Carbon as Catalyst Support

FRANCISCO RODRIGUEZ-REINOSO and ANTONIO SEPULVEDA-ESCRIBANO 

Porous carbon materials constitute a very flexible set of supports for the preparation of heterogeneous catalysts. Their physical and chemical surface properties can easily be tailored to develop a large surface area to disperse the active phases, the proper pore size distribution to facilitate the diffusion of reactants and products to and from the surface, and the acid-base character needed for obtaining the best performance. 

Although carbon-supported catalysts are considered to be the best choice for a great number of reactions, few large-volume processes currently use these systems, and less than 1% of the activated carbon production worldwide is used as catalyst support. This may be due to the lack of reproducibility that sometimes arises with carbon-supported catalysts as a result of the relatively poor knowledge of the properties of these materials that influence their behavior. 

A large variety of carbon materials can and have been used as catalyst supports. The most important are granular and powdered activated carbons and carbon blacks, but there is increasing interest in related materials, such as activated carbon fibers and cloths, nanotubes, and nanofibers [8]. A comprehensive review 

Carbon Materials for Catalysis, Edited by PhUippe Serp and Josd Luts Figueiredo Copyright 2009 John Wiley Sons, Inc. 

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The dispersion and solid-state ion exchange of ZnCl2 on to the surface of NaY zeolite by use of microwave irradiation [17] and modification of the surface of active carbon as catalyst support by means of microwave induced treatment have also been reported [18]. The ion-exchange reactions of both cationic (montmorillonites) and anionic clays (layered double hydroxides) were greatly accelerated under conditions of microwave heating compared with other techniques currently available [19.]... 

In recent years a simplifying attempt to overcome this complexity was to analyze carbon by TPD and to integrate the total CO and CO2 emission and to correlate the results with sample pretreatment and chemical reactivity [33]. The limited validity of such an approach is apparent. As is illustrated below, the chemically complex surfaces which are not described by such crude correlations are those with the highest catalytic activity. In applications of carbons as catalyst support it is immediately apparent that the details of the car-bon-to-metal interaction depend crucially on such details of surface chemistry. This explains the enormous number of carbon supports commercially used (several thousands). A systematic effort to understand these relationships on the basis of modern analytical capabilities is still missing. 

In the preface to their landmark monograph. Mattson and Mark 61 wrote Carbon researchers have, for the most part, considered the surface chemistry of activated carbon to be in such a state of disarray that they want to avoid lengthy discussions of surface phenomena. More than two decades later, with vast improvements in our knowledge of the role of surface chemistry in carbon gasification [37,6181 and in the use of carbons as catalyst supports [22], it is argued here that this is no longer true. 

Since their discovery in 1991 [1], carbon nanotubes have received great attention due to their unique chemical and physical properties which render them attractive in several potential applications [2]. Among them, the use of nanostructured carbon as catalyst support seems to be very promising according to the last results reported in the literature [3-5]. 

L.R. Radovic, C. Sudhakar, (H. Marsh, E.A. Heintz, F. Rodriguez-Reinoso, eds.), Carbon as Catalyst Support Introduction, properties, and applications. In Introduction to Carbon Technologies, University of Alicante, Alicante, Spain, 1997. 

Rodriguez-Reinoso F, Sepulveda-Escribano A (2009) Carbon as catalyst support. In Serp P, Figueiredo JL (eds) Carbon materials for catalysis. Wiley, New Jersey... 

The role of electrochemical surface properties in the use of carbons as catalyst supports and adsorbents was often pointed out in the literature [1,63,66-70]. The surface charge distribution of carbons was most often assessed by two methods, mass titration and electrophoresis. 

Pd supported catalysts on activated carbon (AC) have been widely studied as catalysts for hydrogenation, dehydrogenation and oxidation reactions for the production of fine chemicals [2]. Activated carbons as catalyst supports present several advantages being relatively inexpensive and inert materials [3]. However, they also exhibit a major drawback as their sttrface properties can vary from batch to batch. Fttrthermore, typically Pd/AC catalysts exhibit acidic surface groups, such as carbonyl, carboxylic, phenolic hydroxyl, lactone and quinone groups [4]. It was recently reported that Bransted acid sites enhance the hydrogenolysis of secondary alcohols, such as 1-phenylethanol, in the model reaction [5]. 

H. Juntgen, Activated carbon as catalyst support. A review of new research results, Fuel, 65 (1986) 1436-1446. 

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