Note

The effects of hydrogen on cumene disproportionation and catalyst deactivation on a commercial hydrocracking catalyst

A coke selectivation process, i.e., modification of selectivity, was divided into five separate stages. Each stage was under either N₂ or H₂ as the carrier gas and at different reaction temperatures for various time-on-stream. The spent sample obtained during each stage was characterized by a combination of different techniques, namely thermogravimetric analysis, temperature programmed desorption of ammonia, xenon adsorption, X-ray photoelectron, and ¹²⁹Xe NMR spectroscopy. In the presence of N₂ carrier gas, the carbonaceous deposits formed within the intracrystalline channels were mostly light volatile soft coke, which was effectively removed by a succeeding treatment with H₂ gas. On the other hand, bulkier, more condensed hard coke was formed on the external surface of the zeolite crystallites and is more difficult to be removed by simple hydrogen treatment. After the five-stage selectivation process, most of the coke is found to be on the external surface of the catalyst. As a result, the isomerization of the primary product, para-xylene, was retarded. The para-selectivity was found to increase from ca. 24 to 49% at a slight expense of conversion. A detailed reaction mechanism for the para-selectivity enhancement during the five-stage coke selectivation process is proposed.

The deactivation of acid zeolite catalysts used in hydrocarbon transformations is mainly due to the deposit inside the pores of heavy secondary products generally known as coke. It is shown how the rate of coking and the deactivating effect of the coke molecules are affected by the pore structure and the acidity of the zeolites as well as by the operating conditions. Directives for minimizing the deactivation by coking are proposed: (1) choice of tridimensional zeolites without trap cavities (large cavities with small apertures); (2) adjustment of the density and strength of the acid sites to the lowest values necessary for the selective formation of the desired products; and (3) choice of operating conditions in order to avoid the formation of coke-maker molecules (alkenes, polyaromatics). The regeneration of zeolites is generally carried out through coke combustion under air or oxygen flow. The detrimental effect that water, produced by coke oxidation, has on the zeolite activity can be limited by using a two-stage generation process, the hydrogen atoms of the coke molecules being oxidized at the low temperature of the first stage.

The influence of carrier gases (N₂, H₂, He, and CO₂) on the catalytic activity, stability, and coke formation in zeolite beta during cumene disproportionation reaction is discussed. The reaction intermediates as well as the carbonaceous residues were characterized by¹³C NMR spectroscopy under proton cross-polarization and magic-angle spinning and by thermogravimetric method. The effects of carrier gas dilution on coke formation, catalyst deactivation, and product shape selectivity have also been examined by varying carrier gas (N₂) to reactant molar ratios (0.2–20). The amount of total coke decreases linearly with increasing N₂/cumene ratios above a value near 2. It is also found that the coke induced shape selectivity is notable only at extreme dilution. In the presence of various carrier gases, a notable decrease in catalytic activity has been found to obey the order N₂ > H₂ > He > CO₂, whereas a reverse order was observed for the catalytic stability. Moreover, the amount of coke deposit is found to decrease linearly with the kinetic diameter of the carrier gases. Hence, the incorporation of carrier gases resulted in a decrease in the amount of coke deposition which is mainly due to the transport of coke precursors and less bulky carbonaceous compounds (soft coke). Similarly, as proposed by the transition complex solvation model, the carrier gas molecules stabilize the biphenyl alkane reaction intermediates by van der Walls interactions and prevent them from further dissociation into product molecules. With the exception of H₂, the combination of the carrier gas transport effect and transition complex solvation model is used to describe the observed trends in the activity and stability of the catalyst.

A group of fresh, deactivated, and deactivated-regenerated commercial hydrocracking catalysts was characterized using piperidine hydrogenolysis as a probe reaction at 301C, hydrogen partial pressures of about l6 atm, and initial concentrations of piperidine ranging from 4.14 × 10⁻³ 11.86 × 10⁻³ g mol/liter. The first part of the study examined the effects of the zeolite unit cell size on the piperidine hydrogenolysis activity and catalyst deactivation and revealed that over the range of unit cell size examined (24.35-24.56 Å), adsorption equilibrium constants associated with the acidic and metallic catalyst functions decreased while reaction rate constants increased with increasing unit cell size. The increase of acidic function activity with unit cell size is explained in terms of the total number of potential acid sites present per unit cell. The increase of metallic function activity with unit cell size is suggested to reflect mostly changes of the metallic function upon regeneration. Different types of sites are proposed to exist on the metallic catalyst function in order to account for the results. An inverse relationship between catalyst deactivation and unit cell size was generally observed for both catalyst functions. These results point to the zeolite unit cell size as a possible parameter for correlating catalytic activity as well as selectivity, especially for the acidic catalyst function. The second part of the study examined the effects of wt% sulfur and wt% carbon at low levels of sulfur and coke on the piperidine hydrogenolysis activity and showed that the regenerated catalysts with constant zeolite unit cell size (≈24.40 Å and low levels of coke (<0.8 wt%) had similar acidic function activities. The acivity correlation of the metallic function on these regenerated catalysts with low levels of sulfur (<0.4 wt%) also indicated the presence of two different types of metal sites. One showed a decrease in activity with increasing sulfur content and a possible dependency on the regeneration procedure while the activity of the other site was invariant. The activity of the metallic and acidic functions of the catalyst was also examined at high levels of sulfur and coke and constant unit cell size. For this study a series of deactivated samples was used. The metallic function activity of these catalysts was found to decrease with the commercial time on stream. Between 2.2 and 2.6 wt% sulfur, the activity of the metallic function decreased linearly with the sulfur content on the catalyst. At higher levels of sulfur (>2.5 wt%) the deactivated catalysts had similar metallic function activities. Between 2 and 6 wt% carbon, the activity of the acidic function decreased linearly with the carbon content on the catalyst. At low levels of coke (< 1.5 wt%), the effect of the unit cell size on acidic activity prevailed. The wt% carbon on the deactivated as well as the regenerated catalysts was found to have no effect on the catalyst deactivation rates observed during the reactions.

The formation of coke in the piperidine hydrogenolysis reaction over a fresh sulfided hydrocracking catalyst was examined at temperatures ranging from 281 to 321°C, hydrogen partial pressures of 11.2 to 15.9 atm (1.1 to 1.6 MPa), and initial concentrations of piperidine of 3.94 × 10⁻³ to 11.84 × 10⁻³ g mol/liter using elemental analysis, ¹³C NMR spectroscopy, and ESCA. The results indicated that most of the coke present on the catalyst after 17 h on stream was deposited in the initial 90 min of the reaction. Coke formation and hence catalyst deactivation were found to increase with both reaction temperature and initial concentration of piperidine. Reducing the catalyst instead of sulfiding it had no effect on the final coke content. Nitrogen was found to be present on the catalyst surface after reaction indicating that nitrogen-containing compounds were participating in the formation of coke. The H/C ratio of the coke decreased very slowly with reaction time and was invariant to changes in temperature, initial concentration of piperidine, or catalyst pretreatment method. Comparing the activity of reduced versus sulfided fresh catalysts in the hydrogenolysis of piperidine, it was found that under the present conditions, catalyst presulfidation increased the activity of the metallic catalyst function and decreased the rate of catalyst deactivation. Furthermore, decreasing the partial pressure of hydrogen resulted in an activity decrease of the metallic catalyst function and had a negative effect on the overall catalyst activity maintenance. The intrinsic activity of the acidic catalyst function was not affected by the change in hydrogen partial pressure.

The hydrogenation of benzene to cyclohexane was investigated over a range of nickel-exchanged and nickel-impregnated Y zeolites, varying the nickel content and the nature of the alkali metal co-cation (Li⁺, Na⁺, K⁺, Rb⁺ or Cs⁺). With a view to optimizing benzene conversion levels, the following catalytic parameters were studied: reaction temperature, reaction time, benzene flow rate and coke deposition. The observed catalytic activities are correlated with previously reported physical characterizations. Benzene hydrogenation increased in the order: NiLiY < NiNaY < NiKY < NiRbNaY < NiCsNaY. Catalyst deactivation results from the deposition of involatile coke on the catalyst surface, which is promoted by increasing zeolite acidity. The effects of poisoning the surface Brönsted acid sites by adsorption of ammonia onto the activated reduced zeolites are considered. The results of catalyst regeneration by high temperature oxidation of the coke deposits are also reported.