Dehydrocyclization of n-Hexane over Heteropolyoxometalates Catalysts

The catalytic dehydrocyclization of n-hexane was studied here for the first time using a number of compounds based on H3PMo12O40. The described catalysts were prepared by either replacing the acidic proton with counter-ions such as ammonium or transition metal cations ( , Fe, K), or by replacing Mo with (Ni, Co, Mn) in the polyoxometalate framework, as reported earlier. For comparison purposes, the known (TBA)7PW11O39 catalyst system was used. All reactions were conducted at different temperatures in the range 200 ̊C 450 ̊C. The Keggin structure of these heteropolycompounds was ascertained by XRD, UV and IR measurements. P NMR measurements and thermal behaviour of the prepared catalysts were also studied. These modified polyoxometalates exhibited heterogeneous superacidic catalytic activities in dehydrocyclization of n-hexane into benzene, cyclohexane, cyclohexene and cyclohexadiene. The catalysts obtained by substituting the acidic proton or coordination atom exhibited higher selectivity and stability than the parent compound H3PMo12O40. Catalytic activity and selectivity were heavily dependent on the composition of the catalyst and on the reaction conditions. At higher temperatures, the catalyst exhibited higher conversion efficiency at the expense of selectivity. Using higher temperatures (>400 ̊C) in the presence of hydrogen carrier gas, selectivity towards dehydrocyclization ceased and methane dominated. To explain the results, a plausible mechanism is presented, based on super-acidic nature of the catalyst systems. 4 NH 


Introduction
Catalytic reforming of n-hexane is one of the most widely used processes in oil refining industry.Fuel octane number can be enhanced by increasing concentrations of aromatic compounds [1][2][3].
Catalytic reforming proceeds over bifunctional catalysts having both metallic and acidic functions are currently being considered [4,5].Heteropoly acids (HPAs) are complex proton acids that incorporate polyoxometalate anions (heteropoly anions) having metal-oxygen octahedra as the basic structural units.HPAs have several advantages such as very strong Brönsted acidities approaching the super acid region [6] and high oxidant activities in multielectron redox reactions under mild conditions.Their acid-base and redox properties can be varied widely by changing their chemical compositions.
HPAs have a discrete ionic structure, containing the fairly mobile heteropoly anions and counter-cations (H + , H 3 O + , 5 2 , etc.).This unique structure results in significantly high proton mobility and pseudo liquid H O  phase behaviour.In addition, HPAs are highly soluble in polar solvents and their thermal stabilities are quite high.These properties make HPAs potentially promising acid, redox, and bifunctional catalysts in both homogeneous and heterogeneous systems [7,8].
Heteropolyacids (HPA) usually act as strong acidic catalysts, with the ability to activate alkanes and produce carbonium ion intermediates [9,10].Such intermediates are necessary for alkane cracking, alkylation and isomerization processes [11].The large-scale industrial applications of these reactions drew attention to such catalyst systems for long time [12].
In this work, the compounds have been used in hexane dehydrocyclization reaction for the first time.The reaction may involve other products such as cyclohexane, cyclohexene and cyclohexadiene, in addition to the unwanted methane product.The main goal is to maximize benzene production and minimize methane in the product mixture, by investigating different catalysts, varying temperature, and carrier gas and retention time.

Preparation of Catalysts
Pure heteropolyacid H 3 PMo 12 O 40 •xH 2 O was prepared as described earlier [25].The mixed potassium-iron salt K 0.5 Fe 0.1 H 0.2 PMo 12 O 40 •xH 2 O was prepared as follows: to an aqueous solution of H 3 PMo 12 O 40 (50 ml, 0.08 M) were added successively 9.0 ml of 0.10 M Fe(NO 3 ) 3 and 7.5 ml of 0.80 M KNO 3 .The obtained precipitate was filtered over glass frit and dried at 50˚C under vacuum for 5 h.
(TBA) 7 PW 11 O 39 was prepared by the method described in the literature [27]

Equipment
The UV analysis of the catalyst samples (dissolved in acetonitrile/water 1:1 ratio, as dilute samples solutions 10-5-10-3 M) were performed on a UV-1601PC SHI-MADZU spectrophotometer.IR spectra of the samples were obtained at room temperature with a BIO-RAD FTS 165 FTIR spectrometer using standard KBr pellet technique.Measurements were taken in the wave-number range of 4000 -400 cm −1 , 256 scans, and resolution cm −1 .
The X-ray powder diffraction (XRD) diagrams were recorded on a SIEMENS D5000 equipment in the 5 -45˚ 2θ range, at a scanning speed of 1˚C per minute, using CuK α radiation.The 31 P NMR measurements were performed on a Bruker CXP10 spectrophotometer at MHz.Chemical shifts were referenced to Al(PO 3 ) 3 .
Thermal behaviour of the prepared catalysts was studied using a TGA-DTA SETARAM apparatus.For each experiment 20 mg of powdered sample were heated at a temperature ramp rate of 5˚C/min under air flow.Differential scanning calorimetry experiments (DSC) were run on a DSC 111 SETARAM.Apparatus under similar conditions, with a nitrogen flow.

Catalytic Experiments
The conversion of n-hexane was carried out in a quartz fixed-bed flow reactor at different temperatures from 200˚C to 400˚C, using 0.5 g of catalyst.Catalysts were heated to the desired reaction temperature at 5˚C/min under nitrogen flow and then maintained at this temperature for one hour under nitrogen.A mixture of n-hexane (50 torr) and hydrogen was flowed over the catalyst (0.1 l/h).The reaction mixture was analysed by gas chromatography on a Hewlett Packard 5830A apparatus equipped with two successive separation columns, filled respectively with ® Porapak QS for a TCD detector and 5Å-molecular sieve for FIDCG detector.
Reaction rates (r) were calculated from the following equation: where F is the flow rate of the vector gas (l/h), T the reaction temperature (K), p the partial pressure of the products (torr) and m the mass of the catalyst (g).The activation energy (E a ) was obtained from the Arrhenius plot according to: where R is the perfect gas constant and k the pre-exponential factor.
A. EID ET AL. 84

Characterization of Catalysts
In the UV, the Keggin heteropolyanions show two absorption bands, one around 200 nm and another around 280 nm.According to literature [28], when catalyst concentration inside the suspension decreases, the two bands disappear gradually indicating decomposition of the polyanion with dilution [29].Therefore, intermediate concentrations of different catalyst systems were used here for characterization, as shown in Figure 1.
Values of absorption bands are summarized in Table 1.The Table shows main band observed for each prepared catalyst.The bands are assigned respectively to the vibrations of terminal Mo = Ot and bridging bonds.The bridging bonds are two types: the inter-bridges (M-Ob-M) that occur between two adjacent octahedra, and the intrabridges (M-Oc-M) that occur within same octahedron.
Solid-state FT-IR spectra measured for different prepared catalysts are given in Figure 2. The spectra display similar patterns characteristic for the Keggin structure [30,31].Values of IF bands are summarized in Table 2.The signal is characteristic for the lacunary species K 7 [α-PW 11 O 39 ] [32,33]. 31P NMR measured and literature data are summarized in Table 3.
XRD patterns were measured for all solid catalysts systems in the powder form.Other diffractograms show that the salt FePMo 12 crystallises in a cubic structure with intense (222) peak [16,37].This corresponds to same diffraction plane for ammonium salts of (PMo  140˚C corresponding to the departure of water of crystallization or hydration.A second mass loss at temperatures between 250˚C and 350˚C, was attributed to water content resulting from combination of H + ions and network oxygen.This mass loss leads to a reversible modification of the polyanion. In differential thermal (DT) analysis, the loss of water corresponds to endothermic peaks.An exothermic peak was observed above 350˚C and was attributed to acid decomposition into P 2 O 5 and MoO 3 oxides.TGA analysis for the salts PMo 11 Ni, PMo 11 Co, PMo 11 Mn, FePMo 12 , PW 11 shows three mass losses.A first mass loss occurred around 100˚C, which is interpreted as the start of physisorbed water loss.A second departure is at around 240˚C to 300˚C, which is due to loss of constitution water molecule.Water molecules coordinated to nickel and cobalt ions are involved here.Finally, a loss around 300˚C to 400˚C, corresponding to departure of molecular ammonia or molecular nitrogen, was observed.Salt catalysts exhibit higher thermal stability than H 3 PMo 12 O 40 (Figure 5).The temperature at which their degradation starts depends on the position of the ion M 3+ .When the counter ion involved metal ionic species, the decomposition temperature exceeded 500˚C.On the other hand, the heteropolyacid H 3 PMo 12 O 40 is less thermally stable than the salt, and its decomposition started at 350˚C.

Effect of Catalyst Type
Steady-state activity for catalytic n-hexane dehydrocyclization was reached within about 4 h.The observed products were benzene, (one most preferred product), cyclohexane, cyclohexene, cycloheadiene and methane (least preferred product).As expected, the catalytic activity and selectivity were dependent on the nature of the catalyst.
Figure 6 shows the conversion of n-hexane using different catalysts.The conversion of the solid catalyst was sensitive to the nature of the coordination ion (Mo 6+ , W 6+ , Ni 3+ , Co 3+ , Mn 3+ ) and counter ion H 3 O + , 4 NH  , Fe 3+ ).The replacement of mobile M (Ni 3+ , Co 3+ , Mn 3+ ) ions in the HPA framework increased the activity of the catalyst.Replacing H + ion with Fe 3+ lowered the activity.PW 11 exhibited relatively high activity as well.
Detailed studies on n-hexane isomerization catalysis with HPC are summarized in (Tables 5 and 6).The results show that benzene was the major product at low conversion level while using different catalysts.Cyclohexane, cyclohexene, cyclohexadiene, benzene and methane were also observed.As the conversion level increased benzene product ratio was lowered in ratio of methane product.
The 30% activity for H 3 PMo 12 O 40 (or even 100% for other catalysts excluding FePMo) shows that the protons in anhydrous HPA are accessible to the reactant molecules.The high catalytic activity of HPA may thus be related to high acid strength and high mobility of protons, as reported earlier [38].The resulting protons are responsible of the aromatization of n-hexane.Cyclohexane, cyclohexene cyclohexadiene were also obtained, with variable ratios depending on type of catalyst.An important selectivity obtained by PMoNi catalyst involved cyclohexane 14%, cyclohexene 2%, cyclohexadiene 40%.A lower selectivity with PMoCo and PMoMn catalysts was observed.The results indicate that both relatively strong acidity and metallic propriety are responsible for    catalyst efficiency at lower temperature (lower than 350˚C).The PW 11 catalyst showed specially high efficiency, selectivity and stability towards benzene formation.For example, PW 11 showed high activity (69%) conversion and high selectivity (100%) at 350˚C compared to PMo 12 (30% conversion and 17% selectivity), PMoNi (100% and 11.5%), and PMoCo and PMoMn (100% and 0%).Such behaviour should be ascribed to the Lewis acid properties of the TBA cations, Also, we may conclude that Lewis acid species should be important components of the active site in the isomerization of n-hexane reaction over HPAs, in agreement with other studies [39,40].

Effect of Temperature
The effect of temperature on efficiency and benzene selectivity has been studied for different catalysts.Figure 7 shows how conversion varies with reaction temperature.The temperature curves exhibit linear character.For dif- ferent catalysts, overall hexane conversion increased with temperature.The transformation of n-hexane on the tested catalysts started only above 250˚C and the conversion reached 100% at 350˚C -400˚C.
Until 300˚C, benzene was the unique or major product.The selectivity to benzene decreased with increasing temperature (Tables 4 and 5).The conversion increased from 2 up to 30%, whereas benzene selectivity decreased from 100% to 80% when the reaction temperature was increased from 300˚C to 350˚C when using HPMo 12 O 40 heteropolyacid catalyst.
Methane was obtained with all catalysts at temperature above 300˚C.Methane production selectivity increased with temperature at the expense of benzene.At 400˚C n-hexane was converted completely to methane with all catalysts.The high methane selectivity was with PMoMn catalysts.
At 400˚C the H 3 PMo 12 heteropolyacid was decomposed into Mo 3 and P 2 O 5 .This result agrees with literature [41] which showed that the isomerization of n-hexane was due to surface Bronsted acid sites.

Activation Energy
n-hexane conversion rate were determined over the temperature range 200˚C -450˚C.From (Figures 8(a)-(f) n-hexane dehydrocyclization rate constants were determined over the temperature range 200˚C -450˚C.From Arrhenius plots (Figure 9), Activation energies for the transformation of n-hexane over acidic catalysts were calculated as shown in Table 7.
Values of activation energies for the overall process of dehydrocyclization and aromatization of n-hexane using the acidic catalyst systems (43 to 95.2 kJ•mol −1 ) are much lower than those known for metal type catalysts (typically 230 to 293 kJ•mol −1 ) [42].Acidic catalytic activity is expected to occur by the highly acidic heteropoly compounds.Actually the activation energies are comparable to those found for the transformation of the n-hexane in benzene using acidic catalysts.[43].
Furthermore, the selectivity of the benzene decreases, while the conversion increases, with higher temperatures or longer retention times.In case of all catalysts, the benzene is the secondary product of the reaction.

Effect of Time of the Reaction
The conversion of n-hexane was increase with time for all of the catalysts at 350˚C as it is clear from Figure 10.
The selectivity of benzene only slightly decreased with time on all catalysts Figure 8, when keeping temperature constant at 350˚C.This due to high thermodynamic stability of benzene.

Effect of Type of Carrier Gas
The type of carrier gas affected both conversion and selectivity of the reaction.The overall conversion increased by using H 2 as a carrier gas, while benzene selectivity increased when using N 2 as a carrier gas.This applied to all catalytic systems, as shown in Tables 5 and 6.Comparison between the two Tables shows that for each catalyst system, using H 2 gas gives higher conversion than using N 2 .On the other hand, the Tables show using N 2 gave higher benzene selectivity than using H 2 , for each catalyst.

Mechanism
A plausible mechanism has been suggested to explain the observations discussed above.As shown in Scheme I, two possible products can be expected by the HPA catalyst systems.The Scheme shows that with excess H 2 (carrier gas, as discussed above) the carbonium ion formed by super-acid activation of hexane will be cracked into CH 4 [44].Such a process involves complete saturation of all carbon atoms with hydrogen, via complete cracking of the hydrocarbon chain.On the other hand, in case of nitrogen carrier gas (with no hydrogen) dehydrocylization process is favored to dominate, with no significant carbon chain cracking, as shown in the Scheme.The process is multi-stage involving β-H elimination in each step.This explains the production of cyclohexane, cyclohexene nd cyclohexadiene as ac-a     companying products to benzene.It should also be noted that this logic does not fully work under higher temperatures, where CH 4 production dominates even in the absence of hydrogen gas.In such a case, high temperature experiments without hydrogen yielded high molecular weight aromatics and tars, in parallel to CH 4 formation, which caused some technical difficulties such as reactor blockage during experiments.Therefore, If benzene is the desired product from nhexane, hydrogen gas should not be used, and the reaction must be conducted under mild temperatures (350˚C or lower).Due to their super-acidic nature, the compounds showed catalytic efficiency in the dehydrocyclization of n-hexane.The product distribution (benzene, cyclohexane, cyclohexene, cyclohexadiene and methane) strongly depends on the nature of the catalyst, the type of carrier gas and the reaction temperature.The observed differences in the behaviour of the studied catalysts should be firstly ascribed to the superacidic nature of the catalysts, which can activate saturated alkanes.
FT-IR spectra show that main peaks around 1064, 961, 861 and 786 cm −1 are attributed to P-O a , Mo = O t , Mo-O b -Mo, and Mo-O c -Mo bonds, respectively. 31P NMR spectra were measured for only two catalyst types, as shown in Figures 3(a) and (b). Figure 3(a) shows a single peak at −3.47 ppm for salt Fe 0.1 PMo 12 O 40 , which indicates purity of the sample.The KPW 11 heteropolyanion showed a signal at 10.35 ppm, Figure 3(b).

Figure 7 .
Figure 7. Conversion of n-hexane as function of temperature, (using N 2 as carrier gas).

Figure 9 .
Figure 9. Arrhenuis plots for different catalyst systems (using N 2 as carrier gas).

Figure 10 .
Figure 10.Selectivity of benzene as function of time at 350˚C, (using N 2 as carrier gas).

Table 2 . FT-IR absorption band values for different catalyst systems, together with their assignments.
t Mo-O b -Mo Mo-O c -Mo