고표면적 MgO에 OV(OtBu)3을 함침하여 제조된 Vanadia-Magnesia 촉매에 의한 프로판의 산화적 탈수소화반응

박찬호, Alexis T. Bell* and T. Don Tilley**

삼성종합기술원 MD Lab., *,** 로렌스 버클리 국립연구소, 버클리 캘리포니아 주립대학, *화학공학과, **화학과

Oxidative Dehydrogenation of Propane over Vanadia-Magnesia Catalyst Prepared by Impregnation of OV(OtBu)3 onto High-Surface Area MgO

Chanho Pak, Alexis T. Bell* and T. Don Tilley**

MD Lab., Samsung Advanced Institute of Technology, and *,**Chemical Sciences Division, Lawrence Berkeley National Laboratory and *Department of Chemical Engineering and **Chemistry, University of California at Berkeley

Introduction

A number of catalytic systems have been studied recently as active and selective catalyst for the oxidative dehydrogenation (ODH) of propane [1,2]. These studies have showed that the most catalysts contain vanadia as a catalytic component. The activity and selectivity of these catalysts depend on the local composition and structure of the vanadium-species at the surface, which can be changed by the various preparation methods [1-4].

In the present study, high surface area, V-Mg-O catalysts were obtained by a novel synthetic route. Nanocrystalline magnesia was prepared in a manner previously described by Klabunde [5] and then contacted with a molecular precursor, OV(OtBu)3, dissolved in a non-polar organic solvent. The active form of the catalyst is formed by thermolysis of the precursor. In related work, we have shown that a similar approach leads to well-defined vanadia-zirconia catalysts that exhibit enhanced selectivities and activities for the ODH of propane[3]. These improved properties are associated with higher surface areas and better dispersion of the catalytically active material, relative to more traditional routes based on aqueous impregnation methods.

And then, we have examined the effects of vanadia loading on the activity and selectivity of V-Mg-O catalysts for the ODH of propane to propene. The structure and properties of the catalysts were characterized by various structural analysis methods. Catalyst activity and selectivity are interpreted in terms of the apparent first-order rate coefficients for the ODH of propane (k1), the combustion of propane(k2), and the combustion of propene(k3).

Experimental

High surface area Mg(OH)2 was prepared using a modification of the method reported in ref. [5]. This material was converted into MgO by heating in flowing He while the temperature was raised to 493 K at 1 K/min and maintained at 493 K for 5 h, after which it was raised to 773 K at 1 K/min and held at this temperature for 5 h. V-Mg-O catalyst was prepared by impregnation of MgO with a toluene solution of different amount of OV(OtBu)3. Calcination of the catalyst was performed by heating to 823 K at 1K/min and holding for 6 h in a flow of air. The samples were labeled n V-Mg-O, where n is the nominal weight percentage of V2O5 in the sample.

The experimental details of X-ray diffraction, nitrogen adsorption, temperature programmed reduction, Raman, Diffuse reflectance UV-visible and static solid-state 51V NMR measurements were described elsewhere [3].

Rates of reaction and selectivities for the ODH of propane were measured in a fixed-bed quartz reactor using 15 mg of catalyst diluted with quartz powder (0.50 g) to prevent temperature gradients. A mixture of propane (25 cm3/min), oxygen (9 cm3/min) and nitrogen (2 cm3/min) in He (200 cm3/min) was introduced into the reactor at controlled flow rates. The gaseous reactants and products were analyzed on-line with a Hewlett-Packard 6890 gas chromatograph equipped with both a capillary column (HP-1) and a packed column (HAYESEP-Q). Only C3H6, CO, CO2 and H2O were observed as products. The conversion of propane was measured at total flow rates of 60-235 cm3/min and reaction temperatures of 673, 723 and 773 K.

Results and discussion

As the loading of vanadia increased from 5 wt% to 30 wt%, the BET surface area decreased from 307 to 187 m2/g and the apparent surface density of VOx, assuming all of the vanadium is exposed on the surface, increased from 1.1 to 10.6 VOx/nm2 as listed in Table 1.

The structural analyses indicate that at vanadia loading of 5 wt% (1.1 VOx/nm2) V atoms are present at the surface of MgO as isolated VO4 species exhibiting distorted tetrahedral coordination. These species are characterized by the Raman band at 825 cm-1, the 51V NMR shift of -480 ppm referenced to that of VOCl3 solution, and the UV-Vis  absorption band edge energy of 3.05 eV as shown in Table 1. Temperature programmed reduction of this species in H2 produces a peak at 640 K. For vanadia loadings above 10 wt % (2.2 VOx/nm2), small domains of Mg3(VO4)2 are formed on the MgO support. The presence of this phase is noted by the appearance of Raman bands at 380 and 860 cm-1 and a 51V NMR peak at -560 ppm. With increasing loading, the domains of magnesium vanadate become progressively more difficult to reduce and the peak temperature for their reduction shifts from 813 to 853 K, but remains less than that for bulk Mg3(VO4)2, 903 K.

Sample	Surface area (m2/g)	Apparent surface density of VOx (/nm2)	Absorption edge energy (eV)
5 V-Mg-O	307	1.1	3.05
10 V-Mg-O	299	2.2	2.88
15 V-Mg-O	280	3.5	2.89
20 V-Mg-O	256	5.2	2.90
25 V-Mg-O	227	7.3	2.94
30 V-Mg-O	187	10.6	2.95

Initial propene formation rates normalized per V atom increased up to 3.5 VOx/nm2, and then decreased with further increase in vanadium loading. The decrease in specific activity beyond the maximum has been ascribed to the formation of multilayers of the active form of vanadia, with the consequence that V atoms lying below the surface are inaccessible. The rate of propene formation per unit surface area of the catalysts increases rapidly with increasing V surface density up to about 3.5 VOx/nm2, but then reaches a plateau. Also consistent with the proposed explanation is the recognition that the surface density of VOx for Mg3(VO4)2 is approximately 4.2 VOx/nm2 as described below.

At 673 K, the selectivity to propene increased up to a vanadium surface density of about 7.3 VOx/nm2, after which it remained almost constant at about 80 %. Increasing the reaction temperature to 773K increased the propene selectivity only slightly. The observed trend in selectivity with the apparent surface density of VOx is very similar to that reported previously for vanadia dispersed on MgO [4].

Propane can undergo either ODH to form propene or combustion to form COx combustion whereas propene can undergo combustion to COx. If the conversion of oxygen is low, then the rates of these three processes can be described by pseudo-first-order rate coefficients, k1, k2, and k3 [6]. The value of k1, expressed per unit of catalyst surface area, is shown in Figure 1 as a function of VOx apparent surface density, for different temperatures. For each temperature, the value of k1 increases rapidly up to about 4.0 VOx/nm2, remains constant, and then decreases slightly. For each temperature, the value of k1 increases rapidly up to about 4.0 VOx/nm2, remains constant, and then decreases slightly. The activation energy for propane ODH is 102 kJ/mol for the 5 V-Mg-O catalyst and decreases to about 80 kJ/mol for the catalysts containing 10-30 wt% V2O5. The value of k2/k1 decreases rapidly with rising vanadia surface density but approaches a plateau above a density of 4 VOx/nm2. A similar pattern is seen for k3/k1. The appearance of a plateau in plots of k2/k1 and k3/k1 can be ascribed to the formation of a monolayer of Mg3(VO4)2. The decrease in the values of k2/k1 and k3/k1 observed at apparent surface densities below 4 VOx/nm2 can be ascribed to changes in the reactivity of oxygen at the surface of the catalyst. The temperature programmed reduction spectra show that for temperatures between 673 and 773 K, the reducibility of the catalyst decreases with increasing vanadia loading. This trend is particularly evident for vanadia loadings between 5 wt% and 15 wt%. This suggests that the oxygen associated with isolated VO42-species is much more readily available for combustion of propane and propene than oxygen associated with domains of Mg3(VO4)2. Consistent with this we note that the peak temperature for the reduction of bulk Mg3(VO4)2 is higher than that for 30 V-Mg-O and correspondingly, so are the values of k2/k1 and k3/k1. We note that a similar trend has been reported for the ODH of butane over various metal vanadate catalysts [1]. Taken in the light of the structural characterization discussed above, the increase in k1 seen in Fig. 1 up to a vanadia surface density of about 4 VOx/nm2 suggests that isolated VO42- units are not as active as those present at the surface of Mg3(VO4)2. Based on the (100) surface of Mg3(VO4)2, the surface density of VO42- units is estimated to be 4.2 VOx/nm2. Thus, the data reported here support the view that the most active species for propane ODH are VO42- species existing in a surface structure that is essentially identical to that present in Mg3(VO4)2. This conclusion is identical to that reached by Pantazidis and Miradotos [4] for V-Mg-O catalysts prepared by conventional means. To assess how the area-based activity of bulk Mg3(VO4)2 compares with that of the most active catalysts reported here, we measured the zero-conversion activity of the bulk material for propene formation at 773 K and the same feed composition used throughout our work.  The result is that bulk Mg3(VO4)2 (11 m2/g) has an activity of 3.9 nmol/m2□s, whereas 15 and 25 V-Mg-O catalysts have activities of 136 and 163 nmol/m2□s, respectively. Thus, it appears likely that as the proportion of bulk Mg3(VO4)2 increases, the area-based activity of the catalyst will decrease.  It was suggested, therefore, that the most active V-MgO catalysts are those that contain very thin domains of a Mg3(VO4)2 on the surface of MgO rather than crystallites of bulk Mg3(VO4)2.

Conclusion

A series of V-Mg-O catalysts have been prepared by impregnation of OV(OtBu)3 onto high surface area MgO. After calcination at 823 K, these catalysts have surface areas ranging from 187 m2/g for 30 V-Mg-O to 307 m2/g for 5 V-Mg-O. Structural characterization of the dispersed vanadium shows evidence for isolated VO42- anions for V2O5 loadings below 5 wt%. With increasing V2O5 loading, small domains of Mg3(VO4)2 are observed, but no evidence for either polyvanadate species or V2O5 domains.

The area-based activity of the catalysts prepared in this study were significantly higher than those of V-Mg-O catalysts prepared by conventional methods involving aqueous impregnation of MgO, whereas the propene selectivities were comparable or slightly better than those previously reported.

Consideration of the structural characterization data indicates that the most active form of the catalyst is that in which the surface of the MgO support is covered by small domains of Mg3(VO4)2.  Formation of bulk crystallites of Mg3(VO4)2 are undesirable, since it is found that the area-based activity of the pure phase is approximately 40 times lower than that of the domains of Mg3(VO4)2 formed on the surface of MgO. The values of k2/k1 and k3/k1 decreases with increasing V2O5 surface density but reach a nearly constant value for surface densities above 4 VOx/nm2. These trends show that the activities V-Mg-O catalysts for propane and propene combustion relative to propane ODH declines with increasing surface loading of V2O5.

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