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.     

Ga2O3/HSSZ-13 for dehydrogenation of ethane: Effect of pore geometry of support

Dehydrogenation of ethane to ethylene in the presence of CO₂ was investigated over gallium oxide supported on HSSZ-13 with different Si/Al ratios. These catalysts exhibited higher activity as well as higher selectivity in comparison to HZSM-5 supported ones. 34% ethane conversion with 86.9% selectivity to ethylene could be obtained. The enhanced catalytic performance of Ga₂O₃/HSSZ-13 can be attributed to unique pore geometry of SSZ-13 support. However, the stability is not as good as that of Ga₂O₃/HZSM-5, owing to easy coking in channel with smaller pore-opening size.     

Physicochemical properties and catalytic performance of galloaluminosilicate in aromatization of lower alkanes: a comparative study with Ga/HZSM-5

A series of gallium-containing HZSM-5 zeolites with different Ga contents (Ga/(Al+Ga)?=?0.1?C0.6) were prepared by hydrothermal in situ synthesis and post synthesis. Their catalytic performance were compared in the aromatization of propane, butane and propane/butane mixture (1:1?molar). Galloaluminosilicate obtained via hydrothermal in situ synthesis exhibited high fraction of acidic framework Ga³⁺ with few dispersed extracrystalline Ga₂O₃. Ga/HZSM-5 obtained by post synthesis showed the presence of extracrystalline Ga₂O₃ and/or extra framework gallyl ions. The aromatization performance of Ga-containing HZSM-5 followed the following sequence; galloaluminosilicate > Ga/HZSM-5 (ion-exchange) > Ga/HZSM-5 (impregnation) ? HZSM-5. Optimum aromatization performance over galloaluminosilicate was achieved with Ga/(Al+Ga) ratio of 0.3. Propane conversion reached 50.9?wt% over galloaluminosilicate with Ga/(Al+Ga) of 0.3, as compared to 31.8 and 40.7?wt% for the corresponding Ga/HZSM-5 obtained by impregnation and ion-exchange, respectively, at gas hourly space velocity of 1,600?h^?1, and 540?°C. Comparison of aromatic selectivity at the same conversion level (~10.0?wt%) revealed that galloaluminosilicate is more selective than Ga/HZSM-5. The superior performance of galloaluminosilicate was attributed to the presence of highly dispersed-reducible extra-framework Ga₂O₃ (Lewis-dehydrogenating sites) formed by degalliation in close proximity to zeolitic Br?nsted sites. Thus, hydrothermal in situ approach can thus be considered as an effective method for improving the aromatization performance of HZSM-5.     

Dual effects of supported W catalysts for dehydroaromatization of methane in the absence of oxygen

The screening of a series of W-based catalysts on different supports i.e. HZSM-5, Hβ, USY and Al₂O₃ for the dehydroaromatization of methane (DHAM) revealed that HZSM-5 emerged as the best support. Next, the performance of W/HZSM-5 and W-H₂SO₄/HZSM-5 catalysts for the DHAM reaction was compared to study the effect of acidic treatment in the impregnation method. The results showed that the optimum activity of W-H₂SO₄/HZSM-5 catalyst exceeded that of W/HZSM-5 catalyst. Finally, the influence of Si/Al ratio in the W-H₂SO₄/HZSM-5 catalyst was studied and the catalyst with Si/Al ratio = 30 was found to be the most promising for the DHAM reaction. The remarkable activity of the catalyst is attributed to the presence of dual effects: suitable content of octahedral polymeric and tetrahedral monomeric tungstate species accompanied by proper amount and strength of acid sites in the catalyst.     

A General Method for Preparation of PVP-Stabilized Noble Metal Nanoparticles in Room Temperature Ionic Liquids

Catalysts based on a physical mixture of Ga₂O₃ and MoO₃ have been prepared and evaluated for propane partial oxidation to propene. The Ga₂O₃/MoO₃ catalysts demonstrated propene yields greater than a 6 wt% V₂O₅/TiO₂ catalyst, which is known to be active for the reaction. The higher yield of propene was achieved by the alkane activation properties of Ga₂O₃ and the selective oxidation function of MoO₃ combining in a synergistic manner.     

An evidence of active surface MoOx over MgMoO4 for the catalytic oxidative dehydrogenation of propane

In order to clarify whether MgMo_xO_y catalysts with slight excess of molybdenum relative to the stoichiometric MgMoO₄ compound showed increased activities for propene formation in the propane oxidative dehydrogenation, we investigated the catalytic properties of MoO₃ supported on MgMoO₄ and of MgMo_xO_y catalysts treated with acid or base. Supporting MoO₃ on magnesium-rich MgMo_0.99O_y catalysts which are poorly active, or treating them with acetic acid to remove excess magnesium, resulted in drastic activity increases. On the other hand, the ammonia treatment of molybdenum-rich MgMo_1.05O_y catalysts which are highly active turned out to give a remarkable decrease in activity, because surface MoO_x dissolved in ammonia water. A clear correlation was observed between the catalytic activities for propane oxidation and the dehydration of 2-propanol to propene over the supported catalyst and the treated catalysts. Since the bulk structures were unchanged by supporting or by the treatments, the existence of MoO_x clusters formed on the surface of MgMoO₄ are responsible for the activities in the oxidative dehydrogenation of propane. This revised version was published online in July 2006 with corrections to the Cover Date.     

Propane oxidative dehydrogenation over vanadia catalysts supported on mesoporous silicas with varying pore structure and size

The structural characteristics and the performance of vanadia catalysts (0.7–8 wt.% V) supported on mesoporous (MCM-41, HMS, MCF, SBA-15), microporous (silicalite) and non-porous (SiO₂) silicas in oxidative dehydrogenation of propane were investigated. The structure of vanadia species, the redox and the acidic properties of the catalysts were studied using in situ Raman spectroscopy, TPD- NH₃ and H₂-TPR. The only vanadia species detected on the surface of HMS and MCM-41 for V loadings up to 8 wt.% were isolated monovanadates indicating high vanadia dispersion. Additional bands ascribed to V₂O₅ nanoparticles were evidenced in the case of SBA-15 and MCF supported catalysts while these bands were the only ones identified on the surface of the catalysts supported on silicalite and non-porous silica. The catalysts supported on mesoporous HMS and MCM-41 materials showed the best performance achieving high propane conversions (35–40%) with relatively high propene selectivities (35–47%). Lower activity due to the lower degree of vanadia dispersion, caused by the partial destruction of the pore structure was observed for the SBA-15 and MCF supported catalysts. The degree of dispersion of the V species on the catalyst surface and not the pore size and structure of the mesoporous support or the acidity/reducibility characteristics mainly determine the catalytic activity towards propene production. In addition, it was shown that the pore structure and size of the mesoporous supports did not have any significant effect in the turnover rates (TOF values) of propane conversion (and propene formation at low propane conversion, below ca. 10%). However, the highest propene yield (up to 19%) and stable catalytic behavior was attained for catalysts supported on HMS mesoporous silica, and especially for those combining framework mesoporosity and textural porosity (voids between primary nanoparticles).     

Catalytic cracking of isobutane over HZSM-5, FeHZSM-5 and CrHZSM-5 catalysts with different SiO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ratios

Several systems of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts with different ratios of SiO₂/Al₂O₃ (25,38,50,80, and 150) were prepared and they were characterized by means of X-ray diffraction (XRD), UV–Vis, NH₃-TPD and BET techniques. The results indicated that, compared with uncalcined HZSM-5 zeolites, the total acid amounts, acidic site density and acidic strength of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts obviously decreased, while those of weak acid amounts obviously enhanced with the decrease of SiO₂/Al₂O₃ molar ratio. When the ratio of SiO₂/Al₂O₃ is less than 50, the three systems of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts with same ratio of SiO₂/Al₂O₃ gave similar and high isobutane conversions. However, when the ratio of SiO₂/Al₂O₃ was equal to or greater than 80, these three systems of catalysts possessed different altering tendencies of isobutane conversions, thus their isobutene conversions were different. High yields of light olefins were obtained over the FeHZSM-5 and CrHZSM-5 zeolite catalysts with high ratio of SiO₂/Al₂O₃ (≥80). The ratio of SiO₂/Al₂O₃ has large effects on the surface area, and acidic characteristics of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolites catalysts, and thus further affect their catalytic performances for isobutane cracking. That is the nature of SiO₂/Al₂O₃ ratio effect on the catalytic performances.     

Small-sized HZSM-5 zeolite as highly active catalyst for gas phase dehydration of glycerol to acrolein

The catalytic properties of nanocrystalline HZSM-5 catalysts with high Si/Al molar ratio (ca. 65) were investigated in the gas phase dehydration of aqueous glycerol. Compared with bulk HZSM-5, the small-sized catalyst exhibits greatly enhanced catalytic performance in glycerol dehydration even with very high GHSV (=1438 h^?1). Catalysts with different Si/Al ratios were studied, but it is difficult to separate the influence of Si/Al ratio from that of particle size. However, by varying the proton exchange degree for one mother batch of zeolite, a series of H_xNa_1–xZSM-5 catalysts with same particle size and different Brønsted acid site densities was prepared. The catalytic results for this series of samples show that high density of Brønsted acid sites favors the production of acrolein. Based on these results, small-sized HZSM-5 with high aluminum content appears to be most promising for gas phase dehydration of glycerol.     

Direct synthesis of hydrogen peroxide over Pd nanoparticles embedded between HZSM-5 nanosheets layers

Direct synthesis of hydrogen peroxide (DSHP) was studied over Pd loaded on HZSM-5 nanosheets (Pd/ZN). Pd nanoparticles with average size of ca. 4.3 nm were introduced into the adjacent nanosheet layers (thickness of ca. 2.9 nm) by impregnation method. Pd/ZN with theoretical Si/Al molar ratio of 25 showed the highest selectivity for H₂O₂ among the prepared catalysts, together with highest formation rate of H₂O₂ (38.0 mmol·(g cat)⁻¹·h⁻¹), 1.9 times than that of Pd supported on conventional HZSM-5 zeolite (Pd/CZ-50). Better catalytic performance of nanosheet catalysts was attributed to the promoted Pd dispersion which promoted H₂ dissociation, more Brønsted acid sites and stronger metal-support interaction which inhibited the dissociation of OO bond in H₂O₂. The embedded structure sufficiently protected the Pd nanoparticles by space confinement which restrained the Pd leaching, leading to a better catalytic stability with 90% activity retained after 3 cycles, which was almost 3 times than that of Pd/CZ-50 (30.4% activity retained).     

Benzene Alkylation with Propane over Mo Modified HZSM-5

Benzene alkylation with propane has been studied over HZSM-5 loading 3.1–15.4 wt% Mo in continuous-flow microreactor under 350 °C and atmospheric pressure with the highest activity obtained at 6.7 wt% Mo loading. C_7–9 aromatics were obtained as main products while the total amount of benzene rings kept unchanged. i-Propylbenzene and n-propylbenzene are formed primarily, while toluene, ethylbenzene, and ethyl-toluene are formed secondly from the propylbenzenes. Catalytic performance of 6.7 wt% Mo/HZSM-5(38) partially poisoned by NH₃ shows that the strong acid sites play a crucial role in the alkylation. Low SiO₂/Al₂O₃ ratio of HZSM-5 in the Mo modified catalysts gives high propane conversion. Two hydrothermal treatment methods were applied to the 6.7 wt% Mo/HZSM-5(38) catalyst, caused decrease of propane conversion but result in different product distribution. A possible reaction mechanism concerning bifunctional active centers resulted from combination of loaded Mo species and strong acid centers on HZSM-5 is proposed.     

Oxidation state of platinum clusters during the reduction of NOx with propene and propane

The oxidation state of platinum supported on mesoporous SiO₂ and Al₂O₃ with MCM-41 type structure during the reduction of NO_x with propene or propane was investigated using in situ X-ray absorption spectroscopy. Platinum supported on MCM-41 (SiO₂) was reduced at low and oxidized at high reaction temperatures when propene was used as reducing agent, while it was found to be always oxidized in Pt/MCM-41 (Al₂O₃). When propane was used as reducing agent significant NO conversion was not observed over Pt/MCM-41 (SiO₂) and on both supports platinum was in an oxidized state. At the successive adsorption of the reactants, the prereduced catalysts were oxidized after NO adsorption and reduced after addition of the hydrocarbons. Addition of oxygen re-oxidized the catalysts, while the presence of water vapor did not influence the oxidation state.     

Chromium Oxide Supported on Mesoporous SBA-15 as Propane Dehydrogenation and Oxidative Dehydrogenation Catalysts

The catalytic activity and selectivity of Cr₂O₃ supported on mesoporous SBA-15 for non-oxidative and oxidative dehydrogenation of propane by O₂ and CO₂ have been studied and compared with those of Cr₂O₃/ZrO₂ and Cr₂O₃/-Al₂O₃ catalysts. Cr₂O₃/SBA-15 and Cr₂O₃/ZrO₂/SBA-15 are more selective to propene and more resistant to coking in comparison with Cr₂O₃/ZrO₂ and Cr₂O₃/-Al₂O₃ for non-oxidative dehydrogenation of propane. In oxidative dehydrogenation of propane by O₂ and CO₂, Cr₂O₃/SBA-15 also displays better activity, selectivity and stability than the other two supported catalysts. The propane conversion and propene yield on Cr₂O₃/SBA-15 catalyst for oxidative dehydrogenation of propane by CO₂ at 823 K reach 24.2 and 20.3%, respectively. XPS and TG/DTA have been used to characterize the catalysts before and after reaction. The differences in catalytic behavior of various supported Cr₂O₃ catalysts in the reactions have been discussed on the basis of the characterization results.     

Dehydrogenation of Propane in the Presence and Absence of CO2 Over β-Ga2O3 Supported Chromium Oxide Catalysts

The dehydrogenation of propane to propene in the presence and absence of CO₂ over β-Ga₂O₃ supported chromium oxide catalysts has been studied. The effect of chromium content and feed composition on the dehydrogenation of propane has been investigated. It has been found that deposition of chromium oxide enhances the dehydrogenation activity and resistance to coke deposition. Moreover, it has been shown that CO₂ promotes the dehydrogenation of propane above 848 K. At that temperature in the presence of CO₂ both the yield of propene and conversion of propane were higher than in the inert gas atmosphere.     

Kinetic parameter estimation for supported vanadium oxide catalysts for propane ODH reaction: Effect of loading and support

Kinetic parameters are estimated for a sequential Mars van Krevelan (MVK) reaction model occurring over several supported vanadium oxide (vanadia) catalysts involved in the propane oxidative dehydrogenation (ODH) reaction. The estimated kinetic parameters, pre-exponential factors and activation energies, are used to understand the effect of vanadia loading and oxide support. The pre-exponential factors and vanadia normalized pre-exponential factors vary with vanadia loading and oxide support. The monotonic increase in normalized pre-exponential factors with vanadia loading and the variation of pre-exponential factors with oxide support appears to be related to the change in acidity/basicity of the catalyst and the redox nature of the catalyst, respectively. The activation energy for propene degradation does not significantly change with catalyst; however, the activation energy for propane oxidation is different for the V₂O₅/Al₂O₃ catalyst. It appears that two important considerations are required for the development of an efficient propane ODH catalyst: a high rate constant associated with the propane oxidation reaction, and a high ratio of the rate constant for propene formation to degradation reaction. Based on the observations in the present study it is proposed that a higher TiO₂ support surface area will assist in increasing the propane oxidation activity and propene yield.     

Direct partial oxidation of methane over ZSM-5 catalyst: effects of additives

Previously, it was reported that the direct partial oxidation (DPO) of CH₄ with O₂ over HZSM-5 catalysts produces C₅₊ hydrocarbon liquids when the feed contains a propane or propene additive. This work studies additive effects on C₅₊ production in this system by processing a CH₄/C₃H₈ feed with subsequent removal of the C₃ additive and by processing natural gas feed. Results show C₅₊ production is maintained at constant yields for HZSM-5 catalysts having different zeolitic Al contents after removal of the C₃ additive. Mechanistic implications are discussed. Natural gas DPO consistently produced C₅₊ liquids due to the presence of C₂₊ components in the feed. While C₅₊ yields from natural gas DPO are higher than those observed for CH₄/C₃ feeds, increasing feed O₂ concentration, and thus conversion, deleteriously affected C₅₊ selectivity.     

Catalytic cracking of tricyclo [5.2.1.0] decane over HZSM-5 molecular sieves

The catalytic cracking of a high density hydrocarbon fuel, tricyclo [5.2.1.0^2.6] decane (JP-10) over HZSM-5 molecular sieves with different Si/Al mole ratios of 25, 38, and 50 was investigated at the temperature range from 773 to 873 K. Compared with the thermal cracking and the catalytic cracking over ZSM-5, conversions of JP-10 from the catalytic cracking over HZSM-5 molecular sieves at the same temperature were evidently heightened. The predominant hydrocarbon products from the catalytic cracking, checked at room temperature and atmospheric pressure, were methane, ethane, ethene, propane and propylene in the gaseous phase and benzene, indene, naphthalene and their homologues in the liquid phase. The contents of ethane, propane and propene decrease with increasing Si/Al mole ratio of a catalyst while those of methane and ethene increase simultaneously with the increase of Si/Al mole ratio of HZSM-5. The contents of the main components in the liquid products produced on the catalyst surface at a given temperature also decreased with the increase of Si/Al mole ratio. To keep high yields of alkenes, the HZSM-5 catalyst with high Si/Al mole ratio could be chosen.     

Aromatization of propane over Ga/H-ZSM-5: An explanation of the synergy observed between Ga3+ and H+

The aromatization of propane is investigated for Ga₂O₃, H-ZSM-5 and Ga₂O₃/H-ZSM-5 catalysts, and the results are discussed for a series of ZSM-5 catalysts containing varying SiO₂/Al₂O₃ ratios. It is apparent that on addition of a gallium phase to H-ZSM-5, the yield of methane is significantly decreased. These results are discussed with respect to the mechanism of formation of the initial reaction product from propane. It is proposed that the synergy observed between the gallium compound and the zeolite can be explained in terms of a mechanism in which the role of the gallium phase is to induce C-H bond polarization in the propane, which leads to attack via the Bronsted acid sites of the zeolite, which leads to initial C-H bond cleavage occurring.     

不同硅铝比HZSM-5分子筛对合成气一步法制二甲醚的影响

采用固定床高压反应装置,以工业用（CuO／ZnO／Al2O3）作为甲醇合成催化剂,四种不同硅铝比（硅铝比为25、38、50和150）的HZSM-5作为甲醇脱水催化剂,二者机械混合制备出一步法合成二甲醚双功能催化剂。考察了催化剂中脱水组分（HZSM-5分子筛）的硅铝比对二甲醚合成反应的影响,并通过BET、XRD和NH,-TPD等手段对催化剂进行表征。结果表明,一步法合成二甲醚催化剂中脱水组分HZSM-5的最佳的硅铝比为50。     

多级孔HZSM-5分子筛的制备及其催化丙烷脱氢反应性能

以十八水合硫酸铝、硅溶胶、四丙基氢氧化铵(TPAOH)、硝酸铵(NH_4NO_3)为原料,合成硅铝物质的量比为100的HZSM-5分子筛。采用乙酸钠溶液处理HZSM-5分子筛后加入TPAOH溶液进行二次晶化,通过对处理前、后的HZSM-5分子筛负载Pt,制得Pt/HZSM-5催化剂用于丙烷脱氢反应。利用XRD、SEM、XRF、BET、NH_3-TPD和H_2化学吸附对处理前、后的HZSM-5分子筛进行了表征,考察了不同浓度的TPAOH溶液对HZSM-5分子筛的晶体结构、表面形貌、孔结构、表面酸性及丙烷脱氢性能的影响。结果表明,采用乙酸钠溶液处理后的HZSM-5分子筛在TPAOH溶液中发生了二次晶化,不仅形成了微孔-介孔多级孔结构,还能调变分子筛的表面酸性;当TPAOH溶液浓度为0.3 mol/L、晶化温度为170℃、晶化时间为24 h时,得到的Pt/HZSM-5催化丙烷的初始转化率和丙烯选择性均达到最高值,分别为34.2%和99.0%。