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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Effect of Aluminum Addition and Surface Moisture Content on the Catalytic Activity of Sulfated Zirconia in n‑Butane Isomerization Pengzhao Wang,*,† Siqi Wang,† Chaohe Yang,‡ Chunyi Li,‡ and Xiaojun Bao† †
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National Engineering Research Centre of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou 350002, People’s Republic of China ‡ State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266555, People’s Republic of China S Supporting Information *
ABSTRACT: The effects of aluminum addition and surface moisture on the surface-structure properties and catalytic activity of sulfated zirconia were investigated in n-butane isomerization. Adding 4 mol % Al into zirconia support with coprecipitation method greatly improves the catalytic activity. This is attributed to the monolayer coverage of sulfate species and the maximum of Brønsted acid sites, resulting from the strongly stabilized sulfate species on the tetragonal zirconia (t-ZrO2) surface. The in situ DRIFTS investigations and density functional theory (DFT) calculations show that the dehydration temperature affects both the density and the strength of Brønsted acid sites, and there is a balance between them. The optimal catalytic activity is reached with complete desorption of physical adsorbed water and partial desorption of surface hydroxyl groups. The strongly stabilized sulfate species on appropriately dehydrated surface of t-ZrO2 provide the superacidity and the strong oxidizing ability via a strong electron-withdrawing effect of SO bonds.
1. INTRODUCTION
Numerous papers have reported that the catalytic activity of sulfated zirconia-based catalysts is highly dependent on the preparation, calcination, storage, and activation conditions. It can be seen that all above factors are related to the effect of the degree of hydration and sulfur content of sulfated zirconia on the catalytic activity. In fact, both beneficial and detrimental effects of surface water on the activity have been reported, while most studies indicate that a moderate water content is necessary for high isomerization activity.14 Since it was proposed that the activation treatment leads to the formation of the active sites for n-butane isomerization,15 we believe that there is a generality mechanism about how the water coverage on the catalyst surface affects the catalytic activity. We have concentrated on the synthesis and catalytic properties of sulfated zirconia in n-butane isomerization for several years and have grasped several key factors to maximize the isomerization activity of sulfated zirconia.13,16−19 Actually, the catalytic behaviors are directly related to the number of available active sites on the catalyst surface, which, in turn, closely determined by the catalyst properties, including the crystalline phase of zirconia, the structure and content of the sulfate species, the interaction between surface-sulfated species and zirconia support, and so on. Our work showed that the above-
Sulfated zirconia-based catalysts have attracted much attention and are most promising candidates to replace chlorinated alumina catalysts in normal alkane isomerization. In Hino et al.’s pioneering work, sulfated zirconia was considered to be a superacidic catalyst, based on the Hammett indicator.1 nButane is activated via superacid protolysis to form butyl carbonium ions, followed by hydrogen abstraction. Cheung and co-workers2,3 suggested that sulfated zirconia is a stronger acid than USY zeolite, because it has a higher effectiveness in alkane protonation. The adsorption heat of ammonia4,5 and Ar6,7 also demonstrated stronger acidity of sulfated zirconia than that of zeolites (H-MOR, H-ZSM-5, and H−Y). However, the superacidity of sulfated zirconia has been challenged by the questionable measurement of Hammett acidity functions for solids.8 Some researchers proposed that sulfated zirconia contains only moderately strong acid sites, based on the adsorption-induced chemical shift of the hydroxyl group after adsorption of base molecules, such as acetonitrile,9 pyridine,10 and CO.11 Subsequently, another alternative oxidative mechanism for n-butane activation was suggested.12,13 Partial sulfate species with strong oxidizing ability can convert a small fraction of n-butane into butene via an oxidative dehydrogenation (ODH) route, then the butene can be readily protonated by moderate acid sites to form butyl carbenium ions, which catalyze the n-butane isomerization as the chain carriers. © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
March 11, 2019 July 9, 2019 July 17, 2019 July 17, 2019 DOI: 10.1021/acs.iecr.9b01349 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
equipped with a unique monolithic ellipsoidal reflector permanently fixed in placeeliminating the need for repositioning the focus optics for sample placement. The sample cup (10 mm in diameter and 2.3 mm deep) was also purchased from PIKE Technologies. Approximately 40 mg of catalyst was placed in the sample cup and evacuated at different temperatures before the spectra were recorded. 2.3. Catalytic Activity Tests. The n-butane isomerization assessments were performed in a fixed-bed stainless-steel reactor (inner diameter of 12 mm), and the products were analyzed using a Bruker Model 450 gas chromatography (GC) system that was equipped with a flame ionization detection (FID) device. The trace amount of butene in the butane feed was removed by an H−Y zeolite-containing trap before the isomerization reaction to ensure the concentration of butene remaining in the butane was below the detection limit of the GC equipment (0.001%). About 1.0 g of catalyst (80−180 mesh) was fixed in the middle of the reactor and activated in the flowing air at different temperatures (100−700 °C) for 30 min before the isomerization reactions. Subsequently, the temperature maintains at 200 °C and the reactants (H2:nbutane = 0.6:1, weight hourly space velocity (WHSV) = 1.98 h−1) were fed into reactor under atmospheric pressure. To investigate the influence of surface moisture on the activity of SZA catalyst, 9.5 vol % of water vapor was introduced into the feed stream. Specifically, the reactant (n-butane:H2 = 1:0.6) passed through a bubbler (constant temperature at 45 °C) to carry the water vapor to the feed stream and then was fed into the isomerization reactor. The final partial pressures of nbutane, H2, and water vapor in the feed stream were 57.28, 34.40, and 9.58 kPa. 2.4. Theoretical Calculation and Methods. Deprotonation energy (DPE) values of the hydroxyl groups on the sulfated (101) surface of t-ZrO2 under different degrees of hydration were calculated using the DMol3 program code.20 Geometries were optimized and energies were calculated with the generalized gradient approximation (GGA) of Perdew− Burke−Ernzerhof (PBE)21 and double-numerical quality localized basis-sets were used that explicitly include polarization functions (DNP) and scalar relativistic corrections. All calculations used κ-point meshes of 2 × 2 × 1. All energies, gradient, and displacement were converged to 2.0 × 10−5 hartree, 4 × 10−3 Å, and 5 × 10−3 Å. The details for accuracy assessment of the adopted model are described in the Supporting Information. DPE values reflect the energies required to heterolytically cleave H atoms from the surface hydroxyl groups to form an isolated H+ and a deprotonated conjugate structure. To obtain the DPE values, the geometry optimizations were performed on neutral (ZH) and deprotonated (Z−) structures with the generalized gradient approximation (GGA) of the Perdew− Burke−Ernzerhof (PBE) and double-numerical quality localized basis sets that explicitly included polarization functions (DNP) and scalar relativistic corrections. Thus, DPE values (EDPE) were treated as the energy difference between the resulting deprotonated products and neutral initial structure, as reported by Iglesia et al.:22
mentioned surface and bulk properties of sulfated species are very sensitive to the preparation protocol and conditions of storage, activation, reactions, and regeneration. In this study, we focused on the sensitivity of surface and bulk properties of sulfated zirconia to the aluminum addition and the surface water coverage, as well as their influence on the reactivity in n-butane isomerization. The ZrO2 crystalline phase is highly sensitive to the aluminum addition with coprecipitation method, which, in turn, plays an important role in the stabilization of surface sulfur, the ratio of Brønsted acid sites to Lewis acid sites (B/L), the crystallite size of supports, and the catalytic activity. From our study on the surface structure, we suggest that a highly active sample should contain no physical but moderate chemisorbed water, which provides two types of Zr−OH groups on the surface with different acid strength. The highly covalent SO bonds in sulfate species induce the generation of superacidity via a strong electronwithdrawing effect, which is confirmed by density functional theory (DFT) calculations. The strong oxidizing ability of sulfate species and the superacidity of bridging Zr−OH are responsible for the high isomerization activity of aluminumpromoted sulfated zirconia.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Aluminum-promoted sulfated zirconia catalysts were prepared via a two-step procedure: precipitation and impregnation. At first step, the composite zirconia−alumina hydroxide was synthesized by coprecipitation. ZrOCl2·8H2O and Al(NO3)3·9H2O with a calculated ratio were dissolved in deionized water and NH3·H2O was added dropwise up to pH 10. The obtained composite hydroxide was aged for 6 h at room temperature. Subsequently, the precipitate was filtered, washed with deionized water for three times, and dried at 110 °C for 12 h. The dried powder then was immersed in 0.5 M H2SO4 solution for 0.5 h with a solid-to-liquid ratio of 1:10 (by weight). After filtration, the samples were dried at 110 °C for 12 h and calcined at 650 °C for 2 h in a muffle furnace. The obtained catalysts were designed as SZAx, where “x” is the nominal ratio of Al2O3 to ZrO2 (in moles), expressed as a percentage. The aluminumfree catalyst was synthesized as the same protocol as described above without addition of aluminum and designed as SZA0. 2.2. Catalyst Characterization. The X-ray diffraction (XRD) patterns of catalysts were analyzed by an X’pert PRO MPD diffractometer (PANalytical Company, The Netherlands) with Cu Kα radiation at 40 kV and 40 mA. The specific surface areas were measured by nitrogen adsorption− desorption investigations at 196 °C and calculated using the Brunauer−Emmett−Teller (BET) equation. Prior to the analysis, the adsorbed water on the catalyst surface was removed by evacuating samples at 350 °C for 4 h. The surface sulfur contents of catalysts with different aluminum amount were determined by a high-frequency infrared carbon sulfur analyzer (Model HX-HW8B). The actual aluminum contents of the samples were measured using X-ray fluorescence (XRF) technology. The B/L ratios over different samples were examined by Py-FTIR study, using the same protocols as those reported in the literature.19 The in situ DRIFTS investigations were also performed on a Bruker IR spectrometer (TENSOR 27) at a spectroscopic resolution of 2 cm−1 and a MCT detector. The vacuum treatment and measurements of samples were conducted in a research-grade diffuse reflectance accessory (the PIKE Technologies DiffusIR)
E DPE = E H+ + E Z− − E HZ
where EH+, EZ−, and EHZ are the electronic energies of an isolated proton, a deprotonated structure, and a neutral Brønsted acid, respectively. B
DOI: 10.1021/acs.iecr.9b01349 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
3. RESULTS AND DISCUSSION 3.1. Effects of Aluminum Addition. Our previous study showed that, during a SZA-catalyzed n-butane isomerization reaction, the n-butane conversion always decreased with the time-on-stream of the reaction, indicating the continuous deactivation of the SZA catalyst.13 Here, to investigate the effect of aluminum addition on the catalytic activity of sulfated zirconia, the n-butane conversion and isobutane selectivity over sulfated zirconia catalyst modified with different aluminum additions were measured as the average results of the initial 10 min of reaction. As shown in Figure 1, the rate of n-butane
Figure 2. XRD patterns of catalysts with different Al additions: clover symbols denote t-ZrO2 peaks; solid diamond symbols denote m-ZrO2 peaks.
The sulfate contents and the specific surface areas of samples are compiled in Table 1. Note that the sulfur content of catalyst increases with the Al addition after high-temperature calcination. This indicates that the thermal stability of sulfate species on the t-ZrO2 surface is stronger than those on the mZrO2 surface, which was also supported by the high adsorption energies of SO3 on the surface of t-ZrO2 obtained by our DFT calculation (details are given in the Supporting Information). The specific surface areas of samples also increase with the addition of aluminum, benefitting from the improved thermal stability (Figure 2) and reduced crystallite size (Table 1) of tZrO2 with aluminum addition. The sulfate density on the catalyst surface increases quickly with the addition of small amounts of Al (Al/Zr < 4 mol %), then stabilizes at 3.5 S atoms per nm2, which is slightly lower than the theoretical sulfate density required to a complete monolayer (i.e., four S atoms per nm2 (ref 30). Thus, we concluded that the addition of aluminum primarily improved the thermal stability of t-ZrO2, which, in turn, stabilizes the surface sulfate species and leads to a higher sulfur coverage after high-temperature calcination. Infrared (IR) spectroscopy of adsorbed pyridine was used to determine the ratio of Brønsted and Lewis acid sites (B/L) of the samples. As indicated in Table 1, the catalyst with 4 mol % aluminum addition shows the highest B/L ratio, benefitting from the highest relative crystallinity of t-ZrO2 in sample SZA4. Note that the high density of Brønsted acid sites favors the bimolecular reaction of n-butane, which produces more propane and pentane and results in a decrease in isobutane selectivity.16 Consequently, the optimal isomerization activity of 4 mol % Al-promoted sulfated zirconia can be attributed to the monolayer coverage of sulfate species and the maximum of catalytic Brønsted acid sites, which result from the strongly stabilized sulfate species on the t-ZrO2 surface. This strong interaction between the t-ZrO2 surface and the sulfate species provides superacidity and strong oxidizing ability of catalysts via a strong electron-withdrawing effect of SO bonds.13 In this sense, the aluminum that is introduced to the zirconia support via the coprecipitation method acts as a structure promoter to retard the phase transformation of tetragonal zirconia to monoclinic zirconia. The decrease in isomerization
Figure 1. n-Butane isomerization activity over a sulfated zirconia catalyst modified with different amounts of aluminum addition.
conversion increased with the Al addition from 0 to 4 mol % and then decreased at higher Al amount, and the opposite trend is observed for the selectivity to isobutane. This indicates that adding an appropriate amount of aluminum into the zirconia support with a coprecipitation method makes a big impact on the properties of active sites for n-butane isomerization. To shed light on the key role of aluminum addition on the catalytic properties of sulfated zirconia, the structure and surface properties of samples with different aluminum contents were characterized. The XRD patterns of all samples are plotted in Figure 2. All peaks in Figure 2 are ascribed to the tetragonal and monoclinic zirconia. No Al-related peaks are observed, indicating that the aluminum is well-dispersed into the bulk ZrO2, even at 10 mol % addition. However, the addition of Al leads to the remarkable transformation of monoclinic zirconia (m-ZrO2) to tetragonal zirconia (t-ZrO2). This agrees well with the previous observations that the introduction of Al2O3 into bulk ZrO2 retards the phase transformation of t-ZrO2 during the calcination process.23,24 Furthermore, it has been reported that the catalytic activity of sulfated zirconia-based catalysts was strongly dependent on the sample crystallite size.25−27 Here, the crystallite sizes of tZrO2 were determined by the Scherrer equation, using (101) diffraction peaks at 30.1° 2θ.28 As shown in Table 1, the crystallite sizes of samples obviously decreased with the addition of aluminum using the coprecipitation method, revealing that the well-dispersed aluminum along with the bulk zirconia prevents the growth of zirconia particles to a size necessary to allow the commencement of the transformation into a more-stable monoclinic phase. C
DOI: 10.1021/acs.iecr.9b01349 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Table 1. Textural and Surface Properties of SZA Catalyst with Different Aluminum Addition sample
sulfur (wt %)
aluminum (wt %)
SBET (m2/g)
B/L ratioa
S coverage (nm−2)
relative crystallinity of t-ZrO2
Dcb (Å)
SZA0 SZA2 SZA4 SZA6 SZA8
0.79 1.21 2.12 2.34 2.41
0 0.97 1.66 2.28 3.49
93 114 115 120 122
1.89 2.76 3.61 3.31 3.12
1.6 2.0 3.5 3.7 3.7
0.57 0.91 1.00 0.85 0.82
216 174 172 151 148
a
The ratio of Brønsted and Lewis acid sites (B/L) was determined by Py-FTIR measurement. The values of the integrated molar absorption coefficients used to determine the B/L ratio were 1.67 and 2.22 cm/μmol for the Brønsted and Lewis acid sites, respectively.29 bCrystallite size determined using the Scherrer equation based on the (101) plane of t-ZrO2 at 30.1° 2θ.28
totally inactive for n-butane isomerization (not shown), which also confirms the detrimental effect of excessive moisture. 3.2.2. In Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) Investigation of SZA4. To clarify the effect of pretreatment on the structure of catalyst surface, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements of the SZA4 catalyst were performed after activation at different temperatures (see Figure 4). Because of the strong hydrogen bonding vibrations of
activity of samples with an Al/Zr molar ratio higher than 4% is mainly caused by their decreased B/L ratio. In addition, the smaller crystallite sizes of samples with Al/Zr > 4% may also lead to the decrease in the isomerization activity, since it has been claimed that the highly active sulfated zirconia should have an appropriate range of crystallite sizes.27 3.2. Effects of Water Sorption. 3.2.1. Effects of Activation on the Catalytic Activity of SZA4 in n-Butane Isomerization. Our previous work showed that the optimal activity of n-butane isomerization was obtained when the SZA catalyst was first calcined at 650 °C, which can be attributed to the pure tetragonal zirconia, the monolayer coverage of sulfate species, and the maximum amount of catalytic Brønsted acid sites. When the calcined catalyst was stored in an indoor environment, however, the adsorption of water on the surface of SZA4 sample strongly affects the structure of sulfate species, as well as the acid properties. Thus, an activation pretreatment is necessary to achieve high catalytic activity. The catalytic activity of SZA4 was tested in n-butane isomerization at 200 °C after activation at different temperatures (see Table S1 in the Supporting Information). As shown in Figure 3, the rate of
Figure 4. DRIFTS spectra of the SZA4 catalyst recorded after activation at different temperatures under vacuum.
adsorbed water molecules, the unactivated sample exhibits a broad band at 3700−3000 cm−1. With the increase of activation temperature, the water is desorbed and two types of hydroxyl groups are characterized at ∼3760 and 3660 cm−1, which are assigned to isolated terminal and bridging Zr−OH groups, respectively.31,32 Interestingly, the intensity of the terminal Zr−OH groups reaches a maximum when the sample is activated at 350 °C, at which temperature the catalytic activity of SZA4 also reaches its maximum (see Figure 3). The band intensity of bridging Zr−OH always increases as the activation temperature increases. In addition, the band of the terminal Zr−OH groups shifts to a lower wavenumber (from 3772 cm−1 to 3764 cm−1) with increasing activation temperature, implying a gradual weakening of the O−H bonds. Different from that observed with the Zr−OH groups, the deformation vibration band of water at 1600−1700 cm−1 always decreased in intensity with increasing activation temperature. The relative content of water on the catalyst surface was calculated by integration of the band areas at
Figure 3. n-Butane isomerization activity over SZA4 catalyst after activation at different temperatures under flowing air.
n-butane conversion increased as the activation temperature increased from 100 °C to 350 °C and then decreased at higher temperatures. When activated at
