Acid Sites Of Catalyst Research Articles

Improved surface acidity of CeO2/TiO2 catalyst by Cu doping to enhance the SCR catalytic activity

The catalyst is based on CeO2 cannot be widely used in SCR reaction because of its poor NH3 adsorption performance. In this study, Cu-doped CeTi catalyst was designed. The results show that the CeTiCu0.3 has a wide active temperature window of 200–450 °C in NH3-SCR reaction, and NO conversion is > 80%. This is mainly due to the fact that Cu doping provides more acidic sites on the surface of CeTi catalyst, especially the increase of Lewis acid sites is more obvious. NH3-TPD showed that CeTiCu0.3 had a large NH3 adsorption capacity and was mainly adsorbed at Lewis acid sites. In situ DRIFTs results show that NH3 first adsorbs on the Lewis acid site of catalyst in coordination state and reacts with gaseous NOx, while NOx adsorbed on catalyst surface has low reactivity. Therefore, the CeTiCu0.3 catalyst is mainly controlled by the Eley–Rideal mechanism. More Lewis acid sites, and abunda nt Cu2+/Cu+ and Ce4+/Ce3+ formed Cu2+, Ce3+ and surface reactive oxygen species are the main reasons for the excellent catalytic performance of CeTiCu.

Functionalized construction of multiple active centers for durability controlling during catalytic combustion of chlorinated aromatic hydrocarbons

The catalytic combustion of chlorine-containing VOCs primarily encounters the bottleneck of catalyst poisoning by chlorine, thereby impeding the further industrial application of this technology. The development of multi-active catalytic materials for discerning chlorine fixation, chlorine transfer, deep oxidation during elimination of chlorine-containing VOCs can mitigate catalyst chlorination and enhance stability. Herein, we utilizing LDHs as catalyst precursors and successfully synthesized flower-like Co2FexCr1-xOy catalyst with different functionalized reaction centers. The addition of Cr improved the surface lattice oxygen, oxygen vacancies, and acid sites of catalyst. The removal efficiency of CB can be achieved >90 % at 247 °C (WHSV = 33000 mL · g−1·h−1). The fixation of dissociated chlorine contributed by Cr not only enhances oxidation performance and directional adsorption of chlorine, but also improves carbon deposition resistance and catalyst sintering resistance significantly. Compared with Co2Fe1Oy, Co2Fe0.67Cr0.33Oy catalyst achieves a remarkable threefold reduction in surface carbon deposition. The DFT calculation revealed that water dissociation energy over the Co site is −0.72 eV, which is significantly lower than Fe (−0.59 eV) and Cr (−0.17 eV), facilitating water dissociation and subsequently removes a substantial amount of accumulated chlorine on Cr to form HCl. With the help of In situ DRIFT and GC/MS analysis, we found the efficient removal of chlorine atoms enhances the exposure of active sites on the catalyst surface with hydrolyzed hydroxyl group, which facilitating the conversion of aromatic hydrocarbons into a greater quantity of maleate and carboxylate species. This phenomenon is beneficial for the subsequent mineralization process of chlorobenzene. This work provide valuable insights for multi-active site catalysis mechanism in practical applications aimed at eliminating chlorinated organic waste gases from industrial emissions.

Catalytic depolymerization and hydrodeoxygenation of lignin to high-density fuel precursors using Ni/Nb2O5 catalyst

Converting lignin into renewable fuels via hydrogenolysis is crucial for achieving carbon neutrality. Existing methods primarily produce monomers in the carbon range of 6–9, inadequate for sustainable aviation fuels requiring heavier components. This study introduces a Ni/Nb2O5 catalyst for catalytic transfer hydrogenolysis of organosolv birch lignin with isopropanol, predominantly resulting in dimers and trimers within carbon ranges of 8–16 and 16–30, comprising 24.7 % and 60.5 % of the lignin-oil, respectively. The catalyst's oxygen vacancies and acid sites facilitate efficient C-O and C-C bonds cleavage, preserving a highly aromatic structure with reduced O/C ratio, elevated H/C ratio, and decreased unsaturation. Characterization techniques confirm a significant 55.5 % deoxygenation degree of the lignin-oil, making it suitable for the synthesis of customized fuel across various carbon numbers. This approach offers a practical pathway for obtaining renewable heavier fuel components and precursors from lignin, significantly advancing sustainable fuel technologies.

Effect of acidity of solid acid catalysts during non-oxidative thermal decomposition of LDPE

AbstractThermal decomposition of low-density polyethylene (LDPE) was monitored by thermogravimetry under N2 atmosphere in the presence of solid acid catalysts such as alumina (α-Al2O3, γ-Al2O3), crystalline silica-alumina (SA, molar ratio of Si/Al = 0.19) and amorphous silica-alumina catalysts (ASA, molar ratio of Si/Al = 4.9). Crystal structure and surface area of solid acid catalysts were measured by XRD and BET, respectively. The strength and distribution of acid sites of solid acid catalysts were estimated by NH3-TPD. It was observed that total acidity strength is in the order of ASA (1.77 μmmol NH3/g) > AS (1.42 μmol NH3/g) > γ-Al2O3 (1.06 μmol NH3/g) > α-Al2O3 (0.06 μmol NH3/g). Thermal degradation behavior of LDPE with and without solid acid catalyst was monitored by TGA, where heating rates (β) of 5, 10, and 20 °C/min were employed under an inert atmosphere, and their activation energies (Ea), onset temperatures (Tinitial), decomposition temperatures (Tdecomp) were calculated and compared. The activation energy (Ea) was evaluated using the Coats-Redfern method. Solid acid catalysts with stronger acidity and higher surface area showed a decrease in activation energy and onset temperature. Activation energy of LDPE over ASA catalyst is decreased to 97.3 kJ/mol from thermal decomposition of LDPE without catalyst of 117.2 kJ/mol under heating rate of 10 °C/min. The isothermal decomposition of LDPE was monitored at 300 °C for 3 h with a heating rate of 10 °C/min, where 13.1% and 24.2% wt. loss were observed over SA and ASA, respectively, while only 0.7% wt. loss was observed for LDPE without a solid acid catalyst. Graphical abstract Single step decomposition of LDPE Thermal degradation behavior of LDPE monitored by TGA, with different heating rates (β) of 5, 10, 20 °C/min.

Catalytic co-pyrolysis of woody biomass with polypropylene: Effects of Korean loess hybrid catalysts and pyrolysis temperatures

This paper describes an effort to improve the catalytic characteristics of Korean loess (KL) for the co-pyrolysis of biomass and polypropylene, resulting in a unique multifunctional catalyst. Korean loess was physically and chemically transformed into a microporous structure (ZSM) fitting for co-pyrolysis. Using the catalysts manufactured above, analytical co-pyrolysis was carried out using a Py-GC/MS system with pinewood sawdust and polypropylene mixed at a ratio of 1:1 (w/w). To investigate the impacts of the use of catalysts and the pyrolysis temperatures on the distribution of volatile pyrolytic products, the experiments were designed to mix 2- or 8-fold catalyst mass quantities with feedstock and pyrolyze at temperatures of 500 °C, 600 °C, and 700 °C. Compared to pure Korean loess, the catalytic activity of the optimal hybrid catalyst (0.2KL/ZSM) favored decarbonylation over dehydration, making it more effective than mesoporous ZSM. Pyrolytic products generated using the hybrid catalyst (0.2KL/ZSM) had a higher hydrogen concentration (9.8 wt%) than pyrolytic products produced using ZSM catalysts (6.5 wt%). Because of this, the pyrolytic products were expected to be more compatible as a fuel over 0.2KL/ZSM, with an HHV of 27.4 MJ/kg as opposed to 26.5 MJ/kg over ZSM. Zeolite framework catalysts could promote the production of aromatic chemicals (3.0 mg/g for non-catalyst, 34.7 mg/g for KL, and 53.9 mg/g for 0.2KL/ZSM) in the co-pyrolysis of PP with pine, which synergistically enhance oligomerization, cyclization, and aromatization reactions at catalyst acid sites. Concerning aromatic hydrocarbon yields, 0.2KL/ZSM could be expected to perform very similarly to ZSM in producing 74.7 mg/g of MAH+PAH at 600 °C.

Impacts of ball-milling on ZSM-5 zeolite properties and its catalytic activity in the two-phase glycerol ketalization with acetone

A series of acidic ZSM-5 zeolites were mechanically ball-milled for different periods and further applied as catalysts in the liquid-phase glycerol ketalization with acetone. Structural and textural characterizations showed that the samples had considerable crystallinity and micropore volume loss due to amorphization. On the other hand, due to a comminution effect, the external surface area increased and particle size reduced. TPD and FTIR analyses with probe molecules with different sizes, such as ammonia, pyridine (Py), and 2,6-di-tert-butylpyridine (DTBPy) chemisorbed on the catalyst acid sites, aided in quantifying the various levels of accessibility of chemicals. The treated materials lost acidity due to amorphization or blockage of pores, but the milling enabled higher accessibility of bulkier chemicals to acid sites, as revealed by the chemisorption of Py and DTBPy. A crucial balance of acidity, accessibility, and hydrophobicity was tailored by milling time. The one-hour grounded zeolite had a glycerol conversion 35 % greater than the pristine sample and 67 % greater than the eight-hour milled sample. The results showed that ball milling is a tool to improve the catalytic activity of zeolites with bulky chemicals by fine-tuning the milling time.

Tuning of the acid sites in the zeolite-alumina composite Ni catalysts and their impact on the palm oil hydrodeoxygenation reaction

The hydrodeoxygenation (HDO) of bio-oil is one of the potential approaches to produce green diesel. However, HDO catalyst requires the development of bifunctionality which translates to the simultaneous presence of acidic and metal sites for desired catalytic activity and selectivity. Zeolites and their composites are attractive candidates for the conversion of biomass to fuels. In the present work, a series of Ni-incorporated Al2O3-zeolite beta bifunctional composite catalysts with distinct Al2O3 (25–75 wt%) and Ni contents (5–15 wt%) were synthesized via a facile one-pot method directly from nickel acetate, nano-boehmite (γ-AlO(OH)) and the NH4+ form of beta zeolite (NH4+-BZ). The nano-boehmite particles, due to the positive charges on their surface, electrostatically attract negatively charged beta zeolite crystals, which leads to the assembly of a hierarchical pore structure upon calcination. Interestingly, the composite catalysts synthesized were quite homogeneous with uniform dispersion of Ni particles. All composite catalysts were thoroughly characterized using XRD, SEM-EDX, SEM-mapping, HR-TEM, H2-TPR, H2-TPD, NH3-TPD, XPS, 31P MAS NMR, 27Al MAS NMR and N2 sorption analysis. The synthesized composite catalysts with distinct Al2O3 contents showed diverse textural properties, distinct nature of acid sites and improved performance in hydrodeoxygenation of palm oil. Particularly, the Ni/BZ-Al50 (with BZ:Al2O3 = 50:50) composite catalyst significantly enhanced the conversion of palm oil (up to 90%) and yield of n-C15-C18 hydrocarbons (up to 69%) at moderate temperature of 375 °C as compared to 10Ni/BZ catalyst (Conv. = 75%, yield of n-C15-C18 = 52%). The higher catalytic performance realized with composite catalysts can be ascribed to its hierarchical pore structure, moderate acidity (chemisorption studies), tuned acid sites nature (solid state NMR) and homogeneous distribution of active Ni sites (H2 chemisorption) which helps to improve product selectivity by minimizing side reactions. The time-on-stream (TOS) experiments were carried out up to 20 h which clearly showed that composite catalysts are more stable suggesting the lower amount of coke deposition (TPO studies) and suppression of metal sintering.

Kinetic analysis of monomolecular cracking of normal Alkanes (C4[sbnd]C6) over Brønsted Acid site of Zeolitic type catalyst with energetic evaluation of transition states using Quantum-Chemical modeling

The work aims to determine the kinetic parameters of reactions for production of light olefins via catalytic cracking reactions of C4–C6 n-alkanes based on the energy characteristics of the transition state using quantum chemical calculations. Cracking reactions of C4–C6 n-alkanes proceed via protolytic mechanism on the Brønsted acid sites of zeolite-containing catalysts. For kinetic studies in this work, the thermochemical parameters of the intermediate stages, including hydrocarbon adsorption and transition state were determined, then the activation energies and rate constants were determined over the temperature range of catalytic cracking process from 773 to 903 K (500–630 °C).The results showed that DFT method in combination with B3LYP and ωB97X-D functionals, and 3–21 G basis demonstrated quite high accuracy in determining thermochemical parameters, including enthalpy, entropy and Gibbs free energy at both energetic levels of adsorption and transition state. Then, modeling continued by calculations of activation energies and rate constants of reactions. Obtained kinetic parameters made it possible to determine the reactivity of hydrocarbons with different chain length. It was obtained that the rate constants of butane cracking reactions with the formation of ethylene are 54–90 times higher than the formation of propylene. The rate constants of pentane cracking reactions with the formation of butylene are on average 5 times higher than the formation of propylene. The rate constants for hexane cracking reactions with the formation of butylene are 2.9–3.7 times higher compared to the formation of propylene.

Understanding the effect of spatially separated Cu and acid sites in zeolite catalysts on oxidation of methane

Unraveling the effect of spatially separated bifunctional sites on catalytic reactions is significant yet challenging. In this report, we investigate the role of spatial separation on the oxidation of methane in a series of Cu-exchanged aluminosilicate zeolites. Regulation of the bifunctional sites is done either through studying a physical mixture of Cu-exchanged zeolites and acidic zeolites or by systematically varying the Cu and acid density within a family of zeolite materials. We show that separated Cu and acid sites are beneficial for the formation of hydrocarbons while high-density Cu sites, which are closer together, facilitate the production of CO2. By contrast, a balance of the spatial separation of Cu and acid sites shows more favorable formation of methanol. This work will further guide approaches to methane oxidation to methanol and open an avenue for promoting hydrocarbon synthesis using methanol as an intermediate.

The influence of H2O or/and O2 introduction during the low-temperature gas-phase sulfation of organic COS + CS2 on the conversion and deposition of sulfur-containing species in the sulfated CeO2-OS catalyst for NH3-SCR.

Herein, the typical components of blast furnace gas, including H2O and O2, were introduced to improve the NH3-SCR activity of the sulfated CeO2-OS catalyst during the gas-phase sulfation of organic COS + CS2 at 50 °C. The characterization results demonstrate that the introduction of O2 or H2O during gas-phase sulfation enhances the conversion of organic COS + CS2 on a cubic fluorite CeO2 surface and reduces the formation of sulfur and sulfates in the catalyst, but decreases the BET surface area and pore volume of the sulfated CeO2-OS catalyst. However, the introduction of O2 or H2O during the gas-phase sulfation increases the molar ratios of Ce3+/(Ce3+ + Ce4+) and Oβ/(Oα + Oβ + Oγ) on the sulfated CeO2-OS catalyst surface, thus promoting the formation of surface oxygen vacancies and chemisorbed oxygen, and these properties of the catalyst are further enhanced by the co-existence of O2 and H2O. Furthermore, the reduction of sulfates formed under the action of O2 or H2O decreases the weak acid sites of the sulfated CeO2-OS catalyst, but the few and highly dispersive sulfates present stronger reducibility, and the proportion of medium-strong acid sites of the catalyst increases. These factors help to improve the NH3-SCR activity of the sulfated CeO2-OS catalyst. Thus, there exists a synergistic effect of H2O and O2 introduction during gas-phase sulfation on the physical-chemical properties and catalytic performance of the sulfated CeO2-OS catalyst by organic COS + CS2 at 50 °C.

Hydroisomerization of n‐dodecane over ZSM‐48 with different metal properties

AbstractThe distance between the two active sites of a bifunctional catalyst (acid sites and metal sites) is critical for the hydroisomerization performance. However, the influences of the dehydrogenation/hydrogenation species (Pt or Ni) and the intimacy between the acid sites and the metal sites over the distribution of the isomers have rarely been investigated. Herein, the bifunctional catalysts were prepared by loading the hydrogenation component (Pt or Ni) on γ‐Al2O3 or HZSM‐48 zeolite separately and then mixing them with equal mass of HZSM‐48 molecular sieves and γ‐Al2O3, respectively. The hydroisomerization performance of n‐dodecane was investigated on a fixed‐bed reactor. The loading position of the metal can directly affect the activity of the hydroisomerization reaction. However, the distribution of the isomer products is not affected by the loading position and the metal type (Pt or Ni) obviously, and the middle‐positioned isomer is the main isomer product regardless of the position and the type of the metal. The catalysis results indicate that the isomerization of n‐dodecane mainly follows the “Key‐Lock” catalysis mode on HZSM‐48.

Promotional effect of H2O introduction on the NH3-SCR activity of the gas-phase sulfated CeO2 catalyst by organic CS2+COS: Influence of H2O concentration

Herein, as an indispensable key reactant for the hydrolysis of organic sulfur, H2O was firstly introduced to optimize the NH3-SCR activity of CeO2 by providing the adsorption and reaction spots of catalysis for organic CS2+COS during the low-temperature gas-phase sulfation. The results demonstrate that the introduction of 0.33 vol% H2O is beneficial to enhance the interaction between organic CS2+COS and cube fluorite CeO2, which not only increases the concentrations of Ce3+ ions, chemisorbed oxygen (Oβ) and oxygen vacancies on CeO2-CS2+COS surface, but also effectively enhances the redox cycle of Ce4+/Ce3+ ion pairs and the medium-strong acid sites of catalyst. These all help enhance the promotional effect of organic sulfur low-temperature gas-phase sulfation on the NH3-SCR activity of CeO2 and further improve NOx reduction over the CeO2-CS2+COS catalyst. However, the introduction of 5.0 vol% H2O shows a certain inhibitory effect due to the competitive adsorption of excess water and CS2+COS on the cube fluorite CeO2 surface, which weakens their interaction during the low-temperature gas-phase sulfation, thereby decreases the promotional effect of low concentration water introduction on the NH3-SCR activity of the CeO2-CS2+COS catalyst. Therefore, the results of this research provide a scientific reference for developing the NH3-SCR CeO2-based catalyst in practical applications.

The impact of H-ZSM-5 catalyst on the mechanism of fuel-N conversion during glutamic acid pyrolysis

During the pyrolysis of biomass, Fuel-bound Nitrogen (Fuel-N) is converted into N-containing compounds present in bio-oil and biochar. These compounds are easily oxidized into NOx during subsequent utilization, causing ecological damage. Catalytic fast pyrolysis has emerged as a promising method for denitrogenation in recent years, but the underlying catalyst mechanism still needs to be determined. This study analyzed the impact of catalyst H-ZSM-5 on the mechanism of Fuel-N conversion during the pyrolysis of glutamic acid, a model compound for nitrogenous biomass, through experimental studies and theoretical calculations. According to literature data, the pyrolysis of glutamic acid predominantly yielded the N-containing products pyrrole, glutarimide, and pyrrolidone. The H-ZSM-5 catalyst showed optimum denitrification at 600 ℃, resulting in a 49.62% reduction in the proportion of N-containing compounds. In the catalytic pyrolysis products, the generation of toluene and naphthalene increased. In-situ DRIFTS experiments were consistent with the results of Py-GC-MS experiments, with catalytic pyrolysis promoting deaminations and decarboxylations, decreased imines and nitriles, and increased aromatic hydrocarbons. Additionally, the energy barrier changes of each pyrolysis pathway were calculated based on Density Functional Theory (DFT). The results showed that the H-ZSM-5 catalyst primarily promoted the deamination reaction, followed by the decarboxylation and dehydration condensation reactions, while inhibiting the dehydrogenation reaction. Moreover, the energy barrier for unsaturated hydrocarbon addition reactions was reduced, facilitating the formation of aromatic compounds. The acid sites inside the H-ZSM-5 catalyst weakly interacted with the reactants, making it easier to detach the amino group during catalytic pyrolysis. This study provides a reference for the denitrification of bio-oil and biochar. Subsequent work can realize the regulation of nitrogen conversion by changing the acidity and concentration of catalyst acid sites.

Enhanced stability of Pt@S-1 with the aid of potassium ions for n-hexane and n-heptane aromatization

As a typical high-efficient alkane aromatization catalyst, Pt/KL is prone to deactivation due to the sintering growth of Pt on the outer surface of KL and the channel blockage caused by carbon deposition, which is further exacerbated by the one-dimensional channel structure of KL. Therefore, a series of encapsulated catalysts combining the three-dimensional channel structure advantages of S-1 with the role of potassium are prepared and evaluated. It is shown that the encapsulated catalyst KPt@S-1-02 modified by moderate potassium exhibits extremely superior stability and no significant deactivation is observable after n-heptane aromatization for 180 h under harsh conditions. On the one hand, encapsulation structure enables a strong anti-agglomeration ability of Pt and the topological structure of S-1 endows the encapsulation catalysts less susceptible to channel blockage, which has been demonstrated by HAADF-STEM and N2-adsorption. On the other hand, characterizations involving NH3-TPD, CO-DRIFT and HAADF-STEM verify that the modification by an appropriate amount of K can inhibit acid sites of catalysts and improve Pt dispersion and electron density, thus enhance the stability of catalysts. However, excessive K can serve as weak acid sites, poison of active sites and inhibitor of H2 adsorption on Pt surface, leading to deactivation of the catalyst.

Development of sulfonated carbon-based solid-acid catalysts derived from biorefinery residues and biomass ash for xylan hydrolysis

In this study, biorefinery residues and biomass ash were used to develop sulfonated carbon-based solid-acid catalysts to enhance xylan hydrolysis. Metals extracted from the biomass ash were impregnated into a carbon support prepared from biorefinery residues to synthesize sulfonated ash-impregnated carbon-based catalyst. Metals present in the ash most probably are responsible for the increased acid density in the catalyst. Using this catalyst, 11.4 % xylose yield was produced, an increase of 171 % over the yield obtained from a catalyst without ash impregnation. Using a xylan-to-catalyst ratio of 1:2, 72.9 % xylose yield was obtained, comparable to using 0.5 wt% sulfuric acid under the same reaction condition. Moreover, this catalyst significantly inhibited xylose dehydration, only forming 11.3 % furfural compared to that generated using dilute acid. Hence, these results show the potential for using biomass ash as a metal source to increase the number of acid sites in solid-acid catalysts used for biomass pretreatment.

Insight into the regulating effect of phosphorylation on the NH3-SCR activity and properties of thiourea modified CeO2-TA catalyst

Herein, three qualities of ammonium phosphate were used to investigate the influence of phosphorylation on the NH3-SCR activity of the CeO2-TA catalyst synthesized via the thiourea hydrothermal method. The results demonstrated that when the molar ratio of P/Ce was 0.1, the introduced ammonium phosphate was beneficial to the formation of highly disperse amorphous sulfates and active CePO4 on CeO2-TA surface, thus enhanced the surface total oxygen/adsorbed oxygen concentrations, the redox properties and the intermediate-strong acid sites of catalyst, which might be responsible for the better NH3-SCR activity of P0.1-CeO2-TA than CeO2-TA. However, the introduction of excessive ammonium phosphate promoted the formation of monoclinic CePO4 and reduced the formed quantity of amorphous sulfates on CeO2-TA surface, which were harmful to the redox properties and acid sites of catalyst. Therefore, the introduced ammonium phosphate decreased the NH3-SCR activity of CeO2-TA when the P/Ce molar ratio increased from 0.1 to 1.0.

Efficient Synthesis of Quinolines from 2‐Aminoaryl Ketones with α‐Methylene Carbonyl Derivatives: Exploiting the Dual Acid Sites of Metal Organic Framework Catalyst

AbstractWe report on the use of heterogeneous metal organic framework (MOF) catalysts for the Friedländer synthesis of 2‐aminoaryl ketone with α‐methylene carbonyl derivatives, a versatile and widely used method for the synthesis of quinoline derivatives. The importance of the Lewis and Brønsted acid sites and their spatial proximity have been explored. Postsynthetic modifications were made to introduce the Brønsted −SO3H acid sites in the pristine MIL‐53(Al). Two modifications were carried out, resulting in two different MOFs: MIL‐53(Al)−NH−(CH2)3−SO3H, in which there is a significant spatial distance between the Lewis and Brønsted sites, and MIL‐53(Al)−SO3H, where the two sites are in close proximity. The availability of sulfonic acid groups has a significant impact on their catalytic performance. The sulfonated MIL‐53(Al) material exhibited a high yield of quinoline derivatives under solventless and mild reaction conditions, with a wide substrate scope and good reusability. Mechanistic studies revealed that the dual acidic sites of the catalysts play an important role in the reaction mechanism.

Carbon Deposit Analysis in Catalyst Deactivation, Regeneration, and Rejuvenation.

Hydrocarbon conversion catalysts suffer from deactivation by deposition or formation of carbon deposits. Carbon deposit formation is thermodynamically favored above 350 °C, even in some hydrogen-rich environments. We discuss four basic mechanisms: a carbenium-ion based mechanism taking place on acid sites of zeolites or bifunctional catalysts, a metal-induced formation of soft coke (i.e., oligomers of small olefins) on bifunctional catalysts, a radical-mediated mechanism in higher-temperature processes, and fast-growing carbon filament formation. Catalysts deactivate because carbon deposits block pores at different length scales, or directly block active sites. Some deactivated catalysts can be re-used, others can be regenerated or have to be discarded. Catalyst and process design can mitigate the effects of deactivation. New analytical tools allow for the direct observation (in some cases even under in situ or operando conditions) of the 3D-distribution of coke-type species as a function of catalyst structure and lifetime.

Carbon Deposit Analysis in Catalyst Deactivation, Regeneration, and Rejuvenation

AbstractHydrocarbon conversion catalysts suffer from deactivation by deposition or formation of carbon deposits. Carbon deposit formation is thermodynamically favored above 350 °C, even in some hydrogen‐rich environments. We discuss four basic mechanisms: a carbenium‐ion based mechanism taking place on acid sites of zeolites or bifunctional catalysts, a metal‐induced formation of soft coke (i.e., oligomers of small olefins) on bifunctional catalysts, a radical‐mediated mechanism in higher‐temperature processes, and fast‐growing carbon filament formation. Catalysts deactivate because carbon deposits block pores at different length scales, or directly block active sites. Some deactivated catalysts can be re‐used, others can be regenerated or have to be discarded. Catalyst and process design can mitigate the effects of deactivation. New analytical tools allow for the direct observation (in some cases even under in situ or operando conditions) of the 3D‐distribution of coke‐type species as a function of catalyst structure and lifetime.

Intensified shape selectivity and alkylation reaction for the two-step conversion of methanol aromatization to p-xylene

Two-step conversion of methanol to aromatics via light hydrocarbons can significantly improve the conversion stability compared with direct aromatization of methanol, but it remains a challenge to achieve a high p-xylene (PX) selectivity. Herein, silica coating was firstly used to passivate external acid sites of ZSM-5 catalyst for the aromatization of light hydrocarbons by the chemical liquid deposition method. With the increase of SiO2 deposition, the density of the external acid sites of the catalyst was decreased from 0.1 to 0.03 mmol·g−1, which inhibited the surface secondary reactions and increased the PX/X from 34.6% to 60.0%. In view of the fact that the aromatization process in the second step was partly inhibited as methanol was consumed in advance in the upper methanol-to-light hydrocarbons catalyst layer, part of methanol was directly introduced into the lower aromatization catalyst layer to promote the alkylation process during the aromatization, which decreased the toluene selectivity from 34.5% to 14.3% but increased the xylene selectivity from 40.0% to 55.3%. It was also found that an appropriate external acid density was needed for aromatization catalyst to strengthen the alkylation process and improve the selectivity of xylene under the conditions of methanol introduction.