Next generation PLOT alumina technology for accurate measurement of trace polar in streams

Jaap de Zeeuw, Tom Vezza, Bill Bromps, Rick Morehead and Gary Stidsen

Restek Corporation, Bellefonte, USA

1 Next generation PLOT alumina technology for accurate measurement of trace polar hydrocarbons in hydrocarbon streams

Jaap de Zeeuw, Tom Vezza, Bill Bromps, Rick More head and Gary Stidsen Restek Corporation

Summary In light hydrocarbon analysis, the separation and detection of traces of polar hydrocarbons like , propadiene and methyl acetylene is very important. Using commercial alumina columns with KCl or Na2SO4 deactivation, often results in low response of polar hydrocarbons. Additionally, challenges are observed in response-in time stability. Solutions have been proposed to maximize response for components like methyl acetylene and propadiene, using alumina columns that were specially deactivated. Operating such columns showed still several challenges: Due to different deactivations, the retention and loadability of such alumina columns has been drastically reduced.

A new Alumina deactivation technology was developed that combined the high response for polar hydrocarbon with maintaining the loadability. This allows the high response for components like methyl acetylene, acetylene and propadiene, also to be used for impurity analysis as well as TCD type applications.

Such columns also showed excellent stability of response in time, which was superior then existing solutions. Additionally, it was observed that such alumina columns could be used up to 250 C, extending the Tmax by 50C. This allows higher hydrocarbon elution, faster stabilization and also widens application scope of any GC where multiple columns are used. In this poster the data is presented showing the improvements made in this important application field.

Background In petrochemical arena analysis of volatile hydrocarbons and impurities has become a routine application. In the C1-C5 range many unsaturated hydrocarbons are present (, ) and also these have to be separated and quantified. As hydrocarbons are generally believed to be inert they will elute from a wide variety of stationary phases. Liquid type phases like 100 % polydimethyl siloxanes will separate the saturated hydrocarbons, but selectivity is not present for separating the unsaturated hydrocarbons. Highly selective materials have been developed for the 2 separation of unsaturated hydrocarbons and especially the adsorption chromatography has proven to be very effective. Alumina is one of the most selective materials and is highly valued for this. Most widely used is the aluminum oxide. Aluminum oxides separate all C1-C5 hydrocarbons but has to be deactivated. Special alumina columns have been developed for trace polar hydrocarbon analysis but these columns still do not perform as customer prefers. Biggest challenges are stability of response in time for reactive hydrocarbons c.q. propadiene and methyl acetylene. Also loadability is a challenge which can be improved. Restek developed a new Al2O3 adsorbent called Rt-Alumina BOND/MAPD that shows to be very promising for this application.

This new material is intensively deactivated and maximizes response for polar hydrocarbon, provide a constant response – in time -, and also offers higher retention/loadability and can also be used at temperatures higher then 200ºC.

Test materials and conditions Restek Rt-Alumina BOND / MAPD column (50m x 0.53mm ID x 10um);Varian Select Al2O3/MAPD column (50m x 0.53mm ID x 10um df), Part number – CP7432, S/N 6505409; A Restek Alumina BOND Na2SO4 column (50m x 0.53mm ID x 10um df) (P/N 19756, S/N 971892. Carrier Gas : Helium Sample : DCG custom gas standard S/N 441868 Split Injection : 5ul gas injection Split Vent Flow Rate : 80ml/min Injection Port Temp. : 200°C Detector Temp. : 200°C Oven Temp : 60°C, hold 4 minutes, 10°C/minute to 200°C, hold 5 minutes

Challenges with Present technology Alumina columns show best selectivity for separation of C1-C6 hydrocarbon . Depending on deactivation, the surface will behave more or less polar. Using Na2SO4 deactivation, results in a polar surface. The selectivity of this surface is better then the KCl (less polar) deactivation. It was observed that standard alumina columns showed a different response for polar hydrocarbon. Components like propadiene, acetylene and methyl acetylene showed strong variation in response. As these components have to be measured at low ppm levels, the resulting data was not reliable and better solutions were requested. Besides non reproducible responses, the Na2SO4 deactivated alumina columns also showed drift of response in time. Fig. 1 shows the response for propadiene relative to n- when repeated injections are done. The adsorption of polar compounds is related to the activity of the alumina used, and by deactivation one can make alumina surfaces more inert for the target analytes. One of the commercial solutions developed was the Al2O3/MAPD column.

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Figure 1. Development of response factor for propadiene / butane ratio Na2SO4 deactivated alumina during 50 repeated injections

With this technology a more intense deactivation was applied [ 1 ], making polar hydrocarbons elute with higher response factor. Practically the variation in response factor was improved, but still a linear decrease in time was observed, when repeated injections of hydrocarbon mixture was done, see figure 2. By changing deactivation, another challenge was developing: retention (and loadability) of the alumina columns was greatly reduced, which would directly impact the chromatography.

Figure 2: Development of response factor for propadiene / butane ratio of Agilent CP-Al2O3/MAPD during 50 repeated injections

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This resulted in lower sample capacity and more difficult method optimization as peaks were more affected by concentrations. Figure 3 shows a comparison of 50m x 0.53m Alumina - MAPD columns with similar dimensions. Columns were tested in same GC same temperature and same linear gas velocity. There is a big difference in retention/loadability which is in favor of the Rt-Alumina BOND / MAPD.

New technology Restek developed a new alumina based column, with a new deactivation technology that maintains the absolute retention characteristics of alumina columns, but shows a clear improved stability for polar hydrocarbon response, compared with present commercial solutions. The new column is called Rt-Alumina BOND / MAPD and it has comparable retention/loadability as is obtained with current alumina columns, but it will show maximal responses for polar hydrocarbons. With the deactivation applied, we tried to create a polar surface as these surfaces are preferred. The new Rt-Alumina BOND / MAPD has a slightly lower polarity, but can practically be used for all separation challenges.

The main differences are:

- Constant response for polar hydrocarbons in time ( propadiene, acetylene, methyl-acetylene); - High loadability (peaks stay more symmetrical when level increases); - Extended temperature stability: column is useable up to 250 ºC

Loadability In Gas-solid chromatography, phase overload results in strong tailing peaks. Adsorption surfaces have an additional challenge. It’s not only the amount of active sites that are available for distribution, but also the level of “activity distribution” of the active sites. Ideally all active sites show similar activity, but in reality, there always is a distribution of activity. This is defined by the nature of the alumina, its specific area, impurities, and form how it is used in the PLOT column. That is also why we find differences in behavior between alumina columns.

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Figure 3: Comparison of absolute retention of 50m x 0.53mm Alumina – MAPD columns; Columns tested under identical conditions.

Fig. 3 shows a comparison of 50m x 0.53m Alumina - MAPD columns with similar dimensions. Columns were tested in same GC, same temperature and same linear gas velocity. There is a big difference in retention which is in favor of the Rt-AluminaBOND / MAPD, having almost 2.5 times higher retention. This translates immediately in a higher loadability, which especially affects the polar hydrocarbons.

Impact of loadability on peak shape was tested under 2 types of conditions:

Series A: When the components elute with a comparable retention factor. The Rt-Alumina BOND / MAPD columns was tested at 130 ºC and the CP-Al2O3/MAPD was tested at 100ºC; These conditions also simulate what will practically happen when both columns are used under temperature programmed analysis.

Series B: Comparing the loadability of both columns at the same temperature. This is a different comparison as here we do not have the extra tailing caused by the extra surface activity that is developed at lower test temperature.

6 Series A: Figure 4 shows the peak shape for acetylene, propadiene and butane for both columns. The CP-Al2O3/MAPD elutes the propadiene and acetylene after butane because of the lower oven temperature; As expected there is a big difference in peak symmetry in favor for the new alumina technology.

Figure 4: Overlay of peak shape between MAPD columns. Zoom into peak shape of butane, acetylene and propadiene. To get comparable retention factors, the Rt-Alumina BOND / MAPD was tested at 130ºC; The CP-Al2O3/MAPD was tested at 100ºC. Data see table 1;

The differences are even bigger when testing is done for 1,3 and methyl acetylene, see figure 5. Also note that the retention times are strongly impacted on the amount of component injected. Data of measurements is listed in table 1.

Figure 5: Overlay of peak shape between MAPD columns. Zoom into peak shape for 1,3 butadiene and methyl acetylene. To get comparable retention factors, the Rt-Alumina BOND / MAPD was tested at 130ºC; The CP-Al2O3/MAPD was tested at 100ºC. Data: see table 1;

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Table 1: Retention and area data measured for figures 4 and 5

Restek MAPD (130°C) Tailing Factor (USP) Sample Size (ul) Propadiene Acetylene Methyl Acetylene 5 1.038 1.227 1.083 10 1.040 1.219 1.130 25 1.058 1.248 1.216 50 1.085 1.292 1.388 100 1.094 1.316 1.546 250 1.177 1.481 2.224

Varian MAPD (100°C) Tailing Factor (USP) Sample Size (ul) Propadiene Acetylene Methyl Acetylene 5 1.073 1.298 1.908 10 1.098 1.478 2.743 25 1.165 1.902 4.555 50 1.304 2.580 6.871 100 1.448 3.241 9.208 250 1.979 4.882 15.476

Series B: comparing peak symmetry at the same temperatures. The effect on peak shape will be lower here as peak symmetry in gas-solid chromatography is also impacted by temperature. Figure 6 shows the results. Besides that the peaks have a better symmetry on the Rt-Alumina BOND / MAPD, also the impact on retention time is still clearly much bigger with the conventional technology.

Practically one has to deal mostly with situations as shown in fig.5 as with low retentive columns, the components will elute at lower temperature and peak shape/loadbility will be accordingly.

Figure 6: Overlay of different injected amounts and peak shape between MAPD columns, both columns tested at 130ºC Zoom into peak shape for 1,3 butadiene and methyl acetylene.

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Response for polar hydrocarbons Polar hydrocarbons are often measured at low levels and a high response will benefit sensitivity and detection limit. The response of polar hydrocarbons on PLOT columns has always been a challenge as typical responses are always lower as main objective was to get best possible selectivity. Although eluting peaks are often symmetrical, the absolute response is difficult to predict. Often a calibration is “column-specific”. This is not ideal, but practically its workable, because once implemented, the PLOT column has a very long life time. The new alumina technology, maintain the loadability and produces also high response factors for propadiene, acetylene and methyl acetylene. Figure 7 shows an expanded area showing the response and selectivity for acetylene and propadiene. This response remains also the same when repeated samples are analyzed, simplifying the calibrations.

Fig. 8 shows the stability of response in time for propadiene and methyl acetylene after 50 repetitive injections. The response for both analytes is stable. The same behavior was observed for acetylene.

Figure 7: Response for propadiene and acetylene on Rt-Alumina BOND / MAPD. Both components generate high response, maximizing the signal in trace analysis. Courtesy: Jos Curvers, Bruker, Goes, The Netherlands

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Figure 8: Development of response factor for propadiene and methyl acetylene with new Rt-Alumina BOND / MAPD, during 50 repeated injections all run under identical conditions. Response is very constant which greatly reduces calibration frequency.

Extended maximum Temperature of 250 ºC Alumina PLOT columns typically have a maximum temperature of 200 ºC. Above this temperature, there usually is a systematical shift of polar hydrocarbons. The alumina used for the new MAPD reacts quite differently. We already observed lower reactivity towards halogenated hydrocarbons as well as . We were able to extend the maximum temperature for the Rt-Alumina BOND / MAPD columns to 250ºC.

This allows: o Faster elution of higher hydrocarbons; Extending range of hydrocarbons to C12; o Faster regeneration as adsorbed water can be more easily removed; o If a second column is used in the same oven, it can be operated also at 250C; o Shorter conditioning times required, faster stabilization; o Detection systems can be operated at higher temperatures;

Applications The new Rt-Alumina BOND / MAPD can be used for all common hydrocarbon separations varying from compositional analysis to trace / impurity analysis in , propylene and butylene and butadiene streams. Fig. 9 shows an RGA mixture on the same scale as a crude 1,3 butadiene. Fig. 10 shows a fast RGA analysis, using a 30 m x 0.32mm column. This column was operated at very high optimal gas velocity and an oven program of 50ºC/min. This can only be done in GC’s that are capable to keeping up with such rates. If only 110V is available, one can consider reducing the oven size by using an oven-insert. Fig. 11 shows impurities in 1,2 butadiene. 1,2 butadiene is known to be very reactive on most alumina columns. The alumina surface used for the MAPD columns shows no sign of reactivity. The new technology was successfully applied with all current fused silica column dimensions and diameters.

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Fig. 9: Comparison of the elution profile of crude butadiene with RGA standard; Conditions: Column: 50m x 0.53mm Rt- Alumina BOND / MAPD; Carrier Gas: He, Sample Size: 5ul loop; Split Injection, 45ml/min; Injection Port liner – 2mm straight liner (P/N 20712); Injection Port Temp: 200°C; Detector Temp.: 200°C; Oven Temp: 70°C (hold 5 min), 10°C/min to 200°C (hold10 min), Helium @ 20psi

Fig. 10 Fast analysis of RGA components using 30m x 0.32mm Rt-AluminaBOND / MAPD; Oven : 80C, 0.5 min => 200C, 50C/min; Carrier : H2 , 200kPa; Split: 80 mL/min; Sample : RGA mixture

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Figure 11: Impurities in 1,2-butadiene. Column: 50m x 0.32mm Rt-Alumina BOND / MAPD; Oven: 100C; GC: 5880 Carrier : H2 , 120kPa; Split: 80 mL/min; Sample : 1,2 butadiene

Conclusion A new alumina column is developed that shows to be very promising for light hydrocarbon separations and especially for measuring polar hydrocarbons like propadiene, acetylene and methyl acetylene. By selecting the optimal alumina surface and deactivation, it was possible to maintain the loadability and retention, which is in contrast with commercial solutions. Additionally the new Rt-Alumina BOND /MAPD columns show constant response factors for propadiene, acetylene and methyl acetylene –in time, which is also a big improvement. Lastly we were able to extend the maximum operation temperature of the alumina sorbent from 200 to 250 ºC, allowing this column technology to be used in a much broader application arena.

For the reasons above, the Rt-Alumina BOND MAPD will be a very welcome substitute for many types of commercially available alumina PLOT columns.

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Available Alumina Bond/MAPD columns

19777 Rt-Alumina Bond/MAPD 30m, 0.53mm ID, 10um 19778 Rt-Alumina Bond/MAPD 50m, 0.53mm ID, 10um 19779 Rt-Alumina Bond/MAPD 30m, 0.32mm ID, 5um 19780 Rt-Alumina Bond/MAPD 50m, 0.32mm ID, 5um 79728 MXT-Alumina Bond/MAPD 30m, 0.53mm ID, 10um

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