SBA-16

Jul 20, 2010 - Sarsani , V. R.; Wang , Y.; Subramaniam , B. Toward Stable Solid Acid Catalysts for 1-Butene + Isobutane Alkylation: Investigations of ...
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Ind. Eng. Chem. Res. 2010, 49, 7201–7209

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Alkylation of Isobutane/1-Butene on Methyl-Modified Nafion/SBA-16 Materials Wei Shen,*,†,‡ Yi Gu,† Hualong Xu,† Renchao Che,† David Dube´,‡ and Serge Kaliaguine*,‡ Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials and Laboratory of AdVanced Materials, Fudan UniVersity, Shanghai 200433, PR China, and Department of Chemical Engineering, LaVal UniVersity, Quebec City, QC, Canada G1K 7P4

Three-dimensional mesoporous SBA-16 silica materials were functionalized with perfluorosulfonic acidic resin Nafion using an impregnation method. The intrinsic polarity of the SBA-16 surface was tuned by grafting ethoxytrimethylsilane on its surface. Characterized by N2-physisorption, X-ray diffraction (XRD), and transmission electron micrographs (TEM), all the materials synthesized exhibited ordered three-dimensional Im3m mesoporous structure. Elemental analysis and water adsorption measured by an intelligent gravimetric analyzer (IGA) showed that trimethylsilane is grafted on the surface by capping the -OHs which enhance the hydrophobicity of SBA-16. Elemental analysis and potentiometric titration showed that Nafion resin was incorporated, revealing a three-dimensional mesoporous strong solid acid with a hydrophobic surface. The catalytic alkylation of isobutane/1-butene was thereafter evaluated on each material under specified conditions and compared with the one-dimensional Nafion/SBA-15 and commercial Nafion silica nanocomposite SAC13. The catalyst with three-dimensional mesoporous channels was shown to outperform the one with onedimensional channels. The higher activity of Nafion over methyl modified SBA-16 materials is related to the more hydrophobic surface of support. 1. Introduction Alkylation of isobutane with light (C3-C5) olefins is an important refining operation which produces mixtures of branched alkanes. The formed products, designated as alkylates, are the highest-quality hydrocarbons for the gasoline pool because of their high octane number, low Reid vapor pressure, very low sulfur content, and freedom from aromatics or olefins.1 The current alkylation technology makes use of strong liquid acids (HF or H2SO4). While producing high quality, environmentally benign gasoline components, the process suffers from a number of drawbacks. The obvious environmental risks of an unintentional release upon separation and disposal of these catalysts necessitate their replacement. A variety of strong acidic solids has been investigated, including sulfated zirconia and related materials,2,3 heteropolyacids,4,5 acid resins,6,7 chlorinated alumina,8 and acidic zeolites.9-18 None of them met any commercial success because of their unacceptably rapid deactivation. The alkylation reaction proceeds mainly via addition of n-butene to an isobutyl carbenium ion. The resulting octyl carbenium ion is removed (after possible isomerization) from the active site by hydride transfer from isobutane leading to trimethylpentanes as ideal products and an isobutyl ion, which perpetuates the reaction. On the other side, the octyl carbenium may continue to react with olefins to form a polymer. The polymer is strongly adsorbed on the acid sites and completely fills the pores at the end of the reaction.19 It is generally accepted that a promising catalyst should be acidic enough to form the intermediate carbocations and catalyze hydride transfer. Meanwhile, the pores should be large enough to allow the diffusion of reactants to active sites and products out of the pore system.1 Moreover, a hydrophobic surface which allows a higher paraffin * To whom correspondence should be addressed. Tel.: +86 21 65642401. Fax: +86 21 65641740. E-mail: [email protected] (W.S.). Tel.: +1 418 656 2708. Fax: +1 418 656 3810. E-mail: [email protected] (S.K.). † Fudan University. ‡ Laval University.

steady concentration in the pore system is desired.20 Apart from the above factors, the pore structure of solid acid catalysts also plays very important roles in alkylation. It has a direct influence on the effective diffusivities.21 Yoo et al. reported22 that zeolite ZSM-12 which possesses one-dimensional noninterpenetrating channels outperformed USY with three-dimensional channels and larger pores. The authors thought the linear channels which do not possess any expansions do not allow the formation of bulky carbonaceous materials that could lead to pore plugging. This is in contrast to the common belief that three-dimensional zeolites are less susceptible to pore plugging than onedimensional zeolites. Corma et al. pointed out that tridirectional structures of zeolites which allowed easier diffusion should be preferred to monodirectional ones.1 No research on the influence of pore structure of acid functionalized mesoporous silica on alkylation reaction was yet reported. The strength of perfluorosulfonic acid is comparable to that of pure sulfuric acid.23 Perfluorosulfonic acid functionalized silica showed good activity in isobutane/n-butene alkylation.6,20,24 These materials can be made by immobilization of perfluorosulfonic acid groups on mesoporous materials using postsynthetic grafting strategies20,25 or direct one-step synthesis strategies.26 However, the limited availability of suitable fluorinated silanes hinders the applicability of this kind of materials for large scale production. Supporting the perfluorosulfonic acid resin Nafion on high surface area carriers provided a practical way to produce catalyst with higher surface area, large pore size, and high strength acid sites.27 Wang and Guin reported that impregnation is more beneficial than the sol-gel technique to make Nafion/silica catalyst for etherification of olefins.28 Recently, we synthesized Nafion modified SBA-15 using an impregnation method.29 Nafion resin was supported on the onedimensional mesopores, which still leaves enough space for diffusion of alkylation products. In addition, the hydrophobic/ hydrophilic balance of the catalyst can be tuned by capping the surface -OHs with alkyl trimethoxysilane. This is beneficial to increase the local isoparaffin/olefin ratio in the pore system of catalysts.

10.1021/ie1001873  2010 American Chemical Society Published on Web 07/20/2010

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In this work, Nafion was impregnated on three-dimensional mesoporous SBA-16 silica. The hydrophobicity of the SBA-16 surface was enhanced by capping the surface -OHs. The activity of Nafion modified SBA-16 (denoted as Nafion/SBA16) and hydrophobicity-modified Nafion/SBA-16 (denoted as Nafion/Me-SBA-16) materials was evaluated and compared with Nafion/SBA-15, Nafion/Me-SBA-15, and SAC-13 in the alkylation of isobutane/1-butene. The influence of the structure, hydrophobic nature of support, reaction temperature, isobutane/ 1-butene ratio, and 1-butene space velocity were studied. Nafion modified SBA-16 and Me-SBA-16 materials prepared by impregnation respond to the need of supporting perfluorosulfonic acid resin over materials with three-dimensional mesopore structure in order to provide well-dispersed active sites and allow easier diffusion. Nafion-perfluorosulfonic acid resin has an acidity comparable to that of concentrated sulfuric acid, providing effective active sites in alkylation reactions. Mesoporous SBA-16 support has the desired three-dimensional pore structure, ensuring easier diffusion of products and more surface interacting with reactants. The influence of pore structure on the alkylation reaction can be revealed by comparing the behaviors over Nafion modified mesoporous materials with onedimensional and three-dimensional channels. The surface hydrophobicity of SBA-16 can be adjusted by silylation of the surface -OHs into methylsilane. In this way, the influence of the surface polarity of SBA-16 on the alkylation reaction can be established. 2. Experimental Section 2.1. Catalyst Preparation. SBA-16 mesoporous silica sample was synthesized by the acid-catalyzed hydrolysis and condensation of tetraethyl orthosilicate (TEOS, Aldrich) using the triblock copolymer Pluronic F127 (EO106PO70EO106, BASF) as the structure-directing agent with the aid of K2SO4, according to the procedure described by Zhao et al.30 Because the large surface area of silica decreases the acidity of the sulfonic groups due to the interaction between the sulfonic groups of Nafion resin and silanol groups of the silica,6 a relatively larger TEOS/ F127 ratio was used to synthesize SBA-16 with smaller surfaces. In a typical synthesis, 4 g F127 and 10.48 g K2SO4 were completely dissolved in 120 g of 0.5 M HCl solution under stirring at 38 °C. This was followed by adding 16.8 g TEOS into the solution as the silicon source. After being stirred vigorously for 24 h at 38 °C, the resulting gel was transferred to a Teflon-lined autoclave and heated at 100 °C for an additional 48 h. After cooling to ambient temperature, the solids in the autoclave were recovered by filtering, washing, and drying at 80 °C. Finally, the solids were calcined at 550 °C for 6 h in air to remove the organic surfactant. Silylation of the surface -OHs of SBA-16 was carried out as follows: 2.6 g of SBA-16 was predried under vacuum at 200 °C for 12 h before adding 3.5 g ethoxytrimethylsilane (Aldrich) and 30 mL of dry toluene under argon. The mixture was refluxed at 100 °C for another 12 h. Then, the hydrophobicity-modified material was filtered and washed by toluene and anhydrous ethanol in turn. At last, the solid was dried at 80 °C overnight.31 The supported Nafion catalysts were prepared by impregnating Nafion (5 wt % Nafion in water-alcohol solution, Dupont) on the above prepared materials (pure or hydrophobicitymodified SBA-16), stirring at 60 °C and atmospheric pressure for 6 h. The solid was first dried at room temperature for 12 h in static conditions, and then the water and alcohols were evaporated thoroughly under vacuum at 60 °C for an additional 12 h. The resultant materials were denoted as Nafion(X)/SBA-16

or Nafion(X)/Me-SBA-16, where X indicates the theoretical weight percent of Nafion loading (in this work, two Nafion loadings were studied, i.e. X ) 15 or 30). Me-SBA-16 indicates that the surface -OHs of SBA-16 were silanized by ethoxytrimethylsilane. For comparison, Nafion(X)/SBA-15, Nafion(X)/ Me-SBA-15, and the Nafion resin/silica composite SAC 13, with a resin content of ca. 13 wt % obtained from Aldrich were also studied. 2.2. Catalyst Characterization. Nitrogen adsorptiondesorption isotherms at 77 K were performed using a Micromeritics TRISTAR 3000 apparatus. The samples were degassed at 120 °C and high vacuum prior to the measurements. The BET equation was used to estimate the surface areas of the materials. BJH model was utilized to calculate the mesopore size using adsorption branches of isotherms, and the pore diameter was estimated from the peak position of the BJH poresize distribution. Powder XRD spectra were recorded using a Bruker D4 X-ray diffractometer with nickel-filtered Cu KR radiation (λ ) 1.5418 Å). The tube voltage was 40 kV, while the current was 40 mA. Diffraction patterns were recorded with scan step of 0.02° for 2θ between 0.5° and 5°. Transmission electron micrographs (TEM) were obtained using a JEOL 2011 microscope operated at 200 kV. The samples were prepared by sonication in ethanol and suspended on holey carbon grids. Spatially-resolved data were acquired using an electron energy loss spectrometer (EELS; Gatan tridium) installed on the JEM2100F microscope of Fudan University. This facility can reach a spatial resolution of 0.15 nm and a energy resolution of 0.7 eV. The ion exchange capacities (corresponding to acid site concentration) of the Nafion-modified SBA-16 materials were determined using aqueous solutions of NaCl and titrated potentiometrically by NaOH.32 In a typical experiment, 0.1 g of solid was added to 20 g of 2 M NaCl solution and vigorously stirred at room temperature overnight. The resulting suspension was filtered and washed thoroughly with a total amount of 80 mL 2 M NaCl to retain the hydrogen ions in the solution, and thereafter titrated potentiometrically by a dropwise addition of a 0.1 M NaOH. Water adsorption isotherms at 25 °C were performed using an intelligent gravimetric analyzer (IGA-002, Hiden, UK). Before the measurements, samples (ca. 50 mg each) were degassed at 200 °C for 12 h in a high-vacuum system. IGA is a very accurate, completely computer controlled gravimetric technique which can define the adsorption behavior of the gas-solid system well. The apparatus is an ultrahigh vacuum system. It consists of a fully computerized microbalance which automatically measures the weight of the sample as a function of time with the gas vapor pressure and sample temperature under computer control. Sulfur and carbon content were determined by elemental analysis using a Vario EL III apparatus (CHNS model). 2.3. Reaction Procedure. Liquid phase alkylation of isobutane/1-butene experiments were carried out in an automated stainless steel fixed bed continuous reactor. In each run, 0.35 g of catalyst pellets with diameters of 0.5 to 0.8 mm, obtained by compressing the powder into tablets, crushing, and sieving, were loaded in a reactor tube with internal diameter of 8 mm. The catalyst bed was fixed between two plugs of quartz wool and the remaining empty volume filled with quartz beads. The catalysts were pretreated in situ at 110 °C for 10 h in a 15 mL/ min flow of N2. The reactor was cooled to reaction temperature (either 70 or 100 °C) and a pressure of 3.2-3.4 MPa was

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Figure 1. X-ray diffraction patterns of SBA-16, Me-SBA-16, and their Nafion-modified samples.

established with nitrogen before feeding the reactants. The feedstock consisting of an isobutane/1-butene mixture with a molar ratio of 20/1 and 40/1 was delivered to the reactor using a piston-type pump, and it goes through the catalyst bed located in the middle zone of the reactor. The product stream coming out of the reactor is then collected and stored using a 16-loop sampling valve. In this way, samples at intervals of 1 min can be taken during the run and analyzed automatically once the experiment has been finished. The reaction products were analyzed using a GC (Thermo Trace GC Ultra) equipped with a 100 m capillary column (CP SIL PONA CB) and a flame ionization detector (FID). The individual C4-C8 hydrocarbons were identified by means of available reference standards and GC-MS analysis. The conversion is calculated based on disappearance of butenes, i.e., 1-butene, 2-butene, and isobutene. The selectivity of a product was defined as the ratio between the weight of the product and the weight of converted butenes. In absence of any simultaneous reactions with isobutane-butene alkylation, the theoretical alkylate selectivity value would be 204 wt %. 3. Results and Discussion 3.1. Catalyst Characterization. Powder XRD patterns of SBA-16, Me-SBA-16, and their Nafion-modified samples are depicted in Figure 1, showing the effect of silylation of surface -OHs and impregnation of Nafion resin on the mesostructured SBA-16. It is clearly observed that all the samples show one strong diffraction peak in the range of 0.85-0.90°, assigned to (110). The secondary diffraction peaks of (200) and (211) are slightly showed in the samples of SBA-16, demonstrating the well-defined Im3m mesophase of SBA-16. The intensity of the

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secondary diffraction peaks decreased when the materials are modified by silylation or Nafion impregnation. XRD lines normally decrease in intensity as guest species are loaded in the mesopores, and such an intensity change can be useful as a means of judging the location of the guest species. Therefore, this suggests Nafion resin and methyl groups are located in the mesopores. The obvious peaks of (110) of all samples indicate that the cubic mesostructure is maintained in all cases. The N2 adsorption-desorption isotherms and pore size distributions of SBA-16 and Me-SBA-16 with various Nafion loadings are shown in Figure 2. All the samples exhibit a sharp capillary condensation step on the desorption branch at p/p0 ) 0.45, which are typical features of ordered Im3m mesostructure. The narrow IV-type hysteresis loop of SBA-16 material is maintained after the grafting of trimethylsilane, indicating that the structure is not drastically affected by silylation. The mesostructure is also maintained after impregnation of 15 wt % of Nafion, which means that the Nafion resin, having penetrated into the porous framework, seems to be homogeneously deposited along the three-dimensional mesoporous channels of the materials when the Nafion loading is lower than 15 wt %. With increasing Nafion loading to 30 wt %, the capillary condensation step on desorption branch of Nafion(30)/ Me-SBA-16 becomes broader while no significant broadening is found in the sample Nafion(30)/SBA-16. Meanwhile, the pore size distribution of Nafion(30)/Me-SBA-16 is also a little broader than that of Nafion(30)/SBA-16. Part of the resin on Nafion(30)/Me-SBA-16 may be within the mesoporous structure in the form of polymeric aggregates. The textural parameters deduced from nitrogen isotherms and XRD patterns of the synthesized materials are summarized in Table 1. Impregnation of up to 30 wt % Nafion on SBA-16 has only a slight effect on the BET surface area, pore diameter, and wall thickness. Decreases of BET surface area and pore diameter, and an increase in wall thickness can be observed on Me-SBA-16. This is indicative of the presence of Nafion resin aggregates in the channels of Me-SBA-16. Interestingly, the pore diameter increases from 4.4 (SBA-16) to 4.8 nm (Me-SBA-16) when SBA-16 is silanized by ethoxytrimethylsilane. We believe this is due to a leaching process during the silylation procedure. This is coherent with the decrease of wall thickness from 8.0 to 7.1 nm after silylation. Though the textural parameters changed a little after OH capping and Nafion loading, the threedimensional mesostructure of SBA-16 was not affected, which makes these materials suitable catalysts for alkylation. The mesoporous structure of Nafion-modified SBA-16 materials was characterized using transmission electron microscopy (TEM). Figure 3 shows a typical TEM image of Nafion(30)/ Me-SBA-16. A well-ordered cubic pore structure of ca. 4.5 nm in diameter is clearly observed, which is in a good agreement with the analysis of XRD and N2 physisorption. Figure 4 shows the dark field image of Nafion(30)/SBA-16. The EELS data acquired with 0.5 nm resolution shows that the oxygen from silica is invisible on point A corresponding to the cage with the carbon signal (280 eV) clearly present. The signal at point B indicates complete absence of carbon at cage aperture with intense oxygen peak (540 eV) from the silica wall. These data indicate that the polymer Nafion is present in the cage as an isolated particle not protruding in the pore aperture. Table 2 shows several parameters related to the chemical properties including the acidity and methyl incorporation of the hybrid materials. Methyl content calculated from carbon and sulfur content measured by elemental analysis in all silanized samples is ca. 0.4 mmol/g. It indicates that methyl groups were

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Figure 2. (a) N2 adsorption/desorption isotherms and (b) pore size distributions calculated from the adsorption branch by BJH method. Table 1. Textural Properties for Nafion-Modified Mesostructured SBA-16 Materials

sample SBA-16 Nafion(15)/SBA-16 Nafion(30)/SBA-16 Me-SBA-16 Nafion(15)/Me-SBA-16 Nafion(30)/Me-SBA-16

pore BET pore wall area diametera volumea d110b a0c thicknessd (m2/g) (nm) (cm3/g) (nm) (nm) (nm) 404 392 388 409 356 234

4.4 4.3 4.4 4.8 4.7 4.3

0.33 0.20 0.15 0.20 0.165 0.11

10.2 10.0 10.0 9.7 9.9 9.8

14.4 14.2 14.2 13.7 14.0 13.9

8.0 8.0 7.9 7.1 7.5 7.7

a Total pore size and pore volume of Nafion-modified SBA-16 materials were calculated by the BJH adsorption branch, and the pore diameter was pointed out from the peak position of BJH pore size distribution. b d(110) spacing, measured from small-angle XRD. c a0 ) 21/2d110. d Pore wall thickness ) 31/2a0/2 - pore diameter.

grafted on SBA-16 and no leaching happened during impregnation of Nafion. The experimental Nafion loading of each sample is also evaluated using elemental analysis method. The incorporation yield refers to the percentage of Nafion resin deposited on the support. It ranges from 81 to 85 wt %, which demonstrates that the Nafion loading (between 15 and 30 wt %) and polarity of SBA-16 did not have a great effect on incorporation yield. H+ concentrations determined by dropwise potentiometric titration are similar to sulfur concentrations. The results evidence that the active sites provided by Nafion resin are highly accessible, revealing the effective role of the mesostructured silica support. Methyl capping of surface -OHs, though not providing the active sites as the Nafion resin, can play an important role in tuning the physical property of the surface of the catalyst. It may enhance the hydrophobicity of catalysts, which is beneficial to increasing the surface isoparaffin/olefin ratio in the alkylation reaction conditions, resulting into good catalytic performances. To compare the hydrophobicity of the samples, H2O adsorption

measurements were carried out using an IGA system. Figure 5 depicts water adsorption isotherms of SBA-16, Me-SBA-16, and their Nafion-modified samples. Compared with SBA-16, MeSBA-16 takes less water over the whole pressure range (0-10 mbar), indicating the hydrophobicity of methyl-modified material. A different water adsorption behavior could be observed when these two materials were modified by Nafion resin. The IGA curves for water adsorbed over Nafion(30)/SBA-16 and Nafion(30)/Me-SBA-16 can be divided into two parts. The mass uptake of water is slight higher over Nafion(30)/Me-SBA-16 than over Nafion(30)/SBA-16 when the water vapor pressure is less than 1.8 mbar. This is due to the introduction of perfluorosulfonic acid groups. Water molecules interact strongly with acid sites. They adsorb preferentially on perfluorosulfonic acid groups when the samples are modified with Nafion resin. Because the interaction between acid groups of Nafion and the silanols of the silica leads to a decrease in acid strength,6,33 the acid sites of Nafion(30)/Me-SBA-16 are more acidic and can take more water molecules at low vapor pressure. However, the difference in acidity is not significant; the mass uptake of water over Nafion(30)/Me-SBA-16 is just slight higher. With the increasing of vapor pressure to more than 1.8 mbar, the situation is different. More water was retained over Nafion(30)/ SBA-16. The acid sites were saturated and water molecules adsorbed mainly on surface of SBA-16 or Me-SBA-16. The results indicate that the hydrophobicity is enhanced when the silica is silanized by ethoxytrimethylsilane. 3.2. Catalytic Performance. The alkylation of isobutane with n-butene over solid acid catalysts has been studied intensively over the past two decades.1,34,35 Solid acids deactivate by the buildup of a polymer, which eventually blocks the pores of the catalyst. The pore structure of solid acid catalysts plays very important roles in alkylation. Numerous transfer steps obviously occur in the pores of the solid catalysts. Isobutane

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Figure 3. TEM images of Nafion(30)/Me-SBA-16 showing characteristic planes for a cubic pore structure: (a) [111], (b) [100], (c) [120].

Figure 4. Dark field image of Nafion(30)/SBA-16. The line profile of image intensity is also provided. The brighter point A corresponds to one cage whereas B is located on pore aperture. Table 2. Methyl Incorporation and Acidic-Related Properties of Nafion Modified Mesostructured Materials acidity sample SBA-16 Nafion(15)/SBA-16 Nafion(30)/SBA-16 Me-SBA-16 Nafion(15)/Me-SBA-16 Nafion(30)/Me-SBA-16

methyl content (mmol methyl/g)a

(mmol H+/gcat)b

(mmol S/gcat)c

Nafion incorporation yield wt%d

0.132 0.264

0.126 0.255

84 85

0.130 0.256

0.123 0.243

82 81

0.4 ( 0.1 0.4 ( 0.1 0.4 (0.1

a Methyl content calculated from elemental analysis. b Obtained by cationic-exchange in 2 M NaCl, and titrated potentiometrically with 0.1 M NaOH after filtration and thorough washing. c Sulfur content calculated from elemental analysis. d Yield of incorporation of Nafion was determined by comparing the amount of sulfur deposited and introduced.

and olefins diffuse inward in the pores, and product molecules diffuse outward. The rates of transfer or diffusion vary for different molecules because of differences in size, shape, molecular weight, and interaction with the pore walls.36 In the pores, numerous reaction steps occur. Conditions in the pores are conducive to forming, in addition to alkylate, both conjunct polymers and pseudoalkylate. The diffusion of conjunct polymer is slow. The conjunct polymers would be difficult to diffuse out or may bond with the inner surfaces of the solid catalyst. Diffusion limitations would lead to faster deactivation. Particular

attention should be paid to the role of the structural characteristics of the catalysts. Nafion modified mesoporous materials are solid acids with high surface area, large pore size, and high strength acid sites. They are promising solid catalysts for isobutane/n-butene. It is meaningful to study the effect of pore structure and the hydrophilic-hydrophobic nature of Nafion modified mesoporous materials on the catalytic performance in alkylation. Nafion modified mesoporous materials with different structure and hydrophilic-hydrophobic nature were tested and compared

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Figure 5. Adsorption isotherms of water in SBA-16, Me-SBA-16, Nafion(30)/SBA-16, and Nafion(30)/Me-SBA-16 at 25 °C.

with SAC-13, which was reported to be a good catalyst for isobutane/n-butene alkylation.24 Sample Me-SBA-16 having no impregnated Nafion showed no activity in this reaction. The performances of Nafion(30)/Me-SBA-16 are comparable with those of zeolite Y.20 The main point here is that it is possible to improve the activity of supported Nafion by using hydrophobized mesostructured silica SBA-16 as the support. Figure 6 displays the catalytic performance of catalysts in terms of butenes conversion, TMPs (trimethylpentanes) yield, and C5-C7 (hydrocarbons with 5-7 carbon atoms) selectivity as a function of time on stream. Relevant textural and catalytic properties are summarized in Table 3. It can be observed that the activity of Nafion modified materials with similar H+ concentration (∼0.13 mmol H+/gcat) decreases in the order: Nafion(15)/SBA-16 > SAC-13 > Nafion(15)/SBA-15. The activity of three-dimensional mesostructured Nafion(15)/SBA-16 exhibits much higher initial value (99.1 vs 54.2 wt %) and remains at a higher level in comparison with the one-dimensional Nafion(15)/SBA-15, though Nafion(15)/ SBA-15 contains larger mesopores, and pore plugging is typically anticipated to have only negligible effects on largepore solid acids. We believe that the good catalytic performance of Nafion(15)/SBA-16 for alkylation is mainly due to its pore structure. Three-dimensional mesopore structure is beneficial for isobutane and olefins diffusing inward in the pores and product molecules diffusing outward. At the same time, the slightly lower surface area of Nafion(15)/SBA-16 is also beneficial to its activity. There is more interaction between the Nafion resin and the silica support on a larger surface. In this way, a charge transfer may occur from the protons of the sulfonic groups to the silanol groups of the silica, decreasing the acid strength of the resin and activity of alkylation.6,33 The influence of surface area on acid strength can be confirmed by the initial selectivity of C5-C7, which is the result of cracking and is catalyzed only by the strongest acid sites.2 It can be observed in Figure 6 that the selectivity of C5-C7 increase in the order: Nafion(15)/SBA-15 < Nafion(15)/SBA-16 < SAC13. SAC-13 is a material with the perfluorosulfonic Nafion resin supported over an amorphous silica via sol-gel techniques. It has the smallest surface area and exhibits the highest initial C5-C7 selectivity. Anyway, Nafion(15)/SBA-16 exhibits much better conversion and TMPs yield than SAC-13 and Nafion(15)/ SBA-15. A better availability of acid sites seems to be achieved in the mesostructured silica SBA-16 support. It is reasonable to attribute the good catalytic performance of Nafion(15)/ SBA-16 mainly to the three-dimensional mesopores structure.

Figure 6. Catalytic performance of Nafion modified mesoporous materials. Experiment conditions: T ) 100 °C; olefin WHSV ) 2 h-1; isobutane/1butene (I/O) molar ratio ) 40.

In a recent study from our laboratory, it was found that the diffusivity of small hydrocarbons was systematically 2 orders of magnitude higher in SBA-16 compared to SBA-15.21 Threedimensional structures of mesoporous solid acids which allowed easier diffusion should be preferred to one-dimensional ones. With the increase in Nafion loading on SBA-16 (from 15 to 30 wt %), the number of acid sites which catalyze isobutane/ 1-butene alkylation increases. The butenes conversion of Nafion(30)/SBA-16 remains at a higher level in comparison with

Ind. Eng. Chem. Res., Vol. 49, No. 16, 2010 Table 3. Physical Properties of Catalysts sample Nafion(30)/Me-SBA-16 Nafion(30)/ SBA-16 Nafion(15)/ SBA-16 Nafion(15)/ SBA-15e SAC-13

SBET DPorea VPorea S/gcatb H+/gcatc Cad/gcatd (m2/g) (nm) (cm3/g) (mmol) (mmol) (mmol) 234 388 392 474 200

4.3 4.4 4.3 7.0

0.11 0.15 0.20 0.65 0.43

0.243 0.255 0.126 0.124 0.147

0.256 0.264 0.132 0.130 0.133

0.13 0.18 0.30 0.54 0.45

Table 4. Variation of the Butene Conversion and Product Distribution at Different Reaction Temperatures and Times on Stream over Nafion(30)/Me-SBA-15 and Nafion(30)/Me-SBA-16 Catalystsa catalyst temperature (°C) TOS (min) C4) conv (wt %)

a

Total pore size and pore volume were calculated by the BJH method. b Sulfur content evaluated by elemental analysis. c Obtained by cationic-exchange with NaCl and titration with NaOH. d Increase in carbon content upon catalytic test, evaluated from elemental analysis. e from ref 29.

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Nafion(30)/Me-SBA-16

Nafion(30)/Me-SBA-15

100 1 100

100 1 98.6

100 10 54.0

70 1 88.6

100 10 39.8

70 1 71.4

42.7 48.2 9.1

8.8 65.9 25.3

27.3 64.4 8.0

65.5 33.4 1.1

35.6 50.0 14.4

52.8 37.6 9.6

distribution of C5+ (wt %) C5-C7 C8 C9+

42.2 51.3 6.6

TMP DMH C8)

65.7 30.0 4.3

9.5 66.8 23.7

30.0 62.9 7.1

distribution of C8 (wt %)

the Nafion(15)/SBA-16. High Nafion content favors the formation of TMPs and C5-C7 products. It can also be observed in Figure 6 that the activity and stability of Nafion(30)/SBA-16 are obviously improved after its surface -OHs were capped by ethoxytrimethylsilane. Though both materials give very high initial activity values, Nafion(30)/ Me-SBA-16 gives evidently higher butenes conversion and TMPs yields. The improvement of catalytic performance may be attributed to stronger acidity and three-dimensional pore system. The acid sites of Nafion(30)/Me-SBA-16 are just slightly stronger than those of Nafion(30)/SBA-15 which did not induce evident differences on the C5-C7 selectivity. In other words, the OH capping does not seem to increase activity through increasing the acid strength. Meanwhile, catalyst characterization indicated there are no essential structure changes induced by the capping procedure. We believe that the improved activity and stability of -OHs capped materials for this reaction is a result of their hydrophobic surface. The hydrophobic environment of the capped catalysts allows a higher steady isobutane concentration in the pore system. During isobutane/1-butene alkylation reaction, the octyl carbenium ion is removed from the active site by hydride transfer from isobutane leading to trimethylpentanes as ideal products and an isobutyl ion, which perpetuates the reaction. The high ratio of the rate of hydride transfer vs the rate of oligomerization is crucial for a good catalytic performance, while the isobutane/olefin ratio determines the rate of hydride transfer. To enhance surface carbophilicity by capping, the silanols of SBA-16 increase the I/O ratio in the reaction conditions and therefore improve the catalytic performance of the catalysts. It was checked for both Nafion(30)/SBA-16 and Nafion(30)/Me-SBA-16 that the S/SiO2 ratio did not decrease after catalytic testing. This indicates that no loss of Nafion resin was involved in catalyst deactivation. The fact that the pore surface hydrophobicity is enhanced by -OH capping was confirmed by IGA results. Less water was adsorbed on the surface of Me-SBA-16 and Nafion(30)/MeSBA-16 in comparison to SBA-16 and Nafion(30)/SBA-16. The benefit of three-dimensional mesopore structure and a hydrophobic surface can be confirmed by the amounts of heavy hydrocarbons adsorbed on the used catalysts. It can be observed in Table 3 that less adsorbed hydrocarbons, which are mainly responsible for the catalyst deactivation, were detected on materials with three-dimensional mesopore structure and a hydrophobic surface. The influence of pore structure on the activity and selectivity for the OH capped Nafion modified mesoporous materials is shown in Table 4. On hydrophobic-modified samples with similar Nafion content but with a different pore structure, the catalyst with three-dimensional structure deactivates more slowly. At 100 °C, the butenes conversion decreases from 100 wt % at 1 min TOS to 88.6 wt % at 10 min TOS over Nafion(30)/Me-SBA-16 while it decreases from 98.6 to 39.8

a

h-1.

44.2 41.4 14.4

70.8 25.4 3.7

Other experimental conditions: I/O ratio ) 40, olefin WHSV ) 2

wt % over Nafion(30)/Me-SBA-15. Moreover, a decrease in selectivity to TMPs is also slower on three-dimensional catalysts. The TMPs/C8 ratio decreases from 65.7 wt % at 1 min TOS to 44.2 wt % at 10 min TOS over Nafion(30)/Me-SBA-16 while it decreases from 65.5 to 35.6 wt % over Nafion(30)/Me-SBA15. The three-dimensional mesopores solid acid outperforms the one-dimensional one both in butenes conversion and TMPs selectivity. The adsorption and diffusion of products and reactants are affected by reaction temperature. Compared with liquid acids, a higher reaction temperature is required when solid acids are used. This is attributed to the lower strong acid site concentration of solid acids or the lack of solvation, resulting in higher activation energies for the individual reaction steps. Efficient mobility of products and reactants in the pores of solid acids also requires higher temperature. The poor performance of several solid acids at low reaction temperature is most likely a consequence of the hindered diffusion of bulky molecules under such conditions. The catalyst will be prematurely deactivated by pore blocking. In Table 4, catalytic performances at 100 and 70 °C over Nafion(30)/Me-SBA-15 and Nafion(30)/Me-SBA-16 are also presented. The inferiority of one-dimensional catalyst is intensified at lower temperature. Diffusion problem is more severe for one-dimensional materials. The results obtained suggest that three-dimensional structures, both in a hydrophilic and hydrophobic environment, are capable of diffusing molecules easily and resisting deactivation by pore plugging. Threedimension mesopores structure and hydrophobic surface should be desired for an alkylation catalyst. As with alkylation reaction over other solid acid catalysts, a close interconnection exists between the catalytic performances of Nafion/Me-SBA-16 and the operation conditions. The most important parameters are the reaction temperature, the olefin space velocity, and the feed isoparaffin/olefin ratio. Figure 7 displays the influence of reaction parameters on the catalytic performance of Nafion(30)/Me-SBA-16. It can be seen that with same 1-butene space velocity and isobutane/1-butene ratio, the conversion of butenes increase obviously with temperature. The initial conversion of butenes increases from 88.6 to 100 wt % when the reaction temperature increases from 70 to 100 °C (weight hourly space velocity (WHSV) ) 2 h-1, I/O ) 40). The yield of TMPs is presented in the same manner except the beginning stage (at 1 min TOS). The decline in the TMPs yield with rising temperature during the beginning stage is compensated by an increase in the cracking selectivity.

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The impact of structure of Nafion modified mesoporous materials on activity and selectivity during the alkylation of isobutane with 1-butene was discussed. The three-dimensional mesopore solid acid outperforms one-dimensional ones both in term of butene conversion and TMP selectivity. A threedimensional mesopore structure is capable of diffusing molecules easily and resisting deactivation by pore plugging. Thus, the access of iso-butane to the active sites and its further conversion to TMP is therefore less restricted in the 3D pore lattice of SBA-16 than in the 1D channels of SBA-15. Hydrophobically modified Nafion(30)/Me-SBA-16 shows excellent activity and efficiency in the production of isooctane compared with Nafion modified SBA-15 and SAC-13. It is suggested that the three-dimensional mesopore structure and hydrophobic surface should be desired when designing an alkylation catalyst. Acknowledgment The authors gratefully acknowledge the funding support from the Natural Sciences and Engineering Research Council of Canada (NSERC), Science & Technology Commission of Shanghai Municipality (08DZ2270500), Ministry of Science and Technology of China (“863” key Project No. 2009AA033701, 973 Project Nos. 2007CB936301 and 2009CB930803, 09SG01), and National Natural Foundation of China (Nos. 50872145 and 10776037). Literature Cited

Figure 7. Variation of the conversion of butenes (a) and yield of TMPs (b) with time on stream over Nafion(30)/Me-SBA-16 under varying experimental conditions.

The influence of WHSV is also shown in Figure 7. When the WHSV of 1-butene decreases from 2 to 1.4 h-1 (I/O ) 40, T ) 100 °C), the catalyst decay is much slower. As the WHSV of 1-butene increases, the contact time decreases, resulting in an increase of the C8) and C9+ fractions, which not only decreases the selectivity of TMPs but also makes the catalyst decay faster. The isoparaffin/olefin ratio is a very important parameter in isobutane/n-butene alkylation. Lower I/O ratios lead to a higher rate of oligomerization. It can be seen in Figure 7 that I/O shows an obvious effect on catalytic performance. The conversion of butenes decreases when the I/O ratio decreases from 40 to 20 (WHSV ) 2 h-1, T ) 100 °C). A high I/O ratio obviously favors the yield of TMPs. However, a high I/O ratio will lead to a high cost of separating products from the excess isobutane, which limits the I/O ratio that can be used industrially. 4. Conclusions Mesostructured SBA-16 hydrophobically modified by OH capping and functionalized with Nafion resin by means of impregnation was described. The resulting solid acid catalysts preserve the three-dimensional mesostructure of SBA-16. Capping of surface OH decreases the material’s polarity and provides a hydrophobic environment for the isobutane/1-butene reaction.

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ReceiVed for reView January 26, 2010 ReVised manuscript receiVed April 28, 2010 Accepted June 29, 2010 IE1001873