262005 Conversion of Ethyl Acetate Over Acid-Base Catalysts As a Model Reaction for Biooil Refining: Infrared and Flow Reaction Studies
Several processes are under development to convert biomasses into renewable fuels (Huber et al., 2006; Demirbas, 2008). The production processes of bioethanol and “biodiesel” (fatty acid methyl esters) are already commercial, but they need further improvements to reduce their impact to food and agriculture market as well as to reduce their costs. Several other processes, such as pyrolysis and catalytic conversion of different renewable raw materials (wood, vegetable oils, etc.) and also of wastes are under study. A common problem of these processes is the very high oxygen content of the resulting biooils, which prevents their massive use in several countries such as in European union, because it may overcome the limits by law. Limits to oxygen content are due to the need to reduce the amount of toxic oxygenated VOCs in the waste gas and to improve the cold properties. Particularly noxious is the presence of carboxylic acids in the fuel, because of the high melting point of many acids which reduces cold properties, as well as to the corrosion behaviour associated to their acidity. Carboxylic acids frequently come from incomplete conversion of esters, such as in the case of the conversion of triglyceride-based raw materials.
To improve the qualities of biooils as fuels, further catalytic treatments can be performed (Bulushev and Ross, 2011). Conversion in the presence of acidic catalysts is reported to improve fuels reducing their oxygen content. Solid acids are also reported to be active for the conversion trigliceride-based materials, such as palm oil, into hydrocarbon-rich fuels (Twaiq et al., 2003).
In this work, the conversion of ethyl acetate as a model reaction for biooil refining has been investigated onacid-base catalysts. This ester is much shorter than the typical esters present in vegetable oils, however, it contains the important chemical functionality (R'-O-COR) within a short aliphatic chain (Danuthai et al., 2009). The aim of this work is to have indication on the mechanism involved in the acid- or base- catalyzed conversion of esters.
Siralox 1.5/40 from Sasol was used as the catalyst for catalytic tests.This material has typically the Lewis acidic behaviour of transitional alumina(Bevilacqua et al., 2006).
The catalytic experiments were carried out in a fixed-bed tubular quartz flow reactor, operating isothermally, loaded with 0.5 g of the catalyst (60-70 mesh sieved), feeding 12.5% vol/vol Ethyl Acetate (EA) in Nitrogen with total flow rates of either 40 cc/min or 80 cc/min. The carrier gas (Nitrogen) was passed through a bubbler containing high purity EA (99.5%). The temperature in the experiment was varied stepwise from 473 K to 1073 K.
EA conversion was defined as usual:
While selectivity to product i is defined as follows:
where ni is the moles number of compound i, and νi is the ratio of stoichiometric reaction coefficients.
The outlet gases were analyzed by a Gas Chromatograph 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 GC/MS (Thermo Scientific) was used.
For FT IR studies, 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 experimentIR spectra of the surface species as well as of the gas phase were collected upon increasing temperature in static conditions (pEA~ 4 torr). The FT IR studies of Ethyl Acetate adsorption/desorption have been carried out on several catalysts having different acid-base characters such as Siralox 1.5/40, Puralox SBa-200, Siralox 30/260, Siral 30 and Pural MG 30.
Ethyl Acetate conversion over Siralox 1.5/40 catalyst starts to be significant (1.2 %) at 573 K and increases further by increasing temperature up to be complete at 773 K and above at 96.5 h-1GHSV. Following the thermal evolution, the conversion increases with further formation of decomposition products. C2H4, CH3COCH3 and CH3COOHare the major reaction products, while CO, CO2, CH4, C2H6, C3H8, CH3CHO, C2H5OH, CH3COOH, (CH3CO)2O are minority products. The conversion also increases with decreasing space velocity, whereas conversion decreases with extending time on stream, due to deactivation phenomena at the catalyst surface (Figure 1).
Fig. 1. (a) The conversion as a function of flowrate of 40 cc/min and 80 cc/min; (b)Conversion of Ethyl Acetate over Siralox 1.5/40 as a function of time on stream, at 723 K, total flowrate of 40 cc/min.
The FT IR spectra of Ethyl Acetate adsorbed on Siralox 1.5/40 is reported in Figure2. Adsorption at room temperature gives rise to bands due to molecular EA weakly adsorbed at the surface. The band at 1705 cm-1 can be attributed to the C=O stretching mode of ester groups, while the sharp band at 1377 cm-1 is essentially a CH deformation mode of -CH3 group. The weak and broad CH deformation mode of –CH2 falls at 1450 cm-1, whereas the band at 1280 cm-1 is assigned to a COC stretching mode of the ester group. A similar behaviour can be detected following EA adsorption over other catalysts. The increased basic character of the Pural MG30 sample (71.2% w/w Al2O3, 28.8% w/w MgO) allows an even stronger adsorption of acetate species at the catalyst surface resulting in an increased CO2 formation in the gas phase.
Fig.2. FTIR subtraction spectra of surface species arising from Ethyl Acetate adsorbed over Alumina (a: adsorption at room temperature; b: after outgassing at room temperature; c: 323 K; d: 373 K; e: 423 K; f: 473 K; g: 523 K; h: 573 K; i: 623 K; j: 673 K; k: 723 K; l: 773 K)
The gas phase IR spectra of EA decomposed on acid-base catalysts show ethylene, acetic acid, carbon dioxide, water as the main products. During the IR experiments ketene is also detected amongst the product of EA decomposition in IR cell, possibly due to thermal decomposition.
Similar experiments have been carried out also following adsorption and thermal evolution of intermediate reaction products such as acetic acid and ethanol. The key point of ethyl acetate chemistry at the catalytic surface seems to be the cleavage of the ester group, with the assistance of Lewis sites.
This work was supported by the “fund by EMMA in the framework of the EU Erasmus Mundus Action 2”. EF acknowledges the University of Genova “Progetto di ricerca di Ateneo 2011” for funding.
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