Maciej Trejda*,
Małgorzata Bryś and
Maria Ziolek
Adam Mickiewicz University in Poznań, Faculty of Chemistry, Umultowska 89b, 61-614 Poznań, Poland. E-mail: tmaciej@amu.edu.pl; Tel: +48 618291305
First published on 14th November 2014
MCM-41 mesoporous solids were modified with chromium species using Cr(NO3)3 and CrO3 as precursors. The impact of metal amount and metal sources on the structural and textural parameters of the materials obtained was investigated. Their surface properties were analyzed using XRD, UV-Vis, H2-TPR (the state of Cr species) as well as test reactions (2-propanol decomposition and cyclization and dehydration of 2,5-hexsanedione). The catalytic activity of Cr/MCM-41 materials was tested in methanol partial oxidation. The relationships of reducibility, kind of chromium species, basicity of material surface and the catalysts performance in partial oxidation of methanol were examined and discussed. The impact of basicity on methanol oxidation was clearly documented. Di- and polychromate species were found to be responsible for high yields of formaldehyde.
Partial oxidation of methanol is an important industrial process since it provides valuable products such as formaldehyde, dimethoxymethane (methylal) or methyl formate.8 The production of the first product mentioned above at industrial level has been known for more than 100 years and involves Fe–Mo or Ag containing catalysts. The mechanism of this reaction, depending on the kind of catalyst used, has been described in literature, e.g. ref. 8–18 but some of its aspects are still debatable. Partial oxidation of methanol has been also proposed as a test reaction for different kinds of catalysts basing on mechanisms proposed, e.g. ref. 19 and 20. In our previous work we have shown that partial oxidation of methanol can be combined with other transformation of CH3OH, namely the sulphurisation process.20 As a consequence, these two complementary reactions give more detail information concerning the properties of catalyst surface.
The application of different test reactions or characterization techniques is essential especially when different kinds of species can be formed in course of catalyst preparation. This is often in the case of transition metals, e.g. iron21,22 or chromium,23,24 applied in the catalysts preparation. The latter element was incorporated into different kinds of mesoporous materials (MCM-41, SBA-15 and SBA-3) using CrO3 as a metal precursor.24 We have shown that a kind of support strongly influences chromium species obtained after impregnation. In this paper we focus on MCM-41 support that is modified with different amounts of chromium using CrO3 and Cr(NO3)3 as metal precursors. By changing the amount and sources of chromium we expected to obtain different chromium species on mesoporous support having different properties.
The aim of this study was to examine basicity and redox properties of model catalysts of MCM-41 type containing chromium by the use of selected test reaction (2-propanol decomposition, cyclization and dehydration of 2,5-hexsanedione and methanol oxidation). In particular, we would like to present the correlation of basic properties of Cr/MCM-41 materials (measured by test reactions) and reducibility of chromium species (testified by H2-TPR) with redox properties estimated from methanol oxidation to formaldehyde.
XRD patterns were recorded at room temperature on a Bruker AXS D8 Advance apparatus using CuKα radiation (λ = 0.154 nm), with a step of 0.02° and 0.05° in the small-angle and wide-angle range, respectively.
N2 adsorption/desorption isotherms were obtained on a Micromeritics ASAP equipment, model 2010. The samples (200 mg) were pre-treated in situ under vacuum at 573 K for 3 h. The surface area was calculated using the BET method. The pore diameter and the mesopore volumes were determined from the adsorption branch of the isotherms.
UV-Vis spectra were recorded using Varian-Cary 300 Scan UV-Visible Spectrophotometer. Powdered samples were placed into the cell equipped with a quartz window. The spectra were recorded in the range from 800 to 190 nm. Spectralon was used as a reference material.
The temperature-programmed reduction of the samples was carried out using H2/Ar (10 vol% in Ar) as a reducing agent (flow rate = 32 cm3 min−1). The sample (0.025 g) was introduced to a quartz tube, treated in a flow of helium at 723 K for 2 h (flow rate = 40 cm3 min−1) and cooled to room temperature. Then it was heated at the rate of 10 K min−1 to 1273 K under the flow of the reducing mixture. Hydrogen consumption was measured by a thermal conductivity detector.
Catalysta | Chromium source | Chromium loading, wt% | Colour of sample |
---|---|---|---|
a (o) – obtained from CrO3; (n) – obtained from Cr(NO3)3. | |||
1Cr(o)MCM-41 | CrO3 | 1 | Pale yellow |
3Cr(o)MCM-41 | CrO3 | 3 | Yellow-green |
5Cr(o)MCM-41 | CrO3 | 5 | Dark green |
1Cr(n)MCM-41 | Cr(NO3)2 | 1 | Pale yellow |
3Cr(n)MCM-41 | Cr(NO3)2 | 3 | Yellow-green |
5Cr(n)MCM-41 | Cr(NO3)2 | 5 | Green |
Some conclusions concerning the nature of chromium species can be deduced from UV-Vis measurements. The spectra of samples prepared are shown in Fig. 4. The introduction of chromium on the surface of MCM-41 results in the appearance of some characteristic bands coming from different coordinations of Cr species. In particular the bands at ca. 260 and 350 nm are assigned to the charge transfer from O2− → Cr6+ in monochromate species.28–30 These bands prevail for the samples with the lowest amount of chromium (1 wt%) thus one can postulate that monochromate species are dominant for lower Cr loading on material surface. This conclusion is also supported by the pale yellow (characteristic of Cr6+) color of the samples having the lowest chromium concentration. The introduction of higher than 1 wt% amount of chromium results in the appearance of a band at ca. 465 nm (this band shows a very low intensity for 1 wt% of Cr) that is assigned to di- and poly-chromate species.28 For samples containing 3 and 5 wt% of chromium also a band at ca. 600 nm is formed. Moreover, the intensity of the band at 600 nm is higher when CrO3 is applied as the source of metal for the impregnation. The band mentioned is usually assigned in literature to Cr3+ in α-Cr2O3 particles.28,30 The presence of this species can be additionally testified by the color of the samples (yellow-green for 3 wt% of Cr and green or dark green for 5 wt% of Cr). For the lower concentration of chromium (1 wt%), the XRD peaks do not show the presence of α-Cr2O3, which is in line with UV-Vis results.
Fig. 4 UV-Vis spectra of: (a) MCM-41; (b) 1Cr(o)/MCM-41; (c) 3Cr(o)/MCM-41; (d) 5Cr(o)/MCM-41; (e) 1Cr(n)/MCM-41; (f) 3Cr(n)/MCM-41; (g) 5Cr(n)/MCM-41. |
Fig. 5 presents the H2-TPR profiles of MCM-41 samples containing chromium. For all catalysts one main maximum on H2-TPR profile is observed and according to literature this peak can be assigned to the reduction of Cr6+ species.31 The temperature of Cr6+ reduction depends on metal loading. For both chromium sources applied, the increase of metal loading from 1 wt% to 3 wt% leads to an increase in Cr6+ reducibility. This is in agreement with the formation of lager aggregates, i.e. polychromate species that show lower temperature of reduction.30 However, for samples with 5 wt% of chromium loading the temperature of Cr6+ reduction is higher than for those with 3 wt% of chromium. This fact could be correlated with a decrease in polychromate species in the sample. The latter species decomposes forming α-Cr2O3 observed in XRD patterns (Fig. 3). For the samples that show the presence of α-Cr2O3 (3 wt% and 5 wt% of chromium) a small reduction peak is also visible at ca. 550 K. This peak is the most intense for the highest Cr loading. According to literature, it can be assigned to the reduction of Cr6+ species dispersed on α-Cr2O3.31 Moreover, the amount of hydrogen consumption differs depending on the metal loading and this dissimilarity is not proportional to the chromium content. The highest H2 consumption is observed for 3 wt% of Cr loading independently of chromium source applied for the MCM-41 impregnation. This observation is consistent with XRD and UV-Vis measurements that show the formation of α-Cr2O3 phase especially in samples 5Cr(o)/MCM-41 and 5Cr(n)/MCM. Thus the increase in chromium concentration from 1 wt% to 3 wt% is accompanied by increasing concentration of Cr6+ in the samples, whereas a further increase in the metal content leads mainly to the formation of α-Cr2O3 and limits the amount of different kinds of chromates species (Cr6+) inside or outside the pores of material.
Fig. 5 H2-TPR profiles of: (a) 1Cr(o)/MCM-41; (b) 3Cr(o)/MCM-41; (c) 5Cr(o)/MCM-41; (d) 1Cr(n)/MCM-41; (e) 3Cr(n)/MCM-41; (f) 5Cr(n)/MCM-41. |
All catalysts prepared were tested in cyclization and dehydration of 2,5-hexanedione (2,5-DHN) after activation of the samples at 623 K. The cyclization of 2,5-DHN is a well-known reaction for determination of basicity/acidity of materials and was proposed by Dessau4 and Alcaraz et al.5 as Brønsted acid–base test. The formation of 2,5-dimethylfuran (DMF) occurs on acidic centers, whereas basic centers take part in the production of 3-methyl-2-cyclopentenone (MCP). The ratio of MCF to DMF can be taken as an indicator of acid (<1), basic (>1) or acid-basic properties (∼1).
The results obtained in cyclization and dehydration of 2,5-DHN are presented in Fig. 6 as the MCP/DMF ratio vs. chromium loading in the samples. The samples containing the lowest concentration of Cr species (evidenced on material surface mainly as monochromate ones) show acidic character (MCP/DMF < 1). The impregnation of MCM-41 using higher amount that 1 wt% of Cr species results in an increase in the MCP/DMF ratio. For 3 wt% of metal loading (most pronounced for 3Cr(o)/MCM-41) the MCP/DMF value is the highest, however the introduction of 5 wt% of chromium decreases the basicity of samples. On the basis of these results one can propose to assign the origin of moderate basicity in the materials prepared to the presence of di- and polychromate species, which dominate for the samples with 3 wt% of chromium loading.
The acid–basic properties of materials prepared were also examined in 2-propanol decomposition. In general, propene and diisopropyl ether are formed on acid sites (or on the acid–base pairs), whereas the formation of acetone requires the presence of basic (or redox) centers.6,7 The materials obtained using CrO3 as a metal precursor (xCr(o)/MCM-41) showed negligible activity in the test reaction carried out at 573 K, which was not the case for the catalyst prepared using the other Cr precursor. Therefore in Fig. 7 the selectivity to acetone vs. chromium concentration is presented only for xCr(n)/MCM-41 samples. It can be observed that the profile presented in Fig. 7 shows a volcanic shape (with the maximum for 3 wt% of Cr) similar as for cyclization and dehydration of 2,5-DHN (Fig. 6). These results confirm the moderate basic or redox character of di- and polychromate species on MCM-41 surface.
Table 3 shows the results of methanol oxidation carried out at 523 and 573 K. Unsupported MCM-41 was not active in this reaction. The main product obtained on chromium containing samples is formaldehyde (FA), whose formation demands the presence of moderate acidic and basic centers (the side product detected are methyl formate and CO2). The selectivity to FA does not change systematically with the chromium loading and seems to depend in some extent on a source of chromium. For materials modified with CrO3 the highest loading leads to the lowest selectivity to formaldehyde. This can be correlate with Cr2O3 formation leading to total methanol oxidation. Indeed, 5Cr(o)/MCM-41 sample shows the highest selectivity to CO2 among all catalysts at both reaction temperatures. In case of Cr(NO3)3 applied for MCM-41 modification the selectivity to FA first decrease a little and then does not change significantly. However, the higher amount of polychromates species deduced for samples containing 3 wt% of chromium (UV-Vis, H2-TPR) reflects in the increase of methanol conversion. This in consequence influences the amount of FA formed. The yield of FA depends on the chromium loading as demonstrated in Fig. 8. However, one should take into account that the acid–base properties of catalysts also depend on chromium loading. Irrespectively of the chromium source used for MCM-41 impregnation, as well as the reaction temperature applied, the volcanic shapes of the profiles in Fig. 8 are the same with the maximum for 3 wt% of chromium. The formation of FA requires rather weak acidity (necessary for methanol adsorption and formation of surface methoxy species) and moderate basicity (abstraction of hydrogen from methoxy species). Moreover, the week acidity enhances desorption of formaldehyde from materials surface (a similar effect is observed with increasing reaction temperature). Therefore, the results presented in Fig. 8 suggest the important role of di- and polychromate species (characterized as moderate basic) that participate in partial methanol oxidation process. The nucleophilic character of oxygen linking chromium atoms (Cr–O–Cr) could explain the highest activity of samples containing 3 wt% of chromium. These species are probably responsible for hydrogen abstraction from methoxy species adsorbed on Cr6+ (Lewis acid centers). Therefore, the amount and relative concentration of monochromate, di- and polychromate as well as α-Cr2O3 species is crucial for the results obtained in the methanol partial oxidation process. The relative content of each above-mentioned species can be measured and therefore controlled by the test reactions for acid–base properties and physico-chemical analyses. In consequence the knowledge of the amount, types and properties of active species permits anticipation and design of new catalytic systems.
Catalyst | Methanol conversion (%) | Selectivity (%) | ||||
---|---|---|---|---|---|---|
(CH3)2O | HCHO | (CH3O)2CH2 | HCOOCH3 | CO2 | ||
Reaction temperature: 523 K | ||||||
1Cr(o)/MCM-41 | 7 | Traces | 79 | Traces | 12 | 9 |
3Cr(o)/MCM-41 | 9 | Traces | 78 | Traces | 12 | 10 |
5Cr(o)/MCM-41 | 7 | Traces | 70 | Traces | 11 | 19 |
1Cr(n)/MCM-41 | 14 | Traces | 83 | Traces | 6 | 11 |
3Cr(n)/MCM-41 | 18 | Traces | 72 | Traces | 15 | 13 |
5Cr(n)/MCM-41 | 4 | Traces | 72 | Traces | 14 | 13 |
Reaction temperature: 573 K | ||||||
1Cr(o)/MCM-41 | 18 | Traces | 67 | Traces | 7 | 25 |
3Cr(o)/MCM-41 | 21 | Traces | 74 | Traces | 8 | 18 |
5Cr(o)/MCM-41 | 22 | Traces | 58 | Traces | 8 | 34 |
1Cr(n)/MCM-41 | 26 | Traces | 73 | Traces | 8 | 19 |
3Cr(n)/MCM-41 | 38 | Traces | 69 | Traces | 6 | 25 |
5Cr(n)/MCM-41 | 18 | Traces | 67 | Traces | 11 | 21 |
• The increase in chromium loading on MCM-41 results in an increase in di- and polychromate species and later α-Cr2O3 irrespective of chromium source used for modification.
• Higher number of polychromate species are obtained using Cr(NO3)2 than CrO3 as the precursor, however these species show similar reducibility for the same Cr loading.
• Reducibility of chromium species is influenced by Cr loading – the highest reducibility is observed for the catalysts with 3 wt% of chromium.
• Polychromate species are responsible for basicity of the catalyst tested in 2-propanol decomposition and 2,5-hexanedione transformation. The basic character of the solid is the highest for materials with 3 wt% of chromium loading.
• There is a clear relationship between the basicity of the catalyst surface, reducibility of chromium species and redox properties manifested by the formaldehyde yield in methanol oxidation.
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