Catalyst characterization

The crystallinity and identification of catalyst phases was studied by powder X-ray diffraction (XRD) analysis performed on Rigaku’s Miniflex X-ray diffractometer by means of Ni filtered Cu Kβ radiation (λ = 1.392 Å). The measurements were recorded in the 2θ range of 2 to 80°, scanning at 2° min⁻¹. A beam current of 15 mA and a beam voltage of 40 kV were used for the measurements.

The FTIR spectra of catalysts were recorded in transmission mode on a UATR Perkin Elmer Spectrum Two instrument. Spectra were acquired in the spectral range of 4000–400 cm⁻¹ at a resolution of 4 cm⁻¹ and accumulation of 32 scans. For the analysis of Brønsted and Lewis acid sites in the catalysts, infrared spectra of pyridine adsorbed catalysts (Pyr IR) was acquired in the range of 1600–1400 cm⁻¹. For the pyridine adsorption, the catalysts undergo an hour pretreatment under N₂ at 300 °C for moisture removal followed by adsorption of pyridine at 120 °C for further hour until saturation. The catalysts were then cooled down to 30 °C and pelletized with KBr to record the spectra.

The amount of Pt in all Pt/S-MMT catalysts was quantitatively analyzed by Inductively coupled plasma atomic emission spectrometry (Agilent Technologies-4200MP-AES). The sample preparation includes acid digestion of catalyst (~10 mg in 2 mL aquaregia) at 60 °C and dilution to desired concentration.

Nitrogen adsorption-desorption isotherms were acquired on Quantachrome Autosorb 1 instrument by degassing the samples at 250 °C for 6 h under vacuum and carrying out the analysis at −196 °C under liquid nitrogen. The specific surface areas of catalysts were calculated using the multi-point Brunauer–Emmet–Teller (BET) method. The average pore diameter and pore volumes were measured by adsorption curve analysis using the Barrett–Joyner–Halenda model (BJH) method.

The strength of acid sites in raw MMT, S-MMT and Pt/S-MMT catalysts were determined by NH₃-TPD measurement using Autochem 2910 (Quantachrome) fitted with a thermal conductivity detector. The samples were prepared by outgassing at 200 °C for 1 h under He flow (50 mL min⁻¹). NH₃ gas was passed through the catalyst until complete saturation for about 30 min and then physisorbed NH₃ was removed by passing He (50 mL min⁻¹) for another 30 min. NH₃-TPD analysis for continuous monitoring of desorbed NH₃ was carried out by heating the samples in the temperature range from 100–700 °C at 10 °C min⁻¹ under He flow (50 mL min⁻¹). The volume of desorbed NH₃ was measured using GRAMS/32 software.

The morphology and elemental investigation of the catalysts were performed using a Phenom XL Scanning Electron Microscopy coupled with an Energy Dispersive X-Ray Analyzer (SEM-EDX). Each catalyst sample was deposited on a carbon tape adhered to an aluminum stub prior to analysis by SEM. The micrographs of the catalysts were captured at 10 kV beam voltage and 5,000× magnification using a backscatter electron detector (BSD). The elemental analysis was performed at high resolution (15 kV of beam voltage) and high vacuum pressure (1 Pa) with a secondary electron detector (SED), where the point and mapping analysis for element identification was performed using a Phenom Pro Suite software.

The percentage of platinum dispersion was measured by CO-chemisorption experiments typically carried out in an AutoChem 2910 instrument (Micromeritics) associated with a thermal conductivity detector. Average particle size was also verified during the CO-chemisorption measurements. The experiment begins with reduction of 100 mg of the sample at 300 °C under H₂ flow (50 mL min⁻¹) for 3 h, followed by He treatment at 300 °C for 1 h after which the sample is brought to 30 °C. A number of pulses of 9.96% CO balanced helium were injected in regular intervals over the reduced catalyst to measure the volume of CO uptake by the catalyst.

The skeletal aluminium framework structure of montmorillonite catalysts was examined using ²⁷Al MAS NMR technique performed on DD2 Oxford Magnet AS-500MHz spectrometer (Agilent Technologies) using aluminium oxide (Aldrich) as a reference material. The chemical shifts (δ) are mentioned in ppm.

The microstructure of Pt catalysts was determined by Transmission electron microscopy (TEM) performed on JEOL 2010 electron microscope operating at 200 KV. A thin catalyst suspension is made by ultra-sonicating tiny amount of powder sample in ethanol and a drop of it is dispersed on copper grids. Samples were positioned in the microscope column, at evacuation of less than 1 × 10⁻⁶ Torr.

The thermogravimetric analysis (TGA) of the fresh and spent catalysts was performed on a Perkin Elmer TGA 7 unit. The measurements were performed over a temperature range of 50 °C to 750 °C at 10 °C/min under a constant flow of nitrogen (20 mL/min).