This chapter gives a brief outline of the experimental techniques used for the

Transcription

1 CHAPTER II Experimental Techniques This chapter gives a brief outline of the experimental techniques used for the characterization of catalysts and the test of catalyst activity in this study. 2.1 Thermo-gravimetric Analysis (TGA) Thermal analysis includes a group of techniques in which a physical property of a substance is measured as a function of temperature, while the substance is subjected to a controlled temperature program [1]. In thermo gravimetric analysis (TGA), the change in weight with time or temperature due to dehydration or decomposition is directly measured. Changes in weight result from the rupture and/or formation of various physical and chemical bonds at high temperatures. From the curves, information about the thermodynamics, kinetics, and the reaction mechanisms can be obtained. TGA on the synthesized hydrotalcites and MSU catalysts was performed using a Dupont SDT If not otherwise mentioned, a heating rate of 10 C /min was used for TGA with a gas flow of 100 ml/min. The samples were heated in a flow of air from room temperature to 100 C and held at this temperature for 60 minutes to remove all physically absorbed water before ramping up to 800 C. 33

2 2.2 Phase Determination by X-ray Diffraction (XRD) This method is useful to determine the crystal structure of solids [2]. Atoms in crystals are arranged in a regular, repetitive manner. When irradiated with a monochromatic x-ray beam, every atom in the crystal scatters the x-ray and acts as the source for an outgoing spherical wave. Interference between these waves leads to the diffraction pattern. Atoms located exactly on the crystal planes contribute maximally to the intensity of the diffracted beam. Atoms located halfway between the planes scatter destructively while atoms at some intermediate location interfere constructively or destructively, depending on their exact location. The scattering power of an atom for the x-rays depends primarily upon the number of electrons it possesses. The location of peaks in an x-ray spectrum is determined entirely by the size and shape of the unit cell of the crystal and by the wavelength of the radiation used. The diffraction condition can be given by Bragg s Law: n λ = d sinθ 2 hkl where n is an integer, d is the interplanar spacing, θis the angle of diffraction, and λis the wavelength. For the hexagonal structure (found for example in MSU and MCM41), the unit cell parameter,, can be calculated using the following equation: a 0 d a = cos30 0 a 0 = 2d 100 / 3 34

3 This parameter together with the pore diameter obtained from nitrogen adsorption/desorption measurement,φ, allows one to calculate the wall thickness, t, as: t = a 0 φ Figure 2-1 Hexagonal structure for MSU and MCM41 The intensities of the diffracted beams depend upon the type of atoms in the crystal and the location of the atoms in the unit cell. As no two substances have absolutely identical diffraction patterns considering the direction and intensity of all diffracted beams, it can be used to identify crystalline compounds. The instrument used was a Siemens D5005 X-ray powder diffractometer equipped with Cu- κ radiation (40kV and 40mA) and variable slits. In order to determine the α crystallite size, a small step size of was used to record the profile of a single peak within the range from 2θ=27 to

4 2.3 BET Surface Area Determination The Brunauer-Emmett-Teller (BET) method is the most widely used procedure for the determination of surface area of solid materials. There are two stages in the application of the BET procedure. First, it is necessary to derive the monolayer capacity, a n m defined as the amount of adsorbate required to form a complete monolayer on the surface of unit mass of the adsorbent. The specific surface area, (BET), is then obtained from by taking a value for the average area, a m a s a n m, occupied by the adsorbate molecule in the monolayer. Thus a ( BET ) = n L a s a m m where L is the Avogadro constant. The simplified BET equation using for the determination of surface area is, P P a n 1 P P0 C = + a a nmc nmc P P 0 Where a n is the weight of gas adsorbed at a relative pressure P/P0. The term C, the BET constant, is related to the energy of adsorption in the first adsorbed layer and consequently, its value is an indication of the magnitude of the adsorbent/adsorbate interactions. Nitrogen (at 77K) is generally considered to be the most suitable adsorbate for the determination of the surface area of nonporous, macroporous, or mesoporous solids. It is usually assumed that the BET nitrogen monolayer is closed packed. The surface area is 36

5 measured by determining the quantity of gas that adsorbs as a single layer of molecules on a sample. This adsorption is done near the boiling point of the adsorbate gas. Under specific condition, the area covered by each gas molecule is known within relatively narrow limits. The surface area is thus directly calculable from the number of adsorbed molecules, which is derived from the gas quantity at the prescribed conditions, and the area occupied by each molecule. The textural properties of synthesized solid catalysts (specific surface area, pore volume and pore size distribution) were obtained from nitrogen adsorption-desorption isotherms measured at 77K with a TriStar 3000 (Micromeritics) porosimeter [3]. Surface areas were calculated by the BET method [4]. The Barrett-Joyner-Halenda (BJH) method was used for deriving the pore size distribution [5]. About 30mg sample was weighed into a sample tube and thoroughly dried under a nitrogen flow at 300 C for 3 hours before measurement. The actual sample weight was recorded after cooling. The sample tubes were put into a bath of liquid nitrogen, and adsorption of nitrogen onto the surface of the sample occurs. The point-wise registration of the entire isotherm is under computer control. A complete isotherm (adsorption and desorption branch) needs about 8~10 hours to complete, depending on the surface area of the samples. 37

6 2.4 Temperature-Programmed Desorption of CO 2 (CO 2 -TPD) A schematic diagram of the apparatus for the temperature-programmed desorption (TPD) is shown in Figure 2-2. TPD of CO 2 was used to assess the strength and the number of base sites on the catalysts. Typically, about 300mg of sample was placed in a Pyrex U-tube in the oven and pretreated in helium carrier gas (99.995%, SOXAL) at 450 C for 2 hours before cooling down to room temperature. CO 2 gas was introduced at room temperature for 40 minutes to make sure all the base sites of the catalysts were saturated. Then, the sample was flushed with helium for another 1 hour before starting a temperature ramp. A heating ramp of 20 C /min to 450 C was used. The evolved gases were analyzed using a quadrupole mass spectrometer (Balzers Prisma QMS200) coupled to the reactor by a differentially pumped interface. Figure 2-2 Schematic diagram or apparatus used for TPD: G1, G2=Pressure Gauge (Granvillc Philips; QMS= quadrupole mass spectrometer (Balzers Prisma QMS200); S=Sample; TC1, TC2=Thermocouples; TP=Temperature Programmer (Eurotherm 818P); V1~V5= Fine-metering values and V6=Metal bellow value (Nupro). 38

7 2.5 Transmission Electron Microscopy (TEM) A TEM works much like a slide projector. In a projector, a beam of light shines through (transmits) the slide. The light transmitted is affected by the structures and objects on the slide. These effects result in only certain parts of the light beam being transmitted through certain parts of the slide. This transmitted beam is then projected onto the viewing screen, forming an enlarged image of the slide. TEM work the same way except that a beam of electrons instead of photons (light) shines through the specimen. Whatever part is transmitted is projected onto a phosphor screen for the user to see. Because electrons are charged particles, they can be easily refracted in a magnetic field. Also, it can be accelerated by an electrical potential. The stronger the potential, the faster the electron will move, and as per the de Broglie relationship, the shorter the wavelength the better the resolution. λ = h mv In fact a typical electron microscope at an accelerating voltage of 20kV would have a wavelength less than 5 picometers. This makes the theoretical resolution about hundred thousands times better than that of light, well worth undertaking the engineering effort. Experimentally, this high resolution can not be realized, but it is possible to resolve objects of atomic dimensions (0.1nm) with a transmission electron. 39

8 Figure 2-3 Structure of TEM The samples were vibrated in ultrasonicator for 30 minutes using ethanol as solvent before introduced to the sample plates. Then, TEM images were recorded by JEOL JEM3010 HR TEM (300kV, magnification 1.2 million). 40

9 2.6 Gas Chromatography (GC) Gas Chromatography (GC) is a technique for separating volatile substances by percolating a gas stream over a stationary phase [6]. This separation technique depends upon the adsorptive properties of the column packing to separate volatile samples, primarily gases. Common packing materials used are silica gel, molecular sieve, and charcoal. The column is the heart of any chromatograph. It is in the column that the individual components contained in the mixture are moved away from each other as they passing through, emerging as individual sample bands that can be detected and measured [7]. There are two basic columns for GC: packed columns and capillary columns (Figure 2-4). For the packed column, the column packing consists of two parts. The first part is the solid support; it is very fine solid material with a high surface area. Chemically it is very inert. Chromosorb (an organic polymer), firebrick and diatomaceous earth are common solid supports. The second part of the column packing is a thin film of a high boiling liquid (the stationary phase) coated onto the solid support particles. As the sample components move through the column, they will dissolve in this liquid and be retarded. When the sample is dissolved in the liquid phase, the sample is stationary and does not move. Sample components with a high solubility in the liquid phase spend more time in the liquid phase, move more slowly through the column, and elute than components that have a low solubility. The other phase, a gas, is called the mobile phase, since it flows continuously through the column. It is also called the carrier gas. Nitrogen, helium, and 41

10 hydrogen are the most common carrier gases. The sample will equilibrate between the moving gas phase and the stationary liquid phase. The equilibrium between the two phases is called a theoretical plate [6]. Capillary columns are a thin fused-silica (purified silicate glass) capillary (typically 10~100 m in length and 250µm inner diameter) that has the stationary phase coated on the inner surface. They can be one of two types; wall-coated open tubular (WCOT) or support-coated open tubular (SCOT). Wall-coated columns consist of a capillary tube whose walls are coated with liquid stationary phase. In support-coated columns, the inner wall of the capillary is lined with a thin layer of support material such as diatomaceous earth, onto which the stationary phase has been adsorbed. SCOT columns are generally less efficient than WCOT columns. Capillary columns provide much higher separation efficiency than packed columns but are more easily overloaded by too much sample. Figure 2-4 Schematics of packed and capillary columns [7] 42

11 The analysis of products was conducted in an Agilent 6890N Gas Chromatography. The column used is HP-5 capillary column (Crosslinked 5% PE ME Silicone), 30m 0.32mm 0.25μm film thickness, maximum temperature is 325 C. A flame ionization detector (FID) was used as detector. The injection of samples (reaction solution including unreacted reactants, products, by products and solvents) was made by a syringe of 1μL and 0.1μL sample was injected each time. The flow diagram of the GC system is shown in Figure 2-5. The temperature program and some analysis conditions are as follows: Injector temperature is 250 C, detector temperature is 300 C and carrier gas is Helium. For analysis of the products in isomerization of α -pinene oxide: initial temperature is 60 C, initial time is 2 minutes, rate 20 C /min and final temperature is 200 C, final time is 0 minute; For analysis of the products in the synthesis of monoglycerides: initial temperature is 160 C, initial time is 0 minute, rate 10 C /min and final temperature is 300 C, final time is 10 minutes. Figure 2-5 Flow diagram of the GC system [7] 43

12 2.7 Pyridine Adsorption IR The pyridine adsorption IR method is widely used to investigate the nature of acid sites. To determine the acidity and type of acid sites, IR pyridine adsorption measurements were carried out. The sample was pressed into a thin self-supporting disk of ~10mm diameter. It was placed in a Pyrex cell with NaCl windows and degassed at 300 C for 2 hours under vacuum. After cooling down to room temperature, an IR spectrum of the pellet was measured. The sample was then dosed with pyridine at 22mbar for 15 minutes and evacuated at room temperature for 1 hour before another IR spectrum was conducted. The difference spectrum of two measurements gives the pyridine adsorption of the sample. Subsequently, the sample was heated in vacuum at 100 C and 200 C for 1 hour at each temperature cooled to room temperature before measuring the IR spectra. A Biorad spectrometer was used for recording the spectra at a resolution of 2cm -1 and 32 scans were averaged. According to Emies [8], the molar extinction coefficient of the band at 1545cm -1 due to pyridine on a Brønsted acid site is 1.67cm/ µ mol and for the band at 1455cm -1 due to coordinately bonded pyridine on a Lewis acid site is 2.22 cm/ µ mol. Hence, the relative ratio of Brønsted to Lewis acid sites (B/L) given by following equation, B = L IA( B) 1.67 IA( L) 2.22 Where IA (B, L) is integrated absorbance of B or L band (cm -1 ) 44

13 2.8 Reference 1. M. E. Brown, Introduction to the Thermal Analysis: Techniques and Applications, Kluwer Academic Publishers, Dordrecht, Boston, J. B. Cohen, L. H. Schwarts, Diffraction from Materials, Academic Press, New York, S. J. Gregg, K. S. W. Sing, Adsorption, Surface Area and Porosity, 2 nd Ed. Academic Press, London, S. Brunauer, P. H. Emmett, E. Teller, J. Am. Chem. Soc., 60 (1939) E. P. Barreta, L. S. Joyner, P. P. Halenda, J. Am. Chem. Soc., 73 (1951) H. M. McNair, Basic Gas chromatography, American Chemical Society, Washington, D. C., R. P. W. Scott, Introduction to Analytical Gas Chromatography, 2 nd ED. Marcel Dekker, Inc., C. A. Emeis, J. Catal., 114 (1993)