fulltext(5)

Determination of the nature of distinct catalytic sites in

hydrodenitrogenation by competitive adsorption

M.Jian and R.Prins

Laboratory for Technical Chemistry,Federal Institute of Technology(ETH),8092Zurich,Switzerland

E-mail:prins@tech.chem.ethz.ch

Received8September1997;accepted8November1997

The inhibitive effect of nitrogen-containing compounds on hydrodenitrogenation was studied over NiMo(P)/Al2O3catalysts. From the differences in the adsorption constants it is concluded that at least four distinct catalytic sites are involved in the elemen-tary hydrodenitrogenation steps.The catalytic site for the cleavage of aliphatic C(sp3)^N bonds is acidic and most probably an SH species on a surface Mo or Ni site.The catalytic site for the hydrogenation of a phenyl group is different from that for alkene hydro-genation,the former being more coordinately unsaturated than the latter,with two or three sulphur vacancies.A fourth site is responsible for the hydrogenolysis of the C(sp2)^N bond of anilines.It is characterised by a more reduced environment of Mo and by phosphorus promotion.

Keywords:hydrodenitrogenation,catalytic sites,competitive adsorption

1.Introduction

Hydroprocessing ranks among the most important processes in the petroleum industry.The main hydropro-cessing reactions are hydrodesulphurization,hydrodeni-trogenation,hydrodeoxygenation,and hydrodemetal-ization[1,2].A variety of chemically different reactions are involved in the hydrodenitrogenation(HDN)pro-cess,namely:hydrogenation of alkenes,hydrogenation of carbocyclic or heterocyclic aromatic rings,hydroge-nation of the phenyl group of aniline molecules,aliphatic C(sp3)^N bond cleavage,and aromatic C(sp2)^N bond cleavage.There are clear indications in the literature that these reactions take place on different catalytic sites. Thus,Satterfield and co-workers observed that the pres-ence of H2S in the feed substantially enhanced the C(sp3)^N bond cleavage in the HDN of quinoline,but slightly inhibited the hydrogenation of the quinoline ring over a sulphided NiMo/Al2O3catalyst[3,4].In a com-parative HDN study of aniline,cyclohexylamine,and N-methylaniline over a NiMo/Al2O3catalyst,Shanthi et al.concluded that distinct catalytic sites are needed for C(sp3)^N and C(sp2)^N bond cleavage[5].They found that H2S had a promotional effect on the breaking of the C(sp3)^N bond,but an inhibitive effect on the cleavage of the C(sp2)^N bond.

The necessity of different catalytic sites for C(sp2)^N bond cleavage and hydrogenation of a phenyl group has been demonstrated by the HDN product composition, influence of reaction conditions,and catalyst composi-tion in the HDN of alkylanilines[6,7].A kinetic study of the hydrodeoxygenation of p-cresol which is similar to the HDN of p-toluidine,showed that the inhibitive adsorption constants of ammonia and o-cresol were higher on the active site for C(sp2)^O bond cleavage than on the site for the phenyl ring hydrogenation,and the effect of H2S on the two parallel reactions was different over NiMo/Al2O3and CoMo/Al2O3catalysts[8]. Due to the basic properties of the nitrogen-containing compounds in oil fractions,and the acidic properties of the hydrotreating catalysts,nitrogen-containing mole-cules adsorb strongly during the hydrotreating process. They occupy the active sites on the catalyst surface and inhibit their own as well as other hydrotreating reactions [9^13].A well known inhibitive effect is that of quino-line-type compounds on the HDN of anilines:Pure ani-lines can be converted almost completely to nitrogen-free hydrocarbons under normal HDN conditions,while the presence of quinoline-type compounds strongly inhi-bits this reaction[14^16].The much stronger inhibitive effect of quinoline-type compounds on the HDN of alkylanilines than on their own HDN,suggests that the two reactions take place on distinct catalytic sites.It explains that the concentration of alkylanilines can be much higher in the product of hydrotreatment than in the feed[17].

Notwithstanding all this evidence of different sites for different reactions,it has generally been ignored in kinetic studies.Almost all kinetic HDN studies lump the kinetic equations by assuming that all substrate and intermediate molecules have the same adsorption con-stants.Only a few studies have recognised the possibility that more adsorption constants may be needed,but even they do not go further than a maximum of two distinct adsorption constants[12].

In the present study we show that these simplified assumptions in the kinetic HDN schemes strongly dis-tort the truth.In reality,every single step needs its own

Catalysis Letters50(1998)9^139?J.C.Baltzer AG,Science Publishers

adsorption constant,and thus its own catalytic site. These adsorption constants have been determined by studying the inhibitive effects of nitrogen-containing substrates on a variety of chemically different reactions which are part of the HDN network of quinoline.The nature of the catalytic sites is discussed and correlated with the catalyst properties.

2.Experimental

Catalysts used in this work had a composition of3 wt%nickel,8wt%molybdenum and0or2wt%phos-phorus.Details of the catalyst preparation can be found elsewhere[18].The reactions were carried out in a con-tinuous-flow microreactor.A sample of0.1g catalyst diluted with9.5g SiC was sulphided in situ with a mix-ture of10%(mol)H2S and H2at643K and1.5MPa for 4h.After sulphidation the pressure was increased to3.0 MPa and liquid reactant was fed to the reactor by means of a high pressure pump,with n-octane as the solvent. Dimethyldisulphide was added to the liquid reactant to generate H2S in the reaction stream(usually P H

2S 6X5kPa).

Reaction products were analyzed on-line with a Shimadzu GC-14A gas chromatograph equipped with a 30m DB-5fused silica capillary column(J&W,0.32mm ID,0.25"m film thickness)and a flame ionization detec-tor.Samples were taken after100h on stream at643K when the activity of the catalyst was relatively stable, with n-nonane and n-dodecane as internal standards. The space time was changed by varying the liquid and gaseous reactant flow rates while keeping their ratio con-stant[19].The reproducibility of the measurements is ?10%.

3.Results

The adsorption constant of a molecule on a catalytic site provides a quantitative measure of its strength of adsorption on this site,and a comparison of such adsorption constants gives direct evidence of the similar-ity or difference of the sites for different reactions.When Langmuir^Hinshelwood adsorption is assumed,the reaction rate of substrate A in a heterogeneous catalytic reaction can be written as

àd P A

d(

k A K A P A

1

i

K i P i

k A K A P A

1 K A P0A

Y 1

where(is the space time,P A the partial pressure of reac-tant A(P0A is the initial partial pressure),k A the rate con-stant,and K A the adsorption constant of A.K i and P i represent respectively the adsorption constant and par-tial pressure of compound i in the reaction stream which can adsorb on the surface site.Since the nitrogen-con-taining substrates adsorb much stronger than all other compounds,eq.(1)can be simplified by neglecting the adsorption terms of other substrates,and by assuming a constant adsorption term of reactant A in the denomina-tor at low conversions(x A`157).The adsorption con-stant K A can thus be easily calculated by regression analysis[7].Since hydrogen adsorbs weakly and is in large excess,the effect of hydrogen has been lumped into the rate constant k A.

Equation(1)can also be integrated to eq.(2):

(

àln 1àx A

1

k A K A

1

k A

áP0A X 2

By measuring the conversion x A at different initial reac-tant partial pressures P0A,the adsorption constant K A can be calculated from the slope and intercept of the à(a ln 1àx A versus P0A plot.Figure1gives an example for the HDN of ortho-propylaniline(OPA).The simpli-fied equations(1)and(2)can only be used for relatively simple systems,however,for more complex cases,a complete regression analysis including variation of(as well as P0A has to be performed[20].

The inhibition adsorption constant of a nitrogen-con-taining substrate on the catalytic site on which another weakly adsorbing substrate reacts(such as alkenes)can be determined through simultaneous reactions.At low conversion of the inhibitor A(x A`157),the conver-sion of the alkene B can be written as

à

d P B

d(

k B K B P B

1

i

K i P i

k B K B P B

1 K H A P0A

Y 3 àln 1àx B

k B K B

H

A

A

á(Y 4

where k B,K B,P B,and x B are the rate constant,adsorp-tion constant,partial pressure,and conversion of the weakly adsorbing alkene B,respectively.K H A is the adsorption constant of the strongly adsorbing molecule A on the catalytic site for the hydrogenation of alkene

B.

Figure1.Determination of the kinetic parameters in the HDN of ortho-propylaniline over NiMo/Al2O3at623K and3.0MPa by

eq.(2).

M.Jian and R.Prins/Catalytic sites and competitive adsorption in HDN 10

Since k B K B can be calculated from the reaction of pure

B (P i 0and k B K B (1),K H

A

is then calculated from the slope of the àln 1àx CHE versus (plot in the presence of inhibitor P A .Examples are given in figure 2.This method can also be applied to determine the inhibitive adsorption constant of quinoline-type compounds on the conversion of OPA,an intermediate in the HDN net-work of quinoline whose HDN is strongly inhibited,and for which the inhibitive effect has never been quantified.The experimental results are shown in figure 3.

It proved impossible to distinguish the adsorption constants of quinoline (Q)and 1,2,3,4-tetrahydroquino-line (THQ1),as well as of 5,6,7,8-tetrahydroquinoline (THQ5)and decahydroquinoline (DHQ)in the above measurement due to fast (de)hydrogenation of the het-erocyclic ring of quinoline.The linearity of figure 3also indicates that the inhibition strengths of Q and THQ5(and of THQ1and DHQ)on the hydrogenation of the phenyl group of OPA differ hardly.

Once the inhibitive adsorption constant of DHQ on the catalytic site for C(sp 3)^N bond cleavage is obtained,the inhibitive adsorption constant of anilines on the C(sp 3)^N bond cleavage site can be calculated from eq.(6)by comparing the HDN results of DHQ with those of DHQ OEA (OEA ortho-ethylaniline,similar to OPA)at low conversions,and by assuming that all the quinoline-type compounds exhibit the same adsorption strength on the C(sp 3)^N bond cleavage site:

àd P DHQ Y alone d ( k DHQ K C àN

DHQ áP DHQ

1 K C àN DHQ áP 0DHQ

and

àd P DHQ Y OEA

d ( k DHQ K C àN

DHQ áP DHQ C àN DHQ 0DHQ C àN OEA 0OEA

Y 5 thus

ln 1àx DHQ Y alone

ln 1àx DHQ Y OEA 1 K C àN OEA áP 0OEA 1 K C àN DHQ áP 0DHQ

Y 6

where x DHQ Y alone and x DHQ Y OEA are the C^N bond cleav-age conversions of DHQ in the HDN of DHQ alone and

in the simultaneous reaction of DHQ and OEA,respec-tively;P 0DHQ

and P 0

OEA are the initial partial pressures of DHQ and OEA in the reactant,K C àN DHQ

and K C àN

OEA the inhi-bitive adsorption constants of OEA and DHQ on the catalytic site for the C(sp 3)^N bond cleavage,respec-tively;and k DHQ is the rate constant of the C(sp 3)^N bond cleavage of DHQ.The adsorption constants obtained by the methods described above are presented in table 1.The 95%confidence ranges of these constants are all within ?20%of the reported values.4.Discussion

4.1.The catalytic site for hydrogenation

Hydrotreating catalysts have usually been regarded as acidic and therefore gas phase proton affinities of nitrogen-containing substrates have been correlated with their adsorption constants on the catalysts.Nagai et al.reported a linear correlation between gas phase proton affinities of a series of nitrogen-containing com-pounds and their inhibitive adsorption constants on the hydrodesulphurization of dibenzothiophene over a NiMo/Al 2O 3catalyst [21],although the measured adsorption constants of saturated amines were lower than expected on the basis of their gas phase proton affi-nities.A similar discrepancy exists for the quinoline-type compounds as indicated by figures 2and 3:the adsorp-tion constants of all four quinoline-type substrates are about equal,although the gas phase proton affinities of DHQ and THQ1are much higher than those of Q and THQ5[22].To explain those differences,%-electron

den-

Figure 2.Hydrogenation of cyclohexene at 623K,3.0MPa and P H 2S 6X 5kPa over NiMo/Al 2O 3and NiMoP/Al 2O 3catalysts in the

presence and absence of quinoline (P 0

Q

4X 8kPa,( 0X 038,0.057and 0.11

s).

Figure 3.HDN of ortho-propylaniline over NiMo/Al 2O 3at 623K,

3.0MPa and P H 2S 6X 5kPa in the presence and absence of quinoline

and 5,6,7,8-tetrahydroquinoline (P 0Q

P 0

THQ5 4X 8kPa).M.Jian and R.Prins /Catalytic sites and competitive adsorption in HDN

11

sity has been assumed to influence the adsorption strengths as well [23].

The adsorption constant of OPA is much smaller than those of quinoline-type compounds on the hydrogena-tion site,as expected from the gas phase proton https://www.sodocs.net/doc/f69918027.html, Vopa and Satterfield reported 5^20times larger adsorption constants for quinoline-type com-pounds than for alkylanilines in the hydrodesulphuriza-tion of thiophene [22],which is in good agreement with the present results for the hydrogenation of the phenyl ring of OPA (table 1).The about ten times larger adsorp-tion constant of quinoline-type compounds than of OPA,gives a quantitative explanation for the very low reactivity of anilines in the presence of quinoline-type compounds.The difference between the adsorption con-stants for quinoline-type compounds and alkylanilines is,however,much smaller for the catalytic site of alkene hydrogenation.The results in table 1demonstrate that,although in both cases hydrogenation is the chemical reaction,the hydrogenation of alkene and phenyl groups takes place over different catalytic sites.The catalytic site for the latter is much more sensitive to poisoning than the former.

It is general accepted that the active sites for hydro-treating reactions consist of sulfur vacancies located at the edges and corners of the NiMoS and MoS 2crystal-lites.Thus,Kasztelan et al.emphasized the role of three different coordinatively unsaturated Mo sites [24],and Eijsbouts et al.showed a correlation between the HDN activity of an industrial catalyst and the number of Mo atoms at corners and edges of the MoS 2crystallites [25].Since the unshared electron pair of the nitrogen atom is supposed to be involved in the adsorption on the active sites,the larger adsorption constants of OPA and quino-line-type compounds on the active site for phenyl group hydrogenation than on that for alkene hydrogenation suggests that the former site is more electrophilic.The metal atom in this site may be more uncoordinated,that is more depleted in sulphur ions.The catalytic site for the hydrogenation of a phenyl group may thus be a surface Mo or Ni entity with two or even three sulphur vacan-cies.This makes it very sensitive to poisoning,since a molecule adsorbing on one of the vacancies will com-pletely incapacitate the active site for the hydrogenation of a phenyl group,although the remaining vacancy might still be an active site for alkene hydrogenation.

4.2.The catalytic sites for C^N bond cleavage

Based on the observation that H 2S promotes aliphatic C(sp 3)^N bond cleavage and inhibits hydrogenation,Yang and Satterfield proposed that hydrogenation occurs on a surface molybdenum vacancy and C(sp 3)^N bond cleavage on a Br (nsted acidic site,and that disso-ciative adsorption of H 2S changes the former site into the latter [3,26].This proposal is supported by the fact that sulphidic catalysts catalyse aliphatic C^N bond cleavage by NH 3elimination or nucleophilic attack [13,27].The transformation between the C(sp 3)^N bond cleavage site and the hydrogenation site is also in agree-ment with the fact that a high hydrogenation activity of a NiMo/Al 2O 3catalyst is usually accompanied by a low C(sp 3)^N bond cleavage activity.

Table 1shows that the adsorption constants of quino-line-type compounds on the aliphatic C(sp 3)^N bond cleavage site are much smaller than on the hydrogena-tion sites.This is understandable since a Br (nsted SH group will be a weaker acid than a Lewis metal vacancy,thus the nitrogen-containing compounds will interact more stronger with the metal vacancy site.Table 1also shows that the adsorption constants of alkylaniline on the C(sp 3)^N bond cleavage site is about 5times smaller than that of quinoline-type compounds,and that intro-duction of phosphorus has no influence on the adsorp-tion constants on this site,which is in contrast to that on the hydrogenation sites.This demonstrates the impor-tance of SH groups for C(sp 3)^N bond cleavage.The adsorption of nitrogen-containing molecules occurs probably via these SH groups,and therefore,the other constituents of the catalyst (Ni,Mo,P)will not have a direct influence on the adsorption strengths.

The catalytic site for C(sp 2)^N bond cleavage of aro-matic amines is different from that for aliphatic C(sp 3)^N bond cleavage,because H 2S was found to inhibit the former and to promote the latter,while phosphorus had the opposite effect.Furthermore,in the HDN of alkyla-nilines,the absence of Ni and presence of phosphorus in NiMoP/Al 2O 3catalysts especially favoured the C(sp 2)^N bond hydrogenolysis path [5,6].This indicates that the catalytic site for C(sp 2)^N bond hydrogenolysis is differ-ent from that for hydrogenation of a phenyl group as well.The favourable influence of phosphorus on the C(sp 2)^N bond cleavage activity of NiMo/Al 2O 3cat-

Table 1

Adsorption constants (kPa à1

)of OPA and Q-type compounds for different HDN reactions on different catalytic sites over NiMo(P)/Al 2O 3at

623K and P H 2S 6X 5kPa

Reaction site Catalyst:NiMo/Al 2O 3

NiMoP2/Al 2O 3Inhibitor:

OPA Q-type OPA Q-type C(sp 3)^N cleavage 0.10.50.10.5alkene hydrogenation

0.4 1.60.9 3.4phenyl ring hydrogenation

0.8

7

1.2

13

M.Jian and R.Prins /Catalytic sites and competitive adsorption in HDN

12

alysts is explained by the replacement of some surface S2àby P3àions,creating a more reduced metal site that is responsible for the C(sp2)^N bond cleavage.This is in line with the HDN of quinoline over Mo2N catalysts[28] and the HDS of benzothiophene over reduced sulphided CoMo/Al2O3catalyst[29].In the HDN over Mo2N which has metallic properties,only C(sp2)^N bond cleavage occurred,and reduction of a sulphided CoMo/ Al2O3catalyst favoured the direct hydrogenolysis of sul-phur from benzothiophene.An EXAFS measurement showed that the Mo^S coordination number decreased after reduction of the sulphided CoMo/Al2O3catalyst [29].This suggests that sites active for hydrogenolysis are associated with metallic properties of the catalyst. The necessity for distinct catalytic sites for hydroge-nolysis and hydrogenation is especially clear in the hydrodesulphurization of dibenzothiophene,since the addition of a nitrogen-containing substrate was found to inhibit the hydrogenation path and to promote the hydrogenolysis path[21].Unfortunately,under our reaction conditions it was not possible to determine the adsorption constants of nitrogen-containing com-pounds on the catalytic site for the C(sp2)^N bond cleav-age of OPA due to the relatively low contribution of this hydrogenolysis path(`107)to the total HDN[7].This information may be obtained in future by a careful design of experiment and catalyst.

5.Conclusions

At least four different catalytic sites involved in the HDN reactions over sulphided Mo-based catalysts could be distinguished from competitive adsorption experiments.One site is responsible for the C(sp3)^N bond cleavage of saturated amines;it is promoted by H2S and Ni,and impaired by phosphorus.This site can be an SH group created by dissociative adsorption of H2S under the hydrotreating conditions.Another site is responsible for the hydrogenation of alkenes.It is pro-moted by Ni and phosphorus,but inhibited by H2S.

A third site is required for the direct C(sp2)^N bond cleavage of anilines.This site is promoted by phos-phorus,impaired by H2S,and is characterised by a more reduced environment of Mo site on the surface[6]. Hydrogenation of the phenyl ring needs still another site which resembles that of the NiMoS phase,is promoted by Ni and phosphorus,but poisoned by H2S[7].This site is different from that for the hydrogenation of alkenes, and may be a doubly or triply coordinatively unsat-urated site.

Removal of nitrogen atoms from aromatic nitrogen-containing molecules takes place via a sequence of reac-tion steps:hydrogenation of the aromatic rings,NH3 elimination or C^N bond hydrogenolysis,and olefin hydrogenation.The very different adsorption constants show that these reaction steps cannot be lumped together in one kinetic equation,because every reaction takes place on a different catalytic site which differ in its ability to bind reactant,intermediates and products.

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第14章 数据转换

数 据 转 换 14.1 练习题14及参考答案 1．使用SQL Server 导入和导出向导有什么功能？ 答：使用SQL Server 导入和导出向导可以在支持的数据源和目标之间复制和转换数据。 2. SQL Server 导入和导出向导可以访问哪些数据源？ 答：SQL Server 导入和导出向导可以访问各种数据源。可以向下列源复制数据或从其中复制数据： ? Microsoft SQL Server ? 平面文件 ? Microsoft Office Access ? Microsoft Office Excel ? 其他 OLE DB 访问接口 3. 什么是分离数据库？什么是附加数据库？ 答：分离数据库是指将数据库从SQL Server 实例中删除，但使数据库在其数据文件和事务日志文件中保持不变。之后，就可以使用这些文件将数据库附加到任何SQL Server 实例，包括分离该数据库的服务器。 附加数据库就是将数据库重置为它分离或复制时的状态。 4. 生成SQL Server 脚本向导有哪些功能？ 答：生成脚本向导引导用户完成创建脚本的各个步骤，并允许选择要创建脚本的各种对象。该向导提供各种用于创建脚本的选项，包括是否包含权限、排序规则、约束等。 5. SQL 脚本包含哪些内容？ 答：SQL 脚本包含用于创建数据库及其对象的语句的描述。可以从现有数据库中的对象生成脚本，然后通过在该数据库中运行脚本以将这些对象添加到其他数据库。实际上， 第 章 14

这样做重新创建了完整的数据库结构以及所有的单个数据库对象。 14.2 上机实验题13及操作过程 使用SQL Server管理控制器生成数据库factory的脚本文件H:\DBF\factory.sql，并显示该文件的内容。 操作过程 （1）启动SQL Server管理控制器。 （2）在“对象资源管理器”中展开“LCB-PC”服务器。 （3）展开“数据库”节点。 （4）选中“factory”，单击鼠标右键，在出现的快捷菜单中选择“编写数据库脚本为|CREATE到|文件”命令。 （5）出现“选择文件”对话框，在“保存在”列表中设置文件的保存位置，在“文件名”文本框中输入相应的脚本名称，这里将脚本保存到“H:\DBF\factory.sql”文件中。单击“保存”按钮，开始编写SQL脚本。 （6）在主菜单中选择“文件|打开|文件”命令，在出现的打开文件对话框中选择“H:\DBF\factory.sql”文件，SQL编辑器中显示其内容如下： USE [master] GO /**** 对象: Database [Factory] 脚本日期: 03/11/2008 18:05:45 ****/ CREATE DATABASE [Factory] ON PRIMARY ( NAME = N'Factory', FILENAME = N'H:\SQL Server\Factory.mdf' , SIZE = 3072KB , MAXSIZE = UNLIMITED, FILEGROWTH = 1024KB ) LOG ON ( NAME = N'Factory_log', FILENAME = N'H:\SQL Server\Factory_log.ldf' , SIZE = 1024KB , MAXSIZE = 2048GB , FILEGROWTH = 10%) COLLATE Chinese_PRC_CI_AS GO EXEC dbo.sp_dbcmptlevel @dbname=N'Factory', @new_cmptlevel=90 GO IF (1 = FULLTEXTSERVICEPROPERTY('IsFullTextInstalled')) begin EXEC [Factory].[dbo].[sp_fulltext_database] @action = 'disable' end GO ALTER DATABASE [Factory] SET ANSI_NULL_DEFAULT OFF GO ALTER DATABASE [Factory] SET ANSI_NULLS OFF GO ALTER DATABASE [Factory] SET ANSI_PADDING OFF GO ALTER DATABASE [Factory] SET ANSI_WARNINGS OFF

免费下载论文的数据库

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3.设置全文语言为中文 图2 设置全文语言 在服务器->属性->高级中，设置默认全文语言为2052（中文）。 4.建立数据表 在需要全文检索的数据表中，必须有一列字符型的字段存放文件类型，例如建表语句中的FileType。必须有一列Varbinary(Max)类型的字段存放文件内容，例如建表语句中的FileContent。 建表SQL语句示例： CREATE TABLE SampleBlobTable ( [PKID]int identity(1,1)primary key, [FileName]Nvarchar(255)null, [FileType]Nvarchar(32)null, [FileContent]VARBINARY(MAX)NULL,

[AddTime]datetime default(getdate()) ) 5.建立全文索引 步骤1 建立全文索引 在需要全文检索的数据表上点击右键->全文索引->定义全文索引。

步骤2 选择唯一索引 步骤3 选择表列

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