Characterization Techniques

2.2.1 TPD-TGA measurement

Simultaneous temperature programmed desorption and thermogravimetric analysis (TPD- TGA) measurements was found to be one of the most useful methods available for the initial characterization of the catalysts. The system consisted of a CAHN 2000 microbalance mounted within a vacuum chamber that could be evacuated with a diffusion pump to a base pressure of 1 × 10−5 _{Pa. When evacuated, the partial pressures above the sample could be monitored using an}

SRI quadrupole mass spectrometer (RGA100). A schematic diagram of the TPD-TGA instrument is shown in Figure 2.1.

Figure 2.1 Schematic diagram of the TPD-TGA instrument.

In a typical TPD-TGA measurement, the samples were heated in a vacuum to 823 K and cooled in a vacuum to remove any adsorbed water prior to exposing them to a few hundred Pascal

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of adsorbate of interest at room temperature. After the sample had been exposed to the adsorbate of interest at room temperature, the sample can be evacuated by a diffusion pump, which ensures that the partial pressure above the sample is low enough to minimize secondary reactions. The sample was evacuated for 1 h before the TPD-TGA experiment was started. During TPD-TGA measurements, the sample weights were recorded continuously using the microbalance, which has an accuracy of 0.01 mg and allows the measurement of weight changes as small as 0.01%. The TPD spectra were obtained by ramping the sample temperature and monitoring the partial pressures in the system using the high sensitivity mass spectrometer. The heating rate during desorption was maintained at 10 K/min by a temperature controller. From the TPD-TGA results, one can quantify the desorption amount and determine whether there is a reaction on the sample during desorption.

2.2.2 Calorimetric measurements

Calorimetry is a direct method for measuring heats of adsorption for gaseous adsorbates on acid sites.45 _{Our home-built, Tian-Calvet calorimeter was constructed from five, 2.54 cm square,}

thermalflux meters (International Thermal Instrument Company, Del Mar, CA) placed between a cubic, Pyrex sample cell and a large Al block. The thermopiles had previously been calibrated by passing current through a Pt wire placed between the sample cell and the thermopiles. During the measurement, 1 g zeolite samples, pressed into wafers and placed at the bottom of the Pyrex cube, were covered with quartz chips in order to prevent heats loss due to radiation out the top. The samples were evacuated using a mechanical pump and heated to 573 K overnight in a vacuum to remove water. After cooling, the adsorption experiments were performed at 195 K, a temperature that was maintained by placing the Al-block heat sink in a Styrofoam container with dry ice. The zeolites were then exposed to the gaseous adsorbate of interest from a calibrated GC sample loop. The uncertainty in our measurement of each point was ∼2%, corresponding to ∼0.5 kJ/mol; however, the uncertainty in our calibration factor is larger, so that the uncertainty in the absolute

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values of the differential heats is ∼1 kJ/mol. The adsorption isotherm was obtained simultaneously with the adsorption heats by measuring the amount of gas remaining at equilibrium with the partial pressure above the sample. A schematic diagram of the calorimeter instrument is shown in Figure 2.2.

Figure 2.2 Schematic diagram of the calorimeter instrument.

The data from calorimetric measurements are most reliable when the measurements are performed at the lowest temperatures for which adsorption is reversible. When there is a distribution of site strengths, low temperatures favor having molecules adsorb preferentially in the strongest sites. This is important when the difference between strong and weak sites approaches kBT.

However, adsorption reversibility is critical in order to avoid chromatographic adsorption in the sample and to ensure that adsorption comes to equilibrium. If the measurement temperature is too low so that adsorption is not reversible, the adsorbate will not be able to sample all possible sites. Each gas dose will saturate different parts of the sample, leading to differential heats of adsorption that are constant with coverage, even if the sites are not identical.46 _{Therefore, different}

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verified by comparing isotherms obtained by adsorption and desorption of the sample at a certain temperature.

2.2.3 Fourier Transform Infrared Spectroscopy

Fourier Transform Infrared Spectroscopy (FTIR) is another useful technique to analyze the adsorption properties on the catalyst surface. In this thesis, the FTIR spectra were collected on a Mattson Galaxy 2020 FTIR with a diffuse-reflectance attachment (Collector II) purchased from Spectra-Tech Inc. Our spectrometer allows measurements on powdered samples with control over temperature and atmosphere pressure.

In a typical measurement, the samples were initially heated to 573 K in He that was flowing at 0.5 cm3_{/s to remove any water. After cooling the samples to room temperature, spectra were}

collected at 4 cm−1 resolution. Comparing FTIR spectra before and after adsorption of certain probe molecules provides a convenient way to characterize solid acid catalysts.

2.2.4 XRD

Powder X-ray Diffraction (XRD) was used to determine the structures and the chemical phase composition of the synthesized zeolites. The XRD patterns were recorded on a Rigaku Smartlab diffractometer equipped with a Cu Kα source (λ=1.5405 Å). The sample powders were finely dispersed in 2-propanol by sonication and then drop-cast onto glass slides. The intensities of the diffracted beam were measured while sampling different diffraction angles. The crystallite particle size (d) could be estimated using the Scherrer equation:

d = κ λ

B(2𝜃)𝑐𝑜𝑠𝜃 (Eqs. 2.1)

where κ is a shape factor usually equal to 1, λ is the X-ray wavelength, B is the peak width at half the maximum intensity, and θ is the diffraction angle.

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