The main products were acetaldehyde, diethyl ether, ethyl acetate, 1-butanol, 1,1-diethoxy ethane, 1-hexanol and 1-octanol which typically accounted for over 90% of the formed products. Their relative amounts varied depending on the choice of the catalyst. Also traces of other hydrocarbons were formed during some screening experiments. The higher C-6 and C-8 alcohols could be detected during prolonged experiments indicating that the reaction can proceed further from dimers of ethanol as well as to tri- and tetramers of ethanol. No odd number (C-3, C-5, C-7) alcohols were detected in the screening experiments (based on GC-MS analysis). This kind of product distribution may be related to so called Guerbet reaction which produces branched alcohols from primary ones [15]. The product composition varied greatly over various tested catalysts. Mini-reactor experiments were used to find out the most selective catalyst (Table 2) for ethanol to 1-butanol production. Typical conversion of ethanol (after 3 h of reaction) in the mini-reactor screening experiments was approximately 2–5%. In control experiments without a catalyst, the conversion of ethanol remained around 1% and only acetaldehyde and diethyl ether were formed.

The following qualitative conclusions can be drawn for the formed liquid hydrocarbon products from the catalyst screening experiments which give valuable guidelines for the future catalyst development:

Ru: Commercial 5% Ru on alumina and self-prepared 1-, 2- and 5% Ru on alumina were tested (Table 3). 1-butanol selectivity was 30% when the commercial catalyst was evaluated. The self-prepared Ru-catalysts showed lower selectivity towards 1-butanol when compared to the commercial one but the conversion was much higher. One possible explanation might be the formation of gaseous products over the self-prepared catalysts. A reduced catalyst promoted the formation of diethyl ether. The same catalyst reduction effect was observed for Ni on alumina catalyst as well. For all catalyst screening experiments, the commercial supported metal catalysts were used as received in unreduced state. TEM and XRD were measured over the self-prepared 5% Ru on alumina catalyst (Figure 1).

Rh: Commercial 5% Rh on alumina was tested (Table 4). Rh on alumina produced diethyl ether (S = 30%) and 1-butanol (S = 35%). In comparison to ruthenium (Degussa H 213 B/D), rhodium might have a promoting effect on alumina leading to an increase in the catalyst acidity and, thus, higher diethyl ether formation.

Pd: Commercial Pd on alumina was screened (Table 5) and the results (S_butanol = 21%) follow almost similar behavior with the rhodium, promoting the formation of diethyl ether.

Pt: Commercial Pt catalyst was found to contain a positive impact (S_butanol = 37%) on the desired product (Table 6) and, furthermore, the selectivity towards diethyl ether was much lower in comparison to Pd and Rh.

Ag: The self-prepared silver catalysts were rather inactive towards 1-butanol giving selectivities between 13–20%. However, also a rather high selectivity to acetaldehyde and 1,1-diethoxy ethane were observed (Table 7). TEM and XRD were measured over the self-prepared 4% Ag on alumina catalyst (Figure 2).

Ni: Two types of nickel catalysts were tested (Table 8). Overall, the commercial HTC-500 (20.7% Ni on alumina) was the best catalyst within the whole series of catalysts screened in terms of selectivity (62%). Also self-prepared 20% Ni on alumina (La Roche) showed moderate selectivity (37%) towards 1-butanol but the conversion was almost four times higher in comparison to the HTC-500, producing a significant amount of acetaldehyde. Both nickel catalysts were characterized by means of XRD and TEM (Figure 3 and Figure 4). Interestingly, the self-prepared catalyst shows a considerably smaller particle size distribution and it does not contain any aggregates which are presumed to be inactive parts.

Au: The commercial gold catalyst showed an interesting behavior by producing a moderate selectivity to 1-butanol (35%) and also some amounts of acetaldehyde, ethyl acetate and diethyl acetate (Table 9). This might be due to the ability of gold to promote different and simultaneous reaction pathways.

The reproducibility of the experiments was controlled by repeating experiments with same catalyst at least twice. For each and every test batch of eight (8) mini-bomb reactors, a reference catalyst was used to control the experimental reproducibility. When the product distribution obtained in mini-bomb reactors was compared with the products obtained in the Parr autoclave, under similar conditions and with the same catalyst, it was evident that analogous results were obtained, i.e., the process is scaleable.

Out of all screened catalysts, the catalyst HTC-500 (20.7% Ni on alumina) was clearly the best one giving a good conversion (X = 5%) and highest selectivity (S_butanol = 62%) in the whole series. Nevertheless, it is expected that after optimizing catalyst properties and reaction conditions, the selectivity, activity and ethanol conversion could be notably improved.

Based on the results from the screening experiments (Table 3, Table 4, Table 5, Table 6, Table 7, Table 8 and Table 9) commercial Ni/Al₂O₃ (HTC-500) was chosen for further studies in batch reactor.

The reaction kinetics was investigated in the 300 mL Parr autoclave using the identical catalyst-to-ethanol ratio as in the miniature bomb reactors. Commercial (Crosfield HTC-500, 20.7% Ni on alumina) catalyst was used in the experiments. The temperature profile in the larger batch reactor during the reaction was similar consisting of 15–20 min heating period after which an isothermal reaction was carried out at 250 °C. Analysis of samples taken at time zero and at the time when the desired reaction temperature was reach revealed that negligible ethanol conversion took place during the short (15–20 min) reactor heating step. A representative kinetic plot is illustrated in Figure 5 which shows the conversion of ethanol and the selectivity of 1-butanol among liquid carbon products as a function of time. Moreover, the behavior of side products concentration evolvement is illustrated in Figure 6 and Figure 7, respectively. Interestingly, the behavior of acetaldehyde and 1,1-diethoxy ethane follow a different pattern than that of ethyl acetate, butyraldehyde, diethyl ether and 1-hexanol. Concentrations of the later mentioned by-products illustrate a stable upward trend whereas acetaldehyde and 1,1-diethoxy ethane concentrations seem to peak in early stages of the reaction followed by a sharp decrease. It might be that acetaldehyde and 1,1-diethoxy ethane have a remarkable effect on the reaction in question. The larger scale (300 mL instead of 2 mL) autoclave allowed sampling of the liquid-phase during the course of the reaction under controlled temperature and pressure. The gas-phase composition could be analyzed by MS after the reaction. When the experiments were repeated over the same catalyst it was observed that the reproducibility of the results was within 1%.

Catalyst screening was carried out in small, 2 mL isobaric mini-reactors equipped with magnetic stirring (Figure 10). The reactors were made of stainless steel and could be used for the reaction carried out at pressures up to 100 bar and temperature up to 250 °C (maximum for the Teflon coated stirrer magnet). The catalyst testing procedure was as follows: the reactor was loaded with the catalyst (10 or 50 mg) and ethanol (Etax Aa, 99.5%, 1.5 mL) under inert atmosphere (He) in a glove bag. The reactors were closed and placed in the heating unit allowing 8 reactors to be run simultaneously. The reactors were heated to 250 °C with a heating rate of 20 °C/min. The reaction was allowed to proceed for 3 h. After reaction, the reactors were rapidly quenched (cooled down) in a water bath and the liquid contents were analyzed by gas chromatography (GC). While opening the reactors, it was noted that in many cases notable gas formation had taken place during the experiment (sealed reactors). The gas-phase composition could not be analyzed in the catalyst screening mode; however, it could be done during the kinetic experiments carried out in the standard high pressure Parr autoclave.

Proper kinetic experiments were carried out in a 300 mL high pressure Parr autoclave equipped with a mechanical Rushton turbine type of a stirrer. The reactor was loaded with the catalyst (typically 3.3 g) and flushed with inert gas (He). In some experiments, catalyst reduction under flowing hydrogen, at 400 °C for 3 h, was carried out. The catalyst-to-reactant molar ratio was identical as in the case of catalyst screening experiments in small mini-autoclaves. The ethanol reactant (100 mL) was degassed for 10 min with inert gas before injection to the reactor. The reactor was heated rapidly to 250 °C and the reaction was monitored by taking small liquid samples (0.5 mL). The gas-phase composition was analyzed at the end of the reaction after cooling the reactor back to room temperature (RT). Gas chromatographic analysis (Agilent, DB-Petro 100 m column) was calibrated with commercial standards. The initial compound identification was based in GC-MS identification of the peaks using the same column. The semi-quantitative gas phase analysis was based on a quadrupole mass spectrometer (Balzers). The stirring rate applied throughout the the matrix of catalytic experiments was 1500 rpm thus ensuring that we resided in the kinetic regime. Due to the fact that only liquid reactants (ethanol) were used and that the reaction rates were rather low, the danger of external mass transfer limitations can be regarded as negligible.

A total of 13 catalysts were studied in the present work. Both in-house prepared and commercial catalyst were used. For commercial catalysts, the available characterization data can be found in Table 2, whereas in the case of in-house prepared ones the catalyst preparation is described below and the characterization results are reported in Table 2.

The liquid-phase analysis was carried out with an Agilent Technologies, 6890 N gas chromatograph (GC) equipped with a DB-Petro (122-10A6, 100 m, i.d. 0.250 mm) column. The products were identified by means of an Agilent gas chromatograph coupled to a mass spectrometer (GC-MS) equipped with the same column. Calibration of the GC was carried out with commercial standards using octane (Fluka > 99%) as internal standard. The calibration was carried out using the following commercially available chemicals: ethanol (Altia 99.5%, Aa), butyraldehyde (Lancaster, 99%), diethyl ether (Merck, max 0.0075% H₂O), ethyl acetate (Fluka, >99.5%), 1-butanol (Acros Organics, 99%), 1,1-diethoxy ethane (Fluka, ≥95%), 1-hexanol (Fluka, ≥99%), 1-octanol (Riedel-de Haën, ≥99.5%)

The qualitative gas-phase analysis was carried out with a quadrupole MS. After cooling down the reactor contents to room temperature (RT), the remaining gas phase was analyzed with MS.

Thirteen (13) different catalysts were tested in the screening phase. The catalysts were characterized with TEM, XRD, physisorption and chemisorption. To identify crystal phase and size of the catalyst (and in some cases of the support) X-ray diffraction analyses were carried out using Cu Kα radiation (Siemens D5000 diffractometer equipped with a graphite monochromator to suppress fluorescent and Cu Kβ radiation). The average crystallite size of catalyst particles was estimated using Scherrer’s equation from the halfwidths of the most intensive reflections. Transmission electron microscopy (LEO 912 OMEGA, energy-filtered TEM, 120 kV) has been used to study microstructure and particle size of catalyst samples. Histograms of particle size distribution were obtained by counting typically 100 particles on the micrographs.

The equilibrium composition for the main reaction, i.e., 2 ethanol = 1-butanol + water was calculated in order to check if the conversion would be equilibrium limited. The calculations were performed with the software PRO/II (PRO/II 8.3 invensys Systems, Inc.) using the Gibbs reactor module. At 250 °C and 70 bar the conversion of ethanol to 1-butanol was found to be 98.5%.