CA2110367A1 - Adsorbent for hydrocarbons in exhaust gas - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
This invention discloses an adsorbent for hydrocarbons as organic components contained in an exhaust gas discharged from an internal-combustion engine. The invention more particularly relates to an adsorbent for hydrocarbons which are organic components in exhaust gases discharged from internal-combustion engines, particularly engines in gasoline cars or diesel cars, boilers, and industrial plants. The adsorbent comprises palladium and a zeolite.

Description

ADSORBENT FOR HYDROCARBONS IN EXHAUST GAS
BACKGROUND OF THE INVENTION
Field of the Invention:
Thiq invention relates to an adsorbent for hydrocarbons, namely quch organic components as are contained in the exhaust gas discharged from an internalcombustion engine. It relates more particularly to an adsorbent for hydrocarbons which are organic components entrained by exhauqt gases emanating from internal-~0 combustion engines, particularly engines in gasoline ordiesel engineq, boilerq, and industrial plants.
Description of the Prior Art:
Carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxide~ (NOx~ which are noxious componentq contained 15 in the exhaust gas emanating from the internalcombustion engine as deqcribed above are removed for the most part from the exhaust gas by a noble metal-containing catalyst or the so-called three way catalyst for use in automobileq. It is generally known that among other noxious components 20 mentioned above, hydrocarbons are required to be at a high temperature to be effectively removed by the catalyst. In the case of engineq, ~or example, the exhaust gase-~ these engines emit immediately after they are started contain hydrocarbons in a large amount. Since the~e exhaust gases 25 still have a low temperature, they cannot be treated fully satisfactory with the conventional three way catalyst.
In recent years, the U.S. Government ha~ tightened its ban on exhaust gas, particularly on hydrocarbons. Thus, the removal of the hydrocarbons which are generated in a 30 large amount during the engine start mentioned above (hereina~ter referred to as "cold time") has constituted itself a serious problem.
The hydrocarbon oomponent~ contained in the exhaust ga~ are olefinq, paraffin~, and aromatic hydrocarbons, with 35 ethylene preponderating over other olefins and benzene, toluene, xylene, etc. preponderating over other aromatic hydrocarbons (Society of Automotive Engineer Paper 910174, pages 39-45).
Further, as being clear from the deqcription on pages 41 and 42 of the same, when the exhaust ga~ is evaluated by LA-4 modeq which is a general evaluation method of the exhaust gas, about 20% of total hydrocarbon component_ in the exhaust gas are contained ac more than double bond of C2 (e.g., ethylene, acetylene, etc., and majority is ethylene). Therefore, purification of ethylene 10 is a serious problem in order to increase purification rate of the hydrocarbons. However, ethylene is generally difficult to be adsorbed on the catalyst.
Generally as a method for the removal of these hydrocarbons, the practice of u~ing as an adsorbent for 15 hydrocarbons such a zeolite as posseqseq the effect of a molecular sieve and the quality of an acid has been widely known. As respects the use of these characteriqtic qualities, a method which, by meanq of a monolithic structure comprising a three way catalyst as a main body 20 thereof and a zeolite coating formed on the upqtream side portion of the main body relative to the direction of the stream of the exhau_t ga~, cauqes hydrocarbons to be adsorbed by the zeolite during the cold time and, aq the temperature of the exhaust gas subsequently rises, enables 26 the adsorbed hydrocarbons to be desorbed from the zeolite, and thereafter enables the hydrocarbons to be burnt by virtue of a cataly~t installed at a latter stage is discloqed (JP-A-2-75,327).
It has been a~certained, however, that when a 30 zeolite is used in a simple form as an adsorbent for hydrocarbons, it exhibits a poor performance from the realistic point of view. We continued a study on various adsorbents to find the relation between their ability to adsorb hydrocarbons and the characteristic qualities of 35 hydrocarbons and conqequently found that for the determination of the ability of an adsorbent to adsorb hydrocarbons, the adsorption of ethylene among other hydrocarbon components in the exhauqt gas forms a crucial point. This knowledge clearly indicates that the uqe of a zeolite in a single ~orm as an adsorbent allows no appreciable removal of hydrocarbons from an exhaust gas becau~e the zeolite has a very meager capacity for adsorbing ethylene.
An adsorbent having a catalytic metal carried on a zeolite and exhibiting an ability to ad~orb hydrocarbons and 10 consume them by combustion i~ di~closed in JP-A-2-135,126.
This piece of literature, however, cites no working example involving use of palladium and o~fers absolutely no disclo~ure on the ability of the propoqed adsorbent to adsorb ethylene and the characteristic property of 15 adsorption di~clo~ed at all is not suf~icient. ~-An object of this invention is to provide an adsorbent which is capable o~ efficiently ad~orbing hydrocarbons in an exhaust gas.
Another object of this invention is to provide an 20 adsorbent which i~ capable of efficiently ad~orbing hydrocarbons occurring in a large amount when the temperature of the exhaust gas containing the hydrocarbons is low.
SUMMARY OF THE INVENTION
The objects described above are accompli3hed by an adsorbent which comprise~ palladium and zeolite and effect~
required removal o~ hydrocarbons from an exhauqt gas.
To be ~pecific, this invention i~ directed to an ad~orbent which comprises a zeolite incorporating palladium 30 therein and permit~ removal of hydrocarbon~ from an exhaust ga~. This adsorbent for hydrocarbons haY a palladium content in the range of 0.001 to 10 % by weight based on the amount o~ the zeolite.
We made a diligent study on variou3 adsorbents to 35 find the relation between their ability to adsorb hydrocarbons in an exhaust gas and the compoqition of hydrocarbons. We found as a result of this study that for the determination of the ability of a given adsorbent to adsorb hydrocarbons, the adsorption of ethylene in the exhaust gaq constitute~ itself a crucial point. On the basiq of this knowledge, we continued a study further on the adqorbents concerning their characteri~tic property for adsorption of hydrocarbons in an exhaust gas and consequently found that the use of palladium and zeolite permits efficient adsorption of hydrocarbons. This 10 invention has been perfected as a reqult.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a HC adsorption-desorption spectrum obtained at an elevated temperature in an adqorption test.
In the diagram, a vertical axis is a scale of intensity of a 15 detector and a horizontal axis iQ a scale of temperature elevated at a ~ixed rate.
Fig. 2 i~ a diagram showing a device for experiment.
Fig. 3 iq a HC adsorption spectrum obtained at an elevated temperature in a test in an actual car model 20 atmo3phere. In the diagram, a vertical axis shows a temperature of inlet bed and a concentration of ethylene and a horizontal axis shows reaction time.
Fig. 4 i~ a HC adsorption spectrum obtained at an elevated temperature in a test in an actual car model 25 atmoqphere. In the diagram, a vertical axis shows a temperature of inlet bed and a concentration of toluene and a horizontal axi~ ~hows reaction time.
EXPLANATION OF THE PREFERRED EMBODIMENT
The zeolite to be used effectively in this invention 30 is a zeolite of H form. Though it must be of the H type, it does not need to be particularly restricted a~ to the kind.
As concrete examples of the zeolite of the H type, quch pentacyl type zeolite~ as are represented by mordenite, Y
type, ferrierite, offretite, erionite, and ZSM-5 may be 35 cited. The~e zeolites are either commercially available or capable of being manufactured by methods generally disclosed in literature. The term "zeolites of H type" as used herein means proton-exchanged zeolites, and usually when zeolites ion exchanged by ammonium ion is heat-treated, ammonia liberates at a temperature of 250 to 300 C and surface of zeolites is proton-exchanged to be obtained the zeolites of H type.
Though palladium qources effectively usable herein need not be particularly restricted, nitrates, sulfates, chlorides, and bromide~ of palladium are available, for 10 example. Particularly when the zeolite of Y type to be used happens to be vulnerable to an acid, such a salt as mentioned above is desired to be uqed in the form of an aqueous solution having a pH value adjusted in the range of 5 to 9 , preferably 6 to 8 , with such a basic substance as 15 ammonia or an amine.
Preferably, the palladium is used in an amount in the range of 0.001 to 10 % by weight, preferably 0.001 to 8 % by weight, baqed on the amount of the zeolite. If the amount of palladium is less than 0.001, the produced 20 adsorbent is at a disadvantage in exhibiting an unduly low capacity for adsorption of hydrocarbons,especially ethylene.
Conver~ely, if this amount exceeds 10 % by weight, the disadvanta~e ari~es that adsorption performance decrease, so it is not preferable.
Regarding the relation between palladium and zeolite in the product aq an adsorbent, an aqueous palladium salt solution is caused to impregnate zeolite powder or to ion-exchange to attain deposition of the former on the latter, dried and calcined to obtain palladium-containing zeolite 30 powder. The temperature of this calcination, is in the range of 300 to 600 C, preferably 400 to 550 C.
The powder thus obtained itself may be used as an adsorbent by molding to sphere, cylinder, saddle, column and the like. Further, this adsorbent may be used by wet-35 milling and coating the aqueous qlurry thus obtained on aninert carrier. In this case, when it iq slurrified, a ::

desired amount, e.g., 0.5 to 30 % by weight, preferably 1 to 20 % by weight of a binder such as alumina sol, silica sol, zirconia sol, titania sol and the like may be added to coat on the inert carrier. Furthermore, it is possible to enhance the contact degree with the hydrocarbons by increasing thickness of the coated layer by mixing an oxide such as alumina, silica, zirconia, titania and the like with the adsorbent.
The adsorbent thus containing palladium and zeolite 10 can be molded in a prescribed shape such as, for example, in the shape of spheres, cylinderq, ~addle~, and circular pillars. The substances which are effectively usable for the inert carrier include cordierite, mullite, a-alumina, zirconia, titania, titanium phoqphate, aluminum titanate, 15 betalite, spodumene, aluminoqilicate, and magneqium ~ilicate, ~or example. The monolithic carrierq effectively usable herein are tho~e which are generally referred to as a ceramic honeycomb carrier. Besides, one-piece structure~
made of such oxidation-reQistant refractory alloyq as 20 stainleqs qteel and Fe-Cr-Al alloy can be used a~ inert carriers.
The~e monolithic carrier~ are produced by a method of extrusion molding a bulky material or a method of tightly winding a sheetlike element into a roll. The gas-passing 26 mouth~ (cells) may be in any of various ~hapes quch a~, ~or example, hexagon, square, triangle, and corrugation. For the honeycomb carrier to be fully useful for the adsorbent of this invention, the cell density (number of cells/unit cros~-sectional area) in the range of 150 to 600 30 cells/square inch, preferably 100 to 500 cells/square inch, suffices.
Concerning the manner in which the adsorbent according to this invention is put to use, the adsorbent can be uqed in its simple form when the temperature of the 35 exhaust ga~ being treated continues to be low. Generally, it is used in combination with a three way catalyst because &~

the temperature of the exhaust gas which emanateY from the internalcombustion engine gradually ri~es with the elapqe of time in spite of the fact that the internalcombustion engine is at a low temperature at the start of operation. For 6 example, (1) a method which compriseq setting the adsorbent on the upstream side and the three way catalyst on the downstream qide relative to the stream of the exhaust gac, (2) a method which compriseq dividing an exhau~t pipe into two pipeq, setting a damper at the point of divicion, 10 packing one of the divided pipes with the adsorbent and keeping the other pipe empty both on the downstream qide from the point of division, joining the two divided pipes into one pipe on the further downstream side, Yetting the three way catalyqt on the downstream side, passing the 16 exhaust gas through the pipe packed with the adsorbent while the temperature of the exhauqt gas is low, passing the exhaust gas through the empty pipe while the temperature of the exhaust gas i~ high, suitably feeding the exhaust gas pulsatively through the pipe packed with the ad~orbent 20 thereby desorbing the adsorbed hydrocarbons, and causing the desorbed hydrocarbons to be consumed by combu~tion with the three way cataly~t, and (3) a method which comprises dividing the down~tream side o~ the three way catalyst into two pipeq, packing one of the divided pipeS with the .:
25 adsorbent and keeping the other divided pipe empty, feeding the exhaust gas through the pipe packed with the ad~orbent while the exhaust gas is at low temperature and through the empty pipe while the exhaust gas is at high temperature, introducing the exhauqt gas pulsatively into the pipe packed 30 with the ad~orbent while the exhaust gas is at high temperature, and recycling the exhaust gaq to the three way catalyst for further purification may be cited.
The industrial exhaust gas means a gas containing several tens to several hundreds ppm of hydrocarbons, the 35 adsorbent in accordance with the present invention is effective for using in a plant exhausting hydrocarbons, : ~.

., , . ~ -e~pecially ethylene.
Generally, as e~sential cataly~t components for use in the three way catalyst, such noble metal~ as platinum and rhodium or palladium and rhodium or platinum, palladium, and rhodium and alumina and ceria may be cited, for example.
Optionally, the three way catalyst may additionally incorporate therein at least one oomponent selected from among such rare earth oxides aY lanthanum oxide (La203) and zirconia (ZrO2). The amount of a noble metal to be 10 deposited on the carrier is in the range of 0.1 to 2 g/liter of honeycomb carrier and the amount of alumina or ceria to be deposited on the carrier is in the range of 10 to 300 g/liter of carrier.
As typical examples of the three way catalyst 15 described above, tho~e catalyst~ di~closed in USSN
07/862,967 may be cited.
Now, this invention will be de~cribed more specifically below with reference to working examples.
Example 1 Ten (10) g of H type mordenite in a powdered form was combined with an aqueous palladium nitrate solution used in an amount calculated to account for a palladium content of 0.1% by weight, based on the amount of the mordenite.
- The mixture consequently obtained was thoroughly mixed, 25 dried at 120C, and calcined in air at 500C for one hour, to obtain an adsorbent.
Examples 2 to 4 Adsorbents of Example~ 2 to 4 were produced by following the procedure of Example 1, excepting palladium 30 nitrate was used in amounts calculated to account for palladium contents of 0.001% by weight, 0.01% by weight, and 2.0% by weight respectively, based on the amount of the - mordenite.
Example 5 and 6 Adsorbents of Example 5 and Example 6 were produced by following the procedure oP Example 1, excepting H type ,, ~ . . ~ :. . ,-,,j - . . ., ,., , - . , ": . :

ferrierite and ZSM-5 were used in the place of the H type mordenite.
Controls 1 to 3 Adsorbents of Controls 1 to 3 were produced by 5 following the procedures reqpectively of Example~ 1, 5, and 6, excepting the u e of palladium was omitted.
The compositions of the ~dsorbents obtained as de~cribed above are collectively shown in Table 1.
Table 1 . .__ _ _ Amount of Pd Species of zeolite carried (% by weight) ._. _ . ____ _.
Example 1 H type mordenite 0.1 15 Example 2 H type mordenite 0.001 Example 3 H type mordenite 0.01 Example 4 H type mordenite 2.0 20 Example 5 H type ~errierite 0.1 Example 6 ZSM-5 0.1 .__ Control 1 H type mordenite _ .
Control 2 H type ferrierite _ 25 Control 3 ZSM-5 .
Example 7 The adsorbents obtained in Example~ 1 to 6 and Controls 1 to 3 were pulverized in a mortar and were tested ~or ability to adsorb ethylene by packing 0.05 g of the 30 adsorbents in a conventional ~low tube type reactor and subjecting the sample in the reactor to the conditions shown in Table 2 below. The te~t for ad~orption and de~orption was carried out by a procedure which comprised filling a test tube with a sample adsorbent, introducing a prescribed 35 gas into the test tube, starting to raise the temperature of the test system, and obtain a hydrocarbon ad~orption . ~

pectrum. An FID (flame ion detector) was used for obtaining the spectrum of adsorption and de~orption of hydrocarbonq. The results of the test are shown in Fig. 1.
Table 2 5Synthetic gas composition __ Component-s Concentration . _ Ethylene 10,000 ppm (calculated as C1) 10 H20 10%
_ N2 Balance Total flow volume 150 cc/min Temperature increasing rate 30C/min l~ Example 8 Slurry waq obtained by charging 200 g of an adsorbent obtained by a ~imilar method to Example 1, 20 g of silica qol (silica content 20 % by weight) and 400 g of deionized water to a ball mill and subjected to wet-milling.
20 A monolithic carrier (diameter 24 mm, length 60 mm and cell den~ity 400 cells/~quare inch) made of cordielite was immersed in the slurry thus obtained, and an ad~orbent was obtained by carrying out air blowering to coat a desired amount of ~lurry, drying at a temperature of 120 C and 25 calcining at a temperature of 500 C for 1 hour. An amount of coating was 150 g/liter.
Control 4 A ~imilar method to Example 8 waq carried out, expect that 200 g of H type mordenite, 20 g of silica qol 30 (silica content 20 % by weight) and 400 g of deionized water wa~ charged to a ball mill and was subjected to wet-milling.
An amount of coating was 150 g/liter.
Example 9 Adqorbents obtained in Example 8 and Control 4 were 35 set re~pectively to a holder 1 in a device qhown in Fig. 2, and ethylene, oxygen, nitrogen and ~team in a composition shown in Table 3 were charged respectively by lines 2, 3, 4 and 5 to carry out adsorption experiments. These experiments were modeliæed by a condition between engine starting and after 100 seconds in a cold start (without engine warm-up condition) in LA4 mode determination which was determined in a gasoline automobile.
The procedure of the adsorption experiments were aq follows:
(1) The gase~ to be tested were introduced into the 10 above-mentioned lines and were set in a ~urnace 6 at a temperature of 600 C, (2) The adsorbent was inserted to the holder 1, (3) The ga es were introduced into a line 8 at an ad~orbent side by a valve 7, and - 15 (4) The outlet gas of the ad~orbent holder wa~
analyzed by a FID (flame ion detector) 9 to obtain an adsorption spectrum of hydrocarbon (ethylene).
The re~ults of the determination were ~hown in Fig.
Table 3 Synthetic gas composition ~ ' - Components Concentration . , -Ethylene5,000 ppm (calculated as C1) ~
25 . _ . :
2 0.5%
H20 10%
N2 Balance 30Space velocity49,000 hr~1 Example 10 A similar adsorption experiments to Example 9 were carried out, except that ethylene in the gas composition to 35 be adsorbed wa~ substituted by toluene. The gas compo~ition u~ed in the experiments is ~hown in Table 4.

The re~ult~ of the determination were ~hown in Fig.
Table 4 Synthetic gas compo3ition __ Component~ Concentration _ Toluene 5,000 ppm (calculated as C1) . __ _ .__ 2 0.5%
H20 10%
N2 Balance Space velocity 49,000 hr~

It is generally well-known that zeolite ha~
ad~orption performance to the hydrocarbons. On the other hand, it i9 clear from Figs. 1 and 3 that only zeolite ha~
extremely low adsorption performance to ethylene. On the contrary, the ad~orbent in accordance with the pre~ent 20 invention ha~ remarkablly excellent adsorption performance to the hydrocarbon ~uch a~ ethylene. On the other hand, the adsorbent maintain~ ~imilar ad-~orption performance to aromatic hydrocarbon3 such as toluene to general zeolite.
Further, the adsorbent i~ effective to the exhaust gas 25 during the initial ~tage of the operation of the internal-combustion engine.

Claims (9)

1. An adsorbent for hydrocarbons in an exhaust gas, which adsorbent comprises palladium and zeolite.

2. An adsorbent according to claim 1, which comprises a palladium-containing zeolite.

3. An adsorbent according to claim 2, wherein the palladium content is in the range of 0.001 to 10 % by weight based on the amount of said zeolite.

4. An adsorbent according to claim 2, wherein said zeolite is of an H type.

5. An adsorbent according to claim 2, wherein said palladium-containing zeolite is molded in a bulky form.

6. An adsorbent according to claim 2, wherein said palladium-containing zeolite is deposited on an inert carrier.

7. An adsorbent according to claim 6, wherein said inert carrier is a bulky substance.

8. An adsorbent according to claim 6, wherein said inert carrier has a monolithic structure.

9. An adsorbent according to claim 1 or 2, wherein said hydrocarbons are ethylene.