Dec-2008

TUD-1: a generalised mesoporous catalyst family for industrial applications

The article discusses the synthesis, characterisation and applications of a very unique mesoporous material, TUD-1

Philip J Angevine, Anne M Gaffney, Zhiping Shan, Jan H Koegler and Chuen Y Yeh, Lummus Technology

Article Summary

This amorphous material possesses three-dimensional interconnecting pores with narrow pore size distribution and excellent thermal and hydrothermal stabilities. The basic material is Si-TUD-1; however, many versions of TUD-1 using different metal variants have been prepared, characterised and evaluated for a wide variety of hydrocarbon processing applications. Also, zeolitic material can be incorporated into the mesoporous TUD-1 to take the advantage of its mesopores to facilitate the reaction of large molecules and enhance the mass transfer of reactants, intermediates and products. Examples of preparation and application of many different TUD-1 are described in this article. 

Introduction
Porous materials have a successful history in heterogeneous catalysis. While both microporous and macroporous materials have been used in industry for many decades, academic and industrial scientists have been searching for materials with intermediate pore sizes between the microporous and the macroporous range. These mesoporous materials are defined as having pore diameters between 2 and 50 nm. In addition, materials with hierarchical pore structures — ie, going from larger mesopores to smaller micropores — are also of practical interest. Both types of materials are anticipated to be useful for conversions of higher molecular weight materials. Catalysts based on these materials should have significant benefits for reactions where mass transfer plays a role.

Since the discovery by researchers at Mobil of a new family of crystalline mesoporous materials,1 a large effort has been expended on synthesis, characterisation, and catalytic evaluation.2 MCM-41 is a one-dimensional, hexagonal structure. MCM-48 is a cubic structure with two, nonintersecting pore systems.3 MCM-50 is a layered structure with silica sheets between the layers.4 Many scientists also looked into other mesoporous materials, of note the HMS (Hexagonal Molecular Sieve) family5 and SBA-15 (acronym derived from Santa Barbara University),6 but to date few materials have been both catalytically significant and inexpensive to synthesise.

This article describes the unique properties of TUD-1. We will highlight several organic synthesis applications. We discuss petrochemical and refining applications of TUD-1 in more detail elsewhere.7

TUD-1
A joint research project between Lummus Technology and the Delft University of Technology led to the discovery of a new mesoporous material, named TUD-1.8 TUD-1 is a three-dimensional amorphous structure of random, interconnecting pores. The original emphasis was on the silica version, which has since been extended to about 20 chemical variants (eg, Al, Al-Si, Ti-Si).

Key common properties of TUD-1 are:
• Random, three-dimensional interconnecting pores
• Tunable porosity (pore volumes of 0.3-2.5 cc/g and diameters of 4-25 nm)
• High surface area: 400-1000 m2/g
• Excellent thermal, hydrothermal and mechanical stability.

TUD-1 is an amorphous material. Unlike crystalline structures, it has no characteristic x-ray diffraction pattern. Figure 1 illustrates the pore diameter of TUD-1 in comparison to some major molecular sieves: ZSM-5, Zeolite Y and MCM-41. It is important to note that the pore diameter of TUD-1 can be varied from about 40Å to 250 Å.

One of the early questions raised on TUD-1 dealt with its pore structure: did it have intersecting or nonintersecting pores? At the University of Utrecht, one conclusive characterisation was carried out with a silica TUD-1 with Pt inserted, which was analysed by 3-D TEM (transmission electron microscopy).9 The Pt anchors (not shown) were used as a focal point for maintaining the xyz orientation. As shown in Figure 2, the TUD-1 is clearly amorphous. While not quantitatively measured for this sample, the pores appear rather uniform, consistent with all porosimetry measurements on TUD-1 showing narrow pore size distributions.

General method of synthesis
The original synthesis route (ie, Si-TUD-1) involves a monomeric silica source TEOS (tetraethylorthosilicate), mixing with TEA (triethanolamine) and optionally TEAOH (tetraethylammonium hydroxide). The TEA serves as a template for the mesopore formation. Desirable properties of the TUD-1 template are: physically stable at elevated temperatures (200-250°C), chemically interactive with the inorganic phase and inexpensive. The TEAOH serves as both a source of quaternary cation (to generate some micropores, if required) and a basic environment to accelerate TEOS hydrolysis. The reaction rate increases with pH; to a large extent, this acceleration can also be achieved by increasing the temperature. The second step involves an ageing/drying phase to establish the primary pore structure. The last step, calcination, is required to remove the large quantities of organics. An optional step, between drying and calcination, is a pore modification step employing elevated temperature.

Figure 3 highlights the major structural transformations that take place in TUD-1 synthesis.10 The three major steps are: formation of a homogeneous mixture,  migration of the template to achieve meso-sized aggregation and pore generation. Additionally, if inorganic bases such as TEAOH are used, some micropores are formed in addition to the mesopores. This is another key differentiator from many other crystalline mesoporous materials. In terms of unit operations, the major steps can be divided into six steps: (a1) mixing, (a2) hydrolysis, (b1) ageing, (b2) drying, (c1) heat treatment [optional], and (c2) calcination.

The flexibility of the synthesis method provides for opportunities to incorporate other elements besides silicon. One of the first elements added was Al.11 Soon after the discovery of the silicon based TUD-1, it was found that adding suitable aluminum sources to the above procedure yielded very similar Al-Si-TUD-1 structures. Since then, many other TUD-1 variants have been prepared. Most TUD-1 variants are either Si-TUD-1 or an M-Si version, where M is another element (eg, Ti, Al, Cr, Fe, Zr, Ga, Sn, Co, Mo, V). Non-siliceous versions of Al- and Ti-TUD-1 have also been made. The typical SiO2/ MxOy molar ratio is 20-∞.