Chemicals maker TPC Group on Monday said it received a buyout proposal from Innospec worth as much as $721 million, topping an offer from two private equity firms made in August.

Private equity firms First Reserve and SK Capital Partners in August had offered $627.2 million to acquire TPC.

The deals pit the private equity industry's desire to expand into a high-margin sector against a chemical industry veteran's attempt to expand.

TPC's main product is butadiene, used to make synthetic rubber for tires and other automotive products. But weak demand has pressured its business this year as prices for butadiene have fallen sharply, causing a trough in TPC's market.

Innospec, a specialty chemical maker, could offer $44 to $46 per share in cash, TPC said in a statement on Monday.

First Reserve and SK Capital offered $40 a share in late August.

The shares have consistently traded above the $40 price offered by the private equity firms, and at least two of TPC's top-10 shareholders have said they will vote against the deal.

TPC said it will consider the non-binding proposal from Innospec. It previously said a transaction with First Reserve and SK Capital Partners would be in the best interest of shareholders.

Perella Weinberg Partners is advising TPC.

Blackstone is providing equity financing for Innospec.