Types Of Acid Sites Research Articles

Developing highly stable and reusable WO3 on mesoporous cellular foam silica for dilute bioethanol dehydration to ethylene

This study developed a highly stable and reusable WO3 on mesoporous cellular foam silica catalyst (MCF-Si) for dilute bioethanol (70%v/v) dehydration to ethylene. WO3/MCF-Si exhibited exceptional performance during reaction, maintaining a high yield of ethylene ranging from 79.6% to 76.3% even in the final cycle over five consecutive cycles (20 h each), operating without requiring regeneration. Characterization of the spent catalyst revealed the preservation of key textural properties and a shift in the acidity profile. The strength of acidity transitioned from weak to strong acidity, while the type of acid sites evolved from Lewis to Bronsted acid sites. However, the remaining acidity remained sufficiently high to promote efficient ethylene production. Additionally, WO3/MCF-Si demonstrated outstanding resistance to coke formation, with a coke content measured below 0.9 wt%. In light of these findings, WO3/MCF-Si emerges as a promising candidate catalyst for developing sustainable processes for converting dilute ethanol to ethylene.

Modification of chemical vapor-phase deposition and coking resistance over HZSM-5 catalysts for toluene disproportionation

Abstract To achieve higher catalytic activity of toluene disproportionation and improve the p-xylene selectivity and stability of coking resistance on HZSM-5 catalysts, tetrabutyl titanate, ethyl orthosilicate and diethyl phosphite were used for chemical gas-phase deposition modification on HZSM-5 catalysts. The properties of acid sites on catalysts were characterized by infrared spectroscopy (IR) and temperature-programmed desorption of pyridine (TPD), respectively. The experimental results showed that the type of acid sites and density of weaker acid sites over HZSM-5 zeolites deposited by oxides containing Ti, Si and P in chemical vapor coating did not change while the density of stronger acid sites decreased, in which acid intensity on HZSM-5 zeolite deposited by oxides containing P enhanced obviously. Using HZSM-5 zeolites modified as catalysts of toluene disproportionation, HZSM-5 catalysts deposited by oxides containing Si and P can greatly improve both catalytic activity and coking resistance on catalysts.

Unveiling the intrinsic activity of TiO2 in HMF valorisation

In the context of 5-hydroxymethylfurfural (HMF) valorisation, TiO2 has emerged as a promising support. However, its intrinsic activity and underlying structural parameters remained undiscovered. This study elucidates the close relationship between structural and surface properties of TiO2 (e.g. crystalline size, surface area, and the number and type of acid sites) and its activity in HMF conversion. Lewis acid sites were found to be the exclusive acid sites present in anatase TiO2 samples, and their abundance correlated strongly with the overall surface area of TiO2. HMF conversion over TiO2 led to the simultaneous formation of 5-methylfurfural (5-MF) and 2,5-diformylfuran (2,5-DFF). Their formation was attributed to an intermolecular hydride transfer occurring over the Lewis acid sites. This opens up the possibility of selectively obtaining 5-MF and 2,5-DFF using solely TiO2 active sites. These findings provide valuable insights for designing TiO2-supported metal catalysts for future energy and environmental applications, aligning with green chemistry principles.

Ketalization mechanism of glycerol to biobased glycerol levulinate ketal over HZSM-5: A combined experimental and theoretical study

Methyl levulinate ketal (MLK) is a novel biobased chemical derived from glycerol via ketalization, and exploring its synthesis mechanism is of great significance for the high-value utilization of biomass resources. In this study, the ketalization reaction of glycerol was evaluated using the MLK yield calculated by the GC internal standard method. The acidity of HZSM-5 was determined by NH3-TPD. The catalyst screening experiments demonstrated that HZSM-5 in zeolites and AlCl3 in metal salts exhibited high catalytic activity in the synthesis of MLK. Furthermore, the kinetic results indicated that the HZSM-5 zeolite catalyst exhibited a lower apparent activation energy in catalyzing the synthesis of MLK compared to the metal salt catalyst AlCl3. The ketalization reactions of levulinate and glycerol catalyzed by different types of acidic sites in HZSM-5 were then investigated using the density functional theory method. A comprehensive catalytic path, free energy changes, intermediates, and transition state structures were proposed. Furthermore, the free energy barriers of ethyl levulinate ketal and butyl levulinate ketal catalyzed by HZSM-5 were calculated, and the difference between the steric hindrance effect and the electronic effect of alkyl in levulinate was compared. This study enhances the understanding of ketalization reactions and will be useful in future catalyst design studies.

Consequences of Pore Polarity and Solvent Structure on Epoxide Ring-Opening in Lewis and Brønsted Acid Zeolites.

The structure of solvent molecules within zeolite pores influences the rates and selectivities of catalytic reactions by altering the free energies of reactive species. Here, we examine the consequences of these effects on the kinetics and thermodynamics of 1,2-epoxybutane (C4H8O) ring-opening with methanol (CH3OH) in acetonitrile (CH3CN) cosolvent over Lewis acidic (Zr-BEA) and Brønsted acidic (Al-BEA) zeolites of varying (SiOH) x density. Despite ostensibly identical reaction mechanisms across materials, turnover rates depend differently on (SiOH) x density between acid types. (SiOH) x -rich Zr-BEA (Zr-BEA-OH) provides ∼10 times greater rates than a (SiOH) x -poor material (Zr-BEA-F), while Al-BEA-OH and Al-BEA-F give turnover rates within a factor of 2. Zr-BEA-OH shows more positive activation enthalpies and entropies than Zr-BEA-F across the range of [CH3OH], which reflect the displacement of solvent molecules and lead to greater rates in Zr-BEA-OH due to the dominant role of entropic gains. Measurements of the density and composition of solvent within the pores show that the (SiOH) x nests within Zr-BEA-OH promote hydrogen-bonded solvent structures distinct from Zr-BEA-F, while the Brønsted acid sites confer interactions similar to (SiOH) x nests and give solvent structures within Al-BEA-F that resemble those within Al-BEA-OH. Correlations between apparent activation enthalpies and C4H8O adsorption enthalpies show that interactions with solvent molecules give proportional changes to both C4H8O adsorption and ring-opening transition state formation. The differences in intrapore environment carry consequences for both rates and regioselectivities of epoxide ring-opening, as demonstrated by product regioselectivities that increase by a factor of 3 in response to changes in solvent composition and the type of acid site in the *BEA structure (i.e., Lewis or Brønsted). These results demonstrate the ability to control rates, regioselectivities, and adsorption thermodynamics relevant for industrially relevant liquid-phase reactions through the design of noncovalent interactions among solvating molecules, reactive species, and (SiOH) x functions.

Dehydration of Methanol to Dimethyl Ether—Current State and Perspectives

The main groups of catalytic materials used in the conversion of methanol to dimethyl ether (the MTD process) were presented with respect to their advantages, disadvantages, and the methods of their modifications, resulting in catalysts with improved activity, selectivity, and stability. In particular, the effects of strength, surface concentration, and the type of acid sites, the porous structure and morphology of the catalytic materials, the role of catalyst activators, and others, were considered. The prosed mechanisms of the MTD process over various types of catalysts are presented. Moreover, the advantages of membrane reactors for the MTD process are presented and analysed. The perspectives in the development of effective catalysts for the dehydration of methanol to dimethyl ether are presented and discussed.

Revealing the effect of framework acidity on the catalytic combustion of vinyl chloride over zeolite-based catalysts

The study explored the effect of different types of acid sites on the catalytic combustion of vinyl chloride over zeolite-based catalysts. MFI-type zeolites were synthesized using impregnation method and loaded with the noble metal Ru. Results indicated that the Ru/Sn-MFI catalysts possessed only Lewis acid sites, the Ru/Al-MFI catalysts were primarily governed by Brønsted acid sites, and the Ru/Si-MFI catalysts had no acid sites. The Ru/Sn-MFI catalyst containing Lewis acid sites demonstrated increased catalytic activity, with a T90 of up to 306 °C, and efficiently prevented the formation of chlorinated by-products. In contrast, the Ru-Si/MFI catalyst exhibited inferior catalytic activity. While a significant amount of chlorinated by-products was generated on the Ru/Al-MFI catalyst. The substantial amount of Lewis acid sites on the surface of Ru/Sn-MFI catalyst facilitated the activation of C-Cl bonds and the cleavage of C–C bonds. Furthermore, the Ru/Sn-MFI catalysts had improved low-temperature reducibility and oxygen mobility, which contributed to the deep oxidation of vinyl chloride.

Synthesis of Precursors to Ethylene Glycol via the Acid-Catalyzed Carbonylation of Formaldehyde

Methyl glycolate was synthesized as a precursor to ethylene glycol from the acid-catalyzed carbonylation of formaldehyde, followed by esterification with methanol. Homogeneous acids and different solid acids (e.g., resins and zeolites) were used as catalysts, and the effect of the solvent was examined. Afterward, a carboxylic acid protection strategy was proposed. With sulfolane and acetic acid as the mixed solvent, the solubility of CO increases, and the reaction rate can be accelerated. The rapid reaction between acetic acid and glycolic acid inhibits glycolic acid polymerization and pulls the reaction balance to promote the carbonylation reaction rate. Under the optimal solvent system (a molar ratio of acetic acid to sulfolane of 1:5) and the appropriate reaction conditions, the selectivity of the target product is higher than 85%. Solid acid catalysts with a −SO3H or −CF2SO3H functional group are supposed to be efficient in the carbonylation of formaldehyde, based on which a supported Nafion catalyst with a high surface area and total acid content was designed and synthesized. The novel supported Nafion catalyst presents a high total acid content and high Brönsted–Lewis acid ratio due to the characteristics of modified zeolite and, thus, leads to the high reactivity and very low selectivity of the by-product. A possible reaction mechanism is proposed to explain the product’s distribution by ascribing the formation of different products to different types of acid sites.

Hierarchical zeolite with open tin sites stabilized by metal ions catalyze the conversion of polysaccharides to alkyl lactates

The high yield of alkyl lactate from polysaccharides is attractive and challenging. Here, we report the remarkable catalytic ability of hierarchical tin-containing Beta zeolites for this process. The hierarchical structure, mediated by polystyrene microspheres or starch, improves mass transfer in the polysaccharide conversion. Two types of acidic sites in the zeolite achieved cooperation in the cascade conversion. Brønsted acid associated with Al promoted the hydrolysis of polysaccharides to monosaccharides; the abundant open Sn site stabilized by Ni2+ converted the monosaccharides to alkyl lactate. Polydextrose was used as a substrate in various alcohols, yielding up to 44% for methyl lactate, 37% for ethyl lactate, and 36% for propyl lactate, and the catalytic yields were consistent throughout three runs. More than 90% of the catalytic capability was recovered by the ion exchange and calcination procedures.

Development and Investigation of Carbon-Mineral Catalyst Based on Natural Clay and Tire Crush for Oxidative Decomposition of Nonionic Surfactants by Hydrogen Peroxide in Wastewater

Samples of a carbon-mineral catalyst based on natural clay and tire crumb for the reaction of oxidative decomposition of nonionic surfactants by hydrogen peroxide in wastewater have been developed and studied. The iron content in the samples varied in the range of 2.3–3.9 wt %. The effect of sample pyrolysis temperature in the range of 350–800°С on characteristics, the number and type of acid sites on the surface, and the ζ (zeta) potential of colloidal systems based on prepared samples was studied. Using model solutions under static and dynamic conditions, the effect of sample pyrolysis temperature on catalytic properties in the reaction of oxidative decomposition of H2O2 and nonionic surfactant (NS) nonoxynols N-9 by hydrogen peroxide was studied. The best result in the oxidation of nonoxynols N-9 with hydrogen peroxide was obtained in the presence of a carbon-mineral catalyst sample calcined at 650°C.

One-Pot Conversion of Furfural to γ-Valerolactone over Co- and Pt-Doped ZSM-5 Catalysts

γ-Valerolactone (GVL) is one of the useful biomass compounds produced via different reaction pathways from hemicellulose. In this study, Co- and Pt-doped/ZSM-5 catalysts with different Co loadings (0–10 wt.%) and Pt loadings (0.5–2 wt.%) were prepared by impregnation method and employed in a one-pot conversion of furfural to GVL. The yield of GVL increased with increasing reaction temperature from 100 to 140 °C. At the reaction temperature of 120 °C, higher amounts of secondary products such as AL and IPL can be converted to GVL, especially on the Co- and Pt-modified ZSM-5 catalysts. Compared to the non-modified H-ZSM-5 (GVL yield 35.4%), Co- and Pt-doped ZSM-5 catalysts exhibited much higher yield of GVL with the 1%Pt/ZSM-5 catalyst showing the highest yield of GVL at 85.4% at 120 °C and 1 bar N2 without the use of liquid acid or external H2 supply. The catalyst performances were correlated to the physicochemical properties of the catalysts such as the amount and type of acid sites. The NH3-TPD and in situ FTIR spectra of pyridine adsorption results revealed that Co- and Pt-loaded on ZSM-5 enhanced Lewis and weak acid sites, which are beneficial for the reaction. The results present a simple strategy to obtain high GVL yield under relatively mild conditions.

Improving the methyl lactate yield from glucose over Sn–Al-Beta zeolite by catalyst promoters

Sn–Al-Beta zeolite was prepared through a hydrothermal method and further modified by alkali and alkaline earth (e.g. Sr and Ba) metals not practically altering the textural properties of the catalyst. The catalytic materials were tested in glucose transformation to methyl lactate in a batch reactor at 180 °C allowing to correlate catalytic activity and selectivity with the strength and type of acid sites. The results showed that by suppressing the Brønsted acidity of the modified catalyst with different promoters the yield of methyl lactate can be increased from ca. 20% to over 50%.

Stability and Acidity of Sites at the External Surface and at Point Defects of Faujasite

AbstractFaujasite is one of the most industrially employed zeolite for its catalytic properties. The strong Brønsted bulk bridging Al−OH−Si sites are often thought to be the main origin of the Brønsted acidity of faujasite. However, many reactions also take place at the surface of the material and of its mesopores, where other types of acid sites exist. This study aims at unraveling the nature and strength of faujasite bulk and surface acid sites. Using Density Functional Theory (DFT), we rank the faujasite acid sites based on their stability, investigate their dehydration properties, and study their Brønsted and Lewis acidity via the adsorption of pyridine. Calculations are performed on cells with high (47 for bulk cells and about 300 for surface slabs) and low Si/Al ratio (about 3) and on cells containing defects under the form of silanol nests. These environments generate Brønsted acid sites of various strengths, and strong Lewis acid sites, even in the absence of extra‐framework species.

Studies on the preparation of fly ash-derived Fe-SSZ-13 catalysts and their performance in the catalytic oxidation of NO by H2O2

SSZ-13 zeolite was synthesized from fly ash using a combined alkali fusion hydrothermal method. The optimal synthesis parameters are as follows: NaOH/FA=1.4, SiO2/Al2O3=50, H2O/SiO2=1800, TMAda-OH/Al2O3=7, the amount of SSZ-13-seed=10%, and crystallization time and temperature were 160 °C and 40 h. Based on this, a series of fly ash-based Fe-SSZ-13 catalysts were prepared using three different methods and tested in catalytic oxidation with H2O2 for NO removal. The experiment results showed that the NO removal efficiency catalyzed by Fe-SSZ-13-SSIE, Fe-SSZ-13-LIE and Fe-SSZ-13-IM were 78%, 87% and 84%, respectively. ESR was used to detect hydroxyl radicals, UV-vis, XPS and H2-TPR were applied to test the composition, state and distribution of iron species, while NH3-TPD and Py-FTIR were performed to analyze the intensity and type of acidic sites, and finally to determine the relationship between catalyst structure, active species and catalytic performance in combination with NO removal experiments. The catalytic oxidation mechanism analysis revealed that the excellent catalytic performance can be attributed to the dual function of strong Brønsted acidity on the surface and isolated Fe3+ with high dispersion, which can rapidly decompose H2O2 into ·OH and efficiently achieve NO oxidation. In conclusion, the preparation of fly ash catalysts has great potential for achieving the dual function of waste utilization and pollutant removal.

Formation of PCDD/Fs, PCBs and HCl during catalytic combustion of chlorobenzene over supported transition metal (Cr, V and Cu) oxide catalysts

As an efficient scheme, catalytic combustion can be used to eliminate chlorinated volatile organic compounds (CVOCs). To avoid the formation of more toxic by-products, it is essential to reveal the relationship between the chlorine-containing products and redox properties of catalyst. In the present study, the activity for chlorobenzene (CB) oxidation, HCl production, and formation of by-products polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) and polychlorinated biphenyls (PCBs)were investigated over γ-Al2O3-supported transition metal (Cr, V and Cu) oxide catalysts. The results showed that the 10Cr-Al catalyst with relatively high specific surface areaand more oxygen absorbed than other catalysts exhibited higher catalytic oxidation activity for CB. The temperatures needed for 90% conversion of CB (T90) were 331, 353 and 386 °C for 10Cr-Al, 10V-Al and 10Cu-Al, respectively. The amount and type of acid sites on the surface of catalysts play an important role in the distribution of products and by-products. It was found that the increase of acid sites and the presence of Brønsted acid (B-acid) significantly improve the HCl selectivity of catalysts. More HCl was formed on the 10V-Al catalyst with B-acid sites. Meanwhile PCDD/Fs and PCBs detected in the by-products had a tight relationship with the acidity sites and redox performance of the catalyst. The yields of ∑PCDD/Fs and ∑PCBs were the largest on 10V-Al at T10, reaching 25.60, 66.93 mmol/mol CB, respectively. The obtained results provided theoretical support for the subsequent development of CVOCs catalytic oxidation catalysts and the control of toxic by-products.

Coal fly ash to Y zeolite of great purity and crystallinity: A new and green activation method of combined in situ microwave and ultrasound

Highly crystalline and pure Y (FAU) zeolite were synthesized from coal fly ash (CFA) using a combined in situ microwave and ultrasonic irradiation-assisted method, improving the extraction efficiency of silicon and aluminum from coal fly ash with limited energy usage relative to conventional microwave or ultrasound methods. Significant improvement in the combined method was evident following activation in 4 mol/L sodium hydroxide solution for 1 h at 100°C whereby 39.80 wt% conversion of CFA was attained along with 70.26 mol.% conversion of Si and 68.03 mol.% conversion of Al. Solid-state 29Si and 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were used to investigate the framework chemical coordination environment, while the temperature-programmed desorption of NH3 (NH3-TPD) and pyridine-Fourier transform infrared (Pyridine-FTIR) spectra were employed to quantitatively detect the acidity and types of acid sites of the sample. The Si to Al ratio (SAR) of the as-obtained CFA-based Y zeolite was 4.65 with a specific surface area of 916.10 m2/g, leading towards excellent CO2 adsorption (107.14 cm3/g at 0 °C). Furthermore, the thermocatalytic performance of the CFA-Y sample was analyzed in the conversion of cumene, with high silica commercial ultra-stable Y (USY) zeolite as a reference.

Ion-exchange modification of zeolite for the controllable catalytic transformation of diols and selective conversion of 1,2-butanediol to butanal

For the controllable transformation of 1,2-propanediol (PDO) and 1,2-butanediol (BDO) in ethylene glycol (EG), the acidity of H-Beta zeolite was modified via ion exchange with NaH2PO4 and Na2HPO4. After the modification, the separation factors for PDO and BDO reached 7.7 and 11.6 (vs 3 on the parent H-Beta). The selectivity to butanal-derived acetal declined to 4% (vs 42% on the parent H-Beta). Therefore, EG consumption was greatly reduced. Lower temperatures were favorable to the formation of acetone, butanone, and 1,3-butadiene, while higher temperatures promoted the acetalization and oligomerization reactions. The ion-exchange treatment increased the stability of zeolite crystalline structure. Microcalorimetry and NH3-TPD results indicated that the strong and moderate acid sites with NH3 adsorption heat ≥140–160 kJ/mol and of 100–110 kJ/mol were responsible for the dehydration and butanal-related acetalization reactions, respectively. In contrast, the propanal-related acetalization reactions were less sensitive to the strength and type of acid sites. On the other hand, butanal can be selectively synthesized from BDO with this catalyst. The acidity of zeolite catalysts can be modulated with this simple approach for the improvement of selectivity to desirable products.

Determination of Chemical Kinetic Parameters for Adsorption and Desorption of NH3 in Cu-Zeolite Used as a DeNOx SCR Catalyst of Diesel Engines

Ammonia-based selective catalytic reduction is one of the most effective NOx reduction technologies for diesel engines, but its low NOx reduction efficiency under low-temperature conditions needs further improvement. Previous studies have broadened our understanding of the NH3 adsorption and desorption that occurs in an SCR catalyst of Cu ion-exchanged zeolite. However, many studies conducted to data on the control of the NH3 adsorption and desorption in SCR catalysts have considered a simple chemical reaction related to a single acid site. This study demonstrates a detailed process for determining the chemical kinetic parameters of the adsorption and desorption of NH3 for different types of acid sites of a zeolite catalyst. The determined chemical kinetics parameters will be used for more effective control of the SCR system in future studies.

Dehydration of levoglucosan to levoglucosenone over solid acid catalysts. Tuning the product distribution by changing the acid properties of the catalysts

The effect of the catalytic properties of various types of solid acids, including oxides (ZrO2, SOX/ZrO2, WOX/ZrO2, Nb2O5, NbOPO4, Al2O3, TiO2), silica–aluminas (ZSM5 and USY zeolites) and acidic ion-exchange resins (Amberlyst-70 and Dowex 50wx8) were investigated for liquid-phase levoglucosan (LGA) conversion. H2SO4 was also evaluated as a reference catalyst. The concentration of both Brønsted and Lewis acid sites in the catalysts was measured by FTIR of adsorbed pyridine and titration with NaOH. The catalysts were tested under different reaction conditions (110–170 °C and 30–60 bar of N2) for 1 h using tetrahydrofuran (THF) as solvent. The product distribution obtained from the LGA conversion can be tuned depending on the type of acid site of the catalyst or the reaction conditions used. Catalysts containing only Bronsted acid sites were selective to LA while those with both Bronsted and Lewis acid sites favored the formation of 5-hydroxymethylfurfural (HMF). LGO was mainly produced by the homogeneous catalyst. The USY and HZM5 (SiO2/Al2O3 ratio of 23) promoted the production of HMF and glucose, respectively. A reaction mechanism was proposed taking into account the acid properties of the catalysts. The reactions of dehydration of LGA to LGO or hydration of glucose requires Brønsted acid sites. Catalysts with only Lewis acid sites produced only humins. For the catalysts containing both Brønsted and Lewis acid sites, LGA is mainly hydrated to glucose on the Bronsted acid sites. Then, glucose is preferentially isomerized to fructose on the Lewis acid sites, followed by its dehydration to HMF. The low LA formation shows that the hydration of HMF to LA requires a higher concentration of Brønsted acid sites.

Effect of hydrogen reduction and palladium promotion of tungstate-modified zirconia on isomerization of heptane

The activity of Pd/WO3-ZrO2 catalysts in the reaction of heptane isomerization is determined by a complex of acid sites, primarily by the presence of LAS on the catalysts surface formed by the inclusion of W5+ atoms in their composition. This type of acid sites ensures high heptane conversion and acceptable selectivity in the formation of heptane isomers. The contribution of these acid sites to the activity of catalysts can be optimized at the stages of high-temperature activation in a hydrogen flow. The optimal temperature conditions for H2-activation should be chosen in the average temperature range of 200–300 °С, sufficient to achieve a fraction of W5+ atoms on the catalyst surface of ca. 35% (at. W5+/at. W6+). The second group includes Lewis acid sites, which are formed upon the introduction of palladium into the composition of the tungstated zirconia catalyst. The optimal loading of Pd is ca. 0.1 wt.%; above this amount, the acid sites are covered with dispersed palladium, which leads to a decrease in the total acidity of the catalyst. Stabilization of medium strength acid sites on the catalysts surface after the addition of Pd to the catalyst ensures the heptane isomerization reaction in hydrogen atmosphere at a moderate temperature of 200 °C with a high selectivity of 91.1–91.7%.