The_Catalyst_Review_June_2023 - 10

SPECIAL FEATURE
Future research on waste rubber is needed to increase oil yield. It should focus on dealing with the increasing complexity of modern
tires while optimizing the key variables, notably temperature, heating rate, reactor type, and rubber particle size.
Textile Waste Conversion
Discarded clothing, typically comprised of polyesters and polyethylene terephthalate (PET), represents a substantial proportion of
textile waste. A number of PET recycling technologies are known and are close to commercialization. Polyamides may be solventbased,
while selective dissolution is the most promising method for separating cellulose from other components in polycottons.
Solvents play a crucial role in recycling, but many are toxic and are subject to restrictions in use. Catalysts are not widely used
in textile recycling, but when employed, they have significantly affected the efficiency and selectivity of depolymerization
processes. Ethylene glycol is the most common depolymerization agent as it generates bis-(2-hydroxyethyl) terephthalate (BHET)
depolymerization product. BHET can then be easily polymerized to regenerate PET. In addition, Depolymerization by alcohols such
as methanol can yield dimethyl terephthalate and ethylene glycol, which can subsequently be polymerized to make PET.
Catalysts are not as widely used in textile recycling as in some other areas of waste valorization. Still, when used, they have often
significantly affected the efficiency and selectivity of depolymerization processes. For example, high depolymerization yields are
achieved using zinc acetate.
Several significant challenges remain. While large volumes of textile waste are available at producer sites, there is a need to solve
the logistical challenges associated with consumer waste. In addition, the high cellulose content of various textiles makes them a
renewable feedstock for making platform chemicals, a pathway now being explored by Industry. Increased uses of catalysts and
the employment of better catalysts will play a role in developing this field, especially for the more targeted valorizations such as
producing valuable fuels and platform chemicals.
Organic Waste (Excluding Plastics) Conversion
Readily abundant biowastes are a natural source of numerous noteworthy and valuable chemicals which can be obtained using bulk
biomass extraction or decomposition processes. Currently, most biowaste streams are being valorized as energy sources in pyrolysis.
However, it has been found that adding catalysts to pyrolysis processes reduces the decomposition temperature and presents the
possibility of producing valuable chemicals rather than fuels. For example, using a low-cost catalyst in the flash pyrolysis of woody
waste yields high-value chemicals, including levoglucosenone.
In some cases, liquid catalysts can be replaced by solid catalysts, which offer easier handling and separation and a greater chance of
catalyst reuse, as has been demonstrated by transesterifying food fats and vegetables with methanol to yield biodiesel fuels.
Enzyme catalysis has been found to be an appropriate technology for converting food waste into a valuable fuel such as diesel.
Lipase is commonly used in the transesterification process, but this is a much more expensive catalyst option than the bases (or
acids) used in most industrial processes. Lipase can also be made easier to recover and reuse through immobilization. Another type
of catalyzed conversion of food wastes into fuels involves thermo-catalytic reforming (TCR), which gives a mixture of oil, char, and
(mostly) gas.
A number of conversion technologies are available for single compounds or mixtures that can be further refined downstream, often
including hydrogenation to reduce unwanted functionality. Some processes are designed for crude feedstocks such as sawdust or
mixtures of food waste, although these are best considered as a final option when all the valuable extracts have been removed.
Separating the primary components of biomass can improve downstream processing selectivity, a finding demonstrated by
removing cellulose in the fermentative conversion to bioethanol or other chemicals. Exciting developments in this area include the
dual technology approach of following feedstock fermentation with pyrolysis leading to improved quality products primarily for the
manufacture of liquid fuels.
Conclusions
Catalysis plays an increasingly significant role in the four types of waste streams discussed in this report since it can lower the
biowaste decomposition temperature and simplify the downstream chemistry. Other techniques, such as microwave treatment, may
also enable this. Catalysts are also needed in post-decomposition processes such as hydrogenation that add value by reducing
unwanted functionalization, notably oxygen-based, and removing C-C double bonds. In this way, converting more abundant biowastes
into higher-value product chemical products will help feed the growing market demand for bio-based chemical production.
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The Catalyst Review
June 2023

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