Catalyst Deactivation by Site Coverage and

3 Catalyst Deactivation by Site Coverage and Pore Blockage 

Local value of deactivation function versus site number for a single-ended pore with a deterministic distribution of sites. Parameter At curve 1,0 curve 2,0.02 curve 3,0.50 curve 4, 1.00 curve 5, 2.00 curve 6, oo. From Beeckman and Froment [1979]. 

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M. Sahami and T. Tsotsis, "A Percolation Model of Catalyst Deactivation by Site Coverage Pore Blocking", J. Catalysis. 1986, 26, 552-562. 

The deactivation of a catalyst by site coverage and pore blockage can be expressed in terms of relative activity, R, which is the ratio of the rate of the catalytic reaction at time t to the rate at time zero. Supposing the pore volume to be concentrated in voids (Fig. 2) and using assumptions 1-4, we have (cf. Section III,D)... 

A fundamental kinetic freunework is developed for deactivation by site coverage, coke grovth and blockage in pores and networks of pores. Diffusional limitations are also accounted for The methodology of kinetic studies of catalyst deactivation by coke formation is discussed by means of a number of practical examples. Finally, the effect of catalyst deactivation on the behavior of reactors is illustrated. 

The problem of diffusional limitations in the main reaction and deactivation by site coverage only was solved by Hasamune and Smith [ref. 22] and further investigated by Kan et al, [ref, 6]. when pore blockage also has to be considered, the problem becomes considerably more complex. As dealt with by Beecknan and Froment [ref, 23] the model equation for the ntain reaction closely resembles the second order differential equation for the diffusion and reaction, but also contains the probability S, Further, the blockage affects the distance available for diffusion Since the deactivation is expressed in terms of the coke content of the catalyst, a differential equation relating the evolution of the coke content with time to the rate of coke formation is required. This differential equation contains P and S Both these quantities are governed by additional differential equations The rate of coke formation contains which is not a constant any lAore, because... 

It may be questioned how representative a pore really is for a catalyst particle. No doubt in the future, with growing attention for the more detailed characterization of catalyst particle structures, more elaborate representations of the particle will be used. Beeckman and Froment [1982] developed the theory for reaction and deactivation by coke formation in random networks of pores. The deactivation was considered to occur through site coverage and pore blockage. Diffusion limitations were also included, but only for deactivation by site coverage. 

During induction, catalyst activity and selectivities to aromatics and propene increase steadily. Improvement of catalyst performance is due to increase in Ga dispersion and formation of dispersed Ga species (Gao) which are efficient for the heterolytic recombinative release of hydrogen [18,191. The Ga/H-MFI catalyst then reaches its optimal aromatisation performance (stabilisation). Ci to C3 hydrocarbons productions are at their lowest. The gallium dispersion and the chemical distribution of Ga are optimum and balance the acid function of the zeolite. Reversible deactivation during induction and stabilisation of the catalyst is due to site coverage and limited pore blockage by coke deposition. 

The deactivation of cracking catalysts by coking with vacuum gas oils (VGO) is studied in relation to the chemical deactivation due to site coverage, and with the increase of diffusional limitations. These two phenomena are taken into account by a simple deactivation function versus catalyst coke content. The parameters of this function arc discussed in relation to feedstock analysis and change of effective diffiisivity with catalyst coke content. 

Some cases of catalyst deactivation by over-oxidation platinum leaching, platinum particle growth and site coverage during reductive pretreatment as well as during reaction were presented for the oxidation of ethanol and methyl-a-D glucopyranoside (MGP) in combination with the use of various catalyst characterization techniques. 

J. W. Beeckman and G. Froment, "Catalyst Deactivation by Active Site Coverage and Pore Blockage", Ind. Eng. Chem. Fund.. 1979, lS-3, 246-256. 

Beeckman, J.W. and Froment, G.F. (1979) Catalyst deactivation by active site coverage and pore blockage Ind. Eng. Chem. Fundam. 18, 245-256. 

The deactivation of bifunctional reforming catalysts is mainly due to the deposition of coke on the metal and the acid sites. Coking on the metallic function is responsible for the rapid initial deactivation which is also accompanied by changes in the selectivity of products formed on its surface. The acidic function deactivates more gradually with time as a result of the add site fouling by coke coverage. 

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