Deactivation catalyst

Catalyst deactivation occurs over time during a catalytic process and it is a phenomenon that invariably takes place in most industrial processes. Catalyst deactivation may occur via four main phenomena coke deposition which leads to the blocking of pores and active sites, poisoning via metals (S, As), sintering, and the loss of catalytic active phase. 

To do so, one scales down the process to a laboratory scale. This entails using the same space velocity, pressure, and temperature but a scaled down catalyst mass. Furthermore, to evaluate the catalysts under kinetic control regime one determines the appropriate conditions to minimize heat and mass transfer effects. Comparative evaluations can then be made between the results obtained in laboratory and industry. 

Stability is a key criterion when determining the most appropriate catalyst for industrial use. Among the deactivation phenomena, coke deposition and poisoning may be reversible but sintering is usually irreversible. 

Sintering is a thermodynamically favorable process which decreases the catalyst surface area. Although the catalyst soHd nature and the environment to which the catalyst is exposed influence the process, the sintering phenomenon is basically dominated by temperature. The minimum sintering temperature can be estimated from the melting temperature of the solid (Tmeitmg) expressed in Kelvin  

The melting temperature of nickel is 1726 K. Therefore, according to Equation 19.1, nickel sintering may be carried out above 517 K. 

In the USA, three-way catalysts have to maintain high activity and meet the emission standards of Tab. 10.2 after 50,000 miles or five years. Because catalysts deactivate with use, fresh catalysts are designed such that they perform well below the emission standards. The extent to which a three-way catalyst deactivates depends on many factors. The wide range of vehicle operating conditions due to differences in style of driving is important. 

The most common problem associated with supported metal catalysts for use in oxidation reactions is the overoxidation of the active sites. For example the rate of oxidation is far higher on reduced 

The formation of strongly adsorbed by-products is another major difficulty. Possible contaminants include condensation and oligomerisation products of carbonyl species and the decomposition of alcohol species to yield CO and adsorbed hydrocarbon. Numerous examples of such deactivation scenarios exist giving a genuine challenge to both the catalyst chemist and the chemical engineer to produce systems which minimise these problems. 

Another important consideration in the FCC unit model is the deactivation of catalyst as it circulates through die unit Previous work has used two different approaches to model catalyst activity time on stream and coke on catalyst [49]. Since the 21-lump includes discrete lumps for the kinetic and metal cokes, this work uses a coke-on-catalyst approach to model catalyst deactivation. In addition, this work includes a rate equation in the kinetic network for coke balance on the catalyst. The general deactivation function due to coke, coke is given by Eq. (4.4). 

Detailed kinetic studies revealed an intrinsic deactivation process of the standard phosphine-palladium catalyst [50, 51] both the system Pd(OAc)2/P(C6H5)3 and isolated Pd[P(C6Hs)3]4, employed in the reference reaction of eq. (15), suffer from P-C-bond cleavage, the extent of which seems to increase with temperature. 

It is to be stressed that aryl chlorides do react with Pd complexes such as Pd[P(C6Hs)3]2(dba) or Pd[(C6Hs)3]4 at 140 °C cf. typical Heck conditions [40a]. 

It is P-C-bond cleavage and subsequent isomerizations that are responsible for the deactivation in the case of aryl chlorides and not a missing reactivity for oxidative addition as previously suggested Furthermore, the nature of the anion seems to dominate the subsequent steps in the catalytic cycle. Recently, these problems have been solved by the application of defined catalyst systems such as pallacycles. 

The conversion of alcohols in liquid-phase oxidation on metals does not go to completion, or proceeds at a very slow rate, because catalysts deactivate in the course of reaction. Deactivation could be irreversible when the catalyst structure is modified, e. g. by metal-particle growth or metal leaching, or partially reversible when the metal surface is partially blocked by oxygen or reaction products. 

Metal catalysts could be deactivated by blocking the surface by adsorbed reaction products ( self-poisoning or chemical deactivation ), or by adsorbed oxygen ( over-oxidation or oxygen poisoning ). 

Chemical deactivation by adsorbed impurities or reaction products was identified as a primary cause of catalyst deactivation [42,43,45-48,50]. Deactivation of platinum catalysts in l-methoxy-2-propanol oxidation was attributed to polymeric species formed by aldol-dimerization and detected by chromatographic 

It can be concluded that chemical deactivation and over-oxidation have synergistic effects, because any blockage of surface sites by side products would reduce the rate of reaction and, therefore, the consumption of oxygen, thus favoring the over-oxidation of the surface. 

In designing fixed and ideal fluidized-bed catalytic reactors, we have assumed up to now that the activity of the catalyst remains constant throughout the catalyst s life. That is, the total concentration of active sites, C accessible to the reaction does not change with time. Unfortunately, Mother Nature is not so kind as to allow this behavior to be the case in most industrially significant catalytic reactions. One of the most insidious problems in catalysis is the loss of catalytic activity that occurs as the reaction takes place on the catalyst. A wide variety of mechanisms have been proposed by Butt and Petersen, to explain and model catalyst deactivation. 

Catalytic deactivation adds another level of complexity to sorting out the reaction rate law parameters and pathways. In addition, we need to make adjustments for the decay of the catalysts in the design of catalytic reactors. This adjustment is usually made by a quantitative specification of the catalyst s activity, a(t), In analyzing reactions over decaying catalysts we divide the reactions into two categories separable kinetics and nonseparable kinetics. In separable kinetics, we separate the rate law and activity  

Separable kinetics -r f = fl(past history) X (fresh catalyst). 

When the kinetics and activity are separable, it is possible to study catalyst decay and reaction kinetics independently. However, nonseparability, 

In this section we shall consider only separable kinetics and define the activity of the catalyst at time t, a t), as the ratio of the rate of reaction oi a catalyst that has been used for a time t to the rate of reaction on a fi esh catalyst  

In light of the important roles of H2 and H2O in the CO oxidation reaction, we examined the kinetic isotope effects of these species [5,40]. Similar to H2, D2 in the SCO reaction feed was also effective in preventing deactivation, but the rate of CO oxidation was only about 70% of that in H2. This corresponds to an apparent kinetic isotope effect for CO oxidation of 1.4+ 0.2. Also, the rate of D2 oxidation was much slower than the rate of H2 oxidation, resulting in a selectivity toward CO oxidation of 86% versus 77% in the presence of H2. Moreover, regeneration of the catalyst with the SCO feed containing D2 restored only 65% of the activity that could be restored after treatment with a H2-containing SCO feed for the same exposure time. Similar differences were obtained when we attempted to regenerate a CO-oxidation deactivated catalyst by treatment in a flow of pure, dry D2 at room temperature. The subsequent initial CO oxidation activity was only 70% of that which could be achieved by treatment in H2. 

Acknowledgement. - This research was supported by the EMSI program of the NSF and Department of Energy Office of Science (CHE-9810378) at Northwestern University Institute of Environmental Catalysis. 

Okumura, S. Nakamura, S. Tsubota, T. Nakamura, M. Azuma, and M. Haruta, Catal. Lett. 51 (1998) 53 M. Okumura, S. Nakamura, S. Tsubota, T. Nakamura, M. Haruta, Stud. Surf. Sci. Catal. 118 (1998) 277. 

Horvath, M. Polisset-Thfoin, J. Frissard, and L. Gucci, Solid State Ionics 141 

Margitfalvi, A. Fasi, M. Hegediis, F. Lonyi, S. Gobolos, and N. Bogdan-chikova, Catal. Today 72 (2002) 157. 

---

This was a Hquid-phase process which used what was described as siUceous zeoUtic catalysts. Hydrogen was not required in the process. Reactor pressure was 4.5 MPa and WHSV of 0.68 kg oil/h kg catalyst. The initial reactor temperature was 127°C and was raised as the catalyst deactivated to maintain toluene conversion. The catalyst was regenerated after the temperature reached about 315°C. Regeneration consisted of conventional controlled burning of the coke deposit. The catalyst life was reported to be at least 1.5 yr. 

Amorphous Silica—Alumina Based Processes. Amorphous siHca—alumina catalysts had been used for many years for xylene isomerization. Examples ate the Chevron (130), Mamzen (131), and ICI (132—135). The primary advantage of these processes was their simpHcity. No hydrogen was requited and the only side reaction of significance was disproportionation. However, in the absence of H2, catalyst deactivation via coking... 

Pure dry reactants are needed to prevent catalyst deactivation effective inhibitor systems are also desirable as weU as high reaction rates, since many of the specialty monomers are less stable than the lower alkyl acrylates. The alcohol—ester azeotrope (8) should be removed rapidly from the reaction mixture and an efficient column used to minimize reactant loss to the distillate. After the reaction is completed, the catalyst may be removed and the mixture distilled to obtain the ester. The method is particularly useful for the preparation of functional monomers which caimot be prepared by direct esterification. 

Hydrogenation of the oxides of carbon to methane according to the above reactions is sometimes referred to as the Sabatier reactions. Because of the high exothermicity of the methanization reactions, adequate and precise cooling is necessary in order to avoid catalyst deactivation, sintering, and carbon deposition by thermal cracking. 

Supported aqueous phase (SAP) catalysts (16) employ an aqueous film of TPPTS or similar ligand, deposited on a soHd support, eg, controlled pore glass. Whereas these supported catalysts overcome some of the principal limitations experienced using heterogeneous catalysts, including rhodium leaching and rapid catalyst deactivation, SAP catalysts have not found commercial appHcation as of this writing. 

Desalting is a water-washing operation performed at the production field and at the refinery site for additional cmde oil cleanup. If the petroleum from the separators contains water and dirt, water washing can remove much of the water-soluble minerals and entrained soflds. If these cmde oil contaminants are not removed, they can cause operating problems duting refinery processiag, such as equipment plugging and corrosion as well as catalyst deactivation. 

Because soHd acid catalyst systems offer advantages with respect to their handling and noncorrosive nature, research on the development of a commercially practical soHd acid system to replace the Hquid acids will continue. A major hurdle for soHd systems is the relatively rapid catalyst deactivation caused by fouling of the acid sites by heavy reaction intermediates and by-products. 

Eig. 1. Pathways for catalyst deactivation and ligand decomposition, where R = CgH. ... 

The Snamprogetti fluidized-bed process uses a chromium catalyst in equipment that is similar to a refinery catalytic cracker (1960s cat cracker technology). The dehydrogenation reaction takes place in one vessel with active catalyst deactivated catalyst flows to a second vessel, which is used for regeneration. This process has been commercialized in Russia for over 25 years in the production of butenes, isobutylene, and isopentenes. 

The appHcations of supported metal sulfides are unique with respect to catalyst deactivation phenomena. The catalysts used for processing of petroleum residua accumulate massive amounts of deposits consisting of sulfides formed from the organometaHic constituents of the oil, principally nickel and vanadium (102). These, with coke, cover the catalyst surface and plug the pores. The catalysts are unusual in that they can function with masses of these deposits that are sometimes even more than the mass of the original fresh catalyst. Mass transport is important, as the deposits are typically formed... 

In service, supported catalysts frequentiy undergo loss of activity over a period of time. In many cases, such catalyst deactivation is accompanied by the loss of accessible surface area of the active phase by sintering, by the accumulation of poisons, or by conversion of active sites to inactive species. 

The presence of other functional groups ia an acetylenic molecule frequendy does not affect partial hydrogenation because many groups such as olefins are less strongly adsorbed on the catalytic site. Supported palladium catalysts deactivated with lead (such as the Liadlar catalyst), sulfur, or quinoline have been used for hydrogenation of acetylenic compound to (predominantiy) cis-olefins. 

Catalyst lifetimes are long in the absence of misoperation and are limited primarily by losses to fines, which are removed by periodic sieving. Excessive operating temperatures can cause degradation of the support and loss of surface area. Accumulation of refractory dusts and chemical poisons, such as compounds of lead and mercury, can result in catalyst deactivation. Usually, much of such contaminants are removed during sieving. The vanadium in these catalysts may be extracted and recycled when economic conditions permit. 

The heat released from the CO—H2 reaction must be removed from the system to prevent excessive temperatures, catalyst deactivation by sintering, and carbon deposition. Several reactor configurations have been developed to achieve this (47). 

K. Otto, W. B. Wdhamson, and H. Gandhi, Ceram. Eng. Sci. Proc. 2(6), (May/June, 1981). Good review of various catalyst deactivation processes. 

W. B. Williamson and co-workers. Catalyst Deactivation Due to Glac Formation from Oil-Derived Phosphorus and Zinc, SAE 841406, Society of Automotive Engineers, Warrendale, Pa., 1984. 

In the mass-transfer limited region, conversion is most commonly increased by using more catalyst volume or by increasing cell density, which increases the catalytic wall area per volume of catalyst. When the temperature reaches a point where thermal oxidation begins to play a role, catalyst deactivation may become a concern. 

Catalyst Selection. The choice of catalyst is one of the most important design decisions. Selection is usually based on activity, selectivity, stabiUty, mechanical strength, and cost (31). StabiUty and mechanical strength, which make for steady, long-term performance, are the key characteristics. The basic strategy in process design is to minimize catalyst deactivation, while optimizing pollutant destmction. 

Catalyst Deactivation. Catalyst deactivation (45) by halogen degradation is a very difficult problem particularly for platinum (PGM) catalysts, which make up about 75% of the catalysts used for VOC destmction (10). The problem may weU He with the catalyst carrier or washcoat. Alumina, for example, a common washcoat, can react with a chlorinated hydrocarbon in a gas stream to form aluminum chloride which can then interact with the metal. Fluid-bed reactors have been used to offset catalyst deactivation but these are large and cosdy (45). 

---