ARTICLE pubs.acs.org/JPCC
Selective Isobutene Oligomerization by Mesoporous MSU-SBEA Catalysts Dong Ho Park,†,‡ Seong-Su Kim,‡ Thomas J. Pinnavaia,‡ Francisco Tzompantzi,§ Julia Prince,|| and Jaime S. Valente*,|| †
Department of Biomedicinal Chemistry, Inje University, Gimhae, Korea Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, United States § Departamento de Quimica, UAM-Iztapalapa, Av. San Rafael Atlixco # 186, 09340 Mexico D.F., Mexico Instituto Mexicano del Petroleo, Eje Central # 152, 07730 Mexico D.F., Mexico
)
‡
ABSTRACT: Hydrolytically stable mesostructured aluminosilicates assembled from protozeolitic precursors are examined as acid catalysts in the isobutene oligomerization reaction. Steady state conversions as high as 80% at 60 °C are achieved, depending on the isobutene:butane feed composition and space velocity. Most notably, the product selectivity to dimeric C8= and trimeric C12= olefins is readily fine-tuned simply by changing the feed composition. Undesirable high molecular weight oligomers are not produced in any case. The superior catalytic performance of the mesostructured aluminosilicates in comparison to commercial USY and BEA zeolites is clearly established. The improvement in catalytic performance is attributed to a combination of textural and acidic properties, as judged by N2 adsorption and pyridine adsorption studies.
1. INTRODUCTION Oligomerization of light (C2C5) olefins over phosphoric acid impregnated on solid supports is a process that has been carried out industrially for several decades.1 In particular, oligomerization of isobutene and isobutane in the C4 stream of FCC units has received considerable attention in recent years. Dimerization of isobutene yields C8= branched olefins, mainly 2,4,4trimethyl-2-pentene and 2,4,4-trimethyl-1-pentene, which, after hydrogenation, produces 2,2,4-trimethylpentane, a clean, high octane gasoline (RON = 100, MON = 100) with low sulfur and aromatic content.24 The C12= trimers formed in the process may be used for plastic production and for the synthesis of specialty chemicals such as neo-acids2,5,6 and premium solvents and as diesel and jet fuel additives.6 Owing to increasingly stringent environmental regulations on the use of oxygenated gasoline additives such as MTBE, a surplus of isobutene is expected, as it is the main raw material for MTBE synthesis. In this context, catalytic isobutene oligomerization and its use in the alkylation of isobutane has acquired renewed importance, particularly with regard to processes that may provide high C8= and C12= selectivity while minimizing the formation of high molecular weight oligomers (gC16=). Due to environmental concerns associated with the use of phosphoric acid as a catalyst, alternative processes are being investigated. Sulfonic acid resins have been employed for the simultaneous etherification and oligomerization of isobutene to MTBE and isooctenes, respectively.4 These resins are also r 2011 American Chemical Society
effective for isobutene dimerization in the presence of an alcohol as a C8= selectivity enhancer.79 However, ethers typically are produced as side products. Another alternative process is reactive distillation using a dry ion-exchange resin as a catalyst.10 Several acid solids exhibit catalytic activity for isobutene oligomerization, including sulfated zirconia,11 sulfated titania,2,12,13 nickel-doped zeolites,14,15 nickel supported on sulfated zirconia,5 and NiO-W2O3/Al2O316,17 among others. The reaction is usually performed with gas hourly space velocities (GHSV) in the 520 h1 range in order to attain acceptable conversions. Since isobutene oligomerization requires a regenerable and stable catalyst with both Lewis and Br€onsted acid sites, zeolites are attractive acid catalysts. However, when a typical acidic zeolite such as zeolite Beta (BEA) is used as a catalyst, deactivation is observed after approximately 4 h on stream.18 Although 100% conversion is obtained with ∼60% selectivity to C12= products at a weight hourly space velocity (WHSV) of 10 h1, the conversion steadily decreases at a WHSV of 20 h1.6 H-ZSM-57 zeolite remains stable and provides high conversion and product selectivity over reaction times of several days, but the WHSV is low (between 2 and 7 h1).19 ZSM-5 zeolite also is highly selective for isobutene dimerization, but deactivation is observed at a WHSV of 20 h1.20 Received: November 18, 2010 Revised: February 23, 2011 Published: March 11, 2011 5809
dx.doi.org/10.1021/jp111642b | J. Phys. Chem. C 2011, 115, 5809–5816
The Journal of Physical Chemistry C
ARTICLE
Table 1. Textural Properties, as Determined by N2 Physisorption and Si/Al Molar Ratio sample
BET SA (m2 g1)
MSU-S/WBEA MSU-S/SBEA
a
Langmuir SA (m2 g1)
PV (cm3 g1)
t-plot micro PV (cm3 g1)
PD (nm)
Si/Al molar ratioa
718
1.6b
0.00
4b
43.2
529
b
1.8
0.00
21b
45.0
BEA
811
0.27c
0.07
0.5c
45.7
USY
867
0.27c
0.22
1.6c
2.6
As determined by ICP-AES. b Determined by the BJH method applied to the adsorption branch. c Determined by the HK method.
In oligomerization reactions, the fast deactivation rates observed with zeolites may be due to their microporosity, which limits mass transfer, particularly the departure of bulky oligomer products. Entrapped molecules deposit as a carbonaceous residue that deactivates the catalysts. The development of mesostructured aluminosilicates has intended to overcome diffusional limitations of zeolites. Nevertheless, noncrystalline frameworks display low acidity and poor hydrothermal stability, limiting their catalytic applications.21 An alternative approach that has been explored is the preparation of microporousmesoporous zeolitic composites, so-called hierarchical porous materials, by partial crystallization of preformed mesoporous materials.22 We previously succeeded in preparing mesostructured aluminosilicates, using as framework precursors protozeolitic nanoclusters, or zeolite seeds, which contain secondary subunits of the zeolite structures that they nucleate. These derivatives, denoted MSU-S mesophases, exhibited exceptional hydrothermal stability and strong acidity.2328 Said properties are believed to arise from zeolitic subunits (AlO4 and SiO4 tetrahedra) in the framework walls. Furthermore, the mesoporosity, large specific surface areas, and pore volumes displayed by these materials may facilitate diffusion of bulky hydrocarbon molecules. Thus, mesostructured aluminosilicates possess all the features required for an effective oligomerization catalyst. Here, we examine the catalytic activity, selectivity, and stability of mesostructured aluminosilicates assembled from zeolite Beta seeds for the oligomerization of isobutene. Commercial USY and BEA zeolite catalysts are also tested as a comparison. We show that the product selectivity can be regulated to produce mainly C8= or C12= products simply by varying the feed composition, while avoiding production of undesirable high molecular weight oligomers. High conversions are attained, at higher GHSV in comparison to those reported previously.
2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. Zeolite Beta seeds with a nominal Si/ Al ratio of 44 were prepared according to previously described methods.26,27 Aluminum tri-sec-butoxide was dissolved in a tetraethylammonium hydroxide solution. A tetraethylorthosilicate solution in water was then added under stirring. The mixture was aged overnight and then heated at 100 °C for 6 h. For the preparation of a mesoporous aluminosilicate with a wormhole motif, denoted MSU-S/WBEA, the Beta seeds were added under stirring to a surfactant solution, containing tallow tetraamine, water, and HCl. The mixture was heated at 45 °C for 20 h and then at 80 °C for 20 h. A nonmesostructured analogue, denoted MSU-S/SBEA, was synthesized by digestion of BEA seeds at 150 °C for 20 h in the absence of surfactant as a mesoporous structure directing agent. The final products were recovered by filtration, dried, and calcined at 600 °C for 4 h. As a comparison, we tested commercial BEA and USY zeolites with Si/Al ratios of
45 and 2.5, respectively, containing NH4þ cations, purchased from Zeolyst. Prior to catalytic testing, samples were activated by calcination at 600 °C for 4 h. The Si/Al ratio of all solids was found to be close to the nominal one, within experimental error (Table 1). 2.2. Characterization Techniques. Chemical Analyses. The Si/Al ratio of solids was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a PerkinElmer model Optima 3200 Dual Vision spectrometer. Powder X-ray Diffraction. The X-ray patterns were measured in a D-500 diffractometer, with Cu KR1,2 radiation, and a graphite secondary beam monochromator. The high angle diffraction patterns were measured from 4 to 80° 2θ using a step size of 0.02 and step scan of 2 s. Low angle patterns were recorded from 0.7 to 5° 2θ, using a step size of 0.01 and step scan of 2 s. Nitrogen Physisorption. N2 adsorptiondesorption isotherms were obtained at 196 °C on a Micromeritics Tristar 3000 sorptometer using standard procedures. Calcined MSU-S/ WBEA, MSU-S/SBEA, BEA, and USY were outgassed at 150 °C and 106 Torr for 20 h. BET surface areas were calculated for MSU-S/WBEA and MSU-S/SBEA from the linear part of the BET plot according to IUPAC recommendations. The Langmuir method was employed to determine the zeolites’ surface area. Micropore volume and micropore surface areas were evaluated from t-plots. The BarrettJoynerHalenda (BJH) method was used to estimate the pore size distributions from the adsorption branch of the isotherms for mesoporous solids MSU-S/WBEA and MSU-S/SBEA. The pore size distributions and pore volumes of microporous zeolites were calculated by the HorvathKawazoe (HK) method. Transmission Electron Microscopy. Powdered samples were analyzed through transmission electron microscopy (TEM) in a Jeol 200 Kv JEM-2200FS. The microscope is equipped with a Schottky-type field emission gun in ultra high-resolution (UHR) configuration (Cs = 0.5 mm; Cc = 1.1 mm; point-to-point resolution, 0.19 nm) and in-column omega-type energy filter. Samples were dispersed in ethanol before placing them on the copper grid with Formvar support. Infrared Spectroscopy. FTIR-pyridine adsorption was used to determine the relative strength and abundance of acid sites on calcined samples. Experiments were carried out in a Fourier transform infrared (FTIR) spectrometer Perkin-Elmer model 170-SX. Prior to pyridine adsorption, the sample was heated at a rate of 20 °C/min in a vacuum to a temperature of 400 °C, cooled to room temperature, and exposed for 20 min to pyridine vapor, by breaking a capillary tube containing 100 μL of pyridine. After adsorption, the infrared spectrum of adsorbed pyridine was recorded at the desired temperatures in a vacuum. The concentration of Br€onsted and Lewis acid sites was obtained using a standard procedure based on the Lambert Beer law.29 The absorbency (AI) is determined as the integrated 5810
dx.doi.org/10.1021/jp111642b |J. Phys. Chem. C 2011, 115, 5809–5816
The Journal of Physical Chemistry C
Figure 1. Low angle X-ray diffraction patterns of catalysts MSU-S/ WBEA and MSU-S/SBEA.
area under the curve as follows:
Z
AI ¼ B 3 C 3
ev dv
R where evdv is the extinction coefficient and it is proportional to 0.4343 3 Iv. Iv values were taken from ref 30 as Iv = 1.0086 and 0.9374 cm μmol1 for the bands at 1545 and 1450 cm1, respectively. B is related to the weight/area (g cm2) wafer ratio, and C is the concentration of Br€ onsted or Lewis acid sites, which can be calculated as CBro::nsted ¼
CLewis ¼
AI ð1545 cm1 Þ weight ð0:4343 3 1:0086Þ area 3
AI ð1450 cm1 Þ weight ð0:4343 3 0:9374Þ area 3
2.3. Isobutene Oligomerization. The catalytic activity for the isobutene oligomerization reaction was determined using a fixed bed glass reactor with a volume of 5 mL and using 0.2 g of catalyst. Catalysts were activated at 400 °C for 12 h in flowing (2 cm3 s1) air. Afterward, the temperature was lowered to 60 °C and then a 50:50 or 70:30 (w/w) mixture of isobutane/isobutene was fed into the reactor at atmospheric pressure, in order to ensure that the hydrocarbon mixture was in the gas phase. For the first test reaction, the GHSV was constant (32.8 h1), and a 50:50 (w/w) mixture of isobutane/isobutene was used, for 30 h of reaction. In the second test reaction, the GHSV value was varied at 32.8, 65.7, and 131.5 h1 with a 70:30 (w/w) mixture of isobutane/isobutene, for 30 h of reaction. The analysis of products was made with a Varian Star 3600 CX gas chromatograph equipped with a FID detector and a 24 ft 20% MBEA column coupled to the reactant system. The conversion is reported in terms of the fraction of isobutene depleted, and the selectivity is reported as C8= and C12= mole fractions.
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The morphology, structure, and properties of the MSU-S mesostructured aluminosilicates assembled from protozeolitic precursors, or zeolite seeds, have
ARTICLE
Figure 2. Wide angle X-ray diffraction patterns of indicated catalysts.
been the subject of several previous reports.2328 The main characteristics of the catalysts MSU-S/WBEA and MSU-S/SBEA, and of the commercial catalysts USY and BEA, are summarized here. Surfactant-templated MSU-S/WBEA exhibits a single, broad low-angle Bragg diffraction peak (Figure 1), consistent with the formation of a wormhole-like mesostructure, as expected from the use of a neutral amine surfactant (tallow tetraamine) as the structure directing agent. On the other hand, sample MSU-S/ SBEA displays no diffraction peaks in this region, as anticipated, since no mesoporogen was used in this case. With regard to the wide angle region (Figure 2), the commercial zeolite samples USY and BEA exhibit the characteristic sharp diffraction peaks of the corresponding crystalline zeolites. Conversely, both MSU-S/WBEA and MSU-S/SBEA present only very broad scattering consistent with a proto-crystalline structure with no long-range atomic order. Although the broad reflection centered at about ∼22° 2θ corresponds to the main diffraction peak of zeolite BEA, there are insufficient crystal layers to generate coherent diffraction. Nevertheless, the presence of local order consistent with the secondary subunits of BEA is clearly evident based on IR spectroscopy. The presence of the subunits is verified by vibrational bands in the 550600 cm1 region, characteristic of the five-membered ring subunits of a Beta zeolite structure.24 Additionally, 27Al MAS NMR revealed that more than 90% of the aluminum centers occupy tetrahedrally coordinated sites,24 in agreement with a predominantly protozeolitic framework. Transmission electron micrographs of MSU-S/WBEA and MSU-S/SBEA are presented in Figure 3. For MSU-S/WBEA, the formation of a wormhole-like mesostructure is clearly evident, corroborating low angle XRD observations. Images of sample MSU-S/SBEA corroborate the formation of noncrystalline aluminosilicate nanoparticles in aggregated form. The size of the fundamental particles comprising the aggregates is more or less centered near 20 nm. The aggregation of the said particles creates interparticle mesoporosity. Furthermore, no microcrystalline domains or zeolitic islands were detected by TEM, in agreement with previous observations.27,31,32 Thus, these materials should be distinguished from microporousmesoporous composites, or so-called hierarchical porosity zeolites. TEM images of commercial zeolites BEA and USY are displayed in Figure 4 for comparison purposes. BEA zeolite presents characteristic spheroidal crystals, with cubic lattice fringes clearly distinguishable at 5811
dx.doi.org/10.1021/jp111642b |J. Phys. Chem. C 2011, 115, 5809–5816
The Journal of Physical Chemistry C
ARTICLE
Figure 3. TEM images at different magnifications of catalysts: (a and b) MSU-S/WBEA; (c and d) MSU-S/SBEA.
Figure 5. N2 physisorption isotherms and corresponding pore size distributions for indicated catalysts.
Figure 4. TEM images at different magnifications of commercial catalysts: (a and b) BEA; (c and d) USY.
Figure 6. FTIR spectra of pyridine remaining adsorbed on MSU-S/ WBEA and MSU-S/SBEA after desorption at (a) 50 °C, (b) 100 °C, (c) 200 °C, (d) 300 °C, and (e) 400 °C.
20 nm magnification. USY zeolite exhibits larger crystals with lamellar lattice fringes. Figure 5 presents the N2 physisorption isotherms for the four catalysts tested. Sample MSU-S/WBEA exhibits a type IV isotherm,33 in conformity with a wormhole-like mesostructure with a network of interconnected pores. The uptake step in the region 0.2 < P/Po < 0.4 is indicative of the filling of the framework-confined mesopores, whereas the hysteresis loop at high partial pressure corresponds to textural mesoporosity. The pore size distribution (inset) shows a very narrow distribution
centered around 4 nm. Sample MSU-S/SBEA, synthesized from hydrothermally treated zeolite Beta seeds in the absence of a mesoporogen, presents a sharp increase in adsorbed volume at high partial pressures, corresponding to interparticle porosity. The hysteresis loop is of the H1 type, attributed to uniform pore size distribution.33 In agreement with this, the BJH pore size distribution shows a narrow distribution in the range 330 nm, with a maximum around 20 nm. Microporosity, as analyzed by the t-plot method, was not detected in either MSU-S sample. USY and BEA zeolites display Langmuir type isotherms 5812
dx.doi.org/10.1021/jp111642b |J. Phys. Chem. C 2011, 115, 5809–5816
The Journal of Physical Chemistry C
ARTICLE
characteristic of microporous solids. Surface areas, total and micro pore volumes, and average pore diameters are summarized in Table 1. Infrared spectra of pyridine adsorbed on MSU catalysts are shown in Figure 6; corresponding spectra on BEA and USY catalysts are displayed in Figure 7. Characteristic bands that typically are ascribed to Lewis acid sites appear at approximately 1637, 1621, 1594, 1580, and 1444 cm1.34,35 The band at 1621 cm1 is due to pyridine coordinated to Lewis acid sites of moderate strength; it has also been assigned to tetrahedral aluminum vacancies.36 The bands at 1594 and 1580 cm1 are ascribed to strong and weak Lewis acid sites, respectively. Furthermore, a band around 1544 cm1, characteristic of Br€onsted sites,37 is present in all cases, although the intensity of this band for MSU-S/SBEA is very low. The band at 1490 cm1 is due to the vibration of the pyridine ring adsorbed on either Lewis or Br€onsted sites. As the temperature increases, pyridine is gradually desorbed, and the fraction remaining is attached to the sites with greater chemisorption strength. Therefore, the strength of the acid sites is related to the temperature at which pyridine is retained. The amounts of pyridine (μmol g1) that remain adsorbed upon increasing temperatures are presented in Table 2. Notably, MSUS/SBEA initially has nearly half the Br€onsted sites of MSU-S/ WBEA, but some of these sites are so strong that they retain pyridine up to 400 °C. Commercial BEA zeolite has stronger and more numerous Br€onsted basic sites than either of the two MSUS compositions. Comparatively, USY presents fewer and weaker Br€onsted sites than BEA.
Lewis sites are also more abundant on MSU-S/WBEA than on MSU-S/SBEA. Compared to BEA, both MSU-S catalysts have a larger amount of Lewis acid sites; these sites are of comparable strength in the three samples. The Lewis acidity of MSU-S agrees well with aforementioned 27Al MAS NMR observations. At a temperature of 100 °C, the Lewis to Br€onsted acid site ratio is ∼4.0 for both MSU-S/WBEA and MSU-S/SBEA, while it is rather lower in BEA and USY (0.7 and 1.5, respectively). Br€onsted acid sites are considered as the active sites for oligomerization, because solid phosphoric acid and sulfonic acid resins have been employed as catalysts with some success.1,4,79 The reaction is thought to proceed via a carbenium ion mechanism in the case of sulfonic acid catalysts.38 However, in the case of zeolites and related solid acid catalysts, a high Lewis to Br€onsted acid site ratio has been proposed to increase catalytic activity and stability.13,39 A synergetic interaction between Lewis and Br€onsted acid sites, where the Lewis sites aid stable performance, has been suggested.39 3.2. Isobutene Oligomerization. Figure 8 presents isobutene conversions over mesostructured MSU catalysts and compares them to the conversions achieved for commercial BEA and USY zeolite catalysts. The deactivation constants, calculated according to Levenspiel’s model for independent deactivation assuming a second-order deactivation, are presented in Table 2.40 The pronounced deactivation observed for all catalysts over the first 4 h of reaction very likely is due to extended oligomerization reactions. High molecular weight oligomers may be formed and
Figure 7. FTIR spectra of pyridine remaining adsorbed on commercial zeolites BEA and USY after desorption at (a) 50 °C, (b) 100 °C, (c) 200 °C, (d) 300 °C, and (e) 400 °C.
Figure 8. Isobutene conversion for a 50:50 (w/w) feed of isobutane: isobutene at 60 °C, 760 Torr, and 32.8 h1 GHSV over the indicated catalysts.
Table 2. Br€ onsted (B) and Lewis (L) Acidity, Determined by Pyridine Adsorption Followed by FTIR, and Deactivation Constants amount of pyridine retained (μmol g1) 100 °C
sample
B
MSU-S/WBEA MSU-S/SBEA BEA USY a
200 °C
300 °C
B
400 °C
L
B
L
Lewis to Br€onsted
deactivation constants
acid site ratio
Kda (h1)
L
B
L
165
719
126
114
75
91
0
86
4.4
0.1028
88
395
84
62
33
38
24
46
4.5
0.1338
262 195
175 301
247 98
104 61
204 14
80 29
144 0
67 27
0.7 1.5
0.2060 0.6752
Calculated from isobutene conversion data for a 50:50 (w/w) feed of isobutane:isobutene at 60 °C, 760 Torr, and 32.8 h1 GHSV. 5813
dx.doi.org/10.1021/jp111642b |J. Phys. Chem. C 2011, 115, 5809–5816
The Journal of Physical Chemistry C
Figure 9. Isobutene oligomerization selectivity achieved over zeolitic and MSU-S catalysts at different reaction times (time-on-stream, TOS). The reaction conditions were as follows: 50:50 (w/w) isobutane: isobutene feed ratio; 60 °C; 760 Torr; 32.8 h1 GHSV.
remain strongly adsorbed to the surface. Alternatively, if these large molecules are desorbed, it is possible that they are unable to exit through pore channels. In both cases, the oligomers are deposited as a carbonaceous residue that deactivates the catalysts. As seen in Table 2, the commercial catalysts show the fastest deactivation rates. In these cases, it is likely that internal diffusion limitations play a significant role. USY drops from an initial conversion of 43% to only 5% after 6 h, though the conversion remains nearly stable at 5% for up to 30 h. The deactivation of zeolite BEA is slightly less in comparison to the USY catalyst, as judged by the sustained 10% conversion for over 30 h TOS. This agrees with previous reports that indicate that zeolite BEA is more catalytically stable than USY,6 possibly because of the greater strength of the acid sites. Another possibility is that the smaller crystals of BEA zeolite, compared to those of USY (see Figure 4), facilitate the interparticle molecular diffusion. Notably, mesostructured MSU-S/WBEA and MSU-S/SBEA are significantly more active and stable in comparison to the zeolites. An initial loss of activity is observed in both cases, probably due to the formation of high molecular weight oligomers that remain adsorbed to the surface. It is very likely that the extended oligomerization reactions take place over the most acidic sites. Accordingly, catalyst MSU-S/SBEA, with the strongest Br€onsted acid sites, has a higher initial deactivation rate than MSU-S/ WBEA. However, once the most acidic sites have been deactivated, medium strength sites take over. Thus, both catalysts show stable conversions for at least 30 h on stream. In particular, the wormhole-like mesostructure of MSU-S/WBEA favors high conversions of ∼35%, as these materials provide three-dimensional mesoporosity that reduces pore length and diffusional rate limitations. The improved textural properties of the mesoporous catalysts should have a significant effect on catalyst stability. Larger pores facilitate mass transfer to and from the active sites, whereas in zeolites the reaction is likely controlled by internal diffusion. The acidity promoted by the protozeolitic framework structure plays a determinant role. Since the strongest sites are probably deactivated at the first moments of reaction, the remaining acid sites of both mesostructured samples are strong enough to carry out the reaction with high conversions and
ARTICLE
excellent selectivities. As a comparison, amorphous silica aluminas and Al-containing micelle templated silicas, with comparable surface areas but much weaker Lewis and Br€onsted acidity, have also been tested as catalysts for this reaction. Fast deactivation rates and low conversions were obtained in all cases.41 Also, a high Lewis to Br€onsted acid site ratio has been proposed to increase catalyst stability.13,39 Both MSU-S catalysts have fewer Br€onsted sites and more Lewis sites than the zeolites, as judged by the amount of pyridine retained at 100 °C (Table 2), which gives rise to a higher Lewis to Br€onsted acid site ratio. As we noted above, Lewis sites have been proposed to participate indirectly in this reaction, by means of a synergetic effect between both sites, increasing catalyst stability. Figure 9 presents the time-dependent selectivity of the mesoporous catalysts, in comparison to the commercial zeolite catalysts. Reaction products at these conditions are mainly C8= (2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene isomers), with a selectivity g70% in all cases, and a fraction of C12= isomers (2,2,6,6-tetramethyl-4-methylideneheptane and 2,2,4,6,6-pentamethyl-3-heptene). High molecular weight oligomers (C16= and higher) are not detected in any case. For the commercial USY zeolite, the selectivity to C8= decreases over 24 h of reaction time, and then suddenly increases again, reaching 100% selectivity after 30 h. Selectivity to C8= over BEA is initially 86%, and it continues to increase over time, reaching 99% selectivity after 30 h time on stream. This behavior could indicate that C12= oligomers remain adsorbed on the surface, or are unable to exit through the microporous zeolite channels, depositing as a carbonaceous residue that eventually causes the deactivation of the catalyst. It is possible that high molecular weight oligomers (C16= þ) are formed but unable to leave, remaining undetected. For the mesoporous MSU-S/WBEA catalyst, the C8= selectivity is initially over 80%. With longer reaction times, there is a decrease in C8= selectivity, to ∼70%. On the other hand, catalyst MSU-S/SBEA, lacking in surfactant-templated mesopores, has an average 75% selectivity to C8=, oscillating between 70 and 79% with time on stream. The stable selectivity to C8= and C12= in the case of MSU-S/SBEA could be attributed to the strong Br€onsted acid sites of this sample, and to the large 20 nm average pore size that may facilitate diffusion of products out of the mesopores. Catalyst MSU-S/SBEA was chosen to study the effect of space velocity and feed composition on selectivity and deactivation. The effect of space velocity on conversion is depicted in Figure 10 for a 70:30 (w/w) isobutaneisobutene feed mixture. As expected, the highest conversion, around 80%, is obtained at the lowest space velocity of 32.8 h1 GHSV, and this steady state condition was maintained for up to 30 h on stream. An increase in GHSV to 65.7 h1 decreases conversion slightly, due to the decrease in contact time. With higher space velocities (131.5 h1), catalytic activity is lost rapidly, probably due to the adsorption of high molecular weight oligomers that cannot be desorbed fast enough and block active sites. It is generally accepted that catalytic performance declines rapidly when GHSV is increased. Nevertheless, the lowest GHSV (32.8 h1) used in this work is still much higher in comparison to the GHSV/WHSV values used in previous studies of this reaction.1,2,5,6,1114,1618,39,41 The selectivity to C8= and C12= products under the abovementioned conditions is shown in Figure 11. It is particularly noteworthy that, for a 70:30 isobutane:isobutene feed composition, the reaction is highly selective (∼90%) toward C12= formed 5814
dx.doi.org/10.1021/jp111642b |J. Phys. Chem. C 2011, 115, 5809–5816
The Journal of Physical Chemistry C
Figure 10. Isobutene conversion for a 70:30 (w/w) feed of isobutane isobutene over MSU-S/SBEA, at 60 °C, 760 Torr, and different values of GHSV.
ARTICLE
4. CONCLUSIONS In summary, hydrothermally stable mesostructured aluminosilicates are shown, on the basis of pyridine adsorption studies, to have strong Lewis and Br€onsted acid sites, comparable in strength to those of USY and BEA commercial zeolites. This property as well as the mesoporosity and large surface area that characterize mesostructured materials make them excellent catalysts for isobutene oligomerization. Conversions as high as 80% are achieved at a GHSV of 32.8 h1 with retention of activity for reaction periods over 30 h on stream. The reactivity, selectivity, and longevity of these catalysts are superior to commercial zeolite catalysts and to other acid catalysts that have been reported for isobutene oligomerization. Furthermore, selectivity to the dimeric and trimeric products of interest (C8= or C12=) can be adjusted simply by changing the feed composition. ’ AUTHOR INFORMATION Corresponding Author
*Phone: þ52 (55) 91 75 84 44. E-mail:
[email protected],
[email protected].
’ ACKNOWLEDGMENT J.S.V. gratefully acknowledges the Mexican Academy of Science and the United States-Mexico Foundation for Science for a research grant. ’ REFERENCES
Figure 11. Isobutene oligomerization selectivity for a 70:30 (w/w) feed of isobutaneisobutene over MSU-S/SBEA, at 60 °C, 760 Torr, and different values of GHSV.
through the consecutive reaction iC4= f iC8= f iC12=. Also, the selectivity is not dependent on space velocity, suggesting that undesirable high molecular weight oligomers (C16= and higher) are not formed. The increase in total conversion observed when modifying the feed from a 50:50 to a 70:30 (w/w) mixture of isobutaneisobutene may be due to the accompanying increase in active site to isobutene ratio. Also, when a lower concentration of iC4= is fed to the reactor, there should be a larger fraction of sites available on the surface, which would increase the probability of readsorbing a dimerized C8= molecule, producing C12= in larger amounts. Thus, as the fraction of iC4= in the feed is increased, conversion is decreased, along with a decreased probability of C8= to readsorb. Therefore, higher selectivity to C8= is obtained as the fraction of isobutene in the feed is increased. Notably, the effect of feed composition on product selectivity has not been reported previously, even though numerous experiments have been performed under very different reaction conditions. It has been reported, however, that C8= production increases and C12= formation decreases as the reaction time advances and the catalyst deactivates. This supports the idea that a lower fraction of available sites favors C8= production.42
(1) de Klerk, A.; Leckel, D. O.; Prinsloo, N. M. Ind. Eng. Chem. Res. 2006, 45, 6127. (2) Mantilla, A.; Tzompantzi, F.; Ferrat, G.; Lopez-Ortega, A.; Alfaro, S.; Gomez, R.; Torres, M. Catal. Today 2005, 107, 707. (3) Brown, S. H.; Mathys, G. M. U.S. Patent 2009/01345646A1, 2009. (4) Di Girolamo, M.; Lami, M.; Marchionna, M.; Pescarollo, E.; Tagliabue, L.; Ancillotti, F. Ind. Eng. Chem. Res. 1997, 36, 4452. (5) Tzompantzi, F. J.; Manriquez, M. E.; Padilla, J. M.; Del Angel, G.; Gomez, R.; Mantilla, A. Catal. Today 2008, 133, 154. (6) Yoon, J. W.; Chang, J.-S.; Lee, H.-D.; Kim, T.-J.; Jhung, S. H. J. Catal. 2007, 245, 253. (7) Honkela, M. L.; Root, A.; Lindblad, M.; Krause, A. O. I. Appl. Catal., A 2005, 295, 216. (8) Di Girolamo, M.; Marchionna, M. J. Mol. Catal. A 2001, 177, 33. (9) Di Girolamo, M.; Tagliabue, L. U.S. Patent 6,011,191, 2000. (10) Talwalkar, S.; Mankar, S.; Katariya, A.; Aghalayam, P.; Ivanova, M.; Sundmacher, K.; Mahajani, S. Ind. Eng. Chem. Res. 2007, 46, 3024. (11) Chellappa, A. S.; Miller, R. C.; Thomson, W. J. Appl. Catal., A 2001, 209, 359. (12) Mantilla, A.; Tzompantzi, F.; Ferrat, G.; Lopez-Ortega, A.; Romero, E.; Ortiz-Islas, E.; Gomez, R.; Torres, M. Chem. Commun. 2004, 1498. (13) Mantilla, A.; Ferrat, G.; Lopez-Ortega, A.; Romero, E.; Tzompanzti, F.; Torres, M.; Ortiz-Islas, E.; Gomez, R. J. Mol. Catal. A 2005, 228, 333. (14) Zhang, X.; Zhong, J.; Wang, J.; Gao, J.; Liu, A. Catal. Lett. 2008, 126, 388. (15) Zhang, X.; Zhong, J.; Wang, J.; Zhang, L.; Gao, J.; Liu, A. Fuel Process. Technol. 2009, 90, 863. (16) Tzompantzi, F.; Mantilla, A.; Del Angel, G.; Padilla, J. M.; Fernandez, J. L.; Diaz-Gongora, J. A. I.; Gomez, R. Catal. Today 2009, 143, 132. 5815
dx.doi.org/10.1021/jp111642b |J. Phys. Chem. C 2011, 115, 5809–5816
The Journal of Physical Chemistry C
ARTICLE
(17) Tzompantzi, F.; Valverde-Herrera, M.; Rodríguez-Gonzalez, J.; Gomez, R.; Mantilla, A.; Fernandez, J. L.; Ortiz, H. Chem. Eng. Commun. 2009, 196, 1198. (18) Villegas, J. I.; Kumar, N.; Heikkil€a, T.; Lehto, V.-P.; Salmi, T.; Murzin, D. Y. Top. Catal. 2007, 45, 187. (19) Mathys, G. M. K.; Dakka, J. M.; Mertens, M. M.; Martens, J. A.; Mishin, I. V.; Ravishankar, R.; Eijkhoudt, R. U.S. Patent 2004/ 0006250A1, 2004. (20) Nurminen, M.; Pyhalahti, A.; Siira, P.; Tiitta, M. U.S. Patent 2004/0181106A1, 2004. (21) Corma, A. Chem. Rev. 1997, 97, 2373. (22) Egeblad, K.; Christensen, C. H.; Kustova, M.; Christensen, C. H. Chem. Mater. 2008, 20, 946. (23) Liu, Y.; Zhang, W.; Pinnavaia, T. J. J. Am. Chem. Soc. 2000, 122, 8791. (24) Liu, Y.; Zhang, W.; Pinnavaia, T. J. Angew. Chem., Int. Ed. 2001, 40, 1255. (25) Liu, Y.; Pinnavaia, T. J. J. Mater. Chem. 2002, 12, 3179. (26) Liu, Y.; Pinnavaia, T. J. Chem. Mater. 2002, 14, 3. (27) Liu, Y.; Pinnavaia, T. J. J. Mater. Chem. 2004, 14, 1099. (28) Liu, Y.; Pinnavaia, T. J. J. Mater. Chem. 2004, 14, 3416. (29) Paukshtis, E. A.; Yurchenko, E. N. Russ. Rev. 1983, 52, 3. (30) Emeis, C. A. J. Catal. 1993, 141, 347. (31) Bagshaw, S. A.; Jaenicke, S.; Khuan, C. G. Ind. Eng. Chem. Res. 2003, 42, 3989. (32) Bagshaw, S. A.; Jaenicke, S.; Khuan, C. G. Catal. Commun. 2003, 4, 140. (33) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: London, 1999. (34) Yang, T. S.; Chang, T. H.; Yeh, C. T. J. Mol. Catal. A 1997, 115, 339. (35) Sun, Y.; Yuan, L.; Ma, S.; Han, Y.; Zhao, L.; Wang, W.; Chen, C. L.; Xiao, F. S. Appl. Catal., A 2004, 268, 17. (36) Morterra, C.; Coluccia, S.; Chiorino, A.; Boccuzzi, F. J. Catal. 1978, 54, 348. (37) Parry, E. P. J. Catal. 1963, 2, 371. (38) Alcantara, R.; Alcantara, E.; Canoira, L.; Franco, M. J.; Herrera, M.; Navarro, A. React. Funct. Polym. 2000, 45, 19. (39) Jhung, S. H.; Chang, J.-S. Catal. Surv. Asia 2009, 13, 229. (40) Levenspiel, O. J. Catal. 1972, 25, 265. (41) Chiche, B.; Sauvage, E.; Di Renzo, F.; Ivanova, I. I.; Fajula, F. J. Mol. Catal. 1998, 134, 145. (42) Yoon, J. W.; Lee, J. H.; Chang, J.-S.; Choo, D. H.; Lee, S. J.; Jhung, S. H. Catal. Commun. 2007, 8, 967.
5816
dx.doi.org/10.1021/jp111642b |J. Phys. Chem. C 2011, 115, 5809–5816
