Density Of Bronsted Acid Sites Research Articles

Direct Low-Temperature Oxidative Conversion of Methane to Acetic Acid on Rhodium-Modified Zeolites

Oxidative conversion of methane to acetic acid at 150°С and a pressure of 6.5 MPa with the assistance of carbon monoxide on rhodium-modified ZSM-5 zeolites with different SiO2/Al2O3 molar ratios was studied. A decrease in SiO2/Al2O3 from 300 to 30 leads to a 2.7-fold increase in the acetic acid yield. Such behavior of the catalytic system correlates with an increase in the acidity, primarily with an increase in the density of Bronsted acid sites on the zeolite surface. Introduction of 0.1–0.5 wt % rhodium into the zeolite increases the acetic acid yield by a factor of more than 3. The highest acetic acid yield (0.53 mmol/gcat) is attained on the zeolite with SiO2/Al2O3 = 33 modified with 0.5 wt % Rh. The methane transformation to acetic acid occurs while involving both the acid sites of the zeolite and the sites formed by reactive rhodium particles.

Probing External Brønsted Acid Sites in Large Pore Zeolites with Infrared Spectroscopy of Adsorbed 2,4,6-Tri-tert-butylpyridine

The current work describes the development of a semiquantitative test to measure the density of Bronsted acid sites on the external surface of large pore zeolites. The method uses 2,4,6-tri-tert-bu...

Shape Selectivity in Hydroisomerization of n-Hexadecane over Pd Supported on Zeolites: ZSM-22, ZSM-12 and Beta

The development of bifunctional catalysts with high branched isomers selectivity in hydroisomerization of n-alkanes constantly remains crucial for the manufacture of green biodiesel from vegetable oil. Herein, three bifunctional catalysts (Pd/zeolite) have been prepared by providing 0.5 wt % Pd over the ZSM-12, ZSM-22 and Beta zeolite, respectively. The physicochemical properties of above-prepared zeolites and catalysts were investigated by XRD, SEM, N2 physical adsorption, NH3-TPD, Py-IR measurements and H2 chemisorption methods. The catalytic property for the n-hexadecane hydroisomerization over three bifunctional catalysts was measured which indicate that higher distribution of mono-branched iso-hexadecane is received over Pd/ZSM-22 and Pd/ZSM-12 catalysts based on the zeolites with one dimensional pore channel due to the high shape selectivity. Compared with Pd/ZSM-12, the Pd/ZSM-22 catalyst displays the higher selectivity for iso-hexadecane because of more favorable metal-acid balance caused by the larger CPd/CH+ ratio. Furthermore, the Pd/Beta catalyst shows highest catalytic activity and maximum iso-hexadecane yield of 69.7% when the n-hexadecane conversion is 78.9%, which results from the maximal porosity, Bronsted acid sites density and mild acidity.

Conversion of Methanol to Aromatic-Rich Gasoline over High-Efficiency Bifunctional Catalysts: Green Synthesis of GaZSM-5 Zeolites via Dry-Gel Conversion Strategy

Nanoscale ZSM-5 (AlZ5-D) and Ga-substituted ZSM-5 zeolites ((Al,Ga)Z5-D and GaZ5-D) were synthesized by a green dry-gel conversion strategy. For comparison, ZSM-5 zeolite (AlZ5-T) was also prepared by traditional hydrothermal method. The effects of synthesis method and Ga introduction on the physicochemical property and catalytic behavior of methanol to gasoline (MTG) were studied in detail. The results show that the aromatics selectivity decreased while $$\rm{C}_5^+$$ alkanes selectivity increased for AlZ5-D sample compared with that of AlZ5-T, which was attributed to the significantly reduced density of Bronsted acid sites and weakened strong acidity. After Ga introduction, the acid strength of samples further weakened due to the formation of Si—OH—Ga bridge hydroxyl group by insertion of Ga into the zeolite framework, inhibiting the cracking reaction and thus increasing the gasoline selectivity. Moreover, the extra-framework Ga species, especially in the form of GaO+ in GaZ5-D sample, exhibited high dehydrogenation activity, leading to the highest selectivity of aromatics and gasoline.

A novel UiO-66 encapsulated 12-silicotungstic acid catalyst for dimethyl ether synthesis from syngas

Abstract Dimethyl ether (DME) has received great attention as a promising clean fuel and industrially important intermediate. Tremendous efforts have been made to develop highly active and stable catalysts for DME production. Here, we present the first example of the application of metal-organic framework (MOF) catalyst in DME synthesis from syngas. A Zr-MOF (UiO-66) was selected as the catalyst host and support owing to its exceptionally high stability and porosity. UiO-66 was firstly functionalized by silicotungstic acid (H4SiW12O40, STA) via a one-pot synthesis method and then loaded with Cu by a facile solid grinding method. The prepared catalysts were characterized using XRD, TGA, FT-IR, N2 adsorption and TEM. The high porosity and thermal stability of acidified UiO-66 were maintained after 15% wt STA incorporation without any destruction of the Keggin structure of STA. The catalyst was tested for one step DME synthesis from syngas, showing higher DME selectivity and space-time yield than control catalysts. The superior activity of Cu/STA-UiO was attributed to the uniform dispersion of active centres with close intimacy and high density of Bronsted acid sites, as well as strong metal support interaction between Cu and Zr oxide SBU evidenced by XPS analysis.

Enhanced Catalytic Performance of CuO–ZnO–Al2O3/SAPO-5 Bifunctional Catalysts for Direct Conversion of Syngas to Light Hydrocarbons and Insights into the Role of Zeolite Acidity

Synthesis of light hydrocarbons from synthesis gas using bifunctional catalysts consisting of CuO–ZnO–Al2O3 methanol synthesis catalysts and SAPO-5 were investigated in a fixed bed reactor. The operating results showed that both the temperature and the ratio of CZA/SAPO-5 influenced the CO conversion and the selectivity of the catalysts. The effects of different dehydration component such as HZSM-5, HMOR and SAPO-5 and subsequently the impact of the zeolite acidity on the catalytic performance were also investigated. Experimental results indicated that zeolites in bifunctional catalysts played the crucial role for the distribution of hydrocarbons, and SAPO-5 was superior to the other zeolites in terms of better conversion and C3–C5 selectivity due to its suitable topology and proper acidic property. The efficiency of the CZA/SAPO-5 catalysts was found to be directly proportional to the Bronsted acid sites density of the zeolite and Bronsted acid sites are the likely zeolite active sites for DME dehydration. High time–space yield (461.6 mg mL−1 h−1) and high selectivity (88.1%) of light hydrocarbons (C3–C5) could be achieved on the CZA/SAPO-5-0.4 catalyst at 290 °C.

Effect of acid-metal balance of bifunctional Pt/Beta catalysts on vapor phase hydrodeoxygenation of m-cresol

Abstract Metal/zeolite bifunctional catalysts have been widely explored for hydrodeoxygenation (HDO) of phenolics. However, the effect of acid-metal balance is not clearly understood because it is difficult to decouple this from other effects. In this work, a series of 1 wt.% Pt/Beta catalysts with different Bronsted acid site (BAS) to surface Pt ratios (BAS/Pt) were prepared by varying only the density of BAS, and tested in HDO of m-cresol at 250 °C and atmospheric pressure. Increasing BAS density increases intra-crystallite diffusion constraints of m-cresol/products due to strong chemisorption on BAS, and therefore slows their transportation through BASs toward the Pt sites inside the micropores of zeolite. As a result, both the reaction rate and turnover frequency of HDO are reduced with increasing BAS/Pt ratio. Increasing BAS/Pt ratio increases the yield of ring contraction products but reduces the yields of methylcyclohexane and methylcyclohexanone, because of the increased acid functionality and diffusion constraints of intermediates. A low BAS/Pt ratio is beneficial for improving catalytic stability by reducing coke deposition.

A Comparative Study of n-Butane Isomerization over H-Beta and H-ZSM-5 Zeolites at Low Temperatures: Effects of Acid Properties and Pore Structures

The influences of acidic properties and pore structures of H-Beta and H-ZSM-5 zeolites on the reaction properties of n-butane isomerization at low temperatures were investigated. The results showed that bimolecular pathway of n-butane conversion predominates over H-ZSM-5 zeolites, while the monomolecular and bimolecular pathways occur simultaneously over H-Beta zeolites. The conversion rate of n-butane strongly relies on the amount of strong Bronsted acid sites regardless of zeolite topology. However, the topology of zeolites crucially determines the products distribution, and the density of strong Bronsted acid sites plays a secondary role. The cavities of zeolites, formed in the intersections of channels, provide the places for the bimolecular reaction. The formation of trimethyl C8 intermediates is spatially restricted in the narrow channel intersections of H-ZSM-5 zeolites, resulting in higher contribution of n-butane disproportionation reaction. In addition, the narrow pore channels of H-ZSM-5 zeolite limit the monomolecular isomerization of n-butane molecules and affect the diffusion of heavier products (pentane) produced from bimolecular reaction, leading to the severe secondary reaction and high selectivity to propane. In contrast, the pore channels of H-Beta zeolite allow the monomolecular isomerization of n-butane and the deposition of coke. The topology of zeolites crucially determines the products distribution.

Efficient Oligomerization of Pentene into Liquid Fuels on Nanocrystalline Beta Zeolites

Light alkenes oligomerization, performed in the presence of heterogeneous acid catalysts, is an interesting alternative for the production of clean liquid fuels. The process, when catalyzed by zeolites, is flexible and can be directed to the formation of oligomers in the gasoline, jet fuel, or diesel range by adjusting the reaction conditions and the zeolite’s structure. Herein we show how reducing the crystal size of large-pore Beta zeolites down to 10–15 nm and controlling the number and strength distribution of their Bronsted acid sites leads to highly active and stable catalysts, selective to true oligomers within the naphtha and, especially, the diesel range. The shorter diffusion path lengths in the smaller crystallites and the reduced Bronsted acid site density of the two nanosized beta zeolites (10–15 nm) synthesized with Si/Al = 15 lead to 1-pentene conversion above 80% during the 6 h time on stream (TOS) at a space time (W/F) of 2.8 g·h·mol–1. This value is higher than the olefin conversion obta...

Benzylation of Arenes with Benzyl Chloride over H-Beta Zeolite: Effects from Acidity and Shape-Selectivity

H-Beta zeolites with various Si/Al ratios have been prepared for the liquid-phase benzylation of various arenes with benzyl chloride (BzCl). 29Si and 27Al MAS NMR spectroscopy revealed the incorporation of Al into the silica framework to form catalytically active Bronsted acid sites (BAS). 1H MAS NMR spectroscopy investigations demonstrated the BAS density increased with reducing Si/Al ratio, while the BAS strength decreased as probed by CD3CN molecules. These H-Beta zeolites are highly selective to desired monobenzylation products depending on the shape-selectivity induced by the suitable channel system. The catalytic performance is in line with the nucleophilicity and proton affinity (PA) of arenes (xylene > toluene > benzene) on the same catalyst, typically for Friedel–Crafts reaction. A shape-selective effect has been observed to dominate the reaction, lowering the performance of mesitylene compared to xylene, as well as being selective to only the monobenzylation product. The benzylation performance ...

Synthesis of ordered porous zirconia containing sulfate ions and evaluation of its surface acidic properties

The synthesis and characterization of sulfated ZrO2 with ordered pores as a solid acid catalyst were investigated. Ordered porous ZrO2 containing sulfate ions was synthesized by hydrolysis of zirconium n-propoxide in the presence of HCl, surfactant and (NH4)2SO4 dissolved in distilled water, when the SO4/Zr molar ratio was changed in the range of 0.8–1.4. BET surface area of sulfated ZrO2 thus synthesized was found to be 150–160 m2 g−1. The presence of sulfate ions in ZrO2 was confirmed by an appearance of IR band at ca. 1398 cm−1 in the IR spectra of sulfated ZrO2. Since two IR bands due to surface hydroxyl groups were still observed for sulfated ZrO2, the possibility that sulfate ions are highly dispersed on the surface of ordered porous ZrO2 was suggested. From the evaluation of surface acidity by NH3-TPD measurements, not only the amount of NH3 desorption but also the strength of acid sites related to NH3 desorption temperature were found to be increased with increasing the SO4/Zr ratio up to 1.2. On the other hand, FTIR spectroscopy following pyridine adsorption revealed that the maximum surface density of Bronsted acid sites was attained for sulfated ZrO2 with SO4/Zr ratio of 1.0. Sulfate species interacting strongly with nano-sized tetragonal ZrO2 was considered to be responsible for the creation of Bronsted acid sites. In accordance with the surface density of Bronsted acid sites, sulfated ZrO2 with SO4/Zr ratio of 1.0 showed the highest activity for the skeletal isomerization of 3,3-dimethylbut-1-ene, which predominantly occurs on Bronsted acid sites.

Study of Zn and Ga Exchange in H-[Fe]ZSM-5 and H-[B]ZSM-5 Zeolites

The catalytic properties of H-[Fe]ZSM-5 and H-[B]ZSM-5 were explored after addition of Zn or Ga. TPD–TGA of 2-propanamine adsorbed on Zn- and Ga-exchanged H-[Fe]ZSM-5 showed a decrease in Bronsted-acid site densities and the formation of new dehydrogenation sites, similar to what is observed following exchange in H-[Al]ZSM-5 and in amorphous silica–alumina. Exchanged Zn cations in [Fe]ZSM-5 also exhibited Lewis-acid character, as demonstrated by the appearance of a υ(CN) stretch at 2310 cm–1 upon adsorption of CD3CN. By contrast, the sites in H-[B]ZSM-5 were not capable of protonating 2-propanamine, did not form dehydrogenation sites when Zn or Ga were added, and showed no evidence for sites with Lewis-acid character from the FTIR spectroscopy of CD3CN. Neither H(Zn)-[Fe]ZSM-5 nor H(Zn)-[B]ZSM-5 catalyzed reactions of n-hexane at 773 K, but the TPD–TGA of adsorbed propene on H(Zn)-[Fe]ZSM-5 showed strong interactions between the Zn and olefins that might be responsible for the dehydrocyclization of light ...

Sulfonated char from waste tire rubber used as strong acid catalyst for biodiesel production

A new acid catalyst based in a carbonaceous solid was functionalized using sulfuric acid as source of SO3H acid groups. This carbon‐based material prepared by the pyrolysis of waste tire rubber was used either as catalyst or as catalytic support. The pyrolysis process was performed with a flow of N2 at relatively low temperature to obtain a mesoporous carbon and achieve an effective sulfonation. The sulfonation method of carbon obtained from tire rubber was through direct immersion into concentrated H2SO4 under reflux. The mesoporous solids were characterized by several analytic techniques including an elemental analysis derived from scanning electronic microscopy (SEM). These ones indicated the presence of polycyclic disordered carbon plates in the carbonaceous structure with a low surface area and wide pores that provided many surface acid sites. The high catalytic activity and stability of this catalyst is related to the acid site density of Bronsted acid sites and to its homogeneous distribution. The hydrophobicity presented by this material favorably prevented hydration of hydrophilic OH and SO3H functional groups. The transesterification and esterification of waste oil under sub‐critical methanol mainly in the presence of sulfonated char were achieved. Hence, it was required shorter times, low temperature and significantly, a low amount of methanol compared to other studies. © 2016 American Institute of Chemical Engineers Environ Prog, 36: 619–626, 2017

Transformation of South African coal fly ash into ZSM-5 zeolite and its application as an MTO catalyst

This study presents a way of using South African coal fly ash by extracting metals such as Al and Fe with concentrated sulphuric acid, and then using the solid residue as a feedstock for the synthesis of ZSM-5 zeolite. The percentage of aluminium and iron oxides decreased from 28.0 ± 0.2% and 5.0 ± 0.1% in coal fly ash to 24.6 ± 0.1% and 1.6 ± 0.01% in the acid treated coal fly ash respectively. The fly ash-based zeolite ZSM-5 sample synthesised from the solid residue after extraction of Al and Fe, contained 62% of ZSM-5 zeolite pure phase with a number of Brønsted acid site density of 0.61 mmol per gzeolite. By properly treating the as-prepared coal fly ash-based ZSM-5 zeolite, an active and selective methanol-to-olefins acid catalyst could be designed, leading to full methanol conversion during 15 h on stream. The optimised catalyst exhibited a cumulative methanol conversion capacity of 71 g(MeOHconverted)/g(catalyst) and a light olefin productivity of 21 g(C2=–C4=)/g(catalyst). Cette étude présente une stratégie de valorisation de cendres du charbon (origine : Afrique du Sud) après extraction de métaux tels que le fer ou l'aluminium. Le résidu solide est ensuite utilisé comme matière première pour synthétiser une zéolithe ZSM-5. Cette dernière est obtenue sous la forme d'un matériau composite zéolithe (62%)/cendre, présentant une acidité de Brønsted de 0,61 mmol par gzeolite. Par une conception sur mesure ou rational design, un composite ZSM-5/cendre s'est avéré être un catalyseur actif et sélectif dans la réaction de conversion du méthanol en oléfines légères (MTO). Une capacité totale de conversion du méthanol de 71g/gcatalyseur accompagnée d'une productivité de 21 g (C2=–C4=)/gcatalyseur a ainsi été obtenue.

Screening of Novel Materials for Biodiesel Production Through the Esterification of Oleic Acid

The present contribution screens the specific activity of various inorganic and enzymatic based materials in the esterification of oleic acid that is typically used as a test reaction for the production of biodiesel from high free fatty acid feedstocks. The inorganic materials investigated in this contribution are bulk fosfotungstic heteropoly acid of the Wells Dawson structure H6P2W18O62.nH2O (HPA), as well as dispersed on titanium dioxide 18 % w/w H6P2W18O62/TiO2 and the insoluble cesium salt of the Wells Dawson heteropoly anion Cs2H4P2W18O62. Additionally, the commercial biocatalyst Novozym® 435 (immobilized lipase B of Candida antarctica) and a self-supported lipase of vegetable origin obtained from the latex Araujia sericifera (ASL) were studied among the materials of enzymatic nature. The density and accessibility of Bronsted acid sites have a key role in the specific activity of the fosfotungstic based heteropoly compounds. The HPA dispersed over an oxide support catalyzed the esterification of the fatty acid in a heterogeneous fashion with the highest activity (6.4 µmol mg−1 h−1 at 85 °C) among the inorganic materials. In contrast, the enzymatic materials are more active at lower temperature than the inorganic ones. Particularly, ASL catalyzed the homogenous methanolysis with the highest specific activity (30.7 µmol mg−1 h−1 at 40 °C) at the lowest temperature among the materials assayed.

Coke Formation in a Zeolite Crystal During the Methanol‐to‐Hydrocarbons Reaction as Studied with Atom Probe Tomography

Understanding the formation of carbon deposits in zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, atom probe tomography (APT) has been used to spatially resolve the 3D compositional changes at the sub‐nm length scale in a single zeolite ZSM‐5 crystal, which has been partially deactivated by the methanol‐to‐hydrocarbons reaction using 13C‐labeled methanol. The results reveal the formation of coke in agglomerates that span length scales from tens of nanometers to atomic clusters with a median size of 30–60 13C atoms. These clusters correlate with local increases in Brønsted acid site density, demonstrating that the formation of the first deactivating coke precursor molecules occurs in nanoscopic regions enriched in aluminum. This nanoscale correlation underscores the importance of carefully engineering materials to suppress detrimental coke formation.

Acidity versus metal-induced Lewis acidity in zeolites for Friedel–Crafts acylation

Abstract Acid catalysts including Ni, Ag and Fe-loaded zeolites of different structures were prepared either via cationic exchange or impregnation techniques from pristine H-zeolites (BEA, and MFI). Their catalytic activity was evaluated in the liquid-phase Friedel–Crafts acylation of anisole with propanoic acid. It turned out that, whatever the doping procedure was, the zeolite loaded with transition metals led to considerable decrease in propanoic acid conversion, regardless of the nature or the metal content. However, the extent of this detrimental effect followed the order: Ag + > Ni 2+ > Fe 3+ . Pristine acidic zeolites were not only found to be the most active, but also to be the most selective toward ortho - and para -acylation products. H-ZSM-5 zeolites yielded the highest intrinsic activity, with TOF values of 0.09 h −1 . The catalyst activity proved to be essentially attributed to the density and accessibility of Bronsted acid sites, playing a key role in the activation of the reactants. Bronsted sites are proposed to be the most likely catalytic species for performing this Friedel–Crafts acylation.

Phosphorous Modified ZSM-5 Zeolites: Impact on Methanol Conversion into Olefins

A promising methanol-to-propylene catalyst was designed both at the molecular scale, exhibiting low Bronsted acid site density, and at the microscopic level providing large crystals (25 µm) with few defects. The potential of giant ZSM-5 zeolites synthesized in fluoride medium in the conversion of methanol into propylene led us to investigate the replacement of harmful hydrofluoric acid by non-toxic H3PO4. P-ZSM-5 zeolites were synthesized and thoroughly characterized by XRD, SEM, FT-IR, TPD propylamine, H/D isotope exchange and XRF analyses. The catalytic performance of these acid catalysts was evaluated in the methanol-to-olefins reaction at 673 K. The presence of phosphorous in the zeolite after calcination was ascertained and induced high ethylene selectivity (up to 33 %) and improved catalyst stability with respect to fluoride-mediated zeolites.

Catalyst optimization for enhanced propylene formation in the methanol-to-olefins reaction

ZSM-5 zeolites possessing different chemical compositions, acidities and crystal sizes, were synthesized and characterized by XRD, SEM and FTIR. Those as-prepared acid catalysts were tested in the conversion of methanol into light olefins at 673 K under atmospheric pressure. Propylene-to-ethylene ratios above 5 were achieved over ZSM-5 zeolite catalyst prepared via the fluoride route. This promising methanol-to-propylene (MTP) catalyst was designed both at the molecular scale, exhibiting low Brønsted acid site density , and at the microscopic level since large crystals (25 μm) with few defects were obtained. Aiming in a possible industrial application, the synthesis medium has to be seriously modified, thus avoiding harmful hydrofluoric acid use. Hydrochloric and phosphoric acids were therefore chosen as alternatives and resulted in the same ZSM-5 crystal morphologies and similar catalytic performance. De nos jours, les procédés de conversion du méthanol en oléfines légères (MTO) et plus particulièrement du méthanol en propylène (MTP) sont considérés comme des voies attractives et viables pour la valorisation des ressources en gaz naturel. Notre stratégie est de développer un catalyseur zéolithique de type ZSM-5, via la synthèse en milieu fluorure , ayant des propriétés désirées pour la réaction MTP effectuée à 400 °C. Un catalyseur présentant une densité de sites acides faible et une taille de cristal de 25 μm a démontré des performances intéressantes. De plus, nous avons été en mesure de substituer l’acide fluorhydrique par de l’acide phosphorique dans le gel de synthèse et ainsi développer une voie plus écocompatible pour le design du catalyseur.

Elucidating the olefin formation mechanism in the methanol to olefin reaction over AlPO-18 and SAPO-18

The mechanism of the methanol to olefin (MTO) reaction over AlPO-18 (without Bronsted acid sites) and two SAPO-18 (with different Bronsted acid site densities) catalysts has been investigated. The Bronsted acid site density of AlPO-18 and SAPO-18 catalysts was determined by 1H MAS NMR spectroscopy. Methanol conversion over the catalysts showed that the catalytic activity of the catalysts was strongly influenced by their Bronsted acid site density. Using 13C magic angle spinning (MAS) NMR, we directly observed the pentamethylcyclopentenyl cation (pentaMCP+) over SAPO-18 under real MTO reaction conditions, but no carbenium ion was detected over AlPO-18. Furthermore, analysis of confined organics by 13C MAS NMR and GC-MS clearly demonstrated that higher Bronsted acid site density improved the formation and accumulation of some important and reactive hydrocarbon pool species, such as pentaMCP+ and polymethylbenzenes. With the aid of the 12C/13C-methanol switch technique, the detailed olefin formation mechanism was elucidated. During the MTO reaction, light olefin generation over SAPO-18 mainly followed the aromatic-based hydrocarbon pool mechanism; however, the olefin methylation and cracking mechanism accounted for the production of light olefins over AlPO-18.