Synthesis and Catalytic Performance of Gold Intercalated in the Walls of Mesoporous Silica

Summary

Here, we present a protocol with a sol-gel process to synthesize gold intercalated in the walls of mesoporous materials (GMS), which is confirmed to possess a mesoporous matrix with gold intercalated in the walls imparting great stability and recyclability.

Abstract

As a promising catalytically active nano reactor, gold nanoparticles intercalated in mesoporous silica (GMS) were successfully synthesized and properties of the materials were investigated. We used a one pot sol-gel approach to intercalate gold nano particles in the walls of mesoporous silica. To start with the synthesis, P123 was used as template to form micelles. Then TESPTS was used as a surface modification agent to intercalate gold nano particles. Following this process, TEOS was added in as a silica source which underwent a polymerization process in acid environment. After hydrothermal processing and calcination, the final product was acquired. Several techniques were utilized to characterize the porosity, morphology and structure of the gold intercalated mesoporous silica. The results showed a stable structure of mesoporous silica after gold intercalation. Through the oxidation of benzyl alcohol as a benchmark reaction, the GMS materials showed high selectivity and recyclability.

Introduction

As an emerging technology that has great potential in catalysis applications, nanoscale materials have received intensive research interest in the past decades. Amongst the nanoscale catalysts reported, noble metal catalysts such as Au, Ag, Pd and Pt have attracted world-wide attention ^1-3. Select catalytic reactions include the oxidation of carbon monoxide researchers on Au, Heck reaction on Pd catalysts, and water splitting with Pt. In spite of the promising catalytic potential, nanoscale gold is limited in its applicability due to deactivation from poisoning, coking, thermal degradation, and sintering. It has been reported that gold, as a representative for noble metals, has high selectivity and is less prone to metal leaching, over-oxidation, and self-poisoning⁴. However, the catalytic performance of gold strongly depends on the particle size. Haruta et al. has reported the relationship between catalytic performance and gold cluster diameter, demonstrating the highest activity of gold catalysts with particle size ~ 2.7 nm⁵.

The particle size of noble metals can be controlled by the preparation method^6-9; however, the major hindrance towards broad application remains aggregation and loss of activity. To solve the problem of sintering, a common method is to immobilize nanoscale particles on a support material. Various support materials have been applied including porous silica^10-11, semiconducting metal oxides^12-13, polymers¹⁴, graphene¹⁵ and carbon nanotubes¹⁶. Amongst the materials used, porous silica is an attractive material as a support because it is only mildly acidic, relatively inert, thermally and chemically stable, and can be prepared with very well defined meso-/micro-porosity. The porous structure provides good support for metal particles but also imparts size selective substrate access to the metal catalysts. This selectivity is particularly promising because of the tunability associated with these porous materials. Often, gold particles are found to be extremely mobile on silica surfaces^17-18 and readily form very large (50+ nm) unreactive particles when exposed to high temperatures, thus making it difficult to prepare gold nanoparticles on silica¹⁹. Mukherjee et al. reported immobilization of monodispersed gold nanoparticles on mesoporous silica MCM-41 by 3-aminopropyl-trimethoxysilane and 3-mercaptopropyl-triethoxysilane, and the supported gold nanoparticles were found to be highly active for hydrogenation reactions and no leaching of gold was found in the reaction²⁰.

Following the report of surface modification of mesoporous silica, we reported a method to prepare gold intercalated into the wall of mesoporous silica (GMS). Additionally, the mesoporous silica supported approach offers a scalable approach to potentially independently alter the catalyst and porous environment. Since catalytic processes are of vital economic importance, the benefits could be far reaching. The ability to develop “green” catalysts would have a profound positive impact on the environment and improve the economic feasibility and resource efficiency of important industrial processes.

Protocol

1. Preparation of GMS

1. Use all chemicals in the following process as received.
2. Prepare 75 ml of 2 M of hydrochloric acid (HCl) solution. Weigh 2.0 g of poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123, MW = 5,800) and transfer into the prepared 75 ml of 2 M HCl solution. At RT, apply magnetic stirring to the solution at a speed of 350 r/min until P123 is completely dissolved. The solution will be clear.
3. Weigh 4 g of tetraethoxysilane (TEOS, MW = 208.33) in a small vial and transfer 180 µl of Bis[3-(triethoxysilyl)propyl]-tetrasulfide (TESPTS, MW = 538.94) into the vial. Slowly shake the vial to mix the two chemicals. In another vial, weigh 38 mg of chloroauric acid (HAuCl₄, 99.90%) and dissolve in 1 ml of DI water.
4. Increase the P123 solution temperature to 35 °C in an oil bath with temperature controlled by thermocouple.
5. Add all of the mixture of TEOS and TESPTS prepared in step 1.3 to the P123 solution, and keep the solution at vigorous magnetic stirring of 700 r/min. Keep the solution stirring for 2 min, then add all of HAuCl₄ solution prepared in step 1.3 dropwise within 30 sec.
6. Keep the solution stirring at 700 r/min for 24 hr at 35 °C.
7. After 24 hr, transfer the solution into a sealed bottle and store in an oven set at 100 °C for 72 hr. This is called hydrothermal processing.
8. After the hydrothermal processing, filter the solution with a #1 filtration paper and negative pressure under a funnel, then wash with water two times and ethanol three times to remove remaining HCl. During each wash process, add water or ethanol 1 cm above solid and wait for the material to dry.
9. Transfer precipitation from filtration to a ceramic crucible and calcine at 550 °C for 4 hr. Set the ramp program as follows: 25 °C to 550 °C for 2 hr, keep at 550 °C for 4 hr, then allow the sample to remain in furnace with the door closed until the temperature falls below 40 °C.
10. After calcination, transfer the product to a glass vial with a plastic spatula. The synthesized material has a red color.

2. Catalytic Reaction, Oxidation of Benzyl Alcohol

1. Since the oxidation of benzyl alcohol is a liquid phase reaction without a separate solvent, measure 5 ml of benzyl alcohol (99.8%) and transfer it into a 25 ml three-neck flask, then weigh 10 mg of GMS catalyst and it add to benzyl alcohol.
2. Set up a temperature-controlled oil bath with magnetic stirring to ensure accurate and uniform control of the reaction temperature.
3. Put the flask with benzyl alcohol and catalyst into the oil bath, then set the temperature to 100 °C and stir at 150 r/min.
4. Flow oxygen gas with 99.9% purity into the flask at 2 ml/min controlled by a mass flow controller.
5. When the temperature of the oil bath reaches 100 °C and stabilizes, introduce oxygen gas into the three neck flask.
6. Keep the oxygen flow rate and temperature constant, and allow the reaction to proceed for 6 hr.
7. After the reaction, filter the product with a #1 filtration paper. Collect the liquid phase and transfer an aliquot to a gas chromatography (GC) vial. In the GC vial, mix four parts HPLC grade acetic acid for every one part sample (For example, use 36 µl sample and 144 µl acetic acid.) Put the vial on a gas chromatograph auto sampler for analysis. Wash off the solid precipitate on the filter paper with DI water and ethanol then allow to dry in air. Collect the dried solid with a spatula as recycled catalyst.
8. Repeat the same experiment procedure from step 2.3 through 2.7 with recycled catalyst three times. In each repeat, adjust the amount of benzyl alcohol to match the ratio described in step 2.2.

3. Thermal Treatment of GMS for Testing of Thermal Stability

1. Weigh three separate 300 mg portions of synthesized GMS, and store them in glass vials. These are marked as batch 1, batch 2 and batch 3. Keep batch 1 as control group, and put batch 2 and batch 3 into a furnace for thermal processing.
2. Program the furnace as follows for processing at 400 °C: ramp from 25 °C to 400 °C in 0.5 hr, maintain at 400 °C for 4 hr, allow the sample to remain in the furnace with the door closed until the temperature falls below 40 °C. Put batch 2 in a crucible and start the program.
3. Program the furnace as follows for processing at 650 °C: ramp from 25 °C to 650 °C in 0.75 hr, maintain at 650 °C for 4 hr, allow the sample to remain in furnace with the door closed until the temperature falls below 40 °C. Put batch 3 in a crucible and start the program.

4. Characterization of GMS Materials^21,22

1. On the physisorption instrument, degas GMS materials with the following program: 90 °C for 60 min and then 350 °C for 480 min. Run full-isotherm analysis on the degased materials to obtain physisorption data.
2. Disperse GMS sample on a 200-mesh holey carbon TEM grid and observe the sample under a transmission electron microscope. Restrict magnification under 44,000X to protect the material.
3. Run XRD with Cu Kα radiation (λ = 1.5418 Å). Set tube voltage of 45 kV, and tube current of 40 mA. Collect intensity in the 2θ range between 10° and 90° with a step size of 0.008° and a measuring time of 5 sec at each step.

Results

This method was used to compare the levels of heme synthesis in normal (HBEC30KT) vs. cancer (HCC4017) lung cells. Figure 2 shows a higher level of heme synthesis in cancer cells (HCC4017) than normal lung cells (HBEC30KT). The level of heme synthesis was also measured in normal and cancer cells in the presence of mitochondrial uncoupler carbonyl cyanide 3-chlorophenylhydrazone (CCCP). Cells were treated with 10 μM CCCP for 24 hr before the measurement of heme synthesis levels. As expected, the levels of...

Discussion

Within the synthesis protocol, attention to surfactant concentration, pH of solution and reaction temperature is critical to the successful formation of GMS. The critical steps are 1.2, 1.3, 1.4 and 1.6. The above mentioned parameters control the critical packing parameter and phase of micelles formed from surfactant. The phase and morphology of micelle determines the final state of silica matrix, which serves as the framework for GMS. Also important in the formation process is the sequence and time to add the HAuCl solu...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)	Aldrich	435465-250ML
tetraethoxysilane	TCI	201-083-8
bis[3-(triethoxysilyl)propyl]-tetrasulfide	GELEST	SIB1825.0-100GM
chloroauric acid	Aldrich	520918-1G
benzyl alcohol	Sigma-Aldrich	305197-1L
nitrogen physisorption	Micromeritics	Tristar II
X-ray diffraction	Philips	X'Pert Pro
transmission electron microscopy	Philips	CM200
gas chromatography	Shimadzu	GC-2010