Cr Incorporated MCM-41 Type Catalysts for Isobutane

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Cr Incorporated MCM-41 Type Catalysts for Isobutane Dehydrogenation and Deactivation Mechanism Saliha Kilicarslan, Meltem Dogan, and Timur Dogu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie302543c • Publication Date (Web): 18 Feb 2013 Downloaded from http://pubs.acs.org on February 21, 2013

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Industrial & Engineering Chemistry Research

Cr Incorporated MCM-41 Type Catalysts for Isobutane Dehydrogenation and Deactivation Mechanism Saliha Kilicarslan1, Meltem Dogan1* and Timur Dogu2

1

2

Chemical Engineering, Gazi University, 06570 Ankara, Turkey Chemical Engineering, Middle East Technical University, 06800 Ankara, Turkey * ([email protected])

ACS Paragon Plus Environment

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ABSTRACT Cr incorporated MCM-41 type catalysts, which were prepared by direct hydrothermal

synthesis

and

impregnation

routes,

were

tested

for

dehydrogenation of isobutane. Among the surfactants (C17H38BrN, C19H42BrN, and C32H68BrN) used in the synthesis of MCM-41, C19H42BrN gave the best structure with ordered mesopores. Characterization results proved that chromium was very well dispersed within the MCM-41 lattice of the synthesized catalysts. In all of the synthesized catalysts, presence of Cr+6 (2p3/2) in the form of chromates and Cr+3(2p1/2) in the form of CrOx or Cr2O3 were detected. All chromate types (mono, di and poly) were observed in the catalyst synthesized hydrothermally according to the addition of metal solution simultaneously with silica source (CR_ALS). All characterization methods demonstrated that the highest amount of Cr+6 was present in this catalyst. Catalytic tests of the synthesized catalysts were carried out at 600oC and at atmospheric pressure. The maximum conversion value (~27%) was reached at minute 15 on CR_ALS catalyst. No products other than isobutene and hydrogen were encountered at the reactor outlet. Results proved that coke deposition was negligible over the synthesized catalysts in isobutane dehydrogenation. It was shown that the monochromates were the most active phase among all types of chromates for isobutane dehydrogenation. Catalyst deactivation occured when the tetrahedrally coordinated Cr(VI)O4 type of chromate was converted to inactive octahedrally coordinated Cr(III)O6 groups and Cr2O3 crystal phase. Keywords: isobutane dehydrogenation, Cr-MCM-41, deactivation


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INTRODUCTION

Isobutene is used as a raw material for the production of important chemicals such as butyl rubber and gasoline octane enhancing additives, such as MTBE. Isobutane dehydrogenation is more advantageous than other isobutene production processes in terms of efficiency and the operating costs. Isobutane dehydrogenation is an endothermic equilibrium-limitted reaction. Also high reaction temperatures cause catalyst deactivation and the occurance of side reactions.

MCM-41 attracted significant interest in recent decades, as catalyst and catalyst support with its uniform pore structure in the mesopore range and high surface area. The properties of the synthesized MCM-41 may vary depending on the surfactant/Si ratio in the synthesis solution, the synthesis technique, synthesis conditions, silica source and carbon chain length of surfactant. In the literature, synthesis studies have been conducted using different silica sources such as TEOS, TPOS, sodium silicate etc.1,2,3,4,5,6. In the synthesis accordance with fumed silica, high surface area and proper pore wall thickness were obtained, whereas high thermal stability was achieved by using pure SiO2 5.

The

quaternary ammonium surfactants (CnH2n+1N(CH3)3+) with different alkyl chain lengths (n=10, 12, 14, 16) were used and pore diameters were obtained in the range of 1.7-2.6 nm by Martins and Cardoso7. Synthesis techniques such as micro-wave, ultrasonic radiation, magnetic fields have also been used in recent years, to shorten the synthesis duration and to improve the properties of MCM-418,9,10,11,12. MCM-41 stand alone is quite inert in the reaction applications. However, the catalytic behavior can be improved by introducing the metal cations into the silica matrix 13,14 .

Although Cr-MCM-41 has not been used in isobutane dehydrogenation, there has been the usage in some dehydrogenation, oxidation and polymerization reactions. However, studies on the catalysts prepared with different support


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materials

apart

from

MCM-41

have

been

available

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for

isobutane

dehydrogenation. Studies contributing to the interpretation of our results were summarized in this paragraph. CrOx/ZrO2, CrOx/γ-Al2O3 and CrOx/SiO2 catalysts were prepared with impregnated method by Rossi et al.

15

. Cr(VI),

Cr(V), Cr(III), and small Cr(III) oxide clusters and α-Cr2O3 were defined on the catalyst structure. It was shown that the chromium oxidation states changed depending on the Cr concentration and the support type. The activity of the isobutane dehydrogenation was determined as the highest on the CrOx/ZrO2 catalyst. Cr/SiO2 catalyst containing 1-3% wt. Cr was synthesized using different chromium metal salts by Gaspar et al. 16. It was determined that on the catalyst surface the reduction of Cr(VI) to Cr(III) was changed depending on metal salt source. Cr-SBA-15 and CrOx/SBA-15 catalysts were synthesized with impregnation technique by Zhang et al. 17. It was shown that Cr2O3 crystal phase was formed at high Cr concentration. Alkane dehydrogenation was studied over CrO3/Al2O3 catalyst containing %3-15 wt. Cr by Shee and Sayri18. The chromium states were determined as Cr(III) and Cr(VI) on the catalyst surface. Cr-MCM-41 synthesized by direct hydrothermally synthesis (DHT) and by the template ion exchange (TIE) methods were used in methane oxidation19. It was determined that the synthesized Cr-MCM-41 according to DHT had monochoramate species; whereas the catalyst synthesized with TIE method had different chromate species. The catalyst synthesized with DHT showed higher formaldehite selectivity and methane conversion. M-MCM-41 (M:V, Cr, Fe, Co, Ni, Ga) catalysts were synthesized using hydrothermal method by Takehira et al.

20

. It was shown that Cr-MCM-41 catalyst had the highest propane

dehydrogenation activity. It was indicated that the reduction-oxidation cycle between Cr(VI)O4 and Cr(III)O6 had an important role in propane dehydrogenation. Synthesized M-MCM-41 (M: V, Cr, Fe, Ga) catalysts were used in ethylbenzene dehydrogenation by Ohishi et al. 21. It was determined that Cr(VI)O4 tetrahedral coordination formed monochromate active species on the catalyst surface. However Cr(III)O6 octahedral structure, which was formed by reduction during the reaction, formed the least active polychromate species. It


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was shown that deactivated catalyst could be regenerated with O2 or CO2. Sun et al.22 showed that the samples which had Si/Cr>20 ratio had only Cr(VI) species in catalyst structure. Airaksinen et al.23 indicated that the rate determining step was the adsorption in isobutane dehydrogenation on chromia/alumina catalyst.

Reaction rate was determined depending on the

parameters of hydrogen and isobutene adsorption. Airaksinen and Krause24 indicated that pre-reduction of Cr2O3/Al2O3 with H2 catalyst decreased the dehydrogenation activity.

Literature studies showed that chemical state of chromium oxide was a very important parameter for reaction applications. The main objective of the present study was to determine how chromate types in Cr-MCM-41 catalysts was affected by synthesis method (hydrothermal or impregnation) and the order of the metal source addition to the synthesis solution in hydrothermal method. To determine the factors affecting catalyst deactivation was also aimed in this study. MCM-41 was synthesized hydrothermally using with different carbon chain length surfactants, firstly. Then, Cr-MCM-41 catalyst syntheses were performed over the selected surfactant. The results of the catalytic tests carried out on the synthesized catalysts were evaluated together with the characterization studies.


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EXPERIMENTAL Synthesis of the Cr containing MCM-41 Catalysts MCM-41 synthesis was performed using different surfactants, like C17H38BrN (tetradecyltrimethyl ammonium bromide), C32H68BrN (tetra-n-octylammonium bromide), C19H42BrN (N-cetyl-N,N,N-trimethylammonium bromide) with different carbon chain lengths. Sodium silicate solution was used as the silica source and surfactant:Si (mole/mole) ratio was kept constant as 0.5 in the synthesis solution. Firstly, the surfactant was put into deionized water and the mixture was stirred at 30oC until a clear solution was obtained. Sodium silicate solution (27% wt. SiO2, 8% wt. Na2O, 65% wt. H2O) was added to the mixture. After the addition, pH was set the value to 11 using 4 N H2SO4 solution. Then, the solution was taken into a teflon-lined steel autoclave and it was treated hydrothermally at 120oC for four days. After the mixture was stirred at room temperature and then washing procedure was performed. The mixture was washed at room temperature until the pH value of the filtrate was approximately 7. Washed sample was dried at room temperature for one day, then it was calcined at 600oC for 6 hours under dry air flow (135 ml/min).

Chromium incorporated Cr-MCM-41 catalysts were synthesized hydrothermally by the addition of Cr(NO3)3.9H2O metal salt solution into the synthesis solution. Addition of Cr source to the synthesis solution was achieved following three different routes, namely Cr addition before silica source (CR_BS), addition after silica source (CR_AS) and addition simultaneously with silica source (CR_ALS). In the catalyst synthesis procedure, surfactant:Si ratio (mole/mole) was kept constant at 0.5 and the amounts of the Cr and Si sources were determined to obtain a catalyst containing 3% or 1.5% Cr by mass. After setting of pH to a value as 11 of the obtained mixture, same procedure was followed as the conditions specified in the synthesis of pure MCM-41. CR_ALS1.5 catalyst containing 1.5% Cr by mass was synthesized according to mechanism of the


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addition of metal salt solution to synthesis solution simultaneously with silica source.

Chromium incorporated MCM-41 catalyst containing 3% Cr by mass was also synthesized by an impregnation method, besides one-pot hydrothermal route. The material synthesized by the impregnation procedure is denoted as CR@MCM-41. In the synthesis of this material, Cr(NO3)3.9H2O solution was added to the mixture of aqueous MCM-41 at 40oC drop by drop and then the temperature of the mixture was increased to 60oC. This mixture was mixed for about 2 hours. After the evaporation of water the solid product was dried at 100oC, and then it was calcined at 600oC for six hours under dry air flow (135 ml/min).

Catalyst Characterization Studies

SEM/EDS, N2 adsorpsion/desorpsion, XRD, XPS, Diffuse Reflectance-UV-vis (DR-UV-vis), TPR and TGA analyses were used in the characterization studies. XRD analyses were performed with an X-ray diffractometer (Rigaku D/MAX 2200) using Cu, Ka irradiation source (λ= 1.5406oA). XRD patterns were recorded in the 2θ range of 1-10o for small angle profiles to determine the characteristic peaks of MCM-41 and also in the 2θ range of 10-90o for wide angle profiles to determine different crystalline formations in metal incorporated materials. XPS was employed in order to get information about chemical surrounding of the surface and the oxidation state of chrome metal. XPS analyses were performed in a SPECS instrument at the Central Laboratory of Middle East Technical University.

SEM/EDS analyses were carried out in a Jeol/JSM-6400 instrument. EDS analyses were used in estimating the atomic ratios of metals incorporated into MCM-41 support, SEM photographs were used in analyzing morphological structure of the samples. Surface areas and pore size distributions of the


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synthesized materials were measured in the nitrogen physisorption instrument (Quantochrome, Autosorb-1). DR-UV–vis spectra of the samples in the range of 200–800 nm were measured on a UV-vis spectrophotometer (Perkin Elmer UVvis/ RSA-PE-20) with KBr being standard. TPR (Quantochrome, CHEMBET3000) measurements were performed using hydrogen gas as reducing agent in order to determine the reduction conditions of the synthesized catalysts. TGA analysis (Perkin Elmer Diomond DSC) was employed at 25-900oC under air flow at a rate of 1oC/min in order to determine the amount of C deposition on the catalyst.

Catalytic tests

Catalytic

performance

of

the

synthesized

catalysts

was

tested

in

dehydrogenation of isobutane in a flow reactor. These tests were performed in a quartz glass reactor (length:1m, ID:0.6 cm) which was placed inside a tube furnace. A porous glass filter was welded to the middle of the reactor to support the catalyst. The same amount of catalyst (0.1 g) was placed into the reactor in all tests. Pure isobutane flow at a rate of 20 ml/min was introduced to the system. Catalytic measurements were performed at 600oC and at atmospheric pressure. WHSV (weight hourly space velocity) value was kept at 26 h-1 in experimental studies. Gas samples were taken from the reactor exit at regular intervals. Gas chromatography instrument (SRI 8610C) with silica column was used in product analyses. Catalytic tests were repeated for CR_ALS catalyst and error % was predicted as lower than 5% for each data point.

RESULTS AND DISCUSSIONS

Characterization results of synthesized catalysts

Four different syntheses were conducted to identify the effects of the surfactant carbon chain length on the structure of synthesized MCM-41 using C17H38BrN,


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C19H42BrN, C32H68BrN and C19H42BrN+ C17H38BrN mixture. The synthesis procedure using C32H68BrN as the surfactant was not successful. The fact that no solid product was remained after the calcination step was interpreted as that mesoporous silicate lattice with ordered pores did not form with this surfactant. The XRD diffraction patterns of the MCM-41 samples which were synthesized using C17H38BrN, C19H42BrN and C19H42BrN+ C17H38BrN mixture are given in Figure 1. All four characteristic peaks of MCM-41 at 2θ values of 2.52o, 4.18o, 4.76o and 6.14o are clearly seen in the XRD of the material synthesized using C19H42BrN as the surfactant (Figure 1). In the case of the material synthesized by using C17H38BrN the reflections of the main peak were not quite clear. Characteristic peaks of MCM-41 were not observed in the synthesis achieved with the C17H38BrN and C19H42BrN mixture as the surfactant. Based on these results Cr incorporated materials were synthesized using C19H42BrN as the surfactant.

N2 adsorption/desorption isotherms of MCM-41 samples synthesized with different surfactants are given in Figure 2. The characteristic Type IV isotherms of mesoporous materials with ordered pore structures were obtained in the MCM-41 samples synthesized with C19H42BrN and C17H38BrN as the surfactant. The physical properties of synthesized MCM-41 samples are given in Table 1.

It was observed that MCM-41 samples synthesized using

C17H38BrN and C19H42BrN had quite similar properties. The average pore diameter of the material synthesized by using C19H42BrN was somewhat larger, as expected. C19H42BrN was chosen as the surfactant for further synthesis studies, considering that the synthesized sample had a more uniform structure (as predicted from XRD patterns), suitable average pore diameter, surface area, and wall thickness values.

In the synthesis of Cr-MCM-41 catalysts, three different procedures, which included adding salt solution into the synthesis solution before the silica source (CR_BS), after the silica source (CR_AS) and simultaneously with the silica


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source (CR_ALS) were used. All these samples were synthesized with the objective of obtaining a material which would contain 3 %(wt) Cr. Chromium incorporated MCM-41 was also synthesized using an impregnation procedure and this material was denoted as CR@MCM-41.

The XRD diffraction patterns of the Cr incorporated catalysts are given in Figure 3. It was found that the main peak (2θ=2.52o) of the MCM-41 structure was maintained in all catalysts (Figure 3). Peaks characterizing the Cr2O3 crystals (2θ=24.5o, 33.74o, 36.1o, 50.84 o, 54.6o, 64.04o, 65.8o) are clearly seen in the catalysts CR_AS and CR_BS. Strong peaks of the Cr2O3 crystals were not observed in the CR_ALS catalyst and no peaks corresponding to chromium oxides were observed in the XRD patterns of the catalyst synthesized by impregnation. This result indicated that different species of chromium oxide dispersed very well within the pores of the synthesized MCM-41 and large chromium oxides crystals were not formed.

Cr mapping determined by SEM/EDS analysis for CR_ALS is given in Figure 4. It could be concluded that density of Cr was higher on the bright areas when the mapping was evaluated together with the SEM photograph. Difference in the density of Cr was explained by different chromate types (mono, di and poly) in the CR_ALS catalyst structure.

N2 adsorption/desorption isotherms of Cr incorporated MCM-41 catalysts are given in Figure 5. Type IV isotherm observed in the pure MCM-41 structure was not seen in the prepared catalysts.

XPS curves of the Cr incorporated catalysts which were synthesized following the three different routes described above are given in Figure 6. The band observed at 532.6eV binding energy in all three catalysts corresponds to the O1s of SiO2. The Si2p bands were also observed at 103eV binding energy (Figure 6).17 An examination of Cr2p curves revealed that the peak at 578eV,


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indicates the presence of Cr+6 (2p3/2) in the chromates, in all three of the synthesized catalysts. Again, in all of the synthesized catalysts, the peak at 586eV (2p1/2), which symbolizes the presence of Cr+3 in the CrOx or Cr2O3 form, was observed.17, 18

The Cr/Si atomic ratios of synthesized catalysts determined by EDS and XPS analyses are given in Table 2. It was found that the Cr/Si ratios of those in hydrothermally synthesized catalysts, except for the CR_BS, were higher than that of the synthesis solution (Cr/Si=0.036) both in the surface and in the bulk phase. This case showed that some Si was lost during the washing procedure of catalyst synthesis. EDS and XPS results were quite comparable, XPS results being somewhat smaller than EDS for CR_AS and CR_BS. These results indicated that external surface Cr compositions of these two catalysts were somewhat less than the bulk compositions.

The DR-UV-vis curves of synthesized catalysts are given in Figure 7. The peaks observed at 230 nm in all catalysts, being more pronounced in the CR_ALS catalyst, are due to the interaction of the Cr with the support17. Again, being more pronounced in the CR_ALS catalyst, the peak at about 320 nm expresses the O-Cr(VI) charge transfer in the dichromate 18, 25. Only the peak formation at 380 nm on the CR_ALS catalyst symbolizes the monochromate 17,25,26 .And The peak observed at 430 nm on all synthesized catalysts belongs to polychromates25. The shoulder formation at about 600nm expresses the O-Cr(III) charge transfer in Cr2O318. The DR-UV-vis analysis showed that all chromate types were observed on the CR_ALS catalyst. As expected, different chromate types were predicted for impregnation and hydrothermal methods. Present study also showed that chromate types affected significantly from the order of the metal salt addition to the synthesis solution in hydrothermal method.


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FTIR spectra of the synthesized catalysts are given in Figure 8. Chromium silicate peaks were detected on all catalysts, with a maximum range of 34003450 cm-1 at a band width of 3000-3700 cm-1. This band expresses the Si-O-H in the framework structure and the Si(OH)Cr groups interacting with the adsorbed water molecules. Being lower on the CR_ALS catalyst, the peaks observed at 2925 cm-1 and 2840 cm-1 are due to the organic surfactants remaining in the structure27. Being lower on the CR@MCM-41 catalyst, the band observed at a range of 1618-1620 cm-1 is due to the adsorbed water. And The bands observed at 453-457 cm-1, 784-794cm-1, 1072-1076 cm-1 and 12001300 cm-1 show the presence of Si-O-Si in the frame structure in all catalysts. Being more pronounced on the CR_ALS catalyst and at its lowest on the CR@MCM-41 catalyst, the band observed at 966 cm-1 demonstrated the OCr(VI) structure within the chromates17. This structure was also observed in the CR_AS and CR_BS catalysts.

The TPR curves of all synthesized catalysts are given in Figure 9. An examination of TPR curves of synthesized catalysts based on three different mechanisms demonstrates a single reduction peak, obtained as a result of the reduction of Cr+6 to Cr+3 at the range of 400-700oC

20,28

. That result indicating

that the H2 consumption is the highest on the CR_ALS catalyst shows a higher presence of Cr+6 in the catalyst structure than the others. The reduction temperature of Cr+6 to Cr+3 increased from 434oC to 500oC for the CR_ALS and CR_AS catalysts, respectively. It was thought that the reduction of catalyst including higher amount of monochromates and dichromates was easier because of weak bound of oxygen in the structure. Two reduction peaks were detected on the CR@MCM-41 catalyst synthesized using the impregnation technique. The peaks at 300oC and 460oC were seen a result of the reduction of the Cr+6 at surface and in the support, respectively.


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Catalytic testing

The variation of conversion of isobutane to isobutene as a function of time is shown in Figure 10 for all of the synthesized catalysts (at 600oC). The lowest conversion values were obtained on the Cr impregnated CR@MCM-41 catalyst. Using the CR_ALS catalyst, the maximum conversion value (~27%) was reached at a reaction time of about 15 min. Conversion was quite stable within a reaction between 15-75 min., however, a sharp decline of conversion was observed at 75 min. To explain the maximum value and the decrease in activity, further reaction and characterization studies were carried out. The obtained results are given in the following with comments.

The CR_ALS1.5 catalyst containing 1.5 wt. % Cr was also synthesized hydrothermally by the addition of metal salt solution simultaneously with the silica source. A comparison of the conversion values of the catalysts containing 3 wt. % and 1.5 wt. % Cr is given in Figure 11. It was seen that the conversion values detected using the CR_ALS1.5 catalyst were lower than those detected using the CR_ALS catalyst.

In the catalytic testing performed with all synthesized catalysts, no products other than isobutene and hydrogen were encountered at the reactor outlet. This shows that no unwanted side reactions occurred on the synthesized catalysts. TGA analyses of the used catalysts were performed to check to possibility of coke formation which might explain the reduction in the activity of the catalyst after 75 min. At the end of a two-hour reaction period using the CR_ALS catalyst, TGA analysis indicated less than 0.1 % loss in the weight. Also no pronounced change was observed in the peak intensity of the C1s curve (from XPS analysis) of the fresh and the used catalysts. These results indicated that there was no serious carbon deposition on the catalyst.


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The detection of no serious coke deposition on the catalyst structure using the TGA and XPS analyses suggested that valence and structural change play important roles in catalyst deactivation. The Cr2p curves belonging to the XPS analysis conducted on CR_ALS catalyst after reaction are given in Figure 12. An examination of Figure 12 shows that the Cr+6 (2p3/2) peak in the chromates observed at 578 eV disappeared after the reaction, and the Cr+3 (2p3/2) peak in the CrOx or Cr2O3 form observed in 576 eV was formed. Three types of Cr+3 are expected on the chromia catalyst. These are redox Cr+3 (obtained from the reduction of Cr+6), non-redox Cr+3 in amorphous chromia phase and Cr+3 in crystalline chromia.23 CR_ALS catalyst showing the highest activity had the maximum amount of Cr+6. Decrease in the amount of Cr

+6

showed that these

ions were consumed during the reaction. This study indicated that coordinatively unsaturated Cr+3 ions formed by reduction of Cr+6 ions were active for isobutane dehydrogenation.

The DR-UV-vis diagrams of the samples obtained after 10 min of reaction and also at the end of two hours of the reaction performed using the CR_ALS catalyst are given in Figure 13, together with those of the fresh catalyst. An examination of Figure 13 suggests that at minute 10, the monochromates (380 nm) increased in the catalyst structure compared to the initial state, with a decrease in the dichromates (320nm). It was concluded that the dichromates converted to the monochromates using the water in the structure. In catalytic testing, a maximum conversion value was detected at minute 15 of the reaction study performed using the CR_ALS catalyst. This result showed that the monochromates were the most active phase among all types of chromates for isobutane dehydrogenation. An examination of DR-UV-vis diagram at the end of two hours showed that the monochromates in the structure were mostly consumed. The first step to achieve dehydrogenation reaction is bonding of alkyl group to surface chromium and a hydrogen atom bonding to surface oxygen. It was thought that the first step occured on monochromates was easier and faster than other chromates in the reaction medium.


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The dehydrogenation activity of supported chromium catalysts is due to the types of coordinatively unsaturated Cr+3. The unsaturated types of Cr+3 are formed by reduction of Cr+6. The tetrahedrally coordinated Cr(VI)O4 formed Cr(II)O2 and O2 establishing weak bonds with support after losing its two terminally-bounded oxygen atoms. When there is water in the medium, Cr(II)O2 converts into active Cr(III)O3 and H2. After the reaction, inactive octahedrally coordinated Cr(III)O6 groups form and then as a result of the accumulation of these groups, Cr2O3 crystal phase occurs. The tetrahedrally coordinated structure contains the Cr(3d) and O(2p) orbital mixture, allowing for the 1s-3d electron transfer. And the octahedrally coordinated Cr(III)O6 is symmetrically limited and the Cr in this structure is known to have no electron transfer capabilities.29 The catalyst deactivates as there are more Cr(III)O6 forms in the catalyst structure.

BET surface area values of the synthesized catalysts before and after reaction are given in Table 3. From the table, it was found that serious reductions occurred in the BET surface area values after reactions, especially for CR_AS. In the case of CR_BS and CR_ALS reduction of the surface area was less. Since it was demonstrated that coke formation was insignificant throughout the reaction, the reduction in surface area cannot be explained as coke clogging of the pores. As mentioned above, hydrogen and oxygen are released during the formation of the charge changes of chromium in different types of chromates. It is considered that the released hydrogen and oxygen could result in the formation of water, and the water formed in this manner could damage the MCM-41 support walls that are not very moisture-resistant. It is also considered that the resulting change in the coordination could contribute to a decrease in the surface area. Results indicated that a catalyst support with a higher hydrothermal stability should be preferred for this reaction.


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CONCLUSION

Pure MCM-41 synthesis over surfactants with different carbon chain length and Cr-based MCM-41 catalyst synthesis were performed in the study. Catalytic tests were carried out over all the synthesized catalysts at 600oC and at atmospheric pressure. In the tests, undesired products (other than isobutene and hydrogen) were not detected. At the end of the reaction performed on the CR_ALS catalyst, coke deposition was found as negligible. The catalyst deactivation occured when the inactive octahedrally coordinated Cr(III)O6 groups and the Cr2O3 crystal phase formed. Octahedrally coordinated Cr(III)O6 was symmetrically limited and the Cr in this structure had no electron transfer capabilities. Maximum conversion values were detected when the amount of monochromates reached at to the highest values. This result showed that the monochromates were the most active phase among all types of chromates for isobutane dehydrogenation. It was observed that serious reductions occurred in the surface area values after reactions. Hydrogen and oxygen releasing during the charge changes of chromium to form active Cr species could result in the formation of water, and the water formed in this manner could damage the MCM-41 support walls. It was also considered that change of coordination was cause of decrease of surface area. It was suggested that a catalyst support with high hydrothermal stability should be selected for this reaction. ACKNOWLEDGEMENTS This work was supported by the Scientific and Technological Research Council of Turkey (Project No: 109M403). The Scientific and Technological Research Council of Turkey provided a Ph.D. Student Grant for Saliha Kilicarslan. REFERENCES (1) Yu, J.; Shi, J.L.; Wang, L.Z.; Ruan, M.L.; Yan, D.S. Preparation of High Thermal Stability MCM-41 in The Low Surfactant /Silicon Molar Ratio Synthesis Systems. Material Letters; 2001, 48,112.


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(25) Mahendinan, C.; Sangeetha, P.; Vijayan, P.; Sardhar Basha, S.J.; Shanthi, K. Vapour Phase Oxidation of Tatralin Over Cr And Fe Substituted MCM-41 Molecular Sieves. Journal of Molecular Catalysis A: Chemical; 2007, 275, 84. (26) Marques, F.C.; Canela, M.C.; Stumbo, A.M. Use of TiO2/Cr-MCM-41 Molecular Sieve Đrradiated with Visible Light For The Degradation of Thiophene in The Gas Phase. Catalysis Today; 2008, 133–135, 594. (27) Samanta, S.; Mal, N.K.; Bhaumik, A. Mesoporous Cr-MCM-41 : An Efficient Catalyst For Selective Oxidation of Cycloalkanes. Journal of Molecular Catalysis A:Chemical; 2005, 236, 7. (28) Reddy, E.P.; Davydov, L.; Smirniotis, P.G. Synthesis And Characterization of TiO2 Loaded Cr-MCM-41 Catalysts. Studies in Surface Science and Catalysis; 2002, 141, 487. (29) Elzinga, E.J.; Cirmo, A. Application of Sequential Extractions And X-Ray Absorption Spectroscopy To Determine The Speciation of Chromium in Northern New Jersey Marsh Soils Developed in Chromite Ore Processing Residue. J Hazard Mater.; 2010, 183, 145.


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Table 1. Physical properties of synthesized MCM-41 samples average pore diameter (oA) 25

pore volume (cc/g)

C 17 H 38 BrN

BET surface area (m2/g) 1317

1.2

wall thickness (oA) 14.4

C 19 H 42 BrN C 19 H 42 BrN+C 17 H 38 BrN

1250 531

28 23

1.2 0.4

12.9 10.8

surfactant

Table 2. Atomic ratios of synthesized catalysts determined by EDS and XPS analyses Cr/Si (atomic) Catalyst

XPS

EDS

CR@MCM-41

-

0.049

CR_ALS

0.080

0.067

CR_AS

0.050

0.079

CR_BS

0.020

0.064

*In synthesis solution Cr/Si (molar)=0.036

Table 3. BET surface area values of the synthesized catalysts before and after reaction Catalyst

before reaction

after reaction

2

(m /g)

(m2/g)

CR@MCM-41

400

192

CR_ALS

990

675

CR_AS

567

98

CR_BS

900

743


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FIGURE CAPTIONS

Figure 1. XRD diffraction patterns of synthesized MCM-41 samples Figure 2. N 2 adsorption/desorption isotherms of MCM-41 samples Figure 3. XRD diffraction patterns of synthesized catalysts Figure 4. Cr mapping on CR_ALS catalyst Figure 5. N 2 adsorption/desorption isotherms of the synthesized catalyst Figure 6. XPS curves of the synthesized catalysts Figure 7. DR-UV-vis curves of synthesized catalysts Figure 8. FTIR spectra of synthesized catalysts Figure 9. TPR curves of all synthesized catalysts Figure 10. Conversion of isobutane to isobutene versus time (T=600oC, P=atmospheric, WHSV=26 h-1) Figure 11. Conversion values of catalysts containing 3 wt.% and 1.5 wt.% Cr (T=600oC, P=atmospheric,WHSV=26h-1) Figure 12. XPS analysis conducted on the CR_ALS catalyst after reaction Figure 13. DR-UV-vis diagrams of samples at minute 10 and at the end of two hours of the reaction with CR_ALS catalyst



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C 17 H 38 BrN

C 19 H 42 BrN

C 19 H 42 BrN+ C 17 H 38 BrN 0

1

2

3

4

5

6

7

8

9

2θ

Figure 1. XRD diffraction patterns of synthesized MCM-41 samples


10

Page 23 of 34

C 17 H 38 BrN

adsorpsion desorpsion

C 19 H 42 Br N

V (cc/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C 19 H 42 BrN+ C 17 H 38 BrN

0

0.2

0.4

0.6

0.8

P/Po

Figure 2. N 2 adsorption/desorption isotherms of MCM-41 samples


1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

CR@MCM-41 CR_ALS

CR_AS

CR_BS

0

10

20

30

40

50

60

2θ

Figure 3. XRD diffraction patterns of synthesized catalysts


70

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Figure 4. Cr mapping on CR_ALS catalyst



adsorpsion

CR@MCM-41

desorpsion

CR_ALS V (cc/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

CR_AS

CR_BS

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

P/Po

Figure 5. N 2 adsorption/desorption isotherms of the synthesized catalyst


1

Page 27 of 34

Si2p

O1s

Cr2p CR_BS

intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CR_AS

CR_ALS

560

570

580

590

600

B.E. (eV)

Figure 6. XPS curves of the synthesized catalysts


610


CR@MCM-41 CR_ALS

absorption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 34

CR_AS

CR_BS

200

300

400

500

wavelength (nm)

Figure 7. DR-UV-vis curves of synthesized catalysts


600

Page 29 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


CR@MCM-41

CR_ALS

CR_AS

CR_BS

Figure 8. FTIR spectra of synthesized catalysts ACS Paragon Plus Environment


13

434

12

H2 consumption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

11

CR_ALS

10

9

500 440 460

8

7 200

CR_AS 300

CR_BS Cr@MCM-41

300

400

500

600

T(o C)

Figure 9. TPR curves of all synthesized catalysts


700

Page 31 of 34

30 isobutane conversion(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CR@MCM-41

25

CR_AS

20

CR_ALS

15

CR_BS

10 5 0 0

20

40

60

80

100

time (min)

Figure 10. Conversion of isobutane to isobutene versus time (T=600oC, P=atmospheric, WHSV=26 h-1)


120


30

isobutane conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 34

CR_ALS

25

CR_ALS1.5

20 15 10 5 0 0

20

40

60

80

100

time(min)

Figure 11. Conversion values of catalysts containing 3 wt.% and 1.5 wt.% Cr (T=600oC, P=atmospheric, WHSV=26h-1)


120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


intensity (a.u.)

Page 33 of 34

CR_ALS_fresh

CR_ALS_after reaction

565

570

575

580

585

590

595

600

605

B.E.(eV)

Figure 12. XPS analysis conducted on the CR_ALS catalyst after reaction



CR_ALS_fresh

CR_ALS_10min

absorption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 34

CR_ALS_2hrs

210

310

410

510

610

wavelength (nm)

Figure 13. DR-UV-vis diagrams of samples at minute 10 and at the end of two hours of the reaction with CR_ALS catalyst


Cr Incorporated MCM-41 Type Catalysts for Isobutane

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