Annotation of the results obtained in 2023
1. Layered double hydroxides (LDH) with the ratio (Co+Cu+Mg)/Al = 2, 3, 4, (Co+Cu)/Mg = 0.5, 1, 2, 3 and Co/Cu = 0.5, 1, 2 were obtained by co-precipitation. According to the XRD data, all synthesized systems with the ratio (Co+Cu+Mg)/Al = 2 were characterized by the presence of a single phase of hydrotalcite in their composition, regardless of (Co+Cu)/Mg and Co/Cu. The embedding of cobalt and copper cations into the LDH layers was confirmed by a change in the lattice parameter a. An increase in the (Co+Cu+Mg)/Al ratio contributed to the appearance of an additional CuO phase in the composition of synthesized materials. 2. The morphology of the surface of freshly synthesized CuCoMgAl-LDH with the ratios (Co+Cu+Mg)/Al = 2, (Co+Cu)/Mg = 2, and Co/Cu = 1 and 2, as well as catalysts obtained on their basis (after calcination at 550 °C and reduction at 800 °C) was studied. The surface topology of the freshly synthesized samples did not depend on the Co/Cu ratio and consisted of hexagonal plates oriented in different directions. The morphology of the surface of the finished catalysts was largely determined by the Co/Cu ratio. For catalysts with a Co/Cu = 2 ratio, the initial morphology was preserved after high-temperature treatments. Two types of sites with different surface topologies were observed for samples with Co/Cu = 1. The first one was characterized by an almost smooth surface consisting of barely distinguishable clumped aggregated particles. The initial plates were distinguishable in the sections of the second type. Thus, a slight change in the composition of CoCuMgAl systems contributed to the formation of different catalyst morphologies. 3. The process of formation of the oxide phase from CuCoMgAl-LDH with the ratios (Co+Cu+Mg)/Al = 2, (Co+Cu)/Mg = 2, and Co/Cu = 1 and 2 was studied. It was shown that two areas of mass loss were observed for these systems: The first is in the temperature range of 130-220 ° C with maxima of mass loss at 183 and 195 ° C, and the second is in the range of 240-450 ° C with maxima at temperatures of 283 and 301 ° C, for samples with a ratio of Co/Cu = 1 and 2, respectively. With an increase in the cobalt content in the hydroxides, a shift of the temperature maxima of mass loss to a higher temperature region was observed, which indicates an increase in the thermal stability of this sample even with such a slight change in composition. 4. The process of reduction of cobalt and copper from the corresponding CuCoMgAl-LDH with the ratios (Co+Cu+Mg)/Al = 2, (Co+Cu)/Mg = 2, and Co/Cu = 1 and 2 was studied. According to the TPR data, these systems were characterized by a stepwise nature of metal reduction. The first stage of metal reduction was observed in the temperature range of 150-200 ° C and was characterized by a narrow peak of reduction with high intensity. At the second stage of reduction, a wide low-intensity region of hydrogen absorption was observed at higher temperatures (350-700 °C). This may be due to the presence of oxide phases of different chemical compositions in these systems obtained after calcination of CuCoMgAl-LDH. The reduction of metals from oxides of different types was carried out at different temperatures. An increase in the Co/Cu ratio contributed to a slight shift in the maximum reduction temperature of metals from the corresponding oxides to a higher temperature region. According to the obtained XRD data, regardless of the Co/Cu ratio, the phase composition of the samples after calcination and reduction was a mixture of metallic α-Co (ICDD PDF-2 01-089-4307), Cu (ICDD PDF-2 01-085-1326) and a mixed oxide phase similar in structure to the Co3O4 phase (ICDD PDF-2 00-042-1467). 5. The microstructure of catalysts obtained on the basis of CoCuMgAl-LDH with the ratios (Co+Cu+Mg)/Al = 2, (Co+Cu)/Mg = 2 and Co/Cu = 1 and 2 was studied before and after the furfural hydrogenation reaction by the TEM method. It was found that the surface of the catalysts, regardless of the Co/Cu ratio, was characterized by a core-shell structure. Metal particles Cu and Co with an average diameter of 12.7 nm were present in the "core". The "shell" consisted of mixed nonstoichiometric CuCoMgAl oxides. After the furfural hydrogenation reaction, there was no sintering and no change in the size of metal particles. However, for both samples, carbon deposits formed on the surface after the reaction. According to the XRD data, the phase composition of the sample with a ratio of Co/Cu = 1 remained unchanged after the reaction. At the same time, the sample with a high Co/Cu = 2 ratio turned out to be X-ray amorphous. Consequently, despite the close activity of samples with a Co/Cu ratio of 1 and 2, the structural stability of the catalyst with a high cobalt content turned out to be lower than for a sample with a lower Co/Cu ratio. In the future, this may significantly affect the stability of the catalyst. 6. The properties of catalysts obtained after preliminary high-temperature treatments of CoCuMgAl-LDH (calcination at 550 °C, reduction in hydrogen current at 800 ° C) with different ratios (Co+Cu+Mg)/Al, (Co+Cu)/Mg and Co/Cu were studied in the furfural hydrogenation reaction and 5-hydroxymethylfurfural. At the same time, a relationship was established between the catalytic properties of the samples and their chemical composition, as well as the conditions of the reaction (temperature, pressure, reaction duration, nature of the solvent). It was shown that during the reaction in an aqueous medium for a long time (5 hours), the furfural conversion increased with an increase in the Co/Cu ratio for samples with the same value (Cu+Co)/Mg. Within a group of samples with the same Co/Cu ratio, an extreme dependence of the furfural conversion on the (Co+Cu/)Mg ratio was observed. The catalysts with the ratios (Co+Cu+Mg)/Al =2, (Co+Cu)/Mg = 2 and Co/Cu = 1 and 2 showed the greatest activity in the furfural hydrogenation reaction. The selectivity of furfuryl alcohol formation for all studied catalysts was higher than 99%. The reaction in an ethyl alcohol medium contributed to an increase in the degree of furfural conversion for samples with Cu/Cu = 1 and a decrease for catalysts with a high Co/Cu = 2 ratio. Under mild reaction conditions (90 °C, 20 bar), the catalyst with a Co/Cu = 2 ratio turned out to be twice as active as the sample with a Co/Cu = 1 ratio (reaction time is 1 hour). When using more stringent reaction conditions (150 °C, 30 bar) The activity of the studied catalysts increased sharply and was almost the same. In all cases, the main hydrogenation product was furfuryl alcohol. The selectivity of furfuryl alcohol formation was slightly higher for catalysts with a high Co/Cu ratio. The activity of CoCuMgAl-LDH-based catalysts in the hydrogenation reaction of 5-hydroxymethylfurfural was less than the activity of these samples in the hydrogenation reaction of furfural and increased with a decrease in the Co/Cu ratio. Under the most severe reaction conditions (150 °C, 30 bar), the degree of transformation of the substrate for the catalyst with a ratio of Co/Cu = 1 reached 27 mol. %. Only 2,5-bis(hydroxymethyl)furan was detected as reaction products using the NMR method. It was found that even under harsh reaction conditions, the solvent (water) was not involved in the process of product formation (unlike the furfural hydrogenation reaction under the same conditions in the presence of a catalyst of a similar composition). The catalyst with Co/Cu = 2 had negligible activity, which hardly changed with increasing reaction temperature and pressure. Carrying out the reaction for 5 hours contributed to an increase in the activity of the studied catalysts. The degree of transformation of the raw material was about 75 mol. % with a selectivity of 2.5-bis(hydroxymethyl)furan formation of more than 90%, regardless of the composition of the catalysts.

 

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Annotation of the results obtained in 2022
1. A series of samples of CuAl-layered double hydroxides (LDH) with a molar ratio of Cu/Al = 2, 3, 4, as well as CuCoAl-LDH with a molar ratio of M2+/Al = 2, 3, 4 and Cu/Cu = 0.0, 0.5, 1.0, 2.0 were synthesized by coprecipitation. The optimal conditions for the synthesis of single-phase Cu-containing LDHs (at Cu/Al = 2) have been established. The phase composition of the synthesized systems was studied using the method of X-ray phase analysis (XPA). It has been established that for all samples the hydrotalcite phase is formed. An increase in the Cu/Al ratio in CuAl-LDH leads to the formation of an additional copper hydroxocarbonate phase. All CuCoAl LDHs are single-phase systems regardless of the M2+/Al and Cu/Cu ratios. The calcination of the synthesized samples at 550°C leads to the formation of CuO phases for the CuAl sample, CuO and CoAl oxide with a mixed spinel structure for the CuCoAl sample, and CoAl oxide with a mixed spinel structure for the CoAl sample. After the reduction of the obtained oxides in a hydrogen flow at 800°C, large metallic copper particles were formed with a crystallite size of about 57 nm for CuAl samples and a metallic cobalt phase with a particle size of about 29 nm for CoAl systems. The reduced bimetallic samples (CuCoAl) were a mixture of phases of metallic copper (size 62 nm) and cobalt (size 32 nm), as well as CoAl oxide with a mixed spinel structure. 2. The process of formation of mixed oxides during calcination of CuAl-LDH (Cu/Al = 2), CoAl-LDH (Co/Al = 2), and CuCoAl-LDH (M2+/Al = 2, Cu/Co = 1) was studied by the thermal analysis (TA). It has been established that the decomposition of CuAl-LDH occurs in one stage with the maximum mass loss rate at 165°C. The introduction of cobalt into the composition of the CuAl samples contributed to the two-stage decomposition of the samples, which is typical of most LDHs. The low-temperature mass loss peak is due to the removal of interlayer water, and the high-temperature peak is due to dehydroxylation of layers and destruction of interlayer anions. The only intense peak on the curve of differential thermal analysis (DTG) for CuAl-LDH indicates that the processes of removal of interlayer water and anions and dehydroxylation of layers occur rapidly and simultaneously. With an increase in the content of cobalt in the composition of the samples, the peaks in the DTG curves shifted to higher temperatures. Consequently, the introduction of Co led to an increase in the thermal stability of the samples. 3. The process of reduction of metals (Cu, Co) from CuAl-, CuCoAl- and CoAl-oxides (obtained after calcination of the corresponding LDHs at 550°C) was studied using the temperature-programmed reduction with hydrogen (H2-TPR). It is shown that the reduction of copper from CoAl oxide occurs in one stage. The H2-TPR profile has one broad peak at 230°C, which corresponds to the reduction of Cu from CuO. The large width of the peak in the H2-TPR profile of the CuAl sample may indicate simultaneous processes of copper reduction from different types of oxide particles: finely dispersed CuO particles weakly bound to the Al2O3 surface and therefore more easily reduced, and coarser dispersed CuO particles, which are reduced at more high temperature. CuCoAl and CoAl samples are characterized by a two-stage reduction of metals from the corresponding oxides. The H2-TPR profile shows two peaks in the low- and high-temperature region. An increase in the proportion of cobalt in the composition of CuCoAl oxides led to a shift in the maxima of the reduction peaks to a higher temperature region. The low-temperature peak in the H2-TPR profile of the CuCoAl samples most likely corresponds to the simultaneous reduction of Co3O4 to CoO and/or CuO to Cu0. The second peak of hydrogen absorption, located in the higher temperature region on the H2-TPR profile, may be due to ongoing processes of CoO reduction to Co0 and/or Co reduction from Cu/Co oxide particles. CoAlOx is characterized by the presence of two maxima in the H2-TPR profile: a low-intensity blurred peak with a maximum at 380°C and a narrower and more intense peak at a temperature of 673°C. Most likely, the first broad peak in the H2-TPR profile of the CoAl sample is associated with the reduction of cobalt from Co3O4, and the second is due to the processes occurring during the reduction of cobalt from the CoAl spinel. Thus, the introduction of cobalt into the composition of LDH contributed to an increase in the stability of Co particles. At the same time, the presence of copper in the composition of the CuCoAl samples led to an improvement in the reducibility of Co. 4. The change in the morphology of CuAl- (Cu/Al = 2), CoAl- (Co/Al = 2), and CuCoAl-samples (M2+/Al = 2, Cu/Co = 1) during the preparation of catalysts (after the stages of calcination at 550°C and reduction at 800°C) was studied by scanning electron microscopy (SEM). It has been established that CuAl systems are characterized by a "rosette" surface morphology, which is preserved after all high-temperature treatments. The morphology of CoAl systems (both freshly synthesized and after calcination and reduction stages) is almost the same and is mainly represented by plates oriented in the same plane. The surface of freshly synthesized CuCoAl-LDHs is composed of both "rosettes" and areas of stuck together rounded particles. After calcination, the surface of the CuAl samples was represented by randomly arranged particles, which, after reduction, transformed into a smooth surface of densely packed rounded particles. The morphology of all samples subjected to successive high-temperature treatments became denser, which is additionally confirmed by the lower values of the specific surface area for the reduced samples. In accordance with the data of energy dispersive analysis (EDS), the surface ratio of Cu/Al and Co/Al for CuAl- and CoAl-samples practically corresponds to the volume ratio of metals. At the same time, CoAl systems are characterized by a more uniform distribution of metals over the surface. On the surface of the CuAl samples, there were regions with very low and very high Cu/Al ratios. After the reduction of the CuCoAl sample, its surface was enriched in transition metals, mostly Co. Thus, the introduction of copper facilitated the release of cobalt to the surface of the samples, contributing to an increase in its surface concentration. 5. The properties of the catalysts obtained on the basis of CuAl-, CuCoAl- and CoAl-LDH were studied in the reactions of liquid-phase hydrogenation of furfural and 5-hydroxymethylfurfural. In the hydrogenation reaction of furfural, the influence of the composition of the catalysts, the nature of the solvent (water, ethanol), and the reaction time (1 or 5 h) on the catalytic characteristics was established. For CuCoAl samples, the effect of pretreatment conditions (calcination, reduction), as well as temperature (90 and 150 °C) and pressure (20 and 30 MPa) on their properties in the reaction was studied. The high selectivity of the formation of furfuryl alcohol was shown for all the studied Co-containing systems. It was found that when water was used as a solvent, the degree of furfural conversion increased with an increase in the cobalt content. When ethanol was used as a solvent, a synergistic effect was observed between Cu and Co in the composition of bimetallic catalysts, which manifested itself in its higher activity compared to monometallic samples. For the CuCoAl catalyst tested for 5 hours using ethanol as a solvent, the best catalytic characteristics were achieved: complete conversion of furfural and a selectivity of furfuryl alcohol formation of more than 98%. The study of catalysts based on CoAl-LDH and CuCoAl-LDH with (Co+Cu)/Al = 2 in the reaction of liquid-phase hydrogenation of 5-hydroxymethylfurfural (5-HMF) (T = 90°C, pressure 2 MPa, solvent - water) showed that the degree of conversion increased with increasing cobalt content and reaction time. For the catalyst based on CoAl-LDH, when the reaction was carried out for 5 hours, the degree of conversion of 5-HMF was more than 97%. At the same time, high selectivity (> 96%) for the formation of 2,5-bis(hydroxymethyl)furan was found for all the studied samples.

 

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