(678a) The Acidity Of Aluminas and Silica-Aluminas: Catalytic Activity In The Conversion Of Ethanol

Abstract

The properties of a number of commercial aluminas and silica-alumina have been characterized by XRD, TEM, Surface area and porosity analysis, skeletal IR and Raman spectroscopies, IR studies of adsorbed probe molecules (hydroxy groups, pyridine, CO, CO₂). The catalytic activity of these materials in the dehydration of ethanol to ethylene and diethylether has been studied in a flow reactor as well as in an IR cell. The catalytic activity data have been discussed in relation to the surface chacarterization data. This allowed us to crytically discuss the most popular models for the surface sites of alumina and silica-alumina. 

1. Introduction

Transitional aluminas are among of the most used materials in any field of technologies. However, the details of the crystal structure of the most common polymorph, γ-Al₂O₃, are still matter of controversy, being a defective non stoichiometric spinel [[1],[2]] or a different structure with the occupancy of non-spinel sites [[3]-[4],[5],[6]]. γ-Al₂O₃ starts to convert at ca 500 °C towards other transitional phases: however also the sequence of alumina phases obtained upon calcination is object of disagreement [[7]]; while some authors report the direct conversion of γ-Al₂O₃ into θ-Al₂O₃ near 650 °C, most studies find the formation of slightly different intermediate phases, closely related to that of spinel and of γ-Al₂O₃, formed continuously in the range 500-650 °C. Also in this case, different spinel tetragonal superstructures, such as those denoted as δ-Al₂O₃ [[8]] and γ’-Al₂O₃ [[9]], have been determined by different authors. Only above ca 650 °C the crystal chemistry of alumina becomes well established. In fact, at this temperature θ-Al₂O₃ is formed, whose “beta-gallia” structure is completely determined, that further transforms into α-Al₂O₃, corundum, completely above 1200 °C.

γ-Al₂O₃ and related structures are found in many industrial applications as catalysts and as catalyst supports. Most of reactions performed on alumina are performed in the 200-600 °C temperature range, and imply the use of materials whose real structure is not still established. Several techniques, such as theoretical calculation [1] and selective poisoning [2] are used to propose reliable models for its surface active sites, and, consequently, also detailed mechanisms for the reactions.

Aluminas are used today as the catalyst of the Claus process and for the synthesis of dimethyl ether and methyl chloride from methanol. They have been used in the sixties for producing ethylene from dehydration of bioethanol at temperature > 250 °C. This reaction can become useful again to produce bioethylene from bioethanol.

2       Experimental.

2.1. Catalysts

Four commercial alumina and one silica-alumina samples were investigated (see Table 1).

Table 1. Properties of investigated catalysts

* data from the producers

 2.2. TEM

TEM analysis of samples, prepared in aqueous suspension in an ultrasonic bath, were recorded with a Zeiss EM 900 instrument.

2.3. XRD

The XRD analyses have been performed on a Siemens D-500 Diffractometer (Cu K radiation, Ni filter; 30 mA, 40 KV) equipped with the Diffract AT V3 software package.

2.4. Infrared spectroscopy (IR) experiments

2.4.1. Skeletal studies.

The characterization of catalysts was studies using Nicolet 380 FT-IR spectrometer. The samples were pressed into thin wafers with KBr and spectra where recorded in air.

2.4.2. Pyridine adsorption

The acidity measurements were determined by pyridine adsorption in an in-situ IR-cell using Nicolet 380 FT-IR Spectrometer. The pure powders were pressed into thin wafers and activated in the IR cell connected with a conventional gas-manipulation apparatus at 773 K for 1 h. After contact of activate catalyst samples with pyridine vapor (p_Py~ 1 torr) at room temperature during 15 min, the IR spectra of the surface species were collected  in continuous evacuation at room temperature and upon increasing temperature.

2.4.3. CO adsorption

The CO adsorption/desorption process has been studied by Nicolet 380 FT-IR spectrometer. CO adsorption was performed at 130 K (real sample temperature measured by a thermocouple) by the introduction of a known dose of CO gas inside the low temperature infrared cell containing the previously activated wafers. The sample was saturated with CO using sufficiently high CO pressure (up to 20 Torr), until the intensity of the adsorbed species has raised a maximum. IR spectra were later collected evacuating at increasing temperatures between 130 and 273 K.

2.4.4. Ethanol adsorption

In order to study the mechanism of reaction, the adsorption/desorption process has been studied using Nicolet 380 FT-IR Spectrometer. Pressed disks of the pure catalyst powders were activated in-situ in the IR cell connected with a conventional gas-manipulation apparatus before any adsorption experiment. IR spectra of the surface species as well as of the gas phase were collected upon increasing temperature in static conditions (p_EtOH~ 4 torr).

2.5. Surface area and porosity

The BET surface areas and porosity have been measured with a Micromeritics Gemini 2380 instrument.

2.6. Catalytic experiments

Catalytic experiments have been performed at atmospheric pressure in a tubular flow reactor using 0.5 g catalyst (60-70 mesh sieved) and feeding 7.9% vol/vol ethanol in nitrogen with total flow rate 80 cc/min. The carrier gas (nitrogen) was passed through a bubbler containing ethanol (96%). The temperature in the experiment was varied stepwise from 423 K to 723 K.

Ethanol conversion was defined as usual:

X_EtOH = (n_EtOH(in) – n_EtOH(out))/n_EtOH(in)

While selectivity to product i is defined as follows:

S_i = n_i/(n_i(n_EtOH(in) – n_EtOH(out)))                                

where n_i is the moles number of compound i, and ν_iis the ratio of stoichiometric reaction coefficients.

The outlet gases were analyzed by a gas chromatograph (GC) Agilent 4890 equipped with a Varian capillary column “Molsieve 5A/Porabond A Tandem” and TCD and FID detectors in series. In order to identify the compounds of the outlet gases, a gas chromatography coupled with mass spectroscopy (GC-MS) Thermo Scientific with TG-SQC column (15 m x 0.25 mm x 0.25 mm) was used.

Acknowledgements

TKP acknowledges funding by EMMA in the framework of the EU Erasmus Mundus Action 2.

References