Acidity and catalytic activity of mesoporous ZSM-5 in comparison with zeolite ZSM-5, Al-MCM-41 and silica–alumina

Acidity of mesoporous HZSM-5 prepared using amphiphilic organosilane template molecules was measured. Brønsted acid sites were observed in the prepared sample, and the number and the strength of Brønsted acid sites were determined quantitatively by a method of infrared-mass spectroscopy/temperature-programmed desorption (IRMS-TPD) of ammonia. ΔH for ammonia adsorption as an index of the strength was ca. 150 kJ mol⁻¹ that was almost the same as on usual HZSM-5, but the number was smaller than that of HZSM-5. From the measured acidity, it was concluded that the mesoporous materials contained a smaller concentration of Brønsted acid site notable on the structure of HZSM-5. Measurements of turnover frequency (TOF) in the catalytic cracking of octane supported the conclusion. Density functional calculations showed that the defect sites Al–OH and Si–OH attached to the Brønsted acid site changed the strength of the acid sites to show some possible structures of the weak and strong Brønsted acid sites included in the mesoporous HZSM-5. Acidities of Al-MCM-41 and silica–alumina were also measured, and the difference in the solid acidities of these materials was discussed.     

The role of Brønsted acidity in the SCR of NO over Fe-MFI catalysts

The selective catalytic reduction (SCR) of NO with isobutane and with NH₃ was studied over Fe-MFI catalysts which differ strongly in Brønsted acidity but are similar in Fe content and structure of Fe sites, having shown similar activity in N₂O decomposition in related work. The catalysts were prepared by exchange of Na-ZSM-5 (Si/Al ca. 14) with Fe²⁺ ions formed in situ by acidic dissolution of Fe powder and by steam extraction of framework iron from Fe-silicalite or from H-[Fe]-ZSM-5 (Si/Al ca. 30). The characterization of acidic properties by ammonia TPD and by IR of adsorbed pyridine at different temperatures revealed marked differences in acidity between exchanged and steam-activated samples, the latter being (almost) void of strong Brønsted sites. The structural similarity of the iron sites was confirmed by UV–Vis and EPR spectroscopic results. The weakly acidic samples were inferior both in isobutane-SCR and in ammonia-SCR. With isobutane, dramatic differences over the whole range of parameters studied imply a vital role of Brønsted acidity in the reaction mechanism (e.g. in isobutane activation). In NH₃-SCR, large reaction rates were achieved with non-acidic catalysts as well, but a promoting effect of acidity was noted for catalysts that contain the iron in the most favorable site structure (oligomeric Fe oxo clusters). This suggests that an acid-catalyzed step (e.g. the decomposition of NH₄NO₂) may be rate-limiting at low temperatures.     

Enhanced Stability of HZSM-5 Supported Ga2O3 Catalyst in Propane Dehydrogenation by Dealumination

Gallium oxide catalysts supported on HZSM-5 with different Si/Al ratios were characterized by pyridine adsorption FT-IR, model reactions and XPS studies. As the Si/Al ratio of the support HZSM-5 zeolite rises, the acidity of the supported catalysts decreases accordingly, which comes from two aspects: the loss of acid sites present on HZSM-5 support and the loss of the acid sites present on gallium oxides. The latter was caused by the change in the interaction between Ga₂O₃ and support. The initial activity in the propane dehydrogenation decreases with increasing Si/Al ratio while the stability increases. The enhanced stability is thought to be caused by the decrease of the acidity of the catalysts, resulting in the suppression of the side reactions, such as cracking and oligomerization.     

A novel method for enhancing on-stream stability of fluid catalytic cracking (FCC) gasoline hydro-upgrading catalyst: Post-treatment of HZSM-5 zeolite by combined steaming and citric acid leaching

This article describes a novel modification method consisting of steaming and subsequent citric acid leaching to finely tune acidity and pore structure of HZSM-5 zeolite and thereby to enhance the on-stream stability of the zeolite derived fluid catalytic cracking (FCC) gasoline hydro-upgrading catalyst. A series of dealuminated HZSM-5 zeolites and their derived catalysts were prepared and characterized by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), ²⁷Al MAS NMR, nitrogen adsorption, temperature programmed desorption of ammonium (NH₃-TPD) and infrared (IR) spectroscopy of chemisorbed pyridine. The results showed that the citric acid leaching could preferentially remove the extra-framework Al (EFAl) species formed by steaming treatment and thus reopen the EFAl-blocked pore channels of the steamed zeolite. The steaming treatment at a suitable temperature and subsequent citric acid leaching not only decreased the strength of acid sites to a desirable degree but also increased the ratio of medium and strong Lewis acidity to medium and strong Brönsted acidity, both of which conferred the resulting catalyst with superior selectivity to aromatics, good hydroisomerization activity and gasoline research octane number (RON) preservability, as well as enhanced on-stream stability. The results fully demonstrated that the treatments by steaming and followed citric acid leaching can serve as an important method for adjusting the physicochemical properties of HZSM-5 zeolite.     

Influence of zinc state on the catalyst properties of Zn/HZSM-5 zeolite in 1-hexene aromatization and cyclohexane dehydrogenation

Rational design of Zn-containing HZSM-5 zeolite (Zn/HZSM-5) with high reactivity and excellent aromatization performance for olefin aromatization is crucially desired. We develop a new and uncomplicated method to synthesize Zn/HZSM-5 (IMX/Z5) with superior aromatization performance in the paper. Compared to incipient wetness impregnation (IMP/Z5) and mechanical mixing (MIX/Z5), the as-prepared IMX/Z5 presents a higher amount of surface ZnOH⁺ species (2.87%) while keeping identical bulk zinc content. As a result, more surface ZnOH⁺ favor both the aromatization of 1-hexene and cyclohexane dehydrogenation. For the two olefin aromatization pathways (hydrogen transfer and dehydrogenation), it is the first time found both the hydrogen transfer ability and the dehydrogenation ability increase linearly with the amount of surface ZnOH⁺ species while keeping identical bulk zinc content. We believe that the linear relationships are essential to design next generation olefin aromatization catalysts.     

Structure and Brønsted acid sites of ferrierite

The structure of natural ferrierite (from Kamloops Lake) is determined. The space group is not Immm but I222 (no. 23) and its R-factor is 8.6%. All 18 H₂O molecules, Na⁺, and Mg²⁺ ions are successfully assigned [Mg_1.6(Ca_0.1Ba_0.1Na_1.6K_0.4)Al_5.8Si_30.2O₇₂·18H₂O]. The angle of T(41)–O(5)–T(42) is 168° because the O5 site is not located on an inversion center in I222 space group. We also determined Brønsted acid sites. The Al atom distribution in the framework is also discussed and the T(1), T(2), and T(42) [or T(41)]-sites are the most probable sites.     

Aromatization of propane over Zn/HZSM-5 catalysts prepared by chemical vapor deposition

Zinc was introduced into HZSM-5 by chemical vapor deposition (CVD) to investigate the catalytic properties of Zn/HZSM-5 for propane conversion. Irrespective of catalytic precursor or ion exchange method, protons were found to be replaced by Zn ions. However, Zn species in the zeolite were different depending on the precursors used. Although the addition of Zn decreased the intensity of the BrÕnsted acidity, it did not caused the apparent formation of Lewis acid sites unlike Ga/HZSM-5 catalysts. On Zn/HZSM-5, both Zn and protons intervene in propane activation step.     

Effect of superfine particles of alumina incorporated in the cavities of mordenite in enhancing selectivity

Alumina has been incorporated in the cavities of mordenite by the hydrolysis of Al³⁺ ions in the cavities with moist ammonia gas, following calcination at 450°C. The incorporation of the superfine particles of alumina into the cavities of the mordenite is assessed on the basis of the data of MAS ²⁷Al NMR measurements, X-ray powder diffraction and the amount of saturated adsorption of nitrogen for FAL/MZ. The acid properties, assessed using NH₃ TPD and FTIR studies, show the presence of Brønsted and Lewis acid sites on the catalyst, and an increase in acidic sites on incorporating alumina. The dehydrogenation selectivities of FAL/MZ, Al³⁺/MZ, H⁺/MZ, and Na⁺/MZ have been investigated. Brønsted acid sites largely enhance the ring-cleavage, whereas this incorporation of superfine particles of alumina significantly suppresses the ring-cleavage. Therefore, the incorporation of the superfine particles increases remarkably in the cyclohexene selectivity.     

Effect of reductive and oxidative atmospheres on the propane aromatisation activity and selectivity of Ga/H-ZSM-5 catalysts

The aromatisation of propane was investigated at 823 K on two Ga/H-ZSM-5 catalysts differing by their aluminum content (Si/Al = 18 and 128, respectively), i.e. their Brönsted acid site concentration, and having similar gallium content (1.6 wt%) as a function of successive pretreatments by hydrogen and oxygen. Hydrogen treatment increases activity and selectivity to aromatics while decreasing the formation of methane. Subsequent oxygen treatment enhances further the activity and aromatics selectivity and has little effect on methane selectivity. These improvements of catalytic performance are explained by gallium migration in reducing conditions. The concentration of highly dispersed gallium species is thereby enhanced. These species, in association with Bronsted centers, were reported previously to constitute the dual catalytic sites responsible for alkane activation. Hydrogen-oxygen pretreatment of Ga/H-ZSM-5 catalysts is thus a means to ensure rapid catalyst preactivation and stabilisation in large scale light alkane aromatisation units.     

Isobutane alkylation with C4 olefins on a sulfonic solid acid catalyst system based on laminar clays

Natural clays were modified by means of the controlled attack of strong Brönsted acids in aqueous solution. The variations of the surface acidity of the solids, followed by IR of adsorbed pyridine and hammett indicators, indicated a population of Brönsted type sites above 800 μ mol/g, while the surface area increased from 23 up to 105 m²/g. A series of solids (ATZ-L, ATZ-A and ATZ-G) was activated using trifluoromethanesulfonic acid (CF₃SO₃H), then evaluated at the bench scale in a fixed bed reactor, both in liquid and gas phase, using an iC₄/C₄⁼ molar ratio, which led to 5.5% trimethylpentanes (TMP) against 35.9% of C₈⁼ and 53.4% of heavier C₉⁺ products for the catalysts (ATZ-L) having higher acid strength, i.e. Ho≈−9.3. For the solids having a moderate surface acid strength (Ho≈−4.4) the activation rate after 12 h runs was about 70%, but the solids having a higher acid strength, i.e. Ho=−5.6 and −9.3, showed a deactivation rate almost null after 24 h runs.     

Highly effective P-modified HZSM-5 catalyst for the cracking of C4 alkanes to produce light olefins

A series of HZSM-5 zeolites modified by different amounts of phosphorus (P/HZSM-5) were prepared. The physicochemical features of P/HZSM-5 catalysts were characterized by means of XRD, BET, NH₃-TPD, FT-IR spectra of adsorbed pyridine, etc., and their performances for the catalytic cracking of the mixed C₄ alkanes to produce light olefins were investigated. The results indicated that phosphorus (P) modification not only modulated the amount of acidic sites and the percentage of weak acidic sites in total acidic sites, but also regulated the acid type, i.e., the ratio of L/B (Lewis acid/Brönsted acid). The introduction of P also altered the basic characteristics of HZSM-5 which was evidenced by CO₂-TPD analysis. Consequently, P modification with suitable amount was favorable for enhancing the selectivity to light olefins, especially to propene. At the temperature of 650 °C, the maximum yields of propene and ethene were achieved 25.6 and 33.9%, which were higher than those over parent HZSM-5 by 7 and 4.5%, respectively. Aromatics yield was found to be decreased with the increasing P loading due to the reduction of strong acid and the formation of new basic site which inhibited the hydrogen transfer reaction. All this indicates that P-modified HZSM-5 zeolites are effective catalysts for the cracking of mixed C₄ alkanes to produce more light olefins.     

催化剂结构与焦油模型化合物裂解行为的关联性

以Ni（NO₃）₂·6H₂O为镍源,分别以HZSM-5（Si/Al=25）、HZSM-5（Si/Al=50）、HZSM-5（Si/Al=200）、USY、Al₂O₃为载体制备了5种镍基催化剂,采用XRD、H₂-TPR、BET、NH₃-TPD等方法对其进行了表征,在固定床反应器中考察了上述催化剂对煤焦油模型化合物甲苯+芘的催化裂解性能。结果表明,相比惰性载体石英砂,在催化剂作用下,液相产物中轻质芳烃种类明显增多。3种Ni/HZSM-5催化剂的比表面积和平均孔径较为接近,然而酸性中心数量最多的Ni/HZSM-5（Si/Al=25）,对甲苯+芘的裂解能力最强,体现为液体产物收率最低（仅为15.95%）,气产率和析碳率均最高,分别达16.73%和67.32%。在酸性中心数量相近的Ni/HZSM-5（Si/Al=200）与Ni/Al₂O₃作用下,液体产物收率差别较小,但后者的芘裂解率比前者高41.47%,主要是由于后者的平均孔径比前者高1.64倍,说明平均孔径大有利于重质组分芘的裂解。综合考虑液体产物收率、气产率、析碳率和芘的裂解率,Ni/Al₂O₃更适合甲苯+芘裂解反应体系。     

固体酸表面B酸和L酸与果糖转化制乳酸甲酯产物分布

制备了γ-Al₂O₃、HZSM-5、SnOPO₄、SnZrOPO₄(1:1)、SO₄^2-/ZrO₂5种不同的固体酸催化剂,采用NH₃程序升温脱附、吡啶原位吸附红外对催化剂进行了表征。考察了固体酸催化果糖在甲醇中转化的催化性能,结果表明,果糖的转化率均高于98%,产物分布与固体酸表面L酸、B酸酸量具有显著的相关性,乳酸甲酯的收率随着L酸量的减少而降低,L酸催化剂γ-Al₂O₃催化,主产物只有乳酸甲酯,收率为24.4%。而L酸位和B酸位共存的固体酸,产物中有乳酸甲酯、乙酰丙酸甲酯,并且乙酰丙酸甲酯的收率随着B酸量的增多而升高。最后考察了典型L酸γ-Al₂O₃及B酸L酸共存的固体酸HZSM-5不同反应时间的产物分布,结合气相-质谱联用对产物定性分析,得出了果糖转化过程L酸位催化和B酸位催化的反应路径。     

The effect of Ga and Zn over HZSM-5 on the transformation of palm fatty acid distillate (PFAD) to aromatics

The catalytic conversion of PFAD (palm fatty acid distillate) to aromatics has been studied over HZSM-5, Ga/HZSM-5, and Zn/HZSM-5 catalysts. The presence of both Ga and Zn promoted the aromatization of PFAD. The higher aromatics yield of Zn/HZSM-5 was achieved by the presence of two zinc species; exchanged Zn^2 + promoting the dehydrogenation of paraffins and ZnO promoting the decarboxylation of oxygenates. The shifting from decarbonylation over the Brønsted acid site of the parent HZSM-5 to decarboxylation over ZnO preserved the Brønsted acid site for aromatization, thus increasing the aromatics yield.     

Pyrolysis of an LDPE-LLDPE-EVA copolymer mixture over various mesoporous catalysts

The aim of the present work was to study the performance of mesoporous catalysts in the catalytic cracking of an LDPE+LLDPE+EVA copolymer. Mesoporous catalysts, including MCM-41, Nano-MCM-41, Al-Nano-MCM-41, MMZ-ZSM-5 and Meso-MFI, were applied for this reaction. Also, microporous HZSM-5 was used for a comparison. All of the catalysts showed higher decomposition abilities than thermal decomposition. The catalytic conversion of the LDPE+LLDPE+EVA copolymer was highest with the use of Meso-MFI due to its pore size and strong Br?nsted acidity, with high selectivity for lower olefin and gasoline range hydrocarbon. Both MMZ-ZSM-5 and Al-Nano-MCM-41 have an acid site that induced the decomposition reactions, and thus, produced compounds with lower carbon numbers in liquid products. MCM-41, which exhibits no acidity, showed a similar distribution of liquid products to that via thermal cracking, while Nano-MCM-41 showed better catalytic cracking ability due to its high surface area.     

H MAS studies of acid sites in ZSM-5 type zeolites

Proton magic-angle-spinning nuclear magnetic resonance (¹H MAS NMR) spectroscopy may yield quantitative information about Brønsted acid sites, the extent of structural defects and non-framework species in the intra-zeolitic channels. We show that the extremely high concentration of silanol groups in TPA/ZSM-5 zeolites resonating with a chemical shift of 1.4 – 2.2 ppm should be assigned to a reaction of TPA⁺ ions with the framework during the synthesis and that the dealumination of the ZSM-5 framework leads to the appearance of non-framework AlOH species at 2.6 – 3.6 ppm and strong Lewis centres. The latter give rise to a signal at 6.5 ppm observed in activated samples upon partial rehydration.     

Aromatization of propane over GaHMFI catalysts. Reaction scheme, nature of the dehydrogenating species and mode of coke formation

The kinetic modelling of propane aromatization over a Ga/HMFI catalyst and the comparison of Ga₂O₃-HMFI mixtures with different compositions has allowed us to specify the respective roles of dehydrogenating species and of protonic sites. Propane aromatization over Ga/HMFI catalyst occurs mainly through a conventional bifunctional reaction scheme (with desorbed intermediates) in which the dehydrogenating gallium species are responsible for dehydrogenation of propane, of oligomers and of cyclic olefins and diolefins while protonic sites are responsible for oligomerization, cyclization and cracking. This bifunctional transformation is kinetically limited by propane dehydrogenation. Unfortunately, gallium species also catalyze alkane cracking and hydrogenolysis as well as alkene hydrogenation with formation of the undesired products methane and ethane. From the comparison of the catalytic properties of MFI gallosilicates calcined under air flow at various temperatures, it can be concluded that tetrahedral gallium is inactive in dehydrogenation reactions. Dehydrogenation occurs through a bifunctional mechanism involving the joint participation of gallium oxide (extra framework species) and of protonic sites. Coke formed during propane aromatization causes a blockage of the access to the protonic sites of the zeolite which is responsible for deactivation. Ga decreases the coking/aromatization rate ratio of HMFI and facilitates the coke removal through oxidative treatment, both effects being more pronounced after Ga/HMFI pretreatment at high temperature under hydrogen because of the better dispersion of the Ga species.     

Production of aromatic hydrocarbons by co-cracking of bio-oil and ethanol over Ga2O3/HZSM-5 catalysts

The catalytic cracking of bio-oil is important to produce aromatic hydrocarbons, which can partially replace gasoline or diesel to greatly reduce carbon emissions from transportation. To further promote the formation of aromatic hydrocarbons, this work studied the effects of the preparation method and the acid strength of Ga₂O₃/HZSM-5 on catalytic cracking of the bio-oil distilled fraction systematically. The preparation method of Ga₂O₃/HZSM-5 had an important effect on its catalytic activity: the Ga₂O₃/HZSM-5 prepared by physical mixing showed the low dispersion of active phases and poor pore structure, resulting in its insufficient activity and severe coke deposition; the Ga₂O₃/HZSM-5 prepared by precipitation exhibited the higher activity, while many polycyclic aromatic hydrocarbons unfavorable for the subsequent utilization were in the oil phase; the Ga₂O₃/HZSM-5 prepared by impregnation showed the highest activity and 35.5% (mass) selectivity of the oil phase, including 80.3% monocyclic aromatic hydrocarbons and 12.0% polycyclic aromatic hydrocarbons. The Brønsted acidity of Ga₂O₃/HZSM-5 decreased with Si/Al ratio, leading to the decline in reactant conversion, oil phase selectivity and quality. Meanwhile, the polymerization between monocyclic aromatic hydrocarbons and oxygenates was promoted to produce many polycyclic aromatic hydrocarbons and even coke, causing catalyst deactivation.     

Chemical deactivation of V2O5/WO3–TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils and urea solution: Part II. Characterization study of the effect of alkali and alkaline earth metals

A characterization study on a practice-oriented V₂O₅/WO₃–TiO₂ SCR catalyst deactivated by Ca and K, respectively, was carried out using NH₃-TPD, DRIFT spectroscopy, and XPS as well as theoretical DFT calculations. It was found from NH₃-TPD experiments that strongly basic elements like K or Ca drastically affect the acidity of the catalysts. Detailed DRIFT spectroscopy experiments revealed that these poisoning agents mostly interact with the Brønsted acid sites of the V₂O₅ active phase, thus affecting the NH₃ adsorption. Moreover, these experiments also indicated that the V⁵⁺ = O sites are much less reactive on the poisoned catalysts. XPS investigations of the O 1s binding energies showed that the oxygen atoms of the V⁵⁺ = O sites are affected by the presence of the poisoning agents. Based on these results and on DFT calculations with model clusters of the vanadia surface, the poisoning mechanism is explained by the stabilization of the non atomic holes of the (0 1 0) V₂O₅ phase as a result of the deactivation element. Consequently, V–OH Brønsted acid sites and V⁵⁺ = O sites are inhibited, which are both of crucial importance in the SCR process. The deactivation model also gives an explanation to the very low concentrations of potassium needed to deactivate the SCR catalyst, since one metal atom sitting on such a non-atomic hole site deactivates up to four active vanadium centers.     

Modification of zsm-5 zeolite with trimethyl phosphite part 1. structure and acidity

In the present work we have studied the structure and the acidic properties of HZSM-5 zeolite modified with trimethyl phosphite. The modifications were carried out by using a chemical vapour deposition method as well as impregnation from n-octane solution. Both modification techniques resulted in materials with similar acidic properties. Trimethyl phosphite reacted with both Brönsted acid sites and terminal SiOH groups. The modifications resulted in the total elimination of strong Brönsted acid sites accompanied by the formation of new Brönsted type sites with decreased acid strength. However, the total acidity of the modified zeolites stayed relatively high. A possible structural transformation resulting from the phosphite modifications was proposed to be an insertion of phosphorus species between the structural aluminium- and silicon-oxygen tetrahedra. However, on the basis of ³¹P MAS NMR studies no evidence of the isomorphous substitution of structural T-atoms by phosphorus was found.