Catalytic cracking of n-octane over zeolites with different pore structures and acidities

Catalytic cracking of n-octane has been studied over FAU, FER, MWW, MFI, BEA and MOR zeolites in order to investigate the effects of pore structure and acidity on their catalytic properties. The conversion of n-octane was largely dependent on the number of strong acid sites, while their pore structure determined the rate of catalyst deactivation due to carbon deposit. Continuous and high catalytic activity, therefore, was obtained on the MFI zeolite with sinusoidal pores and small Si/Al molar ratio, because its pores suppress the formation of large intermediate and its large number of strong acid sites accelerates the cracking of n-octane     

干胶法合成多级孔ZSM-5分子筛及其正辛烷催化裂解反应性能

采用干胶转化法制备多级孔ZSM-5分子筛,利用XRD、N_2吸附-脱附、SEM、TEM和NH_3-TPD等考察添加三嵌段共聚物(P123)、十六烷基三甲基溴化铵(CTAB)、十二烷基硫酸钠(SDS)和聚乙烯吡咯烷酮(PVP)等不同软模板剂合成分子筛样品的物化性质。结果表明,利用P123软模板剂合成的多级孔ZSM-5分子筛具有介孔(2.5 nm)-介孔(9.5 nm)-微孔复合的梯级孔结构,且分子筛酸强度有所增强。多级孔ZSM-5分子筛应用于正辛烷催化裂解反应中,具有较高的催化活性和低碳烯烃收率,其优异的催化性能可能归结于强酸性质和优异扩散传质能力之间的协同作用。     

噻吩、苯和正辛烷在Y型分子筛上的选择性吸附

采用重量法测定了温度为373 K、335 K和302 K时,噻吩、苯和正辛烷在NaY分子筛及CeY分子筛上的吸附等温线。结果表明,各温度下,噻吩在NaY分子筛和CeY分子筛上的饱和吸附量基本相同,并且其吸附量相对于苯和正辛烷较大。苯和正辛烷在CeY分子筛上的饱和吸附量很小,且较NaY分子筛上的吸附量明显的下降。CeY分子筛对噻吩有更高的选择性。     

八碳芳烃临氢异构化反应动力学模型

针对某实际工业异构化装置,在已开发的八碳芳烃临氢异构化反应网络的基础上,将系统中的八碳环烷烃和八碳链烷烃作为一个集总组分,提出新的六组分异构化反应网络,由此建立了适用于工业生产的八碳芳烃临氢异构化反应动力学模型.考虑结焦对催化剂活性的影响,提出了一种经验形式的催化剂失活函数,能够合理地描述催化剂失活过程.采用四五阶Runge-Kutta法对模型方程进行数值求解,基于多套稳态平衡数据采用差分变尺度优化算法(BFGS)对动力学参数进行估计,进而在不同操作条件下对模型进行验证.结果表明估计值与工业标定值相当吻合,达到了工业应用的模拟精度要求.     

噻吩、苯和正辛烷在NaY和CeY分子筛上的热脱附

采用智能质量分析仪(IGA),利用程序升温脱附法(TPD)研究了噻吩、苯和正辛烷在NaY和CeY分子筛上的热脱附行为,测定了升温速率为15 K/min时3种吸附质在NaY和CeY上的热重(TG)和微分热重(DTG)曲线。结果表明,3种吸附质在NaY上脱附比较容易,苯和正辛烷在CeY上脱附也很容易,但噻吩在CeY上脱附比较困难,噻吩有一部分不能完全脱附。CeY对噻吩具有较好的吸附选择性,而NaY的吸附选择性不好。     

正辛烷催化裂解反应规律及热力学分析

为了详细分析石脑油中正构烷烃的催化裂解行为,文中以正辛烷为原料,首先进行了单分子裂化反应的热力学分析,然后选择2种不同的催化剂,在固定床微反装置上考察了反应温度对正辛烷催化裂解的影响,计算了裂化机理比率及氢转移系数。结果表明:正辛烷单分子裂化反应易发生在中间的C—C键,直接生成氢气的反应难以发生; ZSM-5分子筛有利于单分子裂化反应,产物中烯烃的选择性较高,但是转化率过低; USY分子筛具有较高的转化率,但是因为氢转移能力较强,产物中烷烃的选择性太高。提高反应温度,有利于单分子裂化反应的发生,同时可以抑制氢转移反应。     

催化裂化汽油临氢异构化/芳构化改质过程的反应特性

采用工业Ni-Mo/Al₂O₃-HZSM-5催化剂在小型固定床加氢微反装置上对催化裂化（FCC）汽油临氢改质过程的反应特性进行了研究,通过考察反应温度、压力、空速和氢油体积比对改质后的FCC汽油烃类组成的影响,分析了汽油中不同烃类的转化性能。结果表明,氢油比对产物组成影响不大,高温、低压、低空速有利于增加芳烃的选择性,低温、高压、高空速则有利于增加异构烷烃的选择性;临氢改质后,FCC汽油的烯烃含量明显降低,芳烃和异构烷烃含量增加,因而产品汽油的辛烷值基本保持不变;全馏分、轻馏分和重馏分FCC汽油临氢改质实验结果表明,烯烃含量较高的轻馏分具有更高的转化活性;在FCC汽油临氢改质过程中,同碳数的端烯烃反应活性高于内烯烃,直链烯烃的反应活性高于支链烯烃。     

Catalytic cycles for hydrocarbon cracking on zeolites

Catalytic cracking of hydrocarbons takes place via elementary reactions such as initiation processes to form carbenium ions, olefin adsorption/desorption steps, isomerization, β-scission, oligomerization, and hydride ion transfer steps. These elementary steps involving reactive carbenium ion surface intermediates are the essential components of catalytic cracking that lead to various catalytic cycles and their corresponding stoichiometric reactions. This review outlines methods used to quantify reaction kinetics for catalytic cracking and describes how the rates of specific catalytic cycles are controlled by catalyst treatment (e.g., steaming) and process conditions (e.g., conversion and temperature). This revised version was published online in June 2006 with corrections to the Cover Date.     

Enhanced hydrocarbon cracking activity of Y zeolites

It has been known for a long time that faujasite zeolites that have been steamdealuminated to form USY zeolite are much more active than the unsteamed parent material for hydrocarbon cracking. A popular model to explain this phenomenon is the generation of active sites of unusually high activity. In this paper, a brief review of recent advances towards understanding this phenomenon is presented. Evidence suggests that steaming does not generate sites of new chemical or catalytic properties for cracking. An alternate model is summarized that incorporates a change in the dominant cracking reaction mechanism as a function of conversion, and raises the possibility that the rates of bimolecular and oligomeric cracking are influenced by pore diffusion.     

Hydroconversion of n-octane over nanoscale HZSM-5 zeolites promoted by 12-molybdophosphoric acid and Ni

12-Molybdophosphoric acid (PMo), Ni and Ni–PMo loaded on nanoscale HZSM-5 zeolites were prepared and characterized by BET, IR, XRD, Py-IR, NH₃-TPD, TG and SEM. The hydroconversion of n-octane over various catalysts was investigated in order to obtain light isomers alkanes and aromatics products with high-octane-number. The acid amounts of the catalysts were modified after the loading of PMo and Ni, and its relation to the activity of the reaction was discussed. It is concluded that the stability of aromatization is improved, and the yield of light iso-alkanes is enhanced due to the introduction of PMo and Ni. The improved activity of n-octane hydroconversion over PMo and Ni loaded nanoscale HZSM-5 zeolites could be attributed to the increase of the acid amounts and the synergetic effect between Brønsted and Lewis acid sites.     

Entropy considerations in monomolecular cracking of alkanes on acidic zeolites

Compensation between adsorption entropies and enthalpies results in less than a two-fold variation in adsorption equilibrium constants for C₃–C₆ alkanes at temperatures relevant for monomolecular cracking; the size-independent activation energy for CC bond activation in C₃–C₆ alkanes indicates that the marked increase in monomolecular cracking turnover rates observed with alkane chain size reflects a concurrent increase in activation entropies. Thermodynamic treatments for non-ideal systems rigorously describe confinement effects within zeolite channels and show that pre-exponential factors depend on solvation effects of the zeolite-host environment through variations in the thermodynamic activity of the zeolitic proton. Observed differences in rates and selectivities of monomolecular alkane activation with zeolite structure, after normalization to intrazeolitic concentrations, reflect differences in intrinsic rate constants.     

正己烷在HZSM-5分子筛上的催化裂解研究

为筛选反应活性和烯烃选择性相对较高的催化剂用于研究吸热型碳氢燃料的催化裂解,以正己烷的催化裂解作为探针反应,探讨其在不同硅铝物质的量比HZSM-5［n(Si)∶n(Al)=25、36、100］分子筛上催化裂解的反应活性和产物分布。结果表明,正己烷在HZSM-5分子筛上的裂解转化率随温度的升高和分子筛中硅铝物质的量比的减小而增大;裂解产物中乙烯、丙烯和总烯烃的选择性均随裂解温度的升高和分子筛中硅铝物质的量比的增加而增加,在（300~550） ℃,HZSM-5［n（Si)∶n(Al)=36］上的总烯烃收率最高,芳烃含量随分子筛中硅铝物质的量比的增加而减小;基于裂解转化率、烯烃和芳烃收率等因素综合考虑,HZSM-5 n(Si)∶n(Al)=36］分子筛为优选催化剂。     

C_8芳烃在SKI-400-40催化剂上的临氢异构化反应动力学

利用固定床积分反应实验装置,在反应温度为365～410℃、空速为4～128 h~(-1)、氢烃物质的量比为3.8、反应压力为1.1 MPa条件下,对C_8芳烃在SKI-400-40催化剂上的临氢异构化宏观反应动力学进行了研究。采用六组分拟一级异构化反应网络,建立了C_8芳烃临氢异构化的宏观反应动力学模型,模型计算值与实验值吻合良好。得到的反应速率常数表明,二甲苯和乙苯间的转化可通过两种方式,一是两者之间的直接转化,二是两者通过C_8N+P的桥接转化,其中第二种方式的贡献更大。     

对二甲苯/间二甲苯在KX和CaX上的吸附平衡和动态吸附性能

通过静态吸附实验,测定了对二甲苯、间二甲苯在分子筛KX和CaX上的吸附速率曲线及吸附等温线;动态吸附穿透实验测定了对二甲苯/间二甲苯混合液在固定床中的连续流出曲线。结果表明,对二甲苯、间二甲苯在CaX上的静态饱和吸附量和动态穿透吸附量均高于KX,但CaX对对二甲苯的吸附选择性不及KX;Langmuir吸附等温线方程可以很好地拟合对二甲苯、间二甲苯在CaX上的等温线;二级吸附速率方程可以较好地描述对二甲苯、间二甲苯的吸附动力学行为。     

Theoretical studies of pentene cracking on zeolites: C–C β-scission processes

The C–C bond-breaking step of pentene adsorbed on a model zeolite cluster is examined using ab initio and density functional theory (DFT/B3LYP) electronic structure techniques as an example of the β-scission process that arises in cracking of alkanes and alkenes. After pentene has been protonated by the acid site, the reactant for the cracking process corresponds to a pentyl cation covalently bound to the oxygen of the zeolite, ZO⁻–C₅H⁺₁₁. The product of the C–C bond-breaking process is propene plus an ethyl cation bound to a neighboring oxygen. The energy of the transition state relative from B3LYP calculations is 60 kcal/mol relative to the pentyl cationic reactant. For the case of the branched olefin methyl pentene, the transition state energy is slightly lower (55 kcal/mol), but the overall reaction energy is essentially the same as for pentene. The results are compared to the case of the gas phase pentyl carbenium ion.     

Sorption kinetics study of the diethylbenzene isomers in MFI-type zeolites

The uptake rates of the three diethylbenzene isomers in MFI-type zeolites were studied with a sorption kinetics apparatus of the barometric type. It was observed that the uptake rates depend crucially on the position of the substituted ethyl groups. While p-diethylbenzene was adsorbed with uptake rates of the same order of magnitude as benzene and the monosubstituted benzenes, diffusion of m-diethylbenzene was approximately four orders of magnitude slower. No sorption uptake was observed for o-diethylbenzene. In the experiments with H-ZSM-5, sorption kinetics were partly determined by a secondary process additional to Fickian diffusion. Comparison to the results obtained with silicalite-1 and earlier results proved that the secondary process is correlated with the existence of acidic sites inside the zeolite and can be tentatively ascribed to an isomerization reaction.     

磷改性ZSM-5沸石的催化裂化性能

用磷改性ZSM-5沸石,并将其制备成催化剂。初步考察了其物化性能的变化,并在固定床重油微反装置上考察了其催化裂化反应特性。研究发现,磷能显著改善ZSM-5沸石的活性、稳定性和选择性。ZSM-5沸石上不同的磷含量和其与不同的Y型分子筛配伍对催化裂化反应有着显著的影响。合理调配催化裂化催化剂的活性组分,适当控制其氢转移能力和烷基化能力,对增产丙烯有着积极的意义。