H3PO4/Al2O3 catalysts: characterization and catalytic evaluation of oleic acid conversion to biofuels and biolubricant

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H₃PO₄/Al₂O₃ catalysts: characterization and catalytic evaluation of oleic acid conversion to biofuels and biolubricant

Lucia Regina Raddi de AraujoI, ^*; Cynthia Fraga Scofield^I; Nídia Maria Ribeiro Pastura^{I, II}; Wilma de Araujo Gonzalez^II

^IUniversidade do Estado do Rio de Janeiro, Inst. Química, Rua São Francisco Xavier, 524, Maracanã, 20559-900 Rio de Janeiro - RJ, Brazil

^IIInstituto Militar de Engenharia, Dep. Engenharia Química, Praça General Tibúrcio, 80, Urca, 22290-270 Rio de Janeiro - RJ, Brazil

ABSTRACT

Al₂O₃ and H₃PO₄/Al₂O₃ catalysts were investigated in the conversion of oleic acid to biofuels and biolubricant at 1 atm and at 623 K. The catalytic tests were performed in a fixed bed and continuous flow reactor, using an oleic acid-catalyst ratio of 4 and N₂ as the carrier gas. The reaction products were analyzed by gas chromatography and acidity measurements. N₂ adsorption-desorption, X ray diffraction, ³¹P nuclear magnetic resonance and FT-IR spectroscopy were also employed to evaluate the textural, structural and acidic properties of the catalysts. The results showed that phosphoric acid impregnation improved the alumina decarboxylation activities, generating hydrocarbons in the range of gasoline, diesel oil and lubricant oil. The best catalytic performance was achieved with the highest surface area alumina impregnated with H₃PO₄, which was the solid that allied high total acidity with a large quantity of mesopores.

Keywords: H₃PO₄ /Al₂O₃, oleic acid, biofuels, biolubricant

1. Introduction

The enormous worldwide use of diesel fuel and the rapid depletion of crude oil reserves have prompted keen interest and exhaustive research into suitable alternative fuels. Biomass sources, particularly vegetal oils, have attracted much attention in recent years thanks to their wide availability and easy renewability. Moreover, biodiesel presents many advantages, such as a high cetane number and the absence of sulfur and aromatics, thus contributing to reduce the load of environmental pollutants¹. Many investigations have focused on improving biofuel produced from vegetal oils and an extensive literature is available in this field^2-6. Most of these researches use the transterification route, which consists of converting the natural triglycerides present in vegetal oils into ethyl or methyl alcohol monoesters. Though easy, this process has disadvantages, e.g., it requires large quantities of ethyl or methyl alcohol and generates large amounts of glycerin as a by-product, thus making the biofuel purification step difficult.

Alternatively, biofuels can be produced by the catalytic triglyceride degradation reaction, which generates a mixture of hydrocarbons very similar to petrodiesel. The catalyst must possess hydrodeoxygenation properties in order to complete the entire process in a single stage. Some catalytic properties such as surface area and acidity are also important^7,8. As for the quality of the fuel, this route is better than the transterification one, since it does not require any engine modification or adaptation. This process can also generate biolubricant, which is a biodegradable fluid and an environmentally friendly product, as opposed to petrochemically derived lubricants.

This research purported to investigate the viability of using H₃PO₄/Al₂O₃ as catalytic agents in the conversion reaction of oleic acid into hydrocarbons in the range of diesel oil (C₁₄ - C₁₈) and lubricant oil (C₁₉ - C₂₂). Oleic acid was chosen as the reactant since it is one of the main constituents of many vegetal oils.

2. Experimental

2.1. Catalyst preparation

Two alumina samples donated by Petrobras Research Center (CENPES), one commercial, Pural 200 (Al₂O₃ - A) and one synthesized (Al₂O₃ - B), were used as support materials. The catalysts were prepared by wetting and impregnating the supports with a 12% (wt. (%)) phosphoric acid solution and these impregnated samples are symbolized by HP/alumina. After homogenization, the catalysts were dried overnight at 373 K and calcine d in a furnace at 623 K for 2 hours.

2.2. Catalyst characterization

The porous properties such as surface area (BET method), average pore diameter and pore volume (t-plot method) were determined by N₂ physical adsorption-desorption at the normal boiling temperature (77 K) using an ASAP 2000 Micromeritics apparatus. Prior to the analysis, the catalyst was subjected to a preheating treatment at 423 K under vacuum. The BET isotherms were measured at pressures between 6 and 21 kPa, using superdry N₂.

Powder X ray diffraction data for structural analysis were collected with a Siemens X ray diffractometer. A Cu Ka (35 kV) anode was used as the X ray radiation source, covering 2q between 2º and 80º. For purposes of identification, the diffraction patterns obtained were compared against the standards of the American Society for Testing of Materials (ASTM).

Acidity measurements were carried out with a Perkin-Elmer 2000 FTIR analyzer at room temperature. The sample, in self-supporting disk form, was initially pretreated in an O₂ flow (30 cm³.min^-1) at 723 K for 1 hour. After a vacuum treatment and cooling to room temperature, a preliminary IR spectrum was measured. The temperature was then increased to 423 K and the sample was exposed to pyridine vapor (4 Torr) under static conditions for 30 minutes. The IR spectrum was measured again after vacuum and cooling to room temperature. The absorbance spectra were obtained by using the interferograms of reduced samples as background references.

The interaction of phosphorous with the support was investigated by ³¹P NMR analysis, using a Bruker DRX-300 Advance spectrometer operating in a resonance frequency of 121.5 MHz.

2.3. Catalytic reaction

The oleic acid conversion was performed at 1 atm and 623 K in a fixed bed continuous flow reactor. A scheme of the catalytic unit is illustrated elsewhere⁹. The catalytic bed comprised 5 g of catalyst and 5 cm of small glass balls. The catalyst was pretreated in situ for 2 hours under a N₂ flow at 623 K. After this treatment, the reactant was pumped into the reactor inlet (reactant flow = 20.5 g.h^-1). The liquid products were collected after passing through a condenser connected to the reactor's outlet. During the reaction, two samples were withdrawn from the reactor effluents at 30 minutes and 60 minutes to determine the acidity (Ma) and for a chromatographic analysis (GC). These analyses were carried out to evaluate the acid decarboxylation properties and the selectivities for biodiesel and biolubricant products.

2.4. Liquid products characterization

The liquid products were analyzed in a Hewlett Packard (HP) 5890 series II gas chromatograph using a PE-1 (100% methylsilicone) capillary column of 50 m and H₂ as the carrier gas. The column temperature was programmed to range from 323 K to 533 K, using a heating rate of 8 K.min^-1.

To measure the acidity, a solution of ethyl alcohol and benzene (volumetric ratio of 1) was initially added to a weighed amount of the liquid product. This mixture was then titrated with an alcohol solution of 0.1 mol.L^-1 KOH, using phenolphthalein as indicator.

3. Results and Discussion

3.1. Porous properties

Table 1 shows the results of the textural characterization of the samples.

As can be seen in Table 1, the microporous volumes of all the samples were almost negligible compared to their mesoporous volumes.

The results indicate that impregnation of phosphoric acid on the two aluminas reduced their surface areas considerably; therefore, this reduction cannot be attributed simply to an increase of the samples' densities due to the incorporation of acid, for both supports clearly underwent partial pore blockage.

As for the HP/Al₂O₃ - A catalyst, impregnation with acid markedly reduced the mesoporous volume, indicating that phosphoric acid may be located mainly on the inner surface of the Al₂O₃ - A mesopores. An increase in the average pore diameter of Al₂O₃ - A was also observed as a result of phosphoric acid impregnation.

A comparison of the Al₂O₃ - B and HP/Al₂O₃ - B catalysts revealed a reduction of the total pore volume, but an increase in the mesoporous volume after the addition of acid to Al₂O₃ - B. One possible explanation may be the condensation of PO₄^3- ions on the external surface of the support, resulting in the formation of superficial phosphate species¹⁰. Similarly to the HP/Al₂O₃ - A catalyst, the average pore diameter also increased after H₃PO₄ impregnation on Al₂O₃ - B.

3.2. X ray diffraction

Figure 1 shows the XRD patterns of Al₂O₃ - A based catalysts. The Al₂O₃ - B based samples presented similar spectra, although the peaks displayed very low intensities, indicating the low crystallinity of these solids.

Figure 1

3.3. Acidity measurements

Figure 2 shows the infrared spectra of adsorbed pyridine obtained in the wave range of 1300 and 2000 cm^-1.

The Al₂O₃ - A and Al₂O₃ - B based catalysts showed similar spectra, although the vibration bands of the Al₂O₃ - A samples were lower than those of Al₂O₃ - B. Pure alumina spectra presented bands at 1450, 1494, 1581 and 1615 cm^-1. The bands at 1450, 1494 and 1615 cm^-1 are likely associated with pyridine bound coordinately to Lewis acid sites¹², while the band at 1581 cm^-1 can be attributed to pyridine adsorbed on Brønsted acid sites¹³.

The infrared spectra showed practically no change after the incorporation of phosphoric acid in the supports. Nevertheless, compared to the pure aluminas, most of the band intensities decreased, with a slight band shift occurring at 1615 cm^-1 towards higher frequencies. The decrease in the peaks' intensities may be ascribed to the lower acid density of these impregnated samples, as evidenced by the strong reduction of surface area due to the phosphoric acid load in the alumina (textural analysis). With regard to the band at 1615 cm^-1, the higher the vibrational frequency the stronger the Lewis acid sites in the sample¹². Therefore, the Lewis acid sites of impregnated samples were stronger than those of pure aluminas. This can be explained by the higher electronegative feature of phosphorous-alumina bonding and also by the decrease in the aluminum's electronic density, resulting in an increase of the acidity of the Lewis sites.

3.4. Nuclear magnetic resonance

Table 2 shows the chemical shifts obtained from the ³¹P NMR analysis of the catalysts.

According to the ³¹P-NMR data, no signal was detected at 0 ppm, indicating the absence of free phosphoric acid in these samples. The spectra signals were attributed to tetra-coordinated phosphorus species¹⁴.

Signals were detected at - 18 and at - 28 ppm in the HP/Al₂O₃ - A catalyst, indicating different phosphorous and aluminum interactions in bridging structures¹⁴, as depicted below:

The peak at - 12 ppm in the HP/Al₂O₃ - B catalyst was possibly related to linear phosphorous and aluminum bonding¹⁴, based on the following simplified scheme:

3.5. Catalytic reaction

Table 3 presents the average results obtained from catalytic conversion reactions at 623 K, where samples 1 and 2 correspond to the aliquots withdrawn after 30 minutes and 60 minutes on stream, respectively.

This table reveals the decarboxylation activity of the catalysts by comparing the products' acidity measurements (Ma), in other words, the lower the acidity the greater the efficiency of the catalyst in the decarboxylation process. The acidity measure of the reactant, oleic acid, is also included in Table 3 for purposes of data comparison.

The catalytic activities are compared as a function of the generation of acidity in the liquid products, i.e., the ability to convert the oleic acid reactant into hydrocarbon products, using the following expression:

For the noncatalyzed reaction, the results suggest the occurrence of a thermal cracking process in the preheating zone of the reactor, which resulted in a small amount of liquid products.

Based on the experimental data, it is clear that the presence of a catalytic agent substantially increased the conversion of oleic acid. Comparing the pure supports, both aluminas presented the same catalytic activity (90% of conversion). The aluminas' catalytic performances were slightly enhanced by impregnating the supports with phosphoric acid, especially in the case of Al₂O₃ - B. As mentioned earlier, the incorporation of phosphoric acid in the aluminas increased their acidity but also decreased of their specific area. Hence, the catalytic promotion effect may be attributed mainly to an increase of the acidic properties of the support.

3.6. Gas chromatography

Table 4 presents the distribution of hydrocarbon products obtained from the catalytic conversion of oleic acid. Quantification of the liquid products was expressed taking into account the different hydrocarbon ranges, which was determined by using standard chromatograms for comparison.

No hydrocarbon product was detected in the noncatalyzed reaction. The presence of the catalyst promoted the formation of compounds in the range of gasoline, diesel oil and lubricant oil (mainly). The addition of H₃PO₄ to Al₂O₃ improved the selectivity for diesel oil.

The best biodiesel selectivity was achieved with HP/Al₂O₃ - B catalyst. The simultaneous analysis of the oleic acid decarboxylation (Table 3) with the chromatographic results allow us to conclude that the HP/Al₂O₃ - B catalyst is the most efficient one for the process, since it showed conversions of up to 96% and 26 w% of diesel oil production. This performance is probably related to its particular properties: it is a solid with a relatively high surface area, with a large quantity of mesopores and strong acid sites. Moreover, the linear phosphorous-aluminum interaction may be inferred to be a beneficial structure for promoting oleic acid decarboxylation.

4. Conclusions

The impregnation of both aluminas with phosphoric acid caused partial blockage of the pores due to the formation of aluminum phosphate, which was detected by XRD. The phosphoric acid was located mainly on the inner surface of the Al₂O₃ - A mesopores and preferentially on the external surface of Al₂O₃ - B support. The phosphorous-aluminum interaction was also dependent on the surface area: in the case of the Al₂O₃ - A support, the predominant interaction was through bridge-type bonding, while the Al₂O₃ - B showed linear phosphorous-aluminum linking. With regard to the catalytic tests, the impregnation of phosphoric acid improved the aluminas' decarboxylation properties. It can be concluded that both textural and acid properties play an important role in the catalytic performance of the conversion of oleic acid into biofuels and biolubricants. The HP/Al₂O₃ - B catalyst, which associated high total acidity with a large quantity of mesopores, presented a better catalytic performance in the oleic acid decarboxylation reaction.

Acknowledgments

The authors are indebted to Deborah Vargas from NUCAT/COPPE/UFRJ for the FT-IR analyses and to Rosane San Gil from IQ/UFRJ for the NMR analyses.

Received: June 27, 2005; Revised: March 9, 2006

• 1. Al-Widyan MI, Al-Shyoukh AO. Experimental evaluation of the transesterification of waste palm oil into biodiesel. Bioresource Technology 2002; 85:253-256.
• 2. Srivastava A, Prasad R. Triglycerides-based diesel fuels. Renewable and Sustainable Energy Reviews 2000; 4:111-133.
• 3. Abreu FR, Lima DG, Hamú EH, Wolf C, Suarez PAZ. Utilization of metal complexes as catalysts in the transesterification of Brazilian vegetable oils with different alcohols. Journal of Molecular Catalysis A: Chemical 2004; 209:29-33.
• 4. Dmytryshyn SL, Dalai AK, Chaudhari ST, Mishra HK, Reaney MJ. Synthesis and characterization of vegetable oil derived esters: evaluation for their diesel additive properties. Bioresource Technology 2004; 92:55-64.
• 5. Alcantara R, Amores J, Canoira L, Fidalgo E, Franco MJ, Navarro A. Catalytic production of biodiesel from soy-bean oil, used frying oil and tallow. Biomass and Bioenergy 2000; 18:515-527.
• 6. Vicente G, Martínez M, Aracil J. Integrated biodiesel production: a comparison of different homogeneous catalysts systems. Bioresource Technology 2004; 92:297-305.
• 7. Fukuda H, Kondo A, Noda H. Biodiesel fuel production by transesterication of oils. Journal of Bioscience and Bioengineering 2001; 92(5):405-416.
• 8. Araujo LRR, Pastura NMR, Rodrigues RC, Picardo MC, Reguera FM, Gonzalez WA. Estudo da descarboxilação do ácido oleico sobre catalisadores de H₃PO₄ suportados. In: Scott C, editor. Proceedings of the 18^th Simposio Iberoamericano de Catalisis [CD-ROM]; 2002 Sept 15-20; Porlamar: Venezuela; 2002. p. 2571-2576.
• 9. Reguera FM, Araujo LRR, Picardo MC, Bello FO, Scofield CF, Pastura NMR et al., The use of niobium based catalysts for liquid fuel production. Materials Research 2004; 7(2):343-348.
• 10. Araujo LRR, Pastura NMR, Scofield CF, Gonzalez WA. Catalisadores H₃PO₄/Al₂O₃ : caracterização e avaliação catalítica na conversão de ácido oleico a óleo diesel. In: PGETEMA PUC-RS, editor. Proceedings of the 16^th Congresso Brasileiro de Engenharia e Ciências dos Materiais [CD-ROM]; 2004 Nov 28-Dec 2; Porto Alegre: RS; 2004. trabalho 26.
• 11. Smith JV, editor. X-Ray Powder Data File. American Society for Testing of Materials 29. Philadelphia: PA; 1960.
• 12. Mekhemer GAH, Nohman AKH, Fouad NE, Khalaf HA. Surface to bulk characterization of phosphate modified aluminas. Colloids and Surface A: Physical and Engineering Aspects 2000; 161:439-446.
• 13. Silva CLT, Camorim VLL, Zotin JL, Pereira MLRD, Faro Jr AC. Surface acidic properties of alumina-supported niobia prepared by chemical vapour deposition and hydrolysis of niobium pentachloride. Catalysis Today 2000; 57:209-217.
• 14. Dixon KR. Phosphorus to bismuth. In: Mason J, editor. Multinuclear NMR 2^nd ed. London, Plenum Press; 1989.

Publication Dates

• Publication in this collection
  06 June 2006
• Date of issue
  June 2006

History

• Reviewed
  09 Mar 2006
• Received
  27 June 2005