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SULFATED NANO-CERIA AS A CATALYST OF HEX-1-ENE OLIGOMERIZATION
S. A. Lermontov1*, A.N. Malkova1, L. L. Yurkova1, A. Ye. Baranchikov2, V. K. Ivanov2'3
1 Institute of Physiologically Active Compounds of the Russian Academy of Sciences,
Chernogolovka, Moscow region, Russia
2Kurnakov Institute of General and Inorganic Chemistry of Russian Academy of Sciences,
Moscow, Russia
3Materials Science Department, Moscow State University, Moscow, Russia
[email protected], [email protected], [email protected], [email protected], [email protected]
PACS 82.65.+r, 81.07.Bc, 81.16.Be
Oligomerization of hex-1-ene over acid catalysts obtained by the impregnation of cerium dioxide by sulfate-containing compounds (sulfuric acid or its salts) was studied. Maximum conversion of hexene-1 over sulfated ceria catalysts was 7-12% at 60°C.
Keywords: sulfated ceria, olefins oligomerization, hex-1-ene. 1. Introduction
Traditional acid-catalyzed industrial processes are often criticized because strong mineral acids are used as catalysts. The low cost of these catalysts does not compensate for the costs associated with the production of corrosion-resistant equipment, the transportation and disposal of liquid acid wastes. To solve this problem, sulfated metal oxides, possessing highly acidic properties were developed as an environmentally-safe alternative to mineral acids [1,2]. Superacidic catalysts based on fluorinated oxides are also known [3].
Solid acids including sulfated titania, zirconia, and stannia are among the most promising heterogeneous catalysts [4]. The acidity of sulfated oxides depends on many parameters, such as the nature of the metal, oxide preparation procedure, sulfation method, calcination temperature, particle size, and surface area [4]. Their acidity is much higher than that of concentrated H2 SO4, with their surface sulfate groups promoting the formation of Lewis and Br0nsted acid sites [5]. The Hammett acidity functions H0 for SO4/ZrO2 and SO4/SnO2 range from -16 to -18; while SO4/TiO2 has a slightly lower value of -14.6 [6]. The SO4/ZrO2 is the most comprehensively studied system, because its acidity is one of the highest among the sulfated oxides [5,7-12]. The main advantages of solid acid oxide catalysts include high stability at elevated temperatures, resistance to deactivation and easy regeneration.
Cerium dioxide (CeO2) is a multifunctional inorganic compound holding a great promise for a wide range of technological applications, including catalysis, ultraviolet shielding materials, sensors, electrochromic devices, anticorrosion coatings, abrasive materials, etc.
Cerium dioxide is widely used in redox catalysis and as an effective antioxidant [13]. Recently, cerium dioxide-based catalysts, wherein CeO2 acts as a so-called oxygen buffer, has been widely used. The relative ease of Ce4+ oCe3+ transition in CeO2 makes it possible to accumulate and release oxygen depending on the O2 content in the environment —
cerium dioxide can accumulate oxygen from oxygen-enriched mixtures and, in the presence of reducing agent, release it forming nonstoichiometric CeO2-x [14].
Despite the considerable information on CeO2 catalytic systems, CeO2 has not been studied yet as a solid acid. In this paper, we present the first use of sulfated cerium dioxide (CeO2/SO4) as a solid acid for hex-1-ene oligomerization. Hex-1-ene was chosen as a model long-chain terminal olefin in the oligomerization reaction catalyzed by modified ceruim dioxide. Previously, hex-1-ene oligomerization was used to probe the catalytic activity of both sulfated zirconia [15] and sulfated stannia [16].
2. Experimental
2.1. Catalysts synthesis
All starting materials used in the experiments were of analytical grade.
Ceria nanopowders were synthesized using a previously-described method [17]. A solution of Ce(NOs) 3-6H2O (0.08 mol L in water/isopropanol mixture (1:1 v/v) was rapidly added under vigorous stirring to an aqueous ammonia (3 mol L-1) solution taken in a fivefold molar excess. The resulting mixture was allowed to stir for 3 h at room temperature. Then, a yellow precipitate was separated by centrifugation, thoroughly washed by distilled water and then dried in air at 50°C.
To prepare each catalyst, 4 g of ceria nanopowder and 100 mL of aqueous 2M NH4F, or 1M (NH4)2SO4, or 0.3M H2SO4, or 3M H2SO4 were stirred for 1 h in a glass flask at room temperature, filtered and dried at 100°C.
2.2. Methods
IR spectroscopy was performed on a Spectrum One (Perkin Elmer, USA) spectrometer from the 4000-350 cm-1 region (KBr pellets, 0.25-0.5 % mass sample content). Samples for IR analysis were prepared as follows: a catalyst sample (~ 0.04 g) was calcined in a dry air flow tube reactor at 475°C. After that, the temperature was decreased to 150°C, a bubbler with 1 mL of pyridine was embedded into an air line and a resulting gas-pyridine mixture was passed over the catalyst sample until all pyridine was evaporated.
Low temperature nitrogen adsorption measurements were conducted using an ATX-6 analyzer (Katakon, Russia). Before measurement the samples were outgassed at 200°C for 30 min under dry helium flow. Surface area determination was carried out by the 8-point Brunauer-Emmett-Teller (BET) method.
Powder X-ray diffraction (XRD) analysis was carried out on a Rigaku D/Max 2500 diffractometer (CuKa radiation) with a rotating copper anode (Cu^« irradiation, 5-80° 29 range, 0.02° step). Particle size was estimated using the Scherrer equation. Diffraction maxima were indexed using the PDF2 database.
Transmission electron microscopy images were taken using Leo 912 AB Omega electron microscope operating at 100 kV. Microstructure of the samples was also studied using a Carl Zeiss NVision 40 scanning electron microscope (micrographs were obtained at 1 kV acceleration voltage) equipped with Oxford Instruments X-MAX energy-dispersive X-ray (EDX) analyzer operating at 20 kV acceleration voltage. The samples were not specially prepared (e.g. coated with conducting material) for TEM and SEM measurements. Before EDX analysis the samples were coated with ~5 nm Au/Pd.
NMR spectra were recorded using a Bruker DPX-200 spectrometer in CDCl3. Tetram-ethylsilane (TMS) was used as an external standard.
2.3. Hex-1-ene oligomerization
1 g of a catalyst sample was pre-conditioned in air at a chosen temperature (400, 475, or 500°C) for 2 h in air and then cooled in a dry atmosphere.
A catalyst (1 g) and 0.1 mol of hex-1-ene were then placed into a water-jacketed glass flask and kept at constant temperature (20-60°C) under vigorous stirring. Samples of the reaction mixture were periodically taken for 1H-NMR and GC-MS analysis. A conversion degree was determined by monitoring the disappearance of vinyl group protons disappearance via 1H-NMR.
3. Results and discussion
3.1. Hex-1-ene oligomerization
We found that sulfated ceria acts as an acid catalyst causing hex-1-ene oligomerization at moderate temperatures:
20-60°C
cat hex-2-ene + hex-3-ene + C12 + Cis + C24 + C30 + C36
Catalytic activity of sulfated ceria samples depended strongly on the method of their preparation and the pre-conditioning temperature. The results of catalytic experiments are presented in Table 1.
Table 1. Hex-1-ene oligomerization
Entry Reaction Reaction Calcination Catalyst Conversion,
time, h temperature,°C temperature,°C %
1 24 rt 0-2
2 2 rt 400 0.3M H2SO4/CeO2 0-2
3 1 40 0-2
4 1 60 7
5 1 60 400 3M H2SO4/CeO2 2
6 1 60 500 0.3M H2SO4/CeO2 2
7 24 rt 0
8 1 40 475 1M (NH4)2SO4/CeO2 2
9 1 60 12
10 24 rt-60 475 2M NH4F/CeO2 0-2
11 1 60 475 CeO2 0
Olefin isomerization and oligomerization are acid-catalyzed processes which were initiated by proton addition to a C=C bond (Fig. 1) to yield a secondary carbocation. This intermediate may then add to another molecule of hex-1-ene, yielding the dimeric carbocation shown, or, the initial intermediate may eliminate an H+ to yield the isomerized internal alkene.
Highly acidic conditions are necessary for this process: ZrO2 (-3.0 < H0 < +1.5 [18]) and TiO2 (H0 < -3.0 [19]) showed no catalytic effect in olefin oligomerization. Table 1 shows that sulfated ceria samples acquire acidic properties which are not inherent to unmodified CeO2. The 1M (NH4)2SO4/CeO2 catalyst appeared to be slightly more active than the 0.3M H2SO4/CeO2 and 3M H2SO4/CeO2. A sample calcined at 500°C was relatively inactive.
Fig. 1. Scheme of hex-1-ene oligomerization
Recently, we showed that fluorinated alumina is highly acidic (-13.75 < H0 < -12.7) and have postulated that fluorination, instead of sulfation, might be used for solid superacid preparation [3], but, surprisingly, fluorinated ceria was practically inactive as an acid catalyst in this reaction (entry 10).
3.2. Physical and chemical properties of sulfated ceria
The specific surface area of the most active 1M (NH4)2SO4/CeO2 sample calcined at 475°C was 75 m2/g and did not differ significantly from that of untreated ceria (79 m2/g).
XRD studies have indicated that only CeO2 phase is present in both samples and that the average particle size, as estimated using the Scherrer equation, was 6.7-6.9 nm. These data were also confirmed by transmission electron microscopy (Fm-3m, PDF #34394). Both of the samples (sulfated and unmodified ceria) were highly aggregated powders, consisting of nearly monodisperse particles ~7 nm in size. Note that particle size in starting CeO2 powders prepared by precipitation from water/alcohol solutions was about 4.5 nm. Thus, heating of starting materials at 475°C resulted in an increase in the particle size, which is in line with our previous studies of ceria nanopowders (for example, see [20]).
EDX analysis revealed the presence of ~ 4 at % sulfur on the surface of sulfated samples prepared by pre-conditioning of sulfated ceria at 475°C.
We could not estimate the H0 value for sulfated ceria using relevant organic indicators because of the intensive yellow color of the samples. To determine the types of acidic centers of the 1M (NH4)2SO4/CeO2 catalyst, we used IR spectroscopy data from the pyridine adsorbed onto the surface of the powders. The IR spectra of sulfated ceria samples calcined at 475 C contained bands which were characteristic for pyridinium ions adsorbed on both Lewis (L) and Bronsted (B) acid centers (cm-1) 1445 (L); 1490 (B+L); 1540 (B); 1639 (L) [21, 22] (Fig. 2).
4. Conclusions
In this paper, we have shown that nanocrystalline sulfated CeO2 possesses acidic properties and demonstrated that it acts as an acid catalyst and promotes the oligomerization and isomerization of hex-1-ene.
Acknowledgment
This work was supported by the Russian Foundation for Basic Research (grant no. 11-03-00981-a) and RAS Presidium Programme N 8.
I_i_l_l_l_l_l_l_
1350 1400 1450 1500 1550 1600 1650
Wavenumber, cm'1
Fig. 2. IR spectroscopy data of pyridine adsorbed onto the surface of the 0.3M H2SO4/CeO2 and 1M (NH4^SO4/CeO2 catalyst
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