Formation Of Protonic Acid Sites Research Articles

Effect of Cr2O3 loading on the properties and cracking activity of Pt/Cr2O3-ZrO2

The effects of Cr2O3 loading ranging from 1 to 12wt.% on the properties, structure and isopropylbenzene (IPB), 1,4-diisopropylbenzene (DIPB) and 1,3,5-triisopropylbenzene (TIPB) hydrocracking activities of Pt/Cr2O3-ZrO2 (Pt/CrZr) were studied. The properties of Pt/CrZr were characterized with XRD, BET, IR and UV–vis spectrometer. The acidity was determined by 2,6-lutidine adsorbed IR spectroscopy. The introduction of Cr2O3 on ZrO2 intensified continuously the tetragonal phase of ZrO2 and bulk crystalline Cr2O3, while the surface area and acidity passed through a maximum of Cr2O3 loading at 8wt.%. The IR study confirmed the presence of CrO stretching bands at 1035 and 1013cm−1 on Pt/CrZr. No dioxo species involving the stretching modes of OCrO existed. H2 and 2,6-lutidine showed different interactions with Pt/CrZr in which the band at 1013cm−1 is more extensively affected with 2,6-lutidine adsorption for all Pt/CrZr and the band at 1035cm−1 is more affected with H2 adsorption for Cr2O3 loading at≤8wt.%. While, at >8wt.% Cr2O3 loading, the H2 is more interacted with the band at 1013cm−1. These results suggested that the absorbance bands at 1013 and 1035cm−1 are assignable to the stretching of the CrO which is connected to the other Cr through O and CrO which is connected to cus Zr4+ through O, respectively. The catalyst with 8wt.% loading performed with a maximum activity in IPB, DIPB and TIPB hydrocracking at 523K which may be due to presence of CrO band at 1035cm−1 which extensively interacted with H2 to form protonic acid sites via a hydrogen spillover phenomenon. In fact, Cr2O3 loading at 8wt.% possessed strong permanent Lewis acid sites which play an important role in the stabilization of an electron during the formation of protonic acid sites.

ISOMERIZATION OF C5-C7 LINEAR ALKANES OVER WO3-ZRO2 UNDER HELIUM ATMOSPHERE

The effect of WO3 on the properties and catalytic isomerization of C5-C7 linear alkanes over ZrO2 was studied under helium atmosphere. The WO3-ZrO2 was prepared by impregnation of Zr(OH)4 with an aqueous (NH4)6[H2W12O40], followed by calcination at 1093 K for 3 h in air. The amount WO3 was 10 wt%. XRD and BET studies showed that the introduction of WO3 stabilizes the tetragonal phase of ZrO2, leading to larger surface area and stronger acidity of ZrO2. Pyridine FTIR study verified the interaction of WO3 with ZrO2 formed strong Lewis and Bronsted acid sites. The presence of WO3 increased the catalytic isomerization of C5-C7 linear alkanes. The conversion of C5, C6 and C7 reached 1.3, 2.6 and 5.1 %, respectively. While the selectivity of isopentane, isohexane and isoheptane reached 15.6, 20.5 and 19.5 %, respectively. The high activity of WO3-ZrO2 was due to the ability of WO3 to adsorb and dissociate linear alkane to form hydrogen and alkane radical in which the atomic hydrogen underwent to the formation of protonic acid sites and hydride. The presence of protonic acid sites and hydride determined the activity of WO3-ZrO2.

Effect of Various Au/Al2O3 Preparations on Catalytic Behaviour during the Continuous Flow Hydrogenation of an Octanal/Octene Mixture

AbstractSupported gold catalysts were prepared on a γ‐Al2O3 support by using the incipient wet impregnation and the deposition–precipitation methods. These catalysts were assessed for the continuous flow, liquid phase hydrogenation of an octanal/octene mixture, and their activity was compared with the activity of a commercially available Au/Al2O3 catalyst. These catalysts were characterised by inductively coupled plasma optical emission spectroscopy, hydrogen chemisorption, BET surface area and pore volume measurements, Raman spectroscopy, temperature‐programmed reduction of hydrogen, energy‐dispersive X‐ray spectroscopy mapping, HRTEM and powder XRD to determine gold crystallites. The results obtained revealed that gold crystallite size, dispersion and the presence of gold chloride had a significant effect on the product selectivity obtained. Large gold crystallites (particle size >15 nm) unevenly distributed on the alumina support obtained by using the wet impregnation method were inactive in hydrogenation. Hydrogen spillover resulted in the formation of protonic acid sites originating from molecular hydrogen, together with the formation of HCl‐catalysed acetal, and yielded 90 mol % selectivity towards the C24 acetal. In contrast, small gold crystallites (particle size <10 nm) evenly distributed on the alumina support obtained by using the deposition–precipitation preparation method facilitated the hydrogenation of the aldehyde and yielded 75 mol % selectivity towards the desired product, octanol.

Relating cumene hydrocracking activity to the acidic center of CrO3–ZrO2

The properties of CrO3–ZrO2 and Pt/CrO3–ZrO2 were studied by IR and ESR spectroscopy to elucidate the active sites in n-heptane isomerization and cumene cracking. Surface analyses showed the presence of cubic, monoclinic and tetragonal phases of ZrO2 with a BET specific surface area of about 150m2/g for both catalysts. 2,6-Lutidine adsorbed IR confirmed the presence of treble doublet bands at 1675+1660, 1650+1625 and 1640+1630cm−1 corresponding to Brønsted acid sites and quartet doublet bands at 1604+1580, 1595+1580, 1590+1580 and 1570+1560cm−1 corresponding to Lewis acid sites. The presence of Pt enhanced the activity of CrO3–ZrO2 in cumene hydrocracking, in which Pt facilitated the formation of protonic acid sites at 1675+1660 and 1650+1625cm−1 through a hydrogen spillover mechanism. The IR study verified that the Lewis acidic center at 1570+1560cm−1 was involved in the formation of protons by trapping electrons. Meanwhile, neither CrO3–ZrO2 nor Pt/CrO3–ZrO2 was active in n-heptane hydroisomerization.

Formation of acidic Brönsted (MoOx)−(Hy)+ evidenced by XRD and 2,6-lutidine FTIR spectroscopy for cumene cracking

2,6-Lutidine adsorbed IR spectroscopy has been employed to study the property of acidic sites on MoO3 and Pt/MoO3. The results showed that both catalysts possess doublet adsorption bands at 1605+1585cm−1, ascribed to Lewis acid sites, and duo-doublet bands at 1660+1650 and 1640+1630cm−1, ascribed to hydroxyl groups; these indicate an OH defect structure of MoO3 and Mo–OH Brönsted acidic sites. All Brönsted acid sites were strong enough to retain outgassing at 473K, while a considerable number of relatively weak and medium acid sites as well as strong Lewis acid sites existed. The addition of Pt slightly altered the ratio of Lewis/Brönsted acid sites and distribution of Lewis acid sites. The XRD result confirmed the formation of molybdenum oxyhydride (MoOx)−(Hy)+ on the hydrogen treated Pt/MoO3, whereas the hydrogen adsorption on 2,6-lutidine pre-adsorbed catalysts showed the formation of protonic acid sites over Pt/MoO3. These results strongly suggested that the interaction of molecular hydrogen with Pt/MoO3 formed acidic Brönsted (MoOx)−(Hy)+ via a hydrogen spillover mechanism. In fact, no (MoOx)−(Hy)+ and protonic acid sites were observed on Pt-free MoO3. The presence of (MoOx)−(Hy)+ enhanced the activity of Pt/MoO3 in the cumene hydrocracking in which the rate conversion of cumene increased by about 30%, while the apparent activation energy decreased by approximately 28kJ/mol.

Effect of iridium loading on the formation of protonic acid sites over Ir/Pt-HZSM5

The Ir/Pt-HZSM5 with different iridium loading (0.3-1.0 wt%) was prepared by impregnation of iridium on Pt-HZSM5. The acidic properties of Ir/Pt-HZSM5 were studied by FTIR spectroscopy, while the activity of the catalysts was tested for n-pentane isomerization in a microcatalytic pulse reactor.The IR results of adsorbed 2,6-lutidine showed that all catalysts possessed strong Brönsted and Lewis acid sites in the outgassing at 473 K and below.When Ir/Pt-HZSM5 was heated in hydrogen, protonic acid sites were formed with concomitant decrease of Lewis acid sites. An increase in iridiumloading continuously decreased the Lewis and Brönsted acid sites and inhibited the formation of protonic acid sites induced by hydrogen. The formationof protonic acid sites induced by hydrogen was also confirmed by the formation of electron detected by ESR spectroscopy. Additionally for n-pentaneisomerization, an increase in iridium loading decreased the yield of isopentane due to the inhibition in the formation of protonic acid sites via hydrogenspillover phenomenon.

Ir/Pt-HZSM5 for n-pentane isomerization: Effect of iridium loading on the properties and catalytic activity

The effects of iridium loading on the properties of Ir/Pt-HZSM5 and n-pentane isomerization were studied. XRD, IR, and NMR results indicated that increasing iridium loading did not much change the properties of catalysts, but eliminated the perturbed silanol groups at 3700 and 3520cm−1, whereas IR and ESR spectroscopy confirmed that increasing iridium loading continuously decreased the permanent Lewis and Brønsted acid sites and inhibited the formation of protonic acid sites induced by hydrogen. At low iridium loading (0–0.3wt%), cracking process proceed through dimerization-cracking step, whereas high iridium loading (0.5–2.0wt%) reduces the contribution of dimerization-cracking step and promotes the contribution of hydrogenolysis. The excessive amount of iridium loading, with the presence of a low amount of active protonic acid sites and hydrogen gas, accelerated the hydrogenolysis process. The activity of Ir/Pt-HZSM5 was marginal in the absence of hydrogen, showing the dependence of activity on promotive effect of hydrogen.

WO3 monolayer loaded on ZrO2: Property–activity relationship in n-butane isomerization evidenced by hydrogen adsorption and IR studies

The property–activity relationship of WO3 supported on ZrO2 (WZ) was evaluated in n-butane isomerization for a series of catalysts with WO3 loading ranging from 5 to 20wt% on ZrO2. The catalysts were prepared by incipient-wetness impregnation of Zr(OH)4 with an aqueous solution of (NH4)6[H2W12O40·nH2O], followed by drying and calcination at 1093K. The introduction of WO3 continuously increased the tetragonal phase of ZrO2, WO3 surface density and coverage. The specific surface area and total pore volume passed through a maximum of WO3 loading at 13wt%; this loading corresponds to 5.9WO3/nm2 and is near the theoretical monolayer-dispersed limit of WO3 on ZrO2. The IR results indicate that the presence of WO3 eroded the absorbance bands at 3738 and 3650cm−1 corresponding to bibridged and tribridged hydroxyl groups up to near the monolayer-dispersed limit of WO3. A new broad and weak band appeared, centered at 2930cm−1, indicating the presence of bulk crystalline WO3 for WO3 coverage exceeding the theoretical monolayer-dispersion limit. In addition to the band at 2930cm−1, two WO stretching bands were observed at about 1021 and 1014cm−1 for all WZ catalysts, confirming the existence of WO connected to coordinative unsaturated (cus) Zr4+ through O and to the other W through O, respectively. Pyridine adsorbed IR and NH3-TPD revealed that the presence of WO3 modified the nature and concentration of acidic sites. The highest acidity was observed with 13wt% loading WO3. The decrease in the intensity of peaks due to increasing WO3 loading was much higher on Lewis acid sites than on Brønsted acid sites. Hydrogen adsorption isotherms and the IR results for hydrogen adsorption on preadsorbed pyridine were used to evaluate the formation of active protonic acid sites from molecular hydrogen. The catalyst with 13wt% WO3 loading showed the maximum hydrogen uptake capacity and formation of protonic acid sites. These results show a direct correlation with the activity of WZ in n-butane isomerization at 573K in which 13wt% WO3 loading on ZrO2 yielded the highest amount of isobutane. It is suggested that the presence of strong Lewis acid sites on monolayer-dispersed WO3 facilitates the formation of protonic acid sites from hydrogen in the gas phase which act as active sites in n-butane isomerization. The presence of permanent Brønsted acid sites could not be directly associated with activity. In fact, no isomerization activity was observed in the absence of hydrogen.

IR study of iridium bonded to perturbed silanol groups of Pt-HZSM5 for n-pentane isomerization

The Ir/Pt-HZSM5 catalyst was prepared by impregnation of iridium on Pt-HZSM5. The activity of Ir/Pt-HZSM5 was tested for n-pentane isomerization under hydrogen stream. The introduction of iridium did not change the bands observed at 3740, 3665 and 3610cm−1 indicating that neither non-acidic terminal silanol groups nor acidic bridging hydroxyl groups interacted with the iridium. Additionally, the peaks corresponding to the perturbed silanol groups at 3700 and 3520cm−1 decreased significantly. X-ray photoelectron spectroscopy (XPS) analysis revealed that the iridium is in the form of IrO2. 27Al MAS NMR confirmed the elimination of distorted tetrahedral aluminum. The presence of iridium slightly increased the acidity of Pt-HZSM5 and its selectivity for iso-pentane. Hydrogen adsorption FTIR indicated that iridium enhanced the formation of protonic acid sites which may participate in the isomerization, and inhibited the formation of hydroxyl groups at 3380, 3600 and 3680cm−1 which may participate in the enhancement of the cracking reaction.

Hydrogen Effects on Cumene Cracking over Zirconium Oxide Promoted by Sulfate Ion and Platinum

The effects of hydrogen on the catalytic activity of zirconium oxide modified with sulfate ion and platinum (Pt/SO2−4–ZrO2) for cumene cracking were studied with a pulse reactor to examine if the catalytically active protonic acid sites are generated from hydrogen molecules present in the gas phase. In a hydrogen stream, a high conversion was obtained and the deactivation with pulse number was not appreciable. In a helium stream, the activity quickly decreased with pulse number, and the activity gradually recovered when the carrier gas of helium was switched into hydrogen. The formation of carbonaceous residue was extended in the absence of hydrogen, but suppressed in the presence of hydrogen. The effects of hydrogen were examined for other catalysts such as zirconium oxide modified with sulfate ion (SO42−–ZrO2) and zirconium oxide modified with platinum (Pt/ZrO2), but no promotion effects of hydrogen on the catalytic activity were observed. The promotion effects of hydrogen for Pt/SO42−–ZrO2are rationalized by the formation of protonic acid sites from hydrogen molecules. The suppression of the carbonaceous residue formation in the presence of hydrogen is also discussed.