3.2. Characterization of Supports and Catalysts
Thermogravimetric analysis was performed with a STA-449C (Netzsch, Selb, Germany) combined thermogravimetric and differential scanning calorimetric (DSC) analyzer coupled with an Aeolos QMS 403 quadruple mass spectrometer (Netzsch, Selb, Germany) in a temperature range of 30–1000 °C at a heating rate of 10 °C. min−1 in a flow of argon. The concentrations of aluminum hydroxide phases were calculated from the amount of water released in their dehydration.
Scanning electron microscopy was performed on an EVO 50 XVP (Carl Zeiss, Oberkochen, Germany) electron microscope.
The elemental composition of the catalysts was determined by X-ray fluorescence spectroscopy on a Clever C31 instrument (ELERAN, Elektrostal, Russia).
Powder X-ray diffraction measurements were carried out using a DRON-2 diffractometer (Burevestnik, Saint Petersburg, Russia). The patterns were obtained using CuKα radiation and graphite monochromator (λ = 1.54187 Å) at 30 kV and 15 mA. The identification of different crystalline phases in the samples was performed by comparing the data with the Joint Committee for Powder Diffraction Standards (JCPDS) files. The crystallite size of the boehmite phase was calculated using the Selyakov–Scherrer Equation. The error in determining the crystallite size was 10%.
Specific surface (Ssp) and pore volume (Vp) of samples were determined from the N2 physisorption measurements at 196 °C using an universal Autosorb-iQ analyzer (Quantachrome, Boynton Beach, FL, USA). Prior to measurement, the sample was outgassed for 1 h at 150 °C (for boehmite precursor) or for 3 h at 300 °C (for alumina supports and catalysts). Ssp and Vp were calculated according to the Brunauer-Emmett-Teller method. The pore diameter distribution was calculated by the desorption branch of isotherm using the standard Barrett–Joyner–Halenda method.
The 29Si MAS NMR spectra of supports were recorded at room temperature on an Avance 400 spectrometer (Bruker, Ettlingen, Germany) operating at frequencies of 79.5 MHz with spectral resolution 48.83 Hz. The sample rotation frequency was 5 kHz.
The IR spectra of supports were measured on a VERTEX 70 (Bruker, Ettlingen, Germany) instrument fitted with a mercury−cadmium−telluride detector. The measurements were done in transmission mode using a Harrick high temperature cell. A background spectrum and the spectra were measured at 480 °C and a residual pressure of less than 10−3 mbar with a resolution of 1 cm−1 and averaged by 128 scans. For the IR analysis, samples were prepared in a tablet-shape of 20 mg; optical density was 20 mg·cm−2.
Hexavalent chromium concentration in the catalyst was determined by dissolution of Cr(VI) in sulfuric acid and subsequent volumetric titration with the iodometric method.
UV-Vis diffuse reflectance spectra of the catalysts were recorded using a V-650 spectrophotometer (Jasco, Tokyo, Japan) equipped with an integrating sphere ISV-722 (Jasco, Tokyo, Japan). A BaSO4 plate was used as the reference. Spectra were recorded in the wavenumber range 12,500–50,000 cm−1 with the spectral resolution 2 nm. UV-Vis-spectra were deconvoluted into Gaussian bands to determine the positions and intensities of the bands’ maximums.
Raman spectra of catalysts were recorded using a dispersion Raman-microspectrophotometer Nicolet Almega XR (Thermo Fisher Scientific, Waltham, MA, USA). The 532 nm line of a Nd-YAG laser was used as an excitation. The spectra were recorded in the wavenumber range 100–1100 cm−1 with the spectral resolution 2 cm−1. Each spectrum was received by averaging 10 exposures on 10 s.
EPR measurements were made at the temperature of −196 °C on a RE-1306 EPR-spectrometer (Institute of Analytical Instrumentation of Russian Academy of Sciences, Saint Petersburg, Russia) with a working frequency 9.37 GHz and 100 kHz magnetic field modulation. Diphenylpicrylhydrazyl (g = 2.0036) was used as reference for g-value determination.
NH
3-TPD and H
2-TPR measurements were carried out on the ChemBET Pulsar TPR/TPD (Quantachrome, Boynton Beach, FL, USA). Before NH
3-TPD measurement the sample was degassed at 600 °C for 2 h in a helium flow. The adsorption step is carried out in an ammonia flow at 100 °C for 30 min. Then the physically sorbed ammonia was removed with helium at 100 °C for 30 min and the sample was cooled to a room temperature in the helium flow. Temperature programmed desorption was performed from room temperature to 700 °C at a heating rate of 10 °C·min
−1. The strength of the acid sites was evaluated by the temperature of ammonia desorption [
44]. A temperature of 175 °C corresponds to ammonia desorption energy of 100 kJ∙mol
−1 and a temperature of 380 °C to the desorption energy 150 kJ∙mol
−1. Acid sites with ammonia desorption energy lower than 100 kJ∙mol
−1 were attributed to weak ones, while the sites with desorption energies of 100–150 kJ∙mol
−1 and higher than 150 kJ∙mol
−1 were attributed to medium and strong sites respectively. The number of weak, medium and strong acid sites was calculated from the area under the NH
3-TPD profiles in the temperature ranges <175 °C, 175–380 °C and >380 °C respectively.
Before H2-TPR measurement the catalyst was heated to 650 °C and held at this temperature for 60 min in a flow of a gas mixture (5 vol% O2 + 95 vol% N2). Then the catalyst was cooled down to room temperature in the helium flow. Temperature programmed reduction was performed from room temperature to 700 °C at a heating rate of 10 °C·min−1. NH3-TPD and H2-TPR profiles were Gauss fitted using the TPRWin software (version 3.52, Quantachrome, Boynton Beach, FL, USA, 2012).
3.3. Catalyst Testing
The catalysts were tested in the reaction of isobutane dehydrogenation in a steel fixed bed reactor of 10 mm internal diameter under atmospheric pressure. An amount of 2 g of fresh catalyst (sieve fraction 40–200 μm) was filled into the reactor. The catalyst was heated at 5 °C·min−1 to 650 °C in an air flow (60 mL·min−1) followed by flushing with air for 30 min at the same temperature. The catalyst was cooled in air to 570 °C and flushed with argon for 15 min at that temperature. Then a mixture of 30 vol% C4H10 in Ar was fed at a rate of 60 mL·min−1 at the same reaction temperature. The reaction was run for 130 min followed by catalyst regeneration in an air flow for 60 min at 650 °C. The regeneration/dehydrogenation cycles were repeated three times.
The hydrocarbon composition of feed and reaction products were analyzed by gas chromatography on a GH-1000 instrument (Chromos, Dzerzhinsk, Russia) with a flame-ionization detector and a capillary VP-Alumina/KCl column (VICI Valco, Houston, TX, USA). The concentrations of H2, CH4, and CO were determined with the use of a column filled by 13× molecular sieves on a GH-1000 apparatus with a thermal conductivity detector.
Based on of the results of chromatographic analysis the rates of i-C
4H
10 dehydrogenation and cracking of hydrocarbons were calculated using Equations (6) and (7), respectively.
where
X(iC
4H
10) is the isobutane conversion, %;
F is the feed rate of isobutane, mole·h
−1;
mcat is the weight of catalyst, g;
CCH4,
CC2H6,
CC2H4,
CC3H8,
CC3H6 are the concentrations of methane, ethane, ethylene, propane, propylene respectively in the reaction products, vol%;
Voutlet is the volumetric flow of the reaction products, mL·h
−1.