Genesis and structure of MoO3/ZrO2 solid acid catalysts of isobutene alkylation
Alkylation of butenes with alkanes results in high octane alkylates with a high octane number (above 94) used for both synthetic motor fuel production and for compounding of low octane number gasoline [1-3]. A promising direction in the development of alkylation processes is the transition from liquid-phase catalysts such as HF and H2SO4 to heterogeneous catalysis thus avoiding problems related to chemical corrosion of equipment, toxicity of waste acids and their disposal.
In the petrochemical synthesis widespread catalysts MoO3 and WO3, which are used in different processes: isomerization, cracking, hydrotreating. Alkylation is a complex process, which also includes the isomerization reaction. Therefore, it is logical to study ZrO2-MoO3 / WO3 catalysts in the alkylation of isobutane with olefins.
For example, [4.5] shows the influence ratio and calcination temperature for ZrO2-WO3 catalyst at the value of the specific surface, ratio of ZrO2 modifications, etc. In experiments on the alkylation of isobutane with olefins , such systems showed better activity and selectivity as compared to the sulfated zircinia (ZrO2/SO4) catalysts. MoO3/ZrO2 have not been studied in this process.
It should be noted that in the preparation of MoO3/ZrO2 catalysts are generally used methods of impregnation and coprecipitation. By mixing the initial components is given little attention.
In this work shows the influence the preparation method (impregnation or mixing), ratio of the initial components, type of MoO3-precursors and calcination temperature on the physicochemical properties of the MoO3/ZrO2 catalysts.
Zirconium hydroxide (Zr(OH)4) was prepared by precipitation from a 10% solution of zirconyl chloride solution at pH=9 by addition of 25% ammonium hydroxide solution. Then the precipitate was kept 1 hr under a mother liquor, then was filtered off on a vacuum filter. The resulting hydroxide is washed off by decantation from chlorine ions at pH=8 at the rate of 30 liters of water per 100g ZrOCl2. The resulting solids were dried in air overnight at 373-383 K.
As MoO3-precursors used: H2MoO4, H3PMo12O40, H4SiMo12O40 and (NH4)6Mo7O24.
Samples of the catalysts by mixing and impregnation were prepared. In the first series - mixing samples - changed Mo-precursor, content of MoO3 (6,6 and 13,2 % wt.) and calcination temperature (500, 600 and 700°C). The catalysts are represented by Zr-XMo(Y)-T, where X represents weight percentage of MoO3, Y represents the type of MoO3-precursor (H - H2MoO4, P - H3PMo12O40, Si - H4SiMo12O40), and T denotes calcination temperature (ºC). In the second series of samples were compared preparation methods, treatment temperature and the MoO3-predecessor. The catalysts are represented by Zr-600Mo(Y)-T, where 600 represents wt. %MoO3 at which MoO3 covers zirconia with a monolayer, Y represents the type of MoO3-precursor (NH4 - (NH4)6Mo7O24, P - H3PMo12O40), and T denotes calcination temperature (ºC).
The speciﬁc surface areas (SSA) of the catalysts were measured by N2 physisorption at liquid nitrogen temperature with a Quantachrome Autosorb 6 iSA and standard multipoint BET analysis methods.
X-ray diffraction (XRD) measurements of the catalyst powder were recorded with a Shimadzu XRD-7000 diffractometer equipped with Ni-ﬁltered CuKα radiation (λ = 1.5418 Å). The volume percentage of the monoclinic phase (Vm) of the calcined samples was estimated with the formula proposed by Toraya et al. .
The nature of the acid sites (Brønsted and Lewis) of the catalyst samples was characterized by in situ FTIR spectroscopy (Shimadzu IrTracer-100) with chemisorbed pyridine. Also functional composition of the samples surface was investigated by adsorption of acid–base indicators with different pKa values ranging from -4.4 to 14.2, which were selectively adsorbed on the surface active sites with the corresponding pKa values a according to the method described in .
Differential Thermal and Thermo Gravimetric Analyis (DTA-TGA) was carried out on a Shimadzu DTH-60H using 0.02 g of sample, and heating in air from 25 to 800°C.
Series 1: Mixture samples
Figure 1 shows an example of XRD for Zr-XMo(Si)-T patterns with different MoO3/ZrO2 ratio. At 500°C and 6.6% wt. MoO3 the phase composition of t-, m-ZrO2 and o-MoO3 is presented. Presumably, with increasing the calcination temperature there is a destruction of crystalline structure of MoO3 and its further spreading of surface ZrO2. With increasing wt.% MoO3 to 13,2% and T=700°C there is formation of the new phase - Zr(MoO4)2 in the hexagonal modification, and also lack of the MoO3 phase. Similar regularity is observed also when using the molybdic acid, however, but at 500°C phase of o-MoO3 is formed, and at 600°C - m-MoO3. When using 12-molybdophosphoric acid formation of Zr(MoO4)2 phase was not observed.
From XRD it is visible that calcination temperature increase results in an increase of the integral intensities m-ZrO2. From Figure 2 shows the dependence Vm on wt.% MoO3 in comparison with the pure ZrO2 with different temperature calcination.
Fig.2. The dependence of the volume fraction (Vm) of m-ZrO2 on the calcination temperature, the content MoO3 and MoO3-precursor 1: ZrO2; 2: Zr-6.6Mo(H); 3: Zr-6.6Mo(Si); 4: Zr-6.6Mo(P); 5: Zr-13.2Mo(H); 6: Zr-13.2Mo(P); 7: Zr-13.2Mo(Si)
Minimum volume fraction falls to 600°C, which is associated with the destruction of the structure of MoO3 phase, its distribution over the surface of the ZrO2 and therefore t-ZrO2 crystal growth delay. Increasing wt.% MoO3 contributes to a greater slowdown in the growth of t-ZrO2 crystals and their further transition to m-ZrO2.
Figure 3 shows the results DTA for mixing samples Zr-13,2Mo(X). It can be seen that the exothermic peak (429°C), which corresponds to the crystallization of the amorphous ZrO2 is not displaced. Consequently when mixing any of the solid acids does not prevent crystallization of the amorphous ZrO2, which is explained by incomplete conversion of the starting Mo-compounds at 429°C.
Series 2: Сomparison mixing and impregnation
In this sample series of content MoO3=4,23% wt., which is calculated according to .
XRD shows that the phase composition of impregnation samples is presented by t- and m-ZrO2, but also mixing additional m-MoO3 phase using ammonium heptamolybdate. Figure 4 shows the results of calculations Vm, that indicate better stabilization of t-ZrO2 when impregnation using ammonium heptamolybdate and somewhat poorer stabilization of t-ZrO2 when impregnation using 12-molybdophosphoric acid.
Fig.4. he dependence of the volume fraction (Vm) of m-ZrO2 on the calcination temperature, preparation method and MoO3-precursor 1: ZrO2; 2: Zr-600Mo(P)mix; 3: Zr-600Mo(NH4)mix; 4: Zr-600Mo(P)imp; 5: Zr-600Mo(NH4)imp
DTA shows that by using the impregnation as the preparation method, there is a delay of amorphous oxide crystallization , i.e. crystallization temperature rises to 462°C (12-molybdophosphoric acid) and 470°C (ammonium heptamolybdate). Such influence preparation method has a positive effect on the value of the SSA (Fig.5).
Result by adsorption-desorption of pyridine (Fig. 6) to mixture sample Zr-13,2Mo(P)-600 shown in Fig. 6 (shows FTIR of the residual adsorbed pyridine at different desorption temperatures Tdes). From the results it follows that the sample surface is represented by different types of acid centers. Among the Lewis acid centers (LAC) is dominated by weak centers. Among the Brønsted acid centers (BAC) is dominated by strong centers. Presumably, LAC corresponds Zr4+ and BAC - Mo-OH and Zr-OH.
Result by adsorption of acid–base indicators with different pKa to impregnate sample Zr-600Mo(P)imp-600 shown in Fig. 7. Its surface is mainly represented by the LAC, but also contains minor amounts of other centers of acidic (BAC, strong and weak) and basic (BBC). A similar result for acid sites shows the adsorption-adsorption of pyridine.
The results obtained indicate a promising catalyst in the alkylation of isobutane with butenes. In the future we plan to explore the best samples in the alkylation of isobutane.
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