이산화탄소의 수소화에 의한 슬러리 반응기 내에서의  Fe/Cu/Al/K 촉매를 이용한 탄화수소로의 전환에 대한 경향 연구

閻世潤*, 홍지숙, 이상봉, 전기원, 최명재, 이규완

한국화학연구소 화학기술단 1팀

* 中國科學院 石炭化學硏究所

An exploratory investigation on carbon dioxide hydrogenation to hydrocarbons over precipitated Fe/Cu/Al/K catalyst in slurry-phase

Yan S. R.*, Hong  J. S., Lee  S. B., Jun  K. W., Choi  M. J. and Lee K.W

Chemical Technology Div. I, Korea Research Institute of Chemical Technology, P.O.Box 107, Yusong, Taejon 305-600

*Institute of Coal Chemistry, Chinese Academy of Sciences

Introduction

Global warming problem is receiving increasing attention, and extensive efforts are being paid to tackle the greenhouse issues. Conversion of greenhouse gases to useful chemicals is one of the possible ways to mitigate the problem. Carbon dioxide, as a main component of industrial flue gas, is the most abundant one among  greenhouse gases: methane, nitrous oxide and CFC's. So the  development of the technology for hydrogenation of CO2 to valuable chemicals is of great significance, which may provide a promising  way not only to curtail the growing amount of industrial carbon dioxide emission, but also to improve  economic benefits.

Although a number of investigations have been made on the CO2 hydrogenation to hydrocarbons in recent years[1,2], the previous studies were exclusively conducted on fixed-bed reactor and at relatively high temperature (573K).A thermodynamic study showed that CO2 hydrogenation to hydrocarbons is very exothermic(100-125KJ/mol) and should be conducted at temperature not higher than 500-550K[3].

In this study, a slurry-bed reactor system which is known to have a better heat transfer efficiency and many other advantages  compared to the fixed-bed reactor was used, and the precipitated Fe-Cu-Al/K catalyst was employed for the CO2 hydrogenation to hydrocarbons.

Experimental

1. Catalyst preparation

The  catalyst preparation involved two steps: preparation of the Fe-Cu-Al precursor followed by potassium impregnation. The constant pH precipitation technique was used to prepare Fe-Cu-Al precursor. After potassium impregnation by incipient wetness technique, the catalyst was dried at 383K for 48 hr, and then calcined at 723K for 5hr. The powder so obtained was crushed and sieved to the required  particle size.

2. Temperature-programmed reduction(TPR) of the catalyst

Reduction of catalyst in-situ is preferable for most of the process. But for slurry-phase  reactor  the catalyst is required be reduced at the temperature much lower than the boiling point of the  reaction medium employed. In order to get the reduction feature of the catalyst and to check whether the Fe-Cu-Al/K catalyst is suitable for in-situ reduction, thermal gravimetric analysis (TGA) of the catalyst during temperature programmed and isothermal reduction were carried out on a SETARAM  TGDTA 92  instrument.

3. Reaction procedures and product analysis

The  hydrogenation reaction were conducted in a 500cm3 continuous stirred tank reactor(CSTR). 15 gram of the catalyst (<200mesh) and 200 gram of the reaction medium of squalane were loaded into the reactor. Prior to reaction the catalyst was reduced in-situ with N2/H2 mixture at 533K and 81Psig. The reduction process was monitored by the on-line GC to determine the optimum reduction time, in which the same N2/H2 mixture was used as reference gas for the detecting of H2 consumption. For the reaction, the flow rate of the reactant H2 and CO2 as well as of internal standard of Ar were controlled by a thermal mass flow controller (LOKAS automation). After flow regulation the three gases were mixed in a 600cm3 pressure vessel prior to entering the reactor. After leaving the reactor, the exit gas passed through a high pressure ice-trap to condense liquid products. The non-condensable gas from ice-trap passed through a back-pressure regulator, then divided into two paths: the one went to vent and another went to on-line gas chromatograph (GC).

The feedgas and non-condensable products were analyzed on-line periodically with a DS 6200 gas chromatography instrument of which Ar, CO2, CO and CH4 were analyzed with a 6-feet carbon sphere column and a thermal conductivity detector, while light hydrocarbons (C1-C8) were analyzed with  a 30-meter GS-Q capillary column and a flame ionization detector. Liquid hydrocarbons and water collected in the trap were weighed and analyzed off-line with a Hewlett Packard 5890 series II gas chromatograph equipped with a SPB -1 capillary column and a flame ionization detector.

Results and discussion

1. Reducibility of the catalyst

TPR profiles of the catalyst are shown in Figure 1. It can be seen that there are  four weight loss peaks in TG-DTA spectra. The first peak should be  attributed to the de-moisture of the catalyst. The ensuing three peaks seems to be formed by the reduction of the catalyst, which is in good accordance with the three-step reduction mechanism of Fe2O3 : Fe2O3→Fe3O4→FeO→Fe[4]. Due to the introduction of the reduction promoter of Cu, the initial reduction temperature was decreased to about 413K. Another point that can be obtained from figure 1 is that the catalyst can be  reduced at about 523K to FeO which is reported to be responsible for the initiation of CO hydrogenation[5]. Based on the above obtained results, the in-situ isothermal reduction of the catalyst with N2/H2 mixture were conducted on slurry-bed reactor at 533K and 81Psig. The consumption of H2 was  monitored and curved continuously (Figure 2). It can be found from the reduction curve of the catalyst that reduction process of the catalyst can be finished  at 533K within  8 hours. This indicates that the catalyst is suitable for in-situ reduction.

2. Reaction performance of the catalyst

CO2 conversion, CO selectivity, light hydrocarbon (HC) distribution and  olefin/paraffin (O/P) ratio results over precipitated Fe/Cu/Al/K catalyst at different temperatures are shown in table 1.

Table 1. CO2 hydrogenation  performance over Fe/Cu/Al/K catalyst at                  different  temperatures

Temp. (K)	CO2 conv. (%)	CO selectivity (%)	Light HC distribution (%, based on Carbon)					O/P ratio (molar)
			C1	C2	C3	C4	C5-C8	C2	C3	C4
533	13.05	35.37	21.45	16.55	20.37	14.99	26.69	1.81	3.08	2.41
543	16.36	26.71	17.73	17.74	22.24	16.93	25.71	2.60	7.28	4.59
553	17.77	25.26	16.21	17.77	22.56	17.59	25.88	3.08	5.68	11.27

P=240psig, W/F=15g.h/mol,H2/CO2=3/1

It can be seen  from table 1 that CO2 conversion  and O/P ratio increase with the increase in reaction temperature; CO and  CH4 selectivity  show a reverse trend. As for the light hydrocarbons' distribution, C2-C4 fraction shows the same response to temperature as that for CO2 conversion.

It is widely accepted that CO2 hydrogenation over iron-based catalysts proceeds via two sequential steps: reverse water gas shift reaction (RWGS) which converts CO2 to CO, and the subsequent CO hydrogenation to hydrocarbons(Fischer-Tropsch reaction), with CO being the intermediate. So the CO selectivity depends not only on the CO2 conversion (RWGS) rate but also on the  CO hydrogenation (F-T reaction) rate. It is well-known that reaction rate should increase with temperature; for a reversible reaction, increasing temperature would favor the reaction toward endothermic direction. It is reasonable that CO2  conversion increases with increasing temperature as observed in table 1, because RWGS is an endothermic reaction. A decreased selectivity to CO with increasing temperature indicates that influence of temperature on  CO hydrogenation  reaction is more pronounce than on the CO2 hydrogenation under the conditions employed. Meanwhile, because the enhanced CO and CO2 hydrogenation  would consume more  adsorbed H species on the catalyst surface, which make the  adsorbed H/C ratio relatively lower. Consequently, further reaction of olefin hydrogenation was inhibited, so an increased O/P ratio and decreased CH4 selectivity were observed. With regards to the HC distribution,  it is easily-understood that selectivity toward C2-C4 fraction increased with  temperature, because Fischer-Tropsch polymerization  reaction is  an exothermic reaction, increase in reaction temperature always shifts the product towards lower carbon number hydrocarbons.

References

1.Pyoung-ho Choi; Ki-won Jun; Soo-jae Lee; Myuong-jae Choi and Kyu-wan Lee, Catalysis Letter 40(1996), 115-118.

2. Jyh-fu Lee; Wen-shing Chern and Min-dar Lee, The Canadian Journal of Chemical Engineering, 70(1992), 511-515.

3.Paushkin,Ya.M; Zhorov,Yu.M.; Lapidus, A.L.; Gorlov,E.G., Dokl.Akad.Nauk.SSSR .298(2) 1988, 374-8.

4.Mehul D. Shroff; Dinesh S. Kalakkad; Kent E. et al., Journal of Catalysis, 156(1995), 185-207.

5.Dragomir B. Bukur;Manoj Koranne; Xiaosu Lang et al., Applied Catalysis, A: General 126(1995), 85-103.

Figure. 1 TPR profiles of the catalyst.

Figure.2 Reduction curve of the catalyst (reduction condition : T=533K, P=81psi, W/F=15g-cat·h/mol(N2/H2), H2 content in the mixed gas 10vol%)