CA1069064A - Adsorption of carbon monoxide using silver zeolites - Google Patents

Abstract

ADSORPTION OF CARBON
MONOXIDE USING SILVER
ZEOLITES

ABSTRACT OF THE DISCLOSURE

Zeolitic molecular sieves having high SiO2/Al2O3 molar ratios and containing Ag+ cations are found to exhibit a high degree of selectivity and affinity for CO
molecules and are capable of separating CO from gas streams even when water vapor is present.

Description

~ 10,533 C
~0~

The present invention relates in general to the method for recovering CO from gas streams, and more par-ticularly to the process for selectively adsorbing C0 from gas streams using zeolitic molecular sieves.
Since carbon monoxide is a product of the in-complete combustion of carbonaceous fuels, it is contained in various amounts in essentially every stack gas stream wherever carbon-containing gases, solids or liquids are burned. Its toxicity to the higher animal forms is well known, snd accordingly it constitutes a serious atmospheric pollution problem in industrial areas.
Because of its strong reducing properties, carbon monoxide is a highly useful industrial gas, and is used commercially in the production of methane, phosgene alco-hols, aldehydes, ketones, organic acids and esters. To recover the CO from waste gas streams for use as a chem-ical reagent, the two methods most commonly employed here-tofore are liquifaction and chemical reac~ion. Liquifaction is suitable only when the C0-containing gas stream has a relatively low concentration of nitrogen since it is not feasible to separate CO from large amounts of nitrogen by liquifaction. Suc~ gas streams must also be essentially free of water vapor and carbon dio~ide since these com-pounds form solids at the temperature conditions involved and would cause plugging of the various apparatus parts.
A recently proposed chemical method involves the use of a composition, CuAlC14, in an aromatic base to ~- - 2 - ~

~ 6 ~ ~ 6 ~
selectively absorb C0 from admixture with nitrogen. The absorbing medium i~ 3aid to "coordinately" complex the C0 but no~ the ni~rogen whlch is dlssolved in the medium only slightly. C0 is recovered from.the C0-enriched medium by raising the temperflture and collecting the evolved product.
The gas streams suitably treated by this procedure must, however, be treated first to red~ce water vapor to extremely low levels since water rea~ts with the absorbing medium. The necessary drying operation is accomplished using adsorption on zeolitic molecular sieves. ~ more detailed discission of the aforesaid process is set forth in CHEMTECH, May, 1975, pg3, 308-311.
Another and older chemical method involves adsorp-tion of the C0 in aqueous solutions of cuprous salts with the formation of complex ions. Either an acid solution of cuprous chloride or an ammoniacal solution of cuprous car-bon~e or formate may be employed. The adsorption step is operated at room temperature under a pressure of approximately 200 atm., and carbon monoxide containing the small amount of hydrogen that ls soluble in the solution is recovered by releasing the pressure and heating the rich liquor above 40 C. The gas may be concentrated by a second cycle and is then scrubbed with water to remove hydrogen chloride or ammonia, and dried.
Despite the obvious advantages a physical adsorp-tion proce~s would have over the aforesaid liquifaction and chemical methods for recoverlng C0 from waste gas streams.
solid adsorbents which exhibit a substantial degree of ~al69V~i4 lo, 533 selectivity for C0 have not been available. This is due in part to the small po~ar character of ~he C0 molecule, and high volatility of C0 and the rè~atively high po~ari~ of H20 vapor and C02 commonly present in gas streams being treated for C0 recovery.
We have now disco~ered, however, that C0 can be ~ele~ively adsorbed for gas streams, even those containing water vapor and C02, by contacting the said stream at a temp-erature of from 0C. to 300C.. preferably 0C to 200C., w~h an adsorben~ mass comprising crystallin0 zeolitic molecular sieves having a framework SiO2/A1203 molar ratio of from 20 to 200, preferably of from 20 ~o 100, and con-taining as structural constituents silver cations. Pressure is not a critical ~actor.
S~nce the zeolite adsorbents used in the present process effectively adsorb C0 even at very low partial pressures o~ the adsorbate, both purification and bulk separation adsorp~ion procedures can be carried out. Gas s~reams containing as little as 10 ppm (volume) of C0 can

2~ be purified by ~he present proce~s.
The composition of the gas stream belng treated or removal o~ carbon monoxide is not critical due to the high degree of affinity the æilver cation has for C0. The silver cation is not oxidized by air) either wet or dry and hence the ambient air in such places as laboratories, ... . : . . .
workshops, factories. tunnels and sealed enclosures such as aircraft and spacecraft is an ideal feedstock for purifica-tion by the present process. Most commonly the gas stream being treated will contain at least one of H20. N2. 2 ~69 ~ ~ ~ 10,533 C

and C0~ in addition to CO. Both H20 and CO2 are morestrongly adsorbed on conventional æeolite adsorbents such as zeolite A and zeolite X and e~fectively block the ad-sorption of appreciable quantities of C0. Surprisingly ~:
neither of these constituents in a gas stream treated by the present process interferes with C0 adsorption.
It will be understood, of course, that althoughthe zeolite adsorbents of the present process have a high affinity for CO as an adsorbate, and are quite tolerant of a wide variety of chemical constituents ln the gas streams used as feedstock, it is nevertheless advantageous to avoid the presence of undue amounts of strongly reducing components, particularly hydrogen, which tend to deactivate the zeolite adsorbents by converting the Ag+ cations to me-tallic silver in cyclic use of the adsorbent mass.
The crystalline zeolitic molecular sieves of the class suitable for use in the practice o~ this in~ention are those species which have pore diameters large enough ~o adsorb C0 and which have or are modi~ied ~o ha~e frame-work Sio2/A1203 molar ratios of from 20 to 200, preferably rom 20 to 100. A number of synthetic zeolite species are available which have sufficiently high SiO2/A1203 molar ratios in the as-synthesized form. These include zeolite :
as defined and disclosed in U.S. Application Serial No.
655,318, filed July 24, 1967, zeolites ZSM-5, ZSM-8, ZSM~
and ZSM-12 as disclosed in detail in U.S.P. 3,702,886.

, ~ .

10,533 There are al~o avallabl?a variety of techniques for increasing the Si/Al ratio~ of zeolite specie~ which have not yet been crystallized in form~ su~ficiently si1iceous for use i~ this invention, One such method involves steam-ing the zeolite whic~ is at least p~rtially in the hydrogen cation form at temperatures within the range of 800 to 1500F. ~ollowed by extraction of the thus-loosened alumina from the zeoli~e s~ructure with dilu~e mineral acids or or~anic chel~ting agents. The procedure is defined in detail in U.S.P. 3,506~400 i~sued April 14, 1970. Another method in w~ich a partialty dicationlzed form of the zeolite i~ treated with acetylacetone to extract framework alumina i8 de~cribed in U.S.P. 3,640,681 issued February 8, 1972.
The zeolite having SiC2/A1203 molar ratios increased by such means to the range of 20 to 200 are sa~lsfactory for the pre~ent process.
The silver catlon forms of the zeolite adsorbents utilized in the presen~ process are readily prepared by conventional ion-exchange technique~ using aqueous solutions of silver salts such as s~lver nitrate.
Example 1.
A sample o zeolite ZSM-S having a dehydrated chemical composition expressed in terms of mole ratios of oxides o 0-9 ~a20: [pr4N]2o A1203: 32 SiO
wherein Pr = n-propyl, was calcined at 600C. in a.stream of air to produce the corresponding hydrogen form of the zeolite. Thereafter the zeolite wa~ ion ex hanged using 10,533 ~(~6~4 an aqueous solution of silver nitrate as the exchange medium, The product zeolite was wash~d with water to remove the exchange solution, dried a~ 100C. in air and activated (dehydrated) at 350C. ~or 3 hours, and cooled to room temperature. A fixed bed of the activated zeolite at 22C.

was employed to treat an air stream-containing 30 ppm CO

and sa~urated with water vapor at 1 atmosphere and 22C.

The zeolite mass adsorbed 0.5 weight-% C0 and reduced the , . . . . . .. ... . .. .. . . . . ... . . . .
CO content of the product gas to ~ 10 ypm. Regeneration of the zeolite mass was readily accomplished by purging with dxy nitrogen at 200C.

Claims (4)

WHAT IS CLAIMED IS:

1. Process for removing carbon monoxide from a gas stream comprising at least 10 ppm CO by volume and at least one other constituent selected from the group consisting of H2O, CO2, N2 and O2 which comprises passing said gas stream at a temperature of from 0 to 300°C. through an adsorbent mass of a crystalline zeolitic molecular sieve having a framework SiO2/Al2O3 molar ratio of from 20 to 200 and having as zeolitic constituents silver cations, whereby CO molecules are adsorbed and a CO-depleted effluent gas stream is obtained.

2. Process according to claim 1 wherein the frame-work SiO2/Al2O3 molar ratio is from 20 to 100.

3. Process according to claim 2 wherein the gas stream is passed through the adsorbent mass at a temperature of 0°C. to 200°C.

4. Process according to claim 3 wherein the gas stream is air containing at least 10ppm CO by volume.