Distribution Of Acid Sites Research Articles

Nickel Poisoning of a Cracking Catalyst Unravelled by Single-Particle X-ray Fluorescence-Diffraction-Absorption Tomography.

Ni contamination from crude oil in the fluid catalytic cracking (FCC) process is one of the primary sources of catalyst deactivation, thereby promoting dehydrogenation-hydrogenation and speeding up coke growth. Herein, single-particle X-ray fluorescence, diffraction and absorption (μXRF-μXRD-μXAS) tomography is used in combination with confocal fluorescence microscopy (CFM) after thiophene staining to spatially resolve Ni interaction with catalyst components and study zeolite degradation, including the processes of dealumination and Brønsted acid sites distribution changes. The comparison between a Ni-lean particle, exposed to hydrotreated feedstock, and a Ni-rich one, exposed to non-hydrotreated feedstock, reveals a preferential interaction of Ni, found in co-localization with Fe, with the γ-Al2 O3 matrix, leading to the formation of spinel-type hotspots. Although both particles show similar surface zeolite degradation, the Ni-rich particle displays higher dealumination and a clear Brønsted acidity drop.

Catalyst design strategies towards highly shape-selective HZSM-5 for para-xylene through toluene alkylation

With the shape selective zeolite catalyst, toluene alkylation with methanol to para-xylene (MTPX) technology could produce highly pure para-xylene (PX) in one step. The lower feedstock cost and less energy consumption in products separation make it more competitive compared to the current toluene disproportionation route. Thus, MTPX is regarded as the most reasonable production route for PX production. This article reviews the strategies that applied to the preparation of high-performance catalysts for MTPX, with special focus on the precise control of pore dimension and acid sites distribution in zeolite to achieve the highest selectivity to PX. The outlook of the MTPX catalyst is also proposed to guide the catalyst development in the field.

Identifying and controlling the acid site distributions in mordenite zeolite for dimethyl ether carbonylation reaction by means of selective ion-exchange

The identification and regulation of acid site distributions in mordenite zeolites are accomplished by using tetramethylammonium ions to selectively exchange with the counter ions in 12-membered channels.

Synthesis of CenTiOx flakes with hierarchical structure and its enhanced activity for selective catalytic reduction of NOx with NH3

• Synthesis of self-assembly hierarchical structure Ce n TiO x flakes for NH 3 -SCR of NO. • Good activity of Ce 0.21 TiO x due to the increasing surface area, Ce 3+ and O α contents. • Good correlation between the percentage of Brønsted acid sites and NO conversion. • An Eley-Rideal (E-R) mechanism dominates the reaction process. Herein, a series of Ce n TiO x (n = 0.068, 0.15, 0.21 and 0.30) flakes with hierarchical structure was prepared by alcoholysis under solvothermal conditions and explored for the selective catalytic reduction (SCR) of NO x with NH 3 . The morphology of the Ce n TiO x flakes was related to the Ce doping content. Remarkably, the flakes morphology of catalyst greatly promoted the SCR activity derived from the strong interaction between the Ce and Ti oxides. Among these samples, the Ce 0.21 TiO x with rod-like hierarchical structures demonstrated good SCR activity that the NO conversion was still over 90% from 270 °C to 450 °C even under SO 2 and H 2 O. Various characterizations clarified that the good activity over Ce 0.21 TiO x was due to the increase of BET surface area (195 m 2 /g), surface Ce 3+ amount and surface O α species. In-situ DRIFTS was conducted to determine the distribution and reactivity of surface acid sites. The results demonstrated that there was a direct correlation between the coverage of the Brønsted acid sites and the NO conversion. Eley-Rideal (E-R) mechanism is the most likely to represent the reaction mechanism on Ce 0.21 TiO x catalyst.

Catalytic upgrading of beech wood pyrolysis oil over iron- and zinc-promoted hierarchical MFI zeolites

The catalytic upgrading of beech wood pyrolysis oil has been carried out over iron- and zinc-promoted hierarchical Mordenite Framework Inverted (MFI) zeolites to produce a blendable stream with fluid catalytic cracking (FCC) feed. The iron and zinc were introduced to the zeolite structure in different loadings through the impregnation technique using metal nitrate solutions with various concentrations. The aim was to investigate the effect of zeolite acidity control along with mesoporosity of HZSM-5 zeolite on catalytic upgrading parameters. The results showed that the metal doping reduced textural properties through coverage of the catalyst surface by metal species. The existence of metals also altered acid site distribution of the hierarchical zeolite retaining crystallinity. The results of bio-oil upgrading experiments revealed a rather significant effect of extra framework metal species, particularly in 2 wt% loading, on the deoxygenation activity, coke formation potential, and selectivity towards desired products. Indeed, the improved accessibility of original and new weak and strong acid sites, as well as controlled acidity by incorporation of metals caused the enhanced conversion of large oxygenates to desired products. Zn-modified hierarchical HZSM-5 showed high selectivity towards desired oxygenates such as ketones via the promotion of carboxylic acids ketonization and reduced the formation of polycyclic aromatics. The analysis of coke also indicated that the coke yield and oxidation temperature decreased by 15% and 10%, respectively, over Zn-promoted catalyst compared with hierarchical sample showing desirable characteristics for cost-effective catalyst regeneration.

A convenient synthesis of core-shell Co3O4@ZSM-5 catalysts for the total oxidation of dichloromethane (CH2Cl2)

Core-shell structured Co3O4@ZSM-5 catalysts were successfully synthesized by a simple two-step microwave hydrothermal method and used for the catalytic oxidation of dichloromethane (DCM). Co3O4@ZSM-5 catalysts exhibited better CO2 yield and lower byproduct yield than pristine Co3O4 and ZSM-5. For pure ZSM-5 and Co3O4, dechlorinated byproducts CH3Cl formed more readily over ZSM-5 due to its higher acidity and lower redox property, while polychlorinated byproducts (CHCl3 and CCl4) formed more readily over Co3O4 due to its lower acidity and higher redox property. Thanks to its core-shell structure, Co3O4@ZSM-5 exhibited appropriate redox ability and a suitable distribution of acid sites, possibly explaining its lower byproduct yields and more efficient formation and desorption of HCl and Cl2. In situ Fourier transform infrared (FTIR) spectra analysis indicated that formaldehyde, formate and methoxy species were the main intermediates of the catalytic oxidation reaction. A pathway for the catalytic oxidation of DCM on 1.5-Co3O4@ZSM-5 was proposed based on the DCM degradation performance and physicochemical property analysis. The addition of water vapor to the DCM degradation process on core-shell structure Co3O4@ZSM-5 was shown to promote DCM conversion and CO2 yield. In addition, we concluded that core-shell catalysts exhibited good thermal stability and durability for DCM oxidation.

Quantitative Relationship Between Activity and Acid Site Distribution in the Oligomerization of Ethylene Over MCM-41 Catalyst

Nickel ion exchanged MCM-41 samples with different Si/Al ratios and Ni-ZSM-5/MCM-41 were prepared and used to catalyze ethylene oligomerization which was performed in a fixed bed model (T = 523 K, P = 2 MPa). A temperature programmed desorption of ammonia (NH3-TPD) method was employed to quantitatively determine the acid strength distribution of Ni-MCM-41 samples with different Si/Al ratios and Ni-ZSM-5/MCM-41. The TPD spectrum was analyzed by an interrupted-TPD method with desorption-control (dc-method) to obtain the desorption activation energy of ammonia, which can be used as acid strength scale for the purpose of correlation with catalytic activity. A quantitative relationship between catalyst acidity and catalytic activity was established by a Bronsted type equation. The site with desorption activation energy of ammonia E = 124 kJ mol−1 was found as the most suitable site for ethylene oligomerization. An approximate linear relationship is obtained between the activity of the ethylene oligomerization reaction and the acid amount corresponding to the acid site with E = 124 kJ mol−1.

Influence of Acid Site Distribution on Dimethyl Ether Carbonylation over Mordenite

Mordenite (MOR) catalysts with different acid site distribution were synthesized. The structural variation and acid site distribution were systematically investigated by different methods. The char...

Catalytic performance and ultrasonic-assisted impregnation effects on WO3/USY zeolites in esterification of oleic acid with methyl acetate

Abstract USY commercial zeolite was used in the esterification of oleic acid and methyl acetate. Ultrasonic-assisted and conventional wet-impregnations of WO3 were compared and influence on catalytic activity and selectivity were evaluated. The reaction was performed with a methyl acetate to oleic acid molar ratio of 10:1, 10 wt% of catalyst and 240 °C. The catalyst samples were characterized using XRD, XRF, SEM, BET, 27Al and 29Si MAS-NMR, FTIR and NH3-TPD and a liquid pyridine adsorption method to identify acid sites nature. Characterization results showed high dispersion of WO3 species. Acid sites nature, strength, and distribution showed that the strongest sites of the parent USY were changed to moderated ones. Bronsted acid sites of USY were partially blocked by tungsten oxide species. Ultrasonic-assisted activation presented the highest selectivity in the esterification of oleic acid with methyl acetate. 10% and 20% WO3/USY/US zeolites reached 79.4 wt% and 80.8 wt% conversion into methyl oleate, showing similar catalytic performance to short-chain alcohols as acyl acceptors.

Investigating the Acid Site Distribution of a New-Generation Methyl Chloride Synthesis Catalyst.

The effect of modifying an η-alumina methyl chloride synthesis catalyst by doping with CsCl and KCl over the concentration range of 0.1–1.0 mmol g(cat)–1 is investigated by a combination of pyridine chemisorption coupled with infrared spectroscopy and mass-selective temperature-programmed desorption measurements. The loading of group 1 metal chloride is equivalent to a titrant that enables selective neutralization of Lewis acid sites present at the surface of the reference η-alumina catalyst. Specifically, a loading of 0.1 mmol g(cat)–1 is sufficient to neutralize the strong Lewis acid sites; a loading of 0.6 mmol g(cat)–1 is sufficient to neutralize the strong and medium-strong Lewis acid sites; a loading of 1.0 mmol g(cat)–1 neutralizes all of the strong and medium-strong Lewis acid sites and partially neutralizes the medium-weak Lewis acid site. These deductions connect with a catalyst design program to develop a methyl chloride synthesis catalyst that exhibits minimal formation of the byproduct dimethyl ether.

Transformation of Propane over ZnSnPt Modified Defective HZSM-5 Zeolites: The Crucial Role of Hydroxyl Nests Concentration

A series of ZnSnPt supported defective MFI zeolites with different SiO2/Al2O3 ratios (30, 110, 700, and ∞) and hydroxyl nests concentration were prepared and characterized by multiple techniques including scanning electron microscopy (SEM), nitrogen physisorption, NH3-TPD, transmission electron microscopy (TEM), hydrogen temperature programmed reduction (H2-TPR), and Fourier transform infrared spectrometer (FT-IR). It was found that Brønsted acid sites (Si(OH)Al) with strong acid strength and the hydroxyl nests with weak acid strength coexisted over the defective ZSM-5 zeolites and ZnSnPt Lewis acid sites preferentially located on the hydroxyl nests. The increase in the concentration of hydroxyl nests and SiO2/Al2O3 ratios apparently improved the distribution of ZnSnPt Lewis acid sites. The hydroxyl nest incorporated ZnSnPt Lewis acid sites showed extraordinary dehydrogenation ability. Specially, operando dual beam Fourier transform infrared spectrometer (DB-FTIR) was applied to characterize the propane transformation under reaction conditions. At low SiO2/Al2O3 ratios, the propane efficiently transforms into propene and aromatics (total selectivity of 93.37%) by the cooperation of Brønsted acid sites and ZnSnPt Lewis acid sites. While at high SiO2/Al2O3 ratios, the propane mainly transforms into propene (selectivity of above 95%) and hydrogen. This study provides guidance for the preparation of highly efficient propane dehydrogenative transformation catalyst.

Dimethyl Ether to Olefins over Modified ZSM-5 Based Catalysts Stabilized by Hydrothermal Treatment

The reaction of dimethyl ether to olefin over HZSM-5/Al2O3 catalysts modified by Zr and Mg and stabilized by hydrothermal treatment has been studied. Regardless of the introduction method and the nature of the metal, the dependence of the key products selectivity on X(DME) over hydrothermally treated steady-state catalysts does not change, and the experimental points are described by the same curves. Metal introduction and the corresponding changes in the acid sites distribution do not change the ratio of main reaction rates, only the absolute values of the formation rate of the products are changed. Zr doping leads to the greatest activity in the DME conversion due to an equable decrease in the total acidity of the sample. On the other hand, the Mg-modified sample has a higher amount of weak acid sites, which reduces activity. At low DME conversion, methanol is one of the primary reaction products which formed from DME simultaneously with propylene in alkene cycle. At high DME conversion, the methanol acts as a main reagent which leads to ethylene formation in the arene cycle. Based on the results, the role of the metal in the reaction chemistry is considered and the mechanism of product formation from DME over steady-state catalyst is proposed, which describes both the participation of DME and the methanol produced.

Catalytic tertiary butylation of phenol over sulfonated mesoporous polymer catalyst (MP-SO3H): Exceptional selectivity towards 2,4-di-t-Butylphenol

Abstract The alkylation of phenol with alcohols/olefins is an important reaction to transform lignin based phenolics to useful chemicals. In the present study, the sulfonated mesoporous polymers exemplified the exceptional selectivity towards 2,4-DTBP. This remarkable activity of the material is well correlated with its textural properties. The characterization data reveals that the presence of intrinsic mesoporosity, high acid strength, uniform distribution of acidic sites over high surface area are the prerequisites to facilitate this unusual selectivity to 2,4-DTBP. The various micro and mesoporous catalysts have been screened to compare the catalytic activity of MP-SO3H. The MP–SO3H–8 surpassed the catalytic activity of all the catalysts screened. The MP–SO3H–8 exhibits 99.5% phenol conversion and 78% selectivity to 2,4-DTBP. To the best of our knowledge, this is the highest yield ever achieved to 2,4-DTBP till date. In addition, leaching & recyclability studies prove that the material is stable to retain its catalytic activity up to three recycles.

The enhanced performance of Ti doped MnOx for the removal of NO with NH3

• Ti doped MnO x show an excellent SCR activity below 200 °C and above 300 °C. • Ti doping yields amorphous oxides, developed pore structure and more Mn 4+ . • Ti doped to MnO x brings more acid sites with proper acid strength. • Mn 1 Ti 1 O x forms more nitrites and shows a better low-temperature SCR activity. • Nitrates and strong NH 3 adsorption over Mn 1 Ti 2 O x delay the deactivation. Ti doped MnO x catalysts for the removal of NO with NH 3 (NH 3 ‒SCR) were prepared by coprecipitation method. The results showed that pure MnO x is composed of dominant Mn 3 O 4 with (101) plane and small amorphous MnO 2 , while TiO x only forms anatase TiO 2 . When Ti is doped into MnO x , as-prepared samples can form amorphous phase, and Mn 1 Ti 2 O x samples achieve 446 m 2 /g of specific surface area and 0.65 cm 3 /g of total pore volume. NH 3 can be adsorbed on MnO x and TiO 2 surface, but the former is weak and the latter is strong. Ti doping changes the acid site distribution and surface adsorbed oxygen of MnO x . Mn 1 Ti 1 O x has more available oxygen vacancies to produce NO 2 and achieves a good deNO x activity below 200 °C. With the increase of reaction temperature, more O 2 can be dissociated into O ‒ , causing that Mn‒containing catalysts show a complete NO removal at 200‒300 °C. When the temperature is higher than 300 °C, adsorption and activation of NH 3 become the rate‒determining step with the strong supply of NO 2 . TiO x can achieve 30% NO removal efficiency while MnO x is 100% at 200‒300 °C. Mn 1 Ti 1 O x samples have 95% of NO removal efficiency at 100 °C and maintain 100% in the range of 150‒320 °C. Nearly 100% N 2 selectivity is attained for all samples in the range of 80–160 °C, but the N 2 selectivity gradually decreases when the temperature is higher than 200 °C.

H-ZSM-5 Materials Embedded in an Amorphous Silica Matrix: Highly Selective Catalysts for Propylene in Methanol-to-Olefin Process

H-ZSM-5 materials embedded in an amorphous silica were successfully synthesized with three different Si/Al ratios (i.e., 40, 45, and 50). The presence of the MFI structure in the synthesized samples was confirmed by X-ray diffraction (XRD), Fourier transform infra-red (FT-IR), and solid state-nuclear magnetic resonance (SSNMR) techniques. The morphology and textural properties of the samples were investigated by scanning electron microscopy (SEM), TEM, and N2-physisorption measurements. Furthermore, acidic properties of the synthesized catalysts have been studied by NH3-TPD and FT-IR spectroscopy of CO adsorption studies. Variation of the Si/Al ratio affected the crystal morphology, porosity, and particle size, as well as the strength and distribution of acid sites. The synthesized zeolite materials possessed low acid-site density and exhibited high catalytic activity in the methanol-to-olefin (MTO) reaction. To study the intermediate species responsible for catalyst deactivation, the MTO reaction was carried out at high temperature (500 °C) to accelerate catalyst deactivation. Interestingly, the synthesized catalysts offered high selectivity towards the formation of propylene (C3=), in comparison to a commercial microporous crystalline H-ZSM-5 with Si/Al = 40, under the same reaction conditions. The synthesized H-ZSM-5 materials offered a selectivity ratio of C3=/C2= 12, while it is around 2 for the commercial H-ZSM-5 sample. The formation of hydrocarbon species during MTO reaction over zeolite samples has been systematically studied with operando UV-vis spectroscopy and online gas chromatography. It is proposed that the strength and type of acid sites of catalyst play a role in propylene selectivity as well as the fast growing of active intermediate species. The effective conversion of methanol into propylene in the case of synthesized H-ZSM-5 materials was observed due to possession of weak acid sites. This effect is more pronounced in H-ZSM-5 sample with a Si/Al ratio of 45.

Catalysts for Synthesizing Liquid Hydrocarbons from Methanol and Dimethyl Ether: A Review

Different ways of increasing the selectivity of catalysts are described for converting oxygenate to liquid hydrocarbons. Analysis of reports suggests that the main factors affecting the selectivity of zeolite-containing catalysts are the structural type and acidic properties of the zeolites. It is shown that the strength and distribution of acid sites depend on the structural type and chemical composition of the zeolite framework, and on the chemical nature of the exchange cations. Different ways of changing the acidic properties of zeolites are discussed, e.g., modifying them with cations of various elements and subjecting them to postsynthesis acidic, alkaline, and steam heat treatment.

Regeneration of deactivated H-ZSM-5 for aromatization by dielectric barrier discharge plasma

A dielectric barrier discharge (DBD) plasma regeneration reaction was conducted at atmospheric pressure and temperature. This method was found to be very effective in removing carbon deposits after a non-oxidative aromatization from acetylene. When a fresh H-ZSM-5 catalyst was used for the aromatization, coke was by-produced and it could be ascribed to the primary reason for deactivation. By applying the DBD plasma to the deactivated catalyst bed at ambient temperature and pressure, the removal of carbon deposition with microdischarge and aeration was very effective; therefore, it is believed that additional thermal activation might not be necessary. More amount of coke was removed by supplying higher power to the catalyst bed. According to the results of XRD, physisorption, 27Al MAS NMR, and NH3-TPD analyses, it was confirmed that the structure and the acid site distribution of H-ZSM-5 were successfully retained even after the plasma treatment. Aromatization over the plasma-regenerated catalyst led to mild deactivation at the early stage and afforded comparable BTX selectivity to fresh H-ZSM-5 catalyst. Compared with the results obtained using the thermally regenerated catalyst, the structural stability was found to be higher and the BTX selectivity was kept higher during most of the reaction time for the plasma-regenerated catalyst.

Heteropolyacids supported on mesoporous AlSBA-15 as efficient catalysts for esterification of levulinic acid

A comparative study of different heteropoly acids supported on mesoporous AlSBA-15 for the synthesis of ethyl levulinate has been undertaken for the first time. The catalysts were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, inductively coupled plasma-optical emission spectrometry, thermo gravimetric analysis, temperature programmed desorption of NH3 and N2 sorption studies. Insights into crucial parameters for the esterification reactions are also furnished. A distinct correlation was observed between acid density and conversion revealing that the distribution of acidic sites and their accessibility by the reactant moieties plays an important role in determining the overall activity. The effects of various reaction parameters were investigated to enhance the catalytic activity and yield of ethyl levulinate. Under the optimized conditions, levulinic acid conversion of 87.4 mol% and ethyl levulinate selectivity of 100% could be obtained with silicotungstic acid supported on AlSBA-15.

Effect of phosphorus and mesopore modification on the HZSM-5 zeolites for n-decane cracking

The objective of this work is to investigate the catalytic effect of a series of modified HZSM-5 zeolites on n-decane cracking at 500 °C and atmospheric pressure. The zeolite catalysts were prepared with phosphorus and/or mesopore modification by excessive impregnation and/or hydrothermal treatment, and were characterized with various methods including SEM, TEM, XRD, FTIR, N2 adsorption-desorption measurement and NH3-TPD. Results show that the addition of phosphorus adjusted the distribution and strength of acid sites in the zeolite crystals. The introduced mesoporous structure improved the diffusion of reactants and products in zeolite pores during n-decane cracking. With appropriate acid amounts and distribution, as well as short channels and high specific surface area, the modified HZSM-5 nanoparticles exhibited higher catalytic activity, better light olefins selectivity and longer lifetime, compared with the original HZSM-5 zeolite. The phosphorus modified HZSM-5 zeolites coupling with MCM-41 matrix was proved to be a promising catalyst for the thermal cracking of n-decane.

Hydrolysis of cellulose to glucose by supercritical water and silver mesoporous zeolite catalysts

The hydrolysis of cellulose into glucose is a critical step for the conversion of lignocellulosic biomass into fuels and chemical products. The combination of supercritical water hydrolysis in ultrafast reactors, with the subsequent hydrolysis of the cello-oligosaccharides on silver-exchanged mesoporous mordenite zeolite, offers the possibility of a clear enhancement in the conversion of cellulose and in glucose formation. Complete dissolution of cellulose is achieved in the supercritical step and 81.8% of formed oligosaccharides are hydrolyzed in the catalytic step, with a yield into glucose of 77.0%. A fraction of Al is octahedrally coordinated, indicating a distribution of acid sites after the silver exchange. A deactivation of the catalyst between the first and third reaction cycle is observed, with a reduction in hydrolyzed carbon from 81.8% up to 45.6%. However, the selectivity to glucose only varies from 94.1 to 81.8%. Afterwards, the activity remains constant up to the fifth cycle. The presence of Ag(0) particles, together with the formation of coke, are responsible for the partial blockage of the pores of the support and loss of catalytic activity.