Partial Regeneration of Spent SAPO-34 Catalyst in Methanol to Olefins Process Via Steam Gasification

The SAPO-34-catalyzed MTO reaction,
working as a non-petrochemical route for producing the lower olefins, has been successfully industrialized
in China.¹ This technology adopts a fluidized bed
reactor-regenerator configuration, in which the continuous catalysts
circulation leads inevitably to the age distribution of catalysts in the
reactor and concomitantly uneven deposition of carbonaceous species onto individual
catalyst particle. This non-uniform coking behavior of catalysts would average,
and, to be more specific, lower down the overall ethene selectivity. Meanwhile,
a common feature of the rather lower initial ethene selectivity raises an issue
that the optimal operation window is too narrow to maximize light olefins for
industrial MTO operation. In this work we studied the partial regeneration of
the spent SAPO-34 catalyst via steam gasification, and showed that this method
could not only reactivate the spent SAPO-34 catalyst, but also prompt the
ethene selectivity and thus the lower olefins selectivity when compared to the partial
regeneration of spent catalyst by traditional air combustion.

Firstly, the
spent SAPO-34 catalyst was partially regenerated by steam gasification and air
combustion, and then the MTO performance was evaluated under the same operation
conditions in a fluidized bed reactor at atmospheric pressure. Figure 1a shows
the catalytic performance over both the partially regenerated catalysts with
nearly the same coke content (4 wt%). The selectivity of ethene over the
SAPO-34-AC-30 min sample was much greater than for fresh SAPO-34 sample but
lower than for SAPO-34-SG-120 min sample. That is, residue coke has a
significant positive impact on ethene formation but depends on the regeneration
gas. In view of this result, partial regeneration method is an efficient way
for controlling the activity-selectivity-deactivation. We also gave the
mechanistic understanding about the different catalytic performance. With several
characterizations results, e.g., TGA, FTIR, GC-MS, indicated that both partial
regeneration samples exhibited similar textual properties, while the SAPO-34-SG-120
min sample possessed lower acidity than the SAPO-34-AC-30 min sample. That is
to say, the difference between two partially regenerated SAPO-34 catalysts in
the catalytic performance in MTO reaction should be irrelevant to their textual
properties. It should be noted that the low acid density favors the selectivity
to propene and higher olefins.² Thus, the difference
between the acidities of both partially regenerated samples does not also
provide a fully satisfactory explanation for the difference in products
distribution. Therefore, the higher selectivity to ethene over the SAPO-34-SG-120
min sample in comparison with the SAPO-34-AC-30 min points to different
reaction mechanisms, as a result of the different property of the residual
coke. As shown in Figure 1b, for SAPO-34-SG-120 min sample, a broad
distribution of species ranging from methylated benzenes to methylated
naphthalenes. Typically, the oxygenated compounds are the dominant components
for SAPO-34-AC-30 min sample, with followed naphthalene and benzene. Methylated benzene and methylated naphthalene acting as active hydrocarbon pool
species are more selective to the formation of ethene than oxygenated compounds,
which has been reported in many literatures.^3-5

Due to the effect of residual coke on the reaction and
deactivation, control of the nature of the residual coke, also the coke content
is an interesting way of improving the MTO process. By doing so, the ethene
selectivity control will be no longer highly dependent on the age distribution
of catalysts, which will beneficially extend the optimal operation window for
industrial MTO operation. Additionally, steam gasification can beneficially transform
invaluable coke to high-valued
syngas (H₂ and CO) and simultaneously reduce greenhouse gas (CO₂)
emissions.

Figure 1. Catalytic performance (a) GC-MS
results (b) over fresh/spent and partially regeneration catalysts by air
combustion (AC) and steam gasification (SG) with nearly the same coke content
(wt 4%).

Reference

1.   Tian, P.; Wei, Y. X.; Ye, M.; Liu, Z. M. Methanol to olefins
(MTO): from fundamentals to commercialization. ACS Catal. 2015, 5,
1922−1938.

2.  
Batholomew, C. H.; Argyle. M. D. Advances in Catalyst Deactivation and
Regeneration. Catalysts. 2015, 5, 949-954.

3.   Liu,
Z.; Dong, X.; Liu, X.; Han, Y. Oxygen-containing coke species in zeolite-catalyzed
conversion of methanol to hydrocarbons. Cata Sci  Technol 2016,
6, 8157-8165.

4.   Wang, C.; Xu, J.; Qi, G.; Gong, Y.; Wang, W.; Gao,
P.; Wang, Q.; Feng, N.; Liu, X.; Deng, F. Methylbenzene hydrocarbon pool in
methanol-to-olefins conversion over zeolite H-ZSM-5. J. Catal. 2015,
332, 127-137.

5.   Song, W.; Fu, H.; Haw, J. F. Supramolecular origins
of product selectivity for methanol-to-olefin catalysis on HSAPO-34. J. Am.
Chem. Soc. 2001, 123, 4749.