Acid Site Density Research Articles

Influence of Aluminum Sources on Synthesis of SAPO-34 and NH3-SCR of NOx by as-Prepared Cu/SAPO-34 Catalysts

The full article will be published in the English version of the journal "Catalysis in Industry" No. 1, 2021.The synthesis of the microporous SAPO-34 molecular sieve goes from a combination of three templates: triethylamine, tetraethylammonium hydroxide, and morpholine under hydrothermal conditions. Two aluminum sources, namely aluminum hydroxide, and aluminum isopropoxide, were used exclusively to synthesize SAPO-34 zeolites. The effects of aluminum sources on the crystallization and physicochemical properties of SAPO-34 were studied thoroughly. The synthesized samples were characterized by using different characterization methods, including XRD, FE-SEM, N2 isotherm, EDS, and NH3-TPD. The results illustrate that the various sources of aluminum used for the synthesis of SAPO-34 materials extremely affect the crystallinity, morphology, and density of acid sites. Besides, the influence of aluminum sources on the performance of NH3-SCR technology was studied with Cu/SAPO-34 catalysts in a fixed-bed flow reactor. The two Cu/SAPO-34 catalysts promoted different NO and NH3 conversions between 200–600 °C though they share similar Cu content, which was loaded by the ion-exchange method in aqueous solution. In addition, the different Cu species in the two catalyst samples are surveyed by H2-TPR, while the EPR method is also used to assess the coordination of the copper element in the two catalysts.

Cooperative catalytic effects between Brønsted and Lewis acid sites and kinetics for production of methyl methacrylate on SO42−/TiO2-SiO2

Either the ethylene or isobutene-based route for industrial production of methyl methacrylate (MMA) contains the esterification step between methacrylic acid (MAA) and methanol, therefore it is necessary to develop highly active and stable catalyst. Herein, series of superacid catalysts of SO42-/TiO2-SiO2 were prepared for esterification of MAA with methanol to produce MMA. The Brønsted and Lewis acid site densities and their ratios in SO42-/TiO2-SiO2 could be modulated by controlling the impregnated (NH4)2SO4 concentration, which were identified by pyridine titration and FT-IR. And no changes in structural skeleton and crystal phase of the SO42-/TiO2-SiO2 during the acid properties regulation process were noticed with FT-IR and XRD. The cooperative catalytic effects between Brønsted and Lewis acid sites were observed through catalytic evaluation. The kinetic studies indicated the catalytic pathway followed L-H mechanism and the activation barrier of forward esterification (48 kJ/mol) was greater than that of reverse hydrolysis (40 kJ/mol) step.

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.

Propylene and aromatics from ethylene conversion over ZSM-5: Effect of zeolite composition

Bioethanol is a green alternative to supply the demand for light olefins (ethylene and propylene) and aromatics (benzene, toluene, and xylenes) that can have a green label. ZSM-5 zeolite is largely used to produce light olefins and aromatics from ethanol or ethylene. The addition of metal species to ZSM-5 can tune the activity and selectivity of the zeolite to specific compounds. The influence of the incorporation of tungsten, iron, and lanthanum species on ZSM-5 was studied. The presence of tungsten species favored the formation of aromatics, while lanthanum promoted the production of light olefins. The density of acid sites and the presence of sites with intermediate strength influenced the ethylene conversion and the product distribution of the metal impregnated zeolites. Moreover, the presence of Lewis acid sites promoted the production of aromatics. However, the role played by the acid properties is dependent on the experimental conditions employed. The reaction routes were elucidated, that is, the aromatics are mainly formed through dehydrocyclization on WHZSM-5, while the olefins are produced from the reaction of ethylene and carbene species (propylene) and the dimerization of ethylene (butenes) on LaHZSM-5.

Synthesis of hierarchical SAPO-34 to improve the catalytic performance of bifunctional catalysts for syngas-to-olefins reactions

Bifunctional process integrating methanol synthesis and methanol-to-olefins conversion provides a new synthetic strategy for lower olefins (C2C4=), whereas designing matchable zeolite components is still challenging due to the harsh reaction conditions. Here, a hierarchical SAPO-34 zeolite with a sandglass-like hollow structure is synthesized from an aluminum-rich precursor gel, which provides an abundance of crystal defects during zeolite crystallization. The bifunctional catalyst obtained by integrating the hierarchical SAPO-34 and ZnOZrO2 oxide offers a C2C4= selectivity of 80% at 25% CO conversion with excellent stability in syngas conversion. We demonstrate that in the presence of high-pressure hydrogen, a moderate density of acid sites is the prerequisite for obtaining high C2C4= selectivity in syngas/methanol conversion, because the olefins are easily hydrogenated into alkanes by excessive Brønsted acid sites. The hierarchical architecture significantly prolongs the lifetime of bifunctional catalysts, by facilitating C2C4= desorption and slowing down the coking rate.

Seed-assisted synthesis of ZSM-48 zeolite with low SiO2/Al2O3 ratio for n-hexadecane hydroisomerization

Synthesis of highly-crystallized ZSM-48 zeolite with low SiO2/Al2O3 ratio was successfully performed via seed-assisted route. The influence of various factors on the seed-assisted synthesis was investigated systematically, including the seeds dosages, different seeds species, adding order of seeds, hydrothermal temperature and time, silica and aluminum sources, and gel compositions. The crystallinity, morphology, textural and acidic properties of ZSM-48 were characterized by various techniques, such as XRD, SEM, FTIR, Py-IR, N2 physisorption, 29Si and 27Al MAS NMR. Aluminum sources posed a critical influence on the crystallinity with NaAlO2 more beneficial to obtain well-crystallized ZSM-48, whereas the crystallinity seemed slightly susceptible to silica sources. SEM results revealed that the crystal size was significantly reduced from ca. 20 μm to around 2 μm after introduced the seeds. Kinetic analysis suggested that the added seeds can significantly reduce the apparent activation energy of nucleation and crystallization, encourage the formation of crystal nuclei and promote crystallization. The Pt-supported seed-assisted samples exhibited considerably higher density of acid sites than that loaded on seed-free sample. The hydroisomerization of n-hexadecane was investigated over various Pt-supported catalysts, and the Pt-supported seed-assisted samples exhibited the highest i-C16 yield of ca. 75 wt% at conversion over 87 wt%.

Nettle leaves-based sulfonated graphene oxide for efficient hydrolysis of microcrystalline cellulose

A green preparation of porous graphitic carbon through iron based catalytic graphitization of nettle (Urtica dioica L.) as precursor for the synthesis of sulfonated graphene oxide to be used as catalyst was developed. The morphological and physicochemical properties of nettle-derived porous graphitic carbon (GI-Ntt), graphene oxide (GO-Ntt) and sulfonated graphene oxide (sGO-Ntt) were assessed with several characterization techniques. Layered soft-structure with surface area of 354.9 m2 g−1, mesopore volume of 0.13 m3 g−1 and presence of high densities of functional groups such as hydroxyl, epoxy, carbonyl, carboxyl and sulfonic acid group with the total acid sites density of 5.47 mmol g−1 on the surface of the sGO-Ntt were observed. Moreover, it is revealed that GO-Ntt and sGO-Ntt both possess extremely thin nanosheet-like structures with an irregular edges and transparent lamella appearance unlike the GI-Ntt. Using the sGO-Ntt as catalyst, hydrolysis of microcrystalline cellulose (MCC) without pretreatment in water as an environmentally benign solvent resulted in the maximum glucose yield and selectivity of 26.3% and 72.8%, respectively. The optimum experimental conditions were sGO-Ntt concentration ratio of 1 g g−1 of untreated microcrystalline cellulose at 160 °C for 3 h.

Nature of acid-base sites on molybdenum nitride catalysts: Effect of nitrogen and oxygen content

The presence of nitrogen and oxygen sites was shown to have a direct impact on the acid-base properties of Mo2N. The gas phase reaction of 2-methyl-3-butyn-2-ol (MBOH), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the Mo2N catalyst. The acid-base character of Mo2N was impacted by synthesis and surface pretreatment methods prior to analysis. The basic character of passivated Mo2N increased after the following pretreatment: degassing in helium < reduction in hydrogen < in-situ unpassivated < renitridation. The CO2 desorbed during TPD increased from 0 to 128.8 × 1016 molecules/m2 for the degassed and renitrided Mo2N, while NH3 desorption decreased from 134 to 13.2 × 1016 molecules/m2, respectively. Acid-base character was linked to N/O atomic ratio using XPS. The renitrided Mo2N surface exhibited the highest N/O atomic ratio, base site density, highest acetylene selectivity, while displaying the lowest acid site density and activity.

Studies on the Binary MgO/SiO2 Mixed Oxide Catalysts for the Conversion of Ethanol to 1,3-Butadiene

The demand for 1,3-butadiene, one of the most important raw materials in the rubber industry, is constantly increasing. The Lebedev process is a classical method of producing 1,3-butadiene from ethanol, which is to be optimized with regard to the mixed oxide catalysts used. In this work, the binary MgO/SiO2 solid system was tested with regard to its optimum chemical composition for the catalytic conversion of ethanol to 1,3-butadiene. Furthermore, novel mesoporous mixed oxides were prepared to investigate their textural, structural, and surface chemical properties as well as the catalytic activity. Nitrogen physisorption, scanning electron microscopy (SEM), and temperature-programmed ammonia desorption (NH3-TPD) measurements were carried out and evaluated. It was shown that the optimum yield of 1,3-butadiene is achieved by using MgO/SiO2 mixed oxide catalysts with 85–95 mol% MgO and not, as suggested by Lebedev, with 75 mol% MgO. The NH3-TPD measurements revealed that the maximum acid-site density is achieved with an equimolar up to magnesium-rich composition. During the synthesis of binary MgO/SiO2 solid systems based on mesoporous MgO, a thermally stable and ordered structure was formed in the autoclave, depending on the carbonate used and on the duration of the treatment.

The critical contribution of oxidation debris on the acidic properties of graphene oxide in an aqueous solution

The contribution of oxidation debris (OD) to the acidity of graphene oxide (GO) was investigated in this study. With Na2CO3 as the titrator base, the Boehm titration results showed that the total acidity of GO in an aqueous solution decreased from 9.72 to 2.74 mmol g−1 after a thorough removal of OD and that the total acidity of OD was 26.45 mmol g−1. Thermogravimetric analyses showed that the mass ratios of OD and residual graphene sheets (named bwGO) were ∼26 % and ∼73 % of the whole pristine GO, respectively. Based on the quantitative relationships between the mass ratio and acid site density, it was concluded that the total acidity of GO was equal to the sum of the acidity from bwGO and the OD contained in GO. Under alkaline conditions, the splitting and stripping of OD was attributed to the combined effect of the cleavage of H-bonds by nucleophilic attack from OH- and the electrostatic repulsion due to the ionization of carboxylic acids, in which the former became dominant when the pH shifted to neutral and weakly acidic. This study provides an explanation for the origin of GO acidity in aqueous solutions and highlights the role of OD in the chemistry of GO.

3D-printed HZSM-5 and 3D-HZM5@SAPO-34 structured monoliths with controlled acidity and porosity for conversion of methanol to dimethyl either

The ability to control the morphology and porosity of zeolite-structured monolith is an important step in the design of zeolite-based catalysts. Herein, additive manufacturing method was used for the rapid synthesis of H-ZSM-5 structured monoliths (3D-HZSM5) with hierarchical porosity (macro-meso-micro) and controlled type and density of acid sites. After 3D-HZSM5 monolith formation, the silicoaluminophosphate (SAPO-34) was grown on the 3D-HZSM5 structured monolith surface via secondary growth method for formation of 3D-HZSM5@SAPO-34 structured monolith. The samples were characterized before and after SAPO-34 growth by XRD, N2 physorption, NH3-TPD, Py-IR and SEM. The characterization results revealed formation of SAPO-34 layer on 3D-HZSM5 structured monolith and also modification also imply that formulation of H-ZSM-5 powder into monolith resulted in slightly less Brønsted acid sites while the density of Lewis acid sites increased. The performance of the 3D-HZSM5 and 3D-HZSM5@SAPO-34 structured monoliths were investigated in catalytic conversion of methanol to dimethyl ether (DME) over a temperature range of 180 to 320 °C for 10 h time on stream. The selectivity to DME was significantly increased as a result of modification in acidity and porosity of the 3D-printed HZSM-5 structured monolith as compared to its powder counterpart and also 3D-HZSM5@SAPO-34 structured monolith. SAPO-34 growth enhanced the density of strong acid sites which caused further conversion of DME to higher hydrocarbons. The highest DME selectivity reached 96% on 3D-HZSM5 structured monolith at 180 °C, which also achieved ~70% conversion of methanol.

Mechanistic Insight into the Framework Methylation of H-ZSM-5 for Varying Methanol Loadings and Si/Al Ratios Using First-Principles Molecular Dynamics Simulations.

The methanol-to-hydrocarbon process is known to proceed autocatalytically in H-ZSM-5 after an induction period where framework methoxy species are formed. In this work, we provide mechanistic insight into the framework methylation within H-ZSM-5 at high methanol loadings and varying acid site densities by means of first-principles molecular dynamics simulations. The molecular dynamics simulations show that stable methanol clusters form in the zeolite pores, and these clusters commonly deprotonate the active site; however, the cluster size is dependent on the temperature and acid site density. Enhanced sampling molecular dynamics simulations give evidence that the barrier for methanol conversion is significantly affected by the neighborhood of an additional acid site, suggesting that cooperative effects influence methanol clustering and reactivity. The insights obtained are important steps in optimizing the catalyst and engineering the induction period of the methanol-to-hydrocarbon process.

Gas phase acylation of guaiacol with acetic acid on acid catalysts

Gas phase acylation of guaiacol on acid solids was studied using catalysts with different acid site density, nature and strength. The main products formed were acetovanillone, 2-acetoxyanisole and catechol using acetic acid as acylating reactant. The product distribution considerably changed with the catalyst nature. Catalysts with Lewis and Brønsted acid sites promoted both the O-acylation route (2-acetoxyanisole formation) and C-acylation route (acetovanillone formation) and formed catechol mainly by demethylation of guaiacol; catalysts with large pore size also favored catechol formation by a bimolecular disproportionation reaction. Catalyst deactivation took place, thus the coke formation was studied suggesting that the Lewis acid sites, responsible of catechol formation, were more affected by deactivation. The effect of reaction temperature, molar reactant ratio and the use of different acylating molecules on guaiacol conversion and products formation were also evaluated. HZSM-5 catalyst, 523 K and acetic acid were appropriate to promote C-acylation during gas phase guaiacol acylation.

Influence of the Acid Properties of the Support on Au‐Based Catalysts for Glycerol Oxidation in Aqueous Medium

AbstractTuning the surface and structural properties of heterogeneous catalysts is important for alcohol oxidation with the aim of obtaining valuable aldehydes and carboxylic acids. In this work, gold nanoparticles (AuNPs) were dispersed and stabilized on two different supports, ZrO2 and TiO2, and the catalytic properties were evaluated in glycerol oxidation as a model reaction. AuNPs were first synthesized through the reverse micelle (RMs) synthetic strategy, which acted as nanoreactor, by using sodium 1,4‐bis(2‐ethylhexyl) sulfosuccinate (AOT). For instance, the growing was governed by the confined environment provided by the AOT micelle allowing obtaining pseudospherical AuNPs with an average diameter of 6 nm. Further deposition of the synthesized AuNPs on ZrO2 and TiO2 supports allowed, after annealing at 400 °C, obtaining AuNPs on ZrO2 and TiO2. Catalytic properties of these AuNPs on glycerol oxidation under oxygen atmosphere and in aqueous NaOH solution at 90 °C, demonstrated that the support type played a key role in the activity of gold species in terms of conversion of glycerol and selectivity, with lactic acid being the main product formed on Au/ZrO2 while glyceraldehyde was the main product formed on Au/TiO2. The differences in catalytic performance of Au/ZrO2 might be correlated to a better dispersion of AuNPs and a higher density of acid sites with middle strength that modified activation of glycerol and the aldehyde intermediates toward the formation of lactic acid.

Octane isomer dynamics in H-ZSM-5 as a function of Si/Al ratio: a quasi-elastic neutron scattering study

Dynamical behaviour of n-octane and 2,5-dimethylhexane in H-ZSM-5 zeolite catalysts of differing Si/Al ratios (15 and 140) was probed using quasi-elastic neutron scattering, to understand molecular shape and Brønsted acid site density effects on the behaviour of common species in the fluid catalytic cracking (FCC) process, where H-ZSM-5 is an additive catalyst. Between 300 and 400 K, n-octane displayed uniaxial rotation around its long axis. However, the population of mobile molecules was larger in H-ZSM-5(140), suggesting that the lower acid site concentration allows for more molecules to undergo rotation. The rotational diffusion coefficients were higher in H-ZSM-5(140), reflecting this increase in freedom. 2,5-dimethylhexane showed qualitative differences in behaviour to n-octane, with no full molecule rotation, probably due to steric hindrance in the constrictive channels. However, methyl group rotation in the static 2,5-dimethylhexane molecules was observed, with lower mobile fractions in H-ZSM-5(15), suggesting that this rotation is less hindered when fewer Brønsted sites are present. This was further illustrated by the lower activation barrier calculated for methyl rotation in H-ZSM-5(140). We highlight the significant immobilizing effect of isomeric branching in this important industrial catalyst and show how compositional changes of the zeolite can affect a range of dynamical behaviours of common FCC species upon adsorption. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Effect of support acidity during selective hydrogenolysis of glycerol over supported palladium-ruthenium catalysts.

We report the role of the acidity of support during the selectivity hydrogenolysis of glycerol over supported bimetallic palladium-ruthenium (PdRu) catalysts. The PdRu nanoparticles were supported on a series of metal oxides and zeolitic supports via the modified impregnation method and tested for the liquid-phase hydrogenolysis of glycerol using gaseous hydrogen. The relative acid site densities of selected catalysts were determined by ammonia temperature-programmed desorption and pyridine desorption experiments. Based on these studies, we report a direct correlation between the catalytic activity (conversion and 1,2 propane diol yield) and two different acid sites (strong acid sites and very strong acid sites). Besides zeolite-supported catalysts, TiO2 supported PdRu nanoparticles exhibit moderate catalytic activity; however, this catalyst shows high selectivity for the desired C-O bond cleavage to produce C3 products over the undesired C-C bond cleavage to produce < C3 products. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Synthesis and characterization of amorphous silica-alumina with enhanced acidity and its application in hydro-isomerization/cracking

Amorphous silica-aluminas with enhanced Brønsted acidity have been successfully synthesized via a novel acetic acid based sol-gel process. The resulting amorphous silica-aluminas possess an irregular spherical morphology, homogeneously element dispersion, and a high external surface area in the range of 374–594 m2/g with little contribution from micropores. By simply adjusting the molar Si/Al ratio in the synthesis precursor solution from 0.5 to 2.0, the total Brønsted acid site density firstly increases and then slightly decreases, consistent with the trend of summarized molar content of four and five-coordinated aluminum species. Among them, the amorphous silica-alumina with initial Si/Al ratio of 1.5 exhibits the highest Brønsted acid site density, and its supported Pt catalyst performs the highest activity resembling that of HBeta-based catalyst in the hydroisomerization of n-hexadecane. Meanwhile, a superior catalytic selectivity to multibranched isomers was received over the amorphous silica-alumina without strong Brønsted acid sites. Moreover, a remarkable stability (>2000 h) was also achieved in the hydrocracking of Fischer-Tropsch wax under industrially relevant reaction conditions. This sol-gel synthesis process provides a facile method to prepare amorphous silica-aluminas with outstanding texture and acid properties for catalytic refining in petrochemical fields.

Interface mediated crystallization of plate-like SAPO-41 crystals to promote catalytic hydroisomerization

Silicoaluminophosphates SAPO-41 is a superior unidimensional zeotype catalyst used in hydroisomerization. Generation of hierarchically structured SAPO-41 provides an effective way to promote its hydroisomerization selectivity, which is nonetheless difficult to achieve due to the formation of impurities. Herein, we report crystallization of plate-like crystals of pure phase SAPO-41 in a toluene-aqueous biphasic media in the presence of both Pr2NH and oleylamine. The obtained material has a larger adjustable range of silicon content (nSi/(nAl+nSi+nP) = 0.02−0.15), intermediate acidity and reduced diffusion path length along the micropore channel direction. Catalytic tests for n-heptane hydroisomerization on Pt loaded plate-like SAPO-41 crystals showed that the yield of the isomerized products could reach up to 68 %, with respect to 57 % for a control sample. The selectivities for isomers were found to increase with decreasing Si contents and enhanced diffusion properties, suggesting side β-scission reaction was disfavored at reduced acid site density and shortened diffusion path length.

Recovery of pentoses-containing olive stones for their conversion into furfural in the presence of solid acid catalysts

Olive stones were employed as feedstock for furfural production in two stages: 1) autohydrolysis of hemicellulosic fraction to recover their pentoses, mainly xylose, and 2) subsequent dehydration of pentoses into furfural. Autohydrolysis step was optimized by using different experimental conditions (temperature: 160−200 °C and time: 30−75 min), giving rise to liquors with different xylose concentrations, since hydrolysis was incomplete in some cases. The combined use of a commercial γ-Al2O3 and CaCl2 led to total hydrolysis of non-hydrolyzed pentosans after autohydrolysis step, and the subsequent dehydration of pentoses into furfural. The maximum values of furfural yield and efficiency were 23 and 96 %, respectively, after only 60 min at 150 °C by using liquor obtained by autohydrolysis at 180 °C and 30 min (L5.1) as source of pentoses. This liquor, L5.1, provided better catalytic results than other liquors which had shown higher xylose concentration after autohydrolysis, probably due to these latter also exhibited a higher concentration of organic acids; thus, the presence of organic acids, such as acetic and lactic acids, could promote undesired reactions leading to lower furfural yields. Finally, γ-Al2O3 was more effective for furfural production under these experimental conditions than other solid acid catalysts, such as mesoporous Nb2O5, Nb-doped SBA-15 and Zr-doped HMS silicas, probably due to alumina has a higher density of acid sites.

Influence of the preparation method on the performance of Ni-based bifunctional catalysts in the one-pot conversion of γ-valerolactone to valeric biofuel

In this work the one-pot production of pentyl valerate (PV) from γ-valerolactone (GVL) and pentanol in liquid phase at 523 K, 10 bar of H2 over Ni-based catalysts supported on SiO2-Al2O3 (SA) was studied. By using incipient wetness impregnation (I) and precipitation-deposition (PD) two different catalysts were prepared, characterized and tested in reaction. With larger Ni0 particles and a significantly higher acid site density, Ni/SA-I was more active and selective to PV, revealing the crucial impact of preparation method on the catalytic performance. After 24 h, a GVL conversion of 83.5% with PV selectivity of 87.8% was reached with Ni/SA-I.