Methods for Carbon Mass Closure in Polyolefin Hydrocracking

Heterogeneous catalytic
hydrocracking of polyolefins is a promising
approach for the processing of postconsumer plastics, but product
quantification methods remain inconsistent across the literature.
In systems that generate a large fraction of vapor-phase products,
typical product capture methods can result in large carbon balance
deficits, exceeding 50%, compromising reported yields and selectivities.
Here, we identify the major sources of product loss and develop enhanced
capture methods to improve the quantification accuracy. Seven supplemental
techniques were evaluated, targeting either increased vapor recovery
(by increasing the volatility or system volume) or enhanced retention
in the liquid phase (by decreasing volatility). Among these, a flow
collection approach using a continuous helium sweep and downstream
gas sampling bag capture yielded the highest recovery, achieving a
96 ± 9.2% carbon balance closure. We show that the efficacy of
these methods is strongly dependent on product distribution. In general,
solvent addition was most effective when condensable species dominate
the product distribution, while flow collection was preferred when
both condensable species and light gases are present in high concentrations.
These results highlight the need for method-specific workup strategies
and demonstrate that no single protocol is universally optimal. We
provide general guidelines for selecting and implementing robust product
capture techniques, enabling accurate yield and selectivity determinations
in polyolefin hydrocracking systems.