슬러리상 CO2 수소화반응을 위한 철촉매계에서 K의 촉진제 영향

염세윤*, 전기원, 홍지숙, 최명재, 이규완

한국화학연구소 화학기술연구부 1팀, *중국과학원 석유화학연구소

Potassium promotional effect on iron-based catalysts for slurry-phase CO2 hydrogenation

Shi-run Yan*, Ki-won Jun, Ji-sook Hong, Myoung-jae Choi and Kyu-wan Lee

Chemical Technology Div. I, Korea Research Institute of Chemical Technology,

*Institute of Coal Chemistry, Chinese Academy of Sciences

1. Introduction

Recently, global warming problem has received increasing attention, and extensive efforts have been paid to explore the possible ways for fixation and disposal of greenhouse gases. Conversion of greenhouse gases to useful chemicals may be one of the promising ways to mitigate the problem. Carbon dioxide is the most important greenhouse gas arising from human activities. Increases in CO2 concentrations contribute over half of the enhanced green house effect, the rest being mainly due to increases in the concentrations of methane, halocarbons and nitrous oxide. Therefore, development of the technologies for hydrogenation of CO2 to valuable chemicals such as hydrocarbons and alcohol is of great significance, which may provide an effective way not only to curtail the growing amount of carbon dioxide emission, but also to improve economic benefits. Previous studies indicated that hydrogenation of CO2 to hydrocarbons proceeds via two consecutive steps. CO2 first reacts with H2 to form CO and H2O, so called reverse water gas shift reaction (RWGS) followed by Fischer-Tropsch (F-T) CO hydrogenation reaction. Since iron-based catalysts are widely used in both WGS and F-T reactions,  iron-based catalysts  may be the potential catalysts for hydrogenation of CO2 to hydrocarbons. On the other hand, numerous studies have shown that the addition of small amount of potassium to iron catalysts used for F-T synthesis affects the performance of such catalysts in a variety of ways. In the present work, effects of potassium promotion on CO2 conversion, reaction selectivity and catalyst stability were investigated. The catalysts were also characterized with BET surface area measurement, X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR) to get some information on the promotional effect of potassium on physical properties and reducibility of the catalysts.

2. Experimental

2.1 Catalyst preparation

The catalyst preparation involved two steps: preparation of the Fe-Cu-Al precursor followed by potassium impregnation. All the catalyst precursor used in this study was prepared in one-batch by continuous precipitation from an aqueous solution of metal nitrates with ammonium hydroxide at a constant pH value. The resulting precipitate was subjected to multiple filtration and washing cycles, then dried in air at 383K for 24 hour. The dried sample was divided into five parts for different amount of potassium loading by incipient wetness technique. After 12 hours impregnation, the impregnated samples were heated at initially 343 K then to 373K to evaporate the remaining water. The samples thus-obtained were moved to oven and dried at 383K for 48 hr, and then calcined at 723K for 5hr.

2.2 Reaction scheme and procedures

In a typical experiment, 15 g of the catalyst (<200mesh) and 200 g of squalane as a reaction medium were loaded into the reactor. Prior to reaction the catalyst was reduced in-situ with N2/H2 (10% of H2) mixture at standard reduction conditions of 533-573K and 81psig. The reason for performing the reduction at an elevated pressure is to minimize the loss of reaction medium during reduction. For the reactions, the flows rates of the reactant gases (H2 and CO2) as well as of internal standard gas (Ar) were controlled by thermal mass flow controllers (LOKAS automation). After flow regulation, the three gases were mixed in a 600-cm3 pressure vessel prior to their entering the reactor. The mixed gases then were introduced into the bottom of reactor through a nozzle tube. After leaving the reactor, the exit gas passed through a high-pressure cold-trap  (275K) to condense liquid products. The non-condensable gases from the cold-trap passed through a back-pressure regulator to reduce the pressure of exit product steam to atmospheric pressure, then divided into two paths: the one vented directly and the other flowed to the on-line gas chromatograph (GC). The flow to the on-line GC sampling valves was kept at 30cc/min. The feed-gas and non-condensable products were analyzed on-line with a DS 6200 GC : Ar, CO2, CO and CH4 were analyzed with a 6-feet carbosphere column and a TCD, while light hydrocarbons (C1-C8) were analyzed with  a 30-meter GS-Q capillary column and a flame ionization detector (FID). Liquid hydrocarbons and water collected in the trap were weighed and analyzed off-line with a Hewlett Packard 5890 series II GC equipped with a SPB -1 capillary column and FID. Because Ar was added as an internal standard, CO2 conversion can be calculated directly by comparison of differences in the CO2/Ar peak ratio in the feed and during the reaction. CO and CH4 flow in the effluent were calculated by using the calibration curve obtained prior to the reaction. The assumption made is that the volume of Ar is the same in the feed gas and in exit gas. The analysis of CH4 was used as the link to correlate the peaks of FID and TCD.

3 Results and discussion

3. 1 Effect of potassium on physical properties and reducibility of the catalyst

Table 1 listed the physical properties of the catalysts. As can be seen in table 1 that precipitated Fe-Cu-Al catalyst showed a high specific surface area (>100m2/g).       Addition of potassium to the catalyst resulted in a slight increase in BET surface area and a decrease in pore volume. With the increasing potassium content, BET surface area increased and passed through a maximum at Fe/K ratio of 100/6 then decreased sharply. The above results can be interpreted as follows: at low levels of potassium loading, potassium may be well dispersed on the surface of Fe-Cu-Al, and the adherence of potassium atom to the precursor surface may increase the total surface area of the catalyst.  When the K: Fe ratio was higher than 6:100, uniform-dispersion was difficult to achieve due to the high concentration of the solution, thus partial plug of micro-pores of the catalyst may occur, so the BET surface area declined naturally. The TPR of the catalysts indicated that two reduction peaks appeared in TPR profiles of all catalysts, as shown in Figure 1. With the addition of potassium and the increase in potassium content in the catalyst, the first peak shifted gradually toward higher temperature and the starting point of the second peak moved to lower temperature direction, which means the potassium addition lead to the retardation of the initial reduction.

Table 1 Physical properties of the catalysts

Cat. No.	#5	# 9	# 6	# 7	# 8
Bulk Fe:K ratio	100: 0	100:2	100:4	100: 6	100:8

BET surface area (m2/g)	136.34	138.29	139.04	165.32	116.21
Pore volume (cc/g)	0.21	0.20	0.20	0.24	0.18
Pore diameter (□)*	60.98	59.23	57.73	59.19	60.28
Surface atomic concentra.(%)** Cu Fe F O K C Cl Al	2.106 16.899 2.549 44.597 27.053 6.796	2.101 16.008 2.928 46.162 1.007 23.753 0.527 7.515	1.489 14.146 3.011 43.318 1.679 29.063 7.294	1.216 15.831 2.977 46.546 2.613 22.280 0.514 8.024	1.427 13.638 3.000 43.102 2.852 29.321 0.576 6.085
Surface Fe::K atomic ratio	100:0	100:6.29	100:11.99	100: 16.51	100::20.91

*: Pore diameter was calculated from BET measurement by D=4V/A

** : XPS results

3.2 Effect of potassium on reaction performance of the catalyst

The reaction results of CO2 hydrogenation over precipitated Fe-Cu-Al-K catalysts containing different concentrations of potassium are summarized in Table 2. It was found that the Fe-Cu-Al catalyst showed a good CO2 conversion activity at the conditions employed. CO and CH4 are the main products. The selectivity to CO and CH4 accounted more than 60 % of the total CO2 converted. As for the hydrocarbon distributions, C1 to C4 took more than 85 % fraction in the total hydrocarbons.

Another point is that hydrocarbons formed on the catalyst without potassium promotion were mainly paraffin. Olefin to paraffin ratios (O/P) was quite low. After small amount of potassium addition, the CO2 conversion did not change obviously, but selectivity to CO and CH4 decreased markedly while the selectivity to heavy hydrocarbons and to olefins increased simultaneously. With the increase of potassium content in the catalyst from Fe:K ratio of 100:2 to 100:6, CO2 conversion increased monotonously while CO and CH4 selectivity showed a reverse trend. The further increase in potassium content led to the CO2 conversion decrease and CO selectivity increase. The potassium loading amount also exhibited influences on hydrocarbon distribution and on O/P ratio. It is found from table 2 that both C5+ selectivity and O/P ratio increased while C1-C4 selectivity decreased with the increasing potassium loading, which is in agreement with the observation in Fischer-Tropsch synthesis. For the stability of the catalyst, a time-on-stream of 100-120 hours was taken for each reaction evaluation. The reaction results showed that CO2 conversion was quite stable for the non-promoted catalyst and for the catalysts with Fe  to K ratio being not higher than 100:4  throughout the investigation period. For the catalyst containing more potassium, catalytic activity began to decrease after 30 hours of time on -stream.  The results above-obtained can be interpreted by the effects of potassium on both reverse water-gas shift reaction and on Fischer-Tropsch reaction.

Fig.1 TPR profiles of the catalysts

Cat No.	5	9	6	7	8
Fe: K (mass)	100:0	100:2	100:4	100:6	100:8
CO2 conv .(%)	13.05	13.18	15.60	16.92	14.44
CO sel (%)	35.48	28.03	22.82	17.97	20.93
CH4 sel. (%)	28.23	16.27	9.97	9.40	8.74
HC distribution (C-at%) CH4 C2H4 C2H6 C3H6 C3H8 C4H8 C4H10 C5+	43.76 0.18 16.93 1.51 15.23 1.05 8.82 12.52	22.09 0.84 12.70 6.24 10.18 2.55 7.31 38.09	12.92 3.25 6.91 10.02 4.63 7.06 4.05 51.16	11.46 4.78 4.14 9.15 3.41 6.43 2.69 57.94	11.05 6.31 2.95 11.34 1.56 7.75 2.42 56.62

Table 2 : Summary of reaction results over catalysts with different K contents

O/P ratio C2 C3 C4	0.01 0.12 0.16	0.06 0.61 0.50	0.47 2.17 1.74	1.16 2.68 2.39	2.14 7.27 3.20

* The results are taken at Time-on-stream of 40hrs; reaction conditions: T=265℃,       P=225psi, GHSV=2.24NL(CO2+H2).h-1.g-cat-1