Conversion of Methane over Ag<Superscript>+</Superscript>-exchanged Zeolite in the Presence of Ethene

Methane is shown to react with ethene over silver-exchanged zeolites at around 673 K to form higher hydrocarbons. Methane conversion of 13.2% is achieved at 673 K over Ag–ZSM−5 catalyst. Under these conditions, H–ZSM−5 does not catalyze the methane conversion, only ethene being converted into higher hydrocarbons. Zeolites with extra-framework metal cations such as In and Ga also activate methane in the presence of ethene. Using ¹³C-labeled methane as a reactant, propene is shown to be a primary product from methane and ethane. ¹³C atoms were not found in benzene molecules produced, indicating that benzene is entirely originated from ethane. On the other hand, in toluene, ¹³C atoms are found in both the methyl group and the aromatic ring. This implies that toluene is formed by the reaction of propene with butenes formed by the dimerization of ethene, and also by the reaction of benzene with methane. The latter path was confirmed by direct reaction of ¹³CH₄ with benzene. In this case, ¹³C atoms are found only in methyl groups of toluene produced. The heterolytic dissociation of methane over Ag⁺-exchanged zeolites is proposed as a reaction mechanism for the catalytic conversion of methane, leading to the formation of silver hydride and CH₃^δ+ species, which reacts with ethene and benzene to form propene and toluene, respectively. The conversion of methane over zeolites loaded with metal cations other than Ag⁺ is also described. The reaction of methane with benzene over indium-loaded ZSM−5 afforded toluene and xylenes in yields of up to 7.6% and 0.9% at 623 K when the reaction was carried out in a flow reactor.     

Decomposition and aromatization of ethanol on ZSM-based catalysts

The adsorption, desorption, and reactions of ethanol have been investigated on pure and promoted ZSM-5 catalysts. FTIR spectroscopy indicated the formation of a strongly bonded ethoxy species on ZSM-5(80) at 300 K. TPD experiments following the adsorption of ethanol on both ZSM-5 and Mo2C/ZSM-5 have shown desorption profiles corresponding to unreacted ethanol and decomposition products (H2O, H2, CH3CHO, C4H10O, and C2H4). The main reaction pathway of ethanol on pure ZSM-5 is the dehydration reaction yielding ethylene, small amounts of hydrocarbons, and aromatics. Deposition of different additives, such as Mo2C, ZnO, and Ga2O3 on zeolite, greatly promoted the formation of benzene and toluene at 773-973 K, very likely by catalyzing the aromatization of ethylene formed in the dehydration process of ethanol. Separate studies of the reaction of ethylene revealed that the previous additives markedly enhanced the selectivity and the yield of aromatics on ZSM-5.     

低硅铝比ZSM-5分子筛上C4烃的催化裂解反应

以低硅铝比(n(SiO2)/n(Al2O3)=20-45)的ZSM-5分子筛为催化剂, 研究了混合C4烃的催化裂解反应, 并对不同硅铝比的ZSM-5分子筛进行了酸性表征. 混合C4烃的催化裂解反应结果表明, 低硅铝比的ZSM-5分子筛具有较高的低温催化活性, 高硅铝比ZSM-5分子筛催化剂上乙烯和丙烯的收率高于低硅铝比ZSM-5分子筛催化剂, 低硅铝比ZSM-5分子筛上苯和甲苯的收率高于高硅铝比ZSM-5分子筛催化剂. 在反应温度为625 ℃时, 硅铝比为20的ZSM-5分子筛催化剂上乙烯、丙烯、苯和甲苯的总收率可达79.42%. 酸性表征结果表明, 硅铝比低的ZSM-5分子筛具有更多的Bronsted(B)酸酸量、Lewis(L)酸酸量及总酸酸量, 这是低硅铝比ZSM-5分子筛具有低温高活性及高的苯和甲苯收率的原因.     

Experimental study and detailed modeling of toluene degradation in a low-pressure stoichiometric premixed CH4/O2/N2 flame

Temperature and mole fraction profiles have been measured in laminar stoichiometric premixed CH4/O2/N2 and CH4/1.5%C6H5CH3/O2/N2 flames at low pressure (0.0519 bar) by using thermocouple, molecular beam/mass spectrometry (MB/MS), and gas chromatography/mass spectrometry (GC/MS) techniques. The present study completes our previous work performed on the thermal degradation of benzene in CH4/O2/N2 operating at similar conditions. Mole fraction profiles of reactants, final products, and reactive and stable intermediate species have been analyzed. The main intermediate aromatic species analyzed in the methane-toluene flame were benzene, phenol, ethylbenzene, benzylalcohol, styrene, and benzaldehyde. These new experimental results have been modeled with our previous model including submechanisms for aromatics (benzene up to p-xylene) and aliphatic (C1 up to C7) oxidation. Good agreement has been observed for the main species analyzed. The main reaction paths governing the degradation of toluene in the methane flame were identified, and it occurs mainly via the formation of benzene (C6H5CH3 + H = C6H6 + CH3) and benzyl radical (C6H5CH3 + H = C6H5CH2 + H2). Due to the abundance of methyl radicals, it was observed that recombination of benzyl and methyl is responsible for main monosubstitute aromatic species analyzed in the methane-toluene flame. The oxidation of these substitute species led to cyclopentadienyl radical as observed in a methane-benzene flame.     

ZSM-5／MOR复合分子筛催化剂对混合C4烃转化反应的催化性能

以ZSM-5/丝光沸石(MOR)复合分子筛为催化剂,对混合C4烃的催化转化反应进行了评价,并采用程序升温脱附和原位红外光谱技术对ZSM-5/MOR的酸性进行了表征. 结果表明,与ZSM-5相比, MOR具有很低的催化活性,但ZSM-5/MOR复合分子筛具有较高的催化活性,随着ZSM-5/MOR复合分子筛中ZSM-5含量的增加, C4烃转化率稍有升高;在C4烃转化率大致相同的情况下,乙烯和丙烯的总选择性比较接近,但苯和甲苯的总收率却快速升高. 随着ZSM-5/MOR复合分子筛中ZSM-5含量的增加,弱酸和中强酸的酸量逐渐减少,强酸的酸量有所增加. 由于ZSM-5/MOR复合分子筛中MOR对ZSM-5起到分散作用而产生更多的L酸中心,且此L酸中心处于分子筛的外表面而具有较高的能量,导致苯和甲苯的总收率升高.     

无第二模板剂法合成多级结构ZSM-5分子筛微球及其在甲烷无氧芳构化反应中的应用

在无第二模板剂的条件下,采用简单的水热晶化法,通过控制条件合成出具有多级结构的ZSM-5(ZSM-5-HW)分子筛,并运用X射线衍射、红外光谱、扫描电镜和N2吸附-脱附等技术对合成的分子筛进行了表征.结果表明,所得样品是由棒状晶体组装而成的具有介孔结构的分子筛微球.用等体积浸渍法制备了Mo/HZSM-5-HW催化剂并用于CH4无氧芳构化反应,表现出较高的催化活性和稳定性.在实验条件下,CH4初始转化率为18.5%,而苯收率最高可达9.5%;反应24h后,两者仍然分别保持在10.2%和5.5%左右.     

ZSM-5/ZSM-57复合分子筛催化剂上混合C4烃的催化转化反应

以ZSM-5/ZSM-57复合分子筛为催化剂, 考察了其对混合C4烃催化转化的反应性能. 采用氨程序升温脱附(NH3-TPD)和吡啶吸附傅立叶变换红外(FT-IR)光谱技术表征复合分子筛的酸性质. 结果表明, 当复合分子筛中ZSM-5的含量较低时, 比ZSM-5具有更高的催化活性及乙烯和丙烯选择性, 这是因为此时复合分子筛酸强度较高、酸量较多, 且小孔ZSM-57有利于乙烯和丙烯的择形反应. 而当复合分子筛中ZSM-5的含量较高时, 具有较高的苯和甲苯选择性, 其原因可能是其孔结构及共晶生长时的结构匹配性对芳构化反应有利.     

反应器型式对甲烷低温等离子体转化制C2烃的影响

就不同反应器对甲烷常压低温等离子体转化制C2烃的影响进行了研究。结果表明,相同的甲烷停留时间和相同甲烷流率下,反应器A和B中反应的主要产物是乙炔,乙烯和乙烷的含量较少,积炭量较多;而反应器C和D中反应的主要产物为乙烷和丙烷,乙烯和乙炔含量较少,积炭量很少。反应积炭对反应器A中甲烷转化率影响很大,对于产物选择性影响不大,而对反应器C中的反应影响较小。根据产物分布可知,在反应器A和B中,由于电子具有很高的能量和密度,甲烷主要解离为碳原子;而在反应器C及D中,由于电子能量和密度较低,甲烷主要解离为CH3自由基。     

电场增强催化甲烷合成碳二烃催化剂影响研究

本研究提出了在常温、常压电场增强等离子体条件下甲烷直接转化合成碳二烃的洁净工艺 ,在不同的放电电压、放电功率、甲烷进料流量和不同的催化剂作用下 ,甲烷能够以不同的转化率和选择性转变为碳二烃。对影响甲烷转化率和碳二烃选择性的因素 :放电电压、放电功率、甲烷进料流量和催化剂进行了研究 ,对催化剂性能进行了比较 ,并探讨了反应机理。结果表明 ,适宜的工艺条件 :放电电压 2 0kV～ 4 0kV ;输入功率 :2 0W～ 4 0W ;合适的甲烷进料流量 :30mL/min～ 70mL/min。在该条件下 ,碳二烃的选择性可以达到 95 % ;催化剂对甲烷转化率的影响顺序为MnO2 /Al2 O3 >Ni/Al2 O3 >MoO3 /Al2 O3 >Ni/NaY >Pd/ZSM 5 >Ni/H4Mg2 Si3 O4>Ni/ZSM 5 >Co/ZSM 5 >无催化剂。     

Methane to Liquid Hydrocarbons over Tungsten-ZSM-5 and Tungsten Loaded Cu/ZSM-5 Catalysts

Metal containing ZSM-5 can produce higher hydrocarbons in methane oxidation. Many researchers have studied the applicability of HZSM-5 and modify ZSM-5 for methane conversion to liquid hydrocarbons, but their research results still lead to low conversion, low selectivity and low heat resistance. The modified HZSM-5, by loading with tungsten (W), could enhance its heat resistant performance, and the high reaction temperature (800℃) did not lead to a loss of the W component by sublimation. The loading of HZSM-5 with tungsten and copper (Cu) resulted in an increment in the methane conversion as well as CO2 and C5 selectivities. In contrast, CO, C2-3 and H2O selectivities were reduced. The process of converting methane to liquid hydrocarbons (C5 ) was dependent on the metal surface area and the acidity of the zeolite. High methane conversion and C5 selectivity, and low H2O selectivity are obtained over W/3.0Cu/HZSM.     

Investigations by (13)C NMR spectroscopy of ethene-initiated catalytic CO hydrogenation

13C NMR spectroscopy shows that the n-alkene and n-alkane products from the catalytic hydrogenation of CO in the presence of (13)C(2)H(4) probes over Ru/150 degrees C, Co/180 degrees C, Fe/220 degrees C, or Rh/190 degrees C (1 atm, CO:H(2) 1:1, "mild conditions") contain terminal (13)CH(3)(13)CH(2)- units. This is consistent with their formation by a regiospecific polymerization of C(1) species derived from CO and initiated by (13)C(2)H(4). Although the activities toward individual products differed somewhat, similar distributions and similar product labeling patterns were obtained over all the four catalysts. 1-Butene and the higher 1-n-alkenes from all the catalysts were largely (13)CH(3)(13)CH(2)(CH(2))(n)()CH=CH(2) (n = 0-3), propene formed over Ru or Co was (13)CH(3)(13)CH=CH(2), while both (13)CH(3)(13)CH=CH(2) and (13)CH(2)=(13)CHCH(3) were formed over Fe or Rh. Comparison of the conclusions from these probe experiments with those from isotope transient experiments by other workers indicates that the ethene initiator does not significantly modify the course of the CO hydrogenation. The reaction products are largely kinetically determined, and the primary products are mainly linear 1-n-alkenes, while the n-alkanes and 2-n-alkenes largely arise via secondary processes. Since the distribution of products and the labeling in them is so similar, it is concluded that one basic primary mechanism applies over all the four metals. Several different reaction paths involving a polymerization of surface methylene, [CH(2(ad))], have been proposed. Although the predictions based on several of these mechanisms agree with many of the results, the alkenyl + [CH(2(ad))] mechanism, initiated by a surface vinyl [CH(2)=CH((ad))], most easily accommodates the experimental evidence. An alternative path involving sequential addition of surface methylidyne and hydride either to a growing alkylidene chain (alkylidene + [CH(ad) + H(ad)]) or to an alkyl chain (alkyl + [CH((ad)) + H(ad)]) has recently been proposed by van Santen and Ciobica. The [CH(2(ad))] mechanism offers an easier explanation for the formation of the various alkenes, the distribution of products, and of the initiation, while the [CH(ad) + H(ad)] mechanism can explain any n-alkanes formed as primary products and not derived from alkenes. At higher reaction temperatures over Ru and Co, considerable (13)C(1) incorporation (from natural abundance in the CO and from cleavage of the (13)C(2)H(4) probe) was found in all the hydrocarbons. Thus, at higher temperatures (13)C(1(ad)) in addition to (13)C(2(ad)) species participate in both chain growth and initiation. In summary, adsorbed CO is transformed very easily into surface C(1(ad)), probably [CH(2(ad))] in equilibrium with [CH((ad))+H(ad)], which act as the propagating species.     

Direct observation of insertion events at rac-(C2H4(1-indenyl)2)Zr(MeB(C6F5)3)-polymeryl intermediates: distinction between continuous and intermittent propagation modes

The propagating species for 1-hexene, propene, and ethene polymerization as catalyzed by [rac-(C2H4(1-indenyl)2)Zr(CH3)][CH3B(C6F5)3] has been intercepted at T < -40 degrees C and characterized by NMR methods. Observation of the propagating species permits direct monitoring of initiation, propagation, and termination processes by NMR. Detailed examination of alkene incorporation into Zr-polymeryl species using 13C-labeled 1-alkenes supports a continuous insertion mechanism whereby the anion re-coordinates after each alkene insertion.     

ReaxFF reactive force field for molecular dynamics simulations of hydrocarbon oxidation

To investigate the initial chemical events associated with high-temperature gas-phase oxidation of hydrocarbons, we have expanded the ReaxFF reactive force field training set to include additional transition states and chemical reactivity of systems relevant to these reactions and optimized the force field parameters against a quantum mechanics (QM)-based training set. To validate the ReaxFF potential obtained after parameter optimization, we performed a range of NVT-MD simulations on various hydrocarbon/O2 systems. From simulations on methane/O2, o-xylene/O2, propene/O2, and benzene/O2 mixtures, we found that ReaxFF obtains the correct reactivity trend (propene > o-xylene > methane > benzene), following the trend in the C-H bond strength in these hydrocarbons. We also tracked in detail the reactions during a complete oxidation of isolated methane, propene, and o-xylene to a CO/CO2/H2O mixture and found that the pathways predicted by ReaxFF are in agreement with chemical intuition and our QM results. We observed that the predominant initiation reaction for oxidation of methane, propene, and o-xylene under fuel lean conditions involved hydrogen abstraction of the methyl hydrogen by molecular oxygen forming hydroperoxyl and hydrocarbon radical species. While under fuel rich conditions with a mixture of these hydrocarbons, we observed different chemistry compared with the oxidation of isolated hydrocarbons including a change in the type of initiation reactions, which involved both decomposition of the hydrocarbon or attack by other radicals in the system. Since ReaxFF is capable of simulating complicated reaction pathways without any preconditioning, we believe that atomistic modeling with ReaxFF provides a useful method for determining the initial events of oxidation of hydrocarbons under extreme conditions and can enhance existing combustion models.     

Reduction of NO with methane over Fe/ZSM-5 catalysts

<正>The catalytic activity of Fe/ZSM-5 for the selective reduction of NO to N_2 with methane in the presence of excess O_2 was studied.Fe/ZSM-5 catalysts with various Fe loadings were prepared by impregnation method.It is well known that methane is inactive when Fe/ZSM-5 as the catalyst for the selective catalytic reduction(SCR) of NO with methane.However,this paper shows that when the content of Fe was about 0.5%,Fe/ZSM-5 showed higher catalytic activity and selectivity of methane,and put forward measurable activation for CH_4 is an important factor for the reaction of removal of NOx with CH_4.     

Activation of arene C[bond]H bonds by a cationic hafnium silyl complex possessing an alpha-agostic Si[bond]H interaction

The reaction of Cp2Hf(SiMes2H)Me (1) with B(C6F5)3 produces zwitterionic Cp2Hf(eta2-SiHMes2)(mu-Me)B(C6F5)3 (2), which is stable for >8 h at -40 degrees C in toluene-d8. Spectroscopic data provide evidence for an unusual alpha-agostic Si-H interaction in 2. At room temperature, 2 reacts with the C-H bonds of aromatic hydrocarbons such as benzene and toluene to produce Cp2Hf(Ph)(mu-Me)B(C6F5)3 (3), isomers of Cp2Hf(C6H4Me)(mu-Me)B(C6F5)3 (4-6), and Cp2Hf(CH2Ph)(mu-Me)B(C6F5)3 (7), respectively. The reaction involving benzene is first-order in both 2 and benzene; rate = k[2][C6H6]. Mechanistic data including activation parameters (DeltaH = 19(1) kcal/mol; DeltaS = -17(3) eu), a large primary isotope effect of 6.9(7), and the experimentally determined rate law are consistent with a mechanism involving a concerted transition state for C-H bond activation.     

Homogeneous catalysis with methane. A strategy for the hydromethylation of olefins based on the nondegenerate exchange of alkyl groups and sigma-bond metathesis at scandium

The scandium alkyl Cp*(2)ScCH(2)CMe(3) (2) was synthesized by the addition of a pentane solution of LiCH(2)CMe(3) to Cp*(2)ScCl at low temperature. Compound 2 reacts with the C-H bonds of hydrocarbons including methane, benzene, and cyclopropane to yield the corresponding hydrocarbyl complex and CMe(4). Kinetic studies revealed that the metalation of methane proceeds exclusively via a second-order pathway described by the rate law: rate = k[2][CH(4)] (k = 4.1(3) x 10(-4) M(-1)s(-1) at 26 degrees C). The primary inter- and intramolecular kinetic isotope effects (k(H)/k(D) = 10.2 (CH(4) vs CD(4)) and k(H)/k(D) = 5.2(1) (CH(2)D(2)), respectively) are consistent with a linear transfer of hydrogen from methane to the neopentyl ligand in the transition state. Activation parameters indicate that the transformation involves a highly ordered transition state (DeltaS++ = -36(1) eu) and a modest enthalpic barrier (DeltaH++ = 11.4(1) kcal/mol). High selectivity toward methane activation suggested the participation of this chemistry in a catalytic hydromethylation, which was observed in the slow, Cp*(2)ScMe-catalyzed addition of methane across the double bond of propene to form isobutane.     

Methane Decomposition and C2 Hydrocarbon Formation under the Condition of DC Discharge Plasma

The infrared emission spectra of methane, H, CH and C2 hydrocarbons in natural gas were measured. The processes of methane decomposition and formation of C2 hydrocarbons were studied. The experiment shows that methane decomposition can be divided into three periods as the reaction proceeds.In the first period, a large number of free radicals were formed. While in the last period, the formation of C2 hydrocarbons and the decrease of free radicals were observed. The time and conditions of methane decomposition and formation of C2 hydrocarbons are different.     

Photoisomerization and photodissociation of toluene in molecular beam

The photodissociation of isotope-labeled toluene C(6)H(5)CD(3) and C(6)H(5)(13)CH(3) molecules at 6.4 eV under collision-free conditions was studied in separate experiments by multimass ion imaging techniques. In addition to the major dissociation channels, C(6)H(5)CD(3) --> C(6)H(5)CD(2) + D and C(6)H(5)CD(3) --> C(6)H(5) + CD(3), the respective photofragments CD(2)H, CDH(2), and CH(3) and their heavy fragment partners C(6)H(4)D, C(6)H(3)D(2), and C(6)H(2)D(3) were observed from C(6)H(5)CD(3) dissociation. Photofragments (13)CH(3) and CH(3), and their heavy fragment partners C(6)H(5) and (13)CC(5)H(5), were also observed from C(6)H(5)(13)CH(3) dissociation. Our results show that 25% of the excited toluene isomerizes to a seven-membered ring (cycloheptatriene) and then rearomatizes prior to dissociation. The isomerization pathway competes with direct C-C bond and C-H bond dissociation. The significance of this isomerization is that the carbon atoms and hydrogen atoms belonging to the alkyl group are involved in an exchange with those atoms in the aromatic ring during isomerization. The dissociation rate of toluene at 193 nm is measured to be (1.17 +/- 0.1) x 10(6) s(-)(1).     

On-line monitoring by membrane introduction mass spectrometry of chlorination of organics in water. Mechanistic and kinetic aspects of chloroform formation

Chloroform formation during the chlorination of simple organic molecules modeling humic substances, such as phenol and di- and trihydroxybenzenes, was studied by on-line membrane introduction mass spectrometry (MIMS). Under the reaction conditions employed, chloroform was rapidly formed from 1,3-dihydroxybenzene, 1, 4-dihydroxybenzene, phenol and 1,2,3-trihydroxybenzene with yields of 17, 13, 7 and 5%, respectively. With the exception of aniline, which afforded a 17% chloroform yield, non-phenolic compounds, such as nitrobenzene, chlorobenzene, toluene, benzene and cyclohexanol, furnished low yields. Mechanistic studies showed that phenol is chlorinated consecutively and produces initially chlorophenol. It is suggested that chloroform might be formed mainly from chlorinated 3, 5-cyclohexadienone-type intermediates. MIMS was also used to determine the reaction rates and to study the kinetics of the chlorination. A good Hammett linear correlation for an electrophilic substitution mechanism was found for the compounds C(6)H(5)X (X = NH(2), OH, CH(3), H, Cl and NO(2)).     

H atom attack on propene

The reaction of propene (CH(3)CH═CH(2)) with hydrogen atoms has been investigated in a heated single-pulsed shock tube at temperatures between 902 and 1200 K and pressures of 1.5-3.4 bar. Stable products from H atom addition and H abstraction have been identified and quantified by gas chromatography/flame ionization/mass spectrometry. The reaction for the H addition channel involving methyl displacement from propene has been determined relative to methyl displacement from 1,3,5-trimethylbenzene (135TMB), leading to a reaction rate, k(H + propene) → H(2)C═CH(2) + CH(3)) = 4.8 × 10(13) exp(-2081/T) cm(3)/(mol s). The rate constant for the abstraction of the allylic hydrogen atom is determined to be k(H + propene → CH(2)CH═CH(2) + H(2)) = 6.4 × 10(13) exp(-4168/T) cm(3)/(mol s). The reaction of H + propene has also been directly studied relative to the reaction of H + propyne, and the relationship is found to be log[k(H + propyne → acetylene + CH(3))/k(H + propene → ethylene + CH(3))] = (-0.461 ± 0.041)(1000/T) + (0.44 ± 0.04). The results showed that the rate constant for the methyl displacement reaction with propene is a factor of 1.05 ± 0.1 larger than that for propyne near 1000 K. The present results are compared with relevant earlier data on related compounds.