Catalyst poisoning and fixed bed reactor dynamics

The poisoning kinetics of thiophene on Ni-kieselguhr catalysts and the deactivation behavior of nonisothermal fixed bed reactors have been studied experimentally using benzene hydrogenation as a model exothermic reaction. The time dependent axial temperature profiles in the reactors were measured and compared with values evaluated from a dispersion model, the parameters of which have been determined in separate experimentation.Poisoning kinetics were measured in a series of differential reactor experiments at atmospheric total pressure, thiophene partial pressures of 0·037-0·19 torr, hydrogen to benzene molar ratios >$\text{8}\text{1}$ and temperatures from 60–180°C. Excellent agreement was found with a power law equation for the rate of change of activity with time, first order in catalyst activity and in thiophene concentration, with an experimental activation energy of 1080 kcal/kmole.This correlation of poisoning kinetics, however, was not able to predict the propagation of the zone of activity (hot-spot) on poisoning of an integral fixed bed reactor. Initial (steady state) temperature profiles were modeled satisfactorally, but the rate of migration of the hot spot was found experimentally to be more rapid than that predicted from the correlation of poisoning kinetics. A semi-empirical two site deactivation model is shown to resolve the discrepancy.     

Deactivation of Copper Metal Catalysts for Methanol Decomposition, Methanol Steam Reforming and Methanol Synthesis

Laboratory and industrial results are reviewed to elucidate the general features of the deactivation of supported copper metal catalysts in various reactions involving methanol as reactant or product. Most catalyst types are based on Cu/ZnO formulations that contain stabilisers and promoters such as alumina, alkaline earth oxides and other oxides. These additional materials have several roles, including the inhibition of sintering and absorption of catalyst poisons. All copper catalysts are susceptible to thermal sintering via a surface migration process, and this is markedly accelerated by the presence of even traces of chloride. Care must be taken, therefore, to eliminate halides from copper catalysts during manufacture, and from reactants during use. Operating temperatures must be restricted, usually to below 300°C.In methanol synthesis involving modern promoted Cu/ZnO/Al₂ O₃ catalysts neither poisoning nor coking is normally a significant source of deactivation; thermal sintering is the main cause of deactivation. In contrast, catalyst poisoning and coking have been observed in methanol decomposition and methanol steam reforming reactions.     

Deactivation of MoS2 catalysts during the HDS of thiophene

Catalytic properties of unsupported MoS₂ catalysts in the thiophene hydrodesulfurization reaction were determined in the temperature range 623–653 K. The catalysts were prepared by ex situ decomposition of ammonium thiomolybdate (ATM) crystals in a mixture of 15% H₂S in H₂ at 673 K. Activity of catalysts decreased very rapidly before reaching a steady state after 15 h on‐stream. The thiophene conversion went down from 10–12 to 3–4% in that time. The surface area of the catalysts also decreased during the catalytic reaction from 40–50 to 8–10 m²/g. Selectivity for hydrodesulfurization, hydrogenation and isomerization reactions was affected distinctly by the deactivation process. By increasing the reaction time, double‐bond isomerization increased, hydrogenation of butenes decreased and hydrodesulfurization remained constant. Results indicate that the main cause of activity decay was surface area loss that was due to sintering of MoS₂ crystallites. Selectivity variation indicates that different active sites are involved for the three reactions. A deactivation model involving diminution of active sites located in edge and rim sites of small MoS₂ particles is proposed to explain the variation of product distribution. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reaction of thiophene on Co?Mo/A12O3 catalysts of various loading at low hydrogen excess

Co-Mo/Al₂0₃ catalysts of various CoMo loadings were examined in the hydrodesulfurization (HDS) reaction of thiophene and tetrahydrothiophene at near stochiometric hydrogen: thiophene ratio. Good compensation plots were found for both reagents and all catalysts. Reversibly bound H₂S blocks active sites of HDS. The most important reaction pathway leads to but-1-ene; more Co gives more but-2-ene. The hydrogenation of buta-1, 3-diene is about 100 times more rapid than that of but-1-ene. The activity of two neighboring sites with twofold coordinative unsaturation is suggested in buta-1, 3-diene hydrogenation. The possible effect of Al₂0₃ support on active sites is discussed.     

Kinetic model and simulation for catalyst deactivation during dehydrogenation of methylcyclohexane over commercial Pt-, PtRe- and presulfided PtRe-Al2O3 catalysts

The catalyst deactivation kinetics for the dehydrogenation of methylcyclohexane were studied with Pt-, PtRe-, and presulfided Presulfided PtRe-Al₂O₃ Catalysts in a fixed-bed differential reactor. The kinetic analysis was made based on a Langmuir-Hinshelwood model assuming that the dehydrogenation of methylcyclohexene into methylcyclohexadiene was the rate-controlling step for the main reaction, and the polymerization of adjacently adsorbed methylcyclohexadiene as coke precursor molecules was the rate-controlling step for the deactivation reaction. The experimental data for the three catalysts at varying partial pressures of methylcyclohexane and hydrogen were well correlated by the hyperbolic deactivation function derived. The number of active sites involved in the rate-controlling step of the deactivation was evaluated to be 2.7 in average. The enhanced activity maintenance by the addition of Re as well as sulfur was attributed to a reduction in the concentration of multiple sites occupied by coke precursors adjacently adsorbed, leading to a decrease in the rate constant of the deactivation reaction.     

Hydrocracking of n-heptane with a NiO-MoO3/HYUS zeolite as catalyst. Kinetic study

The hydrocracking of n-heptane has been carried out in a fixed bed reactor at 2.45 MPa pressure and with a H₂/n-heptane molar ratio of 5.0 using a 4 wt% NiO - 8 wt% MO₃/HYUS zeolite as a catalyst. The W/F₁₀ ratio was varied between 75.3 and 1624 kg · s/kmol at different reaction temperatures: 573, 588, 603 and 623 K. The kinetics of the reaction has been studied by two different procedures due to the slight deactivation of the catalyst. One of them uses the conversion and yield values extrapolated at time on stream to zero and the other uses a kinetic equation where the deactivation of the catalyst has been included. The experimental data were fit well by a pseudo-first order kinetic equation and the kinetic constant values obtained by both methods coincide. The apparent activation energy of the reaction has a value of 105.8 + 5.0 kJ/mol.     

Effect of partial poisoning of acid sites on selectivity in the multiple reactions of mono-substituted-t-Butylbenzenes

The decomposition reaction of p–t-butylcumene and t-butylbromobenzene have been carried out over pyridine poisoned silica-alumina and HY zeolite in order to examine the effect of acid strength on selectivity. Since the catalyst deactivation rate for isomerization over the poisoned silica-alumina was greater than that of de-t-butylation in the reaction of p–t-butylcumene, the selectivity of de-t-butylation gradually improved with process time. In the reaction of t-butylbromobenzene over poisoned HY zeolite, the de-bromination rate was suppressed by the poisoning because the nature of active sites was different between silica-alumina and HY zeolite. The poisoned HY zeolite was found to be favorable for the de-t-butylation of halogenated-t-butylbenzenes.     

Selective hydrogenation of styrene: Poisoning effect of thiophene

The poisoning effect of thiophene on a commercial selective hydrogenation Pd/Al₂O₃ catalyst, using the styrene hydrogenation to ethylbenzene as a test reaction, has been studied. It was found that the addition of thiophene to the reactants modifies the electronic properties of Pd and also prevents (within certain limits) the catalyst from a subsequent poisoning when a second amount of thiophene is added. These modifications on Pd electronic state have been followed by X-ray photoelectron spectroscopy, and the poison addition has been studied by IR spectroscopy.     

Deactivation of Cu/ZnO catalyst during dehydrogenation of methanol

The deactivation of Cu/ZnO catalyst during methanol dehydrogenation to form methyl formate has been studied. The Cu/ZnO catalyst was seriously deactivated under the reaction conditions: various temperatures of 493, 523 and 553 K, atmospheric pressure and methanol GHSV of 3000 ml (STP)/g-cat h. The weight loss due to reduction of ZnO in the Cu/ ZnO catalyst was monitored by a microbalance. X-ray induced Auger spectroscopy of Zn(L₃M_4,5M_4,5) showed the increase in the concentration of metallic Zn on the catalyst surface after the reaction. Temperature-programmed reduction (TPR) of the Cu/ZnO catalyst with methanol demonstrated that the reduction of ZnO in Cu/ ZnO was suppressed by introduction of CO₂ into the stream of helium-methanol. As the concentration of CO₂ in the feed gas increased, the weight loss of the Cu/ZnO catalyst due to the reduction of ZnO decreased. The deactivation of the Cu/ZnO catalyst in the methanol dehydrogenation was also retarded by the addition of CO₂. In particular, oxygen injection into the reactant feed regenerated the Cu/ ZnO catalyst deactivated during the reaction. Based on these observations, the cause of deactivation of the Cu/ZnO catalyst has been discussed.     

镍基芳烃加氢催化剂硫中毒的失活动力学

将某一时刻催化剂表面上活性中心数目与初始时刻活性中心数目之比定义为催化剂表观活性因子,并认为此活性因子包含在主催化反应速率系数的指前因子中。考虑活性中心在催化剂表面分布的不均匀性,在现有级数型失活动力学模型基础上,经推导得到表观活性因子的具体表达式及其极限式,该式在一般情况下为不可分。最后,结合之前报道的具有较高普适性的反应转化率方程式,得到催化反应转化率与运转时间的关系方程,并以Ni-B/SiO₂催化剂在噻吩存在下芳烃加氢反应为例加以验证。结果表明,所提出的失活动力学表达式可以较好地描述镍基催化剂硫中毒的行为,方程参量具有物理意义,方程可以较精确地拟合实验数据。     

Phenol alkylation with methanol: effect of sodium content and ammonia selective poisoning of an HY zeolite

Sodium exchange and ammonia selective poisoning of the acid sites of an HY zeolite (Si/Al=20) were carried out and their effects on the catalytic properties for the alkylation of phenol with methanol (200C, 1 atm and N₂/reactants molar ratio of 4) were evaluated. Results show that the reaction is highly sensitive to the number and strength of the acid sites of the catalyst. A decrease in the number of acid sites by sodium exchange of the protons or by ammonia selective poisoning produces important changes in the selectivity of the reaction. In fact, a high increase in the anisole/cresol ratio is observed as the percentage of exchanged sodium in the zeolite increases, while the ammonia selective poisoning shows that at low desorption temperatures (250C) only anisole is formed while at higher desorption temperatures both anisole and cresols were observed. These results show that anisole formation requires sites with lower acid strength compared to those necessary for cresol formation.     

Sulphide-induced deactivation of Pd/Al2O3 as hydrodechlorination catalyst and its oxidative regeneration with permanganate

Pd-based catalysts have become important in environmental catalysis for their ability to hydrodechlorinate a wide range of chlorinated organic contaminants in water under ambient conditions. The success of their application in the remediation practice, e.g. for groundwater treatment, is often hindered by the sensitivity of Pd to poisoning by sulphur compounds. In this study, the stability and sulphide-induced deactivation behaviour of a highly active Pd/Al₂O₃ catalyst was investigated. The specific activity of Pd for the hydrodechlorination of chlorobenzene corresponds to rate coefficients up to k_Pd = 350 L g⁻¹ min⁻¹. The totally deactivated catalyst, resultant of sulphide poisoning, was regenerated with potassium permanganate. The pH value, as a key parameter which may influence the degree of deactivation as well as the efficiency of catalyst regeneration, was evaluated. Results show that in clean water the Pd/Al₂O₃ catalyst showed no inherent deactivation regardless of the ageing time and the pH value of the catalyst suspension. The degree of catalyst poisoning effected by 1.8–5.4 μM sulphide, corresponding to molar ratios of S:Pd_surface = 1.5–8.5, was observed to be higher under neutral and alkaline than under acidic conditions. The exposure of the catalyst to higher sulphide concentration of 14.2 μM resulted in complete catalyst deactivation regardless of the pH conditions. However, the efficacy of permanganate as oxidative regenerant for the fouled catalyst showed strong pH-dependence. A regeneration time of 10–30 min at low pH was sufficient to recover completely the high catalytic activity of Pd/Al₂O₃ for the hydrodechlorination reaction.     

The effect of sulfur compound on the hydrogenation of tetralin over a Pd–Pt/HDAY catalyst

A composite metal catalyst with a Pd/Pt of 4:1 and an H-form dealuminated Y zeolite support for the hydrogenation of tetralin was investigated. The materials were characterized by XRD, NH₃-TPD, N₂ adsorption, TPR and FTIR. The activity for the hydrogenation of tetralin was investigated with a high-pressure fixed-bed continuous-flow reactor operating at 4.0 MPa. The thiotolerance was tested with feeds containing 200 ppm sulfur in the form of 4,6-dimethyldibenzothiophene and thiophene, respectively. It was found that thiophene was more effective for deactivating the catalyst than the former one. The diffusion barrier of the pores for the sulfur compound is likely the major reason for the different deactivation rate of catalysts. The products other than decalins were mainly decalin isomers and ring-opened products. Selectivity of cracking products for the feed with thiophene was lower than that for the other feed, while that of ring-opened and ring-contracted products with the two feeds was similar.     

Poisoning of Pd–carbon catalysts by sulphur,chloro and heavy metal compounds in liquid phase hydrogenation of o-nitrophenol to o-aminophenol

The poisoning of Pd–carbon (4·1% Pd) catalysts by thiophene, dichloroethane, mercuric chloride and lead, zinc and mercuric acetates at different concentrations (0–5000 g m⁻³) in the liquid phase hydrogenation of o-nitrophenol to o-aminophenol (at 308 K and H₂ pressure of 1508 kPa) in a three-phase stirred slurry reactor has been investigated. The hydrogenation activity of the catalyst is drastically reduced due to the presence of these poisons in the reaction mixture, even at a very low concentration of poison (20 g m⁻³). Among the poisons, mercuric acetate was found to be the most potent. © 1998 Society of Chemical Industry     

Selective hydrogenation of styrene. II. Poisoning of Pd/Al2O3 by thiophene and pyridine

The poisoning of a commercial Pd/Al₂O₃ catalyst by pyridine and thiophene during the selective hydrogenation of styrene to ethylbenzene was studied. It was found that the Pd electronic state is the main factor controlling the interaction between the metal and the poisons. Pyridine added to the non-poisoned feed in a pulse produces a modification in the Pd electronic state and a decrease in conversion, but, within certain limits, inhibits the adsorption of more pyridine or thiophene molecules.     

Analysis of sulfur poisoning on a PEM fuel cell electrode

The extent of irreversible deactivation of Pt towards hydrogen oxidation reaction (HOR) due to sulfur adsorption and subsequent electrochemical oxidation is quantified in a functional polymer electrolyte membrane (PEM) fuel cell. At 70 °C, sequential hydrogen sulfide (H₂S) exposure and electrochemical oxidation experiments indicate that as much as 6% of total Pt sites are deactivated per monolayer sulfur adsorption at open-circuit potential of a PEM fuel cell followed by its removal. The extent of such deactivation is much higher when the electrode is exposed to H₂S while the fuel cell is operating at a finite load, and is dependent on the local overpotential as well as the duration of exposure. Regardless of this deactivation, the H₂/O₂ polarization curves obtained on post-recovery electrodes do not show performance losses suggesting that such performance curves alone cannot be used to assess the extent of recovery due to sulfur poisoning. A concise mechanism for the adsorption and electro-oxidation of H₂S on Pt anode is presented. H₂S dissociatively adsorbs onto Pt as two different sulfur species and at intermediate oxidation potentials, undergoes electro-oxidation to sulfur and then to sulfur dioxide. This mechanism is validated by charge balances between hydrogen desorption and sulfur electro-oxidation on Pt. The ignition potential for sulfur oxidation decreases with increase in temperature, which coupled with faster electro-oxidation kinetics result in the easier removal of adsorbed sulfur at higher temperatures. Furthermore, the adsorption potential is found to influence sulfur coverage of an electrode exposed to H₂S. As an implication, the local potential of a PEM fuel cell anode exposed to H₂S contaminated fuel should be kept below the equilibrium potential for sulfur oxidation to prevent irreversible loss of Pt sites.     

Deactivation Studies of Rh/Ce0.8Zr0.2O2 Catalysts in Low Temperature Ethanol Steam Reforming

Rapid deactivation of Rh/Ce_0.8Zr_0.2O₂ catalysts during low temperature ethanol steam reforming was studied. A significant build-up of reaction intermediates, instead of carbon deposit, was observed at low reaction temperatures. This appears to be the cause of rapid catalyst deactivation. Co-feed experiments indicated that possible intermediate products acetone and ethylene caused more severe catalyst deactivation than other oxygenates such as acetic acid and acetaldehyde.     

Effect of catalyst deactivation on mild hydrocracking reactions

A WNiPd/TiO₂Al₂O₃ mild hydrocracking catalyst deactivation has been studied in a small pilot plant unit. OTMHCK technology can produce low sulfur, high cetane number diesel at constant conversion with acceptable cycle length by increasing initial temperature as a function of time on stream. Three sets of tests were performed for 3 months at constant operating temperature using the same catalyst. At the start and at the end of the run the catalyst activity and selectivity were tested at different temperatures. Spent catalyst was characterized using chemical analysis, ¹³C NMR, XPS, and ammonia adsorption techniques. The hydrocracking reactions were modeled using a lump model including 63-equation reactions, an empirical reactivity function and three distribution functions that included hydrogenation, cracking, and ring opening in liquid phase. Deactivation reaction was simulated using a Levenspiel's two sites type of deactivation mechanism. A Genetic Algorithm tool was used to obtain the deactivation constant using a set of previously determined kinetic constants. This set takes into account the aromatics adsorption on metal and acid sites. The results confirm that deactivation changes the relative rate of hydrogenation/hydrogenolysis, cracking and dealkylation reactions.     

Catalytic decomposition of alcohols over size-selected Pt nanoparticles supported on ZrO2: A study of activity, selectivity, and stability

This article discusses the performance of ZrO₂-supported size-selected Pt nanoparticles for the decomposition of methanol, ethanol, 2-propanol, and 2-butanol. The potential of each alcohol for the production of H₂ and other relevant products in the presence of a catalyst is studied in a packed-bed mass flow reactor operating at atmospheric pressure. All the alcohols studied show some decomposition activity below 200 °C which increased with increasing temperature. In all cases, high selectivity towards H₂ formation is observed. With the exception of methanol, all alcohol conversion reactions lead to catalyst deactivation at high temperatures (T > 250 °C for 2-propanol and 2-butanol, T > 325 °C for ethanol) due to carbon poisoning. However, long-term catalyst deactivation can be avoided by optimizing reaction conditions such as operating temperature.     

Modelling of catalyst pellet poisoning for benzene hydrogenation

Based on the kinetics of benzene hydrogenation, pore diffusion and catalyst deactivation, time-dependent effectiveness behaviour of a single Ni? SiO₂? Al₂O₃ catalyst pellet where the chemical reaction rate is determined by pore diffusion was simulated for different conditions of operation. Poisoning kinetics were measured in a series of differential reactor experiments at atmospheric total pressure at temperatures ranging from 403 to 473 K. A computed effectiveness factor has been compared with experimental values for a catalyst pellet of industrial size. A good degree of correlation between theoretical prediction and the experimental results was found.