II.2.5. Catalytic performance - Carbon dioxide hydrogenation reaction

Catalytic tests were performed in a Microactivity-Reference unit (PID ENG&TECH). 150 mg catalyst was mixed with inactive SiC (Prolabo, 0.5 mm) and placed in a tubular fixed-bed reactor (305 mm long, 9 mm i.d., 316-L stainless steel) up to the volume of approximately 0.5 mL. The temperature was measured by a thermocouple in a direct contact with a mixture of 10 vol% H2/Ar at 300 ˚C for 1.5 h under atmosphere pressure and then increased the pressure to 3 Mbar to re-reduced catalyst for 0.5 h. Afterwards, temperature was decreased to 240 ^oC in pure hydrogen and the catalyst was exposed to a reactant gas mixture CO­­₂/H₂ = 1/3 for ca. 1h and then was the reaction temperature increased to 250 ^oC. After further ca. 15 h reaction, the reaction temperatures was consecutively increased to 260 ˚C and 250 ^oC and maintained for ca. 5 h at each temperature. One more experiment was conducted over Cu/ZnO-2 under 4.5 Mbar with the same other parameters. The products were analyzed on-line with a GC-MS analysis system (Varian 450-GC equipped with a methanizer and TCD and FID detectors).

				300°C			300°C
				1.5 h		0.5 h	0.5 h		injection CO2+H2 (1:3)
			250°C					240°C		250°C
			20 min			increasing		20 min		1 h
		200°C				pressure
		20 min							30 min
	180°C
	20 min
25°C
10min
1 bar							30 bar
Catalyst reduction process H2 + N2 ( 1 : 1 )

Figure 6: Catalyst reduction conditions.

Before hydrogenation reaction all catalysts were reduced in a mixture of 1:1 (molar ratio) of nitrogen and hydrogen in two consecutive procedures. First, reduction was carried out at 1 bar pressure and temperature increased from room temperature to 300 °C, and then at 30 bar and temperature decreased from 300 °C to 250 °C and then was started injection process. Carbon dioxide and hydrogen with molar ratio 1:3 were injected and catalyst was ready to reaction time as shown in consecutive procedures (Figure 6).

				270°C						270°C
				5 h	0.5 h					5 h
			260°C						260°C
			5 h		increasing				5 h
250°C					pressure		250°C
15 h							15 h
30 bar						45 bar
Carbon dioxide hydrogenation reaction CO2 + H2 + N2 ( 1 : 3 : 1 )

Figure 7: Carbon dioxide hydrogenation conditions.

Figure 7 show the conditions of carbon dioxide hydrogenation carried out as follows:. First at 30 bar and temperatures 250 °C, 260 ^oC and 270 °C during 25 h and then at 45 bar pressure at similar temperatures. The procedure were analyzed with a gas chromatography.

On the output from reactor was gas chromatography device witch detect product from this reaction and gave information used for calculation.

Conversion of CO₂ was calculated from equation (1).

CO₂ conversion [%] = [Σ(n_i M_i) / M_CO2 ] 100 (1)

where n_i is the number of carbon atoms in product i, M_i – is the percentage of product i and M_CO2 is the percentage of CO₂ in the syngas feed.

The selectivity (S) to a product was based on the total number of carbon atoms in the products and defined as:

S_i = [ (n_i M_i)/ Σ(n_i M_i)] 100 (2)