Influence of Coke Deposition on Selectivity in Zeolite Catalysis

Abstract

The selectivity of a complex reaction depends on a number of factors, such as the reaction mechanism, operating conditions, catalyst properties and catalyst deactivation. The present work discusses how the selectivity of a complex reaction depends on the formation of coke. For zeolite catalysts, changes in selectivity can be a result of intrinsic selectivity effects or shape selectivity effects. A method is suggested to analyze a complex reaction system with deactivation caused by coke formation, and different cases of selectivity change during deactivation of zeolites are discussed. Transition-state shape selective deactivation is proposed as a mechanism in addition to the deactivation mechanisms suggested by Guisnet and Magnoux (Guisnet, M.; Magnoux, P. Appl. Catal. 1989, 54, 1). By variation of the space velocity, the selectivities to the main products are measured as a function of conversion (optimum performance envelopes). Selectivities at given conversions can then be compared for results obtained with different contact time and with varying degree of catalyst deactivation (space velocity loops). Selective or nonselective deactivation is thereby distinguished. This type of selectivity plot is applied to two different types of reaction, i.e. ethene oligomerization over HZSM-5 and methanol conversion to light olefins (MTO) over SAPO-34. The selectivity of ethene oligomerization was affected only by the decrease in conversion due to coke formation; hence, this is an example of nonselective deactivation. Selective deactivation was found for methanol conversion over SAPO-34.

A methanol to olefins review: Diffusion, coke formation and deactivation on SAPO type catalysts

“…In a similar way, the MeOH diffusivity and the rate constant of methanol conversion were determined as a function of coke content based on the methanol conversion on SAPO-34 with different crystal sizes (0.25-2.5 lm) [49]. The effective methanol diffusivity of 1.1 Â 10 À8 m 2 /s estimated indirectly from the kinetic data is comparable with the steady-state diffusivity of 3 Â 10 À9 m 2 /s measured at low temperatures [49]. The effective diffusivity of methanol decreased by almost three orders of magnitude from the fresh catalyst to that with a coke content of 15 wt.%.…”

“…The effective diffusivity of methanol decreased by almost three orders of magnitude from the fresh catalyst to that with a coke content of 15 wt.%. Such decrease in the effective diffusivity of methanol with increasing coke content has been described by the percolation theory, where changes in the diffusion path with coke blockage of cavities were simulated by Monte Carlo method in a 3-D network of SAPO-34 cubic crystals [49]. Recently, the percolation theory was successfully extended to predict the deactivation behavior of MTO's porous catalyst in a fixed bed reactor [50].…”

“…SAPO-34 has a high catalytic activity for the MTO reaction, but suffers fast deactivation due to coke formation [24,25,34,35,48,49,[52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67]. The rate of deactivation is so high that it is difficult to decouple it from the kinetics of the main reactions.…”

Mathematical modeling of multi-bed adiabatic reactor for the Methanol-to-Olefin process

“…In fact, the decrease of catalyst activity due to coking occurs in the whole MTO process. Coking generation will definitely increase the mass transfer resistance inside the catalyst, as mentioned by Chen et al [17]. However, obvious catalyst deactivation could only be detected after a certain amount of coke is deposited already.…”

Synthesis of Liquefied Petroleum Gas <i>via</i> Methanol and/or Dimethyl Ether from Natural Gas (Part 3) Investigation of Reaction Variables, Ethene Recycling and Catalyst Regeneration at High Conversion from Methanol and/or Dimethyl Ether to LPG

“…Olefins and aromatics are considered to be coke precursors in most zeolitecatalyzed hydrocarbon processes, and transformation of these active intermediates into coke usually depends on the pore structure and acidity of the zeolites, although the relative effects of pore structure and acidity are sometimes difficult to discriminate. Coke formation in the micropores of MFI zeolites is a transition-state shape selective reaction, whereas the formation of fused aromatics may be prohibited by MFI structural constraints 10) . Consequently, the condensed aromatic compounds that easily induce carbon deposition may be formed predominantly outside the zeolite pores.…”