2.1. Synthesis and Characterization of Supports and Catalysts
The procedure for the synthesis of the composite material comprises TEOS hydrolysis, the primary condensation of urea with formaldehyde to form methylolurea, and the subsequent co-polycondensation of the obtained derivatives (Scheme 1
). Further heat treatment of the produced composite (300 °C, Ar) is accompanied by typical reactions of the carbonization process, such as cyclization and condensation, resulting in the formation of a hybrid organic–inorganic material UFSi, the elemental composition of which is presented in Table 1
The overview XPS spectrum of the obtained material (Figure 1
a, Table 2
) shows lines of silicon, oxygen, nitrogen, and carbon. The high-resolution spectra of the element lines were decomposed into components (Figure 1
b,c, Table 2
). The interpretation was based on the published data [25
The data presented suggest that nitrogen on the surface is present in the composition of pyrrole (400.1 eV), pyridine (398.6 eV), and secondary amine functional groups (397.4 eV) [25
]. The C1s XPS spectrum is approximated by the sum of three components with maxima at 283.8, 285.3, and 286.2 eV, which can be assigned to atoms of graphitized carbon (25%) and carbon in C–C/C = C (40%) and C–N/C = N/C = O (35%) bonds [25
The XRD pattern of the obtained sample is a wide halo (Figure 1
d), indicating an amorphous structure.
The SEM study (Figure 2
a) showed that the material has a homogeneous porous structure, which, according to TEM data (Figure 2
b), is formed by agglomerates of particles with a size of about 5 nm.
The isotherm of low-temperature nitrogen adsorption–desorption for the synthesized composite belongs to type IV according to the IUPAC classification, which is inherent in mesoporous materials. At high relative pressures, the isotherm has a rather steep rise, which is typical of materials with near-cylindrical open mesopores (Figure 3
). The surface area calculated by the BET method was 306 m2
/g, and the average pore diameter was 7.6 nm.
The catalyst was obtained by impregnating the support with rhodium trichloride at an elevated pressure of carbon monoxide at a temperature of 100 °C. The catalyst was supported on either the UFSi composite material (Rh-UFSi catalyst), or an amorphous silica sample that had similar textural characteristics but was not doped with nitrogen (Rh-Si catalyst).
The data of low-temperature nitrogen adsorption–desorption (Figure 3
, Table 3
) show that there was no significant decrease in the surface area and pore size of the support during the deposition of rhodium.
presents the IR spectra of the SiO2
and UFSi supports and the Rh-UFSi and Rh-Si catalysts, both fresh and after use in ten and three cycles.
The IR spectrum of the UFSi material shows the absorption bands of the stretching vibrations of siloxane Si–O–Si bonds (1105 cm–1
) and the bending vibrations of Si–OH bonds (968 cm–1
) and Si–O bonds in tetrahedral SiO4
fragments (804 cm–1
); the multiplet band of medium intensity in the range 1590–1750 cm–1
can be assigned to the combined frequencies of the skeletal vibrations of N–H and C–N bonds in pyrrole and pyridine structural fragments. After the deposition of rhodium, intense signals appear in the IR spectrum of the Rh-UFSi catalyst at 2019 and 2079 cm–1
, which are characteristic of carbonyl ligands coordinated to rhodium, and so does a low-intensity band at 1797 cm−1
, which is due to the vibrations of bridging CO groups of carbonyls Rh2
]. In the IR spectrum of the Rh-Si catalyst obtained using unmodified silica gel, the signals of coordinated carbonyl ligands are represented by a wide diffuse band with a maximum at 2069 cm–1
, but the absorption band at 1795 cm–1
(bridging CO groups) is quite intense. After the catalytic experiments, the absorption bands of coordinated carbonyl ligands in the spectrum of the Rh-Si sample are not observed. The disappearance of signals from coordinated carbonyl ligands in the spectrum of the spent Rh-Si catalyst sample is indeed associated with the leaching of rhodium carbonyl complexes from the catalyst surface. The hot filtration test carried out by us after the first and second cycles confirmed the activity of the solution in relation to hydroformylation, which indicates that active compounds, apparently rhodium carbonyls, are present in the solution after catalysis. In the spectrum of the spent Rh-UFSi catalyst sample (Figure 4
), the signals of only bridging carbonyl ligands (1793 cm–1
) disappear, and a couple of bands at 2019 and 2079 cm–1
are retained, which indicates the stable anchoring of rhodium carbonyl complexes.
The electronic configurations of rhodium in both catalysts were studied by XPS (Figure 5
). For the Rh-Si catalyst, the decomposition of the spectrum in the Rh3d region into components shows that, on the surface, there are both Rh(+1) carbonyl complex compounds (with binding energies of 308.8 and 313.5 eV for Rh3d 5/2 and Rh3d 3/2, respectively) and some amount (16%) of Rh(+3) compounds (310.2 and 314.9 eV), probably, residual rhodium trichloride [30
]. For the Rh-UFSi catalyst sample, the spectrum is a doublet of symmetric peaks, the integral intensities of which are in a ratio of 3: 2, and the spin-orbit splitting is 4.7 eV. The electron binding energies of Rh3d are 307.8 and 312.5 eV for Rh3d 5/2 and Rh3d 3/2, respectively, which indicates that Rh is in the +1 oxidation state in the composition of complex compounds with carbonyl ligands and nitrogen complexing groups in the support. An indirect confirmation of the coordination of rhodium to donor nitrogen sites can be the fact that the binding energies for the Rh-UFSi sample turned out to be slightly lower than for Rh-Si.
2.2. Catalytic Activity
1-Octene was chosen as a model substrate for studying the activity of the obtained heterogeneous catalysts in the hydroformylation reaction (Scheme 2
). The process was carried out at a synthesis gas (CO:H2
= 1:1) pressure of 3.0 and 5.0 MPa in the temperature range 80–140 °C.
shows that the rate of conversion of the initial substrate increases significantly with an increase in temperature from 80 to 100 °C; however, simultaneously, there is also a decrease in the ratio of yields of aldehydes with a linear and isomerized carbon chain (l/b ratio) because of an increase in the yield of the latter, which is explained by the acceleration of both the isomerization of the position of the double bond of 1-octene and the hydroformylation of internal alkenes. With a further increase in temperature, the fraction of isomerized products still somewhat increases, the l/b ratio decreases from 0.7 at 100 °C to 0.5 at 140 °C (Table 4
A decrease in the synthesis gas pressure to 3 MPa expectedly leads to a decrease in the yield of aldehydes, whereas the fraction of internal alkenes in the mixture increases (Table 4
, entry 9), which can be explained by the slowing down of CO coordination and incorporation during the formation of the acyl intermediate with a decrease in the partial pressure and, consequently, the concentration of carbon monoxide in the reaction medium, which promotes the isomerization reaction at the position of the olefin double bond.
An analysis of the changes in the composition of the reaction mixture, depending on the conversion of 1-octene and the reaction time at a temperature of 100 °C and a synthesis gas pressure of 5 MPa (Figure 6
), showed that, early in the process, n-nonanal and 2-methyloctanal are relatively rapidly formed, with the yield of the linear aldehyde being twice as high. Simultaneously, a side process of isomerization of the substrate occurs to form a mixture of internal alkenes, which enter into the hydroformylation reaction much more slowly.
An increase in the reaction time makes it possible to achieve a quantitative conversion of the alkene into a mixture of aldehydes; noteworthily, 2-ethylheptanal (III) and 2-propylhexanal (IV) are formed in approximately equal amounts, regardless of the conditions (temperature, time, and pressure).
The isomerizing ability of the support was studied in experiments where it was used as a catalyst (Table 5
, entry 1), and the contribution of the thermal component to the isomerization was estimated by performing the reaction in an inert gas atmosphere (Table 5
, entry 2). In both cases, no 1-octene isomerization was observed at 140 °C for 5 h, which indicates that this process occurs with the participation of the Rh-UFSi catalyst both under hydroformylation conditions and in a hydrogen atmosphere.
Note that the Rh-UFSi catalyst shows virtually no activity in the hydrogenation of the double bond: the yield of octane did not exceed 6% even when the reaction was carried out in an atmosphere of pure hydrogen (Table 5
, entry 5). Enrichment of the CO + H2
gas mixture with hydrogen to a ratio of 1:2 leads to the inhibition of the hydroformylation, and the main products are internal octenes (Table 5
, entry 3). An increase in the partial pressure of carbon monoxide (up to CO:H2
= 2:1) does not have a significant effect, and the yield of aldehydes slightly decreases, while the n-/iso- ratio remains the same. These results are consistent with the published characteristics of homogeneous hydroformylations in the presence of rhodium complexes with organophosphorus ligands [32
]: an increase in the hydrogen partial pressure promotes the formation of hydride complexes responsible for the isomerization. An increase in the substrate/catalyst ratio slows down the hydroformylation, which may be due to the difficulty of carbon monoxide molecules to access the coordination sphere of rhodium because of an increase in the concentration of olefin (Table 4
, entry 8), with the hydroformylation of internal double bonds being inhibited to a greater extent.
The most important characteristics of a heterogeneous catalyst, the stability of operation and the possibility of repeated use, were evaluated in ten consecutive cycles at 120 °C for 2 and 5 h of the reaction. Figure 7
shows that the conversion of the initial substrate and the distribution of the products remain virtually the same, beginning with the third cycle.
Elemental analysis data (Table 6
) show that the metal loss for the Rh-UFSi catalyst after ten consecutive 5 h long cycles is only 7% of the initial content. Additionally, to test the leachability of rhodium in the form of soluble carbonyls, experiments on a homogeneous hydroformylation of 1-octene were performed, in which the catalysts were filtered liquids after hydroformylation with certain Rh-UFSi catalysts. The reactions were carried out under the same conditions as those with the heterogeneous catalyst. After the first two cycles, a noticeable activity of the homogeneous solution was observed: the conversion of 1-octene was about 80% in 5 h, and after the third cycle, the conversion of 1-octene did not exceed 10%. The totality of these observations confirms that there is no significant leaching of the active metal, and the developed catalyst is characterized by excellent stability.
The Rh-Si catalyst obtained using silica gel containing no nitrogen component lost activity already after the second cycle, and the main products were internal octenes. After three cycles, 0.19 wt% rhodium remained on the support (Table 6
The Rh-UFSi catalyst also showed rather high activity in the hydroformylation of linear alkenes with different chain lengths and structures (styrene and cyclohexene); the results are shown in Table 7
. It should be noted that the analysis of the reaction products shows that, in addition to n- and isoaldehydes, internal alkenes with double bond shifts are also formed as isomerization byproducts (with the exception of styrene and cyclohexene hydroformylation). The hydrogenation of alkenes to alkanes was not observed. Under the conditions used, neither the yield of aldehydes nor the n/iso ratio depended on the length of the carbon chain of alkenes. These data are in agreement with the results reported in many articles, for example [33
], using Rh nanoparticles.
The developed Rh-UFSi catalyst showed excellent catalytic characteristics in the hydroformylation of alkenes of various structures, its activity is close to, and in some cases exceeds, the results obtained for catalysts similar in structure presented in the works [34