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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.48 n.2 Concepción jun. 2003

http://dx.doi.org/10.4067/S0717-97072003000200011 

J. Chil. Chem. Soc., 48, N 2 (2003)

CATALYTIC ACTIVITY OF SULFUR RESISTANT CATALYSTS ON
THE ISOMERIZATION OF A MIXTURE OF PINENES POISONED
WITH THIOPHENE AS CRUDE SULFATE TURPENTINE MODEL

Ruby Cid1; Wolfgang F. Hölderich2; Alberto Miranda1; Dominique M. Roberge2; Miguel Zárraga1.

1 Faculty of Chemical Sciences, Department of Organic Chemistry, University of Concepción, Casilla 160-C,
Concepción, Chile.
2 Department of Chemical Technology and Heterogeneous Catalysis,
University of Technology RWTH Aachen, Worringerweg 1, D-52074 Aachen, Germany

( Received : January 1, 2003 ­ Accepted : March 20, 2003)

1. ABSTRACT

The catalytic activity of Pd and Pd-M bimetallic catalysts (M = Cr, Cu, Pt, Ag) on different solid supports for isomerization in gas phase of a mixture of pinenes poisoned with 4000 pmm of sulfur as thiophene was studied. This mixture was used as a model of crude sulfate turpentine (CST), which is a cheap and valuable source of pinenes. The major product of this reaction is p-cymene, an important product in chemical industry. It was found that Pd-Pt/ SiO2-Al2O3 gives a selectivity of 65-70% and a conversion of 98%. Molecular sieves as catalyst support were also investigated but the activity was not as better as the SiO2 or SiO2-Al2O3 for the targeted reaction.

2. INTRODUCTION

p-Cymene is an important product and valuable intermediate in chemical industry. Among others, it is used as a solvent for dyes and varnishes, as a heat transfer medium, as an additive in fragrances and musk perfumes, and as a masking odor for industrial products. The main use of p-cymene is however its transformation to p-cresol. At the moment, p-cresol is mainly produced via the alkylation of toluene with propylene, followed by oxidation and hydroperoxide cleavage1. The alkylation and isomerization steps produce a mixture of cymenes rich in m- and p-isomers. The isomer separation in order to obtain pure p-cresol can be performed with either the cymene or the cresol mixture. For example, UOP developed the Cymex process where m- and p-cymene are separated chromatographically using an appropriate sorbent (molecular sieve) and a desorption medium (toluene)2. Terpenes are naturally occurring renewable materials that can be used as feedstock. It has already been shown that a-limonene3-6, 3-carene7, a-pinene8 and mixtures of terpenes9,10 convert to p-cymene in good yields, producing elementary hydrogen. A commercial use of these materials will, however, be limited from their low price and availability, which must be compared to the market price of toluene and propylene. a-Limonene, for example, is a by product from the citrus industry but has recently become a high demand biodegradable solvent, making it an impractical synthetic raw material11. Crude sulfate turpentine is a cheap source of pinenes, and then its use to produce derivates with higher value is an interesting field that has been recently studied12. Isomerization of pure a-pinene and b-pinene were conducted with success to obtain p-cymene. These reactions occurred with high conversion and selectivity towards the desired compound using catalyst based on palladium supported on silica or alumina. Prior work with CST to produce p-cymene showed that the presence of sulfur in the starting material deactivates the catalyst due to a progressive poisoning12. It is possible to improve the isomerization of pinenes from CST polluted with sulfur using the knowledge developed to obtain sulfur tolerant catalyst for process of hydrogenation of aromatics in diesel fuels. Several works report sulfur tolerant catalyst for hydrogenation processes13-18. These studies have shown that the support acidity and electron deficiency of supported metal are the most important parameters upon the sulfur resistance of the catalyst depend. Impregnated noble metals on higher acidic support were found to have higher sulfur tolerance. The addition on a second metal that induce a electronic deficiency conduce to better catalysts14. On the other hand the acidity of the support has an important influence on the selectivity in the isomerization of a-pinene. Therefore the characteristic of the support and the metals are important parameters considerate in this study.

The main objective of this work was to study the catalytic activity of sulfur tolerant catalysts to obtain p-cymene from a model of CST with 4000 ppm of sulfur as thiophene. Catalytic performance of different transition metals loaded on silica, silica-alumina or molecular sieve in the isomerization of pinene to p-cymene is reported. Transition metals loaded on silica or silica-alumina supports show good sulfur tolerance and catalytic activity. Whereas, when molecular sieves NaHMOR and Yb-USY supports were used a low performance was found.

3. EXPERIMENTAL

3.1. Catalyst preparation

3.1.1. Pd (0,5 wt.%)/SiO2. In the general wet impregnation procedure of Pd an amount of an aqueous Pd(NH3)4(NO3) 2 solution is stirred for a few hours with the desired amount of catalyst in order to obtain a 0.5 wt.% Pd concentration on the catalyst. The water is subsequently removed under vacuum using a rotaevaporator. The catalyst is then dried overnight at 120°C.

3.1.2. Pd(0.5 wt.%)-Ag (0.5 wt.%)/SiO2; Pd (0.5 wt.%)-Cr(0.5 wt.%)/SiO2; Pd(0.5 wt.%)-Cu(0.5 wt.%)/SiO2.The carrier was calcined at 540°C for 6 h with a heating rate of 2°C/min. The support and aqueous Pd(NH3)4(NO3) 2 solution, in the amounts required to obtain 0.5 wt. % Pd, were stirred for few hours. The water was subsequently removed under vacuum using a rotaevaporator. The catalyst is then dried overnight at 120°C. After that, Pd-Ag/SiO2 was prepared by stirring the palladium-supported catalyst in an aqueous solution of AgNO3 with the required concentration to obtain 0.5 wt.% of Ag. The previously mentioned procedure was utilized to prepare both Pd-Cu and Pd-Cr silica supported catalysts.

3.1.3. Pd (0.5 wt. %)/SiO2-Al2O3. The support SiO2-Al2O3 was calcined at 540°C for 6 h with a heating rate of 2°C/min. An amount of aqueous Pd(NH3)4(NO3) 2 solution was stirred for few hours with the desired amount of carrier in order to obtain a 0.5 wt.% Pd concentration of the catalyst. The water is subsequently removed under vacuum using a rotaevaporator. The catalyst is then dried overnight at 120°C.

3.1.4. Pd-Pt(1.5 wt.%)/SiO2-Al2O3 . The carrier was calcined at 540°C for 6 h with a heating rate of 2°C/min. The impregnation was made stirring the support with an aqueous solution of Pd(NH3)4(NO3) 2 and Pt(NH3)4(NO3) 2 necessary to obtain a metal content of 1,05 wt.% of Pd and 0,45 wt. % of Pt. Then the water was removed under vacuum in a rotaevaporator. After that the catalyst was dried overnight at 120°C. Prior to the reaction the catalyst is activated in the reactor under hydrogen flow at 300°C for 2h.

3.1.5. Pd-Pt/NaHMOR and Pd-Pt/Yb-USY. NaHMOR are obtained exchanging the H+ by Na+ with an aqueous solution 0.1 M of NaNO3 for 12 h. The catalyst Pd-Pt/NaHMOR was prepared stirring an aqueous solution of Pd(NH3)4(NO3) 2 and Pt(NH3)4(NO3) 2 with a NaH-MOR (exchanged 72%). The amount of Pd(NH3)4(NO3) 2 and Pt(NH3)4(NO3) 2 was adjusted to obtain a loaded mole ratio Pd:Pt = 4. The water is subsequently removed under vacuum using a rotaevaporator. Then, the catalyst is dried overnight at 120°C. On the other hand, Yb-USY was prepared by impregnation of zeolite with Yb acetate solution (5.0 wt.% on a dry basis), subsequently the bimetallic catalyst was obtained by the same method performed to achieve the Pd-Pt/NaHMOR catalyst

3.2. Catalyst characterization

The chemical compositions of the catalysts (Pt,Pd) were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES), using a Perkin-Elmer Optima 3300DV instrument. BET surface areas were calculated from these isotherms using the BET method in the 0.005­0.25 P/P0 = 0.2 range. In all cases, correlation coefficients above 0.999 were obtained. Metal dispersion of the Pd-Pt catalysts was determined from the amount of chemisorbed CO measured by sing a pulse method. In the calculation of the dispersion, we assumed that the stoichiometry of CO to either Pd or Pt to be unity.


Catalyst

Metal (%)

BET Surface area (m2/g)

Dispersion (%)

Pd/SiO2

Pd (0,5)

235

52

Pd-Ag/SiO2

Pd (0,5); Ag (0,5)

229

36

Pd-Cr/SiO2

Pd (0,5); Cr (0,5)

215

38

Pd-Cu/SiO2

Pd (0,5); Cu (0,5)

219

35

Pd/SiO2-Al2O3

Pd (0,5)

288

58

Pd-Pt/SiO2-Al2O3

Pd (1,05); Pt (0,45)

290

56

Pd-Pt/NaHMOR

Pd (0,9); Pt (0,3)

340

47

Pd-Pt/Yb-USY

Pd (0,9); Pt (0,3)

534

61


Table 1. Properties of the catalysts.

3.3. Reaction conditions

The starting material was a model of "Crude Sulfate Turpentine" (ART-CST). The ART-CST was prepared using amounts of a-pinene, b-pinene and sulfur-containing compound similar to those present in the original CST, that is, 83.5 % of a-pinene, 16.5 of b-pinene and 4000 ppm of thiophene. Prior to the reaction, the catalyst was reduced in situ during the starting-up period by heating it to the reaction temperature at a rate of 5°C/min in the presence of hydrogen (hydrogen flow = 3,9 L/h) for 2 h. The reaction was performed in gas phase at a temperature of 300°C with catalyst granules of 1-1.6 mm packed in a stainless steel tubular reactor with a vaporizing coil. The reactor is placed at the center of a conventional furnace such as isothermal conditions are obtained in the catalyst bed. The reactant (art-cst) was fed into the reactor using hydrogen as carrier gas under atmospheric pressure. The products are condensed at 2°C at the reactor outlet. The weight hourly space velocity (WHSV) was set at 6 h-1 and a H2/ reactant molar ratio equal 4 was used. It is important to indicate that conditions were the same for all experiments carried out.

3.4. Products and reactants analysis

All reaction products and reactants were analyzed by gas chromatography on a HP 6890 GC system equipped with a 60 m OV-1701-CB column (86% methyl-7% cyanopropyl-7% phenylsilicon) and a flame ionization detector. The column is operated at 80°C to maximize the product separation followed by a temperature increase to 240°C for complete desorption of products. The products are identified by GC-MS (Varian Saturn 3) and retention time comparison with known substance (if available).

4. RESULTS AND DISCUSSION

a-Pinene has an ethylenic linkage so placed that two of the bonds of the cyclobutene ring are allylic. As a consequence the pirolysis occurs even below 200C and leads to few products. These are dipentene (1) and alloöcimene (2)19. The formation of both dipentene and alloöcimene has shown to be a first-order reaction20. At somewhat higher temperature, the alloöcimene is isomerized to a mixture of hydrocarbons known as pyronenes (3), and the proportion of pyronenes may reach 50% at 425C (Figure 1)21,22.


Figure 1

The isomerization of a-pinene over "solid acids" is different. It is a well-known reaction leading to bicyclic products of the camphene series and to monocyclic products of p-menthadienic structure23-26. The complexity of the reaction pathways arises from the cyclobutane ring present in a-pinene which has to be selectively opened to a p-menthadienic structure followed by a further dehydrogenation to p-cymene (figure 2).


The isomerization of a-pinene over metallic supported catalysts have been recently studied12. The presence of sulfur in the current CST is the major drawback to use this cheap material as useful feedstock for production of p-cymene. To overcome that inconvenience we studied various metallic supported catalysts to obtain p-cymene and results are discussed below.

4.1. Pd/SiO2. This catalyst showed good yield toward p-cymene in a previous study12. In this study we found that the activity was progressively decaying during the course of the reaction. It was also observed in the isomerization of CST under the reaction conditions here used. The conversion of a-pinene was higher than b-pinene, and the major difference between both pinenes was observed with this catalyst.


Figure 3. Yield of p-cymene, selectivity toward p-cymene and conversion of ART-CST on Pd(0.5 wt.%) SiO2 catalyst at 300° C, WHSV = 6 h-1, H2/reactant molar ratio equal 4 and 1g of catalyst.

4.2. Pd(0.5 wt.%)-Ag (0.5 wt.%)/SiO2; Pd (0.5 wt.%)-Cr(0.5 wt.%)/SiO2; Pd(0.5 wt.%)-Cu(0.5 wt.%)/SiO2. Addition of a second transition metal results in a major sulfur resistance, giving relatively constant yield curves in the time of stream. That fact could be explained by an electronic effect produced by the second metals13. Under the reaction conditions the activity of these catalysts shows a strong decay in the selectivity in the first hour of stream and then the selectivity remains approximately constant. Conversion, selectivity and yield of p-cymene for the targeted reaction with the different new bimetallic catalysts are shown in figures 4-6.


Figure 4. Yield of p-cymene, selectivity to p-cymene and conversion of ART-CST on Pd(0.5 wt.%)-Ag (0.5 wt.%)/SiO2 catalyst at 300° C, WHSV = 6 h-1, H2/reactant molar ratio equal 4 and 1g of catalyst.


Figure 5. Yield of p-cymene, selectivity to p-cymene and conversion of ART-CST on Pd(0.5 wt.%)-Cu (0.5 wt.%)/SiO2 catalyst at 300° C, WHSV = 6 h-1, H2/reactant molar ratio equal 4 and 1g of catalyst.


Figure 6. Yield of p-cymene, selectivity to p-cymene and conversion of ART-CST on Pd(0.5 wt.%)-Cr (0.5 wt.%)/SiO2 catalyst at 300° C, WHSV = 6 h-1, H2/reactant molar ratio equal 4 and 1g of catalyst.

4.3. Pd (0.5 wt. %)/SiO2-Al2O3

This catalyst was chosen because it is known that acidic support has a remarkable influence in the sulfur tolerance of electron deficient noble metals. Pt supported on SiO2-Al2O3 showed to be more sulfur resistant than Pt supported both on SiO2 and on Al2O3 in the aromatic hydrogenation process14,15. On the other hand Roberge and col.12 found that Pd/SiO2 shows a good conversion and selectivity toward p-cymene, and its acidity is comparable with the acidity of Pd/Al2O3. In this study an improvement on the overall catalytic activity with respect to the silica supported catalyst was observed. However, the selectivity was gradually decreasing during the first hour of stream as it is shown in figure 7.


Figure 7. Yield of p-cymene, selectivity to p-cymene and conversion of pure ART-CST on Pd (0.5 wt. %)/SiO2-Al2O3 catalyst at 300° C, WHSV = 6 h-1, H2/reactant molar ratio equal 4 and 1g of catalyst.

4.5. Pd-Pt/NaHMOR and Pd-Pt/Yb-USY.

The sulfur tolerance of Pt-Pd supported on zeolites and modified zeolites has been reported for hydrogenation of aromatics16,17. The high activity of bimetallic catalyst is related to a synergic effect of the two metals18. Modification of USY zeolite with ytterbium by impregnation decrease the number of strong acid sites9,f. On the other hand, Pd/HMOR in previous studies showed a fast deactivation caused by a rapid coke formation in the first minutes of stream8. In this experiment, in order to decrease the acidity of the support a partially exchanged zeolite HNaMOR was used as catalyst support.


Figure 8. Yield of p-cymene, selectivity to p-cymene and conversion of pure ART-CST on Pd-Pt(1.5 wt.%)/SiO2-Al2O3 catalyst at 300° C, WHSV = 6 h-1, H2/reactant molar ratio equal 4 and 1g of catalyst.

Under the described reaction conditions, catalysts supported over molecular sieves shown low performance in the targeted reaction and p-cymene is obtained in low yields. Figure 9 and figure 10 shows that a continuous selectivity decrease is observed while the conversion remains close to 100%.


Figure 9. Yield of p-cymene, selectivity to p-cymene and conversion of pure ART-CST on Pd-Pt/NaHMOR catalyst at 300° C, WHSV = 6 h-1, H2/reactant molar ratio equal 4 and 1g of catalyst.


Figure 10. Yield of p-cymene, selectivity to p-cymene and conversion of pure ART-CST on Pd Pd-Pt/Yb-USY catalyst at 300° C, WHSV = 6 h-1, H2/reactant molar ratio equal 4 and 1g of catalyst.

4.4. Pd-Pt(1.5 wt.%)/SiO2-Al2O3 .

The co-existence of Pt and Pd on SiO2-Al2O3 showed remarkable enhancement in the catalytic activity for hydrogenation of aromatics14. In this experiment we found good conversion of both a-pinene and b-pinene, and the minimal yield in the studied time of stream was of 64%.

5. CONCLUSIONS

This study shows the catalytic performance of different sulfur tolerant catalyst to obtain p-cymene from sulfur containing starting materials. Sulfur tolerant catalysts showed to have good catalytic activity to obtain p-cymene from a model of CST. Under the reaction conditions here used the selectivity is the parameter that most strong effect has on the yield toward p-cymene because conversion remains close to 98% in all experiments. Palladium catalysts supported over molecular sieves showed a weak catalytic activity and a strong decay on the selectivity in the studied time of stream under the investigated reaction conditions. Metallic and bimetallic catalysts supported over mildly acidic solids such as silica an alumina showed the best catalytic activity and p-cymene was obtained in approximately 65%.

ACKNOWLEDGMENT

M. Zarraga O. gives special thanks to the Research Council of University of Concepcion for the support obtained through the project DIUC 97.023.013-1. A. Miranda M. thanks CONICYT for the PhD. studies scholarship granted; German Academic Exchange Service (DAAD) for the research grant within the "sandwich-model" at RWTH-Aachen, Germany; H. Schuster and L. Rios for their valuable collaboration.

REFERENCES

1. H. Fiege, in: Ullmann's Encyclopedia of Industrial Chemistry, Vol. A8, VCH, Weinheim, 1985, p. 25 .         [ Links ]

2. R.W. Neuzil, D.H. Rosback, R.H. Jensen, J.R. Teague, A.J. deRosset, Chemtech, 8 498 (1980).         [ Links ]

3. D. Buhl, D.M. Roberge, W.F. Hölderich, Appl. Catal. A: General 188, 287(1999).         [ Links ]

4. P.A. Weyrich, H. Trevino, W.F. Hölderich, W.M.H. Sachtler, Appl. Catal. A: General 163, 31(1997).         [ Links ]

5. P.A. Weyrich, W.F. Hölderich, Appl. Catal. A: General, 158, 145 (1997).         [ Links ]

6. P. Lesage, J.P. Candy, C. Hirigoyen, F. Humblot, J.M. Basset, J. Mol. Catal. A, 112, 431 (1996).

7. V. Krishnasamy, Aust. J. Chem., 33, 1313 (1980).         [ Links ]

8. A. Stanislaus, L.M. Yeddanapalli, Can. J. Chem. 50, 113 (1972).         [ Links ]

9. D. Buhl, P.A. Weyrich, W.F. Hölderich, Stud. Surf. Sci. Catal., 121, 191 (1998).         [ Links ]

10. D. Buhl, P.A. Weyrich, W.M.H. Sachtler, W.F. Hölderich,Appl. Catal. A: Gen, 171, 1 (1998).         [ Links ]

11. J.O.J. Bledsoe, in: Kirk-Othmer Encyclopedia of ChemicalTechnology, Vol. 23, Wiley, New York, 1996, p. 833.         [ Links ]

12. D. M. Roberge, D. Bhul, J. P.M. Niederer, W. F. Hölderich, Appl. Catal. A: Gen. 215, 111 (2001).         [ Links ]

13. L. Hu, G. Xia, L. Qu, C. Li, Q. Xin, D. Li, J. Mol. Catal. A: Chem. 205, 71 (2001).         [ Links ]

14. T. Fujikawa, K. Idei, T. Ebihara, H. Mizuguchi, K. Usui, Appl. Catal. A: General, 192, 253 (2000).         [ Links ]

15. H. Yasuda, T. Tamoeka, T. Sato, N. Kijima, Y. Yoshimura, Appl. Catal. A: General, 185, L199 (1999).         [ Links ]

16. Y. Yoshimura, H. Yusada, T. Sato, N. Kijima, T. Kamoeka, Appl. Catal. A: General, 207, 303 (2001).         [ Links ]

17. L. J. Simon, J. G. van Ommen, A. Jentys, J. A. Lercher, J. Catal. 201, 60 (2001).         [ Links ]

18. B. Pawelec, R. Mariscal, R.M. Navarro, S. van Bokhorst, S. Rojas, J.L.G. Fierro, Appl. Catal. A: General, 225, 223 (2002).         [ Links ]

19. T.R. Savicha and L.A. Goldblatt, J. Am. Chem. Soc., 67, 2027 (1945)         [ Links ]

20. R. E. Fuguitt and J. E. Hawkins, J. Am. Chem. Soc., 69, 319 (1947).         [ Links ]

21. L. A. Goldblatt and S. Palkin, J. Am. Chem. Soc., 66, 655 (1944);         [ Links ]

22. V. M. Nikitin, J. Gen. Chem. U.S.S.R. (Eng. Transl.), 16, 1041 (1946).         [ Links ]

23. A. Stanislaus, L.M. Yeddanapalli, Can. J. Chem. 50, 61 (1972).         [ Links ]

24. A. Severino, A. Esculcas, J. Rocha, J. Vital, L.S. Lobo, Appl. Catal. A: General 142, 255 (1996).         [ Links ]

25. C.M. Lopez, F.J. Machado, K. Rodriguez, B. Mendez, M. Hasegawa, S. Pekerar, Appl. Catal. A: General 173, 75 (1998).         [ Links ]

26. M. Gscheidmeier, H. Häberlein, H.H. Häberlein, J.T. Häberlein, M.C. Häberlein, US Patent 5826202 (1998), to Hoechst AG.         [ Links ]

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