High Throughput Testing of Catalysts with Different Time Scales of Deactivation for Methanol-to-Hydrocarbons (MTH)

The MTH reaction has been thoroughly investigated with current state-of-the-art catalysts based on ZSM-5 (slow deactivation) and SAPO-34 (fast deactivation) and represents a challenging application for parallel testing of catalysts.^1,2 This challenge was met by appropriate test protocols for both types of catalysts and an advanced analytical setup to detect complex multi-component products in connection with a fully automated data evaluation.

For fast deactivating catalysts such as SAPO-34, a low severity protocol (low MeOH partial pressure, low WHSV) was developed which allows for the formation of all secondary and tertiary stable products at full conversion. Subsequent band aging until MeOH and DME breakthrough delivers additional information on the behavior of catalysts at partial conversion under kinetic control. Slow deactivation of ZSM-5 materials on the other hand requires much higher severity (high MeOH partial pressure, high WHSV) to observe partial conversion in a reasonable time on stream (TOS).

The common variable to compare activity, selectivity and deactivation of both protocols is cumulative MeOH converted on zeolite and the activity, selectivity and deactivation can be differentiated as a function of cumulative MeOH converted on zeolite and reaction parameters (temperature, WHSV, MeOH partial pressure). This is demonstrated in Figure 1, in which the effect of using TOS or cumulative MeOH converted on zeolite as an x-axis is shown for the deactivation curves and olefin yields of various zeolites and mesoporous materials (grey data points) in the MTH reaction in comparison with state-of-the-art catalysts SAPO-34 and ZSM-5.

Figure 1. Breakthrough curves for MTH conversion versus TOS (top) and versus cumulated carbon on zeolite (middle). The olefin yields versus cumulated carbon on zeolite are shown in the bottom chart. The reference catalysts SAPO-34 (green circles) and ZSM-5 (orange squares under low severity conditions / red diamonds under high severity conditions). Transparent data points demonstrate how different catalysts can be differentiated by high throughput catalyst testing.

  

Figure 2. Product yields in carbon percent for SAPO-34 (left) and ZSM-5 (right) for low nad high severity conditions, respectively. From top to bottom, the product yield is shown for different TOS events such as the initial activity at startup (top), the maximum olefin yield (middle) shortly before breakthrough of MEOH and after breakthrough (bottom)

While the MeOH breakthrough under low severity conditions occurs at 13 h TOS for SAPO-34, partial conversion for ZSM-5 is not observed up to 1900 h TOS. Utilizing high severity conditions, the breakthrough of ZSM-5 can be shifted to 75 h TOS.  In the middle and lower part of Figure 1 the conversion and olefin yields are plotted versus cumulated carbon on zeolite, which can be obtained by multiplication of TOS and WHSV. It can be seen that the olefin yield under high severity conditions is in line with the low severity protocol with the advantage of reducing the screening time from > 1900 h TOS to 75 h TOS. Maximum olefin yield is obtained shortly before breakthrough of MeOH for both ZSM-5 and SAPO-34. Each data point contains detailed information on PIANO distribution based on product yields of 95 components. In Figure 2, these product yields are shown for selected TOS events SAPO-34 under low and ZSM-5 under high severity conditions.

It can be concluded that catalysts for fast deactivating systems such as MTH can be successfully tested in parallel fixed-bed reactors and it will be shown that catalysts can be precisely characterized and differentiated by activity, selectivity and deactivation.

 

References

1.         C. D. Chang, Catal. Rev.-Sci. Eng. 25 (1983) 1.

2.         U. Olsbye, , S. Svelle, M. Bjørgen, P. Beato, T. V. W. Janssens, F. Joensen, S. Bordiga, K. P. Lillerud, Angew. Chem. Int. Ed. 51 (2012) 5810.