Synthesis of Solid Acid Catalysts Based on TiO₂- SO₄^2-and Pt/TiO₂- SO₄^2- Applied in n-Hexane Isomerization

Juan Manuel Hernández Enríquez, Luz Aracely Cortez Lajas, Ricardo García Alamilla, Estefanía Ángeles San Martín, Pedro García Alamilla, Edward Brent Handy, Guadalupe Cárdenas Galindo, Luz Arcelia García Serrano

CIEP, Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, Av. Dr. Manuel Nava #6, Zona Universitaria, 78210 San Luis Potosí, San Luis Potosí, México.
Instituto Politécnico Nacional, Centro Interdisciplinario de Investigaciones y Estudios sobre Medio Ambiente y Desarrollo 30 de Junio #1520, Barrio La Laguna Ticomán, 07340 México D.F., México.
Instituto Tecnológico de Cd. Madero, División de Estudios de Posgrado e Investigación, Juventino Rosas y Jesús Urueta S/N, Col. Los Mangos, 89440 Cd. Madero, Tamaulipas, México.
Universidad Juárez Autónoma de Tabasco, División Académica de Ciencias Agropecuarias, Avenida Universidad S/N, Zona Cultura, Colonia Magisterial, 86040 Villahermosa Tabasco, México.

DOI: 10.4236/ojmetal.2013.33006                

Abstract

The physicochemical properties and catalytic activity of pure and sulfated titanium oxide (TiO₂ and ) is described in this work. Titanium hydroxide synthesized by the sol-gel method was impregnated with a 1 N H₂SO₄ solution, varying amount of sulfate ions () in the range from 10 to 20 wt%. Pure and modified hydroxides were calcined at 500°C for 3 h and then characterized by TGA-DTG, XRD, BET, FT-IR, potentiometric titration with n-butylamine and 2-propanol dehydration. Catalytic activity of materials was tested in the n-hexane isomerization at 350°C. The results showed that TiO₂ and mainly developed anatase phase. All have acceptable specific surface area (95-105 m²/g). Potentiometric titration with n-butylamine revealed that showed higher acidity (430-530 mV) than compared to pure TiO₂ (﹣15 mV), indicating that this oxide only has weak acidity. The results showed good relationship between acidity determined by potentiometric titration with n-butylamine and the catalytic activity evaluated by 2-propanol dehydration and n-hexane isomerization. Titanium oxide with 20 wt% ofions was the material that demonstrated the highest catalytic activity for both reactions.

Keywords

Acid Catalysts; Sulfated Titanium Oxide; Physico-Chemical Properties; n-Hexane Isomerization

1. Introduction

Solid acid catalysts have assumed a great importance due to their huge application potential in chemical industry, specifically in the oil refining industry [1-4]. The super-acid liquid catalysts such as HF, H₂SO₄, AlCl₃ and BF₃, which are efficient and selective at low temperatures, are not profitable for such processes due to severe problems of corrosion and their difficult separation from the stream of the products. Chlorinated alumina, zeolites modified by ionic exchange, heteropolyacids, as well as some bifunctional catalysts are examples of solid acid catalysts applied in the types of processes mentioned above and also reported in the literature. Most of these catalysts suffer from various drawbacks such as high working temperature, continuous supply of chlorine and the use of high hydrogen pressure [5]. For these reasons, current researches about heterogeneous acid catalysts have been focused on the synthesis of new materials that are capable of replacing the liquid acid catalysts and halogenated solids mentioned previously. Sulfated oxide supports demonstrate strong acidity and catalytic activity at low temperatures for catalytic cracking, alkylation, and light paraffin isomerization [6-9]. One of the most studied materials for these reactions has been sulfated zirconnium oxide [10-14], although the possibility of using sulfated titanium oxide () for such purposes is not discarded. The sulfate ion can be introduced from H₂SO₄, (NH₄)₂SO₄, SO₂ and H₂S. It was reported that the existence of covalent S-O bonds in sulfur complexes formed on metal oxides is responsible for the generation of acidity [15-17]. The substitution of oxygen atoms in the titania lattice with sulfur and other anionic species [18] is reported to show photocatalytic activity enhancement in visible light [19,20] due to the existence of oxygen vacancies, greater surface area [21-25], and larger fraction of the anatase phase. These physicochemical properties can be controlled from the synthesis process [26]. Although most of these catalytic supports are prepared via the precipitation method [27-29], the sol-gel process offers better control over synthesis parameters, especially to obtain uniform textural and chemical characteristics down to the nanometric level [29-34]. Ohno et al. [35] reported the incorporation of sulfur in titania by sol-gel precipitation using titanium isopropoxide and thiourea and showed the enhancement in photocatalytic degradation of 2-propanol and partial oxidation of adamantane at wavelengths longer than 440 nm. Arata [36] synthesized sulfated titania by exposing Ti(OH)₄ to aqueous sulfuric acid followed by calcination. Ohno et al. [35] studied the hydroxylation of adamantane using sulfated titania as catalyst, a reaction of relevance (green process) using molecular oxygen. It is reported that the activity of sulfated titania could be improved by optimizing the surface area of the sample. In general, it is conceived that Brönsted and Lewis acidity are improved by modifying TiO₂ with H₂SO₄. The superacid properties of sulfated titania were considered to be responsible for the high catalytic activity in various acid-catalyzed reactions such as the alkylation of derivatives of benzene, cracking of paraffins and dimerization of ethylene [37- 41]. The increase in activity for CFC-12 decomposition obtained with sulfated titania was due to the superacid properties of the catalysts [21,42]. Other reactions catalyzed by sulfated titania due to its acidic properties are acylation of aromatics [43], acylation of Friedel-Crafts [44], esterification [45] and transesterification of vegetal oils [46].

In the search for new solid acid catalysts with improved properties, this work focuses on the synthesis and characterization of a sulfated titanium oxide by sol-gel method, and presents a preliminary evaluation of its catalytic properties for the n-hexane isomerization reaction.

2. Experimental

2.1. Supports Preparation

The preparation of titanium hydroxide [Ti(OH)₄] was carried out via sol-gel by dissolving titanium n-butoxide IV (Ti[O(CH₂)₃CH₃]₄; Sigma-Aldrich; 97 wt%) in tertbutyl alcohol ((CH₃)₃COH; Baker; 99.6 %) and the hydrolysis and condensation proceed via the drop-wise addition of a water/tert-butyl alcohol solution. The gel was then aged during 72 h and dried at 100˚C for 24 h. Ti(OH)₄ impregnation with acid agent was done by the incipient wetness method, using a 1 N H₂SO₄ solution and adding the necessary amount of acid reagent to obtain supports with 10, 15 and 20 wt% of ions. Pure and modified hydroxides were dried at 100˚C during 24 h and then calcined in dynamic air atmosphere at 500˚C for 3 h. Taking the calcination temperature and the weight percentage of ions as reference, the synthesized materials were named as follows: T0500, T10500, T15500 and T20500. The method for the preparation of the catalytic supports is illustrated in the flow diagram of Figure 1.

2.2. Catalyst Preparation

To test activity for the light paraffin isomerization, the catalytic supports were impregnated with platinum (Pt) by the incipient wetness method, using an ammonia solution

Figure 1. Flow diagram of synthesis method of titanium oxide and sulfated titanium oxide.

of Pt(NH₃)₂(NO₂)₂; Sigma-Aldrich; 3.4 wt%, followed by a drying at 100˚C for 6 h and a calcination in air flow at 400˚C for 3 h. The theoretical concentration of Pt was fixed at 0.3 wt% for all catalysts. The Pt-containing catalysts are designated: Pt/T0500, Pt/T10500, Pt/T15500 and Pt/T20500.

2.3. Characterization Techniques

Catalytic supports were characterized by thermal analysis, X-ray diffraction, nitrogen physisorption, infrared spectroscopy, potentiometric titration with n-butylamine and 2-propanol dehydration. Thermogravimetric analyses were conducted on a TA Instruments STD 2960 simultaneous DSC-TGA. The samples were analyzed in air flow (10 mL/min) at a heating rate of 10˚C/min from a room temperature to 900˚C. Powder X-ray diffraction patterns were recorded on a Bruker diffractometer using Cu Kα radiation (λ = 1.5406 Å) and a graphite secondary beam monochromator; the intensities of the diffraction lines were obtained in the 2θ range between 20 and 80˚ with a step size of 0.02˚ and a measuring time of 2.7 s per point. The crystallite size of the materials was determined with the Scherrer equation. Nitrogen physisorption was used to determine the specific surface areas of the materials at the temperature of liquid nitrogen (−196˚C) in a Quantachrome Autosorb-1 instrument. Prior to the measurements, samples were outgassed at 350˚C for 2 h. The specific surface area was calculated using the BET equation and the BJH method was used to determine the pore size distributions and the pore volume of the samples. Infrared spectroscopic analyses were carried out in a Fourier Transform spectrometer (Perkin-Elmer Spectrum One) with transparent wafers containing the sample to be analyzed and KBr as a binding agent (9:1 dilution), co-adding 16 scans at a resolution of 4 cm⁻¹. The acidity of the catalytic supports was determined by potentiometric titration with n-butylamine. A small quantity of n-butylamine dissolved in acetonitrile (0.025 M solution) was added to a known mass of solid which was suspended in the solution and stirred for 3 h. After this, the suspension was titrated with the same base at a rate of 0.2 mL/min, and the variation in the electrical potential was measured with a JENWAY-3310 digital pH meter. The potentiometric titration method used in this work has been previously reported by Cid and Pecchi [57]. In order to evaluate the acid-base properties of catalytic materials, 2-propanol dehydration reaction was performed. The reaction was carried out in a fixed-bed reactor operating at 80˚C, atmospheric pressure and WHSV = 5 h⁻¹. Approximately 0.1 g of the catalyst was loaded into the reactor. Prior to the reaction, the catalyst was preheated at 350˚C for 1 h in a purified nitrogen flow. During the reaction, the effluent collected periodically and analyzed by a Varian 3400 gas chromatograph equipped with a FID detector and a column packed with Carbowax 1540 on Chromosorb.

2.4. Catalytic Activity

The n-hexane isomerization was carried out in a conventional flow fixed-bed reactor, operating at 350˚C, atmospheric pressure and WHSV = 0.5 h⁻¹. The catalyst (0.3 g) was reduced at 350˚C for 1 h in a flow of hydrogen prior to running the test reaction. The products were analyzed on-line by a Varian 3400 gas chromatograph equipped with a FID detector and a column packed with 23 SP- 1700 on 80/100 Chromosorb.

3. Results and Discussion

3.1. Thermal Analyses

Four different weight loss stages were identified during thermal analysis of the as-prepared titanium oxide and sulfated titanium oxide materials (Figures 2 and 3). The first two stages, evidenced by two DTG peaks centered at 70˚C and 160˚C correspond respectively to the elimination of water and solvent occluded in the matrix of the inorganic polymer gel. The third stage of weight loss occurs within the range of 250˚C and 400˚C and is more gradual, associated with the transformation of Ti(OH)₄ into TiO₂ and involves the elimination of some terminal hydroxyl groups [47,48]. The sulfated titanium oxide precursors showed a fourth weight loss stage between 400˚C and 900˚C, the DTG peaks being centered around 550˚C and attributed to the decomposition and loss of sulfate ions [49]. It should be noted that samples prepared for catalysis studies were air-calcined at 500˚C, thus sulfate levels in the catalysts may be slightly lower than the theoretical levels assumed from the incipient wetness addition of H₂SO₄. The FT-IR data (see below) clearly show the presence of sulfate groups in all three sulfated TiO₂ materials.

3.2. Textural Properties

The nitrogen physisorption results indicate that the specific surface area of the titanium oxide (35 m²/g) increases when the sulfate ion is incorporated into the matrix of the material, yielding sulfated titanium oxides with specific surface areas in the range 95-105 m²/g (Table 1). Surface area enhancement is likely due to the inhibition of titania particle sintering by the surface sulfate groups. Nitrogen adsorption-desorption isotherms and pore size distributions are shown in Figures 4 and 5, respectively. Pure titanium oxide (T0500) presented a type II isotherm and a very closed hysteresis loop that identifies the presence of pores with shape of cone and/or wedge [50-52]. In contrast, sulfated materials (T10500,

Figure 2. TG profiles developed by TiO₂ and precursors.

Figure 3. DTG profiles developed by TiO₂ and precursors.

Table 1. Textural properties of synthesized titanium oxides.

T15500 and T20500) showed type IV isotherms with H2 hysteresis loops that represent a uniform mesoporous structure [52]. Pore size distributions were obtained by the BJH method. The sulfated titanium oxides (T10500, T15500 and T20500) presented a monomodal distribution with most frequent pore diameters in the 30 to 50 Å range. In contrast, pores in pure titanium oxide (T0500) are considerably larger (200 - 1000 Å) and the distribution is broader.

3.3. X-Ray Diffraction

Figure 6 shows the X-ray diffraction patterns obtained for pure titanium oxide (T0500) and for the sulfated titanium oxides (T10500, T15500 and T20500) calcined at 500˚C. The crystalline anatase (tetragonal) form of titania is evidenced by lines appearing at 2θ = 25.22˚, 36.97˚, 37.88˚, 38.58˚, 47.99˚, 53.90˚, 55.07˚, 62.70˚, 68.84˚ and 70.31˚, which correspond to crystallographic planes (101), (103), (004), (112), (200), (105), (211), (204), (116), and (220), in reference to JCPDS card number 021-1272. In comparison to the pure titania sample, these diffraction lines are noticeably broadened in the diffracttograms of the sulfated titania, indicative of a decrease in crystallite size [53]. Average crystal sizes were calculated using the Scherrer equation to quantify line broadening on the most intense (101) anatase peak, and results are plotted in Figure 7 as a function of sulfate content. The average anatase crystal size decreases to one-half its value upon sulfate addition to pure titania at any of the loadings used. There is no evidence of rutile formation in any of the samples.

Conflicts of Interest

The authors declare no conflicts of interest.

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