Improved Performance of W／HZSM-5 Catalysts for Dehydroaromatization of Methane

The dehydroaramatization of methane over W-supported ZSM-5 with varying degrees of Li^ ion-exchanged catalysts was studied with and without oxygen at 1073 K and atmospheric pressure.Catalyst activity and stability were found to be influenced by the catalyst acidity related to BrSnsted acid sites and by the presence of oxygen in the feed. The NH3-TPD and FTIR-pyridine results demonstrated that partially exchanged of H^ ions by Li^ into the W/HZSM-5 catalysts could be used to control the amount of strong acid sites on the catalyst surface. Without oxygen, the 3WHLi-Z (5:1) catalyst that has strong acid sites equal to nearly 74% of the original strong acid sites in the parent HZSM-5 exhibited the highest methane conversion and selectivity towards aromatics. However, the catalyst deactivated in a five hour period. In the presence of oxygen, the catalyst activity and stability could be improved further.The results of this study revealed that a suitable amount of strong Bronsted acid sites as well as oxygen addition in the feed increased the catalyst activity and stability. The 3WHLi-Z(5:1) catalyst exhibited improved performance in the dehydroaromatization of methane.     

Coking behaviors and kinetics on HZSM-5/SAPO-34 catalysts for conversion of ethanol to propylene

The coke deposition on HZSM-5/SAPO-34 composite catalysts has been studied in the conversion of ethanol to propylene. The HZSM-5/SAPO-34 composite catalysts were synthesized by hydrothermal method(ZS-HS) and fully blending(ZS-MM). The used catalysts were characterized by XRD, N_2 adsorption–desorption, TGA, TPO, elemental analysis, FTIR and XPS. The coking kinetics on both ZS-HS and ZS-MM has been investigated and their coking rate equations were obtained. The used ZS-MM catalyst had higher amount of coke and lower nC:nHthan the used ZS-HS. 90% of the coke was deposited in the micropores of ZS-HS, while almost 45% of the coke located in the micropores of ZS-MM. The coke deposited on ZS-HS catalyst was mainly graphite-like carbon species, whereas dehydrogenated coke species was the major on ZS-MM. The coking activation energy of ZS-MM was lower than that of ZS-HS, and the coking rate on ZS-MM was faster than on ZS-HS. In addition, the regeneration of ZS-MM catalyst showed that it had a good hydrothermal stability. The differences on coking behaviors on the two catalysts were due to their different acidic properties and textures.     

Study on the alkylation reaction of guanine and N-acetylguanine and the synthesis of 1''''-C alkylated carba-DHPG analogues

The alkylation reaction of guanine and N-acetylguanine with model compounds such as isopropyl bromide or 4-heptyl tosylate were studied. The reaction conditions such as temperature, solvent, base, and catalyst were examined for their effects on the reaction rate, and the yield and regioselectivity of the coupling reaction. The highest yield was obtained by using DMSO as the solvent. The reaction proceeded in a homogenous manner to give higher yield of 9-N and 7-N substituted product in a mole ratio of 1:1. The ratio could be raised to 2:1 if dibenzo-18-crown-6 was used as a catalyst. Using the above procedure, three carba-DHPG analogues bearing different 1'-C alkyl side chains were synthesized.     

Methane dehydroaromatization with periodic CH4-H2 switch:A promising process for aromatics and hydrogen

Long-term stability test of Mo/HZSM-5-N catalysts(HZSM-5-N stands for nano-sized HZSM-5) in methane dehydroaromatization(MDA)reaction has been performed with periodic CH4-H2 switch at 1033-1073 K for more than 1000 h.During this test,methane conversion ranges from 13% to 16%,and mean yield to aromatics(i.e.benzene and naphthalene) exceeds 10%.N2-physisorption,XRD,NMR and TPO measurements were performed for the used Mo/HZSM-5 catalysts and coke deposition,and the results revealed that the periodic hydrogenation can effectively suppress coke deposition by removing the inert aromatic-type coke,thus ensuring Mo/HZSM-5 partly maintained its activity even in the presence of large amount of coke deposition.The effect of zeolite particle size on the catalytic activity was also explored,and the results showed that the nano-sized zeolite with low diffusion resistance performed better.It is recognized that the size effect was enhanced by reaction time,and it became more remarkable in a long-term MDA reaction even at a low space velocity.     

Effects of Temperature and Catalyst to Oil Weight Ratio on the Catalytic Conversion of Heavy Oil to Propylene Using ZSM-5 and USY Catalysts

It is useful for practical operation to study the rules of production of propylene by the catalytic conversion of heavy oil in FCC (fluid catalytic cracking). The effects of temperature and C/O ratio (catalyst to oil weight ratio) on the distribution of the product and the yield of propylene were investigated on a micro reactor unit with two model catalysts, namely ZSM-5/Al2O3 and USY/Al2O3, and Fushun vacuum gas oil (VGO) was used as the feedstock. The conversion of heavy oil over ZSM-5 catalyst can be comparable to that of USY catalyst at high temperature and high C/O ratio. The rate of conversion of heavy oil using the ZSM-5 equilibrium catalyst is lower compared with the USY equilibrium catalyst under the general FCC conditions and this can be attributed to the poor steam ability of the ZSM-5 equilibrium catalyst. The difference in pore topologies of USY and ZSM-5 is the reason why the principal products for the above two catalysts is different, namely gasoline and liquid petroleum gas (LPG), repspectively. So the LPG selectivity, especially the propylene selectivity, may decline if USY is added into the FCC catalyst for maximizing the production of propylene. Increasing the C/O ratio is the most economical method for the increase of LPG yield than the increase of the temperature of the two model catalysts, because the loss of light oil is less in the former case. There is an inverse correlation between HTC (hydrogen transfer coefficient) and the yield of propylene, and restricting the hydrogen transfer reaction is the more important measure in increasing the yield of propylene of the ZSM-5 catalyst. The ethylene yield of ZSM-5/Al2O3 is higher, but the gaseous side products with low value are not enhanced when ZSM-5 catalyst is used. Moreover, for LPG and the end products, dry gas and coke, their ranges of reaction conditions to which their yields are dependent are different, and that of end products is more severe than that of LPG. So it is clear that maximizing LPG and propylene and restricting dry gas and coke can be both achieved via increasing the severity of reaction conditions among the range of reaction conditions which LPG yield is sensitive to.     

Coking kinetics and influence of reaction-regeneration on acidity,activity and deactivation of Zn/HZSM-5 catalyst during methanol aromatization

The coking kinetics and reaction-regeneration on Zn/HZSM-5(Zn/HZ) catalyst in the conversion of methanol to aromatics were investigated.The highest initial benzene, toluene and xylene(BTX) yield of ca. 67.7% was obtained on fresh Zn/HZ catalyst, which showed the worst catalytic stability. The cycle of reaction-regeneration significantly modified the texture and acidity of Zn/HZ catalyst, which in turn affected its catalytic performance and coking behavior in methanol conversion to BTX. The residual carbon located on the surface of Zn/HZ catalyst led to the decrease of acid sites and the change on the acid sites distribution, which played an important roles on its activity and deactivation. It was found that the high B/L ratio and the low total acid sites concentration of the Zn/HZ catalyst favored to the high BTX yield and good catalytic stability in methanol conversion.     

Comparative study of fluidized-bed and fixed-bed reactor for syngas methanation over Ni-W/TiO₂-SiO₂catalyst

In this work,syngas methanation over Ni-W/TiO2-SiO2catalyst was studied in a fluidized-bed reactor(FBR)and its performance was compared with a fixed-bed reactor(FIXBR).The effects of main operating variables including feedstock gases space velocity,coke content,bed temperature and sulfur-tolerant stability of 100 h life were investigated.The structure of the catalysts was characterized by XRD,N2adsorptiondesorption and TEM.It is found that under same space velocity from 5000 h 1to 25000 h 1FBR gave a higher CH4yield,lower coke content,and lower bed temperature than those obtained in FIXBR.Ni-W/TiO2-SiO2catalyst possessed excellent sulfur-tolerant stability on the feedstock gases less than 500 ppm H2S in FBR.The carbon deposits formed on the spent catalyst were in the form of carbon fibers in FBR,while in the form of dense accumulation distribution appearance in FIXBR.     

Mo／MCM－49催化剂上甲烷的脱氢芳构化反应

In recent years, the catalytic dehydroaromatization of methane to benzene has been a challenging and intriguing research subject in heterogeneous catalysis and natural gas chemical industry. Since the first report on non-oxidative conversion of methane to benzene on Mo/HZSM-5[1], many attractive studies regarding the development of new catalyst, the catalyst characterization[2～4], the reaction mechanism and the bifunctional role of Mo carbide and HZSM-5 have been reported. However, Mo/HZSM-5 catalyst was thought to have the best catalytic performance for CH4 dehydroaromatization to benzene. Recently, another kind of zeolite, MCM-22, was found to be a promising catalyst support for formation of benzene from methane[5]. Xu et al[6] studied the catalytic performance of MoO3/MCM-49 for this reaction without accounting for coke formation. In this paper, we present MCM-49 as a support, and the catalyst Mo/MCM-49 shows high ratio of benzene to aromatics in selective dehydroaromatization of methane.     

Activation of Mo/HZSM-5 for methane aromatization

The effect of Mo/HZSM-5 pretreatment at 973 K in inert (He), oxidizing (artificial air), and carbu-rizing (CH4/He mixture) atmospheres on its performance in non-oxidative methane dehydroaroma-tization (MDA) was investigated. The effect of post-synthesis silylation on deactivation of external acid sites was also studied. Precarburization resulted in increased aromatic selectivity and im-proved catalyst stability. The benzene selectivity was the highest for the silylated Mo/HZSM-5 cata-lyst (benzene+naphthalene selectivity after 1 h on stream was close to 100%). The deactivation of precarburized zeolites was less pronounced than that of zeolites heated in air or He. During heating in air or He, larger fractions of the molybdenum oxide species diffused into the micropores than during heating in methane. Carburization of the molybdenum oxide species in the micropores dur-ing MDA resulted in the formation of molybdenum carbide particles, and these contributed to pore blocking, making the Br?nsted acid sites inaccessible. The formation of molybdenum carbides dur-ing heating in methane resulted in a less mobile Mo phase. It is argued that the presence of molyb-denum carbide particles in the micropores contributes to rapid catalyst deactivation, in addition to the formation of hard coke on the external surface.     

Effects of P content in a P/HZSM-5 catalyst on the conversion of ethanol to hydrocarbons

A series of P/HZSM-5 catalysts prepared by impregnation method were used for ethanol conversion to lower olefins. The catalysts were characterized by X-ray diffraction (XRD), NH3-temperature-programmed desorption (NH3-TPD) and N2 adsorption-desorption measurements. It was found that the P/HZSM-5 catalysts showed high activity and selectivity toward light olefins. The selectivities of propylene and butylene can be improved with the introduction of phosphorus (P). When the content of P reached 3.0 wt%, more than 18.9% propylene in the gaseous products was obtained over the P/HZSM-5 catalyst at 450 ?C. The introduction of P modified the strong Br?nsted acid sites of the original HZSM-5 catalysts and P/HZSM-5 catalysts could resist coke formation and showed good stability.     

Hydrogen Effect on Coke Removal and Catalytic Performance in Pre-Carburization and Methane Dehydro-Aromatization Reaction on Mo/HZSM-5

1. Introduction As an effective utilization of methane, the methane dehydro-aromatization was focused in the last decade [1-28]. Over the Mo/HZSM-5 bi- functional catalyst at high reaction temperature, methane can be converted into light aromatics (ben- zene and naphthalene) and hydrogen. Mo active species can activate the C—H bond of methane; and HZSM-5 supplies the acid sites for the oligomeriza- tion and cyclization of hydrocarbons to form aromat- ics, and suppresses the deeper condens…     

程序升温技术研究Mo/HZSM-5催化剂上甲烷无氧芳构化反应积炭的化学特性

对经过程序升温表面甲烷无氧芳构化反应(TPSR)后的Mo/HZSM-5催化剂上的积炭进行了程序升温加氢反应(TPH)和程序升温CO₂反应(TPCO₂),并对相应催化剂上的积炭进行了程序升温氧化反应(TPO)和热重(TG)实验.TPH实验及TPCO₂实验表明,H₂主要对高温烧炭峰发生作用.CO₂则可同时对这两种烧炭峰产生影响,对低温峰积炭的影响尤其明显.对不同Mo含量的催化剂进行上述实验的结果表明,Mo担载量的变化及不同的实验过程都会影响催化剂上两种温峰的积炭在总积炭量中所占的比例.由TG实验数据计算出的动力学结果说明,无论是TPH还是TPCO₂实验都可以降低烧炭过程的一级反应起始温度,并减少其活化能.此外,催化剂中Mo担载量的变化也会对开始温度和烧炭活化能产生明显影响.     

Methane dehydroaromatization with periodic CH_4-H_2 switch:A promising process for aromatics and hydrogen

Long-term stability test of Mo/HZSM-5-N catalysts(HZSM-5-N stands for nano-sized HZSM-5) in methane dehydroaromatization(MDA)reaction has been performed with periodic CH4-H2 switch at 1033-1073 K for more than 1000 h.During this test,methane conversion ranges from 13% to 16%,and mean yield to aromatics(i.e.benzene and naphthalene) exceeds 10%.N2-physisorption,XRD,NMR and TPO measurements were performed for the used Mo/HZSM-5 catalysts and coke deposition,and the results revealed that the periodic hydrogenation can effectively suppress coke deposition by removing the inert aromatic-type coke,thus ensuring Mo/HZSM-5 partly maintained its activity even in the presence of large amount of coke deposition.The effect of zeolite particle size on the catalytic activity was also explored,and the results showed that the nano-sized zeolite with low diffusion resistance performed better.It is recognized that the size effect was enhanced by reaction time,and it became more remarkable in a long-term MDA reaction even at a low space velocity.     

HZSM-5型分子筛硅铝比对一步法合成二甲醚的影响

以Cu/Zn/Al(摩尔比为6∶3∶1)甲醇合成催化剂与HZSM-5型分子筛混合，制备了一步法二甲醚合成催化剂。通过改用三种不同Si/Al摩尔比（摩尔比为25、38和50）的HZSM-5型分子筛，考察了催化剂中脱水组分（HZSM-5分子筛）的酸性对二甲醚合成的影响。结果表明，随着催化剂Si/Al摩尔比的降低，分子筛的酸性增强，使得CO单程转化率提高。当催化剂Si/Al＝38时，CO对二甲醚的选择性最高，可达到68.13％，其次是催化剂Si/Al＝50，选择性最差的是Si/Al＝25的催化剂。在553 K、 3 MPa和4 000 h－1的条件下，Si/Al＝25和Si/Al＝38的催化剂CO单程转化率和DME的选择性接近一致。在此条件下，两者的时空产率达到试验的最大值，分别为0.38 gDME/(gcat·h)和0.36 gDME/(gcat·h)，在试验范围内，一步法合成二甲醚催化剂最佳的Si/Al摩尔比为25。     

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.     

W掺杂对Mo/HZSM-5催化甲烷无氧芳构化性能的影响

采用浸渍法制备了Mo/HZSM-5、Mo-W/HZSM-5和W/HZSM-5三种催化剂。通过XRD、BET、Py-FTIR、H2-TPR、XPS、TEM、NH3-TPD、TPO、TG和Raman等技术对催化剂的物化性质进行表征,并考察其在甲烷无氧芳构化反应中的催化性能。结果表明,相比于Mo/HZSM-5,Mo-W/HZSM-5催化剂表现出更高的CH_4转化率、芳烃收率以及催化稳定性。H_2-TPR和XPS结果表明,Mo-W/HZSM-5中存在更易被还原为W~(4+)的正八面体(WO_6)~(n-)前驱体,反应过程中W4+的形成有助于提高CH_4转化率。同时,积炭表征结果表明,石墨型积炭是导致Mo/HZSM-5催化剂快速失活的主要原因,W掺杂可以抑制MoW/HZSM-5催化剂上石墨型积炭的形成,进而提高催化剂的稳定性。     

反应条件对Ni-CaO-ZrO2催化剂上CH4-CO2重整反应及积炭的影响

用共沉淀法制备了Ni-CaO-ZrO₂催化剂,并将其用于CH₄-CO₂重整反应。考察了反应温度、空速和反应物配比对催化剂性能和积炭的影响。通过热力学计算和实验表征研究发现,反应条件对CH₄-CO₂重整反应结果和积炭有重要的影响。由于高温条件同时有利于CH₄裂解和碳物种的及时消除,升高温度可以提高催化剂的活性和稳定性。增大空速则使CH₄的转化率和消碳反应的速率均降低,导致积炭量增加。同时,反应物配比对催化剂表面的积炭量也有很大影响;稍高的CO₂/CH₄摩尔比有利于抑制CH₄-CO₂重整反应过程中的积炭。     

生物质气经由二甲醚两步法制备低碳烯烃

制备出NiSAPO-34及NiSAPO-34／HZSM-5催化剂,考察了其对二甲醚催化转化制备低碳烯烃的性能．利用Cu／Zn／Al／HZSM-55u筛选出的2％NiSAPO—34／HZSM-5催化剂进行生物质气经由二甲醚两步法制备低碳烯烃的实验,结果表明在SAPO-34上添2H2％的Ni不改变其结构,但降低了酸中心数量,并生成了较强的酸中心．添加少量具有稳定酸中心的HZSM-5,该催化剂的活性提高到3h以上,反应进行2h获得了最高的低碳烯烃选择性为90．8％．当把该催化剂应用到两步催化转化过程的第二个反应器中,其高催化活性可达5h以上．当以低氢碳比生物质气（H2／CO／CO2／N2／CH4=41．5／26．9／14．2／14．6／2．89）作为原料时,经两步转化,低碳烯烃的收率达到84．6g／m^3syngas.     

甲烷无氧芳构化反应中Co助剂对Mo／HZSM-5

 甲烷在Co－Mo／HZSM－5催化剂上进行无氧芳构化反应的评价结果表明，Co的添加大大提高了Mo／HZSM－5催化剂在反应过程中的稳定性．BET实验证明，反应后的积炭对Co－Mo／HZSM－5催化剂孔道堵塞的程度较小．对积炭催化剂进行的一系列程序升温表面反应（如TPH，TPCO2和TPO）结果表明，TPO谱上有两个峰温明显不同的烧炭峰，Co的添加明显抑制了高温积炭的生成．H2主要与高温积炭发生反应，这部分积炭是催化剂失活的主要原因；CO2对低温积炭的影响则尤为明显．TEM结果表明，积炭催化剂上存在丝状积炭物种．碳丝不能与H2反应，但能被CO2除去．Co的添加促进了丝状积炭物种的生成，碳丝并不是导致催化剂失活的因素．     

用于甲烷芳构化的Mo/HZSM-5催化剂的活化（英文）

The effect of Mo/HZSM-5 pretreatment at 973 K in inert(He), oxidizing(artificial air), and carburizing(CH4/He mixture) atmospheres on its performance in non-oxidative methane dehydroaromatization(MDA) was investigated. The effect of post-synthesis silylation on deactivation of external acid sites was also studied. Precarburization resulted in increased aromatic selectivity and improved catalyst stability. The benzene selectivity was the highest for the silylated Mo/HZSM-5 catalyst(benzene + naphthalene selectivity after 1 h on stream was close to 100%). The deactivation of precarburized zeolites was less pronounced than that of zeolites heated in air or He. During heating in air or He, larger fractions of the molybdenum oxide species diffused into the micropores than during heating in methane. Carburization of the molybdenum oxide species in the micropores during MDA resulted in the formation of molybdenum carbide particles, and these contributed to pore blocking, making the Brnsted acid sites inaccessible. The formation of molybdenum carbides during heating in methane resulted in a less mobile Mo phase. It is argued that the presence of molybdenum carbide particles in the micropores contributes to rapid catalyst deactivation, in addition to the formation of hard coke on the external surface.