Changes in Ion Exchanger Catalysts after Extremely Long Exposure to

Research Note pubs.acs.org/IECR

Changes in Ion Exchanger Catalysts after Extremely Long Exposure to the Reactor Environment Karel Jeřab́ ek,*,† Libuše Hanková,† Ladislav Holub,† and Hanuš Slavík‡ †

Institute of Chemical Process Fundamentals of the ASCR, v.v.i., Prague, Czech Republic Momentive Specialty Chemicals Inc., Sokolov, Czech Republic

‡

ABSTRACT: This study involved the investigation of morphological changes in ion exchanger catalysts after more than 12 years in an industrial reactor for esterification of acrylic acid with methanol. The results showed that both desulfonation and de/crosslinking of the polymer was quite extensive. These changes were detected to a higher extent in test polymer samples located near the reactor input than in the sample obtained from main reactor charge, indicating that impurities in the reaction mixture, probably peroxidic compounds, could be responsible for the splitting of C−S and C−C bonds.

S

trong acid ion exchangers, such as sulfonated styreneco-divinylbenzene polymers, are important catalysts used in a number of industrial processes, where they replace soluble acids. Their higher cost is offset by the possibility of using them repeatedly or the option to transform batch processes to more efficient continuous ones. In this respect, the stability of their activity plays an important role. It is generally recognized that ion exchanger catalysts may be deactivated as a result of fouling, desulfonation, or neutralization of the acidic centers with metal ions or organic bases.1,2 Fouling of the polymer backbone of ion exchangers with insoluble impurities or high molecular weight byproducts may diminish the accessibility of the acidic centers. Desulfonation is known to proceed at temperatures above 120 °C, especially in the presence of water. Sulfonic groups can be neutralized either by metal ions or organic bases already present as impurities in the reactant input stream or generated by corrosion of the reactor walls. Practical studies investigating deactivation of ion exchanger catalysts are rather rare (e.g., refs 2,3). Recently we had an opportunity to examine samples of ion exchangers having served for more than 12 years as catalysts in an industrial process of esterification of acrylic acid with methanol. An interesting and unusual feature of this particular case is that in addition to the main catalyst bed, the reactor also contained a series of test samples of some other commercially available ion exchangers considered to be possible alternative catalysts for this reaction. The tested alternative ion exchangers were put into flow-through capsules and were inserted into the top (input) layer of the main catalyst charge during the filling of the reactor (see Figure 1). All the catalysts were present in the reactor from May 29, 1999 until November 7, 2011. Before being emptied, the reactor was washed with water and during this operation the catalyst charge was observed to swell to a volume greater than that of the water-swollen fresh catalyst, which indicated unexpected changes in the catalyst morphology. It was therefore decided to investigate these changes using inverse steric exclusion chromatography. Furthermore, an assessment was carried out of the changes in the catalytic activity induced by this very long exposure to the reactor environment. © 2012 American Chemical Society

Figure 1. Location of ion exchanger catalysts in the reactor.

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EXPERIMENTAL PART Table 1 presents basic information of the examined exchanger catalysts. For each of the catalysts removed from the reactor, we Table 1. Examined Ion Exchanger Catalysts main reactor charge Lewatit K 1221 Test samples: Lewatit K1131 Lewatit K1431 Lewatit K2621 Purolite CT110 Relite CFS-H Ostion KSC-2

morphology

estimated DVB content

manufacturer

gel

4%

Lanxess, Germany

gel gel macroreticular gel

2% 8% medium 8%

Lanxess, Germany Lanxess, Germany Lanxess, Germany Purolite, Great Britain

macroreticular macroreticular

low low

Resindion, Italy Spolchemie, Czech Republic

had at our disposal an unused reference sample, albeit not necessarily from the same production batch. Before evaluating their properties, samples of the catalysts taken from the reactor were washed with methanol, water, hydrochloric acid, and finally again with water. The purpose of this washing was to remove all reversibly sorbed impurities Received: Revised: Accepted: Published: 985

October 11, 2012 December 20, 2012 December 27, 2012 December 27, 2012 dx.doi.org/10.1021/ie3027718 | Ind. Eng. Chem. Res. 2013, 52, 985−989

Industrial & Engineering Chemistry Research

Research Note

were obtained at two different reciprocal space velocities and consequently at two conversion levels.

before examining the permanent changes of the catalysts. The exchange capacity of all catalyst was determined by direct titration of a suspension of the polymer in an 0.1 N potassium chloride solution with 0.1 N sodium hydroxide. The swollen-state morphology of all ion exchangers was examined using inverse steric exclusion chromatography (ISEC) as described previously.4,5 Reference samples of exactly the same batches of the ion exchanger catalysts as those used in the reactor were unfortunately not available. Hence, the used catalysts were compared with samples of commercial ion exchanger catalysts of the same type from an archived collection kept in our laboratory. According to our long-term experience of examining commercial ion exchangers, differences in properties of various batches of the same nominal type are usually quite small, and definitely substantially smaller than effects of long-time use in a catalytic reactor. We thus considered it acceptable to use these “fresh” ion exchangers, even though they were not exactly authentic, as references. To assess the differences in the catalytic activity between the used and fresh ion exchanger catalysts, we measured the reaction rates of esterification of acrylic acid with methanol in a glass CSTR-type magnetically stirred reactor with an inner volume of 5 cm3. The experiments were performed at 60 °C with a mixture of acrylic acid and methanol at a molar ratio of 1:1. The weight of the catalyst and the flow rate of the reaction mixture were adjusted to limit the conversion to the range 15− 30%. For each catalyst, two values of steady-state reaction rates

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RESULTS AND DISCUSSION Despite the thorough washing, both neutral and acidic, the exchange capacities of the used catalysts were found to be only a fraction of those of their fresh counterparts. These differences were substantially greater for the research samples which, during use, were located near the reactor input as compared to the lower drop in the exchange capacity found in the sample of the main catalyst charge probably extracted from deeper catalyst layers (Table 2). The difference between the main catalyst charge and the test samples was found also in their swelling behavior. Table 2 presents the specific water-swollen volumes of polymer particles as determined by ISEC experiments from differences between a known volume of an empty chromatographic column and the elution volume of the biggest standard solute corresponding to the volume of the interstitial spaces in a filled column (the “dead” volume). The water-swollen volume of the used main reactor charge was substantially greater than that of the fresh one, but the ability of the test samples to swell in water after their exposure to the reactor environment diminished significantly. This was probably a consequence of their higher desulfonation. The unswollen desulfonated domains restricted the ability of the remaining sulfonated regions to swell in water. Inverse steric exclusion chromatography (ISEC) was used to study the changes in morphologies of the ion exchanger catalysts after long exposures to the catalytic reactor. This method provides information on the swollen, expanded structure of a polymer matrix in the form of a morphology-representative model based on a set of discrete polymer fractions,6 each characterized by a single value of the density of the expanded polymer matrix. The density of the polymer fractions is expressed in units of length (of the polymer chains) per unit of volume.7,8 Values of the nominal densities of the model fractions covered the whole range that could possibly occur in a swollen polymer gel, from 0.1 nm/nm3 representing the density of an almost noncrosslinked polymer mass to 1.5 nm/nm3 characterizing domains in a highly cross-linked polymer into which even the smallest molecules can penetrate with difficulties.9 Such modeling of the swollen polymer morphology was for these materials more relevant than conventional modeling of cylindrical pores commonly used

Table 2. Comparison of Exchange Capacities and Swelling of Fresh and Used Catalysts exchange capacity, meq/g

water-swollen volume, cm3/g

main reactor charge

fresh

used

fresh

used

Lewatit K1221 test samples Lewatit K1131 Lewatit K1431 Lewatit K2621 Purolite CT110 Relite CFS-H Ostion KSC-2

5.35

2.08

2.07

2.66

5.37 5.27 4.92 5.23 5.05 4.65

0.99 0.77 1.16 0.85 0.94 0.79

3.28 1.38 1.54 1.27 1.32 1.28

1.14 1.18 1.27 0.98 1.04 1.09

Figure 2. Comparison of swollen-state morphologies of sulfonated (water-swollen) and desulfonated (THF-swollen) domains in fresh and used ion exchanger catalysts (Lewatit K1221) utilized as the main charge of the reactor. 986

dx.doi.org/10.1021/ie3027718 | Ind. Eng. Chem. Res. 2013, 52, 985−989


Research Note

Figure 3. Comparison of swollen-state morphologies of sulfonated, water-swollen, and desulfonated, THF-swollen domains in fresh and used test samples of ion exchanger catalysts.

for characterization of the morphology of inorganic porous materials. The measurements were performed alternatively in an aqueous environment (0.2-N sodium sulfate solution) and in tetrahydrofuran (THF). In the former, only the sulfonated, hydrophilic parts of the polymer matrix swelled and the unfunctionalized domains collapsed and were “invisible,” whereas with THF as the mobile phase, the sulfonated part of the ion exchanger matrices did not swell in THF and the ISEC evaluation detected only the unsulfonated (or desulfonated) domains in the polymer matrix.10 The fresh, unused ion exchanger catalysts were essentially fully sulfonated and did not contain any domains that could swell in THF. Hence, all the fresh, unused ion exchangers demonstrated a zero porosity when examined by ISEC in THF. On the other

hand, ISEC examination of the used ion exchangers in THF pointed at the presence of a swollen polymer mass indicating desulfonation of a part of the polymer matrix. Figure 2 shows a comparison of the swollen state morphology of the fresh ion exchanger Lewatit K 1221 and that of a sample of the same ion exchanger after more than 12 years of service as the main reactor charge in the process of acrylic acid esterification. In the swollen-state morphology of the fresh Lewatit K1221, only water-swelled (sulfonated) medium-dense polymer matrix fractions were detected, characterized by densities between 0.4 and 0.8 nm/nm3. As already mentioned above, no unsulfonated polymer mass was able to swell in THF. After the prolonged exposure to the reactor environment, some THF-swelled domains appeared. They were the result of desulfonation of a part of the 987



Research Note

The compensation of the negative effect of the desulfonation by the positive effect of a less effective cross-linking significantly reduced the decline in catalytic activity with time. This is the reason that the extremely long lifetime of the reactor charge was possible. In the process of esterification of acrylic acid with methanol, the reaction temperature never exceeded 80 °C. Hence, it is improbable that the observed desulfonation could be of thermal origin. The fact that desulfonation was much more advanced in the test samples of the catalysts located in the reactor near the input (see Table 2) than in the main catalyst charge suggests that an admixture in the reactor feed was responsible for the desulfonation as well as for the reduced cross-linking. A similar effect has already been observed under laboratory conditions during a study of esterification of stearic acid dissolved in edible oil with methanol and catalyzed with a laboratory-prepared sulfonated gel-type ion exchanger catalyst cross-linked with 2% divinylbenzene.11 During a prolonged experiment at a relatively high flow rate of reaction mixture passing through the fixed-bed reactor, that is, 16 kg reaction mixture per gram of the catalyst, we observed a gradual increase of the swollen catalyst bed volume from 1.5 to 4.2 cm3. Moreover, after removing the catalyst from the reactor, its mass was found to have decreased to about 1/3 of the original amount. According to a microscopical examination of the catalyst removed from the reactor and swollen in water, the spherical beads were found to be undamaged but their average diameter was approximately 50% greater than the beads of the unused catalyst. Roughly, this corresponds to the observed increase of the catalyst bed volume during the catalytic experiment. This evident scission of the polymer network was believed to proceed via oxidative decomposition as described by Stahlbush and Strom12a supposition that was supported by the detection of reactive peroxidic compounds in the reaction mixture in the amount 200 μg/g of active oxygen as determined using the ASTM E299-08 method. The content of peroxidic compounds in the reaction mixture for acrylic acid esterification was not followed and hence, there was no positive proof that a similar oxidative mechanism operated here too. However, it seems to be the most probable hypothesis explaining the observed effects. To the best of our knowledge, the observed changes in ion exchanger catalysts after long exposure to the reaction environment have yet to be reported. This is possibly because, in other processes catalyzed with ion exchangers, there are effects of other deactivation processes, namely poisoning with metal ions or stronger bases. The acidic component of the esterification reaction mixture could prevent excessive accumulation of neutralizing ions in the catalyst bed. Nevertheless, when controlling processes catalyzed with ion exchangers, it would be useful to also take into account the possibility of the existence of this type of the catalyst transformation.

polymer matrix, and caused the density distribution to shift to somehow higher polymer chain densities than for the fresh catalyst. On the other hand, the swollen-state morphology of the sulfonated, water-swollen domains pointed at the presence of a substantial volume of much less dense and apparently less cross-linked polymer mass. This explains why during the emptying of the reactor and after washing with water the catalyst charge was observed to have swelled to greater volume as opposed to during the filling of the reactor. Such an increased swelling of the polymer and decrease of the polymer chain density in a part of the matrix was a clear indication of a less effective cross-linking of the polymer. Evidently, there occurred splitting of not only the carbon−sulfur bonds resulting in desulfonation, but also of some of the carbon−carbon bonds of the cross-links. The swollen-state morphologies of the test ion exchanger catalysts (Figure 3) were influenced by more extensive desulfonation than that of the main reactor charge, Lewatit K 1221 (see Table 2). A substantial amount of the desulfonated domains of the polymer matrix collapsed in water which limited the ability of the remaining sulfonated zones to expand in water. Reciprocally, swelling of the desulfonatd zones in THF was restricted by the collapse of the sulfonated zones in this environment. The catalytic activity of all the catalysts was tested by measuring the steady-state reaction rates of esterification of acrylic acid with methanol in the CSTR reactor. For each catalyst, measurements were performed at two conversion levels in the range 15−18 and 24−30%. The activity (reaction rate) ratios of the used and fresh catalysts was evaluated for each conversion level separately or from average values regardless that the conversion differed by no more than 5%. Table 3 presents a comparison Table 3. Comparison of the Catalytic Activities and Exchange Capacities of the Fresh and Used Catalysts average reaction ratea mmol/(h·g cat.)

parameters for the used catalysts in comparison with the fresh ones %

main reactor charge

fresh

used

catalytic activity

exchange capacity

Lewatit K1221 Test samples Lewatit K1131 Lewatit K1431 Lewatit K2621 Purolite CT110 Relite CFS-H Ostion KSC-2

64.2

37.5

58

39

49.9 31.8 38.9 36.6 46.2 40.6

8.6 10.2 7.8 7.3 11.5 9.8

17 32 20 20 25 24

18 15 24 16 19 17

The reaction rates are average values measured at 60 °C with acrylic acid/MeOH mixture in molar ratio 1:1 within the conversion range 15−30%. a

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of average reaction rates evaluated for the fresh and used catalysts with their exchange capacities. This comparison shows that the drop in catalytic activity after more than 12 years of exposure to the reaction environment was in almost all cases significantly lower than the diminution of the exchange capacity. It was especially evident for the main reactor charge, Lewatit K1221. An explanation for this was that the desulfonation diminishing the exchange capacity was accompanied by a splitting of certain cross-links between polymer chains. The resultant expansion of the polymer network improved the accessibility of the remaining catalytically active acid centers. This effect was particularly pronounced in the sample of the main reactor charge, Lewatit K1221, in which the increase of its swelling was quite dominant.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Chakrabarti, A.; Sharma, M. M. Cationic ion exchange resins as catalyst. React. Polym. 1993, 20, 1. (2) Brockwell, H. L.; Sarathy, P. R.; Trotta, R. Synthesize ethers. Hydrocarbon Process. 1991, 70, 133.

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Research Note

(3) Parra, D.; Izquierdo, J. F.; Cunill, F. Catalytic activity and deactivation of acidic ion-exchange resins in methyl tert-butyl ether liquid-phase synthesis. Ind. Eng. Chem. Res. 1998, 37, 3575. (4) Jeřab́ ek, K.; Setínek, K. Strong acidic ion exchangers structure by inverse steric exclusion chromatography. J. Polym. Sci., Part A 1990, 28, 1387. (5) Jeřab́ ek, K. Inverse steric exclusion chromatography as a tool for morphology characterization. In ACS Symposium Series 635; Potschka, M., Dubin, P. L., Eds.; American Chemical Society: Washington, DC, 1996; p 211. (6) Jeřab́ ek, K. Determination of pore volume distribution chromatography data from size exclusion chromatography data. Anal. Chem. 1985, 57, 1595. (7) Ogston, A. G. The spaces in a uniform random suspension of fibres. Trans. Faraday Soc. 1958, 54, 1754. (8) Laurent, J. C.; Klllander, J. J. A theory of gel filtration and its experimental verification. J. Chromatogr. 1964, 14, 317. (9) Jeřab́ ek, K. Characterization of swollen polymer gels using size exclusionchromatography. Anal. Chem. 1985, 57, 1598. (10) Jeřab́ ek, K.; Hanková, L.; Revillon, A. Functional polymers prepared from p-styrenesulfonyl chloride as the functional monomer. Ind. Eng. Chem. Res. 1995, 34, 2598. (11) Jeřab́ ek K. Institute of Chemical Process Fundamentals, Prague, 2008, unpublished results. (12) Stahlbush, J. R.; Strom, R. M. A decomposition mechanism for cation exchange resins. React. Polym. 1990, 13, 223.

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Changes in Ion Exchanger Catalysts after Extremely Long Exposure to

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