Catalytic performance of ternary Mg-Al-Ce oxides for ethanol conversion into 1-butanol in a flow reactor

Olga V.Larina，Karina V.Valihura，Pavlo I.Kyriienko，Nina V.Vlasenko，Dmytro Yu.Balakin，Ivan Khalakhan，Katerina Veltruská，Tomaž Čendak，Sergiy O.Soloviev，Svitlana M.Orlyk

（1.L.V.Pisarzhevsky Institute of Physical Chemistry of the National Academy of Sciences of Ukraine,31 Prosp.Nauky, 03028 Kyiv, Ukraine；2.Institute of Physics of the National Academy of Sciences of Ukraine, 46 Prosp.Nauky, 03028 Kyiv, Ukraine；3.Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science,V Holešovičkách 2, 18000 Prague, Czech Republic；4.National Institute of Chemistry, Department of Inorganic Chemistry and Technology,Hajdrihova 19, SI-1001 Ljubljana, Slovenia）

Abstract: An investigation of hydrotalcite-derived ternary Mg-Al-Ce oxides as catalysts for vapour phase condensation of ethanol to 1-butanol in a flow reactor under atmospheric pressure was carried out.The Mg-Al-Ce oxide systems with Mg/(Al +Ce) ratio from 1 to 4 were synthesized and characterized by XRD, SEM, NMR, and XPS.The study of acid-base characteristics of the systems with different Mg/(Al+Ce) ratio by NH3/CO2 quasi-equilibrium thermal desorption techniques shows that the ratio of the catalyst oxide components (Mg, Al, Ce) can provide acid/base capacity ratio close to 3 for the effectivity of the target process.The highest selectivity 68% is reached over Mg-Al-Ce oxide catalyst with the ratio of Mg/(Al+Ce) = 2.

Key words: ethanol；1-butanol；Mg-Al-Ce hydrotalcite-derived oxides；acid-base properties

1-Butanol (BuOH) is mainly used as a solvent,extractant in pharmaceutical industry and cosmetics,chemical intermediate for the production of methacrylate esters, butyl acrylate, and it is more desirable blend fuel compared to ethanol[1, 2].BuOH is traditionally manufactured by hydroformylation of propylene in the presence of different bases with subsequent hydrogenation of the resulting butanal under high pressure (～30 MPa) using two different catalysts (Co, Rh or Ru in the first step and Ni in the second step)[3].The use of homogeneous system of catalyst and base results in difficult separation, costly preparation of transition-metal complexes and related environmental issues[4].Nowadays catalytic transformation of bioethanol into BuOH via Guerbet coupling reaction over solid catalysts is a sustainable and environmentally friendly alternative to the oil process[5].The main steps of the commonly proposed mechanism for Guerbet coupling[4, 6]are: (1)dehydrogenation to acetaldehyde, (2) aldol selfcondensation to crotonaldehyde, (3) hydrogenation of the condensation product into 1-butanol.

There are two ways of implementation of the EtOH-to-BuOH conversion: the first is condensation at lower temperature and elevated pressure in a batch reactor, and the second is vapour-phase condensation at 523−623 K and atmospheric pressure in a flow reactor[7].The practical interest lays in development of solidphase catalysts enable to work in a flow system with a fixed bed, which can be easily separated from the reaction mixture[5].Both acid and base sites of catalysts take part in Guerbet reaction[8].A major challenge for further development of EtOH-to-BuOH process is finding an optimal ratio between quantity and strength of the sites[9].

A number of studies have shown that MgO-Al2O3oxide systems prepared by thermal treatment of Mg-Al hydrotalcites are active in EtOH-to-BuOH transformation due to their tunable bifunctional acidbase properties, high surface area, and structural stability[10-13].The Mg:Al ratio was found to be one of the critical parameters influencing their catalytic properties[12, 14-16].

Some works have been devoted to a search for promoters of MgO-Al2O3catalysts for Guerbet reaction.Marcu et al[17, 18]concluded that Cu- and Pd-containing catalysts were the most active in terms of ethanol conversion and 1-butanol selectivity amongM-MgOAl2O3(M= Pd, Ag, Mn, Fe, Cu, Sm, Yb) systems obtained from hydrotalcite precursors.A similar positive effect of the modification of MgO-Al2O3with Pd, Rh, Ni, Cu was shown for the synthesis of β-branched alcohols by Guerbet reaction in a batch reactor[19-21].However,introduction of transition metals into Mg-Al hydrotalcite-derived oxides increases reaction rates of step (1) and step (3) of Guerbet coupling, because activation of hydrogen atoms in α-positions of alcohols and hydrogenation by H2occurs easier on metals.To implement EtOH-to-BuOH process in a flow system with fixed bed, it is important to find promoters accelerating the rate-limiting step (2), aldol condensation of acetaldehyde, which takes place over base and acid sites of the oxide systems.

Wang et al[22]showed that the presence of relatively small amounts of lanthanum or yttrium in Mg-Al oxides provided its enhanced basicity in terms of the amount of medium/strong sites and, as a result,higher activity in liquid-phase acetone self-aldolization.Diez et al[23]suggested that Ce4+/Ce3+active sites on ceria surface were both acid-base and redox sites engaged in activation of the reactants (Suzuki-Miyaura cross-coupling of aryl iodides and phenylboronic acid).Wu et al[24]showed a promising approach to useM-CeO2(M= Cu, Fe, Co, Ni, Pd) supported on activated carbon in EtOH-to-BuOH process.Synergistic effect of two components is observed: the first is a metal with high capability to activate hydrogen and the second is CeO2with enhanced basicity for aldol condensation of acetaldehyde.Vlasenko et al[25, 26]showed that doping of zirconia with ceria led to increase in its catalytic activity in EtOH-to-BuOH process towards 1-butanol.

The present work is primarily concerned with investigation of Mg-Al-Ce hydrotalcite-derived oxide systems as catalysts for EtOH-to-BuOH process in a flow system under atmospheric pressure.The effect of Mg/(Al+Ce) ratio on their acid-base properties and catalytic behaviour in EtOH-to-BuOH process was studied.The Mg-Al-Ce oxide catalysts were thoroughly characterized via X-ray powder diffraction (XRD),field emission scanning electron microscopy (SEM)coupled with energy dispersive X-ray analysis (EDX),nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and NH3/CO2quasiequilibrium thermal desorption (QE-TD) techniques.

1 Experimental

1.1 Catalyst preparation

Mg-Al-Ce hydrotalcites were synthesized using co-precipitation method under high supersaturation conditions (pH 10-12).Various amounts of 1 mol/L solutions of Mg(NO3)2·6H2O, Al(NO3)3·9H2O and Ce(NO3)3·6H2O were dropped into 200 mL of a buffer solution containing 1.6 mol/L NaOH and 0.1 mol/L Na2CO3with vigorous stirring at 358 K.The mixtures were kept at 358 K for 24 h.Then, the precipitates were separated using hot filtration, washed several times with warm deionized water until neutral pH was reached, and dried at 393 K for 6 h.The resulting hydrotalcites were calcined at 873 K for 5 h to obtain mixed Mg-Al-Ce oxides.The same procedure was used for parent MgO, Al2O3and CeOxsynthesis.

It should be noted that Al/Ce atomic ratio remained the same (0.9∶0.1) for all Mg-Al-Ce samples,while Mg/(Al+Ce) atomic ratio changed from 4 to 1.Therefore, the obtained samples with Mg/(Al+Ce) of 1−4 were labelled as Mg-Al-Ce-1(2, 4).

1.2 Catalyst characterization

Powdered X-ray diffraction patterns of hydrotalcites, mixed Mg-Al-Ce oxide samples, and parent oxides were recorded using a D8 Advance(Bruker AXS GmbH, Germany) diffractometer with a CuKα radiation (nickel filter,λ= 0.154 nm).

Scanning electron microscopic images were collected with a Tescan MIRA 3 microscope operating at 30 keV electron beam energy.Elemental mapping analysis was performed via energy dispersive X-ray spectroscopy (EDX) on a Bruker XFlash detector mounted directly into the SEM.

The specific surface area (Ssp) of the solids was determined using a chromatographic method by thermal nitrogen desorption studies on a GKh-1 instrument with a gas mixture containing 5% of N2in helium.

Textural characteristics of the Mg-Al-Ce-2 sample were determined by low-temperature (77 K) nitrogen ad(de)sorption measured with Sorptomatic-1990 porous analyser.Before the measurements the sample was evacuated at 473 K for 2 h.Micropore volume were estimated fromt-plot method.The mesopore sizes of the samples were obtained by the Barrett-Joyner-Halenda method.

27Al magic angle spinning NMR spectra were obtained on an Agilent 600 MHz spectrometer at room temperature using 3.2 mm MAS probe.27Al Larmor frequency was 156.4 MHz and AlCl3·6H2O was used as a standard reference (chemical shift 0).The27Al MAS NMR spectra were recorded using a single pulse acquisition with small pulse angle (π/12), at a spinning speed of 20 kHz and with a recycle delay of 4 s.All spectra were normalized towards the under curve area and fitted using a Czjzek model implemented in DMFit software[27].

X-ray photoelectron spectra were recorded on an ultra-high vacuum apparatus equipped with SPECS Phoibos 150 hemispherical analyser using a conventional AlKα source.The samples were pressed into indium foil installed inside the main UHV chamber called Analysis Chamber.The XPS spectra were processed with the KolXPD software subtracting the Shirley background and using Voigt profiles for fitting.The C 1sphotoemission line was used for the binding energy calibration.

Investigation of the oxygen-metal ion chargetransfer bands in the samples was performed by UV-vis diffuse reflectance spectroscopy using spectrophotometer Specord M40 in the range of 200−800 nm.Before the analysis, the samples were carefully grounded and loaded into a cuvette.The measurements were carried out in air at room temperature.

The acid and base characteristics of the catalysts were determined by a quasi-equilibrium thermal desorption of NH3and CO2using vacuum gravimetric apparatus[28].The samples were pre-treated via heating under vacuum (0.133 Pa) to constant weight at 773 K.The adsorption of NH3and CO2was carried out at room temperature until the uptake ending, and the surplus was removed under vacuum.The weight change of the samples was monitored with stepwise temperature increasing.The total concentration of acid and base sites was determined from the adsorbed amount of NH3and CO2on the sample surface at 323 K.The strength of acid/base sites was evaluated by desorption temperature of the probe substance[29].

1.3 Catalytic activity measurement

Catalytic activity tests were carried out in a fixedbed flow quartz reactor with inner diameter of 10 mm and catalysts grains of 0.25−0.5 mm.Ethanol (95.6%,the rest was H2O) was introduced to the hot reactor via a syringe infusion pump for evaporation and carrier gas Ar at a flow rate of 10 mL/min.All catalytic experiments were conducted at fixed weight hourly space velocity(WHSV) of 0.14 g/(gcat∙h) (corresponding alcohol gashourly space velocity (GHSV) was 79.4 h−1), 548 and 573 K and atmospheric pressure.Prior to the experiment,catalysts were treated in Ar at 773 K for 1 h and then cooled down to the experiment temperature.The reactor effluent sampling for analysis was performed via heated tap with interval of 0.5 h during 8 h.The reaction products were analysed on a gas chromatograph(NeoCHROM, Ukraine) equipped with an FID detector and a capillary column (HP-FFAP, 50 m×0.32 mm).

Catalytic behaviour of the catalysts was characterized by EtOH conversion (x), selectivity of products (si), yield of products (yi), specific rate of EtOH conversion (rEtOH) and formation of products (ri):

2 Results and discussion

2.1 Structural and textural characteristics

The XRD patterns of the as-prepared hydrotalcitelike materials are shown in Figure 1(a).The diffractograms are characterized by the presence of reflections located at approximately 11.5°, 23.3°, 34.6°,38.7°, 46.0°, 60.7° and 62.0° which are assigned to(003), (006), (009), (015), (018), (010) and (113)planes of a hydrotalcite structure, respectively[30].Also,the CeO2phase is detected.The peaks at 28.7° and 47.6° are assigned to (111) and (220) reflections of cubic cerianite (fluorite structure)[31].At the same time, the presence of Ce(OH)3or CeCO3OH cannot be excluded[32].

After calcination at 873 K, the hydrotalcite structure disappears with a generation of mixed oxides (Figure 1(b)).Two weak broad reflections positioned at approximately 43.2° and 62.7° correspond to diffraction of (200) and (220) planes of a MgO periclase phase, respectively[33].Whereas the bands at about 28.7°, 33.0°, 47.6°, 56.6° which are assigned to(111), (200), (220) and (310) planes of a fluorite CeO2.The higher ionic radius of Ce3+(Mg2+0.86 Å, Al3+0.67 Å, and Ce3+1.02 Å) may prevent intercalation of larger cerium cations in the hydrotalcite layered structure[22].Therefore, we observe the formation of corresponding segregated CeO2species deposited on the surface of both as-prepared hydrotalcites and calcinated samples[32].Ce3+and Al-containing phases are too poorly crystalline to be identified by XRD.It can indicate a very high dispersion of Mg, Al, and Ce oxides with the formation of mixed oxide phase.

Figure 1 XRD of the samples before (a) and after treatment at 873 K (b)1: Mg-Al-Ce-4; 2: Mg-Al-Ce-2; 3: Mg-Al-Ce-1

Figure 2 shows micrographs of Mg-Al-Cecontaining oxide samples after the treatment at 873 K.The Mg-Al-Ce-4 sample possesses a well-developed platelet structure typical for layered materials[34, 35].The aggregates are composed of plate-like crystals with 150−200 nm length and 15−25 nm width.The micrographs of the Mg-Al-Ce-2 also allow assuming the presence of platelike crystals of ～80 nm in length and ～15 nm in width.Whereas, the Mg-Al-Ce-1 sample consists of more amorphous aggregates, in which plate-like crystals are less noticeable.Since the Al2O3is amorphous, an increase of Al content in the samples causes the disappearing of platelet structure.This effect was also observed in the works[15, 35].The uniform distribution of magnesium,aluminium and cerium atoms on the surface of the samples under study (EDX elemental mapping images in Figure S1, ESI) indicates homogeneity of the prepared Mg-Al-Ce oxide systems.

Figure 2 SEM images of the samples after a treatment at 873 K(a): Mg-Al-Ce-4; (b): Mg-Al-Ce-2; (c): Mg-Al-Ce-1

Textural characteristics of Mg-Al-Ce-2 sample have been investigated by low-temperature nitrogen sorption (Table S1).The sample is characterized by the typical micro-mesoporous structure for hydrotalcitederived materials.The maximum diameter of mesopores is ～17 nm and the proportion of micropore volume in the total pore volume is 15%.

27Al magic angle spinning (MAS) NMR spectra of Mg-Al-Ce-containing oxide samples and the corresponding fit calculated using DMFit software are presented in Figure 3.The fitting results are summarized in Table 1.There are following main signals: the first related to Al3+cations tetrahedrally coordinated to oxygen (Altetra, chemical shiftδ=65−84), the second assigned to Al3+cations pentahedrally coordinated to oxygen (Alpenta, chemical shiftδ= 33−35) and the third assigned to octahedrally coordinated Al3+cations (Alocta, chemical shiftδ=11−18).Herewith, Altetraand Aloctasignals contain two components each.

Figure 3 27Al MAS NMR spectra of the samples after a treatment at 873 K(a) and the corresponding fit calculating using DMFit software (b)1: Mg-Al-Ce-4; 2: Mg-Al-Ce-2; 3: Mg-Al-Ce-1

Table 1 27Al NMR analysis of Mg-Al-Ce oxide compositions

Recently, we have reported that Mg-Al oxide samples with high and low amount of alumina contain segregated γ-alumina phase (27Al NMR signals at chemical shifts of ～11 and ～69)[15].The same tendency is observed for the Ce-containing systems (Figure 3):the samples with high (Mg-Al-Ce-1) and relatively low(Mg-Al-Ce-4) content of aluminium and cerium contain segregated γ-alumina phase.For the Mg-Al-Ce-2 sample,the lines at ～11 and 68−70 are absent or hidden under other broader signals.27Al NMR lines at 14−18 and 80−84 may be assigned to octahedrally and tetrahedrally coordinated aluminium cations involved in a highly dispersed Mg-containing phase[36], while the line at 33−35 is probably attributed to pentahedrally coordinated aluminium cations bonded with cerium species.Such line is not observed in27Al MAS NMR spectra of Mg-Al oxide systems[15].Since the Mg-Al-Ce-1 and Mg-Al-Ce-4 samples contain the segregated Al2O3phase, Al3+is less involved in the interaction with cerium species and, thus, the pentahedrally coordinated aluminium cations are not detected in NMR spectra.

Therefore, all samples under study exhibit the presence of aluminum included in highly dispersed phase of magnesium oxide, regardless of aluminum and cerium content.A segregated phase of γ-alumina is observed in the samples with high and relatively low amount of aluminium and cerium.The effect of Ce doping probably results in the formation of aluminiumcerium oxide bonds as evidenced by the appearance of pentahedrally coordinated Al species in the Mg-Al-Cecontaining oxide samples.

Figure 4 presents Ce 3dXPS spectra of Mg-Al-Ce-containing oxide samples together with parent CeOх.The spectrum of the latter contains three pairs of spinorbit doublet peaks (v-u,v''-u'',v'''-u''') corresponding to Ce4+[37, 38].These Ce 3dpeaks are also observed for Mg-Al-Ce oxide samples indicating the presence of CeO2,in accordance with the XRD data (Figure 1).With an increase in the cerium content (Mg/(Al+Ce) ratio changes from 4 to 1), the intensity of the Ce4+peaks rise.A similar tendency has been observed for the Pt/CeO2-Al2O3catalysts[39].Beside Ce4+components the contributions of Ce3+(v0-u0,v'-u')[37, 40]states are also present for all mixed samples.It may indicate the presence of Ce2O3or mixed Ce-Mg and Ce-Al oxides[41].Summarizing the obtained XPS data, the phases containing both Ce3+and Ce4+cations are concluded to be present in the Mg-Al-Ce samples.The ratio of Ce4+/Ce3+peaks increases with an increase in cerium content.Since the Ce/Al atomic ratio is the same for all Mg-Al-Ce oxide systems and based on the data[42, 43], it can be assumed that Ce3+is stabilized by incorporation into the surface of the Mg-Al-containing phase.In turn,CeO2particles containing Ce4+are formed on the surface of the Mg-Al oxide system.

Figure 4 Ce 3d XPS spectra of the samples after a treatment at 873 K1: Mg-Al-Ce-4; 2: Mg-Al-Ce-2; 3: Mg-Al-Ce-1; 4: CeOx

UV-vis DR spectra of the samples after treatment at 873 K are graphically represented in Kubelka-Munk(R) plots in Figure S2.Pristine ceria shows three characteristic bands at 255, 285, and 340 nm corresponding tocharge transitions, and interband transitions, respectively[44, 45].As can be noted, the samples under study exhibit wide and shifted absorption bands, and a prominent shift can be observed towards higher wavelengths than that of pristine ceria.The changes in the position of these bands may be caused by the interaction of Ce4+with Mg-Al oxide, which leads to the distortions in the CeO2lattice.In turn, Ce3+ions appear to be incorporated into the structure of the Mg-Al-containing phase, which also provokes lowering of symmetry and consequent strain development at the Ce sites[46, 47].The presence of Ce3+← O2-and Ce4+← O2-bands in the UV-vis DR spectra simultaneously justifies the results of XPS measurements (Figure 4).Moreover, according to the ref.[48]the wavelength corresponding to the UV absorption edge of ceria occurring at ＜ 375 nm can be used to probe the presence of finer crystallite size 4.5−8.5 nm, which cannot be detected by XRD[47, 49].Therefore, the results of UV-vis DRS study are also consistent with obtained XRD data (Figure 1), where the presence of Ce2O3is not identified.

Thus, as-synthesized precursors of Mg-Al-Ce oxide systems consist of hydrotalcite crystalline structures; cubic cerianite (CeO2) phase is also detected.After calcination, the hydrotalcite structure disappears with a generation of mixed oxides, while a very high dispersion of Mg, Al, and Ce oxides is observed.NMR data confirm the presence of aluminium atoms embedded in a highly dispersed phase of magnesium oxide.XPS and UV-vis DRS data reveal the presence of Ce3+and Ce4+containing phases.Apparently, CeO2particles are formed on the surface of Mg-Al oxide system and partially Ce atoms diffuse into hydrotalcite structure and interact with Al atoms forming Ce3+states.

2.2 Characterization of acid-base properties

Quasi-equilibrium thermal desorption of ammonia and carbon dioxide was used to determine acid and base characteristics (the concentration and strength of acid/base sites) of Mg-Al-Ce oxide systems and parent CeOx, Al2O3and MgO oxides.The data on acid/base capacity and surface density of the sites are summarised in Table 2.The distribution profiles of acid/base sites as a function of desorption temperature of NH3/CO2are presented in Figure S3.

On the surface of parent MgO there are super weak and weak acid sites (limit temperature of NH3desorption is 473 K).The total acid capacity does not exceed 0.15On the contrary, parent Al2O3has a large number of weak, medium and strong acid sites (limit temperature of NH3desorption is 623 K).The total acid capacity is high (0.94The cerium oxide prepared by the same procedure also has weak, medium, and strong acid sites and the same limit desorption temperature of ammonia as for Al2O3.However, its acid capacity is lower than that of parent Al2O3

We have previously shown that introduction of aluminium cations into MgO leads to an increase in total acid capacity of Mg-Al oxide system due to the formation of a large number of medium and strong acid sites and, as a result, the limit temperature of ammonia desorption for Mg-Al oxide samples reaches 673 K[15].Similarly, in the case of introduction of both aluminium and cerium cations, the formation of stronger acid sites is also observed, while the limit temperature of NH3desorption rises up to 673 K (723 K for Mg-Al-Ce-2 sample).The acidity spectrum of Mg-Al-Ce oxide systems includes four types of sites: super weak, weak,medium and strong.A change in Mg/(Al+Ce) ratio towards an increase in aluminium and cerium content(4 → 1) leads to increase in total acid capacity from 0.56for Mg-Al-Ce-4 to 0.63for Mg-Al-Ce-2 and Mg-Al-Ce-1 samples.Due to inversely proportional dependence of specific surface area on Mg/(Al+Ce) ratio, another tendency is observed for density of acid sites on the surface.Total acid density for Mg-A-Ce oxide systems varies from 5.16 to 7.18in a monotonic sequence: Mg-Al-Ce-1 ＜ Mg-Al-Ce-2 ＜ Mg-Al-Ce-4.

Table 2 Acid-base characteristics of the Mg-Al-Ce oxide catalysts

Regarding base characteristics, it has been found that the limit temperature of CO2desorption does not exceed 673 K for parent MgO and Al2O3, while for CeOxit is 623 K.The addition of small amount of Al3+and Ce4+/Ce3+cations into MgO (Mg-Al-Ce-4 sample)results in a decrease in the total number of base sites(0.18while the limit temperature of CO2desorption remains the same(673 K).Reducing of Mg/(Al+Ce) ratio to 2 causes an increase in the number of base sites (especially weak and medium ones), but at the same time contributes to disappearance of base sites withTdes.= 623-673 K and, as a result, the limit temperature of CO2desorption for the Mg-Al-Ce-2 sample decreases to 623 K.The strong base sites reappear on the Mg-Al-Ce-1 sample.However, with a change in Mg/(Al+Ce) ratio towards an increase in aluminium and cerium content (4 → 1),total base capacity of the samples increases from 0.14 for Mg-Al-Ce-4 to 0.24for Mg-Al-Ce-1.Sample Mg-Al-Ce-2 is characterized by the highest density of strong base sitesBase density changes in a non-monotonic sequence: Mg-Al-Ce-4 ＜ Mg-Al-Ce-1 ＜ Mg-Al-Ce-2.

Table 2 also shows acid/base capacity ratio (ABCR)for all investigated samples.As well as Mg-Al oxide systems, there is a proportional dependence of ABCR on Mg/(Al+Ce) ratio[15].Acid/base capacity ratio for Mg-Al-Ce oxide composition varies from 2.6 to 4.0 in a monotonic sequence: Mg-Al-Ce-1 ＜ Mg-Al-Ce-2 ＜Mg-Al-Ce-4.

Thus, both acid and base sites are present on the surface of all investigated samples.The introduction of aluminium and cerium cations into MgO leads to an increase in acid site content; the highest concentration of such sites is reached for Mg-Al-Ce-2 and Mg-Al-Ce-1 samples.The increase in concentration of base sites is also observed for the samples with large amounts of aluminium and cerium (ratio of Mg/(Al+Ce) = 1-2).

2.3 Catalytic properties

Catalytic behaviour of Mg-Al-Ce oxide catalysts in vapour phase condensation of EtOH to BuOH have been investigated in a flow reactor under atmospheric pressure during 8 h.The conversion of EtOH and selectivity towards the resulting products versus time on stream (TOS) at temperatures 548 and 573 K are presented in Figure 5.The main detected products are BuOH-1-butanol, HeOH-1-hexanol, AA-acetaldehyde,Et-ethylene, DEE-diethyl ether, “Light ”-sum of acetone, ethyl acetate, 1-butene, 1-butanal, 1,3-butadiene and other light fractions, “Heavy ”-other products of condensations: alcohols (2-ethyl-1-butanol,2-ethyl-1-hexanol and 1-octanol), aldehydes, ketones,aromatics.Table S2 represents indices of the process over Mg-Al-Ce oxide systems at different TOS.

Figure 5 Catalytic behaviour of Mg-Al-Ce oxide catalysts in ethanol conversion in a flow reactor during 8 h time on stream(T = 548 and 573 K, atmospheric pressure, WHSV = 0.14 g/(gcat∙h))

For Mg-Al-Ce oxide systems under study at 548 and 573 K EtOH conversion declines noticeably during the first 4 h on stream and then remains quite stable for the next 4 h.In the presence of Mg-Al-Ce-4 samplexEtOHrises with temperature, however for Mg-Al-Ce-2 and Mg-Al-Ce-1 samples the slight increase inxEtOHis observed only at the low time on stream and at high TOS the values are quite similar.The highestxEtOH27.4% is achieved over Mg-Al-Ce-4 at 573 K and 0.5 h TOS.At 548 K and TOS = 4 h EtOH conversion for all samples has similar values: 10.5%, 8.6% and 9.4%,respectively.

According to the literature[4, 6]and the obtained results, the most plausible scheme of Guerbet condensation mechanism of ethanol over Mg-Al-Ce oxide catalysts with the main side reactions is depicted in Scheme S1.As mentioned in the introduction, the main steps of the process include: dehydrogenation to acetaldehyde (1), aldol self-condensation reaction to crotonaldehyde (2), hydrogenation of the condensation product to 1-butanol (3).Also, there is a number of side reactions taking place, i.e.dehydration of EtOH to ethylene (Et) and diethyl ether (DEE), disproportionation of acetaldehyde (AA) to ethyl acetate by Tishchenko reaction, intramolecular hydride shift isomerization with subsequent acetone formation and others.

For the studied Mg-Al-Ce oxide catalysts,acetaldehyde is a primary product of EtOH dehydrogenation (Figure 5) and AA selectivity rises during the 8 h TOS for the investigated samples.Over Mg-Al-Ce-1 the selectivity of AA formation reaches 66.2% at 548 K and TOS = 8 h.Selectivity towards the other side products remains relatively stable during 8 h TOS for Mg-Al-Ce oxide catalysts under study.DEE is more preferable dehydration product under the reaction conditions than Et.Only traces of aldol condensation products, in particular crotonaldehyde, are detected,meaning the aldol-crotonic condensation step occurs rapidly.Acetone, ethyl acetate, 1,3-butadiene, 1-buthene and other by-products are also detected in the product stream.

As observed in our previous work[15], under reaction conditions (T= 573 K, atmospheric pressure)in a flow reactor the resulting BuOH is able to react with itself and with initial reagent EtOH to form higher Guerbet alcohols, i.e.2-ethyl-1-butanol, 1-hexanol(HeOH), 2-ethyl-1-hexanol and 1-octanol (Scheme S2).

Figure S4 shows the yield of the main detected products in the process of EtOH-to-BuOH conversion over Mg-Al-Ce oxide catalysts during different time on stream.As one can see, although the Mg-Al-Ce-4 sample provides the highest yield of BuOH 11.0%(BuOH + HeOH 12.6%) at 548 K and TOS = 0.5 h, this catalyst quickly loses the activity towards the target product formation.Mg-Al-Ce-4 is characterized by a highest specific concentration of acid sites (7.18and high acid/base capacity ratio (4.0),which can be a reason of higher yield of the side products (Light + Heavy) in comparison with Mg-Al-Ce-2 and Mg-Al-Ce-1.For the samples with the ratio of Mg/(Al+Ce) = 2 and 1, ABCR values are lower and close (3.0 and 2.6).However, Mg-Al-Ce-1 sample provides significantly higher AA yield and slightly lower yield of the target product than Mg-Al-Ce-2,which may be caused by the insufficient density of acid and base sites on the catalyst surfaceand 1.97Mg-Al-Ce-2 is the most selective towards target product formation (BuOH selectivity reaches 68.1%) and at high TOS at 548 K it provides the highest BuOH yield 4.6% at TOS = 4 h and 2.7% at TOS =8 h.

It was observed that EtOH conversion and selectivity towards BuOH depend on acid-base characteristics of Mg-Al-Ce oxide systems, namely the Mg/(Al+Ce) ratio.Table 3 depicts the calculated specific rates of EtOH transformation and the formation of the main products.The high content of magnesium and high specific concentration of acid and base sites provide the high rate of EtOH conversion.The highest BuOH formation rate 1.264×10−10is characteristic for Mg-Al-Ce-2 catalyst.Therefore, the optimal ratio of acid/base sites on the catalyst surface is achieved over the Mg-Al-Ce-2 catalyst.Comparing the process indices (Table S2) with QE-TD data (Table 2),an optimum value of ABCR for Mg-Al-Ce oxide systems is concluded to be near 3 for the efficient EtOH-to-BuOH process.

Table 3 Specific rate values for Mg-Al-Ce oxide catalysts in the process of vapour phase of ethanol conversion in a flow reactor (T = 548 K, TOS = 4 h, atmospheric pressure,

Table 3 Specific rate values for Mg-Al-Ce oxide catalysts in the process of vapour phase of ethanol conversion in a flow reactor (T = 548 K, TOS = 4 h, atmospheric pressure,

Formation BuOHHeOH AA Et DEE Mg-Al-Ce-4 11.381 0.775 0.015 4.5570.2240.149 Mg-Al-Ce-2 7.573 1.264 0.043 1.5040.1620.104 Mg-Al-Ce-1 6.514 0.458 0.015 3.4870.2140.117 Sample Rate/(mol·m−2·s−1×1010)EtOH conversion

According to NMR data (Figure 3, Table 1), the samples exhibit the presence of aluminium atoms included in highly dispersed phase of magnesium oxide.A segregated phase of γ-alumina is observed in the samples Mg-Al-Ce-4 and Mg-Al-Ce-1, which are less active towards the formation of the target product than Mg-Al-Ce-2.It can be assumed, in Mg-Al-Ce-2 sample Al-O-Mg species (i.e.Lewis acid-base pairs)located at the surface do not favour the side reactions.Besides, the appearance of pentahedrally coordinated Al atoms could be the evidence of formation of Mg-Al-Ce mixed oxide.Thus, for effective performance in Guerbet condensation of ethanol, the catalyst should have acid and base sites containing Mg, Al, and Ce,and the ratio of these components should provide ABCR close to 3.

The noticeable decrease in EtOH conversion in time can be caused by deactivation of the catalyst[13, 8].The heavy side products can block active sites of aldol condensation of acetaldehyde (carburisation process).Therefore, AA doesn ’t convert in BuOH and the AA selectivity increases significantly during 8 h on stream.However, the catalytic properties of studied Mg-Al-Ce oxide catalysts are fully regenerated after 1 h treatment in the air flow at 773 K, i.e.these catalysts are able to work for many cycles.The XRD analysis of deactivated samples doesn ’t show any differences toward fresh catalysts (Figure S5), meaning the changes with the surface of catalysts are reversible.

3 Conclusions

The results of an investigation of hydrotalcitederived Mg-Al-Ce oxide systems as catalysts for vapour phase condensation of ethanol to 1-butanol in a flow reactor under atmospheric pressure are presented for the first time.The corresponding Mg-Al-Ce hydrotalcites with Mg/(Al+Ce) ratio from 1 to 4 were synthesized using co-precipitation method under high supersaturation conditions (pH 10−12).The hydrotalcite structure provides high dispersion of the oxide phases after treatment at 873 K (confirmed by XRD,SEM/EDX and NMR).XPS data revealed the presence of Ce3+and Ce4+containing phases.The acid and base sites of the Mg-Al-Ce oxide samples have been studied by a quasi-equilibrium thermal desorption of NH3and CO2, and acid/base capacity ratio (ABCR) is used as a characteristic.Activity and selectivity of the catalysts in the ethanol-to-1-butanol process are determined by both the nature of acid/base sites and acid/base capacity ratio.For effectiveness of the target process, the ratio of the catalyst oxide components (Mg, Al, Ce) should provide ABCR close to 3.The highest 1-butanol yield 11.0% (548 K, TOS = 0.5 h) is achieved over Mg-Al-Ce oxide catalyst with Mg/(Al+Ce) = 4, but unfortunately the activity towards 1-butanol formation is decreased in time rapidly.The highest selectivity 68% is reached over Mg-Al-Ce oxide catalyst with the ratio of Mg/(Al+Ce) = 2.The catalyst provides 1-butanol yield of 9.8% (548 K, TOS = 0.5 h) and 4.6%(TOS = 4 h).

Acknowledgements

The authors, in particular O.V.Larina, K.V.Valihura, D.Yu.Balakin, I.Khalakhan and K.Veltruská acknowledge CERIC-ERIC Consortium for access to experimental facilities at Field Emission Scanning Electron Microscope and X-ray Photoelectron Spectrometer at Charles University in Prague (Proposal number: 20192036), and financial support.