Acid Site Strength Distribution Research Articles

Structure and Stability of Mg–HZSM-5/Al2O3 Catalysts for Synthesizing Olefins from DME: Effect of Thermal and Hydrothermal Treatments

Changes in the structural, textural, and acidic properties of Mg–HZSM-5/Al2O3 catalyst subjected to pretreatment (thermal and hydrothermal) of varying temperatures and durations are studied. The catalysts are characterized by XRF, XRD, 27Al solid-state NMR spectroscopy, nitrogen adsorption, DRIFT spectroscopy, and ammonia TPD. The catalytic activity of the samples in converting dimethyl ether (DME) to lower olefins is studied. It is shown that the initial catalytic activity of an Mg-containing zeolite catalyst is determined by the total acidity of the surface and does not depend on the initial acid site strength distribution, while the stability of the catalyst depends on its morphological properties.

Modified Brønsted type equation with ammonia as probe molecule: quantitative acidity-activity relationship for pyridine synthesis with ZSM-5 catalyst

The acid site strength distribution of a series of ZSM-5 zeolite catalyst with different Si/Al ratios is quantificationally measured by temperature-programmed desorption of ammonia. The activation energy for ammonia desorption is not only a practical scale of acidity but also has a definite catalytic meaning in the Bronsted equation of acid catalysis. The activation energy for ammonia desorption was first correlated with experimental catalytic activity data of aldehyde and ammonia condensation reaction over ZSM-5 by means of a quantitative equation. An improved Bronsted equation was established. It can be concluded that different strength acid sites have different effect coefficients for the pyridine synthesis. The acid sites on which the activation energy of ammonia desorption is 150 kJ is the highest active sites for the reaction. The Bronsted-type relations can be used as a way to fast improve catalyst for pyridine synthesis.

Hydrohalogenation of Platinum–Palladium/H-ZSM-5 Catalysts for n-Hexane Hydroconversion

Catalysts containing 0.3%Pt-, 0.3%Pd-, and 0.3%Pt-0.3%Pd-/H-ZSM-5 were prepared and modified via hydrochlorination or hydrofluorination and tested for n-hexane hydroconversion throughout a reaction temperature range of 250–500°C. Bifunctionality parameters of the current catalysts were characterized via acid site strength distribution by NH3-temperature programmed desorption, metal(s) component dispersion by H2 chemisorption, and temperature programmed reduction. Hydrochlorination was found to leach less structural zeolitic Al in the catalyst than hydrofluorination. HCl modification improved n-hexane hydroisomerization activity, whereas HF treatment was deteriorative via causing pore diffusion restriction by excessively formed debris. Acid sites density and strength were higher on hydrofluorination than on hydrochlorination. Surface area and metals dispersion were higher for hydrochlorinated catalyst but lower for the hydrofluorinated one. Maximum isohexanes production (70%) was realized on hydrochlorinated catalyst at 350°C with 100% selectivity. Crystal unit cell d-spacing, obtained by XRD, was smaller by hydrofluorination than by hydrochlorination, whereas such d-spacing was increased through incorporating Pt and increased more via Pd addition during preparation.

Preparation of Mesoporous SiO2-Pillared Lamellar Titanoniobate Catalysts for Bioethanol Dehydration

The lamellar perovskite K0,8Ti0,8Nb1,2O5 was prepared by solid state reaction, and its protonic form was used in a sequence of intercalation steps with n-butylamine, cetyltrimethylammonium bromide (CTABr), and tetraethyl orthosilicate (TEOS). After calcination, a high surface area, mesoporous SiO2-pillared titanoniobate, was obtained. The samples were characterized by XRD, EDX, TG-DTG, N2 adsorption isotherms, and NH3-TPD. The pillarization procedure affected the textural properties, the amount, and strength distribution of acid sites. The influence of the pillarization procedure on the catalytic properties of the lamellar titanoniobates was investigated on ethanol dehydration. High ethanol conversions and ethylene yields (>90%) were obtained in the presence of the SiO2-pillared titanoniobate catalyst, at 350–450°C.

Glycerol dehydration over calcium phosphate catalysts: Effect of acidic–basic features on catalytic performance

This work focuses on investigation of calcium phosphate compounds with different Ca/P ratios (1.39–1.77). Additionally, hydroxyapatite impregnated with tungsten oxide was also investigated. The structural, textural, and surface properties of these materials have been fully characterized using appropriate techniques (low-temperature adsorption–desorption of nitrogen, X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and temperature-programmed reduction (TPR)). Adsorption microcalorimetry of NH3 or SO2 was used to estimate the population, strength and strength distribution of acid and basic sites. The nature of acidic sites was determined through the adsorption/desorption of pyridine, followed by infrared spectroscopy. Catalytic performance of the catalysts was tested in the gas phase dehydration of glycerol with the intention of finding correlations between catalytic activity and surface acid–base features. Results show that Ca/P ratio, beside the already known influence on acidic/basic features, also influences red-ox properties of these materials. The investigation performed here proved that, in order to get acrolein – with high selectivity – which is formed on acid sites, it is not only necessary to provide acidity, but also to hinder basic sites. Our results also show that reducing of number and strength of acid centers increases the yield of other desired product, acetol.

Comparative studies of ethanol to propylene over HZSM-5/SAPO-34 catalysts prepared by hydrothermal synthesis and physical mixture

HZSM-5/SAPO-34 catalysts were prepared by physical mixture (named as ZS(m)-MM-n) and hydrothermal synthesis (named as ZS(m)-HS-n), respectively. Ethanol to propylene over these as-prepared catalysts was investigated under different reaction conditions. In addition, the physicochemical properties of these catalysts were characterized by AAS, XRD, N2 isothermal adsorption–desorption, SEM, NH3-TPD and TPO. The catalytic reactivity of HZSM-5/SAPO-34 catalyst was strongly dependent on its preparation method and composition as well as the reaction conditions. ZS(25)-MM-4 (HZSM-5/SAPO-34=4, weight ratio; HZSM-5 with Si/Al2=25 molar ratio) in isothermal fixed-bed reactor showed the highest propylene yield of ca. 34.5% among these catalysts and the poor catalytic stability. The use of nonisothermal fixed-bed reactor with the temperature configuration (2) significantly improved the catalytic stability of ZS(25)-MM-4. Synergetic effect of HZSM-5 with SAPO-34 occurred in HZSM-5/SAPO-34 catalyst, which modified its properties and catalytic reactivity. The concentration and the strength distribution of acid sites were closely relative to the catalytic reactivity of HZSM-5/SAPO-34 catalyst. For achieving the high yield of propylene, it was necessary that HZSM-5/SAPO-34 catalyst had the moderate concentration and strength distribution of acid sites.

CeO2–Nb2O5 mixed oxide catalysts: Preparation, characterization and catalytic activity in fructose dehydration reaction

In this work, ceria–niobia mixed oxides have been prepared by coprecipitation. The structural, textural, and surface properties of these materials have been fully characterized using appropriate techniques (low-temperature adsorption–desorption of nitrogen, thermogravimetric analysis, X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and temperature-programmed reduction/oxidation (TPR/O)). The acid–base properties were estimated by the adsorption of appropriate probe molecules – NH3 or SO2 were used to estimate the population, strength and strength distribution of acid or basic sites, by means of adsorption microcalorimetry. The nature of acidic sites was determined through the adsorption/desorption of pyridine, studied by infrared spectroscopy. New phases between the oxides were not formed, while there is evidence of interaction between them, as revealed by XRD and Raman spectroscopy. All investigated mixed oxide samples are amphoteric and possess red-ox centers on their surface. Both red-ox and acid–base properties are dependent on the ratio of ceria to niobia in the samples. Both Lewis and Brönsted acid sites are present on the surface of the mixed oxides. In order to check catalytic abilities of these materials, reaction of fructose dehydration has been performed. All the investigated materials are catalytically active in fructose dehydration; conversion of fructose and selectivity to 5-hydroxymethylfurfural (5-HMF) improved with increasing content of niobia in the samples.

Hydrothermally Synthesized HZSM-5/SAPO-34 Composite Zeolite Catalyst for Ethanol Conversion to Propylene

HZSM-5/SAPO-34 zeolite composites were prepared by hydrothermal synthesis (ZS-HS) and mechanical mixture (ZS-MM), respectively. ZS-HS showed higher propylene yield and better catalytic stability in ethanol conversion than ZS-MM as well as HZSM-5 and SAPO-34. In case of ZS-HS, SAPO-34 at least partially overlapped on HZSM-5, which resulted in the interfacial interaction of HZSM-5 and SAPO-34. Such interaction significantly modified the texture and the acidity of ZS-HS, which in turn affected its catalytic reactivity. ZS-HS showed the higher apparent formation rate of propylene, due to its moderate concentration and strength distribution of acid sites. Propylene, ethylene and butylenes might produce through the parallel reaction pathways and the common intermediate on ZS-HS. [Time course of ethanol conversion on ZS-HS and ZS-MM catalysts.]

Reactions of Cyclohexane on Platinum, Palladium, or Iridium-loaded H-ZSM-5 Zeolite Hydrohalogenated Catalysts

The catalytic hydroconversion of cyclohexane using catalysts containing H-ZSM-5 zeolite loaded with 0.35 wt% platinum, palladium, or iridium was studied in a pulse-type microreactor/GC system at atmospheric pressure. These catalysts were also either doped with 3.0% of HCl or HF. The activities of these catalysts and the distribution of the products formed were found to depend on the dispersion of the metallic component as well as on the acidity, acid sites number, and strength in the catalysts. TPD of ammonia and hydrogen chemisorption were applied to evaluate acid site strength distribution and metals dispersion, respectively, in the catalysts. In case of the Pt- and Pd-containing catalysts, hydrochlorination enhanced the isomerization and dehydrogenation activities and selectivities of cyclohexane, but decreased its hydrocracking activity. However, catalyst hydrofluorination resulted in the reverse effects. Nevertheless, for the Ir-loaded catalyst, both hydrohalogenation treatments decreased the isomerization and dehydrogenation of cyclohexane. The Ir/H-ZSM-5 catalyst exhibited higher hydrogenolysis activities than did those acquired by the Pt- and Pd-containing catalysts.

Para-xylene maximization: Part IX—Activation of toluene methylation catalysts with palladium

The importance of increasing para-xylene production as a primary material for producing polyester fibers has activated this research. Although alkylation is an acid catalyzed reaction, we found that Pd can act as a promoter to activate this reaction by virtue of the presence of a vacant d-orbital in the Pd atom. In this work, a series of catalysts containing 0.1, 0.2 or 0.3%Pd in H-ZSM-5 zeolite is examined for alkylating toluene with methanol, aiming to maximize para-xylene at temperature range 300–500 °C in hydrogen using a continuous-flow fixed-bed reactor. The acid site strength distribution of the catalysts was characterized by temperature programmed desorption (TPD) of ammonia, and palladium dispersion in the catalysts by hydrogen chemisorption. The diffusion extent, in the current catalysts during the current reaction was compared via estimation of the Thiele modulus (Φ L). The para-xylene selectivity was found to increase systematically with increasing the Pd content in the catalysts, although, para-xylene production was the highest on the 0.2%Pd/H-ZSM-5 catalyst. The heavy undesired trimethylbenzenes were the lowest to be formed on the 0.3%Pd containing catalyst.

Mild ring-opening coupling of liquid-phase cyclohexane to diesel components using sulfated metal oxides

We have investigated a mild simple synthesis method for ring-opening coupling of liquid-phase cyclohexane to diesel components using various sulfated metal oxides [SO 4 2−/Fe 2O 3 (SF), SO 4 2−/TiO 2 (ST) and SO 4 2−/ZrO 2 (SZ)] under low temperature (333 K) and atmospheric pressure. Neither solvent nor promoters are needed in the reaction system so as to be a clean approach. Operating under these reaction conditions, a maximum activity of 6% was obtained with SF as catalyst, and a significantly high selectivity of 74.5% for nicer diesel components (n-C 14–C 18) was obtained simultaneously. Whereas, ST and SZ displayed low activity for cyclohexane reaction. By utilization of the temperature-programmed desorption of ammonia (NH 3-TPD) measurement and the N 2 adsorption method, the results suggested that a satisfied acid strength distribution and high density of acid sites appeared in SF catalyst in comparison with other catalysts, which may play an important role in the reaction.

Ionone synthesis by cyclization of pseudoionone on silica-supported heteropolyacid catalysts

The liquid-phase cyclization of pseudoionone to ionones (α, β and γ isomers) was studied on silica-supported heteropolyacid (HPA) catalysts containing between 18.8 and 58.5% HPA. The HPA used was tungstophosphoric acid (H 3PW 12O 40). The catalyst surface and structural properties were thoroughly characterized by several techniques. The density, chemical nature and strength distribution of the surface acid sites were determined by adsorbing and monitoring by temperature-programmed desorption and infrared spectroscopy probe molecules such as NH 3 and pyridine. The pseudoionone conversion to ionones increased linearly with the Brønsted acid site density until the HPA loading approached to the monolayer saturation coverage. For higher HPA contents, the pseudoionone adsorption and conversion were hampered because of spatial constraints that diminished the reactant accessibility to the proton active sites. The highest ionone yield, 79%, was obtained on a 58.5 wt% HPA/SiO 2 catalyst at 383 K and is comparable to the best values reported in literature for the homogeneously catalyzed reaction using sulfuric acid. The ionone isomer distribution was modified by varying both the temperature and the reaction time. A reaction mechanism was postulated in which ionone isomers (α, β and γ) are primary products, but γ-ionone is isomerized to α-ionone while β-ionone is not converted in the other isomers.

A FTIR and TPD examination of the distributive properties of acid sites on ZSM-5 zeolite with pyridine as a probe molecule

The amount, type, origin and strength distribution of acid sites on ZSM-5 zeolite were analyzed with pyridine as a probe molecule, and with the aid of FTIR and interrupted-temperature programmed desorption techniques. Desorption activation energy (DAE) was used as a measure of the acid strength. The acid sites were divided into four sets each with a mean strength of DAE 70, 90, 120 and 150 kJ mol −1. Brönsted (B) sites were found dominant in the set with DAE 150 kJ mol −1, while Lewis (L) sites prevail for the DAE 120 kJ mol −1 set. For the other two sets both B and L sites are important. The B sites with DAE 70 kJ mol −1 were attributed to terminal silanol groups, and the B sites with DAE 90 kJ mol −1 were due to bridged hydroxyl and extraframework AlOH groups.

Effect of hydrochlorination and hydrofluorination of Pt/H-ZSM-5 and Pt–Ir/H-ZSM-5 catalysts for n-hexane hydroconversion

Abstract Pt/H-ZSM-5 and Pt–Ir/H-ZSM-5 catalysts were hydrochlorinated or hydrofluorinated with 3.0 wt%HCl or HF, respectively. These mono- and bimetallic catalysts were tested for n -hexane hydroconversion in a pulsed microcatalytic reactor in a flow of H 2 gas. The catalysts were characterized via XRD, metal dispersion via H 2 chemisorption and acid site strength distribution using temperature programmed desorption of ammonia (TPD). Although metallic promoters frequently cause inhibition of the catalytic activities of Pt and since iridium is less active than platinum, fortunately, Ir was found to enhance the hydroconversion activities of the current catalysts, particularly after hydrochlorination.

Effect of Hydrohalogenation of Metal/Zeolite Catalysts for Cyclohexene Hydroconversion ‐Part 3‐ Pd/H‐ZSM‐5 Catalysts

AbstractCyclohexene (CHE) hydroconversion was performed in a flow reactor at atmospheric pressure and temperatures of 50–400 °C using: Pd/H‐ZSM‐5, Pd/H‐ZSM‐5(HCl), and Pd/H‐ZSM‐5(HF) catalysts. These catalysts were characterized for acid site strength distribution via NH3 TPD, Pd dispersion via H2 chemisorption, TPR via reduction of the metal oxide in the catalysts and XRD for tracing crystallinity The hydroconversion steps proceeded as follows: CHE → Cyclohexane (CHA); CHE → Methylcyclopentenes (MCPEs) → Methylcyclopentane (MCPA); CHE → Cyclohexadienes (CHDEs) → Benzene → Alkylbenzenes; CHE and others → Hydrocrackedproducts.The overall hydroconversion of CHE was achieved in the catalyst order: Pd/H‐ZSM‐5 > Pd/H‐ZSM‐5(HF) > Pd/H‐ZSM‐5(HCl). CHE hydrogenation step was the major reaction at low temperatures which significantly inhibited via HCl treatment, but slightly enhanced via HF treatment. At medium temperatures, on all catalysts, isomerisation to MCPEs and MCPA increase to a maximum then a decline with a further increase of temperature. The overall isomerisation of CHE was highest on the untreated catalyst. During the higher temperature range, dehydrogenation, alkylation and hydrocracking were increased with temperature. Dehydrogenation of CHE always yielded larger amounts of 1,3‐CHDE than 1,4‐CHDE. These cyclohexadienes were produced in the catalyst order: Pd/H‐ZSM‐5(HF) > Pd/H‐ZSM‐5(HCl) > Pd/H‐ZSM‐5. In general, benzene alkylation to toluene exceeded that of xylenes, indicating that the second methylation is more difficult than the first. However, the catalytic activities for benzene and toluene production were in the order: Pd/H‐ZSM‐5 » Pd/H‐ZSM‐5(HCl) > Pd/H‐ZSM‐5(HF), whereas for xylenes production, Pd/H‐ZSM‐5 » Pd/H‐ZSM‐5(HF) > Pd/H‐ZSM‐5(HCl). Intrapore diffusion plays an important role during the dehydrogenation reactions as well as during the interconversion of individual aromatic hydrocarbons.

Effect of steam treatment on the activities of Pt/NH 4-MOR catalysts for n-pentane hydroisomerization and hydrocracking

The rationale of this work is to study the effect of using Pt containing catalysts supported on NH 4-MOR zeolite, before or after steaming, to show how steam treatments affect the activities of catalysts used primarily for the hydroconversion of light naphtha to improve its octane rating. Hence, the reactions of the hydroisomerization and hydrocracking of n-pentane, were investigated at temperatures between 250 and 500 °C in a pulsed microcatalytic reactor using catalysts containing 0.25, 0.35 and 0.60 wt.% Pt supported on NH 4-MOR. The effect of steaming on the dispersion of Pt, acid sites strength distribution, intraparticle diffusion resistance and the catalytic activities were investigated. Steaming was found to enhance catalytic activity deterioration. This effect was significantly increased with increasing Pt content in the catalysts. The calculated activation energy and entropy values using the current catalysts were correlated with the activity levels for hydroisomerization and hydrocracking of n-pentane.

Surface acidic properties of supported binary oxides containing CuO coupled with Ga2O3 and SnO2 studied by complementary techniques

Silica–alumina (SA) was used as support for dispersing CuO, Ga2O3, SnO2 oxide phases and the corresponding binary couplings (CuGa/SA and CuSn/SA samples). The acidity of uncovered SA and of SA-supported oxides was studied from a qualitative (nature of acid sites) and quantitative (number, acid strength, and strength distribution of acid sites) points of view by an integrated approach. The adsorption and desorption of basic probes (ammonia and pyridine) onto the acidic surfaces were monitored by the coupled volumetric–calorimetric technique, and by XPS and FT-IR spectroscopy. The results show that the Brönsted acidity of the SA support was converted into predominantly Lewis acidity upon metal oxide deposition. The high dispersion of the copper oxide phase was responsible for this phenomenon. Differences of acid strength distribution were noticed among the samples. Moderate acidity was associated with the Cu sites (100 kJ mol−1 < qdiff < 150 kJ mol−1), whereas the most acidic fraction of the sites (qdiff > 150 kJ mol−1) increased with the presence of Ga and Sn. The experimental results are discussed in the light of the intrinsic acidity of the Cu, Ga, and Sn metal ions derived from electronic parameters.

New method for the study of surface acidity of zeolites by NH3-TPD, using a pH-meter equipped with an ion selective electrode

Desorbed ammonia is captured by an acetic acid solution whose concentration of ammonium ions formed by neutralization, is continuously recorded by a pH-meter equipped with an ion-selective electrode (ISE). This technique allows the study of both the acidity density and the distribution of acid site strengths. Such “strength profile” was obtained by plotting the relative rate of desorbed NH3 versus the temperature. Results obtained can be fairly well correlated with those already published. It is shown that by investigating several H-ZSM-5 zeolites with different SiO2/Al2O3 ratios, the data of acid density obtained by the ISE method are very close to those calculated from the Al content. The slight shift of the desorption peaks towards higher temperature suggests a slightly higher strength for more isolated acid sites. Zeolite structures, which are richer in Al, contain weaker acid sites, as evidenced by the H-Y when compared to the H-mordenite, while the acid sites density of the former is much higher. The ultra-stabilized Y zeolite (H-USY) shows a lower acidity density than the parent H-Y owing to the removal of some of the tetrahedral Al atoms from the framework during the preparation. The strength profile of the H-USY is also quite different from that of the H-Y. The new technique appears to be much simpler, more rapid, more informative and quite accurate when compared to the technique using a gas chromatograph equipped with a thermal conductivity detector (GC-TCD).

Iridium/H-ZSM-5 zeolite catalyst promoted via hydrochlorination or hydrofluorination for the hydroconversion of cyclohexene

The hydroconversion of cyclohexene (CHE) was carried out in a flow reactor at atmospheric pressure and temperatures of 50–400 °C using: Ir/H-ZSM-5, Ir/H-ZSM-5(HCl), and Ir/H-ZSM-5(HF) catalysts. The acid site strength distribution, Ir dispersion, XRD, SEM photography for these catalysts were evaluated. The hydroconversion reaction steps of CHE were found to proceed as follows: CHE → cyclohexane; CHE → methylcyclopentenes (MCPEs) → methylcyclopentane (MCPA); CHE → cyclohexadienes (CHDEs) → benzene; benzene → alkylbenzenes; CHE and others → hydrocracked products. The doping of Ir/H-ZSM-5 catalyst with HCl enhanced its acid sites number and strength but reduced the Ir metal dispersion, whereas doping with HF enhanced acid sites number and strength as well as Ir metal dispersion. The hydrogenation step of CHE to CHA was significantly inhibited via HCl doping of Ir/H-ZSM-5 because of agglomeration of Ir crystallites, but significantly enhanced via HF doping due to improving Ir dispersion and increasing acid sites number and strength of the catalyst. Also, MCPEs hydrogenation and CHE dehydrogenation were highest on the untreated Ir/H-ZSM-5 catalyst because it doesn’t suffer diffusion limitation caused by deposited halo-aluminium debris. CHE isomerisation does not critically depend on acid site parameters or on Ir dispersion in the catalysts. Hydrocracking reactions were more active on the HCl treated catalyst.

Hydroconversion of cyclohexene using H-ZSM-5 zeolite catalysts promoted via hydrochlorination and/or platinum incorporation

Cyclohexene (CHE) was hydroconverted in a flow reactor at atmospheric pressure and temperatures of 50–400 °C using the catalysts: H-ZSM-5, H-ZSM-5(HCl), Pt/H-ZSM-5 or Pt/H-ZSM-5(HCl). The acid sites strength distribution, Pt dispersion, XRD, SEM and TEM photography for these catalysts were evaluated. The reaction steps of CHE hydroconversion were found to be as follows: CHE ↔ cyclohexane; CHE ↔ methylcyclopentenes (MCPEs) ↔ methylcyclopentane (MCPA); CHE ↔ cyclohexadienes (CHDEs) ↔ benzene; benzene ↔ alkylbenzenes; CHE and others ↔ hydrocracked products. Doping of 3.0% HCl in H-ZSM-5 zeolite enhanced acid sites number and strength, whereas its doping in Pt/H-ZSM-5 enhanced acid sites number as well as Pt dispersion in the catalyst but slightly decreased the acid sites strength. The overall hydroconversion of CHE and its hydrogenation step to CHA were enhanced via incorporating Pt and/or doping with HCl. Also, CHE dehydrogenation was enhanced on the Pt/H-ZSM-5(HCl) catalyst, but was affected by diffusion limitation caused by deposited chloro-aluminium debris. The latter catalyst also suffered from diffusion effects during the acid catalysed alkylation and hydrocracking reactions. Indeed, HCl treatment, in the presence of Pt, significantly enhanced the debris lay-down and hence diffusion restriction. However, the isomerisation of CHE does not seem to depend on acid sites parameters or on Pt dispersion in the current catalysts. CHE dehydrogenation to benzene passes through the formation and removal of CHDEs. The magnitude of 1,3-CHDE in product using all catalysts exceeded that of 1,4-CHDE. Benzene formation was enhanced via HCl and/or Pt incorporation in the catalysts.