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The effect of support on RhFe/Al2O3 for ethanol synthesis via CO hydrogenation

The effect of support on RhFe/Al2O3 for ethanol synthesis via CO hydrogenation Different alumina samples prepared with sol–gel, chemical precipitation and hydrothermal synthesis were used as supports of Fe-promoted Rh-based catalysts for ethanol synthesis via CO hydrogenation. The samples were characterized by means of N -adsorpotion, XRD, H -TPR, XPS, STEM, H -TPD, DRIFTS, H and CO chemisorption. The results indicated that the 2 2 2 2 Al O prepared by hydrothermal synthesis exhibited nano-fiber morphology and constituted of mixed crystal phases, while 2 3 Al O prepared by sol–gel and chemical precipitation shows no changes of morphology and crystal phases compared with 2 3 the commercial Al O . In addition, nano-fiber Al O -supported Rh-based catalyst shows higher ethanol selectivity, which 2 3 2 3 is ascribed to the lower metal–support interaction, higher dispersion and stronger CO insertion ability. Keywords Ethanol synthesis · Al O  · Interaction · DRIFTS 2 3 Introduction two opposite findings suggest that the interaction between metal and Al O should be adjusted carefully to meet the 2 3 Ethanol synthesis via syngas directly has attracted a great requirement of ethanol synthesis from syngas. deal of attentions in recent years [1–3]. However, there is The interaction between Al O support and metal 2 3 still a challenge for improving the selectivity of catalysts. depends on the nature of the Al O , such as the texture, 2 3 Fe-promoted Rh-based catalysts have been reported to work crystal and chemical properties. In the reaction of CH well for ethanol synthesis [4–7]. For instance, Fukushima reforming, the good performance was presented on the et al. [8] found the addition of Fe to Rh/SiO can enhance nano-fiber Al O -supported Pt catalysts, Garcı´a-Die´guez 2 2 3 the formation of intermediate species for ethanol synthe- et al. [11] ascribed it to the mixed crystal of the Al O sup- 2 3 sis by high-pressure FT-IR study. A good choice of support port prepared by hydrothermal synthesis. Zhang et al. [12] would be helpful to explore the potentiality of Fe-promoted concluded that different crystal structures of Al O could 2 3 Rh-based catalysts. Al O had been investigated as the decrease the metal–support interaction due to its modi- 2 3 support of Rh-based catalysts since it is extensively used fied hydroxyl group. For supported Rh-based catalyst, it as commercial carrier [9]. Burch et al. [10] found that the is reported that the weakly H-bonded surface hydroxyls of Al O -supported Rh-based catalysts exhibited excellent eth- SiO have an important effect on the interaction between Rh 2 3 2 anol selectivity. However, Chen et al. [9] proposed that the and promoter [13]. Therefore, a comprehensive understand- Al O support was not the best choice for Rh-based catalysts ing of the interaction between support and active metals is 2 3 since it had strong interaction with the metals. The above very important for design of the supports for Rh-based cata- lysts. Moreover, this understanding would be strengthened if we know the effect of support at some curial steps, such * Fang Li as CO adsorption, dissociation and hydrogenation, during zls9390@163.com ethanol formation. In this study, three Al O supports were prepared by School of Chemical and Environment Engineering, Anhui 2 3 Polytechnic University, Wuhu 241000, Anhui, China sol–gel, chemical precipitation and hydrothermal synthe- sis. The commercial γ-Al O was also used as reference. Engineering Research Center of Large Scale Reactor 2 3 Engineering and Technology, Ministry of Education, East The typical physical–chemical properties of samples China University of Science and Technology, No. 130, were characterized by different techniques, such as X-ray Meilong Road, Shanghai 200237, China Vol.:(0123456789) 1 3 306 Applied Petrochemical Research (2021) 11:305–316 diffraction (XRD), nitrogen physisorption, H temperature- calcined at 500 °C for 4 h. The nominal loading of Rh is 2 programmed reduction (H-TPR), H and CO chemisorp- wt% and Fe 4 wt% for all the catalysts. The catalysts sup- 2 2 tion, H temperature-programmed desorption (H -TPD), ported on Al-CM, Al-SG, Al-CP and Al-HS were named 2 2 X-ray photoelectron spectroscopy (XPS) and high resolu- as RhFe/Al-CM, RhFe/Al-SG, RhFe/Al-CP and RhFe/ tion scanning transmission electron microscopy (STEM). Al-HS, respectively. Furthermore, diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) was used to determine CO adsorp- tion, hydrogenation behavior, and surface hydroxyl group Characterization of samples. The N adsorption–desorption isotherms for alumina samples and the corresponding catalysts were measured Experimental in a Micromeritics ASAP 2000 equipment. XRD patterns were recorded on a diffractometer operating with Cu Ka Support and catalyst preparation radiation at 40 kV. The power diffractograms of the sam- ples were collected from 10 to 80° at a rate of 6°/min. H / Alumina samples were prepared by chemical precipitation, CO chemisorption and TPR/ TPD experiments were car- sol–gel and hydrothermal synthesis methods, denoted as ried out on a Micromeritics Autochem 2920 apparatus. Al-CP, Al-SG and Al-HS, respectively. Commercial γ-Al O The H -TPR profile was recorded with a TCD detector 2 3 2 was abbreviated as Al-CM. Al-CP was prepared in the fol- according to H consumption during sample was reduced lowing procedure: 45.0 g of Al (N O ) ·9H O was dissolved from room temperature to 800  °C ramped at a rate of 3 3 2 in 250 mL of de-ionized water with strong agitation to form 10 °C/min under a 10% H /Ar (v/v) f low. For H -TPD 2 2 transparent solution. Simultaneously, the above solution and experiments, typically, 200 mg of sample was placed in aqueous ammonia (2.5 wt %) solution were added dropwise a U-shaped quartz tube and pre-reduced in 50 mL/min into a beaker at constant pH with magnetic stirring. After of 10% H /Ar (v/v) flow at 350  °C for 2  h, and then that, the suspension was aged for 12 h at room temperature, switched to He f low for 30 min. After that, H was intro- and then filtered, washed with de-ionized water many times duced into catalyst bed until saturation, and purged again until no change of pH. At last, the obtained precipitate was by He for 30 min. Finally, the sample was heated up to dried at 110 °C for 12 h, and then calcined at 500 °C for 4 h. 800 °C under He atmosphere at a rate of 10 °C/min, while For the preparation of Al-SG, alumina sol was first prepared. the desorbed products were detected with a TCD detec- Aluminum isopropoxide was added slowly to the water and tor. XPS experiments were performed using a Physical kept at 90 °C for 2 h. Then, nitric acid was added to the Electronics Quantum 2000 Scanning ESCA Microprobe. solution very slowly and kept stirring at 95 °C for 6 h. The STEM measurements were performed on a Tecnai G2 obtained alumina sol was heated in a water bath at 80 °C for F30 S-TWIN electron microscope with 300 kV accelerat- 2 h and cooled to room temperature. The resulting solid was ing voltage via high-angle annular dark-field (HAADF). dried in a vacuum oven at 80 °C for 24 h and then calcined Point energy-dispersive X-ray spectroscopy (EDS) was in a muffle furnace at 500 °C for 4 h. Al-HS was prepared by taken in an area within 5 nm diameter. DRIFTS was car- the following procedure. Typically, 45 g of Al (NO ) ·9H O ried out with a Nicolet 6700 spectrometer equipped with 3 3 2 was dissolved in 250 mL of deionized water and then was infrared cell (PIKE). The DRIFTS experiments contain mixed with 20 wt% TEAOH solution (tetraethyl ammonium CO adsorption, CO temperature-programmed surface hydroxide) under rigorous stirring till pH reached 5.0. The reduction (CO-TPSR) and determination of surface mixture was continuously stirred for 30 min before trans- hydroxyl group. For CO adsorption, first, the sample ferring into an autoclave equipped with a Teflon-liner. The was reduced under pure H f low at 350 °C for 2 h. After hydrothermal treatment was conducted at 170 °C for 72 h. f lushed by N for 30 min, the sample was degassed under –4 The as-prepared sample was calcined at 500 °C for 5 h with vacuum until that 10  mbar was achieved, and then back- a heating rate of 5 °C/min. ground spectra were collected at designated temperature. Fe-promoted Rh-based catalysts supported on the Second, a flow of 5% CO/He (v/v) was introduced to different Al O samples were prepared by incipient reduced catalysts for 30 min. Finally, the IR spectra were 2 3 wetness method. Before impregnation, Al O samples recorded after f lushed by N for 30 min. For CO-TPSR, 2 3 2 were ground and sieved to 40–60 mesh. Aqueous solu- the IR spectra were recorded under 10% H /Ar (v/v) f low tion of Rh (NO ) .2H O (9.35 wt % Rh, Helishi) and Fe with the temperate linearly increased from 30 to 260 °C 3 3 2 (NO ) .6H O were added dropwise to the given weight after CO adsorption. For the determination of surface 3 2 2 of support. The samples were kept stirring for 2 h and hydroxyl group, the sample was f lushed by N at 300 °C –4 then dried at 110 °C for 12 h. Finally, the samples were and then degassed under 10  mbar vacuum for 2 h. The 1 3 Applied Petrochemical Research (2021) 11:305–316 307 IR spectra were collected at room temperature in the of hysteresis loop indicate all the samples are mesopore −1 range of 3200–4000  cm . structure. The type of hysteresis loop for Al-CM, Al-CP, Al-SG, Al-HS is attributed to H4, H4, H2 and H3, respec- Catalytic reaction tively. It is well known that the type of hysteresis loop is related with the special pore structure [14]. For instance, Reaction was carried out in a stainless fixed bed reactor with the presence of H3 hysteresis loop for Al-HS sample indi- an inner diameter of 10 mm. 1.0 g catalyst (1.3 mL) was cates the slit pore formed by the fibrous or sheet particle. diluted with quartz sand (4.2 mL) and then placed in the Table 1 showed the textural properties of the different sam- middle of the reactor. The catalyst was heated to 350 °C ples. Obviously, the surface areas of the samples are as fol- and reduced in H at a flow rate of 60 mL/min for 10 h, lows: Al-CP (398) > Al-SG (336) > Al-CM (256) > Al-HS then cooled down to the reaction temperature (260 °C) in (90). Among the four alumina samples, Al-HS presents the H flow. Subsequently, 60 mL/min syngas (H /CO = 2) was lowest specific surface area. The enlarging pore effect, as a 2 2 introduced to reactor and kept at a pressure of 2 MPa. The result of hydrothermal treatment, may be responsible for its flow rates of gas were controlled by a Brooks 5050 mass low specific surface area. flowmeter. The effluent gas from the reactor was passed Figure 2a presents the XRD profiles of the four Al O 2 3 through a hot trap and a cold trap successively to separate samples. Typical characteristic peaks of γ-Al O were 2 3 gaseous and liquid products. A 24 h liquid sample was col- observed for all the samples. However, besides γ-Al O , 2 3 lected after steady state was achieved. Outlet gases and liq- characteristic peaks of α-Al O, Al O for Al-HS were 2 3 2 3 uid sample were detected with two chromatographs (Agilent observed in Fig. 2b. The results indicate that Al-HS is con- GC7890A), one is equipped with two TCD to analyze CO, stituted of mixed crystal phases, while Al-CM, Al-CP and CO, N and H using a 5-A molecular sieve column online Al-SG are mainly consistent with the γ-Al O crystal phase. 2 2 2 2 3 and the other fitted with one FID and TCD to separate C –C In general, alumina with different forms presents different 1 6 hydrocarbons, oxygenates and water using Plot Q column specific surface areas with the order of α-Al O << γ-Al O 2 3 2 3 offline. [15], which can be responsible for the lowest specific area of Al-HS material. Results and discussion Characterization of catalysts Characterization of supportThe H -TPR profiles of the four catalysts were presented in Fig. 3. It can be observed two peaks of H consumptions in Figure  1 shows the N adsorption isotherms of the four Fig. 3, namely, one low temperature reduction peak (peak Al O samples. The type IV of isotherms and the existence I) located at 50–300 °C and one high temperature board 2 3 peak (peak II) in the range of 350–550 °C. According to the literature [18], the peak II was ascribed to the reduction of Fe O to Fe O and Fe O to Fe speices. 2 3 3 4 3 4 The theoretical and calculated H consumptions of Fe- promoted Rh-based catalysts are listed in Table 2. Therefore, the peak I should be ascribed to the co-reduction of Rh O 2 3 and Fe O species. The area of peak I is in order of RhFe/ 2 3 Al-HS > RhFe/Al-CM > RhFe/Al-SG > RhFe/Al-CP. Dis- tinctly, the RhFe/Al-HS catalyst exhibits the highest reduc- tion degree and RhFe/Al-CM less. In addition, for RhFe/ Table 1 Textural properties of different type of Al O 2 3 Samples BET surface area Pore volume (cm Pore 2 −1 −1 (m g ) g ) diameter (nm) Al-CM 256 0.46 9.0 Al-CP 398 0.48 4.9 Fig. 1 Nitrogen adsorption isotherms and pore distribution for differ - Al-SG 336 0.43 5.0 ent alumina samples. a Nitrogen adsorption isotherms; b pore distri- Al-HS 90 0.33 14.6 bution 1 3 308 Applied Petrochemical Research (2021) 11:305–316 Fig. 2 XRD profiles of different alumina samples the interaction between metal and support got weaker and the contact between Rh and Fe was enhanced for the above two catalysts. Considering with the activity data, the cata- lysts owning stronger Rh–Fe interaction correspond to the higher ethanol selectivity, which indicates that the weaker interaction between Rh and Al may be conductive to the formation of ethanol. The H -TPD profiles of catalysts are presented in Fig.  4. As can be seen, two peaks were presented on all the sam- ples. The peaks at lower temperature (50–200 °C) and higher temperature (300–500 °C) are denoted as H and H , respec- α β tively. H is ascribed to the weak adsorption of hydrogen and H the strong adsorption of it. It is accepted that H β β is responsible for the hydrogenation ability of Rh-based catalysts [16]. Obviously, RhFe/Al-HS exhibits higher peak area and higher temperature of H compared with the other Fig. 3 H -TPR profiles of Fe-promoted Rh-based catalysts catalysts. This indicates that the RhFe/Al-HS has stronger ability to activate hydrogen, which is helpful for promoting the catalytic activity. Al-HS and RhFe/Al-CM catalysts, the reduction temperature Since metal particles cannot be easily distinguished from alumina, dark-field imaging technique (HAADF) was used of peak I shifts to lower temperature in comparison with the corresponding un-promoted ones. It can be concluded that to observe surface metal particle morphology of catalysts Table 2 H consumption of a a Catalysts Reduction Calculated and Catalysts Reduction Calculated and catalysts b b peak (°C) theoretical H consump- peak (°C) theoretical H consump- 2 2 tion (mmol) tion (mmol) a b a b RhFe/Al-CM 111 40 (249 ) Rh/Al-CM 120 36 (58 ) a b a b RhFe/Al-CP 167 37 (249 ) Rh/Al-CP 131 22 (58 ) a b a b RhFe/Al-SG 210 35 (249 ) Rh/Al-SG 160 30 (58 ) a b a b RhFe/Al-HS 114 53 (249 ) Rh/Al-HS 132 48 (58 ) H consumption was calculated with 50 mg CuO as external reference Sample weight, 200 mg 1 3 Applied Petrochemical Research (2021) 11:305–316 309 has a higher Rh dispersion while RhFe/Al-CP and RhFe/ Al-SG shows a lower dispersion compared with RhFe/ Al-CM. However, RhFe/Al-CP and RhFe/Al-SG should have higher Rh dispersion according to STEM results. The possible reason for the inconsistent results given by chemisorption and STEM may be due to that the smaller Rh particles enhanced the interaction with alumina sup- port, resulting in a part of Rh particles incorporation into the alumina skeleton. XPS spectra of fresh catalysts were shown in Fig. 7. The XPS intensity ratios and binding energies (BEs) of Rh 3d and Fe 2p was presented in Table 5. The surface atom con- centrations of Rh/Al are in the order of RhFe/Al-HS > RhFe/ Al-CM > RhFe/Al-CP > RhFe/Al-SG. The ratio of Rh/Fe for RhFe/Al-HS is 0.28, which is even higher than the expected value (0.27). The above indicated that more rhodium spe- cies enrichment on the surface of RhFe/Al-HS. In addition, Fig. 4 H -TPD profiles of Rh-based catalysts supported on different the binding energy of Rh 3d and Fe 2p of RhFe/Al-HS type of Al O 5/2 3/2 2 3 exhibits obvious shift compared to the other catalysts. This further proved stronger interaction exists between Rh and after reduction. Figure 5 shows STEM images of catalysts. Fe for this catalyst. From STEM images (Fig. 5A, D) of the RhFe/Al-CM and The interaction between highly dispersed active metal RhFe/Al-HS, it can be observed that the metal particle sizes and support for different Al O -supported catalysts var- 2 3 of them are both in the range of 1–4 nm with the average ies greatly from the above analysis. Surface hydroxyls of diameter of 2.7 and 3.8 nm, respectively. However, it can be catalysts may be one possible reason for this interaction. seen from the images of RhFe/Al-SG (Fig. 5C) and RhFe/ Figure  8 presents the surface hydroxyl group of catalyst Al-CP (Fig. 5B) that the metal particle size is approximately and supports. The hydroxyl groups on the surface of alu- about or less than 1 nm. The favorable metal particle size mina were classified into I, II, and III, in which type I > type for ethanol formation was reported to be 2–4 nm [17]. In our II > type III in order of frequencies of them [15]. As shown −1 case, the metal particles less than 2 nm may form stronger in Fig. 8, the bands at 3723 (I), 3687 (II), 3572  cm (II or interaction with alumina support leading to decreased III) are ascribed to the absorption of isolated —OH groups, reducible species according to the TPR results. Figure 6 weakly H-bonded and strongly H-bonded OH groups, shows the corresponding EDS spectra of catalysts, in which respectively [18]. It can be seen from Fig. 8a that the inten- one point within the metal particle region (P1) and the other sity of hydroxyl groups for alumina samples is in order of metal particle-free region (P2) was selected at random. Al-SG > Al-CP > Al-CM > Al-HS. The corresponding cata- There exists Rh and Fe signal in the EDS spectra of P1 for lysts also show the same order as can be seen in Fig. 8b. all the samples, indicating Rh and Fe are in close contact. Some researchers believed CO comes from the reaction of However, only Fe signal was observed in the EDS spec- strongly adsorbed CO species with hydroxyl groups of Al O 2 3 tra of P2 for RhFe/Al-CP, RhFe/Al-SG and RhFe/Al-CM support [20]. This is in accordance with our experiments samples, which means some single Fe species distribute results that RhFe/Al-SG shows the highest C O selectivity on the surface of these catalysts. Exceptionally, on RhFe/ with the most amount of surface hydroxyl group. However Al-HS catalyst, there is a weak signal of Rh and Fe within it may be, there is no doubt that the metal–support interac- metal particle-free region on the EDS spectrum, indicating tion increases with the increasing amount of hydroxyl group. that more Fe and Rh species are in intimate contact on the Moreover, it can be observed from Fig. 8b that the ratio of surface of support. different type of hydroxyl groups for RhFe/Al-CP and RhFe/ To get the information of Rh dispersion of catalysts Al-SG catalyst is similar with that of RhFe/Al-CM. How- more accurately, CO and H chemisorption experiments ever, the proportion of type I (isolated hydroxyl) increases in a pulse mode were both carried out. Tables 3 and 4 show for RhFe/Al-HS. the results of the chemisorption of CO and H , respectively. The infrared spectra of the catalysts after CO adsorption −1 The results summarized in Tables 3 and 4 show that Rh at 30 °C are shown in Fig. 9. The 2055  m band can be dispersion calculated by the H chemisorption is higher ascribed to linear adsorbed CO (CO (l)) and double band −1 than the CO chemisorption. Despite this, the same trend at 2084 and 2017  cm can be assigned to the symmet- about Rh dispersion can be gotten. Namely, RhFe/Al-HS ric and asymmetric carbonyl stretching of the dicarbonyl 1 3 310 Applied Petrochemical Research (2021) 11:305–316 Fig. 5 STEM images of Rh-based catalysts supported on different type of Al O . A RhFe/Al-CM; B RhFe/Al-CP; C RhFe/Al-SG; D RhFe/ 2 3 Al-HS + 0 Rh (CO) (CO(gem)) [19].The board band centered at isolated hydroxyl groups is favorable to transfer Rh into −1 + 1860  cm is assigned to bridge bonded CO (CO (b)), Rh [21]. Therefore, we presume that more abundant whose intensity was found to be related with the selectiv- isolated hydroxyl groups are responsible for more Rh ity of CH [10]. It is widely accepted that CO (gem) is on RhFe/Al-HS. Moreover, the bridged adsorbed CO is formed on Rh , which is favorable for CO insertion, while strongest on the RhFe/Al-HS catalyst, which is consist- CO (l) and CO(b) formed on Rh , which is helpful for CO ent with the activity data for methane selectivity of this dissociation [20]. It can be seen from Fig. 9 that the inten- catalyst (see Table 6). sity of CO (gem) is in the order of RhFe/Al-HS > RhFe/ To know the dissociation ability and hydrogenation rate Al-CM > RhFe/Al-CP > RhFe/Al-SG, which means that of the catalyst, CO-TPSR was carried out. As shown in + −1 RhFe/Al-HS has more Rh center. It was reported that Fig. 10, the band at 3015  cm is ascribed to the stretching 1 3 Applied Petrochemical Research (2021) 11:305–316 311 Fig. 6 EDS profiles of Rh-based catalysts supported on different type of Al O . a RhFe/Al-CM; b RhFe/Al-CP; c RhFe/Al-SG; d RhFe/Al-HS 2 3 vibration of gaseous methane, which intensity can be used Table 3 CO chemisorption results of catalysts as a tool to measure CO dissociation ability of catalyst −1 CatalystsCO-chemisorbed/μmol g Metal dis- [22]. The formation of methane for RhFe/Al-CM, RhFe/ persion/% Al-CP, RhFe/Al-SG and RhFe/Al-HS appeared at 220 °C, RhFe/Al-CM 71.2 36.6 210 °C, 200 °C, 190 °C, respectively, which means that RhFe/Al-CP 64.6 32.7 RhFe/Al-HS possessed strongest CO dissociation ability, RhFe/Al-SG 43.1 22.2 whereas the RhFe/Al-CM showed the weakest dissociation. RhFe/Al-HS 116.6 58.6 In addition, it was observed from the spectra in the range −1 of 1800–2200  cm that the hydrogenation rate of adsorbed CO is as follows: RhFe/Al-SG > RhFe/Al-CP > RhFe/ Al-HS > RhFe/Al-CM. Table 4 H chemisorption results of catalysts Table  6 lists the activities data of the catalysts. These −1 Catalysts H -chemisorbed /μmol g Metal data were taken with 2:1 H :CO mix at a total pressure dispersion 2 MPa and a temperature of 260 °C. The results show that /% product contains CO , hydrocarbons, methanol, ethanol and RhFe/Al-CM 92.3 47.4 other oxygenates. Among the by-products, C O, CH and 2 4 RhFe/Al-CP 89.6 40.1 higher hydrocarbons should be inhibited more seriously RhFe/Al-SG 85.2 35.1 due to higher separation cost and lower economic value. RhFe/Al-HS 160.8 82.7 Among the four catalysts, RhFe/Al-HS shows the highest 1 3 312 Applied Petrochemical Research (2021) 11:305–316 Fig. 7 XPS spectra of a Rh 3d and b Fe 2p for the catalysts Table 5 Binding energy and surface composition of catalysts determined by XPS Catalysts Atomic ratio from XPS Binding energy (eV) a a Rh/Al Rh/Fe Rh/FeRh 3dFe 2p 5/2 3/2 RhFe/Al-GM 0.79 0.24 0.27 309.3 710.4 RhFe/Al-CP 0.74 0.22 0.27 309.4 710.4 RhFe/Al-SG 0.64 0.19 0.27 309.4 710.4 RhFe/Al-HS 1.21 0.28 0.27 309.5 710.8 Atomic ratio of Rh/Fe expected in the whole sample Fig. 8 Surface hydroxyl groups of different Al O support and corresponding catalysts a support; b catalysts 2 3 1 3 Applied Petrochemical Research (2021) 11:305–316 313 Al-CP > Al-SG > Al-CM > Al-HS. Among them, Al-HS- supported catalysts have the highest ethanol selectivity although with the lowest specific area, indicating the spe- cific area of the catalysts was not in direct proportion to their catalytic performances. XRD showed the presence of mixed Al O crystalline phases in the Al-HS support pre- 2 3 pared by hydrothermal synthesis. Compared to the Al-CM carrier, the greater specific area for Al-CP and Al-SG supported catalyst may lead to too stronger interaction between meatal and support, which is not good for Rh–Fe interaction. Al-HS carrier with nano-fibrous morphology supported catalysts may weaken the interaction between metal and support, correspondingly, the contact between Rh and Fe is strengthened, which can also be proved by H -TPR and EDS. The lower surface hydroxyl group inten- sity of Al-HS support may be responsible for the above interactions. The higher specific surface area Al-SG- and Fig. 9 FTIR spectra of adsorbed CO on different type of Al O -supported Rh-based catalysts. a RhFe/Al-CM; b RhFe/Al-CP; 2 3 Al-CP-supported catalysts exhibited a higher CO conver- c RhFe/Al-SG; d RhFe/Al-HS sion, but most of CO was converted into hydrocarbon and CO . From the results of TPR, it can be inferred that there exists more single Fe species over the above two catalysts, ethanol selectivity (~ 24.6%) and the lowest C O selectiv- 2 which is favorable for hydrogenation and water gas shift ity (~ 8%), and also, RhFe/Al-CM exhibits higher ethanol reaction. On the other hand, some researchers believed CO selectivity with moderate CO conversion. It can be seen that comes from the reaction of strongly adsorbed CO species Al-CP- and Al-SG-supported RhFe catalysts exhibit higher with hydroxyl groups of Al O support [20, 24]. This is in 2 3 CO conversion but only with 7.3% and 5.8% ethanol selec- accordance with our experiments results that RhFe/Al-SG tivity, respectively. In general, for CO conversion, RhFe/ has strongest hydroxyl group and shows the highest CO Al-SG > RhFe/Al-CP > RhFe/Al-CM > RhFe/Al-HS. And selectivity. for ethanol selectivity, RhFe/Al-HS > RhFe/Al-CM > RhFe/ The formation of Rh is also important for the formation Al-SG > RhFe/Al-CP. of active sites. It is widely accepted that the dicarbonyl spe- 0 + 3+ 2+ It has been reported that (Rh -Rh )–O–Fe (Fe ) is x y cies can only be formed on highly dispersed rhodium [24]. the active site for the formation of ethanol [23]. Therefore, H and CO chemisorptions showed that Rh dispersion was in the synergy effect between Rh and Fe, the formation of order of RhFe/Al-HS > RhFe/Al-CM > RhFe/Al-CP > RhFe/ Rh would be beneficial to form the active site for ethanol Al-SG. This indicated Al-HS-supported catalyst has a higher synthesis. Rh dispersion than others, which also evidenced by XPS. On The characterization and catalytic results show that the other hand, DRIFTS of CO adsorption shows that RhFe/ the Rh–Fe interactions are greatly affected by the sup- Al-HS has more dicarbonyl species, which comes from Rh port. The surface areas of the samples are as follows: sites. Table 6 CO hydrogenation over a Catalysts X /% Product selectivity/C% CO Rh-based catalysts supported on different type of Al O CH C HC MeOH EtOH CO Others 2 3 4 2+ 2 RhFe/Al-CM 25.8 28.6 13.3 15.2 20.5 14.2 8.2 RhFe/Al-SG 38.6 24.1 23.9 5.7 7.3 29.1 9.9 RhFe/Al-CP 35.4 24.8 27.2 5.2 5.8 28.8 8.2 RhFe/Al-HS 13.6 29.1 10.1 19.4 24.6 8.0 8.7 Catalyst:1 g; reaction conditions: T = 260 °C P = 2 MPa, H /CO = 2, space velocity = 3600 mL/(h. gcat) Product selectivity = n M /∑(n M ), where n and M are the carbon atoms number and molar percent of i i i i i i product i, respectively Oxygenates with two or more carbons except ethanol(acetaldehyde, acetone, n-propanol, i-propanol, n-butanol, i-butanol and n-pentanol 1 3 314 Applied Petrochemical Research (2021) 11:305–316 Fig. 10 DRIFTS spectra of CO-TPSR on different type of Al O -supported Rh-based catalysts. a mRhFe/Al-CM; b RhFe/Al-CP; c RhFe/Al-SG; 2 3 d RhFe/Al-HS According to widely accepted mechanism for ethanol syn- Conclusion thesis (see Fig. 11), a good Rh-based catalyst should balance the ability of CO dissociation, CO insertion and CO hydro- Four kinds of alumina were used as supports of Fe- genation. DRIFTS study indicates that RhFe/Al-HS have promoted Rh-based catalysts. The ethanol selectivity of stronger CO dissociation capacity, moderate CO hydrogena- RhFe/Al-HS catalysts supported on alumina prepared tion rate and more Rh sites for CO insertion. Therefore, by hydrothermal synthesis was superior than the other stronger CO dissociation capacity, moderate CO hydrogena- catalysts, which were mainly due to the moderate metal tion rate and stronger CO insertion may be responsible for particle size, the enhanced synergic between Rh and Fe, the good performance of RhFe/Al-HS. and the improved the Rh dispersion. Moreover, moderate CO hydrogenation rate, stronger CO dissociation and CO insertion ability was also favorable to increase the ethanol selectivity. 1 3 Applied Petrochemical Research (2021) 11:305–316 315 Fig. 11 Mechanism of ethanol formation [25] Acknowledgements This work was supported by the National Natural monoxide hydrogenation on silicon dioxide-supported rhodium- Science Foundation of China (No. 31671797). iron catalysts. J Phys Chem 89:4440–4443 9. Chen WM, Ding YJ, Song XG, Wang T, Luo HY (2011) Pro- motion effect of support calcination on ethanol production from Open Access This article is licensed under a Creative Commons Attri- CO hydrogenation over Rh/Fe/Al O catalysts. Appl Catal A bution 4.0 International License, which permits use, sharing, adapta- 2 3 407:231–237 tion, distribution and reproduction in any medium or format, as long 10. Burch R, Hayes MJ (1997) The preparation and characterisation as you give appropriate credit to the original author(s) and the source, of Fe-promoted Al O -supported Rh catalysts for the selective provide a link to the Creative Commons licence, and indicate if changes 2 3 production of ethanol from syngas. J Catal 165:249–261 were made. The images or other third party material in this article are 11. García-Diéguez M, Pieta IS, Herrera MC, Larrubia MA, Malpar- included in the article’s Creative Commons licence, unless indicated tida I, Alemany LJ (2010) Transient study of the dry reforming otherwise in a credit line to the material. If material is not included in of methane over Pt supported on different γ-Al O . Catal Today the article’s Creative Commons licence and your intended use is not 2 3 149:380–387 permitted by statutory regulation or exceeds the permitted use, you will 12. 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The effect of support on RhFe/Al2O3 for ethanol synthesis via CO hydrogenation

Applied Petrochemical Research , Volume 11 (3) – Dec 1, 2021

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Springer Journals
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Copyright © The Author(s) 2021
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2190-5525
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2190-5533
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10.1007/s13203-021-00277-9
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Abstract

Different alumina samples prepared with sol–gel, chemical precipitation and hydrothermal synthesis were used as supports of Fe-promoted Rh-based catalysts for ethanol synthesis via CO hydrogenation. The samples were characterized by means of N -adsorpotion, XRD, H -TPR, XPS, STEM, H -TPD, DRIFTS, H and CO chemisorption. The results indicated that the 2 2 2 2 Al O prepared by hydrothermal synthesis exhibited nano-fiber morphology and constituted of mixed crystal phases, while 2 3 Al O prepared by sol–gel and chemical precipitation shows no changes of morphology and crystal phases compared with 2 3 the commercial Al O . In addition, nano-fiber Al O -supported Rh-based catalyst shows higher ethanol selectivity, which 2 3 2 3 is ascribed to the lower metal–support interaction, higher dispersion and stronger CO insertion ability. Keywords Ethanol synthesis · Al O  · Interaction · DRIFTS 2 3 Introduction two opposite findings suggest that the interaction between metal and Al O should be adjusted carefully to meet the 2 3 Ethanol synthesis via syngas directly has attracted a great requirement of ethanol synthesis from syngas. deal of attentions in recent years [1–3]. However, there is The interaction between Al O support and metal 2 3 still a challenge for improving the selectivity of catalysts. depends on the nature of the Al O , such as the texture, 2 3 Fe-promoted Rh-based catalysts have been reported to work crystal and chemical properties. In the reaction of CH well for ethanol synthesis [4–7]. For instance, Fukushima reforming, the good performance was presented on the et al. [8] found the addition of Fe to Rh/SiO can enhance nano-fiber Al O -supported Pt catalysts, Garcı´a-Die´guez 2 2 3 the formation of intermediate species for ethanol synthe- et al. [11] ascribed it to the mixed crystal of the Al O sup- 2 3 sis by high-pressure FT-IR study. A good choice of support port prepared by hydrothermal synthesis. Zhang et al. [12] would be helpful to explore the potentiality of Fe-promoted concluded that different crystal structures of Al O could 2 3 Rh-based catalysts. Al O had been investigated as the decrease the metal–support interaction due to its modi- 2 3 support of Rh-based catalysts since it is extensively used fied hydroxyl group. For supported Rh-based catalyst, it as commercial carrier [9]. Burch et al. [10] found that the is reported that the weakly H-bonded surface hydroxyls of Al O -supported Rh-based catalysts exhibited excellent eth- SiO have an important effect on the interaction between Rh 2 3 2 anol selectivity. However, Chen et al. [9] proposed that the and promoter [13]. Therefore, a comprehensive understand- Al O support was not the best choice for Rh-based catalysts ing of the interaction between support and active metals is 2 3 since it had strong interaction with the metals. The above very important for design of the supports for Rh-based cata- lysts. Moreover, this understanding would be strengthened if we know the effect of support at some curial steps, such * Fang Li as CO adsorption, dissociation and hydrogenation, during zls9390@163.com ethanol formation. In this study, three Al O supports were prepared by School of Chemical and Environment Engineering, Anhui 2 3 Polytechnic University, Wuhu 241000, Anhui, China sol–gel, chemical precipitation and hydrothermal synthe- sis. The commercial γ-Al O was also used as reference. Engineering Research Center of Large Scale Reactor 2 3 Engineering and Technology, Ministry of Education, East The typical physical–chemical properties of samples China University of Science and Technology, No. 130, were characterized by different techniques, such as X-ray Meilong Road, Shanghai 200237, China Vol.:(0123456789) 1 3 306 Applied Petrochemical Research (2021) 11:305–316 diffraction (XRD), nitrogen physisorption, H temperature- calcined at 500 °C for 4 h. The nominal loading of Rh is 2 programmed reduction (H-TPR), H and CO chemisorp- wt% and Fe 4 wt% for all the catalysts. The catalysts sup- 2 2 tion, H temperature-programmed desorption (H -TPD), ported on Al-CM, Al-SG, Al-CP and Al-HS were named 2 2 X-ray photoelectron spectroscopy (XPS) and high resolu- as RhFe/Al-CM, RhFe/Al-SG, RhFe/Al-CP and RhFe/ tion scanning transmission electron microscopy (STEM). Al-HS, respectively. Furthermore, diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) was used to determine CO adsorp- tion, hydrogenation behavior, and surface hydroxyl group Characterization of samples. The N adsorption–desorption isotherms for alumina samples and the corresponding catalysts were measured Experimental in a Micromeritics ASAP 2000 equipment. XRD patterns were recorded on a diffractometer operating with Cu Ka Support and catalyst preparation radiation at 40 kV. The power diffractograms of the sam- ples were collected from 10 to 80° at a rate of 6°/min. H / Alumina samples were prepared by chemical precipitation, CO chemisorption and TPR/ TPD experiments were car- sol–gel and hydrothermal synthesis methods, denoted as ried out on a Micromeritics Autochem 2920 apparatus. Al-CP, Al-SG and Al-HS, respectively. Commercial γ-Al O The H -TPR profile was recorded with a TCD detector 2 3 2 was abbreviated as Al-CM. Al-CP was prepared in the fol- according to H consumption during sample was reduced lowing procedure: 45.0 g of Al (N O ) ·9H O was dissolved from room temperature to 800  °C ramped at a rate of 3 3 2 in 250 mL of de-ionized water with strong agitation to form 10 °C/min under a 10% H /Ar (v/v) f low. For H -TPD 2 2 transparent solution. Simultaneously, the above solution and experiments, typically, 200 mg of sample was placed in aqueous ammonia (2.5 wt %) solution were added dropwise a U-shaped quartz tube and pre-reduced in 50 mL/min into a beaker at constant pH with magnetic stirring. After of 10% H /Ar (v/v) flow at 350  °C for 2  h, and then that, the suspension was aged for 12 h at room temperature, switched to He f low for 30 min. After that, H was intro- and then filtered, washed with de-ionized water many times duced into catalyst bed until saturation, and purged again until no change of pH. At last, the obtained precipitate was by He for 30 min. Finally, the sample was heated up to dried at 110 °C for 12 h, and then calcined at 500 °C for 4 h. 800 °C under He atmosphere at a rate of 10 °C/min, while For the preparation of Al-SG, alumina sol was first prepared. the desorbed products were detected with a TCD detec- Aluminum isopropoxide was added slowly to the water and tor. XPS experiments were performed using a Physical kept at 90 °C for 2 h. Then, nitric acid was added to the Electronics Quantum 2000 Scanning ESCA Microprobe. solution very slowly and kept stirring at 95 °C for 6 h. The STEM measurements were performed on a Tecnai G2 obtained alumina sol was heated in a water bath at 80 °C for F30 S-TWIN electron microscope with 300 kV accelerat- 2 h and cooled to room temperature. The resulting solid was ing voltage via high-angle annular dark-field (HAADF). dried in a vacuum oven at 80 °C for 24 h and then calcined Point energy-dispersive X-ray spectroscopy (EDS) was in a muffle furnace at 500 °C for 4 h. Al-HS was prepared by taken in an area within 5 nm diameter. DRIFTS was car- the following procedure. Typically, 45 g of Al (NO ) ·9H O ried out with a Nicolet 6700 spectrometer equipped with 3 3 2 was dissolved in 250 mL of deionized water and then was infrared cell (PIKE). The DRIFTS experiments contain mixed with 20 wt% TEAOH solution (tetraethyl ammonium CO adsorption, CO temperature-programmed surface hydroxide) under rigorous stirring till pH reached 5.0. The reduction (CO-TPSR) and determination of surface mixture was continuously stirred for 30 min before trans- hydroxyl group. For CO adsorption, first, the sample ferring into an autoclave equipped with a Teflon-liner. The was reduced under pure H f low at 350 °C for 2 h. After hydrothermal treatment was conducted at 170 °C for 72 h. f lushed by N for 30 min, the sample was degassed under –4 The as-prepared sample was calcined at 500 °C for 5 h with vacuum until that 10  mbar was achieved, and then back- a heating rate of 5 °C/min. ground spectra were collected at designated temperature. Fe-promoted Rh-based catalysts supported on the Second, a flow of 5% CO/He (v/v) was introduced to different Al O samples were prepared by incipient reduced catalysts for 30 min. Finally, the IR spectra were 2 3 wetness method. Before impregnation, Al O samples recorded after f lushed by N for 30 min. For CO-TPSR, 2 3 2 were ground and sieved to 40–60 mesh. Aqueous solu- the IR spectra were recorded under 10% H /Ar (v/v) f low tion of Rh (NO ) .2H O (9.35 wt % Rh, Helishi) and Fe with the temperate linearly increased from 30 to 260 °C 3 3 2 (NO ) .6H O were added dropwise to the given weight after CO adsorption. For the determination of surface 3 2 2 of support. The samples were kept stirring for 2 h and hydroxyl group, the sample was f lushed by N at 300 °C –4 then dried at 110 °C for 12 h. Finally, the samples were and then degassed under 10  mbar vacuum for 2 h. The 1 3 Applied Petrochemical Research (2021) 11:305–316 307 IR spectra were collected at room temperature in the of hysteresis loop indicate all the samples are mesopore −1 range of 3200–4000  cm . structure. The type of hysteresis loop for Al-CM, Al-CP, Al-SG, Al-HS is attributed to H4, H4, H2 and H3, respec- Catalytic reaction tively. It is well known that the type of hysteresis loop is related with the special pore structure [14]. For instance, Reaction was carried out in a stainless fixed bed reactor with the presence of H3 hysteresis loop for Al-HS sample indi- an inner diameter of 10 mm. 1.0 g catalyst (1.3 mL) was cates the slit pore formed by the fibrous or sheet particle. diluted with quartz sand (4.2 mL) and then placed in the Table 1 showed the textural properties of the different sam- middle of the reactor. The catalyst was heated to 350 °C ples. Obviously, the surface areas of the samples are as fol- and reduced in H at a flow rate of 60 mL/min for 10 h, lows: Al-CP (398) > Al-SG (336) > Al-CM (256) > Al-HS then cooled down to the reaction temperature (260 °C) in (90). Among the four alumina samples, Al-HS presents the H flow. Subsequently, 60 mL/min syngas (H /CO = 2) was lowest specific surface area. The enlarging pore effect, as a 2 2 introduced to reactor and kept at a pressure of 2 MPa. The result of hydrothermal treatment, may be responsible for its flow rates of gas were controlled by a Brooks 5050 mass low specific surface area. flowmeter. The effluent gas from the reactor was passed Figure 2a presents the XRD profiles of the four Al O 2 3 through a hot trap and a cold trap successively to separate samples. Typical characteristic peaks of γ-Al O were 2 3 gaseous and liquid products. A 24 h liquid sample was col- observed for all the samples. However, besides γ-Al O , 2 3 lected after steady state was achieved. Outlet gases and liq- characteristic peaks of α-Al O, Al O for Al-HS were 2 3 2 3 uid sample were detected with two chromatographs (Agilent observed in Fig. 2b. The results indicate that Al-HS is con- GC7890A), one is equipped with two TCD to analyze CO, stituted of mixed crystal phases, while Al-CM, Al-CP and CO, N and H using a 5-A molecular sieve column online Al-SG are mainly consistent with the γ-Al O crystal phase. 2 2 2 2 3 and the other fitted with one FID and TCD to separate C –C In general, alumina with different forms presents different 1 6 hydrocarbons, oxygenates and water using Plot Q column specific surface areas with the order of α-Al O << γ-Al O 2 3 2 3 offline. [15], which can be responsible for the lowest specific area of Al-HS material. Results and discussion Characterization of catalysts Characterization of supportThe H -TPR profiles of the four catalysts were presented in Fig. 3. It can be observed two peaks of H consumptions in Figure  1 shows the N adsorption isotherms of the four Fig. 3, namely, one low temperature reduction peak (peak Al O samples. The type IV of isotherms and the existence I) located at 50–300 °C and one high temperature board 2 3 peak (peak II) in the range of 350–550 °C. According to the literature [18], the peak II was ascribed to the reduction of Fe O to Fe O and Fe O to Fe speices. 2 3 3 4 3 4 The theoretical and calculated H consumptions of Fe- promoted Rh-based catalysts are listed in Table 2. Therefore, the peak I should be ascribed to the co-reduction of Rh O 2 3 and Fe O species. The area of peak I is in order of RhFe/ 2 3 Al-HS > RhFe/Al-CM > RhFe/Al-SG > RhFe/Al-CP. Dis- tinctly, the RhFe/Al-HS catalyst exhibits the highest reduc- tion degree and RhFe/Al-CM less. In addition, for RhFe/ Table 1 Textural properties of different type of Al O 2 3 Samples BET surface area Pore volume (cm Pore 2 −1 −1 (m g ) g ) diameter (nm) Al-CM 256 0.46 9.0 Al-CP 398 0.48 4.9 Fig. 1 Nitrogen adsorption isotherms and pore distribution for differ - Al-SG 336 0.43 5.0 ent alumina samples. a Nitrogen adsorption isotherms; b pore distri- Al-HS 90 0.33 14.6 bution 1 3 308 Applied Petrochemical Research (2021) 11:305–316 Fig. 2 XRD profiles of different alumina samples the interaction between metal and support got weaker and the contact between Rh and Fe was enhanced for the above two catalysts. Considering with the activity data, the cata- lysts owning stronger Rh–Fe interaction correspond to the higher ethanol selectivity, which indicates that the weaker interaction between Rh and Al may be conductive to the formation of ethanol. The H -TPD profiles of catalysts are presented in Fig.  4. As can be seen, two peaks were presented on all the sam- ples. The peaks at lower temperature (50–200 °C) and higher temperature (300–500 °C) are denoted as H and H , respec- α β tively. H is ascribed to the weak adsorption of hydrogen and H the strong adsorption of it. It is accepted that H β β is responsible for the hydrogenation ability of Rh-based catalysts [16]. Obviously, RhFe/Al-HS exhibits higher peak area and higher temperature of H compared with the other Fig. 3 H -TPR profiles of Fe-promoted Rh-based catalysts catalysts. This indicates that the RhFe/Al-HS has stronger ability to activate hydrogen, which is helpful for promoting the catalytic activity. Al-HS and RhFe/Al-CM catalysts, the reduction temperature Since metal particles cannot be easily distinguished from alumina, dark-field imaging technique (HAADF) was used of peak I shifts to lower temperature in comparison with the corresponding un-promoted ones. It can be concluded that to observe surface metal particle morphology of catalysts Table 2 H consumption of a a Catalysts Reduction Calculated and Catalysts Reduction Calculated and catalysts b b peak (°C) theoretical H consump- peak (°C) theoretical H consump- 2 2 tion (mmol) tion (mmol) a b a b RhFe/Al-CM 111 40 (249 ) Rh/Al-CM 120 36 (58 ) a b a b RhFe/Al-CP 167 37 (249 ) Rh/Al-CP 131 22 (58 ) a b a b RhFe/Al-SG 210 35 (249 ) Rh/Al-SG 160 30 (58 ) a b a b RhFe/Al-HS 114 53 (249 ) Rh/Al-HS 132 48 (58 ) H consumption was calculated with 50 mg CuO as external reference Sample weight, 200 mg 1 3 Applied Petrochemical Research (2021) 11:305–316 309 has a higher Rh dispersion while RhFe/Al-CP and RhFe/ Al-SG shows a lower dispersion compared with RhFe/ Al-CM. However, RhFe/Al-CP and RhFe/Al-SG should have higher Rh dispersion according to STEM results. The possible reason for the inconsistent results given by chemisorption and STEM may be due to that the smaller Rh particles enhanced the interaction with alumina sup- port, resulting in a part of Rh particles incorporation into the alumina skeleton. XPS spectra of fresh catalysts were shown in Fig. 7. The XPS intensity ratios and binding energies (BEs) of Rh 3d and Fe 2p was presented in Table 5. The surface atom con- centrations of Rh/Al are in the order of RhFe/Al-HS > RhFe/ Al-CM > RhFe/Al-CP > RhFe/Al-SG. The ratio of Rh/Fe for RhFe/Al-HS is 0.28, which is even higher than the expected value (0.27). The above indicated that more rhodium spe- cies enrichment on the surface of RhFe/Al-HS. In addition, Fig. 4 H -TPD profiles of Rh-based catalysts supported on different the binding energy of Rh 3d and Fe 2p of RhFe/Al-HS type of Al O 5/2 3/2 2 3 exhibits obvious shift compared to the other catalysts. This further proved stronger interaction exists between Rh and after reduction. Figure 5 shows STEM images of catalysts. Fe for this catalyst. From STEM images (Fig. 5A, D) of the RhFe/Al-CM and The interaction between highly dispersed active metal RhFe/Al-HS, it can be observed that the metal particle sizes and support for different Al O -supported catalysts var- 2 3 of them are both in the range of 1–4 nm with the average ies greatly from the above analysis. Surface hydroxyls of diameter of 2.7 and 3.8 nm, respectively. However, it can be catalysts may be one possible reason for this interaction. seen from the images of RhFe/Al-SG (Fig. 5C) and RhFe/ Figure  8 presents the surface hydroxyl group of catalyst Al-CP (Fig. 5B) that the metal particle size is approximately and supports. The hydroxyl groups on the surface of alu- about or less than 1 nm. The favorable metal particle size mina were classified into I, II, and III, in which type I > type for ethanol formation was reported to be 2–4 nm [17]. In our II > type III in order of frequencies of them [15]. As shown −1 case, the metal particles less than 2 nm may form stronger in Fig. 8, the bands at 3723 (I), 3687 (II), 3572  cm (II or interaction with alumina support leading to decreased III) are ascribed to the absorption of isolated —OH groups, reducible species according to the TPR results. Figure 6 weakly H-bonded and strongly H-bonded OH groups, shows the corresponding EDS spectra of catalysts, in which respectively [18]. It can be seen from Fig. 8a that the inten- one point within the metal particle region (P1) and the other sity of hydroxyl groups for alumina samples is in order of metal particle-free region (P2) was selected at random. Al-SG > Al-CP > Al-CM > Al-HS. The corresponding cata- There exists Rh and Fe signal in the EDS spectra of P1 for lysts also show the same order as can be seen in Fig. 8b. all the samples, indicating Rh and Fe are in close contact. Some researchers believed CO comes from the reaction of However, only Fe signal was observed in the EDS spec- strongly adsorbed CO species with hydroxyl groups of Al O 2 3 tra of P2 for RhFe/Al-CP, RhFe/Al-SG and RhFe/Al-CM support [20]. This is in accordance with our experiments samples, which means some single Fe species distribute results that RhFe/Al-SG shows the highest C O selectivity on the surface of these catalysts. Exceptionally, on RhFe/ with the most amount of surface hydroxyl group. However Al-HS catalyst, there is a weak signal of Rh and Fe within it may be, there is no doubt that the metal–support interac- metal particle-free region on the EDS spectrum, indicating tion increases with the increasing amount of hydroxyl group. that more Fe and Rh species are in intimate contact on the Moreover, it can be observed from Fig. 8b that the ratio of surface of support. different type of hydroxyl groups for RhFe/Al-CP and RhFe/ To get the information of Rh dispersion of catalysts Al-SG catalyst is similar with that of RhFe/Al-CM. How- more accurately, CO and H chemisorption experiments ever, the proportion of type I (isolated hydroxyl) increases in a pulse mode were both carried out. Tables 3 and 4 show for RhFe/Al-HS. the results of the chemisorption of CO and H , respectively. The infrared spectra of the catalysts after CO adsorption −1 The results summarized in Tables 3 and 4 show that Rh at 30 °C are shown in Fig. 9. The 2055  m band can be dispersion calculated by the H chemisorption is higher ascribed to linear adsorbed CO (CO (l)) and double band −1 than the CO chemisorption. Despite this, the same trend at 2084 and 2017  cm can be assigned to the symmet- about Rh dispersion can be gotten. Namely, RhFe/Al-HS ric and asymmetric carbonyl stretching of the dicarbonyl 1 3 310 Applied Petrochemical Research (2021) 11:305–316 Fig. 5 STEM images of Rh-based catalysts supported on different type of Al O . A RhFe/Al-CM; B RhFe/Al-CP; C RhFe/Al-SG; D RhFe/ 2 3 Al-HS + 0 Rh (CO) (CO(gem)) [19].The board band centered at isolated hydroxyl groups is favorable to transfer Rh into −1 + 1860  cm is assigned to bridge bonded CO (CO (b)), Rh [21]. Therefore, we presume that more abundant whose intensity was found to be related with the selectiv- isolated hydroxyl groups are responsible for more Rh ity of CH [10]. It is widely accepted that CO (gem) is on RhFe/Al-HS. Moreover, the bridged adsorbed CO is formed on Rh , which is favorable for CO insertion, while strongest on the RhFe/Al-HS catalyst, which is consist- CO (l) and CO(b) formed on Rh , which is helpful for CO ent with the activity data for methane selectivity of this dissociation [20]. It can be seen from Fig. 9 that the inten- catalyst (see Table 6). sity of CO (gem) is in the order of RhFe/Al-HS > RhFe/ To know the dissociation ability and hydrogenation rate Al-CM > RhFe/Al-CP > RhFe/Al-SG, which means that of the catalyst, CO-TPSR was carried out. As shown in + −1 RhFe/Al-HS has more Rh center. It was reported that Fig. 10, the band at 3015  cm is ascribed to the stretching 1 3 Applied Petrochemical Research (2021) 11:305–316 311 Fig. 6 EDS profiles of Rh-based catalysts supported on different type of Al O . a RhFe/Al-CM; b RhFe/Al-CP; c RhFe/Al-SG; d RhFe/Al-HS 2 3 vibration of gaseous methane, which intensity can be used Table 3 CO chemisorption results of catalysts as a tool to measure CO dissociation ability of catalyst −1 CatalystsCO-chemisorbed/μmol g Metal dis- [22]. The formation of methane for RhFe/Al-CM, RhFe/ persion/% Al-CP, RhFe/Al-SG and RhFe/Al-HS appeared at 220 °C, RhFe/Al-CM 71.2 36.6 210 °C, 200 °C, 190 °C, respectively, which means that RhFe/Al-CP 64.6 32.7 RhFe/Al-HS possessed strongest CO dissociation ability, RhFe/Al-SG 43.1 22.2 whereas the RhFe/Al-CM showed the weakest dissociation. RhFe/Al-HS 116.6 58.6 In addition, it was observed from the spectra in the range −1 of 1800–2200  cm that the hydrogenation rate of adsorbed CO is as follows: RhFe/Al-SG > RhFe/Al-CP > RhFe/ Al-HS > RhFe/Al-CM. Table 4 H chemisorption results of catalysts Table  6 lists the activities data of the catalysts. These −1 Catalysts H -chemisorbed /μmol g Metal data were taken with 2:1 H :CO mix at a total pressure dispersion 2 MPa and a temperature of 260 °C. The results show that /% product contains CO , hydrocarbons, methanol, ethanol and RhFe/Al-CM 92.3 47.4 other oxygenates. Among the by-products, C O, CH and 2 4 RhFe/Al-CP 89.6 40.1 higher hydrocarbons should be inhibited more seriously RhFe/Al-SG 85.2 35.1 due to higher separation cost and lower economic value. RhFe/Al-HS 160.8 82.7 Among the four catalysts, RhFe/Al-HS shows the highest 1 3 312 Applied Petrochemical Research (2021) 11:305–316 Fig. 7 XPS spectra of a Rh 3d and b Fe 2p for the catalysts Table 5 Binding energy and surface composition of catalysts determined by XPS Catalysts Atomic ratio from XPS Binding energy (eV) a a Rh/Al Rh/Fe Rh/FeRh 3dFe 2p 5/2 3/2 RhFe/Al-GM 0.79 0.24 0.27 309.3 710.4 RhFe/Al-CP 0.74 0.22 0.27 309.4 710.4 RhFe/Al-SG 0.64 0.19 0.27 309.4 710.4 RhFe/Al-HS 1.21 0.28 0.27 309.5 710.8 Atomic ratio of Rh/Fe expected in the whole sample Fig. 8 Surface hydroxyl groups of different Al O support and corresponding catalysts a support; b catalysts 2 3 1 3 Applied Petrochemical Research (2021) 11:305–316 313 Al-CP > Al-SG > Al-CM > Al-HS. Among them, Al-HS- supported catalysts have the highest ethanol selectivity although with the lowest specific area, indicating the spe- cific area of the catalysts was not in direct proportion to their catalytic performances. XRD showed the presence of mixed Al O crystalline phases in the Al-HS support pre- 2 3 pared by hydrothermal synthesis. Compared to the Al-CM carrier, the greater specific area for Al-CP and Al-SG supported catalyst may lead to too stronger interaction between meatal and support, which is not good for Rh–Fe interaction. Al-HS carrier with nano-fibrous morphology supported catalysts may weaken the interaction between metal and support, correspondingly, the contact between Rh and Fe is strengthened, which can also be proved by H -TPR and EDS. The lower surface hydroxyl group inten- sity of Al-HS support may be responsible for the above interactions. The higher specific surface area Al-SG- and Fig. 9 FTIR spectra of adsorbed CO on different type of Al O -supported Rh-based catalysts. a RhFe/Al-CM; b RhFe/Al-CP; 2 3 Al-CP-supported catalysts exhibited a higher CO conver- c RhFe/Al-SG; d RhFe/Al-HS sion, but most of CO was converted into hydrocarbon and CO . From the results of TPR, it can be inferred that there exists more single Fe species over the above two catalysts, ethanol selectivity (~ 24.6%) and the lowest C O selectiv- 2 which is favorable for hydrogenation and water gas shift ity (~ 8%), and also, RhFe/Al-CM exhibits higher ethanol reaction. On the other hand, some researchers believed CO selectivity with moderate CO conversion. It can be seen that comes from the reaction of strongly adsorbed CO species Al-CP- and Al-SG-supported RhFe catalysts exhibit higher with hydroxyl groups of Al O support [20, 24]. This is in 2 3 CO conversion but only with 7.3% and 5.8% ethanol selec- accordance with our experiments results that RhFe/Al-SG tivity, respectively. In general, for CO conversion, RhFe/ has strongest hydroxyl group and shows the highest CO Al-SG > RhFe/Al-CP > RhFe/Al-CM > RhFe/Al-HS. And selectivity. for ethanol selectivity, RhFe/Al-HS > RhFe/Al-CM > RhFe/ The formation of Rh is also important for the formation Al-SG > RhFe/Al-CP. of active sites. It is widely accepted that the dicarbonyl spe- 0 + 3+ 2+ It has been reported that (Rh -Rh )–O–Fe (Fe ) is x y cies can only be formed on highly dispersed rhodium [24]. the active site for the formation of ethanol [23]. Therefore, H and CO chemisorptions showed that Rh dispersion was in the synergy effect between Rh and Fe, the formation of order of RhFe/Al-HS > RhFe/Al-CM > RhFe/Al-CP > RhFe/ Rh would be beneficial to form the active site for ethanol Al-SG. This indicated Al-HS-supported catalyst has a higher synthesis. Rh dispersion than others, which also evidenced by XPS. On The characterization and catalytic results show that the other hand, DRIFTS of CO adsorption shows that RhFe/ the Rh–Fe interactions are greatly affected by the sup- Al-HS has more dicarbonyl species, which comes from Rh port. The surface areas of the samples are as follows: sites. Table 6 CO hydrogenation over a Catalysts X /% Product selectivity/C% CO Rh-based catalysts supported on different type of Al O CH C HC MeOH EtOH CO Others 2 3 4 2+ 2 RhFe/Al-CM 25.8 28.6 13.3 15.2 20.5 14.2 8.2 RhFe/Al-SG 38.6 24.1 23.9 5.7 7.3 29.1 9.9 RhFe/Al-CP 35.4 24.8 27.2 5.2 5.8 28.8 8.2 RhFe/Al-HS 13.6 29.1 10.1 19.4 24.6 8.0 8.7 Catalyst:1 g; reaction conditions: T = 260 °C P = 2 MPa, H /CO = 2, space velocity = 3600 mL/(h. gcat) Product selectivity = n M /∑(n M ), where n and M are the carbon atoms number and molar percent of i i i i i i product i, respectively Oxygenates with two or more carbons except ethanol(acetaldehyde, acetone, n-propanol, i-propanol, n-butanol, i-butanol and n-pentanol 1 3 314 Applied Petrochemical Research (2021) 11:305–316 Fig. 10 DRIFTS spectra of CO-TPSR on different type of Al O -supported Rh-based catalysts. a mRhFe/Al-CM; b RhFe/Al-CP; c RhFe/Al-SG; 2 3 d RhFe/Al-HS According to widely accepted mechanism for ethanol syn- Conclusion thesis (see Fig. 11), a good Rh-based catalyst should balance the ability of CO dissociation, CO insertion and CO hydro- Four kinds of alumina were used as supports of Fe- genation. DRIFTS study indicates that RhFe/Al-HS have promoted Rh-based catalysts. The ethanol selectivity of stronger CO dissociation capacity, moderate CO hydrogena- RhFe/Al-HS catalysts supported on alumina prepared tion rate and more Rh sites for CO insertion. Therefore, by hydrothermal synthesis was superior than the other stronger CO dissociation capacity, moderate CO hydrogena- catalysts, which were mainly due to the moderate metal tion rate and stronger CO insertion may be responsible for particle size, the enhanced synergic between Rh and Fe, the good performance of RhFe/Al-HS. and the improved the Rh dispersion. Moreover, moderate CO hydrogenation rate, stronger CO dissociation and CO insertion ability was also favorable to increase the ethanol selectivity. 1 3 Applied Petrochemical Research (2021) 11:305–316 315 Fig. 11 Mechanism of ethanol formation [25] Acknowledgements This work was supported by the National Natural monoxide hydrogenation on silicon dioxide-supported rhodium- Science Foundation of China (No. 31671797). iron catalysts. J Phys Chem 89:4440–4443 9. Chen WM, Ding YJ, Song XG, Wang T, Luo HY (2011) Pro- motion effect of support calcination on ethanol production from Open Access This article is licensed under a Creative Commons Attri- CO hydrogenation over Rh/Fe/Al O catalysts. Appl Catal A bution 4.0 International License, which permits use, sharing, adapta- 2 3 407:231–237 tion, distribution and reproduction in any medium or format, as long 10. 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Journal

Applied Petrochemical ResearchSpringer Journals

Published: Dec 1, 2021

Keywords: Ethanol synthesis; Al2O3; Interaction; DRIFTS

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