An Introductive Study about CO 2 Hydrogenation into Hydrocarbons Using Iron Catalysts

CO2 hydrogenation reaction was performed on precipitated iron catalysts which were promoted by Si, Zn, K and Cu. The optimum SiO2 content in the catalysts is about 15 wt% relative to Fe2O3 mass. With reaction temperature raised, CO2 conversion is increased continually, but CO and CH4 selectivity only fluctuate in a narrow range which is beneficial to the synthesis of C2+ hydrocarbons. Two kinds of catalyst filling constitution were experimentally compared in order to increase the yield of C5+ hydrocarbons.


Introduction
Greenhouse effect is recognized as one serious threat to the environment.It would increase the global average temperature and make the sea level raised [1].As the results, the land supporting billions of people would be submerged.CO 2 is a main factor for the heightening greenhouse effect, because it contributed about 64% to the total radiative forcings from the long-lived and well mixed greenhouse gases (CO 2 , CH 4 , N 2 O and halogenated compounds) in 2011 [2].Over 32.1 Gt CO 2 was emitted from fossil fuel combustion in 2013 [3].To lessen fossil fuel consumption is a direct and effective method to decrease CO 2 emission, however, it can only be realized when sustainable fuels are supplied to replace the fossil fuels.Methanol, dimethyl ether and hydrocarbons are the products of CO 2 hydrogenation [4]- [12] which are able to be used directly as fuels.
In case H 2 is generated from renewable energy source such as biomass or solar energy [4] [5] [13], the process of CO 2 hydrogenation is CO 2 -neutral besides to supply sustainable fuels.Such process will solve fuel lack of isolate islands by recovering CO 2 from seawater [14].
On iron catalyst, CO 2 can be hydrogenated into hydrocarbons by two steps as shown in the following Equation ( 1) and (2) [9] [10] [11].CO 2 is converted into CO by reverse water-gas-shift (WGS) reaction, and then the produced CO is further hydrogenated to hydrocarbons by Fischer-Tropsch (FT) reaction.
( ) WGS reaction is thermodynamically favored at low temperature [15].Therefore, reverse WGS reaction is favored at high temperature, i.e. high temperature can promote CO 2 conversion to hydrocarbons via the series of Equations (( 1) and ( 2)).In order to find iron catalyst more active for CO 2 hydrogenation, we researched the influences of Zn, K and Cu on precipitated iron catalysts at low temperature (230˚C) [12] [16] [17].
The functions of Zn and K to activate CO 2 into CO were explained based on the experimental data from CO 2 temperature-programmed desorption [16].Recently, the catalysts were evaluated at increased reaction temperature, and the catalyst filling constitutions in the reactor were compared in order to increase hydrocarbon yield as high as possible.

Catalyst Preparation
All the used reagents are analytical grade (Sinopharm Chemical Reagent Co., Ltd.).
Catalysts were prepared in two methods.Method I was to precipitate a mixed solution of Fe(NO 3 ) 3 and potassium silicate with (NH 4 ) 2 CO 3 solution at 50˚C and pH = 6.5 ± 0.5.Method II was to precipitate Fe(NO 3 ) 3 solution with the mixed solution of (NH 4 ) 2 CO 3 and potassium silicate at 50˚C and pH = 6.5 ± 0.5.Then, the precipitate was washed with distilled water and centrifuged at 4500 RPM for three times after 1 h ageing at room temperature.Promoters of Zn, K and Cu were impregnated onto the precipitate with Zn(NO 3 ) 2 , KNO 3 , and Cu(NO 3 ) 2 solution.After the catalyst precursor was spray-dried at 250˚C and calcined at 360˚C for 6 h, it was shaped into particle of 80 -150 μm for activity test.The obtained catalysts are expressed as ZlKmCn/FSr-I or ZlKmCn/FSr-II according to the method to introduce SiO 2 .In the above abbreviation of catalysts, Z, K, C, F, and S represent Zn, K, Cu, Fe, and SiO 2 , respectively.L, m, n, and r are the nominal mass percent of corresponding materials relative to Fe 2 O 3 .

Characterizations
BET surface area, pore volume and average porediameter of the catalysts were measured using ASAP-2020 from Micromeritics at −196˚C.The crystal structure of catalysts was analyzed by PNAlytical diffractometer (X'Pert Pro) with a Cu Kα radiation source (λ = 0.15406 nm).

Activity Test and Product Analysis
The activity of catalysts was tested in a stainless steel fixed bed reactor [12].The catalyst diluted with four-fold quartz particle of equal size was filled into the reactor.After the catalysts were reduced in CO of 50 mL•min −1 at 300˚C for 6 h, it was cooled to room temperature.Then, the feed gas was changed into reactants (64% H 2 /32%CO 2 /4%N 2 ) of 1.6 MPa.The catalyst was heated to reaction temperature in about 3 h and kept at it for 45 h.The detail parameters for activity test are given with the experimental results.
C 5 + hydrocarbons were collected in an ice trap of 0˚C at system pressure.The distribution of C 5 + hydrocarbons was measured off line by GC-9860 (Qiyang Ltd.) with FID detector and OV-1701 capillary column.After the system pressure was released through a backpressure regulator, the exited gas was analyzed on line.The quantities of CO, CH 4 , CO 2 and N 2 were supplied with TCD detector and TDX-01 column.N 2 was used as an internal standard for the quantitation with the GC/TCD.C1 -C5 hydrocarbons were analyzed with FID detector and Al 2 O 3 capillary column [18].

Influence of SiO2 Content in Catalyst
Different to the iron catalysts studied in our previous works [19] [20] [21], SiO 2 is used as structure promoter in this work.Figure 1 contrasts the influence of SiO 2 content on the catalysts' activity for both of CO 2 hydrogenation (CO 2 conversion) and CO hydrogenation (CO conversion).These catalysts were prepared in Method I, and the content of Zn, K and Cu is 8 wt%, 5 wt% and 4 wt%, respectively.The catalyst with 15 wt% SiO 2 is the most active one for CO 2 hydrogenation, while this catalyst shows the lowest CO conversion in CO hydrogenation.
Table 1 gives the data of catalysts' texture.With raised SiO 2 content in the catalysts, For CO hydrogenation:  the specific surface area, pore volume and average pore diameter increase monotonically which do not match to either the volcano shape of CO 2 conversion or reverse volcano shape of CO conversion in Figure 1.The enlarged surface area usually reflects increased dispersive degree of every component in the catalysts. .Therefore, the amount of effective potassium is probably the lowest in Z8K5C4/FS15-I.It results in effective Zn/K ratio higher than the nominal Zn/K ratio calculated by the added amounts of zinc and potassium.We have disclosed that the iron catalyst with higher Zn/K ratio is more active to hydrogenate CO 2 [12] [16].
Table 2 lists the ethene and propene ratio in the C 2 and C 3 hydrocarbons (sum of olefin and paraffin), respectively.For CO hydrogenation, both of the ethene and pro-   The reaction conditions are as same as those shown in the caption of Figure 1.
[9] [10] [11], the C ad is from the CO which was evolved from CO 2 , rather than directly from the adsorbed CO 2 .On the same iron catalyst, the H ad quantity in CO 2 hydrogenation is probably as same as in CO hydrogenation.Therefore, it can be thought that the co-adsorption of CO 2 and CO on iron catalyst induces decreased C ad .However, it is only an assumption at present.We are trying to measure the C ad and H ad quantity after CO and H 2 or CO 2 and H 2 are co-fed to the iron catalysts.

Influence of Reaction Temperature
Catalyst Z2K3C4/FS5-I was evaluated under different reaction temperature.The results are given in Table 3. CO 2 conversion is increased continually with raised reaction temperature.With reaction temperature increased, the equilibrium of Equation ( 1) shifts to the right which is beneficial to supply CO for FT reaction according to Equation (2).It had been found that the extent of CO converted to hydrocarbons increased with raised reaction temperature [22] [25].Both of CO selectivity and CH 4 selectivity show an evident decrease from 230˚C to 240˚C, but there is no monodirectional change under higher temperature.The ethene content in C 2 hydrocarbons increases with raised reaction temperature.Under high temperature, the residence time of adsorbed ethene on the catalysts would be shortened due to the enhanced thermal motion.It leads to decreased probability for the adsorbed ethene to initiate carbon-chain growth or be hydrogenated into ethane.
The observed phenomenon about ethene selectivity in Table 3 is different from the results found in CO hydrogenation, where the ethene selectivity was decreased with increased reaction temperature [25] [26].Combining the above difference between CO 2 hydrogenation (Table 3) and CO hydrogenation [25] [26] with the disparity shown in Table 2, much research is needed to understand the reactive mechanism of CO 2 hydrogenation on iron catalyst.

Comparison of Catalyst Filling Constitution in the Reactor
For CO 2 hydrogenation in a fixed bed reactor, the H 2 O and hydrocarbons produced in the upper part of catalyst bed can absorb on the downstream catalyst, which probably influence the reaction of remained CO 2 and H 2 on the downstream catalyst.The influences are examined using catalyst Z8K3C6/FS5-I and Z8K3C6/FS10-II.Two constitutions are adopted to fill the catalysts as described in Figure 3. Constitution I is to install       hydrocarbons for Constitution I.This phenomenon may result from the olefin re-adsorption [27] which enhances the hydrocarbon growth into long carbon chain.
Considering the simplicity of reactor structure, Constitution I is an optimum type to synthesize liquid hydrocarbons from CO 2 hydrogenation.

Conclusion
Iron catalysts were prepared by co-precipitation of Fe and Si.Zn, K and Cu were impregnated to FeSi precipitate as promoters.For CO 2 hydrogenation reaction, the catalyst with 15 wt% SiO 2 possesses the highest CO 2 conversion.The olefin content in C 2 and C 3 hydrocarbons is lower in CO 2 hydrogenation than in CO hydrogenation which may be influenced by the co-adsorption of CO 2 and CO on the catalyst.CO 2 conversion and ethene selectivity are increased with raised reaction temperature.For fixed bed reactor, the H 2 O and hydrocarbons produced in the upper catalyst may re-adsorb on the downstream catalyst, and it is beneficial for carbon-chain growth by inhibiting H 2 adsorption.

Figure 1 .
Figure 1.Influence of SiO 2 content on catalyst activity in CO 2 hydrogenation and CO hydrogenation.

Figure 2
Figure 2 is the XRD results of the four catalysts.There are peaks around 23.92˚, 33.48˚, 35.88˚, 40.72˚, 49.68˚, 54.08˚, 62.76˚ and 63.76˚ in Z8K5C4/FS5-I, Z8K5C4/ FS10-I and Z8K5C4/FS20-I.They are assigned to Fe 2 O 3 (JCPDS 89-0597).These peaks almost disappear in Z8K5C4/FS15-I.The disappearance of crystalline Fe 2 O 3 indicatesthat iron is highly dispersed by the added SiO 2 in Z8K5C4/FS15-I.This dispersing effect of SiO 2 is happened to the promoters, too.It has been reported that the amount of effective potassium is decreased by the addition of SiO 2 into iron catalysts because of K-Si interaction[22] [23].Therefore, the amount of effective potassium is probably the low-

Figure 2 .
Figure 2. XRD pattern of the catalysts with different SiO 2 content.

Figure 4
Figure 4 compares the distribution of liquid hydrocarbons from the two Constitutions.There is an increase from C 5 hydrocarbons to C 8 hydrocarbons.It does not mean that the selectivity of C 5 , C 6 and C 7 hydrocarbons is less than C 8 hydrocarbons, because some molecule of C 5 -C 7 hydrocarbons cannot be completely condensed in the cold trap and are emitted with the exited gas.The C 5 -C 7 hydrocarbons had been found by the on line GC/FID designed to analyze the gaseous hydrocarbons.The hydrocarbon selectivity of FT reaction usually decreases with elongated carbon chain [27] [28].However, the selectivity for the hydrocarbons in C 20 -C 24 is higher than that of C 15 -C19

Figure 4 .
Figure 4. Influence of catalyst filling constitution on liquid hydrocarbons distribution.

Table 1 .
BET surface area and pore size.
a It is calculated by BJH method from the desorption branch.

Table 2 .
Olefin ratio in C 2 and C 3 hydrocarbons.

Table 3 .
Influence of reaction temperature on CO 2 hydrogenation.

Table 4 .
CO 2 hydrogenation with different catalyst filling constitution.

Table 4 ,
Constitution I has a higher C 5 + yield than Constitution II.