USA) (ATR method) in the range of 400 to 4000 cm^− 1 with resolution of 4 cm^− 1 using pellets, containing 200 mg potassium bromide and 1 mg of the sample.

The specific surface area of synthesized supported nanocatalysts were measured by volumetric method, using BET isotherm model. The BET surface area of the supported nanocatalysts obtained are presented in Table 1. The specific surface area of SiO₂ obtained from serpentines varies within the range of 230–280 m²/g. The specific surface area of Pt/SiO₂ was 250 m²/g, 200 m²/g for Pt/Al₂O₃, 40 m²/g for Pt/kaolinite. After swelling with DMSO, BET analysis revealed a decrease in the surface area (from 40 m²/g to 2.05 m²/g). The increase in specific Pt surface area is probably caused to close the porosity of particle size of support material. XRD patterns of Pt/Al₂O₃ and Pt/SiO₂ nanocatalysts and Al₂O₃, SiO₂ catalyst supports are presented in Fig. 1(a) and 1(b), respectively. The reflections observed in the XRPD patterns indicate the existence of following components: Al₂O₃, SiO₂, Pt. The characteristic reflections (Fig. 1(a) and Fig. 1(b)) of Pt observed at ~ 40.81, 47.14, 68.65 and 82.21^o. In the XRPD pattern of γ-Al₂O₃ low intensity peaks at ~ 37.51, 46.6 and 67.45^o are observed. SiO₂ support obtained from serpentines was found to be amorphous, as a broad peak appears in the SiO₂ sample (Fig. 1(a)). Figure 1(c) and Fig. 1(d) represent XRD patterns of Pt/kaolinite and Pt/kaolinite/DMSO nanocatalysts, respectively. The reflections observed in the XRD patterns are mainly consist of kaolinite, rutile impurities and small amounts of illite. Displacement of the reflections associated to the XRD basal distance d₀₀₁ in kaolinite (7.18 to 11.26 Å), is coherent with a monolayer intercalation of DMSO molecules (Fig. 1(c, d)).

Table 1

BET surface area and composition of supports and nanocatalysts
Supports/nanocatalysts	BET surface area, m²/g	Pt loading, wt.%
SiO₂	230–280	-
Al₂O₃	250	-
Kaolinite	19	-
Pt/SiO₂	250	1
Pt/Al₂O₃	200	1
Pt/kaolinite	40	1
Pt/kaolinite/DMSO	2.05	1

To study the role of DMSO in the process of increasing the interlayer space of kaolinite the adsorption of DMSO on the surface of kaolinite and Al₂O₃ was studied using FTIR spectroscopy. The adsorption of the first portions of DMSO leads to decrease the intensity of the absorption band of kaolinite surface hydroxyl groups. The valence vibrations of C-H groups appear in the spectrum which indicates the presence of DMSO on the surface. To record S = O region (1100 − 1000 cm^− 1) DMSO adsorption was performed on the surface of Al₂O₃. In the gas state the S = O region of DMSO is characterized by an absorption band at 1104 cm^− 1 and a shoulder at 1040 cm^− 1 (Fig. 2). In the liquid state FTIR of DMSO is characterized by an absorption band at 1054 cm^− 1. It should be noted that kaolinite is identified by two absorption bands at 3695 and 3659 cm^− 1 produced by vibrations of the inner-surface hydroxyl groups of kaolinite. The absorption bands observable at 3538 and 3502 cm^− 1 can be assigned to the kaolinite stretching frequencies of inner-surface hydroxyls (Fig. 3a). The absorption band observable at 1121 cm^− 1 corresponds to Si-O bond stretching vibrations. This fact indicates interaction between DMSO and the Si-O surface of kaolinite. The peak at 792 cm^− 1 attributed to out of phase stretching of Al-OH tends to be weaken, and the peak at 687 cm^− 1 describes Al-OH symmetric vibration (Fig. 3b). These IR peaks detail the bonding evolutions of hydroxyl groups at the inner surface of kaolinite upon DMSO intercalation, which leads to strong hydrogen bonds formation between the guest, DMSO molecules, and Al-OH of kaolinite host with and without hydration. Moreover, the IR spectra suggest that the Si-O band of kaolinite is only significantly impacted for DMSO intercalation in the presence of water. The vibration frequency of S = O group decreases from 1104 cm^− 1 to 1060 cm^− 1.

As the concentration of adsorbed DMSO increases the intensity of absorption band at 1104 cm^− 1 increases as well, and the rotational deformation of oscillation frequencies of CH₃ groups which is present in the liquid state (954 cm^− 1 and 932 cm^− 1), does not observed in the adsorbed state.

The SEM micrographs of supports and the synthesized nanocatalysts are presented in Fig. 4. The SEM micrographs of the synthesized Pt/SiO₂ demonstrate some structural changes when loading on 1% platinum. According to the SEM analysis of the synthesized Pt/γ-Al₂O₃ nanocatalyst does not show structural changes when loading on 1% platinum.

SEM images (Fig. 4a, b, c, d) show that the sizes of Pt particles are different. Strong interaction between Pt and the γ-Al₂O₃ support is not observed, but in the case of Pt/SiO₂ a strong interaction can take place. As the temperature increases, C1-C3 hydrocarbons are formed (Fig. 5). At 500–523 K n-heptane conversion on Pt/Al₂O₃ catalyst reaches about 65% (Fig. 6a). At 500–523 K n-heptane conversion on Pt/SiO₂ and Pd/SiO₂ catalysts are inferior in their activity to corresponding catalysts deposited on Al₂O₃ (Fig. 6a, b). However, the formation of isomeric products is observed on these catalysts. An increase in temperature leads to decrease in isomerization selectivity and an increase in the number of cleavage products.

The conversion of n-heptane on Pt/kaolinite and Pt/kaolinite/DMSO catalysts was studied. Increasing the temperature leads to an increase of n-heptane conversion and at 430 K n-heptane conversion on Pt/kaolinite/DMSO catalyst reaches 50% and at 480 K the highest 63% of n-heptane conversion is observed, and an increase in selectivity is observed (Fig. 7) as well. This fact is probably due to the large dispersity of Pt that occurs in the nanosized cavities of kaolinite.

The enthalpy of soaking (Δ_sH) for kaolinite, Al₂O₃ and SiO₂ with H₂O, DMSO and DMSO + H₂O (1:1 v/v ratio) solvents are determined using Calve type DAC-1-1 differential automatic microcalorimeter, and obtained results are presented in Table 2. The values of Δ_sH for all samples in water are lower compared to H₂O + DMSO binary solvent and pure DMSO. These thermal effects in the presence of DMSO could be explained by the formation and penetration of DMSO-H₂O associates of different composition in the interlayer space of catalysts. When soaking with DMSO, this tendency could be probably due to the self-associated structure and penetration of DMSO associates into the interlayer space of catalysts.

Table 2

The values of enthalpy of soaking for kaolinite, Al₂O₃ and SiO₂
Support	H₂O	1H₂O:1DMSO (v/v) Δ_sH, J/g	DMSO
Kaolinite	-1.94	-3.34	-4.94
Al₂O₃	-2.86	-3.31	-5.33
SiO₂	-3.14	-2.67	-4.87

This study provides insight into the structure-function relationship for n-heptane hydrocracking and hydroreforming processes on Pt and Pd supported nanocatalysts. The results showed that Pt catalysts demonstrate higher activity in the catalytic conversion of n-heptane than the corresponding Pd catalysts. It should be emphasized that the n-heptane conversion on Pt/SiO₂ and Pd/SiO₂ catalysts were inferior in their activity to corresponding catalysts deposited on Al₂O₃. Moreover, it has been discovered that swelling kaolinite with polar aprotic solvents, such as DMSO, and impregnating with appropriate metal salts, nanosized metal particles were obtained which show higher catalytic activity on n-heptane hydrocracking and hydroreforming processes. The present results also showed that Pt/kaolinite catalyst was inferior in their catalytic activity to Pt/kaolinite/DMSO catalyst. These studies are of great interest and value for the further development of low prices kaolinite support catalysts for application in environment protection.

Acknowledgements I would like to submit my great thankfulness to Prof. Dr. Miguel A. Rodriguez Barbero (Instituto de Cerámica y Vidrio) for his devotion, efforts, and valuable guidance and discussion throughout this work.

Author Contributions KKK: Investigation, Visualization and Writing—Original draft preparation; TCL: Conceptualization, Methodology, Visualization, Validation and Writing—Reviewing and Editing; GMM: Validation and Data curation; OSO: Conceptualization, Methodology, Visualization and Writing—Reviewing and Editing, Resources, Supervision and Funding acquisition.

Funding This work was supported by the State Committee of Science of the Ministry of Education and Science of the Republic of Armenia.

Conflict of interest The authors declare no conflict of interest.

Data Availability The data that support the finding of this study are available from the corresponding author upon reasonable request.

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No competing interests reported.

Synthesis of Pt and Pd Supported Nanocatalysts. Study of Kaolinite Layered Structure Effect on Their Catalytic Activity

Status:

Version 1

Abstract

Figures

1 Introduction

2 Materials and method

2.1 Materials

2.1.1 Preparation of nanaocatalysts

2.1.1.1 Preparation of supported Pt/Pd nanaocatalysts

2.1.1.2 Preparation of Pt/kaolinite/DMSO nanaocatalyst

2.1.2 Catalytic activity test

2.1.3 Characterization of nanocatalysts

3 Results and Discussion

4 Conclusions

Declarations

References

Additional Declarations

Status:

Version 1