-
何谓积碳
以
含碳化合物为原料的催化反应的第一步通常是在催化剂表面上进行吸附
形成含碳物种,<
/p>
如含碳物种经过分解、
聚合等反应生成碳或焦炭沉积在催化剂表<
/p>
面,
既所谓的积碳或结焦。
积碳会堵塞催
化剂的活性位或催化剂的孔道,
从而导
致催化剂的失活!
常发生积碳的催化反应
催化裂化
?
Pt
重整
?
加氢精制
?
轻油制氢
?
积碳形态
积碳的形态一般分为无定形碳、层状石墨碳、管须状
结晶碳以及粘稠状液
态碳或焦油。
积碳原因
热裂解
?
催化裂解
?
深度脱氢
?
烯烃聚合
?
热裂解积碳
是气相有机原料在高温下热裂解形成烟炱、焦油,它们在催化剂表
面上生成有序的或无
序的碳;
催化积碳
是在催化剂的作用
下,烃类化合物发生催化积碳反应,它与催化剂的
性质密切相关。
氧化物、
硫化物上主要是酸性积碳,
积碳速率与催化剂的表面酸碱性有关;
?
金属上的积碳是通过烃类深度脱氢
和脱氢环化聚合产生积碳,
金属颗粒大
小、分散度、合金化影响
积碳;
?
金属负载酸性载体催化剂的催化积碳则两者同时存在。
?
积碳防止
合金化,如
Pt
重整催化剂中添加第二
组分金属形成合金,可减缓催化剂
的积碳;
?
添加助剂,
Pt
重整催化剂采用少量
Cl
改性,
可以减缓酸性积碳;
?
利用载体活性组分的相互作用,降低积碳;
?
对于酸性催化剂或载体,添加碱性
助剂(
K2O
、
MgO
),减弱酸性积碳
?
通入水蒸气,消碳;
?
添加其他元素如稀土,加速消碳,防止积碳。
?
积碳失活再生——烧碳
<
/p>
催化剂因为积碳而失活,其催化剂再生通常采用烧碳的方法可以部分恢复
< br>催化剂的活性。
烧
碳条件如气氛、温度、气体组分等都会影响到再生后催化剂的结构、物
化性质以及催化性
能,因此必须严加控制。几条注意事项(三步骤):
1.
除催化剂上的烃类和挥发物,必须在惰性气氛下进行;
2.
除去硫和轻质碳,
必须在低温
250-350
度下先使金属氧化除去硫
,
同
时某些聚合物和轻质碳也随之除去;
3.
最后除碳,根据碳的量以及类型在较高温度下进行。
积碳表征
催化剂积碳表征手段一览表
表征手段具体介绍
(
1
)
Temperature-Programmed
Techniques
Temperature-
Programmed Oxidation (TPO)
可获取信息:
?
?
provides direct information regarding
coke oxidation rate
obtain
useful
information
such
as:
location,
composition
(hydrogen/carbon
ratio)
and morphology (highly dispersed or
multidimensional particles).
具体如:
?
?
?
?
?
Determination of H/C Ratio
Determination of Coke
Location.
Determination of
Oxidation Kinetics
Determination of Morphology
Determination of Coke Amount
方法:
?
?
?
?
?
?
detection of
CO2
by a thermal conductivity detector
(TCD)
after
it is separated
from oxygen and water in a GC
column;
detection of CO,
CO2 and hydrocarbonaceous compounds after
methanation;
quantification
of CO2 with a mass spectrometer;
monitoring
temperature
increment
above
a
reference
sample,
in
differentialthermal
analysis (DTA) equipment;
measurement of weight loss in thermal
gravimetric analysis (TGA) equipment.
detection of CO2 by FTIR
Temperature-Programmed Hydrogenation
(TPHy).
尤其适用于<
/p>
H2
为反应物的催化反应——
Chara
cterization of coke by TPHy is of
special interest when hydrogen is one
of the reactants
,
如:
?
?
?
?
reforming,
dehydrogenation
methanol synthesis
Fischer-Tropsch synthesis
Temperature-Programmed
Gasification
与
TPO
相似,采用载气如
He
、
Ar
,也可以采用
CO2
代
替
O2
(
2
)
Electron
Microscopy
The
localization,
nature
and
structure
of
coke
deposits
have
been
examined
with
electron
microscopy
。
Typically,
the
electron
microscopy
alone
does
not
provide
much
information,
and is
generally used in combination with related
spectroscopies
(
3
)
Electron Energy
Loss Spectrocopy (EELS)
EELS
能够提供的信息:
?
?
?
provides
analytical and structural information, similar to
that given by X-ray
absorption
spectroscopy;
detect the
location of the coke
(由于
EELS<
/p>
具有高分辨率——
1nm2
)
;
provides
qualitative information regarding the type of coke
present on the
catalyst
(与参照样
品对比)
.
局限性:
used only in
few cases to characterize coke deposits on
heterogeneus catalysts
(
4
)
Infrare
d Techniques (FTIR, DRIFTS)
能够提供的信息:
?
?
?
obtained with
these techniques is the chemical identity of
compounds that form
the coke, such as
olefinic, saturated or aromatic.
information
regarding
the
location
of
coke
can
be
obtained
by
following
the
signal
of certain catalyst
surface groups, such as Bronsted OH.
observe the deposition of carbonaceous
materials on the working catalyst.
局限性:
limited
information
on
the
nature
of
carbonaceous
deposits
because
of
their
complexity
and
of
the
difficulty
to
assign
unambiguously
an
IR
band
to
particular
species
。
波数归属情况
(
5
p>
)
Laser Raman
Spectroscopy
Classical Laser Raman Spectroscopy
(LRS)
-
-
-
-
-
-
-
-
-
上一篇:电脑BIOS设置(2010年前机型) 翻译中文教程
下一篇:CMOS设置详解