J.M. Parera

INCAPE - Santiago del Estero 2654 - (3000) Santa Fe - Argentina.

Fax: 54-342-4531068. E-mail: incape@fiqus.unl.edu.ar

Catalysts for the production of components for reformulated gasolines are studied, mainly those for reactions using raw materials separated from natural gas (n-C₄H₁₀, n-C₆H₁₀) due to the increase in natural gas consumption and its great reserves. Light alkanes and alkenes isomerization is very important and a new group of catalysts obtained promoting zirconia with oxoanions are active and selective at lower temperatures than former catalysts. New commercial processes use this type of catalysts (1-3).

SO₄^2-, WO₄^2-, PO₄^3- and MoO₄^2- have similar textural and structural promotion on ZrO₂: stabilize the tetragonal structure of ZrO₂ and preserve the high surface area of Zr(OH)₄ after calcination at high temperatures.

The oxoanions of S and W present the best catalytic activity in isomerization of n-C₄H₁₀. The addition of Pt increases the stability, even the metallic properties of Pt are affected by the interaction with the support. Under certain conditions, a bifunctional metal-acid isomerization mechanism can proceed.

There is an antagonism between the conditions needed for optimal acid and metal properties in Pt/SO₄^-2-ZrO₂. The acid support, SO₄^2--ZrO₂, needs to be converted by calcination into an acidic support with tetragonal phase to activate n-butane. Pt should be as Pt° to have its full capacity. It could be obtained by reduction in H₂ if supported on non-crystalline sulfate-zirconia, but for Pt/SO₄^2--ZrO₂, calcination at T>400°C makes Pt to stay in an oxidized (electron deficient) state. The reducibility of Pt is improved and metal properties can be detected when calcination occurs at T<400°C, or calcination of Zr(OH)₄ at T>400°C before sulfation. But the acidic function is completely inactive because SO₄^2--ZrO² is amorphous in the first case and monoclinic in the second.

A similar antagonistic effect is observed when Pt/SO₄^2--ZrO₂ is calcined in N₂ instead of air. Air is necessary to have the maximum isomerization activity but the metallic activity of Pt is the minimum.

Similarly to calcination temperature, the crystalline phases of zirconia have antagonic effects on acid and metal functions. Sulfate promoted tetragonal zirconia is active in n-C₄H₁₀ isomerization but Pt supported on tetragonal zirconia is inactive for dehydrogenation. Sulfate promoted monoclinic zirconia is inactive for isomerization but Pt supported on monoclinic zirconia is active, having all Pt in the metallic state (Pt°).

In the case of Pt/WO_x-ZrO₂ a smaller amount of Pt than in SO₄^2--ZrO₂ is necessary to have metallic function activity due to the lower Pt-support interaction. At low temperatures n-C₄H₁₀ is isomerized through an acid mechanism because of thermodynamic limitations in the formation of n-C₄H₈ and deactivation of the metallic function by the support. Higher temperatures are required to have the benefitial effect of Pt (bifuncional mechanism). Longer chain alkanes, as n-C₇H₁₆, are more easily dehydrogenated and allow the bifunctional mechanism already at 250°C. MoO₃-ZrO₂ is not able to isomerize n-C₄H₁₀, but is active in n-C₇H₁₆ isomerization as a function of Mo content and calcination temperature.

The obtention of light isoalkanes by hydroisomerization-cracking of n-C₈ and higher alkanes is studied using oxoanions promoted zirconia as a bulk catalyst or supported over SiO₂, being Pt/SO₄^2--ZrO₂/SiO₂ very active and stable. Pt was also supported on Al₂O₃ and mixed mechanically.

The isomerization of n-butene is studied using as catalysts promoted ZrO₂, ferrierite and WO_x-Al₂O₃. Ferrierite is the best catalyst and the promotion with WO_x species improves the stability.

For n-C₆H₁₄ isomerization over WO_x-ZrO₂ the presence of Pt is necessary to obtain a good catalyst; very small Pt concentrations (>0.05%) are enought to have a good activity and stability. The concentration of tungsten on zirconia plays a role in catalytic activity. The calcination temperature is important to get a good activity and selectivity; above 600°C appear new species which are important for the catalyst performance. The calcination before the Pt addition enhances the catalytic behavior.

1. U.S. Patent N° 5763713 (1998).

2. C. Gosling, R. Rosin and P. Bullen, Petrol. Technol.-Quaterly, Winter 1997/98, page 55.

3. Chemical Engineering, March 1999, page 19.

 INTEREST IN RESEARCH COLLABORATION

This group of research has several installations for catalytic test reactions (isomerization, reforming, cracking of hydrocarbons, test of metallic function, of acidic functions) and for simple catalyst characterization as XRD, TPR, FTIR but do not have possibilities of using more sophisticated technology.

We should like to have collaborations with other researchers in order to fulfill a research with sound basis joining our experiments with their studies. We are working in reaction of hydrocarbons on acid and bifunctional catalysts. The catalysts more commonly employed contain a metal (Pt, Pd) supported on oxoanion promoted zirconia, mordenite, KL zeolite, ferrierite, and mixtures of these catalysts among them and with Pt/Al₂O₃, SiO₂, etc. For the case of Pt/SO₄^2--ZrO₂ and Pt/WO_x-ZrO₂, the following areas, which are not available to us, are of special interest.

ESR: this method allows the detection of paramagnetic species (Zr³⁺ , O₂^-, O vacancies, hydrocarbon radicals) and should be useful for the system Pt/SO₄^2--ZrO₂ and Pt/WO_x-ZrO₂ in order to study possible active species treating with H₂ or inert.

UV: to study the electronic transitions in the bulk of the solid and in its surface to separate bulk effect from electronic transitions among surface species and relate to active sites.

MS: with controlled temperature in order to monitor the products of combustion, reduction or reaction by means of mass spectrometry.

EXAFS, XPS, AUGER: to see the fine structure of zirconia, of the layers of W in WO_x-ZrO₂, to see the state of Pt to relate with the support interaction.

- New catalyst formulations or methods of catalyst preparation.