The utilization of bio-ethanol for production of 1-butanol catalysed byMg-Al mixed metal oxides enhanced by Cu or Co

Ethanol, as a renewable raw material, is important source for preparation of higher alcohol and other add-value chemicals, which are currently produced from raw fossil materials. The Guerbet reaction is possible way to transformation of ethanol to 1-butanol (important for many kinds of industries), which consists of four steps: dehydrogenation, aldol condensation, dehydration, and hydrogenation. The reaction requires catalysis to favour 1-butanol, because of elimination of side reactions. The transformation was caried out via heterogeneous catalysis (Mg-Al mixed oxides with copper or cobalt) in the micro�ow reactor at three reaction temperatures (280, 300 and 350°C), which was the main aim of this work. The novelty especially lay in the statistically analysis of results from characterisations and catalysis (almost always omitted), which provided new perspective on the catalysis of the Guerbet reaction. The catalyst stability was proved by long term catalytic tests. The higher ethanol conversion, clear correlations of catalytic and characteristic data were achieved by copper dopped catalysts, compared to dopped by cobalt. Moreover, copper is more environmentally friendly, cheaper, and more used metal than cobalt.


Introduction
Ethanol is used as (i) solvent, (ii) fuel for combustion engines as admixture (Costa et al., 2008;Somma et al., 2010). It can be also used for production of compounds currently produced from crude oil such as 1butanol, 1,3-butadiene, isobutylene, acetic acid, ethyl acetate, etc. (Angelici et al., 2013). Therefore, the studding of ethanol as a renewable, alternative source is important to replace the crude oil for renewable one (León et al., 2011;Perrone et al., 2018). Moreover, the producing of biobutanol is in accordance with "Green deal" of EU policy (European Union (The European Green Deal), 2019).
al., 2017) or Mg-Cr and Zn-Al (Padmasri et al., 2002). The mixed metal oxides are mostly based on hydrotalcite like materials (HT) precursors, more speci cally on Mg-Al layered double hydroxide (Rechi Siqueira et al., 2019b;Volli & Purkait, 2016). Structure of Mg-Al mixed metal oxides allows another tuning by transition metals. Pure Mg-Al MOs are mainly considered as acido-basic catalysts, the (de)hydrogenation and acid properties can be tuned by doping with transition metals. From group of heterogenous catalysts the Cu-Mg-Al MOs showed promising results for catalysis of Guerbet reaction, because Cu-Mg-Al dispose of bi-functional properties (Cheng et al., 2018). Despite Co-Mg-Al MOs have been studied less than Cu-Mg-Al MOs, they also provide redox properties to acid-base Mg-Al MO, i.e., Co-Mg-Al can be also classi ed as bi-functional catalyst as Cu-Mg-Al. Also Co-Mg-Al MO reported higher 1butanol selectivity compared to Mg-Al MO for Guerbet reaction (Quesada et al., 2018).
This work is focused on the transformation of ethanol to 1-butanol in the gas phase under solid catalysts and in a ow reactor. Mg-Al was chosen as catalyst because of convenient acid-base properties (Kuljiraseth et al., 2019). Moreover, the doping of Mg-Al MO by copper or cobalt was based on high alcohol dehydrogenation activity (Wu et al., 2017). The metals Cu and Co were convenient candidates to enhance Mg-Al MO. It was possible to prepare bi-functional heterogenous catalysts by this enhancement, which were lling needs for the Guerbet reaction. The concentration series of Cu-Mg-Al and Co-Mg-Al were synthetised to study effect of different copper or cobalt load on (i) MOs properties, (ii) ethanol conversion and (iii) 1-butanol selectivity. MOs were also deeply characterized by several methods such as, X-ray diffraction (XRD), Inductively coupled plasma optical emission spectrometry (ICP-OES), N 2 adsorption.
Larger group of temperature programmed methods, which were temperature programmed reduction (H 2 -TPR) and temperature programmed desorption of CO 2 (CO 2 -TPD) or NH 3 (NH 3 -TPD). Data from these characteristics were corelated with data from the catalysis such as, ethanol conversion and selectivity to 1-butanol.
The main objective was achieving the highest possible ethanol conversion combined with the highest possible 1-butanol selectivity. The novelty is the statistical analysis of outcomes, which able to better understand synergy between structural properties of catalyst and catalytic data (ethanol selectivity and 1butanol selectivity). The relation able to synthesis catalyst with maximum ethanol conversion and butanol selectivity, which will be used as a renewable resource.

Catalysts preparation
Two series of Mg-Al hydrotalcites with different transition metals were synthetized by co-precipitation of metal nitrates: (i) Cu-Mg-Al and (ii) Co-Mg-Al with the constant of molar ratio of Mg : Al at 2:1 and molar ratio of Cu(resp. Co):Al in the range from 0.05 to 0.75. The precipitation was caried out under constant pH The calculated amounts of metal nitrates were dissolved in re-distilled water. The synthesis was carried out in a double jacked batch reactor equipped with a paddle stirrer, peristaltic pump and automatic titrator at 60°C. The amount of 250 ml of re-distilled was poured into reactor and then the solution of metals was added with ow rate 10 ml min − 1 under stirring 360 rpm. After dosing of the whole volume, precipitate aged for one hour. Then precipitate was ltrated from mother solution and ushed with re-distilled water till neutral pH of washing re-distilled water was achieved.

Catalytic tests
The catalytic tests were carried out in a ow reactor (Multireactor catalyst testing unit, Vinci Technologies). The mass of 2 g of catalyst was activated before reaction in the stream of hydrogen gas with ow 10 dm 3 h − 1 and temperature at 450°C. Ethanol ow 9 g h − 1 and pressure 10 MPa. The temperature 300°C (denoted as "300A") was kept for 32 hours, then was increase to 350°C (rate 30°C h − 1 ) and kept for 32 hours. This was followed by cooling to 280°C (cooling rate 30°C h − 1 ) and kept for 32 hours. Finally, the temperature was raised to 300°C with the same heating rate and kept at this value for 32 hours (denoted as "300B").

Analytical methods
The real molar ratios of metals in HTs were determined by ICP-OES, 7900 ICP-OES (Agilent Technologies, USA).
The crystallographic phase composition was studied by XRD for calcinated forms of MOs. Analysis was performed on D8 Advanced diffractometer (Bruker AXS GmbH, USA), which is equipped by Cu K α secondary graphite monochromator.
Step size was 2° and data were collected in the range 1-90°.
The N 2 adsorption isotherms were measured involving ASAP 2020 equipment (Micromeritics, USA) at temperature of liquid nitrogen (77 K) for calcinated forms of MOs. The speci c surface area was calculated by the BET method. The pore diameter, volume and distribution were calculated by NFDLT method from obtained isotherms.
The reducibility of transition metal (Cu or Co) in MOs was determined by H 2 -TPR. The basicity and acidity of MOs were determined by CO 2 and NH 3 -TPD, resp. Entire TP measurements were carried out in Autochem II 2920 equipped with a TCD detector (reduction experiments), and OmniStar™ GSD 320 (Pfeiffer Vacuum, Germany) mass spectrometer (desorption experiments). Around of 100 mg of activated MO was used for each measurement. For measurements, oxygen (99.5%), helium (99.9999%), hydrogen (99.9999%), carbon dioxide (99.9999%) and ammonia (99.9999%) were used. The typical experiment consisted of pre-treatment at the calcination temperature, ushing with inert gas, cooling to RT, and saturation (TPD). The TP experiments were collected with a temperature ramp (10°C min − 1 ) and gas ow (25 ml min − 1 ).

Statistical analysis
The principal component analysis (PCA) was used for correlations among the variables (Statistica 12). The PCA can be described as follows: (i) positive correlation -variables are close to each other, (ii) negative correlation variables are on the opposite sides, (iii) no correlation -angle between variables is at 90°, (iv) distance from the centre of the circle determines how much variable contributes to individual main components (its signi cance) (Hájek, Tomášová, et al., 2018;Hájek, Vávra, et al., 2018) 3. Results And Discussion

Chemical composition
The real molar ratios of metals in HT, which were used for nomenclature of HTs and MOs, are summarized in Table 1. The real contents of metals were close to the theoretical ones. The one only exception was found for HT_Co 0.90 Mg 2 Al 1.16 , exhibiting relatively higher content of Co and Al, which was probably caused by the pH uctuation at the beginning of the HT synthesis (slow homogenization of reaction mixture at the beginning of coprecipitation). After calcination of HTs, the diffraction lines at 2θ ≈ 43.0°, 62.4° were attributed to magnesium oxides, which con rm to decomposition of layered hydrotalcite structure and formation of mixed oxides (

Surface analysis
The adsorption isotherms of N 2 at 77 K were obtained for reduced (activated) forms of MOs (Fig. 3.). All isotherms are of type IV, which is typical for mesoporous materials with capillary condensation (Muttakin et al., 2018). The hysteresis loops were present for each MO, type of these hysteresis loops was between H2 and H3, which pointed out to wide distribution of pore shape. H2 is mostly related to the complex porous structure (with a pore blocking), and H3 is associated with plate-like particles or aggregates with slit-shape pores (Thommes et al., 2015).
Minor changes were observed for adsorption isotherms with increasing amount of copper or cobalt in MOs ( Fig. 3. A) Cu-Mg-Al series, b) Co-Mg-Al series). These changes were observed only in hysteresis region (relative pressure from 0.4 to 1.0) and can be described as shift between H2 and H3 hysteresis loop and growth of plateau at the end of hysteresis loop (relative pressure range from 0.9 to 1.0 isotherms, which is also very well recognized at pore size distribution. Both MOs series had similar changes respectively to amount of transition metal in MOs. But differences were observed by close comparison of the series. Cu-Mg-Al series had larger area of hysteresis loop and larger plateau at the end of hysteresis loop (relative pressure from 0.9 to 1.0) compared to Co-Mg-Al series. This phenomenon was also observed in case of pore size distribution. Co-Mg-Al MOs had narrower pore size distribution compared to Cu-Mg-Al MOs. It can be stated, MOs with wide molar ratios of copper or cobalt were prepared, but structural properties were quite similar for these MOs.
The speci c surface areas and pore volumes were summarized in   Systematic changes were related to the amount of transition metal were observed in both series, such as peak intensity, area, shape, and peak position (Fig. 4). Peak intensity and area under peak were increasing with increasing amount of copper for Cu-Mg-Al series or with increasing amount of cobalt for Co-Mg-Al series. Systematic changes were related to the amount of copper in Cu-Mg-Al series, such as peak intensity, area, shape, and peak position (Fig. 4. a) Cu-Mg-Al series). Peak intensity and area under peak were increasing with increasing amount of copper in MOs from Cu-Mg-Al series.
Change of oxidation number of transition metals in MOs were calculated from the consumptions of H 2 (area under the reduction curve) related to the total amount of the metal in the oxide and taking to account a simple reduction of CuO

Acid-base properties
Basic and acidic properties were described by the desorption of CO 2   TPD record maxima were located around temperature 110°C and range of these curves is from 40°C to 450°C. CO 2 -TPD records were related to overlapped peaks (Fig. 5.) pointing to presence of weak, mediumstrong and strong basic sites on the catalysts surface (Smoláková, Frolich, Troppová, et al., 2017). Pro les of these overlapped peaks were not dependent on the catalyst composition, i.e., amount of copper or cobalt had no in uence on peaks pro le (Fig. 5). It can be concluded that distribution of weak, medium-strong and strong basic sites were the similar for all MOs in the Cu-Mg-Al and Co-Mg-Al series. Amounts of acidic sites ranged from 68 µmol NH 3 g − 1 to 223 µmol NH 3 g − 1 for Cu-Mg-Al series and from 128 µmol NH 3 g − 1 to 250 µmol NH 3 g − 1 for Co-Mg-Al series (Table 4). Single desorption peak was observed in temperature range from 80°C to 300°C with maxima around 175°C (Fig. 5), without any sign of merging peaks, for all materials of Cu-Mg-Al and Co-Mg-Al series. Therefore, the only one type of acidic sites was suggested. On the other hand, temperature range was changing slightly, which could be caused by stronger interactions of ammonia and transition metal in MOs or by diffusion in porous structure of MOs (Dixit et al., 2013).
Co-Mg-Al series reached over all higher amounts of basic and acidic sites compared to Cu-Mg-Al series.
Co-Mg-Al series had also more uniform CO 2 and NH 3 desorption curves compared to Cu-Mg-Al series. It is therefore possible to assume, that Co-Mg-Al series overall recorded higher amounts of basic and acidic sites compared to Cu-Mg-Al series.

Ethanol transformation
The results of catalysis (ethanol conversion (X) and selectivity (S) to 1-butanol) at tree reaction temperatures for reduced forms of Cu-Mg-Al and Co-Mg-Al oxides are summarized in Table 5 and Table 6, respectively.
The ethanol conversion increased with two factors: (i) increasing reaction temperature and (ii) copper content in Cu-Mg-Al MOs or cobalt content in Co-Mg-Al MOs. The ethanol conversion was in large range ( Table 5 and Table 6). The dependency of conversion on the content of transition metal was also well observed. The biggest conversion increase was from 51% for Cu 0.05   The selectivity of 1-butanol decreased with increasing ethanol conversion, i.e., higher ethanol conversion means lower selectivity of 1-butanol, due to higher probability of side products formation. Possible further reaction of 1-butanol to higher alcohols, which may occur, could also lowered selectivity to 1butanol. On the other hand, selectivity of 1-butanol was increased with increasing ethanol conversion for Co 0.05 Mg 2 Al 1.05 . Possible deactivation of catalysts was studied by comparison of ethanol conversion at the same temperatures (300°C) in the beginning and end of reaction. Since the conversion of ethanol was lower for 300B test, Cu-Mg-Al catalysts were related to a certain deactivation. Nevertheless, selectivity of 1-butanol was higher at 300B compared to 300A, which corresponded with observed selectivity to 1butanol dependency on ethanol conversion.
The direct corelation of catalytic data with other studies is relatively di cult, due to (i) high variability of heterogeneous catalysts and (ii) large range of reaction conditions including type of reactor. Many types of catalyst such as zeolites, hydroxyapatite, supported metal catalyst or metal mixed oxides with different metals or metal ratios were studied. The reaction conditions differed in many ways such as type of reactor batch or ow and ration between catalyst and ethanol. The reaction temperatures were in large range from 60°C to 350°C, also pressure was in large range from 0.1 to 10 MPa and time of reaction short 2 hours catalytic test to long term test reaching 150 hours. Ethanol conversion was in large range from 0 to 85%, due to the facts mentioned above (Marcu et al., 2013;Wu et al., 2018).

Statistical correlation
The results were statistically analysed by PCA to discovered of hide relation between all variables (characterisation of catalyst and catalyst activity), because it is multivariable system. Obtained correlation are displayed in Fig. 6 Cu-Mg-Al and Co-Mg-Al series showed similar cluster of variables (Fig. 6), which was H 2 consumption and ethanol conversion (X 280 , X 300A , X 300B , X 350 ). H2-consumption was related to redox properties, which are important in rst and last step of Guerbet reaction. Therefore, redox properties played a major role in ethanol conversion, which was statistically proven by the correlation of the H2 consumption and ethanol conversion. Less dependency of ethanol conversion on redox properties with increasing temperature was observed, because H 2 consumption and ethanol conversion at 280°C (X 280 ) had strong correlation, meanwhile H 2 consumption and ethanol conversion at 350°C (X 350 ) had weaker correlation. Thus, at higher temperatures redox properties of copper or cobalt have played less important role.
Cu-Mg-Al series showed another cluster of variables except H 2 consumption and ethanol conversion (X 280 , X 300A , X 300B , X 350 ), which was basicity, acidity, speci c surface area and selectivity to 1-butanol (S 280 , S 300A , S 300B , S 350 ) (Fig. 6A). Positive corelations of basicity, acidity and 1-butanol selectivity con rmed basic sites catalysed the second step and acidic sites catalysed the third step of the Guerbet reaction. Negative correlation between variables clusters of Cu-Mg-Al was observed. Therefore, redox sites were not involved in second and third step and basic sites along with acidic sites were not involved in rst step of the Guerbet reaction.
Co-Mg-Al series showed another two clusters of variables except H 2 consumption and ethanol conversion (X 280 , X 300A , X 300B , X 350 ): (i) second group is speci c surface area and 1-butanol selectivity (S 280 , S 300A , S 300B ) and (ii) third group is acidity, basicity, and 1-butanol selectivity (S 350 ) (Fig. 6B). Positive corelation of speci c surface area and selectivity to 1-butanol is probably caused by higher steric selectivity at lower reaction temperatures (280°C and 300°C). On the other hand, selectivity to 1-butanol was more dependent on basicity and acidity at higher reaction temperature (350°C).
Four-step mechanism of the Guerbet reaction, which was described in more detail in previous study (Mück et al., 2021), was con rmed by correlation of catalytic and characteristic data. Therefore, mechanism of direct alcohols condensation is less probable (Xu et al., 2014).

Conclusion
The were proved the importantce for the aldolization (2nd step) and dehydration (3rd step). It can be stated, that these corelations may be one of the proofs of the four-step mechanism of the Guerbet reaction.
Promising results allow further development of this ethanol transformation to obtain another renewable source (bio-ethanol) for production of more valuable chemical compounds such as, 1-butanol. Therefore, whole process can be not only reliable, but also more clean and environmentally friendly and less dependent on crude oil.

Supplementary Files
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