Ammonia TPD Research Articles

Characterization and reactivity of 11-molybdo-1-vanadophosphoric acid catalyst supported on zirconia for dehydration of glycerol to acrolein

A series of vanadium-substituted phosphomolybdic acid (HPA) catalysts supported on zirconia were prepared by impregnation method with varying the HPA active phase content from 10 to 50 wt% on the support. The calcined catalysts were characterized by X-ray diffraction, Raman spectroscopy, temperature-programmed desorption of NH3, FT-IR spectra of pyridine adsorption and surface area measurements. XRD results suggest that the active phase of heteropolyacid is present in a highly dispersed state at lower loadings and as a crystalline phase at higher HPA loadings and these findings are well-supported by the results of FT-IR and Raman spectra. Calcination of the samples did not affect the Keggin ion structure of HPA. The ammonia TPD results suggest that acidity of the catalysts was found to increase with increase of HPA loading up to 40 wt% and decreases at higher loadings. FT-IR spectra of pyridine adsorption show that the Brønsted acidic sites increase with increase of HPA loadings up to 40 wt% catalyst. However, Lewis acid sites decrease with increase of HPA loading. Catalytic properties were evaluated during vapour phase dehydration of glycerol to acrolein. The catalyst with 40 wt% HPA has exhibited excellent selectivity towards acrolein formation with complete conversion of glycerol at 225°C under atmospheric pressure. Catalytic performances during dehydration of glycerol are well-correlated with acidity of the catalysts. Zirconia-supported H4PMo11VO40 catalysts have been investigated in the vapour dehydration of glycerol to acrolein. The catalysts are highly active and selective during dehydration of glycerol under mild reaction conditions. The high selectivity of acrolein is attributed to the presence of Brønsted acidic sites in the catalysts.

Eco-friendly nitration of benzenes over zeolite-β-SBA-15 composite catalyst

Direct synthesis of microporous-mesoporous zeolite-β-SBA-15 (ZBS-15) composite catalyst from the synthetic precursors of SBA-15and zeolite-β seeds under acidic hydrothermal conditions through the simultaneous self-assembly of mesoporous silica SBA-15 and zeolite-β has been accomplished and characterized the ZBS-15 catalyst by XRD, N2 sorption, FT-IR, TPD of ammonia and SEM techniques. The activity of the ZBS-15 composite catalyst for the nitration of benzenes under solvent-free conditions has been investigated, which revealed that there is a significant synergistic influence of both zeolite-β and SBA-15 materials on the activity of the ZBS-15 catalyst.

Catalytic activity and selectivity of reusable α-MoO3nanobelts toward oxidation of olefins and sulfides using economical peroxides

The novel catalytic activity of α-MoO3 nanobelts prepared by a new and safe sol–gel method for the epoxidation of olefins and oxidation of sulfides to sulfoxides using H2O2 in ethanol as a safe solvent has been exploited. The reactions also proceeded efficiently in the presence of tert-butyl hydroperoxide (TBHP). Good/high yields and excellent selectivity resulted. The ammonia TPD profile demonstrated strong acidic sites in synthesized α-MoO3 nanobelts, which generated different catalytic activity than the bulk material. The separation and reuse of this heterogeneous nanocatalyst was simple, effective and economical in the presented oxidation methods.

Dehydration of fructose to 5-hydroxymethyl furfural over ordered AlSBA-15 catalysts

5-Hydroxymethyl furfural is an important platform chemical. It is successfully synthesized from renewable carbohydrates using mesoporous AlSBA-15 catalysts under biphasic conditions. Fine tuning of the catalyst acidity is important to drive the reaction to give good yields of furan compound. Al-SBA-15 catalysts with different Si/Al ratios were prepared and characterized by XRD, 27Al MASNMR, SEM, TEM, N2 sorption, ICP-OES and TPD of ammonia. Results show that part of aluminium is substituted into tetrahedral positions. The catalyst with lower acid site density but medium to strong acidstrength favours selective formation of HMF. Under the optimized conditions, HMF selectivity was as high as 88% at 59 mol% conversion of fructose. Leaching of part of aluminium occurs under hydrothermal conditions, if solvent mixtures containing water are used, while activity can be retained if DMSO is used as solvent.

Effect of Sn-MCM-41 hydrothermal synthesis conditions on nopol production

Several catalysts were prepared by hydrothermal synthesis of Sn-MCM-41 at room temperature using SnCl2 •2H2 O as precursor salt. Catalysts were characterized by TPD of ammonia, FTIR of adsorbed pyridine and TPR of H2 . The reactivity of the synthesized materials for the Prins condensation of β-pinene with paraformaldehyde to obtain nopol was explained in terms of the type of acidity. The most active catalysts showed medium strength acidity. Materials synthesized under pH control and by adding the metal source as a solid, exhibited medium strength acidity.

Dehydration of Glycerol Over Niobia-Supported Silicotungstic Acid Catalysts

Liquid-phase dehydration of glycerol to acrolein over nanosized niobia-supported silicotungstic acid catalysts was performed to investigate the effect of the silicotungstic acid loading on the catalytic performance of the catalysts. The catalysts were prepared by following an impregnation method with different HSiW loadings in the range of 10-50 wt%. The prepared catalysts were characterized by N2 physisorption, XRD, FT-IR, TPD of ammonia, and TGA. Dehydration of glycerol was conducted in an autoclave reactor under the conditions of controlled reaction temperatures under corresponding pressure. Increasing HSiW loading rapidly increased the acidity of HSiW/Nb205 catalyst and rate of glycerol conversion, but acrolein selectivity decreased due to enhanced deactivation of the catalyst by carbon deposit. Consequently, it was confirmed that catalytic activity for the dehydration of glycerol to acrolein was dependant on the acidity of catalyst and can be controlled by HSiW loading.

Tungstate promoted vanadium phosphate catalysts for the gas phase oxidation of methanol to formaldehyde

Formaldehyde is an important intermediate in the pharmaceutical, plastic and automobile industries which is produced commercially via methanol oxidation. A series of tungstate promoted vanadium phosphate catalysts were prepared and characterised using a combination of XRD, UV-vis spectrometry, XPS and ammonia TPD. The 25 wt% WO3–VPO catalyst exhibited good activity for the selective gas phase oxidation of methanol with >90% conversion at around 250 °C. This is a considerably lower temperature than used in the current industrial process using either silver or iron molybdate catalysts. This increased activity at low temperatures is ascribed to the increase in Brønsted acidity and redox properties of the tungstate doped materials.

Methanol dehydration to dimethylether over Al2O3 catalysts

The vapor-phase dehydration of methanol to dimethylether (DME) has been investigated over a set of eight commercial and self-prepared Al2O3 catalysts with different physicochemical characteristics. Materials were characterized with respect to their textural properties (B.E.T. and B.J.H. methods), acidity (TPD of ammonia), crystallinity, phase composition and morphology (XRD, TEM). Catalytic activity and selectivity have been evaluated in the temperature range of 150–400°C in the absence and in the presence of water in the feed. The adsorption/desorption properties of catalysts toward methanol and water as well as some aspects of the reaction mechanism have been investigated with the use of transient-MS and in situ DRIFTS techniques. Results show that catalytic activity per gram of catalyst varies by two orders of magnitude from one sample to another in a manner that depends on the physicochemical characteristics of Al2O3. All samples investigated are highly selective to DME at temperature lower than ca. 325°C, whereas small amounts of CO and CH4 are formed at higher reaction temperatures. As a general trend, activity per gram of catalyst increases with increase of SSA (amount of acidic surface sites) and is enhanced over samples characterized by high crystallinity and relatively high porosity. The presence of water vapor in the feed suppresses conversion of methanol at a given temperature due to competition for active sites and thermodynamic limitations. Methanol adsorbs on the Al2O3 surface both molecularly and in the form of methoxy species. Evidence is provided that formation of DME involves weakly adsorbed methoxy species whereas methoxy species that are bonded more strongly on the Al2O3 surface are converted to surface formates, which decompose to yield CO, H2 and CH4 in the gas-phase.

VOx/c-Al2O3 catalyst for oxidative dehydrogenation of ethane to ethylene: Desorption kinetics and catalytic activity

This study reports ethane oxidative dehydrogenation (ODH) using lattice oxygen. Ethane ODH is studied under an oxygen-free atmosphere employing a 10wt.% VOx supported on c-Al2O3. TPR and TPO show that the prepared 10wt.% VOx supported on c-alumina catalyst is a stable catalyst over repeated reduction and oxidation cycles. XRD shows the absence of V2O5 bulk surface species and a high dispersion of VOx on the support surface. Experiments are carried out in the CREC Fluidized Bed Riser Simulator at 550–600°C and pressures close to atmospheric conditions. Reactivity tests show that the prepared ODH catalyst displays 6.5–27.6% ethane conversion and 57.6–84.5% ethylene selectivity in the 550–600°C range. Metal–support interaction is assessed using ammonia TPD. This provides the desorption energy for both the bare c-Al2O3 support and for the VOx/c-Al2O3 catalyst. A slightly increased desorption energy is found when using the V-loaded catalyst. This shows low metal–support interactions and as a result, a well dispersed VOx catalyst phase with high availability of lattice oxygen for ODH. These findings are confirmed with XRD, showing no changes with respect to the XRD for the c-Al2O3 alumina support. This proves that there are no other species formed due to the interaction between the VOx surface species and the Al2O3 support.

Liquid-phase alkylation of benzene with octenes over Y zeolites: Kinetic modeling including acidity descriptors

A fundamental microkinetic model is presented for the alkylation of benzene with octenes in liquid phase over Y zeolites with Si/Al ratios 5.8, 13, and 30. The model describes simultaneously the olefin isomerization and benzene alkylation reactions on the fresh catalysts, that is, in the absence of coke formation. The reaction mechanism considers adsorbed octenes reacting with benzene from the liquid phase. Protonation of phenyloctanes are considered quasi-equilibrated. Protonation of olefins and surface alkylations are considered as non-quasi-equilibrated. Activation energies of 46kJmol−1 and 70kJmol−1 are obtained for olefin protonation and surface alkylation, respectively. Standard protonation enthalpies of −33kJmol−1 and −92kJmol−1 are obtained for the protonation of 1-octene and 2-phenyloctane, respectively. Introduction of two catalyst descriptors allows to describe the influence of the acid properties of the Y zeolites on conversion and product yields. The concentration of acid sites is obtained from ammonia TPD. The change in standard protonation enthalpy relative to a reference Y zeolite, Δ(ΔHpro), accounts for the effect of the zeolite’s average acid strength on the stability of the intermediates and the activation energies and is estimated by regression to experimental data. For the three Y zeolites, a variation in standard protonation enthalpy of 7kJmol−1 is found.

Synthesis of isoprene from formaldehyde and isobutene over phosphate catalysts

Vapor phase Prins condensation of isobutene with formaldehyde has been studied over boron (BP), aluminum (AlP), titanium (TiP), zirconium (ZrP) and niobium (NbP) phosphates. The catalysts were characterized by elemental analysis, X-ray diffraction, low-temperature adsorption of nitrogen, TPD of ammonia and FTIR of adsorbed pyridine. The conversion of isobutene and formaldehyde over phosphates was found to increase in the following order: BP≪AlP<TiP<ZrP<NbP, while the selectivity to isoprene showed the reverse tendency. The main side reaction was formaldehyde decomposition into carbon monoxide and dihydrogen. It is proposed that isoprene is formed on the Brønsted acid sites of the catalyst, while Mδ+Oδ− ion pairs are responsible for competitive carbon monoxide formation. The high resistance of ZrP and NbP catalysts to deactivation is accounted for by the in situ restoration of the Brønsted acid sites in the presence of water directly during the time course of the reaction.

Synthesis of small-sized ZSM-5 zeolites employing mixed structure directing agents

Nano-sized crystals of zeolite ZSM-5 were hydrothermally synthesized using a mixture of two templating agents, tri-ethyl butyl ammonium bromide (TEBA) and ethylene diamine (EDA). Synthesized samples were characterized by XRD, SEM and ammonia TPD. Influences of proportions of the templating agents, silica to alumina ratio (SAR) and crystallization period were have been studied. Crystal morphology and size of the synthesized samples varied with proportions templating agents and SAR. Crystal size in the range of 100–400nm was observed with equimolar templates for an SAR of 200. Crystal size increased to 1 to~3μm with SAR in the range of 400–1200. Crystallization of MFI zeolite in presence of mixed templates was found to pass through an unknown meta-stable intermediate.

Efficient Liquid‐Phase Oxidation of Diphenylmethane to Benzophenone over Vanadium‐Containing MCM‐41 Catalysts

AbstractThe liquid‐phase oxidation of diphenylmethane with tert‐butylhydroperoxide has been studied using vanadium‐containing MCM‐41 materials, which were prepared by direct hydrothermal (V‐MCM‐41) and wet impregnation (V/MCM‐41) methods. These catalysts were characterized in detail by ICP‐AES, N2‐sorption, XRD, FT‐IR, 29Si and 51V NMR, TPD of ammonia, TPR of hydrogen, and chemisorption of oxygen. Both series of catalyst show good catalytic results, which are attributed to their highly ordered mesoporous structure, large BET surface area as well as the presence of easily accessible vanadium‐oxygen species as active centers in the catalyst. Further, V‐MCM‐41 exhibit superior catalytic activity (based on turnover number) than V/MCM‐41 mainly due to well‐dispersed tetrahedral vanadium‐oxygen species with higher oxidation ability. The effect of reaction parameters, i.e., temperature, time, solvent, etc. were investigated. Catalyst recycling test reveals good stability with only slight extent of leaching during the reaction.

Dimerization of 1-butene in liquid phase over H-NK-MM-BEA type embedded mesoporous materials

Dimerization of 1-butene in the liquid phase was studied at relatively mild conditions, 523 K under 20 bar over both microporous and embedded microporous mesoporous catalysts. Embedded catalysts were prepared by introducing Beta zeolite nuclei into the gel synthesis of mesoporous material. Three different embedded catalysts with different acidities were prepared and characterized by XRD, nitrogen adsorption, TEM, SEM, pyridine adsorption desorption and ammonia TPD. The embedded materials were shown to be thermally stable up to 1173 K, containing both micro- and mesopores and uniform pores with regular periodicity. According to ammonia TPD the acidity increased with increasing aluminium content. Due to the relatively low reaction temperature, the highest yields of C8 hydrocarbons, 33 mol% at the conversion level of 81% were achieved over an embedded micro-mesoporous catalyst containing Beta zeolite inside the mesoporous and totally 2.4 wt.% aluminium.

Catalytic dehydration of methanol to dimethyl ether over micro–mesoporous ZSM-5/MCM-41 composite molecular sieves

A series of micro–mesoporous ZSM-5/MCM-41 composite molecular sieves were prepared by combining a microporous zeolite silica source with nano self-assembly methods, and characterized by SEM, TEM, XRD, N 2 adsorption and desorption, TPD of ammonia, and their catalytic performance for the dehydration of methanol to dimethyl ether (DME) in a fixed bed microreactor at atmospheric pressure. Among these catalysts, the ZSM-5/MCM-41 sample alkali-treated with 1.5 mol/L NaOH solution, in which the relative crystallinity of ZSM-5 and MCM-41 are 54.5% and 30.5%, respectively, gave the best activity ( X MeOH > 80%) with 100% selectivity and a long lifetime in a wide range of temperature from 190 °C to 300 °C. From the characterization and activity data, the formation mechanism of the micro–mesoporous composite molecular sieves was proposed to be a liquid-crystal templating mechanism.

Solid acid catalysts for dehydration of glycerol to acrolein in gas phase

The reaction of glycerol dehydration to acrolein was performed at temperatures 220–300 °C over Keggin-type heteropolyacid (H3PW12O40·xH2O), supported on alumina and W-modified SBA-15 (Si/W = 20). The supports and catalysts were characterized by nitrogen adsorption, XRD, IR, UV–Vis DRS, SEM, and TPD of NH3. The acid sites strength, determined according to data from TPD of ammonia, follows the order: HPW/W-SBA-15 > W-SBA-15 > H3PW12O40 > HPW/γ-Al2O3 > γ-Al2O3. It was found that this order well correlates with the catalytic activity of studied samples. The most active and selective sample, HPW/W-SBA-15, showed acrolein selectivity of about 75% at almost 100% conversion of glycerol in all studied temperature interval.

Strong Brønsted acid site in HZSM-5 created by mild steaming

HZSM-5 was treated with water vapor of less than 0.2bar (mild steaming). The acidity of the treated HZSM-5 samples was characterized by usual and IRMS–TPD of ammonia, catalytic cracking activity of octane and hexane was measured, and a correlation between the solid acidity and the catalytic activity was studied. The mild steaming changed the Brønsted acidity; the number was decreased but the strength was increased by increasing the partial pressure of water vapor. The cracking activity showed the maximum conversion against the partial pressure of water vapor, as has already been reported. Therefore, the present study confirmed the principal of the mild steaming for the catalytic activity enhancement. The activation energy for hexane cracking was correlated linearly with the strength of Brønsted acidity; the larger the strength of acid site, the smaller the activation energy. First order dependence of the reaction rate of the cracking on the alkane pressure was confirmed irrespective of the treatment conditions. Thus, the catalytic activity of hexane cracking could be simply explained by proceeding of the mono-molecular reaction on the Brønsted acid site. Dislodged Al-containing species most probably enhances the Brønsted acidity, and accordingly the catalytic activity is enhanced, as long as number of the site is kept sufficiently.

Mechanistic investigation of hydrothermal aging of Cu-Beta for ammonia SCR

The selective catalytic reduction of NOx with NH3 over a Cu-BEA catalyst was studied after hydrothermal aging between 500 and 900°C. The corresponding catalyst was characterized using XPS and XRD techniques in the aging interval of 500, 700 and 800°C. No structural changes during the aging process were observed. However, the oxidation state of copper changed during aging and more Cu2+ was formed. We suggest that one of the deactivation mechanisms is the decrease of the Cu+ species. The NO oxidation and NH3 oxidation activity was decreased with increasing aging temperature. Further, we observed that the ammonia oxidation was decreased faster compared to the SCR reactions at low aging temperatures. The experiments from the calorimeter as well as from the ammonia TPD investigations indicate a trend towards more weakly bound ammonia with higher aging temperatures. From the results of the SCR experiments using different NO2/NOx ratios and ammonia oxidation experiments we suggest that most of the N2O is coming from side reactions of the SCR mechanism and not from reactions between NH3 and O2 alone. Interestingly, we observe that after the 900°C aging a quite large activity remained for the case with 75% NO2/NOx ratio. The N2O production shows a maximum at 200°C, but increases again at higher temperatures. However, the N2O formed at low temperature is decreased after hydrothermal aging while the high temperature N2O is increased. We propose that the different reactions examined in this work do not all occur on the same type of sites, since we observe different aging trends for some of the reactions.

Studies on the Dehydration of Glycerol Over Niobium Catalysts

The dehydration of glycerol over nanosize niobium catalysts was conducted in a stainless steel autoclave reactor. The catalysts were prepared by the calcination of niobium oxalate between 200 and 700 degrees C. Catalysts were characterized by N2 Physisorption, XRD and TPD of ammonia to investigate the effect of the calcination temperature and water on catalytic performance, catalysts' structures and acidity. Acrolein was mainly produced about 51-71% with useful by-products such as acetaldehyde and methanol. Amorphous Nb2O5 catalysts calcined at 200-400 degrees C significantly showed higher conversion of glycerol than the crystallized Nb2O5 catalyst calcined at 500-700 degrees C. Also the conversion of glycerol and selectivity of acrolein was increased with increasing the acidity of catalyst, which can be controlled by calcination temperature.

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.